Doctoral Thesis

Synthesis, Properties of Graphene Oxide, Graphene Oxide
Quantum Dots, and Its Composite: A Cesium Detector for
Environmental Monitoring(࢐ԿήϧϓΥϱɼ࢐ԿήϧϓΥϱྖࢢχρφɼͨ͹෵߻ର͹߻੔ͳ੓࣯ʁ
‫ڧ؂‬ϠωνϨϱή͹ͪΌ͹ιεΤϞ‫ݗ‬ड़‫)ح‬BANGUN SATRIO NUGROHOD192886(Submitted to the Hiroshima University, in fulfilment of therequirements for the degree of Doctor of Philoshopy)

DEPARTMENT OF CHEMISTRYGRADUATE SCHOOL OF SCIENCEHIROSHIMA UNIVERSITY
2022

Keywords
Graphene oxide, Graphene oxide quantum dots, Oxidation debris, Dawson-type
Polyoxometalate, Nanocomposite, Cesium, Radionuclides, Cesium green molecule,
Reflux method, Modified Hummer`s method, C/O ratio, sp2/sp3 carbon domain,
Photoluminescence, Detection system, Fukushima Daiichi Nuclear Power Plant Accident,
Synthesis, Adsorption performance.

iiAbstract
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in
March, 2011, caused a substantial amount of radioactive materials released to theenvironment. Among the radioactive materials released to the environment, radiocesium(134Cs and 137Cs) beside radioiodine (131I) is the most major released volatile radionuclide
having a direct impact on land contamination. The radioactivity can be measured using
radiation measurement techniques. In the present PhD thesis, I tried to captureradiocesium and also tried to form a cesium detection system. Background and objectiveof research are given in detail in Chapter 1.
Chapter 2 shows the Exploration of the Cs Trapping Phenomenon by Combining
Graphene Oxide with α-K6P2W18O62 as Nanocomposite. Graphene oxide (GO) surface has
an interesting interaction using a polar -COO(H) and non-polar (-CH3) group in order toadsorb different kinds of cations (charge, size, complexing capability) by a proposed ionbridging mechanism. In addition, Polyoxometalates (POMs) provide an excellent, robust,and discrete material, which has reversible multi-electron redox properties. Some
researchers have employed POMs as a potential Cs adsorption material by using a cationexchange mechanism. Considering the tremendous properties of GO and α-K6P2W18O62,
it is highly desirable to synthesize a new nanocomposite with Cs adsorption ability using
the GO and POMs. In this chapter, two types of GO samples were employed. One is GO,
which has a large amount of carbon (C), and the other is GO, which has a large amountof oxygen (O). They have a variant sp2/sp3 structure. These GOs can be produced bycontrolling the oxidation degree, which depends on several factors such as the reaction
condition, oxidizing agent, and graphite source. It has been known that the GO structure
is not precisely determined because of a variety of local arrangements of functionalgroups. Each GO sample was incorporated with α-K6P2W18O62. I investigated theinteraction of Cs+ with the nanocomposite by characterizing the surface structure of theiii

composite material and also by considering the adsorption capacity of the nanocomposite
for radioactive Cs. Each property in each material is expected to synergisticallystrengthen the adsorption capacity of the nanocomposite. A high Cs adsorption capacity
was clearly achieved by using the nanocomposite.
Chapter 3 shows the improvement of Cs detection performance and formation of
CsCl and Cs nanoparticles by tuning graphene oxide quantum dot-based nanocomposite.
In recent years, graphene oxide (GO) and graphene oxide quantum dots (GOQDs) are
widely studied in several applications such as metal adsorbent (Cs+, Eu3+, Sr2+) , anddesigned as fluorescence probes by considering their quantum confinement and the edge
effect. In addition, the GO and GOQDs have almost similar properties because they havevarious oxygen functional groups. The functional group can act as a binding point tosynthesize a new nanocomposite to improve their properties. Another important thing isthat the photoluminescence (PL) of GOQDs was independent of pH. It means that theemission wavelength does not shift in different pH condition (only change in the PL
intensity). Further investigation also revealed that the PL of GO has shown a reversible
response to ionic strength and pH. The previous work confirmed that the different
number of layers of GO (single and a few layers) showed different optical responses invarious organic solvent. More currently, Yao et al. have employed graphene quantum
dot-based fluorescence sensing as a dual detector of copper ion (Cu2+) and tiopronin(MPG). They proposed a novel fluorescence turn OFF for the Cu2+ detection and turn ON
for the tiopronin (MPG) detection. Therefore, to facilitate the development of a Cs
detection system, it is highly desirable to create a new nanocomposite using
functionalized graphene oxide quantum dots (GOQDs) with cesium green molecule. In
Chapter 3, I produced a new nanocomposite that has the ability to recognize Cs in waterwith different pH conditions (acidic and basic condition) by considering the turn ON/OFF
response.

iv

Chapter 4 shows the general conclusions. The present findings have provided thefundamental understanding of specific characteristic and structural diversity of GO andits nanocomposite. In particular, the C/O ratio and the GO acidity (influenced by theoxidation debris particle) of the GO sample have significantly influenced the adsorption
capacity, the sensitivity and the accuracy of the detection system of the GO-based
nanocomposite. It can be used as an entry point to design the structure and the function
system for a new advanced nanocomposite. And, the present study offers a bridge
between GO and other potential materials. Also, this work may open an application inenvironmental protection with more deep investigation.

v

List of Publications

1. Potentiality of graphene oxide and polyoxometalate as radionuclide adsorbent
to restore the environment after Fukushima disaster: A mini review.
Bangun Satrio Nugroho, Muh. Nur Khoiru Wihadi, Fabian Grote, Siegfried Eigler,
Satoru Nakashima.
Indones. J. Chem., 2021, 21 (3), 776-786.
DOI: 10.22146/ijc.60493

2. Exploration of Cs trapping phenomenon by combining graphene oxide withK6P2O18W62 as Nanocomposite.
Bangun Satrio Nugroho, Akane kato, Chie Kowa, Tomoya Nakashima, Atsushi
Wada, Muh. Nur Khoiru Wihadi, Satoru Nakashima.
Materials, 2021, 14, 5577.
DOI: 10.3390/ma14195577

3. Improvement of Cs detection performance and formation of CsCl and Cs
nanoparticles by tuning graphene oxide quantum dot-based nanocomposite
Bangun Satrio Nugroho and Satoru Nakashima.
RSC advances, 2022.
DOI: 10.1039/d2ra02091bvi

Acknowledgement
It is with a deep sense of gratitude I would like to greatly thank Prof. Satoru Nakashima
as a supervisor for the encouragement, assistance, personal guidance, remarkable advice
and suggestions enabling me to proceed with my study from zero until the finish. Fiveyears Ph.D program was so challenging. Your valuable knowledge and your attitude
have thought me a lot for research work and provided an excellent basis for life. Under
your guidance, I feel like become a new person.
Most importantly, I also would like to greatly thank (dedicated this work) my beautiful
family for your love, kindness, and your duas. My dear Lia Kumala Dewi (my wife) who
struggled a lot from zero until now, my love Ghaisani Dhuha Sakanti (my daughter), mylove Athawakil Muhammad Cahayanu (my son), my beloved parents, mbah kung, mbahuti, yangkung (alm.), yangti, my sister (Radiati moviana, Mighfar khamida), pakde boy(mas Wiwid), mba Corry, you all are amazing. May every tear that has ever fallen fromyour eyes on my behalf become a river for you in paradise.
I would like to acknowledge the financial support of Phoenix Leader Education Program(for the scholarship), Chemistry department, Hiroshima University. I also acknowledge
the OSIMO Foundation (for the scholarship) and YAMAMOTO company (being a goodworking place for part time job) for the support within 5-years Ph.D program.
A word of thanks should be expressed to my colleagues (chemistry members and LP
members), Triyono Basuki, Habibur Rahman, Heri Wijayanto for helping out in every
single challenged during my study and being my remarkable family.
I also indebted to Dr. Makoto Maeda (Natural Science Centre for Basic Research andDevelopment, Hiroshima University) for the technical support and discussion during theTEM measurement all the time.
I would also like to thank Dr. Takaaki Taniguchi, Dr. Leanddas Nurdiwijayanto as amentor and for the discussion. This work was performed in part by National Institute forMaterials Science internship program (NIMS, Tsukuba, Japan).
I would also like to thank my collaborator Dr. Muh. Nur. Khoiru Wihadi, Prof. Siegfried
Eigler, Fabian Grote, Akane kato, Chie Kowa, Tomoya Nakashima, Atsushi Wada for
their contributions to the publication.
I also would like to thank Dr. Tuswadi, (alm.) Prof. Trijaka, Prof. Jabri, Pak Akbar Dharma
Wahana, who are strongly supported continuing this 5-years Ph.D. program. Finally, my
special appreciation and gratitude goes to Pak Aryo, Bude Donna and family for yourkind help, all the fun we shared, and being remarkable neighborhood.
vii

Table of Contents
Chapter I
1.1.
1.2.
1.3.
Chapter II

2.1.
2.2.

2.3.

General introduction …………………………………………………………………..
Background of study …………………………………………………………………..
The purpose of study …………………………………………………………………..
Reference…………………………………………………………………..
Exploration of Cs trapping phenomenon by combining graphene
oxide
(GO)
with
K6P2O18W62
as
Nanocomposite………………..........................................................................................
Introduction…………………………………
Materials and method…………………………………
2.2.1. Materials…………………………………
2.2.2. Characterizations………………………………...
2.2.3. Synthesis of GO…………………………………
…………………………………
2.2.4. Synthesis of D-K6P2O18W62
2.2.5. Synthesis of GO-POM composite…………………………………
2.2.6. Adsorption experiment…………………………………
Result and Discussion…………………………………
2.3.1. Elemental analysis…………………………………
2.3.2. FT-IR spectra…………………………………
2.3.3. Morphology of GO…………………………………
2.3.4. Morphology of GO-POM composite …………………………………
and its EDS maps
2.3.5. Powder X-ray Diffraction Pattern…………………………………
2.3.6. Raman spectroscopy…………………………………
2.3.7. Cs Adsorption performance…………………………………
3.7.1. Adsorption efficiency and …………………………………
Adsorption capacity of GO andPOM
3.7.2. Adsorption efficiency and …………………………………
Adsorption capacity of GOPOM Nanocomposite

1
1
10
10

16
16
17
17
18
18
19
19
20
20
20
21
22
23
24
25
26
26

30

2.4.

Conclusions…………………………………

36

2.5.

References…………………………………

38

2.6.

Supplementary data…………………………………

44

viii

Chapter III Improvement of Cs detection performance and formation of CsCl 59
and Cs nanoparticles by tuning graphene oxide quantum dot-based
nanocomposite …………………………………………………………………………….
3.1.

Introduction……………………..

59

3.2.

Experimental section……………………..

60

3.2.1. Reagents and materials……………………..

60

3.2.2. Synthesis of GO-GOQDs……………………..

60

3.2.3. Synthesis of GOQD-based nanocomposite……………………..

61

3.2.4. Photoluminescence detection of cesium……………………..

61

3.2.5. Instrumentation and Characterizations……………………..

61

Results and discussion……………………..

62

3.3.1. Characterization of GO and GOQDs……………………..

62

3.3.2. Optical properties of GOQDs and their ……………………..
nanocomposite
3.3.3. Cs detection performance……………………..

63

3.3.

66

3.3.1. Cs detection in acidic condition……………………..

66

3.3.2. Cs detection in basic condition……………………..

69

3.3.3. The detection behaviour of Sr2+, K+……………………..

70
72

3.4.

3.3.4. Cs metal formation on GOQDs and ……………………..
nanocomposite layer
Conclusions……………………..

3.5.

References……………………..

78

3.6.

Supplementary data……………………..

83

Chapter IV General conclusions……………………..

95

ix

76

Chapter I. General Introduction
1.1 Background of study
The Fukushima Daiichi nuclear power plant (FDNPP) disaster (March 2011)
affected the biotic and abiotic environment. FDNPP accident released radioactive
elements that possess long half-lives (30 years for cesium (137Cs)), β-decay to barium(137Ba), and emit γ-radiation. It has been reported that the soil present around FDNPP and
the soil in the neighboring prefectures were contaminated with approximately 100,000
MBq/km2 and 10,000 MBq/km2 of 137Cs, respectively [1]. The soil surface area was the mostcontaminated [2]. It has been predicted that several radionuclides, such as cesium (137Cs,
Cs, 134Cs), strontium (90Sr, 89Sr), uranium (UO2+), iodine (131I), thorium (90Th), barium

136(140Ba), lanthanum (140La), and tritium (3H) are present. Ananalysis by Tokyo ElectricPower Company (TEPCO), an electric power company, revealed that a major part of thefuel (in unit 1 nuclear reactor) was present in its molten state [3]. The radioactive isotopes
released during the Fukushima accident and predominantly responsible for the observed
radioactivity were the radioactive isotopes of Sr and Cs. It has been reported that Sr
exhibits a faster rate of migration compared to other elements [4]. It has been observedthat Cs (137Cs, 136Cs, 134Cs) and 131I become the more radioactive elements that werereleased from the fuel if compared with Sr (90Sr, 89Sr) [5-6].
It is necessary to estimate the quantity of radionuclides and chemicals released
during the accident to determine the immediate and long-term effects of radioactivity on
the environment and health of organisms. Researchers have studied the mobility ofradioactivity, especially the uptake of radioactive nuclides by plants [7]. The uptake ofradioactive nuclides by plants can potentially lead to food contamination that caneventually lead to genome damage or cancer in human beings. Further investigation also
reported that slightly high levels of radioactive cesium were detected in some rice
varieties grown in areas approximately 60 km northwest of the nuclear reactor plant, a
few years after the accident in Fukushima City [8]. Researchers have reported that thetransfer factor (from the soil to the rice) of radioactive cesium depended on the oxidizingor reducing soil atmosphere [9]. The penetration depth (in soil) of radiocesium is
influenced by soil characteristics (such as the particle size, organic carbon content, andclay contents) [10]. One of the key factors influencing the penetration depth ofradiocesium in the soil is the amount of medium-sized sand particles present in the soil
1[11]. Moreover, an investigation was carried out with soil samples collected from the areaaround the Hibara Lake (Yama gun, Fukushima prefecture), situated 100 km away from
FDNPP, revealing that radionuclides (originating from the Fukushima prefecture) were
present in the sediments deposited in the shallow areas of the lake. 134Cs and 137Cs isotopes
were detected (4.5 and 5.2 Bq/kg, respectively) in the soil of these areas. The results
revealed the long-distance migration of radioactive cesium. The high solubility ofradiocesium in water assists soil penetration. Compared to the process of direct
deposition of the nuclides (originating from the exhaust of the accident), the process of
nuclide transport by rivers into lakes (and its subsequent sedimentation) is more likely
to occur [12]. Shizuma et al. investigated the quality of drinking water in the Haramachi
district (Minamisoma City). The authors detected dissolved 137Cs in tap water andgroundwater. The maximum concentration of 137Cs was approximately 300 mBq/L in
August 2013 and 27 mBq/L in September 2014 [13]. The degree of contamination was not
solely dependent on the distance of the place under observation from the area of thenuclear accident. Other factors, such as weather conditions (such as wind direction andprecipitation), also affected the degree of soil contamination [14].
These results revealed that radionuclide contamination, which can potentially
impact human lives, is a severe environmental problem. The contamination had spread
far and wide, away from the Fukushima nuclear power plant. Therefore, it is necessary
to decontaminate the environment effectively. A few decontamination strategies, such as
removing the litter layer and superficial soil layer in the forests, have already been
implemented [15-16]. Decontamination has also been attempted by scraping off the
topsoil and cleaning tree barks. The reverse plowing technique has also been used for thesame aim [17]. Recently, researchers have attempts to synthesize high-quality adsorbent
nanomaterials that can potentially remove radionuclides from the environment.
Graphene oxide (GO) and polyoxometalates (POM) derivatives are the two classes ofmaterials that have recently attracted immense intention. GO has been the focus ofresearch as it has its application in various fields, such as biomedicine [18-19] andenvironmental protection [20- 22]. It has also been used to develop energy storage devices[23], polymer composites [24], and gas sensors [25]. The properties of GO can be tuned
by systematically functionalizing the material. This can lead to improved performance ofthe materials [26]. The synthesis of materials that can function as radionuclide adsorbents,
such as H-titanate nanotubes [27] and phlogopite [28], has been reported. However, not2

all the reported materials are excellent adsorbents. Therefore, a potential material that canfunction as a highly efficient radionuclide adsorbent should be identified.
GO is a promising candidate that can be potentially used for adsorbing radionuclides.
The epoxide and hydroxyl groups are the two major functional groups present on bothsides of the basal plane of the GO sheets [26]. GO contains numerous structural defects.
Other functional groups, such as carbonyl groups, phenol-like groups, and carboxylgroups, are present at the rims of these defects. The particle size of GO plays an importantrole in determining the functional group composition of the material [29-30]. Therefore,
the adsorption capacity of GO is dictated by the accessible surface area. Thus, it isimportant to understand the structural characteristics of GO to gain insight into theadsorption process.

(a) cold(b) medium

(d)

(c)

hot

(e)

Fig. 1. Synthesis of GO using the modified Hummer’s method. (a) Cold stage (Blackish green), (b)Medium stage (Brown), (c) Hot stage (Golden yellow particles of GO), (d) Transmission electron
microscopy (TEM) image of GO, and (e) Scanning electron microscopy (SEM) image of GO. Image is takenfrom personal data based on an experimental.

Recently, researchers have followed the combined method proposed by Grote et al.
and attempted the fabrication of GO. The oxo-functionalized graphene bears an almost
intact hexagonal carbon lattice (unlike the conventional GO material fabricated following
3

Hummer’s method) [31]. The fabrication method involves a three-step process (the cold,
medium, and hot stages). The transition from one stage to the other is accompanied by achange in color of the reaction mixture, which indicates the successful completion of eachstage (Fig. 1(a), (b), and (c)) [32]. The fabrication of a single (or a few) layers of GO can beachieved through the processes of chemical oxidation and exfoliation [33]. The
transmission electron microscopy (TEM) and scanning electron microscopy (SEM)
techniques were used to visualize the surface morphology of GO (Fig. 1(d) and (e)). The
SEM and TEM images revealed the presence of wrinkled and overlapping GO sheets.
These properties were a result of the hydrogen- bonding interactions present between thesurface functional groups. The wrinkled and overlapped samples exhibit different opticaland electrical properties [34]. Guerrero-Contreras and Caballero-Briones reported
that the degree of GO oxidation dictates this particular characteristic, especially thesurface chemistry of the sheets. These properties can be exploited by decorating GO [35].

0.00

Absorbance

-0.05

-0.10

-0.15

-0.20

-0.25

-0.30
4000

3500

3000

2500

2000

1500

1000

500

wavenumber, cm-1

Fig. 2. FTIR spectral profile of GO. Graphic took from personal data based on an experimental study

The characteristic peaks of different functional groups were observed in the Fouriertransform infrared (FTIR) spectral profiles (Fig. 2). The peaks observed at 1057, 1230, 1730,
and 3424 cm-1 corresponded to the stretching vibrations of C–O, C–OH, C=O, and –OH
groups, respectively. The peak corresponding to the unoxidized graphitic area wasobserved at 1618 cm-1 [36].

4The Fukushima disaster of 2011 led researchers to focus on developing new materials
capable of adsorbing radioactive elements. Experiments were conducted to investigate
the effectiveness of GO as radionuclide (especially actinide) adsorbents. The results
revealed that the process of coagulation contributed significantly to the adsorption
process. Cation-induced coagulation leads to the quick precipitation of GO. The
coagulation of GO (in the presence of cations) is affected by pH. Cations with different
charges behave differently under the same pH conditions. For example, sodium (Na +),calcium (Ca2+), and lanthanide (Eu3+), at the same pH, exhibited different criticalcoagulation concentration (CCC). The changes visible to the naked eye were observed.
The interactions present in the GO sheets impart the GO layers with improved adsorption
properties (compared to bentonite clays and activated carbon) [37].
Table 1. Eu(III), Sr(II), and Cs+ sorption capacity of GO and other materials
Nanocomposite

Maximum sorption capacity (mg/g)
Eu (III)Sr (II)

pH range

Ref.

Cs

PS_25GO

-

20.2

-

2.0–8.0[43]

PS_15GO-COOH

-

11.47

-

2.0–7.0[43]

GO nanosheet

175.44

-

-

6.0

[21]

GO nanosheet

161.29

-

-

4.5

[21]

H-titanates nanotubes short

22.8

-

-

4.2

[27]

GO

-

-

528

12

[40]

GO

-

-

465

7

[40]

Phlogopite

-

-

14.8

2

[28]

Ca-phlogopite

-

-

60.4

2

[28]

PANI@GO

-

266.3

121.7

3

[22]

As commonly used for cationic radionuclides, bentonite clays and activated carbon
have different physical characteristics. Bentonite consists mostly of montmorillonite (athree-layered mineral) having octahedral and tetrahedral sheets [38]. Activated carbon isa non-porous material possessing a large specific surface area. However, functionalgroups are absent. Therefore, researchers have attempted to functionalize the surface ofactivated carbon to improve its adsorption ability [39]. Kaewmee et al. reported that thecesium sorption capacity of pristine GO was comparable to the cesium sorption capacityof Prussian blue. It was experimentally demonstrated that the maximum adsorption
capacity was approximately 528 mg Cs/g of GO. The sorption ability was tuned by5

adjusting the pH of the medium [40]. An interesting phenomenon was observed whenthe experiments were carried out under a broad range of pH. It experimentally foundthat the sorption capacity of a magnetic GO nanocomposite, in the pH range of 5 to 9,
exceeded 90% (removal of americium (Am) and plutonium (Pu) from solution) [41]. Yanget al. successfully developed a composite consisting of Prussian blue, Fe3O4 nanoparticles,
and GO. It was reported that GO is an efficient cesium adsorbent. The cesium sorption
capacity of the PB/Fe3O4/GO composite (55.56 mg/g) was higher than that of the PB/Fe3O4
composite (46.30 mg/g). The successful removal of Cs (by H+-exchange or ion trapping)can be attributed to the anchoring of Prussian blue and Fe3O4 nanoparticles on the surfaceof GO. However, peaks corresponding to GO were not observed in the PXRD pattern
because of the low GO content of the samples [42].
Bubenikova et al. reported that the adsorption capacity of GO could be improved via
the carboxylation process (PS_25GO, PS_15GO-COOH, Table 1). The results revealed thatthe uptake kinetics of strontium (II) was fast. A simple method was used forcarboxylation. Multiple carboxylate groups were introduced on the GO surface using asolution of chloroacetic acid in water. Subsequently, a GO-polystyrene (GO-PS)
nanocomposite was fabricated to develop the desired material [43].
Recently, researchers have attempted to tune the properties and control the degreeof oxidation of the graphene-like materials by tailoring their oxygen content (following
specific methods) [44-45]. Novacek et al. reported that a significant increase in thesorption capacity could be achieved by oxidizing GO multiple times. The multiple
oxidation process is the key to achieve high oxygen content and significantly increased
carboxylic acid content, leading to the significantly high sorption capacity of the material[46]. Researchers have also investigated the role of cation bridging in promoting theformation of GO complexes. The Cs cations were better transport inhibitors at higher
concentrations than other monovalent cations. The surface functional groups played an
important role in achieving transport inhibition. GO can form inner-sphere complexes
with Cs cations [47].
Similar

to

GO,

the

POM

derivatives

arehigh-performance

adsorbent

nanomaterials. POMs are anionic metal oxide clusters consisting of tungsten (W),vanadium (V), molybdenum (Mo), and niobium (Nb) in their high oxidation states. POMs
possess interesting properties (such as redox, acidic, photochemical, and magnetic
properties) [48]. To date, many families of POMs have been reported [49-50], such as thephosphotungstate family, which is one of the most popular families of POMs. The

6

Keggin-type compound [PW12O40]3- is a member of the phosphotungstate family
consisting of one phosphate unit surrounded by 12 tungstate units. Other members of thefamily include the Dawson-type compound [P2W18O60]6- formed of two phosphate and 18
tungstate units and the Preyssler-type compound [P5W30O110Na(H2O)]14- consisting of fivephosphates surrounded by 30 tungstates (Fig. 3). Structurally diverse POMs can find theirapplications in a wide range of fields. In this mini-review, we have focused on theremoval of radionuclides using POM. Recently, it has been investigated if POMs
exhibiting the Dawson-type structure can be used to extract thorium and actinium [51].
This work has successfully exploited the coordination chemistry of thorium and actinium
and has conducted experiments under acidic conditions (using ion-exchange resins) to
extract nuclides using the Dawson-type POM. The Keggin-type POM (Fig. 3) exhibits
excellent Bronsted acidity and interesting redox properties. They can be potentially used
for protecting the environment [52]. Reduction-induced Cs+ uptake was achieved by theredox-active ionic crystal [α-SiMo12O40] in the presence of ascorbic acid [53]. In a strongly
acidic solution, the central sodium ion of the Preyssler anion [NaP5-W30O110]14- can be
replaced by Eu3+ [54].

Fig. 3. Members of the Phosphotungstate family of POMs

Therefore, the synergistic effect between POMs and other molecules can assist theefficient removal of nuclides (Fig. 4) [55]. Wang et al. fabricated graphene/POM
composites utilizing covalent bonding. Fabrication of the composites exploiting theconcept of covalent bonding addressed the problem of POM detachment from the carriers.
The report revealed that the surface area of the GO/POM composite was five times larger
than that of the POM-only [56]. Lu et al. reported strong interlayer interactions could be
produced by POMs (on the GO sheets) when epoxy groups are substituted byethylenediamine groups (EN, weak reductant). Abundant hopping sites are generated
that promote the rapid propagation of protons. It has been reported that the anchoring of7

POM nanoparticles onto the GO layers helps to generate mobile protons and flexible
routes for proton transport. Therefore, the protons can exploit the hopping sites to change
their motion path and speed up the process of transportation [57]. The TEM images revealthat the individual clusters of POM appear as black spots on the surface area (when POM
is anchored onto the GO layer). It can be concluded that the oxygen functional group on
the graphene sheet and the corner shared metal-oxygen site of POMs in an aqueous
solution, interacting with each other via electrostatic interactions [58].

Fig. 4. Plausible interaction between Keggin-type POMs and GO

To date, many researchers have attempted to synthesize POM composites that canpotentially serve as adsorbents. Herrmann et al. investigated the effects of chemisorption
and physisorption of POM-ionic liquid (POM-IL) [59]. Liu et al. investigated these effects
in more detail. They successfully fabricated a GO-based nanocomposite by impregnating
POM on the GO layer (following the impregnation method), exhibiting an improved
loading ability of GO. The fabrication process can potentially lead to the strong
electrostatic attraction between the adsorbate and the absorbent [60]. Sures and Nyman
carried out their experiments with A8[Nb6O19]·nH2O (A = Li, K, Rb, Cs), ([Nb2W4O19]4-, and[Nb4W2O19]6-. They concluded that Cs exhibits a strong tendency to form complexes withanionic surfaces. The formation of the complexes can also be mediated by a mediation
sphere [61]. Hitose and Uchida studied compounds I and II (Wells–Dawson-type POMs,[α- P2M18O62]) that exhibit fast adsorption kinetics (1 h to reach equilibrium). They
reported that the compounds exhibited an increased cesium uptake capacity [62].
However, it is challenging to deal with radionuclides (in solution and solid surfaces).
Radionuclides, such as Cs+ that utilize their unusually high coordination number to form
8

complexes, exhibit abnormal behavior (ion-pairing) [61]. The radiation emitted from aradioactive material can potentially lead to the generation of another variable inadsorption materials that can potentially affect their structural stability [63]. Therefore, abetter understanding of the mechanism of action of POMs is required to tune andimprove the functions of POM-based materials. POM, which has different arrangements,
sizes, charges, and structures, can form several types of crystalline structures [58,64].
Their unique structures make them important to stabilize ligands for redox-active highvalance metallic fragments [65]. The interaction between electrons and alkali metals(during their migration) in POM-based porous crystals can be explained by the redox ionexchange phenomenon [66]. Thus, the unique characteristics of POMs eventually lead to
improved properties of the material formed by the interaction between POM and the
support materials.
Based on that, in this thesis, we fabricated two types of a new nanocomposites asradionuclides adsorbents and radionuclides detections to restore environmental
conditions. In the synthesis methodology, Hummer`s method was used as oxidation
method. First, graphene oxide combining with POM material (Dawson-type POM) as Cs
removal. Second, graphene oxide quantum dot (GOQDs) functionalized by cesium green
molecule (C32H33NO8) as Cs detection. As a result, we found that a high Cs adsorption
capacity was achieved by using the nanocomposite. In addition, the nanocomposite has
ability to recognize Cs in water with different pH conditions (acidic and basic) byconsidering the turn ON/OFF response.

91.2. The purpose of study
This thesis has some specific objectives as follows:¾ Investigate the role of the C/O ratio of graphene oxide (GO) specifically to cesium(Cs) adsorption performance after forming the composite¾ Investigate the role of the oxidized zone (sp2/sp3 hybrid carbon cluster) before andafter forming the composite as an important factor in the enhancement of adsorption
capacity¾ Synthesis GO-POM (polyoxometalate) nanocomposite as a Cs removal¾ Investigate the optical properties of GO quantum nano dots and their composite as
cesium (Cs) detection in mixed solution (Distilled water/THF)¾ Synthesis graphene oxide quantum dot-based nanocomposite (combining withcesium green molecule, C32H33NO8) as Cs detector¾ Investigate the Cs cluster formation under electron beam irradiation

1.3. References[1] Yasunari, T.J., Stohl, A., Hayano, R.S., Burkhart, J.F., Eckhardt, S., and Yasunari, T.,2011, Cesium- 137 deposition and contamination of Japanese soils due to the Fukushima
nuclear accident, Proc. Natl. Acad. Sci. U.S.A., 108 (49), 19530–19534.
[2] Nakanishi, T.M., 2017, Research with radiation and radioisotopes to better understandplant physiology and agricultural consequences of radioactive contamination from the
Fukushima Daiichi nuclear accident, J. Radioanal. Nucl. Chem., 311 (2), 947–971.
[3] Tokyo Electric Power Company, 2011, The evaluation status of reactor core damage
at
Fukushima
Daiichinuclear,power
station
unit
1
of
3,
https://www.tepco.co.jp/en/nu/fukushima np/ images/handouts_111130_04-e.pdf.
[4] Wang, X., Chen, L., Wang, L., Fan, Q., Pan, D., Li, J., Chi, F., Xie, Y., Yu, S., Xiao, C.,
Luo, F., Wang, J., Wang, X., Chen, C., Wu, W., Shi, W., Wang, S., and Wang, X., 2019,
Synthesis of novel nanomaterials and their application in efficient removal ofradionuclides, Sci. China Chem., 62 (8), 933–967.
[5] Nishihara, K., Yamagishi, I., Yasuda, K., Ishimori, K., Tanaka, K., Kuno, T., Inada, S.,
and Gotoh, Y., 2015, Radionuclide release to stagnant water in the Fukushima-1 nuclear
power plant 1, J. Nucl. Sci. Technol., 52 (3), 301–307.
[6] Foreman, M.R.S.J., 2015, An introduction to serious nuclear accident chemistry,
Cogent Chem., 1 (1), 1049111.

10[7] Uchida, S., and Tagami, K., 2007, Soil-to-plant transfer factors of fallout 137Cs andnative 133Cs in various crops collected in Japan, J. Radioanal. Nucl. Chem., 273 (1), 205–
210.
[8] Matsuda, N., and Nakashima, S., 2014, Radioactive cesium in water and soil and its
adsorption to rice plant (interim report) (in Japanese), Radiat. Saf. Manage., 13 (1), 84–91.
[9] Nguyen, H.T., Tsujimoto, M., and Nakashima, S., 2019, Study on paddy soil in
Fukushima using Mössbauer spectroscopy, Hyperfine Interact., 240 (1), 122.
[10] Koarashi, J., Atarashi-Andoh, M., Matsunaga, T., Sato, T., Nagao, S., and Nagai, H.,
2012, Factors affecting vertical distribution of Fukushima accident- derived radiocesiumin soil under different land–use conditions, Sci. Total Environ., 431, 392–401.
[11] Tsujimoto, M., Miyashita, S., Nguyen, H.T., and Nakashima, S., 2020, Monthly
change in radioactivity concentration of 137Cs, 134Cs, and 40K of paddy soil and rice plants
in Fukushima Prefecture, Radiat. Saf. Manage., 19, 10–22.
[12] Basuki, T., Miyashita, S., Tsujimoto, M., and Nakashima, S., 2018, Deposition densityof 134Cs and 137Cs and particle size distribution of soil and sediment profile in Hibara Lakearea, Fukushima: An investigation of 134Cs and 137Cs indirect deposition into lake fromsurrounding area, J. Radioanal. Nucl. Chem., 316 (3), 1039–1046.
[13] Shizuma, K., Fujikawa, Y., Kurihara, M., and Sakurai, Y., 2018, Identification andtemporal decrease of 137Cs and 134Cs in groundwater in Minami–Soma City
following the accident at the Fukushima Dai-ichi nuclear power plant, Environ. Pollut.,
234, 1–8.
[14] Steinhauser, G., Brandl, A., and Johnson, T.E., 2014, Comparison of the Chernobyl
and Fukushima nuclear accidents: A review of the environmental impacts, Sci. Total
Environ., 470-471, 800–817.
[15] Ayabe, Y., Hijii, N., and Takenaka, C., 2017, Effects of local-scale decontamination ina secondary forest contaminated after the Fukushima nuclear power plant accident,
Environ. Pollut., 228, 344–353.
[16] Koarashi, J., Atarashi-Andoh, M., Nishimura, S., and Muto, K., 2020, Effectiveness of
decontamination by litter removal in Japanese forest ecosystems affected by the
Fukushima nuclear accident, Sci. Rep., 10 (1), 6614.
[17] IAEA, 2015, The Fukushima Daiichi Accident: Technical Volume 5–Post-Accident
Recovery,International
AtomicEnergyAgency,Vienna,https://wwwpub.iaea.org/MTCD/Publications/PDF/
AdditionalVolumes/P1710/Pub1710-TV5Web.pdf.
[18] del Valle, E.M.M., Galán, M.A., and Carbonell, R.G., 2009, Drug delivery
technologies: The way forward in the new decade, Ind. Eng. Chem. Res., 48 (5), 2475–
2486.
[19] Tabish, M.S., Hanapi, N.S.M., Ibrahim, W.N.W., Saim, N., and Yahaya, N., 2019,
Alginate-graphene oxide biocomposite sorbent for rapid and selective extraction of non11

steroidal anti-inflammatory drugs using micro-solid phase extraction, Indones. J. Chem.,
19 (3), 684–695.
[20] Nor, N.M., Kamil, N.H.N., Mansor, A.I., and Maarof, H.I., 2020, Adsorption analysis
of fluoride removal using graphene oxide/eggshell adsorbent, Indones. J. Chem., 20 (3),579–586.
[21] Sun, Y., Wang, Q., Chen, C., Tan, X., and Wang, X., 2012, Interaction between Eu(III)and graphene oxide nanosheets investigated by batch and extended X-ray absorption finestructure
spectroscopy and by modeling techniques, Environ. Sci. Technol., 46 (11), 6020–6027.
[22] Sun, Y., Shao, D., Chen, C., Yang, S., and Wang, X., 2013, Highly efficient enrichmentof radionuclides on graphene oxide-supported polyaniline, Environ. Sci. Technol., 47 (17),9904–9910.
[23] Ke, Q., and Wang, J., 2016, Graphene-based materials for supercapacitor electrodes –A review, J. Materiomics, 2 (1), 37–54.
[24] Fan, J., Shi, Z., Lian, M., Li, H., and Yin, J., 2013, Mechanically strong graphene
oxide/sodium
alginate/polyacrylamide nanocomposite hydrogel with improved dye adsorption
capacity, J. Mater. Chem. A, 1 (25), 7433–7443.
[25] Debataraja, A., Manurung, R.V., Asri, L.A.T.W., Yuliarto, B., Nugraha, N., andSunendar, B., 2018, Synthesis and characterization of nanocomposites of tin–oxidegraphene doping Pd using Polyol method, Indones. J. Chem., 18 (2), 344–348.
[26] Eigler, S., Dotzer, C., Hof, F., Bauer, W., and Hirsch, A., 2013, Sulfur species ingraphene oxide, Chem. Eur. J., 19 (29), 9490–9496.
[27] Petrov, V.G., Chen, Z., Romanchuk, A.Y., Demina, V.O., Tang, Y., and Kalmykov,
S.N., 2019, Sorption of Eu(III) onto nano-sized H-titanates of different structures, Appl.
Sci., 9 (4), 697.
[28] Tamura, K., Kogure, T., Watanabe, Y., Nagai, C., and Yamada, H., 2014, Uptake ofcesium and strontium ions by artificially altered phlogopite, Environ. Sci. Technol., 48(10), 5808–5815.
[29] Dideikin, A.T., and Vul, A.Y., 2019, Graphene oxide and derivatives: The place ingraphene family, Front. Phys., 6, 149.
[30] Szabó, T., Berkesi, O., Forgó, P., Josepovits, K., Sanakis, Y., Petridis, D., and Dékány,I., 2006, Evolution of surface functional groups in a series of progressively oxidizedgraphite oxides, Chem. Mater., 18 (11), 2740–2749.
[31] Grote, F., Gruber, C., Börrnert, F., Kaiser, U., and Eigler, S., 2017, Thermal
disproportionation of oxo- functionalized graphene, Angew. Chem. Int. Ed., 56 (31),9222–9225.
[32] Hummers, W.S., and Offeman, R.E., 1958, Preparation of graphitic oxide, J. Am.
Chem. Soc., 80 (6), 1339.
12

[33] Ranjan, P., Agrawal, S., Sinha, A., Rao, T.R., Balakrishnan, J., and Thakur, A.D., 2018,
A low-cost non-explosive synthesis of graphene oxide for scalable applications, Sci. Rep.,
8 (1), 12007.
[34] Cote, L.J., Kim, J., Zhang, Z., Sun, C., and Huang, J., 2010, Tunable assembly ofgraphene oxide surfactant sheets: Wrinkles, overlaps and impacts on thin film properties,
Soft Matter, 6 (24), 6096–6101.
[35] Guerrero-Contreras, J., and Caballero-Briones, F., 2015, Graphene oxide powders
with different oxidation degree, prepared by synthesis variations of the Hummers
method, Mater. Chem. Phys., 153, 209–220.
[36] Xu, Y., Liu, Z.W., Xu, Y.L., Zhang, Y.Y., and Wu, J.L., 2014, Preparation andcharacterization ofgraphene/ZnS nanocomposites via a surfactant-free method, J. Exp. Nanosci., 9 (4), 415–
420.
[37] Romanchuk, A.Y., Slesarev, A.S., Kalmykov, S.N., Kosynkin, D.V., and Tour, J.M.,
2013, Graphene oxide for effective radionuclide removal, Phys. Chem. Chem. Phys., 15(7), 2321–2327.
[38] Seliman, A.F., Lasheen, Y.F., Youssief, M.A.E., Abo- Aly, M.M., and Shehata, F.A.,
2014, Removal of some radionuclides from contaminated solution using natural naturalclay: Bentonite, J. Radioanal. Nucl. Chem., 300 (3), 969–979.
[39] Mahmoud, M.R., Sharaf El-deen, G.E., and Soliman, M.A., 2014, Surfactantimpregnated activated carbon for enhanced adsorptive removal of Ce(IV) radionuclides
from aqueous solutions, Ann. Nucl. Energy, 72, 134–144.
[40] Kaewmee, P., Manyam, J., Opaprakasit, P., Le, G.T.T., Chanlek, N., andSreearunothai, P., 2017, Effective removal of cesium by pristine graphene oxide:
Performance, characterizations and mechanisms, RSC Adv., 7 (61), 38747–38756.
[41] Semcuk, S., 2018, Application of graphene oxide based nanocomposites and Šaltiškiai
clay
For radionuclides removal from contaminated solutions, Dissertation, Institute of Physics
of the State Research Institute for Physical Science and Technology, Vilnius University,
Lithuania.
[42] Yang, H., Sun, L., Zhai, J., Li, H., Zhao, Y., and Yu, H., 2014, In situ controllable
synthesis of magnetic Prussian blue/graphene oxide nanocomposites for removal ofradioactive cesium in water, J. Mater. Chem. A, 2 (2), 326–332.
[43] Bubeníková, M., Ecorchard, P., Szatmáry, L., Mrózek, O., Salačová, P., and Tolasz, J.,
2018, Sorption of Sr(II) onto nanocomposites of graphene oxide-polymeric matrix, J.
Radioanal. Nucl. Chem., 315 (2), 263–272.
[44] Morimoto, N., Kubo, T., and Nishina, Y., 2016, Tailoring the oxygen content ofgraphite and reduced graphene oxide for specific applications, Sci. Rep., 6 (1), 21715.

13

[45] Ebajo, V.D., Santos, C.R.L., Alea, G.V., Lin, Y.A., and Chen, C.H., 2019, Regenerable
acidity of graphene oxide in promoting multicomponent organic synthesis, Sci. Rep., 9(1), 15579.
[46] Nováček, M., Jankovský, O., Luxa, J., Sedmidubský, D., Pumera, M., Fila, V., Lhotka,
M., Klímová, K., Matějková, S., and Sofer, Z., 2017, Tuning of graphene oxide composition
by multiple oxidations for carbon dioxide storage and capture of toxic metals, J. Mater.
Chem. A, 5 (6), 2739–2748.
[47] Xia, T., Qi, Y., Liu, J., Qi, Z., Chen, W., and Wiesner, M.R., 2017, Cation-inhibited
transport of graphene oxide nanomaterials in saturated porous media: The Hofmeister
effects, Environ. Sci. Technol., 51 (2), 828–837.
[48] Pope, M.T., and Müller, A., 1991, Polyoxometalate chemistry: An old field with new
dimensions in several disciplines, Angew. Chem. Int. Ed. Engl., 30 (1), 34–48.
[49] Nyman, M., and Burns, P.C., 2012, A comprehensive comparison of transition- metal
and actinyl polyoxometalates, Chem. Soc. Rev., 41 (22), 7354– 7367.
[50] Wihadi, M.N.K., Hayashi, A., Ichihashi, K., Ota, H., Nishihara, S., Inoue, K., Tsunoji,N., Sano, T., and Sadakane, M., 2019, A sandwich complex of bismuth cation and monolacunary α-Keggin-type phosphotungstate: Preparation and structural characterization,
Eur. J. Inorg. Chem., 2019 (3-4), 357–362.
[51] Hatcher, J.L., 2018, Fundamental chemistry related to separations and coordinationof actinium-225, thorium-227, and technetium-99, Dissertation, The Graduate Center,
City University of New York, US. [52] Petit, C., and Bandosz, T.J., 2009, Graphite
oxide/polyoxometalate nanocomposites as adsorbents of ammonia, J. Phys. Chem. C, 113(9), 3800–3809.
[53] Seino, S., Kawahara, R., Ogasawara, Y., Mizuno, N., and Uchida, S., 2016, Reductioninduced highly selective uptake of cesium ions by an ionic crystal based on
silicododecamolybdate, Angew. Chem. Int. Ed., 55 (12), 3987–3991.
[54] Kim, K.C., Pope, M.T., Gama, G.J., and Dickman, M.H., 1999, Slow proton exchange
in aqueous solution. Consequences of protonation and hydration within the central
cavity of Preyssler anion derivatives, [|M(H2O)|‫ـ‬P5W30O110]n-, J. Am. Chem. Soc., 121 (48),11164–11170.
[55] Li, H., Pang, S., Feng, X., Müllen, K., and Bubeck, C., 2010, Polyoxometalate assisted
photoreduction of graphene oxide and its nanocomposite formation, Chem. Commun.,46 (34), 6243–6245.
[56] Wang, R., Dang, L., Liu, Y., and Jiao, W., 2019, Preparation, characterization andphotocatalytic activity of Dawson type phosphotungstate/graphene oxide composites,
Adv. Powder Technol., 30 (7), 1400–1408.
[57] Liu, Y., Liu, S., Lai, X., Miao, J., He, D., Li, N., Luo, F., Shi, Z., and Liu, S., 2015,
Polyoxometalate- modified sponge-like graphene oxide monolith with high protonconducting performance, Adv. Funct. Mater., 25 (28), 4480–4485.
14[58] Kim, Y., and Shanmugam, S., 2013, Polyoxometalate-reduced graphene oxide hybrid
catalyst: Synthesis, structure, and electrochemical properties, ACS Appl. Mater.
Interfaces, 5 (22), 12197–12204.
[59] Herrmann, S., De Matteis, L., de la Fuente, J.M., Mitchell, S.G., and Streb, C., 2017,
Removal of multiple contaminants from water by polyoxometalate supported ionicliquid phases (POM-SILPs), Angew. Chem. Int. Ed., 56 (6), 1667– 1670.
[60] Liu, Y., Luo, F., Liu, S., Liu, S., Lai, X., Li, X., Lu, Y., Li, Y., Hu, C., Shi, Z., and Zheng,Z., 2017, Aminated graphene oxide impregnated with photocatalytic polyoxometalate for
efficient adsorption of dye pollutants and its facile and complete photoregeneration,
Small, 13 (4), 1603174.
[61] Sures, D.J., and Nyman, M., 2017, “Anomalous cesium ion behavior in aqueous
polyoxometalate solutions” in Cesium: Properties, Production and Application, Eds. Hall,
B., Nova Science Publisher, Inc., New York, 119–148.
[62] Hitose, S., and Uchida, S., 2018, Rapid uptake/release of Cs+ in isostructural redoxactive porous ionic crystals with large-molecular-size and easily reducible Dawson-type
polyoxometalates as building blocks, Inorg. Chem., 57 (9), 4833–4836.
[63] Belloni, F., Kütahyali, C., Rondinella, V.V., Carbol, P., Wiss, T., and Mangione, A.,
2009, Can carbon nanotubes play a role in the field of nuclear waste management?,
Environ. Sci. Technol., 43 (5), 1250– 1255.
[64] Ammam, M., 2013, Polyoxometalates: Formation, structures, principal properties,
main deposition, methods and application in sensing, J. Mater. Chem. A, 1 (21), 6291–
6312.
[65] Miras, H.N., Yan, J., Long, D.L., and Cronin, L., 2012, Engineering polyoxometalateswith emergent properties, Chem. Soc. Rev., 41 (22), 7403–7430.
[66] Kawahara, R., Uchida, S., and Mizuno, N., 2015, Redox-induced reversible uptakerelease of cations in porous ionic crystals based on polyoxometalate: Cooperative
migration of electrons with alkali metal ions, Chem. Mater., 27 (6), 2092–2099.

15

Chapter II. Exploration of Cs trapping phenomenon by combining
graphene oxide (GO) with K6P2O18W62 as Nanocomposite
2.1. Introduction
The destruction of the reactor cores of Fukushima Daiichi Nuclear Power Plant(FDNPP) ten years ago has influenced much the environmental condition. Many
researchers predicted the quantity of released material [1] and modelling of the radiation
fallout immediately [2]. It is important to consider the radioactive isotopes delivered toagriculture area [3], tap water [4], and forest area [5]. The subject of considerable concern
by the people is the possibility of the radiation exposure to their body. Both the external
exposure and internal exposure have a possibility to show the radiological consequences
to cause developing cancer [6][7]. Therefore, it is an essential task to treat anddecontaminate the contaminated environment.
137

Cs, which has a half-life of 30 years, is one of the nuclides that have been found in

the contaminated area [8]. The 137Cs was originated from the Tokyo Electric Power
Company (TEPCO) FDNPP accident [9]. Many studies have investigated how to handle
radioactive cesium as a crucial radioisotope [10]. Chemical adsorption methods by using
composite [11], clay [12] to catch the Cs are the techniques suggested in many papers.
According to these methods, the adsorption mechanism emerges as an important concept(like hydroxyl-interlayered vermiculate, 3D microporous composite) and encourages thesynthesis of new material. Therefore, the goal of this work is to create a new sorbent
material, which has an ability to adsorb the Cs with high efficiency. As is well known, theskeleton of GO having different oxygen functional groups can be considered as a bonding
point [13]. Based on that, the chemical modification opens up the idea for tailoring thegraphene oxide (GO) with another compound.
Zilong Liu et al. (2019) investigated that as a novel carbonaceous material GO
surface has an interesting interaction using polar –COO(H) and non-polar (-CH3) groupin order to adsorb different kind of cation (charge, size, complexing capability) byproposed ion bridging mechanism [14]. Another important thing is the solubility of GO
in the diverse range of solvent including water due to the hydroxyl, epoxide functionalgroup and carboxyl group attached on the surface and the sheet edge [15], [16].
On the other hand, polyoxometalates (POMs) provide an excellent, robust anddiscrete material, which has reversible multi-electron redox properties [17]. Some
researchers have employed POMs as potential Cs adsorption material by using a cation
16

exchange mechanism [18]. For example, it was investigated that the interaction of Cs+
with POMs makes it easier to enter and diffuse through the solid-state structurecompared to other type of cation by considering the dehydration enthalpy and hydration
radius of the cations [19]. The K₆P₂W₁₈O₆₂ as a Dawson-type compound has an anionicmolecular framework constructed by two phosphates, 18 tungstates surrounding acentral potassium. POM has a large molecular size [20], high potential to reduce [17], and
it is a structurally well-defined oxide cluster anion. Furthermore, R.D. Gall et al.
confirmed that POM particles could easily be adsorbed on the carbon material [22]. It isalso proved by previous study that the anionic PW12 cluster can be adsorbed on the
reduced GO and can improve the dispersibility of water on the reduced graphene sheet[21].
Considering the tremendous properties of GO and α-K₆P₂W₁₈O₆₂, it is highly
desirable to synthesize a new nanocomposite with Cs adsorption ability using the GO
and POMs [79]. In this study, two types of GO samples were employed. One is GO which
has high carbon (C) element and the other is GO which has high oxygen (O) element.
They have a variant sp2/sp3 structure. These GOs can be produced by controllingoxidation degree, which depends on several factors such as reaction condition, oxidizing
agent, graphite source [23]. It has been known that the GO structure is not precisely
determined because of a variety of local arrangement of functional group [24], [25]. EachGO sample was incorporated with α-K₆P₂W₁₈O₆₂. We investigated the interaction of Cs+
with the nanocomposite by characterizing the surface structure of the composite material
and also by considering the adsorption capacity of nanocomposite for radioactive Cs.
Each property in each material is expected to synergistically strengthen the adsorption
capacity of nanocomposite. High Cs adsorption capacity was clearly achieved by using
nanocomposite.
2.2. Materials and Methods
2.2.1 Materials
All raw chemicals were reagent grade and were used without purification, anddistilled water was used in all experiments. Graphite, Na2WO4·2H2O, CsCl, and methanol
were purchased from Sigma Aldrich Chemistry. NaNO3, H2SO4, HCl, H3PO4, and KCl
were purchased from Fuji Film Wako Pure Chemical Corporation. KMnO4 and H2O2 werepurchased from Tokyo Chemical Industry.

17

2.2.2 Characterizations
Fourier transform infrared spectroscopy (FT-720, HORIBA, Ltd), powder X-ray
diffraction measurement (Rigaku, Thermo plus, XRD-DSC II), Raman spectroscopy(Raman spectrometer HORIBA, Ltd., T64000) were conducted to characterize thefunctional groups, crystallinity, number of layers of GO, size of GO samples, interlayer
distance and the quality of samples. Scanning electron microscope (SEM) (Ultra-highresolution field emission scanning electron microscope device (Hitachi HighTechnologies Corporation, S-5200)), EDAX (Genesis XM2), Transmission electron
microscopy (TEM) (Ultra-high resolution transmission electron microscope (JEOL Ltd.,JEM-2010)) were performed to collect imaging data of the samples. ICP-AES (atomic
emission spectroscopy) (SPS3500, SII Nanotechnology Inc.) was used to calculate theadsorption capability of the samples.
2.2.3. Synthesis of graphene oxide (GO)All GO samples were synthesized using the modified Hummers method [26]. A precooling procedure was implemented by keeping all the reagents and solvent in the
refrigerator for one night before the synthesis process [27]. In order to start the synthesis
process, first, a graphite 1.0 g, NaNO3 1.5 g were put into the vial. Then, a concentrated
H2SO4 (50 ml) was added carefully. Stirred the mixture for 30 minutes at room
temperature. In this step, the colour was blackish green. Put the mixture in the
refrigerator for 30 minutes to keep it in the cooling condition below 10 °C. After that,going to the critical step, added KMnO4 4.0 g slowly and carefully. Stirred for about 20
minutes. Set the temperature at 35 °C under stirring for 2 h. After this reaction, the colourof the solution became brown. Diluted the mixture with 80 ml distilled water with stirringfor 15 minutes. Then, continued stirring for the other 30 minutes at room temperature.
Added 150 ml distilled water slowly and carefully with stirring for 15 minutes. After that,
12 ml of 30% H2O2 was dropped carefully to the solution. A large number of bubbles werereleased. The solution became golden yellow. The solid particle was filtered, washedusing 5% HCl and followed by using distilled water. Dried the solid at 60 °C for 24 h. Inorder to distinguish each GO-sample, we termed these GO samples by index number,which have different amount of C based on elemental analysis. There are two types ofGO-sample based on C/O composition (Table 1). The GOc70 and GOc72 have 70-72 wt% C
element, while GOc39 and GOc40 have ~39 wt% C element.

18

2.2.4. Synthesis of α-KΑP΍WΌΓOΑ΍
The α-KΑP΍WΌΓOΑ΍ was synthesized according to the modified Nadjo Method [28].
The synthesis route started with dissolving the starting material, Na2WO4·2H2O 15 g into
17.5 ml distilled water. Straightforward with vigorous stirring for 15 minutes until thecloudy solution becomes limpid. Followed by acidification process, HCl 4M (12.5 ml) was
added dropwise for 45 minutes carefully. In this process, white precipitate was formed.
In order to avoid agglomeration and to disperse homogenously during the acidification
process, the vigorous stirring was performed. Checked the pH to be around 6-7. Then,
2M H3PO4 (12.5 ml) was added, checked the pH to become around 2-3. The solution was
refluxed for 24 h. And this solution was cooled at room temperature for a while. Then
KCl 7.5 g was added to the solution. Filtered off by paper filtration and the crude material
was obtained. Dissolved it into 32.5 ml distilled water and the solution becomes limpid.
The next reflux process was performed at 80 °C for 72 h. Then, it was cooled at room
temperature. The last step was keeping the solution in refrigerator, and after 24 h, thewell-behaved crystals of α-KΑP΍WΌΓOΑ΍ were produced (6.5 g).
2.2.5. Synthesis of GO-POM composite
The composite sample was distinguished into eight types of composite. Eachcomposite was produced by incorporating GO-sample (GOc70, GOc72, GOc39, and GOc40)
with Dawson-type POM. The GO-POM composite was generated in two different
concentration ratios (concentration ratio GO:POM, 1:8 and 4:1). The constitution issummarized in Table 3. These treatments (different concentration ratio of GO:POM) were
employed in order to investigate the role of GO and Dawson-type POM in the Cs
adsorption and to know the proper composition. In this synthesis process, mixed solvent
was used with the ratio 1:1 (distilled water/methanol). The GO aqueous dispersion in thefirst concentration condition (1:8) was prepared by dispersing 1g GO into the solvent (100ml) with sonification treatment for 5 minutes. Prepared the POM solution by dissolving
1 g α-KΑP΍WΌΓOΑ΍ into the solvent 12.5 ml. Poured the POM solution into the GO
suspension vial, added diluted HCl to adjust the pH in the range 2-3 and then forwarded
to vigorous stirring process for 24 h. In the second concentration condition (4:1), prepared
the GO solution by dispersing GO 0.5 g into the solvent 25 ml with sonification treatment
for 5 minute. POM solution was prepared by dissolving the α-KΑP΍WΌΓOΑ΍ 0.5 g into thesolvent 100 ml. The POM solution was poured into the GO suspension, added diluted
HCl to adjust the pH in the range 2-3, after that stirred for 24 h. Finally, the solid material
was collected by using filter paper and dried at 90 °C for 24 h. Then, the composite
19

product was indexed based on the value of C element of GO (c70, c72, c39, c40 due toelemental analysis) and the concentration ratio of GO/POM (1:8 and 4:1) in order todistinguish each composite’s name, as follows [GO70POM]18, [GO70POM]41, [GO72POM]18,[GO72POM]41, [GO39POM]18, [GO39POM]41, [GO40POM]18, [GO40POM]41. Contant’s groupreported the hydrolytic stability of Dawson-type POM depending on the acidity of thesolution. That is, Dawson-type POM was stable at pH lower than 6 and when the pH
increased above 6, the formation of lacunary species occurs [81]. In the case of our system,
preparation and adsorption experiment were performed at pH lower than 5. Therefore,
the Dawson-type POM was stable in the present solution.
2.2.6. Adsorption experiment
The cesium adsorption was conducted as below. First, each sample (POM, GOsamples, GO-POM composites) 0.08 g was dispersed into distilled water 30 ml withsonification treatment for 5 minutes. Second, 0.6 g of CsCl (3.6 mmol) was added to thesolution. And then the mixture was stirred for 24 h.
The performance of Cs adsorption was analysed by using ICP optical emission
spectroscopy. The solid adsorbent was separated from the solution by using paper
filtration. The measurement was conducted duplicate in each sample solution. The
adsorption efficiency was calculated by equation (1) and the adsorption capacity byequation (2).

Adsorption efficiency % Ads.eff .

Adsorption capacity

C0  Ct
u100
C0% Ads.eff . u C0
W

(1)

(2)

Where C0 (mmol) is the initial concentration of Cs, and Ct (mmol) is the remained
concentration of Cs in the solution after treatment. W (gr) is the dry weight of adsorbent
used in the adsorption experiment.
2.3. Results and Discussion
2.3.1. Elemental analysis

20

Table 1. Elemental analysis of GO sample.
Sample

C%

H%

N%

O%

C/O

GOc70

70.87

7.95

-

21.18

3.34

GOc72

72.04

8.07

-

19.89

3.61

GOc39

39.28

2.76

0.27

57.69

0.68

GOc40

39.61

2.58

0.25

57.56

0.69

Table 1 shows the results of elemental analysis of GOc70, GOc72, GOc39, and GOc40. It canbe seen that the GOc70 and GOc72 have 70-72 wt% C element, while GOc39 and GOc40 have
~39 wt% C element. In the synthesis process, there is one thing that might make thesample a bit different. In the treatment of KMnO4, especially for GOc39 and GOc40, putting
the KMnO4 was treated more carefully (one minute stop, one minute go). It was shownthat the degree of oxidation was controlled by the controlled oxidation process. In
particular, it can be assumed that there is a difference in structure between the two typesof GO. The GOc70 and GOc72 indicating few oxygen defect in the structure have an
important contrast with the GOc39 and GOc40. However, deeper characterization is stillrequired since the oxygen defect is not only the case. As emphasized in the previous
literature, there is no evident relation between oxidation degree and ordered domain thatis exhibited in Raman spectrum [13].
2.3.2. FT-IR spectra
Figures S6, S7 and S8 (see supporting information) show the FT-IR spectra of GO and
Dawson-type compound. The Dawson-type POM was successfully produced by Nadjo
synthesis route. The IR absorptions of the present α-K₆P₂W₁₈O₆₂ are similar with those of
the literature [28]. All GO samples exhibit broad absorption band of O-H stretchingvibration around 3400 cm-1, C=C bond stretching at around 1620 cm-1, C=O vibration ataround 1720 cm-1, C-O vibration band at around 1420 cm-1, and epoxides at 1200-1250 cm1. In addition, the vibration band at around 830 cm-1 – 850 cm-1 was detected especially in

GOc39 and GOc40 samples. This band might be assigned as C-Cl vibration band as reported
in the other work [76]. However, the C-Cl vibration band is unexplained because in thiswork there is no other additional treatment that refers to the addition of Cl. Since the GO21

is nonstoichiometric compound, the structure analysis is quite difficult and the variety ofcomposition depends on the synthesis condition. It can be assumed that the remaining Cl
impurity is coming from laboratory glass material. The difference in the degree ofoxidation was revealed between GOc70, GOc72, and GOc39, and GOc40, i.e., epoxide signal
was seen in GOc39, and GOc40 samples.
2.3.3. Morphology of GO

A1

B1
(a)

(b)

(c)

(d)

(e)

C1

Figure 1. TEM imaging of (a) GOc70 (b) GOc72 (c) GOc39 and SEM imaging of (d) GOc39 (e) GOc40, and SAED
pattern from the location (A1, B1, C1) marked in pict (a, b, c), respectively.

TEM imaging and SAED diffraction are used to draw the structural model in eachGO sample as shown in Fig 1. The 200 kV was applied in each sample for TEM image
measurement. Under this condition, the GO samples were shown that the morphologies
were lamellar, wrinkled and overlaps and found to be stable. The image indicates thatthe some GOs had been exfoliated. This is in agreement with the previous research thatboth wrinkled and overlaps are the essential feature of GO based bulk material [29]. The
sharp spots in an SAED pointed out the crystalline region in GO. Especially GOc72 shows
typical diffraction pattern澳representing the crystalline nature of GO sample. The present
SAEDs are in agreement with the previous work that presented the amorphous region(see Figure 1(A1),(C1)) and crystalline region (see Figure 1(B1)) in GO sample [30].
Particularly, the amorphous zone is associated with the presence of the sp2/sp3 carbon
cluster (as presented in Table 4, see section 3.7.2) in all GO samples [31]. In the otherimaging using SEM measurement, it is shown in the Figure 1 (d) and (e) that the GO
sample consists of several layers (see Table S14, supporting information). Previous workemphasized that the morphology and quality of multilayers of GO depend on several
22

factors such as the source of graphite, degree of oxidation, and reaction condition [32].
These all GO samples are aligned with a few layers of GO in the previous work [33].
2.3.4. Morphology of GO-POM composite and its EDS map

Figure 2. SEM measurement (a) [GO70POM]41, its EDS map (b) Tungsten element, (c) Oxygen element, (d)Carbon K-α X-ray element, TEM image (e) [GO70POM]41, (f) GOc70, showing the formation of layer after
forming composite [GO70POM]41 (E1) compared with original GOC70 (F1) in the same scale image (200 nm).

The SEM image of GO-POM nanocomposite and its EDS map are shown in Fig 2, S1,
and S2 (see supporting information). Fig 2(a) shows the SEM of [GO 70POM]41, and Fig
2(b), (c), and (d) show the W, O, C element distribution, respectively. Tungsten element
is dominantly originated from the α-K₆P₂W₁₈O₆₂ particle, and carbon element is comingfrom the GO. The result suggests that the tungsten, oxygen, and carbon elements were
homogeneously distributed in the GO-POM nanocomposite, showing the successful
combination between GO and POM [34]. This phenomenon was also confirmed by TEM
measurement. Figure 2 (e) show the homogenous POM particle in the surface area of GO
after forming composite. The SEM image of [GO70POM]41 (Fig. 2(e)) is darker than that oforiginal GOC70 (Fig. 2(f)). The formation of layer after forming composite [GO70POM]41

23

(E1) was confirmed by comparing it with original GOC70 (F1) in the same scale image (200nm).
3.5. Powder X-ray diffraction pattern (PXRD)
Powder X-ray diffraction (PXRD) measurement was carried out to characterize theGO, Dawson-type POM, and GO-POM nanocomposite structures. As shown in Figure 4,S4 and S5 (see supporting information), the PXRD patterns of the GOc70, GOc72, GOc39, andGOc40 show diffractions at 2T= 11.58°, 8.66°, 8.91° and 9.13°, respectively. The d-spacing
increases from 0.34 nm of graphite spacing (002) to 0.77 nm – 1.02 nm, which corresponds
to the typical diffraction peak of GO nanosheet (table S14, see supporting information).
The interlayer distance was calculated by the Bragg’s law, as is shown below:d = λ/2sin(ɵ) = 0.154nm/2sin(ɵ)(1)

Where d is the distance between the layers of GO, ɵ is the diffraction angle, λ is the
wavelength of the X-ray beam (λ=0.154nm).
Although the d-spacing of GOc70 is shorter than those of GOc72, GOc39, and GOc40, it
is suggested that oxygen-containing functional groups (epoxide, hydroxyl on the basal
plane and carboxyl groups on the edge of basal plane) successfully formed in different
degree of oxidation. This is the reasonable explanation to the enhancement of theinterlayer spacing of GO [38].
The diffraction peaks of all samples were slightly shifted by forming the composite(Fig 4 and S5 (see supporting information)). That is, d-spacing decreased by forming thecomposite, except for [GO70POM]41 (Table S14, see supporting information). The result of[GO70POM]41 might agree with the previous work that might be assumed as consequenceof a partial intercalation of the POM species and random arrangement of the carbon
layers [39]. The results except for [GO70POM]41 result revealed that the α-KΑP΍WΌΓOΑ΍
species are not embedded in between the GO sheets, however, it could be assumed that
the POM species decorated on the surface of GO as is assumed in the Fig S3 (seesupporting information). It is already known that the molecular size of the α-KΑP΍WΌΓOΑ΍
species is large (ca. 1.03 nm x 1.5 nm) [20] than the d-spacing of each GO-sample (0.77 nm– 1.02 nm).
In addition, we could see in the Table S12 (see supporting information) the out ofplane crystallite size of GO samples. The size was calculated by using the Debye Scherrer
equation:D (nm)= 0.9λß-1 ·(cos(ɵ))-1
24(2)

Where D is the average out of plane crystallite size (nm), λ is the wavelength of the X-ray
beam, ß is FWHM (full width at half maximum), ɵ is XRD peak position.
The in-plane crystallite size of GO samples and the average number of graphene
oxide layers (n) per domain were also estimated by using PXRD data. The following
general formula is described in [42]:L (nm) = 1.84 λ/ ß·cos(ɵ)(3)

n = D/d + 1 = (0.9λß-1 ·(cos(ɵ))-1/d) + 1(4)where D is the average out of plane crystallite size (nm), d is interlayer distance, λ is the
wavelength of the X-ray beam, ß is FWHM (full width at half maximum), ɵ is XRD peak
position, n is the number of layers of GO sample. These calculations provide crucial
information regarding the average crystallite size (reflecting the enhancement or
reduction of graphitic zone and the grain boundaries or lateral defect formation) and theaverage number of GO-layer (see Fig S14, supporting information).
2.3.6. Raman spectroscopy
The Raman spectroscopy was used to characterize the GO material and also it
provides the structural information in detail to draw its quality [43]. From the spectra, we
could see that the D band located at 1356 cm-1, 1352 cm-1, 1352 cm-1, 1347 cm-1 for GOc70,
GOc72, GOc39, and GOc40, respectively (see Fig S9 supporting information). This indicatesthe extensive oxidation of graphite and extended amounts of the sp3-hybridized carbon
atoms [65]. The G band located at 1586, 1581, 1590, and 1590 cm-1 for GOc70, GOc72, GOc39,
and GOc40, respectively, is corresponding to the sp2-hybridized carbon atoms in the
hexagonal framework. The G band is inherently with 2D band (the second order Raman
modes) that appears at 2700 cm-1. The critical thing to fully and accurately characterize
the GO sample is the consideration of the second order Raman modes [44]. The area ratioof the 2D also has a linear correlation with the hole mobility [45]. In addition, the previous
work has explained the packing mechanism of multiple layers GO (bi-, tri- layers) by
STEM-ADF image simulation. It showed that the disorder of graphene sheet and theroughness have been created by the random covalent attachment of oxygen on the top
and bottom surfaces. It influences the lattice distortion and breaks the symmetry of thesystem [80]. Therefore, it can be predicted that the different C/O ratio of GO samples in
our work is sufficient to influence the material structure of the GO-POM composite andthe ability to adsorb the Cs.

25

As presented in Table S13 (see supporting information), the intensity ratio of ID/IG ofeach original material (GOc70, GOc72, GOc39, and GOc40) are 0.93, 0.91, 0.89, and 0.88,
respectively. These values are correlated with the different composition of C/O ratio on
each GO-sample that is already discussed in the previous section. The intensity ratio(ID/IG) tends to increase by forming composite (see Table 4). Therefore, we may conclude
that the anchored process of POM on the GO has affected the structure of GO. In this case,also, there was an opposite trend for GOc70.
2.3.7. Cs adsorption performance
2.3.7.1. Adsorption efficiency and adsorption capacity of GO and POM
Table 2. Adsorption capacity, adsorption efficiency, and adsorption condition.
Materials
POM-CsGOc70-Cs
GOc72-Cs
GOc39-Cs
GOc40-Cs

Adsorption
efficiency
(%)
58.3
58.8
56.8
61.3
56.3

Adsorption
capacity(mmol/gr)
32.8
33.1
31.9
34.5
31.7

Stirring
Time

Sonification
time (minute)

24 h
24 h
24 h
24 h
24 h

5
5
5
5
5

The Cs adsorption capacity of GOc70, GOc72, GOc39, and GOc40 and POM aresummarized in Table 2. Those of GO-POM nanocomposites are presented in Table 3 (seesection 3.7.2). The GO samples were classified into two types according to the elemental
analysis. The GOc70 and GOc72 have 70-72 wt% C element, while GOc39 and GOc40 have ~39
wt% C element. As shown in the Table 2, the result of the present work pointed out that
the Cs+ adsorption by POM, GOc70, GOc72, GOc39, and GOc40 has not much difference among
the samples. The role of different C/O ratio in capturing Cs cannot be revealed. It is also
already strengthened by other researchers that the C/O ratio of GO does not havesystematic correlation with the value of ion exchange capacity [65].
Furthermore, in this work, the interaction between GOc70 and Cs cation occurred
immediately less than one minute after the stirring process (see Figure S16, S17,
supporting information). A brown milky coagulation was formed. The same behaviour
was observed between GOc72 and Cs, however, there was a difference for GOc39, the brownmilky coagulation was formed in small quantity, and the milky coagulation was notformed for GOc40. The difference might be related to the variance in C/O ratio between
GOc70, GOc72, and GOc39, GOc40. Moreover, it was explained that by using surface functional
26

group, the GO can attract the Cs cation by forming inner-sphere complexes [48]. The
metal complexing in the oxidized GO strongly encourages the aggregation behaviour andcan be quantified by the critical coagulation concentrations (CCC) [69]. TEM
measurement shows that there are many Cs clusters in the surface of GOc70 (see Fig 3c).
Especially for this phenomenon, we also considered that the Cs cluster may include Al
impurity (see Fig 3d). It might come from laboratory glass material as reported by otherwork [13]. The black spots also can be seen in the [GO40POM]41, which consists of
GOc40/POM. However, the black spots are not uniformly found in all surface area. It istheoretically explained that the most surface reactions eventuate only on the active siteand do not happen uniformly over the surface area [71]. As proposed by other work, theedge site of graphene (carbene- and carbyne-type carbon atoms) has a significant role foradsorption on the surface of sp2-hybridized carbon [70]. It is also confirmed by previous
O

(a)

(b)

Al

Cs

C

(c)

Cu

C

(d)

Cu
O

W

Cs

Fig 3. TEM image (a) GOc70 after Cs adsorption, many small Cs cluster are seen (b) Elemental
analysis of fig 8a, (c) [GO40POM]41 after Cs adsorption, small black spots are identified (d)Elemental analysis of fig 8c.
27

study that some ions or molecules dissolved in water have a good penetration rate
through the graphene nanocapillaries until the physical size of the ions or molecules
exceeds the critical one [68]. A highly oxidized domain and the hydrophobic domain in
GO have different role. The highly oxidized domain would be responsible for rapid watertransport in the interlayer of GO, while the hydrophobic domain would serve a networkof capillaries [65]. Other researcher also considered the polarizability of graphene in orderto estimate the orientation of interfacial water [66]. And the interfacial water can be used
to observe the surface adsorption behaviour [67]. Based on these analysis, it can beassumed that the interfacial water may support the Cs interaction with GO.
The other important evidence of Cs adsorption is also visible in PXRD
measurement. The sharp and intense diffraction peaks were reduced by introducing Cs(Fig 4 and S5 (see supporting information)). The broad diffraction of GOc70, GOc72, GOc39,
and GOc40 showed the non-uniform surface complexation with Cs cation. PXRD and TEM
show the formation of Cs cluster. This might correspond to the origin of GO acidity,
which contributes to high cation exchange capacity (CEC). The GO acidity is made byoxygen functional groups and the main moieties responsible for the acidic properties arecarboxyl groups [65]. Moreover, as the result by Dimiev et. al. (2012), there is one acidic
site for every six to eight carbon atoms, i.e. 100 g of GO sample contains 500-800 mmol ofactive acidic sites [73]. The adsorption capacity of the present GOs (~3200 mmol/100 g)(Table 2) is larger than the concentration of active acidic site. This is due the formation of
Cs cluster. It is also generally accepted that the basicity and the size of the molecule to beadsorbed are a significant aspect in adsorption process [40] [41]. In this work, the pH of
the solution is around 5-6 when the Cs contacts the GO-POM composite or GO only. Some
previous researchers have proved that the pH condition in GO samples influences theadsorption capacity. It can be explained that at pH above 3.9 (pHpzc), the surface area of
GO is partially negatively charged and the electrostatic attractions between cations andGO are stronger because of deprotonation reaction [63]. It is also caused by the higher
availability of active site on the GO sheets [47].

28

5

10

15

20

25

30

35

72

40

45

50

55

60

5

[GO70POM]18/Cs

48

15

20

25

30

35

42

40

45

50

55

60

[GO72POM]18/Cs

28

24

14

0
144

0
162

[GO70POM]18

108[GO72POM]18

108

72

54

36

0

0
60

57

[GO70POM]41/Cs

40
20
0
116

[GO70POM]41

87[GO72POM]41/Cs

38

Intensity

Intensity

10

58

19
0
153

[GO72POM]41

102
51

29

0

0
48

33
22
11
0
102

GOc70/Cs

32
16
0
510

GOc70

340

GOc72/Cs

GOc72

68

170

34

0

0
5

10

15

20

25

30

35

40

45

50

55

60

5

2Theta

10

15

20

25

30

35

40

45

50

55

60

2ThetaFigure 4. PXRD patterns of GOc70, GOc72, and their GO-POM nanocomposite before and after Cs
adsorption.

As discussed above, these data have a good correlation with the information that isobserved in Raman spectra. Based on Raman analysis (Figure 5 and S9, S10, S11, S12 (seesupporting information)), it was found that the change in spectrum by Cs adsorption was
similar for both the GO-only samples and composite samples. It can be seen that thesecond order peaks have changed after Cs adsorption. In previous studies, it wasstrengthened that the second order peak indicates the crystallite size has changed. This
indicator showed that the spectrum broadens consistently with decreasing the domain
size [46]. Therefore during the chemical processing (Cs adsorption), the significant
structural change of the carbon framework occurred. In addition, based on the sample
experiment using [GO39POM]41, there is a slight shift in the D band from 1347 cm-1 before
Cs adsorption to 1352 cm-1 after Cs adsorption. The same shift also occurred in the othersamples, [GO70POM]18 and [GO70POM]41 (Figure S9, see supporting information). It might
be caused by the interaction of ether hole defect in GO sheets with Cs cation [47]. Thus,
the change that the crystal structure after Cs adsorption has altered by amorphization
could be confirmed.
29

GOc72
GOc72/Cs

3000

5000
2500

[GO70POM]18

Intensity (.a.u.)

Intensity (.a.u.)

4000

3000

2000

[GO70POM]18/Cs
2000

1500

1000

1000

500

0
500

1000

1500

2000

2500

500

3000

1000

Raman shift (cm-1)

1500

2000

2500

3000

Raman shift (cm-1)

Figure 5. Raman spectra before and after Cs adsorption for (a) GOc72 (b) [GO70POM]18.

3.7.2. Adsorption efficiency and adsorption capacity of GO-POM nanocomposite
Table 3. Adsorption capacity, adsorption efficiency, and adsorption condition.
Materials[GOΒ΋POM]18[GOΒ΋POM]41[GO72POM]18[GO72POM]41[GO39POM]18[GO39POM]41[GO40POM]18[GO40POM]41

Composition(GOc70/POM)(GOc70/POM)(GOc72/POM)(GOc72/POM)(GOc39/POM)(GOc39/POM)(GOc40/POM)(GOc40/POM)

Concentrat Adsorption Adsorption
Stirring
ion ratio Efficiency capacity
Time
GO:POM(%)(mmol/gr)

1:8
4:1
1:8
4:1
1:8
4:1
1:8
4:1

84.0
62.7
75.4
78.7
72.7
81.7
70.7
73.5

47.3
35.3
42.4
44.3
40.9
45.9
39.8
41.3

24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h

Sonification time(minute)

5
5
5
5
5
5
5
5

Table 3 shows the result of Cs adsorption by using GO-POM nanocomposite. The GOPOM nanocomposite (Table 3) increased the Cs adsorption capacity effectively comparedto the adsorption capacity of original material (Table 2). The change range is around 6.7%
to 42.9%. This result is in consistent with the previous finding that signified thecharacteristic of surface area showing an important role of POM species with carbon
materials [61], [62]. As consequence, the Cs+ adsorption becomes high. We compared theaverage increment of Cs adsorption of the composite material between GO having 70-72
wt% C element and GO having 39-40 wt% C element. Because the functional groups of
GO provide a binding point for chemical modification [24]. It is seen in Figure 6 that there
is not significant difference between GO having 70-72 wt% C element and GO having 3940 wt% C element.
30

The increment of Cs adsorption (%)

50
45
40
35
30

25
20
15
10
5
0
1;8

1;8

1;8

1;8

4;1

4;1

4;1

4;1

[GO₄₀POM]₁₈[GO₃₉POM]₁₈[GO₇₀POM]₁₈[GO₇₂POM]₁₈[GO₄₀POM]₄₁[GO₃₉POM]₄₁[GO₇₀POM]₄₁[GO₇₂POM]₄₁

GO-POM nanocomposite
Figure 6. The increment of Cs adsorption capacity (%) calculated by using the ratio before and after forming
composite in each GO samples.

We compared the average increment of Cs adsorption of the composite material
between concentration ratio 1:8 and concentration ratio 4:1, but we could not find thesignificant difference. In principle, the adsorption behavior of GO-POM nanocomposite
might be influenced by the significant role of GO as electron acceptor and Dawson-type
POM as the electron donor that led to enhance the attraction between Cs + and thecomposite [53]. The other phenomena authenticated by other researcher confirmed thatthe interaction between POMs and GO can provide more mobile protons and flexible
pathways [54]. Also, Wang’s group reported that the surface charges of GO modified byfunctional groups (GO-COOH, GO-NH2, GO-OCH3) can be used to modulate the chargetransport [55]. Therefore, the presence of Dawson-type POM in the GO system enhanced
the negative charge of GO part in the composite and provided a good adsorption of Cs +ion (Fig S3, see supporting information). The interaction of Cs+ ion with Dawson-type
POM was an ion-pair complex [82]. The anion surface of Dawson-type POM is electronrich and has strong interaction with Cs+. Therefore, the introduction of Dawson-type
POM in our adsorbent system was important to enhance the adsorption of Cs+ ion.
However, further investigation is still needed to find out the mechanism that occurredbehind the phenomenon.

31

(a)

(b)

C

O

Al

Cu
Cs

W

Fig 7. TEM image by using [GO₇₀POM]18 (a) Aggregation process after Cs adsorption (b) Elemental
analysis of fig 7(a).

It iis also
l reportedt d ththatt ththe mechanism
h i
off cation-GOti GO iinteractiontti
iin theth adsorptiondti
behaviour should be considered together with aggregation process [49]. In the present
study, the Figure 7 (a), (b) exhibited that the aggregation occurred after Cs adsorption byusing GO-POM nanocomposite. The results demonstrated that the composite samples
immediately coagulate after Cs adsorption except for [GO₇₀POM]41 that still forms a stable
solution after one hour keeping at ambient temperature (see Figure S15, supporting
information). We predicted that a small granule of Cs was formed and crinkled GO sheets
coating around (see Figure S18, supporting information). It can be signified that astronger bond was formed. It might be predicted that there is a different composition ofoxidative debris (OD) component among the GO-samples. As a previous study
explained, the OD of GO has a significant role in the origin of GO acidity [78]. It is alreadydiscussed in the previous section (section 3.7.1 regarding the contribution of GO acidity
to the cation exchange capacity). However, further investigation is still needed to prove
it and to get a strong evidence. Unfortunately, the elemental analysis of Figure S18(supporting information) was not conducted. Regarding this phenomenon, it can beassumed that there are two important factors which might contribute to the aggregation
behaviour. First, the size of the GO and composite sample. As presented in the Table S12,
the size of each sample is diverse. The range of value is around 4.3 nm to 34.78 nm. It wasstrengthen by the previous study that the size of GO (nano GO, colloid GO, micron GO)
can strongly influence the aggregation behaviour [72]. Second, the other factor is thedegree of acidity. It is strongly related with the functionalities of GO sample. It was
explained in earlier section in this work that there are two types of GO-sample, which has
32different C/O composition. The evidence was shown by the previous study that the GO(single or multilayers) coagulates in highly acidic condition because of losing theirsurface charge [73]. It is also correlated well with other work that identified the effect of
cations on aggregation [48]. Moreover, it was strengthened in the previous theoreticalanalysis that considered an important role of H2O as dipole molecule involving cation-πinteraction. Therefore, the electrostatic interaction would be significantly influenced[50][51]. As calculated in the previous studies, the value of the internuclear distances dionwaterof Cs+ is 0.315 nm [52]. Therefore, the interaction of Cs+ with GO-POM nanocomposite

becomes more easy to diffuse through the interlayer distance of GO-POM nanocomposite
which has d-spacing around 0.69 nm-0.81 nm and 0.77 nm – 1.02 nm for GO sample (seeFig S11, supporting information).

4000

3500

3000

2500

2000

1500

1000

500

4000

(d)

0.15

2500

2000

1500

1000

500

2500

2000

1500

1000

500

(h)

66.0

0.09

62.7

0.06

(c)

-0.03886.4

AbsorbanceAbsorbance

3000

69.3

0.12

-0.076-0.114

67.2
57.6

(b)

-0.156(g)

76.8-0.152(f)

99.2-0.20893.0-0.26086.8-0.312
0.000

80.6

(e)

-0.141-0.054-0.188-0.108-0.162-0.216
4000

3500

72.6

-0.235(a)-0.282

3500

3000

2500

2000

1500

1000

4000

500

3500

3000

wavenumber, cm-1

wavenumber, cm-1

Fig 8. IR spectra for (a)[GO₇₀POM]18, (b)[GO₇₀POM]18 after Cs adsorption, (c)[GO₇₀POM]41, (d)[GO₇₀POM]41
after Cs adsorption, (e)[GO40POM]18, (f)[GO40POM]18 after Cs adsorption, (g) [GO40POM]41, (h)[GO40POM]18
after Cs adsorption.

By FTIR spectra, a specific changes are marked at 1150 cm-1-1350 cm-1 after Cs
adsorption in all GO-POM nanocomposite. Especially for composites which consist of ~39

33

wt% C element, the IR spectrum shows more significant change (see Fig 8 (e)-(h), S7A in
supporting information), whereas the composites which consist of 70-72wt% C element
the peak shifts are not so significant, thus making analysis are quite difficult (see Fig 8(a)-(d), S7B in supporting information). A similar tendency also occurred in all GO
sample after Cs adsorption (see Fig S6 in supporting information). This result indicatesthat there is a change in the interlayer chemistry of the sample and influences the defect(hole) formation and functional group formation. It might be caused by the interaction
between GO functional group (including carboxyl, phenol, multi-group ether) and the Cs
cation [47]. As explained by the other researcher that the pH-driven reversible epoxide
formation in GO (epoxy opening/closing reactions) should be considered as an importantpart of reactivity and properties of GO [77]. The tendency is observed that the interlayer
distance after Cs adsorption in all nanocomposite samples were expanded, even though
the increment was not so high (see Table S14, supporting information). For example, incomposite [GO40POM]18, [GO40POM]41, [GO39POM]18, and [GO39POM]41, the interlayer
distances were changed from 7.6 A to 8.3 A, 7.6 to 7.8 A, 7.4 A to 8.1 A 7.4 to 7.9 A,
respectively. It also happened in composite [GO₇₀POM]18, [GO₇₀POM]41, [GO72POM]41,
and [GO39POM]18, where the d-spacing was expanded from 7.2 A to 7.9 A, 8.1 A to 8.6 A,
7.3 A to 7.9 A and 6.9 A to 7.7 A, respectively. In addition, the other study confirmed that
the CO2 gas is trapped within the multilayers of GO (marked at 2357 cm-1 – 2359 cm-1).
Intentionally, they verified that the intercalated water has an essential role foraccelerating the reaction by some specific mechanism [83].
Another work also confirmed that the functional group of GO can act as a ligand to
replace the water molecule from the metal cations in the first coordination sphere.
Previous work verified that the relaxation time by using NMR relaxation method can beconsidered as an essential variable for identifying the metal complexing mechanism of
GO in different charge, size and electron configuration of the metal as a function of pH
and GO concentration [37]. It was also confirmed by the other study that the presence of
wrinkle-like water tunnels has a strong relation with the water adsorption ability of GO[64].
Table 4. The sp2/sp3 carbon cluster on each composite surface.
Material

Composition

GOc70
GOc72
GOc39

34

Graphiticzone (G%)
69.7
75.4
74.9

Oxidized zone (%)
30.3
24.6
25.1

GOc40[GOΒ΋POM]18[GOΒ΋POM]41[GO72POM]18[GO72POM]41[GO39POM]18[GO39POM]41[GO40POM]18[GO40POM]41

GOc70/POM, 1:8
GOc70/POM, 4:1
GOc72/POM, 1:8
GOc72/POM, 4:1
GOc39/POM, 1:8
GOc39/POM, 4:1
GOc40/POM, 1:8
GOc40/POM, 4:1

74.5
68.4
71.4
67.5
69.0
69.0
69.3
69.1
69.5

25.5
31.6
28.6
32.5
30.9
31.0
30.7
30.0
30.5

Deeper investigation into carbon species in each composite sample was performed.
It was shown in the Table 4, where the graphitic zone can be calculated by using the
integrated intensity of graphitic (2 ɵ = 26.6) and oxidized peaks (2 ɵ = 8.66 - 15) in the
PXRD pattern. The amount of graphitic zone (G%, unoxidized region) was calculatedusing equation 7, as follows:
G% =Igraphitex 100%
Ioxidized + Igraphite(7)

Where Igraphite and Ioxidized are the intensities of graphitic and oxidized diffraction peaks of
GO in the PXRD pattern, respectively [60].
As presented in the Table 4, the diversification of sp2/sp3 carbon cluster on eachsample has been produced. This variance among the samples indicates that there is adifference in the degree of oxidation. However, a peculiar phenomenon was seen in GOc70
sample. The data in Table 4 show that the percentage of the oxidized zone of GOc70 ishigher compared to GOc72, GOc39, and GOc40. The ratio of the oxidized zone is not muchdifferent for GOc72, GOc39, and GOc40. The GOc70 shows the shorter interlayer distance thanGOc72, GOc39, and GOc40. This may cause the peculiarity of GOc70. But, the peculiarity ofGOc70 is still under study. According to the literature, the ratio of the oxidized zone does
not have a linear correlation with oxidation ratio [75]. The unusual characteristic of GOc70
also has a direct effect on the composite material produced by using GOc70 (discussion isprovided at the beginning in section 3.7.2). Further investigation is needed to get more
understanding of the dependence of these factors. Recent study revealed that the ratio
C/O has a big influence on the physical properties because of the change of the surface
speciation [35]. Other investigation also confirmed a strong correlation between thecontent of Lewis acid sites (via basal plane epoxide) and Bronsted acidic carboxylic groupin GO sample with strong acidity and high oxidation degree [36].
35Furthermore, the data show that the oxidized zone (sp2/sp3 hybrid carbon) of eachcomposite sample was enlarged compared to the corresponding original GO. Only
[GO₇₀POM]41, which consists of GOc70 /POM with concentration ratio 4:1 was decreased.
It is correlated well with the in- plane crystallite size of each composite material
that being expanded (see table S14, supporting information). Particularly, GO-POM
nanocomposite with concentration ratio 4:1, shows the in-plane crystallite size larger than
GO-POM nanocomposite with concentration ratio 1:8. It could be assumed that there isan electron transfer from POM species to GO [56][57]. Other researcher also confirmedthat the surface area of GO-POM composite was improved five times due to covalentbond between POM species and GO [34]. In general, the enhancement of adsorption
capacity is in line with the increase of sp2/sp3 carbon cluster on each composite surface.
Based on the result in Figure 6, the lowest value of the percentage increase of Cs
adsorption capacity is observed in [GO₇₀POM]41. It is only 6.7%. This might be caused by
the decreased oxidized zone (sp2/sp3 hybrid carbon) after composite was formed (seeTable 4). From Table 4, it was shown that the value decreased from 30.3% (GOc70) to 28.6%(GOc70/POM composite). It is also confirmed by PXRD pattern, which shows the sametendency in [GO₇₀POM]41, that the diffraction has reduced from 11.57° to 10.99° ifcompared with GOc70 as precursor material. Some researchers have tried to address thecomplexity of GO especially to produce graphene-based hybrid material in diverse
application. For example researcher has attempted to control sp2/sp3 hybrid carbon
structure in GO by simple ethanol solvothermal method [58]. Furthermore, a similar
trend was investigated to enlarge the carbon cluster size by increasing the sonication time[59].
2.4. Conclusions
In this work, the GO-based POM nanocomposite was successfully obtained. The Cs
adsorption capacity increased by forming nanocomposite. The result proposed, first, theC/O ratio of GO sample has significantly influenced the characteristic of GO-POM
nanocomposite for Cs adsorption performance after forming composite. Specifically, GO
sample, which consists of 70-72 wt% C element, is a good precursor material toincorporate Dawson-type POM compared to the GO that has ~39 wt% C element in their
atomic composition. Second, the concentration ratio of GO/POM in forming composite
material can be considered to maximize the adsorption performance. Additionally, thesize and the acidity of the GO also influence the aggregation state. However, it still needs
36further investigation to know the size-tunability and the role of acidic properties of GO
in different C/O composition for Cs adsorption. The further calculation was performed
to quantify the graphitic zone in each sample. The result revealed that the oxidized zone
was enhanced after forming composite in all samples except [GO₇₀POM] 41 that shows adifferent character. Further scrutiny is needed to open the understanding of the role ofoxidative debris (OD) component of GO and the level of hydration of GO for adsorption
capacity.
Supporting Information
Supportive SEM imaging, FT-IR spectra, PXRD data, Raman data, Plausible interaction
GO with Dawson-type POM, Table of the ratio ID/IG of GO-POM nanocomposite, Tableof the crystallite size of GO and composite material.

37References
[1]
[2]
[3]

[4]

[5]

[6]
[7]

[8]

[9]
[10]

[11]

[12]

[13]

[14]

K. Nishihara et al., “Radionuclide release to stagnant water in the Fukushima-1
nuclear power plant1,” J. Nucl. Sci. Technol., vol. 52, no. 3, pp. 301–307, 2015.
R. J. M. Konings, T. Wiss, and O. Beneš, “Predicting material release during a
nuclear reactor accident,” Nat. Mater., vol. 14, no. 3, pp. 247–252, 2015.
S. Uchida and K. Tagami, “Soil-to-plant transfer factors of fallout 137Cs and native
133Cs in various crops collected in Japan,” J. Radioanal. Nucl. Chem., vol. 273, no. 1,
pp. 205–210, 2007.
K. Shizuma, Y. Fujikawa, M. Kurihara, and Y. Sakurai, “Identification and temporaldecrease of 137Cs and 134Cs in groundwater in Minami-Soma City following theaccident at the Fukushima Dai-ichi nuclear power plant,” Environ. Pollut., vol. 234,
pp. 1–8, 2018.
J. Koarashi, M. Atarashi-Andoh, T. Matsunaga, T. Sato, S. Nagao, and H. Nagai,“Factors affecting vertical distribution of Fukushima accident-derived radiocesiumin soil under different land-use conditions,” Sci. Total Environ., vol. 431, pp. 392–
401, 2012.
K. Kamiya et al., “Long-term effects of radiation exposure on health,” Lancet, vol.
386, no. 9992, pp. 469–478, 2015.
A. Hasegawa et al., “Health effects of radiation and other health problems in theaftermath of nuclear accidents, with an emphasis on Fukushima,” Lancet, vol. 386,
no. 9992, pp. 479–488, 2015.
N. Matsuda, S. Nakashima, “Radioactive cesium in water and soil and its
adsorption to rice plant (interim report) (in japanese),” Radiat. Saf. Manage., vol. 13,
no. 1, pp. 84–91, 2014.
TEPCO, “The Evaluation Status of Reactor Core Damage at Fukushima Daiichi
Nuclear Power Station Units 1 to 3,” p. 28, 2011.
T. Yousefi, M. Torab-Mostaedi, M. A. Moosavian, and H. G. Mobtaker, “Potential
application of a nanocomposite:HCNFe@polymer for effective removal of Cs (I)from nuclear waste,” Prog. Nucl. Energy, vol. 85, pp. 631–639, 2015.
S. C. Jang et al., “Synergistically strengthened 3D micro-scavenger cage adsorbent
for selective removal of radioactive cesium,” Sci. Rep., vol. 6, no. June, pp. 1–12,
2016.
K. Tamura, T. Kogure, Y. Watanabe, C. Nagai, and H. Yamada, “Uptake of cesium
and strontium ions by artificially altered phlogopite,” Environ. Sci. Technol., vol. 48,
no. 10, pp. 5808–5815, 2014.
J. Guerrero-Contreras and F. Caballero-Briones, “Graphene oxide powders withdifferent oxidation degree, prepared by synthesis variations of the Hummers
method,” Mater. Chem. Phys., vol. 153, no. March, pp. 209–220, 2015.
Z. Liu, T. Rios-Carvajal, M. P. Andersson, M. Ceccato, S. L. S. Stipp, and T.
38

[15]

[16]

[17]
[18]

[19]
[20]

[21]

[22]

[23]

[24]
[25]

[26]
[27]

Hassenkam, “Ion effects on molecular interaction between graphene oxide andorganic molecules,” Environ. Sci. Nano, vol. 6, no. 7, pp. 2281–2291, 2019.
V. V. Neklyudov, N. R. Khafizov, I. A. Sedov, and A. M. Dimiev, “New insights
into the solubility of graphene oxide in water and alcohols,” Phys. Chem. Chem.
Phys., vol. 19, no. 26, pp. 17000–17008, 2017.
B. Konkena and S. Vasudevan, “Understanding aqueous dispersibility of graphene
oxide and reduced graphene oxide through p K a measurements,” J. Phys. Chem.
Lett., vol. 3, no. 7, pp. 867–872, 2012.
C. Ritchie et al., “Reversible redox reactions in an extended polyoxometalate
framework solid,” Angew. Chemie - Int. Ed., vol. 47, no. 36, pp. 6881–6884, 2008.
R. Kawahara, S. Uchida, and N. Mizuno, “Redox-induced reversible uptake-releaseof cations in porous ionic crystals based on polyoxometalate: Cooperative
migration of electrons with alkali metal ions,” Chem. Mater., vol. 27, no. 6, pp. 2092–
2099, 2015.
S. Uchida, “Frontiers and progress in cation-uptake and exchange chemistry ofpolyoxometalate-based compounds,” Chem. Sci., vol. 10, no. 33, pp. 7670–7679, 2019.
S. Hitose and S. Uchida, “Rapid Uptake/Release of Cs+ in Isostructural RedoxActive Porous Ionic Crystals with Large-Molecular-Size and Easily Reducible
Dawson-Type Polyoxometalates as Building Blocks,” Inorg. Chem., vol. 57, no. 9, pp.
4833–4836, 2018.
H. Li, S. Pang, X. Feng, K. Müllen, and C. Bubeck, “Polyoxometalate assisted
photoreduction of graphene oxide and its nanocomposite formation,” Chem.
Commun., vol. 46, no. 34, pp. 6243–6245, 2010.
R. D. Gall, C. L. Hill, and J. E. Walker, “Carbon powder and fiber-supported
polyoxometalate catalytic materials. Preparation, characterization, and catalyticoxidation of dialkyl sulfides as mustard (HD) analogues,” Chem. Mater., vol. 8, no.
10, pp. 2523–2527, 1996.
S. Eigler and A. Hirsch, “Chemistry with graphene and graphene oxide Challenges for synthetic chemists,” Angew. Chemie - Int. Ed., vol. 53, no. 30, pp.
7720–7738, 2014.
D. R. Dreyer, S. Park, C. W. Bielawski, and R. S. Ruoff, “The chemistry of graphene
oxide,” Chem. Soc. Rev., vol. 39, no. 1, pp. 228–240, 2010.
E. Aliyev, V. Filiz, M. M. Khan, Y. J. Lee, C. Abetz, and V. Abetz, “Structuralcharacterization of graphene oxide: Surface functional groups and fractionatedoxidative debris,” Nanomaterials, vol. 9, no. 8, 2019.
W. S. Hummers and R. E. Offeman, “Preparation of Graphitic Oxide,” J. Am. Chem.
Soc., vol. 80, no. 6, p. 1339, 1958.
P. Ranjan, S. Agrawal, A. Sinha, T. R. Rao, J. Balakrishnan, and A. D. Thakur, “A
Low-Cost Non-explosive Synthesis of Graphene Oxide for Scalable Applications,”
Sci. Rep., vol. 8, no. 1, pp. 1–13, 2018.
39

[28] I. M. Mbomekalle, Y. W. Lu, B. Keita, and L. Nadjo, “Simple, high yield andreagent-saving synthesis of pure α-K 6P2W18O62 · 14H2O,” Inorg. Chem. Commun.,vol. 7, no. 1, pp. 86–90, 2004.
[29] L. J. Cote, J. Kim, Z. Zhang, C. Sun, and J. Huang, “Tunable assembly of graphene
oxide surfactant sheets: Wrinkles, overlaps and impacts on thin film properties,”
Soft Matter, vol. 6, no. 24, pp. 6096–6101, 2010.
[30] K. Krishnamoorthy, M. Veerapandian, K. Yun, and S. J. Kim, “The chemical andstructural analysis of graphene oxide with different degrees of oxidation,” Carbon
N. Y., vol. 53, pp. 38–49, 2013.
[31] S. Shukla and S. Saxena, “Spectroscopic investigation of confinement effects on
optical properties of graphene oxide,” Appl. Phys. Lett., vol. 98, no. 7, pp. 2010–2012,2011.
[32] N. I. Kovtyukhova, “Layer-by-layer assembly of ultrathin composite films frommicron-sized graphite oxide sheets and polycations,” Chem. Mater., vol. 11, no. 3,
pp. 771–778, 1999.
[33] A. B. S. Mohindar S. Seehra, Vishal Narang, Usha K. Geddam, “Correlation
between X-ray diffraction and Raman spectra of 16 commercial graphene-basedmaterials and their resulting classification,” Carbon N Y, vol. 111, no. 5, pp. 380–384,
2017.
[34] R. Wang, L. Dang, Y. Liu, and W. Jiao, “Preparation, characterization andphotocatalytic activity of Dawson type phosphotungstate/graphene oxide
composites,” Adv. Powder Technol., vol. 30, no. 7, pp. 1400–1408, 2019.
[35] T. Szabó et al., “Evolution of surface functional groups in a series of progressively
oxidized graphite oxides,” Chem. Mater., vol. 18, no. 11, pp. 2740–2749, 2006.
[36] V. D. Ebajo, C. R. L. Santos, G. V. Alea, Y. A. Lin, and C. H. Chen, “Regenerable
Acidity of Graphene Oxide in Promoting Multicomponent Organic Synthesis,” Sci.
Rep., vol. 9, no. 1, 2019.
[37] R. R. Amirov, J. Shayimova, Z. Nasirova, A. M. Dimiev, “Chemistry of graphene
oxide. reactions with transition metal cations,” Carbon, 116, 356–365, 2017.
[38] D. C. Marcano et al., "Improved synthesis of graphene oxide," ACS Nano, vol. 4,4806-4814, 2010.
[39] C. Petit and T. J. Bandosz, “Graphite Oxide/Polyoxometalate nanocomposites as
adsorbents of ammonia,” J. Phys. Chem. C, vol. 113, no. 9, pp. 3800–3809, 2009.
[40] N. Mizuno and M. Misono, “Heterogeneous catalysis,” Chem. Rev., vol. 98, no. 1,
pp. 199–217, 1998.
[41] T. Okuhara, T. Nishimura, H. Watanabe, and M. Misono, “Insoluble heteropoly
compounds as highly active catalysts for liquid-phase reactions,” J. Mol. Catal., vol.
74, no. 1–3, pp. 247–256, 1992.
[42] R. Sharma, N. Chadha, and P. Saini, “Determination of defect density, crystallite
size and number of graphene layers in graphene analogues using X-ray diffraction
40

[43]

[44]
[45]
[46]
[47]

[48]

[49]

[50]
[51]

[52]
[53]

[54]

[55]
[56]

and Raman spectroscopy,” Indian J. Pure Appl. Phys., vol. 55, no. 9, pp. 625–629, 2017.
M. S. Dresselhaus, A. Jorio, A. G. Souza Filho, and R. Saito, “Defect characterizationin graphene and carbon nanotubes using Raman spectroscopy,” Philos. Trans. R.
Soc. A Math. Phys. Eng. Sci., vol. 368, no. 1932, pp. 5355–5377, 2010.
A. A. K. King et al., “A new raman metric for the characterisation of graphene oxide
and its derivatives,” Sci. Rep., vol. 6, pp. 1–6, 2016.
C. Y. Su et al., “Highly efficient restoration of graphitic structure in graphene oxideusing alcohol vapors,” ACS Nano, vol. 4, no. 9, pp. 5285–5292, 2010.
R. J. Nemanich and S. A. Solin, “First- and second-order Raman scattering fromfinite size-crystals of graphite,” Phys. Rev. B, vol. 20, no. 2, p. 392, 1979.
P. Kaewmee, J. Manyam, P. Opaprakasit, G. T. Truc Le, N. Chanlek, and P.
Sreearunothai, “Effective removal of cesium by pristine graphene oxide:
Performance, characterizations and mechanisms,” RSC Adv., vol. 7, no. 61, pp.
38747–38756, 2017.
T. Xia, Y. Qi, J. Liu, Z. Qi, W. Chen, and M. R. Wiesner, “Cation-inhibited transportof graphene oxide nanomaterials in saturated porous media: The hofmeister effects,”
Environ. Sci. Technol., vol. 51, no. 2, pp. 828–837, 2017.
K. Yang, B. Chen, X. Zhu, and B. Xing, “Aggregation, Adsorption, andMorphological Transformation of Graphene Oxide in Aqueous Solutions
Containing Different Metal Cations,” Environ. Sci. Technol., vol. 50, no. 20, pp.
11066–11075, 2016.
J. C. Ma and D. A. Dougherty, “The cation-π interaction,” Chem. Rev., vol. 97, no. 5,
pp. 1303–1324, 1997.
Y. Xu, J. Shen, W. Zhu, X. Luo, K. Chen, and H. Jiang, “Influence of the watermolecule on cation-π interaction: Ab initio second order møller-plesset
perturbation theory (MP2) calculations,” J. Phys. Chem. B, vol. 109, no. 12, pp. 5945–
5949, 2005.
Y. Marcus, “Ionic Radii in Aqueous Solutions,” Chem. Rev., vol. 88, no. 8, pp. 1475–
1498, 1988.
D. Zhou and B. G. Han, “Graphene-based nanoporous materials assembled bymediation of polyoxometalate nanoparticles,” Adv. Funct. Mater., vol. 20, no. 16, pp.
2717–2722, 2010.
Y. Liu et al., “Polyoxometalate-Modified Sponge-Like Graphene Oxide Monolithwith High Proton-Conducting Performance,” Adv. Funct. Mater., vol. 25, no. 28, pp.
4480–4485, 2015.
Q. Tu et al., “Effects of surface charges of graphene oxide on neuronal outgrowthand branching,” Analyst, vol. 139, no. 1, pp. 105–115, 2013.
K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prud’homme, I. A. Aksay, and R. Car,
“Raman spectra of graphite oxide and functionalized graphene sheets,” Nano Lett.,
vol. 8, no. 1, pp. 36–41, 2008.
41

[57] Y. Kim and S. Shanmugam, “Polyoxometalate-reduced graphene oxide hybrid
catalyst: Synthesis, structure, and electrochemical properties,” ACS Appl. Mater.
Interfaces, vol. 5, no. 22, pp. 12197–12204, 2013.
[58] S. Wang et al., “The role of sp2/sp3 hybrid carbon regulation in the nonlinear opticalproperties of graphene oxide materials,” RSC Adv., vol. 7, no. 84, pp. 53643–53652,
2017.
[59] S. Il Ahn, “Changing the sp2 carbon clusters in graphene oxide during exfoliation,”Trans. Electr. Electron. Mater., vol. 16, no. 1, pp. 49–52, 2015.
[60] H. Yan et al., “Influence of the surface structure of graphene oxide on the adsorptionof aromatic organic compounds from water,” ACS Appl. Mater. Interfaces, vol. 7, no.
12, pp. 6690–6697, 2015.
[61] M. A. Schwegler, P. Vinke, M. van der Eijk, and H. van Bekkum, “Activated carbon
as a support for heteropolyanion catalysts,” Appl. Catal. A, Gen., vol. 80, no. 1, pp.
41–57, 1992.
[62] A. Khodadadi Dizaji, H. R. Mortaheb, and B. Mokhtarani, “Preparation of
supported catalyst by adsorption of polyoxometalate on graphene oxide/reduced
graphene oxide,” Mater. Chem. Phys., vol. 199, pp. 424–434, 2017.
[63] I. Sánchez-García, A. Núñez, L. J. Bonales, J. M. Perlado, and J. Cobos, “Study of
the adsorption capacity of graphene oxide under gamma radiation in different
media,” Radiat. Phys. Chem., vol. 165, no. July, p. 108395, 2019.
[64] B. Lian, S. De Luca, Y. You, S. Alwarappan, M. Yoshimura, V. Sahajwalla, S. C.
Smith, G. Leslie and R. K. Joshi, "Extraordinary water adsorption characteristics ofgraphene oxide," Chem. Sci., vol. 9, 5106.
[65]
Dimiev, Ayrat.M., Eigler, S., 2017, "Graphene Oxide:Fundamentals andApplications", John Wiley & Son, Ltd, UK.
[66] Ho. Tuan. A., Striolo. A, "Polarizability effects in molecular dynamics simulationsof the graphene-water interface," The Journal of Chem. Phys., vol. 138, 054117, 2013.
[67] J. Sala. E. Guardia, J. Marti, "Specific ion effects in aqueous electrolyte solutions
confined within graphene sheets at the nanometric scale," Phys. Chem. Chem. Phys.,
14, 10799-10808, 2012.
[68] R. R. Nair, H. A. Wu., P. N. Jayaram., I. V. Grigorieva, A. K. Geim, "Unimpeded
permeation Of water through helium-leak-tight-graphene –based membranes,” Science,
335 (6067), 442-444, 2012.
[69] A. Y. Romanchuk, A. S. Slesarev, S. N. Kalmykov, D. V. Kosynkin, J. M. Tour,“Graphene Oxide for effective radionuclide removal,” Phys. Chem. Chem. Phys., 15, 2321,
2013.
[70] L. R. Radovic, “Active site in graphene and the mechanism of CO 2 formation incarbon Oxidation,” J. AM. CHEM. SOC., 131, 17166-17175, 2009.
[71] G. D. Parfitt, C. H. Rochester, 1983, “Adsorption from solution at the solid/liquidinterface,” Academic Press, London.
42[72] T. Szabo, P. Maroni, I. Szilagyi, “Size-dependent aggregation of graphene oxide,”
Carbon, 160, 145-155, 2020.
[73] A. M. Dimiev, L. B. Alemany, J. M. Tour, “Graphene oxide. Origin of acidity, Its
instability in water, and a new dynamic structural model,” ACS Nano, 7, 576-588, 2013.
[74] T. Szabo, E. Tombacz, E. Illes, I. Dekany, “Enhanced acidity and pH-dependent
surface Charge characterization of succively oxidized graphite oxides,” Carbon, 44, 537545, 2006.
[75] K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannet, and A. Zettl, “Determination of
the local chemical structure of graphene oxide and reduced graphene oxide,” Adv. Mater.,
22, 4467-4472, 2010.
[76] D. Bousa, J. Luxa, V. Mazanek, O. Jankovsky, D. Sedmidubsky, K. Klimova, M.
Pumera, and Z. Sofer, “Toward graphene chloride: chlorination of graphene andgraphene oxide”, RSC Adv., 6, 66884, 2016.
[77] T. Taniguchi, S. Kurihara, H. Tateishi, K. Hatakeyama, M. Koinuma, H. Yokoi, M.
Hara, H. Ishikawa, and Y. Matsumoto, “pH-driven, reversible epoxy ring
opening/closing in graphene oxide”, Carbon, 84, 560-566, 2015.
[78] J. Zhang, C. Xiong, Y. Li, H. Tang, X. Meng, and W. Zhu, “The critical contributionof oxidation debris on the acidic properties of graphene oxide in an aqueous solution”,
Journal of Hazardous Materials, 402, 123552, 2021.
[79] B. S. Nugroho, M. N. K. Wihadi, F. Grote, S. Eigler, and S. Nakashima, “Potentialityof graphene oxide and polyoxometalate as radionuclides adsorbent to restore theenvironment after fukushima disaster: A mini review”, Indones. J. Chem., 21 (3), 776-786,
2021.
[80]. K. A. Mkhoyan, A. W. Countryman, J. Sylcox, D. A. Stewart, G. Eda, C. Mattevi, S.
Miller, M. Chhowalla,“Atomic and electronic structure of graphene oxide”, Nano Letter,
9 (3), 1058-1063, 2009.
[81] R. Contant and R. Thouvenot, “A Reinvestigation of Isomerism in the Dawson
structure: syntheses and 183W NMR Structural Characterization of Three New
Polyoxotungstates [X2W18O62]6- (X = Pv, Asv)”, Inorg.Chim.Acta, 212, 41-50, 1993.
[82] A. Misra, K. Kozma, C. Streb, M. Nymann, “Beyond Charge Balance: Counter-cationin Polyoxometalate Chemistry”, Angew. Chem. Int. Ed., 59, 596-612, 2020.
[83] M. Acik, C. Mattevi, C. Gong, G. Lee, K. Cho, M. Chhowalla, Y. J. Chabal, “The roleof intercalated water in multilayered graphene oxide”, ACS Nano, 4 (10), 5861-5868, 2010.

43

Supporting Information
(a)

(b)

(c)

(d)

(e)

Fig S1. SEM measurement (a)α-KΑP΍WΌΓOΑ΍.14H΍O (b)Tungsten element (c)Oxygen element(d)Carbon K- α X-ray element (e)Secondary electron image

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig S2. SEM measurement, [GO70POM]41 (a)Secondary electron image (b)Carbon K- α X-ray element(c)Oxygen element (d)Tungsten element (e)Aluminum element (f)Cesium element (g)Chloride element

44Fig S3. Plausible Interaction between Graphene-Oxide (GO) and Dawson-type Polyoxometalates(POMs). The black and the grey balls corresponds to carbon and hydrogen. The polyhedral model
represents to Dawson-type polyoxometalates. The blue dashes indicated of the presence ofinteraction between GO-POMs.

45

140
120

Intensity

100
80
60
40
20
0
-20
10

20

30

40

50

60

70

80

2ThetaFig S4. Xrd pattern of α-KΑP΍WΌΓOΑ΍.14H΍O
5

10

15

20

25

30

35

51

40

45

50

55

5

60

[GO40POM]18/Cs

34

15

20

25

30

35

40

45

50

55

60

[GO39POM]18/Cs

34
17

17

0
153

0
117

[GO40POM]18

78[GO39POM]18

102
51

39

0

0
57

57

[GO40POM]41/Cs

38[GO72POM]41/Cs

38
19

Intensity

19

Intensity

10

51

0
168

[GO40POM]41

112

0
129

[GO72POM]41

86
43

56
0
51

0
48

GOc40/Cs

34

GOc39/Cs

32
16

17

0

0
150

162

GOc40

100

GOc39

108
54

50

0

0
5

10

15

20

25

30

35

40

45

50

55

60

5

10

15

20

25

30

35

40

45

50

55

60

2Theta2 Theta

Fig S5. XRD pattern of GOc39, GOc40, GOPOM composite before and after Cs adsorption

46

4000
0.099
0.066
0.033
0.000

3500

3000

2000

1500

1000

500

2000

1500

1000

500

(h)

3.32
2.49
1.66
0.83
0.106
0.053
0.000
-0.053
6.0
4.5
3.0
1.5
32.8
28.7
24.6
20.5

(g)

(f)

absorbance

(e)

(d)

95
76
57
38
-0.038-0.076-0.114-0.152
10.4
7.8
5.2
2.6
4000

2500

(c)
(b)

(a)

3500

3000

2500

-1

wavenumber (cm )Fig S6. IR spectra for (a)GOc70 (b)GOc70 after Cs adsorption (c)GOc72 (d)GOc72 after Cs adsorption(e)GOc39 (f)GOc39 after Cs adsorption (g)GOc40 (h)GOc40 after Cs adsorption

47

B

A
4000
-0.021

3500

3000

2500

2000

1500

1000

500

4000

(d)

3500

3000

2500

1500

1000

500

1500

1000

500

(d)

0.15

-0.0422000

0.12-0.0630.09-0.084

Absorbanceabsorbance

0.06

(c)

-0.029-0.058-0.087-0.116-0.076-0.114-0.152-0.105(b)(b)-0.156-0.140-0.208-0.175-0.260-0.210-0.312
0.000

(a)

0.075

-0.0540.050-0.1080.025-0.162

0.000
4000

(c)

-0.038

3500

3000

2500

2000

1500

1000

500

-0.216
4000

wavenumber (cm-1)

(a)
3500

3000

2500

2000

wavenumber, cm-1

Fig S7A. IR spectra for (a) [GO39POM]18 (b) [GO39POM]18 after Cs adsorption (c) [GO39POM]41 (d)[GO39POM]41 after Cs adsorption, S7B. (a)[GOΒ΋POM]18 (b)[GOΒ΋POM]18 after Cs adsorption(c)[GOΒ΋POM]41 (d)[GOΒ΋POM]41 after Cs adsorption.

48

(a) POM(b) POM/Cs
50

Absorbance

40

30

20

10

0
4000

3500

3000

2500

2000

1500

1000

500

-1

wavelength (cm )Fig S8. IR spectra for (a)POM (b)POM after Cs adsorption

49

(a)

Graphite
GOc70
GOc72

6000

GOc39

intensity (.a.u.)

5000

GOc40

4000
3000
2000
1000
0
500

1000

1500

2000

2500

3000

-1

Raman Shift (cm )

1200

(b)

[GO70POM]41[GO70POM]41/Cs

Intensity (a.u.)

1000

800

600

400

200
500

1000

1500

2000

2500

3000

-1

Raman shift (cm )Fig S9. Raman spectra (a)Graphite, GOc70, GOc72, GOc39, GOc40, (b) [GO70POM]41 and after Cs
adsorption

50

4000

Intensity (.a.u.)

3500[GO39POM]41

(a)

3000

[GO39POM]41/Cs

2500
2000
1500
1000
500
500

1000

1500

2000

2500

3000

-1

Raman shift (cm )

(b)

GOc70
GOc70/Cs

5000

Intensity (.a.u.)

4000

3000

2000

1000

0
500

1000

1500

2000

2500

3000

-1

Raman shift (cm )Fig S10. Raman spectra composite (a) [GO39POM]41 before and after Cs adsorption (b)GOc70
before and after Cs adsorption

51

(a)

GOc39
GOc39/Cs

2500

Intensity (.a.u.)

2000

1500

1000

500

0
500

1000

1500

2000

2500

3000

-1

Raman shift (cm )

(b)

GOc40
GOc40/Cs

2000
1800
1600

Intensity (.a.u)

1400
1200
1000
800
600
400
200
0
-200
500

1000

1500

2000

2500

3000

-1

Raman shift (cm )Fig S11. Raman spectra (a)GOc39 before and after Cs adsorption (b)GOc40 before and after Cs
adsorption

52

[GO72POM]41

2500[GO72POM]41/Cs

Intensity (.a.u.)

2000

1500

1000

500

0
0

500

1000

1500

2000

2500

3000

3500

-1

Raman shift (cm )Fig S12. Raman spectra composite [GO 72POM]41 before and after Cs adsorption

53

Table S13.The ratio ID/IG of Graphene oxide and composite before and after Cs adsorption
No.

Material

D bandG bandRatio ID/IG

1

GOc70

1356

1586

0.93

2

GOc72

1352

1581

0.91

3

GOc39

1352

1590

0.89

4

GOc40

1347

1590

0.88

5

[GO39POM]41

1347

1581

0.92

6

[GO39POM]41/Cs

1352

1586

0.90

7

[GO70POM]18

1347

1586

0.96

8

[GO70POM]18/Cs

1352

1577

0.92

9

[GO70POM]41

1357

1592

0.88

10

[GO70POM]41/Cs

1343

1602

0.93

54

Table S14. The crystallize size of GO
SampleConcentration
ratio GO:POM

2Φ (°)FWHM(ß)

Out of
plane

d
(nm)

d
(nm)
after Cs
adsorption

Layer
(n)

In plane
crystallite
size – L(nm)crystallite
size – D(nm)
Graphite

-

26.6

0.28

30.49

0.34

-

90.67

10.27

GOc70

-

11.58

1.93

4.3

0.77

-

6.58

2.59

GOc72

-

8.66

1.69

4.9

1.02

-

5.80

2.53

GOc39

-

8.91

1.57

5.3

0.99

-

6.19

2.56

GOc40

-

9.13

1.47

5.7

0.97

-

6.75

2.61

[GOΒ΋POM]18

1:8

12.28

1.63

5.12

0.72

0.79

-

2.54

[GOΒ΋POM]41

4:1

10.99

1.48

5.64

0.81

0.86

-

2.61

[GO72POM]18

1:8

12.75

1.10

7.60

0.69

0.77

-

3.04

[GO72POM]41

4:1

12.05

1.07

7.80

0.73

0.79

-

3.09

[GO39POM]18

1:8

11.92

1.19

7.02

0.74

0.81

-

2.89

[GO39POM]41

4:1

11.86

1.07

7.80

0.74

0.79

-

3.09

[GO40POM]18

1:8

11.57

0.84

9.94

0.76

0.83

-

3.71

[GO40POM]41

4:1

11.57

0.24

34.78

0.76

0.78

-

11.95

*FWHM was calculated by using origin software

55

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig S15. The composite solution after Cs adsorption. Only Composite [GO70POM]41 (b)that forms stable solution (black color). Another composite solution shows remains
coagulated. The photo was taken after more than one-hour stay at ambient
temperature.

(a)

(b)

(c)

(d)

Fig S16. The GO solution after Cs adsorption. All GO solutions shows remains
coagulated. (a) GOc70 and (b) GOc72 shows different coagulated behavior with (c) GOc39
and (d) GOc40. The photo was taken after more than one-hour stay at ambient
temperature.

56

Dark
Brown

Fig S17. Coagulation process after Cs interaction with (a) GOc70 and (b) GOc72. The photo
was taken after more than one-hour stay at ambient temperature.

57

(b

(a)

Fig S18. SEM image by using composite (a) [GO70POM]41 after Cs adsorption and (b) Cs particle(zoom mode based on figure a).

58

Chapter III. Improvement of Cs detection performance andformation of CsCl and Cs nanoparticles by tuning graphene oxide
quantum dot-based nanocomposite

3.1.Introduction
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) disaster that occurred in 2011
has attracted the attention of researchers. Specifically, the release of radioactive materials
like Cs-137 (with half-life of 30.1 years) has drawn the attention because of the immediateand long-term effects on the environment [1],[2]. Since the radioactive cesium (Cs) has ahigh solubility in water and a long half-life, it can migrate from one pond to the river,move far away to the paddy field [3], and finally come into the groundwater. As reported
in the previous work, the radioactive Cs was detected in the tap water and ground waterwith different degree of contamination in some area of Fukushima [4]. Consequently, it
was restricted to be consumed by all people who stay around the FDNPP area. Therefore,
the detection of Cs is of great importance to address the problem of Cs contamination inwater.
Currently, several types of methods for Cs detection exist including atomic
absorption spectroscopy, electrochemical devices, germanium detector, inductively
coupled plasma mass spectroscopy, and fluorescence-based method. Especially thefluorescence-based method, which is infrequently used in Cs detection system, has apotential to become a non-destructive detector with low-cost. In the previous studysquaraine dye as a mixed solution (DMSO/Water) exhibited a good response in Cs
detection. This compound can work effectively and selectively by quenching thefluorescence intensity after Cs detection. According to this method, the blue fluorescenceof the compound solution turned OFF after the Cs addition[5]. In the other work, T. Moriet al. have successfully designed a new supramolecular material (cesium green,
C32H33NO8) as Cs sensor expecting intermolecular fluorescence approaches (fluorescenceresonance energy transfer (FRET) or exciplex mechanism) [6]. The study of energy
transfer has been reported experimentally for different kinds of nanomaterials (quantum
dots, nanoparticles) that act as energy donors or acceptors [7],[8]. Another researcher
also demonstrated that the Cs can be detected using green fluorescence by cesium green(in methanolic solution) for the freeze-dried stem of plants that have already been treatedwith cesium carbonate[9]. However, the cesium green molecule has several limitations in59

Cs detection application, since it can work effectively only in solid-state, without water,
and in basic condition. Therefore, several technical problems should be addressed tomake it more useful and easily to be used in order to identify Cs in contaminated water.
In recent years, graphene oxide (GO) and graphene oxide quantum dots (GOQDs) are
widely studied in several applications such as metal adsorbent (Cs+, Eu3+, Sr2+)[10],[11],and designed as fluorescence probes by considering their quantum confinement and theedge effect [12],[13]. In addition, the GO and GOQDs have almost similar properties
because they have various oxygen functional groups. The functional group can act as abinding point to synthesize a new nanocomposite to improve their properties [14],[15].
Another important thing is that the photoluminescence (PL) of GOQDs was independentof pH. It means that the emission wavelength does not shift in different pH condition(only change in the PL intensity) [16]. Further investigation also revealed that the PL of
GO has shown a reversible response to ionic strength and pH [17],[18]. The previous work
confirmed that the different number of layers of GO (single and a few layers) showed
different optical responses in various organic solvent [19]. More currently, Yao et al. have
employed graphene quantum dot-based fluorescence sensing as a dual detector of copperion (Cu2+) and tiopronin (MPG). They proposed a novel fluorescence turn OFF for the
Cu2+ detection and turn ON for the tiopronin (MPG) detection [20].
Therefore, to facilitate the development of a Cs detection system, it is highly desirable
to create a new nanocomposite using functionalized graphene oxide quantum dots
(GOQDs) with cesium green molecule. The main purpose of the present study is to
produce a new nanocomposite that has the ability to recognize Cs in water with different
pH conditions (acidic and basic condition) by considering the turn ON/OFF response.
3.2.Experimental Section
3.2.1. Reagents and materials
Graphite, cesium green (C32H33NO8), potassium permanganate (KMnO4), hydrogen
peroxide (H2O2), sulfuric acid (H2SO4), sodium nitrate (NaNO3), sodium hydroxide(NaOH), cesium chloride (CsCl), hydrochloric acid (HCl), and potassium hydroxide(KOH) were analytical grade and were used without any further purification.
3.2.2. Synthesis of GO and GOQDs
Graphene oxide (GO) samples were prepared using modified Hummer`s method withgraphite and the oxidizing agent, KMnO4 (1:4, mass ratio) [21]. In this synthesis protocol,
pre-cooling procedure and one minute stop-one minute go was implemented when
adding the KMnO4 according to our work [10]. The GOQDs were synthesized by using60

as-prepared few-layers GO. In a typical synthesis, the ratio of GO and NaOH was 1:1(mass ratio). First, dispersed the starting material GO (0.19 g) into distilled water followedby sonification for 5 minutes. Then, added NaOH (0.19 g) and stirred for 5 minutes atambient temperature. Second, the solution was refluxed at 80 oC for two hours. After
finished the reflux process, kept the solution at room temperature for a while. Then, cameto the final step, centrifuged the solution for two times (4000 rpm for 10 minutes and 9500
g-force (relative centrifuge force) for 50 minutes). The brown solution was obtained as
GOQDs solution. There is no further treatment for as-obtained GOQDs solution (Fig. 2e).
3.2.3. Synthesis of GOQD-based nanocomposite
The nanocomposite was generated by combining GOQDs solution as precursor material
with cesium green (CG) solution. In this study, the nanocomposite was produced in two
different concentration ratios of GOQDs:Cesium green (concentration ratio 5:1 and 8:1).
In order to obtain the nanocomposite with a concentration ratio of 5:1, first, prepare 25
mL GOQDs solution (distilled water) and prepare the cesium green solution bydissolving 1 mg cesium green in 5 mL THF solvent. After that, mixed the GOQDs solution
with cesium green solution followed by stirring for 30 minutes. A similar treatment was
performed for the nanocomposite with a concentration ratio of 8:1. In order to distinguishboth nanocomposites, the term composite-51 and composite-81 were applied. Each index
number attached to the composite name is corresponded to the concentration ratio of
GOQDs:cesium green molecule.
3.2.4. Photoluminescence detection of cesium
The Cs detection was performed at room temperature. There are two different
concentrations of cesium chloride (CsCl) solution. The CsCl solution was made by adding
0.6 g CsCl in 30 mL distilled water. Based on that, the concentration of the CsCl solution
was 0.12 mmol/L. Then, prepared the second CsCl solution to get a concentration of 0.06
mmol/L. The next step was that the different amounts of CsCl solution (changing volume
from

200

L-800

L)

were

added

into

the

nanocomposite

solution.

Thephotoluminescence spectra were measured and recorded under excitation slit 10 nm,
emission slit 10 nm, PMT voltage 400 V, and excitation wavelength 330 nm. Finally,plotted the data as a standard curve graph (the PL intensity unit is cps as measured fromthe instrument).
3.2.5. Instrumentation and Characterization
Atomic force microscopy (AFM) was conducted by Asylum, MFP-3D origin (Oxford,
UK). Ultraviolet-visible spectroscopy (UV-Vis) measurements were carried out using

61

JASCO

V-650

spectrophotometer(Japan).

Photoluminescence

spectroscopy

measurement was performed by Fluorescence spectrophotometer F-7000, Hitachi(Fukuoka, Japan). Transmission electron microscopy (ultra-high resolution transmission
electron microscope) was performed using JEM-2010 (Tokyo, Japan). Zeta potential
measurements were performed using Zeta potential and particle size analyser ELSZseries 2000 (Otsuka, Japan). Elemental analysis was carried out using Perkin Elmer
CHNS/O 2400II (USA).
3.3. Results and Discussion
3.3.1. Characterization of GO and GOQDs
The transmission electron microscopy (TEM) (Fig. 1a and 1b) and atomic force
microscopy (AFM) (Fig. 1c and 1d) were performed to characterize the morphology andstructure of GO as precursor material and GOQDs after reflux process. It is observed that
the GO sample is wrinkles and overlap (Fig. 1a). For the ratio of graphite to potassium
permanganate (KMnO4) as oxidizing agent, 1:4, the graphite was successfully exfoliated.
The elemental analysis of the GO sample was carried out to confirm the C, H, N, and O
element as displayed in the Table. 1. In addition, the as obtained GO was employed as aprecursor to fabricate GOQDs. As shown in Fig. 1b, the GOQDs image was produced.
The NaOH reflux process has detached well the flake of oxidation debris (OD) from GO
sample. These pieces (several nm) fell down on the GO layer. These are GO quantum dots
(GOQDs). The GOQDs have a heterogeneous particle size distribution as is shown in
TEM imaging (Fig. 1b). As proposed by previous experimental work, these small particles
are adsorbed on the surface of graphene-like sheets [22]. And these small particles can beseparated from GO by base treatment. Rourke at al. used the terminology for this small
particle as `oxidation debris` [23].
Table 1. Elemental analysis of GO sample
Sample
C%
H%
GO
43.82
2.97

N%
0.18

O%
52.4

C/O
0.84

Furthermore, to confirm the thickness of the GO and GOQDs sample, the AFM imaging
was conducted. Fig. 1c exhibited the height of GO is about 2-3 nm. It is also convincedthat the GO material consists of a few layers. In addition, there are two types of dots for
as-obtained GOQDs. The aggregating dots (aggregates) and small dots were confirmedwith an average thickness of about 2 nm and 1.5 nm, respectively (Fig. 1d, S1a, see
62

supporting information). This value (for small dots) is close to the value corresponding
to the thickness of OD as previously obtained by AFM (0.5-1.5 nm) [24] and SNR or
specular neutron reflectivity (1 nm) [25]. It is suggested that the aggregates are composedof many single GOQDs gathered as disc pile up (H-type aggregation) (Fig. S1b, seesupporting information) rather than head to tail arrangement (J-type aggregation).
Another group has reported that there is a unique emission of graphene quantum dots
induced by self-assembled aggregation [26].

a

b

c

d

Fig.1. TEM imaging of (a) GO, scale bars: 200 nm, (b) GOQDs, scale bars: 50 nm.
(c) AFM image of GO, and (d) GOQDs deposited on freshly cleaved silicon
substrate.

3.3.2 Optical properties of GOQDs and their nanocomposite
The optical properties of GOQDs were characterized. UV-Vis absorption spectra (Fig. 2a)show a broad absorption from 800 to 200nm and have one peak at 205nm and two
shoulders at 270nm and 330nm for GOQDs. The photoluminescence (PL) spectra (Fig. 2b)show that the different PL peak intensity appeared before (Fig. 2d) and after
centrifugation process (Fig. 2e). This method also has been considered by anotherresearcher to achieve the tunable optical properties [27]. In particular, the centrifugation
method demonstrated an effective way to decrease the size of functionalized graphene
oxide (GO). This is in good accordance with the characterization data shown in an earlysection (Fig. 1b). The TEM image emphasized a different size of GOQDs. Moreover, in
the Fig. S5a (see supporting information), the larger particle has been isolated. It showed63the presence of large sheets, almost similar in size with the as-produced GO (Fig. 1a). As
a result, it is expected that as-produced GOQDs have an intrinsic PL intensity. The otherexperimental studies were reported to identify the disintegration of GO during the basetreatment [22]. In their work, the concentration ratio of GO to NaOH, 1:1, was used totreat with heating for 1-5 hours. The results showed that the prolonged base treatment
leads to disintegration of the flake of GO into smaller pieces. This work also confirmedthat insignificant deoxygenation would proceed if base treatment was conducted without
heating. On the contrary, heating without base treatment does not bring in detectabledeoxygenation. It is consistent with our result that confirmed the deoxygenation after
base treatment with reflux process. As is shown in Fig. S5b (see supporting information),
the elemental analysis obtained by TEM shows that the oxygen content decreased
significantly.
The photoluminescence (PL) spectra of GOQDs, cesium green (CG) and theirnanocomposite in THF solvent were shown in Fig. 2 and Fig. 3. The PL spectra of GOQDs
are significantly different between acidic and basic conditions. As is reported by aprevious study the GOQDs are strongly sensitive to the deprotonation and protonation
GOQDs
GO

After centrifugation

a

b

DW
Before centrifugationd

GOQDs in Acid
CsGreen in THF
GOQDs in Basic
GOQD

c

e

THF onlyDistilled water

Fig.2. UV-Vis absorbance of (a) GO, GOQDs. Photoluminescence emission spectra of (b)GOQDs before and after centrifugation process, (c) GOQDs in acidic and basic condition,
Cesium green in THF. (d) GO solution after reflux process (before centrifugation). (e) GOQDs
solution after centrifugation.

process [28]. It is suggested that the different concentration in OH- and H+ influences thechange of electronic transition of S-S* and n-S* in GOQDs. It is also amplified by Taniguchi
et al. that the proposed concept of the reversible epoxy formation leads to pH-dependent
on/off PL of GO [18]. They found that the enhancing and quenching PL of GO can be
induced by increasing/decreasing the epoxy concentration and will be influenced by the64presence of charge transfer exciton (CTE). In the present study, the PL intensity of
GOQDs in acidic condition increased compared to that in basic condition. The present
result may be explained by pH-dependent change of functional group. The peak also redshifted from 435nm to 454nm by changing from basic to acidic condition (Fig. 2c).
On the other hand, as shown in Fig. 3, after the nanocomposite was formed, the PL
intensity drastically increased in basic condition (pH 11.01) for both nanocomposites(composite-51 and composite-81), while the PL intensity was slightly quenched in acidic
condition. In this case, the PL of the nanocomposite exhibited an opposite response foracidic and basic conditions compared with the original sample. For original sample, the
GOQDs exhibited enhancing PL intensity in acidic condition and quenching PL intensity
in basic condition. Whilst for the nanocomposites, the decreased PL intensity wasobserved in acidic condition and the increased PL intensity was observed in basic
condition. In the other work the GO showed the reversible response upon acidic and basic
condition [27]. Fig. 3b shows that the spectrum of the nanocomposite is almost theComposite-51
Composite-81

GOQDs

a

b

CsGreen Co
Composite-51
in THF

CsGreenin THF
GOQDs
GO

Composite-81dFig.3. Photoluminescence emission spectra of (a)composite-51, composite-81 in basic
condition, (b) composite-51, composite-81 in acidic condition. (c) TEM image of
GOQDs-based nanocomposite (composite-81). Scale bars: 50 nm. (d) High-resolution
TEM image of individual nanodot (composite-81) labelled by red circle with inset of theFast Fourier Transform (FFT) from the selected region. Scale bars: 5 nm.

superposition of the GOQDs and CG in acidic condition, suggesting the physicalmixture of GOQDs and CG in acidic condition. On the other hand, the PL intensity of thenanocomposite was significantly enhanced under basic condition (Fig. 3a). In basic
condition the chemical bond was suggested between GOQDs and CG. This enhancement
in basic condition is in accordance with PL of conventional carbon nanodots with edge65

located COH/COOH/C=O groups [30]. In the present experiment, the pH condition waschanged from basic condition to acidic condition. Then we also investigated the PL
intensity of the nanocomposite from acidic condition to basic condition. As a result, thepeak moved back to the original position and the intensity significantly increased (Fig.
S13, see supporting information).
As is already reported, the GOQDs-PEG (Polyethylene glycol) interaction exhibited atypical excitation wavelength dependence of the photoluminescence [31], [32]. Fig. 3c,
and Figs. S3, S4 (see supporting information) show the TEM image of the present
composite. It can be seen that the nanodots are combined. Figure 3d shows highresolution TEM image of an individual nanodot (composite-81). This indicates highcrystallinity (localized in lattice fringes labelled by red circle) of GOQDs after forming acomposite. It is also suggested that the cesium green (CG) molecule functionalization of
GOQDs does not cause significant damage to the graphene S system.
3.3.3. Cs detection performance
3.3.3.1. Cs detection in acidic condition
The PL response of the nanocomposite to the Cs cation was tested by mixing ananocomposite solution with an aqueous solution of CsCl. In this experiment, the Cs
treatment was carried out in two different pH conditions (acidic and basic condition) of
the nanocomposite by adding two concentrations of CsCl (0.12 mmol/L and 0.06
mmol/L). Then, the CsCl addition was conducted in the volume range 200 L-800 L.
The results show that the addition of 0.12 mmol/L cesium chloride (200 L-800 L) inacidic condition led to increase the PL intensity gradually (turn ON) by employing
composite-51 (Fig. 4a). For composite-81, using similar method with the case ofcomposite-51, the PL intensity was decreased (turn OFF) at the first time of Cs addition(200 L), and then the PL intensity increased (turn ON) gradually after adding another
200 L CsCl solution (Fig. 4b). The turn ON PL phenomenon also occurred in composite81 by reducing the concentration of CsCl (0.06 mmol/L) (Fig. 4c). This might be explainedby the fact of composite-81 having much GOQDs part compared with the composite-51.
At first GOQDs part reacts with CsCl, leading to the decrease in PL intensity (Fig. 4b) as
is observed in GOQDs (Fig. 4d). After that, turn ON occurs as is the case of composite-51.
We also considered that another factor such as structural defect (holes) effect might
contribute to the PL phenomenon. The presence of holes in nm-size was investigated in
HRTEM studies of GO [33]. Another researcher also considered these tiny vacancy defect66as a part of the oxidized domains [34]. As is explained previously, during the basetreatment the functional groups have been reduced (deoxygenation) from S surface anddisintegration of the flake of GO occurred. In this situation, it can be predicted thatnumerous additional defects were formed. In the present study the evidence of thestructural change was shown through the Zeta potential value change (from -19.67 mV
for GO to -34.63 mV for GOQDs). As a result, these defects can serve as a trap or binding
point since the functional groups (especially the carbonyls) likely exist along the edges of
the holes [35]. Subsequently, a TEM measurement was performed to consider theadsorption behaviour of Cs by nanocomposite. The information obtained from the TEM
image (Fig. 5a) confirmed the small black spots. It is considered as Cs cluster on the
nanocomposite layer by using its eds (Fig. 5b). This might be related to the GO acidity.
Our previous work presented that the different C/O ratio of GO (different in degree ofoxidation) has significant aspect in producing the diversification of sp2/sp3 carbon clusterof GO and influenced the Cs adsorption performance [10]. Another study alsoauthenticated that the oxidation debris (OD) has an important role in GO acidity [36]. The
study also proved that the amount of acid site density of OD is almost 3 times higher thanthat of GO. As we considered in the early section of the present study GOQDs are termed
as the small size of OD produced by reflux process (with base treatment). A chemical
Turn OFF

Turn ON
PL

Turn ONaTurn ON

Turn OFFcd

Fig.4. Photoluminescence emission spectra of (a) composite-51 before and after Cs treatment for 0.12
mmol/L (in acidic condition), (b, c) composite-81 before and after Cs treatment for 0.12 mmol/L and 0.06
mmol/L (in acidic condition), (d) GOQDs after and before Cs treatment for 0.12 mmol/L (in acidic
condition). A colour line (graphic) in each figure indicate the PL response signal by the additionalamount of CsCl solution, with volume range 200 PL (orange), 400 PL (gray), 600 PL (yellow), and 800 PL(blue).

analysis by TEM (Fig. 5d) revealed that Cs rich structure is the main constituent alongwith other species (C, O, Na, Cl). However, it might include the undesired impurities (Si,67

Na, K, S) as revealed in Fig. 5 (b). Si and K might come from the re-used laboratory
glassware as a source. In addition, the remaining Na is originated from a part of NaOH
addition during the base-treatment. Specifically, we assumed that the formation of Cs
cluster is produced as a part of the moiré fringe pattern formation (Fig. 5c) mechanism. It
was verified as a consequence of the decomposition process of the adatoms nanoparticle
under electron beam irradiation [37]. Nakashima`s group also reported the CsCl and Cs
rich particle formation depending on the substrate concentration and the solvent [38].
Further discussion is presented based on several phenomena in the section 3.3.4
regarding the Cs metal formation on GOQDs and nanocomposite layer.
However, as is shown in Fig. 4d, there is no PL enhancement (turn OFF) after 0.12
mmol/L CsCl solution addition by employing original GOQDs solution, and also
quenching occurred in basic condition (Fig. 6a). This is due to the interaction between
GOQDs and Cs+. Some researchers have proved that the fluorescence-quenching
phenomenon occurred after addition of metal and nanoparticles like Fe 3+ ions in GO
solution and unbonded gold nanoparticles (NPs) to graphene quantum dots [39], [40].
Typically, GO and GOQDs can act as both an attractive fluorophores and remarkable
a

b

O
5 nm

Si
Cs

C
Na

Cl
S

K

c

Cs

d

O
Na

Cl

C

Fig.5. (a) TEM image of composite-81 after Cs treatment (in acidic condition), small blackspots are identified with inset of HRTEM from the selected area. Scale bars: 20 nm. (b)Elemental analysis of figure a. (c) HRTEM image of composite-51 after Cs treatment (inacid condition) with inset of the Fast Fourier Transform (FFT) from the selected region.
Scale bars: 5 nm. (d) Elemental analysis of figure c.

68quencher [41]. As enhancing agent, the fluorophores can serve the tunable PL emission
and in contrary, as quencher, the GO and GOQDs revealed an effective quenchingefficiency through either electron transfer or fluorescence resonance energy transfer(FRET) mechanism.
It can be thought that the surface charge has a significant role in the PL of GOQDs
when interacted with Cs cation. As is already shown, Zeta potential was -34.63 mV for
GOQDs and -19.67 mV for GO in the present study. The surface of GOQDs is morenegative than that of GO. In addition, since the GOQDs still have an oxygen functionalgroup, it can be assumed that the interaction between Cs cation and -COOH has induced
PL quenching in the detection system. Another possibility is the -OH on the surface area
can coordinate to Cs cation. Previous study has proposed the concept of dynamic
structural model of GO responsible for binding metal ions [42]. Particularly, the dynamic
structural model is well supported by the conversion process from vicinal diols (formedby opening the epoxides) to enols as the structural transformation of the functionalgroups led to enols being acidic. The other researcher has revealed the reversible epoxy
formation [18]. More current work also strongly suggested a possibility that the otherfunctional groups (not only carboxyl) have played a significant role for binding to metal
ions [43]. The as-formed enols as the product resulted from reorganization of GO
structure might serve as a chelating agent for the metal cations.
3.3.3.2. Cs detection in basic condition
The significant PL quenching was observed in the Cs detection performance in basic
condition. As shown in Fig. 6, the PL intensity of all samples decreased gradually afterCs treatment with different concentration, 0.12 mmol/L and 0.06 mmol/L (200 L-800 L
Cs addition). This quenching phenomenon was clearly different from Cs treatment inacidic condition that shows enhancing phenomenon. It can be seen in the Fig. 6 (b), (c),(d) that no change at 380nm and large change at 470nm. Unlike in acidic condition, thecarboxylic acid groups are present as COO and the surface of GOQDs plane is
negatively charged in basic condition. It is also well known that many metal ions can act
as a PL quencher in several ways such as the effect of complexation, fluorescenceresonance energy transfer, or charge transfer. The previous work emphasized that thequenching reaction can be considered as a longer or shorter distance effect between donor
and acceptor molecules [44], [45]. Furthermore, another study confirmed that in strong
basic condition, GO has a greater interaction for binding metal, which is provoked by asurface modification of GO structure [46].
69In the present study, the modification of GO structure was induced by a chemical
reaction through the base-treatment followed by the reflux process. And as-obtained pH
value of GOQDs solution was 11.60. As was suggested by A. M. Dimiev et al. the basetreatment of aqueous GO solution led to hydrolysis of organic sulfates and trigger thereaction and cause a C-C bond cleavage with formation of ketone at the new edge. Then,the ketones further transform into carboxylates [22]. The other observation additionallyconvinced that the negatively charged GO as a ligand can replace water molecule fromthe first coordination sphere of metal [34]. Therefore, the oxygen functional group of
GOQDs dominantly plays a significant role (activated) for Cs detection as its naturalenvironment even after forming the nanocomposite. The TEM image (Fig. S6 (a, c), seesupporting information) was taken and the interaction between Cs cation andnanocomposite shows a similar tendency with the previous section (in acidic condition),and the CsCl and Cs clusters were observed.
Turn OFF

Turn OFF(a)(b)

Turn OFF

Turn OFF(d(c)

Fig.6. PL emission spectra of (a) GOQDs before and after Cs treatment for 0.12 mmol/L (in basic condition),(b) PL emission spectra of composite-51 before and after Cs treatment for 0.12 mmol/L (in basic condition),(c, d) PL emission spectra of composite-81 before and after Cs treatment for 0.12 mmol/L and 0.06 mmol/L,
respectively (in basic condition). A colour line (graphic) in each figure indicate the PL response signal by theadditional amount of CsCl solution, with volume range 200 PL (orange), 400 PL (gray), 600 PL (yellow), and
800 PL (blue).

3.3.3.3. The detection behaviour of Sr2+, K+
For comparison, the effect of the other cation addition was investigated. Under similar
conditions to the Cs+ treatment, the Sr2+ and K+ treatment was performed in acidic andbasic conditions by using a composites solution. In acidic condition, as exhibited in Fig.
7 (a, b), the addition of KCl solution (from 200uL-800uL) shows a decrease in PL intensity(turn OFF). However, in the case of Sr2+ treatment, the PL intensity increased (turn ON)
for the first 200uL SrCl2 addition both in composite-51 and composite-81, and then, the70PL intensity decreased (turn OFF) gradually after adding another 200uL SrCl 2 solution(Fig. 8 (a, b)).
Moreover, under basic conditions, the presence of K+ and Sr2+ in the composite
solutions exhibited a similar quenching phenomenon (Fig. 7 (c, d) and Fig. 8 (c, d)).
Turn OFF

Turn OFF(b)(a)

Turn OFF

Turn OFF(d)(c)

Fig.7. PL emission spectra of (a) composite-51 before and after K+ treatment for 0.12 mmol/L(in acidic condition), (b) composite-81 before and after K+ treatment for 0.12 mmol/L (inacid condition), (c) composite-51 before and after K+ treatment for 0.12 mmol/L (in basic
condition), (d) composite-81 before and after K+ treatment for 0.12 mmol/L (in basic
condition). A colour line (graphic) in each figure indicate the PL response signal by theadditional amount of KCl solution, with volume range 200 PL (orange), 400 PL (gray), 600
PL (yellow), and 800 PL (blue).
Turn ON

Turn ON

Turn OFF(a)Turn OFF(b)

Turn OFF

Turn OFF(c)(d)

Fig.8. PL emission spectra of (a) composite-51 before and after Sr2+ treatment for 0.12 mmol/L (inacidic condition), (b) composite-81 before and after Sr2+ treatment for 0.12 mmol/L (in acidic
condition), (c) composite-51 before and after Sr2+ treatment for 0.12 mmol/L (in basic condition),(d) composite-81 before and after Sr2+ treatment for 0.12 mmol/L (in basic condition). A colour
line (graphic) in each figure indicate the PL response signal by the additional amount of SrCl 2
solution, with volume range 200 PL (orange), 400 PL (gray), 600 PL (yellow), and 800 PL (blue).

71Especially for Sr2+ treatment, the quenching phenomenon occurred drastically in the first
200uL SrCl2 addition. It might be caused by the Sr2+ having a higher 2+ charge to form
coordination compounds. The precipitation was directly observed after the Sr2+ addition,
which is the cause of drastic decrease in PL. These observations suggested that thenanocomposite in this experiment has a different selectivity and sensitivity to the target
metal ions (Cs+, Sr2+, K+). This feature is an important factor for the detection system. As
is expected, the nanocomposite is more sensitive to the Cs+ by enhancing the PL intensity(turn ON) in acidic conditions compared to other metal cation (Sr 2+, K+). The result also
indicated that the nanocomposite can be used as a promising Sr detection candidate, andfuture investigation is required.
The interaction among GOQDs, cesium green molecule, and some different metal
cations indicated a different PL response signal as displayed in the table S1, seesupporting information. The input-output detection mechanism was constructed to
identify the PL response signal in different pH conditions as applied in the previous work[20]. Here, the different metal cations and cesium green molecule are used as input factor,
and the presence of the input factor is represented by `1` and `0` for the absence of the
input factor. In addition, as output factor, `1` represents the turn ON PL, and `0`
represents the turn OFF PL. In acidic condition, the result shows that, when the input
factor is (0, 0), (1, 1) both for Cs+ and Sr2+ treatment, the PL response signal turns ON,
therefore the output is `1`. This tendency was confirmed by using composite-51 andcomposite-81. On the other hand, the output is `0` for K + treatment by applying (1, 1) as
input factor. A similar phenomenon was observed in the basic condition, i.e., the output
is `1` when the input factor (0, 1) was applied but was not found in the acidic condition
both for composite-51 and composite-81. Furthermore, the quenching phenomenon
appeared in all input factor (0, 0), (1, 1) in basic condition. Therefore, these results
concluded that the presence of different metal cations will activate a different PL response
signal. It is also influenced by the pH condition and the addition of cesium green
molecule in the system.
3.3.3.4. Cs metal formation on GOQDs and nanocomposite layer
An unexpected phenomenon was observed during the TEM measurement. By using acomposite sample, the movement of particle or rearrangement of structure was observedin composite-51 (acidic condition), refer to supporting information, Video S15.1. It can be
attributed to crystal movement between locations or rotation around axis during theelectron beam exposure [37]. The effect of electron beam was considered as a possible72reason for the mobility of the particle during the scanning process [47]. Thus, a moiré
fringe pattern under electron beam exposure was formed (red line (Fig. 5c). As
theoretically proved, the moiré fringe pattern was formed when crystalline surfaces witha lattice mismatch or a rotational misorientation are vertically stacked [48]. It also
corresponded to the crystalline conductive area of small sp2 domains [49]. In the present
study, the lattice size of composite-51 after Cs treatment is 0.39 nm, and the lattice size of
GOQDs after Cs treatment is 0.31 nm (This corresponds to (200) of Cs metal.) (Fig. S7, seesupporting information). It has a larger lattice-size compared with the as obtainednanocomposite before Cs treatment (Fig. 3d) and original GO sample, 0.21 nm (Fig. S8,
see supporting information). This value (for GO sample) is aligned with previously
observed value [50].
Moreover, this phenomenon might be influenced by the role of cesium green (CG)molecule that led to enhance the properties of the nanocomposite. It is correlated wellwith other phenomena that have been proved by using aromatic molecules (which are
non-covalently bound to 2D material), which affected the electronic properties of the 2D
materials significantly [51]. This also confirmed that the moiré superlattices have arisen
in bilayer graphene. It was strengthened by other researchers that the PL of GO was
attributed to the electronic energy transition [52], and theoretically proposed byspeculating the effect of the different oxidation levels of GO [53]. Their calculation
analysis also showed that hydroxyl and epoxy groups have a responsibility for thechange of hybridization of sp2 to sp3. In the present study, a similar behaviour (theadatoms movement) was observed in Figure S6 (a), (b), for composite-81 after Cs
treatment (in basic condition) by the HRTEM image (refer to supporting information,
Video S15.2, S15.3). It is also amplified by other researchers who theoretically suggested
that the formation of CsI (cluster shape) was assigned based on the number of atom orion [54]. For example, a four-ion cluster and a six-ion cluster of CsI were supposed tohave a square planar structure and to form planar hexagon sequentially. It is consistent
with our result (Fig. 5 (c), S7 see supporting information) as discussed in the previous
section, which predicted the interaction between CsCl and the nanocomposite layer
generates a moiré fringe pattern under electron beam irradiation. It can be explained thattwo or more crystals are overlapping with slightly different orientation.
To understand the present phenomenon, the mechanism of the moiré fringe patternformation should be considered together with

73

(b
)

(a)

14.19 1/nm
Inverse space: 14.19/6 = 2.365
Real space (d value): 1/2.365 = 0.42 nm

Cl

C

Cs

(c)

(d
)

(e)

(f)

O

Na
Si
S

K

1

14.23 1/nm

2

Inverse space: 14.23/6 = 2.37
Real space (d value): 1/2.37 = 0.42 nm

3

Fig.9. TEM image of GOQDs after Cs treatment in basic condition (a) soon after TEM
measurement was conducted. Scale bars: 200 nm. (b) Fast Fourier Transform (FFT) to
figure a. (c) Elemental analysis of figure a. (d) After electron beam irradiation was applied
for the second time. Scale bars: 200 nm. (e) Fast Fourier Transform (FFT) to figure d. (f)Specific spot area of the observed sample of figure d. Scale bars: 200 nm.

the formation of two-dimensional nanocrystal on the substrate layer, as is observed in theprevious work [37]. Therefore, another phenomenon is worth mentioning. Through theelectron microscopy characterization by using GOQDs sample after Cs treatment (inacidic and basic condition), the crystal structure transformation was observed. The results
demonstrated the different phenomenon between acidic and basic conditions. In acidic
condition, as shown in Fig. S9 (see supporting information), the crystal structure haschanged from single crystals to amorphous form (Fig. S10, see supporting information).
The FFT of Figure S9 (c) is provided in Figure S9 (d), where (110) reflections of CsCl areobserved at 0.29 nm. In Fig. S9 (b), the elemental analysis revealed that the Cs and Cl are
strongly detected. On the other hand, Cl content decreased (Fig. S10(b)), but still
remained after electron beam exposure. Furthermore, in basic condition, the crystal
74

transformation was also recorded by TEM. As shown in Fig. 9, the sample was further
identified. Soon after TEM measurement the observed crystal formation was recorded(Fig. 9(b)). Continuously, the prolonged electron beam irradiation was applied to confirm
the stability of the sample. In contrast to the Fig. 9(a), the TEM image exhibited a different
morphology (Fig. 9(d)) and different crystal formation (Fig. 9(e)). Following theseobservations, we evaluated the FFT of Fig. 9 (b) and Fig. 9 (e), where (100) reflection of
CsCl at 0.42 nm has similarity for both crystal formations. The observed sample wasdistinguished in three different investigated areas (Fig. 9 (f)). It was found that there arethree different types of crystal formation as depicted in Fig. S11 (a, c, e). These findings
suggest that the characterization of the nature of the active sites of the graphene layers
has an essential aspect to promote different reactions [55]. Also, it is well known
theoretically that the active site is not uniformly found over the surface area. Other
literature obviously investigated that the catalytic activity of GO is affected by the acidand base addition [56]. We predicted that several impurities (chemical contaminations)are determined by the chemical analysis (Fig. S11 (b, d, f)). Interestingly, if compared withbasic condition, the Cl content was significantly decreased in acidic condition.
Our speculation is that the crystal formation phenomenon might be established on thesubstrate layers of GOQDs in three continuous steps. The first step is the decompositionof CsCl nanoparticles under prolonged electron beam exposure (see supporting
information, Video S15.4). From this process, the Cs rich cluster (or residual Cs and
Cl adatoms) is formed. The second step is the rearrangement process of residual Cs and
Cl atoms to form a two-dimensional nanocrystal (Fig. 9 (b, e)). The motion of adatoms(adsorbed Cs and Cl atoms) is a reasonable explanation. The consequence of the electron
beam effect (200 kV) causes the bond breakage of the nanocomposite layer. Thisphenomenon was clearly observed (the layer was broken
then moved around/rotated) in GOQDs sample after Cs treatment (in basic condition),refer to supporting information, Video S15.5. This makes the movement of atom on thebasal plane be easily accelerated, even though the restoration of the graphitic layer ispossible due to the remaining functionalities [35]. This is also found in the present work
(refer to supporting information, Fig. S12, Video S15.6). The interaction of electron beamwith the GOQDs surface after Cs treatment in acidic condition led to the GOQDs layer
being reorganized. The third step is that the interaction between a two-dimensional
nanocrystal and the graphitic layer produces the moiré fringe pattern formation (seesupporting information, Fig. S7 (a, b)). Therefore, the unveiling moiré fringe pattern75

formation has an important role to understand the electronic properties of graphene [57].
Thus, it can significantly influence the PL phenomena (turn ON response) after Cs
treatment in acidic condition.
Additionally, the present experiments also indicated the formation of Cs metal(cluster). It should be noted, however, that more effort is still necessary to get strong
evidence and also to open the understanding of the role of oxygen functional group andthe various defects that act as an active site of the GOQDs for Cs metal formation.
Moreover, since the GOQDs and nanocomposite still include Na originated fromNaOH addition during the reflux process (see Fig. S3 (A3), S4 (A3), supporting
information), the presence of Na cation can be considered as a part of the process of two
dimensional Cs nanocrystal formation via charge transfer between Na atom and aromaticrings of the graphene layer [58]. However, in the present work, the overall mechanism
process is complex, and it is influenced by many conditions such as the composition of
the nanocomposite, the pH, the concentration of CsCl, and the solvent. Therefore, thisresult suggests that the observed cluster was formed because of the migration effect of
the adatoms on nanocomposite layer (specifically trapped to defect site in the graphenelattice) and transformation of the local adsorption environment induced by electron beam
exposure.
Conclusions
The nanocomposite was effectively fabricated by functionalized GOQDs and cesium
green molecule in mixed solution (Distilled water/THF). Until now, no study has
investigated the development of the application of cesium green (CG) molecule for Cs
detection in aqueous solution. In the present study we showed that as-produced
nanocomposite can be essentially used to recognize the presence of Cs cation in the
aqueous solution both in acidic condition and basic condition with turn ON/OFF
photoluminescence (PL) response. There are several important keys that solved in thepresent paper. (1) The unique properties of GOQDs (pH dependent) have a dual role asan enhancer and quencher in composite in the presence of Cs cation. In acidic condition,
the PL behaviour shows turn ON the PL after detecting the Cs cation. On the contrary,
the turn OFF PL occurred in basic condition. (2) Cesium green molecule can be used as
Cs cation detection material not only in solid state (as previous works reported) but also
in aqueous mixed solution. (3) Cesium green molecule can work effectively in basic andacidic condition. (4) The co-solvent application in the nanocomposite system might have76

a contribution to the Cs detection process. (5) The nanocomposites are more sensitive as
Cs detection (turn ON) compared to other metal like Sr and K in acid condition. (6) The
Cs cluster was formed under electron beam irradiation as consequence of adatom
mobility on the substrate layer. In this phenomenon, the pH condition of thenanocomposite system has an important key in the PL response in mixed solvent. In
particular, for the sensitivity and the accuracy of the nanocomposite system further
investigation is needed, since GO structure as precursor material has different local
arrangement of functional group from one batch to the other. Also, it is expected that thiswork as an initial stage of GOQDs-cesium green molecule research offers a new chemical
pathway and widely increases the application in environmental protection with moredeep investigation.

77

References[1]

T. J. Yasunari, A. Stohl, R. S. Hayano, J. F. Burkhart, S. Eckhardt, and T. Yasunari,“Cesium-137 deposition and contamination of Japanese soils due to the Fukushima
nuclear accident,” Proc. Natl. Acad. Sci. U. S. A., vol. 108, no. 49, pp. 19530–19534,
2011, doi: 10.1073/pnas.1112058108.
[2]
T. Basuki, S. Miyashita, M. Tsujimoto, and S. Nakashima, “Deposition density of
134Cs and 137Cs and particle size distribution of soil and sediment profile inHibara Lake area, Fukushima: an investigation of 134Cs and 137Cs indirect
deposition into lake from surrounding area,” J. Radioanal. Nucl. Chem., vol. 316, no.
3, pp. 1039–1046, 2018, doi: 10.1007/s10967-018-5809-1.
[3]
M. TSUJIMOTO, S. MIYASHITA, H. T. NGUYEN, and S. NAKASHIMA, “Monthly
Change in Radioactivity Concentration of 137Cs, 134Cs, and 40K of Paddy Soil andRice Plants in Fukushima Prefecture,” Radiat. Saf. Manag., vol. 19, no. 0, pp. 10–22,
2020, doi: 10.12950/rsm.181219.
[4]
K. Shizuma, Y. Fujikawa, M. Kurihara, and Y. Sakurai, “Identification and temporaldecrease of 137Cs and 134Cs in groundwater in Minami-Soma City following theaccident at the Fukushima Dai-ichi nuclear power plant,” Environ. Pollut., vol. 234,
pp. 1–8, 2018, doi: 10.1016/j.envpol.2017.11.018.
[5]
B. Radaram, T. Mako, and M. Levine, “Sensitive and selective detection of cesium
via fluorescence quenching,” Dalt. Trans., vol. 42, no. 46, pp. 16276–16278, 2013, doi:
10.1039/c3dt52215f.
[6]
T. Mori et al., “Micrometer-level naked-eye detection of caesium particulates in thesolid state,” Sci. Technol. Adv. Mater., vol. 14, no. 1, 2013, doi: 10.1088/14686996/14/1/015002.
[7]
M. Achermann, M. A. Petruska, S. Kos, D. L. Smith, D. D. Koleske, and V. I. Klimov,“Energy-transfer pumping of semiconductor nanocrystals using an epitaxial
quantum well,” Nature, vol. 429, no. 6992, pp. 642–646, 2004, doi:
10.1038/nature02571.
[8]
C. Sönnichsen, B. M. Reinhard, J. Liphardt, and A. P. Alivisatos, “A molecular ruler
based on plasmon coupling of single gold and silver nanoparticles,” Nat. Biotechnol.,vol. 23, no. 6, pp. 741–745, 2005, doi: 10.1038/nbt1100.
[9]
M. Akamatsu et al., “Intracellular imaging of cesium distribution in Arabidopsis
using cesium green,” ACS Appl. Mater. Interfaces, vol. 6, no. 11, pp. 8208–8211, 2014,
doi: 10.1021/am5009453.
[10] B. S. Nugroho et al., “Exploration of the Cs Trapping Phenomenon by Combining
Graphene Oxide with K6P2W18O62 As nanocomposite,” Materials (Basel)., vol. 14,
no. 19, p. 5577, 2021, doi: 10.3390/ma14195577.
[11] B. S. Nugroho, M. N. K. Wihadi, F. Grote, S. Eigler, and S. Nakashima, “Potentialityof graphene oxide and polyoxometalate as radionuclides adsorbent to restore the
78

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]
[21]
[22]

[23]

[24]

environment after fukushima disaster: A mini review,” Indones. J. Chem., vol. 21, no.
3, pp. 776–786, 2021, doi: 10.22146/ijc.60493.
N. Fuyuno et al., “Drastic Change in Photoluminescence Properties of Graphene
Quantum Dots by Chromatographic Separation,” Adv. Opt. Mater., vol. 2, no. 10,
pp. 983–989, 2014, doi: 10.1002/adom.201400200.
D. Kozawa et al., “Excitonic photoluminescence from nanodisc states in graphene
oxides,” J. Phys. Chem. Lett., vol. 5, no. 10, pp. 1754–1759, 2014, doi:
10.1021/jz500516u.
L. Gao, L. Ju, and H. Cui, “Chemiluminescent and fluorescent dual-signal graphene
quantum dots and their application in pesticide sensing arrays,” J. Mater. Chem. C,
vol. 5, no. 31, pp. 7753–7758, 2017, doi: 10.1039/c7tc01658a.
X. Fang et al., “Graphene quantum dot incorporated perovskite films: Passivating
grain boundaries and facilitating electron extraction,” Phys. Chem. Chem. Phys., vol.
19, no. 8, pp. 6057–6063, 2017, doi: 10.1039/c6cp06953c.
ahmadzia sherzad, hakimeh zare, zahra shahedi, F. Ostovari, Y. Fazaeli, and Z.
Pourghobadi, “Effect of pH on Optical Properties of Graphene Oxide Quantum
Dots,” Int. J. Opt. Photonics, vol. 14, no. 2, pp. 135–142, 2020, doi:
10.52547/ijop.14.2.135.
J. L. Chen and X. P. Yan, “Ionic strength and pH reversible response of visible andnear-infrared fluorescence of graphene oxide nanosheets for monitoring theextracellular pH,” Chem. Commun., vol. 47, no. 11, pp. 3135–3137, 2011, doi:
10.1039/c0cc03999c.
T. Taniguchi et al., “PH-driven, reversible epoxy ring opening/closing in graphene
oxide,” Carbon N. Y., vol. 84, no. 1, pp. 560–566, 2015, doi:
10.1016/j.carbon.2014.12.054.
Y. Dong et al., “One-step and high yield simultaneous preparation of single- andmulti-layer graphene quantum dots from CX-72 carbon black,” J. Mater. Chem., vol.
22, no. 18, pp. 8764–8766, 2012, doi: 10.1039/c2jm30658a.
J. Yao and L. Wang, “switching : mechanism and molecular logic gate,” 2021, doi:
10.1039/d1nj01908b.
W. S. Hummers and R. E. Offeman, “Preparation of Graphitic Oxide,” J. Am. Chem.
Soc., vol. 80, no. 6, p. 1339, 1958, doi: 10.1021/ja01539a017.
A. M. Dimiev and T. A. Polson, “Contesting the two-component structural modelof graphene oxide and reexamining the chemistry of graphene oxide in basic
media,” Carbon N. Y., vol. 93, pp. 544–554, 2015, doi: 10.1016/j.carbon.2015.05.058.
J. P. Rourke et al., “The real graphene oxide revealed: Stripping the oxidative debris
from the graphene-like sheets,” Angew. Chemie - Int. Ed., vol. 50, no. 14, pp. 3173–
3177, 2011, doi: 10.1002/anie.201007520.
X. Chen and B. Chen, “Direct Observation, Molecular Structure, and Location of
Oxidation Debris on Graphene Oxide Nanosheets,” Environ. Sci. Technol., vol. 50,
79

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

no. 16, pp. 8568–8577, 2016, doi: 10.1021/acs.est.6b01020.
D. López-Dĺaz, M. D. Merchán, M. M. Velázquez, and A. Maestro, “Understanding
the Role of Oxidative Debris on the Structure of Graphene Oxide Films at the AirWater Interface: A Neutron Reflectivity Study,” ACS Appl. Mater. Interfaces, vol. 12,
no. 22, pp. 25453–25463, 2020, doi: 10.1021/acsami.0c05649.
S. Chen, J. W. Liu, M. L. Chen, X. W. Chen, and J. H. Wang, “Unusual emission
transformation of graphene quantum dots induced by self-assembled aggregation,”
Chem. Commun., vol. 48, no. 61, pp. 7637–7639, 2012, doi: 10.1039/c2cc32984k.
J. Wei and J. Qiu, “Tunable optical properties of graphene quantum dots bycentrifugation,” ASME Int. Mech. Eng. Congr. Expo. Proc., vol. 15, no. November
2013, 2013, doi: 10.1115/IMECE2013-64756.
D. Pan, J. Zhang, Z. Li, and M. Wu, “Hydrothermal route for cutting graphenesheets into blue-luminescent graphene quantum dots,” Adv. Mater., vol. 22, no. 6,
pp. 734–738, 2010, doi: 10.1002/adma.200902825.
S. Kochmann, T. Hirsch, and O. S. Wolfbeis, “The pH dependence of the total
fluorescence of graphite oxide,” J. Fluoresc., vol. 22, no. 3, pp. 849–855, 2012, doi:
10.1007/s10895-011-1019-8.
D. Pan, J. Zhang, Z. Li, C. Wu, X. Yan, and M. Wu, “Observation of pH-, solvent-,
spin-, and excitation-dependent blue photoluminescence from carbon
nanoparticles,” Chem. Commun., vol. 46, no. 21, pp. 3681–3683, 2010, doi:
10.1039/c000114g.
Z. Wang et al., “Synthesis of strongly green-photoluminescent graphene quantum
dots for drug carrier,” Colloids Surfaces B Biointerfaces, vol. 112, pp. 192–196, 2013,
doi: 10.1016/j.colsurfb.2013.07.025.
Y. Lou et al., “Cane Molasses Graphene Quantum Dots Passivated by PEG
Functionalization for Detection of Metal Ions,” ACS Omega, vol. 5, no. 12, pp. 6763–
6772, 2020, doi: 10.1021/acsomega.0c00098.
F. Grote, C. Gruber, F. Börrnert, U. Kaiser, and S. Eigler, “Thermal
Disproportionation of Oxo-Functionalized Graphene,” Angew. Chemie - Int. Ed., vol.
56, no. 31, pp. 9222–9225, 2017, doi: 10.1002/anie.201704419.
R. R. Amirov, J. Shayimova, Z. Nasirova, A. Solodov, and A. M. Dimiev, “Analysis
of competitive binding of several metal cations by graphene oxide reveals thequantity and spatial distribution of carboxyl groups on its surface,” Phys. Chem.
Chem. Phys., vol. 20, no. 4, pp. 2320–2329, 2018, doi: 10.1039/c7cp07055a.
K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, and A. Zettl, “Determination of
the local chemical structure of graphene oxide and reduced graphene oxide,” Adv.
Mater., vol. 22, no. 40, pp. 4467–4472, 2010, doi: 10.1002/adma.201000732.
J. Zhang, C. Xiong, Y. Li, H. Tang, X. Meng, and W. Zhu, “The critical contributionof oxidation debris on the acidic properties of graphene oxide in an aqueous
solution,” J. Hazard. Mater., vol. 402, no. July 2020, p. 123552, 2021, doi:
80

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

10.1016/j.jhazmat.2020.123552.
N. Vats et al., “Substrate-Selective Morphology of Cesium Iodide Clusters on
Graphene,” ACS Nano, vol. 14, no. 4, pp. 4626–4635, 2020, doi:
10.1021/acsnano.9b10053.
T. Basuki and S. Nakashima, “Cs Adsorption and CsCl Particle Formation
Facilitated by Amino Talc-like Clay in Aqueous Solutions at Room Temperature,”ACS Omega, vol. 6, no. 40, pp. 26026–26034, 2021, doi: 10.1021/acsomega.1c02975.
T. K. Mandal, Y. Hou, Z. Y. Gao, H. Ning, W. S. Yang, and M. Y. Gao, “Graphene
oxide-based sensor for ultrasensitive visual detection of fluoride,” Adv. Sci., vol. 3,
no. 12, pp. 1–6, 2016, doi: 10.1002/advs.201600217.
Y. Liu, W. Q. Loh, A. Ananthanarayanan, C. Yang, P. Chen, and C. Xu,“Fluorescence quenching between unbonded graphene quantum dots and gold
nanoparticles upon simple mixing,” RSC Adv., vol. 4, no. 67, pp. 35673–35677, 2014,
doi: 10.1039/c4ra06408a.
P. Zheng and N. Wu, “Fluorescence and Sensing Applications of Graphene Oxide
and Graphene Quantum Dots: A Review,” Chem. - An Asian J., vol. 12, no. 18, pp.
2343–2353, 2017, doi: 10.1002/asia.201700814.
A. M. Dimiev, L. B. Alemany, and J. M. Tour, “Graphene oxide. Origin of acidity,
its instability in water, and a new dynamic structural model,” ACS Nano, vol. 7, no.
1, pp. 576–588, 2013, doi: 10.1021/nn3047378.
J. Shayimova, R. R. Amirov, A. Iakunkov, A. Talyzin, and A. M. Dimiev, “Carboxylgroups do not play the major role in binding metal cations by graphene oxide,”
Phys. Chem. Chem. Phys., vol. 23, no. 32, pp. 17430–17439, 2021, doi:
10.1039/d1cp01734a.
H. Dong, W. Gao, F. Yan, H. Ji, and H. Ju, “Fluorescence resonance energy transfer
between quantum dots and graphene oxide for sensing biomolecules,” Anal. Chem.,
vol. 82, no. 13, pp. 5511–5517, 2010, doi: 10.1021/ac100852z.
R. S. Swathi and K. L. Sebastian, “Long range resonance energy transfer from a dye
molecule to graphene has (distance)-4 dependence,” J. Chem. Phys., vol. 130, no. 8,
2009, doi: 10.1063/1.3077292.
V. S. Talanov, G. G. Talanova, and K. B. Yatsimiskii, “New sulfur-containing
derivatives of graphite oxide for use in fixing silver(I) ions,” Icassp, vol. 21, no. 3,
pp. 295–316, 1997.
T. P. Hardcastle et al., “Mobile metal adatoms on single layer, bilayer, and trilayer
graphene: An ab initio DFT study with van der Waals corrections correlated withelectron microscopy data,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 87, no.
19, pp. 1–16, 2013, doi: 10.1103/PhysRevB.87.195430.
Z. Zou, “Unveiling the Formation of Graphene Moiré Patterns on FourfoldSymmetric Supports: Geometrical Insight,” J. Phys. Chem. C, vol. 125, no. 41, pp.
22705–22712, 2021, doi: 10.1021/acs.jpcc.1c03991.
81

[49]
[50]

[51]

[52]

[53]

[54]
[55]

[56]

[57]

[58]

C. T. Chien et al., “Tunable photoluminescence from graphene oxide,” Angew.
Chemie - Int. Ed., vol. 51, no. 27, pp. 6662–6666, 2012, doi: 10.1002/anie.201200474.
T. Tsugawa, K. Hatakeyama, J. Matsuda, M. Koinuma, and S. Ida, “Synthesis ofoxygen functional group-controlled monolayer graphene oxide,” Bull. Chem. Soc.
Jpn., vol. 94, no. 9, pp. 2195–2201, 2021, doi: 10.1246/bcsj.20210169.
C. R. Dean et al., “Hofstadter’s butterfly and the fractal quantum Hall effect in moiré
superlattices,” Nature, vol. 497, no. 7451, pp. 598–602, 2013, doi:
10.1038/nature12186.
S. Zhu et al., “Strongly green-photoluminescent graphene quantum dots forbioimaging applications,” Chem. Commun., vol. 47, no. 24, pp. 6858–6860, 2011, doi:
10.1039/c1cc11122a.
C. V. Gomez et al., “Structural and Electronic Properties of Graphene Oxide forDifferent Degree of Oxidation,” Mater. Today Proc., vol. 3, no. 3, pp. 796–802, 2016,
doi: 10.1016/j.matpr.2016.02.011.
J. Diefenbach and T. P. Martin, “Model calculations for alkali halide clusters,” J.
Chem. Phys., vol. 83, no. 9, pp. 4585–4590, 1985, doi: 10.1063/1.449029.
S. Navalon, A. Dhakshinamoorthy, M. Alvaro, M. Antonietti, and H. García,“Active sites on graphene-based materials as metal-free catalysts,” Chem. Soc. Rev.,
vol. 46, no. 15, pp. 4501–4529, 2017, doi: 10.1039/c7cs00156h.
A. Dhakshinamoorthy, M. Alvaro, M. Puche, V. Fornes, and H. Garcia, “Graphene
Oxide as Catalyst for the Acetalization of Aldehydes at Room Temperature,”
ChemCatChem, vol. 4, no. 12, pp. 2026–2030, 2012, doi: 10.1002/cctc.201200461.
P. H. Jacobse et al., “Electronic components embedded in a single graphene
nanoribbon,” Nat. Commun., vol. 8, no. 1, pp. 1–7, 2017, doi: 10.1038/s41467-01700195-2.
G. Shi et al., “Two-dimensional Na-Cl crystals of unconventional stoichiometries
on graphene surface from dilute solution at ambient conditions,” Nat. Chem., vol.
10, no. 7, pp. 776–779, 2018, doi: 10.1038/s41557-018-0061-4.

82Supporting Informationab

Fig S1. AFM image of GOQDs (a) Small dots (red line) and (b) Aggregating dots (red circle).

83

(D)

C

(B)

(A)

O

(B)

Si

(E)

C

O
Si

(C)

Fig S2. TEM measurement of Cesium Green in different scale. (A) Cesium green in 200 nm scale, (B) Cesium
green in 1 μm scale, (C) Cesium green in 1 μm scale (in another point of view). (D) Elemental analysis of
figure A. (D) Elemental analysis of figure B.

84

(A.1)

O

(A.2)

(A.3)
Si

C Na
K

S

Fig S3. TEM measurement of GOQDs-based composite (ratio 5:1), (A.1) in 100 nm scale, (A.2) in 50 nm scale,(A.3) Elemental analysis of figure A.1.

(A.1)

(A.2)

(A.3)

O

Na

C
Si
Cl

Al

K
S

Fig S4. TEM measurement of GOQDs-based composite (ratio 8:1), (A.1) in 50 nm scale, (A.2) in 100 nm scale,(A.3) Elemental analysis of figure A.2.

85

(A)

(B)

C

(B)

O

Na Al

200 nmK

Fig. S5. TEM image of (A) isolated fragment (Larger particle of GO) after base treatment with reflux
process followed by double centrifugation process and (B) Elemental analysis of figure A.

(A)

(B)

C

(C)

Cs
O

N
Na

Cl

Fig. S6. TEM image of (A) composite-81 after Cs treatment (in basic condition), Cs cluster are identified, (B)moving adatoms, (C) Elemental analysis of figure B.

86

(A)

2779.74 pm/7 layers =397.11 pm = 0.39 nm(B)

4.39 nm/14 layers = 0.31 nm

Fig. S7. TEM image of moiré fringe pattern (A) Composite-51 after Cs treatment (in acidcondition) under prolonged electron beam irradiation, the lattice size is 0.39 nm and (B) GOQDs
after Cs treatment (in basic condition) under prolonged electron beam irradiation, the latticesize is 0.31 nm.

(a)

(b)

9.44 1/nm

Fig. S8. (a) HRTEM image of GO. Scale bars: 10 nm. (b) Fast Fourier Transform (FFT) from the selected regionof GO sample.

87

(a)

(b)
Cs

C
Cl
O

Na

S
Si

K

(c)

(d)

Fig. S9. TEM image of GOQDs after Cs treatment (in acidic condition), (a) under electron beam irradiation (soonafter TEM measurement was carried out), (b) Elemental analysis of figure a, (c) Investigated area of figure a,(d) Fast Fourier Transform (FFT) corresponding to figure c.

88

(a)
Cs

(b)

C

O

Si
Na

Cl
K

(d)

(c)

Fig. S10. TEM image of GOQDs after Cs treatment (in acidic condition), (a) after prolonged electron beamirradiation was applied, the graphitic layer being reorganized, (b) Elemental analysis of figure a, (c) Investigatedarea of figure a, (d) Fast Fourier Transform (FFT) corresponding to figure c.

89

1

(A)

(B)

C

S

Cs

O Na
Si

2

Cll K
S C

(C)

(D
O
Si

Cs

C
C

Cl
Na
O

S

K

Cs

N
Na
Cl
Si S

3

(E)

K

(F)

Fig. S11. TEM image of GOQDs after Cs treatment (in basic condition), under electron beam irradiation(soon after electron beam irradiation was applied for the second time, (A) Fast Fourier Transform (FFT)
corresponding to figure 9f, (B) Elemental analysis of figure a, (C) Fast Fourier Transform (FFT) corresponding
to figure 9f, (D) Elemental analysis of figure c, (E) Fast Fourier Transform (FFT) corresponding to figure 9f,(F) Elemental analysis of figure e.

90

(a)

(b)

O

Si

Cs

Al

K

Na

C

Cl

(c)

(d)

O

Si
Al

C

Cs

Na

K
Cl

Fig.S12. TEM image of GOQDs after Cs treatment (in acidic condition) under prolonged electron
beam irradiation, (a) before recombination process, (b) Elemental analysis of figure a, (c) after
recombination process, (d) Elemental analysis of figure c.

91

(a)

(b)

(c)
(d)

Fig. S13. Photoluminescence emission spectra of (a) composite-51 (basic condition), (b)composite-81 (basic condition), (c) composite-51 (acidic condition), (d) composite-81(acidic condition). The pH condition was changed from acidic condition to basic
condition.

92

Table S1. The input-output detection mechanism
Input
Precursor
materialpH
condition
AcidicGOQDs solution

Precursor
material

BasicpH
condition
AcidicGOQDs solution

CsCl
0
0
1
0
0
0
0
1
0
0

Basic

CsCl
0
0
1
0
0
0
0
1
0
0

Metal
SrCl2 KCl
0
0
0
0
0
0
1
0
0
1
0
0
0
0
0
0
1
0
0
1
Input
Metal
SrCl2 KCl
0
0
0
0
0
0
1
0
0
1
0
0
0
0
0
0
1
0
0
1

93

Cesium green
molecule(5:1 ratio)
0
1
1
1
1
0
1
1
1
1

OutputCesium green
molecule(8:1 ratio)
0
1
1
1
1
0
1
1
1
1

Output

1
0
1
1
0
0
1
0
0
0

1
0
1
1
0
0
1
0
0
0

SI-videos
S15.1 The movement of particle or rearrangement of structure in composite-51
S15.2, S15.3 The movement of particle or rearrangement of structure in composite-81
S15.4 The decomposition of CsCl nanoparticles under prolonged electron beam exposure
S15.5 The bond breakage of the GOQDs layers (moving or rotating phenomenon of the layer)
S15.6 Recombination of the GOQDs layers

94

Chapter IV. General Conclusions

The Fukushima accident of 2011 has led to numerous environmental problems.
Radionuclides have become one of the significant concerns of many researchers.
Therefore, it is important to design a new nanocomposite that can potentially use toaddress the issue of radionuclides contamination. This thesis explored the tremendous
property of graphene oxide (GO), their derivatives (graphene oxide quantum dots
(GOQDs)), and its composites. Specifically, by considering the dynamic nature of thesurface area of GO, it can be used as an entry point to design the structure and thefunction system. In addition, through a better understanding of the specific characteristic
and the structural diversity of GO (having different C/O ratio), the optimization of GO
properties can potentially generate a new advance nanocomposite. Some methods(covalent bonding, functionalization) and radionuclides removal concepts (ion-trapping,coagulation, electrostatic interaction, and H+-exchange) offers a bridge among GO,
GOQDs and other potential materials. Based on that, chemical modifications open up theidea or tailoring the GO, GOQDs with another compound. In this study, Dawson-type
POM (polyoxometalates) and cesium green molecule (C32H33NO8) were identified. Basedon that, two types of nanocomposites were produced. First, GO-POM nanocomposite as
Cs removal. Second, graphene oxide quantum dot-based nanocomposite (functionalized
by cesium green molecule) as Cs detection.
Moreover, in this thesis, GO and GOQDs as precursor material was synthesis bymodified Hummer`s method. Further measurement was conducted to evaluate thecrystallite morphology and some properties of the sample. Raman spectroscopy,transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic
force microscopy (AFM), UV-Vis spectroscopy, Photoluminescence spectroscopy, PXRD,
FTIR, Zeta potential were measured to obtained some evidences by spectroscopic andmicroscopic data.
As a result, GO-POM nanocomposite increased the Cs adsorption capacity
effectively compared to the adsorption capacity of the original material. The change range
is around 6.7% to 42.9%. An effective increment value of Cs adsorption can be attributed
to several reasons, such as the characteristic of surface area (showing an important roleof POM species with GO surface), as well as the functional group of GO provide a binding95

point for chemical modification. However, the increment value of Cs adsorption bycomparing concentrations 1:8 and 4:1 in the composite material, there is not muchsignificant difference. In addition, it also found that the aggregation process wasobserved in the adsorption behaviour (the mechanism of cation-GO interaction). The
results demonstrated that the composite samples immediately coagulate after Cs
adsorption except for [GO70POM]41, which still forms a stable solution after one hour
keeping at ambient temperature. The unusual behaviour of [GO70POM]41 might bepredicted that there is a different composition of oxidative debris (OD) component
among the GO samples. As is well known that the OD of GO has a significant role in theorigin of GO acidity. Further investigation was evaluated by identified the interaction
GO samples with Cs cation. Immediately less than one minute after the stirring process, A
brown milky coagulation was formed. The same behavior was observed between GOc72
and Cs, however, there was a difference for GOc39, the brown milky coagulation wasformed in a small quantity, and the milky coagulation was not formed for GO c40. The
difference might be related to the variance in C/O ratio between GO c70, GOc72, and GOc39,
GOc40. As evident, TEM measurement shows that there are many Cs clusters in the surfaceof GOc70. The black spots also can be seen in the [GO 40POM]41, which consists of
GOc40/POM. However, the black spots are not uniformly found all over the surface area.
Regarding this phenomenon, it can be assumed that there are two important factors thatmight contribute to the aggregation behavior. First, the size of the GO and composite
sample matter. As resulted, the size of each sample is diverse. The range of values isaround 4.3 to 34.78 nm. Second, the other factor is the degree of acidity. It is strongly
related with the functionalities of the GO sample. It was explained in an earlier section inthis work that there are two types of GO samples, which have different C/O compositions.
Furthermore, since GO material consisted of oxidative debris (OD) component(covalently attached to the oxygen functional groups), in this thesis, we had produced
graphene oxide quantum dot (GOQDs) by separating the OD from the graphene-likesheet with using base (NaOH) treatment and heating process. Continuously, GOQDs
functionalized by cesium green (C32H33NO8) molecule. As a result, the nanocomposite
was eﬀectively fabricated by functionalized GOQDs and cesium green molecule in mixed
solution (distilled water/THF). In this study, as-produced nanocomposite can beessentially used to recognize the presence of Cs cation in the aqueous solution both inacidic condition and basic condition with turn ON/OFF photoluminescence (PL) response.
There are several phenomena observed in this experiment. First, the unique properties of96

GOQDs (pH dependent) have a dual role as an enhancer and quencher in composite in
the presence of Cs cation. In acidic condition, the PL behaviour shows turn ON the PL
after detecting the Cs cation. On the contrary, the turn OFF PL occurred in basic condition.
Second, the nanocomposites are more sensitive as Cs detection (turn ON) compared toother metal like Sr and K in acid condition. Third, the Cs cluster was formed underelectron beam irradiation as consequence of adatom mobility on the substrate layer. In
this phenomenon, the pH condition of the nanocomposite system has an important key
in the PL response in mixed solvent. It should be noted, however, that more eﬀort is stillnecessary to get strong evidence and also to open the understanding of the role of oxygen
functional group and the various defects that act as an active site of the GOQDs for Cs
metal formation. Therefore, based on this phenomenon, it suggests that the observed
cluster was formed because of the migration eﬀect of the adatoms on nanocomposite
layer (specifically trapped to defect site in the graphene lattice) and transformation of thelocal adsorption environment induced by electron beam exposure.
Throughout this study it has been found that GO, GOQDs, and their composite hassome remarkable features that are highly Cs adsorbent and Cs detection. Morespecifically, it shown below:¾ The GO-based POM nanocomposite was successfully obtained. The Cs adsorption
capacity increased by forming a nanocomposite. The result proposed, that C/O ratioof the GO sample and the concentration ration of GO/POM has significantly
influenced the characteristic of the GO-POM nanocomposite for the Cs adsorption
performance after forming the composite.
¾ A new nanocomposite was developed using functionalized graphene oxide quantum
dots (GOQDs) with cesium green molecules for the first time. The results
demonstrated that the unique properties of GOQDs (pH dependent) have a dual roleas an enhancer and quencher in composite in the presence of Cs cation. In acidic
condition, the PL behaviour shows turn ON the PL after detecting the Cs cation. On
the contrary, the turn OFF PL occurred in basic condition. Additionally, the present
experiments also indicated the formation of Cs metal (cluster).
¾ The overall mechanism process is complex, and it is influenced by many conditions
such as the composition of the nanocomposite, the pH, the concentration of CsCl, andthe solvent.
However, since GO structure as precursor material has different local arrangementof functional group from one batch to another batch, the accuracy and the sensitivity of97

the nanocomposite system further investigation is needed. Further scrutiny is alsoneeded to open understanding of the role of degree of the oxidative debris (OD
component) in each GO sample for adsorption capacity and detection system.

98