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Synthesis of surface-modiﬁed iron oxide
nanocrystals using supercritical carbon dioxide as
the reaction ﬁeld
Yasuhiko Orita,* Keito Kariya, Thossaporn Wijakmatee

and Yusuke Shimoyama

In the synthesis of surface-modiﬁed nanocrystals (NCs), a simple and green chemistry approach to reduce
liquid waste, particularly a solventless process, has been desired. In this study, we applied the supercritical
CO2 technology, which is an excellent solventless process, to the synthesis of surface-modiﬁed iron oxide
NCs. The synthesis was performed at 30.0  0.8 MPa of CO2, 18 h and 100  C, where iron(III)acetylacetonate, pure water and decanoic acid were used as starting materials. As a result, thesupercritical CO2 medium gave the NCs of a-Fe2O3 and g-Fe2O3 with unimodal size distribution, where
Received 23rd November 2021
Accepted 25th February 2022

the mean size was 7.8  2.0 nm. In addition, they were self-assembled on the TEM substrate and themean nearest-neighbor spacing was close to the chain length of decanoic acid. Furthermore, FT-IR and
TG analyses indicate that decanoic acid chemically attaches to the surface of iron oxide NCs that are

DOI: 10.1039/d1ra08580h

dispersed in cyclohexane. These results suggest that the supercritical CO2 medium could be the new

rsc.li/rsc-advances

appealing reaction ﬁeld to fabricate densely modiﬁed NCs without liquid waste.

1. Introduction
Surface-modied metal oxide nanocrystals (NCs) have attracted
considerable interest in recent years.1 Although the aggregationof NCs due to their high surface energy is a serious problem forpractical utilization such as thin lm fabrication,2 patterning on
substrates3 and drug carriers,4 surface modication largely
reduces the surface energy and improves their dispersibility tosolvent, thus preventing the aggregation.5 In addition, surface
modication at the synthesis stage allows the inhibition of not
only aggregation but also the control of the size and shape ofNCs that strongly aﬀects their physicochemical properties.6,7
These appealing eﬀects of surface modication have promoted
the research on the direct-synthesis methods for surfacemodied NCs.7–10
The surface-modied NCs has been typically synthesized via
wet-based methods such as sol–gel,8 hot-injection,9 heat-up11
and hydrothermal10 methods because the solvent is required todissolve precursors and surfactants uniformly and to control
the reaction of them.12 However, wet-based methods causea large amount of liquid waste for the synthesis and washing,9–11
where the disposal and the regeneration cost of them is known
as the critical issue. However, the synthesis in supercriticalcarbon dioxide (CO2) can be new appealing candidate to fabricate surface-modied NCs. Supercritical CO2 has unique properties such as high solubility of the metal organic precursor13
Department of Chemical Science and Engineering, Tokyo Institute of Technology,
2-12-1 S1-33, Ookayama, Meguro-ku, Tokyo 152-8550, Japan. E-mail: orita.y.aa@m.
titech.ac.jp7990 | RSC Adv., 2022, 12, 7990–7995

and diﬀusivity, which allows the formation of the homogenous
phase,14 while the synthesis in supercritical CO2 is substantiallythe solventless reaction process. In addition, supercritical CO2
can be used not only for synthesis but also as a washing anddrying solvent for the particle production.14,15 These characteristics allow the simple fabrication process without liquid waste;thus, the supercritical CO2 medium has been applied for thesynthesis of inorganic materials such as metal oxides,16 metalhydroxides15 and metal sulfates.17 However, in the most of case,the products were observed as aggregates or the submicronsized particles aer the synthesis in supercritical CO2.16,18 Thisis probably due to the characteristic of supercritical CO2 that is
non-polar and low viscous solvent. The non-polar properties ofsupercritical CO2 have low compatibility with the metal oxidesurface (generally is hydrophilic), which typically results in theaccelerated aggregation. Furthermore, low viscous properties
generally lead to the vigorous Brownian movement of particles,
which also accelerate the aggregation. The introduction of anorganic surfactant to the synthesis in supercritical CO2 is expected to overcome this serious problem because surface
modication can change the surface properties of metal oxides
from hydrophilic to hydrophobic that is compatible to supercritical CO2.19 In addition, the reduction of surface energy andstearic repulsion between particles can be achieved by surface
modication.4,19 Furthermore, supercritical CO2 has high ability
to dissolve the organic surfactant that is typically used for thesynthesis of surface-modied NCs such as saturated andunsaturated fatty acids.12,20–22 Therefore, supercritical CO2 withthe organic surfactant has the potential to directly synthesize
the surface-modied NCs with good monodispersity. However,© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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the report concerned with the synthesis in supercritical CO2 islimited to the metal oxide without surface modication;15,18,23
thus, the direct synthesis of surface-modied NCs in supercritical CO2 was attempted in this study.
In this study, we report a novel simple synthesis using
supercritical CO2 as a reaction medium for surface-modied
NCs. As a model material, we chose iron oxide that is applied
as the catalyst,24 drug carrier25 and in a magnetic recording
device26 due to its appealing catalytic and magnetic properties.
Iron(III) acetylacetonate and decanoic acid of fatty acids wereused as the precursor and surfactant, respectively, since they arehighly stable and commercially available reagents, which makethem desirable candidates for the synthesis. Herein, thehydrolysis or thermolysis of the iron precursor is a popular
reaction to synthesize iron oxide NCs for conventional hydrothermal and heat-up methods.11,19 The thermolysis reaction of
the iron precursor requires a high temperature above 300  C,
where such a severe environment easily produces the byproductsourced from the thermal decomposition of a high boilingsolvent (commonly dibenzyl ether).27 On the other hand, thehydrolysis reaction of the iron precursor easily proceed under
100  C and yield the well crystalline nanoparticles;28 therefore,a small amount of water was used as a starting material toutilize the hydrolysis reaction for the synthesis of iron oxide
NCs in this study.

2.

Experimental

2.1

Materials

Iron(III) acetylacetonate [Fe(acac)3] (purity > 99%) and decanoic
acid (purity > 98.0%) were purchased from Wako Pure Chemical
Industries, Ltd. CO2 (purity > 99.9%), nitrogen (N2, purity >99.95%) and ultra-high pressure N2 (purity > 99.95%) were
supplied by Fujii Bussan Co., Ltd. Ultra-pure water was
prepared using a Direct-Q UV3 Water Purication System
supplied by EMD Millipore Corp., and the resistivity wasconrmed to be 18.2 MU cm.
2.2

Fe(acac)3, 1.29 g of decanoic acid and 0.45 g of water weretransferred to 76 mL volume of a reaction vessel (TSC-CO2-008;
Taiatsu Glass Corp.). The molality of Fe(acac)3, decanoic acidand water were 0.03, 0.15 and 0.50 mol kg1, respectively, whichwere calculated using the volume of the vessel and CO2 density
under the given reaction conditions. The mixture was stirred for
1 min under ambient conditions. Subsequently, CO2 owed
under 0.5 MPa for 1 min to displace the air in the vessel andliquied CO2 was introduced into the vessel using the HPLC
pump (PU-4386; JASCO Co., Ltd.) until reaching 6.6 to 7.0 MPa.
Aer reaching the appropriate pressure, the vessel was sunk in
the oil bath stirrer, which resulted in the target temperature of
100  C and the pressure of 30.0  0.8 MPa. The vessel was le inthe oil bath stirrer for 18 h, while the enclosed materials werevigorously stirred. Aer the time passed, the vessel was pulled
up and was depressurized at a rate of approximately 0.5
MPa min1 using a metering valve (1315G2Y; HOKE Inc.). Aer
depressurization, the vessel was quenched in a water bath atroom temperature.
The experiments were also performed at N2 of 30.0 MPa andat N2 atmosphere as reference controls. In the experiment at
30.0 MPa, N2 was introduced into the vessel using an ultra-highpressure N2 cylinder and the high-pressure system shown in
Fig. 1 until reaching 22.1 MPa. The cylinder was connected tothe middle point between the pump and the metering valve.
Aer reaching the pressure, the vessel was sunk in the oil bathstirrer, which resulted in the target pressure of 30.0  0.3 MPa.
In the experiment at N2 atmosphere, the vessel was purged by
N2. Subsequently, the inlet and outlet valves were closed and thevessel was sunk in the oil bath stirrer. In both cases, otherconditions and procedures were same as the case using supercritical CO2.
The products were collected by rinsing the reaction vessel
successively with a cyclohexane/ethanol mixture whose volume
ratio was 1 : 4. The products were centrifuged and washed witha mixture of cyclohexane and ethanol (1 : 4) to eliminate theunreacted precursor and surfactant. The solid products weredried in a vacuum oven at room temperature for 24 h.

Synthesis

The high-pressure system shown in Fig. 1 was developed tosynthesize surface-modied NCs in supercritical CO2. 0.53 g ofFig. 1

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2.3

Characterization

The particle yield Y was dened as follows.

Experimental apparatus.

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Y¼

Wcollect 100
Wtheory(1)

where Wcollect is the weight of nal products aer washing anddrying and Wtheory is the maximum particle weight of Fe2O3
when the reaction is complete. X-ray diﬀraction patterns of the
products were obtained via X-ray diﬀraction (XRD)(MiniFlex600-C; Rigaku Corp.) using Cu Ka radiation. The
crystallite size was evaluated using the Scherrer equation, where
0.90 of shape factor was used. The products were observed bytransmission electron microscopy (TEM) (H-7650; Hitachi
Corp.) operated at 100 kV. High resolution TEM (HR-TEM)
images and electron diﬀraction pattern of particle colony wereacquired using a JEOL JEM-2010F electron microscope operated
at 200 kV. The solid products for TEM and HR-TEM analysis
were dispersed in cyclohexane before transferred onto thecopper grids with a organic membrane. The mean size, with
their standard deviation, of the particles was determined byobserving approximately 200 particles for TEM analysis. Thermogravimetric analysis was performed from room temperature
to 600  C at a ramp rate of 10  C min1 and at N2 atmosphereusing a thermogravimetric analyzer (TGA) (TGA-50; Shimadzu
Corp.). The absorbed state of an organic surfactant on the ironoxide surface was investigated using a Fourier transform
infrared spectrometer (FT-IR) (FT-IR4100; JASCO Co., Ltd.).

3.

thus, the hydrolysis reaction of the Fe(acac)3 molecule could beaccelerated in supercritical CO2, which plausibly promotes theparticle formation. Herein, focusing on the diﬀerence ofparticle yields between products 4 and 5, the yield increased
from 52 to 76% by introducing the decanoic acid into thesupercritical CO2 medium. The presence of decanoic acid on
the particle surface possibly raises the weight of particle and theyield. The modication state of the particle surface by decanoic
acid is discussed later.
Fig. 2a–c show typical TEM images of the NCs prepared insupercritical CO2. The products synthesized without decanoic
acid showed aggregation state (Fig. 2a), while the products thatwere synthesized with decanoic acid self-assembled on the TEM
substrate (Fig. 2b and c), where the mean nearest-neighborspacing was calculated to be 2.4  0.7 nm by observing about
100 spacing. The decanoic acid has a carbon chain length of
1.4 nm, thus, the distance between self-assembled NCs can beseen as approximately 2.8 nm.19 This theoretical distance wassomewhat closed to our observed distance of 2.4  0.7 nm,
indicating that the surface of obtained NCs was modied

Results and discussion

Table 1 shows experimental conditions and results. The particle
yields were less than 1% at 30.0 MPa of CO2 without water, at N2
atmosphere with water and at 30.0 MPa of N2 with water, as
shown in products 1 to 3 from Table 1. On the other hand, theywent beyond 50% regardless of the existence of decanoic acid at
CO2 of 30.0 MPa with water. These results clearly indicate thatthe addition of water to the supercritical CO2 medium accelerated the particle formation. The thermolysis rate of Fe(acac)3 is
known to be very small, and the calculated conversion under thereaction conditions (100  C, 18 h) was negligible based on thereport from G. Beech and R. Lintonbon.29 Therefore, thehydrolysis reaction should be key role to form iron oxide.
However, water is not miscible with Fe(acac)3; in addition, highpressure N2 typically has low solubility due to its low density(corresponding to 234 kg m3 at 30.0 MPa and 100  C).30 Thissuggests that Fe(acac)3 and water face the problem of diﬀusion
limit at N2 atmosphere and at N2 of 30.0 MPa. In contrast,
supercritical CO2 (with high density of 662 kg m3) has highsolubility of Fe(acac)3 (ref. 13) and water31 and diﬀusibility;

Table 1(a–c) The typical TEM images of NCs synthesized in supercritical CO2 (a) without decanoic acid and (b and c) with decanoic acid.
(d) Size distribution histogram of NCs synthesized in supercritical CO2
with decanoic acid.
Fig. 2

Experimental conditions and results

Products

Fe(acac)3
(g)

Water
(g)

Decanoic acid(g)

Atmosphere

Pressure (MPa)

Yield (%)

1
2
3
4
5

0.530
0.530
0.530
0.530
0.530

0.00
0.45
0.45
0.45
0.45

1.29
1.29
1.29
0.00
1.29

CO2
N2
N2
CO2
CO2

30.0
0.1
30.0
30.0
30.0

<1
<1
<1
52
76

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enough densely to achieve stearic hindrance. In addition, thesupercritical CO2 medium with decanoic acid gave the NCs with
unimodal and narrow size distribution, as shown in Fig. 2d, andthe mean size of NCs was 7.8  2.0 nm, which supported thatthe aggregation of NCs did not occur. In general, surface
modication by surfactant, such as decanoic acid, enhances
stearic repulsion force between particles, as shown in Fig. 2c,and reduces surface energy.4 In addition, it can change thesurface properties of metal oxide NCs from hydrophilic tohydrophobic, which is compatible to supercritical CO2.19 Theseeﬀects typically allow the inhibition of the aggregation of NCs.
Therefore, it can be concluded that the addition of decanoic
acid to supercritical CO2 eﬀectively modies the surface of NCs,
which allows the inhibition of the aggregation in the synthetic
eld.
High resolution TEM, TEM electron diﬀraction and XRD
analysis were performed to analyze the crystallinity and crystal
structure of NCs synthesized in supercritical CO2 with decanoic
acid, as shown in Fig. 3a–c. In the electron diﬀraction pattern,
the scattered diﬀraction spot and the Debye–Scherrer ring wereobserved, indicating that the obtained NCs have good crystallinity. The good crystallinity of NCs was further conrmed byobserving the atomic arrangement from the HR-TEM image
shown in Fig. 3b. In addition, the crystallite size (calculated
from the XRD peak at 40.2 in Fig. 3c) was 6.0 nm that issomewhat close to the size obtained by TEM, suggesting thatthe as-synthesized NCs have single crystallinity. However, theas-prepared NCs showed a broad XRD pattern (Fig. 3c), which
may be because of the very small particle size and the low
volume portion of iron oxide due to the core–shell structure of
the iron oxide core and decanoic acid shell. A small particle size

RSC Advances(corresponding to small crystallite size) typically broadens thewidths of PXRD peaks. In addition, as stated in the previous
paragraph, Fig. 2c evidentially shows that iron oxide NCs isdensely covered with decanoic acid enough to bring the spacing
between particles, supporting the formation of the core–shellstructure.1 For the core–shell structure, the volume portion ofiron oxide is calculated to be 39.8% using the size of iron oxide
NCs and the carbon chain length of decanoic acid. This low
volume portion causes the reduction of the actual crystallite
volume for PXRD measurement, which results in the decreasing
intensity. Therefore, the small particle size and low volume
portion of iron oxide NCs plausibly result in the broad XRD
pattern. Table 2 lists the lattice spacings calculated by theelectron diﬀraction pattern (shown in Fig. 3a) of obtained NCs,
where the lattice spacings were assigned to the crystal structureof a-Fe2O3 (ICSD: 22505) and g-Fe2O3 (ICSD: 79196).
FT-IR and TG analyses were applied to characterize thesurfactant attached on the surface of NCs synthesized insupercritical CO2 with decanoic acid. Fig. 4a shows the FT-IR
spectra of the obtained NCs and pure decanoic acid. The
characteristic bands at 2850 and 2900 cm1 were assigned toasymmetric and symmetric stretching modes of –CH2– in thealkyl chains of monocarboxylic acid.32 However, the bandassigned to the free carboxyl group (–COOH) of monocarboxylic
acid was not observed, where its band is normally detected atabout 1700 cm1.32 Furthermore, bands at approximately 1530
and 1400 cm1 can be assigned to the asymmetric andsymmetric stretching modes of the carboxylate group (–COO)of monocarboxylic acid.32 These results evidentially show that
decanoic acid does not physically adsorb on the surface but
chemically attaches to the surface of iron oxide NCs.20 Fig. 4bshows the TGA results for the obtained NCs. Increase in theweight loss accelerated from approximately 300  C, which also
supports that decanoic acid chemically attached to the surfaceof iron oxide NCs because increase in the weight loss should be
stopped near the boiling point of decanoic acid (corresponding
to 243  C) in the case of physical absorption.33,34 Herein,chemically bonded dense surfactants on the surface typicallyenables good dispersion of NCs in organic solvents, which areessential for the self-assembly on the substrate.19 In this study,
we could observe well dispersed NCs with a concentration of
0.2% (w/v) in cyclohexane, as shown in Fig. 5b. It was also

Table 2 Lattice spacings and assigned structures of NCs synthesized
in supercritical CO2 with decanoic acid(a) The electron diﬀraction pattern, (b) the high resolution TEM
image and (c) the XRD pattern of NCs synthesized in supercritical CO2
with decanoic acid (products 5 listed in Table 1).
Fig. 3© 2022 The Author(s). Published by the Royal Society of Chemistry

No.

Lattice spacinga [nm]

1
2
3
4
5
6

0.295
0.249
0.224
0.203
0.168
0.148

Assigned structureb

a-Fe2O3
a-Fe2O3
a-Fe2O3
a-Fe2O3

g-Fe2O3
g-Fe2O3
g-Fe2O3
g-Fe2O3
g-Fe2O3a

Lattice spacings were calculated by TEM electron diﬀraction pattern.
The lattice spacings were assigned to the crystal structure of a-Fe2O3(ICSD: 22505) and g-Fe2O3 (ICSD: 79196).
b

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4. Conclusions
In this study, we aimed to synthesize the surface modied ironoxide nanocrystals (NCs) in supercritical CO2 to demonstrate
the feasibility of supercritical CO2 as the reaction eld for thesynthesis of them. To investigate the eﬀects of supercritical
CO2, the synthesis was performed at N2 atmosphere, at 30.0 
0.3 MPa of N2 and at 30.0  0.8 MPa of CO2, where iron(III)acetylacetonate, pure water and decanoic acid were used as
starting materials. As a result, the yield of the solid materials
signicantly increased by using supercritical CO2 comparedwith the case using N2. Moreover, the addition of decanoic acidto supercritical CO2 was revealed to drastically inhibit theaggregation of NCs, which allowed the formation of single-nanosized crystals with the unimodal size distribution. Furthermore,
FT-IR and TG analyses support the decanoic acid chemicallyattach to the surface of the NCs that are dispersed to cyclohexane. These results suggest that supercritical CO2 medium
can be a new appealing reaction eld to fabricate the denselymodied NCs without liquid waste.

Conﬂicts of interest
The authors declare no competing nancial interest.
Fig. 4 (a) FT-IR spectra of NCs synthesized in supercritical CO2 withdecanoic acid (products 5 listed in Table 1) and pure decanoic acid. (b)TG spectra of NCs synthesized in supercritical CO2 with decanoic acid.

Acknowledgements
This work was supported by JSPS KAKENHI grant number
JP21K14454 (Grant-in-Aid for Early-Career Scientists). This work
was supported by Futaba Foundation (The Futaba Research
Grant Program).

References

The optical images of the obtained NCs (products 5 listed inTable 1) with a concentration of 0.2% (w/v) in (a) water and (b)cyclohexane.

Fig. 5conrmed that the NCs were self-assembled on the TEM
substrate shown in Fig. 2b and c by dripping the obtained NCs
dispersed in cyclohexane onto the TEM grid. These results
reversely support that the obtained NCs is densely modied by
the surfactant, which allow some practical applications such as
the patterning and thin lm fabrication.

7994 | RSC Adv., 2022, 12, 7990–7995

1 T. Tomai, N. Tajima, M. Kimura, A. Yoko, G. Seong and
T. Adschiri, J. Colloid Interface Sci., 2021, 587, 574–580.
2 D. Hojo, K. Z. Suzuki, S. Mizukami and T. Adschiri, J. Mater.
Chem. C, 2016, 4, 1263–1270.
3 T. Teranishi, A. Sugawara, T. Shimizu and M. Miyake, J. Am.
Chem. Soc., 2002, 124, 4210–4211.
4 H. Li and Y. J. Zhu, Chem.–Eur. J., 2020, 26, 9180–9205.
5 S. Takami, T. Sato, T. Mousavand, S. Ohara, M. Umetsu and
T. Adschiri, Mater. Lett., 2007, 61, 4769–4772.
6 M. Dejhosseini, T. Aida, M. Watanabe, S. Takami, D. Hojo,
N. Aoki, T. Arita, A. Kishita and T. Adschiri, Energy Fuels,
2013, 27, 4624–4631.
7 C. B. Murray, C. R. Kagan and M. G. Bawendi, Annu. Rev.
Mater. Sci., 2000, 30, 545–610.
8 F. Ansari, A. Sobhani and M. Salavati-Niasari, J. Colloid
Interface Sci., 2018, 514, 723–732.
9 B. Geng, X. Liu, J. Ma and Q. Du, Mater. Sci. Eng., B, 2007,
145, 17–22.
10 J. Zhang, S. Ohara, M. Umetsu, T. Naka, Y. Hatakeyama andT. Adschiri, Adv. Mater., 2007, 19, 203–206.
11 J. Muro-Cruces, A. G. Roca, A. López-Ortega, E. Fantechi,
D. Del-Pozo-Bueno, S. Estradé, F. Peiró, B. Sepúlveda,© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

Open Access Article. Published on 11 March 2022. Downloaded on 3/12/2022 4:10:06 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Paper
F. Pineider, C. Sangregorio and J. Nogues, ACS Nano, 2019,13, 7716–7728.
12 E. Scopel, P. P. Conti, D. G. Stroppa and C. J. Dalmaschio, SN
Appl. Sci., 2019, 1, 1–8.
13 M. Haruki, F. Kobayashi, S. I. Kihara and S. Takishima, Fluid
Phase Equilib., 2011, 308, 1–7.
14 M. Kinoshita and Y. Shimoyama, J. Supercrit. Fluids, 2016,
116, 190–197.
15 M. Kinoshita, T. Kamizato and Y. Shimoyama, J. Supercrit.
Fluids, 2018, 138, 193–199.
16 E. Alonso, I. Montequi, S. Lucas and M. J. Cocero, J. Supercrit.
Fluids, 2007, 39, 453–461.
17 X. Dong, D. Potter and C. Erkey, Ind. Eng. Chem. Res., 2002,
41, 4489–4493.
18 S. W. Kim and J. P. Ahn, Sci. Rep., 2013, 3, 3–7.
19 T. Adschiri, S. Takami, T. Arita, D. Hojo, K. Minami, N. Aoki
and T. Togashi, Handbook Adv. Ceramics: Mater. Appl.
Processing Properties: Sec. Edi., 2013, pp. 949–978.
20 M. Taguchi, N. Yamamoto, D. Hojo, S. Takami, T. Adschiri,
T. Funazukuri and T. Naka, RSC Adv., 2014, 4, 49605–49613.
21 C. E. Schwarz and J. H. Knoetze, J. Supercrit. Fluids, 2012, 66,
36–48.
22 M. Skerget, Z. Knez and M. Habulin, Fluid Phase Equilib.,
1995, 109, 131–138.

© 2022 The Author(s). Published by the Royal Society of Chemistry

RSC Advances
23 J. Wang, Y. Xia, W. Wang, M. Poliakoﬀ and R. Mokaya, J.
Mater. Chem., 2006, 16, 1751–1756.
24 R. Yao, J. Wei, Q. Ge, J. Xu, Y. Han, H. Xu and J. Sun, Catal.
Today, 2021, 371, 134–141.
25 M. Wahajuddin and S. Arora, Int. J. Nanomed., 2012, 7, 3445–
3471.
26 S. Dhara, A. C. Rastogi and B. K. Das, J. Appl. Phys., 1993, 74,
7019–7021.
27 P. Guardia, A. Riedinger, S. Nitti, G. Pugliese, S. Marras,
A. Genovese, M. E. Materia, C. Lefevre, L. Manna and
T. Pellegrino, J. Mater. Chem. B, 2014, 2, 4426–4434.
28 M. A. Dheyab, A. A. Aziz, M. S. Jameel, O. A. Noqta,
P. M. Khaniabadi and B. Mehrdel, Sci. Rep., 2020, 10, 1–8.
29 G. Beech and R. Lintonbon, Thermochim. Acta, 1971, 3, 97–105.
30 J. Chrastil, J. Phys. Chem., 1982, 86, 3016–3021.
31 Z. Wang, Q. Zhou, H. Guo, P. Yang and W. Lu, Fluid Phase
Equilib., 2018, 476, 170–178.
32 K. Nakanishi, Infrared Absorption Spectroscopy: Practical,
Holden-Day, San Francisco, 1962.
33 M. Taguchi, S. Takami, T. Adschiri, T. Nakane, K. Sato and
T. Naka, CrystEngComm, 2011, 13, 2841–2848.
34 M. Taguchi, S. Takami, T. Naka and T. Adschiri, Cryst.
Growth Des., 2009, 9, 5297–5303.

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