九州大学大学院総合理工学府報告第 43 巻 第 2 号 37-41 頁 令和 4 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 43, No. 2, pp. 37-41, FEB, 2022

Spectroscopic Study for UV Irradiation Effect on
Adsorption State of Volatile Aldehyde at Single
Crystalline ZnO Nanowire Surface
Wenjun LI*1,2 Kazuki NAGASHIMA*2,3† Chen WANG*4
Hideto YOSHIDA*5 Takuro HOSOMI*2,3 Tsunaki TAKAHASHI*2,3
Wataru TANAKA*2 Masaki KANAI*6 and Takeshi YANAGIDA*2,6†
E-mail of corresponding author: kazu-n@g.ecc.u-tokyo.ac.jp, yanagida@cm.kyushu-u.ac.jp†(Received January 20, 2022, accepted January 26, 2022)
Ultraviolet (UV) irradiation is a promising approach for enhancing a sensing response in metaloxide-based molecular sensor, however, its substantial impact on the molecular-to-surface
interaction at sensor surface is poorly understood. In this study, we investigated the UV irradiation
effect on adsorption state of volatile aldehyde at single crystalline zinc oxide (ZnO) nanowire surface
by Fourier transform-infrared spectroscopy (FT-IR). A C9 aliphatic aldehyde, nonanal, was employed
as an analyte. We found that oxidation of adsorbed nonanal was significantly promoted by the UV
irradiation. A wavelength dependence implied that the photogenerated carriers play a crucial role in
the enhanced oxidation. Our finding would be beneficial for designing the performances of metaloxide nanowire-based molecular sensing devices.
Key words: Single crystalline, ZnO, nanowires, volatile molecule, aldehyde, nonanal, FT-IR,

adsorption, UV irradiation, molecular sensing

1.

Introduction

Metal oxide nanowires have attracted
considerable research interest for molecularsensor applications due to their uniquestructural properties, variety of chemical
reactions, and material robustness in harshenvironments.1-3) To date, various nanowire
materials have been intensively investigatedwith their surface treatments/modifications
and many excellent sensor properties havebeen successfully demonstrated.4-11) On theother hand, the interactions between
nanomaterial surfaces and analyte molecules
*1

*2
*3
*4

*5

*6

Department of Molecular and Material Sciences,
Interdisciplinary Graduate School of Engineering
Sciences, Kyushu University, Graduate Student
Department of Applied Chemistry, Graduate Schoolof Engineering, The University of TokyoJapan Science and Technology Agency, PRESTO
Department of Electronic and Computer Engineering,
School of Engineering, The Hong Kong University of
Science and Technology
Nanoscience and Nanotechnology Center, The
Institute of Scientific and Industrial Research, Osaka
University
Division of Integrated Materials, Institute forMaterials Chemistry and Engineering, Kyushu
University

have not been understood comprehensively,
which had limited the design of sensing
performance. To overcome this issue, manyefforts have recently been devoted to theunderstandingofmolecule-to-surface
interactions at sensor surface by using
spectroscopic techniques.12-15) We previously
investigated the adsorption states of volatile
molecules at the surfaces of metal oxide
nanowires (i.e., zinc oxide (ZnO) nanowires
and tungsten oxide (WO3) nanowires) byinfrared (IR) spectroscopy, and various
adsorption
statesandchemicaltransformations of adsorbed molecules wererevealed.16-20) Furthermore, according to theacquired knowledge, we modified the nanowire
surfaces and successfully achieved theimproved sensing properties.18-20) As proven inthese
studies,
understandingthemolecule-to-surface interactions at metal oxide
nanowire surfaces is of crucial importance fordesigning the performance of nanowire-based
molecular sensors.
Among various features of molecular sensors,sensitivity (i.e., amplitude of sensing response)is the most important one for detecting analytemolecules at extremely low concentration.

38

Spectroscopic Study for UV Irradiation Effect on Adsorption State of Volatile Aldehyde at ZnO Nanowire Surface

Such demand emerges for example in breath
sensing-based disease diagnosis.21,22) As well
as structural control and surface catalyst
decoration,23-26) ultraviolet (UV) irradiation is
known as a promising approach for enhancing
the sensitivity of metal oxide-based molecularsensor.27-29) The substantial impact of UV
irradiation on molecule-to-surface interaction
at sensor surface is, however, poorly
understood. These backgrounds motivated us
to conduct the spectroscopic study for the UV
irradiation effect on molecule-to-surface
interactions.
In this study, we investigated the UV
irradiation effect on adsorption state ofvolatile aldehyde at single crystalline ZnO
nanowiresurfacebyFouriertransform-infrared spectroscopy (FT-IR), as
schematically illustrated in Fig.1. C9 aliphatic
aldehyde of nonanal, i.e., a biomarker
molecule of lung cancer in exhaled air,30) wasused as an analyte. We found that theoxidationofadsorbed
nonanalwassignificantly promoted by the UV irradiation.
The oxidation mechanism was discussed inconsideration of its wavelength dependence.

2.

Experimental

2.1

Preparation of ZnO Nanowires

ZnO nanowires were synthesized via aseed-assisted hydrothermal synthesis.31-38) A Si
wafer coated with ZnO seed layer was used asa substrate for the nanowire growth. Prior tothe hydrothermal synthesis, a 5 nm-thick ZnO
seed layer was deposited on Si wafer with a 1
nm-thick Ti adhesive layer by radio frequency(RF) sputtering with the RF power of 50 W andthe argon pressure of 0.3 Pa. For the ZnO
nanowire growth, 25 mM zinc nitratehexahydrate(Zn(NO3)2·6H2O),
25
mM
hexamethylenetetramine (HMTA), 2.5 mM ofpolyethyleneimine (PEI) and 3 mL of NH3·H2O(27% in water) were dissolved in 100 mLdeionized (DI) water in this order by stirring
at room temperature. Then the ZnO seedlayer-coated substrate was immersed into thesolution in an up-side-down manner. The
hydrothermal synthesis was conducted at
95 °C for 10 h. As-grown sample was rinsed in
DI water, dried by air blow, and annealed at
600 °C for 1 h in air. The morphology and the
crystal structure of synthesized ZnO
nanowires were characterized by fieldemissionscanning
electronmicroscopy(FESEM) at an accelerating voltage of 15 kV,

Fig.1 Schematic illustration of the spectroscopic study
for the UV irradiation effect on the molecularadsorption state at metal oxide nanowire surface.

transmission electron microscopy (TEM) atthat of 200 kV, and X-ray diffractometry (XRD)
measurement.
2.2

Molecular Adsorption and UV Irradiation

For examining the UV irradiation effect on
the molecular adsorption state, 2 μL liquid
concentrate of nonanal (C9 aliphatic aldehyde),
was dropped on a ZnO nanowires-grownrectangular-shaped substrate (size; 15 mm×15mm) and dried by nitrogen blow to remove theexcess nonanal. Then UV light with threedifferent wavelengths ( = 260 nm, 300 nm,
and 350 nm) was irradiated to thenonanal-adsorbed ZnO nanowires withvarying the irradiation time for 0–40 min. We
performed FT-IR for the nonanal-adsorbed
ZnO nanowires after each UV irradiation time.
All the experiments were performed at room
temperature in air condition.

3.

Results & Discussion

3.1 Characterization of ZnO Nanowires
Figs.2(a) and (b) show the FESEM images of
ZnO nanowires grown on the substrate, whichcorrespond to the 45°-tilted view and thecross-sectional view, respectively. The ZnO
nanowires were grown almost perpendicular tothe substrate. The length and the diameter of
ZnO nanowires were ~6.5 μm and ~80-120 nm,
respectively. Fig.2(c) shows the TEM image ofsingle ZnO nanowire. The nanowire waswell-crystalline, and the surface was
microscopically smooth. Fig.2(d) shows theselected area electron diffraction (SAED)
pattern of the ZnO nanowire in Fig.2(c). The
singlecrystalline
natureandthe[0001]-oriented growth direction of thesynthesized ZnO nanowire were identified in
the SAED pattern. The [0001]-oriented growth令和 3 年度

Fig.2

九州大学大学院総合理工学報告第 43 巻 第 2 号(a,b) FESEM images of ZnO nanowires: (a)45º-tilted view and (b) cross-sectional view. (c)TEM image and (d) SAED pattern of a single ZnO
nanowire.

Fig.4

Fig.3

XRD pattern of ZnO nanowires grown on asubstrate.

of ZnO nanowire originated from i) the
minimization of total interfacial energy during
the nanowire growth31-34) and ii) the (10-10)
plane-selective capping feature of PEI.39) To
statistically analyze the crystal structure ofsynthesized ZnO nanowires, the XRD
measurement was conducted, and the result isshown in Fig.3, In the XRD pattern, the (0002)
peak was seen at 34.42º and no other peaks
were observable. The results confirmed thesinglecrystalline
natureandthe[0001]-oriented growth direction of ZnO
nanowires as consistent with TEM and SAED
results. The diffraction peak could be indexed
to wurtzite ZnO (JCPDS Card No. 36-1451).
Thus, we successfully synthesized the single
crystalline ZnO nanowire array withwell-defined structure, which serves as aplatform to investigate the UV irradiation
effect on the molecular adsorption state.
3.2

39

UV Irradiation Effect on Nonanal
Adsorption State at ZnO Nanowire Surface

The UV irradiation effect on the molecularadsorption state was investigated by FT-IR

Time evolution FT-IR spectra of nonanal adsorbedon the ZnO nanowires (a) under dark condition, (b)with UV irradiation at  = 260 nm, (c) with UV
irradiation at  = 300 nm, and (d) with UV
irradiation at  = 350 nm, respectively.

using nonanal as an analyte molecule. Fig.4(a)shows the time evolution FT-IR spectra ofnonanal adsorbed on the ZnO nanowires under
no UV irradiation (i.e., a dark condition). In
the spectra, several peaks, which can be
assigned as the vibration modes of (C=O),
(COO), (CH2), (CH3) and (CH), wereobserved (as listed in Table 1).40) Complex peak
patterns of (COO) arise from various
adsorption structures such as bridging,bidentate and monodentate.41) Extended UVTable 1 List of observed IR peaks

peak no.

wavenumber (cm-1)

1

1727

2

1711

3

1592

4

1549

5

1530

6

1467

(CH2/CH3)

7

1410

(CH2/CH3)

8

1390

(CH)

9

1399

s(COO)

101378

s(CH3)

vibration modes(C=O)

as(COO)

40

Spectroscopic Study for UV Irradiation Effect on Adsorption State of Volatile Aldehyde at ZnO Nanowire Surface

Fig.5 The variations of peak intensity of (COO) under
the different UV irradiation conditions.

irradiation time resulted in decrease in theintensity of (C=O) peaks and concomitant
increase in that of the (COO) peaks became
larger. The result indicated that the adsorbed
nonanal was gradually oxidized at ZnO
nanowire surface, and unreacted nonanal andoxidized nonanal coexist even after 40 min.
Fig.4(b) shows the time evolution FT-IR
spectra of nonanal adsorbed on the ZnO
nanowires under UV irradiation at the
wavelength of  = 260 nm. We found that the(C=O) peaks fully disappeared after 30 min of
UV irradiation and the intensity of (COO)
peaks remarkably increased. In addition, no
other variations in FT-IR spectra wereobserved by the UV irradiation. These results
implied that the UV irradiation promoted theoxidation of surface-adsorbed nonanal.
3.3

Wavelength Dependence of UV
Irradiation EffectTo gain an in-depth understanding as to thepromoted oxidation of adsorbed nonanal, we
next investigated the wavelength dependenceof UV irradiation. Figs.4(c) and Fig.4(d) show
the time evolution FT-IR spectra of adsorbed
nonanal when performing the UV irradiation
at the wavelength of = 300 nm and = 350 nm,
respectively. Contrary to the result of UV
irradiation at = 260 nm, the (C=O) peaks
remained even after 40 min for both cases.
Also, the increase of (COO) peak intensity
was not so remarkable. Note that the initialstates of nonanal in Figs.4(a)-(d) were largely
different, which originated from the difficulty
to fully remove the excess nonanal from thenanowiresurfaceduringthesamplepreparation. Nevertheless, the observed UV
irradiation effect and its wavelength
dependence cannot be consistently interpretedby the difference of initial states. To directlycompare the progress of nonanal oxidation at
different UV irradiation conditions, the peakintensity of (COO) was plotted as a functionof UV irradiation time in Fig.5. For thiscomparison, the (COO) peak at 1549 cm-1 wasused. The promoted oxidation of adsorbed
nonanal was seen only at = 260 nm, but notat = 300 nm and = 350 nm. Specifically, theintensity of (COO) peak for the UV
irradiation at = 260 nm was about 2 times
higher compared with those for the dark
condition and the UV irradiation at otherwavelengths. Thus, these results indicatedthat the UV irradiation at = 260 nm
significantly promoted the oxidation ofadsorbed nonanal beyond the limit at thegiven temperature condition. Because thesensing of volatile organic compounds closely
correlates with the oxidation of molecules,42)
the enhancement of sensitivity would be
expected in ZnO nanowire-based nonanal
sensing by the UV irradiation at = 260 nm,
which is planned as a future work.
3.4

Discussion of UV Irradiation Effect

Here we discuss why the oxidation of
nonanal was promoted by the UV irradiation.
According to the wavelength dependence, we
considered that the photogenerated carriers in
ZnO nanowires strongly contributed to theoxidation of nonanal. Because ZnO has a widebandgap of ca. 3.3 eV,43) photogenerated
carriers are produced by UV irradiation andits quantum yield tends to become higher at
shorter wavelength. The photogenerated
electrons in conduction band combine withsurface-adsorbed oxygen to form superoxideions and the photogenerated holes in valenceband combine with surface-adsorbed watermolecules to form hydroxyl radicals.44,45) Such
photogenerated species are reactive and thus
might promote the oxidation of adsorbed
nonanal.

4.

Summary and Conclusion

We investigated the UV irradiation effect on
the adsorption state of nonanal at single
crystalline ZnO nanowire surface by FT-IR. We
found that oxidation of adsorbed nonanal was
significantly promoted by the UV irradiation.
A wavelength dependence showed that the UV
irradiation at = 260 nm enhanced theoxidation beyond the limit at the given
temperature condition. We discussed the UV
irradiation-induced promotion of oxidation via
the photogenerated carriers. Our finding inthis study would lead to the in-depth令和 3 年度

九州大学大学院総合理工学報告

understanding of the molecular sensing
mechanism and thus be beneficial for
designing the performances of metal oxide
nanowire sensors.
Acknowledgments
This work was supported by KAKENHI (No.
JP21K18868, JP18H05243) and PRESTO
Program (No. JPMJPR19J7) of Japan Scienceand Technology Corporation (JST). K.N.
acknowledges JACI. This work was partly
supported by the Cooperative Research
Programs of “Dynamic Alliance for Open
Innovation Bridging Human, Environment
and Materials in Network Joint ResearchCenter for Materials and Devices”, “Network
Joint Research Center for Materials andDevices”.
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