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Cite this: J. Mater. Chem. A, 2021, 9,5815

A thermally robust and strongly oxidizing surface of
WO3 hydrate nanowires for electrical aldehyde
sensing with long-term stability†
Guozhu Zhang, ab Takuro Hosomi, ac Wataru Mizukami,cde Jiangyang Liu, abKazuki Nagashima, ac Tsunaki Takahashi, ac Masaki Kanai,ab Takeharu Sugiyama,fTakao Yasui, cg Yuriko Aoki,b Yoshinobu Baba,g Johnny C. Ho bh
and Takeshi Yanagida *ab
Electrical molecular nanosensors require two conﬂicting surface characteristics, which are catalytic activity
for molecular selectivity and thermal stability for long-term data collection. Here, we show that a simple
surface treatment using strong acids creates two such conﬂicting surface properties of WO3 hydrate
nanowires, which enhance the electrical molecular sensing of aldehydes (nonanal, a biomarker). Massspectrometric measurements reveal that the surface treatment using strong acids substantially promotes
both the oxidization of nonanal and the desorption of products, nonanoic acid, from the surfaces at
a temperature of 50  C, which is lower than the 300  C required for untreated surfaces. Spectroscopic
and structural measurements combined with numerical simulations identify two diﬀerent adsorption
structures of carbonyl groups on the surface, where molecules directly bound to coordinatively
unsaturated surface tungsten preferentially proceed to the catalytic oxidization reaction and theReceived 19th November 2020
Accepted 26th January 2021

subsequent desorption process. Furthermore, we conﬁrm the thermal durability (over 10 years) of thecatalytic activity on acid-treated WO3 hydrate nanowire surfaces up to 300  C, which enables us to

DOI: 10.1039/d0ta11287a

demonstrate the long-term stable sensor operations with the sensitivity (4 orders of magnitude)

rsc.li/materials-a

remaining for years.

Introduction
Electrical molecular sensing of various chemical species has
been recently oﬀering new scientic and industrial approaches
to understand inherently complex systems (biological activity,
chemical processing, human metabolism and others) via
a

Department of Applied Chemistry, Graduate School of Engineering, The University ofTokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8654, Japan. E-mail: yanagida@g.ecc.
u-tokyo.ac.jpbInstitute for Materials Chemistry and Engineering, Kyushu University, 6-1 KasugaKoen, Kasuga, Fukuoka, 816-8580, Japanc

JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapandCenter for Quantum Information and Quantum Biology, Institute for Open andTransdisciplinary Research Initiatives, Osaka University, Osaka 560-8531, Japane

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japanf

Research Center for Synchrotron Light Applications, Kyushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 816-8580, Japang
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya
464-8603, Japanh

Department of Materials Science and Engineering, State Key Laboratory of Terahertz
and Millimeter Waves, Centre for Functional Photonics, City University of Hong Kong,
Kowloon 999077, Hong Kong SAR† Electronic supplementary
10.1039/d0ta11287a

information(ESI)

available.

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See

DOI:

analyzing the collected chemical big-data, which can be further
combined with the available existing data obtained using
physical sensors.1–5 Among various molecular sensing methods,
heterogeneous catalysis-based molecular sensing is particularly
promising for such electrical molecular sensing devices forlong-term data collection.6–8 This is because such heterogeneous catalysis on solid surfaces enables molecular transformations with electron transfer,9 which essentially enhance
the molecular sensitivity and selectivity in recorded electricalsignals.9–11 To improve heterogeneous catalysis-based electricalmolecular sensing, there are inherently two requirements,
which are catalytic activity for molecular selectivity and thermalrobustness for long-term data collection.12–14 Because in general
catalytic activity tends to deactivate over time, especially whensensing in a high temperature range.15,16 This would greatly
limit the practical applications of such kinds of molecularsensors for trace molecule (ppb to ppt level) detection anddiscrimination, for example, trace nonanal (aldehyde, a lungcancer biomarker) detection from exhaled breath for lungcancer early screening and diagnosis.17 Thus, satisfying both
requirements of catalytic activity and thermal robustness on
sensor surfaces is essential to design long-term stable molecular nanosensors using catalysis.

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Fig. 1 (a) Schematic illustration and (b) SEM image of a single WO3 hydrate nanowire sensor device. (c) Transmission electron microscopy (TEM)and selected area electron diﬀraction (SAED) images of a WO3 hydrate nanowire. (d) X-ray diﬀraction (XRD) pattern and the Rietveld reﬁnementof the as-grown WO3 hydrate nanowires. Projections of the as-grown WO3 hydrate nanowires along the (e) c axis and (f) the [110] direction. The
red sectors in water “dots” represent the probability of water's presence at the corresponding sites. (g) and (h) show the sensor response data ofboth as-grown and H2SO4-surface treated WO3 hydrate nanowire sensors when introducing 2.67 ppm nonanal at 200  C. Recovery time isdeﬁned as the time required to obtain the 90% original resistance value. (i) Dynamic resistance response of the H2SO4-surface treated WO3nanowire sensor device to various concentrations (from 6.7 ppb to 2.67 ppm) of nonanal at 200  C (Fig. S2†). (j) and (k) show the temperaturedependent sensing response and recovery time of the as-grown and H2SO4-surface treated WO3 nanowire sensor devices to 2.67 ppm nonanal(Fig. S3†). (l) Recovery time and sensing response of the WO3 hydrate single nanowire device at 200  C when varying the surface acid treatment
time (Fig. S5†).

To fulll these two requirements, previous studies onheterogeneous catalysis-based electrical molecular sensing have
utilized various nanostructures, including thermally robust

5816 | J. Mater. Chem. A, 2021, 9, 5815–5824

metal oxides,18,19 noble metal nanostructures supported on
metal oxides20–22 and heterostructures.23–25 For example, nanostructured metal oxides have been tailored by intentionallyThis journal is © The Royal Society of Chemistry 2021

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introducing oxygen vacancies to enhance the catalysis for electrical molecular sensing.26–28 Noble metal nanostructures on
metal oxide surfaces exhibit the so-called spillover eﬀects forgas molecules, which enhance the catalysis and the electricalmolecular sensing.20,29,30 Heterostructures are also promising
candidates to enhance catalysis-based electrical molecularsensing via the presence of a hetero-interface, where a built-in
electrical potential exists.7,23–25 Although these nanostructures
are practically useful for heterogeneous catalysis-based molecular sensing, designing and tailoring of long-term stable catalysts for molecular sensing are still based on a rule of thumbmainly due to the lack of knowledge as to complex moleculartransformation behavior on nanostructured catalyst surfaces
and the correlation between molecular behavior, surface
structures and recorded electrical signals.21–30 For these reasons,various operando methodologies, including diﬀuse reectanceinfrared Fourier transform spectroscopy (DRIFT), Raman, UV/
Vis, X-ray absorption spectroscopy and X-ray diﬀraction, havebeen introduced to understand the mechanisms of heterogeneous catalysis-based electrical molecular sensing.31–35 For
example, the operando DRIFT method is a powerful techniquefor on-line analysis of molecular reaction products on thesensor surface.31,36–39 These operando techniques frequently
sacrice their sensitivity and resolution for in situ operations.31,36–39 We have recently reported a methodology tomonitor surface molecular transformation behavior, which issensitive enough even for relatively low-density molecules
adsorbed on nanostructured oxides, using a combined technique of infrared p-polarized multiple-angle incidence resolution spectroscopy and temperature programmed desorption/
gas chromatography mass spectrometry.40
In this study, we demonstrate thermally robust and catalytically active sensor surfaces of WO3 hydrate nanowires forelectrical aldehyde (nonanal) sensing by monitoring theirsurface molecular transformation behavior under sensing
environments. Mass-spectrometric, spectroscopic and structural measurements combined with numerical simulations
reveal the presence of thermally robust and coordinatively
unsaturated tungstens on WO3 hydrate nanowire surfaces.

Results and discussion
First, we show the eﬀect of surface treatment using sulfuric acid
(H2SO4) on the electrical response data of the WO3 hydrate
nanowire sensor. Fig. 1a and b show the schematic image andscanning electron microscopy image (SEM) of the fabricatedsensor device composed of a single WO3 hydrate nanowire andPt electrodes. The transmission electron microscopy (TEM) andselected area electron diﬀraction (SAED) images of the fabricated WO3 hydrate nanowires are shown in Fig. 1c, revealingthat the as-grown nanowire is a single crystalline hexagonal
crystal structure and the growth direction can be assigned to[001]. Fig. 1d shows the XRD data, which reveal the WO3 hydrate
crystal structure (WO3$nH2O), as illustrated in Fig. 1e and f. In
the single nanowire sensor device, the open-top gap between
electrodes was designed to be 1 mm. The details of sensor device
fabrication processes can be seen in the Experimental section.

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Journal of Materials Chemistry A
We performed a surface treatment of the as-grown WO3 hydrate
nanowires using sulfuric acid (H2SO4 conc.) for 4 h (Fig. S1,†
SEM data). Fig. 1g and h show the comparison between (1) the
sensor device composed of the as-grown WO3 nanowires and (2)the sensor device composed of the H2SO4-surface treated WO3nanowires with respect to the electrical response data whenintroducing 2.67 ppm nonanal. The operating temperature was200  C. As can be seen, the surface treatment signicantly
enhances the sensor electrical response and the recovery rate.
We conrm this signicant eﬀect of surface acid treatment on
both the nonanal concentration dependence data (Fig. 1i) andthe operating temperature dependence data (Fig. 1j and k). The
acid surface treated sensor device exhibits a detectable electricalresponse down to 6.7 ppb, which is two times lower than that of
the as-grown WO3 hydrate nanowire sensor (13.4, Fig. S2†). Notethat the observed nonanal sensing performance of the acidsurface treated WO3 nanowire sensor device is superior insensitivity and ultralow in the detection limit to that of previously reported nonanal sensors as summarized in ESI Table
S1.† In addition, molecular sensing selectivity evaluation
showed that the acid surface treated sensor presents selectivity
to nonanal (Fig. S4†). To further conrm the eﬀect of surfaceacid treatment, the surface treatment time dependence of thesensor properties (sensor response ratio and recovery time data)
are shown in Fig. 1l. Clearly, increasing the surface acid treatment time improves the sensor properties (i.e. increasing thesensor response and decreasing the sensor recovery time).
Thus, the simple surface treatment using H2SO4 signicantly
improves the electrical response behavior of WO3 hydrate
nanowire sensors.
Since the observed electrical response in Fig. 1 is inherently
determined by the surface chemical reaction events, we performed temperature-programmed desorption/gas chromatography mass spectrometry (TPD-GCMS) to analyze the chemicalcompositions of the desorbed molecules from nonanaladsorbed WO3 hydrate nanowire surfaces, as shown in Fig. 2aand b. The temperature-dependent chromatograms show that
not only nonanal (retention time, 19.8 min, Fig. S7†) but also
nonanoic acid (retention time, 31.1 min, Fig. S7†) is included in
the desorbed molecules, which elucidates the progress of
nonanal oxidation on the WO3 hydrate nanowire surface.
Interestingly, there are several distinct diﬀerences in theoxidation and desorption behavior between the as-grown WO3
hydrate nanowires and H2SO4-surface treated WO3 hydrate
nanowires. First, the temperature range where nonanoic acid isdetected in the chromatograms is much lower in the case of the
H2SO4-surface treated WO3 hydrate nanowires (mainly at 25–75
C), when compared to the as-grown WO3 hydrate nanowires(mainly at 250–350  C) (Fig. 2c). Nonanal is almost fully converted into nonanoic acid below 50  C on the H2SO4-surface
treated WO3 hydrate nanowires. It is in contrast to the case of
the as-grown WO3 hydrate nanowires, in which nonanal
remains up to 175  C (Fig. 2d). These results highlight thesuperior oxidative activity of the surface treated WO3 hydrate
nanowires even in a low temperature range. Second, in the hightemperature range (200–350  C), some olens and ketones(Fig. S8†) are desorbed only from the as-grown WO3 hydrate
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Fig. 2 (a and b) TPD-GCMS spectra of desorbed molecules from theas-grown WO3 hydrate nanowire surfaces and H2SO4- treated WO3
hydrate nanowire surfaces when elevating the temperature from 25 to400  C. The desorption products from the as-grown WO3 hydrate
nanowires at retention times of 25.0 min and 30.8 min correspond to
8-heptadecene (I) and 9-heptadecanone (II), respectively. (c) Calculated concentration data of desorbed nonanoic acid (nmol) from theas-grown WO3 hydrate nanowire surfaces and H2SO4-treated WO3
hydrate nanowire surfaces. The nonanoic acid calibration curve isshown in Fig. S9.† (d) Comparison between the as-grown WO3 hydrate
nanowires and H2SO4-surface treated WO3 hydrate nanowires
regarding conversion ratio from nonanal to nonanoic acid as a function of temperature.

nanowires. These compounds are considered to be formed by
the thermal decomposition of nonanoic acid,41,42 which remains
adsorbed even in such a high temperature range. These results
highlight that the adsorption of the molecules is much stronger
on the as-grown WO3 hydrate nanowires than that on thesurface treated WO3 hydrate nanowires. These trends of desorbed chemical species give information about surface chemical events related to the electrical sensor responses in Fig. 1 i.e.,(1) the superior catalytic characteristics of the surface treated
WO3 hydrate nanowires improves the sensing characteristics
and (2) the residual compounds on the as-grown WO3 hydrate
nanowire surface lengthen the recovery time even at higher
temperatures.
Although the above TPD-GCMS data identify the desorbed
chemical compounds related to surface chemical reactions,they do not provide direct information on chemical events on
sensor surfaces. Thus, we performed Fourier transform infrared(FT-IR) spectroscopy measurements on the sensor surface toreveal more directly surface molecular transformation behavior
under sensing environments. Fig. 3a and b show the comparison between the as-grown WO3 hydrate nanowires and H2SO4-

5818 | J. Mater. Chem. A, 2021, 9, 5815–5824

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surface treated WO3 hydrate nanowires with respect to thetemperature dependent FT-IR spectra in the temperature range
from 25 to 400  C under atmospheric environments. Clearly,
there are several distinct diﬀerences between the two spectra.
First, the diﬀerence of the two peaks (n1 and n2) in the 1660–
1740 cm1 region reveals two distinct adsorption states ofcarbonyl groups, which depends on the surface acid treatment.
The two peaks (n1: 1710 cm1 and n2: 1668 cm1) can be associated with the n(C]O) vibration mode.43 Compared to the FTIR spectrum of liquid-phase nonanal (n(C]O): 1726 cm1),40
both n1(C]O) and n2(C]O) show clear red shis, implying
decreases of C–O bond orders via Lewis acid activation.40,44 Our
previous work also revealed that nonanal adsorbed on thesurface –OH of a metal oxide (ZnO) shows adsorption at1709 cm1.40 Thus, we assigned n1(C]O) to nonanal adsorbed
via hydrogen bonding with a surface –OH. On the other hand,
the much larger red shi of n2(C]O) implies that C]O is
activated by stronger Lewis acids. Therefore, we assigned n2(C]O) to nonanal adsorbed by coordination to a surface exposed
W6+ (Fig. 3c). A theoretical calculation based on density functional theory (DFT) also supports this assignment (Fig. S10†).
The wavenumber of a simplied structure of W6+-coordinatednonanal is calculated to be 1658–1688 cm1, which is in good
agreement with the experimental value (1668 cm1). Based onthese peak assignments, we discuss the eﬀect of surface acidtreatment on the n1(C]O) and n2(C]O) peaks. As can be seen in
Fig. 3, at 25  C, the as-grown WO3 hydrate nanowires showed
only the n1(C]O) peak, whereas the H2SO4-surface treated WO3
hydrate nanowires exhibited only the n2(C]O) peak. Thisdistinct diﬀerence can be reasonably interpreted in terms of theabove assignments of the n1(C]O) and n2(C]O) peaks on thesurface molecular structures. On the H2SO4-treated WO3
hydrate nanowire surfaces, stronger Lewis acid sites (baresurface W) exist which show the n2(C]O) peak, whereas for theas-grown WO3 hydrate nanowires, the –OH terminated surface
exhibits the n1(C]O) peak. Thus, these results highlight thattwo distinct adsorption states of carbonyl groups exist, whichstrongly depends on the history of surface acid treatment on
WO3 hydrate nanowires.
Second, the broad peak in the 1550–1600 cm1 region in
Fig. 3, which appeared only for the surface treated WO3 hydrate
nanowires, reveals the presence of carboxylate compounds on
the WO3 hydrate nanowire surface.38,45 Previous studies haveidentied that the broad peak in the 1550–1600 cm1 region
corresponds to an asymmetric stretching vibration mode: nas(COO) of carboxylate compounds, which are strongly bound tosolid surfaces.46,47 This peak was not observable for the as-grown
WO3 hydrate nanowires at all. In other words, such carboxylatecompounds only exist on the H2SO4 treated WO3 hydrate
nanowire surface. To further conrm the eﬀect of H2SO4 treatment on the appearance of surface carboxylate compounds, wemeasured the dependence of FT-IR spectra on H2SO4 treatment
time, as shown in Fig. 3d. As can be seen, the peak intensity inthe 1550–1600 cm1 region tended to increase with the H2SO4
treatment time, revealing the signicant eﬀect of surface
treatment on the appearance of surface carboxylatecompounds. This can also be demonstrated by the time-

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Fig. 3 (a and b) FT-IR spectra of nonanal adsorption on the as-grown WO3 hydrate nanowires and H2SO4-surface treated WO3 hydrate
nanowires when elevating the temperature from 25 to 400  C. (c) Schematic structures of surface-adsorbed nonanal and the corresponding
density functional theory (DFT)-calculated C]O frequencies (see the ESI for details on the theoretical calculations†). (d) FT-IR spectra of nonanal
adsorption on WO3 hydrate nanowires for diﬀerent treatment times with H2SO4.

dependent FT-IR measurement (Fig. S11†). In addition, as seenin Fig. 3b, the surface carboxylate compounds desorbed atrelatively low temperatures below 125  C, which is consistent
with the desorption prole of nonanoic acid for the surface
treated WO3 hydrate nanowires in Fig. 2c. On the other hand, inFig. 3b, the n1(C]O) peak for the as-grown WO3 hydrate
nanowires remains even at higher temperatures up to 350  C.
These temperature dependences of carbonyl groups andcarboxylate compounds observed in the spectroscopic data arein good agreement with TPD-GCMS data in Fig. 2, revealing thedetailed molecular transformation behavior on the WO3
hydrate nanowire surface during the desorption process.
The above mass-spectrometric and spectroscopic results
explain the eﬀect of surface acid treatment on the molecularsensing properties (Fig. 1) in terms of the surface moleculartransformation behavior on the WO3 hydrate nanowire surface.
However, we still cannot answer the following simple question:
what surface structures are created by H2SO4 treatment, which
improve the catalytic characteristics? To identify the mechanisms of catalytic surface structures, rst we focus on thesurface conditions before nonanal adsorption. It is noted thatthe spectroscopic data in Fig. 3 do not contain any experimental
trends as to water molecules on the sensor surface. This isbecause their spectra are intentionally subtracted using a spectrum of the sample before nonanal adsorption to exclude
background moisture eﬀects on the spectra. Fig. 4a–d show the
raw FT-IR spectra of the as-grown WO3 hydrate nanowires andH2SO4-surface treated WO3 hydrate nanowires before nonanal
adsorption. The annealing temperature eﬀects on the spectra
are also shown in Fig. 4a–d, since sensing experiments were
performed at elevated temperatures up to 300  C. As can be

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seen, there are distinct peaks of water molecules, which are in
the range of 3500–3700 cm1: a stretching mode of OH-nOH, and
1595–1650 cm1: a bending mode of OH-dOH.43 Clearly,
increasing the annealing temperature decreases the amount ofwater, and the H2SO4 treatment further enhances the removal ofwater from solid samples due to a strong hydration eﬀect on
SO42.48,49 Thus, there is a diﬀerence of water content between
the as-grown WO3 hydrate nanowires and H2SO4-surface treated
WO3 hydrate nanowires. In addition, at elevated temperaturesof sensing conditions, water related peaks almost disappeared
for the H2SO4-surface treated WO3 hydrate nanowires, but notfor the as-grown WO3 hydrate nanowires. In the 1595–
1650 cm1 region, there are two distinct peaks of dOH. The peakat 1595–1600 cm1 is associated with dOH of WO3$2H2O, and thepeak at 1620–1625 cm1 can be assigned to dOH of WO3$H2O.50
XRD measurements of these samples also show the presence oftungsten trioxide hydrates (WO3$xH2O) (Fig. S12–S14†) and thedehydration behavior when annealing (Fig. S15†) and acidtreatment were performed, as shown in Fig. 4e. Thus, the resultsin Fig. 4a–e consistently indicate that H2SO4 surface treatment
dehydrates WO3 hydrate nanowires more than the as-grown
WO3 hydrate nanowires in an elevated temperature range ofsensing conditions. This diﬀerence of water content in WO3
hydrate nanowires might aﬀect the absorption and desorption
behavior. Therefore, the rst possible model is based on thepresence of water molecules near surfaces. This is because thepresence of water near surfaces might hinder the interaction
between a carbonyl group and a Lewis acid site of surface W. As
seen in the spectra of Fig. 4a–d, the thermal annealing alters thewater content of WO3 hydrate nanowires. Therefore, we can
examine the eﬀect of water content on the molecular

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Fig. 4 (a–d) FT-IR spectra of the as-grown WO3 nanowires and H2SO4-surface treated WO3 nanowires after thermal treatment at diﬀerent
temperatures. (e) X-ray diﬀraction (XRD) pattern and the Rietveld reﬁnement of H2SO4-surface treated WO3 nanowires. (f) Reﬁned crystal
structure unit of the as-grown WO3 hydrates and H2SO4-surface treated WO3 hydrates, where the red sectors in water “dots” represent theprobability of water's presence at the corresponding sites (other viewpoints are shown in Fig. S13†). (g) FT-IR spectra of nonanal adsorbed on the
400  C annealed as-grown WO3 nanowires and H2SO4-surface treated WO3 nanowires.

transformation behavior by using such annealed WO3 hydrate
nanowires. If the water content of WO3 hydrate nanowires solely
dominates the surface molecular transformation behavior of
nonanal, the annealed WO3 hydrate nanowires, which are
dehydrated, should exhibit catalytic and molecular behavior
similar to the H2SO4-surface treated samples. Fig. 4g shows the
spectra of nonanal adsorption on the annealed WO3 hydrate
nanowires. Contrary to our above speculation, the spectra aresimilar to those of the as-grown WO3 hydrate nanowires. As
shown in ESI data (Fig. S16†), the electrical sensing response of
the annealed WO3 hydrate nanowires is also similar to that of
the as-grown WO3 hydrate nanowires in terms of the recovery
time, although the electrical responses are enhanced. Thus, thediﬀerence of water content in WO3 hydrate nanowires alone
cannot explain the occurrence of enhanced catalytic characteristics caused by surface acid treatment.
Here, we focus on the observed diﬀerence between surfaceacid treatment and thermal annealing on the dehydration
process of WO3 hydrate nanowires to clarify the mechanisms of
surface acid treatment. Thermal annealing physically removes
water from a solid at elevated temperatures, whereas surfaceacid treatment chemically dehydrates WO3 hydrate nanowires
at relatively low temperatures by utilizing the strong hydrationeﬀect on SO42.48,49 It is easily inferred that a high temperature
operation of the thermal annealing process creates a thermodynamically stable solid surface structure, while a low temperature operation of chemical surface acid treatment might
produce thermodynamically unstable solid surface structures.51,52 Extended X-ray absorption ne structure (EXAFS)
analysis of the coordination structure of W can reveal such

5820 | J. Mater. Chem. A, 2021, 9, 5815–5824

a surface solid structure diﬀerence.53–55 Fig. 5a shows the normalized W L3-edge X-ray absorption near edge structure(XANES) of the WO3 hydrate nanowire samples aer H2SO4
surface treatment and thermal annealing treatment (400  C). It
can be seen that the peaks of all WO3 hydrate nanowires are
consistent with those of the standard WO3 (10 218.5 eV), indicating that the oxidization states of W of the treated WO3
hydrate nanowire samples do not change. This is also supportedby the XPS characterization (ESI, Fig. S17†). Furthermore, theFourier transform (FT) k3c(k) spectra of the WO3 nanowire
samples exhibit a dominant peak centered at 1.40 
A assignable
to the W–O bond, and the W–W bond peak is centered at 3.60 A(Fig. 5b). In addition, the FT spectra of the surface treated WO3
hydrate nanowires showed that the W–W peaks undergoa downshi to 3.53 
A compared to those of the as-grown WO3
hydrate nanowires, indicating that lattice contraction occurred
due to the dehydration eﬀect. Fig. 5c shows the averaged coordination number data of the rst shell of a W ion from thetting of FT k3c(k) spectra. The analyzed coordination numberof W tends to decrease as the H2SO4 surface treatment timeincreases, while the annealing treatment has negligible eﬀects
on the coordination number of W. These results reveal thepresence of coordinatively unsaturated W for the H2SO4 surface
treated WO3 hydrate nanowires, but not for the thermallyannealed WO3 hydrate nanowires. Thus, all the above spectroscopic results highlight that the presence of coordinatively
unsaturated W of WO3 hydrate nanowires plays an essential rolein promoting the catalytic eﬀects and molecular sensing properties shown in Fig. 1.

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Fig. 5 (a) Normalized W L3-edge XANES spectra of WO3 hydrate nanowires and the standard W, WO2 and WO3 (Sigma Aldrich) as references. (b)Fourier transformed k3-weighted c(k)-function of the EXAFS spectra (circles) and ﬁrst-shell ﬁttings (line) of WO3 hydrate nanowires. The detailed
ﬁtting processes are shown in the ESI.† (c) W coordination numbers (C.N.) of the ﬁrst shell extracted from Fourier transformed EXAFS curve ﬁtting.

Finally, we examine the thermal durability of the surfacetreated WO3 hydrate nanowire catalysts. Fig. 6a shows thethermal durability (air-annealing temperature dependence)
data from the IR spectra of the H2SO4 surface treated WO3
hydrate nanowires under the sensing environment – atmospheric air – and temperature range. The air-annealing time is 1
hour. Fig. 6b shows the extracted thermal durability data of thenas(COO) peak as an index of catalytic properties (oxidization of
nonanal) of WO3 hydrate nanowires. The data are shown as theratio of the nas(COO) peak at the annealing temperature to thatat 25  C. As can be seen, the catalytic properties due to acidsurface treatments can be maintained up to an annealing

temperature of 300  C, which is robust enough for electricalsensing. To further examine the robustness as a time series,
Fig. 6c shows the time series data of the ratio of nas(COO) peaks
during durability tests for various annealing temperatures. The
data are shown as the ratio of the nas(COO) peak at theannealing time to that at 10 minus annealing time. For thetemperature range for molecular sensing experiments around200  C (Fig. 1), the catalytic eﬀect can be maintained at least
over 10 years. Even at a higher temperature up to 300  C, thecatalytic activity remains over a year. Considering the surface
catalytic mechanisms discussed in previous sections, thisobserved thermal robustness of tailored catalytic surfaces

Fig. 6 (a) Temperature dependence of FT-IR spectra of H2SO4 surface treated WO3 hydrate nanowire catalysis. The annealing time is 1 hour. (b)Temperature dependence of the ratio between the intensity of nas(COO) after annealing (I(COO)T) and without annealing (I(COO)25  C), which are
extracted from data from (a). (c) Time series data of catalyst degradation when varying the temperature (I(COO)t, intensity of nas(COO) after
annealing for a designated time; I(COO)10 min, intensity of nas(COO) after annealing for 10 minutes). The raw data of FT-IR are shown in the ESI(Fig. S19 and S20).† (d) Long-term stability of the H2SO4 surface treated WO3 hydrate nanowire sensor device (working at 200  C, nonanal,
2.67 ppm. ESI Fig. S21†). (e) Dynamic resistance response of the H2SO4 surface treated WO3 hydrate nanowire sensor device after storing in air for
1 day and 920 days.

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Journal of Materials Chemistry A
corresponds to the thermal stability of coordinatively
unsaturated W of WO3 hydrate nanowires. In addition, we
examine the long-term stability of H2SO4 surface treated WO3
hydrate nanowire sensors for nonanal, as shown in Fig. 6d ande. As can be seen, the electrical sensing response can be
maintained over 2.5 years with keeping the electrical sensitivity(4 orders of magnitude change). This long-term stable sensing
data are well consistent with the catalytic activity data in Fig. 6c.
Thus, these time series data quantitatively show the thermalrobustness of the surface-treated WO3 hydrate nanowire catalysis for years under sensing environments.

Paper
200  C for 24 h. Finally, the WO3 nanowires were obtained aer
ultracentrifugation and rinsed with DI water.
Acid and annealing treatment of WO3 hydrate nanowires
Concentrated sulfuric acid (H2SO4) was utilized to dehydrate
the as-grown WO3 nanowires. The as-grown WO3 hydrate
nanowires were mixed with 50 mL of H2SO4 and stirred at 90  C
for diﬀerent times (1, 2, 4, and 8 h). Then, the samples were
carefully rinsed with DI water to remove residual H2SO4.
Annealing treatment was conducted at 400  C in air for 1 h.
Structure characterization

Conclusions
In summary, we show that a simple surface treatment using
strong acids enhances both the thermally robust catalysis andelectrical molecular sensing of WO3 hydrate nanowires forcarbonyl compounds. Mass-spectrometric measurements revealthat the surface treatment using strong acids substantiallypromotes both the oxidization of nonanal and the desorption of
products, nonanoic acid, from the surfaces. Spectroscopic andstructural measurements combined with numerical simulations identify two diﬀerent adsorption structures of carbonylgroups on the surface, where the molecules directly bound tocoordinatively unsaturated surface tungsten preferentiallyproceed to the catalytic oxidization reaction and the subsequentdesorption process. Furthermore, we conrm the thermal
durability (over 10 years) of the catalytic activity on acid-treated
WO3 hydrate nanowire surfaces up to 300  C, which enables us
to demonstrate the long-term stable sensor operations with thesensitivity (4 orders of magnitude) remaining for years. Thus,
the present approach will support a better understanding andtailoring of complex nanostructured metal oxide surfaces forfurther sophisticated thermally robust electrical molecularsensing using heterogeneous catalysis.

Experimental
Fabrication of WO3 hydrate nanowires
WO3 hydrate nanowires were fabricated in two steps, which
include the polytungstic acid sol synthesis and the nanowire
growth.56 For the polytungstic acid (H2WO4) sol synthesis,
tungsten powder (4 g) (Sigma-Aldrich, 99.9%) was dissolved inhydrogen peroxide solution (H2O2, 30 wt% in H2O) (FUJIFILM
Wako Pure Chemical Corp., 30.0–35.5%) in an ice bath undercontinuous stirring. When the W powder was dissolved, Pt foil
was added to the solution to catalytically remove the unreacted
H2O2. As no bubbles were generated, the concentration of theas-synthesized W precursor solution was controlled using
deionized (DI) water. Aerward, 50 mL of the W precursor(10.88 mM) was transferred to a 100 mL Teon beaker. Then,
0.5625 g (6.25 mM) of oxalic acid (H2C2O4) (Sigma-Aldrich,
99.0%) was added to the mixture and the pH was carefully
controlled to 1.68 by carefully adding sodium hydroxide (NaOH,
1 M) solution. Finally, the Teon beaker was mounted in an
autoclave system and then transferred to a furnace and kept at

5822 | J. Mater. Chem. A, 2021, 9, 5815–5824

Scanning electron microscopy (SEM) images were obtainedusing a JEOL JSM-7610F microscope. Transmission electron
microscopy (TEM) images were acquired using a JEOL, JEMARM300F. X-ray diﬀraction (XRD) patterns were recorded witha Rigaku SmartLab diﬀractometer (Cu Ka, l ¼ 1.5406 
A, 45 kV,and 200 mA). Fourier transform infrared spectroscopy (FT-IR)
spectra were measured using a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientic) equipped with an MCT detector.
Desorbed gas analysis using gas chromatography-mass
spectroscopy (GC-MS)
A Shimadzu GCMS-QP 2010 Ultra instrument (equipped witha high polar InertCap FFAP capillary column and an OPTIC-4
inlet temperature control system) was used to analyze the desorbed compounds at each temperature. First, the as-grown andacid treated WO3 nanowires (the amount of nanowires wascontrolled at 2 mg) were loaded on a silicon substrate of 0.2 cm
 2.0 cm size and dried at 100  C. Nonanal liquid (4 mL) was
dropped in a 25 mL closed bottle and kept for 30 min tovaporize the nonanal gas. Then, the nanowire sample wastransferred into a bottle lled with nonanal gas for a xed time
(10 min) and immediately transferred into the inlet port of the
GC-MS instrument for further measurements. The inlet
temperature was rapidly increased (60  C s1) to a given
temperature (25–400  C with steps of 25  C) and maintained for
10 min, then immediately cooled to 25  C. The desorbed gas ateach temperature was analyzed by mass spectroscopy. The
measurements for each temperature were conducted sequentially without removing the sample from the inlet port. The
detailed temperature program is shown in Fig. S6.†
FT-IR monitoring of the molecular adsorption progressFirst, the as-grown and acid treated WO3 nanowires (theamount of nanowires was controlled at 50 mg) were loaded ona silicon substrate of 2.0 cm  2.0 cm size and dried at 100  C.
Nonanal liquid (20 mL) was dropped in a 100 mL closed bottle
and kept for 30 min to vaporize the nonanal gas. Then, thesample was transferred into a bottle lled with nonanal gas fora xed time (10 min) and immediately transferred into the FT-IR
chamber for further measurements. The nonanal-adsorbed
WO3 nanowires were heated at a given temperature (25–400  C with steps of 25  C) for 10 min and cooled to room
temperature. The FT-IR spectra of the nanowires were thenThis journal is © The Royal Society of Chemistry 2021

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Paper
recorded at room temperature on a Thermo Fisher Scientic
Nicolet iS50 FT-IR spectrometer equipped with an MCT
detector.

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X-ray absorption spectroscopy
XANES and EXAFS analyses of the W L3-edge were performed intransmission mode at the BL 06 (Kyushu University beamline)of the SAGA-LS (Saga, Japan) with a storage ring operating at anenergy of 1.4 GeV. A monochromator with two Si (111) crystals
was used to scan the energy of the X-ray beam. The photon ux
was 2  1010 photons per second. The intensities of the uorescence X-rays were monitored using a silicon dri detector.
Powder samples of the standard W, WO2, and WO3, and WO3nanowire samples were diluted with boron nitride to obtain the
optimum concentration and then pressed into a tablet witha diameter of 1 cm. The spectra of the prepared pellet samples
were measured in transmission mode. XANES data processing
and r-space tting of XAFS data were carried out using Athenaand Artemis included in the Ifeﬃt package. The detailed tting
processes are shown in the ESI.†
Computational analyses of molecular vibrational infrared
spectraDFT calculations were carried out to compute the vibrationalfrequencies of nonanal molecules bonding on the hexagonal
WO3 (100) plane. We used a simplied cluster model to mimic
the WO3 (100) plane. The cluster model for the W6+ site model
consisted of a single tungsten atom and ve oxygen atoms. We
xed the atomic congurations of these two cluster models
during the geometry optimization. We employed B3LYP-D3 as
the functional. According to the comparison with the resultsfrom other functionals (uB97XD, APFD, B3LYP, or B3LYP-D3),
dependency on the used functional is found to be negligible
in this calculation. A more detailed discussion is provided in the
ESI.†
Fabrication of the single nanowire device and molecularsensing measurement
A nanowire suspension was prepared by mixing the fabricatednanowire samples with 2-propanol with light sonication. Then,the dispersed nanowires were transferred onto a 100 nm thick
SiO2 coated Si (100) substrate with pre-patterned contact Ptelectrodes. Electron beam (EB) lithography (Elionix, ELSS50YND) was performed at an accelerating voltage of 50 kV to
make the electrode patterns for bridging a single nanowire withthe contact electrodes using ZEP520A-7 (ZEON) as a resist. A
800 nm layer of Pt was then deposited by RF sputtering(SEINAN) in an Ar 0.4 Pa atmosphere. Aerward, li-oﬀ (N,Ndimethylformamide) and acetone cleaning processes wereconducted to complete the device fabrication. The gap size ofbridged nanowires was designed to be 1 mm and the nanowires
with ca. 300–400 nm diameter were chosen for all fabricateddevices to eliminate the variation of electrical properties causedby the device geometry. The sensing performance to nonanal
was analyzed by using a self-made chamber combined witha Keithley 4200A-SCS Parameter Analyzer (Tektronix, Inc.).

This journal is © The Royal Society of Chemistry 2021

Journal of Materials Chemistry A
Resistance of the WO3 nanowire device under N2 was recorded
as baseline (RN2). The resistance upon exposure of nonanal gas(produced by bubbling liquid nonanal solution) was noted as
Rgas, and sensing response was dened as RN2/Rgas. The recovery
time was dened as the time required for the recovery of theresistance to 90% of RN2 for the desorption process. All thesensing tests were carried out under a DC bias voltage of 1 V.
Nonanal gas was produced by bubbling liquid nonanal solution
with highly pure N2, and its concentration was calibrated using
GCMS (Shimadzu GCMS-QP2010 Ultra) with a temperature
control system (GL Sciences, OPTIC-4). The ow rates of dry air
and the nonanal gas were controlled at 200 sccm during thesensing performance tests and the testing temperature was set
to vary from 25 to 250  C. The bubbling nonanal gas was diluted
400 times to the ppb level for the sensing test. 10–15 devices
were fabricated for each sample to obtain statistical data.

Author contributions
T. Y. proposed the research direction and guided the project. G.
Z., T. H., J. L., K. N. and T. Y. designed and performed the entire
experiment. W. M. and T. H. carried out computational investigation and provided theoretical analysis. All authors analyzed
and discussed the experimental results. T. S. performed XANES
and EXAFS analyses. G. Z. and T. Y. draed the manuscript. All
the other authors participated in preparing the manuscript andcontributed to the discussion.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
This work was supported by KAKENHI (18H0524) and JSTCREST (JPMJCR19I2). XANES and EXAFS analyses of the W L3edge were performed in transmission mode at the BL 06(Kyushu University beamline) of the SAGA-LS, Saga. The
computation in this work was performed using the facilities of
RIIT, Kyushu University. The study was supported by theGeneral Research Fund of the Research Grants Council of Hong
Kong SAR, China (CityU 11211317).

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