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Yuki Nakagawa, Yasuhiro Shiratsuchi, Tamaki Shibayama, [Masaki Takeguchi](https://orcid.org/0000-0002-0282-6020)

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[Ultraviolet Light-Induced Surface Changes of Tungsten Oxide in Air: Combined Scanning Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy Analysis](https://mdr.nims.go.jp/datasets/3dd30c56-4e57-40f1-ae5a-efb0a9d2429c)

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Ultraviolet Light-Induced Surface Changes of Tungsten Oxide in Air: Combined Scanning Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy AnalysisCitation: Nakagawa, Y.; Shiratsuchi,Y.; Shibayama, T.; Takeguchi, M.Ultraviolet Light-Induced SurfaceChanges of Tungsten Oxide in Air:Combined Scanning TransmissionElectron Microscopy and X-rayPhotoelectron Spectroscopy Analysis.Nanomaterials 2024, 14, 1486.https://doi.org/10.3390/nano14181486Academic Editor: Detlef W.BahnemannReceived: 26 August 2024Revised: 9 September 2024Accepted: 11 September 2024Published: 13 September 2024Copyright: © 2024 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).nanomaterialsArticleUltraviolet Light-Induced Surface Changes of Tungsten Oxide inAir: Combined Scanning Transmission Electron Microscopy andX-ray Photoelectron Spectroscopy AnalysisYuki Nakagawa 1,* , Yasuhiro Shiratsuchi 2, Tamaki Shibayama 1 and Masaki Takeguchi 3,*1 Faculty of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8628, Japan2 Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8628, Japan3 National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan* Correspondence: y-nakagawa@eng.hokudai.ac.jp (Y.N.); takeguchi.masaki@nims.go.jp (M.T.)Abstract: Scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopyanalyses were combined to clarify the ultraviolet light-induced surface changes of WO3 in air. Identical-location STEM (IL-STEM) analysis showed that the WO3 particle surface was covered with an amor-phous thin film after ultraviolet irradiation in air. X-ray photoelectron spectroscopy analysis showedthat hydrocarbon decomposition and the formation of carboxyl/hydroxyl species occurred. Theseresults suggested that the amorphous thin films consisted of photocatalytic oxidative species of hydro-carbon. The IL-STEM analysis could detect small light-induced changes. This technique will be usefulfor the microscopic characterization of photocatalysis or photoinduced hydrophilic conversion.Keywords: tungsten oxide; photocatalysis; identical-location scanning transmission electron mi-croscopy; X-ray photoelectron spectroscopy; ultraviolet light; hydrocarbon decomposition1. IntroductionOxide materials, such as titanium oxide (TiO2), tungsten oxide (WO3) and zinc oxide(ZnO), are widely applied in photocatalysis [1] and sensors [2,3]. Various strategies for im-proving the photocatalytic activity have been used for these oxide photocatalysts, includingintroducing defects [4–6], supporting cocatalysts [7,8], and fabricating heterojunctions [9,10].In order to reveal the correlations between photocatalytic properties and these strategies,transmission electron microscope (TEM) analysis is required. Some studies have focused onthe microscopic analysis of light-induced phenomena [11]. Using irradiation with ultraviolet(UV) light at 360 nm in the TEM, in situ high-resolution TEM observation has been per-formed for the photodecomposition of hydrocarbons on TiO2 films [12]. Additionally, watervapor and environmental TEM have been used for the in situ observation of the reductionof Cu2O to Cu under irradiation at 405 nm [13]. A laser-equipped high-voltage electronmicroscope has been used for the in situ observation of the photocorrosion of ZnO crystalsin an ionic liquid under 325 nm irradiation [14].The photoinduced hydrophilic conversion of a photocatalyst surface can be causedby light irradiation in air. For example, when a TiO2 thin film is exposed to UV light,the water contact angle (CA) decreases over time and finally reaches almost 0◦ to give ahighly hydrophilic state [15]. The characteristics of the highly hydrophilic state have beeninvestigated in previous studies. When the photocatalytic oxidation and photoinducedhydrophilicity of TiO2 and SrTiO3 films were investigated, both films showed almost thesame photocatalytic oxidation activity but a photoinduced highly hydrophilic state was onlyachieved for TiO2 [16]. The relationship between the amount of carbon contamination andCA was investigated using X-ray photoelectron spectroscopy (XPS) and CA measurements.The highly hydrophilic state was obtained even if contaminants remained on the surfaceof the TiO2 film [17]. When the stability of the highly hydrophilic TiO2 surface was investi-gated, ultrasonication in water increased the CA to approximately 10◦ [18] and wet rubbingNanomaterials 2024, 14, 1486. https://doi.org/10.3390/nano14181486 https://www.mdpi.com/journal/nanomaterialshttps://doi.org/10.3390/nano14181486https://doi.org/10.3390/nano14181486https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.comhttps://orcid.org/0000-0001-5020-1458https://orcid.org/0000-0002-0282-6020https://doi.org/10.3390/nano14181486https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.com/article/10.3390/nano14181486?type=check_update&version=1Nanomaterials 2024, 14, 1486 2 of 9increased the CA from 3◦ to 80◦ [19]. Thus, stability can be lost with external stimuli. Theremoval of contaminants by oxidation can make the surface moderately hydrophilic andit appears that the highly hydrophilic state is a metastable state [15]. Structural changeson TiO2 surfaces under UV light have been investigated and this metastable state can beexplained by an increase in the number of hydroxyl (OH) groups on the TiO2 surface [15].The XPS O 1s spectrum for highly hydrophilic TiO2 surfaces shows a broad shoulder peakto the higher binding energy side of the lattice oxygen main peak. This shoulder is fitted bytwo species of hydroxyl groups and physically adsorbed molecular water, and graduallydecreases during storage in the dark [17]. The hydrophilic conversion of a WO3 thin filmhas been also reported in a previous study, where the CA decreased to below 5◦ under UVor visible light irradiation in air. It has been suggested that the presence of oxygen andwater molecules is important in photoinduced hydrophilic conversion [20]. Additionally,a study has shown that photogenerated holes are required to achieve highly hydrophilicconversion [20]. In a cocatalyst system with WO3 and Pt, photoinduced hydrophilic con-version occurred for WO3 films with underlying Pt nanoparticles, but the hydrophilicitydecreased for films with overlying Pt nanoparticles because of the hydrophobic natureof the Pt nanoparticles [21]. In WO3 films fabricated using a W6+ complex salt of citricacid, visible light-induced hydrophilicity was achieved and the interaction between watermolecules and the oxygen-deficient WO3 thin films was important for achieving hydrophilicconversion [22].In the present study, to clarify the UV-induced structural changes of WO3 in air, weperformed an atomic-level observation of WO3 using a spherical aberration-correctedscanning transmission electron microscope (Cs-corrected STEM). The WO3 nanoparticleswere analyzed by identical-location (IL) STEM analysis. In one previous study, oxygenvacancy-induced edge reconstruction was observed at the atomic scale in a high-angleannular dark-field (HAADF) STEM image [23]. In this study, an ozone lamp was used asthe UV light source. In another study, the ozone lamp irradiation of WO3 photoanodes inair yielded a higher surface area for WO3, but the process for this was not clarified [24].Slight structural changes on the surface were observed by IL-STEM after the UV irradiation.We also performed XPS analysis before and after UV irradiation.2. Materials and Methods2.1. Identical-Location-STEM (IL-STEM) AnalysisWO3 nanopowders (particle size: <100 nm, purity: N.A., Sigma–Aldrich Japan G.K.,Tokyo, Japan) were used in this analysis. X-ray diffraction (XRD) measurements wereperformed using a diffractometer with Cu Kα radiation (MiniFlex, Rigaku, Tokyo, Japan).The WO3 particles were dispersed in ethanol and a few drops of the suspension were placedon a microgrid with a carbon-supporting membrane and dried in air. The samples wereobserved using Cs-corrected STEM (JEM-ARM200F, JEOL) with a cold field emission gunoperated at an acceleration voltage of 200 kV. First, HAADF-STEM images were obtained forsome regions of the particles. Then, the TEM grid with WO3 powder was taken out of theTEM and UV light irradiation was performed in air for 4 h. An ozone lamp (GL-4Z, electricpower: 4 W, main wavelengths: 254 nm and 185 nm, Kyokko Denki, Tokyo, Japan) wasused for the UV irradiation. The distance between the lamp and sample was approximately1 cm. The temperature and the humidity during the irradiation were approximately 25 ◦Cand 15%, respectively. To remove ozone species, the irradiation experiment was conductedin a fume hood. After UV irradiation for 4 h, the identical-location of the sample was againobserved by STEM. An atomic structure model of the WO3 was prepared using ReciProsoftware (Ver. 4.876).2.2. XPS AnalysisTungsten plates (purity: 99.95%, 10 mm × 10 mm × 0.2 mm, Nilaco, Tokyo, Japan) werecalcined in air at 800 ◦C for 0.5 h to synthesize WO3 plates. UV irradiation was performed inair using the same ozone lamp as in Section 2.1. The distance between the lamp and sampleNanomaterials 2024, 14, 1486 3 of 9was approximately 1 cm. After UV irradiation, the prepared plates were stored in a vacuumdesiccator. After storage, the plates were analyzed by XPS (JPS-9200, Mg Kα radiation, JEOL,Tokyo, Japan). First, we prepared a pristine WO3 plate (A-1) and a plate irradiated for 12 h(B-1) for XPS analysis (Table 1). The temperature and humidity in the room were recordedusing a digital thermo-hygrometer (PC-7980GTI, Sato Keiryoki Mfg, Tokyo, Japan) beforeand after the light irradiation experiments. For the B-1 sample, the temperature and thehumidity during the experiment were approximately 17.9 ◦C and 30%, respectively. Wethen irradiated the A-1 and B-1 plates again under humid conditions, and these plates werelabeled as A-2 and B-2, respectively. The A-2 and B-2 plates were analyzed by XPS 1 dayafter the UV irradiation (Table 1).Table 1. Sample treatment conditions, number of days that passed between UV irradiation and thedate of XPS analysis, and average humidity and temperature used for the sample treatment.Sample Sample TreatmentThe Number of Days That Passedbetween UV Irradiation and theDate of XPS AnalysisAverageHumidity (%)AverageTemperature (◦C)A-1 None (pristine) – – –B-1 12 h irradiation/dry 108 30 17.9A-2 A-1 + 4 h irradiation/humid 1 72 27.7B-2 B-1 + 4 h irradiation-humid 1 72 27.7The chemical bonding states in the prepared plates were analyzed by XPS. The basepressure during spectra acquisition was better than 5 × 10−6 Pa, which was achievedusing a turbomolecular pump. The area of sample analyzed was 3 mm in diameter. Neithersputter cleaning nor a charge neutralizer was used. The work function method was used forthe calibration of the binding energy values [25]. The work function of WO3 is reportedly5.05 eV [26]; thus, the binding energy of the C 1s peak was calibrated at 284.53 eV in thiswork. The background was subtracted using Shirley’s method. Peak deconvolution andquantification were carried out using SpecSurf software (Ver. 1.9.4.3). A mixed Gaussian–Lorentzian function was used for peak deconvolution. The atomic ratios of C, O, andW were derived from the integrated intensities of the C 1s, O 1s, and W 4f peaks. Therelative sensitivity factors of C 1s, O 1s and W 4f7/2 in the SpecSurf software were used forthe calculation.3. Results and DiscussionAt the magnification used in HAADF-STEM, no change was observed in the mor-phology of the WO3 particles before and after UV light irradiation (Figure 1a,b). For thepristine WO3 powder, the diffraction peaks in the XRD profile (Figure 1c) were indexed toa monoclinic WO3 (γ-WO3, ICDD No. 00-043-1035). In Figure 1c, the XRD profiles of platesamples are also shown, and the results are discussed in the next section. Atomic-scaleHAADF-STEM images of the particle edges were obtained (Figure 2). A HAADF imageof the pristine WO3 particles was obtained at high magnification (Figure 2a). An atomicstructure model of monoclinic WO3 along the [001] zone axis showed overlapping W-Oatomic columns and oxygen atomic columns (Figure 2b). Atomic-scale HAADF-STEMimages of the WO3 particles were observed along the [001] zone axis (Figure 2c–h). BecauseHAADF-STEM imaging is incoherent, and the image contrast is roughly proportional toZ1.6–2.0 (Z: atomic number), the bright dots in Figure 2c–h corresponded to overlappingW-O columns. In the atomic-scale image, some slight changes were observed after UVirradiation in air. We performed IL-STEM analysis immediately after the 4 h UV irradiation.Morphological changes caused by etching were observed in one region (arrow in Figure 2d).Large quantities of carbon species could exist in this region, and this was confirmed by XPSmeasurements, which showed that carbon contamination decreased after UV irradiation(Figure 3b–d and Table 2). The amorphous layer covering the particle edge became thickerNanomaterials 2024, 14, 1486 4 of 9(Figure 2e,g compared with Figure 2f,h). The arrangement of overlapping W-O columns inthe particles was similar before and after UV irradiation, which suggested that the internalcrystal structure was not changed by the UV light. However, the particle surface wascovered by an amorphous layer after UV irradiation in air. The UV light-induced structuralchanges of WO3 in air were characterized by IL-STEM. This analysis minimized the effectof electron irradiation during the observation. Slight structural changes induced by UVlight would be difficult to detect using only ex-situ STEM analysis in different locations.Information about the amorphous layer after UV irradiation is provided later in this section.Nanomaterials 2024, 14, x FOR PEER REVIEW 5 of 9   2 plate (Figure 3h), although the contribution of the lattice oxygen of WO3 was strong, C=O and C–OH peaks were also observed. The results from the XPS spectra were used for quantification (Table 2). For the A-1 and A-2 plates, the C/W and O/W ratios were slightly different. For the B-2 plate compared with the B-1 plate, the C/W ratio decreased and the O/W ratio was increased. Thus, the long period of UV irradiation decreased the amount of carbon and increased the amount of oxygen.  Figure 1. (a,b) HAADF-STEM images of WO3 powders: (a) pristine WO3, and (b) after 4 h of UV irradiation. (c) XRD profile the WO3 powder, plates (A-1, A-2, and B-2), and reference pattern of monoclinic WO3. Figure 1. (a,b) HAADF-STEM images of WO3 powders: (a) pristine WO3, and (b) after 4 h of UVirradiation. (c) XRD profile the WO3 powder, plates (A-1, A-2, and B-2), and reference pattern ofmonoclinic WO3.Nanomaterials 2024, 14, 1486 5 of 9Nanomaterials 2024, 14, x FOR PEER REVIEW 6 of 9    Figure 2. (a) High-magnification HAADF-STEM image of pristine WO3 particles. (b) Atomic struc-ture model of monoclinic WO3 along the [001] zone axis. Gray atoms and red atoms represent W and O, respectively. (c–h) Atomic-scale HAADF-STEM images of WO3 particles before and after UV irradiation.  Figure 3. (a) Photographs of the A-1 and B-1 plates. (b) C 1s XPS spectra of the A-1 and A-2 plates. (c) C 1s XPS spectra of the B-1 and B-2 plates. (d) Normalized C 1s XPS spectra of all the plates. Explanation of each color line was same for that of Figure 3e. (e) Normalized O 1s XPS spectra of all Figure 2. (a) High-magnification HAADF-STEM image of pristine WO3 particles. (b) Atomic structuremodel of monoclinic WO3 along the [001] zone axis. Gray atoms and red atoms represent W and O,respectively. (c–h) Atomic-scale HAADF-STEM images of WO3 particles before and after UV irradiation.Nanomaterials 2024, 14, x FOR PEER REVIEW 6 of 9    Figure 2. (a) High-magnification HAADF-STEM image of pristine WO3 particles. (b) Atomic struc-ture model of monoclinic WO3 along the [001] zone axis. Gray atoms and red atoms represent W and O, respectively. (c–h) Atomic-scale HAADF-STEM images of WO3 particles before and after UV irradiation.  Figure 3. (a) Photographs of the A-1 and B-1 plates. (b) C 1s XPS spectra of the A-1 and A-2 plates. (c) C 1s XPS spectra of the B-1 and B-2 plates. (d) Normalized C 1s XPS spectra of all the plates. Explanation of each color line was same for that of Figure 3e. (e) Normalized O 1s XPS spectra of all Figure 3. (a) Photographs of the A-1 and B-1 plates. (b) C 1s XPS spectra of the A-1 and A-2 plates.(c) C 1s XPS spectra of the B-1 and B-2 plates. (d) Normalized C 1s XPS spectra of all the plates.Explanation of each color line was same for that of Figure 3e. (e) Normalized O 1s XPS spectra of allthe plates. (f) Normalized W 4f XPS spectra of all the plates. Explanation of each color line was samefor that of Figure 3e. (g) C 1s spectra of the B-2 plate deconvoluted into three peaks. (h) O 1s spectraof the B-2 plate deconvoluted into three peaks.Nanomaterials 2024, 14, 1486 6 of 9Table 2. Quantification results (atomic percent [at%] and ratios) from XPS analysis.Sample Sample Treatment C (at%) O (at%) W (at%) C/WRatioO/WRatioA-1 Pristine 23.5 55.0 21.5 1.09 2.56B-1 12 h irradiation/dry 22.8 55.8 21.4 1.07 2.61A-2 A-1 + 4 h irradiation/humid 23.9 54.8 21.3 1.12 2.57B-2 B-1 + 4 h irradiation/humid 19.8 58.4 21.8 0.91 2.68The XRD profiles of the plate sample (A-1, A-2, B-2) are shown in Figure 1c. Thediffraction peaks of all the profiles were indexed to monoclinic WO3. A different peakintensity ratio in the plate samples compared with powder would originate from thepreferred orientation of plates or the coexistence of oxygen deficient phase [5]. The XRDprofiles of A-2 and B-2 were similar compared with that of A-1, indicating that the internalcrystal structures were not changed by UV irradiation. XPS analysis of the WO3 plates wasused to study the surface chemical bonding states before and after UV light irradiation.The pristine WO3 plate (A-1) and the plate after 12 h of irradiation (B-1) were both yellow(Figure 3a), which is a typical color for stoichiometric WO3 [5]. The A-2 and B-2 plateswere also yellow. If the non-stoichiometric oxygen-deficient WO3-x phase formed, thecolor would change to blue [5]. The XPS C 1s spectra (Figure 3b–d) showed peaks forC–H (284.5 eV), C–OH (286.0 eV), C=O (287.3 eV), and C=O–OH (288.7 eV) [27]. Afterthe 4 h irradiation (plate A-2), the C–H peak intensity decreased and the C=O–OH peakof plate A-1 shifted toward the C=O peak (Figure 3b). The XPS C 1s spectrum of the B-1plate (Figure 3c) showed peaks for C–H and C=O–OH. In the XPS spectrum of plate B-2(Figure 3c), the C–H peak intensity decreased and a broad peak for C=O and C–OH wasobserved. The XPS C 1s spectra of all samples were compared by normalizing the spectrato the C–H peak (Figure 3d). The C–H and C=O–OH peaks were observed for the A-1 andB-1 plates. The intensity of the C–H and C=O–OH peak was decreased, and a peak at alower binding energy (C=O peak) appeared for A-2 and B-2. In the normalized O 1s spectra(Figure 3e), a peak corresponding to the lattice oxygen of WO3 was observed at 530.0 eV [5].Compared with pristine WO3, the plates with UV irradiation showed slight increases in thepeak intensities for C=O (531.4 eV) and C–OH (532.2 eV) [28,29]. Plate B-2 (long irradiationperiod) had the strongest peaks. In the normalized W 4f spectra (Figure 3f), all the spectraoverlapped. W 4f5/2 and W 4f7/2 spin-orbit doublet peaks corresponding to W6+ wereobserved at 37.4 eV and 35.3 eV, respectively [5]. These results indicated that the W was inthe form of WO3 for all samples. In the peak fitted C 1s spectra of the B-2 plate (Figure 3g),peaks were observed for C–H, C–OH, and C=O. For the peak fitted O 1s spectra of theB-2 plate (Figure 3h), although the contribution of the lattice oxygen of WO3 was strong,C=O and C–OH peaks were also observed. The results from the XPS spectra were used forquantification (Table 2). For the A-1 and A-2 plates, the C/W and O/W ratios were slightlydifferent. For the B-2 plate compared with the B-1 plate, the C/W ratio decreased and theO/W ratio was increased. Thus, the long period of UV irradiation decreased the amount ofcarbon and increased the amount of oxygen.It is known that the contamination of hydrocarbon is widely used for charge correctionin XPS analysis. The most likely source of hydrocarbon is the air atmosphere. A recentstudy investigated the source of contaminations and suggested that the accumulation ofvolatile organic compounds in air form contaminations on the surface of materials [30]. Inthis study, the existence of contaminations would have also been caused by air exposure.The changes observed in the XPS spectra with UV irradiation (Figure 3) were similar tothose observed for a TiO2 film system in a previous study [31]. In this earlier study, UVirradiation in an oxygen atmosphere was performed for both hydrophilic and hydrophobicTiO2 films. The C 1s spectrum of the hydrophilic film showed that large amounts ofhydrocarbon were removed, and the O 1s spectra for both films showed that hydroxidegroups were adsorbed [31]. This suggests that the hydrocarbon decomposition observedNanomaterials 2024, 14, 1486 7 of 9in the present study (Figure 3b–d) was caused by photocatalytic oxidation on the WO3surface. As shown in the XPS spectra of the A-1 and B-1 samples, carboxyl (C=O–OH)species were detected in the first period of decomposition. The formation of carboxylspecies was also reported for the TiO2 photocatalyst system [32]. For the photodegradationof the acetaldehyde system, the amount of CO2 produced by acetaldehyde decompositionwas low because stable intermediates, such as acetic acid and formic acid, accumulated onthe WO3 surface [33]. Thus, the low photocatalytic activity of pristine WO3 would result inthe existence of carboxyl species in the XPS spectra even with the longer UV irradiationperiod (plate B-1). The humidity during UV irradiation is also important. A comparisonof the C 1s spectra of A-1 and A-2 showed that the peak for C=O–OH species shifted to alower binding energy for A-2. Although the C=O–OH peak was present for the B-1 plate,its intensity was low for the B-2 plate and peaks were observed for C=O and C–OH species.These results showed that the photocatalytic decomposition of C=O–OH species occurredunder humid conditions. The total amount of carbon in A-2 was similar to that in A-1,but the amount in B-2 was significantly lower (Table 2); thus, the full decomposition ofhydrocarbons would be slow for pristine WO3. The surface of the particle was coveredwith an amorphous layer after the UV light irradiation (Figure 2). Considering the UVirradiation conditions in the IL-STEM analysis (4 h under dry conditions) and the XPSresults, the amorphous layer could be formed by intermediates containing carboxyl groupsthat are produced by the photocatalytic oxidation of hydrocarbon species. In future, IL-STEM could be combined with electron energy loss spectroscopy to analyze photocatalystsand clarify their site-dependent photocatalytic decomposition behaviors (e.g., defect sitesand noble metal cocatalyst sites).4. ConclusionsUV light-induced surface changes on WO3 were characterized using IL-STEM andXPS analyses. The XPS analysis showed that hydrocarbon decomposition and hydroxylspecies formation occurred under UV irradiation in air. For the hydrocarbon decomposition,chemical bonding states similar to carboxyl species were observed as intermediates, andthen these species decomposed into C=O and C–OH. For the IL-STEM analysis, UV light-induced surface changes of WO3 in air were observed at the atomic scale. HAADF-STEMobservation along the [001] zone axis showed that the particle surface was covered withan amorphous thin film after UV irradiation; however, the internal crystal structure didnot change. Taking the XPS results into consideration, the formation of amorphous layerscan be induced by the photocatalytic oxidation of hydrocarbon species by UV light. IL-STEM can detect slight morphological changes caused by light-induced reactions. Thismethod will be useful for the atomic-scale analysis of photocatalysis or photo-inducedhydrophilic conversion.Author Contributions: Conceptualization, Y.N. and M.T.; methodology, Y.N. and M.T.; investigation,Y.N., Y.S. and M.T.; writing—original draft preparation, Y.N.; writing—review and editing, T.S. andM.T.; funding acquisition, Y.N. and M.T. All authors have read and agreed to the published versionof the manuscript.Funding: This research received no external funding.Data Availability Statement: Data will be made available upon request.Acknowledgments: A part of this work was supported by “NIMS Joint Research Hub Program”of National Institute for Materials Science, and “Advanced Research Infrastructure for Materialsand Nanotechnology Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Grant Number JPMXP1224HK0013 (Hokkaido University). We thank to KeitaSuzuki and Hiroki Mizuochi for their help with XPS experiments. We thank Gabrielle David fromEdanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.Conflicts of Interest: The authors declare no conflicts of interest.https://jp.edanz.com/acNanomaterials 2024, 14, 1486 8 of 9References1. Kumar, S.G.; Koteswara, K.S.R. Comparison of modification strategies towards enhanced charge carrier separation and photocatalyticdegradation activity of metal oxide semiconductors (TiO2, WO3 and ZnO). Appl. Surf. Sci. 2017, 391, 124–148. [CrossRef]2. Sun, Y.F.; Liu, S.B.; Meng, F.L.; Liu, J.Y.; Jin, Z.; Kong, L.T.; Liu, J.H. Metal Oxide Nanostructures and Their Gas Sensing Properties:A Review. Sensors 2012, 12, 2610–2631. [CrossRef] [PubMed]3. Murakami, S.; Zhang, L.; Jeem, M.; Okamoto, K.; Nakagawa, Y.; Shibayama, T.; Ohnuma, M.; Watanabe, S. Photo- & radio-chromiciron-doped tungstic acids fabricated via submerged photosynthesis. Opt. Mater. 2022, 124, 111966. [CrossRef]4. Khan, S.; Je, M.; Kim, D.; Lee, S.; Cho, S.H.; Song, T.; Choi, H. 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MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.1016/j.carbon.2008.09.045https://doi.org/10.3390/ma15093270https://doi.org/10.1016/j.apsusc.2024.159319https://doi.org/10.1021/la901505mhttps://doi.org/10.1016/S1389-5567(00)00002-2https://doi.org/10.1021/jp0725533 Introduction  Materials and Methods  Identical-Location-STEM (IL-STEM) Analysis  XPS Analysis  Results and Discussion  Conclusions  References