Preparation of Orthorhombic WO 3 Thin Films and Their Crystal Quality-Dependent Dye Photodegradation Ability

Direct current (DC) magnetron sputtering deposited WO3 films with different crystalline qualities were synthesized by postannealing at various temperatures. The in-situ DC sputtering deposited WO3 thin film at 375 ◦C exhibited an amorphous structure. The as-grown WO3 films were crystallized after annealing at temperatures of 400–600 ◦C in ambient air. Structural analyses revealed that the crystalline WO3 films have an orthorhombic structure. Moreover, the crystallite size of the WO3 film exhibited an explosive coarsening behavior at an annealing temperature above 600 ◦C. The density of oxygen vacancy of the WO3 films was substantially lowered through a high temperature annealing procedure. The optical bandgap values of the WO3 films are highly associated with the degree of crystalline quality. The annealing-induced variation of microstructures, crystallinity, and bandgap of the amorphous WO3 thin films explained the various photoactivated properties of the films in this study.


Introduction
Tungsten oxide (WO 3 ), as a wide bandgap semiconductor, has been intensively investigated for various uses in scientific devices [1,2].Among various applications, the photocatalyst application for degrading organic pollutants receives much attention as WO 3 has the advantages of low cost, high chemical stability, and excellent process-dependent reproducibility.In general, pure WO 3 crystal shows five phase transitions at temperatures ranging from −180 to 900 • C [3].Among the five crystal forms, a monoclinic I WO 3 phase is the stable phase at room temperature.However, the orthorhombic WO 3 phase exists only in some WO 3 nanostructures at room temperature, not frequently visible for other morphologies such as a thin-film structure.This is attributable to the fact that the transition temperature of the orthorhombic WO 3 phase for WO 3 nanostructures is generally at room temperature, which is quite lower than that of bulk WO 3 .
WO 3 in a thin-film structure is highly desirable for various device applications because thin solid film can be integrated into various small devices or combined with other materials to form composites for scientific applications.Several methods of manufacturing WO 3 thin films with various microstructures for applications have been reported.For example, the pulsed-laser deposited WO 3 thin films are integrated with TiO 2 thin films to form multilayer films and used for photodegradation of methylene blue (MB) solution.The WO 3 layer in the multilayer structure enhances the photocatalytic ability of the TiO 2 layer [4].The spray pyrolysis synthesized WO 3 thin films have also been used for photodegrading methyl orange (MO) [5].The thermal evaporation deposited WO 3 thin films with

Methods
In this study, the WO 3 thin films were grown on 300 nm-thick SiO 2 /Si and glass substrates by reactive DC magnetron sputtering at 375 • C. The metallic tungsten (purity > 99.99%) target was employed.The sputtering power of tungsten was kept at 40 W and the working pressure was maintained at 1.33 Pa during sputtering.The ratio of Ar/O 2 was fixed at 6:1.The thin film thickness was controlled to be 120 nm.The as-grown WO 3 films were subsequently annealed at 400-600 • C in ambient air for 1 h.
Scanning electron microscopy (SEM; S-4800, Hitachi, Tokyo, Japan) was used to investigate surface morphology of the WO 3 thin films.The surface roughness of various thin-film samples was measured by atomic focus microscopy (AFM; D5000, Veeco Karlsruhe, Germany).Crystal structures of the films were investigated by X-ray diffraction (XRD; D2 Phaser, Bruker, Karlsruhe, Germany).The detailed microstructures of the films were studied by high-resolution transmission electron microscopy (HRTEM; JEM-2100F, JEOL Tokyo, Japan).An X-ray photoelectron spectroscope (XPS; PHI 5000 VersaProbe, ULVAC-PHI, Chigasaki, Japan) was used to understand the chemical binding states of the thin films' elements.The transmittance spectra of the thin films were measured using a UV-Vis spectrophotometer (V750, Jasco, Tokyo, Japan).Comparison of photocatalytic activity of various thin Coatings 2019, 9, 90 3 of 11 films samples was performed using 10 mL methylene blue (MB; 10 −6 M) solution containing various WO 3 thin films under various irradiation conditions.The change of MB solution concentration after photodegradation tests was analyzed by measuring the intensity variation of absorbance spectra using an UV-Vis spectrophotometer.

Result and Discussions
The change in crystal structure features of the WO 3 thin films with various thermal annealing procedures is depicted in Figure 1.The as-grown WO 3 film shows an amorphous structure, as no visible Bragg reflections are found.Figure 1 shows the XRD patterns of the WO 3 films with thermal annealing procedures at 400-600 • C. The orthorhombic crystalline WO 3 phase is formed with distinguishable Bragg reflections (JCDPS. .The more intense Bragg reflections associated with a narrower full-width at half maximum were observed for the thin films annealed at a higher annealing temperature, revealing a higher degree of crystalline quality of the film.Notably, no other peaks of impurities were observed after thermal annealing procedures.The XRD results reveal that the as-grown WO 3 thin films exhibited a polycrystalline feature after postannealing.The crystallite sizes of the annealed WO 3 films were evaluated using the Scherrer formula [12].The crystallite sizes of the WO 3 films annealed at 400, 500, and 600 • C were approximately 24, 32, and 57 nm, respectively.The (001)-oriented crystal dominated the crystal structure feature of the crystalline WO 3 thin films annealed below 500 • C; moreover, the (111)-oriented crystal dominated the crystal structure feature when the film was annealed at 600 • C. Notably, the change of the WO 3 crystal orientation from (001) to (111) at the higher temperature annealing is associated with the surface binding energy among the low index crystallographic planes [13].
Coatings 2019, 9, x FOR PEER REVIEW 3 of 11 containing various WO3 thin films under various irradiation conditions.The change of MB solution concentration after photodegradation tests was analyzed by measuring the intensity variation of absorbance spectra using an UV-Vis spectrophotometer.

Result and Discussions
The change in crystal structure features of the WO3 thin films with various thermal annealing procedures is depicted in Figure 1.The as-grown WO3 film shows an amorphous structure, as no visible Bragg reflections are found.Figure 1 shows the XRD patterns of the WO3 films with thermal annealing procedures at 400-600 °C.The orthorhombic crystalline WO3 phase is formed with distinguishable Bragg reflections (JCDPS. .The more intense Bragg reflections associated with a narrower full-width at half maximum were observed for the thin films annealed at a higher annealing temperature, revealing a higher degree of crystalline quality of the film.Notably, no other peaks of impurities were observed after thermal annealing procedures.The XRD results reveal that the as-grown WO3 thin films exhibited a polycrystalline feature after postannealing.The crystallite sizes of the annealed WO3 films were evaluated using the Scherrer formula [12].The crystallite sizes of the WO3 films annealed at 400, 500, and 600 °C were approximately 24, 32, and 57 nm, respectively.The (001)-oriented crystal dominated the crystal structure feature of the crystalline WO3 thin films annealed below 500 °C; moreover, the (111)-oriented crystal dominated the crystal structure feature when the film was annealed at 600 °C.Notably, the change of the WO3 crystal orientation from (001) to (111) at the higher temperature annealing is associated with the surface binding energy among the low index crystallographic planes [13].Surface morphologies of the WO3 thin films with and without thermal annealing are shown in Figure 2. No distinctly well surface grain features can be seen for the as-grown WO3 (Figure 2a).This might be associated with the amorphous nature of the sample as characterized by the XRD measurement.When the film was annealed at 400 °C, a surface grain feature was visible and the surface grains had an average size of approximately 32 nm (Figure 2b).Further increasing the annealing temperature to 500 °C increased the size of surface grains, and the homogeneity of grain size improved simultaneously.The average surface grain size was approximately 51 nm evaluated from Figure 2c.Notably, the surface grain size was abnormally increased (average grain size of 102 nm) and an uniformly cylindrical crystal feature was obtained when the film was annealed at 600 °C (Figure 2d).The high annealing temperature provides sufficient energy, which might facilitate the coalescence of the adjacent tiny crystals, and therefore large surface grains were formed.
Furthermore, the surface roughness of the various WO3 thin films was further characterized by AFM. Figure 3a exhibits the surface of the as-grown WO3 thin film.The root mean square (RMS) roughness of the as-grown amorphous WO3 thin film was evaluated to be approximately 3.55 nm.Comparatively, the WO3 thin films annealed at 400-600 °C exhibited coarser surface morphology (Figure 3b-d).The RMS roughness values of the WO3 thin films were of approximately 4.02, 4.75, and 9.28 nm corresponding to the annealing temperature of 400, 500, and 600 °C, respectively.This result demonstrated that the surface roughness monotonically increases with increasing annealing Surface morphologies of the WO 3 thin films with and without thermal annealing are shown in Figure 2. No distinctly well surface grain features can be seen for the as-grown WO 3 (Figure 2a).This might be associated with the amorphous nature of the sample as characterized by the XRD measurement.When the film was annealed at 400 • C, a surface grain feature was visible and the surface grains had an average size of approximately 32 nm (Figure 2b).Further increasing the annealing temperature to 500 • C increased the size of surface grains, and the homogeneity of grain size improved simultaneously.The average surface grain size was approximately 51 nm evaluated from Figure 2c.Notably, the surface grain size was abnormally increased (average grain size of 102 nm) and an uniformly cylindrical crystal feature was obtained when the film was annealed at 600 • C (Figure 2d).The high annealing temperature provides sufficient energy, which might facilitate the coalescence of the adjacent tiny crystals, and therefore large surface grains were formed.
temperature because high annealing temperature facilitates the coalescence of the surface grains and therefore rougher surface.The average surface grain sizes of the WO3 thin films annealed at 400, 500, and 600 °C were approximately 26, 43, and 84 nm, respectively.Larger surface grains of the annealed film engendered a rougher surface feature.Similarly, a substantially increased surface grain size, as reported in the CuO film, annealed at the temperature higher than 700 °C [14].The detailed microstructures of the WO3 thin films with and without thermal annealing at 600 °C were investigated by TEM.A low-magnification, cross-sectional TEM image of the as-grown WO3 thin film is shown in Figure 4a.The thickness of the WO3 film was ~120 nm.The film surface is dense and smooth, and no voids can be seen.A high-resolution TEM (HRTEM) micrograph of the as-grown WO3 thin film is depicted in Figure 4b.The random and chaotic lattice fringes with a short-range order are distributed over the area of interest, revealing that the film is in the amorphous phase.Moreover, the selected area electron diffraction (SAED) pattern in Figure 4c exhibits a faint ring-like pattern, revealing that the film without heat treatment is uncrystallized.This is in agreement with the XRD result.Figure 4d depicts the energy-dispersive X-ray spectroscopy (EDS) spectra of the film, confirming that the film's composition consisted of W and O.Moreover, the O/W composition ratio is approximately 2.48, demonstrating oxygen deficiency in the WO3 thin film.This is often observed Furthermore, the surface roughness of the various WO 3 thin films was further characterized by AFM. Figure 3a exhibits the surface of the as-grown WO 3 thin film.The root mean square (RMS) roughness of the as-grown amorphous WO 3 thin film was evaluated to be approximately 3.55 nm.Comparatively, the WO 3 thin films annealed at 400-600 • C exhibited coarser surface morphology (Figure 3b-d).The RMS roughness values of the WO 3 thin films were of approximately 4.02, 4.75, and 9.28 nm corresponding to the annealing temperature of 400, 500, and 600 • C, respectively.This result demonstrated that the surface roughness monotonically increases with increasing annealing temperature because high annealing temperature facilitates the coalescence of the surface grains and therefore rougher surface.The average surface grain sizes of the WO 3 thin films annealed at 400, 500, and 600 • C were approximately 26, 43, and 84 nm, respectively.Larger surface grains of the annealed film engendered a rougher surface feature.Similarly, a substantially increased surface grain size, as reported in the CuO film, annealed at the temperature higher than 700 • C [14].temperature because high annealing temperature facilitates the coalescence of the surface grains and therefore rougher surface.The average surface grain sizes of the WO3 thin films annealed at 400, 500, and 600 °C were approximately 26, 43, and 84 nm, respectively.Larger surface grains of the annealed film engendered a rougher surface feature.Similarly, a substantially increased surface grain size, as reported in the CuO film, annealed at the temperature higher than 700 °C [14].The detailed microstructures of the WO3 thin films with and without thermal annealing at 600 °C were investigated by TEM.A low-magnification, cross-sectional TEM image of the as-grown WO3 thin film is shown in Figure 4a.The thickness of the WO3 film was ~120 nm.The film surface is dense and smooth, and no voids can be seen.A high-resolution TEM (HRTEM) micrograph of the as-grown WO3 thin film is depicted in Figure 4b.The random and chaotic lattice fringes with a short-range order are distributed over the area of interest, revealing that the film is in the amorphous phase.Moreover, the selected area electron diffraction (SAED) pattern in Figure 4c exhibits a faint ring-like pattern, revealing that the film without heat treatment is uncrystallized.This is in agreement with The detailed microstructures of the WO 3 thin films with and without thermal annealing at 600 • C were investigated by TEM.A low-magnification, cross-sectional TEM image of the as-grown WO 3 thin film is shown in Figure 4a.The thickness of the WO 3 film was ~120 nm.The film surface is dense and smooth, and no voids can be seen.A high-resolution TEM (HRTEM) micrograph of the as-grown WO 3 thin film is depicted in Figure 4b.The random and chaotic lattice fringes with a short-range order are distributed over the area of interest, revealing that the film is in the amorphous phase.Moreover, the selected area electron diffraction (SAED) pattern in Figure 4c exhibits a faint ring-like pattern, revealing that the film without heat treatment is uncrystallized.This is in agreement with the XRD result.Figure 4d depicts the energy-dispersive X-ray spectroscopy (EDS) spectra of the film, confirming that the film's composition consisted of W and O.Moreover, the O/W composition ratio is approximately 2.48, demonstrating oxygen deficiency in the WO 3 thin film.This is often observed in oxide thin films prepared by sputtering because the thin film growth condition is in an oxygen deficient environment during sputtering [15].Figure 5a depicts a low-magnification, cross-sectional image of the WO3 film annealed at 600 °C.The film thickness of the annealed WO3 film is homogeneous throughout its cross section.Compared to the as-grown film, the surface and root of the high-temperature-annealed film are more undulated.Figure 5b,c demonstrate HRTEM images of the annealed WO3 thin film.The appearance of visible and ordered lattice fringes in the HREM images indicate that the WO3 film after annealing had a high degree of crystallinity.The atomic lattice fringes with intervals of approximately 0.39, 0.31, and 0.27 nm could be identified and were attributed to the interplanar distances of the WO3 (001), (111), and (021) crystallographic planes, respectively.The boundaries between the adjacent grains were visible.The polycrystalline nature and the orthorhombic structure of the WO3 film were also confirmed by the SAED measurements in Figure 5d.Distinct diffraction spots arranged in centric rings revealed the crystalline WO3 thin film was formed after the 600 °C annealing process.Figure 5e shows the EDS spectra; the spectra revealed that the film mainly composed of W and O.No other impurity atoms were detected.Figure 5a depicts a low-magnification, cross-sectional image of the WO 3 film annealed at 600 C. The film thickness of the annealed WO 3 film is homogeneous throughout its cross section.Compared to the as-grown film, the surface and root of the high-temperature-annealed film are more undulated.Figure 5b,c demonstrate HRTEM images of the annealed WO 3 thin film.The appearance of visible and ordered lattice fringes in the HREM images indicate that the WO 3 film after annealing had a high degree of crystallinity.The atomic lattice fringes with intervals of approximately 0.39, 0.31, and 0.27 nm could be identified and were attributed to the interplanar distances of the WO 3 (001), (111), and (021) crystallographic planes, respectively.The boundaries between the adjacent grains were visible.The polycrystalline nature and the orthorhombic structure of the WO 3 film were also confirmed by the SAED measurements in Figure 5d.Distinct diffraction spots arranged in centric rings revealed the crystalline WO 3 thin film was formed after the 600 • C annealing process.Figure 5e shows the EDS spectra; the spectra revealed that the film mainly composed of W and O.No other impurity atoms were detected.
(021) crystallographic planes, respectively.The boundaries between the adjacent grains were visible.The polycrystalline nature and the orthorhombic structure of the WO3 film were also confirmed by the SAED measurements in Figure 5d.Distinct diffraction spots arranged in centric rings revealed the crystalline WO3 thin film was formed after the 600 °C annealing process.Figure 5e shows the EDS spectra; the spectra revealed that the film mainly composed of W and O.No other impurity atoms were detected.XPS analysis was performed to reveal the elemental binding states of various WO 3 thin films.The annealing temperature-dependent W oxidation state change is shown in Figure 6a-d.From the figures, the intense doublet with binding energies of approximately 35.0 eV (W4f 7/2 ) and 37.2 eV (W4f 5/2 ) are associated with photoelectrons emitted from W 6+ ions of the WO 3 films, while the relatively small peaks at 34.0 and 36.2 eV can be assigned to W4f 7/2 and W4f 5/2 of W 5+ oxidation state in tungsten oxides [16].The presence of W 5+ suggests the existence of crystal defects in the WO 3 film.Comparatively, the area and the height of core level W 5+ decreased after annealing, which implied increased oxidation states of W in the WO 3 film.Notably, the WO 3 film annealed at 600 • C had the smallest features of W 5+ , which indicates the surface tungsten in this film exhibited a larger degree of oxidation state after annealing.No peaks attributed to metallic W were identified in the spectra of all films.Notably, the W/O atomic ratio of the as-grown WO 3 film was approximately 0.4.Moreover, the W/O atomic ratio of the WO 3 films decreased from 0.37 to 0.34 with the annealing temperature increasing from 400 to 600 • C, respectively, evaluated from the XPS analyses.Figure 6e-h show that the XPS spectra of O1s for various WO 3 thin films have an asymmetric curve feature.The O1s spectra of the surface of various WO 3 thin films were fitted by two distributions, centered at approximately 529.3 and 530.8 eV, respectively.The relatively low binding energy peak is attributed to O 2− ions in the oxide lattice.The higher binding energy peak is attributed to the oxygen vacancies in the WO 3 [17].The relative content of the oxygen vacancy for various WO 3 films was evaluated according to the area ratio of these two deconvolution components: (red peak)/(red peak + blue peak).The relative area of the higher energy binding component for the WO 3 films decreased with the annealing temperature.A great amount of vacancy existed in the surface of the as-grown WO 3 film.After annealing, the surface oxygen vacancy content markedly decreased from 34.4% to 25.3% with the annealing temperature increasing from 400 to 600 • C, respectively.The transmittance spectra of the WO 3 thin films with and without annealing are demonstrated in Figure 7a.The light was highly absorbed in the visible region with less than 40% transmittance for the as-grown WO 3 film, attributed to the presence of massive oxygen-related defects.Moreover, the as-grown WO 3 film is seen in semi-transparent bluish color, which shows the amorphous and highly non-stoichiometric natures of the film [18].The highly transparent feature was observed for the WO 3 films conducted with annealing; moreover, no blue colouration appeared in the samples.The enhancement in the transmittance degree of the annealed WO 3 films is due to the reduction of oxygen-related crystal defects, which might play an important role in scattering the incident light.Notably, a clear shoulder feature appeared at approximately 350 nm for the films annealed below 600 • C. That shoulder feature in the optical transmittance spectra is associated with the residual crystal defects associated with oxygen deficiencies in the samples [19].Notably, the shoulder feature completely vanished for the WO 3 film conducted with thermal annealing at 600 • C, indicating that the oxygen-related crystal defects of the film were substantially removed in the annealing process.The bandgap value of various thin-film samples is calculated by plotting (αhv) 1/2 vs. photon energy using the following formula: where α is the absorption coefficient, A is a constant, hv is the energy of an incident photon, E g is the bandgap value.On extrapolating the linear portion of the curves (Figure 7b), the intercept on the energy axis (αhv) 1/2 = 0 gives the value of the indirect bandgap energy.The bandgap values were calculated as 2.48, 3.04, 3.04, and 2.78 eV for as-grown and annealed WO 3 samples at 400, 500, and 600 • C, respectively.The as-prepared WO 3 thin film exhibited the smallest bandgap value and may be associated with a relatively large content of oxygen deficiencies in the film as compared to the annealed ones.Due to the existence of a high density of oxygen-deficient crystal defects, they might form new discrete energy bands below the conduction band, resulting in the relatively low band gap.After annealing at 400 and 500 • C, the E g of two WO 3 thin-film samples originates from the recombination of free carriers from the bottom of the conduction band energy to the valence band energy with the decreased discrete energy bands after thermal annealing.It was noticed that the bandgap value of the WO 3 film further decreased at the highest annealing temperature of 600 • C, assigned to explosive growth in grain size of the WO 3 film [20].7e.A slightly decrease of C/C 0 value was observed for the dark balance of 120 min; this is attributed to the fact that partial MB molecules were absorbed on the surface of the WO 3 thin films under dark balance condition.By contrast, the photodegradation rates of the MB solution with various WO 3 thin films are different.As demonstrated in Figure 8e, the WO 3 film conducted with thermal annealing at 600 • C was most catalytically efficient, giving a photodegradation extent of approximately 45% in 30 min, while other annealed thin films photodegraded only 35% of MB in the same time period.After 120 min irradiation, the WO 3 film annealed at 600 • C still displayed the largest degree of photodegradation toward the MB solution.Notably, under irradiation, WO 3 was photoexcited and the e − /h + pairs were formed.The e − can participate in organic pollutant degradation reactions.The possible formations of the superoxide anion, hydroperoxyl, and hydrogen peroxide (H 2 O 2 ) species in the organic dye solution are advantageous for further degrading the MB dyes [21].However, it has been shown that the position of the conduction band of WO 3 (+0.50V vs. NHE) was below the standard redox potential for the formation of superoxide anion (−0.33 V Coatings 2019, 9, 90 9 of 11 vs. NHE) and hydroperoxyl (−0.046V vs. NHE) [22].Based on the aforementioned, the following reactions are therefore more likely to occur during the MB photodegradation process using WO 3 thin films as photocatalysts:

Conclusions
The WO3 thin films were DC sputtering deposited at 375 °C; moreover, the as-grown films exhibited an amorphous structure because of large composition deviation from the stoichiometric value.The as-grown WO3 thin films were further conducted by thermal annealing procedures at 400-600 °C in ambient air.Structural analyses revealed that the amorphous WO3 thin films crystallized after thermal annealing and demonstrated an orthorhombic structure.The surface grain size and surface roughness of the WO3 films increased with annealing temperature.Moreover, the density of oxygen-deficiency-related crystal defects in the WO3 films decreased with annealing temperature.The optical bandgap of WO3 thin films are highly associated with the crystal quality and this can be controlled by conducting with different thermal annealing procedures.The as-grown WO3 thin film annealed at 600 °C exhibited the highest photodegradation ability toward organic dyes in this study because of its high crystallinity, low crystal defects, and low optical bandgap among the various WO3 thin films herein.
The produced •OH radicals are efficiently degrading species for the MB dyes.Although the band position of the WO 3 is advantageous for the photodegradation mechanism, the microstructure and optical properties should be considered for the final photodegradation efficiency.The WO 3 film with thermal annealing performed at 600 • C was the most active photocatalyst, and the films annealed at lower temperatures were somewhat less active in this work.In contrast, the as-grown WO 3 film had a lower photocatalytic activity.We assume that the deteriorated crystal quality of the WO 3 with the lower annealing temperatures or without annealing was the decisive factor in their inferior photodegradation activity.Fewer oxygen-deficient-related defects in the lattice of the WO 3 film annealed at 600 • C might result in fewer recombination centers in the film, which would be detrimental for the higher photodegradation efficiency in this study.Moreover, the relative lower optical bandgap value of the WO 3 film conducted with thermal annealing at 600 • C among different annealed thin films is another advantageous factor to increase the light harvesting and enhance the degradation ability of crystalline WO 3 film toward MB dyes under irradiation.

Conclusions
The WO 3 thin films were DC sputtering deposited at 375 • C; moreover, the as-grown films exhibited an amorphous structure because of large composition deviation from the stoichiometric value.The as-grown WO 3 thin films were further conducted by thermal annealing procedures at 400-600 • C in ambient air.Structural analyses revealed that the amorphous WO 3 thin films crystallized after thermal annealing and demonstrated an orthorhombic structure.The surface grain size and surface roughness of the WO 3 films increased with annealing temperature.Moreover, the density of oxygen-deficiency-related crystal defects in the WO 3 films decreased with annealing temperature.The optical bandgap of WO 3 thin films are highly associated with the crystal quality and this can be controlled by conducting with different thermal annealing procedures.The as-grown WO 3 thin film annealed at 600 • C exhibited the highest photodegradation ability toward organic dyes in this study because of its high crystallinity, low crystal defects, and low optical bandgap among the various WO 3 thin films herein.

Figure 1 .
Figure 1.XRD patterns of the WO3 films with and without thermal annealing procedures.

Figure 1 .
Figure 1.XRD patterns of the WO 3 films with and without thermal annealing procedures.

Coatings 2019, 9 ,
x FOR PEER REVIEW 5 of 11 in oxide thin films prepared by sputtering because the thin film growth condition is in an oxygen deficient environment during sputtering[15].

Figure
Figure 8a-d show the absorbance spectra of the MB solution in the presence of various WO3 thin films at different irradiation times.After the MB solution was illuminated, the absorbance peak

Figure
Figure 8a-d show the absorbance spectra of the MB solution in the presence of various WO3 thin films at different irradiation times.After the MB solution was illuminated, the absorbance peak

Figure
Figure8a-dshow the absorbance spectra of the MB solution in the presence of various WO 3 thin films at different irradiation times.After the MB solution was illuminated, the absorbance peak intensity at ~663 nm was observed to gradually decrease with duration, implying that MB molecules are photodegraded.The photodegradation degrees (C/C 0 ) of the MB solution containing various WO 3 thin films are summarized in Figure7e.The C 0 is concentration MB solution without irradiation and C is the residual concentration of the MB solution after irradiation at a given duration.Notably, the C/C 0 values of the MB solution containing various WO 3 thin films under various dark conditions are demonstrated in Figure7e.A slightly decrease of C/C 0 value was observed for the dark balance of 120 min; this is attributed to the fact that partial MB molecules were absorbed on the surface of the WO 3 thin films under dark balance condition.By contrast, the photodegradation rates of the MB solution with various WO 3 thin films are different.As demonstrated in Figure8e, the WO 3 film conducted with thermal annealing at 600 • C was most catalytically efficient, giving a photodegradation extent of approximately 45% in 30 min, while other annealed thin films photodegraded only 35% of MB in the same time period.After 120 min irradiation, the WO 3 film annealed at 600 • C still displayed the largest degree of photodegradation toward the MB solution.Notably, under irradiation, WO 3 was photoexcited and the e − /h + pairs were formed.The e − can participate in organic pollutant degradation reactions.The possible formations of the superoxide anion, hydroperoxyl, and hydrogen peroxide (H 2 O 2 ) species in the organic dye solution are advantageous for further degrading the MB dyes[21].However, it has been shown that the position of the conduction band of WO 3 (+0.50V vs. NHE) was below the standard redox potential for the formation of superoxide anion (−0.33 V

Figure 8 .
Figure 8.The intensity variation of absorbance spectra of the MB solution in presence of various WO 3 thin films under different irradiation durations: (a) as-grown, (b) 400 • C, (c) 500 • C, (d) 600 • C, (e) Plot of C/C 0 vs. irradiation time.Meanwhile, the photogenerated h + in the WO 3 might involve the reactions and form •OH radicals through the following equations:H 2 O + h + → H + + •OH(5)OH − + h + → •OH(6)