Vanadium-Doped WO 3 /TiO 2 Microporous Film as Visible-Light Photocatalyst

Abstract

Vanadium (V)-doped WO₃/TiO₂ microporous film on a titanium (Ti) plate was prepared by using the micro-arc oxidation process. Incorporating V and tungsten (W) species into the TiO₂ lattice in one step yields band gap narrowing and strong visible-light (Vis) photoactivity, dominated mostly by incorporation of WO₃ and V₂O₅ oxides, and possibly W_xTi_1−xO₂. Photoactivity under ultraviolet light (UV) irradiation was likely attributable to its WO₃-loaded microporous structure, which favored cationic molecule adsorption on its active sites. Although post annealing enhanced film crystallinity, photocatalytic ability was decreased largely because of changes in doping levels. This suggests that the as-oxidized microporous film is feasible for photocatalytic cationic dye degradation under UV and Vis irradiation.

Introduction

Titanium dioxide (TiO₂) catalysts, known for their environmental friendliness and chemical stability, are among the most suitable semiconductors for various applications such as degrading a wide range of contaminants in air or water (Fujishima et al., 2000). However, because of its wide band gap, the widespread technological use of TiO₂ (3.0–3.2 eV) is hindered by its low utilization rate of solar energy abundant in the visible-light region. In recent years, to extend TiO₂ photocatalytic activities into the visible-light region, various approaches have been employed by coupling semiconductors with small band gaps, such as CdS, CdSe, and CuO (Chi et al., 2010; Lee et al., 2010; Hassan et al., 2013), and by doping the semiconductors with anionic and/or cationic impurities, that is, nitrogen (N), carbon (C), or molybdenum (Asahi et al., 2001; Khan et al., 2002; Yu and Yu, 2009; Cheng et al., 2013). Among many of the cationic dopants used in TiO₂ catalysts, vanadium (V) and tungsten (W) have been the most widely used elements for decreasing band gap energy and for improving photocatalytic activities (Kubacka et al., 2009; Zhou et al., 2010). With a strong red shift threshold wavelength of 426 nm to 650 nm, the V-doped TiO₂ catalyst has shown promising photocatalytic properties (Gu et al., 2008; Kubacka et al., 2009; Bayati et al., 2010b; Zhou et al., 2010). Alternatively, W-loading at the anatase cation position leads to a band gap energy decrease and the enhancement of photoactivity on sunlight excitation (Kubacka et al., 2009; Yu et al., 2010; Li et al., 2012; Yeh et al., 2012).

It has been observed that a heterostructured catalyst can provide a potential driving force for the separation of photogenerated charge carriers, provided that the band edges of the constituent catalysts are properly positioned. Among the various heterostructured oxides, TiO₂/WO₃ has shown promising photocatalytic applications, mostly because of increased surface acidity (He et al., 2009; Yu et al., 2010) and the proper coupling of the WO_x species with TiO₂ for enhancing charge-separation efficiency (Li et al., 2001). For instance, the conduction band (CB) edge of TiO₂ (−0.19 eV vs. NHE) is more negative than that of WO_x (0.3 eV) and the valance band (VB) edge of TiO₂ (3.01 eV) is more positive than that of WO_x (3.5 eV). Under UV illumination, the photogenerated electrons and holes will transfer to the CB of WO_x and to the VB of TiO₂, respectively (Higashimoto et al., 2006; Chen et al., 2008). Similarly, the heterostructured V₂O₅/TiO₂ favors the photogenerated electron transfer to the CB of V₂O₅, due to a proper CB edge of 0.48 eV of V₂O₅ oxide (Dong et al., 2012). As a result, the recombination of the photogenerated carriers in the heterostructured catalyst is suppressed, potentially resulting in higher photocatalytic ability. Additionally, a highly specific surface area reportedly favors photocatalytic activities of the porous anodized TiO₂ oxides (Liu et al., 2009).

Micro-arc oxidation (MAO), also known as plasma electrolytic oxidation, is regarded as a fast electrochemical process that has recently been used for preparing porous TiO₂ layers coupled with vanadia or tungsten oxide (Bayati et al., 2010a, 2010c; Yeh et al., 2012). One of the potential advantages of the MAO process is the possibility of incorporating anionic and/or cationic ions into the TiO₂ layer on a Ti substrate, by controlling the composition and concentration of the electrolyte (Li et al., 2009). The main mechanism in the MAO process involves dielectric breakdown, which causes spark discharge, micro arcing, and anodic oxidation in potentiostatic condition. Under this condition, small luminescent sparks are observed to move rapidly across the surface of the oxide film, facilitating its continued growth. Therefore, porous oxide films continuously form on the anode surface (Yerokhin et al., 1999; Khan et al., 2008; Li et al., 2009). In general, the current density is normally in the region of 0.5–1.0 A/cm² during the MAO process (Yerokhin et al., 1999).

Addition of fluoride into the electrolyte has reportedly increased the electrolyte conductivity, leading to change of the MAO microstructure and properties (Mu and Han, 2008; He et al., 2009). The content of W doped into TiO₂ lattices is affected by the fluorine ion (F⁻) concentration in the electrolyte; the higher content of W generally provides better photocatalytic activity. Note that F-doping does not cause any change in the optical absorption of TiO₂ (Minero et al., 2000; Li et al., 2005) and little change in the surface morphology of MAO films (He et al., 2009). In this study, NaF was added into the electrolyte solution to enhance the MAO properties and hence the photocatalytic activities as well.

We previously determined that, by applying the MAO process at a low applied voltage of 300 V, the loaded WO₃ favored the formation of the well-crystallized anatase with a trace of rutile TiO₂ phase (Yeh et al., 2012). The aim of this study was to elucidate the structural and photocatalytic characteristics of the V-WO₃/TiO₂ layers prepared at a high applied voltage of 380 V. The effects of annealing are also discussed.

Experimental Protocol

Microporous V-WO₃/TiO₂ films were synthesized on Ti sheet substrates (25 mm×75 mm×1 mm) by using an in-house MAO system (MIRDC) that consisted of a high-voltage DC power supply, anode, and cathode. A stainless steel container with stirring and cooling systems was used as an electrolyte bath. The Ti sheet substrate was connected as anode, whereas the stainless steel container was used as cathode. The electrolyte solution (pH=8) comprised Na₂WO₄ (15 g/L), NaVO₃ (5 g/L), and NaF (2 g/L) solutions. The Ti sheet substrates were oxidized using a bipolar pulse current mode, in which the positive voltage, negative voltage, and duty cycle were 380, −10, and 0.2 V, respectively, for a period of 10 min during the MAO process. The electrolyte temperature was maintained at ∼40°C±2°C by the stirring and cooling systems. The as-oxidized MAO (denoted as V-WTO) samples were rinsed with distilled water and then dried in air at room temperature. Some of the as-oxidized V-WTO samples were then annealed at 873 K for 6 h in atmosphere, and were denoted as V_h-WTO herein. For comparison, N,C-codoped TiO₂ (N,C-TiO₂) films were deposited on ITO glass substrate by using a magnetron sputtering technique for a duration of 7.2 h (Wu and Hung, 2009). The thicknesses of the V-WTO and N,C-TiO₂ films were 1.4 and 1.8 μm, respectively.

The crystal structures of the samples were analyzed using a high-resolution X-ray diffractometer (XRD; Rigaku ATX-E). The microstructures and thicknesses of the films were examined using a scanning electron microscope (SEM; JEOL JSM-6700F). A transmission electron microscope (HRTEM; Philips Tecnai 20) equipped with an energy dispersive X-ray spectrometer (EDS) was employed for composition analysis. X-ray photoelectron spectroscopy (XPS) analysis was performed using a high-resolution XPS spectrometer (ULVAC-PHI Quantera SXM/Auger AES 650). The binding energy of XPS spectra was calibrated using the C1s energy of 284.6 eV. The diffused reflectance of the films was measured using a UV–vis–NIR spectrometer (Hitachi U-4100) equipped with an integration sphere. The band gap energy of each sample was calculated using the formula stated by Bayati et al. (2010c).

The photocatalytic activities of the samples were evaluated by immersing the samples with a size of 25 mm×65 mm into 30 mL of aqueous methylene blue (MB) solution with an initial 10 mg/L (C₀) concentration. Prior to every MB degradation test, the samples were soaked in another beaker of aqueous MB solution for 3 h in darkness for equilibrium surface adsorption. Aqueous MB solutions without samples were also illuminated to generate a blank value and found slightly degraded (<10%) under 2 h of UV radiation (Yeh et al., 2012). Two light sources, ultraviolet (UV) lamps (λ=365 nm) and blue-light-emitting diodes (BLEDs; 420<λ<530 nm), were used to provide irradiated light intensities of 2.7 and 10.5 mW/cm², respectively. The degradation rate constant (k) of MB over film was assumed to be a first-order reaction for the evaluation, where the rate constant is expressed from the linear fitting of k=−ln(C/C₀)/t and the degraded MB concentration C (mg/L) at reaction time t (h). The reaction times were set at 0.5-h intervals from 0.5 to 2 h for the UV tests. The experimental details were described in related reports (Wu and Hung, 2009; Yeh et al., 2012).

Results and Discussion

Microstructures

As shown in Fig. 1, the V-WTO diffraction peaks are ascribed to the TiO₂ anatase (101) phase (2θ=25.3°) located at broad and yet to be identified peaks ranging from 2θ=20°–35°, and two weak Ti peaks at 2θ=38.3° and 2θ=40.1°. Moreover, no other peak associated with another phase is found in the probed range. A slight diffraction angle shift, caused by different ion radii of Ti to V and W, has often been observed in the XRD patterns of the anatase TiO₂ (101) phase as W or V doped on titania (Bayati et al., 2010a, 2010c). As seen in the inset of Fig. 1, a slightly low angle shift of the diffraction (101) plane of 2θ=25.32° of TiO₂ phase was observed in the V-WTO, but not in the V_h-WTO. A diffraction angle shift with a broad peak is possibly because of the WO_x monolayer and V₂O₅, or the amorphous phases highly dispersed on the TiO₂ lattice (Sajjad et al., 2009). This suggests that the as-prepared V-WTO is likely doped with W, V, and/or other ions, which have a significant effect on its optical property. The amorphous phases are commonly found in the MAO samples prepared at a low applied voltage (Bayati et al., 2010c). The annealed V_h-WTO exhibits various well-crystallized TiO₂ anatase phases along with crystalline WO₃, V₂O₅, and a trace of TiO₂ rutile phase. An intense diffraction peak at 2θ=23.67° of the crystalline WO₃ (020) plane is observed as shown in the inset of Fig. 1. The other two diffraction peaks at 2θ=29.1° and 2θ=33.5° are assigned to crystalline WO₃ phases. Moreover, the diffraction peaks at 2θ=21.6° and 2θ=26.6° are the predominant (101) and (110) planes of crystalline V₂O₅, respectively. Only two weak peaks at 2θ=38.42° and 2θ=40.17° (JCPDS No. 44-1294) that possibly derive from the Ti substrate are found in both the V-WTO and V_h-WTO. These findings imply that the V_h-WTO is of the crystallized TiO₂ anatase (101) phase incorporated with noticeable contents of crystalline V₂O₅ and WO₃.

FIG. 1.

X-ray diffraction patterns of V-WTO and V_h-WTO samples along with N,C-TiO₂ film.

A significant difference in morphological characteristics is shown in Fig. 2a and b. With randomly dispersed deep pores accompanied by smaller pores inside, the V-WTO surface appears to have less pore density. Such pores constitute the specific surface area of the film's lesser pore density, which is related to a film grown at a high applied voltage, whereas the pore size increases with increased electrolyte concentration (Bayati et al., 2010c). The typical WO₃/TiO₂ microporous film (Fig. 2b) that possessed some submicron pores and colloidal particles on a large cavity was fabricated in electrolyte, comprising Na₂WO₄ (15 g/L) and NaF (2 g/L), without V ions in the solutions at a low applied voltage of 300 V (Yeh et al., 2012). Thus, a high applied voltage of 380 V and a high concentration of NaVO₃ (5 g/L) added in the electrolyte are mostly ascribed to this unique morphological feature of the V-WTO. As expected, the plain SEM image (Fig. 2c) of the annealed V_h-WTO sample shows no noticeable difference in porous appearance to the as-prepared V-WTO sample (Fig. 2a).

FIG. 2.

Scanning electron microscopy images of (a) as-oxidized V-WTO, (b) typical WO₃/TiO₂ microporous films, and (c) annealed V_h-WTO.

XPS analysis

As conveyed in Fig. 3a, the N,C-TiO₂ shows a slightly asymmetric Ti2p peak, whereas the V-WTO and V_h-WTO display reasonably asymmetric peaks. N,C-TiO₂ was rationally resolved into two peaks at 458.8 and 464.4 eV, corresponding to Ti2p_3/2 and Ti2p_1/2, respectively (figure not shown). V-WTO and V_h-WTO were convoluted into two additional Ti2p peaks at 456.0±0.1 eV and 461.1±0.1 eV, assigned to Ti²⁺of TiO phase (McCafferty and Wightman, 1999).

FIG. 3.

High-resolution X-ray photoelectron spectroscopy spectra of (a) Ti2p, (b) O1s with the best fitting of V_h-WTO in the inset, (c) V2p, and (d) W4f core levels of samples.

The O1s spectrum of the V_h-WTO shown in Fig. 3b was resolved into three peaks, as shown in the inset. The first peak, at 531.6 eV, is ascribed to hydroxyl (OH) groups from adsorbed water on the surface. The second peak, at 530.3 eV, is associated with the oxygen bond (O²⁻) in TiO₂. However, the O²⁻ value (530.0±0.1 eV) recorded for the TiO₂ reference materials shifted by ∼0.3 eV to a higher binding energy of 530.3 eV (McCafferty and Wightman, 1999). This may suggest a moderately different oxide state for the W and/or V ions in the V_h-WTO sample. The third peak, at 532.5 eV, is not directly associated with titania, but reportedly corresponds to O ions at a tungsten content of 22 at.% (Kubacka et al., 2010). Moreover, a shift and tail of the O1s spectrum toward the high binding energy is clearly observed in the V_h-WTO, as compared with the N,C-TiO₂ sample. Thus, the incorporation of W and/or V ions in the TiO₂ lattice is related to a binding energy shift at the O1s core level.

The major peak at 517.0 eV is assigned to V⁵⁺, which confirms the formation of V₂O₅, as shown in Fig. 3c. The binding energy at 517.0 eV is slightly lower than the bulk V₂O₅ value and the composite V₂O₅/TiO₂, but is analogous to the reported value for V₂O₅/TiO₂-SnO₂. The weakly minor peak, at 514.2 eV, may correspond to V²⁺ (Zhou et al., 2010). This indicates that the V species, mostly in the form of V₂O₅, exist in the lattice of the V_h-WTO samples. The post annealing effect on the crystallization of V₂O₅ in the film is also apparent in the binding energy increase of V_h-WTO at ∼517.0 eV.

As shown in Fig. 3d, two peaks, at ∼38 and 36 eV of W4f_5/2 and W4f_7/2, respectively, are associated with the oxidation state (6⁺) of pure WO₃ (Li et al., 2001). This indicates that both samples were loaded with a considerably high amount of WO₃ phase. The two other peaks (at ∼33.5 and ∼31.5 eV) are possibly assigned to a different form of tungsten oxide (i.e., WO₂), or nonstoichiometric tungsten oxides such as W₁₂O₃₉ⁿ⁻. In fact, because of the similarity in their ion radii, W⁴⁺ can substitute Ti⁴⁺ in the TiO₂ lattice to form W_xTi_1−xO₂, which is a nonstiochiometric solid solution. Thus, W_xTi_1−xO₂ produces a tungsten impurity energy level (Li et al., 2001). Accordingly, the V_h-WTO W4f spectrum shows higher binding energy intensity than that of the V-WTO. This indicates that purer WO₃ crystals were formed after annealing. The doping concentration of W ions allowed in the anatase TiO₂ lattice is considerably higher than that of V ions, based on the TEM-EDS measurements (data not shown). Additionally, the content of fluorine is so limited to be detected by the TEM-EDS. This is in good agreement with the fact that the content of W ions doped into TiO₂ lattices is affected by the fluorine ion (F⁻) concentration in the electrolyte; the higher content of W generally provides better photocatalytic activity (He et al., 2009). Therefore, the effect of W ions is implicitly the most critical factor in photocatalytic activity.

Absorption spectrum

Displaying broad absorbance across the 400–600 nm UV-Vis regions, Fig. 4 shows the diffused reflection spectra of the V-WTO and V_h-WTO microporous films. Thus, the shift toward the longer wavelengths markedly originates from the band gap narrowing of TiO₂ by coupling with tungsten and vanadium oxides, and possibly W_xTi_1−xO₂. The respective band gap energies are estimated to be 2.41 and 2.25 eV for the V_h-WTO and V-WTO, in comparison with 2.95 eV for the N,C-TiO₂ (Wu and Hung, 2009). These are close to the reported values of the V₂O₅/TiO₂, which has V⁴⁺ transitions at ∼550 nm and V⁵⁺ excitations at ∼450 nm (Kubacka et al., 2009). As discussed in the XRD and XPS analyses, a lesser red shift of the V_h-WTO is closely related to the formation of V₂O₅ and WO₃ after being heated at 873 K. Though, the crystalline bulk WO₃ and V₂O₅ have band gap energies of 2.8 and 2.2 eV, respectively (Higashimoto et al., 2006), about 0.4 eV lower than the corresponding monolayered oxides (Londero and Schröder, 2010). The V-WTO is likely doped with W, V, and/or W_xTi_1−xO₂, which possibly results in a more red shift in visible-light region (Gu et al., 2008; Kubacka et al., 2009; Bayati et al., 2010b).

FIG. 4.

Diffused reflection spectra of microporous V-WTO and V_h-WTO along with N,C-TiO₂ film.

Photocatalytic activities

Figure 5 shows the degraded MB concentrations of the samples under UV and BLED irradiation. The V-WTO exhibits degradation rate constants of 0.58 and 0.36 h⁻¹, which are ∼66% and 53% higher than the planar N,C-TiO₂ film with rate constants of 0.35 and 0.22 h⁻¹ under UV and BLED irradiation, respectively. The photocatalytic activity of both V-WTO and V_h-WTO composite films is enhanced by its WO₃ and V₂O₅ crystallites and its high specific surface area as well (Liu et al., 2009; Yeh et al., 2012). Because of its WO₃-loaded acidic surface, microporous TiO₂ film loaded with WO₃ reportedly favors the adsorption of more MB (a cationic dye) molecules on its active sites, resulting in a higher degradation rate (He et al., 2009; Yu et al., 2010). The sponge-like porous samples can cause a charge carrier to reach the surface more easily than does a typical planar N,C-TiO₂ (Liu et al., 2009). Moreover, the V₂O₅ and WO₃ are mostly loaded onto the TiO₂ matrix, which can generate more electron–hole pairs available for photocatalytic reactions under BLED irradiation, as indicated in the inset of Fig. 5. Thus, the doping of the V and W ions in a form of V₂O₅, WO₃, and/or W_xTi_1−xO₂ is responsible for a strong visible-light photocatalytic activity that is superior to those associated with anionic dopants (i.e., N and C) in the TiO₂ lattice (Kubacka et al., 2009; Wu and Hung, 2009).

FIG. 5.

Degraded methylene blue concentrations of microporous V-WTO and V_h-WTO along with N,C-TiO₂ film under ultraviolet irradiation. Inset: The same samples under blue-light-emitting diode (BLED) irradiation.

In general, crystallinity is one of the most critical factors among several others, including surface area, crystal orientation, surface hydroxyl density, and phase composition, in determining photocatalytic activity (Fujishima et al., 2000; Toyoda et al., 2004; Inagaki et al., 2006; Yang et al., 2008). Surprisingly, the V-WTO shows a slightly higher photocatalytic activity than that of the annealed V_h-WTO. The photoactivity of these two types of V-WTO films tested is really negligible—a slight difference between 3.94 and 4.16 mg/L of degraded MB concentration, that is, k=0.58 and 0.53 h⁻¹, under the same UV treatment conditions, as shown in Table 1. This is because that the annealing process diminished the WO_x monolayer on the crystal surface, while enhanced the crystallinity of TiO₂, WO₃, and V₂O₅ in the V-WTO. The formation of WO₃ crystallites, randomly distributed along TiO₂ nanocrystals, rather than the highly adsorbing WO₃ monolayer or amorphous WO_x, reduces the MB adsorption capacity of WO₃ (Kwon et al., 2000). Therefore, as shown in Fig. 5, a decrease in MB molecule adsorption is observed in the V_h-WTO. This implies that, with no need for post heat treatment, the V-WTO is suitable for photocatalytic degradation. Moreover, all the MAO samples lasted more than 80 test cycles, a total of 160 h, of MB degradation, and ultrasonic washing/cleaning and showed no signs of physical and photocatalytic deterioration.

Table 1.

Degradation Rate Constant of Methylene Blue over V-WTO and V_h-WTO Samples Along with N,C-TiO₂ Under Ultraviolet and Blue-Light-Emitting Diode Illumination

	MB degradation rate constant k (h⁻¹)
Sample	UV	BLED
V-WTO	0.58	0.36
V_h-WTO	0.53	0.34
N,C-TiO₂	0.35	0.22

BLEDs, blue-light-emitting diodes; MB, methylene blue; UV, ultraviolet.

Conclusion

In this study, photocatalytic V-WO₃/TiO₂ microporous films with crystallized anatase TiO₂ phase and network-framed structures were successfully obtained through MAO. The absorption edges of the films were shifted toward the visible wavelengths by incorporating the W and V species into TiO₂ phases. The visible-light photocatalytic efficiency was dominated mostly by the incorporation of WO₃ and V₂O₅ oxides, and possibly W_xTi_1−xO₂. In addition, the microporous film loaded with WO₃ favored the adsorption of more MB molecules on its highly specific surface area. The MAO process, as-oxidized V-WO₃/TiO₂, is one of the most promising methods for efficiently manufacturing photocatalysts for dye degradation.

Footnotes

Acknowledgments

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 98-2221-E-022-004-MY2. The authors wish to thank Jui-Ching Sun of National Kaohsiung Marine University for his sample preparations and experimental tests.

Author Disclosure Statement

No competing financial interests exist.

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