Porous V2O5/TiO2 Nanoheterostructure Films with Enhanced Visible-Light Photocatalytic Performance Prepared by the Sparking Method

Porous V2O5/TiO2 nanoheterostructure films with different atomic ratios of Ti/V (4:1, 2:1, 1:1, and 1:2) were synthesized by a sparking method for the first time. The sparking method, which is a simple and cost-effective process, can synthesize highly porous and composite films in one step. Field-emission scanning electron microscope (FE-SEM) images revealed the porosity morphology of all prepared samples. V2O5/TiO2 nanoheterostructure films were confirmed by Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). The secondary particle size and band gap of the samples were highly correlated to the V2O5 proportion, resulting in enhanced visible-light absorbance. V2O5/TiO2 nanoheterostructure films at an atomic ratio of 1:1 showed the highest photocatalytic performance, which improved the degradation rate up to 24% compared to pure TiO2 film. It is believed that the formed nanoheterostructure and greater portion of V4+ ions are reflected by this ratio.


Introduction
Titanium dioxide (TiO 2 ) is a widely used photocatalyst because of its low cost, environmental friendliness, and stable photocatalytic reaction [1,2]. There are three forms of TiO 2 phases (anatase, rutile, and brookite), in which the anatase phase has higher photocatalytic activity than both the rutile and brookite phases. Moreover, a mixed phase of anatase and rutile was found and is a better alternative [3,4]. However, the wide energy gap of TiO 2 (~3.2 eV) limits use of the visible-light region as the excitation energy for photo-generated electron-hole pairs. At present, there are many methods that have been used to improve the photocatalytic property of TiO 2 .
Surface modification is known to be able to promote photocatalytic performance by providing more effective interfacial properties. In particular, the large surface areas of 1-D structures and porous morphology are preferred in the design [5,6]. Moreover, composite films with a narrow band gap semiconductor are preferred, which can lead to superior performance of the pure TiO 2 films [7][8][9]. Both TiO2 and V 2 O 5 are two important materials due to their excellent electronic, chemical, and optical properties [10,11]. The strong interaction between TiO 2 and V 2 O 5 is a common scientific phenomenon used in catalytic systems. Furthermore, the heterojunction between V 2 O 5 and TiO 2 can improve the

Morphology, Crystal Structure, and Property Investigations
FE-SEM images revealed the porosity morphology of small particles for all prepared samples, which is the highlight of the sparking method. Our previous papers reported that the as-deposited particles produced by the sparking method were small with primary sizes of approximately 1-5 nm [24]. In this paper, the TiO 2 films exhibited interconnected, spherical-like shapes with average secondary particle sizes of 20 nm that were measured by an imageJ program. While the composite films showed the particles, the sizes increased slightly when the proportion of vanadium increased due to the large size of V 2 O 5 particles, as summarized in Table 1. The film s thickness was approximately 300 nm for all prepared samples as confirmed by the cross-section analysis. In addition, the element composition of samples was determined using EDS and revealed that the atomic ratios of Ti/V were approximately 4:1, 2:1, 1:1, and 1:2 for TiV1-4 samples, respectively, as shown in the inset images of Figure 1b-e. Interestingly, the equal proportion of Ti and V wires in the TiV-4 sample did not correlate with the quantity of their atoms due to their own properties (melting point, latent heat, specific heat capacity) [25]. The sparked films in this experiment were very thin and porous; therefore, the large number of oxygen peaks in the EDS results is due to the quartz substrates (SiO 2 ). Figure 2 illustrates the Raman spectra of prepared samples in the range of 100 to 1100 cm −1 . The peaks spiked at 144 and 631 cm −1 , which correspond to E g and B2 g vibration modes, respectively, and confirmed the anatase phase structure of the TiO 2 film [26]. In the case of the V 2 O 5 film, nine strong peaks of V 2 O 5 (145.1, 283.2, 306.5, 406.7, 484.3, 521.9, 696.9, and 990.1 cm −1 ) and two weak peaks of VO 2 (195.6 and 611.2 cm -1 ) were observed indicating the V 2 O 5 -VO 2 mixed phase [27]. The inset image in Figure 2 shows a high-magnification of the overlap between the strong TiO 2 (anatase) and main V 2 O 5 (orthorhombic) peaks, which shifted from 144 to 145 cm −1 for TiV1-4 samples, respectively. The slight shift of peak positions could plausibly be regarded as the result of modifications in the force constants of tetragonal TiO 2 lattices by the added V 2 O 5 , which indicates the formation of a V 2 O 5 /TiO 2 nanoheterostructure [28].   Figure 2 illustrates the Raman spectra of prepared samples in the range of 100 to 1100 cm −1 . The peaks spiked at 144 and 631 cm −1 , which correspond to Eg and B2g vibration modes, respectively, and confirmed the anatase phase structure of the TiO2 film [26]. In the case of the V2O5 film, nine strong peaks of V2O5 (145.1, 283.2, 306.5, 406.7, 484.3, 521.9, 696.9, and 990.1 cm −1 ) and two weak peaks of VO2 (195.6 and 611.2 cm -1 ) were observed indicating the V2O5-VO2 mixed phase [27]. The inset image in Figure 2 shows a high-magnification of the overlap between the strong TiO2 (anatase) and main V2O5 (orthorhombic) peaks, which shifted from 144 to 145 cm −1 for TiV1-4 samples, respectively. The slight shift of peak positions could plausibly be regarded as the result of modifications in the force constants of tetragonal TiO2 lattices by the added V2O5, which indicates the formation of a V2O5/TiO2 nanoheterostructure [28].  Figure 2 illustrates the Raman spectra of prepared samples in the range of 100 to 1100 cm −1 . The peaks spiked at 144 and 631 cm −1 , which correspond to Eg and B2g vibration modes, respectively, and confirmed the anatase phase structure of the TiO2 film [26]. In the case of the V2O5 film, nine strong peaks of V2O5 (145.1, 283.2, 306.5, 406.7, 484.3, 521.9, 696.9, and 990.1 cm −1 ) and two weak peaks of VO2 (195.6 and 611.2 cm -1 ) were observed indicating the V2O5-VO2 mixed phase [27]. The inset image in Figure 2 shows a high-magnification of the overlap between the strong TiO2 (anatase) and main V2O5 (orthorhombic) peaks, which shifted from 144 to 145 cm −1 for TiV1-4 samples, respectively. The slight shift of peak positions could plausibly be regarded as the result of modifications in the force constants of tetragonal TiO2 lattices by the added V2O5, which indicates the formation of a V2O5/TiO2 nanoheterostructure [28].  The XPS survey spectra presented various peaks corresponding to Ti, V, O, Si, and C, which are elements of the prepared films, except only C occurs during the sparking method and Si from quartz substrate, as shown in Figure 3a. There were no impure peaks, which confirms the formation of pure Molecules 2020, 25, 3327 4 of 11 V 2 O 5 /TiO 2 composites films. The different core levels of high-resolution XPS spectra for Ti 2p, V 2p, and O 1s were investigated further. In Figure 3b, the two peaks centered at binding energies of 459.3 and 464.9 eV were assigned to be Ti 2p 3/2 and 2p 1/2 of Ti 4+ , respectively, which are consistent with the typical values of TiO 2 [29]. In addition, the small shift of the Ti 2p peak position towards a higher binding energy indicates the integration of V ions into the TiO 2 lattice [28]. The V 2p spectra displayed that V 5+ and V 4+ ions were uniformly distributed on the composite films, which were centered at 517.8 and 516.7 eV, respectively, as shown in Figure 3c. This affirms the result from Raman spectra that there was some content of VO 2 in the composite films. In Figure 3d, three peaks at binding energies of 530.2, 531.1, and 533 eV observed in O 1s spectra also confirmed the O-Ti, O-H, and O-V bonding, respectively, of Ti-O-V bonds in the composite samples [30,31].
Molecules 2020, 25, x FOR PEER REVIEW 4 of 12 The XPS survey spectra presented various peaks corresponding to Ti, V, O, Si, and C, which are elements of the prepared films, except only C occurs during the sparking method and Si from quartz substrate, as shown in Figure 3a. There were no impure peaks, which confirms the formation of pure V2O5/TiO2 composites films. The different core levels of high-resolution XPS spectra for Ti 2p, V 2p, and O 1s were investigated further. In Figure 3b, the two peaks centered at binding energies of 459.3 and 464.9 eV were assigned to be Ti 2p3/2 and 2p1/2 of Ti 4+ , respectively, which are consistent with the typical values of TiO2 [29]. In addition, the small shift of the Ti 2p peak position towards a higher binding energy indicates the integration of V ions into the TiO2 lattice [28]. The V 2p spectra displayed that V 5+ and V 4+ ions were uniformly distributed on the composite films, which were centered at 517.8 and 516.7 eV, respectively, as shown in Figure 3c. This affirms the result from Raman spectra that there was some content of VO2 in the composite films. In Figure 3d, three peaks at binding energies of 530.2, 531.1, and 533 eV observed in O 1s spectra also confirmed the O-Ti, O-H, and O-V bonding, respectively, of Ti-O-V bonds in the composite samples [30,31]. Normally, V 4+ ions can only integrate within the TiO2 lattice by the replacement of Ti 4+ ions because of the similar radii of V 4+ and Ti 4+ species. The Ti-O-V bond was formed by sharing oxygen atoms of the V 4+ ions in the TiO2 lattice, leading to oxygen vacancy in the lattice and exposure to a high electron generation capacity. Moreover, we know that the greater number of V 4+ ions led to the formation of more O2 radicals, which is the strongest oxidizing species in photocatalysis [32,33]. Thus, the existence of V 4+ plays an important role in improving photocatalytic performance [34], which shows its maximum content in the TiV-3 sample as present by the V 4+ / Vtotal ratio in Figure 4b. The V 4+ /Vtotal ratio were calculated by area under the curve of high-resolution V 2p XPS spectra as shown in Figure 4a. Normally, V 4+ ions can only integrate within the TiO 2 lattice by the replacement of Ti 4+ ions because of the similar radii of V 4+ and Ti 4+ species. The Ti-O-V bond was formed by sharing oxygen atoms of the V 4+ ions in the TiO 2 lattice, leading to oxygen vacancy in the lattice and exposure to a high electron generation capacity. Moreover, we know that the greater number of V 4+ ions led to the formation of more O 2 radicals, which is the strongest oxidizing species in photocatalysis [32,33]. Thus, the existence of V 4+ plays an important role in improving photocatalytic performance [34], which shows its maximum content in the TiV-3 sample as present by the V 4+ / V total ratio in Figure 4b. The V 4+ /V total ratio were calculated by area under the curve of high-resolution V 2p XPS spectra as shown in Figure 4a.
The TEM and HRTEM images in Figure 5a,c confirm that the primary particle size of V 2 O 5 /TiO 2 nanoheterostructure films was less than 20 nm and then agglomerated to secondary particles at larger sizes correlating to the SEM result. These images clearly present V 2 O 5 particle interfaces that are dispersed on the surface of TiO 2 particles, resulting in the formation of a heterostructure. The lattice fringes of 0.35 and 0.43 nm correspond to the (101) lattice distance of anatase TiO 2 and the (001) orthorhombic V 2 O 5 , respectively, [30,31] and were measured to confirm the heterostructure at their particle interface, as shown in Figure 5d. These heterostructures are expected to adjust the charge separation and improve the photocatalytic efficiency. Moreover, Figure 5b represents the indexed selected area electron diffraction (SAED) patterns of the TiV-3 sample, which, again, confirms the presence of anatase-TiO 2 and V 2 O 5 phases and the absence of any other impurity [16].  The TEM and HRTEM images in Figure 5a,c confirm that the primary particle size of V2O5/TiO2 nanoheterostructure films was less than 20 nm and then agglomerated to secondary particles at larger sizes correlating to the SEM result. These images clearly present V2O5 particle interfaces that are dispersed on the surface of TiO2 particles, resulting in the formation of a heterostructure. The lattice fringes of 0.35 and 0.43 nm correspond to the (101) lattice distance of anatase TiO2 and the (001) orthorhombic V2O5, respectively, [30,31] and were measured to confirm the heterostructure at their particle interface, as shown in Figure 5d. These heterostructures are expected to adjust the charge separation and improve the photocatalytic efficiency. Moreover, Figure 5b represents the indexed selected area electron diffraction (SAED) patterns of the TiV-3 sample, which, again, confirms the presence of anatase-TiO2 and V2O5 phases and the absence of any other impurity [16].  Figure 6a illustrates the UV-Vis absorption spectra of the prepared samples. Notably, large absorption in the visible-light region (peaks arise at 420 nm) was only seen in the V2O5, TiV-4, and TiV-3 samples, while other samples exhibited only UV absorption. Moreover, the band gaps (Egap) of samples were calculated from the Tauc plot as shown in Figure 6b [35,36]. The graph shows that the band gaps were approximately 3.28, 3.12, 2.84, 2.63, 2.56, and 2.32 eV for pure TiO2, TiV1-4, and pure V2O5 samples, respectively. These results show that the band gaps of the samples were slightly narrowed with increased vanadium proportion, and this led to enhanced photocatalytic activity  Figure 6b [35,36]. The graph shows that the band gaps were approximately 3.28, 3.12, 2.84, 2.63, 2.56, and 2.32 eV for pure TiO 2 , TiV1-4, and pure V 2 O 5 samples, respectively. These results show that the band gaps of the samples were slightly narrowed with increased vanadium proportion, and this led to enhanced photocatalytic activity under the visible-light region correlated with the previous absorption spectra. The degradation rate of MB was calculated to investigate the photocatalytic activity under visible-light irradiation, as shown in Figure 6c. Calculations show that all composite films had more prominent photocatalytic performances than pure TiO2 that could be due to their narrowed band gap and the heterostructure at the interface between pure TiO2 and pure V2O5 particles [13,18,30]. To quantify the photocatalytic ability of the samples, the photocatalytic reduction process followed the pseudo-first-order kinetic reaction, which can be expressed as (ln(C0/Ct) = kt), where C0, Ct, k, and t represent the initial concentration of MB, the concentration of MB after visible light irradiation, the rate constant, and the reaction time, respectively [37]. As shown in Figure 6d, the k values were 0.0651, 0.0693, 0.0815, 0.0878, 0.0805, and 0.0544 h −1 for pure TiO2, TiV1-4, and pure V2O5 samples, respectively. Especially in the TiV-3 sample, the highest photocatalytic activity at 64.8% and maximum rate constant were achieved due to the large heterojunction interface and the existence of V 4+ ions integrated within the TiO2 lattice [32].

Possible Photocatalysis Process in the Visible-Light-Irradiated V2O5/TiO2 System
To investigate the electronic band structure, the conduction and valence band edge position of TiO2 and V2O5 were calculated through the following formula [38]:

Valence band edge (E ) = E + E
where X is the absolute electronegativity of the semiconductor (TiO2 ~ 5.81 eV, V2O5 ~ 6.10 eV), E e is the energy of free electrons on the hydrogen scale, and Eg is the band gap of the semiconductor [38]. When TiO2 and V2O5 semiconductors with different energy levels contact each other, electric charge is redistributed, leading to Fermi-level alignment. This induces electron and hole transfers at the V2O5/TiO2 interface. After combining TiO2 using V2O5, the band gap width was reduced and led to the absorption of more visible light, which is more conducive to dye degradation. The degradation rate of MB was calculated to investigate the photocatalytic activity under visible-light irradiation, as shown in Figure 6c. Calculations show that all composite films had more prominent photocatalytic performances than pure TiO 2 that could be due to their narrowed band gap and the heterostructure at the interface between pure TiO 2 and pure V 2 O 5 particles [13,18,30]. To quantify the photocatalytic ability of the samples, the photocatalytic reduction process followed the pseudo-first-order kinetic reaction, which can be expressed as (ln(C 0 /C t ) = kt), where C 0 , C t , k, and t represent the initial concentration of MB, the concentration of MB after visible light irradiation, the rate constant, and the reaction time, respectively [37]. As shown in Figure 6d, the k values were 0.0651, 0.0693, 0.0815, 0.0878, 0.0805, and 0.0544 h −1 for pure TiO 2 , TiV1-4, and pure V 2 O 5 samples, respectively. Especially in the TiV-3 sample, the highest photocatalytic activity at 64.8% and maximum rate constant were achieved due to the large heterojunction interface and the existence of V 4+ ions integrated within the TiO 2 lattice [32].

Possible Photocatalysis Process in the Visible-Light-Irradiated V 2 O 5 /TiO 2 System
To investigate the electronic band structure, the conduction and valence band edge position of TiO 2 and V 2 O 5 were calculated through the following formula [38]: Valence band edge (E VB ) = E CB + E g (2) Molecules 2020, 25, 3327 where X is the absolute electronegativity of the semiconductor (TiO 2~5 .81 eV, V 2 O 5~6 .10 eV), E e is the energy of free electrons on the hydrogen scale, and E g is the band gap of the semiconductor [38]. When TiO 2 and V 2 O 5 semiconductors with different energy levels contact each other, electric charge is redistributed, leading to Fermi-level alignment. This induces electron and hole transfers at the V 2 O 5 /TiO 2 interface. After combining TiO 2 using V 2 O 5 , the band gap width was reduced and led to the absorption of more visible light, which is more conducive to dye degradation. Figure 7 illustrates a schematic diagram of the possible photocatalysis process on the heterostructure of V 2 O 5 /TiO 2 under UV-Visible light irradiation. The surface complex can be exited to generate photoelectrons (e − ) and holes (h + ) by V 2 O 5 and TiO 2 semiconductors. The e − in the conduction band of V 2 O 5 , which has a higher valance band edge than that of TiO 2 , is produced by the difference in energy levels and shifts the conduction band of TiO 2 in order to promote photoreduction and produce oxygen ion radicals (O 2 · − ). The h + at the valence band of TiO 2 was produced and then transferred to the valence band of V 2 O 5 to promote photooxidation. Furthermore, the presence of V 4+ , as evidenced by the XPS result in our work, supports and provides the higher active sites. It also provides high electrical conductivity, leading to boosting the separation and transfer of photogenerated charge carriers, which is important for photocatalytic reactions, as reported previously [39][40][41][42]. In this process, it was indicated that photoelectrons and holes were well separated and their recombinations were reduced. It is believed that the enhanced photocatalytic activity could be attributed to the synergism between efficient V 2 O 5 /TiO 2 nanoheterostructures and the existence of V 4+ ions.
Molecules 2020, 25, x FOR PEER REVIEW 8 of 12 conduction band of V2O5, which has a higher valance band edge than that of TiO2, is produced by the difference in energy levels and shifts the conduction band of TiO2 in order to promote photoreduction and produce oxygen ion radicals (O2· − ). The h + at the valence band of TiO2 was produced and then transferred to the valence band of V2O5 to promote photooxidation. Furthermore, the presence of V 4+ , as evidenced by the XPS result in our work, supports and provides the higher active sites. It also provides high electrical conductivity, leading to boosting the separation and transfer of photogenerated charge carriers, which is important for photocatalytic reactions, as reported previously [39][40][41][42]. In this process, it was indicated that photoelectrons and holes were well separated and their recombinations were reduced. It is believed that the enhanced photocatalytic activity could be attributed to the synergism between efficient V2O5/TiO2 nanoheterostructures and the existence of V 4+ ions.

Preparation of Samples
The experiment was carried out by sparking off four pairs of titanium wires (Ti, ø 0.25 mm, purity 99.5%, Advent Research Material Ltd., Oxford, UK) with a high DC voltage of ~3 kV and a limited current of ~3 mA for synthesizing pure TiO2 films. The spark pulse duration and repetition rate were fixed at 0.16 s and 6.2 Hz, respectively. The wires were then placed with 1 mm spacing and 1 mm above the substrate. The sparking head was adjusted to cover all substrates using computer numerical control (CNC) as shown in Figure 8a.

Preparation of Samples
The experiment was carried out by sparking off four pairs of titanium wires (Ti, ø 0.25 mm, purity 99.5%, Advent Research Material Ltd., Oxford, UK) with a high DC voltage of~3 kV and a limited current of~3 mA for synthesizing pure TiO 2 films. The spark pulse duration and repetition rate were fixed at 0.16 s and 6.2 Hz, respectively. The wires were then placed with 1 mm spacing and 1 mm above the substrate. The sparking head was adjusted to cover all substrates using computer numerical control (CNC) as shown in Figure 8a. To synthesize V2O5/TiO2 nanocomposite films, the vanadium wires (V, ø 0.25 mm, purity 99.8%, Advent Research Material Ltd., Oxford, UK) and titanium wires were sparked at the same time. The vanadium concentration was varied by replacing titanium wires with vanadium wires from one pair to four pairs, which were labeled as TiV1-4, respectively, as shown in Figure 8b. Finally, vanadium and titanium nanoparticles were deposited onto the substrate and formed nanocomposite films with different atomic ratios of Ti/V. All as-deposited films were synthesized on quartz glass substrates (10 × 10 × 1 mm 3 ) for 30 min at room temperature and post-annealed at 400 °C for 1 h at atmospheric pressure. The structural, morphological, optical, and photocatalytic properties of the samples were investigated and reported in this paper.

Characterization Methods
The surface morphology of samples was observed by a field-emission scanning electron microscope (FE-SEM, JEOL JSM 6335F, JOEL Ltd, Concord, MA, USA) equipped with an energydispersive spectroscope (EDS, Oxford, Concord, MA, USA) to study the film's elemental distributions. The high-resolution transmission electron microscope (HRTEM) images were analyzed by a JEOL model JEM 2010 instrument (Concord, MA, USA) at an accelerating 200 kV. The structures of samples were investigated by a Raman spectrometer (JOBIN YVON HORIBA T64000, palaiseau, France) at room temperature with 514.5 nm of 150 mW Ar laser and X-ray Photoelectron Spectroscope (XPS, Rigaku Smartlab Japan system, Rigaku Corp., Tokyo, Japan) using an Al K-alpha source at 1487 eV. Optical properties were carried out over a wavelength range from 200 to 800 nm using a UV-Vis spectrometer (Perkin Elmer Instruments, Tokyo, Japan).

Photocatalytic Activity Measurement
The photocatalytic reaction of samples was evaluated by 0.01 mmol/L methylene blue (MB) solution (Ajex Finechem Pty. Ltd., Sydney, Australia) under visible light for 1-5 h. visible irradiation was produced by a 75 W Xenon lamp (100 w/m 2 , Philip, Bangkok, Thailand), which has a spectrum nearly similar to the solar light spectrum. The catalyst films were placed on the bottom of cuvettes with aqueous MB solution, and then variations in the concentration of MB with samples were analyzed at given irradiation times by a UV-Vis spectrometer (Perkin Elmer Instruments, Tokyo, Japan) in absorption mode. The degradation rate of MB, which represents the photocatalytic efficiency of films, can be calculated with the following Equation: where D is degradation rate, C0 is absorption before irradiation, and Ct is absorption after irradiation [43]. To synthesize V 2 O 5 /TiO 2 nanocomposite films, the vanadium wires (V, ø 0.25 mm, purity 99.8%, Advent Research Material Ltd., Oxford, UK) and titanium wires were sparked at the same time. The vanadium concentration was varied by replacing titanium wires with vanadium wires from one pair to four pairs, which were labeled as TiV1-4, respectively, as shown in Figure 8b. Finally, vanadium and titanium nanoparticles were deposited onto the substrate and formed nanocomposite films with different atomic ratios of Ti/V. All as-deposited films were synthesized on quartz glass substrates (10 × 10 × 1 mm 3 ) for 30 min at room temperature and post-annealed at 400 • C for 1 h at atmospheric pressure. The structural, morphological, optical, and photocatalytic properties of the samples were investigated and reported in this paper.

Characterization Methods
The surface morphology of samples was observed by a field-emission scanning electron microscope (FE-SEM, JEOL JSM 6335F, JOEL Ltd, Concord, MA, USA) equipped with an energy-dispersive spectroscope (EDS, Oxford, Concord, MA, USA) to study the film's elemental distributions. The high-resolution transmission electron microscope (HRTEM) images were analyzed by a JEOL model JEM 2010 instrument (Concord, MA, USA) at an accelerating 200 kV. The structures of samples were investigated by a Raman spectrometer (JOBIN YVON HORIBA T64000, palaiseau, France) at room temperature with 514.5 nm of 150 mW Ar laser and X-ray Photoelectron Spectroscope (XPS, Rigaku Smartlab Japan system, Rigaku Corp., Tokyo, Japan) using an Al K-alpha source at 1487 eV. Optical properties were carried out over a wavelength range from 200 to 800 nm using a UV-Vis spectrometer (Perkin Elmer Instruments, Tokyo, Japan).

Photocatalytic Activity Measurement
The photocatalytic reaction of samples was evaluated by 0.01 mmol/L methylene blue (MB) solution (Ajex Finechem Pty. Ltd., Sydney, Australia) under visible light for 1-5 h. visible irradiation was produced by a 75 W Xenon lamp (100 w/m 2 , Philip, Bangkok, Thailand), which has a spectrum nearly similar to the solar light spectrum. The catalyst films were placed on the bottom of cuvettes with aqueous MB solution, and then variations in the concentration of MB with samples were analyzed at given irradiation times by a UV-Vis spectrometer (Perkin Elmer Instruments, Tokyo, Japan) in absorption mode. The degradation rate of MB, which represents the photocatalytic efficiency of films, can be calculated with the following Equation: where D is degradation rate, C 0 is absorption before irradiation, and C t is absorption after irradiation [43].

Conclusions
Porous V 2 O 5 /TiO 2 nanoheterostructure films with various atomic ratios of Ti/V (4:1, 2:1, 1:1, and 1:2 for TiV1-4 samples, respectively) were successfully fabricated by the sparking method in one step. Raman spectra, SAED patterns, and HRTEM were used to confirm the formation of V 2 O 5 /TiO 2 nanoheterostructure films and their particle interface area. XPS revealed the existence of V 4+ that integrated within the V 2 O 5 /TiO 2 nanoheterostructure and had a significant effect on its photocatalytic performance. The interface heterojunctions on the surface of the catalyst are believed to encourage the photocatalytic performance through electron-hole pair production and the easy electron transport mechanism between the valance and conduction bands due to the low band gap of V 2 O 5 /TiO 2 nanoheterostructure samples. Notably, the TiV-3 sample (atomic ratio of V/Ti approximately 1:1) showed strong absorbance in the visible-light region and had large interface heterojunctions that displayed the highest degradation rate of MB up to 24% compared to pure TiO 2 films. The higher electronic conductivity which arise from greater portion of V 4+ /V toatal supports the more active surface reaction and promoting the separation and transfer of photogenerated charge carriers. Thus, this research provides an alternative route for the one-step synthesis of porous V 2 O 5 /TiO 2 nanoheterostructure films and shows that these nanocomposite films can potentially be used in photocatalytic applications.
Funding: This research received no external funding.