Highly efficient water purification by WO3-based homo/heterojunction photocatalyst under visible light

Bandgap engineering has attracted increasing interest in the field of semiconductive materials with specific electrical and optical characteristics for various applications, such as solar energy harvesting, energy-related applications, and photocatalysts. Here, we propose a novel and facile method to obtain a combination of type-II structure and Ohmic contact by the fabrication of WO3/Ti-WOx/TiHyOz homo/heterojunction on a WO3 photocatalyst using ball milling. The morphology and structural characterizations of the material are obtained using UV-vis, SEM/EDS, XRD, Raman spectroscopy, and XPS. It is revealed that titanium hydride (TiH2) facilitates simultaneous WO3 reduction and cation doping, which generates intervalent W ions (W5+, W4+) with oxygen deficiency, leading to the formation of new band structures at the WO3/Ti-WOx/TiHyOz interface. As a result, the homo/heterojunction significantly extends light absorption above 400 nm and facilitates the spatial separation of photoexcited carriers, which significantly enhances the photocatalytic performance to degrade azo-dye water pollutants under visible light irradiation.

In particular, controlling the stoichiometry and introducing oxygen vacancies is a practical approach to extend light absorption and facilitate carrier transfer/separation, which significantly improves photocatalytic performance [1]. In the last decade, various methods were used to prepare nonstoichiometric tungsten trioxides (WO3-x), such as the hydrothermal/solvothermal reaction of inorganic precursors or W [13,16], annealing WO3 in a reducing atmosphere (H2) or vacuum [7,19], and microwave plasma ablation of W in aqueous solution [20]. However, the reported approaches require unique experimental setups for thermal treatment in a vacuum or a hydrogen-rich atmosphere, high-pressure control, and introduction of specific reaction fields.
In this study, we propose a novel and facile bandgap engineering method to fabricate homo/heterojunctions on WO3 at room temperature using ball milling to design a highly efficient visible-light photocatalyst (Figure 1). Here, titanium hydride (TiH2) is significant to facilitate the following simultaneous reactions: (1) element doping and oxygen abstraction from WO3 as a reducing Accepted manuscript_K. Kato et al., Journal of Alloys and Compounds, 901 (2022) 163434. DOI: 10.1016/j.jallcom.2021.163434 agent and formation of specific electronic contacts at the interface of the hybrid material (WO3/Ti-WOx/TiHyOz); (2) easy tunability of the chemical composition in Ti-WOx in the range of 1.39 < x < 2.75, bandgap narrowing, and valence band (VB) edge tailing by controlling the amount of TiH2. As a result, the WO3-based photocatalyst with WO3/Ti-WOx/TiHyOz homo/heterojunction demonstrates a wide range of light absorption above 400 nm and superior spatial carrier separation, which results in the excellent photocatalytic performance (more than three orders of magnitude higher than that of pristine WO3) for the degradation of azo-dye water pollutant under visible light irradiation.
Furthermore, the synergistic effect of the homo/heterojunction structure on the enhancement of photocatalytic activity is discussed in this study.

One pot synthesis
Homo/hetero junction photocatalyst

Experimental
Commercially available pure WO3 powder (>99.5%, Nacalai Tesque Co., Ltd) and TiH2 (>99.9%, Kojundo Chemical Laboratory Co., Ltd.) powder were used as raw materials. High-energy ball milling was performed in a commercial planetary ball mill (Pulverisette-5, Fritsch GmbH) using zirconia balls of 5 mm diameter and zirconia pot of 80 ml. The ball milling was performed at a rotation speed of 300 rpm for 1 h. The mixture powder (volume ratio of WO3 and TiH2; 100-x:x, where x = 0, 1, 2, 4, and 8) was placed into the pot in a N2-filled glovebox before the milling. The synthesized samples were labeled as W-Ti1, W-Ti2, W-Ti4, and W-Ti8, respectively.
The optical properties in the UV to visible range and UV to near-infrared range were analyzed by UV-vis spectrophotometry (V-7100, JASCO. The particle morphology was analyzed using fieldemission scanning electron microscopy (JSM-7000F, JEOL). X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Ultima-IV, Rigaku). Raman spectroscopy was performed photoluminescence (PL) spectra were obtained using a commercial spectrofluorometer (FP-8500, Jasco, Japan) at an excitation wavelength of 350 nm. (2.6 eV) owing to the formation of intervalent tungsten compounds (i.e., W 5+ and W 4+ ) with oxygen vacancies [22]. The trapped electrons in the W 5+ /W 4+ states polarize the WO3 lattice to induce polarons, which are associated with localized surface plasmon resonance (LSPR) [10]. Thus, the strong absorption in the visible and NIR regions can be attributed to the LSPR. Moreover, the high-energy ball milling of the yellow-green WO3 powder with TiH2 results in a dark-blue mixture (Figure 2c).

Particle morphology
Pristine WO3 consisted of 40-50 nm coarse micron-sized particles. However, the mechanical treatment changes the morphology owing to the formation of neck and adhesion, as shown in Figure   S1. In addition, the TiH2 particles with angular shapes were not observed after the milling because they were embedded in the WO3 agglomerates. Figure S2 shows the nitrogen adsorption/desorption isotherms, which are all categorized as type II [24]. The Brunauer-Emmett-Teller (BET) specific However, the phase transformation of WO3 and the generation of new crystalline titanium compounds were not observed in the XRD patterns (Figure 3).
The structural disorder was confirmed by the Raman spectra (Figure 4), according to which the amount of TiH2 strongly depended on the broadening of two primary peaks at 710 and 810 cm -1 (attributed to phonons in crystalline WO3 [25][26][27]). The peak broadening could be ascribed to the oxygen deficiency in the WO3 lattice [7]. To examine the change in the reduction of synthesized Ti-WOx, the Raman maps were obtained at 60 × 60 μm 2 (all results are displayed in Figure S4). Figure   S5a shows the histogram as a logarithmic function of the peak intensity at 810 cm -1 , which can be expressed by a normal distribution with a mean of μ. Figure S5b shows a good relationship between the log-scale μ and the volume fraction of TiH2 in the mixture, suggesting that the reduction occurred with the Ti doping/oxygen abstraction at the WO3/TiH2 interface during high-energy ball milling.
The analysis of the chemical structure of the nanosurface by XPS revealed the reduction state of the Ti-WOx nanoparticles ( Figure 5). In W4f XPS spectra, the shoulder peaks appeared at approximately 37.2/35.3 and 36.4/34.2 eV, which corresponded to W 5+ and W 4+ , respectively [7]. The area fraction attributed to the intervalent W ions (W 5+ and W 4+ ) increased with the increase in the volume fraction of TiH2. Moreover, OH groups and physisorbed H2O molecule-derived shoulder signals [7,14] were detected, as shown in the O1s XPS spectra. In general, oxygen vacancies facilitate the dissociative chemisorption of water molecules in nonstoichiometric transition metal oxides [28]. In particular, the W-O-Ti chemical bond was observed in the W-Ti8 sample. In this case, every signal from Ti was detected by neither a survey nor a narrow scan (Figure S6) [15,29]. Recently, some research groups reported that oxygen deficiency led to the formation of new levels that partially overlapped with the VB [30,31]. This contributed to the VB tailing and bandgap narrowing, which extended the photo-response.

Visible-light photocatalytic activity
The visible-light photocatalytic activity of the WO3-based hybrid nanoparticles is significantly enhanced for the photodegradation of the azo-dye water pollutant, methyl orange (MO; 15 ppm), as shown in Figure 6a. UV-vis spectra of the MO solution before the adsorption equilibrium and during the photodegradation are shown in Figure S7. The photocatalytic degradation kinetics follow the firstorder law, expressed as ln(C0/C) = kt [32], where t is the irradiation time and k is the degradation rate constant. The values of k determined from the slope of the linear curves in Figure 6b, ranged from < 1.00 × 10 -5 (pristine WO3) to 2.54 × 10 -2 min -1 (W-Ti8). Thus, the one-pot mechanical reaction enhanced the photocatalytic degradation rate by three orders of magnitude. Table S1 compares the results of this study with the performance of other WO3 and semiconductor photocatalysts, indicating that the photocatalytic performance of the synthesized photocatalyst is superior even with a lower surface area and earth-abundant elements. However, the excess loading of TiH2 during the ball milling hinders further activity enhancement due to the severe deformation of the WO3 lattice by forming a high concentration of oxygen vacancies. As shown in Figure 6c, the original MO displays strong absorption at 465 and 270 nm, which corresponds to the azo bond (-N=N-) and the benzene ring, respectively [33,34]. The main absorption peak (465 nm) shows a redshift before the adsorption equilibrium (photograph is shown in Figure 6d). Azo dye is commonly used as a pH indicator and MO changes the color from yellow to red with increasing acidity. The redshift indicates that the W-OH (in this case, OH groups formed on the WOx surface, as shown in the XPS results) have Brønsted acidic features [35], which function as the active sites for the photocatalytic reaction. Some research groups have reported that the Brønsted acid sites in semiconductor photocatalysts are significant for activity enhancement (e.g., esterification [36] and degradation of MO [37]). The intensity of the central peak decreased quickly after the visible-light irradiation owing to the cleavage of the conjugated double bond with decolorization by photocatalytic degradation. Moreover, several degraded products were detected, which can be ascribed to amines (<240 nm) [38], sulfanilic acid, and N, N-dimethyl-pphenylenediamine (240-260 nm) [33,34], by the photodegradation. The recombination rates of the photoexcited carriers were obtained by the PL measurements ( Figure S8), where the PL intensity decreased after compositing. The results indicate that the homo/heterojunction formation was significant for enhancing the carrier separation.

Influence of homo/heterojunction on photocatalytic activity
To investigate the influence of the WO3/Ti-WOx/TiHyOz homo/heterojunction on the photocatalytic performance, an additional chemical treatment was performed using NaOH to eliminate a part of the WO3 phase. The obtained material mainly consisted of TiH2 crystals, and the XRD peaks shifted to lower 2-theta values, which could be attributed to the lattice shrinkage caused by the oxidation of TiH2 [39] (Figure 7a). However, in the Raman spectroscopy (Figure S9), the signals originating from WO3 were detected even after alkaline dissolution. In addition, the XPS measurements revealed the presence of intervalent Ti ions and W-O-Ti in the Ti2p and O1s XPS spectra, respectively (Figure 7b). Thus, the results suggested that the strong interaction with TiHyOz at the interface chemically stabilized Ti-WO3-x. Figure 7c displays the EDS mapping of the chemically treated particles, where W is uniformly distributed in the TiHyOz particles. Furthermore, the remaining Ti-WOx nanoparticles exhibited a superior light-absorbing capacity in the visible region and highly efficient spatial carrier separation compared to the chemically untreated particles (W-Ti8) (Figure 7d,e). eV below the CBM [40,41]. Moreover, the electron affinity of WO3 is 5.2-5.3 eV [41]. However, the work function of Ti is 4.3-4.4 eV [42], and this value decreases by ~250 meV by the hydrogen uptake (TiHx, x ≤ 2) [43]. In addition, TiH2 exhibits a metallic feature with an electrical conductivity comparable to that of titanium [32]. Based on the upward tailing of the valence band maximum (VBM) up to 1.5 eV and the optical bandgap narrowing (2.23 eV for W-Ti8) in the formed Ti-WOx, the CBM is located at approximately 5.0 eV relative to the vacuum level, which has a higher energy potential than TiH2 (TiHyOz). A similar value of the Fermi level in WO3 is assumed in Ti-WOx, as shown in the report for transition metal-doped WO3 [44]. Thus, in the heterojunction, the photogenerated electrons on Ti-WOx can be easily transferred to the metallic TiHyOz side by Ohmic contact, which significantly inhibits carrier recombination. Such Ohmic contacts were observed at the interface between the transition metal and n-type semiconductors (e.g., W/WO3 [45], Fe/Fe2O3 [46]). Moreover, the prepared nanocomposites consist of type-Ⅱ [47] homojunction. As charge carrier transfer in the type-Ⅱ structures, the photo-excited electrons will accumulate on the CB of Ti-WOx with weak reduction potential. In contrast, the photo-excited holes will gather on the VB of WO3 with weak oxidation potential, resulting in the spatial charge separation. Considering the possible mechanism of the MO degradation based on previous reports [48,49], the following reactions occur in the synthesized composite photocatalyst: O2 • -+ 2 H + + eCB -→ H2O2 4H2O + 2 hVB + → H2O2 + 2H + (4) •OH/ O2 • -+ MO → Degradation products (6) In general, the excited electrons in the CB of bulk WO3 have less potential to reduce O2 to superoxide anion radicals (O2 • -). However, in this case, many photoexcited holes and electrons could be retained on different counterparts with superior light-absorbing capacity, which enhances the photocatalytic efficiency by a combination of type-II structure and Ohmic contact in the WO3-based homo/heterojunction photocatalyst. As a further experiment, the predominant active species generated in the photocatalytic reaction were detected by scavengers. Adding EDTA-2Na and t-BuOH restrain the photocatalytic reaction significantly (Figure S10)

Conclusions
In summary, we proposed a novel and facile bandgap engineering method with a combination of type-II structure and Ohmic contact by the fabrication of the WO3/Ti-WOx/TiHyOz homo/heterojunction on the WO3 nanoparticles. TiH2 facilitated simultaneous WO3 reduction and cation doping by high-energy ball milling, which offered a wide range and strong light absorption above 400 nm. Furthermore, the chemical composition of Ti-WOx was tunable in the range 1.39 < x < 2.75 by controlling the amount of TiH2. In this case, the Ti element doping and oxygen abstraction by TiH2 generated the intervalent W ions (W 5+ , W 4+ ) with oxygen deficiency, which resulted in the upward shift of the VB edge up to 1.5 eV and bandgap narrowing. Furthermore, the formed Ti-WOx phase exhibited high chemical stability owing to the formation of strong interactions at the interface with TiHyOz as an Ohmic contact (heterojunction). Furthermore, many photoexcited holes and electrons could be retained on different counterparts because the hole on Ti-WOx recombined with the excited electron from the WO3 homojunction as a type-Ⅱ structure (homojunction). The homo/heterojunction facilitated the spatial separation of the photoexcited carriers, which significantly enhanced (more than three orders of magnitude higher than that of pristine WO3) the photocatalytic performance for the degradation of the azo-dye water pollutant (MO) under the visible-light irradiation.