Photocatalytic Performance of Ta2O5/BiVO4 Heterojunction for Hydrogen Production and Methylene Blue Photodegradation

Forming semiconductor heterojunction is promising for improved photocatalytic performance due to synergistic combination of the best properties of each material. The present study reports a simple hydrothermal strategy to form n-n heterojunction of Ta2O5 nanotubes and BiVO4 microstructures. The Ta2O5/BiVO4 heterojunctions were characterized by Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and their photocatalytic activity was evaluated by hydrogen production and photodegradation of methylene blue (MB) dye in aqueous medium under AM 1.5 G (100 mW cm) condition. The heterojunctions have optical absorption in the visible region (200-500 nm) with crystal structures defined as monoclinic for BiVO4 and orthogonal for Ta2O5. For MB photodegradation, the Ta2O5/BiVO4 obtained via hydrothermal route showed a photodegradation of 72.3%, compared to 28.3% presented by the sample produced through the mechanical mixture, with the maintenance of 86.4% of its photocatalytic performance after 3 cycles of photodegradation. For H2 production, hydrothermally prepared Ta2O5/BiVO4 generated 10.2 μmol g of H2 in 3 h; while Ta2O5 nanotubes and mechanical Ta2O5/BiVO4 mixture shows 6.82 and 2.80 μmol g, respectively. The results suggest that Ta2O5/BiVO4 is a promising material for applications in photocatalysis, promoting sustainable energy production through hydrogen and for the treatment of effluents containing cationic dyes.


Introduction
Due to the current serious problems of environmental management and non-renewable energy generation, efficient, clean and low-cost systems for environmental remediation and green energy generation are necessary to guarantee better conditions for the next generations. 1 For power generation, the hydrogen (H 2 ) is an eco-friendly future fuel because it produces water as a combustion product; it is efficient, in addition to being inexhaustible and low cost compared to fossil fuels that cause severe damage to the environment. 2 Industrial pollutants such as organic dyes cause severe damage to the environment. Different processes such as adsorption, coagulation, and electrochemical are commonly used for their removal; 2 however, these methods do not have a leading role due to operational disadvantages. 3 Semiconductor-based photocatalysis is a promising mechanism for solar energy harvesting and pollutant degradation. To obtain efficiency in the process, the semiconductor should have suitable band edges and visible light-harvesting characteristics and present chemical stability and photocorrosion resistance. 4 In addition to these features, the rapid recombination between photogenerated charge carriers (electron/hole pair-e -/h + ) inhibits greater efficiency of different photocatalysts. To suppress the recombination, an alternative route is the formation of heterojunctions which helps decrease the recombination and improve charge transportation; thereby, resulting in greater efficiency. 5 In addition, depending on the composition and bandgaps of the semiconductors, one can synergize the electronic, optical, and structural properties of the semiconductors in the heterojunction for improved photocatalytic performance. 6,7 Tantalum pentoxide (Ta 2 O 5 ) is a widely studied semiconductor for photocatalysis due to its excellent electronic and thermal properties and chemical stability. Ta 2 O 5 has a wide bandgap (3.9 eV, n-type) with an indirect transition which infers its photocatalytic activity once excited with the UV-Vis light. 8 However, due to the restricted absorption properties, structural modifications or semiconductor heterojunctions are needed to improve its performance. 9 Furthermore, due to 1D charge transportation nature and high aspect ratio, nanotubular Ta 2 O 5 morphology displays impressive photocatalytic performance. 10 In recent years, bismuth vanadate (BiVO 4 , energy band gap (Eg) = 2.4 eV, n-type) has drawn considerable attention in the photocatalysis field due to its visible light absorption, good dispersibility, non-toxicity, and corrosion resistance. 11 BiVO 4 , due to lone pair distortion of Bi 6s orbital exhibits a greater photocatalytic performance in visible-light illumination. Additionally, the overlap of O 2p and Bi 6s orbitals in the valence band (VB) increases photocatalytic activity due to the mobility of photogenerated charge carriers. 12 However, low adsorptive capacity and difficulty in migration and separation of the photogenerated charge carriers are the bottlenecks towards its improved photocatalytic performance. 13 The interaction between Ta 2 O 5 and BiVO 4 generates an n-n type heterojunction. For n-n type heterojunctions, the Fermi energies tend to balance to reach the same energy level, enabling the charge transfer between the semiconductors and improving the photocatalytic response of the proposed system. 14 This aspect is efficient in N 2 fixation using g-C 3 N 4 /Cu 3 V 2 O 8 nanocomposites, in the photodegradation of organic compounds using BiOBr-Bi 2 WO 6 , as well as in the H 2 production, using the WO 3 /CoS 2 heterojunction. [15][16][17] It would be interesting to synergize the electronic and absorption properties of both semiconductors to improve the photocatalytic activities through forming an efficient heterostructure between them. It is important to note that the physical mixing of semiconductors may not truly result in the formation of an efficient heterostructure. It is due to non-uniformity and low adsorption at the interfaces inside the heterostructure. 18 In addition, controlling relative amounts of each photocatalyst is another important factor to obtain the synergy between the semiconductors. 7 Thus, efforts are necessary to develop techniques that enable the formation of efficient heterostructures.
To combine the photocatalytic potential and adsorptive capacity of Ta 2 O 5 synergistically with the optical properties of BiVO 4 , this work investigates Ta 2 O 5 /BiVO 4 heterojunction formation through a physical mixture between the semiconductors and also with a hydrothermal heat treatment, applying different molar ratios between the semiconductors. The photocatalysts are applied in the photodegradation of methylene blue (MB) and H 2 production. To the best of our acknowledgment, this is the first work studying this n-n heterojunction.

Chemicals
All chemicals were reagent grade and used without further purification. Tantalum
During the anodization process, a DC power supply (Supplier AC power source) was used to provide a 50 V constant potential for 10 min. Ta and Cu plates were used as anode and cathode, respectively. The area of the electrodes was 15.2 cm 2 each, considering both sides of the Ta and Cu plates, the distance between the anode and the cathode was maintained at 1.0 cm. The electrolyte solution (72 mL) was composed of 95 wt.% of H 2 SO 4 and 5 wt.% of HF. Prior to the anodization, the electrochemical cell was immersed in a hot bath at 54 °C for 15 min to achieve thermal equilibrium and the anodization was conducted at a constant temperature of 54 °C and after anodization, the Ta/Ta 2 O 5 plates were sonicated for 60 min in Milli-Q water for the detachment of the Ta 2 O 5 nanotubes. The powder was centrifuged (10 mg per anodizing on average), dried at a constant temperature (60 °C/24 h), and heated in a furnace at 10 °C min -1 rate up to 800, 850, and 900 °C during 4 h in an oxidizing atmosphere.

Hydrothermal synthesis of BiVO 4
The BiVO 4 synthesis occurred through the co-precipitation method, followed by hydrothermal treatment. Typically, 20 mmol of Bi(NO 3 ) 3  The photocatalytic measurements were performed with a Newport simulator model 69907 equipped with a xenon (Xe) lamp with a maximum power of 300 W. Experiments were carried out under constant irradiation of 100 mW cm -2 , calibrated by a photodiode with known responsiveness and with an 81094 AM 1.5 G filter.
For MB photodegradation experiments, 17 mL of a 15-ppm methylene blue solution were added to a quartz reactor together with 4.25 mg of the tested catalysts (Ta 2 O 5 , BiVO 4 , and Ta 2 O 5 /BiVO 4 ). Initially, the suspension formed with the MB solution and the catalyst remained under magnetic stirring for 30 min in the absence of light, for the MB adsorption process onto the photocatalyst. Under excitation, MB was photodegraded for 180 min, whereas aliquots were taken every 30 min. The aliquots were centrifuged, and the concentration of the supernatant was monitored using an Agilent Cary 300 spectrophotometer (200-800 nm).
For the hydrogen evolution experiments, 4.25 mg of the catalysts were added to a quartz reactor containing 17 mL of a 0.03/0.12 mol L -1 solution of Fe 2+ /Fe 3+ ions used as a hole scavenger system. The quartz reactor was sealed with a rubber septum and purged with argon gas for 30 min. The gaseous products were quantified by gas chromatography using an Agilent Model 7820A gas chromatograph (GC) with 30 m HP-PLOT/Q column and a 30 m HP-MOLESIEVE at room temperature. The generated gases were analyzed simultaneously with a thermal conductivity detector (TCD). Thus, for each time interval (30 min), a 450 μL aliquot was collected from the reactor headspace.

Characterization
The diffuse reflectance spectra of the semiconductors were obtained using a CARY 5000 UV-Vis spectrophotometer (Agilent Technologies) equipped with an integrating sphere, using BaSO 4 as reference. The spectral range was 200 to 800 nm with 1 nm of resolution. X-ray diffraction (XRD) patterns were taken on a Bruker X-ray diffractometer model D8 Advance with Cu Kα radiation (λ = 1.5418 Å) and with the 2θ range from 10 to 80º with a step of 0.02º. The scanning electron microscopy (SEM) images were obtained using an FEI Quanta 200F field emission scanning electron microscope (FESEM) coupled with electron probe energy-dispersive X-ray spectrometer (EDX) for EDS mapping surface. The transmission electron micrographs were recorded with an FEI TECNAI G2 high-resolution transmission electron microscope (HRTEM) at 200 kV. For HRTEM analysis the samples were prepared by dispersing freestanding semiconductor powders in isopropanol at room temperature. One drop was deposited on a 400-mesh holey carbon-coated Cu grid. Raman spectra were obtained with an Ocean Optics QE65000 spectrometer with a 785 nm Ar-Kr laser.

Synthesis and characterization of photocatalysts
The synthesis of Ta 2 O 5 nanotubes was monitored in realtime through the photocurrent density curve (J) vs. time (t) ( Figure S1, Supplementary Information (SI) section). The J-t curve matches well with the previously obtained curves for Ta 2 O 5 anodization. 20 The as-synthesized Ta 2 O 5 nanotubes were heat-treated to increase the degree of structural order. The increase in crystallinity is important as it tends to decrease the recombination rate and makes the material more efficient in photocatalytic processes. 21 In the literature, the orthorhombic phase of Ta 2 O 5 was observed at temperatures from 750 °C, having an increase in crystallinity up to the temperature of 900 °C. Further increasing the temperature resulted in the structural collapse of the nanotubes. 20,22,23 The morphology of the samples was analyzed using SEM ( Figure 1). As observed earlier in the literature, the morphologies of the Ta 2 O 5 nanotubes were sustained even after the thermal treatment at 800 °C (Figure 1a), 850 °C (Figure 1b), and 900 °C (Figure 1c). 10,23 Overall, for the Ta 2 O 5 sample heat-treated at 900 °C, the nanotubes have an approximate length of 7.6 μm and diameter of 100 nm ( Figure S2a, SI section). Figure S2b shows the energydispersive X-ray (EDX) spectra and elemental composition of the synthesized Ta  (0220), and (2111) of the orthogonal phase (COD: 2106064); confirming a complete formation of crystalline Ta 2 O 5 . 24 The better definition of the diffraction peaks presented between 28.0 and 29.0° (Figure 1d), because of the temperature increase (800 to 900 °C). This fact suggests a better structural arrangement of the produced nanotubes.
The UV-Vis absorption spectra of the Ta 2 O 5 nanotubes were obtained by diffuse reflectance spectroscopy (DRS), using the Kubelka-Munk function. 25 Nanotubes obtained through different heat treatment temperatures show similar optical absorption behavior ( Figure S3, SI section), with an intense absorption band (λ onset = 280-310 nm) for all samples. The bathochromic shift in the spectrum of the crystalline Ta 2 O 5 compared to that of the amorphous nanotubes ( Figure S3) depicts the better optical absorption of the solar spectrum for crystalline Ta 2 O 5 nanotubes. The bandgap energy of Ta 2 O 5 was determined by the Tauc plots ((αhν) 1/r vs. hν) (Figure 2), where α is the absorption coefficient, h is the Planck constant and ν is the light frequency. 26 Ta 2 O 5 exhibits indirect allowed electronic transition (r = 2). 27 The estimated bandgap energy for the Ta 2 O 5 nanotubes was 4.10, 3.89, 3.84, and 3.85 eV for the amorphous nanotubes and those treated at 800, 850, and 900 °C, respectively. The cause of the decrease in the band gap energy of Ta 2 O 5 with the heat treatment is due to the change from the amorphous to the orthogonal structure. 28 BiVO 4 synthesis was controlled by pH adjustment before hydrothermal treatment at 180 °C for 6 h without the support of stabilizers. The synthesis occurs through the interaction of Bi 3+ cations with vanadium precursors in an aqueous solution. The VO 4 3ions are stable only in a highly basic medium. After the acidification of the medium, they undergo polymerization, forming VO 4 tetrahedrons connected by corners, developing the species 29,30 Metavanadates produce BiVO 4 at different pH conditions, however, hydrothermal treatment is necessary for the production of nanosystems with phase purity. 31 BiVO 4 morphology was evaluated using SEM (Figures 3a-3c). The hexagon-like morphology is observed in the SEM image (Figure 3), being observed plates with polydispersion of sizes. Figure 3d 32,33 The narrow peaks suggest the highly crystalline nature of the obtained BiVO 4 . The tetragonal structure is characterized by the presence of a single peak at 35°, 34 whereas in the patterns of Figure 3e, two peaks are observed at 34.5 and 35.3°, evidencing the pure monoclinic phase of all prepared BiVO 4 samples. 33 UV-Vis DRS spectra were recorded for the precipitate before hydrothermal treatment and the BiVO 4 samples prepared at pH values of 2.0, 3.5, and 5.0, followed by hydrothermal treatment. The spectrum of the amorphous BiVO 4 sample ( Figure S4, SI section) presents an absorption continuum in the visible region with low intensity and intense band in the UV region, with maxima at 230 and 278 nm. After the hydrothermal treatment, broadened light absorptions towards visible light were observed in all cases, with onsets close to 500 nm. The fundamental electronic transition of BiVO 4 is classified as indirect with a value equal to 2.5 eV, also having a direct transition of 2.7 eV. 27 The Tauc plot (Figure 4a) was applied to determine the bandgap energy of the BiVO 4 . The BiVO 4 presented bandgap energies of 2.13, 2.37, 2.41, and 2.42 eV for amorphous and heat-treated materials at pH 2.0, 3.5, and 5.0, respectively. Despite the enhancement in bandgap energy after hydrothermal treatment (180 °C, 6 h), the significant increase in optical absorption makes the material much more active for photocatalytic processes. In addition, the indirect transition nature of BiVO 4 makes the charge carriers relaxation time longer and the direct transition ensures greater light absorption, making BiVO 4 an impressive semiconductor applied in photocatalysis. 35 Raman spectra were acquired in the range of 0-1000 cm -1 to study the vibrational characteristics of BiVO 4 (Figure 4b).  at 826 cm -1 , is associated with the symmetrical stretching of the V-O connection (A g symmetry) while at 741 cm -1 the asymmetric stretching is present (B g symmetry).
In the Raman spectrum, the position of the bands is sensitive to short-range orders, while the full width at half maximum (FWHM) of the bands are sensitive to the crystallinity degree, defects and disorders, particle size, and aggregation. 31 Figure S6c).
The pseudo-first-order reaction kinetic model of Ta 2 O 5 ( Figure S7a, SI section) and BiVO 4 ( Figure S7b) quantitatively presents the reaction kinetics of MB degradation. 36 The model is defined through equation 1: where C 0 and C are the initial MB concentration and the concentration at time t, respectively. The term k is the first-order rate constant and t is the irradiation time.   Table S3, entry 5). The increase in temperature provides better photocatalytic activity for Ta 2 O 5 due to the improvement in the structural arrangement of the obtained nanotubes, as observed in XRD patterns (Figure 1d). BiVO 4 produced at pH 2.0 is the one with the highest crystallinity and the lowest number of defects, as indicated by Raman spectroscopy (Figure 4b).
The photocatalytic performance of heterojunctions is linked to the crystallinity and low density of structural defects of the semiconductors that constitute it. 25 The best individual performance samples were the Ta 2 O 5 nanotubes heat-treated at 900 °C and BiVO 4 synthesized at pH 2.0, being chosen for the production of heterojunctions. Literature preceding reports on changes in BiVO 4 crystalline phase at harsh acidic conditions. Using hydrothermal treatment at pH 1.7, Zhang et al. 31 obtained the tetragonal phase and demonstrated that the tetragonal phase is not efficient for photochemical reactions when compared to the monoclinic phase of BiVO 4 . The monoclinic phase was obtained preferentially  by increasing the basicity of the reaction medium. In this work, we obtained monoclinic structures, at pH 2.0, 3.5 and 5.0 where the synthesis conducted at pH 2.0 presented the best performance. For the preparation of the Ta 2 O 5 /BiVO 4 heterostructure, a mechanical mixture was subjected to hydrothermal treatment for better interaction between the surface of the semiconductors, being used ratios between the semiconductors Ta 2 O 5 and BiVO 4 equal to 0.5 (Ta 2 O 5 /BiVO 4 -0.5H), 1.0 (Ta 2 O 5 /BiVO 4 -1.0H) and 2.0 (Ta 2 O 5 /BiVO 4 -2.0H). The pH applied during the hydrothermal treatment was adjusted to 2.0 to maximize the efficiency of the Ta 2 O 5 /BiVO 4 heterostructure, justified by its sensitivity to heat treatment pH in its photocatalytic performance (Figure 5b). For comparison criteria, a mechanical mixture (Ta 2 O 5 /BiVO 4 -M) without hydrothermal treatment between semiconductors Ta 2 O 5 and BiVO 4 was prepared, with a molar ratio equal to 1.0.
The studied heterojunctions were initially investigated by SEM (Figures 6a-6d). Mechanical mixing produces a physical approximation between semiconductors (Figure 6a), but without an apparent adhesion on their surfaces. For samples treated via hydrothermal conditions, it is possible to see the BiVO 4 crystals decorated with Ta 2 O 5 grains for the sample of Ta 2 O 5 /BiVO 4 -0.5H (Figure 6b), Ta 2 O 5 /BiVO 4 -1.0H (Figure 6c) and Ta 2 O 5 /BiVO 4 -2.0H (Figure 6d). A better interface between semiconductors can decrease excitonic recombination, being beneficial for the application of heterojunction in photocatalysis. XRD analyses (Figure 6e) present the characteristic peaks described for Ta 2 O 5 (COD: 2106064) and BiVO 4 (COD: 9013437). 24,37 Raman spectroscopy was applied in the characterization of heterojunctions (Figure 6f). The band observed at 78 cm -1 is related to the interaction between the Ta polyhedron and Ta 2 O n 5-2n and/or Ta 6 O 12 6+ . Transitions between 100 < n < 450 cm -1 refer to O-Ta-O bending vibrations of TaO 6 octahedra, where bands are presented for Ta 2 O 5 at 106 and 130 cm -1 . Bands are also observed at 645 and 710 cm -1 , regarding the stretching of the Ta-O bonds present in the Ta 2 O 5 nanotubes. 38,39 Other bands are also identified at 327, 369, and 828 cm -1 (Figure 6f), as presented for BiVO 4 (Figure 4b). The UV-Vis reflectance spectra of the samples Ta  The photocatalytic performance of the mechanical mixture (Ta 2 O 5 /BiVO 4 -M) was evaluated in MB photodegradation ( Figure 7a). As compared to pure BiVO 4 (Figure 5b), this mixture resulted in a worse photodegradation performance (Table S3), reflecting the need for an alternative methodology to form efficient heterojunctions. Thus, we employed the hydrothermal method to form the heterojunction at different conditions   (Figures 6b-6d), justifying the higher value of k. 19,40 By absorbing electromagnetic radiation, Ta In photocatalytic water splitting, despite better absorption in the visible region, BiVO 4 has poor electron transport and inadequate conduction band position to produce H 2 ; however, it is promising for O 2 production; therefore, it has not been individually tested for H 2 production. 42 Firstly, we used Ta 2 O 5 for H 2 production ( Figure S9, SI section). The production rate is sensitive to the heat treatment temperature of Ta 2 O 5 nanotubes, with a production of 4.26, 5.97, and 6.82 μmol g -1 for nanotubes thermally treated at temperatures of 800, 850, and 900 °C, respectively. The hydrogen production results presented are further evidence of crystallinity improvement with higher heat treatment temperatures for Ta 2 O 5 . Furthermore, 900 °C  was also the optimal heat treatment for MB degradation ( Figure 5). After obtaining the optimal condition of MB degradation and H 2 production of individual semiconductors, we applied the Ta 2 O 5 /BiVO 4 heterostructure in photochemical hydrogen generation (Figure 8). Ta (Figures 6b and 6d). However, Ta 2 O 5 /BiVO 4 -2.0H has greater absorption in the visible region (higher content of BiVO 4 ) (Figure 6g), which maximizes light absorption and the number of electrons/holes involved in the photochemical process.
Based on Mulliken's theoretical method of absolute electronegativity, we determined the relative energy levels of the bands referring to the Ta 2 O 5 and BiVO 4 semiconductors (details in SI section) before the heterojunction formation (Figure 9a). It is reported that BiVO 4 and Ta 2 O 5 are n-type semiconductors. 10,43 With the heterojunction formation, there is an alignment of Fermi energies (E F ) at the interface, 44 , the absence of the interface between the semiconductors makes them act individually in the photocatalysis process (Figure 9b). The electrons photogenerated by Ta 2 O 5 are applied in the H 2 production, while recombination of photogenerated charge carriers in BiVO 4 occurs due to the unfavorable energy level for the H 2 production. 46 In the n-n heterojunction formed through hydrothermal treatment (Ta 2 O 5 /BiVO 4 -H, Figure 9c), the excitation of the system produces charge carriers in both semiconductors. It is suggested that, as observed by Xu et al., 44 the electric field formed after semiconductor excitation allows a flow of electrons to the interface region, allowing the improvement of the heterojunction performance in the hydrogen production due to the better  optical absorption of BiVO 4 in the visible region. As the composite is in contact with a sacrificial agent, while the Fe 2+ species consume the photoholes formed at the VB of BiVO 4 , minority carriers (holes) are transferred from the VB of Ta 2 O 5 to the VB of BiVO 4 through the optimized heterostructure, inhibiting the electron-hole recombination in the structure of Ta 2 O 5 . This justifies the low efficiency of the mechanical mixture (Ta 2 O 5 / BiVO 4 -M) for photocatalysis and the improvement shown by the material after hydrothermal treatment. Thus, the hydrothermally-treated Ta 2 O 5 /BiVO 4 presents a great increase in the interface between the semiconductors in addition to enabling the n-n heterojunction produced to be promising in the treatment of environmental effluents and the production of clean energy, contributing to green chemistry and materials engineering.

Conclusions
In summary, the n-n type heterojunction formed by Ta 2 O 5 /BiVO 4 was prepared for the first time and its optical, structural, and photocatalytic properties were successfully investigated. Hydrothermal treatment improves the interface between semiconductors and promotes electronic transport between them, increasing the photocatalytic performance for hydrogen production. The heterojunction formation improves the adsorptive capacity of BiVO 4 , improving the photocatalytic performance of the material for MB degradation. The Ta 2 O 5 /BiVO 4 heterojunction presents improved performance for H 2 production, being 1.49 times higher than that of pure Ta 2 O 5 nanotubes. The n-n Ta 2 O 5 /BiVO 4 heterojunction is a promising material for the photocatalytic conversion of sunlight to produce clean energy through hydrogen and for environmental remediation, as in the photodegradation of the MB dye.