Visible light assisted photocatalytic hydrogen generation by Ta₂O₅/Bi₂O₃, TaON/Bi₂O₃, and Ta₃N₅/Bi₂O₃ composites

Abstract

Composites comprised of two semiconducting materials with suitable band gaps and band positions have been reported to be effective at enhancing photocatalytic activity in the visible light region of the electromagnetic spectrum. Here, we report the synthesis, complete structural and physical characterizations, and photocatalytic performance of a series of semiconducting oxide composites. UV light active tantalum oxide (Ta₂O₅) and visible light active tantalum oxynitride (TaON) and tantalum nitride (Ta₃N₅) were synthesized, and their composites with Bi₂O₃ were prepared in situ using benzyl alcohol as solvent. The composite prepared using equimolar amounts of Bi₂O₃ and Ta₂O₅ leads to the formation of the ternary oxide, bismuth tantalate (BiTaO₄) upon calcination at 1000 °C. The composites and single phase bismuth tantalate formed were characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) surface area measurement, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis diffuse reflectance spectroscopy, and photoluminescence. The photocatalytic activities of the catalysts were evaluated for generation of hydrogen using aqueous methanol solution under visible light irradiation (λ ≥ 420 nm). The results show that as-prepared composite photocatalysts extend the light absorption range and restrict photogenerated charge-carrier recombination, resulting in enhanced photocatalytic activity compared to individual phases. The mechanism for the enhanced photocatalytic activity for the heterostructured composites is elucidated based on observed activity, band positions calculations, and photoluminescence data.

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1. Introduction

After the discovery of titanium dioxide (TiO₂) as an ultraviolet light active photocatalyst for water splitting and degradation of organic compounds in the early 1970's, a large number of semiconductor-based photocatalysts have been extensively studied.^1,2 In a semiconductor photocatalytic system, electron–hole pairs are generated when a photocatalyst is exposed to light with energy larger than that of its band gap (hν ≥ E_g). The photogenerated electron–hole pairs can either recombine with no chemical effect or migrate to the surface of the semiconductor without recombination, where they can be effectively used to carry out specific redox processes.^3,4 Thus, the efficacy of the photocatalytic process depends on (i) the number of charge carriers taking part in the redox reactions and (ii) the effective separation of electron–hole pairs generated by the photoexcitation. High recombination, (i.e. short lifetime of the photo-generated carriers) and limited efficiency under visible light are the two limiting factors in the development of efficient semiconductor-based photocatalysts.

A number of metal oxides and sulfides have been examined as photocatalysts for hydrogen production and decomposition of toxic organic molecules.^5,6 The majority of binary and ternary semiconducting metal oxides (e.g. TiO₂, NaTaO₃) are primarily active under UV irradiation, which represents only 5% of solar spectrum. For better utilization of solar energy, the materials should have band gaps less than 3 eV (λ > 385 nm).⁷ Thus, significant efforts have been invested in the development of new or modified semiconductor photocatalysts capable of being active in the visible region of the electromagnetic spectrum (λ = 400–700 nm).^8–10 The most common approach is the use of cation and/or anion doping to engineer band gaps of UV-active semiconducting oxides by extending their absorption range to better utilize visible light.^3,11–13 Despite the success of this chemically-based strategy, the performance of visible light active doped photocatalysts remains relatively low in comparison to that of UV active photocatalysts. For example, K. Nagaveni et al. has compared the photocatalytic activities of different metal ion (W, V, Ce, Zr, Fe, and Cu) doped and undoped TiO₂ and their results showed that the degradation rates of 4-nitrophenol with doped catalysts were lower than that of the undoped TiO₂ both with UV exposure and solar radiation.¹⁴ The other approach that has generally been applied is to form a semiconductor heterojunction by coupling with a secondary substance (either a dye or another semiconductor).^15–20 This approach was examined in a variety of applications including hydrogen generation and organic molecule degradation. Some studies focused on the enhancement of visible light induced activity of high band gap semiconductors (UV light active) by combining it with small band gap semiconductors (visible light active).^16–18 Other studies used composite photocatalysts made of two or more small band gap semiconductors (both visible light active).^19,20 In general, it is believed that properly designed heterostructured systems (composite photocatalysis) help to separate photogenerated electron–hole pairs, thus increasing the pairs' life time so that they can transfer to the surface to participate in redox processes. Taking into account the above mentioned facts of charge carrier recombination and absorption range extension, we synthesized, characterized, and studied photocatalytic properties of visible light active composite photocatalysts prepared from two semiconducting oxides with suitable band gaps and band positions.

Among the semiconductor oxides, tantalum-based materials attract considerable attention in the field of photocatalytic hydrogen generation because of its unique chemical stability as well as excellent electronic characteristics for water splitting. The conduction band (CB) levels consist of tantalum 5d orbitals (more negative than the titanium 3d orbitals), which gives photogenerated electrons a strong reducing capability. However, practical use of Ta₂O₅ as a photocatalyst is limited because its activity is restricted to UV light as well as its low photocatalytic activity. Many studies focused on improving catalytic activity of Ta₂O₅ by preparing large surface area materials, doping with cations or anions and making composites with other catalysts.^21–24 Another important semiconducting oxide, bismuth oxide, has recently captured considerable attention due to its visible light activity, dielectric permittivity, high refractive index, and thermal stability.²⁵ It is a semiconductor with suitable band positions for water oxidation.²⁶ It has also been proven to be an excellent sensitizer to enhance visible light activity of UV or visible light active photocatalysts. The approach involving the fabrication of heterostructured composites using bismuth oxide as a sensitizer has been successful in reducing the photogenerated charge carrier recombination and optical response extension.^16,27–30 A number of bismuth oxide based heterojunctions, such as Bi₂O₃/BaTiO₃,¹⁶ ZnO/Bi₂O₃,²⁵ and Bi₂O₃/TiO_2−xN_x,²⁹ have shown excellent photocatalytic activity when exposed to visible light. Here, we report the synthesis of new semiconducting heterojunctions formed by combining tantalum oxides, nitrides, and oxynitrides with bismuth oxide. These composites show superior photocatalytic activities compared to pure tantalum oxide, tantalum oxynitride and tantalum nitride for generation of hydrogen in an aqueous solution of methanol using visible light. The enhanced mechanisms of the photocatalytic activity of these heterojunctions are rationalized on the basis of corresponding band gaps, band positions and photoluminescence data. In the case of heterojunction formed of bismuth oxide and tantalum oxide, an equimolar ratio of the two oxides was used to study the photocatalytic activity of composites as well as that of the bismuth tantalate phase (BiTaO₄) which is obtained after calcination. BiTaO₄ is a well-known photocatalyst suitable for water splitting as well as organic molecules degradation.^31–33 This paper details how bismuth oxide may be used to create composites with tantalum-based oxides, oxynitrides and nitrides. The heterojunction between the two components is characterized and discussed.

2. Experimental section

2.1 Synthesis

2.1.1 Synthesis of Bi₂O₃/Ta₂O₅ composite. For the synthesis of the Bi₂O₃/Ta₂O₅ composite, bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, Alfa Aesar, 99%) and tantalum chloride (TaCl₅, Alfa Aesar, 99.9% metals basis) were used as received. A two-step synthesis approach was used to obtain the composite. In the first step, Ta₂O₅ nanoparticles were prepared using anhydrous benzyl alcohol as solvent.³⁴ In the second step, the as-prepared Ta₂O₅ nanoparticles were used as a support during the synthesis of bismuth oxide using bismuth nitrate precursor in benzyl alcohol under reflux conditions. Detailed synthesis of Ta₂O₅ nanoparticles is given in ESI.† In a typical synthesis of the composite, 0.5 g of Ta₂O₅ nanoparticles were transferred to a 100 mL round bottom flask along with 50 mL of anhydrous benzyl alcohol. Then, 1.097 g bismuth nitrate pentahydrate (to make equimolar ratio of bismuth oxide and tantalum oxide) was added to this suspension and refluxed at 150 °C for 8 hours with continuous magnetic stirring. The obtained pale yellow product was washed with ethanol and vacuum dried for 12 hours at 70 °C. The product was then calcined at 400 °C to obtain the Bi₂O₃/Ta₂O₅ composite, BITA-400, as determined by PXRD. The yield of BITA-400 was ∼80%, based upon the weight of bismuth oxide and tantalum oxide. A sample of the product was also calcined at 1000 °C to obtain the phase BiTaO₄ (BITA-1000) as confirmed by PXRD.

2.1.2 Synthesis of Bi₂O₃/TaON and Bi₂O₃/T₃N₅ composites. To better understand the behaviour of these heterostructured composite, two other composites based on tantalum oxynitride (TaON) and tantalum nitride (Ta₃N₅) with bismuth oxide were prepared using the same two-step method described earlier. TaON and Ta₃N₅, were prepared by nitridation of tantalum oxide in ammonia.^35,36 The complete synthesis procedure is described in the ESI.† The surface of TaON and Ta₃N₅ are coated with Bi₂O₃ using the reflux method as described above and calcined at 400 °C for 2 hours to obtain the composites Bi₂O₃/TaON (BITON) and Bi₂O₃/Ta₃N₅ (BITN) respectively.

2.2 Characterization

The products were characterized by powder X-ray diffraction (PXRD) using a Bruker-D8, with Cu-Kα radiation (λ = 0.15418 nm). The morphology and EDAX analysis of the samples were determined by scanning electron microscopy using an SEM (JSM-6380-LA, JEOL, Japan) and transmission electron microscopy (TEM, JEOL JEM2010). The morphologies and structures of the obtained composites were further analysed by a scanning transmission electron microscope (STEM, JEOL 2200-FS) with aberration corrected at 200 kV. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were completed using an SDT-Q600 (TA instrument, USA) from 32–1000 °C under constant airflow and a heating rate of 10 °C min⁻¹. The Brunauer–Emmett–Teller (BET) surface areas were determined from nitrogen adsorption isotherms at 77 K using Autosorb-iQ from Quantachrome. UV–Vis diffuse reflectance spectra (DRS) were collected on a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere. Photoluminescence (PL) spectra were measured on the dual grating Fluorometer-3T (JY Horiba), with right angle configuration of excitation and detection. The excitation was made with 300 nm monochromatic light, with slits on the excitation and detection monochromators set at 5 nm.

2.3 Photocatalytic test

The catalysts were evaluated for their photocatalytic hydrogen generation using 20% (v/v) aqueous solution of methanol under visible light (λ ≥ 420 nm) and UV + visible light irradiation. A 300 W Xenon lamp (Newport Corporation) was used as the light source with a 420 nm cut-off filter to provide visible light irradiation. All experiments were performed at ambient temperature (25 °C) using a water jacket around the photocatalytic cell. In a typical reaction, 50 mg of the as-prepared photocatalyst was dispersed in an aqueous solution of methanol (10 mL of methanol and 40 mL of deionized water). Before illumination, the suspension was degassed using argon gas for 30 min in order to remove any dissolved oxygen. Then, the solution was exposed to visible or UV light irradiation under magnetic stirring. The amount of hydrogen produced was quantified by gas chromatography.

3. Results and discussion

3.1 Characterization of photocatalysts

The crystal structures and phases of the samples studied were investigated by powder X-ray diffraction (PXRD), as shown in Fig. 1. The PXRD pattern of BITA-400 composite (Fig. 1a) confirms the formation of pure tetragonal bismuth oxide (JCPDS-074-1371) and tantalum oxide (JCPDS-079-1375). Similarly, patterns of BITON (Fig. 1b) and BITN (Fig. 1c) show the composite formation of TaON (JCPDS-071-0178) and Ta₃N₅ (JCPDS-089-5200) phases, respectively, with Bi₂O₃ (JCPDS-079-1375). The major peaks in Fig. 1a–c (located at about 2θ = 28.12°, 32.9°, 46.3°, 47.0° and 55.7°) correspond to the tetragonal phase of Bi₂O₃. The other major reflections in Fig. 1a correspond to the orthorhombic phase of Ta₂O₅. The broadening of peaks in Fig. 1a suggests small crystallite size. Using the Scherrer equation, the average crystallite sizes of Bi₂O₃ and Ta₂O₅ are estimated to be 26 nm and 32 nm, respectively. Similarly the crystallite size of TaON and Ta₃N₅ in BITON and BITN are calculated to be 55 nm and 52 nm, respectively. The peaks in Fig. 1b observed at 2θ = 18.11°, 29.5° and 32.0° correspond to the monoclinic form of TaON.^35,36 Besides the peaks of bismuth oxide, major peaks in Fig. 1c correspond to the monoclinic phase of Ta₃N₅. The PXRD shown in Fig. 1d correspond to that of the pure BiTaO₄ (JCPDS-016-0909) obtained after the calcination of BITA composite at 1000 °C.

Fig. 1 PXRD patterns of the samples (a) BITA-400, (b) BITON, (c) BITN and (d) BITA-1000 (BiTaO₄).

Based on the PXRD study, it is confirmed that the crystal structure representation of the individual components of the composites studied differ from each other substantially (Table 1). Ta₂O₅ is orthorhombic while TaON and Ta₃N₅ are monoclinic. These tantalum based compounds are coated with the tetragonal phase of Bi₂O₃ during their composite formation. These individual components of the composite are well known photocatalysts for solar fuel production and light initiated environmental remediation processes.^37–39

Table 1 Crystal structure representation, and electronic structure data of the components of the composites studied. These composites are made with tetragonal Bi₂O₃ and different phases of tantalum based compounds (orthorhombic Ta₂O₅ for BITA-400 composite, monoclinic TaON for BITON composite and monoclinic Ta₃N₅ for BITN composites)

Compounds		Ta2O5	TaON	Ta3N5	Bi2O3
Crystal unit cell representation
Color of samples		White	Yellow–green	Red	Yellow
Band gap, eV		3.80	2.29	2.03	2.78
Calculated band positions, eV vs. NHE	CB	−0.17	−0.17	−0.17	+0.34
	VB	+3.63	+2.12	+1.86	+3.11

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The morphology and microstructure of different samples were revealed by scanning electron microscopy (SEM) and scanning tunnelling electron microscopy (STEM) as shown in Fig. 2 and 3. Additional microscopy images of composites studied can be found in the ESI (Fig. 2S and 3S†). The composites are composed of irregular architectures with micron sized particles. Energy dispersion spectroscopy (EDS) of BITA-400 shows the presence of Bi and Ta only (Fig. 2b). The composites were further analysed by scanning tunnelling electron microscope (STEM) to understand the nature of the heterojunctions. As shown in Fig. 3a and b, the BITA-400 composite consists of block-like Ta₂O₅ nanosize particles with well-defined faces, whereas Bi₂O₃ particles tend to be primarily acicular or spherical shape. Elemental mapping (Fig. 3b), clearly shows that the Bi₂O₃ and Ta₂O₅ phases are segregated from each other with no intimate contact. In contrast, the composite BITON consists of nanosize block-like or cuboidal TaON particles (∼50 nm in size) intimately combined with Bi₂O₃ particles of similar size to form larger porous particles. The elemental mapping shown on Fig. 3d demonstrate the intimate nature of the BITON composite. Similar conclusion is true for the composite BITN. SEM image (Fig. 2d) shows the formation of onion-shaped micron-size particles formed of plate-like crystals. STEM images and elemental mapping shown in Fig. 3e and f indicate formation of intimate mixture of the two individual phases.

Fig. 2 SEM images and EDS spectrum for different samples: (a) and (b) BITA-400, (c) BITON, (d) BITN.

Fig. 3 STEM images of (a) BITA-400, (c) BITON, (e) BITN composites with corresponding mapping analysis (b), (d) and (f).

The optical absorption properties of all samples were measured by UV–Vis absorption spectra transformed from the corresponding diffuse spectra according to the Kubelka–Munk theory and the results are shown in Fig. 4S (ESI†).^40,41 The band gaps obtained from UV–Vis diffuse reflectance spectra were used to calculate the position of the conduction band (CB) bottom (E_CB) using the empirical formula:⁴²

E_CB = X − 0.5E_g + E₀ (1)

where E_g is the band gap energy of the semiconductor, E₀ is a scale factor relating the reference electrode redox level to absolute vacuum scale (E₀ = −4.5 eV for normal hydrogen electrode, NHE), and X is the electronegativity of the semiconductor, which can be expressed as the geometric mean of the absolute electronegativity of the constituent atoms.⁴³ The values of E_CB and E_VB calculated by using this empirical formula are consistent with the reported values.⁴⁴

The DSC–TGA curves of BITA composite are presented in Fig. 4. The first weight loss begins at ∼175 °C, and corresponds to the loss of organics used during synthesis. The loss of organics is complete by ∼400 °C. The weight was almost constant after 400 °C. This is supported by the major exothermic peak in DSC curve as well. Hence, this temperature was used to calcine all synthesized Bi₂O₃ based composites to remove all organic molecules in order to obtain the pure phase of bismuth oxide in all composites. The pure bismuth tantalate phase was obtained after heating at 1000 °C. Additional TGA graphs of composites and individual component are included in the ESI (Fig. 5S†). To understand the phase changes that occur during the heating of the composites, we collected PXRD data of the phases obtained at various temperatures (Fig. 5). The PXRD shown in Fig. 5a corresponds to that of crystalline Ta₂O₅ used as precursor mixed with amorphous Bi₂O₃. After heating at 400 °C, diffraction peaks of both Ta₂O₅ and Bi₂O₃ are present indicating the initiation of the crystallization of Bi₂O₃ and formation of the composite BITA-400 (Fig. 5b). When the compound was heated at higher temperatures, crystalline BiTaO₄ began to form at temperatures as low as 800 °C. Pure BiTaO₄ is obtained at 1000 °C as shown in Fig. 5h.

Fig. 4 DSC–TGA analysis of bismuth oxide and tantalum oxide composite.

Fig. 5 XRD study of BITA composite (Bi₂O₃/Ta₂O₅) heated at different temperatures.

Surface area analysis of the samples are summarized in Table 2 and the adsorption/desorption isotherms are shown in Fig. 6S in the ESI.† The surface area of BITA-400 is higher than that obtained for BITON and the BITN. The smaller particles from BITA-400 are further supported by PXRD and SEM results. This is most likely due to the corresponding tantalum oxynitride and tantalum nitride used during the synthesis of the composites. The oxynitride and nitride compounds were prepared by heating tantalum oxide at 825 °C for 6 hours. This calcination may have increased the particle size and reduced the surface area.

Table 2 Amount of hydrogen evolved from different catalysts with different surface areas in 4 hours of visible light irradiation

Photocatalyst	Surface area (m^2 g^−1)	Visible light irradiated H2 evolution after 4 hour photolysis (μmol g^−1)
BITA-400	25.00	0.5
TaON	2.50	3
Ta3N5	2.05	7
BITON	6.04	68
BITN	5.41	92

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3.2 Photocatalytic testing

The photocatalytic activities of the samples were determined by the amount of hydrogen generated from an aqueous solution of methanol under visible light or UV light irradiation. Fig. 6 and Table 2 display the different amount of hydrogen produced in 4 hours of UV or visible light irradiation by the given samples.

Fig. 6 Amount of hydrogen gas evolved for different samples in 4 hours (50 mg of catalyst in 50 mL of 20% aqueous methanol solution irradiated with visible light or UV + visible light).

Since tantalum oxide and bismuth tantalate do not absorb in the visible light region, there was no hydrogen production when only visible light was used. Furthermore, no hydrogen production was observed when bismuth oxide was used alone because its band position is not suitable for water reduction. The UV light activity of the composite BITA-400 was slightly higher than that of tantalum oxide and the ternary BITA-1000. Only a very small amount of hydrogen was observed when BITA-400 was exposed to visible light irradiation. The photocatalytic hydrogen evolution is significant in composite in comparison to the individual components or phase pure samples. In other words, the composites BITON and BITN display significantly better photocatalytic hydrogen production compared to the individual components of the composites.

It was found that, the visible light activated hydrogen evolution rate is found in the order of BITN > BITON > Ta₃N₅ > TaON > BITA-400, suggesting lower recombination rate of photogenerated carriers in the composites. The higher activity of the composites is presumably due to the intimate contact between bismuth oxide and TaON or Ta₃N₅. The intimate contact between particles of the two composite components allows the photo-generated electrons and holes to reside in two different species and restrict their recombination rate. To understand the lifetime of photogenerated electron–hole pairs, we have conducted photoluminescence (PL) for all samples. PL data (Fig. 7) shows that the PL emission intensity (the main emission peak is centred at about 390 nm) is in the order of BITA-1000 > Ta₂O₅ > BITA-400 > TaON > Ta₃N₅ > BITON > BITN. The emission intensity of the PL spectra for the composites is significantly smaller than that of the individual components. The intensity of the PL spectra is directly proportional to the rate of charge carrier recombination.^45–47 The data suggest that there is significantly less recombination in the composites, which may explain the higher photocatalytic activity observed. Thus, formation of composites has led to better separation of photo-generated electrons and holes, increasing the life time of the electron–hole pairs, which enhances the photocatalytic activity.

Fig. 7 Room temperature photoluminescence (PL) spectrum of as prepared samples (λ_Ex = 300 nm).

3.3 Reaction mechanism

To explain the mechanism of enhanced photocatalytic hydrogen generation for the heterostructured composites, we investigated the relative band positions of two semiconductors because the band edge potentials play a crucial role in determining the flowchart of photo-excited charge carriers in heterojunctions. Fig. 8 shows a schematic representation of band positions of different compounds based on calculated band edges given in Table 1. The band positions of bismuth oxide lay between those of tantalum oxide. Additionally, its valence band position is located below that of TaON and Ta₃N₅.

Fig. 8 Relative band positions of Bi₂O₃, Ta₂O₅, TaON, and Ta₃N₅.

The activities of the composites are directly related to the corresponding band positions of the individual components. The low activity of the BITA-400 composite (in visible and UV light) can be explained using the same facts observed in the ZrO₂/TaON system, as described by K. Maeda et al.⁴⁸ Based on the band positions of bismuth oxide and tantalum oxide, electron transfer from the conduction band of Bi₂O₃ to the conduction band of Ta₂O₅ and transfer of holes from the valence band of Bi₂O₃ to the valence band of Ta₂O₅ should be energetically unfavourable, and the band positions of bismuth oxide are not suitable for hydrogen generation. Hence, it can be concluded that the electrochemical reactions occur at the surface of tantalum oxide and the incorporation of bismuth oxide helps to separate the electrons and holes on the surface, leading to better activity than the individual tantalum oxide. The increased activity of tantalum oxide over single phase bismuth tantalate (BITA-1000) (Fig. 6) can be explained on the basis of surface area. The surface areas of tantalum oxide and bismuth tantalate are 5.8 m² g⁻¹ and 2.05 m² g⁻¹, respectively. The higher surface area brings not only increased surface for contact with transmitted UV light and water molecules for reduction, but also more active catalytic sites. On the other hand, for randomly generated charge carriers, the average diffusion time from bulk to the surface is given by τ = r²/π²D, where r is the grain radius and D is the diffusion coefficient of the carrier.⁴⁹ When the grain radius decreases, diffusion time is reduced, and the recombination probability of photo-generated electron–hole pairs decreases.

The increased visible light activity of the other two composites, namely BITON and BITN (when compared to tantalum oxynitride and tantalum nitride, respectively) can be explained by the trapping of holes due to the presence of bismuth oxide. Under visible light irradiation, both components of each composite become excited, creating electrons and holes in the conduction band and valence band respectively. In principle, it would be expected that electrons from the CB of TaON or Ta₃N₅ would be easily injected in the CB of Bi₂O₃. If electron migration follows this pathway, more electrons would be concentrated in the CB of Bi₂O₃. Based on band position calculations, electrons in the CB of Bi₂O₃ would not be able to reduce protons due to higher positive CB potential in comparison to H⁺/H₂ potential. However, for BITN and BITON composites, H₂ evolution is considerably higher than that of individual Ta₃N₅ or TaON. This result implies that electrons in the CB of Ta₃N₅ or TaON in composite BITN or BITON are responsible for hydrogen generation and the corresponding holes are trapped by the CB electrons of Bi₂O₃, similar to the Z-scheme mechanism as shown in Fig. 9.^15,50–53 This mechanism is further supported by the observed higher activity of BITN compared to BITON. The valence band of TaON lies below that of the valence band of Ta₃N₅, thence trapping of holes in VB of Ta₃N₅ by electrons moving from Bi₂O₃ (Z-scheme) is more energetically favourable than the same process in case of TaON. The significant decrease in PL intensity for BITON and BITN composites (Fig. 7) clearly suggest and support the conclusion that the recombination rate of photo-generated electron–hole pairs in TaON or Ta3N5 is significantly reduced when combined with Bi₂O₃. In all cases, the enhanced activity of heterojunctioned composites containing Bi₂O₃ with tantalum-based simple oxides, oxynitrides and nitrides can be attributed to better electron–hole pair separation.

Fig. 9 Comparative band edge positions and charge transfer process (Z-scheme) in bismuth oxide and tantalum oxynitride or tantalum nitride heterojunction under visible light irradiation.

Conclusions

In summary, tantalum oxide, tantalum oxynitride and tantalum nitride based composites were prepared individually using bismuth oxide. The heterojunctions formed between the two components in each catalyst show favourable conditions for electron–hole separation to generate hydrogen from an aqueous methanol solution. On the basis of the calculated energy band positions, the mechanisms of enhanced photocatalytic activity for the composites were discussed; the enhanced activity is attributed to the effective separation of electron–hole pairs due to the formation of heterojunctions between the two semiconductors, which allows for better photocatalytic activity for the production of hydrogen gas via water splitting under visible light irradiation.

Acknowledgements

A portion of this research was completed as part of a user proposal through ORNL's Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. Zachary D. Hood was supported by Higher Education Research Experiences (HERE) at Oak Ridge National Laboratory. The authors would like to thank Ms. Nacole King from North Carolina State University, Raleigh, NC for her support regarding diffuse reflectance spectra. Dr. Cynthia Day from Wake Forest University, Department of Chemistry, is acknowledged for temperature-dependent PXRD data collection. Support from Phase II Triad Interuniversity Project (TIP) is also acknowledged. Support from the WFU Science Research Fund is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, XRD of tantalum oxide and BITA composite before calcination (Fig. 1S), SEM and EDS spectrum (Fig. 2S), TEM images (Fig. 3S), UV–Vis diffuse reflectance spectra of synthesized products (Fig. 4S), TGA study of BITA composite, Ta₂O₅ and Bi₂O₃ (Fig. 5S), nitrogen adsorption desorption isotherms (Fig. 6S). See DOI: 10.1039/c5ra06563a

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