A Ta₃N₅/BN composite for enhanced photocatalytic water splitting

Abstract

Narrow-bandgap Ta₃N₅ nanorods were supported on a BN substrate to produce a composite photocatalyst using NH₄Cl-based vacuum nitridation. The charge separation efficiency of Ta₃N₅ was effectively enhanced due to electron transfer from Ta₃N₅ to the BN substrate, boosting both the photocatalytic water oxidation and Z-scheme overall water splitting reactions.

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Solar energy conversion into clean hydrogen resources through a photocatalytic process is a promising strategy for sustainable energy systems.^1,2 However, the development of efficient photocatalysts is constrained by limitations in light harvesting, especially in the visible region that makes up 45% of the solar spectrum,³ and photon conversion efficiency, restrained by serious bulk and surface charge recombinations.^4–8 Ta₃N₅ has emerged as a particularly promising photocatalyst due to its bandgap of only 2.1 eV. The conduct band minimum (CBM) and valence band maximum (VBM) of Ta₃N₅ are composed of Ta 5d and N 2p orbitals, respectively, thermodynamically allowing for both photocatalytic and photoelectrochemical water redox reactions.^9,10 In 2002, Hitoki et al. reported photocatalytic water oxidation on Ta₃N₅ in the presence of an electron acceptor.¹¹ However, subsequent studies have demonstrated that the performance of Ta₃N₅ is fundamentally limited by issues directly stemming from the synthesis method. Specifically, Ta₃N₅ is typically prepared through an NH₃-based thermal nitridation process, which meets challenges including the formation of low-valence metal ions and O_N anti-site defects within the crystal structure. These intrinsic defects act as recombination centers that severely suppress the charge separation efficiency.^12,13 Furthermore, high temperature duration leads to particle agglomeration of the products, exacerbating charge recombination issues. As a consequence of these synthesis-related challenges, Ta₃N₅ exhibits unsatisfactory photon conversion efficiencies in photocatalytic reactions.^14–16

Nanostructural engineering, by precisely controlling the morphology and architecture of oxide precursors, has emerged as an effective strategy to improve the charge separation efficiency in Ta₃N₅ photocatalysts by reducing the charge transport distances and extending the carrier lifetimes.¹⁷ Ma et al. applied a Ta₂O₅ and Na₂CO₃ mixture to produce a NaTaO₃/Ta₂O₅ composite, which was then used as a precursor for thermal ammonolysis. This method suppressed Ta₃N₅ agglomeration and enhanced the crystallinity.¹⁴ Wang et al. prepared Ta₂O₅-coated Pt/SiO₂ nanoparticles for the fabrication of hollow Ta₃N₅ nanoshells, and subsequently loaded CoO_x onto the outer surface of Ta₃N₅ with Pt left on the inner shell after removing SiO₂.¹⁸ The nanoshell structure not only provided a structural framework for efficient charge transport but also facilitated directional carrier extraction by spatially separating the reduction and oxidation cocatalysts, thus boosting charge separation. Wang et al. applied KTaO₃ as a vaporous Ta resource to produce Ta₃N₅ nanorod single crystals and accelerate heating of the ammonium flow, which reduced the duration of the nitridation process. The highly crystallized Ta₃N₅ nanorods with fewer defects realized one-step excitation in the overall water splitting reaction under visible light illumination.⁹ Nanostructured precursors enable controlled Ta₃N₅ particle morphology, effectively decreasing agglomeration and shortening the carrier transport distances, while structuring the catalyst/host interface promotes more efficient carrier transport, enhancing the charge separation efficiency and thereby unlocking Ta₃N₅'s potential for photocatalysis.

Our group has developed a NH₄Cl-based vacuum nitridation method to synthesize transition metal (oxy)nitrides such as a Ta₃N₅/Ta thin film,¹⁹ TaON powders²⁰ and ZnO:GaN nanoparticles.^21,22 For the preparation of the Ta₃N₅/Ta thin film, only a Ta wafer and NH₄Cl were used as precursors. Upon heating, in the first step, NH₃ and HCl are produced from the decomposition of NH₄Cl. The metallic Ta reacts with HCl to produce a gaseous TaCl₅ intermediate, which subsequently reacts with NH₃ to form Ta₃N₅ particles on the surface of the Ta plate. The high permeability of the gaseous TaCl₅ and NH₃ reactants not only leads to rapid crystallization of the solid product but also prevents Ta₃N₅ from forming reduced Ta species by limiting exposure to an NH₃ atmosphere.

Herein, we introduce two-dimensional BN as a substrate for the growth of Ta₃N₅ nanoparticles in an NH₄Cl-based nitridation process. The involvement of BN in the precursor led to improved crystallinity and dispersion of the Ta₃N₅ particles. Reductive photodeposition of Ag and Rh suggested that electrons were transferred from Ta₃N₅ to BN under irradiation. Such directional charge migration greatly enhanced the photocatalytic activity of Ta₃N₅ for water splitting, highlighting the benefits of photocatalyst–substrate interaction.

The Ta₃N₅ bulk and Ta₃N₅/BN composite were synthesized by vacuum sealing a mixture of NH₄Cl and metallic Ta powder in a quartz tube with or without adding BN (Fig. 1a, with details shown in the SI). For Ta₃N₅/BN, the atomic ratios of Ta : BN were set as 1 : 10, 2 : 10 and 3 : 10, respectively. Fig. 1b displays the X-ray diffraction (XRD) patterns of the powders prepared with different Ta : BN ratios. All the BN-containing samples exhibit patterns of mixed BN and Ta₃N₅ peaks, with a weak signal at ∼29° assigned to TaB_x impurities. The majority of the peaks of Ta₃N₅/BN are consistent with the standard Ta₃N₅ patterns, indicating the high purity of Ta₃N₅ in the obtained samples. No signals of metallic Ta are observed in the products. Additionally, the full-width half-maximum (FWHM) of the Ta₃N₅ (110) diffraction peaks in the Ta₃N₅/BN samples is lower than that of Ta₃N₅, indicating enhanced crystallinity of Ta₃N₅ when BN is present during synthesis. Fig. 1c shows the UV-vis diffuse reflectance spectroscopy (DRS) of the BN, Ta₃N₅ and Ta₃N₅/BN powders. The BN precursor exhibits negligible UV-visible light absorption due to its wide bandgap of 5.1 eV. The Ta₃N₅ reference powder shows a broad absorption up to 600 nm corresponding to its bandgap (2.1 eV). The observed dark brown color, as opposed to dark red, can be attributed to elevated absorption in the near-infrared region. All of the Ta₃N₅/BN samples exhibit smooth band-to-band absorption features typical of Ta₃N₅, revealing that the optical properties of Ta₃N₅ did not change with the introduction of BN into the precursor, as corroborated by the Tauc plots in Fig. S1. Compared to Ta₃N₅, the relatively lower background levels above 600 nm for the Ta₃N₅/BN samples suggest less absorption associated with intragap defect states. Notably, the Ta : BN = 2 : 10 sample achieves the strongest light absorption, showing that photon absorption does not linearly increase with a higher Ta ratio in the precursor.

Fig. 1 (a) Illustration of the synthesis of the Ta₃N₅/BN composite. (b) XRD patterns of the BN precursor, bulk Ta₃N₅, and Ta₃N₅/BN samples prepared from different Ta/BN ratios. (c) UV-vis DRS of the BN, Ta₃N₅, and Ta₃N₅/BN powders, as well as digital images of the Ta₃N₅ and Ta₃N₅/BN (Ta : BN = 2 : 10) samples. (d) SEM images of the Ta and BN precursors and corresponding Ta₃N₅ and Ta₃N₅/BN products. (e) TEM, (f) HRTEM, and (g) dark field image and EDS mapping images of the Ta₃N₅/BN (Ta : BN = 2 : 10) sample. The referenced patterns of BN and Ta₃N₅ in (b) are ICDD-00-045-1171 and ICDD-04-004-4564, respectively.

The scanning electron microscopy (SEM) images (Fig. 1d) display the morphology of the precursors and the nitridation products. Without incorporating BN during nitridation, large Ta precursor particles are converted into aggregated Ta₃N₅ nanorods. In the presence of BN, the nitridation products show more uniformly distributed and well-isolated Ta₃N₅ particles dispersed on the BN substrate. The SEM images of the different Ta₃N₅/BN samples (Fig. S2) in back scattered electron (BSE) mode show that the dispersity of the Ta₃N₅ nanorods in the Ta : BN = 2 : 10 sample is higher than that in the 1 : 10 and 3 : 10 samples. This demonstrates that the Ta content effectively impacts the growth of Ta₃N₅ particles, highlighting NH₄Cl-based vacuum nitridation as a potential method for constructing regulated nanostructures. The transmission electron microscopy (TEM) image of the Ta : BN = 2 : 10 sample (Fig. 1e) shows Ta₃N₅ nanorods with lengths around hundreds of nanometers lying on the BN substrate. The BN structure remains unchanged after the high-temperature nitridation. The TEM image clearly demonstrates that the Ta₃N₅ particles in the obtained samples adopt a typical nanorod morphology, which is similar to the Ta₃N₅ particles prepared directly from TaCl₅ and NH₃ precursors.²³ The high-resolution TEM (HRTEM) image in Fig. 1f exhibits the interface of the Ta₃N₅ and BN particles. The close stacking of the 0.364 nm lattice fringes assigned to the (110) facet of Ta₃N₅ and the 0.220 nm lattice fringes assigned to the BN (100) facet suggests the growth of Ta₃N₅ on the BN substrate. Fig. 1g shows the TEM dark field image and energy dispersive X-ray spectroscopy (EDS) of the Ta : BN = 2 : 10 sample. The overlapped N and Ta signals further confirm the nanorod morphology of Ta₃N₅ on the BN substrate.

XPS analysis was carried out on the Ta₃N₅, BN and Ta₃N₅/BN particles. The well-defined double peaks at 25.2 and 27.1 eV (Fig. 2a) assigned to Ta 4f_7/2 and 4f_5/2 in Ta₃N₅ reveal that the NH₄Cl-based nitridation suppresses the formation of reduced Ta species. For the Ta₃N₅/BN particles, additional double peaks are observed at high field shift, which are supposed to be TaO_x impurities.²⁴ The O 1s spectrum (Fig. S3) reveals the existence of crystalline O species in Ta₃N₅ and Ta₃N₅/BN, suggesting surface coverage of the TaO_x species. Besides, compared with Ta₃N₅, a slightly higher binding energy of the Ta⁵⁺ signals in Ta–N of the Ta₃N₅/BN samples reflects a possible electron transfer from Ta₃N₅ to BN. Relative to BN, the lower shift in B 1s binding energy for the Ta₃N₅/BN samples also suggests an interfacial electron transfer between the two components (Fig. 2b).

Fig. 2 XPS of the Ta₃N₅/BN samples prepared from different precursor ratios, including Ta₃N₅ and BN for reference. (a) Ta 4f and (b) B 1s spectra.

To verify electron transfer between Ta₃N₅ and BN under illumination, visible light-driven photoreduction of Ag⁺ ions was performed, and Ag particles were loaded on the Ta₃N₅/BN composite photocatalyst. The 0.200 nm lattice fringes attributed to the Ag (103) facet are observed on both the BN and Ta₃N₅ particles. The EDS mappings in Fig. 3c clearly distinguish the Ag nanoparticles loaded away from the Ta₃N₅ nanorods. Considering that BN barely absorbs visible light, this indicates that photogenerated electrons are partially transferred from Ta₃N₅ to BN, as illustrated in the structure diagram of Fig. 3c. As further evidence, by using Rh³⁺ as a precursor, Rh nanoparticles can also be reductively photodeposited on the BN substrate (Fig. S4). Mott–Schottky analysis and valence spectra (Fig. S5) reveal that the Fermi level of BN is closely aligned with that of Ta₃N₅. Such energetic alignment is beneficial for electron transfer from Ta₃N₅ to BN.

Fig. 3 HRTEM images and lattice fringe distances of the Ag particles loaded on (a) Ta₃N₅ and (b) BN in the Ta₃N₅/BN particles. (c) Dark field image, EDS mappings and structure diagram of the Ta₃N₅/BN sample loaded with Ag. (d) Transient absorption spectrum for Ta₃N₅ and Ta₃N₅/BN in water without (top panel) or with L-ascorbic acid (bottom panel). The pump and probe lights in (d) are 355 and 580 nm, respectively. The Ta₃N₅/BN sample is prepared from a Ta : BN = 2 : 10 ratio.

Transient absorption spectroscopy was employed to investigate the electron transfer from Ta₃N₅ to BN under light excitation and in solution. The pump and probe lights were set at 355 and 580 nm, respectively. For BN alone (Fig. S6), no meaningful transient absorption signal is observed as its band gap exceeds the pump light wavelength. In contrast, both Ta₃N₅ and Ta₃N₅/BN show a weak negative absorption at 580 nm (Fig. 3d). To assign this signal, L-ascorbic acid is added as a hole scavenger. Here, the intensity of the negative absorption increases for both samples, confirming that this signal is associated with photogenerated electrons. It is worth noting that the negative absorption for Ta₃N₅/BN is significantly stronger than that for Ta₃N₅, indicating superior charge carrier separation in the former sample.

The catalyst–substrate electron transfer in Ta₃N₅/BN is supposed to promote its photocatalytic performance via improving electron–hole separation. Fig. 4a displays the activities of the photocatalytic oxygen evolution reaction (OER) on Ta₃N₅ or Ta₃N₅/BN in the presence of AgNO₃ as an electron scavenger. An IrO₂ species is loaded on the catalysts to assist the OER process (Fig. S7). The Ta₃N₅ nanorods barely produce oxygen due to rapid passivation by Ag coverage on their active sites.²³ However, Ta₃N₅/BN exhibits linear oxygen evolution in the early stages, revealing that electron interaction slows down the Ag passivation and preserves the photocatalytic activity. Among the Ta₃N₅/BN products, the Ta : BN = 2 : 10 sample has the highest water oxidation rate. This is likely due to better photon absorption (Fig. 1c) and the more uniform distribution of Ta₃N₅ nanorods on the BN substrate (Fig. S2) for this sample.

Fig. 4 (a) Time profiles of photocatalytic OER on Ta₃N₅/BN and bulk Ta₃N₅ samples in AgNO₃ reagent. (b) HER activities of Ta₃N₅/BN and Ta₃N₅ in L-ascorbic acid reagent. (c) H₂ and O₂ production from physical oscillation between Ta₃N₅/BN and 2% Rh:SrTiO₃ at pH 5. (d) Diagram of Z-scheme electron transfer between Ta₃N₅/BN and Rh:SrTiO₃. The Ta₃N₅/BN in (b) and (c) are prepared from a Ta : BN = 2 : 10 ratio and each assessment applied 30 mg of catalyst. 0.75 wt% IrO₂ and 1 wt% Pt were loaded as cocatalysts for OER and HER, respectively.

Fig. 4b displays photocatalytic hydrogen evolution (HER) on Ta₃N₅/BN (Ta : BN = 2 : 10) and Ta₃N₅ assisted with L-ascorbic acid as a hole sacrificial reagent. Pt (1 wt%) is optimized as the HER cocatalyst (Fig. S7). The Ta₃N₅/BN composite structure also shows a considerably enhanced hydrogen evolution rate compared with Ta₃N₅ nanorods, although it remains much lower than the OER rate. This highlights the thermodynamic limitations of Ta₃N₅/BN for proton reduction (Fig. S8). To break such limitations, Fig. 4c displays a Z-scheme overall water splitting reaction by physical collision between Ta₃N₅/BN and Rh-doped SrTiO₃ (Rh:SrTiO₃) particles. Under visible light illumination, stoichiometric hydrogen and oxygen evolution over 10 h is realized for Ta₃N₅/BN, while the Ta₃N₅ exhibits significantly lower activity. Fig. 4d represents the band structure of Ta₃N₅/BN and Rh:SrTiO₃ determined from Mott–Schottky and bandgap assessment in Fig. S9. It is obvious that transfer of photogenerated electrons from Ta₃N₅ to BN favors the Z-scheme electron transfer pathway, effectively overcoming the insufficient reduction driving force of Ta₃N₅/BN.

In summary, a Ta₃N₅/BN composite photocatalyst is synthesized via the NH₄Cl-based vacuum nitridation process. The Ta₃N₅ component exhibits highly crystallized nanorod morphology during growth on the BN substrate. Photodeposition of Ag metal, chemical state analysis and carrier dynamics assessment confirmed electron transfer between Ta₃N₅ and BN. Enhanced photocatalytic performance in water splitting emphasizes the promotion of the charge separation efficiency by electron interaction at the catalyst/substrate interface. Our work not only provides a new method for synthesizing highly crystallized Ta₃N₅ nanorods but also demonstrates an efficient composite structure for enhancing the photocatalytic performance of this promising photocatalyst.

This work was financially supported by the National Natural Science Foundation of China (22572123), Shanghai Natural Science Foundation of China (24ZR1451400), Starting Foundation of ShanghaiTech University, and Double First-Class Initiative Fund of ShanghaiTech University. J. F. Zhang acknowledges financial support from the Shanghai Rising-Star Program (Yangfan Special Project, 24YF2729000), and the Science and Technology Commission of Shanghai Municipality (STCSM). We also acknowledge the support from Analytical Instrumentation Center (no. SPST-AIC10112914) and CℏEM (no. EM02161943), SPST of ShanghaiTech University.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. The detailed SEM images, XPS spectra, experiment conditions were displayed in supplementary information. See DOI: https://doi.org/10.1039/d5cc03833b.

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