Mesocrystalline Ta3N5 superstructures with long-lived charges for improved visible light photocatalytic hydrogen production
Graphical abstract
Introduction
Rapid developments in the semiconductor photocatalysis have elicited considerable attention from researchers of clean energy production and pollutant removal, especially for typical wide bandgap semiconductors [1], [2]. Nowadays, different types of tantalum-based semiconductors, such as Ta2O5 and NaTaO3 catalysts, have successfully been fabricated for improved photocatalysis [2], [3]. However, the wide bandgap tantalum-based semiconductors only respond to UV light because of their ultra-wide bandgaps. And therefore, some modified tantalum-based semiconductors are prepared and show wide spectral response and enhanced visible light photocatalytic performances [4], [5]. However, utilizing wide visible light responsive and efficient tantalum-based semiconductors for hydrogen production are also highly inspired [6], [7], [8].
Tantalum nitride (Ta3N5) is a promising n-type semiconductor photocatalyst with a narrow bandgap of approximately 2.10 eV, which shows very strong visible light absorption up to 600 nm and considerable potential for visible light photocatalytic water splitting [9], [10]. To date, many different types of Ta3N5 semiconductors have been developed and applied in photoelectrocatalysis, such as Ta3N5 thin films [11], Ta3N5 nanorod arrays [12], vertically aligned Ta3N5 nanorod arrays [13]. Besides, the photocatalytic applications of Ta3N5 in energy and environmental fields are also receiving great interests [14], [15]. However, the photocatalytic performances of Ta3N5 photocatalysts are still very weak mainly because of their fast electron-hole recombination and low charge separation efficiency [16], [17], though Ta3N5 shows very strong visible light absorption and excellent chemical stability. How to reduce the recombination of the photo-generated carriers and boost charge separation remains a challenge in the photocatalytic hydrogen production.
Apart from the traditional strategies, such as doping, construction of plasmonic photocatalysts, composites, and dye-sensitized semiconductors [18], [19], [20], [21], the fabrication of superstructure mesocrystals is another way to boost charge transfer and prolong the lifetime of photo-generated carriers [22], [23]. It has been reported that the superstructure SrTiO3 mesocrystals prepared by topotactic epitaxy from TiO2 mesocrystals exhibit rapid charge transport, long-lived charges and a high quantum yield of 6.7% (360 nm) in overall water splitting [24]; Besides, the oxygen-deficient TiO2-x mesocrystals is contributing to enhance the oxidation-reduction abilities, light absorption capability and charge carrier separation efficiency [25], [26], [27]. For tantalum base-nanomaterials, carbon modified NaTaO3 mesocrystal nanoparticles presents a high specific surface area of 90.8 m2 g−1 and excellent efficiency for continuous visible light driven NO gas destruction [28]. The remarkable photocatalytic activity of these prepared mesocrystals is mainly ascribed to rapid charge transfer and prolonged lifetime of photo-generated carriers. Thus, the construction of ordered superstructures is crucial for enhancing the charge separation and photocatalytic performance. Expectations from fabricating Ta3N5 mesocrystals remain very high because of their unique structural features based on the highly ordered superstructures with the alignment of nanoparticle building blocks [24], [29], [30], [31], [32]. To our knowledge, highly ordered Ta3N5 mesocrystals have not been synthesized and employed in the visible-light photocatalytic hydrogen production. Thus, the design and preparation of superstructure Ta3N5 mesocrystals are of significance.
Based on the above considerations, we fabricate a novel sheet-like Ta3N5 mesocrystals by the topochemical transformation through the calcination of superstructure Ta2O5 mesocrystals in the NH3 condition, which is employed as a visible light photocatalyst for hydrogen production from water splitting. The purchased comm-Ta2O5, Ta2O5 mesocrystals and Ta3N5 photocatalyst prepared by comm-Ta2O5 are served as the comparison. The morphologies, phase structures, physical and optical properties and visible light photocatalytic performances are investigated. The results suggest that the prepared Ta3N5 mesocrystals present prolonged lifetimes of photo-generated carriers and improved photocatalytic performance. Finally, the energy band structures and possible visible light photocatalytic mechanisms of the prepared Ta3N5 mesocrystals for hydrogen production are also proposed in this work.
Section snippets
Preparation of Ta2O5 mesocrystals and Ta3N5 mesocrystals
Mesocrystalline Ta2O5 nanosheets (meso-Ta2O5) were prepared according to our precious works [31], [33], [34]. In a typical procedure, 0.30 g of Ta(C2H5O)5 and NH4F (1.6 M) was put into ethylene glycol and stirred for several minutes in a Teflon container. Then, the solution was transferred to another large Teflon container, and the large Teflon container was filled with 8 mL distilled water. The reaction was conducted into an oven for reaction at 160 °C for 24 h. Finally, the prepared
Results and discussion
The crystal structures of the comm-Ta2O5, meso-Ta2O5, comm-Ta3N5 and Ta3N5 mesocrystals were first determined by x-ray diffraction technology, as shown in Fig. 1. The prepared Ta2O5 mesocrystals show five obvious diffraction peaks at 22.902° (d = 3.880 Å), 28.29° (d = 3.150 Å), 36.665° (d = 2.449 Å), 46.686° (d = 1.944 Å), and 55.439° (d = 1.656 Å), which is similar to that of comm-Ta2O5 and could be ascribed to the (0 0 1), (1 1 1 0), (1 1 1 1), (0 0 2), and (0 2 2 1) planes of Ta2O5 (JCPDS
Conclusions
Mesocrystalline semiconductors indicate great prospects in the generation of the efficient charge transport and long-lived charges for the improved photocatalytic hydrogen production. Here, superstructure Ta3N5 mesocrystals are successfully fabricated by the topotactic transformation of Ta2O5 mesocrystals through the calcination treatments. The optimal Ta3N5 mesocrystals show highly enhanced visible light photocatalytic hydrogen production performance, namely, approximately 5.28 times of comm-Ta
Acknowledgements
This work was financially supported by the Program for Innovation Teams in Science and Technology in Universities of Henan Province (No. 20IRTSTHN004) and the Project funded by China Postdoctoral Science Foundation (No. 2019M652526).
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