Achievement of visible-light-driven Z-scheme overall water splitting using barium-modified Ta3N5 as a H2-evolving photocatalyst.

Chemical Science View Article Online Open Access Article. Published on 18 August 2016. Downloaded on 21/03/2017 13:55:36. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE Cite this: Chem. Sci., 2017, 8, 437 View Journal | View Issue Achievement of visible-light-driven Z-scheme overall water splitting using barium-modified Ta3N5 as a H2-evolving photocatalyst† Yu Qi,ab Shanshan Chen,a Mingrun Li,a Qian Ding,ab Zheng Li,ab Junyan Cui,ac Beibei Dong,ab Fuxiang Zhang*a and Can Li*a Ta3N5 is one of the most promising photocatalyst candidates for solar water splitting, but it still remains challenging to achieve overall water splitting via Ta3N5-based photocatalysts regardless of whether it uses a one step or two step method. Here we will address the relatively poor photocatalytic proton reduction of Ta3N5 with an effort for the promotion of charge separation via barium modification. Onepot nitridation of barium nitrate-impregnated Ta2O5 precursor was adopted here for the synthesis of Ta3N5 accompanied with the creation of a Ta3N5/BaTaO2N heterostructure and surface passivation. Due to the synergetic effect of the improved interfacial charge separation and the decreased defect density, Received 22nd June 2016 Accepted 18th August 2016 the photocatalytic H2 evolution rate of barium-modified Ta3N5 is effectively promoted. Encouraged by this, a visible-light-driven Z-scheme overall water splitting system was successfully constructed by using DOI: 10.1039/c6sc02750d the barium-modified Ta3N5 as a H2-evolving photocatalyst, together with a PtOx/WO3 and IO3
/I
pair www.rsc.org/chemicalscience as an O2-evolving photocatalyst and a redox mediator, respectively. Introduction Semiconductor-based photocatalytic overall water splitting for hydrogen production is an ideal way to convert solar energy to chemical energy and has inspired extensive interest in the past few decades.1–5 Towards this, hundreds of semiconductors have been reported for potential solar water splitting, but most of them are only active under UV light irradiation.6–10 To achieve highly efficient solar-to-chemical energy conversion, overall water splitting on photocatalysts harvesting visible light with longer wavelength is desirable. To date, however, the number of wide visible-light-driven overall water splitting systems, regardless of whether they use a one step or two step method, is limited.5,11–18 Tantalum nitride (Ta3N5), with a theoretical solar-tohydrogen conversion efficiency of 15.9%, is one of the most promising candidates for solar water splitting, considering its matched band edge positions (conduction band and valence band edges at ca.
0.4 V and +1.7 V vs. NHE, respectively, at pH ¼ 0), wide visible light harvesting ability (up to 600 nm) and a State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian, 116023, China. E-mail: ; ; Web: http://canli. dicp.ac.cn b University of Chinese Academy of Sciences, Beijing 100049, China c Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun 130021, China † Electronic supplementary 10.1039/c6sc02750d information (ESI) This journal is © The Royal Society of Chemistry 2017 available. See DOI: good photo-stability.19–37 It was rst synthesized in 1973,38 but was not found to be active for the photocatalytic water splitting reaction until 2002.19 Aerwards, Ta3N5 has been widely investigated for water splitting in terms of particulate photocatalysts22–25 and photoanodes.26–31 The increasing research interest and efforts have greatly promoted the water oxidation performance of Ta3N5 for both particulate photocatalyst and photoanode systems. For example, Li et al. fabricated a 1D Ta3N5 nanorod photoanode to achieve a STH of 1.5%.30 Liu et al. achieved Ta3N5 photoanode stability for hours27 and obtained nearly close to the theoretical photocurrent at a potential of 1.23 V vs. RHE under AM 1.5G simulated sunlight.31 Chen et al. reported that the apparent quantum efficiency of the photocatalytic water oxidation activity of the Ta3N5-based particulate photocatalyst can reach 11.3% at 500–600 nm via an interface engineering strategy.24 Compared to the water oxidation, however, the activity of photocatalytic proton reduction from water is much lower or even undetectable in most cases, even though extensive investigations such as forming polymorphic macroporous Ta3N5, reducing the particle size through templates (i.e. SiO2, C3N4) and surface modication have been made.32–37 As a result of the poor proton reduction ability, Z-scheme overall water splitting using particulate Ta3N5 as a H2-evolving photocatalyst is still not reported. Fabricating nanocomposites with another semiconductor to form heterostructures has been extensively adopted for the promotion of photocatalytic performances.16,39–42 A heterostructure can create external bias through interfacial junctions to spatially separate the photogenerated electrons and holes. Chem. Sci., 2017, 8, 437–443 | 437 View Article Online Open Access Article. Published on 18 August 2016. Downloaded on 21/03/2017 13:55:36. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Chemical Science However, it should be pointed out that most of the (oxy)nitride photocatalysts are thermally instable in air, so the fabrication of a heterostructure for (oxy)nitride commonly confronts technical challenges, rendering feasible examples very limited.16 In this work, a barium modication strategy is introduced to address the relatively poor photocatalytic proton reduction activity of Ta3N5 under visible light irradiation. A simple onepot nitridation route was adopted for the synthesis of pristine Ta3N5 and barium-modied Ta3N5, in which a barium nitrateimpregnated Ta2O5 was used as a precursor. It is found that some Ba2+ ions could be doped into Ta3N5 to decrease its defect density. On the other hand, excessive Ba2+ ions will produce BaTaO2N in situ on the surface of Ta3N5 to create a Ta3N5/ BaTaO2N heterostructure. As a result, the photogenerated carrier separation efficiency of Ta3N5 can be promoted aer the barium modication, causing an effectively enhanced H2 evolution rate in the presence of methanol. Finally, the rst example of a visible-light-driven photocatalytic Z-scheme overall water splitting system using the modied Ta3N5 as a H2evolving photocatalyst was successfully constructed. Edge Article 2 h, and then 0.2 g of the annealed sample was immersed in a calculated amount of H2PtCl6 aqueous solution with sonicating for ca. 5 min. Aer complete evaporation in a water bath at 353 K, the resulting powder was collected and annealed in air at 798 K for 0.5 h. Electrochemical analysis For the Mott–Schottky (M–S) measurement, Ta3N5 and BaTaO2N powder were deposited on FTO conducting gl (...truncated)