Date Published: January 03, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Xiuxia Zhao, Jianrui Feng, Jingwei Liu, Jia Lu, Wei Shi, Guangming Yang, Guichang Wang, Pingyun Feng, Peng Cheng.
Developing highly active, recyclable, and inexpensive photocatalysts for hydrogen evolution reaction (HER) under visible light is significant for the direct conversion of solar energy into chemical fuels for various green energy applications. For such applications, it is very challenging but vitally important for a photocatalyst to simultaneously enhance the visible‐light absorption and suppress photogenerated electron–hole recombination, while also to maintain high stability and recyclability. Herein, a metal–organic framework (MOF)‐templated strategy has been developed to prepare heterostructured nanocatalysts with superior photocatalytic HER activity. Very uniquely, the synthesized photocatalytic materials can be recycled easily after use to restore the initial photocatalytic activity. It is shown that by controlling the calcination temperature and time with MOF‐5 as a host and guest thioacetamide as a sulfur source, the chemical compositions of the formed heterojunctions of ZnO/ZnS can be tuned to further enhance the visible‐light absorption and photocatalytic activity. The nanoscale heterojunction ZnO/ZnS structural feature serves to reduce the average free path of charge carriers and improve the charge separation efficiency, thus leading to significantly enhanced HER activity under visible‐light irradiation (λ > 420 nm) with high stability and recyclability without any cocatalyst.
Hydrogen, as a clean and renewable energy carrier, has attracted tremendous attention because of its great potential to replace traditional fossil fuels.1, 2 Hydrogen evolution by photocatalytic water splitting represents one of the most promising technologies due to the use of renewable and clean solar energy.3, 4, 5, 6 As a photocatalyst for hydrogen evolution, its efficiency greatly depends on the catalyst’s capabilities of absorbing visible light and suppressing photogenerated electron–hole recombination.7 Compared with a single catalyst, heterostructured nanomaterials or nanocomposites could combine the strengths of each individual material and solve abovementioned problems.8, 9, 10 Fundamentally, the nanosized structural feature could cause a significant shift in the position of the band edge of the material to achieve the absorption of visible light11, 12 and provide higher surface area so as to potentially increase the accessible reaction sites for the photocatalysis.13 In recent years, numerous efforts have been devoted to the design and fabrication of heterojunction photocatalysts for tailoring band gaps and improving the photocatalytic activity.14, 15 Two most‐used synthesis methods, interfacing different materials via aggregation,16 and epitaxial nucleation of one material on the surface of the other,17 have made a big contribution to this field but tend to lower the accessible active surface area and reduce the number of active sites. Therefore, it is essential to exploit new methods for the synthesis of heterojunctions, especially nanosized heterojunctions, for photocatalytic hydrogen evolution.
A new type of ZnO/ZnS nanostructures has been fabricated by coupling solvothermal synthesis with a two‐step calcination strategy for photocatalytic hydrogen production under visible light. In the heteronanostructures, ZnO and ZnS nanoparticles are homogeneously integrated owning to the uniform sulfidation of the MOF precursor at the molecular level and subsequent oxidization of the nanoscale ZnS@C. This new synthesis method significantly improves the interfacial catalytic active sites and makes it possible to optimize the light absorption capability and charge separation efficiency. In particular, the light‐harvesting ability can be well tuned by regulating the content of ZnO in the ZnO/ZnS heteronanostructures, leading to the optimized ZnOS‐30 that shows excellent photocatalytic HER activity and recoverability under visible light. The strategy reported here, i.e., homogeneous nanoscale ZnO/ZnS heterostructures from a MOF‐assisted route, provides a new approach to synthesize the uniformly integrated heteronanostructures of metal oxides and sulfides for catalysis and energy conversion.
Synthesis of MOF‐5: Zn(NO3)2·6H2O (2.98 g, 10 mmol) was dissolved in 25 mL N,N‐dimethylmethanamide (DMF) to form a solution, which was subsequently poured into 65 mL DMF solution containing 1,4‐benzenedicarboxylate (0.33 g, 2 mmol). After thorough mixing, the solution was refluxed at 100 °C for 12 h. The resulting white precipitates were collected by centrifuging, washed with DMF and methanol in sequence for at least three times, and finally dried in vacuum at 70 °C overnight.
The authors declare no conflict of interest.