Abstract
Photocatalytic water splitting for hydrogen production has been widely recognized as a promising strategy for relieving the pressure from energy crisis and environmental pollution. However, current efficiency for photocatalytic hydrogen generation has been limited due to a low separation of photogenerated electrons and holes. p-n heterojunction with a built-in electric field emerges as an efficient strategy for photocatalyst design to boost hydrogen evolution activities due to a spontaneous charge separation. In this work, we investigated the effect of different preparation methods on photocatalytic hydrogen production over NiO-TiO2 composites. The results demonstrated that a uniform distribution of NiO on a surface of TiO2 with an intimate interfacial interaction was formed by a sol-gel method, while direct calcination tended to form aggregation of NiO, thus leading to an uneven p-n heterojunction structure within a photocatalyst. NiO-TiO2 composites fabricated by different methods showed enhanced hydrogen production (23.5 ± 1.2, 20.4 ± 1.0 and 8.8 ± 0.7 mmolh−1g−1 for S1-20%, S2-20% and S3-10%, respectively) as compared with pristine TiO2 (6.6 ± 0.7 mmolh−1g−1) and NiO (2.1 ± 0.2 mmolh−1g−1). The current work demonstrates a good example to improve photocatalytic hydrogen production by finely designing p-n heterojunction photocatalysts.
Highlights
Over the past decades, environmental pollution and energy depletion have become increasingly critical
A nanocrystal of TiO2 covered by NiO clusters can be obtained during a sol-gel process and the following thermal treatment endows the nanocrystal with high crystallinity
This method leads to a crystal transformation of NiO to form NiTiO3, which has negative effects on photocatalytic hydrogen production [30]
Summary
Environmental pollution and energy depletion have become increasingly critical. NiO is a typical p-type semiconductor and it has widely been combined with TiO2 to form the p-n heterojunction due to its suitable energy band structure, high charge carrier concentration, high chemical stability and low cost [23]. An inner-built electric field is a very efficient manner to improve the transfer of charge carriers between TiO2 and NiO, the commonly fabricated nanocomposites usually suffer an untight contact between these two components, leading to a fast charge accumulation at a nanoparticle surface during the photocatalytic process. These accumulated charges become a recombination center to reduce the photocatalytic quantum efficiency. This work sheds light on improving solar-to-hydrogen conversion by finely designing p-n heterojunction photocatalysts
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