A team at Osaka University in Japan has developed a new material based on gold and black phosphorus to harvest a broader spectrum of sunlight for water-splitting to produce hydrogen.
The three-part composite maximizes both absorbing light and its efficiency for water splitting. The core is a traditional semiconductor—lanthanum titanium oxide (LTO). The LTO surface is partly coated with gold nanoparticles. Finally, the gold-covered LTO is mixed with ultrathin sheets of the element black phosphorus (BP), which acts as a light absorber. The optimum H2 production rates of BP-Au/LTO were about 0.74 and 0.30 mmol g-1 h-1 at wavelengths longer than 420 nm and 780 nm, respectively. A paper on the team’s work is published in the journal Angewandte Chemie: International Edition.
Although H2 can be produced from direct splitting of water using semiconductor photocatalysts under solar light irradiation, the present energy conversion efficiency of STH [solar-to-hydrogen] is too low for the technology to be economically sound. The main barriers are the rapid charge recombination as well as no absorption of traditional semiconductors such as TiO2 in the visible and near-infrared (NIR) light regions.
… One of the options for extending absorption of traditional semiconductors from the UV to visible light region is hybridization with plasmonic metal nanostructures. Plasmonic photocatalysts, recently and rapidly developed novel visible-light-driven photocatalytic systems, possess great potential for improving many intrinsic limitations of conventional photocatalysts using surface plasmon resonance (SPR) properties. Since gold nanoparticles (Au NPs) generally exhibit strong SPR absorption in the visible light region, they have been used most widely.
… More recently, with adjustable band gap from 0.3 eV for bulk to 2.1 eV for monolayer, 2D black phosphorus (BP) has attracted great interest in optical and electronic applications. The band gap properties are promising because of broad solar light absorption from UV to NIR region. Herein, a ternary BP-sensitized Au/LTO (BP-Au/LTO) nanostructure was synthesized firstly and used as an efficient and stable visible and NIR-light-driven photocatalyst for H2 production.—Zhu et al.
The team attributed the enhanced photocatalytic activity to the efficient electron transfer from BP and Au to LTO.
BP is a wonderful material for solar applications, because we can tune the frequency of light just by varying its thickness, from ultrathin to bulk. This allows our new material to absorb visible and even near infrared light, which we could never achieve with LTO alone.—team leader Tetsuro Majima
By absorbing this broad sweep of energy, BP is stimulated to release electrons, which are then conducted to the gold nanoparticles coating the LTO. Gold nanoparticles also absorb visible light, causing some of its own electrons to be jolted out. The free electrons in both BP and gold nanoparticles are then transferred into the LTO semiconductor, where they act as an electric current for water splitting.
Hydrogen production using this material is enhanced not only by the broader spectrum of light absorption, but by the more efficient electron conduction, caused by the unique interface between two dimensional materials of BP and LTO. As a result, the material is 60 times more active than pure LTO.
Zhu M, Cai X, Fujitsuka M, Zhang J, Majima T (2017) “Au/La2Ti2O7 Nanostructures Sensitized with Black Phosphorus for Plasmon-Enhanced Photocatalytic Hydrogen Production in Visible and Near-Infrared Light” Angewandte Chemie International Edition 56 doi: 10.1002/anie.201612315