Mixed-Metal Tungsten Oxide Photoanode Materials Made by Pulsed-Laser in Liquids Synthesis

The conversion of solar energy to storable fuels is of ultimate importance with regards to future global demands. Devices for sunlight-driven water splitting as well as artificial photosynthesis should be composed of components that are made of non-precious elements in a bid to allow for global scalability. Photoelectrochemical devices are beneficial in the sense that the processes of light capture and catalysis are decoupled. The anodic and cathodic elements as well as half reactions are also separated. Although major strides have been made implementing silicon, indium phosphide, and tungsten selenide photocathodes, appropriate Photoanode materials have yet to be realized.

Tungsten trioxide is a promising Photoanode building block for use in acidic media. Unfortunately, the high band gap of its thermodynamically stable γ-monolithic phase limits effective light absorption. Several researchers have focused on shifting its optical band gap to the visible region for improved light absorption, while at the same time avoiding deep traps where charge carriers may recombine.

Dr. Astrid Müller at the California Institute of Technology prepared a series of mixed-metal tungsten oxide photoanode nanomaterials by pulsed laser in liquids synthesis in a bid to analyze the effect of ad-metals on optical as well as photocurrent generation properties. The research work is published in ChemPhysChem.

A pulsed-laser process from metal targets in aqueous ammonium metatungstate solution or water was used to prepare fourteen tungsten-oxide-based nanomaterials, with earth-abundant first-row transition metals, tantalum, or aluminum as ad-metals.

The research team utilized metastable polymorphs of tungsten trioxide and tungsten oxides with different degrees of oxygen deficiency. The authors found that in a series of systematically varied nickel-to-tungsten ratios, nickel-tungsten materials with ≤6% nickel with respect to tungsten developed hollow spheres.

Photoelectrochemistry results of the mixed-metal tungsten oxide photoanode materials in aqueous acid indicated that materials with approximately 5% of nickel or iron performed very well. The authors did not observe correlations of photocurrent generation with the colors and sizes of the laser-made materials. Relative proportions of the ad-metal to tungsten were critical for photocurrent generation and electrocatalytic activities.

The pulsed-laser in liquids synthesis technique in the study allowed for the rapid production of a series of mixed-metal oxide light absorbers with varying optical as well as photoelectrochemical properties for application in aqueous acid. The method also permitted the systematic comparison of the resulting materials, and for quick identification which elemental compositions, morphologies, and crystal phases were important for improved photoanode performance. The authors were therefore able to successfully prepare fourteen sub-µm-sized, differently colored tungsten oxide-based materials through their ultraviolet pulsed-laser in liquids process.