Hybrid Microwave Annealing Synthesizes Highly Crystalline Nanostructures for (Photo)electrocatalytic Water Splitting.

Hydrogen is regarded as an ideal energy carrier for the hydrogen economy that could replace the current hydrocarbon economy in order to achieve global energy security and mitigate climate change. For this purpose, H2 has to be produced from renewable sources (e.g., solar and wind) without producing global-warming CO2. (Photo)electrolysis of water into H2 and O2 is one of the most promising technologies for the production of renewable H2, which requires (photo)electrocatalysts of high efficiency, chemical robustness, and scalability. An essential attribute required for high-efficiency (photo)electrodes is high crystallinity with few defects to facilitate charge transfer without recombination. To this end, fabrication of photoelectrodes is usually completed with high temperature thermal annealing in a furnace. However, conventional thermal annealing (CTA) always results in undesirable crystal sintering, which reduces the surface area, and damage to the transparent conducting oxide (TCO) substrate. An emerging alternative method, hybrid microwave annealing (HMA), offers the beneficial effect of the high-temperature annealing (crystallinity) while minimizing its negative effects of sintering and TCO damage, enabling the fabrication of efficient (photo)electrodes for water splitting. HMA combines direct microwave heating with additional heating from an effective microwave absorber (called a susceptor), thereby avoiding a nonuniform temperature distribution between the interior and exterior of the synthesized material. More importantly, an extremely high temperature of the entire sample can be reached in only a few minutes. Compared with CTA, HMA has several advantages in the preparation of (photo)electrodes: (i) formation of a high-purity phase; (ii) high crystallinity with fewer defects; (iii) preservation of the original nanostructure; (iv) less damage to the TCO substrate for photoelectrodes; (v) smaller nanocrystals and uniform dispersion of catalyst particles. Overall, HMA is a convenient, ultrafast, and energy-economical technology for the synthesis of efficient (photo)electrodes. In this Account, we discuss recent progress made in our laboratory on HMA for preparing photoanodes (Fe2O3, BiVO4, ZnFe2O4, and Fe2TiO5), photocathodes (Cu2O and CuFeO2), and a graphene-based electrocatalyst (MoS2/graphene composite), which exhibit distinctive behavior and efficient performance in (photo)electrocatalytic water splitting. In particular, we have advanced the HMA technique further to synthesize hematite-based photoanodes with core-shell heterojunction nanorods (Nb,Sn:Fe2O3@FeNbO4 and Ta,Sn:Fe2O3@FeTaO4) by solid-solid interface reaction, which simultaneously achieves multiple doping effects (Nb or Ta, Sn) to improve the photoelectrocatalysis of water splitting. Thus, this Account focuses on the synthetic aspects of HMA, which may offer new research opportunities for the synthesis of other metal oxide (photo)electrode materials and hybrid electrocatalysts in the fields of solar energy conversion and storage, secondary batteries, and H2 fuel production.