There is a growing interest in the conversion of water and solar energy into clean and renewable H₂ fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO₂ and pollutant degradation, and electrocatalysts for water splitting.

Recent progress in the preparation, properties and applications of black TiO₂nanomaterials is reviewed.

Over the past decades, there have been many projections on the future depletion of the fossil fuel reserves on earth as well as the rapid increase in green-house gas emissions. There is clearly an urgent need for the development of renewable energy technologies. On a different frontier, growth and manipulation of materials on the nanometer scale have progressed at a fast pace. Selected recent and significant advances in the development of nanomaterials for renewable energy applications are reviewed here, and special emphases are given to the studies of solar-driven photocatalytic hydrogen production, electricity generation with dye-sensitized solar cells, solid-state hydrogen storage, and electric energy storage with lithium ion rechargeable batteries.

The recent discovery of “black” TiO₂ nanoparticles with visible and infrared absorption has triggered an explosion of interest in the application of TiO₂ in a diverse set of solar energy systems; however, what a black TiO₂ nanoparticle really is remains a mystery. Here we elucidate more properties and try to understand the inner workings of black TiO₂ nanoparticles with hydrogenated disorders in a surface layer surrounding a crystalline core. Contrary to traditional findings, Ti³⁺ here is not responsible for the visible and infrared absorption of black TiO₂, while there is evidence of mid-gap states above the valence band maximum due to the hydrogenated, engineered disorders. The hydrogen atoms, on the other hand, can undergo fast diffusion and exchange. The enhanced hydrogen mobility may be explained by the presence of the hydrogenated, disordered surface layer. This unique structure thus may give TiO₂, one of the most-studied oxide materials, a renewed potential.