High-energy-surface engineered metal oxide micro- and nanocrystallites and their applications

Because many physical and chemical processes occur at surfaces, surface atomic structure is a critical factor affecting the properties of materials. Due to the presence of high-density atomic steps and edges and abundant unsaturated coordination sites, micro- and nanocrystallites with high-energy surfaces usually exhibit greater reactivity than those with low-energy surfaces. However, high-energy crystal surfaces are usually lost during crystal growth as the total surface energy is minimized. Therefore, the selective exposure of high-energy facets at the surface of micro- and nanocrystallites is an important and challenging research topic. Metal oxides play important roles in surface-associated applications, including catalysis, gas sensing, luminescence, and antibiosis. The synthesis of metal oxide micro- and nanocrystallites with specific surfaces, particularly those with high surface energies, is more challenging than the synthesis of metal crystals due to the presence of strong metal-oxygen bonds and diverse crystal structures. In this Account, we briefly summarize recent progress in the surface-structure-controlled synthesis of several typical metal oxide micro- and nanocrystallites, including wurtzite ZnO, anatase TiO2, rutile SnO2, and rocksalt-type metal oxides. We also discuss the improvement of surface properties, focusing on high-energy surfaces. Because of the huge quantity and diverse structure of metal oxides, this Account is not intended to be comprehensive. Instead, we discuss salient features of metal oxide micro- and nanocrystallites using examples primarily from our group. We first discuss general strategies for tuning the surface structure of metal oxide micro- and nanocrystallites, presenting several typical examples. For each example, we describe the basic crystallographic characteristics as well as the thermodynamic (i.e., tuning surface energy) or kinetic (i.e., tuning reaction rates) strategies we have used to synthesize micro- and nanocrystallites with high surface energies. We discuss the structural features of the specific facets and analyze the basis for the enhanced performance of the metal oxide micro- and nanocrystallites in water splitting, the degradation of organic pollutants, gas sensing, catalysis, luminescence, and antibiosis. Finally, we note the future trends in high-energy-facet metal oxide micro- and nanocrystallite research. A comprehensive understanding of the properties of metal oxide micro- and nanocrystallites with high-energy crystal surfaces and related synthetic strategies will facilitate the rational design of functional nanomaterials with desired characteristics.