A Generalizable Top-Down Nanostructuring Method of Bulk Oxides: Sequential Oxygen-Nitrogen Exchange Reaction.

A thermal reaction route that induces grain fracture instead of grain growth is devised and developed as a top-down approach to prepare nanostructured oxides from bulk solids. This novel synthesis approach, referred to as the sequential oxygen-nitrogen exchange (SONE) reaction, exploits the reversible anion exchange between oxygen and nitrogen in oxides that is driven by a simple two-step thermal treatment in ammonia and air. Internal stress developed by significant structural rearrangement via the formation of (oxy)nitride and the creation of oxygen vacancies and their subsequent combination into nanopores transforms bulk solid oxides into nanostructured oxides. The SONE reaction can be applicable to most transition metal oxides, and when utilized in a lithium-ion battery, the produced nanostructured materials are superior to their bulk counterparts and even comparable to those produced by conventional bottom-up approaches. Given its simplicity and scalability, this synthesis method could open a new avenue to the development of high-performance nanostructured electrode materials that can meet the industrial demand of cost-effectiveness for mass production.

The influence of surface area, porous structure, and surface state on the supercapacitor performance of titanium oxynitride: implications for a nanostructuring strategy.

A recent surge of interest in metal (oxy)nitride materials for energy storage devices has given rise to the rapid development of various nanostructuring strategies for these materials. In supercapacitor applications, early transition metal (oxy)nitrides have been extensively explored, among which titanium oxynitride stands out due to its great potential for charge storage. Despite recent advances in supercapacitors based on titanium oxynitride, many underlying factors governing their capacitive performance remain elusive. In this work, nanostructured titanium oxynitride is prepared by firing an organic-inorganic hybrid precursor under a hot ammonia atmosphere, and the influence of its physical characteristics on the supercapacitor performance is investigated. New insights into the effects of surface area, porous structure, and surface state of titanium oxynitride on the supercapacitor performance are revealed through which a comprehensive understanding about the capacitive behavior of titanium oxynitride is provided. In addition, the implications of these insights for a nanostructuring strategy striving for higher capacitance and improved stability are discussed.

Control of morphology and defect density in zinc oxide for improved dye-sensitized solar cells.

While zinc oxide (ZnO) with a mesoporous network has long been explored for adsorption of dyes and as an electron-transporting medium in dye-sensitized solar cells (DSSCs), the performance of ZnO-based DSSCs remains unsatisfactory. Despite the importance of understanding the surface characteristics of ZnO in DSSC applications, most of the studies relevant to ZnO-based DSSCs are focused on the synthesis of unique nanostructures of ZnO. In this study, we not only introduce a novel disk-shaped ZnO nanostructure, but also provide insight into the distinctive surface properties of ZnO and its influence on DSSC performance. When utilized in DSSCs, the mesoporous ZnO nanodisk yields 60% better power conversion efficiency (PCE) compared to commercial ZnO nanoparticles, which is attributed to less surface and bulk trap densities as concluded by an in-depth open-circuit voltage decay (OCVD) analysis and electrochemical impedance spectroscopy (EIS). Another aspect that contributes to the higher PCE is the better connectivity of primary particles that join together to form secondary disk-shaped particles. Furthermore, a 40% improvement in the PCE was observed by coating the mesoporous ZnO nanodisk with TiO2, which results from the passivation of the surface defects that aid in suppressing the interfacial charge recombination.

Nanostructuring Matters: Stabilization of Electrocatalytic Oxygen Evolution Reaction Activity of ZnCo2O4 by Zinc Leaching.

Despite the enormous attention paid to cobalt oxide materials as efficient water splitting electrocatalysts, a deep understanding of their activity discrepancy is still elusive. In this work, we showed that stabilization of the internally generated oxygen evolution reaction (OER) active phase (oxyhydroxide) is crucial for ZnCo2O4 electrocatalysts. A systematic evaluation of the bulk and nanostructured ZnCo2O4 system concomitant with nanostructured Co3O4 showed that leaching of Zn is the driving force behind the near-surface transformation to the oxyhydroxide phase. The relative contribution to this near-surface reconstruction was found to be surface-sensitive. The electrochemical observations combined with Raman and impedance spectroscopy revealed that the good catalytic activity could be attributed to the formation of the cobalt oxyhydroxide phase, which was created by the dissolution of Zn from the nanostructured surface. Moreover, this study sheds light on previous contradicting postulates regarding the discrepancy of the OER activity of ZnCo2O4. Our finding regarding the formation of the OER active phase in spinel Zn-Co oxide will motivate researchers to focus more on the near-surface reconstruction behavior of cobalt-based oxide electrocatalysts in the future.