Clean and sustainable hydrogen generation renders a magnificent prospect to fulfill the humans' dream of rebuilding energy supplying systems that work eternally and run without pollution. Water electrolysis driven by a renewable resource of energy, such as wind and solar, is a promising pathway to achieve this goal, which requires highly active and cost‐effective electrode materials to be developed. In this comprehensive review, we introduce the utilization of hierarchical nanostructures in electrocatalytic and photoelectrochemical applications. The unique emphasis is given on the synthetic strategies of attaining these hierarchical structures as well as to demonstrate their corresponding mechanisms for performance improvement. Rather than simply discussing all the methods that can be used in nanofabrication, we focus on extracting the rules for structural design based on highly accessible and reliable methods. Examples are given to illustrate the versatility of these methods in the synthesis and manipulation of hierarchical nanostructures, which are concentrated on nonprecious transition metals or their alloys/compounds. Through this study, we aim to establish valuable guidelines and provide further insights for researchers to facilitate their design of more efficient water splitting systems in the future.

Controlled synthesis of lead halide perovskite (LHP) nanostructures not only benefits fundamental research but also offers promise for applications. Among many synthesis techniques, although catalytic vapor–liquid–solid (VLS) growth is recognized as an effective route to achieve high-quality nanostructures, until now, there is no detailed report on VLS grown LHP nanomaterials due to the emerging challenges in perovskite synthesis. Here, we develop a direct VLS growth for single-crystalline all-inorganic lead halide perovskite (i.e., CsPbX3; X = Cl, Br, or I) nanowires (NWs). These NWs exhibit high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 1012 Jones toward the visible light regime. Field-effect transistors (FET) based on individual CsPbX3 NWs are also fabricated, where they show the superior hole mobility of up to 3.05 cm2/(V s), higher than other all-inorganic LHP devices. This work provides important guidelines for the further improvement of these perovskite nanostructures for utilizations.

Because of sluggish kinetics of the oxygen evolution reaction (OER), designing low-cost, highly active, and stable electrocatalysts for OER is important for the development of sustainable electrochemical water splitting. Here, {112} high-index facet exposed porous CoO nanosheets with oxygen vacancies on the surface have been successfully synthesized via a simple hydrothermal method followed by NaBH reduction. As compared with the pristine and other faceted porous CoO nanosheets (e.g., {110} and {111}), the as-prepared {112} faceted porous nanosheets exhibit a much lower overpotential of 318 mV at a current density of 10 mA cm. Importantly, these nanosheets also give excellent electrochemical stability, displaying an insignificant change in the required overpotential at a current density of 10 mA cm even after a 14 h long-term chronoamperometric test. All these superior OER activity and stability could be attributed to their unique hierarchical structures assembled by ultrathin porous nanosheets, {112} high-index exposed facets with higher ratio of Co/Co and oxygen vacancies on the surface, which can substantially enhance the charge transfer rate and increase the number of active sites. All these findings not only demonstrate the potency of our CoO nanosheets for efficient water oxidation but also provide further insights into developing cost-effective and high-performance catalysts for electrochemical applications.

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