Developing noble metal-free water oxidation catalysts is essential for many energy conversion/storage processes (e.g., water splitting). Herein, we report the facile synthesis of hollow Co₃O₄ microspheres composed of porous, ultrathin (<5 nm), single-crystal-like nanosheets via a novel “self-template” route. The successful preparation of these hollow Co₃O₄ nanomaterials includes three main steps: (1) the synthesis of solid cobalt alkoxide microspheres, (2) their subsequent self-template conversion into hollow cobalt hydroxide microspheres composed of ultrathin nanosheets, and finally (3) thermal treatment of hollow cobalt hydroxide microspheres into the hollow Co₃O₄ material. The as-obtained hollow Co₃O₄ nanomaterial possesses a high BET surface area (∼180 m² g⁻¹), and can serve as an active and stable water oxidation catalyst under both electrochemical and photochemical reaction conditions, owing to its unique structural features. In the electrochemical water oxidation, this catalyst affords a current density of 10 mA cm⁻² (a value related to practical relevance) at an overpotential of ∼0.40 V. Moreover, with the assistance of a sensitizer [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine), this nanomaterial can catalyze water oxidation reactions under visible light irradiation with an O₂ evolution rate of ∼12 218 μmol g⁻¹ h⁻¹. Our results suggest that delicate nanostructuring can offer unique advantages for developing efficient water oxidation catalysts.

Understanding the sensing mechanism of metal oxide semiconductors is imperative to the development of high-performance sensors. The traditional sensing mechanism only recognizes the effect of surface chemisorbed oxygen from the air but ignores surface lattice oxygen. Herein, using in-situ characterizations, we provide direct experimental evidence that the surface chemisorbed oxygen participated in the sensing process can come from lattice oxygen of the oxides. Further density functional theory (DFT) calculations prove that the p-band center of O serves as a state of art for regulating the participation of lattice oxygen in gas-sensing reactions. Based on our experimental data and theoretical calculations, we discuss mechanisms that are fundamentally different from the conventional mechanism and show that the easily participation of lattice oxygen is helpful for the high response value of the materials.

Layered double hydroxide has been used in a variety of areas, including but not limited to catalysis, energy storage, drug or gene delivery, water treatment, etc. Herein, we report a new simple hydrothermal method to prepare a high surface area flower-like Ni–Fe layered double hydroxide (LDH) assembled by nanosheets by using nickel alkoxide and FeSO₄ as the only starting materials. It is free of alkaline solution and other additives for directing or supporting in the synthesis procedure. The formation mechanism of this flower-like LDH formed by ultrathin nanosheets is also discussed. Moreover, the as-obtained LDH material shows increased electrocatalytic activity and stability toward WOR in alkaline media compared with the materials prepared without a Ni alkoxide precursor or Fe precursor, namely α-Fe₂O₃ and Ni(OH)₂, respectively. In addition, the electrocatalytic activity is demonstrated to be related to the molar ratio of Fe and Ni in the final Ni–Fe material, and the best activity is achieved when the ratio reaches 0.52 : 1. The phase compositions of the resulting Ni–Fe_x are discussed. Furthermore, the Ni–Fe LDH material reported herein might be employed as a promising noble-metal-free water oxidation catalyst to replace the IrO_x material—the state-of-the-art water oxidation catalyst.

Design and synthesis of efficient noble metal-free hydrogen evolution catalysts is of paramount importance for the practical application of water-splitting devices. Herein, we report a novel synthetic method to grow dispersed molybdenum carbide (Mo₂C) micro-islands on flexible carbon cloth (CC). This method involves the controlled synthesis of a supramolecular hybrid between cetyltrimethyl ammonium cations and molybdate anions on CC, followed by simple thermal treatment of this supramolecular hybrid in Ar to form Mo₂C on CC in situ. In this synthesis, the presence of cetyltrimethyl ammonium bromide is proven to be important because it effectively immobilizes molybdate ions on CC on the one hand and functions as a carbon source for the formation of Mo₂C on the other. Moreover, the as-prepared Mo₂C/CC composite material can serve as efficient binder-free cathodes toward the hydrogen evolution reaction (HER). The Mo₂C/CC affords a current density of 10 mA cm⁻² at a low overpotential of 140 mV and works stably in acidic media with a Faraday yield of ∼100%. The isolated island architecture of Mo₂C ensures rich active sites to be exposed and allows the easy interaction of reactants (e.g., protons) with the active sites. Also, the strong adhesion between Mo₂C and carbon cloth facilitates electron transport/transfer in the composite material and is helpful for the achievement of excellent catalytic stability.

This review summarizes characterization techniques, modification methods and recent mechanistic (oxygen participation roles) observations for metal–oxide semiconductor-based gas sensor materials and future prospects for advancing sensor technology.