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Hydrogen production via electrochemical water splitting demands high working voltages (>1.23 ​V) and hence incurs high electricity costs, encumbering its large-scale applications. The development of more high-efficiency and electricity-saving systems for hydrogen production is of great significance. Hydrazine oxidation reaction (HzOR)-assisted hydrogen production technology, which is constructed by replacing the anodic oxygen evolution reaction in pure water electrolysis systems with the HzOR, can greatly reduce the working voltage and electricity consumption, and hence shows great application prospects. In recent years, numerous studies have focused on designing various bifunctional electrocatalysts to simultaneously catalyze the cathodic and anodic reactions in HzOR-assisted hydrogen production systems. However, comprehensive reviews summarizing and commenting on this field are scarce. This review provides a systematic and insightful overview of the developments in HzOR-assisted hydrogen production technology from 2017 to the present, primarily focusing on catalyst design strategies, catalytic mechanisms, and economic and application analysis. Additionally, this review discusses several challenges and outlines future research directions in this field to attract more researchers' attention and accelerate the research and potential applications of HzOR-assisted hydrogen production technology.

Abstract Epitaxial growth is one of the most commonly used strategies to precisely tailor heterostructures with well-defined compositions, morphologies, crystal phases, and interfaces for various applications. However, as epitaxial growth requires a small interfacial lattice mismatch between the components, it remains a challenge for the epitaxial synthesis of heterostructures constructed by materials with large lattice mismatch and/or different chemical bonding, especially the noble metal-semiconductor heterostructures. Here, we develop a noble metal-seeded epitaxial growth strategy to prepare highly symmetrical noble metal-semiconductor branched heterostructures with desired spatial configurations, i.e., twenty CdS (or CdSe) nanorods epitaxially grown on twenty exposed (111) facets of Ag icosahedral nanocrystal, albeit a large lattice mismatch (more than 40%). Importantly, a high quantum yield (QY) of plasmon-induced hot-electron transferred from Ag to CdS was observed in epitaxial Ag-CdS icosapods (18.1%). This work demonstrates that epitaxial growth can be achieved in heterostructures composed of materials with large lattice mismatches. The constructed epitaxial noble metal-semiconductor interfaces could be an ideal platform for investigating the role of interfaces in various physicochemical processes.

The electrochemical CO2 reduction reaction (CO2RR) offers a green and sustainable process to convert CO2 into valuable chemical stocks and fuels. Metal is one of the most promising types of catalysts to drive an efficient and selective CO2RR. The catalytic performance of metal nanocatalysts is strongly dependent on their structural features. Recently, phase engineering of nanomaterials (PEN) has emerged as a prominent tactic to regulate the catalytic performance of metal nanocatalysts for the CO2RR. A broad range of metal nanocatalysts with conventional and unconventional crystal phases has been developed, and remarkable achievements have been made. This review summarizes the most recent developments in phase engineering of metal nanocatalysts for the electrochemical CO2RR. We first introduce the different crystal phases of metal nanocatalysts used in the CO2RR and then discuss various synthetic strategies for unconventional phases of metal nanocatalysts. After that, detailed discussions of metal nanocatalysts with conventional and unconventional phases, including amorphous phases, are presented. Finally, the challenges and perspectives in this emerging area are discussed.

Abstract Phase engineering of nanomaterials opens a promising gateway to the construction of noble metal hierarchical heterostructures in a well‐defined manner. Here, by using zero‐dimensional (0D) Pd nanoparticles with hexagonal close‐packed (hcp, 2H type) phase, denoted as 2H‐Pd, as seeds, we report a facet‐selective epitaxial growth method to prepare two‐dimensional (2D)/0D Pd@Rh hierarchical heterostructures, in which two parallel triangular Rh nanoplates selectively grow on two opposite (002)h facets of 2H‐Pd due to the confined growth of Rh along h direction. Systematic characterizations demonstrate that a phase transformation from 2H phase to 2H/face‐centered cubic (fcc) heterophase occurs during the formation of such 2D/0D hierarchical heterostructure with the continuous growth of Rh nanoplates. The obtained 2D/0D Pd@Rh hierarchical heterostructures with a Pd/Rh atomic ratio of ∼39/61, denoted as Pd39@Rh61, exhibit excellent performance toward electrochemical hydrogen evolution reaction (HER) in acid electrolyte. To reach the current density of 10 mA cm–2, the overpotential of only 21.3 mV is required for the 2D/0D Pd39@Rh61, which is comparable to commercial Pt/C and also among the best Rh‐based HER catalysts reported until now. Key Points The rational synthesis of novel 2D/0D Pd@Rh hierarchical heterostructures via the facet‐selective epitaxial growth of Rh nanoplates on Pd nanoparticles with an unconventional 2H phase is realized. Two parallel triangular Rh nanoplates selectively grow on two opposite (002)h facets of 2H‐Pd due to the confined growth of Rh along h direction. 2D/0D Pd@Rh hierarchical heterostructures exhibit excellent performance toward electrochemical hydrogen evolution reaction in acid electrolyte.

The crystal phase-based heterostructures of noble metal nanomaterials are of interest for various applications. Here, the authors report the wet-chemical synthesis of gold nanorods with a well-defined fcc-2H-fcc heterophase, which possess unique optical and catalytic properties.

Elongated β-Si3N4 particles were prepared by gel-cast-foam-assisted combustion synthesis, which is a combination method of gel-casting-foam method and combustion synthesis. Gel-casting-foam method was used to prepare starting powder green body of high porosity and sufficient strength. Combustion synthesis reaction was conducted on the as-made green body. The effects of N2 pressure and diluent content on the combustion velocity, conversion rate and morphology of the resulting particles are studied. With high porosity (~80%), the conversion rate more than >90% was achieved at low N2 pressure (1.5 MPa) and low diluent content (even nil). In the epoxy composites, elongated β-Si3N4 powder was confirmed a better filler than its equiaxed counterpart in improving the thermal conductivities of the resultant composites. [ABSTRACT FROM AUTHOR]