Restructuring is an important yet less understood phenomenon in the catalysis community. Recent studies have shown that a group of transition metal sulfide catalysts can completely or partially restructure during electrochemical reactions which then exhibit high activity even better than the best commercial standards. However, such restructuring processes and the final structures of the new catalysts are elusive, mainly due to the difficulty from the reaction-induced changes that cannot be captured by ex situ characterizations. To establish the true structure–property relationship in these in situ generated catalysts, we use multimodel operando characterizations including Raman spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity to investigate the restructuring of a representative catalyst, Co 9 S 8 , that shows better activity compared to the commercial standard RuO 2 during the oxygen evolution reaction (OER), a key half reaction in water-splitting for hydrogen generation. We find that Co 9 S 8 ultimately converts to oxide cluster (CoO x ) containing six oxygen coordinated Co octahedra as the basic unit which is the true catalytic center to promote high OER activity. The density functional theory calculations verify the in situ generated CoO x consisting of edge-sharing CoO 6 octahedral clusters as the actual active sites. Our results also provide insights to design other transition-metal-based materials as efficient electrocatalysts that experience a similar restructuring in OER.

Glucose detection is a highly important technique for clinical applications. This work reports a new strategy to synthesize the efficient electrocatalyst for electrochemical glucose detection. The electrocatalyst was fabricated by using the conformal film coating technology of atomic layer deposition (ALD) for a pre-electrocatalyst of NiS x , followed by an electrochemical oxidation process to afford Ni(OH) x in alkaline. The afforded Ni(OH) x is highly rough and electrocatalytically active, and it exhibits a superior performance for the glucose sensing. The electrocatalyst material, as well as the glucose sensing conditions, is carefully optimized on flat glassy carbon electrodes, and benefiting from the conformal coating ability of ALD, the material preparation method can be well transferred for a rough high-surface-area substrate. Using high-surface-area carbon cloth (CC) as the ALD substrates and electrodes, we can boost the glucose sensitivity by over 10 times as compared to that on flat glassy carbon substrates. The glucose sensitivity of the obtained Ni(OH) x /CC electrode can achieve as high as 19.42 ± 0.07 mA mM −1 cm −2 , with a decent linear response range of 0.004−0.6 mM and an LOD of 0.47 μM. The Ni(OH) x /CC electrodes are also shown of excellent fabrication reproducibility, good sensing selectivity, and good stability, which all make them highly attractive for practical sensing applications. These results highlight not only the excellent performance of the electrocatalyst for glucose detection but also the high promise of the ALD synthesis approach for fabricating electrochemical sensors in general.

Conspectus The synthesis of monodisperse colloidal nanomaterials with well-defined structures is important for both fundamental research and practical application. To achieve it, wet-chemical methods with the usage of various ligands have been extensively explored to finely control the structure of nanomaterials. During the synthesis, ligands cap the surface and thus modulate the size, shape, and stability of nanomaterials in solvents. Besides these widely investigated roles of ligands, it has been recently discovered that ligands can affect the phase of nanomaterials, i.e., their atomic arrangement, providing an effective strategy to realize the phase engineering of nanomaterials (PEN) by selecting appropriate ligands. Nanomaterials normally exist in the phases that are thermodynamically stable in their bulk states. Previous studies have shown that under high temperature or high pressure, nanomaterials can exist in unconventional phases which are unattainable in the bulks. Importantly, nanomaterials with unconventional phases exhibit unique properties and functions different from conventional-phase ones. Consequently, it is feasible to utilize the PEN to tune the physicochemical properties and application performance of nanomaterials. During wet-chemical synthesis, ligands binding to the surface of nanomaterials can modify their surface energy, which could significantly affect the Gibbs free energy of nanomaterials and thus determine the stability of different phases, making it possible to obtain nanomaterials with unconventional phases at mild reaction conditions. For instance, a series of Au nanomaterials with unconventional hexagonal phases have been prepared with the assistance of oleylamine. Therefore, the rational design and selection of different ligands and deep understanding of their effect on the phase of nanomaterials would significantly accelerate the development of PEN and the discovery of novel functional nanomaterials for diverse applications. In this Account, we briefly summarize the recent progress in ligand-assisted PEN, elaborating the important roles of different ligands in the direct synthesis of nanomaterials with unconventional crystal phases and amorphous phase as well as the phase transformation of nanomaterials. We first introduce the background of this research topic, highlighting the concept of PEN and why ligands can modulate the phase of nanomaterials. Then we discuss the usage of four kinds of ligands, i.e., amines, fatty acids, sulfur-containing ligands, and phosphorus-containing ligands, in phase engineering of different nanomaterials, especially metal, metal chalcogenide, and metal oxide nanomaterials. Finally, we provide our personal views of the challenges and future promising research directions in this exciting field.

Since the discovery of graphene, diverse kinds of 2D nanomaterials have been explored and exhibited great promise for application in electrochemical energy storage and conversion. However, the restacking of 2D nanomaterials severely reduces their exposed active sites and thus impairs their electrochemical performance. Moreover, except for graphene, a large number of 2D nanomaterials normally possess unsatisfactory electrical conductivity. One of the effective strategies to address the aforementioned shortcomings is to hybridize 2D nanomaterials with 3D graphene architectures since large specific surface area and rapid transport pathways for electrons, ions, and mass can be achieved in the obtained hybrid materials. This review summarizes the typical strategies to hybridize 2D nanomaterials with 3D graphene architectures and then highlights the application of these hybrid materials in rechargeable batteries, supercapacitors, and electrocatalytic water splitting. The challenges and future research directions in this research area are also discussed.

Abstract