Search

Structural engineering of nanomaterials offers a promising way for developing high-performance catalysts toward catalysis. However, the delicate modulation of thermodynamically unfavorable nanostructures with unconventional phases still remains a challenge. Here, the synthesis of hierarchical AuCu nanostructures is reported with hexagonal close-packed (2H-type)/face-centered cubic (fcc) heterophase, high-index facets, planar defects (e.g., stacking faults, twin boundaries, and grain boundaries), and tunable Cu content. The obtained 2H/fcc Au99Cu1 hierarchical nanosheets exhibit excellent performance for the electrocatalytic CO2 reduction to produce CO, outperforming the 2H/fcc Au91Cu9 and fcc Au99Cu1. The experimental results, especially those obtained by in-situ differential electrochemical mass spectroscopy and attenuated total reflection Fourier-transform infrared spectroscopy, suggest that the enhanced catalytic performance of 2H/fcc Au99Cu1 arises from the unconventional 2H/fcc heterophase, high-index facets, planar defects, and appropriate alloying of Cu. Impressively, the 2H/fcc Au99Cu1 shows CO Faradaic efficiencies of 96.6% and 92.6% at industrial current densities of 300 and 500 mA cm−2, respectively, as well as good durability, placing it among the best CO2 reduction electrocatalysts for CO production. The atomically structural regulation based on phase engineering of nanomaterials (PEN) provides an avenue for the rational design and preparation of high-performance electrocatalysts for various catalytic applications. © 2023 Wiley-VCH GmbH.

ConspectusAs an important parameter of crystalline materials, the crystal phase describes the periodic atomic arrangement in their structures. For some monoelemental materials, e.g., carbon and phosphorus, they can exist in more than one crystal phase. The different crystal phases of monoelemental materials result in different physicochemical properties and functions. Therefore, engineering the crystal phase of monoelemental materials gives an effective strategy to modulate their properties and functions. Conventionally, the crystal phase of monoelemental materials can be altered under some harsh conditions, e.g., high temperature and high pressure. Recently, with the rapid development of nanotechnology, various monoelemental materials with unconventional crystal phases have been well developed on the nanoscale. For example, our group has successfully achieved the synthesis of Au nanomaterials with unconventional hexagonal close-packed (hcp) 2H and 4H phases by wet-chemical methods, which exhibit distinct optical properties as well as outstanding electrocatalytic performance when compared to those with a thermodynamically stable face-centered cubic (fcc) phase.In this Account, we give a comprehensive overview of the recent development of monoelemental nanomaterials with unconventional crystal phases and their crystal-phase-dependent properties and applications. We first introduce the typical strategies for the synthesis of monoelemental nanomaterials with unconventional crystal phases. By using a wet-chemical reduction method, template-assisted method, and thermal annealing method, monoelemental nanomaterials with unconventional crystal phases can be directly prepared. Besides, unconventional-phase monoelemental nanomaterials can also be obtained via the phase transformation from materials with conventional crystal phases under specific conditions. In addition, some other methods have also been reported for preparing monoelemental nanomaterials with unconventional c

Controlled construction of bimetallic nanostructures with a well-defined heterophase is of great significance for developing highly efficient nanocatalysts and investigating the structure-dependent catalytic performance. Here, a wet-chemical synthesis method is used to prepare Au@Pd core-shell nanorods with a unique fcc-2H-fcc heterophase (fcc: face-centered cubic; 2H: hexagonal close-packed with a stacking sequence of "AB"). The obtained fcc-2H-fcc heterophase Au@Pd core-shell nanorods exhibit superior electrocatalytic ethanol oxidation performance with a mass activity as high as 6.82 A mgPd-1, which is 2.44, 6.96, and 6.43 times those of 2H-Pd nanoparticles, fcc-Pd nanoparticles, and commercial Pd/C, respectively. The operando infrared reflection absorption spectroscopy reveals a C2 pathway with fast reaction kinetics for the ethanol oxidation on the prepared heterophase Au@Pd nanorods. Our experimental results together with density functional theory calculations indicate that the enhanced performance of heterophase Au@Pd nanorods can be attributed to the unconventional 2H phase, the 2H/fcc phase boundary, and the lattice expansion of the Pd shell. Moreover, the heterophase Au@Pd nanorods can also serve as an efficient catalyst for the electrochemical oxidation of methanol, ethylene glycol, and glycerol. Our work in the area of phase engineering of nanomaterials (PENs) opens the way for developing high-performance electrocatalysts toward future practical applications. ©

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 <002>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.

With the development of phase engineering of nanomaterials (PEN), construction of noble-metal heterostructures with unconventional crystal phases, including heterophases, has been proposed as an attractive approach toward the rational design of highly efficient catalysts. However, it still remains challenging to realize the controlled preparation of such unconventional-phase noble-metal heterostructures and explore their crystal-phase-dependent applications. Here, various Pd@Ir core–shell nanostructures are synthesized with unconventional fcc-2H-fcc heterophase (2H: hexagonal close-packed; fcc: face-centered cubic) through a wet-chemical seeded method. As a result, heterophase Pd66@Ir34 nanoparticles, Pd45@Ir55 multibranched nanodendrites, and Pd68@Ir22Co10 trimetallic nanoparticles are obtained via the phase-selective epitaxial growth of fcc-2H-fcc-heterophase Ir-based nanostructures on 2H-Pd seeds. Importantly, the heterophase Pd45@Ir55 nanodendrites exhibit excellent catalytic performance toward electrochemical hydrogen evolution reaction (HER) under acidic conditions. An overpotential of only 11.0 mV is required to achieve a current density of 10 mA cm−2 on Pd45@Ir55 nanodendrites, which is lower than those of the conventional fcc-Pd47@Ir53 counterparts, commercial Ir/C and Pt/C. This work not only demonstrates an appealing route to synthesize novel heterophase nanomaterials for promising applications in the emerging field of PEN, but also highlights the significant role of the crystal phase in determining their catalytic properties. © 2021 Wiley-VCH GmbH

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. © 2022 Wiley-VCH GmbH.

Controlling the architectures and crystal phases of metal@semiconductor heterostructures is very important for modulating their physicochemical properties and enhancing their application performances. Here, a facile one-pot wet-chemical method to synthesize three types of amorphous SnO2-encapsulated crystalline Cu heterostructures, i.e., hemicapsule, yolk–shell, and core–shell nanostructures, in which unconventional crystal phases (e.g., 2H, 4H, and 6H) and defects (e.g., stacking faults and twin boundaries) are observed in the crystalline Cu cores, is reported. The hemicapsule Cu@SnO2 heterostructures, with voids that not only expose the Cu core with unconventional phases but also retain the interface between Cu and SnO2, show an excellent electrocatalytic CO2 reduction reaction (CO2RR) selectivity toward the production of CO and formate with high Faradaic efficiency (FE) above 90% in a wide potential window from −1.05 to −1.55 V (vs reversible hydrogen electrode (RHE)), and the highest FE of CO2RR (95.3%) is obtained at −1.45 V (vs RHE). This work opens up a new way for the synthesis of new heterostructured nanomaterials with promising catalytic application. © 2022 Wiley-VCH GmbH.

Controlled construction of bimetallic nanostructures with a well-defined heterophase is of great significance for developing highly efficient nanocatalysts and investigating the structure-dependent catalytic performance. Here, a wet-chemical synthesis method is used to prepare Au@Pd core-shell nanorods with a unique fcc-2H-fcc heterophase (fcc: face-centered cubic; 2H: hexagonal close-packed with a stacking sequence of "AB"). The obtained fcc-2H-fcc heterophase Au@Pd core-shell nanorods exhibit superior electrocatalytic ethanol oxidation performance with a mass activity as high as 6.82 A mgPd-1, which is 2.44, 6.96, and 6.43 times those of 2H-Pd nanoparticles, fcc-Pd nanoparticles, and commercial Pd/C, respectively. The operando infrared reflection absorption spectroscopy reveals a C2 pathway with fast reaction kinetics for the ethanol oxidation on the prepared heterophase Au@Pd nanorods. Our experimental results together with density functional theory calculations indicate that the enhanced performance of heterophase Au@Pd nanorods can be attributed to the unconventional 2H phase, the 2H/fcc phase boundary, and the lattice expansion of the Pd shell. Moreover, the heterophase Au@Pd nanorods can also serve as an efficient catalyst for the electrochemical oxidation of methanol, ethylene glycol, and glycerol. Our work in the area of phase engineering of nanomaterials (PENs) opens the way for developing high-performance electrocatalysts toward future practical applications. ©

Crystal phase engineering of noble-metal-based alloy nanomaterials paves a new way to the rational synthesis of high-performance catalysts for various applications. However, the controlled preparation of noble-metal-based alloy nanomaterials with unconventional crystal phases still remains a great challenge due to their thermodynamically unstable nature. Herein, we develop a robust and general seeded method to synthesize PdCu alloy nanomaterials with unconventional hexagonal close-packed (hcp, 2H type) phase and also tunable Cu contents. Moreover, galvanic replacement of Cu by Pt can be further conducted to prepare unconventional trimetallic 2H-PdCuPt nanomaterials. Impressively, 2H-Pd67Cu33 nanoparticles possess a high mass activity of 0.87 A mg–1Pd at 0.9 V (vs reversible hydrogen electrode (RHE)) in electrochemical oxygen reduction reaction (ORR) under alkaline condition, which is 2.5 times that of the conventional face-centered cubic (fcc) Pd69Cu31 counterpart, revealing the important role of crystal phase on determining the ORR performance. After the incorporation of Pt, the obtained 2H-Pd71Cu22Pt7 catalyst shows a significantly enhanced mass activity of 1.92 A mg–1Pd+Pt at 0.9 V (vs RHE), which is 19.2 and 8.7 times those of commercial Pt/C and Pd/C, placing it among the best reported Pd-based ORR electrocatalysts under alkaline conditions.