Metal-Particle-Decorated ZnO Nanocrystals: Photocatalysis and Charge Dynamics

Understanding of charge transfer processes is determinant to the performance optimization for semiconductor photocatalysts. As a representative model of composite photocatalysts, metal-particle-decorated ZnO has been widely employed for a great deal of photocatalytic applications; however, the dependence of charge carrier dynamics on the metal content and metal composition and their correlation with the photocatalytic properties have seldom been reported. Here, the interfacial charge dynamics for metal-decorated ZnO nanocrystals were investigated and their correspondence with the photocatalytic properties was evaluated. The samples were prepared with a delicate antisolvent approach, in which ZnO nanocrystals were grown along with metal particle decoration in a deep eutectic solvent. By modulating the experimental conditions, the metal content (from 0.6 to 2.3 at%) and metal composition (including Ag, Au, and Pd) in the resulting metal-decorated ZnO could be readily controlled. Time-resolved photoluminescence spectra showed that an optimal Au content of 1.3 at% could effectuate the largest electron transfer rate constant for Au-decorated ZnO nanocrystals, in conformity with the highest photocatalytic efficiency observed. The relevance of charge carrier dynamics to the metal composition was also inspected and realized in terms of the energy level difference between ZnO and metal. Among the three metal-decorated ZnO samples tested, ZnO–Pd displayed the highest photocatalytic activity, fundamentally according with the largest electron transfer rate constant deduced in carrier dynamics measurements. The current work was the first study to present the correlations among charge carrier dynamics, metal content, metal composition, and the resultant photocatalytic properties for semiconductor/metal heterostructures. The findings not only helped to resolve the standing issues regarding the mechanistic foundation of photocatalysis but also shed light on the intelligent design of semiconductor/metal composite systems to consolidate their utility in photocatalytic fields

Au@Cu₂O Core–Shell and Au@Cu₂Se Yolk–Shell Nanocrystals as Promising Photocatalysts in Photoelectrochemical Water Splitting and Photocatalytic Hydrogen Production

In this work, we demonstrated the practical use of Au@Cu2O core–shell and Au@Cu2Se yolk–shell nanocrystals as photocatalysts in photoelectrochemical (PEC) water splitting and photocatalytic hydrogen (H2) production. The samples were prepared by conducting a sequential ion-exchange reaction on a Au@Cu2O core–shell nanocrystal template. Au@Cu2O and Au@Cu2Se displayed enhanced charge separation as the Au core and yolk can attract photoexcited electrons from the Cu2O and Cu2Se shells. The localized surface plasmon resonance (LSPR) of Au, on the other hand, can facilitate additional charge carrier generation for Cu2O and Cu2Se. Finite-difference time-domain simulations were carried out to explore the amplification of the localized electromagnetic field induced by the LSPR of Au. The charge transfer dynamics and band alignment of the samples were examined with time-resolved photoluminescence and ultraviolet photoelectron spectroscopy. As a result of the improved interfacial charge transfer, Au@Cu2O and Au@Cu2Se exhibited a substantially larger photocurrent of water reduction and higher photocatalytic activity of H2 production than the corresponding pure counterpart samples. Incident photon-to-current efficiency measurements were conducted to evaluate the contribution of the plasmonic effect of Au to the enhanced photoactivity. Relative to Au@Cu2O, Au@Cu2Se was more suited for PEC water splitting and photocatalytic H2 production by virtue of the structural advantages of yolk–shell architectures. The demonstrations from the present work may shed light on the rational design of sophisticated metal–semiconductor yolk–shell nanocrystals, especially those comprising metal selenides, for superior photocatalytic applications

Electronic Interactions and Charge-Transfer Dynamics for a Series of Yolk–Shell Nanocrystals: Implications for Photocatalysis

In recent years, yolk–shell nanocrystals have become the spotlight of research worldwide because of the fascinating structural properties such as a permeable shell, an interior void space, and a movable yolk. Numerous studies have reported various compositions of yolk–shell nanocrystals. Among them, yolk–shell nanocrystals comprising metal yolk and semiconductor shells are particularly interesting because they can be geared to mass transport-related utilizations, for example, photocatalysis. We reported a sequential ion-exchange process to prepare for metal–semiconductor yolk–shell nanocrystals comprising Au yolk associated with various semiconductor shells. The synthetic procedures involved delicate sulfidation on a Au@Cu2O core–shell nanocrystal template, followed by a kinetically controlled cation-exchange reaction that enabled the conversion of the shell composition into various metal sulfides. Four representative yolk–shell nanocrystal samples, including Au@Cu7S4, Au@CdS, Au@ZnS, and Au@Ni3S4, were synthesized for investigation. X-ray photoelectron spectroscopy and photoluminescence spectroscopy were used to explore the electronic interactions and charge-transfer dynamics between the yolk and shell components. Results showed that interfacial charge transfer between the metal yolk and semiconductor shell was significant for the four yolk–shell nanocrystals, leading to pronounced charge-carrier separation that can be utilized to demonstrate a multitude of photocatalysis applications, including environmental purification, hydrogen production, and carbon dioxide reduction

Electronic Interactions and Charge-Transfer Dynamics for a Series of Yolk–Shell Nanocrystals: Implications for Photocatalysis

In recent years, yolk–shell nanocrystals have become the spotlight of research worldwide because of the fascinating structural properties such as a permeable shell, an interior void space, and a movable yolk. Numerous studies have reported various compositions of yolk–shell nanocrystals. Among them, yolk–shell nanocrystals comprising metal yolk and semiconductor shells are particularly interesting because they can be geared to mass transport-related utilizations, for example, photocatalysis. We reported a sequential ion-exchange process to prepare for metal–semiconductor yolk–shell nanocrystals comprising Au yolk associated with various semiconductor shells. The synthetic procedures involved delicate sulfidation on a Au@Cu2O core–shell nanocrystal template, followed by a kinetically controlled cation-exchange reaction that enabled the conversion of the shell composition into various metal sulfides. Four representative yolk–shell nanocrystal samples, including Au@Cu7S4, Au@CdS, Au@ZnS, and Au@Ni3S4, were synthesized for investigation. X-ray photoelectron spectroscopy and photoluminescence spectroscopy were used to explore the electronic interactions and charge-transfer dynamics between the yolk and shell components. Results showed that interfacial charge transfer between the metal yolk and semiconductor shell was significant for the four yolk–shell nanocrystals, leading to pronounced charge-carrier separation that can be utilized to demonstrate a multitude of photocatalysis applications, including environmental purification, hydrogen production, and carbon dioxide reduction