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Hot Carriers in Heterostructure Photocatalysis: History Edit
Subjects: Optics

1. Introduction

With the demand for an eco-friendly and efficient technology for solving the bottlenecks in the fields of environments and fuels, photocatalysis has been studied in the past several decades and has been gradually applied in practice [1,2,3,4,5]. To break through the limits of photon trapping and utilization in conventional semiconductor based photocatalysts [6,7], plasmon mediated photocatalysts, via the modification of plasmonic metal on a semiconductor, is an emerging field for meeting the high requirements of developing high-performance chemical compounds [8,9], new-type fuels [10,11], and green environmental treatment techniques [12,13]. In such a heterostructure, surface plasmons (SPs) [14,15,16], a collective-electrons oscillation confined to the surface of the conductive material excited by an external electromagnetic (EM) field, is generated not only as an optical pathway, which can localize incident light from a far-field at the surface of plasmonic materials and convert photons with lower energy (visible to near-infrared regions) that the semiconductor missed. However, more impressively, SPs can also act as electrical channels for the generation and transportation of HCs (electrons and holes) after light absorption for adjacent semiconductors, for participation in further applications, such as chemical reactions.
Currently, the main mechanisms reported in such plasmon enhanced photocatalysts include (1) light scattering/trapping [17,18], the radiative decay of plasmon to re-emit photons, (2) the process of plasmon induced resonance energy transferring (PIRET) [19], which needs spectrum overlapping between the plasmon and semiconductor, (3) localized electrical field enhancement [20,21], the mechanisms 2 and 3, which can be incorporated as near-field effects, and (4) HCs injections [22,23]. In the mechanisms of 1–3, direct contact is not necessary for the plasmon and semiconductor, and the enhancement is mainly tuned by the energy coupling of light field. In recent years, photocatalytic plasmon-mediated heterostructures based on the effects of HCs injections are becoming a hot topic [24,25,26], mainly from: (1) their capacity to extend the response spectrum of the photocatalyst, as the low energy of the photon beyond the band gap of the semiconductor can be utilized and adjusted adequately; and (2) the efficient suppression of the recombination of electron-hole pairs by the Schottky barrier, built by direct contact. However, the utilization efficiency of HCs that were reported in the past decades is extremely low due to loss of ultrafast electrons-electrons scattering [27,28]. Such bottlenecks mainly result from the low generation efficiency of HCs (inadequate absorption of the energy of photons) and the low extraction efficiency of HCs, such as interfacial defects. In the last studies, researchers have found that the utilization efficiency of HCs are not only related to the selection of plasmonic metals and the semiconductor [29], but also, more importantly, related to other new insights [30,31,32,33], including the polarization of excited light, the selection of a crystal face, the interface engineering and so on; these have emerged for exploring efficient photocatalysts.
Hence, we introduces the recent achievements based on HCs injection mechanisms, referred above, and classifies the optimized methods to design efficient photocatalysts. We will start with the typical process of photocatalytic reactions in a metal-semiconductor heterostructure, in which the intrinsic behaviors of HCs are emphasized. Then, the key factors, reported by the latest research for enhancing the utilization efficiency of HCs, are listed, in view of the process generation and extraction of HCs. In addition, photocatalytic applications for environmental protection based on SPs effects are introduced. Finally, a brief summary and perspectives are also illustrated.
In the past several years, with the development of advanced materials and microscopy, substantial progress has been realized to make deep insights into the kinetics of HCs in PMH based photocatalysts. The efforts for the design of high-performance PMH, including the compositions, morphology, and band structures between metals and semiconductors and the external EM field, have been sufficiently developed for the effective harvesting and transferring of photons. In this review, we presented the progress in the rational design of photocatalysts based on PMH in recent years. The “hot spots” structures, plasmonic cavity and hollow structures for obtaining photon trapping in a broad spectrum are introduced. To realize a fast transfer of HCs, some reliable methods, including clearer interfacial regions, the rational design of electrical transporting channel, a specific contact of the crystal face and the polarization of incident light, are individually detailed. In addition, the typical applications for environmental protections are also listed in order to give a brief framework for researchers to focus on in the present.
There are also some points to consider for future investigations. First, the exact physical model for the process relating to the generation, transportation and injections of HCs needs to be built up. There are arguments that the dominating contributions of electrons (interband transitions with a relatively low energy of electrons or intraband transitions induced by SPs) from photoexcited metals is the improvement of catalytic reactivity, and some works have given different opinions [134,135]. The distinguishing of non-thermal effects (HCs effects) and thermal effects, which allows us to quantify each contribution for the enhancement of the process of photocatalysis, is also of great importance for designing specific structures with different purposes [136,137,138]. Second, in the past years, although numerable technologies on the optimization and design of the rational employment of PMH have emerged [139,140,141,142,143], the crystal growth mechanisms and hetero-assembly techniques still need to be revealed clearly. Furthermore, the optical characteristics of PMH are studied mainly on a macroscopic level; for example, the absorption and reflection in self-assembly films, and the optical phenomena we observed, are integrated effects with millions of nanostructures. Additionally, the microscopic evidence of a single structure must be presented urgently for a comprehensive understanding of light-matter interactions [144,145,146]. In this regard, the in situ characterization technique must be further developed to help solve these limits. Practically, for rich functional systems, the cost, biocompatibility, stability and ease to integrate into other platforms must also be considered.