## 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, this paper 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.