Recent Progress in Photocatalytic Hydrogen Evolution

Abstract

The photocatalytic hydrogen evolution reaction (PHER) has gained much attention as a promising strategy for the generation of clean energy. As opposed to conventional hydrogen evolution strategies (steam methane reforming, electrocatalytic hydrogen evolution, etc.), the PHER is an environmentally friendly and sustainable method for converting solar energy into H2 energy. However, the PHER remains unsuitable for industrial applications because of efficiency losses in three critical steps: light absorption, carrier separation, and surface reaction. In the past four decades, the processes responsible for these efficiency losses have been extensively studied. First, light absorption is the principal factor deciding the performance of most photocatalysts, and it is closely related to band-gap structure of photocatalysts. However, most of the existing photocatalysts have a wide bandgap, indicating a narrow light absorption range, which restricts the photocatalytic efficiency. Therefore, searching for novel semiconductors with a narrow bandgap and broadening the light absorption range of known photocatalysts is an important research direction. Second, only the photogenerated electrons and holes that migrate to the photocatalyst surface can participate in the reaction with H2O, whereas most of the photogenerated electrons and holes readily recombine with one another in the bulk phase of the photocatalysts. Hence, tremendous effort has been undertaken to shorten the charge transfer distance and enhance the electric conductivity of photocatalysts for improving the separation and transfer efficiency of photogenerated carriers. Third, the surface redox reaction is also an important process. Because water oxidation is a four-electron process, sluggish O2 evolution is the bottleneck in photocatalytic water splitting. The unreacted holes can easily recombine with electrons. Sacrificial agents are widely used in most catalytic systems to suppress charge carrier recombination by scavenging the photogenerated holes. Moreover, the low H2 evolution efficiency of most photocatalysts has encouraged researchers to introduce highly active sites on the photocatalyst surface. Based on the abovementioned three steps, multifarious strategies have been applied to modulate the physicochemical properties of semiconductor photocatalysts with the aim of improving the light absorption efficiency, suppressing carrier recombination, and accelerating the kinetics of surface reactions. The strategies include defect generation, localized surface plasmon resonance (LSPR), element doping, heterojunction fabrication, and cocatalyst loading. An in-depth study of these strategies provides guidance for the design of efficient photocatalysts. In this review, we focus on the mechanism and application of these strategies for optimizing light absorption, carrier separation and transport, and surface reactions. Furthermore, we provide a critical view on the promising trends toward the construction of advanced catalysts for H2 evolution.

Fluorinated TiO₂ Hollow Photocatalysts for Photocatalytic Applications

“…These templates are characteristic of narrow size distribution, ready availability in relatively large amounts, availability in a wide range of sizes from commercial sources, and simplicity of their synthesis using well-known procedures. In general, conventional templating methods involve the following three major steps 27,28 : (1) formation of monodispersed template; (2) uniform coating the templates with desired materials or their precursors; and (3) selective removal of the templates to obtain hollow structures. Note that, in most cases, in order to achieve uniform coating in step 2, modification of the template with specific functional groups and/or surface characteristics, including surface charges and porosity, are carried out in advance.…”

“…Advanced photocatalytic technique is one of the most promising strategies in employing solar energy to solve increasingly serious energy crisis and environmental pollution 1,2 . To this end, exploiting advanced semiconductor photocatalysts is a primary step to develop practical and efficient photocatalytic techniques.…”

High Efficiency Electron Transfer Realized over NiS₂/MoSe₂ S-Scheme Heterojunction in Photocatalytic Hydrogen Evolution

“…Semiconductor materials occupied an important position in the field of photocatalysis 11 . Precious metals with excellent electron-transporting capabilities were not conducive to the development of industry because of the relatively large cost of testing, and semiconductor materials have become the main direction dedicated to research by researchers [12][13][14][15] .…”

Carboxyl-Functionalized Graphene for Highly Efficient H₂-Evolution Activity of TiO₂ Photocatalyst

“…Photocatalytic H2 production utilizing semiconductor photocatalytic material is a greatly prospective way toward sustainable and clean energy sources over future energy demands [1][2][3][4][5][6][7][8] . The well-known TiO2 is regarded as a prospective photocatalytic material owing to its chemical stability and nontoxicity [9][10][11][12][13][14] . Nonetheless, pure TiO2 suffers from the speedy quenching of photoexcited carriers to exhibit a finite hydrogenproduction performance [15][16][17][18] .…”