Metal nanoparticles induced visible-light photocatalysis

In the quest to solve solar energy conversion as well as environmental remediation issues, photocatalysis using sunlight has been attracting tremendous attention. Semiconductors with large band gaps have been proven effective under UV light, e.g., TiO2. However, UV light accounts for only ~4 percent while visible light occupies ~43 percent of total sunlight. From the perspective of both chemistry and practical applications, it is undoubtedly important to develop visible-light-responsive photocatalytic materials.

Over the past several years, coinage metal (Au, Ag and Cu) nanoparticles (NPs) photosensitization over semiconductors with a large band gap has emerged as a promising strategy for developing visible-light responsive photocatalytic materials. In this review, mechanisms of metal-induced photocatalysis (MIP) e.g., hot-electron transfer, were first summarized (see Figure 1). Subsequently, the progress toward MIP applications in photocatalytic and PEC water splitting, photoreduction of CO2 and activation of inert molecules such as CH4, N2 were reviewed. Generally, visible-light activity or enhancement was achieved after the introduction of these metal NPs. Nevertheless, for most present metal-induced photocatalytic water splitting under visible light, the obtained apparent quantum efficiency (AQE) was relatively low (i.e. <1 percent). So developing efficient metal semiconductor composite (MSC) materials is still required in this field. To highlight this point, the authors summarized important works in promoting the efficiency of MIP from perspective of achieving broadband or effective light-harvesting, enhancing charge-carrier separation, decoration with cocatalyst, etc.

On the other hand, the particle-size effect was important in MIP systems. Particular attention was paid on this issue and selected works were reviewed, though consensus has not been reached yet. Some researchers claimed that larger metal NPs were favorable, as their strong SPR intensity leads to high electron transfer efficiency, while some others pointed out that smaller ones were better because of their more efficient charge separation. Compromise viewpoints also existed, i.e., both small and large metal NPs were important. So more effort is needed on this issue.

Exploring light absorption of metal NPs in photocatalysis represents a class of novel approaches in exploring efficient visible-light responsive photocatalysts. However, to achieve this goal, there are still many challenges to be addressed. At the end of this review, the authors briefly discuss the challenges and possible development directions of MIP. It includes deeper understanding of the mechanism behind MIP, further improving the efficiency, rational design and precise control of plasmonic metal, etc.

More information:
Lequan Liu et al, Metal nanoparticles induced photocatalysis, National Science Review (2017). DOI: 10.1093/nsr/nwx019

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