Metal nanoparticles induced visible-light photocatalysis

(Nanowerk News) In the quest to solve solar energy conversion as well as environmental remediation issues, photocatalysis using sunlight have been attracting tremendous attention.
Various semiconductors with large band gaps have been proven to be effective under UV light, e.g., TiO2. However, UV light accounts for only ~4% while visible light occupies ~43% of total sunlight. From the perspective of both chemistry and practical applications, it is undoubtedly important to develop visible-light-responsive photocatalytic materials.
Proposed Mechanism for Photoelectrochemistry
Proposed mechanism for the photoelectrochemistry: charges are separated at a visible-light-irradiated Au NPs TiO2 system. (Image: Science China Press) (click on image to enlarge)
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 (National Science Review, "Metal nanoparticles induced photocatalysis"), mechanisms of metal-induced photocatalysis (MIP) were first summarized, e.g., hot-electron transfer (see Figure above).
Subsequently, the progress towards 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%). So, developing efficient metal semiconductor composite (MSC) materials is still highly needed 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, it is undoubted that particle-size effect was important even crucial 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 for its strong SPR intensity leading to high electron transfer efficiency, while some others pointed out that smaller ones were better because of more efficient charge separation could be achieved. Compromise viewpoint 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 and promising 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 discussed the challenges and possible development directions of MIP. It includes deeper understanding the mechanism behind MIP, further improve the efficiency, rational design and precise control of plasmonic metal etc.
Source: Science China Press