Imaging Photochemical Changes in Semiconductor Nanostructures and Their Impact on Catalytic Activity

Abstract:

Our research group uses light to both control the growth and composition of inorganic materials and to probe chemical transformations in semiconductor nanocrystals in situ. We are currently investigating how illumination can introduce crystal defects in inorganic nanostructures and how the concentration and distribution of these defects impacts the resulting electrocatalytic and photocatalytic activity of these materials. For example, oxygen vacancies have been shown to enhance the photocatalytic activity of several metal oxide photocatalysts, including tungsten oxide, titanium oxide, and bismuth oxyhalides. However, the interplay between changes in the electronic structure and surface composition induced by oxygen vacancies and the resulting photocatalytic activity is complex. I will discuss our progress in the use of chemically triggered fluorogenic probes to monitor changes in the photocatalytic activity of individual bismuth oxybromide (BiOBr) nanoplates using single-molecule, super-resolution fluorescence microscopy. Laser irradiation both excites electrons into the conduction band of BiOBr, leading to chemical activation of the fluorogenic probe, and creates oxygen vacancies in the nanoplates. We observe that the catalytic activities of individual nanoplates first increase and then progressively decrease under continuous laser irradiation. To understand the chemical origins of these photoinduced variations in activity we have used a combination of ensemble structural characterization, electronic-structure calculations, and the quantitative spatial correlation of multiple fluorogenic probes that report on orthogonal chemical transformations. While previous work has applied defect engineering to enhance the activity of bismuth oxyhalides and other semiconductor photocatalysts for useful reductive half-reactions (e.g., CO2 or N2 reduction), our results reveal that defect-free regions are needed to promote both oxidation and reduction in fuel-generating photocatalysts that do not rely on sacrificial reagents.