Enhancing Photocatalyst Efficiency through Type-II Core/Crown Nanoplatelets for Improved Charge Separation

Semiconductor nanocrystal-based photocatalysts have garnered significant attention for their potential applications in diverse fields, including hydrogen production, CO2 conversion, and pollutant removal. However, a major hurdle to improving their efficiency arises from the fact that the radiative recombination rate of semiconductor nanoparticles (~hundreds of nanoseconds) is much faster than the rates of photocatalytic reactions (~microseconds to ~milliseconds). This leads to the loss of the majority of charges before they can actively participate in the desired photocatalytic reactions, resulting in low efficiency. Therefore, it is crucial to develop structures that facilitate efficient charge separation to address this efficiency issue in photocatalysts. The most common approach to tackle this challenge involves utilizing core/shell structures to create a type-II band alignment structure. However, this approach has a drawback: while it enhances charge separation, it tends to trap electrons or holes within the core, limiting their participation in photocatalytic reactions.<br/><br/>In this research, we explore the influence of type-II core/crown nanoplatelets (NPLs) on photocatalytic reactions. These NPLs possess a type-II band alignment structure while exposing both the core and crown regions externally, enabling active participation of both electrons and holes in photocatalytic reactions. To maximize the efficiency of charge separation in this open structure, we prepared core/crown nanoplatelets with various compositions, including CdS, CdSe, and CdTe, while carefully controlling the surface area of both the core and crown regions.<br/><br/>We conducted femtosecond laser-based time-resolved photoluminescence measurements and transient absorption measurements to analyze the carrier dynamics of the prepared NPLs. Then, we performed water-splitting experiments under light irradiation to clarify the relationship between charge separation and photocatalytic activity. The results revealed that as the carrier lifetime of the photocatalyst increased, the hydrogen generation rate also significantly increased. This highlights the significance of the charge carrier lifetime as a pivotal parameter impacting photocatalytic activity.<br/><br/>Furthermore, to validate tthe significance of an open structure, such as the core/crown configuration, we prepared type-II core/shell quantum dots with the equivalent composition and similar lifetimes to the core/crown nanoplatelets. Surprisingly, despite the efficient charge separation structure of the type-II core/shell quantum dots, their photocatalytic efficiency decreased. These findings underscore the importance of an open structure that facilitates the active participation of both electrons and holes in photocatalytic reactions while concurrently prolonging the carrier lifetime, ultimately resulting in enhanced photocatalyst efficiency.