Separation of charge carriers and generation of reactive oxygen species by TiO₂ nanoparticles mixed with differently-coated gold nanorods under light irradiation

Abstract

Combinations of semiconductor nanoparticles (NPs) with noble metal NPs enable an increase in the photoactivity of semiconductor NPs into the visible and near-infrared regions. The design rationale of the semiconductor-metal hybrid nanostructures for the optimization of charge carrier separation and reactive oxygen species (ROS) generation remains unclear. In this study, the interactions of Au nanorods (AuNRs) with TiO₂ NPs were modulated by controlling their surface charges. Positively charged AuNRs formed aggregates with the negatively charged TiO₂ NPs (AuNR@CTAB/TiO₂) upon mixing, suggesting that Schottky junctions may exist between Au and TiO₂. In contrast, negatively charged AuNRs (AuNR@PSS) remained spatially separated from the TiO₂ NPs in the mixed suspension (AuNR@PSS/TiO₂), owing to electrostatic repulsion. We used electron spin resonance (ESR) spectroscopy to detect the separation of charged carriers and ROS generation in these two mixtures under simulated sunlight irradiation. We also explored the role of dissolved oxygen in charge carrier separation and ROS generation by continuously introducing oxygen into the AuNR@CTAB/TiO₂ suspension under simulated sunlight irradiation. Moreover, the generation of ROS by the AuNR@CTAB/TiO₂ and AuNR@PSS/TiO₂ mixtures were also examined under 808 nm laser irradiation. Our results show that the photogenerated electrons of excited semiconductor NPs are readily transferred to noble metal NPs simply by collisions, but the transfer of photogenerated hot electrons from excited AuNRs to TiO₂ NPs is more stringent and requires the formation of Schottky junctions. In addition, the introduction of oxygen is an efficient way to enhance the photocatalytic activity of semiconductor NPs/noble metal NPs system combinations.

Keywords:

• gold nanorods
• TiO2 nanoparticles
• light irradiation
• electron transfer
• oxygen consumption

Acknowledgments

This paper is not an official U.S. FDA guidance or policy statement. No official support or endorsement by the U.S. FDA is intended or should be inferred.

Additional information

Funding

This work was supported by the National Key Basic Research Program of China (2016YFA0200903 and 2011CB932802) and a regulatory science grant under the FDA Nanotechnology CORES Program.