Effect of particle size on the reactivity of quantum size ZnO nanoparticles and charge-transfer dynamics with adsorbed catechols

Abstract

The photophysics and reactivity of quantum size ZnO nanoparticles have been studied using static and time-resolved emission and absorption spectroscopy. Size quantization of ZnO nanoparticles is confirmed by steady-state optical absorption and emission spectroscopy. On band gap excitation, ZnO nanoparticles give both exciton and trap-state emission. The emission from ZnO nanoparticles is readily quenched by hole scavengers such as catechol (cat.) and naphthyl catechol (Ncat.). Quenching of ZnO emission has been attributed to the reactivity of ZnO nanoparticles with adsorbed catechols. Studies on particle size variation revealed that the reactivity of ZnO nanoparticles decreases as their size increases. The maximum reactivity of the excited ZnO nanoparticles with the catechols has been observed for the smallest size particles. Time-resolved emission studies of ZnO particles in the presence of the catechols confirmed that the catechols react in the same rate with both shallow and deeply trapped holes. We have also carried out time-resolved absorption studies of ZnO nanoparticles exciting at 355 nm laser light in the presence of the catechols. On laser excitation of ZnO nanoparticles, electrons and holes are generated. It has been observed that the holes react with the catechols and form oxidized species, which in turn convert to the phenoxyl radical of the catechols. The transient spectra of the phenoxyl radical of the catechols are confirmed by pulse radiolysis technique, which shows a strong absorbance peak in UV region and small shoulder at visible region for both the catechols. It has been observed that on the ZnO nanoparticle surface the spectrum of the phenoxyl radical of the catechols are little broader.