Photocatalysis with Pt-Au-ZnO and Au-ZnO hybrids: Effect of charge accumulation and discharge properties of metal nanoparticles

Metal-semiconductor hybrid nanomaterials are becoming increasingly popular for photocatalytic degradation of organic pollutants. Herein, a seed-assisted photodeposition approach was put forward for the site-specific growth of Pt on Au-ZnO particles (Pt@s-Au-ZnO). A similar approach was also utilized to enlarge the Au nanoparticles at epitaxial Au-ZnO particles (Au@Au-ZnO). An epitaxial connection at the Au-ZnO interface was found to be critical for the site-specific deposition of Pt or Au. Light on-off photocatalysis tests, utilizing a thiazine dye (toluidine blue) as a model organic compound, were conducted and confirmed the superior photodegradation and mineralization properties of Pt-Au-ZnO hybrids compared to Au-ZnO. Albeit, Au-ZnO type hybrids were more effective toward two electron photoreduction of toluidine blue to leuco-toluidine blue. It was deemed that photoexcited electrons of Au-ZnO (Au, ~ 5 nm) possessed high reducing power owing to electron accumulation and negative shift in Fermi level/redox potential, however, exciton recombination due to possible Fermi level equilibration slowed down the complete degradation of toluidine blue. In case of Au@Au-ZnO (Au, ~15 nm), the photodegradation efficiency was enhanced, and photoreduction rate reduced compared to Au-ZnO. Pt@s-Au-ZnO hybrids showed better photodegradation and mineralization properties compared to both Au-ZnO and Au@Au-ZnO owing to a fast electron discharge. However, photoexcited electrons lacked the reducing power for the two electron photoreduction of toluidine blue. The ultimate photodegradation efficiency of Pt@-s-Au-ZnO, Au@Au-ZnO and Au-ZnO were 84 %, 66 % and 39 %, respectively. In the interest of effective metal-semiconductor type photocatalysts, the present study points out the importance of choosing the right metal, depending on whether a photoreduction and/or photodegradation process is desired.