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 is put forward for the site-specific growth of Pt on Au–ZnO particles (Pt–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 properties of Pt–Au–ZnO hybrids compared to Au–ZnO. In contrast, Au–ZnO type hybrids were more effective toward 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 the case of Au@Au–ZnO (Au, ∼15 nm), the photodegradation efficiency was enhanced and the photoreduction rate reduced compared to Au–ZnO. Pt–Au–ZnO hybrids showed better photodegradation and mineralization properties compared to both Au–ZnO and Au@Au–ZnO owing to a fast electron discharge (i.e. better electron-hole seperation). However, photoexcited electrons lacked the reducing power for the photoreduction of toluidine blue. The ultimate photodegradation efficiencies of Pt–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.