Efficient Photocatalytic Degradation of Environmental Pollutants with Mass-Produced ZnS Nanocrystals

Photocatalytic degradation of water pollutants using nanometersized semiconductor colloids is an emerging area of environmental remediation. The synthesis of semiconductor nanocrystals (NCs), however, can be costly and result in low product yields. For large-scale photocatalytic application in environmental remediation, cost-effective production of the semiconductor NCs would be ideal. Demonstrated in this report is the efficient photocatalytic degradation of p-nitrophenol (pNP) and Acid Orange 7 (AO7) using ZnS nanocrystals (3 to 5 nm diameter) produced in gram quantities with >50% product yield. The pNP half-life in ZnS nanocrystal photocatalyzed reactions was about 1.95 to 2.45 min, whereas in comparable TiO₂ reactions, the pNP half-lives were in the range of 12 to 15 min. Absorption spectra of the photocatalysis reactions suggested the decolorization of pNP without any noticeable formation of phenolic intermediates, implying a mechanism that involves a pNP ring opening via a radical mediated attack. Likewise, the degradation of AO7 was suggested to occur via an oxidative pathway involving hydroxyl radicals formed at the photocatalyst/liquid interface. Optimum conditions for AO7 degradation such as pH, photocatalyst-to-AO7 ratio, and photocatalyst surface passivation were similar to those for pNP. By demonstrating efficient mineralization of these model pollutants using mass-produced ZnS nanocrystals, we hope to lay the foundations necessary for development of large-scale, field-applicable systems.