A Comparative Study of Gold Impregnation Methods for Obtaining Metal/Semiconductor Nanophotocatalysts: Direct Turkevich, Inverse Turkevich, and Progressive Heating Methods

Mayra Matamoros-Ambrocio,Ernesto Chigo-Anota,Armando Pérez-Centeno,Alejandro Escobedo-Morales,María Ruiz-Peralta,Jesús García-Serrano,Manuel Sánchez-Cantú,Efraín Rubio-Rosas

doi:10.3390/catal8040161

Abstract

ZnO nanostructures decorated with gold nanoparticles (Au-NPs) were synthesized by thermal decomposition of ZnO2 powders and their subsequent impregnation of metal nanoparticles using either the Direct Turkevich Method, the Inverse Turkevich Method, or the Progressive Heating Method. It was found that the impregnation approach influences the resulting microstructure and photocatalytic activity of the obtained materials. While the Direct Turkevich approach gave the highest yield of metal loading, the smallest Au-NPs were obtained by Inverse Turkevich and the Progressive Heating Method. The photocatalytic activity of the pristine support and gold-loaded samples was studied in the decolorization of Rhodamine B solutions using UV- and pure visible-light illumination. All Au-NPs/ZnO samples showed higher photocatalytic activity than the bare support when UV-light was used. This effect is attributed to a charge carrier separation due to electron transfer from ZnO to the metal nanoparticles and the built-in electric field at the interfaces. Contrarily to most reports, visible-light sensitization using plasmonic nanoparticles was not observed. The experimental evidence points against hot-electron injection from Au-NPs to the semiconductor component. This behavior is associated with the height of the Schottky barrier at the metal-semiconductor junctions. The differences in the photocatalytic performance among the samples under UV- and visible-light are explained in terms of the characteristics of the Au-NPs driven by the growth mechanism involved in each impregnation method and the physicochemical properties of the generated interfaces.

Highlights

During the last decades, the water demand associated with anthropogenic activities has increased notoriously
Heterogeneous photocatalysis using semiconductor compounds has proven to be an efficient approach for eliminating recalcitrant pollutants that cannot be removed by other physical, chemical, or biological methods [1,2,3]
Among the most extensively studied photocatalytic semiconductor compounds is zinc oxide (ZnO). This is mainly because the redox potentials to produce O2 − and OH radicals lie between its valence and conduction bands levels [4]

Summary

Introduction

The water demand associated with anthropogenic activities has increased notoriously. The concentration and diversity of pollutants contained in wastewaters have reached alarming levels This situation is compromising the potable water availability worldwide; the development of wastewater treatment technologies capable of handling large volumes at lower time and cost has become a priority. Two different approaches have been explored to overcome this difficulty: (1) modifying the overall composition to introduce allowed states in the forbidden band gap (doping) or even reduce it (solid solutions); and (2) generating heterojunctions with a visible light-harvesting component (e.g., quantum-dots, plasmonic nanoparticles, narrow band gap semiconductors, or dyes) [5,6] In this sense, a notorious increase of the photocatalytic activity of ZnO structures by decorating its surface with gold nanoparticles (Au-NPs) has been reported widely [7,8]

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Results