Heterojunction configuration-specific photocatalytic degradation of methyl orange and methylene blue dyes using ZnO-based nanocomposites

Introduction

Heterostructured photocatalysts have shown an enormous potential in photocatalytic degradation of organic pollutants in wastewater. However, the efficacy of such heterojunction on the photocatalytic degradation behaviors has not yet been fully revealed.

Objectives

This work aims to demonstrate a specific photocatalytic degradation behavior of ZnO-based heterostructured nanocomposites toward methyl orange (MO) and methylene blue (MB) dyes based on a systematically comparative investigation for their physical and chemical properties.

Methods

A series of low-cost and efficient ZnO-based heterostructured nanocomposite photocatalysts including ZnO/CuO, ZnO/TiO₂ and ZnO/SnO₂ with 3 and 10 mol% of CuO/TiO₂/SnO₂ were synthesized by a simple strategy to combine the modified polymer-network gel and traditional sol–gel methods. The physical and chemical properties were analyzed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), X-ray photoelectron spectra (XPS), ultraviolet–visible (UV–Vis) absorption spectra, photoluminescence (PL), surface photovoltage (SPV), electrochemical impedance spectroscopy (EIS) and zeta potential.

Results

Owing to the fast interfacial charge transfer at the heterojunction, all the three ZnO-based nanocomposite catalysts exhibited higher efficient separation of photogenerated electrons and holes, delivering an enhanced photocatalytic activity for the degradation of organic dyes compared with pure ZnO. Three photocatalysts of ZnO/3 %-CuO, ZnO/3 %-TiO₂ and ZnO/10 %-SnO₂ (marking as ZC3, ZT3 and ZS10, respectively) were capable of achieving the complete degradation of 4 mg/L concentration of MB dye within 50 min, and the first two could degrade MO within 80 min. However, the degradation rate of MO by ZS10 became significantly slower. For MO and MB degradation, the active species of photogenerated holes (${\text{h}}_{h\nu }^{+}$${\text{h}}_{h\nu }^{+}$) and superoxide radicals ($·{\text{O}}_2^{-}$$·{\text{O}}_2^{-}$) play the predominant roles, respectively, followed by hydroxyl radicals ($·\text{OH}$$·\text{OH}$). The differences in heterojunction configuration and dominant active species result in a specific photocatalytic degradation behavior of ZnO-based composite nanostructures.

Conclusion

The generation of the active species are influenced by the heterojunction configurations, of which the essence is that the different band alignments can results in the differences of interfacial charge transfer behaviors, and thus selective generation of the active species such as ${\text{h}}_{h\nu }^{+}$${\text{h}}_{h\nu }^{+}$, $·{\text{O}}_2^{-}$$·{\text{O}}_2^{-}$ and $·\text{OH}$$·\text{OH}$. Importantly, this work offers a fundamental understanding for specific photocatalytic degradation of the different heterojunction nanostructures towards the different organic dyes.