Designing ZnS decorated reduced graphene-oxide nanohybrid via microwave route and their application in photocatalysis

Highlights

• ZnS decorated rGO nanohybrid materials were prepared via microwave route.

• HR-TEM micrographs revealed the uniform distribution of ZnS on rGO nanosheets.

• ZnS-rGO nanohybrid exhibits superior photocatalytic activity than bare ZnS.

• The role of rGO in photocatalysis was explained using photoluminescence spectra.

• The role of radicals in the photocatalysis was examined using radical scavengers.

Abstract

In this study, we demonstrated the facile design of zinc-sulphide decorated reduced graphene-oxide (ZnS-rGO) nanohybrid via microwave method and examined their photocatalytic properties. The physico-chemical properties of the ZnS-rGO were analysed using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, high resolution scanning electron microscopy, energy dispersive X-ray spectroscopy and laser Raman analyses, respectively. The photocatalytic activity of the prepared ZnS-rGO nanohybrid was examined by the degradation of two model dyes Methylene blue (MB) and Rhodamine-B (RhB). The experimental results suggested that the designed ZnS-rGO nanohybrid possess superior photocatalytic activity with 1.47 and 2.92 fold higher reaction rates for MB and RhB degradation than that of the pure ZnS nanoparticles. A plausible mechanism for the enhanced properties of ZnS-rGO nanohybrid was discussed using photoluminescence spectra. Further, the role of reactive oxygen species on the photocatalytic properties of ZnS-rGO nanohybrid was investigated using appropriate electron and hole scavengers.

Introduction

The utilization of nanostructured semiconductor for photocatalytic application is an efficient and cost effective approach for the conversion of solar energy to chemical energy [1], [2], [3], [4]. After discovery of photo-electrochemical water splitting on TiO₂ electrode by Fujishima and Honda, many promising applications were initiated in many fields [5]. The light-driven reaction has been significantly increased in pollutant abatement, selective oxidation, artificial photosynthesis, self cleaning membrane, fuel production, lab-on-chip and air purification [6], [7], [8]. In this decade, nanostructured metal sulfides are considered as attractive candidates in the field of electronic, optical, optoelectronic, energy and environmental sectors [9], [10], [11]. Recently, various types of metal sulphide (CdS, ZnS, Ag₂S, NiS, CuInS₂, Sb₂S₃, and PbS) nanostructures have been synthesized with different morphology and structures (nanosheet, nanowires, nanorods, flowers) and examined for application in photocatalysis [12], [13], [14]. Among them, zinc sulphide (ZnS) is one of the pioneer direct bandgap semiconductors which has been used for UV and visible light driven photocatalytic applications [15], [16]. In general, ZnS occurred in two major phases including sphalerite and wurtzite in which the former possess high negative redox potential than the later. The intriguing properties of sphalerite ZnS with higher free radical production rates under light which makes them an ideal candidate for photocatalytic application [17]. The recent study suggested that ZnS photocatalytic efficiency is not as expected level due to the rapid recombination of photo-generated electron and hole pairs [18]. Recently, the researchers are focused to boost up the separation of photogenerated electron-hole pairs to prevent the recombination process which is an essential footstep for photocatalytic process.

In this scenario, different kind of approaches have been made by modifying the semiconductor material with (i) Z-scheme photocatalyst, (ii) ferroelectric material, (iii) piezo materials, and (iv) carbonaceous material for enhancement in semiconductor photocatalysis [19], [20], [21]. Herein, carbonaceous materials based photocatalysis have been attractive among the other methods due to the ease of preparation with the advantage of effective inhibition of electron-hole pair recombination. In this regard, carbon based materials like graphene, graphdiyne, graphene hydroxide, fluorographene, and g-C₃N₄, are recently examined as an additive material for enhancing the semiconductor photocatalysis [2], [22], [23]. Graphene sheets being a two-dimensional material have attracted well among other carbon materials due to its sheet-like morphology, distinct physical and chemical properties, ability to hold semiconductor nanocrystals at the basal planes [24]. The higher electrical conductivity of graphene makes them suitable to transfer the photogenerated electrons from semiconductor material, thereby enhancing the effective degradation [25]. Due to these interesting properties, various graphene-semiconductor nanohybrids are developed and employed for different photocatalytic application such as photocatalytic water splitting, photocatalytic CO₂ reduction, as well as textile waste water degradation, etc [26], [27]. There are few literature demonstrated the effective role of graphene in metal sulfide (ZnS, CuS, Ag₂S, CdS) based semiconductor photocatalysis. Up to now, the photocatalytic properties of graphene-ZnS nanohybrids prepared via hydrothermal, sol-gel, and sonochemical method are reported [28], [29]. Most of the reported procedures for ZnS-rGO system were followed via hydrothermal route with high-temperature and long-time consuming processes. Hence in this study, we propose microwave assisted technique which has the advantages of quick synthesis procedures and low-temperature process comparing with other conventional methods such as heating or hydrothermal. Also this method has dual advantages in which ZnS nanoparticles growth, GO reduction are occurred simultaneously and ZnS-rGO photocatalyst is expected to exhibit higher efficiency in comparison with bare ZnS. So it is essential to undertake a detailed study on the photocatalysis of ZnS-rGO (prepared by microwave method) with mechanistic investigations based on the analysis of reactive oxygen species (ROS) which are not yet reported. Hence, in this study, we demonstrated the synthesis of ZnS-rGO nanohybrid via microwave method and investigated their photocatalytic properties by the degradation of two azo dyes methylene blue and rhodamine B, respectively. In addition, the effective role of ROS on the photocatalytic dye degradation process has been studied and discussed in detail.