Effective visible light-driven ternary composite of ZnO nanorod decorated Bi₂MoO₆ in rGO for reduction of hexavalent chromium

Highlights

• A stable ternary Bi₂MoO₆-rGO-ZnO catalyst for Cr(VI) reduction was developed.

• The catalyst exhibited low photoluminescence and maximum photocurrent density.

• Feasible photocatalytic reduction mechanism using the prepared composite was proposed.

• The photocatalytic mechanism was proposed for Cr(VI) reduction.

Abstract

The construction of ternary composite materials is an excellent approach for achieving effective charge separation and enhanced photocatalytic performance. In this study, a ternary Bi₂MoO₆-rGO-ZnO nanocomposite photocatalyst was prepared by a simple hydrothermal method and characterized by various surface analytical optical techniques. Field emission scanning electron microscopy and high-resolution transmission electron microscopy analyses revealed that rice-like structured Bi₂MoO₆ decorated on ZnO nanorods were dispersed in reduced graphene oxide (rGO) sheets. The photocatalytic efficiency of this nanocomposite was studied by the reduction of hexavalent Cr(VI) under visible-light illumination. The ternary catalyst had a 95.4% reduction of Cr(VI) in 70 min, which is greater than that of Bi₂MoO_6, and ZnO-Bi₂MoO₆ owing to the maximum charge separation and excellent electron transport between ZnO and Bi₂MoO₆ catalyst on the rGO. The prepared ternary catalyst exhibited low photoluminescence and maximum photocurrent density, indicating the suppression of photon-induced electron–hole recombination and high charge separation. The ternary Bi₂MoO₆-rGO-ZnO catalyst was stable for up to three consecutive cycles of Cr(VI) reduction. A feasible photocatalytic reduction mechanism using the prepared ternary composite material was proposed.

Introduction

Heavy metal pollution is one of the most significant environmental issues impacting the world today. Hexavalent chromium (Cr (VI)), a hazardous heavy metal, is prevalent in the effluents of the electroplating, glass, pigments, steel plant, film, photography, and wood preservation industries [1], [2]. To protect human health, there is an urgent need to develop an efficient strategy for the reduction of Cr (VI), which can cause unignorable levels of water pollution. High levels of Cr(VI) may induce toxic, carcinogenic, mutagenic, and teratogenic effects in humans and animals [3], and the toxicity level of Cr(VI) is approximately 500 times higher than that of Cr(III). Cr(VI) is considered to be one of the most toxic pollutants by the Chinese Environmental Protection Board. The maximum allowable Cr(VI) concentration in industrial effluent and drinking water is 0.05–0.10 mg/L. The toxicological effect of Cr(VI) originates from its action as an oxidizing agent, as well as the formation of free radicals during the reduction of Cr(VI) to Cr(III) inside cells. Research on the carcinogenicity of Cr(VI) has focused on the fact that chromate ions quickly pass through cellular and nuclear membranes, often via anion transporter routes, while the trivalent species are slower. In fact, Cr(III) hydrolyzes smoothly at physiological pH to produce insoluble hydroxide [4], [5]. Hence, it is essential to remove Cr(VI) from polluted water before it mixes with uncontaminated soil or water [6], [7]. Several methods, such as electrolysis, microfiltration, ion exchange, nanofiltration, adsorption, liquid–liquid extraction, chemical precipitation, ultrafiltration, and photocatalysis [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] have been used to reduce hazardous Cr(VI) to Cr(III). Apart from photocatalysis, the other techniques have some drawbacks such as an expensive operating system, technical issues, heavy equipment, and lack of environmental friendliness. By contrast, the photocatalysis reduction method is one of the simplest, fastest, and cheapest technologies for industrial wastewater treatment.

Zinc Oxide (ZnO) is widely used owing to its physical and chemical properties, as well as its photostability, low cost, and eco-friendly nature. ZnO is an important n-type semiconducting material that has been extensively used in wastewater treatment because of its low toxicity, earth abundance, and high electron mobility [19], [20]. However, there are hurdles that must be overcome to attain high photocatalytic efficiency owing to i) a wide bandgap (3.37 eV) that limits its visible-light absorption and ii) the predominant photoexcited charge recombination rate [21], [22]. Hence, there is a need to develop an effective method to shift ZnO optical absorption from the UV to visible-light range and to make it a stable photocatalyst for a long life cycle period [23]. Recently, a Bi³⁺ metal oxide material has been shown to exhibit excellent photocatalytic degradation owing to its layered structure and remarkable intrinsic properties. Bismuth molybdate (Bi₂MoO₆) consists of alternative perovskites and layers, which provide a controllable image structure and suitable energy gap to capture visible light [24], [25]. Bi₂MoO₆ is an excellent material to use as a co-catalyst with ZnO, which prevents the photo-induced electron and hole pairs from reconnecting owing to the well-matched energy gap. The heterojunction Bi₂MoO₆-ZnO composite material enhanced the photocatalytic degradation of organic dyes under visible-light sources, as reported by Tian et al. [26], [27].

The morphology of the catalyst plays a vital role in its photocatalytic reduction performance. Three-dimensional, two-dimensional, and one-dimensional materials are of great importance for photocatalytic and adsorption applications [28], [29]. MoO₃ with various morphological structures on TiO₂ nanofibers exhibited a photocatalytic removal efficiency much greater than that of MoO₃ and TiO₂ owing to the 3D porous structure [30]. Based on the literature, 3D structural materials can efficiently enhance photocatalytic performance as 3D structures can utilize light energy in multifold directions produced by the visible source. Three-dimensional materials have a larger surface area and more reactive sites to enhance the photocatalytic reaction and adsorption [31].

Graphene is a carbon crystal allotrope, which has a 2D carbon crystal with a honeycomb structure that has been broadly explored owing to its large surface area, electron mobility, and conductivity [32]. GO is an essential graphene group, consisting of a layered sheet-like structure with reactive oxygen-containing functional groups at the hydrophobic base and hydrophilic properties. The oxygen-containing functional groups in GO produce reaction sites for the reaction with catalytic semiconductors. Furthermore, the oxygen functional groups reduce the movement of electrons in the lattice [33]. Combining rGO with semiconductors reduces the recombination of electron–hole pairs as a result of the electron shuttle from the connected carbon structural network [34]. Moreover, rGO can be utilized as an ideal electron pool owing to its 2D π-combined structure. The photon-generated electrons in the conduction band of the semiconductor can be returned to the valence band and quickly injected into the GO network instead of reconnecting with the positive holes. In addition, the black-body property of materials such as graphene can facilitate the gathering of photons from visible sources [35], [36].

Bi₂MoO₆ with a perovskite structure is a new eco-friendly type of catalytic material with a low bandgap position, high efficiency, and chemical stability, which provides numerous active sites for photocatalytic reactions [37]. Pure Bi₂MoO₆ exhibits a lower photocatalytic performance owing to the lower separation of charge carriers. Therefore, many researchers have used Bi₂MoO₆ as a Supporting material to realize an excellent charge separation and produce a higher photocatalytic efficiency [38], [39], [40], [41]. Recently, a rGO-ZnO@Bi₂MoO₆ nanocomposite synthesized by the solvothermal method was reported for the photodegradation and detoxification of RhB dye [42]. In the reported solvothermal method, GO was added to ZnO@ Bi₂MoO₆ at different ratios via a sonochemical cum solvothermal approach. The prepared nanocomposite was effectively utilized for RhB (10 mg/L) dye degradation under visible-light irradiation. The objective of this study was to prepare a catalyst using another method to obtain a distinct morphological structure. Initially, we prepared ZnO nanorods and decorated Bi₂MoO₆, following which rGO was added. This produced Bi₂MoO₆-rGO-ZnO with an excellent morphology and a higher surface area than rGO-ZnO@Bi₂MoO₆, as reported earlier. The ZnO nanorods modified with Bi₂MoO₆ and rGO were effectively utilized for toxic Cr(VI) reduction under visible light. Our method of preparation is simple and can be utilized for Cr(VI) reduction at high concentrations. Herein, we report the fabrication and characterization of a ternary ZnO nanorod-decorated Bi₂MoO₆-rGO composite material and its photocatalytic performance in Cr(VI) reduction.