Facile synthesis of CuO/CdS heterostructure photocatalyst for the effective degradation of dye under visible light

Highlights

• CuO/CdS composite was fabricated via ultrasound-assisted-wet-impregnation method.

• CuO/CdS exhibited much higher photocatalytic activity than CuO and CdS.

• The inclusion of CuO with CdS improved photogenerated charge separation.

• Dye degradation mostly occurred in the bulk by super oxide ion.

• CuO/CdS composite possessed an excellent reusability.

Abstract

In this work, the photocatalytic property of p-type CuO was tailored by creating a heterojunction with n-type CdS. The CuO/CdS nanocomposite photocatalyst was synthesized by the ultrasound-assisted-wet-impregnation method and the physicochemical and optical properties of the catalysts were evaluated by using N₂ physisorption, X-Ray Diffraction (XRD),X-Ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, Transmission electron microscopy (TEM), Energy dispersive X-Ray (EDX) mapping, Field Emission Scanning Electron Microscope (FE-SEM), UV–Vis and photoluminescence spectroscopy experiments. Detailed characterization revealed the formation of a nanocomposite with a remarkable improvement in the charge carrier (electron/hole) separation. The photocatalytic degradation efficiencies of CuO and CuO/CdS were investigated for different dyes, for instance, rhodamine B (RhB), methylene blue (MLB), methyl blue (MB) and methyl orange (MO) under visible light irradiation. The obtained dye degradation efficiencies were ~93%, ~75%, ~83% and ~80%, respectively. The quantum yield for RhB degradation under visible light was 6.5 × 10⁻⁵. Reusability tests revealed that the CuO/CdS photocatalyst was recyclable up to four times. The possible mechanisms for the photocatalytic dye degradation over CuO/CdS nanocomposite were elucidated by utilizing various scavengers. Through these studies, it can be confirmed that the conduction band edges of CuO and CdS play a significant role in producing O₂⁻. The produced O₂⁻ degraded the dye molecules in the bulk solution whereas the valence band position of CuO acted as the water oxidation site. In conclusion, the incorporation of CuO with CdS was demonstrated to be a viable strategy for the efficient photocatalytic degradation of dyes in aqueous solutions.

Introduction

The textile, leather, plastic, and cosmetic industries discharge large volumes of organic dye-containing wastewater. Consequently, even the presence of a minute dye concentration in the water body (≥5 mg/L) can reduce light penetration through the water surface. This can preclude the photosynthesis of aqueous flora (Gonawala and Mehta, 2014). Most synthetic dyes are toxic (carcinogenic, mutagenic, teratogenic) to fauna (Chequer et al., 2011; Chung, 2016). Additionally, most of the existing dyes possess low acute toxicity (lethal dose (LD₅₀) of 250–2000 mg/kg of body weight) and are classified as ‘dangerous’ by the European Union (de Campos Ventura-Camargo et al., 2013). Rhodamine B (RhB) is a synthetic dye that contains an aromatic ring and ethylated amino groups. It is used as a pigment in the textile industry (Group, 2010; Oyekanmi et al., 2019). It is also commonly used as a model dye in many environmental studies for the evaluation of degradation efficiency. This particular dye is highly harmful if taken orally and irritates the eyes, skin and respiratory tract (Zada et al., 2020). Hence, the removal of RhB from wastewater is crucial for sustainable future (Fendrich et al., 2019). The photocatalytic dye degradation using semiconductors under solar irradiation was found to be an effective strategy for the purification of the wastewater containing recalcitrant dyes (Fendrich et al., 2019). The photocatalytic treatment of wastewater is usually eco-friendly and cost-effective (Chakraborty et al., 2019; Dutta et al., 2017; Li et al., 2011). The crucial steps of photocatalysis comprise of excitation and separation of photoinduced electron-hole pairs (e⁻/h⁺) in the photocatalyst upon solar irradiation; this is followed by charge transfer to the reactants to accomplish the redox reactions (Meng et al., 2017). To harness solar energy, particularly the energy in the visible light region (represents 43% of solar spectrum), various photocatalysts such as CdS (Chen et al., 2019b), MoS₂ (Yang et al., 2017), and Bi₂S₃ (Hao et al., 2014) have been investigated (Chakraborty et al., 2019; Huang et al., 2010). Nevertheless, their photocatalytic activity and stability are low because of the high e⁻/h⁺ recombination rate, and photocorrosion of the catalysts. CdS has recently became popular due to its favourable optical and electronic properties. It possesses a moderate bandgap (~2.4 eV) and can harvest visible light at ≤ 520 nm (Bellamkonda and Rao, 2019). Consequently, it is widely used as a photocatalyst for various reactions (Chen et al., 2019b; Shen et al., 2011; Zhang et al., 2018). However, during the photocatalytic reactions, CdS becomes unstable and this photocorrosivity is attributed to the easy oxidation of its S²⁻ by the photogenerated holes (Xu et al., 2017). Moreover, the easy agglomeration of CdS particles, low specific surface area and fast recombination rate of the photogenerated carriers during photocatalysis limit the lifetime and large-scale utilization of CdS-based catalysts. This has hindered the broad application of CdS (Chen et al., 2019b; Wang et al., 2017a). The abovementioned drawbacks of CdS were partially removed by the incorporation of either metal or non-metal dopants, or by forming heterojunctions for which a remarkable improvement in photocatalytic performance was reported (Hou et al., 2012; Wang et al., 2019a; Yang et al., 2017; Zhang et al., 2018). Fang and co-workers reported the preparation of the CdS/BiVO₄ heterojunction by the solvothermal method and found that this material exhibited higher stability compared to CdS (Fang et al., 2016). For better charge extraction in CdS, it has been combined with various semiconductors, for instance, ZnO (Eley et al., 2014; Lingampalli et al., 2013), Cu₂S (Tan et al., 2014), Bi₂S₃ (Hao et al., 2014), ZnSe (Zhu et al., 2014), and CdTe (Okano et al., 2014), with superior photocatalytic efficiency obtained for the heterostructured catalysts. p-type CuO is widely reported as a photocatalyst for different reactions owing to its excellent photovoltaic properties (Wongratanaphisan et al., 2019), high electroconductivity (Rajeshwar et al., 2013) and low band gap (Li et al., 2014b). Despite all of these advantages, the photocatalytic activity of CuO is still low owing to its high e⁻/h⁺ recombination. The creation of a heterojunction by combining the n-type CdS with p-type CuO may represent an effective strategy for enhancing the stability of the photogenerated e^⁻/h⁺ pairs (Kalamaras et al., 2018). Additionally, the valence band (VB) and conduction band (CB) positions of CuO and CdS match well to form a heterostructure. Therefore, the limitations found in both CuO and CdS can be overcome by coupling these materials to create heterostructure nanocomposites. Zhang et al. (2019b) prepared CuO–CdS nanowire by a one-step solvothermal method in order to obtain enhanced gas-sensing properties. Nonetheless, this preparation method suffers from a relatively high equipment cost and the inability to control the crystal growth (Zaleska-Medynska, 2018). Septina et al. (2017) reported on the CuO/CdS heterojunction synthesis by the chemical bath deposition (CBD) method for H₂ production. They reported that the wastage of the solution after every deposition is the main drawback of this method. Chen et al. (2019a) prepared CuO/CdS composites by the successive ionic layer adsorption and reaction (SILAR) technique for the photodegradation of pollutants in sewage (Chen et al., 2019a). In our earlier work, a CuO–CdS heterojunction was prepared by sol-gel and wet impregnation method for CO₂ reduction and showed significant improved quantum efficiency for methanol production (Tarek et al., 2020). Xia et al. showed that CdS quantum dots modified the CuO synthesized by the SILAR method (Xia et al., 2015). However, the selection of the precursor solutions and the adsorption of ions on the film surface at each SILAR step are fully controlled by the precursor ions. This is the main drawback of the aforementioned method. The preparation procedures for the heterostructured catalysts consist of multiple steps and the control of the morphology of the particles is very sensitive to the variations of the steps; hence, the method lacks robustness that is crucial for bulk synthesis.

In the present work, we report a successful facile synthesis of a CuO/CdS heterojunction photocatalyst by the wet impregnation method. This technique was used due to its ease, simplicity and environmental friendliness. The structural, morphological, optical and electrochemical properties of the as-prepared CuO, CdS and CuO/CdS heterostructure were characterized by various analysis techniques. Excellent photocatalytic performance was attained by the CuO/CdS composite owing to its promising optical properties and appropriate band energy structures. Moreover, the photocatalytic behaviour of these heterojunction catalysts was explored by measuring their ability to degrade RhB, MLB, MB and MO that were used as model dyes. Additionally, the effects of the photocatalysts on the degradation activity were further studied by examining the possible mechanism of this heterostructured composite based on the active species trapping experiments.