Graphitic carbon nitride nanosheets decorated with CuCr₂O₄ nanoparticles: Novel photocatalysts with high performances in visible light degradation of water pollutants

Abstract

In this paper, CuCr₂O₄ nanoparticles were decorated on the surface of g-C₃N₄ nanosheets (g-C₃N₄-NS) by a facile refluxing process. The structure, composition, morphology, optical, textural, and thermal properties were characterized by XRD, EDX, SEM, TEM, UV–vis DRS, FT-IR, XPS, PL, BET, and TGA techniques. The photocatalytic performance of g-C₃N₄-NS/CuCr₂O₄ nanocomposites was assessed by degrading RhB and MB dyes and phenol under visible-light illumination. When the loading amount of CuCr₂O₄ was 10 wt%, the nanocomposite exhibited the highest activity. Activity of the g-C₃N₄-NS/CuCr₂O₄ (10%) nanocomposite refluxed for 3 h and calcined at 520 °C for 4 h was almost 11.8 and 4.8 times greater than those of the bulk g-C₃N₄ and g-C₃N₄-NS photocatalysts in degradation of RhB, respectively. In the prepared nanocomposites, nanosheets of g-C₃N₄ act not only as CuCr₂O₄ support, but also as co-catalyst. The novel visible-light-active photocatalyst has considerable stability and it can be reused for five times without obvious loss of its photocatalytic activity.

Introduction

Nowadays, enhanced concerns about environmental pollution are attributed to growing population, developing different industries, and increasing living standards. Hence, a lot of researches have been devoted for developing efficient environmental remediation technologies [1], [2], [3], [4]. Among various strategies to solve environmental issues, photocatalytic processes have been considered as the most promising solutions, due to their potentials to degrade wide range of pollutants to nontoxic compounds at ambient conditions with low-cost procedures [3], [4]. In these processes, to use effectively the solar energy, a photocatalyst with band gap lower than about 3.0 eV should be applied [5]. However, conventional photocatalysts like TiO₂, ZnO, SnO₂, and ZnS can almost absorb ultraviolet, hence they have poor photocatalytic performance under visible-light illumination [6], [7], [8]. Consequently, there are continuous attempts in the field of searching for robust and efficient visible-light-driven photocatalysts [5], [9], [10]. Recently, a metal-free semiconductor, namely graphitic carbon nitride (g-C₃N₄), has gained great attention from the research communities, due to its low cost, ability to integrate with different semiconductors, appealing electronic properties, high physicochemical stability, and nontoxicity [11], [12]. Importantly, this polymeric photocatalyst is prepared by a facile thermal polymerization of nitrogen-rich precursors including melamine, dicyandiamide, cyanamide, urea, and thiourea [11]. Nonetheless, activity of bulk g-C₃N₄ is not high enough, due to its small surface area, poor optical absorption from visible light, and fast recombination of the photogenerated electron-hole pairs [12], [13], [14], [15].

It is well known that bulk g-C₃N₄ has a layered structure, in which there are strong covalent CN bonds in each layer and weak van der Waals interactions between layers [14], [15]. By exfoliating bulk g-C₃N₄ to nanosheet ones with low thickness, its surface area will increase, providing abundant active sites for photocatalytic reactions [16], [17], [18], [19], [20]. Consequently, bulk diffusion length for the charge carriers will decrease, leading to retardation of electron-hole recombinations. More importantly, in this case, life time of the photogenerated charge carriers will increase, owing to the quantum confinement effect. These new characteristics make remarkable changes in photocatalytic performance of the modified g-C₃N₄ [16], [17], [18], [19], [20]. On the other hand, it has been proved that by combining narrow band gap semiconductors with g-C₃N₄, one can increase visible-light absorption ability and separation capability of electron-hole pairs [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32].

Copper chromite (CuCr₂O₄), as a p-type semiconductor with narrow band gap of nearly 1.40 eV, has been recently applied in different disciplines such as photocatalytic hydrogen evolution, solar absorbing pigments, catalyst for selective oxidation of aromatic CH bonds and cycloalkanes, CN cross-coupling reaction catalyst, selective hydroxylation of benzene to phenol, thermal decomposition of ammonium perchlorate, photocatalytic degradation of some selected organic dyes, ceramic pigments, and supercapacitors [33], [34], [35], [36], [37], [38], [39], [40], [41]. Due to its appropriate band energy structure and narrow band gap, it seems that decoration of CuCr₂O₄ over g-C₃N₄ nanosheets (g-C₃N₄-NS) could improve visible-light harvesting ability and separation of the charge carriers in the prepared nanocomposites, resulting in enhanced photocatalytic performance. Hence, in the present paper, we report preparation of efficient visible-light-driven photocatalysts by decoration of CuCr₂O₄ nanoparticles over nanosheets of g-C₃N₄ to prepare novel g-C₃N₄-NS/CuCr₂O₄ nanocomposites. The prepared samples were characterized by X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform-infrared spectroscopy (FT-IR), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), Brunauer–Emmett–Teller (BET), and thermogravimetric analysis (TGA). Accordingly, the nanocomposites were prepared by refluxing method with different weight percents of CuCr₂O₄ and their photocatalytic activities were investigated by degradation of rhodamine B (RhB) and methylene blue (MB) dyes and phenol under visible-light illumination. It is anticipated that the methodology applied in the present work will bring a new light for the fabrication of different types of g-C₃N₄–based efficient photocatalysts.