Green gelatin-assisted: Synthesis of Co₃O₄NPs@rGO nanopowder for highly efficient magnetically separable methylene orange dye degradation

Highlights

• The synthesis of Co₃O₄ nanoparticles in the presence of gelatin as a reducing agent.

• Graphene oxide as a supporter has been utilized in synthesis of Co₃O₄ microsphere rGO composites.

• Novel support of Co₃O₄ nanoparticles shows efficient performance for photodegradation of organic contaminants.

• Co₃O₄NPs@rGO showed better response compared to the pure Co₃O₄.

Abstract

GO and Co(NO₃)₂ were respectively used as rGO and Co₃O₄ precursors for preparing magnetically separable Co₃O₄NPs attached Co₃O₄NPs@rGO nanocomposites by a straightforward sol–gel technique. To characterize the nanocomposite materials, FESEM, EDX, elemental mapping, XRD, FTIR, Raman spectroscopy, UV–vis, VSM and BET were employed. When exposed to UV rays, the nanocomposite showed extraordinary photocatalytic degradation of MO dye. According to the measurements of photocatalytic activity, the highly efficient photocatalytic efficiency of the nanocomposite could be attributed to preventing electron-hole recombination by highly effective electron transfer between rGO and semiconductor NPs. The nanocomposite succeeded in the efficient degradation of MO dye, even after five photocatalytic cycles.

Introduction

Nowadays, water contamination chiefly owes to organic dyes and their waste material permeating conventional sewage treatment plants staying in water due to their excellent resistance against chemicals, light, microbial substances, and temperature [1], [2]. Persistent organic pollutants (POPs) can be photocatalytically degraded by numerous semiconductor nanoparticles (NPs) [3], [4], [5], [6]. Recent reports have indicated that cobalt oxide (Co₃O₄), a typical p-type semiconductor, has been extensively studied thanks to its extraordinary catalytic, electrical, and magnetic properties, as well as its multitudinous applications in energy storage [7], capacitors [8], materials for field-emission (FE) of electrons [9], li-ion batteries (LIBs) [10], magnetic semiconductors [11], gas sensing [12], and heterogeneous catalysis [13]. In spinel-type Co₃O₄, tetrahedral 8a and octahedral 16d sites are occupied by Co²⁺ and Co³⁺, respectively [14]. Co₃O₄ has a direct optical bandgap of 2.13–3.95 eV [15]. It can be synthesized using a number of certain techniques, such as MW-assisted chemical reactions [16], oxidation–reduction (redox) reactions [17], sequential fuel injection (SFI) [18], chemical vapor deposition (CVD) [19], coprecipitation [20], sonochemistry [21], thermolysis [22], combustion [23], and so on. However, the synthesis techniques above have their own major shortcomings due to extended reaction time, multiplex synthesis procedure at high temperatures, costly apparatus, and high synthesis costs. Likewise, virtually all of them are detrimental to the environment. Hence, green chemistry (a.k.a. sustainable chemistry) has offered a surrogate technique to overcome or diminish the above-mentioned shortcomings. Sustainable chemistry has been adopted as a creative tool for minimizing the use of hazardous and toxic substances while metal oxide NPs are synthesized. It involves using green materials that offer a large number of benefits regarding greenness and compatibility. Furthermore, quasi-one-dimensional (Q1D) nanostructured Co [24] with diameters ranging from tens-hundreds nm is technologically significant, rendering it intriguing scientifically. Within this range, they shall have pronounced couplings and fascinating physical properties far cry from their bulk counterparts. Thus, Co₃O₄ is a highly-efficient photocatalyst applicable to eliminate organic dyes from sewage. The performance of a photocatalyst (a.k.a. photochemical catalyst) in photocatalysis is analogous to that of chlorophyll in photosynthesis (a natural phenomenon) [25]. Photocatalytic systems allow for a reaction or a photoinduced molecular transformation occurring on the catalyst surface. Photocatalytic reactions basically involve generating electron-hole pairs (EHPs). These pairs spread out on photocatalyst surface once light-illuminated with an energy greater than its bandgap, partaking in chemical reactions with electron acceptors and donors. Neighbouring water or oxygen molecules can be converted into free radicals of hydroxyuracil (hoU) with really powerful oxidation properties by these free holes and electrons [25]. Thus, the application of Co₃O₄NPs (as photocatalyst) should be modified to enable excellent protection against UV radiation [26].

As a single-atom-thick, 2D layer of sp²-hybridized carbon atoms with big surface-area-to-volume ratio, extraordinary chemical resistance, and significant specific conductance, graphene can be considered an excellent matrix for nanocomposites. Accordingly, recent years have witnessed a substantial interest in semiconductor-decorated graphene-based composite materials thanks to their multifunctional capabilities [27], [28]. The performance of the aforesaid materials can be significantly enhanced by charge-transfer, electronic, and magnetic interactions between graphene sheets and semiconductor nanostructures (NSs) connected to them [29], [30], [31], [32]. In semiconductors, once excited by photons with an energy equals to or exceeds semiconductor's bandgap energy, electrons can jump from the valence band (VB) to the conduction band (CB), thereby generating EHPs. The generated pairs substantially contribute to the transformation of solar energy and pollutant photocatalytic degradation.

Nevertheless, due to their instability, holes in VB and photoexcited electrons in CB can combine again effortlessly, resulting in a decline in their photocatalytic performance. Once incorporated into semiconductors, graphene sheets may accept photoexcited electrons from CB, thereby suppressing electron-hole recombination [29]. Additionally, graphene nanosheets (GNs) take a role in the growth and dispersion of NPs on the surface of the semiconductor, thus preventing NP aggregation and creating a greater specific surface area for the photocatalyst [33], [34].

The sol–gel technique is believed to be a promising way to help semiconductor nanomaterials to distribute analogously. Moreover, the end-product quality can be enhanced by gelatin, a specific natural surfactant (surface-active agent). Accordingly, this research sought to perform a fast and straightforward sol–gel synthesis of Co₃O₄NPs with a narrow particle-size distribution (PSD), decorated on an rGO sheet in a gelatin environment. Due to its expansion during calcination, gelatin was selected as the natural surfactant and Co₃O₄NPs growth terminator to inhibit particle congregation. The technique above is advantageous in that both Co₃O₄NPs and gelatin are eco-friendly. Likewise, gelatin acts as a reducing agent to reduce GO under moderate conditions [35], and, at the same time, considerably contributes to the stabilization of as-prepared graphene as a capping agent. The by-products were then exploited for photocatalytic removal of methylene orange (MO) dye. Presumably, no one-pot sol–gel (OPSG) syntheses of rGO uniformly decorated with Co₃O₄NPs in a gelatin environment have been recorded. This study pioneers a swift, single-stage, low-cost, green, and OPSG synthesis of rGO uniformly decorated with hierarchical Co₃O₄NPs in a gelatin environment.