Hydrogen peroxide–assisted photocatalytic dye degradation over reduced graphene oxide integrated ZnCr2O4 nanoparticles

Zinc chromite nanoparticles (NPs) and zinc chromite–reduced graphene oxide (ZnCr2O4-rGO) nanocomposite have been synthesized by the combined effects of reflux condensation and calcination processes. The structural properties were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), UV–visible studies, etc. Structural morphology was investigated by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) that indicate the formation of particles in the nanometer regime. The presence of the elements Zn, Cr, O and C has been confirmed by energy-dispersive X-ray spectroscopy (EDX) images which show the purity of the synthesized products. The photocatalytic activities of both as-prepared samples under visible light irradiation were investigated in presence of hydrogen peroxide (H2O2) and the results show that ZnCr2O4-rGO nanocomposite has a quite higher photo-activity response than virgin ZnCr2O4 NPs. The enhanced photo response indicates that, in ZnCr2O4, the photo-induced electrons favor to transfer to the rGO surface and the recombination of electron–hole pairs inhibited for which it results in the significantly increased photocatalytic activity for the ZnCr2O4-rGO photocatalyst and this phenomenon is also supported by the band gap value and photoluminescence results. Our outcomes demonstrate that ZnCr2O4-rGO nanocomposite is a more promising material to build up an efficient photocatalyst for waste water treatment.


Introduction
The heterogeneous photocatalyst gives great promise for environmental contaminant elimination by transfer of photon energy into chemical energy (Nguyen et al. 2016;Kalisamy et al. 2020). The photocatalytic treatment for eradication of hazardous environmental pollutants, such as various organic dyes and chemical compounds, is one of the most efficient and economical concerns (Omidvar et al. 2017a(Omidvar et al. , 2017bNaghdi et al. 2018;Mohazzab et al. 2020;Jaleh et al. 2019;Nasrollahzadeh et al.2019;Ghosh et al. 2021aGhosh et al. , 2021b. These pollutants egress from industries like textile, paper, food, cosmetic, pharmaceutical industries and contaminate water (Dong et al. 2015;Shang et al. 2014). So, the removal of these pollutants from water is very important for living beings (Nasrollahzadeh et al. 2021a(Nasrollahzadeh et al. , 2021bNasrollahzadeh et al. 2020;Nasrollahzadeh et al. 2021aNasrollahzadeh et al. , 2021bDas et al. 2019;Ghosh et al. 2021aGhosh et al. , 2021b. Graphene-based nanocomposites give auspicious results for the removing of the contaminants by catalytic (Anshuman et al. 2018;Tantubay et al. 2020), photocatalytic (Kocijan et al. 2021;Das et al. 2018;An and Yu 2011), adsorption (Chen et al. 2013;Dubey et al. 2015) and other processes due to its unique properties like large surface area (Singh et al. 2021), sp 2 hybridized carbon network (Gao et al. 2018), high electrical conductivity (Rahaman et al. 2020) and electron mobility (Gupta et al. 2020). There are several graphene-based photocatalysts such as, rGO-TiO 2 (Yu et al.2017), rGO-ZnO (Kumar et al. 2021), rGO-CuS/ZnS (Das et al. 2021), rGO-CuO (Gusain et al. 2016), rGO-NiO (Nafiey et al. 2017), rGO-Ag (Sen and Ghosh 2017), etc. for removing of organic dyes like methylene blue, rhodamine B, eosin-Y, congo red and crystal violet by using UV or visible light. But there are very few spinal nanocomposites reported for the removal of organic dyes from contaminant water.
There are much more spinel nanomaterials which showed their enhanced catalytic activities after being combined with graphene or reduced graphene oxide (Padhi et al. 2017;Amer et al. 2017;Gnanamoorthy et al. 2020;Krishnan et al. 2020). But zinc chromite (ZnCr 2 O 4 ) is a less-studied material which behaves as a nanocatalyst in visible light irradiation and also there is no report about the synthesis of graphene or reduced graphene oxide functionalized ZnCr 2 O 4 nanocomposite. Being a mixed oxide, it has a significant role in material science for its physical and chemical properties suitable for various applications. There are fewer literature surveys as described in Table 1 about the applications of ZnCr 2 O 4 such as photocatalysis process for removal of various organic dyes (Dumitru et al. 2018;Sabet and Jahangiri 2018), catalytic application for thermal decomposition (Wang et al. 2020) and sensing property for explosive nitro compounds , humidity sensing (Pokhrel et al. 2003) but no one synthesizes the low-dimensional ZnCr 2 O 4 nanomaterials which can respond in visible light to degrade organic dye. There are several methods for synthesis of ZnCr 2 O 4 nanomaterials, such as thermal conversion (Dumitru et al. 2018), hydrothermal process (Sabet and Jahangiri 2018), sol-gel process (Choudhary et al. 2017), combustion process (Kumar and Chakra 2017) and reflux process (Tajizadegan et al. 2016). Out of these, reflux followed by calcination is a very simple and more cost-effective method to synthesize different nanomaterials.
Here, in this paper, about 4-7nm ZnCr 2 O 4 NPs on reduced graphene oxide (rGO) surface have been synthesized by simple reflux method followed by calcination, and then after characterization by XRD, FTIR, UV-Vis, FESEM, TEM, PL studies the visible light-active photocatalytic activity for the degradation of methylene blue organic dye has been investigated.

Preparation of GO
Graphene oxide (GO) was prepared following the Modified Hummers method (William et al. 1958). Using this method, at first, 1 g natural flake graphite powder was taken in a 500 mL beaker and 23 mL concentrated H 2 SO 4 was added to it. Then the mixture was subjected to magnetic stirrer for 48 h. After stirring, potassium permanganate was added slowly in ice-cold condition and the color has been changed from greenish to brown. Then, the whole dispersion was transferred into ice-cold water and after that hydrogen peroxide was added to make the complete reduction and the color turned from brown to pale yellow. Centrifugation was done to collect the precipitate, then washed several times with

Synthesis of ZnCr 2 O 4 -rGO spinel nanoparticle
ZnCr 2 O 4 -rGO nanocomposites have been synthesized using graphene oxide (GO) sheet as growing substrate in the simple chemical process. At first, 0.364 g GO was dispersed in 120 mL distilled water and sonicated for 40 min for clear dispersion. Twenty milliliters of 0.35 (M) zinc sulfate and 20 mL 0.70 (M) chromium nitrate solution was added separately to this GO dispersion with the continuous stirring condition. The total solution is stirred for the next 20 min and then pH 10 was adjusted by adding liquor ammonia dropwise. Then the whole dispersion was subjected to reflux for 11 h after stirring for 3 h at 80 °C. The obtained product was filtered, washed with distilled water and ethanol, and finally dried at 80 °C under vacuum condition as shown in Scheme 1. Bare ZnCr 2 O 4 nanoparticles was synthesized using polyvinylpyrrolidone (PVP) as a stabilizer instead of GO. The two dried samples were heated for 3 h at 500 °C for calcination.

Characterization techniques
The crystal structures of the composites were determined by the X-ray diffractometer BRUKER D8 ADVANCE and nature of the chemical bonding was investigated by Fourier transform infrared (FTIR) study using SHIMADZU IR Prestige-21. UV-Vis study and the photocatalytic activity studies were carried out by SHIMADZU UV-1800 spectrophotometer. The size and the morphological study of these nanocomposites were investigated by the scanning electron microscope (Model SIGMA-300). Photoluminescence (PL) study was done by Hitachi F-4500 spectrofluorometer.

Photocatalytic activity study
The photocatalytic performances of the two powdered samples ZnCr 2 O 4 NPs and ZnCr 2 O 4 -rGO nanocomposites were investigated by the degradation of methylene blue (MB), an organic dye under a visible light irradiation. Ten milligrams of each sample was added to 50 mL MB dye solution (2.5 mg L -1 ) separately and the mixture was stirred for 20 min to maintain adsorption-desorption equilibrium in dark environment. After 30 min of adsorption-desorption process, H 2 O 2 was added to it and then exposed to visible light 500-W xenon lamp powers with constant stirring. UV-Vis absorption data were taken continuously after certain intervals of time, and it was found that the deep blue color of MB gradually disappears. Measuring the absorbance intensity during the photocatalytic degradation process, degree of degradation was calculated using this relation: The absorption spectra of the ZnCr 2 O 4 -rGO nanocomposite was recorded from 200-800 nm wavelength range as shown in Fig. 2. The peaks at around 270 nm and 355-390 nm can be assigned for the π-π* transition of aromatic C-C bond in graphene network and octahedral Cr 3+ (d 3 ) ions respectively. The hump that arises near the visible region can be attributed to band gap absorption of ZnCr 2 O 4 nanoparticles which is quite left shifted than the reported literature (Naz et al. 2016;Abdullah 2016). The absorption band gap can be estimated using the following Tauc relation (Zanatta 2019):

Results and discussion
where A is a constant and E g is the absorption band gap of the material; n is a number which indicates the nature of electronic transition between valance band and conduction band, which have the values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively. It is well known that ZnCr 2 O 4 responds to UV light but from the above equation, the plot of (αhν) 2 vs. hν will give a divergence at an energy value E g . The estimated band gap value from the plot for ZnCr 2 O 4 -rGO nanocomposite can be obtained by extrapolating the straight line to the energy axis at α = 0. ( The linear part shows that the mode of transition in this nanocomposite is direct in nature and the calculated band gap value was found to be 2.82 eV which is less than that of reported value of virgin ZnCr 2 O 4 nanoparticles. This might be due to the strain that comes for the combination of graphene with nanoparticles after being formation of ZnCr 2 O 4 -rGO nanocomposite. The morphology of virgin ZnCr 2 O 4 and ZnCr 2 O 4 -rGO nanocomposite is analyzed by transmission electron microscopy. Virgin ZnCr 2 O 4 forms by agglomeration of small particles around 15-20 nm as shown in Fig. 3a. The average particle size of ZnCr 2 O 4 NPs in the composite (Fig. 3b) is smaller than that of virgin ZnCr 2 O 4 NPs which is due to the presence of rGO surface. From the Fig. 3c, it is clearly shown that the ZnCr 2 O 4 NPs are spread throughout the rGO sheet and the average particle size is approximately 4-7 nm (Fig. 3d) which is proven by particle size distribution curve as shown in Fig. 3e.
The EDX study of the synthesized samples has been carried out from the SEM images. Table 2 described the obtained results from Fig. 3f and Fig. S1 (supporting info) which showed the presence of the elements Zn, Cr, C and O in ZnCr 2 O 4 NPs and ZnCr 2 O 4 -rGO nanocomposite respectively. Here, the weight percent of carbon is too much less The photoluminescence study of the nanomaterial is one of the important characterizations by which we can demonstrate the efficiency of migration and transfer of charge carriers and gives information about oxygen vacancies and defects as well as the separation and recombination of photo-induced charge carriers (Gao et al. 2019;Qian et al. 2018). Figure 4c shows the PL spectra for ZnCr 2 O 4 and ZnCr 2 O 4 -rGO samples with an excitation wavelength of 249 nm. The main emission peak is centered at about 350-450 nm for the two samples. The PL emission intensity decreases slightly from ZnCr 2 O 4 to ZnCr 2 O 4 -rGO nanocomposite. This suggests the enhanced photocatalytic  performance of ZnCr 2 O 4 -rGO compared to ZnCr 2 O 4 because of lower recombination rate of photo-generated charge carriers in ZnCr 2 O 4 -rGO nanocomposite. Due to presence of rGO sheet in the nanocomposite material, the photo-generated electrons from the conduction band (ZnCr 2 O 4 ) moves to the rGO surface and finally reduces the possibility of recombination of electron-hole pairs and photocatalytic activity increases as a result of lowering in the PL intensity.

Photocatalytic study
In order to evaluate the degradation efficiency of the two samples, the irradiation experiment was conducted by H 2 O 2 -assisted photolysis of MB. In Fig. 4a (Fig. 4a). This is due to the fact that the ZnCr 2 O 4 -rGO sample has a larger surface area than virgin ZnCr 2 O 4 NPs because of the decreased size of ZnCr 2 O 4 NPs after the addition of GO in nanocomposite. The concentration of MB gradually decreases and this reaction follows pseudo-first-order kinetics as shown in Eq. (3), where A 0 is the absorbance at t = 0 (initial MB absorbance), A t is the absorbance at time t = t (final MB absorbance), and k is the rate constant. A linear fit curve was obtained with ln (A 0 /A t ) against illumination time and the  Table 3.

Photocatalytic mechanism
In order to explain the H 2 O 2 -assisted photocatalytic activity, the possible photocatalytic route has been developed in Scheme 2. When the ZnCr 2 O 4 NPs and ZnCr 2 O 4 -rGO nanocomposite photocatalysts were excited with photon energy, the electrons in the valence band (VB) transfer to the conduction band (CB) generating the same number of holes in VB as shown in Scheme 2. The photogenerated electrons can form superoxide radical anions (O 2 ·− ) combining with dissolved O 2 and photogenerated holes transform the HO − into HO · radical. The recombination of holes (h + ) and electrons (e − ) has been considered an adverse process in photocatalysis. So, the probability of photocatalytic process has been improved by retardation of recombination process or increasing photocatalysis performance in the presence of other catalysts. Here H 2 O 2 acts as an important role in this photocatalytic reaction (Baghriche et al. 2016) and it can behave as an electron acceptor and thus can form hydroxyl radicals through the following reaction.
First, the direct photolysis of H 2 O 2 and generation of free radicals occur after the absorption of visible light (Wong et al. 2003) which is to be expected the dominant rate-enhancing mechanism in this process (Eq. 1). Another minor mechanism which has been preferred by Ollis et al. (1991) and Ilisz et al. (1998) may partially affect to the rate enhancement, in which H 2 O 2 is recommended to be a better electron acceptor than oxygen. This would minimize the possibility of electron-hole recombination and can generate one hydroxyl radical as shown in Eq. ii (Gao et al. 2002), rather than the weaker O 2 ·− radical (Eq. v). Finally, these generated radicals (HO · , O 2 ·− ) are the main active species for the degradation of MB dye molecules. In this way of the dual catalysts, the nanocomposite and H 2 O 2 can affect the degradation of MB dye.

Conclusions
Zinc chromite nanoparticles (ZnCr 2 O 4 NPs) and graphene hybridized chromite nanocomposite (ZnCr 2 O 4 -rGO) were synthesized by the simple reflux condensation method. Different characterization techniques have confirmed the formation of the nanomaterial and the nanocomposite. Microscopic methods showed that the particle size of zinc chromite NPs in the nanocomposite is smaller than that of the bare one and this incident has raised due to combination Scheme 2 a, b Pictorial and schematic representation of photocatalytic mechanism respectively of rGO on the nanocomposite sample. The synthesized nanostructures exhibited excellent photocatalytic activity in presence of H 2 O 2 and it is also supported by photoluminescence study and the band gap value. In a fixed time limit, ZnCr 2 O 4 -rGO nanocomposite showed approximately 96% degradation whereas the degradation efficiency is 85% in case of bare ZnCr 2 O 4 NPs. It can be concluded that these nanophotocatalysts could be used extensively for the degradation of pollutants, photocatalytic disinfection and photocatalytic hydrogen generation processes.