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, 2017b; Naghdi et al. 2018; Mohazzab et al. 2020; Jaleh et al. 2019; Nasrollahzadeh et al.2019; Ghosh et al. 2021a, 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, 2021b; Nasrollahzadeh et al. 2020; Nasrollahzadeh et al. 2021a, 2021b; Das et al. 2019; Ghosh et al. 2021a, 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), sp2 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-TiO2 (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 (ZnCr2O4) 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 ZnCr2O4 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 ZnCr2O4 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 (Ghosh et al. 2018), humidity sensing (Pokhrel et al. 2003) but no one synthesizes the low-dimensional ZnCr2O4 nanomaterials which can respond in visible light to degrade organic dye. There are several methods for synthesis of ZnCr2O4 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.

Table 1 Summary of photocatalytic performance of ZnCr2O4 nanomaterials reported in recent papers till date
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Here, in this paper, about 4–7nm ZnCr2O4 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.

Materials and methods

Materials

Zinc sulfate [ZnSO4, 7H2O] (Merck), chromium nitrate [Cr(NO3)3, 9H2O] (Merck), polyvinyl pyrrolidone (PVP) (LOBA), ultrafine graphite powder (Sigma-Aldrich), conc. sulfuric acid (Merck), conc. hydrochloric acid (Merck), potassium permanganate (Merck), liquor ammonia (Merck), hydrogen peroxide, acetone, ethanol, doubled-distilled water, methylene blue dye.

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 H2SO4 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 10% HCl followed by distilled water. Finally, the precipitate was obtained as GO.

Synthesis of ZnCr2O4-rGO spinel nanoparticle

ZnCr2O4-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 ZnCr2O4 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.

Scheme 1
scheme 1

Synthesis diagram of ZnCr2O4-rGO nanocomposites

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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 ZnCr2O4 NPs and ZnCr2O4-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, H2O2 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:

$$D\%=\frac{Ao-At}{Ao} X 100$$
(1)

Results and discussion

X-ray diffraction (XRD) patterns were collected to know the crystallographic structure of the synthesized samples ZnCr2O4 NPs and ZnCr2O4-rGO nanocomposite presented in the Fig. 1a. The characteristic peaks that appeared at 30.2°, 35.5°, 43.5°, 53.8°, 57.4°, 63.4°, and 75.2° correspond to the lattice planes of (220), (311), (400), (422), (511), (440), and (620) respectively. It is observed that all the XRD diffraction peaks for pure ZnCr2O4 sample recorded in the JCPDS 22–1107 are in a perfect match with the diffraction patterns of ZnCr2O4 in the composite material and also in ZnCr2O4 nanoparticles which confirm the formation of ZnCr2O4-rGO nanocomposite.

Fig. 1
figure 1

a XRD patterns of ZnCr2O4 NPs and ZnCr2O4-rGO nanocomposite and b FTIR spectra of GO and ZnCr2O4-rGO nanocomposite

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FTIR spectra of the synthesized sample ZnCr2O4-rGO nanocomposite and GO were recorded from 4000 to 400 cm−1. In Fig. 1b, GO has the characteristic peaks at 3395, 1725, 1620, 1053 and 1230 cm−1 for the corresponding O–H, C = O, C = C, C–O stretching and C–O–C bending respectively (Sharma et al. 2017; Sudesh et al. 2013). But in the nanocomposite sample, the presence of two strong bands at 520 cm−1 and 623 cm−1 indicates the stretching vibrations of Cr–O and Zn–O bonds respectively. At the same time, the appearance of the peaks located at around 1121 (C–O stretching) and 1627 (C = C) cm–1 and lower intensity of the peak due to O–H stretching confirms the formation of the reduced graphene oxide along with the ZnCr2O4 nanoparticles.

The absorption spectra of the ZnCr2O4-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 Cr3+ (d3) ions respectively. The hump that arises near the visible region can be attributed to band gap absorption of ZnCr2O4 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):

Fig. 2
figure 2

a UV–Vis absorbance spectra ZnCr2O4-rGO nanocomposite, b Tauc’s plot

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$$\alpha h\nu = {A(h\nu - {E}_{\mathrm{g}})}^{n}$$
(2)

where A is a constant and Eg 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 ZnCr2O4 responds to UV light but from the above equation, the plot of (αhν)2 vs. hν will give a divergence at an energy value Eg. The estimated band gap value from the plot for ZnCr2O4-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 ZnCr2O4 nanoparticles. This might be due to the strain that comes for the combination of graphene with nanoparticles after being formation of ZnCr2O4-rGO nanocomposite.

The morphology of virgin ZnCr2O4 and ZnCr2O4-rGO nanocomposite is analyzed by transmission electron microscopy. Virgin ZnCr2O4 forms by agglomeration of small particles around 15–20 nm as shown in Fig. 3a. The average particle size of ZnCr2O4 NPs in the composite (Fig. 3b) is smaller than that of virgin ZnCr2O4 NPs which is due to the presence of rGO surface. From the Fig. 3c, it is clearly shown that the ZnCr2O4 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.

Fig. 3
figure 3

FESEM images of a ZnCr2O4 NPs and b ZnCr2O4-rGO nanocomposite, c, d TEM images of ZnCr2O4-rGO nanocomposite, e particle distribution graph of ZnCr2O4-rGO nanocomposite, f EDX of ZnCr2O4-rGO nanocomposite

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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 ZnCr2O4 NPs and ZnCr2O4-rGO nanocomposite respectively. Here, the weight percent of carbon is too much less because less amount of GO was used during the synthesis time.

Table 2 EDX data of bare ZnCr2O4 NPs and ZnCr2O4-rGO nanocomposite
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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 ZnCr2O4 and ZnCr2O4-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 ZnCr2O4 to ZnCr2O4-rGO nanocomposite. This suggests the enhanced photocatalytic performance of ZnCr2O4-rGO compared to ZnCr2O4 because of lower recombination rate of photo-generated charge carriers in ZnCr2O4-rGO nanocomposite. Due to presence of rGO sheet in the nanocomposite material, the photo-generated electrons from the conduction band (ZnCr2O4) 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.

Fig. 4
figure 4

a Photocatalytic degradation efficiency, b plot of ln (A0/At) vs. time, c PL spectra of ZnCr2O4 NPs and ZnCr2O4-rGO nanocomposite

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Photocatalytic study

In order to evaluate the degradation efficiency of the two samples, the irradiation experiment was conducted by H2O2-assisted photolysis of MB. In Fig. 4a, it is shown that after visible light irradiation, the degradation of MB dye increases in the presence of H2O2 but this catalytic reaction enhances in greater extent when NPs are used as a catalyst. During the photocatalytic experiments, the nanocatalysts ZnCr2O4 and ZnCr2O4-rGO achieved 85.48% and 95.69% photodegradation after 70 min of visible light irradiation (Fig. 4a). This is due to the fact that the ZnCr2O4-rGO sample has a larger surface area than virgin ZnCr2O4 NPs because of the decreased size of ZnCr2O4 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),

$$ln ({A}_{0}/{A}_{t}) = kt$$
(3)

where A0 is the absorbance at t = 0 (initial MB absorbance), At is the absorbance at time t = t (final MB absorbance), and k is the rate constant. A linear fit curve was obtained with ln (A0/At) against illumination time and the degradation rate constant was calculated for the samples ZnCr2O4 and ZnCr2O4-rGO as shown in Table 3.

Table 3 Comparison of the degradation efficiency, the rate constant and relative correlation coefficient of different photo degradation experiments
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Photocatalytic mechanism

In order to explain the H2O2-assisted photocatalytic activity, the possible photocatalytic route has been developed in Scheme 2. When the ZnCr2O4 NPs and ZnCr2O4-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 (O2·−) combining with dissolved O2 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 H2O2 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.

Scheme 2
scheme 2

a, b Pictorial and schematic representation of photocatalytic mechanism respectively

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First, the direct photolysis of H2O2 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 H2O2 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 O2·− radical (Eq. v). Finally, these generated radicals (HO·, O2·−) are the main active species for the degradation of MB dye molecules. In this way of the dual catalysts, the nanocomposite and H2O2 can affect the degradation of MB dye.

Conclusions

Zinc chromite nanoparticles (ZnCr2O4 NPs) and graphene hybridized chromite nanocomposite (ZnCr2O4-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 of rGO on the nanocomposite sample. The synthesized nanostructures exhibited excellent photocatalytic activity in presence of H2O2 and it is also supported by photoluminescence study and the band gap value. In a fixed time limit, ZnCr2O4-rGO nanocomposite showed approximately 96% degradation whereas the degradation efficiency is 85% in case of bare ZnCr2O4 NPs. It can be concluded that these nanophotocatalysts could be used extensively for the degradation of pollutants, photocatalytic disinfection and photocatalytic hydrogen generation processes.