Introduction

In the current era, environmental pollution caused by the discharge of textile industries sewage including toxic organic dyes is a matter of serious concern. Therefore, the elimination of organic dyes before the discharge of industrial wastewater is still a challenge for researchers (He et al. 2010). Various technologies have been utilized for wastewater treatment nowadays such as aerobic biodegradation, chlorination, liquid-liquid extraction, activated carbon adsorption, membrane filtration and adsorption, and photocatalytic degradation. Among them, photocatalytic degradation of toxic organic dyes into harmless compounds under light is a simple, green, cost-efficient, and promising process (Ranjith et al. 2019; Dewangan et al. 2020). Semiconductor-based nanoparticles have attracted considerable attention with their potential applications in various fields. The hydrothermal method has been reported for the synthesis of transition metal oxide semiconductor CdO-ZnO nanocomposite. That synthesized nanocomposite showed efficient removal of rhodamine B dye (Mahendiran et al. 2019). Hong et al. have used graphene oxide (GO)/V2O3 and GO/TiO2 composites for hydrogen storage (Hong et al. 2012). Liu et al. have synthesized NiO/MnO2 nanocomposite for flexible supercapacitor application (Liu et al. 2018). Rahman et al. have reported selective chloroform sensing property of NiO/multi-walled carbon nanotubes (MWCNT) nanocomposite (Rahman et al. 2016). Photocatalytic evolution of H2 has been studied by Pt-loaded Zinc vacancies in ZnO–ZnS system (Liu et al. 2021b). Herein, the production of an intermediate energy level takes place because of the presence of zinc vacancies, the introduction of Pt single atoms promotes the type-V electron transport from the conduction band (CB) of ZnO to the intermediate energy level and after that to the Pt atom. The type-V electron transport not only plays a key role in having a high reduction potential of photogenerated electrons but also prohibits the recombination of carriers. The synthesis of the Ni/C/Al2O3 framework and its successful use as a catalyst has been reported for the reduction of perfluoroalkyl carboxylic acid from wastewater (Liu et al. 2021a). The production of H2 via selective oxidation of benzyl alcohol to benzaldehyde has been reported using VC/CdS (vanadium carbide/cadmium sulfide) nanowires (Tayyab et al. 2022). Semiconductors are the most preferred photocatalysts for the degradation of organic dyes due to their chemical properties, physical properties, and energy bandgap (Zhang et al. 2019). There is a formation of electron and hole pair by light radiation on the surface of semiconductor photocatalyst due to its suitable energy bandgap. This electron and hole pair further bring out a radical chain reaction for the photocatalytic degradation of toxic organic dyes into non-toxic compounds (Zhu and Zhou 2019; Abukhadra et al. 2018).

Transition metal oxide semiconductor-based nanocomposites are widely investigated for enhanced photocatalytic degradations of organic dyes. These nanocomposites have been synthesized using different synthesis methods. For example, Co3O4/ZnO nanocomposite-based photocatalyst has been synthesized using a microwave-assisted method (Hassanpour et al. 2017). ZnO/graphene nanocomposite has been synthesized using a one-step kinetic spray process. Synthesized ZnO/graphene nanocomposite has efficiently degraded methylene blue dye solution (Abd-Elrahim and Chun 2021). ZnO/TiO2 photocatalyst has been fabricated by anodizing and calcinations (Hou et al. 2021). ZnO/TiO2 nanocomposite has also been synthesized using the sol-gel method. ZnO/TiO2 nanocomposite has shown photocatalytic degradation of methylene blue dye (Din et al. 2018).

NiO and Cr2O3 transition metal oxides are p-type semiconductors with energy bandgaps of 3.6 to 4.0 eV for NiO and ~3.4 eV for Cr2O3 respectively (Madkour et al. 2016; Rahimi-Nasarabadi et al. 2016). This wide energy bandgap of NiO is making it suitable for photocatalytic degradation of dye molecules (Aminuzzaman et al. 2021). The photocatalytic activity also depends on surface area (morphologies), particle size, and crystalline behavior (Aminuzzaman et al. 2021). Both NiO and Cr2O3 have similar properties (according to energy bandgap). Therefore, nanocomposites of both NiO and Cr2O3 show greater associated functionality than individual NiO and Cr2O3 semiconductors (Al-Hada et al. 2020). Because of this reason, we have tried to synthesize p-p junction NiO/Cr2O3 nanocomposite for enhancing its photocatalytic behavior against organic dyes.

Different approaches for the preparation of NiO/Cr2O3 nanocomposite have been reported in the literature such as a facile hydrothermal method (Maheshwaran et al. 2021), combustion method (Krishna et al. 2018), and co-precipitation method (Zoromba et al. 2019; Ma et al. 2015), evaporation and drying method (Mohammad et al. 2020), etc. These methods of nanocomposite synthesis have some disadvantages such as synthesis at high-temperature, use of hazardous reagents, production of chemical wastes, and others.

In this study, a simple two-step chemical reduction method is used for the synthesis of NiO/Cr2O3 nanocomposite, where harmless tri-sodium citrate is used as stabilizing agent/capping agent and reaction is occurred under low-temperature conditions. The comparative photocatalytic activities of synthesized NiO/Cr2O3 nanocomposite are investigated against crystal violet (CV) and methylene blue (MB) dyes under sunlight irradiation. Photocatalytic stability of synthesized NiO/Cr2O3 nanocomposite has been checked for five cycles. We have also conducted photoelectrochemical measurements such as photocurrent, electrochemical impedance spectroscopy (EIS), and Mott-Schottky plot of synthesized NiO/Cr2O3 nanocomposite.

Experimental section

Materials

Nickel (II) acetate tetrahydrate (Ni(CH3COO)2.4H2O, 98%, extra pure), Chromium (III) acetate anhydrous (Cr(CH3COO)3, 98%, extra pure), Sodium hydroxide (NaOH, 96%), Sodium sulfate anhydrous (Na2SO4, AR grade), Methylene blue (96%), and Crystal violet (96%) were purchased from Central Drug House (P) LTD. Trisodium citrate dihydrate (98%, extra pure) was purchased from Loba Chemie PVT. LTD. Carbon black (super P) and Pyrrolidinone (99 %) were purchased from Thermo Fisher Scientific Pvt. Lid. India. Poly(vinylidene fluoride) was purchased from Sigma Aldrich Chemie USA. All chemicals were used without any further purification and deionized water was used in all experiments.

Synthesis of NiO/Cr2O3 nanocomposite

NiO/Cr2O3 nanocomposite was synthesized through a simple two steps chemical reduction method. In the first step, 200 mM of trisodium citrate dihydrate was added to 50 mL of 0.2 M nickel (II) acetate tetrahydrate aqueous solution. Then, 50 mL of 2.0 M sodium hydroxide aqueous solution was added to the above solution with stirring. The obtained precursor solution was heated at 40 °C with stirring for one hour using a hot plate magnetic stirrer. After that in the second step, a 50 mL solution of 0.2 M chromium (III) acetate anhydrous was added drop-wise into the reaction solution followed by heating for another two hours by maintaining the same reaction conditions. Finally, the obtained sample was washed with deionized water and ethyl alcohol and dried at 80 °C for 5 h. The obtained sample was then placed in a furnace for calcination at 700 °C for 8 h to get NiO/Cr2O3 nanocomposite.

Characterizations of NiO/Cr2O3 nanocomposite

Powder X-ray diffraction (PXRD) data were collected from the Bruker AXS D8 Discover instrument (Cu Kα radiation, λ = 1.54184 Å). Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and elemental mapping analysis were performed by using a JEOL scanning electron microscope (SEM coupled with EDS, Japan model: JSM6610LV) at accelerating voltage and magnification of 20 kV and X 27,000 respectively. Transmission electron microscopy (TEM) analysis was performed by using a JEOL, Transmission electron microscope (JEM-F200) at 100 Kx magnification and 200 kV accelerating voltage, Fourier transform infrared spectroscopy (FTIR) was examined on the Perkin Elmer FTIR spectrometer with ATR & Specular reflectance for the determination of functional groups of the synthesized nanocomposite. Spectramax M2e UV-visible spectrophotometer was used to record the UV-Visible spectrum.

Photocatalytic activity of NiO/Cr2O3 nanocomposite

Photocatalytic activity of synthesized NiO/Cr2O3 nanocomposite was explored against the degradation of crystal violet (CV) and methylene blue (MB) dyes. The degradation experiments were carried out for both the abovementioned dyes separately in the absence and presence of NiO/Cr2O3 nanocomposite. Herein, 50 mL of 10 ppm aqueous dye solutions (both CV and MB separately) was irradiated under sunlight for 30 min in absence of NiO/Cr2O3 nanocomposite. After that, the photocatalytic degradation experiments were performed with 50 mg of NiO/Cr2O3 nanocomposite separately under sunlight within 30 min for CV and 180 min for MB respectively. The former mixtures of dye solutions and NiO/Cr2O3 nanocomposite were stirred in dark for 20 min to achieve adsorption-desorption equilibrium before being irradiated by natural sunlight. Dyes samples were collected at regular intervals of time and analyzed using the UV-visible spectrophotometer (Spectramax M2e UV-visible spectrophotometer). Five cycles of experiments have been run for each dye to understand reusability of synthesized NiO/Cr2O3 nanocomposite as a photocatalyst.

Photoelectrochemical characterizations of NiO/Cr2O3 nanocomposite

Photoelectrochemical characterizations were observed using a CHI 760E electrochemical workstation. NiO/Cr2O3 nanocomposite, carbon black, and poly (vinylidene fluoride) in 8:1:1 ratio were taken in a mortar pestle and crushed together for few minutes. After that, 2 drops of pyrrolidinone were added to the above mixture as solvent and mixed well to get a uniform paste. The above paste was then uniformly spread on sheet of stainless steel having area of 1.0 cm2. This NiO/Cr2O3 nanocomposite modified stainless steel electrode was then dried in a hot air oven at 60 °C. NiO/Cr2O3 nanocomposite modified stainless steel electrode was used as working electrode, Pt wire electrode was used as counter electrode, and Ag/AgCl electrode was used as reference electrode. All the photoelectrochemical measurements were performed in 0.5 M Na2SO4 electrolyte. The one-solar (100 mW/cm2) Xenon lamp was used as light source. The photocurrent experiment was performed for the run time of 240 s at 0.5 V. The electrochemical impedance spectroscopy (EIS) experiment was performed under light at a frequency range of 1–105 Hz with amplitude of 0.005 V. The Mott-Schottky experiment was performed under light at a frequency 1000 Hz with amplitude of 0.005 V.

Results and discussion

Powder X-ray diffraction and UV-visible spectrum analysis of NiO/Cr2O3 nanocomposite

Powder X-ray diffraction (PXRD) analysis has been used to detect the crystalline phase of NiO/Cr2O3 nanocomposite. Figure 1a shows the PXRD pattern of NiO/Cr2O3 nanocomposite. The 2θ peaks have been obtained at 37.96°, 43.57°, 62.42°, 76.46°, and 79.22° respectively. These 2θ values have been indexed corresponding to hkl planes (111), (200), (220), (311), and (222) respectively for synthesized NiO (Srirattanapibul et al. 2022). The 2θ peaks have been determined for synthesized Cr2O3 at 23.91°, 34.21°, 35.33°, 39.98°, 41.51°, 44.52°, 50.40°, 54.83°, 58.97°, 65.44°, 67.26°, 73.43°, 78.44°, and 80.57° respectively. Their corresponding hkl planes have been assigned as (012), (104), (110), (006), (113), (202), (024), (116), (122), (214), (300), (010), (220), and (306) respectively. These data are found to be similar to the reported values in the literature (Fu et al. 2015; Al-Hada et al. 2020). Obtained powder X-ray diffraction patterns of the sample confirm the presence of crystalline phase of nanocomposite NiO/Cr2O3 along with some impurity peaks. Obtained PXRD pattern has been matched with its cubic structure (JCPDS No. 47-1049) for NiO NPs and with the hexagonal structure (JCPDS No. 38-1479) for Cr2O3 NPs.

Fig. 1
figure 1

(a) Powder X-ray diffraction (PXRD) of NiO/Cr2O3 nanocomposite, (b) UV-visible absorbance spectrum of NiO/Cr2O3 nanocomposite, and (c) Tauc’s plots (αhν)2 vs hν of NiO/Cr2O3 nanocomposite

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Figure 1b shows the UV-visible spectrum recorded in the wavelength of range 200–800 nm. The absorption maxima have been obtained at 230 nm and 370 nm for Cr2O3 (Singh et al. 2017) and NiO (Alagiri et al. 2012) respectively. The values of the energy bandgap of the synthesized NiO/Cr2O3 nanocomposite have been determined using Tauc’s equation, as given below.

$$\left(\alpha h\nu \right)=A\ {\left( h\nu -{E}_g\right)}^n$$
(1)

Where α is the absorption coefficient, hν is the photon energy, A is a constant, Eg is the bandgap energy of the material and n is an exponent (n = 1/2 for allowed direct transition and n = 2 for allowed indirect transition). Equation (1) is given in the literature (Barir et al. 2017). Tauc’s plot is shown in Fig. 1c. Calculated bandgap energy values are 2.45 eV for Cr2O3 and 4.06 eV for NiO. Similar values of bandgap energy are also reported in the literature (Al-Hada et al. 2020; Al-Hada et al. 2021). Cr2O3 exhibits a decrease in energy bandgap value than bulk Cr2O3 due to defects. These defects are interstitials defect or vacancy defects (Guillén and Herrero 2021), whereas NiO exhibits a slight blue shift due to the decrease in particle size and quantum confinement (Alagiri et al. 2012).

Scanning electron microscopic, transmission electron microscopic, elemental mapping and energy-dispersive spectroscopic analysis of NiO/Cr2O3 nanocomposite

Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) analysis reveal the surface morphology and size of synthesized NiO/Cr2O3 nanocomposite. Figure 2 represents the scanning electron micrographs and transmission electron micrographs of NiO/Cr2O3 nanocomposite. As it is evident from the SEM images, Fig. 2a and Fig. 2b are the zoom-out parts of an SEM image in Fig. 2c. The synthesized NiO/Cr2O3 nanocomposite has sheet-shaped morphology. These sheets are with rough surfaces and micro-sized to nano-sized cracks. There are also a few external growths on the surface of the sheets. The crack sizes of the NiO/Cr2O3 nanocomposite have been observed in the range of ~25 to ~500 nm (as marked in Fig. 2a and Fig. 2b). The possible reason for these cracks is heating at a high temperature (700 °C). The cracked morphology of synthesized NiO/Cr2O3 nanocomposite is also confirmed by the TEM image presented in Fig. 2d. The selected area electron diffraction (SAED) pattern of synthesized NiO/Cr2O3 nanocomposite with concentric circles reveals its crystalline nature (Rani et al. 2021). Due to the presence of cracks, the surface area of NiO/Cr2O3 nanocomposite has increased which further enhanced the photocatalytic activity against MB and CV organic dyes. The heat treatment has also been used to induce nano-cracks in CeO2-TiO2 hybrid nanostructures (Veziroglu et al. 2019). EDS and elemental mapping analysis also confirm the synthesis of NiO/Cr2O3 nanocomposite. Figure 2f, g, h presents the elemental mapping of O, Ni, and Cr in synthesized NiO/Cr2O3 nanocomposite (selected area for elemental mapping is given in Fig. 2e). The elemental mapping of synthesized NiO/Cr2O3 nanocomposite confirms the uniform distribution of O, Ni, and Cr elements. The EDS analysis of synthesized NiO/Cr2O3 nanocomposite is reported in Fig. 2i. The obtained peaks corresponding to Ni, Cr, and O elements are presented in NiO/Cr2O3 nanocomposite. The percentage elemental composition of NiO/ Cr2O3 nanocomposite has been determined with EDS analysis. Obtained data are given in Table 1. A small amount of Ni than Cr confirms the presence of Cr2O3 on the surface of NiO. Similar EDS data have been reported by Li et al. for ZnO/SiO2 nanocmposite. Herein, SiO2 is found on the surface of ZnO (Li et al. 2009).

Fig. 2
figure 2

(a), (b), (c) SEM images of synthesized NiO/Cr2O3 nanocomposite, (d) TEM image of synthesized NiO/Cr2O3 nanocomposite, (e) is selected area of synthesized NiO/Cr2O3 nanocomposite for elemental mapping, (f), (g), (h) elemental mapping images (O, Ni, and Cr respectively), and (d) energy-dispersive spectroscopy (EDS) of synthesized NiO/Cr2O3 nanocomposite

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Table 1 Energy-dispersive spectroscopy (EDS) analysis
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Fourier transform infrared spectroscopy analysis of NiO/Cr2O3 nanocomposite

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) has been used for studying the functional groups present on the surface of the synthesized NiO/Cr2O3 nanocomposite. The FTIR spectrum of NiO/Cr2O3 nanocomposite has been recorded in the spectral range of 400–4000 cm-1 (Fig. 3). Obtained peaks between 418 and 877 cm-1 are due to metal oxide bond vibrations. The peak located at 1074 cm-1 is due to the bending vibration of water absorbed on the surface of NiO/Cr2O3 nanocomposite. The peaks observed at 1336 cm-1, 1421 cm-1, and 1558 cm-1 correspond to C–O stretching vibrations, C–H stretching vibrations, and C=O stretching vibrations of the carboxyl or carbonyl group respectively. The obtained peak at 1648 cm-1 and the broad absorption peak around 3295 cm-1 are due to OH stretching (Hema et al. 2013; Liang et al. 2012).

Fig. 3
figure 3

FTIR spectrum of synthesized NiO/Cr2O3 nanocomposite

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Evaluation of photocatalytic activity of NiO/Cr2O3 nanocomposite

Sunlight is a very abundant, eco-friendly, freely available and natural source of light (Pirzada et al. 2019). Therefore, we have used natural sunlight as a light source for studying the photocatalytic activity with synthesized NiO/Cr2O3 nanocomposite. Fifty milliliters of (10 ppm) CV and 50 mL of (10 ppm) MB aqueous dye solutions have been used separately to monitor the photocatalytic activity of synthesized NiO/Cr2O3 nanocomposite under sunlight exposure. Two sets of experiments have been performed with each dye. The first experiment was performed in absence of NiO/Cr2O3 nanocomposite and the second experiment was performed in presence of NiO/Cr2O3 nanocomposite with both CV and MB dyes. Samples of dyes have been collected at regular intervals of the time throughout the photocatalytic experiments and degradation observed using UV-visible spectroscopy. The results are shown in Fig. 4a and Fig. 4c respectively. These figures indicate that there is no change in absorption maxima of both the dyes in absence of NiO/Cr2O3 nanocomposite under 30 min sunlight irradiation. This confirms that the degradation of dyes does not occur in the absence of NiO/Cr2O3 photocatalyst under 30 min sunlight exposure. Figure 4b and Fig. 4d demonstrated a significant decrease in the absorption maxima of both CV and MB organic dyes under sunlight irradiation for 30 min and 180 min respectively (absorption maximum of CV is at 580 nm and absorption maximum of MB is at 660 nm). By the Beer-Lambert law, the decrease in absorption maxima confirms the reduction in the concentration of dyes with an increase in sunlight exposure time (Patel et al. 2021; Shubha et al. 2021). The synthesized NiO/Cr2O3 nanocomposite displays magnificent photocatalytic activity towards CV and MB dyes under sunlight irradiation. We have calculated the degradation efficiency of the NiO/Cr2O3 photocatalyst for both CV and MB dyes using Eq. (2). Equation (2) is reported in the literature (Pirzada et al. 2019).

$$\mathrm{Efficiency}\%=\left(\frac{\left({C}_0-{C}_t\right)}{C_o}\right)\times 100$$
(2)
Fig. 4
figure 4

The UV-visible spectrum of (a) CV dye samples in absence of NiO/Cr2O3 nanocomposite under 30 min sunlight irradiation, (b) CV dye samples in presence of NiO/Cr2O3 nanocomposite under 30 min sunlight irradiation, (c) MB dye samples in absence of NiO/Cr2O3 nanocomposite under 30 min sunlight irradiation, and (d) MB dye samples in presence of NiO/Cr2O3 nanocomposite under 180 min sunlight irradiation

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Here, C0 is the initial concentration of dyes, and Ct is the concentration of dyes at different intervals of time in the photocatalytic reaction. The calculated efficiency of NiO/Cr2O3 photocatalyst for CV has been obtained at 58.10 % within 2 min of sunlight irradiation. 88.47 % degradation of CV has been observed after 30 min of sunlight irradiation in presence of photocatalyst. Photocatalytic degradation efficiencies of the above photocatalyst for MB are 59.90 % within 30 min of sunlight irradiation and 93.63 % within 180 min of sunlight irradiation. The results exhibit that NiO/Cr2O3 nanocomposite acts as an efficient photocatalyst for the degradation of both CV and MB dyes under sunlight exposure. Present work is improved in terms of a light source, irradiation time, and degradation efficiency compared to other photocatalysts. For example, SnSe nanocrystal has been reported with 88.68 % degradation of MB dye and 98 % degradation of CV dye under 360 min of UV light irradiation (Patel et al. 2021) and K2Ti6O13 nanoparticles have been reported with 79.23 % degradation of MB dye under 105 min UV light exposure (Somashekharappa and Lokesh 2021). NiO/Co3O4 nanocomposite has also been studied for (89.88%) degradation of methylene blue under sunlight within 360 min (Yadav et al. 2022).

The photocatalytic degradation efficiencies of NiO/Cr2O3 photocatalyst against CV and MB dyes are given in Table 2. A comparative study on the photocatalytic activity of earlier reported photocatalyst with NiO/Cr2O3 photocatalyst against CV and MB dyes is given in Table 3.

Table 2 The photocatalytic efficiency of synthesized NiO/Cr2O3 nanocomposite for CV and MB at different times of sunlight irradiation
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Table 3 Comparative study of earlier reported photocatalytic behavior of various photocatalysts with present work against crystal violet (CV) and methylene blue (MB) dyes
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A possible photocatalytic reaction mechanism can be explained with help of the formation of electron-hole pairs on the surface of the NiO/Cr2O3 photocatalyst. Sunlight exposure excites electrons from the valance band to the conduction band to get an electron-hole pair on the surface of the photocatalyst. In detail, electrons from the conduction band of nanocomposite NiO/Cr2O3 react with an O2 molecule to produce (O2•–) superoxide radical and holes from the valance band react with an H2O molecule to produce (OH–) hydroxyl anion. Hydroxyl anion further reacts with the hole to produce (OH•) hydroxyl radical. Finally, this highly reactive hydroxyl radical is responsible for the degradation of CV and MB dye molecules (Patel et al. 2021; Somashekharappa and Lokesh 2021; Sanakousar et al. 2021). A graphical representation of possible general photocatalytic reaction mechanism for the degradation of organic dyes using metal oxide semiconductors based photocatalysts is given in Fig. 5. Morphology has an important influence on the photocatalytic activity of semiconductor photocatalysts. The reason behind this is the dependence of the surface area/surface active site of a photocatalyst. Herein, NiO/Cr2O3 photocatalyst has high surface active sites (surface area) because of the presence of cracks on its surface. High surface active sites help in increasing the adsorption of dye molecules on the surface of the photocatalyst. This increased adsorption of dye molecules on the surface of NiO/Cr2O3 photocatalyst is responsible for enhanced photocatalytic activity against CV and MB dye solution under sunlight exposure. A similar example is reported in the literature. Where, nano-cracks present in the CeO2-TiO2 hybrid nanostructure show better photocatalytic activity against MB dye than CeO2-TiO2 hybrid nanostructure without nano-cracks (Veziroglu et al. 2019).

Fig. 5
figure 5

A graphical representation of possible general photocatalytic reaction mechanism for the degradation of organic dyes using metal oxide semiconductors based photocatalysts (Tayyab et al 2022), Copyright © 2022 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved, copied with permission

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A kinetic study of photocatalytic degradation of crystal violet (CV) and methylene blue (MB) dye solutions using NiO/Cr2O3 nanocomposite is given in Fig. 6a and Fig. 6b. We have used Eq. (3) for the calculation of photodegradation rates of CV and MB dyes in presence of synthesized photocatalyst. Equation (3) is given in the literature (Somashekharappa and Lokesh 2021). The linear fitted straight lines (Fig. 6a and Fig. 6b) confirm the pseudo-first-order reaction kinetics for the above photocatalytic reaction (Idris et al. 2021).

$$\mathit{\ln}\left(\frac{C_o}{C_t}\right)=- kt$$
(3)
Fig. 6
figure 6

Kinetics of photocatalytic degradation of (a) crystal violet (CV) dye with NiO/Cr2O3 nanocomposite, (b) methylene blue (MB) dye with NiO/Cr2O3 nanocomposite, and (c) C/C0 vs. time of irradiation plot for photocatalytic efficiency NiO/Cr2O3 nanocomposite against CV and MB dyes under sunlight exposure

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Where C0 is the initial concentration of dye solutions, Ct is the concentration of dye solutions at time interval t, k is rate constant (min-1) and t is the time of irradiation of sunlight. The observed rate constant values for NiO/Cr2O3 nanocomposite were 0.04732 min-1 and 0.01179 min-1 against crystal violet and methylene blue organic dyes respectively. Rate constant data suggest that NiO/Cr2O3 nanocomposite has a higher rate of degradation for crystal violet than methylene blue organic dyes under sunlight exposure. A plot between C/C0 vs. time of irradiation is given in Fig. 6c. It is clear from the graph that the degradation efficiency of CV is better than MB dyes in the presence of NiO/Cr2O3 nanocomposite. Finally, the results displayed that NiO/Cr2O3 nanocomposite has excellent photocatalytic activities for both crystal violet and methylene blue dye solutions under the irradiation of natural sunlight. NiO/Cr2O3 nanocomposite shows a better photocatalytic reaction rate for crystal violet dye solution than the methylene blue dye solution.

The reusability of synthesized NiO/Cr2O3 nanocomposite was studied and illustrated in Fig. 7a and b. This photocatalyst has shown 79.68 % of CV dye degradation in second cycle and 74.47 % of CV dye degradation in fifth cycle (within 30 min of sunlight irradiation). Synthesized NiO/Cr2O3 nanocomposite has shown 93.38 % of MB dye degradation in second cycle and 86.34 % of MB dye degradation in fifth cycle (with in 180 min of sunlight irradiation). Also, Fig. 7c and d illustrated the TEM images of synthesized NiO/Cr2O3 nanocomposite after the photocatalytic degradation of CV and MB dyes respectively. The TEM images reveal that the synthesized NiO/Cr2O3 nanocomposite is stable after five cycles of photocatalytic reaction. Consequently, it is clear that synthesized NiO/Cr2O3 nanocomposite exhibited excellent reusability and stability for photocatalytic degradation of both the organic dyes under sunlight exposure.

Fig. 7
figure 7

(a), (b) Five cycles of photodegradation of CV dye and MB dyes respectively using NiO/Cr2O3 nanocomposite, (c) and (d) TEM images of NiO/Cr2O3 nanocomposite after photodegradation of CV dye and MB dye respectively

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Photoelectrochemical measurements of NiO/Cr2O3 nanocomposite

Photoelectrochemical measurements of synthesized NiO/Cr2O3 nanocomposite are given in Fig. 8. The transient photocurrent spectrum (Fig. 8a) reveals that the current of synthesized NiO/Cr2O3 nanocomposite modified stainless steel electrode was increased very fast in solar light irradiation than dark conditions. This increase in photocurrent/photoconductivity is because of rapid formation and movement of electron-hole pair under solar light irradiation. This consequently leads to better photocatalytic activity of synthesized NiO/Cr2O3 nanocomposite. Similar results are given in literature (Cui et al. 2018; Tayyab et al. 2022). The EIS Nyquist plot (Fig. 8b) of synthesized NiO/Cr2O3 nanocomposite shows very small diameter of the semicircle (the semicircle is observed at high frequency region). The small diameter of the semicircle indicated that small charge transfer resistance. This confirms that synthesized NiO/Cr2O3 nanocomposite has increased electron-hole separation for enhanced photocatalytic activity under solar light as reported in literature (Wang et al. 2020). Figure 8c shows Mott-Schottky plot of synthesized NiO/Cr2O3 nanocomposite. The negative slope of Mott-Schottky plot confirms that synthesized NiO/Cr2O3 nanocomposite is a p-type semiconductor. We have also observed the flat band potential (VFB) of synthesized NiO/Cr2O3 nanocomposite by extrapolating the linear portion of Mott-Schottky plot (Wang et al. 2020). The observed VFB value of synthesized NiO/Cr2O3 nanocomposite is +1.01 V with reference to Ag/AgCl electrode. The VFB value of p-type semiconductor is generally considered as approximate equal to the VB edge potential (Wang et al. 2020). The above photoelectrochemical study supports the enhanced photocatalytic activity of synthesized NiO/Cr2O3 nanocomposite against organic dyes degradation under sunlight irradiation.

Fig. 8
figure 8

(a) Transient photocurrent, (b) the EIS Nyquist plot, and (c) electrochemical Mott-Schottky plot of synthesized NiO/Cr2O3 nanocomposite

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Conclusion

In summary, NiO/Cr2O3 nanocomposite has been successfully fabricated by a simple chemical reduction method. Synthesized nanocomposite has been characterized using PXRD, SEM, TEM, EDS, elemental mapping, FTIR, and UV-visible spectroscopy. The PXRD and EDS analysis confirmed the presence of both NiO and Cr2O3 crystal phases in the synthesized photocatalyst. Nano to micro size cracked morphology of synthesized NiO/Cr2O3 nanocomposite was observed by SEM analysis. Synthesized photocatalyst exhibits high photocatalytic efficiencies of 88.47 % for crystal violet in 30 min sunlight exposure and 93.63 % for methylene blue in 180 min sunlight exposure. The photocatalytic degradation occurred for both the dyes and following the pseudo-first-order kinetics. The calculated rate constant values are 0.04732 min-1 for crystal violet degradation and 0.01179 min-1 for methylene blue degradation. We have also performed reusability tests of synthesized photocatalysts up to 5 cycles. NiO/Cr2O3 nanocomposite exhibited excellent reusability and stability for photocatalytic degradation of both methylene blue and crystal violet dyes. The synthesized NiO/Cr2O3 nanocomposite has been characterized photoelectrochemically using the photocurrent, EIS, and Mott-Schottky plot. The photoelectrochemical analysis supported the enhanced photocatalytic behavior of synthesized NiO/Cr2O3 nanocomposite for organic dye degradation under sunlight exposure. Thus, NiO/Cr2O3 nanocomposite is proved to be an effective photocatalyst for the degradation of crystal violet and methylene blue under sunlight irradiation.