Green synthesis of NiO/ZnO nanocomposites for the adsorption of various dyes

A significant waste, including dyes in water, is generated during textile industrial processes, which causes environmental challenges. Herein, various nanocomposites (NC) of nickel oxide (NiO) and zinc oxide (ZnO) were prepared by solvothermal assisted green method where ethanolic extract of spinach leaves were used as a green source. The ultraviolet-visible (UV–vis) spectroscopy revealed that the band gap energies and absorption maxima of NiO/ZnO were 2.25 eV and 371 nm for 1:1 NC, 2.07 eV and 380 nm for 5:1 NC, and 2.02 eV and 385 nm for 1:5 NC. Ultraviolet-visible (UV–vis) spectroscopy, x-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) were employed to investigate the optical and structural characteristics of the NCs. The XRD patterns of NiO/ZnO NCs (i.e., 1:1, 5:1, 1:5) displayed crystallite sizes of 44.6, 52.17, and 42.5 nm, respectively. Associations of different functional groups with the surfaces of NC was confirmed by FTIR. Batch method was used to conduct the NCs-mediated adsorption of methylene blue (MB), methyl red (MR), and methyl orange (MO). Furthermore, several factors, including dye concentration, contact time, and temperature that affected the sorption, are reported. Pseudo-first order and pseudo-second order kinetic models were utilized to examine the adsorption kinetics. For all the dyes studied, pseudo-first order is the one which fitted best to the kinetic data, based on regression coefficient (R2). Indeed, experimental data were found to follow the Langmuir model. The maximum uptake capacity (qm) of MB adsorbed on NiO/ZnO NC (1:1) was 370 mg g−1 which is higher compared to that of values reported in the literature. These findings report a dual method (i.e., solvothermal-green chemistry) contribute to the development of efficient and cost-effective methods for wastewater treatment and environmental remediation.


Introduction
Synthesis of cost-efficient and environmentally non-hazardous nanomaterials has always been a priority in every research field of science (Iqbal et al 2021).Though traditional approaches are commonly employed yet we can't deny their disadvantages include the use of highly toxic reagents and non-biodegradable stabilizing agents, high processing costs, and specific troubleshooting (Zhang et al 2020).For the fabrication of nanoparticles (NPs), biogenic synthesis is regarded as an economic and eco-friendly process.Metal oxides derived from green synthesis are gaining interest because of their relatively low-ecotoxicity, high simplicity, cost-effectiveness (Shuaixuan et al 2022).As plant extract yields bio-reactor agents like phenolic compounds, flavonoids, amino acids, enzymes, polysaccharides, proteins, terpenoids, and alkaloids, which phytochemicals are postulated to act as reducing and capping agents (Flieger et al 2021).Plants have a greater capacity to function as capping as well as reducing agents due to different antioxidants and phytochemicals they contain (Da'na et al 2021).
One of the challenge in the textile industries is the remediation of wastewater containing dyes (Naseem et al 2021).Annually, the environment receives emission of about 50,000 tons of dyes (Nagajyothi et al 2020).The consequences of such a waste released into rivers include pH alteration, reduction in light penetration and oxygen levels, which negatively affect aquatic life (Siddique et al 2021).
MB, MR, and MO are the most commonly used dyes in fabric industries.Methyl red is an azo-linked (-N=N-) organic dye.Hair, flax, paper, wood, and silk industries use the cationic MB dye.MB not only causes respiratory problems such as shortness of breath, but also gives rise to burning, nausea, vomiting, diarrhea, and gastroenteritis if swallowed (Shimosako and Sakama 2021).Hair, flax, paper, wood and silk industries use MB that is a cationic dye (Khan et al 2022).MO is one of the important Azo dyes which is mainly used in paper manufacturing, research labs, printing, textile, pharmaceutical, and food sector.It causes series of behavioral changes along with allergic, mutagenic, and carcinogen effects on the living creatures (Sharf et al 2023).To address water quality issues in the natural environment, various methods such as photocatalysis, nano-filtration, adsorption, and electrochemical oxidation are used (Anantha et al 2020).Among these processes, adsorption has been found to be superior and most efficient technique as compared to others for waste product treatment in terms of cost and simple operation.
A number of metal oxides like Ag 2 O, TiO 2 , ZnO, CuO, NiO and SnO 2 have been reported for dye remediation (Akbari et al 2020, Haq et al 2020, Rashmi et al 2020, Dodoo-Arhin et al 2021, Koppala et al 2021).ZnO, due to its band gap of 3.32 eV, is regarded as a bio-safe semiconductor that finds utility in a variety of fields, such as catalysis as well as chemical and biological sensing (Sharma et al 2022).Moreover, ZnO shows high resistance to microorganisms and carries environmentally non-hazardous mineral (Zn) that is necessary for human beings and there is less toxicity in it because of its longer lifespan (Siddiqi et al 2018).Since it is a p type semiconductor with a band gap of 3.5 eV and exhibits both electric and magnetic properties, nickel oxide (NiO) finds extensive uses.Since it is a p type semiconductor with a band gap of 3.5 eV and exhibits both electric and magnetic properties, nickel oxide (NiO) finds extensive use (Taghizadeh 2016).Applications of NiO NCs are in gas sensor, Li-ion batteries and electrochemical capacitors (Bhatia and Nath 2020).Together with their usage as photocatalysts, gas sensors, super capacitors, UV photodetectors, and adsorbents for the removal of various dyes, mixed metal oxides, such as NiO-ZnO, have evolved into versatile semiconductors (Zhou et al 2021).
The current literature has reported the synthesis of NiO/ZnO NC using various methods for dyes removal.Thereby, a green (e.g., neem leaf extract) and cost-effective preparation of different ratios of NiO/ZnO NC was applied to remove MO from an aqueous suspension (Da'na et al 2021).Also, NiO/ZnO NC, synthesized by a simple microwave method, showed a suitable photocatalytic activity for Rhodamine B and MB (Hassanpour et al 2017).Furthermore, NiO/ZnO NC, fabricated through an affordable co-precipitation approach, was capable of eliminating MB efficiently from industrial wastewater (Paul et al 2021).
In this study, the hydrothermal synthesis of NiO/ZnO NCs was carried out using spinach extract as a green template.To the best of our knowledge, this is the first report using two different approaches, i.e., hydrothermal/solvothermal method and green chemistry for the synthesis of such a NC.Also, spinach extracts have been used for the synthesis of bare ZnO nanostructures and Fe NPs but, has never been used for NiO/ZnO NCs.Hence, this work aims at reporting the crystalline, optical, and structural properties of NiO/ZnO NCs (synthesized at various ratios) and their capacity to adsorb dyes (i.e.MB, MR, and MO) from an aqueous solution using a range of variables, including temperature, contact time, and dye concentration.To study the adsorption kinetics, pseudo first order as well as the pseudo second order kinetic models were used; nevertheless, for the interaction of adsorbate molecules to the surface sites, the Langmuir isotherm model was applied.

Reagents used
Nanocomposites having different molar ratio were synthesized by using Nickel chloride hexahydrate (NiCl 2 •6H 2 O), Zinc sulfate heptahydrate (ZnSO 4 •7H 2 O), Sodium hydroxide (NaOH), and Ethanol (EtOH) purchased from Sigma Aldrich.No purification was performed prior to using the chemicals.For the adsorption purpose, MR, MB, and MO were also acquired from Sigma Aldrich.The leaves of spinach were obtained from the local market, Islamabad, Pakistan.

Synthesis of nanocomposites
NiO/ZnO NCs of various molar ratio were synthesized by solvothermal assisted green method.The method used is ecofriendly and cost effective, as spinach contains vitamin A, C and K, magnesium, manganese, folate and iron.The presence of substantial antioxidant phytochemicals makes the spinach extract as good reducing agent.The extract therefore acts to reduce the cost of expensive reducing agents.In order to accomplish this, fresh spinach leaves were acquired initially, and were cleaned then by washing with tap and deionized water.Finely chopped leaves were put into 10 ml EtOH diluted with deionized water up to 250 ml.Afterwards, the contents were boiled for 30 min under vigorous stirring at 100 °C.In order to synthesize NCs, the extract that was so obtained was utilized as a green template.
NiO/ZnO NCs were produced by reacting ZnSO 4 .7H 2 O (50 mmol) with NiCl 2 .6H 2 O (50 mmol) in 100 ml spinach extract.The mixture was stirred for 20 min and then 20 ml of NaOH (50 mmol) was added slowly.After that, the contents were transferred into an autoclave having a steel lining and were heated at 180 °C for five hours.The equimolar product (1:1 NC) thus obtained was centrifuged at 12000 rpm and then cleaned three times using deionized water.Finally, the obtained NCs were oven dried for entire night at 110 °C.For 1:5 NC, NiCl 2 .6H 2 O and ZnSO 4 .7H 2 O were taken as 10 mmol and 50 mmol whereas for 5:1 NC, 50 mmol NiCl 2 .6H 2 O and 10 mmol ZnSO 4 .7H 2 O were employed.

Adsorption of MB, MO, and MR
Batch method was executed for kinetic adsorption measurement.0.020 g of NiO-ZnO NCs was taken in 7 ml of dye solution for kinetic measurements.Later, it was agitated in a temperature-controlled bath at 298, 308, and 318 K under fixed shaking speed of 130 rpm.The stock solution (500 ppm) of MO, MB, and MR was synthesized through adding 0.125 g of each dye into 250 ml of deionized water.From stock solution, further desired concentrations like 5, 10, 20, 40, 75 and 100 ppm were prepared.These dye solutions after adding the adsorbent were placed in test tubes and agitated for 2 h in end-to-end shaking water bath.Afterwards, the adsorbent was separated to avoid further contact time, and the remaining solutions were scanned by UV-vis spectrophotometer at the λ max of each dye.The adsorption capability (q e ) was calculated by using equation (1) whereas adsorbent's removal efficiency (R %) was found out using equation (2).Here, C i , C e , m, and V represented initial concentration, concentration at equilibrium time, adsorbent's mass, and adsorbate solution's volume, respectively.q e gave value of adsorbed amount of dye on per gram of adsorbent while R% showed percentage removal efficiency (Da'na et al 2021).Before analysis, preliminary dissolution test was performed to check the integrity of the adsorption where no such leaching was detected.Figure 1 depicts the overview of this work.

Results and discussion
3.1.Characterization of NiO/ZnO nanocomposites FTIR, XRD, SEM and UV-visible spectroscopy were used to characterize the NiO/ZnO NCs.The XRD patterns of NCs were recorded on JDX-3532 (JEOL Japan).The XRD pattern for NiO/ZnO NCs are visible in figure 2(a).The major diffraction patterns appearing at 2θ values 34.40°, 48.40°, 36.10°,62.80°, and 67.80°were related to (100), ( 002), ( 102), ( 103), (110) planes of ZnO hexagonal phase that were in accordance with JCPDS card No. (89-1397) while the planes at (111), and (131) with 2θ value 37.40°, and 68.90°were related to cubic NiO phase (JCPDS card No. (89-7130)).In 5:1 NiO-ZnO NC, the high intensity peak appearing at 2θ value 20.30°a ssociated to the plane (202) was for Nickel oxalate.This diffraction peak was found to be less intense in 1:1 NC while in 1:5 NC its intensity was decreased further which was due to difference in NiO: ZnO ratio in those composite materials.From the trend in the diffraction patterns, it was concluded that the diffraction intensities were proportional to the components' concentrations in the NCs.By making use of the Scherrer formula, the nanocomposite's crystallite size was determined (equation (3)): The Bragg angle was represented by θ, full width half maximum by β, and the wavelength of x-ray by λ in this case.The crystallite sizes were found to be 44.6,52.17 and 42.5 nm for 1:1, 5:1 and 1:5 NCs respectively.Keeping in view this trend, it was found that increasing the NiO component in the nanocomposite enhanced the crystallite size (K and K 2016) (Juma et al 2017).Therefore, the desired properties could be achieved by manipulating the NiO and ZnO counterparts of the nanocomposite.Utilizing the KBr pellet technique, the FTIR spectra of NiO/ZnO NCs were acquired on Shimadzu spectrometer.The spectra were obtained from 4000-400 cm −1 .A wide absorption band spanning from 3000 to 3500 cm −1 was linked to OH stretching whereas an absorption band around 1650 cm −1 was assigned to the OH group's bending mode.S-O stretching bands were observed between 1400-1000 cm −1 .A band around 460 cm −1 was assigned to Zn-O linkage whereas, another one at 600-700 cm −1 showed Ni-O stretching.The absorption band at around 850 cm −1 is assigned to the linkage between Ni and O which indicated the successful synthesis of NiO/ZnO NCs.Similar confirmatory band was reported elsewhere (Akash et al 2022).In all three nanocomposites, the bands between 400-800 cm −1 were appeared indicating the interaction of Metal-Oxygen bond.In each spectrum, a shift in absorption bands was detected which was caused by the prepared NiO/ZnO NCs' varying composition as seen from figure 2(b) (Bhatia and Nath 2020).Figure 2(c) represents the surface morphology of the nanocomposites.The images were acquired in SEM model JSM 5910 (JEOL), Japan.The samples were deposited on double stick tape and sputtercoated by gold 90 seconds prior to acquire the images.It was observed that in equimolar NC, the particles are somehow well distributed but still not discrete as in the case of 1:5 NC where uniform distribution of the particles was observed.In 5:1 NC, NiO was found dominantly as a coating material.This behaviour was noticed well in 1:5 NC where the molar concentration of ZnO is 10 times higher than NiO.Thus, in the present investigation, it is the ZnO which controls the morphology of the NC.The particles sizes were found around 80-100 nm in 1:5 and 1:1 NCs, In case of 5:1 NC, NiO was so dominant that it has masked the ZnO particles.
The Optical band gap of NiO-ZnO NCs was measured by employing UV-visible spectrophotometer model 1601 SHIMADZU.Figure 3(a) depicts the UV-visible absorption spectrum for the suspension of NiO-ZnO NCs made in DI water.UV was performed in the region of 250-800 nm.Adsorption maxima for 1:1, 5:1 and 1:5 NCs were found to be 371, 380 and 385 nm, respectively.The adsorption edge was estimated by tracing (αhυ) 2 with photon energy (hυ).The formula for measuring band gaps indirectly as well as directly using tauc plots (equation ( 4)) has been given in figures 3(b)-(d).
MB (q e mg/g) MO (q e mg/g) MR (qe mg/g)  The band gap value was obtained by extrapolating the straight line from the absorption edge to the value of zero absorption coefficient.The band gap for 1:1, 5:1 and 1:5 NCs were 2.25, 2.07 and 2.02 eV, respectively.The band gap energy for 5NiO/ZnO and NiO/5ZnO was decreased, representing the red shift by changing the ratio of starting materials (Yasmeen et al 2019).The FTIR analysis had been carried out in order to examine the bond existing among atoms of NiO-ZnO NCs.

Adsorption of MB, MO and MR
Adsorption of MB, MO and MR was carried out by varying temperature and concentration.The concentration was varied from 5 to 100 ppm and temperature in the series i.e. 298, 308, and 318 K during adsorption process.Both parameters affected the adsorption of dyes.

Effect of concentration and temperature
Temperature, concentration, and the nature of adsorbents' composition have greatly affected the adsorption of dyes.It was found that adsorption of MB, MO and MR (table 1) increased by increasing the initial concentration of dyes.With regard to influence of temperature, dye's adsorption increased as the temperature rose from 298 to 308 K.It indicated the endothermic nature of sorption process.However, with further increase in the temperature (318 K), a drastic decrease in the dye's adsorption was noted.This unusual trend might be due to increased mobility of dyes' ions resulting in the fast collisions of adsorbate-adsorbate (Rápó and Tonk 2021).While comparing dyes' adsorption on NCs, it had been noted that in case of 1:5 NC where the contents of ZnO were high, adsorption capacity (82 mg g −1 upon having 100 mg l −1 initial concentration) was displayed heavily as compared to other two adsorbent systems like NiO/ZnO (5:1) and NiO/ZnO (1:1) NCs, highlighting the predominant role of ZnO in the dye's adsorption which is likely due to the higher surface-to-volume ratio of the NC.

Impact of time on the adsorption of dyes
A crucial factor that influences dye adsorption was time.The shapes of the isotherms in figure 4 show that metal adsorption increased linearly at first, then it gradually levelled off after 3 h of agitation time; later, it slowed down when equilibrium was approached.For the kinetic experiments, 100 mg l −1 of the adsorbate concentration was selected.The availability of adsorbent's vacant sites could describe the conduct of adsorption curves with respect to time.Speedy adsorption which took place on the bare surface was revealed by initial high rate.With time lapse, the presence of vacant sites got decreased as a result the dyes' adsorption remained constant due to competition between adsorbate molecules (Shoukat et al, 2019, Wu et al 2021).For equimolar system (NiO/ ZnO 1:1), fast equilibrium kinetic of dyes sorption was achieved in comparison to the other adsorbent systems.

Adsorption modeling of MB, MO, and MR
Adsorption isotherm describes relationship among amounts of adsorbate, at specific temperature, adsorbed per unit mass of adsorbent.It also explains the process of adsorption.By fitting experimental data with adsorption models, the nanocomposite's adsorption capacity for MB, MO, and MR dye might be determined.However, based on the regression coefficient values, only Langmuir model appeared to be well fitted with the sorption data.The Langmuir equation for finding the adsorbed amount of adsorbate is given below: Where, Langmuir isotherm constant was denoted by K 1 , maximum adsorption capacity of the adsorbent at equilibrium was linked to qe (mg/g), C e represented concentration of adsorbate at equilibrium and q max was maximum adsorption capacity of monolayer formed on adsorbent's surface.When C e /q e was plotted versus C e , a linear plot was obtained which obeyed Langmuir isotherm model (Falahian et al 2018, Haq et al 2019).Langmuir isotherms which describe interaction of adsorbate and adsorbent are shown in the figures 5-7.Adsorption experiment was conducted to test the adsorption efficacy of NiO/ZnO NCs towards MB, MO and MR dyes (Bhatia and Nath 2020).The extracted sorption parameters for equimolar nanocomposite are given below in table 2.
Based on regression coefficient (R 2 ) the Langmuir model fitted well on sorption data.This indicated that all active sites had same affinity towards adsorbate molecules.
For MO, MB, and MR, maximum adsorption capacity (q m ) was observed as 85 mg g −1 , 370 mg g −1 , and 64 mg g −1 , respectively, employing Langmuir model.The realm of interaction between adsorbent and adsorbate was indicated by K 1 .Usually, an immense interaction between adsorbent and adsorbate is depicted by a relatively high K 1 value while its smaller value generally denotes a weaker interaction between those two components (Naseem et al 2021).According to this study, MB had a higher K 1 value than MO and MR, indicating that its adsorbate molecules had a stronger affinity for NiO/ZnO (1:1) NC.

Kinetic models
Lagergren initially proposed pseudo first order kinetics.It was used to find the adsorption tendency of adsorptive.The Pseudo first order equation is given below:  Where qe relates to adsorbed amount of dye at equilibrium.q t shows dye's adsorbed weight at various intervals and ka is Pseudo first order model parameter.Pseudo second order kinetic equation was also applied in the following form: Where K b was the model parameter.The adsorption of dyes on nanocomposite at a given time can be understood by interpreting the kinetic adsorption data.Pseudo first order as well as pseudo second order kinetic models were implemented to match the adsorption data obtained from adsorption capacity in reference to contact time (figures 8 and 9).Pseudo first order kinetic model, according to the values of R 2 , perfectly fit on data.Kinetic order models gave information about the type of adsorption that took place to eliminate dyes.Pseudo first order models revealed the physisorption behavior of adsorbent with adsorbate molecules (Taylor Frey et al 2021).

Spectroscopic investigation of dye adsorption
After adsorption of dyes, there were apparent changes in FTIR spectrum as compared to FTIR spectra NiO-ZnO nanocomposites before adsorption that indicated adsorption of dyes by nanocomposites (figure 10).Bands in some regions appeared which showed some additional functional groups were present and those were due to different dyes.Bands at 1050 cm −1 were observed which appeared due to asymmetric S=O stretching of MO in all three nanocomposites.The band at 1424 cm −1 showed bending of methyl group while 1380 cm −1 band was due to bending of methylene groups (Kumpan et al 2017, Khan et al 2019, Zubair et al 2020, Dey et al 2022).Metal oxygen vibrational bands had shown stronger intensity before adsorption, but their intensity decreased quite a lot after adsorption, which was an evidence of dyes' adsorption upon NCs' surface.

Conclusions
In present study, NiO/ZnO NCs were prepared by utilizing spinach as a green source.The NCs of different combinations were characterized by.FTIR spectroscopy, UV-Vis spectroscopy and XRD.XRD ensured the formation of NiO/ZnO NCs with crystallite sizes of 44.6 nm, 42.5 nm, and 55.17 nm for NiO/ZnO (1:1), NiO/ ZnO (1:5), and NiO/ZnO (5:1) nanocomposites respectively.The adsorption experiments were carried out in a  batch mode.This study showed that the NCs have better dye adsorption capacity.The maximum adsorption capacities for MB, MR and MO were 370, 64 and 85 mg g −1 .The effect of various parameters on dye degradation like time, temperature as well as concentration was studied.The adsorption of dyes was increased by increasing their initial concentration.The adsorption of dyes was also enhanced when the temperature was increased from 298 to 308 K indicating the endothermic behaviour of the process.However, further increase in the temperature to 318 K resulted a drastic decrease in the dye adsorption which might be due to the fast collisions of adsorbateadsorbate ions.Langmuir adsorption model was found better fitted to the adsorption data, whereas pseudo first order model was found well suited to kinetic data.This model revealed the physisorption behavior of adsorbent with adsorbate molecules.The nanocomposite NiO/ZnO (1:5) showed better results as compared to the other two composites against all the three dyes under study.More than 90% of 100 ppm MR was removed by NiO/ ZnO (1:5) NC.Along with the other two ratios, they could have a good prospect of dye removal through adsorption.

Figure 1 .
Figure 1.An overview of the adsorption investigation and the creation of NiO/ZnO nanocomposites.

Figure 4 .
Figure 4. Percentage removal of dyes with the passage of time.

Figure 8 .
Figure 8. Pseudo first order kinetics for NiO/ZnO nanocomposites of various molar ratios.

Figure 9 .
Figure 9. Pseudo second order kinetics for NiO/ZnO nanocomposites of various molar ratios.