Hexavalent chromium reduction by ZnO, SnO 2 and ZnO-SnO 2 synthesized using biosurfactants from extract of Solanum macrocarpon

In this study, ZnO, SnO 2 and their composite (ZnO-SnO 2 ) were synthesized by green route using aqueous extract of Solanum macrocarpon fruit and were used for the photo-reduction of hexavalent chromium. The synthetic route involved a two-step procedure, induced by temperature via calcination at 350 and 600 ºC. The composite was prepared by the treatment of a mixture of the precursor compounds to a temperature up to 800 ºC, and the extension of the temperature to 1000 ºC, resulted in the emergence of ZnO-SnO 2 -ZTO. The nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV–vis spectroscopy. The XRD studies of the binary oxides conrmed a hexagonal wurtzite structure for the ZnO and a cubic structure for the SnO 2 , without any change in the diffraction patterns or supplementary diffraction peaks. The morphology of the nanoparticles indicated fairly spherical shapes for the ZnO, that tend to agglomerate with increase in temperature. The SnO 2 showed rectangular shapes at both temperatures of reaction, while the ZnO-SnO 2 composite showed the presence of both morphologies of the component binary oxides. In the photo-enhanced degradation study, under ultra-violet light, the effect of pH (2–8), concentration of chromium(VI) (2–8 ppm), and photocatalyst dosage (25–150 mg/L) on the reduction of Cr(VI) to Cr(III) were investigated. The reduction showed higher eciency in acidic environment than in the alkaline environment, and also with increase in photocatalyst dosage. The composite exhibited the highest photoreduction eciency, above 90%, at the optimum condition of pH 2, 150 mg/L photocatalyst, 2 ppm chromium solution after 90 min. These low-cost and non-toxic metal oxide and their green synthesized composite have great potentials for Cr(VI) pollution clean-up from waste water.


Introduction
Chromium based materials are receiving considerable attention for their numerous applications such as in green pigments, heterogeneous catalysis, coating materials for thermal protection, and wear resistance. This is due to their excellent corrosion resistance, high hardness and good temperaturestability [1,2]. However, the toxicity of chromium has been the main challenge hampering its potential applications. Chromium's toxicity has been reported to depend on its oxidation states, which ranges from + 6 to − 2; among which hexavalent chromium is the most toxic and second most stable oxidation state found in the environment [3,4]. Hexavalent chromium poses severe threat to human health due to its high toxicity. Unlike organic pollutants, it is not biodegradable and could easily enter the food chain where it accumulates in living bodies over their life span [5,6]. In the past few decades, there has been increasing effort to either remove Cr(VI) from the environment or reduce it to a less toxic Cr(III) with potential industrial applications. Consequently, several techniques have been employed for the removal of Cr(VI) ion from wastewater. Some of these methods include ion-exchange [7], electro reduction [8,9], chemical reduction [10], biological reduction [11], and photocatalytic reduction [12]. Although, chemical routes have been reported to show maximum reduction rates compared to the other methods, it is however expensive and results in the generation of secondary waste, since it requires large amount of reducing agent [13,14]. The photocatalytic reduction of pollutants from wastewater has been widely applauded as an e cient, cost effective and sustainable technology [15].
Recently, there has been an increased attention in the photocatalytic reduction of Cr(VI) to Cr(III) in wastewater treatment. This technology is based on the electron transfer on the surface of a semiconductor, when it is illuminated with light, resulting in the simultaneous redox reaction from photogenerated holes and electrons [16]. During this process, Cr(VI) could undergo photo-enhanced reduction to Cr(III) without the need for any reducing agent, which makes the approach a sustainable one [17]. Semiconductor nanoparticles are suitable for catalytic applications, as they possess high speci c surface area with unique electronic correlation through spins and charge interactions. The broad usage of nanoparticles in environmental applications has resulted to an increased focus on the non-toxic materials which pose no potential hazard to the environment and may not adversely affect human health.
Zinc oxide (ZnO) and stannic oxide (SnO 2 ) are well known n-type, wide band gap metal oxides with band gap energies of ≈ 3.37 and 3.6 eV respectively in the UV region. They are both unique semiconducting materials that exhibits piezoelectric, and pyroelectric properties. Due to their wide bandgap energies, they are excellent candidates for optoelectronic materials and have versatile applications in sensors, piezoelectric devices, spin electronics, solar cells and anti-refractive coatings [18][19][20][21] .
A number of different methods including physical and chemical routes have been devised for the synthesis of metal oxide nanoparticles, and SnO 2 and ZnO in particular. However, most of them employ the use of toxic chemicals [22,23], involve longer reaction procedures and make use of sophisticated instrumentation [24]. The development of environmentally friendly methods for the synthesis of nanoparticles has recently generated high research interest in nanomaterials synthesis. This is due to the increasing awareness of green synthesis route and the need to obtain biologically compatible and environmentally benign materials. The biological routes seem to be in great focus as they serve as alternatives to chemical routes, and are not associated with the use of toxic reagents in the synthesis method [25,26]. The three different templates used in the biosynthesis of nanomaterials includes enzymes, microorganisms and plant extracts [27]. The key reactions which are involved in biosynthesis of nanomaterials are reduction and oxidation processes. The synthesis procedure uses water as the solvent medium which is biocompatible and economical.
Solanum macrocarpon, commonly called garden egg, is a widespread plant genus of the family Solanaceae, mainly found in Asia and Tropical Africa. It is an edible crop and part of folklore remedy, which is used to curb elevated blood sugar, obesity, rheumatism, skin diseases, digestive di culties, and some allergies [28,29]. It contains appreciable amount of phytochemical and nutritive component including vitamins and minerals, which justify its therapeutic and nutritional applications. Some of its phytochemical contents includes high level of phenolic acids, avonoids and anthocyanin with potent antioxidant activity, a potential bene cial action on hypercholesterolemia in humans [30,31]. Apart from the medicinal values of Solanum macrocarpon, it could also be utilized in functionalizing nanomaterials for biomedical and environmental applications. In this study, the synthesis, optical and structural properties of ZnO, SnO 2 nanoparticles and the composite (ZnO-SnO 2 ) using aqueous extract of Solanum macrocarpon was demonstrated and their photocatalytic reduction abilities towards chromium (VI) ion was investigated.

Preparation of the fruit extract
Solanum macrocarpon fruits were collected from Ekiti State, South Western region, Nigeria and were properly identi ed. The fruits were washed with distilled water to remove any dust, cut into pieces, air dried and pulverised. The aqueous extract was prepared by placing 25 g of the powder in 400 mL glass beaker containing 250 mL of distilled water. Thereafter, the mixture was boiled for 30 min to give a brown. The mixture was cooled to room temperature and ltered using Whatman No 1 lter paper, with pore size of 25 µm. The extract was stored in a refrigerator for further use [32]. were collected by centrifuging the solution for 20 min. The product obtained were washed thoroughly with de-ionised water, again subjected to centrifugation for 20 min and then dried in hot air oven at 90°C overnight. Fine particles were obtained by grinding using mortar and pestle, and the powdered sample was calcined using mu e furnace at 350 and 600°C to get pure SnO 2 nanoparticles [33]. ZnO was prepared using similar procedure, but Zinc nitrate [Zn (NO 3 ) 2 ·6H 2 O] was used as the precursor for Zn 2+ ion.

Synthesis of ZnO-SnO 2 nanocomposite
The synthesis of the nanocomposite was carried out by rstly mixing 100 mL of 0.1 M zinc nitrate [Zn(NO 3 ) 2 .6H 2 O] with 50 mL of the fruit extract and of 1 M NaOH was added dropwise with vigorous stirring to adjust the pH to about 7. The solution was heated with continuous stirring for about 2 h, and thereafter 100 mL of 0.02 M tin(IV) chloride pentahydrate [SnCl 4 ·5H 2 O] was added to the solution and heated at 80°C for another 2 h. Then, the solution was centrifuged, washed thoroughly with water and the product was dried in an oven at 90°C for 24 h [33]. The product was grounded, and the powdered ne particles were calcined using mu e furnace at 800°C for 1 h to get ZnO-SnO 2 nanocomposites.

Characterization of the nanoparticles
X-ray diffraction (XRD) analysis was carried using a Bruker-AXS D8 Advance diffractometer with CuK α radiation operating at 40 kV and 30 mA for the determination of the crystal phase and composition of the nanoparticles. Absorption measurements were carried out using a Cary 300 UV-vis spectrophotometer. The particle sizes and morphology were analysed using a TECNAI G2 (ACI) transmission electron microscope (TEM) operated at an accelerating voltage of 200, while the high-resolution TEM (HRTEM) was measured on JEM-2100 JEOL transmission electron microscopy.

Photocatalytic reduction of Cr(VI) to Cr(III)
In a typical photocatalytic experiments, 50 mL of the nanoparticles dispersion (1.0 g/L) containing 10 mg/L aqueous solution of Cr(VI) were stirred for about 30 min in the dark in order to establish the adsorption-desorption equilibrium [34]. In order to initiate the photocatalytic reaction, the solution was irradiated with a LED light (CEL-LED, 365 nm). The reduction of the Cr(VI) to Cr(III) was measured by analysing the concentration of Cr(VI) left in the solution by a UV spectrophotometer at the characteristic wavelength of 540 nm [35].

Synthesis of the nanoparticles
The phyto-components of the plant play a key role in the synthesis of the nanoparticles by acting as both reducing agents and surfactant, to in uence the emergence of the nanoparticles. During the synthesis process, the reaction initiated via the complexation of the plant molecules to the metal ions and the formation of the respective hydroxides. The dehydration of the compounds and subsequent formation of the metal oxides is a thermal process and takes place via calcination. The temperature of the reaction in uences both the size, morphology and crystallinity of the product. Routes (i), (ii) and (iii) in Figure 1 presents the synthesis process for the formation of the ZnO, SnO 2 and the composites respectively. The diffraction patterns of both ZnO and SnO 2 after 1 hr, at different calcination temperatures, showed no prominent differences as both samples showed the same phases and peak positions. However, it could be observed that as the calcination time was increased, a prominent increase in the intensity of the peaks also occurred. The average sizes of the ZnO particles were calculated by the Scherer formula and found to be 64 and 82 nm. The Scherer approach to particle size estimation was not applied to the SnO 2 due to the anisotropic morphology. Figure 4 presents the XRD of the complex metal oxide nanoparticles. The presence of the XRD peaks of both ZnO and SnO 2 con rmed the successful formation of heterojunction between the two metal oxides.
Interestingly and contrary to the observation in the binary oxides, as the calcination temperature was increased to 800 ºC, the intensity of the peaks decreased with increased sharpness but no observable phase change occurred. This was indicative of some increase in the particles' size. Furthermore, as the calcination temperature increased to 1000 °C, a new intermediate phase appeared. In Figure 4b were assigned to zinc stannate (Zn 2 SnO 4 ) with a cubic geometry [38]. Similar observation has been reported by Ali et al. [39].

Morphology
The TEM images, presented in Figure 5, revealed that the ZnO samples consisted of aggregates of particles with fairly spherical morphology, and average sizes that appeared quite close for the samples obtained at 650 ºC (Fig 5b) compared to the samples obtained at 350 ºC (Fig. 5a). As shown in Fig. 6a, the SnO 2 particles had a rectangular shape, which remained unchanged even after calcination at 650 ºC (Fig 6b), although with observable size increase. The micrographs of the mixed metal oxide ZnO-SnO 2 presented in Fig. 7a revealed high agglomeration, perhaps due to the high temperature and increase in surface reactivity. The mixing of two different oxides have been reported to generally result in limiting the growth of both oxides in the system. As could be observed in the micrograph, the mean sizes of the individual metal oxides, ZnO and SnO 2 , presented in Figures 5 and 6 were larger than in the mixed metal oxides ZnO-SnO 2 (Fig. 7). The mixing of both oxides resulted in the reduction of the crystallite size of the mixed oxides, and this was consistent with previous reports on mixed metal oxides of ZnO and SnO 2 [40,41]. Although, the presence of both morphologies of the component binary oxides were still observable, the high degree of agglomeration impeded the correct estimation of particles size.  Figure 7b evidenced the presence of overlapped images rather than images that grew from a common spot to form star shaped morphology. It also evidenced the intergrown lattices of the different metal oxides. Figures 8 and 9 present the absorption spectra of the prepared ZnO and SnO 2 nanoparticles respectively, obtained after 1 h at different temperatures (a and b). In both cases, the samples showed absorption peaks in the UV region. In semiconductor nanoparticles whose dimensions falls within the order of their bulk excition, some unique optical properties are exhibited and these are strongly dependent on the size [42]. In the ZnO nanoparticles, sharp increase in the absorption around 330 nm was observed. This feature in the optical spectrum, which occurred around 300 nm in the spectra of the SnO 2 nanoparticles, is related to the transition between the valence band and the conduction band [43], and is known as the optical absorption edge. The optical band gap (E g ) energies of the nanoparticles were calculated using the Tauc's relation {(α = A(hν-E g ) 0.5 ); where A is a constant, α is the optical absorption and hν is photon energy [44]. The band gap energy E g of the nanoparticles could be obtained by extrapolating the zero absorption coe cient determined from the Tauc relation. In these direct band gap materials, the intercept of the tangent to the plot gave a good estimation of their band gap energies (as shown in the inset of Figures 8 and 9). The optical direct band gap energies (E g ) of the prepared samples were calculated to be 3.75 and 3.62 for ZnO obtained for the product at 350 and 600 ºC respectively. In the SnO 2 , the obtained values were 4.20 and 4.05 eV respectively.

Absorption spectra
Optical band gap decreases with increase in particle size, and this result in a concomitant shift in the absorption edge to a lower energy. The absorption edge in the spectra of ZnO and SnO 2 nanoparticles calcined at 350 and 600 ºC showed a red shift at both temperatures. This shift in the absorption onset is characteristic of particle growth. The band gap energies of the synthesized nanoparticles were larger than their respective bulk values of 3.62 and 3.34 eV for SnO 2 and ZnO respectively [45,46]. The reduction in the band gap values after longer calcination time may be ascribed to the enhanced surface reactivity in the samples during calcination, which resulted into particles growth and higher degree of agglomeration observed in the products. A gradual shift towards macrocrystalline materials with increase in reaction temperature may be proposed, hence the observed shift in the absorption onset.

Photocatalyst reduction of Cr(VI) to Cr(III)
Hexavalent chromium could exist in three different ionic forms, including hydro chromate, chromate and dichromate, with the following ionic formula: (HCrO 4 − ), (CrO 4 2− ), and (Cr 2 O 7 2− ) respectively. Although the chromate ion is reported to predominate in both neutral and basic media, solutions of lower pHs favours the hydrochromium and this indicates the pH dependence of the different species [47].
The amount of Cr(VI) remaining at any given time was calculated using the relationship:

Effects of solution pH
The pH of a solution is crucial in any water treatment process involving the removal of metal ions from water. This is because the chemistry of the metal ions in aqueous solution is mostly in uenced by any change in pH and often signi cantly affect the performance of materials used in water remediation from pollutants. Therefore, it is imperative to investigate the effect of solution pH prior to other process variables. In order to determine the optimal pH, a solution of Cr(VI) with initial concentration of 2 ppm and 100 mg/L dosage of each synthesized catalysts was prepared and the solution pH was varied from 2 to 8, as presented in Fig.10 (a-c). Similar trend could be observed in the performance of all the synthesized catalysts used in the study, with a maximum reduction of Cr (VI) at the solution pH of 2. Since Cr(VI) is present as Cr 2 O 7 2− , HCrO 4 − and CrO 4 2− in aqueous media with an overall negative charge [48], the highest reduction of Cr(VI) at the lowest studied pH values may be due to the development of an overall positive charges on the surface of the catalysts. Consequently, this process would result in the electrostatic attraction of the negatively charged chromium ions. The reduction in the removal of the dichromate ion at higher pH might be associated with the effect of the negatively charged catalysts, which repels the negatively charged chromium ions. Although, the optimum Cr(VI) reduction was observed at pH of 2 in all cases, the nanocomposite catalyst is observed to be more bene cial, perhaps due to the good separation of photogenerated charges, the formation of heterojunction and delayed recombination of photoelectronhole (Fig. 10c) compared to ZnO and SnO 2 nanoparticles [49,50]. Consequently, nanocomposite catalysts was used for the subsequent photocatalysis experiments in this study [51]. The synergetic effect of the two binary semiconductors coexisting in the ZnO/SnO 2 heterojunction, in addition to the small size of the nanocomposite are important factors that are responsible for the observed increased e ciency.

Effect of catalyst loading on Cr(VI)photo reduction
The optimal pH of 2 was selected in order to study the effect of change in the nanocomposite loading with predetermined initial Cr(VI) concentration of 2 ppm [17]. There was considerable increase in the total conversion of Cr(VI) to Cr(III) as the catalysts loading increased from 25 mg/L to 100 mg/L as presented in Fig. 11. This could be attributed to the enhancement in the total surface area with increase in dosage of catalyst, which provided more active sites for interaction with the chromium ions. An initial decrease in the reduction of Cr(VI) could be noticed at the onset, above 100 mg/L catalysts loading, which is an indication that the optimum catalyst loading for 2 ppm is around 100 mg/L [52]. Therefore, the decrease in Cr(VI) conversion to Cr(III) at 125 and 150 mg/L of catalyst loading was an indication that the dosage was super uously based on this operating condition [53,54].

Effect of the initial concentration of Cr(VI) on the removal e ciency of ZnO-SnO 2 nanocomposite
The conversion of Cr(Vl) to Cr(IIl) using different initial concentration of the Cr(VI) was investigated at the optimal pH 2 and dosage of 100 mg/L. The result, presented in Figure 12, showed that the higher the concentration of the Cr(VI) in the solution, the lower the photocatalytic conversion of Cr(Vl) to Cr(IIl). At 8 ppm, only about 0.5 of the initial concentration of the Cr(VI) was totally converted, which was only 50% of the initial concentration of the Cr(VI) in the solution. This could be because at higher concentration of the Cr(VI), the product of the conversion process, Cr(III), could not be quickly separated from the surface of the catalyst. This would, therefore, result in the covering of the active sites of the photocatalyst, thereby causing the lowering of the photocatalytic activity [55]. The results obtained in all the studies involving investigating the effect of the initial concentration of Cr(Vl) were consistent. This is an indication that the rapid separation of Cr(III) from the catalyst surface is an important factor in the improvement of the activity and stability of photocatalytic reduction of hexavalent chromium [56]. In this study, the initial concentration of 2 ppm yielded the optimum conversion of Cr(Vl) to Cr(IIl).) 3.6 Photocatalytic mechanism Figure 13 presents the schematic diagram of the band structure and photogenerated electron-hole (e −h + ) separation in the ZnO-SnO 2 composite. As illustrated in the diagram, the strong combination of the two metal oxides to form nanocomposite had improved activity in the reduction of chromium(VI) to chromium(III). The coupling of two semiconductor nanomaterials with different band gap energies has been demonstrated as one of the effective ways of decreasing the rate of recombination of electron-hole (e−/h+) pairs [57,58].
Both ZnO and SnO 2 absorb in the UV region of the electromagnetic spectrum as a result of their wide band gap energies. The two metal oxides possess suitable electronic properties and are coupled for improved performance. The conduction band potential of ZnO is −0.36 eV, which is lower than that of SnO 2 (−0.11 eV), while the valence band potential of SnO 2 is + 3.44 eV and is higher than that of ZnO (+2.84 eV) (Figure 13 a) [57]. Upon the irradiation of the composite by UV light, Figure 13b, the photogenerated electrons with negative charges migrate from the low potential to high potential energy.
At the same time, the positive holes migrate in the opposite direction. This implies that the electrons migrate from the conduction band of the ZnO to the conduction band of the SnO 2 , whereas the holes migrate from the valance band of SnO 2 to the valence band of the ZnO [59]. Consequently, in the conduction band of SnO 2 is the accumulation of photogenerated electrons, while in the valence band of ZnO is the accumulation of holes. The formation of a junction results in the separation of the electrons and holes that highly restrained the recombination process and effective electron-hole separation is therefore accomplished on the surface of photocatalyst. As a result, more electrons participate in the photoreduction for the effective conversion of Cr(VI) to Cr(III) by the composite semiconductor. This was responsible for the marked enhancement in the photocatalytic activity of ZnO-SnO 2 composite relative to that of pure ZnO and SnO 2 .

Conclusion
In a two-step procedure, using aqueous extract of Solanum macrocarpon fruit, ZnO, SnO 2 and ZnO-SnO 2 nanoparticles (NPs) were successfully prepared. The biomolecules present in the fruit's extract acted as capping as well as complexing agents to form the respective metal hydroxides. appearing at 1000 ºC. A photoreduction of hexavalent chromium to the trivalent specie was evaluated at different solution pH, initial chromium concentration and photocatalyst dosage. The photoreduction e ciency of the nanocomposite showed superior performance compared to the binary oxides, ZnO and SnO 2 . The higher reduction e ciency of the nanocomposite could be ascribed to the improved charge separation and suppressed electron-hole pairs recombination process, which was achieved by the coupling of ZnO and SnO 2 . This method offers an environmental friendly approach to binary oxides and their complex metal oxides, which could also be extended to other metal oxides. Figure 1 Synthetic route for the formation of ZnO, SnO2 and the composites (ZnO-SnO2) using aqueous extract of Solanum macrocarpon.

Figures
Page 16/21    Absorption spectra of SnO2 nanoparticles calcined at (a) 350 °C and (b) 600 °C. Inset show the corresponding plot of (αhν)2 vs photon energy for the SnO2 nanoparticles.

Figure 10
The effect of pH on the photo reduction of Cr(VI) on (a) ZnO (b) SnO2 (c) ZnO-SnO2 nanocomposite synthesised using bio-surfactants from extract of Solanum macrocarpon Figure 11 Page 20/21 The effect of catalyst loading on the photoreduction of Cr(VI) on to ZnO-SnO2 nanocomposite.

Figure 12
The effect of initial concentration of Cr(VI) on photoreduction using ZnO-SnO2 nanocomposite.

Figure 13
Schematic presentation of the band structure and e−-h+ separation in the ZnO and SnO2 before and after compositing