Single step facile synthesis of Cu-SnO2/ZnO nanocomposite photocatalyst for methylene blue dye degradation in aqueous solution

A Cu-SnO2/ZnO nanocomposite was prepared using a single-step facile synthesis method, sol–gel, for photocatalyst application. The XRD of Cu-SnO2/ZnO nanocomposite shows SnO2 and ZnO have tetragonal rutile and hexagonal wurtzite, which is similar to HRTEM and SAED data. The crystallite sizes of SnO2, ZnO, Cu-SnO2, SnO2/ZnO, and Cu-SnO2/ZnO are 8.50 nm, 29.12 nm, 7.10 nm, 6.42 nm, and 3.50 nm, respectively. The calculated energy band gap of SnO2, ZnO, Cu-SnO2, and Cu-SnO2/ZnO from the DRS measurements is 3.60 eV, 3.20 eV, 3.34 eV, 3.48 eV, and 3.09 eV, respectively. The photoluminescence spectroscopy shows that Cu-SnO2/ZnO nanocomposite has a higher defect density than another sample. The Fourier Transform Infrared (FTIR) spectroscopy identifies the functional groups of the Cu-SnO2/ZnO powder samples. The EDS spectra of the synthesized Cu-SnO2/ZnO nanocomposite indicated the existence of the elements of Cu, Sn, Zn, and O, respectively. The photocatalyst activities of Cu-SnO2/ZnO have higher efficiency, ~78%, than other samples.


Introduction
Tin dioxide (SnO 2 ) is an n-type direct band gap semiconductor with a rutile tetragonal structure and outstanding properties due to a large exciton binding energy, electrocatalyst [1], strong adsorption capacity [2], high carrier mobility [3], high thermal stability [4,5], high photosensitivity [6], high abundance, inexpensive and low toxicity.Phil et al 2016 and Pham Van Viet et al 2016, report SnO 2 nanoparticles with higher photocatalytic degradation efficiency of a methylene blue dye than a bulk [7,8].When the SnO 2 nanoparticle is used as a photocatalyst and illuminated by photons with energy greater than or equal to the band gap energy, electrons are migrated from the valence band to the conduction band and leave holes in the valance band [9,10].The photogenerated charge carriers migrate to the surface of the semiconductor and participate in the redox reaction to change the pollutant into water and carbon dioxide.However, this material shows low photocatalytic efficiency due to poor light absorption and photogenerated carrier kinetics [11].
Enhancing strategies like metal doping and making a composite have been employed to overcome poor light absorption and slow photogenerated carriers' kinetic limitations to boost the photocatalytic efficiency of SnO 2 [12,13].Moreover, transition metal doping into SnO 2 lattice acts as a grain growth inhibitor, i.e., the resulting particles may have a high specific surface area.Doping may also decrease the optical band gap of the host material by inducing a defect energy state [14,15].However, merely transition metals doping did not enhance the photocatalytic efficiency of SnO 2 nanoparticles.For instance, Preethi et al (2022) reported that the photocatalytic efficiency of visible light-assisted Fe-SnO 2 nanoparticles catalyst was 43% for the degradation of Methylene blue dye [16].Besides, the construction of SnO 2 /ZnO nanocomposite boosts the photocatalytic degradation efficiency of individual components under UV light irradiation.For instance, Lamba et al 2015 reported that synthesized ZnO-doped SnO 2 nanoparticles showed better performance removing MB dye under UV light illumination [17].Literature report shows, still SnO 2 -based composites exhibit poor photocatalytic efficiency in the visible light spectrum between the range 400-700 nm, which constitutes around 43% of the entire incoming light spectrum.
Incorporation of transition metals and ZnO into the host SnO 2 may suppress the charge carrier recombination, facilitating charge transfer and reducing the band gap.For instance, Olga Długosz & Marcin Banach, 2021 reported that Sn-doped ZnO-SnO 2 photocatalyst showed a higher visible light-assisted photodegradation of rhodamine B [18].Perveen and Akhyar 2017 reported that La metal-doped SnO 2 -TiO 2 nanocomposites exhibited higher photocatalytic efficiency of MB dye degradation under sunlight illumination [19].Asaithambi et al 2021 also published a visible light-assisted Mn-doped SnO 2 @ZnO nanocomposite that showed higher photocatalytic efficiency of 98% for methylene blue dye degradation [20].Therefore, metal ion doping into the binary nanocomposite enhances the photocatalytic dye removal efficiency; that could be due to the reduction of recombination of carriers and the narrowing of the bandgap [18,20].
Tin dioxide based nanocomposites with various morphologies synthesized using a variety of techniques such as precipitation [7], hydrothermal route [8,13,17], co-precipitation [21], sol-gel [22], electrospinning method [23] and polyol route [12].Among these synthesis methods, the sol-gel synthesis method has been employing to synthesize nanocomposite because it is a simple, efficient, cost-effective, and low-temperature technique that controls the product's chemical composition [24,25].In this work, Cu-SnO 2 /ZnO nanocomposite has been synthesized using the sol-gel synthesis method.The optical band gap of the pristine SnO 2 decreased after Cu 2+ and ZnO incorporation in the host lattice.Moreover, PL spectroscopic data show electron-hole recombination decreases due to the defect energy levels formed in the composite.Therefore, a single-step facile synthesized Cu-SnO 2 /ZnO nanocomposite catalyst was used for methylene blue dye degradation in an aqueous solution and exhibited enhanced degradation performance than pristine SnO 2 .

Materials and chemicals
The materials used are digital electronic analytical balance (Model FA2104, China), furnace (Model BK-5-12GJ), magnetic stirrer, hot plate, Ceramic crucible, drying oven (101-0, Biobase Bioindustry, Shandong, Co. Ltd, China) and Whatman cellulose filter paper.Precursor chemicals used for the synthesis of Cu doped SnO 2 /ZnO nanocomposite were tin tetrachloride pentahydrate (SnCl 4 .5HO) were dissolved in distilled water and stirred for 30 min until each solution became transparent.The mixed solution has stirred 360 rev/min at 70 °C for 2 h.While stirring the solution, ammonia solution was slowly added until the pH value reached 8, and white precipitate gel formed and aged for 15 h.Subsequent to aging, filtered and washed with distilled water repeatedly using whatman No.1 cellulose filter paper (125 mm diameter).The filtered gel-like precipitate dried for 24 h in an air oven and subsequently milled using mortar and pestle.The milled powder calcined at 500 °C for 2 h.SnO 2 and ZnO nanoparticles were synthesized by the same procedures using 0.2 M tin tetrachloride pentahydrate and zinc acetate dihydrate as a precursor, respectively.Finally, the samples were ready for further characterization.

Characterization techniques 2.3.1. X-ray diffraction (XRD)
The structural properties of the prepared powder was analyzed using x-ray diffraction (Shimadzu Maxima-7000 x-ray diffractometer, Tokyo, Japan) by generating CuKα monochromatic radiation or wavelength of x-ray (λ = 1.5406Ǻ) .The technique is used to determine the crystallite phase, crystallite size, lattice spacing, dislocation density and the volume of a unit cell of crystals of the synthesized materials.The operation of the x-ray generator was performed at a voltage of 40 kV and current of 30 mA of at room temperature.XRD intensities were measured in scan steps of 0.0020°in 2θ per second, at a scan speed of 3 degrees per minute for 2θ over the range of 10° 2θ 80°in a continuous scan mode.The collected data was plotted using the origin software.The crystallite size of SnO 2 and ZnO, Cu-SnO 2 , SnO 2 /ZnO and Cu-SnO 2 /ZnO nanocomposites were calculated from the three most intense XRD peaks using Debye-Scherer formula (equation (1)): Where D is the crystallite size, β is full width at half maximum in radians, λ is the wavelength of incident x-rays beam (0.1540 nm) for Cu target Kα radiation; θ represents angle of diffraction and K represents the shape factor (0.94).
The lattice parameters for the rutile tetragonal structure of SnO 2 , Cu-SnO 2 , SnO 2 -ZnO, Cu-SnO 2 /ZnO nanocomposites were calculated from peaks 110 and 101 using the Bragg's equation (equation ( 2)): The lattice parameters for the hexagonal wurtzite structure of ZnO nanoparticles were calculated from 100 and 002 XRD peaks using Bragg's equation (equation ( 3)): The volume of unit cell for tetragonal lattice structure of synthesized nanoparticles were calculated using (equation ( 4)): The lattice distortion was calculated using the relation given below (equation ( 5)): Dislocation density (ρ) measures the number of dislocations or defects in a unit volume of a nanocrystalline materials.It was calculated from the values of the crystallite size (equation ( 6)):

UV-vis diffusive reflectance spectroscopy
Uv-visible diffusive reflectance spectroscopy (PerkinElmer Lambda 950 UV-vis spectrophotometer) was used to investigate the optical properties of the synthesized samples in the wavelengths range 250-800 nm.The band gap energies of the synthesized samples were estimated from Kubelka-Munk equation of direct band gap semiconductor.

Fourier transform infrared spectroscopy (FTIR)
The functional groups attached to the synthesized samples were measured with Attenuated Total Reflection FTIR spectroscopy (model IS50 ABX Germany) in wave number ranges of 4000-400 cm −1 .It also utilized to determine the chemical bonds of the samples.Before the sample measurement background spectrum was recorded with the ATR cell.Finally, subtraction of the background of the sample was carried.
Photoluminescence Spectroscopy (PL) The photoluminescence property was studied by fluorescence spectrophotometer (Agilent Cary Eclipse Fluorescence Spectrophotometer) using a 150 W xenon lamp as the excitation source.The grating slits were set at 10 nm for the excitation and 10 nm for the emission while photomultiplier voltage was maintained at medium.Each sample was loaded into a rectangular sample holder using a cuvette and excited at a wavelength of 325 nm.The emission spectra were scanned in the wavelengths range 330-600 nm.

Scanning electron microscopy (SEM)
Scanning electron microscopy (Hitachi S4800) coupled with an EDS detector (Hitachi, Japan) was used to investigated the morphological features of the synthesized samples.The elemental or chemical composition of the synthesized powder was examined with energy dispersive spectroscopy.The concentration and distribution of the elements in Cu-SnO 2 /ZnO nanocomposite was determined by the elemental mapping analysis.

Transmission electron microscope (TEM)
The crystal structure of the synthesized powder was determined using FEI Tecnai F20 transmission electron microscope (Thermo Fischer, USA).High resolution transmission electron microscopy (FEI Tecnai F20 microscope) with a beam energy of 300 kV (Thermo Fischer, USA) was used to determine the lattice spacing using Gatan digital micrograph software.

Photocatalytic MB Dye degradation measurement procedure
The photocatalytic degradation of methylene blue dye was performed under light irradiation using a 150 W tungsten halogen lamp.10.0 mg l −1 of methylene blue dye solution has been prepared for the photocatalytic dye degradation experiment.After optimization, the pH of the solution was adjusted at 10.0 by 0.1 M NaOH and HCl.Then, 100 ml of MB dye solution has taken, and photolysis was measured.Moreover, to examine the catalyst dose for the MB dye degradation performance, 15 mg, 30 mg, and 50 mg nanocatalysts were added.The catalyst-mixed dye solution was magnetically stirred at room temperature for 30 min in the dark to reach its adsorption-desorption equilibrium state.Then, 6 ml of the solution has taken, filtered from the catalyst, and measured its absorbance.Again, after exposing the mixed solution to visible light, the supernatant from the solution during the degradation process under stirring was taken in 20-minute intervals for 140 min.The supernatant was centrifuged for 5 min to remove the photocatalyst.After centrifugation, the absorbance versus wavelength of the filtrates has measured for each sample.Based on the recorded data, the photocatalytic efficiency of the catalyst was measured by degrading methylene blue dye and calculated using equation (7): Where C 0 represents the absorbance before irradiation at time t 0 = min and C t indicates the absorbance measured throughout the irradiation for regular time intervals.
The Langmuir-Hinshelwood model was used to determine the photocatalytic rate constant values of the photocatalysts for low MB concentrations using equation (8): Where C 0 and C t are initial and final concentrations at time, t, respectively.k is the first order rate constant that was determined from slope of graph ln , t is the light irradiation time.

X-ray diffraction analysis
Figure 1 shows the XRD spectra of the sample's diffraction peaks at angles (2θ) of 26 The XRD spectra of SnO 2 /ZnO nanocomposite show almost all the peaks of SnO 2 and ZnO visible and confirm the formation of ZnO and SnO 2 .Moreover, the peak width and intensity of the composite changed compared with the individual materials, and the physical properties change may alter the electronic and catalytic properties of the composite [26].The SnO 2 in Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO have a slightly greater full-width half-maximum (FWHM), which indicates the crystallite size was smaller than pristine SnO 2 .The crystallite size of SnO 2 in Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO, estimated from the most intense peaks using (equation ( 1)) and the pristine SnO 2 shows larger crystallite size than other samples as indicated in table 1.The smaller crystallite size in the doped composite sample might be due to the Cu and ZnO inhibiting the growth of grains.
The lattice constants and unit cell volume of samples are estimated and summarized in table 1.The lattice constant and unit cell volume of prepared SnO 2 and ZnO are almost similar to the standard values.However, the unit cell volume, lattice constant, and distortion of Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO were higher than SnO 2 nanoparticles might be the result of substituting Cu and Zn ions with greater ionic radii in the Sn lattice.Based on the XRD study, in general, the greater the distortion and production of defect states, the smaller the crystallite size, and the higher the light-harvesting and smaller charge carrier trapping, which promotes electron-hole separation and increases the photocatalytic activities of the photocatalyst.
The red shift observed on the absorption peak is due to metal doping and/or heterostructure formation with ZnO, which is similar to the previous report [15,22].The redshift in Cu-SnO 2 nanoparticles is due to the replacement of Sn by Cu ions in the SnO 2 that create a defect energy state and the strain in the Cu-SnO 2 crystal [20,27].In the SnO 2 /ZnO composite, the Zn ions insertion into the Sn lattice might result in oxygen vacancies or defects between bands [12].Similarly, the Cu-SnO 2 /ZnO nanocomposite band gap decreases due to Cu and Zn ions incorporation into the Sn lattice sites [20].Therefore, an energy level created near the conduction band of the materials enhances the density of the photogenerated carriers, increases visible light absorption, and might improve the charge carriers' kinetics and improves the efficiency of the photocatalyst.

Samples
Crystallite size (nm) a b = c unit cell Volume (Å

Photoluminescence spectroscopy analysis
The photoluminescence measurement was carried out to identify the emission peaks associated with band-toband and band-to-defect levels transitions of the photogenerated electrons in the synthesized nanoparticles, as shown in figure 4. The PL spectra of SnO 2 , ZnO, Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO nanocomposites were measured at an excitation wavelength of 325 nm.The SnO 2 , Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO nanocomposites PL spectra show emission peaks at 360 nm which are near-to-absorption band edge and minor emission peaks at 378 nm, 411 nm, and 482 nm attributed to defects energy level between the bands [31].The ZnO PL spectra showed that the primary peak found in the UV region at 390 nm corresponds to the band-toband transition, while minor peaks at 416 nm and 441 nm exist due to induced defects energy states [32].The PL intensities of all samples reduced, indicating the decrease of photogenerated carrier recombination of the Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO nanocomposites.These decreases in PL intensity and recombination of charge carriers were due to lattice distortions, oxygen vacancies, and dislocation density confirmed by the XRD analysis.The HRTEM images showed that Cu-SnO 2 /ZnO nanocomposite contains different grains sizes indicating that it is a polycrystalline.In Cu-SnO 2 /ZnO nanocomposite, the smaller size corresponds to Cu-SnO 2 whereas the larger size represent the ZnO nanoparticles.This was because in addition to the impact of Cu ion the high activation energy of SnO 2 restricts its nanograins growth whereas the grain size of ZnO was bigger due to its low activation energy as reported in literature [33].In figure 7(g), the small angle electron diffraction (SAED) fringes further confirmed that Cu-SnO 2 /ZnO nanocomposite is a polycrystalline material.The interplanar spacings of 0.265 nm and 0.286 nm matched with miller indices of tetragonal rutile SnO 2 (101) shown in figure 7(d) and ZnO (100) in figure 7(e).This further confirms the XRD results.

Photocatalytic MB Dye degradation of Cu-SnO2/ZnO photocatalyst
The photocatalytic activity of SnO 2 , Cu-SnO 2 , SnO 2 /ZnO, and ZnO/Cu-SnO 2 nanocomposite were studied after optimization of the pH and catalyst dosage using methylene blue under illumination of visible light.The optimization was first done for Cu-SnO 2 /ZnO nanocomposite which is the narrower bandgap photocatalyst.

Effect of pH on MB dye degradation of Cu-SnO 2 /ZnO photocatalyst
The adsorption of MB dye molecules on the Cu-SnO 2 /ZnO photocatalyst surface may be affected by pH variations.The photocatalytic experiment has performed in the acidic, neutral, and bases where the pH is 4, 7, and 10, respectively.Figures 8(a)-(d) shows the absorbance versus wavelength of MB dye at the pH of 4, 7, and 10 solutions.When the light illumination time gets longer during the photocatalytic process, the absorbance peak decreases more for the solution with pH 10; It confirms that the concentration of MB dye decreases.The  The better degradation efficiency for the cationic MB dye at a pH of 10 is due to the OH − radicals in the solution.Moreover, more dye molecule adsorption on the active sites of the catalyst may occur in base media than in neutral or acidic media due to electrostatic forces between the MB dye and catalyst surface.

Effect of catalyst dosage
Figures 10(a)-(d) shows the photocatalytic MB dye degradation at various catalyst doses measured for 140 min of visible light irradiation.The MB dye absorbance peak gradually decreases as the illumination time gets longer.The photocatalytic degradation efficiency for catalyst dosages of 15 mg, 30 mg, and 50 mg is 63.4%, 69.3%, and 78%, respectively.The slope of the linearly fitted graph versus light illumination time, in figure 11(d), yields the photocatalytic degradation rate constants for 15 mg, 30 mg, and 50 mg catalysts as 0.00670 min −1 , 0.00842 min -1 , and 0.00957 min −1 , respectively.The photocatalytic degradation effectiveness increased as the catalyst dose increased due to the density of active site enhancement in the Cu-SnO 2 /ZnO photocatalyst absorption and charge carrier density [34].
Moreover, more electrons generated in the conduction band result in more formation of oxygenated species, and more holes created in the valance band also result in more concentration of OH − radicals on the   Cu-SnO 2 /ZnO photocatalyst surface.Therefore, this enhancement of the density of the active site and photogenerated charge carrier results in enhanced dye degradation performance for MB dye.

Degradation kinetics of synthesized samples
The photocatalytic performance of SnO 2 , Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO nanocomposites have been measured and shown in figure 12.In the blank experiment, illuminated without catalyst, the degradation of MB dye was low and not significantly degraded after 140 min implying that the photolysis process has a negligible contribution to the removal of methylene blue dye, as shown in figure 12(a) black color curve.Moreover, the dark reaction with the catalyst was also poor, which indicates the adsorption of MB dye in the photocatalyst surface is small.The photoreactions with the presence of the catalyst show a higher degradation rate of MB dye, as shown in figure 12(a).
Up on addition of SnO 2 , Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO nanocomposite photocatalysts with the assistance of visible light, the absorbance peak of MB dye at 664 nm was slowly quenched with increasing light irradiation time.
Cu-SnO 2 nanoparticles, SnO 2 /ZnO, and Cu-SnO 2 /ZnO nanocomposite photocatalysts showed higher degradation efficiencies and degradation rate constants than the pristine SnO 2 , as indicated in table 2 [20].The Cu-SnO 2 /ZnO nanocomposite exhibits higher degradation efficiency, 78% up on an illumination time of 140 min.It might be the formation of defects in the host materials than others, as confirmed by the XRD and PL analysis.
Moreover, the Cu ion in the host decreases the crystallite size and bandgap, increases the density of defects, and improves the Cu-SnO 2 /ZnO photocatalytic MB dye degradation activities.

Photocatalytic degradation mechanism
The proposed photocatalytic MB dye degradation mechanism for the best photocatalyst, Cu-SnO2/ZnO nanocomposite, is given in figure 13.The band edge position values of Cu-SnO2 and ZnO can be calculated In equations ( 9) and (10), E E and VB CB are valance and conduction band edge potentials, respectively.Whereas E g represents the band gap energy.X represent electronegativity of SnO 2 (6.25 eV) and ZnO (5.79 eV) and E e is the free electron energy (4.50 eV) on the normal hydrogen electrode (NHE) scale.The summarized

X E
, , E and E g VB CB values are given in table 3. Therefore, using the calculated data the MB dye degradation mechanism proposed.
It has been known that the negative charges carriers (or electrons) could be transferred from low potential to high potential whereas the positive charge carriers (or holes) move in the opposite directions.Therefore, the conduction band edge potential of ZnO (−0.31 eV) and Cu-SnO 2 (−0.080 eV) causes the electrons to move from the conduction band of ZnO to the conduction band of Cu-SnO 2 whereas the high valance band edge potential of SnO 2 (3.42 eV) and ZnO (2.89 eV) causes the holes to diffuse towards the valance band of ZnO.The decrease of rapid carrier recombination results from the trapping of electrons at the conduction band of Cu-SnO 2 and holes at the valance band of ZnO.Doping Cu ions into SnO 2 reduces its band gap, which in turn facilitates the formation of electron-hole pairs when the catalyst is exposed to visible light.Moreover, Cu-SnO 2 has a higher valance band edge potential than ZnO, therefore the holes never diffuse back into the valance band and instead carrier on forward reaction to form OH − radical on the surface of ZnO.
The following are the charge transfer and creation mechanisms of the active degrading species or the redox reaction during the photocatalytic processes:    (11)(12)(13)(14)(15)(16) shows that during the photocatalytic reaction, the oxygen molecules received from the air or water combine with the freely mobile photogenerated electrons in Cu-SnO 2 to form peroxide radicals O 2 *− while the photogenerated holes in the valance band of ZnO react with water to produce H + and OH * .The hydroxyl radicals are a stronger degrading species having a high redox potential nearly equals to valance band edge potential (2.89 eV).

Conclusion
The SnO 2 -based photocatalysts were synthesized using a one-step facile sol-gel synthesis method and evaluated for their photocatalytic activity to remove methylene blue dye under visible light irradiation.XRD analysis revealed that the tetragonal rutile SnO 2 and hexagonal wurtzite ZnO coexist in the synthesized Cu-SnO 2 /ZnO composite.The Debye-Scherer formula estimates the crystallite size of the synthesized photocatalysts.Moreover, Cu ion and ZnO incorporation in the lattice of SnO 2 increases the density of defects.The recombination rate of photogenerated charge carriers of the composite sample has decreased, as confirmed by the PL results.The FTIR measurement identifies the functional groups of the synthesized materials.The EDS analysis confirmed that the component elements Cu, Sn, Zn, and O were in the composite sample without any impurities.The TEM or HRTEM images confirmed that the structure of the composite is in good agreement with the XRD result.The lattice spacing determined by the HRTEM is in excellent agreement with the XRD result.Higher photocatalytic efficiency was obtained for a pH value of 10 using a 50 mg Cu-SnO 2 /ZnO composite catalyst.The photocatalytic efficiency of the Cu-SnO 2 /ZnO catalyst was 78%.The enhancement in its performance might be due to narrow bandgap and high defect formation compared to pristine SnO 2 and reduction of charge carrier recombination.
Figures 5(a)-(f) shows SEM images of the synthesized SnO 2 , ZnO, and Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO samples.The morphology of pristine SnO 2 is altered by doping Cu ions and adding ZnO.In SnO 2 nanoparticles, Cu-SnO 2 , SnO 2 /ZnO, and Cu-SnO 2 /ZnO composites, smaller and bigger grains with various irregular morphologies were observed.The bigger grains could be due to the aggregation of smaller crystallites.The EDS measurements for elemental analysis of the synthesized Cu-SnO 2 /ZnO nanocomposite are shown in figure 6(a).

Figure 9 (
photocatalytic efficiency is linearly increased with light irradiation time for pH values of 4, 7, and 10, as shown in figures 9(a)-(c).The degradation efficiency of Cu-SnO 2 /ZnO nanocomposite for degradation of MB dye is 41.4,48, and 78% for pH 4, 7, and 10, respectively.Figure9(d) also displays the photocatalytic degradation rate constants.The photocatalytic efficiency and rate constant are higher at the pH equal to 10 compared to 4 and 7.The better degradation efficiency for the cationic MB dye at a pH of 10 is due to the OH − radicals in the solution.Moreover, more dye molecule adsorption on the active sites of the catalyst may occur in base media than in neutral or acidic media due to electrostatic forces between the MB dye and catalyst surface.

Figure 10 .
Figure 10.UV-vis absorbance spectra shows the effect of catalyst dose on the degradation of MB for (a) 15 mg (b) 30 mg and (c) 50 mg of Cu-SnO 2 /ZnO composite catalyst, and (d) graph of maximum absorbance versus light illumination time in minutes for 15 mg, 30 mg and 50 mg of Cu-SnO 2 /ZnO composite catalyst.

Table 2 .
The photocatalytic efficiency and the degradation rate constants for the synthesized samples are summarized as follows.

Table 3 .
Summary for the absolute electronegativity(X), band gap energy E g (obtained from DRS spectra), conduction band E CB and valance band E VB edge potentials for SnO 2 , Cu-SnO 2 and ZnO.