High performance Co3O4/Sn-ZnO nanocomposite photocatalyst for removal of methylene blue dye

Methylene blue is a toxic, carcinogenic, and non-biodegradable synthetic dye discharged from factories and industries that causes severe harm to human health and environmental pollution. Therefore, in this work, Co3O4/Sn-ZnO nanocomposite was synthesized using a simple sol–gel method for efficient photocatalytic removal of methylene blue dye in an aqueous basic medium. The structural, optical, photoluminescence, morphological, and compositional properties were studied. The XRD result revealed that the crystal size increases as the full width at half maxima (FWHM) decreases when Co3O4 are coupled with Sn-ZnO. From UV-visible diffusive reflectance and photoluminescence spectroscopies, a narrowing of the band gap and a reduction of the charge carrier’s recombination rate were observed, respectively. The photocatalytic efficiency and degradation rate constant of 95.1% and 0.03251 min−1 were recorded for methylene blue dye upon the use of optimized catalyst dosage of 60 mg Co3O4/Sn-ZnO nanocomposite catalyst under an irradiation time of 100 min at room temperature for optimized pH value of 9 in an aqueous basic medium.


Introduction
Industrial wastewater contains many synthetic industrial dyes [1].Among the organic dyes, Methylene blue (MB) is toxic and carcinogenic, and causes problems in human health.Moreover, due to its low biodegradability, MB dye stays longer in the environment and causes severe problems to living things in general [2].To date, the photocatalytic organic dye removal method seems feasible to reduce (or remove) pollutants from aqueous medium using semiconducting metal oxide photocatalysts such as iron oxide (Fe 2 O 3 ) [3], cupric oxide (CuO) [4], zinc oxide (ZnO) [5,6], titanium dioxide (TiO 2 ) [7], and cobalt oxide (Co 3 O 4 ) [8,9].The major drawbacks of these photocatalysts in general are either low visible light absorption capacity or the fast recombination of charge carriers.
Among semiconducting metal oxides, zinc oxide (ZnO) is a non-toxic and inexpensive n-type direct wide bandgap semiconductor.The outstanding properties of ZnO are high carrier mobility [10], high catalytic properties [11], high photosensitivity [12] and ease of synthesis of its crystal [13,14].However, its wide optical bandgap (3.20 eV) leads to poor light absorption in the visible spectrum region, and the fast recombination of charge carriers remains a drawback to use it practically as a photocatalyst for MB dye degradation.For instance, ZnO nanoparticles synthesized by the sol-gel method exhibited a photocatalytic degradation efficiency of 71.7% under UV light irradiation [15].Moreover, ZnO synthesized using simple template-free precipitation techniques showed 24.9% photocatalysis efficiency of MB dye under visible light irradiation [16].This inherent poor photocatalytic properties of ZnO nanoparticles in visible light region can be modified using common strategies such as doping [17,20], making a composite [21,22], and construction of heterojunction [23,24], and combinations of the above strategies at the same time [25,26].
For instance, the photocatalytic degradation efficiency of methylene blue using ZnO was enhanced by doping such as Sb-ZnO synthesized by sonochemical-assisted precipitation and achieved an efficiency of 91% using 300 mg L −1 catalyst under solar irradiation due to reduction of band gap to visible region [17], Ag-ZnO synthesized by co-precipitation route exhibited an enhanced efficiency of 95% for MB dye removal using 500 W halogen lamp [18], Sn-ZnO synthesized by ultrasonication aided co-precipitation method completely removed MB dye under UV light irradiation [19] and hydrothermally synthesized Cu-ZnO showed 90% efficiency of MB dye degradation under UV light [20].The heterojunction of ZnO/NiO shows enhanced photocatalytic MB dye degradation efficiency of 70% compared to pure ZnO (53%), and NiO (2%) due to enhanced charge separation [23].Furthermore, hydrothermally synthesized ZnO/NiFe 2 O 4 heterojunction degrades MB (97%) and Rhodamine B (98%) under visible light irradiation for 3 h due to reduction of the band gap and effectively reducing the recombination rate of e-h pairs [24].
In this article, a combination of strategies such as doping and making composite were used.For this purpose, Co 3 O 4 is one of the suitable p-type semiconducting oxides for coupling with Sn-ZnO because it has an appropriate dual band gap that lies in the visible light spectrum region.The band edge potential of Co 3 O 4 also matches well with ZnO for coupling to increase the visible light harvesting capacity and overcoming the fast recombination of photogenerated carriers.Here, Co 3 O 4 /Sn-ZnO nanocomposite photocatalysts were synthesized by the sol-gel method for improving the dye degradation performance under visible light irradiation.The dopant (Sn +4 ) has a smaller ionic radius than Zn +2 in ZnO and the crystal lattice of the system is affected [27].A defect in energy level is induced between the valance and conduction bands of ZnO due to the higher oxidation state of the dopant.Moreover, when Co 3 O 4 was coupled with Sn-ZnO to produce a Co 3 O 4 /Sn-ZnO nanocomposite the electrons move from the conduction band of Co 3 O 4 to the Sn-ZnO conduction band, and the holes move from the valance band of Sn-ZnO to the valance band of Co 3 O 4 .Thus, the photogenerated carrier recombination is reduced so that it's carrier's lifetime could be extended.The Co 3 O 4 /Sn-ZnO photocatalyst degrades MB rapidly and efficiently, photocatalytic efficiency 95.1% was recorded within 100 min.Therefore, Co 3 O 4 /Sn-ZnO photocatalyst could be used as an alternative low-cost photocatalytic materials for the removal of MB dye from the aqueous solution, and as far as the researchers know it is not yet reported.
The precursor used to synthesize the Co 3 O 4 /Sn-ZnO nanocomposite photocatalyst were tin tetrachloride pentahydrate (SnCl4.5H2O,MW-350.6 g mol −1 , purity >98%, Merck, India), zinc acetate dihydrate (Zn (CH3COO)2.2H2O,MW-219.5 g mol −1 , purity>99, Merck life science Private Ltd, India), ammonium hydroxide (NH3OH, MW-35.046g mol -1 , 30 %), Cobalt chloride (MW-237.93 g mol −1 ), distilled water and ethanol.O) solution was added dropwise into the previous solution, i.e., a solution having a mixture of Sn and Zn precursors.The mixed solution, having the three precursors, was stirred using a magnetic stirrer for 2 h at 360 rev/min at a temperature of 70 °C.Then, ammonia solution was added dropwise to the solution while stirring until the pH value reached 8.0 and a gel precipitate was formed.The precipitated gel was aged for 15 h; after aging, it was filtered and washed with ethanol and distilled water repeatedly using Whatman cellulose filter paper.The filtered gel-like precipitate dried for 24 h in an ambient oven.The dried solid crystal was crushed to powder using a mortar and pestle until a uniform powder was obtained.The powder was calcined at 500 °C for 2 h.Moreover, Sn-ZnO nanoparticles were synthesized similarly using the precursors 0.015 M solution of SnCl 4 .5H 2 O and 0.285 M of Zn (CH 3 COO) 2 .2H 2 O.The Co 3 O 4 /ZnO nanocomposite was synthesized using 0.150 M zinc acetate dihydrate and 0.150 M cobalt chloride precursors.The individual metal oxides ZnO and Co 3 O 4 were synthesized following the same procedures used above by utilizing 0.30 M zinc acetate dihydrate as a precursor and 0.30 M cobalt chloride hexahydrate as a precursor, respectively.

Characterization methods
The crystallography information of the synthesized sample was obtained by x-ray diffraction (Shimadzu Maxima-7000 x-ray diffractometer, Tokyo, Japan) by producing CuKα monochromatic radiation (λ =1.5406 ) operated at 40 kV with 30 mA current and a scan speed of 3 degrees per minute for 2θ over the range of a continuous scan mode.The optical properties of the synthesized materials were measured using Uv-visible diffusive reflectance spectroscopy (PerkinElmer Lambda 950 UV-vis spectrophotometer) with a BaSO 4 coated integrating sphere in the range 250-800 nm.A fluorescence spectrophotometer (Agilent Cary Eclipse Fluorescence Spectrophotometer) coupled with a 150 W xenon lamp was used to investigate the photoluminescence properties in the wavelength range 330-600 nm at an excited wavelength of λ = 325 nm.
The functional groups attached to the synthesized materials were analyzed using attenuated total reflection FTIR spectroscopy (model IS50 ABX, Germany) in wavenumber ranges of 4000-400 cm −1 .The morphology, crystal structure, and elemental analysis-related information of the synthesized materials were studied using scanning electron microscopy (Hitachi S4800) equipped with an EDS detector (Hitachi, Japan) and TEM (FEI Tecnai F20, Thermo Fischer, USA).
Methylene blue dye degradation experiment was performed with 150-watt tungsten halogen lamp as a light source, and in a triple-jacketed glass reactor.First, 100 ml of 10 ppm of MB dye solution was taken and pH of the solution was adjusted at 5.0, 7.0, and 9.0 using 0.01M of HCl and 0.01M of NaOH to optimize it.Moreover, 30 mg, 60 mg and 90 mg of catalysts were added to the dye solution at optimized pH of 9 to optimize the catalyst dosage.At room temperature and in the dark, it was magnetically stirred for 40 min to reach the adsorptiondesorption equilibrium condition.After reaching equilibrium, light was irradiated on the mixed solution for 100 min, and about 3 ml solution sample was taken and centrifuged to measure the dye concentration in 20minute intervals.To identify the more active degrading species in the photocatalytic process, 0.10 mM of isopropyl alcohol (IPA), ethylenediaminetetraacetic acid (EDTA-2Na) and silver nitrate (AgNO 3 ) were added prior to the catalyst to scavenge hydroxyl (OH*), holes (e + ) and electron e - ( ) respectively.The trapping or quenching experiment were carried out using the same procedures and conditions as photocatalytic degradation experiments.

X-ray Diffraction (XRD) analysis
The XRD spectra of the synthesized samples are shown in figure 1(a).The hexagonal wurtizite structure of ZnO, which is in an excellent agreement with the standard JCPDS No. 36-1451, is confirmed [28].The peak intensity of ZnO decreases with the addition of Sn 4+ and Co 3 O 4 .The intense XRD peaks of Co 3 O 4 represent a cubic spinel phase, as confirmed by PDF: 74-2120 [29].No intermediate-phase peaks were observed in the XRD spectra of all the synthesized samples.In the composite sample Co 3 O 4 /Sn-ZnO, peaks representing ZnO and Co 3 O 4 were observed.The presence of the peaks that represent ZnO and Co 3 O 4 in the spectra suggests the existence and formation of composites.However, peaks of Sn do not appear in the XRD spectra, which might be due to its small amount, i.e., below the detection limit.When Zn 2+ is substituted by Sn 4+ in the ZnO host, a small shift towards a higher angle is observed [30], but when it is coupled with Co 3 O 4 , there is no shift observed, as shown in figure 1 The average crystalline size of ZnO, Co 3 O 4 , and Sn-ZnO particles, Co 3 O 4 /ZnO, and Co 3 O 4 /Sn-ZnO nanocomposite were estimated from the three most intense peaks using the Debye-Scherer formula (equation ( 1)) [31]: 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 average crystalline size of ZnO, Co 3 O 4 , and Sn-ZnO nanoparticles, Co 3 O 4 /ZnO, and Co 3 O 4 /Sn-ZnO nanocomposites estimated from the three intense peaks is given in table 1.This XRD analysis showed that crystalline size increased as the FWHM of Co 3 O 4 /Sn-ZnO decreased when Sn 4+ and Co 3 O 4 were incorporated.The lattice parameters for hexagonally structured ZnO can be determined using lattice planes (100) and (002) based on equation (2): Using lattice plane (100): Using lattice plane (002): The lattice parameters for cubically structured Co 3 O 4 can be determined using lattice plane (220) based on the following equation (5): The density of dislocations for the synthesized samples were calculated using (equation ( 6)) and given in table 1: The calculated values of lattice parameters (a, b, c) decrease for Sn-ZnO and Co where A is the proportionality constant depends on the nature of the material, (E g ) is the band gap, h is the Planck's constant ( x J s 6.63 10 ., ) v is the frequency of the photon, and n 2 = in the power index represents the direct transition for direct band gap ZnO and Co 3 O 4 .
The reflectance spectra of ZnO and Sn-ZnO powder showed larger reflectance in the UV region than visible.Moreover, the composite samples have relatively smaller reflectance in the UV region than the bare samples.It is due to the smaller dual band gap of Co 3 O 4 and has a relatively smaller reflectance in the UV-vis region than ZnO & Sn-ZnO.
In this study, the optical bandgap of ZnO negligibly decreased when Sn substitutionally doped into the lattice of ZnO, i.e., bare ZnO = 3.20 eV and the doped one 3.12 eV; it might be due to the incorporation of a small dopant amount [32].The doping of Sn 4+ in ZnO host also induced a donor level near the conduction band and increases the electron density in it.

Photoluminescence spectra analysis
The PL spectra of ZnO, Sn-ZnO, and Co 3 O 4 /ZnO and Co 3 O 4 /Sn-ZnO nanocomposite were measured at an excitation wavelength of 325 nm and shown in figure 4.The photoluminescence intensity of ZnO nanoparticles is higher than that of Sn-ZnO nanoparticles, Co 3 O 4 /ZnO, and Co 3 O 4 /Sn-ZnO nanocomposite.The higher intensity indicates that the recombination rate might be higher in ZnO than in Sn-ZnO, Co 3 O 4 /ZnO, and Co 3 O 4 /Sn-ZnO nanocomposite.In the PL spectra of ZnO, the most intense peak at 390 nm in the UV region corresponds to the band-to-band transition.The weak intensity peaks observed in the visible region at 416 nm and 440 nm might be due to oxygen vacancies, Zn interstitials, or defects [39].In the Sn-doped ZnO nanoparticle, the peak at 356 nm corresponds to a band-to-band transition, whereas the peak at 385 nm could correspond to a transition from the dopant (Sn +4 ) level to the valance band.When Sn was added to ZnO, the intensity was quenched.This means the substitutional doping of Sn +4 into the ZnO lattice reduces the band to

SEM and EDS analysis
The morphology of the synthesized samples is observed by SEM and shown in figure 5. Figure 5 indicated that all of the components of the Co 3 O 4 /Sn-ZnO nanocomposites were present in the spectra with no extraneous impurities.In the EDS spectra, tin (Sn), zinc (Zn), cobalt (Co), and oxygen (O) are identified.This verifies that the atoms were present as expected during the synthesis.Furthermore, the purity of peaks in the EDS spectrum was consistent with the tidy XRD pattern produced for the nanocomposite.In the Co 3 O 4 /Sn-ZnO nanocomposite, the individual color mappings of O, Sn, Zn, and Co are shown in figures 5(h)-(k).Platinum (Pt) peaks might be introduced during coating for EDS sample preparation.All these constituent atoms are distributed uniformly across the surface of the Co 3 O 4 /Sn-ZnO nanocomposite.This also provides further confirmation for the formation of a Co 3 O 4 /Sn-ZnO nanocomposite.

TEM and HRTEM analysis
The TEM image of Co 3 O 4 /Sn-ZnO nanocomposite is given in figure 6(a).The average particle size estimated was about 45 nm, and the particle size distribution ranges from 21 nm to 79 nm, as shown in figure 6  3.6.Photocatalytic performance and mechanism 3.6.1.Photocatalytic efficiency and degradation rate constant of synthesized samples The photocatalytic performance measurement was performed using 30 mg of catalysts and a 10 ppm MB dye concentration at room temperature in a basic medium (pH = 9) to identify the best catalyst.Optimization was done only for the best Co 3 O 4 /Sn-ZnO nanocomposite catalyst.Using the collected data, the photocatalytic efficiency of the methylene blue dye for the photocatalysts were calculated using (equation ( 8)): Where C 0 is the absorbance at time t = 0 min before light exposure, and C t is the absorbance measured during the exposure of light in 20-min intervals.The absorbance versus wavelength recorded using UV-visible spectroscopy during the photocatalytic degradation process of MB dye is given in figure 7.In photolysis (without  nanocomposite is given as 50.33%,65.94%, 75.33%, 93.76%, and 94.32%, respectively.Moreover, using the Langmuir-Hinshelwood model, the photocatalytic rate constant values of the photocatalysts at low MB concentrations were determined by using equation (9): Where C 0 and C t are initial and final concentrations at time t 0 and t, respectively.K is the first-order rate constant that was determined from the slope of graph of ln  catalyst excellent compared with the reported literatures as indicated in table 2. Therefore, it can be used as an alternative photocatalyst for MB dye removal from waste water./ versus light irradiation time, (e) histogram of degradation efficiency versus light irradiation time, and (f) ln (C C 0 t / ) versus light irradiation time using 30 mg Co 3 O 4 /Sn-ZnO nanocomposite catalyst and 10 ppm dye concentration under visible light irradiation for 100 min at room temperature.

Catalyst dosage
It is also one of the major operational parameters that strongly affects the photocatalytic performance of the Co 3 O 4 /Sn-ZnO nanocomposite catalyst for the degradation of MB dye under visible light exposure in terms of its adsorption of the dye molecules and light absorption capacity.Figures 10(a)-(e) shows the effect of variation of catalyst dosage on the degradation efficiency of MB dye at a pH value of 9 for 100 min of light irradiation at room temperature.The addition of 30 mg, 60 mg and 90 mg in a 100-ml dye solution for photocatalytic reactions in a reactor results in a photocatalytic efficiency of 94.3%, 95.1%, and 90%, respectively.There has been no significant difference observed in photocatalytic efficiency between the 30 mg and 60 mg catalyst dosages, but a low efficiency was recorded for the 90 mg Co 3 O 4 /Sn-ZnO nanocomposite catalyst.This might be due to the poor adsorption of dyes on the surface of the catalyst and the poor light penetration through the mixed solution of 90 mg Co 3 O 4 /Sn-ZnO photocatayst and dye solution [42,43].The higher photocatalytic efficiency of 60 mg Co 3 O 4 /Sn-ZnO nanocomposite photocatalyst could be due to better adsorption and good light absorption capacity during the photocatalytic reaction.The rate constants for 30 mg, 60 mg, and 90 mg obtained from the slope of the graph indicated in figure 10(f) are given as K 0.02844 = min −1 , K = 0.03251 min −1 and K 0.02373 = min −1 respectively.The rate constant of 60 mg is higher compared to 30 mg and 90 mg catalyst dosages.This means that at this optimal catalyst dosage, the faster degradation of the MB dye was recorded.where E E and VB CB are valance and conduction band edge potentials, respectively, E g represent the band gap energy, X is a symbol for electronegativity and E e is the free electron energy (4.50 eV) on the normal hydrogen electrode (NHE) scale.The summarized values of the synthesized samples are given in table 3.
In Sn-doped ZnO nanoparticles, a donor level is induced near the conduction band of ZnO, and extra electrons are injected into its conduction band.Moreover, the density of electrons increases, and the Fermi level shifts upward to the conduction band of n-type ZnO.Co 3 O 4 is a p-type semiconductor.The coupling of these semiconductors produces a Co 3 O 4 /Sn-ZnO nanocomposite catalyst.The photocatalytic degradation mechanism of the Co 3 O 4 /Sn-ZnO nanocomposite photocatalyst for the removal of MB dye is given in figure 12.When visible light is produced from a 150-watt tungsten halogen lamp irradiated on Co 3 O 4 /Sn-ZnO nanocomposite, electron-hole pairs are generated on its surface.The electrons could be transferred from a lower to a higher band edge potential, but holes move from a higher to a lower band edge potential.Therefore, in this composite, when electrons are generated and excited by the conduction band of ZnO, in return, the holes generated in its valence band move to the valence band of Co 3 O 4 , whereas the electrons generated in the conduction band of Co 3 O 4 move to the conduction band of ZnO.The photogenerated electrons in the conduction band of ZnO produce peroxide radicals due to a reduction reaction with oxygen molecules (O 2 ) from air, and the holes in the valance band of Co 3 O 4 produce radicals by an oxidation reaction with water (H 2 O).The reaction of these highly active radicals with MB dye results in the MB dye degrading into CO 2 and water.The charge transfer and production mechanisms of the active degrading species, or redox reaction, during photocatalytic activities are given in the following equations:

3. 2 .
Uv-visible diffuse reflectance and FTIR spectra analysis The optical band gap of ZnO, Co 3 O 4 , and Sn-ZnO, Co 3 O 4 /ZnO, and Co 3 O 4 /Sn-ZnO obtained from UV-vis diffusive reflectance spectroscopy data measured at a wavelength range of 200-800 nm using the Kubelka-Munk equation (equation (7)) and shown in figures 2(a)-(e): 37 eV, compared to bare Co 3 O 4 = 2.78 eV and ZnO = 3.20 eV, as shown in figures 2(a), (b), and (d).Moreover, the optical band gap decreased to 2.23 eV for the Co 3 O 4 /Sn-ZnO composite, as shown in figure 2(e).This decrease in band gap could be due to the partial incorporation of Co ions in place of vacancies or Zn interstitials in the ZnO and the lower dual band gap
(a) depicts agglomerated spherical ZnO nanoparticles. Figure 5(b) shows agglomerated spherical Co 3 O 4 nanoparticles; figure 5(c) shows aggregated particles of Sn-ZnO; and figure 5(d) depicts that the spherical and rode-like morphologies were observed in the Co 3 O 4 /Sn-ZnO nanocomposite.Aggregation and agglomeration of particles are also observed on the surface of the Co 3 O 4 /Sn-ZnO nanocomposite.The EDS spectra in figure 5(f) (b).The TEM and HRTEM images in figures 6(c) and (d) depict that the crystal phases of both Co 3 O 4 and ZnO are presented in the Co 3 O 4 /Sn-ZnO nanocomposite.The cubic spinel and hexagonal wurtzite structures correspond to Co 3 O 4 and ZnO, respectively.The co-existence of both Co 3 O 4 and ZnO phases assured the formation of Co 3 O 4 /Sn-ZnO nanocomposite, which agreed very well with the XRD result.In the small area electron diffraction (SAED) in figure 6(d (inset), the bright ring-like fringes confirm that the nanocomposite is polycrystalline.The FFT and IFFT of ZnO and Co 3 O 4 nanoparticles with the corresponding lattice spacing given in figures 6(c) and (d) are obtained from the HRTEM image given in figure 6(d).In figure 6(e (inset)), the lattice distortion observed from the IFFT image might be the effect of Sn 4+ as a dopant.The lattice spacing estimated from the HRTEM image agrees with the XRD result.

Figure 4 .
Figure 4. PL spectra of ZnO, Sn-ZnO nanoparticles, Co 3 O 4 /ZnO and Co 3 O 4 /Sn-ZnO nanocomposite dispersed in DMSO and excited at a wavelength of 325 nm.

Figure 6 .
Figure 6.(a) TEM image and (b) histogram of particle size distribution fitted with a Gaussian distribution curve for Co 3 O 4 /Sn-ZnO nanocomposite; (c) and (d) high-resolution transmission electron microscope (HRTEM) image at different magnification scales; and selected area electron diffraction (SAED) inset in (d), and FFT and their corresponding IFFT of (e) ZnO and (f) Co 3 O 4 of Co 3 O 4 /Sn-ZnO nanocomposite.
(C 0 /C t ) versus light illumination time.The graph of ln (C 0 /C t ) versus light illumination time of blank (without catalyst), ZnO, Co 3 O 4 , Sn-ZnO, Co 3 O 4 /ZnO, and Co 3 O 4 /Sn-ZnO nanocomposite is given in figure 8(d).The degradation rate constants obtained from the slope of the linearly fitted graph are given as 0.0000381 min −1 , 0.00662 min −1 , 0.01152 min −1 , 0.01443 min −1 , 0.02599 min −1 , and 0.02844 min −1 for blank (without catalyst), ZnO, Co 3 O 4 , Sn-ZnO nanoparticles, Co 3 O 4 /ZnO, and Co 3 O 4 /Sn-ZnO nanocomposites, respectively.The highest photocatalytic efficiency and rate constant obtained for Co 3 O 4 /ZnO and Co 3 O 4 /Sn-ZnO nanocomposite were due to the narrower band gap and higher quenching of the photoluminescence intensity.The narrow bandgap obtained from Uv-DRS was associated with the harvesting of more visible light, whereas the higher quenching of intensity observed in PL confirmed a lower recombination rate of charge carriers.The efficiency of Co 3 O 4 /Sn-ZnO nanocomposites

Figure 7 .
Figure 7. Plots of Uv-visible absorbance versus wavelength of (a) blank (without catalyst), and (b) ZnO, (c) Co 3 O 4, (d) Sn-ZnO, (e) Co 3 O 4 /ZnO and (f) Co 3 O 4 /Sn-ZnO nanocomposite using 30 mg catalyst and 100 ml of 10 ppm dye concentration at a pH of 9 under visible light irradiation for 100 min at room temperature.

3. 6 . 2 .
Optimization of photocatalytic factors in Co 3 O 4 /Sn-ZnO nanocomposite 3.6.2.1.Effect of pH The pH value of the solution is one of the operational parameters in photocatalytic activity that impacts the adsorption of the target synthetic dye, and it determines the surface charge of the Co 3 O 4 /Sn-ZnO

Figure 8 .
Figure 8. Plots of (a) C t /C 0 versus light illumination time, (b and c), photocatalytic efficiency versus light irradiation time, and (d) ln (C 0 /C t ) versus light illumination time of blank (without catalyst), ZnO, Co 3 O 4, Sn-ZnO, Co 3 O 4 /ZnO and Co 3 O 4 /Sn-ZnO nanocomposite using 30 mg catalyst, 100 ml of 10 ppm dye concentration and pH value of 9 for visible light irradiation time of 100 min at room temperature.
)-(e) shows the relationship between pH and the photocatalytic performance of the Co 3 O 4 /Sn-ZnO catalyst for the degradation of MB dye in 100 min of visible light irradiation.The adsorption capacity of MB dye on the surface of the Co 3 O 4 /Sn-ZnO nanocomposite catalyst was higher compared to the pH values of 5 and 7 as obtained from the adsorption-desorption analysis in the dark for 40 min.At pH = 5 (acidic medium), the surface of the catalyst is positively charged, and only weak electrostatic repulsion occurred between the MB dye and the catalyst surface, resulting in poor adsorption capacity and photocatalytic efficiency.In the case of pH = 7, due to an equal amount of positive and negative charges, both electrostatic repulsion and attraction of the MB dye occurred on the surface of the Co 3 O 4 /Sn-ZnO nanocomposite catalyst.This results in a better adsorption capacity and photocatalytic efficiency of MB dye compared to a pH of 5 by the Co 3 O 4 /Sn-ZnO nanocomposite catalyst.However, at pH of 9, when MB is dissolved in distilled water, a strong Coulombic attraction occurs on the negatively charged Co 3 O 4 /Sn-ZnO nanocomposite catalyst's surface, but at higher pH, the recombination rate of charge carriers might be increased results in the photocatalytic efficiency to be deteriorated[41].Therefore, the photocatalytic efficiency at pH 5, 7, and 9 are given as 51.1%, 75.4%, and 94.3%, respectively.Moreover, the degradation rate constant of the MB dye by the Co 3 O 4 /Sn-ZnO nanocomposite catalyst was determined by the Langmuir-Hinshelwood model using equation(9) from the slope of the linearly fitting graph of ln (C 0 /C t ) versus light irradiation time shown in figure9(f).The rate constants for Co 3 O 4 /Sn-ZnO nanocomposite catalyst are 0.00775 min −1 , 0.01493 min −1 , and 0.02844 min −1 at pH of 5, 7 and 9 respectively.The higher rate constant was obtained for pH 9 compared to 5 and 7.This means that a faster photocatalytic reaction or rapid decomposition of MB dye was occurred for Co 3 O 4 /Sn-ZnO nanocomposite catalyst at this pH value compared to 5 and 7.

Figure 9 .
Figure 9.Effect of variation of pH value on the photocatalytic degradation performance of MB dye at (a) pH 5, = (b) pH 7, = (c) pH 9, = (d) C C t 0

Figure 10 . 0 /
Figure 10.Effect of of catalyst dosage on the photocatalytic degradation performance of MB dye at (a) 30 mg, (b) 60 mg, (c) 90 mg, (d) C C t 0 / versus light irradiation time, (e) histogram of degradation efficiency versus light irradiation time, and (f) ln (C C 0 t / ) versus light irradiation time using 10 ppm dye concentration at a pH of 9 under visible light irradiation for 100 min at room temperature.

Figure 11 .
Figure 11.Effect of adding various quenchers on the photocatalytic efficiency of MB dye using Co 3 O 4 /Sn-ZnO nanocomposite under visible light irradiation for 100 min at room temperature (a) C t /C 0 versus light irradiation time (b) histogram of degradation efficiency versus light irradiation time using 10 ppm dye concentration at a pH of 9.

Figure 12 .
Figure 12.The photocatalytic degradation mechanism of Co 3 O 4 /Sn-ZnO nanocomposite photocatalyst for removal MB dye from polluted water in aqueous basic medium.

Table 1 .
The average crystalline size (D), calculated lattice parameters, density of dislocations, and band gap of the synthesized samples are given below.Samples D (nm) a = b c Density of dislocations 10 3´-Band gap (eV)

Table 2 .
Comparison of the present work with the reported literatures are reviewed and tabulated as follows.

Table 3 .
The absolute electronegativity (X), band gap energy (E , g ) the valance band E VB ( ) and conduction band E CB ( ) edge potentials for Co 3 O 4 and ZnO are summarized as follows.