STUDIES ON THE PHOTOCATALYTIC ACTIVITY OF METAL OXIDES AND THEIR COMPOSITE FOR DYE DEGRADATION APPLICATION

Ibrahim Tchakala1, K. Tomkouani1, M. Vedhanayagam2, A.‐S. M. Seyf‐Laye3, K.J. Sreeram2, B.L. Moctar1, D.‐B. Gbandi1

1Laboratory of water chemistry, Faculty of Sciences, University of Lomé, Lomé, Togo, ibrahimtchakala@gmail.com

2Chemical Laboratory, central Leather Research Institute (CLRI) Adyar, Chennai 600 020 3Faculty of Science and Technology, University of Kara, Kara, BP 404, Togo

Keywords: methylene blue, metal oxide, nanoparticles, photocatalytic degradation

Abstract In this work, ZnO nanorods and Graphene‐ZnO‐PdO nanocomposites are synthesized through hydrothermal method. Crystallinity of the materials is studied using X‐ray diffraction (XRD), and morphology was analyzed through Field Emission Scanning Electron Microscopy (FE‐SEM) and Transmission electron microscopy (TEM) techniques. The synthesized nanomaterials are used for the photocatalytic degradation of Methylene Blue (MB) under UV‐light illumination (365 nm) and the results are compared with commercial ZnO material. Graphene‐ZnO‐PdO nanocomposites exhibit higher degradation of MB under UV‐light illumination than ZnO nanorods and commercial ZnO material. These nanocomposites materials can so be used for MB photodegradation as for textile wastewater treatment.

1 Introduction Zinc oxide (ZnO) has a wide band gap (3.37eV) and is an n‐type semiconductor [1], which has achieved applications in many fields such as photocatalysis, solar cells, gas sensors, and photo detectors [2, 3]. In general, small size ZnO nanoparticles have an increased specific surface area and a high number of active sites, where the photo generated charge carriers are able to react with adsorbed molecules to form hydroxyl and peroxide radicals [4]. However, small ZnO nano‐particles without capping agents can easily form aggregations, leading to the loss of its active sites and photocatalytic efficiency. Moreover, the high electron‐hole recombination rate also limits the photo‐oxidation rate of organic compounds on ZnO surface, and also aggravates its photo‐corro‐sion in photocatalysis process [5]. Therefore, it is highly desirable to develop a simple and effective technique to overcome the drawbacks of ZnO by improving its photocatalytic efficiency while deterring the formation of aggregations.

Graphene oxide (GO) is a chemically functionalized graphene with hydroxyl and carboxyl groups, which possesses excellent solubility in solvents and thus provides various opportunities for the construction of GO‐based hybrid composites [6]. The new GO‐based hybrid with metal and metal

oxide nanoparticles such as Pt, Au, Ag, TiO2, ZnO, and Fe3O4 have shown potential application in the areas of optics, electronics, catalysts, and sensors [7‐9]. Among these, the fabrication of GO‐based hybrid catalysts, especially those for the photodegradation of organic pollutants, is of the utmost concern at current stage. Moreover, GO shows strong positive effect in the dispersion and stabilization of the photocatalytic nanocrystals without the introduction of organic surface‐tants. All these illustrate the unique electronic properties, extremely high specific surface area, and locally conjugated aromatic system of GO, making it ideal candidates for catalyst carrier or promoter. Several reduced GO (RGO)/ZnO hybridized composites for the enhanced photocatalytic activity in the removal of organic dyes have been reported. However, during the reduction process, GO is easy to be aggregated even with the addition of polymers or surfactants, which limited the production and quality of RGO‐ZnO composite. It is interesting to produce GO‐ZnO composite by simple blending. And then during the photocatalytic process, GO will be reduced into RGO, which can easily produce RGO‐ZnO composite with high photocatalytic activity in the degra‐dation of organic dyes. Moreover, there are still no reports in the long‐term stability of ZnO and graphene based ZnO composites. Recently, Liu et al. [10] fabricated ZnO‐Graphene nanocomposit‐es for photocatalytic reduction of Cr(IV). Zhang Chen et al. [11] reported the synthesis of Graphene –ZnO nanocomposites for application in anti‐photo corrosion and with improved photo activity. Silver modification significantly improved the photocatalytic Rhodamine B degradation capability of the Graphene–ZnO quasi‐shell–core composite.

From the detailed literature survey, the scope of metal oxide as well as of Graphene‐metal oxide nanocomposites and their application in the field of photocatalytic reactivity was well understood. It is worth to be highlighted that this method can successfully anchor uniform size ZnO and PdO nanoparticles on GO sheets. Assembling the ZnO nanoparticles on large GO sheets can remarkably enhance their photocatalytic activity of dye degradation under UV irradiation. Moreover, involving GO sheets into the system could effectively prevent the ZnO nanoparticle from aggregation without additional capping agents. As a result, these high quality GO‐ZnO‐PdO nanocomposites are beneficial for applications in environmental engineering and other fields.

To the best of our knowledge, there is no report on using Graphene‐ZnO‐PdO nanocomposites for photodegradation of Methylene Blue (MB) under UV light illumination. The objectives of the work were to synthesize ZnO nanoparticles and Graphene‐ZnO‐PdO nanocomposites through hydrother‐mal method [11, 12], to study the characteristics of the nanocomposites through XRD, FE‐SEM, TEM techniques, and to use the synthesized nanoparticles/nanocomposites for the photodegra‐dation degradation of MB under UV light illumination. In the present work, we synthesized ZnO nanoparticles and Graphene‐ZnO‐PdO nanocomposites and used for MB dye degradation applica‐tion. The results were compared with standard commercial ZnO particles.

2 Material and Methods 2.1 Chemicals and solvents Zinc acetate dihydrate (Zn(Ac)2

. 2H2O) (≥98%), potassium hydroxide, (≥98%), ethanol (≥99.9%), sodium hydroxide (≥92%), palladium chloride, isopropanol, and ammonium hydroxide were

purchased from Sigma Aldrich Pvt. Ltd, and graphite powder (50 nm Mesh) and commercial zinc oxide were purchased from Merck, Germany. Methylene blue was purchased from SD Fine Chemi‐cals Ltd., Mumbai. These chemicals were used without further purification. Milli‐Q‐water (18 Ω) was used for throughout the experiment.

2.2 Synthesis of ZnO nanorods In a typical procedure, Zn(CH3COO)2

. 2H2O (18.2 mmol, 5 g) was dissolved in a beaker with 60 mL absolute methanol under magnetic stirring at cooling condition for 90 min. At the same time, pow‐dered LiOH (364 mmol, 1.087 g) was dissolved in another beaker with 60 mL absolute methanol and magnetic stirring at room temperature for 45 min. Then, this basic solution was dropped into the zinc acetate solution for few minutes. The mixture was stirred slowly for 30 min at room temperature and dried at 120 °C for 10 hours. After cooling to room temperature for 30 min, the solution was centrifuged at 7500 rpm for 15 min and the obtained material was dried in a vacuum desiccator at 60 °C for 12 h.

2.3 Preparation of Graphene oxide (GO) using the Hummer’s Method Graphite oxide was synthesized from natural graphite powder by a modified Hummer’s method. Graphite powder (3 g) was put into a solution of concentrated H2SO4 (12 mL), K2S2O8 (2.5 g) and P2O5 (2.5 g). The mixture was kept at 80 °C for 4.5 hours using a hot plate. Successively, the mix‐ture was cooled to room temperature and diluted with 0.5 L de‐ionized (DI) water and left over‐night. Then, the mixture was filtered and washed with DI water using 0.2 micron Nylon Millipore filter to remove the residual acid. The product was dried under ambient conditions overnight. This pre‐oxidized graphite was then subjected to oxidation by the Hummer’s method described as follows. Pre‐treated graphite powder was put into cold (0 °C) concentrated H2SO4 (120 mL). Then, KMnO4 (15 g) was added gradually under stirring, and the temperature of the mixture was kept to be below 20 °C by cooling. Successively, the mixture was stirred at 35 °C for 2 h, and then diluted with DI water (250 mL). Because the addition of water in concentrated sulphuric acid medium released a large amount of heat, the addition of water was carried out in an ice bath to keep the temperature below 50 °C. After adding all of the 250mL of DI water, the mixture was stirred for 2 h, and then further 0.7 L DI was added. Shortly after the dilution with 0.7 L of H2O, 20 mL of 30% H2O2 was added to the mixture; the color of the mixture changed into brilliant yellow along with bubbling. The mixture was filtered and washed with 1:10 aqueous HCl solution (1 L) for removing metal ions, followed by 1 L of DI water for removing the excess acid. The resulting solid was dried in air and diluted to make a GO dispersion (0.5% w/w). Finally, it was purified by dialysis for one week in order to remove the remaining metal species. Exfoliation was carried out by sonicating 0.1 mg/mL GO dispersion under ambient condition for 20 min. The resulting homogeneous yellow‐ brown dispersion was tested to be stable for several months and used for further characterization.

2.4 Synthesis of Graphene Oxide‐Zinc Oxide‐Palladium Oxide (GO‐ZnO‐PdO) nanocomposites by hydrothermal method 5 g of Zinc acetate dihydrate was dissolved in 50 mL pure ethanol and kept in stirrer for 30 min. Thereafter, a solution of 1 g LiOH and 50 mL ethanol was added drop wise to the above solution. The overall solution was left to stir for another 30 min. The pH of the solution was adjusted to 9

using NaOH (3 M). 1 mL of PEG was added to the solution after 15 min. In another beaker, sepa‐rate solution was prepared by dissolving 100 mg of palladium chloride in 40 mL isopropanol. The pH of the solution prepared in this second beaker was adjusted to 9 using ammonium hydroxide. Then the two solutions prepared were mixed and kept for 30 min. 100 mg of graphene oxide was added to mixed solution and then stirred continuously for 6 h. Later solution prepared was kept in heating mantle at 80 ˚C for 1 h and cooled to get powder. The powder obtained was transferred to a Teflon lined autoclave and treated in box furnace at 400 ˚C for 2 h. Finally, powdered sample was used for further characterization.

2.5 X‐Ray Diffraction method (XRD) The structure and crystallinity of the synthesized nanoparticles were analyzed by using X‐ray Diffractometer, XRD (Miniflux 11, Rigaku Diffractometer, CuKα radiation, λ=0.1548), Japan. The samples were scanned from 0 to 80o with a scan speed of 1o/min.

2.6 Field Emission Scanning Electron Microscopy (FE‐SEM) The FE‐SEM image was carried out on a Hitachi (SU 6600), Japan. Sample was dissolved in ethanol and then sonicated for 10 min. Samples for FE‐SEM were prepared by drop casting 100 µL of solution dropped on an aluminum foil and dried by keeping overnight under ambient conditions before taking the FE‐SEM images.

2.7 Transmission Electron Microscopy (TEM) Transmission electron microscopy (TEM) and energy dispersive X‐ray analysis (EDAX) spectrum were carried out on a HRTEM (JEOL 3010; 300 KV) or Hitachi (H‐7650; 80KV). Sample was dissolved in ethanol and then sonicated for 10 min. Samples for TEM were prepared by drop casting 100 µL of solution dropped on a carbon coated Cu grid and dried by keeping overnight under ambient conditions before taking the TEM images.

2.8 Molecule model used in the study for photodegradation: Methylene Blue Methylene blue (MB) is the most common dye used in the dyeing of cotton, wood and silk. It can cause eye burns and is responsible for permanent damage to the eyes of human and animals. Inha‐lation can lead to breathing difficulties, and oral ingestion produces a burning sensation, causes nausea, vomiting, sweating, and abundant cold sweats. Treatment of industrial wastewater containing the dye turns of great interest nowadays due to great environmental concerns. Figure 1 shows the molecular structure of the model compound MB.

Figure 1: Molecular structure of Methylene Blue

2.9 Photocatalytic degradation Procedure: Photocatalytic studies were carried out in a multilamp photoreactor (HML MP88, Heber Scientific, India), fitted with eight 8 W medium pressure mercury vapor lamps set in parallel and have emission maximum at 365 nm. It has a reaction chamber with specially designed reflector made of highly polished aluminum and built in cooling fan. The volume of the reaction solution was maintained as 50 mL. Air was bubbled through the reaction solution for effective stirring. The light exposure wavelength was 330 mm. Analysis: the photocatalytic activity of the catalysts was evaluated by monitoring the degradation rate MB in aqueous solution under UV irradiation and continuous stirring. 1 g/l MB was prepared by dissolving 100 mg of MB powder (C16H18ClN3S) in 100 mL DI water. This stock solution was then used to prepare MB working solutions in concentra‐tions varied from 20 to 100 mg/L (namely 20, 40, 60, 80 and 100 mg/L). The MB concentration was determined by measuring the value at 664 nm using a UV‐Vis spectrophotometer. In all cases, 50 mL of the MB solution containing appropriate quantity of the semiconductor powder was magne‐tically stirred in dark for 40 min in order to ensure an adsorption/desorption equilibrium. The reaction suspensions containing MB and ZnO photocatalysts were irradiated by four parallel 8 W medium pressure mercury lamps with continuous stirring. At specific time intervals, 2 mL of sample was withdrawn and nanoparticles were removed by centrifugation. Absorbance measure‐ments were recorded in the range of 200‐800 nm, using a UV‐Vis spectrophotometer. The photo‐catalytic degradation efficiency of the MB solutions was calculated with the following equation:

ϴt = [(Ce – Ct) / Ce] x 100 (%) (1)

Where are Øt the photodegradation efficiency, Ce the MB concentration before and Ct after irradiation, and t irradiation time.

3 Results and Discussion 3.1 Nanoparticle characterization 3.1.1 X‐Ray Diffraction pattern (XRD) Figure 2 shows that the XRD spectrum of ZnO commercial, ZnO nanorods and GO‐ZnO‐PdO nano‐composites. Powder X‐ray diffraction study was carried out on commercial ZnO particles, prepared ZnO nanorod, and GO‐ ZnO‐PdO nanocomposites. From XRD it is very clear that the commercial ZnO peaks can be exactly assigned to a hexagonal ZnO phase (wurzite structure, space group (SG):‐ p63mc) with lattice constants a = 3.2508 Å and c = 5.2069 Å (c/a = 1.60), well matched with the standard card (JCPDS‐36‐1451) [14]. The diffraction peaks are intense and very sharp, indicating that the material is highly crystalline. According to diffraction peaks, no impurities were detected in this sample. The ZnO nanorods exhibit similar peaks of commercial ZnO along with greater intensity of (002) plane than (001) showing that the formation of nanorods is along the C‐axis. GO‐ZnO‐ PdO nanocomposites show XRD spectrum comparable to the ZnO nanoparticle together with additional new peaks. The additional peaks are corresponds to GO and PdO nanoparticles. PdO diffraction peaks marked in Figure 1 indicate the separate phases of PdO as tetragonal (space group D4h9 – P44/mmc), which matches with the standard card (JCPDS 41‐1107) [15]. For graphene, diffraction peaks are also observed at about 2ϴ = 26.15˚, which is well correlated with

the hexagonal structure with p63/mmc (194) space group (JCPDS 411487) [16]. No other impurity peaks were observed. The [001] reflections of GO are not observed in the XRD pattern of GO‐ZnO as a result of the intercalation of ZnO and PdO nanoparticles, which destroyed the regular stack of GO. The XRD analysis further confirms that ZnO and PdO nanoparticles have been effectively intercalated into the GO sheets. The crystalline size of nanoparticle was manipulated by using Debye–Scherrer formula [17]

D = 0.9 λ/βcos (2)

Where, D is the crystal size (in nm), λ is the X‐ray wavelength of source Cu‐K‐α (1.5418 x 10‐10 m), β is the full width at half maximum intensity of the diffraction peak located at 2θ, and θ is the Bragg’s diffraction angle. The crystal size of the nanocomposites is calculated and found to be 50 nm for commercial ZnO, 55 nm x 23 nm for ZnO nanorods, 27 nm for GO, 40 nm for ZnO, and 18 nm for PdO, respectively.

Figure 2: XRD spectra of ZnO commercial, ZnO nanorods and GO‐ZnO‐PdO nanocomposites

3.1.2 FE‐SEM and TEM analysis The morphology of ZnO nanorods and GO‐ZnO‐PdO nanocomposites were investigated by TEM and FE‐SEM analysis, respectively. The morphologies of ZnO nanomaterial exhibit the rod shape structure. ZnO nanorods have a length and diameter in the range of 50 x 19 nm. Figure 3 (b) shows that layer of Graphene oxide sheets are whole covered by ZnO and PdO nanoparticles. ZnO and PdO nanoparticles are uniformly dispersed on the graphene oxide layer, and most of the nanopar‐ticles are round shaped with only few nanoparticles with irregular shapes. The size of GO sheets is over several microns. FE‐SEM image clearly indicates that the ZnO and PdO nanoparticles were successfully anchored onto the GO sheets.

Figure3: (a ‐ left) ZnO nanorods, and (b ‐ right) GO‐ZnO‐PdO nanocomposites

3.2 Photocatalytic degradation of Methylene Blue 3.2.1 Adsorption isotherms

Figure 4: Adsorption isotherms of MB on (a) commercial ZnO, (b) rod ZnO and GO‐ZnO

The adsorption isotherms of MB were carried out without UV light for all samples. Figure 4 shows that when the initial concentration of MB increases, the adsorbed amount increases as well to reach a limit value corresponding to the maximum quantity adsorbed. The commercial ZnO mate‐rial follows the Langmuir type (L‐type) adsorption, whereas GO‐ZnO‐PdO nanocomposites exhibit both Langmuir and Freundlich isotherms according to the classification of Giles et al. [18]. For the commercial ZnO material, the time required to reach the adsorption equilibrium state of MB is 40 min, while in case of GO‐ZnO‐PdO nanocomposites the equilibrium state is reached at 30 min probably due to large surface area (data not shown here).

3.2.2 Effect of nanoparticles on photocatalytic reaction Before photodegradation studies, adsorption of Methylene Blue and its photolysis were evaluated. Figure 5 shows MB removal kinetics in dark with photocatalyst, under UV without photocatalyst,

and under UV with photocatalyst. In dark with photocatalyst and under UV irradiation without photocatalyst, the maximum amount of MB disappearance was about 5% after 60 min.

Regarding photocatalysis (photocatalyst under illumination) efficiency, there was a complete disappearance of MB after 40 min of irradiation. This result shows clearly that ZnO material can be efficiently used for the photocatalytic degradation of MB in aqueous media.

Figure 5: Photodegradation of MB (20 mg/L) vs photolysis and adsorption on ZnO‐C (2 g/L), free pH

3.2.3 Effect of initial dye concentration The influence of initial concentration of the dye solution on the photocatalytic degradation is an important aspect of the study. The kinetic study of the photocatalytic reaction of the dye as a function of the initial concentration was achieved by varying the initial concentration of MB from 20 to 100 mg/L (Figure 6). According to this figure, it is noted that more time is required for MB removal at higher initial concentrations. The photocatalytic degradation decreased with increasing initial concentration of MB. Indeed, for a MB concentration of 20 mg/L, total photodegradation is observed as 40 min, whereas with a concentration of 80 mg/L and 100 mg/L the total elimination is reached after 100 and 120 min of irradiation, respectively, for the commercial ZnO material. The photoactivity studies of synthesized nanomaterials such as ZnO nanorods and GO‐ZnO‐PdO nano‐composites show that these materials were more efficient regarding MB removal. 100% MB removal was observed within 20 min for GO‐ZnO‐PdO nanocomposites, whereas 100% MB removal was achieved in 30 min for ZnO nanorods (20 mg/L dye). For the two later photocatalysts, 100 min were necessary to remove MB from water when concentration does not exceed 40 mg/L.

Figure 6: Effect of initial concentration on the photodegradation efficiency of MB on (a) commercial ZnO, (b) Rod ZnO, and (c) GO‐ZnO‐PdO nanocomposites

More time is needed for the target dye removal when the concentration is over 40 mg/L. Indeed, it was noted that ZnO nanorods exhibited 90% and 50% of degradation for 80 mg/L and 100 mg/L of MB, respectively, while for GO‐ZnO‐PdO nanocomposites only 70% was achieved for 80 mg/L of MB. This result clearly reveals that at lower concentrations of MB, GO‐ZnO‐PdO nanocomposites are more efficient, whereas at high concentrations ZnO nanorods become the more efficient photocatalyst. Nevertheless, increasing the substrate concentration leads to decreasing the degradation rate of the dye. The presumed reason is that when the initial concentration of dye is increased, more and more dye molecules are adsorbed on the surface of photocatalyst. The large amount of adsorbed dye is thought to have an inhibitive effect on the reaction of dye molecules with photo generated holes or hydroxyl radicals, because of the lack of any direct contact between them. Once the concentration of dye is increased, it also causes the dye molecules to adsorb light, and photons never reach the photocatalyst surface, thus the photodegradation efficiency decreas‐es [19]. In addition to this, at higher concentration of nanoparticles GO‐ZnO‐PdO tend to aggre‐gated more than ZnO nanorods that leads to reduce the active surface area of the photocatalyst. There should be an optimum of catalyst amount and the substrate concentration leading to maximum efficiency of MB degradation. Indeed, an increased opacity of the suspension, brought about as a result of excess of photocatalyst particles, would lead to decreased UV absorption.

3.2.4 Kinetics of photodegradation of MB on (a) ZnO commercial, (b) Rod ZnO, (c) GO‐ZnO‐PdO To analyze degradation kinetics, simplified apparent pseudo first‐order kinetics rates were used for MB degradation:

ln(C0/C) = kap. t (3)

Where, kap is the apparent pseudo‐first‐order rate constant of initial degradation (min−1), and C and Ce represent the transient and initial concentrations of MB, respectively. The rate constant kap

was calculated from the slope of ln Ce/C versus t (Figure 7).

Figure 7: Kinetics of photodegradation of MB on (a) ZnO commercial, (b) Rod ZnO, (c) GO‐ZnO‐PdO

As these curves are linear, the degradation kinetic of MB follows well Langmuir kinetic model. The value of initial rate of photocatalytic degradation of MB is calculated using the equation:

r0 = kap.Ce (4)

The results indicate that the initial rate (r0) increases gradually with increasing initial concentration of dye (Table 1). It is noted that at low initial concentrations of MB (20 mg/L) the photocatalyst GO‐ZnO (kap = 0.14676 min‐1) is more active than rod ZnO (kap = 0.646 min‐1), while for the initial concentration higher or equal to 40 mg/L, rod ZnO is more efficient than GO‐ZnO.

Table 1: Initial rate and rate constants at different initial concentrations of the dye

Nanoparticles Constants MB Concentrations

ZnO Commercial kap (min‐1) 0.1866 0.12190 0.0939 0.0716

r0 (mg/L/min) 3.7336 4.8760 5.6320 5.7290

Rod ZnO kap (min‐1) 0.0646 0.0521 0.0338 0.0187

r0 (mg/L . min) 1.292 2.084 2.028 1.496

GO‐ZnO‐PdO kap (min‐1) 0.14676 0.03381 0.01311 0.00492

r0 (mg/L . min) 2.935 1.3524 0.7866 0.3936

4 Conclusions We have successfully synthesized high quality ZnO nanorods and GO‐ZnO‐PdO nanocomposites through hydrothermal method. The degradation of Methylene blue by commercial ZnO, ZnO nanorods and GO‐ZnO‐ PdO nanocomposites exhibits a first order reaction kinetics. The order of photocatalytic activity of nanomaterials is estimated as GO‐ZnO‐PdO > ZnO nanorods > commercial ZnO materials for MB concentration not exceeding 40 mg/L. GO –ZnO – PdO exhibits more efficien‐cy probably due to larger surface area and extended pi‐electron conjugation of GO. Synthesized ZnO nanorods as well as GO‐ZnO‐PdO nanocomposites are good candidate materials for photo‐catalytic removal of dye from water.

5 Acknowledgments We express our sincere thanks to DAAD and the EXCEED SWINDON project for the financial support to attend the Expert Workshop on Water Security, May 15‐20, 2017 in Mekelle, Ethiopia.

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