zinc oxide nanoparticles: Enhanced photocatalysts degrade malachite green dye under visible light conditions

Author’s Accepted Manuscript Platinum/zinc oxide nanoparticles: Enhanced photocatalysts degrade malachite green dye under visible light conditions Reda M. Mohamed, David McKinney, Mohammad W. Kadi, Ibraheem. A. Mkhalid, Wolfgang Sigmund www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)30089-X http://dx.doi.org/10.1016/j.ceramint.2016.02.147 CERI12338

To appear in: Ceramics International Received date: 3 December 2015 Revised date: 14 February 2016 Accepted date: 23 February 2016 Cite this article as: Reda M. Mohamed, David McKinney, Mohammad W. Kadi, Ibraheem. A. Mkhalid and Wolfgang Sigmund, Platinum/zinc oxide nanoparticles: Enhanced photocatalysts degrade malachite green dye under visible light conditions, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.02.147 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Platinum/zinc oxide nanoparticles: Enhanced photocatalysts degrade malachite green dye under visible light conditions Reda M. Mohamed1,3, David McKinney2, Mohammad W. Kadi1, Ibraheem. A. Mkhalid1, and Wolfgang Sigmund4 1

Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Kingdom of Saudi Arabia 2

MM Virtuoso, Gainesville, FL, 32606 (USA)

3

Advanced Materials Department, Central Metallurgical R&D Institute, CMRDI, P.O .Box 87 Helwan, Cairo 11421, Egypt 4

University of Florida, Herbert Wertheim College of Engineering, Department of Materials Science and Engineering, 225 Rhines Hall, P.O. Box 116400, Gainesville, FL, 32611-6400 (USA) Abstract Zinc oxide nanoparticles (ZnO) were prepared via a sol-gel method, and a photo-assisted deposition method was used to prepare platinum on zinc oxide nanoparticles (Pt/ZnO). Several techniques were used to characterize these enhanced photocatalysts: XRD, TEM, UV-Vis spectra, PL spectra, XPS, and BET surface area analysis. As-prepared samples’ photocatalytic performances were studied via degradation of malachite green dye under various visible-lightonly irradiation scenarios. Results demonstrated the following: platinum (Pt) was well dispersed on and in ZnO’s surfaces and pores; as such, Pt/ZnO had less surface area than pure ZnO due to pore blockage; however, advantages gained from enhanced electron-hole separation and decreased band gap width more than made up for this negative effect; moreover, Pt/ZnO prepared with 0.3 wt % Pt exhibited the lowest band gap and the highest photocatalytic activity of the various samples with a solids loading of 0.8 g/l; finally, such samples were recyclable, i.e., photocatalytic performance remained stable even after five uses. 1. Introduction [4-[[4-(dimethylamino)phenyl]-phenylmethylidene]cyclohexa-2,5-dien-1-ylidene]dimethylazanium;chloride, IUPAC’s name for malachite green (MG), is listed in PubChem with CID of 11294, molecular formula of C23H25ClN2, and molecular weight of 364.911 g/mol [1]. Scientists have known about MG’s and leucomalachite green’s toxicities for decades. Foster and Woodbury first reported its use as an effective fish fungicide and antiseptic in 1936, yet even then they warned that MG is highly toxic to fish [2]. Bills et al. documented acute toxicity levels to small fry rainbow trout in the 1970s [3]. Culp and Beland reported in the 1990s that MG promotes tumors and shows marked cytotoxicity in mammals [4]. Moreover, medical case reports on human ingestion showcase unpleasant effects such as diarrhea and abdominal pain [5], as well as severe effects, including blindness, with direct contact [6]. Resultantly, international government entities describe MG as toxic, harmful, hazardous, corrosive, and polluting (United

Nations, European Union, and U.S. National Oceanic and Atmospheric Administration) [1]. Additionally, U.S. Food and Drug Administration nominated MG as a priority chemical for carcinogenicity testing [4]. Though MG’s use is banned in Europe and the United States, industries in many countries prolifically continue to use it. Its application as a dye in the textile industry is particularly frequent and concerning. MG of various quality and purity is used extensively and in large quantities to color wool, cotton, jute, paper, leather, etc. Most alarming, 10-15% of MG dyes is directly lost to wastewater in the process [7]. Furthermore, many studies’ results suggest that MG, its metabolites, and its breakdown products may not be completely removed by wastewater treatments, and then these may be present in sufficient quantities in effluents to cause residues in wild fish populations [8]. Eleven years ago, Srivastava et al. recommended that less controversial, alternative parasiticides be used in aquatic culture and that MG’s uses in other industries be completely avoided so that sum levels of MG and leucomalachite green in edible fish stay below the established zero tolerance of 0.01 mg/kg [9]. Unfortunately, a simple query in Web of Science of MG research published in just the last five years proves the extent that this compound still poses environmental hazards: 1681 articles dealing with MG detection, toxicity levels in fish, environmental remediation, etc. appear. Since MG continues to be used, despite government bans and recommendations to the contrary, it is imperative that researchers find ways to remove it from industrial effluents. Prado and Costa list several processes (and their limitations) that have been applied to treat such contaminated effluents: incineration, biological treatment, ozonation, and adsorption on solid phases [10]. Of greater appeal is the heterogeneous photocatalysis of MG into inert products. It is also cost effective and highly efficient [11]. Furthermore, enhanced nanophotocatalysts can be recycled, as demonstrated herein. There are a variety of nanophotocatalysts, with TiO2 and ZnO perhaps most widely studied, but all require UV light to be effective and efficient in their original forms due to their wide band gaps. Moreover, they also allow a high rate of recombination of charge carriers. Fortunately, several enhancements exist to overcome these deficiencies [12-13]. For example, doping TiO2 with a metal creates a metal-semiconductor junction which facilitates electron-hole (e-h) separation [13]. Zinc oxide is a predominantly n-type semiconductor [14] with a very wide band gap of 3.37 eV in the wurtzite phase. Thusly, it is limited to UV light applications [15]. Next generation zinc oxide nanocomposites exhibit narrower band gaps and improved e-h separation, which is beneficial for applications such as degradation of pollutants in waste water effluents, nitrate reduction [16], nitrite sensors [17], carbon monoxide oxidation [18], methanol steam reforming [19], and improved electrical properties [20]. Recent researchers synthesized various zinc oxide nanocomposites and doped with metals and non-metals to narrow the band gap. Tang et al. synthesized lotus-root Au:ZnO nanostructures with enhanced photocatalytic activity [21]. Reddy et al. made ZnO:RGO/RuO2 nanocomposites with excellent degradation efficiency of methylene blue under simulated sunlight [22]. And Eskizeybek et al. reported photodegradation under natural sunlight of malachite green (MG) and methylene blue dyes using a polyaniline:ZnO nanocomposite [23].

Herein we present a simple methodology for the synthesis of platinum on zinc oxide nanoparticles (Pt/ZnO), which acted as a superb photocatalyst on MG in visible light conditions. Synthesis was carried out via a sol-gel preparation of ZnO, followed by a photo-assisted deposition method that delivered platinum onto their surfaces and into their pores. As-prepared samples were then characterized and tested as nanophotocatalysts in the decolorization of malachite green dye under visible-light-only irradiation conditions. Their performance and kinetic rate constants were compared/contrasted with ZnO as a control. Lastly, recycling studies showed that Pt/ZnO remained a stable and effective nanophotocatalyst even after five uses. To the best of our knowledge, no previous research has reported on the efficacy of Pt/ZnO for the removal of malachite green dye. Also, other recent reports of platinum zinc oxide combinations use carbon nanotube structures [17, 20] or other architecture [18, 19]. Likewise, this report focuses on efficient nanophotocatalytic MG degradation under visible-light-only conditions, a significant achievement beyond other nanophotocatalysis reports that required UV irradiation [11]. 2. Experimental 2.1 Synthesis of pure ZnO and platinum/zinc oxide nanoparticles Initially, zinc oxide nanoparticles (ZnO) were synthesized via a sol-gel technique. In a typical preparation, 20 ml zinc methoxide was mixed with methyl alcohol, ultra pure water (H2O) and nitric acid (HNO3) under vigorous stirring for 1 h. Resultant mixtures were aged at room temperature until they formed gels. Finally, they were dried at 80 °C for 24 h and calcined at 550 °C for 5 h in air. A photo-assisted deposition method was then used to prepare Pt/ZnO. First, ZnO were impregnated in an aqueous solution of platinum chloride of either 0.1, 0.2, 0.3, or 0.4 wt % Pt. These resultant mixtures were exposed to UV irradiation for 24 h. Finally, obtained Pt/ZnO were dried at 60 °C for 24 h. 2.2 Characterization techniques and apparatuses Nanostructure morphology and sample dimensions were measured using JEOL-JEM-1230 transmission electron microscopy (TEM). Samples were suspended in ethanol and ultrasonicated for 30 min. A small amount was then coated with carbon, dried on a copper grid, and loaded into the TEM. Also, N2-adsorption measurements were taken on treated samples (2 h under vacuum at 100 °C) with a Nova 2000 series Chromatech apparatus at 77K to calculate surface area. Crystalline phase was determined by powder X-ray diffraction (XRD) using Bruker axis D8 with Cu Kα radiation (λ = 1.540 Å) at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific K-ALPHA spectrometer. Band gap performance was determined by ultra violet-visible diffuse reflectance spectra (UV-Vis-DRS), measured using a UV-Vis-NIR spectrophotometer (V-570, Jasco, Japan) in air at room temperature to detect absorption over the range 200-800 nm. And photoluminescence emission spectra (PL) were obtained with a Shimadzu RF-5301 fluorescence spectrophotometer. In photocatalysis experiments, visible-light-only irradiation was achieved with a 300 W power xenon lamp equipped with a 420 nm cutoff filter. 2.3 Visible-light-only MG degradation with Pt/ZnO as photocatalysts Pt/ZnO efficiency as nanophotocatalysts was evaluated by testing aqueous MG degradation under visible light illumination. In a typical sequence, a specified amount of photocatalyst was

suspended in MG aqueous solution in a 500 ml reactor. The mixture was stirred for 30 min in darkness to establish the adsorption-desorption equilibrium between photocatalyst and aqueous MG solution. Then a 300 W power Xenon lamp with 0.96 W/cm2 intensity was switched on to simulate sunlight, while a cut-off filter was used to remove UV light (λ < 420 nm). Experiments were carried out with ZnO as control and Pt/ZnO as variables (with 0.1, 0.2, 0.3, and 0.4 wt % Pt). Aliquots were taken from the suspension at different intervals of time and filtrated for analysis. MG photodegradation efficiency was determined with a UV-vis spectrometer by measuring each solution’s absorbance at the absorption wavelength (617 nm) at room temperature at specified time intervals. Essentially, MG concentration and photocatalytic activity are inversely related. As photocatalytic activity improves, MG concentrations lower. The following equation relates decoloring efficiency, D(%), to concentration of MG at time 0 and t (C0 and Ct), where t is irradiation time [11]: ( )

(

)

3. Results & discussion 3.1 Structural and chemical characteristics 3.1.1

X-ray diffraction

The as-prepared samples’ crystal structures were investigated using XRD. As shown in Figure1, all samples gave XRD peaks that match standard diffraction data of zinc oxide’s hexagonal wurtzite crystalline structure (JCPDS card no. 89-0511). These results suggest that zinc oxide’s phase structure remained unchanged during platinum deposition and remained pure [20]. Assuming Pt wt % was within XRD detection limits, then Pt was either merely deposited on the surfaces, or it could have entered ZnO’s lattice interstitially [20]. Mean crystallite sizes of samples calculated with the Scherrer formula were as follows: ZnO, 14 nm; Pt/ZnO 0.1 wt %, 13 nm; Pt/ZnO 0.2 wt %, 12 nm; Pt/ZnO 0.3 wt %, 11 nm; and Pt/ZnO 0.4 wt %, 10 nm. Thus, Pt/ZnO exhibited ever smaller and smaller crystal sizes as platinum content increased. These results align with those in Mujahid’s investigation [20]. Also, such small, quantum dot sizes mean that as-prepared samples should exhibit quantum dot material properties [11], such as shifted emission spectra. 3.1.2

X-ray photoelectron spectroscopy

XPS spectra were used to investigate deposited Pt in Pt/ZnO samples. Figure 2 shows Pt4f7/2 XPS spectra peaks of 0.3 wt % Pt/ZnO at 71.15 eV and 74.45 eV. Textbook matches, the asymmetric peak at 71.1 eV corresponds to Pt metal, and the symmetric peak at 74.45 corresponds to the compound PtO [24-25]. 3.1.3

TEM image

The prepared samples’ morphologies and particle sizes were studied with transmission electron microscopy. Figure 3 presents TEM images of the prepared (A) ZnO and Pt/ZnO with Pt wt % (B) 0.1, (C) 0.2, (D) 0.3, and (E) 0.4. Results show that Pt nanoparticles had an average size of 5 nm. Also, as Pt wt % increased, Pt dispersion on ZnO surfaces also increased. The

addition of Pt wt % up to 0.3 % further improved Pt’s particle size homogeneity on ZnO surfaces. Finally, Pt additions above 0.3 wt % decreased homogeneity, which suggests that there is an optimum content amount for even deposition and size control of Pt onto ZnO. 3.1.4

Surface area analysis

N2-adsorption measurements, taken from treated samples (2 h under vacuum at 100 °C) on a Nova 2000 series Chromatech apparatus at 77K, were used to calculate surface area of all samples. Table 1 lists BET surface areas of the as-prepared ZnO and Pt/ZnO samples. As Pt wt % increased from 0 to 0.4, surface area decreased from 56 to 48 m2/g. It was determined that Pt/ZnO samples exhibited lower total pore volume than ZnO samples because Pt deposits blocked some pores. Mesopores were present in all samples, as confirmed by their similar SBET and St measurements. 3.1.5

Optical properties

Figure 4 compares UV-Vis diffuse reflectance spectra of the ZnO and Pt/ZnO samples. Pt deposition onto ZnO surfaces and into pores red shifted the absorption edge from 387 nm to 468 nm. Obtained UV-Vis spectra were used to calculate the direct band gaps of ZnO and Pt/ZnO with the following equation: Eg = 1239.8/ where Eg is the band gap (eV) and  is the wavelength (nm) of the absorption edges in the spectrum. Table 2 lists the calculated values. Results reveal that increasing Pt wt % from 0.1 to 0.3 decreased the band gap from 3.2 eV to 2.76 eV. However, there was no significant effect on band gap width beyond Pt 0.3 wt %. Both UV-Vis data and band gap calculations correspond to Mujahid’s findings [20]. Therefore, it appears that there is an optimum Pt deposition amount to helps control band gap width. 3.1.6

Photoluminescence emission spectra (PL)

PL spectra for all samples were used to investigate electron-hole (e-h) separation and recombination. The findings corroborated the absorption shift found in UV-Vis data, and PL intensity is observable with Pt additions. Figure 5 shows high resolution scans of the samples’ PL spectra band edge peaks, which confirm the UV-Vis results. PL intensities decreased in the same order as the samples’ UV-Vis absorption edges red shifted with regard Pt wt %. Once more, no significant effect on PL intensity at Pt wt % above 0.3 was found. This, too, agrees with UV-Vis results and suggests that there is an optimum content of deposited Pt which can improve ZnO’s photocatalytic properties. 3.2 Photocatalytic performance 3.2.1

Pt/ZnO versus ZnO

To determine how well the as-prepared samples performed under visible-light-only conditions, photodegradation of MG was measured under several scenarios. For the first tests, 0.3 g weight of the photocatalyst was suspended in 500 ml aqueous solution with MG concentration of 100 ppm. Figure 6 shows the effect of Pt wt % on the samples’ photocatalytic activity, specifically as MG dye decolorization within certain irradiation time intervals.

Photocatalytic activity increased from just 14% decolorization with ZnO to 100% with 0.3 wt % Pt/ZnO after 50 min. Increasing Pt wt % above 0.3 had no significant effect. ZnO samples exhibited very little photocatalytic activity under visible-light-only conditions. As Pt wt % increased, however, the as-prepared samples performed better and better, up to a point. Ultimately, 0.3 wt % Pt/ZnO outperformed other investigated samples, as much as 21 times better than the ZnO control. This result is what we would expect given our characterization findings. We attribute the enhancement to increased absorption in the visible light range, increased e-h separation due to the metal-semiconductor junctions formed from platinum and zinc oxide, and improved proton-coupled electron transfer reactions [26]. Table 3 summarizes the rate constant of reaction kinetics of the various tested photocatalyst types. 3.2.2

Effect of solids loading of 0.3 wt % Pt/ZnO

Next we studied how solids loading of 0.3 wt % Pt/ZnO samples affected decolorization in 1000 ml aqueous solution with MG concentration of 100 ppm under visible-light-only conditions. Figure 7 shows that by increasing the photocatalyst weight from 0.4 g/l to 0.8 g/l, the degradation time was considerably shortened. However, increasing photocatalyst concentration above 0.8 g/l lengthened the degradation time. Specifically, 0.4 g/l solids loading decolored 98% of MG after 50 minutes. 0.6 g/l solids loading decolored 100% of MG after 50 minutes. But the sample of 0.8 g/l solids loading only took 30 min to oxidize the MG. As solids loading went beyond 0.8 g/l, reaction times lengthened once more. The 1.0 g/l and 1.2 g/l samples took 45 and 55 min to complete MG oxidation. The initial increase of photocatalyst amounts helped to speed MG degradation due to more available sites for photocatalytic reaction. However, the higher amounts probably hindered light penetration during the reaction, thus slowing degradation. Table 4 summarizes the rate constant of reaction kinetics of the various tested photocatalyst amounts. 3.2.3

Effect of initial dye concentration

Our third experiments focused on the effect that various initial dye concentrations had on the photocatalytic oxidation of the solutions under visible-light-only conditions. For this set-up, we used 0.8 g/l of the 0.3 wt % Pt/ZnO as photocatalysts, as these were determined to be the optimal parameters established in our earlier tests described above. Figure 8 shows how increased MG concentrations slow down photocatalytic oxidation. The as-prepared photocatalysts performed exceptionally well at the lower MG concentrations. In 25 ppm to 100 ppm tests, the samples took 30 min to completely degrade MG. As MG concentration increased to 150 and 200 ppm, the reaction times to complete oxidation lengthened to 40 and 55 min respectively. We offer the following explanation for this phenomenon: as photocatalytic activity depends on hydroxyl free radicals to reach the catalyst’s surface and react with MG, then an increase in the initial MG concentration also increases the probability of a reaction between the free radicals and MG. Thusly, photocatalytic activity improves. However, higher concentrations of MG decrease photocatalytic activity because MG dye blocks photocatalysts’ active sites, which prevents visible light from reaching them. Table 5 summarizes the rate constant of reaction kinetics of the various tested dye concentrations. 3.2.4 Photocatalyst recycling

Lastly, we recycled one 0.3 wt % Pt/ZnO sample with solids loading amount of 0.8 g/l and tested it multiple times in aqueous solutions with concentrations of 100 ppm MG. We allowed 30 min reaction time in each instance. Amazingly, even after five cycles using the same sample, MG decolorization was still 100%, as seen in Figure 9. Thus, Pt/ZnO is a stable, robust, and easily recyclable and separable photocatalyst. This result makes it a promising material for environmental remediation. 4. Conclusions Herein we detail an efficient, sol-gel with photo-assisted deposition synthesis of Pt/ZnO, which have promising visible-light-only photocatalytic properties and show high efficiency at oxidizing MG, a most serious environmental hazard commonly found in textile wastewater effluents. Depositing platinum onto ZnO surfaces and into their pores narrowed the band gap and prevented electron-hole recombination. From our experiments, we determined 0.3 wt % Pt/ZnO with solids loading 0.8 g/l to be the optimal conditions for achieving 100% MG oxidation after just 30 min. Such samples show promise as applied to solar energy environmental remediation of pollutant dyes. Acknowledgments We acknowledge and thank the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah (No. 51-130-35-RG) for technical and financial support. 5. References 1. National Center for Biotechnology Information. PubChem Compound Database, CID=11294. https://pubchem.ncbi.nlm.nih.gov/compound/11294 (accessed Oct. 15, 2015). 2. F.J. Foster, L. Woodbury. The Use of Malachite Green as a Fish Fungicide and Antiseptic, The Progressive Fish-Culturist 3:18 (1936) 7-9. DOI: 10.1577/15488640(1936)318[7:TUOMGA]2.0.CO;2. 3. T.D. Bills, L.L. Marking, J.H. Chandler Jr. (1977) Malachite green: its toxicity to aquatic organisms, persistence, and removal with activated carbon. Investigations in Fish Control 75 (1977) 1-6. 4. S.J. Culp, F.A. Beland. Malachite green: a toxicological review. J. Am. Coll. Toxicol. 15 (1996) 219–238. 5. R.E. Gosselin, R.P. Smith, H.C. Hodge. Clinical Toxicology of Commercial Products. 5th ed. Baltimore: Williams and Wilkins (1984) II-384. 6. W.M. Grant. Toxicology of the Eye. 2nd ed. Springfield, Illinois: Charles C. Thomas (1974) 431. 7. G. Parshetti, S. Kalme, G. Saratale, S. Govindwar. Biodegradation of malachite green by Kocuria rosea MTCC 1532. Acta Chim. Slov. 53 (2006) 492-498.

8. D. Arnold, B. LeBizec, R. Ellis. Malachite Green. First draft of FAO UN paper (2009) ftp://ftp.fao.org/ag/agn/jecfa/vetdrug/6-2009-malachite_green.pdf (accessed Oct. 15, 2015). 9. S. Srivastava, R. Sinha, D. Roy. Review: Toxicological effects of malachite green. Aquatic Toxicology 66 (2004) 319–329. 10. A.G.S. Prado, L.L. Costa. Photocatalytic decouloration of malachite green dye by application of TiO2 nanotubes. J. Hazard. Mater. 169 (2009) 297-301. 11. H.R. Rajabi, O. Khani, M. Shamispur, V. Vatanpour. High performance pure and Fe3+ion doped ZnS quantum dots as green nanophotocatalysts for the removal of malachite green under UV-light irradiation. J. Hazard. Mater. 250-251 (2013) 370-378. 12. R.M. Mohamed, D.L. McKinney, W.M. Sigmund. Enhanced nanocatalysts. Materials Science and Engineering R 73 (2012) 1-13. 13. A. Charanpahari, S.G. Ghugal, S.S. Umare, R. Sasikala. Mineralization of malachite green dye over visible light responsive bismuth doped TiO2-ZrO2 ferromagnetic nanocomposites. New J. Chem. 39 (2015) 3629-3638. 14. C. Jagadish, S.J. Pearton (ed), Zinc Oxide Bulk, Thin Films, and Nanostructures (2006) New York: Elsevier. 15. Z.L. Wang. Splendid one-dimensional structures of zinc oxide: A new nanomaterial family for nanotechnology. ACS Nano 2 10 (2008) 1987-1992. 16. A.F. Shojaei, F. Golriz. High photocatalytic activity in nitrate reduction by using Pt/ZnO nanoparticles in the presence of formic acid as hole scavenger. Bulgarian Chem. Comm. 47:2 (2015) 509-514. 17. M. Zhang, D. Huang, Z. Cao, Y. Liu, J. He, J. Xiong, Z. Feng, Y. Lin. Determination of trace nitrite in pickled food with a nano-composite electrode by electrodepositing ZnO and Pt nanoparticles on MWCNTs substrate. LWT – Food Sci. Tech. 64 (2015) 663-670. 18. B. Liu, S. Han, K. Tanaka, H. Shioyama, Q. Xu. Metal-organic Framework (MOF) as a precursor for synthesis of platinum supporting zinc oxide nanoparticles. Bull. Chem. Soc. Japan 82 (2009) 1052-1054. 19. L. Arroy-Ramierz, C. Chen, M. Cargnello, C.B. Murray, P. Fornasiero, R. Gorte. Supported platinum-zinc oxide core-shell nanoparticle catalysts for methanol steam reforming. J. Mater. Chem. A. 2 (2014) 19509-19514. 20. M. Mujahid. Synthesis, characterization and electrical properties of visible-light-driven Pt-ZnO/CNT. Bulletin of Materials Science 38:4 (2015) 995-1001.

21. J. Tang, B. Zhou, S. Zhang, Z. Wang, L. Xiong, P. Li. Synthesis and photocatalytic properties of lotus-root like Au-ZnO nanostructures. SCIENCE CHINA Chemistry 58:5 (2015) 858–862. 22. D.A. Reddy, R. Ma, T.K. Kim. Efficient photocatalytic degradation of methylene blue by hetero structured ZnO–RGO/RuO2 nanocomposite under simulated sunlight irradiation. Ceram. Interntl. 41 (2015) 6999–7009. 23. V. Eskizeybek, F. Sarı, H. Gulce, A. Gulce, A. Avci. Preparation of the new polyaniline/ZnO nanocomposite and its photocatalytic activity for degradation of methylene blue and malachite green dyes under UV and natural sun lights irradiations. Appl. Cat. B: Environ. 119– 120 (2012) 197– 206. 24. Thermo Fisher Scientific Inc. http://xpssimplified.com/elements/platinum.php (accessed on October 15, 2015). 25. M.C. Biesinger. http://www.xpsfitting.com/search/label/Platinum (accessed on October 15, 2015). 26. J.N. Schrauben, R. Hayoun, C.N. Valdez, M. Braten, L. Fridley, J.M. Mayer. Titanium and zinc oxide nanoparticles are proton-coupled electron transfer agents. Science 336 (2012) 1298-1301.

Table 1. Texture parameters of ZnO and Pt/ZnO nanoparticles. SBET

St

Smicro

(m2/g)

(m2/g)

3 3 (cm2/g) (cm2/g) (cm3/g) (cm /g) (cm /g) (Å)

ZnO

56.00

56.00

40.00

16.00

0.310

0.209

0.101

25.00

0.1 wt % Pt/ZnO

54.00

54.00

39.00

15.00

0.255

0.186

0.069

30.00

0.2 wt % Pt/ZnO

52.00

53.00

38.00

14.00

0.232

0.165

0.067

35.00

0.3 wt % Pt/ZnO

50.00

50.00

37.00

13.00

0.220

0.160

0.060

40.00

0.4 wt % Pt/ZnO

48.00

49.00

36.00

0.203

0.150

0.053

45.00

Sample

Sext

12.00

Vp

Note: (SBET) ► BET-surface area (St) ► surface area derived from Vl-t plots (Smic) ► surface area of micropores (Sext) ► external surface area (Vp) ► total pore volume (Vmic) ► volume of micropores (Vmes) ► volume of mesopores (r-) ► mean pore radius

Vmicro

Vmeso

r

Table 2. Band gap energies of ZnO and Pt/ZnO nanoparticles.

Sample

Band gap energy, eV

ZnO

3.20

0.1 wt% Pt/ZnO

2.97

0.2 wt% Pt/ZnO

2.85

0.3 wt% Pt/ZnO

2.65

0.4 wt% Pt/ZnO

2.76

Table 3. Rate constant of reaction kinetic for catalyst type’s effect on MG oxidation.

Sample

k x 10-5, min-1.

ZnO

328

0.1 wt % Pt/ZnO

1571

0.2 wt % Pt/ZnO

2394

0.3 wt % Pt/ZnO

5847

0.4 wt % Pt/ZnO

6067

Table 4. Rate constant of reaction kinetic for catalyst amount’s effect on MG oxidation.

Sample

k x 10-5, min-1.

0.4 g/l

605

0.6 g/l

6608

0.8 g/l

9608

1.0 g/l

5244

1.2 g/l

704

Table 5: Rate constant of reaction kinetic for MG concentration’s effect on MG oxidation.

Sample, ppm

k x 10-5, min-1.

25

13401

50

13861

75

9467

100

9744

150

7561

200

4856

Figure captions Figure 1. XRD patterns of ZnO and Pt/ZnO. Figure 2. XPS spectra of Pt 4f from 0.3 wt % Pt/ZnO sample. Figure 3. TEM images of ZnO and Pt/ZnO: Pt wt % is (A) 0.0, (B) 0.1, (C) 0.2, (D) 0.3, and (E) 0.4. Figure 4. UV-Vis absorption spectra of ZnO and Pt/ZnO nanoparticles. Figure 5. Photoluminescence spectra of ZnO and Pt/ZnO nanoparticles. Figure 6. Effect of Pt wt % on MG photocatalytic oxidation. Figure 7. Effect of solids loading of 0.3 wt% Pt/ZnO sample on MG photocatalytic oxidation. Figure 8. Effect of initial MG concentration on MG photocatalytic oxidation with 0.3 wt % Pt/ZnO. Figure 9. Recycling/reusing viability of 0.3 wt% Pt/ZnO photocatalysts for MG oxidation.

0.4 % wt % Pt/ZnO

Intensity, a.u

0.3 % wt % Pt/ZnO

0.2 % wt % Pt/ZnO

0.1% wt % Pt/ZnO

ZnO

10

20

30

40

50

2-theta

Figure 1.

60

70

80

25000

Pt 4f

71.15 eV 74.45 eV

Counts/s

20000

15000

10000

5000

0

65

70

75

Binding energy, eV

Figure 2.

80

85

A

B

C

D

E

Figure 3.

Absorbance, a.u.

ZnO 0.1 wt % Pt-ZnO 0.2 wt % Pt-ZnO 0.3 wt % Pt-ZnO 0.4 wt % Pt-ZnO

200

250

300

350

400

450

500

550

Wavelength, nm

Figure 4.

600

650

700

750

800

50

ZnO 0.1 wt % Pt/ZnO 0.2 wt % Pt/ZnO 0.3 wt % Pt/ZnO 0.4 wt % Pt/ZnO

40

Intensity, a.u.

30

20

10

0

-10 280

300

320

340

360

Wavelength, nm

Figure 5.

380

400

420

0.0

-0.5

Ln(C/Co)

-1.0

ZnO 0.1 wt % Pt/ZnO 0.2 wt % Pt/ZnO 0.3 wt % Pt/ZnO 0.4 wt % Pt/ZnO

-1.5

-2.0

-2.5 0

5

10

15

20

25

30

Reaction time, min

Figure 6.

35

40

45

0.0

Ln(C/Co)

-0.5

-1.0

0.4 g/l 0.6 g/l 0.8 g/l 1.0 g/l 1.2 g/l

-1.5

-2.0

0

5

10

15

Reaction time, min

Figure 7.

20

0.0

Ln(C/Co)

-0.5

-1.0

25 ppm 50 ppm 75 ppm 100 ppm 150 ppm 200 ppm

-1.5

-2.0

0

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17

Reaction time, min

Figure 8.

Figure 9.

Photocatalytic oxidation of malachite green dye, % 100

80

60

40

20

0 1 2 3

Number of cycle 4 5