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Enhancement of photocatalytic degradation of methyl orange by supported zinc oxide nanorods/zinc stannate (ZnO/ZTO) on porous substrates Supamas Danwittayakul, Mayuree Jaisai, Thammarat Koottatep, and Joydeep Dutta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie4019726 • Publication Date (Web): 30 Jul 2013 Downloaded from http://pubs.acs.org on September 5, 2013

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Industrial & Engineering Chemistry Research

Enhancement of photocatalytic degradation of methyl orange by supported zinc oxide nanorods/zinc stannate (ZnO/ZTO) on porous substrates Supamas Danwittayakul1, Mayuree Jaisai2, Thammarat Koottatep2 and Joydeep Dutta2,3*

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National Metal and Materials Technology Center, 114 Thailand Science Park, Klong Nueng, Klong Luang, Pathumthani 12120, THAILAND. 2 Center of Excellence in Nanotechnology, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, THAILAND. 3 Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, P.O. Box 17, 123 Al-Khoudh, Sultanate of Oman,

*Corresponding author. Tel.:+66 25245680; +968 24143266, Fax. +66 25245617; +968 24413532, E-mail address: [email protected], [email protected]

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Abstract Dye wastewater from textile industries are reported to be a major river pollutant. Zinc stannate (ZTO) was grown directly on zinc oxide (ZnO) nanorods coated polyester fiber membranes and porous ceramic substrates by a mild hydrothermal method where the nanorods supplied zinc ions for the zinc stannate growth. Photocatalytic degradation of methyl orange aqueous solution under UV light irradiation was monitored for up to 3 hours duration. Higher photocatalytic activity of ZnO/ZTO catalysts on ceramic substrates was attributed to the large surface area of the nanocomposites. 50% of methyl orange and ~ 95% methyl orange could be degraded within 1 hour and 3 hours of UV light irradiation respectively, by using the porous ceramic supported catalysts (C-ZnO/10ZTO), due to efficient charge separation. Moreover, the formation of ZTO islands on ZnO nanorods led to an enhancement in photocatalytic activity in the exposed areas of electron rich ZnO nanorods.

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1. Introduction Photocatalysis has attracted increasing attention for the treatment of dye waste from textile industries1-3. Photocatalysis is a light induced catalytic process for reducing organic molecules through redox reactions of the process includes the electron-hole pairs creation on the surface of a metal oxide semiconductor by the absorption of photons from a light source and its subsequent loss to an adsorbed molecule4. Titanium dioxide or titania (TiO2) nanoparticles have been widely studied as a potential material for application as a photocatalyst over the last two decades. However, removal of the nanoparticles after the catalysis process and low photocatalytic photo-efficiency are two major obstacles for widespread application of TiO2 nanoparticles as photocatalysts5, 6. Supported photocatalysts that does not require separate treatments for removal from treated water and that perform higher photocatalytic activity are being extensively studied in recent times for large scale photocatalysis applications7. Zinc oxide (ZnO) is a promising candidate as a photocatalytic material since it exhibits higher photocatalytic efficiencies for the degradation of organic contaminants8-10 compared to other metal oxides including TiO211-14. Catalyst surface area is an important factor to enhance photocatalytic activity of metal oxides8,

15, 16

. Larger effective surface area leads to a higher

adsorption of organic molecules leading to a better photocatalytic activity. Diverse nanostructures of ZnO such as nanorods, nanowires and nanoflowers can be prepared simply by controlling reaction conditions during hydrothermal process17. One-dimensional ZnO nanorods as compared to nanoparticles coated on flat substrates can enhance photocatalysis due to an increase in effective surface area18. Cheng et al. and Baruah et al. reported ZnO nanorods grown through a low temperature hydrothermal method showed higher photocatalytic activity than ZnO nanoparticles due to the increased specific surface area5, 19.

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Coupling metal oxides to match the electrochemical potential of semiconductors to direct the electron flow following photogenerated electron-hole pair creation is an alternative mean to enhance photocatalytic activity of pure metal oxides that not only can increase charge separation, but also suppress electron-hole pair recombination, and sometimes extend the spectral response of ZnO into the visible region20, 21. Zinc stannate, also called zinc tin oxide (ZTO), is a ternary oxide semiconductor with wide bandgap (∼3.6 eV) that has been utilized in varying applications including photocatalysis for the degradation of organic pollutants22, 23. ZTO is chemically stable and have a high electron mobility24 essential for enhanced photocatalysis. Synthesis of ZTO nanostructures have been achieved using sol-gel25, thermal evaporation25 and hydrothermal methods23, 27, 28. ZnO and ZTO (ZnO/ZTO) composite oxides have been used as photocatalysts with better activity for the degradation of organic contaminants in water as compared to pure metal oxide materials29. This can be attributed to the formation of hetero-junctions between ZnO and ZTO enhancing the transportation of photoexcited electrons in ZTO to ZnO where holes could be pinned on ZTO surfaces leading to more efficient charge separation thus resulting in an increase in the photocatalytic activity23,

30, 31

. Moreover, the synergistic effect of multi-metal

oxides plays an important role to hinder the electron-hole recombination processes32, 33. Although a lot of researchers have targeted to manipulate composite metal oxides to improve photocatalytic activity, designed charge separation of photoelectrons through the proper engineering of the nanocomposites could further enhance photocatalytic activity. In this work, pure ZnO nanorods were synthesized on polyester fiber membranes and porous cordierite ceramic substrates by a hydrothermal method. Novel structure of ZTO islands were intentionally constructed on ZnO nanorods supported on polyester fiber membranes and porous ceramic substrates by a two step hydrothermal process with varying concentrations of

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precursor solution and the photodegradation of azo dye was studied. Methyl orange is an azo dye having -N=N- which was chosen as a test contaminant throughout this study. Material characteristics and photocatalytic activity of each catalyst was determined by monitoring the reduction of specific absorption peak wavelength of aqueous methyl orange solutions and is reported in this study. 2. Experimental 2.1 Materials All the chemicals used for synthesis of the catalysts were of analytical grade and used asreceived without any further purification. Chemicals used in this work include: zinc acetate dihydrate (Zn(CH3COO)2.2H2O), ethanol (C2H5OH), isopropyl alcohol (C3H8O), sodium hydroxide and zinc nitrate hexahydrate (Zn(NO3)2.6H2O), from Merck; tin tetrachloride pentahydrate (SnCl4.5H2O) and ammonium oxalate monohydrate ((NH4)2C2O4.H2O) were from Univar and hexamethylene tetramine (hexamine: C6H12N4) and benzoquinone (C6H4O2), from Sigma-Aldrich. 2.2 Growth of ZnO nanorods ZnO nanorods were grown on polyester fiber membranes and porous ceramic substrates. Prior to seeding and growth processes, polyester fiber membranes and cordierite honeycomb substrates were prepared and cleaned with ethanol in an ultrasonic bath for 5 minutes to remove surface contaminants. Zero dimensional ZnO nanoparticles were first deposited as a seeding layer. Synthesis of ZnO nanoparticles were carried out by heating equimolar (4 mM) concentrations of zinc acetate dihydrate and sodium hydroxide mixture in ethanol at 60 °C for 3 h in an Erlenmeyer flask. Different seeding procedures of hydrophobic polyester fiber membranes and hydrophilic ceramic substrates were carried out as reported in our previous

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reports34, 35. The ZnO nanorods were epitaxially grown form seeded nanoparticles for 15 h in a chemical bath consisting of equimolar (5 mM) concentrations of Zn nitrate and hexamine kept at 95 °C. The polyester fiber membranes composed of 25 ± 5 µm diameter fibers were treated with 1% dodecane thiol solution in ethanol and then heated at 100 °C for 15 min prior to the ZnO nanoparticle seeding process. The thiolated polyester fiber membranes were dipped into a colloidal suspension of ZnO nanoparticles for 15 min. This cycle was repeated three times and the substrates were heated at 150 °C for 15 min after each dipping to ensure that the seeds were firmly developed34. Seeding of the cordierite ceramic substrates (magnesium iron aluminium cyclosilicate procured from Zhongtian Co. Ltd., China) were done by dipping the substrates into a 2 mM solution of zinc acetate in ethanol followed by heating the substrate to above 350 oC in air until the solvent was completely evaporated. This process was repeated 10 times. ZnO nanorods were then epitaxially grown from the seed layer on the polymer and ceramic substrates through hydrothermal process in a chemical bath containing equimolar concentration (5 mM) of zinc nitrate hexahydrate and hexamethylene tetramine at 95 °C for up to 15 h17. The ZnO nanorods on polyester samples (P-ZnO) were then annealed in the atmosphere at 150 °C and on ceramic substrates (C-ZnO) at 350 °C for 1 h prior to further processing. 2.3 Synthesis of ZnO/ZTO ZnO nanorod on polyester fiber membranes and cordierite substrates used in this section were obtained from a process discussed above. Synthesis of zinc tin oxide (ZTO) on ZnO nanorods to form the composites (ZnO/ZTO) was carried out in a hydrothermal reactor. To achieve ZTO island structure on ZnO nanorods, hydrolysis time for the ZTO growth process on

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the porous substrates were optimized. Aqueous solutions of tin tetrachloride pentahydrate and sodium hydroxide solutions were prepared in deionized water. Molar ratio of NaOH to SnCl4 was controlled at 10:1. ZnO/ZTO composite oxides were synthesized using ZnO nanorods as a precursor of Zn2+ ions with 5 mM and 10 mM Sn4+ solution concentrations. pH of the solutions were controlled at about 10 by titrating with sodium hydroxide solution. ZnO nanorods on polyester fiber membranes and ceramic substrates (P-ZnO and C-ZnO) were immersed in the hydrothermal reactor heated to 120 °C for 4 h, followed by annealing in air for 1 h at 150 °C for polyester fiber membranes and at 350 °C for the cordierite substrate. Details of the synthesis conditions of all the samples reported in this work are summarized in Table 1. 2.4 Materials Characterization Each sample was investigated using field emission scanning electron microscope (FESEM, JEOL-6301) working at 20 KV to record the morphology of nanorods. The length and diameter of the nanorods were determined by considering a sample of 60 nanorods (taken from 3 SEM images) on each substrate using an image analyzer (I-Solution software). Simultaneously the elemental compositions of photocatalysts were determined using energy dispersive spectroscope (EDS, Oxford Instrument; INCAx-sight). X-ray diffraction (XRD) of as-prepared ZnO nanorod samples was performed on a Philips PAnalytical X’Pert PRO X-ray diffractometer with Cu Kα radiation. Optical absorption spectra of the catalyst films were determined using UVVis spectrophotometer (SolidSpec-3700/3700DUV, Shimadzu) and specific surface area of ceramic supported catalysts were determined by gas adsorption technique (BET; Quantachrome, Autosorb-1C) first by outgassing at 300 °C for 5 h followed by nitrogen gas adsorption at 77 K. The characterized conditions of each technique are summarized as in Table 2.

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Table 1 Table 2 2.5 Photocatalysis Photocatalysis experiments were conducted under ultraviolet light irradiation using a representative of azo dye, methyl orange (C14H14N3NaO3S). A 10 mM solution of methyl orange was prepared in deionized water. 10 mL of the solution was poured into a test chamber containing a catalyst support coated with catalyst particles (dimension of ceramic supported catalyst is 1 cm x 1 cm x 1cm) were placed inside the test chamber facing UV light (264 nm, UV-c, 12 W). Optical absorption spectra were determined after varying light exposure durations using a UV/Vis spectrophotometer (Mikropack DH-2000 with USB4000 detector from Ocean Optics) in order to monitor the rate of degradation by recording the reduction in absorption intensity of methyl orange at a maximum wavelength (λmax = 460 nm). The degradation efficiency (DE) was calculated as in equation 1:

DE =

I0 − I C −C ×100 = 0 × 100 I0 C0

(1)

where, I0 is the initial absorption intensity of methyl orange solution at λmax= 460 nm and I is the absorption intensity after photo-degradation. C0 is the initial concentration of methyl orange solution and C is the concentration after photo-degradation. The stability test on the degradation of methyl orange using C-ZnO/10ZTO was conducted by repeating photodegradation (for 3 hours) of 10 mM methyl orange solution under UV light for 5 cycles. The same photocatalyst sample was rinsed with deionized water and a separate photodegradation experiment was conducted during each cycle.

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2.6 Investigation of active species Known concentration of scavengers, namely, benzoquinone, isopropyl alcohol and ammonium oxalate (O2-•, OH• and h+ scavengers respectively) were separately introduced into the MO solution prior to addition of the photocatalyst.

3. Results and Discussion 3.1 Characteristics of photocatalysts Microstructure of ZnO/ZTO on polyester fiber membranes and cordierite ceramic substrates used as photocatalysts for decolorization of methyl orange are shown in Figure 1. It can be observed that non-woven polyester fiber membranes are loosely packed with fibers of ~21 ± 3 µm diameter (Figure 1a). The microstructure of as received cordierite ceramic substrates show a porous structure (Figure 1b) with a pore size distribution of ~164 ± 15 nm and specific surface area of 15.4 ± 0.3 m2.g-1 as determined by the gas adsorption experiments.

Figure 1

Uniform and dense ZnO nanorods firmly attached to the substrates were found to grow on ZnO seeded polyester fiber membranes since ZnO seeded nanoparticles can be securely attached on the thiolated surface34. The average diameter and length of ZnO nanorods grown on polyester fiber membranes from 5 mM growth solution after 15 h of growth were found to be 152 ± 5 nm and 1.8 ± 0.4 µm, respectively (Figure 2a). Dense ZnO nanorods of 134 ± 4 nm in width and 1.6 ± 0.2 µm in length were obtained on porous ceramic substrates by synthesizing under similar conditions (i.e. with equimolar concentration of 5 mM mixture of zinc nitrate and

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hexamine) (Figure 2b). The specific surface areas of ZnO nanorod catalysts immobilized on ceramic substrate could be determined by gas adsorption technique whereas specific surface area of catalysts on polyester fiber membranes could not be measured since N2 gas could not be adsorbed on the polyester fiber membranes due to the limitations in heating the polymer substrate to drive the adsorbed moisture prior to the nitrogen adsorption. The specific surface area (S.S.A.) of ZnO nanorods grown on ceramic substrate was found to be 35.4 m2.g-1 which is more than twice the S.S.A. of the as received ceramic substrates (15.4 m2.g-1). A typical transmission electron micrograph of ZnO nanorods and nanocomposite of ZnO/ZTO collected from the ceramic substrate are shown in Figure 3-4. Diffraction pattern taken on a single ZnO nanorod in the inset of figure 3(a) demonstrate a single crystalline wurtzite structure. High resolution transmission electron micrograph of a typical ZnO nanorod is shown in Figure 3(b) with the crystal plane exposed at the surface being visible. ZTO nanocomposite with ZnO are formed by the dissolution-precipitation reaction wherein Sn4+ and OH- present in the precursor solution react with Zn2+ ions supplied by the ZnO nanorods which we have described in a previous work36 (Figure 4(a)). The lattice fringes of ZTO as shown in Figure 4(b) further show the presence of Zn2SnO4 in the nanocatalyst as observed from the XRD results (Figure 5) confirming the presence of mixed ZnO and Zn2SnO4 (ZTO) phases. Figure 2 Figure 3 Figure 4 Figure 5

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Microstructures of as-synthesized ZnO/ZTO catalysts on polyester fiber membranes were different from ZnO/ZTO nanocomposites synthesized on ceramic substrates as shown in Figure 5. On polyester fiber membranes, with low ZTO fraction (ZTO synthesized with 5 mM concentration of tin tetrachloride pentahydrate), nanoparticles of ZTO of about 50-100 nm sizes were found to be randomly dispersed on ZnO nanorods that did not uniformly cover the ZnO nanorod surfaces (Figure 6a). ZTO particles of 20-50 nm sizes were assembled as islands on the surface of ZnO nanorods when the ZTO synthesis was carried out using 10 mM tin tetrachloride pentahydrate solution (Figure 6b). ZTO synthesized on ceramic substrates were initially formed on ZnO nanorods by the reaction of tin species with dissolved zinc ions in the proximity of the nanorod surfaces leading to the formation of ZTO nuclei (inset of Figure 6c); EDS elemental analysis results in Figure 6e show the presence of zinc and tin species in the grown material. When the nanocomposites were synthesized with higher molar concentrations of tin source solution, most of the ZnO nanorod surfaces were transformed to the nanocomposite of ZnO/ZTO as shown in Figure 5d. This is due to the higher concentration of Sn4+ in the growth solution which leads to increased consumption of Zn2+ ions released from ZnO nanorods exposing the surfaces to the Sn4+ solution to further form ZTO. Although a lot of ZnO nanorods transformed to ZTO as confirmed by EDS elemental analysis carried out on multiple sites on the substrates (Figure 6f), complete reaction of the ZnO nanorod surfaces did not occur since bare ZnO nanorods could be observed under the ZTO coated layer (Figure 6d). This can be attributed to the heterogeneous growth of ZnO nanorods on porous substrate. Since zinc ions are released primarily from the polar surfaces, the nanorods with (311) face perpendicular to the substrates are most accessible to contribute to the ZTO growth37, 38. Moreover, hydrothermal growth period was also optimized to obtain partially ZTO coated ZnO nanorods. Specific surface areas of C-

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ZnO/5ZTO and C-ZnO/10ZTO nanocomposites examined by gas adsorption technique were found to be 36.7 m2.g-1 and 35.5 m2.g-1 which are comparable to the S.S.A. observed for the ceramic substrates coated with ZnO nanorods (35.4 m2.g-1).

Figure 6

Figure 7 shows the optical absorption spectra of the as-prepared samples on microscope glass substrates. As can be observed, ZnO only has a very weak absorption in the UV edge of the spectra while extended UV absorption can be observed in the composites of ZnO/5ZTO and ZnO/10ZTO. With increasing ZTO content, the optical absorption spectra were found to red shift. In general, the red-shift in the absorption band edge and the increase in absorption intensity are attributed to the increased formation rate of electron-hole pairs on the photocatalyst surface, resulting in higher photocatalytic activity39.

Figure 7

The effect of adsorption of methyl orange on the porous substrate, with ZnO nanorods,(C-ZnO), ZnO/10ZTO and (C-ZnO/10ZTO) on the substrates were determined by measuring the change of MO concentration after dipping the samples into a 10 mM MO solution in dark for a 30 min period. We observe that the adsorption of the nanocomposite photocatalyst exhibits highest MO adsorption with almost 7.81% absorption, while C-ZnO and bare substrate show slightly lower adsorption (~6.45% and 5.80% respectively) which is related to the increase in effective surface area of the substrates (Table 3).

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Table 3

3.2 Photocatalytic activity Photocatalytic activity for the degradation of 10 mM of methyl orange aqueous solution upon using pure ZnO nanorods and ZnO/ZTO nanocomposite oxides on both polyester fiber membranes and ceramic substrates synthesized at different conditions were determined with UV light irradiation for up to 3 h duration compared to the control experiment without catalyst. Photocatalytic activities of pure ZnO nanorods on both polyester fiber membranes and ceramic substrates (P-ZnO and C-ZnO) were lower than the composite oxides of ZnO and ZTO (Figure 8). Pure ZnO nanorods on polyester fiber membranes (P-ZnO) exhibited only 32% degradation efficiency after UV irradiation for 3 h (control experiment exhibited 30% degradation efficiency) while 77% degradation efficiency could be obtained upon using ZnO nanorods coated porous ceramic substrates. Almost 58% enhancement in the degradation efficiency of ZnO/ZTO on porous ceramic substrates can be attributed to the enhancement of effective surface area that allow higher contaminant molecules or dye molecules to adsorb for the redox reaction to take place, as reported above5,

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. P-ZnO/5ZTO and P-ZnO/10ZTO nanocomposite oxides on

polyester fiber membranes showed degradation efficiencies of 42% and 49%, respectively (Figure 8) while 81% and 95% degradation efficiencies (10 mM methyl orange) were obtained upon using C-ZnO/5ZTO and C-ZnO/10ZTO nanocatalysts on ceramic substrates attributed to the higher specific surface areas. In addition, degradation efficiency was found to increase by 19% in samples with zinc stannate islands on ZnO nanorods supported on ceramic substrates (CZnO/10ZTO). Nanocomposite of ZnO and ZTO exhibited better photocatalytic activity than pure

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ZnO due to improved charge-separation resulting from the reduction of electron-hole pair recombination on the zinc stannate surfaces32, 33. It was reported that ZnO nanoparticles prepared by thermal decomposition could degrade 0.01 mM methyl orange with 98% efficiency 34, TiO2 coated on activated carbon was found to have maximum 89% degradation efficiency of 1 mM methyl orange35 and mesoporous SrTiO3 prepared by sol-gel method exhibited about 95% degradation efficiency36. Stability of photocatalytic activity on the degradation of 10 mM methyl orange under UV light by using C-ZnO/10ZTO catalyst was studied as shown in Figure 9 (in the revised manuscript). It was observed that the degradation efficiency slightly decreases after 1 h and 2 h irradiation; however, after 3 h of UV irradiation periods, photodegradation efficiency was stable for up to 5 cycles. Figure 8 Figure 9

In the photocatalytic oxidation process, photoinduced active species including h+, OH• and O2-• oxidizes organic molecules following the separation of electron-hole pairs are created by photoexcitation42-44. To determine the role of these active species, three scavengers were introduced prior to the photodegradation test. Separate experiments of MO photodegradation activities were conducted by adding 0.1 mM benzoquinone (BQ), 1 mM isopropyl alcohol (IPA) and 1 mM ammonium oxalate (AO) as the scavengers for O2-•, OH• and h+, respectively45-46. Figure10 shows the MO degradation efficiency with different scavengers. After adding AO, BQ or IPA, the MO degradation efficiency tends to decrease due to the inhibition of the

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scavengers. BQ was found to be the most effective scavenger reducing the photodegradation drastically, indicating that O2-• plays a major role in MO degradation in the composite catalysts47.

Figure 10

The energy band diagram of the ZnO/ZTO showing the possible electron-hole pair separation is given in Figure 11. The presence of the higher bandgap ZTO (3.6 eV) bound to ZnO (3.37 eV) allows the photoexcited electrons to transfer from the conduction band (CB) of ZTO to the CB of ZnO as ZTO is a good electron transport material24, while the hydroxyl groups remain on the ZTO surface, resulting in the reduction of charge recombination30. An increase of photocatalytic activity can be attributed to the morphology of ZTO islands on ZnO nanorods (Figure 6). Electrons transported from the zinc stannate crystals to the uncoated ZnO nanorods would leak to the environment (dye solution) resulting in reduction of the dye. This would enhance the electron density on the uncoated ZnO nanorod surfaces thus leading to more efficient photoreduction reactions.

Figure 11

4. Conclusions One-dimensional ZnO nanorods combined with highly porous ceramic substrate with high specific surface area can enhance photocatalytic activity. Comparative results of photocatalytic degradation studies on methyl orange with UV light demonstrated that porous ceramic supports were 58% more active than polyester fiber membranes. Photocatalytic activity could be further

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augmented by another 19% by coupling ZTO islands to the ZnO nanorods. This enhancement in the photocatalytic activity was attributed to an increase of charge separation due to the coupling of two oxides of different bandgaps leading to a reduction in electron-hole pair recombination and efficient transfer of electrons through the exposed ZnO nanorod surfaces. ZnO/ZTO supported nanocatalysts can be attractive candidates for photocatalysis.

Acknowledgment The authors would like to acknowledge financial support from the National Research Council of Thailand (NRCT), Center of Excellence in Nanotechnology at the Asian Institute of Technology, National Metal and Materials Technology Center (MTEC) and National Nanotechnology Center (NANOTEC) belonging to the National Science & Technology Development Agency (NSTDA), Thailand.

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List of Figures Figure 1 Scanning Electron micrograph (SEM) of catalyst support substrates used in this work; (a) non-woven polyester fiber membranes and (b) porous cordierite ceramic. Figure 2 SEM images of ZnO nanorods coated substrates synthesized via hydrothermal process using equimolar (5 mM) aqueous solution of Zn(NO3)2.6H2O and hexamine at 95 °C for 15 h; (a) grown on polyester fiber membranes and (b) grown on porous ceramic substrate (inset images show high magnification of ZnO nanorods) Figure 3 (a) TEM image of ZnO nanorods obtained hydrothermally at 95°C using 5 mM of zinc nitrate and 5 mM HMT; inset in (a) show the electron diffraction pattern of the as obtained ZnO nanorod and (b) HRTEM image of one ZnO nanorod showing the crystal plane exposed at the surface. Figure 4 (a) TEM image of ZnO/10ZTO nanocomposite obtained hydrothermally at 120°C using 10 mM of tin tetrachloride at pH 10 for 4 hr and (b) HRTEM of Zn2SnO4 formed on ZnO nanorod. Figure 5 X-ray diffraction pattern of ZnO/10ZTO nanocomposite on microscope glass slide showing mixed ZnO zincite and zinc stannate (Zn2SnO4) phases (XRD pattern was referred according to JCPDS card No. 65-3411 and No. 24-1470, respectively). Figure 6 Scanning electron micrographs of microcubes of zinc stannate synthesized on ZnO nanorods coated substrates through hydrothermal process at 120 °C for 4 h with different SnCl4.5H2O solution concentrations (at pH 10); (a) on polyester fiber membranes with 5 mM SnCl4.5H2O, (b) on polyester fiber membranes with 10 mM SnCl4.5H2O, (c) on porous ceramic with 5 mM SnCl4.5H2O and (d) on porous ceramic with 10 mM SnCl4.5H2O (e) Energy dispersive spectrum of selected area from Figure 5c and (f) Energy dispersive spectrum of 17



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selected area from Figure 5d. SEM condition; Filament: LaB6, Accelerating voltage: 20 kV, Aperture: 2, Working distance: 12 mm, Spot Size: 6 mm, analyzing time: 30 sec. Figure 7 Optical absorption spectra of photocatalysts coated on microscope glass substrates operated using absorption mode, medium scan rate, 200-800 nm san range. Figure 8 Degradation efficiency of methyl orange as a function of exposure to UV light on the ZnO nanorods and ZnO/ZTO on different catalyst supports: a) on polyester fiber, b) on porous ceramic, c) Comparison of photocatalysts efficiency on the degradation of methyl orange solution. Figure 9 Repeated Photocatalysis tests with 10 mM methyl orange under UV irradiation for 1 h, 2 h and 3 h. Figure 10 Effect of different scavengers on the degradation of methyl orange in the presence of photocatalyst (C-ZnO/10ZTO). Figure 11 Possible energy band diagram of ZnO/ZTO nanoconposite showing the photoexcited electrons transfer from higher energy level CB of ZTO to lower CB of ZnO.

List of Tables

Table 1 Codes and details of the synthesized photocatalysts Table 2 Characterization conditions Table 3 The adsorption of MO on porous ceramic substrate and photocatalysts on substrates

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Figure1 337x112mm (300 x 300 DPI)



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Figure2 336x112mm (300 x 300 DPI)


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Figure3 244x134mm (300 x 300 DPI)



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Figure4 236x128mm (300 x 300 DPI)


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Figure5 300x129mm (300 x 300 DPI)



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Figure6 133x114mm (300 x 300 DPI)


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Figure7 163x125mm (300 x 300 DPI)



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Figure8. Degradation efficiency of methyl orange as a function of exposure to UV light on the ZnO nanorods and ZnO/ZTO on different catalyst supports: a) on polyester fiber, b) on porous ceramic, c) Comparison of photocatalysts efficiency on the degradation of methyl orange solution. 127x77mm (300 x 300 DPI)


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Figure9 119x77mm (300 x 300 DPI)



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Figure10 182x127mm (300 x 300 DPI)


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Figure11 127x65mm (300 x 300 DPI)


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