Photodegradation of phenol by zinc oxide, titania and zinc oxide–titania composites: Nanoparticle synthesis, characterization and comparative photocatalytic efficiencies

Materials Science in Semiconductor Processing 26 (2014) 603–613

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Photodegradation of phenol by zinc oxide, titania and zinc oxide–titania composites: Nanoparticle synthesis, characterization and comparative photocatalytic efficiencies I. Prabha n, S. Lathasree Department of Chemistry, Sathyabama University, Jeppiar Nagar, Rajiv Gandhi Salai, Chennai 600119, India

a r t i c l e i n f o

Keywords: Nanoparticles ZnO–TiO2nanocomposite particles Photocatalytic degradation Phenol

abstract Nano-ZnO, TiO2 and ZnO–TiO2 composite particles were synthesized by a sol–gel method and were used as catalysts in the photocatalytic degradation. The nanocatalysts were characterized by XRD, SEM, EDX and BET methods. The experimental results showed that the nanoZnO particles are large agglomerates having a hexagonal wurtzite structure of particle size 17.575 nm, nano-TiO2 has uniform particles size of 1575 nm with spherical morphology. The nano-ZnO–TiO2 composite particles are of large agglomerates being embedded inside with particle size of 11.675 nm. The photocatalytic activity of nano-ZnO, TiO2 and ZnO–TiO2 composite particles was evaluated using phenol as a model compound. The effect of initial pH, initial catalyst loading and concentration of phenol under UV light irradiation and direct sun light was studied. The photodegradation of phenol was found to follow pseudo-first order kinetics and the experimental results proved that the nano-ZnO, TiO2 and ZnO–TiO2 composite particles are efficient catalysts for phenol degradation in the presence of UV light. Total organic carbon analysis indicated complete mineralization of phenol in the presence of nano-ZnO, TiO2 and ZnO–TiO2 surface. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction Hazardous organic compounds generated from industries involved in plating, machining, cosmetic production, food processing, textile processing, and also from paint, pesticide, coal conversion, polymeric resin, petroleum and petrochemical industries are discharged into wastewaters. They are the major cause of environmental pollution. Among the pollutants, phenols are generally considered to be one of the most important toxic organic pollutants which when released into the environment causes unpleasant taste and odor to ground and surface waters. The waters polluted with phenol and its derivatives are difficult to treat and are well known for their bio recalcitrant and acute toxicity [1]. Phenol is a toxic and

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Corresponding author. Mobile number: þ 91 90 9403 2390. E-mail address: [email protected] (I. Prabha).

http://dx.doi.org/10.1016/j.mssp.2014.05.031 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

mutagenic substance at high concentrations and may be absorbed through skin [2]. Hence there is a need to develop effective treatment methods to eliminate organic contaminants from wastewaters by conventional chemical treatment systems. Traditional wastewater treatment methods are partly effective, non-destructive, inefficient, costly or just transfers pollutants from water to another phase as secondary pollutant [3]. In pursuit of a better method for the treatment of wastewater, heterogeneous photocatalysis stands superior [4]. The important advantage of this degradation method is that it can be carried out under ambient conditions and results in complete mineralization of the organic contaminants. Heterogeneous catalysis using ZnO, TiO2, WO3, SnO2, ZrO2, CeO2, CdS and ZnS as catalysts in the presence of UV or solar light is used to achieve mineralization of toxic pollutants present in wastewater [5]. Among them, nano-ZnO and TiO2 have been dominating among the photocatalysts [6]. One of

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the major applications of heterogeneous catalysis is photocatalytic oxidation (PCO) or to affect total mineralization of gas phase or liquid phase contaminants into benign substances  3 like CO2, H2O, NO3 , PO4 and halide ions [7]. Nanostructured ZnO and TiO2 have been prepared and tested as effective catalysts in the degradation of organic targets activated by light and have higher photocatalytic efficiency than their commercial counterpart. ZnO, one of the most important multifunctional semiconductor materials, is attractive for its unique performance in electronics, optics and photonics because of its wide direct band gap of 3.37 eV and high excitation binding energy of 60 meV [8]. It is an exceptionally important material for applications in photo-catalysis, antibacterial materials, sensors and dye sensitized solar cells due to its excellent optical, electrical, mechanical and chemical properties [9]. Nano-TiO2 photocatalyst has attracted interest in recent years for its highly active photocatalytic functions, such as the ability to decompose chemical compounds and for its super hydrophilic and antibacterial properties. Nano-TiO2 is most popular as a heterogeneous photocatalyst among the semiconductors due to its excellent optical and electronic properties, low cost, non-toxicity, chemical and thermal stability [10]. Nano-ZnO is sometimes preferred over TiO2 for the degradation of organic pollutants due to its high quantum efficiency [11]. However, it photocorrodes in acidic aqueous suspensions [12]. Under UV light irradiation, both TiO2 and ZnO nanoparticles are highly efficient photocatalysts since their photogenerated electrons and holes are highly oxidizing and reducing agents respectively. In addition, coupled semiconductor photocatalysts provide an interesting way to increase the efficiency of a photocatalytic process by increasing charge separation and extending the energy range of photoexcitation for the system. ZnO–TiO2 nanocomposite particles have been used as a promising photocatalyst in the degradation of organics [13]. In the countries where sufficient amount of sunlight is available, photocatalysis involving sunlight will be economical and preferable. Therefore, there is a need to develop effective photocatalysts which can be used for the photocatalytic degradation of organic pollutants in the presence of sunlight. The present study involves the synthesis of ZnO, TiO2 and ZnO–TiO2 composite nanoparticles by sol–gel method and their characterization by XRD, SEM, EDX and BET methods. The photocatalytic activity of nano-ZnO, TiO2 and ZnO–TiO2 composite particles is investigated using phenol as a model compound. Photocatalytic degradation of phenol was carried out by irradiating UV light and in the presence of sun light. The effect of initial pH, initial catalyst loading and concentration of phenol in the presence of UV light and sunlight on nano-ZnO and TiO2 surfaces was investigated. Kinetic studies of photocatalytic degradation of phenol by nano-ZnO, TiO2 and ZnO–TiO2 composite particles were also attempted. 2. Experimental and analytical 2.1. Materials and methods Zinc acetate dihydrate, oxalic acid dihydrate, ethanol, titanium (IV) isopropoxide, glacial acetic acid, diethanolamine and phenol were of analytical grade purchased from

Merck, SRL, Qualigens. Double distilled water was used for all the measurements. The estimation of phenol was carried out spectrophotometrically using a 4-aminoantipyrene method [14]. 2.2. Synthesis of ZnO nanoparticles ZnO nanoparticles were synthesized by a sol–gel method. In this procedure, 5.49 g of zinc acetate dihydrate was mixed with 150 ml of ethanol in a rotary evaporator at 60 1C and fixed the rotation at 40 rpm. The zinc salt was completely dissolved in 10–15 min. Simultaneously, 6.3 g of oxalic acid dihydrate was dissolved in 100 ml of ethanol by stirring for 10 min at 50 1C using a magnetic stirrer. To the oxalic acid solution, the warm ethanolic solution of zinc acetate was added dropwise with continuous stirring. Stirring was continued for another 45 min after complete addition of zinc acetate solution. A thick white gel of zinc oxide was obtained which was dried in a vacuum oven at 80 1C for 20 h to get xerogel. The xerogel was then calcined at 500 1C in a tubular furnace at a ramp rate of 3 1C/min for 5 h to yield ZnO nanoparticles. 2.3. Synthesis of TiO2 nanoparticles In the sol–gel method of synthesis of TiO2 titanium (IV) isopropoxide, glacial acetic acid and double distilled water were used in a molar ratio of 1:10:350. 18.6 ml of titanium (IV) isopropoxide was hydrolyzed by slowly adding 35.8 ml of glacial acetic acid with constant and vigorous stirring at 0 1C. To this solution, 395 ml of double distilled water was added dropwise under vigorous stirring for 1 h and then continued stirring for 3 h. The prepared titania solution was ultrasonicated for 30 min until a clear solution was formed. The solution was kept undisturbed for 24 h in dark for a nucleation process. It was then gelated in an oven at 70 1C for 12 h to get the xerogel of titania. The titania gel was kept in an oven at 100 1C for its complete dryness. The resulting TiO2 sample was crushed into very fine powder and was then calcined in a muffle furnace at 500 1C for 5 h to get TiO2 nanoparticles. 2.4. Synthesis of nano-ZnO–TiO2 composite particles The synthesis of nano-ZnO–TiO2 composite was also carried out by the sol–gel method. The titanium (IV) isopropoxide was used as a precursor material to synthesize transparent TiO2 sol. 4.32 ml of titanium (IV) isopropoxide was dissolved in 20 ml of ethanol and stirred for half an hour using a magnetic stirrer with medium rotation to get a titania precursor solution. A mixture of 0.26 ml of distilled water, 3.4 ml of glacial acetic acid and 5 ml of ethanol was then added dropwise into the precursor solution at a speed of one drop per second under continuous stirring. The solution was then continuously stirred for  1 h to achieve a yellow transparent sol. The sol was then aged for a period of time. Here, glacial acetic acid was used as an inhibitor to slow down the titanium (IV) isopropoxide hydrolysis. In this case, the pH of the system was determined to be  5 and hence the obtained sol is referred to as an acidic sol. On the other hand, another inhibitor of diethanolamine was used

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and the pH value of the system was determined to be  10. The prepared TiO2 sol in this case is referred to as an alkaline sol. The preparation of ZnO–TiO2 sol could be achieved via direct mixing of the TiO2 sol and the ZnO sol with vigorous stirring. The ZnO sol was prepared as follows: 3.29 g of zinc acetate dihydrate was used as a precursor material to prepare ZnO sol. It was first dissolved in 50 ml of ethanol and was stirred for 5 min at 70 1C to get a precursor solution. To this mixture, 0.26 ml of distilled water, 1.58 ml of diethanolamine and 5 ml of ethanol was then added dropwise into the precursor solution at a rate of one drop per second with vigorous stirring on a magnetic stirrer. The solution was then continuously stirred for 2 h to get a transparent alkaline ZnO sol. The prepared ZnO sol was then directly added to the TiO2 alkaline sol to get ZnO–TiO2 sol. It was gelated in an oven at 70 1C for 12 h and was heated at 100 1C to complete dryness which was then finally crushed into fine powder. The powder was calcined at 500 1C for 5 h in a muffle furnace. After heat treatment, the specimen was cooled to room temperature to get nano-ZnO–TiO2 composite particles. 2.5. Characterization of nanophotocatalysts The crystallite size of nano-ZnO, TiO2 and ZnO–TiO2 composite particles was determined by XRD analysis. Sample for powder X-ray Diffraction (XRD) was prepared by making a thin film of the powder with ethanol on a glass plate and the measurement was performed with a Rigaku Geigerflex X-ray diffractometer with Ni-filtered CuKα radiation (λ¼1.5418 Å, 30 kV, 15 mA). The XRD patterns were recorded in the range of 20–801 with a scan speed of 21/min and the crystallite sizes were calculated using the Scherrer equation. The morphology and size measurement of the nanophotocatalysts were determined by Field-Emission Scanning Electron Microscopy FESEMSUPRA55, CARL ZEISS, Germany and Optical Microscopy (Inverted Metallurgical Microscope, Gx71 Olympus, Japan) coupled with Dispersive Spectroscopy. EDX (Energy Dispersive X-ray) is used for the determination of elemental composition of the specimen. The system works as an integrated feature of the Scanning Electron Microscope (SEM). The surface area (BET method), pore volume (BJH method) of the samples were determined by nitrogen adsorption–desorption isotherms using a Micrometrics ASAP 2020 automated system operated in liquid nitrogen temperature after degassing the samples for 2 h at 200 1C. A Cary-50 ultraviolet spectrophotometer (Varian) was used in the absorbance measurements of phenol in aqueous solution at different concentrations at an incident wavelength, λmax, of 500 nm.

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and the operating conditions for the photocatalytic degradation have been optimized by preliminary trial experiments with respect to (i) the total batch volume of the reactant solution, (ii) the stirring speed and (iii) the time for adsorption equilibrium prior to the exposure to UV light. The typical experimental procedure consisted of aerating a mixture of 250 ml of phenol solution of known concentration and the photocatalyst for 30 min to allow preadsorption followed by irradiation with the UV light. The lamp emits 8 W of UV radiation with a peak wavelength of 254 nm. A sample of 3 ml was withdrawn at periodic intervals of irradiation time followed by centrifugation. The residual concentration of phenol solution was measured at 500 nm using UV–vis spectrophotometer. After the analysis, the sample solutions were returned to the reactor. In the presence of UV light, all the studies have been carried out at 30 1C and the pH of the solution was adjusted to the desired values between 4.0 and 10.0 by using dilute solutions of HCl or NaOH. The radiation intensity of the UV light used in the experimental study, determined by ferri oxalate actinometry, was found to be 3.32  1019 quanta/s. The stability of the nanocatalysts used in the experimental studies was assessed by subjecting the irradiated solutions to atomic absorption spectroscopic (AAS Perkin-Elmer 23800) analysis and the amount of zinc or titanium in dissolution or photo-corrosion was determined. The reusability of the catalyst was evaluated by reclaiming the catalyst after the reaction in the batch mode, washing, drying in air at 110 1C and using it for phenol degradation under similar experimental conditions. For the photodegradation studies in the presence of sunlight, the solutions were illuminated in an open rectangular tray of 16  5  5 cm3 made from borosilicate glass. The slurry was mixed with definite weight of nano-ZnO or TiO2 or ZnO–TiO2 composite particles and stirred for 30 min in dark prior to illumination in order to achieve maximum adsorption of phenol onto the semiconductor surface.

2.6. Photocatalytic reactor in UV light irradiation and sun light The photodegradation study of phenol solution was carried out in a suspended particle vertical catalytic reactor in a batch mode as shown in Fig. 1. Here, nano-ZnO or TiO2 or ZnO–TiO2 particles were suspended in the solution column through aeration. Thereafter, the configuration

Fig. 1. Batch reactor system for photodegradation study of phenol.

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Irradiation was carried out in open air with continuous aeration by a pump to provide oxygen and for the complete mixing of the reaction solution. During the illumination time no volatility of the solvent was observed. The temperature was monitored and the evaporation losses were calculated. The photodegradation studies were carried out in direct sunlight under optimum conditions. Solar light intensity was measured for every 30 min and the average light intensity over the duration of each experiment was calculated. The intensity of the sunlight during the reaction time was in the range of 808–1070 W/m2. The intensity was nearly constant during the experiments. The estimation of TOC was carried out by subjecting the solutions of phenol to photocatalytic degradation in a closed reactor to convert organic carbon to carbon dioxide.

(101) 160

Intensity (a.u.)

606

120

(102) (100)

(103)

80

40

0

10

20

30

40

50

60

70

80

2 theta (degree) Fig. 2. XRD pattern of nano-ZnO–TiO2 composite particles.

3. Results and discussions 3.1. XRD analysis The X-ray diffraction (XRD) peaks obtained for nanoZnO were in good agreement with the wurtzite structure for nano-ZnO and correspond to (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes. The study indicated high purity and crystallinity of ZnO nanoparticles. The average crystallite size of ZnO nanoparticles obtained with the calcination temperature at 500 1C was estimated to be 26.3 nm. This is confirmed by Scanning Electron Microscopy. The XRD pattern for the calcined nano-TiO2 corresponds to the anatase phase which is confirmed by (101), (004), (200) (105) and (211) diffraction peaks. The average crystallite size estimated from the Scherrer equation was found to be around 13.2 nm. During the preparation, the large amount of water used enhances the nucleophilic attack of water on titanium (IV) isopropoxide and suppresses fast condensation of titanium (IV) isopropoxide species yielding TiO2 nanoparticles and the crystallinity of the nanoparticles reduces the less dense anatase phase [15]. The XRD measurements identified the phase composition of ZnO–TiO2 nanocomposite particles as shown in Fig. 2. The XRD pattern for the calcined (500 1C) nanoZnO–TiO2 composite particles corresponds to the anatase phase for the nano-TiO2 and zincite phase for the nanoZnO and it was confirmed by the appearance of peaks at 2θ: 31.7, 36.2, 56.5, 62.9 corresponding to the diffraction peaks of (100), (101), (103) and (112) [16]. It can be seen that both the phase composition and the crystallinity of the composite powder altered obviously with the calcinating temperature. After 2 h of calcination at 500 1C in air, the untreated ZnO–TiO2 composite powder was rather amorphous. With the increase in calcination time from 2 to 5 h, the composite powder crystallized well and the resultant diffraction peaks were quite sharp. From the above investigation it is concluded that the synthesis of nano-ZnO–TiO2 composite particles by direct incorporation of TiO2 sol into ZnO sol forms ZnO–TiO2 composite sol and it is a typically homogeneous distribution of TiO2 and ZnO particles similar to the earlier report [17]. The crystallite sizes of nano-ZnO–TiO2 composite particles are 4.9 nm,

Fig. 3. SEM pattern of nano-ZnO particles.

5 nm and 4.4 nm from the 3 different diffraction peaks and the average crystal size was calculated to be 4.7 nm. 3.2. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) analysis The morphology and the structural properties of the nanoZnO, TiO2 and nano-ZnO–TiO2 composite particles were investigated by Field-Emission Scanning Electron Microscopy (FESEM). Fig. 3 shows the SEM micrograph of nano-ZnO consisting of an average particle size of 17.575 nm with homogeneous and hexagonal shapes with large agglomeration. It is also clear from EDX analysis that the synthesized ZnO nanoparticles composed of only zinc and oxygen with the corresponding elemental percentage composition. Table 1 shows the elemental composition, atomic percentage and weight percentage of nano-ZnO, TiO2 and ZnO–TiO2 composite particles. Fig. 4 shows the SEM micrograph of TiO2 nanoparticles having spherical shape and size uniformity. The particle size of nano-TiO2 is found to be 15 75 nm. From the EDX analysis, nano-TiO2 was found to have the elemental percentage composition of oxygen as 41.66% and Ti as 58.34%. The SEM observation of the nano-ZnO–TiO2 composite particles is shown in Fig. 5. It shows that all Zn and Ti

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Table 1 EDX composition analysis value of nano-ZnO, TiO2 and ZnO–TiO2 composite particles Nanocatalyst

ZnO TiO2 ZnO–TiO2

Composition atom %

Composition weight %

Zn

Ti

O

Zn

Ti

O

41 0 19.59

0 58.34 15.07

59 41.66 65.34

73.95 0 42.01

0 80.74 23.69

26.05 19.26 34.3

Fig. 5. SEM pattern of nano-ZnO–TiO2 composite particles.

volume of the nano-ZnO, TiO2 and ZnO–TiO2 composite particles. The surface area and pore volume of the nanoZnO, TiO2 and ZnO–TiO2 composite particles are calculated from the BET measurements. The order of surface area of the three different nanophotocatalysts is nano-TiO2 4 nano-ZnO–TiO2 composite4nano-ZnO. 3.4. Photodegradability of nanophotocatalysts Fig. 4. SEM pattern of nano-TiO2 particles.

particles are uniformily distributed in the composite material and are composed of large agglomerates being embedded inside and the average particle size is calculated to be 11.675 nm. In addition, it has been reported that the large surface area shows improved photocatalytic efficiency of the nanocatalyst [18]. Accordingly, the ZnO– TiO2 nanocomposite particles, calcined at 500 1C, should have higher photocatalytic efficiency. The EDX analysis indicates that both Ti and Zn are present in the composite particles. 3.3. Surface area and pore volume measurements The surface area of any material is the most important factor influencing its catalytic activity. The surface area (BET method) and pore volume (BJH method) of the samples were determined by nitrogen adsorption–desorption isotherms. The surface area of the nano-ZnO was found to be 21.47 m2/g and the pore volume of nano-ZnO was found to be 0.1194 cm3/g. The pores present in ZnO nanoparticle may allow rapid diffusion of the phenol molecules during photocatalytic reaction and enhance the adsorption of phenol solution and its intermediates on the catalyst surface. The synthesized TiO2 nanoparticles consist of approximately uniform spheres. The BET surface area of the prepared TiO2 is found to be 78.38 m2/g which is higher than that of the TiO2–P25 (50 m2/g) and the pore volume is 0.2472 cm3/g. The surface area of the nano-ZnO– TiO2 was found to be 45.88 m2/g and the pore volume of nano-ZnO was found to be 0.0823 cm3/g. Table 2 shows the comparison of the calcination temperature, average crystallite size, average particle size, surface area and pore

3.4.1. Effect of wavelength of incident light on the photocatalytic degradation of phenol The photocatalytic degradation of phenol was investigated by exposing phenol solution to a light of incident wavelength of 375 and 254 nm and in the presence of ZnO, TiO2 and ZnO–TiO2 composite particles. The photocatalytic degradation of phenol in the presence of nanocatalyst involves light absorption of wavelength (380 nm) which is higher than the band gap energy of 3.2 eV. A sample of 50 ppm of phenol solution upon irradiation with 254 nm light was found to undergo complete mineralization within 90 min. Under similar conditions, with 375 nm light, only 40% of the degradation has been observed. The improved effectiveness of 254 nm light compared to 375 nm light may be due to the fact that the shorter wavelength light is absorbed more strongly by the catalyst particles than the longer wavelength light which is true for any thin film particles. Hence the penetration distance of photons into the particle is shorter with 254 nm light and the photoelectrons and holes are formed closer to the surface of the particle. Therefore they take less time to migrate to the surface of the particle and hence they have less time to participate in energy wasting recombination reactions before useful surface reactions taking place. Many organic molecules are excited by the 254 nm light and get degraded as a result of direct action of light. But the direct degradation rate is very small compared to the rate of degradation in the presence of a photocatalyst. Therefore, photocatalytic degradation studies were carried out using an incident light of wavelength 254 nm. The photodegradability of phenol was investigated by exposing phenol solution to UV light in the absence and in the presence of all three catalysts such as nano-ZnO, TiO2 and ZnO–TiO2 composite particles. In the absence of the

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Table 2 Comparison of average crystallite size, particle size, surface area and pore volume of nano-ZnO, TiO2 and ZnO–TiO2 composite particles. Catalyst

Calcination temperature (ºC)

Average crystallite size (nm)

Average particle size (nm)

Surface area (m2/g)

Pore volume (cm3/g)

ZnO TiO2 ZnO/TiO2 Composite

500 500 500

26.3 13.2 4.7

17.5 7 5 157 5 11.6 7 5

21.47 78.38 45.88

0.1194 0.2472 0.0823

Fig. 6. Photodegradability of phenol. Phenol ¼ 50 ppm; catalyst ¼nanoZnO and TiO2 catalyst (1.0 g/l); absorbance measured at 500 nm; incident wavelength of UV light ¼ 254 nm; temperature¼ 30.0 7 0.1 1C.

Fig. 7. Effect of pH on the initial rate of photocatalytic degradation of phenol. Phenol¼ 50 ppm; catalyst¼nano-ZnO (1.0 g/l); incident wavelength¼ 254 nm; absorbance measured at 500 nm; temperature¼ 30.070.1 1C.

catalyst, phenol was found to be stable upon irradiation with UV light and photodegradation was negligible. In the absence of the UV light and in the presence of nanophotocatalyst, about 10% of phenol was adsorbed on the nanophotocatalyst surface. But in the presence of both UV light and nanophotocatalyst phenol was completely mineralized into carbon dioxide and water. The estimation of TOC was carried out by subjecting phenol to photocatalytic degradation in a closed reactor for the conversion of organic carbon to carbon dioxide [19]. Carbon dioxide formed was determined experimentally and was found to tally with the theoretical value on the basis of molecular weight and the concentrations employed. The percentage of carbon photo-oxidised in aqueous suspension is 98% of the theoretical value.

electrons and holes resulting in improved photocatalytic efficiency [20].

3.4.2. Photodegradability of phenol The photocatalytic activity of the prepared nanocatalysts ZnO and TiO2 were evaluated by the degradation of phenol in aqueous solution. In addition to experiments with the photocatalysts and sun light, blank experiments were also done with the photocatalysts in the absence of sun light and without using photocatalysts in the presence of sun light. Results of the blank experiments showed that the phenol could not be degraded without the photocatalysts or the sun light as shown in Fig. 6. In the presence of sunlight and with nano-ZnO and nano-TiO2, the aqueous phenol was easily degraded within 60 to 70 min. The photo-degradation mechanism of phenol using ZnO could be similar to that of TiO2, which has already been reported that when the photocatalyst is irradiated with ultraviolet (UV) radiation or direct sun light, it produces electron– holes pairs which diffuse to the surface of the catalyst. It can recombine within a time scale of nanoseconds to radiate heat leading to vectorial transfer of the generated

3.5. Photocatalytic degradation study of nano-ZnO and TiO2 3.5.1. Effect of pH The pH of the solution is one of the most important parameter involved in the degradation of aqueous solution of phenol. The effect of pH on the photodegradation of phenol in the presence of nano-ZnO and TiO2 particles was investigated over a pH range of 3.0–10.0. The influence of initial pH generally depends on the type of compound that has to be degraded and the zero point charge (zpc) of the photocatalyst used in the oxidation process [21]. The pH of the solution influences the surface charge properties of the photocatalyst and increases the electrostatic interaction between the nanocatalyst surface and the organic pollutant molecules. Fig. 7 illustrates the effect of pH on the initial rate of photocatalytic degradation of phenol in the presence of the nanocatalyst ZnO. In the presence of nano-ZnO, the initial percentage degradation of phenol was found to decrease with the increase in pH. Under mild acidic and neutral pHs, the percentage degradation was found to be almost the same (15–18%). At higher pH values, phenol exists as negatively charged phenolate species. Therefore, the adsorption of phenol on the catalyst surface decreases. However, at pH 10.0, a slight increase in the degradation rate is observed which may be due to the higher concentration of the hydroxyl ions. Therefore phenol degradation is more favorable in mild, acidic and neutral pH on the surface of ZnO. With nano-TiO2, a very high percentage degradation of phenol was observed at lower pHs between 3.0 to 5.0 as shown in Fig. 8. This increased degradation of phenol can

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Fig. 8. Effect of pH on the initial rate of photocatalytic degradation of phenol. Phenol ¼50 ppm; catalyst ¼nano-TiO2 (1.0 g/l); incident wavelength¼ 254 nm; absorbance measured at 500 nm; temperature ¼ 30.0 7 0.1 1C.

be attributed to the fact that TiO2 is amphoteric in aqueous solution. The point of zero charge (pHpzc) of TiO2 is 6.8. Below this value, the TiO2 surface is positively charged and above it is negatively charged and results in high adsorption of phenol on the catalyst surface [21]. 3.5.2. Effect of catalyst loading The rate of photocatalytic reaction is strongly influenced by the amount of photocatalyst. Heterogeneous photocatalytic reactions are known to show proportional increase in photodegradation with catalyst loading. Generally, in any given photocatalytic application, the optimum catalyst concentration must be determined in order to avoid excess usage of catalyst and to ensure the total absorption of efficient photons. The effect of varying amount of catalysts on the photodegradation of phenol was studied in the range of 0.5– 3.0 g/l. Fig. 9 shows the variation of percentage photodegradation of phenol as a function of catalyst weight. For nano-ZnO, in the weight range of 0.5–3 g/l, the degradation was found to increase from 18% to 83%. At lower loading levels such as 0.5 g/l, photonic adsorption controls the extent of reaction due to the limited availability of the catalyst surface area, and an increase in catalyst loading enhances the process performance. This is probably due to the increased number of available adsorption and catalytic sites. Degradation increases with an increase in the catalyst amount up to the optimum concentration. Also, in the presence of nano-TiO2, degradation reaches up to a maximum of 74% for the initial catalyst loading of 2 g/l. Further increase in the catalyst weight showed a negative effect. The decrease in the initial rate beyond the catalyst loading of 2.0 g/l may be attributed to the screening effect of excess catalyst particles in solution [19]. 3.5.3. Effect of phenol concentration in UV light irradiation The photocatalytic degradation of phenol for different initial concentrations in the range of 40–100 ppm was investigated using nano-ZnO, TiO2 and ZnO–TiO2 composite particles as catalysts. In the presence of nano-ZnO, the degradation was more effective at low initial concentrations of 40–60 ppm. About 90% of phenol was completely

609

Fig. 9. Effect of catalyst weight on the initial rate of photocatalytic degradation of phenol. Phenol ¼ 50 ppm; catalyst ¼ nano-TiO2 and nanoZnO (1.0 g/l); pH¼7.0 70.1; incident wavelength¼ 254 nm; absorbance measured at 500 nm; temperature¼ 30.0 70.1 1C.

degraded within 1 h of irradiation with UV light. Photodegradation of phenol in the presence of nano-TiO2 was more effective at low concentration of 40 ppm and the time required for complete degradation was found to be 90 min. The rate of degradation decreased as the phenol concentration was increased from 50 to 100 ppm. For 60 and 80 ppm of the phenol concentrations, complete degradation occurred within 140 and 160 min respectively. With an increase in the concentration of phenol, the initial rate of the reaction decreases. This may be due to the competition between phenol and hydroxyl ions for adsorption on the catalyst surface. At low initial concentration of 40 ppm, complete degradation occurred within 60 min of irradiation with UV light in the presence of nano-ZnO–TiO2 composite particles. At higher concentrations of 50– 100 ppm of the phenolic solution, about 70–80% of degradation was observed which is shown in Fig. 10. Complete degradation of 100 ppm of phenol solution occurred within 2.5 h of irradiation with the UV light. The degradation efficiency of the composite nanoparticles was higher than that for both nano-ZnO and nano-TiO2. Higher photocatalytic activity of ZnO–TiO2 nanocomposite particles in the degradation of organics has been reported [22]. The fact can be related to the vectorial transfer of electrons and holes, which takes place in coupled semiconductors possessing different redox energy levels, for their corresponding conduction and valence bands [23]. When the ZnO–TiO2 composite particles are agitated by photons with energy higher than the band gap energy (Eg), large number of electrons get promoted from valence band (VB) to the conduction band (CB) of ZnO and TiO2, leading to the generation of electron/hole (e  /h þ ) pairs. The electrons transfer from the CB of TiO2 to the CB of ZnO, and conversely, the holes transfer from the VB of ZnO to the VB of TiO2 decreasing the pairs' recombination rate. Though ZnO possesses an energy band similar to that of TiO2, it still plays an important role in the electron transport. The flat band potential (Vfb) is positively shifted by 0.12 V due to the modification of TiO2 nanoparticle by ZnO [24]. It was observed that, under the same conditions of pH, concentration of phenol and catalyst weight, the nano-ZnO–TiO2 composite particles showed efficient and higher photocatalytic activity compared to the nano-ZnO and TiO2 particles. At higher phenol concentration, the adsorption equilibrium of phenol

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Fig. 10. Effect of concentration on the photocatalytic degradation of the phenol; pH¼ 7.0; weight of nano-ZnO–TiO2 composite particle ¼1 g/L; temperature¼ 307 0.1 1C.

on the catalyst surface active site increases and more and more molecules of phenol get adsorbed on the surface of the catalyst. The degradation rate is reported to occur via hydroxylation [21]. Therefore, competitive adsorption of OH  on the same sites decreases and consequently the amount of OH  and O2 on the surface of the catalyst decreases. Since the generation of.OH does not increase, the probability of phenol molecules to react with OH decreases; therefore, a decrease in the degradation efficiency is observed at higher phenol concentrations [22]. 3.5.4. Comparison of degradation of phenol in UV light irradiation and solar light The effect of concentration of phenol was studied both in the presence of UV light and solar light. Fig. 11 shows the photodegradation of phenol on nano-ZnO surface both in the presence of UV and sunlight. In the presence of UV light, for 50 ppm phenol solution, complete degradation occurred within 80 min. However, for the same initial concentration, the degradation reaction was completed within 60 min in the

presence of solar light. For 100 ppm concentration of phenol, complete degradation occurred within 150 and 100 min in the presence of UV and solar light respectively. In the solar degradation process the color of the phenol solution gradually changed to light brown. After irradiation for about 2.5 h the light brown color also disappeared. The brown color of the photoreaction mixture may be due to the formation of mixture of various reaction intermediates like benzoquinone, hydroquinone and catechol. The mechanism of photodegradation of phenol on ZnO surface was reported by Peiro et al. [25]. The excitation of ZnO by solar energy leads to the formation of an electron–hole pair. The hole combines with water to form OH radicals while electron converts  oxygen to super oxide radical (O2 ), a strong oxidizing species as shown below ZnO-h þ þe 

(1)

H2O þh þ -OHþ H þ

(2)



O2 þe  -O2 

O2 þH þ -HO2

(3) (4)

When phenol molecules are adsorbed on the surface of the excited ZnO particle, there is activation of phenol molecules by reaction with OH radical according to Eq. (2). The hydroxyl radical shows electrophilic character and prefers to attack electron rich ortho or para carbon atoms of phenol. This results in the formation of dihydroxycyclohexadienyl (DCHD) radicals that undergo further reaction with dissolved oxygen to yield dihydroxy benzenes (DHBs) with simultaneous generation of HO2 radicals. DCHD radicals are also converted to phenoxy radical, as shown below.

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3.6. Kinetics studies The observed rate constant values for the degradation of phenol using nano-ZnO, TiO2 and ZnO–TiO2 composite particles with UV light irradiation are given in Table 3. Under constant conditions of pH, catalyst weight and photon flux, the effect of concentration of phenol on its photodegradability has been studied. Photocatalytic degradation of phenol using nano-ZnO, TiO2 and ZnO–TiO2 composite particles was found to follow pseudo-first order kinetics. The degradation rate constant is determined from LnðC 0 =CÞ ¼  kt Fig. 11. Effect of concentration on the photocatalytic degradation of phenol in UV versus solar light. pH ¼ 7.0 7 0.1; weight of nano-ZnO catalyst ¼1 g/L; absorbance measured at 500 nm.

Fig. 12. Effect of concentration on the photocatalytic degradation of phenol in UV versus solar light. pH ¼ 7.0 7 0.1; weight of nano-TiO2 catalyst ¼1 g/L; absorbance measured at 500 nm.

Phenoxy radical exists in three mesomeric forms, as given below

where C0 is the initial concentration of phenol and C is the concentration of phenol after irradiation, t is the time interval and k is the apparent first-order rate constant. A fairly linear relation between phenol concentration and irradiation time was observed for nano-ZnO particles as shown in the plot of log Co/C versus time (Fig. 13). The slope of which on linear regression equals the firstorder rate constant k. The observed rate constants values are 0.0161 min  1, 0.0138 min  1, 0.0115 min  1, 0.0069 min  1 and 0.0046 min  1 for phenol concentrations of 40, 50, 60, 80 and 100 ppm respectively. The rate constant was found to decrease with the increase in phenol concentration. Similar results were also obtained for nano-TiO2 and ZnO–TiO2 composite particles as shown in Figs. 14 and 15 For nanoTiO2, the rate constants are 0.0184 min  1, 0.0161 min  1, 0.0092 min  1, 0.0092 min  1 and 0.0069 min  1 for phenol concentrations of 40, 50, 60, 80 and 100 ppm respectively. It has been reported that the photocatalytic activity of TiO2 catalyst depends on both crystallinity and specific surface area of the material which has been calcined at 500 1C [14]. The Table 3 Comparison of k value for nano-ZnO, TiO2 and ZnO–TiO2 composite particles. Nanophoto catalyst k value 40 ppm 50 ppm 60 ppm 80 ppm 100 ppm

These phenoxy radicals can react with OH to form benzoquinone, hydroquinone, which are colored intermediates and also DHBs. The direct combination of two phenoxy radicals can lead to intermediates with two aromatic rings attached to each other by a single bond. Similar experimental results have been reported earlier [26,27]. The experiments were also conducted with nano-TiO2 particles in the presence of UV and solar light. Fig. 12 shows the photodegradation of phenol on nano-TiO2 particles surface both in the presence of UV light and sunlight. In the case of TiO2 the time required for the complete degradation was almost the same as that of the nano-ZnO for lower concentration of 50 ppm of phenol solution. However, at higher concentration of 100 ppm in the presence of UV light complete degradation of phenol occurred only after 3 h of irradiation. Therefore ZnO was found to be more efficient than TiO2 for phenol degradation at higher concentrations.

ZnO TiO2 ZnO–TiO2 composite

0.0161 0.0184 0.0368

0.0138 0.0161 0.0161

0.0115 0.0092 0.0138

0.0069 0.0092 0.0115

0.0046 0.0069 0.0092

Fig. 13. Plot of Co/C versus time for phenol. pH ¼7.0; weight of nanoZnO¼ 1 g/L; temperature¼307 0.1 1C.

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titanium as titanium ions were more pronounced only after the fifth cycle of reuse. 4. Conclusion

Fig. 14. Plot of Co/C versus time for phenol. pH ¼ 7.0; weight of nanoTiO2 ¼ 1 g/L; temperature¼ 307 0.1 1C.

Fig. 15. Plot of Co/C versus time for phenol. pH ¼ 7.0; weight of ZnO–TiO2 nanocomposite particles¼ 1 g/L; temperature¼ 307 0.1 1C.

particles calcined at higher temperatures showed a rough surface which enhanced the photocatalytic efficiency of the powder. In the case of ZnO–TiO2 nanocomposite the obtained rate constant values were found to be higher than those of nano-ZnO and TiO2 as shown in Table 3. The photocatalytic behavior of ZnO–TiO2 nanocomposite particles is related to crystallinity and particle morphology. The reaction occurs via the formation of OH radicals and the intermediates adsorbed on the catalyst surface are found to decrease the rate of oxidation of phenol as reported earlier [18]. 3.7. Reusability of nanocatalysts Reusability of nanocatalysts for the degradation of phenol by photocatalysis was evaluated. The solution resulting from the photocatalytic degradation of phenol was filtered, washed and the photocatalyst was dried. The dried catalyst samples were used for the degradation of phenol, employing similar experimental conditions. The filtrate was subjected to AAS analysis to assess the loss of Zn2 þ or Ti2 þ into solution as a result of dissolution of nano-ZnO or nano-TiO2 or nano-ZnO–TiO2. Under the present investigation, it was observed that the dissolution of the catalyst was found to be negligible (0.04% loss of zinc or titanium was observed for 2 h of reaction time). Catalyst samples showed considerably reproducible photocatalysis activity up to five cycles for the degradation of phenol. The loss in activity and loss of zinc as zinc ions or

The synthesis, characterization and evaluation of nanophotocatalysts such as ZnO, TiO2 and ZnO–TiO2 composite particles in the photodegradation of phenol were investigated. Complete degradation of phenol using nano-ZnO, TiO2 and ZnO–TiO2 composite particles was observed. From the kinetics data the reaction was found to follow pseudo-first order reaction kinetics. Although phenol is most commonly used in industrial applications, limited literature is available on the remediation of phenol from effluents. The effectiveness of ZnO–TiO2 binary oxide catalyst for phenol degradation is considered to be more effective due to the presence of more surface OH groups than that of the pure nano-TiO2 and ZnO photocatalysts. The nanophotocatalysts could be reused, which means that the adsorption-photocatalytic degradation process could be operated at a relatively low cost. The results obtained in the study demonstrate the use of nanophotocatalysts such as ZnO, TiO2 and ZnO–TiO2 composite particles for phenol degradation and help in designing an up scalable, practical process for treating wastewater containing phenol which would certainly protect the environment.

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