Enhanced photocatalytic activity of Ag doped TiO2 nanoparticles synthesized by a microwave assisted method

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 5489–5496 www.elsevier.com/locate/ceramint

Enhanced photocatalytic activity of Ag doped TiO2 nanoparticles synthesized by a microwave assisted method M.B. Suwarnkar, R.S. Dhabbe, A.N. Kadam, K.M. Garadkarn Nanomaterials Research Laboratory, Department of Chemistry, Shivaji University, Vidya Nagar, Kolhapur 416004, India Received 5 September 2013; received in revised form 2 October 2013; accepted 28 October 2013 Available online 13 November 2013

Abstract Pure anatase TiO2 photocatalyst with different Ag contents was prepared via a controlled and energy efficient microwave assisted method. The prepared material was further characterized by several analytical techniques like X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), surface area measurement (BET), Fourier transforminfrared spectroscopy (FT-IR), diffused reflectance spectroscopy (DRS), and thermogravimetric–differential thermal analysis (TGA–DTA). A 10 nm average crystallite size with nano-crystals of pseudo-cube like morphology was obtained for optimal (0.25 mol%) Ag doped TiO2. The present research work is mainly focused on the enhancement of degradation efficiency of methyl orange (MO) by doping of Ag in TiO2 matrix using UV light (365 nm). A 99.5% photodegradation efficiency of methyl orange was achieved by utilizing 0.25 mol% Ag doped TiO2 (1 g/dm3) at pH ¼3 within 70 min. Recyclability of photocatalyst was also studied, with the material being found to be stable up to five runs. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Calcination; B. Composite; B. Spectroscopy; D. TiO2

1. Introduction Research in the area of nanomaterials is of great scientific interest due to its various applications in different fields like dye sensitized solar cells [1], semiconductors [2–6], photovoltaic cells [7], gas sensors [8], photocatalytic degradation of dyes [9,10] and hydrogen generation [11]. The opto-electronic and physico-chemical properties exhibited by nanomaterials are entirely different from their bulk counterpart which makes them fundamentally as well as technologically important [12]. Since the last two decades, the photocatalytic degradation of numerous organic and inorganic pollutants using oxide semiconductor as a photocatalyst has been widely employed for waste water treatment. A large number of semiconductor oxides show good photocatalytic activity. Among these, nanosized TiO2 is one of the most promising photocatalyst due to its several advantages such as low-cost, non-toxic and photostability towards light [13–16]. The high efficiency of n

Corresponding author. Tel.: þ91 231 2609161; fax: þ 91 231 2692333. E-mail addresses: [email protected], [email protected] (K.M. Garadkar).

TiO2 as a photocatalyst is sometimes limited due to its wide band gap (3.2 eV) as well as its large charge carrier recombination rate (nanosecond) [17–21]. Therefore, it is essential to develop a novel catalyst which holds high potential for photocatalytic activity. The modification of TiO2 with foreign impurity has been considered as an efficient route to increase the lifetime of the charge carrier and tune the band gap to a desired level [22,23]. There are several methods that have been employed for the preparation of TiO2 nanostructure because of its potentially wide-ranging applications. Recently, Abbad et al. [24] reported on preparation of TiO2. In this method TTIP as a titanium precursor with mixture of ethanol and water was used. The final gel was dried at 80 1C for 4 h. The dried powder was calcined at 400, 500, 600 and 700 1C for several hours. The average particle size was found to be 50 nm. Yang et al. [25] report a method wherein TiO2 was calcined at 500 or 650 1C for 2 h; the particle size was observed to be 20 nm. These reported methods required high temperature and longer reaction time, whereas on the other hand, a microwave assisted method is superior with reference to a control over particle size, energy efficiency and shorter reaction time.

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.10.137

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In the present study, we describe the doping of TiO2 with Ag by an energy efficient microwave method. The synthesized nanomaterials were thoroughly characterized by several techniques to predict the structure and composition. The photocatalytic activity of the catalyst was evaluated by using methyl orange as a model pollutant. In order to achieve maximum photocatalytic activity, a series of experiments were carried out which includes the content of silver doping, effect of catalyst loading, and pH. 2. Experimental

3010). The surface morphology and elemental composition of the prepared photocatalyst were obtained by an energy dispersive analysis scope attached to a scanning electron microscope. The surface area of catalyst was studied by using a Micromeritics-2720 model under 30% nitrogen in helium environment at 77 K. Infrared spectra were taken in a KBr pellet, using a Perkin-Elmer spectrum BX-II IR spectrometer. Thermal stability of Ag doped TiO2 was obtained by using a TGA–DTA instrument (SDT Q600V20.9 Build 20 TA Instruments, USA) under N2 atmosphere. The diffused reflectance spectra were recorded on a UV–vis spectrophotometer (Varian Cary-5000).

2.1. Materials 2.4. Photocatalytic activity of Ag doped TiO2 nanoparticles Titanium tetra-isopropoxide (TTIP, 99%) and surfactant cetyl trimethyl ammonium bromide (CTAB, 99%) were purchased from Spectrochem Pvt. Ltd. (India). Absolute ethanol (99.9%) and silver nitrate were purchased from S.D. Fine Chemicals. Ammonia was purchased from Loba Chemie Pvt. Ltd. (India). All analytical grade chemicals were used as received for preparation of solutions. All solutions were prepared in millipore water obtained from a millipore water system (Bangalore, India). 2.2. Preparation of Ag doped TiO2 nanoparticles Titanium tetra-isopropoxide and silver nitrate were used as a source of titanium and silver respectively. Ag doped TiO2 nanocrystalline powder was prepared by controlled addition of 0.1 M TTIP in 100 mL of absolute ethanol with constant stirring to get a clear solution. Further, a sufficient amount of surfactant solutions (1% CTABþ þ 1% SDS) was added with constant stirring. For silver doping of TiO2 the concentration of Ag was varied from 0.12 to 0.5 mol%. The required amount of 0.05 M aqueous solution of silver nitrate was added to get the desired composition. A solution of aqueous ammonia was added dropwise under stirring conditions with special arrangement at room temperature until solution reached up to pH ¼ 8. After complete precipitation, the precipitate was washed with millipore water and acetone several times to remove excess surfactant. The washed precipitate was kept under microwave irradiation for 20 min in a domestic microwave oven (input 900 W, 250 MHz, LG Make) with on–off cycle (20 s on and 40 s off). The dried powder was ground by using an agate mortar and a pestle and calcined at 300 1C for 3 h in a temperature controlled muffle furnace. Nanocrystalline powder of pristine TiO2 was prepared similarly without addition of Ag source. The phase purity and degree of crystallinity of TiO2 and Ag doped nanomaterials were monitored by XRD. 2.3. Characterization of Ag doped TiO2 Powder X-ray diffraction patterns of pristine and doped TiO2 were obtained using XRD (Model X-Pert PRO-1712) with CuKα radiation (λ¼ 1.5406 Å) as the X-ray source. The shape and size of the materials were obtained by using transmission electron microscopy with a model TEM (JEOL

The photocatalytic activity of Ag doped TiO2 was evaluated by testing the degradation of methyl orange as a model pollutant by using UV light (365 nm). To search the highest photocatalytic activity of Ag doped TiO2, catalyst amount was varied from 0.6 to 1.4 g/dm3. The effect of pH on the photocatalytic activity was also studied by varying the pH of solution from 3 to 9 by using HCl and NaOH (1 M each). The quartz photoreactor was kept in a steel container along with magnetic stirrer. A Philips (HPL-N, 250 W) light source was used which had Lumens of 12,750. Outer side of the bulb was broken and just the inside filament is used as a source of light. The distance of lamp was kept 5 cm above the dye solution. In the given experiment the photocatalyst was added in the photoreactor containing methyl orange (100 mL, 20 ppm). Prior to irradiation, dye solution was stirred for 30 min in dark for adsorption–desorption equilibrium. All photodegradation experiments were performed at ambient temperature. At a given time interval, samples were withdrawn and centrifuged to remove the photocatalyst. The absorbance of the clear solution was recorded by using a UV–vis–NIR spectrophotometer (Shimadzu, Model-UV-3600) to monitor the concentration of dye. 3. Results and discussion 3.1. Characterization of as prepared Ag doped TiO2 nanoparticles 3.1.1. XRD X-ray diffraction (XRD) is a reliable and wide spread identification technique especially for crystalline materials [26]. To investigate the changes in the crystal structure of TiO2 affected by Ag doping, XRD analysis was carried out in the range of 2θ¼ 10–901 for different mol% of Ag as shown in Fig. 1. The diffraction pattern reveals the formation of pure anatase phase of TiO2. The strong diffraction peaks at 25.091, 37.651, 48.021, 53.891, 55.071, 62.381, 68.701, 70.041 and 75.001 correspond to the crystal planes [101], [004], [200], [105], [211], [204], [220], [220] and [215] respectively; this is attributed to reflections of anatase TiO2 which is compared with JCPD Card No. (21-1272). It can also be seen that the 2θ peak positions of major diffraction pattern after Ag doping of

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demonstrates that the black dots are for silver metallic particles. The Ag þ ions were reduced by ethanol which is used as a solvent [31]. The average particle size was observed within 10–15 nm. The HRTEM image (Fig. 2b) shows that the interplanar spacing is about 0.35 nm, which corresponds to the [101] plane of anatase TiO2 while 0.23 nm matches with the [111] plane of Ag which is in good agreement with the result obtained from XRD.

Intensity (a.u.)

(101)

(200) (200)

(111)

(105) (211) (204) (215) (116)(220) (311)

0.50% 0.25% 0.12% TiO2

10

20

30

40

50

60

70

80

90

2θ (degree) Fig. 1. X-ray diffraction patterns of a) TiO2, b) 0.12, c) 0.25 and d) 0.5 mol% Ag doped TiO2 nanoparticles.

Table 1 Shifting of ‘d’ values ([101] plane) of TiO2 nanomaterials after Ag doping and surface area of pure and different mol% Ag doped TiO2. Ag content (mol%) 2θ (deg) SBET (m2 g  1)

0.0 (TiO2) 25.36 139

0.12 25.31 137

0.25 25.28 144

0.37 25.23 134

0.50 25.17 128

TiO2 had similar values as that of pristine TiO2, except the changes in the intensities of the peaks [27]. After 0.25 mol% of Ag, the diffraction pattern had additional peaks at 2θ values of 38.011, 44.261, 64.021 and 77.361 which can be attributed to the crystal planes of metallic silver ([111], [200], [200] and [311] respectively) [28]. From the XRD results it is interesting to note that the peak position of TiO2 [101] shifts to a low angle as Ag dopant increases which is shown in Table 1. According to Bragg's law nλ ¼ 2d sin θ

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ð1Þ

where the lesser the value of sinθ, the larger the d spacing. So it can be concluded that the value of ‘d’ gradually increases with increase in Ag contents. This implies that silver ions diffused into the lattice of TiO2 [29]. As the Ag contents increase from 0.0 to 0.25 mol%, the broadening of the peak [101] gradually increases which indicates the smaller crystallite size of the material. The average crystallite size of the material was calculated by using Scherrer's equation [30]. It is observed that the pristine TiO2 has an average size of 15 nm and it decreases up to 10 nm for 0.25 mol% Ag; thereafter the crystallite size of the photocatalyst was more or less constant. 3.1.2. TEM images of Ag doped TiO2 Fig. 2 shows the TEM and HRTEM images for Ag doped TiO2 (0.25 mol%) nanoparticles which show pseudo-cube like morphology and well monodispersed particles. The metal doped TiO2 has many black–gray colored dots in contrast which are displayed in (Fig. 2a). The HRTEM image further

3.1.3. SEM images and EDS spectrum of Ag doped TiO2 nanoparticles In order to investigate the surface morphology of the synthesized Ag doped TiO2 nanoparticles, SEM studies were performed. The particles of pristine and Ag doped TiO2 have pseudo-cube like morphology which is shown in Fig. 3. The SEM images of prepared Ag doped TiO2 compared with those of pure TiO2 confirm that Ag doped TiO2 has a slightly smaller particle size than that of pristine TiO2. The composition is very sensitive for the applications; therefore the elements present in the nanomaterials were scanned by EDS. The EDS spectrum was recorded in the binding energy region of 0–20 keV which is shown in Fig. 4. The signals from the spectrum revealed the presence of Ti, O and Ag at 4.508, 0.525 and 2.983 keV respectively. The atomic percentages of Ti, O and Ag are 33.58%, 66.22% and 0.20% respectively. Though the peaks of Ag are insignificant owing to its content in TiO2 matrix, they indicate the Ag particles present in the catalyst. 3.1.4. Surface area of TiO2 and Ag doped TiO2 Surface area is an important characteristics tool when studying the catalytic efficiency of Ag doped TiO2 nanomaterials. The photocatalytic activity of nanomaterials is known to be a function of their surface properties; as surface area increases, the number of active sites also increases. In order to get better photocatalytic activity, high surface area of the catalyst is often desired. Therefore specific surface area of pure and Ag doped TiO2 was measured by the BET method and it is summarized in Table 1. The optimal 0.25 mol% Ag doped TiO2 had the highest surface area (144 m2/g) which leads to the highest photocatalytic activity towards methyl orange degradation. 3.1.5. FT-IR spectra of pristine and Ag doped TiO2 Fig. 5 shows FT-IR spectra of pristine TiO2 and Ag doped TiO2 nanoparticles. The spectrum (a) shows a broad band centered at 3416 cm  1, which is attributed to OH stretching and band at 1644 cm  1 due to the OH bending mode of water adsorbed on the surface of TiO2 which may have crucial roles in photocatalytic activity. The methylene symmetric and antisymmetric vibrations can be seen in the region of 2923– 2825 cm  1 which arises due to the presence of surfactants. The broad peak at 735–670 cm  1 is the characteristic of Ti–O bending mode of vibrations which confirms the formation of metal oxygen bonding [32]. The intensity of bands at 3471, 1644 and 735 cm  1 shifts towards lower wavenumbers after Ag loading, indicating the formation of Ag–TiO2 composite

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Fig. 2. a) TEM image of Ag doped TiO2 and b) HRTEM image of Ag doped TiO2.

Fig. 3. SEM images of a) pure TiO2 and b) Ag doped TiO2 nanoparticles.

Fig. 5. FT-IR spectra of a) TiO2 and b) Ag doped TiO2 (0.25 mol%). Fig. 4. EDS spectrum of 0.25 mol% Ag-doped TiO2 nanoparticles.

[33]. According to the fundamental transverse optical phonon mode [34] the insertion of Ag into host lattice of TiO2 should result in the downward shift of the peak.

3.1.6. Diffused reflectance spectra of Ag doped TiO2 The diffused reflectance spectra of pristine TiO2 and Ag doped TiO2 with varying mol% of Ag from 0.0 to 0.5 mol%, after being calcined at 300 1C for 3 h are shown in Fig. 6.

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It was found that the absorption onset of Ag doped TiO2 slightly shifted towards the visible region. This is not only due to the metal cluster that gives rise to localized energy in the band gap but also due to surface plasmon absorption owing to Ag nanoparticles. Especially the 0.25 mol% Ag doping showed the largest red-shift. The band gap was calculated by using the plot of (αhv)2 vs. hν which is shown in Fig. 7. By assuming that TiO2 has a direct type of transition ðαhνÞ ¼ Aðhν  E g Þn

ð2Þ

where α, hν, A, Eg and n are the absorption coefficient, photon energy, proportionality constant, band gap and a constant respectively, where n decides the type of transition. From the plot compared to that of pristine TiO2, Eg values of TiO2 nanoparticle have been narrowed from 3.20 to 2.98 eV by doping of Ag in TiO2 matrix.

1.0

Absorbance (a.u.)

3.1.7. TGA–DTA analysis of Ag doped TiO2 nanoparticles The TGA–DTA curves of Ag doped TiO2 nanoparticles calcined at 300 1C are shown in Fig. 8. The TG curve shows three stages of weight loss. The first weight loss 3.51% observed from room temperature to 200 1C corresponds to dehydration of physically adsorbed water. The second weight loss is observed in the range of 200 to 500 1C which can be ascribed to the decomposition of surfactants like CTAB and SDS [35], and finally the third stage weight loss is observed from 500 to 700 1C. This is due to the decomposition of residual organic moieties present in Ag doped TiO2. The total weight loss was found to be 8.35% which is very low. DTA results indicated the appearance of a decalescence peak at 54.17 1C on the curve which corresponds to release of water from the nanoparticles. 3.2. Factors affecting photocatalytic degradation of methyl orange 3.2.1. Effect of concentration on Ag doping Fig. 9 shows the photocatalytic degradation of methyl orange on Ag doped TiO2 photocatalyst with varying Ag concentrations. It was found that the degradation efficiency of TiO2 increased gradually with increase in Ag dopant. The degradation efficiency of 0.25 mol% Ag–TiO2 was found to be the highest. This could be attributed to the following reasons.

b) c)

0.8

5493

a)

0.6

0.4

1) The appropriate amount of Ag doped content in TiO2 can effectively capture the photoinduced electrons; photoinduced electrons can be immediately transferred to oxygen adsorbed on the surface of TiO2 [36]. 2) The amount of surface hydroxyl radical is increased.

0.2

0.0 200

300

400

500

600

Wavelength (nm) Fig. 6. Diffused reflectance spectra of a) TiO2, b) 0.25 and c) 0.50 mol% Ag contents.

Due to these advantages, Ag doped TiO2 photocatalyst significantly improved the photocatalytic activity. However, when Ag content exceeded 0.25 mol%, the number of active sites capturing the photogenerated electron decreased with

20

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αhν2

c)

a)

b) 5

0

-5 2.5

3.0

3.5

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4.5

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Photon Energy (hν) ν) 2

Fig. 7. Variation of (αhv) vs. photon energy. (a) TiO2, (b) 0.25 mol% and (c) 0.50 mol%.

Fig. 8. TGA–DTA curves of 0.25 mol% Ag-doped TiO2 nanoparticles.

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Degradation efficiency (%)

100

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50

80

60

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0

0.0

0.1

0.2

mol % of

0.3

Ag+

0.4

60

0.5

ions

Fig. 9. Effect of Ag doping on photocatalytic activity of anatase TiO2.

increase in particle size of TiO2. Also, the excess Ag can cover the surface of TiO2, leading to decrease in concentration of photogenerated charge carrier and hence photocatalytic performance decreases. Hamadanian et al. [37] reported that methyl orange dye was degraded by TiO2 (87%) and co-doped (Cr and S) TiO2 (80%) degradation within 180 min under UV-light irradiation. In the case of 0.25 mol% Ag doped TiO2, 99% degradation of methyl orange was obtained within 90 min under UV-light which seems to be a better result than the reported one. 3.2.2. Effect of catalyst loading The effect of catalyst loading on the degradation of methyl orange was investigated using optimal (0.25 mol%) Ag doped TiO2 nanoparticles, keeping the other parameters identical. The results of this study are shown in Fig. 10. A series of experiments were carried out by varying the catalyst amount from 0.6 to 1.4 g/dm3 in the aqueous solution of methyl orange. It can be seen that initially the degradation efficiency increases with increase in amount of catalyst. With 1 g/dm3 catalyst, the efficiency of dye degradation was found to be high and then efficiency decreased as the catalyst amount increased. This observation can be explained in terms of the number of active sites available for photocatalytic reactions. It was observed that large amount of catalyst may result in agglomeration of the catalyst. This decreases the number of active sites [36,38]. Another reason for decreased degradation efficiency can be attributed to the increase in the turbidity of suspension due to the large amount of photocatalyst. Scattering leads to the inhibition of photon absorption by the photocatalyst. Fig. 11 shows the UV–visible absorption spectra of methyl orange degradation. The maximum 99% degradation efficiency of methyl orange was observed at 1 g/dm3 of 0.25 mol% Ag doped TiO2 under UV-light within 90 min. 3.2.3. Effect of initial pH In order to study the effect of pH on degradation efficiency, the experiments were carried out at different pH ranging from

80

100

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Catalyst loading (mg/100 mL) Fig. 10. Effect of catalyst loading on degradation of methyl orange.

0 min

2.0

30 min 60 min

Absorbance (a.u.)

Degradation Efficiency (%)

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90 min

1.5

1.0

0.5

0.0

200

300

400

500

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700

Wavelength (nm) Fig. 11. UV–visible absorption spectra of methyl orange degradation under UV-light using Ag-doped TiO2 nanoparticles.

3 to 9 for dye concentration (20 ppm) and catalyst loading (1 g/dm3). Fig. 12 shows the photocatalytic activity of Ag doped TiO2 nanoparticles with different pHs. The maximum 99.5% photodegradation efficiency was obtained at pH 3 within 70 min under UV-light. Liu and Xu [39] reported the photocatalytic activity of Ag doped TiO2 nanoparticles; the optimal doping content was 0.8 wt% Ag; methyl orange could be degraded to 99.3% by Ag–TiO2/CNT within 120 min under optimal conditions using UV light (high pressure lamp 300 W). The results obtained in our case are superior to that reported by Liu and Xu. The adsorption and degradation of dye depending on the surface charge of the catalyst and pH are effective parameters that affect the surface state [40]. The amphoteric behavior of synthesized oxides influences the surface charge of the photocatalyst. The pH of dye solution can vary with the

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electrostatic repulsion between methyl orange and catalyst which results in decrease of degradation efficiency. To investigate the reusability of 0.25 mol% Ag–TiO2 nanoparticles for photocatalysis under UV light irradiation, the catalyst was repeatedly used for five times as shown in Fig. 13. After each cycle the catalyst was collected by centrifugation and washed with distilled water and ethanol until a clear supernatant was obtained. The washed catalyst was dried at 80 1C for 1 h and again used for the degradation of methyl orange dye. The data indicates that the catalyst was reusable (at least for five runs). This indicates the reusability of our catalyst.

100

Degradation Efficiency (%)

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4. Conclusions 0 2

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pH of methyl orange solution Fig. 12. Effect of pH on photodegradation of methyl orange.

1.0

C/C0

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Acknowledgment

e d c b a

0.2

0

30

60

In summary, the TiO2 nanoparticles with different Ag mol% have been synthesized by using a simple and energy efficient microwave assisted method. The average crystallite size of optimal (0.25 mol%) Ag doped TiO2 nanoparticles was found to be 10 nm which was less than that of pristine TiO2 (15 nm). The band gap energy was observed to be decreasing from 3.20 to 2.98 eV, as Ag content increased up to 0.25 mol%, by further doping of Ag; there is no significant change in the band gap. The 99.5% degradation efficiency of methyl orange (20 ppm) was observed at pH 3 within 70 min (0.25 mol% Ag doped TiO2 at 1 g/dm3) which is better than that reported in the literature. The enhanced photocatalytic activity was observed due to large surface area of Ag doped TiO2. The catalyst was reused up to five runs and found to be stable without losing the activity.

One of the authors (KMG) is thankful to DST for financial support under major research project (SR/S1/PC/0041/2010).

90

Time (min.)

References

Fig. 13. Reusability of photocatalyst for photodegradation of methyl orange.

surface charge of the photocatalyst and also shifts the position of redox reaction. [41]. Due to amphoteric nature of TiO2, the following two equilibriums are considered: Ti  OH þ H þ ¼ Ti  OH2þ

ð3Þ

Ti  OH þ OH  ¼ Ti O  þ H þ

ð4Þ

From reactions (3) and (4) the surface of photocatalyst has become positively charged in acidic medium while negatively charged in alkaline medium. Methyl orange is an anionic dye; after ionization it strongly pays attention towards the surface of the catalyst. At pH 3, it shows maximum photocatalytic degradation due to electrostatic attraction between anionic dye and positively charged surface. At pH greater than 7, the surface of catalyst has become negatively charged that leads to

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