Synthesis of Silver Doped TiO2 Nanoparticles for the ...

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Journal of Materials Science and Engineering B 2 (1) (2012) 52-58

DAVID

PUBLISHING

Synthesis of Silver Doped TiO2 Nanoparticles for the Improved Photocatalytic Degradation of Methyl Orange Roshan Nainani1, Pragati Thakur1 and Manohar Chaskar2 1. Department of Chemistry, University of Pune, Pune 411007, India 2. Baburaoji Gholap, College, Pune, India Received: July 22, 2011 / Accepted: September 05, 2011 / Published: January 25, 2012. Abstract: Noble metal doped TiO2 has been emerged as an effective catalyst for improving the activity of TiO2. Here we report the preparation of silver doped TiO2 by photodeposition process. TiO2 and Ag-TiO2 were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, tunnelling electron microscopy, Brunner Emmer Teller analysis. The average particle size of prepared TiO2 and Ag-TiO2 was found to be 7 nm and 20 nm respectively as measured by TEM analysis. BET surface area of prepared TiO2 and Ag-TiO2 was found to be 150 m2/g and 140 m2/g, respectively. From the UV-visible measurement, the absorption in the visible light of the Ag-TiO2 and as prepared TiO2 was found to be improved and was shifted to longer wavelength (visible range). The obtained results show that the photocatalytic efficiency of Ag-TiO2 was better than prepared TiO2 for the photodegradation of methyl orange. Key words: Photocatalysis, degradation, Ag-TiO2, methyl orange.

1. Introduction Heterogeneous photocatalysis is one of the advanced oxidation processes used for removal of organic pollutants. In this process the metal oxide semiconductor absorbs light and generates active species which leads to complete oxidation of organic components present in wastewater. Titanium dioxide is a semiconductor metal oxide with wide band gap (3.2 eV), used as photocatalyst for removal of highly toxic and non-biodegradable pollutants. TiO2 has its advantages over the other semiconductor photocatalysts. TiO2 is chemically stable, environmentally friendly, non-toxic and cheaper [1]. TiO2 is synthesized using various methods such as chemical vapor deposition (CVD), plasma, hydrothermal and sol-gel [2]. Among these various methods, sol-gel is the most simple and sophisticated method [2], in which various Corresponding author: Pragati Thakur, Ph.D., research field: synthesis of metal-oxide nanoparticles for photocatalytic applications. E-mail: [email protected].

material parameters such as the powder morphology, surface area, average crystallite size and phase structure are controlled in determining photocatalytic activity of TiO2. TiO2 has band gap energy 3.2 eV. It is active in UV light, in which electrons are injected into conduction band leaving behind holes in valence band. Thus TiO2 cannot be activated in visible light or sunlight. Two main factors govern the photocatalytic activity of TiO2 first, lowering of band gap energy in visible region and prevention of electron hole recombination rate. One of the methods to improve photocatalytic activity of TiO2 is to dope non-metal or metal impurity in the TiO2 lattice or to couple with lower band gap semiconductor [3-5]. The role of transition metal ion in TiO2 is that transition metal ion offers a way to trap the charge carrier thus improving the efficiency of catalyst. Noble metal doped or deposited on TiO2 shows enhanced photocatalytic activity of TiO2 [6]. They enhance the electron hole separation by acting as electron traps, extend the light absorption into the visible range and

Synthesis of Silver Doped TiO2 Nanoparticles for the Improved Photocatalytic Degradation of Methyl Orange

53

transferred in the photoreactor for photodeposition of silver on prepared TiO2.

enhance the surface electron excitation by plasmon resonance excited by visible light. Among the noble metals, silver ions have attracted interests of several researchers, because of their novel effects on the improvement of photoactivity of semiconductor photocatalysis [7-10]. Silver can be doped with TiO2 using photodeposition process [11-16] chemical reduction of silver ion on TiO2 nanoparticles [17], thermal deposition [18] and pulsed laser deposition [19]. In the present study, we have prepared high surface area TiO2 by sol-gel method, by controlled hydrolysis of the titanium tetra isopropoxide (TTIP) [20]. Prepared TiO2 was then doped with silver by photodeposition to study its enhanced photocatalytic activity for the degradation of methyl orange dye as a model pollutant.

the product was sprinkled on pre-greased glass slide

2. Experiment

(JASCO V-670), equipped with a diffuse reflectance

2.1 Chemicals

substance. UV-visible absorbance spectra of the

Titanium tetra isopropoxide (TTIP) and silver nitrate were purchased from spectrochem India, Isopropanol was purchased from SRL India. Methyl orange and commercial TiO2 anatase were supplied from Merck. Degussa P25 TiO2 was gifted from Evoniks India Pvt limited. All chemicals were used as received without any further purification.

solution

2.2 Preparation of TiO2 and Ag-TiO2 Nanoparticles TiO2 nanoparticles were synthesized using sol-gel method, involving hydrolysis and condensation of TTIP. For this 5 mL of TTIP was diluted with 20 mL of isopropanol. This solution was added dropwise to the distilled water maintained at appropriate pH (2-3) using conc. HNO3. The solution was stirred for 2 h and then kept in water bath maintained at temperature 60 °C-70 °C for 20 h. The obtained TiO2 nanoparticles were dried at 100 °C. 3.6 mL of 1 × 10-4 M AgNO3 solution was added to 200 mL distilled water containing 1 g of prepared TiO2 nanoparticles. The above solution was stirred for an hour and was

2.3 Characterization Nanoparticles

of

TiO2

and

Ag-TiO2

The crystal structure and particle size of doped and undoped TiO2 was determined by powder X-ray diffraction (XRD) analysis (Phillips PW1729, CuKα), and diffractograms were recorded between the angles 20° and 80° sis for the morphological characterization of prepared TiO2 and Ag-TiO2 scanning electron microscopy (Jeol JSM-6360) was used. Particle size was analyzed by transmission electron microscopy Philips (CM200). UV-visible absorbance spectra of the solid materials were recorded between range 200 nm and 800 nm using a UV-visible spectrophotometer sphere and BaSO4 was employed as a reference were

recorded

using

UV-visible

spectrophotometer shimadzu 1800 double beam spectrophotometer. BET surface area was analyzed using micromeritics ASAP 2020 BET surface analyzer. 2.4 Photoreactor Setup The photocatalytic activity of the prepared TiO2 and Ag-TiO2 was studied for the degradation of methyl orange as a model pollutant. The photocatalytic reaction system included a 500 mL cylindrical glass reactor, inside equipped with a UV illumination source (254 nm), which was located axially and held in a quartz immersion tube as shown in Fig. 1. A circulating water jacket was employed to cool the irradiation source, thus preventing any heating of the suspension. Air was bubbled through the reaction solution from the bottom using aerator with constant speed. In a typical photocatalytic test performed at room temperature, 200 mg photocatalyst was added into 500 mL of 5 × 10-5 M methyl orange aqueous solution. Then aliquots were taken out every 1 h, and centrifuged at 6,000 rpm. The

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S Synthesis of Silver S Doped TiO2 Nanopa articles for th he Improved Photocatalyti P ic Degradatio on of Methyl M Orange e

Fig. 1 Photooreactor setup..

Fig.. 2 XRD patteern of the syntthesized TiO2 and a Ag-TiO2.

resultant suupernatant soolutions weree analyzed by b a UV-visible spectrophotoometry (Shiimadzu 16500) at 463 nm.

resp pectively, whhich shows that the paarticles havee sph herical morphhology. EDX X analysis co onfirmed thee pressence of 1 mool% of silver in the doped d TiO2. From F TEM analysis a Figs. 4a and 4b, the averagee partticle size of TiO T 2 and Agg-TiO2 was observed o as 7 nm and 20 nm, respectively. r

3. Results and Discusssion 3.1 Characteerization of TiO T 2 and Ag-T TiO2 The X-rayy diffraction patterns of the silver dooped TiO2 samplees almost coinncides with that t of pure TiO T 2 and show noo diffraction peaks p due to the t silver speccies, thus suggessting that thhe metal paarticles are well dispersed onn the TiO2 suurface. Dopinng with Ag does d not disturb the crystall structure of o anatase TiO T 2 indicating thhat the metal dopant are merely m placedd on the surface of the crysttals without being covaleently anchored innto the cryystal lattice. There are no diffraction pattern p charaactertics of thhe metals inn the XRD patternns. Hence theese metal sitees are expecteed to be below thhe visibility limit of X-ray analysis. The XRD patterrn as shownn in Fig. 2 reveals that the synthesized TiO2 particlles were havving pure anaatase phase whichh was concluded from JCPDF J card NO 21-1272. Thhe particle size s was caalculated throough Debye Scherrer formula and a was foundd to be 7-8 nm m for TiO2 where as for Ag-TiiO2 particle size was founnd to increase to 20 2 nm due too doping. There is absencce of peak of silveer due to its loower proporttion in Ag-TiO O2. The SEM M images aree shown in Figs. 3a andd 3b for the prrepared TiO2 and Ag-TiO O2 nanopartiicles

3.2 Optical Propperties of Prep epared TiO2 and a Ag-TiO2 The T spectrum of TiO2 conssists of a sing gle absorptionn belo ow 370 nm usually u ascribed to charge transfer from m the valence bandd (mainly forrmed by 2p orbitals o of thee oxid de anions) too the conducttion band (mainly formedd by 3d 3 t2g orbitalss of the the Ti4+ cations) [2 21]. The T addition of silver ioons and sub bsequent UV V irraadiation causees significant changes to th he absorptionn spectrum of TiO O2 resulting in high abso orbance from m 400 0 nm to entiree visible regioon which is characteristicss of surface s plasm mon absorption. As seen in n the Kubelkaa Mu unk plot, the band gap abbsorption of silver dopedd TiO O2 catalysts iss slightly shiffted to higherr wavelength.. Thee absorbance in the visiblee region for th he metallizedd systtem shows thhat lower enerrgy transitionss are possiblee. Thiis is because the t metal clussters give risee to localizedd eneergy in the bannd gap of TiO O2 into which valence bandd elecctrons of TiO O2 are excited at wavelengtth longer thann 370 0 nm. The spectra reveals that Ag doping d has a marrked effect on o the absorpption propertties of TiO2. From the diffusee reflectance spectroscopy y as shown inn Fig. 5. absorptioon of TiO2 andd Ag-TiO2 naanoparticles


(a)

(b)

Fig. 3

(a) SEM image of synthesized TiO2. (b) SEM image of synthesized Ag-TiO2.

Fig. 4

(a) (a) TEM image of synthesized TiO2. (b) TEM image of synthesized Ag-TiO2.

Fig. 5 Diffuse reflectance spectra of (---) synthesized Ag-TiO2, (- - -) synthesized TiO2, (…. ) Degussa P25 TiO2 and ( ) Merck TiO2.

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(b)

Fig. 6 Kubelka Munk plot of synthesized Ag-TiO2, synthesized TiO2 and Merck TiO2 and Degussa P25 TiO2.

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Table 1 BET Surface area analysis of various photocatalysts Sr. No.

Catalyst

1

Merck TiO2

2

Prepared TiO2

3

Degussa P25 TiO2

4

Ag- TiO2

2

Surface area (m /g) 6.4 150 50 140

against the concentration of methyl orange. Obtained results show that Ag-doped TiO2 (1.5 mol% optimized) possesses enhanced photocatalytic activity compared to the prepared TiO2. Although the particle size of Ag-TiO2 is increased and BET surface area of the same is reduced, still it shows a good photocatalytic activity than bare TiO2. The better photocatalytic activity shown by Ag-TiO2 over the prepared TiO2 can be explained on the basis of silver being an acceptor impurity in doping of TiO2, it acts as an electron trap and prevents the electron hole recombination, which is important factor in determining the photocatalytic activity as represented in Fig. 9. Compared to other methods our method was simple and cost effective.

Fig. 7 Degradation of methyl orange, using various dopant concentration of Ag on TiO2. experimental conditions: Concentration of methyl orange 5 × 10-5 M Temperature 30 °C, Catalyst dose 0.2 g and volume of methyl orange 500 mL.

has shifted toward longer wavelength. Kubelka Munk plot [22] as shown in Fig. 6. was used to calculate band gap energy of TiO2. For Merck TiO2 (pure anatase) band gap energy was found to be 3.2 eV, for prepared TiO2 it was 2.9 eV, and for Ag-TiO2 it was 2.7 eV. The BET surface area of the prepared TiO2 and Ag-TiO2 was found to be high and is reported in Table 1.

Fig. 8 Degradation of methyl orange, () Ag-TiO2 1.5 mol%, () Prepared TiO2 experimental conditions: concentration of Methyl Orange 1 × 10-4 M temperature 30 °C, catalyst dose 0.2 g and volume of methyl orange 500 mL.

3.3 Photocatalytic Evaluation 5 mL of aliquot was pipette out from the reactor after every hour and centrifuged to separate the catalyst. The absorbance of the supernatant was determined at 463 nm using appropriate blanks. The C/Co was plotted for the comparison of the photocatalytic activity of the prepared TiO2 and silver doped TiO2. The Ag doping concentration in Ag-TiO2 was optimized by photocatalytic degradation of methyl orange. The optimized doping concentration of Ag content in Ag-TiO2 was found to be 1.5 mol% as shown in Fig. 7. Fig. 8 shows the time dependent degradation spectra

Fig. 9 Representation of mechanism for the role of Ag dopant on the TiO2 in the case of photocatalytic degradation of methyl orange.


4. Conclusions TiO2 was prepared by sol-gel method by controlled hydrolysis of TTIP and further Ag-TiO2 was prepared by photodeposition of silver on prepared TiO2. TiO2 and Ag-TiO2 were characterized using XRD, SEM, EDX, TEM, BET and UV-visible spectrophotometer. From XRD it is concluded that, prepared TiO2 is in pure anatase phase. From SEM images, both TiO2 and Ag-TiO2 are found to have spherical morphology. TEM Confirms the nanoparticle size of TiO2 and Ag-TiO2. BET surface area for the prepared TiO2 and Ag-TiO2 were found to be high, i.e., 150 m2/g and 140 m2/g, respectively. Ag-TiO2 shows higher photocatalytic activity compared to prepared TiO2. Enhanced photodegradation of methyl orange when Ag-TiO2 photocatalyst was used clearly indicates the prevention of electron hole recombination which is one of the major limitations of TiO2. Band gap energy of prepared TiO2 and Ag-TiO2 lowered as compared to Merck pure anatase TiO2 as seen in Kubelka Munk plot. The enhanced photocatalytic activity of prepared TiO2 and Ag-TiO2 may also lie in the fact that it absorbs in wider range of UV-visible light, due to lowering of band gap. The measurement of photocatalytic activity in presence of visible light for TiO2 and Ag-TiO2 is under progress.

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

Acknowledgments First author is thankful to University Grants Commission (UGC), New Delhi for Research Fellowship. Corresponding author is thankful to University Grants Commission (UGC), New Delhi and Center for Nanomaterials and Quantum Systems (CNQS), Department of Physics, University of Pune for Research Funding.

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