Correlation of Photodegradation Efficiency with Surface Potential of

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Correlation of Photodegradation Efficiency with Surface Potential of Silver-TiO Nanocomposite Thin Films 2

Avesh Kumar, Arun Singh Patel, and Tanuja Mohanty J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3060137 • Publication Date (Web): 04 Sep 2012 Downloaded from http://pubs.acs.org on September 9, 2012

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The Journal of Physical Chemistry

Correlation of Photodegradation Efficiency with Surface Potential of Silver-TiO2 Nanocomposite Thin Films Avesh Kumar, Arun S. Patel, and T. Mohanty* School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110 067, India

ABSTRACT: The present work focuses on photocatalytic performance of Ag-TiO2 systems. Nanocrystalline thin films of TiO2 and Ag doped TiO2 are grown by sol-gel method followed by spin coating technique on Si surface. The crystallinity and crystal size were measured from X-ray diffraction and Transmission electron microscopy studies. Lateral distribution of work function (WF) was examined through contact potential difference measurement done by Scanning Kelvin Probe microscopy. Local WF of Ag-TiO2 thin films were found to be smaller than that of TiO2 and minimum WF expected from polynomial curve fitting was that of pure Ag. The photocatalytic efficiency of these thin films is estimated from photodegradation of methyl orange analyzed by UV-Vis spectrophotometer. The photodegradation efficiency of Ag-TiO2 nanocrystalline thin films increases up to a certain dopant concentration of silver, beyond which it decreases. The changes in the photodegradation efficiency of these films are correlated with variation in contact potential difference. Keywords: Metal doping, XRD, TEM, Work Function, Charge transfer *Address for correspondence Dr. Tanuja Mohanty School of Physical Sciences Jawaharlal Nehru University New Delhi, India -110 067 E-mail: [email protected] Tel.: +91-11-26738802, FAX: 91-11-26742891

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■ INTRODUCTION Titanium dioxide (TiO2) is a highly studied research material due to its excellent optical transmittance, wide band gap, crystalline structure, chemical stability and strong oxidation power. Its wide band gap structure plays an important role for acting as very good photocatalyst in reduction of organic pollutants

1-3

. For photocatalytic activity

(PCA) study, TiO2 is activated by UV/visible light irradiation having energy equal or higher than the band gap energy (3.2 eV). This process leads to excitation of electrons to conduction band and the positive holes are left behind in the valance band. These electrons and holes are available for reduction and oxidation purpose respectively. The holes present in the highest energy level in the valence band possess strong oxidizing power than the reduction power of electrons excited to the conduction band. These oxidation and reduction process leads to degradation and decomposition of organic pollutants.5-8 However, the major limitation of TiO2 lies in low quantum yield and limited photo response confined to only UV region. For high efficiency in PCA, the surface area of TiO2 needs to be increased by reducing its dimension to nanometer range. Many reports have shown that in addition to nanosize reduction of grain sizes, one can achieve significant improvement of PCA by modification of TiO2 surface with doping of metals like Au, Pt, and Ag etc.9, 10 Among these noble metals, silver is mostly used as dopant because of its low toxicity, bactericidal capability, low cost compared to other noble metals.11 In this paper photocatalytic efficiency of TiO2 and Ag doped TiO2 in reducing organic pollutant like methyl orange (C14H14N3NaO3S) is presented. Different concentration of silver doped TiO2 thin films are deposited on Si (100) substrate by sol

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gel technique and the effect of silver doping on the PCA and the structure of TiO2 thin films are investigated. Although many studies have reported PCA of Ag doped TiO2 thin films, but correlation of surface potential with PCA has not been reported till date. In this work we measure contact surface potential difference (CPD) and hence work function of TiO2 and Ag-doped TiO2 thin films using Scanning Kelvin Probe (SKP) microscope and study the variation of CPD with Ag concentration. We emphasize the role of charge accumulation on the surface of doped thin films in controlling the variation of CPD as well as PCA with silver dopant concentration.

■ EXPERIMENTAL Titanium dioxide and silver doped TiO2 nanocrystalline thin films were grown on Si (100) substrates using sol-gel method followed by spin coating technique. The Ag-TiO2 sol gel solution was prepared by adding 1 mol of titanium isopropoxide (TIP, SigmaAldrich) mixed with 2 mol of Acetic Acid and 2 mol of isopropyl alcohol (Propane-2-ol) at room temperature followed by stirring for 30 minutes. In other solution AgNO3 with 0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3 mol % were dissolved in a mixture of 2 mol of distilled water, 2 mol of acetic acid and 2 mol of isopropyl alcohol with stirring done for 30 minutes. The mixture of both the solution was stirred vigorously on a magnetic stirrer for 60 minutes. The sol was ready for spin coating. The sol was spin coated on Si (100) substrate with 1000 rpm for 60 sec. These films were dried at 100oC for 1 hr and were further annealed at 500oC for 1 hr with heating rate of 1.66oC-min-1. Then the films were cooled down to room temperature. The photocatalytic experiment was performed in a 100 ml glass beaker. The photocatalytic films of TiO2 and Ag-TiO2 were horizontally placed at the bottom in this

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glass beaker containing 6 ml methyl orange solution. The UV light was provided by a 15 W lamp (Phillips, G15T 8). The distance between UV light source and deposited thin films was set at 10 cm and the irradiation intensity on the surface of the sample is 10 W/m2. The methyl orange solution with TiO2 and Ag-TiO2 thin films were irradiated for 1 hr. To determine rate of photodegradation of MO solution, the UV-visible absorption spectra of MO were taken with a UV-Vis spectrophotometer. The crystal structure of Ag-TiO2 thin films was determined by glancing angle Xray diffraction (GAXRD) technique using CuKα radiation. High resolution Transmission electron microscopy (HRTEM) using 200 keV JEOL 2100F TEM was carried out together structural and morphological information of TiO2 nanostructures along with distribution of silver particle in deposit thin films. The work function (WF) of these films doped with varying concentration of silver were measured by Scanning Kelvin Probe (SKP) Microscopy from KP Technology, United Kingdom.12 Degradation properties of TiO2 thin films were studied from UV-Vis absorption measurement in the range from 200 to 550 nm.

■ RESULT AND DISCUSSION The X-ray analysis of the films was carried out using X-ray Diffractometer procured from PAN analytical X'pert PRO. The XRD spectrum of the nanocrystalline TiO2 and silver doped TiO2 thin films are shown in Figure 1. The XRD peaks corresponding to (101), (004), (200), (151), (211) and (204) planes indicate that the TiO2 is in an anatase crystalline phase. But signature of silver related phase was not found in any samples due to presence of low concentration of Ag in TiO2. The size of crystal is an important factor that affects the TiO2 photocatalytic activity.13 All anatase XRD patterns in Figure 1 are

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found to be similar. The crystalline sizes of grains were calculated by using Scherrer equation.14 The calculated crystalline sizes of TiO2 thin films were found to be 15.8, 16.5, 16.3 and 16.1 nm within an experimental error of 2% for 0, 1, 2 and 3 mol % Ag doped TiO2 thin films respectively confirming the formation of nanocrystalline grains. The particle size, morphology and distribution of Ag in TiO2 thin films were studied by Transmission Electron Microscopy. The TEM images of TiO2 and Ag doped TiO2 thin films are shown in Figure 2. The TEM pictures show the formation regular pattern spherical nanocrystallites with approximate sizes of 10-14 nm. The size distribution of Ag-doped TiO2 nanocrystallites are shown in figures 2 (b) to 2(d). The TEM Figure 2(c) of Ag-TiO2 thin film shows the formation of smaller size of Ag particles, which are highly dispersed on the surface of TiO2. The silver particles appeared to have different shapes and sizes in the TEM image in silver doped TiO2. The lattice planes observed are shown in Figure 2(e). The inter planer spacing used for phase determination is 0.141 nm and this value corresponds to lattice spacing of (204) planes. The electron diffraction pattern in Figure 2(f) confirms that the nanoparticles are in crystalline phase. The SKP is used to estimate the work function of the surface of any metal or semiconductor. The work function (WF) of a material is a surface property and depends on both the chemical composition and the surface condition of the material.2,15 It can be expressed as,16

φsample = χ s + (EC − EF )

(1)

Where χs is the electron affinity of the sample, Ec is the conduction band energy and EF is the Fermi energy of semiconductor. Any changes in work function indicate shifting of

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Fermi level either towards conduction band or valence band. The Fermi energy level of n-type semiconductor can be expressed as,17

EF = EC + kT ln(

nc ) Nc

(2)

Where, Nc is density of state in conduction band and nc is density of net ionized states. In this work, the shifting of Fermi level of TiO2 was measured from contact potential measurement (CPD) using SKP unit. The CPD value is equal to the difference between the known WF of a metallic Au tip and unknown WF of the TiO2 sample. It is defined as,2

(φ − φ ) tip sample VCPD = e

(3)

Where Φtip is the work function of Au tip and Φsample is the work function of TiO2. In this measurement, CPD values of these thin films were taken in XYZ directions. The CPD images i.e. relative contact potential differences values of TiO2 and Ag-TiO2 thin films with varying silver concentration are shown in Figure 3. The crests and troughs are observed in the mapping of surface potential over the scanned area in Figure 3 indicating variation on surface morphology. These lateral features do not get significantly affected with varying Ag dopant concentration, but the major change is observed in the average value of CPD with increasing Ag concentration shown in Figure 4. The CPD as well as WF depends on the Fermi level of thin films. For pure TiO2 thin film the CPD value is 0.325 V as shown in Figure 3(a). The work function of nanocrystalline TiO2 thin film estimated to be 4.78 eV as given WF of reference Au tip is 5.1 eV. The measured WF value 4.78 eV for TiO2 thin film lies in the range of 4 to 6 eV

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which matches well with the reported results.2,

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With addition of silver of different

molar concentration to TiO2, there occur flow of electrons between silver and TiO2 due to variation in their work functions.17 Thus there occurs change in CPD value of TiO2 with varying Ag dopant concentrations (Table 1). Table 1. Variation of CPD with Ag dopant concentration Ag (mol %)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

CPD (V)

0.325

0.395

0.445

0.500

0.550

0.576

0.582

It is observed from Table 1 that CPD value of TiO2 thin film increases with increase in Ag concentration. Increase of CPD means decrease of WF of silver doped TiO2. From equation 1, decrease in WF indicates closeness of Fermi level towards conduction band. That is Fermi energy level of Ag-TiO2 nanocomposites get shifted towards the conduction band. This may be due to transfer of electrons from Ag to TiO2 semiconductor which is accumulated within the semiconductor nanoparticles causing shifting of Fermi level.19 It is well known that WF is closely associated with surface effects, so the observed linear increase of CPD with Ag doping upto 2% molar concentration (Figure 4) may be due to increase in concentration surface defects which act as electron trapping centers.20,

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Beyond this limit, the number of surface defects

decreases with increase in size of silver causing retarding trend in rate of increase of CPD. In our previous work, we have observed linear variation of CPD with swift heavy ion beam induced defect concentration in TiO2.2 But in the present case, beyond a certain limit (2% Ag concentration) the CPD variation deviates from linearity. Rather the CPD variation with Ag concentration is very well fitted to a quadratic polynomial (Figure 4). The vortex (X, f(X)) of this quadratic polynomial is calculated from the coefficients of

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this polynomial equation (mentioned in the inset of Figure 4), where X= -B1/2B2 and Y= f(X) = f(-B1/2B2). Thus the maxima of the CPD Vs Ag concentration curve is found to occur at 3.75% Ag dopant concentration with maximum CPD value ~0.6 V. CPD of 0.6 V corresponds to WF ~ 4.5 eV which well matches with that of pure Ag. We can expect that the minimum possible WF of Ag-TiO2 nanocomposite would be same as WF of silver. The photocatalytic activity of the TiO2 thin films were evaluated from degradation studies of Methyl Orange (MO). The UV-visible spectra for MO exhibit two absorption peaks which are shown in Figure 5. The intensity of major absorption peak occurring at 465 nm is used to determine the effect of photocatalysis on the degradation of MO. The peak intensity of MO decreases with increasing concentration of silver in TiO2 thin films up to certain limit (2.0 % Ag doping). Further increase of silver concentration causes increase in the absorption peak intensity. Decrease in the intensity of absorption peak implies decrease in concentration of MO which is due to decomposition of methyl orange. The rate of photo induced degradation was calculated in terms of change in absorbance of MO using the relation.22

Degradation percentage = (1−

At )×100 A0

(4)

Where At is the absorbance of methyl orange exposed by UV- light for time t (hour) and A0 is the absorbance without any UV exposure. In the present case the time of exposure (t) is only one hour. The effect of concentration of silver on photocatalytic degradation for methyl orange is shown in Figure 6. photodegradation percentage increases from 6.3 for undoped TiO2 to 16.6 for 2.0 % Ag doped TiO2. On further increase of silver concentration, the

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photodegradation percentage slowly decreases from 16.6 to 14.7 for 3.0% concentration of silver. The result shows enhancement of photocatalytic activity of Ag-TiO2 films over pure TiO2. The variation of photodegradation is fitted to a cubic polynomial which almost matches with our experimental results indicating the existence of maxima in the peak around Ag dopant concentration close 2%. From this cubic polynomial fitting we can very well extrapolate the rate of photodegradation at other unknown silver concentrations. When TiO2 is exposed to UV-light irradiation, the electrons present in its valence band are excited to its conduction band leaving behind holes in the valence band. These separated e--h+ pairs have a tendency to have fast recombination. With silver doping on TiO2, Ag ions introduces an intraband state close to CB or VB edge of TiO2. The photoexcited electrons present in conduction band of TiO2 are transferred to the surface of silver leading to photon absorption at sub-band gap energies.5 This charge transfer process at interfacial contact between TiO2 and Ag nanoparticle, creates a Schottky barrier due to the difference in WF of Ag and TiO2. This barrier acts as an effective trap for electrons, inhibiting the recombination of electron-hole pairs.23,

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which results in

increase in the concentration of photogenerated holes at the film surface. These trapped electrons are transferred to the absorbed O2 molecules on the surface of TiO2 and produce oxygen radicals (O2·-). These oxygen radicals are finally ·

changed to hydroxyl radicals (OH ).5, 25 On the other hand holes present in the valence band react with OH- or H2O and generate hydroxyl radicals. These hydroxyl radicals react with MO and degrade it. Therefore Ag-TiO2 films show better PCA than pure TiO2 thin film. In our work, as observed in Figure 6, Ag-doped TiO2 show increase in PCA till

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a 2% Ag concentration where we expect that Ag acts as electron hole separation center on the TiO2 surface. With increasing Ag dopant concentration beyond this limit of 2%, more photoexcited electrons are accumulated on the surface of silver, generating more negative space charges. This process increases the probability of capturing holes leading to reduction in the efficiency of charge separation.26,

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In addition higher amount of

silver particles cover the TiO2 nanocrystals surface which is also confirmed from the dark images in TEM picture in Figure 2, thus limiting the UV exposure area. This results in decrease in creation of electron-hole pairs as exposure area of TiO2 decreases. Both these processes contribute to existence of less number of electron-hole pairs for a shorter period. Thus decrease in e--h+ separation results in decreasing in photocatalytic activity of Ag doped TiO2 thin film beyond an optimal limit of Ag concentration. Doping of Ag into TiO2 induces accumulation of charge carriers on its surface. These accumulated charge carriers on one hand shift the Fermi level towards conduction band leading to decrease of WF and increase of CPD. On other side increase in charge accumulation (estimated through CPD measurement) results in improving photocatalytic activity of TiO2 up to a certain limit of Ag concentration due to occurrence of charge transfer process between Ag and TiO2. Beyond 2% doping, increase in size of Ag particles decreases surface defects that traps the electrons. Decrease in electron trapping centers results in deviation from linear growth of CPD with Ag concentration and decrease in photocatalytic activity. Thus 2% Ag concentration acts as a limiting factor beyond which creation of space charge carriers opposes electron trapping leading to decrease of photocatalytic activity.

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■ CONCLUSIONS In this report, effort has been to correlate the variation of surface potential with photocatalytic activity of Ag-TiO2 nanocomposite thin films. The dopant concentration was varied systematically from 0.5% to 3%. It is observed that photodegradation efficiency of Ag-TiO2 thin films as well as their CPD values increase linearly up to the 2.0%

of

silver

concentration.

On

further

increase

of

silver

concentration

photodegradation efficiency of composite film decreases but the CPD value increases at a retarding rate achieving a quadratic polynomial feature. The occurrence of optimal dopant concentration exhibiting maximum photocatalytic activity is explained on the basis of space charge creation, rate of charge carrier recombination and variation in work function value. The charge carrier concentration and hence Fermi level shifting controls the surface potential value of nanocomposite thin films. They are also responsible for observed variation in photocatalytic activity of Ag-TiO2 thin films.

■ ACKNOWLEDGMENTS One of the authors TM would like to thank DAE-BRNS, Mumbai, India, for financial support to purchase Scanning Kelvin Probe unit via Project number 2008/37/9/BRNS. The authors express their gratitude to Dr. Sobhan Sen, of SPS, JNU for UV/Visible absorption study. The authors are thankful to AIRF, JNU for TEM and XRD characterizations. One of the authors Mr. Avesh Kumar is thankful to CSIR, India for providing SRF fellowship.

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Figure Captions Figure 1. XRD patterns of Ag doped TiO2 thin films. Figure 2. (a) TEM image of TiO2 thin film, (b) to (d) TEM images and particle size distribution of Ag doped TiO2 thin films, (e) inter planer spacing and (f) diffraction pattern of TiO2 thin films. Figure 3. CPD image of Ag-TiO2 thin films at different mol % of Ag. Figure 4. The variation of CPD values with varying silver concentrations of Ag-TiO2 thin films. Figure 5. Absorption spectra of MO after UV exposure for 1 h in presence of pure TiO2 and different mol % concentrations of Ag doped TiO2 thin films. Figure 6. Photocatalytic degradation of methyl orange on Ag doped TiO2 thin films (exposure time 1h). Figure 7. Schematic illustration of photocatalytic mechanism in UV region.

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Correlation of Photodegradation Efficiency with Surface Potential of

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