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Photoelectrocatalytic degradation of benzoic acid using sprayed TiO2 thin films
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Photoelectrocatalytic degradation of benzoic acid using sprayed TiO2 thin films V.S. Mohite, M.A. Mahadik, S.S. Kumbhar, V.P. Kothavale, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale
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S0272-8842(14)01545-4 http://dx.doi.org/10.1016/j.ceramint.2014.10.020 CERI9279
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Ceramics International
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Cite this article as: V.S. Mohite, M.A. Mahadik, S.S. Kumbhar, V.P. Kothavale, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale, Photoelectrocatalytic degradation of benzoic acid using sprayed TiO2 thin films, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2014.10.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoelectrocatalytic degradation of benzoic acid using sprayed TiO2 thin films V.S. Mohite, M. A. Mahadik, S. S. Kumbhar, V. P. Kothavale, A.V. Moholkar, K Y. Rajpure, C.H. Bhosale* Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Abstract Transparent TiO2 thin films have been successfully synthesized by chemical spray pyrolysis technique. The effect of substrate temperature on the photoelectrochemical (PEC), structural, morphological, optical and photoelectrocatalytic properties has been investigated. The PEC study shows that both short circuit current (Isc) and open circuit voltage (Voc) at the optimized substrate temperature (450 °C) are relatively maximum (Isc = 1.7 mA and Voc = 770 mV). The tetragonal crystal structure has been confirmed from X-ray diffraction patterns. FESEM study reveals that the film surface is changed from nanogranular to nanorod like morphology. The films exhibit a transmittance of about 80% in the visible region and a sharp absorption edge at 375 nm corresponding to the fundamental absorption edge in UV region. Band gap energy varies from 3.33 to 3.43 eV with substrate temperature. Electron-phonon coupling present in TiO2 films has been analyzed using Raman spectroscopy. The elemental composition and valence states of TiO2 film are studied by using X-ray photoelectron spectroscopy. The effect on the photoelectrocatalytic behavior of the large surface area (64 cm2) TiO2 photocatalyst samples were studied by using photoelectrocatalytic degradation of benzoic acid under UV light illumination. Keywords: Films; Structural applications; Optical microscopy; TiO2 Corresponding author: E-mail:[email protected] Tel.: +91 2312609435; Fax: +91 2312691533.
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1. Introduction TiO2 is a wide band gap, chemically stable and environmental eco-friendly semiconductor with good biocompatibility and stability. It occurs in three phases namely anatase (< 550 °C), rutile (> 550 °C) and brookite (>1200 °C). TiO2 film has the unique characteristics such as high optical transmittance over a wide wavelength range and excellent adhesion to the substrates. The TiO2 thin films are used in a variety of applications such as dye-sensitized solar cells, antireflection (AR) coatings, gas sensors, electrochromic displays, planar waveguides and photocatalytic activity [1-2]. Researchers have used several methods for deposition of the TiO2 thin films viz. sol gel method [3-5], chemical vapor deposition method [6, 7], evaporation method [8], sputtering method [9-11], pulsed laser deposition method [12], electrodeposition method [13] and spray pyrolysis method [14-21]. Amongst these all deposition methods, the most widely studied one is the spray pyrolysis technique. This is very simple, commercially available, cost-effective for mass production with excellent control of chemical uniformity, and stoichiometry. The properties of spray-deposited TiO2 thin films depend on a type of precursor used, because of their thermal decomposition behavior. The precursors like titanyl acetyl acetonate (TiAcAc) [TiC10H14O5], isopropyl titanate [Ti (i-OC3H7)4], titanium tetrachloride (TiCl4), titanium (IV) isobutoxide [Ti ((CH3)2CHCH2O)], peroxo-titanium complex solution, etc. have so far been reported for the deposition of TiO2 thin films of good quality [14–21]. The titanium tetraisopropoxide (TTIP) has been used to prepare TiO2 thin films by spray pyrolysis [22]. Jung et al. [23] deposited titanium dioxide (TiO2) thin films using the sol-gel method and titanium (IV) iso-propoxide Ti{OCH(CH3)2}4 as precursor. It is found that Plasma treated TiO2 showed excellent photocatalytic degradation of phenol and toluene under UV light irradiation. Oh et al [24] synthesized the nanophase TiO2 thin films from (TTIP) by a sol-gel dip-coating method and studied the effect of calcination temperature on the photoactivity of the TiO2 films. Cleveland et al [25] deposited the TiO2 thin films using Atomic layer deposition (ALD) from (TTIP). Wang et al [26] reported the Titanium oxide nanoparticles using the low-pressure spray pyrolysis (LPSD) of titanium tetraisopropoxide (TTIP) and also provided the possible mechanism of particle formation in the LPSP process. Haugen et al [27] reported on the TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors. They studied the effect of precursor concentration by spray pyrolysis 2
technique and also studied the photocatalytic degradation oxidation of methylene blue under UVirradiation. Harra et al [28] synthesized iron oxide–titanium dioxide (-Fe2O3–TiO2) composite nanoparticles by spray pyrolysis process. It is reported that the composite nanoparticles were found as a magnetically separable photocatalyst. Conde-Gallardo et al [29] synthesized the TiO2 thin films using titanium di-isopropoxide as a precursor with spray pyrolysis method and found that, the growth process is closer to chemical vapor deposition than to the splashing mechanisms of spray pyrolysis technique. Conde-Gallardo et al [30] synthesized the TiO2 thin films by using the chemical vapor deposition method and titanium di-isopropoxide as precursor and established the relation between growth rate & surface properties. The attempts are going on to improve photoelectrochemical performance of TiO2 thin film for their photoelectrocatalytic properties. By using titanium isopropoxide precursor we have achieved relatively higher values of short circuit current and open circuit voltage than Titanyl acetyl acetate precursor so these films are of great importance in degradation of organic molecules [31]. In this paper the effect of substrate temperature onto the physicochemical properties has been discussed and the photoelectrocatalytic activity of TiO2 thin films was evaluated by photoelectrocatalytic degradation of benzoic acid under UV light illumination. 2. Experimental The A.R. grade titanium (IV) iso-propoxide Ti{OCH(CH3)2}4, was dissolved in ethanol at room temperature. The resulting 100 ml of 0.1M solution was sprayed onto cleaned corning glass substrates maintained at different substrate temperatures ranging from 400 °C, at an interval of 50 °C to 500 °C. The fine aerosols of the solution sprayed through an atomizer undergo pyrolytic decomposition onto the preheated glass substrates forming a TiO2 thin film. Other preparative parameters like, spray rate (4ml/min) and nozzle to substrate distance (32 cm) were kept constant for all the experiments. Photoelectrochemical (PEC) cell was fabricated using the two-electrode configuration system, containing TiO2 thin film as a photoanode and graphite as a counter electrode, with the 0.1 M NaOH as an electrolyte. The cell was illuminated with 20 W UV OMNILUX lamp with an excitation wavelength of 365 nm for the measurement of short circuit current (Isc) and open circuit voltage (Voc). The structural characterization of deposited TiO2 thin films was carried out, by analyzing the X-ray diffraction patterns obtained under Cu-K radiation from a Bruker D2 Phaser. Transmission spectra were recorded at room temperature using a UV-1800 Shimadzu, 3
UV spectrophotometer and the surface morphological characterization of the films was studied by using FE-SEM (Model: JSM-6701F, Japan). Raman-scattering experiments were performed in air at room temperature with micro Raman system from Jobin Yvon Horibra LABRAM-HR visible from 200-1400 cm-1 having excitation wavelength 532 nm for He-Ne laser source & at 600 and 1800 lines/mm gratings - Detector: CCD detectors were used. The chemical composition and valence states of constituent elements were analyzed by an X-ray Photoelectron Spectroscope (XPS, Physical Electronics PHI 5400, USA) with monochromatic Mg-K (1253.6 eV) x-ray beam. To study photoelectrocatalytic degradation experiment, large area (64 cm2) TiO2 thin film was deposited using a spray pyrolysis technique onto FTO coated glass substrates with sheet resistance of 10-20 /cm2. In single cell phototelectrochemical degradation experiment, TiO2 thin film was used as photoanode and stainless steel disc as a counter electrode placed at a distance of 0.1 cm facing to the photoanode. The active surface area of photoelectrode which is in contact with the pollutant species was 64 cm2. The 1mM benzoic acid in aqueous electrolyte was used as model pollutants in water for degradation studies under UV light illumination in the presence of TiO2 photocatalyst. A fixed amount of electrolyte, that contains the major part of it an external reservoir, was recirculated through the photoelectrochemical cells with a constant flow rate of 12.8 L/h using a Gilson MINIPLUS peristaltic pump, France has silicon tubing.
3. Results and discussion 3.1 Photoelectrochemical (PEC) studies PEC cell formed by typical TiO2 thin film deposited at 450 °C substrate temperature (FTO/TiO2/0.1M NaOH/C) as shown in Fig.1 (a). Under dark there is no change in current upto 1V applied bias but in case of illumination there is a sudden change in photocurrent with small applied bias. Within 0.1 to 1.0 V current graph shows plateau. The optimization of preparative parameters of good quality TiO2 thin films is carried out with PEC technique. These preparative parameters are optimized by taking relatively maximum values of short circuit current (Isc) and open circuit voltage (Voc) of the PEC cell formed with TiO2 working electrode. Fig. 1(b) shows the variation of short circuit current (Isc) and open circuit voltage (Voc) as a function of substrate temperature. The graph shows both Isc and Voc values increase with increase in substrate temperature and attain maximum values Isc = 1.7 mA and Voc = 770 mV for the film deposited at 4
450 °C temperature and then decrease for further increase in substrate temperature. The observed values of Isc & Voc for the PEC cell formed with TiO2 photoelectrode are relatively higher than the values reported earlier, in this case the higher values of Isc = 1.7 mA and Voc = 770 mV are obtained because of variation in the stoichiometry with respect to substrate temperature [30].
3.2 X-ray diffraction studies Fig. 2 shows the X-ray diffraction patterns of TiO2 thin films deposited at different substrate temperatures. The films are nanocrystalline in nature and fit with the tetragonal crystal structure with most intense (101) plane and match with JCPDS card No. 01-075-1537. Some weak reflections such as (004), (200), (211), (204) have also been observed. The reason for comparatively lower peak intensities is due to the lower film thickness, incomplete growth of film and formation of amorphous plus nanocrystalline phase in thin films. As the substrate temperature increases, the crystallinity of the films increases upto 450 °C. Further increase in substrate temperature decreases peak intensity and it is attributed to the lower thickness of the films. The crystallite size of the deposited thin films was calculated by using Scherrer’s formula
D
0.9O E cos T
(1)
Where, D is the crystallite size, is the broadening of the diffraction line measured at half of its maximum intensity (FWHM) and O is the X-ray wavelength (1.5405A). The average crystallite size increases from 40 to 50 nm up to 450 °C and then decrease for higher substrate temperature [31].
3.3 Morphological studies Fig. 3 (a-c) shows FE-SEM images of TiO2 thin films deposited at different substrate temperatures. FE-SEM micrograph of substrate temperatures 400 °C shows nanogranular like morphology, at substrate temperatures 450 °C it shows the nanoground nut like morphology and at substrate temperatures 500 °C it shows nanorod like morphology. Thus as substrate temperature increases the surface morphology changes from nanogranular to nanorod like morphology. So it enhances the large surface area of thin film and is thus useful for photoelectrocatalytic degradation study.
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3.4 Optical properties The optical absorption and transmission spectra of TiO2 films deposited at various substrate temperatures are shown in Fig. 4 (a) and (b) respectively. As the substrate temperature increases the transmittance goes on decreasing. The films show moderate optical transmittance between 60 to 80% at 550 nm. At higher temperature, due to increased defects in films, decrease in transmittance results due to scattering. Fig. 4 (b) shows the plots of (h)2 Vs h of TiO2 thin films deposited at different substrate temperatures. The band gap energy of films deposited at 400 °C is 3.34 eV, at 450 °C is 3.43 eV & at 500 °C is 3.39 eV. This value of band gap energy is slightly greater than the value of energy reported for single crystal TiO2 [32] and comparable with earlier reported values for spray deposited TiO2 thin films [33]. The observed difference in the band gap energy is due to stoichiometric differences in the TiO2 films deposited at various substrate temperatures.
3.5 Raman spectroscopic analysis Fig. 5 shows the Raman spectra of the typical TiO2 thin films deposited at optimized deposition conditions. According to factor group analysis, anatase TiO2 has five Raman active modes (3Eg+2B1g). It shows the slight Raman shift compared to literature [34]. The Raman spectrum of an anatase single crystal has been investigated by Ahti Niilisk, who concluded that the five allowed modes appear at 142 cm-1 (Eg), 197 cm-1 (Eg), 397 cm-1 (B1g), 518 cm-1 (B1g), and 634 cm-1 (Eg). In this study, we assigned and interpreted the Raman bands of the TiO2 using earlier results [35]. Raman bands shift towards higher wave number and their intensities relatively decrease as the particle size decreases. Thus, the observed shift is due to the effect of decreasing particle size.
3.6 X-ray photoelectron spectroscopy The elemental composition and valence states of constituent elements present in TiO2 material are analyzed by X-ray photoelectron spectroscopy. The typical survey scan spectrum of TiO2 thin films is shown in Fig. 6 (a). Photoelectron core peaks of the Ti, O and C elements were recorded. Fig. 6 (b) shows high resolution XPS spectra of the O1s core-level of titanium dioxide thin films. The binding energy of O1s can be fited with their curves appearing at 529.8 eV and 531.9 eV, which can be attributed to Ti-O (529.8 eV) and O-H (531.9 eV) components [36]. 6
Fig.6 (c) shows high resolution XPS spectra of the Ti 2p core level for TiO2 film. Due to spin orbit coupling, the Ti 2p core levels split into 2p1/2 and 2p3/2 components, observed at 464.3 eV and 458.5 eV respectively. The area ratio of these two peaks is equal to 0.5 and the binding energy difference is 5.58 eV, which is in good agreement with reported values attributed to the Ti4+ confirming the formation of the TiO2 compound [37].
3.7 Photocatalytic properties: Photoelectrocatalytic degradation of benzoic acid under UV light illumination is carried out to study the photocatalytic activity of TiO2 photocatalyst. The extinction spectra of benzoic acid solution during the degradation experiment under different reaction times are recorded in the wavelength range from 200 to 400 nm. Fig.7 (a) shows the extinction spectra of benzoic acid solution against reaction time with TiO2 photocatalyst under UV light illumination. During the course of the degradation experiments, the concentration of benzoic acid decreases due to its decomposition. The photocatalytic degradation follows a pseudo first order reaction and its kinetics can be expressed using relation [38],
§ c ln¨¨ © c0
· ¸¸ ¹
kt (3)
Where, t is the time, c the concentration of the solute or the concentration of oxidizable atoms in the organic species; c0 the initial concentration of solute, when COD is used. k can be taken as the first order rate constant of the degradation reaction, Also k is proportional to the area of the electrode. In order to obtain these external parameters, to make a comparison of experimental data obtained under various conditions. We define the equations [39], k ' kV ( cm 3 s 1 ) k' k" ( cm s 1 ) A kVF p k"' (M i ph
( 4) (5) 1
)
(6)
Where, V the volume, A is the area of the electrode, p or k’’’ the rate constant or kinetic parameter, F is Faraday's constant (96,500 mol1), total photocurrent iph, and total liquid volume. 7
The p reflects the efficiency of oxidative degradation of the solute. The plot of ln(c/c0) as a function of reaction time (kinetics of degradation) of benzoic acid is as shown in Fig.7 (b). The photocatalytic degradation of benzoic acid obeys first order kinetics (extinction spectra taken at 230 nm). The slope of this plot gives the rate constant (k). The first order rate constant value of k is found to be 1.97 × 104 s1. The COD value gives the extent of degradation of benzoic acid and is shown in Fig.7 (c). COD study as a function of reaction time gives the concentration of oxidizable matter left in the electrolyte solution. The COD values decrease from 54.3 to 24.0 mg L1 with reaction time. Similar type of work is done in earlier reports. [40-41] 4 Conclusions TiO2 thin films can be successfully synthesized using the chemical spray pyrolysis technique. The films are polycrystalline in nature with tetragonal crystal structure. FE-SEM study reveals that the film surface is covered with nanogranular, nanoground nut and nanorod like morphology thus it enhance the photoactivity of TiO2 electrode. During the photocatalytic degradation of benzoic acid, 3-hydroxybenzoic acids substituted intermediates were observed as initial products. Further OH radical attack on the 3-hydroxybenzoic acids intermediates leading to the formation of phenol. The further oxidation process of the intermediates compounds leds to ring opening followed by complete mineralization of the organic compound. The photoelectrodegradation process follows the pseudo-first order kinetics. Acknowledgement One of the authors (V. S. Mohite) is thankful to the UGC New Delhi, for the financial support through its UGC-Meritorious Fellowship.
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Figure captions Fig. 1 (a) Typical I-V characteristics (in dark and under illumination) of PEC cell formed with spray deposited TiO2 thin film deposited at 450 °C substrate temperature. (b) Variation of short circuit current (Isc) and open circuit voltage (Voc) verses substrate temperatures for PEC cell formed with sprayed TiO2 thin films. Fig. 2 XRD patterns of sprayed TiO2 thin films deposited at 400 °C, 450 °C and 500 °C substrate temperatures. Fig. 3 FE-SEM micrographs (a-c) of sprayed TiO2 thin films deposited at 400 °C, 450 °C and 500 ° C substrate temperatures. Fig. 4 (a) Optical absorption spectra of sprayed TiO2 thin films (b) Optical transmission spectra of sprayed TiO2 thin films. (c) Plot of (h)2 Vs h for sprayed TiO2 thin films. Fig. 5 Raman spectrum of typical sprayed TiO2 thin film deposited at 450 °C substrate temperature. Fig. 6 (a) Typical XPS survey scan spectra of TiO2 thin film deposited at 450 °C substrate temperature (b) Narrow scan XPS spectra of O 1s core level for TiO2 film (c) Narrow scan XPS spectra of Ti 2P core level for TiO2 thin film. Fig.7 (a) Extinction spectra for degradation of benzoic acid using TiO2 thin film (b) Kinetics of degradation (extinction taken at 230 nm) (c) Extent of mineralization by COD.
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Photoelectrocatalytic degradation of benzoic acid using sprayed TiO2 thin films V.S. Mohite, M.A. Mahadik, S.S. Kumbhar, V.P. Kothavale, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale
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PII: DOI: Reference:
S0272-8842(14)01545-4 http://dx.doi.org/10.1016/j.ceramint.2014.10.020 CERI9279
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
31 August 2014 27 September 2014 1 October 2014
Cite this article as: V.S. Mohite, M.A. Mahadik, S.S. Kumbhar, V.P. Kothavale, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale, Photoelectrocatalytic degradation of benzoic acid using sprayed TiO2 thin films, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2014.10.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoelectrocatalytic degradation of benzoic acid using sprayed TiO2 thin films V.S. Mohite, M. A. Mahadik, S. S. Kumbhar, V. P. Kothavale, A.V. Moholkar, K Y. Rajpure, C.H. Bhosale* Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Abstract Transparent TiO2 thin films have been successfully synthesized by chemical spray pyrolysis technique. The effect of substrate temperature on the photoelectrochemical (PEC), structural, morphological, optical and photoelectrocatalytic properties has been investigated. The PEC study shows that both short circuit current (Isc) and open circuit voltage (Voc) at the optimized substrate temperature (450 °C) are relatively maximum (Isc = 1.7 mA and Voc = 770 mV). The tetragonal crystal structure has been confirmed from X-ray diffraction patterns. FESEM study reveals that the film surface is changed from nanogranular to nanorod like morphology. The films exhibit a transmittance of about 80% in the visible region and a sharp absorption edge at 375 nm corresponding to the fundamental absorption edge in UV region. Band gap energy varies from 3.33 to 3.43 eV with substrate temperature. Electron-phonon coupling present in TiO2 films has been analyzed using Raman spectroscopy. The elemental composition and valence states of TiO2 film are studied by using X-ray photoelectron spectroscopy. The effect on the photoelectrocatalytic behavior of the large surface area (64 cm2) TiO2 photocatalyst samples were studied by using photoelectrocatalytic degradation of benzoic acid under UV light illumination. Keywords: Films; Structural applications; Optical microscopy; TiO2 Corresponding author: E-mail:[email protected] Tel.: +91 2312609435; Fax: +91 2312691533.
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1. Introduction TiO2 is a wide band gap, chemically stable and environmental eco-friendly semiconductor with good biocompatibility and stability. It occurs in three phases namely anatase (< 550 °C), rutile (> 550 °C) and brookite (>1200 °C). TiO2 film has the unique characteristics such as high optical transmittance over a wide wavelength range and excellent adhesion to the substrates. The TiO2 thin films are used in a variety of applications such as dye-sensitized solar cells, antireflection (AR) coatings, gas sensors, electrochromic displays, planar waveguides and photocatalytic activity [1-2]. Researchers have used several methods for deposition of the TiO2 thin films viz. sol gel method [3-5], chemical vapor deposition method [6, 7], evaporation method [8], sputtering method [9-11], pulsed laser deposition method [12], electrodeposition method [13] and spray pyrolysis method [14-21]. Amongst these all deposition methods, the most widely studied one is the spray pyrolysis technique. This is very simple, commercially available, cost-effective for mass production with excellent control of chemical uniformity, and stoichiometry. The properties of spray-deposited TiO2 thin films depend on a type of precursor used, because of their thermal decomposition behavior. The precursors like titanyl acetyl acetonate (TiAcAc) [TiC10H14O5], isopropyl titanate [Ti (i-OC3H7)4], titanium tetrachloride (TiCl4), titanium (IV) isobutoxide [Ti ((CH3)2CHCH2O)], peroxo-titanium complex solution, etc. have so far been reported for the deposition of TiO2 thin films of good quality [14–21]. The titanium tetraisopropoxide (TTIP) has been used to prepare TiO2 thin films by spray pyrolysis [22]. Jung et al. [23] deposited titanium dioxide (TiO2) thin films using the sol-gel method and titanium (IV) iso-propoxide Ti{OCH(CH3)2}4 as precursor. It is found that Plasma treated TiO2 showed excellent photocatalytic degradation of phenol and toluene under UV light irradiation. Oh et al [24] synthesized the nanophase TiO2 thin films from (TTIP) by a sol-gel dip-coating method and studied the effect of calcination temperature on the photoactivity of the TiO2 films. Cleveland et al [25] deposited the TiO2 thin films using Atomic layer deposition (ALD) from (TTIP). Wang et al [26] reported the Titanium oxide nanoparticles using the low-pressure spray pyrolysis (LPSD) of titanium tetraisopropoxide (TTIP) and also provided the possible mechanism of particle formation in the LPSP process. Haugen et al [27] reported on the TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors. They studied the effect of precursor concentration by spray pyrolysis 2
technique and also studied the photocatalytic degradation oxidation of methylene blue under UVirradiation. Harra et al [28] synthesized iron oxide–titanium dioxide (-Fe2O3–TiO2) composite nanoparticles by spray pyrolysis process. It is reported that the composite nanoparticles were found as a magnetically separable photocatalyst. Conde-Gallardo et al [29] synthesized the TiO2 thin films using titanium di-isopropoxide as a precursor with spray pyrolysis method and found that, the growth process is closer to chemical vapor deposition than to the splashing mechanisms of spray pyrolysis technique. Conde-Gallardo et al [30] synthesized the TiO2 thin films by using the chemical vapor deposition method and titanium di-isopropoxide as precursor and established the relation between growth rate & surface properties. The attempts are going on to improve photoelectrochemical performance of TiO2 thin film for their photoelectrocatalytic properties. By using titanium isopropoxide precursor we have achieved relatively higher values of short circuit current and open circuit voltage than Titanyl acetyl acetate precursor so these films are of great importance in degradation of organic molecules [31]. In this paper the effect of substrate temperature onto the physicochemical properties has been discussed and the photoelectrocatalytic activity of TiO2 thin films was evaluated by photoelectrocatalytic degradation of benzoic acid under UV light illumination. 2. Experimental The A.R. grade titanium (IV) iso-propoxide Ti{OCH(CH3)2}4, was dissolved in ethanol at room temperature. The resulting 100 ml of 0.1M solution was sprayed onto cleaned corning glass substrates maintained at different substrate temperatures ranging from 400 °C, at an interval of 50 °C to 500 °C. The fine aerosols of the solution sprayed through an atomizer undergo pyrolytic decomposition onto the preheated glass substrates forming a TiO2 thin film. Other preparative parameters like, spray rate (4ml/min) and nozzle to substrate distance (32 cm) were kept constant for all the experiments. Photoelectrochemical (PEC) cell was fabricated using the two-electrode configuration system, containing TiO2 thin film as a photoanode and graphite as a counter electrode, with the 0.1 M NaOH as an electrolyte. The cell was illuminated with 20 W UV OMNILUX lamp with an excitation wavelength of 365 nm for the measurement of short circuit current (Isc) and open circuit voltage (Voc). The structural characterization of deposited TiO2 thin films was carried out, by analyzing the X-ray diffraction patterns obtained under Cu-K radiation from a Bruker D2 Phaser. Transmission spectra were recorded at room temperature using a UV-1800 Shimadzu, 3
UV spectrophotometer and the surface morphological characterization of the films was studied by using FE-SEM (Model: JSM-6701F, Japan). Raman-scattering experiments were performed in air at room temperature with micro Raman system from Jobin Yvon Horibra LABRAM-HR visible from 200-1400 cm-1 having excitation wavelength 532 nm for He-Ne laser source & at 600 and 1800 lines/mm gratings - Detector: CCD detectors were used. The chemical composition and valence states of constituent elements were analyzed by an X-ray Photoelectron Spectroscope (XPS, Physical Electronics PHI 5400, USA) with monochromatic Mg-K (1253.6 eV) x-ray beam. To study photoelectrocatalytic degradation experiment, large area (64 cm2) TiO2 thin film was deposited using a spray pyrolysis technique onto FTO coated glass substrates with sheet resistance of 10-20 /cm2. In single cell phototelectrochemical degradation experiment, TiO2 thin film was used as photoanode and stainless steel disc as a counter electrode placed at a distance of 0.1 cm facing to the photoanode. The active surface area of photoelectrode which is in contact with the pollutant species was 64 cm2. The 1mM benzoic acid in aqueous electrolyte was used as model pollutants in water for degradation studies under UV light illumination in the presence of TiO2 photocatalyst. A fixed amount of electrolyte, that contains the major part of it an external reservoir, was recirculated through the photoelectrochemical cells with a constant flow rate of 12.8 L/h using a Gilson MINIPLUS peristaltic pump, France has silicon tubing.
3. Results and discussion 3.1 Photoelectrochemical (PEC) studies PEC cell formed by typical TiO2 thin film deposited at 450 °C substrate temperature (FTO/TiO2/0.1M NaOH/C) as shown in Fig.1 (a). Under dark there is no change in current upto 1V applied bias but in case of illumination there is a sudden change in photocurrent with small applied bias. Within 0.1 to 1.0 V current graph shows plateau. The optimization of preparative parameters of good quality TiO2 thin films is carried out with PEC technique. These preparative parameters are optimized by taking relatively maximum values of short circuit current (Isc) and open circuit voltage (Voc) of the PEC cell formed with TiO2 working electrode. Fig. 1(b) shows the variation of short circuit current (Isc) and open circuit voltage (Voc) as a function of substrate temperature. The graph shows both Isc and Voc values increase with increase in substrate temperature and attain maximum values Isc = 1.7 mA and Voc = 770 mV for the film deposited at 4
450 °C temperature and then decrease for further increase in substrate temperature. The observed values of Isc & Voc for the PEC cell formed with TiO2 photoelectrode are relatively higher than the values reported earlier, in this case the higher values of Isc = 1.7 mA and Voc = 770 mV are obtained because of variation in the stoichiometry with respect to substrate temperature [30].
3.2 X-ray diffraction studies Fig. 2 shows the X-ray diffraction patterns of TiO2 thin films deposited at different substrate temperatures. The films are nanocrystalline in nature and fit with the tetragonal crystal structure with most intense (101) plane and match with JCPDS card No. 01-075-1537. Some weak reflections such as (004), (200), (211), (204) have also been observed. The reason for comparatively lower peak intensities is due to the lower film thickness, incomplete growth of film and formation of amorphous plus nanocrystalline phase in thin films. As the substrate temperature increases, the crystallinity of the films increases upto 450 °C. Further increase in substrate temperature decreases peak intensity and it is attributed to the lower thickness of the films. The crystallite size of the deposited thin films was calculated by using Scherrer’s formula
D
0.9O E cos T
(1)
Where, D is the crystallite size, is the broadening of the diffraction line measured at half of its maximum intensity (FWHM) and O is the X-ray wavelength (1.5405A). The average crystallite size increases from 40 to 50 nm up to 450 °C and then decrease for higher substrate temperature [31].
3.3 Morphological studies Fig. 3 (a-c) shows FE-SEM images of TiO2 thin films deposited at different substrate temperatures. FE-SEM micrograph of substrate temperatures 400 °C shows nanogranular like morphology, at substrate temperatures 450 °C it shows the nanoground nut like morphology and at substrate temperatures 500 °C it shows nanorod like morphology. Thus as substrate temperature increases the surface morphology changes from nanogranular to nanorod like morphology. So it enhances the large surface area of thin film and is thus useful for photoelectrocatalytic degradation study.
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3.4 Optical properties The optical absorption and transmission spectra of TiO2 films deposited at various substrate temperatures are shown in Fig. 4 (a) and (b) respectively. As the substrate temperature increases the transmittance goes on decreasing. The films show moderate optical transmittance between 60 to 80% at 550 nm. At higher temperature, due to increased defects in films, decrease in transmittance results due to scattering. Fig. 4 (b) shows the plots of (h)2 Vs h of TiO2 thin films deposited at different substrate temperatures. The band gap energy of films deposited at 400 °C is 3.34 eV, at 450 °C is 3.43 eV & at 500 °C is 3.39 eV. This value of band gap energy is slightly greater than the value of energy reported for single crystal TiO2 [32] and comparable with earlier reported values for spray deposited TiO2 thin films [33]. The observed difference in the band gap energy is due to stoichiometric differences in the TiO2 films deposited at various substrate temperatures.
3.5 Raman spectroscopic analysis Fig. 5 shows the Raman spectra of the typical TiO2 thin films deposited at optimized deposition conditions. According to factor group analysis, anatase TiO2 has five Raman active modes (3Eg+2B1g). It shows the slight Raman shift compared to literature [34]. The Raman spectrum of an anatase single crystal has been investigated by Ahti Niilisk, who concluded that the five allowed modes appear at 142 cm-1 (Eg), 197 cm-1 (Eg), 397 cm-1 (B1g), 518 cm-1 (B1g), and 634 cm-1 (Eg). In this study, we assigned and interpreted the Raman bands of the TiO2 using earlier results [35]. Raman bands shift towards higher wave number and their intensities relatively decrease as the particle size decreases. Thus, the observed shift is due to the effect of decreasing particle size.
3.6 X-ray photoelectron spectroscopy The elemental composition and valence states of constituent elements present in TiO2 material are analyzed by X-ray photoelectron spectroscopy. The typical survey scan spectrum of TiO2 thin films is shown in Fig. 6 (a). Photoelectron core peaks of the Ti, O and C elements were recorded. Fig. 6 (b) shows high resolution XPS spectra of the O1s core-level of titanium dioxide thin films. The binding energy of O1s can be fited with their curves appearing at 529.8 eV and 531.9 eV, which can be attributed to Ti-O (529.8 eV) and O-H (531.9 eV) components [36]. 6
Fig.6 (c) shows high resolution XPS spectra of the Ti 2p core level for TiO2 film. Due to spin orbit coupling, the Ti 2p core levels split into 2p1/2 and 2p3/2 components, observed at 464.3 eV and 458.5 eV respectively. The area ratio of these two peaks is equal to 0.5 and the binding energy difference is 5.58 eV, which is in good agreement with reported values attributed to the Ti4+ confirming the formation of the TiO2 compound [37].
3.7 Photocatalytic properties: Photoelectrocatalytic degradation of benzoic acid under UV light illumination is carried out to study the photocatalytic activity of TiO2 photocatalyst. The extinction spectra of benzoic acid solution during the degradation experiment under different reaction times are recorded in the wavelength range from 200 to 400 nm. Fig.7 (a) shows the extinction spectra of benzoic acid solution against reaction time with TiO2 photocatalyst under UV light illumination. During the course of the degradation experiments, the concentration of benzoic acid decreases due to its decomposition. The photocatalytic degradation follows a pseudo first order reaction and its kinetics can be expressed using relation [38],
§ c ln¨¨ © c0
· ¸¸ ¹
kt (3)
Where, t is the time, c the concentration of the solute or the concentration of oxidizable atoms in the organic species; c0 the initial concentration of solute, when COD is used. k can be taken as the first order rate constant of the degradation reaction, Also k is proportional to the area of the electrode. In order to obtain these external parameters, to make a comparison of experimental data obtained under various conditions. We define the equations [39], k ' kV ( cm 3 s 1 ) k' k" ( cm s 1 ) A kVF p k"' (M i ph
( 4) (5) 1
)
(6)
Where, V the volume, A is the area of the electrode, p or k’’’ the rate constant or kinetic parameter, F is Faraday's constant (96,500 mol1), total photocurrent iph, and total liquid volume. 7
The p reflects the efficiency of oxidative degradation of the solute. The plot of ln(c/c0) as a function of reaction time (kinetics of degradation) of benzoic acid is as shown in Fig.7 (b). The photocatalytic degradation of benzoic acid obeys first order kinetics (extinction spectra taken at 230 nm). The slope of this plot gives the rate constant (k). The first order rate constant value of k is found to be 1.97 × 104 s1. The COD value gives the extent of degradation of benzoic acid and is shown in Fig.7 (c). COD study as a function of reaction time gives the concentration of oxidizable matter left in the electrolyte solution. The COD values decrease from 54.3 to 24.0 mg L1 with reaction time. Similar type of work is done in earlier reports. [40-41] 4 Conclusions TiO2 thin films can be successfully synthesized using the chemical spray pyrolysis technique. The films are polycrystalline in nature with tetragonal crystal structure. FE-SEM study reveals that the film surface is covered with nanogranular, nanoground nut and nanorod like morphology thus it enhance the photoactivity of TiO2 electrode. During the photocatalytic degradation of benzoic acid, 3-hydroxybenzoic acids substituted intermediates were observed as initial products. Further OH radical attack on the 3-hydroxybenzoic acids intermediates leading to the formation of phenol. The further oxidation process of the intermediates compounds leds to ring opening followed by complete mineralization of the organic compound. The photoelectrodegradation process follows the pseudo-first order kinetics. Acknowledgement One of the authors (V. S. Mohite) is thankful to the UGC New Delhi, for the financial support through its UGC-Meritorious Fellowship.
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Figure captions Fig. 1 (a) Typical I-V characteristics (in dark and under illumination) of PEC cell formed with spray deposited TiO2 thin film deposited at 450 °C substrate temperature. (b) Variation of short circuit current (Isc) and open circuit voltage (Voc) verses substrate temperatures for PEC cell formed with sprayed TiO2 thin films. Fig. 2 XRD patterns of sprayed TiO2 thin films deposited at 400 °C, 450 °C and 500 °C substrate temperatures. Fig. 3 FE-SEM micrographs (a-c) of sprayed TiO2 thin films deposited at 400 °C, 450 °C and 500 ° C substrate temperatures. Fig. 4 (a) Optical absorption spectra of sprayed TiO2 thin films (b) Optical transmission spectra of sprayed TiO2 thin films. (c) Plot of (h)2 Vs h for sprayed TiO2 thin films. Fig. 5 Raman spectrum of typical sprayed TiO2 thin film deposited at 450 °C substrate temperature. Fig. 6 (a) Typical XPS survey scan spectra of TiO2 thin film deposited at 450 °C substrate temperature (b) Narrow scan XPS spectra of O 1s core level for TiO2 film (c) Narrow scan XPS spectra of Ti 2P core level for TiO2 thin film. Fig.7 (a) Extinction spectra for degradation of benzoic acid using TiO2 thin film (b) Kinetics of degradation (extinction taken at 230 nm) (c) Extent of mineralization by COD.
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