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Photoelectrocatalytic degradation of benzoic acid using Au doped TiO2 thin films
Accepted Manuscript Photoelectrocatalytic degradation of benzoic acid using Au doped TiO2 thin films V.S. Mohite, M.A. Mahadik, S.S. Kumbhar, Y.M. Hunge, J.H. Kim, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale PII: DOI: Reference:
S1011-1344(14)00360-1 http://dx.doi.org/10.1016/j.jphotobiol.2014.12.004 JPB 9885
To appear in:
Journal of Photochemistry and Photobiology B: Biology
Received Date: Revised Date: Accepted Date:
3 September 2014 13 November 2014 1 December 2014
Please cite this article as: V.S. Mohite, M.A. Mahadik, S.S. Kumbhar, Y.M. Hunge, J.H. Kim, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale, Photoelectrocatalytic degradation of benzoic acid using Au doped TiO2 thin films, Journal of Photochemistry and Photobiology B: Biology (2014), doi: http://dx.doi.org/10.1016/j.jphotobiol.2014.12.004
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Photoelectrocatalytic degradation of benzoic acid using Au doped TiO2 thin films V.S. Mohite a, M. A. Mahadik a, S. S. Kumbhar a, Y. M. Hunge a, J. H. Kim b, A.V. Moholkar a, K. Y. Rajpure a, C.H. Bhosale a* a.
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India.
b.
Department of Materials Science and Engineering, Chonnam National University, 300 Yong bong-Dong, Puk-Gu, Gwangju 500-757, South Korea.
Abstract Highly transparent pure and Au doped TiO2 thin films are successfully deposited by using simple chemical spray pyrolysis technique. The effect of Au doping onto the structural and physicochemical properties has been investigated. The PEC study shows that, both short circuit current (Isc) and open circuit voltage (Voc) are (Isc = 1.81 mA and Voc = 890 mV) relatively higher at 3 at % Au doping percentage. XRD study shows that the films are nanocrystalline in nature with tetragonal crystal structure. FESEM images show that the film surface covered with a smooth, uniform, compact and rice shaped nanoparticles. The Au doped thin films exhibit indirect band gap, decreases from 3.23 to 3.09 eV with increase in Au doping. The chemical composition and valence states of pure and Au doped TiO2 films are studied by using X-ray photoelectron spectroscopy. The photocatalytic degradation effect is 49% higher in case 3 at % Au doped TiO2 than the pure TiO2 thin film photoelectrodes in the degradation of benzoic acid. It is revealed that Au doped TiO2 can be reused for five cycles of experiments without a requirement of post-treatment while the degradation efficiency was retained.
Keywords: Spray Pyrolysis; Titanium dioxide; PEC; Structural; XPS spectroscopy Corresponding author: E-mail: [email protected] Tel.: +91 2312609435; Fax: +91 2312691533.
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1. Introduction TiO2 is the most important material as a catalyst for photoelectrochemical purification of wastewater because of its high oxidation potential in its valence band and high chemical stability [1, 2]. TiO2 material has been attracting a great deal of attention amongst researchers because of its unique properties such as high optical transparency, wide band gap energy, high refractive index, high dielectric constant, non-toxicity, abundance in nature and good chemical stability in undesirable environment conditions [3, 4]. Researchers have employed several methods for depositing the TiO2 thin films like sol gel [5-7], chemical vapor deposition [8, 9], evaporation [10], sputtering [11-13], pulsed laser deposition [14], electrodeposition [15], and spray pyrolysis [16]. Out of these deposition techniques, the spray pyrolysis technique is frequently used because of its simplicity, commercial viability, potential for cost effective mass production, excellent control of chemical uniformity, and stoichiometry of thin films. It has been found that TiO2 in anatase crystalline phase is most active photocatalyst. Nevertheless, it has wide band gap energy of 3.2 eV, normal anatase phase TiO2 needs a UV radiation to initiate its photoactivity. Many attempts have been made to extend the light absorption edge of the catalyst to the visible light region by doping gold nanoparticles into TiO2 and to improve its photocatalytic degradation efficiency [17-19]. Noble metal like Au has an ability of producing the highest Schottky barrier potential among the all metals [20]. The Au doped TiO2 thin films are chosen for environmental protection due to their high surface to volume ratio, because in the photocatalytic process the chemical reactions occur at the surface of the catalyst material. Therefore noble metal ion doping enhances the rate of photocatalytic process [21]. Recent studies on gold titanium nanocomposite particles show that metal ion doping extended the response of the photocatalyst into the visible region [22]. The photocatalytic properties of Au based materials depend on the preparation method and particularly on the shape and size of the Au clusters [23] The benzoic acid is used as a model pollutant because it is mainly used as a preservative in food, cosmetics and biological fields. It is suitable organic molecule for studying the photocatalytic degradation of more complex water pollutants with their properties, initially undergoes oxidation leading to the formation phenol and then other intermediates compounds leads to ring opening followed by mineralization of organic compounds [40-41]. So, it is used as the model organic impurity in the photoelectrocatalytic degradation reaction. 2
In this paper the effect of Au doping on the structural, optical, morphological and photoelectrocatalytic properties is investigated. The main objective of this work is to shift the band gap energy of TiO2 in the visible region by Au doping. To study the photocatalytic activity of this photoelectrode for degradation of benzoic acid and study the Chemical Oxygen Demand. 2. Experimental Spray pyrolysis technique: The basic principle involved in spray pyrolysis technique is pyrolytic decomposition of salts of a desired compound to be deposited. Fig. 1 shows the schematic diagram of the spray pyrolysis technique. It mainly consists of spray nozzle, rotor for spray nozzle, liquid level monitor, hot plate, gas regulator value and air tight fiber chamber. In spray pyrolysis, the process parameters like precursor solution, atomization of precursor solution, aerosol transport and decomposition of precursor are very important while studying the structural, optical, morphology and crystallinity of the thin films. Once the sprayed droplet reaching on the surface of the hot substrate undergoes pyrolytic decomposition and forms a single crystalline or cluster of crystallites as a final product. The other volatile by-products and solvents escape in the vapor phase. The substrates provide thermal energy for the thermal decomposition and subsequent recombination of the constituent species, followed by sintering and crystallization of the clusters of crystallites and thereby resulting in coherent film. The required thermal energy is different for the different materials and for the different solvents used in the spray process. The atomization of the spray solution into a spray of fine droplets also depends on the geometry of the spraying nozzle and pressure of a carrier gas. The film thickness depends upon the distance between the spray nozzle and substrate, substrate temperature, the concentration of the precursor solution and the quantity of the precursor solution sprayed [24]. Au doped TiO2 thin films were prepared by using simple, cheap chemical spray pyrolysis technique in a non-aqueous medium onto the FTO (10-20 Ω/cm2)) coated glass substrates. The initial ingredients used were titanium (IV) iso-propoxide Ti {OCH (CH3)2}4 and chloroauric acid (HAuCl4.3H2O). The ( 0.1 M) titanium (IV) iso-propoxide Ti {OCH (CH3)2}4 was dissolved in ethanol at room temperature and then (0.1 M) chloroauric acid solution was mixed into titanium (IV) iso-propoxide solution to achieve the different Au doping percentages. The [Au]/[Ti] ratio calculated on atomic percent used in the starting solution were 1 at %, 2 at %, 3 at %, and 4 at %. The resulting 100 cc precursor solution was sprayed through specially designed glass nozzles 3
onto the preheated glass substrates held at an optimized substrate temperature of 450 °C. The compressed air was used as the carrier gas at a constant spray rate of 4 cc min-1. While varying the doping percentage, other optimized parameters such as solution concentration (0.1 M), the quantity of solution (100 cc), nozzle to substrate distance (32 cm) were kept constant throughout the experiment. Photoelectrochemical cell was fabricated using the two-electrode configuration system, comprising TiO2 thin film as a photoanode and graphite as a counter electrode, with 0.1 M NaOH used as an electrolyte. The cell was illuminated with 20W 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 deposit thin films was carried out, by analyzing the X-ray diffraction patterns obtained under Cu-Kα radiation from a Bruker D2 Phaser. The surface morphological characterization of the films was studied by using FE-SEM (Model: JSM-6701F, Japan). Transmission spectra were recorded at room temperature using a UV-1800 Shimadzu, UV spectrophotometer within wavelength range 300-1100 nm. Valence states of constituent elements were analyzed by an X-ray Photoelectron Spectroscopy (XPS, Physical Electronics PHI 5400, USA) with monochromatic Mg-Kα (1253.6 eV) X-ray beam. Single cell reactor: The Au doped TiO2 electrode used in photoelectrocatlytic degradation experiments were deposited by spray pyrolysis onto large area (10 cm ×10 cm) conducting glass substrate (spray deposited fluorine doped tin oxide on glass, FTO, with sheet resistance of 10–20 Ω/cm2). In single cell phototelectrochemical degradation experiment, large area 3 at % Au doped 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 photoelectrode was illuminated from the backside using UV light. The 1mM concentration of benzoic acid is used as model pollutants in water for degradation studies under UV light illumination in the presence of 3 at % Au doped TiO2 photocatalyst. A fixed amount of electrolyte, that is major part of which contains an external reservoir, was recirculated through the photoelectrochemical cell with a constant flow rate of 12.8 L/h
−1
using a Gilson MINIPLUS peristaltic pump, France with silicon tubing [25]. Using
aliquots withdrawn from the reaction mixture at some intervals, the concentrations of organic impurities in the solutions were determined by measuring the UV–Vis absorbance (extinction) using a UV-1800 Shimadzu, UV spectrophotometer (for the measurement of full spectra). The 4
absorbance (extinction) was measured in 1 cm quartz cell at particular wavelength (where maximum absorption peak is observed) for various impurities. Aliquots extracted from the solutions at various intervals during the degradation reaction were also used for determining chemical oxygen demand (COD) using the standard method of oxidation with an excess of dichromate in concentrated sulphuric acid by digestion at 140 °C. The concentration of the organic solute was calculated from the dichromate extinction at various wavelengths [26]. 3. Results and discussion 3.1 Photoelectrochemical (PEC) studies The PEC characterization is a simple and unique technique to optimize preparative parameters of semiconductor thin films [27, 28]. Here, optimization of Au doping percentage is done with PEC technique. The glass/FTO/Au doped TiO2 substrate as a working electrode and graphite as a counter electrode were immersed in a 0.1 M NaOH solution and the values of short circuit current Isc and open circuit voltage Voc are measured. The Au doped TiO2 thin films exhibited better photoresponse than that of pure TiO2 films. The Isc and Voc values for pure TiO2 thin film sample are (Isc= 1.7 mA and Voc= 770 mV). It is seen that short circuit current (Isc) and open circuit voltage (Voc) are function of Au doping percentage (as shown in Fig.2). The values of Isc and Voc increases with increase in Au doping percentage, attains the relatively maximum values (Isc= 1.81 mA and Voc= 890 mV) at 3 at % and then decrease for further increase in Au doping percentage. The higher values of Isc and Voc values are due to the stoichiometry of the Au doped TiO2 film at optimized condition [29]. 3.2 X-ray diffraction studies Fig.3 shows the X-ray diffraction patterns of pure and Au doped TiO2 thin films deposited at different (1 at %, 2 at %, 3 at % and 4 at %) doping percentages. The films are nanocrystalline with tetragonal crystal structure. The planes match well with the Joint Committee for Powder Diffraction Standards (JCPDS card No. 01-075-1537). When the doping percentage increases, the intensity of (101) and (103) peak increases slightly up to 3 at % and then decreases for higher doping percentage. This is due to the incorporation of Au into the TiO2 films which is able to create more defects in the lattice [30]. Nevertheless, the position of (101) peak is slightly shifted toward lower angle due to atomic radius of Au is greater than that of O and smaller than
5
the Ti, it suggests that Au is substituted on O sites. The crystallite size of the deposited Au doped TiO2 thin films was calculated by using Scherer’s formula as; D=
0.9λ β cos θ
(1)
where, D is the crystallite size, β is the broadening of the diffraction line measured full width at half of its maximum intensity (FWHM) and λ is the X-ray wavelength (1.5405A˚). The crystallite size for 3 at % Au doped TiO2 thin film is 80 nm. 3.3 Morphological study Fig. 4 (a–e) shows the FE-SEM images of Au doped (0 – 4 at % doping percentage) titanium dioxide thin films. The micrographs show compact and homogeneous distribution of grains (rice shaped morphology) with varying doping percentage. The pure titanium dioxide sample show the highly dense, compact and homogeneous distributed. As the doping percentages increases from 1 to 4 at % the grain size is increases upto 3 at % Au doped TiO2 thin film and then decreases for higher doping percentages. The FESEM micrographs looks like a rice shaped morphology effectively increases the surface area of a film and may helpful for enhance the photocatalytic activity. This may be due to it has the well crystallized anatase phase and the high adsorption ability. 3.4 Optical properties The optical transmission spectra of Au doped TiO2 films deposited at various doping percentages is shown in Fig. 5 (a). The transmittance increase upto 3 at % and then decreases for higher doping percentages. The decrease in transmittance at higher doping percentages may be due to the increased scattering of photons by crystal defects formed due to gold doping. The free charge carrier absorption of photons may also contribute to the reduction in optical transmittance [31]. Fig. 5 (b) shows the plots of (αhυ)1/2 Vs hυ for Au doped TiO2 thin films deposited at different doping percentages. The thickness for 1, 2, 3 and 4 at % Au doped TiO2 thin films are 280 nm, 290 nm, 295 nm and 285 nm respectively. The indirect band gap energy decreases from 3.23 to 3.09 eV. It is comparable to the value of 3.23 eV for anatase phase [32-33],[18].
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3.5 X-ray Photoelectron Spectroscopy studies The chemical composition and valence states of constituent elements present in the studied films are analyzed by X-ray photoelectron spectroscopy. The survey scan spectra of pure and 3 at % Au doped TiO2 thin films are shown in Fig. 6. The elements Ti and O are detected from the surface of TiO2 film. The C signal comes from the surface adsorbed carbon dioxide during the sample preparation. Fig.6 (a-b) shows high resolution XPS spectra of the O1s corelevel of pure and 3 at % Au doped titanium dioxide thin films. The binding energy of O1s can be fitted with their curves having peaks appearing at 529.8 eV and 531.9 eV, which can be attributed to the Ti-O (529.8 eV) and O-H (531.9 eV) components. Frequently the O1s peak has been seen in the measured binding energy region of 529–535 eV and for chemisorbed oxygen on the metal surface, binding energy is obtained in the region 530–531 eV, therefore, the binding energy component centered at 530 eV of the O1s spectra is attributed due to chemisorbed oxygen [34]. Fig. 6 (c–d) shows high resolution XPS spectra of the Ti 2p core level as a function of the binding energy of pure and 3 at % Au doped TiO2 film. Due to spin–orbit coupling, the Ti 2p core levels split into 2p1/2 and 2p3/2 components. The binding energies of Ti 2p3/2 and the Ti 2p1/2 peaks are observed at 458.5 eV and 464.3 eV respectively. The binding energy difference is 5.58 eV, which is in good agreement with the value reported for Ti4+, confirming the formation of the TiO2 compound in the studied films [35]. 3.6 Raman spectroscopic analysis Raman spectroscopy is useful for determination of vibrational, rotational and other type of low frequency modes present in a system. Fig. 7 illustrates room temperature Raman spectra of TiO2 and typical 3 at % Au doped TiO2 thin films. According to factor group analysis, anatase phase TiO2 films have five Raman active modes (3Eg+2B1g). Five allowed modes for an anatase single crystal appeared at 142 cm-1 (Eg), 197 cm-1 (Eg), 397 cm-1 (B1g), 518 cm-1 (B1g), and 634 cm-1 (Eg) [36-37]. No other higher order peaks are observed in the region above 1200 cm-1. The all peaks match with those reported in literature and thus it confirm the formation of pure anatase phase of TiO2. 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 Au doping. The absorption of peaks in the higher frequency region becomes weaker and broader in the case
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of the Au doped TiO2 sample. This broadening suggests an increase in crystalline defects in the framework after gold incorporation. 3.7 Photocatalytic properties Photocatalytic degradation of benzoic acid under UV light illumination is carried out to study the photocatalytic activity of Au doped TiO2 photocatalyst. The extinction spectra of benzoic acid solution during the degradation experiment under different reaction time are recorded in the wavelength range from 200 to 350 nm. Fig. 8 (a) shows the extinction spectra of benzoic acid solution against wavelength. The percentage of benzoic acid decreases with time due to its photooxidative degradation. The photocatalytic degradation follows a pseudo first order reaction and its kinetics can be expressed using relation [38].
c ln c0
= −kt
(3)
where, t is the time, c stands for the percentage of the solute or the percentage of oxidizable atoms in the organic species; c0 the initial percentage of solute. k can be taken as the apparent first order rate constant of the degradation reaction. Moreover, k is proportional to the area of the electrode and therefore to the photocurrent. In order to make a comparison of experimental data obtained under various conditions and that of external parameters, it is useful to define [39]. k ' = kV ( cm k ' k " = ( cm A p = k " '=
3
s
s kVF i ph
− 1
−1
)
(4 )
)
(5 ) (M
−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 mol−1). The p reflects the efficiency of oxidative degradation of the solute. During the degradation experiments, due to its photoelectrocatalytic oxidation, the percentage of benzoic acid decreases as a function of time. The plot of ln(Abs/Abs0) as a function of reaction time (kinetics of degradation) of benzoic acid is as shown
8
in Fig. 8 (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), to be 2.05 × 10−5 s−1. The COD value gives the extent of degradation of benzoic acid and is shown in Fig. 8 (c). COD study as a function of reaction time provides the percentage of oxidizable matter left in the electrolyte solution. The COD values decrease from 46.3 to 24.5 mg L−1 with reaction time. Thus it shows 49 % degradation of benzoic acid i.e. greater than pure TiO2 thin films, it shows 37 % degradation of benzoic acid confirming mineralization of benzoic acid, the observed decay constants indicate the annihilation of main pollutant in the water.Similar type of degradation of benzoic acid is done in earlier reports [40-41]. As, we dope Au into TiO2 films it extend the response of the photocatalyst into the visible region and increases the photocatalytic degradation. The mechanism of degradation of benzoic acid:
When light is incident on the TiO2 semiconductor with energy greater than the band gap energy of the semiconductor then and only then it generates valence band holes (hvb+) and conduction band electrons (ecb−). Due to this wide gap energy, TiO2 can be activated by UV light below 385 nm. Further holes and electrons are recombine and liberate heat or make their separate ways to the surface of TiO2 semiconductor, where they can react with species adsorbed on the catalyst surface of TiO2 semiconductor. Valence band holes can react with water and the hydroxide ion to form hydroxyl radicals HO•, while electrons react with adsorbed molecular oxygen reducing it to superoxide radical anion which reacts with protons to form peroxide radicals as shown follow [42]. hvb+ + H 2 O → HO • + H +
(1)
hvb+ + OH − → HO •
(2)
− eCB + O2 → O2• −
O2•− + H + → HO2•
(3) (4)
Organic compounds can then undergo oxidative degradation through their reactions with valence band holes, hydroxyl and peroxide radicals and reductive cleavage through their 9
reactions with electrons yielding various by-products and finally mineral end-products. The proposed possible reaction mechanism of degradation pathway of benzoic acid has been schematically presented in Fig. 1.When benzoic acid react with hydroxyl radical (HO• ) to form 3- hydroxy benzoic acid and immediately loose the CO2 and H2O molecule to form phenol. Then the phenol is reacts with hydroxyl radical (HO• ) with reduction reaction to give cyclohexanol as product. Then further cyclohexanol reacts with oxidizing agent to give cyclohexanone and some several degradation products as mentioned in Fig. 9. It is revealed that Au doped TiO2 can be reused for five cycles of experiments without a requirement of post-treatment while the degradation efficiency was retained. 4. Conclusions
Pure and Au doped TiO2 thin films can be successfully synthesized using simple chemical spray pyrolysis technique. The films are nanocrystalline in nature with tetragonal crystal structure. The indirect optical band gap energy decreases from 3.23 to 3.09 eV. The micrographs show compact and homogeneous distribution of grains (rice shaped morphology) with varying grain size. The rice shaped morphology enhances the photoactivity of Au doped TiO2 films. The 3 at % Au doped TiO2 electrode shows 49 % photocatalytic degradation of benzoic acid under UV light illumination. 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 UGC-Meritorious Fellowship.
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Figure captions Fig. 1 Schematic set-up for spray pyrolysis technique Fig. 2 Variations of Isc and Voc against doping percentage (0- 4 %) for Au-doped TiO2 thin films.
(on FTO)/0.1NaOH/C PEC solar cell. Fig. 3 X-ray diffraction patterns of pure and (1- 4 %) Au doped TiO2 thin films. Fig. 4 (a) FE-SEM images of pure TiO2 thin film, (b) 1 at % Au doped TiO2 thin film (c) 2 at %
Au doped TiO2 thin film (d) 3 at % Au doped TiO2 thin film (e) 4 at % Au doped TiO2 thin film Fig. 5 (a) Optical transmittance spectra of Au doped TiO2 thin films (b) Plot of (αhυ)1/2 Vs hυ of
Au doped TiO2 thin films. Fig. 6 Survey scan XPS spectra of pure and 3 at % Au doped TiO2 thin films. (a) Narrow scan XPS spectra of O 1s core level for TiO2 film (b) Narrow scan XPS spectra of O
1s core level of 3 at % Au: TiO2 film. (c) Narrow scan XPS spectra of the Ti 2P region for TiO2 thin films. (d) Narrow scan XPS spectra of Ti 2P region for 3 at % au doped TiO2 thin films. Fig. 7 Room temperature Raman spectra of pure and 3 at % Au doped TiO2 thin films. Fig. 8 (a) Extinction spectra for degradation of benzoic acid using 3 at % Au doped TiO2 thin
film. (b) Kinetics of degradation (extinction taken at 230 nm). (c) Extent of mineralization by COD. Fig.9 A schematic representation of possible reaction mechanism of degradation of benzoic acid.
15
Fig. 1
16
900
1.80
880
1.78
860 840
1.76
820
1.74
800 1.72 780 1.70 760 0
1
2
3
Au doping percentage (at %) Fig. 2
17
4
Voc (mV)
Isc (mA)
1.82
4at % Au:TiO2
Intensity (counts)
3at % Au:TiO2 (101) (103)
2at % Au:TiO2 1at % Au:TiO2
0at % Au:TiO2 20
30
40
50
2θ (degree) Fig. 3
18
60
70
80
(a)
(b)
(c)
(e)
(d)
Fig 4
19
100 90
Transmittance (%)
80 70 60 50
(a)
(b)
(c)
(d)
40 30 20 10 0 300
400
500
600
Wavelength (nm) 5 (a)
20
700
800
2.0 1.8
1/2
7
(α hv) , 10 (eV/cm)
2
1.6 1.4 (a)
1.2 1.0
(c)
(b)
(d)
0.8 0.6 0.4 0.2 0.0 3.0
3.1
3.2
hυ (eV) 5 (b)
21
3.3
3.4
O 1s
Ti3s Ti3p O2s
C 1s
5
Intensity 10 (cps)
10
3 at % Au:TiO2 Ti2p
Au4p
Ti2s
O KLL
12
TiLMM
14
8 6 TiO2
4 2 0 1200
1000
800
600
400
Binding energy (eV) Fig. 6
22
200
0
1.6
(a)
01s a
TiO2 O1s b
1.2
5
Intensity 10 (cps)
1.4
O1s
O1s c
1.0 0.8 0.6 0.4 536
534
532
530
528
Binding energy (eV) Fig. 6 (a)
23
526
2.0 1.8
5
Intensity, 10 (cps)
01s a
3at % Au :TiO2
(b) O1s b
O1s
1.6 1.4 O1s c
1.2 1.0 0.8 0.6 0.4 536
534
532
530
528
Binding energy (eV) Fig. 6 (b)
24
526
1.2 Ti 2p
5
Intensity, 10 (cps)
1.0
2p3/2
(c)
458.5
TiO2
0.8
2p1/2
0.6
5.58 eV
464.3
0.4
0.2 470
460.88
468
466
464
462
460
Binding energy (eV) Fig. 6 (c)
25
458
456
1.6 (d)
Ti 2p
2p3/2
5
Intensity, 10 (cps)
1.4 1.2
458.5 3at % Au:TiO2
1.0 0.8
2p1/2
5.58 eV
464.3
0.6
460.88
0.4 0.2 470
468
466
464
462
460
Binding energy (eV) Fig. 6 (d)
26
458
456
8000
-1
142 cm (Eg)
Intensity (CPS)
7000 6000
TiO2
5000 -1
4000
-1 634 cm 397 cm 518 cm (Eg) (B1g) (B1g) -1
-1
197 cm (Eg)
3000 2000
3at % Au:TiO2 0
100
200
300
400
500
600
-1 Raman shift (cm ) Fig. 7
27
700
800
900
(a)
4
0 20 40 80 160 240 320 400
Extinction (a.u)
time= 0 min 3
2
time= 400 min
1
0 200
225
250
275
300
Wavelength (nm) Fig. 8 (a)
28
325
350
0.0
(b)
-0.1
Absence of catalyst Absence of light
ln(Abs/Abs0)
-0.2
-5
k=2.05 *10 M
-0.3 -0.4
Presence of catalyst and light
-0.5 -0.6 -0.7 -0.8 0
5000
10000
15000
Time (s) Fig. 8 (b)
29
20000
25000
45
(c)
COD (mg/L)
40
35
30
25
20 0
5000
10000
15000
Time (s)
Fig. 8 (c)
30
20000
25000
HO
O
O
C
OH
OH
OH
OH
+ CO2
+ H2O
OH
Benzoic Acid
3-Hydroxy Benzoic Acid
OH
Phenol O
O
OH
OH
OH Oxidation
Reduction Cyclohexanol
Phenol
Me
n-Caproate
Cyclohexanone
O
O O
C
H
Me
+ Cycobutanone
H
Ethylene
H
Ketene
O
O
H
Hex-5-enal OH hv O
OH Intermediate
H
hv
Fig.9
31
H2C
O C
Ketene
+ Cyclobutane
Research Highlights Au doped TiO2 thin films prepared by chemical spray pyrolysis technique. Effect of Au doping on the structural, morphological and optical properties. Photoelectrocatalytic degradation of benzoic acid under UV light illumination.
32
S1011-1344(14)00360-1 http://dx.doi.org/10.1016/j.jphotobiol.2014.12.004 JPB 9885
To appear in:
Journal of Photochemistry and Photobiology B: Biology
Received Date: Revised Date: Accepted Date:
3 September 2014 13 November 2014 1 December 2014
Please cite this article as: V.S. Mohite, M.A. Mahadik, S.S. Kumbhar, Y.M. Hunge, J.H. Kim, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale, Photoelectrocatalytic degradation of benzoic acid using Au doped TiO2 thin films, Journal of Photochemistry and Photobiology B: Biology (2014), doi: http://dx.doi.org/10.1016/j.jphotobiol.2014.12.004
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Photoelectrocatalytic degradation of benzoic acid using Au doped TiO2 thin films V.S. Mohite a, M. A. Mahadik a, S. S. Kumbhar a, Y. M. Hunge a, J. H. Kim b, A.V. Moholkar a, K. Y. Rajpure a, C.H. Bhosale a* a.
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India.
b.
Department of Materials Science and Engineering, Chonnam National University, 300 Yong bong-Dong, Puk-Gu, Gwangju 500-757, South Korea.
Abstract Highly transparent pure and Au doped TiO2 thin films are successfully deposited by using simple chemical spray pyrolysis technique. The effect of Au doping onto the structural and physicochemical properties has been investigated. The PEC study shows that, both short circuit current (Isc) and open circuit voltage (Voc) are (Isc = 1.81 mA and Voc = 890 mV) relatively higher at 3 at % Au doping percentage. XRD study shows that the films are nanocrystalline in nature with tetragonal crystal structure. FESEM images show that the film surface covered with a smooth, uniform, compact and rice shaped nanoparticles. The Au doped thin films exhibit indirect band gap, decreases from 3.23 to 3.09 eV with increase in Au doping. The chemical composition and valence states of pure and Au doped TiO2 films are studied by using X-ray photoelectron spectroscopy. The photocatalytic degradation effect is 49% higher in case 3 at % Au doped TiO2 than the pure TiO2 thin film photoelectrodes in the degradation of benzoic acid. It is revealed that Au doped TiO2 can be reused for five cycles of experiments without a requirement of post-treatment while the degradation efficiency was retained.
Keywords: Spray Pyrolysis; Titanium dioxide; PEC; Structural; XPS spectroscopy Corresponding author: E-mail: [email protected] Tel.: +91 2312609435; Fax: +91 2312691533.
1
1. Introduction TiO2 is the most important material as a catalyst for photoelectrochemical purification of wastewater because of its high oxidation potential in its valence band and high chemical stability [1, 2]. TiO2 material has been attracting a great deal of attention amongst researchers because of its unique properties such as high optical transparency, wide band gap energy, high refractive index, high dielectric constant, non-toxicity, abundance in nature and good chemical stability in undesirable environment conditions [3, 4]. Researchers have employed several methods for depositing the TiO2 thin films like sol gel [5-7], chemical vapor deposition [8, 9], evaporation [10], sputtering [11-13], pulsed laser deposition [14], electrodeposition [15], and spray pyrolysis [16]. Out of these deposition techniques, the spray pyrolysis technique is frequently used because of its simplicity, commercial viability, potential for cost effective mass production, excellent control of chemical uniformity, and stoichiometry of thin films. It has been found that TiO2 in anatase crystalline phase is most active photocatalyst. Nevertheless, it has wide band gap energy of 3.2 eV, normal anatase phase TiO2 needs a UV radiation to initiate its photoactivity. Many attempts have been made to extend the light absorption edge of the catalyst to the visible light region by doping gold nanoparticles into TiO2 and to improve its photocatalytic degradation efficiency [17-19]. Noble metal like Au has an ability of producing the highest Schottky barrier potential among the all metals [20]. The Au doped TiO2 thin films are chosen for environmental protection due to their high surface to volume ratio, because in the photocatalytic process the chemical reactions occur at the surface of the catalyst material. Therefore noble metal ion doping enhances the rate of photocatalytic process [21]. Recent studies on gold titanium nanocomposite particles show that metal ion doping extended the response of the photocatalyst into the visible region [22]. The photocatalytic properties of Au based materials depend on the preparation method and particularly on the shape and size of the Au clusters [23] The benzoic acid is used as a model pollutant because it is mainly used as a preservative in food, cosmetics and biological fields. It is suitable organic molecule for studying the photocatalytic degradation of more complex water pollutants with their properties, initially undergoes oxidation leading to the formation phenol and then other intermediates compounds leads to ring opening followed by mineralization of organic compounds [40-41]. So, it is used as the model organic impurity in the photoelectrocatalytic degradation reaction. 2
In this paper the effect of Au doping on the structural, optical, morphological and photoelectrocatalytic properties is investigated. The main objective of this work is to shift the band gap energy of TiO2 in the visible region by Au doping. To study the photocatalytic activity of this photoelectrode for degradation of benzoic acid and study the Chemical Oxygen Demand. 2. Experimental Spray pyrolysis technique: The basic principle involved in spray pyrolysis technique is pyrolytic decomposition of salts of a desired compound to be deposited. Fig. 1 shows the schematic diagram of the spray pyrolysis technique. It mainly consists of spray nozzle, rotor for spray nozzle, liquid level monitor, hot plate, gas regulator value and air tight fiber chamber. In spray pyrolysis, the process parameters like precursor solution, atomization of precursor solution, aerosol transport and decomposition of precursor are very important while studying the structural, optical, morphology and crystallinity of the thin films. Once the sprayed droplet reaching on the surface of the hot substrate undergoes pyrolytic decomposition and forms a single crystalline or cluster of crystallites as a final product. The other volatile by-products and solvents escape in the vapor phase. The substrates provide thermal energy for the thermal decomposition and subsequent recombination of the constituent species, followed by sintering and crystallization of the clusters of crystallites and thereby resulting in coherent film. The required thermal energy is different for the different materials and for the different solvents used in the spray process. The atomization of the spray solution into a spray of fine droplets also depends on the geometry of the spraying nozzle and pressure of a carrier gas. The film thickness depends upon the distance between the spray nozzle and substrate, substrate temperature, the concentration of the precursor solution and the quantity of the precursor solution sprayed [24]. Au doped TiO2 thin films were prepared by using simple, cheap chemical spray pyrolysis technique in a non-aqueous medium onto the FTO (10-20 Ω/cm2)) coated glass substrates. The initial ingredients used were titanium (IV) iso-propoxide Ti {OCH (CH3)2}4 and chloroauric acid (HAuCl4.3H2O). The ( 0.1 M) titanium (IV) iso-propoxide Ti {OCH (CH3)2}4 was dissolved in ethanol at room temperature and then (0.1 M) chloroauric acid solution was mixed into titanium (IV) iso-propoxide solution to achieve the different Au doping percentages. The [Au]/[Ti] ratio calculated on atomic percent used in the starting solution were 1 at %, 2 at %, 3 at %, and 4 at %. The resulting 100 cc precursor solution was sprayed through specially designed glass nozzles 3
onto the preheated glass substrates held at an optimized substrate temperature of 450 °C. The compressed air was used as the carrier gas at a constant spray rate of 4 cc min-1. While varying the doping percentage, other optimized parameters such as solution concentration (0.1 M), the quantity of solution (100 cc), nozzle to substrate distance (32 cm) were kept constant throughout the experiment. Photoelectrochemical cell was fabricated using the two-electrode configuration system, comprising TiO2 thin film as a photoanode and graphite as a counter electrode, with 0.1 M NaOH used as an electrolyte. The cell was illuminated with 20W 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 deposit thin films was carried out, by analyzing the X-ray diffraction patterns obtained under Cu-Kα radiation from a Bruker D2 Phaser. The surface morphological characterization of the films was studied by using FE-SEM (Model: JSM-6701F, Japan). Transmission spectra were recorded at room temperature using a UV-1800 Shimadzu, UV spectrophotometer within wavelength range 300-1100 nm. Valence states of constituent elements were analyzed by an X-ray Photoelectron Spectroscopy (XPS, Physical Electronics PHI 5400, USA) with monochromatic Mg-Kα (1253.6 eV) X-ray beam. Single cell reactor: The Au doped TiO2 electrode used in photoelectrocatlytic degradation experiments were deposited by spray pyrolysis onto large area (10 cm ×10 cm) conducting glass substrate (spray deposited fluorine doped tin oxide on glass, FTO, with sheet resistance of 10–20 Ω/cm2). In single cell phototelectrochemical degradation experiment, large area 3 at % Au doped 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 photoelectrode was illuminated from the backside using UV light. The 1mM concentration of benzoic acid is used as model pollutants in water for degradation studies under UV light illumination in the presence of 3 at % Au doped TiO2 photocatalyst. A fixed amount of electrolyte, that is major part of which contains an external reservoir, was recirculated through the photoelectrochemical cell with a constant flow rate of 12.8 L/h
−1
using a Gilson MINIPLUS peristaltic pump, France with silicon tubing [25]. Using
aliquots withdrawn from the reaction mixture at some intervals, the concentrations of organic impurities in the solutions were determined by measuring the UV–Vis absorbance (extinction) using a UV-1800 Shimadzu, UV spectrophotometer (for the measurement of full spectra). The 4
absorbance (extinction) was measured in 1 cm quartz cell at particular wavelength (where maximum absorption peak is observed) for various impurities. Aliquots extracted from the solutions at various intervals during the degradation reaction were also used for determining chemical oxygen demand (COD) using the standard method of oxidation with an excess of dichromate in concentrated sulphuric acid by digestion at 140 °C. The concentration of the organic solute was calculated from the dichromate extinction at various wavelengths [26]. 3. Results and discussion 3.1 Photoelectrochemical (PEC) studies The PEC characterization is a simple and unique technique to optimize preparative parameters of semiconductor thin films [27, 28]. Here, optimization of Au doping percentage is done with PEC technique. The glass/FTO/Au doped TiO2 substrate as a working electrode and graphite as a counter electrode were immersed in a 0.1 M NaOH solution and the values of short circuit current Isc and open circuit voltage Voc are measured. The Au doped TiO2 thin films exhibited better photoresponse than that of pure TiO2 films. The Isc and Voc values for pure TiO2 thin film sample are (Isc= 1.7 mA and Voc= 770 mV). It is seen that short circuit current (Isc) and open circuit voltage (Voc) are function of Au doping percentage (as shown in Fig.2). The values of Isc and Voc increases with increase in Au doping percentage, attains the relatively maximum values (Isc= 1.81 mA and Voc= 890 mV) at 3 at % and then decrease for further increase in Au doping percentage. The higher values of Isc and Voc values are due to the stoichiometry of the Au doped TiO2 film at optimized condition [29]. 3.2 X-ray diffraction studies Fig.3 shows the X-ray diffraction patterns of pure and Au doped TiO2 thin films deposited at different (1 at %, 2 at %, 3 at % and 4 at %) doping percentages. The films are nanocrystalline with tetragonal crystal structure. The planes match well with the Joint Committee for Powder Diffraction Standards (JCPDS card No. 01-075-1537). When the doping percentage increases, the intensity of (101) and (103) peak increases slightly up to 3 at % and then decreases for higher doping percentage. This is due to the incorporation of Au into the TiO2 films which is able to create more defects in the lattice [30]. Nevertheless, the position of (101) peak is slightly shifted toward lower angle due to atomic radius of Au is greater than that of O and smaller than
5
the Ti, it suggests that Au is substituted on O sites. The crystallite size of the deposited Au doped TiO2 thin films was calculated by using Scherer’s formula as; D=
0.9λ β cos θ
(1)
where, D is the crystallite size, β is the broadening of the diffraction line measured full width at half of its maximum intensity (FWHM) and λ is the X-ray wavelength (1.5405A˚). The crystallite size for 3 at % Au doped TiO2 thin film is 80 nm. 3.3 Morphological study Fig. 4 (a–e) shows the FE-SEM images of Au doped (0 – 4 at % doping percentage) titanium dioxide thin films. The micrographs show compact and homogeneous distribution of grains (rice shaped morphology) with varying doping percentage. The pure titanium dioxide sample show the highly dense, compact and homogeneous distributed. As the doping percentages increases from 1 to 4 at % the grain size is increases upto 3 at % Au doped TiO2 thin film and then decreases for higher doping percentages. The FESEM micrographs looks like a rice shaped morphology effectively increases the surface area of a film and may helpful for enhance the photocatalytic activity. This may be due to it has the well crystallized anatase phase and the high adsorption ability. 3.4 Optical properties The optical transmission spectra of Au doped TiO2 films deposited at various doping percentages is shown in Fig. 5 (a). The transmittance increase upto 3 at % and then decreases for higher doping percentages. The decrease in transmittance at higher doping percentages may be due to the increased scattering of photons by crystal defects formed due to gold doping. The free charge carrier absorption of photons may also contribute to the reduction in optical transmittance [31]. Fig. 5 (b) shows the plots of (αhυ)1/2 Vs hυ for Au doped TiO2 thin films deposited at different doping percentages. The thickness for 1, 2, 3 and 4 at % Au doped TiO2 thin films are 280 nm, 290 nm, 295 nm and 285 nm respectively. The indirect band gap energy decreases from 3.23 to 3.09 eV. It is comparable to the value of 3.23 eV for anatase phase [32-33],[18].
6
3.5 X-ray Photoelectron Spectroscopy studies The chemical composition and valence states of constituent elements present in the studied films are analyzed by X-ray photoelectron spectroscopy. The survey scan spectra of pure and 3 at % Au doped TiO2 thin films are shown in Fig. 6. The elements Ti and O are detected from the surface of TiO2 film. The C signal comes from the surface adsorbed carbon dioxide during the sample preparation. Fig.6 (a-b) shows high resolution XPS spectra of the O1s corelevel of pure and 3 at % Au doped titanium dioxide thin films. The binding energy of O1s can be fitted with their curves having peaks appearing at 529.8 eV and 531.9 eV, which can be attributed to the Ti-O (529.8 eV) and O-H (531.9 eV) components. Frequently the O1s peak has been seen in the measured binding energy region of 529–535 eV and for chemisorbed oxygen on the metal surface, binding energy is obtained in the region 530–531 eV, therefore, the binding energy component centered at 530 eV of the O1s spectra is attributed due to chemisorbed oxygen [34]. Fig. 6 (c–d) shows high resolution XPS spectra of the Ti 2p core level as a function of the binding energy of pure and 3 at % Au doped TiO2 film. Due to spin–orbit coupling, the Ti 2p core levels split into 2p1/2 and 2p3/2 components. The binding energies of Ti 2p3/2 and the Ti 2p1/2 peaks are observed at 458.5 eV and 464.3 eV respectively. The binding energy difference is 5.58 eV, which is in good agreement with the value reported for Ti4+, confirming the formation of the TiO2 compound in the studied films [35]. 3.6 Raman spectroscopic analysis Raman spectroscopy is useful for determination of vibrational, rotational and other type of low frequency modes present in a system. Fig. 7 illustrates room temperature Raman spectra of TiO2 and typical 3 at % Au doped TiO2 thin films. According to factor group analysis, anatase phase TiO2 films have five Raman active modes (3Eg+2B1g). Five allowed modes for an anatase single crystal appeared at 142 cm-1 (Eg), 197 cm-1 (Eg), 397 cm-1 (B1g), 518 cm-1 (B1g), and 634 cm-1 (Eg) [36-37]. No other higher order peaks are observed in the region above 1200 cm-1. The all peaks match with those reported in literature and thus it confirm the formation of pure anatase phase of TiO2. 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 Au doping. The absorption of peaks in the higher frequency region becomes weaker and broader in the case
7
of the Au doped TiO2 sample. This broadening suggests an increase in crystalline defects in the framework after gold incorporation. 3.7 Photocatalytic properties Photocatalytic degradation of benzoic acid under UV light illumination is carried out to study the photocatalytic activity of Au doped TiO2 photocatalyst. The extinction spectra of benzoic acid solution during the degradation experiment under different reaction time are recorded in the wavelength range from 200 to 350 nm. Fig. 8 (a) shows the extinction spectra of benzoic acid solution against wavelength. The percentage of benzoic acid decreases with time due to its photooxidative degradation. The photocatalytic degradation follows a pseudo first order reaction and its kinetics can be expressed using relation [38].
c ln c0
= −kt
(3)
where, t is the time, c stands for the percentage of the solute or the percentage of oxidizable atoms in the organic species; c0 the initial percentage of solute. k can be taken as the apparent first order rate constant of the degradation reaction. Moreover, k is proportional to the area of the electrode and therefore to the photocurrent. In order to make a comparison of experimental data obtained under various conditions and that of external parameters, it is useful to define [39]. k ' = kV ( cm k ' k " = ( cm A p = k " '=
3
s
s kVF i ph
− 1
−1
)
(4 )
)
(5 ) (M
−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 mol−1). The p reflects the efficiency of oxidative degradation of the solute. During the degradation experiments, due to its photoelectrocatalytic oxidation, the percentage of benzoic acid decreases as a function of time. The plot of ln(Abs/Abs0) as a function of reaction time (kinetics of degradation) of benzoic acid is as shown
8
in Fig. 8 (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), to be 2.05 × 10−5 s−1. The COD value gives the extent of degradation of benzoic acid and is shown in Fig. 8 (c). COD study as a function of reaction time provides the percentage of oxidizable matter left in the electrolyte solution. The COD values decrease from 46.3 to 24.5 mg L−1 with reaction time. Thus it shows 49 % degradation of benzoic acid i.e. greater than pure TiO2 thin films, it shows 37 % degradation of benzoic acid confirming mineralization of benzoic acid, the observed decay constants indicate the annihilation of main pollutant in the water.Similar type of degradation of benzoic acid is done in earlier reports [40-41]. As, we dope Au into TiO2 films it extend the response of the photocatalyst into the visible region and increases the photocatalytic degradation. The mechanism of degradation of benzoic acid:
When light is incident on the TiO2 semiconductor with energy greater than the band gap energy of the semiconductor then and only then it generates valence band holes (hvb+) and conduction band electrons (ecb−). Due to this wide gap energy, TiO2 can be activated by UV light below 385 nm. Further holes and electrons are recombine and liberate heat or make their separate ways to the surface of TiO2 semiconductor, where they can react with species adsorbed on the catalyst surface of TiO2 semiconductor. Valence band holes can react with water and the hydroxide ion to form hydroxyl radicals HO•, while electrons react with adsorbed molecular oxygen reducing it to superoxide radical anion which reacts with protons to form peroxide radicals as shown follow [42]. hvb+ + H 2 O → HO • + H +
(1)
hvb+ + OH − → HO •
(2)
− eCB + O2 → O2• −
O2•− + H + → HO2•
(3) (4)
Organic compounds can then undergo oxidative degradation through their reactions with valence band holes, hydroxyl and peroxide radicals and reductive cleavage through their 9
reactions with electrons yielding various by-products and finally mineral end-products. The proposed possible reaction mechanism of degradation pathway of benzoic acid has been schematically presented in Fig. 1.When benzoic acid react with hydroxyl radical (HO• ) to form 3- hydroxy benzoic acid and immediately loose the CO2 and H2O molecule to form phenol. Then the phenol is reacts with hydroxyl radical (HO• ) with reduction reaction to give cyclohexanol as product. Then further cyclohexanol reacts with oxidizing agent to give cyclohexanone and some several degradation products as mentioned in Fig. 9. It is revealed that Au doped TiO2 can be reused for five cycles of experiments without a requirement of post-treatment while the degradation efficiency was retained. 4. Conclusions
Pure and Au doped TiO2 thin films can be successfully synthesized using simple chemical spray pyrolysis technique. The films are nanocrystalline in nature with tetragonal crystal structure. The indirect optical band gap energy decreases from 3.23 to 3.09 eV. The micrographs show compact and homogeneous distribution of grains (rice shaped morphology) with varying grain size. The rice shaped morphology enhances the photoactivity of Au doped TiO2 films. The 3 at % Au doped TiO2 electrode shows 49 % photocatalytic degradation of benzoic acid under UV light illumination. 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 UGC-Meritorious Fellowship.
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Figure captions Fig. 1 Schematic set-up for spray pyrolysis technique Fig. 2 Variations of Isc and Voc against doping percentage (0- 4 %) for Au-doped TiO2 thin films.
(on FTO)/0.1NaOH/C PEC solar cell. Fig. 3 X-ray diffraction patterns of pure and (1- 4 %) Au doped TiO2 thin films. Fig. 4 (a) FE-SEM images of pure TiO2 thin film, (b) 1 at % Au doped TiO2 thin film (c) 2 at %
Au doped TiO2 thin film (d) 3 at % Au doped TiO2 thin film (e) 4 at % Au doped TiO2 thin film Fig. 5 (a) Optical transmittance spectra of Au doped TiO2 thin films (b) Plot of (αhυ)1/2 Vs hυ of
Au doped TiO2 thin films. Fig. 6 Survey scan XPS spectra of pure and 3 at % Au doped TiO2 thin films. (a) Narrow scan XPS spectra of O 1s core level for TiO2 film (b) Narrow scan XPS spectra of O
1s core level of 3 at % Au: TiO2 film. (c) Narrow scan XPS spectra of the Ti 2P region for TiO2 thin films. (d) Narrow scan XPS spectra of Ti 2P region for 3 at % au doped TiO2 thin films. Fig. 7 Room temperature Raman spectra of pure and 3 at % Au doped TiO2 thin films. Fig. 8 (a) Extinction spectra for degradation of benzoic acid using 3 at % Au doped TiO2 thin
film. (b) Kinetics of degradation (extinction taken at 230 nm). (c) Extent of mineralization by COD. Fig.9 A schematic representation of possible reaction mechanism of degradation of benzoic acid.
15
Fig. 1
16
900
1.80
880
1.78
860 840
1.76
820
1.74
800 1.72 780 1.70 760 0
1
2
3
Au doping percentage (at %) Fig. 2
17
4
Voc (mV)
Isc (mA)
1.82
4at % Au:TiO2
Intensity (counts)
3at % Au:TiO2 (101) (103)
2at % Au:TiO2 1at % Au:TiO2
0at % Au:TiO2 20
30
40
50
2θ (degree) Fig. 3
18
60
70
80
(a)
(b)
(c)
(e)
(d)
Fig 4
19
100 90
Transmittance (%)
80 70 60 50
(a)
(b)
(c)
(d)
40 30 20 10 0 300
400
500
600
Wavelength (nm) 5 (a)
20
700
800
2.0 1.8
1/2
7
(α hv) , 10 (eV/cm)
2
1.6 1.4 (a)
1.2 1.0
(c)
(b)
(d)
0.8 0.6 0.4 0.2 0.0 3.0
3.1
3.2
hυ (eV) 5 (b)
21
3.3
3.4
O 1s
Ti3s Ti3p O2s
C 1s
5
Intensity 10 (cps)
10
3 at % Au:TiO2 Ti2p
Au4p
Ti2s
O KLL
12
TiLMM
14
8 6 TiO2
4 2 0 1200
1000
800
600
400
Binding energy (eV) Fig. 6
22
200
0
1.6
(a)
01s a
TiO2 O1s b
1.2
5
Intensity 10 (cps)
1.4
O1s
O1s c
1.0 0.8 0.6 0.4 536
534
532
530
528
Binding energy (eV) Fig. 6 (a)
23
526
2.0 1.8
5
Intensity, 10 (cps)
01s a
3at % Au :TiO2
(b) O1s b
O1s
1.6 1.4 O1s c
1.2 1.0 0.8 0.6 0.4 536
534
532
530
528
Binding energy (eV) Fig. 6 (b)
24
526
1.2 Ti 2p
5
Intensity, 10 (cps)
1.0
2p3/2
(c)
458.5
TiO2
0.8
2p1/2
0.6
5.58 eV
464.3
0.4
0.2 470
460.88
468
466
464
462
460
Binding energy (eV) Fig. 6 (c)
25
458
456
1.6 (d)
Ti 2p
2p3/2
5
Intensity, 10 (cps)
1.4 1.2
458.5 3at % Au:TiO2
1.0 0.8
2p1/2
5.58 eV
464.3
0.6
460.88
0.4 0.2 470
468
466
464
462
460
Binding energy (eV) Fig. 6 (d)
26
458
456
8000
-1
142 cm (Eg)
Intensity (CPS)
7000 6000
TiO2
5000 -1
4000
-1 634 cm 397 cm 518 cm (Eg) (B1g) (B1g) -1
-1
197 cm (Eg)
3000 2000
3at % Au:TiO2 0
100
200
300
400
500
600
-1 Raman shift (cm ) Fig. 7
27
700
800
900
(a)
4
0 20 40 80 160 240 320 400
Extinction (a.u)
time= 0 min 3
2
time= 400 min
1
0 200
225
250
275
300
Wavelength (nm) Fig. 8 (a)
28
325
350
0.0
(b)
-0.1
Absence of catalyst Absence of light
ln(Abs/Abs0)
-0.2
-5
k=2.05 *10 M
-0.3 -0.4
Presence of catalyst and light
-0.5 -0.6 -0.7 -0.8 0
5000
10000
15000
Time (s) Fig. 8 (b)
29
20000
25000
45
(c)
COD (mg/L)
40
35
30
25
20 0
5000
10000
15000
Time (s)
Fig. 8 (c)
30
20000
25000
HO
O
O
C
OH
OH
OH
OH
+ CO2
+ H2O
OH
Benzoic Acid
3-Hydroxy Benzoic Acid
OH
Phenol O
O
OH
OH
OH Oxidation
Reduction Cyclohexanol
Phenol
Me
n-Caproate
Cyclohexanone
O
O O
C
H
Me
+ Cycobutanone
H
Ethylene
H
Ketene
O
O
H
Hex-5-enal OH hv O
OH Intermediate
H
hv
Fig.9
31
H2C
O C
Ketene
+ Cyclobutane
Research Highlights Au doped TiO2 thin films prepared by chemical spray pyrolysis technique. Effect of Au doping on the structural, morphological and optical properties. Photoelectrocatalytic degradation of benzoic acid under UV light illumination.
32