Journal of Photochemistry and Photobiology B: Biology 141 (2014) 315–324
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Journal of Photochemistry and Photobiology B: Biology journal homepage: www.elsevier.com/locate/jphotobiol
Oxidative degradation of industrial wastewater using spray deposited TiO2/Au:Fe2O3 bilayered thin films M.A. Mahadik a, S.S. Shinde a, H.M. Pathan b, K.Y. Rajpure a, C.H. Bhosale a,⇑ a b
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Advanced Physics Laboratory, Department of Physics, University of Pune, Pune 411 007, India
a r t i c l e
i n f o
Article history: Received 3 October 2014 Received in revised form 21 October 2014 Accepted 27 October 2014 Available online 1 November 2014
a b s t r a c t The Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 thin films are successfully prepared by the spray pyrolysis technique at an optimised substrate temperature of 400 °C and 470 °C, respectively onto amorphous and F:SnO2 coated glass substrates. The effect of TiO2 layer onto photoelectrochemical (PEC), structural, optical and morphological properties of Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 thin films is studied. The PEC characterization shows that, maximum values of short circuit current (Isc) and open circuit voltage (Voc) are (Isc = 185 lA and Voc = 450 mV) are at 38 nm thickness of TiO2. Deposited films are polycrystalline with a rhombohedral and anatase crystal structure having (1 0 4) preferred orientation. SEM and AFM images show deposited thin films are compact and uniform with seed like grains. The photocatalytic activities of the large surface area (64 cm2) TiO2/Au:Fe2O3 thin film photocatalysts were evaluated by photoelectrocatalytic degradation of industrial wastewater under sunlight light irradiation. The results show that the TiO2/Au:Fe2O3 thin film photocatalyst exhibited about 87% and 94% degradation of pollutant in sugarcane and textile industrial wastewater, respectively. The significant reduction in COD and BOD values from 95 mg/L to 13 mg/L and 75 mg/L to 11 mg/L, respectively was also observed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the semiconductor mediated photocatalytic degradation has been a popular approach to degrade a wide range of organic contaminants like dyes [1]. Among these, synthetic dyes are most notorious organic contaminants that are discharged into the environment from textiles, tanning, leather, paints, and paper processing and pharmaceutical industries [2]. There are more than 100,000 types of dyes commercially available, with over 0.7 million tons of dyestuff produced annually and about 15% of these dyes are reported to get lost in the effluent [3]. Most of the dyes are toxic and carcinogenic compounds; they are also recalcitrant and thus stable in the receiving environment, posing a serious threat to human and environmental health [4]. In the textile industries consumption of water and chemicals for wet processing of textiles is of huge amount. The chemicals used are having variety in chemical composition, ranging from inorganic compounds to polymers and organic products [5]. The presence of very low concentrations of dyes in effluent is highly visible and undesirable [6]. Due to their chemical structure, dyes are resistant to a fading on exposure to light, water and many chemicals. Many dyes are difficult to ⇑ Corresponding author. Tel.: +91 231 2609435; fax: +91 231 2691533. E-mail address: [email protected] (C.H. Bhosale). http://dx.doi.org/10.1016/j.jphotobiol.2014.10.014 1011-1344/Ó 2014 Elsevier B.V. All rights reserved.
decolorize due to their complex structure and synthetic origin. Discoloration of textile dye effluent does not occur when treated aerobically by the municipal sewerage system [7]. Also, there are many varieties of dyes that are coloured, water soluble, reactive and are the most problematic, as they tend to pass through conventional treatment systems unaffected. Accordingly, to protect humans and the receiving ecosystem from contamination, the dyes must be eliminated from industrial effluents before discharging into the environment [8]. Municipal aerobic treatment systems, dependent on biological activity, were found to be inefficient in the removal of these dyes [9]. Other than these, there are various methods used to remove dye from industrial effluents [10]. These methods are not efficient enough because there are various materials in wastewater [11]. Also, advanced oxidation methods such as ozonation, photocatalyst, and photo Fenton are costly and uneconomical. Therefore, there is a need for more effective and cheaper ways of treating textile wastewater which consume the less amount of chemicals and energy [12]. Patil et al. carried out the photocatalytic degradation studies for water soluble hazardous Ponceau-S dye in Nb2O5 aqueous suspension along with commercial activated carbon (CAC) as co-adsorbent. They reported that the 86.2% removal of Ponceau-S dye due to significant photocatalytic degradation along with significant reduction in COD and BOD values [13]. Danwittayakula et al. was synthesized zinc
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Computer Potentiostat Ag/AgCl reference electrode C.E.
R.E. Holding Stand
W.E.
Semiconductor thin film
(b)
Brass rod for contact
FTO/glass (a)
Hamamatsu Photodiode
Optical bench O ring with electrolyte
Glass Optical bench
Constant current source LED
Fig. 1. Photoelectrochemical characterization systems for incident photon to current conversion efficiency (IPCE).
oxide/zinc tin oxide (ZnO/ZTO) nanocomposites using a hydrothermal technique for photocatalytic degradation of organic dyes. They reported that the ZnO/15ZTO showed 50% photocatalytic degradation efficiency and 77% COD removal of textile waste water when irradiated by sunlight [14]. Shavisi et al. studied the photocatalytic degradation of ammonia in petrochemical wastewater under the solar light/TiO2 photocatalysis system. They reported that the ammonia removal efficiency was increased by increasing the pH value, and after solar light irradiation in three days, degradation of ammonia was achieved up to 96.5% [15]. The Au doped Fe2O3 (Au:Fe2O3) is most effective candidates for killing the bacteria and also TiO2 is the most stable materials in aqueous solutions. In the present work, a separate set of Fe2O3, TiO2 and TiO2/Fe2O3 and TiO2/Au doped Fe2O3 (TiO2/Au:Fe2O3) bilayered thin films on amorphous and FTO coated glass substrates have been synthesized. One of the advantages of using photocatalyst in thin film form rather than fine powders or nanoparticles, is to eliminate the filtration and separation steps after the removal of pollutants from water. The deposited thin films are characterized by the PEC, XRD, Scanning electron microscopy, X-ray photoelectron and Raman spectroscopy. As sugarcane and textile factory wastewater may contains both organic impurities and bacteria. The photoelectrocatalytic degradation of sugarcane and textile factory wastewater has been carried out using TiO2/Au:Fe2O3 bilayered thin film photocatalysts in the presence of sunlight illumination has been investigated. The purified water samples were studied for total organic carbon (TOC) and chemical oxygen demand (COD) measurement which gives the extent of mineralization.
same way on these Au:Fe2O3 thin films for the growth of the TiO2/Au:Fe2O3 thin films. To deposit a TiO2 layered on Au:Fe2O3 bilayered thin films; the 0.1 M TiAcAc (C10H14O5Ti) in methanolic solutions was used for spraying. To measure PEC, the graphite was used as the counter electrode and the Fe2O3 based thin films as the working electrode. The measurement of IPCE was done with the cell form with ‘‘O’’ rings, electrolyte, gold counter electrode and Ag/AgCl as a reference electrode as described in Fig. 1(a) and (b). The incident photon to electron conversion efficiency (IPCE) was obtained by measuring the incident photon flux using LED lamps ranging from 360 nm to 420 nm. The light was calibrated using a Hamamatsu photodiode Fig. 1(a). Photocurrent spectra were measured at a constant potential vs. Ag/AgCl using a combination of a 2059 potentiostat AMEL Instruments Italy as shown in Fig. 1(b). Readings were collected at some specific intervals while the monochromatic light was scanned from 320 nm to 580 nm. The IPCE is the accepted measure to calculate the conversion efficiency of incident photons on a PEC cell to photocurrent flowing between the working and counter electrodes. An IPCE of 100% indicates the generation of one photoelectron per each incident photon. However, IPCE of 100% cannot be achieved as there are losses due to the reflection of incident photons, imperfect photon absorption by the photoelectrode and recombination [16,17]. IPCE is also called the external quantum efficiency and can be calculated using following Eq. (1) [18],
2. Experimental
where Ip(k) and P(k) are the photocurrent density (A/m2) and incident power density of light (W/m2) at wavelength k (nm), respectively. The photocurrent was measured with the Hamamatsu photodiode and potentiostat (Amel instrument, Italy). All experiments were carried out at room temperature. These optimized Au:Fe2O3 bilayered thin films were used as working electrode in the degradation experiments of wastewater from the sugarcane and textile factory.
The TiO2/Au:Fe2O3 bilayered thin films were grown by spraying the various quantities of TiO2 on optimized Au:Fe2O3 films deposited on amorphous and F doped SnO2 (FTO) coated glass substrates. Initially, Au:Fe2O3 thin films were deposited by spray pyrolysis technique onto a conductive substrate (FTO) and annealed at 500 °C for 2 h in air. After that, the TiO2 layer was deposited in a
IPCEðkÞ ¼ 1240
Ip ðkÞ PðkÞk
ð1Þ
317
200
330
130
300
120 110
270
100
240
90
2at% Au doped Fe2O3
210 0
(a)
TiO2/Fe2O3
20
40
60
(024)
140
TiO2 /2 at% Au doped Fe2O3
(116)
360
(110)
390
150
(113)
420
160
(101)
450
170
Intensity (a. u.)
180
(-ve) Voc (mV)
Isc (μ A)
480
Isc Voc
190
(104)
(021) (101)
M.A. Mahadik et al. / Journal of Photochemistry and Photobiology B: Biology 141 (2014) 315–324
80
Thickness of TiO2 on Au doped Fe2O3 thin ilms (nm)
20
30
40
50
60
Pure Fe2O3 70
80
2θ (Degree) Fig. 3. The XRD patterns of pure Fe2O3, Au doped Fe2O3, TiO2/Fe2O3 and TiO2/ Au:Fe2O3 bilayered thin films deposited onto glass substrates.
Potenostat
SUN
Reducon 2.1 eV
3.2 eV
Oxidaon
(SS)
Electrolyte
(b)
0.5
pure Fe2O3
Specs GmbH, Germany. The morphological characterization of the films was studied by using JEOL JSM-6360 scanning electron microscope (SEM). Surface topography of the films was further analysed from the AFM images taken by means of the atomic force microscope (AFM, Bruker instrument, Innova1B3BE) operated at room temperature. AFM images were collected in contact mode using a silicon nitride cantilever and the thickness was recorded using a Steller Net. Inc. USA Reflectometer having UV–Vis light source with a CCD detector. The industrial wastewater was obtained from the sugarcane and textile industries nearby Kolhapur city and used to make electrolyte in oxidative degradation experiments.
TiO2 / 2 at % Au doped Fe2O3
0.4
IPCE
3. Results and discussion 0.3
3.1. Photoelectrochemical (PEC) characterization 0.2 0.1 0.0 360 365 370 375 380 385 390 395 400
(c)
Wavelength (nm)
Fig. 2. (a) Variation of Isc and Voc with thickness of TiO2 on Au:Fe2O3 bilayered thin films, (b) schematic representation of the mechanism of charges separation in a photoelectrochemical system operated by coupling a Au doped Fe2O3 semiconductor to a TiO2 electrode, (c) IPCE spectra of the pure Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films.
The photoelectrochemical (PEC) characterization was carried out using PEC cell consisting of Fe2O3 and TiO2/Au:Fe2O3 thin film as photoelectrode, 1 M NaOH solution as an electrolyte and graphite as a counter electrode. The PEC cell was illuminated using visible light. The structural properties of deposited thin films were studied by a Bruker powder diffractometer (AXS) Analytical Instruments Pvt. Ltd. Germany, Model: D2 Phaser (k = 1.5406 Å for Cu Ka) in the range of 20–80°. Raman scattering spectrums were recorded in air at room temperature with micro Raman system from Jobin Yvon Horibra LABRAM-HR visible within 100– 800 cm1. The Raman spectra were excited using the He–Ne 632 nm laser source with 600 and 1800 lines/mm gratings and CCD detector. The XPS measurements on TiO2/Au:Fe2O3 thin films were performed after mounting them on a multi-purpose sample holder using Al Ka X-ray source and Phoibos 100 analyzer from
3.1.1. Effect of TiO2 thickness Fig. 2(a) shows that, as the thickness of TiO2 increase, then the values of photocurrent increases up to 38 nm of TiO2 and attend relatively higher values of Isc and Voc (Isc = 185 lA and Voc = 450 mV) at 38 nm thickness of TiO2. A further increase in thickness of TiO2 on Au:Fe2O3 there is a decrease in both Isc and Voc values. This poor PEC response may be attributed to the following reasons: One is the low absorption intensity; the other is the role of Fe3+. It is suggested that Fe3+ in the composite electrodes may form the deep trap site in the titania matrix to cause the recombination of photogenerated carriers [19]. This could help to optimize the thickness of TiO2 layer. Fig. 2(b) shows the schematic representation of the mechanism of charges separation in a photoelectrochemical system operated by coupling Au doped Fe2O3 semiconductor to a TiO2 electrode. The incident photon to current conversion efficiency (IPCE) values are calculated Eq. (1). Fig. 2(c) shows the IPCE values at 360 nm wavelength are higher than longer wavelengths. The IPCE spectrum of hematite photoanode typically shows IPCE values even below 10% [20,21]. However, the maximum IPCE value of TiO2/Au:Fe2O3 bilayered thin film is about 47% at 360 nm. It indicates that TiO2/Au:Fe2O3 films converts the maximum incident high energy photons to current; however, the IPCE decreased much more rapidly up to 400 nm Thus, IPCE at higher wavelengths near band gap energies is lower, which is a common characteristic of hematite photoelectrode. This is because absorption in this region is due to the d–d transition of Fe3+ and electron–hole pair generated by this transition is localized in Fe3+ and are not used effectively to generate water oxidation photocurrent [22].
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21,000
60,000
(a)
30,000
Intensity (c/s)
40,000
Ti 2P
Fe2P
20,000
Fe2p1/2
600
400
0
200
20,400
20,000 735
0 800
Fe2p3/2
20,600
20,200
Fe3p
10,000
1000
(b)
20,800
O KLL
Intensity (c/s)
50,000
O1s
730
725
Binding energy (eV) 55,000
35,000
(c)
Ti2p3/2
Intensity (c/s)
Intensity (c/s)
(d)
715
710
705
531.9 eV
50,000
30,000
Ti2p1/2
25,000
720
Binding energy (eV)
20,000 15,000
531.5 eV
45,000 40,000 35,000
533.67 eV
30,000 25,000
10,000
20,000 475
470
465
460
455
540
538
536
Binding energy (eV)
532
530
528
526
(e)
7,130
Intensity (c/s)
534
Binding energy (eV)
84.2 eV
87.4 eV
Au 4f7/2
Au 4f5/2
7,120
7,110
7,100
7,090 94
92
90
88
86
84
82
80
78
Binding energy (eV) Fig. 4. (a) The XPS survey scan spectrum of TiO2/Au:Fe2O3 bilayered thin film. (b) Narrow scan spectrum of Fe2p of TiO2/Au:Fe2O3 bilayered thin film. (c) Narrow scan spectrum of Ti2p of TiO2/ Au:Fe2O3 bilayered thin film. (d) Fitted narrow scan O1s spectrum of TiO2/ Au:Fe2O3 bilayered thin film. (e) Narrow scan spectrum of Au 4f of TiO2/ Au:Fe2O3 bilayered thin film.
3.2. X-ray diffraction studies The X-ray diffraction (XRD) patterns of Fe2O3, Au:Fe2O3, TiO2/ Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films deposited on glass substrate at optimized preparative parameters are shown in Fig. 3. It is found that, the TiO2 layer is composed of anatase phase and all the diffraction peaks of the pure Fe2O3 belong to the pure hematite structure of Fe2O3. For the TiO2/Au:Fe2O3 bilayered thin films all the reflection peaks could be indexed to the hematite phase of Fe2O3 and anatase phase of TiO2, there is no appearance of a third phase. It is observed that the (1 1 0) peak increases for Fe2O3, Au:Fe2O3 and TiO2/Fe2O3 bilayered thin films and for the TiO2/Au:Fe2O3 the peak intensity of (1 1 0) decreases and of (1 0 4) increases. This may be due to the crystal reorientation effect. The crystallite size of TiO2/Au:Fe2O3 bilayered thin films is calculated using the well-known Scherer’s relation given in equation [23].
D¼
0:9 k b cos h
ð2Þ
where b is the (FWHM) broadening of the diffraction line measured at half maximum intensity (radians) and k = 1.5406 Å is the wavelength of the Cu Ka radiations. The crystallite size of the TiO2/ Au:Fe2O3 bilayered thin films is found to be 70 nm for (1 0 4) peak. 4.1. Photoelectron spectroscopy (XPS) The oxidation states of the TiO2/Au:Fe2O3 bilayered thin film are revealed in X-ray photoelectron spectroscopy (XPS). Fig. 4(a) shows the survey scan spectrum of TiO2/Au:Fe2O3 bilayered film deposited on glass substrate. X-ray photoelectron spectroscopy (XPS) survey scan spectrum of a TiO2/Au:Fe2O3 film confirms the presence of Ti (Fig. 4(c)) and Au (Fig. 4(e)).The narrow scan spectrum of Fe2p peak positions Fig. 4(b). The Fig. 4(b) and (e) indicat-
200
400
600
1318 cm-1 1318 cm-1
(d) TiO2 /Au doped Fe2O3
(b) 2 at % Au doped Fe2O3
(a) Pure Fe2O3
800
1000
1318 cm-1
(c) TiO2 / Fe2O3
1318 cm-1
611 cm-1 611 cm-1 611 cm-1 611 cm-1
410 cm-1 410 cm-1
410
410 cm-1 cm-1
291 cm-1
225 cm-1 245 cm-1 291 cm-1 225 cm-1 245 cm-1 291 cm-1 225 cm-1 245 cm-1 291 cm-1
Intensity (a. u.)
225 cm-1 245 cm-1
M.A. Mahadik et al. / Journal of Photochemistry and Photobiology B: Biology 141 (2014) 315–324
1200
1400
Raman Shift (cm-1) Fig. 5. Raman spectra of (a) pure Fe2O3, (b) 2 at.% Au:Fe2O3, (c) TiO2/Fe2O3 and (d) TiO2/2 at.% Au:Fe2O3 bilayered thin film excited by 532 nm He–Ne laser.
ing that the chemical states of Fe and Au are +3 and 0, respectively. The XPS data can also confirm the formation of a–Fe2O3. This is in good agreement with the results obtained by Lian et al. [24]. The ratio of the Fe signals (e.g., the Fe2p) and the O1s signal is much lower for the Ti doped samples. This indicates that the Ti atoms present at the surface of the TiO2/Au:Fe2O3 bilayered thin film very
319
likely are bound in an oxide environment. The Ti2p detail spectrum of the TiO2/Au:Fe2O3 bilayered thin film sample is shown in Fig. 4(c). From the peak position and line shape, it is evident that the Ti atoms in the TiO2/Au:Fe2O3 bilayered thin film are not in a metallic environment. The Ti2p3/2 binding energy of 458.6 eV is consistent with typical values reported for TiO2 (458.7 eV and 458.6–459.3 eV) [25]. It is observed that the Ti2p1/2 peak is significantly broader than the Ti 2p3/2 peak due to the presence of a Coster–Kronig decay channel of the Ti 2p1/2 core hole, leading to lifetime broadening of the photoemission line. The Fe2p3/2 line of the TiO2/Au:Fe2O3 bilayered film is at a binding energy of 711.2 eV. The binding energy variations of determination of band gap materials, this value is consistent with typical values observed for Fe2O3 (Fe3+, 710.9 eV or 711.2 eV); FeOOH (Fe3+ 711.2 eV and 711.3–711.9 eV) and Fe3O4 (a mixture of Fe2+/Fe3+, 709.5/711.2 eV and 711.6 eV). The binding energy is not compatible with that of FeO (Fe2+, 709.6 eV and 709.1–709.5 eV) [26,27]. A satellite peak of the Fe 2p3/2 main line is observed at 719.3 eV, i.e., approximately 8.1 eV below the main line. This satellite is most likely indicative of the presence of Fe3+ species. Again, there is no evidence to support the presence of Fe2+, which should give rise to a satellite peak located at 717.2 eV. Peng et al. [28] reported that 706.9 eV related to the Fe2P3/2 binding energy in the TiO2/Fe2O3 thin films. The O1s XPS spectra collected from the TiO2/Au:Fe2O3 films shown in Fig. 4(d). The binding energy of the main line (531.5 eV) for the TiO2/Au:Fe2O3 sample is consistent with the reported value for Fe2O3 (531.9 and 531.6 eV) and Fe3O4 (530.0 and 530.3 eV) [29]. The Au 4f doublet peak of Au is detected by XPS at sufficient intensity; peaks position and shape (Fig. 4(e)) coincide with literature
Fig. 6. Scanning electron micrographs of pure Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films.
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97.24 nm
2000
78.44 nm
2000
(a)
(b) 1500
Y [nm]
Y [nm]
1500
1000
500
1000
500
0 0
500
1000
1500
2000
0
0.00 nm
0
500
X [nm]
64.18 nm
2000
1500
2000
0.00 nm
100.98 nm
2000
(d)
(c) 1500
Y [nm]
1500
Y [nm]
1000
X [nm]
1000
1000
500
500
0 0
500
1000
1500
2000
0.00 nm
X [nm]
0 0
500
1000
1500
2000
0.00 nm
X [nm]
Fig. 7. 2D atomic force micrographs of (a) pure Fe2O3, (b) Au doped Fe2O3, (c) TiO2/Fe2O3 and (d) TiO2/Au:Fe2O3 bilayered thin films.
data [30]. A small shift of Au 4f7/2 peak towards lower binding energy values is found. 4.2. Raman spectroscopy To confirm the phase formation of deposited the thin films the Raman spectra are presented in Fig. 5. The Raman spectra shows that the Raman peaks are well matched to the a–Fe2O3 and the anatase TiO2 phase. The similar results reported by Noh et al. [31]. One sees that the spectra of all these films are similar nature. The intensity of 291 cm1 peak is increased as the TiO2 layer deposited on Fe2O3 thin films. The peaks appearing in the pure Fe2O3 spectra at 225, 245, 291, 410, 495, 611 and 1318 cm1 correspond to the characteristic peaks of a–Fe2O3. The peaks at 225 and 495 cm1 correspond to the A1g mode and the four peaks at about 245, 291, 410 and 611 cm1 are attributed to the Eg mode [32]. According to factor group analysis, anatase has six Raman active modes (A1g + 2B1g + 3Eg). Ohsaka [33] reported the Raman spectrum of anatase single crystal and showed that it shows six modes appearing at 144 (Eg), 197 (Eg), 399 (B1g), 513 (A1g), 519 (B1g), and 639 cm1 (Eg). From Fig. 4, it is seen that intensity and broadening of peak at 291 cm1 is increased for the TiO2/Fe2O3 and TiO2/ Au:Fe2O3 bilayered thin films. This is due to the thickness of TiO2 on the Au:Fe2O3 layer [34]. 4.3. Surface morphology analysis Fig. 6 shows the scanning electron micrographs of pure Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films deposited on glass substrates. It is seen from Fig. 6 that, the surface of the film changes from needle shaped to oval shaped particles with 200–180 nm diameter. It is observed that the grain size determined from SEM images is larger than the value determined by XRD, which may be due to the discrepancy between the mean
dimension of the crystallites perpendicular to diffracting planes by XRD diffraction and the observable aggregates in SEM images. It is seen that, while going from TiO2/Fe2O3 to TiO2/Au:Fe2O3, the surface morphology is essentially changing from granular to oval shaped structure. Fig. 7 shows the two dimensional (2D) AFM images of (a), pure Fe2O3 (b), Au:Fe2O3, (c) TiO2/Fe2O3 and (d) TiO2/Au:Fe2O3 bilayered thin films taken in contact mode with 2 lm 2 lm scan range. From Fig. 7, it is observed that the particle size decreases with increasing roughness of the films. 4.4. Photoelectrocatalytic degradation of sugarcane factory wastewater It is well known that, when semiconductor photoelectrode is irradiated by light with energy higher than or equal to the band gap, an electron (e) in the valence band (VB) can be excited to the conduction band (CB) with the generation of a hole (h+) in the VB at the same time. The photoelectron can be easily trapped by electronic acceptors like adsorbed O2, and form a superoxide radical anion (O 2 ), whereas the photo-induced holes can be easily trapped by such as organic pollutants, to further oxidize organic pollutants [35]. Whenever, the photo-generated electrons and holes recombined, the photocatalytic activity would be decreased. But in oxide semiconductors like ZnO, Fe2O3 and TiO2 the oxygen vacancies can act as the active centres to capture photoinduced electrons and the recombination of photoinduced electrons and holes can be effectively inhibited, so that the photocatalytic activity can be greatly improved [36]. To validate the versatility of the technique, the TiO2/Au:Fe2O3 bilayered thin film is used as a photocatalyst in photoelectrocatalytic degradation of wastewater. The initial experiments showed that sugarcane factory wastewater does not undergo any degradation in the absence of TiO2/Au:Fe2O3 bilayered thin film photocatalyst under direct sunlight illumination only. Also there was no degradation of sugarcane factory
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16 14
Photocurrenet (mA)
Table 1 Physicochemical and bacteriological analyses of sugarcane factory wastewater with respect to time.
(a)
Sr. No.
12 10
Parameters
Result before
Result after
Potable water standards
Unit
1 2
pH Color
4.2 Brown
6.5–8.5
– –
3
Odor
Alcoholic
Odorless
–
4 5 6
Temperature Turbidity Total hardness (as CaCO3) Calcium (as Ca) Total alkalinity (as CaCO3) Chlorides Sulphate (as SO4) Iron (Fe) Fluoride (as F) Nitrates (as NO3) BOD Sodium Potassium Faecal coliforms
22.5 695 700
4.6 Light black Light alcoholic 22 96 600
22 10 300
°C NTU mg/L
200 700
120 200
75 300
mg/L mg/L
315.64 204
285.8 86.32
250 150
mg/L mg/L
6.63 0.1 2.3 120 36.5 22.3 9000
6.03 0.098 1.8 5 34.8 6.8
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology B: Biology journal homepage: www.elsevier.com/locate/jphotobiol
Oxidative degradation of industrial wastewater using spray deposited TiO2/Au:Fe2O3 bilayered thin films M.A. Mahadik a, S.S. Shinde a, H.M. Pathan b, K.Y. Rajpure a, C.H. Bhosale a,⇑ a b
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Advanced Physics Laboratory, Department of Physics, University of Pune, Pune 411 007, India
a r t i c l e
i n f o
Article history: Received 3 October 2014 Received in revised form 21 October 2014 Accepted 27 October 2014 Available online 1 November 2014
a b s t r a c t The Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 thin films are successfully prepared by the spray pyrolysis technique at an optimised substrate temperature of 400 °C and 470 °C, respectively onto amorphous and F:SnO2 coated glass substrates. The effect of TiO2 layer onto photoelectrochemical (PEC), structural, optical and morphological properties of Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 thin films is studied. The PEC characterization shows that, maximum values of short circuit current (Isc) and open circuit voltage (Voc) are (Isc = 185 lA and Voc = 450 mV) are at 38 nm thickness of TiO2. Deposited films are polycrystalline with a rhombohedral and anatase crystal structure having (1 0 4) preferred orientation. SEM and AFM images show deposited thin films are compact and uniform with seed like grains. The photocatalytic activities of the large surface area (64 cm2) TiO2/Au:Fe2O3 thin film photocatalysts were evaluated by photoelectrocatalytic degradation of industrial wastewater under sunlight light irradiation. The results show that the TiO2/Au:Fe2O3 thin film photocatalyst exhibited about 87% and 94% degradation of pollutant in sugarcane and textile industrial wastewater, respectively. The significant reduction in COD and BOD values from 95 mg/L to 13 mg/L and 75 mg/L to 11 mg/L, respectively was also observed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the semiconductor mediated photocatalytic degradation has been a popular approach to degrade a wide range of organic contaminants like dyes [1]. Among these, synthetic dyes are most notorious organic contaminants that are discharged into the environment from textiles, tanning, leather, paints, and paper processing and pharmaceutical industries [2]. There are more than 100,000 types of dyes commercially available, with over 0.7 million tons of dyestuff produced annually and about 15% of these dyes are reported to get lost in the effluent [3]. Most of the dyes are toxic and carcinogenic compounds; they are also recalcitrant and thus stable in the receiving environment, posing a serious threat to human and environmental health [4]. In the textile industries consumption of water and chemicals for wet processing of textiles is of huge amount. The chemicals used are having variety in chemical composition, ranging from inorganic compounds to polymers and organic products [5]. The presence of very low concentrations of dyes in effluent is highly visible and undesirable [6]. Due to their chemical structure, dyes are resistant to a fading on exposure to light, water and many chemicals. Many dyes are difficult to ⇑ Corresponding author. Tel.: +91 231 2609435; fax: +91 231 2691533. E-mail address: [email protected] (C.H. Bhosale). http://dx.doi.org/10.1016/j.jphotobiol.2014.10.014 1011-1344/Ó 2014 Elsevier B.V. All rights reserved.
decolorize due to their complex structure and synthetic origin. Discoloration of textile dye effluent does not occur when treated aerobically by the municipal sewerage system [7]. Also, there are many varieties of dyes that are coloured, water soluble, reactive and are the most problematic, as they tend to pass through conventional treatment systems unaffected. Accordingly, to protect humans and the receiving ecosystem from contamination, the dyes must be eliminated from industrial effluents before discharging into the environment [8]. Municipal aerobic treatment systems, dependent on biological activity, were found to be inefficient in the removal of these dyes [9]. Other than these, there are various methods used to remove dye from industrial effluents [10]. These methods are not efficient enough because there are various materials in wastewater [11]. Also, advanced oxidation methods such as ozonation, photocatalyst, and photo Fenton are costly and uneconomical. Therefore, there is a need for more effective and cheaper ways of treating textile wastewater which consume the less amount of chemicals and energy [12]. Patil et al. carried out the photocatalytic degradation studies for water soluble hazardous Ponceau-S dye in Nb2O5 aqueous suspension along with commercial activated carbon (CAC) as co-adsorbent. They reported that the 86.2% removal of Ponceau-S dye due to significant photocatalytic degradation along with significant reduction in COD and BOD values [13]. Danwittayakula et al. was synthesized zinc
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Computer Potentiostat Ag/AgCl reference electrode C.E.
R.E. Holding Stand
W.E.
Semiconductor thin film
(b)
Brass rod for contact
FTO/glass (a)
Hamamatsu Photodiode
Optical bench O ring with electrolyte
Glass Optical bench
Constant current source LED
Fig. 1. Photoelectrochemical characterization systems for incident photon to current conversion efficiency (IPCE).
oxide/zinc tin oxide (ZnO/ZTO) nanocomposites using a hydrothermal technique for photocatalytic degradation of organic dyes. They reported that the ZnO/15ZTO showed 50% photocatalytic degradation efficiency and 77% COD removal of textile waste water when irradiated by sunlight [14]. Shavisi et al. studied the photocatalytic degradation of ammonia in petrochemical wastewater under the solar light/TiO2 photocatalysis system. They reported that the ammonia removal efficiency was increased by increasing the pH value, and after solar light irradiation in three days, degradation of ammonia was achieved up to 96.5% [15]. The Au doped Fe2O3 (Au:Fe2O3) is most effective candidates for killing the bacteria and also TiO2 is the most stable materials in aqueous solutions. In the present work, a separate set of Fe2O3, TiO2 and TiO2/Fe2O3 and TiO2/Au doped Fe2O3 (TiO2/Au:Fe2O3) bilayered thin films on amorphous and FTO coated glass substrates have been synthesized. One of the advantages of using photocatalyst in thin film form rather than fine powders or nanoparticles, is to eliminate the filtration and separation steps after the removal of pollutants from water. The deposited thin films are characterized by the PEC, XRD, Scanning electron microscopy, X-ray photoelectron and Raman spectroscopy. As sugarcane and textile factory wastewater may contains both organic impurities and bacteria. The photoelectrocatalytic degradation of sugarcane and textile factory wastewater has been carried out using TiO2/Au:Fe2O3 bilayered thin film photocatalysts in the presence of sunlight illumination has been investigated. The purified water samples were studied for total organic carbon (TOC) and chemical oxygen demand (COD) measurement which gives the extent of mineralization.
same way on these Au:Fe2O3 thin films for the growth of the TiO2/Au:Fe2O3 thin films. To deposit a TiO2 layered on Au:Fe2O3 bilayered thin films; the 0.1 M TiAcAc (C10H14O5Ti) in methanolic solutions was used for spraying. To measure PEC, the graphite was used as the counter electrode and the Fe2O3 based thin films as the working electrode. The measurement of IPCE was done with the cell form with ‘‘O’’ rings, electrolyte, gold counter electrode and Ag/AgCl as a reference electrode as described in Fig. 1(a) and (b). The incident photon to electron conversion efficiency (IPCE) was obtained by measuring the incident photon flux using LED lamps ranging from 360 nm to 420 nm. The light was calibrated using a Hamamatsu photodiode Fig. 1(a). Photocurrent spectra were measured at a constant potential vs. Ag/AgCl using a combination of a 2059 potentiostat AMEL Instruments Italy as shown in Fig. 1(b). Readings were collected at some specific intervals while the monochromatic light was scanned from 320 nm to 580 nm. The IPCE is the accepted measure to calculate the conversion efficiency of incident photons on a PEC cell to photocurrent flowing between the working and counter electrodes. An IPCE of 100% indicates the generation of one photoelectron per each incident photon. However, IPCE of 100% cannot be achieved as there are losses due to the reflection of incident photons, imperfect photon absorption by the photoelectrode and recombination [16,17]. IPCE is also called the external quantum efficiency and can be calculated using following Eq. (1) [18],
2. Experimental
where Ip(k) and P(k) are the photocurrent density (A/m2) and incident power density of light (W/m2) at wavelength k (nm), respectively. The photocurrent was measured with the Hamamatsu photodiode and potentiostat (Amel instrument, Italy). All experiments were carried out at room temperature. These optimized Au:Fe2O3 bilayered thin films were used as working electrode in the degradation experiments of wastewater from the sugarcane and textile factory.
The TiO2/Au:Fe2O3 bilayered thin films were grown by spraying the various quantities of TiO2 on optimized Au:Fe2O3 films deposited on amorphous and F doped SnO2 (FTO) coated glass substrates. Initially, Au:Fe2O3 thin films were deposited by spray pyrolysis technique onto a conductive substrate (FTO) and annealed at 500 °C for 2 h in air. After that, the TiO2 layer was deposited in a
IPCEðkÞ ¼ 1240
Ip ðkÞ PðkÞk
ð1Þ
317
200
330
130
300
120 110
270
100
240
90
2at% Au doped Fe2O3
210 0
(a)
TiO2/Fe2O3
20
40
60
(024)
140
TiO2 /2 at% Au doped Fe2O3
(116)
360
(110)
390
150
(113)
420
160
(101)
450
170
Intensity (a. u.)
180
(-ve) Voc (mV)
Isc (μ A)
480
Isc Voc
190
(104)
(021) (101)
M.A. Mahadik et al. / Journal of Photochemistry and Photobiology B: Biology 141 (2014) 315–324
80
Thickness of TiO2 on Au doped Fe2O3 thin ilms (nm)
20
30
40
50
60
Pure Fe2O3 70
80
2θ (Degree) Fig. 3. The XRD patterns of pure Fe2O3, Au doped Fe2O3, TiO2/Fe2O3 and TiO2/ Au:Fe2O3 bilayered thin films deposited onto glass substrates.
Potenostat
SUN
Reducon 2.1 eV
3.2 eV
Oxidaon
(SS)
Electrolyte
(b)
0.5
pure Fe2O3
Specs GmbH, Germany. The morphological characterization of the films was studied by using JEOL JSM-6360 scanning electron microscope (SEM). Surface topography of the films was further analysed from the AFM images taken by means of the atomic force microscope (AFM, Bruker instrument, Innova1B3BE) operated at room temperature. AFM images were collected in contact mode using a silicon nitride cantilever and the thickness was recorded using a Steller Net. Inc. USA Reflectometer having UV–Vis light source with a CCD detector. The industrial wastewater was obtained from the sugarcane and textile industries nearby Kolhapur city and used to make electrolyte in oxidative degradation experiments.
TiO2 / 2 at % Au doped Fe2O3
0.4
IPCE
3. Results and discussion 0.3
3.1. Photoelectrochemical (PEC) characterization 0.2 0.1 0.0 360 365 370 375 380 385 390 395 400
(c)
Wavelength (nm)
Fig. 2. (a) Variation of Isc and Voc with thickness of TiO2 on Au:Fe2O3 bilayered thin films, (b) schematic representation of the mechanism of charges separation in a photoelectrochemical system operated by coupling a Au doped Fe2O3 semiconductor to a TiO2 electrode, (c) IPCE spectra of the pure Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films.
The photoelectrochemical (PEC) characterization was carried out using PEC cell consisting of Fe2O3 and TiO2/Au:Fe2O3 thin film as photoelectrode, 1 M NaOH solution as an electrolyte and graphite as a counter electrode. The PEC cell was illuminated using visible light. The structural properties of deposited thin films were studied by a Bruker powder diffractometer (AXS) Analytical Instruments Pvt. Ltd. Germany, Model: D2 Phaser (k = 1.5406 Å for Cu Ka) in the range of 20–80°. Raman scattering spectrums were recorded in air at room temperature with micro Raman system from Jobin Yvon Horibra LABRAM-HR visible within 100– 800 cm1. The Raman spectra were excited using the He–Ne 632 nm laser source with 600 and 1800 lines/mm gratings and CCD detector. The XPS measurements on TiO2/Au:Fe2O3 thin films were performed after mounting them on a multi-purpose sample holder using Al Ka X-ray source and Phoibos 100 analyzer from
3.1.1. Effect of TiO2 thickness Fig. 2(a) shows that, as the thickness of TiO2 increase, then the values of photocurrent increases up to 38 nm of TiO2 and attend relatively higher values of Isc and Voc (Isc = 185 lA and Voc = 450 mV) at 38 nm thickness of TiO2. A further increase in thickness of TiO2 on Au:Fe2O3 there is a decrease in both Isc and Voc values. This poor PEC response may be attributed to the following reasons: One is the low absorption intensity; the other is the role of Fe3+. It is suggested that Fe3+ in the composite electrodes may form the deep trap site in the titania matrix to cause the recombination of photogenerated carriers [19]. This could help to optimize the thickness of TiO2 layer. Fig. 2(b) shows the schematic representation of the mechanism of charges separation in a photoelectrochemical system operated by coupling Au doped Fe2O3 semiconductor to a TiO2 electrode. The incident photon to current conversion efficiency (IPCE) values are calculated Eq. (1). Fig. 2(c) shows the IPCE values at 360 nm wavelength are higher than longer wavelengths. The IPCE spectrum of hematite photoanode typically shows IPCE values even below 10% [20,21]. However, the maximum IPCE value of TiO2/Au:Fe2O3 bilayered thin film is about 47% at 360 nm. It indicates that TiO2/Au:Fe2O3 films converts the maximum incident high energy photons to current; however, the IPCE decreased much more rapidly up to 400 nm Thus, IPCE at higher wavelengths near band gap energies is lower, which is a common characteristic of hematite photoelectrode. This is because absorption in this region is due to the d–d transition of Fe3+ and electron–hole pair generated by this transition is localized in Fe3+ and are not used effectively to generate water oxidation photocurrent [22].
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21,000
60,000
(a)
30,000
Intensity (c/s)
40,000
Ti 2P
Fe2P
20,000
Fe2p1/2
600
400
0
200
20,400
20,000 735
0 800
Fe2p3/2
20,600
20,200
Fe3p
10,000
1000
(b)
20,800
O KLL
Intensity (c/s)
50,000
O1s
730
725
Binding energy (eV) 55,000
35,000
(c)
Ti2p3/2
Intensity (c/s)
Intensity (c/s)
(d)
715
710
705
531.9 eV
50,000
30,000
Ti2p1/2
25,000
720
Binding energy (eV)
20,000 15,000
531.5 eV
45,000 40,000 35,000
533.67 eV
30,000 25,000
10,000
20,000 475
470
465
460
455
540
538
536
Binding energy (eV)
532
530
528
526
(e)
7,130
Intensity (c/s)
534
Binding energy (eV)
84.2 eV
87.4 eV
Au 4f7/2
Au 4f5/2
7,120
7,110
7,100
7,090 94
92
90
88
86
84
82
80
78
Binding energy (eV) Fig. 4. (a) The XPS survey scan spectrum of TiO2/Au:Fe2O3 bilayered thin film. (b) Narrow scan spectrum of Fe2p of TiO2/Au:Fe2O3 bilayered thin film. (c) Narrow scan spectrum of Ti2p of TiO2/ Au:Fe2O3 bilayered thin film. (d) Fitted narrow scan O1s spectrum of TiO2/ Au:Fe2O3 bilayered thin film. (e) Narrow scan spectrum of Au 4f of TiO2/ Au:Fe2O3 bilayered thin film.
3.2. X-ray diffraction studies The X-ray diffraction (XRD) patterns of Fe2O3, Au:Fe2O3, TiO2/ Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films deposited on glass substrate at optimized preparative parameters are shown in Fig. 3. It is found that, the TiO2 layer is composed of anatase phase and all the diffraction peaks of the pure Fe2O3 belong to the pure hematite structure of Fe2O3. For the TiO2/Au:Fe2O3 bilayered thin films all the reflection peaks could be indexed to the hematite phase of Fe2O3 and anatase phase of TiO2, there is no appearance of a third phase. It is observed that the (1 1 0) peak increases for Fe2O3, Au:Fe2O3 and TiO2/Fe2O3 bilayered thin films and for the TiO2/Au:Fe2O3 the peak intensity of (1 1 0) decreases and of (1 0 4) increases. This may be due to the crystal reorientation effect. The crystallite size of TiO2/Au:Fe2O3 bilayered thin films is calculated using the well-known Scherer’s relation given in equation [23].
D¼
0:9 k b cos h
ð2Þ
where b is the (FWHM) broadening of the diffraction line measured at half maximum intensity (radians) and k = 1.5406 Å is the wavelength of the Cu Ka radiations. The crystallite size of the TiO2/ Au:Fe2O3 bilayered thin films is found to be 70 nm for (1 0 4) peak. 4.1. Photoelectron spectroscopy (XPS) The oxidation states of the TiO2/Au:Fe2O3 bilayered thin film are revealed in X-ray photoelectron spectroscopy (XPS). Fig. 4(a) shows the survey scan spectrum of TiO2/Au:Fe2O3 bilayered film deposited on glass substrate. X-ray photoelectron spectroscopy (XPS) survey scan spectrum of a TiO2/Au:Fe2O3 film confirms the presence of Ti (Fig. 4(c)) and Au (Fig. 4(e)).The narrow scan spectrum of Fe2p peak positions Fig. 4(b). The Fig. 4(b) and (e) indicat-
200
400
600
1318 cm-1 1318 cm-1
(d) TiO2 /Au doped Fe2O3
(b) 2 at % Au doped Fe2O3
(a) Pure Fe2O3
800
1000
1318 cm-1
(c) TiO2 / Fe2O3
1318 cm-1
611 cm-1 611 cm-1 611 cm-1 611 cm-1
410 cm-1 410 cm-1
410
410 cm-1 cm-1
291 cm-1
225 cm-1 245 cm-1 291 cm-1 225 cm-1 245 cm-1 291 cm-1 225 cm-1 245 cm-1 291 cm-1
Intensity (a. u.)
225 cm-1 245 cm-1
M.A. Mahadik et al. / Journal of Photochemistry and Photobiology B: Biology 141 (2014) 315–324
1200
1400
Raman Shift (cm-1) Fig. 5. Raman spectra of (a) pure Fe2O3, (b) 2 at.% Au:Fe2O3, (c) TiO2/Fe2O3 and (d) TiO2/2 at.% Au:Fe2O3 bilayered thin film excited by 532 nm He–Ne laser.
ing that the chemical states of Fe and Au are +3 and 0, respectively. The XPS data can also confirm the formation of a–Fe2O3. This is in good agreement with the results obtained by Lian et al. [24]. The ratio of the Fe signals (e.g., the Fe2p) and the O1s signal is much lower for the Ti doped samples. This indicates that the Ti atoms present at the surface of the TiO2/Au:Fe2O3 bilayered thin film very
319
likely are bound in an oxide environment. The Ti2p detail spectrum of the TiO2/Au:Fe2O3 bilayered thin film sample is shown in Fig. 4(c). From the peak position and line shape, it is evident that the Ti atoms in the TiO2/Au:Fe2O3 bilayered thin film are not in a metallic environment. The Ti2p3/2 binding energy of 458.6 eV is consistent with typical values reported for TiO2 (458.7 eV and 458.6–459.3 eV) [25]. It is observed that the Ti2p1/2 peak is significantly broader than the Ti 2p3/2 peak due to the presence of a Coster–Kronig decay channel of the Ti 2p1/2 core hole, leading to lifetime broadening of the photoemission line. The Fe2p3/2 line of the TiO2/Au:Fe2O3 bilayered film is at a binding energy of 711.2 eV. The binding energy variations of determination of band gap materials, this value is consistent with typical values observed for Fe2O3 (Fe3+, 710.9 eV or 711.2 eV); FeOOH (Fe3+ 711.2 eV and 711.3–711.9 eV) and Fe3O4 (a mixture of Fe2+/Fe3+, 709.5/711.2 eV and 711.6 eV). The binding energy is not compatible with that of FeO (Fe2+, 709.6 eV and 709.1–709.5 eV) [26,27]. A satellite peak of the Fe 2p3/2 main line is observed at 719.3 eV, i.e., approximately 8.1 eV below the main line. This satellite is most likely indicative of the presence of Fe3+ species. Again, there is no evidence to support the presence of Fe2+, which should give rise to a satellite peak located at 717.2 eV. Peng et al. [28] reported that 706.9 eV related to the Fe2P3/2 binding energy in the TiO2/Fe2O3 thin films. The O1s XPS spectra collected from the TiO2/Au:Fe2O3 films shown in Fig. 4(d). The binding energy of the main line (531.5 eV) for the TiO2/Au:Fe2O3 sample is consistent with the reported value for Fe2O3 (531.9 and 531.6 eV) and Fe3O4 (530.0 and 530.3 eV) [29]. The Au 4f doublet peak of Au is detected by XPS at sufficient intensity; peaks position and shape (Fig. 4(e)) coincide with literature
Fig. 6. Scanning electron micrographs of pure Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films.
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97.24 nm
2000
78.44 nm
2000
(a)
(b) 1500
Y [nm]
Y [nm]
1500
1000
500
1000
500
0 0
500
1000
1500
2000
0
0.00 nm
0
500
X [nm]
64.18 nm
2000
1500
2000
0.00 nm
100.98 nm
2000
(d)
(c) 1500
Y [nm]
1500
Y [nm]
1000
X [nm]
1000
1000
500
500
0 0
500
1000
1500
2000
0.00 nm
X [nm]
0 0
500
1000
1500
2000
0.00 nm
X [nm]
Fig. 7. 2D atomic force micrographs of (a) pure Fe2O3, (b) Au doped Fe2O3, (c) TiO2/Fe2O3 and (d) TiO2/Au:Fe2O3 bilayered thin films.
data [30]. A small shift of Au 4f7/2 peak towards lower binding energy values is found. 4.2. Raman spectroscopy To confirm the phase formation of deposited the thin films the Raman spectra are presented in Fig. 5. The Raman spectra shows that the Raman peaks are well matched to the a–Fe2O3 and the anatase TiO2 phase. The similar results reported by Noh et al. [31]. One sees that the spectra of all these films are similar nature. The intensity of 291 cm1 peak is increased as the TiO2 layer deposited on Fe2O3 thin films. The peaks appearing in the pure Fe2O3 spectra at 225, 245, 291, 410, 495, 611 and 1318 cm1 correspond to the characteristic peaks of a–Fe2O3. The peaks at 225 and 495 cm1 correspond to the A1g mode and the four peaks at about 245, 291, 410 and 611 cm1 are attributed to the Eg mode [32]. According to factor group analysis, anatase has six Raman active modes (A1g + 2B1g + 3Eg). Ohsaka [33] reported the Raman spectrum of anatase single crystal and showed that it shows six modes appearing at 144 (Eg), 197 (Eg), 399 (B1g), 513 (A1g), 519 (B1g), and 639 cm1 (Eg). From Fig. 4, it is seen that intensity and broadening of peak at 291 cm1 is increased for the TiO2/Fe2O3 and TiO2/ Au:Fe2O3 bilayered thin films. This is due to the thickness of TiO2 on the Au:Fe2O3 layer [34]. 4.3. Surface morphology analysis Fig. 6 shows the scanning electron micrographs of pure Fe2O3, Au:Fe2O3, TiO2/Fe2O3 and TiO2/Au:Fe2O3 bilayered thin films deposited on glass substrates. It is seen from Fig. 6 that, the surface of the film changes from needle shaped to oval shaped particles with 200–180 nm diameter. It is observed that the grain size determined from SEM images is larger than the value determined by XRD, which may be due to the discrepancy between the mean
dimension of the crystallites perpendicular to diffracting planes by XRD diffraction and the observable aggregates in SEM images. It is seen that, while going from TiO2/Fe2O3 to TiO2/Au:Fe2O3, the surface morphology is essentially changing from granular to oval shaped structure. Fig. 7 shows the two dimensional (2D) AFM images of (a), pure Fe2O3 (b), Au:Fe2O3, (c) TiO2/Fe2O3 and (d) TiO2/Au:Fe2O3 bilayered thin films taken in contact mode with 2 lm 2 lm scan range. From Fig. 7, it is observed that the particle size decreases with increasing roughness of the films. 4.4. Photoelectrocatalytic degradation of sugarcane factory wastewater It is well known that, when semiconductor photoelectrode is irradiated by light with energy higher than or equal to the band gap, an electron (e) in the valence band (VB) can be excited to the conduction band (CB) with the generation of a hole (h+) in the VB at the same time. The photoelectron can be easily trapped by electronic acceptors like adsorbed O2, and form a superoxide radical anion (O 2 ), whereas the photo-induced holes can be easily trapped by such as organic pollutants, to further oxidize organic pollutants [35]. Whenever, the photo-generated electrons and holes recombined, the photocatalytic activity would be decreased. But in oxide semiconductors like ZnO, Fe2O3 and TiO2 the oxygen vacancies can act as the active centres to capture photoinduced electrons and the recombination of photoinduced electrons and holes can be effectively inhibited, so that the photocatalytic activity can be greatly improved [36]. To validate the versatility of the technique, the TiO2/Au:Fe2O3 bilayered thin film is used as a photocatalyst in photoelectrocatalytic degradation of wastewater. The initial experiments showed that sugarcane factory wastewater does not undergo any degradation in the absence of TiO2/Au:Fe2O3 bilayered thin film photocatalyst under direct sunlight illumination only. Also there was no degradation of sugarcane factory
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16 14
Photocurrenet (mA)
Table 1 Physicochemical and bacteriological analyses of sugarcane factory wastewater with respect to time.
(a)
Sr. No.
12 10
Parameters
Result before
Result after
Potable water standards
Unit
1 2
pH Color
4.2 Brown
6.5–8.5
– –
3
Odor
Alcoholic
Odorless
–
4 5 6
Temperature Turbidity Total hardness (as CaCO3) Calcium (as Ca) Total alkalinity (as CaCO3) Chlorides Sulphate (as SO4) Iron (Fe) Fluoride (as F) Nitrates (as NO3) BOD Sodium Potassium Faecal coliforms
22.5 695 700
4.6 Light black Light alcoholic 22 96 600
22 10 300
°C NTU mg/L
200 700
120 200
75 300
mg/L mg/L
315.64 204
285.8 86.32
250 150
mg/L mg/L
6.63 0.1 2.3 120 36.5 22.3 9000
6.03 0.098 1.8 5 34.8 6.8
