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Jun 9, 2012 - Photoelectrocatalytic degradation of oxalic acid by spray deposited nanocrystalline zinc oxide thin films. S.S. Shinde a, P.S. Shinde b, R.T. ...
Journal of Alloys and Compounds 538 (2012) 237–243

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Photoelectrocatalytic degradation of oxalic acid by spray deposited nanocrystalline zinc oxide thin films S.S. Shinde a, P.S. Shinde b, R.T. Sapkal a, Y.W. Oh b, D. Haranath c, C.H. Bhosale a, K.Y. Rajpure a,⇑ a

Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Department of Nano-Engineering, Kyungnam University, Masan 631-701, Republic of Korea c National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110 012, India b

a r t i c l e

i n f o

Article history: Received 1 April 2012 Received in revised form 28 May 2012 Accepted 31 May 2012 Available online 9 June 2012 Keywords: Semiconductors Chemical synthesis Electrical Optical Thermal properties

a b s t r a c t The high quality nano-crystalline zinc oxide thin films are deposited onto corning glasses by spray pyrolysis technique. The influence of reaction temperature onto their photoelectrochemical, structural, morphological, optoelectronic, luminescence and thermal properties has been investigated. The structural characteristics studied by X-ray diffractometry has complemented by resistivity measurements and UV–Vis spectroscopy. The photoelectrochemical activity shows enhancement in short circuit current (Isc = 0.357 mA) and open circuit voltage (Voc = 0.48 V). Direct band gap calculated by considering R & T values of ZnO thin films increases from 3.14–3.21 eV exhibiting a slight blue shift in band edge. Three characteristic luminescence peaks having near band-edge, blue and green emission are observed in the photoluminescence spectra. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant in films. Photocatalytic degradation of oxalic acid followed with reaction mechanism by using zinc oxide photoelectrode under solar illumination has been investigated. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Metal oxides are an important class of materials that find several relevant technological applications such as high-temperature superconductivity, ferroelectricity, ferromagnetism, piezoelectricity and semiconductivity. Zinc oxide (ZnO) being a potential II–VI semiconductor with direct wide band gap (3.3 eV), high exciton binding energy (60 meV) has received enormous scientific attention because of its promising applications in optoelectronic nanodevices [1], piezoelectric nano-generators [2], dye-sensitized solar cells [3], UV-photodetector [4], gas sensors [5], TFTs [6] and photocatalysts for degradation and complete elimination of environmental pollutants [7]. Furthermore, ZnO is an environmental friendly material, which is desirable especially for bio-applications, such as bio-imaging and cancer detection [8]. Currently, intense research is focused on ZnO for several reasons: (i) the abundance of its components in nature (ii) its non-toxicity (iii) and the wide range of its electrical resistivity which can extend from 104 to 1012 X cm according to the deposition conditions. Structures of ZnO have also attracted increasing attention because it can be fabricated in a variety of shapes, such as thin films, nanowires, nanorods and nanoparticles [9–12]. Chakrabarti and Dutta [13] reported photocatalytic degradation of textile dyes in wastewater using ZnO as semiconductor catalyst. ⇑ Corresponding author. Tel.: +91 231 2609435; fax: +91 231 2691533. E-mail address: [email protected] (K.Y. Rajpure). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.124

The effects of process parameters like, catalyst loading, initial dye concentration, airflow rate, UV-radiation intensity, and pH on the extent of photo degradation have been investigated. Substantial reduction of COD, besides removal of color was also achieved. Yassıtepe et al. [14] prepared ZnO plates for photocatalytic degradation of RO 16 and RR 180 textile dyes in aqueous solutions by tape casting method. They observed enhancement in photocatalytic activity due to increased surface area plates. Total organic carbon (TOC) removal was 43% at 180 min for RR 180 solution. Chen [15] reported degradation pathways of ethyl violet by photocatalytic reaction with ZnO dispersions under UV irradiation. Twenty-six intermediates have been detected by HPLC–ESI-MS. Also for decolorization of dyes, the possible photodegradation pathways were proposed. Gaya et al. [16] studied the photocatalytic treatment of 4-chlorophenol in aqueous ZnO suspensions. Kinetic dependence of transformation rate on operating variables such as initial 4-chlorophenol concentration and photocatalyst doses was investigated. A radical mechanism involving o-hydroxylation is proposed to account for the formation of catechol. Many researchers have reported photocatalytic degradation of various organic compounds in aqueous suspension for different zinc oxide [17,18]. The removal of aqueous ZnO particles is very difficult so we tried to use thin films of ZnO for photocatalysis of organic impurities. In recent years, many efforts have been made for achieving ZnO thin films via different approaches. However, due to lack of information about film formation mechanisms, many researches have

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been focused on different possible methods to obtain desired shapes. These methods include hydrothermal [19], solvothermal [20], reverse micelles [21], sol–gel [22], direct chemical synthesis [23], spray pyrolysis [24–26] etc. Among different methods of synthesis, which have been used by several researchers, spray pyrolysis carry some advantages including ease of operation, being fast, low cost and high efficiency. This manuscript deals with the influence of substrate temperature onto the physico-chemical properties (such as photochemical, structural, morphological, optical, luminescent, electrical and thermal properties) of ZnO films. The kinetics of oxalic acid degradation with reaction mechanism has been investigated. 2. Experimental

substrate temperature. Both Isc and Voc increases gradually with substrate temperature, attains a maximum value (Isc = 0.357 mA and Voc = 0.48 V respectively) at 450oC substrate temperature and then decreases for higher temperatures. The lower values of Isc and Voc at low temperatures are due to partial decomposition of spraying solution. The decrease in Isc and Voc after 450 °C temperature is due to decrease in film thickness. Upon illumination of junction in PEC cell, the magnitude of Voc increases with negative polarity towards the ZnO thin film, indicating cathodic behavior of photovoltage which confirms the films are n-type in nature.

3.2. Structural analysis Fig. 3 shows the X-ray diffraction (XRD) patterns of zinc oxide thin films deposited on corning glass substrate for different substrate temperatures from 400–500 °C. The films are polycrystalline and fit well with the hexagonal crystal structure with preferred orientation along (0 0 2) plane. Comparison of standard and observed ‘d’ values of ZnO thin films is carried out using Joint Committee for Powder Diffraction Standards (JCPDS) card No. 05-0664. The reason for relatively lower peak intensities is the lower film thickness and formation of nanocrystalline phase in the films. As the substrate temperature increases, the intensity of (0 0 2) reflection increases and (1 0 0) and (1 0 1) reflections decreases evidently up to 450 °C and then vice versa, showing the crystal reorientation effect [27]. Some weak reflections such as (1 0 0), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) have also been observed but with small

0.40

0.50 Isc Voc

0.35

Isc (mA)

3. Results and discussion

Fig. 1. Schematic experimental setup of PEC reactor.

0.48

0.30

0.46

0.25

0.44

0.20

0.42

0.15

0.40

Voc (V)

Zinc oxide thin films were synthesized by using chemical spray pyrolysis technique onto the ultrasonically cleaned corning glass substrates. The 0.1 M zinc acetate (Zn (CH3COO)22H2O) Himedia, 99.99%, A.R. grade is dissolved in double distilled water. The resulting 100 cc solution was sprayed onto cleaned corning glass substrates for different substrate temperatures ranging from 400 to 500 °C keeping interval of 25 °C assisted with compressed air as a carrier gas. Other preparative parameters viz., solution concentration (0.1 M), spray rate (5 cc/min), nozzle to substrate distance (32 cm) was kept constant for all experiments. The fine aerosols of aqueous zinc acetate after being sprayed through an atomizer onto the preheated glass substrate, was underwent pyrolytic decomposition, forming thereby a thin solid film. PEC cell was fabricated using a two-electrode configuration, comprising n-ZnO thin film as a photoanode and graphite as a counter electrode. The PVA +0.1 M KOH +0.1 M NaOH in the ratio (3:3:4) was used as an electrolyte. This PEC cell was illuminated with 20 W UV OMNILUX lamp with an excitation wavelength of 365 nm for the measurement of short circuit current (Isc) and open circuit voltage (Voc). The structural characterization of deposited thin films were carried out, by analyzing the X-ray diffraction patterns obtained under Cu-Ka (k = 1.5406 Å) radiation from a Philips X-ray diffractometer model PW-1710 and surface morphology was studied using JEOL JSM-6360 scanning electron microscope (SEM). The resistivity of the film was measured using Hall effect technique in van der pauw configuration. Transmission spectra were recorded at room temperature and near to normal incidence using a 119 SYSTRONICS UV–Vis spectrophotometer. The optical reflectance was recorded using a StellerNet Inc USA Reflectometer having UV–Vis light source with CCD detector. The room-temperature photoluminescence (PL) spectra were recorded using Perkin-Elmer Luminescence Spectrometer (Model: LS-55) equipped with a Xenon flash lamp and a grating to select the source of excitation (wavelength is 224 nm). The specific heat capacity and thermal conductivity was measured by using C–T meter made by Teleph Pvt. Ltd., France. Photocatalytic activity has been studied with the help of photoelectrocatalytic reactor model with ZnO photocatalyst. The ZnO electrode used in this study was deposited by spray pyrolysis onto large area (10 cm  10 cm  0.125 cm) conducting glass plates (spray deposited fluorine doped tin oxide on glass, FTO, with sheet resistance of 10–20 X sq1). The photoelectrochemical cell encloses a ‘ZnO electrode coated on FTO substrate’ serving as photoanode and stainless steel disc as a cathode at a distance of 0.1 cm facing the photoanode. The illuminated surface area of the electrode in contact with the organic species was 64 cm2 using external bias voltage of about 1.5 V. These electrodes were illuminated from backside employing solar light with manual inclination and azimuth tracking to avoid photolysis. The experimental setup of photoelectrocatalytic degradation reactor is as shown in Fig. 1. Oxalic acid was a model organic species obtained from s.d. fine chem. Ltd. and used without further purification to make electrolyte in double distilled water. The electrolyte was recirculated through the PEC reactor with a constant flow rate of 8.4 l h1 using a Gilson MINIPLUS peristaltic pump, France with silicon tubing. Using aliquots withdrawn from the reaction mixture at some intervals, the concentrations of impurities in the solutions were determined by measuring the extinction spectra (in 1 cm quartz cell) using a 119 SYSTRONICS UV–vis spectrophotometer.

3.1. Photoelectrochemical (PEC) characterization Optimization of preparative parameters for deposition of goodquality adherent thin films is most essential. It is carried out by measuring the maximum values of Isc and Voc of the PEC cell formed with photoactive ZnO films, PVA +0.1 M KOH +0.1 M NaOH was used as an electrolyte. Fig. 2 shows the variation of short circuit current (Isc) and open circuit voltage (Voc) with respect to

0.10

0.38 400

420

440

460

480

500

Substrate Temperature (oC) Fig. 2. Variation of Isc and Voc against substrate temperature for ZnO thin films.

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intensities. The diffraction angle of the (0 0 2) peak is almost in agreement with the zinc oxide bulk single crystal, implying that no evident residual stress or inclusion-induced lattice distortion has been developed in the film due to temperature. The average crystallite size of ZnO thin films is calculated using the Scherrer’s relation [28],



0:9k ðb cos hÞ

ð1Þ

where k is the wavelength of Cu-Ka line, b is FWHM in radians and h is the Bragg’s angle. The crystallinity of the ZnO thin films decreases up to 450 °C and then increases for higher temperature. Average crystallite size varies from 41–37 nm with substrate temperature confirms nanocrystalline nature of deposited films. Quantitative information concerning the preferential crystallite orientation is obtained from different texture coefficients (hkl) defined by well known relation [28], IðhklÞ Io ðhklÞ TCðhklÞ ¼ X IðhklÞ 1 N

ð2Þ

Io ðhklÞ N

where I(hkl) is measured intensity, I0(hkl) the JCPDS (card No-050664) intensity and N is the reflection number. It is seen that texture coefficient of (0 0 2) plane increases with temperature up to 450 °C and then decreases further (Table 1). The highest achieved value of texture coefficient at (0 0 2) plane is 5.03 at 450 °C temperature due to enhancement in the crystallinity of the film. 3.3. Morphological properties Scanning electron micrograph (SEM) images of zinc oxide thin films deposited at different substrate temperatures are shown in Fig. 4(a–e). The micrographs reveal that substrate is well covered with large number of grains and film surface is uniform, adherent with compact growth. The surface morphology is essentially changing from granular to platelet-rich with increasing substrate temperature. Deposited film surface is smooth with some overgrown particles as seen in (a). As we increase temperature the overgrown particles disappears due to complete decomposition of precursor solution. At higher temperature, films become rough, compactless and grains are randomly grown on surface of

o 500 C

(112)

(110)

(102)

(103)

o 450 C

(101)

(100)

Intensity (A.U.)

(002)

o 475 C

o 425 C o 400 C 30

40

50

60

70

2θ (Deg) Fig. 3. XRD patterns of ZnO thin films deposited on glass substrates at different substrate temperatures.

Table 1 Electrical properties of ZnO thin films deposited for different substrate temperatures. Sub. Temp. (°C)

r,

l,

(X cm)1

(cm2/Vs)

n, 1017, (cm3)

TC (0 0 2) plane

400 425 450 475 500

0.157 0.171 0.177 0.165 0.147

3.33 4.51 6.51 4.86 3.37

2.96 2.38 1.7 2.12 2.74

1.6 2.88 5.03 2.09 1.33

substrate. The film surface becomes relatively smooth, dense and a mixture of large and small hexagonal platelets is seen in the micrograph (c) for film deposited at 450 °C temperature due to an improved surface diffusion. As the grains agglomerated with an increase in the temperature, large flakes like structure are observed in the films. It is observed that the grain size (70– 125 nm) observed by 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. For higher temperatures, these platelets are clustered forming large grains as seen in (d–e). Therefore it can be concluded that the morphology of the particles change and their sizes reduce by increasing the reaction temperature. Theoretically, we know small regions of high strain will evolve as grooves or pits if strain is not homogeneously relieved during the growth [29]. Therefore, we can assume that the uneven component distribution of ZnO which grown at relatively low temperatures could result in local sites with high strain. As a result, the depositing materials diffused away from these high-strain sites, leaving behind the pits on the surface. 3.4. Optical properties Fig. 5 shows the optical reflectance and transmission spectra of ZnO films over spectral range 350–850 nm for the films deposited at different substrate temperatures. The well-developed interference patterns in R and T show that the films are specular to a great extent. As the temperature increases the transmittance goes on increasing attains maximum at 450 °C and then decreases for further increase in temperature. The films show moderate optical transmittance and reflectance between 60–80% and 1–9% at 550 nm respectively. In general, in the visible region of the spectrum, the transmission is very high due to transfer of electrons from valence band to conduction band owing to optical interference effects; it is possible to maximize the transmission. An increase in the transmittance of zinc oxide films at 450 °C can be attributed to the removal of organic and hydroxide species due to optimum substrate temperature. In metal oxides, the metal to oxygen ratio decides the percentage of transmittance. A metal rich film usually exhibits less transparency [30]. At lower temperatures, i.e. 450 °C) indicates the loss in surface smoothness leading to a slight scattering loss. These interference peaks are used to determine the film thickness by fitting observed and calculated transmittance data. Thickness of the film varies from 215–285 nm, with 285 nm being maximum at 450 °C substrate temperature. Direct band gap (Fig. 6) calculated by considering R & T values of ZnO thin films increases from 3.14–3.21 eV up to 450 °C and further decreases for higher temperatures. The spectra clearly exhibit a slight blue shift in band edge due to the variation of substrate temperature, with a

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Fig. 4. Scanning electron micrographs of ZnO thin films deposited at (a) 400 °C, (b) 425 °C, (c) 450 °C, (d) 475 °C and (e) 500 °C substrate temperatures.

strain in deposited zinc oxide films and after increasing the deposition temperature, the band-gap value is decreasing due to the relaxation of the built in strain [29].

1.0

Reflectance, Transmittance

0.9 0.8

3.5. Photoluminescence properties

0.7 o

(a') R 400 C

0.6

o

0.5

(a)

0.4

(b)

(c)

(d)

(a) T 400 C o (b') R 425 C

(e)

o

(b) T 425 C o (c') R 4 50 C o

(c) T 450 C o (d') R 475 C

(e')

0.3

o

(d) T 475 C o (e') R 500 C

(d') (c')

o

(e) T 500 C

(b')

0.2

(a') 0.1 0.0 400

500

600

700

800

Wavelength (nm) Fig. 5. Optical transmittance and reflectance spectra of ZnO thin films deposited at various substrate temperatures.

transparency in the visible range. The band gap of films increases up to 450 °C temperature showing negative strain along c-axis. So the blue shift of the band gap is attributed to the compressive

Generally, ZnO shows four PL emissions [31]: (a) near band edge emission around 390 nm (UV emission) attributed to nearband-gap recombination of free excitons, (b) blue emission around 460 nm is because of intrinsic defects such as oxygen and zinc interstitials, (c) green emission around 540 nm is known to be a deep level emission which is caused by deep-level defects (impurities, a structural defects in the crystal such as oxygen vacancies, zinc interstitials, etc.), and (d) red emission around 630 nm due to oxygen and zinc antisites. Fig. 7 shows the room temperature photoluminescence spectra having 225 nm excitation wavelength of ZnO thin films deposited at different substrate temperatures. Three characteristic luminescence emission peaks having near band-edge emission at 397 nm, blue emission at 446 nm and green emission at 560 nm wavelength are observed in the PL spectra for all temperatures having similar nature. Fig. shows the intensity of the UV–visible emission, as well as the contribution of different observed transitions in the visible spectral range, is clearly dependent on the growth conditions. The intensity of characteristic peaks increases with substrate temperature up to 450 °C and further decreases for higher temperatures. It

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25

V =Zinc vacancy Zn O =Interstitial Oxygen i

1/2 (αhν) , 104 (eV.cm)1/2

o

400 C o 425 C o 450 C o 475 C o 500 C

20

15

V =Oxygen vacancy O O =Anticite Oxygen Zn

Zni=Interstitial zinc

1.65 eV 2.22 eV Eg= 3.21 eV

10

2.40 eV

3.12 eV 2.78 eV VO

5

Oi VZn

0 2.0

2.2

2.4

2.6

2.8

3.0

3.2

OZn

Zni

3.4

Energy (eV) Fig. 6. Variation of (ahm)2 against hm for ZnO thin films.

Fig. 8. Schematic representations of calculated defect levels in ZnO thin films.

is explained that progressive increase of the UV emission relative to the deep level emission suggests the ZnO films have higher crystallization and the low density of defects in the films. The calculated energy levels of the intrinsic defects in ZnO by applying full-potential linear muffin-tin orbital method and the results are depicted in Fig. 8. The strong near band-edge emission at 397 nm is attributed to the recombination of free excitons [32]. They have a larger binding energy of about 60 meV and can give efficient excitonic emission at room temperature. Meanwhile, the energy formation of free exciton stimulated from the light is less than the energy required by the free electron transiting from the valence band to conduction band and this leads to the fact that the probability of free excitons recombination emission is larger than that of the band to band recombination emission and hence the intensity of free excitons recombination emission is much stronger than that of the band to band recombination emission. With increasing temperature, the defect concentration decreased and the crystal quality changed better, resulting in higher concentration of free excitons as a result, the intensity of the UV peak increases. The peak around 446 nm i.e. blue emission is attributed to intrinsic defects such as oxygen and zinc interstitials [31]. The green band emission (560 nm) corresponds

60000

60000

o

397 nm

450 C

50000 40000

40000

o

450 C o 475 C o 425 C o 500 C o 400 C

30000

Intensity/ A.U.

Intensity (A.U.)

50000

30000 446 nm

20000

560 nm

10000 0 350

400

450

500

550

600

650

700

Wavelength/nm

20000 10000 0 350

to the singly ionized oxygen vacancy in ZnO in the bulk of nano-particles and excess oxygen on the surface, which might be in the form of OH ions and results from the recombination of photo-generated hole with single ionized charge state of this defect [33]. However, no blue shift has been observed in photoluminescence spectra due to size effect. This is because the PL emission generally comes from the ZnO nano-crystals and the blue shift in the optical absorption spectra is due to the presence of amorphous phase in the crystalline material [34].

3.6. Electrical properties The dependence of electrical properties of ZnO thin films deposited at various substrate temperatures is as shown in Table 1. The initial increase in conductivity with temperature up to 450 °C is a consequence of increase in mobility due to the improvement in crystalline structure of the films as observed by the XRD analysis. Further, the conductivity decreases after 450 °C temperature due to evaporation of solution and attaining lower film thickness. Since air and water is used as the carrier gas and solvent, there might have been the possibility of chemisorptions of oxygen during deposition of the films. Zinc oxide is an n-type semiconductor in which donor levels are due to oxygen vacancies (VO) and interstitial zinc (Zni) atoms. Hence, one can assume that, the reason for increase in conductivity is due to creation of oxygen vacancies (VO) and zinc interstitial (Zni) during the deposition as confirmed from PL analysis. The electron mobility increases with substrate temperature up to 450 °C, attains 6.51 cm2/Vs value, and then decreases. Further increase in substrate temperature leads to decrease in mobility due to an enhancement in film disorder confirming grain boundary scattering is the dominant mobility limiting mechanism [35]. In addition to that, carrier concentration decreases with respect to substrate temperature showing lowest value 1.7  1017 cm3 at optimum temperature.

3.7. Thermal properties

400

450

500

550

600

650

700

Wavelength (nm) Fig. 7. PL spectra of ZnO thin films as a function of substrate temperature (Inset shows Gaussian fitting of PL spectrum of typical ZnO thin film deposited at 450 °C temperature).

It is essential to understand the thermo-physical properties of the zinc oxide thin films, when working on those industrial and scientific applications that involve not only equipment design but also analysis, modeling and process control, where there are temperature-dependent physical, chemical and biochemical changes.

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Thermal conductivity analysis is carried out with the help of the following relation,

DT ¼

0.7 0 min

0.6

RI2 1 ½lnðtÞ þ C te  L 4pk

ð3Þ 0.5

Extinction

where, R is the resistance of the ring probe, k the thermal conductivity, I the current, t the pulse time, L the length of ring probe and Cte the integration constant. The variation of thermal conductivity and specific heat with respect to substrate temperature is as shown in Fig. 9. Thermal conductivity and specific heat increases up to 450 °C (attains maximum value 0.854 W/mK and 1654 kJ/ m3K respectively) and then decreases. The increase in thermal conductivity is attributed to grain boundary conduction and decrease in thermal conductivity is due phonon conduction behavior in these films [36]. The thermal conductivity for a crystalline solid is due to changes of lattice vibrations, which are usually described in terms of phonons.

0.4 0.3 0.2 0.1

180 min

0.0 200

225

250

275

300

325

Wavelength (nm)

3.8. Photocatalytic activity

Fig. 10. Variation of extinction spectra versus time interval for oxalic acid degradation.

0.0 k =4.28E-4 s

-0.2

-1

-0.4 -0.6

ln (c/c0)

Photocatalytic degradation of oxalic acid under the solar light is performed to investigate the photocatalytic activity of ZnO thin films [37]. Dark experiments showed zero effect on oxalic acid degradation with ZnO electrodes. Initially, oxalic acid does not undergo any degradation under direct solar light illumination in the absence of ZnO electrodes and in the absence of solar light. Variation of extinction spectra versus time interval for oxalic acid degradation in the wavelength range of 200–325 nm is shown in Fig. 10. It is seen that, extinction goes on decreasing with reaction time, but close inspection shows that the extent of this decrease is not the same for all wavelengths, hinting the absorption by intermediate compounds. During degradation process, the concentration of oxalic acid decreases due to its decomposition (photoelectrochemical oxidation). Fig. 11 shows the kinetics of degradation of oxalic acid (i.e. plot of ln(c/c0) w. r. t. reaction time) using ZnO under solar illumination. The linear portion in the plot has a slope of –k (k = 4.28  104 cm3 s1). The kinetic data of the photocatalytic degradation of oxalic acid fit well to the pseudo first-order reaction kinetics. The apparent first-order reaction rate constant k0 , k00 and k’00 are found to be 6.4  102 cm3 s1, 1.003  103 cm s1 442.52 M1, respectively. The calculated kinetic parameter (p) is about 0.00226 M. The degradation efficiency of oxalic acid is observed to be 84% by using single ZnO electrode in 180 min. The possible reaction mechanism for degradation of oxalic acid is as shown in following reactions.

-0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0 0

2000

4000

6000

8000

10000

12000

Time (s) Fig. 11. Plot of ln(c/c0) with respect to reaction time.

At the ZnO working electrode: þ

ZnO þ hm ! e þ h þ ZnO þ

e þ h ! Heat ðrecombinationÞ Thermal Conductivity Sp. Heat



1650

1550

0.80

1500 0.75 1450 1400

0.70

1350 0.65 1300 1250

0.60 400

425

450

475 Substrate Temperature (oC)

COOH ! Hþ þ e þ CO2 " þ

1600 Sp. Heat (KJ/m3K)

Thermal Conductivity (W/mK)

0.85

þ

h þ HOOC  COO !  COOH þ CO2 "

1700

0.90

500

Fig. 9. Variation of thermal conductivity and Sp. heat of ZnO thin films as a function substrate temperature.

h þ OH ! OH HOOC  COO þ OH ! OH þ COOH þ CO2 " 

COOHþ OH ! H2 O þ CO2 "



OHþ OH ! H2 O2 þ

þ

H2 O2 þ 2h ! O2 þ 2h

At the counter electrode: þ

4e þ 4h þ O2 ! 2H2 O    The generated oxidants (HO, HO2 , O 2 ) and reductants (H ,eaq , HO2 ,  O2 ) make possible simultaneous reductions and oxidations in the chemical system. Variation chemical oxygen demand (COD) and total organic carbon (TOC) in course of time is shown in Table 2. A strongly absorbing solution is almost degraded after 180 min and reduction of COD and TOC is obtained from 101.2 to 16.2 and 38.92 to 6.55 mg/l respectively. This technology can be used for

S.S. Shinde et al. / Journal of Alloys and Compounds 538 (2012) 237–243 Table 2 Variation COD and TOC values w. r. t. reaction time. Reaction time (Min)

COD (mg/L)

TOC (mg/L)

0 20 40 80 100 120 150 180

101.2 78.57 74.69 38.86 24.92 22.43 19.54 16.2

38.92 30.22 28.72 14.79 9.53 8.47 7.46 6.55

degradation of pollutants in water treating for oxidizable compounds that are difficult to treat, such as chlorinated, fluorinated hydrocarbons. 4. Conclusions Highly textured and nano-crystalline ZnO thin films have been prepared by inexpensive spray pyrolysis technique. It is found that the substrate temperature plays an important role in depositing high quality ZnO electrodes. The growth and physic-chemical properties are temperature dependent. The PL study reveals the near band-edge, blue, green emissions. Photocatalytic degradation of oxalic acid by using ZnO films under solar illumination with reaction mechanism has successfully studied. Continuous COD and TOC measurements should be performed to follow degree of mineralization during the process. The photochemical advanced oxidation process can results an effective alternative to traditional processes (e.g. chlorination, biological treatment). Acknowledgements The authors are very much thankful to the Defense Research and Development Organization (DRDO), New Delhi, for the financial support through its project No. ‘‘ERIP/ER/0503504/M/01/ 1007’’. Authors also thank to UGC-DSA-I, DST-PURSE and DSTFIST-II programs for financial support. References [1] S.S. Shinde, Prakash.S. Patil, R.S. Gaikwad, R.S. Mane, B.N. Pawar, K.Y. Rajpure, J. Alloys Compd. 503 (2010) 416–421. [2] S. Gupta, P. Kumar, A. Arul Chakkaravathi, D. Craciun, R.K. Singh, Appl. Surf. Sci. 257 (2011) 5837–5843.

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