J Mater Sci: Mater Electron (2016) 27:5851–5859 DOI 10.1007/s10854-016-4502-9
Photocatalytic activity of spray deposited ZrO2 nano-thin films on methylene blue decolouration M. Jothibas1 • C. Manoharan2 • S. Johnson Jeyakumar1 • P. Praveen3 I. Joseph Panneerdoss1
•
Received: 16 November 2015 / Accepted: 8 February 2016 / Published online: 16 February 2016 Ó Springer Science+Business Media New York 2016
Abstract Nanostructured zirconium oxide (ZrO2) thin films were deposited at various substrate temperatures (275–475 °C) through spray pyrolysis technique. The X-ray diffraction pattern of the products was indexed to the tetragonal phase of ZrO2 and the crystallite size of the films increased by increasing substrate temperature. The Raman spectrum confirmed the crystalline structure of the sample. The spherical shape morphology of the materials was obtained by scanning electron microscopy and verified using transmission electron microscopy. The energy dispersive X-ray analysis indicated that the elemental ratio (Zr:O = 33.76:66.24) was similar to ZrO2. The selected area electron diffraction pattern reveals the well-crystallized tetragonal form. From the UV–Vis spectra, the observed band gap values were increased with increase of substrate temperatures. The photoluminescence spectra showed emission peaks corresponding to blue emission. Mainly, the photocatalytic activity of the ZrO2 catalyst was tested using methylene blue (MB) dye as a model contaminant for environmental remediation. The percentage degradation increases rapidly with the increase in time for MB-ZrO2 solution.
& M. Jothibas
[email protected] 1
Department of Physics, T.B.M.L.College, Porayar, Tamilnadu 609307, India
2
Department of Physics, Annamalai University, Annamalai Nagar, Tamilnadu 608002, India
3
PG and Research Department of Physics, St. Joseph’s College of Arts and Science (Autonomous), Cuddalore, Tamilnadu 607001, India
1 Introduction In recent years, the use of thin films for optical applications widely spread. The main research effort has been concentrated in the development of low-cost process and diagnostic technologies that supply the required performances and reliability to determine the quality of the materials to be used in defined applications. In spite of the different nature and applications of these films, they all show properties that are dependent on the technique and conditions of growth. One of these properties is the uniformity, which can compromise the electrical or optical behavior of the devices in which the materials are to be used. Zirconium oxide (ZrO2) mainly contains three polymorphs: monoclinic, tetragonal and cubic. The monoclinic phase is thermodynamically stable from room temperature up to 1170 °C. The tetragonal phase forms at 1170–2370 °C and transforms to cubic phase at temperatures over 2370 °C [1], and phase transformation among them can be induced by high temperature. Thermal annealing is a widely used method to improve crystal quality and to study structural defects in materials. Due to annealing, the structure and the stoichiometric ratio of the material will change. Such phenomena can have major effects on the structural, optical and morphological properties [2]. ZrO2 has several properties that make it a useful material. These properties include high density, hardness, electrical conductivity, wear resistance, high fracture toughness, low thermal conductivity, relatively high dielectric constant and extreme chemical inertness [3, 4]. It is a very useful candidate in optics due to its excellent optical properties, such as a high refractive index, large optical band-gap, low optical loss and high transparency in the visible and near-infrared regions being used for high refractive mirrors, broadband interference filters and active
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electro-optical devices [5]. Several techniques have been used to obtain thin films of ZrO2: metal–organic chemical vapour deposition [6], sol–gel [2] radio-frequency sputtering [7], ultraviolet ozone oxidation [8], pulsed laser deposition [9], spin coating [10], photo-chemical vapour deposition [11], sputtering [12] and spray pyrolysis method etc. However, there are a few reports on the deposited of them using a spray pyrolysis method [13]. The spray pyrolysis method has distinct advantages such as cost effectiveness, thin, transparent, multicomponent oxide layers of many compositions on various substrates, simplicity, excellent compositional control, homogeneity and lower crystallization temperature. In this paper, we have investigated on the structural, optical and morphological properties of ZrO2 nano-thin films deposited at different substrate temperatures (275–475 °C) by spray pyrolysis method. Particularly, we have examined the photocatalytic performance of pure ZrO2 catalyst by the photodegradation of methylene blue under sunlight irradiation.
2 Experimental details 2.1 Spray coating of thin films The ZrO2 films were deposited onto microscopic glass substrate using Spray pyrolysis technique. The precursor of zirconium (IV) chloride (0.1 M) was dissolved in deionized water. A few drops of concentrated hydrochloric acid (con.HCL) was added for complete dissolution and sprayed onto microscopic glass substrates with dimensions of 75 9 25 mm2 at different substrate temperatures (275–475 °C). The substrates were first cleaned with a water bath, followed by dipping in con. HCl, acetone and ethanol successively. Then, the substrates were rinsed with deionized water and allowed to dry in a hot air oven. In the spray unit, the substrate temperature was maintained with the help of heater, controlled by a feedback circuit. During the spray, the substrate temperature was kept constant with an accuracy of ±5 K. Spray head and substrate heater were kept inside a chamber provided with an exhaust fan, for removing gaseous by-products and vapors from the solvent. The spray head was allowed to move in the x–y plane using the microcontroller stepper motor, in order to achieve a uniform coating on the substrate. The spray head could scan an area of 200 9 200 mm with x-movement at a speed of 20 mm/s and y-movement insteps of 5 mm/s simultaneously. In that unit, there was a provision for controlling the spray rate of the solution as well as the pressure of the carrier gas. The microcontroller device was communicated with personal computer (PC) through the serial port; the data of each spray could be stored in the PC. The deposition parameters like solution flow rate, carrier
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gas pressure and nozzle to substrate distance were kept as 2 ml/min, 1.0 kg/cm2 and 20 cm respectively. This spray system can be used for large area of deposition with better uniformity. After the deposition, ZrO2 films were allowed to cool slowly to room temperature and washed with deionized water and then dried. Finally, the film coated glass substrates were kept in a muffle furnace and annealed at 500 °C each to get nanosized ZrO2 thin films. 2.2 Characterization tools The structural characterization of the ZrO2 films was carried out by X-ray diffraction (XRD) technique on Shimadzu˚ ). FT6000 (monochromatic CuKa radiation, k = 1.5406 A Raman spectra were observed on a Bruker: RFS27 instrument using a 100 nm excitation laser at room temperature. The surface morphological study was carried out using scanning electron microscope (SEM: Nano Surf Easy Scan2) and confirmed by transmission electron microscope (TEM: Tecnait20) operating at 200 kv. The dispersion of zirconium and oxygen in the product was characterized by energy dispersive X-ray elemental analysis (EDX: Thermo Super Dry II) equipped with the SEM instrument. Optical absorption spectra were obtained using Jasco V-670 Spectrophotometer. The photoluminescence (PL) spectra were studied at room temperature by Plorolog 3-Horiba Jobin– Yvon with an excitation wavelength of 325 nm.
3 Results and discussion 3.1 Structural analysis The XRD patterns of pure ZrO2 thin films prepared at various substrate temperatures (275–475 °C) and annealed at 500 °C are shown in Fig. 1. All these films exhibit polycrystalline nature with tetragonal crystal structure. The peak positions are in good agreement with the standard ZrO2 (JCPDS: 50-1089). No extra peaks are observed, which indicates the absence of an impurity phase in the films. The four prominent peaks corresponding to (011), (110), (112) and (121) planes reveal the tetragonal ZrO2 (t-ZrO2). The sharp increase in intensity of the above peaks with increasing substrate temperature up to 375 °C is due to the improvement in crystallinity of the films. At and above 400 °C a new phase of monoclinic ZrO2 (m-ZrO2), was observed, accompanying with the decrease in intensity of (011) diffraction of t-ZrO2. At 475 °C, the presence of peaks due to tetragonal (011) and monoclinic (-111) planes are identified. Yanfeng Gao [14] reported that the annealing caused crystallization of the amorphous film into tetragonal ZrO2 at 500 °C and phase transformation from tetragonal to monoclinic ZrO2 at higher temperatures.
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From the Table 1, the lattice parameters ‘a’ and ‘c’ are in concordance with the standard ZrO2 single crystals ˚ and c = 5.095 A ˚ ), which indicate that the (a = 3.642 A quality of ZrO2 films is good crystalline in nature. The values of lattice constants are less than the bulk ZrO2 which is strong indication of stress in the films. The structural parameters are calculated from the following equations [16–18], Micro strain,
e¼
bcosh 4
1 d¼ 2 D " # 2p2 SF ¼ 1 b 45ð3tanhÞ2
Dislocation density, Stacking fault,
IðhklÞ =IoðhklÞ 100 % Texture Co - efficient, TCðhklÞ ¼ P IðhklÞ =IoðhklÞ The structural parameters including dislocation density (d), micro strain (e), stacking fault (SF) and texture coefficient (TC) of tetragonal ZrO2 thin films are summarized in Table 1. The lattice defects like d, e and SF showed a decreasing trend with increasing substrate temperature from 275 to 475 °C which may be due to the improvement of crystallinity as well as the high orientation along (011) direction (Fig. 1). This type of change in d and e might be due to the recrystallization process in the polycrystalline films [19]. The TC represents the texture of a particular plane. Where I(hkl) is the measured relative intensity of a plane (hkl), Io(hkl) is the standard intensity of plane (hkl) taken from the JCPDS data. A sample with randomly oriented crystallite yields TChkl = 1, while the value is larger, the abundance of crystallites oriented in the (hkl) direction is also larger. Hence, the TC clearly indicates that the tetragonal ZrO2 are highly oriented in (011) direction.
Fig. 1 XRD spectra of ZrO2 thin films deposited at different substrate temperatures
The full width at half maximum (FWHM) of the tetragonal (011) diffraction peak increased by increasing substrate temperature, suggesting that the average particle size is increased with substrate temperature. The crystallite size has been obtained from 2h and FWHM of the (h k l) peaks using Scherrer’s relation [15], D¼
0:9k : b cos h
˚ , K is the shape where D is the average crystallite size in A factor (0.9), k is the wavelength of X-ray, h is the Bragg angle and b is the corrected line broadening of the thin films. By applying Scherrer’s formula on the tetragonal (011) diffraction peak, the crystallite size is found to increased with increase of substrate temperature and the results are given in Table 1. The lattice constants can be evaluated from the following expression, Lattice constant,
3.2 Raman spectroscopy Room temperature Raman spectrum of a representative ZrO2 thin film deposited at 475 °C is shown in Fig. 2. Raman bands around 181, 306, 469, 562 and 654 cm-1 are observed. At higher substrate temperature (475 °C), the
1 h2 þ k2 l2 ¼ þ 2 2 d a2 c
Table 1 Structural properties of ZrO2 thin films along (011) direction
Temperature oC
D (nm)
d 9 1015
SF 9 10-3
TC
e 9 10-3
˚) Lattice constant (A a
c
275
14.56
4.71
5.22
1.27
2.58
3.6245
5.1001
325
15.02
4.43
4.85
1.23
2.40
3.6393
5.0905
375
16.25
3.78
4.66
1.20
2.30
3.6304
5.0951
425
16.93
3.48
4.60
1.19
2.26
3.6425
5.0905
475
17.12
3.44
4.57
0.95
2.28
3.6375
5.0823
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tetragonal phase becomes dominant as indicated by the emerging tetragonal ZrO2B1g vibration mode at *306, 469 and 654 cm-1, with a minor fraction of monoclinic phase illustrated by the weak monoclinic ZrO2Ag ? Bg and Ag vibration modes at *186 cm-1, respectively [20]. The high intense Raman active mode from the Si substrate was located at. *562 cm-1. No additional peaks are observed in the Raman spectrum. Therefore, the Raman spectroscopy study supports our XRD results and reveals that the represented sample consists tetragonal phase of ZrO2 which converted into monoclinic phase after the substrate temperature 475 °C. 3.3 Surface morphological and compositional analysis The substrate temperature is the most important spray parameter. Figure 3 shows different film morphologies of ZrO2 films deposited using the spray pyrolysis technique at different substrate temperatures (275–475 °C). The morphology of the film changed with increasing substrate temperature. At, below temperature 325 °C (Fig. 3a, b) the presence of wet layer on the top surface of the film is due to presence of solvent rich in the deposited droplet. The fast drying of the wet layer resulted in stress cracking boundaries and microspores. Apart from this, during annealing films undergo shrinkage resulted in contraction, which leads to the formation of cracks. The droplets are dried above the substrate temperature of 325 °C (Fig. 3c–e), and spherical shaped particles are formed due to slow spreading. The elemental analysis of ZrO2 thin films with 475 °C (representative sample) investigated by EDX spectrum is shown in Fig. 4. It is found that zirconium and oxygen are present in near stoichiometric ratio (Zr:O = 33.76:66.2.).
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The size and morphology were further explored by TEM. The typical TEM photographs and selected area electron diffraction (SAED) pattern of ZrO2 thin film (at 475 °C) was illustrated in Fig. 5. It could be found that the film appeared monoclinic shape with the average particle diameter of about 15.23 nm, demonstrating that the substrate temperature could influence the increase of ZrO2 particle size. This value was consistent to the crystallite size calculated from the XRD line broadening. Moreover, the particle size was slightly larger than its crystallite and it attributed to the reunion of nano-sized crystallites. But some bigger aggregates can still be observed. The SAED pattern of ZrO2 (Fig. 5d) exhibits the well defined electron diffraction spots, confirming the single crystalline nature of the tetragonal phase of zirconia nanocrystals. The appearance of the dot pattern confirms the formation polycrystalline nature of the product. Hence, the discrete bright spots reveal the well-crystallized tetragonal form. 3.4 Optical study Figure 6 shows the optical transmittance spectra of ZrO2 thin films deposited at different substrate temperatures. These films are highly transparent in the visible range with a sharp fundamental absorption edge and exhibit an average transmittance of above 70 %. This sharp fall in transmission near the fundamental absorption edge is an identification of the good crystallinity of the films [20]. The percentage of transmittance is increased with increase of substrate temperature. The high average value of transmittance in the visible region is indicative of homogeneity and chemical purity. Using the absorption data, the band gap energy is estimated using Tauc relationship [21–23] as follows, ðahmÞ2 ¼ AðhmEg ) where hm is the photon energy, Eg is the optical band gap of the film material, A is a constant and n is 1/2, for direct band gap semiconductor. An extrapolation of the linear region of (ahm)2 on the y-axis versus photon energy (hm) on the x-axis give the value of Eg (Fig. 7). The band gap values are increased from 4.66 to 5.14 eV with increase of substrate temperatures. The extinction co-efficient (k) can be obtained from the expression [24], k=
Fig. 2 FT-Raman spectra of ZrO2 thin film deposited at 475 °C
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ak 4p
where a is the absorption co-efficient. Figure 8 depicts the variation of extinction co-efficient values for ZrO2 films with different temperatures. It is found that k value varies in the range of 0.27–0.52. The observed low k value of these films is a qualitative indication of excellent surface smoothness of the films [25]. However, the high value of k
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Fig. 3 SEM micrographs of ZrO2 thin films deposited at various substrate temperatures (a 275 °C, b 325 °C, c 375 °C, d 425 °C and e 475 °C), annealed at 500 °C
is also obtained for these films may be due to the crystallographic defects such as grain boundaries and voids present in the layers. The refractive index (n) is calculated at different wavelengths using the following relation [26], n¼
ð1 þ RÞ1=2 ð1 RÞ1=2
where R is the optical reflectance. The variation of refractive index with wavelength is shown in Fig. 9. It can be noted that the nmax value is nearly constant with the variation in substrate temperature and the n value varies in the range of 1.80–2.65.
3.5 Photoluminescence spectroscopy Figure 10 shows the PL spectra of ZrO2 thin films deposited with different substrate temperatures, annealed at 500 °C. It was seen that with the increase of substrate temperature, the luminescent peak intensity increases and also shifted to lower wavelength region. The variation in the PL intensity with temperature is effected by the crystallinity of the films [27]. The blue shift of the emission peak from 425 to 418 nm for the samples treated at higher temperature is much larger and is believed to the originated from the change of stress in the thin films due to lattice distortions [28, 29].
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Fig. 4 EDX spectrum of ZrO2 thin film deposited at 475 °C
3.6 Photocatalytic activity The photocatalytic activity is known to be dependent on the crystallinity, surface area, and morphology and it may be improved by slowing the recombination of photogenerated electron–hole pairs, extending the excitation wavelength to a lower energy range, and increasing the amount of
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surface-adsorbed reactant species. In general, the process for photocatalysis begins when supra-band gap photons are directly absorbed consequently generating electron–hole pairs in the semiconductor particles. This is followed by diffusion of the charge carriers to the surface of the particle where the interaction with water molecules would produce highly reactive species of peroxide (O2-) and hydroxyl radical (OH) responsible for the degradation of adsorbed organic molecules. The process of photocatalytic degradation of methylene blue over ZrO2 catalyst can be described as follows. The first step involves adsorption of the dye onto the surface of ZrO2 nanostructure sample. Exposure of dye adsorbed ZrO2 nanostructures with sunlight leads to generation of electron–hole (e-–h?) pairs in ZrO2. The photogenerated electrons in the conduction band of ZrO2 interact with the oxygen molecules adsorbed on ZrO2 to form superoxide anion radicals (*O2-). The holes generated in the valence band of ZrO2 react with surface hydroxyl groups to produce highly reactive hydroxyl radicals (*OH). These photogenerated holes can lead to dissociation of water molecules in the aqueous solution, producing radicals. The highly reactive hydroxyl radicals (*OH) and superoxide radicals (*O2-) can react with
Fig. 5 a–c TEM photographs of pristine ZrO2 thin films deposited at 475 °C, d corresponding SAED pattern
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Fig. 6 Transmittance spectra of ZrO2 thin films at different substrate temperatures
Fig. 8 Variation of extinction co-efficient (k) as a function of wavelength for ZrO2 thin films
Fig. 7 Plot of (ahm)2 versus hm for ZrO2 thin films
Fig. 9 Variation of refractive index (n) versus wavelength for ZrO2 thin films
methylene blue (MB) dye adsorbed on ZrO2 nanostructures and lead to its degradation. ZrO2 þ ht ! e ðCBÞ þ hþ ðVBÞ O2 þ e ! O 2 hþ þ OH ! OH hþ þ H2 O ! Hþ þ OH
OH þ Methylene blue ! Degradation products
O 2 þ Methylene blue ! Degradation products
The photocatalytic activity of ZrO2 sample was evaluated by monitoring the photodegradation of MB (C16H18 ClN3S) in aqueous solution. The strong absorption towards metal oxide with good optical absorption and resistance to
light degradation are the reason for the selection of MB. The time between 11 a.m. and 2 p.m. of sunny ray day was selected to carry out the photocatalytic experiment. The 0.03 g of photocatalyst of pure ZrO2 was charged into 100 mL of 10 mg/L MB aqueous solution. The suspension was magnetically stirred for 30 min to attain adsorption– desorption equilibrium between MB dye and ZrO2. The mixed solution was irradiated using sunlight. The solution was then taken out every 30 min (up to 180 min) and the photocatalyst was separated from the mixture solution by centrifugation immediately, and then the UV–Vis absorption of the clarified solutions was analyzed with a UV–Vis spectrophotometer (Shimadzu, UV-1800). The absorbance of MB solution was measured at 664 nm, which corresponds to its maximum absorption wavelength. The
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1200000
1.6
664 nm 1.4
275 oC 325 oC 375 oC 425 oC 475 oC
800000 600000
1.2
Absorbance (a.u)
Intensity (counts)
1000000
0 min 30 min 60 min 90 min 120 min 150 min 180 min
400000 200000
1.0 0.8 0.6 0.4 0.2
0
0.0
350
400
450
500
550
400
600
degradation percentage (%D) of the MB dye can be calculated from the following equation [30], %D ¼
C0 Ct 100 C0
700
800
Fig. 11 UV–Vis absorption spectra of MB with respect to irradiation time using ZrO2 photocatalyst
Table 2 The effect of methylene blue (MB) dye degradation by ZrO2 photocatalyst Time (min) 0
where C0 is the initial concentration of dye and Ct is the concentration of dye after irradiation in selected time intervals (0–120 min). Several experimental results suggest that the rates of photocatalytic oxidation of various contaminants over illuminated ZrO2 occur via pseudo firstorder kinetics [31],
600
Wavelength (nm)
Wavelength (nm)
Fig. 10 PL spectra of ZrO2 thin films with different substrate temperatures, annealed at 500 °C
500
% Degradation of MB dye 0
30
30.82
60
37.46
90
46.03
120
63.70
150
69.07
180
72.13
lnðC0 =Ct Þ ¼ kKt ¼ kt ðorÞ kt
Ct ¼ C0 e
where k is the reaction rate constant and K is the adsorption co-efficient of the reactant. A plot of ln (C0/Ct) versus time represents a straight line; the slope equals the apparent first-order rate constant k. Figure 11 shows the absorption spectra of MB using ZrO2 catalyst as a function of wavelength (400–800 nm) for various time intervals 0, 30, 60, 90, 120, 150 and 180 min. The degradation effect was characterized by monitoring the absorption peak of MB centered at 664 nm. The plots clearly demonstrate that the maximum absorption peak decreases with increasing irradiation time. This illustrates that the MB dye concentration decreases in the presence of ZrO2 catalyst and solar light illumination. The decrease in the absorption of the mixed solution was due to the destruction of the homo and hetropoly aromatic rings present in the dye molecules or due to rapid degradation of MB, which is confirmed by the lower intensities of the absorbance peak of MB. The effect of ZrO2 on percentage degradation of the MB dye has been examined by varying the time interval from 0 to 180 min
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and the results are presented in Table 2. The percentage degradation increases rapidly with the increase in the time for MB-ZrO2 solution. The maximum degradation of MB dye took place in 180 min of irradiation with sunlight using ZrO2 catalyst was 72.13 %. While in the case of time at 30 min, only 30.82 % degradation was observed. According to the pseudo-first-order rate equation, the rate constant (k) for MB degradation by pure ZrO2 was determined. The plot of ln (C0/Ct) as a function of irradiation time gives the rate constant value 0.012 min-1. Moreover, the fitting correlation co-efficient (R2) is also determined to be 0.983.
4 Conclusions In conclusion, we identified that the one step tetragonal-tomonoclinic phase transformation segment in nanosized ZrO2 thin film induced by thermal annealing at 500 °C. The XRD study indicated that the films were polycrystalline in nature with preferred grain orientation along (011) plane and exhibited a tetragonal crystal structure. The crystallite size of the films increases with increasing
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glass substrate temperature. Raman study lends support to our conclusion that the ZrO2 thin film (475 °C) was essentially consists of the both tetragonal and monoclinic crystalline phases. The sphere like formation was observed from the SEM with increasing temperatures. The optical transmittance was high in the visible region and the optical band gap value increase with substrate temperature. The higher values of extinction co-efficient and refractive index were also obtained in the UV region. The blue shift in the PL peak is due to the change of stress in the film. Our results suggest a means to improve the structural and optical properties of highly oriented ZrO2 thin films at various substrate temperatures and its photocatalytic applications.
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