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Fabrication and composition control of porous ZnO-TiO2 binary oxide thin films via a sparking method
Accepted Manuscript Title: Fabrication and Composition Control of Porous ZnO-TiO2 Binary Oxide Thin Films via a Sparking Method Author: T. Kumpika E. Kantarak W. Sroila A. Panthawan P. Sanmuangmoon W. Thongsuwan P. Singjai PII: DOI: Reference:
S0030-4026(17)30023-2 http://dx.doi.org/doi:10.1016/j.ijleo.2017.01.012 IJLEO 58715
To appear in: Received date: Revised date: Accepted date:
14-5-2016 31-10-2016 6-1-2017
Please cite this article as: T.Kumpika, E.Kantarak, W.Sroila, A.Panthawan, P.Sanmuangmoon, W.Thongsuwan, P.Singjai, Fabrication and Composition Control of Porous ZnO-TiO2 Binary Oxide Thin Films via a Sparking Method, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.01.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and Composition Control of Porous ZnO-TiO2 Binary Oxide Thin Films via a Sparking Method T. Kumpika1, E. Kantaraka, W. Sroilaa, A. Panthawana, P. Sanmuangmoonc, W. Thongsuwana,b and P. Singjai,a,b* a
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
b
Materials Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c
Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
*Corresponding author. Tel.: +66 53 941922 ext 610; fax: +66 53 892270. E-mail address: [email protected] ABSTRACT ZnO-TiO2 binary oxide thin films have been deposited on quartz substrate using sparking of zinc and titanium electrodes. The composition ratio of zinc to titanium in the sparked films is inverse correlated with the emission intensity of Zn/Ti ratio during high voltage discharges across the gap of zinc anode - titanium cathode tips. Interestingly, it exhibited a direct correlation for zinc cathode – titanium anode experiment. Moreover, the Zn/Ti atomic ratio in the deposited films is correlated with Zn/Ti electrode mass loss rate ratio. Meanwhile, the mass loss rate from a metal wire corresponds to an amount of energy required to melt the metal electrodes. Furthermore, structural and optical properties of the asdeposited and the annealed films were investigated. The sparking process is a versatile nanocoating process for metal oxides as well as their binary compounds.
Keywords: Sparking deposition; Nano-coating; Binary compounds
1. Introduction Metal oxide binary oxide, such as ZnO–Al2O3 [1], ZnO–In2O3 [2], ZnO-Fe2O3 [3], ZnO–SnO2 [4], and ZnO–TiO2 [5-15], have attracted great attention in the past decade due to their unique photocatalytic, optical and electrical properties. Among these binary compounds, ZnO–TiO2 is one of the most interesting materials because their heterojunction can promote electron–hole separation [5]. The ZnO–TiO2 systems have been investigated for various
2 applications such as gas-sensor [6-7], corrosion and wear resistance [8], humidity sensor [9], lithium ion battery [10], photocatalysis [11], and dye-sensitized solar cells [12]. In some previous studies, a number of preparation techniques have been used to fabricate ZnO-TiO2 binary compound. Due to the importance of the atomic concentration ratios on the structural, optical, and electrical properties [13], the studies tend to focus attention on controlling the Zn/Ti ratio. D. Yu et al. [8] using so-gel process followed by thermal treatment to produced ZnO-TiO2 film with various Zn/Ti atomic ratios. ZnO sol was prepared by mixing of zinc acetate, ethanol, diethanolamine and distilled water whereas TiO2 sol was mixer of Tetrabutyltitanate, ethanol, distilled water and glacial acetic acid. The different ratios of Zn/Ti were achieved by mixed the solutions at different volume ratio. Z.Y. Zhong and T. Zhang prepared ZnO-TiO2 films on glass substrate by RF magnetron sputtering. To control the level of Ti concentration, ZnO powder was mixed with TiO2 power at various wt% to produce the sputtering target [14]. H.F. Hsu and co-workers used the dualtargets cathodic arc plasma deposition process to prepared ZnO-TiO2 films. Metallic Zn and Ti were separated set as cathode targets. A shutter was placed to partly cover the Ti target to control amount of Ti element in the deposited films [15]. The basic underlying idea in the most studies is controlling product composition by fixing the Zn/Ti ratio of the starting materials. To our knowledge, no previous studies have provided the ratio adjustable during the film growth. Therefore, in this paper, we deposited ZnO-TiO2 binary compound in a form of nanoparticle thin films by simple and inexpensive method of sparking process [16] which allowed real-time monitoring of the composition ratio and was easily adjustable by controlling sparking voltage. The deposition is produced by using the tips of zinc and titanium wires as starting materials. An elemental composition ratio of zinc to titanium in the sparked films could be estimated by a Zn/Ti emission intensity ratio. However, the spark emission spectroscopy has been used to determine an alloy composition for many years ago [17-18]. We believe that, the reverse technique to control thin film composition by spark emission intensity ratio relies on the same fundamental principle. Furthermore, structural and optical properties of the asdeposited and the annealed films were investigated. It is believed that, this method is possible to perform various binary compounds of metal oxide nanoparticle thin films with desirable composition ratios and thickness on any arbitrary substrates.
3 2. Experimental details High voltages of 4 to10 kV discharged from 25 nF capacitor were applied at a frequency of 15 times/min across two sharp tip electrodes, titanium wire (Ø 0.25 mm, purity 99.5%, Advent Research materials Ltd) and zinc wire (Ø 0.38 mm, purity 99.97%, Advent Research Materials Ltd). The tips were then placed horizontally at 1 mm spacing and 2 mm above the center of the quartz substrate which was mounted horizontally on a rotating platform. The experiment was performed both zinc anode - titanium cathode and titanium anode - zinc cathode, in air atmospheric pressure at room temperature. During the sparking procedure, spark emission spectra were measured by spectrometer (Avantes, AvaSpec-2048). Surface morphology and the film thickness were characterized by scanning electron microscopy (SEM, JEOL JSM 6335F). Energy dispersive X-ray spectroscopy (EDS, Oxford) were performed for composition determination of the films. Crystal structures of the asdeposited and annealed films at 600 C in air for 1 h were investigated by X-ray diffraction (PANalytical X'Pert Pro MPD) using CuKα radiation with λ=1.54 Å operating at 40 kV, 40 mA. Raman spectra of the samples were obtained with a 514.5 nm argon ion laser at room temperature (Jobin Yvon Horiba T64000).
3. Results and discussion Fig. 1a and b show the top-view SEM images of the sparked ZnO-TiO2 films at magnifications of 30,000 and 10,000, respectively. The as-deposited films have a fluffy morphology due to an irregular stacking of primary nanoparticles, which have secondary particles in sizes of 20–50 nm. It is noted that the morphology is similar for the sparking voltages in the range of 4 to10 kV and also by swapping the anode and cathode electrodes. Fig. 2 shows the relation between Zn,Ti atom percent measured by EDS and the sparking voltage. The Zn/Ti atomic ratio was 95:5 for sparking voltage of 4 kV and decreased to approximately 80:20 with increasing the sparking voltage to 10 kV. It is anticipated that a higher composition of zinc than titanium is attributed to a relatively low energy required to melt the zinc tip. It is known that the zinc electrode has a relatively low melting (419.5 C), a relatively low specific heat capacity (0.39 J/gC) and a relatively low heat of fusion (244 J/g) than those of the titanium (1,725 C, 0.54 J/gC and 702 J/g). Therefore, the amount of the zinc droplets were generated more than those of titanium at the same amount of applied energy. It is noted that these results were even more apparent at a relatively low sparking voltage.
4 In order to confirm the film atom percent depends on the thermal property of the electrodes, the sparking voltage of 10 kV were applied between 1 mm spacing of various types of metal electrodes. The electrode mass loss rate results show in Fig. 3. It is clearly seen that the mass loss rate decreased with increasing energy used to melt the metal. Energy applied to the tips (Eapp) separated to energy loss to the environment (Eloss) and energy used to melt the metal tips described by equation (1) [ (
)
]
(1)
where cp is a specific heat capacity, m is an effective mass of the metal tip, Tm is a melting point, Tr is a room temperature and Lf is a heat of fusion. Considering at the loss rate ratio of Zn/Ti, the ratio of 0.111 mg/min : 0.021 mg/min is close to the Zn:Ti atomic ratio of 80 % : 20 % as shown above in fig. 3. It is concluded that the atomic ratio in the sparked films is close to the electrode mass loss ratio. During the sparking reaction, the light emission spectra were measured as shown in Fig. 4. It is clearly seen that at a higher sparking voltage, the observed spectra are higher peak intensities. Six emission peaks from air molecules were observed, five peaks were ascribed to nitrogen at 444(N II), 491(N I), 500(N II), 519(NII), and 550 nm (NII) and one peak was ascribed to oxygen at 470 nm (O II) [19]. Two peaks at 430 and 453 nm indicate the emission peaks of titanium (Ti I) and the peak at 481 nm indicates that of the zinc (Zn I) [20]. It was also found that for the zinc anode – titanium cathode experiment, the peak height ratio of the zinc emission intensity (IZn) at 481 nm to a summation of the titanium emission intensities (∑ITi) at 430 and 453 nm is inversely correlated to the Zn atom percent. In contrast with the sparking voltage (V), it shows a directly correlation, as shown in Fig. 5, as followed: (
)
[2]
where Zn (at. %) is zinc atom percent and V is the sparking voltage. As the sparking voltage increased from 4 kV to 10 kV,
increased from 0.43 to 0.55, whereas the Zn
atom percent decreased from 96.0% to 79.3%. It is suggested that the emission intensity of Zn/Ti ratio can be used to monitor and estimate the composition ratio of binary compounds of the sparked films. For the example, as shown in Fig. 5, in order to accomplish the zinc atom percent of 82%, the sparking voltage had to be adjusted until the emission of Zn/Ti peak intensity ratio is 0.526. In this case, the estimated sparking voltage is approximately 9 kV.
5 Interestingly, as shown in figure 5 (b), the zinc cathode – titanium anode experiment gave the contrast result with the zinc anode – titanium cathode experiment. The intensity ratios of the Zn/Ti emissions are direct correlation to the Zn/Ti atomic ratio but inversely correlated to the sparking voltage as shown the relation in equation [3]. (
)
(
)
[3]
The relation can be also used to monitor and control the film composition ratio. As the same example of the zinc atom percent of 82%, the fitting ratio of the Zn/Ti emissions is 0.465. Even if the both experiment present the different trend of the emission intensity ratio but at the same requirement of the zinc atom percent (82%) they showed the same estimation voltage of approximate 9 kV. The prepared sample at Zn 80 (at.%) was selected to study its structural and optical properties and effect of the annealing treatment. Fig. 6a-d show SEM cross-section images of the samples deposited at 100, 200, 300 and 400 sparking times, respectively. It is noted that the film deposition rate was approximately 0.7 nm/spark. Fig. 7 shows the XRD spectra of ZnO-TiO2 binary compound of the as-deposited sample and the annealed sample at 600 C in air for 1 h. It is clearly seen that the annealed sample after a crystallinity improvement by the heat treatment shows higher diffraction peaks of ZnO and Zn2TiO4 phases, without the rutile and anatase peaks of TiO2 as found in the asdeposited sample. It is known that a spinel structure of Zn2TiO4 is rapidly formed and high thermodynamic stability [21]. Hence, a solid–solid reaction of ZnO and TiO2 caused the formation of Zn2TiO4 and the vanishing of both TiO2 peaks [22]. Raman spectra of both as-deposited and annealed samples show in Fig. 8. Three peaks located at 341, 440 and 581 cm-1 corresponds to the 3E2H-E2L, E2 high and E1LO mode of ZnO, respectively [23]. The E2 mode is ascribed to a band characteristic of wurtzite phase of ZnO and E1 (LO) mode associated with an oxygen deficiency [24]. After annealing, an increase in intensity of the E2 (high) was observed due to the increase of the wurtzite structure of ZnO. It is clearly seen that the intensity of the E1 (LO) decreased due to the reduction in oxygen deficiency. Moreover, the peak occurred at 720 nm was only observed in the annealed sample which corresponds to the A1g Raman active mode of Zn2TiO4 [25]. A broad plateau at a range of 100-250 cm-1 of mixed amorphous TiO2 was only observed from the as-deposited sample. It is noticed that our Raman result is well agree with that of the XRD.
6 IL spectra measured at room temperature show UV and green emissions for both samples as shown in figure 9a. The higher UV peak located at 382 nm and the broader green peak centered at 518 nm were observed which related to a near band-edge emission and singly ionized oxygen vacancies in ZnO, respectively [26]. An improvement of crystallinity by the heat treatment leads to an increase of signal to noise ratio in the IL spectrum. The optical transmittance spectra of the films as shown in figure 9b, were very high optical transmittance in a visible region (over 90%) and a sharp drop in UV region at approximately 390 nm. The film energy gaps can be determined from the plot of an absorption coefficient () and incident photon energy (h) as shown in figure 9c. The is given by formula:
1 t
ln
(100) T
[4]
where t is the film thickness and T is the optical transmittance [27]. The optical band gap was determined by extrapolating the linear portion of the curve to (αhν)2 = 0. The value of the energy band gaps are 3.80 and 3.69 eV for the as-deposited and the annealed samples, respectively. It is noted that, the obtained band gaps are higher than those of pure ZnO films, 3.43 eV and 3.25 for as-deposited and annealed samples, respectively [14]. This is due to an increasing in the carrier concentration attributed to the substitution of Ti4+ into the sites of Zn2+ ions, known as Burstein-Moss shift [28]. The decreasing of energy gap after the heat treatment is consistent with the decreasing of the oxygen defect intensity. The refractive index, significant factor in designing spectrum dispersion devices, is shown in figure 9d. The refractive index of the films was calculated from the relation [29],
4R 1 R n k2 2 (1 R) 1 R
[5]
where k =λ/4 is the extinction coefficient. In this study were stable for entire visible spectra. The refractive indexes at the wavelength of 550 nm, were 1.76 and 1.83 for the as-deposited and the annealed sample at 600 C, respectively.
4. Conclusions The sparking emission ratio can be used to monitor and estimate zinc and titanium composition ratio in the sparked films. However, the zinc content was limited in the range from 77 to 96 atom percent. The higher atom percent of zinc in the sparked films is due to a less energy consumed to melt the zinc electrode. The obtained films are porous and fluffy,
7 therefore they are suitable for gas sensor and photocatalytic applications. We also suggest that other binary metal oxides such as Co-Fe, In-Sn and Ag-Zn can be sparked to control the atomic ratio by this method.
Acknowledgments We thank the National Research University Project under Office of Higher Education Commission (OHEC) Thailand, Thailand Research Fund (TRF, IRG5780013) and Chiang Mai University (CMU) for the financial support. T. Kumpika is grateful for his postdoctoral fellowship from Chiang Mai University.
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FIGURE CAPTIONS Fig. 1.
SEM images of the ZnO-TiO2 binary compound nanoparticle thin films at the magnification of (a) 30,000x and (b) 10,000x.
Fig. 2.
Plot of the zinc and the titanium atom percent measured by EDS as a function of the sparking voltages.
Fig. 3.
Mass loss rates versus energy required to melt the electrodes in the sparking process of various metals.
Fig. 4.
Spark emissions of the zinc anode – titanium cathode experiment with various sparking voltages.
Fig. 5.
Plot of the zinc atom percent and sparking voltage as a function of the IZn/ITi emission ratios of the zinc anode – titanium cathode experiment.
Fig. 6a-d
Cross-section images of the ZnO-TiO2 binary compound films with sparking at 10 kV for 100, 200, 300 and 400 times, respectively.
Fig. 7.
XRD patterns of the as-deposited and the annealed ZnO-TiO2 films on the quartz substrate.
Fig. 8.
Raman spectra of the as-deposited and the annealed ZnO-TiO2 films on the quartz substrate.
Figure 9(a)
The IL spectra of as-deposited and annealed ZnO-TiO2 thin films on quartz substrates.
Figure 9(b)
Transmittance spectra of the as-deposited and the annealed ZnO-TiO2 thin films on quartz substrates.
Figure 9(c)
Plot of (h)2 versus the photon energy (hfor estimation of the energy gap for as-deposited and annealed samples.
Figure 9(d)
Refractive index of the as-deposited and the annealed ZnO-TiO2 thin films on quartz substrates.
9
Fig. 1a.
Fig. 1b.
10
Fig. 2.
Fig. 3.
11
Fig. 4a.
Fig. 4b.
12
Fig. 5a.
Fig. 5b.
13
Fig. 6.
14
Fig. 7.
15
Fig. 8.
16
Figure 9a.
Fig. 9b.
17
Fig. 9c.
Fig. 9d.
S0030-4026(17)30023-2 http://dx.doi.org/doi:10.1016/j.ijleo.2017.01.012 IJLEO 58715
To appear in: Received date: Revised date: Accepted date:
14-5-2016 31-10-2016 6-1-2017
Please cite this article as: T.Kumpika, E.Kantarak, W.Sroila, A.Panthawan, P.Sanmuangmoon, W.Thongsuwan, P.Singjai, Fabrication and Composition Control of Porous ZnO-TiO2 Binary Oxide Thin Films via a Sparking Method, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.01.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and Composition Control of Porous ZnO-TiO2 Binary Oxide Thin Films via a Sparking Method T. Kumpika1, E. Kantaraka, W. Sroilaa, A. Panthawana, P. Sanmuangmoonc, W. Thongsuwana,b and P. Singjai,a,b* a
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
b
Materials Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c
Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
*Corresponding author. Tel.: +66 53 941922 ext 610; fax: +66 53 892270. E-mail address: [email protected] ABSTRACT ZnO-TiO2 binary oxide thin films have been deposited on quartz substrate using sparking of zinc and titanium electrodes. The composition ratio of zinc to titanium in the sparked films is inverse correlated with the emission intensity of Zn/Ti ratio during high voltage discharges across the gap of zinc anode - titanium cathode tips. Interestingly, it exhibited a direct correlation for zinc cathode – titanium anode experiment. Moreover, the Zn/Ti atomic ratio in the deposited films is correlated with Zn/Ti electrode mass loss rate ratio. Meanwhile, the mass loss rate from a metal wire corresponds to an amount of energy required to melt the metal electrodes. Furthermore, structural and optical properties of the asdeposited and the annealed films were investigated. The sparking process is a versatile nanocoating process for metal oxides as well as their binary compounds.
Keywords: Sparking deposition; Nano-coating; Binary compounds
1. Introduction Metal oxide binary oxide, such as ZnO–Al2O3 [1], ZnO–In2O3 [2], ZnO-Fe2O3 [3], ZnO–SnO2 [4], and ZnO–TiO2 [5-15], have attracted great attention in the past decade due to their unique photocatalytic, optical and electrical properties. Among these binary compounds, ZnO–TiO2 is one of the most interesting materials because their heterojunction can promote electron–hole separation [5]. The ZnO–TiO2 systems have been investigated for various
2 applications such as gas-sensor [6-7], corrosion and wear resistance [8], humidity sensor [9], lithium ion battery [10], photocatalysis [11], and dye-sensitized solar cells [12]. In some previous studies, a number of preparation techniques have been used to fabricate ZnO-TiO2 binary compound. Due to the importance of the atomic concentration ratios on the structural, optical, and electrical properties [13], the studies tend to focus attention on controlling the Zn/Ti ratio. D. Yu et al. [8] using so-gel process followed by thermal treatment to produced ZnO-TiO2 film with various Zn/Ti atomic ratios. ZnO sol was prepared by mixing of zinc acetate, ethanol, diethanolamine and distilled water whereas TiO2 sol was mixer of Tetrabutyltitanate, ethanol, distilled water and glacial acetic acid. The different ratios of Zn/Ti were achieved by mixed the solutions at different volume ratio. Z.Y. Zhong and T. Zhang prepared ZnO-TiO2 films on glass substrate by RF magnetron sputtering. To control the level of Ti concentration, ZnO powder was mixed with TiO2 power at various wt% to produce the sputtering target [14]. H.F. Hsu and co-workers used the dualtargets cathodic arc plasma deposition process to prepared ZnO-TiO2 films. Metallic Zn and Ti were separated set as cathode targets. A shutter was placed to partly cover the Ti target to control amount of Ti element in the deposited films [15]. The basic underlying idea in the most studies is controlling product composition by fixing the Zn/Ti ratio of the starting materials. To our knowledge, no previous studies have provided the ratio adjustable during the film growth. Therefore, in this paper, we deposited ZnO-TiO2 binary compound in a form of nanoparticle thin films by simple and inexpensive method of sparking process [16] which allowed real-time monitoring of the composition ratio and was easily adjustable by controlling sparking voltage. The deposition is produced by using the tips of zinc and titanium wires as starting materials. An elemental composition ratio of zinc to titanium in the sparked films could be estimated by a Zn/Ti emission intensity ratio. However, the spark emission spectroscopy has been used to determine an alloy composition for many years ago [17-18]. We believe that, the reverse technique to control thin film composition by spark emission intensity ratio relies on the same fundamental principle. Furthermore, structural and optical properties of the asdeposited and the annealed films were investigated. It is believed that, this method is possible to perform various binary compounds of metal oxide nanoparticle thin films with desirable composition ratios and thickness on any arbitrary substrates.
3 2. Experimental details High voltages of 4 to10 kV discharged from 25 nF capacitor were applied at a frequency of 15 times/min across two sharp tip electrodes, titanium wire (Ø 0.25 mm, purity 99.5%, Advent Research materials Ltd) and zinc wire (Ø 0.38 mm, purity 99.97%, Advent Research Materials Ltd). The tips were then placed horizontally at 1 mm spacing and 2 mm above the center of the quartz substrate which was mounted horizontally on a rotating platform. The experiment was performed both zinc anode - titanium cathode and titanium anode - zinc cathode, in air atmospheric pressure at room temperature. During the sparking procedure, spark emission spectra were measured by spectrometer (Avantes, AvaSpec-2048). Surface morphology and the film thickness were characterized by scanning electron microscopy (SEM, JEOL JSM 6335F). Energy dispersive X-ray spectroscopy (EDS, Oxford) were performed for composition determination of the films. Crystal structures of the asdeposited and annealed films at 600 C in air for 1 h were investigated by X-ray diffraction (PANalytical X'Pert Pro MPD) using CuKα radiation with λ=1.54 Å operating at 40 kV, 40 mA. Raman spectra of the samples were obtained with a 514.5 nm argon ion laser at room temperature (Jobin Yvon Horiba T64000).
3. Results and discussion Fig. 1a and b show the top-view SEM images of the sparked ZnO-TiO2 films at magnifications of 30,000 and 10,000, respectively. The as-deposited films have a fluffy morphology due to an irregular stacking of primary nanoparticles, which have secondary particles in sizes of 20–50 nm. It is noted that the morphology is similar for the sparking voltages in the range of 4 to10 kV and also by swapping the anode and cathode electrodes. Fig. 2 shows the relation between Zn,Ti atom percent measured by EDS and the sparking voltage. The Zn/Ti atomic ratio was 95:5 for sparking voltage of 4 kV and decreased to approximately 80:20 with increasing the sparking voltage to 10 kV. It is anticipated that a higher composition of zinc than titanium is attributed to a relatively low energy required to melt the zinc tip. It is known that the zinc electrode has a relatively low melting (419.5 C), a relatively low specific heat capacity (0.39 J/gC) and a relatively low heat of fusion (244 J/g) than those of the titanium (1,725 C, 0.54 J/gC and 702 J/g). Therefore, the amount of the zinc droplets were generated more than those of titanium at the same amount of applied energy. It is noted that these results were even more apparent at a relatively low sparking voltage.
4 In order to confirm the film atom percent depends on the thermal property of the electrodes, the sparking voltage of 10 kV were applied between 1 mm spacing of various types of metal electrodes. The electrode mass loss rate results show in Fig. 3. It is clearly seen that the mass loss rate decreased with increasing energy used to melt the metal. Energy applied to the tips (Eapp) separated to energy loss to the environment (Eloss) and energy used to melt the metal tips described by equation (1) [ (
)
]
(1)
where cp is a specific heat capacity, m is an effective mass of the metal tip, Tm is a melting point, Tr is a room temperature and Lf is a heat of fusion. Considering at the loss rate ratio of Zn/Ti, the ratio of 0.111 mg/min : 0.021 mg/min is close to the Zn:Ti atomic ratio of 80 % : 20 % as shown above in fig. 3. It is concluded that the atomic ratio in the sparked films is close to the electrode mass loss ratio. During the sparking reaction, the light emission spectra were measured as shown in Fig. 4. It is clearly seen that at a higher sparking voltage, the observed spectra are higher peak intensities. Six emission peaks from air molecules were observed, five peaks were ascribed to nitrogen at 444(N II), 491(N I), 500(N II), 519(NII), and 550 nm (NII) and one peak was ascribed to oxygen at 470 nm (O II) [19]. Two peaks at 430 and 453 nm indicate the emission peaks of titanium (Ti I) and the peak at 481 nm indicates that of the zinc (Zn I) [20]. It was also found that for the zinc anode – titanium cathode experiment, the peak height ratio of the zinc emission intensity (IZn) at 481 nm to a summation of the titanium emission intensities (∑ITi) at 430 and 453 nm is inversely correlated to the Zn atom percent. In contrast with the sparking voltage (V), it shows a directly correlation, as shown in Fig. 5, as followed: (
)
[2]
where Zn (at. %) is zinc atom percent and V is the sparking voltage. As the sparking voltage increased from 4 kV to 10 kV,
increased from 0.43 to 0.55, whereas the Zn
atom percent decreased from 96.0% to 79.3%. It is suggested that the emission intensity of Zn/Ti ratio can be used to monitor and estimate the composition ratio of binary compounds of the sparked films. For the example, as shown in Fig. 5, in order to accomplish the zinc atom percent of 82%, the sparking voltage had to be adjusted until the emission of Zn/Ti peak intensity ratio is 0.526. In this case, the estimated sparking voltage is approximately 9 kV.
5 Interestingly, as shown in figure 5 (b), the zinc cathode – titanium anode experiment gave the contrast result with the zinc anode – titanium cathode experiment. The intensity ratios of the Zn/Ti emissions are direct correlation to the Zn/Ti atomic ratio but inversely correlated to the sparking voltage as shown the relation in equation [3]. (
)
(
)
[3]
The relation can be also used to monitor and control the film composition ratio. As the same example of the zinc atom percent of 82%, the fitting ratio of the Zn/Ti emissions is 0.465. Even if the both experiment present the different trend of the emission intensity ratio but at the same requirement of the zinc atom percent (82%) they showed the same estimation voltage of approximate 9 kV. The prepared sample at Zn 80 (at.%) was selected to study its structural and optical properties and effect of the annealing treatment. Fig. 6a-d show SEM cross-section images of the samples deposited at 100, 200, 300 and 400 sparking times, respectively. It is noted that the film deposition rate was approximately 0.7 nm/spark. Fig. 7 shows the XRD spectra of ZnO-TiO2 binary compound of the as-deposited sample and the annealed sample at 600 C in air for 1 h. It is clearly seen that the annealed sample after a crystallinity improvement by the heat treatment shows higher diffraction peaks of ZnO and Zn2TiO4 phases, without the rutile and anatase peaks of TiO2 as found in the asdeposited sample. It is known that a spinel structure of Zn2TiO4 is rapidly formed and high thermodynamic stability [21]. Hence, a solid–solid reaction of ZnO and TiO2 caused the formation of Zn2TiO4 and the vanishing of both TiO2 peaks [22]. Raman spectra of both as-deposited and annealed samples show in Fig. 8. Three peaks located at 341, 440 and 581 cm-1 corresponds to the 3E2H-E2L, E2 high and E1LO mode of ZnO, respectively [23]. The E2 mode is ascribed to a band characteristic of wurtzite phase of ZnO and E1 (LO) mode associated with an oxygen deficiency [24]. After annealing, an increase in intensity of the E2 (high) was observed due to the increase of the wurtzite structure of ZnO. It is clearly seen that the intensity of the E1 (LO) decreased due to the reduction in oxygen deficiency. Moreover, the peak occurred at 720 nm was only observed in the annealed sample which corresponds to the A1g Raman active mode of Zn2TiO4 [25]. A broad plateau at a range of 100-250 cm-1 of mixed amorphous TiO2 was only observed from the as-deposited sample. It is noticed that our Raman result is well agree with that of the XRD.
6 IL spectra measured at room temperature show UV and green emissions for both samples as shown in figure 9a. The higher UV peak located at 382 nm and the broader green peak centered at 518 nm were observed which related to a near band-edge emission and singly ionized oxygen vacancies in ZnO, respectively [26]. An improvement of crystallinity by the heat treatment leads to an increase of signal to noise ratio in the IL spectrum. The optical transmittance spectra of the films as shown in figure 9b, were very high optical transmittance in a visible region (over 90%) and a sharp drop in UV region at approximately 390 nm. The film energy gaps can be determined from the plot of an absorption coefficient () and incident photon energy (h) as shown in figure 9c. The is given by formula:
1 t
ln
(100) T
[4]
where t is the film thickness and T is the optical transmittance [27]. The optical band gap was determined by extrapolating the linear portion of the curve to (αhν)2 = 0. The value of the energy band gaps are 3.80 and 3.69 eV for the as-deposited and the annealed samples, respectively. It is noted that, the obtained band gaps are higher than those of pure ZnO films, 3.43 eV and 3.25 for as-deposited and annealed samples, respectively [14]. This is due to an increasing in the carrier concentration attributed to the substitution of Ti4+ into the sites of Zn2+ ions, known as Burstein-Moss shift [28]. The decreasing of energy gap after the heat treatment is consistent with the decreasing of the oxygen defect intensity. The refractive index, significant factor in designing spectrum dispersion devices, is shown in figure 9d. The refractive index of the films was calculated from the relation [29],
4R 1 R n k2 2 (1 R) 1 R
[5]
where k =λ/4 is the extinction coefficient. In this study were stable for entire visible spectra. The refractive indexes at the wavelength of 550 nm, were 1.76 and 1.83 for the as-deposited and the annealed sample at 600 C, respectively.
4. Conclusions The sparking emission ratio can be used to monitor and estimate zinc and titanium composition ratio in the sparked films. However, the zinc content was limited in the range from 77 to 96 atom percent. The higher atom percent of zinc in the sparked films is due to a less energy consumed to melt the zinc electrode. The obtained films are porous and fluffy,
7 therefore they are suitable for gas sensor and photocatalytic applications. We also suggest that other binary metal oxides such as Co-Fe, In-Sn and Ag-Zn can be sparked to control the atomic ratio by this method.
Acknowledgments We thank the National Research University Project under Office of Higher Education Commission (OHEC) Thailand, Thailand Research Fund (TRF, IRG5780013) and Chiang Mai University (CMU) for the financial support. T. Kumpika is grateful for his postdoctoral fellowship from Chiang Mai University.
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FIGURE CAPTIONS Fig. 1.
SEM images of the ZnO-TiO2 binary compound nanoparticle thin films at the magnification of (a) 30,000x and (b) 10,000x.
Fig. 2.
Plot of the zinc and the titanium atom percent measured by EDS as a function of the sparking voltages.
Fig. 3.
Mass loss rates versus energy required to melt the electrodes in the sparking process of various metals.
Fig. 4.
Spark emissions of the zinc anode – titanium cathode experiment with various sparking voltages.
Fig. 5.
Plot of the zinc atom percent and sparking voltage as a function of the IZn/ITi emission ratios of the zinc anode – titanium cathode experiment.
Fig. 6a-d
Cross-section images of the ZnO-TiO2 binary compound films with sparking at 10 kV for 100, 200, 300 and 400 times, respectively.
Fig. 7.
XRD patterns of the as-deposited and the annealed ZnO-TiO2 films on the quartz substrate.
Fig. 8.
Raman spectra of the as-deposited and the annealed ZnO-TiO2 films on the quartz substrate.
Figure 9(a)
The IL spectra of as-deposited and annealed ZnO-TiO2 thin films on quartz substrates.
Figure 9(b)
Transmittance spectra of the as-deposited and the annealed ZnO-TiO2 thin films on quartz substrates.
Figure 9(c)
Plot of (h)2 versus the photon energy (hfor estimation of the energy gap for as-deposited and annealed samples.
Figure 9(d)
Refractive index of the as-deposited and the annealed ZnO-TiO2 thin films on quartz substrates.
9
Fig. 1a.
Fig. 1b.
10
Fig. 2.
Fig. 3.
11
Fig. 4a.
Fig. 4b.
12
Fig. 5a.
Fig. 5b.
13
Fig. 6.
14
Fig. 7.
15
Fig. 8.
16
Figure 9a.
Fig. 9b.
17
Fig. 9c.
Fig. 9d.