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Effect of methanol ratio in mixed solvents on optical properties and wettability of ZnO films by cathodic electrodeposition
Journal of Alloys and Compounds 615 (2014) 327–332
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of methanol ratio in mixed solvents on optical properties and wettability of ZnO films by cathodic electrodeposition Miao Zhang a, Kai Xu a, Xishun Jiang a, Lei Yang a, Gang He a, Xueping Song a, Zhaoqi Sun a,⇑, Jianguo Lv b,⇑ a b
School of Physics & Material Science, Anhui University, Hefei 230601, PR China School of Electronic & Information Engineering, Hefei Normal University, Hefei 230601, PR China
a r t i c l e
i n f o
Article history: Received 26 February 2014 Received in revised form 27 June 2014 Accepted 27 June 2014 Available online 7 July 2014 Keywords: ZnO thin films Methanol ratio Cathodic electrodeposition method Wettability
a b s t r a c t ZnO thin films were prepared in the electrolyte with different methanol ratio by cathodic electrodeposition method. Microstructure, surface morphology, optical properties and wettability of the thin films were investigated by X-ray diffractometer, field-emission scanning electron microscope, ultraviolet– visible spectroscope, fluorescence spectrometer and water contact angle apparatus. Increase of methanol ratio in the solvents may restrain the (0 0 2) plane preferential orientation in some extent. Change of current density curves with the ratio of methanol in the solution play a vital role on electrochemical reaction kinetics, microstructure and/or surface morphology of ZnO thin films. With the methanol ratio increase from 0 to 0.8, the surface morphology changes from nanorods to net-like nanostructure. The adsorbed NO 3 ions on the polar planes hinder the crystal growth along the c-axis and redirect the growth direction along the nonpolar planes. The maximum and minimum band gaps have been obtained in the ZnO thin films with the methanol ratio of 0.4 and 0.6, respectively. Change of contact angle before UV irradiation may be related to surface morphology and oxygen vacancies. The maximum light-induced water contact angle change has been observed in the sample with the methanol ratio of 0.8. The results may be attributed to the higher surface roughness and net-like morphology. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction As an important wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), ZnO has triggered great interest in the past decade due to its great performance and potential applications in electro-optical, ferroelectric, pyroelectric and piezoelectric [1–4]. ZnO thin films can be synthesized by many different methods, such as magnetron sputtering [5–8], spray pyrolysis [9,10], atomic layer epitaxy [11], molecular beam epitaxy [12,13], pulsed laser deposition [14,15], sol–gel [16–18], hydrothermal synthesis [19], cathodic electrodeposition [20–23], and so on. Various kinds of nanostructures of ZnO such as nanorods, nanowalls, and nanowires have been prepared by the above methods [24–26]. Among the various growth methods, cathodic electrodeposition is widely used in the laboratory and industrial production for its rapid and cost-effective approach to the fabrication of ZnO nanostructures. The chemical compound, such as NH4 Cl, NH3, H2O2 and KCl were added into the electrolyte. Effect of addition on surface morphology, microstructure, optical and ⇑ Corresponding authors. Tel.: +86 551 63861767 (O) (Z. Sun), +86 551 63674132 (O) (J. Lv). E-mail addresses: [email protected] (Z. Sun), [email protected] (J. Lv). http://dx.doi.org/10.1016/j.jallcom.2014.06.178 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
electrical properties were investigated by other researchers [27– 29]. However, Effect of methanol addition in the electrolyte on surface morphology and wettability of ZnO thin films has been studied seldom [30]. In the study, the ZnO thin films were prepared by cathodic electrodeposition method. In order to regulate the morphology of ZnO thin films, different amounts of methanol were added to the electrolyte. Microstructure surface morphology, optical properties and wettability of the ZnO thin films were investigated using X-ray diffractometer (XRD), field-emission scanning electron microscope (FE-SEM) ultraviolet–visible spectroscope (UV–Vis), fluorescence spectrometer and water contact angle apparatus. Effects of methanol ratio on surface morphology, optical properties and wettability of the ZnO thin films have been investigated.
2. Experimental ZnO thin films were synthesized by three-electrode system and the electrochemical workstation (CHI600D) was used to control the deposition process. A Pt foil (1 1 cm) and Ag/AgCl (filling solution is KCl of 3.5 mol/L) reference electrode were employed as the counter electrode and the reference electrode respectively. An ITO glass substrate which sheet resistance less than 15X/h (1 1 cm) was used as the working electrode. The ITO glass substrate was cleaned with acetone, ethanol and deionized water for 30 min, successively. Zinc nitrate hydrate (Zn(NO3)26H2O,
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0.05 M) was dissolved in 200 mL of deionized (DI) water and methanol. Methanol ratio x in the mixed solvent was 0, 0.4, 0.6, and 0.8, respectively. All the reaction was carried out at 70 °C on a hot plate. Growth time of the thin films was 60 min. The deposition voltage was controlled by negative DC potential of -0.80 V. After the reaction was completed, the sample was removed from the solution and immediately rinsed in deionized water with strong magnetic stirring to remove excess reagent. Thicknesses of the thin films, which were measured by profile meter, are about 640 ± 14 nm, 648 ± 47 nm, 629 ± 53 nm, and 646 ± 45 nm, respectively. Microstructures of ZnO thin films were characterized by X-ray diffraction (XRD, MAC, M18XHF) employing Cu Ka radiation (1.54 Å). Surface morphology of ZnO thin films were characterized by field-emission scanning electron microscope (FESEM, Hitachi, S4800). The absorption spectra of samples were recorded by an ultraviolet–visible spectrophotometer (UV–vis spectrophotometer, Shimadzu, UV-2550) within the wavelength range of 300–900 nm. Photoluminescence (PL) spectra were investigated by fluorescence spectrometer (F-4500FL) with a xenon lamp as the light source excited at 325 nm. A home-made water contact angle apparatus was employed to measure contact angle (CA) at ambient air. The volume of the water droplets used for the CA measurements was 5 lL. Light induced hydrophilicity was studied by irradiating the samples at specific time intervals by using a 500 W high pressure mercury lamp, which mainly emits ultraviolet light of 365 nm. The distance between the sample and the lamp was 12.0 cm. After each irradiation, a new 5 lL water droplet was placed on the irradiated area then the corresponding contact angle was measured. Water droplets were placed at four different positions for each sample. The average value was adopted as the contact angle and experimental error was calculated.
3. Results and discussion 3.1. Microstructure analysis The current density curves during the electrodeposition process are shown in Fig. 1. In all case, the current density reaches to a maximal value at first (this process corresponds to the initial nucleation of ZnO). After the peak, the current density decreases, finally turns to a relatively stable value (this process corresponds to the grain growth stage of ZnO) [31,32]. The intensity of peaks increase as methanol ratio increases from 0 to 0.6, and then drop remarkably as methanol ratio increases to 0.8. The results indicated that the sample with the methanol ratio of 0.6 has the maximal nuclei density. In addition, time of the current density reach to the peak value decreases as the methanol ratio in the solution increases from 0 to 0.8. The results indicated that the nucleation period becomes shorter gradually. All the circumstance was proved in the following results of SEM. The current density curves indicate that the ratio of methanol in the solution has important effect on electrochemical reaction kinetics, microstructure and/or surface morphology of ZnO thin films [33]. Fig. 2 shows the XRD patterns of ZnO thin films with different methanol ratios and the standard XRD pattern of ZnO with hexagonal wurtzite structure at the top. The diffraction peaks from ZnO thin films with hexagonal wurtzite structure and ITO glass
Fig. 1. Current density curves ZnO thin films with different methanol ratio.
substrate were both observed and denoted by asterisk and pound, respectively. It can be seen that the crystallinity and preferential orientation depend on the methanol ratio. Intensity of (0 0 2) diffraction peak in the ZnO thin film grow at deionized water is much stronger than that of other diffraction peak, which indicates that the ZnO thin film are preferentially grown along the c-axis direction. With the methanol ratio increase from 0 to 0.8, the intensity of (0 0 2) diffraction peak of the thin films decreases gradually and the intensity of (1 0 1) diffraction peak increases first and then decreases. The results indicate that methanol in the solvents may restrain the (0 0 2) plane preferential orientation in some extent. Crystallite size of the thin films was calculated using Scherrer formula as follows [34]:
D ¼ Kk=b cos h
ð1Þ
where K is 0.9, k is 1.5406 Å, b is full width at half maximum of diffraction peak and h is the diffraction angle. The average crystallite sizes of the thin films are listed in Table 1. The change of average crystallite size may be related to the variety of current density as the methanol was added to the mixed solvent. Increase of average crystallite size may be related to the increase of current density. Decrease of average crystallite size, as methanol ratio increase from 0.6 to 0.8 in the mixed solvent, may be due to the decrease of current density and change of the growth mechanism. 3.2. Surface morphology analysis SEM images of ZnO thin films with different methanol ratio are shown in Fig. 3. The results show that the methanol ratio in the solvent has significant effect on morphology of ZnO thin films. As shown in Fig. 3a, nanorods with hexagonal structure can be observed in the ZnO thin film grows on deionized water. When the methanol ratio increases to 0.4, the number of nanorods decrease, the length of nanorods get shorter, and the diameter of nanorods get larger. Therefore, The ZnO thin film is composed of a large number of nanoplates. Upon further increase the methanol ratio to 0.6, the morphology changes to a ridge-like shape with very narrow top surface. As shown in Fig. 3d, very thin nanosheets vertically grown on the substrate as methanol ratio increase to 0.8. The surface morphology of the thin film is net-like nanostructure. There are many air voids between nanosheets. It can be concluded that the surface morphology of ZnO thin films can be controlled by adjusting the methanol ratio. In the electrochemical deposition of ZnO using Zn(NO3)2 as electrolyte, the reaction involves the electrochemical reduction of nitrate in aqueous solution which takes place at the electrode surface under sufficient cathodic bias. These OH ions react with the zinc ions to form zinc oxide by means of the dehydration of zinc hydroxide [35,36]. However, some works show that the formation of ZnO may be produced from the reaction of OH and Zn2+ directly [37]. The general reaction sequence of ZnO thin films can be represented as follows:
NO3 þ H2 O þ 2e ! NO2 þ 2OH
ð2Þ
Zn2þ þ 2OH ! ZnðOHÞ2 ! ZnO þ H2 O
ð3Þ
Zn2þ þ 2OH ! ZnO þ H2 O
ð4Þ
It is well known that the wurtzite structure ZnO consists of polar plane and nonpolar planes. The negative ions Zn(OH)2 in 4 the solution are likely adsorbed on the polar plane by electrostatic force, which enhances the anisotropic growth of the crystal along the h0 0 0 1i direction. When the methanol has been added into the growth solution, the highly electronegative NO 3 ions may be adsorbed preferentially on the polar planes, which hinders the crystal growth along the c-axis and redirects the growth direction
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Fig. 2. XRD patterns of ZnO thin films with different methanol ratios and the standard XRD pattern of ZnO with wurtzite structure. The features marked by asterisks and pound are peaks from ZnO with hexagonal wurtzite structure and ITO glass. Table 1 Average crystalline size and band gap of the ZnO thin films. x
Average crystalline size (nm)
Band gap (eV)
0 0.4 0.6 0.8
33.3 ± 0.9 33.6 ± 0.7 41.7 ± 1.0 33.9 ± 0.8
3.27 3.30 3.09 3.25
along the nonpolar planes. As a result, the surface morphology and crystal growth direction of the ZnO thin films were significantly affected by the addition of methanol [30]. 3.3. Optical properties Fig. 4 presents typical absorption spectra of ITO glass substrate and ZnO thin films with different methanol ratio in the range of
300–900 nm. It can be seen that the average absorption coefficient of the ZnO thin films are larger than that of ITO glass. The larger average absorption coefficient of the samples may be attributed to specific surface nanostructure of ZnO thin films. The ZnO thin films and ITO glass appear a abrupt absorption edge at around 370 nm and 340 nm, respectively. As a direct band gap semiconductor, the relationship between absorption coefficient (a) of ZnO thin films and phonon energy (hm) can be determined by follow equation [38]: 2
ðahmÞ ¼ Aðhm Eg Þ
ð5Þ
where hm is the photo enenergy, a is the absorption coefficient, A is a constant and Eg is the band gap. The band gap can be deduced by the extrapolating of linear part of the function (ahm)2 = f(hm). Fig. 5 shows the (ahm)2 vs hm plots of ZnO thin films. The band gaps of ZnO
Fig. 3. SEM images of ZnO thin films with different methanol ratios (a) x = 0; (b) x = 0.4; (c) x = 0.6; (d) x = 0.8.
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Fig. 4. Absorption spectra of ZnO thin films with different methanol ratios.
thin films are listed in Table 1. It can be seen that the band gap values of the ZnO thin films are less than the standard value 3.37 eV of ZnO and the thin films grow in the mixed solvent with the methanol ratio of 0.4 and 0.6 have the maximum and minimum band gaps. In this paper, the change of band gap may be related to the variety of the phase stoichiometry deficiency and crystallite size in ZnO thin films [39]. Fig. 6 shows the PL spectra of ZnO thin films grown at various methanol ratios. It can be seen that all the samples show a violet emission band at around 400 nm, two weak blue emissions at about 455 and 471 nm and a broad visible emission. In general, the violet emission may be attributed to the exciton transition from the localized level below the conduction band to the valance band [40,41]. Zeng et al. [42] thought that the blue emissions in the range of 440–488 nm may be attributed to the electron transition from extended Zn interstitial (Zni) levels to the valance band top. The electrons have been transferred to the conduction band or Zni level, then relax to extended Zni levels, and finally transit to the valance band. Therefore, the two blue emissions may be attributed to the electron transition from extended Zni levels, which are slightly below the Zni level, to the valance band top. The visible emission is associated with deep level defects of oxygen vacancy (Vo) [43] and interstitial oxygen (Oi) [44]. The changes of peak intensity and position of the visible emission are attributed to different contributions from VO and VZn [45], which may be caused by methanol ratio in the mixed solvent. Therefore, the ZnO thin film grow at the solution with methanol ratio x = 0.6 has the larger violet and visible emissions, indicating the thin film has the major localized states and deep level defects.
Fig. 6. PL spectra of ZnO thin films with different methanol ratios.
Wettability is an important property governed by both of surface free energy and surface roughness of a solid surface. Fig. 7a– d shows the photographs of 5 lL water droplet on the ZnO thin films before UV irradiation. The contact angles of the ZnO thin films are 124°, 109°, 104° and 133° responding to different methanol ratios 0, 0.4, 0.6 and 0.8, respectively. It can be seen that surface of all the thin films exhibit hydrophobic and the CA decreases first and then increases with the increase of methanol ratio. Therefore, variety of hydrophobicity of the ZnO thin films can be explained by the Cassie-Baxter model. According to the model, the surface roughness may enhance the hydrophobicity of the low surface energy material. The net-like thin film, which grows at the solution with methanol ratio x = 0.8, has the maximum CA may be due to the higher proportion of air/water interface in solid and air composite rough surface structure. In addition, water molecules may coordinate into the oxygen vacancies, which lead to dissociative adsorption of the water molecules on the surface [46]. The ZnO thin film, grow at the solution with methanol ratio x = 0.6, has the minimum CA may be due to the major oxygen vacancies on the surface. Therefore, change of CA of the thin films can be attributed to a synergistic effect of surface morphology and oxygen vacancies. Fig. 7e–h shows the photographs of 5 lL water droplet on the ZnO thin films after UV irradiation. The corresponding contact angles of the ZnO thin films are 64°, 27°, 23°and 16° responding to different methanol ratios 0, 0.4, 0.6 and 0.8, respectively. It can be seen that surface of all the thin films exhibit hydrophilic and the CA decreases with the increase of methanol ratio. The change of the wettability after UV irradiation can be explained as follows: Electron–hole pairs will generated in the ZnO thin films by UV irradiation with photon energy, which higher than or equal to the band gap of ZnO. Some of the electrons can react with lattice metal ions (Zn2+) to form Zn+s defective sites (surface trapped electrons), while some of the holes react with lattice oxygen to form surface oxygen vacancies [47]. þ
ZnO þ 2hm ! 2h þ 2e
ð6Þ
Zn2þ þ e ! Znþs
ð7Þ
þ
O2 þ h ! O1 Fig. 5. Plot of (ahm)2 vs photon energy for ZnO films grown at various methanol concentrations.
þ
O1 þ h !
ðsurface trapped electronÞ ðsurface trapped holeÞ
1 O2 þ V O 2
ðoxygen vacancyÞ
ð8Þ ð9Þ
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the intensity of (0 0 2) diffraction peak decreases gradually and the surface morphology changes from nanorods with hexagonal structure to net-like nanostructure. The violet emission may be associated with the exciton transition from the localized level below the conduction band to the valance band and the visible emission may be attributed to deep level defects of oxygen vacancy (Vo) and interstitial oxygen (Oi). The CA before UV irradiation decreases first and then increases with the increase of methanol ratio. The maximum light-induced CA change has been observed in the thin film with the methanol ratio of 0.8. The results may be attributed to the higher surface roughness and net-like nanostructure. Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51072001, 51272001, 51102072), National Key Basic Research Program of China (2013CB632705) and the National Science Research Foundation for Scholars Return from Overseas, Ministry of Education, China. The authors would like to thank Yonglong Zhuang and Zhongqing Lin of the Experimental Technology Center of Anhui University, for electron microscope test and discussion. References
Fig. 7. Photographs of water droplet on the ZnO thin films grown at different methanol ratios: (a) 0, (b) 0.4, (c) 0.6, and (d) 0.8 before and (e) 0, (f) 0.4, (g) 0.6, and (h) 0.8 after UV irradiation. The corresponding CA was plotted in (i).
Water and oxygen may compete to dissociatively adsorb on these defective sites. The defective sites are kinetically more favorable for hydroxyl groups (OH) adsorption than oxygen adsorption. As a result, the water adsorption on the irradiated surface of ZnO thin film is enhanced, and the water contact angle is drastically reduced [47] Therefore, the hydrophilicity of the thin films after UV irradiation can be described by the Wenzel model [48]. According to this model, the water droplets are assumed to wet the entire rough surface, not leave any air pockets [49]. The Wenzel model predicts enhancement of hydrophilicity with increasing roughness. Therefore, the morphologies of the thin films have an important effect on wettability. For the ZnO thin film grown in the mixed solvent with the methanol ratio of 0, the smaller light-induced CA change may be related to the lower surface roughness and nanorods with hexagonal structure. For the thin film grown in the mixed solvent with the methanol ratio of 0.8, the surface to volume ratio is higher in this case, and as a result, the total interface between water and the ZnO thin film is higher. This leads to an effective increase in photoactive defect sites, which are in contact with water molecules [50]. Therefore, the maximum light-induced CA change may be due to the higher surface roughness and net-like nanostructure. 4. Conclusions Microstructure, surface morphology, optical properties and surface wettability of ZnO thin films were characterized by XRD, FESEM, UV–vis spectroscope, FL spectrometer and water contact angle apparatus. With the methanol ratio increase from 0 to 0.8,
[1] S.T. Rajan, B. Subramanian, A.K.N. Kumar, M. Jayachandran, M.S.R. Rao, J. Alloys Comp. 584 (2014) 611–616. [2] K. Kumar, M. Chitkara, I.S. Sandhu, D. Mehta, S. Kumar, J. Alloys Comp. 588 (2014) 681–689. [3] K.W. Liu, M. Sakurai, M. Aono, Sensor. Actuators B – Chem. 157 (2011) 98–102. [4] K. Zhong, J. Xu, J. Su, Y.L. Chen, Appl. Surf. Sci. 257 (2011) 3495–3498. [5] H.S. Al-Salman, M.J. Abdullah, J. Alloys Comp. 547 (2013) 132–137. [6] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943–945. [7] S.S. Shinde, P.S. Shinde, R.T. Sapkal, Y.W. Oh, D. Haranath, C.H. Bhosale, K.Y. Rajpure, J. Alloys Comp. 538 (2012) 237–243. [8] S.H. Jeong, B.S. Kim, B.T. Lee, Appl. Phys. Lett. 82 (2003) 2625–2627. [9] J.M. Bian, X.M. Li, C.Y. Zhang, L.D. Chen, Q. Yao, Appl. Phys. Lett. 84 (2004) 3783–3785. [10] J. Winenke, B.V.D. Zanden, M. Tijssen, M. Zeman, Sol. Energy Mater. Sol. C 92 (2008) 884–890. [11] E.B. Yousfi, B. Weinberger, F. Donsanti, P. Cowache, D. Lincot, Thin Solid Films 387 (2001) 29–32. [12] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270–3272. [13] I. Ohkubo, Y. Matsumoto, A. Ohtomo, T. Ohnishi, A. Tsukazaki, M. Lippmaa, H. Koinuma, M. Kawasaki, Appl. Surf. Sci. 159–160 (2000) 514–519. [14] M. Joseph, H. Tabata, H. Saeki, K. Ueda, T. Kawai, Physica B 302–303 (2001) 140–148. [15] Z.Y. Ning, S.H. Cheng, S.B. Ge, Y. Chao, Z.Q. Gang, Y.X. Zhang, Z.G. Liu, Thin Solid Films 307 (1997) 50–53. [16] K.G. Kanade, B.B. Kale, R.C. Aiyer, B.K. Das, Mater. Res. Bull. 41 (2006) 590–600. [17] S. Akin, G. Karanfil, A. Gultekin, S. Sonmezoglu, J. Alloys Comp. 579 (2013) 272–278. [18] J. Lee, A.J. Easteal, U. Pal, D. Bhattacharyya, Curr. Appl. Phys. 9 (2009) 792–796. [19] C.W. Cheng, G.Y. Xu, H.Q. Zhang, Y. Luo, P.G. Zhang, K. Shen, Mater. Res. Bull. 43 (2008) 3506–3513. [20] G.W. She, X.H. Zhang, W.S. Shi, X. Fan, J.C. Chang, Electrochem. Commun. 9 (2007) 2784–2788. [21] T. Singh, D.K. Pandya, R. Singh, J. Alloys Comp. 509 (2011) 5095–5098. [22] H.B. Zeng, J.B. Cui, B.Q. Cao, U. Gibson, Y. Bando, D. Golberg, Sci. Adv. Mater. 2 (2010) 336–358. [23] Y.J. Kim, H.M. Shang, G.Z. Cao, Sol-Gel. Sci. Technol. 38 (2006) 79–84. [24] Y.F. Gao, M. Nagai, Y. Masuda, F. Sato, K. Koumoto, Cryst. Growth 286 (2006) 445–450. [25] Th. Pauportéa, I. Jirka, Electrochima Acta 54 (2009) 7558–7564. [26] Y. Li, G.W. Meng, L.D. Zhang, Appl. Phys. Lett. 76 (2000) 2011–2013. [27] J.B. Cui, Y.C. Soo, T.P. Chen, U.J. Gibson, J. Phys. Chem. C. 112 (2008) 4475– 4479. [28] T. Pauporte, D.J. Lincot, Electrochem. Soc. 148 (2001) C310–C314. [29] H. Cui, M. Mollar, B. Marí, Opt. Mater. 33 (2011) 327–331. [30] K.Y. Wu, Z.Q. Sun, J.B. Cui, Cryst. Growth Des. 12 (2012) 2864–2871. [31] A. Goux, T. Pauport´e, J. Chivot, D. Lincot, Elecrochim. Acta 50 (2005) 2239– 2248. [32] M. Skompska, K. Zarebska, Elecrochim. Acta 127 (2014) 467–488. [33] S. Patrick, A. Elisabet, Appl. Surf. Sci. 289 (2014) 345–350. [34] L. Sagalowicz, G.R. Fox, Mater. Res. 14 (1999) 1876–1885. [35] M. Izaki, T. Omi, Appl. Phys. Lett. 68 (1996) 2439–2440.
332
M. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 327–332
[36] M.A. Thomas, J.B. Cui, J. Electrochem. Soc. 160 (2013) D218–D225. [37] K.M. McPeak, M.A. Becker, N.G. Britton, H. Majidi, B.A. Bunker, J.B. Baxter, Chem. Mater. 22 (2010) 6162–6170. [38] A.Y. Oral, Z.B. Bahsßi, M.H. Aslan, Appl. Surf. Sci. 253 (2007) 4593–4598. [39] Ü. Özgür, I.Y. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.J. Cho, H. Morkoç, Appl. Phys. 98 (2005) 041301–041404. [40] X.D. Gao, X.M. Li, W.D. Yu, J. Solid State Chem. 177 (2004) 3830–3834. [41] D. Behera, B.S. Acharya, J. Lumin. 128 (2008) 1577–1586. [42] H.B. Zeng, G.T. Duan, Y. Li, S.K. Yang, X.X. Xu, W.P. Cai, Adv. Funct. Mater. 20 (2010) 561–572. [43] C.H. Ahn, Y.Y. Kim, D.C. Kim, S.K. Mohanta, H.K. Cho, J. Appl. Phys. 105 (2009) 013502–013505.
[44] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285–2287. [45] I. Hussain, N. Bano, S. Hussain, O. Nur, M. Willander, J. Mater. Sci. 46 (2011) 7437–7442. [46] J.G. Lv, C.L. Liu, W.B. Gong, Z.F. Zi, X.S. Chen, K. Huang, T. Wang, G. He, X.P. Song, Z.Q. Sun, Sci. Adv. Mater. 4 (2012) 757–762. [47] V. Khranovskyy, T. Ekblad, R. Yakimova, L. Hultman, Appl. Surf. Sci. 258 (2012) 8146–8152. [48] R.N. Wenzel, Ind. Eng. Chem. Res. 28 (1936) 988–994. [49] G. Kenanakis, E. Stratakis, K. Vlachou, D. Vernardou, E. Koudoumas, N. Katsarakis, Appl. Surf. Sci. 254 (2008) 5695–5699. [50] E.L. Papadopoulou, M. Barberoglou, V. Zorba, A. Manousaki, A. Pagkozidis, E. Stratakis, C. Fotakis, J. Phys. Chem. C 113 (2009) 2891–2895.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of methanol ratio in mixed solvents on optical properties and wettability of ZnO films by cathodic electrodeposition Miao Zhang a, Kai Xu a, Xishun Jiang a, Lei Yang a, Gang He a, Xueping Song a, Zhaoqi Sun a,⇑, Jianguo Lv b,⇑ a b
School of Physics & Material Science, Anhui University, Hefei 230601, PR China School of Electronic & Information Engineering, Hefei Normal University, Hefei 230601, PR China
a r t i c l e
i n f o
Article history: Received 26 February 2014 Received in revised form 27 June 2014 Accepted 27 June 2014 Available online 7 July 2014 Keywords: ZnO thin films Methanol ratio Cathodic electrodeposition method Wettability
a b s t r a c t ZnO thin films were prepared in the electrolyte with different methanol ratio by cathodic electrodeposition method. Microstructure, surface morphology, optical properties and wettability of the thin films were investigated by X-ray diffractometer, field-emission scanning electron microscope, ultraviolet– visible spectroscope, fluorescence spectrometer and water contact angle apparatus. Increase of methanol ratio in the solvents may restrain the (0 0 2) plane preferential orientation in some extent. Change of current density curves with the ratio of methanol in the solution play a vital role on electrochemical reaction kinetics, microstructure and/or surface morphology of ZnO thin films. With the methanol ratio increase from 0 to 0.8, the surface morphology changes from nanorods to net-like nanostructure. The adsorbed NO 3 ions on the polar planes hinder the crystal growth along the c-axis and redirect the growth direction along the nonpolar planes. The maximum and minimum band gaps have been obtained in the ZnO thin films with the methanol ratio of 0.4 and 0.6, respectively. Change of contact angle before UV irradiation may be related to surface morphology and oxygen vacancies. The maximum light-induced water contact angle change has been observed in the sample with the methanol ratio of 0.8. The results may be attributed to the higher surface roughness and net-like morphology. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction As an important wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), ZnO has triggered great interest in the past decade due to its great performance and potential applications in electro-optical, ferroelectric, pyroelectric and piezoelectric [1–4]. ZnO thin films can be synthesized by many different methods, such as magnetron sputtering [5–8], spray pyrolysis [9,10], atomic layer epitaxy [11], molecular beam epitaxy [12,13], pulsed laser deposition [14,15], sol–gel [16–18], hydrothermal synthesis [19], cathodic electrodeposition [20–23], and so on. Various kinds of nanostructures of ZnO such as nanorods, nanowalls, and nanowires have been prepared by the above methods [24–26]. Among the various growth methods, cathodic electrodeposition is widely used in the laboratory and industrial production for its rapid and cost-effective approach to the fabrication of ZnO nanostructures. The chemical compound, such as NH4 Cl, NH3, H2O2 and KCl were added into the electrolyte. Effect of addition on surface morphology, microstructure, optical and ⇑ Corresponding authors. Tel.: +86 551 63861767 (O) (Z. Sun), +86 551 63674132 (O) (J. Lv). E-mail addresses: [email protected] (Z. Sun), [email protected] (J. Lv). http://dx.doi.org/10.1016/j.jallcom.2014.06.178 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
electrical properties were investigated by other researchers [27– 29]. However, Effect of methanol addition in the electrolyte on surface morphology and wettability of ZnO thin films has been studied seldom [30]. In the study, the ZnO thin films were prepared by cathodic electrodeposition method. In order to regulate the morphology of ZnO thin films, different amounts of methanol were added to the electrolyte. Microstructure surface morphology, optical properties and wettability of the ZnO thin films were investigated using X-ray diffractometer (XRD), field-emission scanning electron microscope (FE-SEM) ultraviolet–visible spectroscope (UV–Vis), fluorescence spectrometer and water contact angle apparatus. Effects of methanol ratio on surface morphology, optical properties and wettability of the ZnO thin films have been investigated.
2. Experimental ZnO thin films were synthesized by three-electrode system and the electrochemical workstation (CHI600D) was used to control the deposition process. A Pt foil (1 1 cm) and Ag/AgCl (filling solution is KCl of 3.5 mol/L) reference electrode were employed as the counter electrode and the reference electrode respectively. An ITO glass substrate which sheet resistance less than 15X/h (1 1 cm) was used as the working electrode. The ITO glass substrate was cleaned with acetone, ethanol and deionized water for 30 min, successively. Zinc nitrate hydrate (Zn(NO3)26H2O,
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0.05 M) was dissolved in 200 mL of deionized (DI) water and methanol. Methanol ratio x in the mixed solvent was 0, 0.4, 0.6, and 0.8, respectively. All the reaction was carried out at 70 °C on a hot plate. Growth time of the thin films was 60 min. The deposition voltage was controlled by negative DC potential of -0.80 V. After the reaction was completed, the sample was removed from the solution and immediately rinsed in deionized water with strong magnetic stirring to remove excess reagent. Thicknesses of the thin films, which were measured by profile meter, are about 640 ± 14 nm, 648 ± 47 nm, 629 ± 53 nm, and 646 ± 45 nm, respectively. Microstructures of ZnO thin films were characterized by X-ray diffraction (XRD, MAC, M18XHF) employing Cu Ka radiation (1.54 Å). Surface morphology of ZnO thin films were characterized by field-emission scanning electron microscope (FESEM, Hitachi, S4800). The absorption spectra of samples were recorded by an ultraviolet–visible spectrophotometer (UV–vis spectrophotometer, Shimadzu, UV-2550) within the wavelength range of 300–900 nm. Photoluminescence (PL) spectra were investigated by fluorescence spectrometer (F-4500FL) with a xenon lamp as the light source excited at 325 nm. A home-made water contact angle apparatus was employed to measure contact angle (CA) at ambient air. The volume of the water droplets used for the CA measurements was 5 lL. Light induced hydrophilicity was studied by irradiating the samples at specific time intervals by using a 500 W high pressure mercury lamp, which mainly emits ultraviolet light of 365 nm. The distance between the sample and the lamp was 12.0 cm. After each irradiation, a new 5 lL water droplet was placed on the irradiated area then the corresponding contact angle was measured. Water droplets were placed at four different positions for each sample. The average value was adopted as the contact angle and experimental error was calculated.
3. Results and discussion 3.1. Microstructure analysis The current density curves during the electrodeposition process are shown in Fig. 1. In all case, the current density reaches to a maximal value at first (this process corresponds to the initial nucleation of ZnO). After the peak, the current density decreases, finally turns to a relatively stable value (this process corresponds to the grain growth stage of ZnO) [31,32]. The intensity of peaks increase as methanol ratio increases from 0 to 0.6, and then drop remarkably as methanol ratio increases to 0.8. The results indicated that the sample with the methanol ratio of 0.6 has the maximal nuclei density. In addition, time of the current density reach to the peak value decreases as the methanol ratio in the solution increases from 0 to 0.8. The results indicated that the nucleation period becomes shorter gradually. All the circumstance was proved in the following results of SEM. The current density curves indicate that the ratio of methanol in the solution has important effect on electrochemical reaction kinetics, microstructure and/or surface morphology of ZnO thin films [33]. Fig. 2 shows the XRD patterns of ZnO thin films with different methanol ratios and the standard XRD pattern of ZnO with hexagonal wurtzite structure at the top. The diffraction peaks from ZnO thin films with hexagonal wurtzite structure and ITO glass
Fig. 1. Current density curves ZnO thin films with different methanol ratio.
substrate were both observed and denoted by asterisk and pound, respectively. It can be seen that the crystallinity and preferential orientation depend on the methanol ratio. Intensity of (0 0 2) diffraction peak in the ZnO thin film grow at deionized water is much stronger than that of other diffraction peak, which indicates that the ZnO thin film are preferentially grown along the c-axis direction. With the methanol ratio increase from 0 to 0.8, the intensity of (0 0 2) diffraction peak of the thin films decreases gradually and the intensity of (1 0 1) diffraction peak increases first and then decreases. The results indicate that methanol in the solvents may restrain the (0 0 2) plane preferential orientation in some extent. Crystallite size of the thin films was calculated using Scherrer formula as follows [34]:
D ¼ Kk=b cos h
ð1Þ
where K is 0.9, k is 1.5406 Å, b is full width at half maximum of diffraction peak and h is the diffraction angle. The average crystallite sizes of the thin films are listed in Table 1. The change of average crystallite size may be related to the variety of current density as the methanol was added to the mixed solvent. Increase of average crystallite size may be related to the increase of current density. Decrease of average crystallite size, as methanol ratio increase from 0.6 to 0.8 in the mixed solvent, may be due to the decrease of current density and change of the growth mechanism. 3.2. Surface morphology analysis SEM images of ZnO thin films with different methanol ratio are shown in Fig. 3. The results show that the methanol ratio in the solvent has significant effect on morphology of ZnO thin films. As shown in Fig. 3a, nanorods with hexagonal structure can be observed in the ZnO thin film grows on deionized water. When the methanol ratio increases to 0.4, the number of nanorods decrease, the length of nanorods get shorter, and the diameter of nanorods get larger. Therefore, The ZnO thin film is composed of a large number of nanoplates. Upon further increase the methanol ratio to 0.6, the morphology changes to a ridge-like shape with very narrow top surface. As shown in Fig. 3d, very thin nanosheets vertically grown on the substrate as methanol ratio increase to 0.8. The surface morphology of the thin film is net-like nanostructure. There are many air voids between nanosheets. It can be concluded that the surface morphology of ZnO thin films can be controlled by adjusting the methanol ratio. In the electrochemical deposition of ZnO using Zn(NO3)2 as electrolyte, the reaction involves the electrochemical reduction of nitrate in aqueous solution which takes place at the electrode surface under sufficient cathodic bias. These OH ions react with the zinc ions to form zinc oxide by means of the dehydration of zinc hydroxide [35,36]. However, some works show that the formation of ZnO may be produced from the reaction of OH and Zn2+ directly [37]. The general reaction sequence of ZnO thin films can be represented as follows:
NO3 þ H2 O þ 2e ! NO2 þ 2OH
ð2Þ
Zn2þ þ 2OH ! ZnðOHÞ2 ! ZnO þ H2 O
ð3Þ
Zn2þ þ 2OH ! ZnO þ H2 O
ð4Þ
It is well known that the wurtzite structure ZnO consists of polar plane and nonpolar planes. The negative ions Zn(OH)2 in 4 the solution are likely adsorbed on the polar plane by electrostatic force, which enhances the anisotropic growth of the crystal along the h0 0 0 1i direction. When the methanol has been added into the growth solution, the highly electronegative NO 3 ions may be adsorbed preferentially on the polar planes, which hinders the crystal growth along the c-axis and redirects the growth direction
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Fig. 2. XRD patterns of ZnO thin films with different methanol ratios and the standard XRD pattern of ZnO with wurtzite structure. The features marked by asterisks and pound are peaks from ZnO with hexagonal wurtzite structure and ITO glass. Table 1 Average crystalline size and band gap of the ZnO thin films. x
Average crystalline size (nm)
Band gap (eV)
0 0.4 0.6 0.8
33.3 ± 0.9 33.6 ± 0.7 41.7 ± 1.0 33.9 ± 0.8
3.27 3.30 3.09 3.25
along the nonpolar planes. As a result, the surface morphology and crystal growth direction of the ZnO thin films were significantly affected by the addition of methanol [30]. 3.3. Optical properties Fig. 4 presents typical absorption spectra of ITO glass substrate and ZnO thin films with different methanol ratio in the range of
300–900 nm. It can be seen that the average absorption coefficient of the ZnO thin films are larger than that of ITO glass. The larger average absorption coefficient of the samples may be attributed to specific surface nanostructure of ZnO thin films. The ZnO thin films and ITO glass appear a abrupt absorption edge at around 370 nm and 340 nm, respectively. As a direct band gap semiconductor, the relationship between absorption coefficient (a) of ZnO thin films and phonon energy (hm) can be determined by follow equation [38]: 2
ðahmÞ ¼ Aðhm Eg Þ
ð5Þ
where hm is the photo enenergy, a is the absorption coefficient, A is a constant and Eg is the band gap. The band gap can be deduced by the extrapolating of linear part of the function (ahm)2 = f(hm). Fig. 5 shows the (ahm)2 vs hm plots of ZnO thin films. The band gaps of ZnO
Fig. 3. SEM images of ZnO thin films with different methanol ratios (a) x = 0; (b) x = 0.4; (c) x = 0.6; (d) x = 0.8.
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Fig. 4. Absorption spectra of ZnO thin films with different methanol ratios.
thin films are listed in Table 1. It can be seen that the band gap values of the ZnO thin films are less than the standard value 3.37 eV of ZnO and the thin films grow in the mixed solvent with the methanol ratio of 0.4 and 0.6 have the maximum and minimum band gaps. In this paper, the change of band gap may be related to the variety of the phase stoichiometry deficiency and crystallite size in ZnO thin films [39]. Fig. 6 shows the PL spectra of ZnO thin films grown at various methanol ratios. It can be seen that all the samples show a violet emission band at around 400 nm, two weak blue emissions at about 455 and 471 nm and a broad visible emission. In general, the violet emission may be attributed to the exciton transition from the localized level below the conduction band to the valance band [40,41]. Zeng et al. [42] thought that the blue emissions in the range of 440–488 nm may be attributed to the electron transition from extended Zn interstitial (Zni) levels to the valance band top. The electrons have been transferred to the conduction band or Zni level, then relax to extended Zni levels, and finally transit to the valance band. Therefore, the two blue emissions may be attributed to the electron transition from extended Zni levels, which are slightly below the Zni level, to the valance band top. The visible emission is associated with deep level defects of oxygen vacancy (Vo) [43] and interstitial oxygen (Oi) [44]. The changes of peak intensity and position of the visible emission are attributed to different contributions from VO and VZn [45], which may be caused by methanol ratio in the mixed solvent. Therefore, the ZnO thin film grow at the solution with methanol ratio x = 0.6 has the larger violet and visible emissions, indicating the thin film has the major localized states and deep level defects.
Fig. 6. PL spectra of ZnO thin films with different methanol ratios.
Wettability is an important property governed by both of surface free energy and surface roughness of a solid surface. Fig. 7a– d shows the photographs of 5 lL water droplet on the ZnO thin films before UV irradiation. The contact angles of the ZnO thin films are 124°, 109°, 104° and 133° responding to different methanol ratios 0, 0.4, 0.6 and 0.8, respectively. It can be seen that surface of all the thin films exhibit hydrophobic and the CA decreases first and then increases with the increase of methanol ratio. Therefore, variety of hydrophobicity of the ZnO thin films can be explained by the Cassie-Baxter model. According to the model, the surface roughness may enhance the hydrophobicity of the low surface energy material. The net-like thin film, which grows at the solution with methanol ratio x = 0.8, has the maximum CA may be due to the higher proportion of air/water interface in solid and air composite rough surface structure. In addition, water molecules may coordinate into the oxygen vacancies, which lead to dissociative adsorption of the water molecules on the surface [46]. The ZnO thin film, grow at the solution with methanol ratio x = 0.6, has the minimum CA may be due to the major oxygen vacancies on the surface. Therefore, change of CA of the thin films can be attributed to a synergistic effect of surface morphology and oxygen vacancies. Fig. 7e–h shows the photographs of 5 lL water droplet on the ZnO thin films after UV irradiation. The corresponding contact angles of the ZnO thin films are 64°, 27°, 23°and 16° responding to different methanol ratios 0, 0.4, 0.6 and 0.8, respectively. It can be seen that surface of all the thin films exhibit hydrophilic and the CA decreases with the increase of methanol ratio. The change of the wettability after UV irradiation can be explained as follows: Electron–hole pairs will generated in the ZnO thin films by UV irradiation with photon energy, which higher than or equal to the band gap of ZnO. Some of the electrons can react with lattice metal ions (Zn2+) to form Zn+s defective sites (surface trapped electrons), while some of the holes react with lattice oxygen to form surface oxygen vacancies [47]. þ
ZnO þ 2hm ! 2h þ 2e
ð6Þ
Zn2þ þ e ! Znþs
ð7Þ
þ
O2 þ h ! O1 Fig. 5. Plot of (ahm)2 vs photon energy for ZnO films grown at various methanol concentrations.
þ
O1 þ h !
ðsurface trapped electronÞ ðsurface trapped holeÞ
1 O2 þ V O 2
ðoxygen vacancyÞ
ð8Þ ð9Þ
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the intensity of (0 0 2) diffraction peak decreases gradually and the surface morphology changes from nanorods with hexagonal structure to net-like nanostructure. The violet emission may be associated with the exciton transition from the localized level below the conduction band to the valance band and the visible emission may be attributed to deep level defects of oxygen vacancy (Vo) and interstitial oxygen (Oi). The CA before UV irradiation decreases first and then increases with the increase of methanol ratio. The maximum light-induced CA change has been observed in the thin film with the methanol ratio of 0.8. The results may be attributed to the higher surface roughness and net-like nanostructure. Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51072001, 51272001, 51102072), National Key Basic Research Program of China (2013CB632705) and the National Science Research Foundation for Scholars Return from Overseas, Ministry of Education, China. The authors would like to thank Yonglong Zhuang and Zhongqing Lin of the Experimental Technology Center of Anhui University, for electron microscope test and discussion. References
Fig. 7. Photographs of water droplet on the ZnO thin films grown at different methanol ratios: (a) 0, (b) 0.4, (c) 0.6, and (d) 0.8 before and (e) 0, (f) 0.4, (g) 0.6, and (h) 0.8 after UV irradiation. The corresponding CA was plotted in (i).
Water and oxygen may compete to dissociatively adsorb on these defective sites. The defective sites are kinetically more favorable for hydroxyl groups (OH) adsorption than oxygen adsorption. As a result, the water adsorption on the irradiated surface of ZnO thin film is enhanced, and the water contact angle is drastically reduced [47] Therefore, the hydrophilicity of the thin films after UV irradiation can be described by the Wenzel model [48]. According to this model, the water droplets are assumed to wet the entire rough surface, not leave any air pockets [49]. The Wenzel model predicts enhancement of hydrophilicity with increasing roughness. Therefore, the morphologies of the thin films have an important effect on wettability. For the ZnO thin film grown in the mixed solvent with the methanol ratio of 0, the smaller light-induced CA change may be related to the lower surface roughness and nanorods with hexagonal structure. For the thin film grown in the mixed solvent with the methanol ratio of 0.8, the surface to volume ratio is higher in this case, and as a result, the total interface between water and the ZnO thin film is higher. This leads to an effective increase in photoactive defect sites, which are in contact with water molecules [50]. Therefore, the maximum light-induced CA change may be due to the higher surface roughness and net-like nanostructure. 4. Conclusions Microstructure, surface morphology, optical properties and surface wettability of ZnO thin films were characterized by XRD, FESEM, UV–vis spectroscope, FL spectrometer and water contact angle apparatus. With the methanol ratio increase from 0 to 0.8,
[1] S.T. Rajan, B. Subramanian, A.K.N. Kumar, M. Jayachandran, M.S.R. Rao, J. Alloys Comp. 584 (2014) 611–616. [2] K. Kumar, M. Chitkara, I.S. Sandhu, D. Mehta, S. Kumar, J. Alloys Comp. 588 (2014) 681–689. [3] K.W. Liu, M. Sakurai, M. Aono, Sensor. Actuators B – Chem. 157 (2011) 98–102. [4] K. Zhong, J. Xu, J. Su, Y.L. Chen, Appl. Surf. Sci. 257 (2011) 3495–3498. [5] H.S. Al-Salman, M.J. Abdullah, J. Alloys Comp. 547 (2013) 132–137. [6] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943–945. [7] S.S. Shinde, P.S. Shinde, R.T. Sapkal, Y.W. Oh, D. Haranath, C.H. Bhosale, K.Y. Rajpure, J. Alloys Comp. 538 (2012) 237–243. [8] S.H. Jeong, B.S. Kim, B.T. Lee, Appl. Phys. Lett. 82 (2003) 2625–2627. [9] J.M. Bian, X.M. Li, C.Y. Zhang, L.D. Chen, Q. Yao, Appl. Phys. Lett. 84 (2004) 3783–3785. [10] J. Winenke, B.V.D. Zanden, M. Tijssen, M. Zeman, Sol. Energy Mater. Sol. C 92 (2008) 884–890. [11] E.B. Yousfi, B. Weinberger, F. Donsanti, P. Cowache, D. Lincot, Thin Solid Films 387 (2001) 29–32. [12] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270–3272. [13] I. Ohkubo, Y. Matsumoto, A. Ohtomo, T. Ohnishi, A. Tsukazaki, M. Lippmaa, H. Koinuma, M. Kawasaki, Appl. Surf. Sci. 159–160 (2000) 514–519. [14] M. Joseph, H. Tabata, H. Saeki, K. Ueda, T. Kawai, Physica B 302–303 (2001) 140–148. [15] Z.Y. Ning, S.H. Cheng, S.B. Ge, Y. Chao, Z.Q. Gang, Y.X. Zhang, Z.G. Liu, Thin Solid Films 307 (1997) 50–53. [16] K.G. Kanade, B.B. Kale, R.C. Aiyer, B.K. Das, Mater. Res. Bull. 41 (2006) 590–600. [17] S. Akin, G. Karanfil, A. Gultekin, S. Sonmezoglu, J. Alloys Comp. 579 (2013) 272–278. [18] J. Lee, A.J. Easteal, U. Pal, D. Bhattacharyya, Curr. Appl. Phys. 9 (2009) 792–796. [19] C.W. Cheng, G.Y. Xu, H.Q. Zhang, Y. Luo, P.G. Zhang, K. Shen, Mater. Res. Bull. 43 (2008) 3506–3513. [20] G.W. She, X.H. Zhang, W.S. Shi, X. Fan, J.C. Chang, Electrochem. Commun. 9 (2007) 2784–2788. [21] T. Singh, D.K. Pandya, R. Singh, J. Alloys Comp. 509 (2011) 5095–5098. [22] H.B. Zeng, J.B. Cui, B.Q. Cao, U. Gibson, Y. Bando, D. Golberg, Sci. Adv. Mater. 2 (2010) 336–358. [23] Y.J. Kim, H.M. Shang, G.Z. Cao, Sol-Gel. Sci. Technol. 38 (2006) 79–84. [24] Y.F. Gao, M. Nagai, Y. Masuda, F. Sato, K. Koumoto, Cryst. Growth 286 (2006) 445–450. [25] Th. Pauportéa, I. Jirka, Electrochima Acta 54 (2009) 7558–7564. [26] Y. Li, G.W. Meng, L.D. Zhang, Appl. Phys. Lett. 76 (2000) 2011–2013. [27] J.B. Cui, Y.C. Soo, T.P. Chen, U.J. Gibson, J. Phys. Chem. C. 112 (2008) 4475– 4479. [28] T. Pauporte, D.J. Lincot, Electrochem. Soc. 148 (2001) C310–C314. [29] H. Cui, M. Mollar, B. Marí, Opt. Mater. 33 (2011) 327–331. [30] K.Y. Wu, Z.Q. Sun, J.B. Cui, Cryst. Growth Des. 12 (2012) 2864–2871. [31] A. Goux, T. Pauport´e, J. Chivot, D. Lincot, Elecrochim. Acta 50 (2005) 2239– 2248. [32] M. Skompska, K. Zarebska, Elecrochim. Acta 127 (2014) 467–488. [33] S. Patrick, A. Elisabet, Appl. Surf. Sci. 289 (2014) 345–350. [34] L. Sagalowicz, G.R. Fox, Mater. Res. 14 (1999) 1876–1885. [35] M. Izaki, T. Omi, Appl. Phys. Lett. 68 (1996) 2439–2440.
332
M. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 327–332
[36] M.A. Thomas, J.B. Cui, J. Electrochem. Soc. 160 (2013) D218–D225. [37] K.M. McPeak, M.A. Becker, N.G. Britton, H. Majidi, B.A. Bunker, J.B. Baxter, Chem. Mater. 22 (2010) 6162–6170. [38] A.Y. Oral, Z.B. Bahsßi, M.H. Aslan, Appl. Surf. Sci. 253 (2007) 4593–4598. [39] Ü. Özgür, I.Y. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.J. Cho, H. Morkoç, Appl. Phys. 98 (2005) 041301–041404. [40] X.D. Gao, X.M. Li, W.D. Yu, J. Solid State Chem. 177 (2004) 3830–3834. [41] D. Behera, B.S. Acharya, J. Lumin. 128 (2008) 1577–1586. [42] H.B. Zeng, G.T. Duan, Y. Li, S.K. Yang, X.X. Xu, W.P. Cai, Adv. Funct. Mater. 20 (2010) 561–572. [43] C.H. Ahn, Y.Y. Kim, D.C. Kim, S.K. Mohanta, H.K. Cho, J. Appl. Phys. 105 (2009) 013502–013505.
[44] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285–2287. [45] I. Hussain, N. Bano, S. Hussain, O. Nur, M. Willander, J. Mater. Sci. 46 (2011) 7437–7442. [46] J.G. Lv, C.L. Liu, W.B. Gong, Z.F. Zi, X.S. Chen, K. Huang, T. Wang, G. He, X.P. Song, Z.Q. Sun, Sci. Adv. Mater. 4 (2012) 757–762. [47] V. Khranovskyy, T. Ekblad, R. Yakimova, L. Hultman, Appl. Surf. Sci. 258 (2012) 8146–8152. [48] R.N. Wenzel, Ind. Eng. Chem. Res. 28 (1936) 988–994. [49] G. Kenanakis, E. Stratakis, K. Vlachou, D. Vernardou, E. Koudoumas, N. Katsarakis, Appl. Surf. Sci. 254 (2008) 5695–5699. [50] E.L. Papadopoulou, M. Barberoglou, V. Zorba, A. Manousaki, A. Pagkozidis, E. Stratakis, C. Fotakis, J. Phys. Chem. C 113 (2009) 2891–2895.