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Electro-deposition of luminescent molybdate and tungstate thin films via a cell route
Materials Chemistry and Physics 116 (2009) 242–246
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electro-deposition of luminescent molybdate and tungstate thin films via a cell route Lianping Chen ∗ , Yuanhong Gao Jiangxi University of Science and Technology, Ganzhou 341000, China
a r t i c l e
i n f o
Article history: Received 4 August 2008 Received in revised form 16 January 2009 Accepted 19 March 2009 Keywords: Thin films Luminescence
a b s t r a c t Highly crystallized luminescent AMO4 (A = Ca, Sr, and Ba; M = W, Mo) thin films have been prepared by cell electrochemical technique. Researches reveal that the deposition rate can be tailored by adjusting temperatures and pH values. The optimum temperature is between 308 and 333 K, and the pH value is between 12 and 13.5. X-ray diffraction and photoelectron spectra examinations confirm that the obtained films are polycrystalline AMO4 (A = Ca, Sr, and Ba; M = W, Mo). Under cell conditions, the homogeneity of films has been improved markedly due to the disappearance of large crystal clusters. At room temperature, a broad emission band ranging from the violet to visible can be observed with the excitation wavelength at 201 nm. Further analyses show that such a broad band can be divided into two sub-bands. Their centers are situated at 357 and 456 nm, respectively. It is proposed that the longer wavelength emission probably originates from MoO4 2− complex. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Scheelite-structured AMO4 (A = Ca, Sr, Ba; M = W, Mo) are good laser host materials [1–5]. They also show promising applications in the fields of luminescence, displays, electro-optics and detectors due to their blue or green emission. They remain a tetragonal structure over wide range of temperatures [3] and pressures [6]. Hence, they may become the material of choice under some extreme environments. Since thin film phosphors have superior resolution compared to powders, AMO4 (A = Ca, Sr, Ba; M = W, Mo) thin films have received significant attention in recent years. Various methods, such as sputtering [7], vacuum evaporation [8], pulsed laser deposition [9] and citrate complex [10], have been used to prepare such films. But samples prepared by these methods need to be annealed (500–1100 ◦ C). Annealing not only consumes a large amount of energy, but also leads to poor adhesion. Hence, it is urgent to develop new ways to prepare thin films without annealing or with lower temperature annealing treatment. Electrochemical methods, such as galvanostatic or potentiostatic method, have been widely used to prepare AMO4 (A = Ca, Sr, Ba; M = Mo, W) thin films [11–14] due to much less consumption of energy. One of the advantages of electrochemical techniques is that
∗ Corresponding author at: School of Materials and Chemical Engineering, Jiangxi University of Science and Technology, 86, Red Flag Avenue, Ganzhou, Jiangxi, China. Tel.: +86 0797 8312204; fax: +86 0797 8312411. E-mail address: [email protected] (L. Chen). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.03.018
good crystalline films can be obtained from solutions without any annealing treatment. The electro-deposition of AMO4 films mainly depends on the following reactions (taking the formation of BaWO4 films as an example): W + 8OH− → WO4 2− + 4H2 O + 6e−
(1)
Ba2+ + WO4 2− ↔ BaWO4
(2)
The first one is a faradic process, and the latter is a purely chemical one. In fact, in alkaline solutions the first reaction could take place spontaneously [15]. Based on such a phenomenon, cell electrochemical technique [16] has been proposed and used it to synthesize films [15,17,18]. Compared with those widely used electro-deposition methods, cell electrochemical technique has the peculiar advantage that good crystalline films can be obtained with zero or near zero (referring to experiments sometimes being operated near room temperature for some purposes) expenditure of energy. Obviously, it provides an environmentally friendly route to fabricate materials. Since cell method runs without apparatuses such as potentiometers, it is very difficult to tailor its reaction rate. Hence, it is urgent to work out how to control it. In this paper, two ways on how to control the rate of electrodeposition were suggested. Also, some common characteristics of AMO4 (A = Ca, Sr, Ba; M = Mo, W) thin films synthesized by cell electrochemical technique were investigated.
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243
scanning electron microscopy (SEM) (JSM-5000, JEOL, made in Japan), and fluorescence spectrometer (FL-4600, Hitachi, made in Japan). For photoluminescence (PL) measurements, the scan rate was 240 nm per minute, and the corrected spectra and shutter control were off. The slit of excitation and emission was 5.0 and 2.5 nm, respectively. The PMT voltage was 700 V.
3. Results and discussions 3.1. Investigations on how to control the rate of cell electro-deposition It is essential for electro-deposition to be controllable. For electrolysis, the rates of electrode reactions can be easily controlled by adjusting the potentials or current densities given by electrochemical apparatus. However, for a cell electrochemical method, its rate cannot be controlled by adjusting the potentials or current densities, but depends on the characteristics of the cell system. Though an electrochemical workstation was used in the experiments, it was not used to supply potentials or current densities, but used to in situ collect data because of its good and sensitive definition. Due to the particularity of cell electro-deposition, its rate controlling is greatly different from the widely known methods suiting for electrolysis. Hence, it is much more difficult to control its rate. Since AMO4 (A = Ca, Sr, Ba; M = Mo, W) films are insulating, the current would decrease greatly as the deposits become denser. Hence, we define the average current density in the early 5 min as the parameter indicating the rate of change. The following will present two ways of controlling the reaction rate of the cell route.
Fig. 1. Flow chart of synthesis of BaWO4 thin films by cell electrochemical technique.
2. Experimental procedure All the experiments were operated in a sealed three-electrode-system. The diagram of such a device was shown in the literature [17]. The pretreated tungsten or molybdenum plate was used as the working electrode. The platinum foil worked as the auxiliary electrode. An Ag/AgCl (sat. KCl) electrode was used as the reference electrode. The tungsten plate (20 mm × 10 mm × 0.1 mm, with purity of 99.5%), Ba(OH)2 ·8H2 O (with purity of 99.5%) and deionized water were used as the starting materials to prepare BaWO4 thin films. In order to obtain other AWO4 thin films, different electrolytes were used, such as alkaline solutions containing Sr2+ , or Ca2+ ions. Molybdenum and alkaline solutions containing alkaline-earth ions were used as the startings to fabricate AMoO4 thin films. The flow chart of synthesis of BaWO4 thin films was shown in Fig. 1. Additives, such as HCl, NaOH, were used to control the pH values of electrolytes. Sometimes different additives should be adopted in order to control the morphologies of products. Before experiments, solutions were purged with N2 gas for 10 min. In our researches, an electrochemical workstation (PST050, made in France) was used to measure potentials and a personnel computer was used to save the data (except sample 1). The span of measurements was six seconds per point. As to experiments assisted by hydrothermal treatment, the sealed cell was put in a container and the water temperature was controlled by a heater. After treatments, samples were washed with deionized water and dried. By analyzing the in situ data collected by the workstation and computer, the process of preparations can be studied. For example, using the in situ data of currents and potentials of working electrode, the rate determining step during the deposition of SrMoO4 thin films had been investigated [17]. After data processing, the rate of reactions can be deduced. If the rate is not desirable, it can be tailored by controlling the additives. Such a cycle occurs until content. Therefore, the experiment system is adaptable in the rate control. Since the potentials were recorded after they had become balanced. The reproducibility of the process is good. Due to under cell route, the VOLTAMASTER 4 cannot work out the current densities. Thus, the data processing cannot be finished automatically. The films were characterized by X-ray diffraction (XRD) (X’Pert PRO MPD, made in Netherlands), X-ray photoelectron spectrometry (XPS) (XSAM-800, made in UK),
3.1.1. One method: hydrothermal treatments Fig. 2 demonstrates the in situ potentials of electrodes collected and saved by a professional electrochemical workstation system during the fabrication of BaMoO4 thin film at 308 K. Either of Ea and Ek can be divided into three stages. The potential decreased rapidly at the starting stage, almost kept stable during the second stage, and rose swiftly at the end of several minutes. Using the Ea and Ek shown in Fig. 2, current densities were calculated. By integrating the current densities shown in Fig. 2, the average current density in the first 5 min was worked out (shown in Table 1). Table 1 exhibits the influences of temperatures on the rates of deposition of BaMoO4 thin films. When the temperature increased from 298 to 308 K, the rate almost doubled. The higher the temperature rose, the greater the current density occurred. Therefore, the rate of cell electro-deposition is controllable by changing the
Fig. 2. The in situ data of Ea , Ek and calculated current density during the fabrication of BaMoO4 thin film at 308 K.
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Table 1 Effects of temperatures on the rates of deposition of BaMoO4 thin films by cell route. Sample
T (K)
Current density (A m−2 )
1 2 3
298 308 333
1.9596a 3.6833 4.3378
a
Data recorded every minute.
The probable reasons may lie in that the rates of diffusion, electrode reaction and hydrogen desorption have been accelerated because of the increase of temperatures. 3.1.2. Another method: to adjust pH values Processed as the above, the Ea , Ek and the current density during the synthesis of BaWO4 thin film at pH value equaling 13.8 were illustrated in Fig. 3. The instability of potentials after 9800 s may originate from the repeating and instantaneous breakdown and nucleation of deposits at some crystal boundaries. The effect of pH values on the current densities was listed in Table 2. From Table 2 it can be seen that the pH value plays an important role in adjusting the reaction rate. The higher the pH value rose, the faster the reaction proceeded. But some impurities, probably BaCO3 , could be observed in the SEM images of sample
Fig. 3. The in situ data of Ea , Ek and calculated current density during the fabrication of BaWO4 thin film at pH value equaling 13.8.
Table 2 Impacts of pH values on the rates of cell electro-deposition of BaWO4 thin films. Sample
pH
Current density (A m−2 )
1 2 3
13.8 11.6 10.5
3.9363 2.9479 0.6236
hydrothermal temperatures, which is in agreement with other research [15,17]. When the temperature rose above 333 K, the conducting clamps were badly corroded because of the moisture. The literature [15] reported that when the temperature was above 343 K counterpolarization, which should be avoided, would happen. Hence, in order to synthesize thin films quickly the optimum temperature would range from 308 to 333 K.
Fig. 4. XRD patterns of AWO4 (A = Ca, Sr, and Ba) films prepared by a cell electrochemical method.
Fig. 5. XPS of BaMoO4 thin films prepared by cell electrochemical route: (a) Ba, (b) Mo, and (c) O.
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Fig. 6. Effects of preparations on the SEM images of films: (a) BaWO4 , cell route; (b) BaWO4 , I = 1 mA cm−2 ; (c) SrWO4 , cell route; and (d) SrWO4 , I = 1 mA cm−2 .
1 besides BaMoO4 crystals [19]. Since formula (1) has made quite clear that the concentration of OH− ions would affect the formation of WO4 2− or MoO4 2− complex ions dramatically, it is necessary to control the pH value within a proper range (generally above 12 but below 13.5). 3.2. Characterizations 3.2.1. X-ray diffraction analyses Fig. 4 shows the XRD patterns of AWO4 (A = Ca, Sr, Ba) thin films prepared by cell electrochemical technique. The XRD results of AMoO4 (A = Ca, Sr, Ba) thin films had been studied in the literature [18]. Both Fig. 4 and the literature [18] reveal that these alkaline-earth tungstate and molybdate thin films have a tetragonal structure [20]. Their corresponding peaks shift towards high 2Â in the order of Ba, Sr, Ca due to the decrease of their radii. 3.2.2. X-ray photoelectron spectra investigations When tungsten or molybdenum was dipped in alkaline solutions, in which tungstate or molybdate ions were formed, molybdenum or tungsten oxides with other band energies might be produced. Therefore, it is necessary to investigate the composition of films. The following takes the XPS analysis of BaMoO4 thin film as a case. Fig. 5 exhibits the raw and Gaussian fitting (according to the XPSPEAK software) XPS of BaMoO4 thin film prepared by a cell electrochemical method. Since the band energies of Ba3d5/2 and Ba3d3/2 are 779.710 and 795.040 eV, the Ba element in films should exist in +2 [21]; It can be seen that the typical band energies of Mo3d are 232.460 and 235.640 eV, which reveals that the Mo element in films exists in +6 [21]; from Fig. 5, it can also be inferred that the valence of oxygen in the film is −2 [21]. Hence, the deposit should be BaMoO4 , which is in good agreement with the XRD results. Other researches [15,17–19] have also proved that the valences of A, M, and O elements in AMO4 (A = Ca, Sr, Ba; M = W, Mo) thin films obtained by cell route are embodied as +2, +6 and −2, respectively.
3.2.3. Comparisons of microstructures Fig. 6 shows the impacts of preparations on films’ morphologies. From Fig. 6 it can be seen that the homogeneity of BaWO4 and SrWO4 thin films prepared by cell route has been improved markedly. The surface becomes more smooth and homogeneous. The crystal size greatly decreases. Both BaWO4 and SrWO4 crystals grow in a pattern of tetragonal tapers. However, when synthesized by constant current method, some BaWO4 crystals grow in clusters with fourfold symmetry, and SrWO4 clusters seem like dentrites. Such tendencies would become especially clear when films are fabricated by large current densities. Generally speaking, the environment has deep influences on crystal morphologies. Under cell conditions, the rate is relatively slow and the growth unit of crystals can shift to places with lower energies, so that clusters are not apt to form. 3.2.4. Photoluminescence The photoluminescence of CaMoO4 thin films prepared by cell route is presented in Fig. 7. With the light of 201 nm, a broad emis-
Fig. 7. Comparisons of the PL of CaMoO4 and SrMoO4 thin films excited by 201 nm at room temperature.
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So far, the blue-green emission is widely regarded as the intrinsic behavior of MoO4 2− complex [10,22–24]. As to the origination of the violet emission, it remains to be studied. 4. Conclusions
Fig. 8. Gaussian fitting of the PL of CaMoO4 thin film.
sion band ranging from the violet to the green has been observed. The intensity of CaMoO4 is a little stronger than that of SrMoO4 . In general, the PL of molybdates decreases in the order of CaMoO4 , SrMoO4 and BaMoO4 . From Fig. 7 it can be seen that the labeled peaks can be divided into two groups, 434, 469 and 492 nm belong to the blue-green; and 381, 359 and 345 nm belong to the violet. Hence, it can be inferred that the PL of CaMoO4 or SrMoO4 thin film is probably composed of two sub-bands, the violet and the blue-green. The multi-Gaussian fitting of the PL of CaMoO4 thin film is given in Fig. 8. The two sub-bands center at 357 and 456 nm. They are very close the local maximum of 359 and 469 nm. From Figs. 7 and 8 it can be concluded that the emission of CaMoO4 may derive from two centers. One is responsible for the violet emission, and the other for the longer wavelength emission. Compared to the PL excited by the light of 289 nm, the emission of CaMoO4 thin film obtained with the excitation of 201 nm shifts blue. Since PL is a complicated phenomenon, many factors will account for it. In the field of materials science and engineering, it is well known that preparations will impact materials’ structures and performances greatly. Though the XRD of AMO4 thin films prepared by both cell and electrolysis methods hardly shows great differences, defect, which is regarded as one of the most sensitive factors to the PL, is definitely different. Secondly, the reaction between the crystals and phonons also contribute much to the luminescent property. In addition, the PL intensity of phosphors depends strongly on the excitation. Ryu et al. [22] reported that a broad emission band from 280 to 580 nm was observed when BaMoO4 powders, which were synthesized by a microwave-assisted citrate complex method, were excited by 240 nm. Except the strongest peak at 390 nm, many relatively weak peaks could be clearly seen. From Fig. 7 given in the literature [22], it is confident that the PL spectrum of BaMoO4 powders was also made up of the violet and the blue sub-emissions. And the primary was the violet emission.
Cell electrochemical technique is an environmentally friendly process to fabricate luminescent alkaline-earth tungstate and molybdate thin films. Studies show that the deposition rates can be controlled by adjusting temperatures and pH values. The asprepared AMO4 (A = Ca, Sr, Ba; M = Mo, W) polycrystalline films have a scheelite structure and their compositions are consistent with their stoichiometry. Under cell conditions, the complicated clusters have disappeared and the quality of films has been improved markedly. At room temperature, these films show a broad emission band ranging from the violet to the green at the light of 201 nm. Such a broad band can be divided into two sub-bands (peaking at 357 and 456 nm, respectively). The blue-green emission band is primary. It is regarded as the intrinsic behavior of MoO4 2− complex. Acknowledgments This work was funded by National Science Foundation of China (50802036, 50372042 and 50410179), National Science Foundation of Jiangxi Province (2008GQC0025), and Education Foundation of Jiangxi Province (GJJ09512) in China. References [1] G. Bayer, H.G. Wiedemann, Thermochim. Acta 133 (1988) 125. [2] A.G. Page, S.V. Godbole, M.D. Sastry, J. Phys. Chem. Solids 50 (1989) 571. [3] T.T. Basiev, A.A. Sobol, Y.K. Voronko, P.G. Zverev, Opt. Mater. 15 (2000) 205. [4] L.I. Ivleva, T.T. Basiev, I.S. Voronina, P.G. Zverev, V.V. Osiko, N.M. Polozkov, Opt. Mater. 23 (2003) 439. [5] P. Cerny, P.G. Zverev, H. Jelinkova, T.T. Basiev, Opt. Commun. 177 (2000) 397. [6] D. Christofilos, G.A. Kourouklis, S. Ves, J. Phys. Chem. Solids 56 (1995) 1125. [7] Y. Kashiwakura, O. Kanehisa, Japan Patent No. 1-263188 (October 19, 1989). [8] C. Feldman, J. Soc. Motion Pict. Eng. 67 (1958) 455. [9] J.Y. Huang, Q.X. Jia, Thin Solid Films 444 (2003) 95. [10] J.H. Ryu, J.W. Yoon, K.B. Shim, Solid State Commun. 133 (2005) 657. [11] W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, Appl. Phys. Lett. 66 (1995) 1027. [12] M. Yoshimura, M. Ohmura, W.S. Cho, M. Kakihana, J. Am. Ceram. Soc. 80 (1997) 2464. [13] C.T. Xia, V.M. Fuenzalida, R.A. Zarate, J. Alloys Compd. 316 (2001) 250. [14] J. Bi, D.Q. Xiao, D.J. Gao, P. Yu, Cryst. Res. Technol. 38 (2003) 935. [15] L. Chen, Y. Gao, Mater. Res. Bull. 42 (2007) 1823. [16] P. Yu, L.P. Chen, D.Q. Xiao, China Patent No. 200510022471.3 (2005). [17] L. Chen, Y. Gao, Chem. Eng. J. 131 (2007) 181. [18] L.P. Chen, D.Q. Xiao, P. Yu, X.L. Jin, Z.N. Yang, Ferroelectrics 356 (2007) 48. [19] L. Chen, Investigations on the preparations and the photoluminescence of scheelite-structured polycrystalline films by electrochemical methods, Doctorate Dissertation, Sichuan University, Chengdu, China, 2007 (in Chinese). [20] JCPDS Cards No. 72-0746, 85-0587, 77-2233. [21] C.D. Wagner, W.M. Riggs, L.E. Davis, G.E. Muilenberg, A Reference Book of Standard Data for Use in X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation and Physical Electronics Division, Minnesota, USA, 1979. [22] J.H. Ryu, J.W. Yoon, C.S. Lim, K.B. Shim, Mater. Res. Bull. 40 (2005) 1468. [23] M.J. Treadaway, R.C. Powell, J. Chem. Phys. 61 (1974) 4003. [24] J.A. Groenink, C. Hakfoort, G. Blasse, Phys. Status Solidi A 54 (1979) 329.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electro-deposition of luminescent molybdate and tungstate thin films via a cell route Lianping Chen ∗ , Yuanhong Gao Jiangxi University of Science and Technology, Ganzhou 341000, China
a r t i c l e
i n f o
Article history: Received 4 August 2008 Received in revised form 16 January 2009 Accepted 19 March 2009 Keywords: Thin films Luminescence
a b s t r a c t Highly crystallized luminescent AMO4 (A = Ca, Sr, and Ba; M = W, Mo) thin films have been prepared by cell electrochemical technique. Researches reveal that the deposition rate can be tailored by adjusting temperatures and pH values. The optimum temperature is between 308 and 333 K, and the pH value is between 12 and 13.5. X-ray diffraction and photoelectron spectra examinations confirm that the obtained films are polycrystalline AMO4 (A = Ca, Sr, and Ba; M = W, Mo). Under cell conditions, the homogeneity of films has been improved markedly due to the disappearance of large crystal clusters. At room temperature, a broad emission band ranging from the violet to visible can be observed with the excitation wavelength at 201 nm. Further analyses show that such a broad band can be divided into two sub-bands. Their centers are situated at 357 and 456 nm, respectively. It is proposed that the longer wavelength emission probably originates from MoO4 2− complex. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Scheelite-structured AMO4 (A = Ca, Sr, Ba; M = W, Mo) are good laser host materials [1–5]. They also show promising applications in the fields of luminescence, displays, electro-optics and detectors due to their blue or green emission. They remain a tetragonal structure over wide range of temperatures [3] and pressures [6]. Hence, they may become the material of choice under some extreme environments. Since thin film phosphors have superior resolution compared to powders, AMO4 (A = Ca, Sr, Ba; M = W, Mo) thin films have received significant attention in recent years. Various methods, such as sputtering [7], vacuum evaporation [8], pulsed laser deposition [9] and citrate complex [10], have been used to prepare such films. But samples prepared by these methods need to be annealed (500–1100 ◦ C). Annealing not only consumes a large amount of energy, but also leads to poor adhesion. Hence, it is urgent to develop new ways to prepare thin films without annealing or with lower temperature annealing treatment. Electrochemical methods, such as galvanostatic or potentiostatic method, have been widely used to prepare AMO4 (A = Ca, Sr, Ba; M = Mo, W) thin films [11–14] due to much less consumption of energy. One of the advantages of electrochemical techniques is that
∗ Corresponding author at: School of Materials and Chemical Engineering, Jiangxi University of Science and Technology, 86, Red Flag Avenue, Ganzhou, Jiangxi, China. Tel.: +86 0797 8312204; fax: +86 0797 8312411. E-mail address: [email protected] (L. Chen). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.03.018
good crystalline films can be obtained from solutions without any annealing treatment. The electro-deposition of AMO4 films mainly depends on the following reactions (taking the formation of BaWO4 films as an example): W + 8OH− → WO4 2− + 4H2 O + 6e−
(1)
Ba2+ + WO4 2− ↔ BaWO4
(2)
The first one is a faradic process, and the latter is a purely chemical one. In fact, in alkaline solutions the first reaction could take place spontaneously [15]. Based on such a phenomenon, cell electrochemical technique [16] has been proposed and used it to synthesize films [15,17,18]. Compared with those widely used electro-deposition methods, cell electrochemical technique has the peculiar advantage that good crystalline films can be obtained with zero or near zero (referring to experiments sometimes being operated near room temperature for some purposes) expenditure of energy. Obviously, it provides an environmentally friendly route to fabricate materials. Since cell method runs without apparatuses such as potentiometers, it is very difficult to tailor its reaction rate. Hence, it is urgent to work out how to control it. In this paper, two ways on how to control the rate of electrodeposition were suggested. Also, some common characteristics of AMO4 (A = Ca, Sr, Ba; M = Mo, W) thin films synthesized by cell electrochemical technique were investigated.
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243
scanning electron microscopy (SEM) (JSM-5000, JEOL, made in Japan), and fluorescence spectrometer (FL-4600, Hitachi, made in Japan). For photoluminescence (PL) measurements, the scan rate was 240 nm per minute, and the corrected spectra and shutter control were off. The slit of excitation and emission was 5.0 and 2.5 nm, respectively. The PMT voltage was 700 V.
3. Results and discussions 3.1. Investigations on how to control the rate of cell electro-deposition It is essential for electro-deposition to be controllable. For electrolysis, the rates of electrode reactions can be easily controlled by adjusting the potentials or current densities given by electrochemical apparatus. However, for a cell electrochemical method, its rate cannot be controlled by adjusting the potentials or current densities, but depends on the characteristics of the cell system. Though an electrochemical workstation was used in the experiments, it was not used to supply potentials or current densities, but used to in situ collect data because of its good and sensitive definition. Due to the particularity of cell electro-deposition, its rate controlling is greatly different from the widely known methods suiting for electrolysis. Hence, it is much more difficult to control its rate. Since AMO4 (A = Ca, Sr, Ba; M = Mo, W) films are insulating, the current would decrease greatly as the deposits become denser. Hence, we define the average current density in the early 5 min as the parameter indicating the rate of change. The following will present two ways of controlling the reaction rate of the cell route.
Fig. 1. Flow chart of synthesis of BaWO4 thin films by cell electrochemical technique.
2. Experimental procedure All the experiments were operated in a sealed three-electrode-system. The diagram of such a device was shown in the literature [17]. The pretreated tungsten or molybdenum plate was used as the working electrode. The platinum foil worked as the auxiliary electrode. An Ag/AgCl (sat. KCl) electrode was used as the reference electrode. The tungsten plate (20 mm × 10 mm × 0.1 mm, with purity of 99.5%), Ba(OH)2 ·8H2 O (with purity of 99.5%) and deionized water were used as the starting materials to prepare BaWO4 thin films. In order to obtain other AWO4 thin films, different electrolytes were used, such as alkaline solutions containing Sr2+ , or Ca2+ ions. Molybdenum and alkaline solutions containing alkaline-earth ions were used as the startings to fabricate AMoO4 thin films. The flow chart of synthesis of BaWO4 thin films was shown in Fig. 1. Additives, such as HCl, NaOH, were used to control the pH values of electrolytes. Sometimes different additives should be adopted in order to control the morphologies of products. Before experiments, solutions were purged with N2 gas for 10 min. In our researches, an electrochemical workstation (PST050, made in France) was used to measure potentials and a personnel computer was used to save the data (except sample 1). The span of measurements was six seconds per point. As to experiments assisted by hydrothermal treatment, the sealed cell was put in a container and the water temperature was controlled by a heater. After treatments, samples were washed with deionized water and dried. By analyzing the in situ data collected by the workstation and computer, the process of preparations can be studied. For example, using the in situ data of currents and potentials of working electrode, the rate determining step during the deposition of SrMoO4 thin films had been investigated [17]. After data processing, the rate of reactions can be deduced. If the rate is not desirable, it can be tailored by controlling the additives. Such a cycle occurs until content. Therefore, the experiment system is adaptable in the rate control. Since the potentials were recorded after they had become balanced. The reproducibility of the process is good. Due to under cell route, the VOLTAMASTER 4 cannot work out the current densities. Thus, the data processing cannot be finished automatically. The films were characterized by X-ray diffraction (XRD) (X’Pert PRO MPD, made in Netherlands), X-ray photoelectron spectrometry (XPS) (XSAM-800, made in UK),
3.1.1. One method: hydrothermal treatments Fig. 2 demonstrates the in situ potentials of electrodes collected and saved by a professional electrochemical workstation system during the fabrication of BaMoO4 thin film at 308 K. Either of Ea and Ek can be divided into three stages. The potential decreased rapidly at the starting stage, almost kept stable during the second stage, and rose swiftly at the end of several minutes. Using the Ea and Ek shown in Fig. 2, current densities were calculated. By integrating the current densities shown in Fig. 2, the average current density in the first 5 min was worked out (shown in Table 1). Table 1 exhibits the influences of temperatures on the rates of deposition of BaMoO4 thin films. When the temperature increased from 298 to 308 K, the rate almost doubled. The higher the temperature rose, the greater the current density occurred. Therefore, the rate of cell electro-deposition is controllable by changing the
Fig. 2. The in situ data of Ea , Ek and calculated current density during the fabrication of BaMoO4 thin film at 308 K.
244
L. Chen, Y. Gao / Materials Chemistry and Physics 116 (2009) 242–246
Table 1 Effects of temperatures on the rates of deposition of BaMoO4 thin films by cell route. Sample
T (K)
Current density (A m−2 )
1 2 3
298 308 333
1.9596a 3.6833 4.3378
a
Data recorded every minute.
The probable reasons may lie in that the rates of diffusion, electrode reaction and hydrogen desorption have been accelerated because of the increase of temperatures. 3.1.2. Another method: to adjust pH values Processed as the above, the Ea , Ek and the current density during the synthesis of BaWO4 thin film at pH value equaling 13.8 were illustrated in Fig. 3. The instability of potentials after 9800 s may originate from the repeating and instantaneous breakdown and nucleation of deposits at some crystal boundaries. The effect of pH values on the current densities was listed in Table 2. From Table 2 it can be seen that the pH value plays an important role in adjusting the reaction rate. The higher the pH value rose, the faster the reaction proceeded. But some impurities, probably BaCO3 , could be observed in the SEM images of sample
Fig. 3. The in situ data of Ea , Ek and calculated current density during the fabrication of BaWO4 thin film at pH value equaling 13.8.
Table 2 Impacts of pH values on the rates of cell electro-deposition of BaWO4 thin films. Sample
pH
Current density (A m−2 )
1 2 3
13.8 11.6 10.5
3.9363 2.9479 0.6236
hydrothermal temperatures, which is in agreement with other research [15,17]. When the temperature rose above 333 K, the conducting clamps were badly corroded because of the moisture. The literature [15] reported that when the temperature was above 343 K counterpolarization, which should be avoided, would happen. Hence, in order to synthesize thin films quickly the optimum temperature would range from 308 to 333 K.
Fig. 4. XRD patterns of AWO4 (A = Ca, Sr, and Ba) films prepared by a cell electrochemical method.
Fig. 5. XPS of BaMoO4 thin films prepared by cell electrochemical route: (a) Ba, (b) Mo, and (c) O.
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Fig. 6. Effects of preparations on the SEM images of films: (a) BaWO4 , cell route; (b) BaWO4 , I = 1 mA cm−2 ; (c) SrWO4 , cell route; and (d) SrWO4 , I = 1 mA cm−2 .
1 besides BaMoO4 crystals [19]. Since formula (1) has made quite clear that the concentration of OH− ions would affect the formation of WO4 2− or MoO4 2− complex ions dramatically, it is necessary to control the pH value within a proper range (generally above 12 but below 13.5). 3.2. Characterizations 3.2.1. X-ray diffraction analyses Fig. 4 shows the XRD patterns of AWO4 (A = Ca, Sr, Ba) thin films prepared by cell electrochemical technique. The XRD results of AMoO4 (A = Ca, Sr, Ba) thin films had been studied in the literature [18]. Both Fig. 4 and the literature [18] reveal that these alkaline-earth tungstate and molybdate thin films have a tetragonal structure [20]. Their corresponding peaks shift towards high 2Â in the order of Ba, Sr, Ca due to the decrease of their radii. 3.2.2. X-ray photoelectron spectra investigations When tungsten or molybdenum was dipped in alkaline solutions, in which tungstate or molybdate ions were formed, molybdenum or tungsten oxides with other band energies might be produced. Therefore, it is necessary to investigate the composition of films. The following takes the XPS analysis of BaMoO4 thin film as a case. Fig. 5 exhibits the raw and Gaussian fitting (according to the XPSPEAK software) XPS of BaMoO4 thin film prepared by a cell electrochemical method. Since the band energies of Ba3d5/2 and Ba3d3/2 are 779.710 and 795.040 eV, the Ba element in films should exist in +2 [21]; It can be seen that the typical band energies of Mo3d are 232.460 and 235.640 eV, which reveals that the Mo element in films exists in +6 [21]; from Fig. 5, it can also be inferred that the valence of oxygen in the film is −2 [21]. Hence, the deposit should be BaMoO4 , which is in good agreement with the XRD results. Other researches [15,17–19] have also proved that the valences of A, M, and O elements in AMO4 (A = Ca, Sr, Ba; M = W, Mo) thin films obtained by cell route are embodied as +2, +6 and −2, respectively.
3.2.3. Comparisons of microstructures Fig. 6 shows the impacts of preparations on films’ morphologies. From Fig. 6 it can be seen that the homogeneity of BaWO4 and SrWO4 thin films prepared by cell route has been improved markedly. The surface becomes more smooth and homogeneous. The crystal size greatly decreases. Both BaWO4 and SrWO4 crystals grow in a pattern of tetragonal tapers. However, when synthesized by constant current method, some BaWO4 crystals grow in clusters with fourfold symmetry, and SrWO4 clusters seem like dentrites. Such tendencies would become especially clear when films are fabricated by large current densities. Generally speaking, the environment has deep influences on crystal morphologies. Under cell conditions, the rate is relatively slow and the growth unit of crystals can shift to places with lower energies, so that clusters are not apt to form. 3.2.4. Photoluminescence The photoluminescence of CaMoO4 thin films prepared by cell route is presented in Fig. 7. With the light of 201 nm, a broad emis-
Fig. 7. Comparisons of the PL of CaMoO4 and SrMoO4 thin films excited by 201 nm at room temperature.
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So far, the blue-green emission is widely regarded as the intrinsic behavior of MoO4 2− complex [10,22–24]. As to the origination of the violet emission, it remains to be studied. 4. Conclusions
Fig. 8. Gaussian fitting of the PL of CaMoO4 thin film.
sion band ranging from the violet to the green has been observed. The intensity of CaMoO4 is a little stronger than that of SrMoO4 . In general, the PL of molybdates decreases in the order of CaMoO4 , SrMoO4 and BaMoO4 . From Fig. 7 it can be seen that the labeled peaks can be divided into two groups, 434, 469 and 492 nm belong to the blue-green; and 381, 359 and 345 nm belong to the violet. Hence, it can be inferred that the PL of CaMoO4 or SrMoO4 thin film is probably composed of two sub-bands, the violet and the blue-green. The multi-Gaussian fitting of the PL of CaMoO4 thin film is given in Fig. 8. The two sub-bands center at 357 and 456 nm. They are very close the local maximum of 359 and 469 nm. From Figs. 7 and 8 it can be concluded that the emission of CaMoO4 may derive from two centers. One is responsible for the violet emission, and the other for the longer wavelength emission. Compared to the PL excited by the light of 289 nm, the emission of CaMoO4 thin film obtained with the excitation of 201 nm shifts blue. Since PL is a complicated phenomenon, many factors will account for it. In the field of materials science and engineering, it is well known that preparations will impact materials’ structures and performances greatly. Though the XRD of AMO4 thin films prepared by both cell and electrolysis methods hardly shows great differences, defect, which is regarded as one of the most sensitive factors to the PL, is definitely different. Secondly, the reaction between the crystals and phonons also contribute much to the luminescent property. In addition, the PL intensity of phosphors depends strongly on the excitation. Ryu et al. [22] reported that a broad emission band from 280 to 580 nm was observed when BaMoO4 powders, which were synthesized by a microwave-assisted citrate complex method, were excited by 240 nm. Except the strongest peak at 390 nm, many relatively weak peaks could be clearly seen. From Fig. 7 given in the literature [22], it is confident that the PL spectrum of BaMoO4 powders was also made up of the violet and the blue sub-emissions. And the primary was the violet emission.
Cell electrochemical technique is an environmentally friendly process to fabricate luminescent alkaline-earth tungstate and molybdate thin films. Studies show that the deposition rates can be controlled by adjusting temperatures and pH values. The asprepared AMO4 (A = Ca, Sr, Ba; M = Mo, W) polycrystalline films have a scheelite structure and their compositions are consistent with their stoichiometry. Under cell conditions, the complicated clusters have disappeared and the quality of films has been improved markedly. At room temperature, these films show a broad emission band ranging from the violet to the green at the light of 201 nm. Such a broad band can be divided into two sub-bands (peaking at 357 and 456 nm, respectively). The blue-green emission band is primary. It is regarded as the intrinsic behavior of MoO4 2− complex. Acknowledgments This work was funded by National Science Foundation of China (50802036, 50372042 and 50410179), National Science Foundation of Jiangxi Province (2008GQC0025), and Education Foundation of Jiangxi Province (GJJ09512) in China. References [1] G. Bayer, H.G. Wiedemann, Thermochim. Acta 133 (1988) 125. [2] A.G. Page, S.V. Godbole, M.D. Sastry, J. Phys. Chem. Solids 50 (1989) 571. [3] T.T. Basiev, A.A. Sobol, Y.K. Voronko, P.G. Zverev, Opt. Mater. 15 (2000) 205. [4] L.I. Ivleva, T.T. Basiev, I.S. Voronina, P.G. Zverev, V.V. Osiko, N.M. Polozkov, Opt. Mater. 23 (2003) 439. [5] P. Cerny, P.G. Zverev, H. Jelinkova, T.T. Basiev, Opt. Commun. 177 (2000) 397. [6] D. Christofilos, G.A. Kourouklis, S. Ves, J. Phys. Chem. Solids 56 (1995) 1125. [7] Y. Kashiwakura, O. Kanehisa, Japan Patent No. 1-263188 (October 19, 1989). [8] C. Feldman, J. Soc. Motion Pict. Eng. 67 (1958) 455. [9] J.Y. Huang, Q.X. Jia, Thin Solid Films 444 (2003) 95. [10] J.H. Ryu, J.W. Yoon, K.B. Shim, Solid State Commun. 133 (2005) 657. [11] W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, Appl. Phys. Lett. 66 (1995) 1027. [12] M. Yoshimura, M. Ohmura, W.S. Cho, M. Kakihana, J. Am. Ceram. Soc. 80 (1997) 2464. [13] C.T. Xia, V.M. Fuenzalida, R.A. Zarate, J. Alloys Compd. 316 (2001) 250. [14] J. Bi, D.Q. Xiao, D.J. Gao, P. Yu, Cryst. Res. Technol. 38 (2003) 935. [15] L. Chen, Y. Gao, Mater. Res. Bull. 42 (2007) 1823. [16] P. Yu, L.P. Chen, D.Q. Xiao, China Patent No. 200510022471.3 (2005). [17] L. Chen, Y. Gao, Chem. Eng. J. 131 (2007) 181. [18] L.P. Chen, D.Q. Xiao, P. Yu, X.L. Jin, Z.N. Yang, Ferroelectrics 356 (2007) 48. [19] L. Chen, Investigations on the preparations and the photoluminescence of scheelite-structured polycrystalline films by electrochemical methods, Doctorate Dissertation, Sichuan University, Chengdu, China, 2007 (in Chinese). [20] JCPDS Cards No. 72-0746, 85-0587, 77-2233. [21] C.D. Wagner, W.M. Riggs, L.E. Davis, G.E. Muilenberg, A Reference Book of Standard Data for Use in X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation and Physical Electronics Division, Minnesota, USA, 1979. [22] J.H. Ryu, J.W. Yoon, C.S. Lim, K.B. Shim, Mater. Res. Bull. 40 (2005) 1468. [23] M.J. Treadaway, R.C. Powell, J. Chem. Phys. 61 (1974) 4003. [24] J.A. Groenink, C. Hakfoort, G. Blasse, Phys. Status Solidi A 54 (1979) 329.