- Email: [email protected]
Fabrication of AMoO4 (A = Ba, Sr) film on Mo substrate by solution reaction assisted ball-rotation
Materials Research Bulletin 43 (2008) 3155–3163 www.elsevier.com/locate/matresbu
Fabrication of AMoO4 (A = Ba, Sr) film on Mo substrate by solution reaction assisted ball-rotation Dinesh Rangappa, Takeshi Fujiwara, Tomoaki Watanabe, Masahiro Yoshimura * Center for Materials Design, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 226-8503, Japan Received 15 March 2007; received in revised form 15 June 2007; accepted 26 June 2007 Available online 7 January 2008
Abstract Alkaline earth molybdates, such as BaMoO4 and SrMoO4, films have been successfully fabricated on a Mo metal substrate in AOH (A = Ba, Sr) solutions by a ball-rotation-assisted solution reaction, at room temperature. The dissolution of Mo was mainly controlled by the concentration of the H2O2 oxidizing agent and ball-rotation to form MoO42 in the solution. AMoO4 was deposited on the substrate by the reaction between MoO42 and A2+ ions without any high energy or high-temperature treatment. Also, the mass transport of alkaline earth ions onto the solid/solution interface was improved as a result of the vigorous solution agitation by the ball-rotation. Therefore, the rate of deposition of the AMoO4 films was accelerated by the ball-rotation. A decrease in the grain size of the film was observed with an excessive ball-rotation. # 2007 Elsevier Ltd. All rights reserved. PACS : 82.65.J; 43.35P; 68.35G Keywords: A. Optical materials; B. Oxides; C. Surfaces; D. Thin films; E. Chemical synthesis
1. Introduction Alkaline earth molybdates with a Scheelite-type structure are important materials in electro-optics. The green luminescence properties of these materials have attracted the attention of many researchers due to their application in the electro-optical industry [1,2]. An AMoO4 powder is usually prepared using conventional methods consisting of repeated ball-milling, for example, the solid-state reaction between BaCO3 and MoO3 powders [3]. Although films are more important for better performance with regard to industrial applications, problems may occur in the preparation of these films when conventional techniques with high-temperature treatments are used for crystallization of the film. Due to the characteristic volatile nature of MoO3, low-temperature processing is suitable for the synthesis of crystallized luminescent AMoO4 films. In this regard, a novel ball-rotation-assisted solution processing has been developed under Soft Solution Processing (SSP) which was proposed by Yoshimura for the first time [4,5]. This synthetic approach has been used for the preparation of lithium ferrites [6], (Ce, Zr) O2 solid solutions [7] and barium tungstate [8,9].
* Corresponding author. E-mail address: [email protected] (M. Yoshimura). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.06.062
3156
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Generally, mechanically activated reactions are regarded as mechanochemical reactions like mechanical alloying, the production of intermetallic and alloy compounds, nano-crystalline powders, etc. [10,11,12]. These solid-state reactions can be activated by mechanical agitation to form new surfaces, defects, dislocations and/or strains on the surface of the grains. Senna and co-workers proposed a soft mechanochemical method [13], in which the presence of highly reactive surface functional species, notably OH’s, can assist the solid-state reactions. On the other hand, ballrotation assistance can accelerate the solution reactions without any solid-state reactions [6,7]. The vigorous stirring of a solution by ball-rotation will accelerate the interfacial reaction between solid and solution. In our present study, we have successfully applied this method to prepare a crystalline film of AMoO4 at room temperature using simple ballrotation-assisted solution reaction, where, metal substrate is placed in a solution containing alkaline earth ions and stainless steel balls. This paper describes the formation, characterization and the formation mechanism of the AMoO4 (A = Ba, Sr) films during the ball-rotation-assisted solution reaction. 2. Experimental procedure A molybdenum metal substrate with 10 mm 10 mm 0.2 mm dimensions of 99.9 wt.% purity (Niraco, Japan) was mechanically polished and degreased in acetone using an ultrasonic cleaner. Alkaline earth solutions were made from redistilled water using guaranteed reagents with a minimum 98% assay of Ba(OH)28H2O, Sr(OH)28H2O and H2O2 (Kanto, Kagaku, Japan). Distilled water was purged with N2 gas for 30 min in order to remove the dissolved O2 and CO2. In a typical experiment, the molybdenum metal substrate was placed in a 50 ml solution containing 8 mmol of Ba2+ or Sa2+ and 6 wt.% of hydrogen peroxide solution as shown in schematic diagram in Fig. 1. The pH of the solution was adjusted to 12. The starting precursor solution along with metal substrate was placed in a teflon container housed in a stainless steel autoclave. In order to provide the mechanical energy, 40 stainless steel balls of 5 mmØ size were added to the container. Substrate was kept in contact with rotating balls as seen in schematic diagram in Fig. 1. All experiments were performed in a planetary mechanical rotator at the rotation speed of 50–200 rpm at room temperature. Dense crystalline film of BaMoO4 and SrMoO4 were fabricated using this simple set-up of the ballrotation-assisted solution reaction at room temperatures for 3–6 h. The resultant phases of the produced films were characterized by X-ray diffraction and Raman scattering techniques. A standard X-ray diffractometer (Model MXP3VA, MAC Science Co., Ltd., Tokyo, Japan) was used with Cu Ka radiation at 40 kVand 40 mA at the scanning rate of 28/min. The Raman spectra were measured at the 514.5 nm line of an Ar laser at room temperature in a backscattering geometry using microprobe optics (designed by Atago Bussan Co., Ltd., Tokyo, Japan). The laser beam was focused on the sample surface with a spot size of 1 mm. The scattered light was analyzed with a Jobin Yvon/Atoago Bussan T64000 Triple Spectrometer and collected with a liquid-nitrogen-cooled charge coupled device detector. The microstructures of these films were examined by SEM (Hitachi model S-4000, Japan) and ESEM (ESEM-2700, Nikon).
Fig. 1. Schematic diagram of experimental set up for the BaMoO4 fabrication under 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution in a ball-rotation (200 rpm)-assisted solution reaction at room temperature.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3157
Fig. 2. X-ray diffraction patterns of crystalline tetragonal (a) BaMoO4 films and (b) SrMoO4 films formed at different reaction time under 8 mmol alkaline ion solution and 6 wt.% H2O2 solution in a ball-rotation (200 rpm)-assisted solution reaction.
3. Results and discussion 3.1. Characterization of films The X-ray diffraction patterns of the AMoO4 films deposited on the molybdenum substrate shows the single phase of the crystalline film. All films show a Scheelite-type tetragonal structure without any other impurity phases, as shown in Fig. 2. The unit cell parameters of BaMoO4 and SrMoO4 calculated from the XRD data are (a = 0.5579 nm, c = 1.2810 nm) and (a = 0.5401 nm, c = 1.2023 nm), respectively. These lattice parameters are in agreement with the JCPDS (Card No 29-193 and 8-482, PCPDF WIN version 2.01) [14,15] and other published data [16,17] in the literature. The formation rate of AMoO4 was faster when compared to the AWO4 which was reported in our previous study [9]. The BaMoO4 film was nucleated on the Mo substrate within a 30 min reaction time and became a well crystallized film with an increase in the reaction time, whereas the SrMoO4 film formed in about 2 h as shown in Figs. 3 and 4. The obtained Raman spectra for the BaMoO4 film are shown in Fig. 5. The Raman spectra exhibited bands, which corresponded to the tetragonal Scheelite-type BaMoO4 structure, without any impurity phases. 3.2. Reaction mechanism The formation and growth of the AMoO4 crystalline film occurs through the dissolution and precipitation reaction on the Mo substrate. It can be explained by the following reactions: Mo þ 2OH þ 3H2 O2 ! MoO4 2 þ 4H2 O;
(1)
MoO4 2 þ Ba2þ ! BaMoO4 ;
(2)
3158
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Fig. 3. Effect of reaction time on the BaMoO4 film formation at room temperature, a plot of reaction time against XRD intensity ratio (IB/IMo) of BaMoO4 over Mo substrate; fabricated by ball-rotation (200 rpm)-assisted solution reaction containing 8 mmol and 6 wt.% H2O2 in solution.
Fig. 4. A plot of reaction time against XRD intensity ratio (IS/IMo) of SrMoO4 over Mo substrate; fabricated by ball-rotation (200 rpm)-assisted solution reaction containing 8 mmol Sr(OH)2 and 6 wt.% H2O2 in solution.
Fig. 5. Raman spectrum of the BaMoO4 film in a 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution under ball-rotation (200 rpm)-assisted solution reaction.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
MoO4 2 þ Sr2þ ! SrMoO4 :
3159
(3)
First, reaction (1) occurs by the oxidation and dissolution of the Mo substrate, in which the Mo metal substrate surface was selectively attacked by H2O2 and oxidized to MoO42. In addition to this, the vigorous steel ball-rotation transferred these oxidized MoO42 ions onto the solid/solution interface region. It is well known that in presence of alkaline solution, Mo metal has a slight tendency to decompose water with the evolution of hydrogen, dissolving into the hexavalent state as molybdate ions, MoO42 [18]. In present study, the potential difference measured during reactions (1–3) versus Ag/AgCl reference electrode well agreed with the data reported by Pourbaix [18]. This suggests that a necessary chemical potential exists in the reaction even in the absence of an applied electric current. Cho et al. reported the fabrication of AMoO4 and AWO4 films by electrochemical reactions, in which a charge transfer occurs between the solution and the electrode [19–21]. In contrast, reaction (1) is a simple chemical reaction involving oxidation–reduction, in which an electron transfer takes place between the solution and Mo metal. Under this condition, the driving force for reaction (1) was the oxidizing agent, H2O2. Reactions (2) and (3) then follow when the MoO42 species reacts with the Ba2+/Sr2+ ions at the interface of the solid/solution, and precipitate as AMoO4 films. The AMoO4 films grow at the surface of the substrate as a result of the solid and solution interfacial reactions. These reactions are non-faradic processes that are accompanied by no charge transfer. Thus, reactions (2) and (3) are simple solution reactions with a higher concentration of MoO42 and Ba2+/Sr2+ ions at high pH as the driving forces for these reactions. 3.3. Role of ball-rotation The vigorous movement of the solution induced by ball-rotation significantly assisted in the dissolution and precipitation reactions (1) and (2)/(3). Diffusion of the reactant species, such as OH, H2O2 and MoO42, could be accelerated by vigorous solution agitation by the ball-rotation. It could also improve the mass transport of the Ba2+/ Sr2+ ions onto the solid/solution interface. The deposition rate of AMoO4 was enhanced by the increased diffusion rate of all species by decreasing the diffusion layer thickness. A model mechanism depicted for the ball-rotation effect on the BaMoO4 film formation is shown in Fig. 6. It should be noted that no AMoO4 precipitation was observed in the solution. This means that the precipitation reaction was more dominant at the interface than in the solution, probably due to the low solubility product of AMoO4. The MoO42 formed at the surface was almost completely entrapped by the A2+ ions to form AMoO4. Further, when the rotating balls come in contact with the substrate surface, they help to expose a new substrate surface for oxidation by removing the MoO42 from the oxidized surface. This leads to increase in MoO42 concentration. In this way, the rate of AMoO4 deposition could be enhanced by ball-rotation through the continuous supply of alkaline earth ions to maintain their concentration in the solid/solution interface region. It compensates for the interface concentration though there would be a decrease in these ions due to the precipitation. We have observed an increase in the film thickness of BaMoO4 and SrMoO4 with an increase in the rotation speed as shown in Fig. 7. However, an excessive ball-rotation damaged the
Fig. 6. A model for ball-rotation effect on BaMoO4 film formation in a 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution at room temperature under ballrotation (200 rpm)-assisted solution reaction.
3160
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Fig. 7. A plot of film thickness of BaMoO4 and SrMoO4 against the rotation speed in the solution reaction assisted with ball-rotation at room temperature.
deposited film as the substrate will be under contact with rotating balls in the system. Therefore, controlling the ball-rotation time and speed were important factors for fabrication of good quality films. This discussion on the ball-rotation effect based on interface concentration, diffusion rate and molecular transport of the alkaline ions from the solution to the interface are supported by the dissolution/growth rate of potash alum crystals reported by Dejmek and Ward [22]. They reported that the increase in the dissolution/growth rate of the potash alum crystals in solution by increasing the rotation speed. The diffusion layer thickness decreased with an increase in the ions, concentration at the interface [22]. The reaction mechanism proposed in our present study is completely different from that of mechanochemical reactions in which a mechanical activation increases the free energy of the solids, which proceeds with material formation or destruction. This is due to the formation of new surfaces, defects, dislocations and strains in the grains [11,12]. For the soft mechanochemical reaction, the solid-state reaction proceeds in the presence of highly reactive surface functional species, notably OH ones [13]. However, in our present study, we have proposed a solid/solution interfacial reaction particularly at low temperatures like room temperature. Here, no diffusion occurs in the solid, because diffusion in the solid is much slower than in solution. Therefore, the diffusion of alkaline earth ions from the solution to the solid/solution interface could be accelerated by the ball-rotation as already described.
Fig. 8. Effect of different concentrations of H2O2 on BaMoO4 films formation in the solution reaction assisted with ball-rotation at room temperature for 3 h.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3161
Fig. 9. Effect of different concentrations of H2O2 on SrMoO4 film formation in the solution reaction assisted with ball-rotation at room temperature for 3 h.
3.4. Effect of H2O2 on dissolution of Mo and growth of AMoO4 film The addition of a suitable concentration of H2O2 plays an important role in the present solution reaction as reported in our previous studies on the formation of BaWO4 by the dissolution of the W substrate [8]. We observed similar results for the AMoO4 films. When the Mo substrate was immersed in pure water, the oxidation/dissolution of the
Fig. 10. BaMoO4 film formed (a) under excess of ball-rotation and (b) in presence of controlled ball-rotation in a solution containing 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution under ball-rotation (200 rpm)-assisted solution reaction.
3162
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
substrate surface was almost negligible. However, in the presence of H2O2 in solution, the oxidation/dissolution was drastically enhanced, releasing MoO42 ions with a porous substrate surface. These molybdate ions that encountered the barium ions formed the BaMoO4 crystalline film when Ba(OH)2 solution was added to the system. In our previous study [8,9], the addition of H2O2 of more than 12 wt.% resulted in a decreased BaWO4 deposition. However, in our present studies, we found that the deposition rate of BaMoO4 decreased above 24 wt.% H2O2 as shown in Figs. 8 and 9. This was probably due to the rapid precipitation of Ba2+ as barium hydroxide in solution, which could lead to a decrease in the total Ba2+ ions concentration. On the other hand, the addition of H2O2 by less than 6 wt.% leads to a decrease in the dissolution rate of the Mo substrate, which in turn results in the lack of MoO42. For these reasons, controlling the concentration of H2O2 was very important for the AMoO4 film formation in the solution reaction where the externally applied driving forces, such as high-temperature or applied electric current as reported in the literature [3,20], were absent. 3.5. Morphology Scanning electron micrographs of the BaMoO4 and SrMoO4 films synthesized by the ball-rotation-assisted solution reaction revealed different grain sizes. Some BaMoO4 crystals had grown to 6 mm in size with an elongated bipyramidal habit (Fig. 10). Thus, the shape was symmetrical, but they are homogeneous in composition as confirmed by the Raman measurements. An excessive ball-rotation resulted in a decreased grain size from 4–6 mm to 150– 200 nm as shown in Fig. 10. An excessive ball-rotation above 200 rpm damaged the film resulting in peeling off of the film. However, this problem could be solved by controlling the rotation speed and stopping the rotation at 1 h before
Fig. 11. The morphology of BaMoO4 film (a) in the absence of H2O2 and (b) in the presence of H2O2 in a solution containing 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution under ball-rotation (200 rpm)-assisted solution reaction.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3163
the reaction stops. The difference in the morphology of BaMoO4 film in the presence or absence of H2O2 was observed as shown in Fig. 11. In the absence of H2O2, the film was not homogeneous due to the lack of MoO42 ions and low dissolution rate of the substrate. 4. Conclusion Well-crystalline AMoO4 films were fabricated by the ball-rotation-assisted solid/solution interfacial reaction at room temperatures. Growth of the crystals could be due to the dissolution of the Mo metal substrate which was mainly controlled by using a suitable concentration of the H2O2 without any high energy or high-temperature treatment. The mass transport of Ba2+/Sr2+ ions to the solid/solution interface was improved as a result of the vigorous solution agitation by the ball-rotation. The deposition rate of the AMoO4 film was enhanced through the increased rate of diffusion of the alkaline earth ions to the solid/solution interface. The improved transport of these ions is attributed to the thicker deposit due to a decrease in the diffusion layer thickness. The crystal size decreased with an excessive ballrotation. Crystallized alkaline earth molybdate films were fabricated by a simple and low energy consuming process. No diffusion in the solid or a solid-state reaction was observed. Thus, this is an example that firing or high-temperature treatments are not always required to prepare a crystallized ceramic material as long as the appropriate processing and starting materials are chosen. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
E.F. Paski, M.W. Blades, Anal. Chem. 60 (1988) 1224. F.A. Kroger, Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam, 1948, pp. 107. V.V. Vakulyuk, A.A. Evdokimov, G.P. Khomchenko, Russ. J. Inorg. Chem. 27 (1982) 1016. M. Yoshimura, J. Mater. Res. 13 (4) (1998) 796. M. Yoshimura, J. Livage, MRS Bull. 25 (9) (2000) 12. A. Ahniyaz, T. Fujiwara, S.W. Song, M. Yoshimura, Solid State Ion. 151 (2002) 419. A. Ahniyaz, T. Fujiwara, T. Fujino, M. Yoshimura, J. Nanosci. Nanotechnol. 4 (3) (2004) 233. D. Rangappa, T. Fujiwara, M. Yoshimura, Solid State Sci. 8 (2006) 1074. D. Rangappa, T. Fujiwara, T. Watanabe, K. Byarappa, M. Yoshimura, J. Mater. Sci. 41 (2005) 1541. B.D. Stojanovic, J. Mater. Proc. Tech. 143–144 (2003) 78. M.M. Ristic, S. Milosevic, Mechanical Activation of Inorganic Materials, Monographs of SANU, Belgrade, 1998. V.I. Molchanov, O.G. Selezneva, Aktivaciya Materialov pro Izmelchenii, Nedra, Moskva, 1982. E. Avvakumov, M. Senna, N. Kosova, Soft Mechanochemical Synthesis, Academic publishers, Boston, MA, USA, 2001. JCPDS Card No. 29-193. JCPDS Card No. 8-48230. W.S. Cho, M. Yoshimura, Solid State Ion. 100 (1997) 143. W.S. Cho, M. Yoshimura, Jpn. J. Appl. Phys. 35 (1996) 1521. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution, second ed., National Association of Corrosion Engineers, Houston, TX, 1974, pp. 280–285. W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, Appl. Phys. Lett. 66 (1995) 1027. W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, J. Am. Ceram. Soc. 78 (1995) 3110. W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, Appl. Phys. Lett. 68 (1996) 137. M. Dejmek, C.A. Ward, J. Chem. Phys. 108 (20) (1998) 8698.
Fabrication of AMoO4 (A = Ba, Sr) film on Mo substrate by solution reaction assisted ball-rotation Dinesh Rangappa, Takeshi Fujiwara, Tomoaki Watanabe, Masahiro Yoshimura * Center for Materials Design, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 226-8503, Japan Received 15 March 2007; received in revised form 15 June 2007; accepted 26 June 2007 Available online 7 January 2008
Abstract Alkaline earth molybdates, such as BaMoO4 and SrMoO4, films have been successfully fabricated on a Mo metal substrate in AOH (A = Ba, Sr) solutions by a ball-rotation-assisted solution reaction, at room temperature. The dissolution of Mo was mainly controlled by the concentration of the H2O2 oxidizing agent and ball-rotation to form MoO42 in the solution. AMoO4 was deposited on the substrate by the reaction between MoO42 and A2+ ions without any high energy or high-temperature treatment. Also, the mass transport of alkaline earth ions onto the solid/solution interface was improved as a result of the vigorous solution agitation by the ball-rotation. Therefore, the rate of deposition of the AMoO4 films was accelerated by the ball-rotation. A decrease in the grain size of the film was observed with an excessive ball-rotation. # 2007 Elsevier Ltd. All rights reserved. PACS : 82.65.J; 43.35P; 68.35G Keywords: A. Optical materials; B. Oxides; C. Surfaces; D. Thin films; E. Chemical synthesis
1. Introduction Alkaline earth molybdates with a Scheelite-type structure are important materials in electro-optics. The green luminescence properties of these materials have attracted the attention of many researchers due to their application in the electro-optical industry [1,2]. An AMoO4 powder is usually prepared using conventional methods consisting of repeated ball-milling, for example, the solid-state reaction between BaCO3 and MoO3 powders [3]. Although films are more important for better performance with regard to industrial applications, problems may occur in the preparation of these films when conventional techniques with high-temperature treatments are used for crystallization of the film. Due to the characteristic volatile nature of MoO3, low-temperature processing is suitable for the synthesis of crystallized luminescent AMoO4 films. In this regard, a novel ball-rotation-assisted solution processing has been developed under Soft Solution Processing (SSP) which was proposed by Yoshimura for the first time [4,5]. This synthetic approach has been used for the preparation of lithium ferrites [6], (Ce, Zr) O2 solid solutions [7] and barium tungstate [8,9].
* Corresponding author. E-mail address: [email protected] (M. Yoshimura). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.06.062
3156
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Generally, mechanically activated reactions are regarded as mechanochemical reactions like mechanical alloying, the production of intermetallic and alloy compounds, nano-crystalline powders, etc. [10,11,12]. These solid-state reactions can be activated by mechanical agitation to form new surfaces, defects, dislocations and/or strains on the surface of the grains. Senna and co-workers proposed a soft mechanochemical method [13], in which the presence of highly reactive surface functional species, notably OH’s, can assist the solid-state reactions. On the other hand, ballrotation assistance can accelerate the solution reactions without any solid-state reactions [6,7]. The vigorous stirring of a solution by ball-rotation will accelerate the interfacial reaction between solid and solution. In our present study, we have successfully applied this method to prepare a crystalline film of AMoO4 at room temperature using simple ballrotation-assisted solution reaction, where, metal substrate is placed in a solution containing alkaline earth ions and stainless steel balls. This paper describes the formation, characterization and the formation mechanism of the AMoO4 (A = Ba, Sr) films during the ball-rotation-assisted solution reaction. 2. Experimental procedure A molybdenum metal substrate with 10 mm 10 mm 0.2 mm dimensions of 99.9 wt.% purity (Niraco, Japan) was mechanically polished and degreased in acetone using an ultrasonic cleaner. Alkaline earth solutions were made from redistilled water using guaranteed reagents with a minimum 98% assay of Ba(OH)28H2O, Sr(OH)28H2O and H2O2 (Kanto, Kagaku, Japan). Distilled water was purged with N2 gas for 30 min in order to remove the dissolved O2 and CO2. In a typical experiment, the molybdenum metal substrate was placed in a 50 ml solution containing 8 mmol of Ba2+ or Sa2+ and 6 wt.% of hydrogen peroxide solution as shown in schematic diagram in Fig. 1. The pH of the solution was adjusted to 12. The starting precursor solution along with metal substrate was placed in a teflon container housed in a stainless steel autoclave. In order to provide the mechanical energy, 40 stainless steel balls of 5 mmØ size were added to the container. Substrate was kept in contact with rotating balls as seen in schematic diagram in Fig. 1. All experiments were performed in a planetary mechanical rotator at the rotation speed of 50–200 rpm at room temperature. Dense crystalline film of BaMoO4 and SrMoO4 were fabricated using this simple set-up of the ballrotation-assisted solution reaction at room temperatures for 3–6 h. The resultant phases of the produced films were characterized by X-ray diffraction and Raman scattering techniques. A standard X-ray diffractometer (Model MXP3VA, MAC Science Co., Ltd., Tokyo, Japan) was used with Cu Ka radiation at 40 kVand 40 mA at the scanning rate of 28/min. The Raman spectra were measured at the 514.5 nm line of an Ar laser at room temperature in a backscattering geometry using microprobe optics (designed by Atago Bussan Co., Ltd., Tokyo, Japan). The laser beam was focused on the sample surface with a spot size of 1 mm. The scattered light was analyzed with a Jobin Yvon/Atoago Bussan T64000 Triple Spectrometer and collected with a liquid-nitrogen-cooled charge coupled device detector. The microstructures of these films were examined by SEM (Hitachi model S-4000, Japan) and ESEM (ESEM-2700, Nikon).
Fig. 1. Schematic diagram of experimental set up for the BaMoO4 fabrication under 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution in a ball-rotation (200 rpm)-assisted solution reaction at room temperature.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3157
Fig. 2. X-ray diffraction patterns of crystalline tetragonal (a) BaMoO4 films and (b) SrMoO4 films formed at different reaction time under 8 mmol alkaline ion solution and 6 wt.% H2O2 solution in a ball-rotation (200 rpm)-assisted solution reaction.
3. Results and discussion 3.1. Characterization of films The X-ray diffraction patterns of the AMoO4 films deposited on the molybdenum substrate shows the single phase of the crystalline film. All films show a Scheelite-type tetragonal structure without any other impurity phases, as shown in Fig. 2. The unit cell parameters of BaMoO4 and SrMoO4 calculated from the XRD data are (a = 0.5579 nm, c = 1.2810 nm) and (a = 0.5401 nm, c = 1.2023 nm), respectively. These lattice parameters are in agreement with the JCPDS (Card No 29-193 and 8-482, PCPDF WIN version 2.01) [14,15] and other published data [16,17] in the literature. The formation rate of AMoO4 was faster when compared to the AWO4 which was reported in our previous study [9]. The BaMoO4 film was nucleated on the Mo substrate within a 30 min reaction time and became a well crystallized film with an increase in the reaction time, whereas the SrMoO4 film formed in about 2 h as shown in Figs. 3 and 4. The obtained Raman spectra for the BaMoO4 film are shown in Fig. 5. The Raman spectra exhibited bands, which corresponded to the tetragonal Scheelite-type BaMoO4 structure, without any impurity phases. 3.2. Reaction mechanism The formation and growth of the AMoO4 crystalline film occurs through the dissolution and precipitation reaction on the Mo substrate. It can be explained by the following reactions: Mo þ 2OH þ 3H2 O2 ! MoO4 2 þ 4H2 O;
(1)
MoO4 2 þ Ba2þ ! BaMoO4 ;
(2)
3158
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Fig. 3. Effect of reaction time on the BaMoO4 film formation at room temperature, a plot of reaction time against XRD intensity ratio (IB/IMo) of BaMoO4 over Mo substrate; fabricated by ball-rotation (200 rpm)-assisted solution reaction containing 8 mmol and 6 wt.% H2O2 in solution.
Fig. 4. A plot of reaction time against XRD intensity ratio (IS/IMo) of SrMoO4 over Mo substrate; fabricated by ball-rotation (200 rpm)-assisted solution reaction containing 8 mmol Sr(OH)2 and 6 wt.% H2O2 in solution.
Fig. 5. Raman spectrum of the BaMoO4 film in a 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution under ball-rotation (200 rpm)-assisted solution reaction.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
MoO4 2 þ Sr2þ ! SrMoO4 :
3159
(3)
First, reaction (1) occurs by the oxidation and dissolution of the Mo substrate, in which the Mo metal substrate surface was selectively attacked by H2O2 and oxidized to MoO42. In addition to this, the vigorous steel ball-rotation transferred these oxidized MoO42 ions onto the solid/solution interface region. It is well known that in presence of alkaline solution, Mo metal has a slight tendency to decompose water with the evolution of hydrogen, dissolving into the hexavalent state as molybdate ions, MoO42 [18]. In present study, the potential difference measured during reactions (1–3) versus Ag/AgCl reference electrode well agreed with the data reported by Pourbaix [18]. This suggests that a necessary chemical potential exists in the reaction even in the absence of an applied electric current. Cho et al. reported the fabrication of AMoO4 and AWO4 films by electrochemical reactions, in which a charge transfer occurs between the solution and the electrode [19–21]. In contrast, reaction (1) is a simple chemical reaction involving oxidation–reduction, in which an electron transfer takes place between the solution and Mo metal. Under this condition, the driving force for reaction (1) was the oxidizing agent, H2O2. Reactions (2) and (3) then follow when the MoO42 species reacts with the Ba2+/Sr2+ ions at the interface of the solid/solution, and precipitate as AMoO4 films. The AMoO4 films grow at the surface of the substrate as a result of the solid and solution interfacial reactions. These reactions are non-faradic processes that are accompanied by no charge transfer. Thus, reactions (2) and (3) are simple solution reactions with a higher concentration of MoO42 and Ba2+/Sr2+ ions at high pH as the driving forces for these reactions. 3.3. Role of ball-rotation The vigorous movement of the solution induced by ball-rotation significantly assisted in the dissolution and precipitation reactions (1) and (2)/(3). Diffusion of the reactant species, such as OH, H2O2 and MoO42, could be accelerated by vigorous solution agitation by the ball-rotation. It could also improve the mass transport of the Ba2+/ Sr2+ ions onto the solid/solution interface. The deposition rate of AMoO4 was enhanced by the increased diffusion rate of all species by decreasing the diffusion layer thickness. A model mechanism depicted for the ball-rotation effect on the BaMoO4 film formation is shown in Fig. 6. It should be noted that no AMoO4 precipitation was observed in the solution. This means that the precipitation reaction was more dominant at the interface than in the solution, probably due to the low solubility product of AMoO4. The MoO42 formed at the surface was almost completely entrapped by the A2+ ions to form AMoO4. Further, when the rotating balls come in contact with the substrate surface, they help to expose a new substrate surface for oxidation by removing the MoO42 from the oxidized surface. This leads to increase in MoO42 concentration. In this way, the rate of AMoO4 deposition could be enhanced by ball-rotation through the continuous supply of alkaline earth ions to maintain their concentration in the solid/solution interface region. It compensates for the interface concentration though there would be a decrease in these ions due to the precipitation. We have observed an increase in the film thickness of BaMoO4 and SrMoO4 with an increase in the rotation speed as shown in Fig. 7. However, an excessive ball-rotation damaged the
Fig. 6. A model for ball-rotation effect on BaMoO4 film formation in a 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution at room temperature under ballrotation (200 rpm)-assisted solution reaction.
3160
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Fig. 7. A plot of film thickness of BaMoO4 and SrMoO4 against the rotation speed in the solution reaction assisted with ball-rotation at room temperature.
deposited film as the substrate will be under contact with rotating balls in the system. Therefore, controlling the ball-rotation time and speed were important factors for fabrication of good quality films. This discussion on the ball-rotation effect based on interface concentration, diffusion rate and molecular transport of the alkaline ions from the solution to the interface are supported by the dissolution/growth rate of potash alum crystals reported by Dejmek and Ward [22]. They reported that the increase in the dissolution/growth rate of the potash alum crystals in solution by increasing the rotation speed. The diffusion layer thickness decreased with an increase in the ions, concentration at the interface [22]. The reaction mechanism proposed in our present study is completely different from that of mechanochemical reactions in which a mechanical activation increases the free energy of the solids, which proceeds with material formation or destruction. This is due to the formation of new surfaces, defects, dislocations and strains in the grains [11,12]. For the soft mechanochemical reaction, the solid-state reaction proceeds in the presence of highly reactive surface functional species, notably OH ones [13]. However, in our present study, we have proposed a solid/solution interfacial reaction particularly at low temperatures like room temperature. Here, no diffusion occurs in the solid, because diffusion in the solid is much slower than in solution. Therefore, the diffusion of alkaline earth ions from the solution to the solid/solution interface could be accelerated by the ball-rotation as already described.
Fig. 8. Effect of different concentrations of H2O2 on BaMoO4 films formation in the solution reaction assisted with ball-rotation at room temperature for 3 h.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3161
Fig. 9. Effect of different concentrations of H2O2 on SrMoO4 film formation in the solution reaction assisted with ball-rotation at room temperature for 3 h.
3.4. Effect of H2O2 on dissolution of Mo and growth of AMoO4 film The addition of a suitable concentration of H2O2 plays an important role in the present solution reaction as reported in our previous studies on the formation of BaWO4 by the dissolution of the W substrate [8]. We observed similar results for the AMoO4 films. When the Mo substrate was immersed in pure water, the oxidation/dissolution of the
Fig. 10. BaMoO4 film formed (a) under excess of ball-rotation and (b) in presence of controlled ball-rotation in a solution containing 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution under ball-rotation (200 rpm)-assisted solution reaction.
3162
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
substrate surface was almost negligible. However, in the presence of H2O2 in solution, the oxidation/dissolution was drastically enhanced, releasing MoO42 ions with a porous substrate surface. These molybdate ions that encountered the barium ions formed the BaMoO4 crystalline film when Ba(OH)2 solution was added to the system. In our previous study [8,9], the addition of H2O2 of more than 12 wt.% resulted in a decreased BaWO4 deposition. However, in our present studies, we found that the deposition rate of BaMoO4 decreased above 24 wt.% H2O2 as shown in Figs. 8 and 9. This was probably due to the rapid precipitation of Ba2+ as barium hydroxide in solution, which could lead to a decrease in the total Ba2+ ions concentration. On the other hand, the addition of H2O2 by less than 6 wt.% leads to a decrease in the dissolution rate of the Mo substrate, which in turn results in the lack of MoO42. For these reasons, controlling the concentration of H2O2 was very important for the AMoO4 film formation in the solution reaction where the externally applied driving forces, such as high-temperature or applied electric current as reported in the literature [3,20], were absent. 3.5. Morphology Scanning electron micrographs of the BaMoO4 and SrMoO4 films synthesized by the ball-rotation-assisted solution reaction revealed different grain sizes. Some BaMoO4 crystals had grown to 6 mm in size with an elongated bipyramidal habit (Fig. 10). Thus, the shape was symmetrical, but they are homogeneous in composition as confirmed by the Raman measurements. An excessive ball-rotation resulted in a decreased grain size from 4–6 mm to 150– 200 nm as shown in Fig. 10. An excessive ball-rotation above 200 rpm damaged the film resulting in peeling off of the film. However, this problem could be solved by controlling the rotation speed and stopping the rotation at 1 h before
Fig. 11. The morphology of BaMoO4 film (a) in the absence of H2O2 and (b) in the presence of H2O2 in a solution containing 8 mmol Ba(OH)2 and 6 wt.% H2O2 solution under ball-rotation (200 rpm)-assisted solution reaction.
D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3163
the reaction stops. The difference in the morphology of BaMoO4 film in the presence or absence of H2O2 was observed as shown in Fig. 11. In the absence of H2O2, the film was not homogeneous due to the lack of MoO42 ions and low dissolution rate of the substrate. 4. Conclusion Well-crystalline AMoO4 films were fabricated by the ball-rotation-assisted solid/solution interfacial reaction at room temperatures. Growth of the crystals could be due to the dissolution of the Mo metal substrate which was mainly controlled by using a suitable concentration of the H2O2 without any high energy or high-temperature treatment. The mass transport of Ba2+/Sr2+ ions to the solid/solution interface was improved as a result of the vigorous solution agitation by the ball-rotation. The deposition rate of the AMoO4 film was enhanced through the increased rate of diffusion of the alkaline earth ions to the solid/solution interface. The improved transport of these ions is attributed to the thicker deposit due to a decrease in the diffusion layer thickness. The crystal size decreased with an excessive ballrotation. Crystallized alkaline earth molybdate films were fabricated by a simple and low energy consuming process. No diffusion in the solid or a solid-state reaction was observed. Thus, this is an example that firing or high-temperature treatments are not always required to prepare a crystallized ceramic material as long as the appropriate processing and starting materials are chosen. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
E.F. Paski, M.W. Blades, Anal. Chem. 60 (1988) 1224. F.A. Kroger, Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam, 1948, pp. 107. V.V. Vakulyuk, A.A. Evdokimov, G.P. Khomchenko, Russ. J. Inorg. Chem. 27 (1982) 1016. M. Yoshimura, J. Mater. Res. 13 (4) (1998) 796. M. Yoshimura, J. Livage, MRS Bull. 25 (9) (2000) 12. A. Ahniyaz, T. Fujiwara, S.W. Song, M. Yoshimura, Solid State Ion. 151 (2002) 419. A. Ahniyaz, T. Fujiwara, T. Fujino, M. Yoshimura, J. Nanosci. Nanotechnol. 4 (3) (2004) 233. D. Rangappa, T. Fujiwara, M. Yoshimura, Solid State Sci. 8 (2006) 1074. D. Rangappa, T. Fujiwara, T. Watanabe, K. Byarappa, M. Yoshimura, J. Mater. Sci. 41 (2005) 1541. B.D. Stojanovic, J. Mater. Proc. Tech. 143–144 (2003) 78. M.M. Ristic, S. Milosevic, Mechanical Activation of Inorganic Materials, Monographs of SANU, Belgrade, 1998. V.I. Molchanov, O.G. Selezneva, Aktivaciya Materialov pro Izmelchenii, Nedra, Moskva, 1982. E. Avvakumov, M. Senna, N. Kosova, Soft Mechanochemical Synthesis, Academic publishers, Boston, MA, USA, 2001. JCPDS Card No. 29-193. JCPDS Card No. 8-48230. W.S. Cho, M. Yoshimura, Solid State Ion. 100 (1997) 143. W.S. Cho, M. Yoshimura, Jpn. J. Appl. Phys. 35 (1996) 1521. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution, second ed., National Association of Corrosion Engineers, Houston, TX, 1974, pp. 280–285. W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, Appl. Phys. Lett. 66 (1995) 1027. W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, J. Am. Ceram. Soc. 78 (1995) 3110. W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, Appl. Phys. Lett. 68 (1996) 137. M. Dejmek, C.A. Ward, J. Chem. Phys. 108 (20) (1998) 8698.