Fabrication of AMoO4 (A = Ba, Sr) film on Mo substrate by solution reaction assisted ball-rotation

Article abstract of DOI:10.1016/j.materresbull.2007.06.062

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 MoO₄^2- in the solution. AMoO₄ was deposited on the substrate by the reaction between MoO₄^2- 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.

Full text of DOI:10.1016/j.materresbull.2007.06.062

Materials Research Bulletin 43 (2008) 3155–3163
www.elsevier.com/locate/matresbu
Fabrication of AMoO (A = Ba, Sr) film on Mo substrate
4
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,
4
259 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 BaMoO
4
and SrMoO
4
, 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
2
ꢀ
controlled by the concentration of the H O oxidizing agent and ball-rotation to form MoO
in the solution. AMoO was
4
2
2
4
2
ꢀ
2+
deposited on the substrate by the reaction between MoO
4
and A 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 AMoO films was accelerated by the ball-rotation. A decrease
4
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 AMoO powder is usually prepared using conventional methods consisting of
4
repeated ball-milling, for example, the solid-state reaction between BaCO and MoO powders [3]. Although films are
3
3
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 MoO , low-temperature processing is suitable for the synthesis of
3
crystallized luminescent AMoO films. In this regard, a novel ball-rotation-assisted solution processing has been
4
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) O solid solutions [7] and barium
2
tungstate [8,9].
*
Corresponding author.
E-mail address: yoshimura@msl.titech.ac.jp (M. Yoshimura).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.06.062

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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, ball-
rotation 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 AMoO at room temperature using simple ball-
4
rotation-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) ꢂ8H O, Sr(OH) ꢂ8H O and
2
2
2
2
H O (Kanto, Kagaku, Japan). Distilled water was purged with N gas for 30 min in order to remove the dissolved O
2
2
2
2
and CO . In a typical experiment, the molybdenum metal substrate was placed in a 50 ml solution containing 8 mmol
2
2+
2+
of Ba or Sa 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 BaMoO and SrMoO were fabricated using this simple set-up of the ball-
4
4
rotation-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
3
VA
techniques. A standard X-ray diffractometer (Model MXP , 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 BaMoO
200 rpm)-assisted solution reaction at room temperature.
4 2 2 2
fabrication under 8 mmol Ba(OH) and 6 wt.% H O solution in a ball-rotation
(

D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3157
4 4
Fig. 2. X-ray diffraction patterns of crystalline tetragonal (a) BaMoO films and (b) SrMoO films formed at different reaction time under 8 mmol
alkaline ion solution and 6 wt.% H solution in a ball-rotation (200 rpm)-assisted solution reaction.
2 2
O
3
. Results and discussion
.1. Characterization of films
The X-ray diffraction patterns of the AMoO films deposited on the molybdenum substrate shows the single phase
3
4
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 BaMoO and SrMoO calculated from the XRD data are (a = 0.5579 nm,
4
4
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 AMoO was faster when compared to the AWO which was reported in our previous
4
4
study [9]. The BaMoO film was nucleated on the Mo substrate within a 30 min reaction time and became a well
4
crystallized film with an increase in the reaction time, whereas the SrMoO film formed in about 2 h as shown in
4
Figs. 3 and 4. The obtained Raman spectra for the BaMoO film are shown in Fig. 5. The Raman spectra exhibited
4
bands, which corresponded to the tetragonal Scheelite-type BaMoO structure, without any impurity phases.
4
3
.2. Reaction mechanism
The formation and growth of the AMoO crystalline film occurs through the dissolution and precipitation reaction
4
on the Mo substrate. It can be explained by the following reactions:
ꢀ
2ꢀ
Mo þ 2OH þ 3H O ! MoO
þ 4H O;
(1)
(2)
2
2
4
2
MoO4 þ Ba2þ ! BaMoO4;
2
ꢀ

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D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Fig. 3. Effect of reaction time on the BaMoO
4
film formation at room temperature, a plot of reaction time against XRD intensity ratio (I
B
/IMo) of
BaMoO over Mo substrate; fabricated by ball-rotation (200 rpm)-assisted solution reaction containing 8 mmol and 6 wt.% H O
4
2 2
in solution.
Fig. 4. A plot of reaction time against XRD intensity ratio (I /IMo) of SrMoO
S
4
over Mo substrate; fabricated by ball-rotation (200 rpm)-assisted
solution reaction containing 8 mmol Sr(OH) and 6 wt.% H O in solution.
2
2 2
4 2 2 2
Fig. 5. Raman spectrum of the BaMoO film in a 8 mmol Ba(OH) and 6 wt.% H O solution under ball-rotation (200 rpm)-assisted solution
reaction.

D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3159
MoO4 þ Sr2þ ! SrMoO4:
2
ꢀ
(3)
First, reaction (1) occurs by the oxidation and dissolution of the Mo substrate, in which the Mo metal substrate surface
2
was selectively attacked by H O and oxidized to MoO . In addition to this, the vigorous steel ball-rotation
ꢀ
2
2
ꢀ
4
2
transferred these oxidized MoO4 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
2
ꢀ
the hexavalent state as molybdate ions, MoO4 [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 AMoO and AWO films by electrochemical reactions, in which a charge transfer occurs
4
4
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, H O .
2
2
2
ꢀ
2+
2+
Reactions (2) and (3) then follow when the MoO4 species reacts with the Ba /Sr ions at the interface of the
solid/solution, and precipitate as AMoO films. The AMoO films grow at the surface of the substrate as a result of the
4
4
solid and solution interfacial reactions. These reactions are non-faradic processes that are accompanied by no charge
2
ꢀ
2+
2+
transfer. Thus, reactions (2) and (3) are simple solution reactions with a higher concentration of MoO4 and Ba /Sr
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
ꢀ
2ꢀ
4
precipitation reactions (1) and (2)/(3). Diffusion of the reactant species, such as OH , H O and MoO , could be
2
2
2
+
accelerated by vigorous solution agitation by the ball-rotation. It could also improve the mass transport of the Ba /
Sr ions onto the solid/solution interface. The deposition rate of AMoO was enhanced by the increased diffusion
2
+
4
rate of all species by decreasing the diffusion layer thickness. A model mechanism depicted for the ball-rotation
effect on the BaMoO film formation is shown in Fig. 6. It should be noted that no AMoO precipitation was
4
4
observed in the solution. This means that the precipitation reaction was more dominant at the interface than in the
2
ꢀ
solution, probably due to the low solubility product of AMoO . The MoO
formed at the surface was almost
4
4
2
+
completely entrapped by the A ions to form AMoO . 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 MoO
oxidized surface. This leads to increase in MoO4 concentration. In this way, the rate of AMoO deposition could
4
2
ꢀ
from the
4
2
ꢀ
4
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 BaMoO and SrMoO
4
4
with an increase in the rotation speed as shown in Fig. 7. However, an excessive ball-rotation damaged the
4 2 2 2
Fig. 6. A model for ball-rotation effect on BaMoO film formation in a 8 mmol Ba(OH) and 6 wt.% H O solution at room temperature under ball-
rotation (200 rpm)-assisted solution reaction.

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D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
Fig. 7. A plot of film thickness of BaMoO
temperature.
4 4
and SrMoO against the rotation speed in the solution reaction assisted with ball-rotation at room
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 H
for 3 h.
2 2 4
O on BaMoO films formation in the solution reaction assisted with ball-rotation at room temperature

D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3161
Fig. 9. Effect of different concentrations of H
for 3 h.
2 2 4
O on SrMoO film formation in the solution reaction assisted with ball-rotation at room temperature
3
.4. Effect of H O on dissolution of Mo and growth of AMoO film
2 2 4
The addition of a suitable concentration of H O plays an important role in the present solution reaction as reported
2
2
in our previous studies on the formation of BaWO by the dissolution of the W substrate [8]. We observed similar
4
results for the AMoO films. When the Mo substrate was immersed in pure water, the oxidation/dissolution of the
4
Fig. 10. BaMoO
4
film formed (a) under excess of ball-rotation and (b) in presence of controlled ball-rotation in a solution containing 8 mmol
solution under ball-rotation (200 rpm)-assisted solution reaction.
Ba(OH) and 6 wt.% H O
2
2 2

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D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
substrate surface was almost negligible. However, in the presence of H O in solution, the oxidation/dissolution was
2
2
2
ꢀ
drastically enhanced, releasing MoO4 ions with a porous substrate surface. These molybdate ions that encountered
the barium ions formed the BaMoO crystalline film when Ba(OH) solution was added to the system. In our previous
4
2
study [8,9], the addition of H O of more than 12 wt.% resulted in a decreased BaWO deposition. However, in our
2
2
4
present studies, we found that the deposition rate of BaMoO decreased above 24 wt.% H O as shown in Figs. 8 and 9.
4
2 2
2
+
This was probably due to the rapid precipitation of Ba as barium hydroxide in solution, which could lead to a
decrease in the total Ba ions concentration. On the other hand, the addition of H O 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 MoO4 . For these reasons,
controlling the concentration of H O was very important for the AMoO film formation in the solution reaction where
2
+
2
2
2
ꢀ
2
2
4
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 BaMoO and SrMoO films synthesized by the ball-rotation-assisted solution
4
4
reaction revealed different grain sizes. Some BaMoO crystals had grown to ꢃ6 mm in size with an elongated
4
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–
2
00 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
4 2 2 2 2 2
Fig. 11. The morphology of BaMoO film (a) in the absence of H O and (b) in the presence of H O in a solution containing 8 mmol Ba(OH) and
6
wt.% H solution under ball-rotation (200 rpm)-assisted solution reaction.
2 2
O

D. Rangappa et al. / Materials Research Bulletin 43 (2008) 3155–3163
3163
the reaction stops. The difference in the morphology of BaMoO film in the presence or absence of H O was observed
4
2
2
ꢀ
2
as shown in Fig. 11. In the absence of H O , the film was not homogeneous due to the lack of MoO
4
ions and low
2
2
dissolution rate of the substrate.
. Conclusion
Well-crystalline AMoO films were fabricated by the ball-rotation-assisted solid/solution interfacial reaction at
4
4
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 H O without any high energy or high-temperature treatment. The
2
2
2
+
2+
mass transport of Ba /Sr 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 AMoO film was enhanced through the increased rate of
4
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 ball-
rotation. 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.
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