Solid State Sciences 3 (2001) 183–188

Direct fabrication of crystalline vanadates films byhydrothermal-electrochemical method

Tomoaki Watanabe a,*, Woo-Seok Cho a,1, Wojciech L. Suchanek a,2,Masakazu Endo a, Yasuro Ikuma b, Masahiro Yoshimura a

a Center for Materials Design, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,Yokohama 226-8503, Japan

b Department of Applied Chemistry, Kanagawa Institute of Technology, Atsugi 243, Japan

Received 19 September 2000; accepted 26 October 2000

Dedicated to Professor Michel Tournoux on the occasion of his retirement

Abstract

Lithium vanadate films have been prepared on vanadium metal substrates using lithium hydroxide aqueous solutions inthe autoclave by the hydrothermal-electrochemical method. The X-ray diffraction pattern of the fabricated film showed asingle phase of stoichiometric crystalline orthorhombic bII-Li3VO4 without any other impurity phases. As an application ofthis method, we have demonstrated the direct fabrication of europium doped YVO4 films on vanadium substrate forluminescent materials using hydrothermal-electrochemical method. Those films showed sufficient intensity of luminescentproperty at room temperature. Therefore, this processing route may serve as an inexpensive and environmentally friendlyway of the direct fabrication of the vanadate films with controlled crystal morphology and size simply by changing synthesisparameters. © 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Keywords: Li3VO4; YVO4; Film; Hydrothermal; Electrochemical; Direct fabrication; Solution process

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1. Introduction

The vanadium double oxides has many attractiveproperties for optic materials, catalytic materials andcathode materials for Li-ion battery. Generally, theyare first synthesized as powders, and then they arefabricated in desired shape and size for their particu-

lar purposes and/or applications. For example,Li3VO4 have been prepared in powders by classicalsolid state reactions [1–3] and a solution process [4].Single crystals of this material have been preparedby a floating zone technique [5,6]. The rare earthdoped yttrium vanadates, which are well known asluminescent materials, have been synthesized in pow-ders using aqueous solution process [7,8] or wet-chemical method [9]. Using these methods,sometimes said to be low temperature syntheses,‘firing’ at lowest >500 °C seems to be essential tomake certain shapes. Moreover additional steps aregenerally required to fabricate materials with desiredshape and size, i.e. thin/thick films. Those steps alsoconsume a lot of energy. For example, synthesized

* Correspondence and reprints.E-mail addresses: tomo@masa.rlem.titech.ac.jp (T. Watanabe);

woocho@samsung.co.kr (W.-S. Cho), yoshimu1@rlem.titech.ac.jp(M. Yoshimura).

1 Present address: Research center, Samsung Corning, Suwon,Korea.

2 Present address: Rutgers University, Dept. of Ceramic &Material Engineering, 607 Taylor Rd., Piscataway, NJ, USA.

1293-2558/01/$ - see front matter © 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

PII: S1293-2558(00)01126-2

T. Watanabe et al. / Solid State Sciences 3 (2001) 183–188184

powders dispersed as slurries in a solvent are printed/coated and then must be dried. So-called ‘sol–gel’processing also require ‘firing’ to pyrolyze organicpart of precursors after coating or film forming.Chemical vapor deposition (CVD) or physical vapordeposition (PVD) can produce films of double oxidein a single step. However, they need sophisticatedapparatus, precursors and also sufficient heat tocrystallize the compounds on the substrates. Thusthey should cost not only economically but alsoenvironmentally [10,11]. Heating/firing and/or vapor-izing at high temperature often result in composi-tional nonstoichiometries in easy vaporable cations,like alkaline metals and/or in anions and/or physicaldamages of deposited films like cracking, swellingand/or peeling.

In those regards, direct fabrication method ofcrystallized double oxide films which have been de-veloped in our laboratory, as a typical soft solutionprocessing [10,11], should be attractive. We havereported the fabrication of thin films of BaTiO3,SrTiO3 (and their multilayer), BaWO4, SrWO4,CaTiO3, LiNbO3, LiCoO2, LiNiO2 and other materi-als by hydrothermal and/or hydrothermal-electro-chemical methods at low temperatures of 100–200 °C[12–14], sometimes even at room temperatures[15,16].

The present work deals with the direct fabricationof vanadate films on vanadium substrates by hy-drothermal-electrochemical methods in LiOH solu-tions at 150 °C without any post-heating. To the bestof our knowledge this is the first report of the directfabrication of Li3VO4 and europium doped YVO4

films from solutions. This paper consists of two parts.1. Confirmation and characterization of directly

fabricated Li3VO4 films using usual autoclave.2. As an application of the methods, direct fabrica-

tion and surface morphology control of eu-ropium doped YVO4 films on the vanadiumsubstrate for fluorescent materials using hy-drothermal method.

2. Experimental

2.1. Hydrothermal-electrochemical synthesis ofLi3VO4 using a classical autoclave

A vanadium metal plate with 99.5 % purity anddimension of 10×50×0.5 mm3 was used as a working

electrode for film preparation. Pt metal plate (10×20×0.2 mm3, purity 99.98 %) was used as a counterelectrode. Prior to the hydrothermal-electrochemicaltreatment, both electrodes were degreased in acetoneusing an ultrasonic cleaner. A 4 M-LiOH aqueoussolution, which served as the source of lithium, wasprepared by dissolving reagent grade LiOH·H2O(Wako Pure Chemical Industries Ltd., Japan) intodistilled water. The working and counter electrodeswere plunged in the LiOH solution, put into aautoclave and treated under hydrothermal conditionsat 150 °C for 24 h with a current density of 1 mAcm–2 generated by a commercial power source (TypePAD 1K-0.2A, Kikusui Electronics, Tokyo, Japan).The pressure, 15 MPa, was auto-generated watervapor pressure from the LiOH solution. The resul-tant films deposited on the vanadium plate wereanalyzed by an X-ray film diffractometer (MXP3VA,MAC Science, Tokyo, Japan) using CuKa, graphitemonochromator and 40 kV–40 mA. The microstruc-ture of the films was investigated by atomic forcemicroscopy (AFM) (Molecular Imaging/Digital In-strumental, NanoscopeE, Santa Barbara, USA).

2.2. Fabrication and characterization of europiumdoped YVO4 using hydrothermal reaction

The procedures were basically similar to the above-mentioned fabrication procedures for lithium vana-date in the autoclave. In the case of using aflow-system (Fig. 1) [17], the same experimental con-ditions were applied except for a flow rate of 1 mLmin–1 or 10 mL min–1. All hydrothermal treatmentson vanadium plates were performed in a 0.025-Mnitrate solution (Eu:Y=1:9) for 3 h at the tempera-ture of 150 °C. Surface morphology investigation wasdone using laser microscope with CCD detector(Keyence VK8500, Tokyo, Japan). The fluorescentspectra were measured using a UV–vis spectrometer(Hitachi F4500, Tokyo, Japan) at room temperature.

3. Results and discussion

3.1. Phase composition of the films prepared byclassical autoclave

Under the investigated conditions only singlephase bII-Li3VO4 have been prepared on the sub-strate. As shown in Fig. 2 the X-ray diffraction

T. Watanabe et al. / Solid State Sciences 3 (2001) 183–188 185

Fig. 1. Schematics diagram of flow-system for hydrothermal-electrochemical reactions.

pattern of the prepared film shows a single phase ofcrystalline orthorhombic bII-Li3VO4 without anyother impurity phases [18]. The lattice parameters forthe Li3VO4 prepared in this work are identical tothose previously reported for crystals prepared bysolid state reactions of the stoichiometric mixture[18,19] as shown in Table 1. It indicates that ourhydrothermal-electrochemical products should bestoichiometric as well as the solid state reacted prod-ucts at high temperatures. The AFM image of thesurface of the as-prepared Li3VO4 film demonstrateswell-crystallized grains as seen in Fig. 3. The averagegrain size of Li3VO4 in the film was approximately5 mm, and the largest size was about 10 mm.

It should be noted that well-crystallized Li3VO4

films could form and grow on the vanadium sub-strate (=anode) in the aqueous LiOH solution attemperatures as low as 150 °C. According to thePourbaix’s diagram [20], VO4

3– ion can be formed inthe vicinity of the vanadium electrode in alkalinesolutions by anodic dissolution as follows:

2V+8H2O�2VO43– +16H++10e– (1)

The VO43- ion then reacts with Li+ ion in the

solution to precipitate as the double oxide Li3VO4 asionic reactions:

3Li++VO43–�Li3VO4 (2)

As a result of reactions (1) and (2), Li3VO4 cannucleate on the vanadium substrate and then growto form the Li3VO4 crystals, which might finallycover the substrate as a film. That is, the growth ofLi3VO4 crystals can be achieved by the combination

of both the electrochemical reaction (1), which isaccompanied by charge transfer between the solutionand the electrode, and the solution reaction (2),which is not accompanied by any charge transfer inthe electrolytic cell. It means that the size and mor-

Fig. 2. X-ray diffraction pattern of Li3VO4 film prepared on avanadium metal substrate by the hydrothermal-electrochemicaltreatment in the aqueous solution of 4 M LiOH for 24 h at150 °C.

Table 1Lattice parameters of Li3Vo4 prepared by various method

Scheet et al. [19] Our resultWast andGlasser [18]

a [A, ] 6.319 (4) 6.3272 (2) 6.313b [A, ] 5.448 (3) 5.4471 (1) 5.457

4.940 (3) 4.943c [A, ] 4.9483 (1)

170.54 170.3170.0Vol. [A, 3]

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Fig. 3. AFM image of Li3VO4 film surface prepared on a vana-dium metal substrate by the hydrothermal-electrochemical treat-ment in the aqueous solution of 4 M LiOH for 24 h at 150 °C.

3.2. Fabrication and characterization of rear earthdoped YVO4 using hydrothermal-electrochemicalreaction

After confirmation that the hydrothermal-electro-chemical reactions could produce stoichiometricvanadates crystalline films in Li3VO4, we have ap-plied this method to prepare Eu3+:YVO4 films. Un-der the investigated conditions only single phaseEu3+:YVO4 has been prepared both in the autoclaveand the flow-system. As shown in Fig. 4, the X-raydiffraction pattern of the prepared film shows singlephase of YVO4 with tetragonal zircon structure.However, it is difficult to detect doped europium inYVO4 using only X-ray diffraction method.

The luminescent spectra shown in Fig. 5 demon-strates that the Eu3+ has been doped in YVO4

crystal. The most intense transitions observed in theluminescence spectrum originate from the 5D0 levelof Eu3+, which is not split by the crystal field.However, an electric dipole results in multiple transi-tion to 7F sublevels, one sublevel of 7F1, two sublev-els of 7F2, three sublevels of 7F3 and three sublevelsof 7F4, in this spectrum range [22]. In accordancewith Judd-Ofelt theory, transitions to even J-num-bers have much higher intensity than those to corre-sponding neighboring odd J-numbers. The mostintense luminescence lines correspond to the 5D0-7F2

transitions. From these results we can conclude thatdirectly fabricated Eu3+:YVO4 films from the solu-

Fig. 4. X-ray diffraction of Eu:YVO4 film prepared on a vana-dium metal substrate by the hydrothermal-electrochemical treat-ment in the aqueous solution of 0.025 M Eu(NO3)3:Y(NO3)3 (1:9)for 24 h at 150 °C.

Fig. 5. Luminescence spectrum of fabricated Eu:YVO4 film on avanadium substrate by the hydrothermal treatment in the auto-clave at 150 °C, 3 h.

phology of crystals might be controlled by someexperimental conditions such as current density, tem-perature, species, concentration and supply of solu-tions etc. In fact, the crystal size increased at highertemperatures, by higher current densities, in higherconcentrations and flow rates. The details of crystalgrowth of Li3VO4 will be reported elsewhere [21].

T. Watanabe et al. / Solid State Sciences 3 (2001) 183–188 187

Fig. 6. Laser microscope of Eu:YVO4 films on vanadium substrateprepared by different flow rate. (A) 1 mL min–1 (B) 10 mL min–1

(C) 1 mL min–1 with 1 mA cm–2 electric current, respectively.

fabricated by hydrothermal and/or hydrothermal-electrochemical method using the flow-system. Thesolution flow rate were (A) 1 mL min–1, (B) 10 mLmin–1, (C) 1 mL min–1 with 1 mA cm–2 electriccurrent, respectively. The crystals are (A) rod-likewith the size of 1 mm width and 15 mm long, (B)polyhedra with 1 mm size and (C) polyhedra with1 mm size. These results clearly indicate that higherflow-rate and applying an electric current causesdecreases in grain size and changes of crystal mor-phologies. It can probably be explained due tohigher solubility of VO4

3– in hydrothermal conditionsthat might lead to increases of nucleation on thesubstrate. These findings are in opposition to previ-ous research in BaTiO3 (SrTiO3) cases [23,24], whereincreased flow rates resulted in grain growths. It wasmainly due to lower solubility of titanate species;thus the increase of the growth rate required a highersupply of titanate species by increased flow rate.

In conclusion, we have succeeded to control crys-tal morphology of various vanadate films using dif-ferent flow rate and electric current in theflow-system.

4. Summary

We have succeeded to prepare lithium vanadatefilms directly on vanadium metal substrates usinglithium hydroxide aqueous solutions in the autoclaveand/or flow-system reactor at a low temperature of150 °C. The X-ray diffraction patterns indicate thatour hydrothermal-electrochemical products shouldbe stoichiometric as well as the solid state reactedproducts at high temperature.

As an application of the hydrothermal-electro-chemical reaction, we fabricated europium dopedyttrium vanadate films directly on the vanadiumsubstrate. The fabricated Eu3+:YVO4 films showedsufficient intensity of red luminescent by UV-lightirradiation at room temperature. In addition, the sizeand morphology of the Eu3+:YVO4 crystals werecontrolled by changing solution flow rate and electriccurrent density. The laser microscope image of Eu3+

:YVO4 films clearly indicates that higher flow-rateand applying electric current resulted in decreases ofgrain size and changes of crystal morphologies.

The flow system can be closed, enabling easyrecycling of the solutions. This processing route mayserve as an inexpensive, environmentally friendly and

tion process have sufficient photo luminescentproperty.

Moreover, we can control the size and morphol-ogy of Eu3+:YVO4 crystals in the films. Figure 6shows morphology difference of Eu3+:YVO4 films

T. Watanabe et al. / Solid State Sciences 3 (2001) 183–188188

direct fabrication of various materials including thevanadium double oxide system.

Acknowledgements

This research was supported by the ‘Research forthe Future’ Program No. 96R06901 of the JapaneseSociety for the Promotion of Science (JSPS). Theauthors are greatly indebted to Prof. M. Kakihana,S. Mizuno and T. Asai for fluorescent experimentsand to Dr K. S. Han, Dr P. Krtil, Dr S.W. Song, DrT. Fujiwara, Mr R. Teranishi and Mr H. Uono forstimulating discussions.

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