Electrophoretic Deposition of ZnO

A. Dogan, E. Suvaci, G. Gunkaya and E. Uzgur

Anadolu University, Department of Materials Science and Engineering, Eskisehir/Turkey

Keywords: Electrophoretic deposition, ZnO, Thin film

Abstract. Electrophoretic phonemenon is applied in different parts of science. Electrophoretic

Deposition (EPD) is effective, fast and controllable process for obtaining various thin or thick film

layers. In this study application of EPD to functional ceramics especially for sensors is aimed. Sub-

micron ZnO powder was synthesized by using homogeneous precipitation method. A 5 wt% ZnO

aqueous suspension prepared for EPD and an anionic dispersant was used as stabilizer. ZnO powder

is deposited on alumina substrate at various voltages which is made conductive. Coated samples

were sintered at 1200°C for two hours. Characterization of the sintered ZnO deposited substrates

were examined by SEM and XRD techniques.

Introduction

Electrophoretic deposition is a coating technique which is searched increasingly demand. By EPD

thin or thick, porous or nonporous films can be obtained on conductive layers. In this study by using

EPD ZnO coatings on alumina substrates were applied. The advantage of EPD when it is compared

by other techniques like CVD, PVD, spray pyrolysis, sputtering, laser ablation, sol-gel method,

MOCVD, PECVD and LPE is being low cost process, simple, rapid, having a high potential for

depositing on to curved or cylindrical shaped bodies in different scales and thicknesses from 10nm

to 1m [1,2].

ZnO is one of the first and most widely used ceramic material for sensor applications. Some of the

ceramic semiconductors such as tin oxide and zinc oxide with controlled porous bodies show an

electrical conductivity as a response to trace amount of hydrocarbon based gases, hydrogen, and

ammonia etc. at elevated temperature in air [3,4]. Gas sensing in n-type semiconductor gas sensors

like ZnO is achieved by the decrease in resistivity as a response to increasing amount of reducing

gases in the air. The reason for decrease in resistivity is explained by desorption of oxygen absorbed

on the surface and grain boundaries of metal-oxides at high temperatures in air [4]. By using EPD

technique small thin film layered sensors can be produced so low power consumption and easy

integration to other devices is provided. Because of its simplicity and versatility as mentioned above

electrophoretic deposition might have potential for being preferable technique for ceramic sensor

applications. Accordingly there are some studies reporting the use of deposited ZnO films for sensor

application. In this study EPD is used for the formation of ZnO thin films on alumina substrates for

sensor applications.

Experimental Procedure

Sub-micron sized ZnO particles were synthesized by using homogenous precipitation method [5].

The synthesized powders were characterized by using a particle size analyzer. It was observed that

the particles were highly agglomerated even in 4-µm size. To disperse the agglomerated particles

grinding process was employed. In this study, two types of milling, conventional and chemical

added milling (CAM) was used to see the effect of dispersants on particle de-agglomeration during

Key Engineering Materials Vols. 264-268 (2004) pp 269-272Online available since 2004/May/15 at www.scientific.net© (2004) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.264-268.269

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Degree (2θ)

(002)

the milling of ZnO particles. The powder was milled for 22 hours after milling particle size was like

in Fig. 1.

Figure 1. Laser particle size analysis of ZnO powder which is synthesized by using homogeneous

precipitation method.

An anionic dispersant was used for the chemical aided milling to stabilize the 5 wt% ZnO aqueous

suspension prepared for Electrophoretic Deposition (EPD). After milling sedimentation experiments

were performed to test the stability of suspensions from each milling process.

After obtaining well dispersed medium, EPD experiments were conducted under constant DC

voltage. High-grade alumina (Coors Ceramics) with 1 in.2

dimension was used as substrate. The

surfaces of the substrates were electroded with gold-palladium by sputtering process. A Pyrex glass

made deposition chamber due to chemical and electrical inertness was used for EPD studies.

Coating was done altering voltage and time by using stable suspension of ZnO. Coating was carried

out in the range of 5-240 V. Coated samples were dried at room temperature to eliminate surface

tension originated cracks [6]. Samples were sintered at 1200°C for two hours. Characteristics of the

sintered ZnO coated substrates were investigated by scanning electron microscope (SEM) and X-ray

difractometer (XRD).

Results and Discussion

XRD pattern of the synthesized powder shows that the formed phase is pure ZnO Fig.2.

0

50

100

150

200

250

25 30 35 40 45 50 55 60 65 70

Figure 2. XRD chart of the synthesized ZnO powder.

(100)

(101)

(102)

(110)

(103) (112)

Intensity

ZnO

270 Euro Ceramics VIII

Stabilization of ZnO is obtained by anionic polyelectrolyte, well dispersion is achieved by charging

surface of the powder negatively. The anionic polyelectrolyte was chosen because ZnO dissolves in

acid.

While the conventional milled particles settle with in about 10 minutes; the particles in CAM

system deflocculates after 24 hours. Prepared suspensions conductivity was 0.87 mS/cm and

pH=8.4 so the stability was enough for EPD. This study shows that chemical milling is required to

achieve stable suspension of ZnO particles. Coating layers were formed with EPD. However, for

sensor applications rather thin porous layers are preferable. For that reason we focused on coating

layers around 10 µm Fig.3. Fig.4 shows SEM photographs of sintered coatings for 2 hours at

1200°C. As it is seen on the figure it is possible to create porous structure. However the pore

distribution was not homogenous, but for sensor application uniform porosity required. As a next

step of this research, homogeneous distribution of nanopores in EPD coated films will be

investigated.

Figure 3. SEM image of Al2O3 substrate and ZnO coating from a different point of view.

Al2O3

ZnO

20 µm

20 µm 10 µm

Fig. Figure 4. SEM images of porous body by EPD after sintering 1200ºC for 2 hours.

Key Engineering Materials Vols. 264-268 271

Density of the sintered body should be changed by using different powders, which have different

grain sizes and sintering temperatures. In this experiment nearly 5 µm powders and 2 µm pores were

get after sintering as seen on Fig.4.

Summary

It has been demonstrated that the EPD technique can be utilized to form ZnO films with ~10 µm

thickness for sensor applications. A stable suspension is necessary for a successful EPD process.

The stable suspension of submicron ZnO particles can be achieved by chemically-aided milling

approach.

Acknowledgements

The authors gratefully thank to C. A. Randall and J. Van Tassel of The Pennsylvania State

University, MRI for their support. They also would like to thank Cem Caglar for the ZnO synthesis.

This study is sponsored by Turkish State Planning Organization DPT (2003K120170).

References

[1] N. OGATA., J. VAN TASSEL and C.A RANDALL: Mater. Let. Vol. 49, (2001), p. 7

[2] W. K. CHOI., S.K. SONG,. J. S. CHO, Y. S. YOON, D. CHOI, H.-J. JUNG and S. K. KOH: A

Sensors and Actuators, Vol. B 40 (1997), p. 21.

[3] G.S. TRIVIKRAMA RAO. and D. TARAKARAMA RAO: Sensors and Actuators, B 55

(1999), p 166.

[4] W. J MOON., J. H. YU and G. M.CHOI Sensors and Actuators, Vol. B 87 (2002), p. 464.

[5] Y. SAKKA., K. HALADA and E. OZAWA., Ceramic Transactions - Ceramic Powder Science

IIA, (1991), p31.

[6] F. TANG, T. UCHIKOSHI and Y. SAKKA: J. Am. Ceram. Soc.: Vol. 85 (2002), p.2161.

272 Euro Ceramics VIII

Euro Ceramics VIII 10.4028/www.scientific.net/KEM.264-268 Electrophoretic Deposition of ZnO 10.4028/www.scientific.net/KEM.264-268.269

DOI References

[2] W. K. CHOI., S.K. SONG,. J. S. CHO, Y. S. YOON, D. CHOI, H.-J. JUNG and S. K. KOH: A ensors and

Actuators, Vol. B 40 (1997), p. 21.

doi:10.1016/S0925-4005(97)80194-3 [4] W. J MOON., J. H. YU and G. M.CHOI Sensors and Actuators, Vol. B 87 (2002), p. 464.

doi:10.1016/S0925-4005(02)00299-X [6] F. TANG, T. UCHIKOSHI and Y. SAKKA: J. Am. Ceram. Soc.: Vol. 85 (2002), p.2161.

doi:10.1111/j.1151-2916.2002.tb00428.x [2] W. K. CHOI., S.K. SONG,. J. S. CHO, Y. S. YOON, D. CHOI, H.-J. JUNG and S. K. KOH: A Sensors

and Actuators, Vol. B 40 (1997), p. 21.

doi:10.1016/S0925-4005(97)80194-3