1

A ZnO Nanorod Based Layered ZnO/64°

YX LiNbO3 SAW Hydrogen Gas Sensor

A. Z. Sadek*1, W. Wlodarski1, Y. X. Li2, W. Yu2, X. Li2, X. Yu2 and K. Kalantar-zadeh1

1RMIT University, School of Electrical and Computer Eng., Melbourne, AUSTRALIA

Phone: (+61 3) 99253690, Fax: (61 3) 99252007, *Email: sadek@ieee.org

2Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, CHINA

Abstract— A zinc oxide (ZnO) nanorod based surface acoustic wave (SAW) sensor

has been developed and investigated towards hydrogen (H2) gas. The ZnO nanorods

were deposited onto a layered ZnO/64° YX LiNbO3 substrate using a liquid solution

method. Micro-characterization results revealed that the diameters of ZnO nanorods

are around 100 and 40 nm on LiNbO3 and Au (metallization for electrodes),

respectively. The sensor was exposed to different concentrations of H2 in synthetic air

at operating temperatures between 200°C and 300°C. The study showed that the

sensor responded with highest frequency shift at 265°C. At this temperature, stable

base-line and fast response and recovery were observed.

Keywords: ZnO, Nanorod, Gas sensor, H2

2

I. INTRODUCTION

Hydrogen has numerous applications in industry, such as petroleum distillation,

chemical production, cryogenic cooling, semiconductor manufacturing processes, fuel

cell technology, rocket engines [1]. Its wide range of applications has instigated

researchers to develop highly sensitive, stable, specific, robust, selective and

affordable hydrogen sensors that enable its safe and accurate use. Various types of

sensor technologies, such as electrochemical, conductometric, Schottky junction, field

effect, surface and bulk acoustic wave (SAW and BAW), optical, and combination of

these have been developed and utilized. Out of them, conductometric sensors based

on semiconductor metal oxide thin films have been extensively used for gas sensing

based on film conductivity changes caused by interaction with gas molecules [2-3]. The

gas sensing mechanism involves chemisorption of oxygen on the oxide surface

followed by charge transfer during the reaction of chemisorbed oxygen with target gas

molecules [4]. Recently, there is an increasing trend to utilize nanostructured materials

as gas sensing elements [2-3]. The use of nanoparticles, nanobelts, and nanowires in

gas sensing offers high surface to volume ratios and unique structural features that are

expected to enhance the properties and performance of gas sensors [2-3]. For

conductometric chemical sensors, it was observed that the sensing properties

improved with reducing the size of the oxide particles [5]. Nanostructured oxide based

chemical sensors were also found to respond extremely fast due to rapid diffusion of

gaseous species into the materials’ nano- and micro porosities [2].

Among the semiconductor metal oxides, ZnO was one of the earliest discovered and

is the most widely applied oxide gas sensing materials due to its high mobility of

conduction electrons and good chemical and thermal stability under operating

conditions [6]. ZnO gas sensors have been fabricated in various forms, including single

3

crystals, sintered pellets, thick films, thin films and hetero-junctions which were studied

to detect H2 [7], NO2 [8], NH3 [9], CH4 [10], O2 [11], CO [8] and ethanol [12]. It is

envisioned the ordered ZnO nanostructure has the potential to enhance performance

of the gas sensors. As a result, the interest in synthesis well-aligned ZnO nanorods on

substrate keeps growing. Well aligned ZnO nanorods, with a distinct structural

morphology, characterized by a hexagonal or circular cross section has high surface to

volume ratios and its nanoscale dimensions provide a complete diffusion of carriers

inside the rod which increase the intensity of redox reactions. Low dimensional ZnO

nanorod based thin film gas sensors have been studied by a number of researchers

[13-14] and reported that enhance performance has been achieved compared to

coarse micro-grained ZnO sensors. There are numerous vapor-phase methods such

as metal organic chemical vapor deposition (MOCVD) [15], pulse laser deposition [16],

thermal evaporation [17] and solution methods such as cathodic electro-deposition

[18], thermal decomposition [19] and hydrothermal synthesis [20] have been employed

successfully for the growth of well-aligned ZnO nanorods and nanowires.

In this work, we deposited ZnO seed layer on a 64° YX LiNbO3 SAW substrate using

radio frequency (RF) sputtering. Then, hydrothermal growth of high quality densely

packed and well oriented single crystalline ZnO nanorods with average diameters of

about 100 nm on LiNbO3 and 40 nm on gold have been achieved. In SAW device,

change in electrical conductivity perturbs the electro-acoustic properties of the

propagating acoustic wave, resulting in a change in the velocity of the acoustic wave

propagation [21]. Deviations in velocity are monitored by measuring the changes in

frequency. This change in frequency is directly proportional to the gas concentration in

the environment. The developed nanorod based sensor then investigated for its

response towards H2 gas between 200°C and 300°C.

4

II. EXPERIMENTAL

In this work, a layered SAW device is used as the transducing platform. The

substrate is 64° YX LiNbO3 with an intermediate ZnO seed layer. A shear-horizontal

(SH) leaky surface acoustic wave is the dominant mode in this layered substrate. The

sensor consisted of two-port resonators with 38 electrode pairs in the input and output

inter digital transducers (IDT), 160 electrodes in each reflective grating, 700 µm

aperture width and a periodicity of 40 µm. The center-to-center distance between the

IDTs was 1920 µm. The IDTs were formed by patterning an 80 nm layer of gold and a

20 nm titanium layer. The titanium layer is added to improve adhesion of the gold film.

A fabricated dual SAW transducer is shown in Fig. 1.

To form a layered ZnO/64° YX LiNbO3 substrate, a 1.2 µm ZnO layer was

deposited using RF sputtering. The ZnO layer was deposited from a 99.99% pure ZnO

target with RF power of 120 W. The sputtering gas was 40% O2 in Ar with a pressure

of 10-2 torr, a substrate temperature of 260°C and a deposition time of 60 minutes. This

transparent ZnO layer acts as a seeding layer for the subsequent growth of the ZnO

nanorods.

To grow ZnO nanorod on the layered substrate, an aqueous solution of zinc nitrate

hexa-hydrate (Zn(NO3)2.6H2O, 0.0125M) and NaOH (0.5M) with a mole ratio of 1: 40

was prepared as the standard precursor solution. The sputtered ZnO-coated substrate

was rinsed with de-ionized water, and then submerged in the precursor solution. The

growth temperature was 70°C and deposition time was 60 minutes. The procedure was

similar to that described in the literature [22]. After growth, the sample was rinsed by

de-ionized water and dried at 60°C for several hours before characterization.

5

The gas sensor consist of two important physical components: the ZnO nanorods

sensitive layer, which interacts with the gas media by changing conductivity, and the

SAW transducer which changes its operating frequency in response to this conductivity

change. Using the layered SAW device as a positive feedback element in a closed loop

circuit with an amplifier, an oscillator was formed. A Fluke high-resolution counter

(PM66860B) was used to measure the operational frequency of the sensor and found

to be approximately 106.9 MHz in dry synthetic air at 265°C.

The sensor was mounted inside an enclosed environmental cell. Heating for the

device was provided by a micro heater fabricated on a sapphire substrate with a

patterned platinum resistive element. The heater was placed beneath the SAW sensor

and controlled by a regulated DC power supply. A computerized mass flow controller

(MFC) system was used to vary the concentration of H2 in synthetic air. The gas

mixture was delivered at a constant flow rate of 0.2 liters per minute. Gas exposure

time was fixed for each pulse of H2 gas and the cell was purged with synthetic air

between each pulse to allow the surface of the sensor to recover to atmospheric

conditions. The sensor was exposed to a hydrogen gas pulse sequence of 0.05%,

0.1%, 0.15% and 0.1% concentrations at operating temperatures between 200 and

300°C.

III. RESULTS

Structural Characterization: Scanning electron microscope (SEM) images for

sputtered ZnO seed layer growth on bare LiNbO3 (left) and gold (right) are shown in

Fig. 2 [23]. The SEM images reveal that on seed layer, the average diameters of ZnO

particles are 40 and 100 nm on LiNbO3 and gold, respectively. The SEM images of

ZnO nanorods are shown in Figs. 3 to 6 and indicate that the ZnO nanorods density

6

and morphology are different on the gold and LiNbO3 surfaces. The nanorods are

densely packed and vertically grown on top of gold IDTs (Fig. 6). On LiNbO3, the

growth direction of nanorods has an angle with the surface (Fig. 5). The diameters of

ZnO nanorods are around 100 nm on gold and 40 nm on LiNbO3, respectively. The

SEM images also reveal that the surface of the nanorod thin film is highly porous which

is essential for gas sensing.

The mechanism for nanorod growth is similar for almost all solution methods where

a seed layer of ZnO is usually pre-deposited on the substrate to promote nucleation

and subsequent ordered nanorod growth. Supersaturation condition is required for

crystal growth on top of seed layer [22]. It has been reported that in hydrothermal

synthesis the texture of the seed surface has great influence on the morphology and

the alignment ordering of the ZnO nanorod arrays. In addition, the preparing conditions

such as precursor concentration, growth temperature and deposition times also have

an influence on the morphology of ZnO nanorods [20, 24].

Electrical Characterization: The sensor requires an elevated operating

temperature to enhance chemisorption of oxygen species to achieve the optimum

sensitivity [4]. The optimal operating temperature of the ZnO nanorod based sensor for

H2 sensing was found to be 265°C. Dynamic response of the ZnO nanorod based

sensor towards different concentrations of H2 at 265°C is shown in Fig. 7. The sensor

response is defined as the variation in operating frequency of oscillation due to the

interaction with the target gas. The measured response was 274 kHz towards 0.15% of

H2 at 265°C with response and recovery time of 28 and 36 sec, respectively. The

response magnitude variation for the sensor to different H2 concentrations is shown in

7

Fig. 8. Frequency shift increases almost linearly with the increase of H2 concentration.

Frequency shift versus operating temperature for 0.15% H2 is shown in Fig. 9.

The sensing mechanism is based on the reactions which occur at the sensor

surface between the surface of nanorods and the H2 molecules to be detected. It is

well known that in an air environment, oxygen molecules adsorb onto the surface of the

ZnO layer to form O2-, O-, O2- ions depending on temperature by extracting electrons

from the conduction band [4]. The positively charged surface state and negatively

charged adsorbed oxygen ions form a depletion region at the surface. Reducing gas

such as H2 gets oxidized to H2O consuming chemisorbed oxygen from the sensor

surface by releasing electrons into the conduction band. This mechanism results in a

reduction of surface depletion region to increase the film conductivity which

corresponds to the gas concentration.

Short term reproducibility was observed as indicated when a second pulse of 0.1%

H2 was introduced into the sensor chamber. It was found that the ZnO nanorod based

sensor produce repeatable responses of the same magnitude with a stable baseline.

IV. CONCLUSIONS

SAW gas sensor has been fabricated based on ZnO nanorod synthesized by simple

hydrothermal process. The sensor has been investigated for its response to H2 gas at

different operating temperatures between 200 and 300°C. The sensor shows a

repeatable and large response towards H2. Study shows that the optimum operating

temperatures for the sensor are in the range of 260-270°C. Measured sensor response

was 274 KHz towards 0.15% of H2 at 265°C. The results demonstrate that the

8

developed sensors are promising for industrial applications in the respective

concentration range.

REFERENCES

[1] X. Bevenot, A. Trouillet, C. Veillas, H. Gagnaire, M. Clement, Sens. Actuators B:

Chem. 67 (2000) 57.

[2] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z. L. Wang, Appl. Phys. Lett. 81

(2002) 1869.

[3] A. A. Tomchenko, G. P. Harmer, B. T. Marquis, Sens. Actuators B: Chem. 108

(2005) 41.

[4] S. R. Morrison, Chemical sensors, in semiconductor sensors, John Wiley, New

York, 1994.

[5] P. K. Dutta, A. Ginwalla, B. Hogg, B. R. Patton, B. Chwieroth, Z. Liang, P. Gouma,

M. Mills, S. Akbar, J. Phys. Chem. B 103 (1999) 4412.

[6] N. Yamazoe, G. Sakai, K. Shimanoe, Catal. Surveys Asia 1 (2003) 63.

[7] P. Mitra, A. P. Chatterjee, H. S. Maiti, Mat. Lett. 35 (1998) 33.

[8] H. M. Lin, S. Tzeng, P. Hsiau, W. Tsai, Nanostruct. Mater. 10 (1998) 465.

[9] G. Rao, D. Rao, Sens. Actuators B: Chem. 23 (1999) 181.

[10] A. P. Chatterjee, P. Mitra, A. K. Mukhopadhyay, J. Mater. Sci. 34 (1999) 4225.

[11] G. Sberveglieri, P. Nelli, S. Groppelli, Mater. Sci. Eng. B 7 (1990) 63.

[12] B. B. Rao, Mater. Chem. Phy. 64 (2000) 62.

[13] H. T. Wang, B. S. Kang, F. Ren, L. C. Tien, P. W. Sadik, D. P. Norton, Appl.

Phy. Lett. 86 (2005) 1.

[14] J. Xu, Y. Chen, Y. Li, J. Shen, J. Mater. Scien. 40 (2005) 2919.

9

[15] W. I. Park, G. –C. Yi, J. -W. Kim, S. -M. Park, Appl. Phys. Lett. 82 (2003) 4358.

[16] J. H. Choi, H. Tabata, T. Kawai, J. Cryst. Growth 226 (2001) 493.

[17] V. A. Roy, A. B. Djurisic, W. K. Chan, J. Gao, H. F. Lui, C. Surya, Appl. Phys.

Lett. 83 (2003) 141.

[18] M. Izaki, T. Omi, J. Electrochem. Soc. 143 (1996) L53.

[19] L. Vayssieres, Adv. Mater. 15 (2003) 464.

[20] H. Q. Le, S. J. Chua, Y. W. Koh, K. P. Loh, E. A. Fitzgerald, J. Cryst. Growth

293 (2006) 36.

[21] A. Z. Sadek, W. Wlodarski, K. Shin, R. B. Kaner, K. Kalantar-zadeh,

Nanotechnology 17 (2006) 4488.

[22] R. B. Peterson, C. L. Fields, B. A. Gregg, Langmuir 20 (2004) 5114.

[23] K. Kalantar-zadeh, D. A. Powell, A. Z. Sadek, W. Wlodarski, Q. B. Yang, Y. X.

Li, Sensor Letters 4 (2006) 135.

[24] J. Zhao, Z. G. Jin, T. Li, X. Liu, Z. F. Liu, J. Amer. Cer. Soc. 89 (2006) 2654.

10

Figure 1. Typical SAW sensor, showing two sets of identical resonators.

12 mm

12 mm

In put Out put

IDT

11

Figure 2. SEM image of sputtered ZnO seed layer growth on bare LiNbO3 (left)

and gold IDT’s (right) [23].

12

Figure 3. SEM image of ZnO nanorods on ZnO/64° YX LiNbO3 substrate.

13

Figure 4. SEM image of ZnO nanorods growth on the interface

of LiNbO3 and gold IDT’s.

14

Figure 5. SEM image of ZnO nanorods growth on LiNbO3.

15

Figure 6. SEM image of ZnO nanorods growth on gold IDT’s.

16

106460000

106560000

106660000

106760000

106860000

106960000

107060000

0 110 220 330 440 550

Time (second)

Op

era

tio

nal

freq

uen

cy (

Hz)

0.05%0.1%

0.15%

0.1%

Figure 7. Dynamic response of the sensor to different H2 gas concentrations

in synthetic air at 265°C.

17

50

100

150

200

250

300

0.03 0.06 0.09 0.12 0.15 0.18

Hydrogen concentration (%)

Fre

qu

en

cy s

hif

t (k

Hz)

Figure 8. Frequency shift (kHz) versus H2 gas concentration (%)

in synthetic air at 265°C.

18

120

160

200

240

280

320

220 230 240 250 260 270 280 290

Temperature (°C)

Fre

qu

en

cy s

hif

t (k

Hz)

Figure 9. Frequency shift (kHz) versus operating temperature for 0.15% H2

concentrations in synthetic air.