Fabrication and gas sensing property of honeycomb-like ZnO

Chao Li a,b,*, Zhi Shuo Yu a, Shao Ming Fang a, Huan Xin Wang a,Yang Hai Gui a, Jia Qiang Xu a, Rong Feng Chen b

a Henan Province Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry,

Zhengzhou 450002, Chinab Henan Academy of Science, Zhengzhou 450002, China

Received 20 December 2007

Abstract

We report the structural characterization and proposed formation mechanism of honeycomb-like ZnO conglomerations

fabricated by direct precipitation method. X-ray diffraction (XRD), energy-disperse X-ray spectrometry (EDS), scanning electron

microscopy (SEM) showed that the as-prepared ZnO calcined at 700 8C were micron sphere particles with honeycomb-like

structure. In the UV–vis absorbing spectrum, it was observed that there is a new additional absorption band at 260 nm, and it was

speculated that the absorption may be caused by defects on the surface and interface of honeycomb-like ZnO. The as-products

showed high sensitivity and short response time to sulfured hydrogen gas. These results demonstrate that honeycomb-like ZnO

conglomerations are very promising materials for fabricating H2S gas sensors.

# 2008 Chao Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: ZnO; Precipitation; Honeycomb-like; Gas sensing property

As an important wide band-gap semiconductor, zinc oxide (ZnO) has unique applications [1–11,15] in catalysts,

sensors, piezoelectric transducers and actuators, and photovoltaic and surface acoustic wave devices. Because the size

and morphology of ZnO have great effects on its properties and applications, various size and morphology of ZnO

crystals such as wires [2,3], tubes [4,5], rods [5,6], belts [7], prisms [8], towers [9], dandelions [10] and combs [11] in

nano- or microscales have been fabricated by various physical and chemical techniques. These unique morphologies

and structures may have particular applications in nano- or microscaled optoelectronic devices, sensors, and networks

[2–11,15]. In this work, honeycomb-like ZnO conglomerations were prepared by direct precipitation method for the

first time and it was found that the sensors based on the samples showed high sensitivity and short response time to

sulfured hydrogen H2S gas.

In a typical synthesis process, 6 g zinc powder and 923 mL dilute sulfuric acid (0.05 mol/L) were mixed together

under vigorous stirring. The temperature was kept at 60 8C. After the dilute sulfuric acid was depleted completely, the

reaction of zinc and dilute sulfuric acid stopped, and then 230 mL ammonia solution (0.2 mol/L) was added to the

resulting solution drop by drop under continuous stirring. The produced gray-white precipitates were collected and

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Chinese Chemical Letters 19 (2008) 599–603

* Corresponding author at: Henan Province Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou

450002, China.

E-mail address: zzulilc@zzuli.edu.cn (C. Li).

1001-8417/$ – see front matter # 2008 Chao Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2008.03.032

washed repeatedly with ethanol and distilled water, and then the product was dried in an oven at 100 8C for 10 h. The

following calcination procedure was carried out at 700 8C for 1 h. All the chemicals were of analytical grade and

without further purification.

For fabrication of honeycomb-like ZnO, the molar ratio of Zn and H2SO4 was 2:1, in which Zn was excessive. With

the reaction proceeding, the dilute sulfuric acid was depleted completely. In the process, the surface of the excessive

zinc powder was partly dissolved and the new-formed surface had relative high activity, which was unstable and easily

bonded with other atoms. What’s more, the release of hydrogen left small holes on the excessive zinc powder, which

can serve as nuclei sites and pinning centers sticking the other atoms to the zinc surface. It was noticed that the

honeycomb-like ZnO conglomerations cannot be obtained when Zn was not excessive. After the addition of ammonia

solution, Zn2+ in solution was transformed to Zn(OH)2 and attached easily on the active surface of excessive zinc thus

formed Zn/Zn(OH)2 composite particles. During calcination, Zn(OH)2 decomposed to ZnO at the temperature of

125 8C [12], and then Zn/Zn(OH)2 transformed to Zn/ZnO composite particle. Since Zn has a low melting point of

419 8C [12], it was believed that honeycomb-like ZnO were formed by the thermal evaporation of the Zn core from Zn/

ZnO particle during the increase of calcination temperature. With the thermal evaporation of Zn core, some

sublimation of Zn powders may randomly attach on the ZnO shell and immediately oxidized because they were very

active, and so, the surface of honeycomb-like ZnO conglomerations were be with a variety of irregular pores and

surface defects.

The as-prepared samples were characterized by XRD (D/MAX-bA, Rigaku, using Cu Ka, l = 0.15418 nm), SEM

(JEM-5600 electron microscopes with EDS), UV–vis absorption spectrum (T6 spectrophotometer, Beijing Purkin

General Instrument Co., Ltd.). The typical SEM images of as-synthesized ZnO are given in Fig. 1. By above

synthesization procedure, honeycomb-like ZnO samples were obtained. Fig. 1(a) shows the typical honeycomb-like

ZnO conglomeration is a porous micron sphere with a variety of irregular pores and surface defects, which is

analogous to real honeycomb. The enlarged images of the surface of the honeycomb-like sphere are shown in Fig. 1(b)

and (c). Careful observation can reveal that the bore diameters rang from hundreds of nanometers to several tens

nanometers.

The XRD pattern of the ZnO sample is shown in Fig. 2. The diffraction peak positions in the XRD spectrum of the

products can be indexed to a pure hexagonal ZnO wurtzite structure with lattice constants a = 3.24982(9) A,

c = 5.20661(15) A (JCPDS No. 36-1451). No peak due to other phases is detected, indicating Zn(OH)2 has completely

thermal decomposited. Similar result can also be obtained by EDS pattern.

Fig. 3 shows the UV–vis absorption spectrum of obtained ZnO product. The measured onset of the absorption curve

gives a value at about 373 nm (3.26 eV), which is close to the band-gap of ZnO 1s–1s electron transition (3.37 eV).

Meanwhile, it was observed that there is also a weak absorption band at 260 nm in the spectrum. The appearance of

unusual additional absorption bands of ZnO has not been reported to our knowledge. Many authors [13,14] have

systematically investigated additional optical absorption in single crystals of a-Al2O3. According to their results, it is

reasonable to assume, that the new additional absorption band at 260 nm in the spectrum may be caused by the color

center with high concentration converted by numerous defects, such as vacancy sites, cavities and impurities in the

large surface of the as-products.

C. Li et al. / Chinese Chemical Letters 19 (2008) 599–603600

Fig. 1. The typical SEM images of the as-synthesized ZnO.

We selected four kinds of reducing gases as a detecting gas to characterize the gas sensing properties of samples.

The four reducing gases include H2S, CH4, CO and H2, whose concentration are all 50 ppm. The test was operated in a

measuring system of HW-30A (Hanwei Electronics Co. Ltd., Henan, PR China). The gas sensitivity was measured in

static state. The basic fabrication process of gas sensors based on as-prepared samples is as literature [1]. The circuit

voltage was 10 V, and outputs Vout were the terminal voltage of the load resistor. The working temperature of a sensor

was adjusted through varying the heating voltage. The resistance of a sensor in air or test gas was measured by

monitoring Vout. The gas sensitivity (response magnitude) in this paper was defined as S = Ra/Rg, where Ra and Rg

were the resistances of a sensor in air and in a test gas, respectively. The response time was defined as the time required

for the variation in conductance to reach 90% of the equilibrium.

Column chart in Fig. 4 shows the comparison of sensitivity of the sensor to four reducing gases at 300 8C. We can

see that the resistance response is 26 to H2S gas, much higher than other three gases. Hence it can be determined that

the material would have a good interface resistance property as a H2S detection. It is well known that the sensing

mechanism of ZnO belongs to the surface-controlled type [1]. When ZnO was contacted with H2S gas, the strong

reducing gas may react with O2x� easily and put back the electrons to the semiconductor, and thereby the resistance of

ZnO would decrease. Its gas sensitivity is relative to grain size, surface state, oxygen adsorption quantity, active energy

of oxygen adsorption and lattice defects. Compared to the sensors based on non-pores ZnO prepared by direct

precipitation, whose sensitivity values were usually less than 15, the honeycomb-like ZnO sensors exhibited a higher

sensitivity. It is confirmed that the surface activity of the semiconductor material was enhanced as the surface area is

C. Li et al. / Chinese Chemical Letters 19 (2008) 599–603 601

Fig. 2. The XRD pattern of the as-synthesized ZnO.

Fig. 3. The UV–vis spectrum of the as-synthesized ZnO.

increased [1,15]. Fig. 5 shows the sensing and recovery test of ZnO for H2S carried out under the conditions:

Vh = 5.5 V, Rl = 4.7 kV, relative humidity = 48% at room temperature 18 8C. From this curve, we can see that the

response time is 12 s, quick enough to satisfy users’ requirement.

Acknowledgments

The financial support from the National Natural Science Foundation of China (No. 20771095) and He’nan

Outstanding Youth Science Fund (No. 0612002700) is gratefully acknowledged.

References

[1] J.Q. Xu, Y.P. Chen, D.Y. Chen, et al. Sens. Actuators B 113 (2006) 526.

[2] M.H. Huang, S. Mao, H. Feick, et al. Science 292 (2001) (1897).

[3] Q. Xiang, Q.Y. Pan, J.Q. Xu, et al. Chin. J. Inorg. Chem. 23 (2007) 369.

[4] X. Kong, X. Sun, X. Li, et al. Mater. Chem. Phys. 82 (2003) 997.

[5] M. Lucas, W. Mai, R. Yang, et al. Nano. Lett. 7 (2007) 1314.

[6] X. An, C. Cao, H. Zhu, J. Cryst. Growth 308 (2007) 340.

[7] H. Yu, Z. Zhang, M. Han, X. Hao, F. Zhu, J. Am. Chem. Soc. 127 (2005) 2378.

[8] H.Y. Xu, H. Wang, Y.C. Zhang, et al. Ceram. Int. 30 (2004) 93.

[9] F. Wang, L. Cao, A. Pan, et al. J. Phys. Chem. C 111 (2007) 7655.

[10] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 16744.

C. Li et al. / Chinese Chemical Letters 19 (2008) 599–603602

Fig. 4. Sensitivity of the as-synthesized ZnO curve to different gases of sensor.

Fig. 5. Typical response and reversion of ZnO sensor to H2S.

[11] C. Li, G. Fang, F. Su, et al. Cryst. Growth Des. 6 (2006) 2588.

[12] D.R. Lide, Hand Book of Chemistry and Physics, CRC Press, Boca Raton, New York, 1997–1998, pp. 4–97.

[13] W.L. Paul, Phys. Rev. 123 (1961) 1226.

[14] C.M. Mo, L. Zhang, Z. Yuan, Nanostruct. Mater 5 (1995) 95.

[15] J.Q. Xu, Y.P. Chen, J.N. Shen, J. Nanosci. Nanotechnol. 6 (2006) 248.

C. Li et al. / Chinese Chemical Letters 19 (2008) 599–603 603