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Rapid synthesis of ZnO nano-rods by one-step, room-temperature, solid-state reaction and their gas-sensing properties

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2006 Nanotechnology 17 2266 (http://iopscience.iop.org/0957-4484/17/9/032) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 103.55.110.121 This content was downloaded on 03/03/2017 at 10:45 Please note that terms and conditions apply.

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INSTITUTE OF PHYSICS PUBLISHING

NANOTECHNOLOGY

Nanotechnology 17 (2006) 2266–2270

doi:10.1088/0957-4484/17/9/032

Rapid synthesis of ZnO nano-rods by one-step, room-temperature, solid-state reaction and their gas-sensing properties Zhi-Peng Sun, Lang Liu, Li Zhang and Dian-Zeng Jia Institute of Applied Chemistry, XinJiang University, Urumqi, XinJiang 830046, People’s Republic of China E-mail: [email protected]

Received 11 January 2006, in final form 24 February 2006 Published 7 April 2006 Online at stacks.iop.org/Nano/17/2266 Abstract ZnO nano-rods are prepared by one-step solid-state reaction of zinc acetate dihydrate, sodium hydroxide and cetyltrimethylammonium bromide (CTAB) at room temperature. The samples are characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The gas-sensing properties of the prepared material have been investigated. The results indicate that the as-prepared ZnO nano-rods are uniform with diameters of 10–30 nm and lengths of about 150–250 nm. The relatively high sensor signal and stability of sensors made from ZnO nano-rods demonstrate the potential for developing a new class of sensitive sensors. (Some figures in this article are in colour only in the electronic version)

1. Introduction Recently, one-dimensional (1D) nanoscale materials have received considerable attention due to their remarkable properties as applied in optoelectronic and electronic nanodevices [1–3]. In particular, semiconductor nanomaterials such as SnO2 and In2 O3 are very interesting from the point of view of application in gas sensors [4, 5]. Of these, ZnO is recognized as one of the most promising oxide semiconductor materials because of its good optical, electrical and piezoelectrical properties. It can be used in many areas, such as field emission displays, solar cells and gas sensors [6–8]. But up to now many studies have mostly been focused on the gas-sensing properties of ZnO nano-dots [9, 10] and there are still few detailed reports about the gas-sensing properties of ZnO nano-rods. The gas-sensing efficiency of a material depends on its microstructural properties which are related to its method of preparation, the latter playing a very important role with regard to the chemistry, structure and properties of ZnO nanomaterials. In fact, many different methods for synthesizing ZnO nano-rods have been published, such as hydrothermal synthesis, the micro-emulsion hydrothermal process, chemical vapour deposition (CVD) and a catalyst-free CVD method [11–14]. However, these 0957-4484/06/092266+05$30.00

preparation methods are generally complicated and expensive, especially when organo-metallic precursors, catalysts and complex process controls are involved. Micro-emulsion techniques involve the use of large amounts of organic solvents in the reaction medium and are not environmentally friendly. The catalyst-free CVD method, CVD and the hydrothermal method are not suitable for making ZnO nano-rods at lost cost, because the processes are complex and have a high energy consumption. For widespread use of ZnO in many nanoscale devices, an ideal synthesis process should be environmentally friendly and as simple as possible. A novel and rapid preparation technique for nanomaterials, one-step solid-state reaction synthesis at ambient conditions, has been developed in our laboratory [15, 16], which brings the opportunity to study the gas-sensing properties of ZnO nano-rods. In this paper, ZnO nano-rods were prepared by this simple solid-state reaction with the assistance of the surfactant cetyltrimethylammonium bromide (CTAB). The gas-sensing properties of the ZnO nano-rods are also discussed. We found that the process is convenient, environmentally friendly, inexpensive and efficient. As-prepared pure ZnO nanorods had diameters of about 10–30 nm and lengths of 150– 250 nm. Furthermore, the ZnO nano-rods obtained exhibited an excellent gas-sensing sensor signal with test gases.

© 2006 IOP Publishing Ltd Printed in the UK

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Rapid synthesis of ZnO nano-rods by one-step, room-temperature, solid-state reaction and their gas-sensing properties

2. Experimental details 2.1. Synthesis Zinc acetate dihydrate, sodium hydroxide, CTAB and the other reagents used were all analytical grade (from Shanghai Chemical Corp.) without further purification. Manipulations and reactions were carried out in air. In a typical synthesis, zinc acetate dihydrate, CTAB and sodium hydroxide were mixed (molar ratio 1:0.4:3) and ground together in an agate mortar for 50 min at room temperature (25 ◦ C). The reaction started readily during the mixing process, accompanied by the release of heat. The mixture was washed with distilled water in an ultrasonic bath. Finally, the product was dried in air at 60 ◦ C for 2 h.

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2.2. Characterization X-ray diffraction (XRD) was done on a MAC Science MXP18AHF x-ray diffractometer with graphite-mono˚ Transmission chromatized Cu Kα radiation (λ = 1.540 56 A). electron microscope (TEM) images were obtained on a Hitachi H-600 TEM with an accelerating voltage of 100 kV. Scanning electron microscope (SEM) images were obtained on a LEO 1430 VP SEM. 2.3. Measurements of gas-sensing properties Gas sensors made from as-prepared pure ZnO nano-rods, asprepared ZnO nano-rods doped with 0.5 wt% Ag and pure ZnO nano-rods (annealed at 600 ◦ C for 1 h) were designated as Z1, Z2 and Z3, respectively. The gas sensors were made in the conventional way [17–19]. The final powders were dispersed in terpineol which was used as a binder to form pastes. The alumina ceramic tube, which was assembled with platinum wire electrodes for electrical contacts, was dipped into the paste several times to form a gas-sensing film. Then the elements were annealed at 600 ◦ C for 1 h to evaporate the terpineol. Finally, the alumina tube obtained with a Ni–Cd heater fixed inside was welded onto a bakelite substrate. To improve the stability and repeatability, the sensors were aged at 300 ◦ C for 7 days in air prior to use. The gas-sensing properties were examined in a chamber (1500 cm3 ). The resistance of the sensor was measured using a conventional circuit, in which the element was connected with an external resistor in series at a circuit voltage of 10 V. The electrical response of the sensor was measured with an automatic test system, controlled by a personal computer. The sensor signal is defined as β = Rg /Ra , where Rg and Ra is the resistance of the sensor in the testing gases/air mixture and in the air, respectively.

3. Results and discussion 3.1. The structural characterization and synthesis of ZnO Figure 1 shows that the XRD patterns of products Z1, Z2 and Z3, respectively. It can be seen that all of the diffraction peak can be indexed as ZnO with hexagonal phase (JCPDF card file no 361451), No characteristic peaks of impurities such as Zn, CTAB or Zn(OH)2 are observed. The results indicate that ZnO has been successfully prepared by a onestep solid-state reaction at room temperature. The phase of

Figure 2. The SEM images of the ZnO nano-rods.

the sensitive materials is unchanged after doping with 0.5 wt% Ag. Unlike the reactions in the solution, solid-state reactions are carried out directly without solution. So they have a different reaction mechanism from the solution reaction. The products of the solid-state reaction are also different from the products of the solution reaction. Metal oxides are obtained by grinding solid metallic salts with sodium hydroxide, which is different from the results obtained with hydroxides in solution [16]. Considering that hydroxides are decomposed by a strong heat of reaction to produce oxides in the reaction process, we speculate that the mechanism of formation of ZnO under the solid-state reaction condition can be represented as follows: Zn(OH)2 nanoparticles are first obtained from metallic salts and sodium hydroxides. Because they have a smaller diameter and higher reaction activity, freshly produced Zn(OH)2 decomposes to produce ZnO nanoparticles with a strong heat of reaction within a short reaction time. 3.2. The morphology of products A typical SEM image of pure ZnO nano-rods is shown in figure 2. ZnO nano-rods with diameters of 10–30 nm have been synthesized conveniently with this method. The lengths of the ZnO nano-rods are about 150–250 nm. Typical TEM images of ZnO nano-rods before and after annealing are shown in figures 3(a) and (b), respectively. The materials maintain their rod-like morphology even after annealing at 600 ◦ C for 1 h. The length of the nano-rods increases to about 300 nm. 2267

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Figure 3. The TEM images of nano-rods: (a) as-prepared pure ZnO nano-rods, (b) ZnO nano-rods after annealing.

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The formation of the rod-like morphology is due to the presence of a suitable surfactant. Although CTAB is added to the reaction system, it does not react with the reactant and is completely removed by washing with distilled water and alcohol. During the formation of ZnO nano-rods, the CTAB surfactant may provide a long chain reaction interface and induce the nanocrystals to grow in a definite direction. Furthermore, the reaction rate is also slowed down because the surfactant hinders the particles on the two substrates in the reaction system from coming into contact. Therefore, there is enough time for the small particles to assemble into rods. A detailed study of the growth mechanism of metal oxide nanorods is in progress. 3.3. Gas sensor signal of the products In general, for n-type ZnO single crystals the intrinsic carrier concentration is primarily determined by deviation from stoichiometry in the form of interstitial zinc and equilibrium oxygen vacancies, which are predominantly atomic defects. The conduction electrons resulting from the point defects play a major part in the gas sensing properties of the materials. Thus, the electrical conductivity of nanocrystalline ZnO depends strongly on the surface states produced by molecular adsorption that results in changes in the space– charge layer and band modulation. The relation between the resistance and operating temperature of the sensors (in air) is shown in figure 4. In the 240–320 ◦ C region, the resistances (Z1 > Z3 > Z2) of Z1, Z2 and Z3 decrease dramatically. In the 320–380 ◦ C region, the change in resistance is very small. 2268

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Figure 5. The effects of operating temperature on the sensor signal of sensors at 100 ppm C2 H5 OH.

As the conductance of the ZnO nano-rods hardly changes from 330 to 380 ◦ C, Z2 has good thermal stability when its operating temperature is in this range. In addition, the region of thermal stability for Z2 is broader than that for Z1 and Z3. Figure 5 shows that the operating temperatures have an obvious influence on the sensor signal in response to 100 ppm ethanol gas (C2 H5 OH). Each of the curves shows a maximum sensor signal corresponding to an optimum working temperature. The sensors exhibit the highest sensor signal to C2 H5 OH at 332 ◦ C. At an operating temperature of 332 ◦ C, the sensor signal to C2 H5 OH is 10.3, 19.4 and 13.5, for the sensitive materials designated Z1, Z2 and Z3, respectively. If the temperature increases, the sensor signal decreases. The results show that sensor Z2 has a higher sensor signal than Z1 or Z3. Compared to the reported pure ZnO nano-dot sensors, whose sensor signal values are usually less than 5 (to 100 ppm C2 H5 OH) [9]. The sensor signal of pure ZnO nanorods to 100 ppm C2 H5 OH is more than 12. The results indicate that pure ZnO nano-rods have a higher sensor signal than the conventional materials. The sensor signal of ZnO nano-rods doped with 0.5 wt% Ag to various reducing gases (all 100 ppm) like ammonia (NH3 ), hydrogen (H2 ), liquefied petroleum gas (LPG) and C2 H5 OH as a function of operating temperature are presented in figures 6(a)–(c). It is observed that the sensor signals

Rapid synthesis of ZnO nano-rods by one-step, room-temperature, solid-state reaction and their gas-sensing properties 20

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Figure 7. Long-time stability of sensors to 100 ppm C2 H5 OH.

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to different gases go through maxima at different operating temperatures. The maximum sensor signal to C2 H5 OH is obtained by sensor Z2 around 330 ◦ C. Compared to Z2, Z1 and Z3 have a smaller sensor signal to C2 H5 OH around 330 ◦ C. It is evident from figure 6(b) that the noble metal Ag is beneficial for the sensor signal to C2 H5 OH, as Ag usually acts as an efficient catalyst. By controlling the operating temperature of sensor Z2, one can realize the detection of different gases. This type of sensor may find many potential applications in the detection of C2 H5 OH. Most of the semiconductor oxide gas sensors operate on the basis of the modification of the electrical properties of an active element, which is brought about by the adsorption

of an analyte on the surface of the sensor. ZnO nano-rods can adsorb oxygen from the atmosphere, both the O− 2 and the O− species. The adsorption of O− is the most interesting process in sensors, because this oxygen ion is the more reactive and makes the material more sensitive to the presence of reducing gases. At relatively low temperature the surface preferentially adsorbs O2− and the sensor signal of the material is consequently very small. As the temperature increases, the oxygen will be adsorbed on the surface of the ZnO nanorods (denoted as O− ). The adsorbed oxygen and the surface lattice oxygen (O2− ) of the ZnO nano-rods take part in the oxidation of organic molecules. Once the oxidation reaction takes place, electrons will enter into the ZnO nano-rods, resulting in their decreased resistance. The extremely high surface to volume ratios associated with 1D nanostructures may be the reason why the sensor signal of the ZnO nanorod material increases so much. If the temperature increases too much, progressive desorption of all previously adsorbed oxygen ionic species occurs and the sensor signal decreases. The promoting effects of noble metals (or metal oxides) have been widely confirmed in semiconductor gas sensors for inflammable gases [20]. There are two mechanisms of sensitization by metal or metal oxide additives: chemical and electronic sensitization. Chemical sensitization is mediated by a spillover effect, while electronic sensitization is mediated by the direct exchange of electrons between the semiconductor and the metal additives [21–24]. The Ag additive to ZnO nanorod sensors can act as a strong acceptor of electrons from the oxide, and can induce an enlarged surface space charge layer which results in the depletion of electrons near the interface. When the Ag additive is reduced on contact with the target gas, it relaxes the space charge layer by giving electrons back to the oxide. Such a change in the oxidation state of the additives is responsible for the promotion of the gas response. Sensor Z2 shows a higher sensor signal than sensor Z3. The results agree well with the method of electronic sensitization. The long-time stability of those sensors is also detected by repeating the test many times after 30 day’s aging. The results are shown in figure 7. It can be seen that sensors have a nearly constant sensor signal to 100 ppm C2 H5 OH during the test. 2269

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4. Conclusions In summary, ZnO nano-rods have been synthesized using a one-step solid-state reaction technique at room temperature. The key to the formation of the rod shape is the assistance of the surfactant CTAB. SEM and TEM observation revealed that the nano-rods are mainly uniformly short with diameters of 10– 30 nm and lengths of about 150–300 nm. The technique is a convenient, inexpensive and effective method of preparation for ZnO nano-rods. The gas-sensing measurements show that the ZnO nano-rods have excellent potential applications as gas sensors. The sensor signal of sensor Z1 to 100 ppm C2 H5 OH is 10.8. The addition of 0.5 wt% Ag is found to enhance the sensor signal to 100 ppm C2 H5 OH from 10.8 to 19.4, demonstrating that these sensors are applicable to the detection of C2 H5 OH.

Acknowledgments This work was financially supported by the National Science Foundation of China (nos 20366005 and 20462007). The authors wish to thank the referees for their careful reading of the manuscript and helpful suggestions.

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