Microwave assisted hydrothermal synthesis of single crystalline ZnO nanorods for gas sensor application Prabhakar Rai, Hyeon-Min Song, Yun-Su Kim, Min-Kyung Song, Pyong-Rok Oh, Jeong-Mo Yoon, Yeon-Tae Yu ⁎ Division of Advanced Materials Engineering and Research Centre for Advanced Materials Development, College of Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea abstractarticle info Article history: Received 9 August 2011 Accepted 6 October 2011 Available online 13 October 2011 Keywords: Ceramics ZnO nanorods Microwave-assisted hydrothermal synthesis Single crystalline Gas sensing property Single crystalline ZnO nanorods were synthesized via microwave assisted hydrothermal method using zinc hydroxide as starting material, cetyltrimethylammonium bromide (CTAB) as structure directing agents, and water as solvent. As synthesized ZnO was characterized using X-ray powder diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The SEM images showed that the diameter of the nanorods falls in the range of about 100–150 nm and length in the range of 1–2 μm. The TEM study showed that as prepared ZnO nanostructures are single crystalline in nature. Gas sensors were prepared and tested for the detection of CO (1000–200 ppm), ethanol and acetaldehyde (250–50 ppm) in air. Results demonstrate that as synthesized ZnO nanorods can be selectively used for the detection of ethanol at 200 °C testing temperature. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Metal-oxide semiconductor (MOS) based gas sensors have been widely used in industry, ecology, medicine, etc. [1– 3]. ZnO is an im- portant semiconductor material with extensive applications in elec- tronics, photoelectronics, sensors, and optical devices. It is well known for its wide band gap of 3.37 eV, large exciton binding energy of 60 meV at room temperature, and excellent chemical and thermal stability. ZnO has been proved to be a kind of highly sensitive material for the flammable or toxic gas detection [4,5]. Recently, 1D ZnO nano- crystals are considered to be the most promising highly sensing mate- rials of sensors due to the slow electron/hole recombination rate, as well as their high surface-to-volume ratio. Extensive studies have been put on improving the sensing performance of 1D ZnO based gas sensors. Consequently, synthesis of 1D ZnO nanostructure with different morphologies and sizes is of significant importance from the basic fundamental research and the novel devices development [6]. To date, various synthetic approaches have been developed to synthesize 1D ZnO nanocrystals [7–10]. Microwave irradiation as a heating method has found a number of applications in chemistry. As an environmentally benign technology with wide applications, mi- crowave synthesis has the advantages of homogeneous volumetric heating, and high reaction rate. Here, we report a simple microwave assisted hydrothermal meth- od to synthesize ZnO nanorods in short reaction time and investigate its gas sensing properties. The results demonstrate that as prepared ZnO nanorods can be used for the selective detection of ethanol gas at low operating temperature. 2. Experimental ZnO nanorods were synthesized similar to our previous work with a little modification [11]. In a typical procedure, Zn(NO 3 ) 2 ·6H 2 O (Re- agent Grade, 98% Sigma-Aldrich) was dissolved in 100 ml of deio- nized water to make 0.1 M solution. Ammonia water was added to this solution to make pH 7, followed by vigorous stirring for 1 h. A white precipitate (Zn(OH) 2 ) was produced which was collected by centrifugation and washed thoroughly with deionized water. Then the precipitate was transferred into 35 ml water containing 0.1 M of CTAB (Aldrich) and charged in a 100 ml capacity autoclave with Tef- lon liner. Microwave assisted hydrothermal reaction was carried out at 150 °C for 1 h in microwave oven (MARS; CEM). After completion of the reaction, it was cooled to room temperature and powder sam- ple was collected by centrifugation. Powdered sample was thorough- ly washed with deionized water and ethanol. Finally, sample was dried at 80 °C for 12 h. The crystal structure of the powder was studied by powder X-ray diffraction (D/Max 2005, Rigaku), and Raman spectroscopy (Perkin Elmer-Spectrum GX). The particles morphology was investigated by scanning electron microscopy (SEM; JSM-5900, JEOL) and transmis- sion electron microscopy (TEM-JEM-2010, JEOL). Materials Letters 68 (2012) 90–93 ⁎ Corresponding author. Tel.: +82 63 270 2288; fax: +82 63 270 2305. E-mail address: yeontaeyu@chonbuk.ac.kr (Y T. Yu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.10.029 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet To measure the sensor response of the ZnO nanorods for CO/ ethanol/acetaldehyde gases, a sensor device was prepared as follows; ZnO powder (0.1 g) was mixed with α-terpinol (500 μl) and grinded in agate mortar for 30 min. The ZnO paste was pasted by doctor blad- ing method onto the cleaned alumina circuit board with interdigitat- ed platinum electrodes (Fig. S1). The device was dried at 80 °C for 5 h. The ZnO loaded device was sintered at 500 °C in an electric furnace for 5 h. The change in resistance of the device due to the presence of test gas was measured using a high resistance meter (Agilent 34970A). The device was tested at the temperature range of 100 to 400 °C at various concentrations of CO (200–1000 ppm) and etha- nol/acetaldehyde (50–250 ppm) in a temperature-controlled envi- ronment. The balance gas was N 2 , and air was mixed to be 5% of oxygen. The total gas flow rate was 100 ml/min. Before injection of test gas, device was stabilized in air for 2 h, to stabilize the base line. The sensor response (R s ) was calculated using (R a /R g ), where R a is the resistance in air with 5% O 2 , and R g is the resistance in the test and mixed gas. 3. Results and discussion Typical powder XRD pattern of as prepared ZnO nanostructure is shown in Fig. 1a. All the diffraction peaks are indexed to the hexago- nal phase of ZnO (JCPDS 36-1451) and no other crystalline phases are detected. The Raman spectra are sensitive to crystallization, structural disorder, and defects in micro- and nanostructures. As shown in Fig. 1b, the presence of a sharp and strong non-polar optical phonon E 2H mode at 438 cm − 1 confirms that the products formed are wurt- zite hexagonal ZnO. Two very small peaks at 332 and 376 cm − 1 were also obtained which were assigned as E 2H –E 2L (multi-phonon) and A 1T modes, respectively [12]. The absence of the E 1 (LO) mode of ZnO crystal at 583 cm − 1 which is associated with oxygen deficien- cy reveals the high quality of the ZnO nanorods [13,14]. SEM images of ZnO nanorods are shown in Fig. 1c which shows the formation of well dispersed ZnO nanorods. The length and width of these ZnO nanorods varied from 1–2 μm and 100–150 nm respectively. The morphology and structure of the ZnO nanostructures have been char- acterized in further detail using TEM. The TEM images of as prepared ZnO nanostructures are shown in Fig. 1d and their corresponding high resolution TEM (HRTEM) images are shown in Fig. 1e, along with selected area electron diffraction (SAED) pattern (inset). The HRTEM image of as prepared ZnO nanostructures revealed a spacing of 0.281 nm between two fringes, which corresponds to the (01–10) plane of the bulk wurtzite ZnO crystal. From the HR-TEM images and SAED patterns, it is seen that these ZnO nanorods are single crys- talline in nature, with a growth direction along the c-axis. Fig. 2 shows the resistance change of ZnO nanorods at various temperatures and concentrations of test gases. It shows that resis- tance is very high at low temperature and it decreases with increasing testing temperature. This generally happens due to the increase of electron concentration on the surface of ZnO nanorods with increas- ing temperature. Furthermore, the response increased and recovery time decreased with increasing testing temperature as well as gas concentration in case of CO gas while in case of ethanol and acetalde- hyde, response as well as recovery time both increased. These ZnO nanorods show response as low as 100 °C for CO and ethanol gas but for acetaldehyde it does not show response below 200 °C (resis- tance change for CO and ethanol at 100 °C is shown in Fig. S2). Although, the concentration of ethanol is lower than the CO gas but it shows better response at almost every test temperature. However, recovery time for CO is better than the ethanol at every testing tem- perature. Furthermore, reproducibility is an important factor for gas sensor and it is checked by repeating the test three times at every test temperature for 1000 ppm of CO and 250 ppm of ethanol as well as acetaldehyde. As can be seen from Fig. 2, ZnO nanorods show good reproducibility at every testing temperature for these gases. Fig. 3a shows the comparison of response of ZnO for 1000 ppm of CO and 250 ppm of ethanol as well as acetaldehyde at different test- ing temperatures. The highest response for CO gas is 4.93 at 200 °C testing temperature. The response increases from 100 to 200 °C and then sharply decreased at 300 °C while again increased at 400 °C. Fig. 1. (a) XRD pattern of as prepared ZnO nanorods; (b) Raman spectroscopy; (c) SEM; (d) TEM; and (e) HRTEM along with SAED patterns. 91P. Rai et al. / Materials Letters 68 (2012) 90–93 ZnO nanorods show good response even at 150 °C and it was 4.80. However, in case of ethanol the highest response was 6.83 at 400 °C. In this case also response increased with increasing tempera- ture except 300 °C. Similarly, in case of acetaldehyde also, the highest response was obtained at 400 °C and it was 5.30. However, in this case response decreased linearly with decreasing testing tempera- ture. Furthermore, selectivity is an important issue for the real use of metal oxide gas sensor. Therefore, response of ZnO nanorods for 200 ppm of CO ethanol and acetaldehyde is compared at different testing temperatures. The response for ethanol is higher at every test- ing temperature as compared to CO and acetaldehyde as shown in Fig. 3b. For each gas, response increased with increasing testing tem- perature except 300 °C. At 200 °C testing temperature the response for ethanol (3.7) is almost three times higher than the CO (1.36) and acetaldehyde (1.05). Hence as prepared ZnO nanorods can be selectively used for ethanol at 200 °C testing temperature. 4. Conclusions Single crystalline, ZnO nanorods are synthesized by microwave assisted hydrothermal method in short reaction time. These ZnO nanorods are applied for the sensing of three reducing gases namely CO, ethanol and acetaldehyde. These ZnO nanorod gas sensors showed high response for these gases and their response increased with increasing gas concentration as well as testing temperature. For low concentration of gas response for ethanol was better than the CO and acetaldehyde gas at low testing temperature. Fig. 2. Resistance change of ZnO nanorods at various temperatures and concentrations of either CO, ethanol or acetaldehyde gas. Fig. 3. Response of ZnO nanorods at different testing temperatures for (a) CO (1000 ppm), ethanol (250 ppm) and acetaldehyde (250 ppm) (b) 200 ppm of CO, ethanol and acetaldehyde. 92 P. Rai et al. / Materials Letters 68 (2012) 90–93 Acknowledgments This study was supported by the Post BK21 program in the Minis- try of Education, Human Resources Development, and also supported by the Korea National Research Foundation (NRF) grant funded by the Korea Government (MEST) (NRF-2010-0028802, 2010-0019626). Appendix A. 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