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Catalyst-free growth of one-dimensional ZnO nanostructures on SiO2 substrate and in situ investigation of their H2 sensing properties
Journal of Alloys and Compounds 622 (2015) 73–78
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Catalyst-free growth of one-dimensional ZnO nanostructures on SiO2 substrate and in situ investigation of their H2 sensing properties Xiaoguang San a, Guosheng Wang a,⇑, Bing Liang b, Yinmin Song c, Shangyao Gao a, Jinsong Zhang a, Fanli Meng d,⇑ a
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China College of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, China d Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, China b c
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 30 September 2014 Accepted 30 September 2014 Available online 13 October 2014 Keywords: ZnO 1D nanostructures Thermal evaporation Sensing properties H2
a b s t r a c t One-dimensional (1D) ZnO nanostructures were synthesized on SiO2 substrate by a catalyst-free thermal evaporation method using metallic Zn powders as a raw material. The crystal structure and morphology of the ZnO nanostructures were investigated by SEM, XRD and BET. The results showed that the 1D nanostructures obtained on the SiO2 substrate were hexagonal ZnO. Source temperature and Ar flow rate were an important parameter for the growth of the 1D nanostructures and particular type of ZnO nanostructures could be grown in a specific temperature and Ar flow rate. The H2 sensing properties of prepared 1D ZnO nanostructures were in situ investigated. The sensor exhibited high sensitivity and fast response to H2 gas. The highest sensitivity observed upon exposure to H2 at 1000 ppm was 5.3 at 200 °C, which demonstrates the potential application of the 1D ZnO nanostructures for fabricating gas sensors compatible with semiconductor technology. Ó 2014 Published by Elsevier B.V.
1. Introduction One-dimensional (1D) nanostructural materials, such as nanowires, nanorods and nanotubes, have attracted considerable attention due to their unique fundamental properties and potential applications in the fabrication of nanoscale devices [1]. Recently, the sensors based on 1D semiconductor nanostructural materials have been widely studied and showed higher sensitivity, faster response and/or enhanced capability to detect low concentration gases compared with the corresponding thin film materials [2,3]. For example, Shen et al. [4] showed that the SnO2 nanorods exhibited considerable sensitivity and rapid response. Liu et al. [5] reported that the Ga2O3 nanowire sensor showed higher O2 and CO sensitivity and quicker response compared with the corresponding thin film sensors. Zinc oxide (ZnO) is an inexpensive II–VI n-type semiconductor with a wide direct-band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature [6]. ZnO nanostructures owning large surface-to-volume ratio and great surface activity, have attracted considerable attention due to their potential
⇑ Corresponding authors. E-mail addresses: [email protected] (G. Wang), fl[email protected] (F. Meng). http://dx.doi.org/10.1016/j.jallcom.2014.09.224 0925-8388/Ó 2014 Published by Elsevier B.V.
applications in gas sensors [7–11], dye-sensitized solar cells [12,13], UV sensors [14,15], UV light emitters [16], and so on. Among those ZnO nanostructures, 1D ZnO nanostructures have in recent years attracted much attention due to their peculiar characteristics, size effects and the possibility that may be used as building blocks for future electronics and photonics [17]. For example, Liu et al. [18] prepared thornlike hierarchical structured ZnO nanorods via a wet-chemical approach which exhibited highly sensitive gas sensing performance combined with excellent optical properties. Ko et al. [19] reported that treelike hierarchical ZnO nanostructures assembled by ZnO nanowires demonstrated potential application in dye-sensitized solar cell. Over last few years, extensive efforts have been made on the synthesis of 1D ZnO nanostructures. Chemical route such as hydrothermal [20], sol–gel [21], and aqueous solution method [22] have been widely used to synthesize 1D ZnO nanostructures due to their good stoichiometry control, morphology control, low cost infrastructural requirements and easy scalability. However, chemical route sometimes causes contamination in the products affecting various device applications. Thermal evaporation method provides another commonly used methodology for generating 1D ZnO nanostructures [23,24]. The main advantage of this method is the possibility to produce different type of high quality of nanostructures in an easy way by using cheap deposition systems. However, metallic catalyst is
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often used in the synthesis process and it is very difficult to remove its residues completely, which will heavily affect the investigation of their application properties. Although some catalyst-free methods have been developed on sapphire or Si substrate [25,26], little work has been done about catalyst-free growth of 1D ZnO nanostructures on SiO2 substrate which is compatible with semiconductor technology. In the present study, 1D ZnO nanostructures were synthesized on SiO2 substrate by a catalyst-free thermal evaporation method using metallic Zn powder as a raw material. The H2 sensing properties of ZnO nanostructures were in situ investigated. The result demonstrates the potential application of 1D ZnO nanostructures for the integration of gas sensor onto a semiconductor chip. 2. Experimental methods ZnO nanostructures were synthesized by the thermal evaporation method in a horizontal tube furnace system with one side connected to gas inlet and the other side connected to atmosphere, as shown in Fig. 1. In a typical synthesis procedure, metallic Zn powder (300 mg, 99.9% purity) was used as a raw material, which placed in an alumina boat. Silicon substrates covered by a thin silicon dioxide layer with 500 nm in thickness were laid 5 mm above the source for collecting products. No catalysts or additives were introduced. The alumina boat was placed at the center of a quartz tube inserted in a horizontal tube furnace. Ar gas was introduced at a flow rate of 150 ml/min into the quartz tube at ambient pressure. Then, the furnace was increased to the temperature of 900, 975 or 1050 °C. After maintaining the furnace at these temperatures for 1 h, the furnace was cooled naturally to room temperature. Thus, a layer of product was obtained on the substrate. The growth time of 5 and 10 min were also chosen to investigate the growth process at temperature of 975 °C. In order to understand the effect of Ar flow rate on the morphologies, the flow rate was changed from 20 to 200 ml/min at temperature of 900 °C. The structure characterization of ZnO nanostructures was investigated by using an X-ray diffractometer (XRD) (Shimadzu XRD-6100) with Cu Ka radiation and a field emission scanning electron microscope (FESEM) (JEOL JSM-6700F). The surface areas were calculated using the Brunauer–Emmett–Teller (BET, Tristar II 3020M) method using N2 adsorption-desorption. H2 gas sensing properties were investigated. For the gas sensing test, a quartz tube furnace was used, which was inserted in an electric furnace. Synthetic air was used as a carrier gas at a constant flow rate of 200 ml/min, the desired concentration of H2 gas dispersed in synthetic air was introduced and exhausted at an operating temperature from 50 to 350 °C by heating the furnace. The electrical resistance of gas sensors was determined by measuring the electric current, which flowed when a potential difference of 5 V was applied between the Pt electrodes.
3. Results and discussion 3.1. Structure characterizations Fig. 2 shows the low and high magnification typical FESEM images of ZnO nanostructures synthesized at different source temperatures. It is obvious that the temperature play an important role in the morphology of ZnO nanostructures. At temperature of 900 °C, uniformly distributed ZnO nanowires in a high density were obtained, as shown in Fig. 2(a) and (b). These nanowires are smooth and the diameters of the grown nanowires are uniform throughout their length. The diameters and lengths of the nanowires are in the range of 50–120 nm and 1–6 lm, respectively. When the temperature is increased to 975 °C, nanorods with
Fig. 1. Schematic diagram of the apparatus used for the preparation of ZnO nanostructures.
hexagonal shape at their tips were formed, as shown in Fig. 2(c) and (d). These ZnO nanorods are uniformly grown onto the whole surface in a large quantity. The diameter size distribution is wider and in the range of 150–600 nm, while the length is the range of 1– 4 lm. It is interesting to see that very short nanorods having the diameter of 50 nm and the length of 100 nm were observed on the tip of each ZnO nanorod. Furthermore, by increasing the temperature to 1050 °C, rods with hexagonal shape at their tips were also formed, as shown in Fig. 2(e) and (f). Interestingly, these rods possess a typical hierarchical structure, consisting of micrometer sized rods to nanometer sized rods. The diameter of these rods decreases from the root (about 1 lm) to a sharp tip (about 100 nm). Because no metal catalysts were used in our experiment, the growth process of ZnO nanostructures presented here follows the vapor–solid (VS) growth mechanism, which is likely to divide into two stages: nucleation and growth [27]. Here, taking the ZnO nanorods synthesized at 975 °C as an example illustrate the corresponding growth process of ZnO nanostructures, as shown in Fig. 3. Firstly, Zn vapors are generated by evaporated Zn powder at a certain heating temperature. These Zn vapors combine with oxygen, forming ZnO vapors. Deposition of ZnO vapors on the substrate results in the formation of film like structures of ZnO islands as nucleation site for the growth of nanostructures. At the apices of these islands, rod like structures nucleate and they subsequently evolved into nanostructures [28,29]. It is known that the source temperature greatly influences the growth of the ZnO nanostructures synthesize by thermal evaporation [30,31]. The higher temperature, the higher will be the Zn vapor pressure. This induces a high lateral growth rate and a low axial growth rate, resulting in the formation of nanostructures with a large diameter. In addition, the higher temperature can also promote the formation of hierarchical nanostructures. In our case, high Zn vapor pressure generated at high temperature induced the formation of the large rod at the initial stage. After that, a secondary nanorod with a small diameter was formed on the preformed nanorod. Along with the consuming of the Zn vapor, the diameter of the grown nanorod abrupt decreased. As a result, rods possess a typical hierarchical structure was formed at high temperature. The influence of Ar flow rate was also investigated because it may adjust the super saturation degree of Zn source [24,32,33]. Fig. 4 shows the typical FESEM images of ZnO nanostructures synthesized at 900 °C under different Ar flow rates. It is interesting to note that different morphologies of ZnO nanostructures were grown on the SiO2 substrate with changing Ar flow rate. At a low flow rate of 20 ml/min, ZnO particles were obtained and the distribution of diameter was in the range from 100 nm to 1 lm. When the flow rate was increased to 50 ml/min, the particle size increased and some particle agglomerated together. As further increase the flow rate to 100 ml/min, thin nanowires with diameters about 60 nm grew from the surfaces of the particles. When increasing the flow rate to 200 ml/min, the diameter of nanowires was significantly increased. However, there were many curly nanowires. So, the flow rate of 150 ml/min was optimal, under which uniform nanowires with smooth surface were obtained as shown in Fig. 2(a) and (b). Fig. 5 shows the XRD patterns for the 1D ZnO nanostructures synthesized at different source temperatures. It is clearly seen that the diffraction peaks of all ZnO nanostructures agree with the standard card of bulk ZnO with a hexagonal structure (JCPDS Card No. 36–1451). No additional peaks from zinc or other impurities were observed in the XRD patterns, indicating the obtained products are pure ZnO nanostructures. It is also found that the diffraction peak assigned to be as ZnO (0 0 2) is stronger in intensity in all the spectra, indicated that the as-grown nanostructures are highly crystalline and preferentially oriented in the c-axis direction.
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Fig. 2. FESEM images of ZnO nanostructures synthesized at different source temperatures: (a) and (b) 900 °C, (c) and (d) 975 °C, (e) and (f) 1050 °C.
Fig. 3. Schematic illustration of the growth process of ZnO nanostructures. The SEM image is for the product synthesized at 975 °C during different growth time.
3.2. H2 sensing properties Gas sensors were fabricated by directly deposited ZnO nanostructures on the oxidized Si substrates equipped with a pair of interdigitated Pt electrodes. The schematic illustration of the gas sensor and corresponding FESEM image are shown in Fig. 6(a) and (b), respectively. The sensitivity (response value) of the sensor is defined as the ratio of the electrical resistance in air (Ra) to the electrical resistance in the mixture of H2 and air (Rg), which is given by the equation: S = Ra/Rg. The temperature dependence of the sensitivity of ZnO nanostructures synthesized at different source temperatures is shown
in Fig. 6(c). For all the sensors, the sensitivity to H2 increased to a maximum value and then decreased with a further increase in temperature. The sensor based on ZnO nanowires synthesized at 900 °C showed a relatively higher sensitivity than the others. The highest sensitivity is 5.3 for the sensor made of ZnO nanowires obtained at 200 °C. In addition, operating temperature reaching the highest sensitivity is lower than others [34,35]. BET measurements revealed that ZnO nanowires synthesized at 900 °C possessed the highest specific surface area (20.48 m2 g1), while the specific surface areas of the ones synthesized at 975 °C and 1050 °C was 17.26 m2 g1 and 15.65 m2 g1, respectively. The similar results have been also reported in [31,36,37]. According to the
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Fig. 4. FESEM images of ZnO nanostructures synthesized at different Ar flow rates: (a) 20 ml/min, (b) 50 ml/min, (c) 100 ml/min, and (d) 200 ml/min.
Fig. 5. XRD patterns of ZnO nanostructures synthesized at different source temperatures: (a) 900 °C, (b) 975 °C, and (c) 1050 °C.
reports, it is generally accepted that the nanowires owing to great surface activity associated with large surface-to-volume ratio are likely to increase the gas sensitivity and decrease its operating temperature. Fig. 7 shows the typical dynamic responses of ZnO nanowire gas sensor to 1000 ppm H2 at various operating temperatures. It is noted that the resistance in air decreases with increasing operating temperature from 50 to 300 °C. It is known that the chemisorption of oxygen atoms on ZnO surface in air causes electron depletion at the surface and a high energy barrier, leading to a high resistance. With increasing temperature, the electrons trapped by the adsorbed oxygen thermally return to the nanowires, causing high carrier concentration of electrons throughout the bulk, thus causing a decrease in resistance. Upon exposure to H2, the resistance decreased, while the resistance can recover to its initial value after removing H2 above 150 °C. It is also noted that the response and recovery times become short as the operating temperature increases. This is because the response and recovery times of oxide
semiconductor gas sensors are determined by the adsorption– desorption kinetics that depends on the operating temperature. The temporal responses of a sensor made of ZnO nanowires upon exposure to different concentrations of H2 gas measured at 250 °C are shown in Fig. 8(a). It is found that the sensor exhibited good response/recovery cycle to H2 gas pulses and the resistance changes of 2.3, 4, 5.2, 6.8, and 8.4 times with respect to the baseline are observed towards 500, 1000, 1500, 2000, and 3000 ppm H2, respectively. The sensitivity of ZnO nanowires as a function of H2 concentration is shown in Fig. 8(b). It is noted that the sensitivity increases as H2 concentration increases. The dependence of sensitivity on concentration follows the normally achieved law for metal oxide semiconductor sensors, S = A[Gas]B, where A and B are constants for a given gas [38]. In this study, ‘‘A’’ and ‘‘B’’ were found to be 0.106 ± 0.032 and 0.515 ± 0.037, respectively. It is found that the experimental data and the theoretical curve obtained from the empirical model show good agreements. A possible sensing mechanism can be described as follows. The oxygen molecules from the ambient atmosphere are initially adsorbed on the surface of the ZnO nanostructures leading to electrons extraction from the conduction band to form oxygen ions in the following the reactions [39]:
O2 ðgasÞ ! O2 ðadsÞ
ð1Þ
O2 ðadsÞ þ e ! O2 ðadsÞ
ð2Þ
O2 ðadsÞ þ e ! 2O ðadsÞ
ð3Þ
O ðadsÞ þ e ! O2 ðadsÞ
ð4Þ
Thus, these charged oxygen species form an electron depletion layer at the surface of the ZnO nanostructures, showing a high resistance state in air ambient. When ZnO nanostructures are exposed to hydrogen, hydrogen reacts with surface oxygen species, following reactions [40]:
H2 þ 1=2O2 ðadsÞ ! H2 OðgasÞ þ e
ð5Þ
H2 þ O ðadsÞ ! H2 OðgasÞ þ e
ð6Þ
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Fig. 6. (a) Schematic illustration of the gas sensor. (b) FESEM image of the gas sensor between two electrodes. (c) Sensitivity of ZnO nanostructure sensors upon exposure to 1000 ppm H2 gas at different operating temperatures.
Fig. 7. Temporal responses to H2 measured for the sensors made of ZnO nanowires synthesized at 900 °C. The results for various operating temperatures are shown.
As a result trapped electrons are released back to the conduction band of ZnO nanostructures, leading to a decrease of depletion width, which decreases the resistance of the ZnO nanostructures. These changes in the resistance reflect the sensing behavior of ZnO nanostructures. Here, the ZnO nanowires with small diameter compared to other structures own to great surface activity associated with large surface-to-volume ratio, which are likely to increase the gas sensitivity. Therefore, the ZnO nanowire sensor shows the highest sensitivity. It is also noted that the thermal energy is needed for the reaction of hydrogen molecules with the surface adsorbed oxygen species needs. Therefore, the sensor exhibited a lower sensitivity at the low operating temperature. With increasing temperature, the surface reaction is thermally activated, arriving maximum sensitivity. On the other hand, as the temperature further increases, based on the above discussion, the resistance in air largely decreases due
Fig. 8. (a) Temporal responses of ZnO nanowire sensor to H2 with various concentrations at 250 °C. (b) Relationship between the sensitivity and H2 concentration measured at 250 °C. The nanowires used for this measurement were synthesized at 900 °C.
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to the thermal effect and the influence of hydrogen on the resistance of sensor becomes relatively less, and the adsorption coefficient of H2 decreases as well. Therefore, the sensitivity decreases when the operating temperature is too high. By increasing H2 concentration, the sensitivity increases due to the higher surface coverage of gas molecules. 4. Conclusions Different types of 1D ZnO nanostructures were synthesized on SiO2 substrate by a catalyst-free thermal evaporation method using metallic Zn powder as a raw material. The structure characteristics and H2 sensing properties of the ZnO nanostructures were investigated. The results showed that the obtained nanostructures were hexagonal ZnO. The source temperature is an important parameter for the growth of nanostructures and a particular type of ZnO nanostructure can be grown in a specific temperature. The sensors based on ZnO nanostructures showed high sensitivity to H2 at relatively low operating temperatures. The highest sensitivity observed in this study upon exposure to H2 at 1000 ppm was 5.3 at 200 °C, which was measured for a sensor made of the ZnO nanowires. These results suggested that ZnO nanowires have excellent potential application for fabrication high performance H2 sensors. Acknowledgements This work was supported by the National Natural Science Foundation of China – China (61374017) and the Scientific Research Foundation of Shenyang University of Chemical Technology – China. References [1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Onedimensional nanostructures: synthesis, characterization, and applications, Adv. Mater. 15 (2003) 353–389. [2] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by onedimensional metal oxide nanostructures, Annu. Rev. Mater. Res. 34 (2004) 151–180. [3] Y.F. Sun, S.B. Liu, F.L. Meng, J.Y. Liu, Z. Jin, L.T. Kong, J.H. Liu, Metal oxide nanostructures and their gas sensing properties, Sensors 12 (2012) 2610– 2631. [4] Y.B. Shen, X.M. Cao, B.Q. Zhang, D.Z. Wei, J.W. Ma, W.G. Liu, C. Han, Y.T. Shen, Synthesis of SnO2 nanorods and application to H2 sensor, J. Alloys Comp. 593 (2014) 271–274. [5] Z.F. Liu, T. Yamazaki, Y.B. Shen, T. Kikuta, N. Nakatani, Y.X. Li, O2 and CO sensing of Ga2O3 multiple nanowire gas sensors, Sens. Actuators B 129 (2008) 666–670. [6] Z.L. Wang, Zinc oxide nanostructures: growth, properties and applications, J. Phys.: Condens. Matter 16 (2004) 829–858. [7] S.D. Shinde, G.E. Patil, D.D. Kajale, V.B. Gaikwad, G.H. Jain, Synthesis of ZnO nanorods by spray pyrolysis for H2S gas sensor, J. Alloys Comp. 528 (2012) 109–114. [8] S. Öztürk, N. Kılınç, Z.Z. Öztürk, Fabrication of ZnO nanorods for NO2 sensor applications: Effect of dimensions and electrode position, J. Alloys Comp. 581 (2013) 196–201. [9] C. Li, Z. Du, H. Yu, T. Wang, Low-temperature sensing and high sensitivity of ZnO nanoneedles due to small size effect, Thin Solid Films 517 (2009) 5931– 5934. [10] J.Y. Liu, Z. Guo, F.L. Meng, T. Luo, M.Q. Li, J.H. Liu, Novel porous singlecrystalline ZnO nanosheets fabricated by annealing ZnS(en)0.5 (en = ethylenediamine) precursor. Application in a gas sensor for indoor air contaminant detection, Nanotechnology 20 (2009) 125501. [11] S. Tian, D. Zeng, X. Peng, S. Zhang, C. Xie, Processing-microstructure-property correlations of gas sensors based on ZnO nanotetrapods, Sens. Actuators B 181 (2013) 509–517. [12] I. Herman, J. Yeo, S. Hong, D. Lee, K.H. Nam, J. Choi, W. Hong, D. Lee, C.P. Grigoropoulos, S.H. Ko, Hierarchical weeping willow nano-tree growth and effect of branching on dye-sensitized solar cell efficiency, Nanotechnology 23 (2012) 194005. [13] S. Sönmezog˘lu, V. Eskizeybek, A. Toumiat, A. Avcı, Fast production of ZnO nanorods by arc discharge in de-ionized water and applications in dyesensitized solar cells, J. Alloys Comp. 586 (2014) 593–599.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Catalyst-free growth of one-dimensional ZnO nanostructures on SiO2 substrate and in situ investigation of their H2 sensing properties Xiaoguang San a, Guosheng Wang a,⇑, Bing Liang b, Yinmin Song c, Shangyao Gao a, Jinsong Zhang a, Fanli Meng d,⇑ a
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China College of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, China d Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, China b c
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 30 September 2014 Accepted 30 September 2014 Available online 13 October 2014 Keywords: ZnO 1D nanostructures Thermal evaporation Sensing properties H2
a b s t r a c t One-dimensional (1D) ZnO nanostructures were synthesized on SiO2 substrate by a catalyst-free thermal evaporation method using metallic Zn powders as a raw material. The crystal structure and morphology of the ZnO nanostructures were investigated by SEM, XRD and BET. The results showed that the 1D nanostructures obtained on the SiO2 substrate were hexagonal ZnO. Source temperature and Ar flow rate were an important parameter for the growth of the 1D nanostructures and particular type of ZnO nanostructures could be grown in a specific temperature and Ar flow rate. The H2 sensing properties of prepared 1D ZnO nanostructures were in situ investigated. The sensor exhibited high sensitivity and fast response to H2 gas. The highest sensitivity observed upon exposure to H2 at 1000 ppm was 5.3 at 200 °C, which demonstrates the potential application of the 1D ZnO nanostructures for fabricating gas sensors compatible with semiconductor technology. Ó 2014 Published by Elsevier B.V.
1. Introduction One-dimensional (1D) nanostructural materials, such as nanowires, nanorods and nanotubes, have attracted considerable attention due to their unique fundamental properties and potential applications in the fabrication of nanoscale devices [1]. Recently, the sensors based on 1D semiconductor nanostructural materials have been widely studied and showed higher sensitivity, faster response and/or enhanced capability to detect low concentration gases compared with the corresponding thin film materials [2,3]. For example, Shen et al. [4] showed that the SnO2 nanorods exhibited considerable sensitivity and rapid response. Liu et al. [5] reported that the Ga2O3 nanowire sensor showed higher O2 and CO sensitivity and quicker response compared with the corresponding thin film sensors. Zinc oxide (ZnO) is an inexpensive II–VI n-type semiconductor with a wide direct-band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature [6]. ZnO nanostructures owning large surface-to-volume ratio and great surface activity, have attracted considerable attention due to their potential
⇑ Corresponding authors. E-mail addresses: [email protected] (G. Wang), fl[email protected] (F. Meng). http://dx.doi.org/10.1016/j.jallcom.2014.09.224 0925-8388/Ó 2014 Published by Elsevier B.V.
applications in gas sensors [7–11], dye-sensitized solar cells [12,13], UV sensors [14,15], UV light emitters [16], and so on. Among those ZnO nanostructures, 1D ZnO nanostructures have in recent years attracted much attention due to their peculiar characteristics, size effects and the possibility that may be used as building blocks for future electronics and photonics [17]. For example, Liu et al. [18] prepared thornlike hierarchical structured ZnO nanorods via a wet-chemical approach which exhibited highly sensitive gas sensing performance combined with excellent optical properties. Ko et al. [19] reported that treelike hierarchical ZnO nanostructures assembled by ZnO nanowires demonstrated potential application in dye-sensitized solar cell. Over last few years, extensive efforts have been made on the synthesis of 1D ZnO nanostructures. Chemical route such as hydrothermal [20], sol–gel [21], and aqueous solution method [22] have been widely used to synthesize 1D ZnO nanostructures due to their good stoichiometry control, morphology control, low cost infrastructural requirements and easy scalability. However, chemical route sometimes causes contamination in the products affecting various device applications. Thermal evaporation method provides another commonly used methodology for generating 1D ZnO nanostructures [23,24]. The main advantage of this method is the possibility to produce different type of high quality of nanostructures in an easy way by using cheap deposition systems. However, metallic catalyst is
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often used in the synthesis process and it is very difficult to remove its residues completely, which will heavily affect the investigation of their application properties. Although some catalyst-free methods have been developed on sapphire or Si substrate [25,26], little work has been done about catalyst-free growth of 1D ZnO nanostructures on SiO2 substrate which is compatible with semiconductor technology. In the present study, 1D ZnO nanostructures were synthesized on SiO2 substrate by a catalyst-free thermal evaporation method using metallic Zn powder as a raw material. The H2 sensing properties of ZnO nanostructures were in situ investigated. The result demonstrates the potential application of 1D ZnO nanostructures for the integration of gas sensor onto a semiconductor chip. 2. Experimental methods ZnO nanostructures were synthesized by the thermal evaporation method in a horizontal tube furnace system with one side connected to gas inlet and the other side connected to atmosphere, as shown in Fig. 1. In a typical synthesis procedure, metallic Zn powder (300 mg, 99.9% purity) was used as a raw material, which placed in an alumina boat. Silicon substrates covered by a thin silicon dioxide layer with 500 nm in thickness were laid 5 mm above the source for collecting products. No catalysts or additives were introduced. The alumina boat was placed at the center of a quartz tube inserted in a horizontal tube furnace. Ar gas was introduced at a flow rate of 150 ml/min into the quartz tube at ambient pressure. Then, the furnace was increased to the temperature of 900, 975 or 1050 °C. After maintaining the furnace at these temperatures for 1 h, the furnace was cooled naturally to room temperature. Thus, a layer of product was obtained on the substrate. The growth time of 5 and 10 min were also chosen to investigate the growth process at temperature of 975 °C. In order to understand the effect of Ar flow rate on the morphologies, the flow rate was changed from 20 to 200 ml/min at temperature of 900 °C. The structure characterization of ZnO nanostructures was investigated by using an X-ray diffractometer (XRD) (Shimadzu XRD-6100) with Cu Ka radiation and a field emission scanning electron microscope (FESEM) (JEOL JSM-6700F). The surface areas were calculated using the Brunauer–Emmett–Teller (BET, Tristar II 3020M) method using N2 adsorption-desorption. H2 gas sensing properties were investigated. For the gas sensing test, a quartz tube furnace was used, which was inserted in an electric furnace. Synthetic air was used as a carrier gas at a constant flow rate of 200 ml/min, the desired concentration of H2 gas dispersed in synthetic air was introduced and exhausted at an operating temperature from 50 to 350 °C by heating the furnace. The electrical resistance of gas sensors was determined by measuring the electric current, which flowed when a potential difference of 5 V was applied between the Pt electrodes.
3. Results and discussion 3.1. Structure characterizations Fig. 2 shows the low and high magnification typical FESEM images of ZnO nanostructures synthesized at different source temperatures. It is obvious that the temperature play an important role in the morphology of ZnO nanostructures. At temperature of 900 °C, uniformly distributed ZnO nanowires in a high density were obtained, as shown in Fig. 2(a) and (b). These nanowires are smooth and the diameters of the grown nanowires are uniform throughout their length. The diameters and lengths of the nanowires are in the range of 50–120 nm and 1–6 lm, respectively. When the temperature is increased to 975 °C, nanorods with
Fig. 1. Schematic diagram of the apparatus used for the preparation of ZnO nanostructures.
hexagonal shape at their tips were formed, as shown in Fig. 2(c) and (d). These ZnO nanorods are uniformly grown onto the whole surface in a large quantity. The diameter size distribution is wider and in the range of 150–600 nm, while the length is the range of 1– 4 lm. It is interesting to see that very short nanorods having the diameter of 50 nm and the length of 100 nm were observed on the tip of each ZnO nanorod. Furthermore, by increasing the temperature to 1050 °C, rods with hexagonal shape at their tips were also formed, as shown in Fig. 2(e) and (f). Interestingly, these rods possess a typical hierarchical structure, consisting of micrometer sized rods to nanometer sized rods. The diameter of these rods decreases from the root (about 1 lm) to a sharp tip (about 100 nm). Because no metal catalysts were used in our experiment, the growth process of ZnO nanostructures presented here follows the vapor–solid (VS) growth mechanism, which is likely to divide into two stages: nucleation and growth [27]. Here, taking the ZnO nanorods synthesized at 975 °C as an example illustrate the corresponding growth process of ZnO nanostructures, as shown in Fig. 3. Firstly, Zn vapors are generated by evaporated Zn powder at a certain heating temperature. These Zn vapors combine with oxygen, forming ZnO vapors. Deposition of ZnO vapors on the substrate results in the formation of film like structures of ZnO islands as nucleation site for the growth of nanostructures. At the apices of these islands, rod like structures nucleate and they subsequently evolved into nanostructures [28,29]. It is known that the source temperature greatly influences the growth of the ZnO nanostructures synthesize by thermal evaporation [30,31]. The higher temperature, the higher will be the Zn vapor pressure. This induces a high lateral growth rate and a low axial growth rate, resulting in the formation of nanostructures with a large diameter. In addition, the higher temperature can also promote the formation of hierarchical nanostructures. In our case, high Zn vapor pressure generated at high temperature induced the formation of the large rod at the initial stage. After that, a secondary nanorod with a small diameter was formed on the preformed nanorod. Along with the consuming of the Zn vapor, the diameter of the grown nanorod abrupt decreased. As a result, rods possess a typical hierarchical structure was formed at high temperature. The influence of Ar flow rate was also investigated because it may adjust the super saturation degree of Zn source [24,32,33]. Fig. 4 shows the typical FESEM images of ZnO nanostructures synthesized at 900 °C under different Ar flow rates. It is interesting to note that different morphologies of ZnO nanostructures were grown on the SiO2 substrate with changing Ar flow rate. At a low flow rate of 20 ml/min, ZnO particles were obtained and the distribution of diameter was in the range from 100 nm to 1 lm. When the flow rate was increased to 50 ml/min, the particle size increased and some particle agglomerated together. As further increase the flow rate to 100 ml/min, thin nanowires with diameters about 60 nm grew from the surfaces of the particles. When increasing the flow rate to 200 ml/min, the diameter of nanowires was significantly increased. However, there were many curly nanowires. So, the flow rate of 150 ml/min was optimal, under which uniform nanowires with smooth surface were obtained as shown in Fig. 2(a) and (b). Fig. 5 shows the XRD patterns for the 1D ZnO nanostructures synthesized at different source temperatures. It is clearly seen that the diffraction peaks of all ZnO nanostructures agree with the standard card of bulk ZnO with a hexagonal structure (JCPDS Card No. 36–1451). No additional peaks from zinc or other impurities were observed in the XRD patterns, indicating the obtained products are pure ZnO nanostructures. It is also found that the diffraction peak assigned to be as ZnO (0 0 2) is stronger in intensity in all the spectra, indicated that the as-grown nanostructures are highly crystalline and preferentially oriented in the c-axis direction.
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Fig. 2. FESEM images of ZnO nanostructures synthesized at different source temperatures: (a) and (b) 900 °C, (c) and (d) 975 °C, (e) and (f) 1050 °C.
Fig. 3. Schematic illustration of the growth process of ZnO nanostructures. The SEM image is for the product synthesized at 975 °C during different growth time.
3.2. H2 sensing properties Gas sensors were fabricated by directly deposited ZnO nanostructures on the oxidized Si substrates equipped with a pair of interdigitated Pt electrodes. The schematic illustration of the gas sensor and corresponding FESEM image are shown in Fig. 6(a) and (b), respectively. The sensitivity (response value) of the sensor is defined as the ratio of the electrical resistance in air (Ra) to the electrical resistance in the mixture of H2 and air (Rg), which is given by the equation: S = Ra/Rg. The temperature dependence of the sensitivity of ZnO nanostructures synthesized at different source temperatures is shown
in Fig. 6(c). For all the sensors, the sensitivity to H2 increased to a maximum value and then decreased with a further increase in temperature. The sensor based on ZnO nanowires synthesized at 900 °C showed a relatively higher sensitivity than the others. The highest sensitivity is 5.3 for the sensor made of ZnO nanowires obtained at 200 °C. In addition, operating temperature reaching the highest sensitivity is lower than others [34,35]. BET measurements revealed that ZnO nanowires synthesized at 900 °C possessed the highest specific surface area (20.48 m2 g1), while the specific surface areas of the ones synthesized at 975 °C and 1050 °C was 17.26 m2 g1 and 15.65 m2 g1, respectively. The similar results have been also reported in [31,36,37]. According to the
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Fig. 4. FESEM images of ZnO nanostructures synthesized at different Ar flow rates: (a) 20 ml/min, (b) 50 ml/min, (c) 100 ml/min, and (d) 200 ml/min.
Fig. 5. XRD patterns of ZnO nanostructures synthesized at different source temperatures: (a) 900 °C, (b) 975 °C, and (c) 1050 °C.
reports, it is generally accepted that the nanowires owing to great surface activity associated with large surface-to-volume ratio are likely to increase the gas sensitivity and decrease its operating temperature. Fig. 7 shows the typical dynamic responses of ZnO nanowire gas sensor to 1000 ppm H2 at various operating temperatures. It is noted that the resistance in air decreases with increasing operating temperature from 50 to 300 °C. It is known that the chemisorption of oxygen atoms on ZnO surface in air causes electron depletion at the surface and a high energy barrier, leading to a high resistance. With increasing temperature, the electrons trapped by the adsorbed oxygen thermally return to the nanowires, causing high carrier concentration of electrons throughout the bulk, thus causing a decrease in resistance. Upon exposure to H2, the resistance decreased, while the resistance can recover to its initial value after removing H2 above 150 °C. It is also noted that the response and recovery times become short as the operating temperature increases. This is because the response and recovery times of oxide
semiconductor gas sensors are determined by the adsorption– desorption kinetics that depends on the operating temperature. The temporal responses of a sensor made of ZnO nanowires upon exposure to different concentrations of H2 gas measured at 250 °C are shown in Fig. 8(a). It is found that the sensor exhibited good response/recovery cycle to H2 gas pulses and the resistance changes of 2.3, 4, 5.2, 6.8, and 8.4 times with respect to the baseline are observed towards 500, 1000, 1500, 2000, and 3000 ppm H2, respectively. The sensitivity of ZnO nanowires as a function of H2 concentration is shown in Fig. 8(b). It is noted that the sensitivity increases as H2 concentration increases. The dependence of sensitivity on concentration follows the normally achieved law for metal oxide semiconductor sensors, S = A[Gas]B, where A and B are constants for a given gas [38]. In this study, ‘‘A’’ and ‘‘B’’ were found to be 0.106 ± 0.032 and 0.515 ± 0.037, respectively. It is found that the experimental data and the theoretical curve obtained from the empirical model show good agreements. A possible sensing mechanism can be described as follows. The oxygen molecules from the ambient atmosphere are initially adsorbed on the surface of the ZnO nanostructures leading to electrons extraction from the conduction band to form oxygen ions in the following the reactions [39]:
O2 ðgasÞ ! O2 ðadsÞ
ð1Þ
O2 ðadsÞ þ e ! O2 ðadsÞ
ð2Þ
O2 ðadsÞ þ e ! 2O ðadsÞ
ð3Þ
O ðadsÞ þ e ! O2 ðadsÞ
ð4Þ
Thus, these charged oxygen species form an electron depletion layer at the surface of the ZnO nanostructures, showing a high resistance state in air ambient. When ZnO nanostructures are exposed to hydrogen, hydrogen reacts with surface oxygen species, following reactions [40]:
H2 þ 1=2O2 ðadsÞ ! H2 OðgasÞ þ e
ð5Þ
H2 þ O ðadsÞ ! H2 OðgasÞ þ e
ð6Þ
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Fig. 6. (a) Schematic illustration of the gas sensor. (b) FESEM image of the gas sensor between two electrodes. (c) Sensitivity of ZnO nanostructure sensors upon exposure to 1000 ppm H2 gas at different operating temperatures.
Fig. 7. Temporal responses to H2 measured for the sensors made of ZnO nanowires synthesized at 900 °C. The results for various operating temperatures are shown.
As a result trapped electrons are released back to the conduction band of ZnO nanostructures, leading to a decrease of depletion width, which decreases the resistance of the ZnO nanostructures. These changes in the resistance reflect the sensing behavior of ZnO nanostructures. Here, the ZnO nanowires with small diameter compared to other structures own to great surface activity associated with large surface-to-volume ratio, which are likely to increase the gas sensitivity. Therefore, the ZnO nanowire sensor shows the highest sensitivity. It is also noted that the thermal energy is needed for the reaction of hydrogen molecules with the surface adsorbed oxygen species needs. Therefore, the sensor exhibited a lower sensitivity at the low operating temperature. With increasing temperature, the surface reaction is thermally activated, arriving maximum sensitivity. On the other hand, as the temperature further increases, based on the above discussion, the resistance in air largely decreases due
Fig. 8. (a) Temporal responses of ZnO nanowire sensor to H2 with various concentrations at 250 °C. (b) Relationship between the sensitivity and H2 concentration measured at 250 °C. The nanowires used for this measurement were synthesized at 900 °C.
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to the thermal effect and the influence of hydrogen on the resistance of sensor becomes relatively less, and the adsorption coefficient of H2 decreases as well. Therefore, the sensitivity decreases when the operating temperature is too high. By increasing H2 concentration, the sensitivity increases due to the higher surface coverage of gas molecules. 4. Conclusions Different types of 1D ZnO nanostructures were synthesized on SiO2 substrate by a catalyst-free thermal evaporation method using metallic Zn powder as a raw material. The structure characteristics and H2 sensing properties of the ZnO nanostructures were investigated. The results showed that the obtained nanostructures were hexagonal ZnO. The source temperature is an important parameter for the growth of nanostructures and a particular type of ZnO nanostructure can be grown in a specific temperature. The sensors based on ZnO nanostructures showed high sensitivity to H2 at relatively low operating temperatures. The highest sensitivity observed in this study upon exposure to H2 at 1000 ppm was 5.3 at 200 °C, which was measured for a sensor made of the ZnO nanowires. These results suggested that ZnO nanowires have excellent potential application for fabrication high performance H2 sensors. Acknowledgements This work was supported by the National Natural Science Foundation of China – China (61374017) and the Scientific Research Foundation of Shenyang University of Chemical Technology – China. References [1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Onedimensional nanostructures: synthesis, characterization, and applications, Adv. Mater. 15 (2003) 353–389. [2] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by onedimensional metal oxide nanostructures, Annu. Rev. Mater. Res. 34 (2004) 151–180. [3] Y.F. Sun, S.B. Liu, F.L. Meng, J.Y. Liu, Z. Jin, L.T. Kong, J.H. Liu, Metal oxide nanostructures and their gas sensing properties, Sensors 12 (2012) 2610– 2631. [4] Y.B. Shen, X.M. Cao, B.Q. Zhang, D.Z. Wei, J.W. Ma, W.G. Liu, C. Han, Y.T. Shen, Synthesis of SnO2 nanorods and application to H2 sensor, J. Alloys Comp. 593 (2014) 271–274. [5] Z.F. Liu, T. Yamazaki, Y.B. Shen, T. Kikuta, N. Nakatani, Y.X. Li, O2 and CO sensing of Ga2O3 multiple nanowire gas sensors, Sens. Actuators B 129 (2008) 666–670. [6] Z.L. Wang, Zinc oxide nanostructures: growth, properties and applications, J. Phys.: Condens. Matter 16 (2004) 829–858. [7] S.D. Shinde, G.E. Patil, D.D. Kajale, V.B. Gaikwad, G.H. Jain, Synthesis of ZnO nanorods by spray pyrolysis for H2S gas sensor, J. Alloys Comp. 528 (2012) 109–114. [8] S. Öztürk, N. Kılınç, Z.Z. Öztürk, Fabrication of ZnO nanorods for NO2 sensor applications: Effect of dimensions and electrode position, J. Alloys Comp. 581 (2013) 196–201. [9] C. Li, Z. Du, H. Yu, T. Wang, Low-temperature sensing and high sensitivity of ZnO nanoneedles due to small size effect, Thin Solid Films 517 (2009) 5931– 5934. [10] J.Y. Liu, Z. Guo, F.L. Meng, T. Luo, M.Q. Li, J.H. Liu, Novel porous singlecrystalline ZnO nanosheets fabricated by annealing ZnS(en)0.5 (en = ethylenediamine) precursor. Application in a gas sensor for indoor air contaminant detection, Nanotechnology 20 (2009) 125501. [11] S. Tian, D. Zeng, X. Peng, S. Zhang, C. Xie, Processing-microstructure-property correlations of gas sensors based on ZnO nanotetrapods, Sens. Actuators B 181 (2013) 509–517. [12] I. Herman, J. Yeo, S. Hong, D. Lee, K.H. Nam, J. Choi, W. Hong, D. Lee, C.P. Grigoropoulos, S.H. Ko, Hierarchical weeping willow nano-tree growth and effect of branching on dye-sensitized solar cell efficiency, Nanotechnology 23 (2012) 194005. [13] S. Sönmezog˘lu, V. Eskizeybek, A. Toumiat, A. Avcı, Fast production of ZnO nanorods by arc discharge in de-ionized water and applications in dyesensitized solar cells, J. Alloys Comp. 586 (2014) 593–599.
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