- Email: [email protected]
Preparation of ZnO nanorods by microemulsion synthesis and their application as a CO gas sensor
Sensors and Actuators B 160 (2011) 94–98
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Preparation of ZnO nanorods by microemulsion synthesis and their application as a CO gas sensor Sang Kyoo Lim a,∗ , Sung-Ho Hwang a , Soonhyun Kim a , Hyunwoong Park b a b
Division of Nano & Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea School of Energy Engineering, Kyungbuk National University, Daegu 702-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 11 July 2011 Accepted 12 July 2011 Available online 20 July 2011 Keywords: Zinc oxide Nanorod Microemulsion Surface area Gas sensor
a b s t r a c t Zinc oxide nanorods with different surface area were synthesized by surfactant assisted microemulsion method. The alkyl chain length of surfactant would affect aspect ratio of ZnO nanorods. ZnO nanorods synthesized by ethyl benzene acid sodium salt (EBS), which is surfactant with short alkyl chain length, show higher aspect ratio than ones by dodecyl benzene sulfonic acid sodium salt (DBS). These nanorods had diameters in the range of 80–300 nm and length of up to several microns. The Brunauer–Emmett–Teller (BET) surface area of the ZnO nanorods was strongly affected by the morphology of the nanorods. The BET surface area of the nanorods synthesized with EBS was higher than the surface area of the nanorods synthesized with DBS (20.2 and 14.1 m2 /g for EBS and DBS, respectively). The response of ZnO nanorods to CO in air was strongly affected by surface area, defects and oxygen vacancies. The results demonstrate that the microemulsion synthesis is an easy and useful method to synthesize ZnO nanorods with large aspect ratio, which may enhance their gas sensing properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, one-dimensional nanostructured materials such as nanowires, nanofibers, nanorods, and nanotubes have received a great attention for their potential applications in numerous areas due to their special properties, which are distinct from conventional bulk materials. The synthesis of nanometer-sized one-dimensional ZnO materials has attracted considerable attention because of their great potential for fundamental studies of the role of dimensionality and size in the physical properties as well as for many applications such as transparent electrode in solar cells, gas sensors and photoluminescence devices [1–3]. Therefore, strong efforts have been made to fabricate one-dimensional ZnO, which were synthesized, for example, by the high-temperature physical evaporation, the micro emulsion process and the template induced method [4–6]. Among all these methods, the solution based synthesis, by thermal treatment of the reactant in different solvents, can be considered as the most simple and effective way to prepare sufficiently crystallized materials at relatively low temperatures. Also, the benefits of utilizing solution-based method have involved controlling the size and morphology according to the choice of reactant species. On the other hand, many of the previous investigations on ZnO prepared by solution based method, have mainly utilized zinc hydroxide or salt
∗ Corresponding author. Tel.: +82 53 785 3510; fax: +82 53 785 3439. E-mail address: [email protected] (S.K. Lim). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.07.018
as precursors and water or organic solvent as reaction media. Only few publications reported the relationship between the morphology of ZnO nanorod and the structure/type of surfactant, and how to effect on the CO gas sensing property. In this work, we presented microemulsion method toward the growth of well-proportioned and crystallized ZnO nanorods using two types of surfactants as the modifying and protecting agent. Under the same synthetic conditions, ZnO nanorods have been obtained with different aspect ratios, depending on the structure of surfactants. The material was characterized structurally using X-ray diffraction (XRD) and morphologically using scanning electron microscopy (SEM). The gas sensing properties of the ZnO nanorod based sensors to carbon monoxide (CO) in air were analyzed.
2. Experimental All chemicals used in this experiment were of analytical grade and used without further purification. The synthesis of ZnO nanorod was carried out in microemulsion, which were consisting of 5 g of surfactant such as ethyl benzene acid sodium salt (EBS), dodecyl benzene sulfonic acid sodium salt (DBS) and 2 mmol of zinc acetate dihydrate (ZnAc2 ·2H2 O) both dispersed in 60 ml xylene by stirring until a homogenous slightly turbid appearance of mixture was obtained. Then hydrazine monohydrate 2 ml and ethanol 8 ml mixture solution was added drop-wisely to the well-stirred mixture at room temperature by simultaneous agitation. The resulting precursor-containing mixture was subsequently
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
95
Fig. 1. Chemical structure of the surfactants: (a) EBS and (b) DBS.
heated to the 140 ◦ C with refluxed. After refluxing for 5 h, a milkywhite suspension was obtained and centrifuged to separate the precipitate, which was rinsed with absolute ethanol and distilled water for several times and dried in vacuum oven at 70 ◦ C for 24 h. XRD patterns of the samples were obtained from an X-ray diffractometer (Rigaku D/MAX-2500, 18 kV) using Cu K␣1 radiation and a quartz monochromator, in the 2 range of 10–80◦ in steps of 0.05◦ . Sample morphology was studied using an Hitachi S-4200 field emission scanning microscopy (FE-SEM). Optical properties were obtained by the photoluminescence (PL) measurements using SPEX 1403 Spectrophotometer excited by a continuous He–Cd laser with a wavelength of 325 nm. The BET surface areas of the prepared samples were determined from nitrogen adsorption–desorption isotherms at 77 K (ASAP 2020 Micromeritics). The sensor was fabricated by dropping ZnO gels on a sapphire substrate with prepatterned Pt electrodes. The ZnO gel was made by grinding 10 mg of ZnO powder with 0.25 mL of de-ionized water in an agate mortar. A precise auto pipette (Eppendorf) was then used to deposit the solution. After dropping 10 L of ZnO gels, the substrate was allowed to dry. The sensor element was heated at 400 ◦ C for 1 h to prevent crystal growth at sensing temperatures (50–400 ◦ C). The properties of the ZnO nanorods must be well-analyzed to optimize their characteristics for CO gas sensing applications. The sensing measurements were performed n a computer-based testing apparatus (PXI-DAQ system, National Instruments, USA) and the gas concentration was controlled by a pre-calibrated mass flow meter (Brooks 5850E, USA). The source measure unit (4200SCS, Keithley, USA) was used for acquisition of the electrical signal, while a power supply (E3631A, Agilent, USA) was employed to bias the sensor’s built-in back heater. The gas response is defined as the percent change in resistance of the sensor upon CO exposure, {(Ra − Rg )/Ra } × 100, where Ra and Rg are the resistances in air and in the presence of CO, respectively. 3. Results and discussion 3.1. Characterization of ZnO nanorods The growths of ZnO nano-structures in microemulsion with different surfactants were investigated. In order to understand the effects of alkyl chain lengths on the morphology and physicochemical properties of ZnO, two different surfactants, EBS and DBS, were used. Fig. 1 shows the structure of surfactants used in our experiments. XRD patterns indicated that all samples exhibited a wurtzite structure (hexagonal phase, space group P63 mc), as shown in Fig. 2. All the diffraction peaks are well indexed to hexagonal phased ZnO (JCPDS card no. 36-1451). The strong diffraction intensities along the [1 0 1] direction as well as the weak diffraction intensities along the [0 0 2] direction were observed in all the samples, implying the preferred growth orientation of ZnO. The XRD results also show only diffraction peaks of ZnO without any trace of other diffraction peaks. Guo et al. firstly reported this microemulsion synthetic method of ZnO nanorods using DBS as the modifying
Fig. 2. XRD patterns of synthesized ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
and protecting agent [7]. They successfully synthesized regular shaped single crystalline ZnO nanorods, however the synthesized ZnO nanorods contained minor Zn(OH)2 phase. In our experiments, we had changed synthetic conditions, and finally obtained highly pure ZnO phase nanorods. The crystal growth could be understood on the basis of following reactions. Micelles formed in a higher concentration of surfactants might act as micro-reactors which contains zinc precursor. In the presence of the OH− ions [Zn(OOCCH3 )2 ·2H2 O] was dissociated to form Zn(OH)2 in the ethanol solvent at high temperature. Sometimes, such Zn(OH)2 might react with the excess OH− ions to produce [Zn(OH)4 ]2− ions. At the high temperature, Zn(OH)2 and [Zn(OH)4 ]−2 in the microreactors could be transformed to ZnO, which came to play a role as a seed for ZnO crystal growth. On the other hand, rodlike space-confined crystal growth occurs in these microemulsion systems due to collision between micelles. The elevated temperature can make the activity of surfactants enhanced and microreactors to more highly collide with each other during the reaction process. FE-SEM images of prepared samples are shown in Fig. 3. The rodlike shapes of samples are relatively uniform with average diameters of 80 nm and 300 nm for EBS and DBS, respectively, and length of up to several microns for both samples. In Fig. 3, it was clearly seen that both the length and thickness of the ZnO nanorods got to increase with increment with alkyl chain length of surfactants. In special, such a trend was prominently observed in diameter. It can be explained by the fact that EBS with shorter alkyl chain would make the smaller size of micelles in microemulsion system than long alkyl chain of DBS. That is, surfactant DBS is easy to form larger size of microreactors containing ZnO seed than EBS, and so could make the diameter of ZnO nanorods larger in DBS microemulsion systems since collisions between larger micelles would lead to larger crystal growth. Fig. 4 shows the N2 adsorption–desorption isotherms of ZnO nanorods synthesized with EBS and DBS surfactants respectively. ZnO nanorods synthesized with short alkyl chain length of surfactant, EBS, show higher specific surface area than DBS (20.2 and 14.1 m2 /g for EBS and DBS, respectively). It seems to be due to the morphological difference between both ZnO nanorods. ZnO nanorods synthesized with EBS shows larger aspect ratio compare with DBS. Fig. 5 shows the PL spectra of the ZnO nanorods derived from microemulsion synthesis with different surfactants. The nanorods obtained using the EBS shows a broad visible emission around 547 nm, which originates from deep level defect emission associated with oxygen vacancies in the ZnO lattices. As prepared by DBS show a strong UV emission around 383 nm and a week and broad emission around 545 nm. The UV emission is the
96
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
Fig. 5. PL spectra obtained from the ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
Fig. 3. FE-SEM images of synthesized ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
band-edge emission resulting from the recombination of excitonic centers [8]. These emissions are closely correlated with the aspect ratios of the nanorods [9]. In addition, the week emissions from zinc vacancies and zinc interstitial defects in the range 410–470 nm are observed in the all samples. To explain the above PL results, the mechanism behind the origin of the green emission band around 545 nm must be considered. As mentioned above, a lot of effort has been considered to understand the mechanism for the green emission band. Heglein et al. attributed it to surface anion vacancies [10]. Bahnemann and co-workers proposed a mechanism that involves tunneling of surface-bound electrons to pre-existing trapped holes [11]. Mo et al. accounted for the green emission band in terms of defect levels associated with oxygen vacancies [12]. Vanheusden et al. proved the singly ionized oxygen vacancy is responsible for the green emission in the ZnO and emission results from the recombination of a photogenerated hole with an electron occupying the oxygen vacancy [13]. Huang et al. have reported that the progressive increase of green light emission intensity to the UV emission as the wire diameter decrease, which suggests that there is a great fraction of oxygen vacancies in the nanowires [14]. We believe that nanorods with high aspect ratios have more surface and sub-surface oxygen vacancies. The nanorods synthesized by EBS which has large aspect ratios show broad and strong green emission compared to the nanorods synthesized by DBS which has small aspect ratios. It was therefore reasonable to believe that the green light emission from the ZnO nanorods in our work could be attributed to the above mentioned single ionized oxygen vacancy. 3.2. Gas sensor measurement The gas sensor reactions were measured for CO in air. Fig. 6 shows the working temperature dependent response to 100 ppm CO in air for the ZnO nanorods based sensors. The gas sensors showed maximum response at 300 ◦ C, and this result may be explained by an oxygen decomposition and adsorption reaction on the sensor surface. ZnO, which is an n-type semiconducting metal oxide, exhibit a change in the resistance through the thickness of the depletion layer that varies according to the mass of oxygen adsorbed to the surface. This phenomenon is the basic mechanism of the resistive gas sensor. Accordingly, in a gas sensor based on the mechanism above, the adsorption and the form of the oxygen ion have a decisive effect on the response of the sensor: O2(gas) → O2(ads) −
O2(ads) + e → O2 Fig. 4. N2 adsorption–desorption BET isotherms of synthesized ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
O2
−
(ads)
−
(1) −
(ads) (T op
+ e → 2O
−
◦
< 100 C)
(ads) (100
(2) ◦
< T op < 300 C)
O− (ads) + e− → O2− (ads) (T op > 300 ◦ C)
(3) (4)
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
Fig. 6. Response of ZnO nanorods to 100 ppm of CO in air depending on working temperature.
97
surface area provides a larger number of surface sites for adsorption and reaction of CO [17,18]. In addition, above mentioned the photoluminescence results of ZnO nanorods said that the nanorods with high aspect ratios show more defects and oxygen vacancies than nanorods with low aspect ratios. These defects and oxygen vacancies of ZnO nanorods may also be considered to influence the gas sensing property of ZnO nanorods. Although it is very difficult to explain the effect of oxygen vacancy on the sensing properties in n-type semiconducting metal oxide since it can be affected by a variety of complicated factors, there were several previous researches describing on the effect of oxygen vacancy on the sensing response. Leu et al. have reported more oxygen vacancies lead more free electrons to migrate between the sensing materials and the adsorbed gas, which results in a promotion of sensitivity in a gas sensor [19]. Wang et al. suggested concentration of oxygen vacancies increases absorbed oxygen and specific surface area increase [20]. That is, when ZnO nanorods are exposed to air, oxygen will be adsorbed on ZnO surface and acts as electron acceptors. The adsorbed oxygen species will capture electrons from ZnO and form oxygen ions, which leads to surface depletion in the conduction channel of ZnO. The carrier density in the ZnO is decreased and the surface depletion layer is widened, both giving rise to an increase of resistance. On the other hand, the oxygen vacancies in ZnO nanorod, which usually act as an electron donor, can make it easier to adsorb atmospheric oxygen on the surface of ZnO nanorod, which would also lead to more effective sensing reaction [21–23]. Such previous researches about relationship between oxygen deficiency and sensing properties were in good agreement with present our results. 4. Conclusions
Fig. 7. Response of ZnO nanorods to 100 ppm of CO in air at 300 ◦ C: (a) EBS and (b) DBS.
Oxygen can be chemisorbed in various forms including O2 − , O− and O2− , which is determined by the operating temperature of the sensor. At low temperature, oxygen exists as O2 − or O− ions on the surface, reacting with one electron. At high temperature, oxygen exists as O2− ions on the ZnO, reacting with two electrons and causing a rapid change in resistance. At the same time, the spontaneous chemisorption of oxygen, which is an exothermic reaction, also occurs on the surface of the sensor. In this spontaneous reaction, the Gibbs’ free energy is less than 0, but the entropy decreases with; thus the whole reaction is an exothermic reaction where the change in entropy is negative. Accordingly, with a temperature increase, the adsorptive reaction decreases. For this reason, at elevated temperatures the sensitivity is increased because of the formation of O2− ions, but is decreased because of the decrease in the adsorptive reaction. Thus, a gas sensor in which the two factors are balanced must use the optimal operating temperature. This behavior is in agreement with previous results [15,16]. Fig. 7 shows the response transients of ZnO nanorods synthesized with EBS and DBS to 100 ppm CO in air at 300 ◦ C. The electrical resistance changed quickly upon turning-on and -off the CO, although the recovery rates were slightly slower than the response rates. The ZnO synthesize with EBS sensor was more sensitive to CO than DBS. The sensor response is strongly affected by the surface area of the material. The BET surface areas of the ZnO nanorods synthesized with EBS is higher than DBS. It is observed that the sensor response increases with increasing surface area of the ZnO nanorods. The response of ZnO nanorods to CO may depend on the adsorption and reaction of CO on the surface, and their higher
ZnO nanorods were synthesized by microemulsion method with different alkyl length of surfactants. It was found that these ZnO nanorods exhibit a high sensor response to CO in air. The aspect ratios and morphologies of ZnO nanorods could be controlled by alkyl group length of surfactants. The surface area of the ZnO nanorods synthesized by EBS and DBS is 20.2 and 14.1 m2 /g, respectively. The highly elevated sensor response of ZnO nanorods synthesized with EBS at 300 ◦ C to CO in air may be attributed to their high surface area, defects and oxygen vacancies. In conclusion, the surfactant assisted microemulsion method can be used to synthesize metal oxide nanorods with high surface areas that may be potentially useful in gas sensors. Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, also supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (11-NB-03). References [1] C. Chou, J. Huang, C. Wu, C. Lee, C. Lin, Lengthening the polymer solidification time to improve the performance of polymer/ZnO nanorod hybrid solar cells, Sol. Energy Mater. Sol. Cells 93 (2009) 1608–1612. [2] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (2004) 3654–3656. [3] H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Nanowire ultraviolet photodetectors and optical switches, Adv. Mater. 14 (2002) 158–160. [4] Z. Yin, N. Chen, R. Dai, L. Liu, X. Zhang, X. Wang, J. Wu, C. Chai, On the formation of well-aligned ZnO nanowall networks by catalyst-free thermal evaporation method, J. Cryst. Growth 305 (2007) 296–301. [5] R. Zhang, P. Yin, N.L.G. Wang, Photoluminescence and Raman scattering of ZnO nanorods, Solid State Sci. 11 (2009) 865–869.
98
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
[6] W.I. Park, D.H. Kim, S.-W. Jung, G. Yi, Metal organic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods, Appl. Phys. Lett. 80 (2002) 4232–4234. [7] L. Guo, Y.L. Ji, H. Xu, Regularly shaped single-crystalline ZnO nanorods with wurtzite structure, J. Am. Chem. Soc. 124 (2002) 14864–14865. [8] L.E. Greene, L. Matt, G. Joshua, K. Franklin, J.C. Johnson, Y.F. Zhang, R.J. Saykally, P.D. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem. Int. Ed. 42 (2003) 3031–3034. [9] G. Sun, M. Cao, Y. Wang, C. Hu, Y. Liu, L. Ren, Z. Pu, Anionic surfactant-assisted hydrothermal synthesis of high-aspect-ratio ZnO nanowires and their photoluminescence property, Mater. Lett. 60 (2006) 2777–2782. [10] U. Koch, A. Fojtik, H. Weller, A. Henglein, Photochemistry of semiconductor colloids. Preparation of extremely small ZnO particles, fluorescence phenomena and size quantization effects, Chem. Phys. Lett. 122 (1985) 507–510. [11] D.W. Bahnemann, C. Karmann, M.R. Hoffmann, Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study, J. Phys. Chem. 91 (1987) 3789–3798. [12] C.M. Mo, Y.H. Li, Y.S. Liu, Y. Zhang, L.D. Zhang, Enhancement effect of photoluminescence in assemblies of nano-ZnO particles/silica aerogels, J. Appl. Phys. 83 (1998) 4389–4391. [13] K. Vanheusden, W.L. Warren, C.H. Seager, D.K. Tallant, J.A. Voigt, B.E. Gnade, Mechanisms behind green photoluminescence in ZnO phosphor powders, J. Appl. Phys. 79 (1996) 7983–7990. [14] M.H. Huang, Y.Y. Wu, H.N. Feick, N. Tran, E. Weber, P.D. Yang, Catalytic growth of zinc oxide nanowires by vapor transport, Adv. Mater. 13 (2001) 113–116. [15] G. Neri, A. Bonavita, G. Micali, G. Rizzo, E. Callone, G. Carturan, Resistive CO gas sensors based on In2 O3 and InSnOx nanopowders synthesized via starchaided sol–gel process for automotive applications, Sens. Actuators B: Chem. 132 (2008) 224–233. [16] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Selective CO detection by using indium oxide-based semiconductor gas sensor, J. Electrochem. Soc. 143 (1996) L36–L37. [17] G.J. Li, X.H. Zhang, S. Kawi, Relationships between sensitivity, catalytic activity, and surface areas of SnO gas sensors, Sens. Actuators B: Chem. 60 (1999) 64–70. [18] Y. Shen, T. Yamazaki, Z. Liu, D. Meng, T. Kikuta, N. Nakatani, Influence of effective surface area on gas sensing properties of WO3 sputtered thin films, Thin Solid Films 517 (2009) 2069–2072. [19] C.M. Chang, M.H. Hon, I.C. Leu, Preparation of ZnO nanorod arrays with tailored defect-related characteristics and their effect on the ethanol gas sensing performance, Sens. Actuators B: Chem. 151 (2010) 15–20.
[20] D. Wang, J. Jin, D. Xia, Q. Ye, J. Long, The effect of oxygen vacancies concentration to the gas-sensing properties of tin dioxide-doped Sm, Sens. Actuators B: Chem. 66 (2000) 260–262. [21] L. Liao, H.B. Lu, J.C. Li, H. He, D.F. Wang, D.J. Fu, C. Liu, W.F. Zhang, Size dependence of gas sensitivity of ZnO nanorods, J. Phys. Chem. C 117 (2007) 1900–1903. [22] Y.J. Chen, C.L. Zhu, G. Xiao, Reduced-temperature ethanol sensing characteristics of flower-like ZnO nanorods synthesized by a sonochemical method, Nanotechnology 17 (2006) 4537–4541. [23] Y. Zhang, J.Q. Xu, Q. Xiang, H. Li, Q.Y. Pan, P.C. Xu, Brush-like hierarchical ZnO nanostructures: synthesis, photoluminescence and gas sensor properties, J. Phys. Chem. C 113 (2009) 3430–3435.
Biographies Sang Kyoo Lim is working in the Division of Nano and Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST) as senior researcher. His research interests are in the area of metal oxide semiconductor gas sensor and nano-structured materials. Sung-Ho Hwang is working in the Division of Nano and Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST) as senior researcher. His research interests are in the area of metal oxide semiconductor gas sensor and nanostructured materials. Soonhyun Kim received her PhD in Environmental Engineering from Pohang University of Science and Technology (POSTECH) in 2005. Currently she is working in the Division of Nano and Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST) as senior researcher. Her research interests are in the area of photocatalysts, metal oxide semiconductor gas sensor and nano-structured materials. Hyunwoong Park is working in the School of Energy Engineering, Kyungbuk National University as professor. His research interests are in the area of photocatalysts, metal oxide semiconductor gas sensor and nano-structured materials.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Preparation of ZnO nanorods by microemulsion synthesis and their application as a CO gas sensor Sang Kyoo Lim a,∗ , Sung-Ho Hwang a , Soonhyun Kim a , Hyunwoong Park b a b
Division of Nano & Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea School of Energy Engineering, Kyungbuk National University, Daegu 702-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 11 July 2011 Accepted 12 July 2011 Available online 20 July 2011 Keywords: Zinc oxide Nanorod Microemulsion Surface area Gas sensor
a b s t r a c t Zinc oxide nanorods with different surface area were synthesized by surfactant assisted microemulsion method. The alkyl chain length of surfactant would affect aspect ratio of ZnO nanorods. ZnO nanorods synthesized by ethyl benzene acid sodium salt (EBS), which is surfactant with short alkyl chain length, show higher aspect ratio than ones by dodecyl benzene sulfonic acid sodium salt (DBS). These nanorods had diameters in the range of 80–300 nm and length of up to several microns. The Brunauer–Emmett–Teller (BET) surface area of the ZnO nanorods was strongly affected by the morphology of the nanorods. The BET surface area of the nanorods synthesized with EBS was higher than the surface area of the nanorods synthesized with DBS (20.2 and 14.1 m2 /g for EBS and DBS, respectively). The response of ZnO nanorods to CO in air was strongly affected by surface area, defects and oxygen vacancies. The results demonstrate that the microemulsion synthesis is an easy and useful method to synthesize ZnO nanorods with large aspect ratio, which may enhance their gas sensing properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, one-dimensional nanostructured materials such as nanowires, nanofibers, nanorods, and nanotubes have received a great attention for their potential applications in numerous areas due to their special properties, which are distinct from conventional bulk materials. The synthesis of nanometer-sized one-dimensional ZnO materials has attracted considerable attention because of their great potential for fundamental studies of the role of dimensionality and size in the physical properties as well as for many applications such as transparent electrode in solar cells, gas sensors and photoluminescence devices [1–3]. Therefore, strong efforts have been made to fabricate one-dimensional ZnO, which were synthesized, for example, by the high-temperature physical evaporation, the micro emulsion process and the template induced method [4–6]. Among all these methods, the solution based synthesis, by thermal treatment of the reactant in different solvents, can be considered as the most simple and effective way to prepare sufficiently crystallized materials at relatively low temperatures. Also, the benefits of utilizing solution-based method have involved controlling the size and morphology according to the choice of reactant species. On the other hand, many of the previous investigations on ZnO prepared by solution based method, have mainly utilized zinc hydroxide or salt
∗ Corresponding author. Tel.: +82 53 785 3510; fax: +82 53 785 3439. E-mail address: [email protected] (S.K. Lim). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.07.018
as precursors and water or organic solvent as reaction media. Only few publications reported the relationship between the morphology of ZnO nanorod and the structure/type of surfactant, and how to effect on the CO gas sensing property. In this work, we presented microemulsion method toward the growth of well-proportioned and crystallized ZnO nanorods using two types of surfactants as the modifying and protecting agent. Under the same synthetic conditions, ZnO nanorods have been obtained with different aspect ratios, depending on the structure of surfactants. The material was characterized structurally using X-ray diffraction (XRD) and morphologically using scanning electron microscopy (SEM). The gas sensing properties of the ZnO nanorod based sensors to carbon monoxide (CO) in air were analyzed.
2. Experimental All chemicals used in this experiment were of analytical grade and used without further purification. The synthesis of ZnO nanorod was carried out in microemulsion, which were consisting of 5 g of surfactant such as ethyl benzene acid sodium salt (EBS), dodecyl benzene sulfonic acid sodium salt (DBS) and 2 mmol of zinc acetate dihydrate (ZnAc2 ·2H2 O) both dispersed in 60 ml xylene by stirring until a homogenous slightly turbid appearance of mixture was obtained. Then hydrazine monohydrate 2 ml and ethanol 8 ml mixture solution was added drop-wisely to the well-stirred mixture at room temperature by simultaneous agitation. The resulting precursor-containing mixture was subsequently
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
95
Fig. 1. Chemical structure of the surfactants: (a) EBS and (b) DBS.
heated to the 140 ◦ C with refluxed. After refluxing for 5 h, a milkywhite suspension was obtained and centrifuged to separate the precipitate, which was rinsed with absolute ethanol and distilled water for several times and dried in vacuum oven at 70 ◦ C for 24 h. XRD patterns of the samples were obtained from an X-ray diffractometer (Rigaku D/MAX-2500, 18 kV) using Cu K␣1 radiation and a quartz monochromator, in the 2 range of 10–80◦ in steps of 0.05◦ . Sample morphology was studied using an Hitachi S-4200 field emission scanning microscopy (FE-SEM). Optical properties were obtained by the photoluminescence (PL) measurements using SPEX 1403 Spectrophotometer excited by a continuous He–Cd laser with a wavelength of 325 nm. The BET surface areas of the prepared samples were determined from nitrogen adsorption–desorption isotherms at 77 K (ASAP 2020 Micromeritics). The sensor was fabricated by dropping ZnO gels on a sapphire substrate with prepatterned Pt electrodes. The ZnO gel was made by grinding 10 mg of ZnO powder with 0.25 mL of de-ionized water in an agate mortar. A precise auto pipette (Eppendorf) was then used to deposit the solution. After dropping 10 L of ZnO gels, the substrate was allowed to dry. The sensor element was heated at 400 ◦ C for 1 h to prevent crystal growth at sensing temperatures (50–400 ◦ C). The properties of the ZnO nanorods must be well-analyzed to optimize their characteristics for CO gas sensing applications. The sensing measurements were performed n a computer-based testing apparatus (PXI-DAQ system, National Instruments, USA) and the gas concentration was controlled by a pre-calibrated mass flow meter (Brooks 5850E, USA). The source measure unit (4200SCS, Keithley, USA) was used for acquisition of the electrical signal, while a power supply (E3631A, Agilent, USA) was employed to bias the sensor’s built-in back heater. The gas response is defined as the percent change in resistance of the sensor upon CO exposure, {(Ra − Rg )/Ra } × 100, where Ra and Rg are the resistances in air and in the presence of CO, respectively. 3. Results and discussion 3.1. Characterization of ZnO nanorods The growths of ZnO nano-structures in microemulsion with different surfactants were investigated. In order to understand the effects of alkyl chain lengths on the morphology and physicochemical properties of ZnO, two different surfactants, EBS and DBS, were used. Fig. 1 shows the structure of surfactants used in our experiments. XRD patterns indicated that all samples exhibited a wurtzite structure (hexagonal phase, space group P63 mc), as shown in Fig. 2. All the diffraction peaks are well indexed to hexagonal phased ZnO (JCPDS card no. 36-1451). The strong diffraction intensities along the [1 0 1] direction as well as the weak diffraction intensities along the [0 0 2] direction were observed in all the samples, implying the preferred growth orientation of ZnO. The XRD results also show only diffraction peaks of ZnO without any trace of other diffraction peaks. Guo et al. firstly reported this microemulsion synthetic method of ZnO nanorods using DBS as the modifying
Fig. 2. XRD patterns of synthesized ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
and protecting agent [7]. They successfully synthesized regular shaped single crystalline ZnO nanorods, however the synthesized ZnO nanorods contained minor Zn(OH)2 phase. In our experiments, we had changed synthetic conditions, and finally obtained highly pure ZnO phase nanorods. The crystal growth could be understood on the basis of following reactions. Micelles formed in a higher concentration of surfactants might act as micro-reactors which contains zinc precursor. In the presence of the OH− ions [Zn(OOCCH3 )2 ·2H2 O] was dissociated to form Zn(OH)2 in the ethanol solvent at high temperature. Sometimes, such Zn(OH)2 might react with the excess OH− ions to produce [Zn(OH)4 ]2− ions. At the high temperature, Zn(OH)2 and [Zn(OH)4 ]−2 in the microreactors could be transformed to ZnO, which came to play a role as a seed for ZnO crystal growth. On the other hand, rodlike space-confined crystal growth occurs in these microemulsion systems due to collision between micelles. The elevated temperature can make the activity of surfactants enhanced and microreactors to more highly collide with each other during the reaction process. FE-SEM images of prepared samples are shown in Fig. 3. The rodlike shapes of samples are relatively uniform with average diameters of 80 nm and 300 nm for EBS and DBS, respectively, and length of up to several microns for both samples. In Fig. 3, it was clearly seen that both the length and thickness of the ZnO nanorods got to increase with increment with alkyl chain length of surfactants. In special, such a trend was prominently observed in diameter. It can be explained by the fact that EBS with shorter alkyl chain would make the smaller size of micelles in microemulsion system than long alkyl chain of DBS. That is, surfactant DBS is easy to form larger size of microreactors containing ZnO seed than EBS, and so could make the diameter of ZnO nanorods larger in DBS microemulsion systems since collisions between larger micelles would lead to larger crystal growth. Fig. 4 shows the N2 adsorption–desorption isotherms of ZnO nanorods synthesized with EBS and DBS surfactants respectively. ZnO nanorods synthesized with short alkyl chain length of surfactant, EBS, show higher specific surface area than DBS (20.2 and 14.1 m2 /g for EBS and DBS, respectively). It seems to be due to the morphological difference between both ZnO nanorods. ZnO nanorods synthesized with EBS shows larger aspect ratio compare with DBS. Fig. 5 shows the PL spectra of the ZnO nanorods derived from microemulsion synthesis with different surfactants. The nanorods obtained using the EBS shows a broad visible emission around 547 nm, which originates from deep level defect emission associated with oxygen vacancies in the ZnO lattices. As prepared by DBS show a strong UV emission around 383 nm and a week and broad emission around 545 nm. The UV emission is the
96
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
Fig. 5. PL spectra obtained from the ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
Fig. 3. FE-SEM images of synthesized ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
band-edge emission resulting from the recombination of excitonic centers [8]. These emissions are closely correlated with the aspect ratios of the nanorods [9]. In addition, the week emissions from zinc vacancies and zinc interstitial defects in the range 410–470 nm are observed in the all samples. To explain the above PL results, the mechanism behind the origin of the green emission band around 545 nm must be considered. As mentioned above, a lot of effort has been considered to understand the mechanism for the green emission band. Heglein et al. attributed it to surface anion vacancies [10]. Bahnemann and co-workers proposed a mechanism that involves tunneling of surface-bound electrons to pre-existing trapped holes [11]. Mo et al. accounted for the green emission band in terms of defect levels associated with oxygen vacancies [12]. Vanheusden et al. proved the singly ionized oxygen vacancy is responsible for the green emission in the ZnO and emission results from the recombination of a photogenerated hole with an electron occupying the oxygen vacancy [13]. Huang et al. have reported that the progressive increase of green light emission intensity to the UV emission as the wire diameter decrease, which suggests that there is a great fraction of oxygen vacancies in the nanowires [14]. We believe that nanorods with high aspect ratios have more surface and sub-surface oxygen vacancies. The nanorods synthesized by EBS which has large aspect ratios show broad and strong green emission compared to the nanorods synthesized by DBS which has small aspect ratios. It was therefore reasonable to believe that the green light emission from the ZnO nanorods in our work could be attributed to the above mentioned single ionized oxygen vacancy. 3.2. Gas sensor measurement The gas sensor reactions were measured for CO in air. Fig. 6 shows the working temperature dependent response to 100 ppm CO in air for the ZnO nanorods based sensors. The gas sensors showed maximum response at 300 ◦ C, and this result may be explained by an oxygen decomposition and adsorption reaction on the sensor surface. ZnO, which is an n-type semiconducting metal oxide, exhibit a change in the resistance through the thickness of the depletion layer that varies according to the mass of oxygen adsorbed to the surface. This phenomenon is the basic mechanism of the resistive gas sensor. Accordingly, in a gas sensor based on the mechanism above, the adsorption and the form of the oxygen ion have a decisive effect on the response of the sensor: O2(gas) → O2(ads) −
O2(ads) + e → O2 Fig. 4. N2 adsorption–desorption BET isotherms of synthesized ZnO nanorods with different surfactants: (a) EBS and (b) DBS.
O2
−
(ads)
−
(1) −
(ads) (T op
+ e → 2O
−
◦
< 100 C)
(ads) (100
(2) ◦
< T op < 300 C)
O− (ads) + e− → O2− (ads) (T op > 300 ◦ C)
(3) (4)
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
Fig. 6. Response of ZnO nanorods to 100 ppm of CO in air depending on working temperature.
97
surface area provides a larger number of surface sites for adsorption and reaction of CO [17,18]. In addition, above mentioned the photoluminescence results of ZnO nanorods said that the nanorods with high aspect ratios show more defects and oxygen vacancies than nanorods with low aspect ratios. These defects and oxygen vacancies of ZnO nanorods may also be considered to influence the gas sensing property of ZnO nanorods. Although it is very difficult to explain the effect of oxygen vacancy on the sensing properties in n-type semiconducting metal oxide since it can be affected by a variety of complicated factors, there were several previous researches describing on the effect of oxygen vacancy on the sensing response. Leu et al. have reported more oxygen vacancies lead more free electrons to migrate between the sensing materials and the adsorbed gas, which results in a promotion of sensitivity in a gas sensor [19]. Wang et al. suggested concentration of oxygen vacancies increases absorbed oxygen and specific surface area increase [20]. That is, when ZnO nanorods are exposed to air, oxygen will be adsorbed on ZnO surface and acts as electron acceptors. The adsorbed oxygen species will capture electrons from ZnO and form oxygen ions, which leads to surface depletion in the conduction channel of ZnO. The carrier density in the ZnO is decreased and the surface depletion layer is widened, both giving rise to an increase of resistance. On the other hand, the oxygen vacancies in ZnO nanorod, which usually act as an electron donor, can make it easier to adsorb atmospheric oxygen on the surface of ZnO nanorod, which would also lead to more effective sensing reaction [21–23]. Such previous researches about relationship between oxygen deficiency and sensing properties were in good agreement with present our results. 4. Conclusions
Fig. 7. Response of ZnO nanorods to 100 ppm of CO in air at 300 ◦ C: (a) EBS and (b) DBS.
Oxygen can be chemisorbed in various forms including O2 − , O− and O2− , which is determined by the operating temperature of the sensor. At low temperature, oxygen exists as O2 − or O− ions on the surface, reacting with one electron. At high temperature, oxygen exists as O2− ions on the ZnO, reacting with two electrons and causing a rapid change in resistance. At the same time, the spontaneous chemisorption of oxygen, which is an exothermic reaction, also occurs on the surface of the sensor. In this spontaneous reaction, the Gibbs’ free energy is less than 0, but the entropy decreases with; thus the whole reaction is an exothermic reaction where the change in entropy is negative. Accordingly, with a temperature increase, the adsorptive reaction decreases. For this reason, at elevated temperatures the sensitivity is increased because of the formation of O2− ions, but is decreased because of the decrease in the adsorptive reaction. Thus, a gas sensor in which the two factors are balanced must use the optimal operating temperature. This behavior is in agreement with previous results [15,16]. Fig. 7 shows the response transients of ZnO nanorods synthesized with EBS and DBS to 100 ppm CO in air at 300 ◦ C. The electrical resistance changed quickly upon turning-on and -off the CO, although the recovery rates were slightly slower than the response rates. The ZnO synthesize with EBS sensor was more sensitive to CO than DBS. The sensor response is strongly affected by the surface area of the material. The BET surface areas of the ZnO nanorods synthesized with EBS is higher than DBS. It is observed that the sensor response increases with increasing surface area of the ZnO nanorods. The response of ZnO nanorods to CO may depend on the adsorption and reaction of CO on the surface, and their higher
ZnO nanorods were synthesized by microemulsion method with different alkyl length of surfactants. It was found that these ZnO nanorods exhibit a high sensor response to CO in air. The aspect ratios and morphologies of ZnO nanorods could be controlled by alkyl group length of surfactants. The surface area of the ZnO nanorods synthesized by EBS and DBS is 20.2 and 14.1 m2 /g, respectively. The highly elevated sensor response of ZnO nanorods synthesized with EBS at 300 ◦ C to CO in air may be attributed to their high surface area, defects and oxygen vacancies. In conclusion, the surfactant assisted microemulsion method can be used to synthesize metal oxide nanorods with high surface areas that may be potentially useful in gas sensors. Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, also supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (11-NB-03). References [1] C. Chou, J. Huang, C. Wu, C. Lee, C. Lin, Lengthening the polymer solidification time to improve the performance of polymer/ZnO nanorod hybrid solar cells, Sol. Energy Mater. Sol. Cells 93 (2009) 1608–1612. [2] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (2004) 3654–3656. [3] H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Nanowire ultraviolet photodetectors and optical switches, Adv. Mater. 14 (2002) 158–160. [4] Z. Yin, N. Chen, R. Dai, L. Liu, X. Zhang, X. Wang, J. Wu, C. Chai, On the formation of well-aligned ZnO nanowall networks by catalyst-free thermal evaporation method, J. Cryst. Growth 305 (2007) 296–301. [5] R. Zhang, P. Yin, N.L.G. Wang, Photoluminescence and Raman scattering of ZnO nanorods, Solid State Sci. 11 (2009) 865–869.
98
S.K. Lim et al. / Sensors and Actuators B 160 (2011) 94–98
[6] W.I. Park, D.H. Kim, S.-W. Jung, G. Yi, Metal organic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods, Appl. Phys. Lett. 80 (2002) 4232–4234. [7] L. Guo, Y.L. Ji, H. Xu, Regularly shaped single-crystalline ZnO nanorods with wurtzite structure, J. Am. Chem. Soc. 124 (2002) 14864–14865. [8] L.E. Greene, L. Matt, G. Joshua, K. Franklin, J.C. Johnson, Y.F. Zhang, R.J. Saykally, P.D. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem. Int. Ed. 42 (2003) 3031–3034. [9] G. Sun, M. Cao, Y. Wang, C. Hu, Y. Liu, L. Ren, Z. Pu, Anionic surfactant-assisted hydrothermal synthesis of high-aspect-ratio ZnO nanowires and their photoluminescence property, Mater. Lett. 60 (2006) 2777–2782. [10] U. Koch, A. Fojtik, H. Weller, A. Henglein, Photochemistry of semiconductor colloids. Preparation of extremely small ZnO particles, fluorescence phenomena and size quantization effects, Chem. Phys. Lett. 122 (1985) 507–510. [11] D.W. Bahnemann, C. Karmann, M.R. Hoffmann, Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study, J. Phys. Chem. 91 (1987) 3789–3798. [12] C.M. Mo, Y.H. Li, Y.S. Liu, Y. Zhang, L.D. Zhang, Enhancement effect of photoluminescence in assemblies of nano-ZnO particles/silica aerogels, J. Appl. Phys. 83 (1998) 4389–4391. [13] K. Vanheusden, W.L. Warren, C.H. Seager, D.K. Tallant, J.A. Voigt, B.E. Gnade, Mechanisms behind green photoluminescence in ZnO phosphor powders, J. Appl. Phys. 79 (1996) 7983–7990. [14] M.H. Huang, Y.Y. Wu, H.N. Feick, N. Tran, E. Weber, P.D. Yang, Catalytic growth of zinc oxide nanowires by vapor transport, Adv. Mater. 13 (2001) 113–116. [15] G. Neri, A. Bonavita, G. Micali, G. Rizzo, E. Callone, G. Carturan, Resistive CO gas sensors based on In2 O3 and InSnOx nanopowders synthesized via starchaided sol–gel process for automotive applications, Sens. Actuators B: Chem. 132 (2008) 224–233. [16] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Selective CO detection by using indium oxide-based semiconductor gas sensor, J. Electrochem. Soc. 143 (1996) L36–L37. [17] G.J. Li, X.H. Zhang, S. Kawi, Relationships between sensitivity, catalytic activity, and surface areas of SnO gas sensors, Sens. Actuators B: Chem. 60 (1999) 64–70. [18] Y. Shen, T. Yamazaki, Z. Liu, D. Meng, T. Kikuta, N. Nakatani, Influence of effective surface area on gas sensing properties of WO3 sputtered thin films, Thin Solid Films 517 (2009) 2069–2072. [19] C.M. Chang, M.H. Hon, I.C. Leu, Preparation of ZnO nanorod arrays with tailored defect-related characteristics and their effect on the ethanol gas sensing performance, Sens. Actuators B: Chem. 151 (2010) 15–20.
[20] D. Wang, J. Jin, D. Xia, Q. Ye, J. Long, The effect of oxygen vacancies concentration to the gas-sensing properties of tin dioxide-doped Sm, Sens. Actuators B: Chem. 66 (2000) 260–262. [21] L. Liao, H.B. Lu, J.C. Li, H. He, D.F. Wang, D.J. Fu, C. Liu, W.F. Zhang, Size dependence of gas sensitivity of ZnO nanorods, J. Phys. Chem. C 117 (2007) 1900–1903. [22] Y.J. Chen, C.L. Zhu, G. Xiao, Reduced-temperature ethanol sensing characteristics of flower-like ZnO nanorods synthesized by a sonochemical method, Nanotechnology 17 (2006) 4537–4541. [23] Y. Zhang, J.Q. Xu, Q. Xiang, H. Li, Q.Y. Pan, P.C. Xu, Brush-like hierarchical ZnO nanostructures: synthesis, photoluminescence and gas sensor properties, J. Phys. Chem. C 113 (2009) 3430–3435.
Biographies Sang Kyoo Lim is working in the Division of Nano and Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST) as senior researcher. His research interests are in the area of metal oxide semiconductor gas sensor and nano-structured materials. Sung-Ho Hwang is working in the Division of Nano and Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST) as senior researcher. His research interests are in the area of metal oxide semiconductor gas sensor and nanostructured materials. Soonhyun Kim received her PhD in Environmental Engineering from Pohang University of Science and Technology (POSTECH) in 2005. Currently she is working in the Division of Nano and Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST) as senior researcher. Her research interests are in the area of photocatalysts, metal oxide semiconductor gas sensor and nano-structured materials. Hyunwoong Park is working in the School of Energy Engineering, Kyungbuk National University as professor. His research interests are in the area of photocatalysts, metal oxide semiconductor gas sensor and nano-structured materials.