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Article Journal of Nanoscience and Nanotechnology Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America Vol 14, 6261–6265, 2014 www.aspbs.com/jnn Fabrication of ZnO Nanorods for Gas Sensing Applications Using Hydrothermal Method Cam Phu Thi Nguyen1 , Phan Phuong Ha La3 , Thanh Thuy Trinh1 , Tuan Anh Huy Le1 , Sungjae Bong1 , Kyungsoo Jang1 , Shihyun Ahn1 , and Junsin Yi1 ∗ Information and Communication Devices Laboratory, College of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea College of Sciences, Vietnam National University, Ho Chi Minh City, 70000, Vietnam We showed well-aligned zinc oxide (ZnO) nanorod arrays synthesized using hydrothermal method at atmospheric pressure The influence of fabrication conditions such as Zn2+ /hexamethylentriamin concentration ratio, and growth temperature on the formation of ZnO nanorods was investigated Scanning Electron Microscope (SEM) images and X-ray Diffraction (XRD) analysis were used to confirm the single crystal of ZnO nanorods, which showed wurtzite structure with growth direction of [0001] (the c-axis) Photoluminescence (PL) measurements of ZnO nanorods revealed an intense ultraviolet peak at 388.5 nm (3.19 eV) at room temperature The results showed that the ZnO seed layers hadDelivered strong influence on the growth of vertically ZnOof nanorods The gas sensor based by Publishing Technology to: aligned University Southern California IP: 162.251.157.108 On: Tue, 01 Dec 2015 19:01:50 on ZnO nanorod arrays had the most selectivity with n-butanol gas (within surveyed gas: ethanol American Publishers and n-butanol) and showedCopyright: a higher sensitivity of Scientific 222, fast response time of 15 seconds, recovery time of 110 seconds and lower operating temperature of 200–250 C than the sensor based on the ZnO film in the same detecting conditions Keywords: ZnO Nanorods, Hydrothermal Method, n-Butanol Sensing, High Sensitivity INTRODUCTION Since the discovery of carbon nanotubes (CNTs) in 1991 one dimensional (1D) nanomaterials were widely used because of their potential applications As a n-type semiconductor with the wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature,2 ZnO played an important role in the 1D nanomaterials field The recent research focused on electronic and photonic applications such as ultraviolet (UV)/blue lightemitting devices,3 solar cells,4 chemical sensors.5 1D ZnO nanorods have been fabricated by various methods namely metal-organic chemical vapor deposition metal organic vapor phase epitaxy (MOVPE),6 and electrodeposition.7 Nevertheless, these methods required generally high temperature and expensive equipments Recently, hydrothermal method has been used due to their unique advantage of milder conditions and simple equipments.8 Nowadays, ZnO nanorods were used as a material for gas sensors because of their ultrahigh surface-to-volume ∗ Author to whom correspondence should be addressed J Nanosci Nanotechnol 2014, Vol 14, No ratio The main advantages of these sensors are their long life, low cost, and easy for large-scale production N -butanol is highly inflammable and volatile gas It can convert to inflammable gas compound, inhibit the central nervous system, damage liver and kidney and affect reproduction system However, there has been little research about n-butanol gas sensing In this study we presented a comprehensive perspective on surveys involved in the wet solution process to optimize the single crystal ZnO fabrication with wurtzite structure on glass substrate In addition, the application of ZnO single crystal on n-butanol gas sensor was also investigated EXPERIMENTAL DETAILS The well-aligned ZnO nanorod arrays were fabricated using simple wet-solution processes For the first step, zinc acetate dehydrate (Zn(CH3 COO)2 · 2H2 O) was dissolved in a mixture of 2-methoxyethanol ((2ME) H3 COCH2 CH2 OH) and monoethanolamine ((MEA) HOCH2 CH2 NH2 solution to make solution A Solution A were then stirred in 1533-4880/2014/14/6261/005 doi:10.1166/jnn.2014.8819 6261 Fabrication of ZnO Nanorods for Gas Sensing Applications Using Hydrothermal Method Nguyen et al hour and aged at room temperature in 22 hours The reaction for creating the sol solution was presented as: Zn CH3 COO ·2H2 O → ZnCOOCH3 + +H+ +2OH− HOCH2 CH2 NH2 + ZnCOOCH3 → ZnOCH2 CH2 NH2 +CH3 COOH ZnCOOCH3 + + +H +2OH (1) + (2) − → CH3 COOH+Zn OH Zn OH → ZnO+H2 O (3) (4) Figure XRD pattern of ZnO nanorods grown at 80 C, h and The ZnO films were then coated onto the glass substrates Zn2+ /HMTA = 1:1 by dip coating method These films were pre-heated afterwards at 250 C for 20 mins immediately to remove solseed-layer-covered glass The nanorods were grown on vent and then annealed in air at 500 C for hours both substrates Nevertheless, the density of ZnO nanorods In the second step, ZnO nanorods were grown by grown on glass substrates coated with ZnO seed layer hydrothermal method in which zinc nitrate hexalhydrat (Fig 1(b)) was significantly greater and better aligned (Zn(NO3 · 6H2 O) and hexamethylentriamin ((HMTA) than on bare substrates (Fig 1(a)) Because of the exisC6 H12 N4 were dissolved in deionized water to form solutence of ZnO seed layers, the surface roughness of the tion B Substrates coated with ZnO seeds were then soaked glass substrates was higher than that on the bare subin solution B with different conditions of temperature, time strates Therefore, the density of sites which were favorand HMTA concentrations able to form ZnO nuclei in the initial stage was small Finally, the substrates were removed from the aqueous in case of bare substrates.11 These results confirmed that solution, rinsed with double distilled water, and allowed seed layers had a strong influence on the growth of to dry in air at room temperature The morphology of to:the Delivered by Publishing Technology University of Southern California 13 nanorods IP: 162.251.157.108 On: Tue, 01 Dec122015 19:01:50 the ZnO nanorods was observed by scanning electron Copyright: American FigurePublishers shows XRD patterns of the ZnO nanorods microscopy (SEM) X-ray diffraction (XRD) patterns were Scientific grown on the substrates coated with ZnO seed layer The obtained by employing a Siemens Diffraktometer using XRD pattern exhibits a (002) preferred orientation in domCuK radiations ( = 54059 Å) The optical properties inating at 34.589 and no diffraction peaks of other impuof ZnO nanorods were measured by photoluminescence rities phases are found in the samples This suggests that spectroscopy (PL) with a N2 laser as the excitation light the rods virtually aligned with the c-axis which oriented source with wavelength 337.1 nm at room temperature orthogonally to the substrate surface The sharp and narDetails of the sensing measurement and the parameter row diffraction peaks indicate that the ZnO nanorods have definition had been described in our previous article.10 good crystallinity The emission properties of the ZnO nanorods were eval3 RESULTS AND DISCUSSION uated by room-temperature PL (Fig 3) Photolumines3.1 Grown of ZnO Nanorods cence measurements of ZnO nanorods revealed an intense Figure shows the SEM images of ZnO nanorods fabultraviolet peak at 388.5 nm (E = hv = 19 eV) The high ricated on different substrates including bare glass and Figure SEM images of ZnO nanorods fabricated (a) without seeds layer (b) with ZnO seeds layer on glass substrates 6262 Figure PL spectra of ZnO nanorods grown at 80 C, h and Zn2+ /HMTA = 1:1 J Nanosci Nanotechnol 14, 6261–6265, 2014 Nguyen et al Fabrication of ZnO Nanorods for Gas Sensing Applications Using Hydrothermal Method intensity of this peak in ZnO nanorods was related to exciton emission which could be observed in high quality samples.14 This demonstrated high purity of the ZnO hexagonal phase and good crystallinity of ZnO nanorod samples.2 From the results mentioned above, we decided to use ZnO nanorods grown on substrate coated with ZnO seed layer for next surveys The ZnO nanorod growth mechanisms were involved by the following reactions: Zn NO3 → Zn2+ + 2NO− (5) C6 H12 N4 + 6H2 O → 6CH2 O + 4NH3 (6) NH3 + H2 O → NH4+ + OH− (7) Zn 2+ − + 2OH → Zn OH Zn OH 2 → ZnO + H2 O (8) (9) Figure SEM images of ZnO nanorod arrays at different temperature: In aqueous solution, C6 H12 N4 was solvated by water (a) room temperature, (b) 50 C, (c) 80 C, (d) 120 C − to create aquo ions (NH+ and OH ) In a hydrother4 mal process, Zn2+ bonded with OH− ions, producing the 50 C, 80 C and 120 C At room temperature (Fig 5(a)), wurtzite structural ZnO crystal The OH− concentration ZnO nanorods were not formed The size of ZnO nanorods played an important role in the growth process When the increased as the temperature was increased When the HMTA concentration was 0, i.e., there was no OH− in temperature increased to 50, 80 and 120 C, the ZnO the solution, ZnO nanorods were unfabricated as seen in nanorod’s diameters increased to 25, 50, 55 nm, respecFigure 4(a) Figures 4(b) and (c) show that after annealtively During the hydrothermal process, the Zn(OH)2 dising at the concentration ratio Zn2+ /HMTA = 1:1 and 1:2, Delivered by Publishing Technology to: University of Southern California solved with2015 treatment temperature When the temperature the nanorods had diameters of about 80–100 nm and 150– IP: 162.251.157.108 On: Tue, 01 Dec 19:01:50 of solution reached the term of value, ZnO nuclei were 200 nm, respectively Consequently, Copyright: in the same fabriAmerican Scientific Publishers formed spontaneously.15 There was significant difference cating conditions, higher HMTA concentration resulted in in ZnO nanorod size between the sample fabricated at larger diameter 50 C and at temperature higher than 80 C It is due to Figure displays the top-view SEM images of ZnO the thermal energy provided for ZnO nanoparticles Therenanorod arrays on glass substrates at room temperature, fore, temperature of the reactor played an important role on the ZnO nanorod formation 3.2 Gas-Sensing Based on ZnO Nanorods Figure shows the response of ZnO nanorods gas sensor with ethanol and n-butanol measured at 300 C at Figure SEM images of ZnO nanorod arrays at different ratio of Zn2+ /HMTA: (a) 1:0, (b) 1:1, (c) 1:2 J Nanosci Nanotechnol 14, 6261–6265, 2014 Figure Sensitivity S versus of the ZnO nanorods operating in gas environment (Ethanol and n-butanol) at 300 C and the concentration from 50 to 500 ppm 6263 Fabrication of ZnO Nanorods for Gas Sensing Applications Using Hydrothermal Method various gas concentration (50–500 ppm) The response increases with increasing gas concentration The changes in the response is influenced the selectivity of the sensor When ZnO was exposed to air, the oxygen molecules were adsorbed on the surface of ZnO, captured electrons from the conduction band and formed oxygen ions.16 − O2 ads ↔ O− ads ↔ 2O2− ads ads ↔ 2O (10) Nguyen et al These equations show that n-butanol reacts with more O− ion, so the response of n-butanol with the sensitivity of Ra /Rg = 222 is higher than ethanol In the same conditions, the aligned ZnO nanorods had a larger surface area exposed to air than those of other ZnO thin films The response and recovery time of the gas sensors based on the aligned ZnO nanorod arrays and the ZnO thin films were 25 s, 110 s, and 55 s, 170 s, respectively (Fig 7) The response value of the gas sensor based on ZnO nanorods was higher compared to the gas sensor based on ZnO thin films Such results indicated that the response speed of the fabricated sensor was good The behavior of oxygen ions was changed by the operating temperature At the temperature lower than 150 C, the − O− (ads) was the major oxygen species Whereas O (ads) played the major role at the range of temperature from 150 C to 450 C For further increase of the temperature higher than 450 C, O2− (ads) became dominant CONCLUSION In our study, when the temperature at the range from In summary, we fabricated successfully the ZnO nanorods 150 C to 300 C, the O− (ads) would be changed to with good crystalline quality, high density, and well2 (ads) corresponding to the removal of one electron O− aligned on the substrate surface Based on these ZnO from the thin film/rod surface; therefore, it led to increase nanorod arrays, the gas sensors were fabricated with faster the density of charge on the surface and changed the surresponse, higher sensitivity, and lower operating temperaface conductivity Therefore, O− ions had strong influence ture than the gas sensor based on the ZnO film in the same on the ZnO response detecting conditions Well-aligned ZnO nanorod arrays In case the ZnO nanorod sensor was exposed to a reducwere obtained at 80 C for hours with the concentrative gas such as ethanol and n-butanol at operating temtion ratio of Zn2+ /HMTA = 1:1 Finally, the n-butanol gas perature, the test gas reacted with oxygen, chemisorbed sensor based on ZnO nanorod arrays showed high effion the semiconductor surface and released electrons to the ciency with the sensitivity of Ra /Rg = 222, fast response Delivered Publishing University of Southern California conduction band This eventuallybyincreased the Technology conductiv- to:time of 15 seconds, recovery time of 110 seconds These IP:effective 162.251.157.108 01 Dec 2015arrays 19:01:50 ity of ZnO Hence, the more the surface areaOn: is, Tue, ZnO nanorod are a promising material for gas Copyright: American Publishers the more oxygen-adsorption is quantified, and the higher Scientific sensor the sensitivity of ZnO thin film sensors is as shown in Acknowledgment: This research was supported by Figure the New and Renewable Energy Technology DevelopThe reaction between ethanol17 and n-butanol18 and ment Program (No 20113020010010) and the Human ionic oxygen species can be described by: Resources Development program (No 20124010203280) CH3 CH2 OH ads +6O− ads of the Korea Institute of Energy Technology Evaluation − and Planning (KETEP) grant funded by the Korea govern→ 2CO2 +3H2 O+6e (11) ment Ministry of Knowledge Economy CH4 CH9 OH ads +12O− ads → 4CO2 +5H2 O+12e− (12) Figure Sensitivity response curves of ZnO nanorods and ZnO thin film in 500 ppm n-butanol at the optimal operated temperature 300 C 6264 References and Notes S Iijima, Nature 354, 56 (1991) T Mahalingam, K M Lee, K H Park, S Lee, Y Ahn, J Y Park, and K H Koh, Nanotechnology 18, 035606 (2007) L Schmidt-Mende and J L MacManus-Driscoll, Materials Today 10, 40 (2007) I M Tiginyanu, O Lupan, V V Ursaki, L Chow, and M Enachi, Semicond Sci Technol 3, 396 (2011) Z Fan and J G Lu, J Nanosci Nanotechnol 5, 1561 (2005) G.-C Yia, T Yatsuib, and M Ohtsub, Nanoscience and Technology 1, 335 (2010) S P Anthony, J I Lee, and J K Kim, Appl Phys Lett 90, 103107 (2007) S.-J Park, J Qiu, W He, W Namgung, J.-H Lee, Y.-H Hwang, and H.-K Kim, J Nanosci Nanotechnol 9, 6993 (2009) A R Kim, J H Lee, C H Kim, H S Kim, K.-H Baek, and L.-M Do, J Nanosci Nanotechnol 11, 7319 (2011) 10 T T Trinh, N H Tu, H H Le, K Y Ryu, K B Le, K Pillai, and J Yi, Sens Actuators, B: Chemical 152, 73 (2011) J Nanosci Nanotechnol 14, 6261–6265, 2014 Nguyen et al Fabrication of ZnO Nanorods for Gas Sensing Applications Using Hydrothermal Method 11 Z Liu, J Ya, and L E., J Solid State Electrochem 14, 957 (2010) 12 Y Tao, M Fu, A Zhao, D He, and Y Wang, J Alloys Compd 489, 99 (2010) 13 G J Lee, S.-K Min, C.-H Oh, Y Lee, H Lim, H Cheong, H J Nam, C K Hwangbo, S.-K Min, and S.-H Han, J Nanosci Nanotechnol 11, 511 (2011) 14 W Park, Y Jun, S Jung, and G C Yi, Appl Phys Lett 82, 964 (2003) 15 L E Greene, B D Yuhas, M Law, D Zitoun, and P Yang, Inorg Chem 45, 7535 (2006) 16 M Liu, Synthesis of ZnO nanowires and applications as gas sensors, University of Saskatchewan (2010) 17 K Mirabbaszadeh and M Mehrabian, Physica Scripta 85, 035701 (2012) 18 H Zhang, R Wu, Z Chen, Z Zhang, and Z Jiao, CrystEngComm 14, 1775 (2012) Received: 20 May 2012 Accepted: 13 March 2013 Delivered by Publishing Technology to: University of Southern California IP: 162.251.157.108 On: Tue, 01 Dec 2015 19:01:50 Copyright: American Scientific Publishers J Nanosci Nanotechnol 14, 6261–6265, 2014 6265 ... versus of the ZnO nanorods operating in gas environment (Ethanol and n-butanol) at 300 C and the concentration from 50 to 500 ppm 6263 Fabrication of ZnO Nanorods for Gas Sensing Applications Using. . .Fabrication of ZnO Nanorods for Gas Sensing Applications Using Hydrothermal Method Nguyen et al hour and aged at room temperature in 22 hours The reaction for creating the sol solution... Yi, Sens Actuators, B: Chemical 152, 73 (2011) J Nanosci Nanotechnol 14, 6261–6265, 2014 Nguyen et al Fabrication of ZnO Nanorods for Gas Sensing Applications Using Hydrothermal Method 11 Z Liu,

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