Comparative study of gas sensing properties between zno sno2 and zno sno2 nanofibers

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY - PHAN HONG PHUOC COMPARATIVE STUDY OF GAS SENSING PROPERTIES BETWEEN ZnO/SnO2 AND ZnO-SnO2 NANOFIBERS NGHIÊN CỨU CHẾ TẠO VÀ SO SÁNH ĐẶC TRƯNG NHẠY KHÍ GIỮA SỢI NANO ZnO/SnO2 VÀ ZnO-SnO2 MASTER THESIS MATERIAL SCIENCE Hanoi - 2019 MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY - PHAN HONG PHUOC COMPARATIVE STUDY OF GAS SENSING PROPERTIES BETWEEN ZnO/SnO2 AND ZnO-SnO2 NANOFIBERS NGHIÊN CỨU CHẾ TẠO VÀ SO SÁNH ĐẶC TRƯNG NHẠY KHÍ GIỮA SỢI NANO ZnO/SnO2 VÀ ZnO-SnO2 Major: Material Science MASTER THESIS MATERIAL SCIENCE SUPERVISOR Associate professor Ph.D Nguyen Van Duy Hanoi - 2019 CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM Độc lập – Tự – Hạnh phúc BẢN XÁC NHẬN CHỈNH SỬA LUẬN VĂN THẠC SĨ Họ tên tác giả luận văn: Phan Hồng Phước Đề tài luận văn: Nghiên cứu chế tạo so sánh đặc trưng nhạy khí sợi nano ZnO/SnO2 ZnO-SnO2 Chuyên ngành: Khoa học Kỹ thuật vật liệu Mã số SV: CB170310 Tác giả, người hướng dẫn khoa học Hội đồng chấm luận văn xác nhận tác giả sửa chữa, bổ sung luận văn theo biên họp Hội đồng ngày 30 tháng 10 năm 2019 với nội dung sau: - Tên đề tài luận văn chỉnh sửa từ “Nghiên cứu chế tạo sợi nano tổ hợp phương pháp phun tĩnh điện ứng dụng cho cảm biến khí” thành: “Nghiên cứu chế tạo so sánh đặc trưng nhạy khí sợi nano ZnO/SnO ZnO-SnO2” Tên tiếng anh: “Comparative gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers” - Tác giả chỉnh sửa cách diễn đạt, bình luận, đánh giá kích thước hạt đo SEM tính qua cơng thức Debye-Scherrer (trang 32) - Tác giả chỉnh sửa thích hình 1.5, 1.8, độ phân giải hình 3.6 c d - Tác giả xếp bảng kí hiệu viết tắt theo thứ tự abc bổ sung thêm số từ viết tắt PVP, TGA, DTG - Tác giả bổ sung thêm số kết vào phần introduction Hà Nội, ngày 12 tháng 11 năm 2019 Giáo viên hướng dẫn Tác giả luận văn PGS TS Nguyễn Văn Duy Phan Hồng Phước CHỦ TỊCH HỘI ĐỒNG PGS TS Nguyễn Phúc Dương Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers ACKNOWLEDGMENTS I would like to take this opportunity to express my gratitude to all persons who have given me their invaluable support and assistance I am profoundly grateful to Professor Ph.D Nguyen Van Hieu, Associate professor Ph.D Nguyen Van Duy, Associate professor Ph.D Nguyen Duc Hoa, Ph.D Chu Manh Hung for their scientific advice and insightful discussions I would like to thank Associate professor Ph.D Pham Anh Son (HUS-VNU), MSc Ta Ngoc Bach (VAST), MSc Nguyen Quang Hoa (HUS-VNU), MSc Pham Thi Nga (HUS-VNU) for their help in material characterization analysis I am very grateful to my colleague, Ph.D student Nguyen Van Hoang, who has dedicated so much time in helping me during all the time I my thesis I would like to say great thanks to my classmate, MSc student Tran Thi Mai Phuong who was giving me a lot of supports in two years I am doing my Master's degree at ITIMS Finally, but not least, I am deeply thankful to my family, my parents, for their love and encouragement I am heavily indebted to my younger brother and my maternal grandmother who passed away while I have to live far from home to this thesis Dedicated to the memory of my younger brother and my maternal grandfather, who have always believed in my ability to be successful in the academic arena You are gone but your belief in me has made this journey possible I love you so much Hanoi, 30th September 2019 Phan Hong Phuoc PHAN HONG PHUOC i ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers COMMITTAL IN THE THESIS I confirm that this thesis is the result of my personal research and is solely my own work under the guidance of Associate professor Ph.D Nguyen Van Duy I declare that my scientific results are righteous I have responsibilities for my research results in this thesis Author Phan Hong Phuoc PHAN HONG PHUOC ii ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers TABLE OF CONTENTS ACKNOWLEDGMENTS i COMMITTAL IN THE THESIS ii TABLE OF CONTENTS iii List of Abbreviations v List of Figures vi List of Tables ix INTRODUCTION .1 Object for study Research objective Research method Organization of the Thesis Chapter LITERATURE REVIEW 1.1 Metal oxide semiconductor gas sensors 1.1.1 Gas sensors construction 1.1.2 Gas-sensing mechanisms 1.2 Nanofibers for gas sensors .7 1.2.1 Fabrication of nanofibers 1.2.2 Gas sensing mechanisms of nanofibers 10 1.2.3 Review of composite nanofibers for gas sensors 11 1.3 Effect of hetero-junction between ZnO and SnO2 nanofibers on gas sensing properties 13 1.3.1 Zinc oxide and Tin oxide properties .13 PHAN HONG PHUOC iii ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers 1.3.2 1.4 Hetero-junction between ZnO and SnO2 in gas sensing .15 Research orientation 17 Chapter EXPERIMENTAL 18 2.1 Chemicals preparation 18 2.2 Nanofibers synthesis 18 2.2.1 Preparation of the precursor solution for electrospinning 18 2.2.2 Electrospinning process 19 2.3 Material characterization .22 2.4 Gas sensing measurements 22 Chapter RESULTS AND DISCUSSIONS 25 3.1 Materials characterization 25 3.1.1 Thermogravimetric analysis 25 3.1.2 Morphological observation of nanofibers .26 3.1.3 Compositional and crystal properties of the nanofibers .29 3.2 Gas sensing properties of the nanofibers .34 3.2.1 H2S sensing results 34 3.2.2 NO2 sensing results .39 3.3 Gas sensing mechanisms .41 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK .46 LIST OF PUBLICATIONS 47 REFERENCES 48 APPENDIX .56 PHAN HONG PHUOC iv ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers List of Abbreviations No Abbreviation Meaning 1D One-dimensional EDX Energy Dispersive X-ray Spectroscopy FESEM Field Emission Scanning Electron Microscopy HRTEM High-Resolution Transmission Electron Microscopy JCPDS Joint Committee on Powder Diffraction Standards MOS Metal Oxide Semiconductors NFs Nanofibers ppm Parts per million PVP Polyvinylpyrrolidone 10 Ra The resistances in the air 11 Rg The resistances in the gas 12 S Gas response 13 TG-DTA Thermal gravimetric and differential thermal analysis 14 resp Response time 15 recv Recovery time 16 XRD X-Ray Diffraction PHAN HONG PHUOC v ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers List of Figures Figure 1-1 The scheme of a typical resitive sensor system Figure 1-2 Band bending at an n-type semiconductor surface due to oxygen absorbtion [23] .5 Figure 1-3 The schematic of the on-chip fabrication of NF sensors by electrospinning [34] .7 Figure 1-4 The schematic illustration of the effect of increasing the surface charge on drop deformation [36] .8 Figure 1-5 (a) The schematic diagram of the path of an electrospinning jet (b) The schematic illustration of the Earnshaw instability leading to bending of an electrified jet [41],[43] Figure 1-6 The schematic of the gas-sensing mechanism of NFs: (a) in air, and (b) in H2S gas [46] .10 Figure 1-7 (a) SEM images of SnO2/In2O3 hetero-NFs, (b) Gas responses of SnO2, In2O3 and SnO2/In2O3 sensors to 10 ppm formaldehyde as a function of operating temperature [13] 11 Figure 1-8 (a) SEM images of SnO2-ZnO NFs (the inset shows the corresponding high magnification images), and (b) Comparison the gas response of ZnO, SnO2ZnO and SnO2 NFs to 100 ppm ethanol at different operating temperatures (200 °C - 400 °C) [9] 12 Figure 1-9 ZnO unit cell with wurtzite structure [49] 13 Figure 1-10 Bulk structures of the SnO2 polymorphs (gray and red colors represent Sn and O atoms, respectively) [57] 14 Figure 1-11 The schematic diagrams of the energy band structure of ZnO and SnO2: (a) before contact, and (b) after-contact 16 PHAN HONG PHUOC vi ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers Table 3-2 Properties of several gas molecules [80], [81] Gas type H2S NO2 NH3 H2 CO Bond H-SH NO-O H-NH2 H-H C-O Bond energy (kJ/mol) 381 305 435 436 1076 PHAN HONG PHUOC 45 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK  In summary, two types of the composite structure of ZnO-SnO2 NFs and ZnO/SnO2 NFs, as well as pure ZnO NFs and SnO2 NFs were successfully synthesized through facile on-chip electrospinning method  The gas sensing characteristic of NF sensors was investigated carefully towards reducing H2S gas and oxidizing NO2 gas Highly sensitive result of NF sensors were explained by two main combined factors including the modulation of the resistance along with the surface depletion layer and the grain boundaries  The results displayed that the composite NFs with different structures of ZnO-SnO2 NFs and ZnO/SnO2 NFs exhibited higher sensitivity compared to that of pure ZnO NFs and SnO2 NFs, in which, the ZnO-SnO2 NFs indicated the highest sensitivity towards reducing H2S gas and NO2 gas at all operating temperatures from 250 °C - 450 °C Moreover, the ZnO-SnO2 NFs showed the fastest response  Overall, the finding demonstrates that the high response of fibers is mainly governed by potential barriers at the intergranular contact in NFs (internal-junction)  Since the gas sensing mechanisms of n-type MOS and p-type MOS are somewhat contradictory The p-type composite NFs with two structures need to be prepared and investigated to comparare the effects of internal-junction and externaljunction on their gas sensing performance PHAN HONG PHUOC 46 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers LIST OF PUBLICATIONS [1] Phan Hong Phuoc, Chu Manh Hung, Nguyen Van Toan, Nguyen Van Duy, Nguyen Duc Hoa, and Nguyen Van Hieu, “One-step fabrication of SnO2 porous nanofiber gas sensors for sub-ppm H2S detection”, Sensors & Actuators: A Physical, 2019 [2] Phan Hong Phuoc, Nguyen Van Duy, Chu Manh Hung, “H2S gas sensing characteristics of ZnO nanofibers fabricated by electrospinning”, The 11th Vietnam National Conference of Solid Physics and Materials Science, 2019 PHAN HONG PHUOC 47 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers REFERENCES [1] A Mirzaei, S S Kim, and H W Kim, “Resistance-based H2S gas sensors using metal oxide nanostructures: A review of recent advances,” Journal of Hazardous Materials, vol 357 pp 314–331, 2018 [2] R Binions and A J T Naik, “Metal oxide semiconductor gas sensors in environmental monitoring,” Semicond Gas Sensors, no x, pp 433–466, 2013 [3] K Y Dong et al., “Enhanced H2S sensing characteristics of Pt doped SnO2 nanofibers sensors with micro heater,” Sensors Actuators, B Chem., vol 157, no 1, pp 154–161, 2011 [4] C International and C Assessment, “Concise International Chemical Assessment Document 53 HYDROGEN SULFIDE : HUMAN HEALTH ASPECTS,” 2003 [5] National Institute for Occupational Safety and Health, “Threshold Limit Values (TLV) and Immediately Dangerous to Life and Health (IDLH) values,” Saf Heal., p 900, 2005 [6] M M Arafat, B Dinan, S A Akbar, and A S M A Haseeb, “Gas sensors based on one dimensional nanostructured metal-oxides: A review,” Sensors (Switzerland), vol 12, no 6, pp 7207–7258, 2012 [7] A Katoch, S W Choi, H W Kim, and S S Kim, “Highly sensitive and selective H2 sensing by ZnO nanofibers and the underlying sensing mechanism,” J Hazard Mater., vol 286, pp 229–235, 2015 [8] A Katoch, J H Kim, Y J Kwon, H W Kim, and S S Kim, “Bifunctional sensing mechanism of SnO2-ZnO composite nanofibers for drastically enhancing the sensing behavior in H2 gas,” ACS Appl Mater Interfaces, vol 7, no 21, pp 11351–11358, 2015 [9] S H Yan et al., “Synthesis of SnO2-ZnO heterostructured nanofibers for enhanced ethanol gas-sensing performance,” Sensors Actuators, B Chem., vol 221, pp 88–95, 2015 [10] S W Choi, A Katoch, J Zhang, and S S Kim, “Electrospun nanofibers of CuOSnO2 nanocomposite as semiconductor gas sensors for H2S detection,” Sensors PHAN HONG PHUOC 48 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers Actuators, B Chem., vol 176, pp 585–591, 2013 [11] J H Kim, J H Lee, A Mirzaei, H W Kim, and S S Kim, “Optimization and gas sensing mechanism of n-SnO2-p-Co3O4 composite nanofibers,” Sensors Actuators, B Chem., vol 248, pp 500–511, 2017 [12] P Feng, Q Wan, and T H Wang, “Contact-controlled sensing properties of flowerlike ZnO nanostructures,” Appl Phys Lett., vol 87, no 21, pp 1–3, 2005 [13] H Du, J Wang, M Su, P Yao, Y Zheng, and N Yu, “Formaldehyde gas sensor based on SnO2/In2O3 hetero-nanofibers by a modified double jets electrospinning process,” Sensors Actuators, B Chem., vol 166–167, pp 746–752, 2012 [14] H Du, J Wang, Y Sun, P Yao, X Li, and N Yu, “Investigation of gas sensing properties of SnO2/In2O3 composite hetero-nanofibers treated by oxygen plasma,” Sensors Actuators, B Chem., vol 206, pp 753–763, 2015 [15] A Katoch, J H Kim, and S S Kim., “CuO/SnO2 mixed nanofibers for H2S detection,” J Nanosci Nanotechnol., vol 15, no 11, pp 8637–8641, 2015 [16] A Dey, “Semiconductor metal oxide gas sensors: A review,” Mater Sci Eng B Solid-State Mater Adv Technol., vol 229, no July 2017, pp 206–217, 2018 [17] G Korotcenkov, “Metal oxides for solid-state gas sensors: What determines our choice?,” Mater Sci Eng B Solid-State Mater Adv Technol., vol 139, no 1, pp 1–23, 2007 [18] M M Arafat et al., “Metal oxide semiconductor gas sensors in environmental monitoring,” Semicond Gas Sensors, vol 12, no 2, pp 433–466, 2012 [19] Z U Abideen, A Katoch, J H Kim, Y J Kwon, H W Kim, and S S Kim, “Excellent gas detection of ZnO nanofibers by loading with reduced graphene oxide nanosheets,” Sensors Actuators, B Chem., vol 221, pp 1499–1507, 2015 [20] J Y Park, K Asokan, S W Choi, and S S Kim, “Growth kinetics of nanograins in SnO2 fibers and size dependent sensing properties,” Sensors Actuators, B Chem., vol 152, no 2, pp 254–260, 2011 [21] Y Shimizu and M Egashira, “Basic aspects and challenges of semiconductor gas PHAN HONG PHUOC 49 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers sensors,” MRS Bull., vol 24, no 6, pp 18–24, 1999 [22] N Barsan and U Weimar, “Conduction model of metal oxide gas sensors,” J Electroceramics, vol 7, no 3, pp 143–167, 2001 [23] D A Mirabella, C Buono, C M Aldao, and D E Resasco, “Sensors and Actuators B : Chemical Chemisorption and sensitivity at semiconductor sensors revisited,” Sensors Actuators B Chem., vol 285, no October 2018, pp 232–239, 2019 [24] M M Arafat, A S M A Haseeb, and S A Akbar, Developments in Semiconducting Oxide-Based Gas-Sensing Materials, vol 13 Elsevier, 2014 [25] H J Kim and J H Lee, “Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview,” Sensors Actuators, B Chem., vol 192, pp 607– 627, 2014 [26] J Wu et al., “Highly selective gas sensing properties of partially inversed spinel zinc ferrite towards H2S,” Sensors Actuators, B Chem., vol 235, pp 258–262, 2016 [27] X Gao, Y Sun, C Zhu, C Li, Q Ouyang, and Y Chen, “Highly sensitive and selective H2S sensor based on porous ZnFe2O4 nanosheets,” Sensors Actuators, B Chem., vol 246, pp 662–672, 2017 [28] Z Zhang, M haq, Z Wen, Z Ye, and L Zhu, “Ultrasensitive ppb-level NO2 gas sensor based on WO3 hollow nanosphers doped with Fe,” Appl Surf Sci., vol 434, no 2, pp 891–897, 2018 [29] P Rai and Y T Yu, “Citrate-assisted hydrothermal synthesis of single crystalline ZnO nanoparticles for gas sensor application,” Sensors Actuators, B Chem., vol 173, no 2, pp 58–65, 2012 [30] A Mirzaei, B Hashemi, and K Janghorban, “α-Fe2O3 based nanomaterials as gas sensors,” J Mater Sci Mater Electron., vol 27, no 4, pp 3109–3144, 2016 [31] A A Haidry, N Kind, and B Saruhan, “Investigating the influence of Al-doping and background humidity on NO2 sensing characteristics of magnetron-sputtered SnO2 sensors,” J Sensors Sens Syst., vol 4, no 2, pp 271–280, 2015 [32] N G Cho, D J Yang, M J Jin, H G Kim, H L Tuller, and I D Kim, “Highly PHAN HONG PHUOC 50 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers sensitive SnO2 hollow nanofiber-based NO2 gas sensors,” Sensors Actuators, B Chem., vol 160, no 1, pp 1468–1472, 2011 [33] H S SalehHudin, E N Mohamad, W N L Mahadi, and A Muhammad Afifi, “Multiple-jet electrospinning methods for nanofiber processing: A review,” Mater Manuf Process., vol 33, no 5, pp 479–498, 2018 [34] J V Patil, S S Mali, A S Kamble, C K Hong, J H Kim, and P S Patil, “Electrospinning: A versatile technique for making of 1D growth of nanostructured nanofibers and its applications: An experimental approach,” Appl Surf Sci., vol 423, pp 641–674, 2017 [35] D J Lockwood, Electrospun Nanofibers for Energy and Environmental Applications 2014 [36] E Zdraveva, J Fang, B Mijovic, and T Lin, Electrospun nanofibers 2016 [37] Kenry and C T Lim, “Nanofiber technology: current status and emerging developments,” Prog Polym Sci., vol 70, pp 1–17, 2017 [38] G C Rutledge and S V Fridrikh, “Formation of fibers by electrospinning,” Adv Drug Deliv Rev., vol 59, no 14, pp 1384–1391, 2007 [39] K Asokan, J Y Park, S W Choi, and S S Kim, “Nanocomposite ZnO-SnO2 nanofibers synthesized by electrospinning method,” Nanoscale Res Lett., vol 5, no 4, pp 747–752, 2010 [40] F K Ko and Y Wan, Introduction to nanofiber materials, vol 9780521879 2014 [41] S Mohammadzadehmoghadam, Y Dong, and I J Davies, “Modeling electrospun nanofibers: An overview from theoretical, empirical, and numerical approaches,” Int J Polym Mater Polym Biomater., vol 65, no 17, pp 901–915, 2016 [42] D H Reneker and A L Yarin, “Electrospinning jets and polymer nanofibers,” Polymer (Guildf)., vol 49, no 10, pp 2387–2425, 2008 [43] K Wei and I S Kim, Electrospun Nanofibers for Energy and Environmental Applications 2014 [44] D H Reneker, A L Yarin, H Fong, and S Koombhongse, “Bending instability of PHAN HONG PHUOC 51 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers electrically charged liquid jets of polymer solutions in electrospinning,” J Appl Phys., vol 87, no I, pp 4531–4547, 2000 [45] J H Lee, A Katoch, S W Choi, J H Kim, H W Kim, and S S Kim, “Extraordinary improvement of gas-sensing performances in SnO2 nanofibers due to creation of local p-N heterojunctions by loading reduced graphene oxide nanosheets,” ACS Appl Mater Interfaces, vol 7, no 5, pp 3101–3109, 2015 [46] N Van Hoang, C M Hung, N D Hoa, N Van Duy, and N Van Hieu, “Facile onchip electrospinning of ZnFe2O4 nanofiber sensors with excellent sensing performance to H2S down ppb level,” J Hazard Mater., vol 360, no July, pp 6– 16, 2018 [47] J H Kim, A Mirzaei, H Woo Kim, P Wu, and S S Kim, “Design of supersensitive and selective ZnO-nanofiber-based sensors for H2 gas sensing by electron-beam irradiation,” Sensors Actuators, B Chem., vol 293, no January, pp 210–223, 2019 [48] A Katoch, S W Choi, G J Sun, and S S Kim, “An approach to detecting a reducing gas by radial modulation of electron-depleted shells in core-shell nanofibers,” J Mater Chem A, vol 1, no 43, pp 13588–13596, 2013 [49] R Kumar, O Al-Dossary, G Kumar, and A Umar, “Zinc oxide nanostructures for NO2 gas–sensor applications: A review,” Nano-Micro Lett., vol 7, no 2, pp 97– 120, 2015 [50] R Kumar, G Kumar, and A Umar, “Zinc oxide nanomaterials for photocatalytic degradation of methyl orange: A review,” Nanosci Nanotechnol Lett., vol 6, no 8, pp 631–650, 2014 [51] J Y Park, J J Kim, and S S Kim, “Electrical transport properties of ZnO nanofibers,” Microelectron Eng., vol 101, pp 8–11, 2013 [52] X Song, Z Wang, Y Liu, C Wang, and L Li, “A highly sensitive ethanol sensor based on mesoporous ZnO-SnO2 nanofibers,” Nanotechnology, vol 20, no 7, pp 2– 7, 2009 [53] W Y Wu, J M Ting, and P J Huang, “Electrospun ZnO Nanowires as Gas PHAN HONG PHUOC 52 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers Sensors for Ethanol Detection,” Nanoscale Res Lett., vol 4, no 6, pp 513–517, 2009 [54] V H Nguyen, V D Nguyen, Q D Do, T M N Quan, M H Chu, and V H Nguyen, “On-chip ZnO nanofibers prepared by electrospinning method for NO2 gas detection,” Commun Phys., vol 27, no 4, p 317, 2018 [55] S Bai et al., “Synthesis of SnO2-CuO heterojunction using electrospinning and application in detecting of CO,” Sensors Actuators, B Chem., vol 226, pp 96–103, 2016 [56] A Fujisawa, T Nishino, and Y Hamakawa, “Hall-effect measurement on polycrystalline SnO2 thin films,” Jpn J Appl Phys., vol 27, no 4R, pp 552–555, 1988 [57] A A Bolzan, C Fong, B J Kennedy, and C J Howard, “Structural Studies of Rutile-Type Metal Dioxides,” Acta Crystallogr Sect B Struct Sci., vol 53, no 3, pp 373–380, 1997 [58] L Gracia, A Beltrán, and J Andrés, “Characterization of the high-pressure structures and phase transformations in SnO2 A density functional theory study,” J Phys Chem B, vol 111, pp 6479–6485, 2007 [59] J Y Park, K Asokan, S W Choi, and S S Kim, “Growth kinetics of nanograins in SnO2 fibers and size dependent sensing properties,” Sensors Actuators, B Chem., vol 152, no 2, pp 254–260, 2011 [60] J Y Park, S W Choi, and S S Kim, “A model for the enhancement of gas sensing properties in SnO2-ZnO core-shell nanofibres,” J Phys D Appl Phys., vol 44, no 20, 2011 [61] J H Kim, Y Zheng, A Mirzaei, H W Kim, and S S Kim, “Synthesis and Selective Sensing Properties of rGO/Metal-Coloaded SnO2 Nanofibers,” J Electron Mater., vol 46, no 6, pp 3531–3541, 2017 [62] X Kou et al., “Superior acetone gas sensor based on electrospun SnO2 nanofibers by Rh doping,” Sensors Actuators, B Chem., vol 256, pp 861–869, 2018 [63] Q Qi, T Zhang, L Liu, and X Zheng, “Synthesis and toluene sensing properties of PHAN HONG PHUOC 53 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers SnO2 nanofibers,” Sensors Actuators, B Chem., vol 137, no 2, pp 471–475, 2009 [64] Y Zhang, X He, J Li, Z Miao, and F Huang, “Fabrication and ethanol-sensing properties of micro gas sensor based on electrospun SnO2 nanofibers,” Sensors Actuators, B Chem., vol 132, no 1, pp 67–73, 2008 [65] A Katoch, S W Choi, G J Sun, H W Kim, and S S Kim, “Mechanism and prominent enhancement of sensing ability to reducing gases in p/n core-shell nanofiber,” Nanotechnology, vol 25, no 17, 2014 [66] D Zappa, V Galstyan, N Kaur, H M M Munasinghe Arachchige, O Sisman, and E Comini, “„Metal oxide -based heterostructures for gas sensors‟- A review,” Anal Chim Acta, vol 1039, pp 1–23, 2018 [67] C Liangyuan, B Shouli, Z Guojun, L Dianqing, C Aifan, and C Chiun, “Sensors and Actuators B : Chemical Synthesis of ZnO-SnO2 nanocomposites by microemulsion and sensing properties for NO2,” vol 134, no 2, pp 360–366, 2008 [68] X Song, D Zhang, and M Fan, “A novel toluene sensor based on ZnO-SnO2 nanofiber web,” Appl Surf Sci., vol 255, no 16, pp 7343–7347, 2009 [69] W Tang, “Sensing mechanism of SnO2/ZnO nanofibers for CH3OH sensors: Heterojunction effects,” J Phys D Appl Phys., vol 50, no 47, p 475105, 2017 [70] X Song and L Liu, “Characterization of electrospun ZnO-SnO2 nanofibers for ethanol sensor,” Sensors Actuators, A Phys., vol 154, no 1, pp 175–179, 2009 [71] H A Khorami, M Keyanpour-Rad, and M R Vaezi, “Synthesis of SnO2/ZnO composite nanofibers by electrospinning method and study of its ethanol sensing properties,” Appl Surf Sci., vol 257, no 18, pp 7988–7992, 2011 [72] W Tang, J Wang, P Yao, and X Li, “Hollow hierarchical SnO2-ZnO composite nanofibers with heterostructure based on electrospinning method for detecting methanol,” Sensors Actuators, B Chem., vol 192, no 2, pp 543–549, 2014 [73] T T Wang et al., “Preparation of SnO2-ZnO hetero-nanofibers and their application in acetone sensing performance,” Mater Lett., vol 159, no 36, pp 447–450, 2015 [74] L V Thong, L T N Loan, and N Van Hieu, “Comparative study of gas sensor PHAN HONG PHUOC 54 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers performance of SnO2 nanowires and their hierarchical nanostructures,” Sensors Actuators, B Chem., vol 150, no 1, pp 112–119, 2010 [75] K Thangavel, A Balamurugan, T Venkatachalam, and E Ranjith Kumar, “Structural, morphological and optical properties of ZnO nano-fibers,” Superlattices Microstruct., vol 90, pp 45–52, 2016 [76] J Y Park and S S Kim, “Growth of nanograins in electrospun ZnO nanofibers,” J Am Ceram Soc., vol 92, no 8, pp 1691–1694, 2009 [77] H Du, P J Yao, Y Sun, J Wang, H Wang, and N Yu, “Electrospinning heteronanofibers In2O3/SnO2 of homotype heterojunction with high gas sensing activity,” Sensors (Switzerland), vol 17, no 8, 2017 [78] M S Choi et al., “Selective H2S-sensing performance of Si nanowires through the formation of ZnO shells with Au functionalization,” Sensors Actuators, B Chem., vol 289, no July 2018, pp 1–14, 2019 [79] W Zeng, T Liu, and Z Wang, “Sensitivity improvement of TiO2-doped SnO2 to volatile organic compounds,” Phys E Low-Dimensional Syst Nanostructures, vol 43, no 2, pp 633–638, 2010 [80] Z Liu, T Fan, D Zhang, X Gong, and J Xu, “Sensors and Actuators B : Chemical Hierarchically porous ZnO with high sensitivity and selectivity to H2S derived from biotemplates,” vol 136, pp 499–509, 2009 [81] J H Bang et al., “Selective NO2 sensor based on Bi2O3 branched SnO2 nanowires,” Sensors Actuators, B Chem., vol 274, no 2, pp 356–369, 2018 PHAN HONG PHUOC 55 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers APPENDIX (a) (b) 200 µm (c) 200 µm (d) 10 µm 10 µm Figure A1 The comb-type interdigitated Pt electrodes deposited Si/SiO2 substrate at different magnifications (a,c), NFs deposited onto the Pt/SiO2/Si substrate (b,d) PHAN HONG PHUOC 56 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers (a) (b) 100 ZnO/SnO2 NFs H2S@250oC&1ppm H2S@300oC&1ppm o 0.1 H2S@350 C&1ppm Resistance (MW) Resistance (MW) 10 10 ZnO-SnO2 NFs H2S@250oC&1ppm H2S@300oC&1ppm H2S@350oC&1ppm 0.1 o H2S@400oC&1ppm H2S@400 C&1ppm o H2S@450 C&1ppm 0.01 500 1000 1500 H2S@450oC&1ppm 0.01 2000 500 10 SnO2 NFs o H2S@250 C&1ppm o H2S@300 C&1ppm o H2S@350 C&1ppm o H2S@400 C&1ppm Resistance (MW) Resistance (MW) (c) 0.1 1000 1500 2000 Temperature (oC) Time (s) (d) ZnO NFs H2S@250oC&1ppm 0.1 H2S@300oC&1ppm H2S@350oC&1ppm 0.01 H2S@400oC&1ppm o H2S@450oC&1ppm H2S@450 C&1ppm 0.001 200 400 600 800 Time (s) 200 400 600 800 Time (s) Figure A2 Dynamic response of the (a) ZnO/SnO2 NFs, (b) ZnO-SnO2 NFs, (c) ZnO NFs, and (d) SnO2 NFs toward ppm H2S gas at various operating temperatures PHAN HONG PHUOC 57 ITIMS 2017-2019 Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers NO2@10ppm&450oC 10 ppm NO2@10ppm&350oC NO2@10ppm&300oC NO2@10ppm&250oC 10 ZnO/SnO2 NFs (a) Response (Ra/Rg) 100 Resistance (MW) 1000 NO2@10ppm&400oC NO2@10ppm&450oC 10 ppm NO2@10ppm&400oC NO2@10ppm&350oC NO2@10ppm&300oC 100 NO2@10ppm&250oC ZnO-SnO2 NFs (b) 10 100 200 300 400 100 200 Time (s) 100 NO2@10ppm&450oC 10 ppm 300 600 NO2@10ppm&450oC 10 ppm NO2@10ppm&400oC o NO2@10ppm&350oC NO2@10ppm&350 C o NO2@10ppm&300 C o NO2@10ppm&250 C SnO2 nanofibers (c) Resistance (MW) Response (Rg/Ra) 500 o NO2@10ppm&400 C 10 400 Time (s) NO2@10ppm&300oC 10 NO2@10ppm&250oC ZnO nanofibers (d) 1 200 400 600 800 Time (s) 200 400 600 800 Time (s) Figure A3 Dynamic response of the (a) ZnO/SnO2 NFs, (b) ZnO-SnO2 NFs, (c) SnO2 NFs, and (d) ZnO NFs toward 10 ppm NO2 gas at various operating temperatures PHAN HONG PHUOC 58 ITIMS 2017-2019 CO@350 oC &200 ppm H2@350 oC &250 ppm 100 Response (Ra/Rg or Rg/Ra) Response (Ra/Rg or Rg/Ra) Comparative study of gas sensing properties between ZnO/SnO2 and ZnO-SnO2 nanofibers NH3@350 oC & 250 ppm NO2@350 oC &5 ppm H2S@350 oC &1 ppm 10 ZnO/SnO2 nanofibers (a) CO@350 oC &200 ppm H2@350 oC &250 ppm NH3@350 oC & 250 ppm 100 NO2@350 oC &5 ppm H2S@350 oC &1 ppm 10 ZnO-SnO2 nanofibers (b) 200 400 600 800 100 200 CO@350 oC &200 ppm H2@350 oC &250 ppm NH3@350 oC & 250 ppm NO2@350 oC &5 ppm H2S@350 oC &1 ppm 10 300 400 500 600 700 Time (s) Response (Ra/Rg or Rg/Ra) Response (Ra/Rg or Rg/Ra) Time (s) SnO2 nanofibers (c) CO@350 oC &200 ppm H2@350 oC &250 ppm 100 NH3@350 oC & 250 ppm NO2@350 oC &5 ppm H2S@350 oC &1 ppm 10 ZnO nanofibers (d) 1 100 200 300 400 500 Time (s) 100 200 300 400 500 Time (s) Figure A4 Dynamic response of (a) ZnO/SnO2 NFs, (b) ZnO-SnO2 NFs, (c) SnO2 NFs, and (d) ZnO NFs to 200 ppm CO, 250 ppm H2, 250 ppm NH3, ppm NO2, and ppm H2S at 350 °C PHAN HONG PHUOC 59 ITIMS 2017-2019 ... (d) 150 350 ZnO- SnO2 nanofibers ZnO/ SnO2 nanofibers ZnO nanofibers SnO2 nanofibers 300 o T = 400 C 100 50 (e) 250 200 ZnO- SnO2 nanofibers ZnO/ SnO2 nanofibers ZnO nanofibers SnO2 nanofibers T =... ZnO- SnO2 nanofibers ZnO/ SnO2 nanofibers ZnO nanofibers SnO2 nanofibers H2S @ ppm resp (s) 50 40 30 (a) 20 1600 ZnO- SnO2 nanofibers ZnO/ SnO2 nanofibers ZnO nanofibers SnO2 nanofibers H2S @ ppm 1400... 2017-2019 Comparative study of gas sensing properties between ZnO/ SnO2 and ZnO- SnO2 nanofibers ZnO- SnO2 nanofibers ZnO/ SnO2 nanofibers ZnO nanofibers SnO2 nanofibers 100 80 60 Resistance (MW) Resistance