SYNTHESIS AND CHARACTERIZATION OF ZINC OXIDE AND TIN OXIDE BASED NANOSTRUCTURES FOR GAS SENSING APPLICATIONS THARSIKA THABOTHANAYAKAM THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY FACULTY OF ENGINEERING UNIVERSITY OF MALAYA KUALA LUMPUR 2015 UNIVERSITY OF MALAYA ORIGINAL LITERARY WORK DECLARATION Name of Candidate: Tharsika Thabothanayakam (Passport No: N2563261) Registration/Matric No: KHA110101 Name of Degree: Doctor of Philosophy Title of Dissertation (―this Work‖): SYNTHESIS AND CHARACTERIZATION OF ZINC OXIDE AND TIN OXIDE BASED NANOSTRUCTURES FOR GAS SENSING APPLICATIONS Field of Study: Advanced Material / Nanomaterials I do solemnly and sincerely declare that: (1) I am the sole author/writer of this Work; (2) This Work is original; (3) Any use of any work in which copyright exists was done by way of fair dealing and for permitted purposes and any excerpt or extract from, or reference to or reproduction of any copyright work has been disclosed expressly and sufficiently and the title of the Work and its authorship have been acknowledged in this Work; (4) I do not have any actual knowledge nor do I ought reasonably to know that the making of this work constitutes an infringement of any copyright work; (5) I hereby assign all and every rights in the copyright to this Work to the University of Malaya (―UM‖), who henceforth shall be owner of the copyright in this Work and that any reproduction or use in any form or by any means whatsoever is prohibited without the written consent of UM having been first had and obtained; (6) I am fully aware that if in the course of making this Work I have infringed any copyright whether intentionally or otherwise, I may be subject to legal action or any other action as may be determined by UM. Candidate‘s Signature Date: Subscribed and solemnly declared before, Witness‘s Signature Date: Name: Designation: ii ABSTRACT This thesis focuses on the fabrication of one-dimensional ZnO and SnO2 based nanostructures with various morphologies by a single-step carbon-assisted thermal evaporation process and investigates their gas sensing properties. A mixture of zinc oxide, tin oxide and carbon powders was used as a source material which was loaded in a quartz boat and placed at the center of a tube furnace. A gold coated alumina substrate was placed next to quartz boat at downstream of gas flow direction. The furnace was heated to a particular growth temperature for a certain time while maintaining a constant flow of purified Ar gas. Grey and white fluffy deposits formed on Au coated alumina substrate and at the side and top of the quartz boat, respectively. The deposits collected from alumina substrate and the quartz boat were examined and found to be nanostructured in nature. The effects of different parameters such as growth temperature (800 - 950 °C), growth time (15 min - 120 min) and distance between the source and alumina substrate (3 cm - 12 cm) on the structure of the deposits were studied. Crystallographic phase, microstructure and elemental composition of nanostructures were investigated by X-ray diffractometer (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) equipped with an energy dispersive X-ray spectroscopy (EDS). The crystalline quality of samples was analysed by photoluminescence spectroscopy (PL). The nanostructures were used to fabricate resistor-type sensors on a gold interdigitated alumina substrate. The gas sensing properties of these nanostructures based sensors were studied by measuring their sensitivity towards methane, hydrogen, ethanol and carbon monoxide over different gas concentrations and operating temperatures. Results show that single- crystalline Zn2SnO4 nanowires grew at 834 °C on the Au coated alumina substrate. Nanowires have an average diameter of about 15 to 80 nm and length ranging from two iii to several tens of micrometers. The white fluffy mass found on a quartz boat exhibited SnO2-core/ZnO-shell nanowires as well as hierarchical nanostructures. Nanostructures deposited on the quartz boat consisted of pristine SnO2 nanowires with a rectangular cross-section, SnO2-core/ZnO-shell nanowires and SnO2-core/ZnO-shell hierarchical nanostructures at a growth time of 15 min, 30 min, and exceeding 60 min, respectively. The hexagonal shape of ZnO branches grew on the ZnO shell layer in the hierarchical nanostructures. Growth mechanisms of Zn2SnO4 nanowires and SnO2-core/ZnO-shell nanostructures are suggested. Zn2SnO4 nanowires based sensors showed excellent sensitivity and selectivity towards ethanol with quick response and recovery times. SnO2-core/ZnO-shell hierarchical nanostructures deposited at 90 min exhibited 5 folds enhanced sensitivity than that of pristine SnO2 nanowires towards 20 ppm ethanol at 400 °C. This improvement in ethanol sensitivity was attributed to highly active sensing sites and the synergistic effect of the encapsulation of SnO2 by ZnO nanostructures. SnO2-core/ZnO-shell hierarchical nanostructures grown for 90 min showed higher sensitivity of 76 compared with Zn2SnO4 nanowires based sensor of 60.8 for 100 ppm ethanol. Specifically, SnO2-core/ZnO-shell hierarchical nanostructures prepared by the single carbon assisted thermal evaporation method are promising candidates for the detection of ethanol with high sensitivity and selectivity against other gases. iv ABSTRAK Tesis ini memberi tumpuan kepada penghasilan nanostruktur satu-dimensi berasaskan ZnO dan SnO2 dengan pelbagai morfologi melalui proses penyejatan haba satu-langkah dengan bantuan karbon dan menyiasat ciri-ciri pengesanan gas mereka. Campuran zink oksida, timah oksida dan serbuk karbon telah digunakan sebagai bahan sumber yang telah dimuatkan di dalam bot kuarza dan diletakkan di tengah-tengah sebuah relau tiub. Sebuah substrat alumina bersalut emas diletakkan di sebelah bot kuarza di hilir arah aliran gas. Relau dipanaskan pada suhu pertumbuhan untuk masa yang tertentu di samping mengekalkan kadar aliran gas Ar tulen. Deposit berwarna kelabu dan putih bergebu terbentuk di substrat alumina bersalut Au serta di sisi dan atas bot kuarza, masing-masing. Deposit yang dikumpul daripada substrat alumina dan bot kuarza telah diperiksa dan didapati bernanostruktur. Kesan-kesan parameter seperti suhu pertumbuhan (800-950 °C), masa pertumbuhan (15 min - 120 min) dan jarak di antara sumber dan alumina substrat (3 cm - 12 cm) pada struktur deposit telah dikaji. Fasa kristalografi, mikrostruktur dan komposisi unsur struktur-struktur nano telah disiasat oleh X-ray diffractometer (XRD), field-emission scanning electron mikroskopi (FESEM) dan transmission electron mikroskopi (TEM) dilengkapi dengan energy dispersive X-ray spektroskopi (EDS). Kualiti kristal sampel dianalisis dengan photoluminescence spektroskopi (PL). Struktur-struktur nano tersebut digunakan untuk menghasilkan sensor jenis-perintang pada substrat alumina interdigitated emas. Sifat- sifat pengesanan gas sensor berasaskan struktur-struktur nano ini dikaji dengan mengukur kepekaan mereka terhadap metana, hidrogen, etanol dan karbon monoksida dengan kepekatan gas dan suhu operasi yang berbeza. Keputusan menunjukkan nanowayar Zn2SnO4 kristal-tunggal ditumbuhkan pada 834 °C atas substrat alumina bersalut Au. Nanowayar tersebut mempunyai diameter purata kira-kira 15-80 nm dan v panjang di antara dua hingga beberapa puluh mikrometer. Jisim gebu berwarna putih ditemui pada bot kuarza menunjukkan struktur nanowayar SnO2-teras/ZnO-shell serta struktur-struktur nano hierarki. Nanostruktur yang didepositkan pada bot kuarza terdiri daripada nanowayar SnO2 dengan keratan rentas segi empat tepat, nanowayar SnO2- teras/ZnO-shell dan SnO2-teras/ZnO-shell nanostruktur hierarki pada masa pertumbuhan 15 min, 30 min, dan melebihi 60 min, masing-masing. Cawangan ZnO berbentuk heksagon tumbuh pada lapisan shell ZnO dalam struktur-struktur nano hierarki. Mekanisme pertumbuhan nanowayar Zn2SnO4 dan struktur-struktur nano SnO2-teras/ZnO-shell adalah disyorkan. Sensor berasaskan nanowayar Zn2SnO4 menunjukkan sensitiviti dan pemilihan yang sangat baik terhadap etanol dengan tindak balas dan masa pemulihan yang cepat. SnO2-teras/ZnO-shell nanostruktur hierarki didepositkan untuk 90 min menunjukkan peningkatan sensitiviti 5 kali ganda berbanding dengan nanowayar SnO2 terhadap 20 ppm etanol pada 400 °C. Peningkatan dalam sensitiviti etanol adalah disebabkan oleh laman sensing yang sangat aktif dan kesan sinergi daripada pengkapsulan SnO2 dengan nano ZnO. SnO2-teras/ZnO-shell nanostruktur hierarki yang ditumbuh selama 90 min menunjukkan kepekaan yang lebih tinggi, 76 berbanding dengan nanowayar berasaskan Zn2SnO4, 60.8 bagi 100 ppm etanol. Secara khusus, SnO2-teras/ZnO-shell nanostruktur hierarki yang disediakan melalui kaedah penyejatan haba satu-langkah dengan bantuan karbon merupakan calon yang menjanjikan untuk mengesan etanol dengan sensitiviti yang tinggi serta mempunyai pemilihan keutamaan terhadap etanol berbanding gas-gas lain. vi ACKNOWLEDGMENTS First and foremost, I would like to express my most sincere and respectful gratitude to my main supervisor, Professor A.S.M.A. Haseeb, Department of Mechanical Engineering, University of Malaya. He offers me a great opportunity to work under his guidance and for making this project such a pleasant experience. I deeply appreciate the constant support which I have received during the past three years. Together with him, I would like to express my sincere gratitude to my external advisor Professor Sheikh A. Akbar,