MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY - NGUYEN HOANG HUNG IMPROVING THE GAS SENSING PROPERTY OF WO3 NANOMATERIALS MAJOR: ENGINEERING PHYSICS MASTER OF SCIENCE THESIS ENGINEERING PHYSICS SUPERVISOR: Dr DANG DUC VUONG HANOI - 2012 BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI - NGUYỄN HOÀNG HƯNG NGHIÊN CỨU CẢI THIỆN ĐẶC TÍNH NHẠY KHÍ CỦA VẬT LIỆU NANO WO3 CHUYÊN NGÀNH: VẬT LÝ KỸ THUẬT LUẬN VĂN THẠC SĨ KHOA HỌC VẬT LÝ KỸ THUẬT HƯỚNG DẪN KHOA HỌC: TS ĐẶNG ĐỨC VƯỢNG HÀ NỘI - 2011 TABLE OF CONTENT TABLE OF CONTENT TABLE OF CONTENT i LIST OF ABBRIVATION iii LIST OF TABLE iv LIST OF FIGURE v PREFACE vii ACKNOWLEDGMENT ix CHAPTER 1: INTRODUCTION I Chemical sensors and Gas sensors based on metal oxides 1 Chemical sensors Gas sensors based on metal oxides 2.1 Characterized features of gas sensors based on metal oxides 2.1.1 Sensitivity 2.1.2 Response and recovery time 2.1.3 Selectivity 2.1.4 Stability 2.2 Basic scientist approach 2.2.1 The nature of gas sensitivity 2.2.2 Factors affecting the sensitivity Approaches 12 II Motivation and objectives 22 Tungsten oxide among metal oxides for gas detection 22 Structural properties of tungsten oxide 25 Gas sensors based on tungsten oxide 27 CHAPTER 2: EXPERIMENTAL AND METHODOLOGY 33 I WO3 materials synthesis and doping 33 Nguyen Hoang Hung i Master Thesis TABLE OF CONTENT WO3 microsheets synthesis 33 WO3 nanoparticles synthesis 34 Doping 35 II Characterization 35 Electron microscopy 36 1.1 Interaction of high energy electrons with matter 36 1.2 SEM and FESEM 37 Chemical Analysis 38 2.1 Energy dispersive X-ray analysis 38 2.2 X-ray diffraction 38 Gas sensing properties 40 CHAPTER 3: RESULT AND DISCUSSION 43 I Materials synthesis 43 Tungsten trioxides microsheets 43 Tungsten trioxides nanoparticles and doping 45 II Gas sensing properties 46 Tungsten trioxides microsheets 46 Tungsten trioxides nanoparticles and doping 49 CONCLUSION 54 RELATED PRESENTATION 55 OUTLOOK 56 REFERENCE 57 Nguyen Hoang Hung ii Master Thesis LIST OF ABBRIVATION LIST OF ABBRIVATION  HS: Hydrothermal and Solvothermal  XRD: X-ray Diffraction  SEM: Scanning electron microscope  FE-SEM: Field Emission Scanning Electron Microscopy  EDX or EDS: Energy-dispersive X-ray spectroscopy  TEM: Transmission electron microscopy  CNTs: Carbon nanotubes  VLS: Vapor-liquid-solid  PVD: Physical vapor deposition  CVD: Chemical vapor deposition  LEIS: Low energy ion scattering  DA: Depletion approximation Nguyen Hoang Hung iii Master Thesis LIST OF TABLE LIST OF TABLE Table Sign of resistance change to change in gas atmosphere [93] Table Typical deposition techniques 20 Table Known polymorphs of tungsten trioxide 26 Table Occupational Exposure Standards 2000 28 Table Some properties of NH3 28 Table Requirements for NH3 gas detection equipment 29 Table Selected publications on NH3 gas sensors based on WO3 31 Nguyen Hoang Hung iv Master Thesis LIST OF FIGURE LIST OF FIGURE Figure Chemical sensors Figure Cross-section of a chemical sensor Figure An example of resistance change when a reduce gas was introduced Figure Response and recovery time Figure Schematic representation of barrier formation Figure Three mechanisms of conductance 10 Figure Chemical (a) and electronic (b) sensitization schemes 11 Figure Sol-gel processing options 13 Figure Pressure/temperature map of material processing techniques 16 Figure 10 Particle processing by conventional and hydrothermal 16 Figure 11 General purpose pressure autoclave and white Teflon Figure 12 VLS synthesis apparatus 18 Figure 13 Map of temperature variations in furnace Figure 14 Schematic illustration of nucleation and growth of ZnO nanorods Figure 15 SEM images of ZnO nanorods Figure 16 Screen printing technique 21 Figure 17 Spin coating 22 Figure 18 Dip coating Figure 19 Comparison of the papers published on gas sensors 23 Figure 20 Schematic model of crystalline WO3 in the undistorted cubic phase Figure 21 Structural model of the WO3 grain surface 26 Figure 22 NH3’s structure and symmetry axis Figure 23 Some types of ammonia detector Figure 24 Some of commercialized gas sensors head Figure 25 Schematic diagram of WO3 microsheets synthesis 34 Figure 26 Steps of WO3 nanoparticles synthesis 35 Figure 27 Diagram of heat treatment Figure 28 Electron scattering and secondary signal generation 36 Figure 29 Schematic diagram of an SEM 37 Figure 30 Pt interdigitated electrodes and heater used in system Figure 31 Static gas sensing system and principal circuit 41 Figure 32 Dynamic gas sensing system 41 Nguyen Hoang Hung v Master Thesis LIST OF FIGURE Figure 33 SEM images of WO3 microsheets 43 Figure 34 The XRD pattern and EDX pattern of WO3 thin film 44 Figure 35 SEM images of WO3 microsheets Figure 36 FESEM images of WO3 nanoparticles 45 Figure 37 SEM images of WO3 nanomaterials Figure 38 EDX partner of Fe2O3 nanorods doped WO3 1% wt 46 Figure 39 Dependence of the electrical resistance on working temperature Figure 40 Response to NH3 of WO3 nanosheets at 60oC and 255oC 47 Figure 41 Response to NH3 of WO3 microsheets at 160C 47 Figure 42 The sensor response of the materials 48 Figure 43 The sensor response as a function of gas concentration 49 Figure 44 Response to NH3 of WO3 nanoparticles at 55 C 50 Figure 45 Response to NH3 of WO3 nanoparticles at 95 C 50 Figure 46 The dependence of gas response on NH3 51 Figure 47 Response to NH3 of WO3 nanoparticles at 294 C Figure 48 The dependence of the sensor response on operating temperature Nguyen Hoang Hung vi Master Thesis PREFACE PREFACE Nowadays, the pollution level is increasing due to the misuse of chemicals in industry, agriculture as well as in life The presence of inflammable gases, toxic gases that have caused large damage to both people and their property Aims to minimize the risks as well as industrialization and modernization of industrial processes, it is necessary to fabricate a kind of environmentally benign devices capable of detecting gases Since then the term “gas sensor” was born During the last decades of the century, the kind of gas sensor, which was best known, was based on the metal oxide semiconductor In particular, materials such as TiO2, SnO2, WO3, are widely used in gas sensing applications to detect toxic gases The principle for gas sensing applications using metal oxide semiconductor based on the change in resistance of the sensitive layer in presence of gases One of the metal oxide material promising for semiconductor gas sensor applications was tungsten oxide With many advantages such as high sensitivity, low response time, low operating temperature, tungsten oxide material was gradually brought to second place in the world of gas sensor based on metal oxides semiconductor (after SnO2) One of the gases that was widely used and caused great impact on human health is ammonia Recently, ammonia (NH3) is used in many industries, the NH3 gas leak in the pipeline has caused serious consequences to health So, in the gases to be detected, NH3 in one of the most concerned gas and sensitive material to detect this gas that was emphasized by scientists is WO3 Developing in parallel with nanotechnology, WO3 is a sensitive materials even at large sizes, but when the material reach to the size limit, the sensitivity was strongly improved and appear more interesting properties Currently, there are many routes to synthesis WO3 nanomaterials such as ball milling, thermal oxidation, chemical vapour deposition (CVD), physical deposition However, these methods require a rigorous technological processes and conditions It is difficult to obey in Vietnam science condition Recently, wet chemical method combined with hydrothermal technology emerged with many advantages as simple technology, inexpensive, not undemanding on technological process as well as technical conditions Moreover this method allows mass production and variable morphologies could be synthesized The above advantages make wet chemical method has been studying and using more and more in all over the world Nguyen Hoang Hung vii Master Thesis PREFACE In this thesis, the WO3 materials are synthesized and measured in NH3 gas sensor application The morphological form of the material was deposited by wet chemical methods combining hydrothermal technology Gas sensing properties of the materials was improved by reducing in grain size and doping with Fe2O3 nanorods The thesis title: “Improving the gas sensing property of WO3 material” was selected and the results are presented in three main chapters: Chapter I Introduction: A short introduction to chemical sensors based on metal oxides, with a particular emphasis on WO3 This chapter also includes the motivation, targets and organization of this investigation Chapter II Experimental and methodology: Illustrating the experimental details used in this work, method to analyze the structural and morphological properties of material, a gas effective sensing system was also built in this chapter Chapter II Experimental and methodology: Aiming at contributing to the understanding of the whole gas sensing process Final Conclusions and future Outlook are also proposed in this thesis Nguyen Hoang Hung viii Master Thesis RESULTS AND DISCUSSION concentration, the sensors were exposed to NH3 gas with concentrations of 500, 1000, 2000, 3000, and 4000 ppm Figure 43 indicates the response of the sensors based on the microsheets annealed at 400 C at an operating temperature of 60, 130, 200 and 255 C All the three kinds of sensors exhibit good response/recovery cycle to the NH2 gas pulses and the sensor responses increase with the increase of gas concentrations As observed in Figure 43, profiles of the sensor responses as a function of NH3 gas concentrations The sensor responses increase nearly linearly with the increase of NH3 gas concentration o Sensor response 60 C o 130 C o 200 C o 255 C 1000 2000 3000 4000 NH3 concentration (ppm) Figure 43 The sensor response as a function of gas concentration The gas sensing property of the WO3 thin film was also evaluated upon exposure to another gases, such as LPG and ethanol at 255 C and in range of concentration from 500 to 400 ppm The obtained results showed the film resistance changing in the presence of the gas is very low, and almost no change Based on collected data, the ability to detect selective NH3 gas was made as well Tungsten trioxides nanoparticles and doping Gas sensitive properties of WO3 particles was tested on dynamic systems with NH3 concentrations from 0-250 ppm at different working temperatures Gas flow rate that was introduced to the sample surface in this measurement mode is 300 sccm Background gas used was dry air generated from the compressor after passing through the air filter as state previously NH3 has concentration of 1000 ppm The Nguyen Hoang Hung 49 Master Thesis RESULTS AND DISCUSSION concentration of NH3 was controlled by controlling the flow of NH3 and dry air through the GMFC (chapter 2) o o 55 C 250ppm 40 20 200ppm 150ppm 250ppm Resistance (k) 15 30 50ppm 20ppm 15ppm 50ppm 10 25ppm 20 20ppm 15ppm 5ppm 10ppm Resistance (k) 200ppm 150ppm 100ppm 100ppm 10 95 C 2500 5000 Time (s) 7500 10000 10ppm 5ppm 3000 6000 Time (s) 9000 Figure 44 Response to NH3 of WO3 Figure 45 Response to NH3 of WO3 nanoparticles at 55 C nanoparticles at 95 C Figure 44 and 45 show the response curve of WO3 films containing nanoparticles in the presence of NH3 at temperatures 55 C and 95 C In these temperature, WO3 particles expressing p-type semiconductor so the resistance increases when exposed to reduced gas This result is similar when surveyed on WO3 microsheets (Figure 40) One can see that in this case, the response of the materials to NH3 is higher than the previous one (nanosheets) because of the rising in specific surface area The electrical resistance of the material dramatically increases from 1.51 k to 2.82 k in the presence of ppm NH3 at 95 C The corresponding sensitivity of 1.86 was obtained The response curve shows that the resistance of the sensor varies over time with various cyclic tests for different NH3 concentrations (5, 10, 15,…250 ppm).Thus, the as-synthesized materials are potential candidate for gas sensor application The sensor response of products operating at 95oC and 55oC were calculated and shown in figure 46 It can be seen that the response of the sensor to NH3 increases rather steeply with increasing gas concentration For nanoparticles sample, at 95 C, the sensitivity in presence of ppm NH3 is about 1.86 The sensitivity increases to 13.2 when NH3 concentration is 250 ppm and the curve is going to quasi-linearity The change in materials sensitivity is fairly obvious while the NH3 concentration change several ppm This change indicates that the WO3 nanoparticles sensor is particularly suitable for NH3 detection at sub-ppm level In fact, low concentration ammonia detection is actually desired for the development of highly sensitive sensor [105] Nguyen Hoang Hung 50 Master Thesis RESULTS AND DISCUSSION 14 12 o 55 C o 95 c Sensor response 10 0 50 100 150 200 Concentration (ppm) 250 Figure 46 The dependence of gas response on NH3 concentration of nanoparticles When measured at high 1.6 294 C temperature with nanoparticles, the 1.4 changing type of semiconductor also appeared as like as nanosheets as 1.2 shown in figure 47 It can be seen 1.0 that the resistance of materials 0.8 quickly decreased when NH3 was introduced into the chamber and it 5ppm 0.6 10ppm exhibits n-type semiconducting 15ppm 0.4 20ppm The electrical resistance changes 500 1000 1500 2000 of the WO3 nanowires when exposed Time (s) to ammonia at low temperature is Figure 47 Response to NH3 of WO3 believed to be caused by the nanoparticles at 294 C variation of the surface acceptor states density related to the chemisorbed oxygen [105] As presented above, in case ò WO3, the removal of oxygen causes the appearance of these crystallographic shear planes into the crystal along the [1m0] direction [49] This leads to the formation of a family of WO3-x compounds The surfaces of the ultra-thin diameter of the Non-stoichiometric tungsten oxide (The thickness for microsheets) are more active than the conventional bulk fully oxidized tungsten oxide materials The large amounts of oxygen vacancies in the reduced tungsten oxide WO3 can serve as adsorption site The small diameter and large amounts of oxygen vacancies in the WO3 will facilitate the chemisorption of oxygen at a low temperature William has predicted that if the grain size of the materials is Resistance ( k) o Nguyen Hoang Hung 51 Master Thesis RESULTS AND DISCUSSION smaller than the depletion layer thickness (Debye-length), the grain could be considered as completely depleted so that the conductivity would become surface-trap limited [94] Taking into account of the equilibrium between gaseous oxygen and surface oxygen ions, the Debye-length can be defined as: (24) where ε is the material’s relative dielectric constant and ε0 is dielectric constant of vacuum, K is the Boltzmann constant, T is the absolute temperature, e is the electron charge, and ND is bulk donor density The conductivity of such materials can be expressed in terms of the surface acceptor states density formulated as chemisorbed oxygen species: (25) where and KII = pn, where and are electron and hole mobility respectively, p and n are concentration of hole and electron respectively, fA is fraction of acceptors with trapped electrons The interaction of the surface with the gas presented in air will cause the surface acceptor state density change From the above equation it can be seen that the conductivity is not linearly dependent on the surface acceptor state The conductivity will decrease pass through a minimum and then increase again with the increases of NA The minimum occurs when NA is satisfying the following condition: (26) If the bulk donor density ND is too large or too small the achievable surface state density NA could not reach the minimum and the material will exhibit a pure n-type or p-type response to the gas presented However, if the ND is in a proper range that N A could reach the minimum with variation of gas concentrations and a change of the sign in conductivity variation will be observed The abnormal behavior of the response of the WO3 to ammonia can be explained if the diameter of the materials is less than the Debye-length at room temperature when the bulk donor density of the materials is in a proper range By adopting the typical values of and ND = – 6×1015 cm3 for typically sputtered tungsten oxide thin films [51], [30], the Debye length at room temperature is calculated to be about 45–50 nm We cannot use this value directly as the Debye-length for the as-prepared materials since the donor density of the materials is affected by the oxygen vacancies in the particles However, for a reference, the ultra-small diameter and thickness of the particles, sheets and the Nguyen Hoang Hung 52 Master Thesis RESULTS AND DISCUSSION Sensor response conductive sign change observed in the experimental suggest that the diameter of the as-prepared materials may be smaller than the Debye-length of it at room temperature [105] Figure 48 shows the dependence of the sensor response on operating temperature of WO3 nanoparticles doped different 20 concentration of Fe2O3 nanorods, Fe2O3 pure coated onto electrodes and two pure WO3 pure 15 Fe2O3 t doped WO3 sample One can see in this case that, Fe2O3 t doped WO3 the response of materials added 1% wt 10 Fe2O3 nanorods to ammonia is much higher than another one The highest obtained sensitivity of 18.2 in the presence of 250 ppm ammonia The response of the pure WO3 50 100 150 200 250 o 300 350 400 nanoparticles was lower and strongly Temperature ( C) decrease when the concentration of Figure 48 The dependence of the sensor doped substance was %wt One thing response on operating temperature can be seen that the optimum temperature of the sensor is about 100 C with WO3 nanoparticles doped with 1%wt Fe2O3 nanorods It make a promising chance to fabricate a low consumption power sensors and solve the problem of energy for hand held sensor Nguyen Hoang Hung 53 Master Thesis CONCLUSION CONCLUSION This thesis has examined the structural and gas sensing properties of pure and catalyzed WO3 material powders, as well as the species and reactions that occur on their surface This study has led to the following conclusions:  WO3 microsheets has been successfully synthesized by a soft chemical process The obtained materials exhibited 500-700 nm in size with thickness of nano scale (< 100 nm)  Thick-film of material containing WO3 microsheets has successfully developed onto Pt-interdigitated electrode I also have examined the structural properties, electrical properties and possible applications as NH3 gas sensor of the thickfilm  The gas sensor property of the materials was highly improved by reducing in the particles size and doping with Fe2O3 nanorods WO3 nanoparticles has been successfully synthesized under the assistance of hydrothermal technique at 180 C for 24 h The Fe2O3 nanorods was also deposited in right concentration of and %wt as a dopant  It is very necessary to examine the sensors at low concentration of target gas, particularly for ammonia The as-synthesized materials was tested at low concentration by dynamic gas sensing system  The nanomaterials and doped present high responsiveness when compared with NH3 gas and alcohol vapor at the same concentration range  The material’s conductivity changes with temperature have been studied and discussed At low temperature region (near room temperature), WO3 nanomaterials exhibit p-type conductivity with NH3 and turn to n-type conductivity when was measured at high temperatures A speculative explanation was proposed and discussed Main results of the thesis have been transferred to an article entitled “Synthesis of WO3 nanosheets, nanoparticles, nanorods by soft chemical process for NH3 sensor application”, which was one of the oral articles at The 5th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2010) - Hanoi, Vietnam Nguyen Hoang Hung 54 Master Thesis RELATED PRESENTATION RELATED PRESENTATION Dang Duc Vuong, Nguyen Hoang Hung, Nguyen Duc Chien “Tính chất điện đặc trưng nhạy khí NH3 màng mỏng WO3 (Electrical and NH3 sensing properties of WO3 thin films)”, The 6th Vietnam National Conference on Solid State Physics & Materials Science at Danang city, Vietnam 2009, pp 705-708 Nguyen Hoang Hung, Dang Duc Vuong, Nguyen Duc Chien “Synthesis of WO3 Nanosheets, Nanoparticles, Nanorods by Soft Chemical Process for NH3 Sensor Application”, The 5th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2010) - Hanoi, Vietnam, 2010, one of the Oral articles Nguyen Hoang Hung, Dang Duc Vuong, Nguyen Duc Chien “Improved ethanol sensing properties of SnO2 nano-flowers materials by doping CuO” The 7th Vietnam National Conference on Solid State Physics & Materials Science at Ho Chi Minh city, Vietnam 2011 Nguyen Hoang Hung, Nguyen Phan Thang, Dang Duc Vuong, Nguyen Duc Chien “Gas sensing property of CuO doped hollow sea urchin-like α-Fe2O3 material” The 6th Vietnam-Korea International Joint Symposium on Advanced Materials and Their Processing - Hanoi, Vietnam - November 04-05, 2011 Nguyen Hoang Hung 55 Master Thesis OUTLOOK OUTLOOK Many avenues for further research remain for extending the current work, some of which are suggested and described below:  It would be interesting to reduce the grain size under 10 nm in order to improve sensor response One strategy could be to change the temperature of hydrothermal process and pH of pre-treatment solution  The addition of other water soluble chlorides, e.g MnCl2, AgCl and NiCl2, needs to be investigated with the aim of producing Mn, Ag and Ni-doped WO3 nanomaterials, respectively Further, measurement and comparison of their functional properties should allow routes for the controlled doping of nanomaterials to be identified  In order to make a large effective surface area, it is necessary to synthesis another morphologies of the materials 1-D structure such as nanorods and nanowire are two of typical target morphologies in future In addition, recently because hydrothermal seem to be the best method for mass synthesis 1-D structure or nanomaterial at low temperature with good uniform throughout the sample The best route to meet the purpose could be to use some structure-directing templates such as K2SO4, thiourea and pH of pretreatment solvent as diameters controlling agents [82], [80] Nguyen Hoang Hung 56 Master Thesis REFERENCES REFERENCE [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] Aiyer R.C., Ansari S.G., Boroojerdian P., Karekar R.N., Kulkarni S.K., and Sainkar S.R (1997), “Grain size effects on H2 gas sensitivity of thick film 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