Fabrication of zn2sno4 nanostructures for gas sensor application chế tạo vật liệu zn2sno4 cấu trúc nano ứng dụng cho cảm biến khí

Nhan đề : Fabrication of Zn2SnO4 nanostructures for gas sensor application = Chế tạo vật liệu Zn2SnO4 cấu trúc nano ứng dụng cho cảm biến khí Tác giả : Lai Van Duy Người hướng dẫn: Nguyen Duc Hoa Từ khoá : Vật liệu; Vật liệu Zn2SnO4 Năm xuất bản : 2020 Nhà xuất bản : Trường đại học Bách Khoa Hà Nội Tóm tắt : Tổng quan về vật liệu Zn2SnO4, cấu trúc nano; thử nghiệm; kết quả và thảo luận.

MASTER THESIS

Fabrication of Zn2SnO4 nanostructures

for gas sensor application

LAI VAN DUY Duy.LVCA180178@sis.hust.edu.vn Specialized: Electronic materials

Supervisor: Professor Ph.D Nguyen Duc Hoa

Institute: International Training Institute for Materials Science (ITIMS)

HANOI, 6/2020

MASTER THESIS

Fabrication of Zn2SnO4 nanostructures

for gas sensor application

LAI VAN DUY Duy.LVCA180178@sis.hust.edu.vn Specialized: Electronic materials

Supervisor: Professor Ph.D Nguyen Duc Hoa

Institute: International Training Institute for Materials Science (ITIMS)

HANOI, 6/2020

Signature of GVHD

Độc lập – Tự do – Hạnh phúc

BẢN XÁC NHẬN CHỈNH SỬA LUẬN VĂN THẠC SĨ

Họ và tên tác giả luận văn: Lại Văn Duy

Đề tài luận văn: Chế tạo vật liệu Zn2SnO4 cấu trúc nano ứng dụng cho cảm biến khí

Chuyên ngành: Khoa học vật liệu-VLĐT

Mã số SV: CA180178

Tác giả, Người hướng dẫn khoa học và Hội đồng chấm luận văn xác nhận tác giả đã sửa chữa, bổ sung luận văn theo biên bản họp Hội đồng ngày 30/06/2020 với các nội dung sau:

- Bổ sung chú thích hình 3.9, 3.10, 3.13

- Các công thức, phương trình phản ứng được đánh số theo trình tự

- Bảng danh mục chữ viết tắt sắp xếp theo thứ tự alpha b

- Chữ trên trong các hình 3.2, 3.4, 3.6, 3.25 được để ở kích thước lớn hơn

- Phần chú thích hình có dấu chấm sau số thứ tự hình

- Chỉnh sửa các lỗi chính tả, hành văn

Ngày 09 tháng 07 năm 2020 Giáo viên hướng dẫn Tác giả luận văn

GS TS Nguyễn Đức Hòa Lại Văn Duy

CHỦ TỊCH HỘI ĐỒNG

PGS TS Nguyễn Phúc Dương

Chế tạo vật liệu Zn2SnO4 cấu trúc nano ứng dụng cho cảm biến khí Học viên: Lại Văn Duy

Chuyên ngành: Khoa học vật liệu-VLĐT

Giáo viên hướng dẫn

(Ký và ghi rõ họ tên)

GS TS Nguyễn Đức Hòa

First of all, I would like to express my greatest gratitude to Prof PhD Nguyen Duc Hoa for his valuable scientific ideas, guidance and support of favorable conditions for me to complete this thesis His kindness and enthusiasm will be in my heart forever

Simultaneously, I would like to express my sincere thanks to all staffs of the Laboratory for Research, Development, and Application of Nanosensors at ITIMS-HUST has always been enthusiastic about helping, sharing experiences and suggesting many important ideas for me to carry out the research of this thesis Moreover, I am also very grateful to my colleagues, PhD students, the iSensors’ graduated students who have always accompanied and assisted me in two years of doing my master thesis at ITIMS

Finally, I would like to thank all my family, friends and colleagues who have always encouraged and shared me to complete this thesis

SUMMARY OF MASTER THESIS

In this project, we developed high-performance VOC gas sensors for breath analysis by focusing on the controlled synthesis of nanostructured Zn2SnO4

ternary metal oxides to maximize the gas sensitivity To archive the objective, we synthesised hollow structure ternary metal by hydrothermal technique with the assistance of soft template The thickness of the hollow cells was optimised to desire the highest VOC response By hydrothermal method, the author has successfully synthesized many nanostructures of Zn2SnO4 with different morphologies At the same time, the thesis also proves the application potential

of Zn2SnO4 material in the gas sensor VOCs The sensor based on Zn2SnO4

materials could detect various VOCs gases such as acetone, ethanol, and methanol at low concentrations of ppb levels with high sensitivity

STUDENT

Lai Van Duy

CONTENTS

ABBREVIATIONS iii

LIST OF FIGURES iv

LIST OF TABLES viii

INTRODUCTION 1

1 Foundation of the thesis 1

2 Aims of the thesis 3

3 Research object and scope of the thesis 4

4 Research Methods 4

5 The practical and scientific significance of the thesis 4

6 New contributions of the thesis 5

7 The structure of the thesis 5

CHAPTER 1 OVERVIEW 7

1.1 Volatile organic compounds 7

1.2 Overview of Zn 2 SnO 4 material 8

1.2.1 Crystal structure of Zn 2 SnO 4 material 9

1.2.2 Electrical properties of Zn 2 SnO 4 material 11

1.2.3 Application of Zn 2 SnO 4 material in gas sensors 12

1.2.4 Gas sensitivity mechanism of metal oxide for VOCs 17

1.3 Hydrothermal method 21

CHAPTER 2 EXPERIMENTAL APPROACH 25

2.1 The synthesis processes of nanostructured Zn 2 SnO 4 materials with different morphologies by hydrothermal method 25

2.1.1 Equipment and chemicals 25

2.1.2 The synthesis process of Zn 2 SnO 4 nanostructures with different morphologies by hydrothermal method 26

2.2 Sensor manufacturing processes 29

2.3 Morphological and microstructure analysis 30

2.4 Survey of gas sensitivity properties 30

CHAPTER 3 RESULTS AND DISCUSSION 32

3.1 Morphology and crystal structure of zinc Stannate nanomaterials (Zn 2 SnO 4 ) synthesized by hydrothermal method 32

3.1.1 Effect of hydrothermal temperatures on the morphology of Zinc Stannate (Zn 2 SnO 4 ) materials 32

3.1.2 Effect of surfactant P123 on the morphology of Zn 2 SnO 4 material 34

3.1.3 Effect of pH on the morphology of Zn 2 SnO 4 materials 38

3.1.4 Crystal structure of synthesized Zn 2 SnO 4 materials 44

3.2 Gas sensing properties of Zn 2 SnO 4 materials with different morphological structures 49

3.2.1 Methanol gas-sensing properties of the fabricated sensors 50

3.2.2 Ethanol gas-sensing properties of the fabricated sensors 53

3.2.3 Acetone gas-sensing properties of the fabricated sensors 56

CONCLUSIONS AND RECOMMENDATIONS 68

LIST OF REFERENCES 69

LIST OF PUBLICATIONS 78

ABBREVIATIONS

Microscope

for Materials Science

Diffraction Standards

LIST OF FIGURES Figure 1.1 VOCs in exhaled breath can be used as biomarkers for diseases diagnose [47] 8 Figure 1.2 Crystal structures of zinc stannate (Zn2SnO4) [51] 9 Figure 1.3 Sublattices of zinc stannate (Zn2SnO4) 10 Figure 1.4 Schematic representation of the inverse spinel lattice of Zn2SnO4 [49] 10 Figure 1.5 Model explains the n-type semiconductor of Zn2SnO4 material [50] 11 Figure 1.6 A schematic diagram of reaction mechanism of SnO2-based sensor to HCHO: (a) in air, (b) in VOCs [73] 19 Figure 1.7 Schematic energy level diagram of a metal oxide before (a) and after exposure to a VOCs (b) [43] 19 Figure 1.8 A schematic of the sensing mechanism of (a) ZnO NPs and (b) ZnO QDs in air (left) and isoprene (right) [74] 20 Figure 2.1 Photos of some of the main equipment using synthesized Zn2SnO4

nanomaterials by a hydrothermal method such as thermos flask (1), magnetic stirrer (2), pH meter (3), centrifugal rotary machine (4) and annealing furnace (5) 26 Figure 2.2 Process diagram of synthesizing Zn2SnO4 nanomaterials with different morphological structures by hydrothermal method 27 Figure 2.3 The process diagram for making sensors on the basis of nano

Zn2SnO4 material by small coating method 29 Figure 2.4 (A) Gas sensitive measuring system at ITIMS; (B) Diagram of the gas measuring system by static measurement method 31 Figure 3.1 SEM image of Zn2SnO4 samples synthesized by hydrothermal method with different hydrothermal temperature: (A, B) 160 ºC; (C, D) 180 ºC; (E, F) 200 ºC 33 Figure 3.2 General diagram of synthetic Zn2SnO4 materials with different morphology according to changes in hydrothermal temperature 34 Figure 3.3 SEM image of Zn2SnO4 samples synthesized by hydrothermal method with different amount of P123 surface-active agent (A, B) 0 g; (C, D) 0.25 g; (E, F) 0,5 g; (G, H) 1,0 g 36

Figure 3.4 Schematic mechanism of synthesizing Zn2SnO4 materials with different morphology by the concentration of surfactants P123 by hydrothermal method 37 Figure 3.5 SEM image of Zn2SnO4 nanomaterial synthesized by hydrothermal method with different pH conditions: (A, B) pH = 8; (C, D) pH = 9; (E, F) pH = 10; (G, H) pH = 12; (I, K) pH = 13 40 Figure 3.6 General diagram of the synthesis of Zn2SnO4 materials with different morphology according to the pH change of the hydrothermal environment 41 Figure 3.7 TEM (A-D) images of the synthesized hollow cubic Zn2SnO4 Inset

of (D) is correspondent SAED 43 Figure 3.8 (A) STEM image and (B-D) EDS mapping of the hollow cubic

Zn2SnO4 43 Figure 3.9 XRD samples of Zn2SnO4 with condition pH = 8 and pH = 13 at hydrothermal temperature of 180 °C/24h 44 Figure 3.10 XRD patterns of Zn2SnO4 with condition pH = 8 and pH =13 hydrothermal temperature of 180 °C/24h after treatment heat at 550 °C for 2h in air 45 Figure 3.11 Raman and PL spectrum of synthesized Zn2SnO4 46 Figure 3.12 BET spectra of Zn2SnO4: (A) - Octahedron, (B) - Cubic, (C) – Nanoparticles 48 Figure 3.13 I-V curve of the sensor (A) - Octahedron, (B) - Cubic, (C) – Nanoparticles measured in air at 450 oC 49 Figure 3.14 Methanol sensing characteristics of nanoparticles Zn2SnO4

(ZTO_PH8): (A) transient resistance versus time upon exposure to different concentrations of methanol measured at different temperatures; (B) sensor response as a function of methanol; (C) respon and recovery time of sensor 52 Figure 3.15 Methanol sensing characteristics of hollow cubic Zn2SnO4

(ZTOP5_PH8): (A) transient resistance versus time upon exposure to different concentrations of methanol measured at different temperatures; (B) sensor response as a function of methanol; (C) respon and recovery time of sensor 52 Figure 3.16 Methanol sensing characteristics of hollow octahedron Zn2SnO4

(ZTOP5_PH13): (A) transient resistance versus time upon exposure to different

concentrations of methanol measured at different temperatures; (B) sensor response as a function of methanol; (C) respon and recovery time of sensor 53 Figure 3.17 Ethanol sensing characteristics of nanoparticles Zn2SnO4

(ZTO_PH8): (A) transient resistance versus time upon exposure to different concentrations of ethanol measured at different temperatures; (B) sensor response

as a function of ethanol; (C) response and recovery time of sensor 53 Figure 3.18 Ethanol sensing characteristics of hollow cubic Zn2SnO4

(ZTOP5_PH8): (A) transient resistance versus time upon exposure to different concentrations of ethanol measured at different temperatures; (B) sensor response

as a function of acetone; (C) response and recovery time of sensor 54 Figure 3.19 Ethanol sensing characteristics of hollow octahedron Zn2SnO4

(ZTOP5_PH13): (A) transient resistance versus time upon exposure to different concentrations of ethanol measured at different temperatures; (B) sensor response

as a function of ethanol; (C) response and recovery time of sensor 54 Figure 3.20 Acetone sensing characteristics of nanoparticles Zn2SnO4

(ZTO_PH8): (A) transient resistance versus time upon exposure to different concentrations of acetone measured at different temperatures; (B) sensor response as a function of acetone; (C) response and recovery time of sensor 56 Figure 3.21 Acetone sensing characteristics of hollow cubic Zn2SnO4

(ZTOP5_PH8): (A) transient resistance versus time upon exposure to different concentrations of acetone measured at different temperatures; (B) sensor response as a function of acetone; (C) response and recovery time of sensor 57 Figure 3.22 Acetone sensing characteristics of hollow octahedron Zn2SnO4

(ZTOP5_PH13: (A) transient resistance versus time upon exposure to different concentrations of acetone measured at different temperatures; (B) sensor response as a function of acetone; (C) response and recovery time of sensor 57 Figure 3.23 Response to low acetone concentration of detection limit of (A) hollow cubic (ZTOP5_PH8) and (B) hollow octahedron (ZTOP5_PH13)

Zn2SnO4 at 450 °C and (C) - (F) calculation of detection limit of the Zn2SnO4

sensor 60 Figure 3.24 The responses of ZTO_PH8; ZTOP5_PH8; ZTOP5_PH13 gas sensors to 125ppm ethanol, methanol and Acetone measured at 450 ºC 61

Figure 3.25 Stability of sensor ZTO_PH8 (A, B); ZTOP5_PH8 (C, D); ZTOP5_PH13 (E, F) to ethanol and acetone 63 Figure 3.26 Selectivity of sensors ZTO_PH8, ZTOP5_PH8, and ZTOP5_PH13 when surveying with different gases: acetone (100 ppm), ethanol (100 ppm), methanol (100 ppm), NH3 (25 ppm), H2 (50 ppm) and CO (5 ppm) at 450 ºC 64 Figure 3.27 (A, B) transient resistance and (C, D) response value versus time upon exposure to 0.5 ppm acetone measured at 450 ºC in different values of humidity of the hollow cubic, hollow octahedron Zn2SnO4 sensor 65 Figure 3.28 Schematic of the VOCs gas-sensing mechanism of the Zn2SnO4 67

LIST OF TABLES Table 1.1 Comparative VOC gas response of different Zn2SnO4 structure-based sensors 14 Table 2.1 Designation of samples of Zn2SnO4 materials synthesized in the processes 28 Table 3.1 Symbols of selected samples for sensor analysis and fabrication 42

INTRODUCTION

1 Foundation of the thesis

The Fourth Industrial Revolution is fundamentally changing the world via the Internet of Things (IoT), cloud computing, 3D Graphic, Augmented Reality, Machine learning, sensor technology, and Artificial intelligence, …[1, 2] Among others, IoT [3] opens up positive effects in almost aspects of human life, including in the field of health care [4, 5] IoT has grown from the convergence

of wireless technology, micro-mechatronics technology, and the internet, which serve high purposes and more advantages to human’s society For instance, before the IoT, patients, doctors, and managers spent much time and money on health care and medical diagnosis Proper medical diagnosis will also reduce the need for hospitalization IoT has healthcare applications with many utilities for patients, families, doctors, hospitals, and insurance companies Today, IoT products that integrate nanotechnology into smart wearables have been growing strongly [6] Integration of electronics devices with IoT played a huge role in personal health care by using handheld diagnostic sensors, health monitors, chronic disease monitors, therapeutic sensors, etc

The role of sensor types and especially gas sensors is becoming important

in improving the quality of human life in the age of the IoT industry Gas sensors have been developed very strong; they are widely applied to many fields, from agriculture, industries to environmental monitoring, and health care Before

2000, as the industry developed, the risk of air pollution was increasing day by day The applications of gas sensors were largely focused on applications in the toxic gas leakage warning Until the 2000, gas sensor applications were developed with many advanced features such as high sensitivity, fast response time, they are used in the automotive field, including process control Combustion and quality control of emissions It is predicted that the field of application of gas sensors will expand to medicine in the years 2010 - 2020 To apply in this field, new generations of sensors with ability to analyze low concentrations (ppb level) of various gases such as volatile organic gases (VOCs) [7-9]

Research [9] shows that the global wireless gas sensor market in the period from 2015-2020 will grow from 8.21 billion USD to 15.01 billion USD, showing great promise most to take advantage of gas sensors The research and manufacture of gas-sensitive sensors in Industry 4.0, it is necessary to have high-sensitive materials, fast response and recovery time, good selectivity to determine the concentration of VOCs in human breath accurately So far, researches on the fabrication, property surveys, and the applicability of metal oxide nanomaterials have been attracting widespread research interest all over the world [11, 12] Till now, various nanostructures of semiconductor metal oxides have been researched to improve the characteristics of VOCs gas sensors such as nanowires [13, 14], nanoplates [14], nanorods [15], nanoflowers [16], nanotubes [17], heterostructures [18], nanofibers [19] and nanoparticles [20] The semiconductor metal oxide (MOS) gas sensor has many advantages such as small size, low power consumption, quick response, high sensitivity, and good compatibility with silicon chips compared to optical sensors, mass spectroscopy, chromatography sensors and electrochemical sensors for VOCs detection [22-24]

Resistive gas sensors can be a new road for environmental monitoring, disease diagnosis, and patient monitoring because of its simple operation, low cost, and portability [22, 25] Metal and modified oxides, such as SnO2, ZnO, TiO2, In2O3, Fe2O3, WO3, CuO, and NiO, have been investigated as sensing materials for detecting different toxic and VOC gases in new semiconductor sensors [26-28] However, these oxides suffer from limitations, such as low sensitivity, poor selectivity, and instability at low concentrations [26] Besides, for application in breath analysis, high-performance low detection limit VOCs gas sensor is needed because the concentration of VOCs in exhaled breath is shallow, ranging from parts per trillion (ppt) to parts per million (ppm) Despite many attempts have paid to the development of new structural materials for enhancing gas sensing characteristics [21], it is challenging to obtain high performance VOC sensors with high sensitivity, fast reaction and recovery times, low- energy operation, and long-term stability for breath analysis

Recently, the use of complex oxides as gas-sensitive materials has elicited interest because these oxides have many advantages, such as chemical inertness, thermal stability, and environmental friendliness, over common binary oxides [30, 31] Zn2SnO4 is a typical n-type semiconducting ternary oxide [29] that has multifunctional characteristics, including high electron mobility, good thermal stability, high chemical sensitivity, and low-visibility absorption [33, 34], thus suitable for gas sensor applications [35, 36] To be applied in exhaled breath analysis, the gas sensors should have a low detection limit down the ppb level The expansion work was devoted to the preparation of Zn2SnO4 nanostructures with novel morphologies to improve further the response speed, selectivity, and stability of gas sensor devices [28, 32] In comparison with dense particles, porous or hollow structural materials [38] provide more surface activities, high surface-to-volume ratio, and faster diffusion, thus enhancing sensing performance [34] Many approaches have used to fabricate Zn2SnO4 materials, and these include hydrothermal [38, 39], co-precipitation [37], sol-gel [38], and thermal evaporation [35, 42] techniques The hydrothermal method has certain advantages, such as simple fabrication and low cost, and it is commonly employed to synthesize Zn2SnO4 hollow structures [40] However, recent studies involve the use of a sample and two or three steps to synthesize empty structures

Zn2SnO4 [35, 44] The use of a sample or multi-step can lead to a high-cost synthesis and contamination of the final product, thereby causing a loss of purity and material changes [42] Therefore, simple protocols for the fabrication of hollow structures Zn2SnO4 by the secure hydrothermal method are needed to enhance the gas sensor performance Furthermore, the correlation between the material characteristics and gas sensor characteristics of the equipment is also essential in understanding and improving sensor performance However, few researches report on general and application of the Zn2SnO4 hollow block for

VOC gas sensor applications Therefore, this thesis targets to the “Fabrication of

Zn 2 SnO 4 nanostructures for gas sensor application”

2 Aims of the thesis

- To successfully fabricate Zn2SnO4 nanostructures using the hydrothermal method for gas sensor applications

- To investigate the gas sensitivity and electrical properties of the synthesized Zn2SnO4 nanostructures

- To understand the gas mechanism of the Zn2SnO4 nanostructures

3 Research object and scope of the thesis

To implement this study with the above objectives, the thesis focused on researching the following key issues:

the I-V characteristic method of measurement The gas-sensing characteristics of

Zn2SnO4 material-based sensors have studied by static measurement techniques

on the gas sensing characteristics of the Air Sensing Group (iSensor.vn) at the ITIMS Institute-Institute for International Scientific Training on scientific research materials University of Technology-Hanoi

5 The practical and scientific significance of the thesis

The thesis has launched a stable process to produce Zn2SnO4 materials using simple methods, namely the hydrothermal method The author has synthesized the Zn2SnO4 nanostructures with different morphologies for application in gas sensors All studies were carried in the conditions of technology and equipment in Vietnam These processes can allow for the mass manufacturing of sensors, with high repeatability, consistency, and reliability The fabricated sensor has high sensitivity and selectivity, which can detect VOCs

such as methanol, ethanol, and acetone at low concentrations of ppm to ppb The results are very likely and can put into practical applications for healthcare Furthermore, the results of the research have criticized by scientists at home and abroad, published in reputable professional journals such as Sensors and Actuator

A This shows the content of the thesis meaningful scientific and practical

6 New contributions of the thesis

By hydrothermal method, the author has successfully synthesized many nanostructures of Zn2SnO4 with different morphologies At the same time, the results of the thesis also prove the potential application of Zn2SnO4 material in the gas sensor VOCs The sensor based on Zn2SnO4 materials has high sensitivity and can detect some VOCs gas such as acetone, ethanol, and methanol at low concentrations of ppb

The main research results of the thesis were published in one ISI articles:

1 Nguyen Hong Hanh, Lai Van Duy, Chu Manh Hung, Nguyen Van Duy, Young-Woo Heo, Nguyen Van Hieu, Nguyen Duc Hoa*, "VOC gas sensor based on hollow cubic assembled nanocrystal Zn2SnO4 for

breath analysis", Sensors and Actuators A 302 (2020) 111834-111839 [IF2018: 2.73]

Also, there are some published results in national magazines and proceedings of the conference

7 The structure of the thesis

To achieve the proposed goals, the thesis was divided into the following sections:

Chapter 1: Overview

In this chapter, we present an overview of volatile organic compounds and their toxicity, as well as their presentation in breath as biomarkers The introduction of Zn2SnO4 semiconductor metal oxide materials and their applications in the field of gas sensors Synthesis of Zn2SnO4 nanomaterials, a review of some published research results on the gas sensing mechanism of VOCs based on Zn2SnO4 materials

Chapter 2: Experimental approach

In this chapter, we present the technological process of manufacturing

Zn2SnO4 nanomaterials by the hydrothermal method Introducing the method of surveying morphology, gas-sensitive and electric properties of Zn2SnO4 materials used in the thesis

Chapter 3: Results and discussion

In this chapter, we present the results and discuss on the morphology, sensing properties, and the sensitivity mechanism of Zn2SnO4 material structures Details on the effect of synthesis condition on the morphology and gas sensing properties of synthesized materials are reported and discussed

gas-Conclusions and recommendations

In this section, the author has presented the conclusions of the thesis, including the outstanding results that the thesis has achieved, the scientific conclusions about the research content as well as limitations and research directions for the next studies

CHAPTER 1 OVERVIEW

In this chapter, the author presents some general questions about volatile organic compounds, Zn2SnO4 materials and their applicability in gas sensors Author also introduced the hydrothermal methods and gas sensing mechanism of Zinc Stannate materials (Zn2SnO4) for VOCs

The updated definition of the United States Environmental Protection Agency (EPA) is as follows: volatile organic compounds (VOCs) are chemicals that easily penetrate the air in the form of gases or vapors from solid or liquid materials that can evaporate naturally when experiencing atmospheric pressure at room temperature [46, 47] The VOCs are ingredients in many commercial, industrial, and household products Large amounts of VOCs are discharged into the atmosphere from both artificial and natural sources The concentration of the VOCs in the house is much higher than outdoors (up to 10 times higher), and estimates can detect 50 to 300 different VOCs in the atmosphere of homes, schools, offices, and commercial buildings at any time [21]

VOCs, which originate from burning fuel (such as gas, firewood, and kerosene), personal hygiene products (such as aromatherapy oils and hair sprays, cleaning utensils, laundry detergents, paint a house), almost every human activity

in daily life as smoking products, cutting the grass, using pesticides or much simpler breathing, result in the emission of organic compounds such as carbonyls, alcohols, alkanes, alkenes, esters, aromatics, ethers, and amides [10,

43, 45] VOCs include several different chemicals that can cause severe diseases such as respiratory problems (asthma, shortness), allergy, neurological symptoms (cause eye, nose and neck itching, nausea, lethargy, headaches, and depressions), and cancers (leukemia, colon cancer, rectal cancer, and lung cancer) Prolonged exposure to high concentrations may also cause liver, kidney or central nervous system damage VOCs can adversely affect human health and the ecosystem in low concentrations, the presence of these VOCs in the atmosphere is very dangerous because they are environmental pollutants, it is involved in many

reactions that create hazardous substances in the environment, reduce the amount

of ozone in the air, and their inhalation poses a serious health risk

Human breath contains around 500 various VOCs, and the concentration of VOCs in part per million (ppm), part per billion (ppb) range or part per trillion (ppt), depending on the human health condition Most VOCs are, however, not generated in the body (endogenous), but derive from food ingestion, exposure to environmental pollutants (exogenous) or from the metabolization of a drug [21] Accurate detection of specific VOCs during exhalation can provide the information needed to diagnose diseases early on Measurement of blood-borne VOCs occurring in human exhaled breath as a result of metabolic changes or pathological disorders is a promising tool for noninvasive medical diagnoses such as exhaled acetone, ammonia, H2S and toluene measurements in terms of diabetes monitoring, kidney failure, halitosis, and lung cancer [46] Differences

in the concentration of exhaled VOCs can serve as a biomarker for specific diseases that distinguish healthy people from sick people [21] The noninvasive diagnosis that detects various conditions is the main advantage of breathing exhalation techniques [47]

Figure 1.1 VOCs in exhaled breath can be used as biomarkers for diseases

diagnose [47].

1.2 Overview of Zn2SnO4 material

Zn2SnO4, a semiconductor metal oxide material has attracted immense interest, with exciting properties, high strength, stability, high electron mobility,

high conductivity, and low absorption in a visible light area Development of simple fabrication method has been widely studied in the world Zn2SnO4 can apply in many fields such as sensors for the detection of humidity and various combustible gases It can also be used as electrode material for Li-ion battery, solar cells, a photocatalyst for the degradation of organic pollutants, etc

1.2.1 Crystal structure of Zn2SnO4 material

Zinc stannate (Zn2SnO4) has a spinel structure based on a face-centered

cubic of the oxygen ions with space group Fd3m (JCPDS PDF 24-1470), which

belongs to AIIBIVO4 material group, where A is the first metal ion with two valances and B is the second metal ion with four valences This structure can be viewed as the combination of the rock salt and zinc blend structures [48] As other spinel cubic, a unit cell of zinc stannate contains eight formula units or 56 ions, including 32 oxygen and 24 metal ions (Figure 1.2) The oxygen ions are packed quite close together in a face-centered cubic arrangement, and the smaller metal ions occupy the space between them Thus, A and B ions occupy the tetrahedral and octahedral interstitial sites respectively, and the oxygen ions occupy the face-centered cubic close packing structure as shown in Figure 1.3

Figure 1.2 Crystal structures of zinc stannate (Zn 2 SnO 4 )[51]

Figure 1.3 Sublattices of zinc stannate (Zn 2 SnO 4 )

The spinel structure can further classify into typical and inverse spinel structures In the normal spinel structure, the Zn2+ ions occupy the tetrahedral sites, and the Sn4+ ions occupy the octahedral sites, respectively However, not all

of the available sites occupied by metal ions Only one eight of the (Zn) sites and one-half of the [Sn] sites occupied Zn2+ and Sn4+ cations distributed over the sites of tetrahedral (A) and octahedral [B] coordination In the inverse spinel structure, the A2+ ions and half of the B+ ions occupy a bowl-side position together, and the other half of the ion B3+ occupies the quadrangle position, which is governed by the general formula B(AB)O4

Bulk Zn2SnO4 crystals exhibit an ideal inverse spinel structure with a cubic lattice parameter a = 8.6574 Å in which one-half of Zn2+ ions occupy (A) sites, whereas Sn4+ cations and the other half of Zn2+ ions occupy [B] sites, which is governed by the generalized formula (Zn2+)[Sn4+ Zn2+ ]O4 (Figure 1.4) [49]

Figure 1.4 Schematic representation of the inverse spinel lattice of

Zn 2 SnO 4 [49]

1.2.2 Electrical properties of Zn2SnO4 material

Zn2SnO4 is an n-type semiconductor with a wide bandgap (Eg ≈ 3.6 eV) The semiconductor properties of Zn2SnO4 explained by the fact that during the fabrication process, the crystal lattice has oxygen defects Each missing oxygen node in the mesh creates a free electron pair, and therefore, the primary carrier in the material is the electron Each oxygen vacancy will create a free pair of electrons that can participate in the conduction process (Figure 1.4), so Zn2SnO4

semiconductor metal oxide is an n-type semiconductor

The larger number of oxygen locations per unit of volume, the higher the level of electrical power per unit of the volume leads to the conductivity of increased materials or reduced resistance The amount of oxygen defect in the

Zn2SnO4 crystal lattice can be controlled through heat treatment processes at different temperatures or heat-treated in different environments

Figure 1.5 Model explains the n-type semiconductor of Zn 2 SnO 4 material [50]

Figure 1.5 (A) is the atomic structure for the difference of Zn2SnO4

perfectly and shows that the Sn and Zn atoms are arranged in order Figure 1.5 (B) shows two oxygen locations in Zn2SnO4 Around the O – A bonding site are surrounded by 3 Zn atoms and 1 Sn atom, while at around O – B bonding positions are surrounded by 2 Zn atoms and 2 Sn atoms, and the number of Each unit is identical The cation positions are classified into a tetrahedral unit cell, (tet-I) and three octahedral units (oct-I, oct-II and oct-III) The tetrahedral position is always occupied by Zn [50] Oxygen vacancies are formed at vacancies at position O – A and vacancies at position O – B The position of the energy level will depend on the position of oxygen defect in the direction of the main connection O – A and O – B, which determines the properties of the

material Joohwi Lee et al [50] calculated the effect of oxygen vacancies on atomic and electron structure in Zn2SnO4 crystal Based on the spectroscopic spectroscopy, the calculated indicates that the bandgap of Zn2SnO4 material lies

in about 2 eV The analysis of the atomic and electronic motions around the absence of oxygen shows that Sn plays a key role in changing electrical properties Whereas, the electronic state of Zn remained almost unchanged, although atomic displacement was superior to Sn around the oxygen vacancy 1.2.3 Application of Zn2SnO4 material in gas sensors

So far, many different nanostructures based on semiconductor metal oxide

Zn2SnO4 materials have successfully researched and applied widely in many different fields Particularly, in recent years, there have been many studies on the gas-sensitive properties of gas sensors based on Zn2SnO4 materials Still, so far, the understanding of the effects of nanostructures and morphology of Zn2SnO4

materials into their air-sensitive properties are not complete Sensors based on

Zn2SnO4 materials with different morphologies, including of nanoparticles [37], nanowires [51], nanospheres [52], hierarchical quasi-microspheres [41], nanosheets [53], lamellar microspheres [54], octahedra [55], and hollow octahedra [42], have been tested over many gases [55], such as toluene [41], ethanol [52], methane, H2S, NO2, and hydrogen [51], but sensor response still needs to be improved

For instance, Lili Wang et al [53] reported on the synthesis of hierarchical

Zn2SnO4 products with sheet/sphere/cube structure via a hydrothermal route for toluene gas sensor application, where the hierarchical Zn2SnO4 with nanosheet structures-based the sensor exhibits excellent toluene sensing properties, the response value (Ra/Rg) to 100 ppm toluene at 280 °C was 25.2; higher than that

of the Zn2SnO4 sphere (Ra/Rg: 19.2) and Zn2SnO4 cube (Ra/Rg:11.7), followed by ethanol, benzene, Xylol, carbon monoxide, hydrogen, and sulfuretted hydrogen

Y Tie et al [56] reported on the synthesis of Zn2SnO4 nanocube via a hydrothermal route for formaldehyde gas sensor application, where the response value (Ra/Rg) to 50 ppm ethanol at 230 °C was 23,57; and the sensor response at

230 °C was the highest to ethanol, acetic acid, acetone, methanol, and benzene Shaoming Shu et al [57] synthesis of Zn2SnO4/SnO2 materials has a hierarchical

structure with many different morphologies through a simple chemical reaction route with post-heat treatment, morphologies as spherical, surface layered hierarchical octahedral-like and perfect octahedral summed up by adjusting the reaction solution temperature, the sensor based on surface layered hierarchical octahedral-like Zn2SnO4/SnO2 (20 °C) exhibited much more excellent gas-sensing performance toward formaldehyde than other sensors derived from spherical, perfect octahedral structure Zn2SnO4/SnO2, where the response value (Ra/Rg) to 100 ppm formaldehyde at 200 °C was 60; and the sensor response at

200 °C was the highest to ethanol, acetone, methanol, and methylbenzene (14, 6,

5 and 7) Fengjun Liu et [58] reported on the synthesis of novel Zn2SnO4-ZnO hierarchical structures via hydrothermal route for triethylamine gas sensor application, where the Zn2SnO4-ZnO hierarchical structures were composed of 1D ZTO nanowires and ZnO nanosheets, and it was found that the ZTO-ZnO hierarchical structures exhibited excellent sensing performance toward triethylamine; the response value (Ra/Rg) to 100 ppm at 200 °C was 175.5; the response time was approximately 13 s, but the recovery time was quite long (189 s) and 30.8 times higher than that of the pure ZTO and ZnO sensors, respectively; and the sensor response at 200 °C was the highest to butanol, ethanol, ethyl acetate, acetone, and methanol Wang et al [59] reported on the synthesis of Zn2SnO4 nanopowders via a hydrothermal route for ethanol gas sensor application, where the response value (Ra/Rg) to 100 ppm ethanol at 300

°C was 30; and the sensor response at 300 °C was the highest to ethanol, followed by methanol, methylbenzene, and acetone Yang et al [36] used carbonaceous spheres as templates in combination with calcination to synthesize

Zn2SnO4 hollow spheres for acetone gas sensor, where the response value of 153 was obtained to 200 ppm at 200 °C Chen et al [60] prepared highly ordered

Zn2SnO4 three dimensional flowerlike superstructures assembled with nanorods for ethanol sensor, where the device showed the response value of approximately

35 to 100 ppm ethanol at 128 °C

It is clear that the gas sensing performance of metal oxides highly depends

on their morphologies, crystallite size, porosity, defect level, and other features [61, 62] To be applied in the exhaled breath analysis, the gas sensors should

have a low detection limit down to ppb levels The expansion work was devoted

to the preparation nanostructures Zn2SnO4 with novel morphologies to improve further the response speed, selectivity, and stability of gas sensor devices [61, 62] In comparison with dense particles, porous or hollow structural materials [63] provide more surface activities, higher surface-to-volume ratio, and faster diffusion, thus enhancing sensing performance [34] Many approaches have been used to fabricate Zn2SnO4 materials, and these include hydrothermal [35, 36], co-precipitation [37], sol-gel [38], and thermal evaporation techniques [39] The hydrothermal method has certain advantages, such as simple fabrication and low cost, and it is commonly employed to synthesize Zn2SnO4 hollow structures [40] However, recent studies involved the use of a template and two or three steps for the synthesis of Zn2SnO4 hollow structures [32, 41]

Through the overview of the researched works related to Zn2SnO4 materials applied to the gas sensor in Table 1.1, we can draw the conclusion that the properties of the materials such as morphological structure, the size of the material, the temperature at which it is surveyed, the type of gas surveyed, etc significantly influence the sensitivities of the sensor

Table 1.1 Comparative VOC gas response of different Zn 2 SnO 4 structure-based sensors

Spheres

7 Zn2SnO4 Flower-like Formaldehyde 1000 18.9 20 [65]

13 Zn2SnO4/

SnO2

Hierarchical octahedral-like

a research group at Hanoi National University, etc A research group at Hanoi National University of Education, headed by Professor Nguyen Van Minh, Assoc Luc Huy Hoang studied manufacturing and surveying optical properties

of metal oxide material systems with wide bandgaps such as MnWO4 [69], ZnWO4 [70], Zn1-xCoxO [71] and Ti1-xVxO2 [52], etc Spectroscopic survey methods such as Raman and PL spectroscopy have used to study the optical properties of materials [69] The studies of this group often use chemical processes, including sol-gel, microwave, and hydrothermal methods, to prepare micro nanostructured materials [70] The studied materials are usually in the form of nanoparticles, micro nanostructured materials, or bulk samples for photocatalytic applications Non studies of Zn2SnO4 nanoparticles have applied

in gas sensors Recently, a research team headed by prof Nguyen Duc Hoa (Hanoi University of Science and Technology) has succeeded in manufacturing ZnFe2O4 nanofibers by electrospinning method [28], or manufacturing Zn2SnO4

nanowires by thermal evaporation method [32, 39] Previously, this group only focused on manufacturing nanowires, such as ZnO, SnO2, and WO3 by thermal evaporation method This group started to focus on researching and manufacturing plural nanostructures such as ZnWO4, MnWO4, ZnFe2O4, and

Zn2SnO4 etc and examine their gas-sensitive characteristics for applications on nano gas sensors

1.2.4 Gas sensitivity mechanism of metal oxide for VOCs

Gas sensors based on semiconductor metal oxide materials work relying on the change in electrical properties of the material caused by adsorption and release of gas molecules on the surface of the sensing material Semiconductor oxide materials, when placed in the air, will adsorb oxygen on the surface and

electronically conduct from the conduction band for the n-type semiconductor

and top of the valence band for the p-type semiconductor In the framework of the dissertation, we only introduce the gas-sensitive mechanism of n-type semiconductor metal oxide (Zn2SnO4), usually in the oxygen-deficient crystal at the lattice leading to the appearance of loading particles in the crystal the electronics Zn2SnO4, an n-type semiconductor - electrical properties governed

by the concentration of defective oxygen in the lattice or the level of adsorbed oxygen on the surface In the air, oxygen adsorbed on the surface of metal oxides exists in different ionic forms, such as depending on the temperature Typically,

in the lower temperature zone of 150 °C, oxygen adsorbs on the surface of metal oxide in the form of O2- molecules In the temperature range from 150 °C to 500

°C, the adsorbed oxygen on the surface of the material can be in the form of atoms like (O- hoặc O2-) When the temperature is above 500 °C, part of the adsorbed oxygen can diffuse into the material lattice (absorption) [72] The adsorption kinematics are described as follows:

concentration The charge carriers of n-type semiconductor metal oxide are

electrons, so the interaction between the semiconductor metal oxide and the ionic oxygen species causes a decrease in conductivity upon exposure to air As a result, a thick electron depletion layer will form on the surface of Zn2SnO4

nanoparticles, and a high potential barrier is formed between the adjacent nanograins, leading to an increase of resistance in the sensing material

For the n-type semiconductor oxide material, when the semiconductor

metal oxide material puts in the environment with a reducing gas, the reducing gas molecules will react with the surface adsorption oxygen ions, these oxygen ions will release the electron to the zone When the broad area of poverty and barrier height is also assumed, the cause of the material increases or reduces the resistance of materials Based on inadequate regional mechanisms, when gas molecules breakdown to the surface of a sensitive layer they may interact with surface oxygen or interactions (adsorption) directly on the layer surface At the same time, swap the grain load with the gas sensitivity layer This grain exchange can expand or shrink the grain load (or agglomeration area), thereby changing the resistance of the sensor The electrochemical absorbents in the particle surface of the electron exchange, alter the concentration of the charge in the vicinity to change the barrier between the two borders leading to changes in the material conductivity For semiconductor metal oxide sensors, the sensitivity of semiconductor metal oxide gas sensors can be improved by adjusting the microstructure, defects, catalyst, heterojunction, and humidity The gas-sensing performance influenced by the morphology particle size, porosity and gas diffusion ability of the semiconductor metal oxide

Detailed VOCs gas sensitivity mechanism of semiconductor metal oxide sensors can refer to recent publication In the work of Yuxiu Li et al [73] also studied the mechanism of adsorption of SnO2 with formaldehyde, as illustrated in Figure 1.6 When SnO2 particles come into contact with formaldehyde gas, formaldehyde will react with the adsorbed oxygen ions and release the trapped electrons back to the semiconductor metal oxide, increasing the carrier concentration Consequently, the thickness of the space-charge layer is reduced, resulting in a decrease of the potential barrier and resistance, as shown in Figure 1.6

Figure 1.6 A schematic diagram of reaction mechanism of SnO 2 -based sensor

to HCHO: (a) in air, (b) in VOCs [73]

For the gas sensor based on nanoparticles, the formation of a poor region on the surface of the particles and along the particle boundary leads to the formation

of a Schottky barrier between crystal particles

Figure 1.7 Schematic energy level diagram of a metal oxide before (a) and

after exposure to a VOCs (b) [43]

The oxygen ion density on the surface, as well as the width and height of the Schottky barrier, depending on the partial pressure of oxygen in the survey area The reaction of adsorbed oxygen species with adsorbed VOCs can modify the intensity of the Schottky barrier [43], resulting in a variation of conductivity (Fig 1.7)

In the report by Yoonji Park et al., ZnO nanoparticles with different sizes synthesized by wet chemical methods shows that quantum dots of ZnO give an excellent gas sensor performance with isoprene gas Both ZnO nanoparticles and

ZnO quantum dots have gas detection limits below 0.01 ppm In this study, the ability to enhance the gas sensitivity of ZnO nanoparticles attributed to the change in particle size [74]

Figure 1.8 A schematic of the sensing mechanism of (a) ZnO NPs and (b) ZnO

QDs in air (left) and isoprene (right) [74]

As a result, a thick electron depletion layer will form on the surface of

Zn2SnO4 nanoparticles, and a high potential barrier is formed between the adjacent nanograins, leading to an increase of resistance in the sensing material When the sensor is exposed to reducing gas such as methanol, ethanol and acetone at a moderate temperature, the methanol, ethanol and acetone molecules would react with the surface adsorbed oxygen species and the captured electrons will release back to the conduction band, resulting in an increasing conductivity and a deceasing resistance of the sensor The reaction process between surface adsorbed oxygen species and methanol, ethanol, acetone gases are described as Eqs (1.4) – (1.6)[17, 52, 75]:

CH3OH + 4O- → CO2 + 2H2O + 4e- (1.4)

C2H5OH + 6O- → 2CO2 + 3H2O + 6e- (1.5)

CH3COCH3 + 8O- → 3CO2 + 3H2O + 8e- (1.6)

Sensitivity is a parameter reflecting the resistance variation in a certain concentration of target gas For semiconductor metal oxide sensors, the sensitivity of semiconductor metal oxide gas sensors can be improved by adjusting the microstructure, defects, catalyst, heterojunction, and humidity The gas-sensing performance is influenced by the morphology of the semiconductor metal oxide The optimal morphology promotes the adsorption and desorption of the adsorbed oxygen species, which plays a crucial role in the enhancement of the gas-sensing performance Thus, it is necessary to understand the parameters of the microstructure for further improvement of gas sensors The grain size, number of activated adsorption sites and gas diffusion ability are the structural parameters that dramatically affect the gas-sensing performance The reduction in grain size to the nanoscale is one of the most effective strategies for the enhancement of the gas-sensing properties

The large specific surface area also enhances the gas diffusion into the sensor material which improves gas sensor performance Generally, the sensing materials of gas sensors are classified into dense and porous nanostructures In the case of dense nanostructures, gas diffusion can only occur on the surfaces of sensing materials because the gas molecules cannot penetrate the sensing materials In the case of porous nanostructures, the gas molecules can interact with the inner grains because the special structures are conducive to the penetration of gas molecules into the sensing materials Sufficient diffusion can lead to a large change in the resistance of the gas sensor The porous structure is divided into a microporous, mesoporous and macroporous structure based on the pore size [10] However, it is still challenging in synthesis of large specific surface area metal oxide for gas sensors

Zn2SnO4 nanomaterials can be fabricated by various methods such as chemical vapor deposition method (CVD), electrospinning method, thermal evaporation, co-precipitation, sol-gel, etc Each different method has its advantages and disadvantages, can create Zn2SnO4 nanomaterials with different sizes, shapes, properties, and applications To synthesize nanostructured Zn2SnO4

materials, we found that the chemical method was the most appropriate,

specifically the hydrothermal method The hydrothermal method is a simple method that does not require very expensive equipment By the hydrothermal method, the scientists synthesized many types of semiconductor metal oxide nanomaterials with different shapes and sizes such as nanofibers, nanorods, nanoplates, nanoflowers, nanoparticles and objects the material has a porous structure

The hydrothermal method is a method of using any heterogeneous chemical reaction in the presence of a solvent (water or no water), at room temperature and

a pressure above 1 atm in a closed system With advantages over other methods such as: easy to control the substance involved in the reaction, large sample volume obtained in one fabrication, the product has crystallinity, copper size regular, simple process, low cost, saving time This method involves heating the chemical solution in a closed vessel (autoclave), thus increasing the pressure inside the container above atmospheric pressure when the temperature exceeds the boiling point of the solvent This increases the solubility and reaction rate of the precursors used in the synthesis of materials The hydrothermal development process of crystals covers aspects of physicochemical and hydrodynamic principles, solutions, solubility, phase equilibrium, thermodynamics, kinetics, and simulation hydrothermal reactions, etc., the properties of the solvent (water) can vary with an increase in temperature and pressure

The most considered mechanism in the process of crystalline germ formation and development of oxide material in hydrothermal synthesis is the breakdown and recrystallization, which includes the analysis of the substance and the diffusion of the funds the content leads to a crystallinity to form the composition of the desired compound The mechanism of formation of sprouts and the development of a crystal germ is the basis for explaining the evolution of nanomaterials in 3D structure with a heterosexual crystalline structure To guide the event of a one-way structure by hydrothermal method, it is common to add appropriate organic substances or surfactants to support the orientation for the process of abnormal crystal growth

In most cases, the hydrothermal synthesis of 3D nanostructured Zn2SnO4

nanomaterials begins with the formation of ZnSn(OH)6 intermediate precipitates

or other intermediate phases, prepared from a precursor metal salts Zn, Sn, and surfactants in alkaline medium The solution is then kept at high temperatures in the autoclave for a specific time, allowing the decomposition of ZnSn(OH)6 or other intermediate phases to be the final product of Zn2SnO4 Therefore, it is possible to manipulate the synthesis of well-controlled Zn2SnO4 nanostructures

in a variety of shapes by choosing different solutions and by adjusting the concentration of surfactants and pH Systematic studies of experimental parameters show that the reaction temperature, reaction time, and surfactants, along with the pH value of the precursor solution affect morphology, size, and properties of Zn2SnO4 nanostructures obtained

Some reports on the fabrication of Zn2SnO4 by hydrothermal methods using different precursors have been published such as Lili Wang et al successfully fabricated sheets, spheres, and cubic by hydrothermal reaction using cetyl trimethyl ammonium bromide surfactant (CTAB), sodium dodecyl sulfate (SDS) and hexamethylenetetramine (HMT) [53] Xueli Yang et al [35] studied the

processes with the help of the surfactant cetyl trimethyl ammonium bromide (CTAB) Fengjun Liu et al [58] synthesized by hydrothermal method using commercial oxide powder precursors to obtain Zn2SnO4 nanorods Chao Chen et

al [64] successfully developed 3D Zn2SnO4 flower-like by hydrothermal reaction using a cetyl trimethyl ammonium bromide surfactant (CTAB) Tingting Zhou et

al [76] studied manufacturing Zn2SnO4 nanostructures such as nanocubes,

ZnSO4.7H2O, Na2SnO3.3H2O, SnCl4.5H2O, cetyl trimethyl ammonium bromide (CTAB), NH3.H2O

Surveying the state-of-the art investigation on the gas sensors Research problems are still remaining as below:

- Controlling the synthesis of advanced nanoporous Zn 2 SnO 4 materials with ability to control the morphology for enhancement of VOC gas sensing performance

- In deep study on the VOC gas sensing properties of different

nanostructures of Zn 2 SnO 4 for application breath analysis

- Understating the fundamental of gas sensing mechanism of Zn 2 SnO 4

based VOC gas sensors

With the advantages of hydrothermal method in manufacturing nanomaterials, herein, we fabricate Zn2SnO4 nanostructures for applications in gas sensors The Zn2SnO4 nanomaterials were made from the low-cost

(HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H) and NaOH We changed synthesis conditions such as surfactant concentration, hydrothermal temperature, and pH to synthesize Zn2SnO4 nanomaterials with different shapes and sizes We also investigate the VOC gas sensing properties of Zn2SnO4 nanomaterials

CHAPTER 2 EXPERIMENTAL APPROACH

In this chapter, the author introduces details of the synthesis of nanomaterials Zn2SnO4 with different forms, sizes using hydrothermal methods The electrode structure of the sensor, sensor manufacturing process, heat treatment, and construction process, the principle of the gas measuring system also introduced in detail in this chapter The methods of morphological survey, structure, properties of the material also presented Process of sensor manufacturing, structure, the principle of the gas measurement system, also introduced in detail in this chapter

2.1 The synthesis processes of nanostructured Zn2SnO4 materials with different morphologies by hydrothermal method

2.1.1 Equipment and chemicals

The source materials and solvents used for the synthesis of Zn2SnO4

materials by hydrothermal methods include: ZnSO4.7H2O, SnCl4.5H2O, NaOH,

(HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H) are purchased from Sigma_Aldrich Company (USA) All chemicals used are analytical chemicals Equipment: Electronic scales, magnetic stirrer, heat annealing oven, centrifuges, hydrothermal tank, ultrasonic vibratory machine All chemicals and equipment for material synthesis are available at NanoSensor Laboratory, ITIMS, HUST (Figure 2.1)

With the equipment, the synthesis process can be easily carried out in our lab The magnetic stirrer used to dissolve the precursors Centrifuges used to clean and collect the products Electric oven used to perform the hydrothermal at high temperature

Figure 2.1 Photos of some of the main equipment using synthesized Zn 2 SnO 4

nanomaterials by a hydrothermal method such as thermos flask (1), magnetic stirrer (2), pH meter (3), centrifugal rotary machine (4) and annealing furnace (5)

2.1.2 The synthesis process of Zn2SnO4 nanostructures with different morphologies by hydrothermal method

Zn2SnO4 semiconductor metal oxide nanomaterials which have different morphological structures and sizes, were synthesized by the hydrothermal

method according to the process described in the general diagram, including the following steps Figure 2.2

Figure 2.2 Process diagram of synthesizing Zn 2 SnO 4 nanomaterials with different

morphological structures by hydrothermal method

Firstly, ZnSO4.7H2O (8 mmol) and SnCl4 5H2O (4 mmol) were dissolved

in 30 mL deionized water After stirring for 15 min, 20 ml NaOH (32 mmol) solution was added with further stirring for 15 min to adjust the pH value of 8 Then, the above turbid solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave for hydrothermal The hydrothermal process was maintained at 180 ºC for 24 h After natural cooling to room temperature, the precipitate was centrifuged and washed with deionized water for several times The last two times were washed with ethanol solution and collected by centrifugation at 4000 rpm Finally, the white product was obtained and dried in

an oven at 60 ºC for 24 h

To synthesize Zn2SnO4 material of the different morphological structures,

we have to study, adjust the parameters for hydrothermal processes such as: change the surface-active substance mass P123, change the pH of the hydrothermal solution and change temperature of the hydrothermal method Correctly, to synthesize various morphological structures of the nano Zn2SnO4