HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY
..

MASTER THESIS
Fabrication of Zn2SnO4 nanostructures
for gas sensor application
LAI VAN DUY


Specialized: Electronic materials

Supervisor: Professor. Ph.D. Nguyen Duc Hoa
Institute:

International Training Institute for Materials Science (ITIMS)

HANOI, 6/2020


HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

MASTER THESIS
Fabrication of Zn2SnO4 nanostructures
for gas sensor application
LAI VAN DUY


Specialized: Electronic materials

Supervisor: Professor. Ph.D. Nguyen Duc Hoa
Institute:

Signature of GVHD

International Training Institute for Materials Science (ITIMS)

HANOI, 6/2020


CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM
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 cơng 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


ĐỀ TÀI LUẬN VĂN
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


ACKNOWLEDGEMENT
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 Zn2SnO4 material ............................................................................................. 8
1.2.1. Crystal structure of Zn2SnO4 material ........................................................................... 9
1.2.2. Electrical properties of Zn2SnO4 material ................................................................... 11
1.2.3. Application of Zn2SnO4 material in gas sensors.......................................................... 12
1.2.4. Gas sensitivity mechanism of metal oxide for VOCs.................................................. 17

1.3.

Hydrothermal method ......................................................................................................... 21

CHAPTER 2.
2.1.

EXPERIMENTAL APPROACH........................................................................... 25

The synthesis processes of nanostructured Zn2SnO4 materials with different

morphologies by hydrothermal method ............................................................................ 25
2.1.1. Equipment and chemicals ............................................................................................ 25
2.1.2. The synthesis process of Zn2SnO4 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
i


CHAPTER 3.
3.1.

RESULTS AND DISCUSSION ..............................................................................32

Morphology and crystal structure of zinc Stannate nanomaterials (Zn2SnO4)
synthesized by hydrothermal method ................................................................................32
3.1.1. Effect of hydrothermal temperatures on the morphology of Zinc Stannate
(Zn2SnO4) materials .....................................................................................................32
3.1.2. Effect of surfactant P123 on the morphology of Zn2SnO4 material ............................34
3.1.3. Effect of pH on the morphology of Zn2SnO4 materials ...............................................38

3.1.4. Crystal structure of synthesized Zn2SnO4 materials ....................................................44

3.2.

Gas sensing properties of Zn2SnO4 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

ii


ABBREVIATIONS
Number

Abbreviations
and symbols

Meaning

1

ads

Adsorption


2

BET

Brunauer- Emnet-Teller

3

CVD

Chemical Vapour Deposition

4

EDS/EDX

Energy-dispersive X-ray spectroscopy

5

HRTEM

High Resolution Transmission Electron
Microscope

6

IoT


Internet of Things

7

ITIMS

8

JCPDS

9

P123

HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H

10

ppb

Parts per billion

11

ppm

Parts per million

12


Ra

Rair

13

Rg

Rgas

14

S

Sensitivity

15

SEM

Scanning Electron Microscope

16

TEM

Transition Electron Microscope

17


VOCs

Volatile Organic Compounds

18

XRD

X-ray Diffraction

International Training Institute
for Materials Science
Joint Committee on Powder
Diffraction Standards

iii


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

iv


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
v


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
vi



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

vii


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

viii


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].
1


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 highsensitive 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 [2224].
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.
2


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
Zn2SnO4 nanostructures for gas sensor application”.
2. Aims of the thesis
-

To successfully fabricate Zn2SnO4 nanostructures using the hydrothermal
method for gas sensor applications.
3


-

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:
-

Fabrication and survey of the sensible properties of Zn2SnO4
nanomaterials

-

Survey of electrical properties, analysis of factors affecting the gas
sensing characteristics, and explanation the gas sensing mechanism of
Zn2SnO4 material.

4. Research Methods
The thesis was carried out based on experimental methods combined with
theoretical research and surveying the published articles. In details, the Zn2SnO4
nanoparticles were synthesized by the hydrothermal method. Morphology and
structure properties of the material were analyzed by Raman scattering, scanning
electron microscope (SEM), Transmission electron microscope (TEM), X-ray
diffraction (XRD), diffusing X-ray Energy dispersive (EDX) and it is surface
area Measurement (BET). The electrical properties of material analyzed using
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
4


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
5


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, gassensing 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.
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.

6


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.
1.1.

Volatile organic compounds
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

7


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,
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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 (Zn2SnO4)[51].
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Figure 1.3. Sublattices of zinc stannate (Zn2SnO4).

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
Zn2SnO4[49].
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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 Zn2SnO4 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
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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
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