Luận văn fabrication of zn2sno4 nanostructures for gas sensor application chế tạo vật liệu zn2sno4 cấu trúc nano Ứng dụng cho cảm biến khí

Methanol sensing characteristics of hollow cưồic ZzuSnO¿ ZYOP5_PH8: A transient resistance versus time upon exposure to different concentrations of methanol measured at different tempe

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

MASTER THESIS

Fabrication of Zn2SnO4 nanostructures

for gas sensor application

LAI VAN DUY Duy LVCA180178@sis.husteduvn

Specialized: Electronic materials

Supervisor: Professor Ph.D Nguyen Due Hoa

Tastitate: 9 International Traimng Tstitule for Materials Science (TTTMS)

TIANOI, 6/2020

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

MASTER THESIS

Fabrication of Zn2SnQ4 nanostructures

for gas sensor application

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

Supervisor: Professor Ph.D Nguyen Duc Hoa Signature of GVHD _

Institute: = International Training Institue for Materials Science (ITIMS)

HFANOI, 6/2020

CONG HOA XA HOI CHU NGHTA VIET NAM

Độc lận — Tự đo— Hạnh phúc

BẢN XÁC NHẬN CHỈNH SỬA LUẬN VĂN THẠC SĨ

lo và tên tác giả luận văn: Lại Văn Duy

Để tài luận văn: Chế tạo vật liên ZnzSnO; cảu trúc nano img dung cho cảm biến

khí

Chuyên ngành: Khoa học vật liệu-VLĐT

Mã số SV: CA180178

Tác giã, Người hướng đẫn khoa hoc va 116i ding châm luận văn xác nhận tác

giả dã sửa chữa, bổ sung luận văn theo biên bản họp Hội dồng ngảy 30/06/2020 với các nội dụng sau:

- Bễ sung chủ thích hình 3.9, 3.10, 3.13

- Các công thức, phương trinh phần ứng được đánh số theo trinh tự

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

- Chit trên trong các hình 3.2, 3.4, 3.6, 3.25 được để ở kich thước lóm hon

- Phân chú thích hình có đâu chấm sau số thứ tự hình

- Chính sửa các lỗi chính tả, hành văn

y 09 tháng 07 năm 2020

CHỦ TỊCH HỘI ĐỒNG

TGS TS Nguyễn Phúc Dương

ĐỂ TÀI LUẬN VĂN Chế tạo vật liệu #2nsSnŒx câu trúc nano ứng dụng cho cảm biển khí

Học viên: Lại Văn Duy

Chuyên ngành: Khoa học vật liệu-VI ĐT

Giáo viên hướng dân

(Kỹ và ghỉ rõ họ tên)

GS T8 Nguyễn Đức Hòa

ACKNOWLEDGEMENT

First of all, I would like to expross my greatest gratitude to Prof PhD

Nguyen Duc Hoa for his valuable scicutific ideas, guidance

and support of

favorable conditions for me to complete this thesis His kindness and enthusiast

will be in my heart forever

Simultaneously, T would like to express my sincere thanks (o all staffs of

the Laboratory for Research, Development, and Application of Nanosensors at

TTIMS-LIUST has always been enthusiastic about helping, sharing experiences and suggesting many imporlant ideas for me to carry out the research of (his

thesis Moreover, Ï am also very graleful to my colleagues, PhD students, the

iSensors’ graduated students who have always accompanied and assisted me in twa 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

lysis by focusing on the controlled synthesis of nanostructured 7n25nOQ, temary metal oxides to maximize the gas sensitivity To archive the objective, we synthesised hollow structure temary muctal by hydrothermal technique with the

assistance of soft template The thickness of the hollow cells was optimised to

desire the highest VOC response Ry hydrothermal method, the author has

successfully synthesized any nanostructures of ZrSnOq with different

amorphologies AL lhe same Lime, ihe thesis also proves the application potcntial

of ZmSnOy material in the gas sensor VOCs The sensor based on ZmSnO4

materials could detect various VOCs gascs such as acetone, cthanol, and

methanol at low conventzalions of ppb levels with high sensitivily

STUDENT

Lai Van Duy

CONTENTS ABBREVIATIONS

2 Aims of the thesis

CHAPTER 2 EXPERIMENTAL APPROACH

2.1 The synthesis processes of nanostructured Zm,Sn0, materials with different

morphatogies by hydrothermal method

2.1.1, Rquipment and chemicals

2.1.2 The synthesis process of Zn-SnO, nanostructures with different morphologies

‘by hydrothermal method

2.3 Morphological and anicrostructure analysis

2.4, Survey of gas sensitivity proper tics

CHAPTER3 RESULTS AND DISCUSSION =-

3.14, Caystal structure of synthesized Zn:SuO, materials

32 Gas sensing propertics of Zn:SnO, matcrials with different morphological

3.2.2 ihanol gus-sensing properlies of the [abricaled sensors

3.2.3 Acetone gas-sensing properties of the fabricated sensors

Number Abbreviations

and symbols

ads

BET CVD

ADK HRTEM ToT

TTIMS

ICTDS

P23 ppb

Ra

Ry

5 SEM

‘TEM VOCs

XRD

ABBREVIATIONS

Meaning

Adsorption Brunauer- Imnet-Teller Chemical Vapour Deposition Fnergy-dispersive X-ray spectroscopy

High Resolution Transmission Electron Microscope

Tntornet af Things

Tntermational Trainmg Institue

for Materials Science

Join Commitiee an Powder

Diffraction Standards

LIO(CLSCLBO}(CLbCII(CLB)O)(CH:C11:O)zä1l

Parts per billion

Parts per million

Rav

Rass Sensitivity

Scanning Electron Microscope

‘Transition Lilectron Microscope

Volatile Organic Compounds

X-ray Diffraction

LIST OF FIGURES

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

diagnose [47]

Figure 1.2, Crystal structures of zine stannate (4m8nO1) [51] 0

Figure 1.3 Sublattices of vine starmate (79104)

Figure 1.4, Schematic representation of the inverse spinel lattiec of ZmSnO4 [49]

Vignre 1.5 Model explains the n-type semiconductor of Zn SnO4 material [50]

Figure 1.6 A schematic diagram of reaction mechanism of SnO2-based sensor Lo

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

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

exposure to a VOCS (b) [43]

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

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

Figure 2.1 Photos of some of the main cquipment using synthesized ZmSnOa

nanomaterials by a hydrothermal method such as thermos flask (1), magnetic

stirrer (2), pH meter (3), centrifugal rotary machine (4) and annealing firmace (5)

Figure 2.2 Process diagram of synthesizing 7aoSnO¿ nanomalerials wilh

different morphological structures by hydrothermal method

Figure 2.3 The process diagram for making sensors on the basis of nano

7mSnOa material by small coating method

Figure 2.4, (A) Gas sensitive measuring system at ITIMS; (3) Diagram of the

gas measuring system by static measurement method

Figure 3.1 SEM image of ZueSnO4 samples synthesized by hydrothermal

method with different hydrothennal temperature: (A, B) 160 °C; (C, D) 180°C;

(F, F) 200°C

Figure 3.2 General diagram of synthetic ZmSnO4 materials with different

morphology according to changes in hydrothermal temperature

Figure 3.3 SEM image of ZmzSnO¿ samples synthesized by hydrothermal

method with different amount of P123 surface-active agent (A, B) 0 g, (C, D)

36

Figure 3.4 Schematic mechanism of synthosizing ZmSnO; materials with

different, morphology by the concentration of surfactants P123 by hydrothermal

method

Figure 3.5 SEM image of ZmŠnO+ nanomaterial synthesized by hydrothermal

mthod wilh different pH conditions: (A,B) pH 8; {C,D) pH 9; (E, F) pH

10; (G, H) pH = 12; (1, K) pH = 13

Figure 3.6 General điagram of the syrthesis of 7125104 materials with different

morphology according to the pH change of the hydrothermal environment

Ligure 3.7 TM (A-D) images of the synthesized hollow cubic ZmSnO1, Inset

Figure 3.10 XRD pattems of ZngSnO4 with condition pil = 8 and plI =13

hydrothermal temperature of 180 °C/24h atter treatment heat at 580 °C for 2h in

Figure 3.11 Raman and PL spectrum of synthesized Zm2Sn0,

Figure 3.12 BRT spectra of 7m$nOq: (A) - Oelahedron, (B) - Cubic, (C) —

Nanoparticles

Figure 3.13 1-V curve of the sensor (A) -

Nanoparticles measured in air al 450 °C

Figure 3.14 Methanol sensing characteristics of nanoparticles ZmzSnOs

(7.TO_PH8) (A) transient resistance versus lime upon exposure lo different

concentrations of methanol measured at different temperatures, (B) scusor

response as a function of methanol; (3) respon and recovery time of sensor

Figure 3.15 Methanol sensing characteristics of hollow cưồic ZzuSnO¿

(ZYOP5_PH8): (A) transient resistance versus time upon exposure to different

concentrations of methanol measured at different temperatures; (B) sensor

response as a unction of methanol, (C) respon and recovery lime of sensor

Ligure 3.16 Methanol sensing, characteristics of hollow octahedron ZmSn0s

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

v

„ d0 -Ö„ 4]

49

52

concentrations of methanol measured at different temperatures; (B) sensor

response as a function of methanol, (C) respon and recovery lime of sensor 53 Figure 3.17 Dthanol sensing characteristics of nanoparticles ZmSnOs

(7.TO_PHR): (A) transient resistance versus time upen exposure to different

conventralions of ethanol measured al different temperatures; (B) sensor response

as a function of ethanol, (C) response and recovery time of sensor

Figure 3.18 Ethanol sensing characteristics of hollow cubic ZrzSnO4

(ZTOP5_PH8): (A) transient resistance versus time upon exposure to different

concentrations of ethanol measured at different temperatures; (13) sensor response

aa a function of acetone; (C) response and recovery time of sensor 54

Figure 3.19 Kthanol sensing characteristics of hollow octahedron ZmSnO4

(ZTOPS PII13): (A) transient resistance versus time upon exposure to different

concentrations of ethanol measured at different temperatures; (B) sensor response

as a Ñmetion of ethanel; (C) response and recovery time of sensor 34 Figure 3.20 Acetone sensing characteristics of nanoparticles ZmSnO,

(ZTO_PH8): (A) transient resistance versus time upon oxposure 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 Acvlone sensing characterislics of hollow cubic ZmSnO4

(£TOP5_PHB8): (A) transient resistance versus time upon exposure to different

concentrations of acetone measured at different temperatures; (B) sensor

response as a function of acclone, (C) response and recovery time of sensor 5 Figure 3.22 Acetone sensing characteristics of hollaw octahedron ZmSnOx

(ZTOPS_PH13: (A) trarsienl resislanoe versus time upon exposure to different

concentrations of acctone measured at different temperatures; (B) sensor

response as a function of acetone; (C) response and recovery time of sensor

Figure 3.23 Response io low acclone concentration of detcolion Fit of (A)

hollow cubic (ZTOPS_PH8) and (B) hollow octahedron (ZTOPS_PHI3)

ZmSnOq at 450 °C and (C) - (F) calculation of detection limit of the ZneSnO,

Figure 3.24 The responses of ⁄IO PH8; ⁄IOP5 PH#, ZTOP5 PHI3 gas

sensors to 125ppm ethanol, methanol and Acetone measured at 150°C aK

vi

Figure 3.25 Stability of sensor ZTO_PH8 (A B), ZTOPS PHR (C, D),

Figure 3.26 3electivity of sensors ZTO PIIR, ZTOPS F1IR, and ZTOPS PIIH3

when surveying with different gases: acetone (100 ppm), ethanol (100 ppm),

xnothanol (100 ppm), NH: (25 ppm), He (50 ppm) aud CO ($ ppm) at 450 °C 64

Figure 3.27 (A, B) transient resistance and (C, D) response value versus time

upon exposure Lo 0.5 ppm acelone measured al 450 °C im different values of

humidity of the hollow cubic, hollow octahedron ZmSuO4 sensor 6S

Figure 3.28 Schematic of the VOCs gas-sensing mechanism of the ZmSn(i 67

vii

INTRODUCTION

1 Foundation of the thesis

The Fourth Industrial Revolution is fundamentally changing the world via

the Internet of Things (oT), cloud computing, 31D Graphic, Augmented Reality,

Machine leaning, sensor technology, and Artificial intelligence, .[1 2) Among others, lo [3] opens up positive effects in almost aspects of human life, including in the field of health care [4, 5] ToT 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 lor instance,

before the ToT, palionls, doctors, and managers spent much time and money on health care and medical diagiosis 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, oT products that integrate nanctechnology into smart wearables have been growing strongly [6] Integration of electronics devices with IoT played a huge role in porsonal health care by using handheld diagnostic sensors, health monitors, chronic disease monitors, therapeutic sensors, ete

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

in improving the quakily of human life in the age of the ToT 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 warming 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 Tl is predioted that the [ied 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 goneentralions (ppb level) of various gases such as volatile organic gases (VOCS)

17-9].

Research [9] shows that the global wireless gas sensor market in the period

from 2015-2020 will grow from §.21 billion USD to 15.07 bition USD, showing

great promise most ta take advantage of gas sensors The research and

manufacture of gas-sensitive sensors in Industry 4.0, it is necessary fo have high-

sensitive materials, fast respousc and recovery lime, good sclectivily to

determine the concentration of VOCs in human breath accurately So far,

researches on (he fabrication, properly surveys, and the applicalnlity 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 (o improve the characterislics of VOCs gas serisors

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 sensars, mass spectroscapy, chromatography sensors and electrochemical sensors for VOCs detection |22- 24]

Resistive gas sensors can be a new road for environmental monitoring, disease diagnosis, and paticnt monitoring bevause of ils simple operation, low

cost, and portability [22, 25] Metal and modified oxides, such as SuOs, ZnO,

TiQz, In2O3, FexOs, WOs, CuO, and NiO, have been investigated as sensing

T

materials for detevting different toxic and VOC gases in new semiconductor sensors [26-28] Ilowever, these oxides suffer from limitations, such as low

sensilivily, poor selectivily, and instability al 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 tnHion (ppl) to parls per miltion (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 sonsilivily, fast reaction and revovery

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 (hese oxides have many advantages, such as chernical inertness,

thermal stability, and environmental friendliness, over common binary oxides

[30, 31] Zn;SnÖx is a typical n-type semiconducting temary oxide [29] that has

amullifimetional characteristics, including high clectron mobilily, goud thenual stability, high chemical sensitivity, and low-visibility absorption [33, 34], thus suilable 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 ZnzSnO4 nanostructures wilh novel morphologies to improve further he response speed, scleetivity, and stability of gas sensor devices [28, 32] In comparison with dense particles, porous or hollow structural materials [38] provide more surface activities, high

performance [34] Many approaches have used to fabricate ZmSnO4 materials, and these include hydrothermal [38, 39], co-precipitatian [37], sol-gel [38], and thormal evaporation |35, 42| techniques The hydrothermal method has certain advantages, such as simple fabrication and low cost, and it is commonly employed to synthesize ZnzSnO, hollow structures [40] However, recent studies

involve the use of a sample and two or three sleps Lo synthesize empty structures

ZmSn04 [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 matcrial changes [42] Therefore, simple protocols for the fabrication of

hollow structures ZrzSnO, by the secure hydrothermal method are needed to

evhance 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 Liowever, few yescarches report on general and application of the 7mS$nO4 hollow block for VOC gas sensor applications ‘therefore, this thesis targets to the “Fabrication of Zn:SnOs nanostructures for gas sensor application”

2 Aims of the the:

= ‘To successfully fabricate ZnonQs nanostructures using the hydrothermal method for gas sensor applications

- To investigate the gas sensitivity and electrical properties of the

synthesized 7.n2SnQ4 nanostructures

- Tounderstand the gas mechanism of the ZnzSnO, nanostructures

3 Research object and scope of the thesis

To implomont this sludy with the above ubjeclives, the thesis focused on

researching the following key issues:

The thesis was carried out based on experimental methods combined with

theoretical research and surveying the published articles In details, the Zm2SnOs nanoparticles were synthesized by the hydrothermal method Morphology and structure properties of the material were analyzed by Raman scattoring, scanning, electron microscope (SLM), ‘lransmission electron microscope (LM), X-ray

diffraction (XRD), diffusing X-ray Energy dispersive (EDX) and it is surface area Measurement (BET) The electrical properlics of material analyzed using

the -F characteristic method of measurement, The gas-seusing characteristics of

ZmSnO, material-based sensors have studied by static measurement techniques

on the gas sensing characicristies of the Air Sensing Group @Sonsor yn) al the

ITIMS Institute-Institute for International Scientific Trainmg on scientific

research materials University of Teclmology-Hanoi

5 The practical and scientific significance of the thesis

‘The thesis has launched a stable process to produce ZmSnO,4 materials using simple metheds, namely the hydrothermal method The author bas synthesized the ZmSnQ nanostructures with different morphologies for application in gas sensors, All studies were carried in the conditions of technology and equipment in Victiam These processes can allow for the mass

manufachwing of sensors, with high repeatability, consistency, and reliability The fabricated sensor has high sensitivity and selectivity, which can detect VOCs

a

such as methanol, ethanol, and acetone at low concentrations of ppm to ppb The yosults are very likely and carr put ino practical applications for heallhoare

Tirthermore, 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 ZneSnOs with different morphologies At the same time, the results of the thesis also prove the potential application of ZmSnO4 material in the gas sensor VOCs The sensor based on 7.m2$nO4 materials has igh sensitivily

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 TSI articles

1 Nguyen Hong Hanh, Lai Van Duy, Chu Manh Hung, Nguyen Van Duy, Young-Woo Ileo, Nguyen Van Ilieu, Nguyen Due Hoa*, "VOC

gas sensor based on hollow cubic assembled nanocrystal ZmSnO,4 for

breath analysis’, Sersors and Actuators A 302 (2020) 111834-111839

[IF 2018: 2.73) Also, thers are some published resulls 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

tmroducHơn of ZmzSnÖa semiconductor melal oxide tatcnals and their

applications in the field of gas sensors Synthesis of Zm:SnO4 nanomaterials, a review of some published research results on the gas sensing mechanism of

VOCs based on 7.1 $nOq materials

Chapter 2: Experimental approach

In this chapter, we present the technological process of manufacturing 7mSnO4 nanomaterials by the hydrothermal method Introducing the method of surveying morphology, gas-sensitive and electric properties of ZnzSnO, materials

used in the thesis

Chapter 3: Results and discussion

In this chapter, we present the results and discuss on the morphology, gas-

sensing propertis

and the sensitivity mechanism of Znz$nO4 material strucLures

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 bas presented the conclusions of the thesis, inchuding 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 t 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 (he hydrothermal methods anv gas sensing mochanism of

Zinc Stannate materials (⁄nsSnOs) for VOCs

1.1 Volatile organic compounds

The updated definition of the United States Environmental Protection

Agency (UPA) 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

inaterials that can evaporate naturally when experiencing atmospheric pressure at

room temperature [46, 17] The VOCs are ingredients in many commercial,

industrial, and household products Large amounts of VOCS are discharged into

the almosphere from both artificial and tatural sources The conecniration 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 hygicne produets (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 thal 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 Iung cancer) Prolonged exposure to high concontrations may also cause liver, kidney or contral 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-bome

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

1.2 Overview of Zn2SnO4 material

ZmSnO4, a semiconductor metal oxide material has attracted immense

interest, with exciting properties, high strength, stability, high electron mobility,

8

high conductivity, and low absorption in a visible light area, Development of

simple fabrication method has been widely studied in the world ZnzSnO, 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, ete

1.2.1 Crystal structure of Zm2:Sn0, material

Zine stannate (ZmSnO4) 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 A™B'Oy 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

Be

Oxygen

B-atoms octahedral sites

(a) Tetrahedral A site (b) Octahedral 8 site

Figure 1.3 Sublattices of zine stannate (Zn2SnOx)

The spinel structure can further classify into typical and inverse spinel

structures In the normal spinel structure, the Zn?* ions occupy the tetrahedral sites, and the Sn** 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 Zn** and Sn‘ cations distributed over the sites of tetrahedral (A) and octahedral [B] coordination In the inverse spinel

structure, the A** ions and half of the B* ions occupy a bowl-side position

together, and the other half of the ion B** occupies the quadrangle position,

which is governed by the general formula B(AB)O4

Bulk Zn2Sn0O, crystals exhibit an ideal inverse spinel structure with a cubic

lattice parameter a = 8.6574 A in which one-half of Zn?* ions occupy (A) sites,

whereas Sn** cations and the other half of Zn’* ions occupy [B] sites, which is

governed by the generalized formula (Zn?*)[Sn** Zn?* ]O, (Figure 1.4) [49]

Figure 1.4 Schematic representation of the inverse spinel lattice of

Zn28n0.[49]

10

1.2.2 Electrical properties of ZnzSnOx material

ZnzSnO¿ is an n-type semiconductor with a wide bandgap (Eg ~ 3.6 eV)

The semiconductor properties of ZmSnOy 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 ZnzSnO4

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

ZmSnOj crystal lattice can be controlled through heat treatment processes at

different temperatures or heat-treated in different environments

Lo ORG RONG: See es

perfectly and shows that the Sn and Zn atoms are arranged in order Figure 1.5

(B) shows two oxygen locations in ZnSnO4, 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-[, 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

11

material Joohwi Lee et al [50] calculated the effect of oxygen vacancies on

alomie and cleetron structure in ZmSnO4 crystal Based on the spectroscopic

spectroscopy, the calculated indicates that the bandgap of ZnzSnO1 material lies

in about 2 eV The analysis of the atomic and electronic motions around the

absence of oxygen shows thal Sn plays a key role in changing clcctrical properties Whereas, the electronic state of Zn remained almost unchanged,

although atomic displacement w:

superior to Sn around the oxygen vacancy:

1.2.3, Application of ZaSn0 material in gas sensors

So far, many different nanostructures based on semiconductor metal oxide

ZmSnO4 materials have successfully vescarched and spplied widely in many different fields Particularly, in recent years, there have been many studies on the gas-sensitive properties of gas sensors based on Zn2SnOj materials Still, so far,

the understanding of the effects of nanostructures and morphology of Zm2SnO4

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

ZmSnO4 materials with different morphologies, including of nanoparticles [37], nanowires |5l|, nanospheres |52|, hicrarchical quasi-microspheres [41], nanosheets [53], lamellar microspheres [54], octahedra [55], and hollow octahedra [42], have been tested over many pases [55], such as toluene [41],

siructures-based the sensor exhibils excellent tolusne sensing properties, the

response valuc (Rs/R,) to 100 ppm toluene at 280 °C was 25.2, higher than that

of the Znai8nQ« sphere (Re/Ry: 19.2) and Zn;SnO cube (Rz/Rz.11.7), followed by cllumol, benzene, Xylol, carbou monoxide, hydrogen, and sulluretled hydroger

Y Tie et al [56] reported on the synthesis of ZneSnOs nanocube via a hydrothermal route for formaldehyde gas sensor application, where the response

value (R„R,) to SO ppm ethanol at 230 °C was 23,57; and the sensor response al

230 °C was the highest to ethanol, acetic acid, acetone, methanol, and benzene

Shaoming Shu et al [57] synthesis of ZmSnO4/SnOa materials has a hierarchical

12

structure with many different morphologies through a simple chemical reaction youle with posl-heal treatment, morphologies as spherical, surface layerod hierarchical octahedral-like and perfect octahedral summed up by adjusting the

reaction sohition temperature, the sensor based on surface layered hierarchical

octahedral-hke ZmSnO4/Su02 (20 °C) exhibited much more excellent gas-

sensing performance toward formaldehyde than other sensors derived from

spherical, perfect octahedral structure ZSnOQ4/SuO2, where the response value

(RwR,) 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,

S and 7) Fengjun Liu et [58] reporicd on the synthesis of novel Zm2SnO4-70

hierarchical structures via lydrothermal route for triethylamine gas sensor application, where the Zn2SnO+-ZnO hierarchical structures were composed of

1D ZTO nanowires and 7nO© nanosheets, and it was found that the 7TO-72nOQ

hierarchical structures exhibited excellent sensing performance toward

, the

triethylamine; the response value (Ra/Rg) to 100 ppm at 200 °C was 175

response time was approximately 13 s, but the recovery time was quite long (189 s) and 308 times higher than that of the pure ZITO and ZnO sensors,

respectively: and the sensar response at 200 °C was the highest ta butanol,

elhamol, ethyl acctate, acetone, and methanol Wang ef al [59] reported on the

synthesis of ZmSnO4 nanopowders via a hydrothermal route for ethanol gas

sensor appheation, where the response value (R,/R,) to 100 ppm ethanol at 300

°C was 30, and ihe sensor response al 300 °C: was the highest to ethanol,

followed by methanol, methylbenzene, and acetone Yang et al [36] used

carbonaceous spheres as templates ini combination with calcination lo syuthesive

ZmSnO4 hollow spheres for acotone gas sensor, whore the response value of 153 was obtained to 200 ppm at 200 °C Chen et al [60] prepared highly ordered

7mSnO4 three dincnsional Nowerlike superstructures assembled with tnanorods

for ethanol sensor, where the device showed the response value of approximately

35 to L00 ppm ethanol at 128 °C

This

car thal ihe gas scnsing 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

13

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 Zn2SnO, 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 ZnzSnO, hollow structures [32, 41]

Through the overview of the researched works related to ZnzSnO4 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, ete

significantly influence the sensitivities of the sensor

Table 1.1 Comparative VOC gas response of different Zn28nO4 structure-based sensors

53]

52]

56] 57]

58]

59]

60] 66] 67]

33]

15

SnO2

22 Graphene- Nanoparticles Formaldehyde 1000 18.9 20 [65]

ZnSn04

In Vietnam, there are several researching and manufacturing groups

dedicating their studies on the enhancement of gas sensing properties of ABO

oxides For instance, a research group at Hanoi National University of Education,

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 MnWOs [69],

ZnWO¿ [70], Zm„Co,O [71] and Ti,„V,O; [52], ete 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 ZiySnOs 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

ZnFe204 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 WO; by thermal

evaporation method This group started to focus on researching and

manufacturing plural nanostructures such as ZnWOy, MnWOs, ZnFe2O,, and

ZmgSnO4, ete and examine their gas-sensitive characteristics for applications on

nano gas sensors

16

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 (ZmSn0), usually in the oxygen-deficient crystal at the lattice leading 10 the appearance of loading particles im the crystal the electronics ZmSnOs, an mtype semiconductor - electrical properties govemed

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 ‘'ypically,

in the lower temperature zone of 150 °C, oxygen adsorbs on the surface of metal

oxide in the form of 02° molccules 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° hofe 0%) When the temperature is above 500 °C part of the

adsorbed oxygen van diffuse into the malorial Jaltice (absorption) [72] The

adsorption kinematics are described as follows:

Oa (pas) —* Ox (ads) ai O2 (ads) +e" > OF (ads) q2) C# (ads) + e- —+ 2O (ads) 43)

The formation of oxyger species results in the capLurs of electrons from the

conduction band, leading to a change in conductivity of the semiconductor metal oxide, the variation in conductivity results from a change in the charge carrier omecntralion The charge carers of n-type semiconductor metal oxide arc

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

yesult, a thick electron depletion layer will form on the surface of Z1SnO,

nanopaiticles, and a high potential barrier is formed between the adjacent

nanograins, leading to an increase of resistance in the sensing material

17

For the mtype semiconductor oxide material, when the semiconductor metal oxide material puls in Ihe enviroment 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

barner 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 sensifive layer they may imlsract, 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 arca), theroby 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

Dolailed VOCs gas scnsitivity mechanism of semiconductor molal 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 SnOz particles come into contact with formaldehyde gas, formaldehyde will react with the adsorbed oxygen ions and release the trapped

concentration Conscquently, the thickness of the space-charge layer is reduced, resulting in a decrease of the potential barrier and resistance, as shown in l'igure

16

18

(b)

potential barrige FP"

surtace

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

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

For the gas sensor based on nanopatticles, 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

19

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

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

As a result, a thick electron depletion layer will form on the surface of ZmSnO4 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]:

CH30OH + 40° + CO + 220 + 4e (1.4)

C.HsOH + 60° > 2CO2+3H20+6e (15) CH3COCH3 + 80° — 3CO2 + 3H20 + 8e" (16)

20

Sensitivity is a parameter reflecting the resistance variation in a certain concentration of targel gas For semiconductor melal oxide sensors, the

sensitivity of semiconductor metal oxide gas sensors can be improved by

adjusting the microstructure, defects, catalyst, heterojunction, and humidity

The gas-scnsing performance is imfluenced 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 sive, number of activated adsorption siles and gas diffusion ability are the structural parameters that dramatically affect the gas-sensing performance ‘Ihe reduction in grain size ta 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 cecur on the surfaces of sensing materials because the gas molecules cannot penetrate the sensing imatcrals Tn the case of porous nanoalruclures, (he gas molscules can inleracl

with the imer 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 (he resislance of lhe gas sensor The porous structure is

divided into a microporous, mesoporous and macroporous structure based on the

21

specifically the hydrothermal method The hydrothermal method is a simple method thal 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, nanoplatcs, nanoflowers, nanoparticles and objects the material has a porous

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 conlent leads to a crystallinity to fonn 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 helcrosexual eryslatlme structure To guide the event of a one-way structure by hydrothermal method, it is commen to add appropriate organic substances or surfactants to support the orientation for the process of abnormal crystal growth

An most cases, the hydrothermal synthesis of 3D nanostructured “mSn04 nanomaterials begins with the formation of ZnSn(OI[y intermediate precipitates

22

or other intermediate phases, prepared from a precursor metal salts Zn, Sn, and surfactants in alkaline median The solution is then kept al high (emporalures in the autoclave for a specific time, allowing the decomposition of ZnSn(OIDs or

other intermediate phases to be the final product of ZnzSnOQ4 Therefore, it is

possible to manipulate the synthesis of well-controlled ZmSnO4 nanostructures

in a variety of shapes by choosing different solutions and by adjusting the

concentralion of surfactants and pH Systematic siulies of experimental

parameters show that the reaction temperature, reaction time, and surfactants,

along with the pil value of the precursor solution affect morphology, size, and

properties of ZngSnO4 nanostructures blamed

Some reports on the fabrication of ZmSn04 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

trimethy! ammonium bromide surfactant (CTAB), sodium dodecyl sulfate (SDS)

and hexamethylenetetramine (IIMT) [53] Xueli Yang et al [35] studied the manufacture of Znj$nO4 nano octahedrons synthesized by hydrothermal processes with the help of the surfactant cetyl trimethyl ammonium bromide (CTAB) Fengjun Lin et al [58] synthesized by hydrothermal method using commercial oxide powder precursors to obtain Z11SnO4 tanoreds Chaa Chen eL

al [64] successfully developed 31D ZnSnO4 flower-like by hydrothermal reaction

using a cetyl trimethyl ammonium bromide surfactant (CTAB) Tingting Zhou et

al [76) studied manufacturing ZgSnO4 mumostiructures such as nanocubes, nanorods, and nano oetahedrons using precursors Zn(CIICOO):21b0,

¥nSO4.7M2O, Na2SnO3.3A20, SnCly5H2O, cetyl trimethyl ammonium bromide

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

nanostructures of Zn:SnOu for application breath analysis

23

- Understating the fundamental of gas seHsing mechanigm dƒ ZnSnO, based VOC gay sensdrs

With the advantages of hydrothermal method in manufacturing

nanomaterials, herein, we fabricate 7ngSnOQ4q nanostructures for applications in

gas sensors The ZieSnO4 manomalerials were made fiom the low-cosL precursors: Zn8O4.7H2O, SmCA5H2O, surfactants phươưúc P123

(HO(CA2CH20)2e(CH2CH(CH3)}0)}1o(CH2CH20)20H) and NaOH We changed

synthesis conditions such as surfactant concentration, hydrothermal temperature, and pli to synthesize ZneSnQ1 nanomaterials with different shapes and sizes We also investigate the VOC gas sensing properties of Zze2SnO4 nanomaterials

24

CHAPTER 2 EXPERIMENTAL APPROACH

Tn this chapter, the author imtroduecs details of the synthesis of

nanomaterials Zn2SnO 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 ‘he methods of morphological survey,

structure, properlies 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 provesses of nanostructured Zn28n04 materials with

different morphologies by hydrothermal method

2.1.1 Equipment and chemicals

The source materials and salvents used for the synthesis of 7m2SnOQ4

materials by hydrothermal methods include: %n$O4.7H20, SnCl 5Hs0, NaOH,

(HO(CH;CH;O);(CH;CH(CH3)O)o(CH:CHO)øH) am purchacd from

Sigma Aldrich Company (LSA) All chemicals used are analytical chemicals

Equipment: Electronic scales, magnetic stirrer, heat annealing oven, contriluges, hydrothermal tank, ultrasonic vibratory machine All chemicals and equipment for material synthesis are available at NanoSensor Laboratory, I'TIMS,

Figure 2.1 Photos of some of the main equipment using synthesized Zn2SnOx

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 Zm2SnO, nanostructures with different

morphologies by hydrothermal method

ZmSnO4 semiconductor metal oxide nanomaterials which have different

morphological structures and sizes, were synthesized by the hydrothermal

26

method according to the process described in the general diagram, including the

following steps Figure 2.2,

— @›

Figure 2.2 Process diagram of synthesizing Zn28nO nanomaterials with different

morphological structures by hydrothermal method

Firstly, ZnSO4.7H20 (8 mmol) and SnCly 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 °C 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°C for 24 h

To synthesize ZngSnO4 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

27