Fabrication of in2o3 nanowires for self heated gas sensor application

i HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY MASTER THESIS Fabrication of In2O3 nanowires for self-heated gas sensor application NGUYEN THANH DUONG Duong.NT202253M@sis.hust.edu.vn S

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HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

MASTER THESIS

Fabrication of In2O3 nanowires for

self-heated gas sensor application

NGUYEN THANH DUONG

Duong.NT202253M@sis.hust.edu.vn

Specialized: Materials science (Electronic materials)

Supervisor 1: Associate Professor Ph.D Nguyen Van Duy

Unit: International Training Institute for Materials Science (ITIMS)

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DECLARATION

I hereby declare that this thesis represents my work which has been done after the registration for the degree of Master at the International Training Institute of Materials Science – Hanoi University of Science and Technology and has not been previously included in a thesis or dissertation submitted to this or any other institution for a degree, diploma or other qualifications

Hanoi, 22th April, 2022

Nguyen Thanh Duong

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ACKNOWLEDGEMENT

First of all, I am sincerely grateful to my thesis supervisor Assoc Prof Nguyen Van Duy and Prof Nguyen Duc Hoa - International Training Institute of Materials Science, for allowing me this opportunity to be their student; all of their advices, indication, and inspiration during the time I studied and carried out my Master thesis in ITIMS I am very proud to have their whole guidance, encouragement, and insight which have always been invaluable

I would like to show my gratitude to all of teachers and staff not only in ITIMS but also in HUST to support me, I would like to send special thanks to Mr Dang Ngoc Son and Mr Lai Van Duy - ITIMS for sharing me the initial experiences and many useful suggestions relevant to my work

Last but not the least, I would like to thank my family and my friends for their support and encouragement

SUMMARY OF MASTER THESIS

In this work, we focused on the fabrication and testing of the H2S gas sensing characteristic of the self-heated In2O3 nanowires sensor via a one-step CVD technique and drop-casting on the IDE electrode The self-heated In2O3 NWs gas sensor was measured at room temperature with different applied power toward H2S gas This performance was better than the state-of-the-art microheater gas sensor The sensor is a potential candidate for application related to H2S detection such as breath exhaled analysis and environmental monitoring

Hanoi, 20th August, 2022

Master Student

Nguyen Thanh Duong

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CONTENT

DECLARATION ii

ACKNOWLEDGEMENT iii

ABBREVIATIONS vi

LIST OF FIGURES vii

LIST OF TABLES x

INTRODUCTION 1

1 Foundation of the thesis 1

2 Aims of the thesis 2

3 Research object and scope of the thesis 2

4 Research Methods 2

5 New contributions of the thesis 3

6 Structure of the thesis 3

CHAPTER 1 OVERVIEW 4

1.1 Gas sensor 4

1.2 Self-heated gas sensor Error! Bookmark not defined.10 1.2.1 Self-heating effect 10

1.3 In2O3 materials in gas sensor 19

1.3.1 In2O3 materials 19

1.3.2 In2O3 nanowires in gas sensor 21

1.4 Hazardous properties of H2S gas 22

CHAPTER 2 EXPERIMENTAL APPROACH 24

2.1 Synthesis of In2O3 nanowires 24

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2.1.1 Equipment and chemical 24

2.2 Fabrication of In2O3 nanowires 25

2.3 Fabrication of self-heated In2O3 gas sensor 27

CHAPTER 3 RESULT AND DISCUSSION 30

3.1 Morphology of Indium Oxide (In2O3) synthesized by CVD method and In2O3 NWs based sensor fabricate by drop-casting 30

3.1.1 Effect of Sn proportion on the morphology of Indium Oxide (In2O3) materials 30

3.1.2 The distribution of In2O3 NWs in the various isopropanol solvent ratio 32

3.2 The microstructure characterization 33

3.3 Gas sensing properties 35

CONCLUSION AND RECOMMENDATIONS 48

REFERENCES 49

4 EDS/EDX Energy-dispersive X-ray spectroscopy

5 HRTEM High Resolution Transmission Electron Microscope

7 ITIMS International Training Institute

for Materials Science

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LIST OF FIGURES

Figure 1.1 Detection methods of semiconductor gas sensing materials [1] 4

Figure 1.2 Different material classes for gas sensing application [1] 5

Figure 1.3 Sensing mechanism of metal oxide based gas sensor [1] 7

Figure 1.4 Power consumption and temperature characterized of Hwang WJ’s micro-heater [8] 8

Figure 1.5 Sang Chung Gwiy, Jae-Min Young group’s micro heater [9] 9

Figure 1.6 KMHP 100 commercial micro heater 10

Figure 1.7 Single SnO2 NW contacted with electron beam assisted platinum deposition in a four probes configuration before (a) and after (b) a few hours of operating in self-heating mode 19

Figure 1.8: In2O3 crystalline structure 20

Figure 1.9 The response of single oxide and composite sensors to 5 ppm ethanol vapor at 100% RH [36] 22

Figure 2.1 CVD system - ITIMS 24

Figure 2.2 Precursor material (A), Aluminum oxide boat (B) 25

Figure 2.3 Thermal cycle of In2O3 nanowires fabrication process 26

Figure 2.4 FESEM microscope – HUST 27

Figure 2.5 Procedure of self-heated In2O3 NWs based gas sensor 28

Figure 2.6 Gas sensitive measuring system at ITIMS (A), Diagram of the gas measuring system by static measurement method (B) 29

Figure 3.1 Morphology and microstructure of three composite samples at (A),(B): 0 %; (C),(D): 20 %; (E),(F): 50 %; (G),(H): 80 % mass ratio of Sn were observed by SEM scanning electron microscopy 30

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Figure 3.2 Distribution of In2O3 NWs onto silicon substrate with (A)10 ml (B)20 ml and (C) 30ml of Isopropanol solvent 32Figure 3.3 SEM image of In2O3 nanowires dispersion on the electrode with various ratio solvent 33Figure 3.4 (a–d) XRD pattern of 0%, 20%, 50 and 80% SnO2/In2O3 NWs 34Figure 3.5 EDX spectrum of (A) Pure In2O3 NWs and (B),(C) SnO2/In2O3 NWs 35Figure 3.6 The response of self-heated In2O3 gas sensor versus time at different power

of 600, 800, 1200, 1200 μW (a) and the function of response with concentration H2S gas (b) 36Figure 3.7 The response of self-heated 20% wt SnO2/In2O3 NWs gas sensor versus time

at different power of 300, 500 and 700 μW (RT) (a) and the function of response with concentration H2S gas (b) 38Figure 3.8 The response of self-heated 50% wt SnO2/In2O3 NWs gas sensor versus time

at different power of 300, 500 and 700 μW (RT) (a) and the function of response with concentration H2S gas (b) 39Figure 3.9 The response of self-heated 80% wt SnO2/In2O3 NWs gas sensor versus time

at different power of 300, 500 and 700 μW (RT) (a) and the function of response with concentration H2S gas (b) 40Figure 3.10 Response and heating power graph of four fabricated sensors 41Figure 3.11 The response of self-heated 80% wt SnO2/In2O3 NWs gas sensor versus time at different temperature of 200RC, 250 RC, 300 RC and 350 RC and the function of response with concentration H2S gas 42Figure 3.12 Response characteristic of In2O3 – nanowires gas sensor toward 5 ppm H2S

at 250 oC and self-heating with a power consumption of 700 μW 43Figure 3.13 Response characteristic of 80% SnO2/In2O3 NWs gas sensor toward 5 ppm

H2S at 250 RC and self-heating with a power consumption of 700 μW 43

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Figure 3.14 Response to H2S of the 80% wt SnO2/In2O3 NWs sensor used self-heating effect (Orange line) and sensor using the external heater at 200 oC (Blue line) 44Figure 3.15 Stability of sensor A External heater B Self-heated mode 45Figure 3.16 Selectivity of In2O3 NWs gas sensor toward NH3, Ethanol and H2S gas under self-heating mode 46Figure 3.17 In2O3 material H2S gas sensing mechanism 47

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LIST OF TABLES

Table 1.1 Summary of publication reporting quantitative information about self-heated devices based on nanomaterial 11Table 1.2 Publications reported self- heating effects in gas sensor using metal oxide materials 12Table 2.1 Precusor material in this experiment 26Table 3.1: Comparison with previous study at ITIMS with same approach method 44

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1

INTRODUCTION

1 Foundation of the thesis

Since the first device was invented by the Greeks to manage the level of water using a floater, similar to those that are used today in water boxes to keep a water container at a constant level, sensors have been used to gather signals from the environment for more than 2000 years Many other sensors and actuators, include gas sensors, a branch of the sensor family, have been developed after a thousand years of development It's critical

to monitor gases, humidity, and moisture in areas including agriculture, medicine, and industrial processes Gas sensors are frequently used to monitor environmental pollution and detect low concentrations of hazardous, flammable, or explosive gases In many different industries and applications, such as smart building and smart home systems, environmental monitoring, and food quality monitoring, the usage of chemical sensor devices to detect and measure gases has become truly indispensable

Recently, the Fourth Industrial Revolution is dramatically changing the world with Internet of Things (IoT), cloud computing, 3D Graphic, Augmented Reality, Machine learning, sensor technology and Artificial intelligence Among them, IoT brings in a lot of advantages in almost aspects of human life IoT is the result of the fusion of the internet, wireless technology, and micro mechatronics technology, all of which have great utility and are beneficial to human society Gas sensors and sensor nodes are essential parts of cutting-edge communication systems like the Internet of Things Modern sensor requirements for IoT include: (1) dependability; (2) energy consumption; (3) cost; (4) communication ability; and (5) data security With that being said, alongside with the high requirement of energy saving, promote researches of low power consumption devices are critically important

In addition, the metal oxide-based gas sensor requires thermal energy to activate the interaction between the analytic gas molecule and the sensing material Self-heated gas sensors have recently been developed to reduce the device's power usage It has been proved that Joule self-heating effect is nearly ideal for operating NWs gas sensors at

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ultralow power consumption, without external heaters In this thesis, we present an optimal fabrication process of In2O3 multiple NWs as well as the sensing capability of the self-heated networked In2O3 NWs effectively prepared by drop-casting method with

H2S gas Finally, I identify the limitations of these sensors and highlight the most promising approaches to enable the use of these technologies in real-world applications

2 Aims of the thesis

- To successfully fabricate self-heated gas sensor based on In2O3 NWs using the chemical vapor deposition method (CVD) and drop-casting method

- To investigate the microstructure of the synthesized In2O3 NWs as well as comparing self-heated operating mode with using external heater sensor sensitivity toward H2S gas

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 In2O3 NWs and In2O3, SnO2 composite NWs

- Survey of gas sensing properties, analysis of factors affecting the gas sensing characteristics material by using external heaters as well as self-heating operation

4 Research Methods

The thesis was carried out based on experimental methods combined with theoretical research and surveying the published articles In details, the In2O3 NWs were synthesized by the CVD method Morphology and structure properties of the material were analyzed by scanning electron microscope (SEM), X-ray diffraction (XRD) and diffusing X-ray Energy dispersive (EDX) The gas-sensing characteristics of In2O3 NWs -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

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5 New contributions of the thesis

By CVD and drop-casting method, the author has successfully synthesized In2O3nanowires as well as SnO2/In2O3 nanowires which being used to fabricate self-heated gas sensor At the same time, the results of the thesis also prove the potential application

of In2O3 material in the gas sensor, especially with low power consumption advantage

6 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 gas sensor and self-heated gas sensor as well

as introducing the In2O3, SnO2 NWs material

Chapter 2: Experimental approach

In this chapter, we present the technological process of manufacturing In2O3 nanowire

by the CVD method and fabrication of self-heated gas sensor based on In2O3 NWs structure Introducing the method of surveying morphology of the material, gas-sensitive and electric properties of self-heated gas sensor used in the thesis

Chapter 3: Result and conclusion

In this chapter, we present the results and discuss on the morphology, gas-sensing properties, and the sensitivity mechanism of In2O3 and SnO2/In2O3 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

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CHAPTER 1 OVERVIEW 1.1 Gas sensor

According to the mechanism of detection, gas sensors made from various sensing materials may be divided into categories One class of sensing techniques uses variations

in electric characteristics, while others use optic, auditory, chromatographic, and calorimetric gas sensors The primary physical properties of the sensing material, such

as conductivity, permittivity, and work function, vary when the gas sensor is exposed to the environment, as illustrated in Figure 1.1

Figure 1.1 Detection methods of semiconductor gas sensing materials [1]

The transducer - a component of the gas sensor, transforms these physical properties into electrical signal that are measurable as resistance, capacitance, and inductance Thus, interfaces play a crucial role in determining the sensitivity and durability of sensing devices in electrically transduced semiconductor gas sensors because gas molecules directly interact with the sensing material It is necessary to build the sensing material in a way that it has a sizable exposed surface for interacting with gas molecules, adequate active sites for binding these molecules, and the capacity to efficiently translate

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these binding events into detectable signals For simple processing, these materials must also have specific mechanical qualities

Figure 1.2 Different material classes for gas sensing application [1]

Research into various gas sensing materials has been extensive and intense for decades From the very beginning with metal oxides to conducting polymers, to carbon nanotubes and continued with their composites and more recently 2D materials, as shown in Figure 1.2 Among them, metal oxides, commonly referred to as semiconducting oxides, continue to be the most widely used sensing material Numerous studies have employed and described various oxide materials The most important quality indicators of gas sensor performance are sensitivity, selectivity, response and recovery time, stability, and working temperature [2] These parameters of gas sensors based on SMOxs can be significantly improved by reducing the particle size to nanoscale, doping (or modification) of the sensing material, and enhancement of sensor design [3]

Sensitivity indicates a change in the physical and/or chemical properties of the sensitive

material in the presence of gas It is determined as the ratio of sensor’s resistance in the atmosphere of the target gas to its resistance in the air if the target gas is an oxidizing one

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ܵ ൌ ܴ௔௜௥െ  ܴ௚௔௦

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Selectivity: the capacity of the semiconductor layer to differentiate between a mix of

target gases or a single gas in the gas mixture is known as selectivity To increase the selectivity of gas sensors, surface modification or bulk doping with various catalytic additives is used for better adsorption of the target components [4],[5],[6] Previous literature demonstrate that sensing materials based on SnO2 and TiO2 nanostructures yield high selectivity sensors through either their surface modification with NM loading

or bulk doping with redox capable elements thus facilitating the selective gas detection

in mixed gas environments These studies indicate that the improvement of sensing performance in such cases is due to the creation of new active centers on the MOx surface or changing the electronic structure of material [3]

Stability or reproducibility is the ability of the gas sensors to provide repeatability of

measurement results for the prolonged usage The preheat treatment at temperatures above the sensor operating temperatures improves the stability of the sensitive layers

Response time determines the period during which the parameter value changes by a

certain percentage of its initial value at the certain gas concentration

After considering gas sensing properties of metal oxides, it is necessary to reveal the sensing mechanism of metal oxide gas sensor There is ongoing debate on the precise underlying processes that result in a gas response, but essentially trapping of electrons

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at adsorbed molecules and band bending induced by these charged molecules are responsible for a change in conductivity The negative charge trapped in these oxygen species causes an upward band bending and thus a reduced conductivity compared to the flat band situation [7]

Figure 1.3 Sensing mechanism of metal oxide based gas sensor [1]

As shown in Figure 1.3, when O2 molecules are adsorbed on the surface of metal oxides, they would extract electrons from the conduction band Ec and trap the electrons at the surface in the form of ions This will lead a band bending and an electron-depleted region The electron-depleted region is so called space-charge layer, of which thickness

is the length of band bending region Reaction of these oxygen species with reducing gases or a competitive adsorption and replacement of the adsorbed oxygen by other molecules decreases and can reverse the band bending, resulting in an increased conductivity The oxygen species, i.e O−, is the dominant at the operating temperature

of 300450°C which is the work temperature for most metal oxide gas sensors [5] Adsorption phenomena, more specifically, the thermodynamics of gas adsorption on the surface of semiconductor materials, which are connected to temperature, play a key role

in the detection of a particular gas The temperature at which the equilibrium between the adsorption and desorption rates is attained yields for a particular sensor the maximum response for a given gas In addition, the activation energy of the reaction

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taking place during the detection is reached at an optimum temperature, then the optimization of the sensor working temperature by the detection of a target gas is usually used to enhance the selectivity In general, the response and recovery times of a sensor depend also on the temperature because the kinetic reactions between semiconductors and gases are temperature dependent Then, at the low temperature, sensors have a long response and recovery times because the kinetic reaction rate is low Therefore, conductometric gas sensors based on semiconductor especially metal oxide materials are a paradigmatic example of the need for temperature control and the challenges related to achieving it in a power efficient manner [2]

Several methods have been tested to reduce operating temperature as well as power consumption of metal oxide-based gas sensor, such as: surface coating sensor metal oxide layer with catalyst [16] or use of micro-heater as a component of the sensor Sung Hoon Choa and his group has fabricated a micro-heater using a novel design of a poly-

Si in order to improve the uniformity of heat dissipation on the heating plate Temperature uniformity of the micro-heater is achieved by compensating for the variation in power consumption around the perimeter of the heater

Figure 1.4 Power consumption and temperature characterized of Hwang WJ’s micro-heater

[8]

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With the power compensated design, the uniform heating area is increased by 2.5 times and the average temperature goes up by 40 °C Power consumption and sensor temperature characteristic using fabricated micro heater is shown as Figure 1.4

Figure 1.5 Sang Chung Gwiy, Jae-Min Young group’s micro heater [9]

As shown in Figure 1.5, this micro heater consumes a power of 250 mW to heat up the system to temperature of 400oC Sang Chung Gwiy and his co-author reported a micro heater on polycrystalline 3C-SiC suspended membranes, The heater was designed for

an operating temperature up to about 800 °C and can be operated at about 500 °C with

a power of 312 mW The thermal coefficient of the resistance (TCR) of fabricated Pt RTD’s is 3174.64 ppm/°C In commercial field, as shown in Figure 1.6, a MEMS microheater with lower power consumption which is 120 mW at the temperature of

500oC

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Figure 1.6 KMHP 100 commercial micro heater

Although researches in micro heater of gas sensor have gained a lot of achievement, its fabrication processes are still remains complex and requiring expensive equipment

1.2 Self-heated gas sensor

1.2.1 Self-heating effect

It has been understood that the electrical energy lost by a resistive component causes the component's temperature to rise Even when just small amounts of electrical power are used, the self-heating effect is pushed to such a small scale that the temperature increase is remarkable A definition of the Efficient Self-Heating coefficient (ESH) has been proposed:

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ܲwhere ∆T is the temperature increase experimented by the device when it is subject to electrical power dissipation P Different materials and device configurations lead to a broad range of ESH values as shown in Table 1.1

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Table 1.1 Summary of publication reporting quantitative information about

self-heated devices based on nanomaterial.

Material Method Morphology Temp

( o C) S typ

ESH (K/μW) Ref

SnO2 Drop casting Multi NWs 300 95nm x 95nm 0.01 [4] CNF Drop casting Networked

Si/Pd Lithography Single wire 85 100nm x 50nm [16]

1.2.2 Research on nanowire based self-heated gas sensor

Due to their high surface-to-volume ratio, one-dimensional (1D) nanostructures, such as nanowires, nanorods, nanotubes, and nanofibers, have attracted considerable interest for

a wide range of applications, including catalysis, electronic devices, optoelectronic devices, storage devices, and gas sensors Nanowires (NWs) and NW-based heterostructures thanks to their peculiar properties such as high crystallinity, flexibility, conductivity, and optical activity are key components of future sensing devices When electrically driven, research using 1D nanowire has revealed an extra and unexpected benefit Even with the minimal electrical power expended during the electrical probing, they can nonetheless attain relatively high temperatures when put through electrical tests (such as electrical resistance measures The self-heating effect makes it possible to

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reduce the power consumption of nanoscale devices down to the microwatt regime Technically, the self-heating effect is just the consequence of the Joule dissipation of power at a very small scale Simple figures about the power dissipated per unit volume (i.e the power density) can help to dramatic decrease the power consumption when it comes to large device scale As a matter of fact, one nanowire in self-heating operation can easily reach larger power densities than a conventional external heater

A decade of research on self-heated devices has shown that nanowire-like structures have such a huge potential for efficient heating in miniaturized devices, which could be

of application in several fields of sensing and actuation Table 1.2 summarizes some of the works reporting self- heating effects in electronic devices The community mostly used two bottom-up approaches: either transferring a fully grown nanowire to a chip, placing it in a certain position, orienting it, and connecting it; or alternatively, attempting the direct growth of the nanowire in the right position in the final chip

Table 1.2 Publications reported self- heating effects in gas sensor using metal oxide

with gold coating for

20s reach the best

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However, it is evident that the self-heating effect in nanowire-based devices can damage

or destroy them, similarly to well-known equivalent effects in other microelectronic components (Figure 1.7 (a-b)) The self-heated sensors operated with a Joule-heating effect can lead to thermal destruction of the networked NWs upon supplying heating power Therefore, the heating power threshold should determine for long term stability

In any case, special care must be taken during device manipulation, connection, and start-up stages to avoid static discharge effects and power peaks

1.3 In 2 O 3 materials in gas sensor

1.3.1 In 2 O 3 materials

Indium (III) oxide is one of the important metal oxides in the TCO group (Transparent conducting oxides) including: Ga2O3 - In2O3 - SnO2, In2O3 nanomaterials is an essential and interesting nanomaterial for a number of applications, including solar cells, photocatalysts, organic light emitting diodes, architectural glasses, panel displays

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Number of studies on the synthesis of different structured In2O3 like nanotubes, nanowires [26], nanobelts, nanofibers, have been reported for wide applications In recent years, together with materials such as SnO2, ZnO, In2O3 material has received great attention in the manufacture of toxic gas sensors, biosensors[27], along with a number of projects Some recent studies related to In2O3 nanostructured materials increase rapidly [28] In this session, we are given some overviews of the properties of

In2O3 materials

Figure 1.8 : In 2 O 3 crystalline structure

In2O3 is wide band gap transparent n-type semiconductor (Eg a3.6 eV) The crystal structure and lattice parameters of In2O3 crystal structure were studied by X-ray diffraction method, the energy band structure of In2O3 was studied by X-ray emission and absorption spectroscopy In2O3 crystal has a body-centered cubic structure - BCC (body centered cubic), with lattice constant a = 10.118 Å, In2O3 unit cell has 80 atoms

in which 48 O anions are at the vertices, 8 In cations are at positions b-site, 24 cations

In at d-site, 16 O-anion vacancies (the b-site is the position where the two oxygen vacancies lie on the diagonal face of the lattice, and the d-site is the position where 2 oxygen vacancies lie on the block diagonal of the lattice cell) [29]

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1.3.2 In 2 O 3 nanowires in gas sensor

In2O3 NWs has been used for gas sensor fabrication towards various reducing and oxidizing gases such as ethanol [25][26], H2S [30], H2 [31], NO2[32] Well known to improve the response and recovery times of In2O3 nanostructures sensors; their nanostructures size must be reduced or doping with convenient metal nanoparticles [35]

In addition, it is possible to obtain a material with a high sensitivity to gas sensing by controlling the morphology and structure of the material These materials should exhibit

a large surface to volume ratios.There are many parameters of materials for gas sensor applications, for example, adsorption ability, catalytic activity, sensitivity, thermodynamic stability, etc Many different metal oxide materials appear favorable in some of these properties, but very few of them are suitable to all requirements For this situation, more recent works focus on composite materials, such as SnO2-ZnO [33],

Fe2O3-ZnO [34], ZnO-CuO [35] etc

There are several ternaries, quaternary, and complex metal oxides in addition to binary oxides that are of interest for the aforementioned purposes Much research has also been done on the interaction between metal oxides and other substances, such as organic and carbon nanotubes Here, we mostly use composite metal oxides as examples to illustrate how chemical composition may have an impact The composite ZnO-SnO2 sensors exhibited significantly higher response than sensors constructed solely from tin dioxide

or zinc oxide when tested under identical experimental conditions [36] Sensors based

on the two components mixed together are more sensitive than the individual components alone suggesting a synergistic effect between the two components Details about the synergistic effect is still unknown, but de Lacy Costello and co-workers [37] have suggested a possible mechanism Taking SnO2-ZnO binary oxides responding to butanol as an example, they hypothesize that butanol is more effectively dehydrogenated

to butanal by tin dioxide, but that tin dioxide is relatively ineffective in the catalytic breakdown of butanal On the other hand, zinc oxide catalysis the breakdown of butanal extremely effectively A combination of the two materials would effectively dehydrogenate butanol and then subsequently catalyze the breakdown of butanal The