Luận văn thạc sĩ synthesis an voc gas sensing characteristics of nanostructured cofe2o4

2i Synthesis of CoFe,O, nanostructured materials with different structural morphology by hydrothermal method .... The gas-sensitive properties of the imalerials were invesligated by me

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

MASTER THESIS

Synthesis and VOC gas sensing

characteristics of nanostructured CoFe,04

NGUYEN DUC IIOANG LONG

Long NDH202554M@sis hust.edu.vn Specialized: Electronic materials

Signature of Supervisor

Institute: —_ International Training Institute for Materials Science (1 ELMS)

HANOT, 05/2022

SOCIALIST REPUBLIC OF VIETNAM

Indcpendence - Freedom - Happiness

CONFIRMATION OF MASTER’S THESIS ADJUS’

Fullname of author: Nguyen Duc Hoang Long

Synthesis and VOC gas scnsing charactcristics of nanostructured Col'e;04 Major: Materials Science

Student LD: 20202554M

The author, the supervisor, and the Committee confirmed that the author has adjusted and implemented the thesis according to the report of the Committee

on May 19", 2022 with the following contents

‘The thesis has been corrected for typographical errors and printing according to

the opinions of the committee’s members

DECLARATION

I declare that the content of this thesis is my research work under the guidance of

Prof PhD Nguyen Duc Iloa The data and results in this thesis are completely

honest and have ot been published by other authors

Hanoi, 6th May, 2022

Professor PhD Nguyen Duc Hoa Nguyen Duc Hoang Long

ACKNOWLEDGEMENT

First of all, T would like to express my deep gralilude to Prof PhD

Nguyen Duc Hoa He has contributed valuable scientific ideas as well as create favorable conditions for me to do scientific research and complete this thesis

In addition to this, I would like to express my sincere thanks to the teachers, and staff at the Laboratory of Research, Development, and Application

of Nanosensors - Titernational Training Tustitule for Malerials Science (ITTMS)

for enthusiastically guided as well as supported me a lot during my study and

scientific research Studying and researching in a new environment is very

surprising, but thanks 1o the dedicated guidance of the leachers and brothers and sisters, | have leaned lots of salutary knowledge Furthermore, 1 am very grateful

to Dr Nguyen Ilong Ianh and PhD student Lai Van Duy for supporting and

sharing work oxpericnees

Moreover, I would like to thank the International Training Institute for

Materials Science, the Tnslitule of Engincering Physics, and the Training Department - Hanoi University of Science and Technology for creating

conditions for me to study and research

linally, 1 would like to thank my family, colleagues, and friends for their

support and encouragement during difficult times so that I could overcome and

complete this thesis

Hanoi, 6th May, 2022

STUDENT

NGUYEN DUC IIOANG LONG

2 Aims of the thesis

5 The practical meaning of the thesis - - cond

6, ‘The novel statements of the (HiesÏs

1.2.1 Crystal structure of CoFe:O, nanomaterials

1.2.2 CoFe:O, material in gas sensor application - 12

1⁄23 Gas-sensing mechanism of metal oxide semiconducting

The synfhesis process of CoFe,O„ nanostructure by hydrothermai method 2i

Synthesis of CoFe,O, nanostructured materials with different structural morphology by hydrothermal method a A The process of modifying Ag/Pt nanopartides on the surface of CoFe,O,

24

2S

CHAPTER 3 RESULTS AND DISCUSSION

23.3 The process of modifying Ag/Pt uaoparticles on the surface of CoFes0,

Methods of investigation and analysis of materials 229

3.3 Research on surface modificatlan nf CoFe:Q; nanorods by Ag/Pt nanapartirles ta

3.3.1 Morphology and structure of Ag/Pt nanopatticles - 50 33.2 Morphology and microstructure of CoF'eQ, nanomaterials before amd aller

surface decoration with Ag/Pt nanopatticles a 5H 33.3 Gas-sensing properties of Ag/Pt decoraled Colte;O, mamorodi, SỐ

LIST OF REEERANC!

iv

Joint Committee on Powder

Table 3.4 Symbols of selected samples for sensor fabricatlon 43:

vi

Figure 2.1 Diagram of synthesizing CoFe,O nanomaterials with different mnorphology

Figure 3.2 XRD patterns of CoFe:O¿ nanomaterials with different hydrothermal temperatutss: 120; 140; I80 anđ 220 °C in 61L - - - 3

vii

Figure 3.3 SEM images of synthesized CoFe204 nanorods by hydrothermal method with different hydrothermal times: (A, B) 6h; (C, D) 8 h, (E, F) 24h .39 Figure 3.4 (A) XRD patterns and (B) Raman spectra of CokezO¿ nanomaterials wilt đïfTorzmt hydrothermal times: 6h; 8 hand 24 hal 120 °C - - 4I

Eigure 3.8 (A) Acetone sensing transient resistance varsus time upon to different concentrations at various operating temperatures; (B) response value as a function of aoctone; (C) sensor responsc/recovery time of the CFO_120_24 sensor 47

Figure 3.9 (A) ‘Transient resistance, (3) sensor selectivity to 500 ppm various VOCs and (C) stability to 500 ppin avcton: measurad al 350 °C of the CFO_120_24 sonsor 48 Figure 3.10 (A) ‘Transient resistance and () response value versus time upon exposure

fo 100 ppm acctone measured al 380 °C in different values of humidily of the CFO 120 24 SENSO0 oe ccescseteinnneeiimiinettinetnntniiienuesesitenininne 2D Figure 3.11 (A, B) SEM images of Ag/PL nanoparticles - 50 Figure 3.12 (A) EDX spectra and (B) UV-Vis spectrum of Ag/Pt nanoparticles sl Figure 3.13 SEM images of Ag/Pt0-CFO sample (A, B) and (C, D) Ag/Ptl-CFO

Figure 3.14 (A) SEM image and (B-F) EDS mapping of the Ag/Pt1-CFO samplc S3 Figure 3.15 XRD patterns of the Ag/Pi0-CTO and Ag/PU-CFO- 54 Figure 3.16 (A) EDX analysis and (B) Raman spectrum of Ag/PH-CFO .55

Figure 3.17 (A) The 1-V emve measured al different temperatures in air and (B) Arrhenius fit of the In(I) varsus 1/T of hs Ag/PLI-CEO sensor coc 57 Figure 3.18 (A) Transient resistance, (B) response value versus fime to 500 ppm acetone measured at 350 °C and (C) response to 500 ppm acetone of different sensors at

Figure 3.19 (A) Acetone sensing transient resistance versus time upon to different concentrations at various working temperatures; (3) sensor response as a function of

acctone, (C) response/recovery time of Ag/Pt] CFO sensor - él

viii

Figure 3.20 (A) Stability toward S00 ppm acetone and (B) selectivity to 500 ppm đifferent VOCSs offilc Ag/PI1-CFO measurcd at 350 °C eo Ờ

INTRODUCTION

1 Foundation of the thesis

In recent years, science and technology are developing prosperously in the era of Industry 4.0 along with plobalization taking place strangly, it means that people's lives are much improved, especially in the world in terms of health care and monitoring, In addition, the Internet of Things (IoT), is considered the next

generation of Internet-Connected Devices and plays an important role in

transforming [he "traditional desktop computer" to the "pervasive computer" [} | For instance, the adoption of IoT and the development of wireless technology

tnakes 1 possible 10 transmit the palient's health slatus directly 10 the caregiver im

real-time |2] Furthcrmore, sensors that can be directly connected to a mobile

device through an inbuilt application can measure human physiological parameters such as heart rate, breathing rate, or blood pressure Ihrough a onc- touch operation [3]

On the ofher hand, along with the development of society, it is

environmental pollution, climate change, extreme weather, food safety and hygiene, etc due to the activities of machines and people, it has become an urgent global problem that dircotly threatens human Hf According lo the Workd

Health Organization (WHO), the combined effects of indoor and outdoor air

pollution cause about 7 million deaths each year, and this number is projected to

double by 2050 [4] Therefore, the re:

earch, development and applieatien of sensors, especially gas sensors, becomes more and more important at this time Gas sensors have Jong been used in many fields, from agriouliure to cnyironmental monitoring, Among them, medical applications, a market that is

predicted to grow globally to more than 15 billion USD by 2022 shows great promise in taking advantage of the benefits of gas sensors [5] Compared with current diagnostic methods such as slow and invasive blood tests, gas sensors designed for breath analysis are a promising alternative with real-time as well as rapid and accurate diagnosis corpse Indeed, to be able to diagnose disease through breath analysis, the gas sensor must be able to detect volatile organic compounds (VOCs) at low concentrations in a mixed gas environment

Up to now, there has been a lot of research to develop gas sensors based

on metal oxide nanomaterials which are attracting a lot of attention in the world

Among them, metal oxide semiconductor (MOS) nanostructures have been

studied to improve VOCs sensitivity because of their outstanding advantages such as low-cost and simple to produce, small size, low power consumption and

good compatibilily with silicon chips [6], but also allow for a wide selection

possible nanomaterials with different nanostructured dimensions These

structured range from zero-dimensional (0D) nanoparticles [7], one-dimensional

(ID) wanorods [8] and nanowires [9], two-dimensional (2D) narosheets [10],

nanoplates [11] and nanotubes [12], there-dimensional (3D) polycrystals and

ultraporous [13] and even heterostructures [14]

So far, gas sensors based on resistance changes can be a new method for

environmental monitoring and disease diagnosis because of its simplicity, high

minialurvalion potential, law power and low-cost production [15][16], which is

making them more promising when compared to other methods such as GU and

MS There have been many previous studies on gas sensars based on

semiconductor metal oxides such as SnQ2, ZnO, WO3, TiQs, CuO, Co,O,, NiO,

and its structural and moiphological characteristics, size has a great influence on gas-sensing properties [17][18][19] However, sensors based on these materials that require high operating temperatures, and therefore power consumption, and poor long-term stability, present significant challenges [20], hindering the actual application [21] Efforts to improve the above disadvantages are proposed such

as adjusting, controlling the desired morphology, or, above all, developing new materials with unique properties

In the last few decades, the research and application of ternary semiconducting oxide materials with the desired ability to optimize the physical and chernical properties of gas sensors [22] Fspecially spinel {erriles wilh the formula ABO, are oxides that are said to hold great promise for gas sensing applications Up to now, spinel structures have been widely used in gas storage

or soparation [23]|24], batlery vatalysis [25][26] and high (requency applications

as micro-electronic/magnetic devices, but litle is known in the field of gas sensors Besides, there are also studies showing that semiconductor magnetic

oxides are very sensitive to reducing or oxidizing gases because of magneti parameters such as M, (saturation magnetism), M, (remanence magnetization), and H, (coercivity) [27][28][29] A research points out that gas sensors based on

p-type MOS receive relatively little attention compared to n-type MOS [30][19]

Although the response of the p-type MOS is only the square root of the n-type

when comparing the same inleracion with the same gas and the same

morphology [31] The p-type MOS has its advantages, such as being less affected

by environmental humidity [32][33] or high catalytic properties [34], etc Among

them, CoFe:O, is plype semiconducting: teary oxide [35], which has many outstanding properties such as excellent stability in a thermal and chemical

environment, fast response and recovery time, low cost, and simple electronic

slructure [36] Tn addition, materials with a porous sinucture are very suitable for

gas sensors because the adjustable capillary pores allow gas molecules to easily

diffuse and adsorb on the surface of the material, resulting in fast response and

recovery times as well as high sensitivity, and thus ouhance the performance of

the sensor [37][38] Col’e,0, has been studied for application in several different toxic gases such as NH; [39], NO, [40] and VOCs [41][42] In addition, surface modification of CoFe,O, materials with rare earth elements [41] or catalytic

noble metals such as Ag, Au, Pt, Pd, ete can increase gas sensitivity,

selectivity, decrease response and recovery time or decrease working temperature

[11]/43] As mentioned above, there is few research in Ihe world devoted to

nanostructured Col'e;0, materials in the field of gas sensing Therefore, this

thesis focuses on the topic: "Svathesix and VOC gas sensing characteristics of

nanostructured CaFe,0,;”

2, Aims of the thesis

- Suovessfully synthesis of CoKe,O, and Ag/Pt modified Cole,0y

nanostructures for gas sensing applications

- Understanding the gas and electrical sensitivity properties of the sonsors based on the synthesized nanomaterials.

3, Research object and research scope

- Survey and fabricate Col’e,0, nanostructures by hydrothermal method, where the porous structure of materials is desired

- Research on the fabrication of Ag/Pt nanoparticles by chemical reduction and modification of Ag/Pt nanoparticles on the surface of CoFe:O,

nanorads

- Survey and evaluate the VOCs sens

and CoFe,0, and Ag/Pt moditied CoFe,0, materials

€ properties of sensors based on

4 Research method

The thesis is carried oul based on experimental research results and literature studies from intemational publications Spocitically, the material is synthesized by hydrothermal method The morphological and structural properlics of the materials were analyved by scanning cleclrou microscope (SEM), X-ray diffraction pattern (XRD), Raman scattering, Hnergy-disparsive X- ray (EDX), and surface area Measurement (BET) The gas-sensitive properties of

the imalerials were invesligated by measurement techniques on the gas sensing

characteristics of the Sensing Group (iSensor.vn) at the IIMS, HUST

5, ‘The practical meaning of the thesis

The thosis has proposed the basic process of fabrication CoFe:O, materials by hydrothermal method under different reaction conditions for application in gas sensors All research results are carried oul under tschnological conditions in Vietnam Besides, the thesis also contributes important studies on gas-sensitive properties of CoFe,0, and Ag/Pt modified CoFe,0, materials The

fabricated sonsors have a good response to VOC

specially acetone

6 The novel statements of the thesis

The author has synthesized CoFe,Q, wanomaterials with different

morphology by simple hydrothermal method Successfully researched and modified Ag/Pt nanoparticles on the surface of Cole), materials by drop- casting method, and investigated the VOCs-sensilive properties of sensors based

on Cole,0, nanomaterials with different morphology and Ag/Pt-CKO.

7 The structure of the thesis

The thesis has the following structure:

Chapter 1: Overview

In this chapter, the author introduces an overview of volatile organic compounds (VOCs), their toxicity affecting air quality and human health

Structures of metal oxide materials semiconductor CoFe;0, and their application

in gas sensing field are introduced Methods of synthesis and modification of

Cole, materials, gas-sensitive mechanism of the material are also mentioned

Chapter 2: Experimental

This chapter presents technology to fabricale CoFe,Q4 nanomaterials with

diffcrent morphology by hydrothermal method At the same time, the author also presents a method to modify the surface of CoFe,O, materials by Ag/Pt

wanoparticles using a dircel drop-casting method Lo improve the performance of

gas sensors Introduction of morphological, structural, gas-sensitive survey

methods of Colfe,O, and Ag/Pt modified CoFe;0, nanomaterials used in the

thesis

Chapter 3: Results and discussion

In this chapter, the author presents the influence of hydrothermal conditions on the morphology, structure, and gas-sensitive properties of Col’e,04

materials, which have been discussed and reported im detail, From there, the

appropriate Col’e,Q, structure was found to modify Ag/Pt nanoparticles to

improve the sensing performance The thesis also compares the parameters from

the cxperimental results investigating the acctone gas sensilivily of the gas seusor

based on the unmodified CoFe,0, with the Col'e,0, material after surface was

decorated with Ag/Pt nanoparticles for acetone gas, also explaining the role of

modulation to sensor parameters

Conclusions and recommendations

Tn the conclusion of the thesis, Ihe author draws oul the outstanding and

focused results of scientific significance that the thesis has made At the same time, if also mentions the limitations of the thesis that need to be further

researched.

CHAPTER 1 OVERVIEW

In this chapter, the author introduces some general background knowledge about volatile organic compounds, CoFe,0, malcrial structures Tn addition to this, methods for fabrication, application in gas sensor field and gas sensing

mechanism of CoFe,O4 materials were also presented,

1.1 Volatile organic compounds (VOCs)

Nowadays, haze covers all densely populated urban areas as well as indoor smoke and is a major threat to both the climate and human health Taxic gases such sẽ SỐ,, NO, CO), NH, cle in the environment cause many risks

to the ecosystem as well as to public health due mainly to human activities Among g

causing air pollution, the emission of volatile organic compounds

(VOCs) is one of the biggest onvironmental problems today [44] There are several different definitions of VOCs given by mainstream intemational orgattivalions According lo US Environmental Prolection Ageney (US EPA), it can be defined as any compound of carbon [45] The World Health Organization (WIIG) considers VOCs to be organic compounds with a saturated vapor pressure above 133 Pa and a boiling point between 50 and 250 °C al almospheric pressure [46] VOCs are emitted from everyday human activities such as driving, cooking, painting houses, or simply breathing in, to industrial activities in

developed and developing countries [45] When VOCs are released into the

environment and exposed to sunlight, they react with other gases to form terrestrial smog compounds, causing plant disease and inhibiting plant

production |47[ Research indicates thal concentrations of VOCs are much tngher

indoors than outdoors and it is estimated that up to 50-300 different VOCs can be

detected at any given time indoors [48] Long-term human exposure to VOCs can

cause scrinus health problems such as chronic obstructive pulmonary disease, lung cancer, and strokes [49] In addition, some VOCs are believed to be highly

toxic or careinogenic, with short and Jong-lerm health effects on human health

and ocosystems For instance, Benzene is a highly carcinogenic substance to humans (eg, liver, kidney, spleen, and stomach) and the central nervous system

(og, Uke nervous, circulatory, reproductive, immune, cardiovascular, and

respiratory systems) [45] Moreover, an ongoing problem today is sick building syndrome (SBS), which is a phenomenon in which people living in buildings or

high-rise buildings experience health problems such as headaches, nausea,

imitation eyes, coughs, ete., due to spending a lot of time inside the building [50]

Therefore, it is necessary to focus on monitoring indoor air quality

In addition to this, several VOCs present in human breath is involved in

metabolic processes and can be used as an effective non-invasive tool for health

monitoring and diagnosis [20] Most breath consists of nitrogen, oxygen, CO2,

water, inert gases, and volatile organic compounds (VOCs) in concentrations

from one part-per-billion (ppt) to one part-per-million (ppm) [51] Concentrations of endogenous substances including inorganic gases and VOCs

can be altered in the breath of patients with specific diseases [21], However, most

VOCs are not produced in the body (endogenous), but originate from the

ingestion of food, exposure to pollutants (exogenous) or drug metabolism [52] In

addition, VOCs like these are also known as disease-specific breath markers

[53][54], as shown in Figure 1.1 For example, when exhaling acetone, ammonia

or carbon monoxide above the threshold of a healthy person, it corresponds to

diabetes, kidney disease, or lung inflammation respectively [55][S6][57] Thus,

VOC gas sensors are very important in practical applications

== Healthy szzza Unhealthy

Figure 1.1 Select breath markers with their average concentrations for

healthy and unhealthy humans [53]

8

1⁄2 Overview of CoFe;O, nanomaterials

1.2.1 Crystal structure of CoFe,O, nanomaterials

Cobalt ferrite (CoFe,04) has a spinel structure, belongs to the AB,O,

material group with space group Fd3m In general, cobalt ferrite has a structure

in which both the tetrahedral and octahedral sites (the A and B sites respectively)

contain a small fraction of the Co*' and Fe’' cations, however, it is generally

accepted that the majority of the Co* ion is located at the B site and the rest is at

the A site, and that gives the material an inverse spinel structure [58] Besides, in

the cubic siructure, many Co ions will be in the (etrahedral interstitial sites

while some Co” ions still exist in the octahedral interstitial and the remaining

interstitial sites will be of Fe™* ions to create a mixed spinel structure [59][60]

The structure of the material can be represented by the formula [61]

whore x is (he [raction of the tcirahedral siles occupied by Fe" ions, called the degree of inversion ‘This degree depends on various factors such as fabrication method, heat treatment conditions, etc {62] Figure 1.2 shows that the basic structure of spinel CoFc,0, consists of 56 ions or cight formula units in a unit cell, Larger oxygen ions are arranged in a dense structure like the Face Center Cubic (FCC) and smaller ions occupy the space between these oxygen ions

‘These spaces come in two forms, the first being the tetrahedral (A-site) site due

to its position at the center of the tetrahedron, and the angular position occupied

by oxygen ions The remaining space is the octahedral site (B-site) due to the

» oxygen surrounding the Co”* ions

Figure 1.2 Polyhedral model and tetrahedral, and octahedral sites are presented

within the CoFe,0, crystal structure [63]

But the inverse spinel structure of the material is more ideal with all the A

tetrahedral sites occupied by the Fe*’ cation and the B octahedral sites randomly

occupied by the Co” and Fe* cations in equal proportions [64], and crystal size

a = 8.38 A, which is governed by the generalized formula ((Fe’*)[Co”’Fe™"]O,) (Figure 1.3) [65] The octahedral site preference energy of Co?” is greater than

that of Fe!” in CoFe,O, material, Co” preferentially enters the oxygen octahedron and forms the inverse spinel structure [66] In addition, the spinel

structure of cobalt ferrite allows the introduction of different metal ions into its

crystal lattice, thereby altering the structural, magnetic, electrical, and catalytic

properties [62]

10

showed that under oxygen-rich conditions, the probability of formation of cation

vacancies in the octahedral sublattice increased significantly, which suggested that the electronic conductivity was clearly increased [68] The energy formation

of cation vacancies is quite low under oxygen-rich (metal-poor) conditions,

especially with octahedral cations, where the p-type semiconductor properties of CoFe,0, will be shown According to the study by Azouaoui et al [69], the band

structures and density of state (DOS) of CoFe204, are deduced from the region structure calculation (Figure 1.4) Calculations show that the ground state of the

CoFe,0, material has semiconductor properties with a band gap Eg = 0.84 eV, the maximum value of the valence band and the conduction band minimum are

not on the same high symmetry line Another study shows that in the case of the

ideal inverse spinel, Co is in the 2+ state and Fe is in the 3+ state at both the

tetrahedral and octahedral positions Since spin in tetrahedral sites is always antiparallel to spin in octahedral sites, the magnetic moment of Fe3+ is removed,

leaving the only net contribution from Co** The calculation show that cobalt

ferrite becomes insulating with a band gap Eg = 0.72 eV [66]

11

Energy

(eV)

Energy (eV)

Density of states (states/eV)

Figure 1.4 The total density of state (left) and band structure (right) of

CoFe;O¿ [69]

1.2.2 CoFe,0, material in gas sensor application

Up to now, nanostructured CoFe,O, materials have been studied by many

research groups for application in various fields related to magnetic properties

[70], magneto-optic [71], electrical [72], chemical [73], electrochemical [74],

thermal [75], adsorption [76], photoelectro-chemical [77] and thermo-acoustic

[78] In recent years, CoFe,O4 has received more attention in the field of gas

sensing, however, the understanding of the influence of morphology and

structure of CoFe,0, materials on gas-sensitizing properties has not been studied

systematically There have been a few studies in the world to fabricate CoFe,O, materials with different morphologies such as nanoparticles [79], nanowires [80], nanocubes [42] and microspheres [39], tested with gases such as NO; [81], NH;

[82] or VOCs [41][83] For instance, Wang et al used hierarchical CoFe,O,

double-shelled hollow spheres (DSHs) prepared via a self-templating for NH; gas

sensor application [39], where the CoFe,0, based sensor exhibits NHj sensing properties, and the response value (Reay/Ryis) to 100 ppm NH at 240 °C was

0.679, high almost as twice as LPG (0.384) and much more than other gases like

HS, SO2, NO>, CO, and H) (Figure 1.5)

12

Figure 1.5 FESEM and TEM images of the precursors of (A, B) CoFe;O¿ SHSs and (C, D) CoFe,O4 DHSs, respectively; (E) sensor response based on

CoFe,0,4 SHSs and DHSs to 100 ppm various testing gases at 240 °C [39]

A study reported by Xiangfeng et al showed that CoFe,0, nano- crystallines synthesized by hydrothermal method, exhibit both n-type and p-type

semiconductor properties at different sensor operating temperatures [84] This

CoFe,0, nanostructure was used for the ethanol gas sensor, and they found that

the sensor shows a high response (Ray/Reas) was 4 with 10 ppm ethanol at

working temperature of 150 °C and 2 with 10 ppm triethylamine at 190 °C, thus the sensor had good selectivity Prasad et al [85] synthesized CoFe,0, fibers by

electrospinning technique, the result shows that the sensor-based CoFe,0, fibers

exhibited a maximum response of 0.42 at 900 ppm NH; at room temperature, and

better than ethanol, acetone (0.1 and 0.11), as shown in Figure 1.6

13

Figure 1.6 (A, B) TEM images and (C) sensor response of as-spun precursor

nanofibers calcined at 800 °C towards different gas concentrations [85]

Rathore et al [83] reported on the gas sensing properties of CoFe;O,

nanoparticles toward liquefied petroleum gas, ethanol and ammonia It was found

that the maximum response increase when the grain size is reduced from 28.6 to

5.8 mm, and the sensitivity (%) [(Reas/Raiz) X 100] for LPG, ethanol and ammonia

at 250 °C was 72 %, 44 %, and 40 %, respectively Show that the morphology,

structure, temperature and type of tested gas, etc plays an important role in

affecting the gas sensitivity of the sensor In addition, there is research to

improve the gas sensitivity of sensors based on CoFe,O, material by doping rare

earth elements Khandekar et al fabricated CoFe),Ce,O4 nanocrystals (x = 0,

0.04, 0.08) via a molten-salt method, where the gas response of ferrite materials

was evaluated towards the reducing gases such as LPG, ethanol, acetone and ammonia [41] It was found that after Ce incorporation, the response of Ced

ferrite sensor has been improved to 177 % at 225 °C towards 2000 ppm acetone,

which is higher than the pure sample (146% at 350 °C) Furthermore, the Ce4

sensor exhibits good selectivity with 100 ppm of LPG, acetone, ethanol and

ammonia were 119 %, 138 %, 122 %, and 117 % at 200 °C, respectively

Through the overview of reported research related to CoFe,O, materials

applied to gas sensors in Table 1.1, we can draw the conclusion that the

properties of the materials such as morphology, structure, material size, operating

14

temperature or type of gas under investigation, etc have a significant effect on

gas-sensitivity characteristics, In addition, modification also contributes to the creation of superior sensors However, very few studies reported on the use of noble metal alloy for enhancement of gas sensing properties of Cole,0,

materials

Table 1.1 Comparalion of various CoFe,0, structure-based sensors to different

gas

No Materials Morphology Gus Cunc Gas Temp — ReF

(ppm) Response ỨC)

4 CoFe:O, Nanoparticles Lthanol 200 0.39 250 183]

CoFe:0,

powder

8 CoFeiO, Nanocubes — tormldehvde 10 3.718 l61 1

9 Cosy! Nanocnhes —_ Formaldehyde 1 127 275 [42]

advanced nanomaterials for gas sensors Due to the fact that this field is one of

the hottest research topics, that attracts the interest of many researchers For

insince, a research (cam headed by prof Nguyen Due Hoa (Hanon University of Science and ‘lechnology) has successfully fabricated spine! structural materials such as ZnFe;0, and ZnSnO, for application in gas sensors [86][87][88] In

15

addition, there are research groups of prof Nguyen Due Chien, and associate prof Dang Duc Vuong is targeting the synthesis of dioxides, such as e203, NiO, WOs, ete for toxic gas sensor application Prof Nguyen Van Hieu (Phenikaa University) is focusing on the toxic gas sensor, where he used one-dinensional metal oxides as sensitive materials for room temperature as a gas sensor Dr Io Truong Giang at VAST developed an electrochemical sensor using perovskile as sensitive matcrials for toxic gas detection [89] However, very few studies have researched and fabricated CoFe,(, nanomaterials for application in gas sensors field Tn addition, eflective decoration the surface of CoFc,0, nanomaterials [or enhancement the VOC sensing characteristics is still challenging

1.2.3 Gas-sensing mechanism of metal oxide semiconducting

Gas sensors based on semiconductor tactal oxide materials operate on the

basis of a change in electrical properties that causes the adsorption of oxygen on

the surface and the release of gas molecules on the surface of the gas-sensitive

maternal, And the semiconductor metal oxide malenal, placed in the air, will

cause electron withdrawal from the top of the valence band for p-type

semiconductors and the conduction band for n-type semiconduetors

In the framework of this thesis, we only show the gas-sensitive

mechanism of p-type semiconductor metal oxide (CoFe,(,), in the crystal there

will oflen be an excess of oxygen or in other words a lack ef metal ions, causing the appearance of carriers as holes In the p-type semiconductor material

CoFe,O,, their electrical properties are governed by the mobility of the base

caxricr (posilive holes) in ils matrix The receiving impurity level is localed ucar the valence band at room temperature and is filled, leaving errors on the valence band, oxygen will be physicz

Hy adsorbed from the air al the surface by trapping electrons at the surface of the object, material, forming O* ions 190) Thus, a chemical bond is formed between the semiconductor metal oxide and the adsorbed oxygen The existence of these oxygen ions is strongly dependent on the operating temperature Specifically, at low temperature (< 150 °C), oxygen will be adsorbed on the surface of the semiconductor metal oxide material in the form of Oz’ ion molecules, but will disappear rapidly at high temperature than In the temperature range from 150 °C to 400 °C, the oxygen adsorbed on the surface

16

of the material can be in the form of O° or O* ions, and 0" is the most commonly

adsorbed oxygen ion When the temperature is raised further, part of the adsorbed oxygen diffuses into the lattice or in other words is absorbed [91] The

chemical reaction of development of these oxygen ions after taking, electrons are

The capture of cleetrons from the surface of ihe maternal causes bending

of the energy band and an increase in the hole concentration near the interface

Thus, forming a hole accumulation layer on the surface of the metal oxide leads

to a decrease in the resistance of the p-type metal oxide When the metal oxide is exposed to any reducing gas, the molecules are physically adsorbed on the surface of the active layer and will react with the adsorbed oxygen to release

electrons When electrons recombme wilt boles, the width of the accumulation

layer is reduced Thus the resistance of p-type metal oxide increases During the

desorption of gas from the metal oxide surface, holes in the accumulation layer

continue Lo be restored, which reduces the resistance of the sensor |92]|4S|

For p-type semiconductor metal axide materials, the conduction can be explained by the competition between parallel paths through the wide resistive core and along narrow regions The detailed conductivity model and energy band

plot of the p-type semiconductor gas sensor are explained by the report of Barsan

et al, [31], as presented in Figure 1.7 ‘'he author has described that electrons are

introduced into the material through an oxidation reaction between the reducing

gas and the oxygen anions on the surface of the oxide semiconductor which

reduces the concentration of holes in the shell, thereby inercasing the resistance

of the sensor

17

Gas (CO)

Figure 1.7 Simplified gas sensing mechanism of p-type oxide semiconductor

[30]

More specifically, the gas sensitivity mechanism of p-type semiconductor

metal oxide materials in general and CoFe,O, nanomaterials in particular, has

been reported in a number of recent studies For the acetone gas sensor based on

the p-type semiconductor metal oxide materral, when the gas is injected into the measuring chamber, it reacts with the adsorbed oxygen ions and the electrons are

released as follows [93]

CH,COCH, (ads) + 80° + 3CO, + 3H,0 + 8e”

The released electrons are introduced into the sensing layer and the empty

layer accumulation of the p-type sensor is reduced, As a result, the resistance of

the p-type gas sensor increases when exposed to acetone gas (Figure 1.8)

Hole accumulation layer ©

‘ys

Figure 1.8 Schematic illustration of acetone sensing mechanism in p-type

High resistance

Low resistance in air

semiconductor metal oxides [94]

In the work of Wang et al [39], the NH; sensitivity mechanism of the

nanostructured CoFe)O, materials was explained through the adsorption and

desorption of the analyte gas molecules, as displayed in Figure 1.9 When the

CoFe,0, material is exposed to a reducing gas such as NH, the gas molecules

18

can react with oxygen ions adsorbed on the surface of CoEezO¿ to release these electrons back to the conduction band, This process reduces the potential barrier

height and hole thickness through electron and hole recombination,

(A)

Figure 1.9 (A) Schematic diagram and (B) energy-band diagram of the sensing

mechanism of CoFe,O,4 exposed to air and ammonia gas [39]

Besides, the structural characteristics of the material are also an important factor affecting the improvement of the sensor sensitivity For CoFe,O,

nanostructured materials, Co” ions are usually located in octahedral positions

while Fe** ions prefer both octahedral and tetrahedral positions The higher the

concentration of Fe*’, the higher the conductivity, after absorbing oxygen on the

surface of the material, Fe” will be oxidized to Fe’ However, the Fe”

concentration plays an important role in the gas sensitization properties Sutka et

al reported that the gas response of the sensor decreased due to excessive Fe”*

concentration [95] The oxygen gap is considered to be a catalyst of many adsorbents, so many surface reactions are affected by this type of defect Any

hypoxia site can donate two electrons, and the oxygen molecules on the surface

of the sensing material pick up electrons to become oxygen ions [96][97]

As mentioned before, sensitivity is a parameter that reflects the change in resistance in a given gas concentration of the target gas Therefore, the

enhancement of gas sensitivity is very necessary for gas sensors to apply in

practice, Specifically, for sensors based on semiconductor metal oxide materials,

19

microstructure-affected gas sensitivity performance, such as reducing the particle size to the nanoscale is one of the most effective strategies [98] It is recognized

that the gas-sensitive response of the sensor occurs at active sites on the

semiconductor metal oxide surface ‘Iherefore, the mumber of activated

adsorption sites has an effect on improving gas sensing performance, namely

increasing the specific surface area of the material, which will greatly affect the

adsorption capacity of the sonsor [99] Furthermore, the sensing materials of gas

sensors are classified into porous and dense nanostructures In the case of dense

remostrustures, gas diffusion can only occur on the surlaces of sensing materials

due to the fact that gas molecules cannot penetrate into materials But in the case

of porous nanostructures, the gas molecules can interact with the inner grains

cause the structures are comducive to the penetration of gas molecutes into (he

sensing matenals

The calalytic influence on semiconductor metal oxides results in (he acceleration of the interaction between adsorbed oxygen species and target gases

Generally, the noble metal is introduced into semiconductor metal oxides as a

sensilizer or activator [100] This is because the noble melal can affect the mer

granular contact region and thus change the resistance of the gas sensor in two

ways, namely chemical sensitization ard electronic sensitization,

1.3 Synthesis and modification methods of CoFe;O, nanomaterials

1.3.1 Lydrothermal method

CoFe,0, nanostructured materials can be synthesized by many methods

such as eleclrospirming, chemical vapor deposition (CVD), thermal

decomposition, sol-gel, hydrothermal, co-precipitation, ote Each method has its own advantages and disadvamages, and depencing on the specific purpose, which method will he used For example, physical methods can synthesize materials with high purity and accuracy, but require cumbersome, modern equipment, are expensive, and are not suitable for the production of disposable

sensors Therefore, in order to solve the above problems, the author has focused

on researching and manufacturing Coe, materials by chemical method, which

is the most suitable, especially hydrothermal method With outstanding

advantages such as: easy to control precursor ingredients, igh crystallinily

20

products, similar size and morphology, easy to repeat, many products can be obtained in one manufacturing time, low cost, and environmentally friendly In

addition, this method can synthesize many different structures, shapes and sizes

At the same time, research on surface modification of Coe,O, nanostructures by

Ag/Pt metal nanoparticles to improve sensor performance such as sensitivity,

response, selectivily Lowards applications in many other Cields

Indeed, the hydrothermal method is a simple, high-performance method

that does not require high technical requirements as well as expensive equipment

‘The hydrothermal method uses any heterogeneous chemical reaction in the presence of a stttable solvent, at room temperature and a pressure above 1 atm in

a closed system [101] Heats the chemical solution in an autoclave, thereby increasing the pressure inside the vessel above atmospheric when the temperature

exceeds the boiling point of the solvent This speeds up the dissolution and

reacliot of the precursors duriyg the fabrication of the material Decomposilion and recrystallization are most considered during the formation and growth of the

mucleation of the material To guide the event of a one-way structure by

hydrothermal method, il is common lo add appropriale organic substances or surfactants to support the orientation for the process of abnormal crystal growth

Tn many fabricalion processes, CoFe,Q4 nanostructured malerials have been

studied, starting with the formation of Co{OHh and Fe(OH) intermediate

precipitates or other intermediate phases from metal junctions Co, I'e precursors

and surfactants in an alkaline (or acidic) environment Then, the solution is kept

in a closed system al high lemperature for a cerlain tame, Fe(OH),

Ca(OH} or inlermediale phases ofher will react with cach other to form the final product CoKe,(), [102] ‘thus, it is possible to control the synthesis of Col’e,04 nanostructured materials with different morphology by using surlactants, changing the cnvironmental pH There have boon memy studies ot comtolling and improving the size and morphology of CoFe,0, nanomaterials by

precursor salt type, reaction temperature, reaction time, plI and surfactant,

leading to change the properties of materials [102][1 03][84]

Some reports on the fabrication of Col’e,0, nanostructured materials by

hydrothermal method using dilferenl precursors such as M Penchal Reddy of, al

21

successfully synthesized mesoporous Cole; nanospheres with an average of

180 nm by hydrothermal reaction using sodium acetate trihydrate (NaAc) as an electrostatic stabilizer, ethylene glycol as solvent and polyethylene glycol (PEG- 4000) [104] Chandekar et al fabricated Col'e,0, nanoplatelets with the rhomboidal shape of average particle size of 14 to 17 nm by hydrothermal

method with the assistance of surfactanis cetyl trimethyl ammomum bromide

(CTAB) and sodium dodecyl sulfate (SDS) [105] Jalalian ct al synthesized CoFe,(, nanoparticles by hydrothermal process with polyvinyl alcohol (PVA) as surfactant [106]

13.2 Modification method of CoFe,O, material

As is known, sensors based on semiconductor metal oxide often have poor

seloclivity, low response and high operating lomperature [107] Therefors, one of

the solutions to solve this problem has been the use of catalysts or doping

precious metals (Pt, Au, Ag, Pd, Ag-Pt, .) or other metals transition (Ni Mn,

Cu, Fe, ) (108][109] The most important purpose of surface madilication is La

increase the response, increase the sensitivity, reduce the response time as well as

reduce the working temperature of the sensor Indeed, to improve their gas sonsing propertics including selectivily, scusilivity, response (ime and operaling

temperature, metal oxides can be modified on the surface or in the orystal lattice with different materials Metal oxides doped with catalytic metals or precious metal nanoparticles on the surface are commonly used For instance, Devi ct al

reported that with 3 % Mn atoms doped into the structure Col’e,0, nanoparticles

by chemical precipitation method The results showed that the sensor based on

Mn-doped (3 at %) had enhanced sensitivity compared with the pure sample when tested with ethanol gas [110] Khandekar et al successfully synthesized

nanocrystalline CoFe; Ce,0,4 ferrites (x=0, 0.04,0.08) by molten-salt method

[41] The gas response and selectivity strongly depend on the doping content

The Ce0 sample demonstrated the selective response of 146 % acetone at 2000

ppm al the operating lemperalure of 350 °C However, after Ce incorporation, the

sensitivity has been improved to 177 % for Cod sample at 225 °C, while gas response decreased to 157 % at 200 °C for Ce& sample (Figure 1.10) In addition,

compared with (he single denaturation of the malerial, several studios have

22

shown that different metal-doped types significantly improve the gas-

Figure 1.10 TEM images of the (A) undoped Ce0 and (B) Ce4 (4 wt% Ce)

ferrite samples; (C) gas responses of Ce0, Ced and Ce8 ferrite sensors towards

100 ppm concentration of test gases at 200 °C [41]

In chapter 1, the author reviewed the volatile organic compounds VOCs

and the relationship between breath VOCs concentrations and common diseases

in humans Overview of CoFe,O, nanomaterials and their applications in various

fields, especially in gas-sensing application The author also listed several typical

works on the research and fabrication of CoFe,O, nanomaterials applied in gas

sensors However, the above studies still have some unresolved problems

regarding the applicability of CoFe,O, materials to make gas sensors such as:

> The influence of structure, morphology, crystal size, which has not been

systematically studied on CoFe,O4 nanomaterials to the gas-sensitive properties of the sensor

> Improving the gas-sensitive properties of CoFe,O, nanomaterials by surface modification with catalytic metal nanoparticles is still challenging

23

CHAPTER 2 EXPERIMENTAI,

In this chapter, the author presents in detail the processes of synthesizing CoFe,0, nanomaterials by hydrothormal method Next, the author also introduces

the process of making Ag/Pt nanoparticles by chemical method and the process

of madifying Ag/PL nanoparticles on the surface of CoFe,Q,4 materials by direct

small mcthod to apply to gas sensors The structure, manufacturing process of the sensor, and the structure, principle of the gas measuring system are also included

in this chapter In addilion, methods Lo invesligale the morphology, structure, and properties of materials are also mentioned

2.1 The synthesis process of CoFe;Q, nanostructure by hydrathermal

method

2.1.1 Equipment and chemicals

The source materials and solvents used for the synthesis of CoFe,0, materials by hydrothermal methods include: cobalt sulfate heptahydrate (Co80,.7H,0), iron(II) sulfate heptahydrate (FeS0,.7H,0), oxalic acid (C;HzOx), polyvinyl alechol (PVA) surfactants are purchased from China and other solvents like CyII,OIl, deionized water All chemicals used are analytical chemicals with purity above 99.8%

Liquipment: Llectronic scales, magnetic sturer, heat annealing oven,

wal tank, ullrasome vibralory machine Alt chemicals and

2.2 Synthesis af CoFe,O, nanostructured materials with different

structural morphology by hydrothermal method

The semiconducting metal oxide material CoFe,0, was synthesized by the hydrothermal method according to the described procedure, including detailed

steps as demonstrated in Figure 2.1

24

‘Wash centrifuge, dry at 70°C, oxidation a1 600 °C/2h

‘Nanorods CoFe,0,

120 220°C! 6 + 24 b

Figure 2.1 Diagram of synthesizing CoFe,, nanomaterials with different

morphology by hydrothermal method Firstly, CoSO4.7H,O (3.2 mmol), FeSO,.7H,O (6.4 mmol), and 0.5 g

PVA were dissolved in 50 ml DI water at room temperature After stirring the above mixture with a magnetic stirrer for 15 min, and 0.864 g of oxalic acid was added and further stirred for 15 min to create a yellow milky solution The

mixture solution was then poured into a 100 ml Teflon-lined stainless-steel autoclave and placed in an oven to hydrothermally at 120 °C for 6 h After that,

the oven was turned off and allowed to cool down to room temperature naturally

The yellow precipitated product was washed several times with DI water and

ethanol to remove the unreacted reagents by centrifugation at 4000 rpm before

drying at 70 °C for 24 h

In order to synthesize CoFe,0, materials with different structures and

morphology, we have studied and adjusted the parameters of the hydrothermal

process such as temperature change and hydrothermal time

Procedure 1: Hydrothermal temperature changes

To change the hydrothermal temperature, we used a solution consisting of

0.5 g CoSO¿.7H;O, 1.77 FeSOa.7H;O, 1 g PVA and 0.864 g oxalic acid were

dispersed in 80 ml of DI water The hydrothermal process was carried out at 120

°C, 140 °C, 180 °C, and 220 °C for 6h

Procedure 2: Hydrothermal time changes

To change the hydrothermal time, we used a solution consisting of 1 g

CoSO4.7H,0, 1.77 g FeSO4.7H,0, 1 g PVA and 0.864 g oxalic acid dispersed in

25

80 ml DI water The hydrothermal process was carried out at 120 °C for 6, 3 and 24h

Table 2.1 Symbol oŸ samples of CoFesOa materials synfhesized in the prooeases

2.3.1 Process of synthesis Pt nanoparticles by polyol method

In this thesis, Pt nanoparticles were synthesized by polyol method as reported in our proup publication [112], as shown in Figure 2.2 and consisted of

the following 4 steps:

26

Figure 2.2 Process diagram of the Pt particle production by polyol

method Step 1: 20 mL ethylene glycol was stirred at 160 °C in an oil bath for 5

min (cup A)

Step 2: 0.5 g PtCl, was dissolved in 5 mL ethylene glycol (cup B) and

stirred for 5 min,

Step 3: Pour cup B into cup A with vigorous stirring for 5 min to reduce

Pt** into Pt nanoparticles (with refluxing at 160 °C)

Step 4: To prevent the aggregation of Pt nanoparticles, we poured 0.1 g of

PVP dissolved in 20 mL of ethylene glycol (cup C) into cup A with further

stirring for 15 min before stopping and naturally cooling to room temperature The product obtained was calibrated with ethanol to obtain an 8 mg/mL colloidal

Pt nanoparticle solution

2.3.2 The synthesis process of Ag/Pt nanoparticles

The synthesis process of Ag/Pt nanoparticles is almost the same as the

synthesis process of Pt nanoparticles presented in Figure 2.3 and includes the

following 3 steps

27

Figure 2.3 Schematic diagram of the synthesizing of Ag/Pt particle by

polyol method

Step 1: 45 mL ethylene glycol was stirred at 150 °C in an oil bath for 5

Step 2: Add 1 g AgNO; and 0.1 g prepared Pt (12.26 ml) to the above

solution, stir for 5 minutes

Step 3: Dissolve 1 g of PVP in the above mixture, stir for more 15 min

before turning off the magnetic stirrer and letting it cool naturally to room

temperature The solution containing Ag/Pt nanoparticles is blue-black

2.3.3 The process of modifying Ag/Pt nanoparticles on the surface of

CoFe;O¿ nanorods

To improve the performance of the sensor, we decorated the surface CoFe,O, nanorods by Ag/Pt nanoparticles However, we selected only one CoFe,0, nanorods structure obtained by hydrothermal method according to the

procedure of Figure 2.1 The process of modifying Ag/Pt nanoparticles on

CoFe,0, surface by direct drop-casting method includes the following steps:

Step 1: Use a micropipette to take the Ag/Pt nanoparticles solution into the glass and disperse the nanoparticles in the N-vinyl-pyrrolidone (NVP) solvent

by ultrasonic vibration bath

Step 2: Weigh 10 mg of CoFe,0, into 3 Eppendorf tubes Then, add

dispersed Ag/Pt nanoparticles solution into the 3 Eppendorf tubes above with the

weight content of Ag/Pt nanoparticles over CoFe:O, of 0.5%, 1% and 2%,

respectively

Step 3: This mixture is ultrasonically vibrated for 5 minutes to obtain a

colloidal solution containing Ag/Pt nanoparticles and CoFe;O¿

28

2.3.4 Sensor manufacturing process

To evaluate the gas sensitization process depending on the nature of the

size, morphology and structure of the CoFe)O, nanomaterials, we have chosen a drop-casting technique to perform The electrode used is a Pt-toothed electrode fabricated by photolithography on a SiO, substrate The width per tooth comb is

20 jum, and the distance between two consecutive combs on the wire is 20 jum

The sensor manufacturing process is described in Figure 2.4 and specifically

includes the following 4 steps:

Step 1: Disperse 10 mg of the material in 20 ml of ethanol

Step 2: Wash the electrode with ethanol and dry it at 120 °C

Step 3: Use a micropipette to mix the material and ethanol, and drop

mixture on the electrode surface and let it dry at room temperature for about 6 h

Step 4: The sensor is incubated at 500 °C/2 h with a heating rate of 2

°C/min Then, the furnace will tum off automatically and let cool down naturally

The synthesized materials will be morphologically investigated by the Scanning Electron Microscopy (SEM) device (HITACHI $-4800), made at the Vietnam Academy of Sciences The crystal structure, and the phase composition

of the materials were determined by X-ray diffraction method (XRD; Bruker D8 Advance) with wavelength CuKa with wavelength 4 = 0.15406 nm In addition, the elemental composition and a mass ratio are determined by the energy dispersive X-ray spectrometer (EDS) In addition, information about the vibrational energy levels of an atom, molecule, or lattice is based on Raman

29

spectroscopy measurements on the Raman LabRAM HR of HORLBA Jobin Yvon - with an excitation laser beam wavelength of 633 nm at the Institute of Technical Physics - Hanoi University of Science and Technology Determination

of specific surface area of materials by BET analysis of isothermal adsorption lines N; at -196 °C and pore size in the mesoporous to microporous range (inner diameter range of 0-500 A) was determined by analysis of BH or DA isothermal

adsorption gas Nz at 77 K

2.5 Survey of gas-sensing properties

To invesligate the gas-sensilivily charactenstics of the synthesized

materials, we conducted gas-sensitivity measurements in the laboratory of Development research and application of nanosensor — ITIMS, Hanoi University

of Science and Technology The sensor's sensilivily was investigated al different

temperatures and gas concentrations by static measurement Static measurement

is a method of measurement in a closed chamber in which the volume of gas in

the chamber is kept defaulted, while a certain volume of gas is injected into the

measuring chamber I'he gas concentration in the measuring chamber will be

calculated according to the ideal gas formulas Therefore, to minimize errors, the

volume of the measuring chamber is ofien required to be large, so thal when adding more gas to be measured into the measuring chamber does not significantly change the pressure of the chamber The concentration of analytical gas C (ppm) in the chamber will be calculated according to the following

formula:

Cxpm) = 1000 x2 x ¢,

Where !’ (2) is the volume of the chamber, # (yz) 1s the volume of standard

gas injected into the chamber, and Cy (ppm) is the standard gas concentration in the cylinder

The main parts of the gas measurement system are shown a Figure 2.5,

including: gas measuring chamber, Keithley 2450 source, computer with gas

measurement software installed Gas measurement chamber: [he sensor sample

to be measured is placed on the heater connected to the temperature controller to

generate the temperature to be investigated, the maximum temperature of the

30