Luận văn thạc sĩ synthesis and gas adsorption properties of nickel ferrite nanoparticles

SOCTALIST REPUBLIC OF VIETNAM Tndependenee — Freedom — Happiness CONFIRMATION OF MASTER’S THESIS ADJUSTMENT Full name of the author : Cav Xuan Fruong Thesis topic: Synthesis and gas ad

TIANOI UNIVERSITY OF SCIENCE AND TECIINOLOGY

MASTER THESIS

Synthesis and gas adsorption properties of

nickel ferrite nanoparticles

CAO XUAN TRUONG

Truong CX211148M@isis hust edu vn

SOCTALIST REPUBLIC OF VIETNAM

Tndependenee — Freedom — Happiness

CONFIRMATION OF MASTER’S THESIS ADJUSTMENT

Full name of the author : Cav Xuan Fruong

Thesis topic: Synthesis and gas adsorption properties of nickel ferrite nanoparticles

Major: Material Science

Student ID: 20211148M

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 April 28", 2023 with the following contents:

- Literature review outline and content

- Spelling and printing errors

Day Month Year

COMMITTEE'S CHAIRMAN

Prof Nguyen Phuc Duong

Acknowledgement First of all, 1 would like to express my greatest gratitude toward my supervisor, Assoc Prof Nguyen Van Quy ty lor being wm ideal (cacher, mentor, and

thesis supervisor, offering advice and encouragement with a perfect blend of

insight and humor I also desire ta extend my appreciation to Dr Luong Ngoc Anh, Dr Nguyen ‘Thanh Vinh and Dr ‘iran Van Dang for their invaluable

recommendations ard explanations related to my research topic

I would also like to express my special thanks to all lecturers and

employees at TTTMs for creating a wonderful environment while T was on my

course | also thank the project grant number 132021-3KA-04

Sincerely, I would like to thank my lab-mates at Room 202 for their

strong support, endless assistance, regular encouragement and inspirations every

single day

Tast bul niol least, my special gralilude is expressed la my doar family

members, who are always by my side, both fiancial and mental supporlive

during my master program.

Abstract

Industrialization and modemization in today society bring about many

benefits Pollution caused by these processes put people’s health and environmental status at risk It is urgent that a sensor with high sensitivity, stable

operalion, low cost, low energy consumption and mobility is developed lo

monitor the pollution status and prevent potential risk A quartz crystal microbalance (QCM) sensor is researched to meet those requirements This sensor can detect a small concentration of gas by mass change principle ‘To

enhance the adsorption capabilities, metal oxides are deposited on the electrode

of QCM sensor Among the most considerable sensing materials, NiTe:Ox nanoparticles with porous structure, large specific area and various functioning group on its surface can be considered suitable for being a good sensing layer of

QCM sensor The material is fabricated by hydrothermal and co-precipitation

methods The characterization of NiFe:Oa was investigated by some measuring anethods ‘then the QCM sensors are coated and tested their gas sensing ability

by OCM200 system Aller various experiments, il can be assured thal a QCM

coated NiFe2Q1 sensor is capable of detecting SO2, NOz, IbS at room

temperature In addition, the results suggest that the material are most responsive

to SO» and little deviated after a long time operating The mechanism of physisorption of nickel ferrites 18 also presented

STUDENT

Cao Xuan ‘Truong

INTRODUCTION

CHAPTER 1 LITERATURE REVIEW

1.1 Introduction of nickel ferrite (NiFexO,) 14 1.1.1 Overview of the structure of fermites - 14 1.1.2 0 Nickel Ferrite (NiFa2O4)} - - - 16

1.13 Fabrication methods - - H 1.2 Introduction of quartz crystal microbalance (QCM) - 18

121 Piezoelectric Effect - - - 18

1.2.2 — Quartzcrystalmierobalanee "—

1.3 Quartzcrystal microbalance gas sehsor 24

1.31 IntreducHon of QCM gas sensor c seo 214 1.3.2 QƠMsenmsor working prinoiple "—-

CHAPTER 2 EXPERIMENT DETAILS

2.1 Chomical and apparatus

214 Chemical - - 28

2.1.2 Apparatus - - 28

2.2 NiFe2O« nanoparticles fabrication - 28

2.21 Nilfe.O nanoparticles fabrication by hydrothermal method .28

2.2.2 NH'e:Ox nanoparticlas fabrication by co-precipitation method 29 2.3 Characterizatienmethods —-

X-ray Diffaction (XRĐ) ec "¬

.30 31 Scanning Electron Microscope ‘Transmission electron microscope Fourier Transform Initared Spectroscopy (FTTR) Surface area and pore size distribulion measurements 33 2.4 Fabrication of NiFexO4 sensing layer on the QCM electrode and gas

2.4.1 Pebricatien ofNiFe:Oa sensing layer ơn QCM 34 2.4.2 Gas scnsing measureiieii e si 135

CHAPTER 3 RESULTS AND DISCUSSIO!

3.1 Fabrication method iwestigation "—- ST

3.3 Gas sensing properties of QUM coated NiHesOa NEs sao 43

3.3.1 Mass density of NiFeaOx NEs đeposited on the elecưode 43

3.3.2 Inorganic toxic gases adsorption ability - 43 3.3.3 Long-temstability .49

BET Đrunauer — Finmell — Teller

BH Barrett — Joyner —Halenda

BVD Butterworth-Van Dyke

TTR Fourier Transform Infrared Spectroscopy

ICPDS Joint Committee on Powder Diffraction Standards

ITIMS International Training Institute for Materials Science

MFC Masa Flow Controller

ppm Parts per million

QCM Quartz Crystal Microbalance

sccm Standard cubic centimeters per minute

SEM Scanning Electron Microscopy

TEM Transmission Electron Microscope

XRD X-ray Dillrachon

LIST OF FIGURES Figure 1.1 Schematic of a partial unit cell and ferrimagnetic ordering, of spinel ferrite structure [44] TH HH ga ses 14 igure 12 Cation distribution in spinel femites (a) inverted ferrites, ) mumganese ferrites and (¢) zine manganese ferziles [18] - 15

Figure 1.3 Atomic positions in the inverse spinel structure of NFO A portion of connecting (Fe,Ni)Os octahedra around a Fe, tetrahedron is alsa depicted, where “Oc” and “Te” in the suffix indicate the octahedron and tetrahedron 17 Figure 1.4 Typical device in hydrothermal method 18 Figure 1.5 The piezoelectric effect in the material: without piezoelectric polarization (A), the molecules subjected to an external force with charge forming (B), and piczoclectrie cffect on the surface Note that, P denotes

polarization vector, F is applied external force [36] 19

Figure 1.6 Direct and inverse pievoelecinic effect in the material 20 Figure 1.7 Practical application of the piezoelectric material [34] 20

Figure 1.8 The schematic of quartz crystal with clectrode (ab), the strain induced in an AT cut crystal on application of AC! voltage (c), and the amphinde

of vibration varies with the distance from the center of the sehSOI "-

#igure 1%, ‘The quartz crystal structure (A), A'l-cut crystal (3), and “aystal

Tigure LIU The Butterworth-Van Dyke (BVD) equivalent circuit for an unloaded quartz crystal microbalance, QCM under viscous and mass loading (A),

and Ihe device parameter versus frequeticy characlerislic curve (B) [45] 23

Figure 1.11 The diagram of Quartz Crystal Microbalance oscillator 24

Figure 1.13 (a) Schematic top view and cross-sectional view of 2 QCM uncoated and coated with a sensing layer, and (b) the illustration of Frequency decreasing due to active layer coating and during sensing measurements [50] « vee 26 Figure 2.1 NiFexO« NPs hydrothermal method synthesis process 29 Figure 2.2 Nilfez©a NES cơ precipiiation synthesis proG€SS co co 2Ø Figure 2.3 XRD measurement system in ITIMS - 30 Figure 2.4 Working principle of scanning electron microscopy 160] 31 Figure 2.5 General layout of a TEM - 32 Figure 2.6 (a) Schematic diagram of a Fourier transform infrared instrument (b)

Figure 2.7 Spray coating system and spraving "`" sueesseusoou 35 igure 2.8 Sehematic diagram of gas measuring system Xueesseusoou 35 igurc 2.9 Gas measuring syse c onooirriieiroree sao 3Ó

Figure 3.1, XRD spectra of NiFez04 al different, armeating temperature (co-

igure 3.5, SEM image and size distribution figwe of C - NFO sample 40 Figure 3.6 SEM image and size distribution figure of H - NFO sample 40 Figure 3.7 ‘The Fourier transform infrared spectrum of C NEO (a) and H

Figure 3.8 Adsorption and desorption isotherm and pore size distribution of C -

Figure 3.9 Gas mass absorbed on the QCM-C - NEO and QCM-H— NEO 44 Figure 3.10 The relationships betweeu the frequency shifis/adsorbed mass on the QCM C - NFO electrode and target gases concentrations from 5 to 20 ppm of

Figure 3.42 Linear dependence of the sensitivity factor on the SO2, NO2

concentrations between 5ppm and 20ppm of two sensors - - a7

Figure 3.13 The comparison between SƠ; and NOs sensibility of both sensors

Figure 3.14 The long -term stability « oŸ gas adsorption performanee 48

igure 3.15 ‘Ihe response and recover time of C NEO coated sensor at diTerent coneontration of 8O (a) and NÓ (b) M

Figure 3.16 The response and recover time ol’ H — NFO coated sensor al different concentration of SO2 (a) and NO» (b) coe 51 Figure 3.17 Response towards different gases in different concentrations of C -

List OF TABLES

Table 3.1 BET data of C - NFO and H- NFO sample 4 Table 3.2, The mass density of the material deposited on the surface of the gold electrode cocecoeee ¬ _Ô

‘Table 3.3, Mass density absorbed on QCM sensors (ig/emÊ) 4Š Table 3.4 Summarized information of QCM — C— NFO 47

'Table 3.5 Summarized information of QƠM — LI — NHO,

Table 3.6 Comparison of SO, response and recover time of different sensing

material at the same cOrIoentrat1OT ái ĐT

INTRODUCTION

Nowadays, our lives are hecoming more and more convenient thanks to the significant development of industrial sectors and technologies Unfortunately, during these fast-growing processes, manufacturing activities and vehicles uncontrollably discharge many toxic compounds directly into the environment without any treating precautions This is one of our major concems, which puts

both humankind’s health and our environment at risk Indeed, mixing of external

hazardous gases and particulate matter in the air such as S02, NOz, HS, CO,

CO2, NH3 ultimately routes inside our body through inhalation Tw both

children and adults, short-term or long-term exposure to air pollution can lead to decreased lung function, respiratory infections, and worsening asthma Emerging evidence also suggests that ambient air pollution may affect diabetes and neurodevelopment in children [1], [2] To minimize the aapact of the toxic gases

on buman’s health and environment, scientists are developing a wide range of sensors that help people observe and control the polluting problems related to

those bavardous gases Tl can be listed many types of the sensors inchuding

electrochemical, magnetic, optical, MEMs and mass sensitive using quartz crystal microbalance (QCM) [3] [5] Among them, QCM sensors are indicated

to have some excellent advantages like low — energy consumption, casy inlegralion with porlable devices, up lo nanogram sensilivily, fast response and recover rate, and stable operation in room temperature [6], [7]

In the field of mass sensitive sensors using QCM devices, the sensing layer

is the greatest contributor to adsorption properties, Many kinds of the materials are bemg investigated namely carbon groups, polymers, metal oxides [8| [11] Metal oxides seem to attract the scientist by their potential characters such as

various crystalline structure, simple fabrications, non-toxicity, low cost, high

density of functioning group, and environmentally friendly [22] [14] Spinel nickel ferie nanoparticles is widely used in eleetric and cleetronie devices as a magnetic material, and is also applied to the fields of catalysis and gas sensors

due to its semiconducting properties[15] Moreover, Nil‘eza NPs also possess

the potential characterization to be a mass-changing type sensing layer Due to its

inverse spinel structure (etal — oxygen bonding) and large specific arca, the

material is appropriate to exhibit physical adsorption ability

By analyzing the benefits of both QCM sensors and nickel ferrite (NiFe20,)

nanoparticles, a thesis with the topic of “Synthesis and gas adsorption

properties of nickel ferrite nanoparticles” was introduced

Research objectiv

« Successfully fabricate the nickel ferrite (NilexOs) nanoparticles by hydrothermal and co-precipitation methods

« Tuvestigale the characterization of the synthesized material

« Fabricate the QCM coated with nickel ferrite (Niex04} nanoparticles,

© Asscas the gas sensing properties of the fabricated sensors.

Study methods

To attain the research objectives, several experiments are conducted by

some methods which can be listed:

© Fabricate the nickel ferrite (NiFe2O4) nanoparticles by hydrothermal

method and co precipitation method then calcinate in the air at high

temperature

* Camy out some characterization measurements such as XRD, SEM,

FTIR, BET

« Disperse in deionized water then spray coating on the QCM electrode

« Investigate the gas sensing properties of some inorganic toxic gases like

SQ:, NOx, ILS and Ns using the Quartz Crystal Microbalance

oscillator QCM200 and QCM25 controlled by SRSQCM200 software

‘Thesis outiine

‘he theorical and experimental research in this thesis are reported in a sequent order, specifically

Chapter 1: Literature review demonstrates the general knowledge, busic

ciple of the QCM serwors and an overview of NiFe:O4

gas sensing pri

nanoparticles

Chapter 2: Experimental details show the procedure of experiments

conducted to fabricate and investigate the properties of the materials

Chapter 3: Results and Discussion exhibits the results obtained from the

experiments The structure and the gas sensing measurement are analysed and compared, Then the gas seusing mechanism of the material is proposed Finally, Conclusion summarizes main findings presented in the thesis and suggests the future study of the topic.

1.1 Introduction of nickel ferrite (NiFe;Ox}

1.1.1 Overview of the structure of ferrites

The spinel ferrite slructure MeFo2O4, where Me refers to the melal, can be

described as a cubic close-packed arrangement of oxygen atoms, with Me?" and

Fe* at two different crystallographic sites These sites have tetrahedral and

octahedral oxygen coordinalion (termed as A and B-sites, respectively), so the

resulting local symmetries of both sites are different The spinel structure

contains two cation sites for metal cation occupancy here are 8 A-sites in

which the metal cations arc tetrahedrally coordinated with oxygen, and 16 B-sites

which possess octahedral coordination, When the A-sites are aceupied by Me?! cations and the B-sites are occupied by Fe** cations, the ferrite is called a normal

spinel Lf the A-sites are completely occupied by Fe** cations and the B-sites are

yandomly occupied by Me and Fe?* cations, the structure is referred to as an

inverse spinel Tons localed at the telrahedral sites are known as ‘network

formers’, while those located at the octahedral sites are known as “network xmodifiers” [I6] In most spinels, the cation distribution possesses an intermediate degree of inversion where both siles contain a fraction of the Me” and Fe*

cations Magnetically, spinel ferrites display ferrimagnetic ordering The

magnetic moments of cations in the A and B-sites are aligned parallel with yespert to one another Between the A and R-sites the arrangement is antiparallel

and as there are twice as many B-siles as A-siles, there is 2 nel moinernt of spins

yielding ferrimagnetic ordering for the orystal The choice of metal cation and the distribution of ions between the A and 13-sites therefore, offer a tunable magnetic system [17]

“ Actgtrghedral sie

B:actahedral site

© croneen Higure 1.1 Schematic of a partial unit cell and ferrimagnetic ordering of spinel ferrite

structure [44].

According {o the distribution of cations, there are the following types of

forrospinels:

* Normal spinel structure, where all Mc”* ions occupy A- sites; structural

formula of such ferrites is Me?'[Fer!']O4 This type of distribution takes place in zine ferrites Zn?*[Fe?*Fe**]O,2-

s Taversed spinel structure, where all Me* are in B-posilions and Fe?*

tons are equally distributed belween A and B-siles: structural formuta

of these ferrites are Fe?*[Me**Fe?*]O,? Magnetite FejO,, ferrites

NiFe2O4 and CoFc204 have inversed spinel structure

« “Mixed spinel structure, when cations Me?! and Fe!' occupy both A and

B-positions; structural formula of this ferrite is Moi-s*Fos*| Mes” Fors" 104", where & is the degrce of inversion

MnFe2O4 represent this type of structure and has an inversion degree of

6 = 02 and its structural formula therefore is Mnos” Feos"*[Mno2”Fers"JO., Mn—Zn ferrites also have a mixed spine] structure (Zn2* prefers to occupy A-sites) Zn?*Mny? Foray 3

[Mni x f*Feisiy? JOr”, where §=1 x y

At B J Fe? [Me"Fe*10,2-

Figure 1.2 Cation distribution in spinel ferrites: (a) inverted ferrites, (bi manganese

ferrites and (c) zine manganese ferrites [18]

ability to withstand penetration or abrasion Soll materials are easy lo magnetize

and demagnetize, so are used for electromagnets, while hard materials are used for permanent magnets They can also be classified based on their coercive field strength into soft and hard materials With soft magnetic materials the hysteresis loop is small (low courvive ficld strength, independent of magnetic field

amplitude); with permanent magnets however large il is (high coercive leld strength) Iard ferrite magnets are made in two different magnetic forms - isotropic and oriented isotropic magnets are formed to desired shapes, sintered and lhon magnetized These exhibit a modest magnotic Geld and find applivalions

ineyele dynamos and ring magnels Onered magnels are formed to shape under

a strong magnetic field and then sintered ‘hese exhibit a very strong magnetic field and find applications in loudspeakers, magnets of two wheelers like

scooters, ete, [19]

Spmel ferrites have drawn a huge allention of the research world because of their interesting magnetic and electrical properties such as high saturation magnetization, high squareness ratio, large magneto crystalline anisotropy, low coercivity [13], high permeability [20], low eddy current losses [21], high Curie

temperature, and mechanical hardness [22] Spinel ferrites are conventionally

used in the field of microwave absorbers due to their large magnetic losses and large resistivities [23]

1.1.2 Nickel Ferrite (NiFez04)

The first authentic research publication in these ferrites occured in 1953,

when Hastings and Corliss analyzed the chemical and magnetic properties of nickel and zine ferrites by means of neutron diffraction at room temperature[24]

Based on their analysis, it was experimentally verified that nickel ferrites possess

an inverted magnetic structure, with properties that are in good agreement with the Neéel model of ferrimagnetism Nil’e:Os orystallizes in the inverse spinel

structure (Fd3m) with fcc erystal and lattice constants : a = 90°,a =b=e

= 0.834 nm at room temperature [25] The formal chemical formula of Fes can

‘be represented as Fer [Ie’'Fe?']o.O” , where one-third of Te occupies the

tetrahedral (i'e) site in the Fe3+ state, and the remaining Fe?* and Ke’* equally

ovcupy (he octahedral (Ov) sile, Analogous to FesOs, NFO also carries the

inverse spinel structure, where Ni?! replaces the I'e?' at the octahedral site in the

1'd3m structure ‘Ihe inverse spinel structure of NFO is illustrated in, where the tetrahedral (8b) site is occupied by Fe** and the octahedral (1 6c) site is occupied

by Ni?! and Fe?! with a 50% occupancy, as highlighted by the bicolor atoms in the figure The figure also partially depicts that the octahedra are connected to

each other and linkage of the octahedra with a tetrahedral unit through the O

atoms

Figure 1.3 Atomic positions in the inverse spinel structure of NFO A portion of connecting (Fe,Ni) Ox octahedra around a FeOs tetrahedron is also depicted, where

‘Oc” and “Te” in the suffix indicate the octahedron and tetrahedron

Due to their high electric resistivity, low coercivity, moderate saturation magnetization and low hysteresis losses, nickel ferrites are categorized in the

class of soft ferrites These soft magnetic materials also offer other favorable

properties, such as high permeability at high frequency, mechanical hardness, electrochemical stability, reasonable cost and low dielectric and eddy current

losses [21] The material is widely used in diverse applications in many areas

such as rechargeable batteries [26], [27], magnetic recording [28], medicine, and

biology [29], [30], and continues to excite with its complex and intriguing fundamental properties However, in this study, the properties of the material that

need to take into concern is the surface morphology and the function group's

presence, which are important factors of a high physisorption ability [31] With

available -OH groups, porous surface and large specific area, the NiFexO4 nanoparticles can be a promising candidate to be the gas sensing layer of mass

change gas detection sensor like QCM

out in this thesis

Hydrothermal synthesis is one of the most commonly used methods for preparation of nanomaterials It is basically a solution reaction-based approach

In hydrothermal synthesis, the formation of nanomaterials can happen in a wide

temperature range from room temperature to very high temperatures To control

the morphology of the materials to be prepared, either low-pressure or high- pressure conditions can be used depending on the vapor pressure of the main

composition in the reaction Many types of nanomaterials have been successfully synthesized by the use of this approach There are significant advantages of

hydrothermal synthesis method over others Hydrothermal synthesis can generate

nanomaterials which are not stable at elevated temperatures Nanomaterials with

17

high vapor pressures can be produced by the hydrothermal method with

minimum loss of materials The compositions of nanomaterials to be synthesized

can be well controlled in hydrothermal synthesis through liquid phase or multiphase chemical reactions [32] The paper by Z Rak and D W Brenner [33]

presents the fundamental work on the formation of nickel ferrite (NiFe2O.)

nanoparticles under hydrothermal conditions A model was established via a

method that combines results of first principal calculations, elements of aqueous thermochemistry, and experimental free energies of formation Based on

calculations using the model, negative formation energies for the (111) surfaces

and positive free energies for the formation of bulk nickel ferrite were predicted

The combination of the negative surface and positive bulk energies yields thermodynamically stable nickel ferrite nanoparticles with sizes between 30 and

150 nm in the temperature range of 300 to 400 K under alkaline conditions The effect of processing condition on the stability of the nickel ferrite nanoparticle

Figure 1.4 Typical device in hydrothermal method

Coprecipitation is a widely utilized approach in the synthesis of metal oxide

nanoparticles and metal/ceramic nanocomposites Different factors such as the

concentration of starting reagents, the pH, and the heating effect have great influence on the size and shape of the nanoparticles In this method, raw

materials (chloride or nitrate) are dissolved in the solvent to get a homogeneous solution Then, a base (NaOH or NHsOH) is added to the hydroxide solution that

results in the formation of precipitates After the corresponding salts are washed, the heating treatment results in the synthesis of metal oxide nanoparticles

1.2 Introduction of quartz crystal microbalance (QCM)

1.2.1 Piezoelectric Effect

Piezoelectric effect based on the production of an electrical charge when

subjecting a mechanical strain on the specific materials, whereas the piezo represents the presence of pressure and electricity means the electron moving

18

[34] The fundamental of piezoelectricity is regarding to the non-

centrosymmetric distribution of positive and negative charge in the unit cell of

material Basically, when a piezoelectric material is placed with an external

applied stress or mechanical vibration, resulting the modification of displacement

of ion due to the change in the dipole moment of the unit cell, as shown in

Figure 1.5 In other word, there are generated piezoelectric potential, being the

electrical signal output, across the material[35]

The piezoelectric effect composes two main phenomena, that is direct

(DFE) and reverse piezoelectric effect (IPE) Namely, the direct one is the

formation of electric charges on the top of specific insulation material from applied mechanical force For instance, some crystalline minerals is getting

electrically polarized in cases of placing the external stress [36] Additionally,

compression and tensile produced in this situation being opposite polarity

voltages which are proportional with applied force Otherwise, the polarity of

voltages caused by tensile force tend to be opposite with the polarity of voltages generated via compressive force, as illustrated in Figure 1.6 [37] Meanwhile, if

an inverse piezoelectric material is placed in the presence of electric field, it will

be strained Electrical dipoles inside the piezoelectric material act as the creation

of the electrical output or potential difference go through the materials which is

connected with circuit When the material is no external force, the material state

will be neutrally charged, means that the positive charge number is equal with

negative charges, thus there is no electrical output in this circumstance When it

is placed external force, the stress across the material was determined as

following equation: Stress = Force/Cross — Sectional area

Being the modification in the relative position, there is a change in the dipole moment, creating a potential difference As a result, the material

experience deformation This deformation leads to build up positive charge in one end of the material and negative charge within the other side of material The

potential difference can generate the charge to be driven in the circuit and

19

produce the electricity In contrast, the electrostatic attraction or repulsion is created by charge or opposite charge after applying a potential difference

"Original Shape Direct Plezoelectric Effect

‘Output voltage of Output voltage of Applod voltage of Applied voltage of

Potanzation Direction Cee polarity ee ` ees Beier pretest

Figure 1.6 Direct and inverse piezoelectric effect in the material

Piezoelectric materials are classified into different groups, including ceramics, crystals, and polymer There are only 20 points group of the 21 crystal

classes of non-centrosymmetric crystal possess piezoelectric properties, while ten

points group of these crystals belong to non-polar, that is polar capability even no

mechanical strain due to a nonvanishing electric dipole associated Non-polar

piezoelectric materials such as quartz [38] are non-ferroelectric can have no

electric net dipole in the zero-stress state, hence there are no the electric dipole unless applying stress This is caused by the separation of electric charge centers

and induced piezoelectric potential Inversely, the polar piezoelectric materials,

ie, zine oxide, exhibits polarization in the zero-stress state since there is a separation between positive and negative charges [39] Furthermore, a subclass

of piezoelectric materials are ferroelectric perovskites which also possess polar

crystals, for instance, barium titanate [40], [41]

Owing to unique properties allow opportunities for implementing

renewable and sustainable energy through power harvesting and self-sustained smart sensing in buildings [42] The piezoelectric materials used for numerous

applications such as selective deposition, hydrogen production, dye degradation,

self-charging power cells

Figure 1.7 Practical application of the piezoelectric material [34]

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Tu terns of different applications, the selection of’ piezoelectric malerials with properties suitable for the purpose is crucial The essential properties of piezoelectric materials consist of frequency stability, negligible deviation of the circuil constants, stable oscillation A quarlz crystal resonator is considered a

device that meets all the as-mentioned characteristics Firstly, im order to oblain

resonance frequency stability, it must be chosen the right quartz slab cutting angle so that the thermodynamic coefficient is zero Second, quartz œrystal is

known to be one of the most stable inorganic materials about chemical and physical aspects compared to other piezoelectric materials, therefore the wear out

of the material is almost zeros Finally, the quality factor of a quartz orystal needs

to be high so that its oscillation is not affected by other properties Conclusion,

the electronic devices based on quartz crystals mus have long hfe-tine, long-

†em stability, accuracy, high repeatability and low cost

1.2.2 Quarts crystal microbalance

Quartz Crystal Microbalance (QCM) 1s an extremely mass sensitive device that can measure the changes in mass of particles per unit area from nanograms

to micrograms level Lense, it is appropriate for low concentration gas detection applications As shown in Figure 1.8 (A-B), the structure of QCM consists of a

quartz disc Quartz is a piezoelectric material that can be made to oscillate at a

certain frequency when applying into the metal electrodes by a suitable voltage

‘The frequency of oscillation can be varied due to the deposition or removal of small amounts of mass onto the clectrade surface This modulation in frequency can be observed and measured versus real-time that provides the information related to the molecular interaction of reaction happening in the electrode surface (ie, film growth, oxidations and corrosion) In addition to be piezoelectric, quartz, also possesses a distinelive properlios that make it becume promising candidate for ultrasensitive devices specially, the a-quartz phase can act as a xesonator with thermodynamically stable up to 573°C

The quartz discs used in QCMs are almost the AT cut which provides pure

thhekness shear mode oscillation with lwo surfaces of the crystal move in are arli-

parallel fashion When the corresponding altemating current is taken place to the

quartz disc, it undergoes thickness shear mode of oscillation at its resonance frequency fo ‘This frequency is order of MIlz and inversely proportional to the

thickness of crystal As illustrated in Figure 1.8 (Cj, the induced strain on the AT-out under AC voltage AT-out exhibits good stability at room temperature

-with small frequency change in the range of 1-3 IIzC

Figure 1.8 The schematic of quart: crystal with electrode (a-b), the strain induced in

an AT cut crystal on application of AC voltage (c), and the amplitude of vibration varies

with the distance from the center of the sensor

Quartz is one of the polymorphs of silica undergoes the phase transition

from a-phase (low-quartz) via the incommensurate phase at 573°C to B-phase (high quartz) at 574°C, as Figure 1.9 A The c-and B-phase belong to point

group Ds and De, respectively Among of them, [i-phase have been extensively

studied at low temperature due to high piezoelectric effect and stability, as shown

in Figure 1.9C Quartz used for industrial devices was cut into X-cut bar

specimens with bar directions of -30, 0, 30 and 60° from the Y-axis, and Y-cut

bar specimens with bar directions of 30, 45, and 60° from X-axis (Figure 1.9B) There are piezoelectric d-constant di, diz, and dia in the X-cut specimens and das

and dye for Y-cut [43]

In order to generate the resonant frequency for quartz crystal, the

Butterworth-Van Dyke (BVD) model can be used to characterize any

mechanically vibrating system driven by electrostatic field Figure 1.6 A exhibits

the BVD circuit is typically used to describe the unloaded QCM, in-liquid QCM

(i.e, mass loading and viscous loading) [45] In the BVD model, four component

values consist of Co, Ci, Li, and Ri are the static capacitance, motional

capacitance, motional inductance, and motional resistance for unloaded QCM, respectively, While Co for the quartz crystals is usually of a few pF and its typical

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value for the given resonator is specified by the manufacturer [46] In terms of

in-liquid QCM, it presents additional parameter such as inductance Lz and resistance Rj were used to approximate the impedance of the viscous loading The inductance Ls was used to approximate the impedance of the mass loading

and capacitance Cp was used to present for the parasitic capacitance of the test

Figure 1.10 The Butterworth-Van Dyke (BVD) equivalent circuit for an unloaded quart: erystal microbalance, QCM under viscous and mass loading (A), and the device

parameter versus frequency characteristic eurve (B) [45]

The resonant frequency f of the device depends on the inductance and

capacitance of circuit, as following equation:

aE

To determine the resonant frequency of the QCM, the device is connected

to the QCM oscillator circuit including two main components, that is the digital

controller (QCM200) and a crystal oscillator (QCM25) Two parts connected to each other via RJ-45 connector, as shown in Figure 1.11 The QCM200 provides

power for the QCM25, receives the oscillating signals from the QCM25 and displays on the computer screen In addition, the integrated electronic circuit in

the QCM200 can be controlled to reduce the influence of Co, making the QCM

oscillate reach near the resonance frequency fy

23

re 1.11 The diagram of Quartz Crystal Microbalance oscillator

Figure 1.12 Crystal Holder components.[47]

oscillation frequency of approximately SMHz During the oscillation of the

QCM, the mass change on the electrode will affect the Lm and change the

of an adaptor and RF amplifier to maintain the cị

AT-cut, o-quartz with circular

‘al holder consists of a

chemi ant material, an O-ring and a 50 Ohm BNC connector that connect ystal sensor to the oscillator The digital controller cam measure the series resonance frequency at 0.1 s, 1 s and 10 s with resolution of 1,0 Hz, 0.1

Hz, and 0.01 Hz, respectively [48]

1.3 Quartz crystal microbalance gas sensor

1.3.1 Introduction of QCM gas sensor

With the imcrease of human population, industrial renovation releases

numerous toxic gases due to vehicles, pollution, and industrial waste that cause the continuous number of molar rate each year Thus monitoring hazardous gas

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plays a crugial role in protecting human health Basically, there are several approaches to detect gas in the environment, namely mass spectrometers, gas chromatographs, flame ionization detectors, and Fourier transform infrared spectrometers However, this equipment is bulk, tinwe-consuming, and complex operaling which is massive limit for prac

development of devices that exhibit promising characteristic such as compact size, low cost, high precise and reproductivity as well as integration mto the smart device (or being wearable and (exible has been allracied various studies

1 application Therefore, the

Sensors mostly consist of 2 main components uamely receplor and

transductor The first part is usually a material or a material system that changes its properties while interacting with the target gases The transductor then change this effect into recordable signal like electric signal [49] Many gas sensors

operate based on this principle can detect some inorganic gases like CO, HyS,

SO¿, NI, due to both chemical and physical interactions through receptor A\fter that, the transductor transforms the received information into resistance and electric intensity in the conductance change sensor type or frequency in the mass change sensor type These sensors can be widely applied as long as they meet some technical demands Meanwhile, a number of basic prosperities can be listed

as sensitivity, selectivity and stability An ideal sensor is the sensor that can

detec a very low concentration without any response to noise in the

environment

1.3.2 QCM sensor working principle

Quart, microbalance sensers work under the principle of changing

resonance frequency of AT-cut quartz crystalline This frequency is designated as the QCM signal which will change due to the change in mass on the quartz surface, When the QCM working im the unleaded mode, its trequency is a constant, over lime To make the QCM working properly, a sensing layor is

coated on the surface of QCM electrode This layer contains the material that

have the ability to interact with the target gas During this process, the mass (gas

molecules, sensing material ) on (he QCM changes, which creales a pressure

on the electrode and compresses the quartz crystalline The resonance frequency

of the quartz crystalline then increases or decreases depending on the change in amass of gas or material, Figure 1.11 shows the typical top and cross-sectional view of QCM chip with a gold clectrode while Figure 1.11b illustrates the change of the resonance frequency due to sensing layer deposition and sensing process, The sensing materials are commonly coated on the QCM electrode to

detgcl analyles via the adsorplion-desorption process This coaling process leads

to the decrease of the QCM resonant frequency as depicted in Figure 1.11b in step 1 to step 2 Then, the frequency will further decrease due to the adsorption process of analyte molecules onto the surface of the sensing material during measurement (step 2 to step 3)

active layer coating and during sensing measurements [50]

The relationship between mass change and resonance frequency shift is based on the equation published by Sauerbrey in 1959 [51]:

Where Af (Hz) is the resonance frequency shift QCM, Am (1g/emz) is the mass change of the material coated on the sensor’s surface, C, is where Cf is the

sensitivity factor for the crystal of QCM in Hz.1g"!,cm? Sensitivity factor can be

determined by this following equation [6]:

° Ale, where fo is the base resonant frequency (Hz), A is the electrode surface area

(cm*), 4, and p, are the shear modulus (gem™'s*) and density of quartz crystal

(g em), respectively

According to Figure 1.9, both quartz crystalline and precious metal made

electrode don’t demonstrate a high adsorption capability Therefore, it is

impossible to detect a specific gas by letting it interact with the electrode, In

order to solve this problem, all QCM sensors must be coated with sensing layers

to enhance the ability to detect the target gases The gas adsorption mechanism of

the sensing material is the key to sensor’s gas sensing properties (response, sensitivity and selectivity, .) On the whole, the mechanism of the sensing

material adheres to physical adsorption or the combination of chemical and physical adsorption in the room temperature With the material like polymer,

organic, GO, CNT which have many functioning group on the their surface such

as -OH, carboxyl, epoxy, amino, can create adsorbing center positions through

hydro bonding between them and the target gases [9], [10], [52]-{54] The sensitivity of the sensing material in this case depends largely on the number of

hydro bonds and depolarized force of the gas molecules [55] Another way to

improve the adsorbing performance is to dope a small amount of other material into the sensing material This process causes crystal defects, which can be

26

considered as vacancies, operating like a positive charge or creating extra suncomected electrons Ience, the adsorption ability will be improved [11] In some situations, chemical adsorption takes place at the same time as physical adsorplion, In chemisorplion, adsorption happen in adsorbed substance that is

held by chemical bonds Chemisorplion has high specificity thal is il is highly

specific, and it takes place only if there is a chemical bonding between adsorbent and adsorbate Therefore, the selectivity of the sensor with this kind of

mechanism is oflen much beter than that of the sensors using arly physisorption principle However, the recovery rate of them is rather low when measuring in

xoom temperature [56], [57] Besides, the detection of the target gases can be established by double contact of two hydrogen bonds between the adsorbent and

two functioning group working parallel in the sensing waterial, This kind of

adsorption normally appears in humidity sensors[8], [58]

In this thesis, all experiments are camied out under a dry condition atmosphere or relatively stable humidity environment ‘hus, Ug 1.2 is used to assess the results, The basic gas scnsing properties investigated and compared during the thesis can be listed:

« The gas sensing response: ‘The changes of signal correspond to the changes of gas concentrations

« The sensitivity: defined as the ratic of resonance frequency shift on a

CHAPTER 2 EXPERIMENT DETAILS:

Chapter 2 of the thesis will demonstrate the fabrication processes of the Nike nanoparticles in two different methods namely hydrothermal method and coprecipitations In addition, the synthesis of the QTM-based gas sensor will also be presented After that, there come the characterizations method and the

set-up gas sensor systems

2.1 Chemical and apparatus

2.1.1 Chemical

All of the chemicals uscd im the cxperiments of this thesis are the commercial products of Xilong Scientific Co., Lid (Guang Dong, China), which includes: Ni({NOs)2.6120, Fe(NOs)s.9Lh0, NaOlL with the purity of more than 98% Deionized water (D1) is extracted from the ultrahigh water purification

system in the TTTMS’s cleanroom

2.2 NiFe:O, nanopartictes fabrication

2.2 Ni¥exOs nanoparticles fabrication by hydrothermal method

Based on the ionic equation:

Ni* + 2e¥+ 801 —> Nie;Oazt 4Ll2O Fig 24

In the hydrothermal method shown in Figure 2.1, 1,4558g Ni(NO3)2.6H20

mixed with 404g Fe(NO3)9H20 (1:2 molar ratio) are dissolved completely in 50m1 deionized water Dy using magnetic stirrer, the solution was stirred in approximately 20 minutes until its color shift into yellow green ‘Then, 20ml

NaOH 2M was gradually dropped in the starring solution to adjust the pH The

process continually happened till the pH reached 8 After that, 15-minute stirring process was needed to obtain a red-brown solution Next, the mixture was transferred into a teflon coated autoclave and kept at s After the hydrothermal reaction, the product was washed with deionized water and ethanol several times

to eliminate the unexpected substances and an amount of OIF to achieve the plI

of 7 In the next step, the solution was dried in the drying oven at 70°C in 24 hours

Figure 2.1, NiFe2O;NPs hydrothermal method synthesis process

2.2.2 NiFezO4 nanoparticles fabrication by co-precipitation method

An amount of Ni(NOs)) and Fe(NOs); with molar ratio [Ni*] : [Fe**] = 1

2 were dissolved completely in deionized water To obtain the desired ferrite compositions, stoichiometric amounts of nickel nitrate Ni(NO3)2 and ferrous nitrate Fe(NO3); were mixed at 80 °C and then added to the NaOH solution until

pH * 10 (all the chemicals and reagents used were of high purity) Precipitation

and formation of nickel ferrite phases take place by the conversion of metal salts

into hydroxides, which occurs immediately, and followed by transformation of

hydroxides into nickel ferrite The solution was maintained at 95 °C for 2 hours This duration was necessary to ensure the transformation of hydroxides into

nickel ferrite (dehydration and atomic rearrangement involved in the conversion

of intermediate hydroxide phase into ferrite) The fine particles were washed several times with distilled water followed by acetone rinse in a magnetic field

and dry at a temperature of 80 °C for 24 hours In order to stabilize the spinel

structure within the samples and to achieve high degree of crystallinity, a heat- treatment process is necessary The samples of different compositions were

obtained by annealing the as-precipitated products at 600 °C in air for 5 hours

2.3 Characterization methods

2.3.1 X-ray Diffraction (CRD)

XRD is one of the most efficient and widely used method for characterizing the crystal structure of nano materials In order to find the average size of the particle and structure, diffraction techniques are used This type of information

includes variations im crystal structure, phase quantification and identification, shape and size of crystalline, distortion of lattice, size, and periodicity of non- crystalline and orientation, etc., [60] When a single-wavelength X-ray beam

interacts with the sample under a gradually changing angle, the X-ray beam will

be diffracted by the crystal planes of the crystalline solid, and a spectrum of

diffraction intensity will be recorded In this thesis, the sample was investigated

by X-Ray Diffraction (XRD, D8 Advance, Bruker, Germany) in Figure 2.3 using

the Cu-Ka radiation According to Bragg’s law the waves are constructively

added in specific directions

Where d is spacing between diffracting angles; @ is incident angle; n is integer,

and / is wavelength of the beam

The crystal size is also calculated by Scherrer’s equation:

Ø8cosøØ

'Where 2 is wavelength of the beam; k = 0,94; # is the line broadening at half the

maximum intensity and @ is diffracted angle

Figure 2,3 XRD measurement system in ITIMS

2.3.2 Scanning Electron Microscope

Scanning Electron Microscope (SEM) is considered a versatile technique

for micro and nanostructures analysis with a large range of applications SEM falls under the category of a surface-imaging method in which the sample surface

30

scans across an electron beam Tt rellecls the lopographie detail and slomie composition by generating signals with sample interactions SIM can provide several qualitative information of the specimen including its topography, morphology, composition and crystallographic information In other words, it provides information aboul the surface features and texture, shape, sive and arrangement of the particles lying on the sample's surface [61] ‘Ihe resolution attained by SEM is around 1 nm and it mainly depends upon the operating parameters, properties of the specified sample [60

Figure 2.4 Working principle of seanning electron microscopy [60]

2.3.3 Transmission electron microscope

The transmission clectron microscope is a very powerful tool for material

science A high energy beam of electrons is shone through a very thin sample,

and the interactions between the electrons and the atoms can be used to observe features such as the crystal structure and features in the structure like dislocations and grain boundaries Chemical analysis can also be performed TEM can be used to study the growth of layers, their composition and defects in semiconductors High resolution can be used to analyze the quality, shape, size and density of quantum wells, wires and dots

The TEM operales on the same basic principles as the light microscope but uses electrons instead of light Because the wavelength of electrons is much

smaller than that of light, the optimal resolution attainable for TEM images is amany orders of magnilude better than thal from a lighl microscope Thus, TEMs

can reveal the finest details of internal structure — in some cases as small as

individual atoms [62]

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