Synthesis and gas adsorption properties of nickel ferrite nanoparticles

SOCIALIST REPUBLIC OF VIETNAM Independence – Freedom – Happiness CONFIRMATION OF MASTER’S THESIS ADJUSTMENT Full name of the author : Cao Xuan Truong Thesis topic: Synthesis and gas ad

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

MASTER THESIS

Synthesis and gas adsorption properties of

nickel ferrite nanoparticles

CAO XUAN TRUONG

Truong.CX211148M@sis.hust.edu.vn

Materials Science

Supervisor: Assoc Prof Nguyen Van Quy

Institute:

Dr Luong Ngoc Anh

International Training Institute for Materials Science

HA NOI, 04/2023

SOCIALIST REPUBLIC OF VIETNAM

Independence – Freedom – Happiness

CONFIRMATION OF MASTER’S THESIS ADJUSTMENT

Full name of the author : Cao Xuan Truong

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 28th, 2023 with the following contents:

- Literature review outline and content

- Spelling and printing errors

Day Month Year

Assoc Prof Nguyen Van Quy Cao Xuan Truong

COMMITTEE’S CHAIRMAN

Prof Nguyen Phuc Duong

THESIS TOPIC

Synthesis and gas adsorption properties of nickel ferrite nanoparticles

Assoc Prof Nguyen Van Quy

Supervisor

Acknowledgement

First of all, I would like to express my greatest gratitude toward my supervisor, Assoc Prof Nguyen Van Quy for being an ideal teacher, mentor, and thesis supervisor, offering advice and encouragement with a perfect blend of insight and humor I also desire to extend my appreciation to Dr Luong Ngoc Anh, Dr Nguyen Thanh Vinh and Dr Tran Van Dang for their invaluable recommendations and explanations related to my research topic

I would also like to express my special thanks to all lecturers and employees at ITIMs for creating a wonderful environment while I was on my course I also thank the project grant number B2021-BKA-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

Last but not least, my special gratitude is expressed to my dear family members, who are always by my side, both financial and mental supportive during my master program

Abstract

Industrialization and modernization 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 operation, low cost, low energy consumption and mobility is developed to 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, NiFe2O4

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 NiFe2O4 was investigated by some measuring methods Then the QCM sensors are coated and tested their gas sensing ability

by QCM200 system After various experiments, it can be assured that a QCM coated NiFe2O4 sensor is capable of detecting SO2, NO2, H2S at room temperature In addition, the results suggest that the material are most responsive

to SO2 and little deviated after a long time operating The mechanism of physisorption of nickel ferrites is also presented

Cao Xuan Truong

STUDENT

TABLE OF CONTENTS

INTRODUCTION 12

CHAPTER 1 LITERATURE REVIEW 14

1.1 Introduction of nickel ferrite (NiFe 2 O 4 ) 14

1.1.1 Overview of the structure of ferrites 14

1.1.2 Nickel Ferrite (NiFe2O4) 16

1.1.3 Fabrication methods 17

1.2 Introduction of quartz crystal microbalance (QCM) 18

1.2.1 Piezoelectric Effect 18

1.2.2 Quartz crystal microbalance 21

1.3 Quartz crystal microbalance gas sensor 24

1.3.1 Introduction of QCM gas sensor 24

1.3.2 QCM sensor working principle 25

CHAPTER 2 EXPERIMENT DETAILS 28

2.1 Chemical and apparatus 28

2.1.1 Chemical 28

2.1.2 Apparatus 28

2.2 NiFe2O4 nanoparticles fabrication 28

2.2.1 NiFe2O4 nanoparticles fabrication by hydrothermal method 28

2.2.2 NiFe2O4 nanoparticles fabrication by co-precipitation method 29 2.3 Characterization methods 30

2.3.1 X-ray Diffraction (XRD) 30

2.3.2 Scanning Electron Microscope 30

2.3.3 Transmission electron microscope 31

2.3.4 Fourier Transform Infrared Spectroscopy (FTIR) 32

2.3.5 Surface area and pore size distribution measurements 33

2.4 Fabrication of NiFe2O4 sensing layer on the QCM electrode and gas sensing measurement 33

2.4.1 Fabrication of NiFe2O4 sensing layer on QCM 34

2.4.2 Gas sensing measurement 35

CHAPTER 3 RESULTS AND DISCUSSION 37

3.1 Fabrication method investigation 37

3.1.1 Co-precipitation method 37

3.1.2 Hydrothermal method 38

3.2 Characterization 39

3.2.1 X-ray Diffraction 39

3.2.2 SEM images 40

3.2.3 Fourier transform Infrared Spectra 40

3.2.4 BET and BJH analysis 42

3.3 Gas sensing properties of QCM coated NiFe2O4 NPs 43

3.3.1 Mass density of NiFe2O4 NPs deposited on the electrode 43

3.3.2 Inorganic toxic gases adsorption ability 43

3.3.3 Long-term stability 49

3.3.4 Response and recover time 50

3.3.5 Selectivity 52

CONCLUSIONS 54

LIST OF PUBLICATION 62

ABBREVIATIONS

BET Brunauer – Emmett – Teller

BJH Barrett – Joyner – Halenda

BVD Butterworth-Van Dyke

FTIR Fourier Transform Infrared Spectroscopy

JCPDS Joint Committee on Powder Diffraction Standards

ITIMS International Training Institute for Materials Science

MFC Mass Flow Controller

NFO NiFe2O4

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 Diffraction

LIST OF FIGURES Figure 1.1 Schematic of a partial unit cell and ferrimagnetic ordering of spinel

ferrite structure [44] 14

Figure 1.2 Cation distribution in spinel ferrites: (a) inverted ferrites, (b) manganese ferrites and (c) zinc manganese ferrites [18] 15

Figure 1.3 Atomic positions in the inverse spinel structure of NFO A portion of connecting (Fe,Ni)O6 octahedra around a FeO4 tetrahedron is also 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 piezoelectric effect on the surface Note that, P denotes polarization vector, F is applied external force [36] 19

Figure 1.6 Direct and inverse piezoelectric 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 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 22

Figure 1.9 The quartz crystal structure (A), AT-cut crystal (B), and crystal structure of -SiO2 (c) [44] 22

Figure 1.10 The Butterworth-Van Dyke (BVD) equivalent circuit for an unloaded quartz crystal microbalance, QCM under viscous and mass loading (A), and the device parameter versus frequency characteristic curve (B) [45] 23

Figure 1.11 The diagram of Quartz Crystal Microbalance oscillator 24

Figure 1.12 Crystal Holder components.[47] 24

Figure 1.13 (a) Schematic top view and cross-sectional view of 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] 26

Figure 2.1 NiFe2O4 NPs hydrothermal method synthesis process 29

Figure 2.2 NiFe2O4 NPs co precipitation synthesis process 29

Figure 2.3 XRD measurement system in ITIMS 30

Figure 2.4 Working principle of scanning electron microscopy [60] 31

Figure 2.5 General layout of a TEM 32

Figure 2.6 (a) Schematic diagram of a Fourier transform infrared instrument (b) Michelson interferometer [64] 32

Figure 2.7 Spray coating system and spraying gun 35

Figure 2.8 Schematic diagram of gas measuring system 35

Figure 2.9 Gas measuring system 36

Figure 3.1 XRD spectra of NiFe2O4 at different annealing temperature precipitation method) 37

(co-Figure 3.2 TEM image of sample annealed at 600oC (left) and 800oC (right) 38

Figure 3.3 XRD spectra of NiFe2O4 at different annealing temperature (hydrothermal method) 38

Figure 3.4 XRD spectra of NiFe2O4 fabricated by co-precipitation (C - NFO) and hydrothermal method (H - NFO) 39

Figure 3.5 SEM image and size distribution figure 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 – NFO (a) and H –

NFO (b) 41

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

NFO (a) and H - NFO (b) 42

Figure 3.9 Gas mass absorbed on the QCM-C - NFO and QCM-H – NFO 44 Figure 3.10 The relationships between the frequency shifts/adsorbed mass on

the QCM C - NFO electrode and target gases concentrations from 5 to 20 ppm of

SO2, NO2 46

Figure 3.11 The relationships between the frequency shifts/adsorbed mass on

the QCM H - NFO electrode and target gases concentrations from 5 to 20 ppm of

SO2, NO2 46

Figure 3.12 Linear dependence of the sensitivity factor on the SO2, NO2

concentrations between 5ppm and 20ppm of two sensors 47

Figure 3.13 The comparison between SO2 and NO2 sensibility of both sensors 48

Figure 3.14 The long-term stability of gas adsorption performance 49 Figure 3.15 The response and recover time of C – NFO coated sensor at

different concentration of SO2 (a) and NO2 (b) 50

Figure 3.16 The response and recover time of H – NFO coated sensor at

different concentration of SO2 (a) and NO2 (b) 51

Figure 3.17 Response towards different gases in different concentrations of C -

NFO sample 52

Figure 3.18 Response towards different gases in different concentrations of H -

NFO sample 53

LIST OF TABLES

Table 2.1 Spray coating statistic 35

Table 3.1 BET data of C - NFO and H - NFO sample 42

Table 3.2 The mass density of the material deposited on the surface of the gold electrode 43

Table 3.3 Mass density absorbed on QCM sensors (µg/cm2) 45

Table 3.4 Summarized information of QCM – C – NFO 47

Table 3.5 Summarized information of QCM – H – NFO 48

Table 3.6 Comparison of SO2 response and recover time of different sensing material at the same concentrations 52

INTRODUCTION

Nowadays, our lives are becoming 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 concerns, 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 SO2, NO2, H2S, CO,

CO2, NH3… ultimately routes inside our body through inhalation In 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 impact of the toxic gases

on human’s health and environment, scientists are developing a wide range of sensors that help people observe and control the polluting problems related to those hazardous gases It can be listed many types of the sensors including 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, easy integration with portable devices, up to nanogram sensitivity, 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 being 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 [12]–[14] Spinel nickel ferrite nanoparticles is widely used in electric and electronic devices as a magnetic material, and is also applied to the fields of catalysis and gas sensors due to its semiconducting properties[15] Moreover, NiFe2O4 NPs also possess the potential characterization to be a mass-changing type sensing layer Due to its inverse spinel structure (metal – oxygen bonding) and large specific area, the material is appropriate to exhibit physical adsorption ability

By analyzing the benefits of both QCM sensors and nickel ferrite (NiFe2O4)

nanoparticles, a thesis with the topic of “Synthesis and gas adsorption properties of nickel ferrite nanoparticles” was introduced

Research objectives:

• Successfully fabricate the nickel ferrite (NiFe2O4) nanoparticles by hydrothermal and co-precipitation methods

• Investigate the characterization of the synthesized material

• Fabricate the QCM coated with nickel ferrite (NiFe2O4) nanoparticles

• Assess the gas sensing properties of the fabricated sensors

• Carry 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

SO2, NO2, H2S and NH3 using the Quartz Crystal Microbalance oscillator QCM200 and QCM25 controlled by SRSQCM200 software

Thesis outline

The theorical and experimental research in this thesis are reported in a sequent order, specifically:

Chapter 1: Literature review demonstrates the general knowledge, basic

gas sensing principle of the QCM sensors and an overview of NiFe2O4

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 analyzed and compared Then the gas sensing mechanism of the material is proposed Finally,

Conclusion summarizes main findings presented in the thesis and suggests the

future study of the topic

CHAPTER 1 LITERATURE REVIEW 1.1 Introduction of nickel ferrite (NiFe 2 O 4 )

1.1.1 Overview of the structure of ferrites

The spinel ferrite structure MeFe2O4, where Me refers to the metal, can be described as a cubic close-packed arrangement of oxygen atoms, with Me2+ and

Fe3+ at two different crystallographic sites These sites have tetrahedral and octahedral oxygen coordination (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 There are 8 A-sites in which the metal cations are tetrahedrally coordinated with oxygen, and 16 B-sites which possess octahedral coordination When the A-sites are occupied by Me2+

cations and the B-sites are occupied by Fe3+ cations, the ferrite is called a normal spinel If the A-sites are completely occupied by Fe3+ cations and the B-sites are randomly occupied by Me2+ and Fe3+ cations, the structure is referred to as an inverse spinel Ions located at the tetrahedral sites are known as ‘network formers’, while those located at the octahedral sites are known as “network modifiers” [16] In most spinels, the cation distribution possesses an intermediate degree of inversion where both sites contain a fraction of the Me2+ and Fe3+

cations Magnetically, spinel ferrites display ferrimagnetic ordering The magnetic moments of cations in the A and B-sites are aligned parallel with respect to one another Between the A and B-sites the arrangement is antiparallel and as there are twice as many B-sites as A-sites, there is a net moment of spins yielding ferrimagnetic ordering for the crystal The choice of metal cation and the distribution of ions between the A and B-sites therefore, offer a tunable magnetic system [17]

Figure 1.1 Schematic of a partial unit cell and ferrimagnetic ordering of spinel ferrite

structure [44]

According to the distribution of cations, there are the following types of ferrospinels:

• Normal spinel structure, where all Me2+ ions occupy A- sites; structural formula of such ferrites is Me2+[Fe23+]O4 2− This type of distribution takes place in zinc ferrites Zn2+[Fe2+Fe3+]O4 2−

• Inversed spinel structure, where all Me2+ are in B-positions and Fe3+

ions are equally distributed between A and B-sites: structural formula

of these ferrites are Fe3+[Me2+Fe3+]O4 2− Magnetite Fe3O4, ferrites NiFe2O4 and CoFe2O4 have inversed spinel structure

• Mixed spinel structure, when cations Me2+ and Fe3+ occupy both A and B-positions; structural formula of this ferrite is

Me1−δ2+Feδ3+[Meδ2+Fe2−δ3+]O4 2−, where δ is the degree of inversion MnFe2O4 represent this type of structure and has an inversion degree of

δ = 0.2 and its structural formula therefore is

Mn0.82+Fe0.23+[Mn0.22+Fe1.83+]O4 2− Mn–Zn ferrites also have a mixed spinel structure (Zn2+ prefers to occupy A-sites) Znx2+Mny2+Fe1−x−y 3+

[Mn1−x−y2+Fe1+x+y3+]O42−, where δ =1−x−y

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

ferrites and (c) zinc manganese ferrites [18]

Magnetic materials are grouped into two types, soft and hard This is the classification based on their ability to be magnetized and demagnetized, not their ability to withstand penetration or abrasion Soft materials are easy to 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 coercive field strength, independent of magnetic field

amplitude); with permanent magnets however large it is (high coercive field strength) Hard ferrite magnets are made in two different magnetic forms - isotropic and oriented Isotropic magnets are formed to desired shapes, sintered and then magnetized These exhibit a modest magnetic field and find applications

in cycle dynamos and ring magnets Oriented magnets are formed to shape under

a strong magnetic field and then sintered These exhibit a very strong magnetic field and find applications in loudspeakers, magnets of two wheelers like scooters, etc,… [19]

Spinel ferrites have drawn a huge attention 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 (NiFe 2 O 4 )

The first authentic research publication in these ferrites occurred in 1953, when Hastings and Corliss analyzed the chemical and magnetic properties of nickel and zinc 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 Néel model of ferrimagnetism NiFe2O4 crystallizes in the inverse spinel structure (Fd3m) with fcc crystal and lattice constants : α = β = γ = 90°, a = b= c

= 0.834 nm at room temperature [25] The formal chemical formula of Fe3O4 can

be represented as FeTe3+[Fe2+Fe3+]OcO42−, where one-third of Fe occupies the tetrahedral (Te) site in the Fe3+ state, and the remaining Fe2+ and Fe3+ equally occupy the octahedral (Oc) site Analogous to Fe3O4, NFO also carries the inverse spinel structure, where Ni2+ replaces the Fe2+ at the octahedral site in the Fd3m structure The inverse spinel structure of NFO is illustrated in, where the tetrahedral (8b) site is occupied by Fe3+ and the octahedral (16c) site is occupied

by Ni2+ and Fe3+ 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)O 6 octahedra around a FeO 4 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 NiFe2O4

nanoparticles can be a promising candidate to be the gas sensing layer of mass change gas detection sensor like QCM

1.1.3 Fabrication methods

There are variety of methods to synthesize the nickel ferrite nano particles researched by the scientists such as hydrothermal method, co-precipitation method, sol-gel, thermal decomposition, … However, due to their significant advantages, hydrothermal and co-precipitation methods are chosen to be carried 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

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 Rák and D W Brenner [33] presents the fundamental work on the formation of nickel ferrite (NiFe2O4) 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 was discussed

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 NH4OH) 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

[34] The fundamental of piezoelectricity is regarding to the 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

non-of ion due to the change in the dipole moment non-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]

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 piezoelectric effect on the surface Note that, P denotes polarization vector, F is applied

external force [36]

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

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

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, i.e zinc 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]

In terms of different applications, the selection of piezoelectric materials with properties suitable for the purpose is crucial The essential properties of piezoelectric materials consist of frequency stability, negligible deviation of the circuit constants, stable oscillation A quartz crystal resonator is considered a device that meets all the as-mentioned characteristics Firstly, in order to obtain resonance frequency stability, it must be chosen the right quartz slab cutting angle so that the thermodynamic coefficient is zero Second, quartz crystal 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 crystal needs

to be high so that its oscillation is not affected by other properties Conclusion, the electronic devices based on quartz crystals must have long life-time, long-term stability, accuracy, high repeatability and low cost

1.2.2 Quartz crystal microbalance

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

to micrograms level Hence, 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 electrode 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 (i.e, film growth, oxidations and corrosion) In addition to be piezoelectric, quartz also possesses a distinctive properties that make it become promising candidate for ultrasensitive devices Especially, the α-quartz phase can act as a resonator with thermodynamically stable up to 573oC

The quartz discs used in QCMs are almost the AT cut which provides pure thickness shear mode oscillation with two surfaces of the crystal move in an anti-parallel fashion When the corresponding alternating current is taken place to the quartz disc, it undergoes thickness shear mode of oscillation at its resonance frequency f0 This frequency is order of MHz and inversely proportional to the

thickness of crystal As illustrated in Figure 1.8 (C), the induced strain on the

AT-cut under AC voltage AT-cut exhibits good stability at room temperature with small frequency change in the range of 1-3 Hz/ºC

Figure 1.8 The schematic of quartz 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 -phase (low-quartz) via the incommensurate phase at 573ºC to -phase

(high quartz) at 574ºC, as Figure 1.9 A The -and -phase belong to point

group D3 and D6, respectively Among of them, -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 60o from the Y-axis, and Y-cut bar specimens with bar directions of 30, 45, and 60o from X-axis (Figure 1.9B)

There are piezoelectric d-constant d11, d12, and d14 in the X-cut specimens and d25

and d26 for Y-cut [43]

Figure 1.9 The quartz crystal structure (A), AT-cut crystal (B), and crystal structure of

-SiO 2 (c) [44]

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 C0, C1, L1, and R1 are the static capacitance, motional capacitance, motional inductance, and motional resistance for unloaded QCM, respectively While C0 for the quartz crystals is usually of a few pF and its typical

value for the given resonator is specified by the manufacturer [46] In terms of in-liquid QCM, it presents additional parameter such as inductance L2 and resistance R2 were used to approximate the impedance of the viscous loading The inductance L3 was used to approximate the impedance of the mass loading and capacitance Cp was used to present for the parasitic capacitance of the test fixture

Figure 1.10 The Butterworth-Van Dyke (BVD) equivalent circuit for an unloaded

quartz crystal microbalance, QCM under viscous and mass loading (A), and the device

parameter versus frequency characteristic curve (B) [45]

The resonant frequency f of the device depends on the inductance and capacitance of circuit, as following equation:

0

1 1

12

f

L C



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 fs

Figure 1.11 The diagram of Quartz Crystal Microbalance oscillator

Figure 1.12 Crystal Holder components.[47]

The QCM25 consists of an adaptor and RF amplifier to maintain the crystal oscillation frequency of approximately 5MHz During the oscillation of the QCM, the mass change on the electrode will affect the Lm and change the resonant oscillation frequency of the QCM In terms of quartz crystal sensor, the diameter is about 2.54 cm including thin disks, AT-cut, -quartz with circular gold/platinum electrode patterned on both sides The crystal holder consists of a chemically resistant material, an O-ring and a 50 Ohm BNC connector that connects the crystal 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 increase 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

plays a crucial 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, time-consuming, and complex operating which is massive limit for practical application Therefore, the development of devices that exhibit promising characteristic such as compact size, low cost, high precise and reproductivity as well as integration into the smart device for being wearable and flexible has been attracted various studies Sensors mostly consist of 2 main components namely receptor 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, H2S,

SO2, NH3, … due to both chemical and physical interactions through receptor After 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 detect a very low concentration without any response to noise in the environment

1.3.2 QCM sensor working principle

Quartz microbalance sensors 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 in the unloaded mode, its frequency is a constant over time To make the QCM working properly, a sensing layer 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 the QCM changes, which creates 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

mass of gas or material Figure 1.11a shows the typical top and cross-sectional view of QCM chip with a gold electrode 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 detect analytes via the adsorption-desorption process This coating 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)

Figure 1.13 (a) Schematic top view and cross-sectional view of 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]

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

2 02

f

e q q

Nf C

where f0 is the base resonant frequency (Hz), A is the electrode surface area (cm2), q and q are the shear modulus (gcm−1s-2) and density of quartz crystal (g cm-3), 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

considered as vacancies, operating like a positive charge or creating extra unconnected electrons Hence, the adsorption ability will be improved [11] In some situations, chemical adsorption takes place at the same time as physical adsorption In chemisorption, adsorption happen in adsorbed substance that is held by chemical bonds Chemisorption has high specificity that is it 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 often much better than that of the sensors using only physisorption principle However, the recovery rate of them is rather low when measuring in room 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 material This kind of adsorption normally appears in humidity sensors[8], [58]

In this thesis, all experiments are carried out under a dry condition atmosphere or relatively stable humidity environment Thus, Eq 1.2 is used to assess the results The basic gas sensing 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 ratio of resonance frequency shift on a

unit of concentrations

• Selectivity: the ability to respond selectively with a group or a kind of

target gas [59] It can be evaluated by comparing the response of the gas sensor at the same gas concentrations or comparing the sensitivities between a range of concentrations

• Stability: the capability of operating in a period of time with a

competitive repeatability [59]

CHAPTER 2 EXPERIMENT DETAILS

Chapter 2 of the thesis will demonstrate the fabrication processes of the NiFe2O4 nanoparticles in two different methods namely hydrothermal method and coprecipitations In addition, the synthesis of the QCM-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 used in the experiments of this thesis are the commercial products of Xilong Scientific Co., Ltd (Guang Dong, China), which includes: Ni(NO3)2.6H2O, Fe(NO3)3.9H2O, NaOH with the purity of more than 98% Deionized water (DI) is extracted from the ultrahigh water purification system in the ITIMS’s cleanroom

2.1.2 Apparatus

With a view to synthesizing the desired nanostructure materials, basic apparatus in the laboratory used in the experiments can be listed such as beaker, pipettes, funnel, hot plates, magnetic stirrer, pestle, mortar, ultrasonic vibrato and drying oven

2.2 NiFe 2 O 4 nanoparticles fabrication

2.2.1 NiFe 2 O 4 nanoparticles fabrication by hydrothermal method

Based on the ionic equation:

Ni2+ + 2Fe3++ 8OH¯ → NiFe2O4+ 4H2O Eq 2.1

In the hydrothermal method shown in Figure 2.1, 1,4558g Ni(NO3)2.6H2O mixed with 4,04g Fe(NO3)3.9H2O (1:2 molar ratio) are dissolved completely in 50ml deionized water By 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 stirring solution to adjust the pH The process continually happened till the pH reached 8 After that, 45-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 OH- to achieve the pH

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

Figure 2.1 NiFe 2 O 4 NPs hydrothermal method synthesis process

2.2.2 NiFe 2 O 4 nanoparticles fabrication by co-precipitation method

An amount of Ni(NO3)2 and Fe(NO3)3 with molar ratio [Ni2+] : [Fe3+] = 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)3 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

Figure 2.2 NiFe 2 O 4 NPs co precipitation synthesis process

2.3 Characterization methods

2.3.1 X-ray Diffraction (XRD)

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 in 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-Kα radiation According to Bragg’s law the waves are constructively added in specific directions

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

scans across an electron beam It reflects the topographic detail and atomic composition by generating signals with sample interactions SEM can provide several qualitative information of the specimen including its topography, morphology, composition and crystallographic information In other words, it provides information about the surface features and texture, shape, size and arrangement of the particles lying on the sample’s surface [61] The 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 scanning electron microscopy [60]

2.3.3 Transmission electron microscope

The transmission electron 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 operates 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 many orders of magnitude better than that from a light microscope Thus, TEMs can reveal the finest details of internal structure – in some cases as small as individual atoms [62]