Luận văn improving the gas sensing property of wo3 nanomaterials

Selected publications on Ny gas sensors based on WO; 31 Nguyen Hoang Hung l Master Thesis... ‘The principle for gas sensing applications using metal oxide semiconductor based on the ch

NGUYEN HOANG HUNG

IMPROVING THE GAS SENSING PROPERTY OF

WO; NANOMATERIALS

MAJOR: ENGINEERING PHYSICS

MASTER OF SCIENCE THESIS ENGINEERING PHYSICS

SUPERVISOR: Dr DANG DUC VUONG

HANOI - 2012

ee BO GIAO DUC VA BAO TAO ina

TRUONG BAI TIOC BACH KIIOA HA NOL

NGUYEN HOANG HUNG

CUA VAT LIEU NANO WO;

TABLE OF CONTENT

2.1, Characterized features of gas sensors based on metal oxides 4

2.2, Basic scientist approach ccsscssisuisveeseseeseeeetestasinnivanotaseivtnnseet 6

1 Tungsten oxide among metal oxides for gas detection 3

2, Structural propertios oŸ tungsien oxide à occcoseneeorererseooee TỔ

3 Gas sensors based on tungsten oxide eo ¬— 27

Nguyen Hoang Hung ; Master Thesis

1 WO¿ microsheets sựnthesis

3 Gas sensing properties

CIIAPTER 3: RISULT AND DISCUSSION

1 Tungsten trioxides microsheets

2 Tungsten trioxides nanyparticles and doping

LIST OF ABBRIVATION

© HS: Hydrothermal and Solvothermal

¢ XRD: X-ray Diffraction

* SEM: Scanning electron microscope

« FE-SEM: Field mission Scanming Electron Microscopy

¢ EDX or EDS: Energy-dispersive X-ray spectroscopy

« TIM: ‘[ransmission electron microscopy

© CNTs: Carbon nanotubes

© VLS: Vapor-liquid-solid

© PVD: Physical vapor deposition

© CVD: Chemival vapor deposition

« LETS: Low energy ion scattering

* DA: Depletion approximation

Nguyen Hoang Hung it Master Thesis

LIST OF TABLE

‘Table 4, Occupational Lixposure Standards 2000,

'Table 5 3ome properties of NH; kinh tien

Table 6 Requirements for NIh gas detection equipment - 29

Table 7 Selected publications on Ny gas sensors based on WO; 31

Nguyen Hoang Hung l Master Thesis

LIST OF FIGURE

Figure 1 Chemical sensors

Figure 5 Schematic representation of banier foanadien

Figure 6 Three mechanisms of conductance

ure 7 Chemical (a) and electronic (b) sensitization schemes

Figure 9 Prossure/emperature map of inaterial processing techniques

igure 10, Particle processing by conventional and hydrothennal

Figure 11 General purpose pressure autoclave and white Teflon 1

Figure 13 Map of temperature variations in fimace 1

Figure 14 Schematic Mustration of nucleation and growth of ZnO nanorods 1

Figure 19 Comparison of the papers published on gas sensors 2

Figure 20, Schematic model of crystalline W03 in the undistorted cubic phase 1

Figure 21 Structural model of the WO; grain surface - 26

Figure 22 NH3"s structure and symmetry axis

Figure 23 Sore types of armmotiia detector 1

Some of commercialized gas sursors head 1

Figure 25 Schematic diagram of WO; miorosheets synthesis 34

Figure 26 Steps of WO; nanoparticles svnthesis " 35

igure 27 Diagram of heat treatment 1

Figure 28 Electron scattermg and secondary signal generation,

Schematic diagram of an SEM

Pt interdigitated electrodes and heater used in system

Figure 31 Static gas sensing system and principal cireutt

Figure 32 Dynamic gas sensing system,

Nguyen Hoang Hung Master Thesis

Figure 33 SEM images of WOs microsheets vie „48

Figure 34 The XRD pattem and EDX pattem of W0s thin film 4

Figure 36 FESEM images of WO; nanoparticles

Figure 37 SEM images of WO; nanomaterials

Figure 38 EDX partner of #203 nanorods doped WO, 1% wt

Figure 41 Response to NH3 of WOs microsheets at L60°C wae ssssssesssteseneeeee 4

Figure 42 The sensor response of the materials - 48

Figure 43 The sensor response as a function of gas concentration 49

Figure 44 Response to NH3 of WOs nanoparticles at 55 °C

Figure 45 Rosponse to NHy of WO, nanoparticles al 95 °C

Figure 47 Response to NII; of WO nanoparticles at 294 °C 1

Figure 48 The dependence of the sensor response on operating temperature 1

Nguyen Hoang Hung Master Thesis

vi

PREFACE

Nowadays, the pollution level is increasing due to the misuse of chemicals in

industry, agriculuure as well as in life The presence of inflammable gases, toxic gases

that have caused large damage to both people and their property Aims to minimize

the risks as well as industrialization and modemization of industrial processes, it is

necessary to fabricate a kind of environmentally benign devices capable of detecting

gases Since then the term “gas sensor” was bom

TRunng the last decades of the century, the kind of gas se which was best

Tia, SnOz, WOs, are widely used in gus sensing applications to detect toxic gases

‘The principle for gas sensing applications using metal oxide semiconductor based on

the change in resistance of the sensitive layer in presence of gases One of the metal

oxide material promising for semiconductor gas sensor applications was lungster

oxide With many advantages such as high sensitivity, low response time low

operating temperature, tungsten oxide material was gradually bronght to second pk

in the world of gas sensor based on metal oxides semiconductor (after SnO2)

One of the gases that was widely used and caused great impact on human health is

ammonia Recently, ammonia (NH;) is used in many industries, the NH gas leak in

the pipeline has caused serious consequences to health So, in the gases to be detected,

XH; in one of the most concerned gus and sensitive material to detect this gas thal was

emphasized by scientists is WO

Developing in parallel with nanotechnology, WO; is a sensitive materials even at

large sizes, but when the material reach to the size limit, the sensitivity was strongly

improved and appear more interesting properties Currently, there are many roules 1o

synthe

deposition (CVD), physical deposition However, these methods require a rigorous

technological processes and conditions It is difficult to obey in Vietnam science

condition Recently, wet chemical method combined with hydrothermal technology

emerged with many advantages as simple lechnology, inexpensive, nol undemanding

WO) manomaterials such as ball milling, thermal oxidation, chemical vapour

on teclmological process as well as technical conditions Moreover this method allows

mass production and variable morphologies could be synthesized ‘he above

advantages make wet chemical method has been studying and using more and more in

all over the world

Nguyen Hoang Hung vit Master Thesis

In this thesis, the WOs materials are synthesized and measured in NH gas sensor

application The morphological form of the material was deposited by wet chemical

methods combining hydrothermal technology Gas sessing properties of the materials

was improved by reducing in grain size and doping with FeyO, nanorods The thesis

title: “Improving the gus sensing properly of WO, material” was selected and the

Tesults are presented in three mam chapters:

Chapter I Introduction: A short introduction to chemical sensors based on

metal oxides, with a particular emphasis on WOs This chapter also includes the

inctivation, targcts and organization of this investigation

Chapter LL Experimental and methodology: Illustrating the experimental

details used in this work, method to amalyze the structural and morphological

properties of material, a gas effective sensing system was also built in this chapter

Chapter Tl Experimental and methodology: Anning af contributing lo the

understanding of the whole gas sensing process

inal Conclusions and future Outlook are also proposed in this thesis

Nguyen Hoang Hung vili Master Thesis

ACKNOWLEDGMENT

First of all, 1 would like ta thank my advisor, Dr Dang Due Vuong for the

scientific support and the detailed, helpful discussions of the resulis He was not only

dedicated to guide and assist me during the oxperimental process, but also give me

knowledge and the most scientific approaches [esides, he has given me many

valuable ideas as well as point out the shortcomings that helped me complete this

thesis

Tam especially grateful to my colleagues m Department of Flectromie Materials —

School of Engineering Physics, they lave helped me a lols in experiment as well as

always available lo answer me any questions of the basic theory

Finally, | would like to add personal thanks to my family for their love and

supporting to my work, although they have never really known what I was doing

Hanoi, 01" of March , 2012

Nguyen Hoang Hung ix Master Thesis

CHAPTER 1: INTRODUCTION

The purpose of this chapter is to present the general framework where this

investigation is placed Therefore, it will advance from general aspects of chemical

sensors to the more specific details concerning gas sensors based on tungsten oxide

1 Chemical sensors and Gas sensors based on metal oxides

1 Chemical sensors

There are many definitions on

chemical sensors, According to

Göpel and Schierbaum [32] the ¿„ -,.„„ Compe

most simple, understandable and “tise "Soutien

suitable definition on this is:

“Chemical sensors are devices

which convert a chemical state into

an electrical signal” By “chemical

state”, it must be understood different concentrations or partial pressures of molecules

or ions in a gas, liquid or solid phase If it is not specified, it is often assumed that

these chemical sensors are just the primary link of the measuring chain, in other

words, an interface between the chemical world and the electronics

Some typical properties associated with chemical sensors, according to Stetter and

Penrose [81] are:

Vonage Cumeat Resitancr

sosuag IE9Iulet2

Figure 1 Chemical sensors

© asensitive layer is in chemical contact with the analyte

e a change in the chemistry of the sensitive layer (a reaction) is produced after

the exposure to the analyte

the sensitive layer is on a platform that allows transduction of the change to

electric signals

e they are physically “small”

«they operate in real time

¢ they do not necessarily measure a single or simple physical or chemical

property

¢ they are typically less expensive and more convenient than an equivalent

instrument for the same chemical measurements

As stated above, every chemical sensor is divided into two domains: the physical

transducer and the chemical interface layer At the chemical interface, the analyte

interacts chemically with a surface, producing a change in physical/chemical

properties These changes are measured by the transducer domain, which monitors

this change and generates a related electrical signal (Figure 2)

One way to classify chemical sensors is by the transduction mechanism As shown

in [81], the different transduction principles and the magnitudes that can be measured

are:

e Electrochemical: voltages, currents, impedance

© Mechanical: weight, size, shape

¢ Thermal: heat flow, heat content

© Magnetic: field strength, field detection

e Radiant: frequency, intensity

7 Vv

Figure 2 Cross-section of a chemical sensor

Besides, chemical sensors can also be arranged by the chemical reaction that

occurs at the interface This approach is very interesting since chemical parameters,

such as the type of chemical reaction, equilibrium constant, kinetic parameters, will

determine the sensor performance, including sensor sensitivity and selectivity

2 Gas sensors based on metal oxides

Sensors using DC resistance of heated metal oxide semiconductors are members

of the Electrochemical class of chemical sensors, subclass of impedance sensors Gas

sensors based on different metal oxides (SnO2, TiO2, In,O;, WO3) should be identified

as different types within the “class” of electrochemical-impedance sensors and are

often called MOX sensors (from Metal Oxide sensors) In their simplest

configuration, MOX sensors consist of a substrate with a heater, electrodes and a

sensitive layer in contact with the electrodes

A gas sensor based on metal oxide has some advantages that make MOX sensors

appealing for gas sensor users Although highly debatable, some of them are [37]:

Nguyen Hoang Hung Master Thesis

« Low cost, small size and easy to handle (compared to other gas sensors)

Fast sensor response and recovery

* Robust construction and good mechanical strength

« Long operating lite

On the other hand, they have some disadvantages that are still a matter of research

for scientists (some of them being common to most of gas sensors):

* Poor selectivity

Strong influence of humidity

¢ High power consumption (except micromachined-supported)

From the point of view of a user, the sensor response is usually based on the

variation of the DC sensor resistance with gas pressure or concentration, although AC

resistance or lime<derivalive of the resislance can be also measured The typical

ompirical formula describing the variation of conductance (G) willt partial pressure of

the gas (p) for MOX sensors is [68]

Where, Gp and G are conductance before and after expose the target gas, A is a

constanl, § is a factor that was affected by semiconductor type

The relationship between sensor resistance and the concentration of deoxidizing

gas can be expressed by the following equation ever a cerlain range of gas

concentration [68]

where: R, is electrical resistance of the sensor, [C] is gas concentration,

Many other ompirical formulae have been proposed for the detection of certain

gases, to avoid humidity imterference or to compensate the drift of the sensor response

[6].[8],[1 0]

The effect related to changing of electrical resistance of a semiconductor in

presence of nnpurites in tts volume or al the surface was demonstrated for Ge [1 4] for

the first tume in 1953 Later, it was shown that the conductivity of ZnO thin films

heated to ~300°C was sensitive to the presence of traces of reactive gases in the air

[74] Similar properties were reported for SnO,, with higher stability [101] These

results iniliated further development of commer

ial pas sensors The early metal oxide-based sensor materials pos

characteristics, such as high cross-scnsitivity, sensitivity to humidity, long-texm signal

drift and slow sensor response In order to improve sensor performance, a series of

various metal-oxide semiconductors have been tested [60] At first, the poor

sed a number of unpleasant

Nguyen Hoang Hung Master Thesis

understanding of sensor response mechanisms caused the use of trial and error

strategy in the search of an appropriate material The most successful investigations

were connected with SnO,, 7nO, and TiO, Parallel to this approach, the basic

rescarch of metal-oxide materials was carried out in scientific laboratories

211 Sensitivity

Sensitivity is the detection ability of related gas at certain concentration (called the

gas responsibility) On the other hand, Sensitivity is a change of measured signal per

analyte Goncentralion unil, the slope of a calibration graph, Le

In this report, the sensor response was defined as a ratio Rus/

đan:

Where: K,, is the resistance in air

Reo: is the resistance in presence of related gas

In fact, This formula is relative and changeable dependent on target gas The

target gas interacts with the surface of the metal oxide film (generally through surface

adsorbed oxygen ions), which results m a charge in charge carer concentration of

the material ‘his change in charge cartier concentration serves to alter the

conductivity (or resistivity,) of the material An n-type semiconductor is one where

the majority charge carriers are electrons, and upon interaction with a reducing gas an

increase in conductivity occurs Conversely, an oxidizing gas serves to deplete the

sensing layer of charge carrying electrons, resulting in a deerwase in condutivily A

p-type semiconductor is a material that conducts with positive holes being the

majority charge carriers; hence, the opposite effects are observed with the material

and showing an increase in conductivity in the presence of an oxidizing gas (where

the gas has increased the number of positive holes) A resistance increase with a

reducing ges is observed, where the negative charge introduced in ta the matcriat

reduces the positive (hole) charge carrier concentration A summary of the response is

provided in Table | and an example was showed in figure 3

Table 1 Sign of resistimce change to change in gas atmosphere [93]

Classification Oxidizing Gases Reducing Gases

Ttlype Resistance increase Resistance decrease

ptype Resistanee deerease — Resisanee merease

Nguycn Hoang Hung Mastcr Thcsis

Figure 3 An example of resistance change when a reduce gas was introduced

a n-type semiconductor b p-type semiconductor

2.1.2, Response and recovery time

Response time is the time required for sensor to respond to a step concentration

change from initial value to a certain ‘Sonsor response Xs)

concentration value

Recovery time is the time it takes for

the sensor signal to retum to its initial

value after a step concentration change

from a certain value to initial value

In the gas sensor field, the smaller

response and recovery time the higher Figure 4 Response and recovery time

efficiency, Generally, the response and

recovery time could be calculated as the time in that the sensor resistance changes

about 90% of stable value

2.1.3 Selectivity

Selectivity refers to characteristics that determine whether a sensor can respond

selectively to a group of analyte (gas) or even specifically to a single analyte The

appearance of another gas in the environment is not affected to the sensor response

The sensor selectivity is depended on some factors such as: materials, doping,

materials doped concentration and sensor working temperature

For a general MOX sensor, the selectivity is one of their disadvantages because

most of them respond to many analyte The best resolution for this problem concerned

that is doping and fabricating different morphologies of sensitive materials

2.1.4, Stability

Stability is the ability of a sensor to provide reproducible results for a certain

period of time This includes retaining the sensitivity, selectivity, response, and

recovery time

Kabrication methods, heat treatment technology (increasing temperature rate and

stability of temperature) are the main factors that affect to the MOX sensors stability

Besides, il is also strongly depend on the Lamidity (The concentration of vapor in air)

After a long time working in Vietnam chmate condition, a MOX sensors gencrally

lake several days to get their initial stable value

In addition to above feature, there are some important parameters [34], such as:

© Detection limit is the lowest concentration of the analyte thal can be delected

by the sensor under given conditions, patticularly at a given temperature

ion TimL

© Dynamic range is the

valyle concentration range between the de!

and the highest limiting concentration

Linearity is the relative deviation of an experimentally determined calibration

graph [rom an ideal straight Hine

¢ Resolution is the lowest concentration difference that can be distinguished by

sensor

© Working temperature is usually the temperature that comesponds to maximum

semsilivily

¢ Hysteresis is the maximum difference in output when the value is approached

with an increasing and a decreasing analyte concentration range

© Life cycle is the period of time over which the sensor will continuously operate

All of these parameters are used to characterize the properties of a particular

material or device An ideal chemical sensor would possess high sensitivity, dynamic

range, selectivity and stability, low delechon lmuil, good linearity, small hysteresis

and response time: and long life cycle Investigators usually make olTorts to approach

only some of these ideal characteristics, disregarding the others On one hand, this is

because the task of creating an ideal sensor for some gases is extremely difficult, if at

all possible On the other hand, real applications usually do not require sensors with

all perfect characteristics at once For cxample, a sensor device monitoring the

concentration of a component in industrial process docs not need a detection limit at

the ppb level, though the response time at range of seconds or less would be desirable

In case of environmental monitoring applications, when the concentrations of

pollutants nonnally change slowly such as ammonia, the detection limit requirements

ofa few minutes ean be

om be much higher, bul response tin ceptable

2.2 Basic scientist approach

2.2.1 The nature of gas sensitivity in semiconductor metal oxide nanomaterials

Basically, the actual gas sensing process consists of three different parts: receptor,

Nguyen Hoang Hung Master Thesis

transducer and operation mode [79] The receptor is the surface of the metal oxide,

where chemical species undergo adsorption, reaction and desorption Traditionally,

the adsorption of a yas species on a solid has been divided inlo physic-sorplion and

chemsorption Although arguably, a molecule 1s considered to be chemisorbed if

there is an clectronic transfer belween the gas and the solid, whereas the

is no

transfer in the case of physic-sorption Ideally, the imteraction of the gaseous

molecules will induce a change in the depletion layer of the metal oxide grain (see

nexL subsection) These changes are bansduced into an electrical signal depending on

the microstructure of the sensitive film (the transducer) The porosity of the film, the

grain size and the different grain intersections will detennine the output signal, which

takes into account the whole sensitive layer ‘his output signal is usually electric,

although the measurement of the thermo-voltage or of the changes in the sensor

temperature is also possible Excellent reviews providing more details can be found in

[63],[33], Herealter the attention will be focused on the role of oxygen surface

species, the role played by catalytic additives and the microstructure of the sensitive

film

According to Williams and Moseley [13],[31],[63] most target gases are detected

due to their influence on the oxygen stoichiometry of the surface Many studies have

revealed thal the key reachon mvolves modulation of the concentration of surface

oxygen ions ‘The reactions involved in geneiating conductivity changes are reported

to be confined to the first monolayer

The change of electrical properties of the metal-oxide semiconductor due to

adsorption of gas molecules is primarily comected with the chemisorption of oxygen

Molecular oxygen adsorbs on the surface by trapping an electron (ron the conduction

band of the semiconductor At temperatures between 100 and 500°C the ionized

molecular (Oz) and aloe {07 and OF) forms ean be present al the surface [43]

The molecular form dominates below 150°C, whereas above this temperature, ionic

species prevail [7] ‘I'he general reaction equation can be written as:

Here 08" is an oxygen molecule in the ambient atmosphere and e is an electron

that can reach the surface, overcoming the electric field resulting from negative

charging of the surface Their conceutration isn, S denotes unoccupied chemisorption

siles [or oxygen, OfGy is the chemisorbed oxygen with a 1 or 2 for singly or doubly

ionized form and B = 1 ot 2 for atomic or molecular fomn, respectively ‘I'he presence

of charged species on the surface of a semiconductor induces band bending and

Nguyen Hoang Hung Master Thesis

formation of a depletion layer [56] Depending on the type of semiconductor the

concentration of charge carriers in the surface layer can be either increased or

decreased The space charge layer is described by the thickness L, and surface

potential (V,) [47],[11]

For granular metal oxides, the formation of a depletion layer at the surface of

grains and grain boundaries leads to the formation of Schottky barriers between the

oxide crystallites, as depicted in Figure 5 The density of surface oxygen ions and the

height and width of Schottky barriers depend on the oxygen partial pressure in the

surrounding atmosphere The electronic theory of adsorption [95] is in quantitative

agreement with the experimentally observed conductance dependencies of

semiconductor layers on oxygen partial pressure [72]

CXYXXX)

CB

VB

Figure 5 Schematic representation of barrier formation at the grain boundaries due

to the space charge layer

In the figure 5, The shaded part denotes the space charge region (high resistivity);

the un-shaded part denotes the core region(low resistivity) CB and VB are the lowest

edge of the conduction band and the highest edge of the valence band, respectively

Depending on the content of the atmosphere, the concentration of the surface

oxygen ions and therefore the occupation of the surface states can be changed, leading

to the change in conductivity As a measure of gas sensitivity one can use either the

conductivity change of the sample, exposed to the analyte-containing atmosphere in

relation to its conductivity in the reference gas, or the slope of the dependence of

conductivity on analyte concentration [92]

2.2.2 Factors affecting the sensitivity of metal-oxide gas sensor materials

As mentioned above, the requirements for each gas sensor depend on the

particular application It is not necessary to have material with a detection limit of one

molecule if the sensor is designed to work in the 1 — 10% concentration range

Nonetheless, materials with high sensitivity and low detection limit always attract the

attention of scientists and engineers, In this section, the main approaches for

increasing the gas sensitivity of metal-oxide sensor materials are listed, namely those

utilizing the size effects and doping by metal or other metal oxides

and the conductance of the film is limited by Schottky barriers at grain boundaries In

this case, tle sensitivily is pracically independent of D When gram size is

comparable to 2L (D — 2L) every conducting channel in the necks between grains

becomes smtall enough to influence the total conductivity Since the number of necks

is much larger the grain contacts, they govern the conductivity of the material and

define the size-dependence of gas sensitivity If D < 21, every grain is fully involved

in the space charge layer, andl the electron transport is affected by the charge on the

particles surfaces

‘The considered Schottky barmer formation model was developed for the semi-

infinite planar geometry system It can be safely used to describe the barrier formation

in case of large metal-oxide prains Ilowever, for materials with the grain sizes

comparable to the length of the depletion region, the effect of the curvature cannot be

noglee!

‘The shape of a bottom of conductive band for the grains of different size was

studied theoretically Application of the depletion approximation (DA) under

spherical symmetry allowed the calculation of an analytical solution for the potential

Ptr} = Sat where E.y,(1) is the energy of the bottom of of the conductive band at a

since the density of surface stales depends on the grain radius [17,159]

distance r from the center of a grain For a more general case, where DA is not

applicable, the Poisson equation using a complete expression for the charge density

can be solved only numerically The calculated potential shape inside the grains

agrees with the experimentally observed flattening of the band bending for films in air

[S8),59], Thus, by reducing the particle size the conduetion of the sample may be

controlled by the grain boundaries, necks, or grains ‘the latter case is the most

desirable, since it allows achieving the highest resistance change For different

semiconductor oxides the length of depletion layer may vary in the range of 1-100

nm Numerous experimental investigations of nanostructured metal-oxide films

revealed a strong increase im scnsitivily whon the average grain sive was reduced Lo

several nanometers [4311 ],[36],[85],[62] Systematic analysis of size-dependence of

SnQ, sensitivity was presented recently [44],[22

Nguyen Hoang Hung Master Thesis

"CŒXXXX5

Figure 6 Three mechanisms of conductance in metal-oxide gas-sensitive materials

Another prospective approach is to affect the sensitivity by changing the

microstructure and porosity For this purpose the low-temperature vapor co-deposition

of metal and inert gas can be used After removing the gas by annealing, the highly

porous metal structure can be formed Then, metal can be oxidized by reaction with

oxygen, This approach was used for preparation of porous Pb/PbO nanostructures by

co-deposition of Pb vapors with CO; at 80 K followed by annealing [29] SnO) and

TiO, mesoporous powders fabricated using a self-assembly of a surfactant followed

by treatment with phosphoric acid as well as conventional tin oxide powders with

surfaces modified by mesoporous SnO, show higher sensor performance than

corresponding metal oxide powder materials, which have lower specific surface area

[77],{27] Other porous metal oxides also exhibit increased gas sensitivity [75],

[78],{100]

In recent years, a definite trend in using quasi one-dimensional (1D) nano-objects

for gas sensor applications has been observed [19],[20],[23],[42].[48].[70] This is due

partly to expanding opportunities for synthesis and characterization of such structures

[35] Besides, the application of nanowires, nanorods, nanobelts, and nanotubes for

gas sensors can significantly lower the detection limit, since the conductance of 1D

objects is affected by lower amounts of adsorbed analyte than is the case for thin

granular films It was found that SnO; nanowires are sensitive to low CO

concentrations, so the gas sensitivity of SnO) nanobelts (the quasi-1D materials with

defined crystal structure) to polluting gases like CO, NO, and ethanol was tested [19]

An additional increase of sensitivity can be achieved by creating 1D objects with

necks that define the conductivity of the whole nano-object The comparative study of

the sensor response to 0.4 ppm of hydrogen of straight SnO) nanowires with diameter

of ~100 nm and segmented nanowires consisting of thick parts 500 nm in diameter

connected by thin parts 10 nm in diameter [26] was carried out It was found that

response is larger for segmented nanowires, despite the fact that their mean radius is

almost three times larger than that of straight nanowires

li 634

#3gure 7 Chemical (4) and elecwonic (b) sensitication schemes in MOX-doped, gas

sensor [12]

‘The sensitivity of metal-oxide gas sensors can be substantially improved by

dispersing a low concentration of additives, such as Pd [91],[13], Pt [53], Au [98],

[104], Ag [18], Tn [45] and N [64] on oxide surface or in ils volume Although doping

has been used for a long time now in preparation of commercial gas sensors, the

working principle of additive modificd metal oxide materials is still not completely

understood Iwo general schemes of the gas sensing mechanism are depicted in

Figure 7 In the chemical scheme (Fig 7(a)) the reaction takes place at the oxide

surface The role of the additive nangparticles is considered within a spillover process,

increasing the metal oxide surface coverage of the gas, involyed in the sensing

scheme In the electronic mechanism (Fig 7(b)) he reacuon volves dic dopant

atoms, and the oxide material has to transduce the electrochemical changes into a

detectable output signal Moreover, the introduction of additives may lead to the

formation of new donor or acceplor euergy states or influence the gram size and

monoxide [28],[87],[16] It is supposed that the reduction of gas molecules is first

activated hy the metal surface, forming the active surface species that then react via a

spillover process with the charged oxygen molecules, adsorbed ou tin oxide This

reaction leads 10 the re-myjection of the localized electrons back to the bulk, thus

increasing the conductivity of the material For instance, the sensing mechanism

proposed for the PMOX + CO system involves two main processes [28] First, at

elevated temperature, Pt is oxidized by the chemisorbed oxygen

Nguyen Hoang Hung 1 Mastcr Thcsis

Second, exposure to CU leads to a reduction of platinum oxide:

3 Approaches

Tn recent yeas, there are many research group who have tried completing their

research into gas sensors by minimize error or by a systematic research approach, The

goal is to determine a parameter set so that optimized sensors can be manufactured

and applied in practice for fulfilling certain tasks such as monitoring different

substances in different cnvironmental conditions [9] Two of diese parameters will be

briefly described here: Methods for synthesis the sensitive materials and deposition-

preparation the layer on substrate

3.1 Methods for synthesis the sensitive materials

Up to now, there are many successful methods for synthesis of MOX materials

with dilTerent morphologies Some of them are simple, very uscful and suitable in

Vicinamn condition, that make strong attention for scientisis of gas sensor Meld, such

as Sol-gel, Vapor Solid Liquid and hydrothermal

3.1.1 Sol—gel

The sol-gel process is a wet-chemical technique widely used in the fields of

materials science and ceramic engineering Such methods are used primarily for the

labrication of materials (typically metal oxides) starting [rom a colloidal solution (sol)

that acts as the precursor for an integrated network (or gel) of either discrete particles

or network polymers Typical precursors are metal alkoxides and metal salts (such as

chlorides, nitrates and acetates), which undergo various forms of hydrolysis and

polycondensation reactions [40]

Tn this chemical procedure, the “sol” (or solution) A sol is a stable dispersion of

colloidal particles or polymers in a solvent The particles may be amorphous or

crystalline An aerosol is particles in a gas phase, while a sol is particles in a liquid A

gel consists of a three dimensional continuous network, which encloses a liquid phase,

Ina colloidal gel, the network is built from agglomeration of colloidal particles In a

polymer gel the particles have a polymeric sub-struclure made by aggregates of sub-

colloidal particles Generally, the sol particles may interact by van der Waals forces or

hydrogen bonds A gel may also be formed from linking polymer chains In most gel

systems used for materials synthesis, the interactions are of a covalent nature and the

gel process is irreversible The gelation process may be reversible if other interactions

are involved The method has some advantages, such as:

¥ The idea behind sol-gel synthesis is to “dissolve” the compound in a liquid in

Nguyen Hoang Hung 12 Mastcr Thcsis

order to bring it back as a solid in a controlled manner

¥ Multi component compounds may be prepared with a controlled stoichiometry

by mixing sols of different compounds

Y The sol-gel method prevents the problems with co-precipitation, which may be

inhomogeneous, be a gelation reaction,

¥ Enables mixing at an atomic level

¥ Results in small particles, which are easily sinterable

Sol-gel synthesis may be used to prepare materials with a variety of shapes, such as

porous structures, thin fibers, dense powders and thin films

Figure 8 Sol-gel processing options

A sol-gel process could be came from different precursor, in common We can

divided it into three main route, such as:

- Colloid route

In this route, we use metal salts in aqueous solution, pH and temperature control

The salt when dissolved in water will dissociate into ions and the phenomenon of

combination of the ions with water molecules to form complexes will occurs This

hydrolysis process form the single-complexes, After that, the single-complexes

condense with each other to form complex multi-core, also known as colloidal

particles (Equation 7 and 8)

Hydrolysis

M(H,0)§ + [M(H,0),-,0H]@-" + H† Œ) Condensation-polymerization

M(H;O)* © [(H;O);_;M(OH)„M(H,0),,_;]@?=2” + 24* (8)

Salt that was used for this method is usually nitrate (NO),

chloride (Cl) and sulfate (SO,) based salt For synthesizing SnO, nanoparticles, It

often goes from SnCly.5H2O solution reacts with NH,OH, (NHy) 2SO4 or NHyHCO;

[24] For example Group G Saikai, Kyushu University, Japan have used sol-gel

method combining hydrothermal technology from SnCl, in NHyHCO; solution to

form SnO) particles of 6 nm uniform [5] On the other hand, in order to synthesis of a-

FeO; nanorods, the salt is used as Fe(NO3); under the assistance of Na,SO4 The

obtained result is ø-FezO; nanorods like sea-urchim [66] In this report, WO; materials,

the precursor is sodium tungstate (Na,WO,)

- Metal-organic route

The combination of metal cations with organic ligands is used as precursor in this

route The organic ligand consisting of citric acid, carboxylic acid, oleic acid, Phthalic

acid The bonding between the ligands in the complexes is the coordinator bonding

so the binding energy should be smaller than the binding energy of the ion thus the

polarity characteristic decreases It causes the reaction can easily occurs to make high

uniformity and small particle size

- Metal alkoxides route

In addition to above routes, metal alkoxides M(OR), in alcoholic solution and

water addition are also used for sol-gel method Where, M is a metal; (OR) is

alkoxides group and R is usually alkyl groups (R = CHs, C2Hs .) Depending on the

purpose, we choose the different alkoxides based metals The synthesizing of SnO,

materials by this method may come from tin tetra-isopropoxide alkoxide (TTIP)

hydrolyzing in isopropanol, ethanol, methanol, hydroxypropyl cellulose [65] In

addition, we also use the catalyst, nitric acid (HNO3), hydrochloric acid (HCI) for

example, to control the hydrolysis and condensation through the adjustment of pH

For an easily description, for example SnO, materials, sol-gel process occurs

under the following reactions:

(OR),Sn— 0+ (OR},8n—O + (OR),8n—O-Sn—-(OR)3+H,0 (11)

‘This process is repeated, the compound -Sn-O-Sn- linked together to form colloidal

particles During heating and constantly stirring process, the solvent was evaporated,

the colloidal particles will bind to each other Wheu the viscosity of the sal solution is

reducing the gel structure gradually is taken fon In this method, the properties of

Sn0y alkoxides largely depends on the alkoxides concentration iu: solution, the pH of

the solution, water amount, temperature, [24] In general, ‘I's improve stability and

uniformity of the material, we conducted an extra step called hydrothermal

3.1.2 Hydrothermal synthesis

The term ‘hydrothermal’ is of geological origin and has undergone several

changes from the original Greek words ‘hydros’ and ‘thermos’ meaning water and

heat, respectively It was first used by the British geologist, Sir Roderick Murchison

(1792 — 1871), to describe the action of water at elevated temperature and pressure,

leading to the formation of various rocks and minerals in the earth's crust [15]

In chemical nano-world, hydrothermal processing can be defined us any

heterogeneous reaction in an aqueous solvent (or non-aqueous solvent for

solvothermmal processing) under high pressure and temperature conditions, which

induces the dissolution and recrystallization of materials that are relatively insoluble

under ordinary conditions Figure 9 shows a pressure/temperature map of HS in

relation 10 other matenal proces

hydrothermal processing of materials is considered environmentally benign Further,

the hydrothermal technique offers the highly controlled diffusivity of strong solvent

media in a closed system In the context of nanotechnology, the hydrothermal

technique provides an ideal method for producing ‘designer particulates’, i.e mono-

img techniques Tu comparative forms, the

dispersed particles with high purity, high crystallinity and controlled physicochemical

characteristics Such particles are in great demand by industry

Kigure 10 shows the major differences in particle products obtained by ball

milling, sintering firing and hydrothermal methods For example, ball milling involves

breaking down bulk material into small imegular shaped particles, and hence is

considered a crude fabrication method im comparison to the controlled growth provide

by HS ‘Ihe hydrothermal produet particle size can range from a few nanometers up to

several microns, depending on temperature, nucleation seed content, pH and solvent

concentration

The behavior of solvents under hydrothermal canditions allows the development

of crystal structres under sub- and supercritical states (along with pH variations,

viscosity, coefficient of expansion and density, etc.) to be understood in tenns of

varying pressure and temperature Similarly, thermodynamic studies provide valuable

Nguyen Hoang Hung 15 Master Thesis

information on the behavior of solutions with respect to varying pressure and

temperature conditions Some commonly studied aspects are solubility, stability, yield

and dissolution / precipitation reactions, etc However, fundamental understanding of

the kinetics during hydrothermal crystallization is limited This is due to an absence of

data relating to the formation of intermediate phases and the inaccessibility of direct

in situ investigation techniques under conditions of high pressure and temperature

Figure 10 Particle processing by conventional and hydrothermal techniques,

producing irregular shaped particles and “designer particulates”

Nguyen Hoang Hung 16 Master Thesis

Hydrothermal materials processing requires a vessel capable of containing a

highly corrosive solvent, operating under extreme pressure and temperature

conditions The hydrothermal apparatus, commonly known as an autoclave, reactor,

pressure vessel or high pressure bomb, must meet a variety of objectives, processing

conditions and tolerances A generic hydrothermal autoclave should be:

¥ Leak-proof under high pressure/temperature conditions.[15]

¥ Easily assembled/disassembled

¥ Inert to acids, bases and oxidizing agents

¥ Resilient to high pressure and temperature experiments, so that no machining or

treatment is needed after each experimental run

In view of the above requirements, autoclaves are generally fabricated from thick

glass or quartz cylinders and high strength alloys, such as austenitic stainless steel,

iron, nickel, cobalt-based super alloys or titanium and its alloys The primary

parameters to be considered in the selection of a suitable reactor are the experimental

temperature and pressure conditions,

including corrosion resistance in the

pressure/temperature range for a given

solvent Materials processing from

aqueous phosphoric acid media or other

highly corrosive media, ie extreme pH

conditions, require the use of an un-

reactive Teflon lining, as shown in Figure

11 or inert tubes (platinum, gold or silver) Figure 11 General purpose pressure

to protect the autoclave body from @oclave and white Teflon lining used

‘for HS[15]

corrosion

3.1.3 Vapor —Liquid — Solid

The vapor-liquid-solid method (VLS) is a mechanism for the growth of one-

dimensional structures, such as nanowires, from chemical vapor deposition The

growth of a crystal through direct adsorption of a gas phase on to a solid surface is

generally very slow The VLS mechanism circumvents this by introducing a catalytic

liquid alloy phase which can rapidly adsorb a vapor to super-saturation levels, and

from which crystal growth can subsequently occur from nucleated seeds at the liquid—

solid interface The physical characteristics of nanowires grown in this manner

depend, in a controllable way, upon the size and physical properties of the liquid

alloy [41]

INTRODUCTION

For example, to fabricate ZnO nanowires, nanorods, A thin film of gold was

sputter deposited onto Si/SiO) substrates under with the thickness about 10nm The

‘Au islands were formed on the substrates by heating the coated samples in a rapid

ZnO powder

Furnace

Figure 12 VLS synthesis apparatus

(99.99 %) and graphite ZnO powder (99.99 %)

was chosen to be source materials for this

example Equal amounts of ZnO powder and

graphite powder (99.99 %) were mixed well

and placed into a small alumina boat The boat

and Au-coated substrate were placed into a

small quartz tube The substrate was placed

about 2 — 10 cm from the source along the

direction of Ar flow Equal amounts of ZnO

%) were mixed well and placed into a small Fjgure 13 Map of temperature

alumina boat as the source The boat and Au- variations in furnace

coated substrate were placed into a small quartz tube The substrate was placed about

2 — 10 cm from the source along the direction of Ar flow Map of temperature

variations between the boat and substrate are shown in Figure 13 This unit was then

loaded into the greater horizontal quartz tube (4 cm diameter and 110 cm length)

inside the central hot zone of a tube furnace The complete unit was heated at 950 —

11500 C for 30 — 90 min under constant flow of argon gas (~ 20 80 sccm) [38]

Zn, CO, and CO; gases are produced by following chemical reactions (12 and 13),

and transferred to the Au catalyst surface by Ar flow as shown in Figure 14

Zn0, + C, ©> Zn, + CO at ~ 900°C (12)

Zn atoms preferentially adsorb on Au droplet surface due to higher sticking

coefficient of Zn on liquid versus solid Based on low energy ion scattering (LEIS)

measurements, it has been shown that little or no CO, CO) and Oy adsorb on Au

clusters, On the figure 14, Zn atoms condense and attach to the edges of nuclei and

then oxidized by CO/CO,; lateral growth of ZnO nuclei causes the completion of one

zone

Ar-low ếZ“

ZnO nuclel ‘surface diffusion

Figure 14, Schematic illustration of nucleation and growth of

ZnO nanorods by the VLS mechanism [38]

‘As observed in figure 14, we can see the nanorods are grown on Au/SiQ,

substrates The range of nanorods diameter are 40-120 nm In the images, an Au-Zn

alloyed droplets are present at the tips of nanoreds Tlois could be strong evidence that

ZnO nanorods have been grown by the VLS mechanism

This work is one of the bottom up nanometers approaches By changing synthesis

conditions and sourced materials, it is possible to obtain the similar results for other

materials, SnO) for example

3.2 Methods for coating sensitive materials onto the substrates

Table 2 Typical deposition techniques used for the preparation of gas-sensitive

Sputtering Evaporatiun Sol-gel Thenual CVD Šputleing ‘Molecular beam epitaxy

Precipitation Plasma CVD Reactive sputtermg | ‘Thermal evaporation

Screcn printing Laser induced CVD | Cathode sputtcring, | Reactive cvaporation

Dip coaling Electroless plating Ton plating

part in the preparation of gas sensors Three main groups can be distinguished!

powder/slury deposition, chertival vapour deposilion (CVD) and physical vapour

deposition (PVD) [9](able 2) ‘The main difference between powder/slury based

films and CVD or PVD has traditionally been attributed to their different film

thickness While the former lead to sensitive layers of several microns of thickness

(thick films), the layer thickness of the latter varies between 20 and 1000 nm Beyond

ihis classification, there is a fundamental difference in the microstractae of these

thick and thin films Thin films are usually very “compact” (not porous), so the

interaction with gas is limited to the extemal surface of the sensitive layer On the

other hand, gas can penetrate through mast of thick films and so the interaction can

occur throughout the whole layer This has led to some authors argue that thick film

must be more sensitive than dnn films [79], sinee the change of conductivity is not

limited to the outermost zone of the sensitive layer but to the whole layer,

Nevertheless, this classification must be carefully taken For instance, it is well

known that spin-coating techniques, which are actually using a slurry, are able to

obtain “thin films” in the sense of thickness (and the slurry can be obtained by a sol-

Nguyen Hoang Hung Master Thesis

20

gel process, for example), being actually “thick films” in terms of porosity [71] As

regards screen-printing, it must be understood that this technique is a two-step

process: firstly the powder is obtained (by sol-gel, precipitation or any other method)

and then a slurry based on this powder is screen printed

Regarding substrates, thick films have been typically deposited on alumina

substrates provided with electrodes (usually interdigitated) and a heater Thin films

are of course deposited on flatter surfaces, ie silicon, what allows the use of

micromachined gas sensors However, the compatibility between powder technology

and micromachined substrates have been also presented, what opens a new line of low

power-consumption gas sensors with thick sensitive films [79]

3.2.1 Screen printing

Mask

Figure 16 Screen printing technique

Screen printing is a very popular technique for the fabrication of thick film Three

steps of this technique are described in Figure 16 The starting materials are often in

the form of fine powder that was mixed with a suitable solvent to form colloids The

masks had been designed for opening space where need to be covered the materials

The materials were spreads on the surface of the grid by the lever system, which is

then compressed through the chinks of the mask and to be pasted onto the substrate

Finally, the entire substrate is heat treated to stabilize the membrane and remove the

previous organic solvent This technique has the advantage of making the film with

uniform thickness of the membrane, that is exactly the same as thickness of mask

Membrane thickness is usually made from several jum to hundreds of jum

3.2.2 Spin coating

Spin coating has been used for several decades for the application of thin films A

typical process involves depositing a small puddle of a fluid resin onto the center of a

substrate and then spinning the substrate at high speed (typically around 3000 rpm)

Centripetal acceleration will cause the resin to spread to, and eventually off, the edge

of the substrate leaying a thin film of resin on the surface Final film thickness and

other properties will depend on the nature of the resin (viscosity, drying rate, percent

solids, surface tension, etc.) and the parameters chosen for the spin process Factors

such as final rotational speed, acceleration, and fume exhaust contribute to how the

properties of coated films are defined The substrate should be placed on a supporting

plate and is fixed by fitting plate or vacuum system For more detailed, the technique

was shown in figure 17

` solution > rotating =t? đựng > Fmuitilayer structure

2 ay

Figure 17 Spin coating

One of the most important factors in spin coating is repeatability Subtle variations

in the parameters that define the spin process can result in drastic variations in the

coated film

3.2.3 Dip coating

Dip coating is an effective technology of making thin film The process could be

described as follows (figure 18): The

substrate should be fixed to a motor, The

different speed The substrate were t ew

embedded in the membrane solution The |

motor in on low speed model and pull

back the substrate slowly (Figure 18b)

When the substrate go out from the

solution, it will come with a thin layer of

material on the surface (Figure 18c) The

viscosity of the membrane liquid, drag

and rotation speed are adjusted to get the desired film thickness

Figure 18 Dip coating

Il Motivation and objectives

1 Tungsten oxide among metal oxides for gas detection

Since Seiyama and Taguchi used the dependence of the conductivity of ZnO on

the gas present on the atmosphere for gas sensing applications [74], [83], many

different metal oxides have been proposed for gas detection Generally speaking,

these oxides can be divided into binary oxides and more complex oxides, being the

former much more common in gas sensing applications

Among binary metal oxides, tin dioxide (SnO2) is the one that has received by far

more attention since Taguchi built the first tin oxide sensor for Figaro Sensors in 1970

84] This is probably due to its high reactivity to many gaseous species However,

this characteristic has also revealed as a lack of selectivity, and thus investigation on

other metal oxides has been considered necessary

Figure 19 Comparison of the papers published on gas sensors based on ZnO, Fe203,

TiO2, SnO», In;03, and WO3

Besides, developers of electronic noses have experimented with arrays of different

sizes that may include around ten MOX sensors, apart from other types of chemical

sensors The use of different MOX sensors is highly recommended in order to

increase the amount of information Figure 19 displays the number of published

papers belonging to different metal oxides for gas sensing applications It is evident

that tin oxide receives clearly more attention than the rest However, the number of

papers where tungsten oxide is used for gas sensing applications has been increasing

during recent years, leading this material to be the second MOX most studied for gas

sensing applications (The number of papers has been evaluated using the database of

Elsevier Publishing house with a typical search of (WO; OR ftungsien oxide) OR

(tungsten trioxtde}) m topic of gas sensor)

Nguyen Hoang Hung 4 Master Thesis

2 Structural properties of tungsten oxide

Tungsten trioxide exhibits a cubic perovskite-like structure based on the corner

sharing of WOs regular octahedra, with

the O atoms (W atoms) at the corner

(center) of each octahedron [21] (Figure 9 ‘Oxygen

20) The crystal network can also be

viewed as the results of alternating ) tungsten

disposition of O and WO; planes, placed

normally to each main crystallographic

direction This structure is also found in

rhenium trioxide structure (ReO3), from

which takes its common name (ReQ;-

structure) This structure is in itself

rather uncommon However, since it

forms the base of perovskite (one of the most important ternaries), it has in fact chief

importance

Actually, the symmetry of tungsten oxide is lowered from the ideal ReO3

structure by two distortions: tilting of WO, octahedra and displacement of tungsten

the center of its octahedron [97] Variations in the details of these distortions give rise

to several phase transitions In fact tungsten trioxide adopts at least five distinct

crystallographic modifications between absolute zero and its melting point at 1700 °K

When the temperature is decreased from the melting point, the crystallographic

symmetry for WO; changes in the sequence: tetragonal — orthorhombic — monoclinic

~ triclinic — monoclinic Most of the transitions appear to be first order, and they often

display large hysteresis in the transition temperatures A summary of these transitions

is given in Table 3 [29],[52] It is interesting to notice that, as suggested by Table 3

and confirmed experimentally in [52], the coexistence of triclinic and monoclinic

phases in WO; at room temperature is common

Another point worth noting is that the tungsten trioxide structure is likely to host

several kinds of defects One of the most elementary defects, as in most metal oxides,

is the lattice oxygen vacancy, where an oxygen atom is absent from a normal lattice

site In many d° oxides of Ti, V, Nb, Mo and W this sort of point defects are largely

eliminated by the formation of crystallographic shear phases In the case of WO3, the

removal of oxygen causes the appearance of these crystallographic shear planes into

Figure 20 Schematic model of crystalline

WO; in the undistorted cubic phase

the crystal along the [1m0] direction [49] This leads to the formation of a family of

WO} compounds From an electronic point of view, an oxygen vacancy causes the

increase of the electronic density on the metallic (W) adjacent cations, leading to the

formation of donor-like states slightly below the edge of the conduction band of the

oxide, which acquires semiconducting properties [29] Finally, it is important to point

out that in this work, superficial properties of tungsten oxide are of paramount

importance, since that is where gas interaction occurs This important point is

sometimes overlooked in many papers concerning MOX gas sensors, where bulk

properties are extensively reported and little attention is paid at the surface of the

material

Table 3 Known polymorphs of tungsten trioxide (Adapted from [52])

ø-WO; | Tetragonal 1010-1170

ÿ-WO; | Orhorhombic | 600-1170 y-WO; | Monoclinic 290-600 6-WO; | Trichinic 230-290

&-WO; | Monoclinic 0-230

Figure 21 Structural model of the WO; grain surface Left panel: idealized WO;

structure with the (100) fracture planes shown Right panel: two possible states of the

grains surface: in both cases the formation of the reduced tungsten ions W** is

required by the neutrality condition (Adapted from [50])

The way to progress from the above-explained crystalline structure and the

surface has been proposed by Kuzmin et al [50] If a crack along the (100)