Luận văn thạc sĩ synthesis of one dimensional sno2 nanostructure via hydrothermal method for gas sensor application

Syathesis of one-dimensional SwO; nanostructure via hydrothermal method for gas « SEM: Scanning electron microscope « FE-SEM Field Einission Scanning Electron Microscopy « EDX or EDS: Bn

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METHOD FOR GAS SENSOR APPLICATION

MAJOR: ENGINEERING PHYSICS

MASTER OF SCIENCE THESIS ENGINEERING PHYSICS

SUPERVISOR: Dr DANG DUC VUONG

HANOI - 2011

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CHE TAO VAT LIEU NANO SnO; CÁU TRÚC

MOT CHIEU UNG DUNG LAM CAM BIEN KHi

CHUYEN NGANH: VAT LY KỸ THUẬT

LUẬN VĂN THAC SI KHOA HOC

VẶT LÝ KỸ THUẬT

HƯỚNG DẪN KIIOA THỌC: TS BANG ĐỨC VƯỢNG

HÀ NỘI -2011

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ACKNOWLEDGEMENTS

Furst, it’s pleasure to send my sincere thanks to teachers and brothers at

Llectronic Materials Department, School of Engineering Physics about the supports

during my study and research

Fapecially, T want to give my thank Dr Dang Due Vuong, who enthusiastically guided me throughout the course to complete this thesis { also thank the members of the sensor group by the valuable suggestions and enthusiastic support during the

implermentation of the Disserlation:

Finally, during the course, T also received many supports and instructions of internal as well as external laboratories, such as Structural Analysis Laboratory - School of Engineering Physics and Electron Microscopy Laboratory - National

Institute of Hygiene and Fpidemiology Again T sincerely thank about 1

Lanci, Fuly 8% 2011

Author

Vu Xuan Hien

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GUARANTEE

I hereby declare that this is my own research, Results described in the

thesis are true and never published in any works before

Author

Vu Xuan [Een

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SUMMARY OF THESIS

Th

te: “Synthesis of one-dimensional SnO, nanostructure via hydrothermal

method for gas sensor application”

Supervisor: Dr Dang Duc Vuong

Content

SnO, is a special n-type metal oxide semiconductor which possesses many feature properties, namely low cost, high gas sensitivity, good chemical resistance and fast electron mobiliy Therefore, many scientists have paid special allention to this material, especially in Nano-scale Many studies in preparing different morphologies

of SnO, by various methods have been successfully carried out Today, the rod-like

structure of tin dioxide nanomaterial has been attracting many concerns of scicnitisis

because of its huge potential application in manufacturing gas sensor, optical devices, dye-sensitized solar cell and transparent conducting electrodes Among numerous methods, hydrothermal treakuenl is an interesting approach which can support mass synthesize nanomaterials uniformly and cheaply at low temperature ‘Therefore it has been widely choosing for synthesizing SnO, nanorods, recently

Purpose of this work is to investigate the oplimum process of synthesizing SnOz nanorods under hydrothermal treatment as well as propose a possible formation mechanism for the growth Also, the thesis studies about LPG and ethanal sensing properties of SnO, nanorods in order {o orientate the use of SO, nanomaterial in gas sensor application

The thesis focuses on synthesiving SnO, wanorods in powder form using hydrothermal method The material is characterized by FE-SEM, TEM, XRD to investigate the structure and morphology In addition, the study about LPG and ethanol

sensing properties of as-prepared materiel is also conducted.

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Summary: Base on the experimental results, key conclusions of the thesis can be described as follow:

« - SnO; nanerods (5-7 nm diameters and 10-30 nm lengths), nanoflowers (Constructed of nanorods with the diameter is from 40 to approximately 200 nm and the length is from 150 mm to roughly 1 jan) and smicrospheres (irom several nanometers to approximately 2 micrometers diameter) were successfully synthesized

by a low temperature (below 200 °C) process using hydrothermal method Also, the striking feature of the process comes from the uniform and high density all over the powders of the as-prepared materials

In the formation period, hypothetically, the spheres were made up by the isotropic aggregation (in case the hydrothermal temperature, “I” is below 190 °C or

hydrothermal time “1” is under 20 hours) of SnO, crystals, nucle: and clusters, whercas

190°C

the rods and flowers were together constructed via crystallization process (1

and t > 20 hours) The XRD, FI-SEM and TEM results indicate that the anisotropic of SnQ; Mowers is higher hau nanorods and is followed by SuOz microspheres This may result in the better selectivity toward ethanol of SnO, nanoflowers while comparing to other morphologies In addition, the gas testing result again proves the enhancement of LPG and cthanol sensing properties for SnOz nanorods to nanoparticles,

‘Main results of the thesis have been used to write a paper entitled “Sputhesis of SnO; micro-spheres, nano-rods and nano-flowers via simple hydrothermal route”

which has been accepted by Physica FE

kà

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Syathesis of one-dimensional SwO; nanostructure via hydrothermal method for gas

sensor application

CONTENT LIST OF ABBREVIATIONS .cescsesssstsssesssnteeestateietsniatiateeieenseeesee EDL LIST OE TAILU8 kiHư HH HH hư ri re KmeeererrooWV

1.INTRODUCTION s22 222 cerrriiee kererererreroaf

1 SnO, material and its applications -

1.3.3 Solid state gas sensQr nen

2.3.3 Chemical vapor deposition cecseererrrrrrar.TR

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2 Tufluence of technical parameters - - - 32

4, Characterization techniques of SnO; nanomaterial 33

1 Prepare SnO nanorods in large scale - - 39

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« SEM: Scanning electron microscope

« FE-SEM Field Einission Scanning Electron Microscopy

« EDX or EDS: Bnergy-dispersive X-ray spectroscopy

« TEM: Transmission electron microscopy

« AFM: Atomic force microscopy

«+ 1COs: Transparent conducting films

«+ CNIs: Carbon nanotubes

«VIS: Vaportiquid-solid

« PVD: Physical vapor deposition

«CVD: Chemical vapor deposition

« APCVD: Atmospheric pressure CVD

¢ LPCVD: Low-pressure CVD

¢ UHVCVD: Uluahigh vacuum CVD

« AACVD: Acrosol assisted CVD

« DLICVD: Direct liquid injection CVD

« =MPCVD: Microwave plasma-assisted CVD

«+ PUCVD: Plasma-linhaneed CVD

«RPECVD: Remote plasma-cohanced CVD

« ALCVD: Atomic layer CVD

* CCV: Combustion Chemical Vapor Deposition

« IIWCVD: Hotwire CVD

« MOCVD: Metal-organic chemical vapor deposition

« HPCVD, Hybrid Physical-Chemical Vapor Deposition

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sensor application

« RTCVD: Rapid thermal CVD

« VPE: Vapor phase Epitaxy

«KE Radio Frequency

«PAA: Porous Anodic Membmme

«© PLD: Pulsed Laser Deposition

« CTAB; Cetylirimethyl Ammonium Bromide

« PUG: Polyethylene Glycol

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sensor application

LIST OF TABLES Table 1, Physical propertios of tin dioxide, sesjctensnseen cine „3 Table 2, Lixperimental parameters for synthesizing SnÓ¿ nanerods 3] Table 3, Experimental parameters lor sludying the maternal formation 32

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sensor application

LIST OF FIGURES Figure 1 SnQz Rutile structure, wees sssssestssesvientenetstneeieenosaeiatiiasiveensneeied igure 2 XRD pattern of SnQp .ssessasieensstasesenenstnesieene 3 Figure 3 Applications of $nO, as transparent, conduclor and coated layer 5 Vigure 4, Reduction and re-oxidation in the Mars-van-Krevelen mechanism 6 Figure 5 Solid state gas sensor wc sesssessentssersineesenssneeineee 8 Figure 6 Structure of bulk and film types gas sensor based on SnO; materiaL 9 Vigure 7 Energy model for SiOz sssssectssesneneeneenineeneenenaeiateiaeiieenenee

Figure 8 Schottky barrier between gain boundaries - - u Figure 9 Number of papers related lo SnO, multianorphology 13 igure 10, Number of paper related to SnO; nanorods and naaowires 14 Figure 11 Comparison between SnO, nanoparticles and nanorods 15 Figure 12 Apparatus sysiem for VIS synthesis - - 32 igure 13 SHÀI and THM imaees of SnO; nanowires grown by VL8 method [52]

33 Figure 14, Process of Syuthesiving SnQ2 nanorods via template route 24

Vigure 15, SnO, nanowires derived from template route associated with chemical

Figure 16 Diagram of high-pressure pulsed laser systcm 25 Vigure 17 SnO, nanorods synthesized by pulsed laser beam [S3] _—- Tigure 18 Process of synthesizing SnO; nanorods via molten salt method 27 Vigure 19 TEM image of SnO; nanorods synthesized by thermal decomposition [63] (a) and molten salt method [59] (b) sec ¬ Figure 20 SEM images of SnO; nanorods đerived by hydrothermal method with CTAB support [341 à ác ớt rederrareiereesoiuR Tignre 21 SEM images of SnO; nanorods đerived by hydrothermal method with

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Figure 27 Static gas sensing system (a) and principal circuit (b) 7 Figure 28, FE-SEM images o[ 8n; sumnplos Seseoeo 39 Figure 29 VH-SEM images of SnO; nanomaterial at diserete pois of

Figure 36 fnflusnce of sensor response to operating temperalure for microspheres,

nanorods and nanoflowers at 780 ppm C;H;OH and 10000 ppm LPG 48 Figure 37 Influence of sensor response to gases concentration for microspheres,

Figure 38 influence of sensor response to gases concentration for nanorods at

Figure 39 Response-recovery curves of SnOz nanorods, nanoflowers and

xnlerospheres to LƠ at 370 ”C cccieieeiiererrree HrrerderrreioÐE

Figure 41 Process of depositing SnQ2 nanoparticles on a substrale 55

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different morphologies of SnO, by various methods have successfully been

conducted Today, the rod-like structure of tin dioxide nanomaterial has been attracting many concerns of scientists because of its huge potential application in

manufacturing gas sensor, optical devices, dye-seusilzzed solar cell and transparent

conducting clectrodes VLS (vapor-liquid-solid), molten salt and hydrothermal treatment are well-known methods in synthesizing 1-0 nanostructures (nanorods,

nanowires and nanotubes, clc.) In such of those methods, VS is a rare method

which allows 1-D structure to be grown at definite positions, but in small quantity and low uniform Ifydrothermal treatment, inversely, can mass synthesize variety

morphologies of nanomaterials uniformly and cheaply at low temperature

‘Therefore it has been choosing for synthesizing SnO, nanorods, recently

Thesis named “Synthesis of one-dimensional Sn, nanostructure via

hydrothermal method for gas sensor application” has been chosen since the need

of studying and applying SnO, nanorods and nanowires among scientists and engineers has been widely urged Tn this work, the nO, nanorads were riol only successfully prepared, the SnO, microsphere and nanoflowers were also

accidentally synthesized Interestingly, the formation relationship of such

morphology has been introduced and discussed The striking feature of this study comes from the gas sensing results Indeed, Sn, nanorods introduced better LPG and ethanol vapor sensing than nanoparticles whereas SnO; nanoflowers were

setectively sensed with chanol vapor

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Synthesis of one-dimensional SnO} nanostructure via hydrothermal method for gas sensor application

and flowers of tin Cassiterite which is also called tinstone is heavy, metallic and a

major mineral form of SnO} Generally, SnO, is the most important raw material in

tin chemistry It crystallizes with the rutile structure (the symmetry space group is

P4/nmm and the lattice constants are a = b = 3.8029 A and e = 4.8382 Á [44]),

wherein the tin atoms (six coordinate) connect with the oxygen atoms (three

coordinate) by strong ionic bonds Figure 1 illustrates the images of Cassiterite ore,

SnO, powder as well as Rutile structure of SnO;

1.1 Physical and chemical properties

SnO, is usually regarded as an oxygen-deficient n-type semiconductor

Hydrous forms of SnO, have been described in the past as stannic acids, although

such materials appear to be hydrated particles of SnO, where the composition

reflects the particle size

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Synthesis of one-dimensional SnO, nanostructure via hydrothermal method for gas

In nature, it is hard to find the stoichiometry structure of SnO; because its

structure often has oxygen vacancies which lead the material to be an n-type semiconductor Tin dioxide is also known as a transparent material to visible light

because the band gap 3.6 eV is too high to allow SnO, absorb energy of the visible

Figure 2 XRD pattern of SnO>

The structure of SnO;, tetragonal structure which is already known as a strong

structure can be characterized by XRD method Figure 2 indicates the XRD pattern

of SnO) tetragonal structure Typically, the pattern has the strongest peak at 20 =

26.54° and followed important peaks are at 2@ = 33,7° and at 20 = 51,7°

corresponding to (110), (101) and (211) faces, respectively For these bulk

terminated SnO) surfaces, for example surfaces with surface-tin atoms in their bulk Sn“ oxidation state, the (110) surface exhibits the lowest energy surface followed

by the (100), (101) and (001) surfaces

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SnO; + 6 1 > Lb Snlg +2 1h0 a Similarly, Sn, dissolves in sulfuric avid lo give the sulfale

SnQ; + 2 Hạ5Ou — Sn(SO/); + 2 HO @) SnO) dissolves in strong base to give “Stannates”, with the nominal formula Na,Su0s Dissolving the solidified $uO,/NuOH melt in water gives “preparing salt” (Na;[Sn(OH%];) which is used in the dyeing industry

1.2 Typical applications

It is well known that tin dioxide have its place in a class of metal

semiconductor materials, and because of its high electrical conductivity with optical

transparency, SnQz has constituled numerous components for oploclectronic

applications Besides, oxides, in general, are not only used as support materials for

dispersed metal catalysts but also expose calalytical characterislic by thernselves

Morcover, many oxides included tin dioxide reveal sensitivity towards oxidizing and reducing gases by an aberration of their electrical properties In short, there are

thres remarkable applications of tin dioxide thal excited a huge number of scientists

in general research, namely transparent conductor, oxidation catalyst and solid state

gas sensor

1.21 Transparent conductor

Tin dioxide, an n-type semiconductor with approximately 3.6 eV band-gap, tas low electrical resistance with high optical wansparency in the visible range of clectromagnetic spectrum Hence, the oxide material has beon applied strongly in numerous areas, such as electrode materials in solar cells [5], light emitting diode

(LED) [65], fla pancl displays [43] and other oploclectronic devices im which

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Synthesis of one-dimensional SnO» nanostructure via hydrothermal method for gas sensor application

obstructing photons from either arriving or escaping the optical active areas Also,

another interesting characteristic of tin dioxide and other TCOs is an ability of

reflecting infrared light although they are transparent to visible light [47] As a

consequently, SnO) has been, therefore, used as an energy conserving material Indeed, SnO2-coated windows or other parts of a building, for instance, allow the

light come through without absorbing the heat from the sun ray Recently, more

moder architectural windows called smart windows based on electrically contact

electro-chromic films that are able to change their color and transparency by

applying a voltage across the films [16, 33]

Electrode layer

Figure 3 Applications oƒ SnO› as transparent conductor and coated layer

According to intrinsic defects, n-type TCOs generally have high conductivity

Tin dioxide, a typical example, which is a wide ban-gap semiconductor, is a good

insulator in its stoichiometric form Inversely, in non-stoichiometry, where there is deficiency of oxygen in its structure, SnO) represent a conductor In 2002, Kilig and

Zanger [23] proclaimed that the formation energy between oxygen vacancies and

tin interstitials in SnO, is very low, hence leading these defects to form freely This

outcome is worth to explain why pure SnO) often appear in non-stoichiometry and

possess high conductivity In additions, all applications, where these materials are

employed require better conductivity, so doping with extrinsic additives is a

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frequent work Tin dioxide, for example, often has Sb doped as a cation dopant or F

doped as anion dopant [14]

It should be pointed out that, the most common TCOs like SnOp, ZnO, In,O3 (known as ITO if doped with Sn), Ga,O; and CdO [6, 8] are n-type materials In

order to enlarge the kind of TCOs, scientists now are making their motivation on p-

type conducting TCOs [21] because dissimilar materials may possess valuable

properties for different applications

A lot of oxides mostly act as a support material for dispersed metal catalysts

Tin-oxides, however, by themselves are an oxidation catalyst which their oxidation

reactions are supported to follow the Mars-van Krevelen mechanism [4] In this

mechanism, adsorbed molecules (oxidizing, reducing gas or other molecules) are

oxidized by consuming lattice oxygen of the oxide catalysts which in turn is re- oxidized by gas-phase oxygen occurred in outer environment This may be the

consequence of the multivalent oxidation states which transition and post-transition

oxides have, so that it allows the materials to easily give up lattice oxygen to react with adsorbed molecules and subsequently re-oxidized by oxygen Figure 4 is an

example of heterogeneous catalytic reaction in which the surface of the VO,

catalyst reacts with propane to form propene and water [60]

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sensor application

Due to the Mars-van Krevelen mechanism, tin-oxides expose the same behavior to absorbed molecules, thus there is, normally, no selectivity for such kind

of catalyst Nevertheless, by combination of hetero-elements, the aclivily and

selectivity of tin-oxide catalysts can be substantially umproved For cxample, with the copper [7], palladium [51] and chromium [18] added, the catalysts will have their beller oxidation to carbon monoxide and hydrocarbons Although, a summary

of reaction catalyzed by pure and modified SuO, was given by Harrison [19], the role of several additives has not been fully understood since most of the additives are oxidized during operalion of Lhe catalysts Special active siles may be stabilized

at the interface between the additive and SuQ, For example, it was suggested that

Mo*" sites which allocated between Mo(), catalyst and SnO, play and important

role in methanol and ethanol oxidation [28] Bestdes, in lhe most case, the addilives

form clusters support on SnQ, surface Antimony, for instance, is proposed that antimony oxide forms a solid solution with SnQ, or Sb"! surface species may be the

active silcs

Oxidation of organic molecules over oxide catalysts is processed by the consumption of lattice oxygen in the oxidation reaction which is followed by the re- oxidalion of the ealalysls Thus Uhere are two adsorbed redox couples involved the oxidation cycle:

In the first stage, electrons from redox couple are injected into the oxide as the redox couple of the adsorbed molecules is located above the Fermi level and above the bottom of the conduction band The re-oxidation of the catalyst is occurred in the next stage in which electrons are extracted from the oxide to activate adsorbed oxygen [17]

Although many catalysis bave been characterized, il has been challenging to find a fundamental description and fully understanding the additives roles because

of the dynamic change of the catalysts during operation

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3 Solid state gas sensor

Figure 5 Solid state gas sensor

Gas sensing materials are some special materials which have ability of changing their properties in ambient gas Normally, they often have their change in

the electrical conductance (or resistance) while exposing to environmental gases

There are a large number of metal oxides which are suitable for detecting

combustible, reducing and oxidizing gases, namely Cr,03, Mn,O3, Co;Oa, NiO,

CuO, CdO, MgO, SrO, BaO, In,O;, WO¿, TiO;, V;O;, Fe;O;, GeO,, Nb,Os, MoO3,

Ta,Os, La,O3, CeO,, Nd,O; Nonetheless, the most common used materials for gas

sensing purpose till now are ZnO and SnO¿

It is generally known that the phenomenon of varying electrical conductance

of semiconductors due to the ambient atmosphere was investigated in 1953-1954

[3], but until, Seiyama and co-workers applied this knowledge to gas detection

(1962) [49] The most significant contribution into the development of this

technology was provided by Japanese scientists and engineers including Naoyoshi Taguchi [50] who investigated a series of ceramic sensors called Taguchi Gas

Sensors (TGS) not long (about ten years) after the work of Seiyama He established

a company named Figaro which has been delivering annually millions of sensors

Even so, the urge of scientists to studying in this major is not disappeared because the “three S” features (Sensitivity, Selectivity and Stability) exposing in most of gas

sensors are still imperfect

For SnO, gas sensors, two typical categories which are now utilizing widely

are bulk-type and film-type The bulk-type gas sensor (or can also be known as TGS) was the first generation with the structure is illustrate in figure 6 As for the

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figure, the sensing material is coated uniformly on the ceramic tube where two Pt

electrodes are placed on its outer surface and a heating resistance coil is inserted

into the tube to control the working temperature The upsides for this type are low cost and good endurance while working in harsh surrounds Recently, along with

the advance of the micro-electronic technology and Nano-technology, thin and thick

films gas sensors have been studying and manufacturing which was given a hope of

improving the quality of gas sensors

Figure 6 Structure of bulk and film types gas sensor based on SnO) material

The structure of this innovative type is quite simple which has a thick or thin

film of sensitive nanomaterials coated on a Pt or Au comb-like electrode (created by

sputtering Pt or Au onto a Si or Al,O; substrate) as in figure 6 Undeniably, smaller

size and lower power consumption are two striking features of this type which are

potential for portable devices Although SnO, is the best choice among other oxides

for gas sensing applications, low selectivity and sensitivity are two inherent

characteristics of this material However, many studies proclaim that SnO) sensor selectivity can be fine-tuned over a wide range by applying different SnO,

morphology, dopants, contact geometries as well as mode of operations, ete [1]

Still now, the exact fundamental of SnO, sensing mechanism are debated, nonetheless, the trapping of electrons at adsorbed molecules as well as band

bending induced by these charged molecules are responsible for the transformation

in material conductivity, and then becoming the most reliable sensing mechanism

for this material As for Göpel [15] and Madouand Morrison [38], charge transfer in

chemisorption is taken place when the electrons of SnO, surface can be transferred

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sensor application

to adsorbed molecule This is happen while the lowest lying unoccupied molecular

orbitals of the adsorbate complex stay below the Fermi level (acceptor level) of

SnO) Inversely, donor level is established whilst the highest occupied molecules stay above the Fermi level of the material Therefore, the molecular adsorption may

create a net change at the SnO, surface causing an electrostatic field This

consequence leads the energy band of the solid bending, in which the negative

surface charge bends the band upward and pushes the Fermi level into the band gap

Figure 7 Energy model for SnO»

Finally, an electron depletion zone is formed due to the dropping in charge

carrier concentration The surface depletion layer can be expressed by the Debye

length, which is defined as:

where e, is the static dielectric constant, n, is the total carrier concentration, e ¡s the

carrier charge, K is the Boltzmann constant, and T is the absolute temperature

Maximum sensitivity is achieved whenever the Debye length is about half the

particle size

In polycrystalline SnO, material, Schottky barriers are formatted among grain boundaries as illustrate in figure 8 [39] Apparently, electrons which transport from

negative to positive electrode have to pass through the upward band bending or the

Schottky barrier For pure SnO,, charges carriers can only be generated and

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Conduction band Fermi level ==

across grain boundaries

Figure 8 Schottky barrier between gain boundaries

When gases contact and react to the SnO, surface, two different phenomena

occur naturally Reducing gases will replace the adsorbed oxygen at the surface of

SnO, by other molecules and then reverse the band bending, thus increase the conductivity of the material, Oxidizing gases, inversely, tum the conductivity of

SnO; to higher value Several reactions between target gas and oxygen anion on the

SnO) surface can be illustrated as follow:

CH;CH,OH,,, + Oa CHyCHO,4 + H,0 + &° (6) CH¡CHO, + 4O”, + CO + CO) +H,0 + 4e” (7) CyHg + 100° a> 3CO2 + 4H,O + 1007 (8)

iH + 1307 a> 4CO, + SH,O + 13e7 (9)

By monitoring the variation of SnO, conductivity when exposing to gases, the

appearance or even the concentration of those gases can be evaluated

2 Tin dioxide in the Nano-world

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sensor application

technology have been establishing its own place in many areas and applications, since they were bom The accidental discovery of Bucky ball (1985) and carbon-

tianotubes (CNTs) (1991) seemed lo establish a new era of advanced technology in

which scientists expooted nanomaterials as auspicious future materials A mumerous applications related to nanomaterials have taken place because of their excellent

features like smaller, more interesting characteristics and more stable Nowadays,

anost of electronic devices like computer, mobile phone, I'V, regulator, etc are achievements of applying Nano-technology and nanomaterials Moreover, thanks to Nano-technology, numerous medical discoveries are helping palionts out of (heir paint and disease Growing with a break neck speed, Nano-science and its branches

have been standing steady on almost industrial areas in the modern world Thus, the

yequirements of sludying and expanding nanotmalertals are urgent need for

development

The difference between the bulk-like materials and nanomaterials comes from

the quantum offeet occasionally appearing in several nanomaterials The effcot leads

to expand the band gap of nanomaterials comparing with the same materials in

bulk-size In some nanomaterials like SiO, or ZnO (with condition that the particle

sives are smaller dian their Debye lengths), for example, terms like “ireel gap” or

“indirect gap” are not exist ‘herefore, it has been interestingly found that those

aalenals have luminescent characleristic which is never happen in the same bulk-

kind materials In addition, an mportant feature of nanomaterials is the immensely

increased the ratio of surface area to volume while comparing to bulk-like materials

and thus making botler characteristics for calalyst-role materials Morcever, il

should be noted that the dimension of nanomaterials is not only the factor affecting

their characteristics; morphology is another important aspect Clearly, Fullerene,

CNTs (carbon nano tubes) and Graphene are three typical type of carbon in

nanoscale, nevertheless, the behaviors of each type are different If Fullerene has

restricted itself in small number of applications (mainly as superconducting material

with Cs and Rb doped), CNTs with the superior mechanical properties are projected

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to be utilized widely in everyday items like clothes, sports gear to combat jackets

and space elevators [46], transistors in electrical circuits [24], paper batteries [11],

solar cell [9], supercapacitors [45], tip for AFM (atomic force microscopy) [61], etc In 2004, the discovery of Graphene by physicists from University of

Manchester and Institute for Microelectronics Technology, Chernogolovka, Russia

has given a new birth of super conducting material in normal temperature

comparing with other conductors This fantastic invention leads to the prospect in

near future that the semi-metal will be utilized instead of gold (in the role of being the conducting wires for microchips and microprocessors) in order to increase the

transfer rate as well as reduce noise and heat in a single chip

Figure 9 Number of papers related to SnO» multi-morphology

With regard to tin dioxide, by reducing its size to nanoscale, the surface-to-

volume ratio is consequently amplified enhancing the catalytic reactions in a definite area This upside feature lead to the possibility of making strong catalyst

material as well as high sensitive gas sensor It is well known that the optimum dimension of SnO) for sensitivity is 6 nm (twofold the SnO, Debye length which is

approximately 3.07 nm at 293 K) [13] However, almost SnO} gas sensors have

been fabricated with the condition that the SnOQ) particles sizes are above 15 nm to

surge the sensor stability because below 15 nm, the sensor resistance depends

remarkably on temperature [64] Along with many studies of reducing the particle size, recently, many attempts of preparing different morphologies of SnO, have

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sensor application

been carried out, namely nano-wires [25], nano-tubes [36], nano-belts [67], nano-

ribbon [31], nano-rods [48, 62, 35, 68, 58] and nano-flower [66, 34, 10] In general,

a tendency of studying nanomaterials has been established in which assembling new morphology has been received a special concem of scientists

By making small survey on website “http:/Avww sciencedirect.com/” (a huge

resources of sciential and technological journals), a bar chart of papers owning to

Sn) nanostructures has been drawn as figure 9 According to the figure, except particle-like, SnO nanorods, nanowires, nanobelts, nanoflowers and microspheres

have been attracting many concerns of scientists

2.2 One-dimensional SnO, nanostructure

Figure 10 Number of paper related to SnO; nanorods and nanowires

Since production of SnO, nanorods by redox reaction in 2000s, a trend of

studying about the SnO, nanorods and nanowires have been gradually increased

(figure 10) For gas sensing properties only, 1-D nanostructures are promised to

improve the Sensitivity and Selectivity of SnO, material It is well known that the operation of SnO, sensor depends on two basic functions: receptor and transducer

functions [64] Receptor function is the ability of reacting between oxygen anion on

the SnO, surface with target gases, whereas transducer function is the ability of

intact transmitting electrons from negative to positive electrodes of the sensor The

difference of those two functions while using SnO, nanoparticles and nanorods as

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while comparing with other morphologies as in the same amount However, whilst

exposing to gases, the effective surface area of a film which is made of nanoparticle

is not as good as we expected This consequence comes from the lack of spaces that

allow gases particles to pass through the first or second layer of a thin film derived

from nanoparticles Nanorod, inversely, owns the smaller surface area than

nanoparticle Nonetheless, the porous of a thin film which is built up by nanorods is

distinctly better, therefore, the effective surface area of a film made of nanorod, apparently, seems to be better than one made of nanoparticle Base on this

argument, the receptor function of gas sensor manufactured by SnO} nanorods will

be better than one made of SnO; nanoparticles

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sensor application

From another viewpoint, it is generally known that the Schottky barrier is naturally formulated between two SnO, grains Thus, in a sensor of SnO,

twanoparticles, there are numerous barriers established between two electrodes

These barriers, in my point of view, prohibit not only the movement but also a large quantity of carriers from anode to cathode of the sensor hence the twansducer function of the material is estimated to be low Contrasting to the particle-like, rod- like SuO; nanomaterial has better transducer function because there are fewer Schottky barriers shown up between anode and cathode of the sensor Even if carriers Wansler from anode lu cathode via grain boundaries, the roule for carriers im the rod-like also shorter than ones in the particle-like

It was experimentally introduced that oxygen anion species play a vital role in the sensing mechanism of SnO, and they are chemisorbed into vacancies of 8nO; lattices as the process follow [42]: O; (gas) > O, (ad) ++ Oy (ad) 4 O" (ad) = O” (ad) < 07 (lattice) Moreover, It is important pointing out that a single missing latlice oxygen atom, for example in the bridging oxygen row of specific surface like (110), (101) or (100), constructs a site which differs from a surfaces with all the

bridging oxygen atoms removed [2] Those different sites behave differently to

gases dissociation, for example: Cluster and periodic ab initio caleulalions showed

that the stoichiometric SnO, (110) surface is rather unreactive towards CO, [41], whereas the SnQ, (110) surface was investigated as the adsorption sile for methanol

[12] Therefore, study of anisotropic structure like 1-D nanostructure may be

promised as the shortest way to improve the selectivity of SnO2

2.3 Methods for synthesis of nanomaterials

Up to now, there are two distinguishing approaches to synthesis

nanomaterials, namely “top-to-bollom” and “boltom-lo-lop” where nanomalerials

are received by grinding bulk-like materials to nanoscale or by synthesizing atoms

or molecules to form bigger structure, respectively In such approaches, there are a

lol of roules from plysical and chemical methods, for insumice PVD (physical vapor

deposition), CVD (chemical vapor deposition), sol-gel, self-assembly, ball grinding,

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hydrothermal synthesis cle Bach icthod has upsides aud downsides by their own Nonetheless, scientists always make their attempts to synthesize nanomaterials by various routes because the essential differences among methods will determine the quantity and the quality of the final results (dimension, morphology, uniform ete.)

2.3.1 Physical vapor deposition

Physical vapor deposition (PVD) is a variety of vacuum deposition and is a general term used to describe any of a numerous methods to deposit thin films by

the condensation of a vaporized form of the material onlo various surfaces (e.g

onto semiconduetor wafers) ‘I'he coating method involves purely physical processes suoh as high temperature vacuum evaporation or plasma sputter bombardment

vathor than involving a chemical reaction at the surface to be coated asm chemical

vapor deposition Different types of PVD include:

« Evaporative deposition: Material to be deposited is heated to a high vapor pressure by electrically resistive heating in low vacuum

« 1lectron beam physical vapor deposition: Thin film is deposited by electron

bombarding source materials in high vacuum,

« Sputter deposition: A method where glow plasma discharge is created between source material and target The plasma bombards the source

material to vapor which be deposited on the target

« Cathodic Arc Deposition: A high power arc is directed al Ihe source material

and blasts the target away into a vapor

« Pulsed laser deposition: A thin film constructing method where high power laser ablates material from the target into a vapor

PYM is used mm the manufacture of items including semiconductor devices,

aluminized PL’ film for balloons and snack bags, and coated cutting tools for

metalworking Besides PVD method can be used to serve the purpose of building extreme thin fils like atomic layer A good example is mini e-beam evaporator which can deposit monolayers of virtually all materials with melting points up to

3500 °C

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2.3.2 Chemical vapor deposition

Chemical vapor deposition (CVD) is a chemical process used to praduce high-

purty, high-performance sohd materials Also, this method is often used in the

semiconductor industry to produce thin films In a typical CVD process, the substrate is exposed to one or more volatile precursors, which react and/or decompose on the subsirate surface to produce the desired deposit Frequently, volatile by-products are also produced, which are removed by gas flow through the

reaction chamber or burn out

Micro-fabrication processes widely use CVD to deposit malerials in various forms, including: mono-crystalline, polycrystalline, amorphous, and epitaxial

These materials include: silicon, carbon fiber, carbon nanofibers, filaments, carbon

nanotubes, SiO,, silicomgermanium, lingsten, silicon carbide, silicon nitride,

silicon oxynitride, titanium nitride, and various high-k dielectrics ‘The CVD process

is also used to produce synthetic diamonds

A number of forms of CVD are m wide use and are (requently referenced in

the literature These processes differ in the means by which chemical reactions are

initiated (for instance, activation process) and process conditions

Classified by operating pressure:

Classified by physical characteristics of vapor:

Aerosol assisled CVD (AACVD) - A CVD process in which the precursors are transported to the substrate by means of 'a liquid/gas acrosol, which can

be generated ultrasonically ‘his technique is suitable for use with non-

volatile precursors,

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* Direct liquid injection CVD (DLICVD) - A CVD process in which the preewssors are in liquid form (liquid or solid dissolved in a convenicnl solvent)

Plasma methods:

* Microwave plasma-assisted CVD (MPCVD)

«_ Plasma-linbanoed CVD (PHCVD) - CVD processes that utilize plasma to enhance chemical reaction rates of the precursors PECVD processing allows deposilion at lower temperatures, which is ofien critical in the manufacture of semiconductors

* Remote plasma-enhanced CVD (RPECVD) - Similar to PECVD except that the wafer substrate is not directly im the plasma discharge region, Removing the wafer from the plasma region allows processing

temperatures down to room temperature,

Besides, there are numerous other types of CVD, namely Atomic layer CVD (ALCVD), Combustion Chemical Vapor Deposition (CCVD), Holt wire CVD (HWCVD), Metal-organic chemical vapor deposition (MOCVD), Hybrid Physical- Chemical Vapor Deposition (IPCVD), Rapid thermal CVD (RTCVD), Vapor phase Fpilaxy (VPR)

2.3.3 Hydrothermal synthesis

Hydrothermal syuthesis, which inchides the several techniques of crystallizing substances from high temperature aqueous solutions at high vapor pressures in a

closed system, is termed “hydrothermal method” The term “hydrothermal” began

form geologic where gcochemists and mineralogists have studied hydrothermal phase equilibrium since the beginning of the twentieth century Material scientist

used this method (o produce crystals, for instance, in 1839, the German chemist

Robert Bunsen synthesized crystals of barium carbonate and strontium carbonate by contained aqueous solutions in thick-walled glass tubes at temperatures above 00°C and at pressures above 100 bars [30] It was recommended as the first use of

hydrothermal aqueous solvents by means of media

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sensor application

Hydrothermal synthesis can be defined as a method of synthesis of single crystals which depends on the solubility of minerals in hot water under high

pressure The crystal growth is performed in an apparatus consisting of a stect

pressure vessel called autoclave, in which a nutrient is supplicd along with water In

a typical operation, a gradient of temperature is maintained at the opposite ends of the growth chamber so that the hotter end dissolves the nufnent and the cooler end

causes seeds to take additional growth

The crystallization vessels used are autoclaves These are usually thick-walled

steel cylinders wilh a hermetic scal which must withstand high temperatures and pressures for prolonged operation time Furthennore, the autoclave material must be

inert with respect to the solvent The closure is the most important element of the

autoclave Many designs lave been developed for seals where the most famous is

the Lridgman seal In most cases, steel-corroding solutions are used in

hydrothermal experiments Hence, to prevent corrosion of the internal cavity of the

autoclave, proleclive inserls are generally uscd These may have the same shape of the autoclave and fit in the intemal cavity (contact-type insert) or be a “floating”

type insert which occupies only part of the autoclave interior Inserts may be made

of carbor-free iron, copper, silver, gold, platinum, titanium, glass (or quartz), or

‘Teflon, depending on the temperature and solution used

Tis worth pointing out that a large mumber of compounds belonging to

practically all classes have been synthesized under hydrothermal conditions, such as elements, simple and complex oxides, tungstate, molybdates, carbonates, silicates, gormitaics cic Tr addition, hydrothermal synthesis is commonly used to grow synthetic quartz, gems and other single orystals with commercial value Some of the crystals that have been efficiently grown are emeralds, rubies, quartz, alexandrite

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synthesis aud crystal growing The super-saluration is achieved by reducing the temperature in the crystal growth zone The nutrient is placed in the lower part of

the autoclave filled with a specific amount of solvent The autoclave is heated in

order to create two temperature zones The nutrient dissolves in the hotter zone and

the saturated aqueous solution in the lower part is transported to the upper part by omveclive motion of the solution The cooler and denser solution in the upper part

of the autoclave descends while the counter-flow of solution ascends ‘he solution

becomes supersaturated in the upper part as the result of the reduction in

tcmperature and cryslallization sols in,

As for temperature-reduction technique, crystallization takes place without a temperature gradient between the growth and dissolution zones The super-

saturation is achieved by a gradual reduction in temperature of the solution im the

autoclave ‘Ihe disadvantage of this technique is the difficulty in controlling the

growth process and introducing seed crystals For these reasons, this technique is

very seldom used

Metastable-phase technique is based on the difference in solubility between

the phase to be grown and that serving as the starting material The nutrient consists

of compounds thal are thermodyramically unstable under the growths conditions

‘rhe solubility of the metastable phase exceeds that of the stable phase, and the latter

crystallize due to the dissolution of the melastable phase This technique is usually

combined with one of the other two techniques above

Possible upsides of the hydrothermal method over other types of crystal growth include the abilily to create crystalline phases which are nol stable at the smelting point Also, materials which have a high vapor pressure near their melting points can also be grown by the this method Moreover, the method is particularly

suitable for the growth of large good-quality crystals whilst maintaining good

control over the composition Problem of the method comes from the need of

expensive autoclaves and the impossibility of observing the crystal as it grows

during the process Nevertheless, cheap and simple autoclaves may be accepted in

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this method

2.4 Methods for synthesis of SnO, nanorods

As it is mentioned above, nanomaterials can be synthesized by two typical

processes (top-to-bottom and bottom-to-top) in which various methods and

techniques have been introduced Many methods such as vacuum evaporation [29],

RF, sputtering [26], spray pyrohydrolysis [40], ball milling [32], CVD [22] and

hydrothermal method [56] have been used to synthesize SnO, nanoparticles, etc

This type of SnO) nanostructure, nonetheless, has been studied for a long time and

it no longer attracted scientists Recently, SnO, 1-D nanostructure (nanorods,

nanowires) has been involving many concerns because of their interesting characteristics There are several methods to synthesize SnO) 1-D structure, namely

VLS [52, 27], high-pressure pulsed laser deposition [53], molten-salt method [59,

35], template route [70], thermal decomposition [63], oriented aggregation of initial

SnO, nanoparticles [57] and hydrothermal method [69, 54, 37]

2.4.1 Vapor-Liquid-Solid

Figure 12 Apparatus system for VLS synthesis

Among numerous methods of synthesis 1-D structures, vapor-liquid-solid or

VLS seems to be the most effective one to create beautifully oriental nanorods and

nanowires morphologies It should be noted that VLS has been named due to its

growth mechanism in a CVD system It can be also called thermal deposition

method The typical system for VLS method includes a quartz tube located in a

horizontal furnace, alumina boats, substrates and gas in/out pipes (figure 12)

22

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Synthesis of one-dimensional SnO nanostructure via hydrothermal method for gas

sensor application

Common steps of synthesizing SnO; nanorods via VLS method is illustrated

as follow:

catalyst by sputtering next to the source (Pure SnO powder for growing

core NWs or Sn powder for growing outer NWs)

high purity Ar gas

e Increase the furmace temperature from room-temperature to elevate

temperature (regularly 980 °C or 800 °C)

e Add pure oxygen to the quartz tube at slow flow rate (0.3-0.5 sccm),

meanwhile the inside pressure is approximately 2-5 Torr Maintain the growth temperature for desire time

Figure 13 SEM and TEM images of SnO, nanowires grown by VLS method [52] Figure 13 shows the SEM and TEM images of SnO, nanowires derived by VLS method It can be easily seen that high density and quite uniform of SnO, nanowires were created These features are two important advantages of VLS Nevertheless, VLS requires high treatment temperature and other method/technique like sputtering, vacuum pumping and gas flow controller Therefore, it is not simple

to setting up as well as operating a new system which support VLS method in a

laboratory The gold particle, which is observed at the head of SnO, single wire in

TEM image, is closely related to growth mechanism of the method

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242 Hard template

Template route is a wide term for many methods that use template pattern to

build up materials Owning to synthesizing SnO, nanorods/nanowires, template route can be easily understood as a method that uses a porous anodic membrane

(PAA) to trap material in definite positions, and then construct the structure

Porous anodic

‘alumina (PAA) membranes

Figure 14 Process of Synthesizing SnO» nanorods via template route

‘The material may be deposited on the membrane by numerous methods and

techniques, namely sputtering, VLS or some chemical methods etc A typical

process of template route associated with chemical method is described as in figure

14, and following the process

e Dissolve an amount of Chloride of Stannum (SnCly.5H,O) into ethanol

aqueous, and then add urea into the solution

¢ Keep the solution in a reflux system for a several hours After that, a transparent sol is obtained

Immerse the PAA template into a beaker that contained the sol

Sonicate the beaker by ultrasonic wave to accelerate the introduction of the

sol into the channels of PAA

e Take the PAA membrane out and dry it in ambient condition

Place and maintain the membrane in a high temperature furnace (often

above 600 °C) for several hours

The final sample which synthesized by this method is introduced as in figure

15 It can be observed that the nanowires were successfully created, however the

uniform of this sample is not as good as the sample derived from VLS method The

Tiêu đề	Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application
Tác giả	Vu Xuan Hien
Người hướng dẫn	Dr. Dang Duc Vuong
Trường học	Hanoi University of Science and Technology
Chuyên ngành	Engineering Physics
Thể loại	Thesis
Năm xuất bản	2011
Thành phố	Hanoi

Định dạng
Số trang	75
Dung lượng	2,94 MB