Synthesis of one dimensional sno2 nanostructure via hydrothermal method for gas sensor application

Also, the thesis studies about LPG and ethanol sensing properties of SnO2 nanorods in order to orientate the use of SnO2 nanomaterial in gas sensor application.. The thesis focuses on sy

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

-

VU XUAN HIEN

NANOSTRUCTURE VIA HYDROTHERMAL METHOD FOR GAS SENSOR APPLICATION

MAJOR: ENGINEERING PHYSICS

MASTER OF SCIENCE THESIS ENGINEERING PHYSICS

SUPERVISOR: Dr DANG DUC VUONG

TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI

First, it’s pleasure to send my sincere thanks to teachers and brothers at Electronic Materials Department, School of Engineering Physics about the supports during my study and research

Especially, I want to give my thank Dr Dang Duc Vuong, who enthusiastically guided me throughout the course to complete this thesis I also thank the members of the sensor group by the valuable suggestions and enthusiastic support during the implementation of the Dissertation

Finally, during the course, I 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 Epidemiology Again I sincerely thank about it

Hanoi, July 8th 2011

Author

Vu Xuan Hien

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 Hien

Thesis title: “Synthesis of one-dimensional SnO 2 nanostructure via hydrothermal method for gas sensor application”

Author: Vu Xuan Hien Course: 2009

Supervisor: Dr Dang Duc Vuong

Content:

SnO2 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 mobility Therefore, many scientists have paid special attention to this material, especially in Nano-scale Many studies in preparing different morphologies

of SnO2 by various methods have been successfully carried out 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-sensitized solar cell and transparent conducting electrodes Among numerous methods, hydrothermal treatment is an interesting approach which can support mass synthesize nanomaterials uniformly and cheaply at low temperature Therefore it has been widely choosing for synthesizing SnO2 nanorods, recently

Purpose of this work is to investigate the optimum process of synthesizing SnO2nanorods under hydrothermal treatment as well as propose a possible formation mechanism for the growth Also, the thesis studies about LPG and ethanol sensing properties of SnO2 nanorods in order to orientate the use of SnO2 nanomaterial in gas sensor application

The thesis focuses on synthesizing SnO2 nanorods 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 material is also conducted

(Constructed of nanorods with the diameter is from 40 to approximately 200 nm and the length is from 150 nm to roughly 1 µm) and microspheres (from several nanometers to approximately 2 micrometers diameter) were successfully synthesized

by a low temperature (below 200 oC) 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, “T” is below 190 oC or hydrothermal time “t” is under 20 hours) of SnO2 crystals, nuclei and clusters, whereas the rods and flowers were together constructed via crystallization process (T ≥ 190 o

C and t ≥ 20 hours) The XRD, FE-SEM and TEM results indicate that the anisotropic of SnO2 flowers is higher than nanorods and is followed by SnO2 microspheres This may result in the better selectivity toward ethanol of SnO2 nanoflowers while comparing to other morphologies In addition, the gas testing result again proves the enhancement of LPG and ethanol sensing properties for SnO2 nanorods to nanoparticles

Main results of the thesis have been used to write a paper entitled “Synthesis of

SnO 2 micro-spheres, nano-rods and nano-flowers via simple hydrothermal route”

which has been accepted by Physica E

LIST OF ABBREVIATIONS III LIST OF TABLES V LIST OF FIGURES VI

PREFACE 1

I INTRODUCTION 2

1 SnO2 material and its applications 2

1.1 Physical and chemical properties 2

1.1.1 Physical properties 2

1.1.2 Chemical properties 4

1.2 Typical applications 4

1.2.1 Transparent conductor 4

1.2.2 Oxidation catalyst 6

1.2.3 Solid state gas sensor 8

2 Tin dioxide in the Nano-world 11

2.1 Overview of nanomaterials 11

2.2 One-dimensional SnO2 nanostructure 14

2.3 Methods for synthesis of nanomaterials 16

2.3.1 Physical vapor deposition 17

2.3.2 Chemical vapor deposition 18

2.3.3 Hydrothermal synthesis 19

2.4 Methods for synthesis of SnO2 nanorods 22

2.4.1 Vapor-Liquid-Solid 22

2.4.2 Hard template 24

2.4.3 High-pressure pulsed laser deposition 25

2.4.4 Molten-salt 26

2.4.5 Hydrothermal treatment 28

II EXPERIMENT 31

1 Synthesis of SnO2 nanorods 31

2 Influence of technical parameters 32

3 Coating thick film SnO2 nanomaterial 33

4 Characterization techniques of SnO2 nanomaterial 33

4.1 X-Ray diffraction (XRD) 33

4.2 Scanning Electron Microscopy (SEM) 34

4.3 Transmission Electron microscopy (TEM) 36

4.4 Gas sensing properties 36

III RESULTS AND DISCUSSION 39

1 Prepare SnO2 nanorods in large scale 39

2 Influence of hydrothermal time 40

3 Influence of tin (IV) chloride weight 45

4 Influence of hydrothermal temperature 47

5 Gas sensing characteristics 49

CONCLUSION 53

FUTURE PLAN 54

REFERENCES 56

LIST OF ABBREVIATIONS

 IUPAC: International Union of Pure and Applied Chemistry

 XRD: X-ray Diffraction

 SEM: Scanning electron microscope

 FE-SEM: Field Emission Scanning Electron Microscopy

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

 TEM: Transmission electron microscopy

 AFM: Atomic force microscopy

 TCOs: Transparent conducting films

 CNTs: Carbon nanotubes

 VLS: Vapor-liquid-solid

 PVD: Physical vapor deposition

 CVD: Chemical vapor deposition

 APCVD: Atmospheric pressure CVD

 LPCVD: Low-pressure CVD

 UHVCVD: Ultrahigh vacuum CVD

 AACVD: Aerosol assisted CVD

 DLICVD: Direct liquid injection CVD

 MPCVD: Microwave plasma-assisted CVD

 PECVD: Plasma-Enhanced CVD

 RPECVD: Remote plasma-enhanced CVD

 ALCVD: Atomic layer CVD

 CCVD: Combustion Chemical Vapor Deposition

 HWCVD: Hot wire CVD

 MOCVD: Metal-organic chemical vapor deposition

 HPCVD: Hybrid Physical-Chemical Vapor Deposition

 RTCVD: Rapid thermal CVD

 VPE: Vapor phase Epitaxy

 R.F.: Radio Frequency

 PAA: Porous Anodic Membrane

 PLD: Pulsed Laser Deposition

 CTAB: Cetyltrimethyl Ammonium Bromide

 PEG: Polyethylene Glycol

 Acc Voltage: Accumulation Voltage

 JCPDS: Joint Committee on Powder Diffraction

LIST OF FIGURES

Figure 1 SnO2 Rutile structure 2

Figure 2 XRD pattern of SnO2 3

Figure 3 Applications of SnO2 as transparent conductor and coated layer 5

Figure 4 Reduction and re-oxidation in the Mars-van-Krevelen mechanism 6

Figure 5 Solid state gas sensor 8

Figure 6 Structure of bulk and film types gas sensor based on SnO2 material 9

Figure 7 Energy model for SnO2 10

Figure 8 Schottky barrier between gain boundaries 11

Figure 9 Number of papers related to SnO2 multi-morphology 13

Figure 10 Number of paper related to SnO2 nanorods and nanowires 14

Figure 11 Comparison between SnO2 nanoparticles and nanorods 15

Figure 12 Apparatus system for VLS synthesis 22

Figure 13 SEM and TEM images of SnO2 nanowires grown by VLS method [52] 23

Figure 14 Process of Synthesizing SnO2 nanorods via template route 24

Figure 15 SnO2 nanowires derived from template route associated with chemical technique [70] 25

Figure 16 Diagram of high-pressure pulsed laser system 25

Figure 17 SnO2 nanorods synthesized by pulsed laser beam [53] 26

Figure 18 Process of synthesizing SnO2 nanorods via molten salt method 27

Figure 19 TEM image of SnO2 nanorods synthesized by thermal decomposition [63] (a) and molten salt method [59] (b) 27

Figure 20 SEM images of SnO2 nanorods derived by hydrothermal method with CTAB support [54] 28

Figure 21 SEM images of SnO2 nanorods derived by hydrothermal method with PEG support [69] 29

Figure 22 SEM images of SnO2 nanorods derived by hydrothermal method with PEG support [37] 30

Figure 23 Process for preparing SnO2 nanorods by hydrothermal route 31

Figure 24 A modern automated X-ray diffractometer (a) and an XRD pattern of

Fe2O3 sample (b) 33

Figure 25 Hitachi SU-70 (a) and SEM image of human blood (b) 35 Figure 26 Jeol JEM-2100F TEM (a) and TEM image of Sin Nombre virus (b) 36 Figure 27 Static gas sensing system (a) and principal circuit (b) 37 Figure 28 FE-SEM images of SnO2 samples 39

Figure 29 FE-SEM images of SnO2 nanomaterial at discrete points of hydrothermal time 40

Figure 30 Influence of hydrothermal time to SnO2 morphology 41

Figure 31 Illustration of processes in hydrothermal treatment with parameters: time

and temperature 43

Figure 32 Nucleation, aggregation and crystallization 45 Figure 33 Influence of SnCl4.5H2O weight to SnO2 morphology 46

Figure 34 Influence of hydrothermal temperature weight to SnO2 morphology 47

Figure 35 XRD pattern of SnO2 condensed micro-spheres (c), nano-flowers (b) and nano-rods (a) 48

Figure 36 Influence of sensor response to operating temperature for microspheres,

nanorods and nanoflowers at 780 ppm C2H5OH and 10000 ppm LPG 49

Figure 37 Influence of sensor response to gases concentration for microspheres,

nanorods and nanoflowers at 370 oC 50

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

PREFACE

SnO2 is a special n-type metal oxide semiconductor which possesses many feature properties, namely low cost, high gas sensitivity, heat durability, chemical resistance and fast electron mobility Therefore, many scientists have paid special attention to this material, especially in Nano-scale Many studies in preparing different morphologies of SnO2 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-sensitized solar cell and transparent conducting electrodes VLS (vapor-liquid-solid), molten salt and hydrothermal treatment are well-known methods in synthesizing 1-D nanostructures (nanorods, nanowires and nanotubes, etc.) In such of those methods, VLS is a rare method which allows 1-D structure to be grown at definite positions, but in small quantity and low uniform Hydrothermal treatment, inversely, can mass synthesize variety morphologies of nanomaterials uniformly and cheaply at low temperature Therefore it has been choosing for synthesizing SnO2 nanorods, recently

Thesis named “Synthesis of one-dimensional SnO 2 nanostructure via hydrothermal method for gas sensor application” has been chosen since the need

of studying and applying SnO2 nanorods and nanowires among scientists and engineers has been widely urged In this work, the SnO2 nanorods were not only successfully prepared, the SnO2 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, SnO2 nanorods introduced better LPG and ethanol vapor sensing than nanoparticles whereas SnO2 nanoflowers were selectively sensed with ethanol vapor

I INTRODUCTION

1 SnO2 material and its applications

Figure 1 SnO 2 Rutile structure

Tin dioxide is the IUPAC name of an inorganic compound with the formula SnO2 Besides, tin dioxide also has other names like stannic oxide, tin (IV) oxide and flowers of tin Cassiterite which is also called tinstone is heavy, metallic and a major mineral form of SnO2 Generally, SnO2 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 Ǻ and c = 4.8382 Ǻ [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, SnO2 powder as well as Rutile structure of SnO2

1.1 Physical and chemical properties

SnO2 is usually regarded as an oxygen-deficient n-type semiconductor Hydrous forms of SnO2 have been described in the past as stannic acids, although such materials appear to be hydrated particles of SnO2 where the composition reflects the particle size

1.1.1 Physical properties

Several physical properties of SnO2 are described as in table 1

Table 1 Physical properties of tin dioxide

Boiling point

Solubility

in water

Band gap

SnO2

150.71

g/mol

White powder

6.95 g/cm3 1630 °C 1800-

1900 ºC insoluble 3.6 eV

In nature, it is hard to find the stoichiometry structure of SnO2 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 SnO2 absorb energy of the visible light

Figure 2 XRD pattern of SnO 2

The structure of SnO2, tetragonal structure which is already known as a strong structure can be characterized by XRD method Figure 2 indicates the XRD pattern

of SnO2 tetragonal structure Typically, the pattern has the strongest peak at 2θ = 26.54o and followed important peaks are at 2θ = 33,7o and at 2θ = 51,7ocorresponding to (110), (101) and (211) faces, respectively For these bulk terminated SnO2 surfaces, for example surfaces with surface-tin atoms in their bulk

Sn4+ oxidation state, the (110) surface exhibits the lowest energy surface followed

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

1.1.2 Chemical properties

SnO2 is an amphoteric oxide and it is insoluble in water While Cassiterite ore has been known as difficult to dissolve in acids and alkalis, tin oxides are able to dissolve in acids Reaction of SnO2 in refluxing HI for many hours:

SnO2 + 6 HI → H2SnI6 + 2 H2O (1) Similarly, SnO2 dissolves in sulfuric acid to give the sulfate:

SnO2 + 2 H2SO4 → Sn(SO4)2 + 2 H2O (2) SnO2 dissolves in strong base to give “Stannates”, with the nominal formula

Na2SnO3 Dissolving the solidified SnO2/NaOH melt in water gives “preparing salt” (Na2[Sn(OH)6]2) 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, SnO2 has constituted numerous components for optoelectronic applications Besides, oxides, in general, are not only used as support materials for dispersed metal catalysts but also expose catalytical characteristic by themselves Moreover, many oxides included tin dioxide reveal sensitivity towards oxidizing and reducing gases by an aberration of their electrical properties In short, there are three remarkable applications of tin dioxide that excited a huge number of scientists

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

1.2.1 Transparent conductor

Tin dioxide, an n-type semiconductor with approximately 3.6 eV band-gap, has low electrical resistance with high optical transparency in the visible range of electromagnetic spectrum Hence, the oxide material has been applied strongly in numerous areas, such as electrode materials in solar cells [5], light emitting diode (LED) [65], flat panel displays [43] and other optoelectronic devices in which electric contacts are required enough transparency to exhibit the phenomenon of

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, SnO2 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 modern 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]

Figure 3 Applications of SnO 2 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, SnO2 represent a conductor In 2002, Kiliç and Zunger [23] proclaimed that the formation energy between oxygen vacancies and tin interstitials in SnO2 is very low, hence leading these defects to form freely This outcome is worth to explain why pure SnO2 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

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 SnO2, ZnO, In2O3(known as ITO if doped with Sn), Ga2O3 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

1.2.2 Oxidation catalyst

Figure 4 Reduction and re-oxidation in the Mars-van-Krevelen mechanism.

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

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

of reaction catalyzed by pure and modified SnO2 was given by Harrison [19], the role of several additives has not been fully understood since most of the additives are oxidized during operation of the catalysts Special active sites may be stabilized

at the interface between the additive and SnO2 For example, it was suggested that

Mo5+ sites which allocated between MoO3 catalyst and SnO2 play and important role in methanol and ethanol oxidation [28] Besides, in the most case, the additives form clusters support on SnO2 surface Antimony, for instance, is proposed that antimony oxide forms a solid solution with SnO2 or Sb3+ surface species may be the active sites

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-oxidation of the catalysts Thus there 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 catalysts have been characterized, it has been challenging to find a fundamental description and fully understanding the additives roles because

of the dynamic change of the catalysts during operation

1.2.3 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 Cr2O3, Mn2O3, Co3O4, NiO, CuO, CdO, MgO, SrO, BaO, In2O3, WO3, TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3,

Ta2O5, La2O3, CeO2, Nd2O3 Nonetheless, the most common used materials for gas sensing purpose till now are ZnO and SnO2

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 SnO2 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

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 2 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 Al2O3 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 SnO2 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 SnO2 sensor selectivity can be fine-tuned over a wide range by applying different SnO2morphology, dopants, contact geometries as well as mode of operations, etc [1] Still now, the exact fundamental of SnO2 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 SnO2 surface can be transferred

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 SnO2 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 SnO2 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

of SnO2 (figure 7)

Figure 7 Energy model for SnO 2

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:

LD = (eoKT/noe2)1/2 (5) where eo is the static dielectric constant, no is the total carrier concentration, e is 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 SnO2 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 SnO2, charges carriers can only be generated and

received enough energy to overcome the barrier at elevate temperature (often above

200oC)

Figure 8 Schottky barrier between gain boundaries.

When gases contact and react to the SnO2 surface, two different phenomena occur naturally Reducing gases will replace the adsorbed oxygen at the surface of SnO2 by other molecules and then reverse the band bending, thus increase the conductivity of the material Oxidizing gases, inversely, turn the conductivity of SnO2 to higher value Several reactions between target gas and oxygen anion on the SnO2 surface can be illustrated as follow:

CH3CH2OHgas + O−ad ↔ CH3CHOad + H2O + e− (6)

CH3CHOad + 4O−ad ↔ CO + CO2 +H2O + 4e− (7)

C3H8 + 10O−ad↔ 3CO2 + 4H2O + 10e− (8)

C4H10 + 13O−ad↔ 4CO2 + 5H2O + 13e− (9)

By monitoring the variation of SnO2 conductivity when exposing to gases, the appearance or even the concentration of those gases can be evaluated

2 Tin dioxide in the Nano-world

2.1 Overview of nanomaterials

As well as the development of human civilization, science and

Nano-technology have been establishing its own place in many areas and applications, since they were born The accidental discovery of Bucky ball (1985) and carbon-nanotubes (CNTs) (1991) seemed to establish a new era of advanced technology in which scientists expected nanomaterials as auspicious future materials A numerous applications related to nanomaterials have taken place because of their excellent features like smaller, more interesting characteristics and more stable Nowadays, most of electronic devices like computer, mobile phone, TV, regulator, etc are achievements of applying Nano-technology and nanomaterials Moreover, thanks to Nano-technology, numerous medical discoveries are helping patients out of their 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 requirements of studying and expanding nanomaterials are urgent need for development

The difference between the bulk-like materials and nanomaterials comes from the quantum effect occasionally appearing in several nanomaterials The effect leads

to expand the band gap of nanomaterials comparing with the same materials in bulk-size In some nanomaterials like SiO2 or ZnO (with condition that the particle sizes are smaller than their Debye lengths), for example, terms like “direct gap” or

“indirect gap” are not exist Therefore, it has been interestingly found that those materials have luminescent characteristic which is never happen in the same bulk-kind materials In addition, an important feature of nanomaterials is the immensely increased the ratio of surface area to volume while comparing to bulk-like materials and thus making better characteristics for catalyst-role materials Moreover, it 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

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 2 multi-morphology

With regard to tin dioxide, by reducing its size to nanoscale, the 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 SnO2 for sensitivity is 6 nm (twofold the SnO2 Debye length which is approximately 3.07 nm at 293 K) [13] However, almost SnO2 gas sensors have been fabricated with the condition that the SnO2 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 SnO2 have

surface-to-0 20 40 60 80 100 120

been carried out, namely wires [25], tubes [36], belts [67], ribbon [31], nano-rods [48, 62, 35, 68, 58] and nano-flower [66, 34, 10] In general,

nano-a tendency of studying nnano-anomnano-aterinano-als hnano-as been estnano-ablished in which nano-assembling new morphology has been received a special concern of scientists

By making small survey on website “http://www.sciencedirect.com/” (a huge resources of sciential and technological journals), a bar chart of papers owning to SnO2 nanostructures has been drawn as figure 9 According to the figure, except particle-like, SnO2 nanorods, nanowires, nanobelts, nanoflowers and microspheres have been attracting many concerns of scientists

2.2 One-dimensional SnO 2 nanostructure

Figure 10 Number of paper related to SnO 2 nanorods and nanowires.

Since production of SnO2 nanorods by redox reaction in 2000s, a trend of studying about the SnO2 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 SnO2 material It is well known that the operation of SnO2 sensor depends on two basic functions: receptor and transducer functions [64] Receptor function is the ability of reacting between oxygen anion on the SnO2 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 SnO2 nanoparticles and nanorods as

0 5 10 15 20 25 30

sensing material is introduced as in figure 11

Figure 11 Comparison between SnO 2 nanoparticles and nanorods.

Nanoparticle possesses the smallest dimension because of the three-oriental confinement Thus, the kind of structure often introduces the largest surface area 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 SnO2 nanorods will

be better than one made of SnO2 nanoparticles

From another viewpoint, it is generally known that the Schottky barrier is naturally formulated between two SnO2 grains Thus, in a sensor of SnO2nanoparticles, 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 transducer function of the material is estimated to be low Contrasting to the particle-like, rod-like SnO2 nanomaterial has better transducer function because there are fewer Schottky barriers shown up between anode and cathode of the sensor Even if carriers transfer from anode to cathode via grain boundaries, the route for carriers in 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 SnO2 and they are chemisorbed into vacancies of SnO2lattices as the process follow [42]: O2 (gas) ↔ O2 (ad) ↔ O2- (ad) ↔ O- (ad) ↔ O2-(ad) ↔ O2- (lattice) Moreover, It is important pointing out that a single missing lattice 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 calculations showed that the stoichiometric SnO2 (110) surface is rather unreactive towards CO2 [41], whereas the SnO2 (110) surface was investigated as the adsorption site 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-bottom” and “bottom-to-top” where nanomaterials 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 lot of routes from physical and chemical methods, for instance PVD (physical vapor deposition), CVD (chemical vapor deposition), sol-gel, self-assembly, ball grinding,

hydrothermal synthesis etc Each method has upsides and 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 etc.)

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 onto various surfaces (e.g., onto semiconductor wafers) The coating method involves purely physical processes such as high temperature vacuum evaporation or plasma sputter bombardment rather than involving a chemical reaction at the surface to be coated as in 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

 Electron 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 at the 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

PVD is used in the manufacture of items including semiconductor devices, aluminized PET 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 films 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

2.3.2 Chemical vapor deposition

Chemical vapor deposition (CVD) is a chemical process used to produce purity, high-performance solid 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 substrate 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

high-Micro-fabrication processes widely use CVD to deposit materials in various forms, including: mono-crystalline, polycrystalline, amorphous, and epitaxial These materials include: silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2, silicon-germanium, tungsten, 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 in wide use and are frequently 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:

 Atmospheric pressure CVD (APCVD) - CVD processes at atmospheric pressure

 Low-pressure CVD (LPCVD) - CVD processes at sub-atmospheric pressures

 Ultrahigh vacuum CVD (UHVCVD) - CVD processes at a very low pressure, typically below 10-6 Pa (~10-8 torr)

Classified by physical characteristics of vapor:

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

be generated ultrasonically This technique is suitable for use with volatile precursors

non- Direct liquid injection CVD (DLICVD) - A CVD process in which the precursors are in liquid form (liquid or solid dissolved in a convenient solvent)

Plasma methods:

 Microwave plasma-assisted CVD (MPCVD)

 Plasma-Enhanced CVD (PECVD) - CVD processes that utilize plasma to enhance chemical reaction rates of the precursors PECVD processing allows deposition at lower temperatures, which is often critical in the manufacture of semiconductors

 Remote plasma-enhanced CVD (RPECVD) - Similar to PECVD except that the wafer substrate is not directly in 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), Hot wire CVD (HWCVD), Metal-organic chemical vapor deposition (MOCVD), Hybrid Physical-Chemical Vapor Deposition (HPCVD), Rapid thermal CVD (RTCVD), Vapor phase Epitaxy (VPE)

2.3.3 Hydrothermal synthesis

Hydrothermal synthesis, which includes 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 geochemists and mineralogists have studied hydrothermal phase equilibrium since the beginning of the twentieth century Material scientist used this method to 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 200°C and at pressures above 100 bars [30] It was recommended as the first use of hydrothermal aqueous solvents by means of media

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 steel pressure vessel called autoclave, in which a nutrient is supplied 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 nutrient and the cooler end causes seeds to take additional growth

The crystallization vessels used are autoclaves These are usually thick-walled steel cylinders with a hermetic seal which must withstand high temperatures and pressures for prolonged operation time Furthermore, the autoclave material must be inert with respect to the solvent The closure is the most important element of the autoclave Many designs have been developed for seals where the most famous is the Bridgman seal In most cases, steel-corroding solutions are used in hydrothermal experiments Hence, to prevent corrosion of the internal cavity of the autoclave, protective inserts are generally used These may have the same shape of the autoclave and fit in the internal cavity (contact-type insert) or be a “floating” type insert which occupies only part of the autoclave interior Inserts may be made

of carbon-free iron, copper, silver, gold, platinum, titanium, glass (or quartz), or Teflon, depending on the temperature and solution used

It is worth pointing out that a large number of compounds belonging to practically all classes have been synthesized under hydrothermal conditions, such as elements, simple and complex oxides, tungstate, molybdates, carbonates, silicates, germinates etc In addition, hydrothermal synthesis is commonly used to grow synthetic quartz, gems and other single crystals with commercial value Some of the crystals that have been efficiently grown are emeralds, rubies, quartz, alexandrite and others

There are generally three techniques of synthesizing crystal via this method, namely temperature-difference, temperature-reduction and metastable-phase Temperature-difference is the most extensively used method in hydrothermal

synthesis and crystal growing The super-saturation 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 convective motion of the solution The cooler and denser solution in the upper part

of the autoclave descends while the counter-flow of solution ascends The solution becomes supersaturated in the upper part as the result of the reduction in temperature and crystallization sets 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 in the autoclave The 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 that are thermodynamically unstable under the growth conditions The solubility of the metastable phase exceeds that of the stable phase, and the latter crystallize due to the dissolution of the metastable 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 ability to create crystalline phases which are not stable at the melting 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 their 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

this method

2.4 Methods for synthesis of SnO 2 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], R.F sputtering [26], spray pyrohydrolysis [40], ball milling [32], CVD [22] and hydrothermal method [56] have been used to synthesize SnO2 nanoparticles, etc This type of SnO2 nanostructure, nonetheless, has been studied for a long time and

it no longer attracted scientists Recently, SnO2 1-D nanostructure (nanorods, nanowires) has been involving many concerns because of their interesting characteristics There are several methods to synthesize SnO2 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 SnO2 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)

Common steps of synthesizing SnO2 nanorods via VLS method is illustrated

as follow:

 Place a substrates (Si or Al2O3) with a layer of previously deposited Au catalyst by sputtering next to the source (Pure SnO powder for growing core NWs or Sn powder for growing outer NWs)

 Evacuate the quartz tube (roughly 10-2 Torr) and purge it several times with high purity Ar gas

 Increase the furnace temperature from room-temperature to elevate temperature (regularly 980 oC or 800 oC)

 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 2 nanowires grown by VLS method [52]

Figure 13 shows the SEM and TEM images of SnO2 nanowires derived by VLS method It can be easily seen that high density and quite uniform of SnO2nanowires 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 SnO2 single wire in TEM image, is closely related to growth mechanism of the method

2.4.2 Hard template

Template route is a wide term for many methods that use template pattern to build up materials Owning to synthesizing SnO2 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

Figure 14 Process of Synthesizing SnO 2 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:

 Dissolve an amount of Chloride of Stannum (SnCl4.5H2O) 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

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

 Place and maintain the membrane in a high temperature furnace (often above 600 oC) 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