Characterisation and functionality of SnO_1tn2 gas sensors using vibrational spectroscopy [Elektronische Ressource] = Schwingungspektroskopische Charakterisierung der Funktionsweise von SnO_1tn2-Sensoren / vorgelegt von Serpil Harbeck

Serpil Harbeck

Informations

• Publié par: eberhard_karls_universitat_tubingen
• Publié le: 01 janvier 2005
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Characterisation and Functionality of SnO Gas 2
Sensors Using Vibrational Spectroscopy



Schwingungspektroskopische Charakterisierung der
Funktionsweise von SnO -Sensoren 2



DISSERTATION

der Fakultät für Chemie und Pharmazie
der Eberhard-Karls-Universität Tübingen

zur Erlangung des Grades eines Doktors
der Naturwissenschaften


2005


vorgelegt von
SERPIL HARBECK

























Tag der mündlichen Prüfung: 04.03.2005
Dekan: Professor Dr. S. Laufer
1. Berichterstatter: Privatdozent Dr. U. Weimar
2. Berichterstatter: Professor Dr. V. Hoffmann

Content
1 Introduction and Motivation ...........................................................1
1.1 Introduction ..................................................................................................1
1.2 Motivation.....................................................................................................3
2 Basics and Survey .........................................................................5
2.1 Introduction to IR techniques5
2.2 Diffuse Reflectance Spectroscopy DRIFT .........................................................6
2.2.1 Advantages of DRIFT Spectroscopy...........................................................6
2.2.2 Disadvantages .........................................................................................7
2.3 Reflection Types of Electromagnetic Radiation on Mat Surfaces........................8
2.4 Intensity Distribution of Diffuse Reflectance: Law of Lambert.........................10
2.5 Theory of Diffuse Reflectance: Theory of Kubelka-Munk.................................12
2.6 Specular Reflectance....................................................................................15
2.6.1 Methods for Minimizing Specular Reflectance ...........................................16
2.7 Diffuse Reflectance on Absorptive Materials ..................................................18
2.8 Adsorption Processes on Solid Surfaces.........................................................19
2.8.1 Physisorption .........................................................................................19
2.8.2 Hydrogen Bonding..................................................................................20
2.8.3 Chemisorption........................................................................................20
2.8.4 Ionosorption ..........................................................................................22
2.9 Electronic Structure of Semiconductors .........................................................22
2.10 IR Characterisation of Metal Oxide Surfaces ..................................................25
2.11 Adsorption Mechanism of Selected Molecules on MOX surfaces ......................28
2.11.1 Oxygen Adsorption.............................................................................28
2.11.2 Water Adsorption ...............................................................................33
2.11.3 CO Adsorption....................................................................................42
2.12 Influence of Surface Additives on the Sensing Mechanism..............................50
2.13 Correlations between Spectroscopic and Electrical Data .................................51
3 Experimental Section ...................................................................55
3.1 IR Spectrometer..........................................................................................55
3.2 DRIFT-unit ..................................................................................................56
3.3 Sample Chamber .........................................................................................57
3.4 Gas Mixing Bench ........................................................................................58
3.5 Sensor Heating............................................................................................59
3.6 Samples...................................................................................................... 60
3.7 Drying and Thermal Activation of the Sensors ............................................... 62
3.8 Sampling of Gases in the Exhaust for the Photoacoustic IR Spectrometer ....... 63
3.9 Measurement Protocol ................................................................................. 64
3.10 Band Analysis.............................................................................................. 64
4 Results and Discussion ................................................................ 67
4.1 Characterisation of the Samples ................................................................... 68
4.1.1 Characterisation of Differently Prepared SnO Powders at RT.................... 68 2
4.1.2 Characterisation of Un-doped and Pd-doped Sensors at RT....................... 75
4.1.3 Temperature Effects............................................................................... 80
4.1.4 CO Measurements.................................................................................. 90
4.1.5 Comparison of the Un-doped and Pd-doped Sensor................................ 138
5 Conclusions and Outlook............................................................ 143
5.1 Reaction Mechanism at Low Temperature (150°C) ...................................... 144
5.1.1 In the Absence of Humidity .................................................................. 144
5.1.2 In the Presence of Humidity 144
5.2 Reaction Mechanism at High Temperatures (300 and 350°C) ....................... 146
5.2.1 In the Absence of Humidity 146
5.2.2 In the Presence of Humidity 146
5.3 Outlook..................................................................................................... 147
6 References ............................................................................... 149

List of Symbols
B a) Density of radiation
b) Rotation constant
c a) Concentration
b) Speed of light
d a) Thickness of Layer
b) Distance between the interacting particles
e Elementary charge
E Extinction (Law of Lambert-Beer)
E. Attraction energy attr
E Lower edge of conduction band energy C
E Fermi energy F
E Band gap energy g
E Potential energy pot
E. Repulsive energy rep
f(R ) Kubelka-Munk unit for the intensity of the diffuse reflected ∞
radiation
g Scattering anisotropy factor
h Planck’s constant
I Intensity
I Intensity of true specular reflection s
I Intensity of diffuse specular reflectance ds
I Intensity of diffuse reflectance d
J Radiant flux contrary to incoming radiation (x=0)
k a) Linear absorption coefficient
b) Boltzmann constant
K Absorption coefficient
n Ratio of the refraction index of the sample to that of the
surrounding medium
N The density of the free charges in the conduction bands C
N Ionised donor density D
N Number of ions in the space charge area i
n The density of the free charges on the surface s
ppm part per million
R a) Reflection
b) Resistance
r Distance between two atoms in a molecule
s Linear scattering coefficient
S Scattering coefficient
S Irradiance 0
T Transmission
T Temperature
. ..V Oxygen vacancies (single ionised, double ionised) 0 (V0 , V0 )
V Schottky barrier s
Incident angle α
β Observation angle
δ Symbol for the deformation mode of a vibration
ε a) Extinction coefficient
b) Depth of potential
c) Permittivity
ε Permittivity of free space 0
Θ Angle of incident light
κ Absorption index
λ Wavelength
ν a) Frequency
b) Symbol for stretching mode of vibration
~ ~ Frequency of the maxima of the P and R rotation bands ν ,ν Rp
Φ Electrical potential
Φ Potential in the bulk b
Φ Potential in the edge layer x

List of Abbreviations
ATR Attenuated Total Reflection
cus Coordinatively unsaturated sites
DC Direct Current
DRIFT Diffuse Reflectance Infrared Fourier Transform
EPR Electron Paramagnetic Resonance
ES Emission Spectroscopy
FT Fourier Transformation
FTIR Fourier Transform Infrared
HOMO Highest Occupied Molecular Orbital
IR Infrared
IRRAS Infrared Reflection Absorbance Spectroscopy
ISS Ion Scattering Spectroscopy
KM Kubelka-Munk
LEED Low Energy Electron Diffraction
LUMO Lowest Unoccupied Molecular Orbital
MOX Metal Oxide
PAS Photo Acoustic Spectroscopy
r.h. Relative humidity
RT Room Temperature
TPD Temperature Programmed Desorption
UPS Ultraviolet Photoelectron Spectroscopy
XPS X-ray Photoelectron Spectroscopy

Introduction and Motivation
1 Introduction and Motivation
1.1 Introduction
Gas sensors based on metal oxide sensitive layers are playing an
important role in the detection of toxic pollutants (CO, H S, NO , SO etc.) 2 x 2
and inflammable gases (H , CH , hydrocarbons etc.) and in the control of 2 4
industrial processes. Tin dioxide is widely used as a basic material for the
preparation of gas