• TABLE OF CONTENTS
HIDE
 Title Page
 Copyright
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 General introduction
 An XPS investigation of tin oxide...
 An electronic and structural interpretation...
 A study of the dehydration of tin...
 An observation of water adsorption...
 The interaction of polycrystalline...
 General conclusions and recommendations...
 A brief discription of the experimental...
 Computer-interfaced digital pulse...
 Reference
 Biographical sketch
 Copyright






Group Title: Surface characterization and chemisorption properties of polycrystalline systems : SnO2, PtSnO2 and Zr
Title: Surface characterization and chemisorption properties of polycrystalline systems SnO2, PtSnO2 and Zr
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Permanent Link: http://ufdc.ufl.edu/UF00085803/00001
 Material Information
Title: Surface characterization and chemisorption properties of polycrystalline systems SnO2, PtSnO2 and Zr
Physical Description: xiii, 147 leaves : ill. ; 28 cm.
Language: English
Creator: Cox, David Fullen, 1956-
Publication Date: 1984
 Subjects
Subject: Platinum catalysts   ( lcsh )
Chemisorption   ( lcsh )
Crystals   ( lcsh )
Zirconium   ( lcsh )
Chemical Engineering thesis Ph. D
Dissertations, Academic -- Chemical Engineering -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis (Ph. D.)--University of Florida, 1984.
Bibliography: Bibliography: leaves 142-146.
Statement of Responsibility: by David Fullen Cox.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00085803
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000496891
oclc - 12041382
notis - ACR6118

Table of Contents
    Title Page
        Page i
    Copyright
        Page ii
    Acknowledgement
        Page iii
    Table of Contents
        Page iv
        Page v
    List of Tables
        Page vi
    List of Figures
        Page vii
        Page viii
        Page ix
        Page x
        Page xi
    Abstract
        Page xii
        Page xiii
    General introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
    An XPS investigation of tin oxide supported platinum
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
    An electronic and structural interpretation of tin oxide ELS spectra
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
    A study of the dehydration of tin oxide surface layers
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
    An observation of water adsorption on tin oxide using ESD and grazing-exit-angle XPS and AES
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
    The interaction of polycrystalline zirconium with o2, N2, co and N2o
        Page 91
        Page 92
        Page 93
        Page 94
        Page 95
        Page 96
        Page 97
        Page 98
        Page 99
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
        Page 112
        Page 113
        Page 114
        Page 115
        Page 116
    General conclusions and recommendations for future research
        Page 117
        Page 118
        Page 119
        Page 120
    A brief discription of the experimental techniques
        Page 121
        Page 122
        Page 123
        Page 124
        Page 125
        Page 126
        Page 127
        Page 128
        Page 129
        Page 130
    Computer-interfaced digital pulse counting circuit
        Page 131
        Page 132
        Page 133
        Page 134
        Page 135
        Page 136
        Page 137
        Page 138
        Page 139
        Page 140
        Page 141
    Reference
        Page 142
        Page 143
        Page 144
        Page 145
        Page 146
    Biographical sketch
        Page 147
        Page 148
        Page 149
    Copyright
        Copyright
Full Text










SURFACE CHARACTERIZATION AND CHEMISORPTION PROPERTIES
OF POLYCRYSTALLINE SYSTEMS:
Sn02, Pt/SnO2 and Zr


















By


DAVID FULLEN COX


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


1984

































Copyright 1984

by

David Fullen Cox











ACKNOWLEDGMENTS


The author would like to thank Gar Hoflund for the

guidance and encouragement he furnished in his role as

research advisor. Special thanks go to Gar also for

many hours spent in the discussion of academic and purely

nonacademic matters and for invaluable assistance rendered

in the author's quest to master the French (not to mention

the English) language, pate de foie gras! Thanks go

also to Herb Laitinen for the benefit of his expertise

on tin oxide in all its forms and for providing the

laboratory facilities used for sample preparation. Thanks

go to Paul Holloway for his helpfulness in allowing the

use of his SIMS apparatus and for his patience in enduring

the associated visits. A special thank you also goes

to Dick Gilbert at the University of Nebraska for a

multitude of software and hardware contributions which

played such a major role in all the results presented

here.

The author thanks G.B Hoflund and H.A. Laitinen

for financial support supplied through sponsored research

grants. Thanks go also to the Department of Chemical

Engineering and the State of Florida for financial support.

Lastly, the author acknowledges the Florida Guaranteed

Student Loan Program for unilateral support during the

final six months of his degree program.


iii













TABLE OF CONTENTS


PAGE

ACKNOWLEDGMENTS iii

LIST OF TABLES vi

LIST OF FIGURES vii

ABSTRACT xi

SECTION

ONE GENERAL INTRODUCTION 1

Motivation 1
Format 4

TWO AN XPS INVESTIGATION OF TIN
OXIDE SUPPORTED PLATINUM 8

Introduction 8
Experimental 10
Results and Discussion 13
Conclusions 21

THREE AN ELECTRONIC AND STRUCTURAL
INTERPRETATION OF TIN OXIDE
ELS SPECTRA 23

Introduction 23
Experimental 25
Background 26
Results and Discussion 29
Conclusions 56

FOUR A STUDY OF THE DEHYDRATION OF
TIN OXIDE SURFACE LAYERS 58

Introduction 58
Experimental 61
Results and Discussion 62
Conclusions 71







PAGE


FIVE AN OBSERVATION OF WATER ADSORPTION
ON TIN OXIDE USING ESD AND
GRAZING-EXIT-ANGLE XPS AND AES 73

Introduction 73
Experimental 74
Results and Discussion 81
Conclusions 90

SIX THE INTERACTION OF POLYCRYSTALLINE
ZIRCONIUM WITH 02, N2, CO AND N20 91

Introduction 91
Experimental 92
Results and Discussion 93
Conclusions 115

SEVEN GENERAL CONCLUSIONS AND
RECOMMENDATIONS FOR FUTURE
RESEARCH 117

Pt Sn Oxide 117
Zirconium 119

APPENDICES

A A BRIEF DESCRIPTION OF THE
EXPERIMENTAL TECHNIQUES 121

X-Ray Photoelectron Spectroscopy (XPS) 121
Auger Electron Spectroscopy (AES) 124
Electron Energy-Loss Spectroscopy (ELS) 126
Electron-Stimulated Desorption (ESD) 128

B COMPUTER-INTERFACED DIGITAL PULSE
COUNTING CIRCUIT 131

Introduction 131
Circuit Description 132
Time-of-Flight Modification 139
Acknowledgments 140

REFERENCES 142

BIOGRAPHICAL SKETCH 147












LIST OF TABLES


PAGE

TABLE

3-1. Electronic Configurations of Atomic
Tin and Oxygen and the Stoichiometric
Oxides. 40


5-1. Variation in O/Sn Ratio With Emission
Angle. 83













LIST OF FIGURES


FIGURE PAGE

2-1. Pt 4f XPS spectrum of an electro-
chemically platinized substrate.
The lower binding energy doublet
is characteristic of Pt metal,
and the higher binding energy
doublet is characteristic of
a Pt species chemically bonded
to be the tin oxide substrate. 14

2-2. Pt 4f XPS spectra of a sample
prepared by platinum chemisorp-
tion. Spectrum (A) is obtained
immediately after pumpdown,
(B) after high temperature oxida-
tion, (C) after high temperature
reduction and (D) after a high
temperature anneal in vacuo. 17

3-1. Energy level diagram representing
the SnO2 band structure. The
locations of the major occupied
(unoccupied) valence and core
(conduction) states involved
in the energy loss spectrum
are shown. The approximate
locations of the SnO VBM and
CBM are indicated by dashed
lines. 30

3-2. N(E) ELS spectrum of tin oxide
after a high temperature vacuum
anneal. The high primary beam
energy (Ep = 1500 eV) at normal
incidence results in primarily
a bulk sensitivity. The indicated
features are characteristic
of well-annealed SnO2. The
low energy features are due
to VB ----> CB transitions,
and the high energy loss features
are core ----> CB transitions. 33


vii






PAGE


3-3. Variation in the N(E) ELS spectrum
with primary beam energy, Ep.
The set of spectra represent
a depth profile of the annealed
material. The growth of the
27 eV feature is due to an
increasing oxygen deficiency
as the spectra become more surface
sensitive. 36

3-4. EN(E) ELS spectrum for Ep = 50
eV. The main loss feature at
13 eV shows that the annealed
material is essentially SnO
at the surface. 37

3-5. ELS spectra for a sample annealed
at 6000C. The angle of incidence
for the primary beam is 45.
With Ep = 1500 eV it is seen
that the bulk is primarily SnO2.
The Ep = 300 eV spectrum shows
SnO at the surface and evidence
of a defect structure between
bulk SnO2 and the surface. 44

3-6. Valence band XPS spectra after
a (A) 6000C anneal and (B) 2
KeV argon-ion bombardment. The
spectra are more bulk than surface
sensitive. 46

3-7. ELS spectra following 2 KeV argon-
ion bombardment. The valence
band features show the bulk
to be SnO21ike while the core
features reveal a significant
concentration of defects. The
VB features in the more surface
sensitive spectrum illustrate
an amorphous structure at the
surface due to sputtering. 49

3-8. Valence band XPS spectra following
(A) a short 5000C anneal after
sputtering and (B) subsequent
oxygen exposure. The presence
of a mixture of SnO and SnO2
is indicated in (A). 52


viii







PAGE


3-9. ELS spectra corresponding to Figures
3-8(A) and (B) respectively. 54

4-1. Valence band XPS spectra after
(A) hydration by exposure to
atmospheric humidity, (B) a
5000C vacuum anneal for 45 minutes
and (C) a 6000C vacuum anneal
for 30 minutes. 64

4-2. ELS spectra of (A) subsurface
and (B) surface regions after
hydration due to atmospheric
humidity. 65

4-3. ELS spectra of (A) subsurface
and (B) surface regions after
a 5000C vacuum anneal. 68

4-4. ELS spectra of (A) subsurface
and (B) surface regions after
a 6000C vacuum anneal. 70

5-1. Variation in path length with
emission angle. 76

5-2. Deflection circuit for desorption
event initiation. 79

5-3. Time-of-flight spectrum for mass
analysis. 86

5-4. Ion kinetic energy distribution
after sputtering. 88

5-5. Ion kinetic energy distribution
following water exposure. 89

6-1. AES spectra taken after (A) 2
hours of heating and (B) 14
hours of heating below the HCP-to-
BCC transition temperature. 94

6-2. AES spectra of state 1 zirconium
after room temperature exposure
to (A) nitrogen and (B) nitrous
oxide. 98





PAGE


6-3. XPS spectra showing the zirconium
3d peaks for (A) clean zirconium,
(B) N2 exposure, (C) N20 exposure
and (D) oxygen exposure 99

6-4. XPS spectra showing the zirconium
3s peak for (A) clean zirconium,
(B) nitrogen exposure and (C)
N20 exposure. The nitrogen
Is peak appears at 396 eV in
(B) and (C) 100

6-5. XPS spectra showing the oxygen
Is peak after (A) 02 adsorption
on zirconium and (B) N20 adsorp-
tion on zirconium 102

6-6. AES spectrum for clean zirconium
after heating near the melting
temperature for 3 hours. The
AES 175 eV peak is greatly dimi-
nished which is characteristic
of state 2 zirconium 104

6-7. AES spectrum taken after clean
state 2 zirconium is exposed
to CO contamination from the
electron beam for 8 hours. The
carbon peak shows characteris-
tics of both graphite and carbidic
carbon 106

6-8. XPS spectrum of the carbon Is
peak corresponding to the AES
spectrum shown in Figure 6-7.
Both graphitic and carbidic
carbon are present 107

6-9. AES spectra after exposing state
2 zirconium to CO at (A) room
temperature and (B) high tempera-
ture but allowing the sample
to cool during the exposure 108

6-10. XPS spectra of the carbon Is peak
corresponding to the AES spectra
shown in Figure 6-9. The room
temperature adsorption produces
approximately equal amounts
of graphitic and carbidic carbon
as shown in spectrum (A) while
the high temperature adsorption
results in predominantly carbidic
carbon as shown in spectrum
(B) 109







PAGE


6-11. (A) AES spectrum taken after expos-
ing state 2 zirconium to nitrogen
at 5x10-6 Torr for 15 minutes
at room temperature. A small
amount of carbon and oxygen
contamination accumulated during
the long exposure and subsequent
AES run. (B) AES spectrum taken
after exposing state 2 zirconium
to nitrogen initially at high
temperature and then allowing
the sample to cool during the
exposure. (C) AES spectrum
taken after allowing the sample
to remain in vacuum for 3 days
at room temperature. State
2 zirconium has transformed
into state 1 zirconium. (D)
AES spectrum taken after exposing
the state 1 zirconium of spectrum
(c) to nitgrogen at 5x10-6 Torr
for 5 minutes at room temperature. 111

A-1. Photoemission process. 122

A-2. KL1L2 Auger decay process. 125

A-3. Electron energy-loss process. 127

B-l. On-board timer schematic showing
jumper selectable system clock
rates. 133

B-2. Schematic of the control logic
section. 135

B-3. (A) Timing and (B) event counter
schematics. 137

B-4. Layout and wiring diagram. All
unmakred (pull up) resistors
are 10K ohms. 138













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy



SURFACE CHARACTERIZATION AND CHEMISORPTION PROPERTIES
OF POLYCRYSTALLINE SYSTEMS:
Sn02, Pt/SnO2 and Zr

By

DAVID FULLEN COX

December, 1984



Chairman: Gar B. Hoflund

Major Department: Chemical Engineering

X-ray photoelectron spectroscopy (XPS) is used to

characterize platinum supported on tin oxide. A feature

in the platinum 4f XPS spectrum associated with the bond

formed between supported platinum and the tin oxide

substrate is identified. The bond is believed to form

with surface lattice oxygen resulting in a Pt-O-Sn surface

species. This substrate-bonded species appears to act

as a nucleation site for cyrstallite growth in both the

electrochemical desposition of platinum and in the

sintering of supported platinum.

It is demonstrated that electron energy-loss

spectroscopy (ELS) is an acceptable technique for


xii








distinguishing between the different oxides of tin. The

major features in the N(E) loss spectrum are interpreted

as due to collections of optically allowed interband

transitions. It is shown that depth profile information

about tin oxide may be obtained by varying the primary

electron beam energy. Combined ELS and valence band

XPS results indicate that a significant amount of

structural information may be inferred from -the size,

shape and/or position of the N(E) ELS features. Core

level features are found to be quite sensitive to the

presence of defects in an SnO2 lattice with some

specificity as to the type of defect.

The chemisorption properties of polycrystalline

zirconium have been found to vary dramatically depending

on the thermal history of the sample. Chemisorption

on this surface is found to be suppressed by heating

for prolonged periods of time above the HCP-to-BCC phase

transition temperature at 11350K. The chemisorption

behavior can be correlated roughly with the appearance

or disappearance of a zirconium MVV Auger peak. A slow

phase transition at the surface is postulated as the

cause of the variation in chemisorption properties.


xiii












SECTION I
GENERAL INTRODUCTION



Motivation

The primary motivation for the work presented here

is an interest in the catalytic properties exhibited

by tin oxide supported platinum. It has been demonstrated

that platinized tin oxide surfaces display higher catalytic

activities than platinum electrodes in the electrochemical

oxidation of methanol [1-5] and for the reduction of

oxygen in alkaline [6] and 85% phosphoric acid solutions

[7]. A similarity between the electronic properties

of platinum in these supported systems and in the

industrial platinum-tin bimetallic reforming catalyst

supported on alumina has been demonstrated recently [8].

The bimetallic catalyst is known to exhibit improved

stability and higher average catalytic activity than

platinum supported on alumina [9,10].

Tin oxide and its modified forms also exhibit a

significant catalytic activity. Tin oxide has been shown

to be active for the catalytic oxidation of CO [11,12]

and the reduction of NO [12-14]. Chromia-doped tin oxide

is very active for the reduction of NO with CO, H2 and

C2H4 [13]. Antimony-doped tin oxide is known to be active








toward the selective (partial) oxidation of propylene

to acrolein [15] and the oxidative dehydration of butene

to butadiene [16]. Much of this behavior is believed

to be linked to the oxidation and reduction of active

sites on the catalyst surface.

The catalytic properties of the tin oxide support

are believed to play an important role in platinized

tin oxide catalysts. For example, Tseung and Dhara [7]

have postulated that a spillover mechanism may be involved

in the electrochemical reduction of oxygen. Their results

suggest that adsorbed oxygen migrates from the supported

platinum to the tin oxide surfaces before undergoing

reduction. The possible importance of spillover mechanisms

in platinum/tin oxide catalyzed reactions and the apparent

redox behavior of tin oxide surfaces both indicate that

a successful surface characterization of the supported

system must include a determination of the tin oxidation

state.

The main focus of the work presented here is the

application of ultrahigh vacuum (UHV) surface probes

to a fundamental characterization of tin oxide and

platinized tin oxide catalysts. It is hoped that this

characterization will aid in understanding the

physiochemical properties affecting the preparation and

the catalytic behavior of these systems. To this end,

a realistic system is studied which utilizes








polycrystalline tin oxide as the support material. The

polycrystalline nature of the support presents immediate

problems in terms of structural characterization because

the usual surface-structure-sensitive technique, low-energy

electron diffraction (LEED), is not applicable. For

this reason the approach primarily has been to use electron

spectroscopies as probes of the electronic properties

of the materials. The direction taken in the work

presented here has been influenced by some earlier studies

which have already been reported in the literature [17-19].

The earlier results will not be repeated here but will

be referenced when appropriate to the discussion of present

results.

One unforeseen result of the study of tin oxide

has been a broadening of interests to include structural

effects in electronic spectra (see Sections III and IV).

Out of this interest has grown a study of the chemisorption

properties of clean polycrystalline zirconium.

Observations in the literature of anomalous effects on

the Auger electron spectrum of zirconium due to gas

adsorption combined with conflicting results on the uptake

rate of adsorbates are responsible for the selection

of zirconium for study. Though not directly related

to the characterization of platinized tin oxide, these

zirconium results are also presented here.







Format

The results presented here are divided into

independent sections, each of which is complete within

itself. Each section deals with one aspect of the surface

characterization of a platinum-tin oxide catalyst with

the exception of Section VI which deals with the apparent

structural dependence of the chemisorption properties

of polycrystalline zirconium.

Section II presents results on the characterization

of supported platinum on tin oxide. X-ray photoelectron

spectroscopy (XPS) has been used in this characterization

to investigate the valence (oxidation) states of the

supported platinum. In situ thermal and chemical

treatments are used to help identify the various platinum

oxidation states and to investigate the nature of the

chemisorption bond formed between platinum and the tin

oxide substrate.

While XPS has proven useful in determining the valence

states of supported platinum, it has been of limited

use in characterizing the oxidation state of tin in the

support. Though core level XPS can distinguish between

metallic tin and tin oxide, it can not distinguish between

the different oxides of .tin, SnO and SnO2. In view of

the apparent importance of the determination of the tin

valence state, the majority of effort has been devoted

to finding a suitable characterization technique for








the tin oxide support. The chosen technique is electron

energy-loss spectroscopy (ELS).

Section III is a discussion of the use of ELS in

the characterization of tin oxide. An interpretation

of the spectra is given in terms of the excitation of

interband transitions. Using ELS in conjunction with

valence band XPS, it is demonstrated that a significant

amount of structural information about polycrystalline

tin oxide may be inferred from changes in the electronic

structure as probed by ELS.

An understanding of the interaction of water with

tin oxide surfaces is believed to be of primary importance

in elucidating the chemisorption properties of platinum

on tin oxide. This belief is supported by considerable

evidence that the active chemisorption sites are surface

hydroxyl groups [6,19]. In Section IV ELS is applied

to the study of the dehydration of a tin oxide sample

after exposure to atmospheric humidity. The flexibility

of analysis depth provided by this technique affords

a particularly worthwhile characterization of the

subsurface region.

Because of a phenomenon known as electron-stimulated

desorption (ESD), ELS is not useful in studying the

interaction (i.e. adsorption) of water with the surface

of tin oxide. The incident electron beam used for ELS

actually removes the adsorbed species of interest from








the surface. This phenomenon has been observed in the

earlier studies [17-19] which show that significant surface

modification can result from an impinging electron beam.

However, the ESD phenomenon can itself be used to probe

the interaction between surface and adsorbate.

Section V presents some preliminary observations

of the adsorption of water on tin oxide.

Grazing-exit-angle XPS provides a measure of the water

adsorbed from the background vacuum in the UHV system.

Preliminary results are also presented which demonstrate

the potential of ESD in characterizing the interaction

of water with tin oxide surfaces.

Section VI presents a study of the chemisorption

properties of polycrystalline zirconium. The study

focuses on the chemisorption behavior as a function

of the thermal history of the sample. A slow phase

transition in the surface region is postulated as the

cause of a suppression in chemisorption after high

temperature annealing. A zirconium feature in the Auger

electron spectrum is shown to be an indicator of the

chemisorption properties of the surface.

Section VII presents a summary of conclusions for

the entire study along with recommendations for future

research. Two appendices are included after Section

VII. Appendix A contains a brief description of the

physical processes involved in the experimental techniques





7


used in this work. A brief introduction to XPS, AES,

ELS and ESD is given. Appendix B describes the

computer-interfaced digital pulse counting circuit used

for data collection in XPS, ELS and time-of-flight ESD

measurements.













SECTION II
AN XPS INVESTIGATION OF TIN OXIDE
SUPPORTED PLATINUM



Introduction

The study of tin oxide supported platinum is motivated

by the interesting catalytic properties displayed by

mixed Pt-Sn systems. It has been shown that platinized

tin oxide exhibits a catalytic activity 50 to 100 times

greater than that of platinum electrodes for the

electrochemical oxidation of methanol [1-5]. Watanabe

et al. [6]. have studied the effects of platinum loading

on the catalytic activity for oxygen reduction in alkaline

solution. Their results show that the catalytic activity

of highly dispersed platinum on tin oxide may exceed

that of platinum electrodes by a factor of four or more

[6]. An interest in impure H2 fuel cells using 85%

phosphoric acid at 1500C has prompted Tseung and Dhara

[7] to study supported Pt on antimony-doped tin oxide

because of the corrosion resistance and electrical

conductivity exhibited by this system. Their results

show a significant increase in the catalytic activity

for oxygen reduction over that of platinum black. In

addition, the similarity between platinized tin oxide





9

and the Pt-Sn bimetallic reforming catalyst has been

demonstrated recently [8].

The catalytic effects of low coverages of strongly

bound oxygen on Pt single-crystal surfaces has been

demonstrated by Smith, Biberian and Somorjai [20]. They

interpret dramatic, oxygen-coverage-dependent changes

in activity and selectivity for hydrogenation and

dehydrogenation reforming reactions as being due to the

formation of surface Pt oxides. Interestingly, it has

been shown that the formation of Pt-Sn alloys generally

results in lower catalytic activities [21-24]. These

observations suggest that oxygen may be partially

responsible for the catalytic properties exhibited by

Pt/tin oxide systems.

Several X-ray photoelectron spectroscopy (XPS) studies

have been performed on Pt-Sn systems [4,5,8,25]. These

studies all agree that tin is largely present as tin

oxide while platinum is present in the metallic form

and as Pt2+ and Pt4+ in the form of oxides and hydroxides.

These previous XPS studies have examined intimately mixed

systems of tin oxide and platinum oxides and metal. In

the present work a supported platinum system is studied.

The use of this system in conjunction with in situ chemical

and thermal treatments has allowed the assignment of

a Pt "oxidation state" characteristic of the chemical

bond formed between the metal and the tin oxide substrate.








Experimental

The tin oxide substrates are prepared by the thermal

hydrolysis of Sn (IV) from a solution containing 3 M

SnC14 5H20, 1.5 M HC1 and 0.03 M SbC13. The solution

is sprayed onto the hot surface of a titanium foil held

at 5000C in air. The formation of tin oxide occurs

according to:



SnC14 + 2H20 -----> Sn02 + 4HC1



The resulting planar film is a polycrystalline, n-type,

Sn02 semiconductor with the rutile structure. The antimony

is incorporated into the film at a concentration

approximately twice that of the spray, i.e. 2% [26].

This dopant is known to be in solid solution with the

tin oxide [27], and it acts as a donor which raised the

conductivity of the film to a level adequate for

electrochemical studies [26]. Spraying is continued
0
until a tin oxide layer approximately 6000 A to 7000
0
A thick is obtained as determined by the colors of the

interference fringes of the layer. After cooling in

air, the samples are polished with 0.25 pm diamond paste.

The use of alumina as a polishing compound is avoided

because of the overlap of platinum 4f and aluminum 2p

peaks in XPS [8].

A previous Auger electron spectroscopy (AES) and

XPS investigation has shown that the surface of an







antimony-doped tin oxide film prepared in the described

manner may contain a number of surface contaminants [17].

Among these surface contaminants are carbon, chlorine,

potassium, sodium, calcium and sulfur in varying amounts.

Argon ion bombardment and high temperature oxygen

treatments have proven to be effective in removing this

surface contamination, but the effects on the tin oxide

surface (see Section III) and platinum oxidation state

(as shown below) are substantial. Hence, an understanding

of surfaces such as those being used in electrochemical

studies [6,28] may require analysis in the presence of

several types of surface contamination.

Two different techniques are used in the present

study to prepare the supported Pt. The first of these

techniques is electrochemical in nature. In essence,

the platinum is plated from a 5x10-5 M solution of H2PtCl6

buffered at a pH of 6.8. The process is carried out

for varying amounts of time at -0.5 V versus SCE. The

second technique utilizes a chemisorption mechanism.

The substrate is pretreated by exposure to a 10 M NaOH

solution at 900C for 30 minutes. The pretreated substrate

is washed in distilled water and then exposed to an 800C

solution of 0.01 M KOH and 500 ppm Pt (IV) from Na2Pt(OH)6.

The Pt loading is dependent on the exposure time for

both preparation techniques. Regardless of the procedure

used, the samples are washed in distilled water after







platinization and solvent cleaned before mounting in

the vacuum system.

An important consideration in the preparation of

these supported platinum catalysts is the relative rate

of crystallite nucleation to growth. The indications

are that the growth of crystallites is favored over

nucleation in the electrochemical preparation [29]. The

chemisorption technique, however, has been shown to be

capable of producing highly dispersed (>90%) platinum

at low loadings [6]. The alkaline pretreatment is believed

to hydroxylate the surface, thereby increasing the number

of active sites available for Pt chemisorption [6,19].

Electron beam effects on these surfaces can be

dramatic. The removal of carbon, chlorine, oxygen,

hydrogen and sodium by electon-stimulated desorption

(ESD) has been observed previously [17-19]. These beam

effects somewhat limit the usefulness of AES as an

analytical tool on these surfaces making XPS the preferred

technique because of its less destructive nature. However,

an understanding of the ESD phenomenon in terms of an

interatomic Auger decay model [30] shows promise in helping

to unravel the chemistry of these surfaces (see Section

V and ref. 19).

All XPS spectra were collected with a Physical

Electronics double-pass CMA using Mg Ka X-rays as an

excitation source. A pass energy of 50 eV (AE/E = 0.014)





13

was used throughout. All binding energies are referencecd

to the tin 3d 5/2 peak at an assumed energy of 486.4

eV [31]. It has been shown that there is no change in

this core level binding energy for the different oxides

of tin [32-34] making this peak an excellent reference.

The base pressure in the vacuum system for this study

was lx10-9 Torr. Details of the vacuum system have been

given previously [17].



Results and Discussion

Figure 2-1 shows the Pt 4f XPS spectrum of an

electrochemically platinized sample. The plating process

was carried out for 40 minutes at -0.5 V versus SCE.

A fairly high platinum loading of about 40 pg/cm2 is

obtained by this process as estimated from a previous

Rutherford backscattering (RBS) study [35]. Deconvolution

of the spectrum reveals the presence of two platinum

species. In the assignment of these features platinum

chloride species are neglected. A check for surface

chlorine contamination using standard sensitivity factors

[31] showed the concentration to be low (Cl/Pt < 0.1).

The lowest binding energy doublet in Figure 2-1

has the 4f 7/2 peak at 71.2 eV and the 4f 5/2 peak at

74.5 eV. These features are assigned to Pt metal in

agreement with the work of Katayama [5,25]. The binding

energy reported here is about 0.4 eV higher than that







































77 76 75 74 73 72 71 78

BINDING ENERGY CEV)

Figure 2-1. Pt XPS spectrum of an electrochemically plati-
nized substrate. The lower binding energy doublet is
characteristic of Pt metal, and the higher binding energy
doublet is characteristic of a Pt species chemically bonded
to the tin oxide substrate.







generally reported for the bulk Pt metal [31]. This

observation of a higher binding energy for supported

clusters crystallitess) over that of bulk metal is in

agreement with general expectations [36]. This shift

is most likely the result of differences in the reference

levels (work functions) of the bulk metal and the tin

oxide support and/or a decrease in the final-state

extra-atomic relaxtion energy as a result of the change

from bulk metal to small cluster [37].

The doublet at higher binding energies in Figure

2-1 has a 4f 7/2 peak at 72.3 and a 4f 5/2 peak at 75.5

eV. This doublet is shifted about 1.1 eV above Pt metal

and about 0.5 eV below the position expected for a Pt(OH)2

species [5,25,31]. The higher oxides of platinum all

fall to significantly larger binding energies which removes

them from consideration (see below). Examination of

a second substrate electrochemically plated for only

1/4 the time (i.e. 10 minutes) gives a Pt 4f spectrum

(not shown) characterized primarily by this high binding

energy species observed in Figure 2-1. These results

suggest that the higher binding energy doublet in Figure

2-1 may be associated with a platinum species directly

bonded to the tin oxide substrate. Further, the appearance

of Pt metal at longer plating times demonstrates that

this substrate-bonded Pt species acts as a nucleation

site for the growth of metallic crystallites by







electrochemical deposition. These results are consistent

with a model of nucleation and crystallite growth suggested

by earlier work on the electrodeposition of platinum

on tin oxide [29].

Figure 2-2 shows the Pt 4f peaks for a sample prepared

by the chemisorption technique. The pretreated tin oxide

substrate was exposed to the 800C Na2Pt(OH)6 solution

for one hour. The platinum loading is approximately

2 .g/cm2. Because the Pt loading is small, the

signal-to-noise ratio does not justify a spectrum

deconvolution. However, the use of in situ chemical

and thermal treatments allows a manipulation of the Pt

valence state for a more complete determination of the

supported species. Because only a trace of chlorine

was detected, the possibility of platinum chloride species

was again discounted.

Figure 2-2a shows the spectrum obtained immediately

after pumpdown. The position of the doublet indicates

that the substrate-bonded species is the predominant

form of platinum obtained by the chemisorption procedure.

However, the peak widths also suggest the presence of

small amounts of Pt metal at lower binding energies and

Pt(OH)2 at slightly higher binding energies. The

observation of the substrate-bonded species as the primary

form of platinum is consistent with earlier work showing

nucleation is preferred over crystallite growth during

chemisorptive platinization [6].







' I LU I I I 1 1 1f 1 111 I i

PT 4F
PT


PT(OH)m J

PT-O-SN


'I 'l l i I I r i I I I, I I i,


BINDING ENERGY (EV)'


Figure 2-2. Pt XPS spectra of a sample prepared by plati-
num chemisorption. Spectrum (A) is obtained immediately
after pumpdown, (B) after high temperature oxidation,
(C) after high temperature reduction and (D) after a
high temperature anneal in vacuo.






Figure 2-2b shows the effect of a high temperature

(6000C) in situ oxidation in 11 Torr of 02 for 30 minutes.

The oxygen treatment shifts the Pt 4f XPS peaks to higher

binding energies. The presence of the higher oxides,

PtO and Pt02, is clearly indicated by structure on the

high binding energy side of the specturm. The PtO features

are shifted approximately 2.9 eV higher with respect

to Pt metal while the PtO2 features are shifted about

3.9 eV [31]. Though there is no evidence of Pt metal

in Figure 2-2b, the shoulder at 72.3 eV is clear evidence

of the persistence of the substrate-bonded Pt species.

The observation of this species in conjunction with PtO

and Pt02 confirms that the substrate-bonded species is

not simply a stoichiometric platinum oxide.

A 5000C in situ reduction in lxl0-3 Torr of H2 for

30 minutes results in the spectrum shown in Figure 2-2c.

An XPS inspection of the Sn 3d core levels shows no sign

of a reduction of the substrate to bulk Sn metal. However,

the PtO and PtO2 species observed in Figure 2-2b have

undergone a complete reduction leaving primarily Pt metal.

The loss of the higher oxides coupled with the appearance

of a Pt 4f 7/2 peak at 71.2 eV confirms the earlier

assignment of this binding energy to Pt metal. Evidence

of the substrate-bonded species is also found in Figure

2-2c in the form of a shoulder on the high binding energy

side of the 4f 5/2 peak.




19

A subsequent 8000 anneal in vacuo has little effect

on the XPS peak positions as shown by Figure 2-2d. The

platinum remains primarily in the metallic state with

a small contribution due to the substrate-bonded species.

The presence of this substrate-bonded species after high

temperature annealing confirms that these features are

not due simply to a platinum hydroxide or hydrate species.

Decomposition or dehydration of such species would be

expected at significantly lower temperatures.

Before the oxidation-reduction cycle the platinum

from the chemisorption preparation is largely present

in the substrate-bonded form. This observation suggests

a high dispersion as found by Watanabe et al. for samples

prepared in a similar fashion [6]. The high temperature

oxidation-reduction cycle results in a sintering of the

supported species as shown by the large fraction of Pt

metal in Figure 2-2c. The remaining presence of the

substrate-bonded species suggests that a fraction of

these species acts as nucleation sites for the crystallite

growth as was observed in the electrochemical platinization

process.

The constancy of the XPS peak positions for the

substrate-bonded species obtained by either the

electrochemical or chemisorption process indicates a

similarity in the species formed regardless of the





20

procedure used. It has been shown above that this species

may not be identified as simply a PtOx or Pt(OH)y species.

Likewise, the binding energy shift of this species with

respect to Pt metal is not consistent with that observed

in the formation of Pt-Sn alloys [38]. These observations

suggest that the bond formed with the surface occurs

through surface lattice oxygen. The formation of a Pt-O-Sn

substrate-bonded Pt species is postulated. The tenacity

displayed by this species in resisting complete reduction

by chemical and thermal treatments is characteristic

of a species exhibiting such a strong interaction with

the substrate. Komiyama et al. have observed a similar

resistance to reduction by ion bombardment of strongly

interacting rhenium species on iron oxide [39].

Previous work on samples prepared by the chemisorption

technique offers insight into the mechanism of formation

of the substrate-bonded species. Watanabe et al. Have

shown that an alkaline pretreatment of the substrate

prior to platinization results in an increased Pt uptake

[6]. It is believed that the pretreatment hydroxylates

the surface and provides an increased number of active

chemisorption sites for the platinum species in solution.

Earlier studies using secondary-ion mass spectrometry

(SIMS) [40] and ESD [19] lend support to the surface

hydroxylation model by showing significant increases

in surface hydrogen and oxygen after the alkaline







pretreatment. Platinum chemisorption is believed to

occur by replacement of the proton on the surface hydroxyl

group with the loss of a coordinated ligand from. the

platinum solution species. Under the pH conditions used

for chemisorption from a H2PtCl6 solution, the

chloroplatinate undergoes hydrolysis resulting in the

replacement of two chlorines by hydroxyl groups.

Chemisorption should occur via



Sn-OH + Pt(OH)2C142 ----> Sn-O-Pt(OH)C142- + H20



with a subsequent dehydroxylation and loss of chlorine

from the surface complex. For chemisorption from an

alkaline solution of Na2Pt(OH)6 the surface hydroxyl

group is ionized through the loss of the acidic proton.

Chemisorption is expected to occur via



Sn-O- + Pt(OH)62- ----> Sn-O-Pt(OH)52- + OH-



leaving the substrate-bonded species after dehydration

of the surface complex.



Conclusions

XPS has been used to study tin oxide supported

platinum prepared by electrochemical and chemisorption

techniques. Features in the Pt 4f spectrum have been







assigned to a species chemically bonded to the substrate.

The position of these features is independent of the

preparation technique used. In situ chemical and thermal

treatments confirm that this substrate-bonded platinum

is not simply a PtOx or Pt(OH)y species. The platinum

is believed to bond through surface lattice oxygen giving

a Sn-O-Pt surface species. High temperature reduction

results in a sintering of these species, but the inability

to completely reduce the platinum is indicative of the

strong chemical interaction between the platinum and

tin oxide.

A model for the chemisorption of platinum on tin

oxide is proposed. Surface hydroxyl groups are believed

to be the active chemisorption sites for platinum species

in solution. The chemisorption process is believed to

occur through the replacement of the hydroxyl group proton

with the loss of a coordinated ligand from the platinum

species.













SECTION III
AN ELECTRONIC AND STRUCTURAL INTERPRETATION
OF TIN OXIDE ELS SPECTRA



Introduction

The spectroscopic study of tin oxide surfaces is

complicated by the difficulty in distinguishing between

the two oxides of tin, SnO and SnO2. Several x-ray

photoelectron spectroscopy (XPS) studies have failed

to detect any changes in core level binding energies

between SnO and SnO2 [32-34]. Similar problems are

encountered using Auger electron spectroscopy (AES) where

no significant differences in kinetic energies or line

shapes are found [41]. As expected, however, the valence

band spectra of the two oxides do differ. An ultraviolet

photoelectron spectroscopy (UPS) study of tin oxidation

by Powell and Spicer [42] and a valence-band XPS study

by Lau and Wertheim [32] have shown these differences,

but interpretation difficulties associated with analysis

depth have proven to be substantial.

Electron energy-loss spectroscopy (ELS) is a technique

which offers flexibility of analysis depth and is sensitive

to changes in the valence band density of states. Powell

[41] has shown that ELS may be used to distinguish between




24

the two oxides of tin and has given a preliminary

interpretation of the spectra in terms of differences

in plasmon frequencies. For SnO2 a main loss feature

at 19.5 eV was identified while for SnO a main loss feature

was found at approximately 13.5 eV. A combined UPS

and high-resolution electron energy-loss spectroscopy

(HREELS) study of 3% Sb doped and undoped SnO2 has shown

the room temperature occupied conduction bands to be

very free-electron like [43] in agreement with a bulk

tight-binding band structure calculation [44]. For the

heavily doped sample an HREELS loss feature at 0.55 eV

was found. Based on the experimentally determined carrier

concentration and effective mass ratio, the 0.55 eV loss

feature was identified as a surface plasmon loss associated

with conduction band electrons from Sb donors. Since

valence band and core level electrons in Sn02 are not

free-electron like, the higher energy ELS losses in the

present study are not assigned to plasmon losses.

While an interpretation of the ELS spectrum would

be useful for distinguishing between the two oxides of

tin, an additional benefit may be derived due to the

usefulness of ELS measurements in the interpretation

of electron-stimulated desorption (ESD) threshold studies.

It has been shown that core level transitions can be

correlated with desorption thresholds and may specify

adsorbate binding sites [30,45,46]. In particular, the







ability to distinguish between transitions from Sn 4d

and 0 2s core levels which cannot be resolved using XPS

could be most useful in understanding the chemistry of

tin oxide surfaces.



Experimental

The polycrystalline tin oxide films used in this

study were prepared by spraying a solution of 3 M SnCl4

and 1.5 M HC1 onto a titanium foil maintained at 5000C

in air. Unlike the samples used in Section II and in

previous studies [17-19], a high purity (99.998%) anhydrous

SnCl4 reagent was used. The resulting samples were found

to have significantly less surface contamination. Trace

chlorine and carbon contamination was found to be removed

quickly in situ by heating at 5000C in 10 Torr of oxygen

for about 5 minutes. This procedure gave a clean oxide

surface as determined by AES.

The samples were annealed in vacuo initially and

were heated briefly and allowed to cool before each

individual measurement. Using angle-resolved ultraviolet

photoelectron spectroscopy (ARUPS) on an ion-sputtered

SnO2 (001) single crystal surface, Gobby [47] has shown

that the annealing process (5500C to 8350C) strengthens

the primary emission from the valence bands and increases

the sharpness and magnitude of the anisotropic emission

indicating a well ordered crystal. Similar annealing








effects in the sharpness and magnitude of ELS spectra

and on the magnitude of core level emission in XPS have

been observed for polycrystalline tin oxide samples in

this study.

All spectra were collected with a double-pass CMA.

Details of the vacuum system have been published previously

[17]. The ELS data were taken in the retarding (N(E))

mode to allow a comparison with the data of Powell [41].

All ELS spectra were collected with a pass energy of

25 eV ( A E/E = 0.014) with the exception of the 50 eV

primary beam measurement. This spectrum was recorded

in a nonretarding (EN(E)) mode to suppress the large

signal from secondary electrons at near zero kinetic

energies. All ELS spectra were collected using 100 nA

beam currents and pulse counting detection. The XPS

spectra were taken using a Mg K a source and a 50 eV

analyzer pass energy. The base pressure in the vacuum

system for this study was 1 x 10-10 Torr.



Background

For energy losses of the magnitude of electronic

excitations, the inelastic scattering event may be

described in terms of optical (dipole) selection rules

in cases where the primary electron energy is high enough

to justify the Born approximation. It is generally thought

that primary electron beam energies above 100 eV to 200

eV satisfy this criterion [48-51].




27

Consider a primary electron of momentum h K scattered

inelastically into a state h K' resulting in an interband
4.
transition between one-electron states, Ik,l> ---->

k',l'>. Momentum conservation requires AK = k' k
+ -> ->
+ G where AK E K K' and G is a reciprocal lattice

vector. Energy conservation requires h2( K 12- 1K' 12)

= 2m( Ek',l'-k,l) where Eq,l is the eigenenergy of the
4.
one-electron state q,l>. Not only must energy and

momentum be conserved, but the matrix element


+ -+ +
T =



must be nonzero [48,51,52]. Expansion in powers of
.* 4.
(AK*r) yields the selection rules. It has been shown [48,51]

that the monopole term vanishes due to orthogonality

and that retaining only the dipole (linear) term gives



T = i AK < k + AK, 1' r k, 1>



For small AKI Rudberg and Slater [48] have shown a fair

approximation at small energy losses or large IKI may

be obtained by considering only direct transitions, k

= k'. Hence, in the regime where the Born approximation

applies the selection rules are essentially optical in

nature. To a first approximation, the energy dependence

of the loss spectrum should be similar to that measured








in optical absorption [49]. Since the momentum transfer
-4
in the ELS transition, h A K, may be different than in

the optical process, it is expected that a broadening

in the energy dependence of the ELS features will occur

with respect to the optical features [48].

Because the results from this study are for

polycrystalline samples with a random grain size which

is small compared to the excitation volume, the orientation

of K with respect to the crystal axes may be assumed

to be random. Therefore, the ELS spectra presented here

represent an average over the entire Brillouin zone.

The present results should be most comparable to optical

absorption studies of polycrystalline samples.

It should be mentioned that a breakdown in dipole

selection rules is possible for low beam energies and

large energy losses. In this case the expansion of the

phase factor, exp(i A K *r), must be carried to the

quadratic (quadrupole) term to obtain an accurate

description. Ludeke and Koma [50] and Colavita et al.

[51] have taken advantage of this effect to identify

loss features due to quadrupole-allowed transitions between

dipole-unallowed states. No such identifications have

been made in the present work.

Using a generalization of the joint density-of-states

function for optical interband transitions which includes

finite momentum changes, Ludeke and Esaki [53] have shown

that the energy-loss distribution due to transitions








from narrow, filled initial states to empty conduction-band

final states may be proportional to the conduction-band

(CB) density of states. This density-of-states

interpretation requires the initial state to be isolated

with no additional scattering channel existing near the

same energy loss. An additional complication may arise

if there is a significant modulation of the scattering

cross section due to a partial filling of the conduction

bands from a competing scattering channel originating

from a different initial state. In spite of these problems

it should be possible to obtain some picture of the CB

density-of-states in tin oxide if the Sn 4d and O 2s

core levels couple to final states of significantly

different energy.



Results and Discussion

Figure 3-1 is an energy level diagram depicting

the band structure of SnO2. The character of the

electronic states in the valence and lower conduction

bands is due to Robertson [44]. The assignment of Sn

4f character to high lying conduction band states is

due to Gobby [47]. The width of the valence bands and

the location of the three major features therein are

from the available photoemission data [32,47]. The

position of the O 2s and, Sn 4d core levels are from XPS

measurements made in this laboratory with no attempt











SN 4F


SN5P 02P


SNSS


02P LONE PAIR

MIN. BONDING 02P


02P SNSS BONDING







02S SN4D


Figure 3-1. Energy level diagram representing the SnO2
band structure. The locations of the major occupied
(unoccupied) valence and core (conduction) states involved
in the energy loss spectrum are shown. The approximate
locations of the SnO VBM and CBM are indicated by dashed
lines.


25


-22



18




-12

9.5




3.6




-2

-4


--7.5

--9.5


(EV)


"-23






31

at deconvolution. Photoemission results [47] were used

to locate the states in the conduction bands which couple

strongly to various valence and core states as discussed

below. The cut-off position at the top of the conduction

bands was determined form the ELS spectrum in Figure

3-2 based on the interpretation of the high-energy loss

features given below. While all the states are represent

by single horizontal lines, some are quite broad and

may extend over 5 eV or more. The dashed lines in the

band gap and lower conduction bands represent the

approximate location of the SnO valence-band maximum

(VBM) and conduction-band minimum (CBM) respectively.

These assignments are due to photoemission results for

SnO [32] and optical absorption on highly defect laden

tin oxide films [54].

Annealing Effects

Figures 3-2 to 3-4 show the ELS data for a sample

annealed at 7500C in vacuo. Each of these spectra were

recorded for a normal incidence primary beam of specified

energy, Ep. The annealing process was carried out until

the background chamber pressure went through a clear

maximum (about 45 minutes). Giesekke et al. [55] have

shown using thermogravimetric analysis and electron

diffraction that the decomposition of tin (IV) hydroxide

proceeds through four distinct crystalline hydrogen

containing compounds before yielding SnO2 above 6000C.







The observed pressure maximum during the annealing process

is indicative, in part, of this dehydration. The

hygroscopic nature of tin oxide and the study of hydrated

surfaces is discussed in Sections IV and V.

Figure 3-2 is the loss spectrum for a 1500 eV primary

beam. This spectrum may be divided into two parts; the

higher energy loss features above about 28 eV and the

features at lower energy losses. The lower half of the

spectrum consists of two major features at 19.5 eV and

13 eV in agreement with the SnO2 spectrum reported by

Powell [41]. Additionally, extrapolation of the linear

portion of the leading edge of the loss spectrum to the

baseline gives a minimum energy loss of 3.6 eV. This

value is equal to the best available optically determined

band-gap energy for SnO2 single crystals [56,57] and

the calculated lowest energy direct-allowed one-electron

transition ( Fr ----> F ) found by Robertson [44]. Using

constant-intial-states (CIS) ARUPS measurements and

angle-integrated UPS for SnO2 (001), Gobby [47] has shown

that VB-to-CB transitions are dominated by excitations

form an initial state about 1.5 eV below the VBM to final

states near 10 eV, 13 eV and 19 eV to 22 eV higher in

energy as shown in Figure 3-1. Inspection of Figure

3-2 reveals a shoulder in the loss spectrum near 10 eV

as well as the two higher energy features. This 10 eV

loss feature also corresponds to a collection of VB-to-CB




















VACUUM
ANNEAL
7SC

t rT t
46.1 36.2 19.5 3.6

v 12.9
z

ENERGY LOSS
SPECTRUM

Ep= 158 EV
NORMAL INCIDENCE



68 58 48 38 28 18

ENERGY LOSS (EV)

Figure 3-2. N(E) ELS spectrum of tin oxide after a high
temperature vacuum anneal. The high primary beam energy
(Ep = 1500 eV) at normal incidence results in primarily
a bulk sensitivity. The indicated features are
characteristic of well-annealed SnO2. The low energy
features are due to VB ---> CB transitions, and the high
energy loss features are core ---> CB transitions.







dipole-allowed transtions at the r point in the Brillouin

zone for bulk SnO2 as found by Robertson. It is concluded

that the lower energy loss features in Figure 3-2 are

due to collections of optically (dipole) allowed interband

(VB -> CB) transitions.

The loss features above 30 eV are strongly dependent

on the thermal history of the sample and are dominated

by core-to-conduction-band transitions from tin 4d and

oxygen 2s levels. Gobby [47] has shown that these core

levels couple to final states in two energy regimes.

Coupling to CBs which are 32 eV to 36 eV above the core

level is observed easily in UPS while coupling to the

lower CBs (the CB minimum lies approximately 26.6 eV

above the core levels) is not observable due to the

photoemission threshold and large background of secondary

electrons. At higher photon energies coupling to CBs

37 eV and higher relative to the core levels is observed.

This coupling begins to strengthen at 40 eV above the

core level, but higher energies were not used because

of a lack of photon intensity. However, a higher energy

CB final state was identified for an initial state feature

in the lower VBs. This final state falls about 45 eV

above the core level, and Gobby suggests that it is a

Sn 4f derived state (see Figure 3-1). In Figure 3-2

a range of energy-loss features from about 29 eV to 48

eV are visible. The strongest features fall near 36






35

eV and 46 eV in excellent agreement with the photoemission

results of Gobby.

On the basis of the similarities between the

photoemission results for single crystal SnO2 and the

energy-loss spectrum, Figure 3-2 is interpreted as being

characteristic of a well-annealed (though polcrystalline)

SnO2 material. Additionally, these similarities support

the conclusion that the main features observed in the

ELS spectrum are due to single inelastic events possibly

in conjunction with elastic scattering events. Because

of the long mean free path of electrons near 1500 eV,

the spectrum in Figure 3-2 (Ep = 1500 eV) is primarily

due to contributions from the bulk of the material.

Figure 3-3 shows the effect on the loss spectrum

of varying the primary beam energy from 1500 eV to 200

eV. Figure 3-4 shows the EN(E) loss spectrum for a 50

eV primary beam. Decreasing the beam energy decreases

the analyses depth due to a reduction in the electron

mean free path with kinetic energy. The set of spectra

in Figures 3-3 and 3-4, therefore, represent a depth

profile of the vacuum-annealed tin oxide material. In

Figure 3-3 the main change in the valence band region

is seen to be a growth of the 12 eV to 13 eV feature

relative to the 19 eV feature with decreasing beam energy.

This change is most apparent in Figure 3-4 where a feature

near 13 eV dominates the spectrum. Changes in the core










Ep = 1588 EV


Ep 188 EV


Ep = 688 EV


Ep = 488 EV


Ep = 200 EV


NORMAL INCIDENCE


i l 1 111111 11 l lllll l lilllil l ,,,ijji ij jjjjjjjjj jjjj
68 58 48 38 20 10
ENERGY LOSS (EV)
Figure 3-3. Variation in the N(E) ELS spectrum with
primary beam energy, Ep. The set of spectra represent
a depth profile of the annealed material. The growth
of the 27 eV feature is due to an increasing oxygen
deficiency as the spectra become more surface sensitive.


VACUUM
ANNUAL
758


111111111 111-[H 11 111iil 11 1111" 1 1ll ,,llllTl' l jTll If





48 38 28 18

ENERGY LOSS (EV)

Figure 3-4. EN(E) ELS spectrum for Ep = 50 eV. The
main loss feature at 13 eV shows that the annealed material
is essentially SnO at the surface.


z\

z
W


1 111111 1 I ll ff ll rrr i 1 1 1 1 11 11 1 1 1,1 l I I fT


VACUUM
ANNEAL
758C




Ep = 58 EV
13
NORMAL INCIDENCE

27











l l ll,, i lli, ,i l , , ,i ,l , ,i , ,l l


------------


"





38

level region are more dramatic. The core level losses

may be resolved into two features. The large loss feature

at 46 eV is seen to decrease rapidly with beam energy

leaving a separate feature near 36 eV. Concurrent with

the loss of the 46 eV feature, the growth of a feature

at 27 eV is observed.

By comparison to the work of Powell [41], the changing

valence-band derived features in Figures 3-3 and 3-4

may be loosely interpreted as a change in the tin oxide

from a SnO2 compound in the bulk to a more SnO-like

material at the surface. Because the SnO-like feature

near 13 eV dominates the spectrum only for a 50 eV primary

beam, it appears that such a material exists in the near

surface region, possibly in the top few atomic layers.

This interpretation is reasonable in view of the well

documented oxygen loss from tin oxide surfaces during

high temperature annealing [47,58,59]. Such oxygen losses

have been observed frequently in this laboratory.

Decreases in surface O/Sn ratios from near 2 down to

1 on annealing have been monitored with AES and XPS.

The interpretation of changes in the core-to-CB

region of the spectrum leads to the same conclusion as

derived from the VB-to-CB features, but some discussion

of the symmetry of the initial and final states involved

is required. The band structure calculations of Robertson

[44] and Munnix and Schmeits [60] as well as the ARUPS







measurements of Gobby [47] show that the Sn02 valence

bands are mostly 0 2p like with only a small admixture

of Sn derived states. The lower conduction bands are

primarily Sn 5s and 5p like, and within 3 eV to 4 eV

of the CBM these states are 90% Sn 5s like [441. To

a first approximation the atomic character of these states

suggests that the electronic structure of SnO2 may be

considered to be ionic. Within this ionic approximation

the electronic configurations of the atomic and

stoichiometric oxide systems are those given Table 3-1.

For SnO2 the highest occupied states are oxygen

2p like, and the lowest unoccupied states are tin 5s

like in basic agreement with the band structure

calculations. Reduction of SnO2 to SnO populates the

Sn 5s states leaving the lowest unoccupied states more

Sn 5p like. Likewise, the removal of oxygen from SnO2

to form a nonstoichiometric oxide should result in a

mixing of Sn 5s states into the valence bands (possibly

as defect states) leaving a more Sn 5p like CBM. Such

a variation in symmetry near the conduction band minimum

should be apparent in the energy-loss spectrum. In

particular, a Sn 4d core-to-CBM transition will be dipole

unallowed for a Sn 5s dominated CBM, but dipole allowed

(Al=l) for a Sn 5p like CBM. Hence, the loss of oxygen

from Sn02 should result in a change in the Sn 4d

core-to-CBM transition from unallowed to allowed. Notice














Table 3-1. Electronic Configurations of Atomic
Tin and Oxygen and the Stoichiometric Oxides


Atomic Tin


Atomic Oxygen



Stannous Oxide, SnO


Sn : [Kr] 4d10 5s2 5p2


00 : is2 2s2 2p4



Sn2+ : [Kr] 4d10 5s2 5p


02- : 1s2 2s2 2p6


Sn4+ : [Kr] 4d10 5s 5p


02- : Is2 2s2 2p6


Stannic Oxide, SnO9







in Figure 3-1 that little change is expected in the energy

of the CBM between SnO2 and SnO.

The changing nature of states near the tin oxide

CBM may be seen clearly in Figures 3-3 and 3-4. The

46 eV loss feature may be interpreted as a transition

from the Sn 4d core to a high lying Sn 4f-like CB state

[47]. The 27 eV feature may be interpreted as a Sn 4d

core-to-CBM transition [61]. A feature near 27 eV has

been observed in the N(E) loss spectrum for both SnO

and Sn metal [41,62] but not for SnO2. For the case

of metallic tin, this feature may be viewed as a transition

from the Sn 4d core to empty states above the Fermi level.

The growth of the 27 eV loss feature in conjunction with

the decrease in the 46 eV feature may be interpreted

as a change in the CBs. The growth of the 27 eV loss

feature with decreasing beam energy is characteristic

of the changing nature of the CBM due to a deficiency

of oxygen in the surface region. This interpretation

is supported by the relative strengths of the two

transitions. The d ----> f transition is expected to

be stronger than the d ----> p transition [63].

The insensitivity of the 36 eV loss feature to

incident beam energy relative to the Sn 4d core features

discussed above suggests that the atomic origin of this

core derived feature is, significantly different. From

Figure 3-3 the main change in this feature is a gradual







decrease in intensity with decreasing beam energy. The

assignment of this 36 eV loss feature to an 0 2s core-to-CB

transition can explain this trend for a material exhibiting

a decreasing oxygen concentration on moving from the

bulk to the surface. This is precisely the situation

encountered in the present case.

Because the 0 2s and Sn 4d core levels couple to

CB final states of significantly different energy, the

energy-loss distribution due to these transitions may

be viewed as approximately proportional to the CB density

of states over a narrow range. There is certainly some

overlap between the 0 2s and Sn 4d transitions in the

neighborhood of the 0 2s feature. At the extremes,

however, near the 46 eV or 27 eV feature the

density-of-states interpretation should be valid although

substantial matrix element differences are likely between

these two regions. The observation from Figure 3-3

that changes in the core level features at high beam

energies are more dramatic than in the VB loss features

suggests the Sn 4d core features are more sensitive to

low concentrations of crystal structure defects than

the VB features. The Sn 4d core level losses may be

viewed as an strong indicator of the structural order

of the tin oxide material. Supporting evidence is found

from results on ion-sputtered samples.







Sputtering Effects

In order to increase the surface sensitivity of

the ELS measurement, the sample orientation was changed

to give the coaxial electron beam from the CMA an incident

angle of 450 with respect to the sample normal. This

change allowed reasonably surface sensitive measurements

with higher primary beam energies, and it eliminated

the problem of low-energy secondary electrons inherent

in the use of low electron beam energies (50 eV) for

N(E) measurements. Also, the probability of encountering

additional quadrupole-allowed features was minimized.

Assuming a straight line incident and exit path for an

electron scattered through a nominal angle of 137.70

(fixed by the CMA [64]), a very crude estimate of the

ELS analysis depth based on sample orientation and electron

mean free path can be made. Since the main features

observed in the ELS spectrum are due to single inelastic

events possibly in conjuction with elastic events, a

total path length of twice the mean free path of an

electron at the primary beam energy seems appropriate.
0
These assumptions lead to an estimate of 5 to 10 A (2
0
to 4 atomic layers) at Ep = 200 eV and 15 to 20 A (5

to 7 atomic layers) at Ep = 1500 eV. These estimates

should be viewed as qualitative at best.

Figure 3-5 shows the ELS data for a sample annealed

at 600C. The spectrum for Ep = 1500 eV shows the

structure characteristic of a well annealed Sn02 bulk

















Ep = s158 EV


V Ep = 280 EV\

18 13



688C ANNEAL

45 DEGREE INCIDENCE




\lil I\ill\ \i il\ l\lil lil\ il l iil i li il iilll i n1 1 l ifillim
68 58 48 38 28 18

ENERGY LOSS CEV)

Figure 3-5. ELS spectra for a sample annealed at 6000C.
The angle of incidence for the primary beam is 450. With
Ep = 1500 eV it is seen that the bulk is primarily SnO2.
The Ep = 200 eV spectrum shows SnO at the surface and
evidence of a defect structure between bulk SnO2 and
the surface.


1111 11111IHI 11 1111m111111111 1 Im IIiiii II11 1im







material. The more surface sensitive spectrum for Ep

= 200 eV has a sharp structure near 13 eV which is

characteristic of SnO [41]. The broad feature centered

at 18 eV is not characteristic of either SnO or SnO2,

and it most likely comes from a subsurface

nonstoichiometric defect structure accompanying the change

in structure from SnO2 in the bulk to SnO at the surface.

The 27 eV feature is also present indicating a structure

which is oxygen deficient in comparison to SnO2.

Figure 3-6a is the valence band XPS spectrum for

the 6000C annealed sample. The resolution of the VB

XPS data is seen to be poor. This poor resolution is

due to a combination of very low signal intensity, the

x-ray line. width, x-ray satellite emission from Sn 4d

and O 2s core levels and data smoothing. In spite of

these difficulties, the general shape of the VB emission

is similar to that for SnO2 as found by Lau and Wertheim

[32]. The obvious lack of surface sensitivity in this

measurement is not unexpected. Because the kinetic energy

of the valence band photoelectrons is large ( > 1200

eV), the mean free path is correspondingly large.

Additionally, the sample orientation is such that the

angle between the surface normal and the cylinder axis

is very nearly equal to the nominal 42.30 acceptance

angle of the CMA [64]. Since photoemission from

polycrystalline materials is expected to peak at the














(B)
O/SN a 8.96







0/SN U 1.38




VALENCE BAND XPS



i l l I I L I I I 1 1 I I f I |
iS 18 5 0
BINDISG EERGY (EV)
Figure 3-6. Valence band XPS spectra after a (A) 6000C
anneal and (B) 2 KeV argon-ion bombardment. The spectra
are more bulk than surface sensitive.


I I I I I I I I I I I P1 I I II I I I 1 1I







surface normal, the VB XPS results shown here have their

largest contribution from high energy electrons at near

normal emission. Hence, Figure 3-6a is primarily due

to the bulk SnO2 material.

The annealed sample characterized by Figures 3-5

and 3-6a was ion sputtered with 2 KeV argon ions. Figure

3-6b illustrates the change in the VB XPS spectum. AES

and core level XPS show no evidence of a reduction to

metallic tin in this particular case, but the preferential

sputtering of oxygen is demonstrated by a drop in the

O/Sn ratio. Ion sputtering introduces a shoulder on

the VB emission near a binding energy of 2 eV to 3 eV.

A similar feature has been observed in ARUPS and

interpreted as emission from defect states associated

with a deficiency of oxygen [47]. Interestingly, this

sputter-induced feature lies near the same binding energy

as the highest lying SnO VB feature [32]. There is

even a fair correspondence between the SnO VBM as found

by Lau and Wertheim and the low binding energy edge of

the defect emission.

The assignment of the shoulder in Figure 3-6b to

defect states rather than SnO is justified by the ELS

spectra for the sputtered sample in Figure 3-7. The

more surface sensitive spectrum, Ep = 200 eV, shows a

broadening of the characteristic SnO feature at 13 eV.

The entire valence band portion of the spectrum becomes





48

broad and relatively featureless as a result of sputtering.

This broadening may be interpreted as a change from the

SnO structure at the surface to a more amorphous structure

caused by sputtering. For Ep = 1500 eV the loss spectrum

is sensitive to the bulk within the region probed by

the VB XPS measurements. While there is some broadening

and a small shift toward lower energy losses, the VB

features are still very much SnO2 like in approximate

agreement with the VB XPS spectrum shown in Figure 3-6b.

The core level loss features reflect the defect presence

much more strongly than the VB loss features. The absence

of the 46 eV feature and prominence of the 27 eV feature

confirm the change from a well-annealed SnO2 structure

to a more oxygen-deficient defect structure after

sputtering.

The damage induced by ion sputtering is heaviest

in the top few layer's of the solid as illustrated by

Figure 3-7. Thus, the amorphous structure at the surface

implied by the valence band features for Ep = 200 eV

is not unexpected. Sputtering damage in layers deeper

in from the surface may result from ion implantation,

knock-in and other ion-matrix phenomena, but the damage

in these deeper layers should be significantly less than

near the surface. Bearing in mind that ion bombardment

effects become less apparent as the experiment becomes

more bulk sensitive, a comparison of the results between




















Ep = 1588 EV

18

45 27 13

I =

9Ep = 288 EV
z





2 KEV ARGON ION BOMBARDMENT

45 DEGREE INCIDENCE



ll Ii I I1 I iI l i i il i [ 1 11l1 1 1i 1il lii ll i il1 i 11 1 ili ii
68 58 48 38 28 18

ENERGY LOSS (EV)

Figure 3-7. ELS spectra following a 2 KeV argon-ion
bombardment. The valence band features show the bulk
to be SnO2 like while the core features reveal a signifi-
cant concentration of defects. The VB features in the
more surface sensitive spectrum illustrate an amorphous
structure at the surface due to a sputtering.








annealed and sputtered samples suggests that a significant

amount of qualitative structural information may be gained

from the N(E) energy-loss spectrum. The width and

center-of-gravity position of the valence band features

can be used as a gross indicator of the tin oxide

structure. A matrix characteristic of a stoichiometric

form of tin oxide is suggested by sharper, more well

defined valence band loss features near 19.5 eV for SnO2

and near 13 eV for SnO as was found by Powell [41]. A

broadening and shift in energy between these two

characteristic features suggests an increasing structural

disorder. The radical change in core level features

in comparison to VB features suggest a higher sensitivity

to lattice defects. In particular, the 46 eV feature

appears to be an excellent indicator of the SnO2 structure.

Even when the valence band features appear to be very

SnO2-like, the presence of defects is indicated by the

loss or decrease of the 46 eV feature relative to the

VB features. This interpretation of structurally related

changes in the ELS spectrum is strongly supported by

the combined LEED and ELS study of de Fresart et al. [58]

on SnO2 (110).

Oxygen Effects

It is shown above that a growth of the 27 eV feature

and loss of the 46 eV feature reflects a change in tin

oxide away from a well-annealed SnO2 material. Some







distinction between the origins of the changes in these

two core level features can be made. This distinction

requires a measure of the oxygen concentration which

is provided by core level XPS using standard sensitivity

factors [31]. To make comparisons with VB XPS useful,

attention is limited to the more bulk sensitive energy-loss

measurements for 1500 eV primary beam energies. The

quantitation of oxygen levels within the matrix by core

level XPS presents a problem due to a difference in

analysis depth with respect to VB XPS and ELS. This

problem is minimized by using the O/Sn ratios determined

in this manner as only a rough measure of the oxygen

concentration further into the bulk. The O/Sn ratios

are reported within an uncertainty of 0.03 which describes

the reproducibility of the measurements. No uncertainty

in the sensitivity factors is reported. In this regard

trends in the O/Sn ratios are more important than the

absolute values.

For the sputtered sample described by Figures 3-6b

and 3-7, the O/Sn ratio is 0.96. Annealing the sputtered

sample in vacuo at 5000C for 20 minutes repairs some

of the sputter-induced damage. Figure 3-8a shows the

effect on the VB XPS spectrum. A decrease in the defect

feature at low binding energies is observed, and a

splitting in the VB features at 4.5 eV binding energy

appears. This splitting is characteristic of a mixture





15 10 5 8

BINDING EMERSY (EV)

Figure 3-8. Valence band XPS spectra following (A) a
short 5000C anneal after sputtering and (B) subsequent
oxygen exposure. The presence of a mixture of SnO and
SnO2 is indicated in (A).


V
z
2


I1 1 1 1 I 1 1 1 I I I 1 1 I i 1 1 11 I 1 1 11








(A)
0/SN 1 1.34








O/N- 1.14




VALENCE BAND XPS


\1 1 \ \1 1 \ 11 1 1 1 1 1 1 1








of SnO and SnO2 [32]. The short anneal also increases

the O/Sn ratio to 1.14 presumably due to some oxygen

diffusion into the surface region from the bulk. The

energy-loss spectrum in Figure 3-9a also shows more

evidence of structural repair caused by annealing. The

largest valence band feature is sharper and centered

at 19.5 eV, and the presence of the high energy loss

feature near 45 eV is again slightly visible. Both of

these features indicate the presence of an Sn02 structure.

The presence of the 27 eV feature reveals an oxygen

deficiency relative to Sn02, and the size and shape of

the feature near 13 eV suggests the possibility that

SnO is present. However, the 13 eV feature is a

convolution of SnO and Sn02 features which yields little

information by casual inspection.

Subsequent treatment in situ with 11 Torr of 02

at 5000C for 15 minutes results in the addition of a

significant amount of oxygen to the matrix, O/Sn = 1.34.

Figure 3-8b shows the effect on the VB XPS spectrum.

The splitting which is apparent in Figure 3-8a is removed,

and the shape of the VB emission is predominantly that

of Sn02. The addition of oxygen also affects the ELS

spectrum as seen in Figure 3-9b. The change in VB features

is minimal. The main loss feature falls at 19.5 eV as

expected for an Sn02 material, and there is a decrease

in the feature near 13 eV relative to the 19.5 eV feature


















(5)


t
18.5

t 13
27 4
4/>


Ep 15 EV


1 1 1 1 1 1 1 1.1 1 1 111 | | 1 1 111 11 1 1111 11 11 Il l


ifIfuI;;III r u r u I IIrrruI I


60 58 40 38 28 18

EMR6Y LOSS (EV)

Figure 3-9. ELS spectra corresponding to Figures 3-8(A)
and (B) respectively.


n[lir[i I[lllljnlilnrr[rrl 11I1I1I 11111 I1III1IIIII Ilrlnrllm


I







suggesting a loss of the SnO contribution to the spectrum.

The most apparent changes occur in the core level features.

A small increase in intensity of the feature near 36

eV is observed. This increase is consistent with the

assignment of this feature to 0 2s-to-CB transitions.

It can be seen that the 27 eV feature in Figure

3-9b is greatly diminished. The loss of this feature

by annealing in oxygen substantiates the earlier

interpretation that it is associated with a loss of oxygen

from the SnO2 structure and be may interpreted as due

to a change in symmetry of the states near the CBM. It

seems that the growth of the 27 eV feature reflects a

loss of coordinating oxygen or a lowering of the valency

of the tin. This loss may occur through the formation

of defects such as oxygen vacancies in a nonstoichiometric

or amorphous oxide, through the formation of stoichiometric

SnO or through the formation of metallic Sn.

Figure 3-9b demonstrates that the 46 eV and 27 eV

loss features are not strictly interdependent. The weak

intensity of the high-energy loss feature suggests a

sensitivity to defects other than those associated only

with a deficiency of oxygen in a SnO2 lattice. It is

postulated that the high-lying conduction-band final

states associated with this transition are strongly

dependent on the periodic potential of the SnO2 lattice

and easily perturbed by the presence of defects. This





56

strong dependence may occur if the states are less atomic

in nature than the valence and lower conduction bands

while still containing a fraction of tin 4f character

as suggested by Gobby [47].



Conclusions

The use of ELS combined with valence band

photoemission and results of band structure calculations

provides a powerful means for studying tin oxide surfaces.

In this study an assignment of the major features in

the tin oxide N(E) energy-loss spectrum is made. The

loss features are assigned to collections of optically

(dipole) allowed interband transitions based on a previous

photoemission study by Gobby [47]. It is found that

the low-energy portion of the spectrum may be associated

with valence-to-conduction-band transitions, and the

higher energy-loss features are due to

core-to-conduction-band transitions. Of these core level

features, it is possible to distinguish between transitions

from Sn 4d and 0 2s levels even though these features

cannot be resolved in XPS.

It is demonstrated for tin oxide surfaces that depth

profile information may be obtained using ELS. By varying

the primary electron beam energy and hence the analysis

depth, it is shown that a high temperature anneal results

in bulk SnO2 under an oxygen deficient structure which

is essentially SnO at the surface.







By using ELS in conjunction with valence band XPS,

it is found that a significant amount of structural

information may be inferred from the size, shape and

position of the N(E) ELS features. In particular,

distinctions can be made between SnO2, SnO and defect

or amorphous structures. The Sn 4d core level features

are found to be much more sensitive to defects in an

SnO2-like lattice than are the VB features. A loss feature

at 27 eV assigned to transitions from Sn 4d levels to

states near the CBM is associated with atoms in a lower

oxidation state or in a lattice deficient in oxygen

relative to SnO2. An SnO2 loss feature near 45 eV is

shown to be very sensitive to defects not necessarily

associated with oxygen vacancies or deficiencies, but

the specific type(s) of structural defects) associated

with the behavior of this high-energy-loss (45 eV) feature

has not yet been determined.














SECTION IV
A STUDY OF THE DEHYDRATION OF TIN OXIDE
SURFACE LAYERS



Introduction

The chemisorption properties of tin oxide surfaces

can be significantly influenced by the interaction with

water. Kaji et al. have demonstrated that it is possible

to fixate Cu (II) and Pd (II) complex ions on hydrated

tin oxide surfaces in the preparation of propylene

oxidation catalysts [65]. The modification of tin oxide

electrode surfaces by an alkaline pretreatment has been

shown to give enhanced cell emf responses to changes

in pH [28]. This enhancement is thought to occur through

the hydrolysis of surface Sn=O bonds to give Sn-OH surface

species. The specific adsorption of Fe (III) and Pb

(II) cations has been shown to occur on these hydrated

surfaces apparently by replacement of the proton on the

surface hydroxyl groups [66,67]. Similarly, the specific

adsorption of bromine and iodine anions on tin oxide

occurs only on hydrated surfaces [68,69]. Most recently

it has been shown that an increase in Pt uptake rates

during chemisorption from solution occurs on hydrated

tin oxide surfaces [6]. Pt dispersions can exceed 90%








on these surfaces, and the resulting catalytic activity

per surface Pt atom exceeds that of metallic Pt electrodes

for the electrochemical reduction of 02.

Secondary-ion mass spectrometry (SIMS) and

electron-stimulated desorption (ESD) were used in a

previous study to examine alkaline-pretreated tin oxide

for evidence of surface hydroxylation [19]. ESD

demonstrated higher yields of both H+ and 0+ after the

alkaline pretreatment suggestive of significant surface

hydroxylation. A small signal due to OH+ desorption

was also observed. Results using dynamic SIMS showed

no apparent differences in the bulk regardless of

pretreatment. It has become apparent, however, that

hydrogen is a major constituent in most tin oxide films,

and it appears that the actual film composition may be

best described as SnxOyHz. SIMS depth profiles of H+,

O+, OH+ and SnH+ species indicate an excess of hydrogen

and/or hydroxide or hydrated species at the surface of

tin oxide films [40]. A steep concentration gradient

within approximately the outer 30 A of the material

indicates that hydration is not limited strictly to the

outer atomic layer. While. the degree of hydration is

greatest for the alkaline-treated samples, significant

hydration occurs over the same depth for samples exposed

to atmospheric humidity only. This observation is

indicative of the hygroscopic nature of tin oxide surfaces.










The complexity of the interaction of water with

tin oxide is demonstrated by the work Giesekke et al.

on the decomposition of bulk tin (IV) hydroxide [55].

Using thermogravimetric analysis, it was determined that

the decomposition of SnO3H2 leads to the formation of

Sn205H2 above 2500C, Sn409H3 between 3250C and 3600C,

Sn8O06H2 at 5000C and SnO2 above 6000C. Electron

diffraction clearly shows that each dehydration product

is a different crystalline substance. Though an accurate

determination of the structures was not possible, a study

of proton magnetic resonance line shapes shows the

structures to be complex. None of the substances can

be described as simple hydrates or hydroxides.

In Section III it was shown that electron energy-loss

spectroscopy (ELS) is sensitive to electronic changes

in tin oxide and is useful as either a surface or

subsurface probe. With the aid of valence band x-ray

photoelectron spectroscopy (XPS), it was also shown that

certain changes in the ELS spectrum may be related to

structural changes in the material. ELS and valence

band XPS are used in the present study of the hydrated

layer formed on tin oxide by exposure to atmospheric

humidity. A preliminary observation of water adsorption

using grazing-exit-angle 'XPS and ESD is given in Section

V.








Experimental

The preparation of the polycrystalline tin oxide

film used in this study has been described in Section

III. Once prepared the sample was exposed to atmospheric

humidity for several months to allow hydration of the

near surface region. All spectra were collected with

a double-pass CMA. The ELS data were taken in the N(E)

mode to allow for a direct comparison with the data of

Powell [41]. A 100 nA, 0.1 mm diameter primary electron

beam was used. All ELS spectra were recorded with a

25 eV pass energy ( AE/E = 0.014) using pulse counting

detection. The XPS spectra were taken using a Mg Ka x-ray

source and a 50 eV analyzer pass energy. The base pressure

in the vacuum system for this study was lx10-10 Torr.

Details of the vacuum system have been given previously

[17].

The ELS spectra were taken using the coaxial electron

gun in the CMA at an incident angle of 450 with respect

to the sample normal. As shown in Section III, ELS

measurements sensitive to the top few atomic layers can

be obtained in this configuration using a 200 eV primary

beam energy (Ep). Using a 1500 eV primary beam energy

significantly decreases the surface sensitivity of the

ELS measurement. This higher beam energy makes ELS more

sensitive to the subsurface region with an estimated

analysis depth of approximately 20 A. The valence band








(VB) XPS spectra obtained with this sample orientation

are also more sensitive to the subsurface region, and

the VB XPS analysis depth is expected to be similar to

the high energy (Ep = 1500 eV) ELS measurements. Both

the subsurface VB XPS and ELS measurements are sensitive

to the same region in which previous SIMS results [40]

suggest that hydration occurs.



Results and Discussion

Prior to analysis, the sample was cleaned in situ

by heating to 5000C in 10 Torr of 02 for 5 minutes. This

procedure removed carbon and chlorine contamination while

leaving a trace amount of K on an otherwise clean oxide

surface as determined by Auger electron spectroscopy

(AES). This contamination is known to be segregated

at the surface [17], and it may be removed easily by

Ar+ bombardment. However, in order to preserve the

hydrated layer of the sample, no ion bombardment was

used. While the 02 treatment may have affected the very

near-surface region of the sample, the following data

reveal that the subsurface layers were not dehydrated.

Figure 4-1 shows the valence band XPS data for the

sample after various treatments. Figure 4-la is the

spectrum recorded after the in situ cleaning. Figure

4-2a illustrates the effect of a 5000C vacuum anneal

on the spectrum. The annealing process was continued








until the background chamber pressure went through a

distinct maximum (about 45 minutes). Figure 4-ic shows

the result of a similar (30 minute) 6000C anneal. Figures

4-2, 4-3 and 4-4 show the ELS spectra corresponding to

Figures 4-la, 4-1b and 4-lc respectively.

The valence band spectrum in Figure 4-la is

characteristic of a sample hydrated due to exposure to

atmospheric humidity. The large feature near 10 eV binding

energy (7 eV below the valence band maximum (VBM)) is

largely due to this hydration though the form of the

incorporated water is unknown. The 10 eV feature is

similar to that found for water adsorption on other oxides

[70].

Figure 4-2a shows the ELS spectrum (Ep = 1500 eV)

corresponding to Figure 4-la. The features at energy

losses less than about 30 eV are due to

valence-to-conduction-band transitions as discussed in

Section III. The main VB loss feature in Figure 4-2a

falls at 20 eV which is characteristic of a SnO2-like

material (see Section III and ref. 41). Additionally,

a large shoulder associated with the VB loss features

is observed. This feature has not been previously

reported, and it appears to be composed of two loss

features near 24.5 eV and 27 eV. This shoulder is not

characteristic of SnO2. These additional features may

be interpreted as transitions from the hydrate-induced


















ANNEAL
(C)





t2I ANEAL







(A)
HYDRATED




VALENCE BAND XPS

I I I I I i I I I I l l i [ i l 1 I
15 t1 5 8

BINDING EDERY CEV)
Figure 4-1. Valence band XPS spectra after (A) hydration
by exposure to atmospheric humidity, (B) a 5000C vacuum
anneal for 45 minutes and (C) a 6000C vacuum anneal for
30 minutes.




























2I7, (B) Epa zee88 EV

















Figure 4-2. ELS spectra of (A) subsurface and (B) surface
regions after hydration due to atmospheric humidity.








lower valence band feature observed in Figure 4-la to

conduction band. states. This interpretation is in

agreement with the ultraviolet photoelectron spectroscopy

(UPS) measurements of Gobby [47]. The UPS results show

that the lower valence band feature couples strongly

to conduction band states in a range from 25 eV to 30

eV higher in energy. Features at energy losses greater

than about 30 eV are due to core-to-conduction-band

transitions. In particular, the broad feature centered

near 36 eV is due to a set of O 2s-to-CB transitions

(see Section III).

Figure 4-2b (Ep = 200 eV) shows the more surface

sensitive ELS spectrum of the hydrated sample. The main

VB loss feature falls near 19 eV suggestive of an Sn02-like

material. The lack of higher energy-loss VB features

in this spectrum indicates a surface which is dehydrated

relative to the subsurface layers. This dehydration

is most likely the result of ESD from the surface under

the influence of the primary electron beam and/or some

superficial dehydration due to the elevated temperature

used during the oxygen cleaning procedure.

The main VB loss features in Figures 4-2a and 4-2b

suggest that the sample is fully oxidized in both the

surface and subsurface regions. This is confirmed for

the surface region in Figure 4-2b by the lack of a 27

eV Sn 4d core level loss feature which would be








characteristic of a deficiency of oxygen relative to

SnO2 (see Section III). Although the presence of such

a 27 eV feature would be obscured in Figure 2a by the

hydrate-induced VB loss feature, the absence of any low

binding energy structure (2 eV to 3 eV) in Figure 4-la

reveals that no oxygen deficiency exists (see Section

III). It is apparent, however, that the structure of

the material is perturbed relative to a well-annealed

SnO2 rutile structure. This perturbation is evidenced

by the lack of a core level loss feature near 45 eV in

Figure 4-2 (see Section III). For the hydrated subsurface

layers, the perturbation is easily understood as due

to the addition of excess oxygen and hydrogen from the

water of hydration while at the surface beam damage is

the most likely cause.

Figure 4-lb shows the effect of a 5000C vacuum anneal

on the valence bands. The large feature near 10 eV in

Figure 4-la has been greatly reduced suggestive of a

dehydration of the subsurface region, and a spectrum

very similar to that characteristic of SnO2 remains (see

Section III and ref. 32). This change is also reflected

in the ELS spectrum in Figure 4-3a. While the main VB

loss feature remains near 20 eV, the features in the

25 eV to 27 eV region associated with the hydrated oxide

are substantially decreased. The valence band ELS features

in Figure 4-3a are quite SnO2 like in agreement with




















(A) Ep s88 I VE




46 28



(B) Ep 2M88 EV



27 T
13

SM CANEAL



IIIJj 111 w n wmmliltllinfil iim ii inn j jnn i liffit
s o 40 38 28 18is

ENERGY LOSS CE)
Figure 4-3. ELS spectra of (A) subsurface and (B) surface
regions after a 5000C vacuum anneal.








the VB XPS spectrum of Figure 4-1b. Concurrent with

the change in VB features, the appearance of a small

core level loss feature near 45 eV is observed in Figure

4-3a. The weak presence of the 45 eV loss feature

represents the beginning of a change in the subsurface

oxide to a true SnO2 structure (see Section III). However,

a significant perturbation of this structure is still

apparent. At the surface the annealing has resulted

in an oxygen deficiency as illustrated by Figure 4-3b

(Ep = 200 eV). The broad valence band features with

increased intensity at 13 eV show that the surface has

changed from SnO2-like to a more SnO-like material. The

oxygen deficiency in the surface region is confirmed

by the appearance of the small feature near 27 eV (see

Section III).

Further annealing at 6000C has only a small effect

on the VB XPS spectrum shown in Figure 4-lc. The feature

near 10 eV binding energy is completely removed leaving

a spectrum characteristic of SnO2 (see Section III and

ref. 32). Similarly, the ELS spectrum in Figure 4-4a

shows the core level loss features characteristic of

a well-annealed Sn02 material indicating the nearly

complete dehydration of the subsurface region. The

annealing process has, however, effected a reduction

at the surface. The sharp sturcture near 13 eV in Figure

4-4b is 'characteristic of SnO as found by Powell [41].























Ep 1588 EV t
28


45




V Ep = 208 EV
z
18 13



688C ANNEAL







68 58 48 38 28 18

ENERGY LOSS CEV)
Figure 4-4. ELS spectra of (A) subsurface and (B) surface
regions after a 6000C vacuum anneal.





71


The broad feature centered near 18 eV is not characteristic

of SnO or Sn02, and it may be interpreted as due to a

nonstoichiometric defect structure accompanying the change

from subsurface (bulk) SnO2 to surface SnO (see Section

III).

The changes observed in the subsurface layers using

ELS clearly indicate a temperature dependence in the

decomposition of the hydrated near-surface region. The

dehydration product observed at 500C is a forerunner

to the formation of a true SnO2 compound near 6000C in

the subsurface region. These observations are in agreement

with the work of Giesekke et al. [55] on the thermal

decomposition of bulk tin (IV) hydroxide. The formation

of Sn8016H2 could account for the apparent perturbation

of the subsurface crystal structure evidenced by Figure

4-3a while causing only a small variation in the VB density

of states from that expected for SnO2.



Conclusions

The near-surface region of a hydrated polycrystalline

tin oxide film has been studied. A large increase in

the lower VB density of states has been observed for

hydrated subsurface layers using VB XPS and ELS. These

observations are in agreement with SIMS data [40] which

suggests that hydration 'due to exposure to atmospheric
humidity occurs to depths of at least 30 A.
humidity occurs to depths of at least 30 A.




72

The thermal decomposition appears to proceed in

a stepwise fashion. The subsurface hydrated layers yield

SnO2 near 6000C, but the surface undergoes a reduction

to SnO. A comparison with existing data on bulk tin

(IV) hydroxide decomposition leads to an interpretation

consistent with the formation of an intermediate

hydrogen-containing compound in the subsurface region

near 5000C.













SECTION V
AN OBSERVATION OF WATER ADSORPTION ON TIN OXIDE
USING ESD AND GRAZING-EXIT-ANGLE
XPS AND AES



Introduction

As discussed in Sections I and IV, the chemisorption

properties of tin oxide surfaces can be significantly

affected through the interaction with water. In

particular, the chemisorption of platinum on tin oxide

surfaces is believed to occur at hydroxylated surface

sites (see Section II and ref. 6). In Section IV, the

dehydration of tin oxide surfaces has been studied using

valence band x-ray photoelectron spectroscopy (XPS) and

electron energy-loss spectroscopy (ELS). These techniques

have proven particularly useful in studying the subsurface

layers of the material. Though ELS may be made quite

surface sensitive by lowering the primary beam energy,

the study of adsorbates on tin oxide with this technique

is made difficult by the phenomenon of electron-stimulated

desorption (ESD).

Some preliminary observations of water adsorption

on tin oxide are given in this section. ESD experiments

and grazing-exit-angle XPS and Auger electron spectroscopy

(AES) measurements provide this observation. Though







the data presented here is incomplete, it provides an

interesting comparison with the dehydration study in

Section IV, with previous ESD data on alkaline and

non-alkaline treated tin oxide surfaces [19] and with

the work of Giesekke et al. [55] on the dehydration of

bulk Sn (IV) hydroxide. The incomplete nature of the

ESD experiments is due to a prolonged (16 months and

counting) failure of the Physical Electronics double-pass

CMA. Grazing-exit-angle XPS and AES are used because

of the unavailability of a preferred technique, ultraviolet

photoelectron spectroscopy (UPS).



Experimental

The preparation of the polycrystalline tin oxide

film used in this study has been described in Section

III. After preparation, the sample is rinsed in distilled

water and solvent cleaned prior to insertion into the

vacuum system. Before analysis the sample is cleaned

in situ by heating to 5000C in 10 Torr of 02 for 5 minutes.

This procedure removes trace chlorine and carbon surface

contamination leaving a clean oxide surface as determined

by AES. The AES, XPS and ESD data were taken with a

PHI double-pass. CMA. The AES spectra are collected in

the nonretarding (EN(E)) mode using a 3 KeV, 200 mA/cm2

electron beam. For XPS the analyzer was run in the

retarding (N(E)) mode with a pass energy of 50 eV (AE/E








= 0.014). Details of the vacuum system have been published

previously [17]. The base pressure for this study was

5x10-10 Torr.

The surface sensitivity of the electron spectroscopies

(AES and XPS) can be improved by collecting the emission

at angles away from the sample normal, i.e. at a more

grazing exit angle. The path length, L, that an escaping

electron (photoelectron or Auger electron) must travel

through a solid is related to the signal attenuation

due to inelastic collisions. Signal attenuation as a

function of path length can be described by an exponential

decay law with a uniform attenuation length. The

attenuation length, X, is known as the mean free path.

Hence,



I o( exp(-L/X )



where I is the signal intensity. Figure 5-1 illustrates

the increased surface sensitivity obtained at grazing

exit angles for a perfectly flat surface. If an emitting

source (atom) is a fixed distance, D, below the surface,

the path length, L, traversed within the solid increases

with increasing exit angle, 0, as


L'= D/cos 0



























L= D


L = D/COSe


Figure 5-1. Variation in path length with emission angle.








Therefore, at fixed D, the signal intensity decreases

with increasing 9 making the measurement more surface

sensitive. In general, experimentally observed increases

in surface sensitivity are not as large as expected from

the above analysis. Two possible reasons for the deviation

are surface roughness and a decay of total signal with

increasing 0 causing a reduction in the signal-to-noise

ratio [71].

For all measurements the sample was mounted with

approximately a 450 angle between the CMA axis and the

surface normal. This orientation directs the sample

normal into the 42.30 60 acceptance cone of the CMA

[64]. Grazing exit angles are chosen with the 120 angular

acceptance aperture on the angle resolving drum mounted

coaxially within the inner cylinder of the second stage

of the CMA [72]. Using the relationship derived by Gobby

[47], the exit angle for a given drum setting may be

found.

ESD experiments are performed by using the CMA in

a time-of-flight (TOF) mode which allows for a simultaneous

determination of the mass and energy of desorbing ions.

For these measurements the analyzer is operated at a

constant pass energy of about 80 eV. This pass energy

(kinetic energy of the analyzed ions) sets the flight

time of the ions through the analyzer (about 4 psec for

H+). Because the CMA only passes charged particles of








the proper kinetic energy, species of different masses

(but same charge) have different axial velocities through

the CMA. Hence, the flight time of an ion through the

analyzer is directly proportional to the square root

of the mass-to-charge ratio. Traum and Woodruff [73]

have discussed in-depth the analyzer characteristics

which effect the flight time and mass resolution. Unity

mass resolution is possible for mass-to-charge ratios

(m/e) of at least 20.

To operate the CMA in a TOF mode for ESD experiments,

a computer-interfaced digital pulse counting circuit

is used (see Appendix B). The TOF modification to the

pulse counter allows it to perform three functions:

(1) it initiates the desorption event,

(2) it delays for a programmed flight time

(3) and it measures (counts) the signal pulses.

Before beginning the TOF analysis, the coaxial

electron gun in the CMA is configured with a +50 V charge

on the lower deflection plate. This voltage deflects

the electron beam (typically below 200 eV) downward out

of the analysis area (focal region) of the analyzer.

The pulse counter circuit initiates the desorption event

by supplying a 300 nsec TTL pulse to the base of a n-p-n

power transistor in series with the deflection plate

and ground (see Figuae '5-2). This pulse "grounds" the

deflection plate and swings the electron beam into the



























TTL PULSE INPUT


- -+5 V


5K OlHMS


DEFLECTION
PLATE


.HRF427A


Figure 5-2.
initiation.


Deflection circuit for


desorption event


T








analysis region for 300 nsec. The circuit delays for

a programmed flight time before enabling an event counter

which records signal pulses for a similar 300 nsec period.

The count is subsequently read into the computer where

it is stored, and the process is repeated. By scanning

the programmed delay time a TOF (i.e. m/e) spectrum is

obtained. The only real-time constraint is that a total

time span be observed between desorption events at least

equal to the flight time of the most massive species

in the spectrum. This delay clears the analyzer of ions

before the initiation of a new desorption event.

To analyze low energy positive ions like those

obtained in the ESD experiment, the CMA is operated in

an accelerating mode. The inner cylinder and accelerating

grid which are connected internally are set initially

at -70 V, and the sample is biased at +10 V. This

potential difference between the sample and accelerating

grid raises ions initially at zero kinetic energy up

to the 80 eV analyzer pass energy thereby allowing their

detection. By ramping the accelerating grid to more

positive potentials, ions of higher initial energy

(typically 10 to 20 eV) can be measured. If the TOF

analyzer is operated at a fixed flight time, an energy

distribution spectrum of a single desorbing species may

be obtained. In this experiment, part of the accelerating

potential is imposed on the sample to provide a voltage








difference with an outer grid which is grounded to the

magnetic shield of the CMA. In this way. any spurious

signal due to ESD from this grid is shifted to apparent

negative kinetic energies and is easily recognizable

[73].



Results and Discussion

XPS and AES

Measurements have been made following three different

in situ treatments. These treatments include a 5000C

and a 6000C annealing step as was studied in Section

IV. Measurements have also been made following a 2 KeV

argon ion bombardment. Three types of AES and XPS

measurements are reported. Angle integrated results

are obtained with the angle resolving aperture retracted.

Angle dependent results have been obtained at normal

emission (00 40) and at a 700 2.50 grazing exit angle.

The results are presented in terms of O/Sn ratios. For

XPS this determination is made using the area under the

0 Is and Sn 3d 5/2 peaks corrected with standard

sensitivity factors [31]. The AES measurements are made

in a similar fashion using the peak-to-peak heights of

the 0 KLL (512 eV) and Sn MNN (437 eV) transitions [74].

A problem encountered when using the angle resolving

aperture for these measurements is a drop in total signal

and in signal-to-noise ratio. In AES this drop is not








a significant problem because of the magnitude of the

signal, but in XPS the drop is so large that a minimum

seven hour period is required to accumulate enough signal

for a single O/Sn ratio determination. The measurements

were undertaken originally to demonstrate the depletion

of oxygen near the surface due to annealing (as observed

in Section IV), but water adsorption from the background

Vacuum is observed instead because of the extended period

of time required to collect the data. Assuming a

background of water at the base pressure for this study

(5x10-5 Torr), the surface receives a 12.5 Langmuir (lL

= lxl0-6 Torr-sec) dose over a seven hour period. For

a unity sticking coefficient this dose represents about

12 monolayers of water. The water adsorption from the

background vacuum observed here is manifested by an

increased O/Sn ratio in the most surface sensitive (700)

XPS measurement. The adsorption of CO is believed not

to be a contributing factor because no carbon or CO

desorption signal is observed in the subsequent ESD

experiments.

The XPS and AES results are given in Table 5-1.

Angle integrated measurements (with the aperture retracted)

are made quickly after a given treatment before any

significant H20 adsorption occurs. These results indicate

a drop in the O/Sn ratio with increased annealing

temperatures and argon ion bombardment. The angle











Variation in O/Sn Ratio With Emission Angle.


XPS


*1


AES


ANGLE INTEGRATED 1.4
588C NORMAL EMISSION 1.5 1.3
78 DEGREE 2.1




6C ANGLE INTEGRATED 1.3
ANNUAL NORMAL EMISSION 1.4 1.5 1.1 1.2
ANAL 78 DEGREE 1.8 2.3




2KE ANGLE INTEGRATED 1.8
TV NORMAL EMISSION 1.1 1.8
SU R 78 DEGREE 1.5


Table 5-1.








integrated results are consistently similar to, but

slightly lower than, those obtained at normal emission.

This observation illustrates that the signal intensity

is highest at the sample normal as expected for a

polycrystalline material. A similar observation has

been made in Section III regarding the lack of surface

sensitivity in valence band XPS spectra. It is worth

noting that angle integrated measurements taken several

hours after a given treatment show a small increase in

O/Sn ratio like that observed with the seven hour normal

emission measurements.

The increase in O/Sn ratio observed for 70 emission

illustrates a significant uptake of H20 at the surface

from the background vacuum. Indeed, the large O/Sn ratio

of 2.3 observed in one case, suggests the formation of

a hydrated surface. Regardless of the order in which

the data is taken (i.e. the total exposure), the 70

emission always shows a substantially higher O/Sn ratio

indicating H20 adsorption at the surface. Water adsorption

during the normal emission measurements also explains

the small increase observed relative to the angle

integrated measurements.

The AES results given in Table 5-1 show no variation

with exit angle, and the O/Sn ratio is generally lower

than that obtained by angle integrated XPS. The lower

O/Sn value relative to XPS is probably due, in part,

to the increased surface sensitivity of AES. The kinetic

energies of the AES peaks are more than 200 eV less than







are observed in AES even after several hours of exposure

to the background vacuum is due to the ESD phenomenon.

As observed in a previous study [19], the surface

concentration of desorbing species can be rapidly depleted

under an electron beam of high current density. It is

believed that water adsorbed on the surface is quickly

removed by the incident beam used for the AES analysis

and is therefore undetected.

ESD

The first TOF ESD measurements made in this laboratory

are reported here. The data was acquired during the

process of tuning the instrument for the first time.

Unfortunately, an electron gun failure ended this

familiarization procedure before a good rapport could

be developed with the experimental set-up. Therefore,

the results shown here do not represent the full

capabilities of the equipment.

Figure 5-3 is the TOF spectrum of a tin oxide surface

sputtered with 2 KeV argon ions. The desorption of H+

and 0+ is clearly visible at 4.4 usec and 17.6 usec,

respectively. The feature at 18.6 4sec is possibly due

to H20+ desorption, but it is most likely due to O+

desorption with different initial conditions than in

the 17.6 usec peak. These possibilities can be checked

by increasing the accelerating potential and compressing

the flight times in the entrance region of the analyzer.



























0
-I
H

>-











5 1t '15 28 25

ION FLIGHT TIME CMICROSEC)

Figure 5-3. Time-of-flight spectrum for mass analysis.








The 120 angular aperture has been used to select species

desorbing at near normal angles. Traum and Woodruff

[73] have shown that a significant increase in mass

resolution is possible by using the 40 angular acceptance

aperture. With the smaller aperture it should be possible

to resolve 0+ and OH+ species.

Figure 5-4 shows the ion kinetic energy distribution

for the sputtered sample after exposure to a high current

density electron beam for 30 minutes. Figure 5-4a shows

the total ion yield, 5-4b the time-gated (mass resolved)

H+ ion yield and 5-4c the time-gated 0+ ion yield. It

is seen that the CMA may be used for a simultaneous mass

and energy determination.

Figure 5-5 shows the total ion kinetic energy

distribution for the same sample after exposure to the

background vacuum for two hours. No time-gated

distributions were obtained in this case. Though a power

supply problem encountered during the analysis prevents

an accurate determination of the true kinetic energy

scale, the total ion energy distribution is seen to be

very different after H20 adsorption. This variation

suggest that ESD will prove useful in distinguishing

between different forms of hydrogen and oxygen on the

tin oxide surface.




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