NANOSIZED ALKALINE EARTH METAL TITANATES

245 Pages·2006·7.01 MB·English

NANOSIZED ALKALINE EARTH METAL TITANATES: EFFECTS OF SIZE ON
PHOTOCATALYTIC AND DIELECTRIC PROPERTIES

by

DMYTRO V. DEMYDOV

M.S., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1996
M.B.A., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1997
M.S., Pittsburg State University, Kansas, USA, 2002

AN ABSTRACT OF A DISSERTATION

submitted in partial fulfillment of the requirements for the degree

DOCTOR OF PHILOSOPHY

Department of Chemistry
College of Arts and Sciences

KANSAS STATE UNIVERSITY
Manhattan, Kansas

2006
ABSTRACT

A new approach to synthesize nanosized strontium titanate (SrTiO ) and barium titanate
3
(BaTiO ) has been developed. Nanocrystals of mixed metal oxide were synthesized by a
3
modified aerogel procedure from alkoxides.
The textural and surface characteristic properties were studied by nitrogen BET analysis,
transmission electron microscopy, and powder XRD. The crystallite sizes of aerogel prepared
powders can vary from 6 to 25 nm by the use of different solvents. A mixture of ethanol and
toluene was found to be the best binary solvent for supercritical drying, which produced a SrTiO
3
sample with a surface area of 159 m2/g and an average crystallite size of 8 nm, and a BaTiO
3
sample with a surface area of 175 m2/g and an average crystallite size of 6 nm.
These titanates have been studied for photocatalytic oxidation of volatile organic
compounds and acetaldehyde (CH CHO) in particular. The big band gaps of the bulk (3.2 eV for
3
SrTiO and 3.1 eV for BaTiO ) limit their application to a UV light region only. The
3 3
modification of titanates by doping with transition metal ions (partial substitution of Ti ions with
metal ions) creates a valence band or electron donor level inside of the band gap, narrows it, and
increases the visible light absorption.
The enhanced adsorption of visible light was achieved by the synthesis of nanosized
SrTiO and BaTiO by incorporating Cr ions during the modified aerogel procedure. Gaseous
3 3
acetaldehyde photooxidation has been studied on pure SrTiO and BaTiO , and on chromium
3 3
doped Cr-SrTiO and Cr-BaTiO under UV and visible light irradiation, and compared with the
3 3
photoactivity of P25 TiO .
2
SrTiO doped with antimony/chromium shows absorption in visible light and show
3
photocatalytic activity for CH CHO oxidation. The reason for the codoping of SrTiO with Sb/Cr
3 3
was to maintain the charge balance and to suppress oxygen defects in the lattice. This
photocatalyst shows high photoactivity under visible light irradiation even after several
continuous runs. The photoactivity under visible and UV light irradiation was almost identical
for the Sb/Cr-SrTiO photocatalyst.
3
Dielectric properties of aerogel prepared barium titanate samples have being studied and
the bulk resistance values of AP-BaTiO were significantly lower than that of commercial
3
BaTiO , by several orders of magnitude.
3
NANOSIZED ALKALINE EARTH METAL TITANATES: EFFECTS OF SIZE ON
PHOTOCATALYTIC AND DIELECTRIC PROPERTIES

by

DMYTRO V. DEMYDOV

M.S., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1996
M.B.A., Ukrainian State University of Chemical Technology, Dnepropetrovsk, Ukraine, 1997
M.S., Pittsburg State University, Kansas, USA, 2002

A DISSERTATION

submitted in partial fulfillment of the requirements for the degree

DOCTOR OF PHILOSOPHY

Department of Chemistry
College of Arts and Sciences

KANSAS STATE UNIVERSITY
Manhattan, Kansas

2006

Approved by:

Major Professor
Kenneth J. Klabunde
ABSTRACT

A new approach to synthesize nanosized strontium titanate (SrTiO ) and barium titanate
3
(BaTiO ) has been developed. Nanocrystals of mixed metal oxide were synthesized by a
3
modified aerogel procedure from alkoxides.
The textural and surface characteristic properties were studied by nitrogen BET analysis,
transmission electron microscopy, and powder XRD. The crystallite sizes of aerogel prepared
powders can vary from 6 to 25 nm by the use of different solvents. A mixture of ethanol and
toluene was found to be the best binary solvent for supercritical drying, which produced a SrTiO
3
sample with a surface area of 159 m2/g and an average crystallite size of 8 nm, and a BaTiO
3
sample with a surface area of 175 m2/g and an average crystallite size of 6 nm.
These titanates have been studied for photocatalytic oxidation of volatile organic
compounds and acetaldehyde (CH CHO) in particular. The big band gaps of the bulk (3.2 eV for
3
SrTiO and 3.1 eV for BaTiO ) limit their application to a UV light region only. The
3 3
modification of titanates by doping with transition metal ions (partial substitution of Ti ions with
metal ions) creates a valence band or electron donor level inside of the band gap, narrows it, and
increases the visible light absorption.
The enhanced adsorption of visible light was achieved by the synthesis of nanosized
SrTiO and BaTiO by incorporating Cr ions during the modified aerogel procedure. Gaseous
3 3
acetaldehyde photooxidation has been studied on pure SrTiO and BaTiO , and on chromium
3 3
doped Cr-SrTiO and Cr-BaTiO under UV and visible light irradiation, and compared with the
3 3
photoactivity of P25 TiO . SrTiO doped with antimony/chromium shows absorption in visible
2 3
light and show photocatalytic activity for CH CHO oxidation. The reason for the codoping of
3
SrTiO with Sb/Cr was to maintain the charge balance and to suppress oxygen defects in the
3
lattice. This photocatalyst shows high photoactivity under visible light irradiation even after
several continuous runs. The photoactivity under visible and UV light irradiation was almost
identical for the Sb/Cr-SrTiO photocatalyst.
3
Dielectric properties of aerogel prepared barium titanate samples have being studied and the bulk
resistance values of AP-BaTiO were significantly lower than that of commercial BaTiO , by
3 3
several orders of magnitude.
TABLE OF CONTENTS

TABLE OF CONTENTS............................................................................................................v
LIST OF FIGURES...................................................................................................................ix
LIST OF TABLES...................................................................................................................xiv
ACKNOWLEDGEMENTS......................................................................................................xvi
DEDICATION......................................................................................................................xviii
PREFACE................................................................................................................................xix
Chapter 1 : Introduction..............................................................................................................1
1.1 Literature review on titanates.............................................................................................3
1.2 Advantages of nanosized titanates...................................................................................10
1.3 Possible applications .......................................................................................................11
1.4 References.......................................................................................................................13
Chapter 2 : Synthesis of titanates...............................................................................................18
2.1 Introduction.....................................................................................................................18
2.2 Preparation of titanates....................................................................................................18
2.2.1 Solid-state reaction...................................................................................................19
2.2.2 Gas phase reaction....................................................................................................21
2.2.3 Sol-gel technique......................................................................................................29
2.2.4 Aerogel procedure....................................................................................................32
2.3 Temperature treatment.....................................................................................................33
2.3.1 Heat treatment..........................................................................................................35
2.3.2 Calcination...............................................................................................................35
2.3.3 Drying......................................................................................................................36
2.4 Conclusions.....................................................................................................................38
2.5 References.......................................................................................................................39
Chapter 3 : Characterization of titanates....................................................................................41
3.1 Introduction.....................................................................................................................41
3.2 Synthesis of strontium and barium titanates.....................................................................41
3.2.1 Solid–state reaction...................................................................................................42
v
3.2.2 Modified aerogel procedure......................................................................................44
3.3 Structural studies.............................................................................................................47
3.3.1 UV-visible spectroscopy...........................................................................................47
3.3.2 Braunauer-Emmer-Teller analysis (BET)..................................................................50
3.3.3 Powder X-ray diffraction..........................................................................................54
3.3.4 Transmission electron microscopy............................................................................65
3.3.5 Elemental analysis....................................................................................................71
3.3.6 Thermogravimetric analysis......................................................................................71
3.4 Discussion.......................................................................................................................71
3.5 Conclusions.....................................................................................................................73
3.6 References.......................................................................................................................75
Chapter 4 : Photooxidation of acetaldehyde by titanates............................................................76
4.1 Introduction.....................................................................................................................76
4.2 Photoactivity under light irradiation.................................................................................77
4.2.1 Design of photocatalysts of high activity...................................................................82
4.2.2 UV light irradiation...................................................................................................84
4.2.3 Visible light irradiation.............................................................................................87
4.3 Acetaldehyde photodecomposition studies.......................................................................88
4.3.1 Experimental setup for photodecomposition reactions...............................................90
4.3.2 Photoactivity of commercially available and synthesized samples.............................96
4.4 Conclusions...................................................................................................................102
4.5 References.....................................................................................................................103
Chapter 5 : Modification of titanates by doping.......................................................................106
5.1 Introduction...................................................................................................................106
5.2 Doping process..............................................................................................................108
5.2.1 Doping with transition metals in solid-state reaction...............................................112
5.2.2 Doping with transition metals in aerogels................................................................112
5.3 Photoactivity of doped titanates.....................................................................................115
5.3.1 Cr doping and Sb/Cr codoping of aerogel prepared catalysts...................................115
5.3.2 Cr doping and Sb/Cr codoping of solid-state prepared samples...............................134
5.3.3 Cr doping of SrTiO and BaTiO catalysts..............................................................147
3 3
vi
5.4 Discussion and conclusions...........................................................................................157
5.5 References.....................................................................................................................159
Chapter 6 : Surface studies of titanates by FTIR spectroscopy.................................................160
6.1 Introduction...................................................................................................................160
6.2 Acetaldehyde decomposition on the surface of aerogel prepared SrTiO ........................163
3
6.2.1 Acetaldehyde adsorption over SrTiO at 243 K: evidence of H-bonding .................165
3
6.2.2 Warm up effect of adsorbed acetaldehyde: evidence of aldol condensation and
formation of α, β-unsaturated aldehyde (crotonaldehyde, CH -CH=CH-CHO)................168
3
6.2.3 Dark oxidation: influence of dioxygen exposure over preadsorbed acetaldehyde at 243
K.....................................................................................................................................170
6.2.4 Spectral development during photooxidation reaction: influence of dioxygen exposure
over preadsorbed acetaldehyde at 243 K..........................................................................173
6.3 Acetaldehyde decomposition on the 2% Cr doped and 2.5% Sb/2% Cr codoped AP-SrTiO
3
............................................................................................................................................176
6.3.1 Dehydroxylation on 2% Cr doped AP-SrTiO .........................................................176
3
6.3.2 Dehydroxylation on 2.5% Sb/2% Cr doped AP-SrTiO ...........................................177
3
6.3.3 Adsorption, evacuation, and warming prior to dark oxidation of acetaldehyde on 2%
Cr doped AP-SrTiO ........................................................................................................177
3
6.3.4 Adsorption, evacuation, and warming prior to dark oxidation of acetaldehyde on 2.5%
Sb/2% Cr codoped AP-SrTiO .........................................................................................181
3
6.3.5 Attempted dark oxidation of acetaldehyde on 2% Cr-SrTiO ...................................181
3
6.3.6 Attempted dark oxidation of acetaldehyde on 2.5% Sb/2% Cr codoped AP-SrTiO .184
3
6.3.7 Photooxidation of acetaldehyde on 2% Cr doped AP-SrTiO ..................................184
3
6.3.8 Photooxidation of acetaldehyde on 2.5% Sb/2% Cr doped AP-SrTiO ....................187
3
6.4 Mass spectrometry studies on reaction products.............................................................187
6.5 Conclusions...................................................................................................................191
6.6 References.....................................................................................................................192
Chapter 7 : Dielectric studies on titanates................................................................................193
7.1 Introduction...................................................................................................................193
7.2 Dielectric properties of titanates....................................................................................193
7.3 Aerogels for electrical applications................................................................................196
vii
7.4 Synthesis of Ba Sr TiO aerogel................................................................................197
0.5 0.5 3
7.5 Dielectric measurements................................................................................................203
7.5.1 Impedance analysis.................................................................................................203
7.5.2 Raman spectroscopy...............................................................................................210
7.6 Conclusions...................................................................................................................212
7.7 References.....................................................................................................................213
APPENDIX A: XRD analysis.................................................................................................215
APPENDIX B: Elemental analysis..........................................................................................217
APPENDIX C: TGA analysis..................................................................................................218
APPENDIX D: in situ FTIR....................................................................................................220
APPENDIX E: Dielectric measurements.................................................................................223
APPENDIX F: Permission to reproduce materials...................................................................226

viii
LIST OF FIGURES

Figure ‎1.1 Perovskite (ABO ) Unit Cell......................................................................................3
3
Figure ‎1.2 Perovskite Structure (BO and A2+ Layers).................................................................4
6
Figure ‎2.1 Unit Cell of YBa Cu O Oxide...............................................................................20
2 3 7-x
Figure ‎2.2 Principal Scheme for Chemical Transport (Adapted and Modified from Reference 5)
..........................................................................................................................................22
Figure ‎2.3 Principal Scheme of a CVD Reactor for Oxide Film Deposition (MFC – Mass Flow
Controller) (Adapted and Modified from Reference 5).......................................................25
Figure ‎2.4 Production Options for the Sol-gel Process (Adapted and Modified from Reference 5)
..........................................................................................................................................31
Figure ‎2.5 Sintering by diffusion (path 1 - surface diffusion, path 2 - volume diffusion)............34
Figure ‎2.6 Temperature-pressure Diagram for Supercritical Drying, where C - Critical Point,
SCF – Super Critical Fluid, T – Critical Temperature, P – Critical Pressure (Adapted and
c c
Modified from Reference 5 and 13)...................................................................................37
Figure ‎3.1 Solid-state Reactions for SrTiO and BaTiO Synthesis............................................43
3 3
Figure ‎3.2 Modified Aerogel Procedure (MAP) from Alkoxides for SrTiO and BaTiO
3 3
Synthesis...........................................................................................................................46
Figure ‎3.3 UV-visible Spectra of TiO P25 Degussa and Aerogel Prepared SrTiO ...................48
2 3
Figure ‎3.4 UV-visible Spectra of Different SrTiO Samples......................................................49
3
Figure ‎3.5 Five Types of Adsorption Isotherms [2]...................................................................52
Figure ‎3.6 Powder XRD Patterns of Commercial and Synthesized SrTiO with Different
3
Alcohols Used in Synthesis (CM-SrTiO – Commercial, NCM-SrTiO – Commercial
3 3
Nanosized, AP-SrTiO – Aerogel Prepared Samples)........................................................57
3
Figure ‎3.7 Powder XRD Patterns of Solid-state (SSR-SrTiO ) and Aerogel Prepared (AP-
3
SrTiO ) Samples................................................................................................................58
3
Figure ‎3.8 Powder XRD Patterns of AP-SrTiO (Ethanol) Calcined in Air at Different
3
Temperatures.....................................................................................................................60
Figure ‎3.9 Powder XRD Patterns of Commercial and Synthesized BaTiO with Different
3
Alcohols Used in Synthesis (CM-BaTiO – Commercial, NCM-BaTiO – Commercial
3 3
ix
Nanosized, AP-BaTiO – Aerogel Prepared Samples). The XRD Studies of the Samples
3
were Conducted using a Shimadzu XRD 6000 (NanoScale Materials, Inc.).......................61
Figure ‎3.10 Powder XRD Patterns of AP-BATiO (Ethanol) Calcined in Air at 500°C with a
3
BaCO Phase vs. Freshly Prepared Sample of AP-BaTiO (Ethanol) .................................63
3 3
Figure ‎3.11 Calcination of AP-BaTiO (Ethanol) in Air and Oxygen at 500°C..........................64
3
Figure ‎3.12 Transmission Electron Micrographs of SSR-SrTiO Prepared at 1100°C................67
3
Figure ‎3.13 Transmission Electron Micrographs of AP-SrTiO after Synthesis (left) and after
3
Calcination in Air at 500°C (right)....................................................................................68
Figure ‎3.14 Transmission Electron Micrographs of SSR-BaTiO Prepared at 1100°C...............69
3
Figure ‎3.15 Transmission Electron Micrographs of AP-BaTiO (Isopropanol) Samples after
3
Synthesis (left), Heat Treated in Vacuum at 500°C (middle), and Calcined in Air at 500°C
(right)................................................................................................................................70
Figure ‎4.1 Semiconductor Photocatalyst for Water Photolysis...................................................78
Figure ‎4.2 Semiconductor Oxide Band Gaps and Potentials ......................................................80
Figure ‎4.3 Photocatalytic Reactions: Photoinduced Reaction (down hill) and Photon Energy
Conversion Reaction (up hill) [17].....................................................................................81
Figure ‎4.4 Photocatalytic Oxidation of Various Organic Compounds on TiO Surface under UV
2
Light Irradiation [33].........................................................................................................84
Figure ‎4.5 Oxidation of Acetic Acid on the TIO under UV Irradiation and in Oxygen-free
2
Environment [33]..............................................................................................................86
Figure ‎4.6 UV-visible Absorbance of Titanium Based Semiconductor Oxides...........................89
Figure ‎4.7 Experimental Setup for Photocatalysis Reactions......................................................92
Figure ‎4.8 Peak Areas versus Time for CH CHO Decomposition and CO Evolution under UV
3 2
Light for AP-SrTiO ..........................................................................................................93
3
Figure ‎4.9 Concentration versus CH CHO Decomposition and CO Evolution under UV Light
3 2
for AP-SrTiO ...................................................................................................................94
3
Figure ‎4.10 Photocatalytic Decomposition of Acetaldehyde under UV Light for P25 TiO ........95
2
Figure ‎4.11 CO Evolution under UV Light Irradiation for Different Catalyst Samples.............97
2
Figure ‎4.12 CH CHO Decomposition under UV Light Irradiation for Different Catalyst Samples
3
..........................................................................................................................................98
Figure ‎4.13 Defuse Reflectance Spectra of Different SrTiO Samples.....................................100
3
x