INVESTIGATION, DESIGN, AND COMMERCIALIZATION OF ACTIVELY-ENHANCED SOLID-STATE GAS SENSOR ARRAYS By BRYAN MICHAEL BLACKBURN 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 2009 1 © 2009 Bryan Michael Blackburn 2 To my family and friends for always believing in me 3 ACKNOWLEDGMENTS I would like to thank my colleagues and mentors over my many years of study and personal growth. I would also like to thank my parents and grandparents for fostering in me from an early age a philosophy for continuous learning and improvement. This includes the pursuit of knowledge from all fronts: engineering, science, mathematics, business, religion, literature, and more. To my fiancée, Kaitlin, I am thankful for the many years of understanding and patience as I worked towards this and future goals. In and around the lab, I would like to thank Dr. Hee-Sung Yoon, Nick Vito, and Patrick Wanninkhof for helping me with various aspects of my work. For keeping me calm during times of high stress, I would like to thank Cynthia Kan. I would also like to thank my brother, Jeremy, for providing a constant source of healthy competition during our childhood. I would like to thank Dr. Scott Perry for many useful talks regarding surface science and the use of his equipment. Furthermore, I would like to thank his graduate student, Greg Dudder, for working with me on some of the surface science experiments and teaching me about photoelectron spectroscopy. I would also like to thank Dr. Juan Nino and Dr. Enrico Traversa for teaching me valuable concepts during my time as a graduate student. I would be remiss not to thank Dr. Toshi Nishida for serving as the initial source of inspiration while I was an undergraduate student in electrical engineering that lead me to pursue a doctorate degree; he has since continued to motivate me and I look forward to the possibility of working with him on projects in the future. Finally, I would like to thank my advisor Dr. Eric Wachsman and mentor P.N. ‘Vaidy’ Vaidyanathan for invaluable guidance and giving me the opportunity to prove myself as an engineer, scientist, and businessman. 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES.........................................................................................................................10 LIST OF FIGURES .......................................................................................................................11 ABSTRACT...................................................................................................................................17 CHAPTER 1 INTRODUCTION ...................................................................................................................19 1.1 Small, Robust, Low-Cost Selective Multifunctional Gas Sensors ...................................19 1.1.1 Significance............................................................................................................19 1.1.1.1 Energy efficiency.....................................................................................19 1.1.1.2 Pollutant emissions..................................................................................21 1.1.1.3 Process control.........................................................................................22 1.1.1.4 Noninvasive disease diagnostics and monitoring....................................23 1.1.2 Key Applications for Advanced Gas Sensor Technology .....................................23 1.1.2.1 Energy production industry .....................................................................24 1.1.2.2 Transportation, heavy equipment, and materials handling industries .....26 1.1.2.3 Materials and chemical processing industries .........................................28 1.1.2.4 Medical industry......................................................................................29 1.1.2.5 Other important applications ...................................................................31 1.1.3 General Requirements of Gas Sensors...................................................................32 1.1.3.1 Sensitivity................................................................................................32 1.1.3.2 Selectivity................................................................................................32 1.1.3.3 Response time..........................................................................................33 1.1.3.4 Repeatability, accuracy, and resolution...................................................33 1.1.3.5 Signal-to-noise ratio ................................................................................34 1.2 Research Objectives..........................................................................................................34 1.2.1 Sensor Arrays for Detecting Multiple Gas Species ...............................................34 1.2.2 Thermal Enhancements..........................................................................................35 1.2.3 Electric-Field Enhancements .................................................................................35 1.2.4 Surface Science Analysis.......................................................................................35 1.2.5 Improved R&D and Commercialization Efforts....................................................36 2 BACKGROUND AND LITERATURE REVIEW .................................................................39 2.1 Gas Sensors.......................................................................................................................39 2.1.1 General Issues ........................................................................................................40 2.1.2 Solid-State Gas Sensor Technology.......................................................................41 2.1.2.1 Thermal sensors.......................................................................................41 2.1.2.2 Mass sensors............................................................................................41 5 2.1.2.3 Optical sensors.........................................................................................41 2.1.2.4 Electrochemical sensors ..........................................................................41 2.1.3 Electrochemical Gas Sensor Theory......................................................................43 2.1.3.1 Principles of electronic and ionic conduction .........................................43 2.1.3.2 Principles of solid-state electrochemistry................................................43 2.1.3.3 Electrochemical promotion .....................................................................47 2.1.3.4 Semiconducting oxides............................................................................48 2.1.3.5 Mixed potential theory ............................................................................50 2.1.3.6 Differential electrode equilibria theory ...................................................51 2.1.3.7 Electrolyte and gas sensing electrode materials ......................................52 2.2 Related Work....................................................................................................................54 2.2.1 Techniques for Studying Gas Sensor Mechanisms................................................54 2.2.1.1 Conductivity measurements ....................................................................54 2.2.1.2 Controlled-potential techniques...............................................................55 2.2.1.3 Electrochemical impedance spectroscopy...............................................55 2.2.1.4 Mass spectrometry...................................................................................56 2.2.1.5 Kelvin probe work function measurements.............................................56 2.2.1.6 Electron spectroscopic techniques...........................................................57 2.2.2 Electric-Field Effects in Surface Processes and Solid-State Devices....................58 2.2.3 Other Important Electrolytes and Sensing Electrodes ...........................................59 3 DIFFERENTIAL ELECTRODE TEMPERATURE EFFECT ...............................................65 3.1 Introduction.......................................................................................................................65 3.2 Background.......................................................................................................................65 3.3 Experimental.....................................................................................................................67 3.3.1 Sample Preparation ................................................................................................68 3.3.2 Test Setup...............................................................................................................69 3.3.3 Testing Procedures.................................................................................................69 3.3.4 3D Reconstruction of Sensing Electrodes with FIB/SEM.....................................70 3.4 Results and Discussion .....................................................................................................71 3.4.1 Proof of Concept....................................................................................................71