Changing the Wetting Properties of Titanium Dioxide Surfaces with Visible and Near Infrared Light by Samuel B. Rosenthal A dissertation submitted to Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy Baltimore, Maryland September, 2016 © 2016 Samuel Rosenthal All Rights Reserved Abstract TiO2 is known to change its wetting properties under UV light. In this thesis, the photoinduced hydrophilicity (PIH) of TiO2 surfaces is examined with the goal of extending the range of wavelengths to the visible and NIR regions. Chapter 2 examines the wetting properties of sub-stoichiometric titanium dioxide (TiO2-x). Single crystal rutile (110) TiO2 surfaces were reduced (TiO2-x where x<0.06) and characterized using XRD, XPS, UV-Vis spectrophotometry, and AFM. The increase in wetting of the surfaces under ultraviolet (UV) and visible light irradiation was investigated by contact angle analysis. Conversion back to their original hydrophobicity was attained by IR heating. Complete wetting by water was measured for all samples, including a simultaneously tested stoichiometric TiO2 surface, within 5 minutes under UV irradiation. Under visible light, the reduced TiO2-x surfaces showed an increase in wetting behavior while o o the TiO2 surface did not. The contact angles of the reduced surfaces decreased 20 -30 within 1 hour of visible light irradiation, but did not attain a state of complete wetting. The surface with greater reduction attained a lower contact angle than the less reduced surface and lowered its contact angle at a higher rate. The asymptotic approach to finite contact angle values well above complete wetting under visible light irradiation for the TiO2-x surfaces may provide a means to investigate the currently debated mechanism behind TiO2 being able to convert to a state of complete wetting. Chapter 3 examines stable water-in-oil (w/o) Pickering emulsions created using hydrophobically modified TiO2 nanoparticles at the water/oil interface. The emulsions were made unstable by exposure to UV light. The results of the destabilization are compared to macroscopic contact angle studies of the same hydrophobic TiO2 nanopowder in air. The ii results suggest that the Pickering emulsion destabilization is due to the TiO2 photocatalytically degrading the hydrophobic coating and undergoing a contact angle change sufficient to cause the particles to leave the water/oil interface and enter the water phase. Chapter 4 examines a combined surface composed of hydrophobic TiO2 nanoparticles and upconversion nanoparticles (UCNP). Core-shell UCNP were synthesized using an oleates-based synthesis process. The core and shell used a NaYF4 crystalline support structure. The core was doped with Yb, Tm, and Nd and the shell with Y and Nd (NaYF4:Yb,Tm,Nd@NaYF4:Y,Nd) in doping ratios appropriate to create nanoparticles that radiate in the UV range when irradiated with intense near-infrared (NIR) 980 nm or 808 nm radiation. High intensity, 1 W lasers were used to apply the necessary irradiation. The UV emission from the stimulated UCNP was hoped to be of sufficient intensity to cause the hydrophobic coating on the TiO2 nanoparticles to break down due to the photocatalytic activity of TiO2. Combined hydrophobic UCNP/TiO2 surfaces were created by mixing 1%, 5%, and 10% UCNP by weight. The 5% and 10% surfaces were found to respond to NIR irradiation within 30 minutes by a lowering of their contact angles by as much as 15o. Thesis Advisor: Thesis Advisor: Patricia M. McGuiggan, Ph.D. Mark O. Robbins, Ph.D. Professor of Materials Science Professor of Physics iii Contents Abstract ................................................................................................................................................... ii Contents ................................................................................................................................................. iv List of Figures ....................................................................................................................................... vii List of Tables.......................................................................................................................................... ix 1. INTRODUCTION ................................................................................................. 1 Contact Angles ........................................................................................................................................ 2 Contact Angle Definitions ................................................................................................................................................ 2 Wenzel and Cassie-Baxter Wetting ................................................................................................................................. 4 Hysteresis ............................................................................................................................................................................. 6 Titanium Dioxide .................................................................................................................................... 7 Electrical Properties of TiO2 ............................................................................................................................................ 9 Photocatalytic Activity of TiO2 ...................................................................................................................................... 10 Photoinduced Hydrophilicity on TiO2 ......................................................................................................................... 12 Reduced TiO2 ......................................................................................................................................... 15 Pickering Emulsions .............................................................................................................................. 18 Factors Effecting Pickering Emulsion Formation ..................................................................................................... 20 In Situ Pickering Emulsion Destabilization .................................................................................................................. 21 Upconversion Nanoparticles ................................................................................................................. 23 The Upconversion Process ............................................................................................................................................. 23 Upconverting Nanoparticles .......................................................................................................................................... 26 iv 2. WETTING OF REDUCED TIO2 SURFACES: RESPONSE TO VISIBLE AND ULTRAVIOLET IRRADIATION .............................................................................. 29 Introduction .......................................................................................................................................... 29 Experimental Methods .......................................................................................................................... 30 Results ................................................................................................................................................... 36 Discussion ............................................................................................................................................. 54 Conclusion ............................................................................................................................................ 59 3. FORMATION OF A PICKERING EMULSION MADE WITH TIO2 NANOPARTICLES AND IN SITU DESTABILIZATION UNDER ULTRAVIOLET IRRADIATION .............................................................................. 62 Introduction .......................................................................................................................................... 62 Experimental Methods .......................................................................................................................... 65 Results ................................................................................................................................................... 67 Model Sessile Drop Experiment .................................................................................................................................... 67 Solid Stabilized Pickering Emulsions using Uncoated TiO2 ..................................................................................... 70 Solid Stabilized Pickering Emulsions using Hydrophobically Coated TiO2 .......................................................... 77 Discussion ............................................................................................................................................. 83 Conclusion ............................................................................................................................................ 93 4. WETTING OF TIO2 SURFACES UNDER NEAR-INFRARED LIGHT INCORPORATING UPCONVERSION NANOPARTICLES ................................. 95 v Introduction .......................................................................................................................................... 95 Experimental Methods ........................................................................................................................