A FACILE METHOD TO FABRICATE VERSATILE JANUS NANOPARTICLES A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Xiaoxu Lu August, 2016 A FACILE METHOD TO FABRICATE VERSATILE JANUS NANOPARTICLES Xiaoxu Lu Thesis Approved: Accepted: Advisor Department Chair Dr. Younjin Min Dr. Sadhan C. Jana Faculty Reader Dean of the College Dr. Alamgir Karim Dr. Eric J. Amis Faculty Reader Dean of the Graduate School Dr. Bryan D. Vogt Dr. Chand Midha Date ii ABSTRACT Janus colloidal particles have two distinct surface structures or properties. The two faces of Janus particles lead to asymmetric interactions with different phases which depend on their different separations and orientations. Due to this novel property, Janus particles can be used for oil recovery through capturing the oil in the water phase. Although various behaviors of Janus particles at interfaces have been studied, there are still many constraints such as limited surface functionality selections and poor control of fraction of each surface when generating Janus colloidal particles. The scope of this thesis focuses on the fabrication and surface modification of versatile Janus colloidal particles using a selective unmasking and masking process for uniform surface modification through thermal, dipping and chemical vapor deposition (CVD) methods. Particles immobilized on a solid substrate will be first embedded into a polymer matrix through spin coating, then part of the polymer will be etched through plasma treatment so that particles can be exposed partially for the modification. Highly monodisperse spherical silica particles with diameters of 150 to 400nm are prepared using an ethanol based synthesis method where the size of the spheres can be precisely controlled via a seed regrowth method. Monodisperse silica seeds can be synthesized through modified Stöber method. Dilution of tetraethyl orthosilicate (TEOS) with ethanol can improve the monodispersity and iii shape of the silica particles. Highly ordered two-dimensional colloidal monolayer. are fabricated onto silicon substrates by using the Langmuir-Blodgett (LB) deposition technique at the air-water interface. PMMA toluene solution is coated onto monolayers through a two-step spin-coating process and then particles will be partially exposed by plasma etching. The exposed area can be controlled through adjusting plasma treatment time. Afterwards, trimethylchlorosilane (MTS), pentyltrichlorosilane (PTS) and phenyltrichlorosilane (PHTS) are used to modify the exposed area of the particles through thermal, dipping and CVD method. In conclusion, Janus colloidal particles with controllable chemical composition and anisotropic surface properties have been prepared. The CVD method is proven to be most effective to generate relatively homogeneous silane layers on silicon surfaces within 30 minutes. The Janus colloidal particles modified with PTS through the CVD show the most hydrophobic properties as anticipated from its chemical structure and packing density. Looking to the future, this study will be continued to generate Janus nanoparticle based Pickering emulsions for the application of enhanced oil recovery. iv ACKNOWLEDGEMENTS I appreciate all of those people who have helped me to accomplish the project and Master’s thesis. First and foremost, I want to express gratitude towards my advisor, Prof. Younjin Min for her patient and continuous guidance, support and encouragement for my research work and master study. Prof. Min not only provided me with training in experimental and research skills, but also built my scientific qualities, curiosity, skepticism and writing skills. Next I would like to thank Prof. Alamgir Karim for providing Laurell spin coater, Prof. Nicole Zacharia for providing VCA Optima XE contact angle measurement machine, Dr. Rong Bai for Atomic Force Microscopy (AFM) training and Dr. Bojie Wang for Scanning Electron Microscopy (SEM) training. Finally, I would like to thank all of my group members: Mr. Ming Zhang, Mr. Yuanzhong Zhang, Miss Marisa Tukpah, Mr. Tianxin Zhao, Mr. Shihao Wen, and friends who once helped me with experiments. I would have achieved nothing without them. v TABLE OF CONTENTS Page LIST OF FIGURES .......................................................................................... ix CHAPTER I INTRODUCTION ......................................................................................... 1 1.1 Janus particles ....................................................................................... 1 1.2 Previous method of fabrication of Janus particles .................................. 3 1.2.1 Pickering emulsion ................................................................................. 3 1.2.2 Stamping ................................................................................................ 4 1.2.3 Langmuir Blodgett functionalization ........................................................ 6 1.2.4 Masking/unmasking technique ............................................................... 7 1.3 About silane modification ....................................................................... 7 1.3.1 Mechanism of Langmuir-Blodgett ........................................................... 8 1.3.2 Mechanism of spin-coating ..................................................................... 9 1.3.3 Plasma etching ..................................................................................... 10 1.3.4 Mechanism of modification ................................................................... 10 1.4 Physicochemical characterization ........................................................ 12 1.4.1 Scanning electron microscope (SEM) .................................................. 12 1.4.2 Transmission electron microscope (TEM) ............................................ 14 1.4.3 Contact angle (CA) ............................................................................... 15 1.4.4 Dynamic light scattering (DLS) ............................................................. 15 1.4.5 Atomic force microscope (AFM) ........................................................... 16 vi II MATERIAL SPECIFICATION AND EXPERIMENT SECTION ................. 18 2.1 Introduction .............................................................................................. 18 2.2 Material .................................................................................................... 18 2.3 Synthesis of monodisperse bare silica particles ...................................... 19 2.3.1 Smaller particle synthesis (≤ 300nm) .................................................... 19 2.3.2 Larger particle synthesis (> 300nm) ..................................................... 20 2.4 Fabrication of silica particle monolayer .................................................... 21 2.4.1 Suspension and substrate treatment .................................................... 21 2.4.2 Preparation of monolayer ..................................................................... 22 2.5 Masking/unmasking technique ................................................................ 22 2.6 Modification ............................................................................................. 23 2.6.1 Thermal method ................................................................................... 23 2.6.2 Dipping method .................................................................................... 24 2.6.3 Chemical vapor deposition ................................................................... 24 2.7 Particle treatment .................................................................................... 24 2.8 Characterization ...................................................................................... 25 III RESULT AND DISCUSSION .................................................................. 26 3.1 Characterization of silica particles ........................................................... 26 3.1.1 Smaller silica particles (≤300 nm) ......................................................... 26 3.1.2 Larger silica particles (>300 nm)........................................................... 30 3.2 Fabrication of monolayer through Langmuir-Blodgett technique ............. 31 3.2.1 Effect of treatment ................................................................................ 31 3.2.2 Surface pressure-Area per particle isotherm plot ................................. 32 3.2.3 Characterization ................................................................................... 34 3.3 Masking/unmasking through spin-coating and plasma treatment ............ 36 vii 3.3.1 Effect of PMMA concentration .............................................................. 36 3.3.2 Effect of plasma treatment .................................................................... 37 3.4 Modification ............................................................................................. 39 3.4.1 Thermal method ................................................................................... 39 3.4.2 Dipping method .................................................................................... 41 3.4.3 Chemical vapor deposition ................................................................... 41 CONCLUSION AND FUTURE WORK .......................................................... 44 REFERENCES .............................................................................................