Max-Planck-Institut für Kolloid- und Grenzflächenforschung Utilization of Graphitic Carbon Nitride in Dispersed Media Dissertation zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. nat.) In der Wissenschaftsdisziplin „Kolloid- und Polymerchemie“ eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät der Universität Potsdam von Baris Kumru Potsdam-Golm, im Oktober 2018 Published online at the Institutional Repository of the University of Potsdam: https://doi.org/10.25932/publishup-42733 https://nbn-resolving.org/urn:nbn:de:kobv:517-opus4-427339 I Table of Contents Chapter 1. Introduction.......................................................................................................... 1 Chapter 2. Background and Motivation ................................................................................ 3 2.1. Carbon Nitride: A Futuristic Semiconductor for Photo-based Applications ......................................3 2.2. Carbon Nitride in Polymer Chemistry: State of the Art ................................................................. 12 2.3. Biological Tissues and Hydrogels: Strong Relation ........................................................................ 16 2.4. Reinforced Hydrogels: Chemical Methods for Reinforcement ...................................................... 23 2.5. Motivation of the Thesis .............................................................................................................. 27 Chapter 3. Outline ................................................................................................................ 29 Chapter 4. Reinforced Hydrogels via Carbon Nitride Initiated Polymerization .............. 31 Chapter 5. Tough High Modulus Hydrogels Derived from Carbon Nitride via an Ethylene Glycol Co-solvent Route ........................................................................................................ 44 Chapter 6. Enhanced Dispersibility of Graphitic Carbon Nitride Particles in Aqueous and Organic Media via a One Pot Grafting Approach ................................................................. 59 Chapter 7. Extremely Compressible Hydrogel via Incorporation of Modified Graphitic Carbon Nitride......................................................................................................................... 75 Chapter 8. Tough and Lubricant Hydrogels via Carbon Nitride ‘Prepolymer’ ............... 86 Chapter 9. Electrostatic Stabilization of Carbon Nitride Colloids in Organic Solvents Enables Stable Dispersions and Transparent Homogeneous CN-Films for Optoelectronics .................................................................................................................. 98 Chapter 10. Conclusion and Outlook ................................................................................ 111 Chapter 11. Appendix ........................................................................................................ 115 11.1. Materials .................................................................................................................................. 115 11.2. Synthesis Procedures ................................................................................................................ 117 11.3. Characterization ....................................................................................................................... 129 11.4. Appendix Figures ...................................................................................................................... 134 11.5. Abbreviations ........................................................................................................................... 175 11.6. Publication List ......................................................................................................................... 177 11.7. Declaration ............................................................................................................................... 179 Chapter 12. References ...................................................................................................... 180 Chapter 13. Acknowledgements ........................................................................................ 190 II III Chapter 1 1. Introduction From the dawn of civilization, humans have depended on wood fires followed by fossil fuels as their primary energy supply. However, a steadily increasing demand has created awareness that the sustainability of this model is not possible as the sources are rapidly depleting. In addition, the side effects arising from fires and fossil fuel consumption have been exposed, such as a 1 drastic increase in atmospheric CO2 concentration. As the problems surmounted, research into alternative and sustainable energy source that is environmentally benign was necessitated. As a result, the awesome and enigmatic power of the sun was turned to once again, as it always has since the beginning of Homo erectus epoch. The metaphors retrieved from the sun have differed through ages. Firstly, it was worshipped as a supernatural entity in ancient Egypt2 and Sumeria3 as a significant part of their religion. However, later on, the desire to comprehend orbits of the planets and stars placed the sun as an object instead of God-like entity. Observing planetary motion provided hypotheses related to the changing of the seasons and cosmic events.4 Essential scientific clues were retrieved by the studies of numerous scientists, notably, Newton, Herschel, Planck and Fraunhofer, which showed rays from the sun are made up of many colors with different energies creating optical spectrum.4 It was realized that sunlight can be transferred into energy via utilization of semiconductors, and innumerable of innovations came to follow. Of tremendous importance has been the silicon- and TiO2-focussed research, but other metal-containing materials such as CdS has been investigated as well. However, the necessity to significantly increase the yield couple with the inherent toxicity of utilizing metal-based semiconductors initiated the search for other materials. Graphitic carbon nitride (g-CN) has since been discovered having the benefits of being metal-free in composition and can be synthesized from abundant and low cost precursors. Diversified applications of g-CN as a heterogeneous catalyst have being presented, however functionality and processability of g-CN must be enhanced for the synthesis of ideal energy-harvesting devices in the future. Humankind’s innate drive toward development, innovation, explaining seemingly inexplicable events, and designing devices has facilitated today’s technology level. Indeed there were many challenges and obstacles on the way, but humanity has always found a way to proceed even in the 1 Chapter 1 face of adversity. Considering the very short time humans have spent on Earth, the transition from sleeping in caves to skyscrapers, from local limited resources to globalization and an industrial era have been achieved. Similar breakthroughs have occurred in healthcare, i.e. the plague, once killing millions of people, can now be treated with antibiotics. Novel disease treatments have increased average lifetime profoundly. Further, current healthcare studies are overcoming substantial hurdles: It is likely tissue engineering will soon facilitate the synthesis of artificial organs, where synthetic materials will present both mechanical and biological performance similar to natural tissues. Hydrogels, crosslinked hydrophilic polymer networks, are promising as synthetic materials for tissue engineering as high water content of hydrogels are similar to biological tissues. Being mechanically weak under regular synthesis conditions, hydrogel reinforcement can be applied in order to fabricate hydrogels with diverse mechanical features. The synthesis of appropriately reinforced hydrogels will be the first step in manufacturing advanced healthcare materials. In the present thesis, the applications of g-CN in dispersed media utilizing visible light will be emphasized and the potentials from bio based applications to energy harvesting will be investigated. 2 Chapter 2 2. Background and Motivation 2.1. Carbon Nitride: A Futuristic Semiconductor for Photo-based Applications Scientific understanding of sunlight has triggered the research about semiconductors, materials which can be employed to transform visible light into energy. Absorption in the visible light range and decent band gap properties are determining factors for semiconductors, providing photoexcitation which allows facilitating chemical processes. Materials like Si or TiO2 were studied to enhance the knowledge about chemistry and fabrication of energy harvesting devices.5 In addition, various metal including semiconductors (such as ZnO, CdS, Ta3N5, GaP) were reported which expanded the library of semiconductors with different photophysical properties.6 Despite the fact that reported materials have effective early stage results, several problems regarding manufacturing, long-term efficiency and sustainability are still present. The search for sustainable semiconductors encountered carbon nitride (CN), a synthetic polymeric structure based on carbon and nitrogen atoms.7 The metal-free structure is based on repeating units of ’melems’ where tri-s-triazine units are connected via nitrogen bridges in defect- rich polymeric structure with slight negative surface charge (Figure 1.1).8 Theoretically, it was foreseen that novel polymer has 5 different