Inkjet Printing of TiO2 Thesis submitted in accordance with the requirements of The University of Liverpool for the degree of Doctor in Philosophy By Josh Turner September 30, 2019 Abstract This thesis describes the formulation, optimisation, and development of inks for the deposition of TiO2 using inkjet printing. TiO2 is an industrially significant metal oxide (MO) with applications in photocatalysis, gas sensing, dye pigmentation, and self-cleaning materials, to name just a few. For applications that require a directly patterned thin film of TiO2, inkjet printing is an attractive route to deposition. Inkjet printing of MOs, including TiO2, is a process still in its infancy and requires further development. Most inks are based on colloidal suspensions of TiO2, either purchased or synthesised using the sol-gel technique, that are typical in the spin and dip-coating processes. Our work aimed to instead base our inks on the solution precursors used in chemical vapour deposition (CVD), specifically titanium(IV) isopropoxide (TTIP). Due to the strong preference for the anatase TiO2 polymorph in most applications, emphasis was placed on obtaining anatase and reducing the temperature at which this occurred. To this end, several ink formulations were developed including: a solution-based TTIP ink, a hybrid alkoxide/nanoparticle ink, titanium oxo-cluster inks, and niobium doped inks. TTIP is moisture-sensitive, reacting with H2O to ultimately form TiO2 through a series of hydrolysis and polycondensation reactions. This property was exploited to produce a solution-based TTIP ink that reacts with ambient moisture to form TiO2 post-deposition. The use of glycol ethers as stabilising agents was investigated, to inhibit the reactivity of the TTIP during ink storage and printing. 1,2-dimethoxy ethane was identified as the optimum stabiliser when using iPrOH as the carrier. Post-deposition phase analysis showed the films to be amorphous on a glass substrate. An annealing step of 450 °C for 40 minutes yielded anatase. To reduce the annealing temperature required for anatase formation, the use of phase-pure anatase nanoparticles as seed sites for crystallisation was investigated. Addition of anatase nanoparticles to the solution-based TTIP ink was found to reduce the annealing temperature required for anatase formation to 200 °C for 160 minutes. This temperature is compatible with some flexible substrates, such as polyethylene terephthalate (PET), so printing and annealing was also demonstrated on PET. This hybrid alkoxide/nanoparticle ink is, to the best of our knowledge, the first example of a hybrid precursor/nanoparticle ink and the inclusion of crystal seed sites within an ink for inkjet printing. i Titanium oxo-clusters were investigated as a potential titanium source for inkjet inks. Several clusters were synthesised by the controlled hydrolysis of a reactive titanium precursor, such i as TTIP, with H2O. The [Ti11O13(O Pr)18] cluster was identified as yielding the best ink when dissolved in a toluene carrier. An annealing temperature of 350 °C for 40 minutes was required to convert to amorphous TiO2 to anatase, a reduction of 100 °C when compared to the solution-based TTIP ink. The printed oxo-cluster films were less continuous and less homogeneous than those produced with the TTIP and hybrid inks. A niobium-doped Ti(OEt)4 solution was provided by our industrial sponsors, EpiValence, with the intention of use as a potential ink to form a transparent conducting oxide (TCO) thin film. The solution was formulated into an ink and printed onto glass substrates, along with an analogous ink using the solution-based TTIP ink. Despite the inclusion of a niobium dopant, the TiO2 films demonstrated a low transmittance and conductivity measurements could not be obtained. Further work would be required before the films produced by these niobium doped inks would be suitable for applications as TCOs. ii Acknowledgements First and foremost, I would like to thank my supervisors Professor Helen C. Aspinall and Dr Kate Black for their invaluable support, patience and guidance. Helen always provided me with an appropriate style of supervision, whether it be firm or relaxed. Her experience as a supervisor was evident from the start and I am extremely grateful for the numerous pieces of feedback she provided me with. I appreciated the up-front and direct comments that were tremendously beneficial in their honesty and openness. Kate always offered useful support and guidance, often with an alternate view to be explored. Through Kate’s supervision, I was introduced to her expanding research group and the many wonderful individuals it contains. Both Kate and her research group provided the opportunity for friendly interaction in the additive manufacturing lab, the office, and the various meals out and conferences we attended. I wish I’d have taken more advantage of this growing community! It was an absolute pleasure and a privilege to work under the tutelage of both Helen and Kate. I wish you both the best of luck in all of your future endeavours. Secondly, I would like to thank everyone at EpiValence, especially Simon Rushworth. His advice and candour were greatly appreciated. I always enjoyed travelling to the EpiValence site with him, and I valued every long car journey we did together. I am also grateful to all of my colleagues and friends at the University of Liverpool who helped me during my Ph.D, especially the technical training staff and the members of the 4th floor Chemistry laboratory. Special mentions to Josh and Alex whose MCHEM projects were with myself and Helen. I would like to thank Alex and Sean for going through the Ph.D process alongside me, giving us all the understanding required to support one-another no matter the situation. Their continued friendship gave me the motivation to get out of bed when it was needed. I would also like to thank Mike, Steven, Patricia, Penelope, John, and Matthew for being great friends and keeping me social and sane. Finally, the biggest thank you to my family. Hara and Simon, you gave me with life, a home that was far enough away from you to get some work done, and the financial support to continue eating during the last few months. Kris, you provided me with the camaraderie that only a brother could and I am grateful to have you back on the same continent as the rest of your family. iii Nomenclature acac - Acetyl acetone AFM – Atomic force microscopy ALD – Atomic layer deposition CAD – Computer aided design CIJ – Continuous inkjet CMYK – Cyan, magenta, yellow, black CVD – Chemical vapour deposition DFT – Density functional theory DLS – Dynamic light scattering DME – 1,2-dimethoxyethane, CH3OCH2CH2OCH3 DMF – N,N-dimethylformamide, HCON(CH3)2 DOD – Drop-on-demand DOS – Density of states DSSC – Dye-sensitised solar cell EDXS – Energy-dispersive X-ray spectroscopy FTIR – Fourier-transform infrared spectroscopy FTO – Fluorine doped tin oxide, SnO2-SnF2 FWHM – Full width at half maximum intensity ITO – Indium tin oxide (In2-xSnxO3) MO – Metal oxide MOCVD – Metal organic chemical vapour deposition NP – Nanoparticle Oh – Ohnesorge number iv PEN – Polyethylene naphthalate, (C14H10O4)n PET – Polyethylene terephthalate, (C10H8O4)n i PPE – 2-isopropoxyethanol, (CH3)2CHOCH2CH2OH PTFE – Polytetrafluoroethylene, (C2F4)n PZT – Lead zirconate titanate (Pb[ZrxTi1-x]O3 for 0<x<1) Re – Reynolds number RMC – Reactive molecular cluster SEM – Scanning electron microscopy SOS – Second order scattering TCO – Transparent conducting oxide TEM – Transmission electron microscopy TLC – Thin-layer chromatography i TTIP – Titanium(IV) isopropoxide,Ti(O Pr)4 We – Weber number XANES – X-ray absorption near edge spectroscopy XRD – X-ray diffraction v Contents Abstract......................................................................................... i Acknowledgements..................................................................... iii Nomenclature.............................................................................. iv 1 Introduction............................................................................ 1 1.1 Context of Research............................................................................ 1 1.2 Project Overview................................................................................. 1 1.3 Thesis Structure.................................................................................. 2 1.4 Publications and Conference Proceedings......................................... 3 2 Literature Review...................................................................... 4 2.1 TiO2...................................................................................................... 4 2.1.1 Properties of TiO2........................................................................... 4 2.1.2 Applications of TiO2........................................................................ 7 2.2 Deposition of TiO2............................................................................... 7 2.2.1 Common Deposition Techniques.................................................. 7 2.2.2 Titanium Alkoxides....................................................................... 14 2.2.3 TiO2 Deposition Literature Review............................................... 14 2.2.4 Technique Comparison and Summary......................................... 17 2.3 Fundamentals of Inkjet Printing........................................................ 18 2.3.1 Basics of Inkjet Printing...............................................................