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Title: Inkjet printing of TiO₂
Author: Turner, Joshua
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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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. 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 as TTIP, with H2O. The [Ti11O13(OiPr)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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral