Impact of Catalytic Additive on Spray Deposited and Nanoporous Titania Thin Films Observed via in Situ X-ray Scattering: Implications for Enhanced Photovoltaics

Abstract

With the aim of obtaining nanostructured titania thin films for the potential use in hybrid or dye sensitized solar cells, the amphiphilic diblock copolymer polystyrene-b-poly(ethylene oxide) is employed as a structure directing template in combination with sol− gel chemistry. For easy upscaling, spraying is used as a deposition technique. In situ grazing incidence small-angle X-ray scattering (GISAXS) measurements are performed during spraying and show that most titania structures are already formed within the solution prior to deposition. However, structural rearrangement is enabled during the deposition period when small amounts of hydrochloric acid (HCl) are used as a catalytic additive to the spray solution. This behavior is ascribed to an altering of the reaction dynamics and phase separation in the presence of HCl, which significantly improves the templating effect of the employed diblock copolymer. With HCl as an additive the final nanoscale morphologies exhibit smaller pore sizes and strongly enhanced order as compared to thin films sprayed from solutions that do not contain HCl as quantified with atomic force microscopy, scanning electron microscopy, and GISAXS.

Key Factors for Template‐Oriented Porous Titania Synthesis: Solvents and Catalysts

“…However, regarding the condensation process, a further growth and agglomeration process of the condensed titania nanoparticles is prone to occur with a low HCl addition. Since the isoelectric point of TiO 2 lies at a pH of 5–7, charges on the particle surfaces in a weak acidic medium are not sufficient for keeping discrete particles in a dispersed state, which facilitates the formation of randomly arranged large pore structures . A second approach is based on the phase separation behavior of the diblock copolymer in the sol–gel solution.…”

“…As mentioned before, the phase separation of a diblock copolymer can be provoked by solvent evaporation or poor solvent addition. The similar synthetic process in our previous work demonstrated the randomly arranged titania nanostructures formed without HCl addition, which suggested the weak effect of the solvent‐induced self‐assembly for the final titania nanostructures. Accordingly, the structure evolution in Figure can also be assigned to the surface energy variation between PS blocks and surrounding solvent associated with poor solvent addition.…”

“…Cheng et al successfully synthesized nanowires, nanoparticles, flake‐like structures, nanodoughnuts, and worm‐like and foam‐like nanoscale networks by controlling the relative content of precursor, catalyst, and solvent in a polystyrene‐b‐polyethylene oxide (PS‐b‐PEO) based casting solution . Similarly, Hohn et al demonstrated the strong influence of hydrochloric acid (HCl) addition on the resulting thin film morphology . Solvent effects on surfactant templated mesoporous sol–gel silica systems were, e.g., studied by Kruk and co‐workers .…”

“…In the case of organic–inorganic hybrid solar cells, the exciton diffusion length within typical backfilling materials is typically less than 20 nm. Mesoporous titania with uniform pore arrangement provides an excellent basis to suppress recombination losses of excitons . Although many achievements have been reached in the preparation of porous metal oxide materials, current synthetic methods still have several inherent drawbacks from the perspective of thin film nanotechnology, such as time consuming processing conditions and the ill‐defined powder form product .…”

Key Factor Study for Amphiphilic Block Copolymer‐Templated Mesoporous SnO2 Thin Film Synthesis: Influence of Solvent and Catalyst

“…In order to integrate the functional inorganic part into the micro‐phase‐separated block polymer network, a hydrogen bond interaction between the precursor molecules and a specific segment of the block copolymer template is expected. [ 48 ] According to previous studies, the factors that affect the microstructure of the block copolymer‐templated metal oxide thin films include the reaction time, [ 49 ] the component content, [ 50–52 ] the surface conditions of the substrate, [ 53 ] the operational environment, [ 54 ] and the way of removing the polymer template. [ 55 ]…”