Highly Organized Mesoporous TiO₂ Films with Controlled Crystallinity: A Li‐Insertion Study

Abstract

A study of electrochemical Li insertion combined with structural and textural analysis enabled the identification and quantification of individual crystalline and amorphous phases in mesoporous TiO2 films prepared by the evaporation‐induced self‐assembly procedure. It was found that the properties of the amphiphilic block copolymers used as templates, namely those of a novel poly(ethylene‐co‐butylene)‐b‐poly(ethylene oxide) polymer (KLE) and commercial Pluronic P123 (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H), decisively influence the physicochemical properties of the resulting films. The KLE‐templated films possess a 3D cubic mesoporous structure and are practically amorphous when calcined at temperatures below 450 °C, but treatment at 550–700 °C provides a pure‐phase (anatase), fully crystalline material with intact mesoporous architecture. The electrochemically determined fraction of crystalline anatase increases from 85 to 100 % for films calcined at 550 and 700 °C, respectively. In contrast, the films prepared using Pluronic P123, which also show a 3D cubic pore arrangement, exhibit almost 50 % crystallinity even at a calcination temperature of 400 °C, and their transformation into a fully crystalline material is accompanied by collapse of the mesoporous texture. Therefore, our study revealed the significance of using suitable block‐copolymer templates for the generation of mesoporous metal oxide films. Coupling of both electrochemical and X‐ray diffraction methods has shown to be highly advisable for the correct interpretation of structure properties, in particular the crystallinity, of such sol–gel derived films.

Low temperature crystallisation of mesoporous TiO2

“…Using sufficiently high molecular weight BCPs leads to precursor-sol confinement on length-scales that are large enough to allow TiO 2 crystallisation at temperatures up to 700 • C 14,15 . The conversion of the carbon-rich polymer into a sturdy carbonaceous support under an inert atmosphere is a powerful method to further raise the crystallisation temperature to 1000 • C 16 .…”

Improved conductivity in dye-sensitised solar cells through block-copolymer confined TiO2crystallisation

“…19 Transformation into a fully crystalline material at high annealing temperatures leads to the collapse of the structure. 20 This can be attributed to the fact that nanocrystals grow at typical crystallisation temperatures to diameters of 10-20 nm, which by far exceed the length scales of the self-assembled P123 structure and therefore causing the micro-morphology to break down during annealing. Alternative concepts include backfilling the mesopores with carbon or silica 21 or elaborate annealing and crystallisation protocols 14 to enhance temperature stability or the addition of preformed TiO 2 nanocrystals to decrease the crystallisation temperature 22 but to simultaneously achieve high crystallinity and structural integrity remains a challenge.…”

“…The synthesis is rather time-consuming due to the slow self-assembly process and can take several days. 14 In order to prevent structural collapse, calcination is typically carried out at temperatures around 400 • C which leads to poor crystallinity (less than 50% 19,20 ) in comparison to sintered nanoparticle films.…”

Thermal conductivity of cubic and hexagonal mesoporous silica thin films

“…Samples primary mesoporosity was estimated from the measured pore diameters and lattice spacings. On the other hand, cubic films microporosity could be evaluated by comparing the estimated mesoporosity with the total porosity measured by Fattakhova-rohlfing et al [39] using adsorption isotherms of Kr at 77 K on similar titania samples. Cubic films microporosity was estimated to be around 12 to 20% of the mesoporosity which is also in good agreement with microporosities reported in Ref.…”

“…Three different types of polymer or surfactants were used, namely polyoxyethylene (10) stearyl ether (Brij76), poly(ethylene oxide)-block-poly (propylene oxide)-block-poly(ethylene oxide) triblock copolymer (EO 20 PO 70 EO 20 , P123), and poly(ethylene-co-butylene) 89 -b-poly(ethylene oxide) 79 (KLE) [39]. These were used to prepare open pores films with different pore sizes and interpore spacings through evaporation induced self-assembly.…”