Multi-layered mesoporous TiO2 thin films with large pores and highly crystalline frameworks for efficient photoelectrochemical conversion

Abstract

Mesoporous thin films with various compositions are unique architectures for photoelectrochemical (PEC) solar cells. In this paper, we report the synthesis of highly ordered, multi-layered, continuous mesoporous TiO₂ thin films with uniform large pores, crystalline walls and tunable film thickness, via a ligand-assisted evaporation induced self assembly (EISA) method. A Ti(acetylacetone) precursor and a diblock copolymer PEO-b-PS are employed for the controlled assembly of the TiO₂/template mesostructure, followed by a two-step pyrolysis that generates carbon residue as an intermediate protection layer to support the TiO₂ framework and mesostructures during the crystallization. Other transition metal ion dopants (such as Cr, Ni and Co) can be facilely incorporated into the TiO₂ frameworks by co-assembly of these metal acetylacetone precursors during the EISA process. The obtained TiO₂ thin film possesses an ordered monoclinic mesostructure distorted from a (110)-oriented primitive cubic structure, uniform and tunable large pores of 10–30 nm, a large surface area of ∼100 m² g⁻¹ and a high crystallinity anatase wall. The film thickness can be well controlled from 150 nm to several microns to tune the absorption, with the capability of generating free-standing film morphologies. Furthermore, this designed architecture allows for effective post-deposition of other small-bandgap semiconductor nanomaterials inside the large, open and interconnecting mesopores, leading to significantly improved solar absorption and photoconversion. As a proof-of-concept, we demonstrate that the photoanodes made of 4.75 μm thick mesoporous TiO₂ film with deposited cadmium sulfide quantum dots exhibit excellent performance in PEC water splitting, with an optimized photocurrent density of 6.03 mA cm⁻² and a photoconversion efficiency of 3.9%. These multi-layered mesoporous TiO₂-based thin films can serve as a unique architecture for PEC and other solar energy conversion and utilization.