Hierarchically structured nanocrystalline photoanode: Self-assembled bi-functional TiO2 towards enhanced photovoltaic performance

We report a simple strategy towards nanocrystalline photoanodes with structural hierarchies (hn-electrode) and simultaneously enhanced dye-loading capacity and electron conductivity. Totally built up by TiO2 nanocrystallite (5-20 nm), the hn-electrode comprised two major parts: the nanocrystalline matrix, and the bi-functional TiO2 mesostructure (BF-TiO2). BF-TiO2 was obtained from common sot-gel, ambient-drying and annealing processes, modified with very high H2O/Ti ratio (48.9:1) and hexamethylenetetramine additive. Two morphologies were found in BF-TiO2: high-surface-area mesoporous nanograin aggregates, and conductive nanosheets composed of both anatase TiO2 and oxygen-deficient Ti6O11, corresponding to its two functions. The preferential capping of amino groups on high-energy facets of TiO2 and the self-assembly process of nanograins along high-energy facets promoted the formation of nanosheets. Hn-electrodes with BF-TiO2 level of 5-40 % were prepared, which exhibited the maximum improvement of 110% in the dye loading capacity, much lower recombination frequency (8.1 Hz vs. 31.5 Hz), and much longer electron lifetime than the disordered counterpart. The optimal hn-electrode (20% BF-TiO2, similar to 16 mu m) with and without the scattering layer exhibited the conversion efficiency of 8.03% and 7.99%, similar to 29% higher than the control nanocrystalline electrode (6.24%). The work provides a versatile route towards functional-integrating hierarchical materials, and may find vast applications in relevant areas such as lithium batteries, supercapacitors and photocatalysts. (C) 2014 Elsevier Ltd. All rights reserved.