Mesoporous Anatase TiO₂ Beads with High Surface Areas and Controllable Pore Sizes: A Superior Candidate for High‐Performance Dye‐Sensitized Solar Cells

Abstract

Mesoporous titania-based materials with a crystalline framework, high surface area and tunable pore size have received significant research attention due to the range of applications for such materials: photocatalysis, energy storage and conversion, electrochromic and sensing fields.[1] For a variety of practical applications, the fabrication of desired morphologies and textures is important as well as control in crystallinity, porosity and composition.[2] Mesoporous titania films, [3] beads, [4] monoliths, [5] networks, [6] and tubes [7] have been prepared via different synthesis strategies. Of particular interest are monodisperse TiO 2 beads with a submicrometer-sized diameter, because of their relatively high refractive index and comparative particle sizes to optical wavelengths, which make them ideal candidates for photon-related applications such as photocatalysis, [4c-g] dyesensitized solar cells (DSSC), [4h] and photonic crystals.[8]As a promising alternative to conventional silicon-based solar cells, high performance DSSCs have been extensively studied in recent years.[9] For an efficient DSSC, the porous electrode composed of anatase phase TiO 2 nanocrystals (% 20 nm) is essential due to the high internal surface area which maximizes the uptake of the dye molecules, thereby giving rise to DSSCs with large current density and high photon-to-current conversion efficiency.[10] However, such TiO 2 nanocrystal films usually show high transparency and negligible light scattering due to the small particle size and this results in poor light-harvest.[11] One approach to enhance the light-harvesting capability of the TiO 2 electrodes, without sacrificing the accessible surface for dye loading, is the use of submicrometer-sized TiO 2 beads with abundant mesopores. Such purpose-built mesoporous TiO 2 beads are expected to simultaneously promote light scattering and the dye loading of the electrodes, thereby increasing the photon-to-current conversion efficiency. However, to our knowledge, few reports addressing this issue have been documented.[4h]Herein we report the synthesis of crystalline, mesoporous TiO 2 beads with surface areas up to 108.0 m 2 g À1 and tunable pore sizes (pore diameters varying from 14.0 to 22.6 nm) through a facile combination of sol-gel and solvothermal processes. The mesoporous TiO 2 beads have a diameter of (830 AE 40) nm and are composed of anatase TiO 2 nanocrystals. These mesoporous TiO 2 beads have been used in the preparation of the working electrodes for DSSCs and an improved efficiency was observed when compared to cells prepared using standard Degussa P25 TiO 2 electrodes of similar thickness. An overall light conversion efficiency of 7.20% (open-circuit voltage (V oc ) of 777 mV, short-circuit current density (I sc ) of 12.79 mA cm À2 and fill factor (FF) of 0.72) was achieved using the mesoporous TiO 2 bead electrodes. Figure 1 shows scanning electron microscopy (SEM) images of the sol-gel prepared precursor materials and the calcined mesoporous TiO 2 beads prepared after t...

Sandwich-structured TiO2 inverse opal circulates slow photons for tremendous improvement in solar energy conversion efficiency

“…I is the photocurrent density (mAcm -2 ), P is intensity (mWcm -2 ) and λ is the wavelength (nm) of the incident monochromatic light. The transient decay time can be calculated by: 44 (6) where t is the time and τ is the transient time constant which is the time at lnD=-1. The transient decay time can be found from a logarithmic plot of parameter D, given by the equation: 45 (7) where I t is the photocurrent at time t.…”

“…1, 2 Significant research efforts have been made to enhance photon collection by extending absorption width through chemical doping [3][4][5] and increasing the path length of light by scattering and/or resonance. 6,7 In the past decade, photonic crystal (PC) structures have received increased attention for enhancing photochemical energy conversion by intensifying light-matter interactions without chemical modification. 1,8,9 With periodic modulation of the refractive index, PC possesses a stopband which forbids light of certain frequencies from propagation at certain crystallographic direction.…”

Preparationand Characterization of Rutile-anatase Hybrid TiO2Thin Film by Hydrothermal Synthesis

“…TiO2 has three types of crystallographic structures: anatase, rutile, and brookite [9,10]. Anatase is known, despite its metastable characteristics at ambient temperature, to possess the highest photocatalytic activity because of its large surface area compared to rutile and brookite [11][12][13][14]. In addition, the anatase structure has low crystal-lattice packing fraction so that it absorbs light better owing to the reduced refractive index [15].…”

Improvement of Solar Cells Efficiency through Detailed Characterization of TCO/TiO2 Interface

“…This process although being used very often especially for DSSC does not provide a good interface quality of heterostructure of TCO layer and photoanode layer. So far the particles size of TiO 2 was mainly studied in order to increase the adsorption of dye [4], or diffusion and recombination in dye-sensitized TiO 2 solar cells [5]. In our work we mainly focus on increase of the charge transfer in order to increase final efficiency.…”