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...

Monodisperse mesoporous anatase titania beads with high surface areas and tunable pore size and grain diameter have been prepared through a combined sol-gel and solvothermal process in the presence of hexadecylamine (HDA) as a structure-directing agent. The monodispersity of the resultant titania beads, along with the spherical shape, can be controlled by varying the amount of structure-directing agent involved in the sol-gel process. The diameter of the titania beads is tunable from approximately 320 to 1150 nm by altering the hydrolysis and condensation rates of the titanium alkoxide. The crystallite size, specific surface area (from 89 to 120 m(2)/g), and pore size distribution (from 14 to 23 nm) of the resultant materials can be varied through a mild solvothermal treatment in the presence of varied amounts of ammonia. On the basis of the results of small-angle XRD, high-resolution SEM/TEM, and gas sorption characterization, a mechanism for the formation of the monodisperse precursor beads has been proposed to illustrate the role of HDA in determining the morphology and monodispersity during the sol-gel synthesis. The approach presented in this study demonstrates that simultaneous control of the physical properties, including specific surface area, mesoporosity, crystallinity, morphology, and monodispersity, of the titania materials can be achieved by a facile sol-gel synthesis and solvothermal process.

Dye-sensitized solar cells employing mesoporous TiO(2) beads have demonstrated longer electron diffusion lengths and extended electron lifetimes over Degussa P25 titania electrodes due to the well interconnected, densely packed nanocrystalline TiO(2) particles inside the beads. Careful selection of the dye to match the dye photon absorption characteristics with the light scattering properties of the beads have improved the light harvesting and conversion efficiency of the bead electrode in the dye-sensitized solar cell. This has resulted in a solar to electric power conversion efficiency (PCE) of greater than 10% (10.6% for Ru(II)-based dye C101 and 10.7% using C106) for the first time using a single screen-printed titania layer cell construction (that is, without an additional scattering layer).

Submicrometer‐sized (830 ± 40 nm) mesoporous TiO2 beads are used to form a scattering layer on top of a transparent, 6‐µm‐thick, nanocrystalline TiO2 film. According to the Mie theory, the large beads scatter light in the region of 600–800 nm. In addition, the mesoporous structure offers a high surface area, 89.1 m2 g−1, which allows high dye loading. The dual functions of light scattering and electrode participation make the mesoporous TiO2 beads superior candidates for the scattering layer in dye‐sensitized solar cells. A high efficiency of 8.84% was achieved with the mesoporous beads as a scattering layer, compared with an efficiency of 7.87% for the electrode with the scattering layer of 400‐nm TiO2 of similar thickness.

Highly ordered mesoporous silicon carbide ceramics have been successfully synthesized with yields higher than 75 % via a one‐step nanocasting process using commercial polycarbosilane (PCS) as a precursor and mesoporous silica as hard templates. Mesoporous SiC nanowires in two‐dimensional (2D) hexagonal arrays (p6m) can be easily replicated from a mesoporous silica SBA‐15 template. Small‐angle X‐ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images show that the SiC nanowires have long‐range regularity over large areas because of the interwire pillar connections. A three‐dimensional (3D) bicontinuous cubic mesoporous SiC structure (Ia3d) can be fabricated using mesoporous silica KIT‐6 as the mother template. The structure shows higher thermal stability than the 2D hexagonal mesoporous SiC, mostly because of the 3D network connections. The major constituent of the products is SiC, with 12 % excess carbon and 14 % oxygen measured by elemental analysis. The obtained mesoporous SiC ceramics are amorphous below 1200 °C and are mainly composed of randomly oriented β‐SiC crystallites after treatment at 1400 °C. N2‐sorption isotherms reveal that these ordered mesoporous SiC ceramics have high Brunauer–Emmett–Teller (BET) specific surface areas (up to 720 m2 g–1), large pore volumes (∼ 0.8 cm3 g–1), and narrow pore‐size distributions (mean values of 2.0–3.7 nm), even upon calcination at temperatures as high as 1400 °C. The rough surface and high order of the nanowire arrays result from the strong interconnections of the SiC products and are the main reasons for such high surface areas. XRD, N2‐sorption, and TEM measurements show that the mesoporous SiC ceramics have ultrahigh stability even after re‐treatment at 1400 °C under a N2 atmosphere. Compared with 2D hexagonal SiC nanowire arrays, 3D cubic mesoporous SiC shows superior thermal stability, as well as higher surface areas (590 m2 g–1) and larger pore volumes (∼ 0.71 cm3 g–1).

This review article highlights recent progress on the n-type material-based electron transporting layers for high-performance perovskite solar cells.

Mesoporous silica with Ia3̄d structure has been successfully prepared by using mixed surfactants of commercially available nonionic block copolymer P123 (EO20PO70EO20) and anionic sodium dodecyl sulfate (SDS) as structure-directing agents through an acid-catalyzed silica sol−gel process. XRD, TEM, and N2 sorption measurements show that the products have highly ordered bicontinuous cubic mesostructure with high surface area (∼770 m2/g), large pore volume (∼1.5 cm3/g), and uniform pore size (∼10 nm). Effects of preparation parameters on the formation of the mesostructure have been extensively investigated. It is found that the molar ratios of SDS/P123 between 2.1 and 2.5 and that of silicic species to P123 in the range from 40 to 75 are favorable for the formation of highly ordered Ia3̄d mesostructure. Prolonging hydrothermal treatment time leads to almost unchanged cell parameters of the products, whereas there is obvious increase of the pore sizes and pore volume. The results show that resultant template-free mesoporous silica products have excellent thermal stability, and they are more stable in N2 atmosphere than in air. Morphologies of the resultant materials can be further controlled by adding inorganic salt (such as Na2SO4) into the mixed surfactants system. Coral- and petaline-like mesoporous silica with continuous skeletons can be obtained. Understanding this synthesis system might be useful for economical and large-scale production of mesoporous materials with controllable structures.

Device scale-up and long-term stability constitute two major hurdles that the emerging perovskite solar technology will have to overcome before commercialization. Here, a comparative study was performed between ZnO and TiO2 electron-selective layers, two materials that allow the low-temperature processing of perovskite solar cells on polymer substrates. Although the use of TiO2 is well established on glass substrates, ZnO was chosen because it can be readily printed at low temperature and offers the potential for the large-scale roll-to-roll manufacturing of flexible photovoltaics at a low cost. However, a rapid degradation of CH3 NH3 PbI3 was observed if it was deposited on ZnO, therefore, the influence of the perovskite film preparation conditions on its morphology and degradation kinetics was investigated. This study showed that CH3 NH3 PbI3 could withstand a higher temperature on TiO2 than ZnO and that TiO2-based perovskite devices were more stable than their ZnO analogues.