Photovoltaic Property of Anatase TiO 2 3‑D Mesoflowers T. G. Deepak, Devika Subash, G. S. Anjusree, K. R. Narendra Pai, Shantikumar V. Nair, and A. Sreekumaran Nair* Amrita Centre for Nanosciences and Molecular Medicine, Amrita Institute of Medical Sciences, Amrita Vishwa Vidyapeetham, AIMS PO, Ponekkara, Kochi 682041, Kerala, India ABSTRACT: We have synthesized 3-D flower-like mesostructured TiO 2 from one-dimensional electrospun TiO 2 -SiO 2 nanocomposites through a modified titanate route for dye-sensitized solar cell (DSC) application. The TiO 2 3-D mesoflowers with commendable internal surface area, crystallinity, and a good light scattering property satisfy the prerequisites of a DSC photoanode material. The starting TiO 2 -SiO 2 composite, intermediate titanate, and final 3- D mesoflowered TiO 2 were characterized by spectroscopy, microscopy, and surface area measurements. A DSC employing 3-D mesoflowered TiO 2 as the photoanode showed a power conversion efficiency of 8.3% which was 23% higher than that of commercial P-25 (6.37%). KEYWORDS: Dye-sensitized solar cell, TiO 2 , TiO 2 -SiO 2 composite, Light scattering, Electrochemical impedance spectroscopy ■ INTRODUCTION TiO 2 is a material for multifaceted applications in areas such as dye-sensitized solar cells (DSCs), storage devices, photonic crystals, self-cleaning coatings, environmental remediation, water purification, etc. 1-9 DSCs, first presented by O’Regan and Grä tzel in 1991, marked a paradigm shift in the area of renewable energy research. 10 DSC is still considered as one of the options for harnessing solar energy as it employs environmentally benign and relatively cheap raw materials for its making in addition to fabrication feasibilities under nonvacuum conditions. TiO 2 forms the backbone of DSCs as it performs the dual functions of supporting the sensitizers (the dyes) and charge transport. Sensitizer loading on TiO 2 is directly related to its density, surface area, and phase purity, and smooth charge transport is a measure of its crystallinity. Thus, fabrication of dense, crystalline, and high surface area TiO 2 is essential to have efficient DSC devices. Many researchers have investigated the usefulness of electro- spun TiO 2 nanofibers for DSCs but ended up with a lower efficiency in the range of 4-5% in lab-scale research. 11-15 Although nanofibers have beneficial aspects such as semidirected electron transport and the presence of straight pores in one- dimensional architecture that facilitates effective electrode wetting by electrolyte species, their internal surface area is very low, which prevents good dye chemisorption. 14-16 Thus, to promote the use of electrospun TiO 2 in DSCs and photo- catalysis, it is necessary to find ways to improve their surface areas through chemical means. We have found that the titanate route, which is the chemical conversion of TiO 2 into Na 2 Ti 3 O 7 (sodium titanate) by concentrated NaOH/KOH and subsequent conversion of the Na 2 Ti 3 O 7 back into TiO 2 , leads to improved surface areas. 17-19 The route has been demonstrated in the case of P-25 TiO 2 (spherical TiO 2 particles of ∼25 nm average diameter), which with titanate chemistry leads to the formation of TiO 2 nanorods/nanowires. 20,21 We have adopted titanate chemistry in electrospun TiO 2 -SiO 2 composite nanofibers. 18,19 The rationale for incorporating SiO 2 along with TiO 2 in the nanofibers is that SiO 2 is an acidic oxide that can be completely etched by NaOH (during the titanate chemistry), and we demonstrated previously that etching of one of the components from a composite metal oxide system will lead to high surface area for the unetched metal oxide in addition to structural anisotropy. 22,23 When we adopted the titanate route on the electrospun TiO 2 -SiO 2 composite fibers, the result was unexpected 3-D mesoflowers with high surface areas of 166 m 2 /g and good crystallinity. 17 The usefulness of the material was tested only for photocatalysis, and the present manuscript investigates its utility in DSCs. The initial TiO 2 -SiO 2 composite fibers, intermediate Na 2 Ti 3 O 7 , and final TiO 2 mesoflowers were characterized by spectroscopy, microscopy, powder XRD, and surface area measurements. The 3-D mesoflowers were employed in a DSC, which showed a power conversion efficiency of 8.3% (for a square-shaped cell of 0.20 cm 2 area and 14 μm thickness) in comparison to 6.37% for the commercially available P-25 (both had similar electrode parameters). The IPCE measurements of the devices was ∼80% for the 3-D mesoflowers vs ∼66% for P-25. The DSCs were additionally characterized by dye loading and electrochemical impedance measurements. It must be noted that an efficiency of 8.3% from the mesoflowers is an impressive value Received: August 18, 2014 Research Article pubs.acs.org/journal/ascecg © XXXX American Chemical Society A dx.doi.org/10.1021/sc500642c | ACS Sustainable Chem. Eng. XXXX, XXX, XXX-XXX