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ed Single-Crystal Rutile TiO2 Nanowires

Solar Cells DOI:10.1002/anie.201108076 Rapid Charge Transport in Dye-Sensitized Solar Cells Made from Vertically Aligned Single-Crystal Rutile TiO 2Nanowires**

Xinjian Feng, Kai Zhu, Arthur J. Frank, Craig A. Grimes, and Thomas E. Mallouk*

Over the past two decades, dye-and quantum dot-sensitized solar cells and polymer/inorganichybrid solar cells have emerged as promising alternatives to solid-state p-n junction cells for addressing the urgent need for inexpensive, efficient solar power. [1]At the heart of these solar cells is a mesoporous TiO 2nanoparticle (NP)electrode, which not only provides a high surface area for accommodating the light-absorbing sensitizer but also serves as the stable conductor for photo-generated electrons. Fast charge transport in the network is essential for effective charge collection, particularly in solid-state cells in which recombination is very fast. [2]To increase the electron mobility, TiO 21D polycrystalline nanotube arrays [3]and single-crystalline nanowire (NW)arrays [4,5] have recently been proposed and studied as electrode materials for solar cells. However, there is little experimental evidence of substantially enhanced electron transport in these TiO 21D nanostructure-based solar cells. For example, there is no substantial difference in electron transport between 1D TiO 2nanotube and NP-based dye-sensitized solar cells (DSSCs).[3c]

Herein, we report a ketone–HCl solvothermal process for the growth of single-crystal rutile TiO 2NW arrays on F-doped tin oxide (FTO)substrates. The TiO 2NWs begin to grow within 15min and lengths up to 10m m can be obtained within 2h. Using these NW films, we show for the first time that the electron diffusion coefficient of single-crystal rutile TiO 2NWs is more than two orders of magnitude higher than that of rutile NP films at the same photoelectron density. In light of the findings reported here, arrays of 1D single-crystal rutile TiO 2are attractive for solar cell and other optoelectronic applications.

Figure 1a,b show, respectively, cross-sectional field-emis-sion scanning electron microscope (FE-SEM;JEOL 6300) images taken at low and high magnification, which show arrays of uniform and well-separated NWs. NWs grow vertically from the substrate to an average length of about 1.6m m and a diameter of 40nm during a reaction time of 30min. From the grazing angle X-ray diffraction (GAXRD) patterns (FigureS1in the Supporting Information), the crystal phase of the thin base layer and nanowire arrays were identified as tetragonal rutile (JCPDSfile number 21-1276). High-resolution transmission electron microscope (HR-TEM)images (Figure1c) confirm that the NWs are single crystalline and have a (110)interplanar distance of 0.327nm. The NWs grow with a preferred [001]orientation. The length of NWs increases almost linearly with the reaction time. For example, growth of 15min results in around 260nm long NWs and a two hour reaction gives around 9.6m m long NWs (seeFigure S2in the Supporting Information). The NW length can also be adjusted by using different amounts of precursor. For example, for a reaction temperature of 2008C and a duration of 45min NWs with lengths of around 1.1and around 4.4m m can be obtained by using 0.4and 0.8mL of tetrabutyl titanate, respectively (seeFigure S3in the Support-ing

ed Single-Crystal Rutile TiO2 Nanowires

Information).

Figure 1. a and b) FE-SEM cross-sectional images of NW arrays on FTO-coated glass substrate at low and high magnifications, respec-tively. c) HR-TEM image of the as-synthesized single NW.

[*]Dr. X. Feng, [+]Prof. C. A. Grimes, Prof. T. E. Mallouk

Department of Chemistry, Materials Research Institute

The Pennsylvania State University

University Park, PA 16802(USA)

E-mail:tem5@psu.edu

Dr. K. Zhu, [+]Dr. A. J. Frank

National Renewable Energy Laboratory

1617Cole Boulevard, Golden, CO 80401(USA)

[+]These authors contributed equally to this work.

[**]Work at Penn State was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under grant number DE-SC0001087. Work at NREL was supported by the U.S. Department of Energy, under grant number DEAC36-

08GO28308. The Penn State Nanofabrication facility is supported by the National Science Foundation under grant number ECS-0335765. We would like to thank Dr. Bangzhi Liu at the Penn State Nanofabrication facility for his help with FE-SEM, TEM, and HR-

TEM

ed Single-Crystal Rutile TiO2 Nanowires

analyses.

Supporting information for this article is available on the WWW

under http://www.sodocs.net/doc/d09726c4650e52ea541898b7.html /10.1002/anie.201108076.

ed Single-Crystal Rutile TiO2 Nanowires

ed Single-Crystal Rutile TiO2 Nanowires

2727 Angew. Chem. Int. Ed. 2012, 51, 2727–2730 2012Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim

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