Electron Kinetics in Dye Sensitized Solar Cells Employing Anatase with (1 0 1) and (0 0 1) Facets Barbora Laskova a, b, *, Thomas Moehl c , Ladislav Kavan a, b, *, Marketa Zukalova a , Xianjie Liu d , Aswani Yella c , Pascal Comte c , Arnost Zukal a , Mohammad Khaja Nazeeruddin c , Michael Graetzel c a J. Heyrovsky Institute of Physical Chemistry, ASCR, v. v. i. Dolejskova 2155/3, Prague 8, CZ 182 23, Czech Republic b Department of Inorganic Chemistry, Faculty of Science, Charles University, Hlavova 2030/8, Prague 2, CZ 128 43, Czech Republic c Laboratory of Photonics and Interfaces Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology, Lausanne, CH 1015, Switzerland d Linköping University, Department of Physics, Chemistry and Biology, S-58183 Linköping, Sweden ARTICLE INFO Article history: Received 3 December 2014 Received in revised form 17 January 2015 Accepted 3 February 2015 Available online 4 February 2015 Keywords: Titanium dioxide anatase dye-sensitized solar cell electrochemical impedance spectroscopy electron transport nanoparticles ABSTRACT Two phase-pure nanocrystalline anatase materials differing in the exposed crystal facets (0 0 1) or (1 0 1) are studied by electrochemical impedance spectroscopy and by transient photovoltage and photocurrent decay in dye sensitized solar cells. A larger chemical capacitance, indicating larger density of states, is observed for anatase (0 0 1). The presence of deep electron traps in (0 0 1) nanosheets is further confirmed by optical (UV-Vis) and photoemission (XPS, UPS) spectra. The difference in chemical capacitance indicates a slower diffusion of electrons in the (0 0 1) anatase material, but also a higher electron lifetime compared to (1 0 1) anatase material. ã 2015 Elsevier Ltd. All rights reserved. 1. Introduction TiO 2 (anatase) is an attractive semiconductor material due to its low cost, nontoxicity, chemical stability, abundance and facile synthesis. It has high photocatalytic activity and photostability, what is attractive in photocatalysis[1–4] and in dye sensitized solar cells (DSCs) [5–7]. Anatase has also good Li-storage capacity and charging rate, which predetermines it for application in Li-ion batteries [8–13]. The benefits of anatase nanoparticles for all these applications motivated numerous studies addressing phase purity, particle size and nanocrystals shape [14–25]. Typical anatase nanoparticles have the shape of truncated bipyramids with the main exposed crystal facet (1 0 1). The domination of (10 1) facet (>94% of the total surface area of ordinary crystals)[26] is rationalized by its lowest surface energy. The remaining surface on the anatase crystals is usually the (0 0 1) oriented facet [27]. Only rarely, the rhombic-shaped crystals are found, exposing the (0 1 0) and (111) facets [28,29]. Yang et al. [26] synthesized micrometer sized platelets of anatase with 47% of (0 0 1) facets by using HF as a capping agent under hydrothermal conditions. The follow-up studies reported on particles enriched with up to 90% of the (0 0 1) facet [30–32]. The synthesis of anatase enriched with (0 0 1) facets[26] triggered deeper investigations of orientation-dependent effects in nanocrystalline anatase materi- als. They provided various inputs for photocatalysis[33–35], DSCs [36–40] and Li-ion batteries [31,41,42]. These results also activated theoretical studies to explain the observed differences between (1 0 1) and (0 0 1) facets [32,43–46]. However, there are still some discrepancies in the studies of anatase (101) and (0 0 1) [37,38,47,48] which require attention. Here, we focus on a deeper insight into the electron energetics of anatase (0 0 1) and (10 1) facets. The DSCs employing anatase (10 1) or anatase (0 0 1) as semiconductor photoanodes were prepared and their properties were investigated by electrochemi- cal impedance spectroscopy, transient photovoltage and photo- current decay. To the best of our knowledge, there is only one paper reporting on the electron kinetics of anatase nanosheets with 30% of (0 0 1) facets [40]. It concluded that the nanosheets showed higher chemical capacitance compared to anatase nanoparticles (10 1). However, nanosheets with only low percentage of (0 0 1) facet were used in this work, and the used reference (1 0 1) material was P25 (from Degussa/Evonik) which is hardly the optimum standard, because it is a mixture of anatase and rutile [40]. The * Corresponding authors. E-mail addresses: laskova@jh-inst.cas.cz (B. Laskova), kavan@jh-inst.cas.cz (L. Kavan). http://dx.doi.org/10.1016/j.electacta.2015.02.016 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved. Electrochimica Acta 160 (2015) 296–305 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta