Beneficial effect of Nb doping on the photoelectrochemical properties of TiO 2 and TiO 2 -polyheptazine hybrids Bastian Mei a,nn , Harriet Byford a , Michal Bledowski b , Lidong Wang b , Jennifer Strunk a , Martin Muhler a , Radim Beranek b,n a Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universitässtr.150, 44801 Bochum, Germany b Inorganic Chemistry II, Ruhr-University Bochum, Universitätsstr. 150, 44780 Bochum, Germany article info Article history: Received 18 March 2013 Received in revised form 6 May 2013 Accepted 9 May 2013 Keywords: Water splitting Water photooxidation Hybrid materials Visible light Titanium dioxide Doping abstract Nb-doped TiO 2 in pure anatase form prepared by spray drying exhibits enhanced photoelectrochemical performance both in its bare form (under UV irradiation) and when used as an electron collector in TiO 2 - polyheptazine hybrid photoanodes for water photooxidation under visible (λ 4420 nm) light. The optimum Nb-doping concentration was 0.1 at%, and the enhancement of photocurrents was found to be chiefly due to enhanced mobility of electrons in Nb-doped TiO 2 . Accordingly, the beneficial effect of Nb doping on photocurrent generation in hybrid photoanodes was pronounced particularly at longer irradiation wavelengths and lower bias potentials. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Direct conversion of solar energy into chemical energy by sunlight-driven water splitting into hydrogen and oxygen in a photoelectrochemical cell is one of the promising strategies to secure the future supply of clean and sustainable energy [1]. Although very high solar-to-hydrogen efficiency of up to 18% have been already achieved, such cells were typically based on multi- junction photoelectrodes using expensive materials [2,3]. An economically viable solution, therefore, requires the development of new, low-cost materials. Notably, due to the complex chemistry involved in four-electron oxidation of water to dioxygen [4], the major challenge in photoelectrochemical water splitting is the development of cheap, efficient and stable photoanodes. The research efforts have mostly focused on low-bandgap transition metal oxides, like WO 3 (2.6 eV, ∼480 nm) [5,6] or α-Fe 2 O 3 (2.1 eV, ∼590 nm) [7–9], which combine light absorption in the visible with high stability. However, their rather positive conduction band edge normally translates into the need to apply significant external electric bias or use tandem cell configurations in order to achieve proton reduction at the counter electrode. An attractive alternative is to use a nanocrystalline layer of a stable wide-bandgap oxide (e.g., TiO 2 ) sensitized by a visible light absorbing dye coupled to a co-catalyst for water oxidation [10–12]. In this approach the oxide is used essentially as a collector for electrons injected from the dye, whereby the positive charges (holes) are channeled to a colloidal co-catalyst for water oxidation (IrO 2 nanocrystals). Interestingly, such hybrid photochemical assemblies not only resemble the Photosystem II of green plants by exhibiting kinetic charge separation [13], but they also take advantage of the relatively negative potential of the conduction band edge of the wide-gap metal oxide (e.g. -0.15 V vs. RHE for TiO 2 ) which promises significantly reduced need for external bias to drive complete water splitting. Obviously, the stability issue plays a crucial role in this type of devices since many dyes are not stable enough to survive the harsh conditions during water oxidation [10,12,14]. Recently, we have been developing photoanodes (Fig. 1) based on a novel class of visible-light photoactive inorganic/organic hybrid materials—TiO 2 modified at the surface with polyheptazine (also known as “graphitic carbon nitride”) [15–18]. The most attractive feature of these inorganic/organic hybrids is the high thermal (up to 550 1C in air) [15] and chemical stability of polyheptazine-type compounds as compared to conventional organic dyes. Notably, we have shown that the optical absorption edge of the TiO 2 -polyheptazine hybrids is red-shifted into the visible (2.3 eV; ∼540 nm) as compared to the bandgaps of both of Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.05.024 n Corresponding author. Tel.: +49 234 32 29431; fax: +49 234 32 14174. nn Corresponding author. Tel.: +49 234 32 22341; fax: +49 234 32 14115. E-mail addresses: bastian.mei@rub.de (B. Mei), radim.beranek@rub.de (R. Beranek). Solar Energy Materials & Solar Cells 117 (2013) 48–53