CHEMPHYSCHEM 2003, No. 1 ¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/03/04/01 $ 20.00+.50/0 85 Organisation and Reactivity of Nanoparticles at Molecular Interfaces. Part II.  Dye Sensitisation of TiO 2 Nanoparticles Assembled at the Water j 1,2-Dichloroethane Interface David J. FermÌn,* [a] Henrik Jensen, [a] Jacques E. Moser, [b] and Hubert H. Girault [a] KEYWORDS : colloids ¥ dyes ¥ liquid j liquid interfaces ¥ photocurrents ¥ titanium oxide Dye sensitisation of wide bandgap semiconductors has been extensively studied in the fields of photography and photo- voltaics. [1, 2] Strong electronic coupling between dyes featuring suitable anchoring groups and metal oxides can lead to electron injection from the excited state into the solid in the subpico- second domain. [3±9] Time-resolved transient absorption studies of TiO 2 and ZrO 2 nanoparticles sensitised by alizarin have demon- strated that ultrafast injection may occur not only into the conduction band but also into empty surface states located in the bandgap. [4] These studies also showed that the lifetime of the charge separated state in the TiO 2 ± alizarin system exhibits a multiexponential relaxation with a fast component of the order of 430 fs. However, a substantial fraction of the charge separated state feature lifetimes beyond the nanosecond time scale. Previous studies by Moser and Gr‰tzel have shown that the slower component is of the order of 500 ms. [10] This slow back electron transfer has been rationalised in terms of the inverted Marcus region for electron transfer. [10, 11] On the other hand, Haque et al. have indicated that population/depopulation dynamics in traps levels can be the determining factor in the rate of back electron transfer in dye-sensitised mesoporous TiO 2 nanocrystalline photoelectrodes. [12] The lifetime of the charge separated state is one of the key points behind the high quantum efficiencies exhibited by dye sensitised nanocrystalline (DSNC) solar cells. [1] In these systems, the fast injection process is followed by the regeneration of the dye by a redox couple present in the electrolyte phase. This process is equivalent to the cosensitisation reaction employed in photography. Assuming that all injected electrons arriving to the back contact are collected in the external circuit (short-circuit conditions), the photocurrent efficiency is determined by the diffusion of electrons across the mesoporous film and the rate of electron capture by species in solution. Independent measure- ments of both parameters revealed opposite dependencies on the light intensity, which is consistent with the weak relationship between the incident photon-to-current conversion efficiency (IPCE) and the illumination intensity. [13±16] In our previous publication, we explored a fundamentally different approach in which TiO 2 colloids and redox couples are separated by a polarisable liquid j liquid junction. [17] Under potentiostatic (short-circuit) conditions, the origin of the photo- responses is connected to charge transfer processes across the boundary between the two immiscible electrolyte solutions. For instance, the transfer of valence band holes from the particles in the aqueous electrolyte to an electron donor in the organic phase manifests itself by a photocurrent signal. [17] In the present communication, we demonstrate that photocurrent responses originating from the photooxidation of ferrocene can be extended into the visible region by dye sensitisation of TiO 2 particles assembled at the water j 1,2-dichloroethane (DCE) interface. The sensitisation of TiO 2 nanoparticles in the aqueous phase was carried out by homogeneous complexation in the presence of chlorin e-6, catechol or alizarin (Scheme 1), as well as by interfacial complexation involving alizarin in the DCE phase. A representation of the electrochemical cell employed in all measurement is shown in Scheme 2. Cyclic voltammograms in the presence of these dyes as well as TiO 2 colloids do not show any faradaic responses within the polarisable window. The positive and negative limits of the potential window are determined by the transfers of Li  and Cl  ions from water to the DCE phase. [18] Photocurrent spectra for various concentrations of TiO 2 particles in the presence of chlorin e-6 are shown in Figure 1. In the absence of the particles, the photocurrent responses are rather small mainly due to low coverage of the dye at the interfacial boundary. [19] No photoresponses are observed in the absence of ferrocene in the organic phase, indicating that the photocurrent originates from heterogeneous electron transfer involving the organic phase donor. Upon increasing concen- tration of TiO 2 to 1.0 g L 1 , the photocurrent is increased approximately ten-fold at wavelengths around 650 nm. These results contrast remarkably with the photocurrent spectra obtained in the absence of dyes, where the photocurrent onset coincides with the TiO 2 bandgap (413 nm). [17] It is also observed that the photocurrent responses show a weak dependence on the TiO 2 concentration above 0.5 g L 1 , which indicates a saturation of the particle density at the interface. Previous differential capacitance and photocurrent studies have shown that the organisation of TiO 2 at water j DCE boundary is affected by lateral interaction forces that counteract the aggregation and determined a maximum surface coverage of the order of 3  10 10 cm 2 . [17] [a] Dr. D. J. FermÌn, Dr. H. Jensen, Prof. H. H. Girault Laboratoire d'Electrochimie Physique et Analytique Institut de Chimie Mole ¬culaire et Biologique Ecole Polytechnique Fe ¬de ¬rale de Lausanne 1015 Lausanne (Switzerland) Fax: ( 41) 21 ± 693 ± 3667 E-mail : david.fermin@epfl.ch [b] Dr. J. E. Moser Laboratoire de Photonique et Interface Institut de Chimie Mole ¬culaire et Biologique Ecole Polytechnique Fe ¬de ¬rale de Lausanne 1015 Lausanne (Switzerland) [  ] Part I: ref. [17].