Journal of Photochemistry and Photobiology A: Chemistry 132 (2000) 115–120 Surface modification and photosensitisation of TiO 2 nanocrystalline films with ascorbic acid A.P. Xagas a,c , M.C. Bernard b , A. Hugot-Le Goff b , N. Spyrellis c , Z. Loizos c , P. Falaras a,∗ a Institute of Physical Chemistry, NCSR ‘Demokritos’, 153 10 Agia Paraskevi Attikis, Athens, Greece b UPR 15 du CNRS ‘Physique des Liquides et Électrochimie’, Université Pierre et Marie Currie, 75252 Paris, Cedex 05, France c Department of Chemical Engineering, National Technical University of Athens, Iroon Politechniou 9, 157 80, Zografou, Athens, Greece Received 21 December 1999; accepted 4 January 2000 Abstract Semi-transparent microporous TiO 2 films on SnO 2 -coated conductive glass were prepared by applying a sol–gel process. Surface topography analysis was conducted with atomic force microscopy, which revealed a highly textured and rough surface, possessing a high capacity for adsorption of a diverse variety of compounds. Surface modification of the films with ascorbic acid results in the formation of a stable, coloured surface complex which red shifts the absorption threshold of TiO 2 , enhancing utilisation of the solar spectrum. Upon illumination in a liquid electrochemical cell, ascorbic acid injects electrons into the conduction band of the semiconductor, giving rise to electrical current. The incident-photon-to-current efficiency presents a maximum at 415 nm (5.29%). ©2000 Elsevier Science S.A. All rights reserved. Keywords: Ascorbic acid; TiO 2 films; Photosensitisation; AFM; Sol–gel 1. Introduction Wide band gap semiconductors, like TiO 2 , have been proved suitable for a great variety of applications, includ- ing degradation of inorganic and organic pollutants [1–2], lithium insertion rechargeable batteries [3], catalytic and photocatalytic reactions [4–5], metal ion reduction in aque- ous solutions [6] and optical devices [7]. This versatile ap- plicability of TiO 2 can be further enhanced by surface mod- ification, a very convenient method for altering the kinetic, redox and optical properties of the semiconductor and there- fore, affecting the overall chemical affinity and behaviour of the TiO 2 surface [8–9]. Generally, surface modifiers, can affect the semiconduc- tor characteristics by three ways: (a) by inhibiting charge recombination; (b) by expanding the wavelength response range; (c) by changing the selectivity or yield of a partic- ular product. Thus, recent studies have shown that the sur- face modification of TiO 2 with cysteine leads to enhanced rates of Pb 2+ ion photoreduction [10]. On the other hand, dye-modified photoelectrochemical cells based on porous nanocrystalline TiO 2 films sensitised by a number of organic ∗ Corresponding author. Tel.: +30-1-6503644; fax: +30-1-6511766. E-mail address: papi@mail.demokritos.gr (P. Falaras). colorants or transition metal complexes have attracted sub- stantial attention, mainly owing to their ability to differenti- ate light absorption and charge separation and their outstand- ing solar-to-electrical energy conversion efficiencies [11]. There exists a vast body of literature dealing with the pho- tosensitisation of titanium oxide with inorganic [12–14] and organic dyes [15–19]. Optimum performance was achieved with systems where an intimate contact between the dye molecule and a semiconductor surface with high roughness factor is established. This contact is significantly enforced by the presence of suitable interlocking groups, which serve actually as an electronic bridge [20]. The highest efficien- cies to date have been attained using Ru II -bipyridine com- plexes (∼11% under diffuse light), functionalised by car- boxyl [21–22]. It has been postulated that the –COOH group anchors to the TiO 2 films through an ester-like bond with the surface –OH groups [23–25]. The search for alternative sensitisers consists an impor- tant goal for the further development of the solar nanocell. Herein, we report the photosensitisation and surface mod- ification of TiO 2 films with L(+)-ascorbic acid (AA) (widely known as Vitamin C) through the formation of a charge-transfer (CT) complex, which shifts the onset of the photoresponse of TiO 2 up to 600 nm. Along with it, we present a modified sol–gel procedure, which yields nanos- 1010-6030/00/$ – see front matter ©2000 Elsevier Science S.A. All rights reserved. PII:S1010-6030(00)00202-1