Surface Molecularly Imprinted TiO 2 Nanoparticle for Photoreduction of Viologen
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Surface Molecularly Imprinted TiO2 Nanoparticle for Photoreduction of Viologen Takashi Sagawa1,2, Mayu Kudo1, Joachim H. G. Steinke2, and Takashi Morii1 1 Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan 2 Department of Chemistry, Imperial College London, South Kensington, London, England, SW7 2AZ, United Kingdom
ABSTRACT Photo-induced reduction of viologen (V) with surface-modified TiO2 was investigated (Scheme 1). In order to enhance the reduction of V, cavities for molecular recognition of V were fabricated on the surface of TiO2 through sol-gel imprinting method. Once TiO2 had recognition site for the oxidized viologen (V2+) on its surface, it was distinguishable through shape or charge differences from the reduced viologen (V+·) and facilitated the inclusion of target molecule resulted in enhancement of the photoinduced reduction efficiently. The formation of viologen monocation radicals from three different types of viologens (methyl-, hexyl-, and pentacarboxyl-) was monitored spectrophotometrically with time in the presence of the imprinted TiO2 and also using a blank. Higher conversion was observed in every case of imprinted TiO2 compared with the blank one indicating that recognition was not an ill-defined surface effect but involved the bipyridinium part of the viologen. INTRODUCTION When small organic molecules (e.g. amino acids, nucleotides, and so on) are incorporated in metal oxide particles formed from metal alkoxide in the sol-gel process, the organic moieties can be removed readily by solvent washing or by degradative oxidation. Under proper conditions, nano-sized cavities corresponding to the shape of the individual organic moiety are produced within the metal oxide network as molecularly imprinted material such as surface molecularly imprinted TiO2 [1]. We tried to fabricate nano-sized particle of TiO2 [2, 3] to increase the effective contact area for the reactants, and further additional function for molecular recognition was intended to introduce through the
V+ e(Tris)o x
hν h+
Tris
TiO 2 Scheme 1.
.
V 2+
surface modification of the molecular imprinting method [4-7] in order to enhance the photoreduction of viologen. In this context, we prepared viologen imprinted TiO2 as an active and stable photocatalyst for photoreduction of viologen as a well-defined and readily identifiable and reversible electron carrier with appropriate redox potential for photoactivated TiO2. The basic concept of molecularly imprinting in this system is as follows. Oxidized viologen (V2+) is generally twisted and dication molecule. On the other hand, reduced viologen (V+·) is planar and monocation radical (Figure 1). Once
0° 60° ox red
MV+･
MV2+
Figure 1. Optimized geometry of methylviologen (MV). The calculations are based on CASSCF wave functions. The geometry was optimized with HF/3-21G method. TiO2 has such recognition site on its surface, it would be distinguishable through shape or charge recognition or both factors and facilitate the inclusion or exclu
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