Metal clusters [1][2][3] with sizes smaller than 2 nm have discrete electronic structures due to quantum size effects, and exhibit various properties such as optical absorption in the visible and near-infrared regions, [4][5][6] photoluminescence, [7,8] two-photon absorption, [9] and paramagnetism, [10] which are different from those for bulk metal or nanoparticles larger than 3 nm. While the studies on metal clusters have mostly focused on Au because Au clusters are stable and easy to synthesize, [1][2][3][4][5][6][7][8][9][10] other metal clusters such as Pt, [11,12] Pd, [13,14] Ag, [15][16][17] Cu, [18,19] Ni, [20] and Fe [21] have also been synthesized by using thiolates, dendrimers, and reversed micelles as stabilizers or templates. [22] Their optical, catalytic, and magnetic properties have also been reported.
Recently, we found that TiO 2 electrodes modified with glutathione-protected Au clusters can be used for photoelectric conversion [23] and photocatalysis [24] under visible and infrared light. The internal quantum efficiency reaches ~60 % when Au 25 clusters are used. The efficiency as well as stability, catalytic selectivity, and/or cost would be improved by the use of other metal clusters. In the present study, we have synthesized Pt and Pd clusters, and have compared their photovoltaic properties with those of Au clusters. We have employed mercaptosuccinic acid (MSA) and dimercaptosuccinic acid (DMSA) as protecting agents, and have examined the effect of core metals on the properties. While syntheses of MSA- [25] and DMSA-protected [26] Au clusters have been reported, syntheses of Pt and Pd clusters protected by MSA or DMSA have not yet been reported, to the best of our knowledge.
Herein, we report that the Pt and Pd clusters work as sensitizers for TiO 2 under visible light, and that the MSA-protected Pt (Pt:MSA) and DMSA-protected Pd (Pd:DMSA) clusters yield the highest photocurrent among the MSA-and DMSA-protected clusters examined, respectively. The Pt and Pd clusters are