Structural Effects on the Incident Photon-to-Current Conversion Efficiency of Zn Porphyrin Dyes on the Low-Index Planes of TiO<sub>2</sub>

The structural effects of substrates on the incident photon-to-current conversion efficiency (IPCE) of Zn porphyrin (ZnP) dyes (ZnP-ref, YD2, and ZnPBAT) have been studied on well-defined single-crystal surfaces of rutile TiO<sub>2</sub> (TiO<sub>2</sub>(111), TiO<sub>2</sub>(100), and TiO<sub>2</sub>(110)). IPCE of ZnP-ref depends on the structure of the substrates remarkably: TiO<sub>2</sub>(100) < TiO<sub>2</sub>(110) < TiO<sub>2</sub>(111). IPCE of ZnP-ref/TiO<sub>2</sub>(111) is 13 times as high as that of ZnP-ref/TiO<sub>2</sub>(100) at 570 nm. YD2 and ZnPBAT also give the highest IPCE on TiO<sub>2</sub>(111). The relative coverages of the porphyrin dyes give the following order: TiO<sub>2</sub>(111) < TiO<sub>2</sub>(110) < TiO<sub>2</sub>(100). This order is opposite to that of IPCEs. The orientation of the dyes is predicted using density functional theory calculations on simplified models of TiO<sub>2</sub> surfaces. The highest IPCE on TiO<sub>2</sub>(111) is attributed to the high rate of electron transfer through the space due to the fluctuation of the tilt angle of the adsorbed dyes.