DFT Verification of Photoelectrochemical Electrodes of Anatase Nanocrystalline TiO2

Shozo Yanagida^a^,^d, Susumu Yanagisawa^b, Ryota Jono^c, Kouichi Yamashita^c, Hiroshi Segawa^d

^aOsaka University, Japan, FRC, 2-1Yamada-oka,, Suita, 565, Japan

^bUniversity of the Ryukyus,, 1, Senbaru, Nishihara, Okinawa,, Japan

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Shozo Yanagida, 014
Publication date: 5th February 2015

Mesoscopic anatase nanocrystalline TiO₂ (nc-TiO₂) electrodes play in the same essential role both in dye-sensitized solar cells (DSC) and in photoelectrochemical (PEC) water oxidation¹⁾. Smooth electron injection form sensitizing dye molecules to mesoporous nc-TiO₂ layers and effective water oxidation on the UV-irradiated nc-TiO₂electrodes are qualitatively explained by band bending concept using the energy structure of E_c of -0.65V(SCE) and E_v of 2.35V (SCE) of TiO₂ (energy gap 3.0eV), and the equilibrium cell potential of water (1.23eV) in the case of water photooxidation²⁾. In order to gain in-depth insight of electron energy structures created on the electrodes and successive electron transfer from electron-donating substrates, e.g., water molecules, a nc-TiO₂ cluster structure that consists of protonated nine TiO₂ unit and hydroxyl group, OH(TiO₂)₉H, and water cluster model of (H₂O)₃ are simulated as an interfacial model of PEC nc-TiO₂ electrodes (Figure 1). The water cluster will be replaced by dye molecule with redox couple electrolytes in DSC. The interfacial molecular orbital structure of (H₂O)₃&OH(TiO₂)₉H as a stationary PEC nc-TiO₂ electrode model and the radical-cation model of [(H₂O)₃&OH(TiO₂)₉H]^.+ as a working nc-TiO₂ electrode model are simulated using DFT (B3LYP. 6-31G*) in Spartan 14. The stationary model reveals that the model surface provides catalytic H₂O binding sites where hydrogen-bond interactions works, and that (H₂O)₃ with HOMO(-1) (-7.3eV) will be oxidized under UV-irradiated bias conditions in the PEC cell. The working radical cation model discloses energy gap between HOMO and LUMO potentials to be 0.3eV, verifying that nc-TiO₂ electrodes become conductive at photo-irradiated working conditions. Further, DFT-simulation of successive electron transfer from the (H₂O)₃ unit proves that two-electron oxidation leads to hydroxyl radical clusters, which should give hydrogen peroxide as a precursor of oxygen molecule. The nc-TiO₂ electrodes are now confirmed theoretically to become conductive by energy gap photo-excitation, and PEC electron transfer occurs successively via molecular orbitals that are created between the nc-TiO₂ electrodes and interacting molecules such as water clusters and sensitizing dye molecules.
Fig. 1. Simulation of (H2O)3&OH(TiO2)9H as an interface model of PEC nc-TiO2 electrodes
1) Graetzel. M. Photoelectrochemical cells, Nature, 2001, 414, 338-344. 2) Hashimoto. K.; Irie. H.; Fujishima. A. Photoelectrochemical cells, JSAP International, 2006, 24, 4-20.

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