Visible-to-Near-IR Wide-Range Light Harvesting by Interfacial Charge-Transfer Transitions between TiO2 and p‑Aminophenol and Evidence of Direct Electron Injection to the Conduction Band of TiO2

Interfacial charge-transfer (ICT) transitions between wide band gap inorganic semiconductors such as titanium dioxide (TiO2) and organic compounds are characteristic electronic transitions which enable the absorption of visible light even using colorless low-molecular-weight organic compounds and also direct electron injection into TiO2. Because of these features, ICT transitions are expected to be a new mechanism of light absorption and photoinduced charge separation for solar energy conversions such as photovoltaics. However, in fact, almost all ICT transitions reported so far are limited in the visible region. Visible-to-near-IR wide-range light harvesting of solar energy by ICT transitions is an important subject for such applications. In addition, the conventionally used picture of the direct electron injection into the conduction band of TiO2 by ICT transitions still remains to be demonstrated clearly. In order to solve the two issues, in this paper we study ICT transitions in anatase TiO2 nanoparticles chemically adsorbed with o-, m-, and p-aminophenol, which possess an electron-donating amino group at different positions. We demonstrate that visible-to-near-IR wide-range light absorption due to ICT transitions up to ca. 1.1 μm is generated by using p-aminophenol. In addition, we reveal from the spectral analysis of the ICT band that the ICT transitions take place from the highest occupied molecular orbital (HOMO) of p-aminophenol adsorbates to the continuous conduction band of TiO2, not to discrete surface localized levels. This result offers a direct evidence of the direct electron injection into the conduction band of TiO2 by the ICT transitions. Our research provides novel knowledge on the light absorption and electron transfer properties of ICT transitions for their applications to solar energy conversions.