Adsorption, Electronic, and Optical Properties of TiO₂–Pyridine Complexes: General Principles for Interfacial Charge-Transfer Transitions

Abstract

Interfacial charge-transfer transitions (ICTTs) in heterogeneous complexes of organic compounds and inorganic semiconductors have recently gained increasing interest for their potential applications in photoenergy conversions and chemical sensing. ICTTs have been reported in wide band-gap inorganic semiconductors such as anatase TiO 2 nanoparticles adsorbed with negatively charged electron-donating ligands. ICTTs occur from the highest occupied π orbital of the organic component to the conduction band of the inorganic one. To generate ICTTs in the visible region strongly, the following two requirements should be satisfied: the orbital overlap between the highest occupied π orbital and conduction band through a covalent bond and the staggered electronic structure at the organic−inorganic interface. To verify these requirements, here, we report a new type of ICTT system with pyridine (Py) derivatives. A TiO 2 −Py complex is quite different from the reported ICTT systems because Py is an electron-accepting ligand. We have examined the adsorption, electronic, and optical properties of the anatase TiO 2 −Py and anatase TiO 2 −Py derivative complexes. For non-substituted Py, no ICTTs take place in the visible region owing to the deep highest occupied π orbital of the Py adsorbate. By the way, ICTTs appear in the visible region due to the introduction of a strong electrondonating group to Py via the significant elevation of an occupied π orbital of the adsorbed Py, satisfying the above-mentioned two requirements. This result supports the general principles for ICTTs definitively.

Interfacial Charge-Transfer Transitions in Rutile TiO2 Nanoparticles Functionalized with Benzoic Acid Derivatives

“…Recently, ICTTs have been reported in a variety of surface complexes of wide band gap metal-oxide semiconductors (anatase TiO 2 , − SrTiO 3 , BaTiO 3 , , ZnO, − SnO 2 , etc.) with various organic compounds ranging from aromatic Brønsted acids bearing protonic anchoring groups (−OH, − ,, –SH, ,,,, and −COOH − ,, ) to aromatic Lewis bases such as pyridine derivatives . In the surface complexes, ICTTs take place from the highest occupied molecular orbital (HOMO) of the adsorbed molecules to the conduction band of the inorganic semiconductors.…”

“…Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors enable the absorption of visible light and direct charge separation at the organic–inorganic interfaces regardless of the optical properties of the organic and inorganic components, as shown in Figure a. − Based on these unique features, ICTTs provide new mechanisms of visible-light absorption and efficient photoinduced charge separation for colorless organic and inorganic substances to perform photoenergy conversions such as photovoltaic conversion and photocatalysis. Recently, ICTTs have been reported in a variety of surface complexes of wide band gap metal-oxide semiconductors (anatase TiO 2 , − SrTiO 3 , BaTiO 3 , , ZnO, − SnO 2 , etc.) with various organic compounds ranging from aromatic Brønsted acids bearing protonic anchoring groups (−OH, − ,, –SH, ,,,, and −COOH − ,, ) to aromatic Lewis bases such as pyridine derivatives .…”

Interfacial Charge-Transfer Transitions in Phenol-Functionalized SrTiO3 for Highly Reducible Visible-Light Photocatalysts

“…Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors enable the absorption of visible light and the direct charge separation at the organic–inorganic interfaces regardless of the optical properties of the organic and inorganic components. − These features provide new opportunities for colorless organic and inorganic substances to function as photoenergy conversion materials in photocatalytic reactions and photovoltaic conversion. So far, ICTTs have been reported in various kinds of surface complexes of wide band gap metal-oxide semiconductors (anatase TiO 2 , − SrTiO 3 , BaTiO 3 , , ZnO, − SnO 2 , etc.) with organic compounds ranging from aromatic Brønsted acids bearing protonic anchoring groups (−OH, − ,, −SH, ,,,, −COOH − ,, ) to aromatic Lewis bases such as pyridine derivatives .…”

“…So far, ICTTs have been reported in various kinds of surface complexes of wide band gap metal-oxide semiconductors (anatase TiO 2 , − SrTiO 3 , BaTiO 3 , , ZnO, − SnO 2 , etc.) with organic compounds ranging from aromatic Brønsted acids bearing protonic anchoring groups (−OH, − ,, −SH, ,,,, −COOH − ,, ) to aromatic Lewis bases such as pyridine derivatives . In these surface complexes, ICTTs take place from the highest occupied molecular orbital (HOMO) of adsorbed molecules to the conduction band (CB) of inorganic semiconductors, as shown in Figure a.…”

Interfacial charge-transfer transitions enable photovoltaic conversion with CO2-fixation products

“…with organic compounds ranging from aromatic Brønsted acids (–COOH, –OH, –SH) to aromatic Lewis bases such as pyridine derivatives. 7–17 In these complexes, ICTTs occur from the highest occupied molecular orbital (HOMO) of surface-bound organic molecules to the conduction band of inorganic semiconductors, as shown in Fig. 1(a).…”