Linkage Dependence of Interfacial Charge-Transfer Transitions in ZnO: Carboxylate versus Sulfur Linker

Abstract

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted much attention due to the unique features of a wide range of visible light absorption with colorless organic molecules and direct interfacial charge separation for their potential applications in photoenergy conversions and chemical sensing. As the research on ICTT has almost been limited to titanium oxide semiconductors such as TiO2, the exploration of ICTT in other inorganic semiconductors is a high-priority issue. Recently, we demonstrated that ICTT is strongly induced by chemisorption of aromatic thiols on ZnO nanoparticles via the sulfur atom. Here, we report on ICTT in ZnO nanoparticles adsorbed with benzoic acid derivatives and the linkage dependence of ICTT in ZnO. We observed ICTT bands in the visible region upon adsorption of 4-(dimethylamino)­benzoic acid (4-DMABA) and 3,4-dimethoxybenzoic acid (3,4-DMOBA) on ZnO nanoparticles via the carboxylate group. Notably, the ICTT absorption intensities are about 1 order of magnitude lower than those in the ZnO surface complexes with aromatic thiol compounds. Time-dependence density functional theory (TD-DFT) calculations well reproduce the linkage dependence of ICTT. This characteristic linkage dependence of ICTT in ZnO is attributed to the difference in the valence orbital of bridging atoms. The sulfur bridging atom with the larger 3p valence orbitals gives rise to strong electronic couplings between ZnO and adsorbates for ICTT, in contrast to very weak electronic couplings via the smaller 2p valence orbitals of the oxygen bridging atoms in the carboxylate linkage. Our research reveals the important linkage dependence of ICTT in ZnO and elucidates the mechanism.

Interfacial Charge-Transfer Transitions in Weakly Coupled TiO2 Complexes with Aromatic Monothiols for Visible-Light Photocatalytic Reactions

“…Particularly, surface coordination complexes of wide band-gap inorganic semiconductors such as TiO 2 with organic compounds possessing anchoring groups characteristically show interfacial charge-transfer transitions (ICTTs), which enable the absorption of visible light for colorless organic molecules and direct photoinduced charge separation at the organic–inorganic interfaces, as shown in Figure . − Because of these features, ICTTs have attracted increasing interest as a new visible-light absorption mechanism and a direct photoinduced charge-separation mechanism for photoenergy conversions such as photovoltaic conversion ,− and photocatalytic reactions − and for chemical/biological sensing based on direct visualization, ,,− surface enhanced Raman scattering, − and visible-light circular dichroism of colorless organic compounds. So far, ICTTs have been studied intensively in strongly coupled complexes of inorganic semiconductors (TiO 2 , − SrTiO 3 , BaTiO 3 , , ZnO, − SnO 2 , etc.) with aromatic or π-conjugated organic molecules that possess anchoring groups (−OH, − ,,…”

“…So far, ICTTs have been studied intensively in strongly coupled complexes of inorganic semiconductors (TiO 2 , − SrTiO 3 , BaTiO 3 , , ZnO, − SnO 2 , etc.) with aromatic or π-conjugated organic molecules that possess anchoring groups (−OH, − ,, −SH, ,,, −COOH, − ,, −NH–, ,,, etc.). Accordingly, the basic and applied research of ICTTs has focused on the strongly coupled complexes, which can be isolated without remarkable desorption of the adsorbed molecules.…”

Interfacial Charge-Transfer Transitions between ZnO Nanoparticles and Aliphatic Thiols

“…Particularly, interfacial charge-transfer transitions (ICTTs) at the organic–inorganic semiconductor interfaces provide a new strategy for the design and control of light absorption and photocarrier generation in the applications. This is because ICTTs enable the absorption of lower-energy photons than the optical gaps of the organic and inorganic components with a staggered electronic structure and direct photoinduced charge separation without energy loss in contrast to the conventional two-step charge separation (e.g., dye sensitization) comprising light absorption and subsequent interfacial charge transfer, as shown in Figure . − Notably, ICTTs allow the absorption of visible light even using colorless organic and inorganic substances and efficient charge separation between them. Recently, ICTTs have attracted increasing attention for their potential applications in photovoltaic conversions ,,− and photocatalytic reactions ,− and chemical/biological sensing via direct visualization, ,,− surface-enhanced Raman scattering (SERS), − and circular dichroism (CD) .…”

“…To date, ICTTs have been reported in heterogeneous complexes of organic compounds bearing anchoring groups (−COOH, −OH, −SH, etc.) such as phenol and benzenethiol and wide-band-gap semiconductor nanoparticles (TiO 2 , − SrTiO 3 , ZnO, − SnO 2 , etc.). These organic compounds are dissociatively adsorbed onto the inorganic surfaces via the deprotonation of each anchoring group.…”

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

“…Interfacial charge-transfer transitions (ICTTs) in heterogeneous complexes of organic molecules and inorganic semiconductors enable the absorption of photons at lower energies than the optical gaps of the organic and inorganic components with the staggered electronic structure and efficient photoinduced charge separation due to the direct electron transfer in contrast to the conventional two-step charge separation mechanism such as dye sensitization, as shown in Figure a. − Particularly, ICTTs allow visible-light absorption and efficient charge separation even using colorless organic and inorganic substances, providing an innovative strategy for visible-light-induced charge separation in photoenergy conversions and chemical sensing. Recently, ICTTs have attracted increasing attention for their potential applications in photoenergy conversions including photovoltaic conversions ,,− and photocatalytic reactions ,− and chemical/biological sensing via direct visualization, ,,− surface-enhanced Raman scattering, − and circular dichroism .…”