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

We demonstrate that organic-inorganic interfacial charge-transfer transitions enable favourable photovoltaic conversion with CO2-fixation products such as aromatic carboxylic acids, verifying a new possibility of CO2-fixation products in the development of optoelectronic conversion materials.

Efficient visible light photocatalytic NO abatement over SrSn(OH)6 nanowires loaded with Ag/Ag2O cocatalyst

SrSn(OH)₆ (SSOH) possesses a high oxidation potential in the valence band (VB), which is suitable for photocatalytic oxidation removal of pollutants. However, the electrons in the VB of these catalysts are difficult to transition to the conduction band (CB) under visible light, which makes it difficult to utilize sunlight effectively. In this work, Ag/Ag₂O is loaded on the surface of SSOH nanowires, which stimulates the interfacial charge-transfer transition on SSOH. Compared with pure-phase SSOH, the NO abatement ratio of Ag/Ag₂O-SSOH under visible light irradiation is increased to 45.10%. The e^- in the VB of Ag₂O are excited into the CB under visible light, and are further transferred to the Ag to react with O₂ to produce superoxide radicals. The photo-excited e^- in the VB of SSOH enter into the VB of Ag₂O through interfacial charge-transfer transition to recombine with the photo-generated holes in the VB of Ag₂O, thereby leaving photo-generated holes in the VB of SSOH. The holes in the VB of SSOH have sufficient oxidizing ability to oxidize the adsorbed hydroxyl groups into hydroxyl radicals. This work provides a new perspective for photocatalytic removal of pollutants by wide band gap photocatalyst under visible light.

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

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 SnO2 functionalized with benzoic acid derivatives

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted increasing attention for their potential applications in solar energy conversions and chemical sensing due to the unique functions of visible-light absorption with colourless organic molecules and direct charge separation. However, inorganic semiconductors available for ICTT are quite limited to a few kinds of metal-oxide semiconductors (TiO2, ZnO, etc.). Particularly, the exploration of ICTT in inorganic semiconductors with a lower-energy conduction band such as SnO2 is an important issue for realizing a wide range of visible-light absorption for organic adsorbates with the deep highest occupied molecular orbital (HOMO) such as benzoic acid derivatives. Here, we report the first observation of ICTT in SnO2. SnO2 nanoparticles show a broad absorption band in the visible region by chemisorption of 4-dimethylaminobenzoic acid (4-DMABA) and 4-aminobenzoic acid (4-ABA)) via the carboxylate group. The wavelength range of the ICTT band significantly changes depending on the kind of substituent group. The ionization potential measurement and density functional theory (DFT) analysis reveal that the absorption band is attributed to ICTT from the HOMO of the adsorbed benzoic acid derivatives to the conduction band of SnO2. In addition, we clarify the mechanism of ICTT in SnO2 computationally. Our research opens up a way to the fundamental research on ICTT in SnO2 and applications in solar energy conversions and chemical sensing.

Thiol adsorption on metal oxide nanoparticles

The adsorption of 2-naphthalenethiol (2-NPT) and methanethiol (MT) on 13 different metal oxide nanoparticles, of approximately 30 nm average primary particle size, has been investigated. In the case of 2-NPT, which is fluorescent, a screening method to assess adsorption was developed that consists of mixing the nanoparticles with a dilute ethanolic solution of 2-NPT and performing several cycles of centrifuging and rinsing with ethanol. Fluorescence measurements on the re-dispersed particle suspensions were then used to diagnose whether or not adsorption had occurred. Complementary experiments were performed by mounting powder samples of each of the metal oxide nanoparticles onto sample stubs and performing X-ray photoelectron spectroscopy (XPS) before and after in situ dosing with MT. In both cases, adsorption was observed only on ZnO, TiO₂, and In₂O₃. Adsorption did not occur on Al₂O₃, CeO₂, Fe₂O₃, Gd₂O₃, Ho₂O₃, NiO, SiO_x, WO₃, Y₂O₃, and ZrO₂. Density functional theory (DFT) calculations were performed using small metal oxide clusters, assuming that dissociative adsorption occurs by replacement of a hydroxyl group attached to a metal site and the formation of water. The theoretical and experimental results generally agree, suggesting that this is indeed the adsorption mechanism for most of the nanoparticles. The agreement also suggests that the size and geometry of the nanoclusters play a minor role and that the relative strengths of the metal-sulfur and metal-hydroxyl bonds dictate thiol adsorption. This work has important implications related to the functionalization of metal oxide nanoparticles and surfaces.

Interfacial Charge-Transfer Transitions for Direct Charge-Separation Photovoltaics

Photoinduced charge separation (PCS) plays an essential role in various solar energy conversions such as photovoltaic conversion in solar cells. Usually, PCS in solar cells occurs stepwise via solar energy absorption by light absorbers (dyes, inorganic semiconductors, etc.) and the subsequent charge transfer at heterogeneous interfaces. Unfortunately, this two-step PCS occurs with a relatively large amount of the energy loss (at least ca. 0.3 eV). Hence, the exploration of a new PCS mechanism to minimize the energy loss is a high-priority subject to realize efficient solar energy conversion. Interfacial charge-transfer transitions (ICTTs) enable direct PCS at heterogeneous interfaces without energy loss, in principle. Recently, several progresses have been reported for ICTT at organic-inorganic semiconductor interfaces by our group. First of all, new organic-metal oxide complexes have been developed with various organic and metal-oxide semiconductors for ICTT. Through the vigorous material development and fundamental research of ICTT, we successfully demonstrated efficient photovoltaic conversion due to ICTT for the first time. In addition, we revealed that the efficient photoelectric conversion results from the suppression of charge recombination, providing a theoretical guiding principle to control the charge recombination rate in the ICTT system. These results open up a way to the development of ICTT-based photovoltaic cells. Moreover, we showed the important role of ICTT in the reported efficient dye-sensitized solar cells (DSSCs) with carboxy-anchor dyes, particularly, in the solar energy absorption in the near IR region. This result indicates that the combination of dye sensitization and ICTT would lead to the further enhancement of the power conversion efficiency of DSSC. In this feature article, we review the recent progresses of ICTT and its application in solar cells.