Photosensitized hydrogen evolution from water using a single-walled carbon nanotube/fullerodendron/SiO2 coaxial nanohybrid

MoSe2-Sensitized Water Splitting Assisted by C60-Dendrons on the Basal Surface

To facilitate water splitting using MoSe2 as a light absorber, we fabricated water-dispersible MoSe2/C60-dendron nanohybrids via physical modification of the basal plane of MoSe2. Upon photoirradiation, the mixed-dimension MoSe2/C60 (2D/0D) heterojunction generates a charge-separated state (MoSe2⋅+/C60⋅-) through electron extraction from the exciton in MoSe2 to C60. This process is followed by the hydrogen evolution reaction (HER) from water in the presence of a sacrificial donor (1-benzyl-1,4-dihydronicotinamide) and co-catalyst (Pt-PVP). The apparent quantum yields of the HER were estimated to be 0.06 % and 0.27 % upon photoexcitation at the A- and B-exciton absorption peaks (λmax=800 and 700 nm), respectively.

Non-negligible roles of charge transfer excitons in ultrafast excitation energy transfer dynamics of a double-walled carbon nanotube

Herein, we employed a developed linear response time dependent density functional theory-based nonadiabatic dynamics simulation method that explicitly takes into account the excitonic effects to investigate photoinduced excitation energy transfer dynamics of a double-walled carbon nanotube (CNT) model with different excitation energies. The E₁₁ excitation of the outer CNT will generate a local excitation (LE) |out*〉 exciton due to its low energy, which does not induce any charge separation. In contrast, the E₁₁ excitation of the inner CNT can generate four kinds of excitons with the LE exciton |in*〉 dominates. In the 500-fs dynamics simulation, the LE exciton |in*〉 and charge transfer (CT) excitons |out^-in⁺〉 and |out⁺in^-〉 are all gradually converted to the |out*〉 exciton, corresponding to a photoinduced excitation energy transfer, which is consistent with experimental studies. Finally, when the excitation energy is close to the E₂₂ state of the outer CNT (∼1.05 eV), a mixed population of different excitons, with the |out*〉 exciton dominated, is generated. Then, photoinduced energy transfer from the outer to inner CNTs occurs in the first 50 fs, which is followed by an inner to outer excitation energy transfer that is completed in 400 fs. The present work not only sheds important light on the mechanistic details of wavelength-dependent excitation energy transfer of a double-walled CNT model but also demonstrates the roles and importance of CT excitons in photoinduced excitation energy transfer. It also emphasized that explicitly including the excitonic effects in electronic structure calculations and nonadiabatic dynamics simulations is significant for correct understanding/rational design of optoelectronic properties of periodically extended systems.

Hot Electron Extraction in SWCNT/TiO2 for Photocatalytic H2 Evolution from Water

Single-walled carbon nanotube (SWCNT)/TiO2 hybrids were synthesized using 1,10-bis(decyloxy)decane-core PAMAM dendrimer as a molecular glue. Upon photoirradiation of a water dispersion of SWCNT/TiO2 hybrids with visible light (λ > 422 nm), the hydrogen evolution reaction proceeded at a rate of 0.95 mmol/h·g in the presence of a sacrificial agent (1-benzyl-1,4-dihydronicotinamide, BNAH). External quantum yields (EQYs) of the hydrogen production reaction photosensitized by (6,5), (7,5), and (8,3) tubes were estimated to be 5.5%, 3.6%, and 2.2%, respectively, using monochromatic lights corresponding to their E22 absorptions (570 nm, 650 nm, and 680 nm). This order of EQYs (i.e., (6,5) > (7,5) > (8,3)SWCNTs) exhibited the dependence on the C2 energy level of SWCNT for EQY and proved the hot electron extraction pathway.

Mechanistic Study on the Photogeneration of Hydrogen by Decamethylruthenocene

Detailed studies on hydrogen evolution by decamethylruthenocene ([Cp*₂ Ru^II ]) highlighted that metallocenes are capable of photoreducing hydrogen without the need for an additional sensitizer. Electrochemical, gas chromatographic, and spectroscopic (UV/Vis, ¹ H and ¹³ C NMR) measurements corroborated by DFT calculations indicated that the production of hydrogen occurs by a two-step process. First, decamethylruthenocene hydride [Cp*₂ Ru^IV (H)]⁺ is formed in the presence of an organic acid. Subsequently, [Cp*₂ Ru^IV (H)]⁺ is reversibly reduced in a heterolytic reaction with one-photon excitation leading to a first release of hydrogen. Thereafter, the resultant decamethylruthenocenium ion [Cp*₂ Ru^III ]⁺ is further reduced with a second release of hydrogen by deprotonation of a methyl group of [Cp*₂ Ru^III ]⁺ . Experimental and computational data show spontaneous conversion of [Cp*₂ Ru^II ] to [Cp*₂ Ru^IV (H)]⁺ in the presence of protons. Calculations highlight that the first reduction is endergonic (ΔG⁰ =108 kJ mol^-1 ) and needs an input of energy by light for the reaction to occur. The hydricity of the methyl protons of [Cp*₂ Ru^II ] was also considered.

Photo-induced H2 evolution from water via the dissociation of excitons in water-dispersible single-walled carbon nanotube sensitizers

To observe a clear-cut example of the formation of mobile carriers from excitons on semiconducting single-walled carbon nanotubes (s-SWCNTs) surrounded by a medium with a high dielectric constant, water-dispersible s-SWCNT nanocomposites were fabricated by physical modifications using poly(amidoamine) dendrimers that contain an aliphatic core. The evolution of H₂ from water using these s-SWCNT/dendrimer nanocomposites as photosensitizers under irradiation with visible light demonstrated a photo-induced electron transfer from the s-SWCNTs to the co-catalysts.

Heterogeneous catalysis based on supramolecular association

Non-covalent assembly of individual components can develop a material with activity to promote the transformation of substrates into products.

Synthesis of a poly(amidoamine) dendrimer having a 1,10-bis(decyloxy)decane core and its use in fabrication of carbon nanotube/calcium carbonate hybrids through biomimetic mineralization

A new dendritic dispersant of carbon nanotubes (CNTs) was synthesized and applied for the noncovalent functionalization of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The 1,10-bis(decyloxy)decane core of the poly(amidoamine) dendrimer strongly adhered to the sidewalls of CNTs to form CNT/dendrimer supramolecular nanocomposites having many carboxyl groups (–COOH) on the surface. Then, crystallization of calcium carbonate (CaCO3) by the CO2 diffusion technique in aqueous environments using the CNT/dendrimer supramolecular nanocomposites as scaffolds afforded monodisperse spherical CNT/CaCO3 nanohybrids consisting of CNTs and calcite nanocrystals. The morphologies of the SWCNT/CaCO3 hybrids and MWCNT/CaCO3 hybrids were almost the same.

SWCNT Photocatalyst for Hydrogen Production from Water upon Photoexcitation of (8, 3) SWCNT at 680-nm Light

Single-walled carbon nanotubes (SWCNTs) are potentially strong optical absorbers with tunable absorption bands depending on their chiral indices (n, m). Their application for solar energy conversion is difficult because of the large binding energy (>100 meV) of electron-hole pairs, known as excitons, produced by optical absorption. Recent development of photovoltaic devices based on SWCNTs as light-absorbing components have shown that the creation of heterojunctions by pairing chirality-controlled SWCNTs with C60 is the key for high power conversion efficiency. In contrast to thin film devices, photocatalytic reactions in a dispersion/solution system triggered by the photoexcitation of SWCNTs have never been reported due to the difficulty of the construction of a well-ordered surface on SWCNTs. Here, we show a clear-cut example of a SWCNT photocatalyst producing H2 from water. Self-organization of a fullerodendron on the SWCNT core affords water-dispersible coaxial nanowires possessing SWCNT/C60 heterojunctions, of which a dendron shell can act as support of a co-catalyst for H2 evolution. Because the band offset between the LUMO levels of (8, 3)SWCNT and C60 satisfactorily exceeds the exciton binding energy to allow efficient exciton dissociation, the (8, 3)SWCNT/fullerodendron coaxial photocatalyst shows H2-evolving activity (QY = 0.015) upon 680-nm illumination, which is E22 absorption of (8, 3) SWCNT.

Incorporating a TiOx shell in single-walled carbon nanotube/fullerodendron coaxial nanowires: increasing the photocatalytic evolution of H2 from water under irradiation with visible light

The SWCNT/fullerodendron/TiOx coaxial nanowire shows an enhanced photocatalytic activity (Φ = 0.47) for the evolution of hydrogen from water under irradiation with visible light (λ = 450 nm).

Engineering interfacial photo-induced charge transfer based on nanobamboo array architecture for efficient solar-to-chemical energy conversion

Engineering interfacial photo-induced charge transfer for highly synergistic photocatalysis is successfully realized based on nanobamboo array architecture. Programmable assemblies of various components and heterogeneous interfaces, and, in turn, engineering of the energy band structure along the charge transport pathways, play a critical role in generating excellent synergistic effects of multiple components for promoting photocatalytic efficiency.

Programmable photo-electrochemical hydrogen evolution based on multi-segmented CdS-Au nanorod arrays

Programmable photocatalysts for hydrogen evolution have been fabricated based on multi-segmented CdS-Au nanorod arrays, which exhibited high-efficiency and programmability in hydrogen evolution as the photoanodes in the photoelectrochemical cell. Multiple different components each possess unique physical and chemical properties that provide these cascade nanostructures with multiformity, programmability, and adaptability. These advantages allow these nanostructures as promising candidates for high efficient harvesting and conversion of solar energy.

Fullerene assemblies toward photo-energy conversions

Manipulating the molecular interaction and assembly of fullerene derivatives leads to their enhanced photoconductivity and applications in photo-energy (electric and thermal) conversion systems.

Optoelectronic functional materials based on alkylated-π molecules: self-assembled architectures and nonassembled liquids

The engineering of single molecules into higher-order hierarchical assemblies is a current research focus in molecular materials chemistry. Molecules containing π-conjugated units are an important class of building blocks because their self-assembly is not only of fundamental interest, but also the key to fabricating functional systems for organic electronic and photovoltaic applications. Functionalizing the π-cores with "alkyl chains" is a common strategy in the molecular design that can give the system desirable properties, such as good solubility in organic solvents for solution processing. Moreover, the alkylated-π system can regulate the self-assembly behavior by fine-tuning the intermolecular forces. The optimally assembled structures can then exhibit advanced functions. However, while some general rules have been revealed, a comprehensive understanding of the function played by the attached alkyl chains is still lacking, and current methodology is system-specific in many cases. Better clarification of this issue requires contributions from carefully designed libraries of alkylated-π molecular systems in both self-assembly and nonassembly materialization strategies. Here, based on recent efforts toward this goal, we show the power of the alkyl chains in controlling the self-assembly of soft molecular materials and their resulting optoelectronic properties. The design of alkylated-C60 is selected from our recent research achievements, as the most attractive example of such alkylated-π systems. Some other closely related systems composed of alkyl chains and π-units are also reviewed to indicate the universality of the methodology. Finally, as a contrast to the self-assembled molecular materials, nonassembled, solvent-free, novel functional liquid materials are discussed. In doing so, a new journey toward the ultimate organic "soft" materials is introduced, based on alkylated-π molecular design.

Photoconductivity and enhanced memory effects in hybrid C60-graphene transistors

We describe the observation of photoconductivity and enhanced memory effects in graphene devices functionalized with clusters of alkylated C(60) molecules. The alkylated C(60) clusters were adsorbed on chemical vapor deposition-grown graphene devices from an aprotic medium. The resulting alkylated C(60)-graphene hybrid devices showed reproducible photoconductive behavior originating from the electron-accepting nature of the C(60) molecules. Significantly enhanced gate hysteresis was observed upon illumination with visible light, thereby enabling the use of C(60)-graphene hybrid devices in three-terminal photo-memory applications.