Dynamics of Interfacial Charge Transfer to Formic Acid, Formaldehyde, and Methanol on the Surface of TiO₂ Nanoparticles and Its Role in Methane Production

Transient absorption and electron paramagnetic resonance (EPR) spectroscopies were used to study reactions of photogenerated electrons and holes on TiO₂ with methanol, formaldehyde, and formic acid (compounds that, together with methane, have been observed in the photocatalytic reduction of CO₂). The ultrafast dynamics of hole scavenging was found to be an order of magnitude faster on the surface of TiO₂ than in the corresponding homogeneous systems. Additionally, the equilibrium constant for the reaction of photogenerated electrons in TiO₂ with adsorbed CO₂ was estimated to be less than 3.2 M^–1, regardless of the presence of hole scavengers and product molecules. Formic acid serves as both the hole and the electron acceptor, yielding the protonated radical anions (OC^•OH), and formyl radicals, respectively. For methanol and formaldehyde only photooxidation, but no one-electron photoreduction, was observed by EPR spectroscopy; these molecules are either reduced in a two-electron process or act only as hole scavengers

Heteroatom-Transfer Coupled Photoreduction and Carbon Dioxide Fixation on Metal Oxides

Photoactive metal oxides, such as hydrated TiO₂, are known to reduce carbon dioxide to methane, but the mechanism for this photoreaction is insufficiently understood. In particular, it is not known whether the reduction of crucial reaction intermediates, including the formate anion, involves one- or two-electron reactions. In this study, we demonstrate that formic acid and its derivatives can be reduced to the formyl radical via a concerted reaction in which the electron transfer is coupled to oxygen transfer to a Ti³⁺ center on the oxide surface. Several other examples of such heteroatom-transfer reactions are demonstrated, suggesting a general pattern. The implications of these reactions for photocatalytic methanogenesis, perchlorate diagenesis, and planetary chemistry on Mars are discussed

Coupling Titania Nanotubes and Carbon Nanotubes To Create Photocatalytic Nanocomposites

A titania nanotube/single-wall carbon nanotube composite was prepared by a simple hydration dehydration process. These composites were characterized using X-ray diffraction and spectroscopic techniques (UV–visible diffuse reflectance, Raman, photoluminescence, and EPR) as well as electron microscopy (SEM, TEM). SEM and TEM images indicated that single-wall carbon nanotubes (SWCNTs) were interwoven with the titania nanotubes. Raman spectra further confirmed the chemical interaction between the titania nanotube and the SWCNT in the composites. The photoactivity of these composites was tested by the photooxidation of acetaldehyde. The composites showed enhanced photoactivity under both visible and UV light in comparison with conventional titania (P25) and controls. The composite having a mass ratio of 1:50 (SWCNT/TiNT) showed the maximum photocatalytic activity for acetaldehyde decay under visible light. XPS and EPR spectra indicated the creation of Ti–O–C bonds between the titania nanotube and the SWCNTs during the hydration dehydration process, which explains the visible light photoactivity

Photocatalytic Hydrogen Production from Noncovalent Biohybrid Photosystem I/Pt Nanoparticle Complexes

A photocatalytic hydrogen-evolving system based on intermolecular electron transfer between native Photosystem I (PSI) and electrostatically associated Pt nanoparticles is reported. Using cytochrome c₆ as the soluble mediator and ascorbate as the sacrificial electron donor, visible-light-induced H₂ production occurs for PSI/Pt nanoparticle biohybrids at a rate of 244 μmol H₂ (mg chlorophyll)⁻¹ h⁻¹ or 21 034 mol H₂ (mole PSI)⁻¹ h⁻¹. These results demonstrate that highly efficient photocatalysis of H₂ can be obtained for a self-assembled, noncovalent complex between PSI and Pt nanoparticles; a molecular wire between the terminal acceptor of PSI, the [4Fe−4S] cluster F_B, and the nanoparticle is not required. EPR characterization of the electron-transfer reactions in PSI/Pt nanoparticle biohybrids shows blocked electron transfer to flavodoxin, the native acceptor protein of PSI, and presents evidence of low-temperature photogenerated electron transfer between PSI and the Pt nanoparticle. This work demonstrates a feasible strategy for linking molecular catalysts to PSI that takes advantage of electrostatic-directed assembly to mimic acceptor protein binding

Study of Nucleation and Growth Mechanism of the Metallic Nanodumbbells

We propose a general nucleation and growth model that can explain the mechanism of the formation of CoPt₃/Au, FePt/Au, and Pt/Au nanodumbbells. Thus, we found that the nucleation event occurs as a result of reduction of Au⁺ ions by partially oxidized surface Pt atoms. In cases when Au³⁺ is used as a gold precursor, the surface of seeds should be terminated by ions (e.g., Co²⁺, Pb²⁺) that can reduce Au³⁺ to Au⁺ ions, which can further participate in the nucleation of gold domain. Further growth of gold domain is a result of reduction of both Au³⁺ and Au⁺ by HDA at the surface of gold nuclei. We explain the different ability of CoPt₃, Pt, and FePt seeds to serve as a nucleation center for the reduction of gold and further growth of dumbbells. We report that the efficiency and reproducibility of the formation of CoPt₃/Au, FePt/Au, and Pt/Au dumbbells can be optimized by the concentration and oxidation states of the surface ions on metallic nanocrystals used as seeds as well as by the type of the gold precursor

Effect of Metal Ions on Photoluminescence, Charge Transport, Magnetic and Catalytic Properties of All-Inorganic Colloidal Nanocrystals and Nanocrystal Solids

Colloidal semiconductor nanocrystals (NCs) provide convenient “building blocks” for solution-processed solar cells, light-emitting devices, photocatalytic systems, etc. The use of inorganic ligands for colloidal NCs dramatically improved inter-NC charge transport, enabling fast progress in NC-based devices. Typical inorganic ligands (e.g., Sn₂S₆^4–, S^2–) are represented by negatively charged ions that bind covalently to electrophilic metal surface sites. The binding of inorganic charged species to the NC surface provides electrostatic stabilization of NC colloids in polar solvents without introducing insulating barriers between NCs. In this work we show that cationic species needed for electrostatic balance of NC surface charges can also be employed for engineering almost every property of all-inorganic NCs and NC solids, including photoluminescence efficiency, electron mobility, doping, magnetic susceptibility, and electrocatalytic performance. We used a suite of experimental techniques to elucidate the impact of various metal ions on the characteristics of all-inorganic NCs and developed strategies for engineering and optimizing NC-based materials

CO₂ Preactivation in Photoinduced Reduction via Surface Functionalization of TiO₂ Nanoparticles

Salicylate and salicylic acid derivatives act as electron donors via charge-transfer complexes when adsorbed on semiconducting surfaces. When photoexcited, charge is injected into the conduction band directly from their highest occupied molecular orbital (HOMO) without needing mediation by the lowest unoccupied molecular orbital (LUMO). In this study, we successfully induce the chemical participation of carbon dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA). We determine the geometry of CO₂ using a combination of ultraviolet–visible spectroscopy (UV–vis), surface enhanced Raman scattering (SERS), ¹³C NMR, and electron paramagnetic resonance (EPR). We find CO₂ binds on Ti sites in a carbonate form and discern via EPR a surface Ti-centered radical in the vicinity of CO₂, suggesting successful charge transfer from the sensitizer to the neighboring site of CO₂. This study opens the possibility of analyzing the structural and electronic properties of the anchoring sites for CO₂ on semiconducting surfaces and proposes a set of tools and experiments to do so

Controlling Surface Defects and Photophysics in TiO₂ Nanoparticles

Titanium dioxide (TiO₂) is widely used for photocatalysis and solar cell applications, and the electronic structure of bulk TiO₂ is well understood. However, the surface structure of nanoparticulate TiO₂, which has a key role in properties such as solubility and catalytic activity, still remains controversial. Detailed understanding of surface defect structures may help explain reactivity and overall materials performance in a wide range of applications. In this work we address the solubility problem and surface defects control on TiO₂ nanoparticles. We report the synthesis and characterization of ∼4 nm TiO₂ anatase spherical nanoparticles that are soluble and stable in a wide range of organic solvents and water. By controlling the temperature during the synthesis, we are able to tailor the density of defect states on the surface of the TiO₂ nanoparticles without affecting parameters such as size, shape, core crystallinity, and solubility. The morphology of both kinds of nanoparticles was determined by TEM. EPR experiments were used to characterize the surface defects, and transient absorption measurements demonstrate the influence of the TiO₂ defect states on photoinduced electron transfer dynamics