Infrared and Near-Infrared Spectrometry of Anatase and Rutile Particles Bandgap Excited in Liquid

Chemical conversion of materials is completed in milliseconds or seconds by assembling atoms over semiconductor photocatalysts. Bandgap-excited electrons and holes reactive on this time scale are key to efficient atom assembly to yield the desired products. In this study, attenuated total reflection of infrared and near-infrared light was applied to characterize and quantify the electronic absorption of TiO2 photocatalysts excited in liquid. Nanoparticles of rutile or anatase were placed on a diamond prism, covered with liquid, and irradiated by steady UV light through the prism. Electrons excited in rutile particles (JRC-TIO-6) formed small polarons characterized by a symmetric absorption band spread over 10000–700 cm–1 with a maximum at 6000 cm–1. Electrons in anatase particles (JRC-TIO-7) created large polarons and produced an asymmetric absorption band that gradually strengthened at wavenumbers below 5000 cm–1 and sharply weakened at 1000 cm–1. The absorption spectrum of large electron polarons in TIO-7 was compared with the absorption reported in a Sr-doped NaTaO3 photocatalyst, and it was suggested that excited electrons were accommodated as large polarons in NaTaO3 photocatalysts efficient for artificial photosynthesis. UV-light power dependence of the absorption bands was observed in N2-exposed decane liquid to deduce electron–hole recombination kinetics. With light power density P > 200 W m–2 (TIO-6) and 2000 W m–2 (TIO-7), the polaron absorptions were enhanced with absorbance being proportional to P1/2. The observed 1/2-order power law suggested recombination of multiple electrons and holes randomly moving in each particle. Upon excitation with smaller P, the power-law order increased to unity. The unity-order power law was interpreted with recombination of an electron and a hole that were excited by the same photon. In addition, an average lifetime of 1 ms was estimated with electron polarons in TIO-6 when weakly excited at P = 20 W m–2 to simulate solar-light irradiation

Competitive Adsorption on Graphite Investigated Using Frequency-Modulation Atomic Force Microscopy: Interfacial Liquid Structure Controlled by the Competition of Adsorbed Species

The competitive adsorption of long-chain (C₁₈ and C₂₄) carboxylic acids versus <i>n</i>-decanol on graphite was investigated using frequency-modulation atomic force microscopy. A long-range-ordered monolayer of the solute (stearic acid or lignoceric acid) developed in saturated decanol solution, whereas an ordered decanol monolayer was deposited from dilute solutions. The piconewton-order tip–surface force was observed in solutions as a function of the vertical and lateral coordinates, together with the topography of the monolayers. The tip–surface force was periodically modulated, which was interpreted with a solution structure commensurate with the ordered assembly of adsorbed monolayers. These results show that the solution structure at the interface was controlled by the competitively adsorbed species and thus was sensitive to the composition of the bulk solution

Infrared Absorption of Zn_0.5Cd_0.5S Photocatalyst Bandgap-Excited Under an Aqueous Environment

The adoption of infrared and near-infrared spectrometry techniques for mechanistic and kinetic studies on the photocatalytic water splitting reaction has been steadily increasing over the years. Herein, a transition metal sulfide photocatalyst, Zn0.5Cd0.5S (ZCS), which has been proven to be an efficient hydrogen (H2) evolution photocatalyst, was investigated through attenuated total reflection FTIR. Electronic absorption in the wavenumber region of 500–3000 cm–1 was detected, while the photocatalyst was irradiated by a UV light source in a pure water environment. The source of absorption was further characterized by employing scavenging solutions, and it was found that the contribution from photogenerated electrons and holes coincidentally appeared in the same wavenumber window. This is likely to be unprecedented as prior studies generally were not able to observe electronic absorption by holes, or the hole absorption was present close to the visible wavenumber window instead. Nevertheless, the shape of absorption peak associated with holes was noticeably distinctive from that of electrons when their normalized spectra were compared, allowing easy identification of the nature of the detected absorption. These observations suggested that electron and hole trap states were present in ZCS, and it is proposed that the trap states could be innately present due to the formation of physical defects during chemical synthesis, or they were instantaneously created upon photoexcitation where polarons were manifested by the disruption of ionic equilibrium in the valence band and conduction band through injection of foreign photogenerated electrons or holes

Water and 2‑Propanol Structured on Calcite (104) Probed by Frequency-Modulation Atomic Force Microscopy

The structure of liquid water and 2-propanol on the (104) surface of calcite (CaCO₃) was probed by frequency-modulation atomic force microscopy. The microscope tip scanned each liquid to record the tip–surface force perturbed by the liquid structure at the interface. In water, the force distribution on planes cross-sectional to the surface presents a 0.5-nm-thick checkerboard-like pattern matching the corrugated topography of the calcite surface. This provides evidence that the local water density was laterally and vertically modulated. With 2-propanol, a laterally uniform, vertically layered structure was found between the first laterally structured layer and the bulk liquid. These results are consistent with the density distributions of water and ethanol proposed in earlier X-ray and simulation studies

Long-Life Electrons in Metal-Doped Alkali-Metal Tantalate Photocatalysts Excited under Water

Conversion of materials for artificial photosynthesis is completed in milliseconds or seconds by assembling atoms over semiconductor photocatalysts. Band-gap-excited electrons and holes reactive on this time scale are key for efficient atom assembly to yield the desired products. In this study, attenuated total reflection of infrared (IR) light was applied to characterize the electronic absorption of long-life charge carriers excited under water. Under excitation, NaTaO3 and KTaO3 photocatalyst particles doped with Sr or La cations absorbed IR light. A broad absorption band appeared with a maximum at 1400 cm–1, which was enhanced by the addition of hole scavengers (e.g., methanol and Na2SO3) and disappeared in the presence of electron scavengers (e.g., FeCl3, NaIO3, and H2O2). This absorption corresponded to the electronic transition of band-gap-excited electrons accommodated in mid-gap states. In anaerobic n-decane, the electron absorption was enhanced by the excitation light power, P, with absorbance being proportional to P1/2. The observed 1/2-order power law suggested deexcitation via recombination of electrons and holes. When the excitation light was stopped, the absorbance decreased as a function of time with a second-order rate law, as expected in the case of recombinative deexcitation. In addition, the 1/2-order power law and second-order decay rate law were observed in anaerobic water, with an accelerated decay rate, which was possibly due to a water-related electron-consuming reaction. This study demonstrated that long-life electrons contribute to surface redox reactions over semiconductor photocatalysts for artificial photosynthesis

Cross-Sectional Structure of Liquid 1‑Decanol over Graphite

The interface of graphite and liquid 1-decanol was studied using frequency modulation atomic force microscopy (FM-AFM). The topography of epitaxially physisorbed decanol on the substrate was traced with submolecular resolution. The tip–surface force was monitored in the liquid as a function of the vertical and lateral tip coordinates to reveal the cross-sectional structure of the interfacial decanol. Four or more liquid layers were identified by vertically modulated force distributions. The first and second liquid layers were laterally heterogeneous, as evidenced by a force distribution that was periodically modulated along lateral coordinates. A possible structuring mechanism is proposed on the basis of energy gain by hydrogen bonding and van der Waals interactions

Chemical Recognition at an Atomically Flat Surface of Metal Oxide

An adsorbate monolayer composed of acetate (CH3COO-) and formate (HCOO-) was prepared on an atomically flat surface of titanium oxide as a two-dimensional analogue of substituted solid solution. When the mixed-monolayer was exposed to acetic acid (CH3COOH) vapor, an impinging acid molecule recognized a preadsorbed acetate among a lot of formates and replaced one of its formate neighbors. Straight chains of acetate were assembled in the monolayer as a result. A bimolecular intermediate state of the gas-adsorbate exchange was proposed to kinetically control which formate to be replaced

Specific Hydration on <i>p</i>‑Nitroaniline Crystal Studied by Atomic Force Microscopy

The molecular-scale structure of water was studied over the (101) surface of p-nitroaniline crystals using advanced atomic force microscopy. p-Nitroaniline contains two polar groups on opposite ends of the nonpolar benzene ring and presents a surface of controlled heterogeneity. The cross-sectional distribution of force applied to the tip was precisely determined and was related to the local density of the structured water. Force modulations were present on the polar end-groups and absent on the benzene ring, suggesting water localization on the polar end-groups

Double Doping of NaTaO₃ Photocatalysts with Lanthanum and Manganese for Strongly Enhanced Visible-Light Absorption

Perovskite-structured tantalates, NaTaO3 in particular, have been at the forefront of solar energy conversion to generate hydrogen fuel via photocatalytic water splitting. However, their application as a photocatalyst remains impractical due to their inability to absorb long-wavelength light. One promising scheme for extending the material response to long-wavelength light is via the charge compensation of doped cations with lanthanum and a transition metal. In this research, NaTaO3 is doubly doped with lanthanum and manganese, a 3d-block transition metal, for visible-light sensitization. Following the double doping, the light absorption is extended to the near-infrared region. The doubly doped samples not only strongly absorb visible light but also produce electrons under visible-light irradiation (420 < λ < 740 nm). EDX confirms the one-to-one ratio of dopant incorporation, and nanometer-resolution elemental mapping with TEM demonstrating that La and Mn are incorporated at approximately the same location in the particles. EXAFS furthermore reveals that La occupies the Na site, while Mn occupies the Ta site. These findings suggest that La pairs with Mn at the neighboring site to form a LaMnO3–NaTaO3 solid solution

Effect of Etching on Electron–Hole Recombination in Sr-Doped NaTaO₃ Photocatalysts

Sodium tantalate (NaTaO₃) photocatalysts doped with Sr²⁺ produce core–shell-structured NaTaO₃–SrSr_1/3Ta_2/3O₃ solid solutions able to split water efficiently, when prepared via the solid-state method. In this study, the photocatalysts were chemically etched to examine the different roles of the core and shell with respect to the recombination of electrons and holes. Under excitation by Hg–Xe lamp irradiation, the steady-state population of electrons in the core–shell-structured photocatalyst with a bulk Sr concentration of 5 mol % increased by 130 times relative to that of the undoped photocatalyst. During etching for the first 10 min, the shell detached from the top of the core, and the electron population in the uncovered core further increased by 40%. This population enhancement indicates that electrons are excited in the core and recombined in the shell. Etching up to 480 min resulted in the reduction of the electron population. To interpret the population reduction in this stage of etching, a Sr concentration gradient that controls the electron population in the core is proposed