Showing papers on "Absorption spectroscopy published in 2006"

Effects of Structural Variation on the Photocatalytic Performance of Hydrothermally Synthesized BiVO4

Abstract: Highly crystalline monoclinic scheelite BiVO4 powders are synthesized from aqueous Bi(NO3)3 and NH4VO3 solutions over a wide range of pH by a hydrothermal process BiVO4 powders with various morphologies, surface textures, and grain shapes are selectively synthesized by adjusting the pH The dependence of the Raman peak position and intensity on the synthesis conditions indicates that the symmetry distortions in the local structure of the synthesized BiVO4 are affected by the preparation conditions These variations in the local structure result in the modification of the electronic structure of BiVO4, which results in a blue-shift in the UV-vis absorption spectrum of hydrothermally synthesized BiVO4 in comparison with a well-crystallized sample prepared by homogeneous coprecipitation The photocatalytic activities for O2 evolution from an aqueous AgNO3 solution under visible-light irradiation are strongly dependent on the pH used in the synthesis The differences in the photocatalytic activities between BiVO4 samples prepared under various conditions is attributed to the degree of structural distortion, leading to differences in the mobility of photogenerated holes formed in the valence band, which consists of Bi 6s and O 2p orbitals

Synthesis and photovoltaic properties of two-dimensional conjugated polythiophenes with bi(thienylenevinylene) side chains.

Abstract: Three two-dimensional (2-D) conjugated polythiophenes with bi(thienylenevinylene) side chains (biTV-PTs), P1, P2, and P3, were designed and synthesized for application in polymer solar cells. The absorption spectral, electrochemical, and photovoltaic properties of the biTV-PTs were investigated and compared with those of poly(3-hexylthiophene) (P3HT). The biTV-PTs show a broad absorption band from 350 to 650 nm; especially, the absorption spectrum of P3 displays a broad plateau and much stronger absorbance than that of P3HT in the wavelength range from 350 to 480 nm. Cyclic voltammograms reveal that the onset oxidation and reduction potentials of the biTV-PTs positively shifted by ca. 0.2 V in comparison with those of P3HT, indicating that the HOMO energy level of the biTV-PTs is ca. 0.2 eV lower than that of P3HT. Polymer solar cells (PSCs) were fabricated based on the blend of the polymers and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PCBM) with a weight ratio of 1:1. The open circuit voltage of ...

Semiconductor-Metal Nanoparticle Molecules: Hybrid Excitons and the Nonlinear Fano Effect

Abstract: Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent internanoparticle Coulomb interaction and can be viewed as a nonlinear Fano effect which is quite different from the usual linear Fano resonance.

Terahertz dielectric properties of polymers

Abstract: The terahertz dielectric properties of polymers were characterized by transmission terahertz time domain spectroscopy (THz-TDS) in the frequency range extending from 0.2 to 3.0 THz. The terahertz absorption spectra, the refractive indices and the dielectric functions of various polymer materials were measured and compared. The variation of the refractive index of the polymers was less than 6 %, ranging from 1.4 to 1.8, within the investigated frequency range, but the absorption properties of the polymers showed very different frequency-dependent behaviors. The loss mechanism for terahertz radiation in polymers is discussed by correlating the absorption coefficients and the loss tangents (tan {delta}) of the materials.

Characterization and photocatalytic mechanism of nanosized CdS coupled TiO2 nanocrystals under visible light irradiation

Abstract: Nanosized CdS coupled TiO2 nanocrystals were prepared by a microemulsion-mediated solvothermal method at relatively low temperatures The prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), BET surface area analysis, X-ray diffraction (XRD), UV–vis absorption spectroscopy (UV–vis), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) It was found that the CdS coupled TiO2 materials consisted of uniform anatase TiO2 of 6–10 nm with highly dispersed cubic phase CdS nanocrystals The prepared samples exhibit strong visible light absorption at about 550 nm Meanwhile, they have high surface area in the range of 156–263 m2 g−1 and mesoporous character with the average pore diameter of ca 50–65 nm The coupling between the (1 0 1) crystal planes of anatase and (1 1 1) crystal planes of CdS was observed in the HRTEM image Ti3+ signal was observed in the electron paramagnetic resonance (EPR) spectrum of CdS coupled TiO2 nanocrystals under visible light irradiation It provided the evidence of an effective transfer of photo-generated electrons from the conduction band of CdS to that of TiO2 As expected, the nanosized CdS sensitized TiO2 nanocrystal materials showed enhanced activity in the oxidation of methylene blue in water or nitric oxide in air under visible light irradiation The mechanism of photocatalysis on CdS coupled TiO2 nanocrystals under visible light is also discussed

Size-dependent extinction coefficients of PbS quantum dots.

Abstract: We report here on a detailed study on PbS colloidal quantum dots. A characterization via X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) allowed us to reliably determine the diameter and the shape of the nanocrystals. These data, together with second-derivative analysis of the absorption spectra, allowed us to determine the size dependence of seven transitions in the absorption spectrum; some of these transitions were identified on the basis of their normalized confinement energy. The size dependence of the first excitonic transition was best modeled by a four-band envelope approach which considers the anisotropy of the band edges (Andreev, A. D.; Lipovskii, A. A. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15402−15404). The extinction coefficients were calculated using concentrations obtained from inductively coupled plasma atomic emission spectrometry (ICP-AES), and their size dependence was found to follow a power law with exponent equal to ∼2.5. In contra...

Shape-Controlled Synthesis of Ternary Chalcogenide ZnIn2S4 and CuIn(S,Se)2 Nano-/Microstructures via Facile Solution Route

Abstract: We demonstrated in this paper the shape-controlled synthesis of ZnIn2S4, CuInS2, and CuInSe2 nano- and microstructures through a facile solution-based route. One-dimensional ZnIn2S4 nanotubes and nanoribbons were synthesized by a solvothermal method with pyridine as the solvent, while ZnIn2S4 solid or hollow microspheres were hydrothermally prepared in the presence of a surfactant such as cetyltrimethylammonium bromide (CTAB) or poly(ethylene glycol) (PEG). The mechanisms related to the phase formation and morphology control of ZnIn2S4 are proposed and discussed. The UV−vis absorption spectra show that the as-prepared nano- and micromaterials have strong absorption in a wide range from UV to visible light and that their band gaps are somewhat relevant to the size and morphology. The photoluminescence measurements of the ZnIn2S4 microspheres at room temperature reveal intense excitation at ∼575 nm and red emission at ∼784 nm. Furthermore, CuInS2 and CuInSe2 with different morphologies such as spheres, plat...

Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy

Abstract: A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2−60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump−probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core−shell m...

A Facile Synthesis and Photoluminescent Properties of Redispersible CeF3, CeF3:Tb3+, and CeF3:Tb3+/LaF3 (Core/Shell) Nanoparticles

Abstract: CeF3, CeF3:Tb3+, and CeF3:Tb3+/LaF3 (core/shell) nanoparticles were prepared by the polyol method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), UV−vis absorption spectra, photoluminescence (PL) spectra, and lifetimes. The results of XRD indicate that the obtained CeF3, CeF3:Tb3+, and CeF3:Tb3+/LaF3 (core/shell) nanoparticles crystallized well at 200 °C in diethylene glycol (DEG) with a hexagonal structure. The TEM images illustrate that the CeF3 and CeF3:Tb3+ nanoparticles are spherical with a mean diameter of 7 nm. The growth of the LaF3 shell around the CeF3:Tb3+ core nanoparticles resulted in an increase of the average size (11 nm) of the nanopaticles as well as in a broadening of their size distribution. These nanocrystals can be well-dispersed in ethanol to form clear colloidal solutions. The colloidal solutions of CeF3 and CeF3:Tb3+ show the characteristic emission of Ce3+ 5d−4f (320 nm) and Tb3+ 5D4−7FJ (J = 6−3, with 5D4−7F...

Chiral N-isobutyryl-cysteine protected gold nanoparticles: preparation, size selection, and optical activity in the UV-vis and infrared.

Abstract: We have prepared gold nanoparticles covered with N-isobutyryl-l-cysteine and N-isobutyryl-d-cysteine, respectively. These particles with a mean particle size smaller than 2 nm are highly soluble in water and are amenable to chiroptical techniques such as vibrational circular dichroism (VCD) and circular dichroism (CD) spectroscopy. Density functional theory shows that the VCD spectra are sensitive toward the conformation of the adsorbed thiol. Based on the comparison between the experimental VCD spectrum and the calculated VCD spectra for different conformers, a preferential conformation of the thiol adsorbed on the gold particles can be proposed. In this conformation the carboxylate group interacts with the gold particle in addition to the sulfur. The particles could furthermore be separated according to their charge and size into well-defined compounds. The optical absorption spectra revealed a well-quantized electronic structure and a systematic red-shift of the absorption onset with increasing gold core size, which was manifested in a color change with particle size. Some compounds showed basically identical absorption spectra as analogous gold particles protected with l-glutathione. This shows that these particles have identical core sizes (10-12, 15 and 18 gold atoms, respectively) and indicates that the number and arrangement of the adsorbed thiol are the same, independent of the two thiols, which have largely different sizes. Some separated compounds show strong optical activity with opposite sign when covered with the d- and l-enantiomer, respectively, of N-isobutyryl-cysteine. The origin of the optical activity in the metal-based transitions is discussed. The observations are consistent with a mechanism based on a chiral footprint on the metal core imparted by the adsorbed thiol.

Probing the 3d Spin Momentum with X-ray Emission Spectroscopy: The Case of Molecular-Spin Transitions

Abstract: We report X-ray emission spectra of Fe, Fe, and Cospin-crossover compounds in their high-spin and low-spin forms. It is shown that all X-ray emission features are sensitive to the spin state. Variations of the K

Tetrahedral structure or chains for liquid water.

Abstract: It has been suggested, based on x-ray absorption spectroscopy (XAS) experiments on liquid water [Wernet, Ph., et al. (2004) Science 304, 995–999], that a condensed-phase water molecule’s asymmetric electron density results in only two hydrogen bonds per water molecule on average. The larger implication of the XAS interpretation is that the conventional view of liquid water being a tetrahedrally coordinated random network is now replaced by a structural organization that instead strongly favors hydrogen-bonded water chains or large rings embedded in a weakly hydrogen-bonded disordered network. This work reports that the asymmetry of the hydrogen density exhibited in the XAS experiments agrees with reported x-ray scattering structure factors and intensities for Q > 6.5 A−1. However, the assumption that the asymmetry in the hydrogen electron density does not fluctuate and is persistent in all local molecular liquid water environments is inconsistent with longer-ranged tetrahedral network signatures present in experimental x-ray scattering intensity and structure factor data for Q < 6.5 A−1. polarizability tetrahedral liquid x-ray absorption spectroscopy x-ray scattering hydrogen-bonding

Preparation of nitrogen-substituted TiO2 thin film photocatalysts by the radio frequency magnetron sputtering deposition method and their photocatalytic reactivity under visible light irradiation.

Abstract: Nitrogen-substituted TiO2 (N-TiO2) thin film photocatalysts have been prepared by a radio frequency magnetron sputtering (RF-MS) deposition method using a N2/Ar mixture sputtering gas. The effect of the concentration of substituted nitrogen on the characteristics of the N-TiO2 thin films was investigated by UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. The absorption band of the N-TiO2 thin film was found to shift smoothly to visible light regions up to 550 nm, its extent depending on the concentration of nitrogen substituted within the TiO2 lattice in a range of 2.0-16.5%. The N-TiO2 thin film photocatalyst with a nitrogen concentration of 6.0% exhibited the highest reactivity for the photocatalytic oxidation of 2-propanol diluted in water even under visible (lambda > or = 450 nm) or solar light irradiation. Moreover, N-TiO2 thin film photocatalysts prepared on conducting glass electrodes showed anodic photocurrents attributed to the photooxidation of water under visible light, its extent depending on wavelengths up to 550 nm. The absorbed photon to current conversion efficiencies reached 25.2% and 22.4% under UV (lambda = 360 nm) and visible light (lambda = 420 nm), respectively. UV-vis and photoelectrochemical investigations also confirmed that these thin films remain thermodynamically and mechanically stable even under heat treatment at 673 K. In addition, XPS and XRD studies revealed that a significantly high substitution of the lattice O atoms of the TiO2 with the N atoms plays a crucial role in the band gap narrowing of the TiO2 thin films, enabling them to absorb and operate under visible light irradiation as a highly reactive, effective photocatalyst.

Aerosol Absorption Measurement using Photoacoustic Spectroscopy: Sensitivity, Calibration, and Uncertainty Developments

Abstract: Light absorption by aerosols is one of the most uncertain parameters associated with the direct and indirect aerosol effects on climate and is one of the most difficult quantities to measure This article describes the development of a sensitive method of measuring aerosol absorption at 532 nm with excellent time response (detection limit: 008 Mm−1, 60 second average) using photoacoustic absorption spectroscopy An accurate calibration method (accuracy of 1–2%) at atmospherically relevant absorption levels and independent validation of the photoacoustic technique is presented An upper limit to the instrument precision for aerosol absorption measurement is ∼6% (2σ, 30 sec) while instrument accuracy is calculated to be ∼5% A standard for aerosol absorption measurement techniques using well characterized absorbing aerosol is also proposed

Visible Light Absorption by Various Titanium Dioxide Specimens

Abstract: A set of heat-induced and photoinduced absorption spectra of various compositions of Degussa P25 TiO2 and different polymers has been examined The spectra are described as the sum of overlapping absorption bands (ABs) with maxima at 290 eV (427 nm, AB1), 255 eV (486 nm, AB2), and 205 eV (604 nm, AB3); the spectra correlate entirely with the experimentally observed absorption spectra after the reduction of TiO2 Absorption spectra of visible-light-active TiO2 photocatalysts reported recently in the literature have also been analyzed Relatively narrow absorption spectra are very similar and independent of the method of photocatalyst preparation The average absorption spectrum can be described reasonably well by the sum of the two absorption bands AB1 and AB2 It is argued that visible light activation of TiO2 specimens (anion-doped or otherwise) implicates defects associated with oxygen vacancies that give rise to color centers displaying these absorption bands and not to a narrowing of the original band gap of TiO2 (EBG approximately 32 eV, anatase) through mixing of dopant and oxygen states, as has been suggested recently in the literature

Fe L-Edge XAS Studies of K4[Fe(CN)6] and K3[Fe(CN)6]: A Direct Probe of Back-Bonding

Abstract: Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand π* orbitals due to metal-to-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of π back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.

Hyperspectral temperature and salt dependencies of absorption by water and heavy water in the 400-750 nm spectral range.

Abstract: The temperature and salt dependencies of absorption by liquid water (H2O) and heavy water (D2O) were determined using a hyperspectral absorption and attenuation meter (WET Labs, AC-S). Sodium chloride (NaCl) was used as a proxy for seawater salts. There was no significant temperature (ΨT) or salt (ΨS) dependency of absorption at wavelengths <550 nm. At wavelengths >550 nm, ΨT exhibited peaks at ∼604, 662, and 740 nm. A small negative trough in ΨS occurred at ∼590 nm, followed by a small positive peak ∼620 nm, a larger negative trough at ∼720 nm, and a strong positive peak at ∼755 nm. The salt dependency of absorption by heavy water, ΨsH, exhibited a negative power-law shape with very low ΨsH, at wavelengths >550 nm. Our experiments with NaCl, clean open ocean seawater, and artificial seawater support the hypothesis that salts modify the absorption spectra of seawater by modifying the molecular matrix and vibrations of pure water.

A directly fused tetrameric porphyrin sheet and its anomalous electronic properties that arise from the planar cyclooctatetraene core.

Abstract: Oxidation of a directly meso−meso linked cyclic porphyrin tetramer 2 gave a porphyrin sheet 3. The symmetric square structure of 3 is indicated by its simple 1H NMR spectrum that exhibits only two signals for the porphyrin β-protons. The absorption spectrum of 3 displays characteristic Soret-like broad bands and weak Q-bands, and its magnetic circular dichroism (MCD) spectrum exhibits a negative Faraday A term at the 762 nm band as a rare case, indicating the absorption as a transition from a nondegenerate level to a degenerate level. A slightly longer S1-state (1.1 ps) and smaller TPA cross section (2750 GM) than a tetrameric linear porphyrin tape also indicate its unique electronic properties. The porphyrin sheet 3 forms stable 1:2 complexes with guest molecules G1 and G2, whose 1H NMR spectra exhibit remarkable downfield shifts for the guest protons that are located just above the cyclooctatetraene (COT) core of 3, whereas the imidazolyl protons bound to the zinc(II) porphyrin local cores are observed ...

Linear and nonlinear intersubband optical absorption coefficients and refractive index changes in a quantum box with finite confining potential

Abstract: In this work, both the intersubband optical absorption coefficients and the refractive index changes are calculated exactly in a quantum box. Analytical expressions for the linear and nonlinear intersubband absorption coefficients and refractive index changes are obtained by using the compact-density matrix approach. Numerical results are presented for typical GaAs/AlxGa1−x As quantum box system. The linear, third-order nonlinear and total absorption and refractive index changes are investigated as a function of the incident optical intensity and structure parameters such as box-edge length and stoichiometric ratio. Our results show that both the incident optical intensity and the structure parameters have a great effect on the total absorption and refractive index changes.

Electronic transitions involved in the absorption spectrum and dual luminescence of tetranuclear cubane [Cu4I4(pyridine)4] cluster: a density functional theory/time-dependent density functional theory investigation.

Abstract: We present a combined density functional theory (DFT)/time-dependent density functional theory (TDDFT) study of the geometry, electronic structure, and absorption and emission properties of the tetranuclear "cubane" Cu4I4py4 (py = pyridine) system. The geometry of the singlet ground state and of the two lowest triplet states of the title complex were optimized, followed by TDDFT excited-state calculations. This procedure allowed us to characterize the nature of the excited states involved in the absorption spectrum and those responsible for the dual emission bands observed for this complex. In agreement with earlier experimental proposals, we find that while in absorption the halide-to-pyridine charge-transfer excited state (XLCT*) has a lower energy than the cluster-centered excited state (CC*), a strong geometrical relaxation on the triplet cluster-centered state surface leads to a reverse order of the excited states in emission.

Influence of an electric field on the optical properties of few-layer graphene with AB stacking

Abstract: The effect of perpendicular electric field F on optical properties of the AB-stacked few-layer graphene, made up of two, three, or four graphite sheets, is explored through the gradient approximation. In contrast to the featureless optical spectra of graphene, the low-energy absorption spectra of few-layer graphene with AB stacking exhibit many jumping structures, which result from the band-edge states caused by the stacking effect, in the absence of an electric field. Remarkably, F causes the subband anticrossing, changes the subband spacing, produces the oscillating bands, and increases the band-edge states. It, therefore, follows that the field-modulating spectra with sharp peaks are generated. Moreover, the frequency of peak, which is strongly dependent on the layer number and the field strength, is predicted. Above all, the predicted absorption spectra and the associated electronic properties could be verified by the optical measurements.

Probing the collective vibrational dynamics of a protein in liquid water by terahertz absorption spectroscopy

Abstract: Biological polymers are expected to exhibit functionally relevant, global, and subglobal collective modes in the terahertz (THz) frequency range (i.e., picosecond timescale). In an effort to monitor these collective motions, we have experimentally determined the absorption spectrum of solvated bovine serum albumin (BSA) from 0.3 to 3.72 THz (10–124 cm−1). We successfully extract the terahertz molar absorption of the solvated BSA from the much stronger attenuation of water and observe in the solvated protein a dense, overlapping spectrum of vibrational modes that increases monotonically with increasing frequency. We see no evidence of distinct, strong, spectral features, suggesting that no specific collective vibrations dominate the protein's spectrum of motions, consistent with the predictions of molecular dynamics simulations and normal mode analyses of a range of small proteins. The shape of the observed spectrum resembles the ideal quadratic spectral density expected for a disordered ionic solid, indicating that the terahertz normal mode density of the solvated BSA may be modeled, to first order, as that of a three-dimensional elastic nanoparticle with an aperiodic charge distribution. Nevertheless, there are important detailed departures from that of a disordered inorganic solid or the normal mode densities predicted for several smaller proteins. These departures are presumably the spectral features arising from the unique molecular details of the solvated BSA. The techniques used here and measurements have the potential to experimentally confront theoretical calculations on a frequency scale that is important for macromolecular motions in a biologically relevant water environment.

Tetrahedral Co(II) Coordination in α-Type Cobalt Hydroxide: Rietveld Refinement and X-ray Absorption Spectroscopy

Abstract: We report a Rietveld refinement analysis and X-ray absorption study on a green-color Cl(-)-intercalated alpha-type cobalt hydroxide phase. The refinement clearly demonstrated that one-fifth to one-sixth of the Co(II) at octahedral sites was replaced by pairs of tetrahedrally coordinated Co(II) on each side of the hydroxide plane, represented by a structural formula of [Co(octa)(0.828)Co(tetra)(0.348)(OH)2](0.348+)Cl(0.348).0.456H2O. X-ray absorption spectroscopy also indicated that the divalent cobalt were in local neighboring environments of both octahedral and tetrahedral coordination. Furthermore, UV-vis spectroscopic measurements elucidate the typical green/blue color of an alpha-type cobalt hydroxide.