Nb 3d and O 1s core levels and chemical bonding in niobates

A set of available experimental data on binding energies of Nb 3d_5/2 and O 1s core levels in niobates has been observed with using energy difference (O 1sNb 3d_5/2) as a robust parameter for compound characterization. An empirical relationship between (O 1sNb 3d_5/2) values measured with XPS for Nb⁵⁺-niobates and mean chemical bond length L(NbO) has been discussed. A range of (O 1sNb 3d_5/2) values possible in Nb⁵⁺-niobates has been defined. An energy gap ∼1.4–1.8 eV is found between (O 1sNb 3d_5/2) values reasonable for Nb⁵⁺ and Nb⁴⁺ states in niobates.     

Reflection absorption infrared spectroscopy analysis of the evolution of ErSb on InSb

We discuss an ex-situ monitoring technique based on glancing-angle infrared-absorption used to determine small amounts of erbium antimonide (ErSb) deposited on an indium antimonide (InSb) layer epitaxially grown on an InSb (100) substrate by low pressure metal organic chemical vapor deposition (MOCVD). Infrared absorption from the indium–hydrogen (InH) stretching mode at 1754.5 cm^? 1 associated with a top most surface of an epitaxial InSb layer was used to compare varying levels of surface coverage with ErSb. Among four samples of varying coverage of ErSb deposition (7.2 to 21.5 monolayers), detected infrared absorption peaks distinct to InH weakened as ErSb surface coverage increased. In the early stage of ErSb deposition, our study suggests that outermost indium atoms in the InSb buffer layer are replaced by Er resulting in increase in absorption associated with the InH mode. Using this simple ex-situ technique, we show that it is possible to calibrate the amount of ErSb deposited atop each individual InSb substrate for depositions of few to tens of monolayers.     

Extrinsic fluorescence as a sensitive method for studying photo-degradation of high density polyethylene part I

Artificial ageing of high density polyethylene (HDPE) exposed to ultra-violet irradiation has been investigated by fluorescence spectroscopy. In the case of HDPE, fluorescence spectroscopy requires the addition of fluorophore, (rhodamine 101 laser dye). The spectral features of fluorescence are very sensitive to the interaction between the exited molecules and the polymer matrix. Under wavelength close to 254 nm, the diffusion of oxygen can introduce groups such as CO, CO and C(O)O into the molecular chains. Thus entail structural and chemical modifications such as polymeric chain breaking, cross linking and oxidation. In the present work, we clearly illustrate the fact that fluorescence spectroscopy is very worthwhile in particular to follow the early stages of photo-degradation.     

Unusual oxygen re-equilibration kinetics of TiO2−δ

Oxygen re-equilibration kinetics, along with the equilibrium conductivity, have been measured on undoped, single-crystal TiO_2−δ, by a four-probe d.c. conductivity relaxation technique, against oxygen partial pressure in the range of − 16 < log(P_O2/atm) ≤ 0 at different temperatures in the range of 1173 ≤ T/K ≤ 1373. The isothermal conductivity varies as σ ∝ P_O2^m with m ≈ − 1/4, − 1/5 and − 1/6 in turn with increasing P_O2 up to 1 atm, suggesting a sequential variation of the majority ionic disorder types from Ti_i to Ti_i to V_O, respectively. Contrary to the conventional knowledge that due to the local (defect) equilibrium postulate there should be one and only one chemical diffusivity or single relaxation time for a binary oxide, the oxygen re-equilibration kinetics has turned out to be twofold with two different relaxation times depending on oxygen activities. This is interpreted as being due to the independent relaxation of each sublattice of TiO₂ in an oxygen activity gradient applied, indicating a failure of local equilibrium during oxygen re-equilibration. From the two different relaxation times the chemical diffusion coefficients of component Ti and O are separately evaluated and subsequently, their self-diffusion coefficients. The latter are found to be in a good agreement with the literature data.     

Magnetic susceptibility of liquid Na1−xSbxalloys

The magnetic susceptibility of liquid NaSb alloys was measured in a temperature range between 600°C and 1100°C. The NaSb system is supposed to exhibit, as does the CsSb system, a metal to semiconductor transition dependent upon composition. A relatively strong diamagnetic minimum of x = ?26 × 10^?6 cm³/mol was found near the composition of the stoichiometric formula Na₃Sb. This minimal susceptibility can be relatively well described with an ionic type binding model.     

Nonstoichiometry of the perovskite-type solid solution La0.9Ca0.1Cr1−yAlyO3−δ

Nonstoichiometry of the perovskite-type solid solutions La_0.9Ca_0.1Cr_1−yAl_yO_3−δ was studied by high-temperature gravimetry under controlled P(O₂) atmospheres of 1–10^− 23 bar at 1073–1273 K. The observed data were described by a regular solution-like model for the randomly distributed defects of V_O¨, Cr_Cr., Ca_La,, and Al_Cr^X. With the increase in y, V_O¨ formation becomes much easier. For y > 0.8, some fraction of Ca_La, becomes surrounded only by Al_Cr^X and V_O¨ remains around such Ca_La, up to high P(O₂) to reduce the maximum oxygen content below 3.000.     

Dimethyl methylphosphonate decomposition on fully oxidized and partially reduced ceria thin films

The thermal decomposition of dimethyl methylphosphonate (DMMP) on crystalline ceria thin films grown on Ru(0 0 0 1) was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and infrared absorption reflection spectroscopy (IRAS). TPD experiments show that methanol and formaldehyde desorb as the two main products at 575 K, while water, formaldehyde and CO are produced above 800 K. IRAS studies demonstrate that DMMP adsorbs via the phosphoryl oxygen at 200 K, but the PO bond converts to a bridging OPO species at 300 K. DMMP decomposition initially occurs via POCH₃ bond scission to form methyl methylphosphonate (MMP) and methyl phosphonate (MP) between 300 and 500 K; XPS and IRAS data are consistent with a methoxy intermediate on the surface at these temperatures. The more stable PCH₃ bonds remain intact up to 700 K, and the only surface intermediate at higher temperatures is believed to be PO_x. Although the presence of PO_x decreases activity for DMMP decomposition, some activity on the ceria surface remains even after 7 cycles of adsorption and reaction. The ceria films become reduced by multiple DMMP adsorption-reaction cycles, with the Ce⁺⁴ content dropping to 30% after seven cycles. Investigations of DMMP reaction on reduced ceria surfaces show that CO and H₂ are produced in addition to methanol and formaldehyde. Furthermore, DMMP decomposition activity on the reduced ceria films is almost completely inhibited after only 3 adsorption-reaction cycles. Similarities between DMMP and methanol chemistry on the ceria films suggest that methoxy is a key surface intermediate in both reactions.     

Growth and vibrational properties of MnOx thin films on Rh(111)

The growth, structures, and vibrational properties of ultrathin manganese oxide films on Rh(111) had been investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), low energy ion scattering spectroscopy (LEIS) and Auger electron spectroscopy (AES). MnO_x grew in a layer-by-layer fashion on the Rh(111) surface. HREELS phonon features and XPS binding energies showed that an OMnO like tri-layer formed initially. Which was stable on the Rh(111) surface with MnO_x coverage less than one monolayer. At above one monolayer, Mn₃O₄ was preferred as indicated from a four-phonon feature peaked at 13.3, 39, 68 and 83 meV in HREELS. Higher temperature oxidation and annealing were found to improve the long-range order of the MnO_x films.     

Determination of nitrofurans in feeds based on silver nanoparticle-catalyzed chemiluminescence

A highly sensitive chemiluminescence (CL) method for the determination of nitrofurans (NFs) was developed based on the enhancement of CL intensity of luminol–H₂O₂–NFs system by silver nanoparticles (AgNPs). It was supposed that the oxygen-related radicals of OH and superoxide radical (O₂^?) could be produced when NFs reacted with H₂O₂. Furthermore, the enhancement mechanism was originated from the reinforcer of AgNPs, which could catalyze the generation of the OH radical. Then OH radicals reacted with luminol anion and HO₂^? to form luminol radical (L^?) and O₂^?. The excited state 3-aminophthalate anion was obtained in the reaction of L^? and O₂^?, which was the emitter (luminophor) in the luminol–H₂O₂ CL reaction system and the maximal emission of the CL spectrum was at 425 nm. The experiments of scavenging oxygen-related radicals were done to confirm these reactive oxygen species participated in the CL reaction. The limits of detection (LOD) (S/N=3) were 8×10^?7 g mL^?1 for furacilin, 8×10^?8 g mL^?1 for furantoin, 4×10^?8 g mL^?1 for furazolidone and 2×10^?7 g mL^?1 for furaltadone. The proposed method was successfully applied to the determination of NFs in feeds and pharmaceutical samples.     

Structural characteristics of mixed oxides MOx/SiO2 affecting photocatalytic decomposition of methylene blue

A series of photocatalysts based on silica (nanoparticulate) supported titania, ceria, and ceria/zirconia were synthesized and characterized by a variety of techniques including surface area measurements, X-ray diffraction, Fourier transform infrared spectroscopy, zeta potential, surface charge density, and photocatalytic behavior toward methylene blue decomposition. Thermal treatment at 600 °C increases the anatase content of the titania based catalysts detected by XRD. Changes in the infrared spectra before and after thermal treatment indicate that at low temperature there are more SiOTi bonds than at high temperature. As these bonds break upon heating the SiO₂ and TiO₂ separate, allowing the TiO₂ anatase phase to form. This results in an increased catalytic activity for the thermally treated samples. Nearly all titania based samples exhibit a negative surface charge density at pH 7 (initial pH of photocatalytic studies) which aids adsorption of methylene blue. The crystallinity of ceria and ceria/zirconia based catalysts are in some cases limited, and in others non-existent. Even though the energy band gap (E_g) can be lower for these catalysts than for the titania based catalysts, their photocatalytic properties are inferior.     

Infrared diode laser spectroscopy of N212C18O2

The high-resolution infrared spectrum of N₂¹²C¹⁸O₂ has been observed in the ν₃ band (2314 cm^?1) region of ¹²C¹⁸O₂ with diode laser absorption spectroscopy of pulsed molecular beam. The geometry of N₂¹²C¹⁸O₂ is similar to N₂¹²C¹⁶O₂, a T-shaped structure with the nitrogen molecular axis pointing towards the carbon atom. The geometrical parameters of the T-shaped ground-state structure are determined as R_NcmC = 3.7285(5) Å and (90?Θ_NcmCO) = 6.85(3)°. The vibrational band origin of N₂¹²C¹⁸O₂ corresponding to the ν₃ mode of ¹²C¹⁸O₂ shows a shift of 0.52499(10) cm^?1 with respect to that of ¹²C¹⁸O₂.     

Structure of ordered oxide on InAs(100) surface

It was recently found that oxygen induces ordered reconstructions on several III–V surfaces. The most oxygen-rich reconstruction shows (3 × 1) periodicity. Based on first-principles investigations, a detailed atomic model is presented for this reconstruction. The uncommon periodicity is attributed to the highly stable InOIn trilayer below surface which also leads to stabilizing additional bonds within the surface layer. The strain induced by the trilayer is more effectively accommodated within the (3 × 1) reconstruction than within the competing (2 × 1) reconstruction due to smaller number of dimers. It is proposed that the experimentally found semiconductivity is reached by substitutional atoms within the surface layer. Suitable substitution preserves the magnitude of the bulk band gap.     

Crystal growth and characterization of an NLO organic crystal: N′-[(Z)-(4-methylphenyl)methylidene]-4-nitrobenzohydrazide

In this paper, we report the synthesis, growth and characterization of a new organic NLO single crystal of NMPMN (N′-[(Z)-(4-methylphenyl)methylidene]-4-nitrobenzohydrazide), for the first time. The single crystal was grown by slow evaporation method at room temperature. The cell dimensions obtained by single crystal XRD studies reveal that the crystal belongs to triclinic system. It was characterized by different techniques like powder XRD, UV, FTIR, TGA and DSC. The Knoop microhardness test was carried out to measure the mechanical strength of the crystal. Its refractive index was determined by the Brewster’s angle method. The laser damage threshold studies have been carried out for the crystal using a Q-switched Nd:YAG laser of ns pulses at a wavelength of 532 nm. The Kurtz Powder Second Harmonic Generation revealed that the SHG efficiency of the grown crystal is about 50% that of KDP and is found to be phase matchable. The intermolecular OH⋯O, OH⋯N and CH⋯O hydrogen bonds and a π–π stacking interaction between the nitrobenzene and tolyl rings helps to create a delicate balance between the molecular and supramolecular charge transfer processes by creating a noncentrosymmetric structure.     

Time-evolution of thermal oxidation on high-index silicon surfaces: Real-time photoemission spectroscopic study with synchrotron radiation

The initial oxidation on high-index silicon surfaces with (113) and (120) orientations at 820 K has been investigated by real-time X-ray photoemission spectroscopy (Si 2p and O 1s) using 687 eV photons. The time evolutions of the Siⁿ⁺ (n = 1–4) components in the Si 2p spectrum indicate that the Si^2 + state is suppressed on high-index surfaces compared with Si(001). The O 1s state consists of two components, a low-binding-energy component (LBC) and a high-binding-energy component (HBC). It is suggested that the O atom in strained SiOSi contributes to the LBC component. The reaction rates are slower on high-index surfaces compared with that on Si(001).     

Using pulsed wave ultrasound to evaluate the suitability of hydroxyl radical scavengers in sonochemical systems

Hydroxyl radical (OH) scavengers are commonly used in sonochemistry to probe the site and nature of reaction in aqueous cavitational systems. Using pulsed wave (PW) ultrasound with comparative sonochemistry we evaluated the performance of OH scavengers (i.e., formic acid, carbonic acid, terephthalic acid/terephthalate, iodide, methanesulfonate, benzenesulfonate, and acetic acid/acetate) in a sonochemical system to determine which OH scavengers react only in bulk solution and which OH scavengers interact with cavitation bubbles. The ability of each scavenger to interact with cavitation bubbles was assessed by comparing the pulse enhancement (PE) of 10 μM of a probe compound, carbamazepine (CBZ), in the presence and absence of a scavenger. Based on PE results, acetic acid/acetate appears to scavenge OH in bulk solution, and not interact with cavitation bubbles. Methanesulfonate acts as reaction promoter, increasing rather than inhibiting the degradation of CBZ. For formic acid, carbonic acid, terephthalic acid/terephthalate, benzenesulfonate, and iodide, the PE was significantly decreased compared to in the absence of the scavenger. These scavengers not only quench OH in bulk solution but also affect the cavity interface. The robustness of acetic acid/acetate as a bulk OH scavenger was validated for pH values between 3.5 and 8.9 and acetic acid/acetate concentrations from 0.5 to 0.1 M.     

Theoretical estimation of the temperature and pressure within collapsing acoustical bubbles

Formation of highly reactive species such as OH, H, HO₂ and H₂O₂ due to transient collapse of cavitation bubbles is the primary mechanism of sonochemical reaction. The crucial parameters influencing the formation of radicals are the temperature and pressure achieved in the bubble during the strong collapse. Experimental determinations estimated a temperature of about 5000 K and pressure of several hundreds of MPa within the collapsing bubble. In this theoretical investigation, computer simulations of chemical reactions occurring in an O₂-bubble oscillating in water irradiated by an ultrasonic wave have been performed for diverse combinations of various parameters such as ultrasound frequency (20–1000 kHz), acoustic amplitude (up to 0.3 MPa), static pressure (0.03–0.3 MPa) and liquid temperature (283–333 K). The aim of this series of computations is to correlate the production of OH radicals to the temperature and pressure achieved in the bubble during the strong collapse. The employed model combines the dynamic of bubble collapse in acoustical field with the chemical kinetics of single bubble. The results of the numerical simulations revealed that the main oxidant created in an O₂ bubble is OH radical. The computer simulations clearly showed the existence of an optimum bubble temperature of about 5200 ± 200 K and pressure of about 250 ± 20 MPa. The predicted value of the bubble temperature for the production of OH radicals is in excellent agreement with that furnished by the experiments. The existence of an optimum bubble temperature and pressure in collapsing bubbles results from the competitions between the reactions of production and those of consumption of OH radicals at high temperatures.     

IR wavelength-selective laser desorption via OH and CH stretching modes

We present new results on wavelength-selective desorption of solid samples using the resonant interaction between the laser beam and the bulk. The experimental set-up is based on the coupling of three techniques: laser desorption in the near-infrared (IR) (2.7–4 μm) with a tunable IR optical parametric oscillator (OPO), UV multi-photon ionization, and reflectron time-of-flight mass spectrometry. The resonant character of the laser desorption process has been investigated for an ice/polycyclic aromatic hydrocarbon (PAH) mixture, by excitation of the OH and CH stretching modes. Highly preferential desorption has been evidenced, with exclusive desorption of water and PAH molecules at the OH and CH resonances, respectively. Potential analytical (e.g. selective analysis of complex samples) and technological (e.g. dry laser cleaning, DLC) applications are discussed.     

Synthesis,characterization and luminescent property of metal-ion-doped terbium complexes of 2,3-Pyrazinedicarboxylate

Using 2,3-pyrazine dicarboxylate (pzdc^2?) as ligand, a series of new terbium complexes Tb₂L₂(HL)(NO₃)10 H₂O, Tb₂Mg₂L₄(HL)(NO₃)14 H₂O, Tb₂Ca₂L₄(HL)(NO₃)14 H₂O, Tb₂Sr₂L₄(HL)(NO₃)14 H₂O, Tb₂Ba₂L₄(HL)(NO₃)14 H₂O, Tb₂Cd₂L₄(HL)(NO₃)14 H₂O, Tb₂Co₂L₄(HL)(NO₃)14 H₂O, Tb₂Ni₂L₄(HL)(NO₃)14 H₂O and Tb₂Zn₂L₄(HL)(NO₃)14 H₂O (L=pzdc^2-) have been synthesized. The complexes were characterized by elemental analysis, ICP-AES, molar conductivity measurement, TG-DSC analysis, IR spectroscopy and UV absorption spectroscopy. The luminescence spectra, luminescence lifetimes and emission quantum efficiencies of the complexes were measured. The results show that doping alkaline earth metal ions have significantly increased the luminescence intensities and quantum efficiencies of the complexes, and the sequence of the quantum efficiencies of the doped complexes is Ba²⁺>Ca²⁺>Mg²⁺>Sr²⁺. The enhancement of luminescence efficiencies may result from the decrease of the concentration quenching effect of Tb³⁺ ions, intramolecular energy transfer from the ligands coordinated with doped ions to Tb³⁺ ions and the lattice distortion of the complexes. The luminescence efficiencies of the Tb³⁺ ions are also enhanced by doping Cd²⁺ and Zn²⁺ ions. However, the complexes doped with Co²⁺ or Ni²⁺ ions exhibit luminescence quenching, which is caused by the energy consumed by these two ions in the form of d-d electron transitions.     

Thermal activation and reaction of allyl alcohol on Ni(100)

The thermal chemistry of allyl alcohol (CH₂CHCH₂OH) on a Ni(100) single-crystal surface was studied by the temperature programmed desorption (TPD) and the X-ray photoelectron spectroscopy (XPS). The allyl alcohol adsorbs molecularly on the metal surface at 100 K. Intact molecular desorption from the surface occurs at temperatures around 180 K, but some molecules exhibit chemical reactivity on the surface: activation of the OH, CC, and CO bonds produces η¹(O)-allyloxy CH₂CHCH₂O_(a), η²(C, C) allyl alcohol (C_(a)H₂C_(a)HCH₂OH), and η³(C, C, O)-alkoxide (C_(a)H₂C_(a)CH₂ O_(a)) intermediates. Further thermal activation of allyl alcohol on the surface yields propylene (CH₂CHCH₃), 1-propanol (CH₃CH₂CH₂OH), propanal (CH₃CH₂CHO), and combustion and dehydrogenation products (H₂O, H₂, and CO). Propylene desorbs from the surface at temperatures of around 270 K. Hydrogenation to the η³(C, C, O)-alkoxide intermediate leads to the production of propanal which desorbs from the surface around 320 K, while hydrogenation of the η²(C, C) allyl alcohol intermediate produces 1-propanol, which desorbs at around 310 K. The co-adsorption of hydrogen atoms on the surface enhances the formation of the saturated alcohol, while co-adsorption of oxygen enhances the formation of both the saturated alcohol and the saturated aldehydes.     

Enhanced activity for supported Au clusters: Methanol oxidation on Au/TiO2(110)

Gold clusters supported on TiO₂(110) exhibit unusual activity for the oxidation of methanol to formaldehyde. Temperature programmed desorption studies of methanol on Au clusters show that both Au and titania sites are necessary for methanol reaction. Isotopic labeling experiments with CD₃OH demonstrate that reaction occurs via OH bond scission to form a methoxy intermediate. When the TiO₂ surface is oxidized with ¹⁸O₂ before or after Au deposition, methanol reaction produces H₂¹⁸O below 300 K, indicating that oxygen from titania promotes OH bond scission and is incorporated into desorbing products. XPS experiments provide additional evidence that during methanol reaction on the Au/TiO₂ surface, methanol adsorption occurs on TiO₂, given that the titania support becomes slightly oxidized after exposure to methanol in the presence of Au clusters. While the role of TiO₂ is to dissociate the OH bond and form the reactive methoxy intermediate, the role of the Au sites is to remove hydrogen from the surface as H₂, thus preventing the recombination of methoxy and hydrogen to methanol. The decrease in formaldehyde yield with increasing Au coverage above 0.25 ML suggests that reaction occurs at Au–titania interfacial sites; scanning tunneling microscopy images of various Au coverages confirm that the number of interfacial sites at the perimeter of the Au clusters decreases as the Au coverage is increased between 0.25 and 5 ML.