The photodouble ionization of water at about 32 eV excess energy has been investigated both experimentally and theoretically. In an energy and angular resolved photoelectron–photoelectron coincidence experiment, the two photoelectrons in unequal energy sharing (25 and 7 eV) condition, have been detected in a plane perpendicular to the propagation direction of the linearly polarized radiation. The measured angular distributions have been compared with, molecular orientation averaged, triple differential cross sections calculated with a recently developed theoretical model (Randazzo et al 2020 Phys. Rev. A 101 033407). The model uses separable products of orbitals as initial electronic state of the water molecule taken as a two-electron target and describes accurately the correlated two electron continuum. The combination of calculated cross sections corresponding to different dication states capture most of the measured features in terms of the evolution of both the shape and the intensity as a function of the faster electron direction which is set at 0°, 30° and 60° with respect to the polarization vector ε⃗ of the incident radiation. A detailed analysis in terms of the different dication states as well as of the partial wave contributions to the electron pair wave function sheds light on the origin of such features.

Synopsis The photodouble ionization of water has been studied experimentally by photoelectron-photoelectron angular resolved coincidence experiments and theoretically by applying the Generalized Sturmian Functions approach. The measured differential cross sections present a rich angular distribution that can be, in part, reproduced by a first order treatment of the interaction considering a two active electrons target.

The photodouble ionization of the water molecule is studied at 20 eV excess energy in a combined experimental and theoretical investigation. In the experiments, two photoelectrons of equal kinetic energy are detected in coincidence after energy and angular selection. On the theoretical side, a generalized Sturmian function approach is implemented to describe accurately the correlated two-electron continuum, while separable products of Moccia orbitals [J. Chem. Phys. 40, 2164 (1964)] are used for the initial electronic state of the water molecule. The theoretical triple-differential cross sections (TDCSs) are averaged over all possible molecular orientations in order to be compared with the experiments. The measured TDCSs display rich angular distributions that are in large part well reproduced by the adopted first-order treatment of the interaction with a two-active-electron target.

The photodouble ionization of the water molecule has been studied at about 20 eV excess energy by detecting in coincidence two photoelectrons of equal energy. Two types of measurements have been performed. In the first one the photon energy has been scanned in order to reconstruct the spectrum of the H2O2+ states; in the second, the photon energy has been fixed, the direction of one photoelectron has been chosen along the direction of the polarization axis of the incident radiation and the coincidence angular distribution of the two photoelectrons has been measured. The angular distributions show that, at variance with previous measurements in diatomic molecules, the emission of the two electrons along the antiparallel direction is highly favored.

The photocatalytic (PC) activity of anatase titania nanoparticles can be improved through codoping with transition metals and nitrogen. In addition, the PC activity can also be improved by creating monodisperse, mesoporous nanoparticles of titania. The question naturally arose as to whether combining these two characteristics would result in further improvement in the PC activity or not. Herein, we describe the synthesis and photocatalytic characteristics of codoped, monodisperse anatase titania. The transition metals tested in the polydisperse and the monodisperse forms were Mn, Co, Ni, and Cu. In each case, it was found that the monodisperse version had a higher PC activity compared to the corresponding polydisperse version.

The composition of anatase TiO2 was modified by codoping using combinations of a transition metal and nitrogen in order to increase its photocatalytic activity and extend it performance in the visible region of the electromagnetic spectrum. The transition metals (Mn, Co, Ni, Cu) were added during the hydrothermal preparation of mesoporous TiO2 particles, and the nitrogen was introduced by post-annealing in flowing ammonia gas at high temperature. The samples were analyzed by SEM, XRD, BET, inductively-coupled plasma spectroscopy, and diffuse reflectance UV–vis spectroscopy. The photocatalytic activity was assessed by observing the change in methylene blue concentrations under both UV–vis and visible-only light irradiation. The photocatalytic activity of the (Mn,N), (Co,N), (Cu,N), and Ni,N) codoped TiO2 was significantly enhanced relative to (N) TiO2.

There is great interest in improving TiO2’s photocatalytic activity in the visible portion of electromagnetic spectrum. Recent work has shown that co-doping mesoporous TiO2 microparticles with a transition metal and nitrogen, hereby designated as (M,N) TiO2, significantly increases its visible light absorption. However, the hydrothermal method used to produce the microparticles creates a wide distribution in the size of the microparticles, which could affect the absorption properties. Recently, it has become possible to produce monodisperse, mesoporous TiO2 microparticles with engineered sizes using a hybrid sol-gel/hydrothermal technique. Further, it has also been shown that the size of monodisperse TiO2 microparticles affects the the photocatalytic activity. This study investigated whether using mondodisperse (M,N) TiO2 microparticles would further increase visible-light absorption for (M,N)TiO2. The first-row transition metals chosen for this study - Mn, Fe, Co, Ni, and Cu – have been characterized in the earlier (M,N) TiO2 UV-vis study, which was used as a baseline. The doping levels of the transition metals samples were set at the 2.5 percent level previously shown to be optimum for photocatalytic activity.

By exploiting phase-separation in oxide materials, we present a simple and potentially low-cost approach to create exceptional superhydrophobicity in thin-film based coatings. By selecting the TiO2–Cu2O system and depositing through magnetron sputtering onto single crystal and metal templates, we demonstrate growth of nanostructured, chemically phase-segregated composite films. These coatings, after appropriate chemical surface modification, demonstrate a robust, non-wetting Cassie–Baxter state and yield an exceptional superhydrophobic performance, with water droplet contact angles reaching to ∼172° and sliding angles <1°. As an added benefit, despite the photo-active nature of TiO2, the chemically coated composite film surfaces display UV stability and retain superhydrophobic attributes even after exposure to UV (275 nm) radiation for an extended period of time. The present approach could benefit a variety of outdoor applications of superhydrophobic coatings, especially for those where exposure to extreme atmospheric conditions is required.

Nanorods composed of complex oxides have been synthesized using hydrothermal and sol-gel methods, but pulsed-laser deposition (PLD) provides precise, layer-by-layer control of growth, and is the method of choice for synthesizing complex structures. However, producing complex-oxide nanorods by PLD has proved elusive. Here we report on our efforts to produce nanorods composed of the best-understood complex oxide, strontium titanate (STO). The results suggest it is indeed possible to produce STO nanorods via PLD by using a template of MgO nanorods.

Pulsed electron deposition (PED) is an attractive alternative to pulsed laser deposition (PLD) for growing thin films because of PED’s relatively low cost. A potential problem with PED, however, is the generation of particulates that interfere with film growth. The influence of ambient pressure and accelerating potential on the number of and size of particulates appearing on the surface of films was investigated for the barium fluoride-based YBCO precursor process. It was found that the size of the particulates varies exponentially with accelerating voltage. The size of the particulates can be reduced to less than 100nm by increasing the ambient pressure beyond that required for optimum deposition rate. The ability to control the size of particulates could make PED useful for technical applications where the generation of sub-micron sized materials is desired.

We investigated the dependence of critical current density (Jc) on thickness of Yba2Cu3O7−δ (YBCO) films grown by pulsed laser deposition on (100) SrTiO3 (STO) and on rolling-assisted biaxially textured substrates (RABiTS). The thickness of YBCO films varied from 0.19 to 3 μm. The highest Jcs of 5.3 and 2.6 MA/cm2 at 77 K, self-field were obtained for 0.19-μm YBCO films on STO and RABiTS, respectively. Jc was found to decrease exponentially with YBCO thickness on both substrates. However, the results suggest different mechanisms are responsible for the Jc reduction in the two cases. On STO, growth of a-axis grains within c-axis films and broadening of the in-plane texture were observed in thick films. On RABiTS, degradation in cube texture as well as development of a porous surface morphology were found to correlate with film thickness.

Transmission electron microscopy was used to study the microstructure of pulsed laser deposited YBa2Cu3O7−δ (YBCO) layers on four different biaxially textured Ni-substrates with epitaxial oxide buffer layers. The oxide layer sequence, YBCO/YSZ/CeO2/Ni (YSZ signifies yttria-stabilized zirconia) was the same in each sample, but different deposition techniques were used to deposit the buffer layers. The thicknesses of all three oxide layers varied from sample to sample. A columnar sub-grain structure composed of equiaxed sub-grains in the plane of the film was observed in the YBCO layer of each sample. The sub-grain shape and local mosaic spread (∼3°) appeared to be largely insensitive to the surface topography, density, or method of depositing the YSZ layer immediately below. The sub-grain size (0.2–1.2 μm) increased with film thickness (0.2–2.3 μm), but showed no obvious correlation with the microstructure of the underlying YSZ. The measured misorientation angles (θ) at individual sub-grain boundaries suggested that a large fraction of the boundaries in the sub-grain boundary network have θ in the range of 2–3°. For most boundaries, the measured θ was below the 5° “threshold” angle for weak link behavior. Thus, a representative picture of the sub-grain boundary network above a single grain of nickel in a film of sub-micron thickness might be a honeycomb shaped arrangement of 2.5° [001] tilt boundaries spaced 0.25 μm apart and containing dislocations at a spacing of 10 nm. This structure probably has consequences for the structure of grain boundaries in the YBCO that are replicated from the Ni template below. The superposition of the Ni grain boundary network with the grain boundary network due to mosaic spread almost certainly introduces substantial variations of θ along such boundaries on the sub-micron to micron length scale.

A YBa2Cu3O7−δ (YBCO) film with a transport critical current density (Jc) value of 1 mA/cm2 (77 K, 0 T) was grown on a solution deposited NdGaO3 (NGO) buffer layer on (100) SrTiO3 (STO). The 25-nm thick NGO buffer layer was dip-coated onto the STO single crystal from a solution of metal methoxyethoxides in 2-methoxyethanol. Pulsed laser deposition (PLD) was used to grow a 250-nm-thick YBCO film on the NGO. The epitaxial relationships are cube-on-cube throughout the structure when the pseudo cubic and pseudo tetragonal unit cells are used to describe the NGO and YBCO crystal structures, respectively: (001) YBCO∥(001) NGO∥(001) STO and [100] YBCO∥[100] NGO∥[100] STO. High resolution scanning electron microscopy (SEM) of the bare NGO surface revealed ∼40 nm diameter pinholes with number density of ∼2×1013 m−2, corresponding to an area fraction coverage of 2.5%, in an otherwise featureless surface. Cross-sectional transmission electron microscopy (TEM) showed that these pinholes penetrate to the STO; otherwise the NGO layer was uniformly thick to within approximately ±5 nm and defect free. The X-ray diffraction φ- and ω-scans indicated that the YBCO film was highly oriented with a full-width-half maximum peak breadth of 1.14° for in-plane and 0.46° for out-of-plane alignment, respectively. The film contained sparse a-axis oriented grains, an appreciable density of (001) stacking faults and apparently insulating second phase precipitates of the type that typically litter the surface of PLD films. All of these defects are typical of YBCO thin films. High-resolution cross-sectional TEM images indicate that no chemical reaction occurs at the YBCO/NGO interface.

In continuation of our effort to develop single buffer layer architectures for YBCO (YBa 2 Cu 3 O 7-y ) coated tape conductors, we have studied RE 2 O 3 (RE = Y, and rare earths) as candidate materials. Three types of crystal structures including the preferred cubic phase are known for the rare earth oxides. High quality simple cubic RE 2 O 3 buffer layers were grown epitaxially on {100} textured Ni substrates using both reactive evaporation and sol-gel processing. Detailed X-ray studies have shown that the Y 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , and Yb 2 O 3 were grown with a single epitaxial orientation. SEM micrographs indicated that both e-beam and sol-gel grown films were dense, continuous and crack free. High J c YBCO films were grown on RE 2 O 3 -buffered Ni substrates with sputtered cap layers. Two new alternative buffer layer architectures were developed. A high J c of 1.8 MA/cm 2 at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (pulsed laser deposition)/Yb 2 O 3 (sputtered)/Y 2 O 3 (e-beam)/Ni. Also, a high J c of over 1 MA/cm 2 at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (ex-situ BaF 2 process)/CeO 2 (sputtered)/YSZ (sputtered)/RE 2 O 3 (sol-gel or e-beam)/Ni. The performance of sol-gel grown buffers approached the quality of e-beam grown buffers.

Although high critical current density (J c ) YBCO can be deposited on rolling assisted biaxially-textured substrates (RABiTS) with the YSZ/CeO2/Ni architecture, improvement in uniformity is needed due to the presence of two-component YBCO epitaxy and cracking in the CeO2 buffer. We have determined that Yb2O3 is an excellent buffer material that provides a single-component YBCO epitaxy. In addition, crack-free epitaxial Y2O3 can be consistently deposited onto textured Ni substrates. High quality YBCO films have been deposited, and J c as high as 1.8 ×106 A/cm2 at 77 K has been obtained on this Yb2O3/Y2O3/Ni alternative RABiTS architecture.

In an effort to develop alternative single buffer layer architectures for YBCO (YBa2Cu3O7-y) coated conductors, we have studied RE2O3 (RE = Y, and rare earths) as candidate materials. High-quality Y2O3, Gd2O3 and Yb2O3 buffer layers were grown epitaxially on biaxially textured Ni (100) substrates using reactive electron beam evaporation. Using thermodynamic considerations for the formation of metal oxides, we employed both reducing atmospheres and water vapour to oxidize the film in situ to form stoichiometric RE2O3. We have also prevented NiO formation at the substrate-film interface during this process. Detailed x-ray studies have shown that the Y2O3, Gd2O3 and Yb2O3 films were grown with a single epitaxial orientation. The lattice mismatch between YBCO and Gd2O3 was small as compared with that of YBCO with other rare earth oxides. SEM micrographs indicated that ~0.5 µm thick Y2O3 films on rolled-Ni substrates were dense, continuous and crack free. A high Jc of 1.8 × 106 A cm-2 at 77 K and self-field was obtained on YBCO films grown on alternative buffer layers with a layer sequence of YBCO/Yb2O3 (sputtered)/Y2O3 (e-beam)/Ni.

A reel-to-reel, electron beam evaporation system has been developed to continuously deposit epitaxial CeO/sub 2/ and other oxide buffer layers on meter-long lengths of biaxially textured Ni tapes. The deposition system includes two interconnected electron beam evaporation chambers and a chamber in which as-rolled Ni tape is in situ annealed to develop biaxial texture. An integral reel-to-reel system with tension control enables motion of the tape with little or no plastic deformation. When depositing epitaxial oxides on Ni, the formation of unfavorably oriented NiO is difficult to avoid. Oxide free, {100}<100> oriented Ni tapes are prepared by control of the partial pressures of H/sub 2/, H/sub 2/O and O/sub 2/ during Ni annealing. X-ray /spl phi/-scans have been performed as a function of length to determine the crystallographic consistency of the epitaxial CeO/sub 2/ over length. Results of SEM examinations of the CeO/sub 2/ buffer layer microstructure are presented. Results for YBCO films deposited on short segments of these buffered substrates are summarized.

In continuation of our effort to develop single buffer layer architectures for YBCO (YBa 2 Cu 3 O 7- g ) coated tape conductors, we have studied RE 2 O 3 (RE = Y, and rare earths) as candidate materials. Three types of crystal structures including the preferred cubic phase are known for the rare earth oxides. High quality simple cubic RE 2 O 3 buffer layers were grown epitaxiahy on {100} textured Ni substrates using both reactive evaporation and sol-gel processing. Detailed X-ray studies have shown that the Y 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , and Yb 2 O 3 were grown with a single epitaxial orientation. SEM micrographs indicated that both e-beam and sol-gel grown films were dense, continuous and crack free. High J c YBCO films were grown on RE 2 O 3 -buffered Ni substrates with sputtered cap layers. Two new alternative buffer layer architectures were developed. A high J c of 1.8 MA/cm 2 at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (pulsed laser deposition)/Yb 2 O 3 (sputtered)/Y 2 O 3 (e-beam)/Ni. Also, a high J c of over 1 MA/cm 2 at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (ex-situ BaF 2 process)/CeO 2 (sputtered)YSZ sputtered)/RE 2 O 3 (sol-gel or e-beam)Ni. The performance of sol-gel grown buffers approached the quality of e-beam grown buffers.

A reel-to-reel, electron beam evaporation system has been developed to continuously deposit epitaxial CeO2 and other oxide buffer layers on meter long-lengths of rolled Ni tapes. The deposition system includes two interconnected electron beam evaporation chambers and a chamber in which as-rolled Ni tape is in situ annealed to develop biaxial texture. An integral reel-to-reel system with tension control enables motion of the tape with little or no plastic deformation. When depositing epitaxial oxides on Ni, the formation of unfavorably oriented NiO needs to be avoided. Oxide-free, {100}[100]-oriented Ni tapes are prepared by control of the partial pressure of H-2 during Ni annealing. In situ annealed rolled Ni tape over 70 cm long and 1 cm wide has been deposited with an epitaxial CeO2 buffer layer. Critical current density as high as 700 000 A/cm(2) at 77 K in self-field has been achieved for TiBa2Cu3O7-x (YBCO) films deposited on short segments of these buffered substrates. X-ray in-plane phi-scans have been performed as a function of length to determine the crystallographic consistency of the epitaxial CeO2 over length. Results of scanning electron microscopy (SEM) examinations of the CeO2 buffer layer microstructure are presented. (C) 1999 Published by Elsevier Science B.V. All rights reserved.