New Insight into the Gas Phase Reaction Dynamics in Pulsed Laser Deposition of Multi-Elemental Oxides.

The gas-phase reaction dynamics and kinetics in a laser induced plasma are very much dependent on the interactions of the evaporated target material and the background gas. For metal (M) and metal-oxygen (MO) species ablated in an Ar and O2 background, the expansion dynamics in O2 are similar to the expansion dynamics in Ar for M+ ions with an MO+ dissociation energy smaller than O2. This is different for metal ions with an MO+ dissociation energy larger than for O2. This study shows that the plume expansion in O2 differentiates itself from the expansion in Ar due to the formation of MO+ species. It also shows that at a high oxygen background pressure, the preferred kinetic energy range to form MO species as a result of chemical reactions in an expanding plasma, is up to 5 eV.

Portable gas chromatograph calibration with gases of varying viscosities.

The quantitative analysis of gas mixtures with gas chromatography is based on the calibration with certified standards and the determination of a response relationship for each species by regression analysis. The conventional assumption of a constant amount-of-substance injected onto the column, the sample size, for all standards analyzed represents one of the largest sources of uncertainies in this analysis technique. For systems using time-based microinjectors, the sample size injected onto the column is determined by the opening time of a pneumatically actuated microvalve and therefore depends upon the gas velocity developed in the injector's microchannel. For this reason the sample size is not necessarily constant for a given opening time, but strongly correlates - according to fluid mechanic theory - with the gas mixture's dynamic viscosity. Neglecting these sample size variations leads to errors in the analysis up to 30%, depending on the diversity of the standards' viscosities, especially in processes with strongly changing hydrogen contents. A mathematical correction exploiting the inverse proportionality of the sample size and the sample's dynamic viscosity normalizes the injection amounts and minimizes the influence of the sample variations on the calibration regression. The data analysis based on the corrected normalized calibration is no longer viscosity dependent and can be applied to gas mixtures of any composition.

Kinetics and Mechanism of Metal Nanoparticle Growth via Optical Extinction Spectroscopy and Computational Modeling: The Curious Case of Colloidal Gold.

An overarching computational framework unifying several optical theories to describe the temporal evolution of gold nanoparticles (GNPs) during a seeded growth process is presented. To achieve this, we used the inexpensive and widely available optical extinction spectroscopy, to obtain quantitative kinetic data. In situ spectra collected over a wide set of experimental conditions were regressed using the physical model, calculating light extinction by ensembles of GNPs during the growth process. This model provides temporal information on the size, shape, and concentration of the particles and any electromagnetic interactions between them. Consequently, we were able to describe the mechanism of GNP growth and divide the process into distinct genesis periods. We provide explanations for several longstanding mysteries, for example, the phenomena responsible for the purple-greyish hue during the early stages of GNP growth, the complex interactions between nucleation, growth, and aggregation events, and a clear distinction between agglomeration and electromagnetic interactions. The presented theoretical formalism has been developed in a generic fashion so that it can readily be adapted to other nanoparticulate formation scenarios such as the genesis of various metal nanoparticles.

Enhanced Proton Conductivity in Y-Doped BaZrO3 via Strain Engineering.

The effects of stress-induced lattice distortions (strain) on the conductivity of Y-doped BaZrO3, a high-temperature proton conductor with key technological applications for sustainable electrochemical energy conversion, are studied. Highly ordered epitaxial thin films are grown in different strain states while monitoring the stress generation and evolution in situ. Enhanced proton conductivity due to lower activation energies is discovered under controlled conditions of tensile strain. In particular, a twofold increased conductivity is measured at 200 °C along a 0.7% tensile strained lattice. This is at variance with conclusions coming from force-field simulations or the static calculations of diffusion barriers. Here, extensive first-principles molecular dynamic simulations of proton diffusivity in the proton-trapping regime are therefore performed and found to agree with the experiments. The simulations highlight that compressive strain confines protons in planes parallel to the substrate, while tensile strain boosts diffusivity in the perpendicular direction, with the net result that the overall conductivity is enhanced. It is indeed the presence of the dopant and the proton-trapping effect that makes tensile strain favorable for proton conduction.

Double modulation pyrometry: A radiometric method to measure surface temperatures of directly irradiated samples.

The design, implementation, calibration, and assessment of double modulation pyrometry to measure surface temperatures of radiatively heated samples in our 1 kW imaging furnace is presented. The method requires that the intensity of the external radiation can be modulated. This was achieved by a rotating blade mounted parallel to the optical axis of the imaging furnace. Double modulation pyrometry independently measures the external radiation reflected by the sample as well as the sum of thermal and reflected radiation and extracts the thermal emission as the difference of these signals. Thus a two-step calibration is required: First, the relative gains of the measured signals are equalized and then a temperature calibration is performed. For the latter, we transfer the calibration from a calibrated solar blind pyrometer that operates at a different wavelength. We demonstrate that the worst case systematic error associated with this procedure is about 300 K but becomes negligible if a reasonable estimate of the sample's emissivity is used. An analysis of the influence of the uncertainties in the calibration coefficients reveals that one (out of the five) coefficient contributes almost 50% to the final temperature error. On a low emission sample like platinum, the lower detection limit is around 1700 K and the accuracy typically about 20 K. Note that these moderate specifications are specific for the use of double modulation pyrometry at the imaging furnace. It is mainly caused by the difficulty to achieve and maintain good overlap of the hot zone with a diameter of about 3 mm Full Width at Half Height and the measurement spot both of which are of similar size.

Increasing the Sensitivity to Short-Lived Species in a Modulated Excitation Experiment.

The combination of spectroscopic and diffraction methods to study chemical transformations is fundamental for the understanding of reaction mechanisms. The identification of short-lived species, likely active species, is often hindered by the contribution of spectator species not directly involved in the reaction. The present study considers two different approaches to obtain increased sensitivity to transient species for experiments obeying the modulated excitation paradigm and exploiting phase sensitive detection (PSD). First, the variation of the frequency of the external stimulation (ω) during the experiment is considered. We demonstrate using the Fourier analysis that the increase of ω, i.e., the decrease of the modulation period T, enhances the sensitivity to short-lived species. The second alternative is the use of a single stimulation frequency (ω) during the measurement and the variation of the demodulation frequency (nω) during data analysis. The absolute intensity of the phase-resolved signals is reduced by increasing n. However, species with slow kinetics are more attenuated than species with fast kinetics. Thus, transient species possessing fast kinetics are enhanced relative to other components and can be, in principle, discerned with improved sensitivity in the phase-resolved data obtained with n > 1. Experimental results in the field of heterogeneous catalysis are provided that support our findings.

Determination of Conduction and Valence Band Electronic Structure of LaTiOx Ny Thin Film.

The nitrogen substitution into the oxygen sites of several oxide materials leads to a reduction of the band gap to the visible-light energy range, which makes these oxynitride semiconductors potential photocatalysts for efficient solar water splitting. Oxynitrides typically show a different crystal structure compared to the pristine oxide material. As the band gap is correlated to both the chemical composition and the crystal structure, it is not trivial to distinguish which modifications of the electronic structure induced by the nitrogen substitution are related to compositional and/or structural effects. Here, X-ray emission and absorption spectroscopy are used to investigate the electronic structures of orthorhombic perovskite LaTiOx Ny thin films in comparison with films of the pristine oxide LaTiOx with similar orthorhombic structure and cationic oxidation state. Experiment and theory show the expected upward shift in energy of the valence band maximum that reduces the band gap as a consequence of the nitrogen incorporation. This study also shows that the conduction band minimum, typically considered almost unaffected by nitrogen substitution, undergoes a significant downward shift in energy. For a rational design of oxynitride photocatalysts, the observed changes of both the unoccupied and occupied electronic states have to be taken into account to justify the total band-gap narrowing induced by the nitrogen incorporation.

Investigating the behavior of various cocatalysts on LaTaON2 photoanode for visible light water splitting.

We performed a comparative study on the photoelectrochemical performance of LaTaON2 loaded with NiOx, Ni0.7Fe0.3Ox, CoOx and IrOx as cocatalysts. Ni-based oxides lead to the highest improvement on the photoelectrochemical performance, while CoOx and IrOx also enhance the performance though to a lower extent, but they simultaneously introduce more pseudocapacitive current thus resulting in an inefficient utilization of the photo-generated holes. Repetitive voltage cycling between 1.0 VRHE and 1.6 VRHE transforms the NiOx and Ni0.7Fe0.3Ox into oxyhydroxides known to possess higher catalytic activities. However, these oxyhydroxides lead to lower photoelectrochemical performance compared to the as-loaded oxides, most probably due to the decay of the passivation centers at the photoelectrode-cocatalyst interface. High catalytic activities cannot be achieved without sufficient passivation of surface recombination states. Despite that the photoelectrochemical performance of LaTaON2 can be improved by cocatalysts, the maximum achievable photocurrent density is still not comparable to that reported for other oxynitride compounds. Our study suggests that poor electronic conductivity or severe bulk recombination of the photo-generated electron-hole pairs are the main limiting factors for the photon-to-current conversion efficiency in LaTaON2 photoanodes.

Synchrotron-Based Highest Resolution Terahertz Spectroscopy of the ν24 Band System of 1,2-Dithiine (C4H4S2): A Candidate for Measuring the Parity Violating Energy Difference between Enantiomers of Chiral Molecules.

The chiral C2 symmetric molecule 1,2-dithiine (1,2-dithia-3,5-hexadiene, C4H4S2) has been identified as a possible candidate for measuring the parity violating energy difference between enantiomers. We report here the observation and analysis of the low-frequency fundamental ν24 using highest resolution synchrotron-based interferometric Fourier transform infrared (FTIR) spectroscopy in the terahertz range with a band center of ν0 = 6.95375559 THz (ν̃0 = 231.952319 (10) cm-1) and two related hot bands, the (ν13 + ν24) ← ν13 band at ν0 = 6.97256882 THz (ν̃0 = 232.579861 (33) cm-1) and the 2ν24 ← ν24 band at ν0 = 7.01400434 THz (ν̃0 = 233.962001 (14) cm-1). This success in the difficult analyses of the THz spectrum of a complex chiral molecule of importance for fundamental tests on molecular parity violation is enabled by the ideal setup of an FTIR experiment of currently unique resolution with the very stable and bright synchrotron radiation at the Swiss Light Source (SLS).

Highly sensitive SnO2 sensor via reactive laser-induced transfer.

Gas sensors based on tin oxide (SnO2) and palladium doped SnO2 (Pd:SnO2) active materials are fabricated by a laser printing method, i.e. reactive laser-induced forward transfer (rLIFT). Thin films from tin based metal-complex precursors are prepared by spin coating and then laser transferred with high resolution onto sensor structures. The devices fabricated by rLIFT exhibit low ppm sensitivity towards ethanol and methane as well as good stability with respect to air, moisture, and time. Promising results are obtained by applying rLIFT to transfer metal-complex precursors onto uncoated commercial gas sensors. We could show that rLIFT onto commercial sensors is possible if the sensor structures are reinforced prior to printing. The rLIFT fabricated sensors show up to 4 times higher sensitivities then the commercial sensors (with inkjet printed SnO2). In addition, the selectivity towards CH4 of the Pd:SnO2 sensors is significantly enhanced compared to the pure SnO2 sensors. Our results indicate that the reactive laser transfer technique applied here represents an important technical step for the realization of improved gas detection systems with wide-ranging applications in environmental and health monitoring control.

In situ stress observation in oxide films and how tensile stress influences oxygen ion conduction.

Many properties of materials can be changed by varying the interatomic distances in the crystal lattice by applying stress. Ideal model systems for investigations are heteroepitaxial thin films where lattice distortions can be induced by the crystallographic mismatch with the substrate. Here we describe an in situ simultaneous diagnostic of growth mode and stress during pulsed laser deposition of oxide thin films. The stress state and evolution up to the relaxation onset are monitored during the growth of oxygen ion conducting Ce0.85Sm0.15O2-δ thin films via optical wafer curvature measurements. Increasing tensile stress lowers the activation energy for charge transport and a thorough characterization of stress and morphology allows quantifying this effect using samples with the conductive properties of single crystals. The combined in situ application of optical deflectometry and electron diffraction provides an invaluable tool for strain engineering in Materials Science to fabricate novel devices with intriguing functionalities.

Partially reduced graphite oxide as an electrode material for electrochemical double-layer capacitors.

Partially reduced graphite oxide was prepared from graphite oxide by using synthetic graphite as precursor. The reduction of graphite oxide with a layer distance of 0.57 nm resulted in a reduction of the layer distance depending on the degree of reduction. Simultaneously the amount of oxygen functionalities in the graphite oxide was reduced, which was corroborated by elemental analysis and EDX. The electrochemical activation of the partially reduced graphite oxide was investigated for tetraethylammonium tetrafluoroborate in acetonitrile and in propylene carbonate. The activation potential depends significantly on the degree of reduction, that is, on the graphene-layer distance and on the solvent used. The activation potential decreased with increasing layer distance for both positive and negative activation. The resulting capacitance after activation was found to be affected by the layer distance, the oxygen functionalities and the used electrolyte. For a layer distance of 0.43 nm and with acetonitrile as the solvent, a differential capacitance of 220 Fg(-1) was achieved for the discharge of the positive electrode near the open-circuit potential and 195 Fg(-1) in a symmetric full-cell assembly.

Sequential printing by laser-induced forward transfer to fabricate a polymer light-emitting diode pixel.

Patterned deposition of polymer light-emitting diode (PLED) pixels is a challenge for electronic display applications. PLEDs have additional problems requiring solvent orthogonality of different materials in adjacent layers. We present the fabrication of a PLED pixel by the sequential deposition of two different layers with laser-induced forward transfer (LIFT), a "dry" deposition technique. This novel use of LIFT has been compared to "normal" LIFT, where all the layers are transferred in a single step, and a conventional PLED fabrication process. For the sequential LIFT, a 50-nm film of an alcohol-soluble polyfluorene (PFN) is transferred onto a receiver with a transparent anode, before an aluminum cathode is transferred on top. Both steps use a triazene polymer dynamic release layer and are performed in a medium vacuum (1 mbar) across a 15 µm gap. The rough morphologies of the single-layer PFN pixels and the PLED device characteristics have been investigated and compared to both bilayer Al/PFN pixels fabricated by normal LIFT and conventionally fabricated devices. The functionality of the sequential LIFT pixels (0.003 cd/A, up to 200 mA/cm(2), at 30-40 V) demonstrates the suitability of LIFT for sequential patterned printing of different thin-film layers.

Electronic structure and oxygen vacancies in PdO and ZnO: validation of DFT models.

PdO is one of the most important catalytic materials currently used in the industry. In redox catalytic reactions involving PdO, the bulk phase is an additional source of oxygen. This leads to strong transformations not only at the surface of PdO but also in the near sub-surface and bulk regions. The redox process is, therefore, governed not only by the extent of PdO d-band filling, but also depends on the material properties of the PdO crystal--the ease with which its structure can be deformed. Methane oxidation is of key industrial interest, and therein the rate of CH(4) conversion depends strongly on the reversible oxygen defects formation on the surface and in the bulk of the catalyst. The present study gives a first insight into these complex phenomena at the atomistic level. Comparison of different density functional theory (DFT) approaches and their capacity to reproduce experimental values of the heat of formation as well as the band gap of the PdO are discussed in detail. Results from DFT calculations for an oxygen vacancy creation in the bulk and on the surface of PdO are presented and compared at the level of accuracy of the implemented approaches with defect calculations for ZnO. Many different modeling approaches based on functionals and pseudopotentials (non-modified PP and empirically tuned) have been evaluated in their aptness to capture key PdO properties. It was shown that simulations with the PP-115 pseudopotential gave the closest possible agreement to the relevant PdO thermodynamic data and energy of oxygen vacancy formation.

Reaction of SO4˙⁻ with an oligomer of poly(sodium styrene sulfonate). Probing the mechanism of damage to fuel cell membranes.

Clarification of the mechanism of degradation of model compounds for polymers used in polymer electrolyte fuel cells may identify intermediates that propagate damage; such knowledge can be used to improve the lifetime of fuel cell membranes, a central issue to continued progress in fuel cell technology. In proton-exchange membranes based on poly(styrene sulfonic acid), hydroxycyclohexadienyl radicals are formed after reaction with HO˙ and thought to decay to short-lived radical cations at low pH. To clarify subsequent reactions, we generated radical cations by reaction of SO(4)˙(-) with oligomers of poly(styrene sulfonic acid) (MW ≈ 1100 Da). At 295 K, this reaction proceeds with k = (4.5 ± 0.6) × 10(8) M(-1) s(-1), both at pH 2.4 and 3.4, and yields benzyl radicals with an estimated yield of ≤60% relative to [SO(4)˙(-)]. The radical cation is too short-lived to be observed: based on a benzyl radical yield of 60%, a lower limit of k > 6.8 × 10(5) s(-1) for the intramolecular transformation of the aromatic radical cation of the oligomer to a benzyl radical is deduced. Our results show that formation of the benzyl radical, an important precursor in the breakdown of the polymer, is irreversible.

Evaporation of urea at atmospheric pressure.

Aqueous urea solution is widely used as reducing agent in the selective catalytic reduction of NO(x) (SCR). Because reports of urea vapor at atmospheric pressure are rare, gaseous urea is usually neglected in computational models used for designing SCR systems. In this study, urea evaporation was investigated under flow reactor conditions, and a Fourier transform infrared (FTIR) spectrum of gaseous urea was recorded at atmospheric pressure for the first time. The spectrum was compared to literature data under vacuum conditions and with theoretical spectra of monomolecular and dimeric urea in the gas phase calculated with the density functional theory (DFT) method. Comparison of the spectra indicates that urea vapor is in the monomolecular form at atmospheric pressure. The measured vapor pressure of urea agrees with the thermodynamic data obtained under vacuum reported in the literature. Our results indicate that considering gaseous urea will improve the computational modeling of urea SCR systems.

Laser-induced forward transfer of polymer light-emitting diode pixels with increased charge injection.

Laser-induced forward transfer (LIFT) has been used to print 0.6 mm × 0.5 mm polymer light-emitting diode (PLED) pixels with poly[2-methoxy, 5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the light-emitting polymer. The donor substrate used in the LIFT process is covered by a sacrificial triazene polymer (TP) release layer on top of which the aluminium cathode and functional MEH-PPV layers are deposited. To enhance electron injection into the MEH-PPV layer, a thin poly(ethylene oxide) (PEO) layer on the Al cathode or a blend of MEH-PPV and PEO was used. These donor substrates have been transferred onto both plain indium tin oxide (ITO) and bilayer ITO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) blend) receiver substrates to create the PLED pixels. For comparison, devices were fabricated in a conventional manner on ITO substrates coated with a PEDOT:PSS hole-transporting layer. Compared to multilayer devices without PEO, devices with ITO/PEDOT:PSS/MEH-PPV:PEO blend/Al architecture show a 100 fold increase of luminous efficiency (LE) reaching a maximum of 0.45 cd/A for the blend at a brightness of 400 cd/m(2). A similar increase is obtained for the polymer light-emitting diode (PLED) pixels deposited by the LIFT process, although the maximum luminous efficiency only reaches 0.05 cd/A for MEH-PPV:PEO blend, which we have attributed to the fact that LIFT transfer was carried out in an ambient atmosphere. For all devices, we confirm a strong increase in device performance and stability when using a PEDOT:PSS film on the ITO anode. For PLEDs produced by LIFT, we show that a 25 nm thick PEDOT:PSS layer on the ITO receiver substrate considerably reduces the laser fluence required for pixel transfer from 250 mJ/cm(2) without the layer to only 80 mJ/cm(2) with the layer.

Damage to fuel cell membranes. Reaction of HO* with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O(2).

An understanding of the reactivity of oligomeric compounds that model fuel cell membrane materials under oxidative-stress conditions that mimic the fuel cell operating environment can identify material weaknesses and yield valuable insights into how a polymer might be modified to improve oxidative stability. The reaction of HO˙ radicals with a polymer electrolyte fuel cell membrane represents an initiation step for irreversible membrane oxidation. By means of pulse radiolysis, we measured k = (9.5 ± 0.6) × 10(9) M(-1) s(-1) for the reaction of HO˙ with poly(sodium styrene sulfonate), PSSS, with an average molecular weight of 1100 Da (PSSS-1100) in aqueous solution at room temperature. In the initial reaction of HO˙ with the oligomer (90 ± 10)% react by addition to form hydroxycyclohexadienyl radicals, while the remaining abstract a hydrogen to yield benzyl radicals. The hydroxycyclohexadienyl radicals react reversibly with dioxygen to form the corresponding peroxyl radicals; the second-order rate constant for the forward reaction is k(f) = (3.0 ± 0.5) × 10(7) M(-1) s(-1), and for the back reaction, we derive an upper limit for the rate constant k(r) of (4.5 ± 0.9) × 10(3) s(-1). These data place a lower bound on the equilibrium constant K of (7 ± 2) × 10(3) M(-1) at 295 K, which allows us to calculate a lower limit of the Gibbs energy for the reaction, (-21.7 ± 0.8) kJ mol(-1). At pH 1, the hydroxycyclohexadienyl radicals decay with an overall first-order rate constant k of (6 ± 1) × 10(3) s(-1) to yield benzyl radicals. The second-order rate constant for reaction of dioxygen with benzyl radicals of PSSS-1100 is k = (2-5) × 10(8) M(-1) s(-1). We discuss hydrogen abstraction from PSSS-1100 in terms of the bond dissociation energy, and relate these to relevant electrode potentials. We propose a reaction mechanism for the decay of hydroxycyclohexadienyl radicals and subsequent reaction steps.

Neutron reflectometry and spectroscopic ellipsometry studies of cross-linked poly(dimethylsiloxane) after irradiation at 172 nm.

Poly(dimethylsiloxane) (PDMS) was irradiated under ambient conditions in air with a Xe2-excimer lamp. The formation of atomic oxygen and ozone during irradiation in air by V-UV photons results in the transformation of PDMS to silicon oxide. The irradiated surfaces were studied by spectroscopic ellipsometry and neutron reflectometry. The measurements revealed the formation of a rough, i.e., between 11 and 20 nm, oxidized surface layer and a decrease of the total layer thickness. The thickness of the oxidized layer decreased for a given PDMS thickness when the polymer was irradiated for longer times and/or higher intensities. The composition of the oxidized layer after irradiation was not uniform through the layer and consisted of a mixture of original polymer and silicon bonded to three or four oxygen atoms (SiOx). The refractive index n determined by ellipsometry reaches a value similar to values reported for SiO2.

Nanoparticles in energy technology: examples from electrochemistry and catalysis.

Nanoparticles are key components in the advancement of future energy technologies, thus, strategies for preparing nanoparticles in large volume by techniques that are cost-effective are required. In the substitution of fossil-fuels by renewable energy resources, nanometer-sized particles play a key role for synthesizing energy vectors from varying and heterogeneous biomass feedstocks. They are extensively used in reformers for the production of hydrogen from solid, liquid, or gaseous energy carriers. Catalyst activities depend critically on their size-dependent properties. Nanoparticles are further indispensable as electrocatalysts in fuel cells and other electrochemical converters. The desire to increase the activity per unit area, and decrease the necessary amount of the expensive catalytic standard, platinum, has spurred innovative approaches for the synthesis of platinum-alloy nanoparticles by wet chemistry, colloidal routes, or physical techniques such as sputtering.