Low Platinum-Loaded Molybdenum Co-catalyst for the Hydrogen Evolution Reaction in Alkaline and Acidic Media.

Developing an efficient catalytic system for electrolysis with reduced platinum (Pt) loading while maintaining performance comparable to bulk platinum metal is important to decrease costs and improve scalability of the hydrogen fuel economy. Here we report the performance of a novel sputter-deposited molybdenum (Mo) thin film with an extremely low co-loading of Pt, where Pt atoms were dispersed on Mo (Ptd-Mo) as an electrocatalyst for the hydrogen evolution reaction (HER) in either alkaline or acidic media. The Ptd-Mo electrocatalyst presents similar catalytic activity to bulk Pt in alkaline media, while the performance is only slightly decreased in acidic media. Differential electrochemical mass spectrometry (DEMS) results confirm that the Ptd-Mo electrocatalyst produced hydrogen at a rate comparable with that of a pristine Pt sample at the same potential. A comparison with Pt-loaded degenerately doped p-type doped silicon (Ptd-Si) suggests that Mo and Pt work synergistically to boost the performance of Ptd-Mo catalysts. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) before and after 1000 cycles of continuous operation confirm the significant durability of the Ptd-Mo performance. Overall, the Ptd-Mo electrocatalyst, with comparable HER activity to bulk Pt despite an ultra-low Pt loading, could be a strong candidate for hydrogen production in either acidic or basic conditions.

Review: Geometric interpretation of reflection and transmission RHEED patterns.

Reflection high-energy electron diffraction (RHEED) is widely used to characterize the surface structure of single crystals. Moreover, RHEED has become a standard tool to monitor thin film growth in molecular beam epitaxy and is used to monitor other vapor deposition techniques including evaporation, sputtering, and pulsed laser deposition. With the rapid development of the fabrication methods and use of nanoparticles, RHEED operating in the transmission mode is being applied to characterize nanoparticles on surfaces. In this review, the fundamentals needed to interpret RHEED patterns from the top few atomic layers, in its reflection mode, and from nanoparticles and nanofeatures, in its transmission mode, are discussed based on the geometric kinematic approximation. Examples are provided on the interpretation of RHEED patterns from unreconstructed and 2 × 1-reconstructed Si(100), InP(100), highly oriented pyrolytic graphite, indium nanoparticles, and indium growth on Si(100)- 2 × 1.

Carbon multicharged ion generation from laser-spark ion source.

Multicharged carbon ions are generated by using a laser-assisted spark-discharge ion source. A Q-switched Nd:YAG laser pulse (1064 nm, 7 ns, ≤ 4.5 × 109 W/cm2) focused onto the surface of a glassy carbon target results in its ablation. The spark-discharge (∼1.2 J energy, ∼1 µs duration) is initiated along the direction of the plume propagation between the target surface and a grounded mesh that is parallel to the target surface. Ions emitted from the laser-spark plasma are detected by their time-of-flight using a Faraday cup. The ion energy-to-charge ratio is analyzed by a three-mesh retarding field analyzer. In one set of experiments, the laser plasma is generated by target ablation using a 50 mJ laser pulse. In another set of experiments, ∼1.2 J spark-discharge energy is coupled to the expanding plasma to increase the plasma density and temperature that results in the generation of carbon multicharged ions up to C6+. A delay-generator is used to control the time delay between the laser pulse and the thyratron trigger. Ion generation from a laser pulse when a high DC voltage is applied to the target is compared to that when a spark-discharge with an equivalent pulsed voltage is applied to the target. The laser-coupled spark-discharge (7 kV peak voltage, 810 A peak current) increases the maximum detected ion charge state from C4+ to C6+, accompanied by an increase in the ion yield by a factor of ∼6 compared to applying 7.0 kV DC voltage to the target.

Multicharged carbon ion generation from laser plasma.

Carbon ions generated by ablation of a carbon target using an Nd:YAG laser pulse (wavelength λ = 1064 nm, pulse width τ = 7 ns, and laser fluence of 10-110 J cm-2) are characterized. Time-of-flight analyzer, a three-mesh retarding field analyzer, and an electrostatic ion energy analyzer are used to study the charge and energy of carbon ions generated by laser ablation. The dependencies of the ion signal on the laser fluence, laser focal point position relative to target surface, and the acceleration voltage are described. Up to C4+ ions are observed. When no acceleration voltage is applied between the carbon target and a grounded mesh in front of the target, ion energies up to ∼400 eV/charge are observed. The time-of-flight signal is analyzed for different retarding field voltages in order to obtain the ion kinetic energy distribution. The ablation and Coulomb energies developed in the laser plasma are obtained from deconvolution of the ion time-of-flight signal. Deconvolution of the time-of-flight ion signal to resolve the contribution of each ion charge is accomplished using data from a retarding field analysis combined with the time-of-flight signal. The ion energy and charge state increase with the laser fluence. The position of the laser focal spot affects the ion generation, with focusing ∼1.9 mm in front of the target surface yielding maximum ions. When an external electric field is applied in an ion drift region between the target and a grounded mesh parallel to the target, fast ions are extracted and separated, in time, due to increased acceleration with charge state.

Synthesis of Cerium-Doped Titania Nanoparticles and Nanotubes.

Cerium-doped titania nanoparticles and nanotubes were synthesized via hydrothermal processes. X-Ray Diffraction revealed that cerium-doped titania nanoparticles have an anatase crystal structure, while cerium-doped titania nanotubes have an H2Ti3O7-type structure. Scanning electron microscopy and high resolution transmission electron microscopy showed that both types of titania are well crystallized with relatively uniform size distribution. The photocatalytic degradation of methylthioninium chloride known as methylene blue dye was tested and both cerium-doped titania nanoparticles and nanotubes. The preliminary photocatalytic degradation of Methylene Blue data showed significantly improved visible light photocatalytic activities as compared to commercial titania powders.

Spark discharge coupled laser multicharged ion source.

A spark discharge is coupled to a laser multicharged ion source to enhance ion generation. The laser plasma triggers a spark discharge with electrodes located in front of the ablated target. For an aluminum target, the spark discharge results in significant enhancement in the generation of multicharged ions along with higher charge states than observed with the laser source alone. When a Nd:YAG laser pulse (wavelength 1064 nm, pulse width 7.4 ns, pulse energy 72 mJ, laser spot area on target 0.0024 cm(2)) is used, the total multicharged ions detected by a Faraday cup is 1.0 nC with charge state up to Al(3+). When the spark amplification stage is used (0.1 µF capacitor charged to 5.0 kV), the total charge measured increases by a factor of ∼9 with up to Al(6+) charge observed. Using laser pulse energy of 45 mJ, charge amplification by a factor of ∼13 was observed for a capacitor voltage of 4.5 kV. The spark discharge increases the multicharged ion generation without increasing target ablation, which solely results from the laser pulse. This allows for increased multicharged ion generation with relatively low laser energy pulses and less damage to the surface of the target.

High-throughput ultrasensitive characterization of chemical, structural and plasmonic properties of EBL-fabricated single silver nanoparticles.

Electron beam lithography (EBL) has become a popular means to prepare a wide variety of nano-arrays for numerous studies and applications, including photonics and sensors. Their fabrications and characterizations are costly and time consuming, underscoring the importance of developing effective tools to rapidly study their physicochemical stabilities and properties over time. In this study, we characterized EBL-fabricated single silver nanoparticle (Ag NP) arrays over their 12-week exposure to ambient conditions using SEM/EDS, AFM and dark-field optical microscopy and spectroscopy (DFOMS). We found that chemical compositions, structural morphologies and plasmonic optical properties of single NPs altered drastically over the exposure. Single cuboid and triangular-prism Ag NPs degraded at rates of (0.74 ± 0.02) and (0.66 ± 0.02) per week, and their localized surface plasmon resonance (LSPR) spectra showed striking blue-shifts (171 ± 25 and 203 ± 35 nm) over the 12-week exposure, respectively. Plasmonic colors of single NPs changed distinctively from red to green over the 12-week exposure. The LSPR spectra of individual NPs in each array were acquired simultaneously and correlated specifically with their SEM and AFM images, demonstrating that DFOMS can serve as high-throughput, ultrasensitive and non-invasive means to characterize chemical, structural and optical properties of nano-arrays in situ in real time at single-NP resolution.

Localized surface plasmon resonance of single silver nanoparticles studied by dark-field optical microscopy and spectroscopy.

Localized surface plasmon resonance (LSPR) of Ag nanoparticles (NPs) with different shapes and disk-shaped Ag NP pairs with varying interparticle distance is studied using dark-field optical microscopy and spectroscopy (DFOMS). Disk-, square-, and triangular-shaped Ag NPs were fabricated on indium tin oxide-coated glass substrates by electron beam lithography. The LSPR spectra collected from single Ag NPs within 5×5 arrays using DFOMS exhibited pronounced redshifts as the NP shape changed from disk to square and to triangular. The shape-dependent experimental LSPR spectra are in good agreement with simulations using the discrete dipole approximation model, although there are small deviations in the peak wavelengths for square- and triangular-shaped NPs. The LSPR spectra of disk-shaped Ag NP pairs with varying interparticle distances were acquired from five different locations across the pair axis. It was clearly observed that the LSPR wavelength redshifts as the interparticle distance decreases, indicating a strong interaction when two Ag NPs are close to each other.

Identification of single nanoparticles.

The physicochemical properties of nanomaterials significantly depend on their three-dimensional (3D) morphologies (sizes, shapes and surface topography), the surrounding media, and their spatial arrangement. Systematically and precisely correlating these parameters with the related physicochemical properties of specific single nanoparticles (NPs) is a fundamental requirement for the discovery of their novel properties and applications, as well as for advancing the fundamental and practical knowledge required for the design and fabrication of new materials. In this article, the progress in the identification of the specific individual NP is summarized, including the in situ methods and the spatial-localization methods based on plasmonic NPs as model. Identification of single NPs based on local surface plasmon resonance observed by fluorescent inverted optical microscopy, dark-field microscopy, scanning near-field optical microscopy, atomic force microscopy, and transmission electron microscope are reviewed. Recent progress in the investigation of 3D morphology-dependent optical properties by these methods is described. Experimental and theoretical developments in single-NP identification for the purpose of understanding the physicochemical properties are discussed.

Copper cation removal in an electrokinetic cell containing zeolite.

Zeolites are used in environmental remediation of soil or water to immobilize or remove toxic materials by cation exchange. An experiment was conducted to test the use a low electric field to direct the toxic cations towards the zeolite. An electrokinetic cell was constructed using carbon electrodes. Synthetic Linde Type A (LTA) zeolite was placed in the cell. Copper(II) chloride dissolved in water was used as a contaminant. The Cu(2+) concentration was measured for ten hours with and without an applied electric field. The removal of the Cu(2+) ions was accelerated by the applied field in the first two hours. For longer time, the electric field did not improve the removal rate of the Cu(2+) ions. The presence of zeolite and applied electric field complicates the chemistry near the cathode and causes precipitation of Cu(2+) ions as copper oxide on the surface of the zeolite. With increased electric field the zeolite farther away from the cathode had little cation exchange due to the higher drift velocity of the Cu(2+) ions. The results also show that, in the LTA Zeolite A pellets, the cation exchange of Cu is limited to a shell of several tens of micrometers.

Correlation and Characterization of 3D Morphological Dependent Localized Surface Plasmon Resonance Spectra of Single Silver Nanoparticles Using Dark-field Optical Microscopy and Spectroscopy and AFM.

We have developed a new and effective methodology to correlate optical and AFM images of single Ag nanoparticles (NPs), allowing us to study 3D-morphological dependent localized surface plasmon resonance (LSPR) spectra of individual Ag NPs. We fabricated arrays of distinctive microwindows on glass coverslips using photo-lithography method, and created well-isolated individual Ag NPs with a wide variety of shapes and morphologies on the glass coverslips using a modified nanosphere lithography method (NSL). Using distinctive geometries of microwindows, we located individual Ag NPs of interest in their optical and AFM images, enabling us to correlate and characterize the LSPR spectra and 3D morphologies of the same single NPs using dark-field optical microscopy and spectroscopy (DFOMS) and AFM, respectively. We found that LSPR spectra of single Ag NPs, with nearly equal volume [(8.6 ± 0.4) × 10(3) nm(3)], cross-section [(2.2 ± 0.2) × 10(2) nm(3)], and height (39.6 ± 3.6 nm), highly depend on their shapes, showing the red shift of peak wavelength to 629 nm (quasi trapezoidal cylindrical NP) from that of 506 nm (quasi circular cylindrical NP). LSPR spectra of single Ag NPs simulated using discrete dipole approximation (DDA) agree well with those measured experimentally when their shapes and morphologies can be accuractely described in both methods, but differ when they are not. Furthermore, we found location-dependent LSPR spectra on and around a single NP, offering a unique opportunity to characterize multi-mode plasmonic NPs at nanometer resolution for better understanding their plasmonic optical properties and for rational design of single NP optics.

Compact high-pulse-energy ultraviolet laser source for ozone lidar measurements.

An all solid-state Ti:sapphire laser differential absorption lidar transmitter was developed. This all-solid-state laser provides a compact, robust, and highly reliable laser transmitter for potential application in differential absorption lidar measurements of atmospheric ozone. Two compact, high-energy-pulsed, and injection-seeded Ti:sapphire lasers operating at a pulse repetition frequency of 30 Hz and wavelengths of 867 and 900 nm, with M2 of 1.3, have been experimentally demonstrated and their properties compared with model results. The output pulse energy was 115 mJ at 867 nm and 105 mJ at 900 nm, with a slope efficiency of 40% and 32%, respectively. At these energies, the beam quality was good enough so that we were able to achieve 30 mJ of ultraviolet laser output at 289 and 300 nm after frequency tripling with two lithium triborate nonlinear crystals.

Acceleration element for femtosecond electron pulse compression.

An acceleration element is proposed for compressing the electron pulse duration in a femtosecond photoelectron gun. The element is a compact metal cavity with curved-shaped walls. An external voltage is applied to the cavity where a special electric field forms in such a way that the slow electrons in the electron pulse front are accelerated more than the fast electrons, and consequently the electron pulse duration will be compressed. The distribution of the electric field inside the acceleration cavity is analyzed for the geometry of the cavity. The electron dynamics in this acceleration cavity is also investigated numerically. Numerical results show that the electron pulse front and pulse duration can be improved by compensating for the effects of space charge and the initial energy spread of photoelectrons with a Lambertian angular distribution. Depending on the design parameters and the shape of the electron pulse, for a femtosecond electron gun with an electron energy of 30 keV, 10(3) electrons per pulse, and an electron drift length of 40 cm, the electron pulse duration can be reduced from 550 to 200 fs when using a compensating cavity with an average radius of 1.7 and 5.6 cm in length. Electron pulses shorter than 200 fs can be achieved if the length of the drift region is reduced.