A Threshold-Based Approach to Calorimetry in Helium Droplets: Measurement of Binding Energies of Water Clusters

Helium dropletbeam methods have emerged as a versatile technique that can be used to assemble a wide variety of atomic and molecular clusters. We have developed a method to measure the binding energies of clusters assembled in helium droplets by determining the minimum droplet sizes required to assemble and detect selected clusters in the spectrum of the dopeddropletbeam. The differences in the droplet sizes required between the various multimers are then used to estimate the incremental binding energies. We have applied this method to measure the binding energies of cyclic waterclusters from the dimer to the tetramer. We obtain measured values of D0 that are in agreement with theoretical estimates to within ∼20%. Our results suggest that this threshold-based approach should be generally applicable using either mass spectrometry or optical spectroscopy techniques for detection, provided that the clusters selected for study are at least as strongly bound as those of water, and that a peak in the overall spectrum of the beam corresponding only to the cluster chosen (at least in the vicinity of the threshold) can be located

Infrared spectroscopy of the ν3 band of C3 in helium droplets

The C3 molecule is an important species known to participate in key chemical reactions in combustion and astrochemistry. Its occurrence in environments of interest, its intramolecular physics, and its intermolecular reactivity have been areas of extensive and ongoing study. Much of the interest in C3 is related to investigating its interactions with other species relevant to combustion processes or astrochemistry. Helium droplet methods offer a promising route to assemble and study a wide variety of novel complexes, clusters, and adducts made from C3. Here we report the results of our recent efforts to dope cold helium droplets with C3 molecules and record the rotationally-resolved infrared spectrum of the embedded C3. The spectrum consists of P(2), R(0), and R(2) lines well-described by a linear rotor Hamiltonian with ν0 = 2039.09(2) cm-1, B = 0.204(5) cm-1, and T = 0.37 K. The B rotational constant of the C3 molecule is found to be reduced from its gas-phase value by a factor of 2.1 due to rotational following by the helium solvent

Photophysical Properties of CdS Nanoparticles in Thin Films for Opto-Chemical Sensing

In recent years, II-VI compound semiconductor nanoparticles synthesized in a liquid solution have been shown to possess unique optoelectronic properties which are highly attractive for the fabrication of various sensors based on the optical signal readout scheme. The challenge has been to immobilize these nanoparticles into films on solid surfaces, i.e. on a chip, so that they do not suffer any property deterioration as a sensing medium. In the presented work, synthesis of CdS nanoparticles in reverse micelle solution using AOT surfactant as a stabilizer has led to particles with relatively bright photoemission identified as originating from both shallow and deep traps inside the bandgap. Moreover, slightly altering the preparation procedure has produced samples with two distinctive crystal structures. Both types of CdS nanoparticles suspended in commonly utilized solvents such as chloroform and hexane were subject to chemical quenching when various organic compounds were introduced into the solution, demonstrating the sensitivity of trap states to their chemical environment. However, the two structures have shown very different optical properties. While post-synthesis treatment had no effect on one type of particle, the other type was able to undergo a photochemical reaction via prolonged UV irradiation, which resulted in an increased luminescence quantum yield ÖL from 2% to 14%. The same particle type was also responsive to thermal treatment, showing even higher values of ÖL (∼40%). The CdS/AOT particles have been cast into thin films by spin-coating on a Si wafer. Coating parameters have been investigated in order to achieve optimal control over the film thickness, uniformity, overall film durability, etc. These nanostructured films capped with various porous polymeric and sol-gel protective coatings were exposed to a series of organic compounds. Photoluminescence data collected for these samples served for identification of the compounds and their concentrations. This paper offers the discussion of photophysical response in CdS nanoparticle-based thin films with respect to development of novel nanostructured opto-chemical sensors

Sonochemically Assisted Thermal Decomposition of Alane N,N-Dimethylethylamine with Titanium (IV) Isopropoxide in the Presence of Oleic Acid to Yield Air-Stable and Size-Selective Aluminum Core−Shell Nanoparticles

Using sonochemistry to provide the thermal energy and mixing, we demonstrate the ability to synthesize air-stable aluminum nanoparticles of two different size distributions from the titanium-catalyzed thermal decomposition of alane. Characterization data indicate the presence of spherical face-centered-cubic aluminum nanoparticles with average sizes of either 5 or 30 nm that are capped with an organic shell. The average size of the nanoparticles correlates with the concentration of the passivation agent oleic acid, where a higher concentration results in smaller particles. Thermal analysis data demonstrates that at elevated temperatures (\u3e550 °C), these particles react via a typical aluminum oxidation mechanism, whereas at low temperatures (\u3c550 °C), the behavior of these particles is unique and directly related to the presence of the organic shell

Formation of Protein−Metal Oxide Nanostructures by the Sonochemical Method: Observation of Nanofibers and Nanoneedles

The sonochemical reaction of iron pentacarbonyl is explored in water and in water with the protein BSA (bovine serum albumen). In water, the reaction is found to produce spherical nanoparticles of magnetite (Fe3O4) with a particle size distribution of \u3c10 to ∼60 nm. In water with BSA, the reaction produces either nanofibers or nanoneedles, depending on the concentration of BSA. The nanofiber and nanoneedle samples are found to be mixtures of goethite, lepidocrocite, and hematite (α-FeOOH, γ-FeOOH, and α-Fe2O3, respectively). The sonochemical reaction of iron pentacarbonyl with BSA in water is thought to proceed through the thermal decomposition mechanism for iron pentacarbonyl with BSA acting as a templating agent

Chemical Dynamics of Aluminum Nanoparticles in Ammonium Nitrate and Ammonium Perchlorate Matrices: Enhanced Reactivity of Organically Capped Aluminum

Aluminum nanoparticles have been a subject of active investigation in recent years because of their potential to enhance the energy content of energetic materials. The associated kinetics of the chemical reaction and energy release are, in many cases, governed by the properties of the passivation layer protecting the particle rather than those of the underlying metal core. The passivation layer of Al particles is typically an oxide shell several nanometers thick, but other possibilities are now available. We have previously developed synthesis routes to produce air-stable Al nanoparticles that are capped by oleic acid. In the present study, we examine the chemical dynamics of these materials in ammonium nitrate and ammonium perchlorate matrices. For comparison, the analogous experiments were also performed on samples using traditional oxide-protected particles. Reactions are initiated by a 20 μs IR laser pulse and then probed via time-of-flight mass spectrometry of the evolved gases and by emission spectroscopy of the flame. In both ammonium nitrate and ammonium perchlorate matrices, the organically passivated nanoparticles are found to be significantly more reactive and are able to access some reaction pathways unavailable to oxide-protected particles

Spontaneous Hydrogen Generation from Organic-Capped Al Nanoparticles and Water

The development of technologies that would lead toward the adoption of a hydrogen economy requires readily available, safe, and environmentally friendly access to hydrogen. This can be achieved using the aluminum−water reaction; however, the protective nature and stability of aluminum oxide is a clear detriment to its application. Here, we demonstrate the spontaneous generation of hydrogen gas from ordinary room-temperature tap water when combined with aluminum−oleic acid core−shell nanoparticles obtained via sonochemistry. The reaction is found to be near-complete (\u3e95% yield hydrogen) with a tunable rate from 6.4 × 10−4 to 0.01 g of H2/s/g of Al and to yield an environmentally benign byproduct. The potential of these nanoparticles as a source of hydrogen gas for power generation is demonstrated using a simple fuel cell with an applied load

Multispectroscopic (FTIR, XPS, and TOFMS−TPD) Investigation of the Core−Shell Bonding in Sonochemically Prepared Aluminum Nanoparticles Capped with Oleic Acid

Organically capped metal nanoparticles are an attractive alternative to more conventional oxide-passivated materials, due to the lower reaction temperatures and the possibility of tuning the organic coating. Sonochemical methods have been used to produce small (∼5 nm average size) air-stable aluminum nanoparticles capped with oleic acid. In order to understand the nature of the metal−organic bonding in the nanoparticles, we have used FTIR, XPS, and TOFMS−TPD techniques to study the organic passivation layer and its desorption at elevated temperatures. In the present case we find that the organic layer appears to be attached via Al−O−C bonds with the C atom formerly involved in the carboxylic acid functional group