RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations

Prediction of the density and lattice compression properties of the α and γ phases of the hexahydro-1,3,5-trinitro-1,3,5-<i>s</i>-triazine (RDX) crystal and of the low-pressure α → γ phase transition upon pressure increase are general tests used to assess the accuracy of density-functional-theory- (DFT-) based computational methods and to identify the essential parameters that govern the behavior of this high-energy-density material under extreme conditions. The majority of previous DFT studies have analyzed such issues under static optimization conditions by neglecting the corresponding temperature effects. In this study, we extend previous investigations and analyze the performance of dispersion-corrected density functional theory to predict the compression of RDX in the pressure range of 0–9 GPa and the corresponding α → γ phase transition under realistic temperature and pressure conditions. We demonstrate that, by using static dispersion-corrected density functional theory calculations, direct interconversion between the α and γ phases upon compression is not observed. This limitation can be addressed by using isobaric–isothermal molecular dynamic simulations in conjunction with DFT-D2-calculated potentials, an approach that is shown to provide an accurate description of both the crystallographic RDX lattice parameters and the dynamical effects associated with the α→ γ phase transformation. An even more comprehensive and demanding analysis was done by predicting the corresponding shock Hugoniot curve of RDX in the pressure range of 0–9 GPa. It was found that the theoretical results reproduce reasonably well the available experimental Hugoniot shock data for both the α and γ phases. The results obtained demonstrate that a satisfactory prediction of the shock properties in high-energy-density materials undergoing crystallographic and configurational transformations is possible through the combined use of molecular dynamics simulations in the isobaric–isothermal ensemble with dispersion-corrected density functional theory methods

Photoinduced Charge Transfer and Acetone Sensitivity of Single-Walled Carbon Nanotube–Titanium Dioxide Hybrids

The unique physical and chemical properties of single-walled carbon nanotubes (SWNTs) make them ideal building blocks for the construction of hybrid nanostructures. In addition to increasing the material complexity and functionality, SWNTs can probe the interfacial processes in the hybrid system. In this work, SWNT–TiO₂ core/shell hybrid nanostructures were found to exhibit unique electrical behavior in response to UV illumination and acetone vapors. By experimental and theoretical studies of UV and acetone sensitivities of different SWNT–TiO₂ hybrid systems, we established a fundamental understanding on the interfacial charge transfer between photoexcited TiO₂ and SWNTs as well as the mechanism of acetone sensing. We further demonstrated a practical application of photoinduced acetone sensitivity by fabricating a microsized room temperature acetone sensor that showed fast, linear, and reversible detection of acetone vapors with concentrations in few parts per million range

Tunable Lattice Constant and Band Gap of Single- and Few-Layer ZnO

Single and few-layer ZnO(0001) (ZnO­(<i>n</i>L), <i>n</i> = 1–4) grown on Au(111) have been characterized via scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. We find that the in-plane lattice constants of the ZnO­(<i>n</i>L, <i>n</i> ≤ 3) are expanded compared to that of the bulk wurtzite ZnO(0001). The lattice constant reaches a maximum expansion of 3% in the ZnO­(2L) and decreases to the bulk wurtzite ZnO value in the ZnO­(4L). The band gap decreases monotonically with increasing number of ZnO layers from 4.48 eV (ZnO­(1L)) to 3.42 eV (ZnO­(4L)). These results suggest that a transition from a planar to the bulk-like ZnO structure occurs around the thickness of ZnO­(4L). The work also demonstrates that the lattice constant and the band gap in ultrathin ZnO can be tuned by controlling the number of layers, providing a basis for further investigation of this material

Mechanism of Oxygen Exchange between CO₂ and TiO₂(101) Anatase

The mechanism of oxygen exchange between CO₂ and a defective anatase (101) surface was investigated by density functional theory calculations including corrections for long-range dispersion interactions and for on-site Coulomb interactions. The calculations identify a carbonate-like configuration at a surface oxygen defect site as the key intermediate species responsible for the oxygen exchange. The stability of this species, its vibrational frequencies, and the reaction barriers involved in the oxygen exchange mechanism are found to be highly dependent on the specific value of the Hubbard <i>U</i> correction used to describe the on-site Coulomb interactions within the GGA+U procedure. <i>U</i> parameter values that result in CO₂ adsorption energies and reaction barriers for oxygen exchange consistent with the results of room-temperature experiments are smaller (<i>U</i> ≤ 2.5 eV) than those that reproduce the experimental band gap or location of defect states in the band gap of the reduced TiO₂ crystal

Welding of Gold Nanoparticles on Graphitic Templates for Chemical Sensing

Controlled self-assembly of zero-dimensional gold nanoparticles and construction of complex gold nanostructures from these building blocks could significantly extend their applications in many fields. Carbon nanotubes are one of the most promising inorganic templates for this strategy because of their unique physical, chemical, and mechanical properties, which translate into numerous potential applications. Here we report the bottom-up synthesis of gold nanowires in aqueous solution through self-assembly of gold nanoparticles on single-walled carbon nanotubes followed by thermal-heating-induced nanowelding. We investigate the mechanism of this process by exploring different graphitic templates. The experimental work is assisted by computational studies that provide additional insight into the self-assembly and nanowelding mechanism. We also demonstrate the chemical sensitivity of the nanomaterial to parts-per-billion concentrations of hydrogen sulfide with potential applications in industrial safety and personal healthcare

Hybridization of Phenylthiolate- and Methylthiolate-Adatom Species at Low Coverage on the Au(111) Surface

Using scanning tunneling microscopy we observed reaction products of two chemisorbed thiolate species, methylthiolate and phenylthiolate, on the Au(111) surface. Despite the apparent stability, organometallic complexes of methyl- and phenylthiolate with the gold-adatom (RS‑Au‑SR, with R as the hydrocarbon group) undergo a stoichiometric exchange reaction, forming hybridized CH₃S‑Au‑SPh complexes. Complementary density functional theory calculations suggest that the reaction is most likely mediated by a monothiolate RS‑Au complex bonded to the gold surface, which forms a trithiolate RS‑Au‑(SR)‑Au‑SR complex as a key intermediate. This work therefore reveals the novel chemical reactivity of the low-coverage “striped” phase of alkanethiols on gold and strongly points to the involvement of monoadatom thiolate intermediates in this reaction. By extension, such intermediates may be involved in the self-assembly process itself, shedding new light on this long-standing problem

Water Chain Formation on TiO₂(110)

The adsorption of water on a reduced rutile TiO₂(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM measurements show that at a temperature of 50 K, an isolated water monomer adsorbs on top of a Ti­(5f) atom on the Ti row in agreement with earlier studies. As the coverage increases, water molecules start to form one-dimensional chain structures along the Ti row direction. Supporting DFT calculations show that the formation of an H-bonded one-dimensional water chain is energetically favorable compared to monomer adsorption. In the chain, there are H-bonds between adjacent water molecules, and the water molecules also form H-bonds to neighboring bridging oxygens of TiO₂(110). Thermal annealing at <i>T</i> = 190 K leads to the formation of longer chains facilitated by the diffusion of water on the surface. The results provide insight into the nature of the hydrogen bonding in the initial stage of wetting of TiO₂

Diffusion of CO₂ on the Rutile TiO₂(110) Surface

The diffusion of CO₂ molecules on a reduced rutile TiO₂(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO₂ molecule at an oxygen vacancy (V_O) becomes increasingly streaky with increasing temperature, indicating thermally activated CO₂ diffusion from the V_O site. From temperature-dependent tunneling current measurements, the barrier for diffusion of CO₂ from the V_O site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO₂ along Ti rows of only 1.33 kcal/mol

Assessing the Performances of Dispersion-Corrected Density Functional Methods for Predicting the Crystallographic Properties of High Nitrogen Energetic Salts

Several density functional methods with corrections for long-range dispersion interactions are evaluated for their capabilities to describe the crystallographic lattice properties of a set of 26 high nitrogen-content salts relevant for energetic materials applications. Computations were done using methods that ranged from adding atom–atom dispersion corrections with environment-independent and environment-dependent coefficients, to methods that incorporate dispersion effects via dispersion-corrected atom-centered potentials (DCACP), to methods that include nonlocal corrections. Among the functionals tested, the most successful is the nonlocal optPBE-vdW functional of Klimeš and Michaelides that predicts unit cell volumes for all crystals of the reference set within the target error range of ±3% and gives individual lattice parameters with a mean average percent error of less than 0.81%. The DCACP, Grimme’s D3, and Becke and Johnson’s exchange-hole (XDM) methods, when used with the BLYP, PBE, and B86b functionals, respectively, are also quite successful at predicting the lattice parameters of the test set

Indium OxideSingle-Walled Carbon Nanotube Composite for Ethanol Sensing at Room Temperature

Utilizing a sol-gel synthesis, indium oxide is grown on the surface of oxidized single-walled carbon nanotubes (SWCNT) to form a hybrid material with high conductivity and sensitivity toward certain organic vapors. The room-temperature sensing of dilute ethanol and acetone vapors on the surface of indium oxide/SWCNT hybrid material is studied using electrical conductance experiments in a nonoxidizing environment. Through testing of variously calcinated materials, it was observed that the degree of annealing greatly affects the material’s response to acetone and ethanol, such that the intermediate calcination condition yields the best sensitivity. DFT simulations are used to study the interface between defective SWCNT and indium oxide, as well as the interaction between ethanol and acetone molecules with the indium oxide/SWCNT hybrid material