We present the first experimental vibrational spectroscopy study providing direct evidence of a water phase inside single-walled carbon nanotubes that exhibits an unusual form of hydrogen-bonding due to confinement. Water adopts a stacked-ring structure inside nanotubes, forming intra- and inter-ring hydrogen bonds. The intra-ring hydrogen bonds are bulk-like while the inter-ring hydrogen bonds are relatively weak, having a distorted geometry that gives rise to a distinct OH stretching mode. The experimentally observed infrared mode at 3507 cm(-1) is assigned to vibrations of the inter-ring OH-groups based on detailed atomic-level modeling. The direct observation of unusual hydrogen bonding in nanotubes has potential implications for water in other highly confined systems, such as biological channels and nanoporous media.

Since Au turned out to be an active catalyst for CO oxidation at low temperatures, CO adsorption on various Au surfaces has been in the scope of numerous surface science studies. Interestingly, supported particles as well as stepped and rough single-crystal surfaces exhibit very similar adsorption behavior. To elucidate the origin of these similarities, we have performed temperature-programmed desorption and infrared absorption spectroscopy for a whole range of Au surfaces from nanoparticles grown on HOPG to Au(111) surfaces roughened by argon ion bombardment. In line with previous results, we have observed two desorption states at ∼130-145 and ∼170-185 K, respectively, and one infrared peak at around 2120 cm -1 in all cases. In addition to the experiments, we have carried out theoretical studies of CO adsorption on Au(332). The calculations show that CO desorption states above 100 K may be located at step-edges but not on terrace sites. Reducing the coordination of Au atoms further leads to successively higher binding energies with an unchanged anharmonic frequency. Therefore, we conclude that both desorption peaks belong to CO on low-coordinated Au atoms at steps and kinks. For the sputtered Au(111) surface, scanning tunneling microscopy reveals a rough pit-and-mound morphology with a large number of such sites. In annealing experiments we observe that the loss of these sites coincides with the loss of CO adsorption capacity, corroborating our conclusions.

We present theoretical and experimental evidence for CO(2) adsorption on different sites of single walled carbon nanotube (SWNT) bundles. We use local density approximation density functional theory (LDA-DFT) calculations to compute the adsorption energies and vibrational frequencies for CO(2) adsorbed on SWNT bundles. The LDA-DFT calculations give a range of shifts for the asymmetric stretching mode from about -6 to -20 cm(-1) for internally bound CO(2), and a range from -4 to -16 cm(-1) for externally bound CO(2) at low densities. The magnitude of the shift is larger for CO(2) adsorbed parallel to the SWNT surface; various perpendicular configurations yield much smaller theoretical shifts. The asymmetric stretching mode for CO(2) adsorbed in groove sites and interstitial sites exhibits calculated shifts of -22.2 and -23.8 cm(-1), respectively. The calculations show that vibrational mode softening is due to three effects: (1) dynamic image charges in the nanotube; (2) the confining effect of the adsorption potential; (3) dynamic dipole coupling with other adsorbate molecules. Infrared measurements indicate that two families of CO(2) adsorption sites are present. One family, exhibiting a shift of about -20 cm(-1) is assigned to internally bound CO(2) molecules in a parallel configuration. This type of CO(2) is readily displaced by Xe, a test for densely populated adsorbed species, which are expected to be present on the highest adsorption energy sites in the interior of the nanotubes. The second family exhibits a shift of about -7 cm(-1) and the site location and configuration for these species is ambiguous, based on comparison with the theoretical shifts. The population of the internally bound CO(2) may be enhanced by established etching procedures that open the entry ports for adsorption, namely, ozone oxidation followed by annealing in vacuum at 873 K. Xenon displacement experiments indicate that internally bound CO(2) is preferentially displaced relative to the -7 cm(-1) shifted species. The -7 cm(-1) shifted species is assigned to CO(2) adsorbed on the external surface based on results from etching and Xe displacement experiments.

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The chemisorption of NO 2 on carbon nanotubes is studied by modeling the interaction between NO 2 and N 2 O 4 with an infinitely long (8,0) single-walled carbon nanotube, using planewave/pseudopotential-based density functional theory (DFT). In sharp contrast to the case of graphite, for which NO 2 adsorption is physical, a NO 2 radical could readily adsorb on the exterior of an (8,0) tube by addition, similar to the reaction between NO 2 and alkenes. The process is slightly endothermic and reversible with a low energy barrier, with the NO 2 group in either the nitro or nitrite configuration. Adsorption of a second NO 2 is considerably exothermic, and desorption of NO 2 from such a configuration is much more difficult. The chemisorption of NO 2 also increases the conductivity of the (8,0) tube. On the other hand, N 2 O 4 only plays a minor role in the equilibrium between desorption and adsorption processes. These results indicate that the (8,0) tube is more reactive toward NO 2 than graphite, due to the curvature on the rolled graphene sheet.

Ozone is known to react with single-walled nanotubes (SWNTs) to form oxide species on the nanotubes and, upon annealing, to etch the SWNTs. However, the mechanism of ozone attack is not known. We use gradientcorrected density functional theory to compute the potential energy surfaces for O 3 dissociation on the sidewall of a pristine (8,8) SWNT. Two decomposition pathways were considered; the first involves the formation of a Criegee intermediate, with a barrier of 17 kcal/mol, followed by transformations leading to lactone, quinone, and carbonyl functional groups. The activation barriers for these transformations are below 23 kcal/mol. The cleavage of the lactone group, evolving CO and CO 2 , have barrier heights of 39.4 and 49.3 kcal/mol, respectively. This agrees well with experimental findings that the evolution of CO 2 and CO occur at 600 K. The second decomposition pathway involves the direct cleavage of the ozonide, forming a singlet O 2 and an ether or epoxide group on the SWNT. This pathway competes with the Criegee mechanism; the barrier for forming singlet O 2 is 7.9 kcal/mol, which is 9.1 kcal/mol lower than the barrier to formation of the Criegee intermediate, indicating that formation of ether or epoxide groups is kinetically favored. However, formation of ester and carbonyl groups could proceed by subsequent addition of O 3 on newly generated defect sites. Vibrational frequency calculations were carried out on cluster models in order to predict infrared absorption signals of local structures. The calculated results for CdO stretching frequencies agree well with experiments. Analysis of the calculated frequencies indicates that the unassigned experimental band at 1380 cm -1 is due to ester and ether groups, while the unassigned band at 925 cm -1 is due to epoxide groups. The vibrational frequency of the O + -O -stretch in the Criegee intermediate is in the range 1055-1096 cm -1 .

Being a giant bulk Rashba semiconductor, the ambient-pressure phase of BiTeI was predicted to transform into a topological insulator under pressure at 1.7−4.1 GPa [Nat. Commun. 2012, 3, 679]. Because the structure governs the new quantum state of matter, it is essential to establish the high-pressure phase transitions and structures of BiTeI for better understanding its topological nature. Here, we report a joint theoretical and experimental study up to 30 GPa to uncover two orthorhombic high-pressure phases of Pnma and P4/nmm structures named phases II and III, respectively. Phases II (stable at 8.8−18.9 GPa) and III (stable at >18.9 GPa) were first predicted by our first-principles structure prediction calculations based on the calypso method and subsequently confirmed by our high-pressure powder X-ray diffraction experiment. Phase II can be regarded as a partially ionic structure, consisting of positively charged (BiTe) + ladders and negatively charged I − ions. Phase III is a typical ionic structure characterized by interconnected cubic building blocks of Te−Bi−I stacking. Application of pressures up to 30 GPa tuned effectively the electronic properties of BiTeI from a topological insulator to a normal semiconductor and eventually a metal having a potential of superconductivity.

We demonstrate an efficient D-A-π-A sensitizer with a benzothiadiazole-cyclopentadithiophene [corrected] moiety as the spacer in a triphenylamine organic dye for dye-sensitized solar cells. The dye has a broad visible light absorption range up to 800 nm. A power conversion efficiency >9% has been achieved using a [Co(bpy)3](2+/3+)-based electrolyte.