Nitrogen-containing functional groups were generated on the surface of partially oxidized multi-walled carbon nanotubes (CNTs) via post-treatment in ammonia. The treatment temperature was varied in order to tune the amount and type of nitrogen- and oxygen-containing functional groups, which were studied using high-resolution X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The surface defects on CNTs due to the incorporation of nitrogen were investigated by Raman spectroscopy. Deconvoluted XP N1s spectra were used for the quantification of different nitrogen-containing functional groups, and TPD studies were performed in inert and ammonia atmosphere to investigate the surface reactions occurring on the oxidized CNT surfaces quantitatively. Nitrile, lactam, imide and amine-type functional groups were formed in the presence of ammonia below 300 degrees C. When the OCNTs were treated in the medium temperature range between 300 degrees C to 500 degrees C, mainly pyridine-type nitrogen groups were generated, whereas pyridinic, pyrrolic and quaternary-type nitrogen groups were the dominating species present on the CNT surface when treated above 500 degrees C. It was found that about 38% of the oxygen functional groups react with ammonia below 500 degrees C.

The details of ion hydration still raise fundamental questions relevant to a large variety of problems in chemistry and biology. The concept of water "structure breaking" and "structure making" by ions in aqueous solutions has been invoked to explain the Hofmeister series introduced over 100 years ago, which still provides the basis for the interpretation of experimental observations, in particular the stabilization/destabilization of biomolecules. Recent studies, using state-of-the-art experiments and molecular dynamics simulations, either challenge or support some key points of the structure maker/breaker concept, specifically regarding long-ranged ordering/disordering effects. Here, we report a systematic terahertz absorption spectroscopy and molecular dynamics simulation study of a series of aqueous solutions of divalent salts, which adds a new piece to the puzzle. The picture that emerges from the concentration dependence and assignment of the observed absorption features is one of a limited range of ion effects that is confined to the first solvation shell.

We present terahertz (THz) measurements of salt solutions that shed new light on the controversy over whether salts act as kosmotropes (structure makers) or chaotropes (structure breakers), which enhance or reduce the solvent order, respectively. We have carried out precise measurements of the concentration-dependent THz absorption coefficient of 15 solvated alkali halide salts around 85 cm(-1) (2.5 THz). In addition, we recorded overview spectra between 30 and 300 cm(-1) using a THz Fourier transform spectrometer for six alkali halides. For all solutions we found a linear increase of THz absorption compared to pure water (THz excess) with increasing solute concentration. These results suggest that the ions may be treated as simple defects in an H-bond network. They therefore cannot be characterized as either kosmotropes or chaotropes. Below 200 cm(-1), the observed THz excess of all salts can be described by a linear superposition of the water absorption and an additional absorption that is attributed to a rattling motion of the ions within the water network. By providing a comprehensive set of data for different salt solutions, we find that the solutions can all be very well described by a model that includes damped harmonic oscillations of the anions and cations within the water network. We find this model predicts the main features of THz spectra for a variety of salt solutions. The assumption of the existence of these ion rattling motions on sub-picosecond time scales is supported by THz Fourier transform spectroscopy of six alkali halides. Above 200 cm(-1) the excess is interpreted in terms of a change in the wing of the water network librational mode. Accompanying molecular dynamics simulations using the TIP3P water model support our conclusion and show that the fast sub-picosecond motions of the ions and their surroundings are almost decoupled. These findings provide a complete description of the solute-induced changes in the THz solvation dynamics for the investigated salts. Our results show that THz spectroscopy is a powerful experimental tool to establish a new view on the contributions of anions and cations to the structuring of water.

We present a new and alternative interpretation of the structure of the IR vibrational mode (nu(OH) band) of pure water. The re-interpretation is based on the influence of the cooperative hydrogen bonding arising from a network of hydrogen bonds in the liquid. The nu(OH) band has six components that are dominated by differences in their O-H bond lengths but deviate from thermodynamically average values due to interactions with the hydrogen bond network. The physical origin of the structure in the nu(OH) band is directly related to the O-H bond length, and variations in this bond length are caused by the influence of the surrounding hydrogen-bonded network of water molecules.