Acid dissociation and the subsequent solvation of the charged fragments at ultracold temperatures in nanoenvironments, as distinct from ambient bulk water, are relevant to atmospheric and interstellar chemistry but remain poorly understood. Here we report the experimental observation of a nanoscopic aqueous droplet of acid formed within a superfluid helium cluster at 0.37 kelvin. High-resolution mass-selective infrared laser spectroscopy reveals that successive aggregation of the acid HCl with water molecules, HCl(H2O)n, readily results in the formation of hydronium at n = 4. Accompanying ab initio simulations show that undissociated clusters assemble by stepwise water molecule addition in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ringlike undissociated HCl(H2O)3 then spontaneously yields the compact dissociated H3O+(H2O)3Cl- ion pair. This aggregation mechanism bypasses deep local energy minima on the n = 4 potential energy surface and offers a general paradigm for reactivity at ultracold temperatures.

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.

Electrochemical splitting of water to produce oxygen (O) and hydrogen (H) through a cathodic hydrogen evolution reaction (HER) and an anodic oxygen evolution reaction (OER) is a promising green approach for sustainable energy supply. Here we demonstrated a porous nickel-copper phosphide (NiCuP) nano-foam as a bifunctional electrocatalyst for highly efficient total water splitting. Prepared from a bubble-templated electrodeposition method and subsequent low-temperature phosphidization, NiCuP has a hierarchical pore structure with a large electrochemical active surface area. To reach a high current density of 50 mA cm, it requires merely 146 and 300 mV with small Tafel slopes of 47 and 49 mV dec for HER and OER, respectively. The total water splitting test using NiCuP as both the anode and cathode showed nearly 100% Faradic efficiency and surpassed the performances of electrode pairs using commercial Pt/C and IrO catalysts under our test conditions. The high activity of NiCuP can be attributed to (1) the conductive NiCu substrates, (2) a large electrochemically active surface area together with a combination of pores of different sizes, and (3) the formation of active Ni/Cu oxides/hydroxides while keeping a portion of more conductive Ni/Cu phosphides in the nano-foam. We expect the current catalyst to enable the manufacturing of affordable water splitting systems.

We report the infrared spectra of HCl, (HCl)2, and H2O-HCl in liquid helium nanodroplets in the frequency region between 2680 and 2915 cm(-1). For the HCl monomer a line width of 1.0 cm(-1) (H35Cl) corresponding to a lifetime of 5.3 ps was observed. The line broadening indicates fast rotational relaxation similar to that previously observed for HF. For (HCl)2 the free HCl as well as the bound HCl stretching band has been observed. The nu2+ bands of (HCl)2 could be rotationally resolved, and rotational constants were deduced from the spectra. We observed both the allowed and the symmetry forbidden transition. However, the forbidden "broken symmetry" tunneling transition of the mixed dimer shows an intensity that is considerably enhanced compared to the gas phase. Upon the basis of the present measurements we were able to calculate the tunneling splitting in the excited state. The tunneling splitting is found to be reduced by 28% compared to the gas phase. Transitions from the ground state to the Ka=1 level of the free HCl stretch (nu1) are recorded and show considerable line broadening with a line width of 2 cm(-1). The excited state Ka=1 has an additional rotational energy of about 10 cm(-1), thereby allowing fast rotational relaxation by coupling to helium excitations. In addition we observed the HCl stretch of the HCl-H2O dimer, which exhibits an unusually large width (1.7 cm(-1) for H35Cl)) and large red shift (8.5 cm(-1)), compared to the gas-phase values. The large-amplitude motion originating from the libration mode of the HCl-H2O complex is supposed to act as a fast relaxation manifold.