Christoph G. Salzmann

The Preparation and Structures of Hydrogen Ordered Phases of Ice

Two hydrogen ordered phases of ice were prepared by cooling the hydrogen disordered ices V and XII under pressure. Previous attempts to unlock the geometrical frustration in hydrogen-bonded structures have focused on doping with potassium hydroxide and have had success in partially increasing the hydrogen ordering in hexagonal ice I (ice Ih). By doping ices V and XII with hydrochloric acid, we have prepared ice XIII and ice XIV, and we analyzed their structures by powder neutron diffraction. The use of hydrogen chloride to release geometrical frustration opens up the possibility of completing the phase diagram of ice.

Structure of ice crystallized from supercooled water

The freezing of water to ice is fundamentally important to fields as diverse as cloud formation to cryopreservation. At ambient conditions, ice is considered to exist in two crystalline forms: stable hexagonal ice and metastable cubic ice. Using X-ray diffraction data and Monte Carlo simulations, we show that ice that crystallizes homogeneously from supercooled water is neither of these phases. The resulting ice is disordered in one dimension and therefore possesses neither cubic nor hexagonal symmetry and is instead composed of randomly stacked layers of cubic and hexagonal sequences. We refer to this ice as stacking-disordered ice I. Stacking disorder and stacking faults have been reported earlier for metastable ice I, but only for ice crystallizing in mesopores and in samples recrystallized from high-pressure ice phases rather than in water droplets. Review of the literature reveals that almost all ice that has been identified as cubic ice in previous diffraction studies and generated in a variety of ways was most likely stacking-disordered ice I with varying degrees of stacking disorder. These findings highlight the need to reevaluate the physical and thermodynamic properties of this metastable ice as a function of the nature and extent of stacking disorder using well-characterized samples.

Ice XV: a new thermodynamically stable phase of ice.

A new phase of ice, named ice XV, has been identified and its structure determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart of ice VI and is thermodynamically stable at temperatures below approximately 130 K in the 0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure range of the phase diagram have thus been finally mapped, with only hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is antiferroelectric (P1), in clear disagreement with recent theoretical calculations predicting ferroelectric ordering (Cc).

The polymorphism of ice: five unresolved questions

Our recent discovery of three new phases of ice has increased the total number of known distinct polymorphs of ice to fifteen. In this Perspective article, we give a brief account of previous work in the field, and discuss some of the particularly interesting open questions that have emerged from recent studies. These include (i) the effectiveness of acid and base dopants to enable hydrogen-ordering processes in the ices, (ii) the comparison of the calorimetric data of some of the crystalline phases of ice and low-density amorphous ice, (iii) the disagreement between the experimental ice XV structure and computational predictions, (iv) the incompleteness of some of the hydrogen order/disorder pairs and (v) the new frontiers at the high and negative pressure ends of the phase diagram.

A second distinct structural “state” of high-density amorphous ice at 77 K and 1 bar

High-density amorphous ice (HDA), further densified on isobaric heating from 77 K to 165 (177) K at 1.1 (1.9) GPa, relaxes at 77 K and 1 bar to the same structural “state” with a density of 1.25 ± 0.01 g cm−3. Its density is higher by ≈9% than that of HDA, and thus it is called very-high-density amorphous ice (VHDA). X-ray diffractogram and Raman spectrum of VHDA clearly differs from that of HDA, and the hydrogen-bonded O–O distance increases from 2.82 A in HDA to 2.85 A in VHDA. Implications for the polyamorphism of the amorphous forms of water are discussed.

Fabrication of carbon-nanotube-reinforced glass–ceramic nanocomposites by ultrasonic in situ sol–gel processing

The addition of carbon nanotubes (CNTs) to ceramic or glass matrices has the potential to provide composites with novel properties but composites with a uniform dispersion of undamaged CNTs have proved difficult to make. This paper reports a processing method that overcomes these problems by using a powderless sol–gel route to produce a low-melting point aluminoborosilicate glass matrix, in combination with a dispersion method for the CNTs that is compatible with the sol. Single-walled or multi-walled CNTs were first functionalized by treatment with nitric acid followed by an ammonia–ethanol solution and were then dispersed in a glass precursor sol via ultrasonic processing. After gelation and calcination, the powder was hot pressed to make dense, well dispersed CNT–borosilicate glass composites with CNT contents from 0.5 to 5 wt%. Raman spectroscopy showed that the CNTs had suffered little damage during processing. The formation of cristobalite initially lead to microcracking but this was successfully suppressed by further additions of alumina. The CNTs suppressed cracking around hardness indentations and substantial crack bridging by the CNTs was observed.

Stacking disorder in ice I

Traditionally, ice I was considered to exist in two well-defined crystalline forms at ambient pressure: stable hexagonal ice (ice Ih) and metastable cubic ice (ice Ic). However, it is becoming increasingly evident that what has been called cubic ice in the past does not have a structure consistent with the cubic crystal system. Instead, it is a stacking-disordered material containing cubic sequences interlaced with hexagonal sequences, which is termed stacking-disordered ice (ice Isd). In this article, we summarise previous work on ice with stacking disorder including ice that was called cubic ice in the past. We also present new experimental data which shows that ice which crystallises after heterogeneous nucleation in water droplets containing solid inclusions also contains stacking disorder even at freezing temperatures of around -15 °C. This supports the results from molecular simulations, that the structure of ice that crystallises initially from supercooled water is always stacking-disordered and that this metastable ice can transform to the stable hexagonal phase subject to the kinetics of recrystallization. We also show that stacking disorder in ice which forms from water droplets is quantitatively distinct from ice made via other routes. The emerging picture of ice I is that of a very complex material which frequently contains stacking disorder and this stacking disorder can vary in complexity depending on the route of formation and thermal history.

Removal of amorphous carbon for the efficient sidewall functionalisation of single-walled carbon nanotubes

The sidewall functionalisation of carbon nanotubes using the standard nitric acid treatment can be greatly enhanced by first removing the amorphous carbon present in the sample.

The Role of Photoinduced Defects in TiO2 and Its Effects on Hydrogen Evolution from Aqueous Methanol solution

The hydrogen evolution from aqueous methanol solutions was found to follow two stages of zero order kinetics during photoreactions using TiO 2 as the photocatalyst. Maximal hydrogen evolution was found at the 10% (v/v) methanol solution. X-ray photoelectron spectroscopy (XPS) shows that Ti(1566) defects are formed on the surface of TiO 2 and X-ray powder diffraction (XRD) indicates that Ti(1566) defects are also formed in the bulk after photoreaction. Formation of defects is also shown by broadening of Bragg peaks and blue shifts and peak broadening in Raman spectroscopy. The defect disorder results in the increase of hydrogen evolution. UV-vis diffuse reflection spectra confirm that new absorptions in the visible light region are related to the defect content. At high methanol concentration, XPS implies that the active sites of the surface are blocked by hydroxyl groups, which results in the decrease of hydrogen evolution. TEM images showed that the photoreaction occurred on the surface of the photocatalyst as the surface of the TiO 2 became rough after the photoreaction.

2D ice from first principles: structures and phase transitions

Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ∼2  GPa, at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width.