Advanced materials

Arno Kraft
Heriot-Watt University, Edinburgh, UK

Artificial water channels for desalination

Desalination involves the removal of NaCl from seawater and presents an attractive but challenging option to tackle water shortages across the world. The most common method for desalination is reverse osmosis, which is, however, a very energy-demanding process and requires specialised membranes. More technological advances are needed.

A recent paper has looked at artificial water channels that mimic aquaporines, which are cellular membrane proteins, to transport water across a membrane and with high efficiency (W. Song et al., Nat. Nanotech., 2020, 15, 73).

The authors proposed a hybrid[4]arene macrocycle reminiscent of a calixarene to which they attached tripeptide chains (Scheme 1). The hybrid[4]arene macrocycle has an inner pore large enough to hold a molecule of water. The 8 attached phenylalanine tripeptide chains face outward and form β-strands, which hold their extended linear conformation in place through hydrogen bonds, promote aggregation between neighbouring channels, and do not hydrogen-bond to water molecules.

Scheme 1 Structure of a hybrid[4]arene macrocycle with appended tripeptide chains (red arrows indicate triPhe peptide chains) 

The hybrid[4]arene molecules were incorporated into phosphatidylcholine liposomes where each molecule spanned the entire membrane and created an artificial water channel. They were studied by atomic-force microscopy, fluorescence quenching following photobleaching, and molecular dynamics simulations.

A single hybrid[4]arene molecule showed a permeability of 4 x 109 water molecules per second at 25°C, which is comparable to aquaporins. In addition, the water/salt selectivity approached 109 that far surpassed the selectivity (~16,000) seen in standard desalination membranes.

Bioinspired artificial water channels with such high efficiency and selectivity could offer an attractive new approach for designing desalination membranes.

Polyethylene thermoplastic elastomers

Thermoplastic elastomers (TPEs) show the elastic properties of rubber but can be melt-processed just like other thermoplastics. This makes TPEs potentially recyclable – unlike rubber, which has to be irreversibly crosslinked to become elastic. A characteristic feature of TPEs are their alternating regions of ‘hard’ and ‘soft’ segments. Hard segments have a high glass transition or melting temperature, whereas soft segments are amorphous and have a glass transition usually well below room temperature.

A recent paper has looked at the potential of making TPEs by the copolymerisation of ethylene and a bio-derived polar comonomer, such as methyl 10-undecanoate or 10-undecenoic acid (2), which can be obtained from extraction and subsequent pyrolysis of castor oil (S. Dai et al., Macromolecules, 2020, 53, 2539).

Early transition-metal catalysts polymerise ethylene to a linear, virtually unbranched polyethylene, which is generally highly crystalline. Radical polymerisation of ethylene gives access to a branched polyethylene with both occasional long and short-chain branches, which is still sufficiently crystalline that it does not show any rubber-elastic properties. Late transition-metal catalysts also polymerise ethylene but are prone to chain-walking, which leads to a hyperbranched structure where branches form on other branches.

The authors opted for just such a late transition metal catalyst (1) well known for both its chain walking tendency and good copolymerisation capability with polar comonomers (Scheme 2). After activation, the Pd diimine complex (1) was able to produce a polyethylene with about 60 branches per 1000 carbon atoms. Although care had to be taken as the addition of a polar comonomer decreased the activity of the catalyst and reduced the molecular weight, under suitable conditions high molar masses of up to 340kg/mol were obtained. A moderate (ca. 1%) incorporation of the comonomer gave the polymer good elastic recovery.

The work illustrates the potential of making a TPE in a single step from ethylene and a bioderived polar comonomer.

Scheme 2 Structure of diimine Pd catalyst 1 for the copolymerisation of ethylene and 10-undecenoic acid 2

Alternating current photovoltaic effect

The photovoltaic effect converts light into a modest electrical current and a voltage. Solar cells, which make use of the photovoltaic effect, have become an attractive sustainable energy source. The only snag is that the photovoltaic effect generally produces a direct current. Z. L. Wang and co-workers have recently demonstrated that, under certain conditions, an alternating current can be obtained (Adv. Mater., 2020, 32, 1907249). They achieved this by periodically varying the light intensity from high to low, for example, by placing a rotating optical chopper into the path of the light beam.

When the light hit a typical photovoltaic material such as a p-doped silicon/titanium dioxide junction with aluminium and indium-tin oxide as electrodes on either side, it generated a direct current through the conventional photovoltaic effect. Superimposed on this were spikes of different polarities each time the light came on or shut off (Scheme 3).

When the chopper rotation frequency was increased to several hundred hertz, the periodic spikes became the dominant feature and surpassed even the current generated by the conventional photovoltaic effect.

The peak current reached values of 200µA with a maximum voltage of slightly over 20mV. The current output could be doubled by placing two devices in parallel, whereas the voltage increased 2-fold when the two devices were in series. The effect was seen for a variety of junctions, even a commercial solar cell, but not in organic photovoltaic materials since their initial carrier concentration is generally zero.

The authors explained the observed phenomenon by a short-time shift and realignment in the Fermi levels at the semiconductor junction every time the light comes on or off. The effect showed no decrease even after millions of cycles. The same device worked also as a highly sensitive photodetector under zero bias.

The reported AC photovoltaic effect could have applications ranging from a power source to a light sensor.

Scheme 3 Alternating current produced by a p-doped SI/TIO2 junction exposed to a periodically flashing light

Mg3Sb2 thermoelectrics

Thermoelectric materials offer the possibility to convert waste heat, or more precisely a heat gradient, into electricity. Over the last few years, perovskites have become a promising new group of thermoelectrics due to their simple synthesis, flexible structure and good overall performance.

B. B. Iverson and co-workers have reported a fast and low-cost synthesis of n-type Mg3Sb2 thermoelectrics through spark plasma sintering, which compacted the material at the same time (Angew. Chem. Int. Ed., 2020, 59, 4278).

Doping with small amounts of Sc or Te improved the figure of merit ZT to 0.4-1.5 across a wide temperature range (300-725K). These are respectable values comparable with other high-performance thermoelectrics but obtained through a much easier and faster approach than the multistep syntheses previously described.