Carbon dioxide reduction: A cheap and selective route
Solar-driven electrochemical reduction of CO2 can be a valuable approach to produce chemical energy in the form of fuels. A major hurdle, however, is to produce a specific product, such as CO, formic acid, methane, ethylene or ethanol with high selectivity, high yield and low cost. Now, Schreier et al. report an inexpensive electrocatalyst made of Earth-abundant elements, which is capable of catalysing the reduction of CO2 to CO with selectivity up to 97%.
The researchers fabricate the electrocatalyst by atomic deposition of SnO2 onto CuO nanowires. After just two to five cycles of deposition, the selectivity of the catalyst towards CO production is maximized. According to Schreier et al., such high selectivity could be attributed to the fact that SnO2 decreases the adsorption energy of CO and hydrogen to the surface, preventing further reduction of CO. Aside from high selectivity, this SnO2-modified CuO nanowire electrocatalyst is also bifunctional: it catalyses both CO2 reduction and water oxidation. Consequently, the researchers fabricate a CO2 reduction device powered by a triple-junction (GaInP/GaInAs/Ge) photovoltaic cell, using the bifunctional electrode for both CO2 reduction and oxygen evolution, water as electron source and a bipolar membrane as separator. The photo-driven conversion efficiency of CO2 to CO peaks at 13.4% under simulated solar illumination.
2D Materials: Nano-tiles at large
Large-scale fabrication of devices based on 2D materials requires scalable and low-cost deposition techniques that go beyond traditionally used mechanical transfer and high-temperature growth. Solution-based methods offer quick and simple ways of thin-film deposition but do not provide sufficient control of the thickness and morphology of coatings. Matsuba et al. now report a spin-coating method to obtain dense large-area mono- and multilayers formed of neatly tiled 2D nanosheets.
The researchers disperse several types on nanosheets including graphene oxide and its reduced form into DMSO/tetrabutylammonium (TBA) hydroxide solution. When spin-coated under optimized conditions, the suspensions form a uniform and ordered monolayer with edge-to-edge packed nanosheets showing negligible overlaps. Due to its high viscosity, DMSO acts as a binder for the nanosheets, preventing them from spreading out during the deposition process, whereas TBA cations connect the basal planes of adjacent nanosheets, promoting a monolayer formation. The uniformity of the obtained monolayer films is preserved within a radius of 8 mm. A layer-by-layer deposition of thicker films can be achieved by successive spin-coating steps. The films require post-deposition annealing to ensure better adhesion to the substrate.
Monolayer tiling of nanosheets can be applied to other 2D materials, although further optimization is required for industrial-scale production.
Sensing: Red-flagging counterfeit food
DNA barcoding allows unambiguous species identification through genetic sequencing and can serve as an analytical tool to prevent food fraud. However its routine application is hampered by strict requirements regarding sample treatment and the need for specialized equipment and handling personnel. Valentini et al. have now developed a method for rapid, naked-eye detection of counterfeit food. Similar to barcoding, their technique — called NanoTracer — is based on the identification of species-specific DNA sequences through common molecular biology methods, but the detection is a colorimetric assay that exploits the plasmonic properties of gold nanoparticles, requiring simple laboratory techniques.
The researchers use ordinary DNA extraction kits to isolate the DNA from samples of perch and saffron, two food items that are often mislabelled or contaminated. With a specific polymerase chain reaction, they amplify the DNA regions that are unique to the investigated species and generate single-stranded fragments (amplicons). For authentication, Valentini et al. amplify a DNA region from perch, whereas to detect spice contamination they select the DNA of known saffron contaminants. The generated amplicons contain, by design, not only the DNA sequence of interest but also a universal tag that can hybridize to complementary oligonucleotide sequences attached to gold nanoparticles. DNA coupling triggers nanoparticle aggregation and induces a red-to-violet colour change of the solution — a visual indication of the presence of the desired species.
The entire test, from preparation of the food matrix to the colour-change determination, takes three hours and can detect the presence of contaminants down to 1% by weight.
Surface science: Molecular rotors en marche
Molecules on a surface can assume switchable configurations that can be toggled by applying a voltage pulse using a scanning tunnelling microscope (STM) tip. This motion might be exploited for computation if the switching could be coupled among adjacent molecules. Now, Wasio et al. show that a self-assembled array of molecular rotors exhibits correlated switching in response to a voltage pulse.
The researchers prepare a two-dimensional crystal in which two brominated ethylbenzene molecules are linked through a Cu adatom on the surface. This interaction forces the ethyl rotor groups to assume either an up or down configuration. A high-voltage pulse supplied by an STM tip localized on top of one rotor causes its configuration to switch. By imaging the surface, Wasio et al. observe that adjacent rotors, namely the next closest rotor and the third rotor over, also change their configuration as a result of the high-voltage pulse. According to Monte Carlo simulations, this correlated switching is due to the intermolecular interaction among the molecules and the crystal packing on the surface. Using molecules with different packing arrangements, the researchers show that correlated switching occurs at different places with respect to the location of the triggering pulse.
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Bubnova, O., Moscatelli, A., Pastore, C. et al. Our choice from the recent literature. Nature Nanotech 12, 721 (2017). https://doi.org/10.1038/nnano.2017.171