New graphene application in supercapacitors
30 March 2012 - Forget batteries
|Forget batteries: think supercapacitors. You can even power a bus with it. Key thing to remember: compared to a regular battery electron density is modest but charging a supercapacitor is a lot faster with much lower losses and more endurance. In a recent Science article El-Kady et al. describe the use of graphene as a potential electrode in a supercapacitor device (DOI). Graphene is a great conductor with potentially a high surface area. Problem to solve: in conventional graphite oxide to graphene conversion, the graphene sheet restack as fast as they exfoliate. |
El-Kady deposited an aqueous solution of graphite oxide on a DVD disk, this disk was then laser-irradiated in a conventional DVD drive , converting the GO layer to a graphene layer and then the laser-scribed graphene (LSG) layer was peeled of. This material did not display restacking and a supercapacitor setup was constructed by sandwiching an ion porous PP sheet with an ionic liquid as electrolyte with two LSG sheets and then two PET sheets.
This device is able to deliver 4 volts with an energy sufficient to power a LED lamp for 30 minutes. Charging takes a couple of seconds, it does not lose capacitance with over 10,000 charging cycles. It also endures bending to up to 180 degrees without complaining which is important in potential applications in wearable technology.
29 March 2012 - The chemical dictionary
|Concept: Multiplet theory|
Invented by: Aleksei Balandin 1929
Principle: catalytic reaction takes place when key atoms in a substrate are optimally positioned in relation to key metal atoms. There is no distinct difference between homogeneous catalysis (one metal center) and heterogeneous catalysis (several metal centers). The fields blur with advent of nanoparticles, nanosalts (incorporating heteroatoms) and homogeneous catalyst fixation (heterogenization), metal atoms are known to leach into solution from a solid catalyst. Likewise nanoparticles are known to form in mononuclear meta complexes. Ananikov & Beletskaya call this a cocktail of catalysts.
Main challenges: experimental evidence: all dynamic nanoscale processes, indirect kinetic evidence.
Literature: Ananikov & Beletskaya Organometallics 2012 (DOI). Relevant quote: ''we argue that research efforts should be concentrated on developing a unified concept of catalysis rather than spreading the field into repeating reports of one more
example of catalyst or ligand''
24 March 2012 - Catalysis
|Ilija Coric and Benjamin List must have made the biggest chiral catalyst ever. The thing did not even fit on one of the pages of the letter to nature (DOI)and even in the supplemental information it looks highly distorted. Only enzymes are bigger but that is exactly what Corin and List had in mind: creating a synthetic catalyst that works like an emzyme (biomimetics) , rigid, completely encapsulating the substrate with an internal cavity that is as small as possible for maximum chiral induction. The main building block is the well-known chiral binol. The core is a imidodiphosphoric acid that has a basic site as well as a acidic site. When this Brønsted acid catalyst donates a proton the residue (X-) is a C2 symmetric chiral anion (concept introduced by Toste in 2007). The substrate is a vinyl ether and after accepting a proton the intermediate is a oxocarbenium ion. The delta-hydroxy group attached to it can add from the top of the plane or from below the plane depending on the chirality of the catalyst. The resulting spiro compound in this enantioselective synthesis is unremarkable except that it is a sex pheromone for the olive fruit fly. The (R) isomer attracts only the males and the (S) isomer only the females. We can only hope the List laboratory is a safe distance away from any olive orchard. |
Homogeneous catalysis not always what it seems
|Remember the good old days when heterogeneous catalysis was heterogeneous catalysis and homogeneous catalysis was homogeneous catalysis?. Take for example palladium: the metal itself is a well known heterogeneous catalyst and there is tris(dibenzylideneacetone)dipalladium(0), the soluble and homogeneous cat. Well not anymore!. At least according to Zalesskiy & Ananikov in the journal Organometallics (DOI). They found that commercially available Pd2(dba)3 contains up to 40% palladium nanoparticles and this is relevant because it a) is a catalyst superstar and b) researchers rarely bother to make their own. Zalesskiy & Ananikov investigated the catalyst in a chloroform solution with proton NMR which contrary to expectation yields very complex spectra. Therefore they looked at the DOSY variation and also at 2D-NMR. This allowed detection and quantification of free dba and two yet to be identified Pd2(dba)3 isomers and according to the researchers, if there is free dba then there has to be free Pd (as nanoparticles) as well. In commercial samples the ratio can vary between 1:0.09 and 1:0.56. |
The finding may explain why Pd2(dba)3 catalysed reactions are notoriously inconsistent. Take for example OS 68:47 with a typical Pd2(dba)3 application where the supplier was Strem Chemicals but where The submitters noted that distinctly lower enantioselectivity was obtained when the reagent of other suppliers was used.
Luckily Zalesskiy & Ananikov offer a recipe for fresh and reliable Pd2(dba)3. So lazy chemists pay attention! Take palladium acetate, sodium acetate and dibenzylideneacetone in methanol (the reducing agent). Stir at 40°C for 3 hours. Isolate the brown solid.
The continuous random network challenge
10 March 2012 - Unsolved problems in chemistry (V)
|Remember the good old days when an amorphous material was just amorphous and nothing else? Sorry, no long range order! Just last month we have been able to view for the first time amorphous silica on graphene via transmission electron microscopy (DOI). More recently though, Treacy & Borisenko have analysed amorphous silicon and find inhomogeneous paracrystalline structures containing local cubic ordering at the 10 to 20 angstrom length scale (DOI). |
In the classical model amorphous silicon forms a tetracoordinated continuous random network (CRN), consisting of a random network of 5,6 and 7-membered rings. It is metastable towards crystalline silicon that only has hexacycles. The coordination number for silicon is slightly less then than 4 for amorphous silicon compared to that of crystalline silicon and therefore also slightly less dense. A so-called reduced density function can be obtained from traditional electron diffraction and by modelling and usually the CRN model agrees with both methods.
Treacy & Borisenko manufactured a layer of amorphous silicon by Si ion implantation in a layer of crystalline Si and analysed it using a TEM technique called fluctuation electron microscopy which can measure local variations in density. Then they measured and modelled (Metropolis algorithm) reduced density functions of their own and found experiment and theory supported each other.
The continuous random network was first proposed in 1927. A paracrystalline model dates back to 1962. Figuring out what exactly amorphous silicon looks like may be relevant as ion implantation is an important tool in industrial semiconductor device fabrication.
|Today, on my way to a meeting I happened to pass the new (2007) Openbare Bibliotheek Amsterdam. I had some time to kill so I went in to have a look. They have some fantastic lamps in one of the main area's! Incidentally or purposely they are all hydrocarbon shaped. They must have some of the biggest anatomically correct butane models in the country. But who designed them? And where to buy my my personal cyclohexane lamp?|
04 March 2012 - Unsolved problems in chemistry
|Remember the good old days when carbon atoms could bond together by not more than a bond order of three as in acetylene?. Sason Shaik et al. in this month's Nature chemistry argue that for dicarbon it can be four as well giving C2 a quadruple bond (DOI).|
In the conventional scheme of things each of the carbon atoms in the dicarbon MO diagram is bringing a total of 6 electrons to the table. The 1s and 2s electrons combine and fill out both bonding and antibonding MO's and therefore do not contribute to bonding. The 2p electrons combine and fill out two degenerate pi MO's making the bond order 2. That is an unusual case of 2 pi bonds without any sigma bonds but with two sigma lone pairs. In an alternative picture C2 is a diradical with bond order 3.
In the new calculations Shaik arrives at a bond order of 4 with an energy of the fourth bond of between 11 and 14 kcal/mole compared to 100 kcal/mole for the sigma bond and 95 kcal/mole for the pi bonds. Valence bond theory and MO theory support each other on this one. This bond is called an inverted bond because just as in tetrahedrane the pi-shaped lobes are actually pointing away from each other.
Realistic? Just how the electrons in this bond are supposed to interact is unclear. The article hints at an experimental confirmation for the energy involved from spectroscopy but a review is cited and not a research article. As far as future experimental evidence Shaik suggests an investigation into the radical properties.
C2 by the way is just one in a whole sick list of troublesome molecules. In the accompanying editorial Jörg Grunenberg explains why. Compared to their colleagues of twenty years ago, these days computational chemists have all the computing power they need.
|The IBM Zurich people have been treating us to a pic of naphthalocyanine at the nanoscale exposing its electronic picture (DOI). |
But what exactly do we see? First of all, naphthalocyanine is a curious molecule, first investigated as a molecular logic gate. The molecule looks like the letter X with one axis (SW to NE) definitely aromatic and full of electrons and the other axis non-aromatic (compare to isoindole but look at all resonance structures) and devoid of electrons. The axis can interchange by a process of tautomerism where the two inner hydrogen atoms change position.
This molecule has already been scanned with regular atomic force microscopy and the new images are obtained by an AFM variation called Kelvin Probe Force Microscopy where a voltage is applied between the AFM tip and the surface (Where does Kelvin come in?). The naphthalocyanine molecules are isolated from the surface by double monolayer of NaCl.
As the electrons inside the molecule will interfere with the voltage, the local electron density can be measured. In addition to that, when the applied voltage matches the LUMO energy, a tautomerisation is induced in the molecule and the electron density of both states can be compared. The images clearly confirms high/low electron density where you would expect it. The amazing thing is that the contrast is maximized not by closing up on the molecule but by hovering over it from a small distance. Contrast therefore is mainly determined by the electron density at a certain height above the molecule. In the article round-up the IBM people expect in the near future to be able to monitor electron distribution changes in actual chemical reactions between molecules taking place on surfaces.
Synthetic caffeine is good for you
|This blog is not going to actually read Caffeine in your drink: natural or synthetic? by Lijun Zhang et al. (DOI). They imagine health issues with bad synthetic versus good natural caffeine and came up with a crazy over-the-top analytical tool ( high-temperature reversed-phase liquid chromatography / isotope ratio mass spectrometry if you need to know) to detect synthetic caffeine. According to Rajendrani Mukhopadhyay in C&EN it is against the law in some countries to replace natural with synthetic without mentioning it on the label. |
And what is so bad about this synthetic caffeine anyway. Compared to sourcing from coffee beans it is cheaper to manufacture, does not involve slave labour, is eco-friendly, does not require pesticides and quality control is easier.
But how is synthetic caffeine made? Wikipedia for some reason denies it exists. Thanks to a Ruhr University Bochum lab handout we have a clue. One method was invented by Wilhelm Traube already in 1900. Starting materials are dimethylurea and cyanoacetic acid. Reaction steps are acylation, then ring-closing with a Pinner reaction variation, then nitrosation (nitric acid), then reduction (sodium dithionite), then another ring-closing with formamide to theophylline and finally alkylation with iodomethane and a base. Modern industrial methods are adaptations. So remember next time at Starbucks: demand synthetic!