Platinum in all shapes and sizes

14 May 2011 - Nanotechnology

platinum nanotech.jpgOne of the main themes in nanotechnology is crystal morphology tinkering. The challenge is to take any inorganic crystalline material and come up with novel shapes at the nanolevel and of course built from the ground up. And in the field of heterogeneous catalysis any new metal catalyst morphology holds the promise of new catalytic activity. A fair share of the work concerns platinum as the next three examples from the recent literature illustrate. And what did you say the reducing agent was?

First up: the research group led by C.B. Murray (DOI). Their idea: as multifaceted crystals tend to grow from the faces with the highest energy and as surfactants can selectively adhere to a particular face lowering this energy, different surfactants can help to grow differently shaped nanocrystals. The advantage of using peptides as the surfactant component is that they can be tailored by their specific monomer sequence.

In their recipe chloroplatinic acid is reduced by two reducing agents: sodium borohydride and ascorbic acid. The first one (fast) is added as a single-shot and will initiate platinum nucleation as truncated tetrahedron. The second one takes it slower and reduction is continued with platinum atoms growing from the crystal face lowest in energy depending on what surfactant is used. The peptide Ac-Thr-Leu-Thr-Thr-Leu-ThrAsn-CONH2 for example produces cubes and Ac-Ser-Ser-Phe-Pro-Gln-Pro-Asn-CONH2 produces tetrahedrons. These peptides were selected from a large commercially available chemical library of peptides. Procedural details are fuzzy (journal editors: pay attention!) but the library (how big?) was exposed somehow to Pt cubes and octahedrons sythesised from established methods and in some manner the relative peptide affinity for either shapes was measured at some stage involving infecting Escherichia coli (no, the supplementary information did not help).

Another novel development is the platinum nanolawn as recently described by Shen et al (DOI). To create one apparently all you have to do is heat a solution of Na2Pt(OH)6 in water (chloroplatinic acid does not work) at 300°C on a titanium oxide surface. The resulting lawn density is 80 nanowires per 100 square micron, with wires diameter 34 nanometer and height up to 6 micron. Shen et al. claim platinum is reduced by electrons generated from TiO2, photocatalytically at first and then thermally. This is somewhat confusing since the whole procedure never mentions a light source.

In the meanwhile Cavaliere et al. (DOI) have been busy doing the opposite: TiO2 nanofibers covered by platinum. Take a solution of PVP (carrier polymer), Pt(II) 2,4-pentanedionate, titanium(IV)isopropoxide, niobium ethoxide (Nb doping required) and make fiber by electrospinning. Heating to 500°C removes the carrier polymer and deposits Pt nanoparticles as little nanobugs. Again the hunt was on for the nature of the reducing agent, trace alcohol left or carbon monoxide from polymer decomposition were suggested.