A Salt-Templated Synthesis Method for Porous Platinum-based Macrobeams and Macrotubes

This protocol offers a simple, relatively fast method for synthesizing high surface area, high aspect ratio platinum in platinum alloys macrobeams and macrotubes with a square cross-section. The salts and plating method allows control of the template metal ion ratio and resulting mass composition, and of the macrobeam and macrotube nano structures. Macrobeam and macrotube pressed films may address the need for integral three-dimensional electrodes for catalysis and sensing applications.

The ability of Magna salt derivatives to be chemically reduced to form macrobeams and macrotubes, suggest that the salt templating synthesis method may be applied to a wider range of metal salts. To prepare Magnus salts with a one to zero to one platinum two positive to platinum two negative ratio, add 0.5 milliliters of 100 millimolar potassium tetrachloroplatinate into a microfuge tube, and forcefully pipette 0.5 milliliters of 100 millimolar tetraammineplatinum(II)chloride hydrate in water into the tube. The resulting one milliliter volume salt needle template solutions will exhibit an opaque like green color.

To prepare a one to one to zero platinum-palladium salt needle template, add 0.5 milliliters of tetraammineplatinum(II)chloride hydrate in water to a microcentrifuge tube and forcefully pipette 0.5 milliliters of 100 millimolar sodium tetrachloropalladate to the tube. To prepare a two to one to one platinum-palladium salt needle template, add 0.25 milliliters of 100 millimolar sodium tetrachloropalladate and 0.25 milliliters of 100 millimolar potassium tetrachloroplatinate to a microfuge tube. Vortex the tube for three to five seconds, before forcefully pipetting 0.5 milliliters of 100 millimolar tetraamminePlatinum(II)chloride hydrated water to the tube.

To prepare a three to one to two platinum-palladium salt needle template, pipette 0.167 milliliters of 100 millimolar sodium tetrachloropalladate and 0.333 milliliters of 100 millimolar potassium tetrachloroplatinate to a microfuge tube. After vortexing, forcefully pipette 0.5 milliliters of 100 millimolar tetraamminePlatinum(II)chloride hydrated water to the tube. Salt-templates with a higher platinum ratio should yield a greener color, while templates with increasing palladium contents result in more orange, pink, and brown colors within the solution.

To prepare a one to zero to one salt ratio copper-platinum salt needle template, add 0.5 milliliters of 100 millimolar potassium tetrachloroplatinate to a microfuge tube and forcefully add 0.5 milliliters of 100 millimolar tetraammineCopper(II)sulfate in water to the microfuge tube. To prepare a three to one-to-two salt ratio copper-platinum salt needle template, add 0.167 milliliters of 100 millimolar tetraamminePlatinum(II)chloride hydrated water and 0.333 milliliters of 100 millimolar tetraammineCopper(II)sulfate in water to the tube. After vortexing, forcefully add 0.5 milliliters of 100 millimolar potassium tetrachloroplatinate to the tube.

To prepare the two to one-to-one salt ratio copper-platinum salt needle template, add 0.25 milliliters of 100 millimolar tetraamminePlatinum(II)chloride hydrated water and 0.25 milliliters of 100 millimolar tetraammineCopper(II)sulfate in water to a microfuge tube, and vortex the microfuge tube for three to five seconds. Then, forcefully pipette 0.5 milliliters of 100 millimolar potassium tetrachloroplatinate to the tube. To prepare the one to one to zero salt ratio copper-platinum salt needle template, pipette 0.5 milliliters of 100 millimolar tetraamminePlatinum(II)chloride hydrated water to a microfuge tube, and forcefully pipette 0.5 milliliters of 100 millimolar potassium tetrachloroplatinate into the tube, to obtain a one milliliter salt needle template solution.

The combination of copper and platinum ions will result in the formation of a purple cloudy solution that is not as opaque as the platinum and palladium solutions. To perform a chemical reduction of the platinum-palladium salt-templates, add 50 milliliters of 0.1 molar sodium borohydride solution into each of four 50 milliliter conical tubes, and add the entire one milliliter volume of one platinum-palladium salt-template solution to each tube. To perform a chemical reduction of the copper-platinum salt-templates, add 50 milliliters of 0.1 molar DMAB solution into each of four 50 milliliter conical tubes, and add the entire one milliliter volume of one copper-platinum salt-template solution to each tube under a fume hood.

After 24 hours, slowly decaf the supernatant from each reduced solution into a waste container, taking care, not to pour out the samples and transfer the precipitates into new 50 milliliter tubes. Fill each tube with 50 milliliters of deionized water and incubate the tightly capped tubes for 24 hours with gentle rocking. The next day, place the tubes upright in a tube rack for 15 minutes to allow the samples to sediment before slowly pouring off the supernatant.

Refill each tube with 50 milliliters of the ionized water, and rock the samples for an additional 24 hours. At the end of the incubation, place the tubes in a rack for 15 minutes before decanting as much of the clear or gray supernatants as possible. To prepare macrotube and macrobeam films, use a pipette or a spatula to gently transfer the precipitant material from each to tube onto individual glass slides, and consolidate the samples into uniform piles, approximately 0.5 millimeters.

Then place the slides in a location that will not be disturbed by air currents for 24 hours. When the samples have dried, place a second glass slide onto each dried reduced sample, and manually apply approximately 200 kilopascals of force to the top slide to create a thin film of macrotubes, or macrobeams on the bottom slide. For scanning electron microscopy of the samples, use carbon tape to fix the thin film to a scanning electron microscopy sample stub, and set the initial accelerating voltage to 15 kilovolts, and the beam current to 2.7 to 5.4 picoamps.

Then zoom out to a large sample area, and collect an energy dispersive X-Ray spectrum to quantify the elemental composition of the sample. For X-Ray diffract-dimetric analysis, place the thin film sample slide onto the scanning stage and perform X-Ray diffractometry scans for diffraction angles to theta, from five to 90 degrees at 45 kilovolts and 40 milliamps with Copper K-alpha radiation, a two theta step size of 0.0130 degrees and 20 seconds per step. To normalize the electrochemical measurements by milligrams of active materials, transfer the samples into individual electrochemical vials, and gently add 0.5 molar sulfuric acid to each sample for a 24 hour incubation at room temperature.

The next day place the lacquer coated wire with a one millimeter exposed tip from individual three electrode cells in contact with the top surface of the film at the bottom of each electrochemical vial. And perform electrochemical impedance spectroscopy from one megahertz to one millihertz with a 10 millivolt sine wave at zero volts. Then perform cyclic voltammetry using a voltage range of negative 0.2 to 1.2 volts, with scan rates of 10, 25, 50, 75 and 100 millivolts per second.

The addition of oppositely charged square planar noble metal ions results in near instantaneous formation of high aspect ratio salt crystals. The chemical reduction of Magnus salts formed with a one-to-one ratio of platinum positive to platinum negative ions, and reduced sodium borohydride results with macrotubes with generally a hollow inner cavity and porous sidewalls. The macrotubes generally conform to the geometry of the salt needle templates, with flat sidewalls and a square cross-section, DMAB reduced copper-platinum macrotubes present the most distinct and largest square cross-sections, with approximately three micrometers sides.

The DMAB be reduced copper-platinum macrotube sidewalls also demonstrate a highly textured surface without a significant porosity. Platinum and platinum-palladium macrotube and macrobeam chemical composition can be initially characterized with X-Ray diffraction. X-Ray diffraction analysis of DMAB reduced macrotubes reveal super-imposed peaks that shift towards either platinum or copper, depending on the relative salt-template stoichiometry, suggesting an alloy composition.

Sodium borohydride reduced copper-platinum macrobeams, exhibit distinct copper and platinum X-Ray diffraction peaks suggesting a bimetallic composition. X-Ray photo electron spectra for Platinum macrotubes indicate little evidence of an oxide species, suggesting a catalytically active surface. X-Ray photo electron spectra for platinum-palladium macrobeams also present no indication of metal oxide content.

Well, the pressed films can be manipulated with tweezers. Care must be taken when transferring the films into the electrochemical vials to prevent fracturing. Given the ability to press macrobeams and macrotubes into integral films, mechanical characterization to determine elastic and flexible modular can also be performed.

Salt-templates, for the synthesis of pores, high surface area materials should enable researchers to explore a wider range of metal salts and resulting metal, alloy and multi metallic materials.