Name: a salt-templated synthesis method for porous platinum-based macrobeams and macrotubes
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Description: Watch this Scientific Journal Video about A Salt-Templated Synthesis Method for Porous Platinum-based Macrobeams and Macrotubes at JoVE.com

This work was funded by a United States Military Academy Faculty Development Research Fund grant. The authors are grateful for the assistance of Dr. Christopher Haines at the U.S Army Combat Capabilities Development Command. The authors would also like to thank Dr. Joshua Maurer for the use of the FIB-SEM at the U.S. Army CCDC-Armaments Center at Watervliet, New York.

CAUTION: Consult all relevant chemical safety data sheets (SDS) before use. Use appropriate safety practices when performing chemical reactions, to include the use of a fume hood and personal protective equipment. Rapid hydrogen gas evolution during electrochemical reduction can cause high pressure in reaction tubes causing caps to pop and solutions to spray out. Ensure that reaction tube caps remain open as specified in the protocol. Conduct all electrochemical reductions in a fume hood.
1. Mag...

<ol>
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This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials. The use of Magnus&#8217; salt derivatives as high aspect ratio needle shaped templates provides the means to control resulting metal composition through salt-template stoichiometry, and when combined with choice of reducing agent, control over the nano...

Numerous synthesis methods have been developed to obtain high surface area, porous platinum-based materials primarily for catalysis applications including fuel cells1. One strategy to achieve such materials is to synthesize monodisperse nanoparticles in the form of spheres, cubes, wires, and tubes2,3,4,5. To integrate the discrete nanoparticles into a porous structure for a functional device, polymeric binders and carbon additives are often required6,7. This strategy requires extra processing steps, time, and can lead to a decrease in mass specific performance, as well as agglomeration of nanoparticles during extended device use8. Another strategy is to drive the coalescence of synthesized nanoparticles into a metal gel with subsequent supercritical drying9,10,11. While advancements in the sol-gel synthesis approach for noble metals has reduced gelation time from weeks to as fast as hours or minutes, the resulting monoliths tend to be mechanically fragile impeding their practical use in devices12.
Platinum-alloy and multi-metallic 3-dimensional porous nanostructures offer tunability for catalytic specificity, as well as address the high cost and relative scarcity of platinum13,14. While there have been numerous reports of platinum-palladium15,16 and platinum-copper17,18,19 discrete nanostructures, as well as other alloy combinations20, there have been few synthesis strategies to achieve a solution-based technique for 3-dimensional platinum alloy and multi-metallic structures.
Recently we demonstrated the use of high concentration salt solutions and reducing agents to rapidly yield gold, palladium, and platinum metal gels21,22. The high concentration salt solutions and reducing agents were also used in synthesizing biopolymer noble metal composites using gelatin, cellulose, and silk23,24,25,26. Insoluble salts represent the highest concentrations of ions available to be reduced and were used by Xiao and colleagues to demonstrate the synthesis of 2-dimensional metal oxides27,28. Extending on the demonstration of porous noble metal aerogels and composites from high concentration salt solutions, and leveraging the high density of available ions of insoluble salts, we used Magnus&#8217; salts and derivatives as shape templates to synthesize porous noble metal macrotubes and macrobeams29,30,31,32.
Magnus&#8217; salts assemble from the addition of oppositely charged square planar platinum ions [PtCl4]2- and [Pt(NH3)4]2+&#160;33. In a similar manner, Vauquelin&#8217;s salts form from the combination of oppositely charged palladium ions, [PdCl4]2- and [Pd(NH3)4]2+&#160;34. With precursor salt concentrations of 100 mM, the resulting salt crystals form needles 10s to 100s of micrometers long, with square widths approximately 100 nm to 3 &#956;m. While the salt-templates are charge neutral, varying the Magnus&#8217; salt derivatives stoichiometry between ion species, to include [Cu(NH3)4]2+, allows control over the resulting reduced metal ratios. The combination of ions, and the choice of chemical reducing agent, result in either macrotubes or macrobeams with a square cross section and a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Macrotubes and macrobeams were also pressed into free standing films, and electrochemically active surface area was determined with electrochemical impedance spectroscopy and cyclic voltammetry. The salt-template approach was used to synthesize platinum macrotubes29, platinum-palladium macrobeams31, and in an effort to lower material costs and tune catalytic activity by incorporating copper, copper-platinum macrotubes32. The salt-templating method was also demonstrated for Au-Pd and Au-Pd-Cu binary and ternary metal macrotubes and nanofoams30.
Here, we present a method to synthesize platinum, platinum-palladium, and copper-platinum bi-metallic porous macrotubes and macrobeams from insoluble Magnus&#8217; salt needle templates29,31,32. Control of the ion stoichiometry in the salt needle templates provides control over resulting metal ratios after chemical reduction and can be verified with x-ray diffractometry and x-ray photoelectron spectroscopy. The resulting macrotubes and macrobeams may be assembled and formed into a free-standing film with hand pressure. The resulting films exhibit high electrochemically active surface areas (ECSA) determined by electrochemical impedance spectroscopy and cyclic voltammetry in H2SO4 and KCl electrolyte. This method provides a synthesis route to control platinum-based metal composition, porosity, and nanostructure in a rapid and scalable manner that may be generalizable to a wider range of salt-templates.

<table>
	<tbody>
		<tr>
			<td>50 mL Conical Tubes</td>
			<td>Corning Costar Corp.</td>
			<td>430290</td>
			<td></td>
		</tr>
		<tr>
			<td>Ag/AgCl Reference Electrode</td>
			<td>BASi</td>
			<td>MF-2052</td>
			<td></td>
		</tr>
		<tr>
			<td>Cu(NH3)4SO4&#159;&#8226;H2O</td>
			<td>Sigma-Aldrich</td>
			<td>10380-29-7</td>
			<td></td>
		</tr>
		<tr>
			<td>dimethylamine borane (DMAB)</td>
			<td>Sigma-Aldrich</td>
			<td>74-94-2</td>
			<td></td>
		</tr>
		<tr>
			<td>K2PtCl4</td>
			<td>Sigma-Aldrich</td>
			<td>10025-99-7</td>
			<td></td>
		</tr>
		<tr>
			<td>Miccrostop Lacquer</td>
			<td>Tober Chemical Division</td>
			<td>NA</td>
			<td></td>
		</tr>
		<tr>
			<td>Na2PdCl4</td>
			<td>Sigma-Aldrich</td>
			<td>13820-40-1</td>
			<td></td>
		</tr>
		<tr>
			<td>NaBH4</td>
			<td>Sigma-Aldrich</td>
			<td>16940-66-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Polarized Optical Microscope</td>
			<td>AmScope</td>
			<td>PZ300JC</td>
			<td></td>
		</tr>
		<tr>
			<td>Potentiostat</td>
			<td>Biologic-USA</td>
			<td>VMP-3</td>
			<td>Electrochemical analysis-EIS, CV</td>
		</tr>
		<tr>
			<td>Pt wire electrode</td>
			<td>BASi</td>
			<td>MF-4130</td>
			<td></td>
		</tr>
		<tr>
			<td>Pt(NH3)4Cl2&#159;&#8226;H2O</td>
			<td>Sigma-Aldrich</td>
			<td>13933-31-8</td>
			<td></td>
		</tr>
		<tr>
			<td>Scanning Electron Microscope</td>
			<td>FEI</td>
			<td>Helios 600</td>
			<td>EDS performed with this SEM</td>
		</tr>
		<tr>
			<td>Shelf Rocker</td>
			<td>Thermo Scientific</td>
			<td>Vari-Mix&#8482; Platform Rocker</td>
			<td></td>
		</tr>
		<tr>
			<td>Snap Cap Microcentrifuge Tubes, 1.7 mL</td>
			<td>Cole Parmer</td>
			<td>UX-06333-60</td>
			<td></td>
		</tr>
		<tr>
			<td>X-ray diffractometer</td>
			<td>PanAlytical</td>
			<td>Empyrean</td>
			<td>X-ray diffractometry</td>
		</tr>
		<tr>
			<td>X-ray photoelectron spectrometer</td>
			<td>ULVAC PHI - Physical Electronics</td>
			<td>VersaProbe III</td>
			<td></td>
		</tr>
	</tbody>
</table>

a salt-templated synthesis method for porous platinum-based macrobeams and macrotubes

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed by rinsing and supercritical drying steps, often resulting in mechanically fragile materials. Here, a method to synthesize nanostructured porous platinum-based macrotubes and macrobeams with a square cross section from insoluble salt needle templates is presented. The combination of oppositely charged platinum, palladium, and copper square planar ions results in the rapid formation of insoluble salt needles. Depending on the stoichiometric ratio of metal ions present in the salt-template and the choice of chemical reducing agent, either macrotubes or macrobeams form with a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Elemental composition of the macrotubes and macrobeams, determined with x-ray diffractometry and x-ray photoelectron spectroscopy, is controlled by the stoichiometric ratio of metal ions present in the salt-template. Macrotubes and macrobeams may be pressed into free standing films, and the electrochemically active surface area is determined with electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials.

The addition of oppositely charged square planar noble metal ions results in near instantaneous formation of high aspect ratio salt crystals. The linear stacking of square planar ions is shown schematically in Figure 1, with the polarized optical microscopy images revealing salt needles that are 10&#8217;s to 100&#8217;s of micrometers long. A concentration of 100 mM was used for all platinum, palladium, and copper salt solutions. While the salt needle templates are charge neutral in that th...

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

A synthesis method to obtain porous platinum-based macrotubes and macrobeams with a square cross section through chemical reduction of insoluble salt-needle templates is presented.

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed ...

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.

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