This method can help answer key questions in the aerogel field about the possibility of directly synthesizing metal aerogels from aqueous solutions rather than from preformed nanoparticles. Other methods, such as sol gels, require much longer times for nanoparticle coalescence. The implications of this technique extend toward catalysis and sensing applications due to the high surface area of the noble metal aerogels and the lack of support materials.
Though this method can provide insight into direct aerogel synthesis, it can also be applied to other synthesis techniques, such as biotemplating, to achieve better shape control over aerogel monoliths. To begin the procedure, prepare two milliliters each of 0.1 molar solutions of gold(III)chloride trihydrate and sodium tetrachloropalladate in deionized water. Vigorously shake and vortex the solutions to dissolve the salts.
Let the palladium solution sit for a few hours to ensure complete dissolution. Prepare two milliliters of 0.1 molar potassium hexachloroplatinate in a one to one by volume mixture of deionized water and ethanol. Vigorously shake and vortex the mixture until the salts have dissolved.
Prepare 10 milliliters each of 0.1 molar solutions of dimethylamine borane and sodium borohydride in deionized water. Vortex the solutions for one to two minutes to ensure complete dissolution. Then, transfer 0.5 milliliters of the gold solution to a 1.7-milliliter or two-milliliter microcentrifuge tube.
Forcefully pipette 0.5 milliliters of DMAB into the tube to rapidly mix the salt and reducing agent. Place the microcentrifuge tube in a rack, leaving the cap open to allow hydrogen gas to escape. In the same way, combine 0.5 milliliters of the platinum solution with 0.5 milliliters of DMAB and 0.5 milliliters of the palladium solution with 0.5 milliliters of sodium borohydride.
Approximately five minutes after starting the reaction, cap the microcentrifuge tubes, and gently invert them three to five times to aid coalescence of metal particles not part of the metal gel. Immediately uncap the tubes, and return them to the rack after inversion. Leave the nascent metal gels in the reducing agent solutions for three to six hours to allow complete reduction of metal ions and surface free energy minimization to occur.
Then, carefully remove excess reducing agent from each tube, ensuring that enough solution remains so that the meniscus does not contact the gel. Slowly pipette deionized water into the microcentrifuge tubes until they are filled completely. Then, fill three 50-milliliter conical tubes with deionized water.
Submerge each microcentrifuge tube in a tube of deionized water, and allow each gel to slide from its microcentrifuge tube into the larger tube. Gently move the gel with a spatula if needed. Cap the tubes, and leave the gels in deionized water for 24 hours.
Replace the water after 12 hours by solvent exchange. Do not allow the meniscus to contact the gels while replacing the water. To begin the EIS procedure for wet metal gels, remove as much water as possible without the meniscus contacting the rinsed gels.
Add 50 milliliters of 0.5 molar potassium chloride to each tube, and allow the gels to soak for 24 hours. Then, use a fine bristle brush to coat a one-millimeter potassium wire electrode with non-reactive lacquer, leaving four to five millimeters exposed at the tip. Allow the lacquer to dry for 20 minutes.
Apply at least one more coat of lacquer in the same way. Next, cut a 50-milliliter conical tube in half to serve as the electrochemical vial. Insert a saturated silver-silver chloride reference electrode and a 0.5-millimeter potassium wire counter electrode through a perforated cap or gasket for the modified tube.
Connect the three electrodes to a potentiostat. Transfer one of the gels and its electrolyte to the modified tube by gently pouring the electrolyte and equilibrated gel into the tube without letting the gel contact the liquid-air interface. Ensure that the gel is settled at the bottom of the tube, and add electrolyte if necessary.
Then, place the cap on the tube, and adjust the reference and counter electrodes as needed. For the gold gel, gently insert the bare tip of the lacquer-coated working electrode into the gel. For the palladium and platinum gels, place the lacquer-coated working electrode in the conical vial along the inner surface, and rest the metal gel on top of the bare platinum wire tip.
Perform potentiostatic EIS scans with frequencies between 100 megahertz and one millihertz using a 10 millivolt amplitude sine wave. In the event of current overflow, use a galvanostatic EIS with the same frequency range and a 100 to 200 milliamp sine wave amplitude. Following electrochemical characterization, rinse the gels with deionized water for 24 hours as described at the end of the gel synthesis.
Then, remove as much of the rinse as possible without letting the meniscus contact the gels. Freeze the gels at negative 80 degrees Celsius for 30 minutes. Lastly, freeze dry the gels in a freeze dryer with a pressure set point of four pascals or lower to obtain the metal aerogels.
The gold, palladium, and platinum aerogels all showed characteristic hydrophobicity and had properties comparable to gels formed by slower synthesis techniques. Scanning electron microscopy showed that gold aerogels had large nanopores and smooth ligaments. Palladium and platinum aerogels had a beads-on-a-string structure with fused nanoparticles and narrower ligaments.
The platinum aerogel had a greater range of macropore structure diameters than the palladium aerogel, which was attributed to platinum nanoparticle stability requiring the use of ethanol to drive coalescence and the consequent ease of large hydrogen gas bubble evolution during gel formation. X-ray diffraction showed characteristic peaks for the respective metals with no detectable oxides. Nitrogen gas physisorption isotherms were Type IV, which is characteristic of mesoporous materials.
The pore size distribution and cumulative pore volume modeled from the isotherms showed pore sizes predominantly in the two to 50 nanometer range, confirming that the aerogels were mesoporous. The electrochemical impedance spectra were fit with a transmission line model based on a modified Randle's equivalent circuit model. The specific capacitances and specific surface areas were calculated from the EIS data.
These values were consistent with the specific capacitances and surface areas determined from cyclic voltammetry experiments. We first developed the idea for this method based on combining 100 millimolar noble metal solutions and reducing agents for other projects prior to discarding them and observing the rapid formation of metal gels. Once mastered, this technique can result in noble metal gels within minutes prior to rinsing and drying steps.
While attempting this procedure, remember to avoid contact of the metal gels with the liquid-air interface in order to prevent gel compaction. Following this procedure, other methods like biotemplating may be performed to address the challenges of aerogel shape control.
