This method using cellulose nanofiber biopolymer covalent hydrogels to achieve a palladium metal aerogel composite may be generalizable to a wide range of biopolymer templates and metals. This composite aerogel synthesis method uses cellulose nanofibers as a biotemplate to achieve control over both pallium metal nanostructure and macroscopic aerogel monolith shape. The shape control and mechanical integrity of biotemplated metal aerogels should facilitate applications for catalysis, energy storage, and sensing.
This method can be applied to further develop biopolymer carbon metal templates and to achieve better control of three-dimensional nanostructures in composite aerogel materials. To prepare a cellulose nanofiber solution, first mix 1.5 grams of carboxymethyl cellulose nanofibers with 50 milliliters of deionized water. After shaking, vortex the solution for one minute, followed by a 24-hour incubation in a bath sonicator at ambient temperature to ensure complete mixing.
The next morning, add 0.959 grams of EDC and 0.195 grams of MES buffer to 2.833 milliliters of deionized water. Then, adjust the final volume to 10 milliliters and a pH of 4.5 with one-molar hydrochloric acid and deionized water. Next, transfer 0.25 milliliters of the 3%cellulose nanofiber solution into each of six microfuge tubes, and sediment the nanofibers by centrifugation.
Use a pipette to aspirate the excess water above the compacted nanofibers while avoiding contact with the top surface. Add one milliliter of the EDC and diamine crosslinking solution above the compacted cellulose nanofibers in each microfuge tube. Wait at least 24 hours for the crosslinking solution to diffuse through the gels to crosslink the cellulose nanofibers.
Then, aspirate the crosslinking solution supernatant from the microfuge tubes, and immerse the microfuge tubes in one liter of deionized water for at least 24 hours with the caps open to remove any excess crosslinking solution from within the nanofiber hydrogels. The next day, add approximately 0.5 milliliters of one 3%cellulose nanofiber solution in deionized water to the sample stage of a Fourier-transform infrared spectrometer, and scan the percent transmittance for 650 to 4, 000 reciprocal centimeters. To prepare the palladium solution, vortex 10 milliliters of one-molar palladium ammonium chloride for 15 seconds before diluting the solution to one-milliliter volumes at one, 10, 50, 100, 500, and 1, 000-millimolar concentrations in deionized water.
Next, add one milliliter of each dilution to the top of individual cellulose nanofiber hydrogel samples, and allow the palladium solutions to equilibrate within the hydrogels for 24 hours. The next day, prepare 60 milliliters of two-molar sodium borohydride and pipette 10 milliliters into each of six 15-milliliter conical tubes in a fume hood, and transfer the tubes of palladium equilibrated cellulose nanofiber hydrogels to the fume hood. Wearing the appropriate personal protection equipment, invert one microcentrifuge tube and gently tap the tube to remove the hydrogel, using flat tweezers to transfer the hydrogel into one of the tubes of sodium borohydride.
After 24 hours, transfer each reduced hydrogel into a second 24-hour, 0.5-molar sodium borohydride reduction solution before rinsing the cellulose nanofiber-palladium composite gels in 50 milliliters of deionized water in new conical tubes. Exchange the deionized water after 12 hours, and allow the gels to rinse for at least an additional 12 hours. Then, use flat tweezers to transfer the rinsed cellulose nanofiber-palladium gels to successive 50-milliliter volumes of 25%50%75%and 100%ethanol solutions for at least six hours per solution.
After the last solvent exchange, dry the hydrogels in a supercritical dryer with carbon dioxide, with a set point of 35 degrees Celsius and 1200 pounds per square inch. When the drying is complete, allow the chamber to equilibrate for at least 12 hours before opening the dryer for removal of the aerogels. To characterize the composite aerogels by scanning electron microscopy, use a razor blade to cut each gel into one-to two-millimeter-thick sections, and use carbon tape to fix the thin film sample onto a scanning electron microscope sample stub.
Load the stub onto the microscope, and use an initial accelerating voltage of 15 kilovolts and a beam current of 2.7 to 5.4 picoamps to image the sample. To analyze the aerogels by x-ray diffractometry, place the cellulose nanofiber-palladium aerogel in a sample holder, and align the top of the aerogel with the top of the holder. Then, perform x-ray diffraction scans for diffraction angles two theta from five to 90 degrees at 45 kilovolts and 40 milliamps with copper K-alpha radiation, a two theta step size of 0130 degrees, and 20 seconds per step.
For thermal gravimetric analysis, place an aerogel sample in the instrument crucible, and perform the analysis by flowing nitrogen gas at 60 milliliters per minute with heating at 10 degrees per minute from ambient temperature to 700 degrees Celsius. For nitrogen gas adsorption-desorption, degas the samples for 24 hours at room temperature before using nitrogen at minus 196 degrees Celsius as the test gas with equilibration times for adsorption and desorption of 60 and 120 seconds, respectively. For electrochemical characterization, immerse the aerogel samples in 0.5-molar sulfuric acid electrolyte for 24 hours before placing a lacquer-coated wire with a one-millimeter exposed tip in contact with the top surface of the aerogel at the bottom of the electrochemical vial.
Then, use a three-electrode cell with a silver/silver chloride 0.5-millimeter-diameter platinum wire auxiliary counter electrode and a lacquer-coated 0.5-millimeter-diameter platinum working electrode to perform electrochemical impedance spectroscopy from one megahertz to one millihertz with a 10-millivolt sine wave and cyclic voltammetry using a voltage range of minus 0.2 to 1.2 volts with scan rates of 10, 25, 50, and 75 millivolts per second. Fourier-transform infrared spectroscopy can be performed as demonstrated to confirm cellulose nanofiber hydrogel crosslinking. Here, covalently crosslinked cellulose nanofiber hydrogels before and after equilibration across a range of palladium ammonium chloride or sodium palladium chloride concentrations are shown.
Here, reduced cellulose nanofiber-palladium gels are shown before and after supercritical aerogel composite drying. In general, the aerogels present interconnected fibrillary ligaments with an increasing nanoparticle size, correlating with an increase in palladium solution concentration by scanning electron microscopy. X-ray diffractometry spectra for palladium and palladium hydride become more convoluted with increasing palladium synthesis concentration until the spectra are no longer distinguishable at 1, 000 millimolar, correlating with the increase in nanoparticle diameters observed by scanning electron microscopy.
Thermogravimetric spectra analysis reveals an increasing metal content in the cellulose nanofiber-palladium composite aerogels as the synthesis palladium solution concentration increases. The physisorption data demonstrates a Type IV adsorption-desorption isotherm, indicating a mesoporous and macroporous structure, and Barrett-Joyner-Halenda pore size analysis indicates a decreasing frequency of mesopores as the aerogel palladium content increases. Electrochemical impedance spectroscopy spectra illustrate the low charge transfer resistance and double layer capacitance for the cellulose nanofiber-palladium composite aerogel.
Further, cyclic voltammetry scans indicate hydrogen adsorption and desorption at potentials less than zero volts, as well as characteristic oxidation and reduction peaks for palladium greater than 0.5 volts. Remember to rinse the gels with incremental concentrations of water and ethanol as the osmotic swelling from large concentration differences may rupture the hydrogel. Incorporating other materials such as graphene and carbon nanotubes for composite biotemplates may be possible to achieve greater mechanical durability and conductivity of the aerogels.
The use of cellulose nanofiber covalent hydrogels to achieve porous metal composite aerogels offers a synthesis route for other noble and transition metal materials in a variety of form factors. High concentrations of aqueous sodium borohydride results in the production of flammable hydrogen gas. It is important to electrochemically reduce the samples in a well-ventilated area, away from open flames.
