A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

Summary

A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Abstract

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. Conducting the reduction step in a microcentrifuge tube or small volume conical tube facilitates a proposed nucleation, growth, densification, fusion, equilibration model for gel formation, with final gel geometry smaller than the initial reaction volume. This method takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step, and as a consequence of reagent concentrations. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. After rinsing and freeze drying, the resulting aerogel structure is examined with scanning electron microscopy, X-ray diffractometry, and nitrogen gas adsorption. The synthesis method and characterization techniques result in a close correspondence of aerogel ligament sizes. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

Introduction

A wide range of energy storage and conversion, catalysis, and sensor applications benefit from three-dimensional metallic nanostructures which provide control over chemical reactivity, and mass transport properties^1,2,3,4,5. Such 3-dimensional metallic nanostructures further enhance conductivity, ductility, malleability, and strength^8,9. Integration into devices necessitates that materials be free-standing or combined with support materials. Incorporation of nanomaterials onto support structures provides a means of minimizing active material, but may suffer from weak adsorption and eventual agglomeration during device operation^10,11.

While there are a variety of synthesis methods to control individual nanoparticle size and shape, few approaches enable control over contiguous 3-dimensional nanomaterials^12,13,14. Noble metal 3-dimensional nanostructures have been formed through dithiol linkage of monodisperse nanoparticles, sol-gel formation, nanoparticle coalescence, composite materials, nanosphere chains, and biotemplating^15,16,17,18. Many of these approaches require synthesis times on the order of days to weeks to yield desired materials. Noble metal nanofoams synthesized from the direct reduction of precursor salt solutions have been prepared with a faster synthesis timescale and with short-range order of hundreds of micrometers in length, but require mechanical pressing for device integration^19,20.

First reported by Kistler, aerogels provide a synthesis route to achieve porous structures with high specific surface areas that are orders of magnitude less dense than their bulk material counterparts^21,22,23. Extending 3-dimensional structures to the macroscopic length scale of bulk materials offers an advantage over nanoparticle aggregates or nanofoams that require support materials or mechanical processing. While aerogels provide a synthesis route to control porosity and particle feature size, however, extended synthesis times, and in some cases the use of capping agents or linker molecules, increases overall processing steps and time.

Here a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented²⁴. Combining various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. The use of a microcentrifuge tube or small volume conical tube takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step facilitating a proposed nucleation, growth, densification, fusion, equilibration model for gel formation. A close correlation in aerogel nanostructure feature sizes is determined with scanning electron microscopy image analysis, X-ray diffractometry, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

Protocol

CAUTION: Consult all relevant 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 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.

1. Metal Gel Preparation

1. Preparation of metal ion solutions.
   1. Prepare 2 mL of 0.1 M solutions of the following salts: HAuCl₄•3H₂O and Na₂PdCl₄ in deionized water. Prepare 2 mL of 0.1 M K₂PtCl₆ in a 1:1 (v/v) water and ethanol solvent. Vigorously shake and vortex solutions to aid in the dissolution of the salts.
2. Preparation of reducing agent solutions.
   1. Prepare 10 mL of 0.1 M solutions of the following reducing agents: dimethylamine borane (DMAB) and NaBH₄ (sodium borohydride).
3. Preparation of Au gels.
   1. Pipette 0.5 mL of 0.1 M HAuCl₄•3H₂O solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of DMAB into the microcentrifuge tube with gold solution to ensure a rapid mix of salt and reducing agent solutions. Once the solutions are mixed, place the microcentrifuge tube vertically in a tube rack with the tube cap open.
      Note: If the tube cap is left closed, hydrogen gas evolution will cause the pressure inside to force the cap to pop open and potentially spray the reduction mixture.
4. Preparation of Pd gels.
   1. Pipette 0.5 mL of 0.1 M Na₂PdCl₄ solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of NaBH₄ into the microcentrifuge tube with palladium solution. Place the microcentrifuge tube vertically in a tube rack with the tube cap open.
5. Preparation of Pt gels.
   1. Pipette 0.5 mL of 0.1 M K₂PtCl₆ solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of DMAB into the microcentrifuge tube with platinum solution. Place the microcentrifuge tube vertically in a tube rack with the tube cap open.
6. Tube inversion.
   1. At approximately 5 min, cap the microcentrifuge tubes and gently invert 3-5 times to aid in the coalescence of metal particles not part of the metal gel. Ensure that the tube caps are immediately uncapped after inverting tubes, and replace tubes in a rack to maintain the vertical orientation of the tube.
7. Equilibration.
   1. While Au, Pd, and Pt gels initially form within minutes, leave nascent gels in reducing agent solution for 3 – 6 h to allow for complete reduction of metal ions and for surface free energy minimization to occur.
      Note: Metal gels occupy a smaller volume than the initial volume of mixed metal ion and reducing agent solution. Some additional slight volume contraction may be observed during equilibration time, and is more pronounced for gold gels and believed to be due to Ostwald ripening²⁵.
8. Gel rinsing.
   1. For Au, Pd, and Pt gels after the equilibration period, remove excess reducing agent solution, but leave enough solution volume so that the metal gel remains submerged. Ensure that the solution meniscus does not come in contact with the metal gel.
      Note: Although the metal gels are stable enough to transfer between solutions with a spatula, capillary forces due to contact with the solution meniscus will deform and compress the gels resulting in an increase in the final aerogel density. This requires that some reducing agent solution remains in the tube with the gel submerged when transferring to deionized water.
   2. Slowly pipette deionized water to the top of the reaction microcentrifuge tubes. Submerge the microcentrifuge tube in a 50 mL conical tube full of deionized water and allow the gel to slide out of the microcentrifuge tube.
   3. Leave the gel in the deionized water for 24 h, and replace the water at 12 h. Do not to allow a liquid meniscus to come in contact with the gel.

2. Electrochemical Surface Area (ECSA) Characterization of Wet Metal Gels

Note: Electrochemical characterization is performed on wet metal gels prior to conducting freeze drying. The resulting ECSA is then an estimate of the surface of the final aerogel structure. Nitrogen adsorption measurements are used to estimate the surface area of the dried aerogels.

1. Solvent exchange.
   1. Remove as much of the deionized water from the Au, Pd, and Pt rinse solutions as possible and ensure that the liquid meniscus does not come in contact with the gel.
   2. Add 50 mL, 0.5 M KCl to the conical tubes in order to exchange the deionized water with supporting electrolyte within the gel pores. Leave the gels in KCl solution for 24 h.
2. Working electrode preparation.
   1. Coat a 1 mm platinum wire electrode with non-reactive lacquer using a fine bristle brush or other application device leaving a 4-5 mm length of the wire tip exposed.
   2. Allow 20 minutes for the lacquer to dry.
   3. Apply at least two coats of lacquer.
3. 3-electrode cell set-up.
   1. Use a 3-electrode cell set-up with an Ag/AgCl (3 M saturated) reference electrode, a 0.5 mm diameter Pt wire auxiliary/counter electrode, and the lacquer coated working electrode.
   2. Cut a plastic 50 mL conical tube in half and use as an electrochemical vial.
   3. Contact the gel with the working electrode with one of two methods: 1) impaled gel, or 2) contact mode.
      1. Working electrode - impaled gel.
         1. With the gel at the bottom of the modified 50 mL conical tube, gently insert the lacquer coated electrode into the gel.
            Note: The impaled gel method proves more effective with Au gels, whereas Pd and Pt gels fracture more frequently upon electrode insertion.
      2. Working electrode - contact mode.
         1. Insert the lacquer coated working electrode into the conical tube along the inner surface and rest the metal gel on the top of the exposed Pt wire of the working electrode.
4. Electrochemical impedance spectroscopy (EIS).
   1. Perform potentiostatic EIS scans with frequencies between 100 MHz and 1 mHz using a 10 mV amplitude sine wave. In the event of current overflows, use galvanostatic EIS with the same frequency range and a 100 – 200 mA amplitude sine wave.
5. Determination of electrochemical surface area (ECSA) from EIS data.
   1. For Z", the imaginary component of impedance, at the lowest EIS frequency f of 1 mHz, and dividing by the sample mass, m, use the following equation to determine specific capacitance, C_sp:
      C_sp = 1/(2πfZ"m)              (1)
      Note: Given that the ECSA is determined from a wet gel prior to freeze drying in Step 3 below, determine mass by assuming all of the metal ions in solution are reduced to form the gel. Based on this assumption, any actual yield less than 100% will result in underestimating C_sp.
6. Cyclic voltammetry (CV).
   1. Use scan rates of 100, 75, 50, 25, 10, 5, and 1 mV/s for CV measurements. Use voltage ranges of -0.2 to 0.2 V (vs Ag/AgCl) for Au gels, and select 0.1 to 0.4 V for Pd and Pt gels to avoid hydrogen adsorption and desorption, and oxidation-reduction of the metals.
7. Determination of electrochemical surface area (ECSA) from CV data.
   1. Use the slowest CV scan rate of 1 mV/s, and calculate specific capacitance with the equation:
      C_sp = (∫ivdv) / (2μmΔV)              (2)
      Note: Here i and v are the current and potential in the CV scan (A and V), scan rate is μ (V/s), mass of the gel is m (g), and ΔV is the potential window of discharge (vs Ag/AgCl).

3. Aerogel Preparation and Characterization.

1. Remove the deionized rinse water for Au, Pd, and Pt gels in Step 1.8 and ensure that the water meniscus does not come in contact with the metal gels.
2. Place the gels in a -80 °C freezer for no less than 30 min. Transfer the frozen metal gels to a freeze dryer with a set point pressure of 4 Pa or lower.

Results

The addition of metal ion and reducing agent solutions together results in solutions immediately turning a dark black color with vigorous gas evolution. Observation of the reaction progress suggests the proposed gel formation mechanism shown in Figure 1. Gel formation proceeds through five steps of 1) nanoparticle nucleation, 2) growth, 3) densification, 4) fusion, and 5) equilibration. The first four steps are observed to occur during the first few minutes o...

Discussion

The noble metal aerogel synthesis method presented here results in the rapid formation of porous, high surface area monoliths that are comparable to slower synthesis techniques. The 1:1 (v/v) metal ion solution to reducing agent solution ratio is critical in facilitating the proposed gel formation model. The rapid hydrogen gas evolution as a by-product of the electrochemical reduction of metal ions serves as a secondary reducing agent and facilitates the densification, and fusion of growing nanoparticles during gel forma...

Materials

Name	Company	Catalog Number	Comments
HAuCl4Ÿ•3H2O	Sigma-Aldrich	16961-25-4
Na2PdCl4	Sigma-Aldrich	13820-40-1
K2PtCl6	Sigma-Aldrich	16921-30-5
Pd(NH3)4Cl2	Sigma-Aldrich	13933-31-8
K2PtCl4	Sigma-Aldrich	10025-99-7
Pt(NH3)4Cl2Ÿ•H2O	Sigma-Aldrich	13933-31-8
dimethylamine borane (DMAB)	Sigma-Aldrich	74-94-2
NaBH4	Sigma-Aldrich	16940-66-2
NaH2PO2Ÿ•H2O	Sigma-Aldrich	10039-56-2
Ethanol	Sigma-Aldrich	792780
Snap Cap Microcentrifuge Tubes, 2.0 mL	Cole Parmer	UX-06333-70
Snap Cap Microcentrifuge Tubes, 1.7 mL	Cole Parmer	UX-06333-60
Conical Centrifuge Tubes 15mL	Stellar Scientific	T15-101
Ag/AgCl Reference Electrode	BASi	MF-2052
Pt wire electrode	BASi	MF-4130
Miccrostop Lacquer	Tober Chemical Division	NA
Potentiostat	Biologic-USA	VMP-3	Electrochemical analysis-EIS, CV
Freeze Dryer	Labconco	Freezone 2.5 Liter	Aerogel freeze drying
XRD	PanAlytical	Empyrean	X-ray diffractometry
Surface and Pore Analyzer	Quantachrome	NOVA 4000e	Nitrogen gas adsorption
ImageJ, Image analysis software	National Institute of Health	NA	SEM image analysis