Synthesis of Self-Supporting Porous Au Sponges Using Polymer Gel Templates

Porous metal structures can be applicable in wide range of fields including electrochemical or acid-base catalyst, heat dissipation, and bio filtration. Porous template mediated metal structures have higher surface area (1 m²g^-1) than pure metal structures. Though the template method has been advanced rapidly using starch gel, dextran, pumice, and alumina, sacrificial template method has been seldom investigated. Here, sponge-like porous Au structures were synthesized using agarose gel, HAuCl₄, and NaBH₄ as sacrificial soft template, Au precursor, and reductant with simple heating method. First, agarose gel has a 3D cross-linked pore structure, which is easy to control the porosity, shape, and scale. This implies that the use of agarose as a soft sacrificial template enables synthesis of pure Au sponges with purposed porosity and shape. To synthesize the porous Au structure, agarose gel was formed by mixing agarose with water at elevated temperature around 95 °C. The Au precursor was penetrated into the agarose gel by a capillary force at room temperature for 18 h in a sealed container. The metal ions were easily soaked into the 3D pores of agarose gel, as it has hydrophilic characteristics, which supply high loading amount of metal. The metal ions in agarose gel was reduced by NaBH₄. The agarose is composed of many aldehyde groups, which promotes the reduction of metal ions ad nucleation of metal clusters. Because of the reduction of Au ions in the pores of agarose, the agarose was getting darkened. The resulting Au-filled agarose gel was dried at room temperature and calcined for 5h at various temperatures to remove the agarose template and to form 3D Au structure. To understand the decomposition behavior during calcination step, thermal gravimetric analysis (TGA) was conducted. The weight loss observed from 200°C to 550°C by the decomposition of agarose template and the weight loss was around 80.5 wt %, which implies that the approximately 19.5 wt % of pure Au was formed in agarose gel. To investigate the structural change depending on the temperature, the Au-agarose bi-continuous structure was calcined at various temperatures of 200, 300, 500, 70, and 800 °C for 5 h. The porous Au showed continuous interconnected open-pore structure maintaining the intrinsic porous nature of the agarose matrix under 500 °C. In addition, the EDS showed that the synthesized Au had negligible impurities. However, the Au nanoparticles were aggregated showing dough-like structure without pore at the temperatures greater than 500 °C. The chemical states of the Au structure calcined at 200 °C are identical to that of pure Au crystal, demonstrating peak of Au (0) 4f_7/2 and Au (0) 4f_5/2 at 83.5 eV and 87.2 eV. The crystal structure of the Au sponge was also examined by XRD and Rietveld refinements. The measured lattice distance was 4.0705 Å, which is similar value to that of bulk Au (4.07 Å) calculated by DFT calculations. The chemical features at the interface between Au and carbon from agarose were investigated using TEM-EELS analysis. The EELS spectra of agarose template are identical to that of amorphous carbon with two peaks at 285 eV, and 290 eV, corresponding to the transition energy to the π* and σ* molecular orbitals of sp² bonds. Near the Au-carbon interface, slight peak shift of amorphous C signal was observed, indicating negligible interaction or bonding between Au and agarose template. In the Au region, the EELS spectra demonstrated strong localized electron energies. Based on the above results, the agarose template takes an excellent roll as a sacrificial template providing porous structure without interaction with Au nanoparticles and retaining the porous structures of Au sponges. In summary, the agarose is highly attractive material for the production of macroporous pure metal sponges with simple heating method. Furthermore, agarose gel is nature abundant, bio-degradable, and mechanically strong compared with other natural polymers.