Mechanically robust, macroscopic-sized low-density gold foams with unique morphology of hollow gold shells were prepared by the synthesis of gold-coated polystyrene coreshell particles followed by thermal removal of the polymer core. Here, we reported a simple, highly efficient electroless gold plating procedure to form ultrathin, conformal gold coatings on the surface of polystyrene particles. A modified, scalable casting approach to form uniform, large monoliths (up to 1 cm diameter) and a baking condition to minimize undesirable densification of final foams were also developed. To demonstrate the mechanical stability and integrity of the Au foams, a representative monolithic sample (~0.9 g/ cm3 ) was successfully cut and shaped into a gold foam “washer” by a series of precision machining and processing. A depth-sensing indentation test (nanoindentation) as well as characterizations on other useful properties such as surface areas and electrical conductivity of the Au foams were carried out to understand the mechanical behavior and potential applicability.

•    Gold; Metal Foams; Electroless Gold Plating; 
Mechanical Properties

Kim SH, Shin SJ, Bhandarkar SD, Baumann TF (2021) Preparation of Mechanically Robust, Macroscopic Low-Density Gold Foams by Highly Efficient Electroless Gold Plating. JSM Chem 8(1): 1056.

Despite a promise of nanostructured porous metals or metal foams for potential uses in electronic, optical, and biomedical applications, it has been a major difficulty to produce largearea continuous films or macroscopic monoliths (~millimeters and above) with precisely controlled porosity (density), pore geometry, and uniformity [1-3]. Several successful synthetic routes to develop monolithic (nano) porous, macroscopic gold materials have been recently reported [4-6]. For example, Tappan et al. reported a synthesis of catalytically active, macroscopic nanoporous gold foams (< 0.1 g/cm3 , ∼0.5 % relative to full density Au) via combustion synthesis [4]. Fang et al. fabricated ultralow density gold aerogels (6 -23 mg/cm3 , ∼0.05- 0.1 % relative density) by freeze-casting of gold nanowire suspensions [5]. Traditionally, dealloying of bimetallic gold alloys such as Ag/Au was widely used for synthesis of nanoporous gold foams with relatively higher density (∼15-40 % relative density) [7,8]. However, studies to fill the big gap in densities between ultralow density gold and high-density dealloyed gold have been rarely reported. Thus, the development of a synthetic route to low density gold foams in the range of ∼1 − 10 % relative density (~0.2 to 2.0 g/cm3 ) will expand the scope of possible morphologies and help gold foams to find widespread technological applications. Gold foams in the density range are also highly desirable for studies in laser and plasma physics from fundamental understanding of the nature and universe to future energy sources and next generation accelerator technology [9, 10].

Previously, we reported the synthesis of low-density copper foams(∼0.8 g/cm3 , ~10 % relative density) by slip-casting of 1 µm -diameter copper-coated polystyrene core-shell particles followed by thermal removal of the polystyrene core [11], Nyce et al. and others synthesized hierarchical porous gold foams (0.28 g/cm3 , 1.5 % relative density) by combining gold/silvercoated core-shell particles (with a diameter of 10µm) with a dealloying approach [12-14]. While the dealloying was able to lower the density, the gold foams were very fragile, and easily broke or cracked during handling and processing, despite a great potential of those pioneering works for applications to catalysis [15]. Recently, we developed a simple, highly efficient electroless gold plating procedure to form ultrathin, conformal gold coatings on 3D-printed polyacrylate substrates [16]. Based on this work, we present a synthetic route to low-density gold foams composed of a very thin, continuous gold coating, compared to previous works on highly porous, dealloyed gold shells. This morphological transition from highly porous shells to a thin, continuous layer help resultant gold foams to maintain a better mechanical stability without collapsing and enables us to synthesize mechanically robust, macroscopic low density gold foams.

Carboxylate-terminated polystyrene beads (PS-COOH, 10µm diameter, 2.7 % solids-latex) and non-functionalized PS beads (PS, 10 µm diameter, 2.5 % solids-latex) were purchased from Polysciences Inc. Gold (III) chloride trihydrate (HAuCl4 ⋅3H2 O), hydroxylamine hydrochloride (NH2 OH⋅HCl), and Polyvinylpyrrolidone (PVP) were purchased from Sigma-Aldrich.

All the chemicals were used as received unless otherwise noted.

Density of the cylindrical metal foam samples was determined from the measurement of the weight, diameter, and height:

where ρ is the density, W is the weight, d is the diameter, and h is the height.

Morphology of fractured samples was investigated with a Jeol JSM-7401F Scanning Electron Microscope (SEM) and ThermoFisher Scientific Apreo SEM without any additional conductive coating. X-ray diffraction analysis (XRD) was performed on powder samples with Bruker D8 Advance X-ray diffractometer fitted with a Cu Kα source. BET surface area was measured by nitrogen absorption porosimetry with a Micromeritics Instrument ASAP 2000 after degassing at 70 °C for 12 h.

Mechanical property of gold foams was characterized with an MTS XP Nanoindenter equipped with a 200 µm-diameter spherical indenter. Before indentation, foam samples were attached to silicon wafers with epoxy. A series of load-unload indents were carried out in laboratory air at room temperature. The loading rate was continuously adjusted to keep a constant representative strain rate of 10-2 s-1. For every cycle, the unloading rate was kept constant and equal to the maximum loading rate of the cycle.

Electrical resistivity and conductivity of a typical gold foam sample (∼0.9 g/cm3 ) were measured with a Keithley 2400 source meter and determined by using following equations:

where R is the electrical resistance of a material, ρ is the electrical resistivity, L is the length, A is the cross-sectional area, and σ is the conductivity, the inverse of resistivity.

Preparation of Gold Nanoparticles

Gold nanoparticles (AuNP) stabilized with 4-dimethylaminopyridine were prepared, as similarly to the literature procedure [17]. Typically, HAuCl4 ⋅3H2 O (0.35 g) was dissolved in water (30 mL) and added to tetraoctylammonium bromide (1.1 g) dissolved in toluene (80 mL). Then, freshly prepared NaBH4 (0.4 g) dissolved in water (25 mL) was slowly added to the mixture and stirred for ~1 h. After the reaction, the organic layer was taken and washed with H2 SO4 (0.1 M, 100 mL), NaOH (0.1 M, 100 mL), and water (100 mL). For the phase transfer, 4-dimethylaminopyridine (DMAP, 0.98 g) was dissolved in water (80 mL) and mixed with the as-prepared gold nanoparticle mixture in toluene. After the completion of spontaneous phase transfer, the aqueous phase was taken, diluted with additional water (240 mL), and stored in a refrigerator until further use.

Preparation of Gold Nanoparticle-Seeded Polystyrene (Seeding)

The AuNP (~5 nm diameter) were positively charged within pH range of 7-12. To take advantage of the strong electrostatic interaction during the seeding process, commercially available, carboxylate-terminated polystyrene beads (PS-COOH, 10 µm diameter, 2.7 % solids-latex, Polysciences Inc.) were used as a polymer template. Typically, aqueous solution of AuNP (5 g) was added to the PS-COOH (2 mL) and mixed with a vortex mixer. The mixture was allowed to sit overnight. Then the crude gold nanoparticle-seeded polystyrene (PS-AuNP) beads were separated by centrifugation at 6000 rpm for 5 min (Fisher Scientific Accu Spin 400) and the supernatant was decanted. The PS-AuNP was repeatedly (2X) washed by re-suspension in DI H2 O (7 mL) and isolation by centrifugation. After the final washing, the PS-AuNP beads were re-suspended in 2 mL H2 O.

Preparation of Gold-Coated Polystyrene (Au Coating)

Electroless gold plating reactions to form Au coatings on the PS-AuNP were carried out by improving Brown and Natan’s hydroxylamine method using aqueous mixture of HAuCl4 ⋅3H2 O and NH2 OH⋅HCl [18]. Typically, four of the seeded PS-AuNP (8 mL) were first dispersed in water (400 mL) under stirring, and NH2 OH⋅HCl (11.2 g) was added. Then, HAuCl4 ⋅3H2 O (1.0 g) was added to the mixture. The plating reaction was completed in 1-2 hr. The gold-coated polystyrene (PS-Au) was isolated by a membrane filter and was repeatedly (2-3X) washed with excess DI water. Finally, the PS-Au was moved to a vacuum oven at room temperature for complete drying

Preparation of PS-Au Monoliths (Casting)

Prior to the casting to form a PS-Au monolith, a plaster-ofParis mold was prepared. For the preparation of the mold, H2 O (22 g) was added to plaster-of-Paris (40 g) in a Teflon cup and mixed with a Teflon spoon. The mixture (45 g) was placed in a small cup, and a Teflon tube (~4.8 mm or 1.1 cm) was inserted into the center of the cup. The wet mold was dried at a benchtop for a few hours and moved to an oven at 60°C for complete drying.

For the preparation of a PS-Au monolith, the dried PS-Au was first powdered into fine individual particles by gently pressing them with a small lab spatula. Then, a pre-determined amount of PS-Au powder (~150−200 mg and ~2 g for a standard and a large sample, respectively) was inserted into a plaster-of-Paris mold. Then, a small amount of water (2-4 mL) was pipetted into the Teflon tube of the mold, and the mold was transferred into an ultrasonic bath (Branson 2510). Once the water was completely removed from the suspension (~1 h), a PS-Au monolith cast in the mold was dried for a few days at the benchtop. Then, the monolith was carefully separated from the Teflon tube and then further dried.

Formation of Au foams (Baking)

The PS-Au monolith was placed in a small ceramic crucible and put into a tube furnace. After attaching all fittings to the quartz tube, N2 flow was adjusted and purged for a few minutes prior to the baking. For the baking of samples, the samples were heated to 350 °C at a rate of 5 °C/min, held at 350 °C for 4 hr., and then cooled to RT at a rate of 10 °C/min.

Figure 1

Figure 1: Schematic representation of our synthetic procedure for gold foams: (i) seeding of polystyrene particles with gold nanoparticle; (ii) Au coating of the seeded particles by electroless gold plating; (iii) casting to form PS-Au monolith; (iv) baking to remove the polymer for Au foam formation. Lower left image is a photo of a cylindrical Au foam (0.9 cm (diameter)×1.4 cm (height)). Scale bar is 1 cm.

summarized our synthetic procedure for gold foams: (i) incorporation of gold nanoparticle (AuNP) seeds into polystyrene beads (seeding); (ii) electroless gold plating of the seeded PS-AuNP (Au coating); (iii) assembling gold-coated PS-Au into a PS-Au monolith (casting); and (iv) thermal removal of PS templates to form Au foams (baking). By using a different size of tubes in a plaster-of-Paris mold, we tuned the size of resultant Au foams (~4.8-11 mm in diameter) without a significant variation in densities of gold foams.

Seeding to incorporate appropriate catalysts (typically noble metals, e.g., Pd, Pt, Au) onto the surface of a substrate is an important step to form high-quality metal deposits and coatings on non-conductive polymers by electroless plating [19]. To overcome the inherent lack of adhesion between catalytic metal seeds and polymers, we explored the potential of electrostatic interaction between polystyrene beads (substrates) and gold nanoparticle (seeds) in this study. We used positively charged gold nanoparticles (AuNP) and negatively charged polystyrene (PS-COOH) beads and compared the resultant gold coating with that of a non-functionalized counterpart (PS) (Figure 2)

Figure 2: Effect of surface functionality of polystyrene beads on the formation of gold coatings. (a) Polystyrene microspheres functionalized with carboxylic groups (PS-COOH) and non-functionalized PS were used. (b, c) Photos of a suspension of gold nanoparticles (AuNP) in water before and after mixing with an aqueous suspension of PS-COOH, and (d-e) SEM images of seeded particles (PS-AuNP) and gold-coated particles (PS-Au). (f) gold-coatings on plain PS beads (non-functionalized) for a comparison purpose. Scale bars are all 10 µm.

 Aqueous colloidal solution of AuNP (purple) was stable in a refrigerator for months without aggregation. However, addition of PS-COOH (light gray) into the solution quickly precipitated dark-colored particles, and the solution became completely colorless in a few minutes, as demonstrated in figure 2b and 2c. Due to the small size of gold nanoparticles, it was difficult to precisely map the distribution of individual gold nanoparticles on the polymer beads. However, the SEM image in figure 2d (prepared with PS-COOH and AuNP) clearly showed tiny seeds as well as some larger aggregates on the entire surface of the PS-AuNP beads. Electroless gold plating on the seeded PS beads (prepared with either PS-COOH or PS) was carried out by using the hydroxylamine method [18]. The resulting PS-Au samples clearly demonstrated the effects of surface functionality of polystyrene beads and the seeding of AuNP on the formation of gold deposits and coatings, as shown in figure 2e and 2f. More uniform gold coatings with finer Au deposits were observed in the PS-Au sample prepared with PS-COOH. Without proper surface interactions, weaklyadsorbed Au-seeds and large Au deposits seemed to be easily detached during washing steps. Thus, coarse Au deposits or poor-quality, non-uniform coatings were produced in plain PS beads. It is worth mentioning that extensive washing steps still caused some loss of gold deposits and coatings from the PS-Au prepared with the PS-COOH, despite a significant improvement compared to the non-functionalized PS.

Precise control over the morphology of metal deposits and coatings on a substrate was another important aspect for a successful low density foam development. Figure 3a-3e 

Figure 3: Morphology of the gold coatings on PS particles. Low-/high-magnification SEM images of PS-Au samples (a, b) prepared with addition of polyvinylpyrrolidone (PVP) during Au electroless plating, and (c, d) without addition of PVP. (e) XRD patterns of the samples. Scale bars in the images are all 1 µm.

showed SEM images of the PS-Au at low and high magnifications, respectively, which was prepared with a similar procedure as in the literature (as well as in Figure 2e) [12-14]. The gold coatings had morphology of uniform, fine grains in size of 100 nm. Considering that the hydroxylamine method was initially developed to grow existing Au nanoparticles (seeds) into larger particles based on the dramatically accelerated reduction of Au3+ on the Au surface, large Au particles observed after electroless plating reflect the initial distribution of the seeds adsorbed on the substrate [18]. This morphology, however, poses a concern about collapse and densification of the resultant gold foams when the PS-Au monoliths are treated at high temperature to remove the polymer template. Without forming a continuous layer, simple reduction in the amount of metal salts in the solution could cause a serious collapse of the metal foams after baking. In our previous study of low-density Cu foams, we showed that increasing the reaction rate by using a larger amount of HCHO (reducing agent) during electroless copper plating reaction played a crucial role in controlling the morphology of metallic Cu deposits, which ranged from separated large grains to small particles, and finally to almost continuous thin films [11]. In this study, we observed a similar morphological transition of gold coatings from smaller grains (Figure 3a and 3b) to almost continuous thin films (Figure 3c and 3d). The key change from other earlier formulations for electroless gold plating is the removal of polyvinylpyrrolidone (PVP), in conjunction with the increased addition of NH2 OH⋅HCl (reducing agent). The PVP was widely used as a polymer stabilizer added to minimize particle aggregation during gold deposition [12-14,20]. When the PVP was added, we observed that the electroless gold plating reaction became slower. More fragile, higher-density Au foams (above ∼1.7 g/cm3 ) were typically obtained due to a larger shrinkage during the baking process although the same amount of Au salts was used. To further investigate the effect on gold crystal growth, both PS-Au samples prepared with and without PVP addition were compared by an X-ray diffraction (XRD) technique. Positions of the diffraction peaks match well with those of polycrystalline face centered cubic (fcc) Au crystals and the broader peak widths of the PS-Au sample with PVP reflect the small grain sizes of the Au coatings (~100 nm), consistent with SEM images in Figure 3b [21].

Monolithic Au foams were prepared by casting dried PS-Au particles in a mold followed by thermal removal of the polymer. Typically, for a conventional slip-casting method, a well-dispersed suspension of metal-coated polymer particles would be ideal, but it was not possible to uniformly re-disperse gold-coated PSAu aggregates into water. It is worth mentioning that we tested with several approaches to re-disperse gold-coated polystyrene particles into water including use of different pH of water, dilute KI/I2 as a gentle gold etching, and thiol-functionalized polymer (e.g., PEG-SH) as a dispersing agent, but none of them worked for our purpose. The final Au foams completely collapsed during baking step. Instead, we observed that PS-Au precipitates were weakly aggregated, and they could be easily made into powder by gently pressing them with a small lab spatula after drying in a vacuum oven. This powdering step caused the formation of some small holes or fractured debris attached at the surface of PS-Au particles (see ∼1 µm-diameter open hole in Figure 3c). For casting, the PS-Au powder was put into a plaster-of-Paris mold. Small amount of water (∼2 mL) was added to flow through a tube of the mold and the powder was dried in air. After a careful separation from the mold, well-packed PS-Au monoliths with nearly identical tube diameters were successfully obtained, similarly to a conventional slip-casting using a suspension.

Baking the PS-Au monolith at high temperature was carried out to remove the PS template, promote inter-particle bonding and improve mechanical stability of the metal foams at the same time. Figure 4a-d

Figure 4: Formation of Au foam. (a, b) TGA curves of PS-Au and Au foam. (c, d) SEM images of the Au foam at low- and high-magnifications. Scale bars are all 10 µm

showed typical Thermogravimetric Analysis (TGA) curves of PS-Au samples under a flow of N2 or air. Here, a simpler decomposition behavior of PS under N2 was preferred because the two-stage decomposition under air started earlier, but it also required higher temperature for complete removal of the PS. After several attempts, we optimized the baking condition for preparation of Au foams. When the Au foam was prepared by baking at 350 °C for 4 h in a flow of N2 , the baked sample of Au foam did not show any additional weight loss, as shown in figure 4b. Baking samples at higher temperatures induced a sudden increase in the density (not shown here). figure 4c and 4d are selected SEM images of a fractured Au foam (∼0.9 g/cm3 )sample. From the image of broken shells, we clearly observed that the PS was completely removed after the calcination process. It also showed the existence of open holes at the Au shell and shell fragments generated from our modified casting procedure.

This study has been motivated by our need to fabricate a low-density gold foam specimen for various physics experiments [9-14]. They require typical Au foam monoliths (shown in Figure 1) to be cut (machined) into a precise shape, for which good mechanical stability and integrity are necessary. Figure 5a-c

Figure 5: Mechanical properties of Au foams (∼0.9 g/cm3 ). (a) a photo of an Au foam sample after being precisely machined, (b) Nanoindentation load-displacement curves obtained with a spherical indenter, and (c) stress-strain curve.

showed a photo of Au foam “washer” sample (∼0.9 g/cm3 ) after being successfully machined by a series of processing steps: briefly, the monolithic Au foam was first cut to have a fixed outer diameter, then inserted into a tubular mold, and finally bored to have a specific inner diameter and wall thickness. To further understand the improved mechanical property of this gold foam, we carried out nanoindentation testing using a 200 µm-diameter spherical indenter and compared the result with that of our previous 3D-Au foam prepared using a 3D-printed polyacrylate substrate as a template [16]. While the 3D-Au foam showed a sudden jump in the displacement (sign of fracture) or an abrupt increase in load (sign of densification), this Au foam did not show such an extreme behavior, as shown in load-displacement (P-h) and stress-strain curves of figure 5b and 5c. Compared to previous 3D-Au foam, this Au foam showed mechanical behaviors like a full-density material for a wider range of strain (%). We believe this distinction could be attributed to a unique morphology of this Au foam (composed of more uniform, smaller (∼10 µm) hollow spherical pores), as opposed to that of the 3D-Au foam (∼150 µm tubular pores, 3D-printed morphology). Further detailed discussion is beyond the scope of this paper. More details on experimental procedures and the data analysis could be found elsewhere [22,23].

In addition, the BET surface area of this Au foam was determined to be 3.1×105 m2 m−3 (0.34 m2 /g at 0.9 g/cm3 ). This was slightly lower than previously reported hierarchical porous gold foam (1.48 m2 /g at 0.28 g/cm3 ) [12]. It might be due to the non-porous, smooth surface structure of this Au foams. But it still exceeds commercially available metal sponges by a factor of ∼1000. Figure 6

Figure 6: Electrical conductivity of gold foams. Measurement of the resistance of a sample using a Keithley 2400 source meter. Scale bar is 5 mm.

showed the measurement of the electrical resistivity and conductivity of a typical gold foam sample (∼0.9 g/ cm3 ). The Au foam of this study was so conductive that it was not possible to directly measure a resistance using a typical multimeter. We used a Keithley 2400 source meter to measure the resistance difference (0.019 Ohm = 0.317 (with a sample)-0.298 (wire only)). The length (0.72 cm) and area (=3.14× (0.44/2)2 cm2 ) were measured from the image analysis. The resistivity and conductivity of the sample were determined to be 39×10-6 ? m and ∼27.8 × 103 S/m, respectively. The very high electrical conductivity of this Au foam suggests a good connectivity among the Au spheres.

In summary, low-density gold foams (~5% relative to full density Au, ∼0.9 g/cm3 ) with a unique pore structure were synthesized using gold-coated polystyrene core-shell particles prepared by electroless Au plating, followed by thermal removal of the polystyrene. Optimization of chemistry for highly efficient seeding and electroless gold plating allowed the synthesis of gold-coated polystyrene core-shell particles with continuous, ultrathin gold layers to minimize undesired densification of the final metal foams. The improved mechanical stability of the Au foam enabled us to successfully fabricate a challenging shape of gold foam “washer” by a series of precision machining and processing. The mechanically robust, macroscopic Au foams are of great interest as an ideal candidate for many challenging physics experiments. In addition, a simple, highly efficient synthetic route to high surface area, highly conductive gold foam materials and large-area continuous gold coatings will open many exciting new opportunities to benefit both academia and industries in areas of materials science, electronics, energy, and other applications.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. We thank Dr. Jae-Hyuck Yoo for measuring the electrical conductivity of the Au foam sample.

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