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Porous Silica-Coated Gold Sponges with High Thermal and Catalytic Stability Min-Jae Lee, Shin-Hyun Kang, Jahar Dey, and Sung-Min Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04811 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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ACS Applied Materials & Interfaces

Porous Silica-Coated Gold Sponges with High Thermal and Catalytic Stability

Min-Jae Lee, Shin-Hyun Kang, Jahar Dey and Sung-Min Choi*

Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea

KEYWORDS : gold sponges, porous silica coating, thermal stability, catalysts, 4-nitrophenol reduction

ACS Paragon Plus Environment

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Abstract A method to fabricate porous silica-coated Au sponges which show high thermal and catalytic stability has been developed for the first time. The method involves dense surface functionalization of Au sponges (made by self-assembly of Au nanoparticles) with thiolated poly(ethylene glycol) (SH-PEG) which provides binding and condensation sites for silica precursors. The silica coating thickness can be controlled by using SH-PEG of different molecular weights. The silica-coated Au sponge prepared by using 5 kDa SH-PEG maintains its morphology at temperature as high as 700 °C. The calcination removes all organic molecules, resulting in porous silica-coated Au sponges which contain hierarchically connected micro- and meso-pores. The hierarchical pore structures provide an efficient pathway for reactant molecules to access the surface of Au sponges. The porous silica-coated Au sponges show an excellent catalytic recyclability, maintaining the catalytic conversion percentage of 4-nitrophenol by NaBH4 to 4-aminophenol as high as 93 % even after 10 catalytic cycles. The method may be applicable for other porous metals which are of great interests for catalyst, fuel cell, and sensor applications.


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1. INTRODUCTION In recent years, gold (Au) sponges have attracted great interests due to their high surface area, electrochemical and plasmonic properties, which make them excellent candidates for potential applications such as catalysts,1–4 fuel cells,5,6 sensors7–9 and optical devices.10,11 Various methods have been successfully developed for the synthesis of Au sponges, using dealloying of Au-Ag12 or Au-Cu13 alloys, self-assembly of Au nanoparticles (Au NPs)14–16 and template-mediated electrochemical deposition.17,18 However, the practical applications of Au sponges have been dampened by their structural instability at high temperature or catalytic reaction conditions which results in the loss of their unique physical properties.19–21 Different approaches to overcome the structural stability problem of Au sponges have been reported. Adding a small amount of Pt into AuAg alloy22 before dealloying or plating a thin layer of Pt on the surface of Au sponges23 made the structure of Au sponges stable up to 400 and 300 °C, respectively. More recently, atomic layer deposition (ALD) of TiO2 or Al2O324,25 on Au sponge surface has been used to stabilize Au sponges up to 600 and 1000 °C, respectively. While the ALD method has improved the thermal stability of Au sponges significantly, it has been demonstrated with Au sponges made by the dealloying method only. While Au sponge made by self-assembly of Au NPs provides thinner ligaments and higher specific surface area than those made by dealloying, it is more susceptible to elevated temperature and becomes significantly coalesced even at 100 °C. Therefore, the ALD method which requires temperature higher than 100 °C for deposition (125 °C for Al2O3 and 110 °C for TiO2) may not be quite applicable for Au sponges made by the self-assembly method, although it provides thermally most stable Au sponges reported so far. To make full use of Au sponges made by the self-assembly method which is considered as


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a more energy-efficient and eco-friendly method than dealloying, therefore, a new method to improve their thermal stability should be developed. Here, we report a new method to make Au sponges (prepared by self-assembly of Au NPs) stable up to 700 °C by coating the surface of Au sponges with porous silica. In this method, the Au sponges are functionalized with thiolated poly(ethylene glycol) (SH-PEG) which provides binding and condensation sites for silica precursors. The silica coating thickness can be easily controlled by using SH-PEG of different molecular weights, which determines the thermal stability of Au sponges. Upon calcination, all organic molecules are removed and porous silica-coated Au sponges containing hierarchically connected micro- and meso-pores are formed. The porous silicacoated Au sponges show a high catalytic activity and excellent recyclability, maintaining the catalytic conversion percentage of 4-nitrophenol by NaBH4 to 4-aminophenol as high as 93 % even after 10 catalytic cycles. To the best of our knowledge, this is the first report of Au sponges coated with porous silica of controlled thickness which provide high thermal and catalytic stability.

2. EXPERIMENTAL SECTION 2.1. Materials: Gold (III) chloride trihydrate (HAuCl4·3H2O), citric acid trisodium salt (anhydrous), p-nitrophenol and tetraethyl orthosilicate (TEOS) were used as purchased from Sigma-Aldrich. Thiolated polyethylene glycol (SH-PEG) with different molecular weights (1, 2 and 5 kDa) was used as purchased from Creative PEGWorks. 2.2. Synthesis of Citrate-Stabilized Au NPs: Au NPs were synthesized by using a reported method.26 500 ml of 1 mM HAuCl4·3H2O in water was boiled with stirring in a 1 liter round flask equipped with a condenser. 50 ml of 38.8 mM trisodium citrate aqueous solution was quickly added to the boiling solution, resulting in a color change from yellow to burgundy red. Boiling


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was continued for 10 minutes with stirring, followed by removal of a heating mantle to cool down the solution at room temperature. 2.3. Synthesis of Porous Silica-coated Au Sponge: Au sponges were prepared by using the method previously reported by our group.14 0.8 ml of 20 μM 5 kDa SH-PEG aqueous solution was added to 100 ml of 8 nM Au NP aqueous solution. Upon shaking for 3 hours, the supernatant of the solution becomes transparent, leaving the Au sponge precipitates at the bottom. The assynthesized Au sponges in aqueous solution were mixed with 25 mg of SH-PEG of different molecular weights (1, 2, and 5 kDa) and the mixture was shaken for 24 hours with aluminum foil wrapping. The Au sponges densely functionalized with SH-PEG were washed with water 3 times to remove the unbounded SH-PEGs and freeze-dried. 25 mg of Au sponge functionalized with SHPEG (1, 2 and 5 kDa) was added to 1 ml of water, to which 1.5 mmol of TEOS was added under stirring at 300 rpm. The mixture was continuously stirred for 2 days. This resulted in silica-coated Au sponges with different coating thickness depending on the molecular weight of SH-PEG. The samples were washed 3 times with water and dried before any characterization or further treatment. The silica-coated Au sponge prepared with 5 kDa SH-PEG was calcined at 450 °C for 4 hours under helium to remove organic molecules, resulting in porous silica-coated Au sponges. 2.4. Thermal Stability Measurements: The as-prepared and the silica-coated Au sponges (without calcination) were annealed in a tube type furnace (Dae-Heung Science Inc) at 100, 300, 500 and 700 °C for 2 hours under helium gas. The heating rate for the desired temperature of 100, 300, 500 and 700 °C was 10 °C/min. After the thermal annealing at different temperatures, the samples were cooled down in the furnace. 2.5. Catalytic Activity and Recyclability Measurements: A 15 ml of 0.18 mM 4-NPh aqueous solution was mixed with 12 ml of 0.36 M NaBH4 aqueous solution in a vial. To this mixed solution,


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25.7 mg of Au-PS-5 was added under stirring at 300 rpm, respectively. To monitor the progress of reaction with time, small amount (0.2 ml) was taken from the mixture at different reaction time and placed into a 1 mm path length quartz cell for which UV-vis absorption spectroscopy (Car Zeiss spectrometer, MCS 601 UV-NIR C) measurements were performed. After each UV-vis absorption measurement, the sample taken from the mixture was returned to the mixture vial to keep the total amount of mixture maintained throughout the reaction. The same procedure was used to monitor the progress of reaction for the as-prepared Au sponges. In this case, 15 mg of asprepared Au sponges (which is the amount of gold in 25.7 mg of Au-PS-5) was used. To study the catalytic recyclability of Au-PS-5 and as-prepared sponge, the catalysts were washed 3 times with water after each 60 min (or longer) of reaction, and re-used for subsequent cycles under the same reaction conditions as described above. 2.6. Characterizations: To determine the amount of SH-PEG functionalized on the surface of Au sponges, thermo-gravimetric analysis (TGA) measurements were performed using thermogravimetric analyzer (TGA-92-18, Setaram) under the nitrogen atmosphere with 5 °C/min heating rate. The morphologies of the samples were characterized by Tecnai (200kV) transmission electron microscope (TEM) and Magellan 400 scanning electron microscope (SEM). The x-ray photoelectron spectroscopy (XPS) analysis of silica-coated Au sponges was performed by using a Thermo VG Scientific K-alpha XPS with Al Kα x-ray source. Pore size distribution in porous silica-coated Au sponges was characterized by Ar adsorption and desorption measurements at 87 K with a surface area and porosity analyzer (Tristar II, Micrometrics Inc.). The inductively coupled plasma-optical emission spectroscopy (ICP-OES, iCAP 6300 Duo) measurement was performed to determine the Au contents in porous silica-coated Au sponges. The UV-vis-NIR measurements


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were performed using a Car Zeiss spectrometer (MCS 601 UV-NIR C) to monitor the catalytic reaction with time.

3. RESULTS AND DISCUSSION The process of coating Au sponges with porous silica is schematically illustrated in Figure 1. Au sponges were synthesized using self-assembly of Au NPs as reported by our group elsewhere.14 In brief, the citrate-stabilized Au NPs of 12.3 nm (Figure S1) in aqueous solution (8 nM) are selfassembled into mesoporous Au sponges with randomly interconnected 3D network structures by addition of a very small amount of 5 kDa SH-PEG (SH-PEG/Au NP molar ratio of 20) and 3 hours of shaking at 300 rpm. The as-prepared Au sponges in aqueous solution are densely functionalized by SH-PEG by adding an excessive amount of SH-PEG of different molecular weights (1, 2, and 5 kDa). To estimate the amount of SH-PEG functionalized on the surface of Au sponge, TGA measurements were performed after removing unbound SH-PEG by washing the samples 3 times with water (Figure S3). The weight fractions of SH-PEG in the functionalized Au sponge are 4.0, 5.4, and 5.9 wt% for 1, 2 and 5 kDa SH-PEG, respectively. The areal number densities of SH-PEG on Au sponge surface estimated from the weight fractions of SH-PEG and the specific surface area of as-prepared Au sponges (11.6 m2 g-1)14 are 2.22, 1.52 and 0.67 nm-2, respectively. This clearly indicates that the Au sponge surface is well functionalized with SH-PEG. It should be noted that as the molecular weight of SH-PEG increases, the weight fraction of SHPEG increases although the areal number density decreases. The Au sponges functionalized with SH-PEG (25 mg, freeze-dried after 3 times of washing with water) immersed in 1 mL of water are mixed with tetraethyl orthosilicate (TEOS, 1.5 mmol) under stirring at 300 rpm, followed by 2 days of continuous stirring. The hydrolyzed TEOS bind to ethylene oxide (EO) units of SH-PEG


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grafted on the surface of Au sponges by hydrogen bonding and silica species polymerize27 to form a silica shell on Au sponge surface. All organic molecules are removed by calcination at 450 °C for 4 hours in helium gas, resulting in porous silica-coated Au sponges. TEM and SEM images of the as-prepared Au sponges show randomly interconnected 3D networks of ligaments with a large number of open pores (Figure 2a and b). The TEM and SEM images of Au sponges after silica coating with TEOS (without calcination) clearly show that the surfaces of Au sponges are fully covered with silica layer, and the morphologies of Au sponges are well maintained during silica coating process (Figure 2c-h). The thickness of silica coating layer estimated from TEM images increases with the molecular weight of SH-PEG which provides binding sites for hydrolyzed TEOS (ca. 1.5, ca. 4.5 and 10-15 nm for samples prepared with 1, 2, and 5 kDa SH-PEG, respectively). It should be noted that while the silica coating thickness is fairly uniform for the samples prepared with 1 and 2 kDa SH-PEG, it is less uniform for the sample

Figure 1. Schematics for the synthesis procedure for porous silica-coated Au sponges.


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Figure 2. TEM and SEM images of as-prepared Au sponge (a, b) and silica-coated Au sponges prepared with SH-PEG of different molecular weights, 1 kDa (c, d), 2 kDa (e, f) and 5 kDa (g, h). The SEM images at low magnification are shown in Figure S2.


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prepared with 5 kDa SH-PEG. This can be attributed to the fact that short chain SH-PEG molecules (1 and 2 kDa) functionalize the surface of Au sponges more densely (resulting in more uniformly defined PEG layer) than long chain SH-PEG (5kDa). XPS spectra of the silica-coated Au sponges (without calcination) were measured and compared with that of as-prepared Au sponges (Figure S4). In all the cases, the Au 4f spectra show two peaks at 87.8 and 84.1 eV which are characteristic for metallic gold Au0.28 The Si 2p spectra of silica-coated Au sponges show a peak at 103.9 eV which is characteristic for Si4+.29 This confirms the formation of SiO2 layer. The XPS spectrum of as-prepared Au sponge shows no peak in the Si 2p spectral region. To investigate the thermal stability of the as-prepared and the silica-coated Au sponges (without calcination), samples were annealed at different temperatures for 2 hours under helium gas, for which SEM measurements were performed (Figure 3). The ligaments of as-prepared Au sponges become significantly coarsened even at 100 °C, showing the thickness of 30 - 60 nm, although the overall 3D network structures are maintained. With further increase of temperature, the ligaments of as-prepared Au sponge become rapidly thicker and fully collapsed. This coarsening of ligaments occurs to reduce the surface energy, which is facilitated by the increased surface diffusion of gold atoms with temperature.20 The morphologies of silica-coated Au sponges prepared with 1, 2 and 5 kDa SH-PEG (which have silica coating thickness of ca. 1.5, ca. 4.5 and 10-15 nm, respectively) are well maintained up to 100, 300 and 700 °C, respectively, although slight increases of ligament thickness were observed. This clearly indicates that the thermal stability of Au sponges is significantly enhanced by silica coating and the degree of enhancement increases with the silica coating thickness. The silica shell, which has the higher melting point (1713 °C) than that of gold


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Figure 3. SEM images of as-prepared Au sponges and silica-coated Au sponges annealed at different temperatures for 2 hours under helium gas. All the scale bars are 50 nm. The SEM images at low magnification are shown in Figure S5.


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(1064 °C), may restrict the surface diffusion of gold atoms, resulting in the suppression of coarsening of ligament and pores. As temperature becomes higher, the mechanical stress induced by the deformation of gold ligaments would increase, making cracks in the silica shell eventually.30 This leads to the leakage of gold, destroying the morphologies of Au sponges. The increase of thermal stability with the silica coating thickness can be attributed to the enhanced mechanical strength of silica shell with its thickness. The micro- and meso-porous silica shell on nanoparticles provides efficient channels for reactant molecules to reach the surface of nanoparticles as well as high thermal stability of nanoparticles.31,32 The silica-coated Au sponges prepared with 5 kDa SH-PEG were calcined for 4 hours at 450 °C under helium to remove the PEG molecules. This resulted in porous silica-coated Au sponges (denoted as Au-PS-5). The TEM and SEM measurements of Au-PS-5 (Figure 4) show that the morphologies of Au sponges are well maintained after the calcination with the ligaments thickness of 12-26 nm. The Au loading in Au-PS-5, measured by ICP-OES, is 58.4 % by weight. Argon gas adsorption and desorption isotherms were measured for Au-PS-5 (Figure 5a). The adsorption isotherm of Au-PS-5 exhibits a sharp increase in the low pressure region (P/Po < 0.1) which indicates the presence of micropores. The isotherm in the range of 0.4 < P/Po < 0.9 shows a type-IV hysteresis which is typical for mesoporous structures.33 The pore size distributions evaluated from the adsorption curve by using the density functional theory method show a bimodal distribution,34 one peak centered at ca. 1.8 nm (micropores) and another at ca. 5.7 nm


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Figure 4. TEM (a) and SEM (b) images of Au-PS-5 after calcination at 450 °C for 4 hours under helium.

(mesopores) (Figure 5b). The porosity is estimated to be 70.4 %. The micropores can be attributed to the pyrolysis of PEG molecules in the silica shell during calcination. The mesopores originate from the 3D network structures of Au sponges coated with silica as shown in the inset of Figure 5b. This hierarchical pore structure is an important feature to allow fast access of reactant molecules to the surfaces of Au sponges.

Figure 5. (a) Argon sorption isotherms of Au-PS-5 measured at 87 K and (b) pore size distribution of Au-PS-5. The inset shows a schematic view of micro- and meso- pores.


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The catalytic activity and recyclability of Au-PS-5 were investigated using the liquid phase reduction of 4-nitrophenol (4-NPh) by NaBH4 to 4-aminophenol (4-APh) as a model reaction. The 4-NPh is hazardous pollutant which is produced as by-products during manufacturing of pesticides, herbicides, and synthetic dyes.35,36 The 4-APh, the reduction product of 4-NPh, can be widely used as dying agent, analgesic and antipyretic drug.37,38 The catalytic reduction of 4-NPh by NaBH4 to 4-APh in the presence of Au catalysts is typically explained by the Langmuir-Hinshelwood mechanism or the Eley-Rideal mechanism.35 In both mechanisms, the surface-hyrdogen species are formed on the surface of Au catalysts by the adsorption of BH4−. Subsequently 4-NPh (either adsorbed on the surface or colliding from the solution without being adsorbed, depending on the mechanisms) is reduced by the surfacehydrogen species. A 15 ml of 0.18 mM 4-NPh aqueous solution was mixed with 12 ml of 0.36 M NaBH4 aqueous solution. The UV-vis spectrum of the mixed solution shows an absorption peak at ca. 400 nm due to the formation of yellow colored 4-nitrophenolate ions. To this mixed solution, 25.7 mg of AuPS-5 was added under stirring, resulting in a gradual change of color from yellow to transparent within several minutes (Figure S6). The progress of reaction with time was monitored by UV-vis absorption spectroscopy measurements (Figure 6a and b). As the reaction proceeds with time, the absorption peak at ca. 400 nm decreases while a new absorption peak at ca. 300 nm corresponding to 4-APh shows up and increases. The catalytic conversion of 4-NPh by NaBH4 to 4-APh in the presence of Au-PS-5 was completed within ca. 8 minutes. As a comparison, the catalytic reduction of 4-NPh using as-prepared Au-sponges (15 mg, the amount of gold in 25.7 mg of Au-PS-5) as catalysts was also monitored with the UV-vis absorption spectroscopy (Figure 6b). Here, the mixed solution conditions and the total amount of gold in the catalysts were kept the same as the


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Figure 6. UV-vis absorption spectra monitoring the catalytic reduction of 4-NPh by NaBH4 to 4APh with time after the addition of Au-PS-5 (a) and as-prepared Au sponge (b). (c) Plots of the ln(Ct/C0) against reaction time, where k is a rate constant obtained from linear fitting. (d) Recyclability of Au-PS-5 and as-prepared Au sponge as catalysts for the reduction reaction of 4NPh by NaBH4 to 4-APh. The conversion percentages of Au-PS-5 and as-prepared Au sponge were measured 8 and 25 minutes after the addition of catalysts, respectively.


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reaction with Au-PS-5. The catalytic conversion using as-prepared Au sponges was completed within ca. 60 minutes. This shows that the catalytic activity of Au-PS-5 is much higher than that of as-prepared Au sponges. To quantitatively compare the catalytic activities of Au-PS-5 and as-prepared Au sponge for the reduction of 4-NPh by NaBH4, the rate constants were evaluated. Since an excess amount of NaBH4 was used, the concentration of BH4― can be considered effectively constant during the reaction. Therefore, a pseudo-first-order reaction kinetics was used to evaluate the reaction rate constant (k).39 The concentration ratio of Ct/C0, where Ct and C0 are the concentration of 4-NPh at time t and zero, respectively, was obtained from the absorption peak intensity at 400 nm (at time t and zero) which is proportional to the concentration of 4-NPh. The plots of ln (Ct/C0) versus reaction time for Au-PS-5 and as-prepared Au sponges are linear (Figure 6c), confirming that the reaction is a pseudo-first-order reaction. The rate constants for the reduction of 4-NPh estimated from the slopes of the plots are 9.70 × 10-3 s-1 and 1.28 × 10-3 s-1 for Au-PS-5 and as-prepared Au sponge, respectively. The rate constant of Au-PS-5 is ca. 7.6 times higher than that of as-prepared Au sponges. This difference can be understood as following. During the fabrication of Au-PS-5, 4 hours of calcination at 450 °C removed all organic species on the surface of Au sponges, allowing the Au surface easily accessible by reactant molecules.40 On the other hand, the residual organic molecules on the surface of as-prepared Au sponge such as SH-PEG and citrate, which are known as strong stabilizers, limit the access of reactant molecules to the Au surface, hampering the catalytic reaction.38 The catalytic recyclability of Au-PS-5 was investigated by performing 10 catalytic cycles and compared with that of as-prepared Au sponges. After each catalytic reaction at the same condition as described above, the catalysts were washed 3 times with water before re-use. The catalytic


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conversion percentage of 4-NPh for Au-PS-5 is as high as 93 % after 10 catalytic cycles (Figure 6d), which confirms the excellent recyclability of Au-PS-5. This is in stark contrast with the catalytic conversion percentage of 4-NPh for as-prepared Au sponges which is significantly

Figure 7. TEM and SEM images of Au-PS-5 (a, b) and as-prepared Au sponges (c, d) after 10 catalytic cycles.


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decreased with cycle. The recyclability of Au-PS-5 is superior than that of the commercial catalyst Pd/graphite hybride for the reduction of 4-NP which shows a conversion percentage of ca. 70 % after 5 cycles.41 The TEM and SEM images of Au-PS-5 and as-prepared Au sponges after 10 catalytic cycles show that the overall 3D network structure of Au-PS-5 is well maintained (although the ligaments became slightly thicker), while as-prepared Au sponges shows significant coarsening and heavily collapsed regions (Figure 7). This shows that the structural stability of AuPS-5 during catalytic reaction provided by porous silica coating is the key for its high catalytic recyclability. It has been reported that the interaction of the reactant molecules with the active surface Au atoms increases the mobility of Au-containing species, enhancing the surface diffusion of Au atoms which results in the coarsening of as-prepared Au sponges.21 In the case of Au-PS-5, the silica shell may restrict the surface diffusion of Au atoms, suppressing the coarsening of Au sponges.

4. CONCLUSION We developed a new facile method for coating Au sponges (made by self-assembly of Au NPs) with porous silica of controlled thickness for the first time. The porous silica-coated Au sponges show high thermal stability and excellent recyclability. The method involves dense surface functionalization of Au sponges with SH-PEG which provides binding and condensation sites for silica precursors. The silica coating thickness can be controlled by using SH-PEG of different molecular weights. The silica-coated Au sponge prepared by using 5 kDa SH-PEG maintains its overall morphology at temperature as high as 700 °C. The calcination removes all organic molecules including SH-PEG, resulting in porous silica-coated Au sponges which contain hierarchically connected micro- and meso-pores. The hierarchical pore structures provide an


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efficient pathway for reactant molecules to access the surface of Au sponges. The porous silicacoated Au sponges show an excellent catalytic recyclability, maintaining the catalytic conversion percentage of 4-NPh to 4-APh as high as 93 % even after 10 catalytic cycles. The method can be applicable for Au sponges made by dealloying method and other porous metals which are of great interests for catalyst, fuel cell, and sensor applications.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS publication web site at https://pubs.acs.org. A TEM image and SAXS intensity of citrate-stabilized Au NPs; TGA measurements of Au sponges functionalized with SH-PEG of different molecular weights (1, 2 and 5 kDa); XPS spectra of silica-coated Au sponges prepared with SH-PEG of different molecular weights (1, 2 and 5 kDa) and as-prepared Au sponge; SEM images of as-prepared Au sponge and silica-coated Au sponges annealed at different temperatures for 2 hours; Photographs showing the color change with the catalytic reduction of 4-NPh by NaBH4 to 4-APh ca. 8 minutes after the addition of Au-PS-5 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the NRF grants funded by the Ministry of Science and ICT of the Korean government (NRF-2017M2A2A6A01021366 and NRF-2017R1A2A1A05001425) and the KUSTAR-KAIST Institute, KAIST, Korea. We thank Prof. J. I. Yun and S. Choi at KAIST for providing a UV-vis spectrometer used in this study. We also thank the Pohang Accelerator Laboratory for providing beam time at the beamline 4C.

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