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Scientific Reports Volume 5, Article Number: 17726 (2016) Cite this article

The in-situ uniform dispersion of precious metals in three-dimensional graphene sheets is a key strategy for generating macrostructures, which is desirable in practical applications such as electromagnetic interference shielding and catalysts. We report a one-step green method for developing 3D graphene/precious metal (Pt and Ag) nanocomposite porous structures. The obtained graphene/noble metal nanocomposite material has ultra-low density, excellent elasticity and good conductivity. In addition, in order to clarify the advantages of 3D-graphene/noble metal nanocomposites, their electromagnetic interference (EMI) shielding and electrocatalytic properties were further studied. Compared with bare graphene, the synthesized 3D graphene/precious metal nanocomposite exhibits excellent EMI shielding effect; in the 8.2–12.4 GHz X-band range, the efficiency is 28 dB on average. In the electrooxidation of methanol, the 3D-graphene/platinum nanocomposite also exhibits significantly enhanced electrocatalytic performance and stability compared with reduced graphene/platinum oxide and commercial platinum/carbon.

Graphene is a carbon material with a two-dimensional structure. Due to its exceptionally high mechanical strength, ultra-high specific surface area and unique electrical conductivity, it has attracted great attention from the scientific community1,2. These properties make it a promising material for potential applications, such as energy storage 2, super capacitor 1, 3 fuel cell electrocatalyst 4, 5, 6, water split 7, solar cell 8, and electromagnetic interference (EMI) shielding9. In addition, in order to meet the requirements of practical applications, current research is mainly focused on doping with nitrogen 4, 5, 6 and using metals 10, 11, 12, metal oxide 6 and various composite materials 5, 13. The synergistic combination of these foreign substances and graphene sheets enables them to produce composite materials with excellent performance in various applications, such as energy storage 2, fuel cell electrocatalysts 4, 5, 6, catalysis 14, water splitting 7, sensors 15, Super capacitors 16, 17, solar cells 18, high-performance lithium-ion batteries 19 and EMI shielding 20. Among these composite materials, graphene powder (ie, chemically exfoliated graphene oxide (GO) and reduced graphene oxide (RGO)) 7, 13, 21, 22 is considered a support material. However, the high levels of aggregation and surface defects in GO and RGO result in a significant reduction in surface area and changes in electrical conductivity. Therefore, it is necessary to develop an effective and reliable method to improve the performance of graphene-based support materials.

Recently, three-dimensional (3D) graphene components have been used as support materials for many applications because they have better properties (increased conductivity, surface area, and mass transfer efficiency) compared to graphene powder. Many different methods have been used to manufacture 3D graphene components 24, 25, 26, 27, 28, 29. However, the manufacturing process is very expensive and harmful to the environment. A major challenge is to manufacture well-defined mesoporous microporous 3D graphene structures with low cost and environmentally friendly processes. Qiu Ling et al. It has been reported that an ultra-light 3D graphene-based honeycomb monolith using a cost-effective freezing casting process has ultra-low density, super elasticity, good electrical conductivity and high energy absorption efficiency30. This unusual carbon-based honeycomb material opens up many opportunities in a wide range of innovative applications. Here, our goal is to use this 3D graphene monolithic structure to make 3D graphene/noble metal (Pt or Ag) nanocomposites for EMI shielding and methanol fuel cell applications.

In the current work, we have developed a more environmentally friendly, efficient and low-cost one-step method to mass-produce 3D graphene/precious metal (Pt or Ag) nanocomposites by freezing and casting part of RGO and metal salt solutions. The obtained 3D graphene/noble metal (Pt or Ag) nanocomposite material was characterized by various spectroscopic studies and correlated with microscopic and surface area analysis. The structure, electrical conductivity and mechanical properties of the synthesized 3D graphene/noble metal (Pt and Ag) nanocomposites are also studied. The resulting 3D graphene/precious metal (Pt and Ag) nanocomposite shows a combination of ultra-low density, excellent elasticity and good electrical conductivity. In addition, the enhanced conductivity 3D-graphene/noble metal (Pt and Ag) nanocomposites have been extended to EMI research, where enhanced conductivity is a necessary standard. Interestingly, the 3D graphene/noble metal (Pt and Ag) nanocomposite shows an enhanced shielding effect compared to the bare 3D graphene. In addition, 3D-graphene/platinum nanocomposite materials are used for the electrocatalytic activity study of methanol electrooxidation. Here, the 3D-graphene network provides large and accessible pores of variable size, which helps to quickly transport the reactants to the electroactive site; in addition, the high conductivity of the catalyst is maintained overall. Therefore, in the electrooxidation of methanol, the 3D-graphene/Pt nanocomposite exhibits excellent electrocatalytic properties, such as high electrocatalytic performance, abnormal toxicity tolerance and reliable stability, and Pt/reduced graphene Compared with higher grade oxide nanocomposites and commercial Pt/C.

Natural graphite powder with a particle size of 45 μm (purity 99.99%), hexachloroplatinate (H2PtCl6.6H2O) and silver nitrate (AgNO3) were purchased from Sigma-Aldrich. All other chemicals used are of analytical grade and purchased from Merck Specialties Private Limited, India. Commercial Pt/C (Pt on graphitized carbon, 20 wt%) was obtained from Sigma-Aldrich. All chemicals used are used as is, without any further purification.

The aqueous GO dispersion was prepared by using the reported literature method 30,31, and was further stripped by using a bath sonicator. In a typical method of preparing 3D-graphene/noble metal (Pt and Ag) nanocomposites, 1.5 ml of GO solution (5 mg/ml) and the corresponding noble metal salt (H2PtCl6.6H2O = 4.5 mg/AgNO3 = 2.5 mg) The mixture was put into a cylindrical glass bottle with a capacity of 4 ml. Add ascorbic acid (15 mg) to it and keep it in a heating furnace at 100 °C for 30 minutes to obtain partially reduced GO dispersion and metal ion reduction. Each vial was then frozen in a dry ice bath for 0.5 hours. After thawing (at room temperature), the vial was placed in a heating oven at 100°C for 4 hours to further reduce the GO to obtain a gel. The gel is then dialyzed (to remove dissolved materials), freeze-dried, and then dried at 50°C for 24 hours. The detailed schematic diagram of the synthesis process is shown in Figure 1. For comparative study, in the presence and absence of ascorbic acid, naked 3D-graphene and RGO/Pt (20 wt%) nanocomposites were synthesized by using ascorbic acid as a reducing agent. Respectively the freezing casting procedure.

Schematic diagram of the freezing casting method of 3D-graphene/precious metal nanocomposite and its application in EMI shielding and methanol electrocatalytic oxidation.

The structure of the synthesized sample was checked by using powder XRD diffractometer (Philips PW 3040/60) and Cu Kα radiation (λ = 1.541 Å); FTIR spectra were recorded by standard KBr particle method using Magna-IR spectrometer-50 (Nicolet). Raman scattering is performed by RAM HR 800 miniature laser Raman system and 519 nm Ar laser. Use XPS (MULTILAB from Thermo VG Scientific, monochromatic Al Kα X-ray (1486.6 eV)) to analyze the chemical state and surface composition of the synthesized sample. Use Phillips-CM 200 to check the morphological electron microscope of the sample through HRTEM. Through ICP-AES (Prodigy, Teledyne Leeman Labs) analyzed the composition of the prepared samples. The specific surface area, pore volume and size distribution of the synthesized samples were analyzed by the Micromeritics instrument ASAP 2020. The specific surface area was determined by the multipoint Brunauer-Emmet-Teller (BET) method. The samples were analyzed before analysis. Degas at 60°C for 4 hours. Cylindrical shape (approximately 10 mm in height and 12 mm in diameter). The sample is ready for conductivity measurement and compression test. Broadband dielectric spectrometer (Novocontrol) and Alpha-A frequency analyzer are used to study the AC conductivity y of synthetic samples at frequencies ranging from 1 Hz to 10 MHz. Conduct conductivity measurement by placing the sample between gold-plated copper electrodes. The compression test was performed in an Instron (Micro Tester, 5848). The instrument uses a load cell (10 N) with a strain rate of 100% per minute. Measure the EMI shielding characteristics in the X band by using the PNA network analyzer N5222A (Agilent). The catalyst ink was prepared by dispersing 5 mg of each catalyst in a mixed solution of 5 mL Millipore water, 1 mL isopropanol, and 10 μL Nafion® solution, and sonicating for 20 minutes. 20 μL of catalyst ink (well dispersed) was dripped on the glassy carbon electrode (0.07 cm2) and named as the modified glassy carbon electrode. Electrochemical measurement uses the potentiostat/galvanostat Autolab-302N with GPES 4.9 software. The electrochemical measurement is performed in a three-electrode system. The platinum wire and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. A modified glassy carbon electrode (3 mm diameter) was used as the working electrode. All measurements are performed at room temperature. Before the electrochemical scan, purge the volumetric solution with high-purity nitrogen for 5-15 minutes. In the manuscript, all electrochemical potentials are reported for the reversible hydrogen electrode (RHE).

We demonstrated the fabrication of 3D graphene/precious metal (Pt and Ag) nanocomposites by using low-cost wet molding technology (often called freeze casting). During this process, a phase separation occurs between the solid particles and the formed ice, while the rejected particles are installed between the developing ice templates 32, 33. When the fraction of particles exceeds the permeation limit and further sublimation of ice results in a porous solid monolith, the trapped particles form a 3D network. Recently, the freezing casting method has been widely used to prepare various porous materials 28, 30, 33, 34, 35, 36, 37, 38. In the current work, we have implemented the principle of the freezing casting process used to form 3D graphene/precious metal (Pt and Ag) nanocomposites. Together with precious metals, GO was selected as the initiator of the preparation of porous 3D monoliths.

In the presence of ascorbic acid, GO and metal salt solutions produced 3D porous components during the freezing casting process. In order to understand the mechanism, a control experiment was carried out using the process of reporting. It has been observed that compared with bare 3D graphene monoliths, in the presence of precious metals, the gelation time and annealing temperature are reduced by about half and a quarter of the synthesis process, respectively. The chelation of precious metals with the unreduced functional groups of graphene oxide sheets and water molecules promotes the formation of 3D gels. Tang et al. also observed this type of phenomenon in GO/precious metal sponges (hydrothermal method). Group 25. In addition, the chelation of graphene with precious metals is demonstrated by FTIR spectroscopy and XPS, and is discussed in the following section. In the freezing step, the partially reduced GO flakes chelated with the metal are excluded from the formed ice and trapped between adjacent ice templates to form a continuous network structure. The 3D-graphene/precious metal composite is forced to align with the growth direction of ice crystals. In addition, the chelation of graphene sheets on precious metals facilitates the assembly process and increases the π-π interaction between the graphene layers. It is found that the resulting 3D network structure is very stable and can maintain its structural integrity during thawing, reduction, dialysis and freeze-drying processes. The characteristic of the developed material is the use of various complex analysis techniques to explore its characteristics and applications.

XRD is used to explain the phase and structural parameters of the synthesized GO and 3D-graphene/noble metal (Pt and Ag) nanocomposite samples; the results are shown in Figure 2. The diffraction peaks at 2θ=10.8° and 43° in Figure 2(a) correspond to the (002) and (100) planes of graphite oxide and the hexagonal structure of carbon, respectively. However, in Figure 2(b) the diffraction peaks of bare 3D-graphene In the XRD pattern, the peak (2θ = 10.8°) disappeared and reappeared at ~2θ = 24°, indicating that the graphite oxide was effectively reduced and there were π -π stacked between the graphene sheets, resulting in an ordered crystal structure 40. As shown in Figure 2 (c, d), the diffraction patterns of Pt and Ag correspond to the (111), (200), (220), (311) and (222) planes of the fcc structure in 3D-graphene/platinum and 3D-graphene/silver nanocomposite material. Third, the diffraction peaks appearing in stacked graphene sheets almost disappeared in 3D-graphene/Pt and 3D-graphene/Ag nanocomposites, indicating an obstacle to re-stacking in graphene sheets, which may be due to Pt and Ag nanoparticles The surface 41 of the graphene sheet is attached to the graphene sheet. The average crystallite size of Pt and Ag in the 3D-graphene/noble metal (Pt and Ag) nanocomposite derived from Schiller's formula is 7 and 16 nm, respectively. In addition, the XRD pattern of the RGO/Pt nanocomposite is shown in Figure S1 (supporting information). The diffraction peaks show the fcc structure of Pt (JCPDS 04-0802) in the RGO/Pt nanocomposite.

(a) GO, (b) bare 3D-graphene, (c) 3D-graphene/Pt and (d) XRD patterns of 3D-graphene/Ag nanocomposite.

Raman spectroscopy is widely used to observe the electronic and structural properties of carbon-containing materials. Figure S2(A) shows the characteristic Raman spectra of GO, bare 3D-graphene and 3D-graphene/noble metal (Pt and Ag) nanocomposites. All spectra indicate the presence of the characteristic D, G, and 2D bands of graphene/graphene oxide. In the case of GO, for bare 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag, the G band is centered at 1592 cm-1 and moved to 1577, 1579 and 1580 cm-1, respectively. Close to the value of original graphene (Figure S2(B)). The movement of the G-band confirms that GO was successfully reduced during the chemical reduction process. The D bands of GO, bare 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites are concentrated at 1355, 1345, 1346 and 1345 cm-1, respectively, and the defects and in-plane size sp2 are predicted area. The change in the relative intensity of the D and G bands (ID/IG) is related to the change in the average size of the sp2 domain. The ID/IG ratios of GO, bare 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites are calculated to be 0.82, 1.1, 1.11, and 1.13, respectively. The increase in ID/IG ratio of bare 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites indicates that the average size of sp2 domains decreases. Furthermore, the size of the new sp2 domain created by the reduced GO is smaller than the domain that existed in the GO before the reduction.

The FTIR spectra of the prepared 3D graphene-based precious metal (Pt and Ag) nanocomposites and GO are shown in Figure S3 (supporting information). Figure S3(a) shows the characteristic band of GO. GO characteristic bands are observed at 3400 cm-1 (OH stretching), 1730 cm-1 (COOH group C=O stretching vibration), 1405 cm-1 (tertiary C-OH OH deformation vibration), 1222 cm -1 (CO stretching vibration of phenolic C-OH) and 1045 cm-1 (CO stretching vibration of epoxy group). The band at 1612 cm-1 is related to the flexural vibration of adsorbed water molecules or the skeletal vibration of unoxidized CC bonds. In 3D-graphene noble metal (Pt and Ag) nanocomposites, except for the C=O stretching vibration of -COOH and the OH stretching vibration and bending vibration of water, the characteristic bands of almost all functional groups have disappeared in the FTIR spectrum. The C=O stretching vibrations of GO, 3D-graphene/Pt and 3D-graphene/Ag are measured to be 1730, 1717 and 1725 cm-1, respectively. As the mass of the precious metal increases, the shift of the stretching vibration peak to a low wave number indicates the chemical interaction between the metal ion and the graphene sheet. In addition, the OH bending vibration peak of adsorbed water molecules becomes more prominent in the 3D-graphene/noble metal nanocomposite, and shifts to a low wave number as the mass of the precious metal increases. For GO, 3D-graphene/Pt and 3D-graphene/Ag monoliths, the OH bending vibration peaks of H2O are observed at 1612, 1568 and 1572 cm-1, respectively. The significant shift of wavenumber to the low side in 3D-graphene/precious metal nanocomposites may be due to the chelation of water molecules with precious metals.

The surface chemistry of GO, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites was studied by XPS analysis. Figure 3(a,b) shows the high-resolution spectra of the C 1s characteristic peak region of GO and 3D-graphene/Pt nanocomposites. This area shows a different arrangement of carbon and oxygen combinations. It has been noted that various oxygen-containing functional groups (-CO = 286.6 eV, -C=O = 286.9 eV, -COO = 288.4 eV) exist on the higher binding energy side of the C1s region of GO45. Compared with the intensity of sp2 carbon peak (284.5 eV), in the case of 3D-graphene/Pt nanocomposites, the intensity of various oxygen-containing functional groups and the intensity of sp3 carbon peak (285.1 eV) are effectively reduced. It was further confirmed that GO was successfully reduced to graphene in the 3D-graphene/Pt nanocomposite by the freezing casting method. Figure 3(c,d) shows the high-resolution XPS spectra of Pt and Ag in 3D-graphene/Pt and 3D-graphene/Ag nanocomposites, respectively. In Figure 3(c), the Pt 4f signal is deconvolved into two components (4f5/2 and 4f7/2). The strongest bimodal Pt 4f at 71.1 eV (4f7/2) and 74.6 eV (4f5/2) is designated as metallic Pt0, and the second bimodal Pt 4f peak at 72.1 and 77.51 eV is designated as PtII . The appearance of PtII may be due to the partial oxidation of Pt0 on the graphene surface. In addition, it can be seen from Figure S4 (supporting information) that the binding energy of oxygen 1s in the 3D-graphene/Pt nanocomposite is blue-shifted compared with the oxygen 1s of GO. The 1s blue shift of oxygen in the 3D-graphene/Pt nanocomposite indicates that Pt is covalently connected to the sp2 carbon of graphene through oxygen (Pt-OC)46. In Figure 3(d), the peaks near 368.1 eV and 374.4 eV belong to Ag 3d5/2 and Ag 3d3/2, respectively, indicating the presence of metal Ag0 47.

(a) GO, (b) 3D-graphene/Pt, (c) Pt 4f7/2 and Pt 4f5/2 3D-graphene/Pt peaks and (d) Ag 3d5/2 and 3d3/2 peaks XPS spectra In 3D-graphene/silver nanocomposite.

The surface morphology of the 3D-graphene/precious metal nanocomposite was inspected by FEG-SEM. Figure 4(a,b) shows a 3D porous arrangement with continuous openings, ranging from submicron to several microns. Careful observation revealed that various nanoparticles were uniformly distributed on the surface of the 3D graphene nanosheets. TEM images of different 3D-graphene/noble metal (Pt and Ag) nanocomposites are shown in Figure 4(c,d). Noble metal nanoparticles (Pt and Ag) are highly monodisperse and uniformly distributed on the entire surface of the graphene sheet. This result indicates that the graphene sheet effectively prevents the aggregation of ultrafine metal nanoparticles, and confirms the strong interaction between the noble metal nanoparticles and the graphene sheet. For Pt and Ag, the average size of different noble metal nanoparticles is 5 and 13 nm, respectively. Before TEM characterization, all 3D graphene/precious metal (Pt and Ag) nanocomposites were subjected to ultrasonic treatment for 15 minutes. The precious metal nanoparticles are still attached to the graphene sheet, indicating that they are firmly fixed on the graphene surface. The HRTEM images in the insets of Figure 4 (c, d) show that the noble metal nanoparticles (Pt and Ag) in the graphene sheet exhibit high crystallinity, and the lattice spacing is about 0.228 nm and 0.236 nm, corresponding to (111) respectively Pt and Ag crystal plane spacing. It is worth mentioning that the amount of precious metal nanoparticles loaded on the graphene sheet, because it has a direct impact on the morphology and catalytic performance. The loading amount of different noble metal nanoparticles was analyzed by ICP. It was found that the loading amount of Pt and Ag nanoparticles was 20%.

(a) FEG-SEM images of 3D-graphene/Pt and (b) 3D-graphene/Ag nanocomposites; (c) 3D-graphene/Pt and (d) 3D-graphene/Ag nanocomposites TEM and HRTEM images.

Figure 5 shows the measured AC conductivity of bare 3D-graphene and 3D-graphene/precious metal (Pt and Ag) nanocomposites. As shown in the figure, the 3D-graphene/precious metal nanocomposite material exhibits enhanced conductivity in the frequency range of 1 to 106 Hz due to the inclusion of noble metal (Pt and Ag) particles in the 3D-graphene nanosheets . The real part of the AC conductivity is derived from the complex conductivity using the following relationship:

The frequency-dependent real part of the AC conductivity of 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites.

Where and are the real and imaginary parts of the complex conductivity.

Figure 5 shows that the conductivity of 3D graphene increases with increasing frequency, following a power law. The total AC conductivity related to frequency is expressed by the power law:

Where ω is the angular frequency, and the exponent s is a function of frequency, limited to between 0.7 and 0.9. σdc is the frequency-independent conductivity or direct current conductivity (when ω=0), and A is a temperature-dependent constant. The addition of precious metal particles to 3D graphene shows a significant increase in conductivity, indicating that the behavior is independent of frequency. The AC conductivity (σ' (ω)) is almost constant in the frequency range of 3D-graphene/Ag and 3D-graphene/Pt nanocomposites, which is attributed to the DC conductivity, where the electrode polarization is negligible. Similar behavior was previously observed in composites based on carbon nanotubes48.

The dispersion of precious metal (Pt and Ag) particles on the 3D building blocks of the nanosheets increases the conductivity of 3D-graphene/Ag to 0.032 S/cm, and the conductivity of 3D-graphene/Pt nanocomposites to 0.01 S /cm. The electrical conductivity of 3D-graphene/precious metal nanocomposites (Ag and Pt) has important implications for the previously reported compressed graphene nanosheets and graphite oxide 43,49. In the current work, noble metal (Pt and Ag) particles act as bridges between nanosheets by forming coordination bonds or covalent bonds and leading to increased π-π interactions between graphene sheets. The uniform distribution of noble metals on graphene sheets and the presence of large electron clouds in complex metals may increase the charge transport in the 3D graphene/noble metal nanocomposite monolith, rather than bare 3D graphene. However, due to the porous 3D graphene nanosheets, conductivity loss was observed in the developed samples. In addition, the enhanced conductivity 3D-graphene/noble metal nanocomposite is extended to EMI research, where enhanced conductivity is a necessary standard.

The compression test of 3D-graphene/precious metal (Pt and Ag) nanocomposites at 50% strain was studied to determine the stress-strain behavior under cyclic loading. The 3D-graphene/precious metal (Pt and Ag) nanocomposite is compressed to a strain of up to 50% under pressure; it returns to its original shape immediately after the pressure is released (Figure 6(ac)). The compressive stress-strain curve of the nanocomposite is recorded for up to 10 cycles under cyclic loading. The compressive stress-strain curves of bare 3D-graphene (inset), 3D-graphene/Pt and 3D-graphene/Ag nanocomposites at 50% strain are shown in Figure 6 (d, e). In all cases, the unloading curve almost returned to the initial point after 10 cycles of 50% strain fatigue loading. This shows that, unlike exposed 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites also have excellent superelasticity and complete shape recovery, and will not undergo plastic deformation after pressure is released. After 10 compression cycles, the compressive stress dropped to 92% (3D-graphene), 87% (3D-graphene/Pt) and 91% (3D-graphene/Ag) of the original value, respectively, confirming 3D- Graphene has good elasticity and flexibility. Graphene/precious metal nanocomposites. Similar behavior was observed earlier in carbon-based aerogels50, 51. After the compressive stress is released, the morphology study of the developed sample is carried out (Figure S5, supporting information). The pores of 3D-graphene elongated longitudinally under the restored lateral load. However, the porous surface of the 3D-graphene/noble metal (Pt and Ag) nanocomposite did not have any elongation or cracks, indicating that it can be restored to its original state more quickly (Figure S5, supplementary video). The stress-strain curve of the 3D-graphene/Pt composite shows a linear behavior at ε <35%, with a modulus of elasticity of 16 kPa, and then a nonlinear behavior, with the slope increasing to 50% and the modulus of 49 kPa. Similarly, 3D-graphene/Ag exhibits an elastic modulus of 11 kPa in the linear state and 45 kPa in the nonlinear state, while 3D-graphene/Ag exhibits an elastic modulus of 3 kPa in the linear state. It exhibits 5 kPa in the non-linear state. The increase in elastic properties of 3D-graphene nanocomposites is due to the strong attraction between metal nanoparticles and graphene sheets, resulting in high load transfer efficiency, thereby increasing the toughness of elastic behavior. Earlier literature proved that nanocomposites have similar types of enhanced elasticity. In addition, these excellent mechanical properties of 3D-graphene/precious metal nanocomposites can be used in various applications of elastic conductors, flexible electrodes and pressure sensors.

The compressibility of 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites.

(Ac) A digital image of a typical compression process of 3D-graphene/precious metal nanocomposites. (d) Compressive stress-strain curves of 3D-graphene/Pt and (e) 3D-graphene/Ag nanocomposites for 10 cycles (loading and unloading) (inset: 3D-graphene compressive stress-strain curve ).

In order to support the conduction research, the shielding parameters of 3D-graphene and 3D-graphene/precious metal nanocomposite materials were studied in the X band (8.2-12.4 GHz) by using the vector network analyzer to measure the shielding parameters (Figure 7). The shielding effectiveness (SE) is calculated by using the relationship 53

EMI shielding efficiency (dB) of 3D-graphene, 3D-graphene/Pt and 3D-graphene/Ag nanocomposites.

Where PI corresponds to the input power, and PT corresponds to the transmit power.

The shielding performance of the 3D-graphene/noble metal nanocomposite was compared with that of the main body 3D-graphene. The total SE in composite materials is dominated by absorption phenomena. Interestingly, 3D-graphene/Ag nanocomposites show better shielding performance than 3D-graphene/Pt and 3D-graphene. The SE of the 3D-graphene/silver nanocomposite is 28 dB, the average film thickness is 0.8 mm, and the shielding ability is about 99.6%. In addition, the SE of 3D-graphene/Pt is enhanced by 24 dB compared to 3D-graphene (~18 dB). The total SE of the absorption phenomenon is mainly due to the interaction of the electric dipole and the radiated electromagnetic field and the functional group of the functionalized 3D-graphene obtained by adding precious metal (Pt, Ag) particles. Therefore, the incorporation of fillers into 3D graphene plays an important role in improving the structure and conductivity. Our earlier research also revealed this. The particles are uniformly dispersed on the surface of the 3D building block nanosheets, and the skin depth is reduced by improving the absorption loss, thereby obtaining a higher surface; this further enhances the SE.

The SE in terms of permittivity (ε), conductivity (σ') and permeability (μ') is expressed as:

By showing that the proportional increase in the conductivity of the 3D-graphene/silver nanocomposite material results in a significant enhancement of the shielding effect. Similar results have been reported before, in which carbon foam is embedded with nickel particles to improve the shielding performance55.

In order to further explore the potential applications of 3D-graphene/precious metal nanocomposites as electrocatalysts, we chose 3D-graphene/Pt nanocomposites to study its catalytic activity in methanol oxidation. Figure 8(a) shows the cyclic voltammetry (CV) curves of commercial Pt/C (20 wt%), RGO/Pt nanocomposite and 3D-graphene/Pt nanocomposite in 0.5 M H2SO4 solution. Scan the CV from -0.2 to 1.0 V at a scan rate of 50 mV s-1 in a 0.5 M H2SO4 (N2 saturated) solution. It has been observed that there are three potential areas on the CV of commercial Pt/C (20 wt%), RGO/Pt nanocomposites, and 3D-graphene/Pt nanocomposites. The first potential region from -0.2 to 0.05 V exhibits hydrogen adsorption/desorption. The second potential region from 0.1 to 0.3 V represents the electric double layer capacitance region. The third potential region from 0.4 to 0.9 shows the metal oxidation/reduction region. Compared with commercial Pt/C and RGO/Pt nanocomposites, 3D graphene/Pt nanocomposites show higher double-layer capacitance. This is because 3D graphene has a larger surface area than RGO and carbon black. Brunauer-Emmett-Teller (BET) surface area analysis confirms the larger surface area of ​​3D-graphene, which indicates that the specific surface areas of carbon black, RGO and 3D graphene are 94, 122, and 155 m2/g, respectively (Figure S6, supporting information ),respectively. The electrochemically active surface area (ECSA) of Pt in the three different catalysts was calculated from the integrated hydrogen adsorption area. It is observed that the ECSA of Pt in 3D-graphene/Pt nanocomposites is 20.7 m2/g, which is about 66% higher than Pt/C (12.5 m2/g) and 36% higher than RGO. /Pt nanocomposites (15.2 square meters) M/g). The higher ESCA of Pt in 3D-graphene/Pt nanocomposites may be due to the uniform distribution of Pt nanoparticles on 3D-graphene (as shown by TEM) and the unique structure of 3D-graphene (independent, interconnected) Graphene sheet).

(a) CV of 3D-graphene/Pt, RGO/Pt nanocomposite and Pt/C electrode in 0.5 M H2SO4 solution; scan rate: 50 mV s−1, (b) 3D-graphene/Pt, RGO CV of /Pt nanocomposite and Pt/C electrode in 0.5 M H2SO4 1.0 M methanol solution; scan rate: 50 mV s-1 and (c) 3D-graphene/Pt nanocomposite, RGO/Pt nanocomposite And Pt/C electrode in 0.5 M H2SO 4 1.0 M methanol solution current-time curve at 0.7

The electrochemical activity of commercial Pt/C (20 wt%), RGO/Pt nanocomposite and 3D-graphene/Pt nanocomposite on the oxidation of methanol was compared by CV, and the subsequent results are shown in Figure 8(b). The peaks at 0.7 V and 0.5 V in the forward and reverse scans correspond to the direct electro-oxidation of methanol adsorbed on the catalyst surface and the oxidation of the intermediate carbonaceous residues produced on the catalyst surface in the forward scan 56, 57, 58 respectively. Therefore, the catalytic performance of electrocatalysts is usually estimated by forward scanning. Compared with Pt/C and RGO/Pt nanocomposites, 3D-graphene/Pt nanocomposites exhibit a higher specific current density at 0.7 V. The higher current density indicates that the 3D-graphene/Pt nanocomposite exhibits higher catalytic activity (20 wt%) and RGO/Pt nanocomposite than Pt/C. This is because of the unique structure of 3D graphene. The 3D structure of graphene promotes the mass transfer efficiency of reactants, products and electrolytes during the reaction, thereby improving the electrocatalytic performance of the 3D-graphene/Pt nanocomposite. According to the ratio of the forward scan peak current (If) to the reverse scan peak current (Ib), that is, the value of If/Ib, the tolerance of the catalyst to the accumulation of intermediate carbonaceous substances (mainly carbon monoxide) is estimated at 59,60. Pt/ The If/Ib values ​​of C, RGO/Pt and 3D-graphene/Pt are 0.85, 1.06 and 1.3, respectively, indicating that 3D-graphene/Pt nanocomposites have higher efficiency in forward methanol oxidation.

The long-term catalytic activity of Pt/C, RGO/Pt nanocomposite and 3D-graphene/Pt nanocomposite was measured by steady-state chronoamperometry. Figure 8(c) shows the chronoamperometry data of Pt/C, RGO/Pt nanocomposites and 3D-graphene/Pt nanocomposites at 0.5 V in a solution containing 1M CH3OH and 0.5 H2SO4 for 2000 seconds. The initial current decay in all cases is due to the development of double-layer capacitors. Subsequently, the current decays slowly, which can be attributed to the small amount of carbon monoxide (COads) adsorbed on the surface of the catalyst during the electrooxidation of methanol. The attenuation of the current comes from the adsorption of SO42− species on the catalyst surface, which inhibits the reactive sites of the catalyst. After a long time of operation, the current slowly reaches a quasi-equilibrium steady state. Compared with Pt/C and RGO/Pt nanocomposites, 3D-graphene/Pt nanocomposites showed a higher steady-state current during methanol electrooxidation, which was in good agreement with the CV results. Therefore, both the steady-state current and CV results indicate that the 3D-graphene/Pt nanocomposite has better catalytic performance in the electrooxidation of methanol.

In this study, we demonstrated a universal and clean method to fabricate 3D graphene/precious metal (Pt and Ag) nanocomposites, which are composed of partially reduced graphene oxide and metal precursors, and are easy to use And cost-effective freezing casting method. The synthesized 3D graphene/noble metal (Pt and Ag) nanocomposite is composed of highly interconnected porous structures, and the 3D graphene building block incorporates noble metal nanoparticles (Pt and Ag), thus exhibiting enhanced Mechanical properties and electrical conductivity. Further study their electrocatalytic performance and electromagnetic interference (EMI) shielding effect. Compared with bare 3D graphene, the synthesized 3D graphene/precious metal (Pt and Ag) nanocomposite has excellent EMI shielding effect and can be used for electromagnetic radiation or as a radar absorbing material in civil and military applications. The electrocatalytic activity of 3D-graphene/platinum nanocomposite in methanol oxidation has been proven. Compared with RGO/Pt nanocomposite and Pt/C, it shows high electrocatalytic activity, reliable stability and exceptionally high Tolerance of toxicity. The resulting 3D graphene/precious metal (Pt and Ag) nanocomposites have highly interconnected micro- and macro-pore structures and good electrical, mechanical and catalytic activities, and are expected to be photocatalysts, pressure sensors, actuators and The application of electrode materials opens up a new way.

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The author thanks the Indian Ministry of Science and Technology (DST) Nano-Mission and IITB-Monash Research Academy for financial support.

IITB-Monash Institute, Indian Institute of Technology Mumbai, Mumbai, 400076, India

PK Sahoo & D. Bahadur

Department of Materials Engineering, National Defense Advanced Technology Institute, Pune, 411025, India

Radhamanohar Aepuru and Himanshu Sekhar Panda

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DB and HSP conceive work equally, while PKS conducts all experimental work including sample synthesis. RA conducted research on conductivity and electromagnetic interference (EMI) shielding characteristics. DB, HSP, and PKS analyzed the data, and PKS wrote the manuscript. Each author has made an indispensable contribution to different aspects of this work.

The author declares that there are no competing economic interests.

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Sahoo, P., Aepuru, R., Panda, H. etc. Ice template synthesis of multifunctional three-dimensional graphene/noble metal nanocomposites and their mechanical, electrical, catalytic and electromagnetic shielding properties. Scientific Report 5, 17726 (2016). https://doi.org/10.1038/srep17726

DOI: https://doi.org/10.1038/srep17726

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