2020 Volume 61 Issue 5 Pages 1045-1048
In this study, monolithic micro/nanoporous copper is prepared by sintering and dealloying, that is, the micro/nanoporous copper is obtained by chemical dealloying of a sintered microporous Cu–Mn alloy. The porosity and Brunauer–Emmett–Teller (BET) surface area are 68% and 6.77 m2·g−1, respectively. The micropore and nanopore sizes are 2.44 ± 0.48 µm and 69 ± 16 nm, respectively. The mechanical property is investigated by nanoindentation. The hardness and elastic modulus are 282 ± 65 MPa and 8.25 ± 1.63 GPa, respectively. Indentation creep occurs with a creep depth of 44 ± 7 nm. The electrocatalytic property towards the oxidation of methanol and glucose are investigated using cyclic voltammetry and chronoamperometry. The monolithic micro/nanoporous copper exhibits high electrocatalytic activity and stability. The oxidation peak current density in methanol or glucose alkaline solution is 17 or 51 times that of smooth copper.
Fig. 1 (a) XRD spectra of MP-CuMn and MNP-Cu, (b), (c) SEM images of MP-CuMn, (d)–(f) SEM images, and (g) nitrogen adsorption and desorption isotherm of MNP-Cu.
Nanoporous copper (NP-Cu) with a three-dimensional bicontinuous structure can be prepared at a low cost using a chemical dealloying method that does not involve noble metals, and has potential applications in the fields of catalysts, sensors, etc., due to its high porosity, specific surface area, and electrical and thermal conductivities.1) The introduction of micropores can favor mass transfer in catalytic or sensing processes, and the construction of a micro/nanoporous structure is expected to enhance its performance.2) Therefore, obtaining monolithic micro/nanoporous copper (MNP-Cu) is a challenge.
Cu–Al alloy with high content of Al was employed to prepare the MNP-Cu. The one-step chemical dealloying of α-Al and Al2Cu (Al11Cu5Mn3) led to the formation of micropores and nanopores.3,4) Furthermore, powder metallurgy technology is generally applied in the preparation of metallic foams. The copper alloy with a microporous structure can be obtained before dealloying. The presence of micropores facilitates the infiltration of the corrosion solution and dealloying process. The micropores and nanopores can be prepared and regulated separately. In addition, compared with Cu-based intermetallic compounds,5–7) the Cu–Mn solid solution favors the chemical dealloying to obtain monolithic NP-Cu.
In this research, the monolithic MNP-Cu was prepared by sintering and dealloying. That is, a microporous Cu–Mn alloy (MP-CuMn) was initially prepared by sintering without a space holder, and subsequent chemical dealloying was performed to prepare MNP-Cu. The morphology, mechanical property, and electrocatalytic property towards the oxidation of methanol and glucose were investigated.
Cu (purity >99.9%, average size 3 µm) and Mn (purity >99.9%, average size 2 µm) spherical powders, purchased from Shanghai Yunfu Nanotechnology Corporation, were mixed for 8 h with an atomic ratio of 4:6. The mixed powders were unidirectionally cold pressed in a cylindrical stainless steel die with an internal diameter of 12 mm and at a pressure of 10 MPa. The green compact was loaded into a horizontal quartz tube furnace, which was then evacuated to a pressure of 10−2 Pa before filling it with pure argon to prevent oxidation. MP-CuMn was prepared by sintering at 870°C for 4 h under the flow of argon, which was then polished using fine grit paper to reduce the thickness to 1 mm for dealloying. MNP-Cu was prepared by the dealloying of MP-CuMn in 0.1 mol/L HCl solution at 70°C in a water bath.
The phase compositions and morphologies of MP-CuMn and MNP-Cu were analyzed by X-ray diffraction (XRD-7000) and scanning electron microscopy (SEM, JSM-6700F), respectively. The average pore size was calculated from the SEM images, and the porosity was calculated based on the measured mass and volume. The Brunauer–Emmett–Teller (BET) surface area of MNP-Cu was measured by nitrogen adsorption (ASAP 2000). The mechanical property of MNP-Cu was measured by constant load nanoindentation (Agilent G200) using a Berkovich indenter with a curvature radius of 20 nm. The hold time was 10 s at an applied maximum load of 10 mN. The electrocatalytic activity and stability of MNP-Cu were evaluated by cyclic voltammetry (CV) and chronoamperometry (CA) in methanol or glucose alkaline solution on a CHI760E electrochemical workstation in a three-electrode electrolytic cell with MNP-Cu as the working electrode, a Pt mesh as the counter electrode, and a saturated calomel electrode as the reference electrode. The scan rate was 10 mV/s.
Figure 1(a) shows the XRD spectra of MP-CuMn and MNP-Cu, respectively. The three peaks from MP-CuMn and MNP-Cu correspond to the (111), (200), and (220) lattice planes of (Cu, γMn) (JCPDS Card No. 65-5589) and Cu (JCPDS Card No. 4-836), respectively. This indicated that a (Cu, γMn) solid solution phase formed after sintering at 870°C and transformed to a copper phase after dealloying in HCl solution.
(a) XRD spectra of MP-CuMn and MNP-Cu, (b), (c) SEM images of MP-CuMn, (d)–(f) SEM images, and (g) nitrogen adsorption and desorption isotherm of MNP-Cu.
Figure 1(b), (c) shows the SEM images of MP-CuMn. The micropore size and porosity were 2.34 ± 0.73 µm and 20.5%, respectively. In addition to the formation of a microporous structure, a number of submicropores were observed, which was attributed to the Kirkendall effect.
Figure 1(d)–(f) shows the SEM images of MNP-Cu. The micro-pore size was 2.44 ± 0.48 µm; the increase in size was not apparent after dealloying. The removal of Mn led to the formation of a nanoporous structure and dramatic increase in porosity to 68%. The nanopore and nanoligament sizes were 69 ± 16 and 108 ± 20 nm, respectively. Figure 1(g) shows the nitrogen adsorption and desorption isotherm of MNP-Cu. The type-IV curve with an H3 hysteresis confirmed the presence of a nanoporous structure.8) The BET surface area was 6.77 m2·g−1.
Figure 2(a) shows the nanoindentation load-depth curves of MNP-Cu. The hardness and elastic modulus were 282 ± 65 MPa and 8.25 ± 1.63 GPa, respectively. The high ratio of the residual indentation depth to maximum indentation depth (0.86) corresponded to the low ratio of hardness to elastic modulus (0.03), which indicated the high degree of plastic deformation, small elastic recovery, high recovery resistance, and high local energy dissipation.9)
(a) Nanoindentation load-depth curves and (b) variations of creep depth as a function of load-holding time of MNP-Cu.
As Fig. 2(a) shows, the increase in depth during load holding indicated that indentation creep occurred. The creep depth was 44 ± 7 nm. Figure 2(b) shows the variations in creep depth as a function of load-holding time. MNP-Cu exhibited a typical nanoindentation creep behavior. The creep consisted of two stages, namely, transient creep and steady state creep, which are also observed in porous noble metals.10,11) In consideration of the relative density, the Gibson-Ashby shear mechanism may be dominant for the creep of MNP-Cu.12)
Figures 3(a), (b) and 4(a), (b) show the CV curves of MNP-Cu in various methanol or glucose alkaline solutions, respectively. The corresponding variations in the oxidation peak current density as a function of concentration of NaOH and methanol or glucose are shown in the insets in Figs. 3(a), (b) and 4(a), (b), respectively. The maximum peak current density at 0.25 mol/L methanol and 1.25 mol/L NaOH was observed in the methanol alkaline solution rather than in the glucose alkaline solution. The oxidation peak current density of MNP-Cu in methanol or glucose alkaline solution was 17 or 51 times that of smooth copper, as shown in Figs. 3(c) and 4(c), respectively. The CA curves, shown in Figs. 3(d) and 4(d), depicted the stability of MNP-Cu in methanol or glucose alkaline oxidation.
(a), (b) CV curves of MNP-Cu in various methanol alkaline solution, (a) 0.25 mol/L NaOH, (b) 0.25 mol/L methanol; insets in (a), (b) show variations of oxidation peak current density in corresponding CV curves as a function of concentration of methanol or NaOH, (c) CV and (d) CA curves of MNP-Cu in 0.25 mol/L methanol + 1.25 mol/L NaOH solution; inset in (c) shows CV curve of smooth copper.
(a), (b) CV curves of MNP-Cu in various glucose alkaline solution, (a) 0.25 mol/L NaOH, (b) 1 mol/L glucose; insets in (a), (b) show variations in oxidation peak current density in the corresponding CV curves as a function of concentration of glucose or NaOH, (c) CV and (d) CA curves of MNP-Cu in 0.25 mol/L glucose + 0.25 mol/L NaOH solution; inset in (c) shows CV curve of smooth copper.
The micro/nanoporous copper exhibited high electrocatalytic activity and stability towards the oxidation of methanol and glucose. The bicontinuous interconnected pore-ligament structure favored mass transfer and electron conduction, and the introduction of micropores enhanced the ability and promoted the adsorption and desorption of methanol, glucose, and reaction intermediates.13) The monolithic structure supported the long-term stability in the electrocatalytic oxidation process.
Monolithic micro/nanoporous copper was prepared by sintering and dealloying. The porosity and BET surface areas were 68% and 6.77 m2·g−1, respectively. The micropore and nanopore size were 2.44 ± 0.48 µm and 69 ± 16 nm, respectively. The hardness and elastic modulus were 282 ± 65 MPa and 8.25 ± 1.63 GPa, respectively. Indentation creep occurred with a creep depth of 44 ± 7 nm. The oxidation peak current density in methanol or glucose alkaline solution was 17 or 51 times that of smooth copper.
This work was supported by National Natural Science Foundation of China (Grant No. 51471132) and Shaanxi Province Science Fund for Distinguished Young Scholars (2018JC-027).
