Magnetically Aligned Ultrafine Cobalt Embedded 3D Porous Carbon Metamaterial by One‐Step Ultrafast Laser Direct Writing

Abstract Spatial manipulation of nanoparticles (NPs) in a controlled manner is critical for the fabrication of 3D hybrid materials with unique functions. However, traditional fabrication methods such as electron‐beam lithography and stereolithography are usually costly and time‐consuming, precluding their production on a large scale. Herein, for the first time the ultrafast laser direct writing is combined with external magnetic field (MF) to massively produce graphene‐coated ultrafine cobalt nanoparticles supported on 3D porous carbon using metal–organic framework crystals as precursors (5 × 5 cm2 with 10 s). The MF‐confined picosecond laser scribing not only reduces the metal ions rapidly but also aligns the NPs in ultrafine and evenly distributed order (from 7.82 ± 2.37 to 3.80 ± 0.84 nm). ≈400% increment of N‐Q species within N compositionis also found as the result of the special MF‐induced laser plasma plume. (). The importance of MF is further exmined by electrochemical water‐splitting tests. Significant overpotential improvements of 90 and 150 mV for oxygen evolution reaction and hydrogen evolution reaction are observed, respectively, owing to the MF‐induced alignment of the NPs and controlled elemental compositions. This work provides a general bottom‐up approach for the synthesis of metamaterials with high outputs yet a simple setup.


Experimental Section
Synthesis of ZIF-67: 5.82 g (0.02 mol) Co(NO 3 ) 2 ·6H 2 O (Sigma Aldrich, NO. 203106) and 6.16 g (0.075 mol) 2-methylimidazole (Sigma Aldrich, NO. M50850) were dissolved in 75 ml methanol (Sigma Aldrich, NO. 34860) under magnetic stirring for 5 min to form clear solutions, respectively. Then the 2methylimidazole solution was poured into Co(NO 3 ) 2 ·6H 2 O solution under magnetic stirring for 5 min. The above mixed solution was aged at room temperature for 24 h. The purple precipitates were collected by centrifuging at 3000 rpm. The final ZIF-67 was obtained by washing with methanol for 3 times and dried at 60 C for 12 h.
Laser system setup: A picosecond pulse fiber laser (Fianium HE-1060) was used as laser source. The wavelength was 1064 nm and the pulse width was 5 ps with the waveforms digitally controlled by computer. The laser spot size was focused to ca. 50 µm for the scanning processing by galvo mirrors. The writing speed of the laser is fixed to 10 mm s −1 .
Various laser energies were applied with/without MF involvement, and the corresponding products are electrodes were cycled at 10 mV s -1 until a stable cyclic voltammetry (CV) was achieved before we collected the data. Electrochemical capacitance was measured using CV measurements. The currents were measured in a narrow potential window that no faradaic processes were observed. CVs were collected at different scan rates: 5, 10, 20, 30, 40, and 50 mV s -1 . The measured current in this non-Faradaic potential region should be mostly due to the charging of the double-layer. By plotting the capacitive currents against the scan rate and following with a linear fit, the double layer capacitance C dl is around half of the slope.

Statistical Analysis:
Co NP size distribution: HRTEM images ( Figure S2-7) of ZIF67 samples are imported into the ImageJ to measure particle sizes. Sample size (n) is shown below: Measured particle sizes are grouped and separated by 1 nm to plot the pore size distribution by Origin 2020 in stacked column graph (0-1 nm, 1-2 nm,...,9-10 nm). The average size is calculated by the sum of the set of numbers divided by the count which is the number of the values being added. The standard deviation is calculated as the square root of variance by determining each data point's deviation relative to the mean. The curve ontop of the column graph is derived by the nonlinear curve fit (Gaussian) by Origin 2020.
XPS peak analysis: the atomic composition are collected directly from XPS data. The N1s composition is processed by XPSPEAK 4.1 peak optimization.

Theoretical Ananlysus of MF-induced plasma plume change:
In this paper, a hypothesized explanation is provided based on theoretical analysis: [1] The high intensity laser beam would generate a plasma plume at the surface of ZIF-67 crystals, which would expand. When the external MF exists, it will induce electric current and hence electromagnetic force in the plasma, and the current density J and EM force F are given by: (1) and (2) = (−∇ + × )

= ×
Where is the plasma electric conductivity, is the electrostatic potential, is the material velocity in plasma, and is the MF. If the term of ∇ is neglected (for the simplicity of analysis), then the current density can be given by: 1c) shows a schematic for the effect of magnetic field on plasma induced by laser irradiation, where the MF is assumed to be in +Z direction. Equations (2) and (3) imply that the motion of plasma material in any direction (except the ±Z direction) would induce electric current and thus electromagnetic force that would constrain the motion. Hence, the electromagnetic force would confine the expansion of the plasma in any direction that is not parallel to the Z-axis. For example, it will confine the plasma expansion in ±X as shown in the lower part of Fig. 1c).
The plasma might absorb a huge amount of energy from the laser beam, and it might also transfer part of its energy to the workpiece through thermal conduction and other mechanism. A very rough estimation of the heat conduction flux, q, from the plasma to the ZIF-67 surface could be given by: where is the average plasma thermal conductivity, is the plasma average temperature, is the workpiece surface temperature, and L is the average distance from plasma to the workpiece surface in X direction.
The confinement of the plasma expansion in +X direction due to the electromagnetic force induced by the MF would decrease the value of L. The confinement effect would convert part of the plasma kinetic energy to its internal energy, and hence may increase the plasma temperature . A higher plasma temperature would also increase the plasma thermal conductivity . All of these would increase the value of the heat conduction flux from the plasma to ZIF-67, which could be seen from Eq. (4).             Figure S12. ZIF67-13, ZIF67-13M and ZIF67-A under external MF.