Electrochemical Conversion of Cu Nanowire Arrays into Metal-Organic Frameworks HKUST-1

Template-based fabrication of Cu nanowire arrays

Conversion of Cu nanowires to HKUST-1

Characterization

Optical microscopy of the samples shows a pronounced color change from initially orange to brown and finally greenish depending on the conversion time of the Cu nanowires into HKUST-1 (Supporting Information Fig. S3). The corresponding SEM images at low magnification presented in Supporting Information Fig. S4 illustrate the increase from small to large and from sparse to dense coverage of the surface with nanostructures.

SEM images at higher magnification of different samples and their elemental distribution by EDX are presented in Fig. 1. The as-grown Cu nanowires have a length of ∼10 μm and their diameter is ∼210 nm (Fig. 1a). Some neighboring nanowires touch each other at the tips. We ascribe this effect to bending of the initially parallel wires during the dissolution process of the polycarbonate matrix. High-resolution TEM (HRTEM) and selected area electron diffraction (SAED) images indicate that the as-grown Cu nanowires are crystalline. The distance between adjacent planes is 0.128 nm, which corresponds to the (220) plane of face-centered cubic (fcc) copper (Supporting Information Fig. S5). Figure 1b shows the sample after 100 s conversion. At the top of the nanowires, few octahedral particles have formed with sizes ranging from 400 nm to 4 μm, while the diameter and morphology of the Cu nanowires have not changed significantly. After 300 s of conversion, the number of MOF particles has increased, while the size is still distributed between 400 nm and 4 μm (Fig. 1c). The diameter of the nanowires remains the same, but the nanowire tips start to fray. At this stage, the MOF particles and the nanowires seem to form a composite structure with the Cu nanowires being wrapped around the MOF particles or even passing through them. After 600 s conversion, the number and size of the MOF particles have further increased, some particles reach 7 μm (Fig. 1d) and the originally smooth surface of the nanowires is rather rough. After 1200 s, the nanowires are severely degraded and many of them collapsed (Fig. 1e). This collapse may be due to the wire degradation and/or weight of the MOF particles. With conversion times getting longer, the nanowires begin to break (Fig. 1f) and after 3000 s, the nanowires are completely broken into small sections (Fig. 2g). After 5000 s, almost no nanowires remain and MOF particles with size larger than 10 μm are formed (Fig. 2h). After 8000 s, the Cu nanowires are completely transformed into MOF particles (Fig. 1i). Elemental analysis by EDX provides clear evidence that the observed particles are indeed HKUST-1 which consists of the elements copper, carbon, and oxygen.

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Figures 2a–2d presents TEM images of samples after 0 s, 300 s, 3000 s, and 8000 s conversion, clearly illustrating that the nanowires gradually disappear and eventually transform completely into HKUST-1 particles. At the intermediate stages of the transformation hybrid Cu-nanowire@HKUST-1 structures are formed as revealed in Fig. 1c. Figures 2e–2g are EDX mapping images measured of a representative structure formed after 300 s conversion displaying signals corresponding to the elements C, O and Cu and confirming the results from SEM-EDX analysis, demonstrating the formation of the hybrid Cu-nanowire@HKUST-1 composites.

The Cu nanowire to HKUST-1 conversion process was also confirmed by XRD analysis. XRD diffractograms recorded at different reaction times are shown in Fig. 3. We observe three Cu reflections, namely (111), (200), and (220), whose intensities systematically decrease with increasing conversion times. When the conversion time reaches 5000 s, these reflections have almost disappeared, which is consistent with the progressing dissolution and disappearance of the Cu nanowires on the SEM images (Fig. 1). After 300 s conversion time, a series of characteristic reflections appears at low diffraction angles, which are in agreement with simulated single-crystal HKUST-1 XRD data ^44,45 shown at the bottom of Fig. 3. The increase of their intensity with longer conversion times suggests that more and more HKUST-1 particles are formed. The narrow width of the reflections indicates that the synthesized HKUST-1 particles are crystalline and of high purity. During the whole transformation process, no cuprous oxide phase is found, which is different from previous Cu to HKUST-1 transformations are conducted in an etched ion-track template. ²⁰ The four high-intensity reflections at 38.2°, 44.4°, 64.6°, and 77.6° correspond to the Au back electrode, which remains unchanged throughout the conversion process. The evolution of the X-ray diffractograms is in excellent agreement with the morphology observations from scanning electron microscopy.

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Based on the morphological and structural analysis presented above, we propose two main stages of the HKUST-1 growth mechanism: In a first step, the HKUST-1 crystals are initiated by the applied 2.5 V potential versus the Cu counter electrode, the Cu nanowires are oxidatively dissolved to Cu²⁺ cations and the BTC³⁻ anions of the ligand coordinate with Cu²⁺ cations on the Cu nanowire surface, where HKUST-1 nucleation sites built up. In the second stage, the MOF crystals grow thermodynamically, forming a fcc network (Fm3m) with octahedron morphology. The shape is determined by the coexistence of slower and faster growth facets. ^46,47 In HKUST-1, {100} facets have a higher surface energy and thus a faster growth rate than {111} facets making the {111} facets to be exposed on the crystal surface to minimize the total surface energy and consequently exhibiting an octahedron morphology. ^48,49

Figure 4a shows the Raman spectra of samples recorded after different conversion times. To complement the experimental data and provide information on vibrational properties of HKUST-1, wavenumbers of the Raman modes are studied by DFT calculations. Relevant calculation parameters are described in Supporting Information (Fig. S6). Ignoring the water molecules in the crystal, HKUST-1 has the symmetry of the O_h space group. So, the vibrational modes of irreducible representation E_g and T_2g are Raman active. Considering that the signal of T_2g modes are relatively weak and difficult to be observed, we only compare the energies of E_g irreducible representations with the experimental Raman data (Fig. 4b). Apart from some minor deviations, the calculated results overall agree well with the experimental values. E_g irreducible representations are doubly-degenerate, so each E_g mode corresponds to two vibrational modes. Animations of the listed E_g vibrational modes are illustrated in the Supporting Information (Vibration Modes), where vibration modes 01 to 19 correspond to wavenumbers from low to high. The vibrations of heavy atoms tend to occur at lower and not at higher energy levels. In our spectra, wavenumbers below 490 cm⁻¹ are ascribed to vibrations of the joint contribution of carbon, oxygen and copper atoms, while vibrations with wavenumbers above 490 cm⁻¹ are attributed to the benzene ring and oxygen atoms without a contribution from Cu atoms. Some of the bands in our Raman spectra cannot be described by a specific vibration, but reflect the vibration of the whole lattice. Our Raman findings differ slightly from previous works. ^37–39

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In summary, we demonstrated that free-standing Cu nanowires arrays can be converted into Cu-based metal–organic framework HKUST-1 by electrochemical oxidation. By systematically increasing the conversion time, we visualized the growth process of HKUST-1 particles from small to large and from sparse to dense, finally forming the characteristic octahedron structures. The analyses performed at different stages of the conversion process did not reveal any evidence of the presence of Cu₂O. Comparison of the measured Raman spectra with DFT calculations revealed that some Raman modes cannot be described simply by individual bond vibrations, but in fact reflect the vibration of the entire lattice. In the future, the influence of ligand concentration, electrolyte temperature, and applied potential on the conversion process will be studied. We believe that this method can be applied to control the position and dimensions of the HKUST-1 crystals by optimizing the density and adjusting the dimensions of the free-standing Cu nanowires.

This work was supported by the International Postdoctoral Exchange Fellowship Program (No. 2020018) hosted and financial supported by The Office of China Postdoctoral Council (OCPC) and the Helmholtz Association (GSI Helmholtz Center for Heavy Ion Research). Dr. Hang Liu (University of Stuttgart) is acknowledged for the discussion on our experimental results. The preparation of the samples employed for this work is based on an UMAT irradiation experiment, which was performed at the beamline X0 at the GSI Helmholtz Centre for Heavy Ion Research, Darmstadt (Germany) in the frame of FAIR-Phase 0.

See the supporting information for the current-time curves of Cu nanowires deposition and conversion process of Cu nanowires into HKUST-1 particles, more morphological and structural characterization, DFT calculation parameters, and the animations of vibration modes.

All authors contributed to the review and editing of the manuscript. In addition, Jia Luo: conceptualization, investigation, formal analysis, writing–original draft, writing–review & editing; Mu Lan: formal analysis, writing–review & editing; Michael Wagner: investigation; Nils Ulrich: investigation; Peter Kopold: investigation; Ioannis Tzifas: investigation; Hongyan Wang: formal analysis, writing–review & editing; Christina Trautmann: funding acquisition, writing–review & editing; Maria Eugenia Toimil-Molares: conceptualization, funding acquisition, writing–review & editing.