Unraveling a two-step oxidation mechanism in electrochemical Cu-MOF synthesis

Abstract

To employ the full potential of electrochemical (ec) synthesis to grow metal–organic frameworks (MOFs) in more complex organizations at the mesoscale, it is vital to understand the underlying crystallization reaction pathway. For the MOF most typically grown electrochemically, CuBTC, we systematically investigated the role of oxygen species in the synthesis.

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Metal–organic frameworks (MOFs) attract significant interest as versatile materials for gas separation¹ and storage,² and more recently as light harvesters for novel energy conversion schemes.^3,4 Their topology (crystal structure, pore size, pore connectivity) and functionality (chemistry at reactive sites, luminescence/absorption properties of framework, guest donor/acceptor moieties) can, in principle, be tuned to desire by making use of the wealth of metal ion and organic linker combinations the chemist's tool kit provides. While classically, MOFs are synthesized solvothermally at elevated pressure and/or temperature,^5,6 recently, novel anodic and cathodic^7–12 electrochemical (ec) fabrication protocols are emerging as electrosynthesis offers milder conditions, greater energy efficiency and energetic control over the MOF growth.^7,13,14

It has been shown that by applying an appropriate synthesis potential, the size of MOF crystals can be directly controlled in the sub- to 5 μm size range,^7,8 or that different linkers can be built in on demand to fabricate multi-functionality MOFs in situ.¹⁰ These cases demonstrate the extraordinary possibilities electrosynthesis offers for controlled MOF growth. However, generalizing the ec approach requires a detailed understanding of the underlying reaction mechanisms which have to date only been speculated on.^7,15 Questions regarding the chemical species involved, the role of the substrate surface and the energetics and kinetics of the overall reaction pathway of ec MOF formation need to be answered to mature ec synthesis into a readily available tool for rational MOF design.

In this work, we unravel the reaction mechanism of the ec formation of a showcase MOF, CuBTC (HKUST-1; Cu 1,3,5 tricarboxylic acid). Prepared according to the original protocol from 2005, electrosynthesized CuBTC, finds large-scale application e.g. for carbon dioxide/methane¹⁶ or propylene/propane¹⁷ gas mixture separation, or in novel sensors and electronic devices.¹⁸ Despite CuBTC being one of the most extensively studied MOFs,^7,19 its ec formation mechanism has not yet been unraveled, hindering full exploitation of electrosynthetic MOF design possibilities. It has been postulated that, Cu is anodically oxidized in one step to Cu²⁺ followed by linker coordination in solution.^7,15 Other groups, however, have observed MOF crystals intergrown with the surface that suggest on-surface growth.²⁰

To solve the prevailing controversy and open a route for rational MOF topology and functionality design, we set out to identify the chemical species involved in CuBTC electro-synthesis and unravel the reaction mechanism. We systematically varied the experimental conditions such as the Cu source, the presence of O₂ and the applied potential (Table 1), generally following the protocol found in literature:^7,21 a Cu plate was immersed at open circuit in a mixture of 100 mL absolute EtOH, 3 g BTC linker and 1 g methyl tributyl ammonium methyl sulfate electrolyte. After electrode immersion, a potential of E = 1 V vs. Ag/AgCl was applied for 20 min. To prevent Cu₂O formation on the Cu electrode from ambient aerobic corrosion in O₂-free experiments, EtOH was degassed with Ar for 20 min prior to Cu immersion, and the ec cell was constantly kept under Ar. To remove the natural oxide layer (comprised mainly of Cu₂O and a small amount of CuO and Cu(OH)₂²²), the Cu electrode was etched with 25% HNO₃ and 25% HCl²³ before transfer to the synthesis cell.

Table 1 Experimental conditions, sample overview

Sample type	Cu source	O2 presence	E /reaction time	CuBTC formation
a Potentials reported vs. Ag/AgCl/3 M KCl. b SEM/Raman characterization in ESI.
A	Cu with natural oxide layer	Yes	1 V/20 min	Yes
B	Cu, oxide free	No	1 V/20 min	No
C	Cu with artificial oxide layer	No	1 V/20 min	Yes
D	Cu2O	Yes	No/16 hours	Yes
E	Cu2O	Low amount	No/16 hours	Yes (truncated)
F^b	CuO	Yes	No/16 hours	No

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The resulting topography and chemical composition of each sample type was characterized to determine whether CuBTC grows under the respective conditions. CuBTC is known to form octahedral crystals that can be seen with scanning electron microscopy (SEM).²⁴ Raman in-plane ring bending vibrations at 743 and 825 cm⁻¹ of the BTC moiety in CuBTC confirm the presence of CuBTC (Fig. S2 and Table S1, ESI†).²⁵ X-ray diffraction (XRD) was used for CuBTC identification where a sufficient amount of MOF crystals was obtained (see ESI,† for experimental details).

Fig. 1 shows the SEM and Raman results for the different sample types under investigation. Samples A were fabricated under synthesis conditions similar to the ec synthesis protocols found in literature,^7,21i.e. starting from a Cu-electrode with a natural oxide layer and taking no precautions to avoid the presence of O₂. As expected, octahedral crystals of CuBTC of <1 μm to 5 μm in diameter form (Fig. 1A). Electrode coverage is incomplete at ∼20 particles per 100 μm², likely due to the low potential of 1 V. An increased synthesis time should improve surface coverage. The corresponding Raman spectrum in the inset of Fig. 1A displays the characteristic bending vibrations of CuBTC at 743 and 825 cm⁻¹, and XRD diffractograms confirm the successful synthesis of CuBTC (Fig. S1A, ESI†). Note that the Raman spectrum also shows a significant contribution of Cu₂O (see Fig. S3A, ESI,† broad peaks at 525 and 626 cm⁻¹).

Fig. 1 SEM micrographs and Raman spectra of Samples A to D. Scale bar: 10 μm.

Samples B were produced under oxide- and O₂-free conditions, thus void of all components unnecessary for the Cu²⁺ dissolution-coordination mechanism proposed in literature. The SEM image (Fig. 1B) shows parallel Cu tracks due to the acidic etching of the surface. With exception of only about 1 particle per 100 μm² of ca. 0.5 to 1 μm diameter, no CuBTC crystals are visible. The CuBTC response in the Raman spectra can hardly be distinguished from the noise (Fig. 1B inset), and no XRD signal of CuBTC was obtained (Fig. S1B, ESI†).

To find the smallest set of components required to enable CuBTC synthesis, we electrochemically oxidized the surface of the Cu plate on purpose by immersing it in the electrolyte solution and applying a potential of 1 V vs. Ag/AgCl for 25 min prior to O₂-free MOF synthesis (Samples C). SEM, Raman and XRD confirm the successful synthesis of CuBTC (Fig. 1C and Fig. S1C, ESI†); the Raman spectrum shows a significant Cu₂O contribution (Fig. S3C, ESI†). Samples C are covered with a mixture of about 5 crystals per 100 μm² of octahedral crystals of 3 to 5 μm diameter and ca. 50 particles per 100 μm² in the sub-μm diameter range. At some spots, the smaller crystals cover the larger particles.

From experiments A to C, we learn that oxygen plays an important role in ec CuBTC synthesis as CuBTC crystals do not grow under O₂-oxygen free conditions and Cu₂O is always present on the electrodes. To further investigate the role of oxygen, we used pure oxides as precursors for CuBTC growth. For Samples D, we immersed Cu₂O powder in an ethanolic solution of 0.15 M BTC without any other additions for 14 hrs under ambient conditions. No potential was applied. A blue powder was isolated by centrifugation and characterized. The SEM micrograph in Fig. 1D shows octahedral crystals of 1 to 5 μm diameter that are intergrown into larger agglomerates of 10 to 15 μm length. Raman spectra (Fig. 1D inset) and XRD (Fig. S1D, ESI†) confirm CuBTC synthesis. Interestingly, an analogous experiment with CuO powder as starting material did not produce any CuBTC (Samples F, ESI†).

To identify the necessary oxidation agent for the oxidation of Cu(I) to Cu(II) in absence of an applied potential, Samples E were synthesized like Samples D, but in an O₂-deficient environment. The solution was degassed for 15 min, but during transfer into a glove box, some O₂ could re-dissolve. The SEM micrographs show about 10% of the Cu₂O surface covered with octahedral crystals of diameters between 0.3 and 0.7 μm, and partly with what seem to be incomplete CuBTC crystals with diameters of 100 to 200 nm (Fig. 2, red). This is the only Sample for which we observe incomplete CuBTC octahedrons. It is unlikely that these crystals deposited onto the Cu₂O substrate during centrifugation; rather they must have grown at the surface, and their growth was halted prematurely due to lack of O₂. Similarly, also the few octahedral crystals (Fig. 2, blue) are visibly attached to the Cu₂O substrate. From these results, we conclude that O₂ is the necessary oxidant for the oxidation of Cu₂O to CuBTC without ec potential. Furthermore, the visible connection between the CuBTC and the Cu₂O indicates that CuBTC nucleation likely takes place directly at the Cu₂O solid/liquid interface. Summing up, the first step to ec CuBTC formation is the oxidation of Cu to Cu¹⁺ achieved either by ec oxidation of the Cu anode or by providing O₂ and/or H₂O (H₂O enables CuBTC synthesis even under inert conditions.²⁰) as reactant. Cu₂O is the predominant oxidation product of Cu in ethanol.²⁶ Note that also Cu(OH)₂ readily converts into CuBTC in presence of the linker.²⁷ While Cu(OH)₂ as an intermittent, short-lived reaction intermediate cannot be excluded, our spectroscopic data does not indicate stable Cu(OH)₂ formation (i.e. no Raman band at 460 cm⁻¹, Fig. S2b, ESI†). Furthermore, CuBTC forms directly from Cu¹⁺₂O powder while Cu²⁺O does not convert to CuBTC under the given experimental conditions.

Fig. 2 SEM of Samples E. Red: truncated crystals; blue: intergrowth with surface. Scale bar: 500 nm.

As known from literature, Cu₂O can be produced from Cu by the following oxidation reactions with H₂O or O₂:^22,28

2Cu + H₂O ↔ Cu₂O + 2H⁺ + 2e⁻ (1)

4Cu + O₂ ↔ 2Cu₂O (2)

A positive applied potential facilitates the net release of electrons in reaction (1). Excess electrons are transferred to the counter electrode where a not-further specified counter reaction takes place. Reaction (2) represents the aerobic oxidation of Cu that takes place in the presence of O₂. In Samples B, both (1) and (2) were blocked by using dry EtOH and by degassing the electrolyte with Ar. Since all surface oxides were removed by acidic etching, no Cu₂O – and thus no CuBTC – could be formed.

Instead of using O₂ or H₂O as oxidant, Cu can be electro-chemically oxidized. In absence of O₂ and H₂O, the amount of available Cu⁺¹₂O is expected to limit the CuBTC yield. Indeed, for Samples C, we did not observe continuous detachment of large MOF crystals from the electrode into the solution phase, indicating that CuBTC growth stopped after all provided Cu₂O had been consumed.

For the second oxidation step of Cu¹⁺₂O to Cu²⁺BTC, we propose the following ec half reaction:

3Cu₂O + 4H₃BTC ↔ 2Cu₃BTC₂ + 3H₂O + 6e⁻ + 6H⁺ (3)

H₂O is a likely product that can be produced from the oxygen released from Cu₂O and the protons released from BTC. In the ec synthesis, this oxidation half-reaction takes place at the Cu anode. (A corresponding, not further specified reduction reaction takes place at the counter electrode.) With the production of H₂O, the reaction should be self-sustaining after a certain threshold H₂O concentration is reached. For aerobic Cu₂O oxidation (Samples D), CuBTC growth occurs at the Cu₂O/solution interface. CuBTC synthesis from Cu₂O without applied potential is only possible in the presence of O₂, as shown by the greatly reduced amount of CuBTC in Samples E. Likely, O₂ is reduced to H₂O in a counter reaction

O₂ + 4H⁺ + 4e⁻ ↔ 2H₂O (4)

Eqn (3) as the key step in ec CuBTC synthesis as derived from our results is fully consistent with synthesis facts reported in literature: Van Assche et al. showed that CuBTC cannot be produced electrochemically in electrolytes that contain more than 50% H₂O.²⁰ The suppression of the reaction by H₂O is consistent with H₂O formation on the product side of (3), leading to an increase of the oxidation potential of the reaction while increasing the propensity for the formation of H₂O-containing catena-triaqua-μ-CuBTC.²⁰

Direct oxidation of solid Cu₂O to CuBTC at the ec solid/liquid interface to start crystal nucleation likely explains the tight attachment of CuBTC crystals to the electrode as previously observed⁷ as well as the incomplete CuBTC crystals visibly bound to Cu₂O that we observe under oxygen-limited conditions; however, a dissolution process with Cu₂O as intermittent cannot be completely excluded. It remains unclear whether further crystal growth happens at the Cu₂O–CuBTC–electrolyte interface or continues through a solution-based process with Cu²⁺ ions attaching to the outer facets of the growing crystal. Campagnol et al. show that electrosynthesized Cu isonicotinate grows at the MOF–electrolyte interface.²⁹ The significant amount of water (50%)³⁰ that makes Cu²⁺ ions more soluble in the form of [Cu(H₂O)₆]²⁺ could explain a solution-based process. The lack of H₂O in our system, however, makes a direct comparison difficult.

Let us summarize the reaction mechanism for the ec oxidation of Cu⁰ to CuBTC: in presence of O₂ and/or H₂O, Cu is oxidized in a one-electron step to Cu₂O. A second oxidation step from Cu¹⁺ to Cu²⁺ in the presence of BTC directly converts Cu₂O to CuBTC. Involvement of an intermediate Cu(I) reaction step has not been proposed so far and opens up new pathways to increase control of the synthesis.

Our newly gained knowledge that CuBTC crystallization proceeds through Cu₂O formation allows us to suggest a novel method to fabricate patterned MOF devices (Fig. 3A). As a proof of principle, we prepared a Cu substrate with both Cu and Cu₂O present by ec oxidizing the surface in the presence of NaOH.³¹ Then, we selectively removed Cu₂O in a spot of ∼1 cm diameter in the centre of the sample by pipetting a drop of 10% HCl. After removal of that drop and subsequent rinsing with EtOH, the Cu plate was covered with Cu₂O except for the acid-etched blank Cu area. Under synthesis conditions B (O₂ exclusion, 1 V), CuBTC was grown solely on the oxide-covered part of the sample as discernible in the white-light image (Fig. 3B). The left part of the image where surface oxides were removed by acidic etching shows the pristine Cu surface. To the right of the image, outside the area of the acid droplet, CuBTC has visibly grown and is spectroscopically detectable. Raman band integration of the 800 to 850 cm⁻¹ region provides the relative amounts of CuBTC at different sample positions (Fig. 3C; error bars are standard errors of the mean of three measurements taken at roughly the top, middle and bottom of the white-light image). This facile oxide-based pre-patterning approach can serve as starting point to develop improved synthesis protocols for CuBTC patterning. Current patterning approaches require the use of Cu electrodes in the desired shape (i.e. printed circuit boards⁸ or Cu meshes²⁰). Our approach offers the possibility to subsequently activate parts of a Cu substrate. It is easy to imagine how established ways to deposit Cu₂O as nanoparticles or films^32,33 or to selectively remove Cu oxide, as shown here, can be converted into a versatile basis for the fabrication of arbitrarily patterned surfaces on nm to mm length scales to create devices covered with MOF of controlled sizes at predetermined spots.

Fig. 3 A: Scheme for the fabrication of patterned CuBTC devices by oxide patterning; B: light-microscopy image of a Cu sample (left) partly covered with CuBTC (right). Scale bar: 200 μm; C: normalized amount of CuBTC across sample.

To conclude, we unravelled CuBTC electrosynthesis to proceed in a two-step oxidation mechanism at the electrode surface: Cu is first oxidized to Cu¹⁺₂O in the presence of H₂O or O₂. Cu₂O is further oxidized to Cu²⁺BTC in presence of the linker at the cuprite–electrolyte interface. The MOF octahedrons nucleate directly at the electrode surface and not in solution, rendering the ec mechanism different from the one of solvothermal syntheses with metal salts.^34,35 We demonstrate how the gained knowledge can be used for a novel quick and versatile approach to produce patterned CuBTC devices.

We thank G. Glasser for SEM measurements, M. Steiert and B. Norder for XRD measurements and the Max Planck Graduate Center and the Studienstiftung des deutschen Volkes for funding. KFD acknowledges generous support through the Emmy Noether Program of the DFG (#DO1691/1-1).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and methods (Raman, XRD, SEM data); characterization of Samples F. Current transients. See DOI: 10.1039/c6cc00534a

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