Selective formation of biphasic thin ﬁ lms of metal – organic frameworks by potential-controlled cathodic electrodeposition

Cathodic electrodeposition lends itself to the formation of biphasic metal – organic framework thin ﬁ lms at room temperature from single deposition baths using potential bias as the main user input. Depending on the applied potential, we selectively deposit two di ﬀ erent phases as either bulk mixtures or bilayer ﬁ lms.

Electrochemical methods are best suited to address the challenge of interfacing MOFs with electrode surfaces. 29 Generally, electrodeposition methods nd wide utility in industrial settings, because they enable lm deposition only at the electrode-solution interface, do not require line-of-sight instrumental setups, as physical vapor deposition techniques do for instance, and can therefore be used to build conformal coatings on electrodes of virtually any geometry and surface area. Furthermore, because any exposed electrode surface is electrochemically active, the deposition of non-conductive, electrode-passivating lms such as MOFs allows for in situ repairing of defects such as cracks and pinholes. Finally, the electrochemical nature of the process offers additional advantages in that the deposition progress can be monitored by the amount of passed charge, giving control over lm thickness. Indeed, because of scalability, ease of processing, and the ability to work at room temperature, electrochemical synthesis based on anodic dissolution of the metal component-Cu, Al, Zn 18,30 -is the method of choice for the large-scale production of some of the commercially available MOFs. However, because the anode provides the metal ions for the MOF and is necessarily corroded during this process, anodic methods offer limited choices in terms of electrode surfaces, have thus far yielded only single-phase MOFs, and may not be best suited for the formation of more complex lms. To address these challenges, we recently reported that electrodes can be used as chemically inert spectators and only as sources of electrons when employed in cathodic MOF electrodeposition schemes. 31 We initially surmised that aqueous reduction of oxoanions such as NO 3 À , which produces hydroxide, would raise the pH of the solution near the cathode and induce crystallization of MOFs in an electrolysis bath containing the respective ligand and metal precursor by slowly deprotonating the ligand. We showed that this method was indeed effective for the deposition of Zn 4 O(BDC) 3 (MOF-5), which was crystallized selectively on the surface of a uorine-tin-oxide (FTO) electrode upon biasing a solution containing Zn(NO 3 ) 2 and H 2 BDC at a sufficiently negative potential.
Our initial results hinted at a general method whereby any conductive surface could function as the electron reservoir and, more importantly, any electrochemical half-reaction involving the reduction of a probase molecule would generate a base, increase the local pH near the electrode, and induce MOF crystallization (see Scheme 1). In fact, we surmised that nitrate may be a poor choice as a probase, because it typically requires large reduction overpotentials in both aqueous solutions 32 and in our system (see also Fig. S1 †). To circumvent this problem, we aimed to replace it with one of the reactions in eqn (I-III), which were potential candidates for inducing cathodic electrodeposition of MOFs. In particular, the reduction of triethylammonium, Et 3 NH + , to H 2 and triethylamine, Et 3 N, was chosen because H 2 is a relatively inert and insoluble molecule that would not interfere with MOF formation. Moreover, the pH buffering pair Et 3 NH + /Et 3 N could add another addressable handle to our deposition protocol. The latter was especially attractive, because previous studies had suggested that phase selection in MOF synthesis can be highly dependent on pH. 23,33 We hypothesized that we could selectively deposit multiple MOF phases by simply controlling the Et 3 NH + concentration and dialing the electrochemical deposition potential. Because the rate of an electrochemical reaction is proportional to the current density, which in turn is logarithmically dependent on overpotential, 34 we expected that increasing the deposition potential would generate Et 3 N faster and increase the local pH at the electrode surface, thus modulating the phase of the deposited MOF. Finally, Et 3 N had been employed previously as a base in the original synthesis of MOF-5 35 and therefore had good precedent in constructing such materials.
To minimize the overpotential required for the reduction of Et 3 NH + , our probase, we chose Pt, a well-known catalyst for proton reduction and an otherwise inert metal, as our working electrode. As shown in the cyclic voltammogram (CV) in Fig. 1, the onset of proton reduction from a solution of Et 3 NHCl in DMF occurred at approximately À0.5 V (vs. Ag/Ag(cryptand) + ) and reached a peak at E p,c ¼ À1.05 V when using a Pt button electrode and a scan rate of 100 mV s À1 . Et 3 N was therefore formed below the reduction potential for Zn deposition, which occurred at approximately À1.0 V (see also Fig. S2 †). Thus, MOF crystallization may occur in the absence of Zn plating, in contrast to what had been observed when using NO 3 À as a probase. 31 Addition of H 2 BDC to the Et 3 NHCl/DMF solution only increased the peak current for H + reduction, as expected for an increased proton concentration, but did not shi E p,c , con-rming that the electrochemical event is indeed proton reduction (Fig. 1). Attempts to electrodeposit MOFs from electrolysis baths containing Zn(NO 3 ) 2 , H 2 BDC, and concentrated Et 3 NH + ($300 mM) at À1.00 V (see also Fig. S3 †) yielded a white crystalline lm that adhered to the Pt gauze working electrode. As shown in Fig. 2, powder X-ray diffraction (PXRD) of this white lm revealed a pattern that matched that of the anionic framework (Et 2 NH 2 ) 2 Zn 3 (BDC) 4 , 36 where Et 3 NH + ions likely replace the Et 2 NH 2 + charge-balancing ions present in the reported structure (see also Fig. S4 and S5 †). No Zn plating was observed, and the only crystalline deposit under these conditions was (Et 3 NH) 2 Zn 3 (BDC) 4 . Scanning electron micrographs (SEMs) of this phase revealed distinctive featherlike crystallites of $5 mm width and sub-micron thickness (Fig. 2). Surprisingly, Zn plating was not observed even at more negative potentials if the concentration of Et 3 NH + was maintained at or above 300 mM, and the only phase deposited under these conditions even at À1.50 V was (Et 3 NH) 2 Zn 3 (BDC) 4 (see also Fig. S6-S8 †). This contrasted with previous depositions from Zn(NO 3 ) 2 and H 2 BDC solutions devoid of Et 3 NH + , which at À1.60 V produced composites of MOF-5 and Zn metal on FTO. 31 We reasoned that two effects may come into play when large concentrations of Et 3 NH + are present in the electrodeposition bath: (1) any Zn that could plate is etched away by the Et 3 NH + acid according to eqn (IV), and (2) the presence of a large concentration of Et 3 NH + effectively buffers the pH and may never allow the accumulation of enough Et 3 N to induce the formation of a different crystalline phase such as MOF-5. This remarkable shi from one MOF phase to another depending on the concentration of Et 3 NH + presented the intriguing prospect of selectively depositing two different MOF phases from a single solution of low/intermediate Et 3 NH + concentrations, with phase selection depending on the applied deposition potential. We surmised that lowering the Et 3 NH + concentration would have no effect on the phase deposited at more positive potential, that is (Et 3 NH) 2 Zn 3 (BDC) 4 , but would reduce the buffering capacity, thereby enabling the accumulation of Et 3 N, the increase in pH, and perhaps the formation of a second crystalline MOF phase at a more negative potential.
Indeed, bulk electrolysis at À1.10 V of a solution containing only 100 mM of Et 3 NH + gave exclusively (Et 3 NH) 2 Zn 3 (BDC) 4 , while deposition at À1.50 V from an identical electrolysis bath gave a mixed lm composed of MOF-5, Zn metal, and (Et 3 NH) 2 Zn 3 (BDC) 4 , as identied by PXRD in Fig. 3. This represents the rst demonstration of the simultaneous deposition of biphasic MOF lms from a single precursor solution. Shiing the deposition potential cathodically to À1.70 V virtually eliminated the deposition of (Et 3 NH) 2 Zn 3 (BDC) 4 Fig. S12 and S13 †), attesting that lms grown by cathodic deposition maintain porosity and should be effective in the many applications proposed for MOFs that require surface attachment. This prospect is currently under investigation in our laboratory in the context of gas separation membranes.
Encouraged by the phase control enabled by potential modulation, we sought to deposit not just heterostructured mixed lms as above, but also bilayer structures by sequential electrolyses at two different potentials. Notably, the electrolysis of a 100 mM Zn(NO 3 ) 2 , 50 mM H 2 BDC, and 100 mM Et 3 NHCl solution in DMF at À1.10 V for 6 h followed by deposition at À1.50 V for only 5 min gave lms whose PXRD patterns revealed the presence of both (Et 3 NH) 2 Zn 3 (BDC) 4 and MOF-5 (Fig. 4). Prolonged electrolysis at this potential, up to 20 min, shied the relative ratio of (Et 3 NH) 2 Zn 3 (BDC) 4 to MOF-5 with a signicant increase in the latter, observable by PXRD. SEMs of these lms  revealed remarkable bilayer structures, wherein the Zn/MOF-5 composite, grown at À1.50 V, was deposited underneath the distinctive feather-like crystalline layer of (Et 3 NH) 2 Zn 3 (BDC) 4 grown at À1.10 V. This was anticipated because the insulating (Et 3 NH) 2 Zn 3 (BDC) 4 lm was expected to passivate the Pt surface, but suggests that the anionic phase can be penetrated by the precursors required to form MOF-5. More importantly, the deposition of bilayer/biphasic MOF lms had not been demonstrated before outside the more laborious layer-by-layer approach, 9,26,28 and is enabled here from a single precursor solution through simple potential modulation.
When the second deposition step was held at more cathodic potentials, such as À1.70 V, similar bilayer constructs could be observed. However, the rapid growth of the bottom layer bulges and ruptures the top MOF layer (Fig. S17 †) even with the deposition time kept short at this more negative potential. Although the exact mechanisms that allow selective deposition of one phase over another at various potentials are still unclear, we propose that more negative potentials allow the accumulation of base equivalents and in turn promote the formation of the m 4 -oxo atom required for the nucleation of {Zn 4 O} clusters and MOF-5 (see eqn (V)), where L is a coordinating ligand and 3 # x # 5. At sufficiently negative potentials, NO 3 À reduction also takes place and may be necessary ‡ for MOF-5 formation. 37 Furthermore, Zn metal may be required for MOF-5 formation either for mediating NO 3 À reduction or for templating the crystallization of MOF-5. These are fascinating mechanistic questions worthy of more in-depth examination. Regardless, these results demonstrate that the formation of MOF heterostructures-an important goal for the technological implementation of MOFs-is enabled by cathodic electrodeposition with unprecedented selectivity and minimal user input.

Conclusions
Numerous applications proposed for MOFs depend critically on the development of selective, generalizable, and scalable methods for their growth as thin lms, membranes, or composites. We expect that the versatility of the cathodic deposition approach, demonstrated here, will enable the formation of complex MOF architectures for using these emerging materials in a range of important applications. Many challenges remain before these techniques can be translated to industrial settings, not least of which are concerns regarding the orientation of the crystallites, which could be addressed by surface functionalization for instance, 7,13 and the extension of this method to other metalligand systems, both of which are currently being addressed in our lab. Overall, the results presented herein provide a potential roadmap for a large-scale fabrication methodology for heterostructured multiphasic and multilayered MOF thin lms and membranes.