Functional conductive nanomaterials via polymerisation in nano-channels: PEDOT in a MOF

Poly(3,4-ethylenedioxythiophene) (PEDOT) is formed inside a metal–organic framework (MOF). MOF removal leads to sub-millimetre structures of the nanostructured conducting polymer.


Polypyrrole deposition on Stainless steel
Polypyrrole (PPy) was polymerised on a stainless steel by electrochemical deposition. In this method, deposition solution is a composition of 0.05 Lithium perchlorate (LiClO 4 ), 0.06 M distilled pyrrole and 1% (v/v) distilled water in acetonitrile. A piece of stainless steel mesh served as a counter electrode and a AISI 403 stainless steel substrate was used as a working electrode. Deposition was done at -20 o C by applying a constant current (0.125 mA/cm 2 ) for 8 hours. During the polymerisation process, polymerised pyrrole or PPy is deposited on the stainless steel substrate as working electrode. The one side of the stainless steel was masked with Kapton © tape. After deposition, the PPy film was rinsed with pure acetonitrile for 3 times followed by rinsing with DI water. The sample was dried under ambient condition.

MOF Stabilized on PPy Coated Stainless Steel
Suppl. Fig. S1.3: PPy coating synthesised on a stainless steel substrate via the described electrochemical method.
Suppl. Fig. S1.4: Most MOF crystallites will be localized at the PPy coated part of the stainless steel substrate after MOF synthesis in an autoclave.
Dynamic heating rate thermogravimetric analysis (TGA) was performed with a TA Instruments Q500 TGA. The dynamic heating rate TGA is an improved TGA method which can give high-resolution mass-loss profile with improved separation for mass-loss steps. 1 Though the MOFndc used here has copper as metal linker instead of zinc, the MOFs have very similar unit cell parameters with almost equal monomer absorbed. 2 The monomer absorbed determined by copper-based MOFndc is comparable with that for zinc-based MOFndc. The TGA for pure MOFndc can be found in 3 , which shows that residual DMF can be removed at ca. 150 o C. Fig. S2.1 shows the TGA for a MOFndc sample collected after being immersed in EDOT monomer. The sample was thermally treated at 150 o C to remove residual DMF before soaking the EDOT. TGA steps indicate different chemical reactions and/or physical transitions with unique activation energies. Starting from room temperature, the first step was due to vaporisation of extra EDOT outside the MOF. Such vaporisation occurs mostly at ca. 100 o C indicated by the peak for deriv. weight (weight loss for incremental temperature increase). Therefore, for the real synthesis, EDOT-infiltrated MOF was treated at 100 o C to remove the monomer outside.

Suppl.
The second step in TGA was due to desorption of EDOT inside the MOF. These two processes have different activation energies, as EDOT is likely to require higher activation to overcome interactions within the nano-channels than just evaporation. The second step is between ca. 125 o C and ca. 270 o C is used to estimate the amount of EDOT absorbed by the MOF. Above ca. 270 o C, the MOF is considered to start decompose.
Since the molecular weight of the MOFndc (Cu 2 (1,4-ndc) 2 (dabco)) unit cell is 667.55 g/mol, the mass of EDOT that is loaded into 1 mol MOF unit cell can be obtained from the weight loss in the second step, which is 8.1 mg shown in Suppl. As EDOT has a molecular weight of 142.18 g/mol, the number of EDOT loaded in a MOF unit cell is: The number is comparable with the number for styrene (2.2) and methyl methacrylate (2.3) monomers absorbed by the same copper-based MOFndc and those number for zinc-based MOFndc (2.1 styrene monomer and 2.3 methyl methacrylate monomer) 2 . Both monomers show the shape retention after polymerising the monomers and removing the MOF. 4

XRD analysis
Suppl. Fig. S3.1: PXRD (powder X-ray diffraction) overview for MOFndc/PEDOT system. No significant peak is found in the PXRD pattern for nano-PEDOT. One of the reasons is that the amount of sample with the nano-scaled alignment is insufficient. From the TEM images, we observed that the majority of nano-PEDOT structures show no chain alignment. Without perpendicular structures connecting parallel fibrils (acting as crosslinkers 5 ) to keep them aligned, the structure is likely to suffer collapse upon removing the MOF template.
The as prepared (DMF containing) MOFndc crystallises in the tetragonal space group P4/mbm. 6  Upon drying in air (partial loss of DMF guest molecules) the material changes its structure as indicated by the unsuccessful Pawley refinement of the corresponding PXRD pattern using the initial unit cell parameters and space group (Suppl. Fig. S3.3). However, the pattern can be fit tolerable to a distorted orthorhombic unit cell in space group Pmmm (Suppl. Fig.  S3.4). This indicates that the square channels of MOFndc distort to rhomb-shaped channels upon air-drying, as has been reported for various other MOFs of this family. 7,8 Suppl. Fig. S3.3: Attempt of a Pawley fit to the diffraction pattern of air-dried MOFndc (grown on a PPycoated stainless steel substrate) in the space group P4/mbm and unit cell parameters similar to literature values. The data cannot be fitted satisfactory in this space group, indicating a significant distortion of the structure as a consequence of (partial) loss of DMF upon drying in air. Experimental, calculated and difference patterns are shown in green, red and grey, respectively. Positions of allowed Bragg peaks are shown as blue tick marks. Fig. S3.4: Attempt of a Pawley fit to the diffraction pattern of air-dried MOFndc (same pattern as in Suppl. Fig. S3.3) in the orthorhombic space group Pmmm (a = 16.653(4) Å, b = 13.828(5) Å, c = 9.660(3) Å; R p = 10.75%, R wp = 18.32%, R exp = 1.05%). A reflection at approx. 16.4 degrees 2theta could not be fit with the chosen unit cell and space group, and may originate from an impurity. Experimental, calculated and difference patterns are shown in green, red and grey, respectively. Positions of allowed Bragg peaks are shown as blue tick marks.

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Evidently, there are significant changes to the structure upon (partial) loss and/or exchange of the guest molecules. The MOF hosting EDOT exhibits much lower crystallinity than the DMF-containing as-prepared sample. The pattern features significantly broader peaks but could be fit satisfactory using a simple tetragonal unit cell in the high symmetry space group P4/mmm (Suppl.  Fig. S3.1). Indexing the diffraction pattern with the pattern indexing routine in the TOPAS academic program package was unsuccessful as well. This leads to the conclusion that the pattern of the PEDOT-MOF composite does not correspond to a single phase. It rather may be that several structurally slightly different phases of the composite coexist. However, the major peaks for MOFndc are still approx. at the same positions as for the as-prepared material, which indicates that structurally intact MOFndc is still present after the polymerisation of EDOT. The additional peaks may correspond to decomposition products of MOFndc due to the treatment with acidic FeCl 3 (aq) solution.

Optical Images and Additional Characterizations on Materials Chemistry
Suppl. Fig. S4.1: Optical images for MOFndc, PEDOT-MOF composite and nano-PEDOT. All the scale bars represent 50 μm. Fig. S4.2: SEM images (left column), 30 kV cathodoluminescent (CL) images (right column) and the corresponding CL spectra for MOFndc, PEDOT-MOF composite, nano-PEDOT and a reference PEDOT film made with the same condition. All the samples are on the PPy-coated stainless steel substrate. Samples were rinsed with methanol for a few time to remove FeCl3. The peak's redshift from ca. 420 nm (black) to ca. 470 nm (red) further proved that PEDOT is inside the MOF leading to the local chemistry change.

Suppl.
Suppl. Fig. S4.3 Fig. S5.1), which is consistent with laterally-aggregated fibrils after removal of the MOF. Previously, such highly aligned nanostructures were observed by Distefano et al. 5 for MOF-templated polystyrene, but we demonstrate such alignment in a MOF-templated conducting polymer for the first time. However, the chemistry identity of the fibrils is still an open question. TEM sample preparation procedures for nano-PEDOT are illustrated in Suppl. Fig. S5.2. Briefly, the PEDOT-MOF composite was placed in HCl (aq) followed by NaOH (aq). Those composite with PEDOT only formed in some part (mostly close to the surface) of MOF was attacked by the acid and the base gradually. Since those semi-formed composites have limited PEDOT connection to the substrate, they started to detach the substrate after the MOF part was sufficiently dissolved. The detached material was collected by centrifugation and immersed in fresh acid and base respectively to completely remove the MOF. It was then rinsed with water and methanol. The prepared sample was suspended in methanol and loaded on the TEM copper grid by drop casting.