Development of a UiO-Type Thin Film Electrocatalysis Platform with Redox-Active Linkers

Article abstract of DOI:10.1021/jacs.7b13077

Metal-organic frameworks (MOFs) as electrocatalysis scaffolds are appealing due to the large concentration of catalytic units that can be assembled in three dimensions. To harness the full potential of these materials, charge transport to the redox catalysts within the MOF has to be ensured. Herein, we report the first electroactive MOF with the UiO/PIZOF topology (Zr(dcphOH-NDI)), i.e., one of the most widely used MOFs for catalyst incorporation, by using redox-active naphthalene diimide-based linkers (dcphOH-NDI). Hydroxyl groups were included on the dcphOH-NDI linker to facilitate proton transport through the material. Potentiometric titrations of Zr(dcphOH-NDI) show the proton-responsive behavior via the OH groups on the linkers and the bridging Zr-μ₃-OH of the secondary building units with pK_a values of 6.10 and 3.45, respectively. When grown directly onto transparent conductive fluorine-doped tin oxide (FTO), 1 μm thin films of Zr(dcphOH-NDI)@FTO could be achieved. Zr(dcphOH-NDI)@FTO displays reversible electrochromic behavior as a result of the sequential one-electron reductions of the redox-active NDI linkers. Importantly, 97% of the NDI sites are electrochemically active at applied potentials. Charge propagation through the thin film proceeds through a linker-to-linker hopping mechanism that is charge-balanced by electrolyte transport, giving rise to cyclic voltammograms of the thin films that show characteristics of a diffusion-controlled process. The equivalent diffusion coefficient, D_e, that contains contributions from both phenomena was measured directly by UV/vis spectroelectrochemistry. Using KPF₆ as electrolyte, D_e was determined to be D_e(KPF₆) = (5.4 ± 1.1) × 10^-11 cm² s^-1, while an increase in countercation size to n-Bu₄N⁺ led to a significant decrease of D_e by about 1 order of magnitude (D_e(n-Bu₄NPF₆) = (4.0 ± 2.5) × 10^-12 cm² s^-1).

Full text of DOI:10.1021/jacs.7b13077

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Article
Development of a UiO-type thin film
electrocatalysis platform with redox active linkers
Ben A. Johnson, Asamanjoy Bhunia, Honghan Fei, Seth M. Cohen, and Sascha Ott
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13077 • Publication Date (Web): 09 Feb 2018
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Development of a UiO-type thin film electrocatalysis platform with re-
dox active linkers.
Ben A. Johnson,[a] Asamanjoy Bhunia,[a] Honghan Fei,[b]† Seth M. Cohen,[b] and Sascha Ott*[a]
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[a]Department of Chemistry Ångström Laboratory, Uppsala University Box 523, 75120 Uppsala (Sweden) E-mail:
sascha.ott@kemi.uu.se
[b]Department of Chemistry and Biochemistry, University of California, La Jolla, San Diego, CA, 92023-0358, USA.
ABSTRACT: Metal organic frameworks (MOFs) as electrocatalysis scaffolds are appealing due to the large concentration
of catalytic units that can be assembled in three dimensions. To harness the full potential of these materials, charge
transport to the redox-catalysts within the MOF has to be ensured. Herein, we report the first electroactive MOF with the
UiO/PIZOF topology (Zr(dcphOH-NDI)), i.e. one of the most widely used MOFs for catalyst incorporation, by using
redox-active naphthalene diimide-based linkers (dcphOH-NDI). Hydroxyl groups were included on the dcphOH-NDI
linker to facilitate proton transport through the material. Potentiometric titrations of Zr(dcphOH-NDI) show the proton
responsive behavior via the –OH groups on the linkers and the bridging Zr-μ3-OH of the secondary building units with
pKa values of 6.10 and 3.45, respectively. When grown directly onto transparent conductive fluorine-doped tin oxide
(FTO), 1 ꢀm thin films of Zr(dcphOH-NDI)@FTO could be achieved. Zr(dcphOH-NDI)@FTO displays reversible elec-
trochromic behavior as a result of the sequential one-electron reductions of the redox-active NDI linkers. Importantly,
97% of the NDI sites are electrochemically active at applied potentials. Charge propagation through the thin film pro-
ceeds through a linker-to-linker hopping mechanism that is charge-balanced by electrolyte transport, giving rise to cyclic
voltammograms of the thin films that show characteristics of a diffusion-controlled process. The equivalent diffusion
coefficient, De, that contains contributions from both phenomena was measured directly by UV/Vis spectroelectrochem-
istry. Using KPF6 as electrolyte, De was determined to De(KPF6) = 5.4±1.1x10−11 cm2 s−1, while an increase in counter cation
size to n-Bu4N+ led to a significant decrease of De by about one order of magnitude (De(n-Bu4NPF6) = 4.0±2.5x10−12 cm2 s−1).
charge transport to the MOF-embedded catalysts.19-20 As a
consequence, the realization of effective electrocatalytic
INTRODUCTION
Metal-organic frameworks (MOFs) are a class of po-
rous, crystalline materials made from polydentate organic
linkers and inorganic metal nodes, so-called secondary
binding units (SBUs).1-3 In some cases, the organic linkers
can be functionalized using postsynthetic modification
(PSM),4-5 postsynthetic exchange (PSE),6 or via direct
solvothermal synthesis7 to incorporate catalytic units into
the framework. Linkers constructed from molecular tran-
sition metal complexes8-14 or open coordination sites at
the SBUs15-16 often serve as catalytic centers. The resulting
materials exhibit many appealing properties for electroca-
talysis applications,17-18 such as permanent porosity that
facilitates fast substrate diffusion, and site isolation of
molecular catalysts inside the framework that inhibits
bimolecular decomposition pathways. Moreover, MOFs
provide exceptionally high surface areas with potential
catalyst loadings that are orders of magnitude higher than
those of corresponding monolayers on electrode surfaces.
However, the potential of molecular redox catalysis in
MOFs has been exploited only sporadically, a shortcom-
ing that is due, in part, to the fact that MOFs are tradi-
tionally composed of redox-inactive organic linkers and
metal clusters (e.g. d0 ZrIV, d10 ZnII) that result in poor
MOFs has remained a challenge.
MOF architectures that exhibit electroactive behavior
or high conductivities typically are constructed from spe-
cialized linkers and have confined structures that en-
hance conduction through the framework. Recent reports
of conducting 3D MOFs21-22 utilize either through-space
charge transport with aromatic π-stacked linkers,23-24
through-bond charge transport via covalent linkages of
metal-sulfur bonds (-M-S-) in semi-infinite chains,25 or
induced charge transport by guest molecules inserted
between the SBUs.26-27 These materials exhibit high con-
ductivities (10−7 – 10−3 S cm−1),21 but are limited in terms of
porosity and chemical stability, which are crucial for ca-
talysis.17 Thus, reports of electrocatalysis in MOFs are
limited to primarily porphyrin-based frameworks8-11 or 2-
D metal-organic layers28-29 where the linkers serve dual
roles as charge transport mediators and catalysts. Unfor-
tunately, all of these materials have limited options for
postsynthetic modification, which makes catalytic func-
tionalization and tuning of these frameworks a significant
challenge.
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Conversely, highly stable, porous, and modular frame- ples of a redox-active UiO-type MOF, which has the po-
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works, like the UiO series (UiO = University of Oslo) of
MOFs30-31, make excellent vehicles in which to immobilize
molecular catalysts. They typically exhibit excellent ther-
tential to be readily utilized for electrocatalysis.
mal30 and chemical stability32-33 even under catalytic con-
ditions (i.e. under irradiation,7,
at high and low pH,
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and at applied oxidative or reductive potentials).34-35 In
addition, the linkers can be easily exchanged or appended
with catalytic units by postsynthetic methods.36 In partic-
ular, the mild conditions required for PSE in MOFs with
UiO connectivity are advantageous when the catalyst is
not compatible with solvothermal conditions37-38 or when
incorporating catalysts into MOF thin films.34, 39 Despite
of all these advantages, the suitability of UiO-type MOFs
as electrocatalysis platforms is currently limited because
the Zr-based SBUs, as well as their phenyl carboxylate
linkers, are redox-inactive within a reasonable potential
window. Even when doped with an electroactive transi-
tion metal complex, they are essentially non-
conducting.38 UiO-type MOFs have been used in numer-
ous catalytic applications, but to date none feature redox
active organic linkers, which could be utilized for charge
transport to catalytic sites during electrocatalysis.
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Figure 1. MOF design strategy for developing a redox-active
framework featuring UiO-topology with proton responsive
functionalities.
RESULTS & DISCUSSION
Synthesis and Characterization. The carboxylate
functionalized NDI-linker dcphOH-NDI was obtained by
the condensation of 1,4,5,8-napthalenetetracarboxylic
dianhydride and 4-amino-3-hydroxybenzoic acid in DMF.
Addition of ZrCl4 to dcphOH-NDI with acetic acid as a
modulator in DMF under solvothermal conditions (120
°C) resulted in the new material, Zr(dcphOH-NDI). Up-
on inspection of the PXRD pattern, it is clear that this
material is highly crystalline (Figure 2a). SEM images
reveal the bulk microcrystalline powder to consist of in-
tergrown octahedral shaped crystallites ~2 μm in length
(Figure 2b). Zr MOFs with linear carboxylate ligands gen-
erally assume a typical topology containing Zr6O4(OH)4
clusters connected by 12 linkers, characteristic of the UiO
series. In this case, the PXRD pattern closely resembles
the pattern simulated from the crystal structure of a re-
cently reported UiO-66 analogue Zr-MOF with a two-fold
Herein, we report an electroactive MOF, which com-
bines the attractive properties of the UiO structure type,
i.e. high surface area, porosity, chemical stability, and
facile PSE, with charge transport pathways at the operat-
ing potential of many proton reduction catalysts. Naph-
thalene diimide (NDI) was selected as the redox-active
functional unit and was appended with carboxylic acid
groups (dcphOH-NDI, Figure 1) for UiO-type MOF fabri-
cation. The NDI moiety is a common electron-acceptor in
organic photovoltaics, and n-type organic semiconduc-
tor,40-42 and features two one-electron reductions at po-
tentials with sufficient driving force for electron transfer
to common proton reduction catalysts. In addition, fur-
ther proton responsive components (hydroxyl substi-
tutents) were added to the design of the framework to
enhance proton conductivity or allow for proton-coupled
electron transfer (PCET) pathways43-44 during catalysis.
Several MOFs utilizing NDI-type linkers have been re-
ported with interesting optical and electrochemical prop-
erties;45-53 however, as described above, many suffer from
low surface areas, minimal tunability, and limited stabil-
ity. Installing the relatively large (length ~19 Å) NDI link-
er in a UiO-type framework overcomes these limitations
and results in a new material, Zr(dcphOH-NDI) (Figure
1) with high stability, porosity, and charge transport prop-
erties needed for electrocatalysis. When grown on con-
ducting substrates (fluorine-doped tin oxide, FTO), a
redox active thin film cathode material is obtained
(Zr(dcphOH-NDI)@FTO). In contrast to prior porphy-
rin-based MOFs, the concept proposed herein decouples
the redox hopping pathways from the sites of catalysis.
The framework reported here, Zr(dcphOH-NDI) and its
corresponding thin film on FTO, is one of the first exam-
interpenetrated structure (Zr-L6; L6
=
4,4'-[1,4-
naphthalene-bis(ethyne-2,1-diyl)]-dibenzoate),54 contain-
ing naphthalene-based linkers of approximately the same
size (length ~19 Å) (Figure S1). MOFs synthesized with
extended linkers often result in instability, such as pore
collapse upon solvent removal. Behrens and co-workers
have recently introduced a highly-stable class of porous
interpenetrated Zr-organic frameworks (PIZOFs) with
extended linkers.55-56 PIZOFs contain SBUs with the same
structure and connectivity as in UiO MOFs (Zr6O4(OH)4
nodes with twelve-fold connectivity); however, the
framework comprises two independent interpenetrating
UiO networks. Additionally, the frameworks were found
to be stable under ambient conditions as well as towards
hydrolysis.55
Examples
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Figure 2. Characterization of bulk microcrystalline powder Zr(dcphOH-NDI) by: (a) PXRD, (b) SEM imaging, (c) FTIR-ATR
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spectroscopy, and (d) TGA from 25-800 °C.
PIZOF analogs in the literature have shown their suitabil-
ity for PSM57-58 and potential for catalytic applications.59-60
A comparison of the PXRD patterns, establishes that the
structure of Zr(dcphOH-NDI) adopts UiO/PIZOF-
connectivity with Zr6O4(OH)4 nodes and two-fold inter-
penetration (i.e. PIZOF analog). Stability tests were per-
formed by incubating the as-synthesized material in or-
ganic solvent (DMF), H2O, and under ambient conditions
in air for several days (Figure S2). Identical PXRD patterns
were obtained before and after exposure to these condi-
tions, which confirms the stability of framework. The
incorporation of the NDI-linker inside the framework was
probed by FTIR-ATR, where the bulk powder
Zr(dcphOH-NDI) exhibits vibrational bands similar to
the free ligand, although shifted slightly due to coordina-
tion with the Lewis acidic Zr4+ metal centers (Figure 2c
and Figure S3).13 Notably the C=O stretches occur be-
tween 1700-1600 cm−1 and a broad –OH stretching band is
Zr(dcphOH-NDI) at 100–200 °C, which is most likely due
to removal of guest solvent molecules (DMF, etc…) from
the framework. The material begins to decompose at
~400-450°C as evidenced by the large mass loss in that
range.
N2 sorption analysis showed Zr(dcphOH-NDI) to have
a high degree of porosity and resulted in a Brunauer-
Emmet-Teller (BET) surface area of 1531 m2 g−1 with a total
pore volume (VT) of 1.53 cm3 g−1 after degassing under
dynamic vacuum at 85 °C (Figure 3a). These values are in
accordance with typical PIZOFs that have large internal
diameters (19 Å), resulting in BET surface areas on the
order of 1250 m2 g−1.55 Type-IV isotherms were observed
for Zr(dcphOH-NDI) with a hysteresis loop characteris-
tic of hierarchically structured micro-mesoporous materi-
als. A recent example of a PCN-160 derivative, in which a
hierarchical pore structure was achieved by missing linker
defects introduced via linker labialization, displays type-
IV isotherms with hysteresis.61 This behavior was assigned
to the presence of both micropores and mesopores. Simi-
lar results were also obtained for a series of hierarchically
structured polyMOFs, which also show a hysteresis loop
as a result of an interlaced framework structure.62-63 Fur-
observed centered around ~3175 cm−1. Additionally, H
1
NMR confirmed the presence of the NDI linker in the
framework. After digesting Zr(dcphOH-NDI) with HF,
peaks at chemical shifts matching dcphOH-NDI (Figure
S4) were observed. Thermogravimetric analysis (TGA)
measurements (Figure 2d) show an initial mass loss for
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thermore, Zr(dcphOH-NDI) exhibits pore sizes of ~13 Å in the microporous range as well as a low portion of
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Figure 3. (a) N2 sorption isotherm at 77 K of Zr(dcphOH-NDI) and (b) corresponding pore width distribution.
mesopores, which were observed around 25 Å and 70 Å
(Figure 3b). Thus, in the present case, it is likely that a
combination of missing linker defects and the interpene-
trated framework topology of Zr(dcphOH-NDI) produc-
es the hierarchical structure observed in the isotherms
and pore size distribution. The PXRD pattern before and
after the gas sorption measurement was unchanged (Fig-
ure S5), indicating that the material retains its crystallini-
ty and structural integrity even upon desolvation. Based
on previous reports, small pore sizes and relatively low
where they proposed the Zr-oxo nodes act as proton ac-
ceptors under operating conditions at pH = 7, where the
corresponding homogeneous complex is catalytically
inactive due to the lack of sufficient Brønsted base.
This prompted us to examine the possibility of proton-
responsive behavior in Zr(dcphOH-NDI) at either the Zr-
nodes or the hydroxyl groups on the linker (Figure 1).
Hupp and co-workers70 recently developed an effective
method to gauge the pKa values using potentiometric
titrations of terminal –OH and –OH2 on the SBUs of vari-
ous Zr-MOFs as well as that of defect sites within the
framework. Following this procedure, the acid-base titra-
tion curve for Zr(dcphOH-NDI) is shown in Figure 4.
Two equivalence points can be observed that correspond
to pKa values of 3.45 ± 0.02 and 6.10 ± 0.05, which were
assigned to the Zr-μ3-OH and NDI-OH protons of the
SBU and linker, respectively. The first pKa value matches
well with the reported pKa for the bridging Zr-μ3-OH
proton of UiO-66 (pKa = 3.52) and UiO-67 (pKa = 3.44),70
while the second pKa value was assigned based on the
titration curve of the free ligand in solution, which yield-
ed a pKa for the hydroxyl proton of 6.27 (Figure S6).70 The
PXRD pattern of the material after the titration is un-
changed from before (Figure S7), indicating the frame-
work is stable. It is clear from these experiments,
Zr(dcphOH-NDI) exhibits proton-responsive behavior at
both the Zr-SBUs and the hydroxyl-functionalized linker
porosities have the potential to be limiting under catalytic
12, 20, 64-66
conditions,7,
as this obstructs the diffusion of
substrates, products, and redox-mediators (counter-ions,
oxidants, or reductants). The high surface area and large
pore size of Zr(dcphOH-NDI) is thus advantageous for
catalytic applications where large open channels are re-
quired.
Protonation Behavior. Managing the transfer of mul-
tiple protons during redox reactions is essential for effi-
cient catalysis. In analogy to natural enzymatic systems,
the local environment and secondary coordination sphere
around the catalytic active sites have profound influences
on the reactivity and observed rates of catalytic reactions.
For example, numerous reports have found that local
hydroxyl groups near the binding site of a Fe-porphyrin-
based CO2 reduction catalyst greatly enhance the TOF
during electrocatalysis.67-68 MOFs present a unique oppor-
tunity to incorporate proton-responsive functionalities
near the catalyst active sites within the framework. Sever-
al proton conductive MOFs have been reported for cata-
lytic applications, including a NU-1000 film composed of
MOF nanorods grown on a Ni-S catalyst film for electro-
catalytic proton reduction.69 The Zr-OH and Zr-OH2 ter-
minal groups on the NU-1000 SBUs assist in the proton
transport to the catalyst on the surface of the electrode,
which decreases the overpotential by 200 mV. In another
example, Morris et al.11 reported a Ni-metallated PCN-224
MOF thin-film with porphyrin linkers for water oxidation,
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and SEM (Figure 5b) confirmed the formation of the MOF
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thin film on FTO. The PXRD pattern is consistent with
that of the bulk powder with minor changes in the inten-
sity of some reflections, indicating a degree of orientation
of the crystallites when grown on FTO. SEM images re-
vealed the film is uniform and composed of interpene-
trated crystallites in a ~1 μm thick layer (Figure S8). The
SAM-modified FTO and Zr(dcphOH-NDI)@FTO were
also characterized by XPS (Figure 5c,d). Absorption of the
monolayer was confirmed by the N1s peak observed at a
binding energy of 399.9 eV, arising from the NDI core of
dcphOH-NDI.71 The spectra of Zr(dcphOH-NDI)@FTO
displayed new peaks at 182.3 eV and 184.7 eV correspond-
ing to Zr 3d of the UiO-framework.72-73 Additionally, both
the film and monolayer showed a C1s peak around 288.0
eV, which can be associated with the carbonyl groups of
the NDI core (Figure S9).74-75 The C1s and N1s peaks at-
tributed to NDI are slightly shifted towards higher bind-
ing energy in the case of Zr(dcphOH-NDI)@FTO, pre-
sumably due to coordination of the NDI linker to the Zr-
oxo components with the SBUs within the framework.
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Figure 4. Acid-base titration curve for Zr(dcphOH-NDI)
(red) and first derivative (blue).
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over a pH range of ~3.5-6.10. In this pH range, there
would be both proton acceptor (SBUs) and proton donor
units (linkers) present, which could be beneficial for pro-
ton transport under operating conditions in electrocata-
lytic applications.
Electrochemical and Optical Properties. A summary
of the electrochemical and optical properties of the NDI-
MOF thin films is shown in Table 1. Cyclic voltammo-
grams (CVs) of Zr(dcphOH-NDI)@FTO thin films and
dcphOH-NDI in solution for comparison were recorded
in DMF using 0.8 M KPF6 as the supporting electrolyte
(Figure 6a) and clearly show two reversible couples in the
Thin Film Characterization. To explore the electro-
chemical and optical properties of this new material, thin
films of Zr(dcphOH-NDI) were grown on transparent
conducting substrates. Thus, a self-assembled monolayer
(SAM) of the NDI linker on FTO (SAM@FTO) was
formed by soaking a FTO slide in a 1 mM solution of
dcphOH-NDI in DMF for 18 h. Decreasing the amount of
ZrCl4 and dcphOH-NDI used in the reaction as com-
pared to the bulk solvothermal synthesis resulted in thin
films of Zr(dcphOH-NDI)@FTO. Both PXRD (Figure 5a)
cathodic scan at E1/2 = −0.96 V and −1.36 V vs. Fc+/0 con-
,
sistent with sequential one-electron reductions on the
NDI core (NDI/NDI∙− and NDI∙−/2−) These values match
well with the half-wave potentials recorded for the linker
in solution (Figure S10).
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Figure 5. (a) PXRD of Zr(dcphOH-NDI)@FTO films.(b) SEM image of Zr(dcphOH-NDI)@FTO. XPS core-level spectra
Zr(dcphOH-NDI)@FTO films (red) and SAM-modified FTO (black) showing the (c) Zr3d and (d) N1s peaks.
Table 1. Summary of Electrochemical and Optical Properties of Zr(dcphOH-NDI)@FTO thin films
λmax [nm]
364, 382
359, 379
E1/20/∙−[V] (∆Ep [mV])[a]
−0.96 (166)
E1/2∙−/2−[V] (∆Ep [mV])[a]
−1.36 (175)
Zr(dcphOH-NDI)@FTO
dcphOH-NDI
−0.93 (67)
−1.30 (75)
[a] Redox potentials were measured in DMF with 0.8M KPF6 at 50 mV s−1.
Additionally, because the Zr(IV) centers have a d0 elec-
tron configuration and are redox inactive, the Faradic
response in the CVs is exclusively attributed to the NDI
linkers. During the first successive scans, the current
density slowly increases until reaching a maximum after
20 cycles (Figure 6b). This conditioning effect has been
shown in other MOF-modified electrodes and can be
explained as the ingress of supporting electrolyte into the
framework as the result of an applied potential, which
increases the local concertation of available counter
ions.46 The integrity of the framework was maintained
during electrochemical conditioning, as shown by PXRD
patterns of the films before and after cycling (Figure S11).
Coulometry measurements (Figure S12) estimated the
amount of electroactive NDI molecules in the film and
resulted in an electroactive surface coverage of 3.1x10−7
mol cm−2. By comparison, integration of the current from
a CV of only the SAM on FTO resulted in a surface con-
centration of 7.9x10−11 mol cm−2 (Figure S13).
It is well known that MOFs in electrochemical applica-
tions often operate by an electron hopping mechanism
between fixed sites, i.e. redox-active linkers,76-80 where
charge transport can be viewed as the diffusion of elec-
trons through the material.81-82 In addition, the transport
of ions or supporting electrolyte is required to balance
charge upon oxidation or reduction of the framework.
Combined with the hopping mechanism of localized
charge carries, this creates a situation of diffusion-like
charge transport through the porous material, depending
on an equivalent diffusion coefficient, De.83-85 In the case
of electrocatalytic MOFs,17 if the catalytic reaction and
substrate diffusion within the framework are compara-
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tively fast, the catalytic current will be limited by De.83-84
Thus, it is vitally important for MOFs in electrocatalytic
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applications to have fast electron transport as well as
facile movement of counter ions throughout the material.
Notably, the CVs of Zr(dcphOH-NDI)@FTO films take
the form of diffusion waves. Upon varying the scan rate
(Figure 7), this was confirmed by plots of peak current
density (jpc) vs. the square root of the scan rate (ν1/2) in
Figures 7c and d, where a linear relationship was obtained
at the first and second reduction. When linear diffusion is
the sole mode of transport, ipc is proportional to ν1/2, and
the voltammetric response is described by eq. (1),
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ꢋꢌꢍ
ꢀꢁꢂ ꢃ 0.446ꢄꢅꢆꢇ
ꢉ
(1)
ꢈ
ꢊ
ꢎꢏ
where F is Faraday’s constant, ν is the scan rate, A is the
surface area of the electrode, C° is the concentration of
redox active species within the film, R is the universal gas
constant, T is the temperature, and De is the equivalent
diffusion coefficient. It follows that the process of elec-
tron transfer through the film is diffusion limited via the
combined effect of electron hopping between NDI linkers
and mass transport of counter ions through the pores of
the framework, giving rise to De (vide infra). The large
non-zero peak separation ∆Ep = 166 mV and 175 mV for
the first and second reduction respectively also reflects
slow electron transfer through the framework A rapid
surface-confined process will display a ∆Ep = 0, and in the
ideal case of a fully reversible (one-electron) diffusion-
controlled process, ∆Ep = 57 mV at 25 °C.
UV-vis spectroelectrochemistry was used to observe the
spectral changes of Zr(dcphOH-NDI)@FTO upon reduc-
tion and probe the kinetics of electron transfer within the
film. Steady-state UV-vis spectra of Zr(dcphOH-
NDI)@FTO thin films display characteristic π-π* absorp-
tion bands of the NDI core at 364 and 382 nm (Figure
S14).
Figure 6. CVs of Zr(dcphOH-NDI)@FTO thin films at (a)
50 mV s−1 and (b) multiple scans at 100 mV s−1 showing in-
creasing current density on progressive scans.
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Figure 7. Scan rate dependent CVs of Zr(dcphOH-NDI)@FTO upon: (a) reversing the scan after the first reduction and (b) after
the second reduction at scan rates from 5 to 100 mV s−1. Plots of jpc vs. ν for (c) the first reduction wave and (d) the second reduc-
tion wave. The peak current density is linearly proportional to ν1/2.
At applied potentials, we see the disappearance of these
bands and the appearance of new absorptions in the visi-
ble region, resulting in defined color changes in the film
(Figure 8). When the potential is held at −1.26 V, new
bands are seen at 475, 608, and 780 nm, assigned to the
formation of the NDI∙− species. At −1.76 V, the absorption
at 475 nm decreases, while a new pair of bands appears at
394 and 416 nm, concomitant with an increase in intensi-
ty at 608 nm. These new transitions suggest the for-
mation of NDI2−, and all assignments were corroborated
by UV-vis spectra obtained from the free NDI linker at
similar potentials (Figure S15). Additionally, by comparing
the initial concentration of NDI inside the framework to
the final concentration of reduced NDI∙−, we determined
that approximately 97% of the NDI linkers in the film are
redox-active at an applied potential of −1.26 V (see Sup-
porting Information). In previously reported UiO-based
electrodes, the fraction of electroactive linkers ranges
from 1% to 32%.34-35 The electrochromism of the films
upon reduction is reversible and the color returns when
oxidized back to the NDI0 species. To demonstrate this,
the potential was stepped between 0 V and −1.3 V for 5
cycles while the absorbance change (ΔAbs) at 475 nm was
monitored (Figure S16). This demonstrates that the re-
versible electrochromic behavior is retained after multiple
cycles.
The kinetics of electron transport through the films
could be followed by spectroelectrochemical measure-
ments. By monitoring the absorbance at 475 nm corre-
sponding to the reduced form of the NDI linker, the con-
centration of reduced species inside the framework can
be directly measured as a function of time. Plotting ΔAbs
vs. t1/2 then results in a linear relationship and allows for
measuring the equivalent diffusion coefficient via the
form of the Cottrell equation76, 86 given in eq. 2 where,
ꢗ ꢙ
∆ꢅꢐꢑ ꢃ ꢒꢓ
(2)
ꢈ
ꢔꢕꢖ
ꢘ
ꢚ
ꢛ
ꢜ
√
Amax is the absorbance maximum, t is time in second, and
df is the thickness of the film. For Zr(dcphOH-
NDI)@FTO thin films, this resulted in an equivalent
diffusion coefficient De(KPF6) = 5.4±1.1x10−11 cm2 s−1 (Figure
S17). This is considerably higher than other MOF-based
electrodes with redox-active linkers measured in non-
aqueous solvents (10−12 - 10−13 cm2 s−1),8-9, 76, 87 and indicates
that electron hopping and mass transport through the
films is relatively fast. The effect of mass transport within
the film was further investigated by varying the size of
counter cation in the supporting electrolyte, which must
be transported through the pores during reduction of the
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reduction wave at −0.24 V vs. NHE (−0.90 V vs. Fc+/0 (Fig-
framework to allow charge equilibration. Examining the
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same set of films
ure S18a)) in 0.8 M KCl(aq). Previous reports of homogene-
ous NDI derivatives have shown a solvent-dependent
anodic shift of the NDI∙−/NDI2− couple in the presence of
K+, Li+ or Mg2+ cations that result from a stabilizing inter-
action between the NDI2− species and the cation at the
imide oxygen atoms.90-92 At high cation concentrations,
the two one-electron reduction waves were observed to
merge into a single two-electron process.91 This situation
reflects that in the pores of Zr(dcphOH-NDI)@FTO, and
we propose a similar interaction between the K+ ions and
the NDI linkers in an aqueous environment to give rise to
the observed single reduction wave in the CVs.
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Upon cycling the film between 0.7 V and −0.8 V vs.
NHE at 100 mVs−1 (Figure S18b), the current density de-
creases over the first few scans, presumably due to partial
film delamination. After 20 cycles, the current density
stabilizes and overlapping successive scans are recorded.
From scan rate dependent CVs (Figure S19), the peak
current density varies linearly with ν1/2, indicating a diffu-
sion-controlled charge transfer process also in aqueous
solution. Remarkably, spectroelectrochemical measure-
ments (Figure S20) reveal an order of magnitude increase
in the diffusion coefficient, De(KCl(aq)) = 2.1x10−10 cm2 s−1,
with a comparable electroactive surface concentration
(1.2x10−7 mol cm−2) as compared to that measured in DMF
with K+ as the cation. Digesting the MOF thin films after
measuring the electroactive surface concentration by
coulometry yielded the percentage of redox-active linkers
as 87% (see Supporting Information). In addition, the
number of transferred electrons was calculated as n = 1.7,
indicating that the single reduction wave observed in the
CVs is indeed a two-electron process.
Figure 8. UV-vis spectroelectrochemical measurements of
Zr(dcphOH-NDI)@FTO using 0.8 M KPF6/DMF as the
supporting electrolyte (top). Photos of Zr(dcphOH-
NDI)@FTO thin film electrodes (bottom) at various applied
potentials (a) 0 V, (b) −1.26 V, and (c) −1.76 V vs. Fc+/0
under identical conditions while only changing the sup-
porting electrolyte from KPF6 (ꢝꢞ = 1.33 Å)88 to n-
ꢟ
Bu4NPF6 (ꢝꢠꢡꢢꢣ ꢥ = 4.94 Å),89 the equivalent diffusion
ꢟ
ꢤ
coefficient decreased significantly by about one order of
magnitude (De(n-Bu4NPF6) = 4.0±2.5x10−12 cm2 s−1), con-
cordant with the increase in size of the cation (Figure
S16c). Interestingly, when using KPF6, plots of the ∆Abs at
475 nm vs. t under −1.31 V applied potential (Figure S16b)
show the absorbance reaching a plateau after 400s, corre-
sponding to a maximum concentration of NDI∙− in the
film. Conversely, the same films measured in solutions
with n-Bu4NPF6 show a continual increase in absorbance
at 475 nm over 400 s, suggesting an incomplete reduction
of the NDI linkers on the time scale of the experiment.
This indicates that the large size of n-Bu4N+ impedes
transport within the pores of the framework, reflected by
the decreases in De, and consequently the film is only
partially reduced. Thus, we have shown that charge and
mass transport through redox-active MOF films are po-
tential current-limiting phenomena that must be carefully
controlled by experimental parameters (counter ion size,
porosity, redox activity, film thickness, etc.) for such films
to function optimally in electrocatalytic applications.
CONCLUSION
Designing redox-active MOFs that can be synthesized
as thin films is a key challenge towards their application
in electrocatalysis. Here, we have developed a novel
MOF-based thin film platform that provides electron
transport pathways close to the operating potential of
many proton reduction catalysts. Importantly, this system
is derived from a robust, Zr-based MOF that exhibits a
high surface area and large pore size, necessary for effi-
cient transport of substrates, products, and counter ions.
The pKa of both the proton-responsive NDI linker and the
Zr-node demonstrate potential use in PCET over a wide
pH range (3.4 – 6.1). When grown on conducting sub-
strates, this material forms thin films that display reversi-
ble electrochromic behavior. Electrochemical experi-
ments showed this corresponds to the sequential one-
electron reductions of the redox-active NDI linkers within
the framework. Importantly, nearly all (97%) of the NDI
sites are electrochemically active at applied potential. The
method of charge transport within the films was deter-
mined to be diffusion limited, in agreement with the
electron hopping model common to many MOF-based
thin films. Finally the MOF thin films were found to func-
tion efficiently in aqueous solutions, adding to their po-
tential usefulness in electrocatalytic applications.
As the electrocatalytic applications that we envisage for
our platform in the future (for example proton reduction)
may operate in aqueous solutions, we also tested the
electrochemical behavior of Zr(dcphOH-NDI)@FTO
films using an aqueous 0.8 M KCl (pH = 6.5) solution as
supporting electrolyte. Interestingly, the two sequential
waves that are observed in the CV of Zr(dcphOH-
NDI)@FTO films in DMF collapse to a single reversible
This work represents a significant advancement in the
development of redox active MOFs and MOF thin films.
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To date, reports of catalytic MOFs featuring redox active (177.3 mg, 0.35 mmol), and 0.602 mL (10.5 mmol) of gla-
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linkers are almost exclusively based on porphyrins with
limited routes for linker exchange or functionalization.8-9,
cial acetic acid were combined in 4 mL DMF. The con-
tents of the vial were sonicated for 10 min. before placing
in a pre-heated 120 °C oven for 72 h. The reaction was
allowed to cool to room temperature, and the pale-yellow
precipitate was collected by centrifugation and washed
three times with DMF, followed by incubation in DMF for
24 h. The solvent was then exchanged for MeOH by wash-
ing three times and incubation in MeOH until further
use. The bulk microcrystalline powders were activated by
degassing under vacuum at 85 °C for 12 h.
76
Attainment of the UiO/PIZOF-type structure with
Zr(dcphOH-NDI)@FTO confers unprecedented versatil-
ity to utilize postsynthetic methods to introduce catalytic
units into a robust and widely studied framework that can
now be rendered redox active. This is enabled by decou-
pling the redox hopping mechanism through the film
from the sites of catalysis. Ongoing work is underway to
incorporate molecular catalysts into Zr(dcphOH-
NDI)@FTO thin films for the electrocatalytic conversion
of small molecules.
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Zr(dcphOH-NDI)@FTO MOF thin film synthesis.
FTO slides were cut into 2x1 cm2 slides and cleaned by
sonication successively in solutions of Alconox, ethanol,
and acetone. Cleaned FTO slides were immersed in a
solution of 1 mM dcphOH-NDI in DMF overnight to form
a self-assembled mono-layer (SAM). A solution of ZrCl4
(23.3 mg, 0.10 mmol), dcphOH-NDI (53.8 mg, 0.10 mmol)
and 171 μL of glacial acetic acid (3 mmol) in 8 mL of DMF
was prepared in a 20 mL scintillation vial and sonicated
for 10 min. A single slide of SAM-modified FTO was in-
serted and the vial was placed into a pre-heated 120 °C
oven for 72 h. The vials were allowed to cool to room
temperature, and the films were washed with DMF and
ethanol and allowed to soak in DMF until further use.
EXPERIMENTAL SECTION
Materials and Methods. All solvents and commercial-
ly supplied chemicals were reagent grade and used as
received without further purification. ZrCl4 (99.99%),
tetrabutylammonium hexafluorophosphate (n-Bu4NPF6,
for electrochemical analysis, ≥99.0%), potassium hex-
afluorophosphate
(KPF6,
≥99.0%),
4-amino-3-
hydroxybenzoic acid (97%), and fluorine-doped tin oxide
(FTO) substrates (7 Ω/sq) were purchased from Sigma-
Aldrich. Naphthalene-1,4,5,8-tetracarboxylic dianhydride
(≥98.0%) was purchased from TCI. Glacial acetic acid,
and N,N-dimethylformamide (DMF) (99.9%) were pur-
Powder X-ray Diffraction. Powder X-ray diffraction
patterns (PXRD) were obtained using a Simons D5000
Diffractometer (Cu Kα, λ = 0.15418 nm) at 45 kV and 40
mA, using a step size of 0.02°. The diffractometer was
equipped with parallel beam optics (mirror + mirror) for
grazing-incidence XRD measurements.
1
chased from VWR. H-NMR spectra were measured using
a JEOL 400 MHz spectrometer at 293 K. The chemical
shifts given in ppm are internally referenced to the resid-
ual solvent signal. MOF samples were digested prior to
NMR measurement by addition of 25 µL of HF to a sus-
pension of 10 mg of MOF in 0.575 mL of DMSO-d6. ESI-
MS data were collected on a Thermo LCQ XP Max mass
spectrometer in negative mode with electrospray ioniza-
tion. Electronic absorption spectra were measured using a
Varian Cary 50 UV-Vis spectrophotometer. ATR-FTIR
data were collected on a Bruker ALPHA FTIR Spectrome-
ter from 4000 cm−1 to 450 cm−1 at room temperature.
Scanning Electron Microscopy. Scanning electron
microscopy (SEM) images were obtained using a Zeiss
1550 Schottky field-emission scanning electron micro-
scope equipped with an InLens detector at 5kV accelera-
tion voltage. MOF thin film and powder samples were
anchored to conductive carbon tape on a sample holder
disk and coated using a Pd-Ir-sputter coater for 30 s.
dcphOH-NDI. 3-Hydroxy-2-[7-(4-carboxy-2-hydroxy-
phenyl)-1,3,6,8-tetraoxo-3,6,7,8-tetrahydro-1H-benzo
[lmn] [3,8] phenanthrolin-2-yl]-benzoic acid (dcphOH-
Electrochemistry. Cyclic voltammetry (CV) and
chronocoulometry were performed using
a
one-
NDI) was synthesized following a reported procedure.93
A
compartment, three-electrode configuration connected to
an Autolab PGSTAT100 potentiostat controlled with GPES
4.9 software (EcoChemie). For thin film measurements,
the electrode setup included an auxiliary glassy carbon
disc (0.071 cm2) working electrode, which was used to
monitor the solution between scans of Zr(dcphOH-
NDI)@FTO thin films, a glassy carbon rod counter elec-
trode, and a non-aqueous Ag/Ag(NO3) (0.01 M in acetoni-
trile) reference electrode, referenced to the ferroceni-
um/ferrocene couple (Fc+/0). Zr(dcphOH-NDI)@FTO thin
films were measured in 5 mL DMF solutions as the work-
ing electrode using 0.8 M KPF6 or n-Bu4NPF6 as the sup-
porting electrolyte. Solution-phase measurements of the
linker were performed at a concentration of 1 mM in DMF
using a GC disk working (0.071 cm2) electrode with 0.1 M
n-Bu4NPF6 as the supporting electrolyte. UV-Vis spectroe-
lectrochemistry experiments were performed on the thin
film samples in a 10 mm pathlength quartz cuvette using
round bottom flask attached to a reflux condenser was
charged with 4-amino-3-hydroxybenzoic acid (1.01 g, 6.6
mmol) and naphthalene-1,4,5,8-tetracarboxylic dianhy-
dride (0.804 g, 3 mmol). DMF (20 mL) was added and the
reaction was placed under Ar atmosphere. The resulting
dark-red solution was refluxed overnight. After the reac-
tion mixture was cooled to room temperature, 5 mL of 1
M HCl was added, and the product was precipitated by
adding the solution dropwise into ice-cooled water (250
mL). The pale-yellow precipitate was collected by centrif-
ugation and washed with ethanol, water, and ether (25
1
mL each) and dried under vacuum. Yield: 1.15 g (71%). H
NMR (400 MHz, DMSO-d6) δ ppm: 7.43 - 7.49 (m, 2 H)
7.50 - 7.56 (m, 2 H) 7.56 - 7.60 (m, 2 H) 8.59 - 8.88 (m, 4
H) 10.17 (s, 2 H). ESI-MS(−) (MeOH/THF 1:1 v/v): [M-H]−,
m/z = 536.99 (calc. m/z = 537.06); [M-COOH]−, m/z =
493.25 (calc. m/z = 493.07).
a
Pt wire counter electrode, and
a non-aqueous
Zr(dcphOH-NDI) MOF bulk synthesis. In a 20 mL
scintillation vial ZrCl4 (82 mg, 0.35 mmol), dcphOH-NDI
Ag/Ag(NO3) (0.01 M in acetonitrile) reference electrode in
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experiments of MOF thin films. This material is availa-
DMF with either 0.8 M KPF6 or n-Bu4NPF6. Solution-
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ble free of charge via the Internet at http://pubs.acs.org.
phase spectroelectrochemistry experiments were carried
out in a 1 mm pathlength cuvette using a 1 cm2 Pt mesh
working electrode, Pt wire counter electrode, and a non-
aqueous Ag/Ag(NO3) (0.01 M in acetonitrile) reference
electrode in DMF with 0.1 M n-Bu4NPF6. For experiments
in aqueous solution, 0.8 M KCl was used as the support-
ing electrolyte. Potential were reference to a Ag/AgCl (3
M KCl) reference electrode (+0.210 V vs. NHE). Conver-
sion to vs. Fc+/0 was performed using a reported value.94
AUTHOR INFORMATION
Corresponding Author
*sascha.ott@kemi.uu.se.
Notes
The authors declare no competing financial interests.
9
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Gas adsorption. N2 adsorption isotherms were meas-
ured at 77 K (liquid N2 bath) using a Micromeritics ASAP
2060 micropore physisorption analyzer. The bulk powder
samples were activated under dynamic vacuum (1 × 10–4
Pa) at 85 °C, using a Micromeritic SmartVacPrep sample
preparation unit. The cold and warm free space of the
sample tubes was determined using He gas.
†Shanghai Key Laboratory of Chemical Assessment and Sustain-
ability, School of Chemical Science and Engineering, Tongji
University, Shanghai 200092, P.R. China.
ACKNOWLEDGEMENT
Financial support from the Swedish Research Council, the
Swedish Energy Agency and the European Research
Council (ERC-CoG2015-681895_MOFcat, S.O.) is grateful-
ly acknowledged. The Wenner Gren Foundation is
acknowledged for a sabbatical stipend to S.O. to visit
S.M.C. This work was also supported by a grant from the
Department of Energy, Office of Basic Energy Sciences,
Division of Materials Science and Engineering under
Award No. DE-FG02-08ER46519 (S.M.C.).
Potentiometric Titrations. Acid-base titrations of
dcphOH-NDI and Zr(dcphOH-NDI) were performed
according to a reported procedure.70 ~50 mg of MOF
sample (ground with a mortar and pestle) was suspended
in 60 mL of 0.01 M NaNO3 (aq.) overnight. The solution
was placed in a glass electrochemical cell (100 mL)
equipped with a SevenCompact S220 pH/ion meter (Met-
tler Toledo) and a stir bar. Prior to titration, the meter
was calibrated with commercial (Mettler Toldeo) buffer
solutions at pH = 4.01, 7.00, and 9.21. The pH of the sus-
pension was adjusted with 0.1 M HCl to an initial pH of 3.
Then aliquots of 25 μL of 0.1 M NaOH were added while
stirring and the pH was recorded once the voltage stabi-
lized to a final pH of 8.6. The addition rate was approxi-
mately 0.024 mL min−1. In the case of the free linker
(dcphOH-NDI), 17 mg of compound in a 25 mL solution
of 0.01 M NaNO3 (aq)/DMF (1:1; v/v) was used, and titrat-
ed to a final pH of 11. Equivalence points were calculated
from local maxima in the first derivative of the titration
curve, and the corresponding pKa values are then equal to
the pH at 0.5 times the volume of NaOH added to reach
the equivalence point.
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Thermogravimetric Analysis. Thermogravimetric
analysis (TGA) was performed on a TA instruments Q500
thermogravimetric analyzer under a flow of air (50 mL
min−1) from 25 °C to 800 °C at a rate of 5 °C min−1. Prior to
measurement MOF samples were dried at 120 °C for 20 h
under N2.
(12)
Chambers, M. B.; Wang, X.; Elgrishi, N.; Hendon, C.
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ASSOCIATED CONTENT
Supporting Information. Additional characterization of
MOF bulk powder and thin films; additional spectro-
scopic and electrochemical data for linker; CV of
SAM@FTO; coulometry and spectroelectrochemical
Hoffeditz, W.; Klahr, B. M.; Klet, R. C.; Pellin, M. J.; Farha, O. K.;
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