Effect of Linker Distribution in the Photocatalytic Activity of Multivariate Mesoporous Crystals

Article abstract of DOI:10.1021/jacs.0c09015

The use of Metal-Organic Frameworks as crystalline matrices for the synthesis of multiple component or multivariate solids by the combination of different linkers into a single material has emerged as a versatile route to tailor the properties of single-component phases or even access new functions. This approach is particularly relevant for Zr6-MOFs due to the synthetic flexibility of this inorganic node. However, the majority of materials are isolated as polycrystalline solids, which are not ideal to decipher the spatial arrangement of parent and exchanged linkers for the formation of homogeneous structures or heterogeneous domains across the solid. Here we use high-throughput methodologies to optimize the synthesis of single crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to produce a family of UiO-68-TZDC% binary frameworks, which respect the integrity and morphology of the original crystals. Our results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling crystals for a uniform mixture or the formation of core-shell domains. We also demonstrate how the possibility of generating an asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.

Full text of DOI:10.1021/jacs.0c09015

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Article
Effect of Linker Distribution in the Photocatalytic Activity of
Multivariate Mesoporous Crystals
́
Belen Lerma-Berlanga, Carolina R. Ganivet, Neyvis Almora-Barrios, Sergio Tatay, Yong Peng,
Josep Albero, Oscar Fabelo, Javier Gonzalez-Platas, Hermenegildo García, Natalia M. Padial,*
́
and Carlos Martí-Gastaldo*
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ABSTRACT: The use of Metal−Organic Frameworks as
crystalline matrices for the synthesis of multiple component or
multivariate solids by the combination of different linkers into a
single material has emerged as a versatile route to tailor the
properties of single-component phases or even access new
functions. This approach is particularly relevant for Zr6-MOFs
due to the synthetic flexibility of this inorganic node. However, the
majority of materials are isolated as polycrystalline solids, which
are not ideal to decipher the spatial arrangement of parent and
exchanged linkers for the formation of homogeneous structures or
heterogeneous domains across the solid. Here we use high-
throughput methodologies to optimize the synthesis of single
crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single
linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light
sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to
produce a family of UiO-68-TZDC% binary frameworks, which respect the integrity and morphology of the original crystals. Our
results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling
crystals for a uniform mixture or the formation of core−shell domains. We also demonstrate how the possibility of generating an
asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but
controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.
alcohol, or alkyl derivatives with subsequent changes to the
porosity or physical properties of the pristine material.5−7
The availability of single crystals is particularly important in
understanding the dynamic exchange reactions often used to
produce multivariate frameworks by postsynthetic (PSE)8 or
solvent-assisted linker exchange (SALE).9 The porosity of the
framework and the concentration of the exchanged linker
control the diffusion of the last. As a result, these variables can
impose changes to the microstructure and distribution of
exchanged and parent linkers in the framework. Both features
might modify the physical properties and performance of the
MOF in specific applications. This phenomenon was originally
demonstrated for microporous MOF-5, UMCM-8, and UiO-
66,10 but there are no precedents for mesoporous frameworks.
INTRODUCTION
■
The discovery of the family of Zr(IV) UiO-type Metal−
Organic Frameworks (MOFs) by Lillerud and co-workers in
2008 introduced a versatile platform for the design of MOFs
with excellent stability.1 Since then, the use of monocarboxylic
acids as modulators2 combined with the flexibility of
[Zr6O4(OH)4(CO2)12] clusters acting as inorganic nodes,
has enabled us to gain chemical control over the design of
isoreticular frameworks with linear dicarboxylic linkers for a
huge family of porous materials with tailorable micro/
mesoporosity and application in catalysis, gas storage, or
separation technologies.3,4 Unfortunately, most of these
materials can be only synthesized as polycrystalline solids,
highlighting the difficulties in isolating single crystals. This
problem is even more severe for the mesoporous derivatives,
which require the use of elongated linkers with very poor
solubility in the organic solvents used in the synthesis. This is
possibly why the synthesis of single crystals of UiO-68 using
H2TPDC ([1,1′:4′,1″-terphenyl]-4,4″-dicarboxylic acid) link-
ers still remains elusive, and all crystals available rely on amine,
Received: August 26, 2020
Published: January 12, 2021
https://dx.doi.org/10.1021/jacs.0c09015
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Built upon our recent experience for the challenging chemistry
of titanium,11−13 here we use High-Throughput (HT)
synthetic methodologies14 to optimize the conditions required
for isolating single crystals of pristine UiO-68 and its
photoactive analogue UiO-68-TZDC, based on 4,4′-(1,2,4,5-
tetrazine-3,6-diyl)dibenzoic acid (H2TZDC). The availability
of crystals is used to analyze the effect of TZDC concentration
in the formation of binary frameworks by SALE. Our results
confirm that the crystals evolve from an ordered core−shell
microstructure of TZDC/TPDC linkers into a more
homogeneous distribution for increasing equivalents of the
exchanged linker. By correlating the electronic structure of
UiO-68-TZDC% solids with their photocatalytic activity, we
demonstrate the impact of linker distribution over the
photocatalytic performance of multivariate mesoporous
crystals.
RESULTS AND DISCUSSION
■
High-Throughput Synthesis of UiO-68 Single Crys-
tals. We designed a first experiment for optimizing the
formation of UiO-68 crystals that accounted for systematic
variations of temperature (90, 100, or 120 °C), reaction time
(48 or 72 h), and equivalents of acid modulator (trifluoroacetic
acid (TFA), acetic acid (AcOH), benzoic acid (BA), or formic
acid (FA)). The metal/linker stoichiometry, zirconium source,
and solvent were fixed according to the protocols adopted by
the community to 1:1, ZrCl4 and N,N-dimethylformamide
(DMF)12 for a library of over 80 reactions. We used a FLEX
SHAKE HT workstation from Chemspeed for robotic
dispensing of solids and liquids combined with a 96-well
Powder X-Ray Diffraction (PXRD) plate. This helped in
maintaining a high reproducibility and accelerating the
systematic screening of reaction parameters based on the
crystallinity and PXRD fingerprints of the products. See
Supporting Information, SI, Section S2 for more experimental
details. As shown in the ternary diagrams in Figure S4, crystals
of UiO-68 were only formed by reaction with 40 equiv of TFA
at 100 °C over 72 h. The solid was isolated as pale-yellow
crystals with octahedral morphology and sizes ranging from 20
to 50 μm (Figure 1a left). To challenge the versatility of these
synthetic conditions we next synthesized H2TZDC by
replacing the central phenyl ring in H2TPDC with an s-
tetrazine ring. Tetrazine derivates are electroactive molecules
capable of undergoing reversible transformation into stable
anion radicals. As a result, their fluorescence can be
electrochemically switched for applications in light generation,
sensors, and polymers.15,16 These features can also be
translated to MOFs for generating photoactive frameworks
by reticulation of tetrazine derivatives.17 Despite the poorer
solubility of the tetrazine linker, the conditions optimized for
TPDC only required a slight reduction of the modulator
equivalents, from 40 to 10, to isolate UiO-68-TZDC as pink
octahedral crystals with sizes ranging from 10 to 20 μm (Figure
1a right). These conditions are substantially different to the
reported synthesis of polycrystalline UiO-68-TZDC for
particle sizes near 1 μm.18
Figure 1. (a) (Left) size and morphology of UiO-68. Scale bars
correspond to 50 μm. (Right) size and morphology of photoactive
UiO-68-TZDC crystals. Scale bars correspond to 25 μm. (b)
Structure of UiO-68 along the c axis showing the accessible volume
corresponding to octahedral (pink sphere) and tetrahedral voids
(green). (c) Overlay of the two crystallographic-distinct linker
structures in (011) showing the slight differences that result from
the replacement of TPDC with TZDC.
1b,c). As for the internal structure of the linkers, the
carboxylate groups remain coplanar in both cases but the
central ring presents a certain degree of disorder. We collected
crystallographic data at 100 K (UiO-68) and at room
temperature (UiO-68-TZDC). The disorder is present in
both cases which suggests this is intrinsic to the crystallo-
graphic packing of the solids and not thermal in origin.
Porosity and Chemical Stability: Effect of the Linker.
The effect of the tetrazine core is more notable in the thermal
UiO-68 and UiO-68-TZDC crystallize in the cubic space
group Fm-3m (a = 32.71 and 32.47 Å, respectively). They
display an expanded cubic closed-packed structure for the
characteristic 12-connected fcu topology of the UiO family.1
See SI Section S3 for more details. The impact of the linker
replacement in the porosity metrics is almost negligible for
octahedral and tetrahedral voids of 27.3 and 19.3 Å (Figure
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Figure 2. (a) Optical microscope images of UiO-68-TZDC% samples. Scale bars correspond to 50 μm. (b) TZDC incorporated versus equivalents
used in the synthesis. Inset: reduction of cell parameters as result of TZDC incorporation. (c) N2 sorption isotherms at 77 K and PSD for UiO-68-
TZDC% samples. (Filled marker were used for adsorption and empty marker for desorption). (d) PXRD in logarithmic scale of intensity of the
solids after soaked water: methanol (4:1, v/v%) for 24 h.
and chemical stability of the frameworks. Compared to UiO-68
that decomposes in air above 500 °C, the thermogravimetric
analysis of UiO-68-TZDC shows a gradual decomposition
from 250 to 450 °C indicative of the oxidation of the tetrazine
core at high temperatures. The hydrolytic stability of the solids
was evaluated by incubation in water for 24 h. The PXRDs
after this treatment suggest that the structure of UiO-68
remains intact whereas UiO-68-TZDC undergoes partial
amorphization (SI Section S4). The negative effect of TZDC
on the strength of the framework linkages against hydrolysis is
also confirmed by ICP studies, that show a superior leaching
(22.4 μg·L−1) of zirconium to solution upon incubation of the
solids in water/methanol mixtures (Figure S55). This agrees
well with the weakening of the elongated coordination bonds
in the last for feebler metal joints more prone to hydrolysis.
We next analyzed the effect of the linker in tuning the
structural robustness of the solids for optimizing the process of
activation, a critical step to access the expected surface areas.19
We selected a range of solvents to examine the effect of
changes in coordinating ability, polarity, or surface tension in
the collapse of porosity (Table S5, Figures S15 and S17).20
Methanol exchange leads to amorphization in both cases likely
due to its ability to react with Zr6 nodes.21 In turn, acetone and
DMF do not alter the structure of UiO-68 but provoke the
collapse of UiO-68-TZDC. This suggests a negative effect of
the electron-deficient heterocycle in maximizing the interaction
with polar solvents for a framework more susceptible to the
capillary-forces generated in the activation process. To confirm
this point, we used hexane for minimizing the interaction with
the framework due to its low surface tension (17.9 mN·m−1 at
25 °C) and small dipole moment (0.08 D). Exchange with
hexane for 5 days, followed by drying in anhydrous conditions,
for avoiding humidity and reducing capillary forces22 is
respectful with the structure of both materials. See SI Section
S4.6 for more details. Both samples were degassed overnight at
60 °C and 10−6 Torr before gas analysis. They display a
nonhysteretic, type-I N2 adsorption isotherms with multipoint
BET surface areas in excellent agreement with the expected
values, 4028 and 4289 m2·g−1 for UiO-68 and UiO-68-TZDC,
respectively.1
The Pore Size Distributions (PSDs) calculated by using
Nonlinear Density Functional Theory (NLDFT) models
reveal a narrow distribution of two types of mesopores
centered at 2.0 and 3.0 nm, consistent with the pore size values
calculated from the crystallographic data available (Table S6).
Additional measurements collected for samples exchanged with
wet hexane or methanol show a significant drop in surface area
of 30% and 80% for UiO-68-TZDC, confirming the
importance of minimizing solvent-framework interaction to
access the expected porosity. Note that this is only critical for
the activation step. After that, the solid can be exposed to open
air with no impact over its surface area. CO2 adsorption
measurements reveal a modest uptake at 273 K of around 50
cm3·g−1. The calculated isosteric heats of adsorption confirm
the positive effect of the nitrogenated core in UiO-68-TZDC
for stronger interaction with the guest (Qst = 31 kJ·mol−1). The
properties of the single-linker phases reveal the necessity of
combining both units in the same material to produce a
photoactive material without compromising its chemical
stability or porosity, both important to photocatalytic
applications.
Multivariate Mesoporous Crystals by Solvent-Assis-
ted Linker Exchange. We decided to investigate a HT library
of reactions to produce multivariate MOFs23 by direct
combination of TPDC and TZDC with systematic control of
ratio, temperature, concentration, and mixture of solvents. All
attempts resulted in solids with reduced crystallinity and
negligible incorporation of TZDC, suggesting important
differences in the reactivity of the linkers with Zr(IV) (SI
Section S5.1). On the basis of their structural similarity, we
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Figure 3. (a) Total (DOS, black) and projected (PDOS for each atoms) density of states for pristine UiO-68 (top) and UiO-68-TZDC4%
,
(bottom) calculated using PBE functional, this is a qualitative approach; the electronic structures could not be computed at the HSE06 level
because of the system size. Inset: orbitals contributing to the HOCO (yellow) and LUCO (cyan) localized on the ligand. (b) Experimental and
computed band gap values as a function of the TZDC incorporated. (c) Spatial linker arrangement in UiO-68-TZDC% samples (pale yellow area
for core−shell distribution and pink area for a more uniform distribution). (d) Fluorescence confocal microscope’s images of polish UiO-68
samples reveal the resultant microstructures. Scale bars correspond to 25 and 10 μm for UiO-68 counterparts and UiO-68-TZDC, respectively.
decided to use postsynthetic linker exchange methods instead.
By using UiO-68 crystals as precursors, we prepared a family of
UiO-68-TZDC% multivariate solids by SALE of varying
equivalents of the tetrazine linker in DMF at 80 °C for 3 h.
We used a base (Et3N) to deprotonate H2TZDC and favor the
linker exchange (SI Section S5.2).24 UiO-68-TZDC% crystals
UiO-68 crystals allows for reaching a fine balance between
chemical stability and photoactivity. We soaked UiO-68-
TZDC3−50% crystals in water/methanol mixtures (4:1, v/v%)
for 24 h. These methanolic solutions are often used to test the
photocatalytic activity of MOFs in hydrogen evolution
reactions.11−13 Compared to UiO-68-TZDC that becomes
amorphous, all solids retain their original structure with
negligible changes in their PXRD patterns regardless the
TZDC% incorporated (Figure 2d). Compared to minimum
changes in structural stability, the impact of tetrazine doping
on the hydrolytical stability of the multivariate frameworks is
more acute. ICP analysis shows an increasing rate of metal
leaching upon incorporation of TZDC. Linker sensitization is
an efficient way to maximize the photocatalytic activity of
MOFs with visible light by taking advantage of their synthetic/
structural tunability. This strategy is dominated by porphyr-
ins,26 functionalized organic linkers,27,28 or the incorporation
of ruthenium or iridium complexes.29 s-Tetrazine linkers are
also excellent candidates in this regard due to the combination
of one-electron reversible reduction behavior and visible-light
sensitization.
Impact of Tetrazine Doping in the Electronic
Structure of the Framework. To understand the effect of
tetrazine doping, we calculated the electronic structure of the
hybrid frameworks by using density functional theory (DFT, SI
Section S6.1). Figure 3a shows a comparison of the electronic
density of states diagram of pristine UiO-68 and UiO-68-
TZDC4%, that corresponds to the incorporation of one TZDC
linker per unit cell. In UiO-68, the position the highest
occupied crystalline orbitals (HOCO) and the lowest
unoccupied crystalline orbital (LUCO) is dominated by the
aromatic 2p orbitals of TPDC. The introduction of a single
TZDC linker per cell yields a LUCO that is also dominated by
the linker but shows clear differences. The contribution of the
1
were digested and analyzed by H NMR (Nuclear Magnetic
Resonance). The integration of the resonances for aromatic
protons confirm the presence of both linkers and reveal a fine
control over the exchange rate from 3 to 50%, concomitant
with the color of the crystals (Figure 2a). The FT−IR spectra
of the samples show a shift of 25 cm−1 in the υC=O vibration
compared to the free linker. This rules out the trapping of
TZDC in the mesopores of the MOF and confirms that the
NMR quantification is representative of the actual incorpo-
ration of the linker to the framework. We used these
techniques to analyze the maximum exchange ratio possible.
Beyond UiO-68 TZDC50%, reactions with more than 20 equiv
of TZDC show clear evidence of ineffective linker exchange in
the final products. As shown in Figure 2b, UiO-68-TZDC3−50%
crystals show the same logarithmic rate of TZDC incorpo-
ration with the number of equivalents observed for other Zr-
MOFs.25 Phase purity was preliminary evaluated by PXRD Le
Bail refinement of all solids. We observe a compression of the
unit cell parameters that correlates well with the TZDC%
incorporated. All solids display permanent porosity with
negligible changes to their surface area and PSD compared
to the single-linker phases (Figure 2c). SEM analysis rules out
any morphological damage during linker exchange. All crystals
retain the octahedral morphology and size of the UiO-68 seed,
suggesting that the incorporation of TZDC operates in the
crystal and does not involve their dissolution/recrystallization.
Besides respecting the structure and porosity of the pristine
solids, the coexistence of TPDC and TZDC in multivariate
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Figure 4. (a) Photocatalytic methyl viologen reduction yields after 1 min for UiO-68-TZDC10% and UiO-68-TZDC35%. (b) Photocatalytic H2
production after 6.5 h of irradiation for UiO-68-TZDC10% (pink circles) and UiO-68-TZDC35% (deep pink triangles). (c) (top) Accumulated H2
production after 6.5 h of UiO-68, UiO-68-TZDC and multivariate UiO-68-TZDCx solids (x = 3, 10, 35, and 50%) prepared at 3 (filled circles) and
24 h (empty square). (bottom) XPS analysis of the changes in relative concentration of TZDC units in the surface of the crystals at a penetration
depth of 10 nm. Solid lines are only a guide to the eye.
2p N and C orbitals from the tetrazine ring in UiO-68-
TZDC4% reduces the band gap to 1.37 eV compared to the
2.52 eV in UiO-68. We carried out additional calculations by
systematically increasing the TZDC content up to 50% (12
linkers per unit cell). As shown in Figure 3a, the position of the
LUCO is insensitive to increasing levels of doping and a single
linker replacement is sufficient to dominate the electronic
structure and optical properties of the hybrids. This effect is
very similar to the modification of the band gap with
monoaminated terephthalic linkers reported for MIL-125,
that also saturates for the replacement of one linker per unit
cell.27
To confirm the value of our computational simulations we
used UV−vis solid reflectance measurements to estimate the
optical band gap of UiO-68-TZDC3−50% by using the
Kubelka−Munk function. We observe an excellent match
between the trend described by experimental results and
computed band gap values, that further confirm the
effectiveness of TZDC sensitization at very small doping levels
(Figure 3b) (SI Section S6.2).
Chemical complexity in multivariate frameworks can be
investigated with different techniques. Diffraction methods
cannot account for linker or metal gradients across the crystal,
but spectroscopic studies have been quite useful in solving this
question. The spatial arrangement of organic crystals at
different resolution can be elucidated with microscopic
Attenuated Total Reflectance (ATR) infrared spectroscopy,30
Solid-State Nuclear Magnetic Resonance (SS-NMR),31 Photo-
thermal Induced Resonance (PIR),32 2D Raman spectrosco-
py,10 fluorescence confocal microscopy,33 or fluorescence
lifetime imaging.34 Besides organic linkers, the spatial arrange-
ment of multimetallic Secondary Building Units (SBUs) in
multivariate frameworks has been also deciphered by using a
combination of X-ray Photoelectron Spectroscopy (XPS) and
UV−vis diffuse reflectance spectroscopy,35 Scanning Trans-
mission Electron Microscopy High-Angle Annular Dark Field
imaging (STEM-HAADF),36,37 or with atomic resolution by
using atom probe tomography.38 To analyze the spatial
arrangement of parent and exchanged linkers in this case, we
carried out fluorescence studies with a confocal microscope.
Images were taken at different heights to account for the three-
dimensional distribution of active TZDC and inactive TPDC
linkers when excited with a green filter (λ = 559 nm).
Comparison of UiO-68-TZDC at 3 and 35% doping levels
suggests a higher concentration of TZCD linkers at the surface
of the first based on more intense fluorescence at the edges and
vertices of the crystal (SI Section S7 and Movies S1 and S2).
For a clearer view of this phenomenon, we used an
ultramicrotome to dissect and polish the section of the
crystals. This experiment confirms that the crystals show an
asymmetric distribution of linkers for the formation of core−
shell/TPDC-TZDC architectures between 3 and 20% whereas
this distribution becomes more uniform from 35% (Figure
3c,d). A closer analysis of the confocal images of the crystals
after 3 and 10% doping also suggests that higher concen-
trations of TZDC seem to favor the formation of slightly
thicker shells. To confirm if this asymmetric distribution of
linkers was kinetically controlled, we also synthesized and
characterized another set of UiO-68-TZDC10% crystals by
prolonging the exchange time to 6, 12, and 24 h while fixing
the stochiometric equivalents of H2TZDC in solution (SI
Section S5.4). Compared to the core−shell distribution
present after 3 h, reaction after 24 h seems to favor the
diffusion of TZDC across the crystal for a more homogeneous
distribution not distinguishable with confocal microscopy. XPS
analysis of all samples confirms how longer reaction times
induce and exponential decrease of the superficial concen-
tration of TZDC, that seems to be kinetically controlled. Our
data reveal how shorter reaction times are more likely to
maximize the asymmetric distribution of linkers in multivariate
frameworks prepared by linker-exchange reaction.
Overall, this confirms the ability of UiO-type materials to
produce chemically diverse arrangements of functionalities
after PSE as previously demonstrated with Raman and
fluorescence imaging for UiO-66 and -67, respectively.10,34
Our results point to a chemically diverse set of crystals with
comparable optical band gaps ideal to investigate the effect of
linker distribution over their photocatalytic activity. To the
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best of our knowledge, this still remains an open question for
multivariate MOFs.
after 3 h, the uniform distribution of photoactive linkers
attained at longer reaction times is detrimental to overall
performance.
Interplay between Linker Distribution and Photo-
catalytic Activity. To investigate this possibility, we
evaluated the photocatalytic performance of UiO-68, UiO-
68-TZDC, and multivariate UiO-68-TZDCx solids (x = 3, 10,
35, and 50%) for the reduction of methyl viologen (MV) and
the hydrogen evolution reaction (HER). As shown in Figure
4a, the different distribution of photoactive TZDC and inactive
TPDC linkers promotes a much more efficient reduction of
MV for UiO-68-TZDC10% despite the reduced concentration
of TZDC. This same trend is observed for the HER in a
methanolic mixture (Figure 4b). Compared to the negligible
activity of UiO-68-TZDC35%, UiO-68-TZDC10% shows a linear
increase in the H2 production up to a maximum of ∼100 μmol·
g−1 after 6.5 h. This boost in performance is concomitant to an
increase in the quantum efficiency of close to 15 times higher
that might be associated with the formation of core−shell
domains. It is worth commenting that related MOF-based
photocatalysts, such as UiO-66, have shown negligible H2
production in the absence of Pt nanoparticles or other
CONCLUSIONS
■
Compared to the synthetic design of multiple component
MOFs, which often relies only on the nature and relative
percentage of the linkers combined as synthetic variables to
tune the properties of the resulting materials, our results
demonstrate also the impact of linker distribution as an
efficient tool to code their function as previously demonstrated
with porosity gradients.42 These results confirm the
importance of concentration and reaction time in controlling
the relative ratio and distribution of mixed components in
multivariate frameworks prepared by linker exchange reactions
by using single crystals as a template. The reaction time seems
to be particularly relevant in our case, as it seems to control
linker diffusion to enable the formation of core−shell
architectures under kinetic control. We are currently exploring
this possibility to maximize the sensitization of multivariate
titanium frameworks. Compared to Zr-MOFs for which
photostimulated processes are mainly restricted to the linker,43
Ti is more prone to induce ligand to metal charge transfer for
higher photocatalytic efficiencies.13,28
cocatalysts, in good agreement with the production observed
39−41
for UiO-68-TZDC35%
.
We also observe a clear difference
on the experimental BET value of both solids after the tests.
Whereas UiO-68-TZDC10% maintains a surface area close to
the expected value, the 35% sample displays a clear reduction
indicative of partial collapse of the structure or chemical
damage (SI Section S8).
EXPERIMENTAL SECTION
■
Synthesis of UiO-68. H2TPDC (18 mg; 0.055 mmol) was
suspended in a mixture of 3.5 mL of N,N-Dimethylformamide (DMF)
and 150 μL of Trifluoroacetic Acid (TFA) (40 equiv) in a 5 mL
Teflon vial. Subsequently, the ZrCl4 (12 mg; 0.051 mmol) was added
to the suspension. The bottle was sealed and heated in an oven at 100
°C for 72 h. After cooling down to room temperature, this results in
the formation of deep yellow octahedral crystals that were isolated by
centrifugation and washed with 45 mL of DMF (3 × 15 mL) and 45
mL of methanol (3 × 15 mL) in the centrifuge tube. The product was
soaked in hexane for 3 days, after which it was dried at room
temperature.
To investigate this effect in detail, we tested and compared
the photocatalytic performance of UiO-68, UiO-68-TZDC,
and multivariate UiO-68-TZDCx solids (x = 3, 10, 35, and
50%). As shown in Figure 4c, the accumulated H2 production
after 6.5 h increases with the TZCD% up to a maximum at
10% from which it goes sharply down to negligible values. This
drastic change suggests that performance is somewhat
controlled by the density of TZDC in the surface of the
crystals. This was estimated for each case using XPS as the N/
Zr ratio calculated from the integrated areas of the N1s and
Zr3d survey spectra peaks. The XPS analysis reveals an increase
in the nitrogen surface concentration consistent with the
progressive replacement of parent TPDC by TZDC units for
doping levels equal or below 10%, compared to the complete
surface exchange that takes place from 35%. These results
suggest a complex scenario in which performance is controlled
not only by photoactive linker concentration but also by its
distribution inside the crystal. On the basis of the analysis of
pristine UiO-68-TZDC (vide supra), complete exchange might
compromise the chemical and structural stability of the surface
of the crystal at the conditions used in the photocatalytic tests.
Compared to 35 and 50% for which complete surface exchange
likely induces partial decomposition for an activity drop, the
partial exchange at the surface of UiO-68-TZDC10% is more
suitable to reach a fine balance between photoactivity and
stability. The stability of the frameworks was analyzed by ICP
measurements of the solutions in conditions comparable to the
photocatalytic tests. The rate of metal leaching remains below
10 μg·L−1 for doping levels below 35% but displays an increase
from this point with the TZDC% in the crystal, suggesting
reduced chemical stability for higher doping levels (SI Section
S8). The poor H2 production displayed by UiO-68-TZDC10%
prepared at 24 h also confirms the effect of TZDC surface
concentration in the photocatalytic activity of this family of
frameworks. Compared to the core−shell architecture formed
Synthesis of UiO-68-TZDC. H2TZDC (18 mg; 0.055 mmol) was
suspended in a mixture of 3.5 mL of DMF and 40 μL of TFA (10
equiv) in a 5 mL Teflon vial. Subsequently, the ZrCl4 (12 mg; 0.051
mmol) was added to the suspension. The bottle was sealed and
heated in an oven at 100 °C for 72 h. After cooling down to room
temperature, this results in the formation of deep pink octahedral
crystals that were isolated by centrifugation and washed with 45 mL of
DMF (3 × 15 mL) and 45 mL of methanol (3 × 15 mL) in the
centrifuge tube. The product was soaked in hexane for 7 days, after
that it was dried at room temperature
Synthesis of Multivariate Solids. As-made UiO-68 (140 mg)
was immersed in 54 mL of H2TZDC solution in DMF and 40 μL of
trimethylamine (Et3N) at 80 °C under stirring. Using different
equivalents of H2TZDC (0−17 equiv) for 3 h of reaction produced a
controllable UiO-68-TZDC%. After time reaction, the exchanged
MOF was thoroughly washed with fresh DMF (40 mL × 5). The
resultant solids were kept in hexane.
DFT Calculations. All structural calculation was performed using
dispersion-corrected density functional theory (DFT-D3) with the
Vienna Ab initio Simulation Package (VASP),44,45 and employing the
Perdew−Burke−Ernzerhof (PBE) functional.46 The recommended
GW PAW pseudopotentials47 were used for all geometry and
electronic calculations. A plane wave basis set was employed with a
kinetic energy cutoff of 500 eV, and a Γ-point grid was used to sample
the Brillouin zone. Electronic calculations was moreover performed
with a 2 × 2 × 2 Γ-centered grid and PBE functional, although this
functional is a qualitative approach, hybrid functional calculations on
Zr-MOF have shown PBE to display the correct trends.48,49 To
investigate the effect of tetrazine doping has on the electronic
structure, molecular substitutions were manually installed to the
1803
https://dx.doi.org/10.1021/jacs.0c09015
J. Am. Chem. Soc. 2021, 143, 1798−1806

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
TPDC linker on experimentally determined unit cell of UiO-68,
[(Zr6O4(OH)4)4(TPDC)24], and both lattice parameters and atomic
positions were further relaxed.
de València, 46980 València, Spain; orcid.org/0000-
0001-5269-2705
Sergio Tatay − Functional Inorganic Materials Team, Instituto
de Ciencia Molecular (ICMol), Universitat de València,
46980 València, Spain; orcid.org/0000-0003-0785-
866X
Yong Peng − Instituto Universitario de Tecnología Química
CSIC-UPV, Universitat Politècnica de València, 46022
València, Spain
Josep Albero − Instituto Universitario de Tecnología Química
CSIC-UPV, Universitat Politècnica de València, 46022
València, Spain
Photocatalytic Methyl Viologen Reduction. Photoinduced
electron transfer experiments were carried out using MOF dispersions
(0.2 mg/mL) in water/methanol mixtures (4:1, v/v%) containing
MV2+ (0.15 mM). The MOF dispersions containing MV2+ were
sonicated, placed in a quartz cuvette capped with rubber septum and
Ar purgued for 15 min prior irradiation with a 300 W Xe lamp. The
UV−vis absorbance of the cuvettes was measured at different
irradiation times. The photoinduced electron transfer measurements
were followed by the decrease of the MV2+ absorption band centered
at 260 nm as well as the increase of the MV+• radical cation
absorption bands at 395 and 605 nm.
Photocatalytic H2 Production. A quartz photoreactor equipped
with a manometer was loaded with sonicated dispersions of the
corresponding MOFs at 1 mg/mL concentration in water/methanol
(4:1, v/v) mixture. Prior irradiation the dispersions were purged with
Ar for 15 min and pressurized at a final pressure of 1.3 bar. The
photoreactor was located under the spot UV−vis light of an optic
fiber from a 300 W Xe lamp at 110 mW/cm2 for 6.5 h. The H2
evolution was followed by injecting 250 μL of the reactor gases in an
Agilent 490 MicroGC with a TC detector and Ar as carrier gas
(MolSieve 5A column). Quantification of the percentage of each gas
was based on prior calibration of the system injecting mixtures of H2
and Ar with known percentage of gases. Quantum efficiency
measurements were carried out measuring the lamp photon flux
using a calibrated Si diode and taking into account the produced H2.
Oscar Fabelo − Institut Laue Langevin, Grenoble, Cedex 9
38042, France; orcid.org/0000-0001-6452-8830
́
Javier Gonzalez-Platas − Departamento de Física, Instituto
Universitario de Estudios Avanzados en Física Atómica,
Molecular y Fotónica (IUDEA), MALTA Consolider Team,
Universidad de La Laguna, La Laguna, Tenerife E-38204,
Spain; orcid.org/0000-0003-3339-2998
Hermenegildo García − Instituto Universitario de Tecnología
Química CSIC-UPV, Universitat Politècnica de València,
46022 València, Spain; orcid.org/0000-0002-9664-
493X
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c09015
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c09015.
Notes
■
sı
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the EU (ERC Stg Chem-fs-MOF
714122) and Spanish government (CTQ2017-83486-P,
RTI2018-098568-A-I00, RYC-2016-1981, CEX2019-000919-
M, PID2019-106383GB-C44/AEI/10.13039/501100011033
and RTI2018-098568-A-I00). B.L.-B. thanks the Spanish
government for a FPU (FPU16/04162). S.T. thanks the
Synthetic and experimental details; physical character-
ization and supporting tables and figures (PDF)
Movie S1, confocal images of UiO-68 TZDC3%
showing the evolution of the fluorescence across the
crystal (MP4)
Movies S2, confocal images of UiO-68 TZDC35%
showing the evolution of the fluorescence across the
crystal (MP4)
́
Spanish government for a Ramon y Cajal Fellowship (RYC-
2016-60719817). N.M.P. thanks the European Union for a
Marie Skłodowska-Curie Global Fellowship (H2020-MSCA-
IF-2016-GF-749359-EnanSET). J.G.P. thanks to the SIDIX at
X-ray crystallographic data for UiO-68 (CIF)
́
Servicios Generales de Apoyo a la Investigacion (SEGAI) at La
X-ray crystallographic data for UiO-68-TZDC (CIF)
Laguna University. We also thank BSC-RES for computational
resources (QS-2020-2-0024) and the University of Valencia for
research facilities (Tirant and NANBIOSIS).
AUTHOR INFORMATION
Corresponding Authors
■
Carlos Martí-Gastaldo − Functional Inorganic Materials
Team, Instituto de Ciencia Molecular (ICMol), Universitat
de València, 46980 València, Spain; orcid.org/0000-
0003-3203-0047; Email: carlos.marti@uv.es
Natalia M. Padial − Functional Inorganic Materials Team,
Instituto de Ciencia Molecular (ICMol), Universitat de
València, 46980 València, Spain; orcid.org/0000-0001-
6067-3360; Email: nmpadial@ugr.es
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