Journal of the American Chemical Society
Article
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
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
J. Am. Chem. Soc. 2021, 143, 1798−1806