Sustained Solar H2 Evolution from a Thiazolo[5,4-d]thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Water

Article abstract of DOI:10.1021/jacs.9b03243

Solar hydrogen (H₂) evolution from water utilizing covalent organic frameworks (COFs) as heterogeneous photosensitizers has gathered significant momentum by virtue of the COFs' predictive structural design, long-range ordering, tunable porosity, and excellent light-harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as the co-catalyst for water reduction, which appears to be the bottleneck in the development of economical and environmentally benign solar H₂ production systems. Herein, we report a simple, efficient, and low-cost all-in-one photocatalytic H₂ evolution system composed of a thiazolo[5,4-d]thiazole-linked COF (TpDTz) as the photoabsorber and an earth-Abundant, noble-metal-free nickel-Thiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial electron donor. The high crystallinity, porosity, photochemical stability, and light absorption ability of the TpDTz COF enables excellent long-Term H₂ production over 70 h with a maximum rate of 941 μmol h^-1 g^-1, turnover number TON_Ni > 103, and total projected TON_Ni > 443 until complete catalyst depletion. The high H₂ evolution rate and TON, coupled with long-Term photocatalytic operation of this hybrid system in water, surpass those of many previously known organic dyes, carbon nitride, and COF-sensitized photocatalytic H₂O reduction systems. Furthermore, we gather unique insights into the reaction mechanism, enabled by a specifically designed continuous-flow system for non-invasive, direct H₂ production rate monitoring, providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results presented here open the door toward the rational design of robust and efficient earth-Abundant COF-molecular co-catalyst hybrid systems for sustainable solar H₂ production in water.

Full text of DOI:10.1021/jacs.9b03243

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Article
Sustained Solar H2 Evolution from a Thiazolo[5,4-d]thiazole-Bridged
Covalent Organic Framework and Nickel-Thiolate Cluster in Water
Bishnu P. Biswal, Hugo A. Vignolo-González, Tanmay Banerjee, Lars Grunenberg, Gökcen
Savasci, Kerstin Gottschling, Jürgen Nuss, Christian Ochsenfeld, and Bettina V. Lotsch
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03243 • Publication Date (Web): 20 Jun 2019
Downloaded from http://pubs.acs.org on June 20, 2019
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†
†,‡
†
†,‡
Bishnu P. Biswal, Hugo A. Vignolo-González, Tanmay Banerjee, Lars Grunenberg, Gökcen
†,‡
†,‡
†
‡,§
,†,‡,§,ǁ
Savasci, Kerstin Gottschling, Jürgen Nuss, Christian Ochsenfeld, and Bettina V. Lotsch*
†Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany
‡Department of Chemistry, University of Munich (LMU), Butenandtstraße 5-13, 81377 Mꢀnchen, Germany
§Center for Nanoscience, Schellingstraße 4, 80799 Mꢀnchen, Germany
ǁNanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Mꢀnchen, Germany
ABSTRACT: Solar hydrogen (H ) evolution from water utilizing covalent organic frameworks (COFs) as heterogeneous pho-
2
tosensitizers has gathered significant momentum by virtue of their predictive structural design, long range ordering, tunable
porosity and excellent light harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as
the co-catalyst for water reduction, which appears to be the bottleneck towards economical and environmentally benign solar
H
2
production systems. Herein, we report a simple, efficient and low cost all-in-one photocatalytic H evolution system com-
2
posed of a thiazolo[5,4-d]thiazole linked COF (TpDTz) as the photoabsorber and an earth-abundant, noble-metal-free nickel-
thiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial elec-
tron donor. The high crystallinity, porosity, photochemical stability and light absorption ability of TpDTzCOF enablesexcellent
−1 −1
long-term H
projected TONNi > 443 until complete catalyst depletion. The high H
catalytic operation of this hybrid system in water, surpasses that of many previously known organic dyes, carbon nitride (CNx)
and COF-sensitized photocatalytic H O reduction systems. Furthermore, we gather unique insights into the reaction mecha-
nism, enabled by a specifically designed continuous flow system for non-invasive, direct H production rate monitoring,
2
production over 70 h with a maximum rate of 941 μmol h g and a turnover number, TONNi > 103 and a total
2
evolution rate and TON, coupled with long-term photo-
2
2
providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results pre-
sented here open the door towards the rational design of robust and efficient earth-abundant COF-molecular co-catalyst hy-
brid systems for sustainable solar H production in water.
2
INTRODUCTION
towards scalable, economical solar H production. In addi-
2
tion, the use of nanoparticulate Pt co-catalysts precludes de-
tailed insights into the nature of the catalytic sites and the
intricacies of the photocatalytic cycle. Inspired by natural
Conversion and storage of solar energy in the form of chem-
ical bonds in “solar fuels” like H through light-driven water
reduction has evolved into a key technology over the last
decade due to the fast depletion of fossil energy sources and
rapid global climate change. To drive the proton reduction
half reaction in an efficient way, the major challenge is to
find a robust and highly active, but at the same time low-cost
and earth-abundant catalytic system in combination with a
strongly absorbing, chemically stable photosensitizer (PS).
In this regard, covalent organic frameworks (COFs) have re-
2
1
1
photosynthesis, this shortcoming motivated researchers
worldwide to search for single-site, earth-abundant, non-
precious metal-based co-catalysts with well-defined cata-
lytic centers. So far, only one molecular co-catalyst–COF sys-
1
tem for photocatalytic H
2
evolution has been demon-
1
2
2
strated. This system is based on an azine-linked COF (N2-
1
0b
3
COF) acting as the PS and a cobaloxime molecular proton
reduction catalyst, which shows a H evolution rate of 782
2
cently emerged as an exciting class of photoactive materials
−
1
−1
μmol h g and a TONCo of 54.4. However, the limited pho-
tostability and especially the utilization of an organic sol-
vent (acetonitrile/water mixture; 4:1) was a major con-
for light-driven H
2
production due to their tunable light har-
4
5
vesting and charge transport properties. In contrast to
other porous materials, COFs are known for being mechan-
ically robust and offering large accessible surface areas. By
virtue of their modular geometric and electronic structure,
COFs have attracted significant interest for a range of appli-
12
cern.
Notably, a majority of molecular catalysts decompose dur-
ing prolonged catalysis, are inherently insoluble in water,
and require the addition of organic solvents to accomplish
6
cations including adsorption, storage and separation,
chemical sensing, electronics, and catalysis. In spite of
their versatility, there are only few reports on COFs utilized
7
8
9
13
water reduction. With cobaloxime-based systems for ex-
ample, the catalyst often converts to an inactive form within
1
0
as photoabsorbers for photocatalytic H
2
evolution so far.
a few hours (<6 h) of H
decomposition or hydrogenation.
2
evolution, possibly due to ligand
14
Although rare and expensive, all except one of these works
has employed metallic platinum (Pt) as the co-catalyst to re-
duce water efficiently, which appears to be the bottleneck
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Figure 1. Synthesis and structural characterization of TpDTz COF. a) Schematic representation of TpDTz COF synthesis; b) Space
filling models of TpDTz COF pores with π-π stacking of successive 2D layers (grey: C; blue: N; red: O; yellow: S; and white: H); c)
Indexed PXRD patterns of TpDTz COF with corresponding Pawley refinement (red) showing good fit to the experimental data (blue)
with minimal differences (cyan), the inset shows close-up of the indexed experimental (blue) and simulated (black) PXRD patterns
based on Pawley fits [Final Rwp = 2.59% and Rp = 1.89%].
To overcome these issues, the development of a scalable,
earth-abundant and low-cost co-catalyst system which is
soluble in water and can be coupled efficiently to a molecu-
larly defined heterogeneous photoabsorber is in high de-
photoabsorber and a molecular Ni-thiolate cluster (NiME)18
assembled in situ from a Ni(II) salt and 2-mecaptoethanol
(ME). The combination of the NiME cluster co-catalyst and
TpDTz COF enables sustained H evolution with an excel-
2
−
1
−1
mand. In this regard, Ni based synthetic photocatalytic H
2
lent rate (941 μmol h g ) and a TONNi > 103 (70 h) in the
presence of triethanolamine (TEoA) as the sacrificial elec-
tron donor (SED) in water under AM 1.5 light illumination.
We thus report a single-site heterogeneous COF based pho-
tocatalyst system that operates with a noble-metal free co-
catalyst in water as the solvent. We further carve out struc-
ture-property-activity relationships by comprehensively
screening the parameter space of this heterogeneous-ho-
mogeneous hybrid photocatalytic system, including pH,
SED, co-catalyst metal centres, different N/S containing che-
lating ligands for co-catalysts, and a variety of photosensi-
tizers. Also, our study is built on a continuous flow photo-
catalytic reactor system which enables a non-invasive and
evolution catalysts15 have attracted significant interest be-
cause of their robust and oxygen tolerant nature and im-
portantly, their structural similarity to the active site in
1
6
[
Ni−Fe] hydrogenase .
Likewise, small molecules and polymers containing fused
bi)heterocyclic thiazolo[5,4-d]thiazole (TzTz) moieties
(
have received much attention as semiconductors in organic
electronics lately because of their n-type character featuring
high oxidative stability and their rigid planar structure.17a
The latter enables efficient intermolecular π–π overlap that
affords high electron and hole mobility.17 Such TzTz moie-
ties further feature excellent photoabsorbing ability, which
is likewise beneficial for photocatalysis.2a Nevertheless,
direct monitoring of the H evolution rate with high accu-
2
racy, in contrast to the routinely used standard photocata-
lytic batch reactors, and this allows gathering unique in-
sights into the photocatalytic reaction modeling and kinet-
ics. The results and understanding presented here thus con-
tribute towards the rational development of robust and effi-
cient single-site hybrid photocatalytic systems as a sustain-
1
0a,b
10c
able solution for solar H production in water.
2
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Figure 2. Structural characterization of TpDTz COF. a) 13C and 15N CP–MAS solid-state NMR spectrum of TpDTz COF. Calculated
NMR chemical shifts for the TpDTz-NMR model (Figure S49) obtained on B97-2/pcS-2//PBE0-D3/def2-TZVP level of theory (Table
S4 & S5) are shown as grey dashes; b) Argon adsorption-desorption isotherm for TpDTz COF recorded at 87 K; inset: calculated pore
size distribution of TpDTz COF according to the QSDFT method; c) TEM image of TpDTz COF showing the hexagonal pore structure
with a periodicity of ~3.3 nm (scale bar 100 nm); d) UV-vis diffuse reflectance (DR) spectrum for TpDTz COF measured in the solid
state, insets: plot showing the Kubelka-Munk function to extract the direct optical band gap, and photograph of TpDTz COF powder;
e) Cyclic voltammogram (CV) of a TpDTz COF-modified FTO working electrode in 0.1M NBu
4
PF as the supporting electrolyte in
6
anhydrous acetonitrile at a scan rate of 100 mV/s.
RESULTS AND DISCUSSION
parameters of TpDTz COF were extracted by Pawley refine-
ment in the hexagonal space group P6/m (a = b = 39.27 Å, c
COF synthesis and characterization. The precursor 4,4'-
=
3.46 Å, α = β = 90º and γ = 120º) (Figure 1c). The relatively
(thiazolo[5,4-d]thiazole-2,5-diyl)dianiline (DTz) was syn-
high level of order observed with PXRD may originate from
effective π–π stacking interactions facilitated by the planar-
ity of the DTz linker and thus, the 2D layers. The measured
pore aperture is ~3.4 nm and the π–π stacking distance be-
tween individual layers is ∼3.5 Å for TpDTz COF, as ob-
tained from the structural model.
thesized as described in the supplementary information and
characterized using single crystal X-ray diffraction, nuclear
magnetic resonance (NMR) spectroscopy, Fourier transform
infrared (FTIR) spectroscopy and mass spectrometry.
TpDTz COF was synthesized by solvothermally reacting Tp
(1.0 eq.) and DTz (1.5 eq.) in the presence of 6M aqueous
acetic acid using an o-dichlorobenzene and N,N-dimethyla-
cetamide solvent combination in a high precision glass vial,
which was sealed and heated to 120 ºC for 3 days (Figure 1
and Section S2, Supporting Information). Following a simi-
lar protocol, TpDTP COF with a similar pore size was syn-
The FTIR spectrum of the as-synthesized TpDTz COF
-
1
-1
shows bands at ~1254 cm (─C‒N), ~1571 cm (C=C) and
-1
~1618 cm (C=O) (Figure S9), which confirms the for-
mation of the proposed β-ketoenamine-linked framework.
The TzTz moiety was identified by appearance of C=N vibra-
tions (~1660 cm ) and C-S stretching bands between 650
and 700 cm . The structural composition of TpDTz COF was
further confirmed by C cross-polarization magic-angle
spinning (CP-MAS) NMR spectroscopy (Figure 2a). The
spectrum shows signals corresponding to the heterocyclic
TzTz ring of the DTz building unit ( = ~151 ppm), together
with a characteristic signal of the carbonyl carbon (C=O) at
1
9
-1
thesized as a reference, with the DTz linker replaced with
the linear terphenyl linker.
-1
1
3
To verify crystallinity and phase purity, the as-synthesized
TpDTz COF was analyzed via powder X-ray diffraction
(
2
PXRD). The PXRD pattern exhibits an intense first peak at
.56° 2θ corresponding to the 100 reflection along with
other diffraction peaks at 4.41, 5.23, 6.90 and 9.10° 2θ, at-
tributed to the 110, 200, 210 and 220 reflections, respec-
tively. In addition, at ~26° 2θ a broad set of reflections is vis-
ible, with 00l being the most intense, which corresponds to
the π-π stacking of the 2D layers (Figure 1c). The experi-
mental PXRD pattern is in good agreement with the simu-
lated AA eclipsed stacking model (Figure S6.2). The lattice
~
184 ppm, which further supports formation of the β-ke-
15
toenamine moiety. N NMR spectroscopy confirms the
presence of two different kinds of nitrogen atoms with
chemical shifts of – 93 ppm and – 243 ppm corresponding
to the TzTz and enamine (=C‒NH‒) moieties, respectively
(Figure 2a). All assignments are supported by quantum-
chemical calculations of NMR chemical shifts (Table S4, S5)
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at the B97-2/pcS-2//PBE0-D3/def2-TZVP level using the The fluorescence decays can be fitted with triexponential
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FermiONs++ program package20a,b based on a selected mo-
lecular model system (Figure S49). The corresponding
structures were optimized at the RI-PBE-D3/def2-TZVP
level using Turbomole (version 7.0.3)20c,d. Scanning electron
microscopy (SEM) images of TpDTzCOF reveal a flower-like
morphology composed of flakes with 1-3 μm lateral dimen-
sions (Figure S11). ransmission electron microscopy
functions and the amplitude weighted average lifetimes for
TpDTz and TpDTP COFs are 94 ps and 115 ps, respectively
(Figure S20). Emission intensities were too low to measure
accurate absolute emission quantum yields. The short ex-
cited state lifetimes together with low emission quantum
yields suggest more pronounced non-radiative rates in the
COF systems, relative to the radiative rates, similar to our
1
0b,f
previously reported N3-COF system.
(TEM) images confirm the layered morphology of the crys-
talline network with clearly visible 2D honeycomb-type
pores oriented perpendicular to the crystallographic c axis
with a periodicity of ~3.3 nm (Figure 2c). In order to evalu-
ate the thermal stability of TpDTz COF, we further per-
formed thermogravimetric analysis (TGA) in air. The TGA
profile suggests that the COF pores are guest free and the
material is thermally stable up to ~ 400 ºC (Figure S10).
Cyclic voltammograms (CV) of TpDTz COF films were
measured to estimate the band positions and the thermody-
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namic driving force for H evolution. The voltammogram of
2
a TpDTz COF-modified FTO working electrode22 shows an
irreversible reduction wave with an onset potential of Ered,on-
set  –1.24 V vs. saturated calomel electrode (SCE) (Figure 2e
and Figure S22). From the optical bandgap (E = 2.07 eV)
g
The permanent porosity of TpDTz COF was assessed by Ar
adsorption analysis measured at 87 K (Figure 2b and Figure
S13). A Brunauer-Emmett-Teller (BET) surface area of 1356
determined from the UV-vis DR spectrum, the valence band
(VB) and conduction band (CB) edges of TpDTz COF can be
estimated to be ECB = –3.46 eV and EVB = –5.53 eV vs. the vac-
2
-1
uum level, following the equations E = –(Ered,onset vs. SCE + 4.7)
m g was obtained for TpDTz COF, which is comparable to
CB
2
3
some of the most porous β-ketoenamine-based porous COFs
eV and EVB = ECB – Eg,opt. Quantum-chemical calculations of
vertical ionization potentials and electron affinities on a
TpDTz pore model (Figure S53), cut from a supercell built
using the 2D periodic optimized unit cell of the TpDTz COF
(Figure S52), support these findings (Table S8). By compar-
ing these values with the oxidation potential of TEoA (0.57
previously synthesized via solvothermal methods.10c,d,21 The
experimental pore size of 3.4 nm obtained fromthe adsorp-
tion isotherm using the quenched solid state density func-
tional theory (QSDFT) cylindrical-slit adsorption kernel for
carbon (inset of Figure 2b) is in excellent agreement with
the pore size obtained from the structure model (~3.4 nm)
and TEM (~3.3 nm). Further, the measured water adsorp-
2
3d
V vs. SCE) and the reduction onset potential of the NiME
molecular catalyst system (–0.75 V vs. SCE) (Figure S21), it
is likely that TpDTz COF can transfer electrons to the NiME
co-catalyst system forming a reduced Ni(I) center and
3
-1
tion isotherm (total uptake - 309 cm g , 25 wt% at STP) of
TpDTz COF suggests its relatively hydrophilic nature, in-
duced by the polar N/S containing TzTz group and should
18,23a
thereby enabling H
2
evolution in successive steps.
Also,
thus lead to higher dispersibility of the COF in water during
TEoA can efficiently quench the photoexcited holes in the
COF thereby replenishing its photoactivity.
photocatalysis,10d,e as opposed to the non-TzTz TpDTP COF
3
-1
(
total uptake 75 cm g , 6 wt% at STP) with similar pore
sizes (Figure S15). This fact is also supported by the higher
CO uptake for TpDTz COF compared to TpDTP COF (Figure
S16).
Since chemical stability is a crucial criterion for any mate-
Owing to the planar and conjugated structure, the electron
deficient nature of the heterocyclic backbone, optimal band
gap and hence light absorption ability, the TzTz-linked
TpDTz COF was investigated as the heterogeneous photo-
2
absorber for photocatalytic H evolution in combination
2
rial to be considered for practical applications, we investi-
gated the chemical stability of TpDTz COF under strongly
acidic (12M HCl) conditions, and in boiling water up to 7
days. The retention of all characteristic peaks in the PXRD
pattern suggests a high chemical stability under the tested
conditions (Figure S7). It is important to note that TpDTz
COF is stable only under mild basic conditions (1M KOH) for
up to 3 days, while at harsher basic conditions (12M KOH
for 7 days) the framework decomposes. The high chemical
tolerance of TpDTz COF is ascribed to the combined effect
with the Ni-thiolate hexameric cluster (NiME) co-catalyst in
water. The NiME cluster co-catalyst has a cyclic hexameric
structure composed of six Ni(II) ions forming a planar ring
and the Ni centers are bridged by twelve ME units, which
has been confirmed by DFT calculation and single-crystal X-
1
8,24b
ray diffraction analysis by others (Figure S35).
NiME cluster co-catalyst has been shown to produce H
tively when sensitized by an organic xanthene dye (Erythro-
This
2
ac-
1
8
sin B). However, the generation of unstable PS radical spe-
cies upon photochemical quenching of the excited-state dye
(PS*) leads to a fast decomposition and hence poor photo-
chemical stability of organic dye photosensitizers which
hinders long-term performance of the photocatalytic sys-
2
1
of the stabilizing enol-to-keto-tautomerism, and the pla-
narity of the TzTz moiety,17 which allows for strong π-π in-
teractions between the layers.
1
5a,18
tem.
The strategy of combining a photochemically sta-
Opto-electronic properties and photocatalysis. The UV-
vis diffuse reflectance (DR) spectrum of TpDTz COF reveals
efficient light absorption extending into the orange parts of
the visible spectrum with an absorption edge at ~598 nm
ble COF photoabsorber with the Ni-thiolate cluster co-cata-
lyst in water could thus be a viable path to impart better
long-term stability for H production.
2
(
Figure 2d). Kubelka-Munk analysis yields a direct optical
The beauty of the aforesaid NiME complex lies in its sim-
ple, quick in situ synthesis in water upon addition of a Ni(II)
salt and ME at room temperature. This in-situ assembling
strategy is different from most other Ni(II) and Co(II) co-
catalyst complexes featuring arduous ex situ synthesis and
purification of a water soluble analogue, thus adding to the
cost-effectiveness of the NiME cluster co-catalyst ap-
band gap of ~2.07 eV. In contrast, TpDTP COF shows a blue-
shifted absorption band edge at ~531 nm, corresponding to
a larger optical band gap of ~2.28 eV (Figure S18), due to
the absence of light harvesting TzTz units. The measured
photoluminescence (PL) spectra (Figure S19) reflect this
trend; TpDTz COF has a significantly red-shifted emission
1
2,15
proach.
(max = 690 nm) compared to TpDTP COF (max = 630 nm).
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Figure 3. Continuous flow photocatalytic reactor design. Schematic diagram of the designed continuous flow photocatalytic re-
actor system (red streamlines are the continuous flow pathway of gas). In contrast, the batch configuration involves mass flow con-
trollers as dead-ends after back purging the initial headspace, and replaces the autosampler by a septa-port or a manual sampling
valve.
In addition, this cluster has been shown to be a potent H
evolution co-catalyst producing H immediately after light
illumination in the presence of a PS and SED, and hence does
not require any photo-deposition nor does it show an acti-
vation time, contrary to Pt based photocatalytic systems.10
2
In a typical photocatalytic experiment using our hybrid
system, 5 mg of TpDTz COF was dispersed in 10 mL of H
containing TEoA (10 vol.%) as the SED and the pH was ad-
justed to 8.5 by adding HCl. Ni(OAc) .4H O (10 wt.%, 0.5
2
2
O
2
2
mg) and ME (10 eq., 1.4 μL) were then charged to instanta-
neously form the brown coloured NiME cluster co-catalyst.
For better measurement accuracy and to gain insights into
the photocatalytic mechanism, we developed a continuous
2
When irradiated with 100 mW/cm AM 1.5 radiation, the re-
sulting mixture produces H actively over a period of at least
2
flow system to monitor the H evolution performance of the
2
70 h - with ~40% of the highest production rate still pre-
served after this time - in a single run without adding addi-
hybrid photocatalytic system (Figure 3 and Figure S23). In
this measurement system, the molar flow entering the sys-
tem, the pressure (0.5 bar) and temperature (25 °C) of the
reactor are continuously controlled, while bypassing some
of the out-flow to an open sampling loop gas chromatograph
tional TEoA or co-catalyst (Figure 4a). A maximum H
2
evo-
−
1
−1
lution rate of 941 μmol g h with a TONNi > 103 (70 h), and
-1
a TOF= 2.3 h when the system is fully active were obtained.
A mathematically projected (Section S8, Supporting Infor-
(GC) autosampler. In this way, the rate of H
2
production (R 2)
H
mation) TONNi > 443 (890 μmol of total H
2
evolution) can be
can be monitored directly using only two experimental in-
puts: Fin from the mass flow controller, and xH2,ppm from an
obtained for the photocatalytic H evolution performance
2
corresponding to a complete depletion of the co-catalyst.
The relation between co-catalyst, SED and observed activity
loss of the system in time was confirmed by in-situ addition
of loss-equivalent amounts of ME or TEoA independently af-
ter 72 hours of illumination, which did not change the deac-
tivation trends observed (Section S8, Supporting Infor-
mation). It must be noted that the TONNi mentioned above
is only a lower limit calculated based on the total amount of
Ni(II) salt used for the photocatalysis experiment. Under
identical conditions the Erythrosin B (EB) dye sensitized
on-line GC (BID) detection system (Eqn. 1), where Fin is the
carrier gas (in this case He) flow in to the system and xH2,ppm
is the molar fraction of H
2
at the outlet.
퐹푖푛 × 푥퐻2,푝푝푚 × 10−6
푅퐻2
=
Eqn. ꢁ1ꢂ
1 ꢀ 푥퐻2,푝푝푚 × 10−6
Typically, the run-to-run error with this method is below
%, compared to at least 15-20% error with a standard
(
)
3
batch system. Also, this continuous flow system is inde-
pendent of experimental conditions, and does not require
human intervention when sampling or local derivative ap-
proximations as with regular batch system measurements
system18 produces H
−1 (attained in 1 h), however, the rate rapidly drops off
2
with a maximum rate of 49297 μmol
−1
g h
and the whole system becomes completely inactive within 7
(
Figure S34). In addition to the better accuracy of this
method in monitoring kinetic trends in the photocatalytic
evolution process, the method also keeps the media un-
h. A TONNi >36.5 (73 μmol of total H evolution) was ob-
2
tained after 7 h (TOF = 31.9 h-1), which is 12 times lower
Ni
H
2
than the value projected for the TpDTz COF sensitized sys-
tem (Figure 4a). These results demonstrate the added value
of using a heterogeneous photosensitizer to stabilize charge
transfer in photocatalytic hybrid systems.
perturbed - since the presence of the GC sampling line does
not affect the hydrogen balance - thus completely eliminat-
ing typical sampling losses and mathematical and experi-
mental uncertainties associated with batch photocatalytic
reactor systems.
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Figure 4. Photocatalytic H
2
evolution. a) A comparison of photocatalytic H
2
evolution rates in water (H O) and deuterium oxide
2
(
D
2
O), using TpDTz COF over 72 h and EB dye under AM 1.5 light irradiation [COF photosensitizer: 5 mg of TpDTz COF in 10 ml of
H
2
O/D
2
O with 10 vol.% TEoA, 0.5 mg of Ni(OAc)
and 1.4 μL of ME at a final pH of 8.5]; b) Light on-off cycles for photocatalytic H
evolution with TpDTz COF in water using different co-catalysts;
evolution with TpDTz COF in water using different metal-ME co-catalysts; e) photocatalytic H evolution from
water using different photosensitizers; f) Overlay of the UV-vis DR spectra of TpDTz COF with apparent quantum efficiency (AQEs)
for the photocatalytic H evolution reaction with TpDTz COF at four different incident light wavelengths.
2
and 1.4 μL of ME at a final pH of 8.5; dye photosensitizer: 1.33 mg of EB in 10 ml of
H
2
O with 10 vol.% TEoA, 0.5 mg of Ni(OAc)
2
2
evolution
experiments with TpDTz COF in water over 26 h; c) photocatalytic H
d) photocatalytic H
2
2
2
2
g−1 h ), TP-BDDA/Pt (324 μmol g h ), N2-COF/Pt
−1
25b
−1
−1 10c
Also, the TpDTz COF-NiME photocatalytic system produces
at a 17% higher maximum rate and has a TON nearly
eightfold as high as our previously reported N2-COF-co-
−
1
−1 10b
H
2
(480 μmol g h ), crystalline poly(triazine imide)/Pt
−
1
−1 25a
−1
−1 25c
(864 μmol g h ),
sg-CN-Ni (103 μmol g h ),
−
1
−1
−1 −1 25d NCN
baloxime based system (782 μmol h g , TONCo = 54.4),
Ni12
P
5
/g-C
3
N
4
(536 μmol g h ),
CN -NiP (763 μmol
x
while operating in water.12 The H
evolution rate and the
g
g
−1
h ), and carbon quantum dots (CQDs)-Ni (398 μmol
−1 25e
2
−
1
−1
h )25f.
sustained activity of this simple TpDTz COF-NiME system is
competitive and even superior compared to many COF-
based photocatalytic systems (Table S3) and other bench-
mark photocatalytic systems involving metallic Pt or molec-
Control experiments were performed by sequentially re-
moving one of the components, i.e. TpDTz COF, TEoA,
Ni(OAc) .4H O and ME, at a time from our photocatalytic
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2
2
ular Ni co-catalysts. Examples include g−C
3
4
N /Pt (840 μmol

Page 7 of 12
Journal of the American Chemical Society
system to identify their importance and role for the H
2
evo-
catalyst and TEoA at pH = 8.5 over a period of 24 h as com-
pared to that with NiME. The significant difference between
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lution. Indeed, no H evolution was observed for a period of
2
12 h unless all individual components act in concert, signi-
fying that each is essential for the photocatalytic system to
H
2
evolution of the molecular co-catalyst and photo-depos-
ited Pt nanoparticles is difficult to explain by a single ef-
fect.
1
2,25f
work and efficiently produce H
2
(Figure S27).
However, it may be argued that the higher activity
of the NiME co-catalysed system in contrast to the surface
bound Pt nanoparticles (Figure S48) is due to a more effec-
tive blocking of charge carrier recombination since the co-
catalyst is physically separated from the framework (phy-
sisorbed), which may support better charge separation. We
Furthermore, a 1:10 metal-to-ligand molar ratio and 10
wt.% of catalyst with respect to the PS were observed to
elicit the best photocatalytic performance (Figure S28 and
S28). To confirm water as the source of H
lytic reaction was performed in D O under identical condi-
tions (Figure 4a). A similar production rate of D was ob-
served over 72 h as for H in H O, taking batch-to-batch var-
iations into account. This result suggests that water is the
hydrogen source responsible for the production of H , as-
2
, the photocata-
2
further screened the H evolution activity of other transition
2
2
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metal-ME complexes, such as CoME and CuME, with TpDTz
COF as the PS following a similar method as that of NiME.
2
2
Although all systems produced H
2
, they do so with a much
2
−1 −1
lower rate following the order NiME (941 μmol g h ) >
suming that no significant proton/deuterium exchange pro-
cesses in the individual components are at play. This finding
was further confirmed by an almost complete disappear-
−1 −1
−1 −1
CoME (85 μmol g h ) > CuME (52 μmol g h ). This could
be due to the poor solubility of CoME and CuME clusters in
water compared to the NiME, which is in accordance with
ance of the m/z =2 signal for H
measurement of the headspace gas of the photocatalytic re-
action performed in D O (Figure S26). Note that D is
evolved with a time lag compared to H , which is likely due
to the kinetic isotope effect of deuterium as described be-
low. Further, H evolution experiments performed under
multiple light-on/off cycles over a period of 26 h (Figure 4b)
suggests a purely light driven H evolution process in water.
Once the catalytic system is fully active, H evolution activity
2
in a mass spectrometric
1
8
the reported dye sensitized molecular system (Figure 4d).
We then evaluated the H evolution ability of the NiME
cluster co-catalyst with a variety of photoabsorbing materi-
2
2
2
2
1
9
10b
als; TpDTP COF, N3-COF, an amorphous porous poly-
2
7
2
mer containing TzTz groups (TzTz-POP-3), and the dia-
mine linker DTz were tested under identical conditions.
Even though N3-COF is considered one of the most active
2
2
COFs for photocatalytic H
2
generation (reported rate of
−1
−1
10b
is seen to be restored even after a prolonged light-off period.
1700 μmol g
h when co-catalyzed by Pt), with NiME
co-catalyst it produces H
2
only at a very low rate of 40 μmol
(Figure 4e). Under similar conditions TpDTP COF
SED and the reaction pH are known to have a profound in-
−
1
−1
1
0b,c,12
g
h
fluence on the activity of many H
In our case, we observed a similar effect; the rate of H
erated from the photochemical reaction is the highest (941
2
production systems.
−
1
−1
produces H at a rate of 160 μmol g h , which is nearly 6
2
2
gen-
times less compared to that of the TpDTz COF sensitized
system. The marked difference in photocatalytic activity be-
tween TpDTz COF and TpDTP COF may in part be rational-
ized by the reaction conditions which were not optimized
specifically for TpDTP COF, but also by their different pho-
ton absorption characteristics. A redshift of ~67 nm is ob-
served for TpDTz COF with respect to TpDTP COF, which
indicates that TpDTz COF absorbs photons more effectively
in the visible range. This said, increased reactivity is only ex-
pected if the conduction band is not significantly lowered to
maintain the thermodynamic driving force for the HER. In
addition to that, the higher crystallinity, and the higher BET
−
1
−1
μmol g h ) at pH 8.5 using TEoA as SED. However, at acidic
conditions (pH 6.5) there was negligible H evolution (16
2
−1 −1
μmol g h ). This could be attributed to the protonation of
TEoA or due to inhibition of proton loss from one-electron
+ 13c
oxidized TEoA . Notable H
2
evolution is observed over 24
h under alkaline conditions (pH 11), albeit at lower rates
−
1
−1
(308 μmol g h ) as compared to pH 8.5 (Figure S30). This
is possibly due to the reduced driving force for protonation
of the Ni hydride intermediate co-catalyst species at higher
pH to subsequently generate H
Na S were also explored as potential SEDs. Interestingly,
they produce H
2
. triethylamine (TEA) and
2
2
−1
2
−1
surface area of 1356 m g for TpDTz COF versus 736 m g
2
but with significantly lower rates of 84
and 7 μmol g h , respectively (Figure S31).
−
1
−1
−1 −1
for TpDTP COF, along with a better dispersibility of the
more hydrophilic TpDTz COF in aqueous solution, are likely
determining factors for the enhanced photocatalytic activ-
ity. Notably, the amorphous polymer TzTz-POP-3 and the di-
μmol g
h
Higher TEoA concentrations were found to decrease H
lution rates; a TEoA concentration of 10 vol.% in water was
2
evo-
observed to result in the maximum H
ure S32).
2
production rate (Fig-
amine linker DTz are completely inactive at producing H
2
with the NiME co-catalyst (Figure 4e). Overall, the signifi-
cantly lower reactivity of other photosensitizers in produc-
H evolution rates of the photocatalytic systems containing
2
TpDTz COF PS and different Ni(II) co-catalysts were meas-
ured (Figure 4c). Different sulfur-containing compounds,
such as thiourea (TU) and 2-mercaptophenol (MP), were ex-
plored as potential ligands for in situ formation of Ni(II) co-
catalyst complexes. However, neither NiTU nor NiMP pro-
ing H with the NiME co-catalyst is rationalized by a com-
2
bined effect of unfavourable charge transfer processes, re-
duced light harvesting, low crystallinity and surface area,
and poor dispersibility in water.
The apparent quantum efficiency (AQE) was calculated us-
ing four different bandpass filters with central wavelengths
duced any H with TpDTz COF, possibly due to unfavourable
2
complexation of the ligands with Ni(II) in water: TU and MP
are known to be poorer complexation agents as compared
(
±20 nm) at 400, 500, 550, and 600 nm to quantify the spec-
2
6
tral contribution towards H evolution activity of the TpDTz
2
to ME. Also, a reported ex situ synthesized Ni(abt)
2
com-
evolution co-catalyst
under our experimental conditions, but no H evolution was
seen most likely due to its poor solubility in water. It is also
interesting to note that TpDTz COF produces H with a sig-
nificantly smaller rate of 23 μmol g h with metallic Pt co-
15c
COF photoabsorber. Figure 4f shows that TpDTz COF has a
maximum AQE of 0.2% at 400 nm. Under AM 1.5 illumina-
tion, the AQE was estimated to be 0.044%, which is higher
plex was studied as a potential H
2
2
than that of our previously reported N2-COF-cobaloxime H
evolution system (0.027%).
2
2
12
−
1
−1
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Figure 5. Reaction limitations insights. a) A general schematic of the proposed pathway for H
2
evolution (color code: grey; C, red;
O, yellow; S, blue; N and light pink/white; H); b) proposed key steps of the photocatalytic H
2
evolution reaction with TpDTz COF and
NiME cluster co-catalyst. [Ni--L] denotes a ligand-coordinated co-catalyst state which is attained fast compared to the [R] state, [R]
denotes the catalyst resting state, which is catalytically active nickel cluster species, [D] denotes the deactivated species and [I] de-
notes an intermediate reduced catalyst species able to run the HER step.
We further verified the photochemical stability of TpDTz
COF after a 72 h long photocatalysis experiment. The iso-
lated TpDTz COF sample was fully characterized using
PXRD, ssNMR, SEM and TEM, and it was found that the
framework structure, crystallinity, and morphology of
TpDTzCOF is retained (Section S9, Supporting Information)
thus supporting the high chemical stability of this COF (vide
initial activation time as with H O (Figure 4a). A possible re-
2
action model is outlined in Figure 5b. The absence of an ac-
tivation time during light on/off cycles (Figure 4b) suggests
a light enhanced formation of a catalyst resting state [R] of
the NiME complex upon illumination, as seen by the initial
activation time required for the system to reach maximum
efficiency. The reaction network can then be reduced to the
following core steps using a microkinetics analysis. For the
heterogeneous (COF) fast cycle: COF photoexcitation (hv),
exciton recombination in the COF (krec), reductive quenching
supra). A small additional signal at 56.6 ppm in the 13
C
ssNMR spectrum possibly corresponds to trapped ME mol-
ecules inside the TpDTz COF pore. This, together with the
observation of traces of Ni in the post-photocatalytic TpDTz
COF using SEM-EDAX (Figure S46) may hint to chemisorp-
tion of small amounts of co-catalyst to the COF walls. How-
•
—
of the COF (k
q
), and electron transfer from COF to [R] to
form the active intermediate spcies [I] (k
a
). Quantum-chem-
ical calculations on the TpDTz pore model system (Figure
S55) identify the lowest photoexcitation energy to be
2.30 eV (Table S9), the difference density of this excited
state (Figure S57) visualizing the exciton. The spin density
of the radical anion as a result of the reductive quenching of
this state is shown in Figure S58. For the homogeneous (cat-
alyst) cycle: formation of a rapidly coordinated complex [Ni-
-L] (Keq), slow assembly of the catalytically active species
15
ever, N ssNMR of the TpDTz COF sample does not show
any noteworthy difference in the chemical shifts of the N sig-
nals before and after photocatalysis (Figure S41), suggest-
ing that there is no substantial direct interaction between
the residual Ni and the nitrogen centers of the COF. Also, the
as-recovered TpDTz COF sample does not produce any H
under identical photocatalytic conditions except for the ab-
sence of Ni(OAc) .4H O and 2-ME ligand. This suggests that
2
-1
2
2
(k
[I] (k
closing of the catalytic cycle via a dark step that produces H
T
, k
T
), an apparent first order activation step from [R] to
the interaction between TpDTz COF and NiME is mostly
physical and no lasting chemical interaction exists between
the two components. Our finding thus suggests an outer
sphere electron transfer to be at play, which nevertheless is
efficient enough to allow facile charge transfer from the
photoabsorber to the NiME co-catalyst (Figure 5a).
a
), an irreversible deactivation step [D] (k ), and the
d
2
-1
(kHER). If in such a system k
T
and k
T
are significantly slower
compared to the rest of the steps, the on/off behaviour can
be explained, because in the absence of light the dark equi-
librium is slow and the amount of [R] and [I] will not change
significantly in time. Furthermore, the absence of HER in the
dark during the light on/off cycles observed in Figure 4b
suggests that a kHER limited homogeneous cycle is unlikely.
Then, once nickel enters the cycle as [R] more rapidly due to
a light shifted equilibrium, [R] will build up until the rate of
To obtain deeper insights into the photocatalytic mecha-
nism, an overall coarse-grain mathematical model (Eqn.
S6.2) of the photocatalytic reaction was developed (Section
S8, Supporting Information for details) by taking advantage
of the quantification of hydrogen evolution rates in our flow
detection platform. Our model was based on three primary
experimentally observed trends: the activation time re-
quired by the photocatalytic system to reach maximum
rates in the first run (Figure 4a), the absence of this activa-
tion time during light on/off cycles or long dark periods be-
fore illumination (Figure 4b), and the kinetic isotope effect
[D] leaving the system irreversibly is equal to the rate of for-
mation of [R], [I] being stationary. This will lead to an ex-
pression for activation of the photocatalytic system such
that the activation curve is an exponential asymptote with
time constant t
rate k , a linear deactivation with an apparent time constant
, an HER apparent kinetic constant RH2,max, and the appar-
ent transient time t only which corresponds to the initial
ACS Paragon Plus Environment
a
, which is dominated by electron transfer
a
t
d
in D
2
O, namely, smaller deuterium evolution rates and de-
0
layed response, together with the observation of a similar

Page 9 of 12
Journal of the American Chemical Society
time delay necessary for the the intermediate [I] to be
pseudo-stationary (Eqn. S6.2). Our model further provides
an accurate fit both to the data obtained in H O and D O, in
line with an expected trend of a kinetic isotope effect (KIE).
and t changed as expected, but the
time constant for the activation step (t ) being independent
of the isotopic mass is almost unchanged (Table S2), further
corroborating our proposed rate limiting steps (RLS). Our
reaction model not only explains these qualitative trends
but also provides an accurate fit with standard errors below
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Bishnu P. Biswal: 0000-0002-8565-4550
Tanmay Banerjee: 0000-0002-4548-2117
Lars Grunenberg: 0000-0002-6831-4626
Gökcen Savasci: 0000-0002-6183-7715
Jürgen Nuss: 0000-0002-0679-0184
2
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In this case, RH2,max , t
d
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Christian Ochsenfeld: 0000-0002-4189-6558
Bettina V. Lotsch: 0000-0002-3094-303X
5
% for different data sets. It is important to note that acquir-
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All authors have given approval to the final version of the
manuscript.
ing detailed insights into the H /D evolution reaction
2
2
mechanism for the TpDTz COF-NiME photocatalytic system
became possible only with the use of a flow reactor system.
The authors declare no competing financial interests.
Our reaction modelling results suggest that as long as a
slow catalyst activation time is observed, the RLS of the sys-
tem is seemingly the electron transfer from the COF to the
NiME complex. While this outcome is fully consistent with
the assumed outer sphere electron transfer process, it rein-
forces the idea of studying the kinetics of such processes in
more detail as this will be crucial to improve the HER rate
by rational design of the COF – co-catalyst interface.
B.P.B. acknowledges the Alexander von Humboldt foundation
for a research fellowship. B.V.L. acknowledges financial support
by an ERC Starting Grant (project COF Leaf, grant number
6
39233), the CRC 1333 of the University of Stuttgart, and the
Max Planck Society. B.V.L and C.O. acknowledge further support
by the cluster of excellence e-conversion (EXC 2089), and the
Center for Nanoscience (CeNS). C.O. acknowledges financial
support as a Max-Planck-Fellow at the Max Planck Institute for
Solid State Research, Stuttgart. We thank Viola Duppel for re-
cording the SEM and TEM images, Igor Moudrakovski for solid-
state NMR measurements, Marie-Luise Schreiber for elemental
analysis, Prof. Dr. Ulrich Starke and Dr. Kathrin Müller for XPS
data.
CONCLUSIONS
We report the first COF photosensitizer and noble-metal-
free molecular co-catalyst photocatalytic system for sus-
tained solar H production from water. This single–site sys-
2
tem comprises the newly designed N/S containing TpDTz
COF photosensitizer that absorbs strongly in the visible re-
gion of the solar spectrum and is robust for long-term hy-
drogen evolution. In combination with an earth-abundant
Ni-thiolate cluster co-catalyst which self-assembles in wa-
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2
evolution rates as high as 941 μmol
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benchmark photcatalytic H evolution systems based on
−
1
−1
2
2
COFs and carbon nitrides. To map out the parameter space
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2
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This platform does not only provide higher accuracy in
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