Clean Donor Oxidation Enhances the H2Evolution Activity of a Carbon Quantum Dot–Molecular Catalyst Photosystem

Article abstract of DOI:10.1002/anie.201604355

Carbon quantum dots (CQDs) are new-generation light absorbers for photocatalytic H₂evolution in aqueous solution, but the performance of CQD-molecular catalyst systems is currently limited by the decomposition of the molecular component. Clean oxidation of the electron donor by donor recycling prevents the formation of destructive radical species and non-innocent oxidation products. This approach allowed a CQD-molecular nickel bis(diphosphine) photocatalyst system to reach a benchmark lifetime of more than 5 days and a record turnover number of 1094±61 mol_H2(mol_Ni)^?1for a defined synthetic molecular nickel catalyst in purely aqueous solution under AM1.5G solar irradiation.

Full text of DOI:10.1002/anie.201604355

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201604355
German Edition: DOI: 10.1002/ange.201604355
Carbon dots
Clean Donor Oxidation Enhances H2 Evolution Activity of a Carbon
Quantum Dot–Molecular Catalyst Photosystem
Benjamin C. M. Martindale, Evelyne Joliat, Cyril Bachmann, Roger Alberto, and Erwin Reisner*
Abstract: Carbon quantum dots (CQDs) are new-generation
light absorbers for photocatalytic H2 evolution in aqueous
solution, but the performance of CQD-molecular catalyst
systems is currently limited by the decomposition of the
molecular component. Clean oxidation of the electron donor
by donor recycling prevents the formation of destructive
radical species and non-innocent oxidation products. This
approach allowed a CQD-molecular nickel bis(diphosphine)
photocatalyst system to reach a benchmark lifetime of more
than 5 days and a record turnover number of 1094 Æ
61 molH2 (molNi)À1 for a defined synthetic molecular nickel
catalyst in purely aqueous solution under AM1.5G solar
irradiation.
C
arbon quantum dots (CQDs) have recently emerged as an
exciting new allotrope of carbon because of their ability to
absorb UV/Vis light and fluoresce. These properties have
found use in a variety of applications, from biosensing and
fluorescence probes, to chemical sensing and light-emitting
devices.[1–5] More recently, CQDs have been employed as light
harvesters in photocatalytic applications, such as the co-
sensitization of metal oxides in solar cells, photodegradation
of organic dyes, and as photosensitizers in solar fuel
synthesis.[6–13]
A recent study showed that carboxylate-terminated
amorphous CQDs produced by a bottom-up synthetic
method could photosensitize the water soluble molecular
nickel H2-evolving catalyst NiP (Figure 1).[13] This unique
example of a CQD-molecular catalyst hybrid system made
use of a sacrificial electron donor (ethylenediaminetetraacetic
acid (EDTA)) to quench the holes formed upon generation of
the photoexcited state, but only achieved a final TONNi of 64
with a lifetime of 4 hours. This limited stability is assigned to
the degradation of NiP during catalytic turnover and/or
decomposition through unwanted products generated by the
overall reaction scheme—in this case the oxidation products
of EDTA.
Figure 1. Representation of solar H2 production using CQD-NiP. The
electron-donor systems, TCEP/AA or EDTA, quench the photoinduced
vacancies in the CQDs. EDTA forms radicals upon oxidation and
causes decomposition of the catalyst and cessation of H2 evolution.
DHA recycling by TCEP prevents the formation of large quantities of
reactive oxidation products. Two bromide anions in NiP are omitted
for clarity.
Sacrificial electron donors (and acceptors) are extensively
used to isolate half-reactions in photocatalytic processes
because it is challenging to couple oxidative and reductive
catalysts in
a full photoredox cycle under the same
conditions.[14] Triethanolamine (TEOA), triethylamine
(TEA), and EDTA are commonly used sacrificial electron
donors, but they undergo one-electron oxidation reactions
that result in potentially destructive radical species. Ascorbic
acid (AA) is a common proton and electron donor, but its
oxidation product, dehydroascorbic acid (DHA), is known to
self-inhibit the electron donor ability of AA.[15] As an
effectively unlimited resource, H2O is often considered the
ideal donor molecule. However, upon oxidation it can
produce intermediates such as the reactive oxygen species
OHC and H2O2, and even the final product O2 can be damaging
to the components of the reductive half-reaction.[16] NiP was
shown to be irreversibly inhibited by O2 through oxidation of
[*] B. C. M. Martindale, Dr. E. Reisner
Christian Doppler Laboratory for Sustainable SynGas Chemistry,
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: reisner@ch.cam.ac.uk
Homepage: http://www-reisner.ch.cam.ac.uk/
E. Joliat, Dr. C. Bachmann, Prof. Dr. R. Alberto
Department of Chemistry, University of Zꢀrich
Winterthurerstrasse 190, 8057 Zꢀrich (Switzerland)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201604355.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

Angewandte
Communications
Chemie
its phosphine ligands.[16–18] Furthermore, even in cases where
O2 is not detrimental to the system, it would rarely be
considered a commercially useful product and requires
separation from mixtures of gaseous products.
systems,[17,22,25] whereas the majority of Ni-P2N2 complexes are
active only in acidic organic solutions.[26,27] NiP has a low
overpotential (h ꢀ 200 mV) for H2 evolution, which is a
significant advantage when using photosensitizers with an
excited state potential closely matched to the thermodynamic
potential for proton reduction. When NiP is used in con-
junction with a molecular ruthenium dye (E(RuP/RuPÀ) =
À1.08 V vs. NHE), a high initial H2 evolution rate (TOFNi of
460 hÀ1) but low stability (final TONNi of 723) is observed.[22]
Photocatalytic systems comprised of CQDs (10 mg) as the
photosensitizer, NiP (10 nmol) as the catalyst, and the
regenerating donor TCEP/AA, were assembled and placed
A suitable electron donor for a photocatalytic system
should generate an oxidized product that is stable and
innocent, causing no adverse effects on the components of
the system. A donor system consisting of AA (proton/
electron relay) and tris(carboxyethyl)phosphine (TCEP)
was previously shown to extend the lifetime of a solar H2
production system using molecular ruthenium and rhenium
dyes, improving performance by two- to three-times by
preventing electron back-transfer from the photosensitizer
to DHA.[19] Herein, we employ the TCEP/AA donor system
with CQDs to avoid the formation of intermediates or
products that are destructive to the molecular catalyst
(Figure 1). Photocatalytic H2 generation has been studied
with CQDs and TCEP/AA in combination with NiP and
a series of molecular cobalt catalysts, and a benchmark
performance for the CQD-NiP photosystem is reported.
Carboxylate-terminated amorphous CQDs (7 nm
diameter), NiP (Figure 1) and the cobalt catalysts 1–3
(Figure 2) were synthesized according to established
procedures.[13,20–24] The water-soluble DuBois-type catalyst
NiP is active in aqueous electrocatalytic and photocatalytic
under simulated solar light irradiation (100 mWcmÀ2
,
AM1.5G; Figure 3). The results obtained with these
components indicate that the CQD-NiP photocatalyst
system is remarkably more stable in the presence of the
TCEP/AA donor than in EDTA, which can be attributed to
clean formation of the stable product tris(carboxyethyl)phos-
phine oxide (TCEPO); TCEPO does not act to degrade NiP,
in contrast to the destructive radicals formed during EDTA
oxidation (Supporting Information, Figure S1).[28]
The maximum catalytic rates (expressed as TOFNi of 41
and 53 hÀ1 for EDTA and TCEP/AA, respectively) are
similar for both systems (see inset of Figure 1), but activity
decreases after the first hour in EDTA, leading to a final
TONNi of 64 Æ 4. By comparison, the TCEP/AA system
continues almost linearly for around 24 hours, giving rise to
a TONNi of 1094 Æ 61 (a 17-fold increase). This is a record
TON for NiP,[13,22,25] and indeed any water-soluble DuBois-
type catalyst,[29] achieved by replacing the most expensive
component of the previous benchmark system, the ruthenium
dye,[22] with the low-cost, scalable and photostable CQDs.
This CQD-NiP photosystem also achieves the highest turn-
over with a defined synthetic molecular nickel catalyst in
purely aqueous solution, and is competitive with the best-
performing systems in molecular photocatalysis, which make
Figure 2. Chemical structures of the molecular cobalt catalysts 1, 2,
and 3.
Figure 3. a) Photocatalytic H2 generation using CQD (10 mg) and NiP (10 nmol) in an aqueous solution containing TCEP/AA (0.1m each, pH 5,
3 mL, solid traces) or EDTA (0.1m, pH 6, 3 mL, dashed traces) under 1 sun UV/Vis (l>300 nm, black traces) and visible-only (l>400 nm, green
traces) irradiation. The inset shows the first 4 hours of activity. b) Time-resolved H2 and TCEPO formation using CQD (10 mg) and NiP (10 nmol)
with TCEP/AA and c) H2 generation using CQD (10 mg) and NiP (10 and 100 nmol) with TCEP/AA (0.1m each, pH 5, 3 mL) under 1 sun UV/Vis
irradiation. All experiments thermoregulated at 258C, under an N2 atmosphere (containing 2% CH4) and irradiated with simulated solar light
(100 mWcm–2, AM1.5G).
2
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
use of toxic, expensive, and fragile photosensitizers, as well as
stable, with linear performance for the first 2 days (TONNi
organic co-solvents.[30–34] Although an impressive TONNi was
previously reported in a photocatalytic system using thiol-
capped CdSe QDs and nickel ions, this system utilizes toxic
components and does not make use of a well-defined
molecular catalyst.[35]
825 Æ 183 after 3 days). A decreased H2 evolution rate
(TOFNi) was observed with visible-only irradiation compared
to UV/Vis irradiation, which is due to decreased light
absorption (Figure 3a; Supporting Information, Figure S7).
Other possible degradation pathways for NiP are degra-
dation of the ligand framework during catalytic turnover, or
quenching of holes in the CQD excited state by NiP. As
described above, the latter pathway is viable because CQDs
are able to oxidize the phosphine TCEP under irradiation
(Supporting Information, Figure S2). We infer that the
phosphine ligands of NiP can be oxidized in a similar way,
by holes in the presence of water, albeit more slowly
(Supporting Information, Figure S5). Oxidized NiP has also
previously been formed by O2 in solution, and was shown to
be inactive as an H2 evolving catalyst.[16–18] Nevertheless,
decomposition of NiP by radical oxidation products in EDTA
is the dominant pathway (Figure 1). Hence, the overall
lifetime of the CQD-NiP photosystem is vastly increased
when using the TCEP/AA donor system, which has a stable
TCEPO oxidation product.
Control experiments using either TCEP (0.1m, pH 5) or
AA (0.1m, pH 5) showed much lower photoactivity (Support-
ing Information, Figure S2). With AA the system stability was
low (4 hours), in agreement with previous reports describing
re-reduction of DHA into AA by the photosensitizer,
effectively creating a short circuit in the system (TOFNi
20 hÀ1, TONNi 50). When only TCEP was used the stability
was longer (> 12 hours) but the initial rate of activity was
lower (TOFNi 10 hÀ1) and a TONNi of only 143 was obtained.
This observation suggests that although the direct photo-
oxidation of TCEP by CQD is possible, its kinetics are slower
than AA oxidation, and the primary quenching of photo-
induced holes is mediated by AA, followed by subsequent
irreversible reduction of DHA back to AA by TCEP
(Figure 1).[19]
Proof that TCEP is the ultimate source of electrons in the
photocatalytic system comes from quantitative analysis of the
product TCEPO using 31P NMR spectroscopy. Time-resolved
measurements confirmed a 1:1 ratio of H2:TCEPO through-
out the photocatalytic reaction (Figure 3b). These results also
confirmed the presence of TCEPO as the only detectable
product of TCEP oxidation, in agreement with the quantita-
tive and irreversible formation of TCEPO and the absence of
radical breakdown products (Supporting Information,
Figure S3). Thus, the products of reduction (H2) and oxidation
(TCEPO) can accumulate over prolonged periods of time in
this closed photosystem in the gas and solution phases,
respectively. The absence of apparent quenching of compen-
sating half-reactions, and clean product separation, is
remarkable and emphasizes the benefit of organic substrate
oxidation rather than water oxidation in a single compart-
ment. Classical water splitting would result in O2 generation,
which gives rise to product separation issues and interference
with the reductive half-reaction.
A long lifetime of approximately 1 day is observed for this
system with a low catalyst loading (10 nmol). We sub-
sequently studied the stability of NiP in TCEP/AA solution
by UV/Vis spectroscopy; in the dark, under visible light, and
under UV/Vis solar irradiation (Supporting Information,
Figure S4). The absorption spectrum of NiP shows two
bands: a weak band at 499 nm, characteristic of square-
planar complexes and a stronger charge transfer band below
350 nm.[36,37] Monitoring the peak at 499 nm reveals negligible
loss of NiP in the dark or with visible light irradiation (l >
400 nm), but under UV/Vis solar irradiation (l > 300 nm)
there is a 17% reduction in the NiP signal after 24 hours
(Supporting Information, Figure S5), which is presumably
due to ligand displacement from the metal center. The ligand
substituted Ni2+ in TCEP/AA solution is not an active
catalyst, as demonstrated by control experiments using
NiCl2 under these conditions (Supporting Information,
Figure S6). As a result of the higher NiP stability under
visible-only irradiation the H2-photosystem was also more
When using a 10-fold increased loading of NiP
(100 nmol), the amount of H2 produced during the first
24 hours is the same as that obtained at a lower loading,
indicating that the optimal loading has been reached for the
concentration of CQD and light intensity used (Figure 3c).
However, an increased amount of NiP resulted in almost
linear H2 evolution over days, and the system was still active
when the experiment was halted after 5 days. These observa-
tions clearly demonstrate the long-lived stability of CQDs as
light absorbing components in such photosystems; they offer
significant advantages over organic, and some precious metal
based molecular dyes, which have poor photostability under
solar irradiation.[22,38]
Complexes 1, 2, and 3 are established cobalt H2 evolving
catalysts bearing polydentate sp2-nitrogen, pentapyridyl,
pyrphyrin, and diimine–dioxime ligands, respectively
(Figure 2). Previously, only molecular rhenium and
ruthenium dyes with a high excited state potential were
employed as photosensitizers for compounds 1 and 2 because
of the large overpotential of cobalt polypyridyl complexes
(h ꢁ 800 mV).[39] However, the increased stability of these
ligand frameworks leads to long-term activity over a period of
days with a final TONCo of 33300 and 21900, respectively.[19,24]
Catalyst 3 has a lower overpotential (h ꢀ 400 mV) than 1 and
2,[40] but displays a lower catalytic rate.[41] Photocatalytic
systems involving catalyst 3 previously showed a lower
maximum TONCo of 90.[15,21] These three cobalt cataysts
were employed to establish CQDs more widely as a general
photosensitizer for molecular catalysts other than NiP, and to
examine the relationship between CQD and catalyst over-
potential.
Analogous photocatalytic systems comprised of a CQD
photosensitizer and a cobalt catalyst from the aforementioned
series were tested for solar H2 production in the TCEP/AA
donor system (Figure 4). All the systems performed with
a significantly lower rate of H2 production than those using
the NiP catalyst; the highest rate was observed using
1 (TOFCo of 8 hÀ1). Complex 2, while slower, has greater
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

Angewandte
Communications
Chemie
established an important selection criterion for catalytic
function with low-cost and scalable CQDs.
In summary, we report a photocatalyst, CQD-NiP, which
benefits from the donor recycling system TCEP/AA. This
photosystem exhibits a benchmark photostability of over
5 days and a record TONNi for photocatalytic H2 evolution of
1094 Æ 61, using a defined synthetic molecular nickel catalyst
in purely aqueous solution. Several cobalt catalysts were also
successfully photosensitized using CQDs, demonstrating that
CQDs can be widely used in conjunction with molecular
catalysts for H2 evolution (and likely other fuel-forming
reactions). This study revealed that the presented CQDs are
ideally suited to sensitize low overpotential (h < 500 mV)
catalysts and activities can be achieved that are comparable or
even higher than those with precious metal-based molecular
photosensitizers.
The use of the TCEP/AA donor system was the key to
unlock this great potential. The clean two-electron oxidation
of TCEP into TCEPO circumvented inhibition of the system
and prevented the rapid radical-mediated decomposition of
the catalyst, which is usually observed with conventional
sacrificial electron donors. This concept can be extended to
incorporate valuable clean synthetic transformations in place
of phosphine-to-phosphine oxide conversion, which will
enable coupling of solar fuel with solar chemical synthesis.
Future adaptation of the photosystem into a flow system, by
integration of the CQDs in a flow reactor, will lead to efficient
separation of gaseous and solution phase products.
Figure 4. H2 generation using CQD (10 mg) and various cobalt-
containing molecular catalysts (10 nmol) in an aqueous solution
containing TCEP/AA (0.1m each, pH 5, 3 mL) under 1 sun UV/Vis
irradiation (AM1.5G) under an N2 atmosphere (containing 2% CH4) at
258C. The structures of the catalysts 1, 2, and 3 are shown in Figure 2.
stability and some activity persists even after 4 days irradi-
ation, leading to similar final TONCo values in both cases
(141 Æ 13 and 120 Æ 11 for 1 and 2, respectively). These
systems display lower performance compared to the same
catalysts using precious metal-based molecular dyes, indicat-
ing that high driving force photosensitizers are required to
gain the best performance out of these high overpotential
molecular cobalt catalysts. Complex 3 is shown to be an
inferior catalyst with both a low initial rate and stability that
only lasts about 24 hours (TONCo 56 Æ 12). However, this
overall performance is broadly similar to that observed using
high driving force rhenium dyes because of the lower
overpotential requirement of this type of catalyst. The
molecular catalytic mechanism of the cobalt-based catalysts
was confirmed by control experiments with either CoCl2 or in
the absence of catalyst; negligible H2 was evolved in both
cases (Supporting Information, Figure S6).
Cyclic voltammetry of the soluble CQDs in aqueous
solution showed a wave centered at around À550 mV (vs.
RHE), which is assigned to the potential of the CQD/CQDÀ
couple (Supporting Information, Figure S8).[42] It is likely that
there is a heterogeneous distribution of surface chromo-
phores on the CQD, therefore this half-wave gives a rough
estimate of the average redox potential and the thermo-
dynamic driving force of a photoexcited electron in the CQD,
which follows the expected reductive quenching of the charge
separated excited state by the electron donor.[19] An excited
CQD (estimated average h ꢀ 550 mV for proton reduction)
gives ample driving force for NiP (h ꢀ 200 mV) and 3 (h
ꢀ 400 mV) to produce H2, but limited driving force for the
stable cobalt-based catalysts 1 and 2 (h > 800 mV). Hence,
catalysts with an overpotential lower than that determined for
the excited CQD (such as NiP and catalyst 3) display
excellent performance, which matches or even exceeds that
of ruthenium/rhenium dyes. On the other hand, the cobalt
catalysts 1 and 2 require a higher overpotential for catalysis
and the catalytic rates are therefore slow. Thus, we have
Acknowledgements
This work was financially supported by an Oppenheimer
Ph.D. scholarship, the Christian Doppler Research Associa-
tion (Austrian Federal Ministry of Science, Research and
Economy, and the National Foundation for Research, Tech-
nology and Development), the OMV Group, the Swiss
National Science Foundation Sinergia Program CRSII2-
136205/1 and the University Research Priority Program
(URPP) LightChEC. Georgina A. M. Hutton, Prof.
Christine A. Caputo and Janina Willkomm are gratefully
acknowledged for helpful discussions. The boron-doped
diamond electrode was made by David W. Wakerley using
facilities at the Warwick Electrochemistry and Interfaces
Group.
Keywords: carbon quantum dots · hydrogen evolution ·
molecular catalysis · photocatalysis · photosensitizers
[1] L. Cao, X. Wang, M. J. Meziani, F. Lu, H. Wang, P. G. Luo, Y.
Lin, B. A. Harruff, L. M. Veca, D. Murray, S.-Y. Xie, Y.-P. Sun, J.
Am. Chem. Soc. 2007, 129, 11318 – 11319.
[2] C. Ding, A. Zhu, Y. Tian, Acc. Chem. Res. 2014, 47, 20 – 30.
[3] F. Wang, Y. Chen, C. Liu, D. Ma, Chem. Commun. 2011, 47,
3502 – 3504.
[4] C. Yu, X. Li, F. Zeng, F. Zheng, S. Wu, Chem. Commun. 2013, 49,
403 – 405.
4
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
[5] H. M. R. GonÅalves, A. J. Duarte, J. C. G. Esteves da Silva,
[25] C. A. Caputo, M. A. Gross, V. W. Lau, C. Cavazza, B. V. Lotsch,
Biosens. Bioelectron. 2010, 26, 1302 – 1306.
[6] P. Mirtchev, E. J. Henderson, N. Soheilnia, C. M. Yip, G. A. Ozin,
J. Mater. Chem. 2012, 22, 1265 – 1269.
[7] J. Briscoe, A. Marinovic, M. Sevilla, S. Dunn, M. Titirici, Angew.
Chem. Int. Ed. 2015, 54, 4463 – 4468; Angew. Chem. 2015, 127,
4544 – 4550.
E. Reisner, Angew. Chem. Int. Ed. 2014, 53, 11538 – 11542;
Angew. Chem. 2014, 126, 11722 – 11726.
[26] A. D. Wilson, R. H. Newell, M. J. McNevin, J. T. Muckerman, M.
Rakowski DuBois, D. L. DuBois, J. Am. Chem. Soc. 2006, 128,
358 – 366.
[27] U. J. Kilgore, M. P. Stewart, M. L. Helm, W. G. Dougherty, W. S.
Kassel, M. Rakowski DuBois, D. L. DuBois, R. M. Bullock,
Inorg. Chem. 2011, 50, 10908 – 10918.
[8] D. Tang, H. Zhang, H. Huang, R. Liu, Y. Han, Y. Liu, C. Tong, Z.
Kang, Dalton Trans. 2013, 42, 6285 – 6289.
[9] H. Yu, Y. Zhao, C. Zhou, L. Shang, Y. Peng, Y. Cao, L.-Z. Wu,
C.-H. Tung, T. Zhang, J. Mater. Chem. A 2014, 2, 3344 – 3351.
[10] W. Wu, L. Zhan, K. Ohkubo, Y. Yamada, M. Wu, S. Fukuzumi, J.
Photochem. Photobiol. B 2015, 152, 63 – 70.
[11] L. Cao, S. Sahu, P. Anilkumar, C. E. Bunker, J. Xu, K. A. S.
Fernando, P. Wang, E. A. Guliants, K. N. Tackett, Y.-P. Sun, J.
Am. Chem. Soc. 2011, 133, 4754 – 4757.
[12] S. Sahu, Y. Liu, P. Wang, C. E. Bunker, K. A. S. Fernando, W. K.
Lewis, E. A. Guliants, F. Yang, J. Wang, Y.-P. Sun, Langmuir
2014, 30, 8631 – 8636.
[13] B. C. M. Martindale, G. A. M. Hutton, C. A. Caputo, E. Reisner,
J. Am. Chem. Soc. 2015, 137, 6018 – 6025.
[28] H.-S. Chang, G. V. Korshin, J. F. Ferguson, Environ. Sci. Technol.
2006, 40, 5089 – 5094.
[29] A. S. Weingarten, R. V. Kazantsev, L. C. Palmer, M. McClendon,
A. R. Koltonow, A. P. S. Samuel, D. J. Kiebala, M. R. Wasielew-
ski, S. I. Stupp, Nat. Chem. 2014, 6, 964 – 970.
[30] H. Rao, Z.-Y. Wang, H.-Q. Zheng, X.-B. Wang, C.-M. Pan, Y.-T.
Fan, H.-W. Hou, Catal. Sci. Technol. 2015, 5, 2332 – 2339.
[31] M. P. McLaughlin, T. M. McCormick, R. Eisenberg, P. L. Hol-
land, Chem. Commun. 2011, 47, 7989 – 7991.
[32] Z. Han, W. R. McNamara, M.-S. Eum, P. L. Holland, R.
Eisenberg, Angew. Chem. Int. Ed. 2012, 51, 1667 – 1670;
Angew. Chem. 2012, 124, 1699 – 1702.
[14] A. Harriman, Philos. Trans. R. Soc. London, Ser. A 2013, 371,
20110415.
[15] M. Guttentag, A. Rodenberg, R. Kopelent, B. Probst, C.
Buchwalder, M. Brandstꢀtter, P. Hamm, R. Alberto, Eur. J.
Inorg. Chem. 2012, 59 – 64.
[33] L. Chen, G. Chen, C.-F. Leung, S.-M. Yiu, C.-C. Ko, E.
Anxolabꢁhꢂre-Mallart, M. Robert, T.-C. Lau, ACS Catal. 2015,
5, 356 – 364.
[34] Y. Xu, X. Yin, Y. Huang, P. Du, B. Zhang, Chem. Eur. J. 2015, 21,
4571 – 4575.
[16] D. W. Wakerley, E. Reisner, Energy Environ. Sci. 2015, 8, 2283 –
2295.
[35] Z. Han, F. Qiu, R. Eisenberg, P. L. Holland, T. D. Krauss, Science
2012, 338, 1321 – 1324.
[17] D. W. Wakerley, M. A. Gross, E. Reisner, Chem. Commun. 2014,
50, 15995 – 15998.
[18] J. Y. Yang, R. M. Bullock, W. G. Dougherty, W. S. Kassel, B.
Twamley, D. L. DuBois, M. Rakowski DuBois, Dalton Trans.
2010, 39, 3001 – 3010.
[19] C. Bachmann, B. Probst, M. Guttentag, R. Alberto, Chem.
Commun. 2014, 50, 6737 – 6739.
[20] C. X. Guo, D. Zhao, Q. Zhao, P. Wang, X. Lu, Chem. Commun.
2014, 50, 7318 – 7321.
[36] D. Nakane, S. Kuwasako, M. Tsuge, M. Kubo, Y. Funahashi, T.
Ozawa, T. Ogura, H. Masuda, Chem. Commun. 2010, 46, 2142 –
2144.
[37] G. J. Leigh, D. M. P. Mingos, J. Chem. Soc. A 1970, 587 – 589.
[38] M. Yin, S. Ma, C. Wu, Y. Fan, RSC Adv. 2015, 5, 1852 – 1858.
[39] N. Queyriaux, R. T. Jane, J. Massin, V. Artero, M. Chavarot-
Kerlidou, Coord. Chem. Rev. 2015, 304 – 305, 3 – 19.
[40] P.-A. Jacques, V. Artero, J. Pꢁcaut, M. Fontecave, Proc. Natl.
Acad. Sci. USA 2009, 106, 20627 – 20632.
[21] B. Probst, M. Guttentag, A. Rodenberg, P. Hamm, R. Alberto,
Inorg. Chem. 2011, 50, 3404 – 3412.
[41] V. Artero, J.-M. Saveant, Energy Environ. Sci. 2014, 7, 3808 –
3814.
[22] M. A. Gross, A. Reynal, J. R. Durrant, E. Reisner, J. Am. Chem.
Soc. 2014, 136, 356 – 366.
[42] L. Hutton, M. E. Newton, P. R. Unwin, J. V. Macpherson, Anal.
Chem. 2009, 81, 1023 – 1032.
[23] C. Bachmann, M. Guttentag, B. Spingler, R. Alberto, Inorg.
Chem. 2013, 52, 6055 – 6061.
[24] E. Joliat, S. Schnidrig, B. Probst, C. Bachmann, B. Spingler, K. K.
Baldridge, F. von Rohr, A. Schilling, R. Alberto, Dalton Trans.
2016, 45, 1737 – 1745.
Received: May 4, 2016
Revised: May 20, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Carbon dots
Re-cycling carbon dots: A recyclable
ascorbate donor system coupled to clean
two-electron phosphine oxidation cir-
cumvents the radical-mediated decom-
position of molecular catalysts that limit
sacrificial photocatalytic systems. When
paired with a carbon quantum dot light
absorber and DuBois-type nickel catalyst,
the resultant hybrid photosystem sus-
tained H2 evolution for 5 days and
reached a record TONNi of 1094Æ61 in
aqueous solution under AM1.5G solar
irradiation.
B. C. M. Martindale, E. Joliat,
C. Bachmann, R. Alberto,
E. Reisner*
&&&& — &&&&
Clean Donor Oxidation Enhances H2
Evolution Activity of a Carbon Quantum
Dot–Molecular Catalyst Photosystem
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!