Efficient Hole Trapping in Carbon Dot/Oxygen-Modified Carbon Nitride Heterojunction Photocatalysts for Enhanced Methanol Production from CO2 under Neutral Conditions

Article abstract of DOI:10.1002/anie.202105570

Artificial photosynthesis of alcohols from CO₂ is still unsatisfactory owing to the rapid charge relaxation compared to the sluggish photoreactions and the oxidation of alcohol products. Here, we demonstrate that CO₂ is reduced to me

Full text of DOI:10.1002/anie.202105570

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Efficient Hole Trapping in Carbon Dot/Oxygen-Modified Carbon
Nitride Heterojunction Photocatalysts for Enhanced Methanol
Production from CO2 under Neutral Conditions
Authors: Yiou Wang, Robert Godin, James Durrant, and Junwang
Tang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202105570
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10.1002/anie.202105570
Angewandte Chemie International Edition
RESEARCH ARTICLE
Efficient Hole Trapping in Carbon Dot/Oxygen-Modified Carbon
Nitride Heterojunction Photocatalysts for Enhanced Methanol
Production from CO2 under Neutral Conditions
Yiou Wang[a], [d], Robert Godin[b], [c]*, James R. Durrant[b] and Junwang Tang[a]*
[a]
Dr. Y. Wang, Prof. Dr. J. Tang,
Department of Chemical Engineering, UCL, Torrington Place, London, WC1E 7JE, UK. Email: junwang.tang@ucl.ac.uk.
Dr. R. Godin, Prof. Dr. J. R. Durrant,
[b]
Department of Chemistry and Center for Plastic Electronics, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
Dr. R. Godin, Department of Chemistry, The University of British Columbia, Kelowna, BC, V1V 1V7, Canada. Email: robert.godin@ubc.ca.
Dr. Y. Wang, Chair for Photonics and Optoelectronics, Nano-Institute Munich, Ludwig-Maximilians-Universität München, Königinstr. 10, 80539, Munich,
Germany
[c]
[d]
Supporting information for this article is given via a link at the end of the document.
Abstract: Artificial photosynthesis of alcohols from CO2 is
a
recombination and enhance the light absorption via the novel
design of materials.[9]
The engineering of the linker/terminal groups is believed to be
promising route to provide sustainable fuels. The performance is still
unsatisfactory mainly due to the rapid charge relaxation compared to
the sluggish photoreactions and the oxidation of alcohol products.
Here, we demonstrate that CO2 is reduced to methanol with 100%
selectivity using water as the only electron donor on a carbon nitride-
like polymer (FAT) decorated with carbon dots. The quantum
efficiency of 5.9% (λ = 420 nm) is 300% higher than the previously
reported carbon nitride junction. Using transient absorption
spectroscopy, we observed that holes in FAT could be extracted by
the carbon dots with nearly 75% efficiency before they become
unreactive by trapping. Extraction of holes resulted in a greater
density of photoelectrons, indicative of reduced recombination of
shorter-lived reactive electrons. This work offers a unique strategy to
promote photocatalysis by increasing the amount of reactive
photogenerated charges via structure engineering and extraction
before energy losses by deep trapping.
a promising route to improve the efficiency of polymeric
10]
photocatalysis.[7b,
As previously reported, nitrogen
linkers/terminals can be replaced by oxygen in CN.[7b] Thus, the
band positions, bandgaps, and hydrophilicity of heptazine-based
semiconductors can be easily tuned to stepwise enhance the
charge separation, visible-light absorption, and the contact with
water. The superior properties resulted in an enhancement of
~20 times in hydrogen and oxygen production from water
compared with CN.[10d, 11] The lifetime of photoelectrons is more
demanding in the reduction of CO2 to methanol, a six-electron
process, compared to a two-electron process of H2 production in
water splitting. Interestingly, the carbon-dot (CD) as a selective
hole acceptor was very recently reported by us, resulting in
photoelectrons with an extended lifetime.[6a]
Herein, using transient absorption spectroscopy (TAS), we
distinguish the nature of charge trapping in a linker-controlled
polymer
(FAT)
synthesised
from
formic-acid-treated
Introduction
dicyandiamide from that of pristine CN. We find that CN mainly
traps electrons while FAT mainly traps holes based on the
amplitudes of the deconvoluted components. Coupling this FAT
polymer with efficient hole-accepting CD led to selective CO2
reduction to methanol by water under neutral conditions. O2 is
Natural photosynthesis provides sustainable energy for life
and maintains an ecological balance on the planet by fixing CO2
into organics.[1] Continuous efforts have been put into economic
artificial photosynthesis to convert CO2 using inexhaustible solar
energy.[2] The efficiency of CO2 reduction has been improved
through different routes over the last decades, including the
the only oxidation product and is produced in
a near
stoichiometry ratio. The reaction proceeds with nearly 6%
internal quantum yield (IQY) at 420 nm, which sets a new
benchmark for metal-free systems and is superior to metal-
containing systems (Table S1).[6a] The change to fast hole
trapping in FAT is key since the CD can subsequently extract
holes on the sub-microsecond timescale. With CN, trapped
electrons accumulate, which increases the charge carrier
density and accelerates the rate of recombination, leading to a
lower activity under continuous irradiation according to our
previous report.[12] The CD/FAT system shows how selective
extraction of holes with a propensity for trapping can lead to
significant improvements in activity. This novel strategy to
promote both internal charge separation and to incorporate an
efficient hole acceptor provides an efficient approach to artificial
photosynthesis, thus paving a sustainable and clean avenue to
economic CO2 conversion and solar energy storage using metal-
free materials.
photoelectrochemical
processes,[3]
the
optimisation
of
cocatalysts[4], and the fabrication of junction photocatalysts.[5]
These processes use either expensive and unsustainable
sacrificial reagents or abundant water as electron/proton
donors.[6] The latter involves CO2 reduction by photoelectrons
coupled with water oxidation by photoholes, competing with the
typically much faster charge recombination and the reduction of
protons to H2. Therefore, it is much more challenging to couple
CO2 fixation with pure water oxidation.
The intrinsic properties of a photocatalyst are crucial for
photon-driven chemical processes. Carbon nitride (CN) is one of
the most promising metal-free photocatalysts due to its high
visible-light-driven performance for hydrogen production and
CO2 reduction.[7] However, rapid charge recombination and
limited visible light absorption due to its wide bandgap (2.7-2.9
eV) still limit its performance.[8] It is crucial to optimise the
photocatalytic activity of CN by reducing such severe charge
1
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10.1002/anie.202105570
Angewandte Chemie International Edition
RESEARCH ARTICLE
Results and Discussion
of the interplanar stacking distance has been linked to increased
mobility in CN[14] and may contribute to the enhanced activity
(vide infra).
Treating dicyandiamide by formic acid creates an oxygen-
containing precursor[6a, 13], which is then heated to produce FAT
with oxygen- and nitrogen-linked heptazine-domains (Figure 1a
Raman spectroscopy was used to examine the backbone of
materials (Figure 1d). Characteristic vibrations of the heptazine-
based 2D structure have been found on FAT, CN, CD/FAT and
CD/CN, with the peaks in the regions of 1200-1700 cm-1, 980
cm-1 and 690 cm-1 corresponding to the disordered graphitic
carbon-nitrogen vibrations, the symmetric N-breathing mode of
heptazine and the in-plane bending, respectively.[15] For CD
synthesised at different microwave powers, CD200 and CD250
could not show distinct graphitic signals. Only CD300/FAT has a
clear graphitic structure of D-band around 1350 cm-1 and G-
band around 1600 cm-1, implying the CD300 still maintains a
relatively integrated sp2 structure of graphene even after forming
the heterojunction.
and 1b).[13] CN, as
a reference, was synthesised from
dicyandiamide. The efficient CD cocatalyst was prepared from
citric acid and urea via a microwave-assisted method.[6a] The CD
samples synthesised under different synthetic microwave
powers are denoted as CDx, where x stands for the power in W.
When not specified, CD refers to CD300. The CD was loaded on
FAT and CN by suspending CD with the polymer in 10 mL DMF,
drying and annealing the samples again at 500oC for four hours.
Detailed preparation procedures and other characterisation
methods are listed in the Electronic Supplementary Information
(ESI).
Fourier-transform infrared spectroscopy (FTIR) was carried
out to identify the assignment of the proposed structure (Figure
1e). FAT shows similar trends to CN except that signals related
to C-NHx at 1200 cm-1 and 1450 cm-1 are weaker, and the peaks
of NHx around 3000-3300 cm-1 change to a broad band of O.[10d]
Besides C-C vibrations at 1200 cm-1 and 1450 cm-1, the CD also
displays weak signals around 1250-1550 cm-1 of C-N vibrations
a
b
Red=O
Blue=N
Grey=C
White=H
and
a
peak of C=O around 1730 cm-1, suggesting the
CN
FAT
incorporation of N and O atoms. As for CD/FAT and CD/CN, one
can still see the less pronounced C-N vibrations of CD (1250-
1550 cm-1), which are different from heptazine characters,
confirming the co-existence of CD and polymers. C=O peak in
CD disappeared in the junctions likely due to their reaction
during the annealing. The HRTEM image of CD in CD/FAT
nanocomposite (Figure S1) further confirms the coexistence of
CD and the polymer, and the CD in our study is crystallised
graphite phase, consistent with Raman spectra in Figure 1d.
One can observe the regions of fringes with a characteristic d-
spacing of 0.32 nm across the sample, which agrees with the
(002) values calculated from XRD patterns of CD/FAT. Based on
the previous observation,12 the crystal structure of CN was
relatively more sensitive to the electron beam irradiation and
was therefore damaged after a short time observation, while that
of CD remained intact. Such regions of graphitic fringes are thus
attributed to CD.
According to UV-vis spectroscopy, the brownish CD powder
(Figure 1f and inset) absorbs light from UV to the near IR. X-ray
photoelectron spectroscopy (XPS) was used to identify different
elements (C, N, O) and the chemical states in CD, FAT, CN and
their junctions (Figure S2-S3, Table S2-S4). XPS survey
spectrum of CD shows its surface chemical composition of at.
11.86% N, 61.99% C, and 26.15% O. Peaks of C-NHx, C-OH,
and C=O detected in CD confirm a large portion of the N/O
dopants are located outside the graphitic framework and are
present as surface groups, correlating with the proposed sp2
graphene structure from Raman spectroscopy.[16] Obvious C=O
signals are only observed in CD, which agrees with the FT-IR
spectrum. The compositions of C and N states are similar to
those previously reported.[10d] The O 1s XPS spectrum indicates
that all samples have C-OH on the surface, which favours the
intimate interaction between CD and polymer layers during
junction fabrication due to hydrogen bonds and increases
hydrophilicity to promote the contact with water.[16]
Figure 1. Structure of (a) CN and (b) FAT polymers. Characterizations
of CD, CN, FAT, CD/CN and CD/FAT. (c) PXRD pattern, (d) Raman,
(e) FT-IR and (f) UV-vis spectra. Inset: image of CD, FAT and CD/FAT.
Firstly, thorough characterisations have been carried out to
investigate the structure and composition of all photocatalysts.
According to the X-ray diffraction (XRD) patterns (Figure 1c),
FAT and CN exhibit the typical diffraction signals of (002) and
(100) at 27.1o (3.29 Å), 13.3o (6.65 Å) and 27.4o (3.26 Å), 13.0o
(6.82 Å), respectively.[7b, 10d] The shifts of FAT from pristine CN
are due to more distorted structure and smaller size of unit
cells.[10d] CD shows a broadened (002) band near that of
graphene reported in the literature (26.5o, 3.36 Å), indicating a
graphene-derived structure. Surprisingly, after coupling the
polymers with CD to form junctions, the interlayer distances of
both CD/FAT and CD/CN have been compacted to 3.20 Å
(27.8o), suggesting the formed junction possesses stronger
interaction and attraction. Electronic effects might reduce the
repulsion between layers and potentially indicates some
perpendicular arrangement between CD and polymer. Reduction
After
understanding
the
structure
of
synthesised
photocatalysts, their performance for artificial photosynthesis
2
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10.1002/anie.202105570
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RESEARCH ARTICLE
2-
CO3
.
An acidic condition resulted in decreased activity,
presumably because the CO2 solubility declines sharply at low
pHs.
The internal quantum yield (IQY) was calculated to determine
the efficiency of harvesting photons to redox reactions (Figure
2c). Both junction samples show their highest IQY at 420 nm,
indicating the activity mainly comes from the blue spectral
region. CD/FAT exhibits an IQY of 5.9% at 420 nm, which is 2.8
times that of CD/CN (2.1%). At longer wavelengths of 500 and
600 nm, CD/FAT still shows activity, and it is a little higher than
CD/CN, probably due to the broadened optical window of FAT.
As CN could not harvest 500–600ꢀnm photons by itself, the
activity at longer wavelengths is due to the light absorption by
CD or CD/CN interfacial states. We further measured the
quantum efficiency under the light irradiation of a 365 nm LED
(please see ESI for details), which was determined as 18.6%,
another remarkable value for a metal-free system for CO2
conversion.
To further confirm the conversion of CO2, we have carried out
the CO2 reduction using 13C labelled CO2. The peak of
13CH3OH+ (m/zꢀ=ꢀ33) were observed at m/zꢀ=ꢀ30, 31, 32 and 33,
which were assigned to 13CH3OH+ and three fragments
produced during the MS measurement. Such signals are clearly
distinguished from those measured in 12CO2 photoconversion.
The evidence indicates that the evolved products originate from
the photoreduction of 13CO2. The stability of a photocatalyst is a
crucial factor that determines its long-term application. Three
consecutive runs have also been carried out, and the average
rates of methanol production were 1.7, 1.5 and 1.7 μmol/h for
three runs, respectively. The photocatalytic activity of the sample
did not show a decrease over 17 hours, indicating the stability
(Figure S6a). We also assessed our new junctions’ stability by
post-testing Raman characterisation (Figure S6b). The negligible
change was observed before and after reactions, which further
proves the stability.
To prove the production of oxygen from water, we also carried
out the oxygen production tests on FAT and CD/FAT in the
presence of the electron acceptor AgNO3 (Figure S6c). Pristine
FAT only produced a small amount of oxygen. We attribute the
decline in the total amount of oxygen after 1.5 hour is due to the
back reaction of oxygen reduction by electrons on Ag metal
deposited on the FAT during Ag+ reduction reaction and the
shielding effect of the metal Ag.[18] The CD/FAT could produce
oxygen at a much higher activity compared to FAT alone, in line
with improved photocatalytic activity, which is due to the
improved charge separation and catalytic effect of CD. Hence,
we confirm that FAT and CD/FAT have the thermodynamic
driving force to oxidise water and can produce oxygen in the
presence of Ag+.
Previous theoretical calculations showed that water preferred
to adsorb on CD (where holes accumulate). In contrast,
methanol favoured adsorption to CN (where electrons
accumulate), hence the localisation of the charges was
unfavourable for the undesired oxidation of methanol product.[6a]
Similarly here, we found that methanol cannot be oxidised on
CD/FAT, even in the presence of O2 gas, avoiding oxidation of
the produced methanol and providing a high selectivity to
methanol (Figure 3a and 3b). In Figure 3b, CN is shown to
oxidise methanol to CO under the light. After the loading of CD,
both the CD/CN and CD/FAT junctions do not show CO
production under the light. The fact that CD/CN no longer
oxidises methanol supports our interpretation that holes transfer
to CD and that holes on CD do not effectively oxidise methanol.
Interestingly, pristine FAT polymer does not oxidise methanol to
CO on its own, indicating that the surface of FAT does promote
methanol reaction with holes.
Figure 2. Photocatalytic CO2 conversion to methanol. The photocatalytic activity of
(a) CD/FAT measured under visible light (λ> 420 nm). (b) Control experiments on
CD, FAT, CD/CN. Inset: image of CD, FAT and CD/FAT. (c) IQY of CD/FAT and
CD/CN measured at atmospheric pressure under nearly one sun irradiation
condition. (d) Mass spectra of the product 13CH3OH from 13CO2 photoconversion
by the CD/FAT photocatalyst.
was evaluated at 1 bar condition under irradiation of 300 W
Xenon light source with a light intensity of 100 mW/cm2 (420 nm
< λ < 710 nm) to simulate practical environmental conditions.
Methanol is selectively produced from CO2 and water at a rate of
24.2 μmol▪g-1h-1 on CD/FAT at neutral pH (Figure 2a). Moreover,
the methanol: O2 product ratio of 2:2.64 is close to the expected
2:3 stoichiometry of formula 2CO2 + 4H2O → 2CH3OH + 3O2.
The reason for the smaller amount of O2 might be attributed to
some side reactions related to O2. No product of CO from
methanol oxidation (by O2 or photoholes) could be detected
(Figure 3b), indicating the surface of FAT and CD/FAT is
favourable to the selective reduction of CO2 instead of the back
reaction of methanol oxidation. In comparison, CD/CN shows
two thirds of the activity for methanol production at a rate of 13.9
μmol▪g-1▪h-1 (Figure 2b). Control experiments showed that no
product such as methanol or CO was detected under dark
conditions, with only argon as the feed gas or using solely CD or
FAT photocatalysts (Figure 2b).
Carbon dots are usually a mixture of many structures.[17] To
identify the possible functional structure of the carbon dots in our
study, we carried out the photocatalytic CO2-to-methanol on FAT
decorated with CD synthesised under different microwave
powers (CD200, CD250, CD300 for 200, 250, 300 W,
respectively; Figure 1b). We observe that the trend in activity
(Figure S5a) agrees with the degree of crystallisation in the
Raman spectra and HRTEM. It indicates that more crystallised
graphite structure is favourable to the photocatalytic
performance. The similarity of 2D structure between FAT and
CD benefits the stacking of layers as noted in the reduced layer
distance in FAT, which enhances the contact between interfaces
for charge transfer. The weight ratio between CD and FAT has
been optimised to be 10% (Figure S5b) since a higher
concentration could result in the shielding of light. The starting
pH of the system influences the state of CO2 in water. In this
study, CD/FAT junction works stably under neutral pH (Figure
S5c). No activity was detected at pH = 8.5 when CO2 forms
Photoluminescence spectroscopy (PL) was undertaken to
examine the radiative recombination of the samples (Figure S7).
3
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10.1002/anie.202105570
Angewandte Chemie International Edition
RESEARCH ARTICLE
CN exhibits two signals relative to π–π* and n–π* transitions.
The major emission peaks of FAT are shifted from 500 nm to
around 550 nm, and the 450 nm peak decreased due to
bandgap narrowing, consistent with the previous report.[10d]
Although CD shows an emission around 435 nm (Figure S7b),
the junctions did not demonstrate enhanced PL signals around
435~450 nm, suggesting that the major absorption comes from
the FAT polymer. The signal intensity of CD/FAT is one order of
magnitude lower than pure FAT, indicating an enhanced charge
separation due to the junction structure.
The TA spectra of FAT could be well reproduced using the
CN-derived electron and hole spectra (Figure S9). Spectral
deconvolution confirmed that the TA spectrum of CN is
dominated by electrons and that of FAT is dominated by holes
(Figure 4; also see Figures S10 and S11). Forming the CD/FAT
heterojunction notably reduces the intensity of the 550 nm peak
(Figure 3d). This is quantified as an average ~ 3-fold decreased
hole contribution from spectral deconvolution and corresponds
to a 75% hole transfer efficiency to CD. There is an associated
average ~1.5-fold increase in the electron contribution. Spectral
deconvolution also clearly shows a similar decrease of residual
hole signal and increase of electron signal for CD/CN compared
to CN. These changes in charge carrier populations are
consistent with the hole-accepting nature of these CD.[6a] The
hole-accepting function of CD leads to enhanced charge
separation, which will reduce the rate of recombination,
beneficial for photocatalytic activity.
b
70
60
50
40
30
20
10
0
a
e- e- e-
e-
CH3OH
CO2
CH3OH
CO
H2O
O2
h+ h+ h+
FAT
CD
The electrons monitored on the microsecond and longer
timescales are trapped in low energy states and do not transfer
to CO2 since the much more reactive Ag+ does not affect the
kinetics of the FAT electron decay (Figure S11), consistent with
observations with CN.[9a] The similar and slow recombination
timescale indicates that the observed holes are also trapped. A
lower trapped electron density has previously been linked to
improved photocatalytic efficiency for CN.[10b, 12, 20] Considering
the initial TAS signal amplitude at 3 µs, the lower electron signal
in FAT vs CN and the improved photocatalytic efficiency of FAT
are consistent with these observations and suggest that
detrimental electron trapping is much less prominent in FAT.
Instead, increased hole trapping occurs in FAT vs CN. This is in
line with the inability of FAT to oxidise methanol whereas CN
can drive this photoreaction. As observed in Figure 3b, even in
the presence of O2 gas, methanol can not be oxidised by FAT.
Taking into account the TAS analysis which shows holes are
readily trapped in FAT, it is believed that trapped holes in FAT
have a less positive oxidation potential. Therefore, due to both
the unfavourable adsorption site on FAT for methanol and the
small oxidation potential of the trapped holes in FAT, the
methanol produced on CD/FAT can not be oxidised back to
CO/CO2, resulting in high selectivity to methanol. The
detrimental effect on photocatalytic activity of hole trapping is
mitigated in the CD/FAT heterojunction since the majority of
holes are extracted to the CD on the sub-microsecond timescale
before they can populate trap states (Figure 4b).
CN CD/CN FAT CD/FAT CD/FAT (O2)
c
d
FAT
CD/FAT
1.0
1.0
5 ms
10 ms
1 ms
100 ms
10 ms
100 ms
0.5
0.0
0.5
0.0
550
750
950
550
750
950
Wavelength (nm)
Wavelength (nm)
Figure 3. (a) Scheme of CD/FAT junction. (b) Methanol oxidation test on CD,
CD/CN, FAT and CD/FAT. TAS measurements of FAT and CD/FAT. Diffuse
reflectance TAS spectra for (c) FAT and (d) CD/FAT.
To understand the origin of the superior performance of
CD/FAT photocatalyst, we investigated the electron-hole
dynamics of FAT and CD/FAT aqueous dispersion at µs-s
timescales by TAS using CD/CN as a reference. We observed
very broad TAS features, presumably due to a range of energies
for trap states.[19] In our previous study on CN, we assigned the
TAS signal near 500 nm to holes and signal > 600 nm to
electrons, supported by the amplitude increase (at 500 nm) and
decrease (at > 600 nm) in the presence of Ag+ ions as electron
scavengers (Figure S8a).[6a] Similarly, the photoinduced signal
observed in FAT around 550 nm is mainly assigned to
photogenerated holes and the broad signal observed around
700-900 nm is assigned to photogenerated electrons (Figure
3c). The strong peaking near 550 nm at early timescales (< 100
µs) is notable and indicative of a higher signal contribution from
holes. A quantitative analysis was performed by deconvoluting
the experimental spectra to extract the electron and hole signals.
The reference electron spectrum was obtained from CD/CN at
100 µs based on 1) the hole-accepting function of CD and 2) the
lack of spectral evolution on longer timescales. The reference
hole spectrum was that of CN + 10 mM AgNO3 at 100 ms based
on 1) the electron-accepting function of Ag+ ions and 2) the
decay of the electron signal > 600 nm during shorter timescales.
These reference spectra were fixed to obtain the best-fit weight
of each component that can reproduce the experimental spectra
(Figure S8a). The intensity of the TAS signals of isolated CDs is
about 2 orders of magnitude smaller than that of CN and FAT
(signals ~ 0.5%) (Figure S8b). Due to the difference in the
intensity, we could not see a TAS signal contribution from CD in
the composites and the TAS spectra of CD/CN and CD/FAT
resemble that of the CN and FAT rather than the isolated CD.
Replacing the linker and terminal N atoms in CN by O atoms
a
b
0.015
0.010
0.005
0.000
Electrons
Holes
FAT
CD/FAT
CN
CN/FAT
0.010
0.005
0.000
FAT
CD/FAT
CN
CD/CN
10-6 10-5 10-4 10-3 10-2 10-1
10-5
10-4
10-3
10-2
10-1
Time (s)
Time (s)
Figure 4. Charge carrier populations of (a) electrons and (b) holes
determined from spectral deconvolution of the TAS spectra for FAT, CD/FAT,
CN, and CD/CN samples.
in FAT influenced the dynamics of trapped charges. The half-life
time (t50%) of the electron signal observed at 800 nm increases
by eight folds from 40 µs in CN to 320 µs in FAT (Figure S12).
The FAT polymer is thought to be composed of O-rich and N-
rich domains (Figure 1b) where the electrons and holes,
10d]
respectively, tend to separate.[7b,
We thus attribute the
4
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10.1002/anie.202105570
Angewandte Chemie International Edition
RESEARCH ARTICLE
reduced rate of recombination of trapped charges in FAT to the
intrinsic spatial separation of electrons and holes and expect
that the recombination of shorter-lived reactive charges will also
be reduced.
grant (291482). J.T. thanks the Leverhulme Trust (RPG-2012-
582, RPG-2017-122), EPSRC (EP/S018204/2) and Royal
Society-Newton Advanced Fellowship grants (NA170422). We
also acknowledge Mr. Linzhong Wu for his assistance on PL
measurements and Dr. Markus Döblinger for his expertise on
HRTEM.
We do not observe an increased trapped electron lifetime in
CD/FAT compared to FAT on the timescales monitored, which is
informative considering the changes in charge carrier densities.
A four-fold increase in the electron lifetime was indeed observed
for CD/CN vs CN.[6a] While the origin of the distinct behaviour of
FAT and CN when forming a junction with CD is unclear, it is
likely that the intrinsic charge separation in FAT mitigates the
impact of CD on the recombination of trapped charges. The
increased signal of trapped electrons at 3 µs in CD/FAT vs FAT
indicates reduced recombination in the sub-microsecond
timescale, likely relevant to the reactive charges. We, therefore,
attribute the improved photocatalytic efficiency in CD/FAT vs
FAT to the interfacial transfer of reactive holes to CD prior to
deep trapping and reduced sub-microsecond charge carrier
recombination. The TAS measurements lead us to conclude that
holes are the dominant trapped species in FAT while electrons
are the dominant trapped species in CN (Figures 4 and b).
When CD extracts holes from FAT, hole accumulation in FAT is
not severe in the CD/FAT composite under irradiation and
charge recombination remains low. On the other hand, although
the CD extracts holes from CN, a higher density of electrons still
accumulate in CN when the CD/CN is irradiated and results in
higher rates of recombination compared to CD/FAT.
Keywords: Carbon dioxide fixation • Photocatalysis • Time-
resolved spectroscopy • Carbon dots • Charge Trapping
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Conclusion
In summary, for the first time, we have demonstrated a unique
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methanol by altering the terminal and linker groups in carbon
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Replacing some N atoms in CN to O atoms in FAT resulted in a
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Spectroscopic investigations also showed that that CD could
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Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
A modified carbon nitride with O-containing linkers decorated with carbon dots selectively produces methanol from CO2 and water
with the benchmark quantum yield of ca. 6%. Transient absorption spectroscopic investigation shows that extraction of holes before
trapping by the carbon dots is key for the remarkable performance.
Institute and/or researcher Twitter usernames: @solar_spec, @Tang_group_UK, @kotol12345.
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