Sub-3 nm Ultrafine Cu2O for Visible Light Driven Nitrogen Fixation

Article abstract of DOI:10.1002/anie.202013594

Cu₂O, a low-cost, visible light responsive semiconductor photocatalyst represents an ideal candidate for visible light driven photocatalytic reduction of N₂ to NH₃ from the viewpoint of thermodynamics, but it remains unexplored. Reported here is the successful synthesis of uniformly sized and ultrafine Cu₂O platelets, with a lateral size of <3 nm, by the in situ topotactic reduction of a Cu^II-containing layered double hydroxide with ascorbic acid. The supported ultrafine Cu₂O offered excellent performance and stability for the visible light driven photocatalytic reduction of N₂ to NH₃ (the Cu₂O-mass-normalized rate as high as 4.10 mmol g (Formula presented.) ^?1 h^?1 at λ>400 nm), with the origin of the high activity being long-lived photoexcited electrons in trap states, an abundance of exposed active sites, and the underlying support structure. This work guides the future design of ultrafine catalysts for NH₃ synthesis and other applications.

Full text of DOI:10.1002/anie.202013594

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Sub-3 nm Ultrafine Cu2O for Visible Light-driven Nitrogen
Fixation
Authors: Shuai Zhang, Yunxuan Zhao, Run Shi, Chao Zhou, Geoffrey
I.N. Waterhouse, Zhuan Wang, Yuxiang Weng, and Tierui
Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202013594
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10.1002/anie.202013594
Angewandte Chemie International Edition
RESEARCH ARTICLE
Sub-3 nm Ultrafine Cu2O for Visible Light-driven Nitrogen Fixation
Shuai Zhang,[a], [b] Yunxuan Zhao,*[a] Run Shi,[a] Chao Zhou,[a] Geoffrey I.N. Waterhouse,[c] Zhuan Wang,[d]
Yuxiang Weng,[d], [e], [f] and Tierui Zhang*[a], [b]
[a]
S. Zhang, Dr. Y. Zhao, Dr. R. Shi, Dr. C. Zhou, Prof. T. Zhang
Key Laboratory of Photochemical Conversion and Optoelectronic Materials
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences
Beijing, 100190 (China)
E-mail: tierui@mail.ipc.ac.cn; yunxuan@mail.ipc.ac.cn
S. Zhang, Prof. T. Zhang
Center of Materials Science and Optoelectronics Engineering
University of Chinese Academy of Sciences
Beijing, 100049 (China)
Prof. G. I.N. Waterhouse
School of Chemical Sciences
The University of Auckland
Auckland, 1142 (New Zealand)
[b]
[c]
[d]
[e]
[f]
Dr. Z. Wang, Prof. Y. Weng
Beijing National Laboratory for Condensed Matter Physics, Laboratory of Soft Matter Physics
Institute of Physics, Chinese Academy of Sciences
Beijing, 100190 (China)
Prof. Y. Weng
School of Physical Sciences
University of Chinese Academy of Sciences
Beijing, 100049 (China)
Prof. Y. Weng
Songshan Lake Materials Laboratory
Dongguan, Guangdong, 523808 (China)
Supporting information for this article is given via a link at the end of the document.
Abstract: Photocatalytic N2 fixation to NH3 with water as the reducing Introduction
agent represents a promising future strategy for ammonia synthesis,
motivating the discovery of efficient photocatalysts that offer high
sunlight utilization and catalytic efficiency to N2 fixation. Cu2O, a low-
cost, visible light-responsive semiconductor photocatalyst represents
an ideal candidate for visible light-driven photocatalytic reduction of
N2 to NH3 from the viewpoint of thermodynamics, but remaining
The catalytic reduction of nitrogen (N2) to ammonia (NH3)
represents one of the most important industrial processes in
today’s chemical industry, with the NH3 produced being an
indispensable feedstock for the synthesis of nitrogen-based
fertilizers and various commodity chemicals.[1] In recent years, the
photocatalytic N2 reduction reaction (pNRR) has emerged as an
exciting new research field, harnessing renewable solar energy
and a semiconductor photocatalyst to drive the chemical reaction
of N2 to NH3 using H2O as a proton source. Such an approach is
desirable from a green chemistry perspective relative to the
incumbent and energy-intensive Haber-Bosch process.[2]
However, NN triple bonds are extremely strong (941 kJ mol-1),
making the kinetics of pNRR too slow to demand serious industry
interest at the present time. In order to improve pNRR kinetics,
efficient N2 activation on the surface of photocatalysts and fast
transfer of energetic electrons to adsorbed N2 are demanded.[3]
Generally, the limited availability of active sites on the surface of
photocatalysts for N2 adsorption is rate-limiting.[4] Electron-hole
pair recombination in the semiconductor photocatalyst also limits
the availability of electrons and holes for N2 reduction and H2O
oxidation, respectively, on the surface of the photocatalyst.[5]
These two issues are current bottlenecks to efficient pNRR,
motivating the search for new photocatalysts or strategies for
circumventing these bottlenecks.
unexplored in this field yet. Noticeably,
a majority of Cu2O
photocatalysts synthesized to date with large lateral sizes (typically
tens to hundreds of nanometers) generally suffer from severe
electron-hole recombination and limited surface sites, restricting its
photocatalytic activity for potential N2 fixation. Ultrafine photocatalysts
with lateral dimensions ~1-3 nm offer distinct advantages over
conventional nanoparticle-based photocatalysts in this context,
though the controlled synthesis of ultrafine photocatalysts remains
challenging. Herein, we report the successful synthesis of uniformly
sized and ultrafine Cu2O platelets with lateral size < 3 nm via the in-
situ topotactic reduction of a Cu(II)-containing layered double
hydroxide with ascorbic acid. The supported ultrafine Cu2O offered
excellent performance and stability for the visible light-driven
photocatalytic reduction of N2 to NH3 (the Cu2O-mass-normalized rate
as high as 4.10 mmol gCu2O-1 h-1 at  > 400 nm), with the origin of the
high activity being long-lived photo-excited electrons in trap states, an
abundance of exposed active sites and the underlying support
structure. This work guides the future design of ultrafine catalysts for
NH3 synthesis and other applications.
1
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RESEARCH ARTICLE
Copper oxides due to their multi-electron transfer properties
show great promise in solar energy conversion and
heterogeneous photocatalysis.[6] Cuprous oxide (Cu2O) is a low-
cost and visible light-responsive material, already finding
widespread application in photoelectrochemical/photocatalytic
conversion processes.[7] Cu2O possesses appropriate conduction
and valence band level for driving N2 reduction and water
oxidation, respectively, making it a near-ideal candidate for visible
light-driven photocatalytic reduction of N2 to NH3 in pure water.
However, to our knowledge, no prior work examining the
photocatalytic performance of Cu2O for N2 reduction, motivating
an exciting attempt and a detailed investigation. It is worth noting
that although the intrinsic properties of Cu2O make it very suitable
as a NRR photocatalyst, the photocatalytic performance of Cu2O
remains suboptimal owing to the large size of most Cu2O
photocatalysts synthesized to date (typically tens to hundreds of
nanometers). Accordingly, surface active sites needed for N2
adsorption are limited, whilst bulk recombination consumes a high
percentage of photogenerated electrons and holes,[8] severely
restricting the photocatalytic activity of Cu2O for N2 fixation.
Ultrafine nanostructured photocatalysts with lateral size of less
than 10 nm offer a promising approach for addressing the
aforementioned challenges,[9] due to the ~1 order-of-magnitude
increase in exposed surface sites relative to conventional
nanoparticle photocatalysts and improved charge migration
efficiency.[10] These attributes allow very efficient conversion of
reactant molecules.[11] By analogy, the design and synthesis of
ultrafine Cu2O represent a prudent strategy for improving the
photocatalytic activity of Cu2O for pNRR.[12] Whilst ultrafine Cu2O
would potentially deliver superior pNRR activity, the direct
synthesis of ultrafine Cu2O has proved challenging to date.[13]
Discovering a reliable synthetic strategy towards ultrafine Cu2O
represents a prized research target.
ultrafine Cu2O due to the high dispersity of Cu2+ ions in the host
layers and the spatial confinement of any formed Cu2O (the latter
being especially important for which suppressing Cu2O excessive
grain growth and agglomeration which prevents the synthesis of
freestanding ultrafine Cu2O).[20] Moreover, the LDH support will
ensure that the ultrafine Cu2O generated will have good
dispersibility in the aqueous reaction medium used for pNRR.[21]
Combining intrinsic properties of Cu2O with structural advantages
of ultrafine photocatalysts, we hypothesized that supported
ultrafine Cu2O prepared by in-situ reduction of a Cu(II)-containing
LDH should offer excellent pNRR performance compared to
conventional bulk Cu2O photocatalysts with large particle sizes
and most of NRR photocatalysts reported. Experimental work
presented below validated this hypothesis.
In this work, we describe the successful synthesis of sub-3
nm ultrafine Cu2O by the in-situ topotactic transformation
(controlled reduction) of CuZnAl-LDH (denoted here as LDH)
nanosheets (Scheme 1), which represents one of the finest Cu2O
synthesized to date. The ultrafine Cu2O was obtained via a mild
reduction process at ambient temperature, which used ascorbic
acid as the reducing agent. The evolution of ultrafine Cu2O was
controlled by varying the concentration of ascorbic acid and the
reduction time. The obtained LDH-supported ultrafine Cu2O was
applied to photocatalytic reduction of N2 to NH3 for the first time,
exhibiting superior pNRR activity compared with supported bulk
Cu2O nanocrystals of larger size and most of pNRR
photocatalysts reported recently (the ultrafine Cu2O afforded a
-1
visible light-driven pNRR activity of 4.10 mmol gCu2O h-1, ~64
times higher than
a bulk Cu2O reference photocatalyst).
Advanced spectroscopic characterization studies, including time-
resolved photoluminescence spectra (TPL) and time-resolved
absorption spectroscopy (TAS) analysis, allowed the enhanced
photocatalytic performance of the sub-3 nm ultrafine Cu2O to be
rationalized in terms of the generation of long-lived photo-excited
electrons and highly exposed surface sites for N2 adsorption and
activation.
Freestanding Cu2O nanocrystals with defined shapes and
sizes have previously been prepared through the direct oxidation
of metallic Cu nanoparticles or the reduction of self-assembled
Cu(II)-containing nanomaterials.[14] However, reports of the
synthesis of well-dispersive Cu2O nanocrystals with a size below
50 nm are rare.[13a, 15] To realize ultrafine Cu2O, it is generally
Results and Discussion
necessary to prepare Cu2O by direct growth on a support,[16]
a
procedure which demands precise control of the addition of the
Cu precursor and reaction conditions. For instance, 10 nm Cu2O
particles assembled on BiOBr nanoplates were obtained using
ethylene diamine tetraacetic acid and cetyltrimethyl ammonium
bromide as capping agents. Further, some 5 nm Cu2O particles
were grown onto graphene-based supports.[17] A facile route for
the synthesis of ultrafine Cu2O with lateral size below 5 nm (even
less than 3 nm) under ambient conditions has yet to be reported.
Scheme 1. Synthetic strategy used to prepare LDH-supported sub-3 nm
ultrafine Cu2O.
Layered double hydroxide (LDH) is
a family of two-
dimensional layered clays. Over the past 5 years, LDHs have
been extensively studied as catalyst supports or catalyst
precursors owing to their tunable divalent and trivalent cation
composition and their ability to undergo topotactic transformations
with heating.[18] Cu(II)-containing ternary LDHs, incorporating
Cu(OH)6 units in the layered 2D sheets composed of MO6
octahedra, serve as tantalizing precursors for the preparation of
ultrafine Cu2O via the in-situ reduction of Cu(II) species using mild
reducing agents.[19] Here, only Cu2+ cations in the host layers are
reduced, whilst the remaining divalent and trivalent metal cations
are unaffected thus preserving the basic LDH structure. We
postulated that through this strategy it should be possible to obtain
A one-step in-situ reduction approach, using ascorbic acid as
the reducing agent, was applied to the synthesis of supported
ultrafine Cu2O from ternary metal LDH precursor. The morphology
of the partially reduced sample (denoted herein as u-Cu2O-
0.05M-2h) was first investigated. High-resolution transmission
electron microscopy (HRTEM) showed the uniform dispersion of
ultrafine Cu2O particles (black dots) with an average size of 2.22
nm on the LDH nanosheets (Figure 1a-c), which was one of the
finest Cu2O synthesized to date. Closer observation with HRTEM
(Figure 1b) revealed lattice fringes with a d-spacing of 0.246 nm,
which could readily be assigned to the (111) plane of cubic-phase
2
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10.1002/anie.202013594
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RESEARCH ARTICLE
Cu2O, in good agreement with the X-ray diffraction (XRD) patterns
for the same sample (Figure S1a-b). Results indicate the
successful synthesis of ultrafine Cu2O with the general
preservation of the LDH structure after the mild ascorbic acid
reduction process. Atomic force microscopy (AFM) (Figure 1d-e)
revealed that the u-Cu2O-0.05M-2h possessed a relatively
uniform thickness (~4.83 nm), with energy-dispersive X-ray
0.01M-2h (1.99 nm), with the corresponding size distributions
shown in Figure 1c and Figure S3a-b. To better underline the
advantages of ultrafine Cu2O, bulk Cu2O with an average size of
61.01 nm and bulk Cu2O grafted onto LDH nanosheets were
prepared as control samples (these denoted herein as b-Cu2O
and b-Cu2O/LDH, respectively).[13a] TEM, size distributions and
XRD for the control samples were presented in Figure S3c-d and
S4-S5. Detailed examination of the XRD patterns (Figure 2b) for
u-Cu2O-c-2h showed that the intensity of Cu2O (111) diffraction
peak increased with ascorbic acid concentration, following the
order u-Cu2O-0.01M-2h < u-Cu2O-0.05M-2h < u-Cu2O-0.5M-2h,
suggestive of a corresponding increase in the concentration of
ultrafine Cu2O.[24] The optical properties of u-Cu2O-c-2h and bulk
counterparts were investigated by UV-DRS (Figure 2c). The u-
Cu2O-c-2h showed three absorption peaks: (1) a band at 200-318
nm due to the intrinsic absorption of LDH involving the ligand-
metal and metal-metal charge transfer;[25] (2) an absorption peak
spectroscopy (EDS) mapping (Figure 1f) showing
a
homogeneous distribution of metal elements within the sample
suggesting that the LDH structure was largely maintained with
ultrafine Cu2O formation.
2
around 800 nm associated with Eg→2T2g (D) d-d transitions of
Cu2+ ions in LDH;[26] and (3) an absorption feature at ~475 nm
associated with ultrafine Cu2O which intensified in the order u-
Cu2O-0.01M-2h < u-Cu2O-0.05M-2h < u-Cu2O-0.5M-2h. Both b-
Cu2O and b-Cu2O/LDH showed similar absorption signals in the
200-600 nm range (Figure 2c and S6). Cu K-edge X-ray
absorption near-edge spectroscopy (XANES) was used to
examine the local structure of Cu atoms in the ultrafine Cu2O and
bulk Cu2O reference samples (Figure 2d and S7). The pre-edge
feature at around 8983.8 eV was assigned to a 1s→4p transition
of a Cu(I) species, with the intensity of this spectral feature
increasing in the order u-Cu2O-0.01M-2h < u-Cu2O-0.05M-2h < u-
Cu2O-0.5M-2h.[27] The amounts of ultrafine Cu2O loaded were
further determined by inductively coupled plasma tests on the
basis of “coordinating etching” strategy (Table S1). Results again
demonstrated that the amount of sub-3 nm ultrafine Cu2O was
dependent on the ascorbic acid concentration used in the in-situ
reduction process, consistent with the earlier HRTEM, XRD, UV-
DRS and XANES findings. By tuning concentration of ultrafine
Cu2O, the sub-3 nm ultrafine Cu2O with the optimized exposed
surface sites along with the favorable light absorption was
anticipated to dramatically expedite the photocatalytic ammonia
synthesis of Cu2O.
Figure 1. (a-b) HRTEM images at different magnifications of u-Cu2O-0.05M-2h
and (c) the corresponding size distribution histogram. (d) AFM image and (e)
AFM height profiles of u-Cu2O-0.05M-2h (the numbers 1 to 3 in (d) correspond
to the profiles 1 to 3 in (e)). (f) EDS element maps for u-Cu2O-0.05M-2h.
To explore the evolution of ultrafine Cu2O during the in-situ
reduction process, the effects of ascorbic acid concentration and
ascorbic acid reduction time were systematically studied.
Samples obtained by reacting LDH with 0.05 mol L-1 of ascorbic
acid for different times (t = 0.2, 0.5, 1, 2, or 3 h) are denoted herein
as u-Cu2O-0.05M-t. As shown in the UV-vis diffuse reflectance
spectra (UV-DRS) of Figure S1c, the absorption peak at around
475 nm associated with Cu2O intensified with reduction time up to
2 h, but showed no further noticeable change on increasing the
reduction time to 3 h.[13a, 14a] Results suggested that the formation
and growth of ultrafine Cu2O were completed within 2 h. This was
further studied by capturing the morphological evolution of
ultrafine Cu2O (Figure S2). The HRTEM images showed that
ultrafine Cu2O with a mean size of ~2.20 nm emerged after only
0.2 h, thereafter the yield of ultrafine Cu2O increased and
approached a maximum after 2 h, implying reaction completion
had been achieved.[22] To summarize, ultrafine Cu2O formed
continuously for a specific period of time from LDH, whilst
maintaining a sub-3 nm of particle size. The size control was likely
due to the spatial confinement of Cu2O by the LDH laminate
structure.[20a, 23]
Next, the effect of ascorbic acid concentration was explored,
using a fixed in-situ reduction time of 2 h. Samples were
synthesized using ascorbic acid concentrations of 0.01, 0.05, 0.5
mol L-1, with the resulting samples being denoted herein as u-
Cu2O-c-2h (i.e. u-Cu2O-0.01M-2h, u-Cu2O-0.05M-2h and u-
Cu2O-0.5M-2h, respectively), as depicted graphically in Figure 2a.
HRTEM (Figure 2a) indicated that the density and size of the
ultrafine Cu2O on the nanosheets followed the sequence: u-Cu2O-
0.5M-2h (2.66 nm) > u-Cu2O-0.05M-2h (2.22 nm) > u-Cu2O-
Figure 2. (a) Schematic illustration and HRTEM images of the supported
ultrafine Cu2O prepared through in-situ reduction of LDH using different
concentrations of ascorbic acid. (b) XRD patterns from 2 = 32-41 for different
samples. The Cu2O (111) reflection is highlighted. (c) UV-DRS spectra and (d)
Cu K-edge XANES spectra for LDH, u-Cu2O-c-2h (X = 0.01, 0.05 and 0.5 M),
b-Cu2O and b-Cu2O/LDH.
3
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To ascertain the merits of ultrafine Cu2O for pNRR, we tested
the performance of the u-Cu2O-c-2h (c = 0.01, 0.05 and 0.5 M)
against bulk Cu2O (b-Cu2O and b-Cu2O/LDH) and pristine LDH in
the N2-saturated water (without sacrificial agents) under visible
light irradiation, using rigorous detection methods for product
quantification. As shown in Figure 3a, all the samples except LDH
were active for photocatalytic N2 reduction indicating that Cu2O
was the light-absorbing/active species responsible for pNRR.
Photons absorbed by LDH in the visible light region (involving d-
d transitions of the Cu2+ ions) could not be utilized to drive pNRR.
The rates of NH3 generation followed the trend of: u-Cu2O-0.05M-
2h (30.31 μmol g-1 h-1) > u-Cu2O-0.01M-2h (22.01 μmol g-1 h-1) >
u-Cu2O-0.5M-2h (18.67 μmol g-1 h-1) > b-Cu2O (3.04 μmol g-1 h-1)
> b-Cu2O/LDH (1.29 μmol g-1 h-1) > LDH (trace). Significantly, the
Cu2O-mass-normalized NH3 production rate of u-Cu2O-0.05M-2h
-1
can reach to 4.10 mmol gCu2O h-1, which is superior to most of
benchmark photocatalysts reported (Table S2), indicating an
outstanding photocatalytic activity and atomic utilization of sub-3
nm ultrafine Cu2O. Again, u-Cu2O-0.05M-2h afforded the highest
NH3 generation rate normalized with specific surface area (0.78
µmol m-2 h-1), compared to the other ultrafine Cu2O and bulk Cu2O
photocatalysts (Figure S8-S9). The lower than expected
photocatalytic activity of u-Cu2O-0.5M-2h was likely due to
ultrafine Cu2O aggregation during the tests, thus decreasing the
availability of ultrafine Cu2O. Further time-dependent pNRR
experiments were performed for u-Cu2O-0.05M-2h (Figure 3b),
alongside rigorous blank experiments. The amount of NH3
generated increased almost linearly with the visible light
Figure 3. (a) Photocatalytic NH3 production rate for LDH, u-Cu2O-c-2h (c = 0.01,
0.05 and 0.5 M), b-Cu2O and b-Cu2O/LDH in N2-saturated water under 1 h
visible light irradiation. (b) Time course study of photocatalytic N2 reduction and
various control experiments over u-Cu2O-0.05M-2h. (c) pNRR cycling test data
for photocatalytic N2 reduction over u-Cu2O-0.05M-2h. (d) Wavelength-
dependent apparent QE for N2 photoreduction over u-Cu2O-0.05M-2h.
To gain a deep understanding of the factors contributing to
the enhanced pNRR activity of ultrafine Cu2O, we examined three
principal properties that underpin efficient photocatalytic
processes: (i) the generation of photo-excited charge carriers by
light absorption; (ii) the separation and transfer of photo-excited
charge carriers to the surface; (iii) the charge carriers participating
in surface catalytic reactions.[28] u-Cu2O-0.05M-2h and b-
Cu2O/LDH were selected as representative examples of ultrafine
Cu2O and bulk Cu2O, respectively, thus allowing the particular
advantages of ultrafine Cu2O to be gauged. From the analysis of
UV-DRS spectra (Figure 2c), the b-Cu2O/LDH showed stronger
visible-light absorption (400-650 nm) than u-Cu2O-0.05M-2h, but
much worse pNRR performance. It can thus be concluded that
the light absorption capacity was not a key factor for the enhanced
pNRR activity of u-Cu2O-0.05M-2h. Charge separation and
charge transfer efficiencies following photoexcitation are known
to play an important role in overall photocatalytic performance.
Electrochemical impedance spectra (EIS) and transient
photocurrent (TPC) response measurements were therefore
+
irradiation time, confirmed by ion chromatography data for NH4
quantification (Figure S10). Negligible amounts of NH3 were
produced when N2 was replaced with Ar under visible light
irradiation, or when experiments were conducted with N2 in the
dark. Results provided strong evidence that ultrafine Cu2O was
able to photocatalytic convert N2 to NH3. Further, isotope labeling
experiments were performed using 15N2 with the indophenol
product containing 15N detected at m/z 199.05 (Figure S11), which
unambiguously confirmed that the NH3 detected in this current
study originated from N2 photofixation, rather than some other
nitrogen source. Moreover, the u-Cu2O-0.05M-2h was structurally
robust and photocatalytically stable, as evidenced by a negligible
change in activity, morphology or physico-chemical properties
during pNRR cycling tests (Figure 3c, S12-S14). As shown in
Figures S15-S17, in addition to O2, other potential by-products
-
carried out. u-Cu2O-0.05M-2h offered
a more depressed
(including NO3 , N2H4 and H2) were not detected during the pNRR
semicircle in the Nyquist plot (lower interfacial charge transfer
resistance) and a higher TPC density (Figure 4a) compared to b-
Cu2O/LDH, indicating that charge separation and transport were
more efficient in u-Cu2O-0.05M-2h. Moreover, TPL spectroscopic
experiments (Figure 4b) revealed that the short (1), long (2) and
average decay time () were all longer in u-Cu2O-0.05M-2h
compared with b-Cu2O/LDH, providing solid evidence that
ultrafine Cu2O could effectively inhibit charge carrier
recombination (thus increasing the availability of charges for
surface photoreactions). The presence of the long-lived charge
carriers in u-Cu2O-0.05M-2h motivated a detailed investigation of
charge carrier kinetics in the photocatalyst.
experiments, indicating the high selectivity of the ultrafine Cu2O
for NH3 synthesis. To determine the apparent quantum efficiency
(QE) of pNRR over u-Cu2O-0.05M-2h, we performed
photocatalytic N2 reduction experiments under different
monochromatic light irradiation (400, 450, 500, 550, 600, 650, and
700 nm) (Figure 3d). Monochromatic light of wavelengths 400 and
450 nm afforded the best N2 photoreduction activity with QEs of
0.14% and 0.11%, respectively. u-Cu2O-0.05M-2h demonstrated
almost no photocatalytic activity for N2 reduction under
monochromatic light of wavelengths 700 nm (a wavelength
outside the absorption range of Cu2O).
Femtosecond TAS kinetics analysis data for u-Cu2O-0.05M-
2h were presented in Figure 4c-d. A pump-probe configuration
equipped with pump laser (480 nm) and infrared continuum probe
was adopted here for TAS measurements. The pump laser was
used to excite electrons from the valence band (VB) to conduction
band (CB). Initially, u-Cu2O-0.05M-2h exhibited a more protracted
4
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build-up process (build-up = 444 ± 33 fs) compared with b-
Cu2O/LDH (build-up = 131 ± 18 fs). Such a sub-picosecond build-
up process reflects intraband relaxation (or cooling down
processes) arising from electron-phonon coupling in the photo-
excited Cu2O.[29] Fitting the subsequent biexponential decay
curve yielded 1 = 5.35 ± 0.65 ps and 2 = 47.16 ± 8.99 ps for u-
Cu2O-0.05M-2h, and 1 = 3.02 ± 0.16 ps and 2 = 18.73 ± 2.45
ps for b-Cu2O/LDH. Using ultrafine Cu2O therefore increased 1
and 2 by ∼1.8 and 2.5 times, respectively, compared to the bulk
Cu2O counterpart. These two relaxation processes (1 and 2)
resulted from photo-excited electron relaxation mediated by rich
trap states, with such states being in abundance in the ultrafine
Cu2O.[30] In brief, the fast decay component 1 arises from the
trapping of electrons from the CB into trap states within the
bandgap, whilst the much slower decay component 2 is due to
recombination processes involving CB electrons/trapped
electrons and VB holes.[31] Considering the above data, the
superior pNRR activity of u-Cu2O-0.05M-2h can be rationalized.
The sub-3 nm ultrafine Cu2O possesses an abundance of trap
states that are efficient in capturing photo-excited electrons (thus
suppressing electron-hole recombination), with the ultrafine size
of u-Cu2O-0.05M-2h benefitting the multi-electron-transfer
processes involved in surface N2 reduction to NH3.
reactions, with the underlying LDH simply serving as a structural
support in the pNRR process.[33] This coincides with what was
observed in the photo-deposition experiments (Figure S21), Pt
and Au nanoparticles formed next to the ultrafine Cu2O (i.e. the
reduction of Pt2+ to Pt0 or Au3+ to Au0 would preferentially occur in
the vicinity of Cu2O rather than on the exposed LDH nanosheets),
thus confirming simultaneously that ultrafine Cu2O acted as the
34]
light absorption unit and active species for pNRR.[8a,
The
reduction potentials of N2 on u-Cu2O-0.05M-2h and b-Cu2O/LDH
were determined from electrochemical measurements (Figure
S22). A much higher current density was observed for u-Cu2O-
0.05M-2h in N2-saturated solution compared with an Ar-saturated
solution, confirming u-Cu2O-0.05M-2h was indeed active for N2
reduction.[35] Compared with the bulk Cu2O, the onset potential for
N2 reduction over ultrafine Cu2O was shifted to positive potentials,
along with
a markedly enhanced current density. Results
suggested that the supported ultrafine Cu2O enhanced pNRR
activity by lowering the energy barrier for N2 reduction.[36]
Conclusion
In summary, sub-3 nm ultrafine Cu2O was successfully
synthesized by the partial reduction of a CuZnAl-LDH precursor
with ascorbic acid at room temperature and was exploited to
efficient photocatalytic reduction of N2 to NH3 for the first time. The
density of the ultrafine Cu2O on the surface of LDH support could
be controlled by varying the concentration of the ascorbic acid or
the reduction time. Reducing the lateral dimensions of Cu2O to
less than 3 nm endowed the ultrafine Cu2O with highly exposed
surface sites for N2 adsorption and ample trap states for long-lived
electrons, attributes that were both highly beneficial for the
photocatalytic N2 reduction to NH3 under visible light irradiation.
The introduction of a new family of Cu2O photocatalysts open up
new opportunities for photocatalytic N2 fixation and the facile route
described here for the creation of ultrafine Cu2O from a LDH
precursor is expected to be widely adopted in the future by other
researchers for the synthesis of high-performance Cu2O-based
catalysts.
Acknowledgements
Figure 4. (a) EIS Nyquist plots for u-Cu2O-0.05M-2h and b-Cu2O/LDH (the inset
shows the periodic on/off TPC response). (b) TPL decay curves for u-Cu2O-
0.05M-2h and b-Cu2O/LDH. TAS kinetics analysis for (c) u-Cu2O-0.05M-2h and
(d) b-Cu2O/LDH.
The authors are grateful for financial support from the National
Key Projects for Fundamental Research and Development of
China (2018YFB1502002, 2017YFA0206904, 2017YFA0206900,
2018YFA0208701), the National Natural Science Foundation of
China (51825205, 51772305, 52072382, 21871279, 21802154,
21902168, 21633015, 11721404), the Beijing Natural Science
Foundation (2191002, 2182078, 2194089), the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB17000000), the Royal Society Newton Advanced Fellowship
(NA170422), the International Partnership Program of Chinese
Academy of Sciences (GJHZ1819, GJHZ201974), the K. C.
Wong Education Foundation, and the Youth Innovation Promotion
Association of the CAS. The XAFS experiments were conducted
in 1W1B beamline of Beijing Synchrotron Radiation Facility
(BSRF). The isotopic labeling experiments were conducted in
BioNMR facility, Tsinghua University Branch of China National
Center for Protein Sciences (Beijing). GINW acknowledges
The final step in the pNRR process is catalytic redox
reactions on the surface of ultrafine Cu2O, which encompasses
the adsorption of N2 and its reduction by photo-excited electrons.
N2 adsorption is an important prerequisite in pNRR.[32] Sub-3 nm
ultrafine Cu2O with highly exposed surface sites was expected to
enhance N2 adsorption. This was confirmed by the experiment
(Figure S19), where the ultrafine Cu2O sample offered a higher
N2 adsorption capacity than its bulk Cu2O counterpart.
Subsequently, the photo-excited electron transfer pathway was
probed. From the Tauc analyses (Figure S20), the bandgaps for
Cu2O and LDH were estimated to be ~2.03 and 3.08 eV,
respectively. Thus, it would be reasonable to infer that upon
visible light irradiation of U-Cu2O-2, only Cu2O would be
photoexcited to generate electrons and holes for surface redox
5
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10.1002/anie.202013594
Angewandte Chemie International Edition
RESEARCH ARTICLE
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6
This article is protected by copyright. All rights reserved.

10.1002/anie.202013594
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
Sub-3 nm ultrafine Cu2O was synthesized through a facile in-situ
reduction of LDH, exhibiting superior performance for the visible
light-driven N2 reduction to NH3, nearly a 64-fold increases in NH3
production rate compared to bulk Cu2O and is superior to most of
the reported benchmark photocatalysts. The remarkable activity
can be attributable to the high availability of surface sites and the
generation of long-lived photo-excited electrons.
7
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