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\begin{document}

\title{Facile Design of Oxide-Derived Cu Nanosheet Electrocatalyst for
CO\textsubscript{2} Reduction Reaction}



\author[1]{Juhee Jang}%
\author[1]{Shangqian Zhu}%
\author[1]{Ernest Delmo}%
\author[1]{Tiehuai Li}%
\author[1]{Qinglan Zhao}%
\author[1]{Yinuo Wang}%
\author[2]{Lili Zhang}%
\author[1]{Jingjie Ge}%
\author[1]{Minhua Shao}%
\affil[1]{Hong Kong University of Science and Technology School of Engineering}%
\affil[2]{Huaiyin Normal University Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\selectlanguage{english}
\begin{abstract}
Despite of superior performance of the oxide-derived copper (OD-Cu) in
producing valuable hydrocarbons during CO\textsubscript{2}RR, its
fabrication process is still ambiguous and complicated. In this work, we
develop a simple microwave-assisted method to synthesize the
oxide-derived Cu nanosheet (OD-Cu NS) and reveal that the oxidation
state of Cu species is controlled by varying the Cu precursor amount.
Notably, the simultaneous formation of nano-sized Cu domains influence
the surface roughness of OD-Cu NS. The partially oxidized Cu surface
exhibits a superior faradaic efficiency (FE) of C\textsubscript{2+}
products up to 72\%, along with a partial current density of 55 mA
cm\textsuperscript{-2} in a neutral KHCO\textsubscript{3} solution. More
importantly, the as-obtained OD-Cu NS shows a synergetic effect on
dissociating of CO\textsubscript{2} molecules by the strong binding
energy and promoting of C\textsubscript{2+} compounds productivity by
the enlarged electrochemical surface area. This work provides a new
insight for designing efficient OD-Cu catalysts towards
CO\textsubscript{2}RR.%
\end{abstract}%



\sloppy


Facile Design of Oxide-Derived Cu Nanosheet Electrocatalyst for
CO\textsubscript{2} Reduction Reaction

Juhee Jang\textsuperscript{1}, Shangqian Zhu\textsuperscript{1}, Ernest
Pahuyo Delmo\textsuperscript{1}, Tiehuai Li\textsuperscript{1}, Qinglan
Zhao\textsuperscript{1}, Yinuo Wang\textsuperscript{1}, Lili
Zhang\textsuperscript{2}, Jingjie Ge \textsuperscript{1,3,5,*} and
Minhua Shao\textsuperscript{1,3,4,*}

\textsuperscript{1}Department of Chemical and Biological Engineering,
The Hong Kong University of Science and Technology, Clear Water Bay,
Kowloon, Hong Kong, China

\textsuperscript{2}Jiangsu Key Laboratory for Chemistry of Low-Dimension
Materials, Huaiyin Normal University, Huaian 223300, Jiangsu, China

\textsuperscript{3}Energy Institute, The Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, Hong Kong, China

\textsuperscript{4}Chinese National Engineering Research Center for
Control \& Treatment of Heavy Metal Pollution, The Hong Kong University
of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

\textsuperscript{5}HKUST Jockey Club Institute for Advanced Study, The
Hong Kong University of Science and Technology, Clear Water Bay,
Kowloon, Hong Kong, China

*Corresponding authors

jjge@ust.hk; kemshao@ust.hk

\textbf{ABSTRACT}

Despite of superior performance of the oxide-derived copper (OD-Cu) in
producing valuable hydrocarbons during CO\textsubscript{2}RR, its
fabrication process is still ambiguous and complicated. In this work, we
develop a simple microwave-assisted method to synthesize the
oxide-derived Cu nanosheet (OD-Cu NS) and reveal that the oxidation
state of Cu species is controlled by varying the Cu precursor amount.
Notably, the simultaneous formation of nano-sized Cu domains influence
the surface roughness of OD-Cu NS. The partially oxidized Cu surface
exhibits a superior faradaic efficiency (FE) of
C\textsubscript{2+}products up to 72\%, along with a partial current
density of 55 mA cm\textsuperscript{-2} in a neutral
KHCO\textsubscript{3} solution. More importantly, the as-obtained OD-Cu
NS shows a synergetic effect on dissociating of CO\textsubscript{2}
molecules by the strong binding energy and promoting of
C\textsubscript{2+} compounds productivity by the enlarged
electrochemical surface area. This work provides a new insight for
designing efficient OD-Cu catalysts towards CO\textsubscript{2}RR.

\textbf{Introduction}

Fast industrialization and economic growth have aroused huge consumption
of fossil fuels and released a large amount of carbon dioxide
(CO\textsubscript{2}) into the atmosphere,\textsuperscript{1,2} which
has caused global warming issues and serious climate change. The
electrochemical CO\textsubscript{2}reduction reaction
(CO\textsubscript{2}RR), which converts CO\textsubscript{2} to various
value-added chemicals, has drawn wide interest as a promising approach
for the management of CO\textsubscript{2} emission. Particularly,
C\textsubscript{2+}molecules are regarded as highly valuable products
due to their applications in wide industrial field.\textsuperscript{3-6}
Since Hori et al. firstly reported that Cu foil has unique properties to
generate various hydrocarbons products beyond 2e\textsuperscript{-}
reduction pathway,\textsuperscript{7,8} Cu-based catalysts have been
explored extensively. The constraints in the bulk Cu
foils,\textsuperscript{9}such as low selectivity, high overpotentials,
insufficient stability, and strong competition from the hydrogen
evolution reaction (HER), were addressed by Cu-based nanocatalysts with
modified structures,\textsuperscript{10,11} tuned crystal
facets,\textsuperscript{12} regulated the compositions with other metal
elements and/or organic molecules, etc.\textsuperscript{13-15}

Among various types of Cu-based nanocatalysts, the oxide-derived copper
(OD-Cu) has shown great potential for producing the value-added
multi-carbon compounds.\textsuperscript{16} Notably, the chemical state
of Cu\textsuperscript{+}, i.e., the partially oxidized Cu on the surface
of OD-Cu, could modify the electronic structure of active sites, thereby
lead to a strong binding energy with one of the most crucial
intermediates *CO, and further promote those dimerization to produce
multicarbon compounds.\textsuperscript{17,18} Therefore, various methods
have been developed to fabricate the partially oxidized
Cu\textsuperscript{+} state for enhancing the productivity of
C\textsubscript{2+} hydrocarbons. For instance, a thin
CuO\textsubscript{x} sheath on the surface of OD-Cu nanowires was formed
by slowly oxidized in air atmosphere and reached up to 78\% of FE for
C\textsubscript{2+} products.\textsuperscript{19} In addition, by using
the O\textsubscript{2} plasma treated Cu nanocubes, the selectivity of
C\textsubscript{2}H\textsubscript{4} and the current density in
CO\textsubscript{2}RR were enhanced compared to that under Ar plasma
treatment.\textsuperscript{20} Besides the oxidation state, the role of
surface roughening on the OD-Cu was also examined to enrich the
catalytic active sites.\textsuperscript{21-25} The roughened surface
possesses abundant interfaces, grain boundaries, and low coordinated
sites, which are generally regarded as the active sites to trap the key
intermediate *CO and initiate the dimerization step.\textsuperscript{19}
However, the existing post-treatments on the pristine Cu such as plasma
irradiation,\textsuperscript{10} air-oxidation,\textsuperscript{26} and
electropolishing\textsuperscript{27} are still difficult to
well-controllably synthesize OD-Cu. A facile method to efficiently
fabricate the OD-Cu nanocatalysts is highly
desired.\textsuperscript{18,28}

Herein, we explored a simple microwave heating procedure to synthesize
OD-Cu nanosheets (OD-Cu NSs) and revealed how adding extra Cu precursor
affected the development of the surface Cu\textsuperscript{+} state by
increasing the surface roughness. Impressively, the optimized OD-Cu NS
with 30 wt\% of extra-added Cu precursor, denoting as Cu-30, achieved
the faradaic efficiency (FE) of 72\% and the geometric current density
of 31 mA cm\textsuperscript{-2} for C\textsubscript{2+} products at -1.1
V\textsubscript{RHE}. Furthermore, the electrochemical surface area
(ECSA) of the as-obtained Cu-30 with a rough surface is about five times
over the pristine CuO NS. This works provides insights about the
relationship between the binding energy of *CO intermediate on the
surface of Cu-30 with the specific chemical state of the partially
oxidized Cu by the means of inspection of
OH\textsuperscript{-}adsorption.

\textbf{Experiments}

\emph{Chemicals and Materials} . All chemicals were used without further
purification. Deionized water (DI water) with a resistivity of 18.2
M\selectlanguage{greek}Ω\selectlanguage{ngerman}·cm (Millipore) was used. Copper chloride dihydrate
(CuCl\textsubscript{2}\selectlanguage{english}[?]2H\textsubscript{2}O, 99.0\%), aqueous hydrogen
peroxide (H\textsubscript{2}O\textsubscript{2}, 30\% w/w in
H\textsubscript{2}O), potassium hydrogen carbonate
(KHCO\textsubscript{3}, 99.7\%), 5\% Nafion solution, isopropanol,
ethanol, ethylene glycol (anhydrous, 99\%), dimethyl sulfoxide
(anhydrous, 99.5\%), and phenol (\selectlanguage{english}[?]99\%) were supplied by Sigma Aldrich.
Copper nitrate trihydrate (CuNO\textsubscript{3}\selectlanguage{english}[?]3H\textsubscript{2}O,
99.99\%) and sodium hydroxide (NaOH, 99.99\%) were purchased from
Aladdin and VWR Chemicals, respectively. D\textsubscript{2}O (99.9\%
atom\%D) was purchased from J\&K Chemicals.

\emph{Synthesis of OD-Cu NS} . The pristine CuO nanosheets (NSs) were
synthesized by following a procedure previously reported by Zhang et
al.\textsuperscript{29} 85.2 mg of
CuCl\textsubscript{2}\selectlanguage{english}[?]2H\textsubscript{2}O was dispersed into 10 ml of 3
M NaOH solution by vigorous stirring, then transferred into the
Teflon-liner stainless steel autoclave. After kept at 100 \selectlanguage{english} for 12 hours
in the oven and then cooled down to room temperature, the obtained
product was washed by DI water and ethanol for four times respectively
and dried at 80 \selectlanguage{english} under the vacuum oven for overnight. To prepare the
OD-Cu NS, 20 mg of the as-synthesized CuO NS with 30 wt\% (Cu-30) or 100
wt\% (Cu-100) Cu(NO\textsubscript{3})\selectlanguage{english}[?]3H\textsubscript{2}O was suspended
into 10 ml of ethylene glycol, then transferred into a Pyrex glass
bottle. The mixture solution was ultrasonicated in ice water for 1 hour,
then moved into the digestive microwave (Milestone, Ethos One) with the
power of 1050 W for 210 s under an ambient atmosphere. The dark brownish
colored precipitate was cooled to room temperature and then collected by
centrifugation. Subsequently, the dark brownish precipitate was
collected by a centrifuge process at 15000 rpm for 10 mins, and washed
four times by ethanol. After dried under a vacuum oven at 60 \selectlanguage{english}
overnight, the obtained OD-Cu NS were collected for further usage.

\emph{Physical Characterizations} . Transmission electron microscopy
(TEM) images were recorded on a JEOL JEM-2010F (JEOL, Tokyo, Japan).
Aberration-corrected TEM was operated on JEOL JEM-ARM200F at 200kV
acceleration voltage. The samples used for TEM characterization were
dispersed in absolute ethanol firstly, then dropped onto carbon coated
Cu or Ni grid and dried completely at room temperature. The X-ray
diffraction (XRD) patterns were recorded at room temperature with an
X'pert Pro K-Alpha diffractometer (PANalytical, Almelo, Netherlands),
using Cu K\selectlanguage{greek}α \selectlanguage{english}radiation (\selectlanguage{greek}λ\selectlanguage{english}=1.5406 \selectlanguage{ngerman}Å). The XRD samples were prepared by
dropping the as-prepared nanoparticles (NPs) suspensions onto a glass
slide. The X-ray photoelectron spectroscopy (XPS) was measured by a
Physical Electronics 5600 with an Al K\selectlanguage{greek}α \selectlanguage{english}source (h\selectlanguage{greek}ν \selectlanguage{english}= 1486.6 eV), and the
fresh samples were used for the XPS measurements to avoid over-oxidation
issue.

\emph{Electrochemical Measurement} . To prepare the working electrode, 2
mg of each catalyst (commercial Cu NP, pristine CuO NS, and OD-Cu NSs)
was suspended into 100 \selectlanguage{greek}μ\selectlanguage{english}L of absolute ethanol with 5 \selectlanguage{greek}μ\selectlanguage{english}L of 5\% Nafion
solution (Anhydrous, Sigma Aldrich) by ultrasonication. After that, 5 \selectlanguage{greek}μ\selectlanguage{english}L
of the dispersion was loaded onto a cleaned glassy carbon electrode with
a diameter of 5 mm, and dried in ambient atmosphere. For the
electrochemical CO\textsubscript{2}RR measurement, a two-component
H-type cell was assembled with a proton exchange Nafion 117 membrane
(Aldrich) in the middle. Then each component was filled with 50 ml of
0.1 M KHCO\textsubscript{3} electrolyte, afterwards
CO\textsubscript{2}gas (99.999\%, Asia Pacific Gas Enterprise Co., LTD)
was purged into the solution for 1 hour in advance. Platinum (Pt) mesh
(20 \selectlanguage{ngerman}× 20 mm\textsuperscript{2}) and an Ag/AgCl (saturated into 3.0 M
KCl, BASi) electrode were used as a counter and a reference electrode,
respectively. The reference electrode was calibrated with 0.1 M
HClO\textsubscript{4} solution (pH 1.1) by reversible hydrogen electrode
(RHE) and the potential E was converted to the RHE reference scale in
terms of the equation:

E (versus RHE) = E (versus Ag/AgCl) + 0.197 V + 0.059 V × pH.

As connecting the gas-tight H-cell to the potentiostat (CHI 627E, CH
Instruments, Inc.), iR compensation was conducted automatically at a
level of 85\%, whereas the rest of 15 \% IR drop was manually
compensated during data processing. CO\textsubscript{2} was continuously
fed into both parts of the electrolyte via mass flow controllers
(Sevenstar, Beijing) at a rate of 30 SCCM. Since trapping air bubbles on
the working electrode could cause high resistance, the component
containing the working electrode was stirred magnetically at 1500 rpm.
The gas products were analyzed by a gas chromatograph (GC2060, Ramiin,
Shanghai) equipped with a thermal conductivity detector (TCD) for
hydrogen (H\textsubscript{2}) quantification and a flame ionization
detector (FID) for carbon monoxide (CO), methane (CH\textsubscript{4}),
and ethylene (C\textsubscript{2}H\textsubscript{4}). The calibration
curve was obtained by a series of standard gas mixtures
(H\textsubscript{2}, CO, CH\textsubscript{4}, and
C\textsubscript{2}H\textsubscript{4} balanced in Ar, Shanghai Haizhou
Special Gas Co., LTD). Finally, four gas samples at 15, 28, 41, and 54
min were collected and quantified in average for 1 hour electrolysis.
Liquid products were collected after the measurement and analyzed by
using a Varian 500 MHz \textsuperscript{1}H nuclear magnetic resonance
(NMR) spectra. The NMR samples were prepared by mixing 450 \selectlanguage{greek}μ\selectlanguage{english}L of
electrolyte with 50 \selectlanguage{greek}μ\selectlanguage{english}L home-made internal standard solution containing
500 \selectlanguage{ngerman}× 10\selectlanguage{english}\textsuperscript{-6} M phenol, and 100 x 10\selectlanguage{english}\textsuperscript{-6}M
dimethyl sulfoxide in D\textsubscript{2}O. The calibration curves of
internal standard solution were established by several standard
solutions (0, 50, and 100 \selectlanguage{greek}μ\selectlanguage{english}M of formate, methanol, ethanol, acetate, and
iso-propanol in 0.1 M KHCO\textsubscript{3} solution).

The electrochemical activity was examined by linear sweep voltammetry
(LSV) from -0.1 V\textsubscript{RHE} to -1.8 V\textsubscript{RHE} in
CO\textsubscript{2} or Ar-saturated 0.1 M
KHCO\textsubscript{3}electrolyte and the electrochemical surface area
(ECSA) was measured by cyclic voltammetry (CV) between 0 and 0.4
V\textsubscript{RHE} with different scan rates in
CO\textsubscript{2}-saturated 0.1 M KHCO\textsubscript{3} electrolyte
after 1 hour electrolysis by each catalyst. To compare the binding
energy of OH\textsuperscript{-}adsorption, LSV curves were obtained from
0.1 to 0.6 V\textsubscript{RHE} after the electrolysis in Ar-saturated
0.1 M KOH electrolyte.

\textbf{Results and Discussion}

The OD-Cu NSs were obtained by a facile microwave irradiation method
with a reduction process. In a typical synthesis of Cu-30 and Cu-100, by
adding 30 and 100 wt\% of
Cu(NO\textsubscript{3})\textsubscript{2}[?]3H\textsubscript{2}O to CuO NS,
the OD-Cu NSs with a rough surface were formed in a large scale under
the microwave irradiation with the power of 1050 W for 210 s. Figure
1a-d show TEM images of the pristine CuO NS (Figure 1a,b), as well as
the microwave treated Cu-30 (Figure 1c) and Cu-100 (Figure 1d). The
pristine CuO NS has an edge length of hundreds nanometer and smooth
surface (Figure 1a,b). After the microwave irradiation, the OD-Cu NSs
were obtained exclusively and maintained the original 2D morphology
(Figure 1c,d). Interestingly, the nanosized domains (light grey circles
in Figure 1c,d) were observed on the surface of Cu-30 and Cu-100 NS. The
nuclei of Cu precursor salt could be randomly distributed on the
nanosheet due to the polarity of ethylene glycol in a homogeneous
mixture, then the copper precursor was rapidly reduced to metallic Cu
under the microwave heating and formed the rough surface on the pristine
CuO NS.\textsuperscript{30}

To figure out their crystal structures, XRD measurements were conducted
on the pristine CuO NS, Cu-30, and Cu-100 (Figure 1e). It is clearly
shown that the pristine CuO NS consists of pure CuO phase (denoted as
\#). In comparison, a mixture of metallic Cu and Cu oxide diffraction
patterns were observed in Cu-30 and Cu-100. Specifically, the peaks
located at 43.3\selectlanguage{ngerman}º and 50.4º predominantly in metallic Cu facets (denoted
as *) are corresponding to Cu (111) and (200), respectively. The peaks
at 35.5º and 38.7º assigned as CuO (002) and CuO (111) indicated the
remains of CuO phase after the microwave process. Because CuO species
have a higher dissociative energy than Cu precursors during the
microwave-assisted reduction, the XRD patterns of Cu-100 show almost
equal peak intensities of metallic Cu and CuO species, indicating that
the ratio of oxide phase in Cu-100 was intensified by increasing the
amount of
Cu(NO\textsubscript{3})\textsubscript{2}\selectlanguage{english}[?]3H\textsubscript{2}O.\textsuperscript{31,32}Beyond
the dissociation energy, the insufficient reductant could further impede
the reduction process of CuO NPs since ethylene glycol is rapidly
consumed from the preferential reaction with
NO\textsubscript{3}\selectlanguage{english}\textsuperscript{-} and produced H\textsubscript{2}O
at the initial stage.\textsuperscript{32} Additionally, the reductive
rate between two distinct Cu species can be affected by the presence of
additional Cu salt, resulting in the co-existence of Cu and Cu oxide
phases in the OD-Cu NSs.

X-ray photoelectron spectroscopy (XPS) characterization was conducted to
identify the surface valence states of Cu (Figure 2) and O (Figure S1).
As shown in Figure 2a, all the samples disclosed three distinguished
deconvolutions in the spectra of Cu 2p\textsubscript{3/2}, which can be
assigned to Cu\textsuperscript{0}/Cu\textsuperscript{+},
Cu\textsuperscript{2+}, and Cu(OH)\textsubscript{2} species,
respectively.\textsuperscript{33,34} It was notable that different Cu
oxidation states co-existed on the surface of OD-Cu NS samples (Cu-30
and Cu-100). More importantly, the intensity ratio of
Cu\textsuperscript{0}/Cu\textsuperscript{+} to Cu\textsuperscript{2+}for
Cu-30 (1.48) is 8 times higher than that for Cu-100 (0.18),
demonstrating that Cu-30 surface predominantly consists of
Cu\textsuperscript{0}/Cu\textsuperscript{+} rather than
Cu\textsuperscript{2+} species. Furthermore, the Cu
L\textsubscript{3}M\textsubscript{4,5}M\textsubscript{4,5} Auger region
around 568 eV was analyzed to clarify a specific chemical state of Cu
element in Figure 2b, since the deconvolution of Cu\textsuperscript{0}or
Cu\textsuperscript{+} species is difficult to separate due to their
similar binding energies.\textsuperscript{29,35} The peak position at
569.7 eV in Cu-30 sample corresponds to Cu\textsuperscript{+} species,
whereas the Cu-100 exhibits a positive binding energy shift to
Cu\textsuperscript{2+} state at 568.8 eV. These Auger XPS results
demonstrate the surface of Cu-30 contains a high content of
Cu\textsuperscript{+} species despite the absence of
Cu\textsubscript{2}O signal in the Cu-30 XRD result. Considering
previous reports revealed the relationship between the grain size and
detection limitation for XRD inspection, the size of
Cu\textsubscript{2}O grain was probably too small to be
detected.\textsuperscript{36} Therefore, the targeted Cu oxidation state
was achieved by simply adjusting the Cu salt amount under the microwave
irradiation. Due to the surface oxidation from exposure to air, the
commercial Cu NPs exhibit a chemical valence of Cu\textsuperscript{+} at
569.6 eV in Cu LMM peak (Figure 2b, black line). Moreover, as shown in
Figure S1, the peak of oxygen species in the Cu\textsubscript{2}O
lattice at 530.5 eV on Cu-30 is distinctly presented in comparison with
that of Cu-100 in the O 1s XPS spectra.\textsuperscript{37-39}

The electrochemical properties of each catalyst (commercial Cu NPs, CuO
NSs, and OD-Cu NSs) were first examined by linear sweep voltammetry
(LSV), which are shown in Figure 3a and S2. The LSV curves of these
catalysts evidently show higher current densities in the
CO\textsubscript{2}-saturated 0.1M KHCO\textsubscript{3} than in the
Ar-saturated solution, illustrating the benefits from the
electrocatalytic activities for CO\textsubscript{2}RR. The catalytic
CO\textsubscript{2}RR performance of the OD-Cu NSs was assessed in the
potential window between \selectlanguage{english}-0.9 and \selectlanguage{english}-1.4 V\textsubscript{RHE} in a 0.1 M
KHCO\textsubscript{3} electrolyte. In comparison, the commercial Cu NPs
and CuO NSs were also tested. In Figure 3b and S3, the overall FE was
presented at each potential and ethylene was detected as the main
product for each catalyst. Remarkably, the maximum
FE\textsubscript{C2H4} of 47\% was obtained on Cu-30 sample at a low
applied potential (both at \selectlanguage{english}-1.1 and \selectlanguage{english}-1.2 V\textsubscript{RHE}), which is
obviously higher than those of Cu NP (26\%), CuO NS (40\%), and Cu-100
(40\%) at the same potential, i.e., \selectlanguage{english}-1.1 V\textsubscript{RHE} (Figure
4c). Furthermore, as shown in Figure 4c, Cu-30 could reach the highest
FE of 72\% at \selectlanguage{english}-1.1 V\textsubscript{RHE} for total
C\textsubscript{2+}products (C\textsubscript{2}H\textsubscript{4},
C\textsubscript{2}H\textsubscript{5}OH,
C\textsubscript{3}H\textsubscript{7}OH, and acetate), while only 41\%,
58\% and 60\% were achieved on the commercial Cu NPs, CuO NSs, and
Cu-100, respectively. The current density was also improved by Cu-30
during the CO\textsubscript{2}RR (Figure 4e and S4). Specifically, the
partial current density for C\textsubscript{2+} products
(\emph{j\textsubscript{C2+}} ) of Cu-30 achieved 29 mA
cm\selectlanguage{english}\textsuperscript{-2} at \selectlanguage{english}-1.1 V\textsubscript{RHE}, which is 5 times
higher than that of the pristine CuO NSs. The maximum
\emph{j\textsubscript{C2+}} of Cu-30 and Cu-100 reached up to 55 and 52
mA cm\selectlanguage{english}\textsuperscript{-2} at \selectlanguage{english}-1.4 V\textsubscript{RHE}, respectively,
which are higher than 42 mA cm\selectlanguage{english}\textsuperscript{-2}catalyzed by the
pristine CuO NSs, illustrating that the metallic Cu phase in OD-Cu
produced with the microwave treatment could promote higher conductivity.

Stability is another crucial parameter to evaluate the catalytic
performance. The FE on Cu-30 was monitored through continuous
electrolysis for 8 hours at \selectlanguage{english}-1.1 V\textsubscript{RHE}. As shown in
Figure S5, the FE\textsubscript{C2H4} remained \textgreater{} 40\% for 8
hours stability test while the sum of FEs for CO and
CH\textsubscript{4}constantly kept less than 10\% , suggesting that the
production of low-valued products was effectively suppressed on the
OD-Cu NSs surface. According to the aforementioned findings, the
partially oxidized Cu species on the surface of Cu-30 could promote the
*CO dimerization process, leading to the highest FE for ethylene.

To figure out the catalytic active sites and influential factors toward
the improvement of FE\textsubscript{C2+} on Cu-30 during
CO\textsubscript{2}RR, the electrochemical surface area (ECSA) was
calculated by cyclic voltammetry with different scan speeds under
CO\textsubscript{2}-saturated 0.1 M KHCO\textsubscript{3} solution after
1 hour catalysis in Figure 4a and S6. By calculating within the double
layer charge region, the capacitance values of commercial Cu NPs, CuO
NSs, Cu-30, and Cu-100 are 0.48, 1.4, 2.7, and 1.9 mF, respectively.
Assuming the theoretical specific capacitance of polycrystalline Cu
(pc-Cu) to 29 \selectlanguage{greek}μ\selectlanguage{english}F, the surface area of Cu-30 was significantly enhanced
to 93 cm\textsuperscript{2}\textsubscript{ECSA}, much higher than 16.5
cm\textsuperscript{2}\textsubscript{ECSA} of commercial Cu NPs, 48
cm\textsuperscript{2}\textsubscript{ECSA} of pristine CuO NSs and 65.5
cm\textsuperscript{2}\textsubscript{ECSA} of
Cu-100.\textsuperscript{40}Although the surface morphologies of CuO NSs
and Cu-30 are comparable, the ECSA of Cu-30 is twice as large as that of
CuO NSs due to the unique partial oxidation state of
Cu\textsuperscript{+} surface and the abundant defect sites provided by
the nano-domains.\textsuperscript{19}Accordingly, the enlarged
nano-domains on Cu-30 were obtained after 1 hour electrolysis (Figure
S7) and achieved the highest roughness factor of 93 based on defining
pc-Cu as one.\textsuperscript{41}

It is notable that the dissociation of CO\textsubscript{2} molecules is
one of the rate determining stages, also the energy barrier of
CO\textsubscript{2} decomposition (the absorbed dissociative CO2 - *CO
intermediate + the absorbed O) is highly related to the surface
oxidative state, especially Cu\textsuperscript{+} and intrinsic
defective sites.\textsuperscript{42,43} In parallel, the crucial step
for the formation of multicarbon products, C-C coupling, is associated
with strong binding energy of *CO intermediates on Cu surface.
Considering the dissociative CO\textsubscript{2} is thermodynamically
unstable in the electrolyte, the analysis of
OH\textsuperscript{-}adsorption is a surrogate to inspect the binding
energy of dissociative CO\textsubscript{2} adsorption on Cu
surface.\textsuperscript{42-46} A further investigation for
OH\textsuperscript{-} adsorption was characterized by the
electrochemical measurement in an Ar-saturated 0.1 M KOH electrolyte in
the potential window of 0.3-0.6 V\textsubscript{RHE}. As shown in Figure
4b, except for the commercial Cu NPs, the Cu(100) peak (0.2-0.3
V\textsubscript{RHE}) and the Cu(111) shoulder peak around 0.48
V\textsubscript{RHE} were observed on the samples of CuO NSs, Cu-30, and
Cu-100.\textsuperscript{43,45} The blue dash arrows obviously pointed
out that the corresponding adsorption potential of Cu-30 negatively
shifted compared with those of CuO NSs and Cu-100. This shift indicates
a stronger adsorption energy with the dissociative CO\textsubscript{2}
on Cu-30 surface, which facilitated the C-C coupling process due to the
immobilization of the dissociative CO\textsubscript{2}
intermediate.\textsuperscript{44,45,47,48} It is well matched with
previous reports that the presence of partially oxidized
Cu\textsuperscript{+} species on the surface generally lowers the
activation energy to adsorb the dissociative
CO\textsubscript{2}.\textsuperscript{42,49} Therefore, we conclude that
the rough Cu surface formed on Cu-30 by the microwave treatment has
great impact on the enlargement of surface area and the formation of the
partially oxidized Cu species as active sites, promoting the C-C
coupling process to enhance the CO\textsubscript{2}RR performances.

\textbf{Conclusion}

Herein, we developed a facile microwave irradiation method to prepare
the oxide-derived Cu NSs. Extra-addition of Cu precursor generated a
rough surface with nano-domains deposited on the pristine Cu NSs. The
partially oxidized Cu\textsuperscript{+} state could be distinctively
observed on the XPS spectrum of Cu-30. Due to the large surface area and
the presence of Cu\textsuperscript{+} species, the
FE\textsubscript{C2+}of Cu-30 could be improved up to 72 \% and the
partial current density for C\textsubscript{2+} products could reach up
to 55 mA cm\textsuperscript{-2}. This facile microwave-assisted
fabrication method will give a direction to cost-effective design of
oxide-derived Cu catalyst for efficient synthesis of C\textsubscript{2+}
products by CO\textsubscript{2}RR.

\textbf{Acknowledgements}

This work was supported by the Research Grants Council (16310419,
16309418, and 16304821), Innovation and Technology Commission (Grant No.
ITC-CNERC14EG03) of the Hong Kong Special Administrative Region, and
Hong Kong Branch of National Precious Metals Material Engineering
Research Centre,~City University of Hong Kong.

\textbf{Conflict of Interest}

The authors declare no conflict of interest.

\textbf{Keywords}

Copper nanosheet [?] electrochemical CO\textsubscript{2} reduction [?]
C\textsubscript{2+} selectivity [?] microwave irradiation

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\textbf{Figure 1.} TEM images of (a-b) CuO NS, OD-Cu NSs (c) Cu-30, and
(d) Cu-100 (the marks of white circles mean the nano-domains.),
respectively. (e) XRD patterns of CuO NS, Cu-30, and Cu-100 (the label
of star (*) and hashtag (\#) represent metallic Cu and CuO phase,
respectively)

\textbf{Figure 2} . XPS spectra at (a) Cu 2p\textsubscript{2/3} and (b)
Cu LMM Auger region of commercial Cu NP, CuO NS, Cu-30, and Cu-100

\textbf{Figure 3.} Electrochemical CO\textsubscript{2}RR performance of
commercial Cu NP, CuO NS, Cu-30, and Cu-100. (a) LSV curves of Cu-30 in
the CO\textsubscript{2}- and Ar- saturated 0.1 M
KHCO\textsubscript{3}electrolyte after 1hr electrolysis. (b) Overall
Faradaic efficiencies at a potential of \selectlanguage{english}-1.1 V vs. RHE, partial faradaic
efficiency (FE) for (c) C\textsubscript{2}H\textsubscript{4} and (d)
C\textsubscript{2+}products, (e) partial current density of total
C\textsubscript{2+}products.

\textbf{Figure 4.} (a) Double layer charge capacitance in
CO\textsubscript{2}-saturated 0.1 M KHCO\textsubscript{3} electrolyte
and (b) OH\textsubscript{ads} peaks from LSV curves obtained in
Ar-saturated 0.1 M KOH electrolyte of commercial Cu NP, pristine CuO NS,
Cu-30, and Cu-100. The inset image shows a magnified section of the
(111) shoulder peak between 0.42-0.53 V\textsubscript{RHE}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} TEM images of (a-b) CuO NS, OD-Cu NSs (c) Cu-30, and
(d) Cu-100 (the marks of white circles mean the nano-domains.),
respectively. (e) XRD patterns of CuO NS, Cu-30, and Cu-100 (the label
of star (*) and hashtag (\#) represent metallic Cu and CuO phase,
respectively)\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2} . XPS spectra at (a) Cu 2p\textsubscript{2/3} and (b)
Cu LMM Auger region of commercial Cu NP, CuO NS, Cu-30, and Cu-100\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} Electrochemical CO\textsubscript{2}RR performance of
commercial Cu NP, CuO NS, Cu-30, and Cu-100. (a) LSV curves of Cu-30 in
the CO\textsubscript{2}- and Ar- saturated 0.1 M
KHCO\textsubscript{3}electrolyte after 1hr electrolysis. (b) Overall
Faradaic efficiencies at a potential of -1.1 V vs. RHE, partial faradaic
efficiency (FE) for (c) C\textsubscript{2}H\textsubscript{4} and (d)
C\textsubscript{2+}products, (e) partial current density of total
C\textsubscript{2+}products.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4.} (a) Double layer charge capacitance in
CO\textsubscript{2}-saturated 0.1 M KHCO\textsubscript{3} electrolyte
and (b) OH\textsubscript{ads}peaks from LSV curves obtained in
Ar-saturated 0.1 M KOH electrolyte of commercial Cu NP, pristine CuO NS,
Cu-30, and Cu-100. The inset image shows a magnified section of the
(111) shoulder peak between 0.42-0.53 V\textsubscript{RHE}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Table-of-Contents/Table-of-Contents}

\end{center}
\end{figure}

\selectlanguage{english}
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