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(NO3)2∙3H2O 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.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 43.3º 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(NO3)2∙3H2O.31,32Beyond 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
NO3− and produced H2O
at the initial stage.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 2p3/2, which can be
assigned to Cu0/Cu+,
Cu2+, and Cu(OH)2 species,
respectively.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
Cu0/Cu+ to Cu2+for Cu-30 (1.48) is 8 times higher than that for Cu-100 (0.18),
demonstrating that Cu-30 surface predominantly consists of
Cu0/Cu+ rather than
Cu2+ species. Furthermore, the Cu
L3M4,5M4,5 Auger region
around 568 eV was analyzed to clarify a specific chemical state of Cu
element in Figure 2b, since the deconvolution of Cu0or Cu+ species is difficult to separate due to their
similar binding energies.29,35 The peak position at
569.7 eV in Cu-30 sample corresponds to Cu+ species,
whereas the Cu-100 exhibits a positive binding energy shift to
Cu2+ state at 568.8 eV. These Auger XPS results
demonstrate the surface of Cu-30 contains a high content of
Cu+ species despite the absence of
Cu2O 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
Cu2O grain was probably too small to be
detected.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+ 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 Cu2O
lattice at 530.5 eV on Cu-30 is distinctly presented in comparison with
that of Cu-100 in the O 1s XPS spectra.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
CO2-saturated 0.1M KHCO3 than in the
Ar-saturated solution, illustrating the benefits from the
electrocatalytic activities for CO2RR. The catalytic
CO2RR performance of the OD-Cu NSs was assessed in the
potential window between −0.9 and −1.4 VRHE in a 0.1 M
KHCO3 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
FEC2H4 of 47% was obtained on Cu-30 sample at a low
applied potential (both at −1.1 and −1.2 VRHE), which is
obviously higher than those of Cu NP (26%), CuO NS (40%), and Cu-100
(40%) at the same potential, i.e., −1.1 VRHE (Figure
4c). Furthermore, as shown in Figure 4c, Cu-30 could reach the highest
FE of 72% at −1.1 VRHE for total C2+products (C2H4,
C2H5OH,
C3H7OH, 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 CO2RR (Figure 4e and S4). Specifically, the
partial current density for C2+ products
(jC2+ ) of Cu-30 achieved 29 mA
cm−2 at −1.1 VRHE, which is 5 times
higher than that of the pristine CuO NSs.
The
maximum jC2+ of Cu-30 and Cu-100 reached up to 55
and 52 mA cm−2 at −1.4 VRHE,
respectively, which are higher than 42 mA cm−2catalyzed 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 −1.1 VRHE. As shown in
Figure S5, the FEC2H4 remained > 40% for 8
hours stability test while the sum of FEs for CO and CH4constantly 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 FEC2+ on Cu-30 during
CO2RR, the electrochemical surface area (ECSA) was
calculated by cyclic voltammetry with different scan speeds under
CO2-saturated 0.1 M KHCO3 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 μF, the surface area of Cu-30 was significantly enhanced
to 93 cm2ECSA, much higher than 16.5
cm2ECSA of commercial Cu NPs, 48
cm2ECSA of pristine CuO NSs and 65.5
cm2ECSA of Cu-100.40Although 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+ surface and the
abundant defect sites provided by the nano-domains.19Accordingly, 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.41
It is notable that the dissociation of CO2 molecules is
one of the rate determining stages, also the energy barrier of
CO2 decomposition (the absorbed dissociative CO2 → *CO
intermediate + the absorbed O) is highly related to the surface
oxidative state, especially Cu+ and intrinsic
defective sites.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 CO2 is thermodynamically
unstable in the electrolyte, the analysis of OH−adsorption is a surrogate to inspect the binding energy of dissociative
CO2 adsorption on Cu surface.42-46 A
further investigation for OH− 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
VRHE. As shown in Figure 4b, except for the commercial
Cu NPs, the Cu(100) peak (0.2−0.3 VRHE) and the Cu(111)
shoulder peak around 0.48 VRHE were observed on the
samples of CuO NSs, Cu-30, and Cu-100.43,45 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 CO2 on Cu-30 surface, which facilitated the
C−C coupling process due to the immobilization of the dissociative
CO2 intermediate.44,45,47,48 It is
well matched with previous reports that the presence of partially
oxidized Cu+ species on the surface generally lowers
the activation energy to adsorb the dissociative
CO2.42,49 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 CO2RR performances.