Inorganic Chemistry
Article
only the peaks assigned to MOF-199.10 Energy-dispersive X-
ray spectroscopy (EDS) results, including the elemental
mapping shown in Figure S2, verify the existence and the
uniform distribution of Zn and Cu elements in the obtained
octahedral nanoparticles, indicating the successful doping of
Zn element in the Cu-MOF-199 crystalline structure.
According to the thermogravimetric analysis (TGA, Figure
S3) of the Zn/Cu-MOF-199, the annealing temperature has
been set at 300 °C to ensure the complete conversion of MOF
structure to its derivative. After the calcination treatment, the
low- and high-magnification SEM images (Figure 1b and c) of
the obtained sample show a well-maintained octahedral
morphology, though with rough surfaces and a smaller size
distribution (about 1.1 mm, Figure S1b). The XRD pattern
(Figure 1a2) of the derivative shows only peaks assigned to
Cu2O, indicating the possibility of the doping of Zn inside the
Cu2O crystalline structure. According to the XRD pattern, no
obvious shift of peaks is observed in Zn/Cu2O@C after the
atomic doping, and this phenomenon has also been reported
by other studies on Zn-doped Cu2O.5 In order to explain this
phenomenon, we have simulated the XRD pattern of the Cu2O
crystal structure with one Cu atom replaced by Zn (Figure S4),
which shows no shift, confirming the possibility of no shift in
the XRD peaks after the atomic doping.
The transmission electron microscopy (TEM) image has
also been investigated to obtain more detailed structure
information on Zn/Cu2O@C. The low- and high-magnifica-
tion TEM images (Figures 1d and 1e) of Zn/Cu2O@C
octahedral particles show that the octahedral particles have a
porous structure composed of large amounts of nanoparticles
surrounded by carbon layers. The size distribution curve of the
nanoparticles inside Zn/Cu2O@C gives an average size of
about 11.5 nm. The existence of a carbon layer has been
further revealed by Raman spectroscopy (Figure S5), which
shows the characteristic peaks of graphic carbon, i.e., a D band
and G band. The porous structure can be revealed by N2
sorption measurement (Figures 1f and S6), which provides a
large BET surface area of 108.1 m2/g, and the pore size
distribution mainly centers at the range of 1−10 nm, which can
be attributed to the accumulation of the nanoparticles. A
further high-resolution TEM image (Figure 1g) displays the
crystal fringe spacing of 0.213 nm in a monodispersed
nanoparticle, which can be assigned to the (200) planes of
Cu2O. The corresponding selected area electron diffraction
(SAED) pattern displays a set of concentric rings, which also
matches well with Cu2O phase (Figure S7). The doping of Zn
can be confirmed by the elemental mapping of the Zn/Cu2O@
C shown in Figure 1h−h4, where the Cu, Zn, O, and C
elements are uniformly distributed. ICP (inductively coupled
plasma) results of Zn/Cu2O@C provides an accurate mass
fraction of Zn as 4.07%, and the calculated ratio of Zn/Cu is
about 8.2%. According to the above results, the annealing
product, Zn/Cu2O@C, has a porous octahedral structure
composed of carbon layer-coated Zn-doped Cu2O nano-
particles.
Figure 2. (a) XPS survey and high-resolution (b) Cu 2p spectrum of
Zn/Cu2O@C particles. (c) Comparison of Cu 2p spectra between
Zn/Cu2O@C and Cu2O@C. (d) High-resolution Zn 2p spectrum of
Zn/Cu2O@C.
(932.5 and 952.33 eV) and Cu2+ (934.6 eV, 954.3 eV, and
satellite peaks), where the Cu+ should be attributed to Cu2O
and the Cu2+ should be attributed to the surface oxidation of
Cu2O to CuO. There is only one peak at 916.6 eV (Cu+) in
the Auger Cu LMM curve of Zn/Cu2O@C (Figure S9),
confirming the main formation of Cu element as Cu2O. As
shown in Figure 2c, there is an obvious red shift of the Cu 2p
spectrum in Zn/Cu2O@C compared with undoped Cu2O@C
(obtained by pure Cu-MOF-199 as the precursor, Figures S10
and S11), which should be attributed from copper-deficient
regions inside Cu2O caused by cation doping of Zn.5 The high-
resolution Zn 2p spectrum shows peaks assigned to Zn2+
(Figure 2d), confirming the cation doping of Zn inside Cu2O.
The photocatalytic performance (shown in Figure 3) of Zn/
Cu2O@C with the structural features of cation doping of Zn
and coating carbon layer has been first evaluated by the
hydrogen evolution reaction from water. As shown in Figure 3a
and 3b, Zn/Cu2O@C can afford a hydrogen evolution rate of
26 μmol/g/h and shows a good stability during the five cycles
of photocatalytic processes under the irradiation of simulated
solar light. Compared with contrast samples with different
doping amounts of Zn, including Cu2O@C, Zn/Cu2O@C-l (l
represents less doping amount of Zn), and Zn/Cu2O@C-m (m
represents more doping amount of Zn) (see details in Figures
much higher activity toward the hydrogen evolution reaction
during the reaction time of 10 h due to the optimized amount
of cation doping. Zn/Cu2O@C-l and Zn/Cu2O@C have
shown highly enhanced activity than undoped Cu2O@C, while
Zn/Cu2O@C-m has shown decreased photocatalytic activity
(Figure 3a and Figure S13), indicating the positive influence of
doping Zn in Cu2O, though there is a limitation for the doping
amount. Nevertheless, the stability of calcination products with
different doping amounts of Zn has been maintained well after
five cycles and without obvious decrease in the hydrogen
evolution rate (Figure S14). Moreover, the positive influences
of carbon layers on both the activity and stability have been
highlighted by the significantly decreased separation efficiency
of photoinduced charge carriers, the hydrogen generation rate
X-ray photoelectron spectroscopy (XPS, Figure 2) has been
utilized to investigate the chemical states of elements in Zn/
Cu2O@C and to confirm the doping of Zn inside Cu2O. The
survey spectrum (Figure 2a) has demonstrated the coexistence
of Cu, Zn, O, and C elements in the obtained Zn/Cu2O@C.
The high-resolution O 1s and C 1s spectrum of Zn/Cu2O@C
is shown in Figure S8. The high-resolution Cu 2p spectrum
shown in Figure 2b displays two sets of peaks assigned to Cu+
C
Inorg. Chem. XXXX, XXX, XXX−XXX
Yuan, Yusheng
