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Journal of Materials Chemistry A
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of less than 20 nm, which is favourable for electron migration pattern, corresponding to the {111} crystalline planes of Au. The
from interior to surfaces of the MOFs.
morphological change of quasi-squareDPOPI:F1-03.1_0139/nCa9nToAs0h18e4e0tAs
cannot be found after loading of AuNPs. The loading amount of
AuNPs on PPF-3_1 can be controlled by tuning their original
mass ratio. For example, if increasing the mass ratio of AuNPs
to PPF-3_1 from 1:10 to 1:1 in the self-assembly system, the
high dense AuNPs dispersed on the surfaces of PPF-3_1
nanosheets were found (designated as Au/PPF-3_1B); in
contrast, if decreasing their mass ratio to 1:50, the sparse
AuNPs were on the PPF-3_1 nanosheet surfaces ((designated as
Au/PPF-3_1C, Fig. S9). By careful observation, AuNPs were not
completely adsorbed on the surfaces of PPF-3_1 nanosheets,
and a few AuNPs were scattered in blank areas while the mass
ratio of AuNPs to PPF-3_1 was 1:1.
Fig. 1 (a. b) TEM images with different magnifications, and (c) SEM and (d) AFM images
of PPF-3_1; (e) SEM and (f) TEM images of Au/PPF-3_1A. Inset is the profile of a PPF-3_1
nanosheet in the AFM image.
Electrostatic interaction originated from their different surface
charges is one of effective driving forces to achieve self-
assembly of materials.37 In the research, citrate modified AuNPs
with a SPR absorption peak of ~525 nm were prepared by
sodium citrate reduction of HAuCl4 (Fig. S2†).38 Negatively
charged surfaces were further analyzed by zeta potential of ~-
19.4 mV (Fig. S3†). Thereby, positively charged surfaces of PPF-
3_1 were needed to form Au/PPF-3_1 hybrid through an
electrostatic interaction. However, zeta potential of PPF-3_1
was -5.9 mV, revealing negatively charged surfaces. Citrate
modified AuNPs were difficultly loaded on the surfaces of PPF-
3_1 nanosheets (Fig. S4†). PPF-3_1 was thermally treated firstly
Fig. 2 High-resolution Co 2p XPS spectra of (a) treated PPF-3_1 and (b) Au@PPF-3_1A.
X-ray photoelectron spectroscopy (XPS) measurement was used
to analyze surface chemical state of Co element, whose redox
potential can greatly determine photoreduction ability of PPF-
3_1. From the high-resolution Co 2p XPS spectrum of treated
PPF-3_1 (Fig. 2), Co 2p3/2 and Co 2p1/2 core level photoemission
peaks at 781.0 eV and 796.3 eV, respectively, were found,
accompanied with two satellite peaks at 785.8 and 802.0 eV
corresponding to Co2+ shakeup excitation.39, 40 The presence of
the two satellite peaks confirms PPF-3_1 consisting of Co2+
species. Obvious shifting of Co binding energy was not found
after loading of AuNPs, and Co 2p core level region showed two
peaks at 780.95 eV and 796.2 eV together with two satellite
peaks at 785.9 and 802.0 eV in Au/PPF-3_1A. Hence,
photocatalytic reduction of CO2 could be related to redox
o
in DMF at 80 C for 12 h to expect alteration of its surface
properties. In the event, its zeta potential was changed into +4.9
mV, signifying successful transformation of PPF-3_1 nanosheet
surface properties from negative charge to positive charge. We
speculate that this experimental observation is possibly
attributed to (i) desorption of PVP from PPF-3_1 nanosheet
surfaces although PVP-related signal cannot be found in Fourier
transform infrared spectroscopy (FTIR) spectra of PPF-3_1
possibly due to small amount of PVP (Fig. S5†), and (ii) Co2+ ions
diffusion to surfaces. If the treated PPF-3_1 was re-dispersed
into PVP solution in DMF, its zeta potential was negatively
shifted (Fig. S3†). The morphology and the structure of PPF-3_1
nanosheets were not obviously changed after thermal
treatment in DMF (Fig. S6†). Both materials of AuNPs and PPF-
3_1 nanosheets (mass ratio: 1:10) were separately dispersed
into ethanol, and then were mixed together with gentle stirring
for 24 h. The purple precipitate was separated from the final
mixture that was kept for 8 h without disturbance, leaving a
colorless supernatant (Fig. S7†). In UV-Vis absorption spectrum
of the supernatant, SPR characteristic peak of AuNPs cannot be
found. These results manifest that AuNPs are completely
adsorbed on PPF-3_1 nanosheet surfaces in the suspension.
AuNPs were uniformly dispersed on PPF-3_1 nanosheet
surfaces when the mass ratio of Au and treated PPF-3_1 in the
self-assembly media was 1:10 (designated as Au/PPF-3_1A; Fig.
1e, 1f, and S8). A diffraction peak at 38.1o was appeared in XRD
2+
potential of ECo
/
Co
+ that was evaluated to be -0.64 V (vs NHE)
by cyclic voltammetry measurement of PPF-3_1 (Fig. S10†).
Noted that the position of Co 2p binding energy of PPF-3_1
cannot be alerted by thermal treatment, but the intensity is
weakened (Fig. S11†). The result reveals the change of surface
properties of PPF-3_1 is partly related to Co2+ ions migration.
The optical absorption ability is an important factor for
photocatalytic activity of Au/PPF-3_1 hybrid. UV-Vis absorption
spectra of treated PPF-3_1 and Au/PPF-3_1A showed strong
optical absorption in the range of UV to visible light (Fig. 3).
These absorption peaks were assigned to Soret band at low
wavelength and Q band between 500 and 700 nm related to
TCPP.41 Moreover, absorbance of Au/PPF-3_1A in the
wavelength region larger than 520 nm was significantly
increased compared to treated PPF-3_1, possibly due to SPR
absorption of AuNPs. In addition, photoluminescence (PL)
emission of TCPP was quenched in the treated PPF-3_1 and
Au/PPF-3_1A (Fig. 3b). The possible scenario is attributed to
photo-induced electron transfer (PET) from the excited TCPP to
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