3.6 Photocatalytic H2 evolution activity
measurements
Fig. 7a depicts the photocatalytic hydrogen evolution performance of
different samples under visible light irradiation (λ ≥ 400 nm) with
lactic acid (LA) as the hole scavenger. The original ZnCdS nanoparticles
are visibly active to catalyze the splitting of water under 5 W LED
light irradiation, giving a H2-evolving rate of 8.2 mmol
h-1 g-1.
However, after coupling 0.5 wt.%
CoP polyhedron or 0.5 wt.% Co3O4 QDs
with ZnCdS nanoparticles, the as-obtained
ZCS/CoP and
ZCS/Co3O4 QDs hybrids show an improved
hydrogen evolution activity, which manifests that the introduction of
both CoP polyhedron and Co3O4 QDs can
accelerate the separation and migration of photo-excited charges from
ZCS. More importantly, while 0.5 wt.% CoP polyhedron and 0.5 wt.%
Co3O4 QDs were introduced concurrently,
the constructed hierarchical
CoP/ZnCdS/Co3O4QDs (800>40>4.5 nm) bi-heterostructure cages
exhibits more high-performance photocatalytic hydrogen production, which
strongly indicates that the parallel pathways of electron migration was
successfully established for more rapider charge transport. Fig. 7b
shows the comparison of hydrogen evolution rate of ZCS/CoP with
different wt.% of CoP polyhedron. It can be clearly observed that
ZCS/CoP with 1.5 wt.% CoP exhibits higher hydrogen generation rate
(12.4 mmol h-1 g-1) which is 1.5
times that of pristine ZCS, again demonstrating the significant role of
CoP for promoting the hydrogen evolution activity of ZCS. Fig. 7c
displays the hydrogen production activities after assembling different
content of
Co3O4QDs on ZCS/CoP-3. As observed, the O/ZCS/P-3 with 1.5 wt.%
Co3O4 QDs exhibits the highest
photocatalytic hydrogen evolution property (24.2 mmol
h-1 g-1) under 5 W LED light
irradiation, which is about 2.9 times that of original ZCS. Further,
O/ZCS/P-3 possesses more prominent hydrogen generation performance under
300 W Xe lamp irradiation close to sunlight (Fig. 7d), and a large
number of bubbles can be clearly observed in a closed reaction vessel as
shown in Fig. 7g, which seriously reveal the ultrahigh photocatalytic
performance of the hierarchical
O/ZCS/P-3 bi-heterostructure cages for hydrogen evolution. In order to
more intuitively evaluate the photocatalytic hydrogen evolution rate of
the bi-heterostructured O/ZCS/P-3 cages, the reaction vessel was
equipped with H2-collection-device
(HCD). Dynamic bubbles in hydrogen
evolution reaction is provided in a separate document
(H2-evolution video). While the hydrogen evolution
reaction proceed for 0 h, the HCD of easy expansion is downcast. After 5
h, it was obviously bulging. However, it’s completely propped up after
10 h. Both lots of bubbles and the rapid bulging of elastic HCD indicate
the ultrahigh photocatalytic hydrogen evolution performance of the
as-constructed O/ZCS/P-3 bi-heterostructure cages. Additionally, the
purity of evolved H2 also was tested via ignition
experiment, bright flame means the high purity of H2from O/ZCS/P-3 driven water splitting under visible light irradiation.
Undoubtedly, the hierarchical
O/ZCS/P-3 bi-heterostructure cages
should be one of the world-level photocatalysts for efficient
H2 evolution up to now. The hydrogen generation
performance of O/ZCS/P-3 under
different wavelength was carried out (1 h), the results are shown in
Fig. 7e. Obviously, the decrease of photocatalytic hydrogen evolution
rate is drastic at the wavelengths greater than 500 nm, which indicates
that the water splitting reaction for H2 evolution is
indeed titillated by the light excitation of the O/ZCS/P-3
photocatalyst. And the H2 evolution rate of O/ZCS/P-3 is
the fastest at the wavelength of 450 nm. Furthermore, it also can be
observed from Fig. 7f that the hierarchical O/ZCS/P-3 bi-heterostructure
photocatalyst not only exhibits ultrahigh H2 production
properties but also very good stability, which are mainly contributed to
the construction of the two efficient parallel approaches applied to the
fast migration of photo-induced charges in the architecture of
hierarchical O/ZCS/P-3 bi-heterostructure cages.