2.1 Catalyst preparation
2.1.1 Preparation of ZIF-67: ZIF-67 was prepared according to
the reference with some modifications [33]. In a typical
preparation, two solutions were prepared by
dissolving
Co(NO3)2·6H2O (0.873 g)
in 30 mL of methanol and 2-methylimidazole (0.984 mg) in 10 mL of
methanol, respectively. And they were completely dissolved with
stirring. Subsequently, the two solutions were mixed rapidly and aged
for 24 h at room temperature. The synthesized blue precipitate was
washed with ethanol 3 times and then dried at 60 oC.
2.2 Preparation of ZIF-67 derived CoP polyhedron: CoP
polyhedrons were prepared by annealing the ZIF-67 (0.05 g) with
NaPO2H2•H2O (0.5 g) in a
N2 flow at 300oC for 2 h with a
heating rate of 5 oC min-1.
NaPO2H2•H2O was placed
upstream of N2, and ZIF-67 was placed downstream.
2.1.2 Preparation of Co3O4Quantum Dots: Co3O4 quantum dots were
prepared by reference to published literature [30,31]. Typically,
0.249 g (1 mmol) of cobalt (Ⅱ) acetate tetrahydrate
[Co(Ac)2·4H2O] was put into round
bottom flask containing 7 mL benzyl alcohol. And then the mixture was
kept stirring at ambient temperature till
Co(Ac)2·4H2O was completely dissolved.
Afterward, 7 mL ammonium hydroxide (25%) solution was dropwise added
into the above open reactor under vigorous stirring. The vessel with
reddish-brown solution was kept heating in an oil bath set at
165 oC. Then, the
solution was getting immediately boiling with a large number of bubbles,
and it was kept at 165 oC for 2 h under continuous
stirring. Later, a black suspension was formed. Finally, 7 mL diethyl
ether was put into the finished reaction suspension under continuous
stirring. Subsequently, the black precipitate was washed with ethanol
several times via centrifugation until the washing solution becomes
clear, and then it was dried at 60 oC.
2.1.3 Preparation of ZnCdS: According to the references
[34,35], Cd0.5Zn0.5S solid solution
was prepared by a precipitation-hydrothermal means. Typically, both
1.0975 g (5 mmol) zinc acetate dihydrate and 1.3326 g (5 mmol)
cadmium acetate dihydrate were
dissolved into 40 mL of distilled water in a PTFE vessel (100 mL), and
0.9391 g (12.5 mmol) thioacetamide (TAA) also was added into the vessel
under vigorous stirring. After they were completely dissolved, 10 mL
NaOH aqueous solution (4 M) was put into the above container and then
stirred for 1 h. Subsequently, the sealed autoclave was heated and kept
at 180 oC for 24 h. After that, the resulting yellow
samples was washed with distilled water and ethanol three times, and
then dried at 60 oC.
2.1.4 Fabrication ofCoP/ZnCdS/Co3O4Quantum Dots: First, two solutions were prepared by
dispersing
Co3O4 quantum dots
(0.01 g) and CoP polyhedrons (0.01 g)
in 100 mL ethanol, respectively. And the two solutions were dispersed
thoroughly and evenly through ultrasonic and then stirring treatments.
Later, 0.1 g ZnCdS was dispersed completely in 60 mL ethanol via
ultrasonic treatment, then 15×1000 μL of
CoP suspension (1.5 wt.% CoP) was
continuously added into the above ZnCdS suspension under vigorous
stirring. After it was stirred for half an hour, 15×1000 μL of
Co3O4quantum dots suspension (1.5 wt.% Co3O4quantum dots) also was continuously put into the vessel, and then it was
placed in water bath set as 80 oC with vigorous
stirring until precipitate was completely separate out. This assembled
Co3O4 quantum dots (1.5 wt.%)
/ZnCdS/CoP (1.5 wt.%) was labeled O/ZCS/P-3. And according to the added
content of Co3O4 quantum dots, the named
O/ZCS/P-1, O/ZCS/P-2, and O/ZCS/P-4 correspond to 5×1000 μL (0.5 wt.%),
10×1000 μL (1.0 wt.%), and 20×1000 μL (2.0 wt.%) of
Co3O4quantum dots suspension, respectively. And the content of CoP in O/ZCS/P
samples were arranged as 1.5 wt.% (15×1000 μL of CoP suspension).
2.1.5 Fabrication of ZnCdS/CoP: The synthesis of
ZnCdS/CoP was same as those of
O/ZCS/P in the absence of Co3O4 quantum
dots. 5×1000 μL (0.5 wt.%), 10×1000 μL (1.0 wt.%), 15×1000 μL (1.5
wt.%) and 20×1000 μL (2.0 wt.%) of CoP suspension was added into 60 mL
ZnCdS (0.1 g) suspension, respectively. After the same experiment as
above, the obtained ZnCdS/CoP
samples were named ZnCdS/CoP-1, ZnCdS/CoP-2, ZnCdS/CoP-3, and
ZnCdS/CoP-4 in turn.
2.2 Characterization of catalysts: The crystalline and phase
information were obtained by taking a Rigaku RINT-2000 diffractometer.
The geometric features of the obtained catalysts were investigated by
JSM-6701F JEOL FESEM (Field-Emission Scanning Electron Microscope) and
JEM1200EX JEOL TEM (Transmission Electron Microscopy). The
Brunauer-Emmett-Teller (BET) specific surface area
(SBET) of powers were analyzed on an ASAP 2460 analyzer
equipped with Vac Prep 061 sample degas system. X-ray photoelectron
spectroscopy (XPS) of elements were performed on an ESCALAB 250Xi.
Uv-visible diffuse reflectance spectra (UV-vis DRS) were studied on
UV-2550 spectrophotometer. The fluorescence curves for the resulting
photocatalysts were tested on FLUOROMAX-4 fuorescence spectrometer. The
photoelectrochemical tests (I-T, EIS, LSV) for were measured on
VersaStat4-400 electrochemical workstation equipped with a standard
three electrode cell, and Pt sheet with 2*2 cm2 is for
counter electrode, saturated calomel electrode (SCE) is for reference
electrode, 0.2 mol/L Na2SO4 aqueous
solution is to be electrolyte, ITO
(1*2 cm2)
drop-coating photocatalyst (1*1 cm2) was acted as
working electrode, respectively. A light source for I-T measures is to
be 300W xenon lamp.
2.3 Photocatalytic hydrogen evolution experiments: The
assessment of photocatalytic hydrogen evolution performance of the
obtained catalysts were proceeded in anaerobic system. Typically, 10 mg
of sample was ultrasonically suspended into 30 mL of aqueous solution
containing lactic acid solution (10 vol.%) as
the sacrificial reagent. Prior to
exerting light, the reactor was degassed with ready N2to thoroughly exclude the air and the dissolved oxygen in reaction
system. Then, the enclosed reactor was paced in a nine-channel
photocatalytic reaction system equipped with 5 W LED as a light to
generate H2, and the produced H2 was
detected on a Tianmei GC7900 gas chromatograph (TCD, 13Xcolumn).