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).