Abstract
ZnFe2O4 is a magnetic and visible light response photocatalyst material, but was rarely studied in photocatalytic water splitting for hydrogen production. In this study, we first used ZnFe2O4 aerogels and ZnFe2O4/CdS composites as the photocatalyst for photocatalytic water splitting under visible light irradiation. The high surface area, 3-D connected pore structure, and large porosity are the advantageous for aerogels to serve as the photocatalyst. We prepared ZnFe2O4 aerogels under different annealing conditions and tried to find out the relationship between the hydrogen production rate and annealing condition. In the part of composite material, we utilized the magnetic property of ZnFe2O4 and the compatible energy band structure of ZnFe2O4 and CdS for our photocatalytic water splitting purposes. We also used varius catalyst material analyses to interpret the experimental results. The photocatalytic water splitting reaction of ZnFe2O4 aerogels was carried out in a methanol solution under 400W high pressure Hg light source. The methanol served as the sacrificial agent. Under the full spectrum of radiation, the hydrogen evolution rate of ZnFe2O4 aerogels, calcined at 500 ° C for 10 hours, was 9840μmol / g • hr. The data showed that the rates of hydrogen evolution increased as the crystallinity improved. In our experiments, a co-catalyst, Pt, was loaded via a photodeposition method to give a Pt/ZnFe2O4 aerogels photocatalyst (1 wt %), but it did not show significant improvements in the rate of hydrogen evolution. In the part of visible light irradiation, 1M NaNO2 was circulated through the reactor jacket to filter out the UV light (λ<400 nm). After 6 hours of continuous light irradiation, the hydrogen evolution became saturated. The best performance was achieved by the ZnFe2O4 aerogels calcined at 500 ° C and held for 10 hours, and the hydrogen evolution rate was 7.43μmol/g•hr. Another part of this research focused on the performance of CdS/ZnFe2O4 composite materials in photocatalytic water splitting. In this part, we used a Na2S /Na2SO3 solution as the sacrificial reagent. About 20wt.% of CdS was loaded onto the ZnFe2O4 aerogel. Here, aerogels provided high specific surface area and appropriate pore sizes for loading CdS, leading to 25.8μmol/g•hr in hydrogen evolution rate under visible light irradiation, and showed good stability in the cycle experiment. The photocatalyst of 20wt.% ZnFe2O4 decorated on CdS nanorods were characterized by XRD, BET, XPS, SEM, PL, and TEM. The result proved that ZnFe2O4 was successfully decorated on CdS nanorods, and made the photocatalyst magnetic for easy recycle. The conduction band of ZnFe2O4 is more negative than that of CdS, and thus the electron can move from ZnFe2O4 to CdS for hydrogen reduction. The valence band of ZnFe2O4 is less positive than that of CdS, so that the holes generated in CdS can move to ZnFe2O4 to protect CdS from photocorrosion. In the long term experiment, the ZnFe2O4 decoration not only hampered the photocorrosion of CdS, but also improved the activity of hydrogen evolution. The highest hydrogen production rate of 20wt.% ZnFe2O4 decorating on CdS nanorod was 2811μmol/g•hr.