| Photocatalytic water splitting into H2 is one of the effective ways to solve the energy crisis and environmental pollution.Despite great efforts have been devoted to developing efficient photocatalytic H2-evolution systems,photocatalysts still suffer from low efficiency due to serious charge-carrier recombination.In this thesis,graphite carbon nitrides?g-C3N4?with different morphologies were prepared.And the photocatalytic H2 production from triethanolamine aqueous solution?10 vol%?under visible light irradiation??>420 nm?was investigated after loaded with different cocatalysts.The thesis includes the following three aspects:1.Preparation of Au/g-C3N4 photocatalyst.G-C3N4-H+Cl-nanosheet was first synthesized by calcining dicyandiamide at 550°C,followed by treatment with concentrated hydrochloric acid.Then Au/g-C3N4 composite photocatalyst was obtained through the electrostatic interaction between g-C3N4-H+and AuCl4-1 and the subsequent reduction by NaBH4.The prepared Au/g-C3N4 composite produced H2 at a rate of 3.7?mol·h-1,which is 4 times of that of g-C3N4.Moreover,after adding fluorescein in the photocatalytic reaction system,the rate of H2 generation drastically increased to 84.1?mol·h-1 in the presence of Au/g-C3N4,which is more than 25 times higher than that of FL+g-C3N4.The highly-improved performance is mainly ascribed to the following two points:?1?The loading of Au can greatly entend absorpting in visible light region and improve the electron-hole separation efficiency;?2?The efficient transfer of photo-generated electrons among photo-excited fluorescein molecule,g-C3N4 and Au,which is supported by photoluminescence spectra..2.Synthesis of Ni/g-C3N4 hollow spheres.G-C3N4 hollow spheres?denote as CNHS?were first prepared by calcining a mixture of melamine and cyanuric acid?mass ratio 1:1?at 550°C.Then,a certain amount of nickel acetate hexahydrate was added into the CNHS suspension,and Ni-CNHS composite photocatalyst was obtained after UV irradiation for 1 h.We found that the photocatalytic H2-production performance was dependent on the content of Ni in the Ni-CNHS composite.The highest rate was 19.1 mol·h-1 when the mass ratio of Ni to CNHS was 0.47 wt%.The improvement of photocatalytic acitvity is attributed to the fact that Ni increases the electron-hole separation efficiency and the hollow structure provides more active sites.The transient photocurrent response and photoluminescence spectra of Ni-CNHS composite support our proposed mechanism.3.Synthesis of Ni@C/g-C3N4 quantum dots.According to reference,g-C3N4quantum dots?denoted as CNQDs?were first synthesized by calcination of dicyandiamide at 550°C,followed by treatment with concentrated sulfuric acid,dimethylformamide and water.Ni nanoparticles with a small content of Ni3C wrapped by carbon shell?denoted as Ni@C?were obtained by calcination of nickel acetate at450°C.Finally,Ni@C/CNQD composite was obtained by hydrothermal reaction of CNQD and Ni@C.Compared with Ni/CNQD composite photocatalyst,the photocatalytic H2-production performance of Ni@C/CNQD increased by 1.7 times.The improvement of photocatalytic performance may be attributed to the fact that the good conductive C shell captures the photogenerated electrons,resulting in the effective separation of electron-hole pairs.Our speculation is confirmed by the transient photocurrent response and electrochemical impedance spectroscopy of photoelectrode assembled from Ni@C/CNQD. |
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