Study On Preparation Of Graphitic Carbon Nitride Thin Films And Their Photoelectrochemical Properties

Graphitic carbon nitride（g-CN）has become a new type of photocatalytic/photoelectrocatalytic material,and attracted much attention due to its suitable band gap,low cost and excellent chemical stability.The preparation of high-quality g-CN film is of fundamental significance for the wide application in the field of photoelectrocatalysis;however,because of its high chemical stability,the preparation of g-CN thin films has many difficulties.Thermal evaporation deposition polymerization is a newly developed vapor-phase growth method to prepare g-CN thin films,and the related experimental parameters need to be further improved.In this dissertation,a new two-step thermal evaporation deposition polymerization method has been developed to prepare high-quality g-CN thin films on different substrates,and their photoelectrochemical properties have also been systematically studied.On the basis of the conventional one-step thermal evaporation deposition polymerization method,an additional step of pre-pyrolysis treatment of the precursor under the lower temperature has been employed,by which the quality of the consequently deposited g-CN thin films has been remarkably improved.The differences in light transmittance,surface morphology and roughness,and thickness of the prepared g-CN films prepared by the conventional one-step and improved two-step methods have been compared.The structure,composition and surface chemical bonding states of the two-step-deposited g-CN thin films have also been carefully characterized.The results show that the two-step-deposited g-CN films are more uniform and transparent,with lower surface roughness,and free from obvious particle boundaries.Because the polymerization process is more sufficient,the optical absorption edge of the two-step-deposited g-CN films shows a slight red-shift with respect to the conventional one-step-deposited films.Theπ-electron delocalization in this system is more pronounced,leading to a narrowed band gap;therefore,the position of the main peak in the fluorescence spectrum also shows a corresponding red shift with a notably reduced fluorescence intensity.We performed a comparative study on the photocurrent response characteristics of the g-CN films prepared by these two methods.The results show that the photocurrent density of the two-step-deposited g-CN film is 2.9 times higher than that of the one-step-deposited film.Furthermore,the effect of deposition temperature on the photoelectrochemical properties of the two-step-deposited g-CN films has been systematically studied.The results show that the increase of the deposition temperature promotes more complete polymerization in the g-CN film,and its light absorption and photo-generated carrier separation efficiency are simultaneously improved,thereby significantly improving the photoelectrochemical performance.When the deposition temperature was 590℃,the photocurrent density of the film sample was found to be 55μA cm^-2 under an AM 1.5illumination and a bias voltage of 1.1 V/RHE,and its incident photon-to-electron conversion efficiency was measured as high as 11%under a monochromatic wavelength at 350 nm.The two-step thermal evaporation deposition polymerization method developed in this dissertation can prepare high-quality g-CN thin films,holding the great potentials as photoelectrode materials in photoelectrochemical and related applications.