Study on Photoelectrochemical Properties of Colorful Carbon Nitrides Synthesized in Liquid-Phase

Reducing the band gap value and obtaining an ordered two-dimensional microstructure are crucial to improving the photoelectrochemical performance of carbon nitrides.By adjusting the ratio of urea and citric acid and ripening the precursor at the room temperature, we synthesize the carbon nitride materials with different colors of orange, green and blue.The resultant blue sample has a narrowed band gap of 1.74 eV and the strong absorption to the light in 550～700 nm wavelength range.The photoelectrochemical solar cell is fabricated using the synthesized carbon nitrides as light absorber, TiO2 as electron transporter and I3-/I -as the redox shuttle in electrolyte.The orange carbon nitride has typical photoelectric conversion performance with Jsc＝0.65 mA•cm -2 , Voc＝530 mV and ff＝0.64.The blue carbon nitride has obvious enhanced Jsc of 1.03 mA•cm -2 owing to the obvious enhanced visible light absorption, while the deeply decreased ff of 0.33.The cyclic voltammetry tests demonstrate the electrode reaction properties of the fabricated carbon nitride-based cell under illumination.It turns out that the cell based on the blue carbon nitride has obvious photo-induce pseudocapacitive behavior with an area specific capacitance of 4.9 mF•cm -2 .TEM (Transmission electron microscope), XRD (X-ray diffraction), FTIR (Fourier transform infrared spectroscopy) and XPS (X-ray photoelectron spectroscopy) reveal the microstructure characteristics of the colored carbon nitrides.The orange one is carbon nitride quantum dots with the particle size less than 10 nm.The blue one is a carbon nitride polymer containing graphite domains with oxidized boundaries, forming three-dimensional porous structure constructed by an ordered two-dimensional network.The green one has a mixture microstructure as well as the medium photoelectrochemical properties between the former two, which are both photoelectric conversion and photo-induced charge storage performance.