H2 and O2 Evolution from Water Half-Splitting Reactions by Graphitic Carbon Nitride Materials

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• 作者：A. Belen Jorge ; David James Martin ; Mandeep T. S. Dhanoa ; Aisha S. Rahman ; Neel Makwana ; Junwang Tang ; Andrea Sella ; Furio Cor脿 ; Steven Firth ; Jawwad A. Darr ; Paul F. McMillan
• 刊名：The Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：April 11, 2013
• 年：2013
• 卷：117
• 期：14
• 页码：7178-7185
• 全文大小：459K
• 年卷期：v.117,no.14(April 11, 2013)
• ISSN：1932-7455

文摘

Graphitic carbon nitride compounds were prepared by thermal treatment of C鈥揘鈥揌 precursor mixtures (melamine C₃N₆H₉, dicyandiamide C₂N₄H₄). This led to solids based on polymerized heptazine or triazine ring units linked by 鈭扤鈺?or 鈭扤H鈥?groups. The H content decreased, and the C/N ratio varied between 0.59 and 0.70 with preparation temperatures between 550 and 650 掳C due to increased layer condensation. The UV鈥搗is spectra exhibited a strong 蟺鈥撓€* transition near 400 nm with a semiconductor-like band edge extending into the visible range. Samples synthesized at 600鈥?50 掳C showed an additional absorption near 500 nm that is assigned to n鈭捪€* electronic transitions involving the N lone pairs. These are forbidden for planar symmetric s-triazine or heptazine structures but become allowed as increased condensation causes distortion of the polymeric units. Photocatalysis studies showed there was no correlation between the increased visible absorption due to this feature and H₂ evolution from methanol used for the anodic reaction. In the absence of any cocatalyst the sample synthesized at 550 掳C showed 1.5 渭mol h^鈥? H₂ evolution with UV鈥搗is irradiation, but this dropped to 0.23 渭mol h^鈥? when the UV spectrum was blocked. Use of a Pt cocatalyst was required to observe H₂ evolution from the other samples. Using a more powerful (300 W) lamp led to higher H₂ production rates (31.5 渭mol h^鈥?) with visible illumination. We suggest the distorted N sites caused by increased polymerization result in electron/hole traps that counter the photocatalytic efficiency. Issues concerning sample porosity are also present. Photocatalytic O₂ evolution was determined for RuO₂-coated samples using the 300 W lamp with aqueous AgNO₃ solution as the sacrificial agent. The materials all showed production rates 9 渭mol h^鈥?. A highly crystalline compound containing polytriazine structural units ((C₃N₃)₂(NH)₃路LiCl) prepared in this study did not show measurable photocatalytic activity.