Photocatalytic Degradation of Diclofenac in Water under Simulated Solar Light Using Tubular Carbon Nitride Synthesized by Self-Assembled Supramolecular Precursor
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摘要: 石墨相氮化碳(g-C3N4，GCN)作为一种新型无金属二维材料，因在可见光驱动下能够降解水中新有机污染物而备受关注. 但传统石墨相氮化碳存在比表面积小与活性位点少的弊端，严重限制了其应用前景. 为改良氮化碳性能，利用超分子自组装前驱体热聚合方法成功制备了微米级管状石墨相氮化碳(TCN)，使用扫描电镜(SEM)、傅里叶变换红外光谱(FT-IR)、X射线衍射(XRD)、紫外-可见漫反射光谱(UV-vis DRS)等技术手段，对TCN的形貌、元素组成、晶体结构、电化学性能等进行表征. 研究还选取双氯芬酸(DCF)作为目标污染物，探索其降解行为与机理. 结果表明：①TCN基本结构单元为七嗪环，但比表面积(20.9 m2/g)较GCN增加了1倍以上. ②TCN (100)晶面暴露增强，晶面调控暴露出更多七嗪环边缘氮原子的孤对电子，利于光生电子激发和载流子分离，从而增强光催化活性. ③TCN能带带隙为2.48 eV，小于GCN (2.69 eV)，说明TCN对可见光的吸收能力提升. ④莫特-肖特基曲线、光电流、阻抗谱图和扫描伏安谱图等电化学性能测试结果表明，TCN的光生电子转移效率大幅提升，有利于抑制光生空穴-电子对(h+-e-)的复合. ⑤TCN在模拟太阳光驱动下降解双氯芬酸(DCF)的动力学试验中准一级动力学常数(k1)达6.99×10-2 min-1，是GCN的5.5倍. ⑥电子自旋共振谱(ESR)和自由基淬灭试验证实，体系中超氧根自由基(·O2-)是最重要的活性氧物种，光生空穴(h+)也对DCF的降解有贡献. 研究显示，以超分子自组装的方式制备石墨相氮化碳的前驱体将有助于促进氮化碳可见光吸收、加速载流子分离，并提升光催化活性.Abstract: Graphite carbon nitride (g-C3N4, GCN), as a novel metal-free two-dimensional (2D) semiconductor, can efficiently photodegrade emerging organic pollutants in water under visible light. Thus, the development of new g-C3N4 photocatalysts has attracted more and more attention. However, low specific surface area and active sites limit the potential applications of bulk g-C3N4. In this study, self-assembled supramolecular precursor was initially synthesized by hydrothermal treatment of melamine, and then tubular carbon nitride (TCN) was prepared by thermal polymerization. TCN was characterized by SEM, FT-IR, XRD, UV-vis DRS, Mott-Schottky plot, photocurrent, electrochemical impedance spectroscopy (EIS) response, and linear sweep voltammetry (LSV). Diclofenac (DCF) was selected as target pollutant to study photocatalytic degradation mechanism. The results showed that: (1) TCN had an interconnected tri-s-triazine structure (same with GCN), but the specific surface area of TCN (20.9 m2/g) was 2 times that of GCN. (2) TCN achieved greatly enhanced (100)-facet exposure through control of crystal structure, which exposed more lone pair electrons of nitrogen atoms on the edge of tri-s-triazine, thus facilitating excitation of photogenerated electrons and the separation of charge carriers to improve photocatalytic activity. (3) The energy band gap (Eg) of TCN was 2.48 eV, which was lower than that of GCN (2.69 eV). The narrowed energy band gap verified that TCN improved the absorption of visible light. (4) Photoelectric characterizations, including Mott-Schottky plot, photocurrent, EIS, and LSV, indicated that TCN had enhanced electron transfer efficiency, which was beneficial to inhibit the recombination of photogenerated holes and electron pairs (h+-e-). (5) Under simulated solar light, the pseudo-first order kinetic constant (k1) of the photocatalytic degradation of diclofenac (DCF) by TCN (6.99×10-2 min-1) was 5.5 times that of GCN (1.28×10-2 min-1). (6) Electron spin resonance (ESR) spectra and scavenger quenching tests further demonstrated that the superoxide radicals (·O2-) were the dominant species for DCF photodegradation, and photogenerated holes also contributed to DCF degradation. This study proved that using self-assembled supramolecular precursors for g-C3N4 synthesis could enhance visible light absorption and carrier separation rate, and further improve photocatalytic activity.
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Key words:
- self-assembled supramolecular precursor /
- tubular g-C3N4 /
- photocatalysis /
- diclofenac /
- radicals
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图 4 TCN与GCN的莫特-肖特基曲线、阻抗谱图、光电流谱图和扫描伏安曲线
Figure 4. Mott-Schottky plot, EIS, photocurrent response, and LSV of TCN and GCN
图 5 DCF降解动力学与pH对60 min内DCF光催化降解速率的影响
Figure 5. Photodegradation kinetic of DCF and pH effects on DCF photodegradation at 60 min
图 6 TCN与GCN超氧根自由基的电子自旋共振谱图与淬灭试验
Figure 6. ESR spectra of TCN and GCN for superoxide radical and scavenger effect on DCF degradation
表 1 GCN与TCN降解DCF的矿化率与动力学拟合结果
Table 1. DCF mineralization and pseudo 1st order kinetic model fitting results
催化剂 DCF矿化率/% k1/min-1 R2 GCN 21.8 1.28×10-2 0.961 TCN 51.9 6.99×10-2 0.998 
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