Preparation of CQDs-rGO/ZnO composites and photocatalytic degradation of metronidazole
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摘要: ZnO的低可见光活性及光生载流子迁移率低且容易复合的缺点限制了其在光催化领域的实际应用。为此，本文采用超声辅助浸渍法制备了碳量子点(CQDs)-还原氧化石墨烯(rGO)/ZnO三元复合催化剂，通过XRD、SEM、TEM、X射线能量散射光谱(XEDS)、XPS、BET、UV-Vis DRS、VB-XPS、光致发光(PL)、瞬态光电流响应(TPR)及电化学阻抗(EIS)等表征手段对其晶体结构、形貌和光电性能进行了表征。以模拟抗生素废水甲硝唑(MTZ)为降解对象，考察了CQDs-rGO/ZnO复合催化剂的光催化活性。结果表明：rGO和CQDs的引入，能够优化ZnO的能带结构、增强其对可见光的吸收。rGO和CQDs的强导电性，可促进光生载流子的快速转移和分离，有效提升CQDs-rGO/ZnO复合催化剂的光催化活性。淬灭实验表明，羟基自由基(•OH)和超氧自由基(${\text{•}}{\rm{O}}_2^ - $)是反应过程中的主要活性物质。在CQDs复合量为1%时，CQDs-rGO/ZnO的光催化活性最好，可见光照射2.5 h，对MTZ的降解率可达87.8%。经历4次循环后，依然可以降解75.2%的MTZ，说明CQDs-rGO/ZnO复合催化剂理化性能稳定。Abstract: The low visible light activity, low photogenerated carrier mobility and easy recombination of ZnO limit its practical application in the field of photocatalysis. To overcome these limitations, a carbon quantum dots (CQDs)-reduced graphene oxide (rGO)/ZnO ternary composite catalyst was prepared by an ultrasonic-assisted impregnation method and characterized for its crystal structure, morphology, and photoelectric properties using XRD, SEM, TEM, X-ray energy dispersive spectrometry (XEDS), XPS, BET, UV-Vis DRS, valence band (VB)-XPS, photoluminescence (PL), transient photocurrent response (TPR), and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of CQDs-rGO/ZnO composite catalyst was investigated with simulated antibiotic wastewater metronidazole (MTZ) as degradation object. The results show that the introduction of rGO and CQDs can optimize the band structure of ZnO and enhance its absorption of visible light. The strong electrical conductivity of rGO and CQDs can promote the rapid transfer and separation of photogenerated carriers, and effectively improve the photocatalytic activity of CQDs-rGO/ZnO composite catalysts. The quenching experiments show that hydroxyl radical (•OH) and superoxide radical (${\text{•}}{\rm{O}}_2^ - $) are the main active substances in the reaction process. When the recombination amount of CQDs is 1%, the photocatalytic activity of CQDs-rGO/ZnO is the best, and the degradation rate of MTZ can reach 87.8% under visible light irradiation for 2.5 h. After four cycles, 75.2% of MTZ can still be degraded, indicating that the physicochemical properties of the CQDs-rGO/ZnO composite catalyst are stable.
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Key words:
- ZnO /
- CQDs /
- rGO /
- photocatalysis /
- metronidazole
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图 2 ZnO (a)、rGO/ZnO (b)、CGZ-3 (c)的SEM图像
Figure 2. SEM images of ZnO (a), rGO/ZnO (b), CGZ-3 (c)
图 3 CGZ-3复合催化剂的XEDS能谱和SEM映射图
Figure 3. XEDS spectra and SEM mapping images of CGZ-3
σ—Standard deviation
图 4 (a) CQDs的TEM及HRTEM图像；CGZ-3的TEM图像(b)、HRTEM图像(c)
Figure 4. (a) TEM and HRTEM images of CQDs; TEM image (b) and HRTEM image (c) of CGZ-3
图 5 ZnO、rGO/ZnO和CGZ-3的N2吸附-解吸等温线
Figure 5. N2 adsorption-desorption isotherm of ZnO, rGO/ZnO and CGZ-3
BET—Brrunauer-Emmett-Teller
图 6 ZnO和CGZ-3的XPS图谱：(a) 全扫描谱；(b) Zn2p；(c) O1s；(d) C1s
Figure 6. XPS spectra of ZnO and CGZ-3: (a) Full survey spectra; (b) Zn2p; (c) O1s; (d) C1s
图 7 ZnO、rGO/ZnO和CGZ-3的UV-Vis DRS光谱(a)、Tauc's plot法计算的带隙Eg (b)、价带(VB)-XPS光谱(c)和能带位置变化示意图(d)
Figure 7. UV-Vis DRS spectra (a), band gap Eg evaluation by Tauc's plot (b), valence band (VB)-XPS spectra (c), schematic representation of the position variation of the energy bands (d) of ZnO, rGO/ZnO and CGZ-3
CB—Conduction band; α—Absorption coefficient; hν—Photon energy; NHE—Normal hydrogen electrode; EVB—VB potential
图 8 ZnO、rGO/ZnO和CGZ-3的光致发光(PL)光谱(a)、瞬态光电流响应(TPR)曲线(b)、电化学阻抗谱(EIS) (c)
Figure 8. Photoluminescence (PL) spectra (a), transient photocurrent response (TPR) curves (b) and electrochemical impedance spectroscopy (EIS) (c) of ZnO, rGO/ZnO and CGZ-3
图 9 不同样品对甲硝唑(MTZ)的可见光催化降解效果(a)和动力学曲线(b)
Figure 9. Photocatalytic activity (a) of the different samples through degrading metronidazole (MTZ), and kinetics profiles (b)
Ct—Concentration of MTZ after time t; C0—Initial concentration of MTZ
图 10 (a) CGZ-3对MTZ的循环使用效果；(b) CGZ-3使用前后的XRD图谱；(c) CGZ-3使用后的SEM图像
Figure 10. (a) Recycle effect of CGZ-3 on MTZ; (b) XRD patterns of the fresh and used CGZ-3; (c) SEM image of the used CGZ-3
图 11 不同淬火剂对MTZ降解的影响
Figure 11. Effect of different trapping agents on MTZ degradation
IPA—Isopropyl alcohol; SOD—Superoxide dismutase; TEOA—Triethanolamine; FFA—Furfuryl alcohol; KSO—Potassium peroxodisulfate
图 12 CQDs-rGO/ZnO的光催化机制示意图
Figure 12. Schematic diagram of the photocatalytic mechanism of CQDs-rGO/ZnO
λ—Wave length
表 1 CGZ-X样品及主要试剂使用量
Table 1. CGZ-X samples and main reagent dosage
Sample rGO/ZnO/mg CQDs
(0.1 mg/mL)/ mLCQDs:rGO/ZnO
mass ratioCGZ-1 200 10 0.50 CGZ-2 200 15 0.75 CGZ-3 200 20 1.00 CGZ-4 200 25 1.25 Notes: CQDs−Carbon quantum dots; rGO−Reduced graphene oxide. 
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