Controllable construction of ZnO@SnO2 heterojunction composite nanotubes and their photocatalytic properties
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摘要: 性能优异的功能纳米材料的设计构筑对于光催化应用而言至关重要。基于模板自刻蚀机制，利用两步溶剂热技术，以一维ZnO纳米棒为模板，在无需附加酸刻蚀的条件下成功制备一维圆顶状ZnO@SnO2异质结纳米管复合材料(Heterojunction domed nanotubes，HDNs)。由于ZnO与SnO2具有匹配的能级结构，在纳米管界面处可形成促进载流子分离的内建电场，赋予该材料优异的光催化与稳定性能。通过控制实验过程中自产生的碱性强弱，实现两性氧化物ZnO的自刻蚀，从而实现ZnO@SnO2 HDNs的管壁厚度可控调控与催化性能的调节。借助SEM、TEM、STEM及PL等表征手段对材料的微观形貌、元素组成、生长机制与性能进行了考察。以甲基橙、亚甲基蓝、曙红等为污染物模型，光催化污染物降解实验结果表明，获得的ZnO@SnO2 HDNs具有优良的光催化性能，光照60 min内对亚甲基蓝、曙红的降解率可达到95%，表明构筑的纳米管异质结极大地促进了载流子的分离，抑制其复合，提高了光催化性能。同时，循环稳定性能测试说明构建的异质结纳米管催化剂具有良好的稳定性能，在染料降解方面具有广阔的应用前景。Abstract:
The design and construction of functional nanomaterials with excellent properties is very important for photocatalytic applications. The ZnO@SnO2 heterojunction domed nanotubes (HDNs) were successfully prepared without additional acid etching step using one-dimensional ZnO nanorods as templates by two-step solvothermal technology based on the template self-etching mechanism. The built-in electric field which can promote carrier separation can be formed at the nanotube interface owing to the matching energy level structure between ZnO and SnO2, endowing the material excellent photocatalytic and stability properties. By controlling the intensity of self-generated alkaline during the experimental processes, the amphoteric oxide ZnO can be etched, achieving the controllable regulation of the thickness of tubes and their photocatalytic performance. The morphology, element composition, growth mechanism and properties of the ZnO@SnO2 HDNs were investigated by means of SEM, TEM, STEM and PL. Taking methyl orange, methylene blue and eosin as pollutant models, the experimental results of photocatalytic pollutant degradation showed that ZnO@SnO2 HDNs had excellent photocatalytic performance, and the degradation rate of methylene blue and eosin can reach 95% within 60 min. These results indicate that the constructed one-dimensional heterojunction can greatly promote the separation of carriers and inhibit their recombination, thereby improving the photocatalytic performance. At the same time, the cycle stability test showes that the heterojunction nanotube photocatalyst has great stability and broad application prospects in dye degradation. -
Key words:
- ZnO@SnO2 /
- heterojunction /
- charge separation /
- photocatalysis /
- dye degradation
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图 1 ZnO@SnO2异质结圆顶状纳米管(HDNs)的生长示意图
Figure 1. Schematic of preparation processes of ZnO@SnO2 heterojunction domed nanotubes (HDNs)
图 2 ZnO@SnO2 HDNs样品的SEM图像(插图为ZnO@SnO2 HDNs的高分辨率SEM图像) (a) 、横截面SEM图像 (b) 及元素分布图 (c)
Figure 2. SEM image (Inset is the high-resolution SEM image of ZnO@SnO2 HDNs) (a), corresponding cross-sectional SEM image (b) and elemental mappings (c) of ZnO@SnO2 HDNs
图 3 ZnO@SnO2 HDNs的低倍TEM图像 ((a)、(c)) 和高倍TEM图像 (d)；(b) ZnO纳米棒的低倍TEM图像；ZnO纳米棒 (e)和ZnO@SnO2 HDNs (f)的EDS图谱
Figure 3. Low magnification TEM images ((a), (c)) and high-resolution TEM image (d) of ZnO@SnO2 HDNs; (b) Low magnification TEM image of ZnO NRs; EDS spectra of ZnO NRs (e) and ZnO@SnO2 HDNs (f)
图 5 ZnO NRs在溶剂热生长SnO2壳前后的SEM图像：当SnO2前驱体浓度为0 mmol/L、2 mmol/L, 7 mmol/L和15 mmol/L时样品的SEM图像 ((a)~(d)) 及相应的断面SEM图像 ((e)~(h))
Figure 5. SEM images of ZnO NRs before and after SnO2 shell coating with hydrothermal method: SnO2 precursors concentration are set at 0 mmol/L (as-prepared ZnO NRs), 2 mmol/L, 7 mmol/L, 15 mmol/L, and the plane SEM images are shown in ((a)-(d)) and present the corresponding cross-sectional SEM images, respectively ((e)-(h))
图 6 ZnO@SnO2 HDNs随时间变化的生长过程：ZnO@SnO2 HDNs在反应时间为10 min (a)、20 min (b)、40 min (c) 和80 min (d) 时的TEM图像
Figure 6. Time-dependent growth process of ZnO@SnO2 HDNs: TEM images of the ZnO@SnO2 HDNs grown at the reaction time of 10 min (a), 20 min (b), 40 min (c) and 80 min (d)
图 7 ZnO、SnO2和ZnO@SnO2 HDNs的稳态PL (a)和光电流响应曲线(b)
Figure 7. Steady state PL (a) and photocurrent responses (b) of ZnO@SnO2 HDNs (a), ZnO (b) and SnO2 (c)
a—ZnO; b—SnO2; c—ZnO@SnO2 HDNs
图 8 ZnO@SnO2 HDNs的带隙结构图及其电子-空穴分离示意图
Figure 8. Schematic diagram showing band configuration and electron-hole separation at interface of ZnO@SnO2 HDNs under UV irradiation
Ef—Fermi level; Eg—Band gap; VB—Valence band; CB—Conduction band
图 9 ZnO@SnO2 HDNs对曙红 (a)、甲基橙 (b) 和亚甲基蓝 (c) 的降解效果和上述3种有机染料的光催化降解速率图 (d)
Figure 9. UV-vis absorption spectra of eosin red (a), methyl orange (b) and methylene blue (c) in the presence of ZnO@SnO2 HDNs and the photocatalysis degradation rate of the three organic dyes (d)
C—Real-time concentration of organic dyes; C0—Initial concentration of an organic dye
图 10 汞灯照射不同时间下ZnO纳米棒对曙红 (a)、亚甲基蓝 (b) 和甲基橙 (c) 的降解效果
Figure 10. Variations of adsorption spectra of organic dye solutions in the presence of the ZnO irradiated by a mercury lamp for varying times: (a) Eosin red; (b) Methylene blue; (c) Methyl orange
图 11 ZnO@SnO2 HDNs对亚甲基蓝光催化降解的循环稳定性能
Figure 11. Cycling tests of photocatalytic activity of the ZnO@SnO2 HDNs for methylene blue degradation
表 1 本工作中ZnO@SnO2 HDNs与以往报道复合光催化剂对甲基橙光催化降解性能的比较
Table 1. Comparison of photocatalytic performance of ZnO@SnO2 HDNs toward methyl orange in this work with previously reported composite photocatalysts
Sample Light source Experimental condition Degradation time Ref. ZnO/SnO2 nanocomposites 300 W Hg lamp 200 mg; 100 mL, 6×10−5 mol/L 100 min/56% [31] SnO2 aerogel/rGO nanocomposite 40 W UV lamp
(370 nm)100 mg L−1; 1×10−5 mol/L 60 min/84% [32] SnO2-CNT nanocomposites 9 W eight UV-vis
lamps (365 nm)200 mg; 100 mL, 3×10−5 mol/L 180 min/79% [33] Chitosan-SnO2 8 W mercury lamp 100 mg; 5×10−5 mol/L 100 min/92% [34] Cu-doped SnO2 QDs 200 W xenon lamp 100 mg; 100 mL, 6×10−5 mol/L 180 min/99% [35] ZnO@SnO2 HDNs 500 W xenon lamp 50 mg 80 min/95% This work Notes: rGO—reduced graphene oxide; CNT—carbon nanotube; QDs—quantum dot. 
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