飞秒激光辐照氧化石墨烯的纳结构与电化学性能研究 下载: 965次
Objective Due to the advantage of high electrical conductivity, thermal conductivity, and superior surface area ratio, graphene has become the current research focus in the application of flexible energy storage and sensor. Comparing the chemical vapor deposition, mechanical exfoliation, and epitaxial growth, the direct reduction of graphene oxide (GO) can satisfy the demand for graphene production in the industrial field. Currently, the methods of GO reduction are chemical, thermal, and photon reductions. Based on reduction efficiency and cost-benefit, laser irradiation is an efficient way to remove the surface oxygen group for GO reduction without special physical and chemical conditions. Thus, laser reduction can be considered a highly effective method for graphene production. Some study has focused on different methods of GO through laser reduction, such as KrF excimer, ultraviolet, and femtosecond laser. Despite these investigations on GO reduction, simultaneous reduction and nanopattern of GO through laser irradiation are still challenging. To further investigate the morphology and structural properties of reduced GO, this study compares the morphology of the reduced GO with different nanostructures through femtosecond laser irradiation with 1030 nm and 257 nm. Besides, the influence of different laser-induced nanostructures on the electrochemical impendence will be discussed.
Methods GO can be obtained using Hummers methods. Different from graphene, surface oxygen-containing groups located at the surface and margin of GO nanosheets improve the hydrophilia property. By the preparation of GO dispersion, spray coating was used to form a uniform GO film on the polyethylene terephthalate (PET) substrate. After that, a femtosecond laser with 1030 nm and 257 nm was irradiated on the GO surface to construct the nanostructure. The morphology and characteristics of nanostructure were compared to show the difference of GO through femtosecond laser irradiation. The all-solid-state interdigital micro-supercapacitors were constructed with the assistance of PVA/H2SO4 to obtain the electrochemical performance of GO by femtosecond laser.
Results and Discussions The surface ablated morphology of GO using femtosecond laser irradiation was observed. The comparison results showed that the morphology evolution in GO has not followed the linear change with an increase in the incident laser energy and pulses number (Figs. 3 and 5). Under the 1030 nm laser irradiation, the ablated region of GO occurred in the layered annular structure, resulting from energy deposition and thermal diffusion. However, a large number of nanosheets located at the ablated margin of GO were obtained by 257 nm laser irradiation. The photochemical effect plays a significant role in laser irradiation. Two surface laser-induced nanostructures were further investigated to obtain the mechanism in the morphology evolution of GO (Fig. 7). Femtosecond laser-induced periodic surface structures (LIPSSs) with a high and low spatial frequency contributes to the surface plasmon polaritons (SPPs) on the GO surface. The coupling effect of SPPs and LIPSSs can result in the formation of nanostructure by 1030 nm femtosecond laser irradiation. The photomechanical effect induced by photochemical action is the main reason for the groove nanostructure’s formation by 257 nm laser irradiation. Combined with the results of the Raman spectrum of GO (Fig. 8), the ratio of the intensity of D and 2D peaks relative to that of G peak was calculated. Thus, the 1030 nm laser irradiation is essential for improving the transformation of graphite structure from sp 3 to sp 2 and removing surface oxygen-containing groups. Through the electrochemical impendence spectra (Fig. 9), the impendence spectra of different nanostructures induced by laser irradiation with 1030 nm and 257 nm display apparent distinct. The ohmic resistance value nanostructure with LIPSSs or stripe is 40 Ω, which is lower than that of the nanostructure with groove morphology. According to the test data fitting, the nanostructure with LIPSSs or stripe morphology demonstrates the process of charge transformation at the high frequency and ion diffusion at the low frequency. The results suggested that the nanostructure by femtosecond laser irradiation with 1030 nm can improve the electrochemical action of micro-supercapacitors.
Conclusions In this study, the morphology and characteristics of GO nanostructures were investigated using femtosecond laser irradiation. Under the 1030 nm laser irradiation, the interference effect of SPPs and incident laser results in the formation of stripe nanostructure with the period of subwavelength. The groove nanostructure by 257 nm laser irradiation contributes to the photochemical effect. Based on the analysis of the Raman spectra of GO by femtosecond laser irradiation, the GO reduction level by 1030 nm femtosecond laser irradiation is higher than that of GO by 257 nm laser irradiation. Compared with the results of electrochemical impendence of different nanostructures by femtosecond laser irradiation, the GO nanostructure by 1030 nm laser irradiation improves the rate of ion diffusion of electrodes and decreases the ohmic resistance. This study will strengthen the practical application of simultaneous reduction and nanopatterning of GO by femtosecond laser in microelectronic devices.
1 引言
石墨烯由于其高导电性、高热导率和高比表面积的优势[1-3],逐步成为柔性储能器件[4]和传感器[5]等领域的研究热点。相比于化学气相沉积[6]、石墨机械剥离[7]和外延生长[8]等方法,氧化石墨烯还原更易于满足实际生产对于石墨烯产量的需求。目前氧化石墨烯还原的方式主要以化学还原[9]、热还原[10]和光还原[11]为主,从还原方法的效率和可控性角度看,采用激光辐照方法[12-14]可以在不需要特殊的物理或化学条件下,直接去除表面的含氧官能团,实现氧化石墨烯的快速还原,因此被认为是最具优势的石墨烯制备方法。
近年来,针对激光还原氧化石墨烯也进行了广泛的研究。Guo等[15]利用双束激光干涉方法同步还原和表面刻划氧化石墨烯,通过改变入射激光功率,可以方便地控制氧化石墨烯的还原程度和表面形貌特征,并且基于所获得的多级形貌结构的激光还原氧化石墨烯,成功制备出具有快速响应率和灵敏度的柔性湿度传感器。Kang等[16]详细研究了连续激光和飞秒激光对于氧化石墨烯还原的影响,发现脉冲频率10 kHz的飞秒激光辐照更有助于还原氧化石墨烯的电导率提升,非线性的光化学作用对于氧原子去除起主导作用,凭借这种方法可以实现石墨烯薄膜的快速制备和无损伤加工。Trusovas等[17]采用皮秒激光辐照氧化石墨方法,研究了还原氧化石墨烯的特征拉曼峰的面积和位置变化,结果表明,当入射激光功率为50 mW,扫描速度为30 mm/s时,表面结构缺陷最小且还原程度最高,此外他们还进一步分析了激光还原氧化石墨过程中温度场的变化,认为辐照局部产生1200 K左右的高温有助于表面含氧官能团的去除。Kasischke 等[18]分析了在超快激光辐照过程中还原氧化石墨烯表面形貌演变特征与化学成分变化,他们认为超快激光与氧化石墨烯作用中自组装效应直接导致了表面周期结构的成型,采用这种方法可以同步获得带有周期结构的还原氧化石墨烯。但是直到目前,人们对于飞秒激光还原氧化石墨烯的研究还不足,表面纳结构形貌和结构特征尚需要进一步探究。因此,本文主要针对氧化石墨烯的飞秒激光纳结构成型与电化学阻抗特性开展研究,分析入射激光的波长、脉冲个数和能量对于氧化石墨烯烧蚀特性的影响,对比1030 nm 和257 nm波长的激光辐照产生的纳结构形貌与结构特征,进一步探讨不同表面纳结构对于电化学阻抗特性的影响。
2 实验方法
在进行飞秒激光超快作用氧化石墨烯实验之前,首先需要制备氧化石墨烯薄膜。基于常用的Hummers方法[19],首先在干燥的烧杯中加入适量的浓硫酸(H2SO4, 体积分数为98%),搅拌过程中加入石墨粉(2 g)和硝酸钠粉末(1 g),再依次加入高锰酸钾(KMnO4, 6 g),搅拌并控制温度不超过20 ℃,继续搅拌并升温至35 ℃保持2 h。然后缓慢加入去离子水,通过适量的双氧水(H2O2)还原残余的氧化剂,洗涤并过滤烘干。结合真空冷却方式获得氧化石墨烯粉末。相比于石墨烯,氧化石墨烯表面和边缘处含有大量的含氧官能团(羟基、羧基等),使其具有很好的亲水性,易于制备不同浓度的分散液,因此采用超声喷涂方法可以获得厚度均匀的氧化石墨烯薄膜。首先将200 mg的氧化石墨烯粉末溶解到100 mL的去离子水中,超声分散3 h后获得质量浓度为2 mg/mL的氧化石墨烯分散液。喷涂基底选择聚对苯二甲酸乙二醇酯(PET)薄膜,为了保证喷涂薄膜表面的一致性,喷涂过程中需要控制流量和次数,
表 1. 喷涂氧化石墨烯薄膜的参数
Table 1. Parameters of graphene oxide by spray coating
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图 1. 氧化石墨烯薄膜的形貌。(a)表面形貌;(b)侧面形貌
Fig. 1. Morphology of graphene oxide film. (a) Surface morphology; (b) lateral morphology
所使用的飞秒激光微细加工系统构成如
图 2. 飞秒激光辐照氧化石墨烯的系统原理图
Fig. 2. Schematic illustration of the graphene oxide by femtosecond laser irradiation
3 分析与讨论
3.1 氧化石墨烯的烧蚀特性
在研究飞秒激光辐照氧化石墨烯表面纳结构成型之前,首先针对氧化石墨烯在不同入射激光参数下的烧蚀特性进行分析。烧蚀试验采用激光单点辐照方法,通过控制激光辐照的脉冲能量和脉冲个数,分析氧化石墨烯表面烧蚀特性。为了避免过高的激光能量,导致氧化石墨烯基底的损伤。试验过程中辐照的激光脉冲能量低于100 nJ,脉冲个数在5~100范围内。最终激光辐照所形成的表面形貌利用扫描电子显微镜(SEM)观察获得。
图 3. 1030 nm波长激光辐照下氧化石墨烯的烧蚀区域面积演变
Fig. 3. Evolution of the ablated area of graphene oxide by 1030 nm femtosecond laser irradiation
图 4. 1030 nm飞秒激光辐照下氧化石墨烯表面烧蚀形貌。(a) 5脉冲;(b) 20脉冲;(c) 50脉冲;(d) 60脉冲;(e) 80 脉冲;(f) 100脉冲
Fig. 4. Ablated surface morphology of graphene oxide by 1030 nm femtosecond laser irradiation. (a) 5 pulses; (b) 20 pulses; (c) 50 pulses; (d) 60 pulses; (e) 80 pulses; (f) 100 pulses
应该指出,激光烧蚀的表面形貌特征与入射波长有关。
图 5. 257 nm波长激光辐照下氧化石墨烯的烧蚀区域面积演变
Fig. 5. Evolution of the ablated area of graphene oxide by 257 nm femtosecond laser irradiation
图 6. 257 nm飞秒激光辐照氧化石墨烯表面烧蚀形貌。(a) 5脉冲;(b) 20脉冲;(c) 40脉冲;(d) 60脉冲;(e) 80 脉冲;(f) 100脉冲
Fig. 6. Ablated surface morphology of graphene oxide by 257 nm femtosecond laser irradiation. (a) 5 pulses; (b) 20 pulses; (c) 40 pulses; (d) 60 pulses; (e) 80 pulses; (f) 100 pulses
3.2 表面纳结构成型特征研究
为了进一步研究飞秒激光辐照产生的纳结构形貌特征,采用激光线扫描方式在氧化石墨烯薄膜表面进行处理。
图 7. 1030 nm和257 nm激光辐照下纳结构表面与侧面形貌。(a) 1030 nm 激光辐照表面;(b) 1030 nm激光辐照侧面;(c) 1030 nm激光辐照放大图;(d) 257 nm激光辐照表面;(e) 257 nm激光辐照侧面;(f) 257 nm激光辐照侧面放大图
Fig. 7. Surface and side morphology of nanostructures irradiated by 1030 nm and 257 nm laser . (a) Surface morphology irradiated by 1030 nm laser; (b) lateral morphology irradiated by 1030 nm laser; (c) enlarged lateral morphology irradiated by 1030 nm laser; (d) surface morphology irradiated by 257 nm laser; (e) lateral morphology irradiated by 257 nm laser; (f) enlarged lateral morphology irradiated by 257 nm laser
图 8. 飞秒激光辐照氧化石墨烯的拉曼光谱
Fig. 8. Raman spectra of the graphene oxide by femtosecond laser irradiation
从图中可以看出,1030 nm激光辐照产生的表面条纹纳结构相对于原始氧化石墨烯表面的D峰与G峰强度比(ID/IG)下降,2D峰与G峰(I2D/IG)强度比增加。由还原氧化石墨烯的拉曼光谱峰位变化可知,1030 nm激光作用后不仅使得表面形貌结构发生变化,而且碳网结构中碳原子轨道由sp3杂化转变成sp2杂化,实现了氧化石墨烯的还原。但257 nm激光辐照下表面纳结构的2D峰位强度却不明显,表明氧化石墨烯碳网结构中sp3杂化并没有发生明显变化。拉曼光谱测试结果说明,在相同激光能量辐照下,1030 nm激光相比于257 nm激光更有助于氧化石墨烯内部的碳网结构转变,氧化石墨烯的还原程度相对更高。
3.3 电化学阻抗测试与分析
利用飞秒激光微细刻划方法,在两种纳结构的还原氧化石墨烯薄膜上分别加工出指状电极。
图 9. 1030 nm 和257 nm飞秒激光辐照氧化石墨烯的电化学阻抗谱
Fig. 9. Electrochemical impendence spectra of graphene oxide irradiated by 1030 nm and 257 nm femtosecond laser
4 结论
氧化石墨烯在飞秒激光辐照下的表面纳结构的形成与入射激光参数直接相关。1030 nm激光辐照氧化石墨烯过程中,入射激光与表面等离激元发生干涉作用,进而形成纳周期条纹结构。而257 nm激光辐照产生的沟槽微纳结构主要由光化学作用所致。通过对氧化石墨烯表面产生的纳结构的形貌与结构表征分析,发现1030 nm 激光辐照下氧化石墨烯的还原程度要高于257 nm激光辐照。基于电化学特性分析,对比两种表面纳结构电极的阻抗谱,发现具有条纹周期纳结构的电极的接触电阻小于沟槽纳结构电极的接触电阻。因此,在入射激光能量相同时,1030 nm 飞秒激光辐照更有助于氧化石墨烯的还原以及在微型电极方面的应用。
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