碳包覆纳米铜的原位热解法制备及其稳定性

1993年R.S.Rouff等[1]首次用电弧放电法制备出碳包覆LaC2的纳米复合材料，发现惰性的碳壳有许多优点[1, 2]：碳材料在酸碱条件下比较稳定，能保护被包覆的金属核不受环境的影响，使其抗氧化能力提高；提高了纳米粒子在极性溶剂中的分散性，阻碍其团聚；提高了材料的导电性能 自此碳包覆金属纳米材料受到了极大的关注，在光学、锂离子电池电极材料、超级电容器、生物医药、催化化学及环境工程等领域得到了应用[3~10] 目前制备碳包覆纳米材料的方法有十余种，除了Ruoff 等采用的电弧放电，还有化学气相沉积法、激光辐照蒸发、溅射和热解法等[11~15] 热解法的特点是，制备装置简单、成本低、一次产物较多、节能

糖类或有机大分子碳水化合物等原料绿色环保、来源广泛，备受研究者们的青睐 以淀粉[16]、纤维素、蔗糖、葡萄糖等[2, 12, 17]大分子量有机物为碳源，可热解制备碳包覆金属纳米材料 但是在制备过程中，为了金属源还原需加入适当的还原剂或引入还原性气氛 本文以天然棉纤维为模板，不引入外加还原剂或还原性气氛，采用一步热解法在氮气气氛中原位制备NCCC，研究了碳壳的形成对纳米铜的抗氧化性的影响

1 实验方法1.1 NCCC的制备

先将脱脂棉置于60℃烘箱中干燥至恒重，备用 取适量质量比为10∶1的脱脂棉和五水硫酸铜 将五水硫酸铜溶解在适量的纯水中，加入脱脂棉充分吸附硫酸铜溶液后静置12 h 将吸附了硫酸铜的脱脂棉(Cotton@Cu)置于管式炉(BTF-1200C-4ZL)中，在氮气气氛下以10℃/min的速率升温到390℃并保温1 h，自然冷却到室温，得到NCCC样品

1.2 商业纳米铜/微米铜(Nano-Cu/Micro-Cu)的预处理

将适量的商业纳米铜和微米铜分别置于马弗炉中，以10℃/min的速率升温到300℃并保温1 h，自然冷却到室温后得到Nano-Cu-air/Micro-Cu-air样品

1.3 碳包覆纳米铜/微米铜(Nano-Cu/Micro-Cu)用品的制备

将适量的Cotton@Cu和Nano-Cu-air/Micro-Cu-air置于管式炉中，在流量为100 mL/min的氮气氛中以10℃/min的速率升温到390℃并保温1 h，自然冷却到室温后得到Nano-Cu-air-Cotton/Micro-Cu-air-Cotton样品 为了比较，不加Cotton@Cu，只用适量的Nano-Cu-air/Micro-Cu-air，制备出Nano-Cu-air-N2/Micro-Cu-air-N2样品

1.4 材料性能的表征

用透射电子显微镜(Tecnai G2 F20 S-Twin，FEI)观察用品的微观结构，用X射线衍射分析仪(D8 Advance，Bruker AXS)测试用品的XRD谱，用型号为SDT Q600，TA的热重分析仪进行热分析，用激光显微共聚焦拉曼光谱仪(DXR 2xi，ThermoFisher)测试样品的拉曼光谱

2 结果和讨论2.1 NCCC用品的核壳结构

从图1a可以看出，NCCC中纳米颗粒均匀地镶嵌在碳基体中，粒径分布在15~50 nm，少量颗粒的粒径大于50 nm 图1b给出了根据高分辨TEM照片测得铜的(111)晶面间距d=0.209 nm 可以看出，在球状铜颗粒表面较为均匀地覆盖了一层厚度约为3 nm的碳壳 铜颗粒周围的碳层具有明显的石墨化结构 从图1c可见，粒径分布在15~50 nm的颗粒占比约为91%

图1



图1NCCC的TEM照片和NCCC中铜颗粒粒径的统计

Fig.1TEM images of NCCC (a, b) and particle size of copper particles (c)

为了确认NCCC的物相组成，测试其XRD谱 从图2可以看出，在390℃碳化后棉纤维吸附的硫酸铜已还原成铜(JCPDS Card No.04-0836)和少量的氧化亚铜(JCPDS Card No.05-0667) 棉纤维在氮气气氛下热降解的过程中产生大量的醛、酮、CO等还原性物质 这些还原性物质将二价铜离子还原为铜单质和少量的氧化亚铜 在XRD谱中除了铜和氧化亚铜的衍射峰外，在10°~30°还出现了两个凸包峰，说明棉纤维在390℃的碳化产物大部分为无定型碳；未出现石墨的特征峰，表明石墨化的碳量极少 根据TEM照片和XRD谱给出的结果，NCCC是典型的碳包覆纳米铜核壳结构的材料

图2



图2NCCC的XRD谱

Fig.2XRD patterns of NCCC

2.2 碳包覆商业纳米/微米铜的组成

从图3a中的Nano-CuTEM照片可见，纳米铜颗粒为球状，粒径为130~220 nm 图3b表明，纳米铜表面没有层壳 图4c表明，商业纳米铜并不是纯的纳米铜，还有少量的氧化铜和氧化亚铜

图3



图3Nano-Cu、Nano-Cu-air、Nano-Cu-air-Cotton和Nano-Cu-air-N2的TEM照片

Fig.3TEM images of Nano-Cu (a, b), Nano-Cu-air (c, d), Nano-Cu-air-Cotton (e, f) and Nano-Cu-air-N2 (g, h)

图4



图4Nano-Cu和Micro-Cu热处理前后的拉曼谱和XRD谱

Fig.4Raman spectrum and XRD patterns of Nano-Cu and Micro-Cu before and after treatment

为了解Nano-Cu的热稳定性，分别在氮气气氛和空气气氛中对其进行热重分析，结果在图5中给出 从图5a可以看出，在263.8℃出现一个很小的放热峰，可能是在Nano-Cu的制备过程中有机溶剂挥发或分解所致；从热重曲线可以看出，Nano-Cu的质量损失为0.82%，是水分蒸发和Nano-Cu表面的有机溶剂辉发所致 从图5b可见，在260.7℃和351.4℃分别出现放热峰 260.7℃处的放热峰与氮气气氛下出现的放热峰基本相同 351.4℃处的放热峰对应Nano-Cu的氧化

图5



图5Nano-Cu热重分析图

Fig.5TGA curves of Nano-Cu, (a) tested in N2 atmosphere (10℃/min), (b) tested in air atmosphere (10℃/min)

从图3c，d可以看出，在300℃空气预处理后Nano-Cu-air有明显的团聚，有的颗粒已经熔化，金属颗粒表面无壳 从图3e可见，Nano-Cu-air-Cotton呈现团聚状态，金属颗粒熔化，金属颗粒表面产生了不规则的壳，厚度为5~50 nm 从图3f可清晰地看出，Nano-Cu-air-Cotton表面有一层很均匀的壳，厚度约为5 nm 从氮气气氛中对照样Nano-Cu-air-N2的TEM照片可见，金属颗粒表面未形成壳 这表明，棉纤维热解产生的气氛是材料表面形成壳的直接原因 为了证实热解气氛还原后材料表面的壳为碳壳，对Nano-Cu-air-Cotton进行了拉曼分析 从图4a Nano-Cu-air-Cotton的拉曼谱图可以看出，在1376 cm-1和1592 cm-1处出现了特征峰，对应于碳的D峰和G峰 根据IG/ID的比值可衡量碳材料的有序度[18~20] 代表无序结构的D峰比较平缓，代表有序结构的G峰比较尖锐 IG/ID的比值为2.14，表明Nano-Cu-air-Cotton中有一定量的石墨化碳 与Nano-Cu-air的拉曼谱对比表明，经过热解碳源还原后得到Nano-Cu-air-Cotton，其表面确实有碳壳 将图4c给出的物相组成与Nano-Cu-air-N2相比可见，Nano-Cu-air-Cotton在棉纤维的热解气氛中发生了还原(表1) 从XRD物相组成可以看出，CuO已还原为氧化亚铜或铜单质 这也进一步证实了棉纤维的热解气氛的还原作用，解释了NCCC中单质铜的形成

Table 1

表1

表1商业纳米铜处理前后的物相组成

Table 1Phase composition of Nano-Cu before and after treatment

Sample name	Phase composition
	Cu	Cu2O	CuO
Nano-Cu	√	√	√
Nano-Cu-air	√	√	√
Nano-Cu-air- N2	√	√	√
Nano-Cu-air- Cotton	√	√	×

为了探究相同实验条件下棉纤维热解气氛对微米铜的影响，对微米铜样品进行相同的预处理 从图6a可以看出，Micro-Cu为不规则颗粒，出现了明显的团聚，颗粒尺寸为8 μm×3 μm；图6b给出了在300℃空气中预处理样品的的TEM照片，可见处理前后样品表面都没有壳层 经过棉纤维热解气氛还原后，微米铜颗粒周围包裹了壳层(图6c) 将图6c的照片放大(图6d, e)，可见颗粒表面覆盖了均匀的壳层 从氮气气氛对照样Micro-Cu-air-N2的TEM照片可见，颗粒的周围没有壳层 这个结论和纳米铜的验证结果一致，进一步证实棉纤维热解气氛是金属颗粒壳层形成的决定性因素 为了探究颗粒表面的壳层是否为碳壳，对其进行了拉曼分析 在图4b给出的Micro-Cu-air-Cotton拉曼谱中1365 cm-1和1598 cm-1处分别出现了对应于碳的D峰和G峰 IG/ID的比值为2.00，结合Micro-Cu-air-Cotton的TEM照片，表明Micro-Cu-air-Cotton中有一定量石墨化的无定型碳 与Micro-Cu-air的拉曼谱对比表明，Micro-Cu-air-Cotton表面的壳确实为碳壳 XRD谱给出的物相分析结果(图4d和表2)表明，Micro-Cu经空气热处理后单质铜氧化为氧化亚铜和氧化铜，经棉纤维热分解气氛热处理后得到Micro-Cu-air-Cotton，氧化铜被还原 而对照样品Micro-Cu-air-N2中仍然有氧化铜，证实了棉纤维热分解气氛的还原作用

图6



图6Micro-Cu、Micro-Cu-air、Micro-Cu-air-Cotton和Micro-Cu-air-N2的TEM照片

Fig.6TEM images of Micro-Cu (a), Micro-Cu-air (b), Micro-Cu-air-Cotton (c, d, e), and Micro-Cu-air-N2 (f, g)

Table 2

表2

表2商业微米铜处理前后的物相组成

Table 2Phase composition of Micro-Cu before and after treatment

Sample name	Phase composition
	Cu	Cu2O	CuO
Micro-Cu	√	×	×
Micro-Cu-air	√	√	√
Micro-Cu-air- N2	√	√	√
Micro-Cu-air- Cotton	√	√	×

上述原位热解实验结果表明，棉纤维热解气氛为商业纳米铜和微米铜核壳结构的形成提供了碳源，热解气氛具有一定的还原作用 这就给出了NCCC核壳结构中碳壳、单质铜和氧化亚铜的形成机理

2.3 核壳结构纳米铜的稳定性

为了探究NCCC的稳定性，将NCCC在室温敞口放置180 d，发现其物相组成没有变化(图7a中的曲线g) 另一组将NCCC放置在水中，7 d后氧化亚铜的相对含量提高了，浸渍14~35 d后其物相组成不变(图7a中的曲线b~f) 两组实验结果都表明，NCCC物相组成非常稳定

图7



图7碳包覆纳米铜在不同环境放置不同时间后的XRD谱

Fig.7XRD patterns of carbon-capsulated nano-copper particles stored in different environment for different time (a) NCCC stored in different environment for different time; (b) Nano-Cu stored for one day and 120 d; (c) Nano-Cu-air-Cotton stored for 1 d and 120 d

为了进一步验证核壳结构对材料抗氧化性能的影响，将Nano-Cu在密封环境中放置120 d，而将Nano-Cu-air-Cotton敞口放置于室温环境中120 d，并分析放置当天的样品和120 d后样品的物相 从图7b可以看出，Nano-Cu在密封环境下放置了120 d后氧化亚铜的峰趋于平缓，而氧化铜的峰更加尖锐，即氧化铜的相对含量提高了，表明材料被氧化了；而从图7c可以看出，Nano-Cu-air-Cotton放置120 d前后物相没有变化 这些结果表明，碳壳的生成对Nano-Cu-air-Cotton稳定性有非常关键的作用

3 结论

(1) 用一步热解法原位制备的核壳结构的纳米铜碳材料(NCCC)，单质铜纳米颗粒均匀地镶嵌在碳基体中，大部分粒径为15~50 nm 以浸泡了硫酸铜的棉纤维为热解碳源、以商业纳米铜和微米铜为铜源可原位制备碳包覆纳米/微米铜

(2) 用一步热解法制备碳包覆金属颗粒材料，核壳结构中的碳壳使材料的稳定性和抗氧化性提高

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Core-shell materials for advanced batteries

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" />

Manganese dioxide powders were firstly prepared via electric pulse assisted redox method with KMnO4 and MnSO4 as raw material, then MnO2/C composite materials coated with different amounts of carbon were fabricated via liquid phase sintering with glucose as a carbon source. The effect of amount of coated carbon on the morphology, structure and electrochemical properties of the MnO2/C materials were investigated. Results show that the coated carbon could induce the transformation of crystallographic structure of MnO2 from γ-type into α-type. Under heating conditions glucose decomposed and coated on the surface of MnO2 particles, which could inhibit the grain growth and thus refine grains. When the preparation with the process parameters: glucose concentration was 1.5 g/L and the current density was 2 A·g-1, the prepared MnO2/C material presented the specific capacitance of MnO2 of 722.2 F·g-1, in other words, the carbon coating could increase the specific capacitance by 80%, in comparison with that of the blank ones. Furthermore, after 4000 charge-discharge cycles, the capacitance retention rate could still maintain 74.72%, displayed good electrochemical performance and cycling performance.

潘 双, 庄 雪, 王 冰 等.

碳包覆改性二氧化锰电极材料的制备和性能

[J]. 材料研究学报, 2019, 33(7): 530

[11]

Xu B S, Guo J J, Wang X M, et al.

Synthesis of carbon nanocapsules containing Fe, Ni or Co by arc discharge in aqueous solution

[J]. Carbon, 2006, 44: 2631

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Su L W, Jing Y, Zhou Z.

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[J]. Nanoscale, 2011, 3: 3967

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Strong charge-transfer doping of 1 to 10 layer graphene by NO2

[J]. ACS Nano, 2012, 6(2): 1865

PMID

We use resonance Raman and optical reflection contrast methods to study charge transfer in 1-10 layer (1L-10L) thick graphene samples on which NO(2) has adsorbed. Electrons transfer from the graphene to NO(2), leaving the graphene layers doped with mobile delocalized holes. Doping follows a Langmuir-type isotherm as a function of NO(2) pressure. Raman and optical contrast spectra provide independent, self-consistent measures of the hole density and distribution as a function of the number of layers (N). At high doping, as the Fermi level shift E(F) reaches half the laser photon energy, a resonance in the graphene G mode Raman intensity is observed. We observe a decrease of graphene optical absorption in the near-IR that is due to hole-doping. Highly doped graphene is more optically transparent and much more electrically conductive than intrinsic graphene. In thicker samples, holes are effectively confined near the surface, and in these samples, a small band gap opens near the surface. We discuss the properties and versatility of these highly charge-transfer-doped, few-layer-thick graphene samples as a new class of electronic materials.

[20]

Ammar M R, Galy N, Rouzaud J N, et al.

Characterizing various types of defects in nuclear graphite using Raman scattering: heat treatment, ion irradiation and polishing

[J]. Carbon, 2015, 95: 364

Single-crystal metals encapsulated in carbon nanoparticles

2

1993