Fabrication of CuO Nanowires on Copper Foams by Thermal Oxidation and Investigation of Their Photocatalytic Properties
Yu Jingjing Liao Bin Zhang Xu Zhou Fuzeng Fu Kaihu Wu Xianying
Key Laboratory of Beam Technology and Material Modification of Ministry of Education,College of Nuclear Science and Technology,Beijing Normal University
Abstract:
A three-dimensional CuO nanowires( CuO NWs) network was synthesized by thermal oxidation of copper foams. Morphology,composition and structure of the CuO NWs were characterized by scanning electron microscopy( SEM),X-ray diffraction( XRD)and X-ray photoelectron spectroscopy( XPS),respectively. The effects of different temperatures on the morphologies of CuO NWs,and on average crystalline sizes of oxides were investigated. Growth mechanism of the CuO NWs was also introduced. Based on the experimental data,it was clearly that the diameter of CuO NWs increased with the temperature. The density of CuO NWs decreased with the increasing average crystalline sizes of oxides. It was obvious that the optimal growth temperature was under 400 ℃ with an average diameter in the range of 50 ~ 120 nm,in addition to a length up to 5 μm,which performed the maximum aspect ratio. Meanwhile,the photocatalytic capability of CuO NWs towards methyl orange under visible and ultraviolet light was also tested in this paper. The results showed that CuO NWs possessed excellent photocatalytic ability with the degradation efficiency η of 76. 6% and 87. 2% under visible and ultraviolet light,respectively.
采用日本Hitachi S-4800型号的扫描电镜(SEM)对样品的表面和截面形貌进行分析;采用荷兰PANalytical公司的X'Pert PRO MPD X射线衍射仪(XRD,Cu Kα)进行物相分析;采用VGES-CALABMKⅡ型X射线光电子能谱(XPS Al Kα)对化学成分进行分析;WFJ 7200型可见分光光度计测量吸光度。
图3为相同条件下(400℃,氧流量40 ml·min-1,30 min),在泡沫铜和铜片衬底制备的样品,泡沫铜上的纳米线密度和长度大于铜片上的。图4为Cu O NWs的直径和长度随温度的变化曲线,直径随温度而增大,长度随温度先增大而减小。由以上分析,可得如下结论:以温度为变量,在300~700℃之间,随着温度的上升,Cu O层晶粒尺寸变大,Cu O NWs的密度和长度先增后减,直径逐渐增大,最佳生长温度400℃。
图2 不同温度下,泡沫铜在40 ml·min-1的O2中保温30 min样品的SEM图像Fig.2 Surface SEM images of samples prepared at different temperatures in 40 ml·min-1O2for 30 min
(a)300℃;(b)400℃;(c)500℃;(d)600℃;(e)700℃;(f)Magnified view of image at 500℃
图3 在相同氧化条件下不同衬底上制备样品的SEM图Fig.3 Surface SEM image of samples at same condition of oxidation but at different substrates
(a)Copper foam;(b)Copper foil
图4 不同温度下制备的Cu O NWs的直径和长度随温度的变化曲线Fig.4 Average diameter(D)and length(L)of Cu O NWs prepared at different temperatures(T)as a function of temperature
图5 经不同温度热氧化样品的XRD图谱及Cu2O和Cu O的平均晶粒尺寸随温度的变化曲线Fig.5 XRD patterns of samples at different temperatures(a)and average crystalline size of oxide layers as a function of temperature for Cu2O(b)and Cu O(c)
图6 泡沫铜在500℃,40 ml·min-1的O2中保温30 min所得样品的截面SEM图像Fig.6 Cross-section SEM images of samples prepared on copper foam(500℃,40 ml·min-1O2,30 min)
(a)Top-view of cross-section SEM image;(b)Magnified side-view of cross-section SEM image
2.3 Cu O纳米线的能谱分析(EDS)和XPS表征
图6为在500℃,40 ml·min-1的O2中保温30min,所制得样品截面的SEM图像。从图6(a)看到局部Cu O NWs从泡沫铜骨架上脱落下来,把白色区域放到高倍下可观察到截面部分,即图6(b)所示,在泡沫铜衬底之上有3层不同结构不同厚度的氧化物,底层A较厚为粗大的柱状晶粒,中间层B较薄为细小的晶粒,顶层C为纳米线,且纳米线垂直于样品表面生长),为了确定各层的成分,在每一层做了EDS图谱分析,结果如表1~3所示:A层,B层和C层氧化物中铜元素与氧元素的原子比分别与氧化亚铜,氧化铜和氧化铜匹配。此结果与之前的一些研究相一致,泡沫铜的氧化过程包括两步反应
[15] :
表1 底层A的EDS分析结果Table 1 EDS analysis result of bottom layer A(%) 下载原图
表1 底层A的EDS分析结果Table 1 EDS analysis result of bottom layer A(%)
表2 中间层B的EDS分析结果Table 2 EDS analysis result of intermediate layer B(%) 下载原图
表2 中间层B的EDS分析结果Table 2 EDS analysis result of intermediate layer B(%)
表3 顶层C的EDS分析结果Table 3 EDS analysis result of top layer C(%) 下载原图
表3 顶层C的EDS分析结果Table 3 EDS analysis result of top layer C(%)
为了进一步确定纳米线的成分,又将制备的样品进行了XPS分析。如图7所示,图7为样品中Cu 2p峰的XPS谱图,可以观察到4个峰,其中934.1和954.0 e V的两个峰分别是氧化铜的Cu2p3/2和Cu 2p1/2特征峰。此外,在943.3和962.5e V处的两个卫星峰来自于Cu2+,由此,可以说明纳米线的成分为Cu O
[4] 。
图7 Cu O纳米线的电子能谱图Fig.7 XPS spectrum of Cu O NWs
2.4 纳米线生长的微观解释
近年来,许多科研小组已经利用热氧化方法制备了Cu O NWs,他们对纳米线的生长机制和驱动力进行了详细讨论
[6,7,8,9,10,11,12,13,14] 。Goncalves等
[7] 认为Cu O NWs是由于铜离子在Cu2O层的短路扩散引起的。Yuan等
[10] 提出的纳米线的生长机制是因铜氧化生成的氧化物之间晶格不匹配,产生了压应力,驱动铜离子沿晶界向外扩散,然后再通过快速地表面扩散,形成纳米线。实验方法与他们类似,但不同之处在于采用了泡沫铜基底,泡沫铜受热时,由于其热传导系数很低,温度分布很不均匀,外部的温度高于内部,外部将会膨胀,导致压应力产生
[16] ;另外,当泡沫铜氧化时,形成三层结构:Cu/Cu2O/Cu O层,由于各摩尔体积顺序为Cu<Cu2O<Cu O,所以Cu2O下表面受到来自Cu层的压应力,同时上表面受到Cu O层的张应力,Cu O底层受到Cu2O的压应力。因此在Cu/Cu2O/Cu O界面产生了压应力梯度,促使铜离子向压应力小的方向迁移
[17] 。所以,泡沫铜相对铜片等基底来说,氧化时具有更充足的驱动力,更容易生成纳米线,如图3,得到了很好的验证。
图8 Cu O纳米线生长机制示意图Fig.8 Schematic diagram of growth mechanism for Cu O NWs(Cu ions diffusing outward via grain boundary diffusion driven by compression stress)
光催化结果如图9。由图9(a)和(b)的数据分析可知,不管是可见光还是紫外光条件下,单独使用Cu O NWs光催化时,由于光生载流子复合率高,使其催化效率几乎为零;而H2O2本身具有强氧化性,能够氧化甲基橙;在Cu O NWs和H2O2协同使用时,光照Cu O NWs,产生光生电子-空穴对,H2O2是很好的电子接受体,能够捕获光生电子,有效分离电子-空穴对,大大降低复合率,从而提高Cu O NWs的光催化降解效率
[6] ,其在可见光和紫外光照射下降解效率分别高达76.6%和87.2%,紫外光下的k是可见光的11倍左右。
图9 MO降解随时间变化的一级动力学模型Fig.9 Pseudo-first order kinetics of MO photodegradation with time evolution(k and k'respectively representing reac-tion rate constants on photocatalytic degradation of MO under Vis and UV)
(a)Vis;(b)UV
3 结论
利用简单的热氧化方法在泡沫铜基底上成功制备了三维网状的Cu O NWs,纳米线垂直于泡沫铜骨架生长,最佳生长温度在400℃,直径在50~120 nm,长度高达5μm,直径随温度升高而增大,密度随Cu2O平均晶粒尺寸的增大而减小,验证了纳米线的晶界扩散生长机制。同时,光催化实验证明,三维网状的Cu O NWs和H2O2协同使用光催化降解甲基橙时,在紫外光下催化效率能高达87.2%,降解速率常数较大。