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The Phoenix-like Noble Metal: Cu Hui Yan, Rukang Zhang, Long Feng, Heng Li, and Jiwen Liu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01317 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on January 4, 2018

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Crystal Growth & Design

The Phoenix-like Noble Metal: Cu Hui Yan*,†,‡, Rukang Zhang†, Long Feng†, Heng Li§, Jiwen Liu*,† †

Tianjin Key Laboratory of Photoelectric Materials and Devices, National Demonstration Center

for Experimental Function Materials Education, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ‡

Key Laboratory of Display Materials and Photoelectric Devices, Tianjin University of

Technology, Ministry of Edu-cation, Tianjin 300384, China §

Fujian Provincial Key Laboratory of Semiconductors and Applications, Collaborative

Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen 361005, China KEYWORDS: Cu2O nanostructures, spinodal decomposition, self-assembly

ABSTRACT: Copper is one of the least reactive metals under atmospheric condition. By heating copper in air to hundreds degrees centigrade, its surface is oxidized to black CuO. Interestingly, the black CuO surface layer peels off automatically when the temperature of the sample is lowered to room temperature. Three-dimensional red self-assembled Cu2O nanostructures are observed in the new exposed surface (This phenomenon is compared to a phoenix reborn from the ashes.). A simple extension of the spinodal decomposition to single phase system is proposed to account quantitatively for the self-assembled behavior of Cu2O nanostructures. The presented analysis is also useful to understand similar behaviors of other single phase systems.

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INTRODUCTION Copper was one of the first metals ever extracted and used by humans. It has vital contributions to sustaining and improving society since the dawn of civilization. Thousands years later, copper is still very important to be a material of choice for a variety of domestic, industrial, and high-technology applications due to its thermal conductivity, electrical conductivity,

malleability,

and

stability.

Since

the

discovery

of

high-temperature

superconductivity in LaBaCuO by Bednorz and Müller,1 copper and its oxides have attracted much more attention due to their fascinating physics.2-4 Among the useful properties of copper, the stability is excellent and outstanding: copper is resistance to corrosion and is one of the least reactive metals under atmospheric condition.5 Recently, the oxidation of copper and copper surfaces are considered to be a promising research focus because of their potential for a wide range of both fundamental scientific importance and technological applications.6 Copper oxidation has always been taken as a typical model system to understand the metal oxidation in general.6-8 Notable studies about copper oxidation have been published over the past decades since it was included in the original work of Cabrera and Mott.911

However, the oxidation of copper and the physical properties of the resulting oxides are

fundamental scientific problems which are still not completely understood today. Since there has currently been a surge of interest in the use of copper oxidation for corrosion, catalysis, optoelectronics, gas sensing, and thin-film processing,12-14 the need for a detailed understanding of the surface structures of these resulting oxides has become even more pressing. Along with the development of modern experimental techniques as well as computational simulation methods, much more details about the oxidation process of this important metal can be revealed.


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In this paper, we present a study of oxidation of Cu. By heating Cu foil in air to hundreds degrees centigrade, its surface is oxidized to black CuO nanowire films. A remarkable phenomenon observed is that the black CuO surface layer peels off automatically when the temperature of the sample is cooled down to room temperature. Three-dimensional (3D) red selfassembled Cu2O nanostructures appear on the new exposed surface of the Cu foil. This observed phenomenon is compared to a phoenix reborn from the ashes and the Cu is tentatively denominated as phoenix-like noble metal. This is an interesting phenomenon that both CuO and Cu2O nanostructures formed during this facile process, and they separated from each other naturally, meaning great potential applications. We attribute the peeling off of CuO surface layer to the effect of self-assembled behavior of Cu2O nanostructures. Understanding the principles behind the spontaneous formation of structured morphologies is of interest as a fundamental scientific question. A simple extension of the spinodal decomposition to single phase system is applied to account quantitatively for the self-assembled behavior of Cu2O nanostructures. The presented analysis is also useful to understand self-assembled behaviors of other single phase systems.15,16 The spinodal decomposition is usually applied to explain self-assembled domain patterns of a binary mixture A and B.14, 17-19 The essential physics explored in this model is that the domain patterns arise from the competition between short-range interatomic attractive interaction, which is delineated by boundaries with line tension in two-dimentional (2D) system, and long-range dipolar repulsive interaction induced by electric, magnetic, or elastic fields.14 Usually, the relative composition of A and B phases (or area fraction in 2D system) is selected as the order parameter to characterize the domain patterns. Although the spontaneous formation of organized structures is a well studied subject, new phenomena are still being uncovered recently.20-26


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EXPERIMENTAL SECTION In brief, the copper foils with the copper purity of 99.999% were cleaned in aqueous HCl solution (PH=3) for ~ 60 s, followed by repeated rinsing with distilled water, and then the copper foils had been dried in a Nitrogen flow. The prepared Cu foils were placed in an alumina boat and heated to the set-point temperature under ambient conditions immediately. X-ray diffraction (XRD) measurements were performed on a Shimadzu 6000 X-ray diffractometer with CuKa1 radiation (0.15406 nm) to study the composition of the samples. The morphologies of the nanostructures were studied by scanning electron microscopy (SEM) (Hitachi S-4800, 5 kV), and atomic force microscope (AFM) (Veeco, Multimode 8). The chemical compositions of the nanostructures were determined by an energy dispersive spectrometer (EDS) attached to the SEM. RESULTS AND DISCUSSION Fig. 1(a) shows a typical image of a Cu foil after heating in air at 400 oC for 3 hours. The black CuO layer peels off automatically and red Cu2O is observed on the surface of Cu foil when the temperature of the sample is cooled down to room temperature (Please watch the video of supporting materials for this process). This behavior is compared to a phoenix reborn from the ashes. Fig. 1(b) is a typical low magnification SEM image of the foil at the boundary of residual CuO layer. Fig. 1(c), (d), and (e) show high magnification SEM images of different surface regions marked as c, d, and e in the plane (b). There are serried CuO nanoribbons with hundreds nanometers width and up to several µm in length on the CuO layer, as shown in plane (d). In plane (e), the new exposure surface of Cu foil is composed chiefly of gibbous Cu2O nanostructures. Fig. 1(f) shows the schematic structural model of the Cu foil after heating


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procedure. The product is composed of three parts: black CuO surface layer, red Cu2O middle layer, and orange Cu substrate.

Figure 1. (a) A digital camera image of a Cu foil after heating in air at 400 oC for 3 hours. The black surface is the residual CuO layer. The light red surface is the Cu2O nanostructures. (b) A typical low magnification SEM image of the foil at the boundary of residual CuO layer. (c), (d), and (e) show high magnification SEM images of different surface regions marked as c, d, and e in the plane (b). There are serried CuO nanoribbons with hundreds nanometers width and up to several µm in length on the CuO layer, as shown in plane (d). In plane (e), many gibbous Cu2O nanostructures are observed on the surface of Cu foil. (f) The schematic structural model of the Cu foil after heating procedure. The crystal structures and elemental analysis of the different parts of the sample were further studied by XRD and EDS measurements. The diffraction patterns of (a) the Cu foil substrate before the growth procedure, (b) the black surface layer, and (c) the red middle layer were shown


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in Figure 2. Red, blue and purple crystal face indices indicate the diffraction peaks of Cu (JCPDS 04-0836), CuO (JCPDS 48-1548), and Cu2O (JCPDS 05-0667) respectively. There exist diffraction peaks of Cu2O in the XRD result of CuO layer. This is because when the Cu foils were oxidized in air, CuO nanowires were not formed directly, instead Cu2O appeared firstly and some act as a precursor to the finally formation of CuO. After the growth procedure, there still exist Cu2O on the surface of the CuO layer.27-29 Since the Cu2O and CuO layer are ultrathin, the diffraction peaks of Cu in the XRD results of both is due to the Cu substrate.28,29 The typical SEM results of the black CuO surface layer and the red Cu2O middle layer are shown in Figure 3(a) and 3(b). Figure 3(c) shows the corresponding EDS results of the red frame in panel Figure 3(a), and the insert indicates the composition with about 44.09 atom % of Cu and 55.91 atom % of O, little difference with the stoichiometric value for CuO. And the redundant O element is due to the absorption of oxygen in air. Figure 3(d) shows the corresponding EDS results of the red frame in panel Figure 3(b), revealing the composition with about 74.22 atom % of Cu and 25.78 atom % of O as illustrated in the insert. Since the initial Cu2O layer is ultrathin and it is much thinner than before at the positions between the Cu2O nanograins after annealing, the redundant Cu element is due to the signature of the copper substrate. No other element other than Cu and O could be detected in the EDS spectra of the nanostructures, indicating their high purity. The physical mechanism model of the heating process has been elaborated in our previous work in detail.28 During being cooled down to the room temperature, the CuO surface layer peels off automatically. While the sample cooled down to the room temperature very slowly, the black CuO surface layer also peels off automatically from the Cu substrate. So this interesting phenomenon can not be simply ascribed to the sensible thermal expansion coefficient difference between Cu2O and CuO.


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Figure 2. XRD patterns of (a) Cu foil substrate before growth, (b) the black CuO surface layer, and (c) the red Cu2O middle layer.

Figure 3. Typical SEM results of (a) the black CuO surface layer and (b) the red Cu2O middle layer. (c) and (d) show the corresponding EDS results of the red frame in panel (a) and (c) respectively. The insert in (c) and (d) illustrate the quantitative analysis of the percentage of Cu and O elements. This spontaneous formation that the ultrathin Cu2O film synthesized in the heating process organized to be nanostructures (as shown in Fig. 1e and Fig. 3b) while the temperature decreasing can be identified as a self-assemble progress.29-32 Fig. 4 is a typical AFM image of the


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two-dimensional self-assemble Cu2O nanostructures formed after the cooling procedure. The width of the Cu2O cubic like nanograins ranges from dozens to hundreds nanometres. And the nanograins show varying surface height less than ~ 60 nm. These uneven 3D Cu2O nanograins formed in the annealing process are the key factor that dominates the CuO surface layer peeling off automatically from the substrate.

Figure 4. A typical AFM image of the Cu2O nanoparticles formed after the annealing procedure. The right scale bar indicates the vertical fluctuation of the surface. AFM images were recorded under ambient atmosphere at room temperature using the Scan-Asyst mode. The spinodal decomposition mechanism is adopted to explain Cu2O structure reorganization during the annealing process. In this single phase Cu2O system, there are mainly three forms that modulate the surface topography. One is the local elastic energy which induced the reconstruction of the 2D thin film, decreasing the energy of the system. The other two competition forms are the local boundary energy and the long-range elastic relaxation energy, which make the system stable.


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Figure 5. Schematic diagrams of the self-assembly processes of Cu2O nanograins. The black grids are a guide to the eyes. During the annealing process, the Cu2O layer subject to the mass conservation law, so we assume that the formation of the uneven Cu2O self-assembled nanostructures is due to the migration of the surface molecules of the Cu2O layer, which has been proposed in the literature of the spinodal decomposition theory.17 The detailed schematic diagrams of the self-assembly processes of Cu2O on the copper substrate are illustrated in Figure 5. Figure 5(a) shows the Cu2O ultrathin film formed in the heating process. The vaporized Cu gas molecules oxidized into Cu2O vapor and fell down to the surface of the copper foil forming a thin layer on the substrate, as


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elaborated in our previous work.28 Figure 5(b) shows the early stage of the self-assembly processes, the Cu2O molecules tend to assemble at some point to form small gibbous nanostructures, generating fluctuations in the thin film. As the reorganization continued, more and more Cu2O molecules accumulated on the points until the migration stage stopped. At last, lots of nanograins formed on the Cu substrate, as shown in Figure 5(c). The reorganization of the Cu2O ultrathin film is mainly embodied in the variation of the thickness, so we choose the dimensionless film thickness as the order parameter: d=D/Dc. Here, D is the local thickness of the film. Before annealing, the initial film thickness D0 of Cu2O is considered to be uniform in order to simplify the problem. Taken the surface roughness, D0 is the average thickness of the initial film. And Dc is a critical thickness whether to consider the local elastic energy or not.When the film thickness reaches Dc, the local elastic energy caused by defects and mismatch significantly considerably reduce and can be ignored. In this single phase system, the free energy density can be written as:17,29,33 G = U f + U b + U el .

(1)

Where Uf is the elastic energy density owing to film-substrate (Cu2O-Cu) mismatch, Ub is the boundary energy density caused by the defects of the interface, and Uel is the long-range elastic relaxation energy density as a result of the non-uniform surface stress, all of which can be described by the order-parameter field. From the supporting materials, the elastic energy density Uf can be written as:29,34

U f = α d (1 − ed −1 )2 . Here, α = 2 µe

(2)

E 1 +ν D c f 2 , µe = . v is the Poisson ratio, E is the Young modulus, and f is 1 −ν 2(1 +ν )

the mismatch of the epitaxial growth film defined as: f = ( as − ae ) / ae . as and ae are the lattice


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constant of the substrate and the film respectively. Uf induces the formation of the surface structure, which enlarges the fluctuations of the order parameter d and has a tendency to decrease to two regions: d→0 or d→dm ( dm >Dc). Ub and Uel are both generated by the appearance of boundary. Using the Cahn-Hilliard mode of the two-dimensional system,35-40 the boundary energy Ub can be expressed by the gradient of the order parameter d: U b = h ( ∇d ) . 2

(3)

Here, h is a positive constant in connection with the fluctuation of the order parameter and the local boundary energy. The long-range elastic relaxation energy density due to the surface stress has been described by W. Lu et al.41-43 Taking the isotropy of the Cu2O ultrathin film into consideration, Uel can be written as:

(1 −ν )φ d πE 2

U el = −

Where, φ = 2 µe

2

( x1 − ξ1 )

∫∫ 

∂d ∂d + ( x2 − ξ 2 ) ∂x1 ∂x2

x −ξ + x −ξ ( 1 1 ) ( 2 2 ) 2

2 32

 

dξ1dξ2 (4)

1 +ν Dc f . 1 −ν

Up to now, the detailed description of the local free energy has been obtained. When the fluctuation of the order parameter is enlarged by Uf, the rest two terms of the free energy density will have an important influence on the evolution of the surface structure. Since Ub has a positive value, it will increase the energy of the system by decreasing the length of the boundary, hence the surface structure will become large and sparse; on the contrary, Uel has a negative value, and the surface structure will become small and dense due to its effect. So Ub and Uel obey a competition mechanism determining the stable state of the surface structure. Above all, the phase


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local elastic energy tends to coarsen the morphology, while the local boundary energy and the long-range elastic relaxation energy tend to refine the morphology. When the thickness of the film reaches a certain value, Uf can be large enough to generate defect such as mismatch dislocation in the system, which will make Uf and Uel decay rapidly in return, and then Ub will play a dominant role. That means any boundary will increase the energy of the system, so the result is that the system will evolve from the original nanostructures to the atomically-flat two-dimensional plane at the surface.29 Then, the evolution equation of the surface structure of the film is established (see the supporting material), which is a non-liner Cahn-Hilliard equation and can be numerically computed by Matlab program. In order to reduce the computation, we first transformed the space variable and time variable to reduce the number of the parameters in the equation.41,44,45 Besides, based on the Fourier transform, the numerical integration of the formula was replaced by simple numerical calculation in the reciprocal space. Furthermore, the evolution equation was discretized by the semi-implicit method.46 At last, we obtain the evolution equation in the reciprocal space: dˆ ( k1 , k2 , Tn ) − ∆T k 2 Pê n ( k1 , k2 , Tn ) ˆ d n +1 ( k1 , k2 , Tn +1 ) = . 1 + 2 ∆ T ( k 4 − k 3Q )

(5)

Here, k = k12 + k22 , and dˆ ( k1 , k 2 , Tn ) describes the order parameter field at the time Tn. The constant Q characterizes the relative value between Ub and Uel. Then this equation can be simulated by the computer directly and efficiently.


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Figure 6. The simulation images of the Cu2O reconstruction nanostructures. The suitable parameter used in the simulation is : ∆X=0.050, ∆T=0.049, Q=0.4568. (a) d=0.0646, D0=3.2 nm. The morphology is small 3-dimensional islands. (b) d=0.2121, D0=10.5 nm. The surface turn to be elongated and distorted compact islands, which is correspond with our experimental measurements. (c) d=0.3571, D0=17.6 nm. The surface topography is serpentine interconnected net-like structure. (d) d=0.5152, D0=25.4 nm. When the order parameter d is large enough, the topography of the film after annealing will turn to be an atomically-flat two-dementional plane. XRD pattern of the samples have shown that the Cu and Cu2O exhibit the cube structure and the lattice constant are: aCu = 0.3615 nm (JCPDS 04-0836), aCu2O = 0.4269 nm (JCPDS 05-0667). The mismatch strain energy density is caused between by the Cu (111) substrate and the Cu2O (111) thin film. Numerical simulation was calculated through the corresponding procedure with MATLAB. In the simulation, the physical quantities of Cu and Cu2O are shown in the reference.47 The time step ∆T, the space step ∆X, and the constant Q are the parameters used in the simulation, which can be obtained by comparison with the experimental results. According to our experience, the ranges of the three parameters are: ∆X ranges from 0.05 to 0.3, ∆T ranges from 0.001 to 0.1, and Q ranges from 0.1 to 2, which is useful in other simulations. By


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attempting and comparing with the experiment results many times in this work, we obtain three suitable parameters: ∆X=0.050, ∆T=0.049 and Q=0.4568. From the calculations, the critical thickness DC is ~ 49.29 nm. The range of the initial order parameter d 0 in the simulation is from 0 to 1. By the computer simulation, we find that the reconstruction surface topography images turn to be four obvious different structures, which are shown in figure 6. Each of these images is obtained by the computer iterated 1000 times. In figure 6(a), the topography simulated is 3D islands with the order parameter 0.0646, and the corresponding film thickness before annealing is 3.2 nm. In figure 6(b), the morphology is elongated and distorted compact islands, and the order parameter d is 0.2121. The elongated and distorted compact islands ranges from dozens to hundreds nanometers, and the surface height of each island is also variable with maximum about 50 nm. This morphology is in good accord with the Cu2O SEM and AFM experiment results. The initial film thickness calculated is about 10.5 nm. With increasing d, the surface topography can also turn to be serpentine interconnected net-like structure as shown in figure 6(c), and d is 0.3571. The initial film thickness calculated is about 17.6 nm. When the order parameter d is large enough, the topography of the film after annealing as displayed in figure 6(d) is an atomically-flat 2D plane (d=0.5152). Here, Uf is large enough and any boundary will increase the energy of the system, so the system evolves from the original nanostructures to the atomicallyflat plane. Further, since it is difficult to measure the thickness of the Cu2O ultrathin film formed during the growth procedure in situ in the tube furnace, the simulation can help us speculate the initial thickness of the 2D Cu2O film indirectly. And it also predicts theoretically that the initial film thicknesses of all the possible typical performance of the morphology. CONCLUSIONS


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In summary, the experimental phenomenon that the peeling of CuO nanowires surface layer can be attribute to the self-assembled of Cu2O nanostructures. Understanding the principles behind the spontaneous formation of structured morphologies is of interest as a fundamental scientific question. In this paper, we apply the spinodal decomposition theory of the mixtures to the single-component Cu2O system in order to explain Cu2O structure reorganization during the annealing process. According to the numerical simulation, we found that the self-assembly of this single-component system on the substrate forms obviously four different patterns by the competition of Uf, Ub and Uel. The simulated result accounts quantitatively for the self-assembled behavior of Cu2O nanostructures. This presented analysis is also useful to understand similar behaviors of other single phase systems. It is worth mentioning that both CuO nanowires and Cu2O nanostructures can be formed during the different procedure of the facile synthesis process, and Cu can be denominated as a phoenix noble metal.

ASSOCIATED CONTENT

Supporting Information. The video of the process of the annealing procedure and the derivation of the formula (PDF). The Supporting Information is available free of charge. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. *E-mail: [email protected].


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (11604242), Natural Science Foundation of Fujian Province of China (2015J01028), and Fundamental Research Funds for the Central Universities (20720170084). REFERENCES (1) Bednorz, J. G.; Muller, K. A. Z. Phys. B 1986, 64, 189-193. (2) Lee, Patrick A.; Nagaosa, Naoto; Wen, X.-G. Rev. Mod. Phys. 2006, 78, 17-85. (3) Damascelli, A.; Hussain, Z. ; Shen, Z.-X. Rev. Mod. Phys. 2003, 75, 473-541. (4) Dagotto, E. Rev. Mod. Phys. 1994, 66, 763-840. (5) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238-240. (6) Yang, J. C.; Zhou, G.; Micron 2012, 43, 1195–1210. (7) Gattinoni, C.; Michaelides, A.; Surf. Sci. Rep. 2015, 70, 424–447. (8) Yin, D.; Wu, M.; Cen, W.; Li, H.; Yang, Y.; Fang, H.; Appl. Surf. Sci. 2016, 378, 451–459. (9) Cabrera, N.; Mott, N. F.; Rep. Prog. Phys. 1948, 12, 163-184. (10) Luo, L.; Kang, Y.; Liu, Z.; Yang, J. C.; Zhou, G.; Phys. Rev. B 2011, 83, 155418. (11) Devine, B.; Shan, T.-R.; Cheng, Y.-T.; McGaughey, A. J. H.; Lee, M.; Phillpot, S. R.; Sinnott, S.B.; Phys. Rev. B 2011, 84, 125308. (12) Song, J.; Wang, L.; Zibart, A.; Koch, C.; Metals 2012, 2, 450-477. (13) Reitz, J. B.; Solomon, E.I.; J. Am. Chem. Soc. 1998, 120, 11467–11478.


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ECu2O = 30.12 ×109 Pa ,ν Cu O = 0.455 , f = −0.1532 . 2


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For Table of Contents Use Only

The Phoenix-like Noble Metal: Cu Hui Yan*,†,‡, Rukang Zhang†, Long Feng†, Heng Li§, Jiwen Liu*,† †

Tianjin Key Laboratory of Photoelectric Materials and Devices, National Demonstration Center

for Experimental Function Materials Education, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ‡

Key Laboratory of Display Materials and Photoelectric Devices, Tianjin University of

Technology, Ministry of Edu-cation, Tianjin 300384, China §

Fujian Provincial Key Laboratory of Semiconductors and Applications, Collaborative

Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen 361005, China

After heating copper in air to hundreds degrees centigrade, the oxidized black CuO surface layer peels off automatically when the temperature is lowered down. This interesting phenomenon can be attributed to the formation of red self-assembled Cu2O nanostructures in the new exposed


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surface. A simplified model based on spinodal decomposition was proposed to model the selfassembled behavior of Cu2O nanostructures.


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The Phoenix-like Noble Metal: Cu - ACS Publications - American

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