Summary of the simulation of XANES results for Cu2O/CuO core-shell nanowires at D=4. The XANES of finite size core-shell model is convoluted with the Gaussian instrumental resolution function
1. Introduction
The control of the core-shell structure at the nanometer is one of the most fundamental challenges in condensed matter science [Deville et al., 2009; Woodley et al., 2008; Bannin, 2007; Ji et al., 2007; Lauhon et al., 2002]. One of the fundamental concepts regarding the synthesis of nanowires via the spontaneous self-organization of individual nanoparticles, called spontaneous organization nanocrystals, refers to the formation of two-dimensional (2-D) and 3-D arrays of nanoparticles or nanowires [Tang et al., 2002; Lu et al., 2002; Balazs et al., 2006; Tang et al., 2006]. The copper-oxide based system, such as Cu2O [Huang et al., 2012; Hong et al., 2011; Yao et al., 2010], CuO [Wu, 2012; Cheng et al., 2007; Chou et al., 2008], and Cu2O@CuO [Yec et al., 2012], has been known to facilitate oxidation reactions in the bulk material, which may allow it to be a cost effective substitute for noble metals in various catalytic systems. With recent developments in nanostructures synthesis leading to the ability to control size, reproducibility and structural complexity [Lauhon et al., 2002; Fan et al., 2006; Lu et al., 2005], it becomes worthwhile and possibly paramount to define specific target structures for the nanoparticles based on an understanding of the mechanism of the self-organization mechanism. Upto now, many complex procedures, such as vapor-liquid-solid, chemical vapor deposition, thermal evaporation, and chemical reactions, have been developed for the synthesis of one-dimensional materials of Copper-based nano compounds [Huang et al., 2012; Hong et al., 2011; Yao et al., 2010]. In addition to these methods, solution chemical route including solvothermal, hydrothermal, self-assembly, and template-assisted chemical vapor deposition has become a promising option for large –scale production of nanomaterials, due to the simple, fast, and less expensive virtues [Xin et al., 2002; Li et al., 1999; Lu et al., 2000; Hung et al., 2004; Roy et al., 2003; Baxter et al., 2003]. However, a new type of immersing nanoparticles into liquid alcohol to form core-shell nanowires at room temperature has been difficult, primarily because control of nucleation and growth is still a challenge.
In the present work, we report on a simple method for bottom-up fabrication by means of template-free growth of high density Cu2O/CuO core/shell nanowires. These can be spontaneously self-assembled by the oxidation of ultra-small copper nanoparticles simply immersed in ethyl alcohol. This leads to the formation of Cu2O/CuO core/shell nanowires. Then nanosized growth effects of these nanowires are observed experimentally. We find that the formation of nanowires may be far more due to chemical reactions than previously realized.
2. Synthesis method
The initial Cu nanoparticles used as a precursor in the synthesis of Cu2O/CuO core/shell nanowires, are fabricated by employing the gas condensation method, as shown in Fig. 1. High purity Cu ingots (~ 0.5 g, 99.999% pure and 3 mm in diameter) are heated by a current source of 90 A. They are evaporated at a rate of 0.01 Å/s in an Ar pressure of 0.1 Torr. The evaporated Cu nanoparticles are collected on a non-magnetic quartz plates (1 cm x 1 cm), maintained at liquid nitrogen temperature. After restoration to room temperature, the nanoparticles, which are only loosely attracted to the collector, are stripped off. The resultant samples were in the form of dried powder, consisting of a macroscopic amount of individual Cu nanoparticles. The combined effect of high gas flow rate, low deposition pressure and relatively low substrate temperature resulted in the deposition of copper nanoparticles. The moving Cu nanoparticles were washed and stripped off from quartz plates using ethyl alcohol(99.5%) in argon gas environment (chemical hood), as shown in Fig. 2(a). They were then kept in a glass container and in a sealed quartz capillary (Fig. 2(b) and 2(c) for few days) for
3. Results and discussion
The
where <d> is the mean size,
The morphology of selected samples at D=1, 4 and 7, was characterized by FE-SEM, as shown in Figs. 5(a) to (c). As can be seen in Fig. 5(a), it is clearly evident that the nanoparticles are spherical and stick together due to an electrostatic effect as well as an artifact of the drying of aqueous suspensions. It can be seen that the Cu2O/CuO nanowires grew homogeneously in a large area to form straight nanowires, as shown in Fig. 5(b). The diameters ranged from 100-500 nm and the lengths were up to several tens of microns. As can be seen in Fig. 5(c), the Cu2O/CuO core-shell nanowires are long and straight, ranging from tens of micrometers to more than a hundred micrometers. The growth of the nanowires is clear. An evaluation of the diameter distribution can be obtained and calculated from a portion of the SEM image, as shown in the Figs. 5(d) to 5(f). They are quite asymmetric and can be described using a log-normal distribution function defined as follows:
where <d> is the mean value and
The results of an early study (x-ray patterns) show that the prepared core-shell nanowires are crystalline. We further characterized the structure of sample using TEM microscopy. Figure 6 shows an example of the Cu2O/CuO core-shell nanowires taken at D=4, which has a uniform coating of amorphous CuO (light contrast) surrounding a single-crystal Cu2O core (dark contrast), forming core-shell Cu2O-CuO nanowires. The chain-like configuration of the Cu2O nanoparticles intertwined with the nanowires can also be observed. The results are in good agreement with the NVT Monte Carlo (MC) method [Phillies, 1974] results for the intermediate state of pearl necklace-like aggregates. In the MC simulation gives the formation of nanowires involves a process where Au nanoparticles could self-assemble into chain-like intermediate states then recrystallize into one-dimensional nanowire. The dipole-dipole-interaction in the Cu2O nanoparticles is strong and long-range, leading to the formation of the pearl necklace-like nanostructures. A significant surface CuO layer (a few nms in thickness) was observed, indicating the existence of an amorphous CuO shell. In order to determine the amorphous CuO and the surface characteristics of the core-shell nanowires, we have utilized XANES technique to examine the existence of surface CuO thickness, simulating through an intensity ratio of Cu2O-core and CuO-shell by using a proposed core-shell nanowire model.
For a cylindrically multilayered structure without consider the effect of diameter distribution. An electron being photoionized into the surface CuO has a probability of escaping through the surface without suffering any inelastic processes. Therefore, it can be detected as a photoelectron. In generally, the probability is conveniently expressed in terms of the mean free path for surface layer
where L is the length of the nanowire,
The mean free paths of
In order to clarify the contribution from Cu2O- or CuO- intensity, we have defined a theoretical integrated intensity ratio
The above ratio cannot be evaluated analytically, and we can compute numerically by varying these parameters of R1 and R2, respectively, to match the experimental integrated intensity ratio of γ=ICu2O/ICuO. The electronic geometric structure around Cu in the as-synthesized material was characterized with X-ray absorption near edges(XANES) measurements. XANES spectroscopy of Cu L2,3–edge was performed using the 6-m HSGM beam-line (BL20A1) of the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. The recorded spectra were corrected for the energy dependent incident photon intensity as well as for self-absorption effects and normalized to the tabulated standard absorption cross sections in the energy range of 900-1020 eV for the Cu L-edge. The radiation was monochromatized using a 6m SGM monochromator and the photon energy was calibrated by measuring X-ray absorption spectrum of Cu metal foil by using the Cu L3 white line. Figure 8 shows XANES spectra of as-synthesized material and copper based references such as Cu2O (red curve), CuO (green curve) bulk. The main peak of Cu 2P3/2 at 932.3 eV reveals the presence of Cu2O phase. It is accompanied by a second peak at 935.1 eV related to the CuO phase, and can be regarded as characteristic of Cu+ and Cu2+ (
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932.3 | 3.423 | 935.1 | 1.037 | 3.302 | 3.36 | 99 | 101 | 2 |
4. Conclusion
A new type of spontaneous organization method was successfully utilized to grow Cu2O/CuO core/shell nanowires by immersing copper nanoparticles into liquid alcohol at room temperature. The microstructure of the nanowires during the nucleation and growth process from CuO nanoparticles into Cu2O/CuO core-shell nanowires can be examined using time dependent
Acknowledgment
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC-100-2112-M-259-003-MY3. We would also like to thank Professor Yuan-Ron Ma of National Dong Hwa University and Dr. Ting Shan Chan of National Synchrotron Radiation Research Center (Taiwan) for their valuable contributions and discussions in this work
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