Article pubs.acs.org/crystal
Rayleigh Instability Driven Nodular Cu2O Nanowires via Carbothermal Reduction of CuO Nanowires Fei Wu, Yoon Myung, and Parag Banerjee* Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, Missouri 63130, United States S Supporting Information *
ABSTRACT: Nodular cuprous oxide (Cu2O) vertical nanowire (NW) arrays with a length of more than 20 μm and diameter of ∼300 nm have been synthesized by the carbothermal reduction of thermally grown cupric oxide (CuO) NWs. The transformation initiates at a low temperature of 350 °C. Two important effects are observed: First, the CuO → Cu2O reduction occurs by oxygen out diffusion under a highly reducing atmosphere afforded by the presence of carbon monoxide (CO). Second, the surface reduction creates instabilities, which propagate and grow into a string of nodules along the length of the Cu2O NWs. This effect is determined to be due to Rayleigh instability, but initiated via Cu2O phase transformation.
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NWs.14 However, CuO NWs were only partially reduced, forming small Cu2O nanoparticles on CuO NWs. The interaction of carbon monoxide (CO) with CuO has been investigated recently with the primary motivation of using CuO to catalyze the water gas shift reaction: CO + H2O → CO2 + H2. CO is able to reduce bulk CuO powders from 236 to 500 °C.15,16 Under high concentrations of CO, CuO directly converts to Cu. However, when CO concentrations are low, the reduction process occurs as CuO to Cu2O to Cu. The removal of oxygen and the forming of an off-stoichiometric and disordered Cu2O1+δ was also reported.15 In this article, we synthesize vertically aligned, dense Cu2O NWs arrays via an alternate, but simple carbothermal reduction of CuO NWs. Periodic nodule formation is observed in the Cu2O NWs, which is determined to be driven by Rayleigh instability during the phase transformation process. This approach can thus lead to the formation of a diversity of hierarchical, one-dimensional Cu2O nanostructures, previously not reported. The CuO NWs are first obtained via thermal oxidation of Cu foils. Our group has demonstrated oxidation of 26 μm Cu foils, converting the entire substrate to phase pure CuO by oxidizing at 700 °C for 10 h. The process consumes all metallic Cu while maintaining dense and viable CuO NWs on top.17 This structure is taken as the starting point for the carbothermal reduction of CuO NWs to Cu2O NWs. We systematically study the NW phase transformation and morphology evolution. We
INTRODUCTION
Thin films of Cu2O have received much attention in a variety of applications such as solar energy conversion,1,2 water splitting,3 photocatalytic degradation of pollutants,4 and gas sensing.5 However, it is known that structuring semiconductors into vertically aligned nanowires (NWs) can provide inherently efficient architectures that maximize optical path lengths for photon absorption, increase surface area for charge transfer processes, and minimize electron and hole diffusion lengths toward radially clad, charge collecting electrodes.6,7 Therefore, synthesis of vertically aligned Cu2O NWs can significantly enhance and diversify the portfolio of Cu2O-based applications. Many synthesis strategies for obtaining Cu2O NWs have been attempted. Randomly distributed single crystal Cu2O NWs have been synthesized by hydrothermal method at 180 °C and an aqueous solution method at 95 °C.8,9 Linear sweep voltammetry has been used to reduce copper duplex oxides using 6 M KOH + 1 M LiOH.10 Porous nanotemplate based electrochemical techniques have been shown to yield high density Cu2O NWs ∼200 nm in diameter, but limited in their length to 4 μm, due to diffusion limited electrolyte transport in narrow nanopores.11 Among gas phase techniques, reduction of CuO NW arrays is a possible way to get Cu2O. Consequently, reducing in H2 atmosphere at around 280 °C has been attempted.12 However, phase control remains an issue. N2/H2 mixed plasma has also been used to reduce CuO NWs.13 Porous polycrystalline NWs have been observed after the reduction process. The ratio of Cu2O/CuO can be adjusted by the ratio of N2/H2. High vacuum (2 × 10−6 Torr) has been employed to reduce CuO © XXXX American Chemical Society
Received: May 29, 2014 Revised: July 16, 2014
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observe an “antenna effect” in mass transport where CuO NW arrays with large specific surface area leads to faster phase transformation compared to planar films. Continuous oxygen out diffusion stabilizes the Cu2O NW from fragmenting and collapsing and therefore results in dense arrays of vertically aligned nodular Cu2O NWs.
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EXPERIMENTAL SECTION
Pure CuO foils with NWs on the surface were used as starting samples.17 These samples do not contain any residual Cu or Cu2O phases. The detailed synthesis process of pure CuO NW samples have been reported in our previous paper.17 These samples were held in a graphite cylinder, which was machined and capped at the terminal ends. The cylinder was inserted in a tube furnace. This ensured that the samples were exposed to a CO ambient during the thermal step. The furnace was pumped down to a pressure of ∼10 mTorr or better using an Edwards RV12 and pumped continuously during the reduction process. Samples were reduced at 350 and 400 °C. The time of reduction was varied from 1 to 10 h. Furnace temperature ramp down to ambient was done in vacuum as well. The NW morphology and cross sections were characterized by scanning electron microscopy (SEM, JEOL-7001LVF). High resolution transmission electron microscopy (HRTEM) images were collected using JEOL 2100F with an operating voltage of 200 kV. Fast Fourier transform (FFT) of HRTEM images and additional measurements on Cu2O NWs were conducted by ImageJ software. Raman spectra of the samples were obtained using a Renishaw In Via Raman Microscope with a spot size 2πRo = 680.8 ± 188.4 nm. Comparing this predicted periodicity with the one in Table 1, the 1 and 2 h reduction samples show discrepancy with the calculation, while the 5 and 10 h samples seem to satisfy the criteria. In order to explain these results, recall that for the 1 and 2 h samples, there is residual CuO remaining at the bottom of the E
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Table 1. Periodicity (in nm) of the Nodules for Cu2O NWs Obtained at 400 °C 1, 2, 5, and 10 h; to Obtain Standard Deviation, at Least 30 Nodules Were Measured Per Sample periodicity (nm)
400 °C, 1 h
400 °C, 2 h
400 °C, 5 h
400 °C, 10 h
406 ± 82.8
336 ± 47.2
1328 ± 317.6
670.0 ± 157.1
Figure 7. Schematic of proposed mechanism for transformation of CuO NWs to Cu2O NWs under carbothermal reduction conditions. (a) Starting foil, (b) incipient stage of the transformation process, (c) full conversion of the NWs and partial conversion of the underlying oxide film, and (d) complete conversion of the CuO NWs and underlying oxide film.
underlying oxide film (Figure 3e). This implies that volume out diffusion of oxygen occurs, given the chemical potential gradient set up between the fully converted Cu2O NWs on top and the underlying CuO film. In contrast, negligible oxygen diffusion occurs for the 5 and 10 h samples because of the uniformity in composition (i.e., fully converted Cu2O). Additionally, stress gradients are produced during the transformation CuO NW → Cu2O NW as evidenced by the warpage of NW during the reduction process (Figure 1d). Under conditions of stress, volume diffusion effectively reduces perturbation wavelengths.31 Therefore, it seems reasonable that λo for 1 and 2 h samples is lower than the 5 and 10 h samples. Variability in the starting NW radius and compositional heterogeneity (CuO, Cu2O, and Cu2O1+δ phases) in the NW may also lead to additional complications and discrepancy observed in the Rayleigh criteria. Crystallization and Phase Transformation. The predominance of the {110} set of growth planes, as shown in Figure 2, both on the Cu2O NW surface and the nodules is intriguing since it is well-known that the Cu2O (111) plane is the lowest energy plane.32 This effect can be explained first, by considering the CuO → Cu2O reduction process. It has been shown through structural studies using X-ray absorption spectroscopy15 that the CuO reduction under CO lean conditions occurs via formation of oxygen vacancies in the CuO or the presence of excess oxygen in Cu2O. This initial surface depletion of oxygen creates a chemical potential gradient between the NW surface and the sample and results in the continuous out diffusion of oxygen, initially from the NW and subsequently from the underlying film. Our results reiterate this aspect of the reduction process by (1) demonstrating that loss of oxygen occurs at the NW/ ambient interface and subsequently proceeds into the film for longer reduction times and (2) consistently observing the presence of Cu2O1+δ phase every time Cu2O is formed (Figure
3). Thus, oxygen (or its lack of) plays a crucial role in determining the kinetics and energetics of the reduction process. When the number of oxygen atoms per unit area in Cu2O atomic planes are considered, we find that this density varies as 2.78 to 5.89 to 8.83 nm−2 for (100), (110), and (111) planes, respectively.33 Given the arguments in the previous paragraph for CuO reduction, it becomes obvious that planes with the least oxygen atomic densities will be favored during a rapid reduction process (kinetically controlled). Thus, from a pure kinetic consideration, the growth rates of the Cu2O planes during carbothermal reduction should be given as (100) > (110) > (111). From an energetics point of view, however, a different picture emerges. This is given by recent first-principle studies of the stability of Cu2O surfaces,32 wherein the surface energy of the Cu2O (110) neutral surface under reducing atmosphere is 0.026 eV/Å2 as compared to the Cu-rich Cu2O (100) surface (0.098 eV/Å2). On the basis of these values, we surmise that initially the Cu2O carbothermal reduction process favors both (100) and (110) planes, being the two lowest oxygen density planes. That this is a nonequilibrium process is confirmed by reports of electrochemically grown Cu2O NWs where (100) and (110) oriented planes have been reported under large overpotentials, i.e., nonequilibrium conditions.34 However, under extended periods of reduction (eg., 400 °C 1 h and more), the (100) plane grows the fastest and disappears in favor of the energetically stable (110) plane. This is further borne out by the characteristic pyramidal protrusions in the [100] directions in Figure 2a.35 Phase Transformation Induced Rayleigh Instability. One can combine the Rayleigh instability and phase transformation of CuO to Cu2O NWs to develop a complete picture for the results observed in this article (Figure 7). First, the CO reducing environment creates a conducive atmosphere to F
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induced shrinkage of NWs under phase transformations. Structures like the one presented in this work can form the basis of novel energy harvesting and storage devices and may be useful for catalytic reactions as well.
remove oxygen from the CuO lattice in the NWs. The high surface area of the NWs accelerates the kinetics and is therefore the starting point for the phase transformation. The formation of Cu2O1+δ phase is observed using Raman (Figure 2) and indicates that extra, interstitial oxygen is present.15 Surface stress generation occurs, and we show, using kinetic and energetic arguments, the possibility of both (100) and (110) planes to be formed initially on the surface of the NWs. This phase transformation therefore leads to surface protrusions as shown in Figure 5b. The protrusions are the starting points for Rayleigh instability to propagate. When only oxygen out diffusion is considered in the NWs with periodic protrusions consisting of an outer Cu2O shell layer of thickness τ over an inner CuO NW, we obtain using Cu ion conservation the following relationship for NW radius shrinkage (see Supporting Information Figure S2): ε 2(t ) ⎛ ρCu 2O ZCu 2O ⎞⎛ MWCuO ⎞ ⎟⎟⎜⎜ ⎟⎟τ − ⎜⎜ R CuO = R − 4R ⎝ MWCu 2O ⎠⎝ ρCuO ZCuO ⎠
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ASSOCIATED CONTENT
S Supporting Information *
Reaction free energy and Rayleigh instability equation assumptions and derivations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(P.B.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding from McDonnell Academy Global Energy and Environment Partnership (MAGEEP) and the International Center for Advanced Renewable Energy and Sustainability (ICARES) is acknowledged. Microscopy facilities were provided by the Institute of Materials Science and Engineering (I-MSE), Washington University in St. Louis. We thank Professor Srikanth Singamaneni (Mechanical Engineering and Materials Science) for the Raman microscope and Professor Shankar Sastry (Mechanical Engineering and Materials Science) for providing the rolling machine for mechanically thinning the copper foil. Discussions with Prof. Avner Rothschild (Technion) are acknowledged.
(2)
where ρx = density of material x, MWx = molecular weight of x, Zx = ratio of cation/anion in x, and Z = 1 for CuO and Z = 2 for Cu2O. Compared to the original eq 1, we notice that in eq 2 an additional negative third term is introduced, which enhances the shrinkage rate of the NW. This term depends on the product of the thickness of the Cu2O film (τ) and the ratio of densities and molecular weights of CuO and Cu2O (using appropriate values for CuO and Cu2O, this factor is 1.06). That is to say that Rayleigh instability will be more pronounced in NWs in which phase transformations with associated volume changes occur. The growth of the protrusions then is simply a result of (1) volumetric out diffusion of oxygen, which continues to transform CuO → Cu2O and stabilize the NW structure, thus preventing it from fragmenting, and (2) surface diffusion, which continues to be driven by the instability criteria.
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CONCLUSIONS In this article, we have studied the phase transformation of thermally grown CuO NWs to nodular Cu2O NW using carbothermal reduction. The carbothermal reduction can occur at temperatures as low as 350 °C and is given by the reaction 2CuO + CO → Cu2O + CO2. We find that (1) the reduction front initiates at the NW surface and proceeds through the wire and into the underlying film, (2) the oxygen out diffusion is the primary process by which CuO to Cu2O reduction occurs, creating an intermediate nonstoichiometric phase given as Cu2O1+δ, and (3) the presence of NWs, i.e., extra surface area, enhances the rate of reduction. This reduction scheme leads to Rayleigh instabilities and growth of periodic nodules on the NW surfaces. Fragmentation is avoided and vertically aligned, viable Cu2O NWs are formed. Alternately, under a Cu chemical potential gradient and Ar ambient, Cu can be made to diffuse into the CuO NWs with Cu2O phase and nodule formation as CuO + Cu → Cu2O. The crystallographic relationship of the nodules with respect to the NW shows that the nodules have higher growth rates due to the differing stabilities of the Cu2O (100) and (110) planes, both of which form during the reduction process, but the faster growth along the [100] and associated surface stress leads to nodule formation. This argument was shown to be consistent with kinetic and energetic considerations. An analytical equation is derived, which predicts faster rate of Rayleigh G
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