Comparison Study on the Stability of Copper Nanowires and Their

Comparison Study on the Stability of Copper Nanowires and Their Oxidation Kinetics in Gas and Liquid Liang Xu, Yuan Yang, Zeng-Wen Hu, and Shu-Hong Yu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, CAS Center for Excellence in Nanoscience, Hefei Science Center of CAS, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: The unsaturated “dangling” bonds on the surface of nanomaterials are extremely sensitive to the external environment, which gives nanomaterials a dual nature, i.e., high reactivity and poor stability. However, studies on the long-term effects of stability and reactivity of nanomaterials under practical conditions are rarely found in the literature and lag far behind other research. Furthermore, the long-term effects on the stability and reactivity of a nanomaterial without coating under practical conditions are seriously long-neglected. Herein, by choosing copper nanowire as an example, we systematically study the stability of copper nanowires (CuNWs) in the liquid and gas phase by monitoring the change of morphology, phase, and valence state of CuNWs during storage. CuNWs exhibit good dispersibility and durable chemical stability in polar organic solvents, while CuNWs stored in water or nonpolar organic solvents evolve into a mace-like structure. Additionally, fresh CuNWs are oxidized into CuO nanotubes with thin shells by heating in air. The activation energies of oxidation of CuNWs in the gas phase are determined by the Kissinger method. More importantly, the different oxidation pathways have significant effects on the final morphology, surface area, phase, optical absorption, band gap, and vibrational property of the oxidation products. Understanding the stability and reactivity of Cu nanostructures will add value to their storage and applications. This work emphasizes the significant issue on the stability of nanostructures, which should be taken into account from the viewpoint of their practical application. KEYWORDS: Cu nanowires, dispersibility, chemical stability, oxidation kinetics, storage affected by oxidation.18 Semiconductor nanoparticles (CdSe and CdTe) could be oxidized and assemble into selenium and tellurium nanowires by a stabilizer-depleted process.19,20 Xiong et al. found Ag nanocubes can evolve into nanospheres by oxidative etching.21 Hence, we should attach importance to the oxidative stability of nanomaterials. Due to its high thermal and electrical conductivity and good ductility, copper is the most commonly used material in cables and electrical and electronic components and also can be used as building materials. Humans have used copper and its alloys for thousands of years, but people still have tremendous interest in copper-based materials even up to now, especially nanoscale materials of copper. Recently, our group has used Cu nanowires (CuNWs) as a template to synthesize PtPdCu- and PtCu-alloy nanoparticle nanotubes with efficient and durable electrocatalytic performance.22 In addition, compared to other metal

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s we all know, nanomaterials always have a large surface-to-volume ratio,1 resulting in the increase of surface atoms, which leaves chemical bonds “dangling” outside the solid and gives them a higher reactivity than that in the bulk.2 These unsaturated “dangling” bonds are extremely sensitive to the external environment and interact with the environment dynamically to reduce the surface energy.3 Hence, nanomaterials are not changeless after synthesis, but they adapt to the changing environment; they change dynamically and are unstable. This feature gives them a dual nature, i.e., high reactivity and poor stability. While nanomaterials with large surface energy show exceptional performances in catalysis and energy storage,4−7 their properties related to stability are still not well understood, such as dispersibility,8−10 thermostability,11,12 structural stability,13,14 and oxidative stability.8,15 Among these, oxidative stability is a key factor in the long-term storage and performance retention of nanomaterials. Moreover, surface oxidation is enhanced in nanomaterials due to the large surface-to-volume ratio.16 Stouwdam et al. found that the optical properties of PbSe nanocrystals are highly sensitive to oxidation.17 The surface chemistry of PbSe nanocrystals is also © XXXX American Chemical Society

Received: January 28, 2016 Accepted: March 2, 2016

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DOI: 10.1021/acsnano.6b00704 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. SEM images of morphology evolution of CuNWs during storage in water at room temperature after (a) 0 h; (b) 6 h; (c) 15 h; (d) 1 day; (e) 2 days; (f) 5 days; (g) 10 days; (h) 2 weeks; (i) 1 month. Insets are corresponding TEM images of CuNWs. All scale bars are 500 nm.

synthesis processes or post-treatment processes, which may complicate the operations and increase cost. Additionally, the long-term effects on the stability and reactivity of Cu nanomaterials without coating under practical conditions are seriously long-neglected. The comprehensive and quantitative investigations on the stability and reactivity of Cu nanomaterials in different environments are crucial for their storage and application. Herein, by choosing copper nanowires as an example, we systematically study the stability of CuNWs in the liquid and gas phase. For the liquid phase, the impact of solvents on the dispersibility and chemical stability of CuNWs was investigated. Fresh CuNWs showed good dispersibility in polar solvents but heavily aggregated in nonpolar solvents. Polar organic solvents can prevent CuNWs from oxidizing, while CuNWs stored in water or nonpolar organic solvents were heavily oxidized and finally evolved into a mace-like structure. In the gas phase, CuNWs were oxidized into CuO nanotubes with thin shells by heating. The oxidation of CuNWs in the gas phase follows two different stages. The activation energy at each stage was determined by the Kissiger method to quantitatively evaluate the stability of CuNWs. More importantly than that, the different oxidation pathways have significant effects on the final morphology, optical absorption, band gap, and vibrational property of the oxidation product.

materials (Au, Ag, and so on), CuNWs are also a promising material for transparent conducting electrodes (TCEs) due to their excellent conductivity, low cost, and high abundance.16,23−25 Porous Cu nanowire aerosponges with high porosity, high electrical conductivities, and mechanical robustness can be used in many areas, such as heat sinks, catalysis, or energy storage.26 However, one major problem in utilizing CuNWs is their inherent tendency to be oxidized in ambient conditions because of the relatively low Cu0/Cu2+ redox potential (+0.34 V). The main way to obtain air-stable Cu nanomaterials is minimizing the exposure of Cu nanomaterials to oxygen by coating them with proper protective layers such as long-chain surfactants, organic polymers, inorganic materials, or carbon-based materials.27 Kanninen et al.28 found that the oxidation resistance of the thiol-capped Cu nanoparticles increases with the chain length of the thiol. Similarly, the molecular weight of the polyvinyl pyrrolidone (PVP) capping agent chemisorbed on the Cu surface influences the formation of the surface oxide layer significantly.29 Moreover, Ag, Ni, and Pt are the most common coating materials for obtaining airstable Cu nanomaterials, which are always realized by forming a core−shell structure.30−32 Ahn et al. synthesized a copper nanowire−graphene core−shell nanostructure, which shows remarkable thermo-oxidative and chemical stability caused by the tight encapsulation of CuNWs with gas-impermeable graphene shells.33 Liu et al. also found graphene could be a long-term oxidation barrier for copper nanowires.34 Such coating-stabilized Cu nanomaterials always need complex B


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RESULTS AND DISCUSSION Stability of CuNWs in Water. Due to poor stability, CuNWs are prone to being oxidized by dissolved oxygen in liquid during storage.15 The scanning electron microscope (SEM) and transmission electron microscope (TEM) images in Figure 1 show the morphology evolution of CuNWs during storage in water at ambient conditions. Fresh CuNWs had rough surfaces with small undulations and constant diameters in the range 60−160 nm (Figure 1a and Supporting Information Figure S1). The rough surface may cause many defects on the surface. Some defects would lead to the chemical bonds of surface Cu atoms dangling outside the solid. These unsaturated “dangling” bonds are extremely sensitive to the external environment and interact with the environment dynamically to reduce the surface energy. In addition, due to washing and redispersion, some Cu atoms on the surface could lose their stabilizers (surfactants, solvents, and other absorbed substances) or have a weaker interaction with stabilizers. These phenomena gave these Cu atoms higher reactivity than others because of their higher unsaturated degree. Thus, CuNWs were highly sensitive to oxygen and quickly oxidized by dissolved oxygen within only 6 h (Figure 1b). Oxidation of CuNWs with oxygen preferentially occurred at some sites on the surface of CuNWs rather than the whole surface, suggesting there are some reaction sites (such as unsaturated copper atoms) on the surface that may have higher reactivity than others (Figure 1b− d). These high reactive sites can react with oxygen first under low concentration of dissolved oxygen (about 8.2 mg/L at 25 °C in water). With prolonging the storage time, the oxidation spread to the entire surface quickly (Figure 1e−i and Figure S2). The oxide flakes of copper, meanwhile, became sharper and larger, resulting in the increase of diameter of the oxidized CuNWs (∼200 nm) (Figure 1b−i and Figure S2a−c). These sharp oxide flakes coarsened through Oswald ripening after a long enough storage time (Figure S2d). Generally, CuNWs could be converted to tubular CuO and CuS by heating in air12 or reacting with thiourea in EG,35 because copper moves preferentially toward the surface, i.e., the Kirkendall effect.36−38 However, as shown in TEM images (insets in Figure 1 and Figure S2), distinct hollow structures were hardly found during oxidation. We think the main reason is that the defects on the surface make the oxidation preferentially occur at some sites in different directions, resulting in hardly obtaining sufficient net directional flow of matter to form a distinct hollow nanostructure.39 Finally, an indistinct hollow nanostructure with a few small pores was formed after a long storage time (insets in Figure 1d−i and Figure S2). The phase transformation of CuNWs during storage was investigated by X-ray powder diffraction (XRD). The XRD pattern of fresh CuNWs exhibited sharp and strong reflections at 43.34°, 50.47°, and 74.17°, respectively, which are indexed to Cu crystals (JCPDS No. 65-9743) (Figure 2). XRD patterns of the sample after storing for 3 days, 7 days, 14 days, 21 days, 1 month, and 2 months under ambient conditions are shown in Figure 2 and Figure S3 (Supporting Information). After storing for less than a week, the weak reflection at 36.42° is attributed to the (111) crystal plane of Cu2O (JCPDS No. 05-0667). This indicates that the generation of crystalline copper oxides only needs a few days; CuNWs are highly unstable in water at room temperature. After storage for 2 weeks, the phase of the sample included three parts: one major phase with strong reflection peaks located at 43.34°, 50.47°, and 74.17° can be assigned to

Figure 2. Evolution of XRD patterns for CuNWs during oxidation in water at room temperature after 0 h; 3 days; 2 weeks; 3 weeks; 2 months. Cu2O and CuO are marked with round dots and rhombi, respectively.

those of Cu2O crystals (JCPDS No. 05-0667), and two minor phases are attributed to Cu crystals (weak reflection peaks at 43.34° and 50.47°) (JCPDS No. 65-9743) and CuO crystals (strong reflection peaks at 38.71° and 35.54°) (JCPDS No. 481548). All the diffraction peaks can be indexed to monoclinic copper oxide (JCPDS No. 48-1548) after being stored for more than 2 weeks. Hence, the oxidation of CuNWs could be divided into two steps, i.e., first forming cuprous oxide and further being oxidized into copper oxide. The UV−vis absorption spectrum was used for in situ analysis of the change of CuNWs in the liquid phase. Figure S4 shows the evolution of the UV−vis absorption spectrum of CuNWs during storage. Fresh CuNWs had a remarkable peak centered at 579 nm, which is attributed to the plasma excitation in CuNWs.40,41 As the storage process continued, CuNWs were generally oxidized, and the corresponding UV−vis absorption spectrum exhibited two broad peaks at 350−450 nm and 550− 650 nm (Figure S4). In general, Cu2O nanocrystals have a maximum absorption range from 485 to 560 nm with an increase of size,42,43 and CuO nanocrystals exhibit a maximum absorption at about 280−340 nm.44,45 The storage sample, according to the above XRD analysis, was a mixture of Cu, Cu2O, and CuO, so its UV−vis absorption spectrum combined all three as a composite spectrum: the broad peak at 350−450 nm was assigned to the oxides of copper (i.e., Cu2O and CuO); the other at 550−650 nm was for copper. With the prolonging of the storage time, the broad absorption peak of Cu crystals at 550−650 nm finally disappeared, while the absorption peak of the oxides of copper at 350−450 nm increased, indicating that CuNWs were heavily oxidized during storage. Stability of CuNWs in Other Solvents. Solvents are important for the dispersibility and chemical stability of nanomaterials.8,46 We investigated the solvent effect on the stability of CuNWs by storing CuNWs in different solvents at room temperature. Figure 3 shows the SEM image of CuNWs in various solvents after storage for 1 month. The surface of CuNWs stored in water, n-hexane, or cyclohexane consisted of sharp oxide flakes and showed a mace-like structure (Figure 3a, C


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Figure 3. SEM images of the sample being stored in different solvents at 20 °C after 1 month: (a) H2O; (b) ethylene glycol; (c) ethanol; (d) methanol; (e) 2-propanol; (f) N,N-dimethylformamide; (g) dimethyl sulfoxide; (h) cyclohexane; (i) n-hexane. (j) Photographs of CuNW solution just after dispersion (top) and after storage for 24 h (bottom) at room temperature. All scale bars are 500 nm.

Ethylenediamine (EDA), as a highly polar molecule, plays an important role in the synthesis of CuNWs.15,51 The EDA molecules on the CuNW surface are of great significance in the dispersibility and chemical stability of CuNWs. Due to its high polarity, EDA can easily dissolve in polar solvents and shows poor solubility in nonpolar solvents. Once fresh CuNWs were redispersed into different solvents, the interaction between EDA and solvent molecule would determine the dispersibility of CuNWs in solution. As shown in Figure 3j and Figure S5, CuNWs were well dispersed in polar solvents because of the similarity-intermiscibility theory. On the contrary, nonpolar solvents caused serious aggregation of CuNWs. However, during storage, EDA molecules absorbed on the surface would be dissolved in the solvent with similar polarity due to the concentration gradient between the surface of the nanomaterials and the solvent; meanwhile, the oxidation of CuNWs also caused the abscission of EDA from the surface, resulting in the reduction in coverage for EDA molecules on the surface.18,39 All the samples showed poor dispersibility and settled to the bottom of the sample bottle after storage for 24 h (Figure 3j). Compared to other samples, CuNWs precipitated in water turned dark red, indicating that the CuNWs were quickly oxidized. Compared to centrifugal force and intense pressure, which always compact substances to form a dense and hard lump, the force derived from the immiscible phenomena, which caused CuNWs to aggregate in nonpolar organic solvents, and the weak gravity of CuNWs, which caused CuNWs to precipitate in polar solvents, are milder and more powerless. Thus, the structures of the aggregate and the precipitate of CuNWs are fluffy, would not totally prevent oxygen diffusion during storage, and have a roughly equivalent impact on the oxygen diffusion (Scheme 1). In essence, the oxidation of CuNWs is a gas−solid reaction that involves liquid, gas, and solid phases. Due to the consumption of oxygen during

h, and i). The affecting factor for the oxidation of CuNWs in water or in nonpolar organic solvents (n-hexane and cyclohexane) is different. Atmospheric carbon dioxide can dissolve into deionized water to make it slightly acidic (always about pH = 6.5). Based on the Nernst equation, E = 1.229 + (0.0592/4) lg((pO2[H+]4)/1), the slight acidity of water would increase the O2/H2O potential, which is conducive to the oxidation of CuNWs. As for nonpolar organic solvents, based on the similarity-intermiscibility theory47 and the previous reported information48−50 (as shown in Table 1), the main reason for Table 1. Solubility of Oxygen in Various Solvents at 298.15 K and a Partial Pressure 101.33 kPa48−50 solvent

χ × 105, mole fraction

H2O EGa ethanol

2.293 6.78 58.25

solvent


solvent


methanol 2-propanol DMFa

41.5 78.2 35.1

DMSO cyclohexane hexane

15.7 123 225.8

a

Calculated by interconversion of the reporting solubility parameters for ethylene glycol (EG) and N,N-dimethylformamide (DMF).

serious oxidation is the higher dissolved oxygen concentration, which is several times to hundreds of times higher than that in polar solvents. The sample stored in polar organic solvents still kept the pristine one-dimensional structure except for several oxide burrs of copper due to low dissolved oxygen concentration (Figure 3b−g). The dispersibility of nanomaterials was mainly determined by several factors: their size, polarity and amount of molecules (surfactants, solvents, and other absorbed substances) coated on the surface of the nanomaterials, polarity of solvent being used to disperse nanomaterials, and so on. D


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ACS Nano Scheme 1. CuNW oxidation during storage.

2Cu + O2 → 2CuO

(1)

2Cu(EDA)x + O2 → 2CuO + 2x EDA

(2)

The phase transformation of CuNWs stored in different solvents after 1 month was determined via XRD (Figure 4a). The sample stored in water exhibited strong reflection peaks at 35.54°, 38.71°, and 48.72°, which are attributed to monoclinic copper oxide (JCPDS No. 48-1548), and the strong reflections at 36.42° can be indexed to the (111) plane of Cu2O (JCPDS No. 05-0667). No characteristic peak of copper could be detected, indicating that CuNWs were completely oxidized after 1 month. As for CuNWs stored in nonpolar organic solvents, n-hexane and cyclohexane, the XRD patterns not only included the characteristic of oxides (Cu2O and CuO) but also exhibited strong reflections at 43.34° and 50.47°, which are attributed to Cu crystals (JCPDS No. 65-9743). This suggested that the oxidation rate of CuNWs in nonpolar organic solvents was slower than that in water. Serious aggregation of CuNWs in nonpolar organic solvents could restrain the diffusion of oxygen to the internal CuNWs to protect them from oxidation. It is surprising that all the samples stored in polar organic solvents were confirmed as being almost phase-pure copper (JCPDS No. 65-9743) without any phase of copper oxides. However, XRD is inappropriate for analyzing the trace surface oxides of CuNWs because of the amorphous and extremely thin oxide layers. As shown in Figure 3b−g, a few small oxide burrs of copper grew on the surface of CuNWs, but the corresponding XRD patterns exhibited no signal of oxides (Figure 4a). To probe the extent of oxidation in depth, X-ray photoelectron spectra (XPS) analysis was performed, which is very sensitive to the oxidation state of copper (e.g., Cu2+, Cu+, and Cu0).43 XPS spectra of Cu 2p3/2 and Cu LMM measured for the samples are displayed in Figure 4b,c. The characteristic binding energy (BE) values can directly identify elements that exist in or on the surface of the material being analyzed. BEs and kinetic energies of different oxidation states of copper are listed in Table S1; the data are obtained from the NIST X-ray

oxidation, the concentration of oxygen near the surface of CuNWs is relatively lower than that in solvent. Thus, there are three oxygen concentration gradients in this reaction system: the concentration gradient between air and solution, the concentration gradient caused by the diffusion of oxygen in solution, and the concentration gradient between solvent and CuNW surfaces (Supporting Information Scheme S1). As being the bridge for oxygen transmission, the oxygen solubility of the solvent can directly affect the oxidation rate of CuNWs. A higher oxygen solubility is beneficial to the diffusion of oxygen (concentration gradient between solvent and CuNWs’ surfaces) and maintaining more oxygen near the surface of the CuNWs, which improves the oxidation. In addition, EDA molecules absorbed on the surface of CuNWs also could influence the oxidation kinetics of the CuNWs. Unlike naked Cu atoms, the oxidation of Cu atoms stabilized by EDA molecules is more complicated (eq 1 and eq 2). First, steric hindrance resulted from the molecules absorbed on the surface of CuNWs against the diffusion of oxygen from solvent to the CuNW surface. Second, the binding between EDA molecules and the CuNW surface may shield some effective collisions between oxygen molecules and copper atoms, which may lead to the oxidation of CuNWs.

Figure 4. (a) XRD patterns, (b) Cu 2p3/2 spectra, and (c) Cu LMM spectra of the sample being stored in different solvents after 1 month. Cu and Cu2O are marked with •• and • in (a), respectively. The gray dotted lines in (b) and (c) indicate the peak position of different valence states of Cu. E


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CuNWs stored in H2O, 2-propanol, and cyclohexane. The deconvolution of XPS spectra of other samples is shown in Figure S6. CuNWs stored in water or cyclohexane displayed a main peak observed at 933.7 eV, which was attributed to CuO, and a small peak (932.2 and 932.1 eV), which could be assigned to Cu/Cu2O. However, the corresponding O 1s spectra of these samples only displayed a main peak for CuO (529.75 and 529.65 eV) and a side peak for OH− and CO32− (531.20 and 531.05 eV). Hence, the oxidation state of copper on the surface of these samples was dominated by Cu2+. As for CuNWs stored in 2-propanol, the Cu 2p3/2 BE of 931.7 eV and O 1s BE of 530.90 eV agreed with the values of the Cu2O phase, and the Cu 2p3/2 BE of 933.9 eV and O 1s BE of 529.95 eV agreed with the values of the CuO phase. Due to the larger peak area of the fitting peak of Cu2O, Cu2O was the main surface composition for this sample. Therefore, compared with water and cyclohexane, 2-propanol (i.e., polar organic solvents) is conductive to slow down the oxidation rate of CuNWs during storage. Stability of CuNWs in the Gas Phase. In order to obtain quantitative descriptions of the stability of CuNWs, thermogravimetric analysis (TGA) is used to investigate the oxidation of CuNWs in the gas phase. Figure 6a and b show the α−T and DTG curve at different heating rates during oxidation. Conversion ratio α at a given temperature during thermogravimetric analysis is defined as

Photoelectron Spectroscopy Database. Obviously, Cu (932.2− 932.7 eV) and Cu2O (932.0−932.6 eV) have an overlapping Cu 2p3/2 BE range, so it is improper to distinguish Cu and Cu2O by only using XPS spectra of Cu 2p3/2. Auger electron spectra of Cu LMM are powerful in distinguishing the oxidation states of copper.52 Here, we combined these two spectra of copper to discuss the surface oxidation of CuNWs. As shown in Figure 4b, all the samples exhibited two peaks at about 932.0 and 933.5 eV in the Cu 2p3/2 region. CuNWs stored in water, ethylene glycol (EG), cyclohexane, and nhexane had a strong peak at about 933.5 eV. According to Table S1, the main surface composition of these samples was CuO. The strong peak at about 932.0 eV of CuNWs that were stored in polar organic solvents except EG revealed their main surface composition was Cu/Cu2O. As for auger electron spectra of Cu LMM (Figure 4c), there were two strong peaks at about 916.8 and 917.7 eV and one extremely weak side peak at about 918.9 eV, according to Table S1, which are assigned to Cu2O, CuO, and Cu, respectively. The weak side peak at about 918.9 eV (Cu) demonstrated that oxides are a dominant part of the surface of CuNWs for all the samples. Compared with other samples, a majority of the samples stored in polar organic solvents exhibited the strongest peak at about 916.8 eV, indicating the main surface composition of these samples is Cu2O. Hence, polar organic solvents are beneficial to attenuate the degree of oxidation of CuNWs during storage. For obtaining in-depth and comprehensive information about the surface composition of CuNWs, we discuss the oxidation states of copper on the surface in detail by XPS-peakdifferentiation-imitating analysis. Figure 5 shows the deconvolution of XPS spectra in the Cu 2p3/2 and O 1s region for

α(T ) =

WT − W0 W∞ − W0

(1)

where W0 is the initial weight of the sample, W∞ is the final weight of the sample, and WT is the weight of the sample corresponding to a temperature T. Based on α−T and DTG curves, the oxidation of CuNWs could be divided into two different stages, and the horizontal dash line in Figure 6a is the boundary of stage 1 and stage 2. All DTG curves of CuNW oxidization at different heating rates exhibited a slight inflection point at about 120−200 °C (Figure 6b), which may be caused by the degradation of EDA molecules. For stage 1, the final conversion ratio α was about 50% (Figure 6a), and the corresponding XRD pattern of the sample exhibited one major phase pattern assigned to Cu2O crystals (JCPDS No. 05-0667) with strong reflections at 43.34°, 50.47°, and 74.17° (Figure 6c) and one minor phase attributed to CuO crystals (JCPDS No. 65-2309) (strong reflections at 35.66°, 38.85°, and 48.95° that are marked with five-pointed stars in Figure 6c). After stage 1, CuNWs showed a hollow tubular structure with a rough surface (Figure 6e). However, CuNWs with diameters about 25 nm still keep a solid wire-like morphology after stage 1 in the previous report.53 The reason is that the larger diameter of CuNWs in this study could provide enough diffusion distance to obtain sufficient net directional flow of matter to form a hollow nanostructure.39 For stage 2, the final conversion ratio α reached about 100% (Figure 6a), and the corresponding XRD pattern was attributed to CuO crystals (JCPDS No. 65-2309) (Figure 6c). Finally, CuNWs were transformed to indistinct tubular structures with a diameter of about 100 nm because of the deformation derived from the Ostwald ripening and the restructuring of the nanostructures during heating (Figure 6f).35 In order to quantitatively evaluate the stability of CuNWs, the activation energies of different stages were determined by the Kissinger method. The Kissinger method is one of the most commonly used methods for determining kinetic parameters by

Figure 5. Deconvolution of XPS spectra in the Cu 2p3/2 and O 1s region for the samples stored in H2O, 2-propanol, and cyclohexane after 1 month. F


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Figure 6. (a) α−T curves of CuNWs oxidized at different heating rates. (b) DTG curves of CuNWs oxidized at different heating rates. (c) XRD patterns of CuNWs before oxidation, after stage 1, and after stage 2. (d) Arrhenius plots based on the Kissinger equation for stage 1 and stage 2. (e) TEM image of oxidized CuNWs after stage 1. (e) TEM image of oxidized CuNWs after stage 2.

thermal analysis, and it is based upon a series of experiments in which small milligram quantities of the reacting material are heated at several heating rates (β) while the reaction exothermic peak is recorded.54 The Kissinger equation is ⎛ β ⎞ Ea AR ⎟ = ln − ln⎜ 2 Ea RTmax ⎝ Tmax ⎠

(2)

where A is the Arrhenius frequency factor, Tmax is the temperature of the peak maximum on the DTG curve, β is the heating rate, and R is the gas constant. The activation energy (Ea) can be determined from the slope of the line (ln[β/ Tmax2] versus 1/Tmax). Figure 6d shows Arrhenius plots based on the Kissinger equation of stage 1 and stage 2. The activation energy of different stages was 100.93 kJ/mol for stage 1 and 127.53 kJ/mol for stage 2. The low activation energy revealed the unstable nature of CuNWs. Due to the relatively low Cu0/ Cu2+ redox potential (+0.34 V), the oxidation of copper is a spontaneous reaction in thermodynamics (Table S2). However, in fact, the reaction rate of copper oxidation is slow, which is determined by kinetic factors. We think that thermodynamics is the driving force of oxidation of CuNWs, and kinetics control the oxidation rate of CuNWs.55 Properties of CuO Generated from Different Oxidation Pathways. In order to compare the impact of different morphologies on the property of the oxidation product, the oxidation product obtained from cyclohexane was completely oxidized into copper oxide by heating in air. Figure 7a shows the SEM image of CuNWs after storage for 1 month in cyclohexane. One-dimensional CuNWs stored in cyclohexane formed a mace-like structure. After heating, the oxide flakes of copper became dull and thick, and some indistinct hollow pores were also found in the annealed mace-like oxidized CuNWs (Figure 7c). Fresh CuNWs evolved into CuO nanotubes with thin shells from smooth nanowires after an annealing process (Figure 7b and d). Figure 7e,f show the N2 adsorption/ desorption isotherm and the Barrett−Joyner−Halenda pore size distribution plot of CuO from the different oxidation

Figure 7. SEM images of (a) CuNWs after storage for 1 month in cyclohexane and (b) fresh CuNWs before heating. TEM images of (c) CuNWs oxidized in cyclohexane after 1 month and (d) fresh CuNWs after heating in air at 400 °C for 3 h. Nitrogen adsorption desorption isotherms of (e) CuNWs oxidized in cyclohexane after 1 month and (f) fresh CuNWs after heating. Insets in (e) and (f) are the corresponding pore size distribution plots.

pathways. The shape of the isotherm of mace-like CuO seems to nearly be a type III isotherm according to the IUPAC classification, and that of CuO nanotubes derived from fresh G


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Figure 8. (a) XRD patterns, (b) UV−vis absorption spectra, (c) plots of (αhυ)2 vs Ephot, (d) Raman spectra, (e) overview FTIR spectra, and (f) enlargement FTIR spectra (700 cm−1 to 400 cm−1) for CuO from different oxidation pathways (CuO-L: CuNWs oxidized in cyclohexane; CuO-G: CuNWs oxidized in air).

CuNWs can be classified as type V, indicating the presence of mesopores. The surface area of these samples determined by the BET method was 6.93 m2 g−1 for mace-like CuO and 6.07 m2 g−1 for CuO nanotubes. The wide distribution of the pore size is due to the voids between the aggregated copper oxide. These results clearly show that the different oxidation pathways can heavily influence the final morphology and surface area of copper oxide. As shown in Figure 8a, the XRD patterns of CuO from different oxidation pathways were almost the same except for the intensity of the strong reflections at about 38° and 35°. The XRD patterns of CuO-G (CuNWs oxidized in air) and CuO-L (CuNWs oxidized in cyclohexane) could be attributed to monoclinic copper oxide (JCPDS No. 65-2309) and monoclinic copper oxide (JCPDS No. 48-1548), respectively. No peak from an impurity such as Cu or Cu2O was observed; the heat-treated samples were pure copper oxide. UV−vis absorption was used to reveal the energy structure and optical absorption property of CuO-L and CuO-G (Figure 8b,c). Both samples had a significant absorption edge at ∼650 nm, indicating that the as-obtained CuO samples may have photocatalytic activity under natural light.56,57 The plot of (αEphoton)2 − Ephoton is used to estimate the band gap of these samples. The band gap of as-obtained CuO samples was estimated to be 2.2 eV for CuO-L and 2.6 eV for CuO-G according to prolongation of the linear section, which is similar to the reported values for CuO nanocrystals.45,56,58,59 The obtained CuO samples were further examined by Raman and IR analysis to investigate their vibrational properties. On the basis of the group theory and the fact that CuO belongs to the C62h space group with four atoms in the primitive cell,59,60

the following zone center modes are given by a factor-group analysis: Γvibr = A g + 2Bg + 4A u + 5Bu

Among these modes, there are three acoustic modes (Au + 2Bu), six IR-active modes (3Au + 3Bu), and three Raman active modes (Ag + 2Bg). As shown in Figure 8d, Raman spectra of both samples exhibited three main Raman phonon modes at about 290, 341, and 630 cm−1, which are assigned to Ag, Bg, and Bg modes, respectively. As for FTIR spectra (Figure 8e), a broad absorption band at about 3460 cm−1 could be assigned to the stretching mode of water molecules and/or surface hydroxyls. The weak absorption band at about 1631 cm−1 could correspond to the bending mode of O−H. Compared with the CuO-G sample, the CuO-L sample exhibited two extra absorption bands at 1116 and 880 cm−1, which can be assigned to epoxides. This suggests the cyclohexane molecules absorbed on the surface of the oxides were oxidized by oxygen during heating. The strong absorption in the range 700−400 cm−1 is attributable to the vibration of different facets of CuO (Figure 8f). There are three main absorption peaks, observed at ∼429, 496, and 599 cm−1, which are assigned to the Au, Bu, and Bu modes, respectively.59−61 The high-frequency mode at 599 cm−1 could be the Cu−O stretching vibration along the [−101] direction. The mode located at about 496 cm−1 corresponded to the Cu−O stretching vibration along the [101] direction. The remaining peak at about 534 cm−1 could be a splitting peak, which is attributed to the enhancement of polarity of CuO along the [101] direction.62 Compared with the CuO-L sample, the CuO-G sample exhibited a much smoother FTIR spectrum in the range 700−400 cm−1. In addition, the Cu2O H


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ACS Nano

Figure 9. (a) Photographs of CuNW solution stored under the protection of N2H4·H2O (in water), N2H4·H2O (in EG), ascorbic acid, citric acid, and glucose (from left to right). (b) SEM images of CuNWs stored for 1 month under the protection of (b) N2H4·H2O (in water), (c) N2H4·H2O (in EG), (d) ascorbic acid, (e) citric acid, and (f) glucose.

Figure 10. (a) XRD patterns, (b) Cu 2p3/2 spectra, and (c) Cu LMM spectra of the sample stored for 1 month under the protection of different reducing agents.

IR-active mode at about 615 cm−1 was not found in both samples, indicating the obtained CuO was a pure phase. Storage of CuNWs in the Liquid Phase. For the application of nanomaterials, an important problem is how to keep nanomaterials with a high reactivity and a good dispersibility for a long time. Figure 9a shows photographs of a CuNW solution under the protection of different reducing agents after different storage times. Obviously, all the samples displayed excellent dispersibility after stirring and ultrasonic dispersion; however, after a short storage time (only 24 h), all CuNWs settled to the bottom (Figure 9a and Figure S7). CuNWs stored in EG under the protection of N2H4·H2O were settled more thoroughly than those stored in EG without N2H4· H2O (Figure 3j), indicating the addition of reducing agents had no effect on the improvement of dispersibility of CuNWs. On prolonging the storage time, CuNW sediments still remained red under the protection of N2H4·H2O and ascorbic acid, but CuNWs stored under the protection of citric acid or glucose exhibited a black color, which was assigned to oxides of copper. Additionally, a CuNW solution under the protection of ascorbic acid turned yellow due to the degradation of ascorbic acid, and a CuNW solution stored with citric acid turned blue. All of the CuNWs under the protection of N2H4·H2O or ascorbic acid exhibited a smooth surface without any oxide burrs after storage for 1 month (Figure 9b−d), while there were many pits on the surface of CuNWs stored with citric acid after storage for 1 month (Figure 9e). These thickly dotted pits and the blue color of the CuNW solution could be caused by the reaction between

citric acid and oxides of copper. Hence, citric acid is unsuitable for the protection of CuNWs, because it can etch CuNWs during storage. Due to the serious etching, the collected oxidation product from the CuNWs stored with citric acid was hardly enough for subsequent analysis of XRD and XPS. On the contrary, many convex protrusions were found on the surface of CuNWs stored with glucose after storage for 1 month (Figure 9f). Figure 10a shows the XRD patterns of the different samples. All the samples exhibited sharp and strong reflections at 43.34°, 50.47°, and 74.17°, which are attributed to those for Cu crystals (JCPDS No.65-9743). However, the black color demonstrated the existence of oxides of copper in the sample stored with citric acid or glucose, but no signal of oxides of copper was present in the corresponding XRD patterns because of the powerlessness of XRD for analyzing trace surface oxides. To probe the extent of surface oxides of copper in depth, XPS analysis was performed on every sample. As shown in Figure 10b, all the samples exhibited one strong peak at about 932.0 eV in the Cu 2p3/2 region except the sample stored with glucose, which was assigned to Cu/Cu2O. The sample stored with glucose exhibited a broad peak at about 933.3 eV in the Cu 2p3/2 region, which was assigned to CuO. Figure 10c shows Cu LMM spectra of CuNWs stored for 1 month under the protection of different reducing agents. Most of the samples exhibited two peaks at about 917.0 and 918.7 eV, which were attributed to Cu2O and Cu, respectively, so the main surface composition of these samples was Cu and Cu2O. The broad peak at about 933.3 eV of CuNWs stored with glucose I


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ACS Nano

The mass of CuNWs in each oxidation experiment was controlled between 3 and 5 mg. Storage of CuNWs. A ∼10 mg amount of CuNWs was redispersed in 20 mL of deionized water (or EG) by vigorous magnetic stirring to form a homogeneous suspension. Then 17.5 mmol of hydrazine hydrate was added to the above suspension. After that, the suspension was stored at 20 °C in a constant-temperature and -humidity chamber. Other experimental samples were prepared through the same procedure by replacing hydrazine hydrate with glucose, citric acid, and ascorbic acid, respectively. Measurement and Characterization. The morphology was examined with a JEOL JSM-6700F scanning electron microscope. Transmission electron microscopy observations were performed on a JEOL-2010 microscope operated at an acceleration voltage of 200 kV. X-ray powder diffraction patterns were obtained from a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.541 78 Å). X-ray photoelectron spectra were recorded on an ESCALab MKII X-ray photoelectron spectrometer, using Mg KR radiation as the exciting source. UV−vis spectra were recorded on a UV-2501PC/2550 spectrometer (Shimadzu Corp., Japan) at room temperature.

demonstrated that CuO was a dominant part of the surface. Hence, the reducing capacity of glucose was not enough to prevent CuNWs from oxidizing, and N2H4·H2O and ascorbic acid are optimized reducing agents for the protection of CuNWs during storage. Due to the high reactivity of CuNWs, low Cu 0 /Cu 2+ redox potential (+0.34 V), and many unsaturated surface atoms, the slight oxidation of CuNWs on the surface is inevitable, and thus we should take this into consideration when we use CuNWs.

CONCLUSION In summary, the stability of CuNWs in both liquid and gas phase has been investigated systematically. EDA molecules covered on CuNWs are of great significance in the dispersibility of CuNWs. Based on the similarity-intermiscibility theory, CuNWs show good dispersibility in polar solvents, but nonpolar solvents will cause serious aggregation of CuNWs. CuNWs are highly sensitive to oxygen and can be quickly oxidized by dissolved oxygen within several hours. Polar organic solvents could prevent CuNWs from oxidizing, while CuNWs stored in water or nonpolar organic solvents are heavily oxidized and evolve into a mace-like structure. In the gas phase, CuNWs were oxidized into CuO nanotubes with thin shells by heating. More importantly, the different oxidation pathways have significant effects on the final morphology, surface area, phase, optical absorption, band gap, and vibrational property of the oxidation product. We found that adding suitable reductants is an effective way to prevent the oxidation of fresh CuNWs during storage. However, slight oxidation of CuNWs on the surface is inevitable, and thus we should take this into consideration when we use CuNWs. This work emphasizes an emerging crucial issue on the stability and reactivity of nanostructures, which should draw attention in the future from the viewpoint of practical applications.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00704. SEM and TEM image and XRD patterns of CuNWs during storage; photographs of a CuNW solution stored in different solvents; binding energies (eV) of Cu 2p3/2, O 1s, and Cu LMM; deconvolution of XPS spectra in the Cu 2p3/2 and the O 1s region and Cu LMM spectra for the sample stored in other solvents; photographs of a CuNW solution stored with different reducing agents. (PDF)

AUTHOR INFORMATION Corresponding Author

METHODS

*E-mail: [email protected].

Chemicals. Cu(NO3)2·3H2O, NaOH, ethylenediamine, hydrazine hydrate (N2H4·H2O) (85 wt %), glucose, citric acid, ascorbic acid, ethylene glycol, methanol, ethanol, 2-propanol, dimethyl sulfoxide (DMSO), N,N-dimethylformamide, n-hexane, and cyclohexane were purchased from Shanghai Chemical Reagents Co. Ltd. All the chemical reagents were used as received without further purification. Synthesis of CuNWs. CuNWs were synthesized using the method previously reported by the Zeng group.15 First, 240 g of NaOH was dissolved in 400 mL of deionized water with suitable magnetic stirring, and then the solution was cooled to room temperature naturally. After that, 10 mL of Cu(NO3)2·3H2O (0.2 M) and 3 mL of EDA were added into the above solution, and the mixture was preheated at 60 °C for 30 min under vigorous magnetic stirring. A 200 μL amount of N2H4·H2O was then quickly added, stirring was stopped, and the mixture was maintained at 60 °C for 3 h. When the reaction was completely finished, the product was collected and washed several times with deionized water. Stability of CuNWs in the Liquid Phase. A ∼10 mg amount of CuNWs was redispersed in 20 mL of deionized water by vigorous magnetic stirring to form a homogeneous suspension. Then the suspension was kept at room temperature and examined at certain time intervals. Other experimental samples were prepared through the same procedure by replacing water with ethylene glycol, methanol, ethanol, 2-propanol, dimethyl sulfoxide, N,N-dimethylformamide, nhexane, and cyclohexane, respectively. Stability of CuNWs in the Gas Phase. The stability of CuNWs in the gas phase was tested by a TA-Q600 SDT differential thermal analysis instrument. TGA oxidation curves were obtained with 40 mL/ min air at different heating rates from room temperature to 600 °C.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the funding support from the National Natural Science Foundation of China (Grant 21431006), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), the National Basic Research Program of China (Grants 2014CB931800, 2013CB931800), the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS (2015HSC-UE007, 2015SRG-HSC038), and the Chinese Academy of Sciences (Grant KJZD-EW-M01-1). REFERENCES (1) Goesmann, H.; Feldmann, C. Nanoparticulate Functional Materials. Angew. Chem., Int. Ed. 2010, 49, 1362−1395. (2) Baraton, M.-I. Fourier Transform Infrared Surface Spectrometry of Nano-Sized Particles. In Handbook of Nanostructured Materials and Nanotechnology; Nalwa, H. S., Ed.; Academic: London, 2000; pp 90− 91. (3) Cademartiri, L.; Ozin, G. A. Concepts of Nanochemistry; John Wiley & Sons: Weinheim, 2009; pp 12−19. (4) Zhou, Z. Y.; Tian, N.; Li, J. T.; Broadwell, I.; Sun, S. G. Nanomaterials of High Surface Energy with Exceptional Properties in Catalysis and Energy Storage. Chem. Soc. Rev. 2011, 40, 4167−4185. J


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Comparison Study on the Stability of Copper Nanowires and Their

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