Facile Solvothermal Synthesis of Phase-Pure Cu4O3 Microspheres

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Facile Solvothermal Synthesis of Phase-Pure Cu4O3 Microspheres and Their Lithium Storage Properties Lirun Zhao,†,‡ Han Chen,† Yingli Wang,† Hongwei Che,† Poernomo Gunawan,§ Ziyi Zhong,§ Hong Li,⊥ and Fabing Su*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island, Singapore 627833 ⊥ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: Phase-pure Cu4O3 microspheres were synthesized for the first time via a facile solvothermal method, using Cu(NO3)2·3H2O as the precursor. A formation mechanism was proposed based on the observation of a series of reaction intermediates. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, temperature-programmed reduction and oxidation, X-ray photoelectron spectroscopy, and nitrogen adsorption. It was found that the composition of the prepared products were highly dependent on the synthesis conditions, particularly the hydrate water content in the copper precursor of Cu(NO3)2. Pure Cu4O3 microspheres with a diameter of 2−10 μm could be obtained via the symproportionation reaction (2CuO + Cu2O → Cu4O3), which was regarded not being feasible in aqueous media under mild synthesis conditions. The electrochemical properties of the Cu4O3 microspheres as anode materials for Li-ion batteries were also investigated. Compared to the simple physical mixture of CuO and Cu2O with an equivalent atomic ratio of 2:1, the as-prepared Cu4O3 exhibited unique lithium storage behaviors at a low voltage range and superior electrochemical performances as an anode material for Li-ion batteries. The successful preparation of pure Cu4O3 material could provide opportunities to further explore its physicochemical properties and potential applications. KEYWORDS: phase-pure Cu4O3 microspheres, solvothermal synthesis, characterization, anode materials, Li-ion batteries

1. INTRODUCTION In order to tackle new challenges met in the development of modern industry and society, it is imperative to develop materials with high energy and power density, particular electrodes in Li-ion batteries.1 In recent years, many efforts have been undertaken to improve the battery materials by employing nanostructured materials,2 which presents new opportunities for higher energy density, faster charge and discharge rates, and better cyclability.3 Transition-metal oxides (TMOs, where TM = Co, Fe, Ni, Cu, etc.) have been regarded as potential candidates for anode materials.4,5 This is because lithium can be stored reversibly in TMOs through a heterogeneous conversion reaction: Li + TMO → Li2O + TM, which involves the formation and decomposition of Li2O via the reduction and oxidation of metal nanoparticles, different from the classical Li insertion/desertion or Li-alloying processes.5 Compared to conventional carbonaceous materials with a theoretical capacity of 372 mA h/g,6 TMO anodes have the advantages of high capacity and good safety. Therefore, © 2012 American Chemical Society

many nanostructured TMO materials have recently been explored as promising anodes for Li-ion batteries.7 On the other hand, micrometer-sized materials with a spherical morphology are actually favorable in conventional electrode fabrication art,8,9 because of their high packing density for high volumetric energy and power density, as well as good particle mobility to form a uniformly compact electrode layer. To date, TMO microspheres such as CuO,10 Cu2O,11 MnO,12 Co3O4,13 Fe2O3,14 and NiO15 have been synthesized for anode materials. Although CuO (tenorite or cupric oxide) and Cu2O (cuprite or cuprous oxide) have been extensively investigated as potential anode materials for Li-ion batteries,16−20 the electrochemical lithium storage properties of Cu4O3 have never been reported. Cu4O3 (paramelaconite) was first discovered in the 1870s at the Copper Queen mine located at Bisbee, AZ, USA.21 A pure Received: December 1, 2011 Revised: February 15, 2012 Published: February 23, 2012 1136

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Table 1. Synthesis Conditions Used for Sample Preparation sample

copper concentration (M)

DMF volume (mL)

ethanol volume (mL)

temperature, T (°C)

reaction time (h)

product derived from XRD

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13

0.01 0.07 0.15 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07

10 10 10 0 5 15 30 10 10 10 10 10 10

20 20 20 30 25 15 0 20 20 20 20 20 20

130 130 130 130 130 130 130 130 130 130 120 150 180

9 9 9 9 9 9 9 6 7 15 9 9 9

Cu2O + CuO Cu4O3 Cu2O Cu2(OH)3NO3 Cu2(OH)3NO3 + Cu2O + CuO + Cu4O3 Cu4O3 + Cu2O Cu2O Cu2(OH)3NO3 Cu2O + CuO Cu2O Cu2(OH)3NO3 + Cu2O + CuO + Cu4O3 Cu4O3 + Cu2O Cu2O

transferred into a 50-mL Teflon-lined stainless steel autoclave. The autoclave was sealed, maintained at 130 °C for several hours, and then cooled naturally to room temperature. The product was collected by centrifugation and washed with deionized water and ethanol several times. The collected dark solid was dried in a vacuum oven at 60 °C for 8 h. Theoretically, Cu4O3 is a mixed-valence compound with monovalent and divalent Cu ions that are equally present, which can be expressed as 2CuO·Cu2O. For a comparative investigation, we also prepared a physical mixture containing CuO and Cu2O with a molar ratio of 2:1 (called a “2CuO + Cu2O” composite), a Cu2O sample, which was denoted as S10 in Table 1, and a CuO sample obtained by heating sample S10 in air at 400 °C for 4 h. Characterization. The morphology of the samples was observed with field-emission scanning electron microscopy (SEM) (Model JSM6700F, JEOL, Tokyo, Japan) and high-resolution analytical transmission electron microscopy (TEM) (Model JEM-2010F, JEOL, Tokyo, Japan) at 200 kV. The compositional analysis was conducted using energy-dispersive X-ray (EDX) spectroscopy (Shimadzu). The crystal structure was studied on an X-ray diffractometer (X’Pert PRO, PANalytical) using Cu Kα radiation (λ = 1.5418 Å) at an accelerating voltage of 15 kV and a current of 20 mA. Thermogravimetric analysis (TGA) was carried out on a Seiko Instruments EXSTAR TG/DTA 6300 using a heating rate of 10 °C/min in a stream of air or nitrogen. The temperature-programmed reduction (TPR) and oxidation (TPO) measurements were carried out on Automated Chemisorption Analyzer (chemBET pulsar TPR/TPD, Quantachrome). Upon loading 0.1 g sample into a quartz U-tube, the sample was first degassed at 325 °C for 60 min under helium. After the temperature was reduced to 20 °C, the gas was changed to 9.91% H2/Ar or 3.99% O2/He and subsequently heated up to 800 at 10 °C/min with a gas flow rate of 30 mL/min. The surface chemical composition of the samples was determined via X-ray photoelectron spectroscopy (XPS) (Model VG ESCALAB 250 spectrometer, Thermo Electron, U.K.), using a nonmonochromatized Al Kα X-ray source (1486 eV). The operating pressure in the analysis chamber was maintained below 1 × 10−9 Torr. Wide-scan spectra in the binding-energy range of 1100−0 eV were recorded using a step size of 1 eV step and a pass energy of 50 eV. High-resolution spectra of the elemental signals were recorded in 0.05 eV steps with a pass energy of 20 eV. The calibration of binding energy (BE) of the spectra was based on the C 1s electron binding energy at 284.5 eV. After the linear baseline for nonmetal element signals (using the Shirley baseline for metal element signal) was subtracted, curve-fitting was performed using the nonlinear leastsquares algorithm, assuming a Gaussian peak shape. The surface atomic composition ratio was calculated using the Schofield sensitivity factors (16.73 for Cu 2p3/2 and 2.93 for O 1s) after normalization of the individual peak’s areas. Particle size distribution was measured using a laser particle size analyzer (Model BT-9300Z, Bettersize Instruments, Ltd., China). Nitrogen adsorption−desorption isotherms were carried out on a nitrogen adsorption apparatus (Model NOVA

Cu4O3 phase is difficult to synthesize in bulk or particle form, because stabilizing both the Cu(II) and Cu(I) atom simultaneously via the conventional aqueous chemistry approaches is difficult;22−24 thus, the synthesis of Cu4O3 is rarely reported via reduction of the CuO phase or oxidation of the Cu2O phase.25 However, thin films that contain pure or partial Cu4O3 had been prepared via reactive sputtering processes26−28 and chemical vapor deposition.29 Also, a small amount of Cu4O3 was observed in copper oxide mixtures obtained via the chemical extraction process of copper oxides using aqueous ammonia,25 the thermal decomposition of copper(II) acetate,30 and ultrasound irradiation in aqueous solution of copper salt and aniline.31 Cu4O3 nanoparticles could be produced from the decomposition of CuO under irradiation of an electron microscope beam32 or from laser ablation of Cu metal targets in water.33 However, only microscopic quantities can be generated via these methods. In addition, the aforementioned samples are often contaminated with CuO, Cu2O, or other species that cannot be effectively removed to achieve the required high purity. Therefore, facile synthesis of pure Cu4O3 phase and investigation on its properties and applications remain a big challenge, although some properties have been explored using thin films or composites, including magnetic properties,34 thermal stability,28,35−37 optical band gap,22,26 and catalytic activity.29 Herein, we report a new facile solvothermal method to synthesize phase-pure Cu 4 O 3 microspheres using Cu(NO3)2·3H2O as the precursor in the presence of N,Ndimethylformamide (DMF) and ethanol. Compared to the previously reported methods, this solvothermal method exhibits many advantages, including a milder condition, easier morphological control, simpler operation, macroscopic quantity, and greener chemistry.38 Meanwhile, the electrochemical properties of the pure Cu4O3 microspheres as anode materials for Li-ion batteries are investigated for the first time.

2. EXPERIMENTAL SECTION Synthesis. All the chemicals were of analytical grade and used without further purification. The solvothermal method was used to synthesize the materials. To optimize the synthesis conditions and investigate the product formation mechanism, the parameters such as the reaction temperature, time, and reactants amount were varied while keeping other experimental parameters constant (see Table 1). In a typical synthesis, 0.07 M Cu(NO3)2·3H2O was dissolved in 30 mL of a DMF−ethanol mixed solvent (the volume ratio of DMF to ethanol is 10:20), stirred for 15 min to form a clear solution, and then 1137

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Figure 1. (a) XRD pattern, (b) SEM image, (c) TEM image, (d) SAED, (e) Cu 2p3/2 XPS spectrum, and (f) EDX spectrum of Cu4O3 microspheres (sample S2 in Table 1). 3200e, Quantachrome, USA). The sample was degassed at 200 °C prior to measurements. The Brunauer−Emmett−Teller (BET) specific surface area (SBET) was determined by a multipoint BET method, using the adsorption data in the relative pressure (P/P0) range of 0.05−0.25. Electrochemical Measurement. The working electrode was prepared by mixing pure Cu4O3 (or the 2CuO + Cu2O composite) as active material, acetylene black, and polyvinylidene fluoride (PVDF) with a weight ratio of 75:15:10 in dry N-methylpyrrolidone (NMP) solvent. The resulting slurry was cast onto copper current collectors and then dried at 120 °C under vacuum for 12 h. The foils were rolled into 30-μm thin sheets, and then cut into disks 14 mm in diameter. CR2016 coin-type cells were assembled in an argon-filled glovebox, using lithium foils as counter electrodes and polypropylene microporous films (Celgard 2400) as separators. The liquid electrolyte is 1 mol/L LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v). The galvanostatic charge and discharge tests were carried out at room temperature, using a Model CT2001A LAND testing instrument in a voltage range between 0.01 V and 3.0 V at a current rate of 0.1 C (1 C = 529 mA/g).

correspond to the lattice planes of (200), (202), (220), (400), (206), and (422) in tetragonal Cu4O3 (JCPDS Fiel Card No. 49-1830), respectively. This XRD pattern is also consistent with that of the previously reported pure Cu4O3.25 Since there are not any other peaks that can be assigned to CuO or Cu2O, it is concluded that Cu4O3 with high purity is obtained. The average Cu4O3 grain size is calculated to be 25 nm, based on the peak at 35.8°. Figure 1b displays a SEM image of Cu4O3, showing microspherical morphology and particle diameters of 2−10 μm, which are in good agreement with the particle size distribution derived from the laser particle size analysis (see Figure S1 in the Supporting Information). Considering the calculated crystal size of Cu4O3, it can be inferred that these Cu4O3 microspheres are polycrystalline. The high-resolution TEM image (Figure 1c) shows a lattice distance of ∼0.25 nm, which is consistent with that of the (202) plane of tetragonal Cu4O3 derived from the XRD pattern.33 The inset of Figure 1c shows a dense internal structure of Cu4O3 microspheres. The electron diffraction pattern in Figure 1d provides further evidence that the product is paramelaconite.39 The Cu 2p3/2 XPS spectrum is presented in Figure 1e, and qualitative inspection of the peaks indicates that the obtained sample contains both Cu+ and Cu2+. The peaks at 931.7 and 933.1 eV can be assigned to Cu+(2p3/2) and

3. RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of the obtained copper oxide (sample S2 in Table 1). Diffraction peaks at 2θ values of 30.7°, 35.8°, 44.0°, 63.9°, 65.0°, and 75.5° are identified, which 1138

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decomposition at 300 °C (Figure S4 in the Supporting Information) indicates that the solid product consists of Cu4O3, Cu, and Cu2O, which is consistent with the reported data.25 These results thus demonstrate the metastablility of Cu4O3, implying that Cu4O3 with high purity is hardly achieved by physical methods, such as vapor deposition,43 of which synthesis processes are always carried out at elevated temperature (>250 °C). TPO curves of the Cu2O and the 2CuO + Cu2O composite in Figure 2c shows only one oxygen consumption peak at 395 °C, which can be attributed to the oxidation of Cu2O. For Cu4O3, a negative peak at ∼250 °C is due to the removal of oxygen, suggesting a decomposition of Cu4O3 via the reaction 2Cu4O3 = 4Cu2O + O2,37 which is consistent with the weight loss of the TG curve in air (250−300 °C) (see Figure 2b). Oxidation of Cu4O3 sample begins at ∼320 °C and finishes at 520 °C, which is in good agreement with its TG profile in Figure 2b. The maximum position of the oxidation peak is located at 470 °C, which is a much higher temperature than that of the 2CuO + Cu2O composite. Moreover, the TPO curve of Cu4O3 microspheres is composed of multiple peaks, implying a more-complex oxidation process. The calculated oxygen consumption of Cu4O3 is 0.52 times that of the Cu2O used as a reference, which is close to the theoretical value of 0.50. TPR curves of various samples are shown in Figure 2d. A single peak at 320 °C is observed for the Cu2O reference, while two peaks are observed for the 2CuO + Cu2O composite: (1) the reduction of CuO at 260 °C and (2) a shoulder peak at 320 °C, which is possibly due to the reduction of Cu2O. Thus, we infer that the peak at 260 and 320 °C can be assigned to the reduction of the Cu2+ and Cu+ compounds, respectively. Correspondingly, Cu4O3 also exhibits two distinctive peaks, at 265 and 325 °C, which can be assigned to the reduction of Cu2+ and Cu+ compound, respectively. In addition, the calculated hydrogen consumption of Cu4O3 is 1.45 times that of Cu2O, which is close to the theoretical value of 1.50. The calculated hydrogen consumption of the 2CuO + Cu2O composite is also close to that of Cu4O3. Hence, the above results indicate the high purity of the Cu4O3 microspheres obtained in our work. The synthesis conditions such as copper precursor concentration (Figure S5 in the Supporting Information), volume ratio of DMF to ethanol (Figure S6 in the Supporting Information), synthesis temperature (Figure S7 in the Supporting Information), and reaction time (Figures S8 and S9 in the Supporting Information) were extensively investigated. The possible reaction steps taking place during the reaction (reactions 1−5) are listed below. Initially, the hydrolysis of Cu(NO3)2·3H2O leads to the formation of Cu2(OH)3NO3 (reaction 1) in pure ethanol (Figure S6a) or in a 10:20 (volume ratio) mixture of DMF and ethanol mixture (Figure S7a). With the increase of the reaction time up to 7 h in this mixed solvent, CuO and Cu2O (Figure S8b) are obtained, in which CuO is generated by the decomposition of Cu2(OH)3NO3 (reaction 2).44 Since DMF could react with H2O to form HCOOH that can act as a weak reducing agent (reaction 3), a fraction of CuO is further reduced to Cu2O (reaction 4).45,46 Upon prolonging the reaction time to 9 h (Figure S8c), Cu4O3 is produced via the symproportionation reaction 2CuO + Cu2O → Cu4O3 in the molecular or nanoscale level (reaction 5).30,31 By further extending the reaction period to 15 h, complete reduction of CuO to Cu2O is

Cu2+(2p3/2),40 respectively. The Cu+:Cu2+ atomic ratio derived from Figure 1e and from the O 1s XPS spectrum (see Figure S2 in the Supporting Information) is 0.9:1, which is slightly smaller than the theoretical value (1:1) of Cu 4 O3 (equal to 2CuO·Cu2O). This may be attributed to the partial surface oxidation of Cu+ to Cu2+ in air.41 Accordingly, the atomic ratio of Cu to O is ∼1.31 (the stoichiometric ratio is 1.33:1). In addition, EDX compositional analysis shown in Figure 1f suggests that the atomic ratio of Cu to O is 1.28:1, which is indeed close to the stoichiometric ratio of 1.33:1. The measured surface area of pure Cu4O3 microspheres is ∼3.0 m2/g. Figure 2a shows the TG curves of the Cu4O3 microspheres and the 2CuO + Cu2O composite as the reference sample

Figure 2. (a) Thermogravimetry (TG) curves in nitrogen; (b) TG curves in air; (c) temperature-programmed oxidation (TPO) curves of Cu4O3, Cu2O, and the 2CuO + Cu2O composite; and (d) TPR curves of Cu4O3, Cu2O, and the 2CuO + Cu2O composite.

performed in N2. The XRD pattern and SEM image of the 2CuO + Cu2O composite are shown in Figure S3 in the Supporting Information. Three major weight losses are observed for Cu4O3. The first weight loss occurs at 96% after 35 cycles at a rate of 0.5 C.51 The electrochemical impedance spectra of the two samples are shown in Figure 3d;

(1)

Cu2(OH)3 NO3 → 2CuO + H2O + HNO3

(2)

HCON(CH3)2 + H2O → HCOOH + NH(CH3)2

(3)

2CuO + HCOOH → Cu2O + H2O + CO2 ↑

(4)

2CuO + Cu2O → Cu 4O3 ↓

(5)

The formation of pure Cu4O3 phase is our crucial concern. It has been reported that the occurrence of the symproportionation reaction is unlikely at low temperature and even less likely at high pressures.25 However, highly pure Cu4O3 (sample S2 in Table 1) is successfully prepared in the current reaction system, which thus opens a question on how the species is stabilized. Since the Gibbs free energy of Cu4O3 (40.0 kJ/mol/atom) is between that of CuO (45.9 kJ/mol/atom) and Cu2O (39.2 kJ/ mol/atom),27 the synthesis of pure Cu4O3 in bulk is very difficult, because the nucleation or the growth barrier for the formation of Cu4O3 is reasonably higher for the thermally activated process.37 High current ion beam irradiation37,47 and high energy ultrasonic irradiation in the liquid31 may kinetically lower the barrier and allow the formation of this phase. On the other hand, exact control of the amount of oxygen atoms derived from gaseous oxygen22,28,37,48 or from the dissociation of H2O33 is regarded as the critical parameter to form crystalline Cu4O3 particles. In our work, high-temperature ethanol could create a relatively high-pressure reaction environment in a closed system. H2O molecules released from the hydrate Cu(NO3)2·3H2O should also play a key role in the hydrolysis of Cu(NO3)2 to Cu2(OH)3NO3 (Figure S5 in the Supporting Information) and the formation of the weak reducing compound HCOOH, which may generate CuO and Cu2O in an ideal stoichiometric ratio, which is a prerequisite that ultimately results in the symproportionation reaction occurring in such a closed autoclave system. The role of H2O can be further confirmed by the addition of trace amount of H2O (0.1 g) in the precursor solution, which leads to the formation of Cu2O phase without any Cu4O3 (Figure S10 in the Supporting Information). In short, the proposed formation mechanism of pure Cu4O3 phase could be illustrated in Scheme 1. Lithium storage behaviors of the obtained pure Cu4O3 and the 2CuO + Cu2O composite are compared in Figure 3, which shows different voltage profiles. It is obvious that Cu4O3 shows 1140

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Figure 3. Electrochemical properties of Cu4O3 and the 2CuO + Cu2O composite: (a) initial discharge and charge curves (0.0−3.0 V and 0.1 C), (b) CV curves in the first cycle (0.0−3.0 V and 0.1 mV/s scan rate), (c) rate and cycling performance, and (d) electrochemical impedance spectra.

physicochemical properties and potential applications, and will be applicable to the synthesis of some other metastable metal oxides that cannot be obtained under conventional synthetic conditions.

each of them shows a high-frequency semicircle, a mediumfrequency semicircle, and a low-frequency inclined line, which is typical for the existence of SEI film, the charge-transfer and double layer, and the lithium-diffusion process within electrodes, respectively.52,53 Obviously, the Cu4O3 electrode shows lower impedance than that of the 2CuO + Cu2O composite. This could explain the lower polarization and superior rate performance of the Cu4O3 material, although the intrinsic kinetic properties of Cu4O3 require further investigation. Although we have successfully obtained pure Cu 4 O 3 microspheres with a mass of ∼0.1 g in each 50-mL autoclave, more work about gas and liquid in situ analysis during the synthesis process would be necessary to clarify the detailed formation mechanism. Scale-up experiment is also underway for mass production in order to investigate their other properties and wide applications.

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ASSOCIATED CONTENT

S Supporting Information *

The particle size distribution curve, XPS spectrum, XRD patterns, and SEM images of the samples obtained at different conditions. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-82544850. Fax: +86-10-82544851. E-mail: fbsu@ mail.ipe.ac.cn. Notes

4. CONCLUSIONS In summary, we have demonstrated a facile solvothermal method to synthesize pure polycrystalline Cu4O3 microspheres with diameters of 2−10 μm, using Cu(NO3)2·3H2O as the precursor. The composition of the resulting products is highly dependent on the synthesis conditions. Hydrate water released from the copper precursor plays a key role in the generation of CuO and Cu2O with an ideal stoichiometric ratio, resulting in the symproportionation reaction (2CuO + Cu2O → Cu4O3) in a closed system. Compared to the physically mixed 2CuO + Cu2O composite, the pure Cu4O3 microspheres exhibit superior electrochemical performance with higher reversible capacity, columbic efficiency, cyclability, and rate performance as anode materials in Li-ion batteries. However, its unique lithiumstorage mechanism is still not clear and requires further clarification. It is expected that this successfully developed solvothermal route to the macroscopic preparation of purephase Cu4O3 will provide opportunities to further explore its

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Hundred Talents Program of the Chinese Academy of Sciences (CAS), State Key Laboratory of Multiphase Complex Systems (No. MPCS-2011D-14), National Natural Science Foundation of China (No. 21031005), CAS-Locality Cooperation Program (No. DBNJ2011-058), and China Postdoctoral Science Foundation (No. 20110490597 and 20110490594).

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dx.doi.org/10.1021/cm203589h | Chem. Mater. 2012, 24, 1136−1142