Ultrasmall Cu3N Nanoparticles: Surfactant-Free Solution-Phase

New chemical pathways are of fundamental interest for materials synthesis. Here, we report a novel surfactant-free, solution-phase, low-temperature ro...
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Ultrasmall Cu3N Nanoparticles: Surfactant-Free Solution-Phase Synthesis, Nitridation Mechanism, and Application for Lithium Storage Rupali Deshmukh,*,† Guobo Zeng,† Elena Tervoort,† Malwina Staniuk,† David Wood,†,‡ and Markus Niederberger† †

Laboratory of Multifunctional Materials, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom

‡

S Supporting Information *

ABSTRACT: New chemical pathways are of fundamental interest for materials synthesis. Here, we report a novel surfactant-free, solution-phase, low-temperature route to crystalline, ultrasmall (∼2 nm) Cu3N nanoparticles via a one-step reaction between copper(II) methoxide and benzylamine. We propose a reaction mechanism for Cu3N formation based on the gas chromatography− mass spectrometry (GC−MS) analysis of the organic reaction byproducts. The reaction pathway involves reduction of the Cu(II) to Cu(I) by benzylamine, in situ generation of ammonia, and finally, the reaction between Cu(I) and ammonia to form Cu3N. We tested the Cu3N nanoparticles as an anode material for Li-ion batteries (LIBs). According to cyclic voltammetry, the Cu3N nanoparticles quickly undergo a phase transformation to Cu2O, but then stably deliver a capacity of ∼290 mAh/g at 1 A/g in the following 150 cycles.

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catalytic applications such as Huisgen cycloaddition13 and electrochemical oxygen reduction reactions,14 resistive randomaccess memory chips,15 and optical storage devices.16 Although Cu3N has drawn considerable attention in a wide range of applications, synthesis strategies to prepare Cu3N nanoparticles are still limited. In the past, Cu3N in the form of thin films have been mainly synthesized by gas-phase techniques such as DC (direct current) magnetron sputtering,15 RF (radio frequency) reactive sputtering,17 and ion-assisted vapor deposition.18 Other synthesis methods for Cu3N include ammonolysis of molecular precursors,19,20 reaction of copper or copper oxide with ammonia at higher temperatures,21,22 and solvothermal copper azide precursor decomposition.23 Recently, there have been few reports about the synthesis of colloidal Cu3N nanoparticles. They are all based on the thermal decomposition of copper precursors in media of high boiling coordinating organic solvents.14,24−26 The use of long chain coordinating solvents allowed the shape-controlled synthesis of colloidal Cu 3N nanoparticles; however, these methods employed very high reaction temperatures (often ≥240 °C), and the synthesized nanoparticles were >10 nm in size. Moreover, the long chain surfactants bonded on the Cu3N nanoparticle surface can lower the accessibility of active sites on the nanoparticle surface, resulting in a less effective utilization

INTRODUCTION Nanoscale materials exhibit size- and shape-dependent physical and chemical properties that are difficult to achieve by their bulk counterparts. Consequently, to fully exploit the potential of nanoparticles, the synthesis of nanoparticles with sizes in the range of 1−3 nm is desirable. The unique properties of ultrasmall nanoparticles make them attractive for applications in energy conversion and storage, biomedical imaging, and catalysis.1−3 Recently, the development of viable synthesis routes toward ultrasmall nanoparticles of metals, metal oxides, and metal sulfides has received great attention from the perspective of their unique physiochemical properties.1,4 However, studies focusing on the synthesis of ultrasmall colloidal transition metal nitride nanoparticles and their applications are still scarce. Transition metal nitrides are fundamentally and technologically important materials with useful electronic, catalytic, and magnetic properties.5−7 Traditional synthesis routes to transition metal nitrides usually involve higher temperature nitridation of metals, metal oxides, or metal halides with N2 or NH3.8 Nanocrystalline metal nitrides can be obtained by thermal transformations of various metal oxide nanoparticles by using urea or cyanamide as nitrogen source.9 Recent progress in the synthesis of transition metal nitrides have been achieved by Giordano and co-workers using thermal decomposition of metal-urea complexes under N2 flow at elevated temperature.10,11 Copper nitride, Cu3N, is a semiconductor that has been proposed as a promising material for solar energy conversion,12 © 2015 American Chemical Society

Received: September 3, 2015 Revised: November 11, 2015 Published: November 16, 2015 8282

DOI: 10.1021/acs.chemmater.5b03444 Chem. Mater. 2015, 27, 8282−8288

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Chemistry of Materials

reaction mixture was cooled down to room temperature. The product was precipitated by adding 40 mL of hexane and collected by centrifugation at 4000 rpm for 15 min. The product was washed two more times using 40 mL of hexane and finally dried under nitrogen flux. The yield of the reaction was >90% with respect to Cu(OMe)2. To obtain stable colloidal dispersions of Cu3N nanoparticles, the wet products obtained after washing were directly dispersed in chloroform, THF, and NMP using ultrasonication for 10 min. The nanoparticle dispersion in chloroform was used for TEM sample preparation. Materials Characterization. X-ray powder diffraction (XRD) patterns were recorded on an X’Pert Pro (PANalytical B.V., Netherlands) powder diffractometer operating in reflection mode, equipped with a diffracted beam curved graphite crystal monochromator, with Cu Kα radiation (45 kV, 40 mA). Transmission electron microscopy (TEM), scanning TEM (STEM), and highresolution TEM (HRTEM) images were recorded on a FEI Talos F200X operated at 200 kV. TEM specimens were prepared by placing a drop of a particle suspension onto a conventional TEM support Cugrid covered either with just a lacey amorphous carbon foil or with lacey carbon foil in turn covered with a graphene sheet. The latter was essential for the visualization of the smallest particles by increasing the signal-to-noise ratio of the image. For STEM imaging, the electrode particles, after the first electrochemical cycle, were scratched off and deposited onto the Cu-grid. Attenuated total reflectance-infrared spectroscopy (ATR-IR) was measured on a Bruker Alpha FT-IR Spectrometer with a diamond ATR optics. Thermogravimetric analysis (TGA) was measured on a Mettler Toledo SDTA851e analyzer from 40 to 600 °C at a heating rate of 5 °C/min under nitrogen. GC−MS analyses were performed using a gas chomatograph (Thermo Scientific Trace 1300) coupled to an ISQ single quadrupole mass spectrometer. Samples for GC−MS analysis were prepared as follows: 5 mL of reaction solution was dispersed in 45 mL of hexane, and the precipitated nanoparticles were separated from the solution by centrifugation. The resulting solution was filtered through a polytetrafluoroethylene (PTFE) filter with pore size of 0.1 μm. Then, 0.5 μL of the filtered solution was injected into the chromatographic column by means of an AI-AS 1300 GC autosampler. The injection port was heated to 250 °C. The column temperature program was as follows: hold at 60 °C for 1 min, ramp to 115 °C at 5 °C/min, ramp to 250 °C at 20 °C/min, and hold for 4.5 min, and finally ramp to 300 °C at 30 °C/min and hold for 4 min. Helium was used as a carrier gas, and the column flow rate was 0.8 mL/min. Mass spectra were measured by means of the electron impact technique. Electrochemical Measurement. 20 wt % of carbon black (Super P, TIMCAL) and 10 wt % of binder (PVDF, Aldrich) were mixed under mechanical stirring in 0.7 mL of NMP, to which 0.7 mL of the as-prepared colloidally stable Cu3N (70 wt %) suspension in NMP was added and mixed. The resulting suspension was transferred onto Petri dish-like Ti current collectors. After drying at 80 °C overnight under vacuum, circular electrodes with diameters of 13 mm were assembled into Swagelok-type cells in an Ar-filled glovebox. Average loading of the active material was ∼3 mg/cm2. Lithium metal (99.9%, Alfa-Aesar) served as both reference and counter electrode. A glass fiber or Celgard 2400 separator was soaked with electrolyte (1 M LiPF6 in 1:1 wt % ethylene carbonate (EC)/dimethyl carbonate (DMC), Novolyte). All electrochemical measurements were performed using a Biologic instrument (VMP3) at room temperature. The electrodes were cycled between 0.01 and 3 V vs Li/Li+ for varying specific current rates. All the electrochemical results were based at least on two cell tests.

of the particle surface. Despite extensive efforts on the synthesis of Cu3N nanoparticles, it remains a great challenge to access colloidal ultrasmall Cu3N nanoparticles using a surfactant-free solution-phase route without the need for high temperature and high pressure. Due to the high demand for portable electrochemical power sources with higher energy and power density, the search of new electrode materials for improved capacity performance and rate capability of Li-ion batteries (LIBs) is ongoing.27,28 Nanostructured metal oxides exhibit good electrochemical properties and are regarded as promising anode material for high performance LIBs.29 In addition to metal oxides, metal nitrides have also received broad attention as an emerging anode for high-performance LIBs or Na-ion batteries.5,30,31 Binary metal nitrides like VN, CrN, Zn3N2, Fe3N, Co3N, and Cu3N electrochemically react with Li by conversion reaction, resulting in the formation of metal nanoparticles embedded in a Li3N matrix.5,32,33 Among various nitrides, copper nitride is of particular interest owing to its earth-abundance, inexpensiveness, and environmental friendliness. In 2003, Tarascon and coworkers showed a capacity of ∼300 mAh/g at a current rate of 224 mA/g for submicrometer Cu3N particles.32 Meanwhile, there have been no further reports on the lithium storage performance of Cu3N nanoparticles, to the best of our knowledge. Several studies have shown that the size of electrochemically active material has great influence on their electrochemical performance.34,35 Therefore, small-sized Cu3N nanoparticles with enhanced surface area would be an ideal model system to study the influence of nanosizing on lithium storage performance. Here we report a novel surfactant-free, solution-phase route to ultrasmall, crystalline Cu3N nanoparticles via a one-step reaction between copper(II) methoxide and benzylamine. Up to now, benzylamine has only been used for the preparation of metal oxide nanoparticles.36,37 The synthesis protocol is simple and offers the benefit of significantly lower synthesis temperature. The as-prepared nanoparticles without further surface modification can be readily dispersed in a polar organic solvent to form a stable colloidal dispersion. We propose a reaction mechanism for Cu3N formation based on the gas chromatography−mass spectrometry (GC−MS) analysis of the organic reaction byproducts. We investigated the as-prepared Cu3N nanoparticles as anode material for LIBs. The electrochemical results show that the pristine Cu3N nanoparticles undergo a phase transformation already during the first cycle, which is probably due to the extreme surface activity of Cu3N. After the phase change, the resulting electrode demonstrates stable rate performance and high capacity retention.

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EXPERIMENTAL SECTION

Chemicals. Copper(II) methoxide [Cu(OMe)2, 97%], chloroform (≥99.8%), tetrahydrofuran (THF, ≥99.9%), and hexane (≥95%) were purchased from Sigma-Aldrich. Benzylamine (≥99%) was purchased from Fluka-Chemie AG. N-Methyl-2-pyrrolidone (NMP, 99%) was purchased from Acros Organics. All chemicals were used as received without further purification. Synthesis of Cu3N Nanoparticles. In a glovebox under argon atmosphere (O2 < 0.1 ppm and H2O < 0.1 ppm), 50 mg of Cu(OMe)2 was added into a 10 mL glass vial. Afterward 5 mL benzylamine was added and the vessel was sealed with a Teflon cap and taken out of the glovebox. The reaction vessel was transferred into a preheated oil bath set at 140 °C and held at that temperature for 15 min under vigorous magnetic stirring. During heating, after about 3 min, the color of the reaction solution changed from dark blue to red. Afterward, the

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RESULTS AND DISCUSSION Synthesis and Characterization of Ultrasmall Cu3N Nanoparticles. Surfactant-free synthesis of Cu3N nanoparticles involved solution-phase reaction between Cu(OMe)2 and benzylamine. Surfactant-free here does not necessarily mean naked or ligand free nanoparticles; however, it indicates that the synthesis of the Cu3N nanoparticles was performed in the absence of any additional surfactants. Figure 1 inset shows 8283

DOI: 10.1021/acs.chemmater.5b03444 Chem. Mater. 2015, 27, 8282−8288

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Nanoparticles obtained by solution routes often contain organic residues bonded to the particle surface. To examine the ligands present on the Cu3N nanoparticles, we measured the Fourier transform infrared (FTIR) spectrum of as-synthesized Cu3N nanoparticles (Figure S2). The broad peak at 3280 cm−1 corresponds to the N−H stretching vibrations and the bands at 748 and 695 cm−1 correspond to the −NH2 wagging vibrations.37 The peaks observed at 1441, 1488, and 1600 cm−1 can be assigned to aromatic CC stretching vibration from benzene ring.38 The two peaks observed at 3057 and 3022 cm−1 correspond to the asymmetric and symmetric C−H stretching modes.37,38 The absorption peak occurring at 646 cm−1 is attributed to Cu−N stretching vibrations.39 These FTIR features indicate that benzylamine is indeed bound to the Cu3N nanoparticle surface. The role of benzylamine as surface modifier makes the addition of surfactant superfluous. The surface functionality of Cu3N nanoparticles is important for their redispersion in polar organic solvents. The as-synthesized nanoparticles were readily dispersible in THF and NMP. The colloidal nanoparticle dispersion in THF was stable for 1 day and in NMP for one month. We performed thermogravimetric analysis (TGA) on Cu3N nanoparticles to provide an estimate about the composition of the nanoparticles and to predict their thermal stability in inert atmosphere. Figure S3 shows the TGA results obtained by heating the Cu3N nanoparticles in nitrogen atmosphere from 40 to 600 °C. There are three main losses of mass; the first, a reduction of about 7% up to 150 °C can be attributed to adsorbed solvents and moisture onto the surface of the particles. The main decomposition step occurs between 150 to 270 °C, with a mass loss of 25%, corresponding to the decomposition of the organics that remained on the surface of the particles. Finally, a mass loss of 8% from 270 to 500 °C is the result of the thermal decomposition of Cu3N into 3Cu and 1/2N2. The mass loss is a little higher than the calculated value of 6.8% presumably due to the continued decomposition of organics on the nanoparticles surface. Influence of the Reaction Parameters on the Average Crystallite Size of Cu3N Nanoparticles. We performed XRD analysis to investigate the effect of reaction temperature and reaction time on the average crystallite size of Cu3N nanoparticles. Figure S4 shows the powder XRD patterns of the products synthesized at different temperatures ranging from 80

Figure 1. Experimental powder XRD pattern of the Cu3N nanoparticles with standard diffraction pattern (JCPDS No. 47−1088). The inset picture shows the dark red color of the as-prepared Cu3N nanoparticles in benzylamine.

the as-prepared Cu3N nanoparticle suspension in benzylamine. The powder XRD pattern of the product is shown in Figure 1. The diffraction pattern matches well with the standard diffraction data for Cu3N with an anti-ReO3 structure (JCPDS No. 47−1088). The average crystallite size calculated from the (100) peak using Scherrer equation is 2.3 nm. Figure 2a,b shows representative TEM images of assynthesized Cu3N nanoparticles. The TEM images clearly reveal distinct, nonagglomerated nanoparticles with sizes of around 2 nm. The high crystallinity of the nanoparticles is evident in the HRTEM image (Figure 2b, inset). The observed lattice spacing of 2.6 Å corresponds to the (110) plane of antiReO3 structured Cu3N. Nanoparticles commonly tend to agglomerate in the absence of a surfactant. The as-synthesized Cu3N nanoparticles do not show tendency to agglomerate, which clearly indicates the role of benzylamine as a surfactant that protects the nanoparticles against agglomeration. To get the size distribution of Cu3N nanoparticles, we evaluated TEM images obtained at different magnifications. The size distribution histogram shown in Figure S1 reveals an average size of 2.2 ± 0.5 nm, which is in good agreement with the crystallite size calculated by the Scherrer equation from the XRD pattern.

Figure 2. (a,b) Representative TEM images of Cu3N nanoparticles. The inset of (b) shows the HRTEM image of a single Cu3N nanoparticle. 8284

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Chemistry of Materials Scheme 1. Proposed Reaction Mechanism for the Main Processes Involved in the Formation of Cu3N Nanoparticles

to 180 °C (in steps of 20 °C) for 1 h. All diffraction patterns correspond to the anti-ReO3 structure of Cu3N (JCPDS No. 47−1088). The average crystallite sizes show an increasing trend from 1.8 to 2.8 nm with increase in reaction temperature from 80 to 180 °C (Table S1), which can be easily monitored by the narrowing of the (100) reflection. Furthermore, XRD analysis of the Cu3N nanoparticles obtained at 140 °C for different reaction times in the range of 5 to 60 min revealed that already after 5 min phase pure Cu3N is formed (Figure S5). Prolonging heat treatments just resulted in crystallite growth from 2.2 to 2.5 nm as shown in Table S2. It is clear from the XRD analysis that both reaction temperature and reaction time have little influence on the average crystallite size of Cu3N nanoparticles. In comparison to conventional synthesis methods, the present route gives access to Cu3N nanoparticles at much lower temperature and time.14,19,26 Proposed Reaction Mechanism. We propose a reaction mechanism for Cu3N formation (as depicted in Scheme 1) based on the GC−MS analysis of the organic compounds in the final reaction mixture (Table 1). Figure S6 shows the gas

Garnweitner et al. also reported the formation of Nbenzylidenebenzylamine during the solvothermal treatment of titanium isopropoxide in benzylamine.37 The N-benzylidenebenzylamine appears in considerable amount in the GC with retention time of 21.70 min. Furthermore, we observed trace quantities of benzonitrile in the GC with retention time of 12.32 min. The benzonitrile formation can be explained as a further molecule of benzenemethanimine oxidizes to benzonitrile by reaction with Cu(II) ions (eq 3, Scheme 1). However, the Cu(II) ion concentration should be large enough to prevent competitive addition of benzylamine to benzenemethanimine (eq 2). During the last step, the in situ produced ammonia immediately reacts with CuOMe to form Cu3N and methanol (eq 4, Scheme 1). The strong basicity of methoxide ions provides the driving force for the nitridation reaction. Electrochemical Performance of Cu3N Nanoparticles. Cu3N reversibly reacts with Li+ via conversion reaction: Cu3N + 3Li+ + 3e− → 3Cu + Li3N

which gives a theoretical capacity of 390 mAh/g.32 The conversion reaction requires a heterogeneous charge transfer at the interfaces, Cu−N bond cleavage and Li+ diffusion in solid state.40 The kinetics of such reaction is highly dependent on the pristine particle size, even if the reaction is thermodynamically feasible. Thus, smaller particles can favor the reaction. Indeed, a study on CuO proved that the smaller CuO particles (20−50 nm) undergo much more complete conversion reaction compared to large particles (670 nm).41 To explore the potential of our Cu3N nanoparticles as anode material for LIBs, we first measured cyclic voltammetry (CV) as shown in Figure 3. The Li insertion starts from a main cathodic peak at 0.58 V corresponding to the conversion reaction with Li. The location of the peak slightly differs from the peaks observed for submicrometer Cu3N particles by Tarascon et al.,32 where the insertion of Li upon the first reduction cycle proceeds mainly in two stages at 0.48 and 0.41 V, respectively. The particle size has significant effect on the working potential of the electrode. The observed potential equals the potential at equilibrium minus the polarization (ohmic and electrochemical). Smaller particle size can largely lower the polarization upon Li insertion; therefore, the observed voltage toward Li reaction for Cu3N in our study is slightly higher than the one for larger particles. Finally we can also observe an additional peak at around 0 V on the cyclic voltammogram. The origin of this peak is still not clear. This extra feature is always much more pronounced for small nanoparticles, which might be associated with (a) polymeric/gel-like film formation on the surface of nanoparticles, which is rooted at low potential,32 (b)

Table 1. Organic Fractions Observed in the GC−MS Spectrum of the Final Reaction Solution retention time (min)

assigned structure

4.65 12.32 13.50 21.70

methanol benzonitrile benzylamine N-benzylidenebenzylamine

chromatogram obtained from the final reaction liquid from the synthesis of Cu3N nanoparticles, and Figures S7−S10 show the corresponding mass spectra. Three reaction steps lead to Cu3N formation under given reaction conditions. In the first step, a benzylamine molecule reacts with two Cu(II) ions to form methanol, Cu(I) and benzenemethanimine (eq 1, Scheme 1), out of which methanol is observed in the GC with retention time of 4.65 min. During this step, the high Lewis basicity of benzylamine effectively solvates the Cu(II) ions and the high oxidation efficiency of Cu(II) ions is responsible for oxidation of benzylamine to benzenemethanimine. In the present case, Cu(OMe)2 offers a quite exceptional reactivity since Cu(II) salts usually provide stable complexes with amines. For example, we observed that Cu(II) chloride forms a blue solution with benzylamine and fails to produce Cu3N under the same reaction conditions. The in situ produced benzenemethanimine reacts further with benzylamine to form Nbenzylidenebenzylamine and ammonia (eq 2, Scheme 1). 8285

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the moisture sensitivity, high reactivity, and electrochemical instability of metal nitrides toward electrolyte under the local electrochemical environment may lead to the formation of intermediate copper oxynitride phases and evolve into copper oxides finally.32 Independent of the exact phase, whether it is copper nitride or copper oxynitride, Cu nanodomains embedded in a Ncontaining matrix form upon Li uptake. This matrix is expected to be electronically more conductive than Li2O (which would form in the case of metal oxides as active material), leading to better rate performance than copper oxide. We measured galvanostatic cycling with and without a potential limitation step for the as-prepared Cu3N nanoparticles. A potentiostatic step was added after galvanostatic cycling to help to complete the discharge/charge, particularly at high current rates.42 Figure 4a shows the results of galvanostatic cycling with a potential limitation. On the initial discharge at 0.1 A/g, Cu3N delivers a total capacity of 718 mAh/g. On charge, 437 mAh/g can be recovered, which gives the first Coulombic efficiency of 60.9%. The low efficiency mainly comes from solid electrolyte interphase (SEI) formation that is linearly associated with the total particle surface area. Considering that the pristine Cu3N has a particle size of around 1−3 nm, the low initial Coulombic efficiency is reasonable. Total capacity of the second cycle at 0.1 A/g is 427 and 416 mAh/g for discharge and charge, respectively, with enhanced Coulombic efficiency of 97.4%. At 1 A/g, the total capacity delivered is 413 mAh/g from which the galvanostatic part contributes 332 mAh/g on discharge, while on charge the total capacity is 410 mAh/g in which 342 mAh/g is given galvanostatically. Even at 10 A/g, 111 mAh/g is galvanostatically cycled on discharge and 187 mAh/g on charge, and under the help of the potentiostatic step the total capacity can be maintained around 425 mAh/g. To explore the longterm cyclability, the Cu3N electrode was also galvanostatically cycled at 1 A/g, but without the potentiostatic step (Figure 4b). It demonstrates excellent battery performance retention. It goes through a slight capacity fading from ∼270 mAh/g down to 267 mAh/g in the first 15 cycles. After that, the Cu3N electrode is activated, and the specific charge gradually increases back to 300 mAh/g in the following 80 cycles and can be stably

Figure 3. First three cyclic voltammetry scans of the as-obtained Cu3N anode at the scan rate of 0.05 mV/s.

intercalation of Li into carbon black, or (c) the microstructure of the electrode that possesses mesoporosity. 42 Upon delithiation, two weak and broad peaks appear at ∼1.2 and ∼2.4 V, respectively. The peak at 1.2 V is similar to that for the oxidation of Cu(0) to Cu(I) in the case of Cu3N electrode,32 and 2.5 V is more associated with oxidation of Cu(0) to Cu(I) in the case of Cu2O.43 During the second cycle, the reduction peaks shift to 1.86 and 0.77 V, respectively, and on the third cycle, the peak located at 1.86 V shifts to 1.79 V. Evidently, the intensity of the peaks at ∼1.8 V on the second and third cycles is much more pronounced compared to the one of the first cycle. These values agree well with the redox features of Cu2O, suggesting a phase transformation from Cu3N to Cu2O after the first cycle, which was confirmed by ex situ XRD analysis (Figure S11). A STEM image (Figure S12) of the Cu3N electrode after the first cycle shows that the ultrasmall nanoparticles are embedded uniformly in the matrix. The phase change from Cu3N to Cu2O after the first cycle is in sharp contrast to Tarascon’s work, where such phase transformation occurs after extended cycling (200 cycles) and at elevated temperature. Most probably, the phase transformation is accelerated by the ultrasmall size of the Cu3N nanoparticles. The copper nanoparticles generated during the Li/Cu3N conversion process are highly reactive. Additionally,

Figure 4. (a) Galvanostatic cycling with a potentiostatic limitation (GCPL) at different current rates for the pristine Cu3N electrode. Note: The potentiostatic steps were manually shifted to avoid overlap among different lines. (b) Capacity retention performance for the pristine Cu3N electrode under galvanostatic condition without a potentiostatic step. 8286

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assisting with the XRD measurements. We thank ETH Zurich and the Swiss National Science Foundation (projects 200020_144437, 20PC21_155658 and 200021_137637) for financial support.

maintained at this level for about 150 cycles in total with average Coulombic efficiency of 99.9%. The loss and gain in capacity upon initial dozens of cycles is often observed on nanoparticles,44−46 and it can be ascribed to the unstable SEI evolution. The repetitive volume expansion/contraction during cycling can initially fracture the SEI layer and expose new active surfaces for SEI growth. Therefore, chemical degradation associated with side reactions between the electrode surface and electrolyte and the formation of unstable SEI can degrade the battery performance. However, after a certain number of cycles, the electrode and the SEI self-reconstruct and stabilize, which may help to form more open structures that can effectively buffer the volume expansion and mitigate the stress impact. Finally a stable SEI forms without further fracture. In comparison to the results on Cu3N reported by Tarascon and co-workers, we obtained much better rate performance and capacity retention. However, one has to keep in mind that our Cu3N nanoparticles undergo a phase transformation during the electrochemical tests. Nevertheless, these results emphasize the critical relationship between the particle size and electrochemical behavior of transition metal nitrides.

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(1) Kim, B. H.; Hackett, M. J.; Park, J.; Hyeon, T. Synthesis, Characterization, and Application of Ultrasmall Nanoparticles. Chem. Mater. 2014, 26, 59−71. (2) Chen, Y.; Song, B. H.; Lu, L.; Xue, J. M. Ultra-small Fe3O4 nanoparticle decorated graphene nanosheets with superior cyclic performance and rate capability. Nanoscale 2013, 5, 6797−6803. (3) Zhu, Z. Q.; Wang, S. W.; Du, J.; Jin, Q.; Zhang, T. R.; Cheng, F. Y.; Chen, J. Ultrasmall Sn Nanoparticles Embedded in NitrogenDoped Porous Carbon As High-Performance Anode for Lithium-Ion Batteries. Nano Lett. 2014, 14, 153−157. (4) Kharissova, O. V.; Kharisov, B. I.; Jimenez-Perez, V. M.; Flores, B. M.; Mendez, U. O. Ultrasmall particles and nanocomposites: state of the art. RSC Adv. 2013, 3, 22648−22682. (5) Dong, S. M.; Chen, X.; Zhang, X. Y.; Cui, G. L. Nanostructured transition metal nitrides for energy storage and fuel cells. Coord. Chem. Rev. 2013, 257, 1946−1956. (6) Oyama, S. T. Preparation and Catalytic Properties of TransitionMetal Carbides and Nitrides. Catal. Today 1992, 15, 179−200. (7) Fang, C. M.; Koster, R. S.; Li, W. F.; van Huis, M. A. Predicted stability, structures, and magnetism of 3d transition metal nitrides: the M4N phases. RSC Adv. 2014, 4, 7885−7899. (8) Mazumder, B.; Hector, A. L. Synthesis and applications of nanocrystalline nitride materials. J. Mater. Chem. 2009, 19, 4673−4686. (9) Buha, J.; Djerdj, I.; Antonietti, M.; Niederberger, M. Thermal transformation of metal oxide nanoparticles into nanocrystalline metal nitrides using cyanamide and urea as nitrogen source. Chem. Mater. 2007, 19, 3499−3505. (10) Giordano, C.; Erpen, C.; Yao, W. T.; Milke, B.; Antonietti, M. Metal Nitride and Metal Carbide Nanoparticles by a Soft Urea Pathway. Chem. Mater. 2009, 21, 5136−5144. (11) Giordano, C.; Antonietti, M. Synthesis of crystalline metal nitride and metal carbide nanostructures by sol-gel chemistry. Nano Today 2011, 6, 366−380. (12) Zakutayev, A.; Caskey, C. M.; Fioretti, A. N.; Ginley, D. S.; Vidal, J.; Stevanovic, V.; Tea, E.; Lany, S. Defect Tolerant Semiconductors for Solar Energy Conversion. J. Phys. Chem. Lett. 2014, 5, 1117−1125. (13) Lee, B. S.; Yi, M.; Chu, S. Y.; Lee, J. Y.; Kwon, H. R.; Lee, K. R.; Kang, D.; Kim, W. S.; Bin Lim, H.; Lee, J.; Youn, H. J.; Chi, D. Y.; Hur, N. H. Copper nitride nanoparticles supported on a superparamagnetic mesoporous microsphere for toxic-free click chemistry. Chem. Commun. 2010, 46, 3935−3937. (14) Wu, H. B.; Chen, W. Copper Nitride Nanocubes: SizeControlled Synthesis and Application as Cathode Catalyst in Alkaline Fuel Cells. J. Am. Chem. Soc. 2011, 133, 15236−15239. (15) Zhu, W.; Zhang, X.; Fu, X. N.; Zhou, Y. N.; Luo, S. Y.; Wu, X. J. Resistive-switching behavior and mechanism in copper-nitride thin films prepared by DC magnetron sputtering. Phys. Status Solidi A 2012, 209, 1996−2001. (16) Maruyama, T.; Morishita, T. Copper nitride and tin nitride thin films for write-once optical recording media. Appl. Phys. Lett. 1996, 69, 890−891. (17) Kim, K. J.; Kim, J. H.; Kang, J. H. Structural and optical characterization of Cu3N films prepared by reactive RF magnetron sputtering. J. Cryst. Growth 2001, 222, 767−772. (18) Asano, M.; Umeda, K.; Tasaki, A. Cu3N Thin-Film for a New Light Recording Media. Jpn. J. Appl. Phys. 1 1990, 29, 1985−1986. (19) Nakamura, T.; Hayashi, H.; Hanaoka, T.; Ebina, T. Preparation of Copper Nitride (Cu3N) Nanoparticles in Long-Chain Alcohols at 130−200 °C and Nitridation Mechanism. Inorg. Chem. 2014, 53, 710− 715.

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CONCLUSIONS In conclusion, we developed a novel, low-temperature, surfactant-free, solution-phase route to highly crystalline, ultrasmall Cu3N nanoparticles. The as-prepared Cu3N nanoparticles form stable colloidal dispersions in polar organic solvents without any surface modification. In addition to the novel synthesis, mechanistic investigation provides understanding about the role of the different reactants during the reaction. Inspired by the ultrasmall size with reduced Li ion diffusion lengths and the better electronic conductivity offered by metallic Cu and Li3 N, we investigated the Cu 3N nanoparticles for lithium ion storage. Owing to the high reactivity associated with ultrasmall particles, phase transformation accelerates from Cu3N to Cu2O. Despite the phase change, the resulting electrode display enhanced rate performance and capacity retention. It stably delivers a capacity of ∼290 mAh/g at 1 A/g with average Coulombic efficiency of 99.9%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03444. Size distribution histogram and FTIR spectra of Cu3N nanoparticles, temperature- and time-dependent XRD analysis, gas chromatogram and mass spectra of organic reaction byproducts, ex situ XRD patterns of Cu3N cycled in LIBs, and STEM image of Cu3N electrode after the first cycle (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge ScopeM for providing the TEM facilities and in particular Dr. Alla Sologubenko for her help with TEM and STEM studies. We are also thankful to Dr. Niklaus Kränzlin for 8287

DOI: 10.1021/acs.chemmater.5b03444 Chem. Mater. 2015, 27, 8282−8288

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DOI: 10.1021/acs.chemmater.5b03444 Chem. Mater. 2015, 27, 8282−8288