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A Hierarchical Phosphorus-Nanobarbed Nanowire Hybrid#Its Structure and Electrochemical Properties Dan Zhao, Beibei Li, Jinying Zhang, Xin Li, Dingbin Xiao, Chengcheng Fu, Lihui Zhang, Zhihui Li, Jun Li, Daxian Cao, and Chunming Niu Nano Lett., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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A Hierarchical Phosphorus-Nanobarbed Nanowire Hybrid:Its Structure and Electrochemical Properties Dan Zhao, †,‡ Beibei Li, † Jinying Zhang,*,† Xin Li, † Dingbin Xiao, † Chengcheng Fu, † Lihui Zhang, † Zhihui Li, † Jun Li, † Daxian Cao, † Chunming Niu*,† †

Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation

and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710054, Shaanxi, China. ‡

Department of Mechanical and Aerospace Engineering, Hong Kong University of Science &

Technology, Clear Water Bay, Kowloon, Hong Kong KEYWORDS: hierarchical phosphorus hybrid, red phosphorus, barb shaped graphene nanosheets, in situ, Li-ion battery

ABSTRACT. Nanostructured phosphorus-carbon composites are promising materials for Li-ion and Na-ion battery anodes. A hierarchical phosphorus hybrid, SiC@graphene@P, has been synthesized by chemical vapor deposition of phosphorous on the surfaces of barbed nanowires, where the barbs are vertically grown graphene nanosheets and the cores are SiC nanowires. A temperature gradient vaporization-condensation method has been used to remove the

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unhybridized phosphorous particles formed by homogeneous nucleation. The vertically grown barb shaped graphene nanosheets and high concentration of edge carbon atoms induced a fibrous red phosphorus (f-RP) growth with its {001} planes in parallel to {002} planes of nanographene sheets and led to a strong interpenetrated interface interaction between phosphorous and the surfaces of graphene nanosheets. This hybridization has been demonstrated to significantly enhance the electrochemical performances of phosphorus.

Though it is produced in large scale and used extensively in various industrial applications, phosphorus has been largely out of the attention screen of physicists and chemists for a long time. Recent preparation of phosphorene by exfoliating black phosphorus (BP)1 and subsequent discovery of its excellent optical and electronic properties have attracted great attention, and renewed interest to phosphorus.2-10 In addition to BP, red phosphorus, another allotrope of phosphorus,11 has also been found to possess fascinating semiconducting,12 photocatalysis13-15 and energy storage16,

17

properties. Phosphorus stores Li or Na according to the reaction of

3Li/Na + P → Li3P/Na3P with a high theoretical capacity of 2596 mAh/g.18 Unfortunately, the electrochemical performance is hindered by the low electronic conductivity (∼10−14 Scm-1) of red phosphorus19, 20 and structural pulverization induced by large volume change (up to 300% or 400%) in charging and discharging cycles. To alleviate these problems, carbon based skeletons2126

have been incorporated to simultaneously buffer volume change and provide a conducting path

for electron transport because of their large surface area, high electrical conductivity and chemical stability.27-30 Various composites from liquid phase,10, 31-33 mechanical20, 34-41 or vapor16, 17, 19, 42-44

mixing of red phosphorus, amorphous phosphorus (a-P), or BP with carbon

nanomaterials have been demonstrated as anode materials for lithium ion batteries (LIBs) and sodium ion batteries (SIBs).10, 16, 17 Mechanical mixing is a simple way for the preparation of

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carbon-phosphorous composites. However, it is almost impossible to totally eliminate separated phosphorous phase by mechanical mixing. The aggregation of phosphorus during sublimationcondensation is also a problem for vapor phase sublimation method. The electrochemical performances are dramatically restricted by the weak connections between phosphorus and carbon skeleton, resulted from phosphorus aggregation.

Scheme 1. Schematic illustration of the synthesis of SiC@graphene@P. Here, we describe the synthesis and characterizations of a hierarchical phosphorus-nanobarbed nanowire (NW) hybrid, SiC@graphene@P, where SiC NW core is wrapped by vertically grown barbed graphene (VG) and further by phosphorus (Scheme 1). The SiC NWs were adopted as skeleton due to their excellent chemical and thermal stabilities,45, 46 which were prepared by direct conversion from multi-walled carbon nanotubes (MWCNTs). To make SiC conductive and enhance the interaction with phosphorous, the barbed graphene nanosheets were deposited onto the surfaces of SiC NWs (SiC@graphene). The phosphorus was then in situ grown on the surfaces of barbed graphene nanosheets to form SiC@graphene@P by a vaporizationcondensation method. The SiC@graphene@P hybrid products fused with extra homogeneous nucleated phosphorus when cooled in homogenous temperature chamber (THC) are denoted as THC-SiC@graphene@P (Figure S1a, right). A temperature gradient cooling method (TGC) was adopted to remove extra homogeneous nucleated phosphorus to yield TGC-SiC@graphene@P (Figure S1a, left). Both SiC@graphene@P products have been characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area diffraction (SAED), X-ray diffraction (XRD), Raman scattering, elemental

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mapping, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Brunauer−Emmett−Teller (BET) analysis. Both amorphous phosphorus (a-P) and triclinic crystalline red phosphorus, type IV or fibrous red phosphorus (f-RP)47, were found to be in situ grown on the VG nanosheets of SiC@graphene hybrids. The VG nanosheets with high surfaceto-volume ratio and large concentration of highly active edge carbon atoms (instead of common inert basal carbon surface)48,49 provide effective accommodation for a-P and f-RP during Li+ insertion/extraction. The removal of unhybridized phosphorus from SiC@graphene@P hybrids by temperature gradient vaporization-condensation method has significantly improved the electrochemical performances of phosphorus.

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Figure 1. SEM and TEM images of (a), (d) SiC@graphene; (b), (e) THC-SiC@graphene@P; and (c), (f) TGC-SiC@graphene@P (inset: SAED pattern); Enlarged HRTEM images of (g) SiC@graphene and (h) TGC-SiC@graphene@P. The SiC NWs with diameters of 20-60 nm were prepared from MWCNTs by a vapor phase reaction using silicon monoxide as silicon source, where the MWCNTs with uniform diameters of 10 ± 5 nm were prepared by a chemical vapor deposition (CVD) method (Figure S2).50 All

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XRD peaks (marked by ∗), except one weak peak (marked by ♠), of the SiC NWs (Figure S3a) can be indexed as 3C-SiC (JCPDS card no. 29-1129). The weak peak with 2θ at 34° might be from 6H-SiC (JCPDS card no. 75-1541). The HRTEM image with clear lattice fringe of 2.5 Å, corresponding to {111} lattice planes of 3C-SiC, confirmed the high crystallinity of SiC NWs (Figure S3b, inset). The Raman features of SiC NWs have not been detected due to the strong





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Figure 2. (a) XRD patterns and (b) Raman spectra of SiC@graphene (blue), commercial red phosphorus (green), TGC-SiC@graphene@P (black), and THC-SiC@graphene@P (red), the black vertical lines are simulated XRD pattern based on the crystallographic data CSD-391323 (● from graphene, ♠ from 6H-SiC; * from 3C-SiC; ◊ from medium-range ordered red

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phosphorus structure; ♦ from triclinic f-RP); (c) Experimental and fitted XPS spectra for phosphorus 2p from the TGC-SiC@graphene@P and red phosphorus, the 2p1/2 and 2p3/2 doublet is split by 0.85 eV with an integrated intensity ratio of 1:2; (d) TGA analysis of SiC@graphene in air (blue), THC-SiC@graphene@P (red) and TGC-SiC@graphene@P (black) in N2. To encapsulate SiC NWs, a few layer graphene nanosheets were deposited on their surfaces by a CVD process at 1200℃ under atmospheric pressure using methane as carbon source. The structure of SiC@graphene hybrids was observed by SEM (Figure 1a) and TEM (Figure 1d&g). The SiC NWs are fully covered by graphene nanosheets coating which is comprised of two distinctive regions, one is conformed coating with a-b plane of graphene nanosheets in parallel to the axis of SiC NWs and the other is VG with a-b plane in perpendicular to the axis of SiC NWs (Figure 1g). The barbed graphene nanosheets and high concentration of edge carbon atoms provide strong interpenetrated interfaces for phosphorous (Figure 1d) integration. In addition to SiC, two additional peaks with 2θ at 26° and 43° (marked by ● Figure 2a, blue) associating with {002} and {101} reflections of graphene were detected in the XRD pattern of SiC@graphene, further confirmed the formation of highly crystalline graphene coating. The strong photoluminescence of SiC was quenched by the graphene wrapping (Figure 2b, blue) in SiC@graphene hybrids, which further confirmed that the SiC NWs were well covered by graphene. About 63.22 wt.% graphene was detected in the SiC@graphene hybrids by TGA (Figure 2d). Phosphorus was then in situ deposited on the surfaces of barbed graphene nanosheets of SiC@graphene hybrids by a vapor transportation and condensation process19, 52 to obtain SiC@graphene@P hybrids. Phosphorus clusters were generated by decomposition of red phosphorus at 500℃and condensed onto barbed graphene nanosheets of SiC@graphene. Both

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homogeneous and heterogeneous nucleation of phosphorus have been found to take place during deposition processes. The heterogeneous nucleation led to the formation of uniform dense phosphorus coating around SiC@graphene to yield SiC@graphene@P hybrids. And the homogeneous nucleation led to the formation of unhybridized phosphorus particles which fused SiC@graphene@P hybrids together into a solid mass of THC-SiC@graphene@P product as observed by SEM and HRTEM (Figure S1a right side, Figure 1b, e). The unhybridized homogeneous nucleated phosphorus in THC-SiC@graphene@P was removed by a temperature gradient cooling process to yield TGC-SiC@graphene@P hybrid powder (Figure S1a, left side). Typically, a sealed sample in glass tube was cooled with one end about 50~100℃ lower than the other end where the SiC@graphene sample was set. The homogeneous nucleated phosphorus was condensed to the cooler end of the tube, while heterogeneous nucleated phosphorus on the surface of SiC@graphene was kept in the hotter end of the tube to give a loose TGCSiC@graphene@P powder sample as shown in the left side of Figure S1a. Individual SiC@graphene@P hybrid was easily observed by SEM and HRTEM from TGCSiC@graphene@P products (Figure 1c, f, h). After washing with carbon disulfide, 49.17 wt.% and 30.16 wt.% phosphorous were detected by TGA to be contained in THC-SiC@graphene@P and TGC-SiC@graphene@P (Figure 2d), respectively. A high percentage of heterogeneous nucleation which led to the formation of uniform phosphorus coating was obtained, which can be attributed to the strong interaction between phosphorus and nanographene on the surface of SiC NWs. The structural change of the samples from SiC@graphene to TGC-SiC@graphene@P was clearly reflected by surface area evolution of the samples, specific surface areas of 110.320, 8.533 and 17.204 m2/g for SiC@graphene, THC-SiC@graphene@P and TGCSiC@graphene@P, respectively.

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The phosphorus was well grown on the barbed graphene nanosheets with an average thickness around 30 nm. Barbed nature of nanographene resulted in an interpenetrated interface between phosphorus and nanographene (Figure 1h). Both a-P and crystalline f-RP structures were observed from SiC@graphene@P hybrids. Lattice fringe distance of 5.8 Å (Figure 1h) corresponding to {001} lattice planes of triclinic f-RP was observed from polycrystalline SAED pattern of SiC@graphene@P hybrids (Figure 1f, inset). The result confirmed the XRD analysis that an intensive {001} reflection with 2θ at 15.23° was obtained in the XRD patterns of THCand TGC-SiC@graphene@P products as shown in Figure 2a. The {001} lattice planes with lattice fringe distance of 5.8 Å from f-RP are parallel to {002} lattice planes with lattice fringe of 3.4 Å from nanographene sheets (inset of Figure 1h and Figure S4), indicating a selective growth of f-RP to graphene sheets. Strong interaction between phosphorus and nanographene was confirmed by the XPS analysis of phosphorus 2p spectra as shown in Figure 2c, where the weak 2p1/2 and 2p3/2 peaks at 131.1 eV and 130.2 eV are associated with P-C bond.53-55 Raman features of phosphorus (broad band from 300 cm−1 to 600 cm−1) in addition to graphene were observed from SiC@graphene@P products (Figure 2b). Relative intensity of Raman features decreased from THC-SiC@graphene@P to TGC-SiC@graphene@P samples, which is in consistent with the removal of homogenous nucleated phosphorus from THC-SiC@graphene@P hybrids by the temperature gradient cooling method. The XRD diffraction pattern (Figure 2a, red), substraction of peaks from SiC and graphene, of THC-SiC@graphene@P is well consistent with the simulated pattern of f-RP (black vertical lines at the bottom of Figure 2b, based on crystallographic data CSD-39132347, 56). The XRD peaks with 2θ at 15.2°, 15.9°, 17.2°, 22.9°, 26.4°, 27.8°, 28.6°, 30.0°, 30.8°, 32.1°, 33.3°, 34.0°, 34.2°, 39.6°, 52.3°, 53.2°, 56.7° and 65.6° (marked by ♦) can be indexed as {001}, {200} {201}, {301} {201}, {041}, {202}, {241},

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{002}, {400}, {301}, {340}, {242}, {441}, {543}, {641}, {182} and {803} reflections of triclinic f-RP, respectively. The detailed information of peaks associated with triclinic f-RP is shown in Table S1 and the corresponding structural model is shown in Figure S5. It is very interesting that only two additional broad peaks centered at 15.2° and 30.8° except from SiC@graphene were detected from TGC-SiC@graphene@P sample (Figure 2a, black), which is distinguishable from the medium-range ordered red phosphorus precursor57 (marked by ◊ in Figure 2a, green). This is in consistent with HRTEM observation that a preferred orientation growth of phosphorus with its {001} planes in parallel to {002} planes of nanographene sheets, a result of strong interfacial interaction between highly crystallized nanographene and phosphorous. Two types of f-RP growth, unhybridized homogeneous nucleated phosphorus with random orientation and hybridized heterogeneous nucleated phosphorus with preferred orientation were clearly evidenced by the difference between XRD patterns of THCSiC@graphene@P and TGC-SiC@graphene@P.

Figure 3. (a) Representative HAADF-STEM image and line profiles and the corresponding elemental mappings of P (b), Si (c), and C (d) of a SiC@graphene@P hybrid. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used (Figure 3) to further confirm the structure of SiC@graphene@P hybrids. Elemental distribution profiles of P (purple), C (green) and Si (blue) of line scan across the diameter of a

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hybrid NW is superimposed on its dark field image (Figure 3a). The profiles confirmed the hierarchical structure of SiC@graphene@P hybrids, SiC NW cores wrapped by graphene nanosheets and the graphene surfaces further covered by phosphorous. The elemental mapping of P, Si and C (Figure 3b-d) shows that their distribution is in consistent with TEM observation (Figure 1c, f, h), the phosphorous is intimately bonded to the surfaces of graphene nanosheets (Figure 1h). 3.0

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The electrochemical performances of SiC@graphene@P hybrids as anodes for LIBs have been investigated using a half-cell configuration. The charging-discharging profiles at a current density of 0.2 A/g and cyclic voltammograms at a scan rate of 0.2 mV/s of TGCSiC@graphene@P are shown in Figure 4a & 4b. The cyclic voltammograms (Figure 4b) agree well with the correlative potential plateaus of the discharge/charge voltage profiles of TGCSiC@graphene@P anodes (Figure 4a). The three-step lithium intercalation process of black phosphorus has been illustrated by using ex situ XRD.8 However, the lithiation profiles of red phosphorus, especially at first cycle, vary significantly from different reported data. Three peaks at ~ 1.0, 0.5 and 0.35 V were detected from the first lithiation process of TGCSiC@graphene@P anodes (Figure 4b). The first delithiation process showed a strong peak at 1.41 V surrounded by two small shoulder peaks near baseline at 1.08 and 1.75 V. The lithiation profile is significantly different from those reported works16, 20, 34 due to the structural nature of red phosphorus which is an intermediate phase between the white and violet phosphorus and most of its properties vary by a wide margin. The lithiation profile was dramatically changed after first cycle, reflecting the destruction of the crystallinity of red phosphorus and formation of solid electrolyte interface (SEI). Most of the lithiation capacity is represented by two overlapped peaks at 0.18 and 0.28 V and two other peaks, one small at 0.6 V and the other weak at 1.28 V. The peak positions are typical for most red phosphorus composites.20, 58, 59 The relative intensity of the peak at 0.6 V is significantly lower due to the hybridization effect in the structure of TGCSiC@graphene@P. This shift of storage capacity to low voltage improves storage quality. The four lithiation peaks can be attributed to the gradual Li+ insertion activation process of phosphorus to form various lithium phosphate phases (LixP).58,

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delithiation profile were unchanged from the first cycle, remained at 1.08, 1.41 and 1.75 V, which represent stepwise delithiation process from LixP to elemental phosphorus.58-61 The electrochemical performances (capacities and stability) of TGC-SiC@graphene@P anodes are much better than those of THC-SiC@graphene@P anodes (Figure 4c&d), especially when based on phosphorus (Figure S6a&b), due to the hybridization effect of phosphorus with barbed graphene nanosheets. An initial discharge specific capacity of 2724 mAh/g (Figure 4d) from TGC-SiC@graphene@P, higher than that of THC-SiC@graphene@P (1461 mAh/g) anodes, was obtained at a current rate of 0.2 A/g, which might be associated with the higher specific surface area of TGC-SiC@graphene@P (17.204 m2/g) than that of THC-SiC@graphene@P (8.533 m2/g). The 10th-cycle discharge capacities of 734, 572, 453, and 346 mAh/g (Figure 4c, 1886, 1406, 1042, and 714 mAh/g based on phosphorus weight, Figure S6a) (Coulombic efficiency > 98 %) were obtained from TGC-SiC@graphene@P anodes at current rates of 0.1, 0.2, 0.5, and 1 A/g, respectively. Both anodes have been found to have fast degradation at the beginning. The degradation was slow down after 5-6 cycles for TGC-SiC@graphene@P anodes reaching a discharge specific capacity of 1009 mAh/g (Figure 4d, 3079 mAh/g based on phosphorus weight, Figure S6b). However, the degradation for THC-SiC@graphene@P anodes continued, quickly dropping to a capacity of 471 mAh/g (821 mAh/g based on phosphorus weight, Figure S6b). A specific capacity of 553 mAh/g (Figure 4d, 1362 mAh/g based on phosphorus weight, Figure S6b) with Coulombic efficiency of 98.6 % was still maintained for TGCSiC@graphene@P anodes after 100 cycles, which is better than reported porous carbon/red phosphorus composite,42 activated carbon/phosphorus composite,60 ball-milled red phosphorus initiated BP/graphite composite,18 as well as CNT scaffold/amorphous red phosphorus composite59 and comparable to red phosphorus /graphite/activated carbon composite61 as well as

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exfoliated BP/graphene sheets composite33 even with SiC cores without capacity contribution. The specific capacities of reported phosphorus carbon composites are shown in Table S2. The specific capacity of THC-SiC@graphene@P was degraded to 245 mAh/g (Figure 4d, 244 mAh/g based on phosphorus weight, Figure S6b) with Coulombic efficiency of 98.78 % after 100 cycles. The hybridization of phosphorus with graphene in SiC@graphene@P has been demonstrated to improve the electrochemical performances of phosphorus. Phosphorus structures attached to SiC@graphene were still observed after 160 cycles (Figure S7). In summary, a hierarchical hybrid of SiC@graphene@P has been synthesized and characterized by HRTEM, SEM, SAED, XRD, BET, XPS and Raman scattering. Both f-RP and a-P were in situ grown on the surfaces of barbed graphene nanosheets surround SiC NWs which were produced from MWCNT templates. The barbed graphene nanosheets with a high concentration of edge carbon atoms to interact with phosphorus induced a f-RP phosphorus growth with its {001} planes in parallel to {002} planes of nanographene sheets and led to formation of a strong interpenetrated interface between nanographene and phosphorous, which has been demonstrated by HRTEM and XRD measurements. Two types of phosphorus nucleation, homogeneous and heterogeneous ones, have been observed. The homogeneous nucleation led to the formation of unhybridized phosphorus particles. A temperature gradient vaporization-condensation method has been used to remove the unhybridized phosphorus particles

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TGC-SiC@graphene@P anodes, which is more than twice as that of THC-SiC@graphene@P anodes. ASSOCIATED CONTENT Supporting Information. Experimental details; HRTEM image of pristine MWCNTs; HRTEM image, Experimental Raman spectra and XRD pattern of SiC NWs; HRTEM image, TGA and electrochemical performances of SiC@graphene; Optical image of THC- and TGCSiC@graphene@P; XRD reflection parameters and schematic of crystal structure of triclinic fibrous phosphorus; BET measurements data, extra electrochemical performances of the assynthesized nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Jinying

Zhang,

Email:

[email protected],

Tel:

+86-29-83395372.

*Chunming Niu, Email: [email protected], Tel: +86-29-83395372. Author Contributions ┴

Dan Zhao†,‡ and Beibei Li† contributed equally.

Funding Sources This work has been supported by National Natural Science Foundation of China (51302210, 51521065), Natural Science Fundation of Shaanxi Province (2015JQ5148). ACKNOWLEDGMENT

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The TEM work was done at International Center for Dielectric Research (ICDR). The authors also thank Mr. C. Ma and Dr. L. Lu for their help in using TEM. This work has been supported by National Natural Science Foundation of China (51302210, 51521065), Natural Science Fundation of Shaanxi Province (2015JQ5148), and the Fundamental Research Funds for the Central Universities. J. Zhang is supported by the Cyrus Tang Foundation through Tang Scholar Program. ABBREVIATIONS BP, black phosphorus; a-P, amorphous phosphorus; LIBs, lithium ion batteries; SIBs, sodium ion batteries; SiC, silicon carbide; NWs, nanowires; MWCNTs, multi-walled carbon nanotubes; VG, vertically grown barbed shaped graphene; SEM, scanning electron microscopy; HRTEM, highresolution transmission electron microscopy; SAED, selected area diffraction; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; BET, Brunauer-Emmett-Teller; TGA, thermogravimetric analysis; f-RP, fibrous red phosphorus; CVD, chemical vapor deposition; HAADF-STEM, high angle annular dark-field scanning transmission electron microscopy. REFERENCES 1.

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