Graphene-Oxide-Assisted Synthesis of GaN Nanosheets as a New

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Graphene Oxide Assisted Synthesis of GaN Nanosheets as a New Anode Material for Lithium-ion Battery Changlong Sun, Mingzhi Yang, Tailin Wang, Yongliang Shao, Yongzhong Wu, and Xiaopeng Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07277 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Applied Materials & Interfaces

Graphene Oxide Assisted Synthesis of GaN Nanosheets as a New Anode Material for Lithium-ion Battery Changlong Sun,† Mingzhi Yang,† Tailin Wang, Yongliang Shao, Yongzhong Wu and Xiaopeng Hao*

State Key Lab of Crystal Materials, Shandong University, Jinan 250100, Shandong, P. R. China

*E-mail: [email protected]

[†] These authors contributed equally to this work.

KEYWORDS: graphene oxide; sacrificial template; GaN; nanosheet; anode; lithium-ion battery

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Abstract: As the most studied III-nitride, theoretical researches have predicted the presence of gallium nitride (GaN) nanosheets (NSs). Herein, a facile synthesis approach is reported to prepare GaN NSs using graphene oxide (GO) as sacrificial template. As a new anode material of Li-ion battery (LIBs), GaN NSs anodes deliver the reversible discharge capacity above 600 mA h g−1 at 1.0 A g−1 after 1000 cycles, and excellent rate performance at current rates from 0.1 to 10 A g−1. These results not only extend the family of 2D materials but also facilitate their use in energy storage and other applications.

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Li-ion batteries (LIBs) have received widespread attention due to their high energy density and rechargeability.1 In contrast to the intercalation reaction for graphite, metal nitrides (MN) are attractive materials as prospective anodes.2,3 As typical MN, III-nitrides possess higher carrier mobility and electrical/ionic conductivity. Currently, as potential candidates for energy storage devices and photocatalytic properties nanostructure, III-nitride semiconductors have attracted considerable attention.4-9 Moreover, the inherent chemical stability of III-nitrides also favors their use in the harsh electrochemical reaction environment. During lithium ion insertion/desertion, high electrical conductivity and low volume change are required for the electrode materials, and metal nitrides can meet these requirements.10 However, the morphology and surface chemistry of metal nitrides often play a crucial role in determining their electrochemical properties. Conventional lithium storage is typically employed in the form of particles. This leads to inferior cyclability and poor rate capability which is caused by insufficient electrode–electrolyte contact and surface-interface interactions.11 Hence, the rational construction for a uniform nanostructure to improve the electronic and ionic conductivity is highly required. As a novel class of soft matter, two-dimensional (2D) nanosheet-like crystals have attracted much attention. Theoretical researches have shown few atomic layers wurtzite gallium nitride (GaN) can be achieved by reconstructing into a 2D graphitic structure with a thickness-dependent energy bandgap (Eg).12,13 Recently flower-like GaN nanosheets were synthesized through the nitridation of hydrothermal metastable γ-Ga2O3 nanosheets.14 Atom-thick GaN NSs grown via migration enhanced encapsulated method exhibits covalent bonding to substrate, which greatly restricts its further development.15 Therefore, the synthesis of GaN NSs is still a challenge. Motivated by the guiding and collecting role of graphene oxide (GO),16 we develop a facile 3



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approach to synthesize GaN NSs via a wet chemical method. As a novel anode material of LIBs for the first time, GaN NSs anode reveals fine electrochemical performance. Our results not only open up new possibilities for the preparation of 2D metal nitride nanostructures but also facilitate their use in energy storage and other applications.

Figure 1. Schematic illustration for the synthesis and structure of GaN NSs. Because of the highly hydrophilic nature, and oxygen-containing functional groups with strong affinity to the inorganic ions, GO sheets can be used for superior 2D template to guide and stabilize the preparation of the 2D structure.17 As shown schematically in Figure 1, ammonia (NH3·H2O) is used as the OH− supplier; the released OH− react with Ga3+ (Figure S1). GaOOH (Figure S2) is anchored onto the GO sheets via functional groups by a homogeneous precipitation method. GaN NSs are achieved by calcining in NH3 (Figure S3) and air atmosphere sequentially. Thermogravimetric analysis (TGA) also reveals that GO disappeared during thermal annealing (Figure S4). A detailed description is provided in the supporting information (S1.1). Due to the confinement effect of GO sheets, the growth of GaN was restricted. To reveal the important role of GO in the preparation process, a comparison experiment was designed. Without the guidance of GO sheets, GaOOH formed precipitate directly (Figure S5). Thus, GO sheets must act as the 4


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heterogeneous nuclei of the hydroxide of Ga.18

Figure 2. (a) SEM image of the as-prepared GaN NSs. (b) XRD pattern of GaN NSs and powder GaN. (c) HRTEM image of GaN NS. The selected area electron diffraction (SAED) pattern in the inset shows hexagonal GaN structure. (d) AFM image and height profile of the GaN NSs. Scanning electron microscopy (SEM) image (Figure 2a) and corresponding EDS microanalysis (Figure S6) show that the uniform hexagon GaN layers are remained after GO removed, with lateral sizes ranging from 1 to 5 µm. The uniform morphology of the as-obtained GaN NSs demonstrates the general and effective nature of the proposed synthesis method. The X-ray diffraction (XRD) pattern of the GaN NSs shown in Figure 2b confirms the formation of hexagonal wurtzite GaN. Compared with powder GaN, the (100), (002), and (101) peaks of the as-obtained GaN NSs are puny and broad, indicating the ultrathin nature of these NSs. The HRTEM image (Figure 2c) reveals the lattice fringes with interplanar spacing of 2.76 Å, corresponding to the (100) lattice planes of 5



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hexagonal GaN. The selected area electron diffraction (SAED) pattern further indicates the highly ordered hexagonal structure. The atomic force microscopy (AFM) image (Figure 2d) reveals that the height of the GaN NS is approximately 14.5 nm in thickness, ~ 3 µm in size. The Raman spectra change of the relative strength of A1 (LO) and defect related 659 cm‒1 mode indicates that the microstructure changes in GaN NSs (Figure S7). The nitrogen adsorption/desorption measurements reveal that the surface area of GaN NSs (124 m2 g−1) is higher than powder GaN (67 m2 g−1), as shown in Figure S8 and S9.

Figure 3. High resolution XPS spectra of GaN NSs: (a) Ga 3d XPS spectra. (b) N 1s XPS spectra. (c) Ga K-edge EXAFS k3c(k) oscillation functions k3c(k) of GaN NSs and powder GaN, respectively. (d) Corresponding Fourier transforms (FT) curves of Ga K-edge EXAFS spectra. The high resolution spectrum of Ga 3d is deconvoluted with Ga-N (26.0 eV), Ga-O (28.1 eV) bonds (Figure 3a and b).19 The content of Ga-O is 1.5 % (atomic percent), which was formed in the nitridation process. The N 1s spectrum of the GaN NSs can be well fitted by the N-Ga bond at 397.4 6


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eV.5 The synchrotron radiation X-ray absorption fine structure spectroscopy (XAFS) measurements at the Ga K-edge were carried out, using the powder GaN for reference sample (Figure S10). As shown in Figure 3c, the Ga K-edges k3c(k) oscillation curves for GaN NSs have a spectral shape similar to that of powder GaN, and this is accounted for the preservation of wurtzite GaN structure. A significant reduction of the amplitude is seen around 1 – 15 Å−1 for GaN NSs. This is associated with the distinct local atomic arrangement. As shown in Figure 3d, the peak near 1.6 Å corresponds to the first shell scattering and the peak near 2.9 Å corresponds to the combined second and third shell contributions.20 The coordination peaks of higher nearest neighbors (NN) shells can be seen in the R-space region of 3.8 – 6 Å. Intensities of the Ga−N pairs of the two samples are almost identical. But the intensity of the Ga−Ga peak of GaN NSs shows a significant decrease. The decreased peak intensity in GaN NSs is due to the surface structural disorder and Ga coordination missing at the Ga site.21 The weak peaks for Ga–O at ~2.08 Å and GaO6 at ~1.92 Å confirm the existence of infinitesimal Ga–O in GaN NSs.22 As shown in the X-ray absorption near-edge spectra (XANES) (Figure S11), the maximum Ga K-edge spectral peak of GaN NSs is 1.04 eV lower than that of powder GaN. The Ga−Ga coordination number decreased from 11.5 for the bulk to 10.7 for nanosheet (Figure S12), suggesting that the nature of the nanosheet resulted in the presence of a large amount of dangling bonds. XAFS results disclose that there are significantly higher levels of microstructure change in the GaN NSs.20 This suggests that the reduced size resulted in noticeable distortion on the surface of GaN NSs.

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Figure 4. Electrochemical performance of GaN NSs. (a) Cyclic voltammetry in the first four cycles at 0.1 mV s−1. (b) Discharge/charge profiles plotted for the 1st, 2nd, 10th, and 100th at 0.1 A g−1. (c) Cycle performance at 0.1 A g−1 for 100 cycles. (d) Rate performance with increasing charge rate from 0.1 to 10 A g−1. (e) Cycling performance and corresponding Coulombic efficiency at 1.0 A g−1 for 1000 cycles. Due to the excellent electrical conductivity, high theoretical capacity and the low and flat potentials close to that of lithium metal, metal nitrides are emerging as a new and promising electrode material for electrochemical storage devices.23 Therefore, the electrochemical properties of the GaN NSs anodes were studied. The cyclic voltammetry (CV) analysis was employed at the scan 8


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rate of 0.1 mV s−1, Figure 4a. In the first cathodic process, the irreversible peak (~0.45 V) can be attributed to the formation of solid electrolyte interphase (SEI).24 In the second cathodic process, the peak located at 0.45 V disappears, but a pronounced peak at around 0.9 V appears. These peaks may correspond to Li insertion/extraction reaction with the nitrides.25 Meanwhile, the peak located at 0.54 V has not changed significantly. After the first cycle, the anodic/cathodic peaks in the second to fourth CV curves become very stable, indicating the lithiation/delithiation reaction is highly reversible. As shown in Figure 4b, the GaN NSs anodes exhibit typical discharge/charge profiles with low voltage polarization, indicating the high reversibility. The initial discharge and charge capacities are 922.16 and 559.43 mA h g−1, respectively. The initial irreversible capacity loss might be due to the inevitable formation of SEI.3 The charge/discharge curves show that GaN NSs anodes exhibit an excellent charge/discharge property as electrode materials. By contrast, powder GaN was synthesized with the same method as GaN NSs only without GO. Without the guidance of GO sheets, small particles (~ 100 nm) of GaN cohered to form powder GaN (~ 500 nm), Figure S14a. The powder GaN anodes exhibit a relatively low capacity (Figure S13). The initial discharge and charge capacity of the powder GaN anodes are only ~ 367.8 and 187.7 mA h g−1, respectively. The cycling performance of the GaN NSs anodes at 0.1 A g−1 after the first 100th cycle are displayed in Figure 4c. After 100 cycles, the discharge capacity reaches about 702 mA h g−1, which is probably due to the formation of a polymeric gel-like film on the active material.26 For comparison, the capacity of GaN powder is 189 mA h g−1 after 148 cycles (Figure S14b). The enhanced capacity of GaN NSs electrode can be ascribed to the defects in the host lattice and the distortion on the surface of GaN NSs, as shown in Figure S7 and S12. In addition, in despite of a little Ga-O content, the effect of Ga-O on the electrochemical performance is still limited (Figure S15). Furthermore, the 9



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rate capability further highlights the advantages of this unique GaN NSs structure. As shown in Figure 4d, GaN NSs based anodes exhibit excellent rate performance at current rates from 0.1 to 10 A g−1. At a rate of 0.1 A g−1, the GaN NSs anodes deliver the reversible capacity of ~590 mA h g−1. Even at the higher rates of 0.5, 1.0, 2.0, 5.0 and 10.0, the discharge capacities still remain at stable values of 515 mA h g−1, 470 mA h g−1, 380 mA h g−1, 303 mA h g−1, and 205 mA h g−1, respectively. More importantly, a capacity over 580 mA h g−1 could be recovered when the current density was reduced back to 0.1 A g−1. Especially, even when the current increased to one hundred times of the original, the discharge specific capacity still is maintained 34 % of the original value. This result demonstrates that GaN NSs has great potential as a high-rate anode material for LIBs. The long-term cycling performance of GaN NSs anodes was tested at 1.0 A g−1 (Figure 4e). During the cycling procedure, the Coulombic efficiency quickly increases and stabilizes at 100% in the first few cycles. It is worth noting that the specific capacity of GaN NSs anodes display a distinct increase after a certain number of discharge/charge cycles. After that, a slow increase in the specific capacity occurs and the capacity continues to climb, reaching 612 and 607 mA h g−1 at the 673th cycle, during discharge and charge, respectively. Then discharge capacity stays above 600 mA h g−1 with a slightly decline, demonstrating that after initial cycles for stabilization the composite material exhibits high electrochemical performances. A similar behavior over the cycling has also been reported.27-29 The first possible reason for the increased capacity may be the ability of interfacial lithium storage which is associated with the unique 2D structure.28 The second reason can be the improved Li-diffusion kinetics by an activation process, which indicates the high accessibility for lithium insertion and extraction.30 These results clearly indicate its superior and stable cycling performance of GaN NSs anodes. The SEM images of the discharged/charged GaN NSs electrodes in the 1000th cycles are 10


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shown in Figure S16. After 1000 cycles, the morphology of GaN NSs is well preserved, indicating good structural stability during the discharged/charged process. To more clearly illustrate the electrochemical performances, Tables S1 shows the comparison of material, morphology, and capacity of GaN NSs anode and other previously reported metal nitrides anode. To the best of our knowledge, the GaN NSs anodes deliver the favorable capacities and long-term cycling performance among these results at 0.1 and 10 A g−1, respectively. In conclusion, we have demonstrated GO-templating method for the synthesis of GaN NSs. The approach constructs GaN NSs through only several facile solution based steps. As a new anode material of LIBs, GaN NSs based electrodes deliver a discharge capacity up to 702 mA h g−1 after 100 cycles at 0.1 A g−1 and above 600 mA h g−1 after 1000 cycles at 1.0 A g−1. More importantly, GaN NSs based electrodes exhibit ultrahigh rate capability. This method can expand the family of 2D materials and controllably produce high-quality GaN NSs even for nonlayered compounds as high performance energy storage devices.

ASSOCIATED CONTENT

Supporting Information

Experimental methods regarding preparation of GaN nanosheets, and equipment and characterization techniques; SEM images of intermediate processes; Tyndall effect; XRD pattern, Raman spectra; Thermogravimetry curves; EDX spectrum; nitrogen adsorption/desorption measurements; Ga K-edge XANES spectra; Local structure parameters around Ga; discharge/charge profiles and cycle performance of powder GaN; comparisons of the capacity between GaN NSs anode and other

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previously reported metal nitrides. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions †

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work is supported by the National Natural Science Foundation of China (Contract No. 51572153, 51602177). The authors especially thank the Beijing Synchrotron Radiation Facility (BSRF) for kindly providing the beam time for XAFS experiments.

REFERENCES

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