Article pubs.acs.org/JPCC
Facile Synthesis of Ultrathin ZnO Nanotubes with Well-Organized Hexagonal Nanowalls and Sealed Layouts: Applications for Lithium Ion Battery Anodes Keon Tae Park,† Fan Xia,† Sung Woong Kim,† Seong Been Kim,† Taeseup Song,‡ Ungyu Paik,‡ and Won Il Park*,† †
Division of Materials Science and Engineering and ‡WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Korea S Supporting Information *
ABSTRACT: We report a new facile route to synthesize the ZnO nanotubes by thermal annealing of solid nanorods in ambient NH3. The unique characteristic of this approach allows achievement of ultrathin nanotubes with well-organized hexagonal nanowalls and sealed layouts. On the basis of our experimental observations, we developed a nanotube formation mechanism illustrating the following: (i) energetically active nanorod surfaces could be readily passivated to form a few-atoms-thick Zn3N2 layer and (ii) nanopores generated from the seed layer were extended to the inside of nanorod bottoms and then propagated upward until they reached the tops of the nanorods. On the basis of key features of these tubular structures, we assessed the electrochemical performance of the nanotubes as anode materials in lithium ion batteries, demonstrating significant improvements in cycling performance over counterparts made of solid nanostructures.
■
INTRODUCTION Because of its multiple properties of semiconductivity, piezoelectricity, and pyroelectricity, ZnO, particularly in nanostructured forms, has been extensively studied in the past few years.1 Among its diverse nanostructures, 1D ZnO nanostructures have attracted increased attention because of the variety of possible morphologies as well as wide potential applications.1−3 In particular, the highly specific surface areas of 1D nanostructures offer surface sensitivity and reactivity, whereas the high aspect ratios allow reliable electrical contact and efficient transport of charge carriers.4 Consequently, they are expected to be useful as high-performance sensors, electrochemical cells, and solid-state energy conversion devices.5,6 However, the as-synthesized 1D ZnO nanostructures still have thicker diameters, often larger than several tens of nanometers, and thus their properties are not far from those of thin film structures. Accordingly, the conversion of solid nanostructures to tubular nanostructures is promising because their enhanced specific surface area and porosities may lead to advantages over solid nanostructures. Moreover, the existence of inner voids offers additional applications in various fields such as nanofluidics, ion-exchange, catalysts, and storage− delivery−release systems.7−9 The synthesis of ZnO nanotube structures has been achieved by many different techniques including vapor phase transport, metalorganic chemical vapor deposition (MOCVD), the hydrothermal process, and electrodeposition. These processes © 2012 American Chemical Society
can be categorized into three major classes based on synthetic strategies: (1) template direct approaches,10 (2) direct and selforganizing processes,11 and (3) conversion of solid nanowires/ rods into hollow structures.12 The first approach utilizes conformal coating of pre-existing templates, such as porous anodic alumina membranes,10 followed by selective etching of these templates. The choice of appropriate template and coating layer thickness can determine the dimensions of the nanotubes, but this strategy generally yields poor crystallinity. The direct and self-organizing process is based on the spontaneous formation of a metastable embryo and subsequent growth of a stable ZnO sheathing layer,13,14 which can produce the growth of single-crystal nanotubes. However, the formation mechanism is still not clear, and dimensions are hard to control. In the last approach, the differences in chemical activities by surface polarity15 or surfactant yield preferential etching of the core along the c axis of the ZnO crystals,16 eventually converting the solid nanorods into tubular structures. This process generally occurs in aqueous solution, allowing facile and mass production of ZnO nanotubes at low cost. However, this process is necessarily accompanied by the reduction of the initial aspect ratio (the shrinkage of the length) as a consequence of a lack of etching selectivity. Received: October 22, 2012 Revised: December 12, 2012 Published: December 26, 2012 1037
dx.doi.org/10.1021/jp310428r | J. Phys. Chem. C 2013, 117, 1037−1043
The Journal of Physical Chemistry C
■
Article
RESULTS AND DISCUSSION Figure 1 shows SEM images of the ZnO nanostructures arrays before and after thermal annealing in ambient NH3. Typically,
In this report, we demonstrate the spontaneous conversion of solid ZnO nanorods to ultrathin nanotubes via a gas-phase chemical process. Notable findings include that this reaction process involves not only the initiation of chemical etching from the ZnO crystal but also the formation of very thin zinc nitride (Zn3N2) layers on the crystal surfaces. Because of its strong etch resistance, the Zn3N2 layer can effectively prevent etching from the outsurface of ZnO nanorods and eventually produce ultrathin ZnO nanotubes with preserved outward appearance and single-crystal structure. More interestingly, the resulting nanotubes maintain well-organized hexagonal nanowalls with sealed layouts, which is distinctly different from previous solution-based approaches that yield open-end geometry. Such unique layouts enable significant improvements in related properties compared with the configurations of 1D solid nanostructures. To explore a representative example, we investigated the potential use of ZnO nanostructures as anode materials for lithium ion batteries (LIBs).
■
EXPERIMENTAL SECTION For the synthesis of ZnO nanotubes, hydrothermally grown ZnO nanorods were thermally annealed in ambient NH3. For the synthesis of pristine ZnO nanorods, a thin seed layer was initially deposited on the substrate (generally, drops of zinc acetate solution in ethanol were spin-coated at 4000 rpm several times to form ZnO nanoparticulate seed layer). Optionally, a poly(methyl methacrylate) (PMMA) layer was coated on the seed layer, and a regular array of circular holes with a diameter 500 nm was defined by electron beam lithography to achieve ZnO nanorod array. The sample was then immersed into an aqueous solution containing equimolar (0.025 to 0.1 M) zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma-Aldrich) and hexamethylenetetramine (C6H12N4, SigmaAldrich). The equimolar solution was placed in an oven at 85 °C for 6−10 h, producing ZnO nanorods on the exposed ZnO seed layer surface. Finally, the sample was rinsed repeatedly with deionized (DI) water and dried with nitrogen blowing.17 To convert the solid ZnO nanorods to hollow ones, the sample was placed horizontally in the middle of a quartz tube (1 in. in diameter and 700 mm in length) that was inserted in a horizontal tube furnace. After evacuating the tube system well below 10 mTorr (for 10 min), the temperature was elevated to 660 °C, and the chamber was filled with a flux of 250 sccm ammonium (NH3) gas. While keeping the pressure at 1 Torr, the sample was thermally annealed for 15−35 min and then quickly cooled to room temperature. Structural and optical examinations of the resulting ZnO nanostructures were performed by scanning electronic microscopy (SEM, JEOL JSM-7600F), transmission electron microscopy (TEM, JEOL 2100F at 200 kV) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. ZnO nanorods and nanotubes were directly formed on the Cu current collectors to estimate the electrochemical performance.9 The masses of the synthesized ZnO electrode materials were accurately measured using a microbalance (by checking the total mass before and after ZnO material deposition). The coin-type half cells (2032R type) were prepared in an argonfilled glovebox. Pure lithium metal foil was employed as the counter electrode. The electrolyte was 1.0 M LiPF6 with ethylene carbonate/diethylene carbonate (EC/DEC, 1:1 vol %, Cheil Industries, Korea). The coin-type half cells were cycled at a rate of 0.5 C between 0.01 and 2.5 V.
Figure 1. SEM images of patterned arrays of ZnO nanorods (a) and nanotubes (b−e).
the pristine, hexagonal-shaped ZnO nanorods with diameter of 100−300 nm and length of ∼1 μm are readily prepared via a hydrothermal process (Figure 1a). Even after the etching process, ZnO nanotubes retain their original outward appearance (hexagonal shape), although they have shell layers thin enough for electron beams to penetrate the layers. Therefore, distant nanotubes and nanotube bottoms are clearly identified in SEM images (Figure 1b,c). Higher aspect-ratio nanotubes can also be achieved by the same method (Figure 2d,e). To better understand the etching process, we investigated time-dependent morphology development (Figure S1 of the Supporting Information). After a postannealing process under ambient NH3, the voids initiated from the nanorod roots propagated upward steadily with increasing time (Figure S1a,b of the Supporting Information). After 35 min, the original solid structures completely turned into tubular structures, whereas the outer shells maintained their hexagonal shapes (Figure S1c of the Supporting Information). We also performed TEM analysis on the samples at each step. As with the SEM results, pores generated from the insides of nanorod bottoms were extended to the tops of nanorods, whereas etching at the outer surface of the ZnO nanorods rarely occurred, eventually producing nanotube structures with ultrathin walls (Figures S1d−f of the Supporting Information). With further reaction time (t ≥ 40 min), the nanotubes became dented and lost their original shapes but still sustained their tubular structures (Figure S2 of the Supporting Information). The nanotube formation is distinctly different from the conventional gas-phase etching processes that generally yield isotropic etching from the outer surface to produce thinner nanorods. It is also distinguished from the electrochemical process in an aqueous solution, yielding nanotubes with ultrathin walls nearing 10 nm in thickness and, more importantly, with nanotube heads sealed with hexagonal caps. Further structural analysis of the ZnO nanotubes was performed by high-resolution TEM (HR-TEM). ZnO nanotubes have monolithic shells with typical thickness of 5−12 nm (Figure 2). As shown in Figure 2b,c, the single-crystal structure 1038
dx.doi.org/10.1021/jp310428r | J. Phys. Chem. C 2013, 117, 1037−1043
The Journal of Physical Chemistry C
Article
ZnO seed layer may have been preferentially etched or have served as penetrating paths for etching agents of H 2 molecules.20 Previous studies on the hydrothermal growth of ZnO crystals have revealed the formation of fine ZnO crystal bundles in the initial stage, which subsequently coalesce to produce bigger hexagonal rods. Accordingly, it is expected that the etching process initiated in the seed layer, moved to the lower part of the rods, and finally ended at the top of nanorods. When substantial pores were generated, NH3 molecules could also permeate into the ZnO rods. These NH3 molecules become the protective layer on the inner wall surface, thereby reducing the sudden rapid attack (etching) by H2 so that the resulting ZnO nanotubes preserve their initial outward appearance with ultrathin tube walls. To confirm the formation of a Zn3N2 layer on the ZnO crystal surfaces, we performed XPS analysis on as-grown ZnO nanorods and the resulting ZnO nanotubes. XPS spectra of the core levels of Zn 2p, O 1S, and N 1s peaks for the samples are shown in Figure 3. In the Zn 2p spectra (Figure 3a,b), both curves exhibit two distinct peaks, including characteristic Zn 2p1/2 (1045 eV) and Zn 2p3/2 (1022 eV) peaks. Their binding energies are close to the reference values,21 and there was no peak shift associated with the interstitial Zn (Zni) in the ZnO state. ZnO nanotubes exhibited the distinct N 1s peak that did not appear in the as-grown ZnO nanorods (Figure 3c,d). The binding energy of the N 1s peak is located at 396.4 eV. When compared with free amine at ∼398.8 eV, the N 1s peak shows a chemical shift of 0.4 eV toward a lower binding energy, representing the formation of very strong ionic Zn−N bonds.22 The XPS spectral features of the ZnO nanotubes are also quite close to those of Zn3N2 films, providing strong evidence of the formation of Zn3N2 layer on the nanotube surface. However, the Zn3N2 layer was not clearly identified in the TEM images, illustrating that its thickness is extremely small. A slight change in the spectral feature was also observed in the O 1s signals. To make this point clear, the O 1s signals were resolved into two peaks by a Gaussian curve fitting and the results are shown in Figure 3e,f. Most importantly, the nanotube formation reaction resulted in a slight increase in the relative intensity of the lower binding energy peak as compared with the higher binding energy peak. The peak with the lower binding energy of ∼530.8 eV corresponds to O2− in normal wurtzite ZnO single crystals, whereas the peak with the higher binding energy of ∼532.7 eV can be attributed to O2− around the nonstoichiometric defects in ZnO, such as oxygen-deficient defects (e.g., oxygen vacancies) and impurities.23 A substantial number of unreacted residues remained in the hydrothermally grown ZnO rods, which might also be associated with the higher binding energy peak. We believe that the reduction of the higher binding energy peak, accompanied by tube formation, shows that the etching process preferentially occurs in the defective regions, eliminates defects, and thereby improves crystal quality. We also evaluated the validity of our model with controlled experiments. First, we performed etching of the ZnO nanorods with the use of H2 under similar conditions. As shown in Figure S3 of the Supporting Information, many pores are formed from the outer surface of the nanorods and with time become enlarged and penetrate into the nanorod cores. In the end, the nanorods lose their original shape by excessive etching. These results are distinct from those achieved through the NH3 etching process, emphasizing the surface passivation effect of the Zn3N2 layer formed on the NH3 treatment that prevents etching from the outside.24 Second, to investigate the role the
Figure 2. TEM images of the ZnO nanotube. (a) Low-magnification TEM image of the ZnO nanotube. (b,c) HR-TEM images taken at the closed cap (b) and wall (c) of a ZnO nanotube with corresponding FFT image (insets). (d) Enlarged HR-TEM image taken from the square in panel c confirming that the ZnO nanotube grows along the c axis orientation.
of the ZnO nanotube was confirmed by the highly ordered crystal lattice images at the nanotube side wall and cap and the corresponding fast Fourier transform (FFT) images. Slight lattice distortions were appeared in Figure 2c, but it is rather due to the shrinkage of the very thin wall (Figure S5 of the Supporting Information). Additionally, the spacing between adjacent lattice planes stacked along the axial direction was estimated to be 0.52 nm (Figure 2d), which corresponded with the interplanar distance of the (0001) plane in wurtzite ZnO,18 indicating that ZnO nanotubes grow along the c axis. The outer wall surfaces of the nanotubes maintain their original surface characteristics whereas their inner surfaces are corrugated and rough, illustrating that etching occurs mainly inside of the tube. According to our structural analysis, we propose a mechanism for the transformation from the solid ZnO nanorods to ultrathin nanotubes that hypothesizes the formation of an etching-protecting layer on the gas-phase chemical process. Initially, the surfaces of the ZnO nanorods were nitrided under ambient NH3 to form very thin Zn3N2 by the following reaction:19 3ZnO(s) + 2NH3(g) → Zn3N2(s) + 3H 2O(g)
(1)
Simultaneously, etching of ZnO occurred in the presence of NH3 that was decomposed to N2 and H2 at an elevated temperature above 600 °C in an oxygen-free atmosphere by the following reactions 2NH3(g) → N2(g) + 3H 2(g)
(2)
ZnO(s) + H 2(g) → Zn + H 2O(g)
(3)
Because the Zn3N2 layer is chemically stable in the presence of hydrogen even at high temperatures, etching of ZnO from the nanorod surface was prevented. Defects such as grain boundaries, vacancies, and voids persisting in the underlying 1039
dx.doi.org/10.1021/jp310428r | J. Phys. Chem. C 2013, 117, 1037−1043
The Journal of Physical Chemistry C
Article
Figure 3. Comparison of XPS spectra of the core levels of Zn 2p (a,b), N 1s (c,d), and O 1s (e,f) peaks for ZnO nanorods and ZnO nanotubes, respectively.
seed layer plays in initiating etching, we introduced a wellorganized ZnO hexagonal rod array grown on epitaxial seed layer and performed a time-resolved analysis on the morphology evolution at the initial etching stage (Figure S4 of the Supporting Information).25 In this case, the seed layer became rough and porous, which was followed by etching from the rod bottom. This observation supports our model, which assumes that the etching process involves the permeating of the etching agents (i.e., NH3 or H2 molecules) from the seed layer to the inside of the rod via defects or voids. The etching rate of the hexagonal rods on the high-quality epitaxial seed layer was less than half of that on the polycrystalline layers, offering further evidence in support of our model. To characterize the changes in optical properties accompanied by the etching process, we performed PL measurements on a series of the ZnO nanostructures, using a He−Cd laser line of 325 nm as the excitation source. Figure 4 shows the logarithm plot of the resulting PL spectra of the as-grown ZnO nanorods together with the ones treated by NH3 for 15, 25, and 35 min, respectively. All samples exhibited dominant ultraviolet peaks and broad bands in the visible spectral range (450−700 nm), corresponding to a near-band-edge (NBE) emission associated with free exciton transition and defect-related, deeplevel transition (DLT), respectively.26 The intensities of both NBE and DLT peaks decreased with increasing etching time,
Figure 4. Evolution of the room-temperature PL spectra accompanying the conversion from the solid ZnO nanorods to ultrathin nanotubes by thermal annealing in the NH3 ambient for 15, 25, and 35 min. Inset: IDLT/INBE as a function of the thermal annealing (etching) time.
which can be easily understood by considering the reduced excitation volume. Meanwhile, the relative intensities of the DLT peaks decreased with increasing etching time, contrary to our expectation that etching processes can suppress the bandto-band-transition (i.e., enhance the DLT process) by 1040
dx.doi.org/10.1021/jp310428r | J. Phys. Chem. C 2013, 117, 1037−1043
The Journal of Physical Chemistry C
Article
generating surface and defects. To make this point clear, we plotted the intensity ratio of the DLT to NBE peak (IDLT/INBE) versus etching time in the inset of Figure 4, which illustrates that the IDLT/INBE ratio is significantly reduced and approaches zero when nanorods were converted fully to nanotubes. This result is somewhat consistent with the XPS study, which showed the reduction of the higher binding energy peak for the O 1s signal. The result indicates an elimination of the defects in the as-synthesized ZnO nanorods upon etching in NH3, which eventually improves the luminescence properties of ZnO nanotubes. A slight NBE emission peak shift (∼3 nm blue shift) appeared, and the quantum confinement effect in very thin nanotube walls could be considered to be the cause of this blue shift. To overcome the low specific capacity of conventional graphite anode in LIBs,27 a variety of candidate materials, such as silicon28,29 and metal oxides,30 have been considered. The theoretical specific capacity of ZnO is 978 mAh/g,31,32 almost three times higher than that of graphite, and thus it can in principle increase the energy capacity of the LIBs. However, the cyclic capacity retention of ZnO anode materials is often found to be dissatisfactory because it suffers from a severe volume variation during cyclic insertion/extraction of Li ions, leading to the electrode pulverization and subsequent electrical disconnection from the current collector.31,33,34 On the basis of the unique characteristics of sealed tubular layouts and ultrathin tube wall with high aspect ratios, our ZnO nanostructures provide attractive features that can overcome the critical challenges associated with huge volume changes. Lithium ion half cells containing randomly grown ZnO nanorods and nanotubes on entire Cu foil as anode electrodes were assembled, and their reversible capacity and cycle characteristics were investigated. The first discharge and charge capacities of the ZnO nanotubes were measured to be 932 and 621 mAh g−1, respectively (Figure 5a), resulting in an initial Coulombic efficiency of 66.6%. The ZnO nanorods exhibited a similar electrochemical performance at the first cycle, with first discharge and charge capacities of 929 and 598 mAh g−1, respectively, producing Coulombic efficiency of 64.4%. The lower Coulombic efficiency in the initial stage is the characteristic feature of the metal oxide anode material and can be attributed to the irreversible process of Li2O formation.35 Figure 5b shows the voltage profile of the ZnO nanotube electrode. The flat voltage plateau at 0.4 to 0.5 V in the first discharge curve corresponds to the reduction of ZnO into Zn and Li2O formation; meanwhile, the plateau at 0.15 V corresponds to the decomposition of electrolyte and the formation of lithium−zinc alloy, which is elaborated in its cyclic voltammogram (Figure S6 of the Supporting Information).33,34,36 Despite similar performances in the initial stages, the discharge−charge capacities of the nanotube anode ran ahead of those of ZnO nanorods in the following cycles, and capacity disparity became larger with cycle number. After 50 cycles, ZnO nanotubes exhibited reversible capacity of 386 mAh g−1, which is about five times higher than that of ZnO nanorods (83 mAh g−1). We ascribe the improvement of cycle performance to the intrinsic characteristics of the ZnO nanotube structures. First, the porous structure ensures easy electrolyte permeability, allowing for adequate penetration into active materials. This allows for maximization of the specific surface involving the alloy/dealloy reaction. Furthermore, pores play an important role in stabilizing the capacity.37,38 Second, not only the
Figure 5. (a) Cycle performances and Coulombic efficiency of pristine solid ZnO nanorods (red) and ZnO nanotube (porosity, ∼70%) (blue) at 0.5 C (red solid/hollow circles, discharge/charge capacities of the nanorods; blue solid/hollow circles, discharge/charge capacities of the nanotubes; red/blue triangles, Coulombic efficiency of nanorods/nanotubes). (b) Galvanostatic charge−discharge profiles of ZnO nanotubes for 1st and 20th cycles at 0.5 C. (c) Plot of charge capacity for several specific cycles at 0.5 C as a function of porosity percentage of ZnO nanostructures (0%, pristine ZnO nanorods; 50%90%, porous ZnO nanotubes).
intertubular spaces but also the voids inside the nanotubes provide additional ability to accommodate the large volumetric expansion during lithiation. As shown in Figure S7 of the Supporting Information, the ZnO nanotubes were not pulverized and appeared to remain in contact with the current collector even after cycling 50 times. Third, the ultrathin wall thickness with high aspect ratio can accelerate the ion diffusion through active material, indicating that the electrochemical reaction is promoted kinetically.39On the basis of this analysis, we compared several types of samples with various porosity depending on etching times of NH3 (Figure 5c), where the 1041
dx.doi.org/10.1021/jp310428r | J. Phys. Chem. C 2013, 117, 1037−1043
The Journal of Physical Chemistry C
Article
(6) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455−459. (7) Daiguji, H.; Yang, P.; Majumdar, A. Nano Lett. 2004, 4, 137−142. (8) Seayad, A. M.; Antonelli, D. M. Adv. Mater. 2004, 16, 765−777. (9) Song, T.; Xia, J.; Lee, J. H.; Lee, D. H.; Kwon, M. S.; Choi, J. M.; Wu, J.; Doo, S. K.; Chang, H.; Park, W. I.; et al. Nano Lett. 2010, 10, 1710−1716. (10) Lee, J. S.; Sim, S. K.; Min., B.; Cho, K.; Kim, S. W.; Kim, S. J. Cryst. Growth. 2003, 267, 145. (11) Xing, Y. J.; Xi, Z. H.; Xue, Z. Q.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Song, Y.; Zhang, S. L.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 1689. (12) Shen, X. P.; Yuan, A. H.; Hu, Y. M.; Jiang, Y.; Xu, Z.; Hu, Z. Nanotechnology 2005, 16, 2039−2043. (13) Sun, Y.; Riley, D. J.; Ashfold, M. N. R. J. Phys. Chem. B 2006, 110, 15186−15192. (14) Pyun, Y. B.; Yi, J.; Lee, D. H.; Son, K. S.; Liu, G.; Yi, D. K.; Paik, U.; Park, W. I. J. Mater. Chem. 2010, 20, 5136−5140. (15) Elias, J. R.; Zaera, T.; Wang, G. Y.; Claude, L. C. Chem. Mater. 2008, 20, 6633. (16) Wang, H.; Li, M.; Jia, L.; Li, L.; Wang, G.; Zhang, Y.; Li, G. Nanoscale Res. Lett. 2010, 5, 1102−1106. (17) Lee, W. W.; Yi, J.; Kim, S. B.; Kim, Y. H.; Park, H. G.; Park, W. I. Cryst. Growth Des. 2011, 11, 4927−4932. (18) Park, W. I.; Yi, G. C.; Kim, M.; Pennycook, S. J. Adv. Mater. 2002, 14, 1841−1843. (19) Khana, W. S.; Caoa, C.; Butta, F. K.; Alia, Z.; Safdarb, M.; Panc, L.; Rafiquec, M. Y.; Ainc, Q.; Usmana, Z.; Nabia, G. Appl. Surf. Sci. 2011, 257, 7314−7319. (20) Wu, J. J.; Wen, H. I.; Tseng, C. H.; Liu, S. C. Adv. Mater. 2004, 14, 806−810. (21) Jin, Y.; Cui, Q.; Wen, G.; Wang, Q.; Hao, J.; Wang, S.; Zhang, J. J. Phys. D: Appl. Phys. 2009, 42, 215007. (22) Xing, G. Z.; Wang, D. D.; Yao, B.; Ah Qune, L. F. N.; Yang, T.; He, Q.; Yang, J. H.; Yang, L. L. J. Appl. Phys. 2010, 108, 083710. (23) Wei, X. Q.; Man, B. Y.; Liu, M.; Xue, C. S.; Zhuang, H. Z.; Yang, C. Physica B 2007, 388, 145. (24) Zervos, M.; Karipi, C.; Othonos, A. Nanoscale Res. Lett. 2012, 7, 175. (25) Kim, S. B.; Lee, W. W.; Yi, J.; Park, W. I.; Kim, J. S.; Nichols, W. T. ACS Appl. Mater. Interfaces 2012, 4, 3910−3915. (26) Lee, W. W.; Kim, S. B.; Yi, J.; Nichols, W. T.; Park, W. I. J. Phys. Chem. C 2012, 116, 456−460. (27) Megahed, S.; Scrosati, B. J. Power Sources 1994, 51, 79−104. (28) Chan, C. K.; Peng, H.; Liu, G.; Mcllwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. Nanotechnol. 2008, 3, 31−35. (29) Wu, H.; Chan, G.; Choi, J. K.; Ryu, I.; Yao, Y.; McDowell, M. T.; Lee, S. K.; Jackson, A.; Yang, Y.; Hu, L.; et al. Nat. Nanotechnol. 2012, 7, 310−315. (30) Varghese, B.; Reddy, M. V.; Yanwu, Z.; Lit, C. S.; Hoong, T. C.; Rao, G. V. S.; Chowdari, B. V. R.; Wee, A. T. S.; Lim, C. T.; Sow, C. H. Chem. Mater. 2008, 20, 3360−3367. (31) Li, H.; Huang, X.; Chen, L. Solid State Ionics 1999, 123, 189− 197. (32) Liu, J.; Li, Y.; Ding, R.; Jiang, J.; Hu, Y.; Ji, X.; Chi, Q.; Zhu, Z.; Huang, X. J. Phys. Chem. C 2009, 113, 5336−5339. (33) Wang, J.; King, P.; Huggins, R. A. Solid State Ionics 1986, 20, 185−189. (34) Belliard, F.; Irvine, J. T. S. J. Power Sources 2001, 97−98, 219− 222. (35) Fu, Z. W.; Huang, F.; Zhang, Y.; Chu, Y.; Qin, Q. Z. J. Electrochem. Soc. 2003, 150, A714−A720. (36) Ahmad, M.; Shi, Y.; Nisar, A.; Sun, H.; Shen, W.; Wei, M.; Zhu, J. J. Mater. Chem. 2011, 21, 7723−7729. (37) Kim, H.; Han, B.; Choo, J.; Cho, J. Angew. Chem., Int. Ed. 2008, 47, 10151−10154. (38) Wang, X. L.; Han, W. Q.; Chen, H. Y.; Bai, J. M.; Tyson, T. A.; Yu, X. Q.; Wang, X. J.; Yang, X. Q. J. Am. Chem. Soc. 2011, 133, 20692−20695.
porosity was measured by analyzing the electron microscopy images, as shown in Figure S1 of the Supporting Information. As the pore density in the nanostructures increases, the anodes show better cyclability. Such a tendency continues up to ∼70% porosity; however, the capacity level decreases slightly again with increasing porosity. We believe that immoderate etching may accelerate the pulverization of ZnO at the interface of the nanotubes and the current collector, giving rise to an electrical disconnection from the current collector and eventually a faster capacity fading.
■
CONCLUSIONS We developed a facile route to convert solid ZnO nanorods to ultrathin nanotubes with well-organized hexagonal nanowalls and sealed layouts. Time-dependent investigation of the evolution from solid nanorods to nanotubes illustrated the pore generation at the inside of nanorod bottoms and propagation to the top of nanorods. On the basis of this result, we proposed a transformation mechanism by hypothesizing the formation of a passivation layer (a few-atoms-thick Zn3N2) at the outer nanorod surface, which was further confirmed by XPS analysis. The core-etching process involved the elimination of defects in the as-synthesized ZnO, which eventually improved the luminescence properties of the ZnO nanotubes. Moreover, the tubular structures not only kinetically promote the ion diffusion through the active materials but also can effectively accommodate the large volumetric expansion upon full lithiation, thereby improving the cycling performance as an anode in LIB.
■
ASSOCIATED CONTENT
S Supporting Information *
SEM and TEM images showing the morphological evolution from solid ZnO nanorods to porous and tubular structures after thermal annealing in NH3 ambient and H2 ambient. This material is available free of charge via the Internet at http:// pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +82-2-2220-0504. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (2012-001442) and by National Research Foundation of Korea (NRF) through grant no. K20704000003TA050000310, Global Research Laboratory (GRL) Program provided by the Korean Ministry of Education, Science and Technology (MEST).
■
REFERENCES
(1) Wang, Z. L. MRS Bull. 2012, 09, 814−827. (2) Yi, G. C.; Wang, C.; Park, W. I. Semicond. Sci. Technol. 2005, 20, S22−S34. (3) Wang, Z. L. J. Phys. Condens. Mat. 2004, 16, R829−R858. (4) Lieber, C. M. MRS Bull. 2011, 36, 1052−1063. (5) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat.Nanotechnol. 2008, 3, 31−35. 1042
dx.doi.org/10.1021/jp310428r | J. Phys. Chem. C 2013, 117, 1037−1043
The Journal of Physical Chemistry C
Article
(39) Song, T.; Jeon, Y.; Samal, M.; Han, H.; Park, H.; Ha, J.; Yi, D. K.; Choi, J. M.; Chang, H.; Choi, Y. M.; et al. Energy Environ. Sci. 2012, 5, 9028−9033.
1043
dx.doi.org/10.1021/jp310428r | J. Phys. Chem. C 2013, 117, 1037−1043
