Bottom-up Approach Design, Band Structure, and Lithium Storage

Subscriber access provided by Northern Illinois University

Article

Bottom-up Approach Design, Band Structure and Lithium Storage Properties of Atomically Thin #-FeOOH Nanosheets Yun Song, Yu Cao, Jing Wang, Yong-Ning Zhou, Fang Fang, Yuesheng Li, Shang-Peng Gao, Qin-Fen Gu, Linfeng Hu, and Dalin Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05506 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

ACS Applied Materials & Interfaces

Bottom-up Approach Design, Band Structure and Lithium Storage 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Properties of Atomically Thin γ-FeOOH Nanosheets Yun Song,† Yu Cao,† Jing Wang,† Yong-Ning Zhou,† Fang Fang,† Yuesheng Li,† Shang-Peng Gao,† Qin-Fen Gu,‡ Linfeng Hu,† Dalin Sun†*

†

Department of Materials Science, Fudan University, Shanghai 200433, P. R. China ‡

Australia Synchrotron, 800 Blackburn Road, Clayton, 3168, Australia *Corresponding author: E-mail: [email protected]

ACS Paragon Plus Environment


Page 2 of 24

ABSTRACT: As a novel class of soft matter, two-dimensional (2D) atomic nanosheet-like crystals have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

attracted much attention for energy storage devices due to the fact that nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. Herein, atomically thin γ-FeOOH nanosheets with a thickness of ~1.5 nm are synthesized in a high yield, and the band and electronic structures of the γ-FeOOH nanosheet are revealed using density-functional theory calculations for the first time. The rationally designed γ-FeOOH@rGO composites with a hetero-stacking structure are used as an anode material for lithium-ion batteries (LIBs). A high reversible capacity over 850 mAh g-1 after 100 cycles at 200 mA g-1 is obtained with excellent rate capability. The remarkable performance is attributed to the ultrathin nature of γ-FeOOH nanosheets and 2D hetero-stacking structure, which provide the minimized Li+ diffusion length and buffer zone for volume change. Further investigation on the Li storage electrochemical mechanism of γFeOOH@rGO indicates that the charge-discharge processes include both conversion reaction and capacitive behavior. This synergistic effect of conversion reaction and capacitive behavior originating from 2D heterostacking structure casts new light on the development of high-energy anode materials. KEYWORDS: Unilamellar γ-FeOOH nanosheet; band structure; 2D hetero-stacking; lithium storage; electrochemical mechanism


Page 3 of 24


1. INTRODUCTION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Iron oxide-hydroxide (FeOOH) has been reported to be a promising electrode material for Li-ion batteries (LIBs) due to its high theoretical capacity (905 mAh g-1), natural abundance, low cost, nontoxicity and environmental friendliness.1-3 However, FeOOH is difficult to be fully activated due to the restructuring process during Li+ insertion/extraction,

4-9

for example, more than 300 cycles is required for β-FeOOH

nanorods. 6 In order to shorten the activation period of FeOOH, it is necessary to design a stable structure which can quickly adapt to the restructuring process during lithiation and delithiation. Recently, two-dimensional (2D) materials, which are molecularly thin with great flexibility and diversity of compositions, structures and functionalities, have been reported as a new class of high-energy and high-power energy storage materials. 10-12 For example, graphene or reduced graphene oxide (rGO), the most popular 2D material, 13-15 can not only facilitate electron transport and Li+ diffusion in FeOOH, but also accommodate the strain/stress generated in the restructuring process upon cycling. Benefiting from the introduction of rGO, the activation period of FeOOH nanoparticles is shortened to approximately 100 cycles. 16

However, it is still too long to meet the demand of commercial application. The reason for the long

activation process of FeOOH nanoparticles/rGO composite may be ascribed to the unfavorable point contact between the nanoparticles and rGO. In order to further shorten the activation process, we propose a 2D hetero-stacking structure of FeOOH nanosheets and rGO, in which FeOOH nanosheets contacted with rGO matrix face-to-face. Compared to the point contact between FeOOH nanorods or nanoparticles and rGO, the face-to-face contact between FeOOH nanosheets and rGO can better utilize the surface area of rGO and result in much stronger interactions between these two components. 17, 18 In order to obtain the 2D hetero-stacking structure, the critical step lies in the rational synthesis of the atomically thin 2D FeOOH nanosheets. Previous work of ultrathin lepidocrocite FeOOH has been reported by a hydrolysis route, but the yield rate of the product is too low to realize electrochemical application. The high-yield synthesis of atomically thin FeOOH nanosheets remains a challenge up to date. 19 The main reason for the low yield rate of reported FeOOH sheets is the strong chemical bonds between the layers of FeOOH which are hard to break using a facile method.20 Herein, we develop an alternative method with a high yield rate, namely a bottom-up approach, to ACS Paragon Plus Environment


Page 4 of 24

produce atomic γ-FeOOH nanosheets with a thickness of ~1.5 nm. The band and electronic structure of the γ1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FeOOH nanosheet has been revealed using density-functional theory calculations for the first time, which enriches the database of FeOOH material. The γ-FeOOH@rGO composites with the 2D hetero-stacking structure are further synthesized by the hydrothermal method. Electrochemical tests in Li half cell indicate that the γ-FeOOH@rGO composites yield a reversible capacity of 855 mAh g-1 upon 100 cycles at a current density of 200 mA g-1. It is found that the synergism of conversion reaction and capacitive behaviour leads to a high specific capacity and excellent rate capability of the γ-FeOOH@rGO composites.

2. RESULTS AND DISCUSSION 2.1 Preparation and characterization of γ-FeOOH nanosheets. As depicted in Scheme 1, the unilamellar γ-FeOOH nanosheets were synthesized by a facile bottom-up growth assisted by CuO template. FeCl2·4H2O was added into the CuO nanoplate suspension and the mixture of CuO and FeCl2·4H2O was aged at room temperature for 24 h, and the color of suspension gradually became brown-yellow. The product was collected by a centrifugation process and dried at 80ºC. High-Resolution X-Ray Diffraction (HRXRD, λ = 0.6884 Å) was employed to examine the crystal structure and the composition of the as-synthesized γ-FeOOH nanosheets as shown in Figure 1a. All the diffraction peaks can be indexed to an orthorhombic structure of γFeOOH (JCPDS card, No. 44−1415), indicating a high purity of the as-prepared γ-FeOOH. The space group of the synthesized γ-FeOOH nanosheets is Cmc21 with lattice constants of a = 12.52 Å, b = 3.87 Å and c = 3.07 Å. Figure 1b shows a typical photograph of aqueous suspension of γ-FeOOH nanosheets with an orange color. A clear Tyndall light scattering is observed, indicating the homogeneous colloidal nature of the suspension. Moreover, the γ-FeOOH colloidal suspension is very stable without sediment upon long-term standing. Transmission Electronic Microscope (TEM) images in Figure 1c demonstrate that the γ-FeOOH sample consists of numerous 2D sheet-like morphology with a mean lateral size ranged from 50 to 100 nm. An individual γ-FeOOH sheet as shown in Figure 1d exhibits very faint but homogeneous contrast, reflecting its uniform thickness. The SAED pattern (inset in Figure 1d) of an individual sheet exhibits rectangularly arranged spots which can be indexed to the [100] zone axis pattern. Since the nanosheets generally lay flat on the copper grid perpendicular to the electron beam,Plus the Environment observed diffraction spots of the [100] zone axis ACS Paragon

Page 5 of 24


indicate that each γ-FeOOH nanosheet is a single crystal with the top (100) surface. Figure 1e shows the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

High-Resolution TEM (HRTEM) images of γ-FeOOH nanosheets with lattice fringe distance of 0.20 nm corresponding to the (020) plane, which is well consistent with the SAED pattern. The atomic force microscopy (AFM) image in Figure 1f shows 2D sheet-like objects with lateral dimensions similar to those observed in the TEM image. The height profile in Figure 1g reveals that the nanosheets have a flat surface with an average thickness of ~1.5 nm.

Scheme 1. The schematic illustration of the growth of atomically thin γ-FeOOH nanosheets: (a) CuO template dispersed in deionized water, (b) addition of FeCl2, (c) addition of NH3·H2O.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

Figure 1. (a) Indexing of the synchrotron diffraction pattern of γ-FeOOH nanosheets from 2° to 40°, (b) Photograph of a colloidal suspension of γ-FeOOH nanosheets, TEM image of (c) multiple accumulation of γFeOOH nanosheets and (d) an individual nanosheet, the inset in d value is the corresponding SAED pattern, (e) HRTEM image of the γ-FeOOH nanosheet, (f) Tapping-mode AFM image of the γ-FeOOH nanosheets deposited on a Si wafer, (g) The height profile along the broken line from X to Y.

To further reveal the crystallographic details of our γ-FeOOH nanosheets, the unit-cell of γ-FeOOH orthorhombic crystal is shown in Figure 2a. As confirmed by the aforementioned SAED characterization, the top surface of an individual nanosheet corresponds to (100) plane (Figure 2b). As shown in Figure 2c, the crystallographic thickness (D0) containing two adjacent (100) planes projected along [010] zone can be approximatively calculated as follows: D0 = dO-O +2ROH = 1-2xOH ×a+2ROH = 1.37 nm

(1)

where do-o is the distance of the typical oxygen atoms at the top and down surface of the γ-FeOOH, xOH (0.075) is the fractional coordinates of the OH group along a axis (Table S1), a (1.25 nm) is the lattice parameter along the a axis of the γ-FeOOH, and ROH (0.074 nm) is the radius of OH group. One can see that the thickness of the γ-FeOOH nanosheet (1.5 nm) is very close to D0 value. The slight deviation between the ACS Paragon Plus Environment

Page 7 of 24


experimental value and the crystallographic thickness is attributed to the possible adsorption of some 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

counterions on the surface, which has been observed in other previously reported unilamellar nanosheets such as layer rare-earth hydroxide and layer double hydroxide. 21, 22

Figure 2. (a) Unit-cell of the γ-FeOOH crystal structure, (b) top-view and (c) side-view of an individual γFeOOH nanosheet. The iron, oxygen/OH group atoms are represented by green and red balls respectively, (d) Schematic illustration of the thickness of unilamellar γ-FeOOH nanosheets.

The preparation method of these γ-FeOOH nanosheets is similar to the one used for the preparation of half-unit-cell α-Fe2O3 nanosheets. 23 Firstly, FeCl2 hydrolysis to form Fe(OH)3 and the initial nanosheet seeds nucleate at the CuO template surface. Then, the adsorbed ferrous ions enable oriented growth at the template interface by a slow CuO etching process at a low temperature of 25 °C. The low temperature is beneficial for synthesis of atomically thin nanosheets. After etching out the CuO template by NH3·H2O, freestanding γFeOOH nanosheets are finally obtained in a stable colloidal suspension form (Figure 2a). Furthermore, our experimental result shows that the concentration of FeCl2 plays a key role in the morphology of the γ-FeOOH. The optimized addition of FeCl2 with a concentration of 0.3−0.5 g L–1 yields atomically thin nanosheets. Further increase of FeCl2 concentration up to 0.6−0.8 g L–1 would result in a large number of small-sized nanoparticles as shown in Figure S1. The general reactions for the formation of γ-FeOOH can be simply written as follows: ACS Paragon Plus Environment


Page 8 of 24

°

hydrothermal, 15O C

2CuCl + 2NH3 · H2 O 2CuO + 2NH+4 + 2Cl1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

againg, R. T.

FeCl2 + 3H2 O + O2 FeOH 3 CuO template + 3Cl- + 3H+ againg, R. T.

FeOH 3 CuO template γ-FeOOHCuO template + H2 O NH3 ·H2 O

γ-FeOOH(CuO template) γ-FeOOH

(2)

(3)

(4)

(5)

2.2 Calculation of band structure. The spin-polarized band and electronic structure of our γ-FeOOH nanosheet with a thickness of one unit cell is first illustrated by density-functional theory (DFT) calculations, as shown in Figure 3a and b, respectively. The result indicates an indirect band gap character with the valence band maximum (VBM) at Q point (0, 0.5, 0.5) and the conduction band minimum (CBM) at Γ point of the γFeOOH nanosheet. The top valence bands and the bottom conduction bands are coming from opposite spin states. A DFT-GGA band gap of 0.420 eV is predicated for the 2D γ-FeOOH nanosheet which is slightly smaller than the band gap of 0.532 eV for bulk γ-FeOOH (Figure S2). For 3D γ-FeOOH bulk, the lowest band energy at a k-point near F along Γ─F line is nearly identical (only 0.005 eV higher) to the CBM at G, whereas for 2D γ-FeOOH nanosheet, the lowest band energy near the F point shift upwards slightly and is about 0.03 eV higher comparing to the CBM. Note that DFT in the standard Kohn-Sham formalism with the local density approximation (LDA) or the generalized gradient approximation (GGA) for exchangecorrelation functional will underestimate the band gap. The band dispersion and the relative trend of band gap value should be relatively more reliable. DOS analysis depicted in Figure 3b indicates that the electronic states near the CBM are mainly contributed by d states of Fe atom and the electronic states near the VBM are mainly composed of p states of O atom. The O atom bonded with four Fe atoms (O1) have larger net spin charge than its counterpart bonded with two Fe atoms and one H atom (O2). Such a DFT calculation is further verified by the optical absorption coefficients. Experimental measured absorption coefficients of γFeOOH nanosheet dispersion and theoretical coefficients from DFT-GGA calculation are presented in Figure 3e and f for comparison, respectively. A 0.15 eV smearing and a 0.20 eV scissor operator have been applied ACS Paragon Plus Environment

to obtain the theoretical spectra. Features from A to E in experimental spectra are well consistent with the

Page 9 of 24


theoretical results. Bearing in mind that the experiment is performed using solution of γ-FeOOH which can 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

be regarded as a diluted polycrystalline containing a lot of small crystals of γ-FeOOH nanosheets oriented randomly separated by solvent, whereas the theoretical results are corresponding to the polycrystalline condition without any dilution. Therefore, the absolute magnitude of absorption coefficients from experiment can be several orders smaller than the theoretical results.

Figure 3. (a) Band structure of γ-FeOOH nanosheet, α and β spins are displayed by blue and red lines respectively, (b) Total and projected density of states (DOS) for γ-FeOOH nanosheet.O1 denotes an O atom bonded with four Fe atoms whereas O2 denotes an O atom bonded with two Fe atoms and one H atom, (c) Unit cell employed in the band structure and DOS calculations. Fe, O, and H atoms are depicted by grey large spheres, red medium spheres, and white small spheres respectively, (d) Reciprocal lattice and special kpoints adopted to create the Brillouin zone paths in the band structure calculation, (e) Experimental optical absorption coefficients measured by an ultraviolet-visible spectrophotometry for aquous dispersion of γACS Paragon Plus Environment

FeOOH nanosheets, (f) Theoretical optical absorption coefficients with calculation geometry of γ-FeOOH


Page 10 of 24

polycrystalline. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.3 Hetero-stacking of ultrathin γ-FeOOH nanosheets on rGO matrix. The calculation of band structure has revealed the semiconductor nature of γ-FeOOH nanosheet, which implies poor electronic conductivity of γ-FeOOH. The poor electronic conductivity is unfavorable for application in LIBs. Therefore, the assynthesized γ-FeOOH nanosheets were subsequently interacted with rGO matrix by a hydrothermal method at a moderate temperature of 110 °C. According to our initial design, the mass ratio of FeOOH : GO is 6 : 4. After the reduction process, the actual ratio of FeOOH is slightly higher than 60 wt.%. The SEM image of γFeOOH@rGO nanocomposite in Figure 4a shows large amounts of sheet-like objects, indicating γ-FeOOH nanosheets are dispersed uniformly on the surface of rGO. The γ-FeOOH nanosheets, of which lateral size is much smaller than that of the matrix rGO nanosheets, are lined out in Figure 4b to distinguish γ-FeOOH nanosheets from rGO sheet more clearly. The existence of γ-FeOOH nanosheets is further confirmed by the Energy Dispersed X-ray Spectrometry (EDS) spectra of two different selected areas are shown in Figure 4d and e, respectively. Furthermore, two sets of SAED pattern from the γ-FeOOH and rGO matrix can be distinguished easily in Figure 4c, also demonstrating rGO prevents atomically thin γ-FeOOH nanosheets from self-stacking during sample preparation. We employed FT-IR spectroscopy to investigate the chemical interaction between the FeOOH nanosheets and rGO, as shown in Figure S3. For pure FeOOH nanosheets, the bands at 1618 and 3379 cm-1 are related to the vibration of –OH groups. These two peaks shift to a lower wavenumber for the FeOOH@rGO sample, indicating the electronic conjunction between the FeOOH and rGO sheets. X-ray Photoelectron Spectroscopy (XPS) (Figure S4) confirmes that only Fe, O and C elements are present in the sample. Two peaks at 713.8 and 727.6 eV can be assigned to Fe 2p3/2 and Fe 2p1/2 main peaks, respectively. The appearance of satellite bands near the Fe 2p main peaks is generally regarded as an indicator of Fe3+ valence state.

24

In addition, the C1s spectrum of FeOOH@rGO nanocomposites can be

resolved into two peaks centered at 284.5 and 287.8 eV, assigning to C-C and C=O bonds, respectively. The intensity of C-C bond is much stronger than that of C=O bond, demonstrating the successful reduction of GO to rGO in our process. In addition, it should be mentioned here that a higher hydrothermal temperature of 170 ACS Paragon Plus Environment

°C is disadvantageous to the homogenous dispersion of γ-FeOOH on rGO, as shown in Figure S5.

Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Typical (a) SEM, (b) TEM image and (c) SAED pattern of the γ-FeOOH@rGO nanocomposite, EDS spectra of two selected areas from (d) γ-FeOOH nanosheets and (e) rGO matrix, respectively.

2.4 Electrochemical properties of γ-FeOOH@rGO nanocomposites. The electrochemical properties of γFeOOH@rGO sample were characterized in coin cells with Li foil as the counter electrode. Figure 5a shows the cyclic voltammetry (CV) curves of the γ-FeOOH@rGO, which are collected at a scan rate of 0.1 mV s-1 in a potential window of 0.01−3.00 V versus Li+/Li. In the first discharge process, a small cathodic peak at 1.68 V indicates an insertion process of Li+ into γ-FeOOH lattice with the reduction of Fe3+ to Fe(3-x)+ (Equation 6). A broad peak is observed at 0.58 V, which can be attributed to the formation of solid electrolyte interface (SEI) layers and further reduction of Fe(3-x)+ to Fe0 (Equation 7). 6, 16 In the first charge process, an oxidation bump located in the range of 1.60−1.87 V refers to the oxidation of Fe0 to Fe3+. It can be observed that the broad peak at 0.58 V in the CV curves of the γ-FeOOH@rGO nanocomposites is lower than that of the pure γ-FeOOH nanosheets (0.72 V) (Figure 5c). This may result from the tuning effects of graphene layers to the formation potential of SEI layers. In the second cycle, the main cathodic peak shifts to about 0.7 V due to the irreversible structural change of γ-FeOOH@rGO nanocomposites after the first discharge. In the subsequent cycles, theACS reduction peaks oxidation peaks overlap well with those of the Paragon Plus and Environment


Page 12 of 24

second cycle, indicating excellent reversibility. The electrochemical reactions of the γ-FeOOH@rGO with 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

lithium can be described as follows: 6 FeOOH + xLi+ + xe- ↔ Lix Fe(3-x)OOH

(0

Bottom-up Approach Design, Band Structure, and Lithium Storage

Recommend Documents