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Mesoporous SnO2 nanostructures of ultrahigh surface areas by novel anodization Haidong Bian, Yayuan Tian, Chris Lee, Muk Fung Yuen, WenJun Zhang, and Yang Yang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09795 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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ACS Applied Materials & Interfaces

Mesoporous SnO2 nanostructures of ultrahigh surface areas by novel anodization Haidong Bian a,b, Yayuan Tian a,c, Chris Lee a,c, Muk-Fung Yuen a,c, Wenjun Zhang a,c, Yang Yang Li a,c,d,*

a

Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Kowloon, Hong Kong, China b Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China c Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong, China d City University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park, Nanshan District, Shenzhen, China * E-mail: [email protected]

Abstract Here we report a novel type of hierarchical mesoporous SnO2 nanostructures fabricated by a facile anodization method in a novel electrolyte system (an ethylene glycol solution of H2C2O4/NH4F) followed by thermal annealing at a low temperature. The SnO2 nanostructures thus obtained feature highly porous nanosheets with mesoporous pores well below 10 nm, enabling a remarkably high surface area of 202.8 m2/g which represents one of the highest values reported to date on SnO2 nanostructures. The formation of this novel type of SnO2 nanostructures is ascribed to an interesting self-assembly mechanism of the anodic tin oxalate, which was found to be heavily impacted by the anodization voltage and water content in the electrolyte. The electrochemical measurements of the mesoporous SnO2 nanostructures indicate their promising applications as lithium-ion battery and supercapacitor electrode materials.

Keywords: anodization, tin oxalate, tin oxide, mesoporous structures, Li-ion batteries, supercapacitors

ACS Paragon Plus Environment


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1. Introduction

Tin dioxide (SnO2), as an important n-type semiconductor, has been intensively studied due to its chemical stability, abundance, low cost, facile fabrication and environmental friendliness. Various applications based on tin dioxide have been reported, such as gas sensors,1-4 dye-sensitized solar cells (DSSCs),5, 6 lithium/sodium ion batteries (LIBs/NIBs),7-13 field effect transistors,14, 15 photocatalysts,3, 13

electrochemical catalysis16 and supercapacitors.3,

17

Considering that nanostructured materials

facilitate interesting properties and often outperform their bulk counterparts, SnO2 nanomaterials of rational structural designs would be highly desirable. The most common fabrication strategy for SnO2 nanomaterials is through the hydrothermal or solvothermal methods. Using these methods, SnO2 nanostructures of different morphologies have been successfully fabricated, including nanoparticles,18 nanosheets,22,

23

core-shell

hollow

nanospheres,5,

19

nanowires,4

nanotubes,20

nanorods,21

and 3D hierarchical architectures.24 Notably, the SnO2 nanostructures produced

using the hydrothermal/solvothermal methods generally possess a fairly low specific surface area (< 50 m2 g-1). To increase the surface area, some organic surfactants or capping reagents16,

25

are

required to reduce the nanoparticle aggregation during hydrothermal process. However, a post thermal treatment at high temperature is often needed to remove the surfactants/capping reagents for further application studies, resulting in easy collapse of the nanostructures. Other methods, such as synthesis,5

template

sol-gel,26

chemical

vapor

deposition

(CVD),2,

15

calcination-dissolution-dissolution (CDD) strategy,27 hot-bubbling synthesis28 and microwave synthesis,25 have also been developed for fabricating different SnO2 nanostructures. However, they generally entail either complex fabrication procedures or expensive facilities.

Recently, an anodization method has been reported for fabricating vertically aligned nanochanneled SnO2 directly grown on the tin substrate, presenting a simple, facile and low-cost approach to obtain SnO2 nanostructures.29 Furthermore, the direct bonding of the SnO2 nanostructures with the metal substrate is particularly attractive for potential applications such as gas sensor,30,

31

DSSCs,32 supercapacitors17 and Na-ion batteries.33 Nevertheless, the anodic SnO2

nanostructures reported to date display a relatively low surface area (~ 70 -- 90 m2 g-1).


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In this study, we report a novel type of hierarchical mesoporous SnO2 nanostructures fabricated by first anodizing tin in a novel electrolyte system (an ethylene glycol solution of H2C2O4/NH4F) to produce tin oxalate nanosheets which then undergo thermal decomposition at low temperature. The SnO2 nanostructures thus obtained feature highly porous nanosheets with mesoporous pores well below 10 nm, enabling a remarkably high surface area of 202.8 m2/g which represents one of the highest values reported to date on SnO2 nanostructures. The fabricated mesoporous SnO2 nanostructures were found promising LIB and supercapacitor electrode materials. Moreover, the mesoporous SnO2 exhibits high adsorption of dye molecules, indicating great promise for photocatalytic and DSSC applications.

2. Experimental

2.1. Anodization The anodization experiments was performed in a home-made two-electrode cell controlled using Keithley 2400 Sourcemeter with a Sn foil (0.15 mm thick, purity 99.7%, Fuxin metal products co., LTD, Baoji, 2.4 cm2 exposed to the electrolyte) as the anode and a Pt coil as the cathode. Prior to anodization, the tin foils were ultrasonically cleaned in acetone and deionized water, rinsed with ethanol, and then dried in a nitrogen stream. The electrolyte used was an ethylene glycol (EG, less than 0.2 wt.% H2O, International Laboratory, USA) solution of H2O, NH4F (≥ 98.0 wt%, Sigma-Aldrich) and oxalate acid (≥ 98.0 wt%, Sigma-Aldrich). The anodization voltages applied range between 2 and 40 V. The molar concentrations of NH4F and H2C2O4 varied between 0 M-0.1 M and 0.4 M-0.1 M, respectively. The as-anodized samples were carefully rinsed with deionized water and ethanol, and dried with N2.

2.2. Thermal decomposition The annealing process was carefully conducted in air at 220 oC for 10 h with a heating rate of 5 o

C/min (notice the low melting point of tin at ~232 °C). SnO2 structures anodized in an electrolyte of

different water content (typically, 0, 3, 15, 30, 60, or 100 vol.%) were denoted as “SnO2 (from w vol.% H2O)”, where w is the volume percentage of H2O in the electrolyte, e.g., SnO2 (from 15 vol.% H2O) stands for the SnO2 structures anodized in an electrolyte containing 15 vol.% of H2O.



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2.3. Material characterizations Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations were conducted on field effect SEM (Philips XL-30 FESEM) and TEM (JEOL TEM, 2100F FEG), respectively. Powder X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (D2 Phaser) with Cu-Kα radiation (λ= 1.5405 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG ESCALAB 220i-XL X-ray photoelectron spectrometer using a monochromatic Al Kα X-ray beam (1486.6 eV). The binding energy (BE) of the elements was calibrated to a carbon binding energy of 284.8 eV. The surface area and total pore volume were determined using a nitrogen adsorption and desorption instrument (Quantachrome Autosorb iQ). Brunauer Emmett Teller model (BET) was used to calculate the surface area and Barrett-Joyner-Halenda (BJH) model was employed to analyze the pore size distribution of fabricated mesoporous SnO2 nanostructures. For BET tests, the as-annealed anodic films were scratched off the substrate.

2.4. Electrochemical characterizations LIB tests were carried out using CR2023 coin cells with Li foil as the counter and reference electrode and Whatman GF/F glass microfiber membrane as a separator. The mesoporous SnO2 nanostructures, carbon black and polyvinylidene fluoride (PVDF) were mixed at a weight ratio of 80:10:10 to form slurry, and subsequently pasted onto a copper foil and dried at 100 oC overnight in vacuum. An ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 vol.%) solution of LiPF6 (1 M) was used as the electrolyte. The cyclic voltammetry (CV) performance was tested on a CHI660E electrochemical workstation (CH Instruments, Inc.) with a scan rate of 0.2 mV s-1 in the range of 0 to 2.0 V versus Li+/Li for 4 cycles. The charge-discharge measurements were carried out in a potential window of 0 - 2.0 V vs. Li+/Li at a current rate of 0.5 C using an electrochemical testing system (Neware Technology Co., China). The rate performance of the electrode was evaluated at different current rates from 0.1 C to 10 C.

The capacitance tests were carried out using a three-electrode system with a platinum sheet as the counter electrode, an Ag/AgCl reference electrode, and the SnO2 nanostructures grown on the tin


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substrate as the working electrode. The backside of the working electrode was covered with nail polish. NaOH aqueous solution (0.1 M) was used as the electrolyte. Cyclic voltammetry (CV) characterizations were performed between -0.6 and 0.4 V vs. Ag/AgCl at different sweep rate (5, 10, 20, 50, 100, and 200 mV s-1) on a CHI660E electrochemical workstation. Galvanostatic charge-discharge measurements for supercapacitor were performed at different current densities of 0.1, 0.2, 0.5, 1 and 2 mA cm-2 between -0.6 and 0.4 V vs. Ag/AgCl. Cycling stability was tested by galvanostatic charge-discharge (GCD) measurements at 0.5 mA cm-2 between -0.6 and 0.4 V vs. Ag/AgCl. All the electrochemical experiments were carried out at room termperature.



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3. Results and discussion 3.1. Fabricated mesoporous SnO2 nanosheets Mesoporous SnO2 triangular nanosheets were directly formed on the tin substrate via a facile anodization method followed by thermal decomposition at low temperature. Usually, previous studies on anodic SnO2 are limited to the morphology of self-aligned nanochannels, fabricated by anodization in aqueous solution. In this study, an exotic triangular nanosheet structure was first synthesized through anodization in a viscous NH4F/H2C2O4 (0.1 M/0.1 M) ethylene glycol solution containing 3 vol% of water. The triangular SnC2O4 nanosheets (~ 3 µm of side length) were self-assembled and stacked on the tin substrate, exhibiting a fish scale-like morphology (Fig. 1a). After a low-temperature (220 oC) thermal annealing process to decompose SnC2O4, SnO2 nanosheets with slightly changed appearance were obtained (Fig. 1b). Some small dark spots were observed on the SnO2 nanosheets (Fig. 1b), indicating a change of the inner structure, which was confirmed by the TEM results (Fig. 1 c and d). The triangular SnO2 nanosheets were not found to be compact, but consisted of loosely interconnected SnO2 nanoparticles. The SAED measurements (inset of Fig. 1c) revealed the polycrystalline nature of the SnO2 nanosheets. The particle size distribution was fitted to the Gaussian curve and the average particle size was found to be ~4 nm, whereas the interparticle pores are around 2.5 nm wide. The TEM measurements (Fig. 1d) confirmed that the interconnected nanoparticles rendered a fairly porous structure, in good agreement with the BET measurements (which shows remarkably high specific surface area, and will be discussed in the following part). Well-defined lattice fringes with the lattice spacing of 0.27 nm were observed in the high resolution TEM (HRTEM) images (Fig. 1f), corresponding to the (101) lattice plane of rutile SnO2.3

The as-anodized material appeared to be a white film grown on the tin substrate (inset of Fig. 2a), which proved to be tin oxalate (JCPDS card no. 51-0614) through XRD measurements (Fig. 2a(i)). After thermal treatment (220 oC 10 h), the XRD peaks of tin oxalate disappeared whereas two sets of new peaks arose: one set at 26.6, 33.9 and 51.8° indexed to the (110), (101) and (211), planes of rutile SnO2 (JCPDS card no. 41-1445), respectively, and the other set at 18.3, 29.9, 33.3, 37.2, 47.8, 50.8 and 57.4° indexed to (001), (101), (110), (002), (200), (112) and (211) planes of SnO (JCPDS card no. 06-0395), respectively. Note that the presence of SnO phase is not resulted from the thermal decomposition of SnC2O4, but the partial thermal oxidization of the Sn substrate treated.


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This was confirmed by the XRD pattern collected on the film scratched off the substrate after thermal annealing (Fig. S1), showing only SnO2 peaks. The film color change upon thermal annealing from white to yellow (Fig. 2 insets) also indicates the conversion from SnC2O4 to SnO2.

The chemical states of the as-annealed triangle nanosheets were studied using XPS (Fig. 2 b and c). The Sn 3d spectrum (Fig. 2c) displayed the 3d5/2 and 3d3/2 peaks at the binding energy of 495.7 eV and 487.2 eV, 8.5 eV apart from each other, indicating that the film surface is tin dioxide, not tin monoxide.17 The nitrogen adsorption and desorption isotherms indicated a BET surface area of 203 m2 g-1 for the as-annealed sample (Fig. 2d), which is remarkably higher than the previously reported anodic porous SnO2 nanostructures.29, 30 The measured isotherms exhibited Type IV characteristics, which is typical for mesoporous materials. From the pore size distribution calculated using the Barrett-Joyner-Halenda (BJH) method from the adsorption branch (Fig. 2d inset), it can be concluded that the pores are generally smaller than 5 nm (most of the pores are around 2.5 nm), in good agreement with the TEM results. The total pore volume is 0.4004 cm3 g-1 at P/P0 = 0.99442 and the porosity is calculated to be 73.6%. The findings here are compared in Table 1 with other previously reported SnO2 nanostructures in terms of morphology, fabrication method, surface area and surfactant usage.



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Table 1. Comparison with previously reported SnO2 nanostructures. Morphology

Mesoporous SnO2 nanosheets

Surface

Fabrication method

Surfactants

Ref.

203

Anodization method followed by thermal annealing

/

Our work

area (m2 g-1)

Hierarchical SnO2 nanostructures Quintuple-shelled SnO2 hollow microspheres Double-shelled SnO2 yolk-shell structure

27.8

Hydrothermal method

Sodium citrate

3

38.74

Hard template method

/

5

13

One-pot spray pyrolysis method

/

7

SnO2 nanoboxes

60

Template-engaged coordinating etching process

/

11

Porous SnO2 nanowire bundles

27.72

Precipitation method

PEG600

13

SnO2 nanoparticles

263

Template-assisted hydrothermal method

Tetradecylamine

16

Hierarchical SnO2 nanosheet arrays

28.76

Hydrothermal method

/

22

SnO2 nanocrystals

189.4

Hydrothermal method

Acetic acid

25

Sol-gel method

Triblock copolymer

26

Mesoporous SnO2 powders

253

Ultrasmall SnO2 nanocrystals

90.78

Hot-bubbling Synthesis method

Oleic acid

28

Porous SnO2 nanochannels

99.1

Anodization method

/

29

Porous SnO2 nanostructures

72

Anodization method

/

30

Hierarchical SnO2 octahedra

27.5

Rapid sonochemical process

/

34

Porous tin oxide nanosheets

37.39

Calcining SnS2 nanosheets

/

35

Hierarchical SnO2 microspheres

19

Solvothermal process

/

36

Hollow nanotubular SnO2

185.15

Template assisted methods

/

37

Hierarchical SnO2 microspheres

22

Solvothermal method

TBAOH

38

Porous SnO2 hierarchical structures

28

Hydrothermal method

/

39

3.2. Growth mechanism The proposed growth mechanism of the mesoporous SnO2 nanosheets is illustrated in Fig. 3.


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Time-dependent structural evolution (Fig. 4) suggested that, surface etching of the Sn substrate first took place upon the application of electric field, followed by gradual formation of the triangular SnC2O4 nanosheets. The surface etching is the result of Sn releasing Sn2+ cations under the electric field. In the previously reported aqueous anodization systems, the Sn2+ cations further react with the OH- anions in water that were drawn to the Sn anode, generating a compact tin(II) oxide hydroxide40 or tin oxide layer.29, 41 By contrast, this work uses the ethylene glycol (EG) solution (of oxalic acid, NH4F and a small amount of water), which contains abundant C2O42- ions but few OH- ions. Furthermore, the high resistivity and viscosity of the EG solution largely restricted the mobility of Sn2+ ions in the electric field. Thus, the Sn2+ ions are more likely to react with C2O42- and form insoluble SnC2O4 precipitates (eq. 1). As shown in Fig. 4c, after 10 sec of anodization, tiny nanosheets were found scattered on the surface where the etching pits were located, which quickly grew into self-assembled nanosheets within minutes (Fig. 4d and e). Sn2+ + C2O42− → SnC2O4 ↓

(1)

The geometric restriction among the neighbor nanosheets, together with the applied electric field, affected the growth directions of the nanosheets, leading to triangular nanosheets arrays (Fig. 4f-h). The SnC2O4 nanosheets are then converted to mesoporous SnO2 through a low-temperature (220 oC, below the melting point of tin) thermal annealing process in air (eq. 2). The mild thermal treatment not only effectively produces highly porous SnO2 nanostructures (Fig. 1c, d) with the aid of the CO2 gas produced from the thermal decomposition of SnC2O4, but also largely restricts grain growth so that the triangular nanosheet morphology can be well maintained (Fig. 1b). SnC 2O4 + O2 → SnO2 + 2CO2 ↑

(2)

3.3. Effects of anodization voltage and F- content The morphology of anodic SnC2O4 is affected by various anodization parameters, such as the anodization voltage and electrolyte composition. Different SnC2O4 morphologies were observed at different anodization voltages from 2 to 40 V (Fig. 5). At lower voltages of 2 and 5 V, the rates of Sn dissolution and Sn2+ migration are greatly lowered, producing SnC2O4 nanosheets deposited loosely on the film surface (Fig. 5a and b). Notice that, for the samples anodized at 5 V, vertically standing SnC2O4 nanosheets were fabricated. Upon increasing the voltage to 10 V, self-assembled SnC2O4



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nanosheet arrays were finally obtained. However, when the voltage further boosted to 40 V, only chaotic granular SnC2O4 (ranging from 100 nm to 50 µm, Fig. 5d) were obtained, indicating too rapid for a production of SnC2O4 allowing no time for self-assembly.

The concentration of F- is another important factor. Previous studies have shown that fluorine atoms absorbed onto a specific crystalline facet can lower its surface free energy.42-44 As a result, growth along the particular direction is greatly restricted and 2-D nanosheet structures can be obtained. Similarly, in this work, F- was found to facilitate anodic SnC2O4 to form the triangular nanosheets structures. Four types EG/water electrolyte of 0.1 M H2C2O4 but different NH4F concentration (0, 0.05, 0.1, and 0.4 M) were compared (Fig. S2). When there was no NH4F in the electrolyte, only a compact layer was obtained. With 0.05 M NH4F, granular triangular nanostructures were generated on the substrate surface. When the NH4F concentration was increased to 0.1 M, self-assembled triangular nanosheet structures were produced, whereas with excess NH4F at 0.4 M, a mixture of nanorods and granular particles were resulted.

3.4. Effect of water content Water content in EG solution is another important factor controlling the anodic SnC2O4 structures. Water plays a very important and complicated role during the formation of tin oxalate. The interactions between the dissolved ions and the dipolar water molecules (or EG molecules) form hydration shells, driven by the ion-dipole force and hydrogen bonds. The hydration shells are affected by the ion charge, radius, and concentration. The migration of the hydrated ions under the electric field is greatly affected by the conductivity and viscosity of the electrolyte (Table S1), exerting great impact on the final morphology of the anodic product (e.g., SnC2O4). Here different water contents (0, 3, 15, 30, 60, and 100 vol%) of the EG solution were tested (Fig. 6). With no water in the electrolyte, tin dissolution took place slowly, allowing sufficient time for SnC2O4 to self-assemble into nanosheet structures, with each nanosheet of several µm big and ~200 nm thick (Fig. 6a). These nanosheets were aligned orthogonally to each other with cross interconnection, generating a rugged surface morphology. When the water content was increased to 3 vol.%, a more compact layer consisting of merging nanosheets was observed (Fig. 6b). A higher water content (15 or 30 vol.%) resulted in more rapid diffusion and reaction of the Sn2+ and C2O42-


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ions, producing granular SnC2O4 with wide-ranging particle sizes (~50 nm to ~10 µm) (Fig. 6c and d). With the water content further increased to 60 or 100 vol%, vigorous anodization reactions created a thin film comprised of granular aggregates. It was found that the morphology of the SnC2O4 structures anodized with different water content maintained after the heat treatment (220 oC for 10 h). The corresponding SEM and TEM images of the samples anodized without water addition are presented in Fig. S3 (others only showing small granular structure changes are not shown). The annealed samples were characterized by XRD (Fig. S4). The peaks at 26.6, 33.9 and 51.8° can be indexed as rutile SnO2 (JCPDS card no. 41-1445), the thermal decomposition product of SnC2O4. Other XRD peaks were attributed to SnO (JCPDS card no. 06-0395) which was generated due to oxidation of the tin substrate (as discussed earlier). The nitrogen adsorption-desorption isotherms of the annealed samples (Fig. S5) reveals their surface areas of 189.5, 70.8, 81.1, 59.1 and 52.9 m2 g-1, corresponding to samples anodized with 0, 3, 15, 30, 60, and 100 vol%, respectively (Table S2). A comparison of surface area for samples of different morphologies shows that the nanosheets, no matter whether triangular or rectangular, enable much larger surface area and pore volume than other morphologies, indicating that the nanosheet morphology can better facilitate the fabrication of mesoporous structures from thermal decomposition. As will be presented next, the mesoporous SnO2 structures with large surface area, particularly those obtained with 3 vol% water in the electrolyte, are promising for potential applications in LIB and supercapacitors.

3.5 Application studies Due to its large surface areas and interconnected pores, the novel mesoporous SnO2 nanostructures reported here are potentially attractive for a range of applications, such as, dye adsorbents (testing details are shown in supporting information and Fig. S6), LIBs, and supercapacitors.

3.5.1. For Li-ion battery application SnO2 is a very promising LIB anode material due to its high theoretical capacity (790 mA h g-1).45, 46 Herein, the mesoporous SnO2 nanosheets (anodized with 3 vol% H2O), denoted as denoted



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as “SnO2 from 3% H2O”) readily assembled into a coil cell as an anode material for LIBs. Fig. 7a shows the cyclic voltammograms (CV) profiles for the first four cycles at a scan rate of 0.2 mV s-1 with the potential window of 0.0 - 2.0 V. During the first cathodic scan, a notable reduction peak at 0.78 V was observed (disappeared in the following cycles), which can be attributed to the irreversible reduction of SnO2 to Sn (eq. 3). Another apparent peak at 0.08 V indicated the formation of LixSn alloy (eq. 4).20, 21, 24 The cathodic peaks appeared in the following cycles can be ascribed to formation of the solid electrolyte interface (SEI) layer and the alloy reactions (eq. 4).20 Two anodic peaks emerged at 0.6 and 1.25 V, corresponding to reversible dealloying of LixSn (eq. 4) and the partially reversible decomposition of the SEI layer, respectively. SnO 2 + 4 Li + + 4e - → Sn + 2Li 2 O Sn + xLi + + xe - ↔ Li x Sn (0 < x < 4.4)

(3) (4)

Fig. 7b shows galvanostatic discharge/charge (GCD) profiles up to 50 cycles at a current rate of 0.5 C. The discharge and charge capacities of the second cycle are 1466 and 1225 mA h g-1, respectively. This extra high capacity (larger than theoretical value of SnO2) may be attributed to the SEI layer and irreversible formation of Li2O.47, 48 Sloping plateaus at ~0.9 and 0.5 V due to partially reversible formation of the SEI layer and Li+ insertion/extraction can be observed from the GCD profiles, in good agreement with the CV results. Long-term cycling performance of the SnO2 anode was tested at 0.1 C for the first cycle and then at 0.5 C for the rest cycles in the voltage range of 0.0 – 2.0 V vs. Li+/Li (Fig. 7c). The electrode exhibited a capacity of 2500 mA h g-1 for the 1st cycle at 0.1 C, which dropped to 1466 mA h g-1 after the 2nd cycle at 0.5 C, and gradually decreased to ~ 540 mA h g-1 after 50 cycles, The observed lithium ion storage capability on mesoporous SnO2 outperforms the previously reported SnO2 nanoparticles25 and SnO2 nanotube arrays49 (details are shown in Table S3). The Coulombic efficiency was low for the first few cycles (~ 45% and 84% for the 1st and 2nd cycles, respectively), consistent with the corresponding capacity loss observed, which can be attributed to the irreversible formation of Li2O (eq. 3) during the initial charging processes. In the subsequent cycles, the Coulombic efficiency gradually rose to 97% and above, indicating good reversibility of the electrochemical reactions. The stepwise measurements at 0.1, 0.2, 0.5, 1, 2, 5 and 10 C revealed the discharge capacities of 2064, 1300, 876, 605, 453, 321, and 136 mA h g-1, respectively. With the discharge current rate returning to 0.5 C, the discharge capacity recovered to


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724 mA h g-1, ~ 83 % of the original capacity at the same discharge rate. For comparison, other SnO2 nanostructures anodized with other water contents (0, 15, 30, 60, 100 vol%, denoted as “SnO2 from v% H2O” where v is the volume percentage of H2O in the electrolyte) were tested as LIB anodes (Fig. S6). The electrode based on “SnO2 from 0% H2O” (nanosheet morphology) delivered a retaining capacity of 465 mA h g-1 after 50 cycles at 0.5 C, slightly lower than “SnO2 from 3% H2O”. The other electrodes exhibited much lower capacities and stability (146, 240, 148, and 118 mA h g-1 after 50 cycles at 0.5 C, for 15, 30, 60, 100 vol%, respectively). The outstanding performance observed on the mesoporous SnO2 nanosheets, fabricated with a low water content (0 or 3 vol%), can be ascribed to their interesting morphology, with each nanosheet made from nanoparticle (~ 4 nm) assemblies, featuring high surface areas and porosity (Table S2), which provide a large working surface, easy access to the electrolyte, and short pathway for Li+ diffusion in the electrode. Furthermore, the mesoporous structures can effectively accommodate the volume change during the lithiation/delithiation process. By contrast, the other SnO2 films obtained with higher water contents possess lower surface areas and porosities, more vulnerable to pulverization and electrode degradation.

3.5.2. For supercapacitor application SnO2, possessing high conductivity and exhibiting a pseudo-capacitive behavior, has been investigated as supercapacitor electrode materials.3,

17, 50, 51

In this study, the mesoporous SnO2

nanosheets grown on the tin foil (anodized at 20 V for 10 min with 3 vol% H2O) were directly used as supercapacitor electrodes without further treatment, whose capacitive performance was evaluated by CV and charge/discharge profiles (Fig. 8). The CV curves of the SnO2 electrodes were characterized at various scan rates ranging from 5 to 200 mV s-1 between -0.6 and 0.4 V in 0.1 M NaOH (Fig. 8a). The CV curves of the electrodes show pseudo-capacitive behaviors, attributed to the electrolyte cations (Na+) adsorbed onto the SnO2 surface (eq. 5):52 SnO2 + Na+ + e- ↔ (SnO2- Na+ )surface

(5)

The corresponding area specific capacitances were calculated to be 43, 31, 25, 17 and 14 mF cm-2 at 5, 10, 20, 50, 100, 200 mV s-1, respectively (Fig. 8b). The fabricated mesoporous SnO2 electrode exhibits a much higher capacitance than the electrochemically deposited SnO2 film



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structure (118.4 µF cm-2),52 SnO2 nanowires (12 mF cm-2),53 and SnO2 nanobranch (40.5 µF cm-2).54 This dramatic improvement is possibly due to the greatly enhanced surface area which provides more active sites for the electrolyte ions. The specific capacitance achieved by the mesoporous SnO2 nanosheets is considered very attractive, compared with the previously reported anodic or deposited systems of similar film thickness, such as, the Fe3O4@SnO2 hybrid structures (7.013 mF cm-2),50 TiO2 nanotube arrays (62.5 µF cm-2)55 and H-doped black TiO2 nanotube arrays (3.24 mF cm-2).56 The galvanostatic charge/discharge (GCD) measurements were carried out at different current densities (0.1, 0.2, 0.5, 1 and 2 mA cm-2) between -0.6 and 0.4 V vs. the Ag/AgCl standard electrode (Fig. 8c). Sloping potential plateaus were observed around -0.45 V, particularly at low current densities, revealing pseudo-capacitive behaviors of the electrode. No obvious IR drop appeared for the discharge curves, indicating a low internal resistance of the electrodes. The long-term cycling stability of the electrodes was examined at 0.5 mA cm-2 (Fig. 8d), which exhibited an initial capacitance retention rate boost (from 85.1% to 92.6%), possibly due to the gradual ion (mainly Na+) adsorption onto the mesoporous surface during the first few cycles. The capacitance then decreased mildly till reaching a fairly stable stage, achieving a final retention rate of 93.4% after 8000 cycles. The above-tested mesoporous SnO2 nanosheet structures (anodized with 3 vol% H2O) were found particularly favorable for supercapacitor electrode applications. The other SnO2 structures (obtained with other water contents) displayed drastically poorer performance, except the sample of “SnO2 from 0% H2O” which delivered slightly lower performance with its area specific capacitances calculated to be 37, 27, 21.8, 17.6, 15.2 and 12.6 mF cm-2 at 5, 10, 20, 50, 100, 200 mV s-1, respectively (Fig. S7).

4. Conclusion In summary, hierarchical mesoporous SnO2 nanostructures have been conveniently synthesized using the new anodization method, which displays a very impressive surface area of 203 m2 g-1, a largest specific surface area ever reported on SnO2 nanomaterials. These unique triangular SnO2 nanosheets offer attractive benefits for LIB and supercapacitor applications, such as, more electroactive sites assessable by the electrolyte, short diffusion or migration path for the electrolyte ions, and solid interconnection between the building nanoparticles, enabling a high LIB capacity (540 mA h g-1 at 0.5 C after 50 cycles) and an attractive supercapacitor performance (43 mF cm-2,


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with 93.4% capacitance retained after 8000 cycles). Furthermore, the promising dye (methylene blue and Rhodamine B) adsorption capability of the novel mesoporous SnO2 indicates its great potential in photocatalytic and DSSC applications.

Acknowledgements This

work

was

jointly

supported

by

Shenzhen

Basic

Research

Project

No.

JCYJ20150630140546704, the Research Grants Council of Hong Kong (Project 9042231 (CityU 11302515)), and the Centre for Functional Photonics at the City University of Hong Kong.

ASSOCIATED CONTENT Supporting Information Experiment details on dye adsorption; the detailed XRD result of scratched SnO2 from the substrate (Fig. S1); SEM images of as-anodized samples with different F- contents (Fig. S2); SEM and TEM images of as-fabricated SnO2 nanosheet structures without water addition (Fig. S3); XRD patterns (Fig. S4), nitrogen adsorption-desorption isotherms (Fig. S5) and LIBs results (Fig. S7) of as-fabricated samples with different water contents; supercapacitance performance of as-fabricated SnO2 nanosheets without water addition (Fig. S8). The material is available free of charge via the Internet at http://pubs.acs.org.

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Fig. 1. SEM images of the samples anodized at 20 V for 10 min in the NH4F (0.1 M) and H2C2O4 (0.1 M) ethylene glycol solution containing 3 vol% of water: a) as-anodized SnC2O4; b) as-annealed SnO2 (heated at 200 oC for 10 h); c,d) low and high magnification TEM images of SnO2, with the inset showing the SAED diffraction rings which can be indexed to rutile SnO2; e) histogram showing typical particle size distribution from the TEM measurements; f) HRTEM image of nanoparticles in the SnO2 nanosheet.



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Fig. 2. a) XRD patterns of the as-anodized SnC2O4 (i) and the as-annealed SnO2 (ii) on tin substrates (anodized at 20 V for 10 min in the NH4F (0.1 M) and H2C2O4 (0.1 M) ethylene glycol solution containing 3 vol% of water). The inset optical photographs show the colour change after thermal annealing. b,c) Survey and high-resolution Sn 3d XPS spectra of the as-annealed SnO2. d) Nitrogen adsorption-desorption isotherms of the as-annealed SnO2 with the inset showing the corresponding pore size distribution.


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Fig. 3. Schematic illustration of the self-assemble process of SnC2O4 nanosheets grown on the tin substrate by anodization, which are then converted to mesoporous SnO2 through thermal decomposition treatment. Mesoporous SnO2 nanosheets shown by schematic graphs a) and TEM images b).



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Fig. 4. SEM images of tin substrate anodized in an EG solution (3 vol.% H2O) of NH4F (0.1 M) and H2C2O4 (0.1 M) at 20 V for different time: a) 0 s, b) 5 s, c) 10 s, d) 30 s, e) 60 s, f) 180 s, g) 5 min, h) 10 min, and i) 20 min. Scale bars: 5µm.


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Fig. 5. Surface morphologies of the samples anodized at 2 (a) , 5 (b), 10 (c), and 40 (d) V for 10 min in NH4F (0.1 M) and H2C2O4 (0.1 M) ethylene glycol solution containing 3 vol% of water.



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Fig. 6. SEM images of the samples anodized at 20 V for 10 min in NH4F (0.1 M) and H2C2O4 (0.1 M) ethylene glycol solution with different water content: 0 (a), 3 (b), 15 (c), 30 (d), 60 (e), and 100 (f) vol %. Scale bars: 10 µm.


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Fig. 7. Electrochemical performance of the LIB anode based on the mesoporous SnO2 nanosheet structures (anodized at 20 V for 10 min in the NH4F (0.1 M) and H2C2O4 (0.1 M) ethylene glycol solution containing 3 vol% of water): a) The first four cyclic voltammetry curves of the electrode at a scan rate of 0.2 mV s-1 between 0.0 and 2.0 V vs. Li+/Li, b) the charge/discharge profiles for the 2nd, 5th, 10th, 20th, 30th, 40th and 50th cycles between 0.0 and 2.0 V at a current rate of 0.5 C, c) the cycling performance at 0.5 C and the corresponding Coulombic efficiency of the electrode, d) the rate discharge capabilities of the electrode.



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Fig. 8. Electrochemical performance of the supercapacitor electrode based on the mesoporous SnO2 nanosheets grown on tin foil (anodized at 20 V for 10 min in the NH4F (0.1 M) and H2C2O4 (0.1 M) ethylene glycol solution containing 3 vol% of water): a) Cyclic voltammetry curves at different scan rates (5, 10, 20, 50, 100, 200 mV s-1) between -0.6 and 0.4 V vs. Ag/AgCl; b) the area specific capacitance at different scan rates; c) galvanostatic charge-discharge curves of the electrode at different current densities; d) the cycling stability at 0.5 mA cm-2 with the insert showing charge-discharge curves of the final cycles.


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TOC Graphic

A novel type of hierarchical mesoporous SnO2 nanostructures, with a remarkably high surface area of 202.8 m2/g, was fabricated by a facile anodization method in a novel electrolyte system.


Mesoporous SnO2 Nanostructures of ... - ACS Publications

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