Synthesis of Monodisperse Mesoporous TiO2 Nanospheres from a

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Synthesis of Monodisperse Mesoporous TiO2 Nanospheres from a Simple Double-Surfactant Assembly-Directed Method for Lithium Storage Hongwei Zhu, Yesheng Shang, Yunke Jing, Yang Liu, Yupu Liu, Ahmed Mohamed El-Toni, Fan Zhang, and Dongyuan Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06534 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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ACS Applied Materials & Interfaces

Synthesis of Monodisperse Mesoporous TiO2 Nanospheres

from

a

Simple

Double-

Surfactant Assembly-Directed Method for Lithium Storage Hongwei Zhu†, Yesheng Shang†, Yunke Jing†, Yang Liu†, Yupu Liu†, Ahmed Mohamed El-Toni‡, Fan Zhang†*, Dongyuan Zhao†* † Laboratory of Advanced Materials and Department of Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China. ‡King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia

ABSTRACT Exploring facile and reproducible methods to prepare mesoporous TiO2 nanospheres is crucial for improving the performance of TiO2 materials for energy conversion and storage. Herein we report a simple and reproducible double-surfactant assembly-directed method to prepare monodisperse mesoporous TiO2 nanospheres. A double surfactant system of n-dodecylamine (DDA) and Pluronic F127 is adopted to control the hydrolysis and condensation rates of tetrabutyl titanate in a mixture of waterand alcohol at room temperature. In this process, the diameter size of mesoporous TiO2 nanospheres can simply be tuned from ~ 50 to 250 nm by varying the concentration of H2O and 1

ACS Paragon Plus Environment


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surfactants. The double-surfactant system of n-dodecylamine (DDA) and F127 acts a great role in determining the size, morphology, and monodispersity of mesoporous TiO2 nanospheres to reduce agglomeration during the sol-gel process. The resultant mesoporous anatase TiO2 nanospheres after solvothermal treatment at 160 °C are built of interpenetrating nanocrystals with a size of ~ 10 nm, which are arranged to obtain a large number of connecting mesopores. Mesoporous TiO2 nanospheres with a small diameter size of around 50 nm possess a high surface area (~ 160 m2/g) and mesopores with sizes of 4 - 30 nm. The small diameter size, high crystallinity and mesoporous structure of TiO2 nanospheres lead to excellent performance in the cycling stability and rate capability for lithium ion battery. After 500 cycles, the monodisperse mesoporous TiO2 nanospheres exhibit a charge capacity as high as 156 mAhg-1 without obvious fade, and the coulombic efficiency can reach up to 100 %.

KEYWORDS: Mesoporous Materials; Titania Colloids; Nanospheres; Synthesis; Lithium Storage; Surfactant Assembly; Sol-gel process

INTRODUCTION Mesoporous TiO2 materials have attracted a great deal of attention in the past decade due to their outstanding advantages such as high surface area, good chemical and thermal stability.1-11 The mesostructure could supply more active sites with higher accessibility during reaction process and promote the diffusion of products as well as reactants in comparison with bulk materials.12–23 Furthermore, mesoporous TiO2 materials with a large precision, adjustable particle size and morphology are also desirable for applications, because they influence the structural characteristics related to physical and chemical properties significantly.24-28 Among the various morphologies, spherical TiO2 materials are favorable for the fabrication of uniform and close-packed particulate networks, showing excellent performance in environmental and

energy fields.29-31

Many methods have been developed to prepare the mesoporous TiO2 spheres, such as hydro/solvothermal, sol-gel, and templating approaches.32-35 TiO2 spheres with a large 2


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diameter size on micrometer scale have been realized in previous research.36-37 But large diameter sizes increase the diffusion length of reactants and weaken the application performance greatly. So reducing the diameter size of mesoporous TiO2 spheres to nanoscale is of great significance in enhancing their performance. Particularly, in the fields of lithium battery, small particle size can decrease the Li-ion diffusion length and facilitate electrolyte penetration to improve the performance of materials. However, to our best knowledge, there is no report on the synthesis of monodisperse mesoporous TiO2 nanospheres with a controllable particle size on nanoscale via simple and reproducible process to date. The classical Stöber method is well-known to synthesize colloidal SiO2 nanospheres by hydrolysis and condensation of silicon alkoxides (TEOS) in an alcohol/water solution.38-39 In the process, the diameter size and spherical morphology of the resultant silica colloids can be tuned excellently by changing the synthesis parameters. So the sol-gel process is an attractive method because of its many advantages, such as simplicity, ultra homogeneity and low proceeding temperature. However, the sol-gel process of TiO2 precursors is greatly different from that of SiO2 ones due to their higher chemical reactivity. The hydrolysis and condensation reaction rate of TiO2 precursors is too fast to control and results in undesired phase separation to produce polydispersed amorphous particles with serious agglomeration.40-46 It is a significant challenge to develop a easy and reproducible sol-gel method to control hydrolysis and condensation reaction rate of the precursors and prepare monodisperse mesoporous TiO2 nanospheres. In this paper, we demonstrate a double-surfactant assembly-directed method to prepare the monodisperse TiO2 colloidal nanospheres by hydrolysis and condensation of tetrabutyl titanate (TBOT) in a mixture of water and alcohol at room temperature. The diameter size of mesoporous TiO2 nanospheres can be tuned from ~ 50 to 250 nm by changing the concentration of H2O and surfactants. Moreover, the morphology and monodispersity can also be adjusted by the double-surfactant system in the process. Solvothermal treatment of the TiO2 colloid nanospheres can result in a high surface area. The resultant mesoporous TiO2 nanospheres with a diameter size about 50 nm possess a high surface area (~ 160 m2/g) and mesopores size of 4 - 30 nm. As a result, the monodisperse mesoporous TiO2 nanospheres deliver a high capacity of ~ 209 mAh g−1 3



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for 1 C, showing superior high-rate performance and long-term life (a retention capacity of 156 mAh g−1 up to 500 cycles at the current density of 1 C).

EXPERIMENTAL SECTION Materials. Poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymers Pluronic F127 (Mw = 12600, PEO106PPO70PEO106) and P123 (Mw = 5800, PEO20PPO70PEO20) were got from Acros Corp. n-dodecylamine (DDA) and tetrabutyl titanate (TBOT) were purchased from Sigma-Aldrich. Ethanol (analytical grade) was got from Shanghai Chemical Corp. Deionized water was used for all experiments. All chemicals were used as received without further purification. Preparation of Mesoporous TiO2 Nanospheres. In a typical synthesis, 0.93 g of n-dodecylamine and 1.5 g of Pluronic F127 were mixed and soluted in 150 ml of ethanol. After stirring for 1 h, 15 ml of water was added into the above solution. Then, 1.2 g of TBOT was added dropwise into the solution under vigorous stirring at room temperature. After other stirring for 1 h, the white colloid product remained static for 12 h and then centrifuged. The as-made TiO2 nanospheres were washed for three times using ethanol and then dried at 60 °C in air. The crystallization and removal process of templates for the TiO2 products were achieved by calcination at 500 °C for 3 h or solvothermal treatment. In the solvothermal treatment process, 1.0 g of the amorphous as-made TiO2 nanospheres was dispersed into a mixture of ethanol (30 mL) and water (30 mL). The reaction mixture was sealed within a Teflon-lined autoclave (80 mL) and heated for 12 h at 160 °C. In order to remove the solvent and byproducts during the solvothermal treatment, the samples was further treated at 400 °C for 3 h before other measurements and characterization. Electrochemical measurements. The electrochemical measurements were carried out by using a CR2016-type coin cell. The active materials (i.e., the mesoporous TiO2 nanospheres after calcination at 500 °C), carbon black (Super-P-Li), and polymer binder (polyvinylidene fluoride, PVDF) in a weight ratio of 50:30:20 or 70:20:10 were used for preparing the working electrode. The active material loading was about 1 mg/cm2 in battery testing. Lithium foil was used as 4


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both the counter electrode and the reference electrode. The elecrolyte was 1 M LiPF6 solution in ethylene carbonate/diethyl carbonate (EC: DEC = 1:1, v/v). Cell assembly was carried out in an Ar-filled glovebox. Both moisture and oxygen concentrations in glovebox were all below 1.0 ppm. Cyclic voltammetry measurements were carried out on a Electrochemical workstation (CH Instrument 660A, CHI Company). A battery test system (Land CT2001A, Wuhan Jinnuo Electronic Co. Ltd.) was used for the galvanostatic charge-discharge tests. Characterization. Fourier-transform infrared spectra (FT-IR) were performed by a Spectrometer (IR Prestige-2.1) using compressed KBr disc technique. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA-SDTA851 analyzer (Switzerland) in the temperature range of 35 to 900 °C under O2. The heating rate is about 5 °C/min. Micromeritcs Tristar 2420 analyzer was used to measure the nitrogen sorption isotherms at 77 K after the samples were degassed at 180 °C for 6 h.. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method using adsorption data in a relative pressure range of 0.05 to 0.25. The pore size distributions and pore volumes were derived by the Barrett–Joyner–Halenda (BJH) model from the adsorption branches of isotherms. Transmission electron microscopy (TEM) were measured by JEOL JEM-2100F microscope (Japan) at acceleration voltage of 200 KV. Before the TEM measurements, The samples were dispersed in ethanol and a holey carbon film supported onto a holey carbon film on a Cu grid. Hitachi S-4800 microscope was used to get the field-emission scanning electron microscopy (FESEM) images by observing the samples directly. X-ray diffraction (XRD) patterns were measured in the range of from 10° to 90° by a Bruker D8 X-ray diffractometer with Cu Kα radiation.

RESULTS AND DISCUSSION FESEM images show that the TiO2 samples obtained using the double-surfactant assembly-directed method are uniform spherical in large domains (Figure 1a). The sample prepared by using a high water/TBOT ratio of 120: 1 contains monodisperse TiO2 spheres with a diamater size of approximately 50 nm (Figure 1b). TiO2 colloid nanospheres are separated without agglomeration. What`s more, HRSEM images also 5



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disclose that the surface of the as-made mesoporous TiO2 nanospheres is much rough. TEM images (Figure 1c) further confirm that the as-made mesoporous TiO2 nanospheres are uniform and the mean diameter is about 50 nm. HRTEM images (Figure 1d) show many wormhole-like mesopores in the as-made TiO2 colloid nanospheres. Moreover, the amorphous nature of the TiO2 nanospheres is revealed by the selected-area electron diffraction (SAED) pattern (Figure 1d, inset).

Figure 1. FESEM (a,b) and TEM (c-d) images of TiO2 colloid nanospheres synthesized by the double- surfactant assembly-directed method at a high water/TBOT ratio of 120:1 in an alcohol/water solution at room temperature. The corresponding SAED pattern taken from a single nanosphere is shown by the inset in (d), revealing the amorphous feature of the TiO2 nanospheres. As shown in TG curves (Figure S1), the weight loss from 200 to 500 °C is mainly caused by the removal of both surfantants DDA and Pluronic F127. The weight loss of the samples prepared by Pluronic F127 is much more than that without F127, further demonstrating that the triblock copolymers remain in the amorphous mesoporous TiO2 nanospheres. The oxidation of the amorphous precursor with inorganic/organic hybrid spherical structure can lead to small weight increament at about 470 °C. So the removal of organic surfactants and crystallization of samples can be achieved by calcination 6


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treatment at 500 °C. XRD pattern of the mesoporous TiO2 nanospheres can be matched with anatase structure (JCPDS card No. 21-1272) well (Figure S2C). There is a hysteresis loop in the N2 adsorption/desorption isotherms of the mesoporous TiO2 nanospheres. It is a typical type IV pattern . Moreover, the good homogeneity of TiO2 nanospheres can be revealed by the sharp increase in adsorption volume of N2 . It is mainly caused by the capillary condensation. Using the BJH method, the BET surface area of TiO2 nanospheres is calculated to be 26 m2/g (Figure S2D). Only a few disordered mesopores (intercrystalline pores) can be found on the surface of the TiO2 nanospheres after calcination at 500 °C (Figure S2A, B). Comparing with the the as-made mesoporous TiO2 colloidal nanospheres with many wormhole-like mesopores, sintering and crystallization occur together during the calcination process, which can lead to the collapse of the mesopore structure. So a solvothermal treatment was used to treat the samples instead of calcination. After the solvothermal treatment, SEM and TEM images (Figure S3) reveal that the mesoporous TiO2 nanospheres retain good monodispersity with a little size shrinkage (from 50 to 42 nm). But the surface becomes much rougher compared to the as-made amorphous TiO2 nanospheres. There are many mesopores on the surface. As shown in TG curves (Figure S4), the weight loss (200 - 700 °C) decreases from 35 % to 8 % for the TiO2 nanospheres obtained after solvothermal treatment. The result confirm the removal of templates during the solvothermal process. The elemental mapping images (Figure S5) of the mesoporous TiO2 nanosphere obtained after the solvothermal treatment at 160 °C also show nearly no C element. Raman spectrum was measured on TiO2 nanospheres obtained after solvothermal treatment to determine the absence of carbon residue, which is much sensitive to analyse the absence of traces of carbon residue in material. No peaks are observed for carbon residue in the Raman sprctrum (Figure S6), revealing that TiO2 nanospheres obtained after solvothermal treatment do not have any carbon residual. Mesoporous TiO2 nanospheres after the solvothermal treatment are built of interpenetrating nanocrystals with a size of about 10 nm, arranged to obtain a large number of interconnecting mesopores. The lattice fringe spacing of the TiO2 nanocrystals is measured from HRTEM technique to be 0.35 nm, corresponding to the d101 spacing of anatase sturucture(Figure 2a, b). The XRD pattern (Figure 2c) of the mesoporous TiO2 nanospheres after the solvothermal treatment can also be indexed to anatase crystalline of 7



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TiO2. The polycrystalline feature can also revealeded by the corresponding SAED pattern (Figure 2b, inset) al. with there is also a hysteresis loop at relatively high pressure in the N2 adsorption/desorption isotherms (Figure 2d), showing the typical characteristic of mesoporous sturucture. The BET surface area is calculated by the BJH method to be about 160 m2/g. In addition, the samples possess mesopores with sizes of 4 - 30 nm. The pore size distribution is centered at 20 nm (Figure 2d, inset). The solvothermal treatment not only enhances the crystallization of TiO2, but also prevents the collapse of mesoporous structure.

Figure 2. (a-b) HRTEM images, (c) XRD and (d) N2 adsorption/desorption isotherms of the mesoporous TiO2 nanospheres after a solvothermal treatment. The inset in (d) is the corresponding pore size distribution. Monodispersed TiO2 colloidal nanospheres with a diameter size of 250 nm can be prepared by the double-surfactant assembly-directed method at water/TBOT molar ratio of 40:1 in an alcohol/water solution at room temperature (Figure 3a). As the water/TBOT molar ratio changes to 80:1, the diameter size of the as-made mesoporous TiO2 colloid nanospheres decreases to 100 nm (Figure 3c). If the water /TBOT molar ratio increases to 8


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200:1, the diameter size distribution can be wide into more polydisperse (Figure S7). More aggregated nanoparticles can be found in the samples. So the water/TBOT ratio is very important for controlling the diameter of the nanospheres. The size and monodispersity of the mesoporous TiO2 colloidal spheres can also be adjusted by F127/TBOT molar ratio. Without using F127, the mesoporous TiO2 spheres with a wide range of diameters size (40 - 360 nm) can be obtained (Figure S8a). But, when the F127/TBOT molar ratio is increased to 0.004, the aggregated TiO2 colloidal nanospheres can be prepared by the process (Figure S8b). The diameter size is about 150 nm, which is larger than that synthesized at F127/TBOT molar ratio of 0.002. The optimal amount of F127 is important for the synthesis of monodisperse mesoporous TiO2 colloidal nanospheres. While using another surfactant Pluronic P123 which has the same PPO (70) block as F127, the aggregated amorphous TiO2 nanospheres with a large diameter size can be obtained (Figure S9), indicating that the longer hydrophilic PEO block can reduce agglomeration of the TiO2 colloid nanospheres effectively to get monodisperse samples.

Figure 3. SEM (a, c) and TEM (b, d) images of the monodisperse TiO2 nanospheres synthesized by the double-surfactant assembly-directed method by varying the water /TBOT mole ratios: (a, b) 40:1, (c, d) 80:1. Moreover, the DDA/TBOT molar ratio is used to control the morphology and 9



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monodispersity of mesoporous TiO2 nanospheres. Aggregated small TiO2 nanoparticles can be prepared without using DDA (Figure S10). A wide range of molar ratios of DDA/TBOT (0.3 - 1.0) can be used to obtain uniform mesoporous TiO2 nanospheres. The diameter sizes do not change very much when the molar ratio of DDA/TBOT is varied in the range of 0.3 - 1.0 (Figure S10). It suggests that DDA play a significant role in the formation process of spherical structure, but less affects on tuning their diameter. The monodisperse mesoporous TiO2 nanospheres without aggregation can also be prepared at a relative lower temperature (0 °C). But the diameter size of TiO2 nanospheres increases to about 120 nm. However, if the temperature increases to 60 °C, the aggregated mesoporous TiO2 nanospheres with a wide particle size distribution (30 380 nm) can be obtained (Figure S11). To understand the formation mechanism, the synthesis process is tracked by investigating the products at different intervals of reaction time via TEM technique. Many tiny nanoparticles about 5 nm in diameter can be found after 5 min (Figure S12). As the reaction time prolongs to 20 min furtherly, the diameter of the particles increase to 10 nm. It is noteworthy that many mesopores can be observed in the particles then. The size of the nanospheres formed is increased to more than 20 nm after 30 min, and nearly 50 nm after 60 min, which consist of many packed tiny nanoparticles with 5 nm in diameter.It is interesting to note that the worm-like pores are throughout the nanospheres. Based on the above results, a formation process of the mesoporous TiO2 nanospheres is proposed (Figure 4) to illustrate the role of double-surfactant F127 and DDA to control size, morphology and monodispersity of mesoporous TiO2 colloidal nanospheres. In this process, surfactant F127 molecules can form spherical micelles with the hydrophobic center of PPO and hydrophilic surface of PEO in alcohol/water solution at room temperature (Figure 4a, b). Therefore, the hydrophobic long-chain alkyl groups of DDA can penetrate into the hydrophobic center of PPO in the F127 spherical micelles, but its hydrophilic -NH2 groups remain on the hydrophilic PEO surface of F127 in the micelle formation process.47 Thus, F127 and DDA assemble into cooperative spherical micelles in the alcohol/water solution at room temperature. Secondly, as for hydrolysis of TBOT, the resultant Ti(OCH2CH2CH2 CH3)4-x(OH)x species can interact with -NH2 groups of DDA or PEO segments of F127 through hydrogen-bonding (Figure 4c), which is revealed 10


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clearly by the FTIR spectra of the TiO2 nanospheres (Figure S13).

48

The

F127-DDA-titania oligomer composite spherical micelles are formated by collaborative assembly with Ti precursors driven by hydrogen-bond interaction. As the Ti oligomers further polymerize, the concentration of the composite spherical micelles increases. To minimize the surface free energy, increasing composite spherical micelles aggregates and packs into large spheres on the interface under stirring.49 In this process, the water/TBOT ratio is very important for controlling the diameter of TiO2 nanospheres. So large amount of water can be used to prepare TiO2 nanospheres with a small diameter size due to more composite spherical micelles obtained by high hydrolysis and condensation rate of TBOT precursor. F127 and DDA can form a stronger lipophilic interaction on the surface of the composite spherical micelles, which could enhance the driving force for self-assembly of them to decrease the surface tension, leading to a fast phase separation process to form small-sized TiO2 nanospheres. After colloidal nanospheres are formed in the solution, long hydrophilic PEO of F127 micelles can be highly entangled and penetrated into the walls of the inorganic-organic composites spheres to reduce the surface tension and prevent aggregation of them. Monodisperse mesoporous TiO2 nanospheres with a small diameter size can be obtained after solvothermal treatment. (Figure 4d). So tiny nanoparticles with 5 nm in diameter can be found in the reaction solution after 5 min in the process. As the reaction time is extended, the diameter size of TiO2 nanospheres increases gradually (from 10 to 50 nm).

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Figure 4. Schematic representation of the synthesis process of monodisperse mesoporous anatase TiO2 nanospheres through double-surfactant assembly-directed method. Step 1 A self-assembly of DDA and F127 as the structure-directing agents to form cooperative spherical micelles. Step 2 Collaborative assembly with Ti precursor: Formation of the F127-DDA-titania oligomer composite spherical micelles with hydrogen-bond interaction. Step 3 Packing of the F127-DDA-titania oligomer composite spherical micelles into large spheres on interface of water-rich phase, which is driven by the increasing concentration of the spherical micelles and the requirement of minimization of interface energy. Step 4 Surfactant removal and crystallization. The electrochemical properties of the mesoporous TiO2 nanospheres for LIBs were investigated. Two voltage plateaus can be observed during the discharging and charging processes in representative voltage profiles within a voltage window of 1.0-3.0 V (Figure 5a). For the mesoporous TiO2 nanospheres obtained after solvothermal treatment, the lithium insertion/de-insertion process are correspond to the two voltage plateaus at approximately 1.7 and 1.9 V each other. The initial discharge capacity is as high as 209.7 and charge capacity is as high as 169.7 mAhg-1. So the initial coulombic efficiency is about 80.3 %. During the first 5 cycles, the high capacity fading could be attributed to irreversibly inserted lithium. The large consumption of lithium ions is mainly caused by the interfacial reaction among active material and the electrolyte. Moreover, the intercalation of lithium ions into irreversible sites of the TiO2 crystal framework also probably lead to high capacity loss.50-51 After 5 cycles, the coulombic efficiency of the 12


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monodisperse mesoporous TiO2 nanospheres (50 nm) obtained after solvothermal treatment reaches up to 100 %, suggesting an excellent cycling stability. The rate capabilities of the monodisperse mesoporous TiO2 nanospheres obtained after solvothermal treatment or calcination at 500 °C from 1 C to 10 C is shown in the Figure 5c. No obvious decrease of its charge capacity is noted during the five cycles at different rates. At a low current density of 1 C, mesoporous TiO2 nanospheres obtained after solvothermal treatment exhibits a charge capacity of 155 mAhg-1. As the current density increases to 10 C, the specific capacity is still 107 mAhg-1. The specific capacity can return to 153 mAhg-1 after the current is reduced back to 1 C. TiO2 nanospheres obtained after calcination at 500 °C exhibit a charge capacity of 129 mAhg-1 at current density of 1 C. The specific capacity decreases to only 85 mAhg-1 at current density of 10 C. Among various materials, the mesoporous TiO2 nanospheres obtained after solvothermal treatment with diameter size of 50 nm exhibit the highest capacity and best reversibility (Figure 5b). Even after 500 cycles, it exhibits a charge capacity as high as 156 mAhg-1 without obvious fade. The specific capacity for the TiO2 nanospheres obtained after calcination at 500 °C is only 119 mAhg-1 after 500 cycles. Moreover, the TiO2 nanospheres with 250 nm in diameter exhibit a charge capacity of 125 mAhg-1 only after 100 cycles. But the charge capacity of these materials is much higher than the commercial TiO2 nanoparticles (P25) (Figure S14). All these results clearly reveal that the mesoporous TiO2 nanospheres obtained after solvothermal treatment with a small particle size exhibit excellent cyclic stability, high charge/ discharge capacities and rate capabilities. The electrochemical properties of the mesoporous TiO2 electrodes obtained after solvothermal treatment with low content of conductive carbon black (20 %) and polymer binder (10 %) were also investigated (Figure S15). It exhibits a charge capacity of 140 mAhg-1 after 500 cycles at current of 1 C, only a little lower than that obtained from the content of conductive carbon black (30 %) and polymer binder (20 %).

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Figure 5. Electrochemical performances of the monodisperse mesoporous TiO2 nanospheres with a small diameter of 50 nm as an anode in LIBs (1, the mesoporous TiO2 nanospheres after solvothermal treatment; 2, the mesoporous TiO2 nanospheres obtained after calcination at 500 °C): (a) Discharge-charge voltage profiles for the mesoporous TiO2 nanospheres after solvothermal treatment. (b) Cycling performance and corresponding Coulombic efficiency at a current of 1 C. (c) Rate performance at various current rates (1-10 C, 1 C = 173 mAg-1). (d) Schematic representation for the electrochemical reaction paths and transport pathway of lithium ions.

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Figure 6. (a) Electrochemical impedance spectroscopy of the cells based on different TiO2 materials; (b) the SEM image of the monodisperse mesoporous TiO2 nanospheres with a small diameter of 50 nm as an anode in LIBs after 200 cycles, showing that the nanospheres remain well-defined spherical structures. In the moderate frequency region, Nyquist plots of different kinds of TiO2 materials have a depressed semicircle. It is relevant to the process of charge transfer (Figure 6a). The mesoporous TiO2 nanospheres with a diameter size of 50 nm before cycles present a lower charge resistance than that with a larger diameter size of 250 nm or P25, as shown with a smaller diameter of the semicircle in the moderate frequency region. Lower charge transfer resistance can lead to a higher rate of lithium ion diffusion in of the electrode. The charge resistance is reduced after 200 cycles shown by a smaller diameter of the semicircle, due to the better wetting of the electrolyte to TiO2 active materials, leading more sufficient contact between TiO2 and the electrolyte, making lithium ions and electron transfer effectively. TiO2 nanospheres with a small diameter size of 50 nm electrode after 200 cycles present well-defined spherical structures (Figure 6b). Moreover, the sizes of the mesoporous TiO2 nanospheres do not increase very much, clearly indicating that they are stable without great expansion after lithium ion intercalation/ deintercalation process.52-54 N2 adsorption/desorption isotherms of the mesoporous TiO2 nanospheres after 200 cycles were studied to examine the porosity of materials (Figure S16). The pore size of TiO2 nanospheres after cycles does not change much. It is a little smaller than that of the fresh samples due to penetration and wetting of electrolytes. The large capacity, rate capabilities and excellent cycle stability of the mesoporous TiO2 nanospheres with a small diameter size in this study are comparable to that for the reported TiO2 nanostructure electrodes in previous research.55-57 The excellent electrochemical performance of mesoporous TiO2 nanospheres with a small diameter size as an anode marerial for lithium ion battery can be attributed to their unique features. Small mesoporous TiO2 nanospheres possess a high specific surface area (160 m2 g−1), a wide pore size distribution(4 - 30 nm), and a mesostructure. The connecting mesoporous structure of the TiO2 nanospheres can facilitate the diffusion of the electrolyte inside the nanospheres, improving the interaction between the electrode material and the electrolyte (Figure 5d). Furthermore, small anatase crystalline grains also facilitate lithium-ion intercalation reaction at the interface and shorten the distance of 15



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lithium-ion and electron diffusion.58-61

CONCLUSIONS In summary, monodisperse mesoporous TiO2 nanospheres have successfully been prepared by the facile and reproducible double-surfactant assembly-directed method. The diameter size of the monodisperse TiO2 nanospheres can be controlled from ~ 50 to 250 nm precisely by simply varying the concentration of H2O and surfactants during the synthesis process. The double- surfactant system plays a great role in controlling the morphology, size and monodispersity of TiO2 colloid nanospheres. The anatase TiO2 nanocrystals with small size are arranged in nanospheres to obtain a large number of connecting mesopores. The mesoporous TiO2 nanospheres with a small diameter size of around 50 nm possess a high surface area (~ 160 m2/g) and mesopore size of 4 - 30 nm. When the TiO2 nanospheres are used for the lithium-ion battery, it demonstrates an excellent cyclic stability, high charge/discharge capacities and rate capabilities. The charge capacity remains up to 156 mAhg-1 without obvious fade even after 500 cycles. The TiO2 colloidal nanospheres are expected to be used as high performance anode materials for lithium ion batteries. The simplicity and reproducibility of the double-surfactant assembly-directed method in this work can provide vast opportunities of uniform TiO2 colloids for further applications.

ASSOCIATED CONTENT Supporting Information SEM and TEM images; Nitrogen adsorption–desorption isotherms of the samples obtained with different reaction conditions. FT-IR spectra and TGA curves of TiO2 nanospheres; This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E–mail: [email protected]; [email protected] 16


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ACKNOWLEDGMENT This work was supported by the State Key Basic Research Program of the PRC (2012CB224805

and

2013CB934104),

Shanghai

Nanotech

Promotion

Centre

(0852nm00100), NSF of China (21210004, U1463206 and 21322508) Shanghai Sci. & Tech. Committee (14JC1400700), and the authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding this work through Research Group No. RG–1435–002.

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Table of Contents (TOC)

Hongwei Zhu, Yesheng Shang, Yunke Jing, Yang Liu, Yupu Liu, Ahmed Mohamed El-Toni, Fan Zhang*, Dongyuan Zhao* Synthesis of Monodisperse Mesoporous TiO2 Nanospheres from a Simple Double-Surfactant Assembly-Directed Method for Lithium Storage

Monodisperse mesoporous TiO2 colloidal nanospheres with diameter size on nanoscale were synthesized by a simple and reproducible double-surfactant assemblydirected method at room temperature. The obtained TiO2 nanospheres possess a high surface area and mesopores with a size of 4 - 30 nm. The small diameter size, high crystallinity and mesoporous structure of the TiO2 nanospheres lead to excellent performance in the cycling stability and rate capability of lithium ion battery.

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Synthesis of Monodisperse Mesoporous TiO2 Nanospheres from a

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