Novel porous anatase TiO2 nanorods and their high lithium electroactivity
Introduction
TiO2 (anatase) has been extensively studied during the past decade [1]. This interest arises from its potential application in photovoltaic cells [1], [2], [3], catalysts [4], [5], [6], gas sensing [7], electrochromic devices [8], and rechargeable lithium ion batteries [1], [9], [10], [11], [12]. Especially, in recent years, one-dimensional nanostructured TiO2 such as nanotubes and nanowires have attracted considerable academic and industrial interest due to their superior properties and unique features arising from their nanoscale structures [13], [14]. To realize these promising high-performance applications, the control of the physicochemical properties through tailoring their nanostructures is a great challenge.
Because the synthesis of one-dimensional nanostructured TiO2 is very important for understanding its fundamental properties, considerable effort has recently been concentrated on exploring the various synthetic methods ranging from vapor-phase techniques to solution-growth processes [13], [15], [16], [17]. At present, the main approach to fabricate TiO2 nanotubes and nanowires is the hydrothermal method which provides access to uniform and distinct morphologies with excellent reliability, selectivity and efficiency [18], [19], [20]. However, the method is complicated and restricted by a high reaction pressure and its completion in an autoclave. Although the sample obtained using the hydrothermal method has a good performance, the complicated equipment is considered to be a big obstacle for commercialization. Therefore, it is of significance to explore an alternative method to synthesize nanostructured TiO2 with homogeneous particle morphology in which the process is simple and controllable.
Here we report a novel nanostructure TiO2 synthesis method. The method involves the synthesis of the sample in a solution of molten mixed potassium hydroxide and sodium hydroxide eutectic at ∼200 °C and normal atmosphere without using an organic dispersant or a capping agent. This provides a convenient, low-cost and mass-production route for the fabrication of nanostructures of functional oxide materials with various structure types [21]. Although the method has been widely applied in preparing of complex oxide nanostructures of scientific and technological importance, it has never been used to synthesize nanostructured TiO2.
The results of nitrogen adsorption and desorption experiments and HRTEM analysis indicated that the TiO2 nanorods prepared in binary eutectic mixture system has a large specific surface area and a porous structure. Recently, nano-sized materials with porous structure have attracted great research interest in the field of catalysts, molecular sieves, host materials and battery materials, especially in electrochemistry-related applications [22], [23], [24], essentially because of their substantial advantages in terms of mass transport. Transport in porous nanostructured system typically encompasses shorter lengths for electrons and Li+ transport, higher electrode–electrolyte contact area, and a better accommodation of the strain of Li+ insertion/extraction.
Furthermore, nanosized TiO2 has been widely investigated as a key material for lithium ion batteries, because it is not only a low-voltage insertion host for Li, but also a fast Li insertion/extraction host [25]. Hence, in this work, we mainly report a novel porous TiO2 nanorods synthesized from a binary eutectic mixture system; at the same time we test their electrochemical performance as a negative electrode material of the Lithium ion battery.
Section snippets
Preparation of materials
All the chemicals were of analytical grade and used without further purification. The preparation process is described as follows: (1) an amount of mixed hydroxides (NaOH/KOH = 1:1) was placed in a 50 mL Teflon vessel with a cover for preventing dust, and then, added 1 g TiO2 (commercial TiO2 nanopowder, Aldrich) was added on the top of the hydroxide in the vessel; (2) the vessel was put into a furnace at 200 °C for 3 h, the covered vessel was shaken to ensure that the samples were mixed completely
Results and discussion
A possible reaction mechanism as follows could happen in this experiment. Although the melting points of both sodium hydroxide and potassium hydroxide are over 300 °C (Tm = 323 °C for NaOH and Tm = 360 °C for KOH), the eutectic point at NaOH/KOH = 51.5:48.5 is only about 165 °C [21]. This is likely to be the key for synthesizing the sample at ∼200 °C or lower. During the reaction process, hydroxides played a role not only as the solvent but also as the reactant for decreasing the reaction temperature. In
Conclusions
Novel TiO2 nanorods with a porous structure have been synthesized by using a binary eutectic mixture system. The novel materials exhibited a high-specific surface area with a ca. 5 nm pore-size distribution, uniform and regular rod-shaped structures. The good cycleability and a high rate capability coupled with the low cost and environmentally benign nature of the preparation method may make this material attractive for large applications. Further research to optimize synthesis conditions,
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