A Polymer-Assisted Hydrothermal Approach to Titanium Dioxide Thin Films

Epitaxial anatase TiO2 thin films on LaAlO3(001) substrates are prepared by a novel polymer-assisted hydrothermal method. The titanium ions binding with ethylenediaminetetraacetic acid (EDTA) and polyethyleneimine (PEI) result in a homogeneous clear stable precursor solution followed by hydrothermal for thin film deposition. High resolution transmission electron microscopy analysis shows that the film has two layers with one densely deposits onto the substrate while the other contains nanopillars stacking onto the former layer. The phase, epitaxy, and film thickness can be controlled by metal precursor concentrations, pH, temperature, and reaction time. In addition, TiO2 and SiO2-embedded in TiO2 composite films directly grown on Ni foam are investigated as anodes for lithium-ion battery application.


Introduction
Due to its numerous applications in a wide variety of fields, there has been intensive research on the growth of titanium dioxide (TiO 2 ) . Several techniques have been applied to grow thin films, such as sputtering [12,13], pulsed-laser deposition [14,15], molecular beam epitaxy [16,17], electron-beam evaporation [18], and chemical vapor deposition [19,20]. However, high vacuum and high temperature of these processes have limited the potential application of films. Polymer-assisted deposition [21][22][23], sol-gel, hydrothermal and other solution approaches [7,24,25] have become more attractive alternatives for thin film deposition because of their low cost, large area, and low temperature fabrication [22].
Hydrothermal favors the formation of thicker thin films than other solution spin-coating methods. Liquid phase hydrothermal deposition has been applied to prepare epitaxial oxide thin film [24][25][26][27]. However, the precursor solution for thin film growth by hydrothermal was not clear solution and the films were formed by stacking oxide powders, which created voids inside the films. A variety of high quality epitaxial metal oxides and oxide nanocomposite thin films without voids can be grown by polymer-assisted deposition [21][22][23]28], where the metal-polymer solutions are clear and stable solutions, however, the thickness of resulted thin films is usually less than 100 nm even by controlling the solution concentration and multiple spin-coats. In this paper, we combine polymer-assisted deposition with hydrothermal method, named polymer-assisted hydrothermal, in order to achieve thicker films of metal oxides and oxide nanocomposites. We report dense epitaxial TiO 2 thin films and epitaxial SiO 2 -TiO 2 composites grown on single crystal LaAlO 3 (LAO) (001) substrates. This method provides a safe and environmental friendly path for thin film deposition and avoids the use of fluoride ligand in the liquid phase deposition.
Since TiO 2 nanostructures have been widely studied as anode material for lithium ion batteries [29][30][31][32][33][34][35][36][37][38], we further use nickel foam instead of single crystal LAO substrate in the polymer-assisted hydrothermal synthesis to prepare TiO 2 and SiO 2 -TiO 2 composites for the anodes. Since nickel foam is a polycrystalline substrate, not single crystal substrate, therefore the films on nickel foam are polycrystalline films, not epitaxial films. This one-step binder-and-conductive carbon-free process makes the contact between TiO 2 and current collector (nickel foam) more stable and is expected to improve the battery performance.

Synthesis of anatase TiO 2 thin film on LAO substrate
The titanium solution (0.5mL titanium tetrachloride was added dropwise into 3mL of 30 wt% hydrogen peroxide) was added to a solution containing 2g of 50 wt% polyethylenimine (PEI), 1g of ethylenediaminetetraacetic acid (EDTA), and 40mL of deionized (DI) water to form a bright orange solution. During this process, the pH of the solution was maintained at 7.5. The as-prepared titanium precursor solution was transferred into a Teflon-lined autoclave and the final pH of the solution was adjusted by ammonium hydroxide or hydrogen chloride.

2
The LAO substrate was washed by a mixture of 50% of concentrated hydrogen chloride and 50% of hydrogen peroxide with sonication for 10min to remove contaminants on the substrate. Then the substrate was placed on the Teflon holder with its polished side facing down in a Teflon-lined stainless steel autoclave with 12mL precursor solution. The reaction temperature, time, pH, concentrations were examined to explore the optimum condition for epitaxial thin film deposition. Detailed reaction conditions are listed in Table 1. After the hydrothermal process, the films were washed with deionized water and dried at room temperature.

Synthesis of SiO 2 -embedded in TiO 2 thin film on LAO substrate
The precursor solution was prepared by mixing different concentration of silica (The commercial Ludox silica nanoparticles with sizes around 10 nm bind to PEI, the concentration of silica is 460mM) with titanium solution. LAO substrates were placed into the Teflon-lined stainless steel autoclave as mentioned above for thin film deposition. For a complete deposition, the reaction conditions were controlled at 200°C for 24 h.

Characterization of thin films on LAO
The structures of the products were characterized by X-ray diffraction (XRD) with a Rigaku Miniflex II X-ray powder diffractometer with CuKα (λ=0.15406nm) radiation. A Hitachi High Technologies H-7650 transmission electron microscope (TEM) operated at 80kV, a Hitachi Model S-3400N Type II scanning electron microscope (SEM) and an atomic force microscope (AFM) were used to characterize the morphology and microstructure of samples. The composition of SiO 2 -embedded in TiO 2 thin film was analyzed by energy dispersive X-ray spectroscopy (EDS).

Preparation and electrochemical performance of lithium-ion batteries
The nickel foam was used as the substrate to grow TiO 2 and SiO 2 -TiO 2 films. The nickel foam was immersed in the titanium precursor solution with or without SiO 2 nanoparticles into an autoclave and treated at 200°C for 24h. To prepare the lithium ion battery, coin cells were assembled in an argon-filled dry glove box. Metal lithium was used as a counter electrode, whereas the electrolyte contained 1M LiPF 6 in a mixture of ethylene carbonate (EC) / dimethyl carbonate (DMC) with 1:1 volume ratio. Galvanostatic charge/discharge cycling performance were evaluated on an LAND Battery Tester CT2001A at a room temperature in a potential range of 0.005-3.00V (versus Li+ / Li) at different current densities.

Result and Discussion
Figure 1(a) shows the XRD θ-2θ scan of TiO 2 on LAO substrate by hydrothermal reaction. The preferential (004) diffraction peak from anatase TiO 2 indicates the thin film deposits on (001) LAO single crystal substrate with highly c-axis orientation. Figure 1 (b) shows the in-plane ϕ-scan between TiO 2 (101) and LAO (101)   ing onto the former layer. This interesting structure can be explained by the lattice mismatch between the thin film and the substrate. Since the lattice mismatch between LAO (a = 3.791Å) and anatase TiO 2 (a = 3.7852Å) is only -0.2%, there should be a small strain in the interface. As the film grows thicker, the film will maintain its own crystal structure rather than distorting itself for epitaxial growth, which cause the formation of nanopillar to release the epitaxial strain. Figure 3 shows the SEM image of the TiO 2 thin film surface, which confirms ordered nanopillar alignment. It was found that Ti 4+ was stabilized below pH of 2.5 in the Ti-EDTA complex, and above pH, the Ti-EDTA complex tended to hydrolyze to colloidal TiO 2 . Herein this experiment, by adding PEI into the precursor solution, no precipitates were detected in the solution with a wide range of pH from 1 to 10, which is a clear evidence that PEI served as binding regent to stabilize titanium complexes. Also, in the hydrothermal reaction, when the reactor was heated to 200°C, the pressure in autoclave increased significantly. Though the Ti-EDTA-PEI complex was very stable at ambient conditions, relative high temperatures and high pressures could accelerate the hydrolysis of titanium. Figure 4 shows the XRD patterns of TiO 2 thin film prepared in different reaction conditions. The thin film can be identified from the existence of anatase (004) peak in the XRD pattern. The hydrothermal reaction time varies from 6 to 24 h and the reaction temperature varies from 150 to 200ºC. Thin film was not formed when the reaction time was less than 12 hours or reaction temperature was under 200ºC. It can be seen that longer reaction time and higher reaction temperature increase the thickness of the TiO 2 thin film. Further  Table 1.
increasing the growth time to 24h, the thickness of the thin film can reach 2.4μm, which is much thicker than epitaxial thin film prepared by traditional sol-gel spin-coating or dipcoating [22]. Compared with other reported hydrothermal to thin films [24][25][26][27], where the precursor solutions are not clear solution, this polymer-assisted hydrothermal method provides clear and table solution and has obvious advantages on obtaining films with varied thickness without repeating spincoatings. Meanwhile, the influence of solution pH was also investigated. In acidic condition with pH at 4, no thin film was formed. It can be expected that strong acidic condition may induce severe and fast hydrolysis of titanium precursor, in which the nucleation of TiO 2 nanoparticles could be competitive to the thin film deposition. In neutral condition, the titanium precursor could mildly hydrolyze to deposit on the substrate. When the pH went up to 10, only part of the substrate was covered with TiO 2 thin film. The Ti-EDTA complex is very stable at this condition, which may weaken the attraction to PEI. As we expect, PEI could serve as template in the thin film formation, the hydrolysis process become unpredictable when the template effect is weaker. Besides, the precursor concentration effect was also investigated. When the concentration was 200 mM, no thin film was found on the substrate. The aforementioned nanoparticles nucleation may play an important role in the reaction process. At higher concentration, the hydrolyzed precursor can form more nuclei, which directly induced the formation of nanoparticles rather than thin film deposition.
SiO 2 -embedded TiO 2 thin films were deposited on LAO substrates by using precursor solutions with different SiO 2 concentration to titanium concentration ratios. It is noted 4 that amorphous SiO 2 nanoparticles won't affect the epitaxial growth of TiO 2 on LAO substrates [28], therefore, the SiO 2 nanoparticles embedded in TiO 2 films are still epitaxial. The energy dispersive X-ray spectroscopy (EDS) was carried on SiO 2 -embedded TiO 2 thin films to analyze its composition. With a sample prepared from precursor solution in which the concentration of SiO 2 is approximately half of that of Ti4+, the detected weight ratio of Si/Ti is 26.7%, corresponding to the atomic ratio of Si/Ti is 45.6%. The AFM images of pure and SiO 2 -embedded TiO 2 thin film on LAO substrate are shown in Figure 5. As observed, the pure TiO 2 formed a uniform thin film on LAO substrate without any detectable microcracks, and the root-mean-square surface roughness is 16nm. With adding SiO 2 , both of the grain size and roughness increase compared to those in pure TiO 2 thin films. TiO 2 directly grown on nickel foam was prepared by the same method for the anode of lithium ion batteries. This one-step synthesis method gives a convenient way to prepare anode materials. In addition, this binder-and-conductive carbon-free process made the contact between TiO 2 and current collector (nickel foam) more stable and improved the volumetric utilization efficiency [30]. Figure 6 shows the capacity as a function of cycle numbers at a current density of 100mA/g. It is observed that pure TiO 2 has a capacity of 144mAh/g after 30 cycles, and SiO 2 -embedded TiO 2 has a much higher capacity 270 mAh/g. This can be understood by considering the reaction mechanism and the structure difference bringing by SiO 2 mixing. TiO 2 is one of the transitional metal oxides following Li insertion storage mechanism [38]. With embedded SiO 2 , the crystalline structure of TiO 2 was destroyed to some degree, which induced a large amount of structure disorders and defects supplying more spaces for insertion of Li ions [31]. Therefore, the Li-diffusion of SiO 2 -embedded TiO 2 is much higher than that of pure TiO 2 . In addition, the specific capacity of SiO 2 -embedded TiO 2 increases slightly as cycle number increases. The structure defects caused by embedded SiO 2 make the crystal lattice unstable and therefore much easier to be influenced by the lattices around. As the charge-discharge processes go on, more TiO 2 were activated and involved in reaction. The electrochemical performance of SiO 2 -embedded TiO 2 is much better than that of other nanostructured TiO 2 . Mesoporous TiO 2 -C nanospheres deliver a capacity of ~170mAh/g at a current density of ~96mA/g [33]. Nanostructures TiO 2 -graphene hybrid has a capacity of ~190mAh/g at a current density of 84mA/g and ~160 mAh/g at 168mA/g [37]. TiO 2 mesoporous nanocrystalline microspheres demonstrate capacities of 265, 234, 182, 175, and 156mAh/g at the discharge rates of 0.06, 0.12, 0.3, 0.6, and 1.2 C (1 C corresponds to 170mA/g) [34].

Conclusion
Pure and SiO 2 -embedded epitaxial TiO 2 thin film on LAO substrates have been synthesized via a novel polymer-assisted hydrothermal approach. An interesting two-layer structure was found with well-aligned nanopillars to release the strain in epitaxial growth. The nanoparticles nucleation is shown to be a strong competitive process in the thin film growth, which could be suppressed by lowering the precursor concentration and adjusting the precursor solution to a neutral pH condition. With SiO 2 mixing, the grain size and roughness of the thin film increase. Moreover, this one-step hydrothermal synthesis process can be used to prepare anode materials for lithium ion batteries as well. This binder-and-conductive carbon-free process made the contact between anode material and current collector more stable and improved the volumetric utilization efficiency. Since EDTA and PEI can bind to most of the metal ions to produce stable complex, this polymer-assisted hydrothermal approach could be generalized to other metal oxide systems in epitaxial thin film synthesis and also in the preparation of polycrystalline anode materials for lithium ion batteries.