Nanostructured microtubes based on TiO2 doped by Zr and Hf oxides with the anatase structure

The nanostructured microtubes based on TiO2 have been prepared on the carbon fiber template using the sol-gel method. The microtubes consist of nanoparticles of metal oxides: TiO2/ZrO2 and TiO2/HfO2. The dependence of microtubes morphology and nanoparticles structure on the synthesis conditions has been studied using the methods of SEM, SAXS, and Raman spectroscopy. It has been demonstrated that at the stoichiometric ratio of up to 0.04 for Zr/Ti and up to 0.06 for Hf/Ti microtubes consist of uniform nanoparticles with the anatase structure. Along with further increase of the dopants content in the microtubes composition, nanoparticles acquire the core-shell structure. It has been suggested that nanoparticles have a core composed of the solid solutions Ti1-xZrxO2 or Ti1-xHfxO2 and a shell consisting of zirconium or hafnium titanate. The fabricated Zr- and Hf-doped TiO2 materials were investigated in view of their possible use as anode materials for Li-ion batteries. Charge- discharge measurements showed that the doped samples manifested significantly higher reversibility in comparison with the undoped TiO2. The method opens new prospects in synthesis of nanostructured materials for Li-ion batteries application.


Introduction
Recently times, a substantial attention has been paid to nanosized titanium and zirconium dioxides. The applications of these materials in the industry are diverse and include many branches: production of pigments for paint-and-varnish industry, manufacture of catalysts and their substrates, fabrication of glass, ceramics and refractory materials, electrical engineering, and photocatalysis [ [1], [2], [3]]. The fabrication of modified forms of TiO 2 substantially extends the range of application of these materials [ [4], [5]].
Up to present, many methods of synthesis of highly disperse materials based on titanium dioxides have been developed [ [2], [6], [7], [8]]. One of the well-known methods of fabrication of nanosized oxides is the sol-gel technique including a number of consecutive stages: preparation of the precursor solution, sequential transformation of it into sol and, thereafter, into gel (as a result of hydrolysis and condensation processes), drying, and thermal treatment. The advantages of the sol-gel technique consist in the possibility to produce nanoparticles of multi-component oxides with high degree of chemical homogeneity and to change the nanoparticles phase composition, size, and shape. The main disadvantage of the synthesis method is related to the loss of control over the nanoparticles size and shape during gel drying and thermal treatment [ [2]].
In the present paper, the nanostructured oxide microtubes of the composition TiO 2 /ZrO 2 and TiO 2 /HfO 2 have been prepared by means of the of template sol-gel method. The relationship between the morphology, structure, and synthesis conditions of the materials was studied. The possibility of application of the syntehsized TiO 2 -based microtubes as anodes for Li-batteries was discussed.

Synthesis
The scheme of synthesis is shown in Figure 1. The method of fabricating the microtubes is based on a slow hydrolysis in aqueous solutions of the inorganic precursors of titanium, zirconium, and hafnium accompanied with deposition of oxides on the template surface [[9]]. In the synthesis process, the following precursors were used: TiCl 4 , ZrOCl 2 , and HfOCl 2 . Busofit T055 carbon fiber manufactured at the Belogorsk Chemical Fibers Plant (Belarus) was used as a template. The specific surface of the carbon fiber was determined using an ASAP 2020 V3.04 H (Micrometrics, USA) spectrometer from isotherms of low-molecular adsorption of nitrogen by the BET method: it was equal to 960 m 2 g -1 . The porosity distribution was evaluated using the original density functional theory (DFT). The major part of the surface (780 m 2 g -1 ) is formed d by micropores of a diameter of 2 nm. The template removal was carried out through slow annealing at a temperature of 500°C in air. The carbon template annealing was controlled using Raman spectroscopy. In the course of the extra thermal treatment at 550-850°C in air for 30 min, nanocrystallization of oxide tubes structural units was observed. Since the initial carbon fiber contains ~0.01 wt. % of silicon, some amount of SiO 2 is always present in the materials. The decrease of the SiO 2 concentration in the sample was achieved through autoclave pretreatment of the initial template by ammonium bifluoride. In the samples of as-prepared materials, the SiO 2 content was about 2-5 wt. %. The effect of the SiO 2 content in the above range on the microtube structure and morphology has not been identified. The proposed synthesis technique allows regulating over a broad range the phase composition, size, and shape of the nanostructural units by changing the synthesis conditions (solution pH, component concentrations, and annealing temperature).

Methods of study
The content of titanium, zirconium, hafnium, and silicon oxides in the materials was determined by the energy-dispersive X-ray analysis on an EDX 700 (Shimadzu, Japan) spectrometer. The morphology was investigated by the method of scanning electron microscopy (SEM) using high-resolution S5500 and TEM-3000 (Hitachi, Japan) microscopes. The sizes of microtubes nanoparticles were determined by the method of small angel X-Ray scattering (SAXS). The measurements were carried out on an S3-MicroPIX (Hecus X-ray systems, Austria) spectrometer connected to a Xenocs GeniX generator (France) operated at 50 kV and 1 mA with using CuKα radiation (λ = 0.154 nm).
The Raman spectroscopy method was used to determine more accurately the structure and structural effects of separate components in the obtained material. The registration of the spectra was carried out on an RFS-100/S Fourier-Raman spectrometer (Bruker, Germany) with high-sensitive germanium detector. The Nd:YAG laser with the wavelength of 1064 nm was used as the excitation source.
The working electrode consisted of the active material, carbon black, and polyvinylidene fluoride binder at a weight ratio of 80 : 10 : 10. A lithium metal disc was applied as both counter and reference electrodes. 1 M solution of LiBF 4 salt in the mixture of propylene carbonate and dimethoxyethane at a volume ratio of 3:1 was used as an electrolyte. The design of the electrochemical half-cell is described in detail in [ [10], [11], [12]]. The electrochemical performance was evaluated using a Solartron 1470E (UK) potentiostat/galvanostat. The parameters were measured by the galvanostatic discharge-charge at a current rate of 0.1C (C = 335 mAh g -1 ) in the range from 3 to 1 V during 20 cycles. Within the scopes of the present work, due to half-cell applying, the lithiation process corresponds to discharge while the delithiationto charge.

Morphology
The morphology study by the SEM method demonstrated that all the fabricated materials consist of microtubes ( Figure 2).
The diameters of oxide tubes vary from 1 to 5 m, their lengthsfrom 10 to 100 m, whereas the tube wall thicknesses change in the range of 100 to 500 nm. The microtubes are composed of nanoparticles of sizes from 15 to 50 nm.
The lengths, external and internal diameters, and porosity of microtubes as well as the nanoparticle sizes depend on the content of dopants and treatment temperature. Along with increase of the content of dopants and treatment temperature increase from 550С to 850°С, the decrease in the oxide microtubes lengths was observed and the degree of their destruction increased. One can see that TiO 2 /ZrO 2 microtubes are characterized by a lesser diameter than TiO 2 /HfO 2 ones.  To estimate the sizes of nanoparticles forming the tubes, the SAXS method was used. Based on the SAXS spectra, the pair distribution functions P(r) of the radii of gyration for scattering particles (R g ) were calculated, which allowed revealing the regularities in the arrangement of neighboring atoms, molecules, or atomic groups at a distance of r. The program GNOM was used [ [13]]. The obtained P(r) and R g for the materials are presented in Figures 3, 4. For Zr-doped microtubes, the type of P(r) curve corresponds to scattering from spheroids at Zr/Ti ratio of lower than 0.04 (doping level) (Figure 3а). In this concentration range, P(r) has one maximum. The dependence of R g for nanoparticles on the Zr/Ti ratio shows that the average sizes of nanoparticles decrease nonlinearly (Figure 4). The decrease in average sizes of nanoparticles is probably related to the increase in heterogeneity of the nanoparticles' crystalline lattice and growing microstress in them due to dopant inclusion [ [14]]. The calculated maximal diameters of the scattering particles vary from 25 to 35 nm (Figure 3a).

Nanoparticle sizes
If the Zr/Ti ratio is higher than 0.04, the second maximum increasing along with the increase of the ZrO 2 content emerges on the P(r) curve while R g of nanoparticles increases. As was found earlier, the binary form of the P(r) curve corresponded to the formation of nanoparticles with the core-shell   [15]]; in this case, the nanoparticle comprises a shelled nanocrystallite. The calculated maximal diameter of scattering particles varies in the range from 35 to 50 nm depending on the doping level (Figure 3b).
The character of changes in the sizes of nanoparticles forming the basis of Hf-doped microtubes differs from that of Zr-doped ones. At the annealing temperatures of 650°C and the doping level of up to 0.06, the average sizes of particles do not virtually change being equal to about 25 nm (Figure 4). Further increase of the Hf/Ti ratio results in the decrease in the nanoparticles sizes and the emergence of the second maximum on the P(r) curve (Figure 3b).

Structure of nanoparticles
To determine the structure of nanoparticles forming the microtubes and reveal the role of dopants in the synthesized materials, their Raman spectra were studied. Ramаn spectroscopy is known to enable one to identify changes in the matter crystalline state even if the particle sizes are small.
In the Raman spectra of the materials under study, the bands conforming to the structure of anatase appear at a Zr/Ti ratio of up to 0.04 and a Hf/Ti ratio of up to 0.06 ( Figure 5). Figure 6 shows the dependence of the bands maxima ν 4 , ν 6 , ν 8 in the spectra on the Zr/Ti ratio (0 -0.04) and Hf/Ti ratio (0 -0.06). Along with the increase in the content of the dopant, a shift of the bands ν 4 , ν 6 and ν 8 towards lower frequencies is observed. A number of studies of the reasons of the bands' location change in the Raman spectra of nanosized materials were performed [ [17], [18], [19]]. The nanocrystallite size change was identified as the main reason of the above processes. As was shown on the example of nanodispersed TiO 2 (anatase), low-frequency shift of bands in the spectrum was observed along with the increase of the sizes of nanocrystallites [ [17]]. According to the data of SAXS (Figure 4) and Raman spectroscopy, the above-mentioned dependence is not valid for materials examined in the present paper. It is generally accepted that the other cause of the low-frequency shift of the ν 4 , ν 6 , ν 8 bands in the spectra is related to the presence of structural defects [ [18], [19]], which can include incorporation of the dopant atoms into the material structure that results in changes of lattice parameters. In the spectra of Zr-doped materials, the more noticeable shift of the ν 4 band (as compared to that of Hf-doped materials) was observed ( Figure 6). Taking into account similarity of the Zr 4+ and Hf 4+ ionic radii, the different behavior of the bands in the spectra can be related to the difference of their atomic masses and the emergence of the relativistic effect for hafnium. The dependence of the bands' locations on the atomic masses of cations was demonstrated on the example of spectra of ZrO 2 and HfO 2 crystals [ [20]].
Thus, according to the data of Raman spectroscopy and SAXS, the materials within the range of 0-0.04 for Zr/Ti ratio and 0-0.06 for Hf/Ti ratio consist of the Ti 1-x Zr x O 2 or Ti 1-x Hf x O 2 nanoparticles and have the structure of anatase. Further increase of the dopant contents in the studied materials results in changes in the Raman spectra. In addition to bands of anatase, the bands corresponding to the phase of zirconium or hafnium titanate emerge ( Figure 5) [ [21]]. Therefore, one can assume the formation, along with the increase of the doping level (Zr/Ti>0.04 and Hf/Ti>0.06), of nanoparticles having a core (nanocrystallite) composed of solid solutions of Ti 1-x Zr x O 2 or Ti 1-x Hf x O 2 with the anatase structure and a shell consisted of both anatase and zirconium or hafnium titanate. The emergence of the second maximum on the P(r) curve ( Figure 3b) and changes in the Raman spectra ( Figure 5) are in agreement with this assumption. According to the SAXS data, the shell growth takes place along with increase in the doping level. i.e., in changes of the maxima ratio on the P(r) curve changes. These conclusions agree well with different temperatures of crystallization of tentative structural components, since crystallization of the undoped TiO 2 can occur in the range 250-500°C while crystallization of ZrTiO 4in the range 600-1300°C depending on preparation conditions [ [14], [22]].

Electrochemical investigations
To evaluate the electrochemical performance of nanostructured Zr-and Hf-doped anatase TiO 2 materials as anodes for Li-ion batteries, preliminary galvanostatic discharge-charge measurements were carried out. Figure 1 shows the results of 20-fold cycling for undoped, Zr-doped (Zr/Ti = 0.01; 0.02; 0.03), and Hf-doped (Hf/Ti = 0.01; 0.02; 0.04; 0.06) anatase TiO 2 electrodes at a 0.1C-rate. , and 140 mAh g -1 , respectively, demonstrating better capacity retention of the doped electrodes in comparison with undoped TiO 2 that yielded only 65 mAh g -1 . It seems that enhanced capacities of doped samples are mainly related to facilitating of Li + ions diffusion due to the difference in the values of ionic radii of Zr 4+ and Ti 4+ ions leading to changes in lattice parameters of anatase TiO 2 . In addition, according to electrochemical impedance spectroscopy, the charge redistribution in Zr-doped TiO 2 lattice results in the conductivity increase by two orders of magnitude from 4.4·10 -11 S cm -1 (undoped) to 2.2·10 -9 S cm -1 (Zr/Ti = 0.03) that promotes the electron transport [ [23]].
The is directly depends on the particle size distribution, parameters of the crystal lattice, the number of oxygen vacancies, etc. So we suggest that the inferior capacity enhancement for Hf-doped TiO 2 (as compared to Zr-doped one) is the effect of larger nanoparticles (Figure 7). Note that ZrTiO 4 and HfTiO 4 ) are electric insulators [ [24]]. Hence the formation of Zr 1-x Ti x O 4 (or Hf 1-x Ti x O 4 ) solid solution deteriorates the conductivity of the doped material leading to inferior electrochemical parameters of its application as Li-ion battery anode.

Conclusions
New TiO 2 /ZrO 2 and TiO 2 /HfO 2 materials of varying compositions have been fabricated by the template sol-gel method. The oxides consist of nanostructured microsized tubes formed by nanoparticles of sizes varying from 15 to 50 nm. TiO 2 -based Hf-doped microtubes have larger diameters and consist of larger nanoparticles as compared to Zr-doped ones.
In the ranges of 0 to 0.04 for the Zr/Ti ratio and 0 to 0.06 for the Hf/Ti ratio, the materials are formed by nanoparticles composed of Ti 1-x Zr x O 2 or Ti 1-x Hf x O 2 and have the structure of anatase.
The increase in the material doping level above 0.04 (Zr/Ti) and 0.06 (Hf/Ti) yields changes in the structure of nanoparticles. Probably, the nanoparticles have a core (nanocrystallite) formed by solid solutions of Ti 1-x Zr x O 2 or Ti 1-x Hf x O 2 with the anatase structure and a shell consisted of both anatase and Zr x Ti 1-x O 4 or Hf x Ti 1-x O 4 at the above-mentioned content of dopants.
The promising electrochemical properties of both as-prepared Zr-and Hf-doped anatase TiO 2 samples as anodes for Li-ion batteries were identified. A good reversibility of the Li + intercalation into and deintercalation from the nanostructured Zr-and Hf-doped TiO 2 results from lattice parameters changes and conductivity increase and, thus, provides the improved dynamics for both electron and ion transport. To sum up, the method of synthesis is efficient in terms of the electrochemical performance of the battery systems.