Optimization of Silica Content in Initial Sol−Gel Grain Particles for the

Optimization of Silica Content in Initial Sol-Gel Grain Particles for the Low Temperature Hydrothermal Synthesis of Titania Nanotubes M. Alam Khan*,‡ and O-Bong Yang* School of Semiconductor and Chemical Engineering, Chonbuk National UniVersity, Jeon-Ju, Korea 561-756

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1767–1774

ReceiVed July 31, 2008; ReVised Manuscript ReceiVed January 18, 2009

ABSTRACT: High purity anatase titania nanotubes were prepared by a low temperature hydrothermal process from anatase TiO2 nanoparticles synthesized by a sol-gel process, and the optimization of titania/silica ratios at initial synthetic sol-gel processes is elucidated in detail. It was observed that a high titania/silica ratio (50:50) in the initial sol-gel synthetic process does not allow titania nanotubes to form; rather, a lamellar structure of protonic lepidocrocite titanates is obtained after hydrothermal treatment with 8 M aqueous alkali solution, and without silica in the initial grains at the sol-gel stage, high crystalline shorter nanorods with dominant rutile phases were observed. However, with titania/silica ratios of 90:10, only pure anatase titania nanotubes of diameter ∼8 nm and length of 500-900 nm were observed. The anatase nanotubes were of good quality and clean surface with no phase observation of sodium titanate and protonic titanates. The nanotube purity was confirmed by field-emission scanning electron microscopy, high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis in TEM, X-ray diffraction, Raman spectroscopy, photoluminescence, and BET surface area techniques.

1. Introduction Of the materials prepared so far, TiO2 remains one of the most promising because of its chemical stability, biologically inertness, nontoxicity, high photocatalytic efficiency, activity, reactivity, low cost, and chemical inertness.1-6 Studies have shown that extremely small TiO2 powders have high photocatalytic activities7,8 due to the high surface area and quantum sized effects9 owing to the modifications of electronic band states as well as the closer existence of photoformed electron and hole pairs and their efficient contribution to the reaction sites. Furthermore, the photocatalytic activities are mainly determined by the high crystallinity and high surface area anatase titania nanostructures. Therefore, a large surface area and anatase crystal phase are essential for efficient and high photocatalytic activities. In recent years, there has been renewed interest in the synthetic routes and processes for nanosize material preparation10-12 leading to materials with highly specific properties. The fundamental physical properties of such nanosized novel materials and large surface areas can lead to unexpected or dramatically different properties.13,14 So the need for high quality systems in order to achieve a quantative understanding of the content and crystal phase effects on physical properties of titania nanotubes is also essential. Crystalline nanotubes with uniform diameters and nanoporous structures probably would be ideal substitutes for such materials with high photocatalytic properties. However, the synthesis of titania nanotubes by Kasuga et al.15 by a mixture of TiO2-SiO2 (80:20 ratio) powder produced by sol-gel treated nanoparticles under aqueous alkali solution by hydrothermal treatment is still unclear despite intensive investigations. Du et al.16 assigned these nanotubes as layered H2TinO2n+1 titanates that are neither anatase nor rutile tubes based on their X-ray diffraction (XRD) patterns. Zhang et al.17 synthesized TiO2 rutile nanotubes. TEM observations indicated the existence of layered structures, and selected area electron diffraction patterns (SAED) were pre* Corresponding authors. E-mail: [email protected] (O.-B.Y.), [email protected] (M.A.K.). Phone: +82-63-270-2306. Fax: +82-63-2702306. ‡ Present affiliation: Yeungnam University, Gyeongsan, Korea.

sented as evidence of layered anatase phases,15 but no reported phase of titania had layered structures similar to titanates and hydrous titanates.18 These nanotubular structures are still in dispute but are widely accepted as scrolling of an exfoliated TiO2 derived nanosheet with crystallographic descriptions varying from tetragonal anatase19 to monoclinic H2Ti3O7.20,21 And the controversy still goes on. Clearly, the size, the phase and the composition of nanocrystalline titania nanotubes, the structure and the degree of defects in the prepared samples have a decisive influence on the usefulness of these nanocrystalline titania nanotubes as photocatalytic materials. It is thus important to reveal the influence of initial silica content at starting sol-gel nanoparticles synthesis, which is the basis for titania nanotubes after hydrothermal treatment and their structural similarity/differences. In this paper we report controlled silica addition and its optimization for the formation of only anatase titania nanotubes. The initial grain phases of nanoparticles during hydrothermal treatment (anatase or rutile) are studied. It is clear from the data that the crystallinity and phase of the starting nanomaterial as well as the silica content in the initial grains considerably influence the final morphology and crystal phase of synthesized nanotubes.

2. Experimental Procedures (a) Synthesis of titania nanoparticles: titanium tetraisopropoxide 99% (Junsei Chemical Co.), tetraorthosilcate 99% (Acros Organics), ethanol (Aldrich), hydrochloric acid (36%, Showa Chemical Co.) were purchased. All chemicals were reagent grade and used as received. Nanoparticles were prepared by a sol-gel route. First, the mixture of TiO2/SiO2 mol ratio of 90:10 was obtained by mixing 52 mL of titanium isopropoxide (TTIP, Ti[OCH(CH3)2]4) and 5.2 mL of tetraethyl orthosilicate (TEOS, Si(OC2H5)4) and dissolving in 52 mL of ethanol (99.5%). After the first mixture solution was refluxed at room temperature for 1 h, the second mixture of 52 mL of ethanol and 40.6 g of 4 M aqueous HCl was added slowly to the first mixture solution to form a gel, and it was further refluxed at ∼1500 rpm at room temperature for 1 h; after the hydrolysis the sol was gelled in an incubator for 48 h at 80 °C with a relative humidity of 60%. Then the sample was calcined at 600 °C in air for 3 h. Then it was ground, pulverized, and passed to a ∼38 µ sieve. White fine nanopowder was obtained. Similarly, titania and silica ratios of (50:50), (80:20), and (100:0) were prepared, respectively.

10.1021/cg800837f CCC: $40.75  2009 American Chemical Society Published on Web 03/02/2009

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Figure 1. FE-SEM images and XRD patterns (insets) of titania nanoparticles with initial grain particles containing a titania/silica ratio of (a) 90:10, (b) 100, (c) P-25 degussa, (d) P-25 nanotubes. (b) Synthesis of titania nanotubes: 2 g of the above-synthesized powders respectively was taken in 8 M aqueous NaOH solution into the Teflon vessel (Reaction Engineering, Korea) autoclave at high pressure and 130 °C for 20 h with continuous stirring at 200 rpm; then after the product was naturally cooled at room temperature, it was sonicated for 20 min, then washed with 0.1 N HCl solution and with copious amounts of distilled water many times. The resulting product was dried at 80 °C and calcined in air at 250 °C for 2 h before characterization. These products were numbered as samples (a), (b), (c), and (d) respectively. (c) The details of the structure and morphological analysis of titania samples were examined by field emission scanning electron microscopy (FE-SEM, Hitachi 4700), high-resolution transmission electron microscope (HR-TEM, Philips Technai 160 kV, three focused step, bright field), X-ray diffraction (XRD, Rigaku, with Cu KR radiation), energy dispersive X-ray analysis in TEM (TEM-EDX, JEOL JEM-2010, high voltage range of 80-200 kV), photoluminescence (PL, fluorospectroscopy 15S), Raman spectra (Renishaw, RenCam CCD detector with spectral resolution of 2 cm-1) and BET surface area (Micromeritics ASAP 2100) techniques.

3. Results and Discussion Figure 1 shows FE-SEM images and corresponding XRD patterns (inset) of synthesized nanoparticles used for hydrothermal treatment in aqueous alkali solutions in the autoclave. For comparison commercial P-25 nanoparticles which contain both anatase (80%) and rutile (20%) phases were also used to synthesize titania nanotubes by the same process and under the same conditions. However, the nanotubes were formed by employing P-25 nanoparticles under hydrothermal treatment

having rough surfaces aggregated with hetero phase containing anatase and rutiles in the final product (XRD). The surfaces of nanotubes were not uniform, and smaller aggregated particles on the nanotube surface were observed indicating a lower aspect ratio than the titania-silica nanoparticles as observed from the FE-SEM images. Figure 2 depicts FE-SEM images of synthesized titania products after hydrothermal treatment of sol-gel derived nanoparticles calcined at 600 °C, denoting 50, 20, 10, and 0.0% of silica content at initial grain particles. It shows that when the silica content was 50% (a) the formation of nanotubes was hampered and a dense lamella was observed. When the silica content was 20 or 10% (b) and (c) clear nanotubes ca. 500-900 nm were observed; however, in the case of 20% silica content clean surface nanotubes were in stacks of bundles and aggregated perhaps due to the presence of a high content of silica. With 10% silica content, long clean nanotubes, individually scattered were observed with no contamination attached to their surfaces. In the case of 0.0% silica used in the sol-gel processing, exclusively rutile phase short titnania nanorods were observed. Here two points might be noticed that (1) silica acts as a good support in 10 and 20% mixing and prevents rutile phase formation and hence acts as a good promoter and binder and (2) without silica content different morphologies such as shorter nanorods are formed with rough surfaces. Figure 3 shows the HR-TEM images of individual nanoscale materials after hydrothermal treatment at different silica content

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Figure 2. FE-SEM images of synthesized titania products with initial grain particles containing a titania/silica ratio of (a) 50:50, (b) 80:20, (c) 90:10, and (d) 100.

levels in a sol-gel process; sample (a) reveals the image of 50% silica content with no nanotube formation observed; instead, lamellar sheets were observed showing that a higher content of silica support does not help in the formation of nanotubes. However in (b) and (c) clear nanotubes were formed. The single nanotubes were open at both ends with an inner diameter of ∼5.1 nm and an outer diameter of ∼8 nm with multiwalled structures and one side is slightly thicker than the other. These nanotubes have few defects as seen in the shell layers of the tubes. The electron micrograph projected along the channel axis presents a scroll structure, which suggests that the titania nanotubes were formed by rolling of a single sheet of titanium oxide. HR-TEM images observed are well-defined structures growing along the [001] direction. Two sets of lattice fringes can be observed in the lattice resolved images. The fringes parallel to tube axis along the [001] direction correspond to an interplanar distance of ca. 0.78 nm, which is characteristic of an anatase crystal phase.22 Another set of fringes with a smaller interlayer spacing ca. 38 nm along the [101] plane in the direction of the axis is characteristic of an anatase crystal structure. The formation of Na2Ti3O7 can be excluded, as the interlayer spacing in Na2Ti3O7 is ca. 84 nm different from our findings. It is also notable that in the case of nanotubes prepared by 20% concentration of silica few nanotubes with corrugated sides are observed, and these corrugated sides are not previously reported in the case of titania nanotubes which might provide evidence of cluster-cluster growth at rearrangements of chemical bonds in strong alkali solutions. In the case of sample (d), the short solid rutile titania nanorods were formed. There were no walls and hollow channels; instead, scrolling edges were observed showing nanorods formed by scrolling. The sides of nanorods are corrugated and have not been previously reported.

In Figure 3 inset is shown the corresponding selected area electron diffractions (SAED) patterns of samples (a), (b), (c), and (d). These SAED patterns clearly indicate that with a high content of silica the diffraction patterns were blurred and no clear ring pattern was observed perhaps due to the excess amount of silica. In the case of 20 and 10% of silica content primary and secondary ring patterns were observed, which can be indexed as (101) and (002) corresponding to 2θ of 25.1° and 48.9° anatase phase of titania, but still the patterns were not distinct showing low crystallinity as well as layered structures. However, in the case of rutile nanorods the patterns are more clear and prominent showing higher crystallinity which is very consistent with the XRD patterns. Figure 4 shows the TEM-EDX results taken from the place where dense amounts of nanotube clusters were observed to analyze the content of silica, Ti, oxygen, and sodium in all the samples after hydrothermal treatments. The details of at. % of Ti/O2/Si/Na in all the samples are given in Table 1. It is shown that a higher silica content (50%) acts as an unstable support, and substantial amounts of silica were observed after hydrothermal conditions with strong aq. alkali solution. Sodium content was absent in this sample, which was probably flushed out after washing and a lamellar shape with no intercalation layer is formed. In samples (b) and (c) the sodium and silica content of 3.33 at. %, 1.59% and 2.7%, 0.05% respectively was observed where sodium ion may be intercalated in layers of nanotubes; however, in the case of sample (d) the atomic wt % of sodium was observed in much smaller quantities, which means that sodium is physically attached with high crystalline nanorods and is flushed away by washing with dilute acids and distilled water. In our all samples, no phases of sodium titanate were observed, and from the TEM-EDX data the sodium to

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Figure 3. HR-TEM images of synthesized titania products with initial grain particles containing a titania/silica ratio of (a) 50:50, (b) 80:20, (c) 90:10, (d) 100. Inset is the corresponding SAED pattern.

titanium ratio could not be assigned to sodium titanates. BET surface area and pore size of the prepared samples (a), (b), (c), and (d) were analyzed and found to be 110, 98, 95, and 94 m2/g and 6.3, 4.9, 5.2, and 4.2 nm, respectively. Figure 5 depicts the X-ray diffraction patterns of synthesized titania products after hydrothermal treatment. All of the diffraction patterns were compared with the data listed in JCPDS 21-1272 of anatase TiO2. Prominent and sharp broad characteristic peaks in 50 and 20% silica content were observed at 2θ values of 24.4°, 28.6°, 48.3°, and 61.9° corresponding to anatase, (101), (200), (204) phases, respectively; however, a broad peak at 28.6 appeared due to rutile characteristics. In both cases a rutile peak at 28.6° degrees was observed in addition to the broad anatase peaks. The broadness of the [101] peak is due to the nanometer size effect. Du et al. assigned it as titanate nanotubes because no single anatase or rutile phases were dominant, but when the silica content was 10% only two peaks at 2θ values of 25.1° and 48.9° were observed, which were purely anatase, and the rutile peak at 28.6° was absent. Thus, we assigned it as titania nanotubes. However, other anatase peaks are not expressed possibly due to the very low crystallinity of the products. The low crystallinity of nanotubes prepared are consistent with a less prominent diffraction pattern of SAED patterns because the incomplete destructive interference in scattering directions may result in the absence of less intense peaks at (004), (105, 211 overlapped) in the nanotubes. It is noteworthy to observe here that the peaks in the XRD pattern of our synthesized nanostructures located at 2θ values of 24.4°,

28.6°, and 48.3° can be ascribed to the 110, 130, and 200 peaks, respectively, of lepidocrocite titanates. Hence, it is speculated only by the XRD patterns that the sample (a) and (b) might be assigned as an orthorhombic lepidocrocite (HxTi2-x/40x/4O4 (x ∼ 0.7, 0: vacancy)) titanate structure.23-25 In the case of sample (d) where the source of grain was exclusively rutile, after hydrothermal treatment dominant rutile with a few anatase peaks was observed. Rutile at 2θ values of 27.6°, 36.1°, 39.3°, 41.3°, 44.1°, 54.5°, 56.7°, 62.8°, 64.1°, 69.18° corresponding to (110), (101), (200), (111), (210), (211), (220), (002), (310), (301), and anatase peak at 24.5° and 48.3° is observed; thus it shows the importance of initial grain particle phases before hydrothermal treatment is crucial, and the appearances of few anatase peaks in pure rutile phases is conjectured as due to rearrangement and recrystallization of Ti-O to O-Ti at low hydrothermal treatment, and because of the lower surface energy of anatase crystals it is obvious that some anatase peaks appear in the nanorods. All the peaks observed showed broad longrange order with obvious nanosized width of (101) peaks in all the samples. Figure 6 shows Raman spectra of the synthesized samples to evaluate the purity and crystal phases of synthesized titania materials. Generally, the anatase phase is tetragonal (D194h) and has six Raman active modes (A1g + 2B1g + 3Eg), and rutile is tetragonal (D144h) has four Raman active modes (A1g + B1g + B2g + Eg).26 In the case of sample (a), three broad peaks with less intensity were observed at 279, 445, and 702 cm-1, and a shoulder at 197 cm-1 (Eg), 397 cm-1 (B1g), and 642 cm-1 (Eg)

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Figure 4. TEM-EDX spectra of synthesized titania products with initial grain particles containing a titania/silica ratio of (a) 50:50, (b) 80:20, (c) 90:10, and (d) 100. Table 1. Showing Details of Chemical Composition and BET Surface Areaa

samples Ti/Si Ti/Si Ti/Si Ti/Si a

(50:50) (80:20) (90:10) (100:0)

BET pore surface size Ti O2 Si Na area (m2/g) (nm) (at. wt %) (at. wt %) (at. wt %) (at. wt %) 110 100 98 94

6.3 4.9 5.2 4.2

31.13 33.08 35.0 35.38

59.44 63.00 62.25 64.28

9.43 1.59 0.05 0.0

0 3.33 2.7 0.34

Obtained from TEM-EDX.

Figure 6. Raman spectra of synthesized titania products with initial grain particles containing a titania/silica ratio of (a) 50:50, (b) 80:20, (c) 90:10, and (d) 100.

Figure 5. XRD patterns of synthesized titania products with initial grains particles containing a titania/silica ratio of (a) 50:50, (b) 80:20, (c) 90:10, (d) 100.

can be assigned as protonic lepidocrocite titanate as reported in the literature.23-25 Therefore, it is more reasonable to assign it as protonic lepidocrocite titanates as the data are consistent with XRD patterns. In the case of samples (b) and (c), Raman peaks at 197 cm-1 (Eg), 397 cm-1 (B1g), 514 cm-1 (A1g and B1g, unresolved), and 642 cm-1 (Eg) all are typical to anatase broadband peaks close to those in bulk anatase phase; in addition

to anatase peaks, a Raman peak at 279 cm-1 was observed in synthesized nanotubules (b) and (c) samples, which is assigned due to oxygen defects in the samples. High intensity broad peak of 240 to 290 cm-1 is assigned to lattice-disorder, which in our case is assigned as a deficiency of oxygen defect-induced disorders in nanotubes are very consistent with TEM-EDX.27,28 A Raman peak at 447 cm-1 (Eg) is also observed in all samples and was the least intense in sample (c), which is of a rutile peak, and is evidence of rutile phases even though dominant anatase phases as confirmed by XRD profiles (shown) after hydrothermal treatment rutile peak were observed in titania nanotubes except in sample (c), and where this peak is a weak shoulder, we did not find Raman active peaks for any impurities such as chemical reaction products in sample (c). Therefore, sample (b) cannot be assigned as protonic lepidocrocite titanates even though the XRD patterns are similar. That means anatase phases were dominant and are of good quality. Although the

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may be due to high surface areas along with good surface energy distribution with more uniformity with high quality interfaces.

4. Discussion

Figure 7. PL spectra of synthesized titania products with initial grains particles containing titania/silica ratio of (a) 50:50, (b) 80:20, (c) 90: 10, (d) 100:0.

diameter of the nanotubes was ∼8 nm, no significant shift of Raman spectra owing to nanosize or pressure effect were observed. It is envisaged that an optimum silica content of 10% could be assigned for anatase titania nanotubes. In the case of sample (d) where no silica was used, the dominant rutile nanorods observed are consistent with XRD patterns where significantly high intensity and broad Raman peaks at 447 cm-1 (Eg), 612 cm-1 (A1g) and broad peak at 244 and 279 cm-1 result from a second-order process,27,28 which reveals that defect-induced disorders in relation to oxygen deficiency are consistent with TEM-EDX determination. These spectral modifications have two origins, namely, an oxygen deficiency and (or) the phonon confinement effect, which is related to the nanosize effect. According to Barsani et al.29,30 and Zhang et al.31 the spectral modification observed is due to phonon confinement and partly due to oxygen deficiency; however, in our case because the nanosize particles all the peaks are broad and hence we assign it to oxygen deficiency rather than to phonon confinement. In our samples it is hard to say any sample is Na2Ti3O7/H2Ti3O7 as the Raman measurement of Na2Ti3O7/ H2Ti3O7 was first studied by Bambarger et al.,32 who found sharp peaks at a lower wavenumber regime of below 400 cm-1, which is absent in our nanotube Raman spectra. Photoluminescence (PL) is an extremely useful tool for obtaining information about the electronic, optic, and photoelectric properties of materials. PL spectra were measured for all samples excited at 310 nm at room temperature as shown in Figure 7. In general PL spectra of anatase titania material are usually due to three kinds of origins: self-trapped excitons,33 surface states,34 and oxygen vacancies.33 The spectra of samples (b) and (c) showed a sharp and high intense peak at 360 nm as compared to samples (a) and (d). A broad PL spectrum of rutile nanorods (d) may be due to various kinds of prevailing surface defects. The main spectral bands of sample (a), (b), (c), and (d) at 338, 360, 360, and 363 nm are ascribed to self-trapped excitons localized on TiO6 octahedra, respectively.33 Serpone et al.35 reported that the PL bands of titania anatase nanocrystals at the long wavelength range (442, 455, 465, and 502 nm) were attributed to oxygen vacancies. So the peaks in sample (d) at 426 and 465 are assigned as oxygen vacancies peaks consistent with TEM-EDX and Raman results. The PL peak of sample (a) has less intensity and blue-shifted to 22 nm compared to 10 and 20% silica content sample possibility due to the substantial amount of silica content and oxygen deficiency in the product and weak hetero bonding of Ti-O-Si interfaces.36 From the spectral lines it was clearly observed that the PL spectrum of nanotubes (c) was much stronger and sharper and high intensity

Titania/silica is a high thermal stability system, and the addition of a small amount of silica never allows the rutile phase to form due to the stabilization of the anatase phase (high photoactive than rutile) by surrounding silica through Ti-O-Si interfaces, and the strong interaction between the tetrahedral Ti species and the octahedral Ti sites in anatase are thought to prevent the transformation to rutile phases.37 At lower silica content, practically all titania interacts with silica; however, higher silica concentrations Ti4+ to O2- octahedra are misbalanced leading to a decrease in binding energies,38 so the optimization of silica to titania is thought to be a necessary ingredient to synthesize flawless anatase phase titania nanotubes by the hydrothermal process. In our samples, the formation of nanotubes were independent of the crystal sizes but highly dependent on initial grain crystal phases and their crystallinity, which leads to the different morphological phases of product such as nanotubes, rods, and lamellas. The results indicate that the formation of disordered intermediate phases and the generation of initial seed is a crucial step in the formation of nanotubes. This initial disorder phase may contain a substantial amount of sodium and protons, and it is speculated that at this primitive stage crystallinity and crystal phases act as an inherent generator of initial seeds which rearrange and contribute to the nucleation and growth of nanosheets. From the above results, we found that with dominant anatase nanoparticles we obtained anatase peaks in the products and where rutile is in the dominant phases we obtained rutile nanorods. The mechanism of lamella formation can be explained in Figure 8a as the early stages of disordered phases nanoparticles react with aq. NaOH solution, and because of the presence of excess Na+ ions, which cannot intercalate in weakly formed nanosheets, these nanosheets do have not enough surface energy to withstand the intercalated Na+ ions due to the weak bonds of Ti-O-Ti octahedra owing to the high presence of silica content (TEM-EDX) which destabilize the formation of stable sheets. With the boiling of aq. NaOH solution at 130 °C in a sealed autoclave these disorder stages of single sheets of titania layers peeled off from the nanocrystals like eucalyptus tree bark shedding forming the lamellar structures due to low surface energy. The formation mechanism of 20 and 10% silica containing samples with aq. NaOH solution can be explained in Figure 8b. A substantial amount of Ti-O-Ti building units in TiO2 particles are broken and form Ti-O-Na and Ti-OH bonds forming nanosheets. However, a few of the anatase titania particles are still not dissolved fully and remain in the solution and the crystallinity may acts as a inherent seed for nanosheet formation in the product. These grains are TiO6- and prefer edge sharing owing to the excess amount of hydroxyl ions. It is reported in the literature that the anatase titania (101) peak is the most stable and prefers edge sharing in TiO6- octahedra,39 and our XRD results show that (101) peak was observed where anatase nanoparticles were used as initial grains for nanotube synthesis. To form the Ti-O-Ti dimer by sharing edges, sodium ions and hydroxyl groups in Ti-O-Na and Ti-OH bonds are removed by the reaction of acids and water after washing.40 These dimers are condensed to form skewed chains as a result of an oblation process41 due to partial charge balance.42 The anatase surface energy can be quenched easily in the hydrothermal treatment process because of the lower surface energy leading to preferable growth for the anatase phase

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Figure 8. Probable mechanism of the formation of (a) lamella, (b) nanotubes, (c) nanorods.

(a ) b ≈ 3.78 Å, c ≈ 9.25 Å with longer c axis) rather than the rutile phase in titania nanotubes.43 It is also supported by Raman spectra results in which the crystalline nanotubes show very similar peaks at the position of anatase titania. Bambarger32 first reported the Raman spectra of sodium titanate (Na2Ti3O7), and notable features in these Na2Ti3O7/H2Ti3O7 materials adsorbent peaks below 400 cm-1 were observed; however, in our case typical anatase broadband peaks were observed. PL spectra also show sharp and intense bands showing nanometer sizes with better quality interfaces in the 10% silica containing sample. The change in crystal morphology could be rationalized based on the selective interaction of the intermediate phases in high basic solutions showing the growth of specific lattices planes (most stable) in the final crystal forms. In the case in which only rutile titania is used as initial grain particles, where rutile is the dominant phase in initial grains with (110) phase it is thermodynamically most stable and corner sharing is preferred in TiO6- octahedral.44 In Figure 8c it can be suggested that a few of the rutile titania particles do not dissolve fully and remain in the solution which inherently acts as a seed for the nanosheet formation. These grains are TiO6-

octahedra and prefer corner sharing owing to the polarity developed due to H+ and Na+ ions. During the hydrothermal treatment single surface layers are generated due to the bigger nanoparticles, and an asymmetrical chemical environment occurs due to the imbalance of H+ or Na+ ion concentrations on two sides of a nanosheet giving rise to excess surface energy, resulting in bending. When the formed nanosheets have unequal proton distribution, then both sides have different values of free surface energy, and, in order to compensate imbalance in surface tensions, the plane bends toward the surface forming shorter nanorods with same phases and crystallinities. TEM images (Figure 3) also show corrugated surfaces were observed on nanorods. The presence of a few anatase peaks arising in rutile nanorods is due the low surface energy and high stability of anatase crystals during the process of dissolution/crystallization of nanosheets as observed in the XRD patterns. In a simple approach, the excess energy of strong selective surface interactions involves a change in the crystal morphology by changing the phases and aggregated nanoparticles rearrange their orientation to decrease the energy of the system favoring the growth of nanorods. It is clear from the data that in the absence of

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silica, the length, morphology, and phase is affected in the titania nanotubes/nanorods synthesized by a hydrothermal process.

5. Conclusion In conclusion we have successfully synthesized and optimized silica concentrations in the formation of titania nanotubes where the silica content in initial grains, its crystallinity, and phases before hydrothermal treatment have a profound influence on the products and optimized silica content (90:10) only can be assigned as anatase titania nanotubes evidenced by XRD, Raman, and HR-TEM images and can be synthesized in bulk with the clean surfaces. Raman spectra of sodium titanate (Na2Ti3O7) and the notable features in Na2Ti3O7/H2Ti3O7 are absent showing typical anatase broadband peaks. However with 50% silica content the lamella can be ascribed as protonic lepidocrocite titanates. Titania nanorods are obtained by rutile grain particles, which have no walls and hollow space channels with corrugated surfaces. These anatase nanotubes/nanorods might have many potential applications in photocatalysis, solar cells, and photoelectronics. Acknowledgment. M.A.K. thanks Yeungnam University for granting a faculty position and BK21 program for financial support.

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