Flower-shaped anatase TiO₂ mesostructures with excellent photocatalytic properties

Abstract

The present paper reports a simple route to fabricate high surface area anatase TiO₂ mesoflowers from electrospun TiO₂–SiO₂ composite nanostructures. Electrospun fiber- and rice-shaped TiO₂–SiO₂ composite nanostructures upon treatment with concentrated alkali (NaOH) under hydrothermal conditions (180 °C) result in chemical transformation of the TiO₂ and in situ etching of SiO₂ to give sodium titanates. The sodium titanate upon acidification followed by a low temperature sintering (180 °C) results in 3D TiO₂ mesoflowers. The material is found to be superior to the commercial P-25 in photocatalysis.

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Titanium dioxide (TiO₂) is a material for multifaceted applications in areas such as dye/quantum dot-sensitized solar cells (DSCs),¹ Li-ion batteries,² environmental remediation,³ self-cleaning applications,⁴etc. The performance of TiO₂-based devices depends on its crystallinity, morphology and phase dimension.⁵ As a result; there is an increasing interest in fabricating TiO₂ with novel morphologies and better crystallinity for desired applications. Though several methodologies are available for fabrication of zero- and one-dimensional (1-D) TiO₂ nanostructures, fabrication routes for three-dimensional (3-D) TiO₂ nano/mesostructures such as the flower-shaped ones remain uncommon^6a and there are only very limited reports, mostly based on brookite and rutile TiO₂.^6b,c It is worth noting that amongst anatase, rutile and brookite, anatase is the preferred material for the applications mentioned above.^1–6 In this communication, we present a chemical route to fabricate flower-shaped TiO₂ of high surface area and crystallinity through titanate route.

Titanate route, first reported by Kasuga et al.,⁷ has been used as a versatile method for increasing the surface area of TiO₂ by several scientists. The process involves chemical transformation of the spherical TiO₂ (P-25) particles into 1-D nanorods/nanowires by treating with concentrated alkali followed by acidification, washing and sintering.⁸ However, the titanate route when applied to electrospun TiO₂ nanostructures gives interesting new morphologies of TiO₂.⁹ We have previously shown that electrospun TiO₂–SiO₂ composites when treated with conc. NaOH at 80 °C (under ambient pressure) followed by hydrothermal treatment with acid (HCl) leads to high surface area 1-D titanates/TiO₂.⁹ However, when the reaction sequences were reversed, i.e., when the TiO₂–SiO₂ composites were first hydrothermally treated (in an autoclave at high pressure and 180 °C) followed by acidification and low temperature sintering (180 °C), the result was highly crystalline, high surface area anatase 3-D TiO₂ mesoflowers, which is the subject matter of the present paper. The material showed excellent photocatalytic property.

Experimental details are given in ESI 1–4.†Fig. 1A shows the SEM image of the as-electrospun TiO₂–SiO₂–PVAc composite nanofibers. The fibers were smooth with diameters varying between 80 nm and 400 nm. The as-spun fibers upon sintering (at 450 °C for 3 h) results in the degradation of the polymer forming rice-shaped TiO₂–SiO₂ composites (Fig. 1B). The average dimensions of the rice-shaped structures were ∼400 nm in length and ∼90 nm in diameter. The bottom and top insets of Fig. 1B shows a highly-resolved SEM and TEM image, respectively, of a single rice-like structure revealing its porous nature. Fig. 1C shows an EDS spectrum recorded from the sample showing the elemental composition (Ti, Si and O, the presence of Au was from the sputter coating). Fig. 1D shows a lattice-resolved TEM image indicating the TiO₂ lattice fringes corresponding to 0.35 nm (implying anatase nature of the TiO₂). Powder XRD spectrum of the material showed prominent peaks of the anatase TiO₂ alone (ESI-5†), implying the amorphous nature of the SiO₂ (this is further evident from the TEM image in Fig. 1D where the crystalline regions could be TiO₂ and the amorphous ones SiO₂, marked with dotted circles of different colors). The peaks are indexed in the spectrum itself. The material was further characterized by XPS. The survey spectrum (ESI-6†) and high-resolution spectra of elements (ESI-6†) show elemental composition and oxidation state of the elements. The high-resolution spectra of the O (the spectrum of the O could be de-convoluted into two peaks at 529.2 eV and 532.8 eV, implying the presence of Ti–O–Ti and Si–O–Si bonds)¹⁰ and Si imply the existence of SiO₂ into the TiO₂ matrix. The SEM image of the sodium titanate (Na₂Ti₃O₇) obtained from the rice-shaped TiO₂–SiO₂ composite (before acid treatment and sintering) is shown in Fig. 2A. The initial rice-shaped morphology has been totally lost and sheet/ribbon-like structures appeared. The sheet-like, layered structure of the titanates is evident from the TEM images (Fig. 2B & C, respectively). The spacing between the individual layers was ∼0.8 nm (ref. 11) as evident from the high-resolution image (Fig. 2C). Fig. 2D shows the powder XRD of the titanates showing the relevant diffraction peaks.^7,8,11 The BET surface area of the titanate was found to be 182 m² g⁻¹.

Fig. 1 (A) SEM image of the as-electrospun TiO₂–SiO₂–PVAc composite nanofibers. (B) SEM image of the fibers after sintering showing the rice-shaped TiO₂–SiO₂ composites (top and bottom insets: TEM and SEM images). (C) EDS spectrum acquired from (B) showing the elemental composition. (D) lattice-resolved TEM image of the rice-shaped TiO₂–SiO₂ composite.

Fig. 2 (A) SEM image of the sodium titanate. (B) TEM image of the sheet-like titanate. (C) High resolution TEM image of the titanate sheet. (D)XRD spectrum of the titanate (JCPDS File no. 31-1329).

Fig. 3A & B show a large area and enlarged SEM images, respectively, of the flower-shaped TiO₂ obtained after acidification and low temperature sintering (180 °C) of the sodium titanate. It is evident that the sheet or ribbon-like structures seen for the titanates got completely transformed into flower-shaped mesostructures. Acidification and washing stages help in exchange of Na⁺ with H⁺ (resulting in H₂Ti₃O₇) and subsequent low temperature sintering was enough for conversion of hydrogen titanates to TiO₂.^7,8,11Fig. 3C shows a TEM image of a part of the flower-like TiO₂ revealing that the TiO₂ is actually made-up of small spherical particles of ∼5 nm diameters. High-resolution image of a single particle is shown in the inset of Fig. 3C showing the 0.35 nm lattice spacing corresponding to the (101) phase of anatase TiO₂. Fig. 3D shows EDS spectrum of TiO₂ additionally confirming the elemental composition of the TiO₂ mesoflowers.

Fig. 3 (A & B) SEM images ((A) large area, (B) resolved) of the flower-shaped TiO₂ obtained by the titanate route. (C) TEM image of a wing of the flower. Inset of (C) a lattice resolved TEM image. (D) EDS spectrum of the TiO₂ showing its elemental composition.

ESI-7A† shows the XRD pattern of the flower TiO₂ showing its anatase and highly crystalline nature. Major peaks are indexed in the spectrum itself. This further implies the phase purity of the TiO₂ produced by the route (absence of rutile phase, other impurity, etc.). The average size of the crystallites estimated using the Debye Scherrer equation was ∼6.1 nm in agreement with TEM results. The elemental composition and phase purity of the TiO₂ was assessed by XPS spectroscopy as well. ESI-7B† shows the survey spectrum revealing the elemental composition (Ti & O, respectively). The binding energies of Ti 2p3/2 and Ti 2p1/2 were centered at 458.14 eV and 463.82 eV, respectively (ESI-6C†), corresponding to a spin–orbit coupling of 5. 68 eV. The O1s (ESI-7D†) showed a single peak implying the absence of impurities. The BET surface area of the material was 132 m² g⁻¹. The average pore size and pore volumes of the TiO₂ were found to be 1.3 nm and 0.11 cm³ g⁻¹, respectively. The TiO₂ was further characterized by Raman spectroscopy (ESI-8†). The peaks at 158 cm⁻¹, 201 cm⁻¹ and 644 cm⁻¹, respectively, correspond to the Eg modes and the remaining two peaks at 402 cm⁻¹ and 525 cm⁻¹ correspond to the B1g and A1g modes, respectively.¹² The band-gap of TiO₂ was found to be 3.2 eV (from diffuse reflectance spectroscopy measurement).

From the experimental observations, a tentative mechanism of formation of the flower-shaped structures could be proposed as follows: we believe that the alkali treatment of the TiO₂–SiO₂ composite results in Ti–O–Na bond (amorphous) formation⁸ with the in situ dissolution of SiO₂ from the TiO₂ matrix. The Ti–O–Na bond formation is accompanied by the formation of sheet/ribbon-like structures (SEM image in Fig. 2A). The sheet-like structures upon acidification followed by sintering self-assemble and results in the formation of flower-shaped TiO₂ mesostructures (Fig. 3). The structural rearrangement of the TiO₂ and the in situ dissolution of SiO₂ have contributed to the enormous increase in surface area for the final TiO₂ (from 38 m² g⁻¹ for the TiO₂–SiO₂ composite to 132 m² g⁻¹ for the final TiO₂). A tentative mechanism of the overall formation process is shown in the Schematic in Fig. 4. However, complete unravelling of the mechanism needs further in-depth investigations which are in progress.

Fig. 4 A schematic representing the formation of 3-D TiO₂ mesoflowers (figure not drawn to scale).

We have checked whether the same mechanism operates in electrospun fiber-shaped TiO₂–SiO₂ composites (see ESI-2, 3 & 9†) under the same conditions. SEM images of the fiber-shaped TiO₂–SiO₂ composite (ESI-9†) and the final cauliflower-like TiO₂ are shown in ESI-10.† The anatase nature of the TiO₂ is evident from the powder XRD data in ESI-11.† Thus it is clear that the chemistry operates nearly in the same manner in both the cases, however, the morphology of the final TiO₂ slightly differs from that of the rice-shaped TiO₂–SiO₂ composite. This further implies that the morphology of the final TiO₂ by the route depends on the morphology of the initial TiO₂–SiO₂ composite as well.

To demonstrate the potential use of the material for photocatalytic applications, we have tested the degradation of a drop of methyl orange (MO) on thin films (the films had same area of 2 cm² and average thickness of ∼30 μm fabricated by screen printing technique on glass slides) of the respective TiO₂. The films carrying MO drops (single drop from a dropper) were irradiated for 20 min and digital photographs of the films were taken at an interval of 5 min each (Fig. 5). As could be seen from Fig. 5, the dye degraded faster in the case of the film made out of TiO₂ mesoflowers (near complete degradation in 20 min) while that in presence of P-25 was slower. We have also investigated the degradation behavior of the dye in the absence of light where the effect was found to be negligible. The superior photocatalytic activity of the mesoflowers could be attributed to its high surface area and crystallinity in comparison to that of the P-25.

Fig. 5 A comparison of the photocatalytic degradation of MO dyes on thin films (of same thickness) of the mesoflowers (left panel) and the P-25 (right panel). Images (a), (b), (c), (d) and (e) correspond to images of the films at 0, 5, 10, 15 and 20 min, respectively, of UV irradiation.

In summary, we have found a simple route for fabricating high surface area 3-D TiO₂ mesoflowers from TiO₂–SiO₂ composites. The TiO₂–SiO₂ composite upon treatment with 5 M NaOH under hydrothermal conditions followed by acidification with HCl and low temperature sintering result in flower-shaped anatase TiO₂. The methodology works with rice- and fiber-shaped TiO₂–SiO₂ composites. The mesoflowers were characterized thoroughly by spectroscopy and microscopy. The material showed excellent photocatalysis property.

Acknowledgements

The authors acknowledge financial assistance from Ministry of New and Renewable Energy (MNRE), Govt. of India for financial assistance.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental and characterization details and SEM images of the TiO₂ obtained from TiO₂–SiO₂ composite nanofibers. See DOI: 10.1039/c3ra45021j

‡ These authors contributed equally.

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