Molten salt assisted synthesis of black titania hexagonal nanosheets with tuneable phase composition and morphology

Abstract

A facile, high yield ZnCl₂/KCl molten-salt route is developed to fabricate black titania hexagonal nanosheets under atmospheric pressure and low temperature (400 °C). After post-annealing, the black titania possesses a tunable phase composition and enhanced visible light photocatalytic activity, accompanied with a controllable morphology transformation from hexagonal nanosheets to nanorods.

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Titanium dioxide (TiO₂) as an important semiconductor has attracted extensive interest for its potential use in environmental purification and hydrogen generation.^1,2 However, the wide band gap renders it only responsive to a small portion of sunlight in the UV light region and TiO₂ captures less than 5% of solar energy. Many investigations have attempted to obtain TiO₂ nanostructures with broad spectral response for full utilization of solar energy. Recently, black titania has attracted particular attention.³ As initially reported by Chen et al., black titania exhibited strong visible light absorbance and superior solar photocatalytic performance through a high hydrogen-pressure process.⁴ Hydrogen thermal treatment has been extensively explored since then,⁵ however, most of the studies reported so far have used high pressure or high temperature.^6,7 Though other synthetic routes including other reduction methods (Al, Zn) and some oxidation methods have been proposed,^8–12 an innovative synthesis approach providing easy access to black titania nanostructures with high yield and photocatalytic activity under mild conditions (e.g. low temperature and ambient pressure) still remains a challenge.

Recently, salt molten synthesis (SMS) has emerged as a convenient, low-cost, nontoxic and mass-production route for the synthesis of functional oxide materials.¹³ Another big advantage of SMS lies in the potential of morpho-synthesis of controlled nanostructures due to the highly polar character of the special solvent.¹⁴ As is known, morphology control is of paramount importance for the performance of materials, as properties of materials at nanoscale strongly depend on their shape and dimension, especially that the 2-D nanosheets and 1-D nanorods show unique properties.^15–17 High yield preparation of 1-D single-crystalline SnO₂, Cu₂O and ZnO nanostructures through the SMS method using NaCl as molten salt has been reported.¹⁸ The molten salt, which acts as the reaction medium as well as the reactant speeding up ion transmission, can also lead to well-developed 2-D layered structures. Molten salt preparations often lead to highly anisotropic materials, probably due to preferential interaction of crystallographic planes with the melt ions.¹⁹ Inspired by the chemical oxidation of TiH₂ to prepare Ti³⁺ self-doped black titania,²⁰ we believe that black titania with controlled morphologies and nanostructures could be effectively synthesized by controlled oxidation of TiH₂ under molten salt atmosphere with trace of oxygen.

In this study, we report the facile synthesis of black titania hexagonal nanosheets through oxidizing TiH₂ in a simple eutectic ZnCl₂/KCl salt melt. The proposed molten-salt route requires neither high pressure nor high temperature, and the phase composition and morphology of obtained black titania can be controlled. The initial structure of black titania was tiny rutile hexagonal nanosheets (4–10 nm) embedded in large amorphous hexagonal nanosheets matrix up to several hundred nanometers. After post annealing under Ar atmosphere at different temperatures, the anatase TiO₂ phase appeared, accompanied with an interesting morphology transformation from nanosheets to nanorods. The excellent property of the as-prepared black titania is exemplified by photocatalysis activity in decomposing organic dyes.

In a typical reaction, 2 g TiH₂ was mixed with a eutectic composition of ZnCl₂/KCl (13 g/12 g), and then the powder was homogenized by grinding. During the grinding process, ethanol was added to accelerate the homogenization. Afterwards, the powder mixture was loaded into a muffle furnace, the system was ramped at 5 °C min⁻¹ to 400 °C and kept at this temperature for 3 h. After naturally cooled to room temperature, the product was washed with a large amount of water to remove the salts and dried at 80 °C overnight, the as obtained black product was 3.0 g and denoted as T-400. The black product was further annealed at 450 °C, 500 °C under Ar atmosphere, and denoted as T-450, T-500 respectively. The characterizations of the black titania are discussed in ESI† file.

The structure of the as-synthesized samples has been confirmed by X-ray diffraction (XRD). The commercial TiH₂ is typically fcc TiH_1.971 crystal phase. No diffraction peak associated with TiH₂ is observed, demonstrating that TiH₂ was completely transformed to titania after calcination in molten salt. The eutectic ZnCl₂/KCl salt melt provides a mild atmosphere with trace of oxygen, TiH₂ is firstly dissociate to release metal ions (Ti²⁺) which then gradually oxidized into Ti³⁺. The complete conversion of TiH₂ into black titania is ascribed to its good dispersion in the molten salt system which prevents the caking of the sample. The diffraction pattern in Fig. 1a clearly indicates that the initial sample T-400 is rutile phase (PDF card 75-1755, JCPDS) with rather a small amount of anatase. After post-annealing under Ar atmosphere, the anatase phase (PDF card 21-1272, JCPDS) clearly appeared in T-450. As the annealing temperature increased to 500 °C, the peak intensity of anatase significantly increased, indicating the increase of anatase phase in T-500. The relative rutile/anatase phase ratios calculated by XRD intensity ratio are 95/5, 55/45, 40/60 in T-400, T-450 and T-500 respectively. These results are further evidenced by the Raman spectra of black titania, which show the typical vibration modes from rutile to anatase phase. The Raman spectra of the black titania are presented in Fig. 1b, the two bands at 391, 511 cm⁻¹ confirms the presence of rutile phases and the peaks at 443, 605, 633 cm⁻¹ demonstrate the presence of anatase phases in the annealed black titania, respectively.²¹ XRD results together with Raman spectrum definitely show that the phase composition of our black titania can be controlled.

Fig. 1 (a) XRD spectra and (b) Raman micrographs of the initial synthesized black titania at 400 °C (T-400), annealed black titania at 450 °C (T-450) and 500 °C (T-500) (A = anatase; R = rutile).

Fig. 2a and b shows the representative transmission electron microscope (TEM) and scanning electron microscope (SEM) images of the black titania (T-400). The sizes of the hexagonal nanosheets are typically around 350 nm. Closer observation of Fig. 2a clearly shows that the large hexagonal nanosheets around 350 nm are composed of tiny hexagonal nanosheets less than 10 nm, further evidenced by the high magnification TEM microscopy in Fig. 3a and b. Those crumpled nanosheets indicate that these nanosheets of black titania are ultrathin.²²

Fig. 2 Overview microstructure of the initial synthesized black titania (T-400) by (a) low magnification of transmission electron microscopy (TEM), inset: the distribution of sizes of nanosheets and (b) low magnification of scanning electron microscopy (SEM) image showing hexagonal nanosheet structures.

Fig. 3 (a) and (b) TEM and HRTEM images of the synthesized black TiO₂ (T-400), (c) and (d) TEM and HRTEM images of annealed black TiO₂ at 450 °C (T-450), inset: the enlarged TEM image showing that the nanotubes are consist of hexagonal nanosheets, (e) and (f) TEM and HRTEM images of annealed black TiO₂ at 500 °C (T-500).

The high-resolution TEM images with corresponding HRTEM of the black titania is shown in Fig. 3. The initially synthesized T-400 consists of regular hexagonal nanosheets (4–10 nm) and shows well resolved (110) and (101) lattice planes of rutile phase with plane distance of 0.32 nm and 0.25 nm, and the measured intersection angle between the two planes agrees well with the calculated value of 117°. As the angle in regular hexagon is 120°, there must be dislocations around the tiny rutile phase hexagonal nanosheets. As is shown in Fig. 3b, around these highly crystallized hexagonal nanosheets is amorphous area. Combined with the XRD results, it's concluded that the initial structure of black titania (T-400) was tiny small crystalline rutile phase hexagonal nanosheets (4–10 nm) embedded in amorphous hexagonal nanosheets matrix up to several hundred nanometer. Ionic melts as reaction media provide an alternative to the aqueous chemistry by changing the solubility or the reactivity of species.²³ Similarly to the reported K₂Ti₂O₅ and K₂Ti₄O₉,²⁴ the amorphous nanosheets matrix in our black titania are made of TiO₅ pentahedra or TiO₆ octahedra linked with the K atoms. It's reported that less OH⁻ amount and higher acidity were favorable for the formation of corner-sharing structure, namely the rutile structure.^25,26 Thus, during the heat preserving process, parts of the amorphous phase gradually crystallize into tiny small crystalline rutile hexagonal nanosheets which embedded in the amorphous matrix. Initial tiny small crystalline rutile phase hexagonal nanosheets (4–10 nm) in T-400 grow into larger nanosheets (around 15 nm) for T-450 and orient in line because of annealing. During the annealing process in Ar, the tiny rutile nanosheets grow and simultaneously act as seeds for anatase formation, which occurred on the rutile edges because of dislocations. Crystalline anatase phase with plane distance of 0.35 nm corresponding to (101) lattice planes is observed on the edge of rutile nanosheets in Fig. 3d. Combined with the XRD and Raman results, we can reasonably conclude that the amorphous phase crystallizes into anatase phase during the annealing process. As the annealing temperature increases up to 500 °C, the morphology of T-500 transforms from nanosheets to 1-D nanorods due to the higher energy of the nanosheets. It is noteworthy that the initial intersection angles in hexagonal nanosheets reserves as joint angles between nanorods which are definitely shown in Fig. 3e. The HRTEM displays larger area of anatase phase, in accordance with the XRD and Raman results. These TEM results, in combination with the above XRD and Raman analyses, clearly demonstrate that the synthesized black titania undergoes a crystallization from amorphous into anatase accompanied with the interesting morphology transformation from nanosheets into nanorods during the annealing process.

Electron paramagnetic resonance (EPR) is employed to detect paramagnetic species containing unpaired electrons, which has been widely used to characterize the existence of Ti³⁺ and oxygen vacancies. As indicated in Fig. 4a, commercial P25 contains almost completely Ti⁴⁺ and presents no EPR peaks, meaning neither pure anatase nor pure rutile accounts for the EPR peaks observed in the samples. It was reported that the surface Ti³⁺ tended to adsorb atmospheric O₂, which would be reduced by Ti³⁺ to O₂⁻ and shows an EPR signal at g = 2.02.²⁷ As prepared black titania exhibits strong response at a g value of ∼2.02, verifying the existence of the Ti³⁺ and oxygen vacancies. Given that the signal area is proportional to the amount of Ti³⁺, it can be concluded that the initial T-400 contains the most Ti³⁺.

Fig. 4 (a) EPR spectra and (b) XPS Ti 2p spectrum of the black titania samples (T-400, T-450 and T-500) as compared to that of commercial P25.

After annealing, the EPR signal intensity of T-450 and T-500 halved along with the increased temperature due to the unavoidable oxidation during the annealing process. To further investigate the surface chemical bonding of the black titania samples, X-ray photoelectron spectroscopy (XPS) is adopted. Commercial P25 with Ti 2p3/2 and 2p1/2 peaks centered at binding energies of 458.3 eV and 464.1 eV, which are consistence with the typical pattern of Ti⁴⁺–O bonding. According to the previous reports, the binding energies of Ti³⁺ located at 457.6 eV and 463.5 eV respectively.²⁸ As shown in Fig. 4b, the Ti 2p peaks of T-400 is located at 457.5 eV and 463.4 eV, implying the existing of Ti³⁺ in the surface of T-400. The unavoidable oxidation during the annealing process results in the shift of Ti 2p peaks in T-450 and T-500. All the above XPS results are in consistence with the EPR results, evidenced the existence of the Ti³⁺ and oxygen vacancies. Note that the unique structure with tiny small crystalline hexagonal nanosheets embedded in large amorphous hexagonal nanosheets matrix is considered to contribute to the superior stability.

Fig. 5a displays the diffusion reflectance spectra of P25 and molten salt synthesized samples. As the synthesis temperature increases from 450 °C to 500 °C, the light absorption exhibits nearly no decrease, which is in good agreement with the colors displayed below. All the black titania synthesized at different temperatures display black color indicating the enhanced visible light absorption. The enhanced light absorption is attributed to Ti³⁺ and oxygen vacancies in the samples as confirmed by EPR and XPS results. Unlike surface oxygen vacancies which are unstable enough in air as the Ti³⁺ is easily oxidized, as synthesized black titania exhibits extremely high stability, with the black color remaining for months in air.²⁹

Fig. 5 (a) UV-vis-NIR diffuse reflectance and (b) photograph of black titania samples synthesized at various temperatures (T-400, T-450 and T-500).

The excellent property of as-prepared black titania is exemplified by photocatalysis activity in decomposing organic dyes under the simulated sun light. For different samples, the MO adsorption is nearly the same. Under solar-light irradiation, the photocatalytic activities of the black titania are not as well as P25, due to that the defects in the amorphous matrix may act as recombination centers for photo-induced electron–hole pairs. While under visible light, the black titania shows much better photocatalytic activity than P25 because of its enhanced visible light absorption, as displayed in Fig. 6b. Additionally, the photoactivity shows a regular enhancement for T-400, T-450 and T-500. The amorphous phase crystallization into anatase phase and thus the formation of anatase–rutile hetero-structure may contribute to the efficient electron–hole separation, which is crucial in photocatalysis.³⁰ As a result, T-500 shows the best performance that the MO degradation is accomplished in 60 min under visible light irradiation.

Fig. 6 Plots of C/C₀ for MO solutions vs. the reaction time in the presence of various photocatalysts (T-400, T-450 and T-500) under (a) light irradiation and (b) visible light irradiation (λ > 400 nm).

In summary, we developed a simple, high yield, solution process to fabricate hexagonal nanosheets of black titania. The procedure relies on a salt melt, which enables working under rather mild conditions, that is, atmospheric pressure and relatively low temperature, when compared to other process. The oxidation rate of TiH₂ can be regulated by molten salt atmosphere with trace of oxygen, and the morphology of the black titania kinetically controlled at the same time. Tuning the composition is easily performed by post-annealing under Ar atmosphere, accompanied with an interesting morphology transformation from nanosheets to nanorods, which showed enhanced visible light photocatalysis activity.

Acknowledgements

This work is financially supported by NSF of China (Grant no. 51125006, 91122034, 61376056), STC of Shanghai (Grant no. 13JC1405700, 14520722000 and 14YF1406500), and Key Research Program of CAS (Grant No. KGZD-EW-T06).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17558e

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