Brookite TiO2 Nanosquares Synthesized by a Novel Green Hydrothermal Method

doi:10.21203/rs.3.rs-2722451/v1

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Brookite TiO2 Nanosquares Synthesized by a Novel Green Hydrothermal Method

https://doi.org/10.21203/rs.3.rs-2722451/v1

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published 24 Jul, 2023

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Brookite TiO₂ nanopsquares were synthesized by using a mixture of TiBALDH and TiOCl₂ and low-temperature hydrothermal method at 150℃. The process does not need any template, organic additive, or organic solvent. It is proved to be a clean, simple, time-saving, and mild reaction condition route to prepare brookite TiO₂ nanopsquares. The results of X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) prove that a pure brookite TiO₂ nanosquares with mean edge length of ∼51 nm were successfully prepared. The obtained brookite nanosquares show outstanding photoactivity toward the decomposition of Rhodamine B in water under UV light irradiation which is even better than Degussa P25.

TiO2

Brookite

Hydrothermal

Photocatalyst

Nanoparticles

X-ray techniques

In recent years, the environmental pollution caused by people’s daily life (vehicle emissions, domestic garbage, etc. ), agriculture (pesticide, plastic film, etc.) and industry is becoming more and more serious and now threatening people's life and health^[1]. It is necessary and urgent work to abate these pollutions as soon as possible. In the past years, various kinds of techniques have been applied to eliminate these hazardous contaminants^{[2, 3]}. Among them, the photocatalytic degradation technology provides us a clean, effective, and energy conservation way to degrade pollutants in air and water^{[4, 5]}, while the photocatalyst is the key point. Amongst those developed photocatalysts, titanium dioxide (TiO₂), as the oldest and shinest star, is the most extensively studied UV-light-responsive catalyst because of its excellent properties^[6–8] such as non-toxic, high chemical stability, low cost, and easy availability et al..

Naturally, most titania crystallizes in three crystalline polymorphs: anatase, rutile and brookite^{[9, 10]}. Anatase is usually considered to possess the highest photocatalytic activity, and rutile is the most stable phase. The study of preparation and applications of anatase, rutile and/or their mixtures have been well-explored because all of them are easily prepared. While brookite TiO₂ is the rarest phase in nature and the hardest one to be prepared in the lab because of its thermodynamic instability. Usually, brookite exist as a by-product in the preparation of rutile or anatase process in the lab ^{[11, 12]}.

Little research work about the synthesis and properties of single brookite TiO₂ has been done for a long time because it is thermodynamically metastable and difficult to get pure phase. It is even marked as an inert photocatalyst in the past years^[13]. In the last few years, the research interest in brookite has been aroused and brookite is even considered as a promising active photocatalyst^{[14, 15]}. Theoretical studies of brookite TiO₂ also proved that this special polymorph possesses better photocatalytic activity per surface area than that of anatase and rutile ^[16–18].

Although a multitude of efforts have been devoted in the research work of TiO₂ photocatalyst, there is really not too much concerning about the brookite TiO₂. Moreover, the reported synthesis process of brookite TiO₂ usually needs organic additives and solvent, long reaction time, high temperature and pressure and often involving complex time-consuming steps^{[11,12,15−19]}. Therefore, till now, it’s still a great challenge to prepare single-phase brookite TiO₂ nanostructures in a clean, simple, time-saving, and mild reaction condition route. As a result, the photocatalytic properties of pure brookite without any other crystalline polymorphs of titania have been not fully studied, either.

Water-soluble titanium complexes have been used as precursors for the preparation of high-quality brookite TiO₂ nanostructures by precursor-directed approaches^[20–23], which has already been approved an effective way. Tomita et al.^[24] have successfully prepared impurity-free brookite nanoparticles using this kind of titanium complex as the precursor. They found that the oduced brookite nanoparticles showed high crystallinity, but the morphology can not be controlled. In our previous work^[25], uniform anatase TiO₂ quantum dots have been successfully prepared by using water-soluble titanium compounds as precursors.

In this paper, high-quality pure brookite TiO₂ nanosquares were straightly synthesized using a mixture of two water-soluble titanium precursors simultaneously in aqueous solution without any organic solvent and stabilizers in mild one-step hydrothermal process. As far as we know, this is the first work addressing the synthesis of brookite nanosquare with two kinds of titanium precursors in such a mild, simple and green conditions. The photocatalytic ability of the prepared brookite TiO₂ nanosquares has been evaluated by the decomposition of Rhodamine B in water under ultraviolet light radiation. The results of the photocatalytic experiment indicate that the photodegradation rate of obtained brookite TiO₂ nanosquares is significantly higher than commercial P25.

Rhodamine B (RhB, ≥ 95%), methyl orange (MO), 37% HCl, Titanium(IV) bis(ammonium lactato) dihydroxide (TiBALDH) solution( 50 wt. % in H₂O) and Titanium(IV) oxychloride-hydrochloric (TiOCl₂) acid solution (~ 15% Ti basis, 38–42% HCl basis) were supported by Sigma- Aldrich. Lab-made deionized water (DI-water, 18MΩ) was used in the experiment.

Both TiBALDH and TiOCl₂ were simultaneously used as titanium precursors in our experiment. In a typical synthesis process, 0.05 ml TiOCl₂ solution and 2.00 ml 37% HCl were mixed with 10 ml DI-water, then 1.50 ml TiBALDH solution was poured into the above solution with stirring. The mixed solution was stirred for 5 mins at ambient temperature and then it was transferred into a 23 ml Teflon-lined stainless steel autoclave and maintained at 150°C for 3 h. Then the autoclave was naturally cooled to room temperature. This process led to produce a white precipitate which was centrifuged and washed with DI water and absolute ethanol 3 times, respectively. After that, the powder was dried at 60°C for 24 h in an oven to remove residual solvent. At last, the sample was collected and ground for further investigation.

The TEM, high-resolution transmission electron microscopy (HRTEM) images, and Selected-area Electron Diffraction (SAED) pattern of the products were examined with a transmission electron microscope (TEM, FEI Tecnai F20 S/TEM) operated at 200 kV. The crystal structures of the produced sample were obtained with X-ray diffraction (XRD, Shimadzu XRD6000) with Cu-Kα radiation (λ = 1.5418 Å) scanning range (2θ) from 10° to 80°. Raman analysis was studied in a Micro-Raman HORIBA HR LabRAM, equipped with a 632.8 nm He-Ne laser. X-ray photoelectron spectroscopy (XPS) was detected by ESCA LAB 250 (VG scientific). The UV-vis diffuse reflection spectra of the product were measured by using UV/Vis spectrophotometer (Labomed UV 2500, Labomed Inc. USA).

The photocatalytic properties of the obtained brookite TiO₂ nanosquares were evaluated by degrading Rhodamine B (RhB) irradiated with a 125 W mercury lamp (365 nm) in an aqueous solution at ambient conditions. The experiments were conducted with 10 mg of brookite TiO₂ nanosquares suspended in 100 mL of RhB water solution (10 mg•L^− 1). The absorption equilibrium of the dye on brookite TiO₂ nanosquares achieved after 30 minute stirring in the dark. Later the mixture was irradiated by a UV light to degrade the dye with continuous stirring. In the process of degradation, 2 mL of dye aqueous suspension was taken out every 30 min. The concentration of RhB was measured with UV mini-1285 spectrophotometer (Shimadzu, Japan). To better understand the photocatalytic property of the prepared brookite nanosquares, the commercial TiO₂ P25 (Degussa, Germany) was used as a control.

The brookite nanosquares were synthesized by a novel mild one-step hydrothermal process as illustrated in Scheme 1. Titanium cations of water-soluble titanium compounds usually exist as 6-fold-coordinated, charged complexes [TiL₆]^z± in an aqueous medium. The nature of the ligand L is in connection with the pH value and the species that can coordinate with Ti⁴⁺ in the solution. In our case, two kinds of water-soluble titanium precursors were used simultaneously in the synthesis process. As suggested by the literature, the experiment involved two complexes: one is [Ti(OH)₂Cl₂(OH₂) ₂]⁰ octahedron formed ^[26] when TiOCl₂ was added into HCl water solution; the other spicies are [Ti(C₃H₄O₃)₂(OH)₂]²⁻ octahedrons generated via dissociation of titanium-contained complex TiBALDH^[27]. The formations of these octahedron complexes have been formed by the reaction equations (1) and (2) as follows:

TiOCl₂ + 3H₂O → [Ti(OH) ₂Cl₂(OH₂) ₂] ⁰ (1)

(NH₄)₂[Ti(C₃H₄O₃)₂(OH)₂] → 2 NH₄⁺ + [Ti(C₃H₄O₃)₂(OH)₂]²⁻ (2)

Under hydrothermal conditions and a strong acid environment, along with the condensation reactions, these two kinds of octahedron complexes can form dimers, trimers, small chains of octahedra, and then layered titanate of brookite. Tomita et al. ^[28] have found that metastable phases often crystallize first in chimie douce methods at low temperatures because their nucleolus only needs lower supersaturation. Once the nucleation of the brookite phase had occurred, the growth of the crystallites would continue because of the lower activation energy than the nucleation of another new phase. As a result, with the proceeding of further condensation, solid oxide appeared through the formation and growth of the nuclei and finally crystallized to brookite TiO₂. Previous theoretical calculations revealed that the {210} surface energy of brookite TiO₂ is 0.70 J/m², which makes this surface one of the most stable surfaces^[29]. In our case, the exposed surface of the obtained brookite nanosquare is also {210} (seen in Fig. 2e).

Figure 1a depicts the XRD patterns of the as-prepared powder. It is found that all diffraction peaks of the product in XRD can be indexed to the diffraction peaks of orthorhombic brookite (JCPDS 65-2448). And there are no reflections assignable to anatase or rutile TiO₂ phases which guarantees the high purity of the obtained powder. Therefore, the synthesized samples are concluded to be pure brookite phase. The mean crystallite size was determined to be 50.7 nm from the diffraction patterns using the Scherrer equation.

On the basis of the theory of space group, brookite lattice possesses lower symmetry (D_2h) and larger unit cells which lead to a large amount of Raman active phonons. Raman spectroscopy is considered a very effective way to identify the existence of brookite in a sample because brookite reveals a characteristic complicated vibrational spectrum compared to the other TiO₂ polymorphs. Figure 1b shows the Raman spectra of the prepared TiO₂ sample. The Raman spectrum of the brookite TiO₂ usually shows 15 vibration bands in the range of 100 ~ 700 cm^− 1. These bands can be assigned to the modes of A_1g (155, 194, 247, 412, 636cm ^− 1), B_1g (213, 322, 501 cm^− 1), B_2g (366, 395, 460, 583 cm^− 1), and B_3g (172, 287, 545 cm^− 1), respectively^{[30, 31]}. In our case, the spectra shows the characteristic bands of brookite include the most intense A_1g mode (155 cm^− 1) and other 12 weaker peaks at 126, 194, 213, 248, 287, 322, 365, 412, 461, 500, 545, 583, and 636 cm^− 1 assigned as A_1g, B_1g, B_2g, and B_3g vibrations, respectively. And there are no apparent vibration bands related to the anatase and rutile in the spectrum. XPS experiments were applied to analyze the chemical binding states of brookite. The Ti 2p XPS spectra of is presented in Fig. 1(c). Ti 2p _1/2 and Ti 2p _3/2 peaks locate at the binding energies of 458.5 and 464.3 eV, respectively, and the datas are correspond to the lattice chemical bonding of TiO₂ as the earlier reports^{[12, 16]}. XRD, XPS and Raman studies confirm that the obtained product is pure brookite phases.

To obtain the morphological features of the as-prepared TiO₂ powders, the sample was characterized by using TEM, SAED, and HRTEM, and the results are displayed in Fig. 2. Figure 2a and 2b show that the major morphological feature of the sample is a regular square shape in the almost same size with smooth edges. To examine the uniformity of the synthesized brookite TiO₂ nanosquares, the mean edge length of nanosquares is statistically analyzed. The results show that the average value of the square edge length is ∼51 nm with a narrow size distribution from 30 to 70 nm(Fig. 2c). It is consistent with results from the the crystallite size calculated by using the Scherrer equation.

The HRTEM image (Fig. 2d, 2e) reveals that the lattice spacing is 0.351 nm, which is coherent with the early reported value of brookite (210) planes^{[29, 30]}. The clear lattice fringes indicate the high crystallinity of the prepared brookite nanosquares. The SAED pattern is shown in Fig. 2f which represent the diffraction rings with light point indexed to (210), (211), (221), (321), and (400) planes of TiO₂ brookite structure. This result proves that the prepared TiO₂ is brookite nanosquares and which is consistent with XRD and Ramman results.

Figure 3a depicts the UV-vis diffuse reflectance absorption spectra (DRS) of the obtained brookite TiO₂ nanosquares. The absorption band (band edges at 377.6 nm) at 290 nm is attributed to the intrinsic bandgap absorption of crystalline brookite TiO₂. The bandgap energy (Eg) for the indirect transition of the brookite TiO₂ nanosquares can be estimated through a Tauc plot^{[32, 33]} (inset in Fig. 3a), which is 3.28 eV. The bandgap energy of the brookite TiO₂ nanosquares was consistent with the previous investigation ^{[34, 35]}.

The photocatalytic activity of the as prepared brookite TiO₂ nanossquares was evaluated under UV irradiation using Rhodamine B as a probe molecule and the degradation curve was shown in Fig. 3b. When blank experiments were carried out, the self-photolysis of RhB was negligible in degradation time. About 70% of RhB is degraded by P25 within the prescribed time. Comparatively, brookite nanosquares showed excellent photo-catalytic properties in the decolorization of RhB with 96% in 180 min. The results indicate that the synthesized brookite nanosquares exhibit better activity on RhB degradation than Degussa P25. The better photocatalytic property of the prepared brookite TiO₂ nanosquares could be attributed to the higher bandgap, and conduction band potential^{[36, 37]}, improved charge transport properties and effective charge separation^[38–39], which prevents recombination of photogenerated electron-hole pairs.

In summary, pure-phased brookite TiO₂ nanosquare with a uniform edge size of about 51 nm, was prepared by using a mixture of two kinds of titanium precursors and a facile one-step hydrothermal process in mild reaction conditions. The result of XRD, Raman, and SAED confirm the brookite phase nature of the obtained product. TEM reveals the nanosquare morphology. The prepared brookite nanosquares show outstanding photoactivity toward the degradation of organic contaminants which is even better than Degussa P25. The study provides a feasible way for the massive synthesis and applications of pure brookite in the future.

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Financial interests: The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Zhigang Wu, Yulei Wang, Le Wang and Aiwei Zhang. The first draft of the manuscript was written by Zhigang Wu, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Brookite TiO2 Nanosquares Synthesized by a Novel Green Hydrothermal Method

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