Facile and Large-scale Synthesis of Defective Black TiO_2−x(B) Nanosheets for Efficient Visible-light-driven Photocatalytic Hydrogen Evolution

Abstract

In the work, we firstly report the facile and large-scale synthesis of defective black TiO_2−x(B) nanosheets via a dual-zone NaBH₄ reduction method. The structure, physico-chemical, and optical properties of TiO_2−x(B) nanosheets were systematically characterized by powder X-ray diffraction, Raman spectroscopy, UV-Vis absorption spectroscopy, and X-ray photoelectron spectroscopy, etc. The concentration of Ti³⁺ can be well tuned by NaBH₄ reduction. With increasing the mass ratio of NaBH₄ to TiO₂(B), the generation of Ti³⁺ defects gives rise to the increased intensity of a broad band absorption in the visible wavelength range. It is demonstrated that the TiO_2−x(B) photocatalyst synthesized with the mass ratio of NaBH₄ to TiO₂(B) of 3:1 exhibited an optimum photocatalytic activity and excellent photostability for hydrogen evolution under visible-light irradiation. By combining the advantages of 2D TiO₂(B) nanosheets architecture with those of Ti³⁺ self-doping and simultaneous production of oxygen vacancy sites, the enhanced photocatalytic performance of the defective TiO_2−x(B) nanosheets was achieved.

1. Introduction

Photocatalytic semiconducting materials for hydrogen evolution via water splitting have attracted considerable interest [1,2,3,4]. The anatase-, rutile-, and brookite-type TiO₂ are the most widely studied photocatalysts. However, their potential applications are hindered severely by their large band gaps and consequently their limited visible-light-harvesting properties. Therefore, several approaches to enhance visible-light photoactivity and inhibit charge carrier recombination in TiO₂-based photocatalysis have been developed, such as co-catalysts deposition [5,6], hetero/self-doping [7,8,9,10], junction composite [11,12], crystal facet engineering [13,14], surface disordering [15,16], etc. Especially, the intrinsic defects in the TiO₂ matrix have been proved to trigger the visible-light activity of TiO₂ [17,18,19,20,21,22]. The hydrogenation-induced defect-rich black TiO_2−x displays remarkable stability and photoactivity. Meanwhile, the theoretical results also clearly demonstrate that a vacancy band state is formed as a result of the high vacancy concentration, thus achieving a narrow band gap (about 1.0 eV).

Apart from these TiO₂ polymorphs, the crystalline structure of the metastable TiO₂(B) nanosheet is found to be a layered and perovskite-like with lattice channels. The synthesis process is facile with ethylene glycol solutions and 140–180 °C, and only involves a one-step hydrolysis reaction of TiCl₃ or TiCl₄ [23,24]. Therefore, extensive efforts have been contributed to investigate the applications of the TiO₂(B) nanosheet in the photocatalysis field [23,25,26,27,28]. However, due to the metastable structure and character of TiO₂(B), almost no work has been reported on the facile and large-scale synthesis method and concurrently controllable structure and optical properties for defect-rich TiO_2−x(B).

In this paper, we offer a facile dual-zone reduction approach to produce defect-rich TiO_2−x(B) nanosheets by using NaBH₄ as reductant agents. By adjusting the mass ratio of NaBH₄/TiO₂(B), the formation of Ti³⁺ and oxygen-vacancy defects can be well controlled. The as-synthesized defective TiO_2−x(B) exhibited a broad absorption in the visible-light range, achieving the visible-light photoactivity for H₂ evolution.

2. Results and Discussions

2.1. Fabrication of Defective Black TiO_2−x(B) Nanosheets

The TiO₂(B) phase commonly suffers from phase transformation into the stable TiO₂ phase (anatase or rutile) in high-temperature annealing conditions, owing to the thermodynamically metastable structure. Thus, this work provides a new and simple dual-zone NaBH₄ reduction approach to produce defective TiO_2−x(B) nanosheets (Figure 1), showing the potential to replace the dangerous high-temperature hydrogenation method by directly using H₂ as reductant reagent. Moreover, the byproducts residues from decomposition reactions of NaBH₄ can be also avoided. Taking the advantage of the dual-zone reduction synthetic procedure, the phase transformation of TiO₂(B) was inhibited, achieving self-doping of Ti³⁺ and simultaneous formation of oxygen vacancy sites.

2.2. Material Characterizations

Figure 2 shows the powder XRD patterns of the defective TiO_2−x samples. The observed diffraction peaks at 2θ = 25.0°, 28.6°, and 48.6° can be assigned to the [110], [002], and [020] planes of the TiO₂(B) phase (JCPDS No.74-1940), indicating that no phase transformation was observed on the as-synthesized TiO_2−x(B) samples. The results show that under the mild reduction conditions, the simultaneous self-doping of Ti³⁺ and generation of oxygen vacancy sites were achieved, thus inhibiting the phase transformation of TiO₂(B). In addition, the diffraction peaks intensity was decreased, not only indicating the decreased crystallinity of TiO_2−x(B) along with increasing the mass ratio of NaBH₄ to TiO₂(B), but also shows that the TiO₂(B) prepared by simple hydrothermal method possesses a low crystallinity and the reduction process at 200 °C further reduces the crystallinity.

Raman scatterings (shown as Figure 3) were measured to further examine the structure of the obtained TiO_2−x(B) samples. No obvious change was observed on the Raman spectra of the as-synthesized TiO_2−x(B) samples. However, with increasing the mass ratio of NaBH₄ to TiO₂(B), an obvious decrease in the Raman signal intensity was observed, indicating the lower crystallinity of the TiO₂(B) phase, which is consistent with XRD results.

Figure 4 shows the UV-Vis absorption spectra (a) and Tauc plots (b) for the defective TiO_2−x(B) nanosheets compared to the pristine TiO₂(B). It can be seen that, with increasing the mass ratio of NaBH₄ to TiO₂(B), the UV-Vis absorption of defective TiO_2−x(B) samples is enhanced to expand to the entire visible-light region, in accordance with distinct color change from white BT into black BT3 (inset of Figure 4b).The results confirm the assumption that a new vacancy band state, located below the conduction band minimum (CBM) of TiO₂(B), can be formed by high concentration of Ti³⁺ doping. [17]

As shown in Figure 5a, the XPS survey spectra show the similar surface components of Ti and O elements in BT and BT3. The narrow scan XPS spectra of Ti 2p show that the binding energies of the spin doublet with Ti 2p_3/2 and 2p_1/2 are 458.4 and 464.1 eV, respectively (Figure 5b). The result indicates that the Ti species mainly exist as Ti⁴⁺, which is in good accordance with the literature results [29]. Moreover, the devolution of the Ti 2p XPS spectrum for the BT3 sample results in the obvious peaks belonging to Ti³⁺, for which the Ti 2p_1/2 and Ti 2p_3/2 peaks are located at about 457.5 and 463.2 eV, which confirms the generation of surface Ti³⁺ in TiO₂(B). Such differences can be ascribed to the reduction of Ti⁴⁺ into Ti³⁺ after the NaBH₄ treatment. As represented in Figure 5c, the deconvoluted peaks of O1s at 529.7, 531.0, and 532.8 eV are due to the lattice oxygen (Ti–O–Ti), surface hydroxyl group/O–defective matrix (Ti–O–H), and Ti–O–C groups, respectively [30,31,32]. The observation of Ti–O–C groups can be attributed to the surface-absorbed ethylene glycolate [26]. More interesting, the amount of lattice Ti–O–Ti bonds relative to surface Ti–O–H bonds/O–defective matrix was notably increased in BT3, as compared to those of BT. The results indicate the simultaneous production of oxygen vacancy sites [26,32].

2.3. Photocatalytic Activity for H₂ Evolution

The amounts and rates of H₂ evolution from aqueous methanol solution under visible-light were measured to represent the photocatalytic activity of the defect-containing TiO_2−x(B) with photodeposition of 0.03 wt.% Rh as co-catalysts, as shown in Figure 6. No H₂ gases were evolved from pristine BT. Interestingly, by an intermittent visible-light irradiation (every 30 min radiation followed by 30 min interval), the stable and continuous H₂ evolution (nearly linear correlation between the evolved hydrogen amount with the irradiation time) was observed on the as-synthesized defective TiO_2−x(B) samples (Figure 6a), indicating no significant deactivation of H₂ evolution and excellent stability of defective TiO_2−x(B) nanosheets. With increasing the mass ratio of NaBH₄/BT, the H₂ evolution from BT1 increased drastically, reached the maximum average rate of 0.58 μmol·g⁻¹·h⁻¹ for BT3, and then decreased (Figure 6b). The results confirm that the appropriate amount of self-doped Ti³⁺ defects along with the production oxygen vacancy sites are the key factors, leading to enhanced photocatalytic performance of the defective TiO_2−x(B) nanosheets.

3. Experimental

3.1. Chemicals

Titanium tetrachloride (TiCl₄, ≥99%) and Na₃RhCl₆ were purchased from Alfa Aesar. Ethylene glycol (EG, ≥99%), sodium borohydride (NaBH₄, ≥99%), and methanol (MeOH, ≥99.9%) were purchased from Sigma-Aldrich.

3.2. Photocatalysts Preparation

A modified hydrolysis method was used to synthesize the TiO₂(B) nanosheet powders [23]. Typically, a desired amount of deionized water was added into the pre-mixture of TiCl₄ and ethylene glycol, and then heated at 150 °C for 6 h in a Teflon-lined stainless-steel autoclave. Finally, centrifugation, washing by deionized water and ethanol, and drying at 80 °C all night were performed to obtain products (denoted as BT).

The BT as precursor and the NaBH₄ as reductant agent at different mass ratios of 1 to 4 were separately placed in a dual-zone quartz tube furnace. The reduction processes were carried out in an argon atmosphere with a flow rate of 10 mL/min (5.0 quality) at a heating rate of 5 °C/min. The NaBH₄ and BT samples were annealed at 500 and 200 °C for 1 h, respectively. After being cooled to room temperature, the samples were thoroughly washed with deionized water and ethanol several times, and dried at 80 °C overnight. The obtained products were accordingly denoted as BT1, BT2, BT3, and BT4, with respect to the different mass ratio of 1 and 4.

3.3. Characterization

The X-ray diffraction (XRD) patterns of all samples were recorded using a PANalytical MPD diffractometer, with the radiation source of Cu-Kα (λ = 0.1541 nm) X-ray emission, and the scan range was set to 10° to 70° (2θ), with step of 0.05°. Raman spectra of all samples were collected using a Renishaw Raman microscope equipped with a 514 nm excitation laser. Taking BaSO₄ as a reference, the UV-Vis absorption spectra were measured using a Shimadzu UV-2450 spectrophotometer. The X-ray photoelectron spectroscopy (XPS) measurements were carried out in Thermo ESCALAB 250XI System (ThermoFisher Scientific, Waltham, MA, USA), consisting of the Mg-Kα X-ray radiation source (hν = 1253.6 eV) operating at 250 W (14 kV) and a high resolution hemisphere energy analyzer.

The base pressure of about 5 × 10⁻¹⁰ mbar was maintained in the measurement chamber. To obtain an overall energy resolution of 0.25 eV, the fixed transmission mode and pass energy of 93.9 eV were adopted during the measurements. The charging effects were compensated by a flood gun. A piece of carbon tape (Nisshin EM Co. Ltd, Tokyo, Japan) was used to manually mount samples in the sample holder. The XPS peak deconvolution was performed using the Casa software (Version 2.3.15 RUB license, Casa Software Ltd, Teignmouth, UK, 2009) with Shirley background subtraction and Gaussian–Lorentzian broadening function.

3.4. Photocatalytic Activity Tests

Photocatalytic hydrogen production reactions were conducted in a homemade eight-parallel multi-zone reaction system with air-tight quartz reactors. A 500 W mid-pressure Hg lamp was equipped as light irradiation source, with a water filter and a 420 nm cut-off filter. The evolved gases were determined by gas chromatography method using a thermal conductivity detector (GC7900, Techcomp Ltd., Beijing, China, MS-5A column and high-purity N₂ (5.0 quality) as carrier gas). Typically, 30 mg powders were dispersed in 30 mL 10 vol.% aqueous methanol solution (MeOH), and then the ultrasonication was carried out for 10 min. Subsequently, for in-situ photodeposition of optimum amount of 0.03 wt.% Rh as co-catalyst, the Na₃RhCl₆ solution was added as precursor. Before irradiation, including the photocatalysts, the whole system was purged with N₂ (5.0 quality) to remove air completely.

4. Conclusions

In summary, a facile and large-scale approach of a dual-zone NaBH₄ reduction method was used for preparing defective black TiO_2−x(B) nanosheets. We demonstrate that the mass ratio of NaBH₄ to TiO₂(B) plays a critical role in controlling the self-doped Ti³⁺ defects and simultaneously produced oxygen vacancy sites towards engineering the defective TiO_2−x(B). The presence of Ti³⁺ and oxygen vacancy defects gives rise to the significantly increased intensity of the broad band absorption in the visible wavelength range. Under visible-light irradiation, the photocatalytic performance of defective TiO_2−x(B) photocatalysts was greatly enhanced with excellent stability for hydrogen evolution, as compared to the non-photoactive pristine TiO₂(B). The synthetic approach for synthesis of defective TiO_2−x(B) shows great significance for developing a highly efficient catalytic system.