Synthesis, characterization of B-doped TiO₂ nanotubes with high photocatalytic activity

Abstract

B-doped TiO₂ nanotubes (B/TiO₂ NTs) were prepared by the combination of sol–gel process with hydrothermal treatment. The prepared catalysts were characterized by XRD, TEM and XPS. The photocatalytic activity of B/TiO₂ NTs was evaluated through the photodegradation of aqueous methyl orange. The results demonstrated that the 1.5% B/TiO₂ NTs calcined at 300 °C possessed the best photocatalytic activity. Compared with pure TiO₂ nanotubes, the doping with B significantly enhanced the photocatalytic efficiency.

1 Introduction

Semiconductor photocatalysis is a promising technology in air purification, water disinfection, hazardous waste remediation, and water purification [1]. Among various candidates, TiO₂ is proved to be the most suitable catalyst in view of its strong oxidation activity. However, its photocatalytic oxidation rates for many target pollutants are too slow to be of practical utility [2–4]. Doping with suitable nonmetals has been attempted to improve its performances. Many nonmetals, such as nitrogen [5], carbon [6, 7], sulfur [8], phosphorus [9], fluorine [10] and boron [11], etc. have been used to modify the properties of TiO₂.

Among them, boron doping is observed to be a potential method and improved performance can be obtained. For example, In et al. [12] reported that an optimal level of boron doping induces a larger absorption of visible light and enhanced photocatalytic activity. Su et al. [13] revealed that B-doped TiO₂ nanotubes that were prepared by chemical vapor deposition (CVD) displayed stronger absorption in both UV and visible range. The experimental results of Chen et al. [14] indicated that all B-doped TiO₂ nanoparticles prepared by sol–gel method showed higher photocatalytic activity than pure TiO₂ sample in the photocatalytic reaction of nicotinamide adenine dinucleotide (NADH) regeneration under UV light irradiation. Yang et al. also reported that B-doped anatase models were investigated by spin-polarized density functional theory. It is reported that the radii of the boron atom is very small and can site at an interstitical position. In this B-doped TiO₂ system, that B replacing an O atom induces some gap states close to the bottom of the conduction band, and may account for redshift of the absorption edge [11]. From these researches, it can be inferred that the photocatalytic activity of TiO₂ can be improved by the doping of boron.

Recently, extensive researches have been conducted on the synthesis and characterization of TiO₂ nanotubes because of their novel properties, such as unique shape, size confinement in radial-direction and large specific surface area [15–20]. Furthermore, the high sedimentation rate of the nanotubes facilitates their reuse as photocatalysts [21]. By modifying with different ions, the photocatalytic activity of the tubular TiO₂ can be improved. In our group, different elements, such as gold [22], platinum [23], palladium [24], nitrogen [25], Chromium [26], fluorine [10], have been used to modify the TiO₂ nanotubes and enhanced photocatalytic activity is obtained. However, to our knowledge, photocatalysis upon anatase B/TiO₂ NTs has not been reported. Higher photocatalytic activity can be expected on the B/TiO₂ NTs. In this paper, B/TiO₂ NTs were prepared by using B-doped anatase TiO₂ powder as precursors. The products were characterized by XRD, TEM and XPS. Their photocatalytic activity under UV–Vis light irradiation was evaluated by the degradation of methyl orange.

2 Experimental

Tetrabutyl titanate (C₁₆H₃₆O₄Ti, AR), which was used as titanate (TiO₂) precursor, was purchased from Tianjin Kermel chemical Co., Ltd, China. Boric acid (H₃BO₃, AR), which was used as the doped agent, was purchased from Tianjin Guangfu chemical Co., Ltd, China. Ethanol (C₂H₅OH, AR), which was used as a solvent, was purchased from Tianjin Guangfu chemical Co., Ltd, China. All the reagents were used without any further purification.

2.1 Preparation of B/TiO₂ NTs

B-doped TiO₂ powder was prepared by the sol–gel route using tetrabutyl titanate and boric acid as the precursors. Under constant stirring, ethanol solution of tetrabutyl titanate was added drop-wise to the mixture of ethanol and a desired amount boric acid solution. The transparent sol was obtained when the mixture was hydrolyzed at room temperature for 2 h under vigorous stirring. Then, gel was obtained by aging the sol for 12 h at room temperature. The resulting gel was dried at 80 °C in oven for 12 h to remove the solvents and then calcined at 500 °C in muffle furnace for 3 h. Then the B-doped TiO₂ powder was obtained. The calculated boron concentrations were 1.0, 1.25, 1.5, 1.75%, respectively. Pure TiO₂ powder was synthesized with the same process except that distilled water instead of boric acid solution is used. Pure titanate nanotubes and doped nanotubes with different boron concentration were synthesized via a simple hydrothermal chemical process. 1.0 g B-doped TiO₂ powder or pure TiO₂ powder was mixed with 50 mL 10 mol/L NaOH aqueous solution in a Teflon vessel and maintained at 150 °C for 12 h. The resulting materials were washed with 0.1 mol/L HNO₃ solution and distilled water, respectively, and then dried at 80 °C overnight in an oven. After the prepared material was calcined at 300 °C for 2 h, B/TiO₂ NTs were obtained. The corresponding B concentrations obtained in the photocatalysts were expressed as wt%.

2.2 Samples characterization

The powder X-ray diffraction (XRD) experiments were carried out at room temperature using a Rigaku D/Max-2500 X-ray diffractometer (CuKα λ = 0.154 nm) to identify the crystal phase of the products. TEM images were obtained with a Philips T20ST transmission electron microscopy working at 200 kV. The charge state of doped TiO₂ at the surface was investigated by X-ray photoelectron spectroscopy (XPS) using an Al X-ray source (Al Kα-150W, Kratos Axis Ultra DLD).

2.3 Photoactivity measurement

The photocatalytic activity under UV–Vis light irradiation of the prepared catalysts was evaluated by the degradation rates of methyl orange (20 mg/L) in an aqueous solution (100 mL) containing 50 mg of sample. The reaction mixture was ultrasonically dispersed for 30 min, and then irradiated by using a 300 W High-Pressure Mercury Lamp under stirring. The radiation range of the High-Pressure Mercury Lamp is from 200 to 1,000 nm, and the full spectrum was used for experiments. After every given irradiation time, a sample of 5 mL suspension was withdrawn, centrifugated and filtered. The obtained solution was then measured for checking the residual concentration of methyl orange with a UV–Vis spectrophotometer (TU-1901) at 463.8 nm, which is the maximum absorption of methyl orange. The results are corrected for the decomposition of methyl orange in the absence of catalysts and for absorption of methyl orange on the catalyst. In the irradiation experiments, air or O₂ bubbling was not used.

3 Results and discussion

Figure 1 shows the XRD patterns of 1.5% B/TiO₂ NTs and pure TiO₂ nanotubes calcined at 300 °C for 2 h. Diffractions that are attributable to anatase TiO₂ are clearly detectable in pure TiO₂ nanotubes and B/TiO₂ NTs (JCPDS 21-1272). The diffraction peaks for boron or boron containing phases were not observed, which indicated that boron was highly dispersed on TiO₂, or XRD was not sensitive enough to detect such minor changes to TiO₂.

Fig. 1

XRD patterns of pure TiO₂ nanotubes (a) and 1.5% B/TiO₂ NTs (b) calcined at 300 °C

Figure 2 shows TEM images of 1.5% B/TiO₂ NTs. Tubular structures are clearly observed. Figure 2a indicates that nanotubes are hollow and open-ended. Their diameters are nearly uniform and their length is more than hundreds of nanometers, similar to the results reported by Kasuga et al. [15, 16]. Figure 2b shows the TEM image of the obtained B/TiO₂ NTs calcined at 300 °C for 2 h. It can be seen that most of the nanotubes keep their tubular shape after the calcination process.

Fig. 2

TEM images of 1.5% B/TiO₂ NTs without calcination (a) and calcined at 300 °C (b)

Figure 3 shows the XPS survey spectra before and after Ar⁺ etching of B/TiO₂ NTs with 1.5 wt% boron calcined at 300 °C for 2 h. There are Ti, O, B, and C on the surface of B/TiO₂ NTs. The presence of C element should mainly come from residual carbon from the organic precursors used in the sol–gel method and the adventitious hydrocarbon from XPS. It should be noted that the intensity of peak corresponding C 1s appeared in the sample after etching was obviously weaker than that appeared in the sample before etching, indicating that the adventitious hydrocarbon from XPS itself was partly removed by Ar⁺ etching. The doping concentration of B analyzed by XPS was 1.15 wt% before etching and 0.61 wt% after etching, which implies that B dopant has a much higher concentration at the exterior than that at the interior of TiO₂ nanotubes. After the hydrothermal process, the B-doped nanoparticles turned into tubular structures and the doped boron was mainly dispersed in the outer layer of the nanotubes, which results in the difference of the boron concentration before and after Ar⁺ etching. However, the measured concentrations are smaller than the calculated one. It should be attributed to the boron loss during the hydrothermal process.

Fig. 3

XPS survey spectra of B/TiO₂ NTs before and after Ar⁺ etching

Figure 4 shows the high resolution XPS spectra of Ti 2p. The Ti 2p_1/2 and Ti 2p_3/2 spin-orbital splitting photoelectrons are located at binding energies of 464.1 and 458.3 eV, respectively, which was in agreement with the values in the reported literatures [27, 28], showing the presence of Ti⁴⁺. However, the surface spectrum of the etched sample revealed two shoulder regions. The peak at 456.7 and 455.2 eV should be ascribed to Ti³⁺ 2p3/2 and Ti²⁺ 2p3/2 in the sample, respectively [27]. The presence of low valence Ti might be attributed to Ar⁺ etching.

Fig. 4

High resolution XPS spectra of the Ti 2p region for the B/TiO₂ NTs before and after Ar⁺ etching

Figure 5 shows the O 1s XPS spectra of B/TiO₂ NTs. The O 1s spectrum was asymmetric. The main peak of O 1s located at about 529.6 eV, corresponding to lattice oxygen of TiO₂. The shoulder located at 531.2 eV indicates a great amount of surface hydroxyl groups and chemical adsorbed oxygen [27, 28]. After Ar⁺ etching treatment, the shoulder peak approximately disappeared. That indicates the removal of water and adsorbed oxygen from the TiO₂ surface.

Fig. 5

High resolution XPS spectra of the O 1s region for the B/TiO₂ NTs before and after Ar⁺ etching

Figure 6 shows the B 1s XPS spectra of B/TiO₂ NTs before Ar⁺ etching. The B 1s region contains one peak at 191.6 eV. The standard binding energy of B 1s is 193.0 eV (B–O bond) in B₂O₃ or H₃BO₃ and 187.5 eV (B–Ti bond) in TiB₂. So the binding energy of B 1s is between that of B₂O₃ and TiB₂. This indicates the boron atom was probably incorporated into TiO₂ and the chemical environmental surrounding might be Ti–B–O [14, 29].

Fig. 6

High resolution XPS spectra of the B 1s region for the B/TiO₂ NTs before Ar⁺ etching

B-doping obviously affects light absorption characteristics of TiO₂, as shown in Fig. 7. The absorption spectra of the 1.5% B/TiO₂ NTs calcined at 300 °C for 2 h showed a stronger absorption in the UV–Visible range. Meanwhile, the absorption edge is changed from 407 nm (pure TiO₂ nanotubes) to 422 nm for B/TiO₂ NTs. It suggested that the incorporation of boron with TiO₂ induces an obvious red shift and makes the band gap energy smaller [30]. Yang et al. [11] reported that substitution B replacing an O atom induces some gap states close to the bottom of the conduction band and this may account for the redshift of the adsorption edge in TiO₂. The narrower band gap will facilitate the excitation of an electron from the valence band to the conduction band in the doped TiO₂ catalyst, and enhance the photocatalytic performance of TiO₂ consequently.

Fig. 7

UV–Vis absorption spectra of pure TiO₂ nanotubes (a) and B/TiO₂ NTs (b)

Figure 8 shows the degradation rates of methyl orange for 1.5% B/TiO₂ powder and 1.5% B/TiO₂ NTs calcined at 300 °C for 2 h. 78.2% of the methyl orange is degraded after 2 h irradiation in the presence of B/TiO₂ powder, while 97% of the methyl orange is degraded in the presence of B/TiO₂ NTs. Obviously, the photocatalytic activity of the B/TiO₂ NTs is better than that of B/TiO₂ powder. The reason is that TiO₂ nanotubes have their novel properties, such as unique shape, size confinement in radial-direction and large specific surface area [15–20]. These induce the better photocatalytic activity of the B/TiO₂ NTs than that of B/TiO₂ powder.

Fig. 8

Photocatalytic activity of 1.5% B/TiO₂ powder (a) and 1.5% B/TiO₂ NTs (b)

Calcination is an effective treatment method to enhance the photocatalytic activity of nanosized TiO₂ photocatalysts. Figure 9 shows the photocatalytic performance of B/TiO₂ NTs without calcination and calcined at different temperatures. As shown in Fig. 9, 60.3, 66.3, 99.7 and 87.4% of the methyl orange is degraded after 3 h irradiation in the presence of 1.25% B/TiO₂ NTs without calcination, calcined at 200, 300 and 400 °C, respectively. It is obviously that B/TiO₂ NTs without calcination show much lower photocatalytic activity. It can be attributed to the existence of titanate. When calcined at 200 °C, B/TiO₂ NTs show higher photocatalytic activity, which should due to the formation of anatase. After the calcination treatment, titanate in the nanotubes turn into anatase TiO₂ and higher photocatalytic activity is obtained. The B/TiO₂ NTs calcined at 300 °C possess the best photocatalytic activity. This result is also in accordance with the result reported by Zhang et al. [20]. The photocatalytic activity of B/TiO₂ NTs decreases with a further increase in calcination temperature (Fig. 9d). That is probably due to the agglomeration and sintering damage of nanotubes caused by calcination at high temperature [31].

Fig. 9

Photodegradation of methyl orange by 1.25% B/TiO₂ NTs without calcination (a) and calcined at 200 °C (b), 300 °C (c), and 400 °C (d)

Figure 10 shows the photodegradation of methyl orange for B/TiO₂ NTs as a function of B-doping concentration. All the catalysts were calcined at 300 °C. 69.1, 76.5, 99.7, 99.9, 86.1% of the methyl orange is degraded after 3 h irradiation in the presence of TiO₂ NTs with 0, 1.0, 1.25, 1.5, and 1.75% boron doping, respectively. It can be observed that B/TiO₂ NTs exhibit much higher photocatalytic activity for the photodegradation of methyl orange than pure TiO₂ nanotubes. The results reveal that the photocatalytic performance of TiO₂ NTs can be improved by the boron doping.

Fig. 10

The photocatalytic activity of B/TiO₂ NTs with various boron doping concentrations

As has been reported, the doped-B atoms can promote the formation of oxygen vacancies [12]. The role of oxygen vacancies is to directly provide the sites of the formation of active species for photocatalytic reaction. The formation of O ^·−₂ upon chemisorbed oxygen or OH^· upon adsorbed water requires the presence of surface oxygen vacancies. The presence of Ti³⁺ also can apparently enhance the photocatalytic activity. The boron doping converts some Ti⁴⁺ to Ti³⁺ by charge compensation. The existence of a certain amount of Ti³⁺ can act as an active site to assist the adsorption of reactant and trap the photogenerated electron to reduce the recombination of photoexcited electrons and holes [14]. As a result, the photocatalytic activity of TiO₂ nanotubes is improved by boron doping.

4 Conclusions

B/TiO₂ NTs are prepared by the combination of sol–gel process with hydrothermal treatment. The prepared nanotubes are hollow and open-ended, and their length is more than hundreds of nanometers. Most of the nanotubes keep their tubular texture after the calcination process. B/TiO₂ NTs with 1.5% boron doping in the initial solution calcined at 300 °C possess the best photocatalytic activity. The high photocatalytic activity of B/TiO₂ NTs is ascribed to several beneficial effects produced by B-doping: intense absorption in the UV–Visible range and a red shift in the band gap transition, creation of oxygen vacancies, presence of Ti³⁺, and so on.