Enhanced photocatalytic performance of novel electrospun BN/TiO₂ composite nanofibers

Abstract

High activity boron nitride/titanium dioxide (BN/TiO₂) composite nanofiber photocatalysts were synthesized for the first time via the electrospinning technique. The as-spun nanofibers with a controlled ratio of boron nitride nanosheets (BN) were calcined under air at 500 °C for 4 hours. Their morphological, structural and optical properties were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), BET surface area, Fourier-transform infrared (FTIR), Raman spectroscopy, UV-Visible spectrophotometry and room temperature photoluminescence (PL). The effect of loading different BN sheet amounts on the photocatalytic degradation of methyl orange (MO) was investigated. The results indicated that the presence of BN sheets improved the separation of the photo-induced electron–hole pairs in TiO₂ and increased the band gap energy and the specific surface area compared to pure TiO₂ nanofibers. BN/TiO₂ (10 wt%) composite nanofiber photocatalytic activity is enhanced to 99% compared to 60% and 65% for P25 and TiO₂ nanofibers, respectively. Thus, the BN/TiO₂ composites significantly increase the UV light photo-response and improve the separation of photo-induced electron–hole pairs of TiO₂.

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Introduction

The availability of safe drinking water is a high priority issue for human existence and quality of life. Now, water pollution is becoming more serious with the emission of large amounts of organic pollutants due to the rapid development of agriculture and industry. Therefore, it is desirable to develop an effective method to deal with organic pollutants. In this context, photocatalysis has gradually developed as a new environmental protection technology since the 1970s because it can play an important role in solar energy conversion and the degradation of organic pollutants.¹ Titanium oxide (TiO₂) has drawn the greatest attention as a photocatalyst, due to its relatively low cost, high stability and low toxicity.² TiO₂ has been widely used in many applications including dye-sensitized solar cells, gas sensors, biofuel cells, biomaterials and most notably as a photocatalyst in the UV spectrum.^3–6 However, the fast electron–hole pair recombination of TiO₂ significantly limits the efficiency of the photocatalytic reaction.⁷ This limitation has often been overcome by doping TiO₂ with noble metals or other semiconductor materials such as Ag,⁸ Pt,⁹ NiO,¹⁰ SnO₂¹¹ and graphene.¹² This is one of the most effective methods to increase the separation efficiency of photogenerated electron–hole pairs in order to enhance the photocatalytic activities of TiO₂.

Researchers have recently devoted much attention to 2D boron nitride nanosheets, which have a structural similarity to graphene. The white graphene (monolayer of BN) is an individual BN basal plane in which C atoms have been replaced by alternating B and N atoms.^13,14 Many extraordinary properties can arise from the BN nanosheets due to the high surface area and the reactive edge structure.¹⁵ BN nanosheets demonstrate a high temperature, high chemical stability, a high onset temperature of oxidation (>850 °C), high electrical resistance and a wide band gap (>5.5 eV).^16,17 These properties make BN nanosheets very appealing in a wide range of applications such as nanoelectronic and optoelectronic devices, and nanocomposites.¹⁵ Recently, Fu et al.¹⁸ prepared the h-BN/TiO₂ composite photocatalyst by the ball milling method.¹⁹ They reported that due to the electrostatic interaction, the charged h-BN can promote the migration of h⁺ from TiO₂ bulk to its surface and consequently improve the photocatalytic activity. Tang et al.²⁰ synthesized nanocrystalline TiO₂ attached to BN nanotubes using hydrolysis of TiCl₃ aqueous solution at 80 °C in the presence of esterified BN nanotubes.

Various TiO₂ nanostructures (0D, 1D, 2D and 3D) have been synthesized such as nanotubes,²¹ hollow hemispheres,²² nanorods²³ and nanofibers.²⁴ Several techniques have been developed for the fabrication of these TiO₂ nanostructures, such as the hydrothermal method, vapor-thermal treatment, the colloidal templating method, the solvothermal method and electrospinning.^22–28 Tian et al.²⁹ synthesized for instance 3D hierarchical flower-like TiO₂ nanostructures via a facile solvothermal method combining a calcination process. Reddy et al.^30,31 used an electrospinning technique for fabricating 1D TiO₂ nanofibers for photocatalytic applications. Among all these techniques, electrospinning is a simple and cost-effective technique. It permits the synthesis of nanofibers with controllable diameters ranging from a few to several hundred of nanometers, depending on the polymer and processing conditions.^32,33

In the present work, we report on novel photocatalytic composite nanofibers BN nanosheets/TiO₂ prepared via electrospinning. However, to our knowledge, there is no study concerning the preparation of BN nanosheets/TiO₂ composite nanofibers. Exfoliated boron nitride sheets were obtained by our original method based on high power ultrasonication of boron nitride nanopowder with gelatine.^13,15 The BN/TiO₂ composite nanofibers with different amounts of boron nitride (0, 3, 5, 7 and 10 wt% of the mass of Ti) have been elaborated by using the electrospinning technique. All the samples were annealed at 500 °C in air for 4 hours. The structural, morphological and optical properties of the prepared samples were analysed. The performance of these materials for photocatalytic degradation of methyl orange under UV light has also been evaluated.

Experimental

Chemicals and materials

Commercial boron nitride (95%, 325 mesh), titanium tetraisopropoxide (TTIP) Ti{OCH(CH₃)₂}₄ (97%), polyvinylpyrrolidone (PVP) (M_w = 1 300 000), acetic acid (98%), gelatine from porcine skin, methyl orange and absolute ethanol were purchased from Sigma Aldrich, and used without any further purification.

Exfoliation of boron nitride

Boron nitride sheets were exfoliated from boron nitride powder.^13,15 Briefly, 20 g of gelatine was dissolved in 80 ml of hot water (75 °C). After gelatine dissolution, 1 g of boron nitride powder was added to the mixture. The solution was kept in an ultrasonic homogenizer overnight at 50 °C. Exfoliated BN was separated by centrifugation at 2697g for 30 minutes and the supernatant was decanted away. The resultant precipitates were dried at 80 °C for 48 hours and then calcined at 600 °C in air for 2 hours in order to obtain the pure exfoliated boron nitride.

Preparation of TiO₂ and BN composite nanofibers

The BN-incorporated TiO₂ nanofibers were synthesized by an electrospinning technique. The spun solution was prepared as follows: different amounts of boron nitride (0, 3, 5, 7 and 10 wt% of the mass of Ti) and 2 ml of absolute ethanol were homogenized for 12 h to disperse boron nitride into the ethanol solution. Titanium tetraisopropoxide was dissolved in a mixture of 2 ml of acetic acid and 3 ml of ethanol mixed with 0.3 g of polyvinylpyrrolidone (PVP). The precursor mixture was stirred for 1 h at room temperature to obtain sufficient viscosity required for electrospinning. Finally, the BN solution was added to the precursor solution under vigorous magnetic stirring (150 rpm) for 30 min. The homogeneous solution was typically extruded through a stainless steel nozzle with a diameter of 0.7 mm at a constant flow rate of 1 ml h⁻¹. The feeding rate was controlled by a syringe pump. The temperature level inside the electrospinning chamber was 38 ± 5 °C. Nanofibers were collected on a rotating coil covered with an aluminium foil with a rotation speed of 400 rpm. The distance between the tip of the needle and the aluminium foil was maintained at 20 cm, and a DC voltage of 25 kV was applied. In the electrospinning process, a high applied voltage was achieved by connecting the positive and ground terminals to the nozzle and collector, respectively. During electrospinning, the applied electric field overcomes the surface tension of the polymeric solution, thereby ejecting a continuous jet which upon subsequent solvent evaporation and bending produces nanofibers on the collector surface. The electrospun BN–TiO₂ composite nanofibers were collected and then calcined in a tubular furnace at 500 °C with a heating rate of 5 °C min⁻¹ for 5 h in air.

Physical and chemical characterization

X-ray diffraction (XRD) measurements were carried out using a PANAlytical Xpert-PRO diffractometer equipped with an X’celerator detector using Ni-filtered Cu-radiation (λ = 1.54 Å). Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet 370 FTIR spectrometer using an ATR system. Raman spectra have been obtained from Horiba, λ = 659 nm. Scanning electron microscopy (SEM) images were taken with a Hitachi S4800, Japan. Energy-dispersive X-ray spectroscopy analysis (EDX) and elemental mapping were taken with a Zeiss EVO HD15 microscope coupled with an Oxford X-MaxN EDX detector. The UV-VIS absorbance spectra were recorded by a Jasco V-570 UV-VIS-NIR spectrophotometer. The surface area of samples was determined from nitrogen adsorption–desorption isotherms at liquid nitrogen temperature using Micromeritics ASAP 2010 equipment (outgassing conditions: 200 °C – 12 h). Room temperature photoluminescence (PL) was measured in the range of 370–800 nm. The PL was excited with nitrogen Nd:YAG laser (266 nm, 10 mW, 1 kHz) and recorded with an optical fiber spectrometer (Ocean Optics usb2000).

Photocatalytic activity measurement

Methyl orange (MO) was used as a reference for organic pollutants to evaluate the photocatalytic activity of the BN/TiO₂ composite nanofibers. The photocatalytic performance of the samples was evaluated by analyzing the decrease in concentration of the MO during exposure to UV light irradiation. The reaction temperature was kept constant at 25 ± 0.2 °C by circulating water in a cylindrical tube surrounding the photo-reactor during the entire experiment. The decomposition was carried out in several beakers containing a suspension of 10 mg of each sample in 25 ml of MO solution (10 mg l⁻¹) under visible light irradiation (obtained from a 48 W light source, emission wavelength <400 nm). The distance between the lamp and the dye solution was maintained at 10 cm. First, the solution mixture was magnetically stirred for 30 min without irradiation to reach the adsorption–desorption equilibrium of MO. Then the solution was irradiated with visible light for 75 minutes. Every 15 min, 3 ml of the sample solution was taken out and centrifuged to remove the catalyst. The centrifuged solutions were analyzed by a UV-VIS spectrometer. The absorbance spectra of MO (major absorption band around 462 nm)³⁴ were recorded to measure the change in the concentration of MO.³⁵ After irradiation, the photocatalytic degradation efficiency was calculated as follows:

Degradation efficiency (%) = (C₀ − C)/C₀ × 100 (1)

where C₀ and C are the initial and final concentrations of the dye before and after photo-irradiation respectively.³⁶ This equation shows the dye photocatalyst degradation percentage.

Results and discussion

The TiO₂ and BN/TiO₂ NFs with different amounts of boron nitride were obtained by electrospinning of a solution of polymer with titanium tetraisopropoxide as precursor. Scanning electron microscopy was used to investigate the morphological features of TiO₂ and BN/TiO₂ nanofibers. Fig. 1 shows the SEM images of the obtained TiO₂ and BN-containing electrospun nanofibers after calcination in air at 500 °C for 4 hours. The formation of highly interconnected networks of continuous, randomly oriented nanofibers can be clearly seen. As illustrated in Fig. 1, the nanofibrous morphology was largely preserved after the heat treatment process. The fiber diameters were measured from the SEM images using image analysis software (Image J1.29X). The average diameter was measured on 100 randomly chosen nanofibers of each sample (Fig. 1). After calcination, we measured average diameters of (187 ± 5 nm), (229 ± 7 nm), (269 ± 10 nm), (290 ± 13 nm) and (314 ± 15 nm) for samples with BN weight amount of 0, 3, 5, 7 and 10 wt%, respectively. In fact, the average diameter of the nanofibers increased with increases in the boron nitride amount. This can be attributed to the increase of the solution viscosity induced by inclusion of BN in the electrospinning solution.² This increase reveals the successful incorporation of BN in TiO₂ nanofibers.

Fig. 1 Scanning electron microscope images of TiO₂ and BN/TiO₂ annealed composite nanofibers in air for 4 h at 500 °C.

An energy dispersive X-ray spectrum (EDX) of BN/TiO₂ nanofibers recorded along with elemental analysis is presented in Table 1. The EDX analysis reveals the molar ratio of Ti and O to be 1 : 2 for the above described fibers, which is in good agreement with the stoichiometric ratio of TiO₂. Also, the BN doped fibers have a molar ratio of 1 : 1 for boron and nitrogen. The increase of the atomic percentages of B and N in BN (3 wt%)/TiO₂, BN (5 wt%)/TiO₂, BN (7 wt%)/TiO₂ and BN (7 wt%)/TiO₂ can be clearly seen from Table 1. Fig. 2 shows elemental mapping images on BN (5 wt%)/TiO₂ nanofibers. These images clearly show that Ti, O, B and N elements are evenly distributed over the entire area of the sample confirming good dispersion of BN nanosheets in the TiO₂ nanofibers. Based on these data, the BN/TiO₂ composite nanofibers were successfully fabricated using the electrospinning method.

Table 1 EDX data showing the composition of TiO₂, BN (3 wt%)/TiO₂, BN (5 wt%)/TiO₂, BN (7 wt%)/TiO₂ and BN (10 wt%)/TiO₂ annealed nanofibers under air

Samples	Atomic percentages
	Ti	O	C	B	N
TiO2	33.42	61.28	5.3	—	—
BN (3 wt%)	26.59	53.67	4.16	8.4	7.18
BN (5 wt%)	24.81	52.45	6.31	9.23	7.2
BN (7 wt%)	23.84	48.25	5.48	10.59	11.84
BN (10 wt%)	25.05	46.19	5.13	12.14	11.49

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Fig. 2 Elemental mapping images of the BN (5 wt%)/TiO₂ composite nanofibers.

Table 2 presents the specific surface area (BET) of prepared BN/TiO₂ nanocomposites. The specific surface area varies between 19 and 50 m² g⁻¹. The increase in the surface area with increasing amounts of BN sheets is clearly seen from Table 2. It is commonly accepted that TiO₂ with a large surface area will have superior photocatalyst activity, because a larger surface area offers more active adsorption sites.³⁷

Table 2 BET surface area of TiO₂ nanofibers and BN/TiO₂ composite nanofibers

Samples	Surface area (m^2 g^−1)
TiO2 NFs	19.66
BN (3 wt%)/TiO2	31.75
BN (5 wt%)/TiO2	34.41
BN (7 wt%)/TiO2	48.31
BN (10 wt%)/TiO2	49.64

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X-ray diffraction (XRD) was employed for analyzing the crystalline phase of all the obtained nanofibers. The sharp peaks in the diffraction pattern of all samples (Fig. 3A) can be assigned to the (101), (004), (200), (105), (211) and (204) crystal plane of pure anatase phase of TiO₂ which correspond to 2θ = 25.5°, 38.2°, 48.2°, 54.1°, 55.3° and 62.9°, respectively.⁶ It is worth noting that the diffraction peak at 2θ = 26° of hexagonal BN related to the (002) direction is probably overlapped by the diffraction peak of TiO₂ at the same position.³⁸ The crystallite average sizes of TiO₂ and BN/TiO₂ with different amounts of BN were estimated using the Scherrer equation.³⁹ The calculated results show that the grain fineness numbers of BN/TiO₂ fibers are increased as compared to pure TiO₂ fibers. The crystallite sizes for all samples are shown in Table 3.

Fig. 3 (A and B) XRD patterns of (a) TiO₂, (b) BN (3 wt%)/TiO₂, (c) BN (5 wt%)/TiO₂, (d) BN (7 wt%)/TiO₂ and (e) BN (10 wt%)/TiO₂ annealed nanofibers in air for 4 h at 500 °C.

Table 3 Grain size of TiO₂ NFs and BN/TiO₂ composite nanofibers

	TiO2	BN (3 wt%)/TiO2	BN (5 wt%)/TiO2	BN (7 wt%)/TiO2	BN (10 wt%)/TiO2
Grain size (nm)	16.4	24.3	25.2	26.3	28.7

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As shown in Fig. 3B, the main TiO₂ peak was shifted from the initial position after BN doping in all samples. We may suppose that BN incorporation increases the lattice strain in the TiO₂ cell and it can shift atoms from the initial position and can reduce the formation of the self-trapped excitons (STE) localized on the TiO₆ octahedral.²⁴ At the same time, it might cause vacancy formation. These results were confirmed later by photoluminescence analysis.

Raman spectroscopy is a powerful tool to detect molecular vibrations in order to identify our materials. As shown in Fig. 4, the Raman spectrum of TiO₂, BN (3 wt%)/TiO₂, BN (5 wt%)/TiO₂, BN (7 wt%)/TiO₂ and BN (10 wt%)/TiO₂ was recorded in the range of 100–1500 cm⁻¹. For all samples, we identified the six Raman active modes corresponding to the anatase phase of TiO₂ at 144 cm⁻¹ (E_g), 197 cm⁻¹ (E_g), 399 cm⁻¹ (B_1g), 513 cm⁻¹ (A_1g), 519 cm⁻¹ (B_1g) and 639 cm⁻¹ (E_g) (Fig. 4A).⁴⁰ Besides, the Raman spectra of BN/TiO₂ samples showed the broadened characteristic frequency located at ∼1365 cm⁻¹ referring to the hexagonal BN related to E_2g mode (Fig. 4B).⁴¹ In addition, the increased intensity of the peak corresponding to h-BN with the amount of doping is visibly noticeable. Thus, Raman spectroscopy confirmed the incorporation of the boron nitride in the TiO₂ nanofibers.

Fig. 4 (A and B) Raman shift of (a) TiO₂, (b) BN (3 wt%)/TiO₂, (c) BN (5 wt%)/TiO₂, (d) BN (7 wt%)/TiO₂ and (e) BN (10 wt%)/TiO₂ annealed nanofibers in air for 4 h at 500 °C.

The successful incorporation of boron nitride sheets in the TiO₂ nanofibers was revealed by the typical FTIR spectrum (Fig. 5). For pure TiO₂ nanofibers (Fig. 5a), the characteristic absorption band at 650–950 cm⁻¹ is attributed to the Ti–O bond.⁴² While for the composite BN/TiO₂ nanofibers with different amounts of BN sheets (Fig. 5b–e), two main absorption peaks of hexagonal BN were observed around 800 and 1370 cm⁻¹, ascribed to the out-of-plane bending of sp²-bonded B–N–B and the in-plane stretching of sp²-bonded B–N, respectively.⁴³ In addition, it can be clearly seen, that the intensity of these two peaks related to the bands B–N–B and B–N increases by increasing the BN amounts.

Fig. 5 FTIR spectra of (a) TiO₂, (b) BN (3 wt%)/TiO₂, (c) BN (5 wt%)/TiO₂, (d) BN (7 wt%)/TiO₂ and (e) BN (10 wt%)/TiO₂ annealed nanofibers in air for 4 h at 500 °C.

Fig. 6 shows the UV-VIS absorption spectra of TiO₂ nanofibers and BN/TiO₂ composite nanofibers with different BN amounts. The energy of the band gap was calculated using the following equation:

E_g = hc/λ (2)

where h is Planck's constant, c is the speed of light and λ is the cut-off wavelength where the absorbance value is a minimum. The UV-VIS absorption spectrum of the pure TiO₂ nanofibers, BN (3 wt%)/TiO₂, BN (5 wt%)/TiO₂, BN (7 wt%)/TiO₂ and BN (10 wt%)/TiO₂ composite nanofibers indicates that it absorbs light at 380, 348, 345, 344 and 342 nm corresponding to a band gap of 3.2, 3.56, 3.59, 3.6 and 3.62 eV, respectively. Compared to the pure TiO₂ nanofibers, the absorption edges of BN/TiO₂ composite nanofibers are blue shifted. These facts suggest that introducing BN sheets may increase the photocatalytic activity of TiO₂ nanofibers under UV light.

Fig. 6 UV-VIS absorption spectra of (a) TiO₂, (b) BN (3 wt%)/TiO₂, (c) BN (5 wt%)/TiO₂, (d) BN (7 wt%)/TiO₂ and (e) BN (10 wt%)/TiO₂ annealed nanofibers in air for 4 h at 500 °C.

PL spectra of TiO₂ nanofibers are shown in Fig. 7. The significant increase of the PL intensity was observed for BN (5 wt%)/TiO₂, BN (7 wt%)/TiO₂ and BN (10 wt%)/TiO₂ samples. The PL spectrum of sample BN (3 wt%)/TiO₂ is approximately equal to the initial TiO₂ PL spectrum. The PL spectra have been deconvoluted to individual peaks with Gaussian fitting using Origin software (Fig. 8). The summary of the PL analysis is shown in Table S1 (ESI†). We can distinguish three groups of peaks, centred at 415–430 nm, 544–571 nm and 688–731 nm.

Fig. 7 Photoluminescence of TiO₂ and BN/TiO₂ NFs annealed in air at 500 °C.

Fig. 8 One example of a deconvoluted PL spectrum of BN/TiO₂ NFs annealed in air at 500 °C.

In our previous work, we have reported on the detailed analysis of the PL spectra of TiO₂ based nanostructures.^24,44 The observed peak at 415–430 nm corresponds to the self-trapped excitons (STE) localized on the TiO₆ octahedral.^44,45 The peaks in the range of 544–571 nm correspond to oxygen vacancies, localized on the surface, and can also be considered as recombination of free electrons with trapped holes. Peaks at 688–731 nm correspond to structural defects (Ti³⁺, Ti³⁺ interstitials, under coordinated Ti atoms) and can be related to the recombination of free holes with trapped electrons.^24,45 It is worth noting that no peaks related to rutile phase (800–815 nm) were found.

The PL of TiO₂–BN nanostructures have not been reported before. Therefore, the analysis of the PL properties will be based on other photo-induced properties of TiO₂–BN nanostructures. M. Shanmugam et al. reported on BN induced passivation of the surface states on TiO₂ when BN–TiO₂ heterojunctions are formed for solar cell applications. It was discussed that BN passivates dangling bonds on the TiO₂ surface and improves the photocatalytic activity of TiO₂ nanostructures.^46,47 The improved photocatalytic activity is based on the charge separation at the TiO₂/BN interface and the flow of holes to the BN upper layer. Thus, a lower electron–hole recombination rate in BN doped TiO₂ nanostructures is expected. On the basis of the above mentioned BN–TiO₂ interaction the PL intensity in the BN doped TiO₂ nanostructures should decrease.^37,48 However, in our case, the enhancement of the TiO₂ nanofibers PL was observed when increasing the doping level of BN.

The mechanism of BN doping can be explained from comparison of the deconvoluted TiO₂ PL peaks (Fig. 8). Table 4 shows that the normalized integrated STE peak intensity (I1) decreased with the BN doping ratio. From XRD analysis, it was found that additional lattice strain appeared in the TiO₂ lattice after BN doping. The induced strain reduces the formation of the self-trapped excitons (STE) localized on the TiO₆ octahedral and therefore quenches STE emission. The emission related to surface defects (I3) has a tendency to decrease because of the BN passivation of the dangling bonds, related to the unsaturated surface atoms and the photo-generated holes migration to BN.^46,48 We also suppose that BN induced lattice strain in the TiO₂ lattice might cause oxygen vacancy formation. Therefore, the increase of the absolute PL value of peaks, related to oxygen vacancies was observed as BN can passivate the surface state of non-radiative recombination and enhance the PL intensity of the whole spectrum. We also assume that due to charge separation on the BN–TiO₂ interface, the observed PL emission is due to bulk emission centers rather than surface centers. Thus, the observed results show good correlation between optical and structure properties of BN–TiO₂ nanofibers.

Table 4 Normalized integrated intensity of the TiO₂ PL peaks

Doping level	I1	I2	I3
BN (0 wt%)/TiO2	0.01481	1	0.05784
BN (3 wt%)/TiO2	0.01898	1	0.04002
BN (5 wt%)/TiO2	0.00841	1	0.835766
BN (7 wt%)/TiO2	0.01279	1	0.15328
BN (10 wt%)/TiO2	0.00558	1	0.05248

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The quantum yield of the samples was calculated as following:

QY = N_em/N_abs (3)

where N_abs and N_em are numbers of absorbed and emitted photons. Taking into account that all photons are absorbed by TiO₂ at a wavelength of 266 nm, no correlation coefficients for N_abs were used. The N_abs and N_em were calculated on the basis of the obtained experimental values, such as laser power, repetition frequency, sensitivity of fiber optic spectrometer per integration time and the measured signal. The obtained values are shown in the Table 5. Typical QY of TiO₂ nanostructures is in the range of 0.001–0.2, as reported by Li et al.⁴⁹ According to the data, obtained in the present work, the QY of TiO₂ nanofibers is one order lower than in other sources. It points to a high ratio of charge splitting and non-radiative transitions through surface states⁵⁰ which is significantly good for photocatalytic applications.

Table 5 Quantum yield of the pure TiO₂ and BN/TiO₂ composite nanofibers

	TiO2	BN (3 wt%)/TiO2	BN (5 wt%)/TiO2	BN (7 wt%)/TiO2	BN (10 wt%)/TiO2
QY (×10^−6)	128	137	210	227	246
Error (×10^−5)	3.24	3.48	5.34	5.76	6.25

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The photocatalytic activities of BN/TiO₂ with different BN amounts were evaluated by degradation of methyl orange (MO) under UV light (wavelength < 400 nm). MO was selected as a model organic pollutant. Fig. 9(a–g) shows the UV-Vis absorbance spectra of MO solution (major absorption band around 462 nm) with different time intervals 0, 15, 30, 45, 60 and 75 min after 30 min storage in dark to exclude interference from adsorption. For comparison, the photocatalytic activities of TiO₂ nanofibers and commercial TiO₂–P25 were also tested under identical experimental conditions. The results indicate that methyl orange is stable and difficult to be photodegraded in the absence of a photocatalyst. After irradiation for 75 min, MO was degraded up to 65% in the presence of TiO₂ nanofibers, compared to 60% degradation by commercial granular TiO₂–P25 (Fig. 10). Therefore, in case of pure TiO₂, it is well known that TiO₂ nanofibers have a larger active surface area compared to the commercial TiO₂–P25 nanoparticles which increases its photocatalytic activity.⁶ As shown in Fig. 10, MO was degraded up to 73%, 80%, 93% and 99% in the presence of BN (3 wt%)/TiO₂, BN (5 wt%)/TiO₂, BN (7 wt%)/TiO₂, BN (10 wt%)/TiO₂, respectively. It is well known that the photocatalytic activity is mainly governed by crystalline phase, specific surface area, light absorption capacity and separation efficiency of electron–hole pairs. Therefore, in BN/TiO₂ samples, owing to the pronounced electrostatic interactions, the surface naturally negatively charged boron nitride sheets could lead to the transfer of h⁺ from the activated TiO₂ to the BN sheets when they are connected. Thus, more photo electrons were available compared to pure TiO₂ nanofibers, to take part in the photodegradation process.^18,37 Thus the recombination of e⁻ and h⁺ is inhibited as confirmed previously by photoluminescence. However, considering the specific surface area results of BN/TiO₂ nanofibers presented above, a large increase has been achieved compared to the pure TiO₂ nanofibers. From the XRD analysis, the crystallite average size of TiO₂ is increased after introducing BN sheets. As confirmed by UV-VIS, compared to the pure TiO₂ nanofibers, the absorption edges of BN/TiO₂ composite nanofibers are blue shifted. From the obtained results, it is evident that BN sheets improve the separation efficiency of e⁻ and h⁺ and therefore enhance the photocatalytic activity of TiO₂ under UV light.

Fig. 9 UV-Vis spectra of the photocatalytic degradation of MO under UV light: (a) MO without catalyst, (b) P25, (c) TiO₂ nanofibers, (d) BN (3 wt%)/TiO₂, (e) BN (5 wt%)/TiO₂, (f) BN (7 wt%)/TiO₂ and (g) BN (10 wt%)/TiO₂ composite nanofibers.

Fig. 10 Photodegradation of MO by P25, TiO₂ nanofibers and BN/TiO₂ composite nanofibers.

Fig. 11 shows the different kinetic linear curves. The photodegradation reactions follow a Langmuir–Hinshelwood first-order kinetics model.¹ The explanation is described as follows:

r = dC/dt = kKC/(1 + KC) (4)

where r is the degradation rate of methyl orange (MO) (mg (l min)⁻¹), C is the concentration of the MO solution (mg l⁻¹), t is the irradiation time, k is the reaction rate constant (mg (l min)⁻¹), and K is the adsorption coefficient of MO (mg l⁻¹). Since the initial concentrations (C₀ = 10 mg l⁻¹) of the MO solutions are very low in the present experiments, the relationship between ln(C₀/C) and the reaction time t shows that the decomposition of MO with different photocatalysts accords with a pseudo first order kinetic:¹

ln(C₀/C) = kKt = k_at (5)

where k_a is the apparent first-order rate constant (min⁻¹) and C is the concentration at time t. k_a is obtained from the linear dependence between ln(C₀/C) and time are reported in Table 6. The increasing order of the rate constants in the samples is: BN (10 wt%)/TiO₂ > BN (7 wt%)/TiO₂ > BN (5 wt%)/TiO₂ > BN (3 wt%)/TiO₂ > TiO₂ NFs > P25. The rate constant exhibits a maximum of 0.0586 min⁻¹ for BN (10 wt%)/TiO₂, which is 5 and 3.8 times higher than that of P25 and TiO₂ nanofibers, respectively. In addition, all BN/TiO₂ composite nanofibers show higher photocatalytic activity than that of P25 and TiO₂ nanofibers under UV light, which is represented by larger value of k_a. Even though, in the case of BN (10 wt%), the line of [ln(C₀/C) vs. time] is not completely linear, it is still considered as approximately following the Langmuir–Hinshelwood model. This behavior is possibly due to the presence of a higher concentration of BN. When the rate of the reaction is increased with additional BN sheets, the adsorption–desorption equilibrium will be slightly disturbed and the Langmuir behavior is therefore shifted from a perfect linear first order. In our case, the disturbance is relatively small and the [ln(C₀/C) vs. time] curve still gives an acceptable value of k_a. From the obtained results it is evident that introducing BN sheets improves the separation of photo-induced electron–hole pairs of TiO₂ and significantly increases its photocatalytic activity under UV.

Fig. 11 Kinetics of methyl orange degradation by P25, TiO₂ nanofibers and BN/TiO₂ composite nanofibers.

Table 6 Kinetic parameters for photocatalytic activities of the pure TiO₂ nanofibers and BN/TiO₂ composite nanofibers

	k a (min^−1)	R ^2
MO	0.00031	0.9743
P25	0.0117	0.9791
TiO2 NFs	0.0153	0.9706
BN (3 wt%)/TiO2	0.0173	0.9842
BN (5 wt%)/TiO2	0.0203	0.9900
BN (7 wt%)/TiO2	0.0319	0.9541
BN (10 wt%)/TiO2	0.0586	0.8979

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Conclusions

In summary, pure TiO₂ nanofibers and BN/TiO₂ composite nanofibers with different BN nanosheets amounts were prepared by electrospinning. All the samples were annealed at 500 °C in air for 4 hours. The SEM images showed an increase in the average diameter with the increase of BN amount and confirmed that a one-dimensional morphology was maintained. EDX, Raman and FTIR revealed the successful incorporation of the boron nitride in the TiO₂ nanofibers. The specific surface area measurements (BET) confirmed the increase of the surface area with increasing amounts of BN sheets. A boost in the grain size from 16.4 to 28.7 nm was recorded for the composite BN (10 wt%)/TiO₂ compared to the pure TiO₂ nanofibers. The increase in the TiO₂ nanofibers band gap energy from 3.2 eV to 3.62 eV being due to the presence of BN sheets was confirmed by UV-VIS analysis. XRD and photoluminescence spectra confirmed that BN incorporation increases the lattice strain in the TiO₂ cell and reduces the formation of the self-trapped excitons. Owing to the electrostatic interaction, BN sheets can promote the immigration of h⁺ from the bulk of TiO₂ to its surface and consequently improve the photocatalytic activity of TiO₂ under UV light due to efficient charge separation and reduction of charges carriers' recombination. The photocatalytic degradation of methyl orange under UV light was 5 and 3.8 times higher than that of commercial TiO₂–P25 and TiO₂ nanofibers, respectively. In conclusion, the mixing of TiO₂-based photocatalysts with BN sheets is a challenge worth investigating to enhance efficiency in a simple and economically beneficial manner and could be used as catalysts in the degradation of different molecules for photocatalytic application.

Acknowledgements

M. Nasr would like to thank the Lebanese University for the PhD funding. This study was partially supported by the ANR project ANR-14-CE07-0011 “BONALD”.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj03088b

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