Synthesis of NiTiO₃–Bi₂MoO₆ core–shell fiber-shaped heterojunctions as efficient and easily recyclable photocatalysts

Abstract

Development of efficient and recyclable photocatalysts has drawn more and more attention. Herein, we have prepared a kind of core–shell fiber-shaped NiTiO₃–Bi₂MoO₆ heterojunction as an efficient and easily recyclable visible-light-driven photocatalyst. NiTiO₃ nanofibers have been obtained by an electrospinning–calcination method; and in situ growth of Bi₂MoO₆ is conducted on the surface of NiTiO₃ by a simple solvothermal method. NiTiO₃–Bi₂MoO₆ heterojunctions are composed of NiTiO₃ nanofibers with a diameter of about 150 nm whose surfaces are decorated by Bi₂MoO₆ nanoneedles with a diameter of ∼8 nm and length of ∼35 nm. Under visible light irradiation, NiTiO₃–Bi₂MoO₆ heterojunctions exhibit significantly high photocatalytic activities compared with NiTiO₃, Bi₂MoO₆ and a mechanical mixture (22 wt% NiTiO₃ + 78 wt% Bi₂MoO₆) when rhodamine B (RhB) or 4-chlorophenol (4-CP) are selected as model pollutants. For example, NiTiO₃–Bi₂MoO₆ heterojunctions achieve the highest photodegradation efficiency of RhB (97.2%) compared to 21.9% by NiTiO₃, 40.3% by Bi₂MoO₆ and 45.0% by the mechanical mixture. The enhanced photocatalytic activities can be mainly attributed to the efficient separation of photogenerated electron–hole pairs. During the photocatalytic degradation of RhB, superoxide radical ions (˙O₂⁻) and photogenerated holes (h⁺) are detected as the main active species. Moreover, NiTiO₃–Bi₂MoO₆ heterojunctions can be recycled by sedimentation and still possess excellent stability. Therefore, NiTiO₃–Bi₂MoO₆ heterojunctions have great potential to act as efficient and recyclable photocatalysts in practical applications of environmental purification.

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1. Introduction

Environmental problems, especially organic pollutants (such as benzoapyrene, hexachlorocyclohexane and 4-chlorophenyl) in water, which pose a severe threat to human health, are attracting more and more attention.^1–3 To solve this serious problem, photocatalysis technology has been developed due to its low cost, stability and moderate reaction conditions, which offers an environmentally friendly and energy-efficient method for environmental purification.⁴ The key of photocatalytic technology is to develop efficient visible-light-driven (VLD) photocatalysts.^1,5,6 Currently, many kinds of VLD photocatalysts have been well designed and prepared, including simple oxides (m-Bi₂O₄⁷), metal sulfides (Ag₂S–BiOCOOH⁸), complex oxides (Bi₂WO₆^9–12) and non-mental materials (C₃N₄^13–15). Among these photocatalysts, nickel titanate (NiTiO₃¹⁶) with a band gap of 2.18 eV has been demonstrated to be a promising VLD photocatalyst. Powder-shaped NiTiO₃ photocatalysts have been synthesized, including nanorods,¹⁷ nanoparticles¹⁸ and nanowires.¹⁹ For example, NiTiO₃ nanorods can degrade 70% of nitrobenzene after 2 hours under visible light irradiation.¹⁷ For the development of their practical photocatalytic application, it is still necessary to optimize the VLD photocatalytic activity of NiTiO₃ nanomaterials.

It is well known that the construction of heterojunctions between different materials (TiO₂ and C₃N₄)^20,21 is an efficient way to improve the photocatalytic performance. To improve the photocatalytic activity of NiTiO₃, some NiTiO₃-based heterojunctions have been developed, such as NiTiO₃–Ag₃VO₄ nanoparticles,²² NiTiO₃–CdS nanoparticles,²³ mesoporous NiTiO₃–C₃N₄,²⁴ and NiTiO₃–Fe₂O₃ nanofibers.²⁵ These NiTiO₃-based heterojunctions exhibit enhanced photocatalytic activity compared to pure NiTiO₃ photocatalysts. For example, Yang Qu et al. have reported that NiTiO₃–CdS heterojunctions can degrade 90% of Cr(VI) after 2 h under visible light irradiation, which is higher than that (70%) by pure NiTiO₃ nanorods.²³ However, these powder-shaped photocatalysts also suffer from several drawbacks, such as complex preparation process, recycling difficulty and unsatisfactory photocatalytic activity.

Recently, bismuth-based photocatalysts have been reported to have excellent photocatalytic activities under visible light irradiation.^26–28 Among the bismuth-based semiconductors, Bi₂MoO₆ with a band gap of about 2.66 eV has been prepared as a promising VLD photocatalyst to degrade organic pollutants due to its broad visible-light range and long photogenerated electron-pair lifetime.^11,28–30 Meanwhile, Bi₂MoO₆ has been widely used to form heterojunctions with other materials to enhance the photocatalytic efficiency, such as Bi₂MoO₆–sulfide (Bi₂MoO₆–CdS¹⁴), Bi₂MoO₆–metal oxide (Bi₂MoO₆–TiO₂³¹) and Bi₂MoO₆–carbon (Bi₂MoO₆–graphene²⁶). These heterojunctions show a significant increase in photocatalytic activity for degrading organic pollutants. Bi₂MoO₆ may be a potential candidate as a co-catalyst for NiTiO₃ due to its well-matched band gaps, which can suppress the recombination of photoinduced charge carries. To our knowledge, there is no report on the synthesis of NiTiO₃–Bi₂MoO₆ heterojunctions.

The electrospinning method has been recognized as an efficient technique to prepare nonwoven cloth and nanofibers which are easily recyclable. By using electrospinning-assisted methods, our group has prepared several efficient and easily recyclable photocatalysts, including Ta₃N₅–Pt nonwoven cloth,³² Fe₂O₃–AgBr nonwoven cloth³³ and Ta₃N₅/Bi₂MoO₆ core–shell nanofibers.³⁴ However, these are just preliminary works to fabricate composite photocatalysts by the electrospinning method. It is of great importance to further develop other inexpensive, efficient and recyclable photocatalysts. Herein, we developed novel NiTiO₃–Bi₂MoO₆ core–shell fiber-shaped heterojunctions as an efficient and easily reusable photocatalyst. NiTiO₃–Bi₂MoO₆ core–shell fiber-shaped heterojunctions were obtained through an electrospinning–calcination–solvothermal method. Importantly, NiTiO₃–Bi₂MoO₆ heterojunctions exhibited higher photocatalytic activity in degrading organic pollutants (such as RhB and 4-CP) under visible light irradiation than pure NiTiO₃ or Bi₂MoO₆. Furthermore, the mechanism of the advantageous photocatalytic activity was also proposed.

2. Experimental section

2.1 Synthesis of NiTiO₃

NiTiO₃ nanofibers were prepared by an electrospinning–calcination method. Typically, C₄H₆O₄Ni·4H₂O (3 mmol) was dissolved in a mixture of N,N-dimethylformamide (5 mL) and ethylene glycol (5 mL). After 30 min of magnetic stirring, C₁₆H₃₆O₄Ti (1 mL) and PVP (0.7 g) were added into the above solution. After magnetically stirring for 24 h, the solution was then loaded into a plastic syringe. A voltage as high as 14 kV was applied between the needle and ground at a distance of 16 cm. The feeding rate of the syringe pump was set at 0.4 mL h⁻¹ constantly, with a room temperature of 28 °C and constant humidity at 30%. The resulting nonwoven material was collected and followed by calcination at 700 °C for 6 h in a muffle furnace to produce NiTiO₃ nanofibers.

2.2 Synthesis of NiTiO₃–Bi₂MoO₆ heterojunctions

The in situ growth of Bi₂MoO₆ on the surface of NiTiO₃ nanofibers was realized by a simple solvothermal method. Typically, Bi(NO₃)₃·5H₂O (2 mmol) and Na₂MoO₄·2H₂O (1 mmol) were added into a mixture of ethylene glycol (30 mL) and ethanol (10 mL) aqueous solution respectively. Then NiTiO₃ nanofibers (1 mmol, ∼150 mg) were added into the solution and magnetically stirred for 6 h. The solution was then transferred into an autoclave, followed by a solvothermal treatment at 160 °C for 24 h. The obtained sample was centrifuged and rinsed with ethanol four times and then dried for another 24 h. By comparison, Bi₂MoO₆ nanoparticles were also prepared by using the above method without NiTiO₃ nanofibers. The characterizations of the photocatalysts are shown in the ESI.†

2.3 Measurements of photocatalytic activity

In photocatalytic tests, 30 mg of photocatalyst was dispersed into 50 mL RhB aqueous solution (10 mg L⁻¹) or 50 mL 4-CP aqueous solution (1 mg L⁻¹), respectively.¹⁴ Firstly, the solution was stirred in darkness for 30 min to ensure that organic contaminants and photocatalysts reached an adsorption–desorption equilibrium. Then, the photocatalytic activities were measured by degrading the RhB and 4-CP contaminants at room temperature under a Xe lamp equipped with a UV-cutoff filter (wavelength λ > 400 nm). During the photocatalytic process, 1 mL of solution was collected every 20 min and then centrifuged. A UV-3101PC UV-vis spectrometer (Shimadzu) was used to measure the concentration of RhB; and high-performance liquid chromatography (HPLC, Agilent 1100 series) was used to measure the concentration of 4-CP. Tests of total organic carbon (TOC), radical trapping and photocatalytic stability were carried out and are shown in the ESI.†

3. Results and discussion

3.1 Preparation and characterization of NiTiO₃–Bi₂MoO₆

NiTiO₃–Bi₂MoO₆ heterojunctions were obtained by an electrospinning–calcination–solvothermal method (Fig. 1). Firstly, NiTiO₃–PVP nonwoven cloth was prepared by an electrospinning method. The as-prepared nonwoven cloth was green, and it became yellow after the calcination at 700 °C for 6 h. The sizes and morphologies of the calcinated nonwoven cloth were further studied by SEM and TEM images (Fig. 2). The calcinated nonwoven cloth is composed of plenty of nanofibers with diameters of ∼150 nm and lengths of 1∼2 μm (Fig. 2a). In fact, these nanofibers are constructed from nanoparticles with diameters of 50–70 nm (Fig. 2b). The TEM image (Fig. 2c) further confirms that the nanofibers consist of nanoparticles. The high-resolution TEM image (Fig. 2d) clearly displays one set of lattice fringes. The lattice fringe with a spacing of 0.25 nm is in good agreement with the value of the NiTiO₃ (110) plane, which indicates the formation of NiTiO₃ nanofibers.

Fig. 1 Schematic illustration of the preparation of NiTiO₃–Bi₂MoO₆ heterojunctions.

Fig. 2 SEM (a and b) and TEM (c and d) images of NiTiO₃ nanofibers.

The second step was to grow Bi₂MoO₆ on the surface of NiTiO₃ nanofibers by a solvothermal method to form NiTiO₃–Bi₂MoO₆ heterojunctions. The SEM images (Fig. 3a and b) reveal that NiTiO₃–Bi₂MoO₆ heterojunctions are also composed of nanofibers. Obviously, one can find that there are many small nanoneedles on the surface of the NiTiO₃ nanofibers. The diameter of the nanofibers increases to ∼185 nm, due to the coating of the nanoneedles. Further information about the NiTiO₃–Bi₂MoO₆ heterojunctions can be obtained from TEM images (Fig. 3c and d). The TEM images confirm that the sample is composed of nanofibers whose surface is decorated with plenty of nanoneedles. These nanoneedles have a diameter of ∼8 nm and length of ∼35 nm. The high-resolution TEM image (Fig. 3d) taken from the nanofiber demonstrates that the lattice spacings are 0.25 nm and 0.315 nm, which are in accordance with the values of the NiTiO₃ (110) plane and Bi₂MoO₆ (113) plane, respectively. The above results confirm the formation of NiTiO₃–Bi₂MoO₆ core–shell fibers.

Fig. 3 SEM (a and b) and TEM (c and d) images of NiTiO₃–Bi₂MoO₆ heterojunctions.

The XRD patterns of the NiTiO₃–Bi₂MoO₆ heterojunctions, NiTiO₃ nanofibers and Bi₂MoO₆ nanoparticles were further studied for investigating the phase of the as-prepared photocatalysts (Fig. 4). The diffraction peaks of NiTiO₃ at 24.1°, 33.1°, 35.7°, 40.9°, 49.4° and 54.0° are assigned to the (012), (104), (110), (113), (024), (116) and (018) crystal planes (JCPDS Card No. 33-0960) respectively. In addition, the diffraction peaks of Bi₂MoO₆ at 28.3°, 32.6°, 46.7° and 55.5° are assigned to the (131), (002), (202) and (331) crystal planes (JCPDS Card No. 76-2388) respectively. It is obvious that the XRD patterns of the NiTiO₃–Bi₂MoO₆ heterojunctions can be indexed to a mixture of NiTiO₃ and Bi₂MoO₆, and no trace of any impurity peak was observed in the XRD pattern of NiTiO₃–Bi₂MoO₆ which indicated the high purity of these heterojunctions.

Fig. 4 XRD patterns of NiTiO₃–Bi₂MoO₆ heterojunctions, NiTiO₃ nanofibers, Bi₂MoO₆ nanoparticles. For comparison, the standard XRD pattern of NiTiO₃ (JCPDS Card No. 33-0960) and Bi₂MoO₆ (JCPDS Card No. 76-2388) were also supplied.

The nitrogen adsorption–desorption isotherms of NiTiO₃ nanofibers, Bi₂MoO₆ nanoparticles and NiTiO₃–Bi₂MoO₆ heterojunction samples were also investigated (Fig. 5). The Brunauer–Emmett–Teller (BET) surface area of the NiTiO₃ nanofibers is calculated to be 3.6 m² g⁻¹, while that of the Bi₂MoO₆ nanoparticles is 34.6 m² g⁻¹. Interestingly, after the decoration of Bi₂MoO₆ nanoneedles on the surface of the NiTiO₃ nanofibers, the resulting NiTiO₃–Bi₂MoO₆ heterojunction exhibits a greatly enhanced BET surface area (∼37.0 m² g⁻¹) compared with the NiTiO₃ nanofibers, which should result from the introduction of Bi₂MoO₆ nanoneedles with large surface area. Furthermore, we also calculated the pore size distribution from the desorption branches, which reveals the existence of nanopores. The NiTiO₃ nanofibers, Bi₂MoO₆ nanoparticles and NiTiO₃–Bi₂MoO₆ heterojunction samples have a broad pore size distribution centered at about 31 nm, 22 nm and 22 nm respectively. The presence of nanopores may serve as transport paths for organic pollutant molecules.

Fig. 5 Nitrogen adsorption–desorption isotherms of NiTiO₃, Bi₂MoO₆ and NiTiO₃–Bi₂MoO₆ heterojunctions.

The optical absorption of NiTiO₃–Bi₂MoO₆ heterojunctions as well as NiTiO₃ fibers and Bi₂MoO₆ was measured by a UV-vis-NIR spectrometer (Fig. 6). The pure NiTiO₃ nanofibers exhibit an intense absorption in the visible light region with an absorption edge around 545 nm, which is similar to a previous report.¹⁸ Two absorption bands at about 450 nm and 510 nm are observed due to the crystal field splitting of NiTiO₃ with the 3d⁸ band associated with Ni²⁺ splitting into two sub-bands, which can be called Ni²⁺ → Ti⁴⁺ charge-transfer bands.³⁵ In addition, the pure Bi₂MoO₆ exhibits an absorption edge around 460 nm. After the decoration of Bi₂MoO₆ on the surface of NiTiO₃, NiTiO₃–Bi₂MoO₆ heterojunctions display a broad phtotoabsorption from ultraviolet to visible light with an edge around 540 nm. This fact reveals that NiTiO₃–Bi₂MoO₆ heterojunctions have a broad photoabsorption region, resulting in great potential as an excellent VLD photocatalyst.

Fig. 6 UV-vis diffuse reflectance spectra of NiTiO₃–Bi₂MoO₆ heterojunctions, NiTiO₃ and Bi₂MoO₆.

3.2 Photocatalytic performance under visible light irradiation

RhB, which is a kind of dye with color, was chosen as a representative pollutant to test the photocatalytic performance of NiTiO₃–Bi₂MoO₆ heterojunctions. For comparison, NiTiO₃ nanofibers, Bi₂MoO₆ nanoparticles and a mechanical mixture (22 wt% NiTiO₃ nanofibers + 78 wt% Bi₂MoO₆ nanoparticles) were also applied to test their photocatalytic performance under visible light irradiation. RhB displays a major absorption band centered at 554 nm, which can be used to measure the photocatalytic degradation efficiency (Fig. 7a). After 30 min of magnetic stirring in darkness, the NiTiO₃ nanofibers, the Bi₂MoO₆ nanoparticles and the mixture can adsorb 2.1%, 4.6% and 4.9% RhB, respectively. The low adsorption efficiency (2.1%) of RhB by NiTiO₃ nanofibers is associated with their low surface area (3.6 m² g⁻¹), while the low adsorption efficiency (4.6%) by Bi₂MoO₆ nanoparticles should be attributed to the fact that Bi₂MoO₆ nanoparticles are easily aggregated and there are very limited macropores (as shown in Fig. S1, ESI†). Interestingly, the adsorption efficiency (∼40%) of RhB by NiTiO₃–Bi₂MoO₆ heterojunctions is greatly higher than that from other samples, which should result from the higher surface area (37.0 m² g⁻¹) and hierarchical pores consisting of nanopores among the nanoneedles and/or nanofibers as well as macropores among the nanofibers (confirmed by Fig. 2a, b and 5). Under visible-light irradiation, the blank control indicates that no RhB can be degraded without photocatalysts, and NiTiO₃ can degrade 21.9% RhB after 120 min. With Bi₂MoO₆ as the photocatalyst, 40.3% RhB can be degraded in 120 min. Interestingly, when NiTiO₃–Bi₂MoO₆ heterojunctions are applied as photocatalysts, the photocatalytic efficiency is as high as 97.2% after 120 min under visible light irradiation. Obviously, NiTiO₃–Bi₂MoO₆ heterojunctions have the highest photocatalytic activity among all the above materials. To further reveal the role of the heterojunction, RhB degradation by the mechanical mixture (22 wt% NiTiO₃ + 78 wt% Bi₂MoO₆) was also conducted. The mechanical mixture reveals a weak photocatalytic activity, degrading 45.0% of RhB after 120 min under visible light irradiation. This fact shows that the formation of heterojunctions between NiTiO₃ and Bi₂MoO₆ is crucial for the enhancement of photocatalytic performance. To quantitatively evaluate the kinetics of the RhB degradation reaction by all prepared materials, degradation rates are fit by using the pseudo-first-order mode which is in the type of Ln(C₀/C_t) = kt^36,37 (Fig. 7b). C₀ and C_t in the above formula represent the original concentration of RhB and concentration of RhB at the specific time t. k in the above formula is a kinetic rate constant in the unit of min⁻¹ which may vary with different photocatalysts. The kinetic rate constant of NiTiO₃–Bi₂MoO₆ heterojunctions (0.02592 min⁻¹) is much higher than those of NiTiO₃ fibers (0.00178 min⁻¹), Bi₂MoO₆ (0.00392 min⁻¹), the mechanical mixture (0.00467 min⁻¹). Based on the above results, one can conclude that NiTiO₃–Bi₂MoO₆ heterojunctions are much more active than NiTiO₃ fibers, Bi₂MoO₆ and the mechanical mixture.

Fig. 7 (a) Adsorption and degradation efficiencies of RhB (10 mg L⁻¹, 50 mL) and (b) kinetic rate constants (k) of RhB photocatalytic degradation over different photocatalysts under visible light irradiation.

To further demonstrate that the photocatalytic performance actually results from the excitation of the NiTiO₃–Bi₂MoO₆ photocatalyst instead of a dye sensitization mechanism, colorless organic pollutant 4-CP was chosen as the example pollutant. Degradation of 4-CP was measured by high-performance liquid chromatography (Fig. 8a). NiTiO₃, Bi₂MoO₆, and the mechanical mixture can adsorb 1.9%, 4.8% and 6% 4-CP respectively after 30 min with magnetic stirring in darkness. NiTiO₃–Bi₂MoO₆ heterojunctions adsorb 27% 4-CP under the same conditions, resulting from their high surface area (37.0 m² g⁻¹) and hierarchical pores. In the subsequent photocatalytic process, the blank control indicates that no 4-CP is degraded without photocatalysts. With pure NiTiO₃ or Bi₂MoO₆ as photocatalysts, the photocatalytic degradation efficiency of 4-CP is 13% or 28% after 120 min, respectively. The degradation efficiency of the mechanical mixture (22 wt% NiTiO₃ + 78 wt% Bi₂MoO₆) is only 30% after 120 min. Importantly, when the NiTiO₃–Bi₂MoO₆ heterojunction is used as the photocatalyst, 91% 4-CP can be degraded under visible light irradiation after 120 min, which is better than NiTiO₃, Bi₂MoO₆ and even the mechanical mixture. The kinetic curves are shown in Fig. 8b. The kinetic rate constant of the NiTiO₃–Bi₂MoO₆ heterojunctions (0.01957 min⁻¹) is much higher than those of the NiTiO₃ fibers (0.00117 min⁻¹), Bi₂MoO₆ (0.00231 min⁻¹), mechanical mixture (0.00179 min⁻¹). Obviously, NiTiO₃–Bi₂MoO₆ heterojunctions exhibit the best photocatalytic activities. In addition, GC/MS analysis reveals that by using NiTiO₃–Bi₂MoO₆ photocatalysts, there are six predominant intermediate products at 30 min of the photodegradation of 4-CP (Fig. S2 and Table S1, ESI†).

Fig. 8 (a) Adsorption and degradation efficiencies of 4-CP (1 mg L⁻¹, 50 mL) and (b) kinetic rate constants (k) of 4-CP photocatalytic degradation over different photocatalysts under visible light irradiation.

Mineralization of organic pollutants is an important aspect in pollutant treatment. A typical way to determine the mineralization degree of organic pollutants is to measure the total organic carbon (TOC). The mineralization degree of RhB was studied by adding 250 mg NiTiO₃–Bi₂MoO₆ photocatalyst in 250 mL RhB aqueous solution (50 mg L⁻¹) with magnetic stirring under visible light irradiation. The TOC value continuously decreases with the increase of irradiation time, which indicates that RhB is steadily mineralized (Fig. 9). After 7 h of reaction, the TOC value decreases from 43.48 mg L⁻¹ to 23.95 mg L⁻¹, revealing that RhB mineralization reaches a high ratio of 45%. This fact shows that NiTiO₃–Bi₂MoO₆ photocatalysts can efficiently mineralize organic pollutants under visible light irradiation.

Fig. 9 TOC removal during RhB (50 mg L⁻¹, 250 mL) degradation in the presence of NiTiO₃–Bi₂MoO₆ heterojunctions (250 mg) under visible light irradiation.

To further explore the mechanism of photocatalytic degradation, it is crucial to find out the major active species responsible for the degradation of organic pollutants with NiTiO₃–Bi₂MoO₆ heterojunctions during the photocatalytic process. Isopropyl alcohol (IPA), benzoquinone (BQ), ammonium oxalate (AO) and AgNO₃ are commonly used to capture hydroxyl free radicals (˙OH), superoxide radicals (˙O₂⁻), photogenerated holes (h⁺) and electrons (e⁻), respectively.³⁸ By adding four different scavengers in RhB solution with NiTiO₃–Bi₂MoO₆ heterojunctions, trapping experiments were conducted to find out the most effective species (Fig. 10a). When IPA or AgNO₃ is added, the degradation efficiency of RhB remains 92.4% or 93.6%, which is close to the result (∼97.2%) without scavengers. This fact indicates that ˙OH and e⁻ are not the major active species responsible for RhB degradation. On the contrary, with the addition of BQ or AO, the degradation efficiency of RhB decreases to 37.1% or 47.1%, indicating that the photocatalytic activities can be significantly suppressed. The kinetic rate constant decreases dramatically from 0.02592 min⁻¹ to 0.00142 min⁻¹ and 0.00392 min⁻¹ respectively (Fig. 10b). These facts verify that O₂⁻ and h⁺ are the major active species in the photocatalytic degradation of RhB with NiTiO₃–Bi₂MoO₆ heterojunctions.

Fig. 10 (a) Adsorption and degradation efficiencies of RhB (5 mg L⁻¹, 50 mL) by NiTiO₃–Bi₂MoO₆ heterojunctions in the presence of silver nitrate (AgNO₃), ammonium oxalate (AO), isopropyl alcohol (IPA) or benzoquinone (BQ) under visible light irradiation (λ > 400 nm); and (b) the kinetic rate constants of RhB photocatalytic degradation in the presence of AgNO₃, AO, IPA, BQ.

The stability of the NiTiO₃–Bi₂MoO₆ heterojunctions is a critical factor for practical application. Four consecutive runs of photocatalytic tests were conducted to measure the stability (Fig. 11). After each cycle, NiTiO₃–Bi₂MoO₆ heterojunctions were separated by centrifugation from solution, washed thoroughly with deionized water and then dried for the next use. The recycling efficiency of NiTiO₃–Bi₂MoO₆ heterojunctions is determined to be 95.1% in each cycle. After four cycles, the degradation efficiency of RhB has a very slight decrease from 97.2% at the first cycle to 93.3% at the fourth cycle (Fig. 11). This slight decrease in degradation efficiency should result from the imperfect centrifugation efficiency (95.1%) instead of the loss of photocatalytic activity. Furthermore, we investigated the chemical stability of NiTiO₃–Bi₂MoO₆ heterojunctions by analyzing the Ni, Ti, Bi, and Mo concentration in the clear solution after the centrifugation. Inductively coupled plasma results (Table S2, ESI†) indicate that the concentrations of all metal elements (Ni, Ti, Bi, Mo) are below 0.01 μg mL⁻¹, which suggests that NiTiO₃–Bi₂MoO₆ heterojunctions have very low solubility in water. Therefore, these facts indicate that NiTiO₃–Bi₂MoO₆ heterojunctions can be easily recyclable and have high stability.

Fig. 11 Cycling photocatalytic test of NiTiO₃–Bi₂MoO₆ heterojunctions.

The electronic structures and energy band of NiTiO₃ and Bi₂MoO₆ were studied. On the basis of their energy band diagram, the photocatalytic process of the NiTiO₃–Bi₂MoO₆ heterojunction system can be proposed (Fig. 12). Both NiTiO₃ and Bi₂MoO₆ can generate not only electrons in their conduction band but also holes in their valence band under visible light irradiation. Based on the fact that the CB and VB of NiTiO₃ are lower than those of Bi₂MoO₆, the Fermi levels of NiTiO₃ and Bi₂MoO₆ tend to move up and down respectively when the heterojunction structure forms between NiTiO₃ and Bi₂MoO₆.^17,39 As a result, the photogenerated electrons in the CB of Bi₂MoO₆ tend to flow into that of NiTiO₃. Meanwhile, photogenerated holes are available for the transfer from VB of NiTiO₃ to that of Bi₂MoO₆. Thus, an equilibrium is formed (Fig. 12). Under these circumstances, electrons gathering on the CB of NiTiO₃ are successfully forced to produce more O₂⁻ that can be effectively utilized to decompose organic pollutants (such as RhB and 4-CP). The holes stored in the VB of Bi₂MoO₆ can also oxidize organic pollutants directly. Therefore, the formation of heterojunctions between NiTiO₃ and Bi₂MoO₆ can promote the separation of photoelectron–hole pairs, which further leads to high photocatalytic activity.

Fig. 12 Schematic diagram of electron–hole pair separation and possible reaction mechanism of NiTiO₃–Bi₂MoO₆ heterojunctions under visible light irradiation.

4. Conclusions

In summary, NiTiO₃–Bi₂MoO₆ heterojunctions have been successfully prepared by an electrospinning–calcination–solvothermal method. They are composed of NiTiO₃ nanofibers which are coated with Bi₂MoO₆ nanoneedles. NiTiO₃–Bi₂MoO₆ heterojunctions exhibit enhanced photocatalytic activity compared with NiTiO₃ and Bi₂MoO₆ under visible light irradiation, due to the efficient separation of electron–hole pairs. In addition, the photocatalytic process is dominantly governed by ˙O₂⁻ and h⁺. Moreover, NiTiO₃–Bi₂MoO₆ photocatalysts can mineralize organic pollutants with good stability and can be easily recycled which demonstrates their great potential for practical application in environmental purification.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21477019), the Fundamental Research Funds for the Central Universities, and the DHU Distinguished Young Professor Program.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj03367b

‡ These authors contributed equally to this work.

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