Synthesis of ionic liquid-modified BiPO₄ microspheres with hierarchical flower-like architectures and enhanced photocatalytic activity

Abstract

In this work, hierarchical flower-like BiPO₄ microspheres were successfully synthesized by a microwave-assisted hydrothermal reaction of bismuth nitrate with [C₄mim][PF₆] (1-butyl-3-methylimidazolium hexafluorophosphate) in water at 160 °C, where the ionic liquid could act as a phosphorus source, surface modified agent and template. The photocatalytic activities of the as-prepared BiPO₄ samples were evaluated by decolorization of rhodamine B in aqueous solution under UV light irradiation. It was found that the BiPO₄ sample modified with ionic liquid (IL-BiPO₄) exhibited significantly enhanced photocatalytic activity in comparison with the unmodified one prepared in the absence of [C₄mim][PF₆]. In addition, the effect of ionic liquid modification on the improved photocatalytic activity was investigated in detail and a possible photocatalytic mechanism was proposed for IL-BiPO₄. It is suggested that the significantly improved photocatalytic activity of IL-BiPO₄ could be mainly attributed to trapping of the photoinduced electron at the conduction band of IL-BiPO₄ and thus effectively improving the separation efficiency of photoinduced electron–hole pairs due to ionic liquid modification.

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1. Introduction

Since the 1990s much attention has been paid to photocatalytic technology because of its potential applications in the degradation of organic contaminants in wastewater and the atmosphere for environmental remediation.^1–4 Photocatalysis especially for TiO₂-based photocatalytic technology exhibits powerful oxidation in the treatment of toxic and bioresistant organic pollutants such as dye wastewater by converting them into non-hazardous species such as CO₂ and H₂O. Nevertheless, low solar energy conversion efficiency and high recombination of photoinduced electron–hole pairs are the main problems for TiO₂ photocatalysts, which gives high running costs and greatly restricts their industrial application.^5–8 To overcome these drawbacks, two main strategies have been exploited to improve the photocatalytic performance of photocatalysts. The first one is based on modification of TiO₂, including metal or nonmetal elements doping, surface modification, sensitization and coupling with other semiconductors, etc.^9–11 Another one is to develop new photocatalyst with efficient photocatalytic activity.

As an oxoacid salt photocatalyst, BiPO₄ was firstly discovered by Zhu's group and found to exhibit superior photocatalytic activity to P25.^12,13 Subsequently, many approaches were explored for the synthesis of various BiPO₄ nanostructures with efficient photocatalytic activity. For example, monoclinic phase BiPO₄ nanorods and hexagonal phase BiPO₄ nano-cocoons were selectively synthesized through solvothermal method by Qian and his co-workers.¹⁴ By using different solvents, BiPO₄ with various morphologies such as nanoparticles, needle-like, rod-like and rice-like nanostructures were successfully synthesized by Chen et al.¹⁵ Pan and Zhu synthesized well dispersed BiPO₄ nanocrystals with a diameter of about 9 nm by a high-temperature hydrolysis reaction.¹³ Although many preparation methods and controllable morphologies of BiPO₄ have been investigated so far, little attention has been focused on the surface modification of BiPO₄ with organic compounds for the construction of surface-modified photocatalytic systems which has been deeply researched in the photocatalytic systems of TiO₂.¹⁶

Ionic liquids (ILs), as greener and recyclable solvents, have been widely used in the synthesis of inorganic nanomaterials because of their unique properties such as good dissolving ability, extremely low volatility, high ionic conductivity, high thermal stability and designable structures and properties.^17,18 In the inorganic synthetic procedures, ionic liquids have been employed as reactants, templates, and solvents for the synthesis of inorganic materials with various novel morphologies and improved properties.¹⁹ To date, nanomaterials with various morphologies have been synthesized via various ILs-assisted processes (e.g. TiO₂,²⁰ ZnO,²¹ BiOI¹⁸ and Bi₂WO₆ (ref. 22)). Besides that, the using of ILs during the synthetic procedure can also adjust the physicochemical properties of the as-obtained nanomaterials through the in situ modification of ILs. For examples, Wang et al. synthesized [Bmim]OH modified TiO₂ catalyst and the results indicated [Bmim]OH modification could enhance the visible light absorption.²³ Zhang and his co-workers found the modification of [Bmim]⁺ cation onto the surface of BiOI was beneficial for the separation of charge carriers.¹⁸ Hence, it can be expected that morphology-controlled BiPO₄ catalyst with enhanced photocatalytic activity might be synthesized in the existence of imidazolium-based ionic liquids.

In this work, hierarchical flower-like BiPO₄ microspheres were successfully prepared via a microwave-assisted hydrothermal route by using [C₄mim][PF₆] as reactant and template. The effect of ionic liquid on the enhanced photocatalytic activity is studied and discussed in detail. Then the photocatalytic mechanism of IL-BiPO₄ has been investigated and the result demonstrates that the enhanced photocatalytic activity of IL-BiPO₄ can be mainly ascribed to the in situ modification of an ionic liquid [C₄mim][PF₆] during the hydrothermal process.

2. Experimental

2.1 Preparation of the photocatalyst

All reagents were of analytical grade and used without further purification. Deionized water was used in the whole experiment. In a typical procedure, 1.0 mmol of Bi(NO₃)₃·5H₂O was dissolved into 30 mL deionized water. The mixture was stirred for 30 min to dissolve Bi(NO₃)₃·5H₂O completely. Subsequently, 1.0 mmol of [C₄mim][PF₆] was added slowly into the above solution and the pH value of the mixture was adjusted to 0.5 by aqueous nitric acid. The as-formed colorless transparent aqueous solution was transferred into a 100 mL autoclave and then heated by a microwave-assisted hydrothermal system. The system was operated at 2.45 GHz frequency with 80% of output power of the microwave (1000 W). The temperature and exposure time were programmed. The reaction temperature was raised from room temperature to 80 °C within 2 min, then increased to 160 °C within 10 min and maintained at this temperature for 15 min. After cool-down to room temperature, a white precipitate was obtained by centrifugation and washed with deionized water and ethanol thoroughly. Finally, the product (denoted as IL-BiPO₄) was dried under vacuum at 80 °C for 8 h. For comparison, the BiPO₄ samples obtained in the absence of [C₄mim][PF₆] were also prepared by reacting bismuth nitrate with sodium dihydrogen phosphate (denoted as NH-BiPO₄), while the other experimental conditions remained the same.

2.2 Characterization

The crystallographic phase of the samples was determined by X-ray diffraction (XRD) (Bruker D8 Advance, Germany) using graphite monochromatic copper radiation (Cu Kα). Morphologies and structures of the samples were analyzed by scanning electron microscopy (SEM) (JEOL JSM-63901, Japan) and transmission electron microscopy (TEM) (JEOL JEM-2100, Japan). The Brunauer–Emmett–Teller (BET) specific surface area was measured, in terms of the N₂ adsorption on the power, using a volumetric adsorption apparatus (NOVA Surface Area Analyzer Station A, USA). Surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS) (Kratos, UK) with an Al Kα X-ray source. The optical absorption spectra of the samples were recorded by UV-vis spectrophotometer (Cary 5000 UV-vis-NIR, USA) using BaSO₄ as a reference. Fourier transform infrared spectroscopy (FT-IR) studies were carried out in the 500–4000 cm⁻¹ frequency range (Avatar 360E.S.P. FTIR, USA). Photoluminescence (PL) spectra were measured by using a fluorescence spectrophotometer (FP-6500, Japan) equipped with a xenon lamp at an excitation wavelength of 300 nm. The scanning speed was 1200 nm min⁻¹ and the photomultiplier voltage was kept at 700 V. The slot widths of the excitation and emission slits were both of 5.0 nm. Time-resolved photoluminescence spectroscopy was obtained at room temperature with a FLsp 920 Fluorescence spectrometer (Edinburgh Instruments).

2.3 Photocatalytic test

All experiments were carried out in a photoreaction apparatus as reported in our previous study.²⁴ A 125 W high-pressure mercury lamp with the strongest emission at 365 nm was used as light source. In each experiment, 0.1 g of photocatalyst was added into 100 mL of Rhodamine B (RhB) solution (5 mg L⁻¹). Before illumination, the suspension was sonicated for 20 min and stirred for 30 min in the dark to reach the adsorption–desorption equilibrium. At given time intervals, about 3 mL of the suspension was sampled and centrifuged to separate the catalyst before analysis. The concentration of RhB was recorded using UV-vis spectrophotometer at λ_max of 552 nm.

3. Results and discussion

3.1 Structural characteristics of BiPO₄ samples

Fig. 1 shows the XRD patterns of the as-prepared samples. The diffraction peaks of both NH-BiPO₄ and IL-BiPO₄ samples are in good accordance with the standard data of the pure monoclinic phase BiPO₄ (JCPDS 15-0767). No characteristic peaks of the other impurities can be detected in both of the XRD patterns, indicating the high purity of products. Besides, no obvious change in peak shape and position has been observed in NH-BiPO₄ and IL-BiPO₄ samples, which suggest that [C₄mim][PF₆] modification does not change the crystalline structure of the as-prepared BiPO₄. However, the intensity ratio of the (200) peak to the (120) peak of IL-BiPO₄ is obviously larger than that of NH-BiPO₄. This may be due to the fact that [C₄mim][PF₆], as a reactant and template, prefers to adsorb on the (100) facet of IL-BiPO₄ via Coulomb coupling force and thereby inhibits crystalline growth in the [100] direction during the synthesis of IL-BiPO₄.

Fig. 1 XRD patterns of the as-prepared IL-BiPO₄ and NH-BiPO₄.

The morphologies of the as-prepared BiPO₄ were characterized by SEM and TEM techniques. As shown in Fig. 2a, NH-BiPO₄ product is composed of octahedron-like microcrystals and amorphous nanoparticles without a discernable morphology. While, IL-BiPO₄ products (Fig. 2b) exhibit well-assembled hierarchical flower-like microspheres constructed by a plenty of octahedrons, suggesting that the existence of [C₄mim][PF₆] plays a vital role for the formation of microsphere structures. The HRTEM image of IL-BiPO₄ performed on an individual octahedron is illustrated in Fig. 2c. The lattice interplanar spacing was calculated to be 0.328 nm, corresponding to the (200) lattice plane of monoclinic phase BiPO₄. Moreover, clear lattice fringes only appear in certain parts of the HRTEM image at a given time, indicating the polycrystalline microstructure of IL-BiPO₄,²⁵ which is confirmed by the corresponding diffraction ring of the selected area electron diffraction (SAED) pattern in Fig. 2d.

Fig. 2 (a) SEM image of NH-BiPO₄. (b) SEM image (inset is an individual microsphere), (c) HRTEM image and (d) SEAD pattern of IL-BiPO₄.

To investigate the effect of ILs with different organic cations and/or different alkyl chain lengths on the morphology of the product, the BiPO₄ was also prepared in the presence of diverse ILs. As shown in Fig. S1a and b (ESI material†), no obvious changes in the morphology can be observed when the organic cation [C₄mim]⁺ is replaced by pyrrolidinium or piperidinium cation, indicating that the organic cation of ILs is not the determining factor for the morphological modulation of the final products. As the C2 alkyl chain was applied (Fig. S1c†), the as-obtained BiPO₄ shows analogous morphology to that prepared in the presence of [C₄mim][PF₆], but individual octahedron can also be observed in the product. When the alkyl chain was lengthened to C8, the BiPO₄ product exhibits sphere-like structure but the microsphere is composed of irregular blocks instead of octahedron (Fig. S1d†). As the length of the alkyl chain varies, the morphological variation of the product might be associated with the changes in aggregation and the non-polar domains of ILs and further studies are needed to illustrate this phenomenon.²⁶

The morphologic variety with increasing amount of ILs is supported by SEM observation (Fig. S2†). As the [C₄mim][PF₆] concentration increases to [[C₄mim][PF₆]] : [Bi(NO₃)₃] = 1.5 : 1, irregular spheres, octahedrons and column-like crystals are coexisted in the as-prepared BiPO₄ sample, suggesting that the morphology can be varied at condensed ILs concentrations. The increasing amount of ILs can result in the increase in viscosity and consequently slow down the diffusion rate of the primary BiPO₄ particles, which will favor an anisotropic particle growth and the self-assembly into different morphologies.^27,28

The microwave-assisted ionic liquid method has been proven to be suitable for the rapid synthesis of a variety of elemental and compound nanostructures.²⁹ To demonstrate the effect of microwave irradiation on the morphology of the sample, drying oven was used as heating apparatus to substitute microwave, while the other conditions were similar to the preparing process described above. When the mixture of 1.0 mmol of Bi(NO₃)₃·5H₂O and 1.0 mmol of [C₄mim][PF₆] was treated at 160 °C for 15 min, no precipitates of BiPO₄ was obtained. As the reaction time was prolonged to 24 h,³⁰ the as-obtained BiPO₄ is composed of octahedron-like microcrystals and irregular aggregates of particles (Fig. S3†), suggesting that microwave irradiation also plays key role for the formation of microspheres structures. Due to the high ionic conductivity and polarizability, ILs are excellent microwave absorbents and exhibit highly susceptible to microwave irradiation. Hence, the combination of microwave heating and ILs result in rapid heating rate and can significantly shorten reaction time.²⁹ The polarization and movement of ions resulted from the fast changing electric field of microwave give rise to the transient, anisotropic micro-domains for the reaction system, which thereby improve the anisotropic growth of various BiPO₄ structures.³¹

To understand the formation process of the flower-like BiPO₄ microspheres, a series of time-dependent experiments were conducted at 160 °C for 1, 5, 10 and 15 min, respectively. When the reaction time was processed for 1 min, the as-obtained products consisted of microspheres constructed by irregularly shaped polyhedra with an average crystallite size of about 2.3 μm as building blocks (Fig. S4a†). With a longer reaction time (5–10 min), the products still remained the microspheres structures. However, irregularly shaped polyhedra together with octahedrons composed the microspheres structures (Fig. S4b and c†). Finally, hierarchical flower-like BiPO₄ microspheres constructed by octahedrons were obtained when the reaction time increased to 15 min (Fig. S4d†).

Based on the above experimental results, a possible formation process of flow-like BiPO₄ microspheres is proposed. Initially, BiPO₄ nuclei are formed through the reaction between Bi cations and P anions originating from the ILs [C₄mim][PF₆] in the microwave-hydrothermal system. Then, BiPO₄ nuclei grow to nanocrystals via a process of classical crystal growth. To reduce the surface energy of system, these nanocrystals tend to self-assemble into microsphere structures. During the self-assembly process, the adjacent BiPO₄ nanocrystals in the microspheres will spontaneously modulate themselves to share the common crystallographic orientation, thereby resulting in the oriented attachment of these nanocrystals to form polyhedra.^32–34 Finally, flower-like BiPO₄ microspheres constructed by octahedrons are obtained by further oriented aggregation of nanocrystals. In the synthesis process of BiPO₄ microspheres, the [C₄mim]⁺ can adsorb onto a certain crystallographic plane of BiPO₄ nanocrystals via Coulomb coupling force, which prevent the BiPO₄ nanocrystals from growing into individual octahedron.

In order to illustrate the effect of ILs modification on the photocatalytic activity of the as-prepared BiPO₄, we focused our attention on the hierarchical flower-like IL-BiPO₄ microspheres obtained in the presence of [C₄mim][PF₆]. Surface compositions and chemical state of the as-prepared IL-BiPO₄ photocatalysts were investigated using XPS. The binding energies obtained from the XPS analysis were corrected by referencing C 1s to 284.60 eV. As shown in Fig. 3a, the peaks of Bi 4p, Bi 4d, Bi 4f, Bi 5d, O KLL, O 1s, O 2s, P 2s, P 2p, N 1s, and C 1s can be observed in the XPS survey spectrum of IL-BiPO₄ sample. As for IL-BiPO₄, two peaks at about 159.6 eV and 164.9 eV are attributed to the binding energies of Bi 4f_5/2 and Bi 4f_7/2, respectively, which are in good accordance with the state of Bi³⁺ in BiPO₄ (Fig. 3b).^35,36 A broad signal peak at about 132.9 eV can be detected in the XPS spectra in P 2p region (Fig. 3c), indicating the existence of a pentavalent oxidation state for P.¹ For the O 1s (Fig. 3d), the dominant peak at 530.8 eV is ascribed to the crystal-lattice oxygen in IL-BiPO₄, while the other peak at 532.5 eV may be assigned to O–H, C–O and C–O–O bonds arising from the adsorbed oxygen on the surface of IL-BiPO₄.³⁷ The C 1s of IL-BiPO₄ consists of two peaks at 284.6 and 286.1 eV (Fig. 3e), which are ascribed to the surface adventitious carbon and C–N groups of imidazolium, respectively.^38,39 All these analytic results of XPS confirm that the ionic liquid is modified onto the surface of IL-BiPO₄. Moreover, the binding energies of Bi 4f_5/2 and Bi 4f_7/2 of IL-BiPO₄ display positive shifts compared to those of NH-BiPO₄ (158.9 eV for Bi 4f_5/2 and 164.2 eV for Bi 4f_7/2), while the binding energy of P 2p of IL-BiPO₄ shifts to a lower position than that of NH-BiPO₄ (133.5 eV for P 2p). The reverse shift might be attributed the Coulomb coupling force between [C₄mim]⁺ cation modified onto the IL-BiPO₄ surface and P anion, which cause some electrons to transfer from Bi to P. Similar results have been observed in previous studies.^18,40 In addition, the chemical composition of the IL-BiPO₄ sample was determined using the relative sensitivity factors and the result indicated that atomic ratio of Bi, P and O is about 1 : 0.93 : 4.08, which is close to the theoretical stoichiometry of BiPO₄.

Fig. 3 (a) XPS survey spectra of the as-prepared BiPO₄ samples. High-resolution XPS spectra of (A) IL-BiPO₄ and (B) NH-BiPO₄ in the regions of (b) Bi 4f, (c) P 2p, (d) O 1s, and (e) C 1s.

To further verify the existence of [C₄mim][PF₆] on the modified surface of the photocatalyst, IL-BiPO₄ and NH-BiPO₄ samples were characterized with the help of FT-IR spectroscopy (Fig. S5†). In the spectrum of NH-BiPO₄, the absorption bands at 3483 and 1616 cm⁻¹ may be assigned to the O–H stretching and H–O–H bending vibrations of the water adsorbed on the surface of the sample.^41,42

Four bands observed at 1070, 1001, 954, 925 cm⁻¹ can be assigned to ν₃ stretching vibration bands of (PO₄) group.⁴³ The bending vibration peaks of O–P–O linkages appear around 607, 550 and 528 cm⁻¹.^14,44 In addition, these peaks belonging to the characteristics of NH-BiPO₄ are still presented in the spectrum of IL-BiPO₄, indicating that no crystalline structure change occurs after the modification of BiPO₄ by [C₄mim][PF₆]. This result is in consistence with the analysis of the XRD patterns. Interestingly, several peaks can only be observed in the FT-IR spectrum of IL-BiPO₄ but absent in the spectrum of NH-BiPO₄. Among these peaks the one at 3138 cm⁻¹ can be assigned to the C–C bond in position four and five of imidazolium ring.⁴⁵ Another peak around 2878 cm⁻¹ is attributed to the symmetric stretch of the H–C–H bond in butyl group. The strong peak around 1385 cm⁻¹ is due to the typical stretching modes of C–N heterocycles.⁴⁶ The differences of FT-IR spectra between IL-BiPO₄ and NH-BiPO₄ samples also confirm that [C₄mim][PF₆] is modified onto the surface of IL-BiPO₄, which is consistent with the result of the XPS analysis.

The UV-vis diffuse reflectance spectra of the as-prepared samples were presented in Fig. S6.† The steep shapes of spectra suggested that the light absorption of both BiPO₄ samples is not due to the transition from the impurity level but to the intrinsic band-gap transition. The intense band centered at 248 nm can be attributed to the electronic transition from O 2p states to Bi 6p states.⁴⁷ The band gap energy of IL-BiPO₄ is approximately 3.95 eV, slightly smaller than 4.06 eV for NH-BiPO₄. This slightly smaller band gap energy of IL-BiPO₄ might be related to the collective effects associated with such large size and hierarchical assembly structure. In addition, IL-BiPO₄ exhibits a relatively enhanced optical absorbance than NH-BiPO₄ in the range of 300 to 600 nm, which allows the IL-BiPO₄ catalysts to utilize enough light during the photocatalytic process, especially for visible light. Therefore, modification of BiPO₄ surface by [C₄mim][PF₆] can effectively extend the absorption of BiPO₄ to the visible light region, which is similar to the case of ionic liquid-modified TiO₂ nanoparticles reported by Wang et al.²³

PL emission spectra are widely used to evaluate the efficiency of migration, transfer and recombination of the photoinduced electron–hole pairs in semiconductor particles, since PL emission mainly arises from the recombination of free carriers.⁴⁸ Generally, the higher PL emission intensity indicates the higher recombination efficiency of the photogenerated carriers and the lower photocatalytic activity.⁴⁹ The comparison of the PL spectra of the as-prepared IL-BiPO₄ and NH-BiPO₄ products is illustrated in Fig. 4. It can be seen that the PL emission spectra of both samples exhibit the main peaks at similar positions but with different intensities. The emission intensity of NH-BiPO₄ is much higher than that of IL-BiPO₄, suggesting that ionic liquid modified onto the surface of BiPO₄ can effectively inhibit the recombination of photogenerated electron–hole pairs and thereby improve the photocatalytic activity of IL-BiPO₄ on the degradation of organic contaminants.

Fig. 4 PL spectra of IL-BiPO₄ and NH-BiPO₄.

The lifetime of the charge carriers for IL-BiPO₄ and NH-BiPO₄ was further investigated by time-resolved fluorescence decay spectra, and the results were shown in Fig. 5. Clearly, in contrast to NH-BiPO₄, IL-BiPO₄ exhibits slow decay kinetics. Each decay curve can be well fitted by a bi-exponential function: PL(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂), where τ₁ and τ₂ represent the decay time of the PL emission, A₁ and A₂ stand for the relative weights of the decay components at t = 0. Based on the fitting results, two radiative lifetimes with different percentages have been determined. Generally, the shorter lifetime is several nanoseconds, which is ascribed to the initially populated core-state recombination. The longer lifetime is on the time scale of tens of nanoseconds, which is attributed to the surface-related radiative recombination of carriers. In our case, the shorter lifetime of 1.81 ns for NH-BiPO₄ increases to 2.35 ns for IL-BiPO₄, though its percentage decreases from 89.15% to 87.84%. On the other hand, the longer lifetime increases from 11.86 ns for NH-BiPO₄ to 13.21 ns for IL-BiPO₄ and its percentage increases from 10.85% to 12.16%. These results indicate that the radiative lifetime of all the charge carriers have been effectively lengthened by modifying ionic liquid on the surface of BiPO₄, which play an important role in increasing the possibility of electrons or holes participating in photocatalytic reactions.^50,51

Fig. 5 Time-resolved fluorescence decay spectra of IL-BiPO₄ and NH-BiPO₄.

3.2 Photocatalytic properties of BiPO₄ samples

Photocatalytic activities of different photocatalysts were evaluated by decolorization of RhB under UV light irradiation with otherwise identical conditions. The results were presented in Fig. 6a, where C₀ was the concentration of the initial dye and C was the concentration of RhB after irradiation. A blank test (RhB solution without any photocatalyst) was carried out to estimate the effects of dye self-degradation on the overall photocatalytic performances of BiPO₄. It can be seen that the decolorization efficiency is much lower in the absence of photocatalyst, indicating that the self-degradation of RhB is almost negligible. However, the decolorization of RhB became obvious when photocatalysts were added to the solution. After 60 min irradiation, the photocatalytic decolorization efficiencies of RhB are about 94% and 75% for IL-BiPO₄ and NH-BiPO₄ photocatalysts, respectively. Obviously, IL-BiPO₄ catalyst exhibits higher photocatalytic activity than that of NH-BiPO₄ photocatalyst. To investigate the photocatalytic decolorization kinetics of RhB, the photocatalytic data were fitted to the apparent pseudo-first-order kinetics and the results were shown in Fig. 6b. It can be observed that the photocatalytic decolorization process follows pseudo-first-order kinetics, and the apparent reaction rate constants k calculated on the basis of the slope of the fitting line are 0.044 and 0.021 min⁻¹ for IL-BiPO₄ and NH-BiPO₄, respectively. In other words, the photocatalytic activity of IL-BiPO₄ is twice than that of NH-BiPO₄.

Fig. 6 (a) Effects of IL on the photocatalytic activity of BiPO₄. (b) Kinetic fit for the decolorization of RhB with NH-BiPO₄ and IL-BiPO₄. (c) Photocatalytic decolorization of RhB over IL-BiPO₄ with the addition of EDTA and TBA. (d) Schematic illustration of RhB photodecolorization over IL-BiPO₄.

In addition to high photocatalytic activity, the stability and recyclability of photocatalyst play extremely important role in practical application. The cycling runs for the photodecolorization of RhB by IL-BiPO₄ were performed under UV irradiation (Fig. S7†). After every photodecolorization run of 60 min, the catalyst was separated by centrifugation, washed with deionized water and dried in oven. It was found that the photocatalytic efficiency only slightly decreased after four repeated runs, indicating the high stability of IL-BiPO₄ photocatalyst for RhB photodecolorization.

To illustrate the role of ILs on the photocatalysis of BiPO₄, the comparison of photocatalytic activities of IL-BiPO₄ and NH-BiPO₄ with the addition of ILs in the solution (denote as IL + NH-BiPO₄) was carried out under the identical conditions. The molar amount of ILs added to the solution is kept to be equal to that of NH-BiPO₄. As shown in Fig. 6a, it is clearly found that the IL + NH-BiPO₄ sample exhibits similar photocatalytic activity with NH-BiPO₄, which is obviously lower than that of IL-BiPO₄. This indicates that the addition of ILs in the solution has no positive influence on the enhancement of the photocatalytic performance of BiPO₄. Therefore, the enhanced photocatalytic performance of IL-BiPO₄ is not due to the electrostatic attraction between the negatively charged dye (RhB) and [C₄mim]⁺, but to the in situ ionic liquid modification during the hydrothermal process. It is well known that the photocatalytic activity is affected by various factors such as the surface area, band gap and separation efficiency of photoinduced electrons and holes. The BET surface area of IL-BiPO₄ (0.03 m² g⁻¹) is much lower than that of NH-BiPO₄ (0.99 m² g⁻¹). Generally, a low BET surface area leads to the low photocatalytic performance. However, many other factors can also influence the photocatalytic performance of photocatalyst. In our case, IL-BiPO₄ sample exhibits relatively small band gap and enhanced optical absorbance in the range of 300 to 600 nm (Fig. S6†), which means that more light can be harvested during the photocatalytic process. However, this driving force caused by the difference between the optical property of IL-BiPO₄ and IL-BiPO₄ is not large enough to completely illuminate why the photocatalytic activity of IL-BiPO₄ is much higher than that of IL-BiPO₄. Hence, it is reasonable to deduce that there should be some other more important reasons for its improved activity, which may be related with the separation efficiency of photogenerated electron–hole pairs based on the classic photocatalytic theory. Hence, the improved photocatalytic activity of IL-BiPO₄ may be mainly due to its relatively low recombination efficiency of photogenerated electron–hole pairs by trapping photoinduced electrons via surface ionic liquid modification.

To further reveal the photocatalytic mechanism of IL-BiPO₄, the trapping experiments were carried out to determine the main active species in the photocatalytic process (Fig. 6c). The decolorization efficiency of RhB decreased slightly when 2 mM tert-butyl alcohol (TBA, a hydroxyl radicals scavenger) was added, which confirms that ˙OH should not be the main active species in RhB photodecolorization. However, when 2 mM disodium ethylenediaminetetraacetate (EDTA, a scavenger for photogenerated holes) was added, the decolorization behavior of RhB is significantly suppressed. This result suggests that photogenerated holes are the main active species for the decolorization of RhB while hydroxyl radicals play an assistant role.

On the basis of the above experimental results, a possible mechanism for the enhanced photocatalytic performance of IL-BiPO₄ was proposed as illustrated in Fig. 6d. Under UV light irradiation, IL-BiPO₄ can be excited according to eqn (1) below and generate photoinduced electron–hole pairs. The photoinduced electrons at the conduction band (CB) of IL-BiPO₄ can be trapped by surface modified ILs molecules via Coulomb coupling force (eqn (2)), thus leaving holes in the valence band (VB) of IL-BiPO₄ and decreasing the recombination rate of photogenerated electrons and holes. Meanwhile, the lifetime of the excited holes is also prolonged during the trapping process. Thereafter, hydroxide ions resulted from the self-ionization of water molecule will combine with photogenerated holes to produce the most reactive ˙OH radical (eqn (3)). The holes and as-formed ˙OH radical may induce some oxidation process (eqn (4) and (5)) and decompose the organic pollutant directly. It should be noted that the ˙OH radicals can also be produced by the photogenerated electron through multistep reduction of O₂ in BiPO₄ photocatalytic systems.³⁵ The trapping of photogenerated electron via surface ionic liquid modification will directly reduce the contribution of ˙OH to photocatalytic efficiency for RhB decolorization, which is detrimental to the enhancement of photocatalytic activity of IL-BiPO₄. However, as shown in Fig. 6d, the trapping of photogenerated electron on the surface of IL-BiPO₄ could prolong the lifetime of photogenerated holes and result in the content of photogenerated holes to be much higher than that of NH-BiPO₄. The remaining high content of photogenerated holes on IL-BiPO₄ can significantly enhance the photocatalytic decolorization rates of organic dye simultaneously, which is in good accordance with the above inference that the photogenerated holes are the main active species instead of ˙OH radicals after BiPO₄ modified with [C₄mim][PF₆]. Consequently, IL-BiPO₄ photocatalyst exhibits much higher photocatalytic activity than that of NH-BiPO₄. The whole process is described as follows:

BiPO₄ + hν → e_CB⁻ + h_VB⁺ (1)

e_CB⁻ + [C₄mim]⁺ → [C₄mim] (2)

h_VB⁺ + OH⁻ → ˙OH (3)

˙OH + RhB → degradation products (4)

h_VB⁺ + RhB → degradation products (5)

4. Conclusions

In summary, hierarchical flower-like BiPO₄ microspheres constructed by a plenty of octahedrons have been successfully synthesized by a facile and effective IL-assisted hydrothermal route. The photocatalytic mechanism of ILs-BiPO₄ has been investigated in detail and the result indicates that the enhanced photocatalytic activity of IL-BiPO₄ may be related with its high separation efficiency of charge carriers, relatively small band gap and enhanced optical absorbance in the range of 300 to 600 nm, especially in terms of separation efficiency of charge carriers. The in situ modification of ionic liquid [C₄mim][PF₆] onto the surface of BiPO₄ can trap the photogenerated electron at conduction band and thereby effectively enhance the separation and transfer of photogenerated electron–hole pairs, which consequently result in the improved photocatalytic activity of IL-BiPO₄ in the decolorization of organic contaminants. This study not only provides a facile and effective method for the preparation of BiPO₄ hierarchical structures, but also presents a step forward in the design of modified photocatalyst with enhanced photocatalytic activity.

Acknowledgements

The authors gratefully acknowledge the financial support from The National Natural Science Foundation of China (Grant No. U1204503), Youth Teachers Supporting Project of Henan Normal University (Grant No. 01036500610) and the key projects of science and technology of Henan Educational Committee (Grant No. 14B150049).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: SEM images of BiPO₄ prepared in the presence of different ionic liquids; SEM image of BiPO₄ synthesized with increasing amount of ionic liquids; SEM image of BiPO₄ prepared by hydrothermal route; SEM image of IL-BiPO₄ prepared for different time; FT-IR spectra of samples; UV-vis diffuse reflectance spectra of samples; cycling runs in photocatalytic decolorization of RhB by IL-BiPO₄. See DOI: 10.1039/c5ra14626g

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