A eutectic mixture of choline chloride and urea as an assisting solvent in the synthesis of flower-like hierarchical BiOCl structures with enhanced photocatalytic activity

Abstract

Flower-like hierarchical BiOCl structures were successfully synthesized in the presence of a eutectic mixture of choline chloride and urea through a solvothermal process. The as-prepared sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), N₂ adsorption–desorption analysis, photo-electrochemical analysis and UV-vis diffuse reflectance spectroscopy (DRS). During the reaction process, the eutectic mixture of choline chloride and urea acted not only as the solvent and the template, but also as a chlorine source for the fabrication of flower-like BiOCl. In addition, the photocatalytic activity toward the degradation of RhB in aqueous solution was also investigated. The results showed that the flower-like BiOCl exhibited enhanced photocatalytic activity for the degradation of RhB under sunlight illumination. Meanwhile, the flower-like BiOCl catalyst performed without obvious decrease in activity after five cycles. Furthermore, the main reactive species in the catalytic oxidation reaction were detected through radical trapping experiments.

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1 Introduction

The advantages of TiO₂-based photocatalysts have been widely recognized in past decades, such as the rapid elimination of contaminants in water, cheapness, nontoxic nature, and operation at ambient temperature.^1–3 However, their low quantum yields and the fact that pure TiO₂ can only be activated by UV-light prevents their widespread application. In order to effectively utilize solar energy, the first strategy is to extend modification of TiO₂ by doping, coupling, and sensitization, but this still does not satisfy the requirements for practical application. The other strategy is to develop visible-light-driven photocatalysts;^4,5 among semiconductors, BiOCl, as an efficient photocatalyst in decomposing organic pollutants into inorganic substances under sunlight irradiation, has attracted considerable attention owing to its open crystalline layered structures and its indirect optical transition, and these result in a high photocatalytic performance.^6–9

As we know, the synthesis method can affect the morphology, size and surface area of the photocatalyst, which affects its photocatalytic activity. Hence, the preparation of BiOCl nanoparticles with various morphologies has become a very active research area in recent years.¹⁰ For example, Henle synthesized BiOCl nanoparticles in reverse microemulsions, consisting of heptane, nonionic surfactants, and aqueous salt solutions.¹ Deng fabricated single-crystalline bismuth oxyhalide BiOCl nanoplates, nanosheets, and microsheets using convenient hydrothermal methods.¹¹

Compared with organic solvents, ionic liquids (ILs) are regarded as “green solvents” due to their low melting points, wide liquid ranges, negligible vapour pressures, good solubility characteristics and so on.^12,13 Recently, many different hierarchical nano-structured BiOCl materials have been successfully synthesized in ILs. For instance, Xia and coworkers recently fabricated uniform flower-like hollow BiOCl microsphere and porous nanosphere structures using an ethylene glycol (EG)-assisted solvothermal method in the presence of ionic liquid [C₁₆mim]Cl and an ionic liquid–PVP system. During the reaction process, the ionic liquid not only acted as a Cl source but also as the solvent and template for the fabrication of BiOCl porous microspheres, which exhibited significant photocatalytic activity.^14–16 However, the high cost of ILs is one of the most important issues hampering their large-scale application.

More recently, mixtures of substituted quaternary ammonium salts, such as choline chloride with urea (carboxylic acids), shown to produce eutectics that are liquid at ambient temperature and that have unusual solvent properties, have attracted the attention of researchers because this type of eutectic mixture is sustainable, biodegradable and a large number of variants can be produced from readily available materials. Sheu presented the first use of a eutectic mixture of choline chloride–malonic acid as a solvent in the synthesis of two metal oxalatophosphates.^17,18 The study shows that the eutectic mixture plays a vital role in the crystallization of one desired compound. Up to now, little literature makes reference to BiOCl materials synthesized with eutectic mixtures. So, the potential of eutectic solvents in the synthesis of BiOCl remains to be explored.

On the basis of the above summarizations, we were prompted to develop a eutectic mixture-assisted solvothermal method for the synthesis of BiOCl. In this work, flower-like hierarchical BiOCl structures were obtained through a solvothermal synthesis assisted by eutectic mixtures in ethylene glycol-mediated conditions. During the process, the eutectic mixture of choline chloride and urea acted not only as the solvent and the template, but also as a chlorine source for the fabrication of flower-like BiOCl. It was found that the as-prepared flower-like BiOCl particles exhibited higher photocatalytic activity under sunlight illumination in comparison to plate-like BiOCl particles, which were also synthesized under the same conditions without the presence of a eutectic mixture of choline chloride and urea, using NaCl as the Cl source.

2 Experimental section

2.1 Materials

All reagents in this experiment were purchased from Aladdin Chemical Regent Co., Ltd (analytical grade) and were used directly without further purification.

2.2 Synthesis of flower-like BiOCl and plate-like BiOCl

Eutectic mixtures of choline chloride and urea were prepared according to the procedure described in the literature.¹⁹ The flower-like BiOCl was synthesized as follows: 0.001 mmol of Bi(NO₃)₃·5H₂O was dissolved in 20 mL ethylene glycol solution with magnetic stirring at room temperature, followed by dissolution of 10 mmol eutectic mixture into the above solution. After continuous stirring for 30 min, the mixture was transferred into a 30 mL Teflon-lined stainless steel autoclave up to 80% of the total volume. The autoclave was then heated at 140 °C for 24 h in a static state. The resulting white solid particles were separated and successively washed with water and alcohol, and dried at 50 °C in vacuum for 24 h. For comparison, plate-like BiOCl particles were also synthesized under the same conditions without the presence of a eutectic mixture of choline chloride and urea, using NaCl as the Cl source.

2.3 Characterization

The crystal structure of the samples was recorded on an X-ray diffractometer (XRD, Bruker D8 ADVANCE) using a Cu-Kα X-ray source (λ = 0.15418 nm). Infrared (FT-IR) spectra were recorded on Tensor 27 (BrukerT27) equipment in the 4000–400 cm⁻¹ range. X-ray photoelectron spectroscopy (XPS) measurements were made on a Thermo ESCALAB 250 instrument (USA) using nonmonochromatic Al Kα (hv = 1486.6 eV) radiation. The morphologies of BiOCl were recorded with a scanning electron microscope (SEM, JEOL JSM-6510LV), which was operated at an acceleration voltage of 10 kV. Photo-electrochemical analysis was carried out on a CHI660E workstation in a standard three-electrode configuration with 0.5 M Na₂SO₄ solution as the electrolyte. The ultraviolet-visible light (UV-vis) spectra of the BiOCl samples were recorded on a UV-2450 spectrophotometer (Shimadzu Corporation, Japan). The N₂ adsorption and desorption isotherms were performed at 196 °C on an ASAP 2020 (Micromeritics USA). Prior to measurements, the sample was degassed at 120 °C for 10 h. The specific surface area was determined from the linear part of the BET equation (P/P₀ = 0.05–0.25). The pore size distribution was derived from the desorption branch of the N₂ isotherm using the Barrett–Joyner–Halenda (BJH) method.

2.4 Photocatalytic activity measurement

The photocatalytic properties of the prepared BiOCl were evaluated in terms of the degradation of RhB. The photodegradation experiments were performed in a slurry reactor containing a solution of RhB (200 mL, 10 mg L⁻¹) and the photocatalyst. A Xe lamp was used as a sunlight source. Prior to irradiation, the suspension was kept in the dark under strong magnetic stirring for 30 min to ensure the establishment of an adsorption/desorption equilibrium. At given time intervals, aliquots of about 8 mL were collected from the suspension and centrifuged immediately, and the concentration of RhB after illumination was determined by checking the absorbance at 553 nm using a UV-vis spectrophotometer.

3 Results and discussion

3.1 XRD analysis

The crystal structure and phase purity of the as-synthesized BiOCl samples were first characterized by X-ray diffraction. As shown in Fig. 1, it was found that all the identified peaks for flower-like BiOCl and plate-like BiOCl can be well-indexed to the tetragonal structure of BiOCl (JCPDS no. 06-0249). The major X-ray diffraction peaks at 2θ values of 12.06°, 25.95°, 32.50° and 33.67° correspond to the (001), (011), (110) and (102) crystalline planes of BiOCl, respectively. No other peak corresponding to Bi₂O₃ or BiCl₃ was detected.²⁰ Thus, this important result indicated that all the products are composed of single-phase BiOCl with high purity.

Fig. 1 XRD patterns of synthesized BiOCl ((A): flower-like BiOCl; (B): plate-like BiOCl).

3.2 XPS analysis

To investigate the states of the ions and the composition of the BiOCl, it was examined using XPS. The survey XPS spectrum (Fig. 2a) suggests that the sample contains only the elements Bi, O, C and Cl; the carbon peak came from adventitious carbon on the surface of the sample. It can be seen that the two strong peaks with binding energies of 158.9 eV and 164.05 eV, as displayed in Fig. 2b, are for the Bi 4f 7/2 and Bi 4f 5/2 regions of BiOCl, respectively. Meanwhile, the peak binding energy of 529.5 eV, shown in Fig. 2c, was assigned to O 1s. On the other hand, the Cl 2p peak, shown in Fig. 2d, is deconvoluted into two peaks (198.2 and 200.2 eV), which are assigned, respectively, to the Cl 2p 3/2 and Cl 2p 1/2 region for BiOCl, and to the Cl 2p 3/2 and Cl 2p 1/2 region for BiOCl.^15,21

Fig. 2 XPS spectra of the flower-like BiOCl. (a) Survey of the sample; (b) Bi 4f; (c) O 1s; (d) Cl 2p.

3.3 SEM analysis

SEM images were taken in order to observe the microstructure of the BiOCl photocatalysts in detail. Fig. 3a and b show typical SEM images of the BiOCl samples prepared in the presence of eutectic mixtures of choline chloride and urea through a solvothermal process. From the SEM images, it is clearly demonstrated that the morphologies of the samples are flower-like hierarchical structures, with an average diameter of 1–2 μm, and the sizes are uniform, assembled from a large number of BiOCl nanosheets. For the purpose of comparison, BiOCl samples were also prepared through a solvothermal process using NaCl as the Cl source without a eutectic mixture of choline chloride and urea (Fig. 3c and d). It is found that the BiOCl prepared with NaCl is sphere-like or flake-like in structure. Moreover, Fig. 3b displays that the entire flower-like BiOCl hierarchical structures are composed of BiOCl nano-plates on the surface, different from the BiOCl microspheres synthesized without a eutectic mixture of choline chloride and urea. Therefore, aside from being the Cl source, the eutectic mixture of choline chloride and urea also acted as a soft template and a stabilizer, inducing the growth of flower-like hierarchical BiOCl structures.

Fig. 3 SEM images of the BiOCl photocatalyst.

On the basis of the above characterization, the possible mechanism of the formation of flower-like BiOCl (Fig. 4) might be divided into three steps: (1) the formation of BiOCl and its growth into flake-like structures in the early stages; (2) these primary BiOCl nanosheets form loose flower-like microspheres in the presence of a eutectic mixture of choline chloride and urea; (3) the formation of flower-like regular hierarchical BiOCl microspheres through a dissolution–recrystallization process of the preformed nanoparticles. The detailed formation mechanism of flower-like BiOX in the presence of eutectic mixtures of choline chloride and urea mixture through a solvothermal process needs further investigation.

Fig. 4 Schematic illustration of the proposed formation mechanism of flower-like BiOCl.

3.4 Optical absorption properties of BiOCl

The optical properties of flower-like BiOCl was studied by UV-vis diffuse reflectance spectroscopy. Fig. 5A is the typical UV-vis diffuse reflectance spectroscopy of flower-like BiOCl structures. Due to the band-gap transition, the prepared BiOCl samples indicated an obvious increase in absorption with wavelengths lower than about 350 nm. A classical Tauc approach was further employed to estimate the E_g value of the BiOCl crystals; the energy intercept of a curve of (ahv)² vs. hv yields E_g for a direct transition.²² As shown in Fig. 5, the band gap of flower-like hierarchical BiOCl structures synthesized in the presence of a eutectic mixture of choline chloride and urea was calculated to be 3.05 eV, which was less than the plate-like BiOCl (3.35 eV) and the Degussa TiO₂ (3.2 eV). This result supported the conclusion that BiOCl with flower-like hierarchical structures has a suitable band gap to be activated by visible light for the photocatalytic decomposition of organic contaminants. Thus, it can make the photocatalytic process more efficient compared to plate-like BiOCl and the Degussa TiO₂.

Fig. 5 UV-vis diffuse reflectance spectra and (ahv)² vs. hv curve of BiOCl ((A): flower-like BiOCl; (B): plate-like BiOCl).

3.5 Nitrogen adsorption–desorption analysis

The N₂ adsorption–desorption isotherms together with the corresponding pore size distribution of the flower-like BiOCl and plate-like BiOCl are shown in Fig. 6. It was observed that both the flower-like BiOCl and plate-like BiOCl exhibited type IV curves according to the IUPAC nomenclature with two hysteresis loops in the relative pressure range of 0.4–1.0, implying bimodal pore-size distributions in the mesoporous and macroporous region.² The first hysteresis loop at a low relative pressure (0.4–0.8) corresponds to the intra-aggregated pores of the BiOCl crystals. The second hysteresis loop at a high relative pressure (0.8–1.0) corresponds to the larger pores that could be formed between secondary particles due to the aggregation of nanosheets into flower-like hierarchical architectures. The surface areas are calculated to be as high as about 26.79 m² g⁻¹ and 20.38 m² g⁻¹ for the flower-like BiOCl and plate-like BiOCl samples, respectively.

Fig. 6 N₂ adsorption–desorption isotherms and corresponding pore size distribution curves (inset) ((A): flower-like BiOCl; (B): plate-like BiOCl).

3.6 Photocatalytic properties

Firstly, the photocatalytic activities of the flower-like BiOCl were evaluated by degradation of RhB in aqueous solution under sunlight irradiation as a test reaction. As shown in Fig. 7, it is displayed that the intensity of the absorption peak of RhB decreases gradually with increasing irradiation time, accompanied by a blue-shift of the main absorption peak (∼553 nm), indicating the photoinduced degradation of RhB. It is also found that the process of photocatalytic degradation can be divided into two stages. In the first stage, the RhB is firstly adsorbed on the surface of BiOCl due to its high specific surface area and then changed to its active state (RhB*) under sunlight irradiation; thus the adsorption efficiency became the dominant factor. At the second stage, electrons were injected from the active RhB into the conduction band of the flower-like BiOCl catalyst, and the electron was subsequently trapped by molecular oxygen. During this stage, photocatalytic efficiency is the controlling factor. In short, the flower-like BiOCl served as a RhB absorber and electron-transfer medium in this photocatalytic process. Therefore, the flower-like structure of the BiOCl catalyst with a high surface area could absorb many RhB and active species on its surface, which could enhance its photocatalytic activities.

Fig. 7 The photocatalytic properties of the flower-like BiOCl.

As a comparison, RhB degradation with a BiOCl photocatalyst using plate-like BiOCl and commercial photocatalyst p25 were also performed. As shown in Fig. 8, the photocatalytic activities of the flower-like BiOCl is much higher than that of the plate-like BiOCl photocatalyst and p25. The photodegradation efficiency of RhB by flower-like BiOCl reached 98.5% after 75 min. Interestingly, 84.7% and 65.2% of RhB is photocatalytically degraded by the plate-like BiOCl and p25, respectively. This is because the flower-like BiOCl possesses plenty of pores between nanoplates, which can contribute to the transport of RhB molecules to get to the active sites. In addition, the intermeshed nanoplates on the surface of the flower-like BiOCl wall and hollow interior cavity can allow multiple reflections of visible light, which enhances light-harvesting and thus increases the quantity of photogenerated electrons and holes available to participate in the photocatalytic decomposition of RhB.

Fig. 8 Degradation profile of RhB over different BiOCl ((A): flower-like BiOCl; (B): plate-like BiOCl) and p25.

The photocatalytic properties of the flower-like BiOCl with hierarchical structures were further studied by photocurrent experiment, a unique method to confirm the efficient separation of photogenerated carriers in the photocatalytic system. Fig. 9 displays the photocurrent densities of flower-like BiOCl under sunlight irradiation where the photocurrent was measured at open circuit potential in 0.5 M Na₂SO₄ aqueous solution. The current densities of the flower-like BiOCl as a function of time were recorded by switching sunlight on and off with durations of 50 s. Photocurrents of the flower-like BiOCl rapidly decrease to zero as soon as the light is turned off and recover when the light is turned on, reflecting the movements of photo-generated electrons.

Fig. 9 Photoelectrochemical chronoamperometry results for flower-like BiOCl.

3.7 Possible photocatalytic oxidation mechanism

With an aim to elaborate the photodegradation mechanism of RhB in the presence of flower-like BiOCl photocatalysts, the main reactive species in the catalytic oxidation reaction were detected through radical trapping experiments. The scavengers used in this experiment were ammonium oxalate (AO) for h⁺, isopropanol (IPA) for ˙OH and N₂ for ˙O²⁻, respectively.²³ The effects of a series of scavengers on RhB conversion over the flower-like BiOCl are shown in Fig. 10. The photocatalytic conversion of RhB decreased significantly from 98.6 to 10.8% after adding N₂, indicating that ˙O²⁻ is the main active species in the photocatalytic oxidation process. When AO was added, the photocatalytic conversion of RhB decreased from 96.5 to 17.4%, respectively, which indicates that h⁺ also play an important role in the photocatalytic oxidation of RhB. In summary, the results indicated that the dominant reactive species involved in the photocatalytic degradation of RhB in aqueous solution are ˙O²⁻ and h⁺.

Fig. 10 Effect of different scavengers on the degradation of RhB.

3.8 Reusability of the flower-like BiOCl

It is well known that the stability of photocatalysts is a vital concern for practical application.^24–27 The reusability of flower-like BiOCl was evaluated by repeating photodegradation of RhB under identical conditions; after each photocatalytic degradation reaction, the flower-like BiOCl was easily separated by centrifuge from the solution system, washed with deionized water, and dried at 80 °C before the next run. As shown in Fig. 11, the flower-like BiOCl catalyst performed without obvious decrease in activity after five cycles of photodegradation of RhB, highlighting the stability and reusability of the flower-like BiOCl. Meanwhile, as shown in Fig. 12, there was no obvious difference between the FT-IR spectra of the flower-like BiOCl and the recycled flower-like BiOCl, and the SEM images of the recycle BiOCl photocatalyst exhibited flower-like hierarchical structures (Fig. 13), which indicates that the flower-like structure of BiOCl was unchanged after five cycles of photodegradation of RhB.

Fig. 11 The photodegradation of RhB in solution for 5 cycles using flower-like BiOCl.

Fig. 12 FT-IR spectra of the BiOCl ((A): flower-like BiOCl; (B): recycled BiOCl).

Fig. 13 SEM images of the BiOCl photocatalyst after 5 cycles.

4 Conclusions

We have demonstrated a method for synthesising a flower-like BiOCl photocatalyst in the presence of a eutectic mixture of choline chloride and urea through a solvothermal process. The flower-like BiOCl fabricated by this process exhibited enhanced photocatalytic properties toward the degradation of RhB in comparison to plate-like BiOCl particles, which were also synthesized under the same conditions without the presence of eutectic mixtures of choline chloride and urea using NaCl as the Cl source, and to p25. During the reaction process, the eutectic mixture of choline chloride and urea acted not only as the solvent and the template, but also as a chlorine source for the fabrication of flower-like BiOCl. The uniformly distributed flower-like BiOCl had an average diameter of 1–2 μm, and uniform size, assembled from a large number of BiOCl nanosheets, and the band gap of flower-like BiOCl was calculated to be 3.05 eV. Moreover, investigation of the photocatalytic oxidation mechanism revealed that the dominant reactive species involved in the photocatalytic degradation of RhB in aqueous solution are ˙O²⁻ and h⁺. Meanwhile, the flower-like BiOCl catalyst exhibited without obvious decrease in activity after five cycles. Furthermore, we expect that this new system can pave the way for the purposeful synthesis of other nanosized photocatalysts with high photocatalytic activity.

Acknowledgements

This work was supported by the Natural Science Foundation of Universities of Anhui Province (KJ2014A069), Postdoctoral Science Foundation of Anhui Province (2013DG125) and Natural Science Foundation of Hainan Province (414194).

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