Fabrication and photocatalytic performances of BiOCl nanosheets modified with ultrafine Bi₂O₃ nanocrystals

Abstract

BiOCl nanosheets modified with ultrafine Bi₂O₃ nanocrystals were synthesized by chemical deposition with the assistance of ascorbic acid and Pluronic F127. The structure, morphology, elemental composition and optical absorption performance were characterized using X-ray diffractometry, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis diffuse reflection spectroscopy, respectively. Square-like BiOCl nanosheets with ultrafine Bi₂O₃ nanocrystals evenly distributed on the surface were obtained. The photocatalytic activity of the composite was tested by degrading a 40 mg L⁻¹ methyl orange solution under UV light illumination. A Bi₂O₃/BiOCl composite prepared with a Bi(NO₃)₃ concentration of 0.01 M achieved the highest photocatalytic rate of 100% in 6 min under UV light illumination. Repeated experiments on the degradation processes and recycling experiments on the photocatalysts were conducted under the same conditions. The results show that the as-synthesized Bi₂O₃/BiOCl composites possess excellent photocatalytic activity and outstanding recyclability. The photocatalytic mechanism of the composite is discussed.

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1 Introduction

With the development of industry, environmental contamination has become more and more serious and it is extremely urgent to control pollution. As a rapidly growing green method for treating polluted water and air, photocatalytic degradation using a semiconductor as a catalyst has attracted more and more attention.^1,2 TiO₂ is one of the earliest and most widely studied semiconductors used as a photocatalyst owing to its excellent photocatalytic properties.^3,4 Currently, more and more other photocatalysts have been studied as alternatives to TiO₂, such as metallic oxides,⁵ metal sulfides⁶ and ternary system compounds.⁷ Among these semiconductors, bismuth-containing compounds have attracted a lot of attention. Numerous studies have concentrated on Bi₂O₃,⁸ BiOCl,⁹ Bi₂S₃,¹⁰ Bi₄Ti₃O₁₂,¹¹ Bi₂MoO₆ (ref. 12) and other bismuth compounds, revealing that bismuth-containing compounds are promising in the photocatalytic degradation of organic compounds and photocatalytic water splitting.

BiOCl is the most widely studied bismuth system photocatalyst owing to its high photocatalytic activity and high chemical stability.¹³ Double layers of Cl atoms and [Bi₂O₂] layers along the c-axis are arranged alternately and form the layered structure of BiOCl. The structure can also be expressed as [Cl–Bi–O–Bi–Cl],¹⁴ which possesses enough space to polarize the corresponding atoms and atomic orbitals. The induced dipole moment could separate photoinduced electrons and holes effectively, which can decrease the recombination rate of photoinduced electrons and holes, resulting in the high photocatalytic activity of BiOCl.

In order to improve the photocatalytic properties of BiOCl, numerous studies have concentrated on the modification of BiOCl including control of the crystal planes¹⁵ and modification with metal nanoparticles¹⁶ and semiconductors.^17–19 Single-crystalline BiOCl nanosheets with exposed {001} and {010} facets were selectively synthesized by Zhang et al. via a facile hydrothermal route and were found to have different photocatalytic properties.¹⁵ Ao et al. prepared square-like BiOCl nanoplates with Ag@AgCl nanoparticles deposited via an in situ ultraviolet (UV) reduction method and their photoactivity was enhanced fivefold compared with that of pure BiOCl.¹⁶ Li et al. synthesized BiOCl–TiO₂ heterostructures with TiO₂ nanoparticles deposited on the surface of BiOCl nanoplates via a hydrothermal method. The obtained BiOCl–TiO₂ heterostructures exhibited much higher photocatalytic efficiency than that of single-phase BiOCl and TiO₂.¹⁷ Bi₂O₃/BiOCl heterojunctions were synthesized via the reaction of Bi₂O₃ and HCl and displayed excellent photocatalytic performance in the visible-light region.²⁰

In our previous study, Bi and Bi₂O₃ nanocrystals were successfully deposited on BiOCl nanosheets by in situ reduction and oxidation, which can eventually enhance the photocatalytic performance.^21,22 To further control the deposition process, Bi₂O₃ nanocrystals were obtained on BiOCl nanosheets by a facile and simple method with the assistance of ascorbic acid and Pluronic F127. Well-dispersed Bi₂O₃ nanocrystals can be controlled to be less than 10 nm in size, which is beneficial for promoting charge transfer between BiOCl and Bi₂O₃ and significantly enhancing the photocatalytic performance. Heterojunctions of Bi₂O₃/BiOCl restrict the recombination of photogenerated electron–hole pairs, resulting in an enhancement in the photocatalytic properties of BiOCl nanosheets.

2 Experimental

2.1 Synthesis

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), sodium chloride (NaCl), ethylene glycol (EG), mannitol, ascorbic acid (AA) and terephthalic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Pluronic F127 was purchased from Sigma-Aldrich Co. All reagents were of analytical grade and were used as received without further purification.

BiOCl nanosheets were synthesized by a solvothermal method in EG solution. In a typical process, 2 mmol Bi(NO₃)₃·5H₂O was dissolved in 25 mL EG, and 2 mmol NaCl was dissolved in 25 mL deionized water. The aqueous solution of NaCl was added dropwise into the Bi(NO₃)₃ solution with continuous stirring. After being stirred for 30 min, the mixed solution was transferred to a Teflon-lined stainless-steel autoclave. The solvothermal synthesis was performed at 150 °C for 10 h in an electric oven. The resulting precipitate was centrifuged and washed with ethanol and deionized water several times to remove the chemical residues. Finally, white BiOCl powder was obtained by drying the precipitate at 40 °C in air.

Deposition of Bi₂O₃ was performed via the ordinary reduction and oxidation of Bi(NO₃)₃ in an aqueous solution of mannitol containing AA and F127, in which 0.0388 g to 0.776 g Bi(NO₃)₃ was added to a 20 mL mannitol solution with a concentration of 0.1 M. The solution was stirred continuously for full dissolution, and then mixed with 40 mL Pluronic F127 with a concentration of 1 mM.

Then, 0.2 g BiOCl nanosheets were put into the mixed solution and dispersed sufficiently. Next, 20 mL AA solution (0.1 M) was added to the solution and reacted for 60 min under ultrasonication. The resulting precipitate was washed with deionized water and ethyl alcohol several times to remove the chemical residues. Finally, a Bi₂O₃/BiOCl composite was obtained by drying the precipitate at 50 °C in an electric oven. Bi₂O₃/BiOCl composites synthesized with different Bi(NO₃)₃ concentrations of 1, 5, 10, 15 and 20 mM were labeled as Bi₂O₃/BiOCl-1, Bi₂O₃/BiOCl-5, Bi₂O₃/BiOCl-10, Bi₂O₃/BiOCl-15 and Bi₂O₃/BiOCl-20, respectively. As a comparison, yellow Bi₂O₃ nanoparticles were prepared by the same method without adding BiOCl nanosheets.

2.2 Characterization

The phase structure of the samples was measured by an X-ray diffractometer (D/MAX2500V) using Cu Kα radiation with 2θ ranging from 10° to 70°. The surface morphologies of the samples were observed with a SU8020 field emission scanning electron microscope (FESEM). A JEM-2100F high-resolution transmission electron microscope (HRTEM) was used to observe the morphologies and structures of the samples. X-ray photoelectron spectroscopy (XPS) analysis of the samples was performed using an ESCALAB 250 photoelectron spectrometer with a monochromatic Al Kα X-ray beam (1486.60 eV). The optical absorption performance of the samples was evaluated by a diffuse reflectance spectrometer (UV3600, Shimadzu) using BaSO₄ as a reference.

2.3 Photocatalysis

The photocatalytic degradation of MO solution was used to evaluate the photocatalytic properties of the samples. The photocatalytic tests were conducted on an XPA-7 photochemical reactor (Nanjing Xujiang Machine-electronic Company, China) using a 300 W high-pressure mercury lamp as the UV light source (maximum emission wavelength of 365 nm). The distance between the solution and the lamp was kept at 10 cm, and the solution was continuously stirred during the photocatalytic degradation process.

In a typical procedure, 0.01 g photocatalyst was put into 10 mL MO solution with a MO concentration of 40 mg L⁻¹. The suspension was vigorously stirred for 30 min in the dark to achieve adsorption/desorption equilibrium. After being illuminated every 2 min, a 5 mL solution was collected from the suspension and centrifuged. Then, the upper clear solution was tested by a UV1800 spectrometer (Shimadzu, Japan) to record the residual concentration of MO in the solution being photodegraded.

To further study the effect of the active species h⁺, ˙O₂⁻ and HO˙ on the photocatalytic performance of BiOCl nanosheets and Bi₂O₃/BiOCl composites, the three sacrificial agents NaI, BQ and IPA with concentrations of 10 mM were added, respectively, to the MO solution. The changes in the degradation rate reflect the effects of the corresponding active species on the photocatalytic properties of the photocatalysts. The concentration of HO˙ in the solution during the photocatalytic process was also detected by the terephthalic acid oxidation method.²³ Terephthalic acid can react with HO˙ produced by photocatalysts illuminated with a 300 W high-pressure mercury lamp as the UV light source (maximum emission wavelength of 365 nm), and the product, dihydroxyterephthalic acid, exhibits excellent photoluminescence properties with an emission peak wavelength at around 420 nm. A F4500 fluorospectrophotometer (Hitachi) was used to determine the photoluminescence of terephthalic acid after being treated under UV light irradiation with the respective photocatalysts.

3 Results and discussion

3.1 Characterization

Fig. 1 shows the XRD patterns of products obtained in different conditions. Fig. 1(i) compares the phase structures of the products obtained by chemical deposition, solvothermal synthesis and a combination of the two methods. The chemical precipitation of Bi³⁺ ions in the presence of AA and Pluronic F127 gives rise to a broad diffraction peak at 28.2°, which fits the (111) lattice plane of the cubic phase of Bi₂O₃ (JCPDS file no. 27-0052), which indicates the existence of Bi₂O₃ with an ultrafine grain size in the precipitated products. The mean grain size of Bi₂O₃ can be calculated by the Scherrer equation to be 7.8 nm with a full width at half maximum of 1.25°.

Fig. 1 XRD patterns of products obtained in different conditions: (i) comparison of Bi₂O₃ nanocrystals, BiOCl nanosheets and Bi₂O₃/BiOCl composites; (ii) comparison of Bi₂O₃/BiOCl composites prepared with different concentrations of Bi(NO₃)₃ ranging from 1 to 20 mM.

The solvothermal method is a commonly used method for the synthesis of BiOCl, and the product can be confirmed to be BiOCl (JCPDS no. 06-0249) by the XRD pattern of curve (b). The narrow peaks in curve (b) fit well with the standard diffraction peaks of BiOCl, which indicates the high crystallinity of the product. Curve (c) is the XRD pattern of the product (Bi₂O₃/BiOCl-10) obtained by a combination of the two methods. Only the diffraction peaks of BiOCl crystals can be observed in curve (c) owing to the small amount and low crystallinity of the deposited Bi₂O₃. The similarity of the high background noise between curve (c) and curve (a) indicates that the BiOCl nanosheets are covered with a heterogeneous composition.

Fig. 1(ii) shows the XRD patterns of Bi₂O₃/BiOCl composites prepared with Bi(NO₃)₃ concentrations ranging from 1 to 20 mM. Almost the same diffraction patterns with diffraction peaks of a single BiOCl phase can be observed, which indicates that no Bi₂O₃ can be detected in the composite products owing to the small amount and low crystallinity of Bi₂O₃.

SEM was used to investigate the morphologies of the as-synthesized samples. Fig. 2 shows the SEM morphologies of BiOCl, Bi₂O₃ nanocrystals and Bi₂O₃/BiOCl composites prepared with different Bi(NO₃)₃ concentrations of 5, 10, 15 and 20 mM, respectively. Fig. 2(i) shows the morphology of the solvothermal product without further modification, in which approximately square-like nanosheets with a width of 100–300 nm and a thickness of 30 nm can be observed, and the surface is quite smooth.

Fig. 2 SEM morphologies of BiOCl (i), Bi₂O₃ (ii) and Bi₂O₃/BiOCl composites prepared with different Bi(NO₃)₃ concentrations of 5 (iii), 10 (iv), 15 (v) and 20 mM (vi), respectively.

The morphology of the chemical precipitation of Bi³⁺ ions in the presence of AA and Pluronic F127 is shown in Fig. 2(ii). A reticulation-like structure composed of nanowires can be observed, and some of the nanowires are intertwined to form spheres of a size of hundreds of nanometers.

Fig. 2(iii)–(vi) show the surface morphologies of the composites prepared with different Bi(NO₃)₃ concentrations. The results show that all the samples are mainly composed of approximately square-like nanosheets similar to the unmodified BiOCl nanosheets. However, the surfaces of these nanosheets are not smooth with ultrafine nanoparticles, which are too small to be characterized clearly by SEM. The amount of these nanoparticles increases with an increase in the Bi(NO₃)₃ concentration.

The morphologies and microstructures of the as-synthesized samples were further characterized by high-resolution transmission electron microscopy, of which the results are shown in Fig. 3. Fig. 3(i) shows the TEM morphology of the solvothermal product, in which smooth nanosheets with a width of about 100–200 nm can be observed. The smooth surface of the nanosheets indicates that there are no impurities on the nanosheets. The HRTEM morphology of a BiOCl nanosheet is shown in the inset of Fig. 3(i), demonstrating clear lattice fringes that were measured to be 0.278 nm, corresponding to the (110) lattice planes of the tetragonal phase of BiOCl.

Fig. 3 TEM morphologies of BiOCl nanosheets (i), Bi₂O₃ nanocrystals (ii), and Bi₂O₃/BiOCl-10 (iii) and Bi₂O₃/BiOCl-20 (iv) composites. The insets show the HRTEM morphologies of the respective products.

Fig. 3(ii) shows the TEM morphology of the products of chemical precipitation with the assistance of AA and Pluronic F127. The synthesized products are composed of a large amount of nanowires, and some of the nanowires aggregated into a sphere with a size of about 300–400 nm. In the top-right inset of Fig. 3(ii) the TEM morphology of the nanowires with high magnification is presented, in which it can be observed clearly that the nanowires are composed of many nanoparticles with a size of a few nanometers. A HRTEM analysis of one of the nanoparticles is shown in the bottom-right inset of Fig. 3(ii), indicating clear lattice fringes of a single crystal with an interplanar spacing of 0.328 nm, which corresponds to the (111) lattice plane of cubic Bi₂O₃ crystals.

Fig. 3(iii) and (iv) show the morphologies of the composites prepared in Bi(NO₃)₃ solutions with different Bi³⁺ concentrations of 10 and 20 mM, respectively. The composite samples display similar nanosheets to the as-synthesized BiOCl nanosheets. However, the surfaces of these nanosheets are not smooth but have many nanoparticles on them. These nanoparticles are well distributed with uniform size. In comparison, the quantity of the nanoparticles in the Bi₂O₃/BiOCl-20 composite is greater than that in the Bi₂O₃/BiOCl-10 composite. The insets in both Fig. 3(iii) and (iv) show the HRTEM morphologies of the respective samples, in which a nanoparticle with a size of approximately 10 nm can be observed on the surface of a nanosheet. The clear lattice fringes show that the interplanar spacing is about 0.328 nm, which corresponds to the (111) lattice plane of the cubic Bi₂O₃ phase (JCPDS file no. 27-0052). Also, the clear and regular lattice fringes indicate the single-crystalline structure of the Bi₂O₃ nanocrystals. The interplanar spacing of the substrate is 0.277 nm, which corresponds to the (110) lattice plane of the tetragonal BiOCl phase (JCPDS file no. 06-0249). The results of TEM confirmed that Bi₂O₃ nanocrystals were successfully synthesized and well distributed on the surface of BiOCl nanosheets.

XPS spectra were recorded to reveal the elemental compositions of the samples. Fig. 4 shows the XPS spectra of as-synthesized BiOCl nanosheets, Bi₂O₃ nanocrystals and the Bi₂O₃/BiOCl-10 composite, and the survey spectra of all samples are shown in Fig. 4(i). The peak of C 1s at 284.7 eV is present for all samples and originated from the testing process. The main elemental components of the BiOCl nanosheets and Bi₂O₃/BiOCl-10 nanocomposite are Bi, O and Cl with binding energy peaks at 163 eV (Bi 4f), 442 eV (O 1s) and 198 eV (Cl 2p), respectively, whereas the Bi₂O₃ nanocrystals only contain Bi and O elements with no Cl element.

Fig. 4 XPS spectra of BiOCl nanosheets, Bi₂O₃ nanocrystals and the Bi₂O₃/BiOCl-10 composite, (i) survey spectra; (ii) high-resolution spectra of Bi 4f electrons.

The high-resolution XPS spectra of Bi 4f electrons of the three samples are shown in Fig. 4(ii). Double peaks with binding energies of 158.9 and 164.2 eV can be observed in the spectrum of as-prepared BiOCl nanosheets, which correspond to the 4f_7/2 and 4f_5/2 electron levels of Bi³⁺ ions in BiOCl. Double peaks in the spectrum of Bi₂O₃ nanoparticles are observed at 159.3 eV and 164.6 eV, according to the binding energy of the 4f_7/2 and 4f_5/2 electron levels of Bi³⁺ ions in Bi₂O₃. However, the spectrum of the Bi₂O₃/BiOCl-10 nanocomposite is composed of two broadened peaks at about 159 eV and 164.3 eV, which can each be divided into two peaks by a Gaussian decomposition method. The four peaks agree with the 4f_7/2 and 4f_5/2 electron levels of Bi³⁺ in BiOCl and Bi₂O₃, which reveals that the sample is composed of BiOCl and Bi₂O₃, which can further confirm that Bi₂O₃ nanocrystals were successfully synthesized and deposited on BiOCl nanosheets via chemical deposition with the assistance of AA and Pluronic F127.

3.2 Growth mechanism discussion

Ascorbic acid is a commonly used reductant for synthesizing Ag nanomaterials.²⁴ As a complexing agent, AA can react with Bi³⁺ ions to form bismuth ascorbate, and the Bi³⁺ ions can be reduced to Bi⁰ by AA with the assistance of ultrasonication according to the following reaction:

3C₆H₈O₆ + 2Bi³⁺ → 2Bi⁰ + 3C₆H₆O₆ + 6H⁺ (i)

The reduced Bi⁰ is not stable in aqueous solution and can be reoxidized by H₂O according to the following reaction:

2Bi⁰ + 3H₂O → Bi₂O₃ + 3H₂ (ii)

Bi₂O₃ nanocrystals were obtained by the reduction and oxidation reactions. The presence of Pluronic F127 restrained the growth of the nanocrystals owing to its PEO-PPO-PEO triblock structure. The triblock copolymer formed micelles due to multimolecular aggregation spontaneously in aqueous solution. The component of PPO provided a local hydrophobic microenvironment in the aqueous phase. So the Bi³⁺ ions could not move easily, but assembled along long chains of Pluronic F127. That is why we observe nanowire structures of Bi₂O₃ in the SEM and TEM morphologies.

However, no nanowires or spheres composed of Bi₂O₃ nanocrystals can be observed in the Bi₂O₃/BiOCl composites, which indicates that Bi₂O₃ nanocrystals grew on the surface of BiOCl nanosheets but did not form nanowires or spheres by self-assembly. When BiOCl nanosheets were added to the solution, the nanocrystals tended to nucleate on the surface of the nanosheets. Thus, composites of BiOCl nanosheets deposited with Bi₂O₃ nanocrystals were obtained.

3.3 Photocatalytic activity

Fig. 5 shows the photocatalytic performance of as-prepared BiOCl nanosheets and Bi₂O₃/BiOCl composites synthesized in Bi(NO₃)₃ solutions with different concentrations. Before irradiation, adsorption/desorption equilibrium was achieved by stirring vigorously for 30 min in the dark. Fig. 5(i) shows the absorption rate in the dark and the photocatalytic degradation rate of MO solution irradiated with UV light using BiOCl nanosheets and Bi₂O₃/BiOCl-1, Bi₂O₃/BiOCl-5, Bi₂O₃/BiOCl-10, Bi₂O₃/BiOCl-15 and Bi₂O₃/BiOCl-20 composites as the photocatalyst, respectively. The as-prepared BiOCl nanosheets with smooth surfaces displayed relatively poor performance with an adsorption rate of less than 1% after 30 min. All the adsorption rates of the Bi₂O₃/BiOCl composites were less than 10% and a little higher than that of the BiOCl nanosheets.

Fig. 5 Photocatalytic performance of BiOCl nanosheets and Bi₂O₃/BiOCl composites prepared with different concentrations of Bi³⁺ ions under UV light illumination: (i) degradation rates for BiOCl nanosheets, Bi₂O₃/BiOCl-1, Bi₂O₃/BiOCl-5, Bi₂O₃/BiOCl-10, Bi₂O₃/BiOCl-15 and Bi₂O₃/BiOCl-20 composites; (ii) recycling tests on the Bi₂O₃/BiOCl-10 composite. The MO concentration is 40 mg L⁻¹.

When under illumination with UV light, the concentration of MO decreased rapidly, which indicates that all the samples are extremely sensitive to UV light and can be stimulated by UV light. After irradiation for 8 min, the degradation rate of MO using as-synthesized BiOCl nanosheets was 46%. The photocatalytic activity of Bi₂O₃/BiOCl-1 was similar to that of BiOCl owing to the low quantity of deposited Bi₂O₃ nanocrystals. However, when the concentration of the Bi(NO₃)₃ solution increased to 5 mM or above, the photocatalytic activity of the Bi₂O₃/BiOCl composites increased rapidly. All the Bi₂O₃/BiOCl composites, except for Bi₂O₃/BiOCl-1, can photodegrade 40 mg L⁻¹ MO solution completely in 8 min under UV irradiation, which is a higher rate than that of the Bi₂O₃/BiOCl composites obtained by in situ reduction and oxidation in our previous work.²² Among these, the Bi₂O₃/BiOCl-10 nanocomposite achieved the highest photocatalytic performance and could degrade MO completely in 6 min under UV light.

As a comparison, the photocatalytic performance of TiO₂ nanoparticles (P25) under the same conditions was tested, and the results are shown in Fig. 5(i). The photocatalytic performance of P25 was similar to that of unmodified BiOCl nanosheets, with a degradation rate of 48% after being illuminated with UV light for 8 min. Therefore, the optimized Bi₂O₃/BiOCl-10 nanocomposite displayed much higher photocatalytic activity than that of commercial TiO₂ nanoparticles.

The kinetics of the photocatalytic degradation of MO can be described as follows:²⁵

here k is the reaction rate constant, and t is the irradiation time. The calculated rate constants k of the different samples are listed in Table 1.

Table 1 k values of BiOCl nanosheets and Bi₂O₃/BiOCl composites illuminated with UV light

Sample	BiOCl	Bi2O3/BiOCl-1	Bi2O3/BiOCl-5	Bi2O3/BiOCl-10	Bi2O3/BiOCl-15	Bi2O3/BiOCl-20
k/min^−1	0.0748	0.0703	0.4980	0.5135	0.5085	0.4067

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From Table 1, the kinetic constant of BiOCl nanosheets is 0.0748 min⁻¹, and all the kinetic constants of the Bi₂O₃/BiOCl composites are much higher than that of BiOCl nanosheets, except for the Bi₂O₃/BiOCl-1 composite. The Bi₂O₃/BiOCl-10 composite possesses the highest kinetic constant with a value of 0.5135 min⁻¹, which is about seven times that of BiOCl nanosheets, which indicates the excellent photocatalytic activity of the sample.

To evaluate the stability of the Bi₂O₃/BiOCl-10 nanocomposite in the photocatalytic process, the Bi₂O₃/BiOCl-10 nanocomposite was reclaimed and reused for 10 cycles under the same conditions. Fig. 5(ii) shows the curves for MO degradation when the Bi₂O₃/BiOCl-10 composite was reused 10 times. The first time, MO could be completely degraded by the Bi₂O₃/BiOCl-10 nanocomposite in 6 min and the next photocatalytic process needed 8 min to completely degrade MO. The photocatalytic activity of the sample displayed a slight decline. When the photocatalytic process was repeated for a further eight times, the photocatalyst retained the same degradation rate and degraded MO completely in 8 min, which indicates the excellent recyclability and outstanding photocatalytic performance of the Bi₂O₃/BiOCl-10 composite under UV light illumination.

3.4 Photocatalytic mechanism discussion

3.4.1 Optical absorption performance. UV-vis diffuse reflectance spectroscopy (DRS) is an effective method for characterizing the optical absorption performance of as-synthesized samples. Fig. 6 shows the UV-vis absorption spectra of the BiOCl nanosheets and Bi₂O₃/BiOCl-10 and Bi₂O₃/BiOCl-20 composites in the wavelength range from 200 to 800 nm. All samples exhibit strong absorption in the UV region. The absorption edge of the samples lies at approximately 330 to 360 nm. In addition, the BiOCl nanosheets are inert regarding optical absorption in the visible-light region, whereas the absorption intensities of the Bi₂O₃/BiOCl composites in the visible region are a little higher than that of the BiOCl nanosheets. However, the values of absorption are still very low in the entire visible-light region, which may be ascribed to the low amount of Bi₂O₃ nanocrystals deposited on BiOCl nanosheets.

Fig. 6 UV-vis absorption spectra of BiOCl nanosheets and Bi₂O₃/BiOCl-10 and Bi₂O₃/BiOCl-20 composites.

Considering that the optical absorption of the Bi₂O₃/BiOCl composites displays no evident increase in the UV region, this indicates that optical absorption is not the major reason for the enhancement of the photocatalytic performance of BiOCl nanosheets in the UV region when deposited with Bi₂O₃ nanocrystals.

3.4.2 Surface area analysis. A key factor that affects adsorption performance is the surface area of samples. The surface areas of the samples are shown in Table 2. The surface area of unmodified BiOCl nanosheets was measured to be 18.1 m² g⁻¹. When modified with Bi₂O₃ nanocrystals, the surface area increased a little to 18.3, 19.2, 19.6, 19.4 and 19.4 m² g⁻¹ for the Bi₂O₃/BiOCl-1, Bi₂O₃/BiOCl-5, Bi₂O₃/BiOCl-10, Bi₂O₃/BiOCl-15 and Bi₂O₃/BiOCl-20 composites, respectively, resulting in a slight increase in the adsorption rate, as shown in Fig. 5(i). However, the surface area is not the main factor that enhances the photocatalytic performance.

Table 2 BET surface areas of BiOCl nanosheets and Bi₂O₃/BiOCl composites

Sample	BiOCl	Bi2O3/BiOCl-1	Bi2O3/BiOCl-5	Bi2O3/BiOCl-10	Bi2O3/BiOCl-15	Bi2O3/BiOCl-20
Surface area (m^2 g^−1)	18.1	18.3	19.2	19.6	19.4	19.4

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3.4.3 Charge transfer. A schematic diagram of the photocatalytic process with Bi₂O₃/BiOCl composites is shown in Fig. 7. The photocatalytic processes of BiOCl nanosheets can be described as follows: when BiOCl nanosheets are excited by UV light with energy higher than the band gap, electrons are transferred from the valence band to the conduction band of BiOCl and form photogenerated electron–hole pairs. Photogenerated electrons and holes will be transferred to the surface of BiOCl, and some of them will recombine during the charge transfer process. Electrons on the surface of the photocatalyst can be captured by dissolved O₂ in solution to produce the superoxide radical ˙O²⁻, which can degrade organic compounds into CO₂, H₂O and other inorganic substances. Also, electrons on the surface of BiOCl will enable hydrogen evolution when captured by H⁺ ions. Holes can react with OH⁻ ions to produce the strongly oxidizing hydroxyl radical HO˙, which is efficacious for degrading organic compounds. Also, holes can degrade organic compounds directly when they are captured by an organic compounds on the surface of BiOCl.

Fig. 7 Schematic of the photocatalytic process with Bi₂O₃/BiOCl composites.

Well-combined Bi₂O₃/BiOCl composites lead to the transfer of photogenerated electrons and holes between Bi₂O₃ and BiOCl. The CB of BiOCl (−1.1 eV vs. NHE) is more negative than that of Bi₂O₃ (0.33 eV), which results in the transfer of electrons from the CB of BiOCl to that of Bi₂O₃. The photogenerated holes will be transferred from the VB of Bi₂O₃ to that of BiOCl due to the VB of Bi₂O₃ (3.13 eV) being more positive than that of BiOCl (2.3 eV). The different transfer directions of photogenerated electrons and holes result in charge separation and a low recombination rate of photogenerated electron–hole pairs. Therefore, the photocatalytic performance of the Bi₂O₃/BiOCl composites is enhanced.

3.4.4 Active species during the photocatalytic process. As analyzed above, there are three active species, hydroxyl radicals (HO˙), holes (h⁺) and superoxide radicals (˙O₂⁻), in the photocatalytic process of the degradation of MO. In order to reveal which is the key active species in the photocatalytic process, three sacrificial agents were added to the MO solution to consume the corresponding active species rapidly, namely, NaI for h⁺, BQ for ˙O₂⁻ and IPA for HO˙. Fig. 8 shows the photocatalytic performance of as-synthesized BiOCl nanosheets and the Bi₂O₃/BiOCl-10 composite for the degradation of MO with the addition of different sacrificial agents. All the degradation rates of MO decreased when the three sacrificial agents were added, respectively, which indicates that the three active species play partial roles in the photocatalytic process. For BiOCl nanosheets, when NaI and BQ radicals were added to the MO solution, the degradation rate decreased from 51.7% to 18.3% and 19.5%, respectively, which indicates that h⁺ and ˙O₂⁻ radicals would play key roles in the photocatalytic process. When IPA was added to the MO solution, the photocatalytic rate decreased from 51.7% to 40.9%. The decrease in the photocatalytic degradation rate was only about 10%, which indicates that HO˙ radicals would play a secondary role in the photocatalytic process. However, the changes in the degradation rate were very different for the Bi₂O₃/BiOCl composite. Decreases of only 17.7% and 8.7% in the degradation rate occurred when NaI and IPA were added to the MO solution, respectively, which indicates that h⁺ and HO˙ radicals would not play a key role in the photocatalytic process. When BQ was added to the MO solution as an inhibitor of ˙O₂⁻ radicals, the decrease in the degradation rate reached 82.4%, which indicates that ˙O₂⁻ radicals would play a major role in the photocatalytic process. The results show that the main photocatalytic process is the oxidation of ˙O₂⁻ produced by the Bi₂O₃/BiOCl composite, but not the degradation by h⁺ and HO˙ radicals. Considering that the photogenerated electrons and holes are equivalent in quantity, some of the holes may take part in the oxygen evolution reaction, and the dissolved O₂ captures electrons to form ˙O₂⁻ radicals. This process was confirmed by the fact that the degradation rate changed little when N₂ was pumped continuously to maintain a N₂ atmosphere in the photocatalytic process.

Fig. 8 Photocatalytic performance of BiOCl nanosheets and Bi₂O₃/BiOCl-10 composite for degradation of MO with the addition of different sacrificial agents: NaI for h⁺, BQ for ˙O₂⁻ and IPA for HO˙. The concentrations of all added sacrificial agents are 10 mM. As a comparison, N₂ was pumped into MO solution for 1 h to remove the dissolved O₂.

The concentrations of HO˙ during the photocatalytic process with BiOCl and Bi₂O₃/BiOCl nanosheets were also determined by the terephthalic acid oxidation method. Terephthalic acid would be transformed into dihydroxyterephthalic acid when reacted with HO˙. The emission intensities of dihydroxyterephthalic acid, which can indicate the HO˙ concentrations during the photocatalytic process, were measured. The photoluminescence patterns of a terephthalic acid solution after being illuminated with UV light for 15 min with BiOCl and Bi₂O₃/BiOCl nanosheets are shown in Fig. 9. The weak peak at 430 nm for BiOCl nanosheets reveals the low concentration of HO˙ when the photocatalyst was illuminated with UV light. The peak for Bi₂O₃/BiOCl is a little larger than that for BiOCl and reached about 300. However, the value for P25 (TiO₂) is about 13 700, which is about 45 times higher than that for the Bi₂O₃/BiOCl nanocomposites and about 140 times higher than that for BiOCl nanosheets, which reveals that the HO˙ concentration was much higher than that of BiOCl and the Bi₂O₃/BiOCl nanocomposites. It is well known that the photocatalytic properties of BiOCl are almost the same as or better than that of P25 (TiO₂)²⁶ under UV light illumination, so that the photocatalytic process with BiOCl and Bi₂O₃/BiOCl nanocomposites is very different to that with P25. HO˙ is not the major oxidizing active species, which is consistent with the analysis of Fig. 9.

Fig. 9 Photoluminescence patterns of terephthalic acid after being illuminated with UV light with BiOCl, Bi₂O₃/BiOCl-10, and Bi₂O₃/BiOCl-20 nanosheets and TiO₂ nanoparticles.

4 Conclusions

A facile method was used to deposit Bi₂O₃ nanocrystals on BiOCl nanosheets with the assistance of ascorbic acid and Pluronic F127. Bi₂O₃ nanocrystals were distributed evenly on the surface of BiOCl nanosheets with a size of approximately 10 nm. Bi₂O₃/BiOCl composites possess excellent photocatalytic activity with high stability under UV light illumination, which is much higher than that of unmodified BiOCl nanosheets. The optimized Bi₂O₃/BiOCl-10 composite achieved the highest photocatalytic degradation activity and could degrade 40 mg L⁻¹ MO completely in 6 min under UV light illumination. The calculated reaction rate constant (0.5135 min⁻¹) is seven times that of unmodified BiOCl nanosheets (0.0748 min⁻¹). The enhancement mechanism of the modification of Bi₂O₃ nanocrystals has been discussed by analyzing the entire photocatalytic process, including optical absorption, surface area, charge transfer and surface reactions. The charge transfer between BiOCl and Bi₂O₃ restricts the recombination of photogenerated electrons and holes, which results in higher photocatalytic activity than that of unmodified BiOCl nanosheets. Photogenerated electrons on Bi₂O₃ nanoparticles play the dominant role in the photocatalytic process by forming ˙O₂⁻ radicals, and photogenerated holes take part in this reaction by providing dissolved O₂.

Acknowledgements

This work was supported by the Natural Science Foundation of China (51102071, 51172059 and 51272063), Fundamental Research Funds for the Central Universities (2013HGQC0005), and the Natural Science Foundation of Anhui Province (1408085QE86).

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