Synthesis of novel TiO₂/BiOCl@HHSS composites and its photocatalytic activity enhancement under simulated sunlight

Abstract

A novel TiO₂/BiOCl@HHSS photocatalyst (TiO₂/BiOCl heterostructured semiconductor supported on hierarchical hollow silica spheres (HHSS)) was synthesized by a facile one-pot method, and further characterized by XRD, HRTEM, SEM, N₂ adsorption–desorption analysis and UV-vis diffuse reflectance spectra. Structural characterization indicated that the as-synthesized TiO₂/BiOCl@HHSS photocatalyst possessed a special small-hollow-sphere@big-hollow-sphere structure with a diameter of 90–150 nm and a high specific surface area of 414.3 m² g⁻¹. The TiO₂/BiOCl nanoparticles with smaller crystallite size were well dispersed on the HHSS, due to its unique hierarchical sphere structure which composed of much smaller hollow spheres (∼20 nm) covered by external wall. The photocatalytic activity of the photocatalyst was investigated by the degradation of reactive brilliant red (RBR) under simulated sunlight. The TiO₂/BiOCl@HHSS composite shows the highest photocatalytic activity among TiO₂/BiOCl, TiO₂@HHSS and BiOCl@HHSS catalysts, due to its special hollow structure, high surface area and heterostructure. A possible mechanism is proposed to explain the degradation of organic dye molecules over the BiOCl/TiO₂@HHSS nanocomposite photocatalyst. The proposed grafting strategy on porous support for the formation of this hybrid photocatalyst can also be applied to other semiconductor candidates resulting in improved photocatalytic performance.

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1. Introduction

In recent decades, sewage release from industries has been causing severe hazard to aqueous environments.^1–3 The discharge of waste water often includes organic dyes, which are usually hazardous to human health.^4,5 Therefore, it is necessary to remove this contaminant from wastewater before releasing into the environment. As is known, many technologies have been used to deal with this serious phenomenon, such as biodegradation, physical adsorption, and chemical oxidation methods. However, these technologies may be producing incomplete degradation, secondary pollution, and other dissatisfactory treatments.^6–9 Very recently, one of the most promising technologies, photocatalytic degradation, is the most effective method of converting toxic organic contaminants into carbonaceous products. Semiconductors are important for light harvesting and have excellent potential to offer solutions to organic pollutants degradation.^10,11 Recently, bismuth oxychloride (BiOCl), as one of the most important bismuth oxyhalides semiconductor materials, has been developed as an excellent photocatalyst for degradation of organic compounds in wastewater. BiOCl has an indirect transition band gap of 3.05–3.55 eV, which makes it a more efficient candidate for photocatalysis under UV light.¹² TiO₂ has been recognized as an interesting semiconductor material for a long time, owing to its potential in photoelectron-chemical and solar-energy conversion with a high degree of efficiency, photochemical stability, nontoxicity and low cost. As photocatalysts, heterostructured semiconductor materials have become a research highlight in recent years due to their excellent photocatalytic performance for the degradation of organic pollutants.^13,14 The heterostructured systems with appropriate band structures favor the separation of photoinduced electrons and holes, improving the photocatalytic efficiency significantly. A number of composite photocatalysts, including ZnO/TiO₂,¹⁵ ZnO/ZnS,¹⁶ CdS/TiO₂,¹⁷ WO₃/g-C₃N₄,¹⁸ Ag₂CO₃/BiOBr,¹⁹ Cu₂O@TiO₂,²⁰ BiOCl/BiVO₄,²¹ etc., have been prepared and their photocatalytic activity were investigated. The TiO₂/BiOCl nanocomposite photocatalysts demonstrated the higher photocatalytic activity attributing to the formation of the TiO₂/BiOCl heterojunction at the interface.²² The results of those studies have shown that the heterostructured composite photocatalysts have higher photocatalytic activity than individual ones.

Surface properties and band gap of a material typically affect the photocatalytic activity of the material, which in turn is related to the composition and morphology of the catalyst.^23–25 In particular, the fabrication of mesoporous photocatalysts has become a research hotspot because of their high photocatalytic activities due to the increase in the number of surface reactive sites and improved mass transport, resulting from the large surface areas, ordered porous structures and large pore volumes.^26–28 Other factors, such as surface energy of the exposed facets, high surface area and optical absorption, are also key parameters that influence photocatalysis.²⁹ Exposed facets with higher surface energy can increase the number of catalytic active sites, which enhances the photocatalytic degradation of organic dyes.³⁰ Various solid supports, such as polymers, carbon materials and metal oxides, have been used in the designing of composite photocatalysts to improve the catalytic activity and stability of the material.^31–35 Metals oxides (TiO₂, Bi₂O₃, CuO, WO₃) supported on mesoporous silica (e.g. MCM-41, SBA-15) have been used as photocatalysts to degrade organic dyes.^36–41 Other photocatalysts supported on hollow silica spheres have been used as photocatalysts.^42,43 However, photocatalysts which are supported on a mesoporous frame or have an intrinsic mesoporous structure, face a obstacles: the excess nanocatalyst supported on the mesoporous frame can block the pores thus reducing the number of active sites and the surface area.⁴⁴

Generally, the development of a photocatalytic system includes two parts. First, the design and synthesis of a heterostructured semiconductor photocatalyst and second, engineering a photocatalytic system that creates a large active area and facilitates effective mass transfer during the course of the reaction. Thus, in this study, we have chosen hierarchical hollow silica spheres (HHSS) with shells composed of self-assembled hollow silica nanospheres (HSNS) as a catalyst support, owing to it has a large inner core and shells of special structure demonstrate desired low toxicity, low density, large specific surface area, high absorption as well as high chemical and mechanical stabilities.^45–48

In this work, a novel TiO₂/BiOCl@HHSS photocatalyst has been successfully prepared by a facile one-pot method. The as prepared samples were explored by the degradation of Reactive Brilliant Red (RBR), which showed superior photocatalytic performance in comparison to TiO₂/BiOCl, TiO₂@HHSS and BiOCl@HHSS under simulated sunlight. A tentative photocatalytic mechanism for the enhanced photocatalytic performance is also discussed in detail.

2. Experimental

2.1. Chemicals

Tetraethyl orthosilicate (TEOS, A.R), aqueous ammonia solution (28 wt%), and ethanol (A.R) were purchased from Xilong Chemical Company, China; n-octane (A.R), cetyltrimethylammonium bromide (CTAB, A.R), and reactive brilliant red (RBR, A.R) are from Tianjin Fuchen Chemical Reagents Factory, China; Tetra-butyl ortho-titanate (TBOT, A.R), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), and sodium chloride (NaCl, A.R) are from Guizhou jinjinle Chemical Co., Ltd, China. All materials were used as received without any further purification.

2.2. Preparation

Synthesis of HHSS. Hierarchical hollow silica sphere (HHSS) was prepared in an alkaline solution according to our previous research.⁴⁵ In a typical synthesis, 1.38 g CTAB was dissolved in a mixture of 66 mL H₂O and 14 mL ammonia (1 mol L⁻¹) to make an emulsion, and then 20 mL octane was added. After the mixture was vigorously stirred for 0.5 h at room temperature, 7.2 mL TEOS was quickly added dropwise into the mixture. The resulting mixture was vigorously stirred at room temperature for another 0.5 h. Then the mixture was transferred into a Teflon-lined autoclave and heated at 373 K for 24 h under autogenous pressure. The product was separated by filtration, washed with deionized water, and dried at 333 K in air for 6 h. The as-synthesized solid product was collected and calcined at 823 K for 6 h in air to remove CTAB and other organic components.

Synthesis of TiO₂/BiOCl@HHSS. The TiO₂/BiOCl@HHSS photocatalyst (the loading amount of TiO₂/BiOCl was set to 40 wt% with the TiO₂ : BiOCl molar ratio of 1 : 1) was prepared by the following procedure: 1 g HHSS was dissolved in 80 mL absolute ethanol, then 20 mL octane was added, and 30 minutes latter 0.5 mL deionized water was added. After the mixture was vigorously stirred for 2 h at room temperature, the TBOT (0.43 mL) was added and stirred for 2 h. Then 0.068 g NaCl was dissolved in water and the solution was added to the above mixture. After stirring for 30 minutes, 0.559 g Bi(NO₃)₃·5H₂O was dissolved in acetic acid (10 mL) and the solution was added to the mixture. After stirring for 1 h, the PH of the mixture was adjusted to 6–7 with ammonia. Then the mixture was transferred into a Teflon-lined autoclave and heated at 373 K for 24 h under autogenous pressure. The product was separated by filtration, washed with deionized water and absolute ethanol, and dried at 333 K in air for 6 h. The as-synthesized solid product was collected and calcined at 673 K for 3 h to get obtain the target crystal.

2.3. Characterizations

X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2500 VBZ+/PC diffractometer using Cu-Kα radiation (k = 0.1541 nm). N₂ adsorption–desorption isotherms were obtained using a Micromeritics ASAP2020M instrument. The materials were degassed in vacuum at 573 K for 6 h before the measurements. The specific surface area (S_BET) was estimated using Brunauer–Emmett–Teller (BET) equation. The pore size distribution was calculated from the desorption branch of the isotherm using Barrett–Joyner–Halenda (BJH) method. The morphology of the sample was examined by high-resolution transmission electron microscope (HRTEM) on a JEM-3010 with an accelerating voltage of 200 kV. The scanning electron microscope (SEM) photographs of the samples were obtained using a Hitachi S-4700 electron microscope and the elemental analysis was performed by an energy dispersive X-ray spectroscopy (EDX) and elemental Mapping attached to the SEM. UV-vis diffuse reflectance spectra of the samples were recorded on a Cary 500 Scan UV-vis NIR spectrophotometer with an integrating sphere attachment.

2.4. Evaluation of the photocatalytic performance

Photocatalytic activity of the TiO₂/BiOCl@HHSS photocatalyst was evaluated by the degradation of RBR under simulated sunlight. 200 mg of photocatalyst was dispersed in 800 mL of 20 mg L⁻¹ RBR aqueous solution, ultrasonication for 30 min to ensure the establishment of an adsorption–desorption equilibrium between the photocatalyst and the RBR. A 500 W xenon lamp (CHF-XM-500W, Beijing Trusttech Co., Ltd., China) was used as the light source. During the photocatalytic experiment, before illumination, the suspension was sonicated for 10 min and magnetically stirred for 30 min in the dark to ensure the establishment of an adsorption–desorption equilibrium. Then the suspension was exposed to visible light irradiation under magnetic stirring. At each time interval, 8 mL suspension was sampled and centrifuged to remove the photocatalysts powder and light absorption of the RBR solution at 538 nm was used to evaluate its degradation as measured by the UV-2700 spectrophotometer. The as-synthesized BiOCl, TiO₂@BiOCl, TiO₂@HHSS and BiOCl@HHSS were also performed under identical conditions. All of the measurements were carried out at room temperature.

3. Results and discussion

The morphological evolution of various samples was examined by SEM and TEM (Fig. 1). The SEM (Fig. 1a) and HRTEM images (Fig. 1c and d) of the resulting material show that the HHSS is constructed of the spherical particles with a diameter between 90 and 150 nm having a small ball@big-ball structure. The shell of the big-hollow-sphere is composed of small hollow spheres (∼20 nm), and the small hollow spheres are covered with an external wall (Fig. 1d). Most of the hollow spheres remain spherical and intact with smooth outer surface. From the broken spheres, the hollow interior is clearly observed. TiO₂/BiOCl@HHSS (Fig. 1b, e and f) are kept the size and morphology of HHSS and it was hardly to see big size TiO₂ and BiOCl particles (Fig. 1b and e). This shows that the TiO₂ and BiOCl particles are uniform dispersion and their sizes are very small. The lattice fringes with the spacing of d = 0.27 nm and d = 0.35 nm correspond to the (110) crystallographic plane of BiOCl, and (101) crystallographic plane of TiO₂, respectively. Moreover, a contact interface and continuity of the lattice fringes between the TiO₂ and BiOCl can be observed (Fig. 1f), suggesting that the intact heterojunction was indeed formed between TiO₂ and BiOCl in the TiO₂/BiOCl@HHSS heterostructured photocatalyst. Fig. S1a† shows the morphology of as synthesized BiOCl was nanoplate. The samples are composed almost uniform nanoplates with the size about 100–300 nm and thickness of several tens of nanometers are clearly seen in the pure BiOCl samples (Fig. S1a†). The irregular flake-like particles are found in the images of the BiOCl/TiO₂ composite (Fig. S1b†), It is mainly because of the TiO₂ particles were uniformly dispersed on the BiOCl nanoplates. The detailed morphology of BiOCl, BiOCl/TiO₂, BiOCl@HHSS, and TiO₂@HHSS nanocomposite photocatalysts are further investigated using TEM and HRTEM. Fig. S2a and b† shows that pure BiOCl powders are composed of numerous sheet-like nanostructures with 100–300 nm width and several tens of nanometers thickness. These values are consistent with the SEM images of BiOCl (Fig. S1a†). Fig. S2c and d† show that the morphology of the nanocomposites BiOCl/TiO₂. That is, the nanocomposites contain sheet-like BiOCl nanostructures and anatase TiO₂ nanoparticles, some anatase TiO₂ nanoparticles are attached onto the surface of sheet-like BiOCl nanostructures. The HRTEM images of the BiOCl, BiOCl/TiO₂, BiOCl@HHSS, and TiO₂@HHSS were shown in Fig. S2b, d, f and h,† respectively, provided the detailed information about the structure of the heterojunction wrapped on HHSS shell. The SEM and HRTEM images of BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS reveal that the nanocomposite photocatalysts are constructed by the hollow spherical particles with a diameter between 90 and 150 nm having a small-hollow-sphere@big-hollow-sphere structure. Due to the loading of BiOCl, TiO₂ and BiOCl/TiO_2, the small hollow spheres becomes blurred. The smooth outer surface of hollow spheres in SEM and TEM image also reveals that the BiOCl and TiO₂ particles are well dispersed and silica hollow spheres are encapsulated by external silica wall. This unique morphology had great significance in adsorbing the dye molecules during the photocatalytic degradation process.

Fig. 1 SEM and HRTEM image of HHSS (a, c and d) and TiO₂/BiOCl@HHSS (b, e and f).

EDX spectra of TiO₂/BiOCl@HHSS (Fig. 2a) was recorded for elemental analysis, which confirmed the presence of Si, O, Bi, Cl and Ti. The molar ratio of Bi : Ti estimated from the peak area is 1.7 : 1.85, which is close to that of the standard stoichiometric composition of TiO₂/BiOCl@HHSS photocatalyst (with the TiO₂ : BiOCl molar ratio of 1 : 1). Elemental mapping in the SEM micrograph clearly shows the presence of these elements (Fig. 2b). No aggregation is observed, which indicates that Bi, Cl and Ti are dispersed uniformly on the HHSS. These values are in agreement with the TEM and SEM images of TiO₂/BiOCl@HHSS (Fig. 2b, e and f).

Fig. 2 EDX (a) and elemental mapping (b) of TiO₂/BiOCl@HHSS composites.

The as-synthesized samples of BiOCl, BiOCl/TiO₂, BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS were characterized by XRD techniques and their crystallographic structures are presented in Fig. 3. The results show that HHSS was amorphous.⁴³ From XRD of the materials (Fig. 3a), all diffraction peaks were consistent well with the BiOCl standard (JCPDS no. 06-0249). The intense diffraction peaks indicate all the samples were well-crystallized in the tetragonal phase. No diffraction peaks belonging to the impurity phases were detected, indicating the purity of BiOCl. The strongest diffraction peak at 32.5° corresponds to the (110) plane of BiOCl. Fig. 3b shows the XRD patterns of TiO₂/BiOCl (with the molar ratio of 1 : 1) nanocomposite photocatalysts. All the diffraction peaks in the XRD patterns match with the tetragonal phase of BiOCl (Fig. 3a–c and e). It can be observed that the diffraction peaks of BiOCl slightly decrease with increasing the amount of anatase TiO₂ in the composite. TiO₂ in the BiOCl/TiO₂ composites displayed no obvious diffraction peaks, which due to the low crystallinity of the samples thermal treated at the temperature of 400 °C. The XRD peaks of TiO₂@HHSS (Fig. 3d) can be indexed to the phases of anatase (JCPDS card no. 21–1272). The peaks of anatase TiO₂ appeared after the TiO₂ coating was grown on HHSS and desiccated at 400 °C. The TiO₂/BiOCl@HHSS composite photocatalysts simultaneously displayed all the peaks contributed from TiO₂, BiOCl, and HHSS.

Fig. 3 XRD patterns of (a) BiOCl, (b) BiOCl/TiO₂, (c) BiOCl@HHSS, (d) TiO₂@HHSS and (e) TiO₂/BiOCl@HHSS.

Fig. 4 shows the corresponding N₂ adsorption–desorption isotherms for pure BiOCl, BiOCl/TiO₂, BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS nanocomposite photocatalysts. As is shown, the isotherms of pure BiOCl (Fig. 4a) and TiO₂/BiOCl (Fig. 4b) nanocomposite photocatalysts are typical type-IV N₂ adsorption–desorption isotherms with H1 hysteresis, according to IUPAC classification, indicating the existence of mesoporous structure. The BET surface areas of BiOCl and TiO₂/BiOCl are 15.8 m² g⁻¹ and 70.3 m² g⁻¹, corresponding pore diameter are 105.9 nm and 97.5 nm, respectively (Table 1). Probably, the mesoporous structure is thought to be resulted from the aggregation of BiOCl nanosheets and TiO₂ nanoparticles. The nitrogen adsorption–desorption isotherms of BiOCl@HHSS (Fig. 4c), TiO₂@HHSS (Fig. 4d) and TiO₂/BiOCl@HHSS (Fig. 4e) exhibit a type IV pattern (Fig. 4) with two capillary condensation steps occurring in the high relative pressure range of 0.7–0.95, suggesting the presence of bimodal large nanopores.⁴⁵ These values are consistent with the TEM and SEM images (Fig. 1, S1 and S2†). The BET surface areas of BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS are 386 m² g⁻¹, 482.7 m² g⁻¹ and 414.3 m² g⁻¹, corresponding pore volumes are 0.819 cm³ g⁻¹, 1.064 cm³ g⁻¹ and 1.025 cm³ g⁻¹, respectively (Table 1). The BET surface areas and corresponding pore volumes of composite photocatalysts are lower than HHSS as shown in the Table 1. The S_BET value reduced with increasing the amount of anatase TiO₂ or BiOCl nanoparticles in the nanocomposite photocatalyst. Therefore, the TiO₂@HHSS nanocomposite photocatalyst is expected to have higher adsorption capacity, with the same sample weight of BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS, but the crystal particles are not the same, which make the S_BET different.

Fig. 4 Nitrogen absorption–desorption isotherms for (a) BiOCl, (b) BiOCl/TiO₂, (c) BiOCl@HHSS, (d) TiO₂@HHSS and (e) TiO₂/BiOCl@HHSS.

Table 1 BET surface area and Pore diameter of HHSS, BiOCl, BiOCl/TiO₂, BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS

Sample	BiOCl	BiOCl/TiO2	BiOCl@HHSS	TiO2@HHSS	TiO2/BiOCl@HHSS	HHSS
BET surface area/m^2 g^−1	15.8	70.3	386.0	482.7	414.3	743.5
Pore diameter/nm	105.9	97.5	4.24	4.406	4.98	8.3
Pore volume/cm^3 g^−1	0.084	0.343	0.819	1.064	1.025	2.200

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The optical properties of the prepared catalyst materials were investigated by UV-vis diffuse reflectance spectroscopy in Fig. 5. As can be seen, pure BiOCl nanopowders show a clear absorption edge at about 340 nm (Fig. 5a), exhibiting the fundamental absorption in the UV region. The anatase TiO₂@HHSS nanoparticles show absorption in the wavelength range of 200–400 nm (Fig. 5d). BiOCl has a band gap of 3.4 eV that can only be photoexcited by UV light, BiOCl@TiO₂ has the same absorption band in the UV region. Fig. 5e shows the UV-vis absorption spectra of the TiO₂/BiOCl@HHSS catalyst compared with BiOCl and BiOCl/TiO₂ samples in the range of 240–380 nm. All the nanocomposite photocatalysts samples show a mixed absorption property of BiOCl and TiO₂.

Fig. 5 UV-vis absorption spectra of (a) BiOCl, (b) BiOCl/TiO₂, (c) BiOCl@HHSS, (d) TiO₂@HHSS and (e) TiO₂/BiOCl@HHSS.

The photocatalytic activity of different photocatalysts was evaluated by the decolorization of RBR under simulated sunlight. Fig. 6 shows the variation in the RBR concentrations (C/C₀), where C₀ is the initial concentration of RBR and C is the concentration of RBR at time t. As can be seen from the blank experiment (Fig. 6), RBR self-degradation is almost negligible in the absence of a photocatalyst (Fig. 6 blank). The blank test in the absence of photocatalyst shows that the concentration of RBR was decreased very slightly during 18 min of simulated sunlight irradiation, indicating the stability of RBR under simulated sunlight. During the initial adsorption process, we first studied the adsorption behavior of the samples using 20 mg L⁻¹ of RBR aqueous solution in the dark for 30 min to ensure the adsorption–desorption equilibrium of RBR on the photocatalyst. The concentration of RBR was decreased obviously. i.e., the BiOCl, BiOCl/TiO₂, BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS sample adsorbed about 8.2% 10.9%, 20.0%, 25.2% and 22.4% of RBR molecules, respectively (Fig. 6 dark). It seems the amount of dye adsorption for the five sample increases in the order of the trend of the material surface area and total pore volume (Table 1). From Fig. 6, it can be concluded that pure BiOCl and TiO₂/BiOCl nanocomposite photocatalysts have lower adsorption ability for RBR dye molecules than those composite photocatalysts supported on HHSS (BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS). It is because the HHSS process high surface areas and has special small hollow-sphere@big hollow-sphere structure. After exposing the samples and dye under simulated sunlight for 18 min (Fig. 6), the RBR concentrations (20 mg L⁻¹; 200 mg photocatalyst) decreased dramatically with the BiOCl, BiOCl/TiO₂, BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS. i.e., the concentration of the RBR dye were degraded to 24.5%, 17.9%, 8.1%, 12.3% and 0 for the BiOCl, BiOCl/TiO₂, BiOCl@HHSS, TiO₂@HHSS and TiO₂/BiOCl@HHSS, respectively, after 18 min of simulated solar light irradiation. It is found that the TiO₂/BiOCl@HHSS has higher photocatalytic activity than other four samples.

Fig. 6 Photodegradation of RBR dye by (a) BiOCl, (b) BiOCl/TiO₂, (c) BiOCl@HHSS, (d) TiO₂@HHSS and (e) TiO₂/BiOCl@HHSS as a function of irradiation time using the same weight.

The repeatability and photostability of the TiO₂/BiOCl@HHSS composites have also been studied. As shown in Fig. 7, there was no obvious decrease after five consecutive degradation of RBR under simulated sunlight, indicating that the TiO₂/BiOCl@HHSS composite has good repeatability and photostability.

Fig. 7 Cycles photodegradation efficiency of RBR over TiO₂/BiOCl@HHSS photocatalyst.

As shown above, the TiO₂/BiOCl@HHSS nanocomposite has better photocatalytic activity for the degradation of RBR under simulated solar light. On the basis of the results shown above, possible mechanisms are discussed here to explain the enhancement of the photocatalytic properties of the TiO₂/BiOCl@HHSS nanocomposite photocatalyst. According to the BET results previously reported, the enhancement of the photocatalytic activity of TiO₂/BiOCl@HHSS can not only be ascribed to an increase in the specific surface area (which decreases for TiO₂/BiOCl@HHSS compared to that of TiO₂@HHSS). However, the TiO₂/BiOCl@HHSS nanocomposite photocatalyst has higher efficiency of the decomposition of RBR than TiO₂/BiOCl nanocomposite photocatalyst. Meanwhile, the decrease in the crystal size of the anatase TiO₂ and BiOCl in the composite could also exert a positive effect. It is known that the photocatalytic activity is sensitive to the crystallite size; higher activities are typically achieved for smaller crystal sizes.^49–51 Therefore, it is suggested that HHSS has a strong adsorption capacity (possess a large specific surface area), the dye molecules first adsorbed onto the ball. Instead, the observed higher efficiency of the adsorption RBR can be mostly attributed formation of a heterojunction between the two components of the catalyst, BiOCl and TiO₂. Scheme 1 shows the most probable relative energy band positions between bulk BiOCl and TiO₂.⁵² It is well known that simulated solar light is composed of UV, visible and infrared light. In fact, pure BiOCl nanosheets and anatase TiO₂ nanoparticles can only absorb UV light due to their wide band gaps. As illustrated in Scheme 1, BiOCl and TiO₂ do not directly absorb visible light under simulated solar light. The RBR dye molecules first adsorbed on HHSS, then absorbed the light energy to produce singlet and triplet states, and the electron injection from the excited states of the absorbed dye molecules into the CB of BiOCl and TiO₂ resulted in the conversion of RBR into the radical cation RBR⁺ and the formation of BiOCl (e⁻). Later, the holes [photogenerated holes, h⁺] in BiOCl are guided toward TiO₂ because of the difference between the valence band edges of the two semiconductors. The electrons could be transferred to the CB of BiOCl by electron injection, and the holes could be transferred to the VB of TiO₂, which facilitated the charge separation of electron–hole pairs before recombination, leading to a faster charge-carrier transport and a decrease in the charge recombination. Therefore, the heterostructures created in the TiO₂/BiOCl@HHSS nanocomposite probably favor the separation of electrons and holes, together with the inhibition of its recombination, which might make a great contribution to the improvement in the photocatalysis efficiency. This is why the TiO₂/BiOCl@HHSS nanocomposite photocatalyst shows more efficient photocatalytic activity than other four photocatalysts under simulated solar light.

Scheme 1 Possible degradation mechanism of organic dye molecules over the BiOCl/TiO₂@HHSS nanocomposite photocatalyst under simulated sunlight.

4. Conclusions

A novel TiO₂/BiOCl@HHSS photocatalyst with hierarchical hollow structure has been successfully prepared by a facile one-pot method. TiO₂ and BiOCl nanoparticles were found to be highly dispersed on the HHSS, and the obtained photocatalyst possesses a high specific surface area of 414.3 m² g⁻¹ and a hollow spherical shell structure composed of the smaller hollow nanospheres. Due to its special structure, high surface areas and smaller TiO₂/BiOCl crystallite size, the TiO₂/BiOCl@HHSS has higher photocatalytic activity than TiO₂/BiOCl under simulated sunlight. Also, the results of those studies show that the composite photocatalysts (TiO₂/BiOCl@HHSS) of heterostructured semiconductor supported on HHSS have higher photocatalytic activity than individual ones (TiO₂@HHSS and BiOCl@HHSS). A similar strategy could be used to fabricate other heterosemiconducting structures with excellent photocatalytic activity.

Acknowledgements

This work was supported by the Joint Project of Science and Technology Department of Guizhou Province (LH [2014]7621) and the talent introduction Program of Guizhou University (GDRJH2014-23 and GDRJH2014-21). Natural Science Foundation of China (No. 21663009), Breeding Program from Education Department of Guizhou Province (QJK No. 024).

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Footnote

† Electronic supplementary information (ESI) available: The synthesis methods of BiOCl, BiOCl/TiO₂, BiOCl@HHSS and TiO₂@HHSS, and their SEM and HRTEM images. See DOI: 10.1039/c6ra11904b

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