Controlled Synthesis of BiOCl Hierarchical Self-Assemblies with Highly Efficient Photocatalytic Properties

Herein, we report the controlled synthesis of bismuth oxychloride (BiOCl) hierarchical self-assemblies under hydrothermal conditions and demonstrate their high photocatalytic properties. An interesting morphological evolution from microplates to nanoplate assemblies with microsphere-, microdisk-, and microflower-like structures is investigated by adjusting the amounts of surfactant poly(vinyl pyrrolidone) (PVP). It has been found that three types of three-dimensional (3D) BiOCl micromaterials are formed layer-by-layer from a large number of interconnected 2D nanoplates with a mean side length of about 20 nm. A possible crystal growth and formation mechanism is proposed as a plausible mechanistic interpretation for the self-assembly of nanoplates into the observed microstructures that is based on the detailed experiments. Furthermore, the photocatalytic properties of the obtained samples are investigated by the photodegradation analysis of Rhodamine B and methylene orange (RhB and MO) dyes, thus indicating that the microsphere-like BiOCl hierarchical structure has a higher photocatalytic activity than the microdisk-like and microflower-like BiOCl structures, owing to its novel structure with a high surface area.

Introduction

It is well known that the physical and chemical properties of inorganic materials are related to their shape, size, and structure.1 As a result, great research efforts have been made on the fabrication of nanometer- and micrometer-size anisotropic materials with controlled shape of the architectures, which are important for many applications in optics, electronics, photocatalysis, and energy owing to their unique and desirable properties.2, 3 In particular, hierarchical functional materials assembled from zero-dimensional (quantum dots or nanoparticles), one-dimensional (nanorods, nanowires, nanotubes, etc.), or two-dimensional (nanosheets, nanoplates, etc.) are desired and important for practical devices.4–6 Furthermore, three-dimensional (3D) microscale architectures composed of nanoplates have the advantages of both a microstructure and a nanostructure, such as their high crystallinity, high surface-to-volume ratio, anti-aggregation ability, and abundant electronic transport paths, which can be expected to have enhanced physical and chemical performance.7, 8 Although some significant attempts have been made in the controllable synthesis of hierarchical nanomaterials, the fabrication of 3D structures through the assembly of 2D nanoplates as the building blocks is still a big challenge.

BiOCl, as one of the important main group muticomponent V-VI-VII semiconductors, has attracted a great deal of attention because of its excellent physical and chemical properties in various fields such as ferroelectric materials, selective oxidation catalysts, ionic conductors, pigments, and so forth.9, 10 Since the discovery of the UV-light-driven photocatalytic activities of BiOCl for the degradation of organic compounds because of its unique layered structure and high chemical stability, a great deal of research has been conducted to study its photocatalytic performance under UV irradiation.11, 12 BiOCl is known to be a tetragonal layered structure consisting of [Cl-Bi-O-Bi-Cl] sheets stacked together by nonbonding interactions through the Cl atoms along the c-axis.9 The strong internal static electric fields perpendicular to the Cl layer and the bismuth oxide-based fluorite-like layer enable the effective separation of the photoinduced electron–hole pairs, and this results in a high photocatalytic performance. Inspired by the unique properties and promising photocatalytical applications, many research groups have carried out studies on BiOCl nano/microstructures. Up to now, several methods have been reported for the preparation of many kinds of BiOCl with various morphologies such as nanoflakes,13 nanobelts,14 nanosheets,15, 16 microspheres,17, 18 flower-like hierarchical structures,11 and nanolamellas.12 It has been reported that photocatalysts with special 3D morphologies could not only enhance the absorbability to increase the photoabsorption efficiency but also reduce the recombination opportunities of the photogenerated electron–hole pairs, thus they could transfer to the surface rapidly to degrade the organic molecules.19, 20 Moreover, compared to the nanoscale photocatalysts, 3D-structured photocatalysts could be more easily separated and recycled. Although BiOCl microspheres have been reported in the literature,11–18 the exploration of the controlled construction of complex 3D BiOCl architectures by a chemical self-assembly route is still crucial to better understand the relationship between the morphologies and the photocatalytic activities. Therefore, considering the high-energy conversion efficiencies, light-harvesting capacities, and easy solid/liquid separation, it is necessary to synthesize BiOCl with hierarchical architectures to meet the potential photocatalytic applications from the practical point of view.

Herein, a facile PVP-mediated hydrothermal synthesis of BiOCl is proposed to obtain a variety of hierarchical structures with morphologies ranging from microplates to nanoplate assemblies such as, microsphere-, microflower-, and microdisk-like structures. It is found that the PVP capping ligand in the reaction mixture can prevent the random aggregation of nanoplates and promote the delicate assembly of these nanoplates into 3D microspheres. Based on the results of the controlled experiments, which include reaction time and the amount of PVP, a possible mechanism for the formation of BiOCl with different hierarchical microstructures under hydrothermal conditions is proposed. The band gap energy of the microspheres is determined to be in the range from 3.05 to 3.32 eV by optical absorption. In addition, their photocatalytic activities are also investigated. It is demonstrated that the microsphere-like BiOCl shows the highest photocatalytic activity for the RhB and MO degradation among the BiOCl samples with various morphologies.

Results and Discussion

The BiOCl products with hierarchical nanostructures such as microsphere-, microflower- and microdisk-like, and microplate structures were obtained by the reaction of Bi(NO₃)₃, HNO₃, KCl, and hexamethylenetetramine (HMT) with the addition of different amounts of PVP (0.6 g, 0.5 g, 0.4 g, and 0). For convenience, the products obtained with PVP (0.6 g, 0.5 g, 0.4 g, and 0) are designated as sample I, sample II, sample III, and sample IV, respectively. The X-ray diffraction (XRD) patterns of the four BiOCl products are displayed in Figure 1. It can be observed that all the products can be indexed to the tetrahedral structure, which is consistent with the values in the standard card (JCPDS card No. 6-249). The major XRD diffraction peaks at 2θ=11.81°, 25.61°, 32.41°, and 33.52° corresponding to the (001), (101), (110), and (102) planes, respectively, can be seen clearly. No other impurities are detected, thereby indicating that the products obtained with different amounts of PVP are all pure BiOCl. The sharp peaks on the XRD pattern indicate that the as-synthesized products have a high degree of crystallization. The strongest diffraction peak at around 33.52° corresponded to the (102) plane. It can be seen that the {001} facets family, such as (001) and (002), have peak intensities that gradually decrease with an increase in the amount of PVP relative to the bulk tetragonal phase BiOCl, which could be attributed to the different crystallographic directions under various preparation conditions.

Figure 1. XRD patterns of the as-synthesized products obtained in the presence of different amounts of PVP: a) 0.6 g of PVP, b) 0.5 g of PVP, c) 0.4 g of PVP, and d) the product obtained without PVP.

To investigate the surface compositions and chemical states of the as-prepared BiOCl product in the presence of 0.6 g of PVP, X-ray photoelectron spectroscopy experiments (XPS) were carried out and the results are shown in Figure 2. The full scan spectrum in Figure 2 a shows the presence of the C, Bi, O, and Cl peaks, thereby indicating the presence of these elements in the products. The carbon peak comes from the adventitious carbon on the surface of the sample. After fitting, the two strong peaks at the Bi region (Figure 2 b) at 158.8 and 164.2 eV are respectively assigned to Bi 4f_7/2 and Bi 4f_5/2, which corresponded to Bi³⁺ according to the previous results.21–23 The O 1s core level spectrum (Figure 2 c) can be fitted well with two peaks at 530.0 and 531.7 eV. The sharper peak at 530.0 eV belongs to O²⁻ from a bismuth–oxygen bond in BiOCl. The other peak at 531.7 eV is mainly attributed to the chemisorbed H₂O or OH⁻ on the surface of the samples.24, 25 The peaks with binding energies of 198.5 eV and 200.1 eV correspond to Cl 2p_3/2 and Cl 2p_1/2, respectively; this is characteristic of Cl⁻ in the BiOCl product (Figure 2 d).

Figure 2. XPS spectra of the BiOCl product in the presence of 0.6 g of PVP: a) survey XPS spectrum, b) Bi 4f, c) O 1s, and d) Cl 2p.

Figure 3 shows scanning electronic microscopy (SEM) images of the as-synthesized BiOCl product by using 0.6 g of PVP. A panoramic SEM image shown in Figure 3 a shows that the obtained BiOCl presents microsphere-like morphologies on a large scale. Almost all of them possess the same morphology with a diameter ranging from 8 to 10 μm. Moreover, the surface of the BiOCl products is quite rough (Figure 3 b). It can be observed from an individual sphere-like microstructure that the edge section is higher than the central part. Higher magnfication SEM images, as shown in Figure 3 c–e provide more detailed structural information. A typical sphere-like microstructure is shown Figure 3 c, and more details can be found in Figure 3 d, e. A close-up observation of the rough surface of the microsphere (Figure 3 c) clearly reveals that the microsphere is built from numerous orderly packed nanoplates, and the nanoplates are stacked together through side-by-side and plane-to-plane conjunctions to form plane conjunctions that form microsphere-like architectures. Side and central views of the microsphere (Figure 3 d) indicate that it is actually constructed by several layered structures built from 2D nanoplates with an average thickness of 20 nm, and the layered structures are assembled almost perpendicular to each other to form integrated multilayered microsphere-like structures. It is worth mentioning that these hierarchical mirosphere-like structures are sufficiently stable, therefore they cannot decompose into dispersed nanoplates even after long periods of ultrasonication. These nanoplates are arranged at progressively increasing angles to the radial axis and are highly directed to form arrays in a regular fashion. The side view of an individual microsphere structure supports the conclusion that such a microarchitecture is composed of densely packed nanoplates with an average thickness of about 20 nm.

Figure 3. SEM images of the microsphere-like BiOCl obtained in the presence of 0.6 g of PVP: (a, b, and c) low magnified SEM images, and (d and e) high magnified SEM images of the central and side views of the microsphere-like microstructure.

To gain further information about these microsphere-like BiOCl hierarchical microstructures, the product was further investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Owing to the large size of the BiOCl, the TEM image in Figure 4 a shows the edged part of a single BiOCl microsphere, thus indicating that the building blocks are nanoplates. Figure 4 b is a lattice-resolved HRTEM image taken from the edge of a single sample. The clearly resolved lattice fringes are 0.27 nm, and correspond to the d-spacing of the (110) diffraction of tetragonal BiOCl (0.2753 nm), which is in accordance with the selected-area electron diffraction (SAED) result (Figure 4 c). This result indicates that the preponderant growth direction of the BiOCl microspheres is the [001] orientation, which is parallel to the (110). The SAED can be indexed to a pure tetragonal phase. The appearance of periodic diffraction spots indicates that these nanostructures are self-assembled into highly oriented aggregates as a single crystal. The as-prepared product was also determined by energy-dispersive X-ray spectroscopy (EDS) analysis. The corresponding EDS pattern (Figure 4 d) indicates that the sample contains only Bi, O, and Cl, and no impurity peaks were observed (the peak near 8 keV corresponds to the presence of Cu, which comes from the substrate). In addition, the atomic ratio of Bi/O/Cl is 1:0.93:0.91, which is in agreement with the standard stoichiometric composition, further indicating that the composition of the as-synthesized product is BiOCl.

Figure 4. a) TEM and b) HRTEM images of the microsphere-like BiOCl, c) corresponding SAED pattern of the nanosheets, and d) EDS of the as-prepared sample.

To confirm that the surfactant PVP plays a key role in the formation of BiOCl with hierarchical structures, comparative experiments were carried out and the resulting morphologies were analyzed by SEM, TEM, and HRTEM. With the addition of 0.5 g of PVP, the product is almost entirely composed of a large quantity of uniform monodisperse BiOCl with an average diameter distribution from 4–6 μm (Figure 5 a). No other morphologies can be observed, thus indicating a high yield of these microstructures. In the enlarged SEM image (Figure 5 b), almost all of them possess the same sphere-like architectures and the peripheral surfaces of the microspheres are not smooth. The enlarged image (Figure 5 c) shows the clear structure of the individual microsphere. It can be seen that the microspheres are flower-like shapes, which are organized from numerous thin nanoplates in an interwoven arrangement. Further observations of the side and central parts of the microspheres depict that these nanoplates interweave together to form an open porous structure, as shown in Figure 5 d, e. Especially, careful examination reveals that the BiOCl nanoplatelets have a thickness of about 20 nm, and a relatively large width of dozens of nanometers. Figure 5 f shows the representative TEM image of the microflower-like BiOCl microstructures. This confirms that the microstructures have an average diameter of about 4 μm, which is in agreement with that revealed by the SEM image shown in Figure 5 b. The obvious contrast between the dark edge and the relatively brighter center confirms that the BiOCl microspheres should be loose in the middle part, indicating the porous nature. The HRTEM image of the BiOCl hierarchical nanostructures (Figure 5 g) shows the lattice image obtained at the edge of the particle. The typical lattice fringe spacing is determined to be 0.27 nm, and corresponds to the (110) spacing of the BiOCl. The regular and clear square diffraction spot arrays (Figure 5 h) shown by the corresponding SAED pattern demonstrate that the hierarchical nanostructures consist of the single crystalline nanoplates.

Figure 5. SEM and TEM images of the microflower-like BiOCl samples prepared in the presence of 0.5 g of PVP: a) low magnified SEM image, b, c) enlarged SEM images, d, e) detailed central and side views of an individual sphere, respectively, f) TEM image of an individual sphere, g) side view of the individual sphere, and h) HRTEM image (the inset shows the corresponding SAED).

A further decrease in the amount of PVP to 0.40 g gives rise to quite different product morphologies. Figure 6 a is a low-magnification SEM image of the sample, and it indicates that the product is composed of large-scale microdisks with a uniform diameter of about 3 μm. The thickness of the microdisks is about 50 nm from the SEM image (Figure 6 b) at a perpendicular angle. Figure 6 c, d shows a typical individual BiOCl microdisk structure and the magnified SEM image of the edge part, thereby showing detailed information about the secondary structures of the microdisks. Interestingly, the BiOCl microdisk is composed of hundreds of nanoplates with lengths of hundreds of nanometers in a parallel fashion to form integrated multilayered disk-like architectures (Figure 6 c). A closer inspection further reveals that each nanoplate is made up of numerous tiny nanosheets. The primary structures of these nanosheets are attached side-by-side into nanoplates (Figure 6 d). In addition, the same product was further investigated by TEM, as shown in Figure 6 e. No obvious difference between the edge and the central parts further confirms their solid nature, which is different from the porosity of samples II and III. Combined with the SEM image, it is reasonable to conclude that the central section of the microdisk has the same thickness as the edges of such crystals. The upright microdisks in Figure 6 f indicate that they are bending, which is in agreement with the SEM image in Figure 6 b.

Figure 6. SEM and TEM images of the microdisk-like BiOCl samples prepared in the presence of 0.4 g of PVP: a) low magnified SEM image, b, c) enlarged SEM images, d) detailed view of an individual sphere, e, f) TEM images, and g) HRTEM image (the inset shows the corresponding SAED)

It is known that the addition of surfactants into the reaction system is an effective way to adjust the size and shape of nanomaterials.26 In general, surfactants are required in the self-assembly, which is believed to play a key role in directing the growth and the morphology of these nanomaterials.27 To understand the as-prepared products with various morphologies in more detail, PVP surfactant-dependent experiments were employed; it is found that the amount of PVP that is added plays the most important role in controlling the morphologies of the products. As shown in Figure 7 a, the product obtained in the absence of surfactant PVP consists of a large number of irregular microplates with the size ranging from 2 to 10 μm. Also, the inset in Figure 7 a shows that the thickness of the microplates is about 280 nm, which is much larger than that of the nanoplates as the building blocks in the BiOCl hierarchical structures. By adding a small amount of PVP (0.3 g), BiOCl keeps the plate morphologies, while the size decreases to about 2 μm (Figure 7 b) and the thickness is reduced, compared to the microplates prepared in the absence of PVP. Furthermore, it can be observed that such nanoplates show relatively loose aggregation in a perpendicular fashion and have a tendency to form a flower-like structure. Once the amount of PVP increases to 0.45 g, as shown in Figure 7 c, there are mainly two kinds of hierarchical structures. The first kind is microdisks with a multilayered structure. These nanoplates are assembled in a parallel fashion. The second kind is microspheres that are composed of curved nanoplates. The tilted sheets prefer to assemble at the edges of the flat disks and interconnect with each other around the edges, thereby resulting in porous microspheres. These two kinds of morphologies are similar to those obtained in the presence of 0.4 g and 0.5 g of PVP, respectively. Maybe this is an intermediate of spherical superstructures. This phenomenon has also been observed in the product obtained by using 0.55 g of PVP. In the presence of 0.55 g of PVP, the products are composed of many spherical superstructures. The sizes of these spheres are not uniform. Their diameters are in the range of several micrometers to 10 μm. It is found that the smaller microspheres are composed of nanoplates. The larger ones are assembled from nanoplate aggregates, and this is similar to the microsphere-like hierarchical structure. By further increasing the amount of PVP, the larger microspheres gradually become more and more, thereby driving the disappearance of smaller microspheres. When the amount of PVP is increased to 0.6 g, the smaller microspheres completely disappear, and all the morphologies are microsphere-like hierarchical structures (Figure 3).

Figure 7. The BiOCl products obtained by using different amounts of PVP: a) 0 g, b) 0.3 g, c) 0.45 g, d) 0.55 g, and the BiOCl products obtained by using 0.6 g of PVP under various hydrothermal reaction stages: e) 1 h, f) 3 h, and g) 10 h. The insets of SEM images are their magnified parts, respectively (Figure 7 a, and g).

To further reveal the evolution process of the BiOCl microsphere-like structure, the samples collected at different stages of the hydrothermal reaction, under otherwise identical conditions used for preparing samples, were investigated by XRD and SEM (Figures S1 and S2 in the Supporting Information), and the corresponding results are shown in Figure 7 e–g. As shown in Figure 7 e, some nanoplates orientedly aggregate and many nanosheets self-assemble to form a neat rose-like nanostructure with some interior structures. When the reaction time is prolonged to 3 h, the rose-like morphologies nearly disappear and the dominant products are microspheres, in which the nanoplates as the building blocks aggregated tightly. In addition, some irregular dispersive nanoplates coexist in the system. As the reaction proceeds to 10 h, no single nanoplates are found and a large number of microspheres with a size of 4 μm are present. These microspheres have a similar shape and size to those observed in Figure 5. In addition, it can be detected that some neighboring microspheres tend to organize (marked by the arrows) at the edge and interconnect with each other around the edges, thereby giving rise to microsphere-like sections of a size of 12 μm (Figure 7 g). A closer inspection of the inset in Figure 7 g shows that the outer part of the microsphere-like hierarchical structure is composed of several microspheres in a circular arrangement. At the same time, some microspheres are found to be attracted to the surface of quasi microspheres, which means that a spherically symmetrical structure as a whole will be obtained and the recrystallization is still underway. With a further increase in the hydrothermal treatment time to 18 h, microspheres continuously aggregate, and the 3D architectures become significantly more swollen. Eventually, no single microspheres are detected and well-assembled hierarchical microsphere-like architectures constructed by numerous 2D square nanoplates are formed when the reaction lasts more than 18 h (Figure 3). Therefore, it is concluded that the BiOCl evolves from nanoplates to microspheres with an extension of the reaction time, thus revealing that the reaction time plays an important role in the formation of the hierarchical structures.

It is found that there is an impurity in the product when the amount of PVP is further increased to 0.8 g. This could be attributed to the oxidized product of PVP in the hydrothermal process, as PVP possesses a certain reduction activity in the reaction (see Figures S3 and S4 in the Supporting Information).28, 29 To identify the phase of the impurity, the product obtained by using 1.2 g of PVP was analyzed by XRD and SEM; it reveals that the product has microspheres constructed by nanoplates. Compared with the standard cards (Nos. 6-249 and 41-1448), the product is composed of BiOCl and Bi₂O₂CO₃, and the CO₃²⁻ might be formed by the oxidization of PVP in the hydrothermal system. As an increase in the amount of PVP leads to an increase in the PVP reduction activity, the Bi₂O₂CO₃ is relatively easily formed.29 This phenomenon has also been reported by Zhang et al.29 The important role of PVP in the development of the specific morphology has been further investigated through comparative synthetic experiments in the presence of other surfactants such as cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), while keeping other reaction conditions unchanged. When PVP was substituted by a mass equivalent of CTAB and SDS, the products both presented thick microplate morphologies (Figure S5 in the Supporting Information). The formation of microplates is strongly related to the intrinsic crystal structure of BiOCl. These results further prove the importance of PVP in the formation of BiOCl hierarchical nanostructures. Therefore, our results indicate that the PVP not only controls the morphologies but also influences the purity of the products. Furthermore, HNO₃ could decrease the pH value of the precursor solution to dissolve Bi(NO₃)₃ before the hydrothermal reaction. During the hydrothermal process, HMT hydrolyzed into formaldehyde and ammonia, which could form ammonium hydroxide and support OH⁻.30 In acidic aqueous solutions, the OH⁻ could neutralize the H⁺ to promote the formation of BiOCl nuclei. Then the BiOCl crystal continued to grow as the reaction proceeds. Meanwhile, a HMT molecule containing four N atoms could act as a tetradentate ligand. A Bi^III complex with a HMT ligand has been reported.30, 31 NaOH instead of HMT, was used in our reaction system; no such hierarchical microsphere structures were observed. When the amount of HMT was lower than 8 mmol in the solution, no products were obtained, as not enough OH⁻ could neutralize the H⁺ to release the BiOCl products.

Up to now, hierarchical nanostructures have stimulated many investigators to study their structures and growth mechanism, and various models have been proposed for the growth process of the various nanoplate assemblies. Generally, the method for generation of ordered geometries is to kinetically and/or thermodynamically control the initial nucleating stage and subsequent crystal growth stage through changes in reaction parameters such as temperature, reaction time, concentration, and so on.32, 33 It is generally believed that macromolecules absorbed on the surface of nanoparticles have a great influence on their self-assembly behavior.34 Selective adsorption on the various crystal planes and subsequent anisotropic competition at interfaces play important roles in rotating adjacent nanoparticles so that they result in hierarchical nanostructures. In the past years, PVP has been used for capping organic molecules to control the crystal nucleation and growth in the reaction system to produce nanomaterials with various morphologies.35, 36 In this study, as a determinative factor for the final nanoplate assemblies, PVP played an important role in the formation of integrated nanosheets and multilayered structures. To provide evidence for the interaction between PVP and the products, IR spectra of the products were investigated, including PVP, PVP/Bi(NO₃)₃, and the products obtained by using PVP, CTAB, and SDS (see Figure S6 in the Supporting Information). It is found that the free CO stretching band of pure PVP is at 1671 cm⁻¹. This band disappears and a new band at 1583 cm⁻¹ appears after introducing Bi(NO₃)₃. This is attributed to the coordination interactions between bismuth ions and carbonyl oxygen.37 The shift in the free CO band to a lower wavenumber originates from the loosening of the CO double bond by Bi³⁺-coordination. After completion of the hydrothermal process, a new band at 1643 cm⁻¹ appears in the BiOCl obtained by using PVP, while no such band appears in the products obtained without using PVP and using CTAB and SDS. This is thought to be associated with the weak interactions between the bismuth ions of the product and the carbonyl oxygen atoms of PVP.38 The IR measurement further indicates that PVP has interactions with the products, while CTAB and SDS have no such interactions. Therefore, PVP might play double roles in controlling the superstructure morphology. First, PVP could act as a potential crystal face inhibitor in the system, which encourages the formation of oriented nucleation, thereby leading to the construction of anisotropic growth of the nanplates. Second, the PVP stabilizer might be adsorbed onto the surfaces of the BiOCl nanoparticles by coordinating interactions between bismuth ions and carbonyl oxygen atoms in the polar pyrrolidone groups. This could cause steric hindrance and prompt the formation of hierarchical architectures from individual nanoplates as PVP has multiple coordinating sites.

Therefore, based on the analytical results, a formation process of BiOCl with microsphere-, microdisk-, and microflower-like structures is proposed in Scheme 1. The formation process might be divided into three steps in sequence: (1) At the beginning of the hydrothermal reaction, the Bi³⁺ can complex with PVP. As the reaction proceeded, the formation rate of BiOCl was very fast and some nuclei were formed as the concentration of reactants was comparatively high. (2) On the basis of these primary particles, the BiOCl nanoparticles were absorbed by PVP and then continued to grow, and this led to the formation of nanoplates naturally, as a result of an intrinsic lamellar structure of the BiOCl phase characterized by [Bi₂O₂] layers interleaved by double slabs of halogen atoms (see Figure S8 b in the Supporting Information).11 (3) In the subsequent assembly process, the free polymer molecules PVP were preferentially absorbed on the primary nanoplates and functioned as potential crystal face inhibitors in the system. Driven by the minimization of the total energy of the system and van der Waals interactions between polymer molecules, the nanoplates aggregated together to form submicro-scaled solid spheres, because the spherical structure conserves the lowest surface energy compared with other possible geometric structures.39 In addition, some free polymer molecules might absorb on the face of the submicro-scaled solid spheres if the amount of PVP is 0.6 g. With prolonged hydrothermal treatment, the crystallization of BiOCl began from these nucleation sites on the surface anchoring with PVP, which might provide many high-energy sites for further growth. Then, submicro-scaled solid spheres gradually aggregated and subsequently dissolved the small solid spheres from the inner side of the aggregates toward the outside, which led to the formation of the microsphere structure. As there are no interactions between CTAB and SDS with BiOCl, it is therefore rational that the products obtained by using CTAB and SDS and without using PVP all have microplate morphologies, further indicating the important role of PVP in the formation of nanoplate-assembled microstructures.

Scheme 1. a) Schematic illustration of the proposed formation process of BiOCl hierarchical nanostructures, b) molecular structural formula of PVP, c) the complex of PVP and Bi³⁺, d) crystal structure of BiOCl.

The Brunauer–Emmett–Teller (BET) specific surface areas of the BiOCl samples with different morphologies (sample I, II, III, and IV) were investigated by using nitrogen adsorption–desorption isotherms. The surface area values are shown in Table 1. The BET specific surface area of the sample was calculated to be 61.8 m² g⁻¹, which was much larger than the other structures of BiOCl. When the amount of PVP decreases, the BET values decrease. With no addition of PVP, the BET value of the BiOCl microplates is the least 4.1 m² g⁻¹. Considering the observed morphology of the products, the pores can be attributed to the crystal growth process and the space between the intercrossed nanosheets. The BET specific surface areas of the BiOCl samples indicate that the microsphere-like hierarchically microsturcture has a relatively high surface-to-volume ratio, which is helpful for the photocatalysis application.

Table 1. Morphologies and Brunauer–Emmett–Teller (BET) surface areas of the BiOCl with different microstructures.

Sample	I	II	III	IV
Morphology	microsphere	microflower	microdisk	microplate
BET [m^2 g^−1]	61.8	53.5	13.5	4.1

As is known, the optical absorption properties of semiconductors, which are determined by the electronic structures, are recognized as important factors to evaluate their photocatalytic activity. The typical UV/Vis diffuse reflection spectra of the nanoplate-assemblies BiOCl hierarchical structures (sample I, sample II, and sample III) and BiOCl microplates (sample IV) were measured by UV/Vis spectroscopy (Figure 8). From the spectra, it can be seen that the BiOCl samples show strong absorption in the ultraviolet region. It is known that the optical absorption of a crystalline semiconductor near the band edge follows the formula (αhν)ⁿ=B(hν−E_g), where α, hν, E_g, and B are the absorption coefficient, photon energy, band gap, and a constant, respectively.40 Among them, n decides the characteristics of the transition in a semiconductor, which is either 2 for a direct interband transition or 1/2 for an indirect interband transition. Given the steep sharpness of the absorption onset, the band gap of BiOCl is considered to be indirect and thus the value of n is 1/2 for BiOCl.11 The inset in Figure 8 shows the (αhν)^1/2 versus photon energy (hν) curve for the samples. The band gap (E_g) energy values of as-synthesized BiOCl samples, by extrapolating the straight portion of (αhν)^1/2 versus the photon energy (hν) plot to the point α=0, are 3.05 eV, 3.12 eV, 3.18 eV, and 3.32 eV for samples I, II, III, and IV, respectively. These values are close to the reported values.11 A small difference in the E_g values among the four samples might be attributed to the morphology, size, and specific structure. Generally, the thin self-assembled nanoplates would increase the amplitude of atomic vibrations, thereby leading to larger interatomic spacing.41 So the band gap energy of semiconductors tends to decrease. Furthermore, the nanoplate-assembled structures could allow multiple scattering of UV/Vis light, thereby resulting in a longer optical path length for light transporting through BiOCl than that for the BiOCl microplates.19 It is expected that such photoscattering could increase the optical response in the UV/Vis region. In addition to the high surface area and surface permeability, the obtained BiOCl microsphere-like structure has the appropriate band gap, which could endow the as-prepared microsphere-like BiOCl with potential applications of effective photocatalysis.

Figure 8. UV/Vis spectra of the BiOCl samples (I: microsphere-like, II: microflower-like, III: microdisk-like, and IV: microplates) and the corresponding (αhν)^1/2 versus hν curves.

It is known that RhB and MO dyes are usually chosen as model pollutants to simulate the actual photocatalytic degradation of organic pollutants. Figure 9 shows the photocatalytic degradation of RhB and MO in the presence of BiOCl hierarchical structures and commercial P25 under ultraviolet light irradiation. Slight color changes in the RhB and MO dyes were not observed for the blank samples. In the process of photodegradation of RhB and MO by microsphere-like BiOCl, the remainder of the RhB and MO dyes are 9.8 % and 10.0 % in the presence of samples after 50 and 70 min of ultraviolet light, respectively. However, the remainder of RhB and MO dyes photodegraded by microflower-like BiOCl obtained at 50 min and 70 min are 25.0 % and 22.0 %, respectively. The initial reaction rate constants for the samples are listed in Table S1 in the Supporting Information.

Figure 9. UV light photocatalytic activities of a) RhB and b) MO dyes for blank and BiOCl samples with various morphologies: (I) microsphere-like, (II) microflower-like, (III) microdisk-like, (IV) microplates, blank, and P25.

It was clearly seen that under identical conditions, the microsphere-like structures have superior photocatalytic activity over the samples with other morphologies and the commercial P25. Several reasons may account for this observation. Firstly, the BiOCl material with layered structures has good photocatalyst activity.17 When one electron is excited by one photon from the Cl 3p state to the 6p state in BiOCl, one pair of a hole and an excited electron appear.15 The layered BiOCl structure can provide sufficient space to polarize the related atoms and orbitals, which can effectively separate the photoinduced electron–hole pairs, thereby assisting a high photocatalytic activity. Secondly, the BiOCl microspheres show a higher BET value of 61.8 m² g⁻¹ than the P25 (about 50 m² g⁻¹). It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst.42, 43 It can be clearly seen that the microsphere-like BiOCl can absorb about 30 % of the dye molecules, which is higher than those of P25, BiOCl microflowers, microdisks, and microplates (see Figure S7 in the Supporting Information). The adsorption–desorption data of the samples is consistent with their BET specific surface area. A large specific surface area allows more dye molecules to be absorbed onto the surface of the photocatalyst. Such a high adsorption ability of dyes is favorable for contact between the material and dyes, which can bring the decreased recombination rate and then accelerate the photocatalytic efficiency.19 Furthermore, the higher specific surface area can result in more unsaturated surface coordination sites exposed to the solution.44, 45 This helps to promote the separation efficiency of the electron–hole pairs in photocatalytic reactions, leading to a higher photocatalytic activity. Thirdly, the hierarchical microsphere structure with the highest surface area has plenty of smaller pores, which can be considered as a mode of transport for reactant and product molecules to move in or out of the material, so the chemical reaction could occur more easily, thus enhancing the photocatalytic efficiency. Fourthly, the mirosphere-like hierarchical architecture can allow multiple reflections of light owing to the low band gap, which enhances light-harvesting and thus increases the quantity of photogenerated electrons and holes available to participate in the photocatalytic reaction.46 On the basis of the above analysis, it is concluded that the enhanced photocatalytic activity of the BiOCl is attributed to the unique hierarchical structures, high adsorption ability, and relatively low band gap. Furthermore, the photocatalytic stability of the microsphere-like BiOCl demonstrated that the photocatalytic activity did not show any significant loss after five recycles for the photodegradation of RhB and MO dyes (see Figure S8 in the Supporting Information).

We also evaluated the photocatalytic activities of BiOCl samples and P25 under visible light (λ>420 nm), and found that the BiOCl microspheres only degraded 40 % and 38 % of MO and RhB dyes in 50 and 70 min, respecitively (Figure 10). These degradation efficiencies are largely lower than that under UV/Vis light, but still higher than that of the P25, which degraded only 25 % and 22 % of RhB and MO dyes in the same time. The reason for the low degradation efficiencies is that the visible light could not excite BiOCl and P25 to produce hydroxyl radicals owing to their large band gaps. The degradation process mainly proceeds through adsorption and a photosensitization pathway under visible light irradiation.18 Therefore, the higher BET specific surface area corresponds to a hihger photocatalytic performance.

Figure 10. Visible light (λ>420 nm) photocatalytic activities of a) RhB and b) MO dyes for blank and BiOCl samples with various morphologies: (I) microsphere-like, (II) microflower-like, (III) microdisk-like, (IV) microplates, blank, and P25.

Conclusions

In summary, nanoplate-assembled BiOCl with microsphere-, microflower-, and microdisk-like structures have been successfully prepared by a hydrothermal process in the presence of PVP. Based on the controlled experiments, the formation of BiOCl with various morphologies mainly went through an oriented aggregation-based process and self-assembly growth mechanism. The highest surface area of the hierarchical nanostructures was close to 61.8 m² g⁻¹. Moreover, the photocatalytic activities of BiOCl samples on the degradation of RhB and MO dyes under UV light irradiation showed that the BiOCl with microsphere-like structures showed higher photocatalytic activity than that of BiOCl with microflower-, microdisk-like structures, and microplate structures, and this was attributed to the high BET surface area, high surface-to-volume ratios, and more light absorbance. This BiOCl with microsphere-like structures is a very promising UV-light photocatalyst for degrading organic pollutants and for other applications.

Experimental Section

Materials

Bismuth(III) nitrate pentahydrate (Bi(NO₃)₃⋅5 H₂O), potassium chloride (KCl), nitric acid (HNO₃, 70 %), polyvinylpyrrolidone (PVP, M_w=1300 000), hexamethylenetetraamine (HMT), cetyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfate (SDS) were purchased from Sigma–Aldrich. Commercial TiO₂ (P25, a mixture of 30 % rutile and 70 % anatase) was obtained from Degussa AG, Germany. All the reagents were used without further purification.

Synthesis of BiOCl

In a typical synthesis of the microsphere-like BiOCl, Bi(NO₃)₃⋅5 H₂O powder (1 mmol, 0.487 g) was dissolved in concentrated nitric acid (4 mL) to afford a transparent bismuth aqueous solution. Then, deionized water (30 mL), KCl (1 mmol, 0.074 g), and PVP (0.6 g) were added to the solution, and the mixture was vigorously stirred for 30 min to ensure that all reagents were dispersed homogeneously. Finally, HMT (0.85 g) was added into the solution and stirred at room temperature for 4 h. The resulting solution was transferred to a dried Teflon autoclave, and then kept at 180 °C for 18 h. After being cooled to room temperature, the precipitate was collected by centrifugation, washed with deionized water and ethanol several times, and dried at 60 °C. To understand the role of PVP and the formation process of the BiOCl hierarchical architectures, controlled experiments were carried out by varying the amount of PVP added (0.55, 0.5, 0.45, 0.4, 0.35, 0.3, and 0 g) and the reaction times (1, 3, and 10 h), while keeping other experimental conditions unchanged.

Characterization

The crystalline structures of the products were analyzed with an X-ray diffractometer (XRD, Y-2000) with Cu_Kα radiation (λ=1.5418 Å) at a scan rate of 0.04° s⁻¹ over the range of 10–80°. The morphologies of the as-prepared product were characterized by field-emission scanning electron microscopy (FESEM, JEOL, JSM-6700F) operated at an acceleration voltage of 5.0 kV. TEM and HRTEM observations were carried out on a Philips Tecnai F20 microscope with an accelerating voltage of 200 kV and equipped with an energy-dispersive X-ray spectrometer (EDS). XPS was detected by using a Kratos Axis Ultra DLD electron spectrometer at an accelerating voltage of 13 kV and a pass energy of 35.75 eV (PHI, PHI5300 system), in which the XPSPEAK software was used for the fitting of the XPS spectrum. The nitrogen adsorption–desorption isotherm and Barrett–Joyner–Halenda (BJH) methods were analyzed on a Micromeritics ASAP 2010 analyzer (accelerated surface area and porosimetry system). Fourier-transform infrared (FT-IR) spectra were obtained using a Perkin–Elmer Spectrum GX instrument using KBr pellet technique at a resolution of 1 cm⁻¹. A room temperature UV/Vis absorption spectrum was recorded on a UV-2550 spectrophotometer in the wavelength range of 200–800 nm. The complexes with PVP/Bi(NO₃)₃ were prepared by dissolving the corresponding components in H₂O. The solution was then cast on a Teflon glass plate and dried under vacuum.

Photocatalytic Measurements

RhB and MO dyes were chosen to evaluate the photocatalytic properties of the BiOCl hierarchical architectures under UV light by using a 300 W high-pressure mercury lamp. In a typical reaction, 0.1 g of each sample was dispersed in a Pyrex glass reactor (with the cross section of 10 cm² and the height of 10 cm) containing 100 mL RhB solutions with a concentration of 20 mg L⁻¹. Before illumination, the suspensions were magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption–desorption equilibrium between the photocatalyst and RhB. Then the solution was exposed to UV light irradiation under magnetic stirring. The degradation of RhB dyes was monitored by UV/Vis spectroscopy. For the photodegradation of MO dyes, the test conditions were the same as that for the photodegradation of the RhB dye. Quantities of the used photocatalysts and the MO dyes were 0.1 g and 100 mL of a 20 mg L⁻¹ MO solution, respectively. As a comparison, the photodegradation experiments under visible light irradiation were similar to those under UV/Vis light. The used light source is a 500 W halogen-tungsten lamp and a 420 nm cutoff filter.

Acknowledgements

This work was supported by Singapore NRF-CRP grant on “Nanonets for harnessing solar energy and storage” and also NUS and NTU for providing facilities to carry out the research.