BiOCl Sub-Microcrystals Induced by Citric Acid and Their High

Article pubs.acs.org/crystal

BiOCl Sub-Microcrystals Induced by Citric Acid and Their High Photocatalytic Activities Kun Zhang,† Jie Liang,† Shan Wang,‡ Jie Liu,† Kuaixia Ren,† Xiao Zheng,† Hui Luo,† Yingjie Peng,† Xing Zou,† Xu Bo,† Jihong Li,† and Xibin Yu*,† †

Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry, Shanghai Normal University, Shanghai, 200234, P. R. China ‡ Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *

ABSTRACT: Bismuth oxychloride (BiOCl) sub-microcrystals with tunable morphologies from nanoflakes to hollow microspheres (HMSs) have been synthesized by hydrolyzing a hierarchical precursor (BiCl3) in a solution of water and ethanol with the addition of poly(vinylpyrrolidone) (PVP) and citric acid. The obtained BiOCl possessed sub-microcrystals from single crystals to polycrystals. The formation of the nestlike and hollow structure was found to be induced by citric acid and PVP. The crystal growth and morphology control of BiOCl were explored. Interestingly, citric acid was utilized both as a crystal-growth-inducing agent and a structure-directing agent. The morphology and compositional characteristics of BiOCl were investigated by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman, and UV−vis spectra. The photocatalytic activities of BiOCl with different structures have also been investigated by the degradation of Rhodamine-B (RhB) dye under ultraviolet light irradiation. The as-prepared BiOCl exhibited much higher photocatalytic activity than the comzmon one. In particular, the three-dimensional hierarchical structure such as microflowers and HMSs can effectively improve photocatalytic activity. The results show that BiOCl sub-microcrystals have promise as a novel material for photocatalytic applications.

1. INTRODUCTION The controlled synthesis of inorganic micro-/nanomaterials with tailored morphologies and patterns has long been a topic of research interest among materials scientists because their properties and performances are largely dependent on not only the shape and structure of their primary building blocks and the way in which the building blocks are assembled or integrated but also their structure, phase, shape, size, size distribution, and crystal facet.1−6 In particular, the activities of micro-/nanocrystals strongly depend on their structure and surface properties. The reactivity and selectivity of micro-/nanoparticles can be tuned by controlling the morphology because the different exposed surfaces of the particles and different structures exhibit intrinsic geometric structures, reactivity, and surface properties associated with the crystallographic orientation.6−9 Preparation of crystals with different structures has always been a challenge. Bismuth oxychloride (BiOCl), one of the most important bismuth oxyhalides, prefers to crystallize in the tetragonal matlockite structure with lattice constants of a = b = 3.891 Å and c = 7.369 Å, a layer structure characterized by [Bi2O2] slabs interleaved by double slabs of halogen atoms.10 Therefore, it is very difficult to control the morphologies of the BiOCl crystals. The availability of BiOCl nano-/microstructures with welldefined morphologies, dimensions, and specific crystal facets © 2011 American Chemical Society

may enable new types of applications and/or enhance the performance of currently existing photoelectric devices. Ye et al. reported that BiOCl nanosheets with 87% of exposed {001} facets by hydrolyzing a hierarchical flowerlike molecular precursor (Bin(Tu)xCl3n, Tu = thiourea) showed superior photoreactivity, compared to that of P25 as a benchmarking material.6 Up to now, nanoparticles,11,12 nanoplates,13−16 nanobelts,17 nanotubes,17 fibers,18 solid microspheres,10,19,20 and nanoflowers21,22 of BiOCl have been successfully synthesized using hydrothermal/solvothermal or other methods which used special raw material, high pressure, and high temperature. However, there were a few reports for preparing different structures of BiOCl crystals by adding poly(vinylpyrrolidone) (PVP) and citric acid. Meanwhile, BiOCl has been used as a selective oxidation catalyst,23−25 photocatalyst,10,13,14,19,26 photoluminescence material,15,21 ferroelectric material,27 and pigment28 for many years. In principle, photoactivity of metal oxyhalide semiconductors is based on the photoreaction of the contaminants on the semiconductor surface. For this reason, both the crystal sizes and the structures of the metal oxyhalides determine the sensitivity of photoReceived: August 24, 2011 Revised: December 13, 2011 Published: December 20, 2011 793

dx.doi.org/10.1021/cg201112j | Cryst. Growth Des. 2012, 12, 793−803

Crystal Growth & Design

Article

adsorption measurements. The Brunauer−Emmett−Teller (BET) method was utilized to calculate the specific surface areas (SBET) by using the adsorption data in the relative pressure (P/P0) range of 0.05−0.25. By using the Barrett−Joyner−Halenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms. Raman spectra, UV−visible diffuse reflectance spectra (DRS), and photoluminescence spectra (PLS) were conducted on a Dilor Super LabRam II, a Cary 500 UV−vis−NIR, and a Varian Cary-Eclipse 500, respectively. 2.3. Photocatalytic Test. In all experiments, the reaction temperature was kept at room temperature to prevent any thermal catalytic effect by using a double-wall glass beaker with the circulating water circuit between the two walls. The photocatalytic activities of the as-obtained BiOCl powders were evaluated by the degradation of Rhodamine B (RhB) dye in an aqueous solution at ambient temperature under ultraviolet light irradiation. In a typical experiment, 100 mg of BiOCl powders was dispersed into 100 mL of RhB (20 mg/L) solution. A 100 W high-pressure mercury lamp was employed as a light source with a main emission wavelength of 365 nm. The distance between the center of light and the bottom of double-wall glass beaker was 25 cm. Before illumination, the suspension was strongly magnetically stirred in the dark for 2 h to reach the adsorption− desorption equilibrium of RhB on catalyst surfaces. At given time intervals, 5 mL of solution was continually collected from the suspension and immediately centrifuged, and the concentration of RhB after illumination was determined at 554 nm using a UV−vis spectrophotometer (XIN MAO, UV−vis 7502).

activity performance. The effective integration of small crystal sizes, special structures, and favorable exposed reactive surfaces is expected to be necessary. However, such a strategy to improve the sensitivity of BiOCl photoactivity has not yet attracted much attention up to now, possibly due to the difficulty in synthesizing metal oxyhalide micro-/nanomaterials with different structures. So far, only a few literature reports are about the synthesis of layered BiOCl crystals with nanosheets and microflowers,12,15,16,19 and therefore it is still a challenge to prepare different structures of BiOCl with high photoactivity. Citric acid is well-known for its nontoxicity, stableness, and role as a biological ligand for metal ions (Bi3+, Al3+, Ca2+, Fe3+, Zn2+, and Mg2+ ions) and has been utilized as a common green organic acid for fabricating inorganic nanomaterials.29−33 It has been proven that citric acid is a good capping agent for adjusting the relative activity of the cations and retarding the grain growth of semiconductors. Huo et al.30 have reported that citric acid could assist the solvothermal synthesis of BiFeO3 hollow microspheres which had high visible-light photocatalytic activity. In our work, we report a facile low-temperature onepot route for the fabrication of BiOCl by directly hydrolyzing a hierarchical precursor (BiCl3) in the solution of water and ethanol. Interestingly, with the change of the concentration of citric acid in the reaction solution, controllable BiOCl micro-/ nanostructures such as nanofakes, nanosheets, microflowers, and hollow microspheres were obtained. In the reaction, citric acid was utilized as both a capping agent for inducing the growth of crystals and a structure-directing agent for the formation of different structural architectures. The influence of PVP for the formation of a hierarchical BiOCl nestlike and hollow structure was also investigated. Both PVP and citric acid play important roles in the formation of the BiOCl nestlike and hollow nanostructure. Moreover, we present an experimental investigation to understand photoreactivity of BiOCl with different shapes.

3. RESULTS AND DISCUSSION Figure 1a−d shows the XRD patterns of as-synthesized BiOCl products obtained at 80 °C for about 3 h with different moles

2. EXPERIMENTAL SECTION 2.1. Synthetic Process of BiOCl Samples. All reagents were analytical grade from Aladdin Reagent (China) Co., Ltd. and were used as received without further purification. Distilled water was used throughout. 37.5 mmol of bismuth chloride (BiCl3) was added into 210 mL of double distilled water, and then 24.6 mL of hydrochloric acid (36−38%) was added into the above solution. The mixed solution was stirred until the BiCl3 was completely dissolved at room temperature. Subsequently, the concentration of BiCl3 solution was adjusted to about 0.15 M by transferring it into a volumetric flask (250 mL) and adding a certain amount of distilled water. In the typical synthesis of BiOCl, a series of aqueous solutions were prepared by mixing 42 mL of deionized water, 50 mL of absolute alcohol, 0.01 g of PVP (K30), and a predetermined amount of citric acid monohydrate (0 mmol, 0.45 mmol, 0.75 mmol, 1.20 mmol, respectively). After the aqueous solutions were heated to 80 °C for about 5 min, 8 mL of 0.15 M BiCl3 solution (1.2 mmol) was added dropwise into the above solutions under continuous stirring at 80 °C for 3 h. After the reaction was over, the precipitates were collected by centrifugation, then washed with deionized water and absolute ethanol several times to remove residual ions in the products, and then dried in vacuum at 80 °C for 8 h. 2.2. Characterization. X-ray diffraction (XRD) patterns, obtained on a D/MAX-2000 (Rigaku, Japan) using Cu Kα radiation at a scan rate (2θ) of 4° min−1, were used to determine the phase identity and the size of crystallites. The morphology observation was performed on an JEOL JSM-7500F field emission scanning electron microscopy (FESEM, Hitachi, Japan, operated at 15 kV) linked with an Oxford Instruments X-ray system and transmission electronic micrograph (JEOL JEM-2100, operated at 200 kV). N2 adsorption−desorption isotherms were measured at 77 K with a Quantachrome NOVA 4000 e analyzer. All samples were degassed at 170 °C prior to nitrogen

Figure 1. XRD patterns of the as-synthesized BiOCl products obtained at 80 °C for about 3 h in the presence of different moles of citric acid: (a) 0 mmol (n(citric acid)/n(Bi) = 0), (b) 0.45 mmol (n(citric acid)/n(Bi) = 0.375), (c) 0.75 mmol (n(citric acid)/n(Bi) = 0.625), and (d) 1.20 mmol (n(citric acid)/n(Bi) = 1).

of citric acid. From this figure, we can clearly see that all samples can be indexed to tetragonal matlockite BiOCl phase with lattice constants of a = b = 3.891 Å, and c = 7.369 Å (JCPDS: 06-0249). No other phases and peaks of impurities such as Bi2O3 and BiCl3 were detected, indicating that assynthesized BiOCl products have very high-purity and singlephase. In addition, the intense and sharp diffraction peaks suggest that the as-synthesized products are well-crystallized. As illustrated in Figure 1, the schematic representation of the BiOCl crystal structure can be seen in Scheme 1a. BiOCl crystal has a layered structure characterized by [Bi2O2] slabs which are interleaved by double slabs of Cl atoms, and the dominating 794



Article

Scheme 1. (a) Schematic Representation of the BiOCl Crystal Structure, (b) {001} and (c) {110} Facets of the BiOCl Supercell Structure (Facets Inset as Red Plane)

facets of BiOCl crystal are (001), (110) (Scheme 1b,c).6,34 In the absence of citric acid (Figure 1a), the diffraction peak of the (001) plane is much stronger than those of other planes. For sample (a), the strong peak corresponds to the (001) plane. The relative intensity of the peaks corresponding to the (110)/ (001) and (102)/(001) planes varied significantly from the literature values (JCPDS card: 06-0249), which indicates the different tropism of the products. It indicates that the basal planes of BiOCl synthesized by directly hydrolyzing a hierarchical precursor (BiCl3) with no citric acid in solution should be dominated by {001} facets, and the {001} planes tend to be preferentially oriented parallel to the surface of the substrate; otherwise the preferred orientation of sample (a) (Figure 1a) along the (001) direction could be properly explained on the basis of the crystallographic nature and morphology of the BiOCl crystals. However, in the presence of 0.45 mmol of citric acid (Figure 1b), the diffraction intensities of (101), (110), and (102) facets increase and become much stronger than those of the {001} facets family (Figure 1b) such as (001), (002), and (003). Interestingly, when the BiOCl product (Figure 1c) was obtained in the presence of 0.75 mmol of citric acid, the diffraction intensities of different planes changed a lot and the ratios of the diffraction intensities between the other planes and (110) planes such as (001)/ (110), (002)/(110), and (003)/(110) became much lower, compared to those of the as-prepared samples without adding citric acid (Figure 1a). However, with a further increase in the moles of citric acid (1.20 mmol) (Figure 1d), (101) and (110) diffraction peaks are sharper and stronger and the (001) one is relatively weak, and the relative intensity of the peaks corresponding to the (110)/(001) plane is highest for the hollow microspheres sample (d) (Figure 1d) but gradually decreases from microflowers sample (c) (0.75 mmol) (Figure 1c) to nanoflakes sample (a) (0 mmol) (Figure 1a). In the case of one-dimensional (1D) BiOCl microcrystals, growth is in the longitudinal direction, that is, the growth direction is (001), whereas the side facets of the BiOCl nanoflakes were composed of the (110) family of planes and the other planes. The SEM and TEM studies (Figure 2) discussed in the following section reveal that sample (a) was composed of BiOCl microcrystalline flakes. The {001} facets of the sample (a) were more exposed to the XRD beam, resulting in the strongest (001) peak of sample (a), whereas the more (110), (101), and (102) planes of samples (b−d) were exposed to the XRD beam. Thus, the diffraction intensities of (101), (110), and (102) facets increase and become much stronger than those of the {001} facets

Figure 2. FESEM and TEM patterns of the BiOCl nanoflakes obtained at 80 °C for 3 h with no citric acid: (a) panoramic and (b) magnified FESEM image of the as-synthesized BiOCl nanoflakes; (c) low magnification TEM image of the as-synthesized thin BiOCl nanoflakes; (d) typical HRTEM image of the edge of the BiOCl nanoflakes. Inset: its corresponding SAED pattern of nanoflakes.

family (Figure 1b−d) such as (001), (002), and (003). According to the above discussion, it indicates that all these samples may exist as different crystals-oriented, specific exposed surfaces and different structures to the XRD beam, and therefore different diffraction intensities of facets were detected in the XRD pattern for all of the samples (a−d). As shown in Figure 1, it is also worth noting that the relative intensity of the peaks corresponding to the (110)/(001) and (102)/(001) planes varied significantly from the literature values (JCPDS card: 06-0249), a similar change of XRD diffraction intensities (BiOCl) has been observed between the {001} facets family and other facets, such as the relative intensity of (110)/(001), (101)/(001), and (102)/(001) planes.6,16 Because different shapes might present different exposing planes to the XRD beam. From the above discussion, the relative intensity of (110)/(001), (101)/(001), and (102)/(001) planes has changed (Figure 1), it was known that morphologies of BiOCl microcrystals transformation induced by citric acid might exist. In the current study, BiOCl microstructures with different morphologies could be obtained by directly hydrolyzing BiCl3 in a solution of water and ethanol with different concentrations of citric acid under appropriate reaction conditions. The experiments revealed that the BiOCl morphologies were 795



Article

structure. The diffraction spots of the typical tetragonal phase could be indexed to {110} and {200} panels of the BiOCl. The clear lattice stripe also indicates that these nanoplates are well crystallized and the distance of the lattice fringes is 0.274 nm, which is consistent with the {110} planes of the tetragonal phase. Further increasing citric acid up to 0.75 mmol caused a fancy morphology, as shown in Figure 4a. The overall FESEM image

intensively dependent on the concentration of citric acid, indicating that citric acid was a structure-directing agent for the formation of different structural architectures. However, the reaction time did not influence the final microstructures. As shown in Figure 2, with no citric acid in the reaction system, the morphology of as-prepared BiOCl nanoflakes was characterized by FESEM and TEM techniques. A panoramic FESEM image shown in Figure 2a demonstrates that the product is almost entirely composed of a large quantity of asymmetric round nanoflakes with a diameter of about 0.4− 2 μm and a high yield. Higher magnification FESEM image shown in Figure 2b provides more detailed structural information. Further observation shows that the average thickness of BiOCl nanoflakes with a smooth surface is about 18 nm and the edge is unsmooth and ruleless. The delicate structure of BiOCl nanoflakes was further characterized by TEM (Figure 2c,d). The corresponding selected area electron diffraction (SAED) pattern image of BiOCl (inset of Figure 2d) clearly exhibits the single crystalline nature of the nanoflakes, and the layered structure is perpendicular to the c axis. The high-resolution transmission electron microscopy (HRTEM) image of a single BiOCl nanoflake (Figure 2d) exhibits good crystalline and clear lattice fringes. The lattice interplanar spacing is 0.275 nm, corresponding to the {110} planes of the tetragonal system of BiOCl. The HRTEM observation also confirms that these BiOCl lamellae are perpendicular to the c axis, which is consistent with the XRD result (as illustrated in Figure 1a). Interestingly, in the presence of 0.45 mmol of citric acid, the nanoplate structures were maintained. However, their images are a little different from those shown in Figure 2. As shown in Figure 3, the main morphology is two-dimensional (2D)

Figure 4. FESEM and TEM patterns of the BiOCl nestlike or flowerlike architecture obtained at 80 °C for about 3 h in the presence of 0.75 mmol of citric acid. FESEM images of the as-synthesized BiOCl nestlike structure: (a) panorama, (b) and (c) magnification; (d) a magnified image of single typical nestlike structure. Inset: its corresponding SAED pattern and HRTEM image of the edge of the BiOCl nestlike structure.

demonstrates that the as-obtained product consisted of numerous monodispersed microstructures with a good uniformity and narrow size distribution. The average size of the microstructures is about 1.5 μm. No other morphologies can be observed, indicating a high yield of these microstructures. FESEM images observed from different angles of view are shown in Figure 4b,c. It can be found that the microstructures actually exhibit a hierarchical nestlike architecture and a concave part exists in the middle section from the top view of an individual nest (Figure 4c). A careful observation (Figure 4c) reveals that the BiOCl hierarchical nestlike architectures, acting as the secondary structure, are constructed from four sides by many 2D nanosheets with a thickness of about 30 nm and a width of about 0.5−1.5 μm. The primal structures of nanosheets are densely packed with each other to form a nest through self-assembly. The TEM image (Figure 4d) also confirmed this flower-like structure. Figure 4d shows the HRTEM pattern of a single microflower. The clear lattice stripe indicates that these building blocks are well crystallized and the interplanar distance of the lattice is 0.275 nm, which is also consistent with the {110} planes of the tetragonal system of BiOCl. This result is consistent with the XRD, as illustrated in Figure 1c. With the addition of 1.20 mmol of citric acid, monodispersed hierarchical BiOCl hollow microspheres (HMSs) of 1−2 μm in size were obtained at 80 °C for about 3 h (Figure 5). Higher magnification microscopy (Figure 5b) exhibits detailed surface information of the three-dimensional (3D) BiOCl HMSs. The shell was composed of numerous radically grown nanosheets with a thickness of about 20 nm, forming a porous and loosepacked surface with an open hierarchical structure. The broken

Figure 3. FESEM and TEM patterns of the as-synthesized BiOCl nanoplates at 80 °C for about 3 h in the presence of 0.45 mmol of citric acid. FESEM images of the as-synthesized BiOCl nanoplates: (a) panorama and (b) magnification; (c) low magnification TEM image of the as-synthesized BiOCl nanoplates; (d) a magnified image of single typical nanoplate. Inset: its corresponding SAED pattern and HRTEM image of the edge of BiOCl nanoplate.

nanoplates with an average size of 20 nm in thickness, and these nanoplates have a rough surface and tend to form a square structure (Figure 3c,d). The TEM image (Figure 3c) also confirms the nanoplate structures. Figure 3d shows the HRTEM pattern of an individual nanoplate. The clear SAED pattern (inset in Figure 3d) confirms the well-crystallized 796



Article

Information, only sphere-like BiOCl structure with an average diameter of 1.5−2.5 μm (Figure S1g, Supporting Information) was obtained in the presence of 1.20 mmol and without PVP. The microspheres were composed of numerous radically grown nanoflakes with an average size of about 300 nm in length and with an average size of 40 nm in thickness (Figure S1h, Supporting Information). This result demonstrated that PVP played a key role in the formation of hierarchical BiOCl nestlike and hollow structure. The X-ray diffraction (XRD) pattern of the obtained samples can be also indexed as a BiOCl phase with tetragonal structure (JCPDS 06-0249) (Figure S2, Supporting Information), with lattice constants of a = b = 3.891 Å and c = 7.369 Å. Interestingly, in the absence of PVP and citric acid, the diffraction intensities of {001} facets family such as (001), (002), and (003) are much stronger than those of other facets (Figure S2a, Supporting Information), which is similar to those in the XRD in Figure 1a. With increasing the moles of citric acid, the diffraction intensities of (101), (110), and (102) facets increase and become much stronger than those of the {001} facets family such as (001), (002), and (003) (Figure S2b−d, Supporting Information). However, the diffraction intensities of different planes (Figure S2b−d, Supporting Information) changed a little, and the ratios of the diffraction intensities between (110) planes and the other planes such as (110)/ (001), (110)/(002), and (110)/(003) became much lower, compared to those of the as-prepared samples without adding citric acid (Figure S2a, Supporting Information). As shown in Figure S1, Supporting Information, in the presence of different moles of citric acid and without adding PVP, different structures were obtained, and all these products with different structures may exist as different crystals-oriented, specific exposed surfaces and different structures to the XRD beam, and therefore different diffraction intensities of facets were detected in the XRD pattern for all of the products (a−d) (as shown in Figure S2, Supporting Information). From the above discussion, as shown in Figure S1, Supporting Information, without adding PVP in the reaction system, only BiOCl nanoplates and microspheres were obtained. However, with adding PVP and different concentrations of citric acid in the reaction system, BiOCl nanoflakes (Figure 2), nanoplates (Figure 3), nestlike microflowers (Figures 4 and 6), and hollow microspheres (Figures 5 and 7) were obtained. In this reaction system, citric acid appears to be a good capping agent for adjusting the relative activity of the cations (Bi3+) and retarding the grain growth of semiconductors. It also indicated that the surfactant (PVP) present in the solution played a key role in the typical attachment of the nanomaterials for assembling a nestlike and hollow structure, and this typical attachment was termed oriented attachment (IOA). Therefore, we speculate that, in the present case, the formation of the may be attributed to the strong synergetic effect between citric acid and PVP. And many researchers37,38 have reported that a synergetic effect is very important for synthesizing nanomaterials with different shapes. In order to investigate the formation mechanism of hierarchical nestlike and hollow mircospheres of BiOCl, the growth process has been studied by taking FESEM images of the products at different intervals of reaction time. As shown in Figure 6, with increasing time, it clearly shows that only a nestlike BiOCl structure built by small 2D nanosheets was obtained. The hatch of these nests is square, and the average diameter of the sides is about 1.5 μm. A concave part can be observed in the middle section from the top view of an

Figure 5. FESEM and TEM patterns of the as-synthesized BiOCl hollow microspheres (HMSs) at 80 °C for about 3 h with 1.20 mmol of citric acid: (a) and (b) FESEM image of the as-synthesized BiOCl hollow microspheres (HMSs); (c) an overall TEM image of the assynthesized BiOCl hollow microspheres (HMSs); (d) a magnified image of hollow microspheres (HMSs). Inset: its corresponding SAED pattern.

microsphere (Figure 5b) also reveals its hollow feature. Transmission electron microscopy (TEM) images (Figure 5c,d) further display the apparent hollow microspheres with a shell thickness of about 150 nm. The obvious contrast between the dark and relatively bright parts also confirms their hollow nature (Figure 5d). The corresponding diffraction ring of the SAED pattern (inset of Figure 5d) identifies the polycrystalline nature of BiOCl HMSs. A representative TEM micrograph (shown in Figure 5c,d) indicates the resulting microspheres possess a hollow structure with an average diameter of 1−2 μm and an average wall thickness of 150 nm. The large hollow space inside the microspheres greatly decreases the density of BiOCl HMSs, so they can be easily suspended in the water. In the past few years, PVP has been applied as an important surfactant for the synthesis of nanomaterials, and various nanostructures were successfully fabricated with the assistance of PVP.35,36 It was proposed that the surfactant (PVP) present in the solution played a key role for the typical attachment of the nanomaterials, and this typical attachment was termed as imperfect oriented attachment (IOA). 36 In this study, the influence of PVP as a surfactant for the formation of hierarchical BiOCl nestlike and hollow structure was investigated. In the absence of surfactant PVP and citric acid, only BiOCl nanoflake structure with an average diameter of 1− 2 μm (Figure S1a,b, Supporting Information) was obtained, and the surface structure is different from that in Figure 2. The BiOCl products are asymmetric round nanoflakes with a smooth surface and increased average thickness. Interestingly, in the presence of 0.45 mmol of citric acid and without PVP (as shown in Figure S1c,d, Supporting Information), the main morphology is 2D nanoplates with an average size of 20 nm in thickness. It can be observed that these nanoplates have a rough surface and tend to form a square structure which is the same as those shown in the images of Figure 3a,b. However, with a further increase of the citric acid (0.75 mmol), the images (Figure S1e,f, Supporting Information) of the obtained square nanoplate structures are the same as those shown in Figure S1c,d, Supporting Information, but are different from those shown in Figure 4. As shown in Figure S1g,h, Supporting 797



Article

(3 h), the small nanoparticles on the surface of BiOCl hollow microspheres (Figure 7a) started to dissolve into the solution and grew onto large nanoplates of BiOCl via a process known as Ostwald ripening.40 However, this phenomenon is not so distinct. From what has been discussed above, it would be reasonable to believe that BiCl3 can be easily hydrolyzed to form a self-assembly structure induced by citric acid. As shown in Scheme 1a, the BiOCl structure is one of the simplest members of the Sillen family expressed by [M2O2][Clm] or [M3O4+n][Clm] (m = 1−3) where bismuth oxide based fluorite-like layers, [M2O2][Clm] or [M3O4+n][Clm], are intergrown with [Bi2O2] slabs interleaved by double slabs of halogen atoms such as (Cl−Bi−O−Bi−Cl)n (herewith n is an integer).21,41 For these reasons, the oxyhalide compounds can easily form the 2D laminar structure. To further probe the vibrational and structural properties of BiOCl crystals, the Raman spectra (Figure 8) are presented for BiOCl in the range Figure 6. FESEM images of the BiOCl products collected at different reaction time intervals in the presence of citric acid (0.75 mmol) at 80 °C. (a) 1 min; (b) 4 min; (c) 3 h; (d) 6 h.

individual nest (Figure 6). A careful observation reveals that all of the BiOCl hierarchical nestlike architectures are constructed from four sides by many 2D nanosheets with a thickness of about 30 nm and a width of about 0.5−1.5 μm. Interestingly, the BiOCl hollow microspheres were formed in the presence of 1.20 mmol citric acid, as shown in Figure 7. Only a sphere-like

Figure 8. Raman spectra of the as-synthesized BiOCl products obtained at 80 °C for about 3 h in the presence of different moles of citric acid: (a) 0 mmol, (b) 0.45 mmol, (c) 0.75 mmol, and (d) 1.20 mmol.

of 50−500 cm−1. As mentioned above (Figure 1), the isostructural BiOCl is a tetragonal PbFCl-type structure with space group P4/nnm. For such a structure of space group D4h,7 with two molecular formulas per unit cell, the Raman active modes are two A1g, B1g, and Eg. For exploring the correlation method of this kind of structure, the optical modes are brought forward by Fateley et al.42 and Cao et al.21 as follows:

Figure 7. FESEM images of the BiOCl products collected at different reaction time intervals in the presence of 1.20 mmol of citric acid at 80 °C. (a) 1 min; (b) 4 min; (c) 3 h; (d) 6 h.

Γ = 2A1g + 2A2u + B1g + 3Eg + 2E u where the g modes are Raman-active only and the u modes are infrared-active only. Figure 8 shows the Raman spectra of the as-synthesized BiOCl products obtained at 80 °C for about 3 h in the presence of different moles of citric acid. As shown in Figure 8, the Raman spectrograms consist of three distinguished bands (59.03 cm−1, 142.3 cm−1, 198.9 cm−1) and one weak band (394.89 cm−1). Since symmetric vibrations usually give rise to more intense Raman bands than asymmetric vibrations, the bands at 59.03 cm−1 are assigned to the A1g internal Bi−Cl stretching mode, and the bands at 142.3 cm−1 are also assigned to the A1g internal Bi−Cl stretching mode. The band of 198.9 cm−1 can be assigned to the Eg internal Bi− Cl stretching mode, while Eg external Bi−Cl stretching is probably masked by the strong band at 142.3 cm−1. In addition, there is a weak and readily unnoticeable band at 398.6 cm−1

BiOCl structure with an average diameter of 1.5 μm was obtained at different intervals of reaction time. However, from the FESEM image of the sample produced at 1 min (shown in Figure 7a), it can be clearly observed that many nanoplates adsorbed on the surfaces of hollow microspheres. When the reaction time was extended to 3 h, the nanoplates on the surfaces disappeared. From the above experimental observations, we believe that formation of the BiOCl hollow microspheres can be explained by a kinetically controlled nucleation− dissolution−recrystallization mechanism.39 At the initial synthesis process, the ultrafine precursor grains tended to deposit due to their poor solubility in the water, and then they further agglomerated into hollow microspheres for minimizing the surface tension (Figure 7a). With increasing reaction time 798



Article

which is assigned to the Eg and B1g modes brought by the motion of the oxygen atoms. The band wavenumbers of BiOCl shown in Figure 8 are smaller than the reported values43 (A1g at 146 and 60 cm−1, Eg at 202 and 400 cm−1, B1g at 400 cm−1), which probably derives from the stronger orientation of crystal structure and structure change (from nanoflakes, nanoplates, microflowers to HMSs). Also, the band intensities of the samples obtained in the presence of citric acid increased remarkably. The band intensity increase is probably due to the sizes and different morphologies between them (nanosheets, nanoplates, nestlike, HMSs), and the same phenomenon was observed in the plate-like and flower-like of BiOCl.14 UV−visible diffuse reflectance spectroscopy (DRS) was used to characterize the electronic states of the as-prepared BiOCl samples. Figure 9 shows that all the BiOCl samples prepared Figure 10. Nitrogen adsorption−desorption isotherms (A) and pore size distributions (B) of the as-synthesized BiOCl products obtained at 80 °C for about 3 h in the presence of different moles of citric acid: (a) 0 mmol, (b) 0.45 mmol, (c) 0.75 mmol, (d) 1.2 mmol, and (e) a common BiOCl product obtained at 80 °C for about 3 h without adding citric acid and PVP.

80 °C for about 3 h in the presence of different moles of citric acid. The isotherms can be nearly categorized as type IV with a distinct hysteresis loop observed in the range of 0.5−1.0 P/P0, which is characteristic of mesoporous materials, as shown in the inset image in Figure 10. The BET specific surface areas of the samples synthesized in the presence of different moles of citric acid were calculated from N2 isotherms at 77 K and were found to be 10.08 m2/g (nanoflakes), 14.81 m2/g (nanoplates), 19.39 m2/g (nestlike), and 22.21 m2/g (hollow microspheres), respectively. However, the common BiOCl sample synthesized without adding citric acid and PVP has a very low surface area (4.16 m2/g). The corresponding average pore diameter of products are 11.49 nm, 15.65 nm, 20.23 nm, 20.59 nm, and 19.34 nm, determined by the Barret−Joyner−Halenda (BJH) method (inset in Figure 10). Such BiOCl hollow microspheres with large surface areas and a 3D connected pore system play an important role in catalyst design due to their capacity to improve the molecule transport of reactants and products. Also, the pores on the shell surface can serve as the light-transfer paths for distribution of photo energy onto the inner surfaces of porous frameworks, so the light activation can be significantly enhanced due to the multiple reflections of light within the interior cavity (as shown in Scheme S1, Supporting Information). Such multiple reflections can improve the photoabsorption efficiency of the catalyst. Thus, a more efficient utilization of light can be obtained for improving photocatalytic activity. A similar phenomenon has been reported.45 The schematic representation of the BiOCl crystal structure can be seen in Scheme 1a. BiOCl crystal has a layered structure characterized by [Bi2O2] slabs which are interleaved by double slabs of Cl atoms, and this layered structure provides the large space to separate and conduct the photogenerated hole−electron pairs efficiently, which can enhance the photocatalytic activities.46 On the basis of the above discussions, the speculated process for the formation of BiOCl can be brought out as the following equations. First, Bi3+ was produced after BiCl3 was dissolved in hydrochloric acid (eq 1). When BiCl3 solution was added dropwise into the reaction system, the

Figure 9. UV−vis absorbance spectra of the as-synthesized BiOCl products obtained at 80 °C for about 3 h in the presence of different moles of citric acid: (a) 0 mmol, (b) 0.45 mmol, (c) 0.75 mmol, and (d) 1.20 mmol. Inset: plots of (A × hν)1/2 versus energy (hν) for the band gap energy of corresponding samples.

with the addition of different moles of citric acid exhibited stronger light adsorptions, compared to the sample prepared without adding citric acid. This indicates that the citric acid can be utilized as a structure directing agent for fabricating nanoflakes, nanoplates, microflowers, and hollow microspheres to effectively increase the light adsorption. The band energy gaps of the as-prepared samples could be calculated by using (αhν)n = k(hν − Eg), where α is the absorption coefficient, k is the parameter that is related to the effective masses associated with the valence and conduction bands, n is 1/2 for a direct transition, hν is the absorption energy, and Eg is the band gap energy.34,44 Plotting (αhν)1/2 versus hν based on the spectral response in the inset image of Figure 9 gave the extrapolated intercept corresponding to the Eg value. The optical band energies of the BiOCl hierarchical samples are 3.11, 3.12, 3.15, and 3.27 eV, respectively. The enhanced capacity of hierarchical porous BiOCl hollow microspheres to absorb ultraviolet light makes them promising effective photocatalysts. The specific surface area and porosity of the BiOCl nanoflowers were investigated by using nitrogen adsorption and desorption isotherms. Figure 10 shows representative N2 adsorption−desorption isotherms and distributions of pore diameters of the as-synthesized BiOCl products obtained at 799



Article

mixed solution became colorless and transparent under stirring. Free Bi3+ can combine with citric anions via the chelating effect to produce [C6O7H5]3−Bi3+ (eq 2).30,31 When BiCl3 solution was added dropwise into the reaction system, Bi3+ will be released slowly from [C6O7H5]3−Bi3+ and gradually converted into BiOCl via a self-hydrolysis process (eq 3) since BiOCl is more stable than [C6O7H5]3−Bi3+ in solution. The intrinsic nucleation and anisotropic growth of the BiOCl can form different microstructures with various surface areas. However, in the absence of citric acid, owing to no chelating effect and the higher pH value,47 the hydrolysis of Bi3+ occurs rapidly (eq 3), resulting in the nonuniform crystal growth and larger size of BiOCl nanoflakes with obvious agglomeration (as shown in Figure 2).

BiCl3 → Bi 3 + + 3Cl−

(1)

Bi 3 + + C6O7 H8 → [C6O7 H5]3 − Bi 3 + + 3H+

(2)

Scheme 2. Formation of Different BiOCl Nano-/ Microstructures under Various Concentrations of Citric Acid

[C6O7 H5]3 − Bi 3 + + H2O + H+ + Cl− the presence of the citric acid (0.45 mmol), citrate molecule adsorbed selectively on the surface of BiOCl nanoparticles. Hydrogen bonds were formed between the carboxyl of citric acid and the hydroxyl ions in [C6O7H5]3−Bi3+ (eqs 2 and 3); otherwise hydrogen bonds were also formed between the hydroxyl ions in [C6O7H5]3−Bi3+.30,31 The hydrolysis speed of Bi3+ could be controlled. However, only a few hydrogen bonds were formed, allowing the BiOCl crystallites to grow along one direction to form square nanoplates (as shown in Figure 3). With increasing the [C6O7H5]3− concentration (0.75 mmol), it is believed that the integrating force in the superstructure is the strong hydrogen bonds between the contacting lateral surfaces among the nanoplates. This led the primal nanoplates to densely pack with each other from four sides to form a nest through self-assembly for minimizing the surface tension (Figure 4). However, further increasing the citric acid (1.2 mmol) resulted in more hydrogen bonds. The binding on the surface of nanoplates was too large and it inhibited the growth along one direction to some extent. Consequently, smaller nanoplates were formed (Figure 5). What is more, the interactions between the nanoplates grew strong enough for self-assembling into 3D BiOCl HMSs (Figure 5b) because of the hydrogen bonds between nanoplates. Otherwise, it was reported35,36 that PVP was a very important structure-directing agent. In our experiment (Figure S1, Supporting Information), only BiOCl nanoplates and microspheres were obtained without adding PVP in the reaction system, which means that PVP played a key role in the formation of hierarchical nestlike and hollow structures. In presence of PVP and different concentrations of citric acid, the morphology of BiOCl gradually changed from nanoflakes, nanoplates, nestlike to hollow microspheres (Figure S1, Supporting Information). Therefore, we speculate that the surfactant (PVP) present in the solution has played a crucial role in the formation of the interactions between the nanoplates for self-assembling flower-like and hollow microspherical superstructures. In the present case, the formation of the nestlike and hollow structure may be attributed to the strong synergetic effect between citric acid and PVP. Determining the exact nature of the growth mechanism will require further theoretical and experimental work. The photocatalytic decomposition of RhB using various BiOCl samples was investigated under ultraviolet light

→ BiOCl + C6O7 H8 (3) Citric acid is an important biological ligand for metal ions and can form strong complexes with Bi3+, Al3+, Ca2+, Fe3+, Zn2+, Mg2+ ions, etc.29−33 It has been utilized as a common green organic acid for fabricating inorganic nanomaterials such as BiFeO3 hollow microspheres,30 Ni(OH)2,48 calcite,49 doughnut-shaped ZnO microparticles,33 etc. Self-assembly is believed to be an effective strategy to form hierarchical superstructures. Some researchers reported that self-assembly of nanocrystals is driven by van der Waals forces and hydrogen bonding among the certain organic molecules on the surface of nanomaterials.50,51 For synthesis of BiOCl crystals, citric acid is one main agent that affects the nucleation, the growth behavior of crystals, and the self-assembly of microcrystals because citric acid as an organic acid can be hydrolyzed to [C6O7H5]3− and H+; it has been suggested that [C6O7H5]3− ions may chelate with Bi3+ to produce [C6O7H5]3−Bi3+,30,31 in the reaction system hydrogen bonds were formed between the carboxyl of citric acid and the hydroxyl ions in [C6O7H5]3−Bi3+ (eqs 2 and 3), and meanwhile hydrogen bonds were also formed between the hydroxyl ions in [C6O7H5]3−Bi3+ (eqs 2 and 3); otherwise citric acid could absorb on the surface of nanoparticles or nanosheets,50 and the surface citric acid could also combine with [C6O7H5]3−Bi3+ in the reaction solution to form hydrogen bonds. Therefore, the growth behavior of crystals and the selfassembly of BiOCl microcrystals could be controlled by adjusting the concentration of citric acid. Some researchers have reported the effect of the concentration of citric acid in producing nanocrystals.29,32 In our experiment, we believe that the BiOCl microcrystals were formed through a threestep mechanism. The crystal growth seemed to involve the nucleation of BiOCl nanoparticles, the growth of sheets microcrystal through the oriented attachment of BiOCl nanoparticles, and the formation of a 3D hierarchical structure (nestlike and HMSs) by the self-assembly of BiOCl nanoplates. The BiOCl crystals presented here have uniform shapes and sharp edges, revealing the equilibrium states of the BiOCl crystals. The plausible growth mechanism of BiOCl microcrystals by the oriented attachment and self-assembly is illustrated in Scheme 2. In the absence of citric acid (0 mmol), BiCl3 quickly reacted with deionized water and produced to form tetragonal matlockite BiOCl nanoflakes. In 800



Article

irradiation at room temperature. Figure 10A displays the temporal evolution of the absorption spectra of an RhB solution degraded by BiOCl hollow microspheres under ultraviolet light irradiation. The characteristic absorption of RhB at 553 nm was used to monitor the photocatalytic degradation process. It can be found that the intensity of the absorption peak of RhB decreased gradually with the increase of irradiation time and almost completely disappeared after 75 min, accompanied by a shift of the main absorption peak to lower wavelength. Meanwhile, the color of the RhB solution changed from initially red to light yellow-green, and then to transparent (inset of Figure 10A). The hypsochromic shift of the major absorption peak may be attributed to a step-by-step de-ethylation of RhB.52 At last, no new absorption bands show the complete photodegradation of RhB. The ratio of RhB concentration C and initial RhB concentration C0 (C/C0) versus the degradation time was shown in Figure 11B. It can be found that RhB concentration kept constant with no BiOCl added (Figure 11B(a)) and decreased remarkably with BiOCl added under exposure to ultraviolet light due to high photoactivities of the BiOCl. Compared to the common BiOCl (Figure 11B(b)), the hollow microsphere BiOCl (Figure 11B(f)) has the highest photoactivity. The photocatalytic decomposition process is rapid in the early stage, with a degradation of about 50% RhB after only 15 min. After irradiation for 75 min, the conversion is close to 99.5% for the hollow microsphere BiOCl and 60% for common BiOCl. Additionally, the nanoflakes (Figure 11B(c)) and nanoplates (Figure 11B(d)) show lower photoactivity, compared to the assembled hierarchical nestlike structure (Figure 11B(e)). Besides the higher activity, the hollow BiOCl microspheres also show strong durability during liquid phase photocatalytic degradation of RhB. As shown in Figure 11C, the catalyst can be used repetitively for more than 4 times. A slight decrease in activity could be mainly attributed to the loss of catalyst during recycling and washing process. The excellent durability can be attributed to the high thermal stability and high photoabsorption efficiency. The remarkable contrast of the photocatalytic activities between the flake-like, plate-like, and hierarchical (nestlike, hollow-sphere) samples is owing to the hierarchical structure assembled from nanoplates, which has the ability to increase their interfacial area by incorporating RhB into the interlayer space.53 It is also interesting that the photocatalytic performance of the nestlike structure sample is a little different from that of the hollow one. The building blocks of the nestlike sample are self-assembled from four sides by nanoplates (Figure 3b,c), while the hollow microspheres are cross-link nanoplates (Figure 4a,b). This special hierarchical structure may eventually result in higher photoactivity. Photoluminescence (PL) spectra were further used to investigate the efficiency of charge carrier trapping, migration, and transfer of the as-prepared samples to understand the fate of electron−hole pairs in semiconductor particles. 54 As shown in Figure 12, the PL spectra of the BiOCl samples obtained with an excitation wavelength of 320 nm display an emission peak in the range of 340−420 nm. The peak intensities are strongly dependent on the recombination between photoinduced electrons and holes.55 Among them, the hollow BiOCl microsphere sample shows the lowest PL intensity. This implies that the electron−hole recombination could be effectively pro-

Figure 11. (A) The temporal evolution of the absorption spectra of the RhB solution with BiOCl hollow microspheres obtained in the presence of 1.2 mmol of citric acid under exposure to ultraviolet light (inset: photo images of degraded RhB solutions at different time intervals). (B) Photocatalytic performances of various samples: no BiOCl (a); common BiOCl obtained without adding citric acid and PVP (b); BiOCl obtained in the presence of 0 mmol of citric acid (nanoflakes) (c), 0.45 mmol of citric acid (nanoplates) (d), 0.75 mmol of citric acid (nestlike) (e), and 1.2 mmol of citric acid (hollow microspheres) (f), respectively. (C) Cycling degradation rate (C0 = 20 mg/L, 0.1 g of BiOCl).

hibited, so the quantum efficiency of hollow BiOCl samples is improved. 801



■

4. CONCLUSIONS In summary, we present a simple and effective method for the synthesis of bismuth oxychloride (BiOCl) sub-microcrystals with tunable morphologies. The morphology of BiOCl crystals strongly depends on the concentration of citric acid and can be directed from nanoflakes, nanoplates, nestlike to hollow microspheres. Citric acid as a structure-directing agent for producing different structures of BiOCl microcrystals was discussed. PVP also plays a key role in the formation of hierarchical nestlike and hollow structure. The photocatalytic activity of the BiOCl sub-microcrystals largely depends on the morphology of the products. This work also provides a novel and easy pathway for the synthesis of semiconductors with controllable structures, serving as a promising method for preparing other metal oxyhalide, such as BiOF, BiOBr, and BiOI, etc. Meanwhile, these BiOCl crystals with tunable morphologies show promise as building blocks in catalysts, sensors, and other devices. ASSOCIATED CONTENT

S Supporting Information *

Additional FESEM images of BiOCl, XRD, light multireflections in the hollow structured, morphology of BiOCl crystals. These materials are available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (2) Sun, S. H.; Murray, C.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (3) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (4) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 7055, 121. (5) Han, J. T.; Huang, Y. H.; Wu, X. J.; Wu, C. L.; Wei, W.; Peng, B.; Huang, W.; Goodenough, J. B. Adv. Mater. 2006, 18, 2145−2148. (6) Ye, L. Q.; Zan, L.; Tian, L. H.; Peng, T. Y.; Zhang, J. J. Chem. Commun. 2011, 47, 6951−6953. (7) Geng, B. Y.; Ma, J. Z.; You, J. H. Cryst. Growth Des. 2008, 8, 1443−1447. (8) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (9) Jiang, Z. Y.; Kuang, Q.; Xie, Z. X.; Zheng, L. S. Adv. Funct. Mater. 2010, 20, 3634−3645. (10) Zhang, X.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z. J. Phys. Chem. C 2008, 112, 747−753. (11) Dellinger, T. M.; Braun, P. V. Scr. Mater. 2001, 44, 1893−1897. (12) Henle, J.; Simon, P.; Frenzel, A.; Scholz, S.; Kaskel, S. Chem. Mater. 2007, 19, 366−373. (13) Ma, J. M.; Liu, X. D.; Lian, J. B.; Duan, X. C.; Zheng, W. J. Cryst. Growth Des. 2010, 10, 2522−2527. (14) Lei, Y. Q.; Wang, G. H.; Song, S. Y.; Fan, W. Q.; Zhang, H. J. CrystEngComm 2009, 11, 1857−1862. (15) Deng, Z. T.; Tang, F. Q.; Muscat, A. J. Nanotechnology 2008, 19, 295705. (16) Deng, Z. T.; Chen, D.; Peng, B.; Tang, F. Q. Cryst. Growth Des. 2008, 8, 2995−3003. (17) Deng, H.; Wang, J. W.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Eur. J. 2005, 11, 6519−6524. (18) Yuan, R. S.; Lin, C.; Wu, B. C.; Fu, X. Z. Eur. J. Inorg. Chem. 2009, 2009, 3537−3540. (19) Zhu, L. P.; Liao, G. H.; Bing, N. C.; Wang, L. L.; Yang, Y.; Xie, H. Y. CrystEngComm 2010, 12, 3791−3796. (20) Zhang, J.; Shi, F. J.; Lin, J.; Chen, D. F.; Gao, J. M.; Huang, Z. X.; Ding, X. X.; Tang, C. C. Chem. Mater. 2008, 20, 2937−2941. (21) Cao, S. H.; Guo, C. F.; Lv, Y.; Guo, Y. J.; Liu, Q. Nanotechnology 2009, 20, 275702. (22) Peng, H. L.; Chan, C. K.; Meister, S.; Zhang, X. F.; Cui, Y. Chem. Mater. 2008, 21, 247−252. (23) Kijima, N.; Matano, K.; Saito, M.; Oikawa, T.; Konishi, T.; Yasuda, H.; Sato, T.; Yoshimura, Y. Appl. Catal. B: Gen. 2001, 206, 237−244. (24) Ghosh, R.; Maiti, S.; Chakraborty, A. Tetrahedron Lett. 2004, 45, 6775−6778. (25) Briand, G. G.; Burford, N. Chem. Rev. 1999, 99, 2601−2658. (26) Wang, C. H.; Shao, C. L.; Liu, Y. C.; Zhang, L. N. Scr. Mater. 2008, 59, 332−335. (27) Kusainova, A. M.; Lightfoot, P.; Zhou, W. Z.; Stefanovich, S. Y.; Mosunov, A. V.; Dolgikh, V. A. Chem. Mater. 2001, 13, 4731−4737. (28) Buxbaum, G.; Pfaff, G. Industrial Inorganic Pigments; WileyVCH: Verlag GmbH, 2005. (29) Gao, F.; Chen, X. Y.; Yin, K. B.; Dong, S.; Ren, Z. F.; Yuan, F.; Yu, T.; Zou, Z. G.; Liu, J. M. Adv. Mater. 2007, 19, 2889−2892. (30) Huo, Y. N.; Jin, Y.; Zhang, Y. J. Mol. Catal. A: Chem. 2010, 331, 15−20. (31) Parkinson, J. A.; Sun, H. Z.; Sadler, P. J. Chem. Commun. 1998, 881−882. (32) Luo, W.; Zhu, L. H.; Wang, N.; Tang, H. Q.; Cao, M. J.; She, Y. B. Environ. Sci. Technol. 2010, 44, 1786−1791. (33) Liang, J. B.; Liu, J. W.; Xie, Q.; Bai, S.; Yu, W. C.; Qian, Y. T. J. Phys. Chem. B 2005, 109, 9463−9467. (34) Zhang, K. L.; Liu, C. M.; Huang, F. Q.; Zheng, C.; Wang, W. D. Appl. Catal. B: Environ. 2006, 68, 125−129. (35) Huang, J. H.; Gao, L. Cryst. Growth Des. 2006, 6, 1528−1532. (36) Jyotiranjan., Ota; Roy, P.; Srivastava, S. K.; Nayak, B. B.; Saxena, A. K. Cryst. Growth Des. 2008, 8, 2019−2023.

Figure 12. PL spectra of common BiOCl obtained without adding citric acid and PVP (a) and BiOCl obtained in the presence of 0 mmol of citric acid (nanoflakes) (b), 0.45 mmol of citric acid (nanoplates) (c), 0.75 mmol of citric acid (nestlike) (d), and 1.2 mmol of citric acid (hollow microspheres) (e), respectively.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-64324528. Fax: +86-21-64322511. E-mail: xibinyu@ shnu.edu.cn.

■

ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from Innovation Program of Shanghai Municipal Education Commission (10YZ70, 09ZZ136), Science Foundation of Shanghai Normal University (SK201002), Shanghai Science and Technology Development Fund (No. 09520500500), and the Key Laboratory of Resource Chemistry of Ministry of Education of China. 802



Article

(37) Han, Y.; Liu, S.; Han, M.; Bao, J.; Dai, Z. Cryst. Growth Des. 2009, 9, 3941−3947. (38) Zhang, K.; Zhang, D. Q.; Liu, J.; Ren, K. X.; Luo, H.; Peng, Y. J.; Li, G. S.; Yu, X. B. CrystEngComm 2012, 14, 700−707. (39) Xi, G. C.; Xiong, K.; Zhao, Q. B.; Zhang, R.; Zhang, H. B.; Qian, Y. T. Cryst. Growth Des. 2006, 6, 577−582. (40) Tian, X. L.; Li, J.; Chen, K.; Han, J.; Pan, S. L. Cryst. Growth Des. 2009, 9, 4927−4932. (41) Kusainova, A. M.; Zhou, W. Z.; Irvine, J. T. S.; Lightfoot, P. J. Solid State Chem. 2002, 166, 148−157. (42) Fateley, W. G.; McDevitt, N. T.; Bentley, F. F. Appl. Spectrosc. 1971, 25, 155−173. (43) Davies, J. E. D. J. Inorg. Nucl. Chem. 1973, 35, 1531−1534. (44) Butler, M. A. J. Appl. Phys. 1977, 48, 1914−1920. (45) Li, H. X.; Bian, Z. F.; Zhu, J.; Zhang, D. Q.; Li, G. S.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 8406−8407. (46) Shang, M.; Wang, W. Z.; Zhang, L. J. Hazard. Mater. 2009, 167, 803−809. (47) Shang, M.; Wang, W. Z.; Sun, S. M.; Zhou, L.; Zhang, L. J. Phys. Chem. C 2008, 112, 10407−10411. (48) Meyer, M.; Bée, A.; Talbot, D.; Cabuil, V.; Boyer, J. M.; Répetti, B.; Garrigos, R. J. Colloid Interface Sci. 2004, 277, 309−315. (49) Meldrum, F. C.; Hyde, S. T. J. Cryst. Growth 2001, 231, 544− 558. (50) Feng, Y. L.; Lu, W. C.; Zhang, L. M.; Bao, X. H.; Yue, B. H.; lv, Y.; Shang, X. F. Cryst. Growth Des. 2008, 8, 1426−1429. (51) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (52) Fu, H. B.; Pan, C. S.; Yao, W. Q.; Zhu, Y. F. J. Phys. Chem. B 2005, 109, 22432−22439. (53) Osterloh, F. E. Chem. Mater. 2007, 20, 35−54. (54) Dong, F.; Zhao, W. R.; Wu, Z. B. Nanotechnology 2008, 19, 365607. (55) Tang, H.; Berger, H.; Schmid, P. E.; Lévy, F.; Burri, G. Solid State Commun. 1993, 87, 847−850.

803


BiOCl Sub-Microcrystals Induced by Citric Acid and Their High

Recommend Documents