A. Synthesis

All chemicals (Sinopharm Chemical Reagent Co., Ltd.) were of analytical reagent grade and used as received without further purification. For a typical synthesis, Bi₂O₃ and Bi(NO₃)₃·5H₂O were used as raw materials, and BiOCl appeared as an intermediate product. The synthesis of BiOCl was described as below. Firstly, 0.02 mole of Bi(NO₃)₃·5H₂O was dissolved in 20 ml of glacial acetic acid (CH₃COOH). After being magnetically stirred for 15 min, solution I was obtained. Secondly, NaCl (0.02 mole) and sodium acetate (CH₃COONa, 0.03 mole) were put into deionized water (200 ml). The mixture solution was magnetically stirred for 40 min and solution II was obtained. Thirdly, solution I was added into solution II drop by drop and the mixture solution was magnetically stirred for 12 h, a white suspension was formed. Finally, the suspension was centrifugally separated and washed with alcohol and deionized water for three times, respectively. After being dried at 60 °C for 10 h, white BiOCl powder was collected. The Bi₁₂O₁₇Cl₂ photocatalyst was prepared by a solid-state reaction as follows. As-prepared BiOCl powder (0.008 mole) and Bi₂O₃ powder (0.02 mole) were ground for 5 h and calcined at 600 °C in air for 12 h. The resulting yellow Bi₁₂O₁₇Cl₂ product was collected and ground for 15 min before further characterization.

B. Characterization

Formation of Bi₁₂O₁₇Cl₂ was confirmed by a Rigaku D/Max 2200 X-ray diffractometer (CuKα radiation λ = 1.5406 Å). Morphology of the Bi₁₂O₁₇Cl₂ powder was observed by a field-emission scanning electron microscope (JSM-6700F). Microstructures of Bi₁₂O₁₇Cl₂ sample were characterized by transmission electron microscopy (TEM, JEM-2100), high-resolution transmission electron microscope (HRTEM), and selected-area electron diffraction (SAED) with a 200 kV accelerating voltage. To confirm light absorption range of Bi₁₂O₁₇Cl₂ sample, diffuse reflectance spectra (DRS) in wavelength of 300–800 nm was measured by UV–vis spectrophotometer (Lambda 35).

C. Photodegradation performance test

The photocatalytic activity of the obtained Bi₁₂O₁₇Cl₂ sample was evaluated by degradation of MO under visible light. The photocatalyst-dye mixture solution was put in a quartz cell with a circulating water jack outside, which ensures the reaction proceeding at room temperature to prevent any thermal catalytic effect. The visible-light illumination was obtained by placing a 420 nm cut-off filter under a 300 W Xenon lamp. The concentration of initial MO solution is 10 mg l⁻¹, and the photocatalyst concentration in the MO solution is 1 mg ml⁻¹. Before irradiation, 50 ml suspension was ultra-sonicated for 10 min and magnetically stirred for 20 min, respectively, in dark for the achievement of adsorption–desorption equilibrium of MO on catalyst surfaces. During visible-light irradiation, about 5 ml suspension was taken from the reaction cell at given time intervals. All slurry samples containing catalyst were centrifuged for 5 min at the rate of 8000 rpm to remove the catalyst powder. MO concentration of the filtrate was determined using the UV–vis spectrometer, and the characteristic absorption of MO at 464 nm was used to monitor the photocatalytic degradation.

A. Morphology and microstructure

The morphology and microstructure of the Bi₁₂O₁₇Cl₂ sample was characterized by SEM and TEM. In Figure 1(a), it was observed that as-prepared Bi₁₂O₁₇Cl₂ is of regular plate-stacking structure constructed of dozens of nanosheets, which are about 10–50 nm in thickness and 200–1500 nm in in-plane size. These nanosheets with smooth surface and edges are described as secondary nanosheets in this study. Furthermore, the microstructures of the well-defined primary nanosheets are observed by TEM at higher magnifications. As shown in Figure 1(b), the distinct contrast of light and dark at the edge of a typical secondary nanosheet reveals that the secondary nanosheet is actually consisted of a parallel array of ultrathin nanosheets, which are named as primary nanosheets. The thickness of the primary nanosheets was in several nanometers and their boundaries are marked as “A–A”, “B–B”, and “C–C” in the inset illustration [Figure 1(b)]. To sum up, the as-synthesized Bi₁₂O₁₇Cl₂ nanomaterial was firstly observed as being composed of secondary nanosheets regularly arranged. And more significantly, the relatively thicker secondary nanosheets are plate-stacked of ultrathin primary nanosheets, which are parallelly aligned and closely superimposed. Herein, this distinct hierarchical Bi₁₂O₁₇Cl₂ microstructure in our work is designated as a novel plate-stratiform nanostructure.

Figure 1.

(colour online) (a) SEM and (b) TEM images of plate-stratiform nanostructured Bi₁₂O₁₇Cl₂.

B. Crystal imaging and diffraction

The X-ray diffraction (XRD) pattern of as-prepared sample is shown in Figure 2, which can be indexed to tetragonal Bi₁₂O₁₇Cl₂ with lattice parameters of a = 5.443 and c = 35.2 Å (JCPDS No. 37-0702). It can be seen that the sample is well crystallized. Meanwhile, few small peaks were unable to be indexed and denoted as impurity peaks. Further, the above-narrated plate-stratiform Bi₁₂O₁₇Cl₂ nanostructure was studied by SAED patterns and HRTEM images. Figure 3(a) is the SAED pattern of the typical secondary nanosheet shown in Figure 2(b). It is indexed as three zone axes [011], $[0\bar 71]$ , and $[05\bar 3]$ of tetragonal Bi₁₂O₁₇Cl₂ (JCPDS No. 37-0702). Three sets of diffraction spots are respectively allocated to three axes, which are identified by A, B, and C subscripts. In addition, arrays of other relatively weak diffraction spots could be explained by secondary diffraction principle, which is caused by the distinct plate-stratiform structure. The SAED pattern proves that the secondary nanosheet is stacked of several single-crystalline primary nanosheets and confirms the nature of layer-stacked structure. In the HRTEM image given in Figure 3(b), the interplanar distance of 0.272 nm is consistent with (200) planes, which are common to three sets of SAED diffraction spots marked in Figure 3(a). At the same time, a typical primary nanosheet is analyzed by HRTEM and SAED, as shown in Figure 4. Clear diffraction spots and lattice fringes reveal that the primary nanosheet is highly single-crystalline. Figure 4(a) is an ultrathin primary nanosheet located at the corner of a secondary nanosheet as pointed by the arrow. The corresponding SAED pattern [Figure 4(b)] is along [001], displaying (020), and (220) planes with an angle of 45°. In Figure 4(c), the lattice fringe spacing of 0.195 nm is consistent with (200) planes of tetragonal Bi₁₂O₁₇Cl₂.

Figure 2.

(colour online) XRD pattern of as-prepared Bi₁₂O₁₇Cl₂ sample.

Figure 3.

(colour online) SAED pattern (a) and HRTEM image (b) of a typical Bi₁₂O₁₇Cl₂ secondary nanosheet.

Figure 4.

(a) TEM image of a typical Bi₁₂O₁₇Cl₂ primary nanosheet as pointed by the arrow; (b) the corresponding SAED pattern; and (c) HRTEM image.

C. Optical absorption

DRS is often used to characterize optical absorption property, which is considered as the criteria for photocatalytic activities of semiconductors. Figure 5 exhibits the UV–vis diffuse reflectance spectrum of as-prepared Bi₁₂O₁₇Cl₂ nanomaterial, which indicates that light absorption originates from the UV region and extends up to visible-light region. The light absorption edge is up to 590 nm, consisting of yellow color of the Bi₁₂O₁₇Cl₂ powder. The DRS measurement suggests that as-prepared Bi₁₂O₁₇Cl₂ has the potential to be photoactive under visible-light irradiation. Meanwhile, the band gap energy of the Bi₁₂O₁₇Cl₂ nanomaterial was calculated by the formula αhν = A(hν − E _g) ^n/2, where α, ν, E _g, and A are the absorption coefficient, light frequency, band gap energy, and a constant, respectively (Butler, Reference Butler1977). Besides, n is a constant that depends on the optical transition in a semiconductor. Optical transition of Bi₁₂O₁₇Cl₂ is indirect and its n value is 4. From the plot of (αhν)^1/2 vs. hν (inset of Figure 5), E _g of Bi₁₂O₁₇Cl₂ sample is estimated to be 2.1 eV by the intercept of the tangent to the plot.

Figure 5.

(colour online) UV–vis absorption spectra of Bi₁₂O₁₇Cl₂ nanomaterial.

D. Photocatalytic activity

Since the layer-stratiform structured Bi₁₂O₁₇Cl₂ nanomaterial has a band gap as low as 2.1 eV and its visible-light absorption extends to 590 nm, it is expected that the Bi₁₂O₁₇Cl₂ may exhibit superior visible-light-responsive photocatalytic performance. In photodegradation experiments, MO was adopted as the target pollutant. Figure 6 presents the variation of MO concentration with irradiation time under visible light. In the absence of Bi₁₂O₁₇Cl₂ catalyst, photodegradation of MO under visible light is negligible, which suggests that the existence of Bi₁₂O₁₇Cl₂ catalyst is the prerequisites for MO photodegradation. When adopting Bi₁₂O₁₇Cl₂ as photocatalyst, MO concentration decreases promptly as visible-light irradiation time increases and the efficiency of MO decomposition is nearly 97% in 90 min, as demonstrated in Figure 6. The MO photodegradation experiments verify the as-synthesized Bi₁₂O₁₇Cl₂ photocatalyst as an advanced visible-light-responsive photocatalyst.

Figure 6.

(colour online) Photocatalytic activity and TOC decrement rate of Bi₁₂O₁₇Cl₂ photocatalyst for MO degradation under visible-light irradiation.

The concentration of total organic carbon (TOC) was measured as a mineralization index to characterize MO degradation. As shown in Figure 6, 85% of TOC in the MO solution was eliminated after 90 min of visible-light irradiation in the presence of the Bi₁₂O₁₇Cl₂ photocatalyst, indicating that most of MO was mineralized. The variation trend of TOC removal efficiency is consistent with that of photodegradation efficiency. In addition, the photostability of Bi₁₂O₁₇Cl₂ photocatalyst was evaluated through recycling experiments for MO degradation under visible light, as shown in Figure 7. The catalyst was collected after degradation by filtration and reused for two times under the same conditions. Compared with the first run, there was gradual decline in degradation efficiency in the following two runs and the degradation rate was 86% in 90 min in the third run, which was 89% of that of the first run, showing that there is slight loss in activity for photocatalytic degradation of MO. The recycling experiments and TOC removal efficiency demonstrate that as-synthesized Bi₁₂O₁₇Cl₂ is promising in future practical application.

Figure 7.

Recycling properties of Bi₁₂O₁₇Cl₂ photocatalyst for MO degradation under visible-light irradiation.

E. Photocatalytic mechanism

During the dye photodegradation over semiconductor photocatalyst, there are three possible mechanisms: photolysis, dye photosensitization, and photocatalytic process. The photolysis process mainly involves the photoexcitation of dye molecules and the combination of electrons excited with O₂ to produce single oxygen atoms, which serves as an oxidant for pollutant photodecomposition (Lin et al., Reference Lin, Huang, Huang, Wang and Shi2006). The photolysis process depends on the structure stability of dye molecules. As to the dye photosensitization process, there involves three steps: the dye molecules are photoexcited to induce electrons, electrons induced are injected to the conduction band of semiconductor photocatalysts, and electrons injected are captured by molecular oxygen to generate active species for dye photodegradation (Hu et al., Reference Hu, Xu and Zhao2004). For the photocatalytic mechanism (Li et al., Reference Li, Chen, Zhou and Shen2011a, Reference Li, Wang, Yao, Dang and Li2011b), semiconductor photocatalysts are directly excited and produce electrons and holes, as illustrated in Figure 8. A portion of the induced electrons and holes recombine in the surface of photocatalyst particles, the rest migrate to the surface of catalyst particles and react with dye molecules, subsequently, the photodegradation is performed. The photocatalytic process mainly lies on light absorption of semiconductor photocatalysts.

Figure 8.

(colour online) Proposed direct photocatalytic mechanism over Bi₁₂O₁₇Cl₂ under visible-light irradiation (CB: conducting band; VB: valence band).

On the basis of UV–vis diffuse reflectance spectrum, Bi₁₂O₁₇Cl₂ photocatalyst could be excited by visible light to generate electrons and holes, which suggests that MO photodegradation under visible-light irradiation is dominated by direct photocatalytic mechanism. To provide additional information, a blank experiment (Figure 6) of MO degradation without any photocatalysts under visible light proves that MO is highly structural stable and MO decomposition by the photolysis mechanism can be neglected. For the role of photosensitization mechanism in MO degradation under visible light, Lin et al. studied the cut-off wavelength-dependent photocatalytic activity of TiO₂ (Lin et al., Reference Lin, Huang, Huang, Wang and Shi2007). It was reported that the MO degradation over TiO₂ at the cut-off wavelength of 450 nm is 4.0 and 86.5% under full arc. Since it is widely accepted that TiO₂ cannot respond to visible light of λ > 450 nm, photosensitization mechanism plays the only role in the 4% portion of MO degradation and the portion of photosensitization process in the total MO degradation is about 4.6%. Therefore, the superior MO photodegradation performance of the as-synthesized Bi₁₂O₁₇Cl₂ photocatalyst under visible-light irradiation is mainly attributed to the direct photocatalytic mechanism.

More significantly, the advanced photocatalytic performance is closely related with the morphology of Bi₁₂O₁₇Cl₂ photocatalyst. As-synthesized Bi₁₂O₁₇Cl₂ photocatalyst has a distinct hierarchical plate-stratiform nanostructure, which is composed of secondary nanosheets regularly arranged, with 10–50 nm in thickness and 200–1500 nm in in-plane size. Further, the secondary nanosheets are parallelly stacked by ultrathin primary nanosheets with the thickness in several nanometers. The contribution from the morphology and nanostructure to their photocatalytic activity is discussed as illustrated in Figure 8. Firstly, this plate-stratiform nanostructure has higher specific surface area, which could induce a much stronger dye adsorption capability on the surface of catalyst. From Figure 6, high adsorption capability of about 45% was achieved when the adsorption–desorption equilibrium of MO on catalyst surface was reached. Secondly, the plate-to-plate structure is believed to accelerate the transport of electrons and holes from catalyst to surface reaction sites and improve effective separation of photoinduced e–h pairs and thereby enhances the photocatalyst activity. Thirdly, the absorption edge of 590 nm is much larger than the data reported in the literature (Xiao et al., Reference Xiao, Liu, Hu, Zuo, Nan, Li and Wang2012, Reference Xiao, Jiang and Zhang2013), which leads to better light absorption in visible-light region. The increase of the absorption edge is attributed to the decrease of corresponding band gap energy value, which is very sensitive to the morphology and structure of photocatalysts. In this way, the Bi₁₂O₁₇Cl₂ nanomaterial exhibits highly enhanced photocatalytic activity for the successful photodegradation of MO under visible-light irradiation.