Preparation and characterization of BiOCl/TiO₂/MMT composite materials

MMT was purchased from the Chifeng Hengrun Industry and Trade Co., Ltd. (Inner Mongolia, China); its chemical composition is shown in Table 1. Tetrabutyl titanate (C₁₆H₃₆O₄Ti), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), and potassium chloride (KCl) were purchased from Shanghai Yien Chemical Technology Co., Ltd. (Shanghai, China). Anhydrous ethanol (C₂H₅OH) and glacial acetic acid (CH₃COOH) were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). Sodium hexametaphosphate ((NaPO₃)₆) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Potassium iodide (KI), tert-butanol (C₄H₁₀O), and p-benzoquinone (C₆H₄O₂) were purchased from Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). Deionized water was used in all of the experiments.

First, BiOCl was prepared by the hydrolysis method: 12 mmol Bi(NO₃)₃ · 5H₂O and 12 mmol KCl were dissolved in 20 mL nitric acid and 40 mL deionized water, respectively (The solutions will be referred to as mixed solution A and B, respectively). Thereafter, the mixed solution A was dripped into the mixed solution B, stirred for 2 h and stood for 2 h. The reaction solution was filtered, washed with both water and absolute ethanol three times each, and the filter cake was dried in an oven at 80°C for 24 h to obtain the BiOCl sample.

Then, BTMC composite materials were prepared using the sol-gel method [21] combined with the calcining (calcination) procedure and the preparation procedure shown in Figure 1. The details of the preparation method were as follows: 10 mL anhydrous ethanol was fully mixed with 5 mL glacial acetic acid, then 13.6 mL tetrabutyl titanate was slowly added and stirred for 30 min to form mixed solution C. A certain amount of MMT was dissolved in 50 mL deionized water to obtain the MMT suspension, and a certain amount of BiOCl was dispersed in the MMT suspension according to the molar ratio of Bi/Ti. The mixed solution C was slowly added to the MMT suspension, stirred for 3 h and aged for 24 h. The aged gel was dried in an oven at 80°C for 12 h, ground through a 200-mesh sieve, and then placed in a muffle furnace for calcining at 600°C for 2 h. The calcined samples are BTMC composite materials. The method of preparing TiO₂/MMT composite materials (TMC) is the same except that no BiOCl is added. When the Bi/Ti molar ratio is 10%, 15%, 20%, 25%, and 30%, it is denoted as BTMC-10, BTMC-15, BTMC-20, BTMC-25, and BTMC-30 in sequence.

Fig. 1.

The phase composition of the composite material was analyzed using the Bruker D8 Advance X-ray diffractometer (XRD, Germany). The composite material was irradiated by Cu Kα (λ = 1.5418 Å) at 40 kV and 40 mA at a scanning rate of 5°/min. The measurement range of 2θ is 10°~80°. The specific surface area and pore structure characteristics of the composite material were measured by using the Micromeritics APSP 2460 specific surface and porosity analyzer (BET, American). The surface morphology of the composite material was observed using a Zeiss Merlin compact scanning electron microscope (SEM, Germany). Microstructure of the composite was analyzed using the G2 F20 S-TWIN TMP transmission electron microscopy (TEM, American). The Thermo Scientific K-Alpha X-ray photoelectron spectrometer (XPS, American) and Al Ka monochromatized sources were used to analyze the elemental composition and elemental valence state of the composite material, respectively. A UV-3600 (UV-Vis DRS, Japan) spectrophotometer was used to analyze UV-Vis absorption spectroscopy. The wavelength ranged from 200 to 800 nm, and the measurement interval was 1 nm.

Figure 2a shows the XRD images of MMT. As can be seen from the figure, the diffraction peaks located at 22.1°, 29.2°, and 36.2° are the same as the standard card peaks of cristobalite (JCPDS No. 39-1425). At the same time, the diffraction peaks located at 6.50°, 19.9°, 26.7°, and 62.0° can also be accurately indexed to the standard XRD card of sodium montmorillonite (JCPDS No. 29-1498). The XRD results show that MMT belongs to sodium montmorillonite and the impurity is cristobalite, which is consistent with the chemical composition analysis results of MMT (shown in Table 1).

Fig. 2.

Figure 2b shows the XRD patterns of BiOCl, TiO₂, TMC, BTMC-10, BTMC-15, BTMC-20, BTMC-25, and BTMC-30. It can be seen from Figure 2a that the diffraction peaks located at 25.4°, 38.0°, 48.3°, 54.1°, 55.2°, 62.6°, and 62.9° in the TMC sample belong to the characteristic peaks of anatase phase TiO₂ (JCPDS NO. 21-1272), indicating that TiO₂ supported on the surface of MMT exists in the form of anatase phase. The characteristic diffraction peaks of the rutile-phase TiO₂ were not observed in the TMC sample after calcining at 600° for 2 h, which indicated that MMT can restrain the conversion of anatase to rutile [22]. By comparing the diffraction peaks of TiO₂ and TMC at 25.4°, it can be found that the addition of MMT significantly reduces the diffraction peak intensity corresponding to the (101) crystal plane of TiO₂, indicating that MMT has a certain inhibition effect on the crystallization of TiO₂, thus affecting the photocatalytic activity of the composite material [23, 24].

To better observe the influence of BiOCl on the structure of BTMC, Figure 2b was locally enlarged (Figure 2c). As can be seen from Figure 2c, with the increase of BiOCl content, the intensity of the diffraction peak corresponding to the (101) crystal plane in the TiO₂ gradually weakens and the (101) crystal plane slowly moves to a low angle. This is because Ti⁴⁺ replaces Bi³⁺, creating a lattice defect [25]. In order to further explore the effect of Ti⁴⁺ replacing Bi³⁺ on the crystal structure of BTMC, the Scherrer formula [26] (Eq. 1) was used to estimate the crystal size of BTMC, and the results are shown in Figure 2d. With the gradual increase of BiOCl content, the grain size of the material gradually increased. In addition, when the Bi/Ti molar ratio is 20%, the diffraction peak corresponding to the (101) crystal plane in BiOCl appears, indicating that the degree of combination for BiOCl and TiO₂ is optimal at this time, and increasing further BiOCl content will inhibit the growth of TiO₂ crystal, thus decreasing the photocatalytic activity of the composite materials [27, 28]. 1 D=Kλ/Bcosθ $$D=K\lambda /B\cos \,\theta $$ Where D is the grain diamter perpendicular to the crystal plane, nm. K is Scherrer’s constant, which generally is 0.89. λ is the wavelength of incident X-ray, which is 0.15406 nm. B is the half-peak width of diffraction peak, and rad. θ is the degree of the Bragg diffraction angle.

The specific surface area and pore structure are important factors affecting the photocatalytic performance of composite material. Therefore, N₂ pore size distribution of five samples. As can be seen from the International Union of Pure and Applied Chemistry (IUPAC), the N₂ adsorption– desorption isotherm curve of MMT (Figure 3a) belongs to a type III isotherm with an obvious hysteresis loop, indicating that the presence of mesoporous material (2 nm~50 nm) inside the MMT. When P/P₀<0.4, the adsorption capacity slowly increases and presents a platform, indicating that no micropores exist in the MMT. When 0.4<P/P₀<0.9, the adsorption volume began to rise gradually. When P/P₀>0.9, the adsorption volume rises sharply, indicating that MMT had multilayer adsorption and capillary condensation. The micropores and small pores were filled with a large amount of N₂, and there were more mesoporous and macroporous structures inside the MMT. The adsorption-desorption isotherm curves of TiO₂ (Figure 3b) and BiOCl (Figure 3c) also exhibited a typical type III isotherm: the characteristics of the curves are similar to MMT, but there is no obvious hysteresis loop, indicating that they are also mesoporous material and that there are many mesoporous and macroporous structures inside them. After TiO₂ is loaded onto MMT, the N2 adsorption-desorption isotherm curve changes from type III isotherm to type IV isotherms with a more obvious hysteresis loop, indicating that there are many mesoporous structures inside TMC [29]. In Figure 3d, the N₂ adsorption-desorption isotherm curve of BTMC-20 belongs to type IV isotherms with a more obvious hysteresis loop.

Fig. 3.

Figure 3a₁-d₁ and Table 2 show pore size distribution curves and the BET data of the materials, respectively. In Figure 3a₁-c₁, the pore size of MMT, TiO₂, and BiOCl is mainly distributed between 2.75 nm~24 nm, which is consistent with their average pore sizes (MMT:19.84 nm; TiO₂:18.43 nm; BiOCl:14.01 nm). The pore diameter of TMC and BTMC-20 was centered at 11.05 nm and 9.52 nm (Figure 3d₁), respectively, again showing the mesoporous structure in the composite materials. Furthermore, the pore size of the BTMC-20 was slightly reduced after adding BiOCl, which was attributed to the presence of BiOCl inhibiting the growth of TiO₂ crystals [30].

As can be seen from Table 2, the specific surface area of MMT, TiO₂, and BiOCl are 39.20 m²/g, 6.09 m²/g, and 0.59 m²/g, respectively. It is not difficult to see that the specific surface area of MMT is the largest, which is consistent with its physical properties. After adding TiO₂, the specific surface area increased to 97.46 m²/g, an increase of 58.26 m²/g. Compared with TMC, the specific surface area of BTMC-20 (109.02 m²/g) increased more significantly. The increase of the specific surface area is conducive to the absorption of photons by the material, thus enhancing the photocatalytic activity [31]. This shows that loading TiO₂ and BiOCl onto MMT can effectively improve the photocatalytic performance of the material.

Figure 4 shows the SEM images of MMT, BiOCl, TMC, and BTMC-20. It can be seen from Figure 4a that the MMT has a clear layered structure, and its lamellar layer is stripped to a certain extent, which accords with the micromorphological characteristics of MMT [32]. As Figure 4b shows, BiOCl has an obvious two-dimensional layered structure and an accumulation of layers with layers, indicating that BiOCl nanosheets have high crystallity and a large specific surface area, which is conducive to the separation of photogenerated electron-hole pairs, thus improving the photocatalytic activity of the samples [33]. In comparison with the SEM image of MMT, the micromorphology of TMC was obviously changed, and a slit mesopore appeared (Figure 4c), which is consistent with the result that TMC has an H₃-type hysteresis ring. In addition, a large number of TiO₂ particles were loaded on the MMT, which destroyed its original layered structure, and this is consistent with the disappearance of MMT diffraction peaks in the XRD pattern. This is attributed to the polyhydroxy-cationic Ti⁴⁺ entering the middle of the MMT layer, which expands the crystal plane spacing and generates serious peeling phenomena between the interlamellars and then results in the formation of different sizes of slit mesoporous structures in the process of calcining and crystallization [34]. As can be seen from Figure 4d, a large number of BiOCl nanoparticles can be observed in the BTMC-20. TiO₂ is the shape of cotton batting, and its particle size became significantly smaller, indicating that the addition of BiOCl can inhibit the growth of TiO₂ crystal. The flocculent TiO₂ on the surface of BiOCl nanosheets is adsorbed onto the surface of MMT, which indicates that BiOCl/TiO₂ heterojunction has formed and is supported on the surface of MMT.

Fig. 4.

Figure 5 shows the EDX mapping of BTMC-20, and it was obtained by surface energy spectrum analysis of the graph interval of BTMC-20 in Figure 4d. BTMC-20 is mainly composed of Ti, O, Bi, Cl, and Si elements, indicating that BiOCl, TiO₂, and MMT coexist in BTMC, which further confirms the successful preparation of BTMC-20. Comparison of the Ti and Bi elements shows that the distribution of Ti elements is more uniform, while the distribution of Bi elements is more concentrated, indicating that the lamellar accumulation of BiOCl greatly improves the dispersion of TiO₂ particles.

Fig. 5.

Figure 6 shows the TEM images of BTMC-20. From Figure 6a and 6b, it can be observed that the MMT with a flake structure was supported by a large number of black linear and granular materials on its surface, indicating that BiOCl and TiO₂ have been successfully supported on the surface of the MMT. To further determine the composition of black linear and granular materials, part of Figure 6c was enlarged to get Figure 6d. It can be observed that the lattice fringe with an interplanar spacing of 0.352 nm corresponds to the (101) crystal plane of BiOCl, and the lattice fringe with an interplanar spacing of 0.362 nm represents the TiO₂ (101) crystal plane of anatase phase [35], which demonstrates that the black linear material is BiOCl, and the granular material is anatase phase TiO₂. This also provides strong support for the successful preparation of the BTMC-20. Moreover, it can be seen from Figure 6c that TiO₂ and BiOCl are closely combined and exist in the form of accumulation and plane assembly, indicating that the heterojunction structure formed between BiOCl and TiO₂, which is conducive to the separation of photogenerated electron-hole pairs.

Fig. 6.

Figure 7a shows the XPS of BTMC-20. The electron-binding energy peaks of O 1s, Ti 2p, C 1s, Cl 2p, Bi 4f, and Si 2p can be clearly observed, and correspond to O, Ti, C, Cl, Bi, and Si, respectively. This is consistent with the element composition of BTMC-20 determined in Figure 5a–e. Moreover, the peaks of other impurities were not observed in the figure, indicating that the purity of BTMC-20 is relatively high. Figure 7b–f shows the fine spectra of the Ti 2p, Bi 4f, Si 2p, O 1s, and Cl 2p, respectively. As can be seen from Figure 7b, there are two strong peaks located at 458.75 eV and 464.51 eV derived from the Ti 2p_3/4 and Ti 2p_1/2 orbital of TiO₂, respectively, which indicate that the Ti element exists in the form of a +4 valence state in BTMC-20 [36]. From Figure 7c, we can see that there are two strong peaks located at 159.40 eV and 164.71 eV, corresponding to the characteristic peaks of Bi 4f_7/2 and Bi 4f_5/2, respectively, indicating that the Bi element exists in the form of a +3 valence state in BTMC-20 [37]. The electronbinding, energy-binding peak at 103.22 eV is attributed to the typical Si-O bond (Figure 7d). In Figure 7e, the four strong peaks appear at 529.97, 530.34, 531.57, and 532.83 eV, corresponding to [Bi₂O₂]²⁻, Ti-O-Ti, surface OH, and Si-O-Si bond [38], respectively. From the Cl 2p spectra (Figure 7f), it can be found that two peaks located at 198.10 eV and 199.71 eV correspond to the characteristic peaks of Cl 2p_3/2 and Cl 2p_1/2, respectively, indicating that the Cl element exists in the form of a −1 valence state in BTMC-20.

Fig. 7.

Figure 8 shows the ultraviolet diffuse reflectance spectra (DRS) and band gaps of BiOCl, TiO₂, TMC, and BTMC-20. The band-edge absorption of BiOCl and TiO₂ are 364 nm and 389 nm, respectively, both of which are in the ultraviolet region, indicating that BiOCl and TiO₂ have good absorption capacity in the ultraviolet region, which is consistent with previous reports [39, 40]. Compared with pure TiO₂, the band-edge absorption of TMC (394 nm) showed a weak redshift, indicating that the addition of MMT can improve the light absorption capacity of the composite material. After compounding with BiOCl, the band-edge absorption is redshifted to 417 nm (BTMC-20). Compared with BiOCl, TiO₂, and TMC, the light absorption range of BTMC-20 extends to the visible light region, which indicates that the light-response range of BTMC-20 was broadened and its visible light absorption capacity was enhanced, thus enhancing the photocatalytic activity of BTMC-20.

Fig. 8.

The band gaps of BiOCl, TiO₂, TMC, and BTMC-20 are calculated according to formula (Eq. 2), and the results are shown in Figure 8b. 2 (αhv)1/n=A(hv−Eg) $${{\left( \alpha hv \right)}^{1/n}}=A\left( hv-{{E}_{g}} \right)\ $$ Where α is the absorption coefficient, hv is the binding energy, eV. A is absorbance (abs), E_g is the band gap, eV. n is the power index, which is determined by the type of electronic transition (for direct bandgap materials, n = 1/2, and for indirect bandgap materials, n = 2). Here, the value of n is taken as 2 [41]. The band gaps of BiOCl, TiO₂, TMC, and BTMC-20 are 3.27 eV, 3.06 eV, 2.89 eV, and 2.61 eV, respectively. The band gap of TMC is lower than that of TiO₂, which indicates that the addition of MMT can improve the photocatalytic activity of TMC. Among them, BTMC-20 has the smallest band gap (2.61 eV), which indicates that it has higher photocatalytic activity and stronger light response. The main reason is that a heterojunction (type II) can be formed between BiOCl and TiO₂ because of their different conductance and valence band potential [42]. In the meantime, in the process of calcining–crystallization, BiOCl and TiO₂ replace Bi (III) and Ti (IV) elements with each other to form impurity levels, thus reducing the band gap [43].

It can be seen from Figure 9a and 9b that the valence band potential values (E_VB) of TiO₂ and BiOCl are 2.65 eV and 2.91 eV, respectively. The band gaps of TiO₂ and BiOCl are 3.06 and 3.27, respectively, as calculated by the band gap of DRS (Figure 8b). So, the conduction band potential values (E_CB) of TiO₂ and BiOCl can be calculated as −0.41 eV and −0.36 eV according to the formula E_VB = E_CB + E_g [44]. The above analysis demonstrated that BiOCl combines with TiO₂ on the MMT to form a type II heterojunction, thus accelerating the transfer of photogenerated electrons and holes [45].

Fig. 9.