Xiaoli
Yang
,
Shaodong
Sun
*,
Jie
Cui
,
Man
Yang
*,
Qing
Yang
,
Peng
Xiao
and
Shuhua
Liang
*
Engineering Research Center of Conducting Materials and Composite Technology, Ministry of Education, Shaanxi Engineering Research Centers of Metal-Based Heterogeneous Materials and Advanced Manufacturing Technology, Shaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, Shaanxi, People's Republic of China. E-mail: sdsun@xaut.edu.cn; myang@xaut.edu.cn; liangsh@xaut.edu.cn
First published on 6th July 2021
Heterojunction engineering is an effective strategy to enhance the photodegradation activity via improving the spatial charge separation. However, the poor interface interactions and stability limit the photocatalytic activity and stability of traditional heterojunctions. Herein, robust BiOCl/ZnO p–n heterojunctions with semi-coherent interfaces were prepared by a one-pot hydrothermal method to improve the activity and stability toward photocatalytic degradation than that of the counterpart, in which the semi-coherent interfaces exhibited lower phase boundary energy, resulting in highly-stable interfaces between BiOCl and ZnO as well as the formation of the built-in electric field in this robust p–n heterojunction for enhanced charge separation. The cycle test results verified that the BiOCl/ZnO heterojunctions with semi-coherent interfaces can maintain the photocatalytic degradation activity at the initial level even after 10 cycles, while deactivation of the sample without semi-coherent interfaces occurred after 3 cycles only. Optical and electrical properties revealed that BiOCl/ZnO heterojunctions with semi-coherent interfaces possessed the highest electron migration and charge separation efficiency, resulting in the highest photodegradation activity. Density functional theory (DFT) calculations and electron spin-resonance (ESR) results verified that the enhanced charge separation was assigned to the type-II photocatalytic mechanism, leading to the enhancement of ˙OH and ˙O2− reactive oxygen species. This work would provoke the development of one-step construction of new highly active BiOX (X = Cl, Br, and I)-based heterogeneous photocatalysts with stable semi-coherent interfaces.
Because the redox potentials play an important role in spatial charge separation for a heterojunction,25,26 the composition of the n-type semiconductor building block in a BiOX-based p–n heterojunction is crucial to achieving the high photocatalytic performance. If an n-type ZnO semiconductor with high oxidation potential27–29 is incorporated into the as-mentioned BiOCl with high oxidation potential, the separation efficiency of photogenerated e− and h+ would be highly improved due to the formation of a p–n heterojunction. For instance, Vaizoğullar et al. have fabricated BiOCl/ZnO/Bent materials with an effective degradation of 3-chlorophenol (3-CP) by a successive synthesis method.30 Similarly, Zhang et al. have demonstrated that BiOCl/ZnO/C3N4 photocatalyst exhibited superior photodegradation performance for RhB and methyl orange (MO) to BiOCl/ZnO, BiOCl and C3N4.31 Additionally, Liu et al. have prepared BiOCl/ZnO heterostructures with oxygen-rich vacancies using a one-step microwave technique, which can effectively enhance photocatalytic performance for the degradation of tetracycline hydrochloride (TC-HCl) than those of pure BiOCl and ZnO. This study showed that oxygen vacancy and interfacial electric field derived from heterostructures were responsible for the improvement of light harvesting and the separation efficiency of photogenerated charge carriers.32 However, there are still some drawbacks, such as the poor-matched interface and poor stability as well as complex fabrication strategy in the current BiOCl/ZnO-based heterojunctions. Therefore, it is meaningful to explore a new method for promoting the interface interaction and stability in a BiOCl/ZnO heterojunction.
Herein, we present a novel one-pot hydrothermal strategy for the synthesis of robust BiOCl/ZnO heterojunctions with semi-coherent interfaces and compare the stability of catalysts fabricated by the above strategy with the successive process method. As expected, the well-matched BiOCl/ZnO heterojunctions exhibit significantly higher photocatalytic activities toward the degradation of RhB and tetracycline (TC) as well as ciprofloxacin (CIP) solution than the sample prepared by the successive synthesis method, which can be attributed to the formation of the built-in electric field in the robust p–n heterojunction for improved charge separation. Significantly, the cycle test results confirm that the BiOCl/ZnO heterojunctions with semi-coherent interfaces synthesized by the one-step hydrothermal method maintain the photocatalytic degradation activity at the initial level even after the 10th cycle, while deactivation of the sample without semi-coherent interfaces prepared by a successive process occurred after the 3rd cycle only. The enhanced photocatalytic mechanism was proposed in detail based on the systematic characterizations and density functional theory (DFT) calculations, which belonged to the type-II pathway. This work might provide a theoretical reference for the one-pot construction of new heterogeneous photocatalysts with well-matched interfaces.
In order to confirm the crystal phases of these photocatalysts, XRD was performed. As shown in Fig. 2, the XRD spectrum of the BZ-0.67 sample displayed the diffraction peaks located at (002), (101), (102), (112), (200), (113), (211), (104), (212), (114), and (214) crystal planes, respectively, corresponding to the tetragonal BiOCl crystal phase (JCPDS No. 06-0249). The diffraction peaks of the ZnO crystal (JCPDS No. 75-1526) were also detected, which were indexed to (100), (002), (101), (102), (103), (112), (201), and (202) crystal planes. The results demonstrated that BiOCl and ZnO were synthesized successfully by this one-pot hydrothermal method. Moreover, when the Bi/(Bi + Zn) mole ratio is 1, the XRD pattern was in good accordance with BiOCl (JCPDS No. 06-0249) (Fig. S1(a)†). When the Bi/(Bi + Zn) mole ratio is 0, the XRD pattern could be indexed to the composite phase of Zn2SnO4 (JCPDS No. 24-1470) and ZnSn(OH)6 (JCPDS No. 73-2384) (Fig. S1(b)†).
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the morphology and microstructure of BZ-0.67. As shown in the SEM images in Fig. 3, the BiOCl was composed of square-shaped nanosheets, while the abundant ZnO was well-dispersed on the surface of BiOCl as uniform nanoparticles. The results suggested the formation of BiOCl/ZnO heterojunctions. Notably, the pure BiOCl consisted of nanosheets, and the ZTO consisted of nanocubes, as given in Fig. S2.† The TEM of the top view (Fig. 4(a) and (b)) and the side view (Fig. 4(d)) images of BZ-0.67 further confirmed the ZnO nanoparticles were attached on the surface of the BiOCl nanosheet, indicating the BiOCl/ZnO heterojunction was constructed successfully, which was in good agreement with their SEM images. The selected area electron diffraction (SEAD) pattern displayed periodic diffraction spots with identified crystal planes of (200), (110), and (10) of tetragonal BiOCl as well as diffraction rings with crystal planes of (002) and (110) of hexagonal ZnO (Fig. 4(c)), indicating the formation of single-crystalline BiOCl and poly-crystalline ZnO, respectively. The results were in good agreement with the XRD pattern. High-resolution TEM (HRTEM) was also conducted to further clearly investigate the microstructure between BiOCl and ZnO interface of BZ-0.67. Fig. 4(e) showed the lattice fringe with spacings of 0.19 nm and 0.26 nm, corresponding to the (200) and (002) facets of BiOCl and ZnO, respectively. Particularly, the corresponding Inverse Fast Fourier Transform (IFFT) image exhibited a profound interfacial connection between BiOCl and ZnO in BZ-0.67 (Fig. 4(f)). The intimate interfacial connection between BiOCl and ZnO leads to the establishment of the heterojunction. The HRTEM result clearly demonstrated an obvious semi-coherent interface between the BiOCl and ZnO heterojunction, rather than the simple physical mixture, which further confirmed the formation of the well-matched BiOCl/ZnO heterojunction.
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Fig. 3 (a) Low-magnification SEM image of BZ-0.67, (b) high-magnification of the rectangular section (red color) as shown in (a). |
To investigate the special photocatalytic activity and stability of BiOCl/ZnO heterojunctions with semi-coherent interfaces obtained from the one-pot preparation method, the BZ-0.67, BZ-0.67-S, ZTO, and BiOCl were selected to evaluate the photocatalytic degradation performance of RhB solution under a full spectrum illumination condition. Notably, the BZ-0.67-S was prepared by the successive synthesis method for comparison.33 The XRD spectrum of BZ-0.67-S in Fig. S3† had similar characteristic peaks to the BZ-0.67, indicating that BiOCl and ZnO were constructed. SEM and TEM were employed to characterize the morphology and microstructure of BZ-0.67-S. The SEM images of BZ-0.67-S in Fig. S4† indicated the morphology of BiOCl was also the square-shaped nanosheets, and the ZnO nanoparticles were randomly dispersed on the surface of BiOCl nanosheets. However, the size of ZnO particles in BZ-0.67-S was larger than that in BZ-0.67, suggesting ZnO aggregates to larger particles by a successive synthesis process, which might decrease the active sites. From the TEM images of BZ-0.67-S in Fig. S5,† it can be seen that the interfaces between BiOCl and ZnO were physical contacts and incoherent, which indicated that an unmatchable BiOCl/ZnO heterojunction without semi-coherent interfaces was formed in BZ-0.67-S. The photocatalytic results are shown in Fig. 5(a) and (b), respectively. It was found that BZ-0.67 exhibited a much higher photocatalytic activity than those of the BZ-0.67-S, pure ZTO, and pure BiOCl. After 16 min, the RhB solution can be degraded to 99.7% of the initial concentration over BZ-0.67, while 22.1%, 18.6%, and 33.9% of the initial concentration over BZ-0.67-S, pure ZTO, and pure BiOCl, respectively. The apparent rate constants (k) of the photocatalytic reaction are 0.190, 0.014, 0.013, and 0.024 min−1 for BZ-0.67, BZ-0.67-S, pure ZTO, and pure BiOCl, respectively. The catalysts prepared by the one-pot hydrothermal method with various Bi/(Bi + Zn) mole ratios were also employed for the photocatalytic degradation of RhB. The results demonstrated that BZ-0.67 displayed superior photocatalytic activity in comparison to other samples (Fig. S6†). Besides, we have compared the photocatalytic performance of BZ-0.67 with other BiOCl/ZnO catalysts reported in the literatures (Table S1†). One can see that the BZ-0.67 in our work exhibits a relatively high activity among BiOCl/ZnO catalysts reported in the literature. Furthermore, to eliminate the sensitization effect of RhB on photocatalysis, photocatalytic degradation of TC and CIP were evaluated over BZ-0.67. It was found that BZ-0.67 possessed excellent photocatalytic degradation activities for both TC and CIP (Fig. 5(c) and (d)), suggesting it is photocatalysis rather than photosensitization for dye degradation over BZ-0.67. These results indicate that the construction of robust BiOCl/ZnO heterojunction can accelerate the charge separation and transfer than pure BiOCl and ZnO. Moreover, the preparation methods have an obvious influence on photocatalytic activity. The heterojunction prepared by the one-pot method (BZ-0.67) is much better matched and preferable to the charge separation and transfer than that prepared by the successive synthesis method (BZ-0.67-S).
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Fig. 5 (a and b) Photocatalytic degradation of RhB solution by BZ-0.67, BZ-0.67-S, ZTO, and BiOCl samples; (c and d) photocatalytic degradation of TC and CIP solution by BZ-0.67 sample. |
In order to further investigate the photocatalytic stabilities of BZ-0.67 and BZ-0.67-S, cycle tests were performed. As shown in Fig. 6, the BZ-0.67 almost maintained its photocatalytic degradation activity of RhB at the initial level even after 10 cycles. In contrast, the BZ-0.67-S nearly lost two-thirds photocatalytic degradation activity of RhB at the initial level after 3 recycles. Notably, a series of characterizations of BZ-0.67 after the recycling test was carried out. Both the SEM image and XRD pattern demonstrated that the structure of BZ-0.67 did not change before and after the recycling test (Fig. S7†). The ICP results of the solution after the cycle test in Table S2† demonstrated that no metal was leached after catalytic reaction, further demonstrating the stability of BZ-0.67. These results proved excellent photocatalytic stability of BZ-0.67 with the well-matched semi-coherent interfaces between BiOCl and ZnO in heterojunction.
The optical and electrical properties were conducted to examine the separation of intrinsic photoinduced charge carriers in BiOCl/ZnO heterojunction. As displayed in Fig. 7(a), the BZ-0.67 presented a distinctly higher photocurrent density than BZ-0.67-S, pure ZTO, and pure BiOCl. The results indicated that the BZ-0.67 heterojunctions possessed the highest electron–hole separation efficiency, which was consistent with the trend of photocatalytic activities. In addition, the intrinsic band-edge PL was conducted to detect the recombination of photogenerated electrons and holes. The weakest photoluminescence (PL) quenching of BZ-0.67 suggested that it had the least photogenerated charge recombination among the as-prepared catalysts, which was also in accordance with the trend of photocatalytic activities (Fig. 7(b)). Based on the above results, the BZ-0.67 heterojunction has the highest electron migration and charge separation efficiency for enhanced photocatalytic activity.
To understand the photocatalytic mechanism of the excellent performance over the BZ-0.67 heterojunction, the photocatalytic degradation activities were evaluated with different sacrificial reagents including ethanol (EA), tertiary butyl alcohol (TBA), and ascorbic acid (AA) for capturing h+, ˙OH, and ˙O2−, respectively, in order to determine the active species participating in the photodegradation reaction. As shown in Fig. 8(a) and (b), all three sacrificial agents have a certain inhibitory effect on the photocatalytic degradation activity of RhB in the BZ-0.67 catalyst system. However, in the presence of TBA and AA, the degradation activity decreased dramatically to 56.0% and 4.9% after 16 min, respectively. It proved that ˙OH and ˙O2− were generated and active for the degradation process. Besides, ESR measurement was conducted to confirm the existence of ˙OH and ˙O2−. The 5,5-dimethyl-1-pyrroline (DMPO) was employed as the spin-trapping reagent for the capturing of ˙OH and ˙O2−. As shown in Fig. 8(c) and (d), in the BZ-0.67 catalyst system, ˙O2− signal and weak ˙OH signal were detected, which proved that ˙O2− was the main reactive oxygen species in BZ-0.67, and ˙OH played the second important role in the degradation process. Obviously, it was found that both the ˙O2− and ˙OH signals were weaker in BZ-0.67-S than those of BZ-0.67, which meant that the BZ-0.67 heterojunction was beneficial to producing ˙OH and ˙O2− species than those in BZ-0.67-S.
DFT calculations were applied to further investigate the enhanced photocatalytic mechanism. The Fermi energy of BiOCl and ZnO were calculated as 5.30 eV and 4.68 eV, respectively (Fig. 9(a) and (b)). Besides, the CB potentials and the VB potentials of BiOCl and ZnO can be calculated by the following equations.34
EVB = X − Ee + 0.5Eg |
ECB = EVB − Eg |
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Fig. 9 The calculated Fermi energy of (a) BiOCl and (b) ZnO; (c) schematic diagram of the proposed mechanism for the photocatalytic degradation under light irradiation. |
Footnote |
† Electronic supplementary information (ESI) available: Additional XRD, SEM, DRS and photocatalytic degradation curves of RhB solution. See DOI: 10.1039/d1na00396h |
This journal is © The Royal Society of Chemistry 2021 |