Wenjing Dongab,
Tianyi Xiea,
Zhilun Wud,
Haiyi Penga,
Haishen Rena,
Fancheng Meng*c and
Huixing Lin*ab
aKey Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China
bEducation Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai, 200234, China. E-mail: huixinglin@mail.sic.ac.cn
cCollege of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China. E-mail: mengfancheng@cqut.edu.cn
dUnited Microelectronics Center Ltd, China
First published on 3rd December 2021
Bismuth oxychloride (BiOCl) is a promising photocatalyst material for water purification to remove organic pollutants. However, BiOCl materials can only degrade pollutants under ultraviolet-light owing to their wide band gap. Herein, we propose a simple synthesis route based on Bi2O3–B2O3–ZnO–SrO–Na2O (BBZSN) glass to fabricate 3D hierarchical-structured BiOCl materials with rich oxygen vacancies (OVs), which were introduced from BBZSN glass and inhibited the recombination of electron–hole pairs and adjusted the band structure. The photocatalytic activity of the obtained 3D hierarchical-structured BiOCl photocatalyst was evaluated by the degradation of Rhodamine B (RhB) under ultraviolet light and visible light. The experimental results suggested that the as-fabricated flower-shape BiOCl–NaCl could effectively degrade RhB under ultraviolet light (92.7%/20 min) or visible light (71.4%/20 min, 92.8%/100 min) respectively, which indicates its potential to be applied in environmental remediation.
The existing current research suggests that there are three effective strategies for the fabrication of BiOCl materials with high catalytic performance:3,5,6 crystal facet tailoring, 3D hierarchical structure control, and evolution of the band gap.
First, crystal facet tailoring is an effective method of optimizing the photocatalytic properties of semiconductor materials.3 The exposed facets on BiOCl play crucial roles in charge-carrier separation and transportation, impacting the selectivity and rate of photocatalysis reactions.13 The preparation of BiOX with specific exposed crystal facets was first realized using H+ as a capping agent.13–16 Jiang et al. conducted a comparative study on the photoactivity of BiOCl nanosheets with main surfaces through adjusting the pH.17 It was confirmed that BiOCl single-crystalline nanosheets with exposed facets exhibited higher activity for direct semiconductor photoexcitation pollutant degradation under UV light, whereas its counterpart with exposed facets possessed superior activity for indirect dye photosensitization degradation under visible light.18
Second, photocatalysts with hierarchical structures usually exhibit better performance than bulk photocatalysts, which can be attributed to their high specific surface area, efficient light harvesting, accessibility, and easy transport of the reactants of the former catalysts.5 Therefore, the design and synthesis of 3D hierarchical-structured BiOCl materials have attracted much attention.18,19 Through modification of the polar organic molecules (thiourea) at room temperatures, Hou et al. prepared BiOCl materials with an adjustable 3D layered structure featuring the characteristics of a comparatively positive valence band, high specific surface, area and efficient EHPs' separation.20 Under visible light, the reaction between photogenerated h+ and water molecules directly generated sufficient ˙OH; thereby effectively degrading tetracycline and RhB, for which the efficiency was approximately 20 times better than using pure BiOCl. Wu et al.21 used protonated branched polyethyleneimine (BPEI) to prepare BiOCl with facets. The interaction between the protonated positive and negative charges was utilized to induce the shape evolution of BiOCl and the facet exposure of BiOCl nanosheets. As the concentration of BPEI decreased, the shape of the resulting BiOCl accordingly transformed from 2D nanosheets into 3D assembled flower-like shapes; thereby realizing the efficient removal of Cr(VI) under sunlight.
Last but not least, the band structures, such as valence band (VB), conduction band (CB), and band gap, commonly determine the inherent photoexcitation, migration of carriers, and the redox capacities of electron–hole pairs. Research results show that oxygen vacancies (OVs) enriched in BiOX (X = Cl, Br, and I) materials can extend the long-wave photoresponse to visible light. BiOCl materials with a band gap of about 3.3 eV only show ultraviolet absorption, whereas adding OVs produces visible light-driven photocatalytic activity. Wang et al.22,23 synthesized BiVO4 materials with oxygen-rich vacancies by a calcination method to enhance the light absorption intensity and carrier separation efficiency, which greatly improved hydrogen production for the photoelectric catalytic decomposition of water. Ye et al.24 synthesized BiOCl nanoplates and nanosheets with the assistance of polyvinyl pyrrolidone and mannitol solution via a hydrothermal method. When the thickness of the BiOCl nanosheets decreased from 30 nm down to the atomic level (2.7 nm), the type of vacancy changed from V′′′ Bi to V′′′Bi V′′′Bi associated vacancies, apparently narrowing the band energy of BiOCl from 3.25 to 3.00 eV. Huang et al.25 prepared BiOI nanosheets with oxygen vacancies by electroreduction. After introducing oxygen vacancies, the prepared material exhibited a red-shift of ∼100 nm in the light absorption band, which led to a tenfold increase in the photocatalytic activity for MO degradation compared to untreated BiOI nanosheets.
In addition, the OVs on BiOX materials could serve as potential centers for the photogenerated electrons on the CB; thus impeding the direct recombination of EHPs.26–28 Meanwhile, OVs can improve the electron-transfer rate through strong coupling, which is beneficial for capturing typical molecules (O2, H2O, CO2, and N2). OV-mediated photocatalysis technology can not only realize the sustainable activation of oxygen molecules, but also overcome the spin forbidden reaction of oxygen in the triplet state. Therefore, oxidants (such as O2) can directly generate reactive oxygen species (ROS) to degrade pollutants, which is conducive to elevating the photocatalytic performance of the BIOX surface.29,30
Although the above research is beneficial to improving the photocatalytic activity of BiOCl materials, the conventional solvothermal and hydrothermal methods usually need polyvinylpyrrolidone, ethylene glycol, and mannitol to be introduced in a tedious treatment to obtain BiOCl materials with oxygen vacancies or a hierarchical structure,20,21,26,31,32 which is difficult to achieve in industrial production. In contrast to the aforementioned works, Bi2O(OH)2SO4 prepared by the sulfuric acid corrosion of BBZSN glass containing oxygen vacancies33–35 was innovatively used as the bismuth source and chlorine source to assemble the BiOCl material in the present study. The effects of the bismuth source morphology changes and chlorine source types on the 3D hierarchical structure and catalytic performance of BiOCl materials were studied. The morphological structure, band gap structure, oxygen vacancies, and photocatalytic performance of the prepared BiOCl materials were characterized, and the mechanism of enhancing the photocatalytic activity was determined and explained herein.
All the used chemicals needed no further purification. The characteristics of the intermediate product Bi2O(OH)2SO4 featuring fibrous and agglomerated bundle-shaped structures are shown as follows:
One-step method: first, 5 g BBZSN glass powder was added into 100 mL of 6 wt% H2SO4 solution, and then the mixture was stirred for 2 h to obtain a Bi2O(OH)2SO4 suspension of the fibrous intermediate product. The remaining suspension was allowed to stand to wait for its settling. When the precipitate appeared, the upper layer of waste liquid containing sulfuric acid was discarded, and the remaining was washed to neutrality with deionized water and ethanol. After that, either 100 mL of 2 wt% HCl solution or 100 mL of 3 wt% NaCl solution was added into the washed solid-state precipitate, with the former mixture then magnetically stirred at room temperature of about 25 °C for 2 h, while the latter solution was magnetically stirred at 60 °C for 4 h; thus preparing two kinds of synthetic BiOCl powder (BiOCl–HCl and BiOCl–NaCl), respectively. The products were washed to neutrality with deionized water and then dried at 100 °C for 4 h.
Two-step method: first, 3 g of dried Bi2O(OH)2SO4 powder (agglomerated bundle-shaped structure) was added into either 100 mL of 2 wt% HCl solution or 100 mL of 3 wt% NaCl solution, with the former mixture magnetically stirred at room temperature for 2 h, while the latter was magnetically stirred at 60 °C for 4 h; thereby preparing two kinds of synthetic BiOCl powder (BiOCl–HCl-2 and BiOCl–NaCl-2), respectively. The products were washed to neutrality with deionized water and then dried at 100 °C for 4 h.
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Fig. 1 SEM images of the as-prepared BiOCl samples: (a and b) BiOCl–HCl; (c and d) BiOCl–NaCl; (e and f) BiOCl–HCl-2; (g and h) BiOCl–NaCl-2. |
Interestingly, the agglomerated bundle-shaped structure of Bi2O(OH)2SO4 was shaped after drying its fibrous form, after which bundles of Bi2O(OH)2SO4 were added to the HCl solution to partially decompose the agglomerated structure; thereby forming sheet-structured BiOCl–HCl-2 approximately 1.5 μm in size. When the agglomerated bundle-shaped Bi2O(OH)2SO4 was added to the NaCl solution, its initial structure remained still, during which the bundle-shaped structure of BiOCl–NaCl-2 approximately 10 μm in size was subsequently formed by the directional arrangement of the nanosheets. A schematic diagram illustrating the formation process of the BiOCl sample is shown in Fig. S1.†
The crystal structure of the BiOCl material was studied by XRD. All the peaks of the as-prepared BiOCl samples could be indexed into the tetragonal phase of BiOCl with clear and narrow reflection (Fig. 2), from which no other characteristic peaks were obtained; thereby proving the purity of the as-prepared BiOCl samples.
The samples of BiOCl–HCl and BiOCl–HCl-2 exhibited an obvious diffraction intensity of the (001) crystal plane, which could be attributed to the following explanation. Under acidic conditions, large amounts of H+ ions are easily adsorbed on the O-terminated (001) surface of BiOCl. Thus, the strong binding interaction between the H+ ions and terminated oxygen on the (001) surface favors the formation of BiOCl materials enclosed with dominant facets. When the solution approximates neutrality, the H+ ions' content decreases, thereby weakening the bonding interaction with the terminated oxygen on the (001) surface and inhibiting the growth of the (001) plane; thereby allowing the appearance of the38 faceted surface during the growth of BiOCl materials.17 This is consistent with the fact that BiOCl–NaCl and BiOCl–NaCl-2 had higher (101) and (110) crystal plane diffraction intensities. In addition, a larger lateral dimension implies a stronger exposure of the (001) crystal planes, which means the lateral dimension positively correlates with the exposure degree of the crystal planes. Thus, BiOCl–HCl and BiOCl–NaCl had a large lateral dimension, whereby the (001) crystal plane was noticeably exposed.
Fig. 3 shows the detailed structural information of the as-prepared BiOCl samples obtained by transmission electron microscopy (TEM). As shown in Fig. 4(a), (c), (e), (g), and (i), the TEM images further verified the special morphology of the as-prepared BiOCl samples, with the results showing good consistency with the SEM results. As shown in the high-resolution TEM images in Fig. 4(b), (d), and (f), the lattice fringes featured a 0.367 nm lattice spacing, corresponding to the (002) atomic plane of BiOCl. The corresponding selected area electron diffraction (SAED) mode in the inset shows the single-crystal nature of the as-prepared BiOCl samples. The included angle between two neighboring point series of two different particles marked in the SAED pattern was 45°, which was the included angle between the (102) and (002) planes.
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Fig. 3 TEM images of the as-prepared BiOCl samples: (a and b) BiOCl–HCl; (c and d) BiOCl–NaCl; (e and f) BiOCl–HCl-2; (g and h) BiOCl–NaCl-2. |
As shown in Fig. 4(b), Raman spectra were obtained to test the normal mode vibration of the as-prepared BiOCl samples. The peaks at 60.2, 144.8, and 199.4 cm−1 were paired with the vibration of the Bi–Cl bond, in which the peak at 403.5 cm−1 was caused by the motion of oxygen molecules.43 In addition, all the as-prepared BiOCl samples exhibited a new diffraction peak at 115.3 cm−1, through which the significant defects of these samples were therefore proved. Nevertheless, appropriate defects help improve the catalytic efficiency of the proposed material.20
The surface element compositions of the as-prepared BiOCl samples were analyzed by XPS. The entire measurement spectrum (Fig. 5(a)) showed the composition of Bi, O, and Cl elements. The detailed element compositions of BiOCl are listed in Table S1.†
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Fig. 5 (a) XPS patterns; and XPS spectra of the (b) Bi 4f region, (c) Cl 2p region, and (d) O 1s region of the as-prepared BiOCl samples. |
For all the as-prepared BiOCl samples, the proportion of Bi and Cl was approximately 1:
1, illustrating that the BiOCl substance existed in all samples. As shown in Fig. 5(b), the binding energies of Bi 4f5/2 and Bi 4f7/2 were located at 165 and 159 eV and validated the existence of Bi ions.44,45 It can be seen from Fig. 5(c) that the Cl 2p3/2 and Cl 2p1/2 binding energies of the as-prepared BiOCl samples exhibited a similar shift tendency. The XPS spectrum of O 1s is displayed in Fig. 5(d). The O 1s range of the as-prepared BiOCl samples at 530 eV indicated the existence of O2− from the Bi–O bond in BiOCl and the peak at 532 eV was related to the O–H bond, corresponding to the hydroxyl group adsorbed on the surface. The as-prepared BiOCl samples showed obvious hydroxyl peaks, which were due to Bi2O(OH)2SO4 being used as a template to introduce more hydroxyl active components; thereby improving the photocatalytic activity of the as-prepared BiOCl samples.1
To investigate the existence of oxygen defects in the as-prepared BiOCl samples using different preparation methods, we used electron spin resonance (EPR) measurements.46 As the typical g factor of electrons is approximately 2.0, the EPR signal can reflect the existence of unpaired electrons in the as-prepared BiOCl samples.44 Fig. 6(a) shows that all the as-prepared BiOCl samples exhibited significant signals at g = 2.003, suggesting that the EPR signals detected from the as-prepared BiOCl samples originated from the inherent oxygen vacancies.47,48
In order to further ascertain the originating source of the oxygen vacancies existing in the as-prepared BiOCl samples, the EPR was conducted on BBZSN glass and on the intermediate product Bi2O(OH)2SO4. As shown in Fig. 6(b), it was found that the signals occur at g = 2.003; thereby validating that the existing oxygen vacancies in the as-prepared BiOCl samples originated from the common oxygen vacancies in BBZSN glass.33–35 In Fig. 6(c) and (d), it can be seen that under UV irradiation, the intensity of the as-prepared BiOCl samples increased when g = 2.003. This situation is attributed to the oxygen vacancies formed by the adsorbed hydroxyl groups detaching away from the sample surface,1 which shows good consistency with the results for O 1s by XPS. The existence of oxygen vacancies helps improve the catalytic efficiency of BiOCl materials.49–51
hν = A(hν − Eg)n/2 | (1) |
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Fig. 7 (a) UV-vis diffuse reflectance spectra; (b) curves of (αhν)1/2 with respect to hν; (c) XPS valence band spectra; (d) band gap structures of the as-prepared BiOCl samples. |
Fig. 7(b) shows the plots of (αhν)1/2 versus photon energy (hν), through which the corresponding band gaps (Eg) of BiOCl–HCl, BiOCl–HCl-2, BiOCl–NaCl, and BiOCl–NaCl-2 were calculated to be approximately 3.16, 3.12, 3.08, and 3.02 eV, respectively. In addition, VB-XPS was used to determine the VB values of BiOCl–NaCl-2, BiOCl–HCl-2, BiOCl–NaCl, and BiOCl–HCl, which were 2.32, 2.25, 2.21, and 1.86 eV, respectively (Fig. 7(c) and (d)). Then, the CB bottom potentials can be obtained using the following equation:56
ECB = EVB − Eg | (2) |
Accordingly, the calculated minimum ECB values of BiOCl–NaCl-2, BiOCl–HCl-2, BiOCl–NaCl and BiOCl–HCl were −0.7, −0.87, −0.87, and −1.3 eV, respectively.
The variations of the maximum absorption wavelength and of band gap could be attributed to the morphological transformation of the as-prepared BiOCl samples. Normally, the VB edge of the (110) crystal plane is 0.36 eV higher than that of the (001) plane.57 In addition, the decrease in thickness of the BiOCl materials also enabled the upward shift of the VB and CB.58 Therefore, BiOCl–HCl exposing stronger (001) crystal planes and possessing a smaller thickness had the smallest VB value. The decrease in Eg was caused by the oxygen vacancies in the as-prepared BiOCl samples, by which Eg therefore narrowed.51,59 The corresponding band gap structures are illustrated in Fig. 7(d).
In order to examine the catalytic performance of the as-prepared BiOCl samples, the commonly used RhB dye that has been widely applied for industrial purposes was adopted as the simulated pollutant.
As previously discussed, the mixture containing the photocatalyst and RhB needed to be stirred in the dark for 60 min to achieve the adsorption–desorption equilibrium between the catalyst and the dye. It can be seen from Fig. 8 that BiOCl–NaCl featuring a flower-shaped structure possessed the highest adsorption capacity among the as-prepared samples, reaching up to 20.98%. Similarly, the adsorption capacities of the agglomerated bundle-shaped BiOCl–NaCl-2 (15.46%) and of the discoid-shaped BiOCl–HCl (12.76%) were also higher than that of the sheet-structured BiOCl–HCl-2 (8.98%), which can be attributed to the layered porous structure formed and the high BET.20 In addition, with respect to BiOCl materials, the adsorption capacity enjoys higher priority than the conduction band in terms of degradation, for which reason a higher adsorption efficiency was more prioritized in improving the photocatalysis efficiency of the prepared materials.21
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Fig. 9 Absorption curves of RhB with respect to the irradiation time for the as-prepared BiOCl samples (a), (c), (e) and (g) under Ultraviolet light and (b), (d), (f) and (h) under visible light. |
Interestingly, the as-prepared BiOCl samples still retained excellent catalytic performance for RhB degradation under visible-light irradiation. For an exposure time of 100 min under visible light, the RhB degradation efficiency of BiOCl–NaCl-2, BiOCl–NaCl, and BiOCl–HCl reached up to 93.3%, 92.8%, and 84.4%, respectively. In addition, the RhB degradation efficiency of BiOCl–NaCl reached up to 71.4% when exposed under visible light for 20 min, whereas BiOCl–HCl-2 with a larger sheet structure only reached an efficiency of 61.8% and after 100 min exposure under visible light.
The data were replotted on the basis of a pseudo-first-order kinetic model to quantitatively obtain the reaction kinetics of RhB degradation with the different samples (Fig. 10).
ln(C0/C) = kt | (3) |
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Fig. 10 Photocatalytic activities (a) and (c) and kinetics (b) and (d) of the as-prepared BiOCl samples for RhB degradation. |
In the above equation, k denotes the rate constant, t denotes the irradiation time, and C0 and C are the RhB concentrations when the time is 0 and t, respectively. Under UV light, BiOCl–NaCl possessed the maximum k of 0.16016 min−1, which was approximately 1.17, 1.31, and 2.03 times higher than that of BiOCl–HCl, BiOCl–NaCl-2, and BiOCl–HCl-2, respectively. Interestingly, BiOCl–NaCl-2 possessed the maximum k of 0.02221 min−1, which was about 1.34, 1.75, and 2.8 times higher than that of BiOCl–NaCl, BiOCl–HCl, and BiOCl–HCl-2 under visible light, respectively. The rate constants of all the as-prepared samples are shown in Table S3.†
In order to investigate the stability of the BiOCl–NaCl photocatalyst, we conducted a cycle test for the RhB degradation under UV-light irradiation. It can be seen from Fig. S3† that no obvious decrease was observed in photocatalytic activity after conducting four cycles; thereby proving the stability of BiOCl–NaCl during the degradation experiment. In addition, micro-sized structures can be easily and conveniently recycled from solution, through which the high stability may be facilitated to some extent.
The above results suggest that BiOCl–NaCl possessed excellent photocatalytic activity under both visible-light and UV-light irradiation. In contrast to other BiOCl-based or BiOCl-based composite materials (Table S4†), BiOCl–NaCl prepared using bismuth glass exhibited excellent photocatalytic performance.60–66 With respect to the as-prepared BiOCl samples, their photocatalytic activity and ability to degrade pollutants can be attributed to their properties, including their special hierarchical structure, adjustable energy band structure, high specific surface area, and controllable crystal face changes, and to the defects being introduced in the raw material bismuth glass.67,68
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Fig. 11 EPR signals of (a and b) DMPO-˙OH and (c and d) (DMPO)-˙O2− in aqueous dispersions of the as-prepared BiOCl samples. |
Typically, the ˙O2− derived from the O2 reduction by electrons can be detected by probing the 5,5-dimethyl-pyrroline N-oxide (DMPO)-˙O2− complex in methanol medium.69 The ˙OH originating from the OH−/H2O oxidation in water by holes can be detected by probing the DMPO-˙OH in water medium.40 It can be observed here from Fig. 11(a) and (c) that no obvious EPR signal could be observed in BiOCl–NaCl in the dark environment. Under UV-light irradiation, an EPR signal was observed that could be attributed to (DMPO)-˙O2− and DMPO-˙OH, which indicated that both electrons and holes were generated in BiOCl–NaCl. Meanwhile, as shown in Fig. 11(b) and (d), the as-prepared BiOCl samples had obvious EPR signals of (DMPO)-˙O2− and DMPO-˙OH. Due to the square of the EPR signal intensity being approximately proportional to the number of free radicals, the as-prepared BiOCl samples could produce more light-excited electrons and holes, thereby generating more ROS.44,69 Furthermore, the change in the crystal plane also imposed certain impacts on the generation of ROS. Compared with the (010) crystal plane, the (001) crystal plane is more inclined to produce a large amount of ˙O2− and ˙OH.30
It is generally recognized that the photocatalytic dye degradation is realized via two ways: direct semiconductor photoexcitation and indirect dye photosensitization.17 According to the DRS results, the absorption limits of all the as-prepared BiOCl samples were less than the visible-light range (400 nm). However, the as-prepared BiOCl samples still exhibited excellent photocatalytic performance under visible-light irradiation, which was mainly due to the effect of indirect dye sensitization.17
The as-prepared BiOCl samples with a high specific surface area could adsorb a large amount of colored dyes, such as RhB, to which the absorption limit of the samples were shifted toward high wavelength, thereby realizing catalytic decomposition of the dye under visible light.70–72 The difference in catalytic efficiency of the as-prepared BiOCl samples under visible-light irradiation was mainly due to the change in the crystal plane. Under the condition of UV-light irradiation through direct semiconductor photoexcitation, the (001) facet-dominant BiOCl nanosheets exhibited higher photocatalytic activity than the (010) facet-dominant BiOCl nanosheets in terms of RhB degradation. Conversely, the (010) facet-dominant BiOCl nanosheets exhibited higher photocatalytic activity than the (001) facet-dominant BiOCl nanosheets under visible-light irradiation via indirect dye photosensitization.73–75
In addition, a higher VB potential implies a stronger oxidation capacity of photogenerated holes in removing different pollutant molecules, thereby emphasizing the critical role of h+ throughout the photodegradation process,55 in which the detailed photocatalytic process of BiOCl system can be expressed as follows.76
BiOCl + hν → h+ + e− | (4) |
O2 + e− → ˙O2− | (5) |
˙O2− + ˙O2− + 2H+ → H2O2 + O2 | (6) |
H2O2 + e− → ˙OH + OH− | (7) |
RhB + h+, ˙O2−, ˙OH → degradation product | (8) |
Fig. 12 shows the PL spectra of the as-prepared BiOCl samples at an excitation wavelength of 300 nm, through which the recombination rate of e− and h+ could be obtained. It can be seen from Fig. 12 that the peak intensities of the flower-shaped BiOCl–NaCl and of the discoid-shaped BiOCl–HCl in the PL spectra were significantly lower than that of the other samples, by which the samples' low recombination rate of e− and h+ was therefore validated; thus further suggesting a sufficient catalytic effect of e− and h+ throughout the catalytic process.19
The difference in the PL spectra of the as-prepared BiOCl samples depended primarily on the samples' oxygen vacancies and morphology, among which the former could be used as active sites to improve the efficiency of carrier separation. More oxygen vacancies impose more significant encumbrance on the recombination of e− and h+. Similarly, excellent interface contact is also conducive to a faster interface charge transfer, making it more difficult for e− and h+ to recombine.30
The transient photocurrent response and EIS of the as-prepared BiOCl samples are shown in Fig. 13(a) and (b). It is commonly known that the photocurrent of the ITO electrode can be neglected. Among all the as-prepared BiOCl samples, the discoid-shaped BiOCl–HCl exhibited outstanding transient photocurrent response and small resistance characteristics, to which its special layered structure and (001) crystal plane can be primarily attributed.
As mentioned above, BiOCl has an interestingly unique layered structure, which is characterized by the [Bi2O2] plates interlacing with double anion [Cl2] plates. The strong intralayer covalent bonding of the [Bi2O2] plate together with the van der Waals force interaction between the thin layers of the [Cl2] plate jointly cause a nonuniform charge distribution between the cation [Bi2O2] and anion [Cl2] layers, by which the constituent atoms and orbitals are therefore polarized; thus forming an internal electrostatic field (IEF).77
Considering that the IEF direction is perpendicular to the (001) crystal plane but parallel to the (010) crystal plane, therefore, the large charge transfer promoted by IEF possesses more advantages for the (001) crystal plane than the (010) crystal plane due to the shorter diffusion distance. That said, the transient photocurrent response of the (001) crystal plane was far higher than that of the (010) crystal plane, which indicated a higher charge-separation efficiency of the (001) crystal plane under light. In addition, the intensity of the IEF and the separation efficiency of charge carriers could be further improved by increasing the exposure of the (001) crystal plane in the as-prepared BiOCl samples.77
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra06961f |
This journal is © The Royal Society of Chemistry 2021 |