In the past few decades, environmental pollution has become increasingly serious and was caused by organic pollutants, especially organic dyes produced in chemical production and antibiotics widely used in medical treatment^[1-3]. Wastewater containing organic dyes is carcinogenic and mutagenic to the human body after contact^[4,5]. Antibiotics will not only cause chemical pollution but also enhance the drug resistance of bacteria after entering the water quality, thus posing a threat to human public health^[6]. So far, various technologies have been developed to solve the problem of wastewater pollution, such as adsorption, electrochemistry, photocatalysis, ozone oxidation, and precipitation^[7-11]. It is optimal to use green and efficient photocatalysis technology to treat wastewater.

In recent years, BiOCl is considered a promising photocatalyst because of its chemical stability and typical layered crystal structure^[12-15]. However, the low visible light utilization rate and wide band gap greatly limit the application in the visible light photocatalysis field. The construction of the heterojunction is a potential method to overcome these shortcomings^[16-21]. The heterojunction is usually constructed by coupling two semiconductors together to promote the separation and transmission of charge and adjust the energy band gap of the material to improve the utilization rate of visible light. At present, there have been some reports about the heterojunctions of BiOCl with enhanced photocatalytic performance, such as BiOCl/Mn₃O₄^[22], BiOCl/CuO^[23] and BiOCl/BiPO₄^[24]. These heterostructures design two-photon excitation paths to improve the separation and transmission efficiency of photo-generated carriers, such as type-II or Z scheme heterostructures^[8,25-27]. However, in principle, the photon utilization rate of the two-photon excitation pathway will decrease by 50% under the irradiation of a monochromatic light source. Thus, the one-photon excitation pathway (Fig. 1(a)) is more favorable, which can be obtained by forming a heterojunction between a semiconductor with a suitable bandgap and a semiconductor with a large bandgap^[28,29]. Moreover, most traditional heterojunctions have poor photo-excited charge separation/transport characteristics than 2D in-plane heterojunctions (Figs. 1(b) and1(c))^[30,31]. Based on the above analysis, the construction of a 2D in-plane heterojunction with a one-photon excitation pathway is a promising method to improve photocatalytic efficiency under visible light. Metal-organic frameworks (MOFs) and their derivatives have become a research hotspot because of their porous crystalline structure and abundant surface properties^[32]. In 2016, CAU-17 was first synthesized and considered as the preparation cost which is the lowest among Bi-MOFs^[33,34]. Zhuet al. obtained a BiOBr/CAU-17-2h photocatalyst with CAU-17 as the precursor, which showed excellent performance in removing 90% rhodamine b within 50 min^[35]. Yanget al. designed the Bi₂WO₆/CAU-17 composite photocatalyst, and the best sample showed the photocatalytic degradation performance of 90% methyl blue in 60 min^[36]. Therefore, BiOCl nanosheets can grow in situ on CAU-17 micron-rod via a halogenation process to avoid the aggregation of individual BiOCl nanosheets.

Herein, in-plane Bi₂O₃/BiOCl (i-Cl) heterostructures were synthesized for boosted photocatalytic degradation of RhB and TC by a one-photon excitation pathway of visible light. The novel in-plane heterostructure structure endows the BiOCl-based photocatalyst with efficient spatial carrier separation and intense light absorption capability. As a result, the optimized i-Cl composite shows enhanced photocatalytic efficiency compared to those of pure BiOCl and Bi₂O₃, as well as mechanistically mixed Bi₂O₃/BiOCl samples. The RhB and TC degradation activities of the optimum i-Cl sample show a remarkable 265.08-fold and 4.08-fold enhancement compared with pristine BiOCl. This work provides a logical strategy to construct other 2D in-plane heterojunctions with a one-photon excitation pathway with enhanced performance.

1,3,5-benzene tricarboxylic acid (H₃BTC), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), N, N-dimethylformamide (DMF), methanol anhydrous (MeOH), ethanol, ammonium bromide (NH₄Br), ammonium iodide (NH₄I), and ammonium chloride (NH₄Cl), were all untreated pure samples. The experimental water is deionized.

According to the previous report^[37], CAU-17 was synthesized by the solvothermal method. Firstly, 55 mL MeOH and 5 mL DMF were added into a 100 mL beaker with 1 mmol Bi(NO₃)₃·5H₂O. Then, 3 mmol H₃BTC was dissolved into the reaction mixture and stirred at room temperature for 30 min. The mixed solution was sealed into a 100 mL polytetrafluoroethylene-lined autoclave for 24 h with the oven temperature set at 120 °C. The resulting samples were washed with MeOH and DMF and dried in an oven for 12 h at a temperature of 60 °C.

In-plane Bi₂O₃/BiOX (X = Cl, Br, I) photocatalysts were obtained via the water bath method. Firstly, 50 mL of deionized water was added into a 100 mL triangular bottle with 10 mmol of NH₄X (X = Cl, Br, I). Under stirring, 0.5 g of CAU-17 was added into the above triangular bottle and placed in a water bath pot for 1h, and the temperature was set at 90 °C. The obtained sample was washed several times with pure water and dried. After that, the resulting powder was placed in a muffle furnace and kept at 450 °C for 2 h, and the heating rate was 2 °C /min. The products were named i-X (X = Cl, Br, I), respectively. In addition, the Bi₂O₃ sample was obtained by directly calcining CAU-17.

For comparison, BiOCl was synthesized by primitive hydrothermal method. Then, the traditional Bi₂O₃/BiOCl sample was obtained by a simple electrostatic-driven self-assembling method, which was named t-Cl. A certain amount BiOCl and Bi₂O₃ were added into 60 mL of ethanol and stirred at 60 °C until the solvent evaporated. The molar ratio of BiOCl and Bi₂O₃ was 5 : 1, which was consistent with the proportion in the i-Cl sample.

The photocatalytic performance of the prepared samples was evaluated by photocatalytic degradation of RhB (10 mg/L), TC (10 mg/L), and the mixed solution of RhB (10 mg/L) and TC (10 mg/L) under visible-light irradiation. In the degradation system, a 300 W Xe lamp with a 420 nm cut-off filter was used as the light source. Meanwhile, 10 mg of the photocatalyst was added into 50 mL RhB (10 mg/L), TC (10 mg/L) aqueous solution, and a mixed aqueous solution of RhB (10 mg/L) and TC (10 mg/L), respectively. Then, the mixture should be stirred in the dark for 30 min to reach the adsorption-desorption equilibrium. Then, the beaker was irradiated under visible light and 3 mL of water was extracted every 5 min. After that, dye and antibiotic solution concentration was determined on a UV-vis spectrophotometer at the wavelengths of 554 and 357 nm for RhB and TC, respectively.

In addition, the synthesis of BiOCl, DFT calculation, and characterizations were introduced in the supporting information.

As shown inFig. 2(a), samples including CAU-17, Bi₂O₃/BiOCl (i-Cl), Bi₂O₃/BiOBr (i-Br) and Bi₂O₃/BiOI (i-I) were prepared. The XRD analysis was performed to analyze the structure of CAU-17, Bi₂O₃, i-Cl, BiOCl, i-I, and i-Br samples. The XRD pattern of CAU-17 was almost consistent with the previously reported literature^[37], demonstrating the successful synthesis of CAU-17. As shown inFig. 2(b), the pattern of Bi₂O₃ without halogenation treatment is consistent with that of Bi₂O₃ (JCPDS#65-2366), which indicates that bismuth-base CAU-17 was converted into Bi₂O₃ during the calcination process. The major characteristic peaks located at 11.92°, 25.38°, 31.86°, 32.84°, 39.94°, 45.68°, 48.38°, 52.78°, 57.19° marked with the diamond symbol of i-Cl are ascribed to BiOCl (JCPDS#06-0249)^[38] and the peak located at 27.71° marked with plum blossom symbol vest in Bi₂O₃^[39]. Moreover, it can be seen fromFig. 2(c) that the major peaks of i-I and i-Br are highly corresponding to Bi₅O₇I (JCPDS#40-0548) and BiOBr (JCPDS#85-0862), respectively^[40,41].

The morphologies of i-X (X = Cl, Br, I) samples were observed by a scanning electron microscope (SEM). As shown in Fig. S1, the morphology of CAU-17 displayed rod-like morphologies, which was consistent with the previous paper^[34]. The rod-shaped frame was retained and the nanosheets with a thickness of 20 nm were accumulated on the rod-shaped surface in layers. Moreover, the transmission electron microscope (TEM) and high-resolution transmission electron microscope (HR-TEM) images were obtained to further analyze the lattice structure of the i-Cl sample. The lattice spacings of 0.275 and 0.32 nm corresponding to the (110) lattice plane of BiOCl and (201) crystal plane of Bi₂O₃^[42], respectively, appeared in the central area of the nanosheet (Fig. 3(c)), confirming the formation of in-plane Bi₂O₃/BiOCl heterostructures. The EDS mapping was carried out and the images were presented inFigs. 3(d)–3(f). The images indicated the existence of Bi (blue), O (green), and Cl (red) elements, and all the elements were distributed on the CAU-17 rod frame. The atomic fraction and mass fraction of the three elements were recorded in Table S1. The molar ratio of BiOCl to Bi₂O₃ in the i-Cl sample can be calculated to be about 5 : 1. The energy dispersive X-ray (EDX) line profile of the i-Cl sample (Fig. 3(g)) shows that the spatial distribution of the three elements is obvious inhomogeneous. There were obvious differences after the fabrication of core-shell structure, which further confirms that the i-Cl sample is heterogeneous. Fig. S2 shows the SEM images of the i-I and i-Br samples. Their morphologies all retained the rod-shaped frame of CAU-17 and the nanosheets accumulated on the rod-shaped surface. Atomic structures of BiOCl (110) and Bi₂O₃ (201) were shown inFigs. 3(h) and3(i).

X-ray photoelectron spectroscopy (XPS) was conducted to elucidate the element composition and chemical state of the samples.Fig. 4(a) shows the survey spectra of BiOCl, Bi₂O_3, and i-Cl samples. As shown inFig. 4(b), for pure BiOCl, the two peaks of Bi 4f_5/2 and Bi 4f_7/2 at 165.21 and 159.90 eV demonstrate Bi is +3 oxidation state^[43]. Compared with BiOCl, the Bi 4f peaks of i-Cl show similar two symmetrical peaks and shift to the smaller binding energy. The Bi 4f spectra of Bi₂O₃ show the peaks at 164.17 and 158.89 eV indicating that Bi exists in a +3 oxidation state^[44]. As shown inFig. 4(c), the migration trend of O 1s is similar to that of Bi 4f. The spectra of Cl 2p are shown inFig. 4(d). It can be seen that the peaks at 200.19 and 198.59 eV for BiOCl are assigned to Cl 2p_1/2 and Cl 2p_3/2. For i-Cl, the Cl 2p peaks shift to the smaller binding energy indicating that the built-in electric field will be generated between BiOCl and Bi₂O₃, and the charge will be transferred from Bi₂O₃ to BiOCl^[45].

The photocatalytic performances of the prepared samples were investigated by RhB, TC, and mixed wastewater with TC and RhB degradation under visible light. The photocatalytic degradation comparison results of i-X (X = Cl, Br, I) samples are shown in Figs. S4 and S5. Among the three samples, i-Cl has the best degradation rate constant. The RhB degradation rate constant of i-Cl is 4.25-fold that of i-Br and 2.72-fold that of i-I, respectively. The TC degradation rate constant of i-Cl is 2.41-fold that of i-Br and 2.17-fold that of i-I, respectively. Photocatalytic degradation comparison results of i-Cl, Bi₂O₃, BiOCl, and traditional Bi₂O₃/BiOCl (t-Cl) are shown inFigs. 5 and6. As shown inFig. 5(a), the concentration of RhB decreased with increasing reaction time overall photocatalysts. The RhB degradation rate of the four samples from low to high is BiOCl, Bi₂O₃, t-Cl, and i-Cl, and the degradation rate reached 3%, 7%, 22%, and 90% in 20 min, respectively. It can be seen that BiOCl and Bi₂O₃ samples can hardly degrade RhB, the RhB degradation rate of the t-Cl sample was increased by 15% and the degradation efficiency of the i-Cl sample was greatly improved compared with t-Cl samples due to its good interface effect.Fig. 5(b) shows the degradation rate of RhB follows quasi-first-order kinetics. The kinetic constant (k) of i-Cl is 9.3-fold that of t-Cl.Fig. 5(c) shows the TC degradation rate of the four samples. The TC degradation rate of the four samples from high to low is i-Cl, t-Cl, Bi₂O_3, and BiOCl, and the degradation rate reached 25%, 30%, 40%, and 70% in 20 min, respectively. In addition,Fig. 5(d) shows that the TC degradation rate of all photocatalytic systems follows quasi-first-order kinetics. Thek value of i-Cl is 2.14-fold that of t-Cl. The photocatalytic performance of i-Cl was compared with some BiOCl-based catalysts previously reported in Tables S2 and S3. It is found that i-Cl has a higher degradation rate constant than these catalysts. Therefore, the remarkable activity of i-Cl makes it a valuable photocatalyst for removing organic pollutants.

The photocatalytic mixed wastewater with RhB and TC degradation rates of various samples are shown inFig. 6. As shown inFigs. 6(a) and6(c), the photocatalytic RhB and TC degradation rates of i-Cl samples are 64% and 65% in 20 min. Compared with the degradation of RhB and TC alone, the degradation rate of RhB decreased by 26% and the degradation rate of TC decreased by 5%. It can be seen that the i-Cl sample can degrade RhB and TC simultaneously under visible-light irradiation. Compared with the i-Cl sample, the TC degradation rate of the t-Cl samples was almost unchanged, and the degradation rate of RhB was reduced by 20%. BiOCl and Bi₂O₃ samples still cannot degrade RhB, but the degradation rate of TC was basically unchanged.Figs. 6(b) and6(d) showed the corresponding k values of RhB and TC degradation in the mixed wastewater photocatalytic systems. In summary, the TC degradation rates of the four samples are all almost unchanged. BiOCl and Bi₂O₃ cannot degrade RhB and the RhB degradation rate of BiOCl and t-Cl samples decrease by over 20%, which may be that TC has more advantages in the competitive adsorption on the surface of photocatalyst^[46]. Fig. S6 shows the UV-vis curves of different aqueous solutions over i-X (X = Cl, Br, I) samples. In addition, the cycling ability of the i-Cl sample on mixed wastewater with TC and RhB degradation was evaluated. As shown in Fig. S7(a), the degradation activity decreased only slightly after four cycles. Fig. S7(b) shows the comparison of XRD patterns of the i-Cl sample before and after the recycling process. It can be seen that the peak position has shifted, but the major peaks only changed a little. These results indicate i-Cl sample has good stability in the photocatalytic degradation of mixed wastewater with TC and RhB.

The results of UV-vis DRS were exhibited inFig. 7(a). It can be seen that the obvious absorption edges of BiOCl and Bi₂O₃ are located at 368 and 443 nm, respectively. Compared with the pristine BiOCl sample, the i-Cl and t-Cl samples exhibit an obvious red shift and enhanced UV and visible-light harvesting, which is due to the existence of Bi₂O₃ with strong light-harvesting capability. This extended light absorption is beneficial to the photocatalytic reaction.

The photoluminescence (PL) spectra were tested inFig. 7(b). It can be seen fromFig. 7(b) that the pristine BiOCl displays a prominent emission peak, suggesting the intense recombination of carriers inside BiOCl. After combining with Bi₂O₃, the PL intensity of i-Cl and t-Cl heterojunctions decrease slightly compared to pristine BiOCl, implying that the presence of Bi₂O₃ could retard the electron-hole recombination. t is worth noting that the PL intensity of the i-Cl heterojunction is lower than that of the t-Cl heterostructure, revealing the superiority of monodispersed and in-plane heterostructure for suppressing the recombination of photocarriers^[47]. Surface photovoltage (SPV) spectra of BiOCl, Bi₂O_3, and i-Cl samples were shown inFig. 7(c). It can be seen that i-Cl showed a stronger photovoltage in the visible range compared with BiOCl, which indicated that i-Cl can respond under visible light irradiation. And i-Cl showed higher photovoltage than Bi₂O₃ due to lower electron-hole recombination^[48]. Photocurrent responses and electrochemical impedance spectra (EIS) test are recorded inFigs. 7(d) and7(e). Among all the samples, the i-Cl sample exhibited the highest photocurrent response and smallest semicircle, revealing the most accelerated photocarrier separation efficiency.

According to the absorption spectra of BiOCl and Bi₂O₃, the corresponding band gaps of a semiconductor were calculated by the Tacu Plot formula^[49]:

1(αhν)1/2=A(hv−Eg).

Therefore, the band gap of Bi₂O₃ and BiOCl can be calculated as 2.80 and 3.37 eV (Fig. 8(a)).Fig. 8(b) shows the valence band (VB) values of Bi₂O₃ and BiOCl. Then, the conduction band (CB) values are 0.35 and –0.29 V of Bi₂O₃ and BiOCl according to the coming formula:

3ECB=EVB−Eg.

The band structures of Bi₂O₃ and BiOCl samples before and after contact are shown inFig. 8(c). When BiOCl contacts Bi₂O₃, the built-in electric field will be generated at the interface between BiOCl and Bi₂O₃. To know the direction of the carrier transfer of the internal electric field (IEF) between the Bi₂O₃/BiOCl heterojunction, the charge density difference of the BiOCl/Bi₂O₃ heterojunction and work functions of Bi₂O₃ (201) and BiOCl (110) were calculated by DFT simulation. It can be seen fromFig. 8(d) that charge transfer from BiOCl to Bi₂O₃. It can be seen fromFigs. 8(e)–8(f) that the fermi level of Bi₂O₃ (201) is higher than that of BiOCl (110), which further demonstrated that charge transfer from BiOCl to Bi₂O₃ when they are closely contacted by IEF between BiOCl and Bi₂O₃.

The reactive species trapping experiment of the i-Cl sample was carried out to analyze the major active substances during the mixed wastewater degradation system. Benzoquinone (BQ), disodium EDTA (EDTA-2Na), and isopropyl alcohol (IPA) are used as scavengers for superoxide radical (⋅O2−)^[50], photoinduced holes (h⁺)^[51] and hydroxyl radical (·OH)^[52], respectively. As shown inFig. 9(a), IPA and EDTA-2Na had a great effect on the degradation system, but BQ hardly inhibited the degradation of TC and RhB. The results indicated that h⁺ and ·OH are the main active substances in the photocatalytic degradation system. To further confirm the production of the active radical, electron spin resonance (ESR) test was carried out^[53]. As shown inFig. 9(b), no DMPO-·OH signal was detected in this system without visible-light irradiation while the DMPO-·OH signal can be detected under visible light. In addition, the intensity of the DMPO-·OH signal was increased gradually with the irradiation time gone. The results indicated that ·OH was produced after illumination.

Fig. 9(c) shows a plausible mechanism based on the one-photon excitation pathway for the photocatalytic degradation over the i-Cl sample. Under the irradiation of visible light, the photoexcited electrons jumped from the valence band (VB) of Bi₂O₃ to the conduction band (CB) of Bi₂O₃. While the BiOCl fails to be excited by visible light. As the VB position of Bi₂O₃ is more positive than that of BiOCl and the CB position of Bi₂O₃ is more negative than that of BiOCl, the photogenerated holes in the VB of Bi₂O₃ can migrate quickly to the VB of BiOCl and the electrons reserved in the CB of Bi₂O₃, which is beneficial to the effective spatial separation of photogenerated carriers. Our experimental photocatalytic performance for degrading RhB and TC demonstrates that this one-photon excitation on Bi₂O₃ coupled with BiOCl with a large bandgap can make full use of photons, and it is expected to achieve higher theoretical photon utilization efficiency.

Then, the holes in the CB of BiOCl easily react with H₂O to produce hydroxyl radicals (·OH) that serve as reactive oxygen species for the degradation of organic pollutants. The possible degradation steps are as follows:

4i−Cl+hυ→e−+h+,

5H2O+h+→H2O+→h++OH−,

6OH−+h+→⋅OH,

7TC/RhB+⋅OH→Products.

In conclusion, the 2D in-plane heterogeneous i-Cl composite composed of BiOCl and Bi₂O₃ was synthesized through one-step calcination of halogen-doped CAU-17 for photocatalytic degradation of RhB and TC by a one-photon excitation pathway. In the design of this in-plane heterojunction, the chemical bond at the interface will induce a strong intrinsic electric field, which will promote the charge separation and transmission of photogenerated carriers. In addition, the matched band structures and the strong intrinsic electric field between BiOCl and Bi₂O₃ accelerate the charge transfer from Bi₂O₃ to BiOCl, constructing a one-photon excitation pathway in nanocomposite photocatalysts for efficiently degrading dyes and antibiotics. The i-Cl sample exhibited enhanced photocatalytic performance of degrading TC and RhB aqueous solutions and mixed wastewater containing TC and RhB compared with pure BiOCl, Bi₂O_3, and a traditional t-Cl composite composed of BiOCl and Bi₂O₃. The degradation rate of the RhB solution reached 90% in 20 min and that of the TC solution reached 70% in 20 min. And in the photocatalytic degradation system for mixed wastewater with TC and RhB, the i-Cl sample also has an excellent RhB and TC removal efficiency of up to 64% and 65% at the same time within 20 min. This work provides a logical strategy to construct other 2D in-plane heterojunctions with a one-photon excitation pathway with enhanced photocatalytic performance.

This work was supported by the National Natural Science Foundation of China (11874314, 12174157, and 12074150), the Natural Science Foundation of Jiangsu Province (BK20201424), the Modern Agricultural Equipment and Technology Collaborative Innovation Project (XTCX2025), and the Graduate Research and Innovation Projects of Jiangsu Province (KYCX22_3602).