Highly efficient photocatalytic degradation over rose-like 1D/2D La(OH)₃/(BiO)₂OHCl heterostructures boosted by rich oxygen vacancies and enhanced interfacial charge transfer

Abstract

In this work, novel rose-like photocatalysts were successfully fabricated by constructing 1D/2D La(OH)₃ nanorod/(BiO)₂OHCl nanosheet heterojunctions. The optimal sample, BOL-2, synthesized with a Bi/La molar ratio of 4 : 1 showed significantly boosted visible-light-induced photocatalytic activity. Its kinetic rate constant (k) in the photocatalytic degradation of tetracycline (TC) and o-nitrophenol was 3.5 and 6.4 times that of (BiO)₂OHCl. The existence of rich surface oxygen vacancies (OVs) and 1D/2D heterojunctions together contributes to enhanced photocatalytic performance. The OVs broadened the light absorption range and enhanced visible light absorbance, while the formation of the La(OH)₃/(BiO)₂OHCl heterojunction by close 1D/2D interface contact improved the charge separation and transfer efficiency of BOL-2. The free radical scavenging tests and DMPO spin-trapping technology further affirmed that more ˙O₂⁻ and ˙OH as active species were generated in the presence of BOL-2 than in the presence of (BiO)₂OHCl, thereby leading to remarkably improved photocatalytic performance. BOL-2 also exhibited excellent stability in photocatalytic degradation, and the degradation efficiency only declined by 2.5% in the fourth cycle. Possible photocatalytic mechanisms as well as degradation pathways of TC over BOL-2 were explored. This work will provide new insights into the rational design of efficient photocatalysts by 1D/2D heterojunction construction and OV engineering for wastewater remediation.

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Environmental significance

Antibiotics are frequently detected in natural water resources due to the discharge of a large amount of wastewater containing antibiotics. The long-term presence of antibiotics in a natural environment largely accelerates the occurrence of antibiotic-resistant genes/bacteria and the spread of antibiotic drug resistance, which poses a serious threat to the security of ecological systems. Therefore, it is urgent to explore an efficient way to remove antibiotics from wastewater. This work has reported the fabrication of novel rose-like La(OH)₃ nanorod/(BiO)₂OHCl nanosheet photocatalysts by 1D/2D heterojunction construction and OV engineering for highly efficient removal of antibiotics from wastewater.

Introduction

Recently, frequent detection of various antibiotics in natural water resources, e.g., surface water and ground water, has attracted extensive public attention.¹ Most of these antibiotics cannot be completely degraded by traditional treatment processes in wastewater treatment plants, leading to inevitable discharge of antibiotics into natural water bodies.^2,3 The long-term presence of antibiotics in a natural environment largely accelerates the occurrence of antibiotic-resistant genes/bacteria and spread of antibiotic drug resistance, which becomes a serious threat to the security of ecological systems and even the health and safety of human beings.^3,4 Therefore, it is urgent to explore an efficient way to remove antibiotics from wastewater.

Various technologies including membrane processes, microbial/enzymatic degradation, ozonation, and Fenton process have been studied to remove antibiotics.⁵ Among them, microbial/enzymatic degradation processes can decompose antibiotics into small harmless molecules, such as H₂O and CO₂, using microbial communities or enzymes. However, they show quite limited antibiotic removal efficiency at 9–20% in biological wastewater treatment.⁶ The membrane process shows high removal efficiency of antibiotics; however, it has disadvantages of high energy consumption and high operational cost.⁷ By contrast, semiconductor-based photocatalysis with the utilization of renewable solar energy is considered as one of the green and effective wastewater treatment strategies.^1,4,8 Bismuth oxychloride (BiOCl) has been regarded as an attractive semiconductor with distinctive features of good photocatalytic activity, high chemical stability, low toxicity and corrosion resistance.^8,9 The unique lamellar structure of BiOCl featured by [Bi₂O₂]²⁺ slabs sandwiched between two Cl⁻ slabs enables the formation of internal static electric fields between the layers.⁸ When Cl⁻ ions in BiOCl are partially replaced with OH⁻ ions, a new material, (BiO)₂(OH)Cl, is generated; it has stronger light absorption capability and a narrower band gap than bare BiOCl.^10,11 These substituted OH⁻ anions can not only capture photo-generated holes to form hydroxyl radicals preferentially but also enhance the internal static electric fields in (BiO)₂OHCl, thus improving the photocatalytic activity.¹⁰ Nevertheless, the visible light absorption ability of (BiO)₂OHCl still needs improvement.

Due to the unique electronic structure and low energy of surface Bi–O bonds, surface oxygen vacancy (OV) engineering has been regarded as an outstanding strategy to improve the light absorption capacities of Bi⁻ based photocatalysts and promote their photocatalytic activity.^12,13 The introduction of surface OVs can create a new discrete energy band between its conduction band (CB) and valence band (VB), which enables the OV-rich photocatalysts to harvest the light with a longer wavelength.¹⁴ Moreover, the OVs can not only act as adsorption centers for benefiting O₂ adsorption onto the catalysts, but also act as electron (e⁻) traps for accelerating the production of superoxide radicals (˙O₂⁻) by reacting adsorbed O₂ with the trapped e⁻.¹³ The above-mentioned process leads to the extremely enhanced separation of photogenerated electrons and holes, and in turn, boosts the photocatalytic performance of OV-rich photocatalysts.

It is noted that morphology control has been widely selected to improve the photocatalytic performances of semiconductors.^15–17 Among various architectures, the flower-like microstructure stands out for enhanced photocatalytic activity due to some extraordinary advantageous features including abundant porous and channel-rich structure, large specific surface area, and multiple reflection of light.^16,17 These are beneficial features to promote the mass transfer of contaminants, provide more exposed active sites, and enhance the light absorption of flower-like photocatalysts.^18,19 However, the construction of heterojunctions with two semiconductors having matched band energy structures can dramatically promote the spatial separation and transfer of photogenerated electron–hole pairs, thereby enhancing the photocatalytic activity.^20–24 In particular, flower-like heterostructures assembled by binary or ternary semiconductor heterojunctions have gained great research interest due to the joint advantages from both morphology control and heterojunction construction.^16,25 For example, Etogo et al. controllably synthesized BiOCl/(BiO)₂CO₃/Bi₂O₃ heterostructures with a flower-like morphology, which showed remarkably enhanced photocatalytic degradation efficiency towards organic pollutants, as compared with the mechanical mixture of (BiO)₂CO₃, Bi₂O₃, and BiOCl.²⁵ However, until now, only a few types of (BiO)₂OHCl-based heterojunction catalysts such as (BiO)₂OHCl@Bi₂₄O₃₁Cl₁₀ (ref. 26) and g-C₃N₄/(BiO)₂(OH)_xCl_2−x (ref. 27) have been reported with enhanced photocatalytic performance.

As a typical rare earth hydroxide, 1D La(OH)₃ has a unique electronic structure and a controllable morphology with a large specific surface area, which has been regarded as an emergent photocatalyst, and then drawn increasing research interest.^11,23 The incorporation of La(OH)₃ into heterojunctions has attracted significant attention because of its eminent properties such as reduction of band gap, generation of highly active hydroxyl radicals, and remarkable pollutant adsorption during the photocatalytic process.²⁸ Various heterojunctions such as La(OH)₃/BiOCl,¹¹ La(OH)₃/BiOI,²³ La(OH)₃@BaTiO₃,²⁴ and La(OH)₃/sGO-Ag₃VO₄/Ag (ref. 28) have been fabricated with La(OH)₃, and they showed boosted photocatalytic performances.

In this work, novel rose-like heterostructures were fabricated via constructing 1D/2D La(OH)₃ nanorod/(BiO)₂OHCl nanosheets by a facile and template-free hydrothermal method. The formed heterojunctions featuring with a multi-dimensional interface are expected to further improve the photocatalytic performance of the resulting composites by integrating fast electron transport along 1D nanorods and superior electron mobility in 2D nanosheets, both of which are beneficial for accelerating the transfer and separation of charge carries.^29,30 Moreover, the combined 1D and 2D hierarchical structure would likely enlarge the interface contact area and then provide abundant active sites for superior photocatalytic performance.^31,32 In our study, the visible-light-induced photocatalytic performances of 1D/2D La(OH)₃/(BiO)₂OHCl catalysts were optimized by selecting tetracycline hydrochloride (TC) and o-nitrophenol (o-NP) as target pollutants. The mechanisms underlying enhanced photocatalytic performance were explored in detail, which were supported by inherent characteristics including crystal, textual, optical, and photoelectrochemical properties of samples. It is believed that the improved photocatalytic performances were ascribed to the combined effect of flower-like heterojunctions and OVs. The possible photocatalytic degradation pathways of TC were also explored on the basis of identified intermediates.

Experimental

Preparation of photocatalysts

All the materials used are described in ESI† (ESI, Text S1). The synthesis of La(OH)₃/(BiO)₂OHCl heterojunction photocatalysts is schematically shown in Scheme 1. Typically, 0.97 g Bi(NO₃)₃·5H₂O was dissolved in 20 mL deionized (DI) water, followed by the addition of 0.149 g KCl. An appropriate quantity of concentrated ammonia solution was used to adjust the pH of the solution to 10.0. Our previous results have indicated that BiOCl was formed when the pH of the solution was adjusted to 7.0.⁹ Herein, under alkaline conditions with a solution pH of 10.0, Cl⁻ between the [Bi₂O₂]²⁺ layers of BiOCl could be partially replaced by OH⁻ in the solution via an ion-exchange process, leading to the formation of (BiO)₂OHCl.²⁶ After continuous stirring for 5 min, a certain amount of La(NO₃)₃·6H₂O (0.05 g, 0.1 g, 0.2 g or 0.3 g) was added, and the obtained mixture was stirred for another 1 h at room temperature before transferring to a polytetrafluoroethylene reactor for reaction at 180 °C for 20 h. Once the sealed reactor was cooled to room temperature, solid powder materials were obtained by centrifugal separation, followed by repeated washing with DI water and ethanol. The final product was dried at 60 °C for 12 h and named BOL-x (x = 1–4), corresponding to Bi/La molar ratios of 8 : 1, 4 : 1, 2 : 1 or 4 : 3, respectively.

Scheme 1 Synthesis of (BiO)₂OHCl/La(OH)₃ heterojunction photocatalysts.

For comparison, the samples of pure (BiO)₂OHCl and La(OH)₃ were also prepared. Pure (BiO)₂OHCl was synthesized following the above-mentioned procedures but without the addition of La(NO₃)₃·6H₂O. La(OH)₃ nanorods were synthesized based on a previous study.²³ Besides, mechanically mixed (BiO)₂OHCl/La(OH)₃ with a Bi/La molar ratio of 4 : 1 was synthesized for comparison. Briefly, 2 mmol (BiO)₂OHCl and 1 mmol La(OH)₃ were dispersed in 30 mL absolute ethyl alcohol and stirred for 12 h. The resulting sample was denoted as m-BOL-2.

Photocatalytic activity of catalysts

Tetracycline hydrochloride (TC) and o-nitrophenol (o-NP) were selected as typical pollutant models to study the photocatalytic activity of our catalyst materials. In brief, 50 mg photocatalyst was suspended in 80 mL aqueous solution containing TC (5–100 mg L⁻¹) or o-NP (30 mg L⁻¹) in a double-walled beaker reactor at room temperature. A 420 nm LED lamp was used to provide simulated visible light on top of the reactor. Before applying light irradiation, the suspension was stirred in the darkness for 30 min in order to reach the adsorption–desorption equilibrium. During light irradiation, 3 mL of the suspension was taken out and centrifuged at 10 000 rpm to analyze the residual concentration of TC or o-NP in the supernatant.

The trapping experiments were carried out to investigate the roles of reactive species, i.e. superoxide radical (˙O₂⁻), holes (h⁺), and hydroxyl radicals (˙OH), in the photocatalytic degradation of TC over BOL-2. 1,4-Benzoquinone (PBQ), ethylenediamine tetraacetic acid disodium salt dihydrate (EDTA-2Na) or tert-butanol (TBA) was added as the scavenger for ˙O₂⁻, h⁺, or ˙OH, respectively. The experimental procedures were similar to the above-mentioned tests on photocatalytic activity. The reusability of BOL-2 was carried out by collecting and using it for four cycles. After each cycle, the spent catalyst sample was collected by centrifugation at 10 000 rpm for 10 min and then washed with distilled water. After drying at 80 °C overnight, it was reused in the next cycle of photocatalytic degradation. Approximately 85.4% of used catalyst was collected after each such centrifugation-washing-drying.

Results and discussion

Phase structures

Fig. 1a shows the XRD spectra of the as-prepared samples. For pure (BiO)₂OHCl, the XRD peaks located at 2θ of 11.61°, 26.01°, 33.24° and 47.72° are ascribed to the (001), (101), (110) and (200) crystal planes of (BiO)₂OHCl (JCPDS # 41-0708).¹⁰ Furthermore, the peaks at 2θ of 15.66°, 27.97°, 39.47° and 48.64° for pure La(OH)₃ correspond to the (100), (101), (201) and (211) crystal planes of La(OH)₃ (JCPDS # 83-2034).³³ Note that the intensities of La(OH)₃ characteristic peaks in BOL-x composites increase along with increased addition of La³⁺ during synthesis, from sample BOL-1 to BOL-4. Obvious diffraction peaks referred to La(OH)₃ can be observed in the XRD pattern of BOL-4, whereas the La(OH)₃ peaks in the XRD patterns of BOL-1 and BOL-2 are relatively weak, probably due to the smaller amount of La(OH)₃ formed. However, La(OH)₃-related peaks can still be clearly observed when the XRD pattern of BOL-2 is separately compared with La(OH)₃ and (BiO)₂OHCl standard cards (Fig. S1a†). This indicates that La(OH)₃/(BiO)₂OHCl heterojunctions have been successfully synthesized. Besides, Fig. S1b† shows the magnified XRD pattern of the 2θ region from 25° to 28°. It was found that the peaks of the (101) crystal plane for (BiO)₂OHCl in BOL-x shift to higher values, which is believed due to lattice distortion caused by the incorporation of La³⁺ into (BiO)₂OHCl. A similar phenomenon was also found by La(OH)₃/BiOCl heterojunctions with high OVs.¹¹

Fig. 1 (a) XRD patterns and (b) FT-IR spectra of pure La(OH)₃, (BiO)₂OHCl, and BOL-x (x = 1, 2, 3, and 4).

The FT-IR spectra of samples are displayed in Fig. 1b. The absorption peaks at 3424 cm⁻¹ and 1632 cm⁻¹ are ascribed to the vibrations of the O–H bond from surface adsorbed water. For (BiO)₂OHCl and BOL-x composites, the absorption peak at 3546 cm⁻¹ is assigned to the stretching vibration of the O–H bond arising from (BiO)₂OHCl, indicating the presence of hydroxyl groups in (BiO)₂OHCl.³⁴ The two absorption peaks at 551 cm⁻¹ and 1388 cm⁻¹ are identified as the vibrations of the Bi–O bond and Bi–Cl bond.¹⁰ For La(OH)₃ and BOL-x composites, the absorption peak at 3610 cm⁻¹ matches the vibration of the O–H bond in La(OH)₃, and the band at 654 cm⁻¹ corresponds to La–OH bending vibration in La(OH)₃.³⁵ Obviously, the FT-IR spectra of BOL-x composites include characteristic absorption peaks of Bi–O and La–OH originating from (BiO)₂OHCl and La(OH)₃, strongly confirming the successful synthesis of La(OH)₃/(BiO)₂OHCl composites.

Morphological characteristics

The morphology of (BiO)₂OHCl, La(OH)₃ and BOL-2 was investigated by SEM (Fig. 2). The sample (BiO)₂OHCl exhibits rose-like microflowers assembled by interlaced 2D nanosheets (Fig. 2a and b), while La(OH)₃ displays a stacked 1D rod-like structure (Fig. 2c). A similar rose-like morphology is also observed for BOL-2 (Fig. 2d). When these rose-like architectures are further magnified for investigation under SEM, several 1D La(OH)₃ nanorods can be found to rationally grow on 2D (BiO)₂OHCl nanosheets. Such close 1D/2D interface contact of La(OH)₃/(BiO)₂OHCl can also be obviously found in various parts of other BOL-2 microflowers (Fig. S2a and b†), indicating the formation of heterojunctions between La(OH)₃ and (BiO)₂OHCl with effective contact. Fig. S2g† shows the energy-dispersive X-ray spectra (EDS) of BOL-2, which reveals that all the O (20.37 wt%), Cl (5.80 wt%), La (19.84 wt%) and Bi (53.98 wt%) elements are present in BOL-2. This is further confirmed by the SEM mapping shown in Fig. S2c–f.†

Fig. 2 SEM images of (a and b) (BiO)₂OHCl, (c) La(OH)₃, and (d–f) BOL-2.

In Fig. 3a and b, the 1D nanorods are deposited on the surface of 2D nanosheets. The close contact between the 1D nanorods and 2D nanosheets leads to the formation of 1D/2D heterojunctions. Fig. 3c and d shows the HRTEM images of selected nanorod and nanosheet areas in Fig. 3b. As can be seen, the d space of 0.32 nm is attributed to the (101) plane of La(OH)₃, while the d space of 0.34 nm is ascribed to the (101) plane of (BiO)₂OHCl. Besides, the selected area electron diffraction (SAED) image of BOL-2 shows the (101), (110) and (200) crystal planes of (BiO)₂OHCl and the (202) crystal plane of La(OH)₃ (Fig. 3e). The TEM image in Fig. 3f and the corresponding elemental mapping images exhibit that Bi, Cl, O and La elements are well distributed on BOL-2. This observation further supports the successful construction of heterojunctions between 2D (BiO)₂OHCl nanosheets and 1D La(OH)₃ nanorods.

Fig. 3 TEM images (a and b), HRTEM images (c and d), SAED image (e), and elemental mapping image (f) of BOL-2.

Textual properties

The N₂ adsorption–desorption isotherms and corresponding pore size distribution plots of (BiO)₂OHCl, La(OH)₃ and BOL-x (x = 1–4) are shown in Fig. S3.†Table 1 lists the summary of specific surface area (S_BET), average pore size and pore volume for each sample, which have been suggested as important factors affecting the photocatalytic property of the material.³⁶ The S_BET of the BOL-x composite gradually increases from BOL-1 (8.37 m² g⁻¹) to BOL-4 (14.79 m² g⁻¹), by increasing the molar ratio of La/Bi during synthesis. The slight increase in S_BET values of composites should be the result of the formation of La(OH)₃ nanorods on rose-like (BiO)₂OHCl nanosheets; this is supported by the SEM observation in Fig. 2. It is also noted that the N₂ adsorption–desorption isotherm curves for (BiO)₂OHCl and BOL-x can be classified as type IV with H3-type hysteresis loops (Fig. S2a†), indicating the presence of slit-shaped pores aggregated from flake-like materials.²⁷ The average pore sizes of the samples are in the range of 15–25 nm, as shown in the pore size distribution diagrams in Fig. S2c.† The presence of large mesopores may be due to the agglomeration of La(OH)₃ nanorod/(BiO)₂OHCl nanosheet heterojunctions in the rose-like hierarchical architecture. Such beneficial features are expected to provide channels for pollutants to reach more accessible active sites, thereby improving the photocatalytic activity of BOL-2.

Table 1 Specific surface area (S_BET), average pore size and pore volumes of samples

Samples	S BET (m^2 g^−1)	Average pore size (nm)	Pore volume (cm^3 g^−1)
(BiO)2OHCl	9.42	28.33	0.067
La(OH)3	38.05	20.39	0.190
BOL-1	8.37	18.26	0.038
BOL-2	9.51	28.86	0.069
BOL-3	11.59	28.67	0.083
BOL-4	14.79	22.38	0.083

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Surface composition

The XPS survey spectrum of BOL-2 confirms the presence of Bi, O, Cl, and La elements (Fig. S4†). In the high-resolution XPS spectrum of Bi 4f (Fig. 4a), the two peaks located at binding energies of 159.28 eV and 164.58 eV are ascribed to Bi 4f_7/2 and Bi 4f_5/2, suggesting that bismuth exists in the form of Bi³⁺ in BOL-2.^10,37 Compared with pure (BiO)₂OHCl shown in Fig. 4b, these two Bi 4f peaks are found to slightly shift to higher bonding energy regions, possibly resulting from the interaction between La(OH)₃ and (BiO)₂OHCl.²⁷ Besides, two adjacent peaks with a lower intensity located at binding energies of 158.18 and 163.68 eV are referred to low-valence state Bi^3–x due to the presence of OVs.²⁶ In Fig. 4c, two sets of bimodal peaks ascribed to La 3d_3/2 and La 3d_5/2 of BOL-2 can be fitted into four peaks at binding energies of 851.83/855.44 eV and 834.80/838.79 eV, respectively, suggesting the presence of La³⁺ in the form of La(OH)₃.^11,38 Compared with La 3d of pure La(OH)₃ shown in Fig. 4d, all these four peaks in BOL-2 are observed to shift to higher binding energies, indicating the formation of La(OH)₃/(BiO)₂OHCl heterojunctions in BOL-2.¹¹ The O 1s spectra for BOL-2 can be fitted into three peaks at 530.06 eV, 531.16 eV and 532.76 eV (Fig. 4e), which are ascribed to lattice oxygen of Bi–O/La–O bonds, the presence of oxygen vacancies (OVs), and oxygen of OH groups,^10,11 respectively. Compared with (BiO)₂OHCl (Fig. 4f) and La(OH)₃ (Fig. 4g), these three peaks also undergo a shift. It is notable that the intensity of peaks ascribed to OVs in BOL-2 is much higher than that in (BiO)₂OHCl and La(OH)₃, with a calculated content of 26.37%, as compared to 13.61% and 23.98% for (BiO)₂OHCl and La(OH)₃ respectively. This indicates the higher OV concentration in BOL-2 than in (BiO)₂OHCl and La(OH)₃. As for the Cl 2p spectrum of BOL-2 (Fig. S4†), the two fitted peaks centered at 198.09 eV and 199.64 eV could be attributed to the bonding energies of Cl 2p_3/2 and Cl 2p_1/2.¹¹ Similarly, in contrast to (BiO)₂OHCl, these two peaks have a slight shift. Therefore, compared with pure (BiO)₂OHCl and La(OH)₃, the shifts of Bi 4f, La 3d, Cl 2P, and O 1s in BOL-2 confirm the successful formation of La(OH)₃/(BiO)₂OHCl heterojunctions with good interaction in BOL-2, which agrees well with the results of SEM, TEM, and EDX.

Fig. 4 High-resolution XPS spectra of (a and b) Bi 4f for BOL-2 and (BiO)₂OHCl, (c and d) La 3d for BOL-2 and La(OH)₃, (e–g) O 1s for BOL-2, (BiO)₂OHCl and La(OH)₃; (h) EPR spectra of La(OH)₃, (BiO)₂OHCl and BOL-2.

To further confirm the existence of OVs, low-temperature electron paramagnetic resonance (EPR) was applied to provide persuasive evidence. The EPR spectra of BOL-2, (BiO)₂OHCl and La(OH)₃ are displayed in Fig. 4h. All the samples clearly show a symmetric resonance at about g = 2.001, which has been identified as paramagnetic species resulting from oxygen vacancy trapping and unpaired electrons, confirming the existence of OVs.⁸ Moreover, the peak intensity of BOL-2 is much stronger than that of La(OH)₃, indicating that the amount of OVs in BOL-2 is much greater than that in La(OH)₃. The EPR spectra of BOL-2 and m-BOL-2 are compared, as shown in Fig. S5a.† Notably, m-BOL-2, which was synthesized by mechanical mixing, exhibits significantly weaker peaks at around g = 2.001 than BOL-2, indicating a much lower concentration of OVs in m-BOL-2.

Optical properties

Fig. 5a shows the UV-vis diffuse reflectance spectra (UV-vis DRS) of the as-prepared samples, the technique of which has been commonly used to study the optical properties of catalyst materials.^32,39 Clearly, the light absorption capacity of La(OH)₃ within the UV region is much lower than that of (BiO)₂OHCl and BOL-x, although its light absorption edge is recorded at ∼430 nm. As compared to (BiO)₂OHCl, the BOL-x composites show enhanced UV adsorption and visible light response, with the red-shift of light absorption edge. Notably, the sample BOL-2 exhibits an obvious exponentially decaying absorption tail with improved absorption across the entire visible light region, which may be due to the presence of OVs in heterojunctions,^11,39 later convinced by the XPS and EPR spectra shown in Fig. 4. This indicates the promising photocatalytic activity of BOL-2 induced by visible light. Besides, the UV-vis DRS spectra of m-BOL-2 and BOL-2 are compared in Fig. S5b.† Obviously, the sample of BOL-2 exhibits a wide light absorption range and enhanced light absorption capacity, indicating the importance of OVs in the optical property of La(OH)₃/(BiO)₂OHCl heterojunctions.

Fig. 5 (a) UV-vis DRS spectra and (b) band gaps of (BiO)₂OHCl, La(OH)₃ and BOL-x (x = 1–4).

The band gap energy (E_g) can be estimated from the tangent intercept of (αhν)²vs. hν plots, according to the formula:^10,34αhν = A(hν − E_g)^n/2, in which α, h, ν, E_g and A are the absorption coefficient, Plank constant, optical frequency, band gap energy and constant value, respectively. The results are shown in Fig. 5b and Table S1.† The E_g value of pure (BiO)₂OHCl was estimated to be 3.04 eV. It is noteworthy that the band gap of BOL-2 is approximately 2.51 eV, which is the narrowest among BOL-x (x = 1–4) samples; this material is expected to possess enhanced photocatalytic activity.

Photocatalytic activities

In order to examine the removal efficiency of catalysts, tetracycline hydrochloride (TC) and o-nitrophenol (o-NP) were selected as representative pollutants (Fig. 6). The adsorption of TC and o-NP was first studied in the darkness for 130 min (Fig. S6†). It can be seen that the removal rates of TC and o-NP over the as-prepared catalysts are almost unchanged from 30 min to 130 min, suggesting that adsorption equilibrium has been reached within 30 min. Fig. 6a presents the TC residual concentration (C) vs. its initial concentration (C₀ = 30 mg L⁻¹) along adsorption in the darkness and subsequently photocatalytic degradation under visible light irradiation. As can be seen, the change in TC concentration in the solution without any catalyst is almost negligible, indicating the good stability of TC in the darkness and under visible light. The samples of BOL-x (x = 1–4) show higher adsorption capacities in the darkness than both (BiO)₂OHCl and La(OH)₃, which are beneficial for highly efficient contaminant elimination.^40,41 In Fig. 6a, the total removal efficiency of BOL-x (72.7–91.3%) is greater than that of pure (BiO)₂OHCl (50.5%) and La(OH)₃ (53.1%). In particular, BOL-2 shows the highest TC removal, reaching 91.3%. Moreover, higher degradation efficiencies, 96.7% and 98.3%, are achieved using BOL-2 in the TC solution with a lower initial concentration (10 mg L⁻¹ and 5 mg L⁻¹) (Fig. 7b and S7b†). Even at a relatively high initial concentration of 100 mg L⁻¹, BOL-2 can still remove ∼58% of TC (Fig. S7a†). Such promising performances suggest the great potential application of BOL-2 in removing TC from raw domestic sewage (typically TC ranging from 100 ng L⁻¹ to 5 mg L⁻¹) and pharmaceutical/hospital wastewater (typically TC in the range of 100–500 mg L⁻¹). It should be noted that the removal efficiency of m-BOL-2 (80.6%) is lower than that of BOL-2 (91.3%) under the same experimental conditions (Fig. S5d†), verifying the important role of OVs during the visible-light-induced photocatalytic process. Table S2† compares the removal efficiency of BOL-2 towards antibiotics with other bismuth-based catalysts^11,24,26,34 reported in the literature. As can be seen, BOL-2 synthesized in this work exhibits a higher TC removal efficiency than most of our selected Bi-based photocatalysts under simulated visible light.

Fig. 6 TC (a) and o-NP (c) removal by adsorption in the dark and photocatalytic degradation under visible light (C₀ = 30 mg L⁻¹); Pseudo-first-order kinetic plots of photocatalytic degradation of TC (b) and o-NP (d); comparison of kinetic rate constants k for TC (e) and o-NP (f) photocatalytic degradation.

Fig. 7 (a) Comparison of the percentage of TOC removal in the photocatalytic degradation of TC using BOL-2 and (BiO)₂OHCl (C₀ = 30 mg L⁻¹). (b) Photocatalytic degradation of TC in DI water and river water using BOL-2 (C₀ = 10 mg L⁻¹).

Furthermore, almost no o-NP can be removed in the solution (C₀ = 30 mg L⁻¹) without catalyst. In the dark environment, the adsorptive removal of o-NP using BOL-x, (BiO)₂OHCl, or La(OH)₃ is less than 25%, due to the presence of limited functional groups on o-NP molecules and then their weak interaction with our prepared catalysts. However, it is clear that the photocatalytic degradation mainly accounts for the removal of o-NP. In particular, the photocatalytic degradation efficiency of BOL-2 is 93.1%, which is the highest among all the samples, further confirming the optimal photocatalytic activity of BOL-2.

The photocatalytic degradation kinetics of TC and o-NP was further analyzed using the pseudo-first-order reaction kinetic equation (eqn S3†).⁴² Their fitting curves are shown in Fig. 6b and d. Fig. 6e and f compare the kinetic rate constants (k) of samples, with the corresponding k values and correlation coefficients (R²) listed in Table S3.† Obviously, BOL-2 shows the highest k for TC (0.0169 min⁻¹) and o-NP (0.0235 min⁻¹) among all the samples. Moreover, the value of k for TC is 3.5 times and 2.7 times that of pure (BiO)₂OHCl and La(OH)₃, respectively, indicating the optimized photocatalytic activity of BOL-2 by rationally constructing (BiO)₂OHCl/La(OH)₃ heterojunctions.

The change of total organic carbon contents (TOC) in the solution during photocatalytic degradation of 30 mg L⁻¹ TC is shown in Fig. 7(a). The value of TOC/TOC₀ gradually decreases with the prolonged reaction time. After 80 min photocatalytic reaction, the percentage of TOC removal by utilizing BOL-2 reaches 84.9%, whereas that of pure (BiO)₂OHCl is only 43.3%. Undoubtedly, BOL-2 has better capability to mineralize TC into CO₂ and H₂O during the photocatalytic degradation process.

The TC photocatalytic degradation efficiencies of BOL-2 in deionized (DI) water and river water were compared, as shown in Fig. 7(b). It is well known that natural water contains various inorganic ions and dissolved organics, which may suppress the photocatalytic activity of catalysts due to their competition for active sites and consuming part of reactive species. However, no obvious inhabitation was observed in the river water (Zhang jiang River, Jiangxi Province in China) sample containing 10 mg L⁻¹ TC, indicating that a degradation efficiency of 91.6% was achieved after 120 min visible light irradiation, as compared to that of 96.7% in DI water. This proves that BOL-2 has a good photocatalytic activity in real water environment, which can be applied for practical wastewater remediation.

Fig. 8a shows the percentage of removal in the repetitive runs of 30 min dark adsorption and subsequently 120 min visible-light-initiated photocatalytic degradation of TC using BOL-2, respectively. As observed, a slight drop of 9.3% TC adsorptive removal is observed in the second run in the darkness, which is probably due to the occupation of active sites by intermediate products during TC photocatalytic degradation.⁴³ However, the percentage of TC removal by photocatalytic degradation under visible light irradiation is almost unchanged in four cycles, indicating the good photocatalytic reusability of BOL-2. Fig. 8b compares the XRD patterns of fresh and spent BOL-2 before and after four cycles. There is no significant change observed in the XRD patterns, and the peaks assigned to (BiO)₂OHCl and La(OH)₃ can be clearly seen after four cycles, which indicates that BOL-2 has good stability.

Fig. 8 (a) Percentage of TC removal by BOL-2 in dark and under visible light irradiation in four consecutive cycles (C₀ = 30 mg L⁻¹). (b) XRD patterns of BOL-2 before and after four cycles.

In order to further study degradation and mineralization pathways of TC using BOL-2, the HPLC-MS analysis was used to study the intermediate products during photocatalytic degradation (Fig. 9 and S8†). There are 15 intermediates observed at main mass peaks of m/z = 403–74 (Fig. S5†), which are produced after the decomposition of specific functional groups, ring opening reaction, and hydroxylation of TC.^44–46 According to their molecular weights and structures, it is inferred that two possible degradation pathways are involved (Fig. 9). In pathway I, the first intermediate product TC₁ (m/z = 403) is produced by the deamidation of (–C=ONH₂) of TC,⁴⁶ while further demethylation at the –N(CH₃)₂ bond of the TC molecule and dehydration reaction lead to the formation of TC₂ (m/z = 362).⁴⁷ Next, the carbon ring open reaction along with deamination and dehydration reactions yields TC₃ (m/z = 318), and then TC₄ (m/z = 274) is formed by the decarboxylation reaction.⁴⁷ Meanwhile, the intermediate TC₅ (m/z = 362) is formed by losing a dimethylamino group from TC₁ (m/z = 403). Further, TC₆ (m/z = 318) and TC₇ (m/z = 274) are formed due to the ring opening reaction at the TC₅ molecule and followed by the elimination of the formyl group (–C=OCH₃).⁴⁶ In degradation pathway II, the intermediate TC₈ (m/z = 395) is formed via demethylation at the –N(CH₃)₂ bond of the TC molecule, further dehydration process and loss of amino group; the following deamination and dehydroxylation processes lead to the formation of TC₉ (m/z = 339), which is converted into TC₇ (m/z = 282) by a ring-opening reaction.^48,49 With the prolonged reaction time, the intermediates of TC₄, TC₇, and TC₁₀ are further oxidized into TC₁₁ (m/z = 230) by demethylation and dehydration processes, and the loss of functional groups (such as hydroxyl and methyl alcohol).⁵⁰ When the reaction progresses further, some intermediates with relatively low molecular weights, i.e. TC₁₂ (m/z = 182), TC₁₃ (m/z = 149) and TC₁₄ (m/z = 142), are generated by undergoing a ring-opening reaction,^50,51 which are further oxidized into small molecules of TC₁₅ (m/z = 74).⁴⁴ Finally, some of the intermediate products are mineralized into CO₂, H₂O, NO₃⁻ and NH₄⁺.

Fig. 9 Intermediates and possible photocatalytic degradation pathways for TC removal using BOL-2.

As observed, our newly developed catalyst, BOL-2, has been proven to achieve high TC removal at different initial concentrations (e.g., 5, 10, 30 and 100 mg L⁻¹) and, meanwhile, mineralize TC into CO₂ and H₂O during photocatalytic degradation. In particular, no apparently reduced performance was observed in the sample of river water, indicating BOL-2 to possess good photocatalytic activity in a real water environment. Moreover, BOL-2 exhibited excellent stability and good reusability in photocatalytic degradation. Due to the high TC removal rate, excellent stability, and good reusability of developed BOL-2, it shows promising potential for use in real wastewater treatment.

Electrochemical measurements

To reveal the internal origin of photocatalytic performance enhancement in the BOL-x samples, the separation, recombination, and transfer of charge carries were measured by several photoelectrochemical tests. Photocurrent transient response plots of BOL-x (x = 1–4) are shown in Fig. 10a, along with (BiO)₂OHCl and La(OH)₃ included for comparison. Generally, higher photocurrent indicates a higher separation efficiency of photogenerated electron/hole pairs.⁵² The photocurrent intensity decreases in the order of BOL-2 > BOL-3 > BOL-4 > La(OH)₃ > BOL-1 > (BiO)₂OHCl. Obviously, BOL-2 has the highest photocurrent intensity value, indicating the higher electron–hole (e⁻/h⁺) pair separation efficiency in BOL-2 than in other BOL-x samples, as well as (BiO)₂OHCl and La(OH)₃.⁵³ This reveals that the rational construction of La(OH)₃/(BiO)₂OHCl heterojunctions by introducing proper amounts of La(OH)₃ nanorods to (BiO)₂OHCl nanosheets can greatly improve the separation efficiency of photo-generated e⁻/h⁺ pairs in the resulting catalyst.

Fig. 10 (a) Photocurrent transient response plots and (b) EIS Nyquist plots of (BiO)₂OHCl, La(OH)₃ and BOL-x (x = 1–4). (c) Photoluminescence (PL) spectra of (BiO)₂OHCl, La(OH)₃ and BOL-2. (d) Time-resolved fluorescence decay spectra of (BiO)₂OHCl, La(OH)₃ and BOL-2.

The electrochemical impedance spectroscopy (EIS) Nyquist plots of (BiO)₂OHCl, La(OH)₃ and BOL-x (x = 1–4) are displayed in Fig. 10b. In general, a smaller arc radius of Nyquist semicircle in the high-frequency region suggests lower interfacial charge transfer resistance and faster e⁻/h⁺ separation efficiency.⁴⁰ As can be seen, the radius of La(OH)₃/(BiO)₂OHCl heterojunctions increases in the order of BOL-2 < BOL-3 < BOL-4 < BOL-1. It should be noted that the arc radius of BOL-2 is the smallest, indicating the lowest interfacial charge transfer resistance in BOL-2. This is consistent with the results from photocurrent transient response plots (Fig. 10a). Besides, the EIS plots of m-BOL-2 and BOL-2 are compared in Fig. S5c,† and the arc radius of m-BOL-2 is found much larger than that of BOL-2. The above-mentioned results further confirmed the most efficient charge separation and transfer ability in BOL-2, which contribute to its greatly enhanced photocatalytic activity.

Photoluminescence (PL) spectroscopy is used to detect the recombination status of photogenerated electron–hole pairs in catalysts. Fig. 10c shows the PL spectra of (BiO)₂OHCl, La(OH)₃ and BOL-2. La(OH)₃ possesses a much higher PL intensity than those of pure (BiO)₂OHCl and BOL-2 composites, indicating the highest recombination rate of photogenerated electron–hole pairs among three samples.³⁶ Meanwhile, it is noted that the intensity of the PL emission peak for BOL-2 is much lower than that of La(OH)₃ or (BiO)₂OHCl, indicating the lowest charge recombination rate among the three samples.²⁸

The time-resolved fluorescence emission decay spectra of (BiO)₂OHCl, La(OH)₃ and BOL-2 were used to study the lifetime of photo-generated charge carriers, as shown in Fig. 10d. The fitted parameters are summarized in Table S4.† As can be observed, the average lifetime (τ) of La(OH)₃, (BiO)₂OHCl and BOL-2 is 0.94 ns, 1.92 ns and 2.25 ns, respectively, which indicates that the life expectancy of photoinduced charge carriers in BOL-2 is 0.33 ns and 1.31 ns longer than that of (BiO)₂OHCl and La(OH)₃. Such extension of lifetime further discloses the enhanced interfacial charge separation efficiency by forming 1D/2D La(OH)₃/(BiO)₂OHCl heterojunctions.^54,55

The charge separation of BOL-2 and (BiO)₂OHCl was further analyzed by a surface photovoltage (SPV) technique, as shown in Fig. S9.† The SPV response of BOL-2 is much higher in peak intensity than that of (BiO)₂OHCl, which further proves more efficient separation of the photo-generated e⁻/h⁺ pair in BOL-2 than in (BiO)₂OHCl.⁵⁶ Besides, BOL-2 shows a positive photoelectric response in the band range of 330–440 nm, while the photovoltage response range of (BiO)₂OHCl is between 320 nm and 360 nm. The band range of BOL-2 which is broadened to the visible light region implies the greatly enhanced absorption and utilization of visible light by BOL-2, which is consistent with the results from DRS analysis.

Photocatalytic mechanisms

To elucidate the role of active species during the photocatalytic process, the trapping experiment of TC over BOL-2 and pristine (BiO)₂OHCl was conducted (Fig. 11a and S10†), with the use of PBQ, EDTA-2Na, or TBA as the quencher for superoxide radicals (˙O₂⁻), holes (h⁺), or hydroxide radicals (˙OH), respectively.⁵⁷ As shown in Fig. S10,† when (BiO)₂OHCl is used as a photocatalyst, the introduction of TBA or EDTA-2Na induces negligible impacts on its TC degradation efficiency. However, the addition of PBQ leads to a sharp reduction in the TC degradation with the use of (BiO)₂OHCl, from 38.75% to only 3.09%. This indicates that ˙O₂⁻ plays a vital role in the (BiO)₂OHCl photocatalytic system. In case of BOL-2 (Fig. 11a), the photocatalytic degradation of TC is most apparently weakened by 47.7% in the presence of PBQ, while a slight inhibition of ∼20% is observed in the TC degradation process when EDTA-2Na or TBA is used as the quencher. This suggests that ˙O₂⁻, h⁺ and ˙OH all participate in the TC degradation in the BOL-2 photocatalytic system; in particular, ˙O₂⁻ is the main active species.

Fig. 11 (a) Effect of different trapping agents on the photocatalytic degradation of TC on BOL-2; ESR spectra of BOL-2 (b) DMPO–˙O₂⁻ and (c) DMPO–˙OH. Mott-Schottky plots of (d) (BiO)₂OHCl and (e) La(OH)₃. (f) Energy band structures of (BiO)₂OHCl and La(OH)₃.

The ESR technology was then applied to further confirm the production of ˙O₂⁻ and ˙OH by BOL-2 and pure (BiO)₂OHCl under visible light irradiation. As shown in Fig. 11b, no obvious DMPO–˙O₂⁻ characteristic signal is observed for either (BiO)₂OHCl or BOL-2 in the darkness. However, typical DMPO–˙O₂⁻ characteristic signals are observed after light exposure (e.g. 5–10 min); the intensity of DMPO–˙O₂⁻ signals gradually increases with the prolonged exposure time. Moreover, the intensity of DMPO–˙O₂⁻ signals for BOL-2 is significantly higher than that of (BiO)₂OHCl, suggesting that more ˙O₂⁻- radicals can be generated by BOL-2 during the photocatalytic reaction.⁴⁰ This could be ascribed to the more efficient interfacial charge separation efficiency and the presence of enriched surface OVs, which can act as traps to capture photo-generated electrons and promote the conversion of adsorbed oxygen into ˙O₂⁻ radicals.¹³ Similarly, no ESR signal of DMPO–˙OH adducts is found in the darkness (Fig. 11c), while an increase in the intensity of DMPO–˙OH characteristic peaks is detected over BOL-2 by extending its exposure time from 5 min to 10 min. It is also noted that BOL-2 possesses extremely stronger DMPO–˙OH signals than (BiO)₂OHCl, suggesting the generation of more ˙OH by BOL-2 during the photocatalytic reaction.

The band positions of (BiO)₂OHCl and La(OH)₃ were studied using the Mott–Schottky plots (Fig. 11d and e), in which the linear part is a positive slope proving both of them as n-type semiconductors.⁵⁸ As can be seen, the flat band potential (E_fb) of (BiO)₂OHCl and La(OH)₃ is −0.49 eV and −0.52 eV vs. Ag/AgCl, respectively, which is equivalent to −0.29 eV and −0.32 eV vs. the normal hydrogen electrode (NHE).⁵⁹ Generally, the conduction band potential (E_CB) of an n-type semiconductor is about 0.1 eV more negative than its E_fb.⁶⁰ Therefore, the E_CB value of (BiO)₂OHCl and La(OH)₃ was estimated to be −0.39 eV and −0.42 eV vs. NHE, respectively. Combined with the band gap value (E_g) derived from the DRS measurement in Fig. 5, the valence band potential (E_VB) of (BiO)₂OHCl and La(OH)₃ was calculated as 2.65 eV and 2.53 eV vs. NHE, respectively, based on the empirical equation E_VB = E_CB + E_g.⁶¹Fig. 11f shows the detailed energy band structures of as-prepared (BiO)₂OHCl and La(OH)₃. Even though the wide band gap (3.04 eV) of (BiO)₂OHCl seems inactive to visible light, the surface OVs enable the formation of an intermediate defect level between the CB and VB states, which can act as springboards to facilitate the excitation of photogenerated charges and enhance the light absorption property.⁸ The visible-light-induced photocatalytic activities of (BiO)₂OHCl and La(OH)₃ are due to the presence of OVs, which has been verified by their degradation capabilities towards TC and o-NP (Fig. 6). Moreover, due to the matched band energy structure (Fig. 11f), it is obvious that a type-II heterojunction can be formed by integrating (BiO)₂OHCl with La(OH)₃, which significantly enhances the charge transfer ability of BOL-2 and boosts the photocatalytic degradation performance, as evidenced in Fig. 6.

Therefore, based on the aforementioned observation, possible photocatalytic degradation mechanisms of TC and o-NP over BOL-2 are proposed, as shown in Scheme 2. Thanks to the rose-like architecture, 1D La(OH)₃ nanorods are hierarchically distributed on 2D (BiO)₂OHCl nanosheets to form a close contact interface, which significantly reduces the photo-induced carrier migration distance and, in turn, effectively inhibits the recombination of photo-generated e⁻–h⁺ pairs.⁶² Due to the presence of enriched surface OVs, the OV-induced intermediate level enables the production of photo-generated e⁻/h⁺ by both (BiO)₂OHCl and La(OH)₃, so that the excited e⁻ transit to the OV level and then from the OV level to the CB level.¹³ The e⁻ at the CB of La(OH)₃ can migrate to the CB of (BiO)₂OHCl, since E_CB of La(OH)₃ (−0.42 eV vs. NHE) is more negative than that of (BiO)₂OHCl (−0.39 eV vs. NHE). Meanwhile, the h⁺ in the VB of (BiO)₂OHCl can transfer to the VB of La(OH)₃, because E_VB of (BiO)₂OHCl (2.65 eV vs. NHE) is greater than that of La(OH)₃ (2.53 eV vs. NHE). As E_CB of (BiO)₂OHCl is more negative than E₀ (O₂/˙O₂⁻) (−0.33 eV vs. NHE) and E_VB of La(OH)₃ is more positive than E₀ (OH⁻/˙OH) (1.99 eV vs. NHE), the accumulated e⁻ in the CB of (BiO)₂OHCl possesses the ability of reducing O₂ to ˙O₂⁻, as well as h⁺ in the VB of La(OH)₃ has the ability to oxidize OH⁻ and then to yield ˙OH.^63,64 Therefore, in this way, the e⁻ and h⁺ can be efficiently separated, and the formed ˙O₂⁻, ˙OH, along with h⁺ as active substances participate in the photocatalytic degradation process to achieve an excellent elimination effect of both TC and o-NP from wastewater. The organic pollutants are finally degraded into CO₂, H₂O, and other small compounds.

Scheme 2 Photocatalytic degradation mechanisms of pollutants over BOL-2.

Conclusions

In this study, a series of rose-like photocatalysts with the assembly of 1D/2D La(OH)₃ nanorod/(BiO)₂OHCl nanosheet heterojunctions have been successfully synthesized. The novel La(OH)₃/(BiO)₂OHCl heterojunctions featured with rich OVs and close interface contact, which enhanced the visible light absorption capacity and improved the charge separation efficiency. The optimal sample BOL-2 showed superior removal efficiency for TC and o-NP by synergistic adsorption in the darkness and visible-light-induced photocatalytic degradation. The kinetic rate constant k of TC photocatalytic degradation over BOL-2 was 3.5 times and 2.7 times that of pure (BiO)₂OHCl and La(OH)₃, respectively. A TOC removal of 84.9% was achieved by BOL-2, as compared to only 43.3% over (BiO)₂OHCl. It is believed that the unique rose-like heterostructure with large mesopores in BOL-2 was beneficial for improved adsorption of TC, while the formation of 1D/2D La(OH)₃/(BiO)₂OHCl heterojunctions in type-II scheme enhanced the interface charge separation efficiency, prolonged the lifetime of charge carries, and broadened the light absorption region. The free radical scavenging tests along with ESR analyses revealed the main role of ˙O₂⁻ along with the participation of h⁺ and ˙OH in the BOL-2 photocatalytic system. The catalyst BOL-2 showed excellent stability in four consecutive cycles. This work has provided a blueprint for the highly efficient removal of organic pollutants from wastewater by designing novel bismuth-based photocatalysts with the coupling effect of OV engineering and morphology control of heterojunctions.

Author contributions

Ying Tan & Qin Zhou: investigation, writing – original draft. Weiya Huang: conceptualization, supervision, writing – review and editing. Kangqiang Lu: writing – review and editing. Kai Yang: resources, writing – review and editing. Xunjun Chen: data curation, project administration. Hua He: formal analysis, resources. Dan Li: writing – review and editing. Dionysios D. Dionysiou: writing – review and editing.

Conflicts of interest

The authors declare that there are no competing financial interest.

Acknowledgements

The research was financially supported by National Natural Science Foundation of China (21962006), China Postdoctoral Science Foundation (2021M692456), Jiangxi Provincial Education Department Project (GJJ200819), Jiangxi Provincial Academic and Technical Leaders Training Program – Young Talents (20204BCJL23037), Jiangxi Province “Double Thousand Plan”, Jiangxi Provincial Natural Science Foundation (20212BAB213016) and Qingjiang Youth Talent Program (JXUSTQJYX20170005). D. D. Dionysiou also acknowledges support from the University of Cincinnati through the Herman Schneider Professorship in the College of Engineering and Applied Sciences.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2en00792d

‡ These authors contributed equally to this work.

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