Carving the shell thickness of tungsten trioxide hollow multi-shelled structures for enhanced photocatalytic performance†

Xing Zhang ^a, Yilei He ^a, Yanze Wei *^b and Ranbo Yu *^ac
aDepartment of Physical Chemistry, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30, Xueyuan Road, Haidian District, Beijing 100083, China. E-mail: ranboyu@ustb.edu.cn
^bState Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, 1 North 2nd Street, Zhongguancun, Haidian District, Beijing 100190, China. E-mail: yzwei@ipe.ac.cn
^cKey Laboratory of Advanced Material Processing & Mold, Ministry of Education, Zhengzhou University, Zhengzhou 450002, P. R. China

First published on 9th September 2021

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Introduction

Solar energy conversion through a photocatalytic process, photosynthesis or photocatalysis, provides new opportunities to divert the environmental crises and meet the ever-increasing demand for energy.^1–4 The photocatalytic overall water-splitting reaction, which provides hydrogen and oxygen simultaneously, has received tremendous attention from various industries.^5–7 Water oxidation is a primary process to achieve the overall water-splitting.^8,9 Designing highly active and robust water oxidation photocatalysts is the decisive step in developing an efficient system, but it is facing immense challenges.¹⁰ Tungsten trioxide (WO₃) exhibits strong oxidizing ability that originates from a sufficiently positive valence band level; therefore, it is an excellent candidate for photocatalytic water oxidation.^11,12 Moreover, it shows distinguished stability under acid conditions and possesses a moderate bandgap of approximately 2.7 eV, which indicates the use of a significant part in the visible light region.^13–15 The construction of nanostructures, including zero-dimensional (0D) nanoparticles,^16,17 one-dimensional (1D) nanowires,¹⁸ two-dimensional (2D) nanosheets and three-dimensional (3D) hollow structures^19–22 has been an encouraging strategy to improve the photocatalytic efficiency of WO₃. Particularly, 3D hollow structures with unique optical and electrical properties have captured special interest.^23–26

A hollow multi-shelled structure (HoMS) is a rising platform for photoactive materials.^27,28 As an assembly of multiple shells with intervening spaces between the individual shells, HoMSs provide integrated benefits for the entire photocatalytic proceedings including light absorption, charge diffusion, and surface reactions.^29–31 Specifically, from the micro- to the nano-scale, the hierarchical 3D architectures of HoMSs are premium for light multiple-scattering, thereby improving the light-harvesting ability; porous shells and flexible inter-shell spacings provide high specific surface areas and multilevel spaces are beneficial for accelerating the surface reactions;^32–35 the tight sintering links connecting the individual shells are helpful for inter-particle charge transfer;³⁶ and nano-particle subunits are advantageous for reducing the diffusion length of carriers and suppressing their recombination.^37,38 To further optimize the performance of HoMSs, geometrically controlled synthesis has been extensively studied, which focuses on shapes, diameters, shell numbers, and inter-shell spacings.³⁹ Notably, shell thickness imposes significant influence on the light harvesting, charge separation, and surface reactions of photocatalysis, which could apparently affect the optical and electrical properties of HoMSs.⁴⁰ However, systematic research focusing on adjusting the shell thickness and unraveling its effects on the above-mentioned processes has remained uncultivated. Hence, creating HoMSs in WO₃ with tunable shell thickness to combine the intrinsic properties of WO₃ and the advantages of HoMSs becomes an intriguing approach to photocatalytic water oxidation.

Herein, WO₃ HoMSs with acknowledged good performance for solar water oxidation are chosen to explore the influence of shell thickness on optical, electrical and photocatalytic performance.⁴¹ By skilfully exploiting the sequential templating approach (STA) process, accurate shell thickness control of WO₃ HoMSs could be achieved by adjusting the solvent composition for precursor synthesis (Fig. 1). Light harvesting, charge separation, charge transfer efficiencies, and surface redox reactions of both thin- and thick-shelled WO₃ HoMSs clarified that the thin triple-shelled WO₃ HoMSs with improved charge-carrier dynamics win out with outstanding photocatalytic activities of up to 907 μmol g⁻¹ h⁻¹ for O₂ evolution under visible light irradiation, which gives the highest value for the pure WO₃ photocatalyst without modification.

Results and discussion

To obtain WO₃ HoMSs, a recently developed STA was employed owing to its synthetic freedom for accurately controlling the structure of HoMSs.^42–44 Aqueous solutions of tungsten precursors suffer from heavy hydrolysis effects, which would lead to quick precipitation of the tungsten species.⁴⁵ Therefore, the ethanol (EtOH) based tungsten solutions were chosen alternatively. As expected, the EtOH environment successfully suppressed the hydrolysis of tungsten hexachloride (WCl₆) precursors and afforded the carbonaceous microsphere (CMS) templates with increased adsorption ability of tungsten ions, which led to the smooth fabrication of WO₃ hollow structures (Fig. 2a and b). In particular, by soaking sucrose-based CMSs (∼2 μm) into 0.05 M WCl₆ EtOH solution for 6 (25 °C) or 12 h (40 °C), single- or double-shelled WO₃ hollow structures with a shell thickness of approximately 60 nm were obtained after calcination at 600 °C, respectively (see Experiments and Table S1, ESI†). Unfortunately, the attempt to get higher shell numbered WO₃ hollow structures failed using this synthetic method. In Fig. 3a and b, the scanning transmission electron microscopy (STEM) images of slices of CMSs after absorption indicated that the tungsten ions were mainly adsorbed on the outer part of the CMS templates. The energy dispersive spectrometry (EDS) line scan results in Fig. 3c also give a clear illustration. The intensity of tungsten ions mainly concentrated on the surface of CMS slice, with barely no signal in the core areas. Although dynamic factors of the adsorption process, including metal cation concentration, adsorption temperature and time have been systematically regulated, tungsten in the core zone of CMSs was still inadequate. In addition, the high calcination temperature of 600 °C was necessary for good crystalline WO₃ HoMSs, which is critical for a high photocatalytic performance. However, the growth of grain size under this temperature would easily lead to the collapse of the innermost shell. These factors greatly hindered the construction of WO₃ HoMSs. It was not until CMSs with a larger size of ∼2.6 μm were used that the triple shelled WO₃ HoMSs were obtained. The distribution of tungsten cations along CMSs radius was still uneven, so that the final products possess the thickest outer shell of 90 nm, with the shell thickness decreases from the outside to the inside (Fig. 2c).

From the above syntheses, it can be easily understood that the CMSs can adsorb enough tungsten ions through electrostatic interaction. However, the tungsten precursors could not diffuse into the inner part of the CMSs to give a homogeneous spatial distribution for further formation of HoMSs. To clarify the ion adsorption mechanism and realize the better-controlled synthesis of WO₃ HoMSs, the adsorption system was studied systematically. It is acknowledged that when WCl₆ is dissolved in EtOH, a continuous alcoholysis reaction will take place (eqn (1) and (2)).⁴⁶

WCl6 + xC2H5OH → [W(OC2H5)x](6−x)+ + (6 − x)Cl− + xHCl	(1)

WCl6−x(OC2H5)x + (6 − x)C2H5OH → W(OC2H5)6 + (6 − x)HCl	(2)

This process was confirmed by the time-dependent color changing phenomenon and the dynamic light scattering (DLS) results of the EtOH solution of WCl₆ (Fig. S1a and b, ESI†). In the adsorption process, the W⁶⁺ preferred to alcoholize in EtOH to form [W(OC₂H₅)_x]^(6−x)+, and then, these alcoholized ions would be adsorbed by negatively charged CMSs templates (Fig S2, ESI†). These alcoholized ions [W(OC₂H₅)_x]^(6−x)+ are larger than W⁶⁺(Fig. S3, ESI†), which rendered them gathering easily in the outer part of the CMSs and difficult to diffuse inside the CMSs. Correspondingly, precursors with tungsten concentration gradient along the CMSs radius were obtained. Upon heating, these precursors accumulated around the surface of CMSs would easily transform into thick-shelled WO₃ HoMSs.

Thus, to avoid the alcoholysis of tungsten ions, introducing another solvent with distinct polarity is necessary. A common solvent, acetone, meets the requirement, as two equivalents of acetone can react with the WCl₆ precursors to form W(_VI) oxo-derivatives WOCl₄(OCMe₂), which would hamper the alcoholysis of W⁶⁺ in an EtOH environment (Fig. S1b and S3, ESI†).⁴⁷ Furthermore, the penetration of inevitable [W(OC₂H₅)_x]^(6−x)+ into the CMSs could be favored owing to the lower surface tension of the mixed solvent. By further adjusting the ratio of acetone/EtOH, both the adsorption amount, and the radial distribution of tungsten within the CMSs could be controlled (Fig. S5, ESI†). Results in Fig. 3d–f strongly suggest the boosted diffusion of tungsten precursors into the inner part of CMSs. Moreover, single-, double-, and triple-shelled WO₃ HoMSs were successfully fabricated by varying the adsorption conditions as expected (Fig. 2d–f, Table S1, ESI†).

On the other hand, ion adsorption in CMSs have been discovered to have close relationships with the surface tension of the solvent. The lower surface tension of acetone benefits the tungsten ion adsorption. A small amount of acetone could increase the tungsten ion adsorption in CMSs critically, but a larger amount of acetone (acetone/EtOH > 2) would make the adsorption of tungsten ions in CMSs quickly reach the saturated amount, and lead to condensed WO₃ spheres without any shell structures upon calcination (Table S1, ESI†). Considering the minor difference in surface tension between acetone (σ_acetone: 18.8 mN m⁻¹) and EtOH (σ_ethanol: 21.8 mN m⁻¹), that the surface tension was not the key factor for enhanced tungsten ion adsorption in the case of thin-3 WO₃ HoMS formation. To verify this deduction, dimethylformamide (DMF) with a high surface tension (σ_DMF: 25.7 mN m⁻¹) was tested instead of acetone. As expected, under a certain ratio of DMF/EtOH (1/2), CMSs with homogenous tungsten ions can be obtained, and triple-shelled WO₃ HoMSs could be synthesized (Fig. S6, ESI†), which further confirmed that controlling alcoholysis of the tungsten ion was the key to desirable ion adsorption in CMSs. This strategy makes synthesis of WO₃ HoMSs with definitive, adjustable shell number and thickness feasible (Fig. S7, ESI†).

Taking advantage of the mechanism above, precise control of shell thickness in uniform WO₃ HoMSs from 35 to 90 nm can be realized, and detailed morphology and structural characterizations are shown in Fig. 2 and Fig. S8 (ESI†). Although the thicknesses of outer and inner shells of a certain HoMS sample showed a difference because of the distinct shell diameters, the trend in the thickness variation among WO₃ HoMSs obtained from different precursor solutions remained clear (Fig. S9 (ESI†) and the approximate statistics of the shell thicknesses are listed in Table S3, ESI†). For samples obtained in a pure ethanol environment, tungsten ions mainly distribute on the outer part of CMSs template, yielding thick-shelled WO₃ HoMSs which were denoted as thick-1, thick-2, and thick-3 WO₃ HoMSs. As for the samples with the thinnest shells, the introduction of acetone gives rise to the formation of thin-shelled HoMSs with shell numbers 1 to 3, and were labelled as the thin-1, thin-2, and thin-3 WO₃ HoMSs.

Besides the apparent difference in shell thickness between the thick- and thin-shelled WO₃ HoMSs, the comparison of composition and structure parameters of thick-3 and thin-3 WO₃ HoMSs were then comprehensively explored (Fig. 2g and h). The X-ray diffraction (XRD) patterns showed the similar monoclinic structure (PDF No: 83–0950) of WO₃ in HoMSs, which was also proved using the selected area electron diffraction (SAED) (Fig. 2i and Fig. S10, ESI†). All thick- and thin-series WO₃ HoMSs showed sharp and intense XRD peaks indicating the formation of a highly crystalline WO₃ phase. Elemental mapping of both thick- and thin-shelled HoMSs confirmed the pure WO₃ composition. The high-resolution transmission electron microscopy (HRTEM) images of a single shell subunit of thin or thick-shelled WO₃ HoMSs showed the typical lattice fingers of the (022) and (002) crystal faces attributed to monoclinic WO₃ crystals. Using the focused ion beam (FIB) technique, scanning electron microscopy (SEM) images of opened thick- and thin-3 WO₃ HoMSs gave a direct vision of the thickness (90 nm for thick-3 and 35 nm for thin-3, respectively). Large-scale FIB-SEM images provided more statistical information about the shell thicknesses (Fig. S8, ESI†). The shells of thick-3 WO₃ HoMSs possessed more wrinkles as there were more subunits in the shell, which indicated a more condensed shell structure than thin-shelled ones (Fig. S11 and 12, ESI†). The inner shells of both thick-3 and thin-3 WO₃ HoMSs confirmed the sintering points, which could facilitate the electron transfer among shell structures in catalytic applications. Raman spectroscopy was also employed, and the similar results of WO₃ HoMSs with different shell thickness strongly implied that the regulation strategy precisely affected the shell thickness of WO₃ HoMSs, with limited influence on their crystalline structure (Fig. S13, ESI†). With the aid of X-ray photoelectron spectroscopy (XPS) analysis, additional information regarding to the chemical environments of atoms were studied in WO₃ HoMSs with thick or thin shells (Fig. S14, ESI†), showing a similar chemical environment in thick- and thin-shelled WO₃ HoMSs.

To investigate the effect of the shell thickness on the photocatalytic performance of WO₃ HoMSs, the photocatalytic activities of water oxidation of different types of WO₃ HoMSs were evaluated under visible light in the presence of AgNO₃ as the sacrificial agent. Because of the integrated benefits as mentioned above, all WO₃ HoMSs showed higher photocatalytic activities than that of WO₃ nanoparticles (Fig. S15, ESI†). Results are shown in Fig. 4a and b revealed that the shell number and thickness had a noticeable influence on the photocatalytic activity. Notably, thin-3 WO₃ HoMSs exhibited the highest photocatalytic oxygen evolution activity among all the samples, including thick-3 WO₃ HoMSs. More specifically, for thin-shelled WO₃ HoMSs, the photocatalytic activity increased dramatically with the increase of shell number, and the highest oxygen evolution rate was up to 907 μmol g⁻¹ h⁻¹ for thin-3 WO₃ HoMSs, which was almost three times that of the thin-1 WO₃ hollow spheres (276.7 μmol g⁻¹ h⁻¹). For thick-shell WO₃ hollow structures, thick-2 and thick-3 WO₃ HoMSs exhibited a similar oxygen evolution rate of (673.9 μmol g⁻¹ h⁻¹ and 648.1 μmol g⁻¹ h⁻¹), higher than that of the thick-1 sample, but significantly lower than that of the thin-3 WO₃ HoMSs.

To identify the basic reason for the huge difference of oxygen evolution rates between thick- and thin-shelled WO₃ HoMSs, the apparent quantum efficiency (AQY) was investigated with high photocatalyst loading (Fig. S16, ESI†), which could give a more objective comparison of the photocatalytic activities between thick-3 and thin-3 WO₃ HoMSs. AQYs of thin-3 WO₃ HoMSs for O₂ evolution under monochromatic light at 365 and 400 nm corresponded to 6.13% and 1.64%, respectively, which were obviously higher than 4.86% and 1.1% obtained on thick-3 WO₃ HoMSs under the same wavelengths. These results were consistent with their photocatalytic performances under visible light irradiation. Apparently, thin-shelled WO₃ HoMSs were more advantageous for photocatalytic oxygen evolution. In addition, after long-term photocatalytic testing, the morphology and crystal structure of thin-3 WO₃ HoMSs maintained well, confirming its high stability (Fig. S17 and S18, ESI†).

The deep insight into the different light-harvesting abilities referring to the shell thickness was conducted as a priority, as the changes in the structure easily arouse intuitively interest in their influence on the light absorption process. First, the light-harvesting capability of thick- and thin-WO₃ HoMSs with different shell numbers was experimentally measured using ultraviolet-visible diffuse reflectance spectra (UV-Vis-DRS). Fig. 4c clearly illustrates the enhanced light harvesting ability with the increase in shell numbers in both thin- and thick-shelled WO₃ HoMSs, which could be mainly attributed to the multiple scattering of incident light within WO₃ HoMSs. Second, the shell thickness indeed affected the light-harvesting capacity of WO₃ HoMSs, as thick-shelled samples exhibited stronger light-harvesting ability than that of thin-shelled samples using the same amount of sample (Fig. 4d). The experimental data were further confirmed by the optical simulations based on density function theory (DFT) and finite-difference time-domain (FDTD) method. With the help of DFT calculations, intrinsic optical features of WO₃ HoMSs were obtained for FDTD simulations on a larger spatial scale (Fig. S19, ESI†). FDTD results indicated that single thick-3 WO₃ HoMSs showed better light absorption ability than single thin-3 WO₃ HoMSs. However, a similar variation trend with the actual absorption curves appeared after considering the quantity of HoMSs with differed shell thicknesses (Fig. 4d inset). Additionally, the light absorption of single WO₃ HoMS under monochromatic light at λ = 300, 365, 400 and 425 nm was investigated (Fig. 4e and f and Supporting movies). With deep UV light (∼300 nm), scattering would occur in thin-WO₃ HoMSs, which might account for the enhanced absorption for thin-WO₃ HoMSs in this region. As the wavelength increased, the scattering of the incident light within the single HoMS was obviously favored. Single thick-3 WO₃ HoMS showed much stronger electric field intensities than thin-3 one at most employed wavelengths, which also provided good accordance with the experimental results. Consequently, thick shells showed better light-harvesting ability in most regions of the action spectrum (320–800 nm), but the different quantities of thin-3 and thick-3 WO₃ HoMSs might have a more profound influence on the photocatalytic performances. The amount of surface active sites, inter-particle light scattering effects would play an important role in promoting the AQY of thin-3 WO₃ HoMSs.⁴⁸

Charge carrier dynamics is one of the key factors limiting the photocatalytic efficiency. Thus, the influence of shell thickness on the charge-carrier separation and transfer efficiency was investigated. As shown in Fig. S21 (ESI†), the steady-state photoluminescence (PL) emission intensity of thin-3 WO₃ HoMSs was much weaker than that of thick-3 WO₃ HoMSs, which indicated the inhibited intrinsic radiative recombination of photogenerated charge carrier in thin-3 WO₃ HoMSs owing to the shortened charge diffusion paths to the shell surfaces. To further clarify the surface and bulk charge-transfer efficiencies, electrochemical impedance spectra (EIS) and transient photocurrent (TPC) response were conducted, respectively. First, according to the Nyquist plots shown in Fig. 5a, thin-3 WO₃ HoMSs exhibited a much smaller semicircle diameter and remarkably reduced interfacial charge-transfer resistance as compared to thick-3 WO₃ HoMSs in potassium phosphate buffer solution (pH = 7) under visible light irradiation, indicating the observed improvement of interfacial charge-carrier transfer on the surface of thin-3 WO₃ HoMSs.⁴⁹ Furthermore, TPC density measurements were performed to explore the bulk charge transfer in thick-3 and thin-3 WO₃ HoMSs (Fig. 5b). Specifically, we decoupled the surface charge recombination in thick-3 and thin-3 WO₃ HoMSs by using Na₂S and Na₂SO₃ mixed aqueous solution as electrolyte, in which the photoexcited holes on the surface of photocatalysts were captured rapidly and easily. In this case, the TPC density of thin-3 WO₃ HoMSs is ∼15% higher than that of thick-3 WO₃ HoMSs, which directly reflects an improved bulk charge separation efficiency within the shells of thin-3 WO₃ HoMSs.⁵⁰ In addition, the EIS curves of thin-3 WO₃ HoMSs showed great difference under dark (424.4 Ω) and illuminated (338.7 Ω) conditions, confirming both the formation of abundant light induced excitons and boosted charge transfer capability in thin-3 WO₃ HoMSs (Fig. S22, ESI†).

The surface reaction of O₂ evolution catalyzed by reactive sites on shells is the last step in photocatalysis, which is a pivotal procedure to determine the final photocatalytic performance. To study the influence of shell thickness on the efficiency of the last steps, the specific surface area, pore size distribution, and pore volume were consequently determined using the N₂ sorption analysis (Fig. 6a, b and Table S4, ESI†). The Brunauer–Emmett–Teller (BET) areas of thin-3 WO₃ HoMSs (62 m² g⁻¹) were higher than those of thick-3 WO₃ HoMSs (41 m² g⁻¹), respectively, both showing reversible type IV isotherms. The larger surface area of thin-3 WO₃ HoMSs implied that more active sites exist on the surface of thin-3 WO₃ HoMSs, which can greatly increase the possibility of surface redox catalytic reactions. This was also confirmed by the high-resolution XPS spectra of O 1s orbitals, which indicated that the surface-active oxygen counterparts were 25.2% and 35.1% for thick- and thin-3 WO₃ HoMSs, respectively (Fig. S14, ESI†). Moreover, as illustrated in Fig. 6b, the average pore diameter of thin-3 WO₃ HoMSs was also higher than that of the thick ones. With larger pores, mass transport would be accelerated: the ions and water molecules can easily diffuse into the inner cavities of HoMSs, while the produced oxygen can quickly escape from the core areas. Liquid–solid and gas–solid contact angles in Fig. 6c–f gave additional evidence. It could be clearly observed that the thick-3 WO₃ HoMSs had a larger liquid–solid contact angle (4.2° versus 3.8°) and a smaller gas–solid contact angle (132.2° versus 137.6°) as compared with the thin-3 ones. The enhanced hydrophilic and aerophobic properties of the thin-3 WO₃ HoMSs in the macroscopic view successfully reflect the favored ion/gas diffusion under microscopic conditions. Additionally, the spherical surface of HoMSs was crucial for anti-precipitation of Ag byproducts throughout the photocatalytic process (Fig. S18, ESI†).⁵¹

In the above analysis, a comprehensive comparison between thick-3 and thin-3 WO₃ HoMSs in morphology, structures, optical properties, charge transfer dynamics, and their photocatalytic activities was conducted through experiments and calculations. The structural influence on the optical, electrical, and photocatalytic performance of thick- and thin-series WO₃ HoMSs helped us to draw a picture of structure–performance correlations regarding to the shell thickness of WO₃ HoMSs (Fig. 6g). Thick-3 WO₃ HoMSs exhibit stronger light-harvesting ability than thin-3 WO₃ HoMS owing to the higher shell thickness, whereas thin-3 WO₃ HoMSs express much higher charge-carrier separation/transfer efficiency and better surface catalytic kinetics owing to the architectural advantage. Furthermore, based on the results obtained by decoupling light-harvesting, charge separation, and interfacial charge-carrier transfer, and surface catalytic redox reaction processes, the remarkably enhanced performance of thin-3 WO₃ HoMSs could be attributed to the higher charge-carrier separation efficiency, lower interfacial charge transfer resistance, and more active sites on thin-3 WO₃ HoMSs.

Conclusions

In summary, by optimizing the distribution of tungsten ions in CMSs, highly crystalline WO₃ hollow spheres with accurately controlled shell thickness and varied shell numbers have been successfully synthesized. A solvent dominated adsorption mechanism of CMS templates for metal ions was proposed and experimentally validated. Restraining the alcoholysis of tungsten ions by adjusting the composition of the solvent will enhance the deep penetration of tungsten ions into the CMS templates to form an ideal precursor for WO₃ HoMS construction. This clarity of the mechanism provides an opportunity for the synthesis of WO₃ HoMSs with precisely controllable shell thickness in the range of 35 to 90 nm. All WO₃ HoMS photocatalysts showed enhanced light harvesting abilities and photocatalytic activities toward water oxidation. More importantly, an elaborate comparison of thick- and thin-3 WO₃ HoMSs revealed the effects of shell thickness on the overall photocatalytic proceeding. The higher charge-carrier separation ability, faster mass-transfer efficiency and larger specific surface area of thin-3 WO₃ HoMSs collectively ensure its superb photocatalytic activity on water oxidation up to 907 μmol g⁻¹ h⁻¹ and AQY of 6.3%. Our research clarifies the mechanism for tailoring the shell thickness of WO₃ HoMSs and further explicates the effects of the shell thickness of HoMSs in photocatalysis processes. This study will inspire more reflections in important but negligible issues in photocatalysis and provide a valuable reference for designing and synthesizing efficient nanostructures for solar energy conversion.

Author contributions

Xing Zhang: data curation, formal analysis, methodology, writing – original draft, writing – review & editing; Yilei He: data curation methodology, formal analysis; Yanze Wei: data curation, formal analysis, methodology, software, writing – review & editing; Ranbo Yu: conceptualization, funding acquisition, writing – original draft, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51872024 and 51932001). Dr Y. Wei thanks the Shandong Provincial Natural Science Foundation (ZR2020QB109).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and figures and tables. See DOI: 10.1039/d1qm01124c

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