Solvent and temperature-mediated nanoarchitectonics of hierarchical Bi₂MoO₆ for efficient antibiotic degradation

Abstract

Bismuth molybdate (Bi₂MoO₆) is typically synthesized via a solvothermal method. Elucidating the governing effects of the solvent type and reaction temperature on their phase, morphology, and properties is essential for the rational design of tailored materials. In this study, Bi₂MoO₆ was prepared through a solvothermal approach employing four distinct solvents: water, ethanol, N,N-dimethylformamide (DMF), and ethylene glycol. The influence of reaction temperature on the material's structure evolution was also systematically investigated. The microstructure of Bi₂MoO₆ was found to be strongly dependent on the boiling point of the solvent employed. In particular, solvents with higher boiling points led to an increased concentration of oxygen vacancies within the Bi₂MoO₆ structure. When low-boiling-point solvents were used, a phase transition occurred, yielding bismuth oxide as a by-product. Furthermore, the effect of solvothermal temperature on the structural characteristics of Bi₂MoO₆ was examined. At lower reaction temperatures, the product exhibited an amorphous nature. With increasing temperature, the crystallite size of Bi₂MoO₆ grew progressively until metallic bismuth emerged. Notably, Bi₂MoO₆ synthesized at 140 °C demonstrated optimal degradation efficiency toward ciprofloxacin, which can be attributed to its relatively small crystallite size and a suitable concentration of oxygen vacancies. This study offers valuable guidance for the rational regulation of solvent-mediated catalytic materials.

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1. Introduction

Over the past few decades, photocatalysis has shown considerable promise as an environmentally benign strategy for pollutant elimination.^1,2 Commonly investigated photocatalysts include metal oxides,² metal sulfides,³ g-C₃N₄,⁴ bismuth materials,⁵ and metal–organic frameworks.⁶ Among them, bismuth-based photocatalysts have emerged as a prominent research field, owing to their efficient visible light absorption, stable chemical properties, and moderate conduction and valence band positions. Among these materials, bismuth molybdate (Bi₂MoO₆), a member of the Aurivillius family of layered oxides, has garnered significant interest because of its distinctive layered architecture, remarkable physicochemical properties, and potential applications.^7,8 The special layered structural arrangement endows Bi₂MoO₆ with a tunable structure and morphology.⁹ The structure can be controlled through strategies such as elemental doping, defect engineering, surface modification, and dimensionality reduction.

These modification methods are expected to significantly enhance light absorption capability, photogenerated carrier separation efficiency, and surface reaction activity by regulating the band structure, increasing active sites, and optimizing charge transport pathways, thereby further elevating the photocatalytic performance.^8,10 Heterojunction photocatalysts such as CdIn₂S₄/Bi₂MoO₆,¹¹ g-C₃N₄/Bi₂MoO₆,¹² and Bi₂MoO₆/CoWO₄¹³ were prepared to construct an internal electric field. Chang et al.¹⁴ synthesized indium-doped Bi₂MoO₆ with oxygen vacancies, confirming that the introduction of oxygen vacancies leads to the size reduction and agglomeration of nanosheets, while the additional indium ions help further reduce the grain size of Bi₂MoO₆. Oxygen vacancies play a crucial role in photocatalysis.⁵ Materials enriched with oxygen vacancies, such as Bi₂MoO₆/BiOCl¹⁵ and ZnIn₂S₄/Bi₂MoO₆,¹⁶ exhibited high photocatalytic performance. Dong et al.¹⁷ prepared Bi₂MoO₆ using a solvothermal method, introducing a large number of oxygen vacancies via subsequent calcination. The controlled introduction of oxygen vacancies regulates the valence band position of Bi₂MoO₆, improves the photoelectric performance, and increases the photogenerated carrier separation efficiency and active sites.

In the synthesis of bismuth molybdate, the most commonly used method is solvothermal.^18–21 In this method, the type of solvent has a significant influence on the morphology and structure of Bi₂MoO₆. Our previous studies have shown that Bi₂MoO₆ with a nanoplate structure can be prepared by using water as the solvent.²² Liu et al.²³ investigated the effect of the ethylene glycol (EG) concentration on the structure of Bi₂MoO₆. Their findings revealed that as the concentration of EG increased, the size of Bi₂MoO₆ gradually decreased, and its morphology changed from nanoplates to self-assembled hydrangea-like microspheres. Additionally, a higher EG fraction in the solvent promoted the formation of oxygen vacancies. Li et al.²⁴ found that the introduction of oxygen vacancies not only enhances the adsorption of molecular oxygen by Bi₂MoO₆, but also facilitates the O–O bond dissociation of molecular oxygen, thereby promoting the activation of molecular oxygen. Oxygen vacancies can act as electron traps, capturing electrons produced by light and thus accelerating charge transfer. Collectively, the above research demonstrates that the structure of bismuth molybdate can be regulated by changing the type of reaction solvent. In solvothermal reactions, solvents commonly used by researchers include water, ethylene glycol, ethanol, and DMF. These solvents have different polarities and boiling points, providing various preparation environments in the synthesis and affecting the growth process of materials significantly. For instance, Bi et al.²⁵ suggested that EG exhibits chelating ability and steric hindrance, thereby suppressing the agglomeration of nanoparticles. Furthermore, the phase of Bi₂MoO₆ can also be changed by converting the solvent from nitric acid to EG.²⁶

Therefore, this study aims to further investigate the regulation of the Bi₂MoO₆ structure using solvent systems. Specifically, the effects of solvent type and solvothermal temperature on the structure of Bi₂MoO₆ were discussed. The obtained 3D hierarchical structure of Bi₂MoO₆ was constructed from self-assembled nanosheets. The photocatalyst enriched with oxygen vacancies, using EG as the solvent, exhibited excellent performance. This work thus provided a pathway for the rational design of bismuth-based materials.

2. Experimental

The chemical reagents used in the experiment and the characterization methods for the samples are shown in the SI.

2.1. Synthesis of photocatalysts

First, 5 mmol of bismuth nitrate pentahydrate and 2.5 mmol of sodium molybdate dihydrate were separately dissolved in 30 mL of ethylene glycol. After ultrasonic treatment, the sodium molybdate solution was added dropwise into the bismuth nitrate solution under continuous stirring. The resulting mixed solution was stirred thoroughly and transferred into a 100 mL polytetrafluoroethylene reaction vessel. The vessel was heated at 180 °C, 160 °C, 140 °C, and 120 °C for 12 h, respectively. Finally, the obtained powders were washed several times and dried at 80 °C for 4 h. Similarly, samples prepared with different solvents can be obtained by replacing ethylene glycol with water, DMF, and ethanol.

2.2. Photocatalytic property test

The entire photocatalytic reaction was conducted under visible light irradiation. Prior to irradiation, 30 mg of the photocatalyst was dispersed into 100 mL of ciprofloxacin solution (CIP, 10 mg L⁻¹). During the experiment, magnetic agitation was maintained continuously to ensure the uniform dispersion of the photocatalyst. Before light exposure, it is necessary to stir in the dark to achieve an adsorption–desorption balance. Afterward, the light was turned on, and 3 mL of the dispersion was collected at regular intervals. The supernatant was filtered using a 0.22 µm membrane filter, and the CIP concentration was analyzed using a UV-vis spectrophotometer.^27,28

3. Results and discussion

3.1. Physicochemical characterization

The bismuth molybdate samples synthesized with EG as the solvent at different solvothermal temperatures were characterized by XRD, with the results presented in Fig. 1a. In comparison with standard bismuth molybdate, the samples synthesized at 140 °C and 160 °C exhibited well-defined crystallinity, characterized by peaks at the diffraction angles of 2θ = 28.3°, 32.6°, 47.1°, and 55.6°.²⁹ This indicated that the solvothermal method using EG as the solvent is capable of producing pure bismuth molybdate with high crystallinity within this temperature range. In contrast, the sample synthesized at 120 °C displayed only one weak peak, indicating a poorly defined crystal structure. This observation suggests that the low reaction temperature may have led to insufficient reaction, thereby affecting the nucleation and crystallization process and resulting in the failure to form a distinct crystal structure. The characteristic peaks of metallic Bi (27.2°, 38.0°, 39.6°, and 48.7°)³⁰ appeared when the solvothermal temperature increased to 180 °C. This phenomenon can be attributed to the fact that at higher temperature, the pressure in the reactor increases, enhancing the reactivity of ethylene glycol. As a reducing agent, EG can reduce bismuth in bismuth molybdate to metallic Bi. These findings indicate that the phase of bismuth molybdate can be modulated by adjusting the solvothermal temperature when EG is employed as the solvent.

Fig. 1 (a) XRD patterns and (b) crystallite size of Bi₂MoO₆ at different solvothermal temperatures.

The crystallite size of the catalyst plays a crucial role in determining its catalytic activity. The Scherrer formula was applied to calculate the average crystallite size of bismuth molybdate, with the results presented in Fig. 1b. At a solvothermal temperature of 140 °C, the average crystallite size of bismuth molybdate was found to be 10.6 nm, which increased with higher reaction temperatures. Generally, the smaller-sized catalysts exhibited enhanced catalytic activity due to a larger surface area that accelerates reaction rates.

The bismuth molybdate prepared using EG, water, DMF, and EtOH as solvents at 140 °C was also characterized by XRD, as shown in Fig. 2a. The samples obtained using ethylene glycol or water displayed characteristic peaks corresponding to pure bismuth molybdate. Notably, the water-based sample exhibited narrower peaks than the EG-based sample, indicating that the former has superior crystallinity. In contrast, the characteristic peaks of both bismuth molybdate and bismuth oxide were detected in the samples prepared with DMF and EtOH, indicating partial conversion of bismuth molybdate into bismuth oxide. Fig. 2b provides an enlarged view of the XRD results. It can be observed that the position of the characteristic peak corresponding to the (131) crystal plane of bismuth molybdate shifted. Specifically, the EG-prepared bismuth molybdate showed a lower diffraction angle, suggesting increased interplanar spacing within bismuth molybdate layers.

Fig. 2 (a) XRD patterns, (b) enlarged view, and (c) crystallite size of Bi₂MoO₆ for different solvents.

The observed variations among samples prepared under different solvent conditions may be related to the boiling point of the solvents. Lower-boiling point solvents, such as EtOH, induced higher internal pressures during the solvothermal process, resulting in larger crystallite sizes. Conversely, when using a solvent with a higher boiling point, such as EG, the pressure in the reactor was lower, leading to a smaller crystallite size. Fig. 2c presents crystallite sizes calculated using Scherrer's formula. As mentioned above, the crystallite size of the sample synthesized with EG is 10.6 nm (corresponding to bismuth molybdate), whereas those prepared with ethanol yielded crystallite sizes of 25.2 nm (bismuth molybdate) and 41.9 nm (bismuth oxide). It should be noted that although the solvent boiling point may influence the crystallization process, it is not the sole factor governing the process.

SEM was employed to investigate the micromorphology of Bi₂MoO₆ and its correlation with photocatalytic performance. Fig. 3 illustrates the micromorphology of bismuth molybdate prepared with ethylene glycol at different solvothermal temperatures. The sample synthesized at 120 °C (Fig. 3a and e) consisted of irregular spherical aggregates approximately 300 nm in size, characterized by a smooth surface. This can be attributed to the poor crystallinity of the sample at low temperatures, leading to a small specific surface area and few active sites. When the reaction temperature increased to 140 °C (Fig. 3b and f), a notable morphological transformation occurred, resulting in the formation of flower-like microspheres composed of self-assembly nanosheets. The diameter of these microspheres was about 0.7 µm, with the individual nanosheets measuring about 120 nm. In the formation of 3D nanospheres, ethylene glycol acts as a crosslinker, linking the bismuth and molybdenum precursors together.²³ As the reaction temperature increased to 160 °C (Fig. 3c and g), the diameter of the flower-like microspheres increased to about 1 µm, accompanied by an increase in nanosheet dimensions. When the reaction temperature increased to 180 °C (Fig. 3d and h), spheroids with smooth surfaces and diameters between 0.5 and 1 µm are attributed to metallic Bi, consistent with the XRD results shown in Fig. 1a. In summary, the optimal temperature for synthesizing flower-like microspheres and abundant active sites was determined to be 140 °C. Dynamic light scattering (DLS) was used to obtain a more statistically reliable particle size distribution of the samples (Fig. S1). As the temperature increased from 140 °C to 180 °C, the particle size gradually decreased from about 500 nm to 220 nm. This trend may be attributed to the changes in nucleation and crystal growth dynamics at different temperatures. At higher temperatures, faster nucleation rates and controlled growth could lead to the formation of smaller particles. In addition, EG can act as a more effective structure-directing or capping agent, potentially suppressing excessive particle agglomeration and growth.

Fig. 3 SEM micrographs of Bi₂MoO₆ with different solvothermal temperatures: (a) and (e) 120 °C, (b) and (f) 140 °C, (c) and (g) 160 °C, (d) and (h) 180 °C, (i) EDS mapping at 140 °C, and (j) element content at 140 °C.

Fig. 3i displays the EDS spectrum of the sample synthesized at 140 °C. The results revealed a uniform distribution of Bi, Mo and O elements, demonstrating good sample homogeneity. Fig. 3j shows that the molar ratio of elements Bi : Mo : O is 1.92 : 1 : 2.60. In order to more accurately determine metallic stoichiometries, ICP-OES analysis was conducted to quantify the metallic elements (Bi and Mo). The measured mass fractions of Bi and Mo are 65.34% and 14.79%, respectively. The ratio is remarkably close to the theoretical stoichiometry of 2 : 1 for bismuth molybdate.

Fig. 4a and b illustrate the micromorphology of samples synthesized using water as the solvent. In contrast to the samples synthesized with ethylene glycol, those prepared with water exhibited a nanoparticle structure approximately 100 nm in size, with good dispersion and a smooth surface. The EDS spectrum (Fig. 4c) indicates a uniform distribution of all constituent elements, confirming the homogeneity of the sample. Elemental analysis (Fig. 4d) reveals a molar ratio of Bi : Mo : O of 1.77 : 1 : 5.11. The measured mass fractions of Bi and Mo (ICP-OES) are 68.72% and 15.45%, respectively. The ratio is also close to the theoretical stoichiometry of 2 : 1 for bismuth molybdate.

Fig. 4 (a) and (b) SEM micrographs, (c) EDS mapping, and (d) element content of Bi₂MoO₆ using water as the solvent.

Fig. S2 presents the XPS spectra of bismuth molybdate synthesized using water or ethylene glycol as the solvent. The spectra confirm the presence of Bi, Mo, and O elements in both samples. The high-resolution XPS spectrum of Bi 4f shows that distinct peaks are observed at approximately 164.7 eV and 159.4 eV for both samples, corresponding to Bi 4f_5/2 and Bi 4f_7/2 of Bi³⁺ in Bi₂MoO₆, respectively.¹² The peak positions for bismuth molybdate synthesized with the two solvents show a negligible variation. Deconvolution of the high-resolution O 1s spectrum reveals two peaks located at around 530.3 eV and 531.7 eV, which correspond to lattice oxygen and chemically adsorbed oxygen/oxygen vacancies in Bi₂MoO₆, respectively.³¹ Notably, the bismuth molybdate prepared with ethylene glycol exhibits a more intense peak with chemically adsorbed oxygen, indicating a higher concentration of surface oxygen vacancies, which is closely related to the differences in the crystal structure. To substantiate the existence of oxygen vacancies (O_Vs), EPR measurements were conducted, and the results are shown in Fig. S3. A prominent peak situated at g = 2.004, which corresponds to O_Vs, was discernible. It is noteworthy that the utilization of ethylene glycol during the synthesis process could induce crystal lattice distortions in Bi₂MoO₆.

Solid-state diffuse reflectance spectroscopy was performed on samples synthesized at different solvothermal temperatures, and the UV-Vis absorption spectra are illustrated in Fig. S4a. Within the wavelength range of 200–700 nm, all samples exhibited differentiated visible light absorption. Notably, as the solvothermal temperature increased, the samples displayed a concomitant enhancement in visible light absorption capacity. As the synthesis temperature increases, the crystallinity of the Bi₂MoO₆ samples significantly improves, as evidenced by the sharpening and intensification of the XRD diffraction peaks. The growth of larger and more perfect crystals at higher temperatures typically results in a narrower band gap due to reduced quantum confinement and a more delocalized electronic structure.

The band gap value (E_g) of the material was determined using the Tauc plot method: αhν = A(hν − E_g)^n/2, extrapolated from the linear region of the plot of (αhν)²versus hν, where α, hν, and A are the absorption coefficient, the photon energy, and the constant, respectively. Given that bismuth molybdate is a direct band gap semiconductor (n = 1), the calculated band gaps of samples were 3.14, 2.97, 2.93, and 2.67 eV, respectively. Notably, higher temperatures were associated with lower band gap values. The band gap of a semiconductor dictates the energy range of absorbed photons, directly influencing the photocatalytic efficiency. A larger band gap results in a shorter wavelength threshold of light absorption, thereby limiting sunlight utilization. Conversely, an excessively small band gap may promote the recombination of photogenerated electron–hole pairs.

The UV-vis absorption spectra of samples prepared using different solvents are shown in Fig. S4c, indicating similar visible light absorption properties among all the samples. Within the wavelength range of 400–460 nm, the samples synthesized in ethylene glycol and water demonstrated favorable absorption characteristics. At longer wavelengths beyond 460 nm, the sample obtained with DMF exhibited better absorption performance. The band gap energies were further calculated using the Tauc method. As illustrated in Fig. S4d, the band gap of samples synthesized with DMF was found to be larger (3.02 eV) compared with that of the samples prepared using water (2.83 eV).

To evaluate the energy positions of the valence band (VB) in the samples, valence band X-ray photoelectron spectroscopy (VB-XPS) was utilized to determine these values via the formula: E_NHE = φ + E_VB-XPS − 4.44 eV.^32,33 In this equation, E_NHE represents the standard hydrogen electrode potential of the valence band, E_VB-XPS signifies the VB value obtained directly from VB-XPS, and φ denotes the instrumental work function, assigned a constant value of 4.2 eV for the given experimental setup. Based on the above analysis, the band structure of bismuth molybdate can be deduced, as presented in Fig. 5b. The bismuth molybdate synthesized with ethylene glycol displayed more positive (or negative) VB (or CB) positions relative to the counterpart prepared with water. This variation resulted in an enhanced redox capability of the generated radicals. Furthermore, Fig. 5c indicates that the bismuth molybdate synthesized with ethylene glycol exhibits a superior surface photocurrent response, signifying greater efficiency in the separation of photogenerated electron–hole pairs. Radical trapping experiments (Fig. 5d) confirmed that superoxide radicals are the primary active species in the degradation process catalyzed by bismuth molybdate.

Fig. 5 (a) VB-XPS spectra, (b) band structure, (c) transient photocurrent response, and (d) scavenger experiment.

3.2. Photocatalytic adsorption performance

To investigate the differences in photocatalytic performance among samples prepared at different temperatures with ethylene glycol as the solvent, the photocatalytic degradation of CIP was tested. The results are presented in Fig. 6a. Prior to illumination, the removal of CIP primarily relied on the adsorption capabilities of the materials. Significant differences in the adsorption performance were observed across samples prepared at different temperatures. Notably, the bismuth molybdate prepared at 140 °C exhibited the highest adsorption capacity. Upon illumination, the sample prepared at 120 °C displayed negligible photocatalytic performance, which can be ascribed to its poor crystallinity and the absence of bismuth molybdate phase formation at this temperature. In contrast, when the solvothermal temperature reached 180 °C, the resulting bismuth molybdate demonstrated diminished catalytic performance due to the formation of elemental Bi solid spheres, leading to a reduction in active sites hindering the degradation of pollutants. After illumination, the samples prepared at 140 °C and 160 °C exhibited pronounced photocatalytic performance. Notably, the bismuth molybdate synthesized at 140 °C featured smaller crystallite sizes, enabling the exposure of more active surfaces, thus demonstrating enhanced photocatalytic performance. By comparing the adsorption and degradation rates (Fig. 6b), it can be observed that materials with higher adsorption capacities also exhibited higher catalytic activities. The degradation process was fitted using a kinetic model (Fig. 6c), which revealed that the bismuth molybdate prepared at 140 °C possessed the highest apparent kinetic constant (0.060 min⁻¹). In summary, the photocatalytic performance of samples is governed by both the crystallite size and the degree of exposed active surfaces.

Fig. 6 (a) and (d) Adsorption/degradation curves of CIP, (b) and (e) removal rate, and (c) and (f) kinetic curves.

Fig. 6d illustrates the adsorption–degradation curves of CIP by bismuth molybdate synthesized with different solvents. Among the samples, the bismuth molybdate prepared with ethylene glycol exhibited superior adsorption capacity. Upon illumination, it also exhibited the fastest degradation rate toward CIP. After 20 min of illumination, the degradation rate of CIP by bismuth molybdate prepared with ethylene glycol reached 85.65%, surpassing that of bismuth molybdate prepared with water (55.44%), DMF (43.15%), and ethanol (9.05%) as solvents. This enhanced performance can be attributed to the smaller crystallite size and increased availability of oxygen vacancies in the bismuth molybdate prepared with ethylene glycol, which facilitate the exposure of active surfaces and the separation of photogenerated electron–hole pairs. Conversely, bismuth molybdate prepared with water exhibited larger crystallite sizes, while samples prepared with DMF or ethanol underwent partial conversion to bismuth oxide, resulting in inferior catalytic performance under visible light compared to bismuth molybdate. The kinetic fitting results (Fig. 6f) further corroborate that the bismuth molybdate synthesized with ethylene glycol exhibits a higher degradation rate.

Fig. S5a illustrates the influence of various impurity ions and humic acid on the catalytic performance of bismuth molybdate. Specifically, the presence of Na⁺, K⁺, Cl⁻, or HCO₃⁻ ions in the reaction system diminished the degradation efficiency of bismuth molybdate towards pollutants. Conversely, Fe³⁺ exhibited oxidative properties that facilitate pollutant degradation throughout the system. Humid acid was found to have an insignificant impact on the degradation of CIP by bismuth molybdate. Fig. S5b demonstrates the effect of solution pH on pollutant degradation, indicating that bismuth molybdate exhibited optimal performance under neutral conditions. In either acidic or alkaline environments, the presence of H⁺ and OH⁻ ions inhibited the degradation process. To evaluate the catalytic stability of bismuth molybdate, adsorption–degradation data were obtained after four consecutive cycles of the catalyst, as shown in Fig. S5c. The bismuth molybdate prepared in this study exhibited excellent cycling stability. Furthermore, the XRD patterns of the catalyst before and after the reaction, presented in Fig. S5d, reveal that the catalyst remains largely unchanged following the degradation of pollutants (Fig. 7).

Fig. 7 Proposed mechanism of photodegradation by Bi₂MoO₆ microspheres.

4. Conclusions

In summary, the effects of water, ethylene glycol, ethanol, and DMF on the microstructure of bismuth molybdate were systematically investigated. Ethylene glycol was identified as the optimal solvent, producing bismuth molybdate with smaller crystallite sizes and abundant active sites compared to water, ethanol, and DMF. The reaction temperature also significantly influenced the structure and morphology of bismuth molybdate. At lower temperatures, no well-defined crystalline phase was observed. However, as the temperature increased, the crystallinity of bismuth molybdate improved and crystallite sizes increased. When the reaction temperature reached 180 °C, metallic Bi was formed due to the reducing action of ethylene glycol. At a solvothermal temperature of 140 °C, bismuth molybdate exhibited a three-dimensional nanosphere structure composed of nanosheets, featuring more exposed surfaces and enhanced ciprofloxacin adsorption capacity. The presence of oxygen vacancies in the synthesized bismuth molybdate facilitated effective separation of photogenerated electron–hole and improved catalytic degradation performance, leading to an 85.65% degradation rate of ciprofloxacin within 20 min of light exposure.

Author contributions

Shilin Li: investigation, methodology, and writing – original draft. Yunhui Tian: writing – review and editing. Guangxin Zhang: conceptualization, methodology, resources, writing – original draft, and writing – review and editing. Xiangnan Wang: methodology.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5nj03957f.

Acknowledgements

The authors gratefully acknowledge the financial support provided by Science and Technology Special Project of Qingdao City (No. 25-1-5-cspz-9-nsh) and the project ZR2020QE142 supported by Shandong Provincial Natural Science Foundation. The authors extend their gratitude to Mr Qiang Li (from Scientific Compass https://https://www.shiyanjia.com) for providing invaluable assistance with the XPS analysis.

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