A Novel Molten Salt Mediated Synthesis of Mesoporous Metal Oxides with High Crystallization

The controlled synthesis of mesoporous metal oxides remains a great challenge because the uncontrolled assembly process and high-temperature crystallization can easily destroy the mesostructure. Herein, we develop a facile, versatile, low-cost, and controllable molten salt assisted assembly strategy to synthesize mesoporous metal oxides (e.g., CeO2, ZrO2, SnO2, Li2TiO3) with high surface area (115–155 m2/g) and uniform mesopore size (3.0 nm). We find this molten salt mediated assembly enables the desolvation of the precursors and forms bare metal ions, enhances their coordination interaction with the surfactant, and promotes their assembly into a mesostructure. Furthermore, the molten salt assisted crystallization process can lower the collision probability of the target metal atom, inhibit its further growth into large crystals, and achieve a well-maintained mesostructure with high crystallization. Furthermore, this method can be expanded to synthesize various structured mesoporous metal oxides, including hollow spheres, nanotubes, and nanosheets by introducing the carbon template. The obtained mesoporous CeO2 microspheres loaded with Cu species exhibit excellent antibacterial performance and superior catalytic activity for the hydrogenation of nitrophenol with high conversion and cycling stability.

mortar and grinding for 3 minutes.The mixture is transferred to the crucible and placed into the muffle furnace for calcination.The heating up procedure is as follows: first, the temperature rises from room temperature to 160 ℃ at 2 ℃/min, keeps at 160 ℃ for three hours and then rises to 400 ℃ at 2 ℃/min with preservation for three hours.Finally muffle furnace naturally cools to room temperature.Then, the salt is removed by centrifugation and washing with deionized water for more than four times, and finally the mCeO2 microspheres are dried in an oven at 70 ℃.

Synthesis of mCeO2 hollow spheres:
The polydopamine microspheres, were calcined in N2 at 350 ℃ for 3 h in order to obtain a pre-carbonisation.The obtained carbon microspheres (0.1 g), F127(0.3g), 1 mmol Ce(SO4)2• 4H2O were added to the mortar and grinding for 3 minutes.Then, 10 mmol nitrate (molar ratio KNO3: LiNO3=0.57:0.43)were added to the above mortar and grinding for 3 minutes, then the mixture is transferred to the crucible and placed into the muffle furnace for calcination.The temperature rise procedure is as follows: first, the temperature rises from room temperature to 160 ℃ at 2 ℃/min, keeps at 160 ℃ for three hours and then rises to 400 ℃ at 2 ℃/min with preservation for three hours.
Finally muffle furnace naturally cools to room temperature.Finally, the mCeO2 hollow spheres were dried in oven at 70°C after removing the salt by centrifugation and washing with deionized water more than four times.

Synthesis of mCeO2 nanotubes:
Soak carbon nanofibers in hydrogen peroxide for pretreatment (sCNF).0.3 g of F127, 0.1 g of sCNF and 1 mmol Ce(SO4)2• 4H2O were added to the mortar and grinding for 3 minutes, 10 mmol nitrate (molar ratio KNO3: LiNO3=0.57:0.43)were added to the above mortar and grinding for 3 minutes, then the mixture is transferred to the crucible and placed into the muffle furnace for calcination.The temperature rise procedure is as follows: first, the temperature rises from room temperature to 160 ℃ at 2 ℃/min, keep at 160 ℃ for three hours and then rises to 400 ℃ at 2 ℃/min with preservation for three hours.Finally muffle furnace naturally cools to room temperature.Finally, the hollow mCeO2 nanotubes were dried in oven at 70°C after removing the salt by centrifugation and washing with deionized water more than four times.

Synthesis of mCeO2 nanosheets:
Firstly, the graphite powder is immersed into dilute nitric acid (40%) for pretreatment (sGP).0.3 g of F127, 0.1 g of sGP and 1 mmol Ce(SO4)2• 4H2O were added to the mortar and grinding for 3 S3 minutes, then 10 mmol nitrate (molar ratio KNO3: LiNO3=0.57:0.43)were added to the above mortar and grinding for 3 minutes, then the mixture is transferred to the crucible and placed into the muffle furnace for calcination.The temperature rise procedure is as follows: first, the temperature rises from room temperature to 160 ℃ at 2 ℃/min, keep at 160 ℃ for three hours and then rises to 400 ℃ at 2 ℃/min with preservation for three hours.Finally muffle furnace naturally cools to room temperature.Finally, the mCeO2 nanosheets were dried in oven at 70°C after removing the salt by centrifugation and washing with deionized water more than four times.

Characterization:
The absorbance of the material is tested by Fourier Transform infrared spectroscopy (FTIR), ranging from 400-4000 cm -1 (Nicolet Nexus 470).The X-ray diffraction (XRD) of the powder was measured in the ARL EQUINOX 1000 with Cu Kα (λ = 0.154 nm) in the range of 10-80°.The N2 adsorption/desorption curve is characterised on ASAP2460 after vacuum degassing at 180° for 8h.SEM (Zeiss Gemini 300), TEM (JEM-1400FLASH 120kV), HRTEM and EDS-mapping (Tecnai G2 F20 S-TWIN, FEI 200kV) are used to characterise the morphology of the samples.X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha 250Xi) equipped with Al-Kα source and hemispherical chemical analyser.ICP-OES (Agilent 720ES) was used to measure metal concentrations (ppm level).Thermogravimetric analysis (TG 209F3) curve by heating of sample in air from room temperature to 800°C (rate: 5°C/min).The Raman (Horiba LabRAM HR Evolution) test is performed with a 540 nm laser in the range of 50 to 4000 cm -1 .

Antibacterial assay
Two bacterial strains, Escherichia coli (E.coli, ATCC 25922) and Staphylococcus aureus (S.aureus, ATCC 25923) were tested for antibacterial properties of the materials.Specifically, the materials were treated with bacterial suspension (500 μL, 1×105 CFU/mL) into EP tubes to achieve a concentration of 100 ug/ml of the materials, and then hydrogen peroxide was added to the experimental groups to achieve a concentration of 1mM.All groups were incubated at 37 °C for 300 minutes.Afterwards, bacterial suspension was collected from each EP tube to LB agar plate at 37 ° C for 12 hours to obtain bacterial colonies.

Catalytic reduction of nitrophenols
In general, 15 mg of catalyst was added to 50 mL of freshly prepared aqueous 4-NP solution (0.03 mmol), followed by the addition of 0.05 g of NaBH4, with constant stirring at room S4 temperature.At regular intervals, 2.5 mL of the reaction solution was withdrawn and filtered through a 0.2 μm membrane, and the absorbance was measured using a UV-Vis spectrophotometer.In the cyclic test, after each reaction, the catalyst was washed by centrifugation and then dried in a vacuum oven before the next reaction.

Figure S12 .
Figure S12.The optical photographs.(a) precursor after milling; (b) as-made sample after reaction; (c) the obtained mCeO2 with a g-scale yield; (d) SEM image of the mCeO2.

Figure S17 .
Figure S17.The characterizations of mCeO2 with different Cu loadings.(a) XRD patterns; (b) the content of Cu in Cu-mCeO2 (the values were calculated based on ICP results); (c) XPS survey spectrum; (d-f) XPS spectra of Ce 3d, O 1s and Cu 2p.

Figure S18 .
Figure S18.(a) UV-vis absorption spectrogram of the reaction solution at various time in the catalysis of mCeO2 microspheres; (b) UV-vis absorption spectrogram of the reaction solution at various time in the catalysis of the Cu-cCeO2; (c) Cu-mCeO2 catalyses the conversion of 4-NP to 4-AP for 5 cycles.

Figure S19 .
Figure S19.(a) UV-vis absorption spectrogram with and without NaBH4 for o-Nitrophenol, (b) UV-vis absorption spectrogram of Cu-mCeO2 at various time for o-Nitrophenol.

Figure S20 .
Figure S20.(a) XRD pattern and (b)TEM pattern of Cu-mCeO2 after the cycling.