Fabrication of TiO₂@MIL-53 core–shell composite for exceptionally enhanced adsorption and degradation of nonionic organics

Abstract

Herein, we report a new strategy based on non-aqueous sol–gel reaction for in situ fabrication of TiO₂@MIL-53 core–shell composites, and systematically show the excellent performance of the composites for exceptionally enhanced adsorption and degradation of nonionic organics, including Sudan red III, Sudan red IV, Sudan red I and vat pink.

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Environmental and energy problems have aroused attention from all over the world. In the huge production of modern industry, organics, which are difficult to be removed, cause irreversible damage to all forms of life due to their toxicity and wide distribution.^1–3 Nonionic organic dyes, including Sudan red series, vat series (vat pink, yellow and blue), which are widely used in textile, leather, painting, and plastic industries, pose overall environmental hazards. However, novel multi-functional materials provide great possibilities in removing and decomposing such organics, which may bring this tough trouble into brightness.

As an emerging class of inorganic–organic hybrid materials, metal–organic frameworks (MOFs) have gained growing attention as advanced porous materials.^4–9 Owing to their exceptionally high surface areas, MOFs have been widely used for adsorption-based applications, including high-efficient adsorption from small metal ions,^10,11 organic molecules^12,13 to large guest molecules such as organometallic complexes,¹⁴ enzymes¹⁵ and peptides.¹⁶ Among the various MOFs, the Material Institut Lavoisier (MIL) series are of special interest. MIL-53, one of the MIL materials first reported by Férey et al., has one-dimensional lozenge-shape tunnels with especially good thermal and chemical stability, which is capable for post-synthesis.¹⁷ Titanium dioxide (TiO₂) has been long recognized as an important class of photoactive materials for various energy and environmental applications.^18–22 Recently, it has been realized that the surface area of the TiO₂ nanocrystals is positively correlated with their photocatalytic performance.²³ Efforts have been made to design and synthesize hollow and/or nanosized anatase TiO₂ with high surface area, to improve the photocatalytic activity.²⁴ The high surface area of MOFs and the superiority of MOFs in adsorption together with the superiority of TiO₂ in photocatalysis provide great opportunity in the pre-treatment of organic contaminations. In light of these facts, investigating the TiO₂@MOF composite is a promising approach that possibly exhibits both strong adsorption and photocatalytic abilities. However, to the best of our knowledge, investigations on the synergism of TiO₂ and MOF based on the TiO₂@MOF composite for adsorption and decomposition of organics have never been reported before.

DeKrafft et al.²⁵ and Gu et al.²⁶ have reported a strategy for constructing MOF–TiO₂ core–shell composite using a similar method based on an aqueous sol–gel reaction. Herein, we report a new approach for preparation of TiO₂@MIL-53 core–shell composite via a non-aqueous sol–gel reaction, which involves anhydrous ethanol as solvent, hydrolysis agent, and stabilization agent, titanium butoxide (TBOT) as titanium precursor, and MIL-53 as additional agent for crystallization of TiO₂ (Scheme 1 Step A). To demonstrate the proof-of-concept, investigations have been made to show the synergism of TiO₂@MIL-53 core–shell composite for the adsorption (Scheme 1 Step B) and the decomposition of nonionic organics (Scheme 1 Step C). After activated at 300 °C, the fabricated TiO₂@MIL-53 core–shell composite which possessed larger surface area compared to the pure MIL-53 exhibited better adsorption capacity for Sudan red III (SR_III) based on the porous structure of TiO₂@MIL-53 (Fig. 1 dark). Furthermore, the fabricated TiO₂@MIL-53 core–shell composite had strong ability to degrade SR_III under UV irradiation on basis of the outer TiO₂ coatings (Fig. 1 UV light). In addition, the fabricated TiO₂@MIL-53 composite has been found to be versatile adsorbent and catalyst for removing various nonionic organics in solution.

Scheme 1 Schematic for the fabrication of TiO₂@MIL-53 core–shell composite and its adsorption, degradation abilities for organics.

Fig. 1 Performance of the synthesized TiO₂@MIL-53 core–shell composite for adsorption (in the dark) and degradation (under UV light irradiation) of Sudan red III. The inset image represents the UV adsorption spectrum of Sudan red III after adsorption and/or degradation for a pre-determined time corresponding to the point plot on the left.

The structure of MIL-53 was confirmed by X-ray diffraction experiment (Fig. S1†). The appearance of the characteristic peaks of anatase TiO₂ at 2θ = 25.43°, 37.92°, 48.09°, 54.58°, 62.81° and 78.89° (Fig. 2A) and the existence of the characteristic signal of MIL-53 at 2θ = 5–35° in the XRD pattern of TiO₂@MIL-53 (Fig. 2B) confirmed the successful fabrication of TiO₂@MIL-53 composite. Thermogravimetric data showed good thermostability of MIL-53, and the evacuated MIL-53 crystals were stable up to about 550 °C (Fig. S2†). The MIL-53@TiO₂ composite could be activated under the temperature of 550 °C to avoid the decomposition of the framework in MIL-53. SEM images and TEM images were further used to confirm the formation of TiO₂@MIL-53 core–shell structure (Fig. 2C and D and S3†). The synthesized MIL-53 crystal, which was in a representative cubic shape had smooth surfaces (Fig. 2C and S3A†). However, after treating with titanium butoxide (TBOT) solution, the MIL-53 crystal had rough surfaces (Fig. 2D and S3B†). The TiO₂ crystals were coated homogeneously on MIL-53 crystal, which indicated the successful formation of MIL-53@TiO₂ core–shell structure. Nitrogen adsorption isotherms of MIL-53 and TiO₂@MIL-53 composite are shown in Fig. S4,† and the BET surface area (S_BET) and total pore volume (V_total) are summarized in Table 1. Interestingly, the TiO₂@MIL-53 composite activated at 300 °C possessed the highest surface area and pore volume compared to the pure MIL-53 and the lower-temperature-activated-TiO₂@MIL-53. Normally, MIL-53 is activated upon heating at 300–320 °C to produce a fully open framework.²⁷ Although, the sublimation point of the unreacted ligand, 1,4-benzenedicarboxylic acid (H₂BDC), is approximately 300 °C, a number of free H₂BDC ligands remained in the pores of MIL-53 after activation at 300 °C.²⁶ However, the following procedure of solvent thermal treatment and calcination at higher temperature (300 °C) for TiO₂ coating and crystallization may facilitate the sublimation of unreacted H₂BDC ligands in the framework of MIL-53, which may be helpful for producing larger BET surface area and total pore volume.

Fig. 2 (A) Comparison of the XRD patterns of the prepared TiO₂@MIL-53 core–shell composite with the synthesized TiO₂; (B) comparison of the XRD patterns of the prepared TiO₂@MIL-53 with the simulated MIL-53 structure data; (C) SEM image of the synthesized MIL-53 crystal; and (D) SEM image of the prepared TiO₂@MIL-53 core–shell composite. The scale bar refers to 2 μm.

Table 1 Textural properties of MIL-53 and TiO₂@MIL-53

Sample	SBET [m^2 g^−1]	Vtotal^a [cm^3 g^−1]
a Vtotal was measured at P/P0 = 0.99.
MIL-53	919.74	470.2
TiO2@MIL-53 activated at 300 °C	1139.32	513.9
TiO2@MIL-53 activated at 200 °C	712.98	418.4
TiO2@MIL-53 activated at 100 °C	687.73	390.2

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To study the effect of activation temperature on adsorption, three activation temperatures (100, 200 and 300 °C) of TiO₂@MIL-53 were evaluated. The time-dependent adsorption of SR_III on MIL-53 and TiO₂@MIL-53 was investigated at the initial concentration of 30 mg L⁻¹ at the temperature of 30 °C (Fig. 3A and S5†). When pure MIL-53 was utilized as adsorbent, the concentration of SR_III decreased significantly in the first 10 min and reached equilibrium after 140 min (Fig. S5†). The adsorption capacity of SR_III on MIL-53 was about 18 mg g⁻¹ after equilibrium (Fig. 3A). However, more time was needed to reach adsorption equilibrium when adsorbed on TiO₂@MIL-53 core–shell composite at the same initial concentration of SR_III (about 240 min) (Fig. S5†), showing a slower dispersion velocity of SR_III on TiO₂@MIL-53 composite compared to the pure MIL-53. It was obvious that the outer TiO₂ coatings of the synthesized TiO₂@MIL-53 core–shell composite might have played as barriers that delayed the adsorption equilibrium. The adsorption capacity of SR_III on TiO₂@MIL-53 showed different results when activated at different temperatures (Fig. 3A). Activated at 100 °C or 200 °C, the maximum adsorption capacity of TiO₂@MIL-53 composite was less than 16 mg g⁻¹, which was smaller than the original pure MIL-53 materials. However, after activated at 300 °C, the maximum adsorption capacity of TiO₂@MIL-53 composite was a little bit larger than 20 mg g⁻¹ at the same conditions, which is significantly higher than that of the pure MIL-53. The exceptionally enhanced adsorption ability of 300 °C-activated TiO₂@MIL-53 composite was further revealed by the determination of the adsorption isotherms of SR_III on MIL-53 and TiO₂@MIL-53 in the concentration range of 10–50 mg L⁻¹ at the temperature of 30 °C (Fig. 3B). The maximum adsorption capacities of TiO₂@MIL-53 activated at 300 °C were higher than that activated at 100 °C and the pure MIL-53 materials at each initial concentration, showing the larger adsorption capacity of this high-temperature-activated TiO₂@MIL-53 absorbent. The abovementioned results were in accordance with the data of BET surface areas (Table 1). For each absorbent, the adsorption capacities increased with the initial concentration of SR_III, showing the favorable adsorption of SR_III at higher concentrations and the superiority of 300 °C-activated TiO₂@MIL-53 for the removal of SR_III at higher concentrations. The lack of plateau in the adsorption isotherms indicated multi-layer adsorption and higher adsorption capacity for SR_III.

Fig. 3 (A) Time-dependent adsorption of Sudan Red III on MIL-53, 100 °C-activated TiO₂@MIL-53, 200 °C-activated TiO₂@MIL-53 and 300 °C-activated TiO₂@MIL-53; (B) adsorption isotherms for the adsorption of Sudan red III at the temperature of 30 °C; (C) photocatalytic activity of TiO₂ crystals activated under different temperatures; and (D) the apparent reaction rate constant versus UV irradiation time.

Photocatalytic activities of the synthesized pure TiO₂ activated at different temperatures were evaluated by their uses in the degradation reaction of SR_III (Fig. 3C and D). According to the results shown in Fig. 3C, the synthesized TiO₂ activated at 300 °C showed improved photocatalysis. The percentage of dye degradation increased from 65% to 97% after 40 min of UV-light illumination with increasing temperature from 100 °C to 300 °C. We plotted the results by ln(C/C₀) versus reaction time, and the results showed a good linear relationship (with r² > 0.99), which indicated that the reaction was in good agreement with the first-order reaction, and the reaction rate could be evaluated by the apparent reaction rate constant obtained from the slope of the linear fit of the results. As shown in Fig. 3D, the reaction rate of 300 °C-activated TiO₂ was approximately two-fold that of the 100 °C-activated TiO₂, showing the better photocatalytic efficiency of 300 °C-activated TiO₂.

The abovementioned results showed that 300 °C was the optimized activation temperature for TiO₂@MIL-53 core–shell composites on both adsorption and photocatalytic processes. Consequently, the synergism of TiO₂@MIL-53 core–shell composites for the adsorption and photocatalysis was further investigated simultaneously (Fig. 4 and S6†). As shown in Fig. 4A, after adsorption in dark for 50 min, about 50% of SR_III was absorbed by 300 °C-activated TiO₂@MIL-53, while only 10% of SR_III was absorbed by 100 °C-activated TiO₂@MIL-53. Images 1 to 5 in Fig. 4B and C were images of the resulted SR_III solution after absorbed by 100 °C-activated TiO₂@MIL-53 (Fig. 4B) and 300 °C-activated TiO₂@MIL-53 (Fig. 4C) for 0, 10, 20, 30 and 50 min (numbered as 1, 2, 3, 4 and 5), sequentially. Then, the photocatalytic process was carried out under UV light illumination. After irradiation for 30 min, only 2% of SR_III remained in the solution in the group of 300 °C-activated TiO₂@MIL-53, showing the high efficiency of this high-temperature-activated TiO₂@MIL-53 material for efficient removal of SR_III. In addition, the fabricated 300 °C-activated-TiO₂@MIL-53 core–shell composite exhibited large adsorption capacity and excellent photocatalytic activity to a wide range of nonionic organics. As shown in Fig. 5, 300 °C-activated TiO₂@MIL-53 showed excellent ability for both adsorption and decomposition of nonionic organic dyes, including Sudan red IV, vat pink and Sudan red I. After adsorption for 40 min and UV irradiation for another 30 min, only 1.5%, 4.3% and 13% of Sudan red IV, vat pink and Sudan red I, respectively, remained in the solution.

Fig. 4 (A) Comparison of TiO₂@MIL-53 core–shell composites activated at different temperatures for the adsorption and degradation of Sudan red III; (B and C) images of Sudan red III solution after adsorption for 0, 10, 20, 30 and 50 min and then UV light irradiation for 10, 20 and 30 min based on 100 °C-activated TiO₂@MIL-53 (B), and 300 °C-activated TiO₂@MIL-53 (C).

Fig. 5 Performance of the synthesized 300 °C-activated TiO₂@MIL-53 core–shell composite for adsorption (in the dark) and degradation (under UV light irradiation) of Sudan red III, Sudan red IV, Sudan red I and vat pink.

In summary, we report a simple method for preparation of TiO₂@MIL-53 core–shell composite based on a non-aqueous sol–gel reaction for highly efficient adsorption and degradation of nonionic organics, including Sudan red III, Sudan red IV, Sudan red I and vat pink. The fabricated TiO₂@MIL-53 combines both the advantages of the excellent adsorption property of MIL-53 and the photocatalytic ability of the anatase TiO₂. Furthermore, the high-temperature activated TiO₂@MIL-53 showed exceptionally enhanced adsorption capacity compared to the pure MIL-53. We believe that with the intrinsic characteristics of MOFs, such as high surface area and excellent photoactivity of anatase TiO₂, the MOF–TiO₂ composite will be attractive for the removal of organics, and can be further used in water purification.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21305102, 21475095), Tianjin Natural Science Foundation (15JCQNJC02700, 15JCTPJC55300), the National Training Program of Innovation and Entrepreneurship for Undergraduates (201410058026), and Tianjin High School Science & Technology Fund Planning Project (2013512).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13397e

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