TiO₂ nanocrystals with the {001} and {101} facets co-exposed with MIL-100(Fe): an egg-like composite nanomaterial for efficient visible light-driven photocatalysis

Abstract

The exposure of a specific crystal face to a specific composition or a suitable carrier composition with synergistic effects can effectively improve the photocatalytic activity of the material and enhance its practical value. For choosing an ideal carrier, the primary factor is a large specific surface area. Herein, by using MIL-100(Fe) as the carrier, an egg-like TiO₂/MIL-100(Fe) composite was successfully prepared, for the first time, via a facile two-pot hydrothermal method. XRD, SEM, TEM and other characterization methods showed that when the molar ratio of Ti : Fe was 0.3 : 1, the morphology of the TiO₂/MIL-100(Fe) composite was completely egg-like. The TEM results showed that the {001} and {101} facets of TiO₂ in the TiO₂/MIL-100(Fe) composite were co-exposed. The BET results showed that the TiO₂/MIL-100(Fe) composite had a large specific surface area and pore size. The larger pore size provided an effective channel for the photocatalytic degradation of MB and the interfacial effect of TiO₂ and MIL-100(Fe). The separation efficiency of the photogenerated electron–hole pairs was effectively improved. The efficiency of 30% TiO₂/MIL-100(Fe) in the photocatalytic degradation of MB reached 99.02% in 30 min under visible light. All these findings showed that the composite of the effectively charge-separated photocatalytic semiconductor and the porous MOF with a high specific surface area had a high potential application value for the photocatalytic degradation of organic pollutants.

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1. Introduction

Nowadays, environmental pollution caused by organic dyes in wastewater is becoming more serious.^1–5 The design and self-assembly of novel, efficient, and highly stable photocatalysts are critical to solving the current environmental energy problems.^6–9 TiO₂ has been widely used for water decomposition and the photocatalytic degradation of organic pollutants because of its chemical stability, strong oxidizing power, non-toxicity and low-cost.^2–5,10 Large-scale commercial applications are still difficult in research at present. Moreover, the adsorption capacity of TiO₂ is poor, the specific surface area is small, and the rapid recombination of photoexcited electron–hole pairs is also a factor that causes its low photocatalytic efficiency.^11,12 Researchers have used many methods to improve and enhance the photocatalytic performance of TiO₂. For instance, precious metal loading, quantum dot doping, semiconductor recombination, and different heterojunction structures improve the photocatalytic properties of materials to some extent.^13–15 In particular, the synergistic effect of the {001} and {101} crystal facets of anatase TiO₂ and the crystal face heterojunction theory have proven that the photocatalytic properties of the facet-controlled TiO₂ material are improved.^{14–17,44–46} However, high-efficiency TiO₂-based photocatalytic materials have been rarely reported thus far.

Metal-organic frameworks (MOFs) are bridged by metal ions or metal clusters with organic ligands containing O or N, forming 1D, 2D or 3D coordination porous crystal materials, which together possess the advantages of organic and inorganic compounds.^18–21 An ideal carrier is a solid matrix with a large specific surface area.²² The advantages of MOFs as photocatalysts are their diversity, ideal ordered structure and high surface area, and they have considerable application potential in adsorption and catalysis.^20–25 Notably, several MOF species, such as MIL-101(Fe),^26–28 NH₂-UiO-66 (ref. 29–32) and NH₂-MIL-125(Ti),^33–36 and various MOF-semiconductor composites, such as TiO₂@MIL-101,^37–39 CsPbBr₃ dots/UiO-66(NH₂),⁴⁰ ZnIn₂S₄@NH₂-MIL-125(Ti)⁴¹ and Fe₃O₄@MIL-100(Fe),⁴² are widely studied in the photocatalytic degradation of pollutants. The sandwich-like TiO₂NS@MIL-100(Fe) nanocomposite obtained by the self-assembly of TiO₂NS on porous MIL-100(Fe) not only has a porous channel to adsorb organic molecules, but the interface of the composite also provides a platform for photoexcited electrons. Consequently, the photoelectron–hole separation increased, the photocatalytic performance of the material was enhanced, and the adsorption performance was also improved.⁴³ The MIL-100(Fe)/P25 composite photocatalyst was formed by a process in which MIL-100(Fe) was grown on the TiO₂ surface, and several Ti atoms were replaced by Fe atoms, thereby resulting in a change in the electron density distribution and defect levels.⁴ In the photocatalytic process, the excited electrons were trapped at the defect level, which considerably suppressed the recombination of the charge carriers and increased the number of charge carriers, thereby improving the photocatalytic efficiency. When special TiO₂@MIL-101 core–shell nanocomposites were used as adsorbents and catalysts, the adsorption capacity was outstanding and good catalytic performance was also exhibited.³⁷

Herein, an egg-like TiO₂/MIL-100(Fe) composite nanomaterial was successfully synthesized, for the first time, via a facile two-pot hydrothermal method, and anatase TiO₂ had the {001} and {101} facets co-exposed. The synthesised TiO₂/MIL-100(Fe) composite could efficiently photocatalyse the organic dye methylene blue (MB) under visible light (λ = 664 nm). The outstanding photocatalytic properties of the composite can be attributed to the introduction of facet-controlled TiO₂ in porous MIL-100(Fe) with a high surface area. At the same time, the increased surface area and average pore size of the TiO₂/MIL-100(Fe) composite effectively improved its photocatalytic properties, which was different from those reported in previous studies. Amongst these composites, the 30% TiO₂/MIL-100(Fe) composite exhibited the most complete egg-like morphology and had high photocatalytic performance. The efficiency of MB degradation could reach 99.02% in 30 min, and the kinetic rate was 0.158 min⁻¹, which is 2.1 and 4.3 times those of pure TiO₂ (0.036 min⁻¹) and MIL-100(Fe) (0.077 min⁻¹), respectively.

2. Experimental details

2.1 Synthesis of MIL-100(Fe)

The MIL-100(Fe) was synthesized via a simple hydrothermal method.^43,47 FeCl₃·6H₂O (18 mmol), H₃BTC (12 mmol) and Na₂CO₃ (25 mmol) were added into Milli-Q water (36 mL) and stirred well. Then, the mixture was poured into a 100 mL Teflon-lined steel autoclave at 160 °C for 12 h. After naturally cooling to room temperature, the light orange solid was recovered by centrifugation. In order to remove the unreacted H₃BTC, which was present both inside and outside the pores of the material, the reaction product was purified by consequent washing with hot Milli-Q water and hot ethanol several times. After centrifugation, the product was finally dried overnight at 60 °C. To prevent any experimental error, the synthesis of each material was repeated several times, and the samples were subjected to the same characterization techniques.

2.2 Synthesis of the {001} and {101} co-exposed TiO₂/MIL-100(Fe) composite

The synthesis method of the {001} and {101} co-exposed TiO₂/MIL-100(Fe) egg-like composite was similar to the above method for MIL-100(Fe). The only difference was that {001} and {101} co-exposed TiO₂ (1.8, 3.6, 5.4, and 7.2 mmol) was added to the reaction precursor during the synthesis process. The purification process was adopted from the above method.

2.3 Characterization

X-ray powder diffraction analysis (XRD) was performed on a German Bruker D8 type. The experimental conditions were Cu Kα radiation, the tube pressure and tube flow were 35 kV and 30 mA, respectively, and the scanning range was 2θ = 5–70°. Scanning Electron Microscopy (SEM) was performed using the model MAIA 3 XMH. The transmission electron microscope (TEM) model JEM-2100F with an acceleration voltage of 200 kV and produced by the Japanese Electronics Society was employed. The UV-Vis-DRS model UV-2600 produced by Shimadzu Corporation of Japan was used with BaSO₄ as the standard reference with a scanning wavelength range of 200–800 nm and a scan step size of 1 nm. X-ray photoelectron spectroscopy (XPS) was conducted on an EXCALAB210 produced by the American Thermoelectric Corporation. The UV-Vis spectrophotometer (model UVmini-1240) manufactured by Shimadzu Corporation of Japan was used for measuring the absorbance of methylene blue solution in the wavelength range of 200–800 nm.

2.4 Evaluation of photocatalytic activity

The photocatalytic performance of the TiO₂/MIL-100(Fe) photocatalyst was evaluated by the photodegradation of methylene blue dye (MB). 5 mg of TiO₂/MIL-100(Fe) photocatalyst was mixed with 100 mL of MB aqueous solution (10 ppm). A xenon lamp was used as the analog light source, and the dark adsorption experiment was performed for 1 h before the light was turned on, and the adsorption–desorption equilibrium was achieved between the photocatalyst and the contaminant. 3 mL of the suspension was centrifuged at 8000 rpm for 5 min, and the supernatant was taken for the measurement of absorbance at λ = 664 nm using a UV-visible spectrophotometer (UV-2550, Shimadzu).

3. Results and discussion

3.1 Structural and morphological characterization

The XRD patterns of bare TiO₂ nanoparticles, MIL-100(Fe) and TiO₂/MIL-100(Fe) with different molar ratios of components 100 (Fig. 1) depicted characteristic structures. All the peaks of pure TiO₂ at 25.28°, 37.80°, 48.05°, 53.89° and 55.06° corresponded to the (101), (004), (200), (105) and (211) crystal facets of anatase TiO₂. They depicted the characteristic tetragonal structure of anatase TiO₂ (JCPDS card no. 21-1272). The characteristic peak of MIL-100(Fe) was consistent with those in the previous reports.^43,47 After the simple hydrothermal TiO₂ modification on MIL-100(Fe), the XRD patterns of the TiO₂/MIL-100(Fe) composites with different molar ratios contained unique peaks for both TiO₂ and MIL-100(Fe) (e.g. 2θ = 11.0°, 25.2°, 37.8°, 48.0°, and 53.8°). With the increase in added TiO₂, the unique peak of TiO₂/MIL-100(Fe) at 25.28° was evident, thereby corresponding to the {101} crystal plane of anatase TiO₂. This result demonstrated the successful modification of TiO₂ on the MIL-100(Fe) materials. The XRD pattern of the TiO₂/MIL-100(Fe) material also retained the characteristic peak of MIL-100(Fe), indicating that the composite retained the typical crystal structure of MIL-100(Fe).

Fig. 1 XRD patterns of {001} and {101} co-exposed TiO₂ and 10–50% TiO₂/MIL-100(Fe) composites; the inset is the enlarged XRD pattern of MIL-100(Fe) in the region of 2θ = 5–30°.

The BET results indicated the porous nature of MIL-100(Fe) and the 30% TiO₂/MIL-100(Fe) composite. Fig. 2 shows the specific surface area and pore size distribution curves of the MIL-100(Fe) and 30% TiO₂/MIL-100(Fe) materials. According to the BDDT classification, both samples belonged to type II isotherm, which also indicated that all the materials prepared had porous structures. Surprisingly, with the addition of TiO₂, the specific surface area did not decrease and the pore size increased slightly, which is in agreement with the XRD results. According to previous reports, the average specific surface area of pure P25 is 53 m² g⁻¹.⁴ The average specific surface area of the 30% TiO₂/MIL-100(Fe) composite was 460.56 m² g⁻¹. The porous 30% TiO₂/MIL-100(Fe) composite with high specific surface area and large pore size can adsorb additional organic molecules when used as a photocatalyst and accelerate the photogenerated carrier separation, thereby improving the photocatalytic performance of the material (Table 1).

Fig. 2 N₂ adsorption–desorption isotherms of {001} and {101} co-exposed TiO₂ and the 30% TiO₂/MIL-100(Fe) composite.

Table 1 BET surface area of the {001} and {101} co-exposed TiO₂ and 30% TiO₂/MIL-100(Fe) composite

Samples	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)
MIL-100(Fe)	431.9545	0.2380	5.7786
30% TiO2/MIL-100(Fe)	460.5602	0.2809	6.0852

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The SEM and TEM images illustrated the influence of MIL-100(Fe) introduction on the morphological characteristics of the {001} and {101} co-exposed TiO₂ samples (Fig. 3, S1 and S2†). TiO₂ with co-exposed of {001} and {101} facets was successfully loaded on the surface of MIL-100(Fe), and the egg-like TiO₂/MIL-100(Fe) was established well (Fig. 3(a)–(d), S1(a), (b) and S2†). Evident lattice fringes could be observed by HRTEM (Fig. 3(d)). The lattice fringe was 0.354 nm in size, corresponding to the {101} crystal plane of TiO₂, and d = 0.235 nm corresponded to the {001} facet of TiO₂. The crystal faces demonstrated the co-exposure of the {001} and {101} facets of TiO₂ in the TiO₂/MIL-100(Fe) composites. The element distribution maps further showed that the Fe and Ti elements were uniformly distributed on the TiO₂/MIL-100(Fe) samples, thereby indicating that TiO₂ was uniformly distributed on the MIL-100(Fe) carrier, which is consistent with the SEM results. The appearance of the characteristic peaks of Fe, Ti and O on the EDS map further demonstrated the successful preparation of TiO₂/MIL-100(Fe) composites. TiO₂ was also a factor in accelerating the photogenerated electron–hole pair separation due to the synergistic effect of the crystal plane between the {001} and {101} facets, which further explains the efficient photocatalytic performance of the {001} and {101} co-exposed TiO₂/MIL-100(Fe) composites.

Fig. 3 TEM images of the 30% {001} and {101} co-exposed TiO₂/MIL-100(Fe) composite. (a and b) Different perspectives, (c) edge position, (d) lattice spacing.

The chemical composition and binding status of neat TiO₂ and TiO₂@30%MIL-100(Fe) were measured by XPS. The survey spectrum (Fig. 4(a)) showed the presence of Ti, Fe, O and C. The high resolution XPS spectrum of O 1s consisted of three component peaks at 531.4 eV, 530.4 eV and 528.8 eV (Fig. S3(a)†). The characteristic binding energy peaks at 458.8 eV and 464.5 eV corresponded to Ti 2p_3/2 and Ti 2p_1/2 of Ti atoms, which belong to Ti⁴⁺ in a tetragonal structure (Fig. S3(b)†).¹⁴ An additional peak appeared at 458.1 eV in the presence of the Fe–O–Ti bond. The Fe–O–Ti bond is formed during the growth of TiO₂/MIL-100(Fe), in which some of the Ti atoms are replaced by Fe atoms. As shown in Fig. 4(b), the peaks of Fe 2p_3/2 and Fe 2p_1/2 were observed at 711.9 eV and 724.8 eV, respectively, thereby indicating that the Fe element in TiO₂/MIL-100(Fe) existed in the form of Fe³⁺.⁴⁷

Fig. 4 XPS pattern measurements of neat TiO₂ and 30% TiO₂/MIL-100(Fe): (a) survey and (b) Fe 2p binding energy spectra.

In order to further explore the factors affecting the photocatalytic activity of TiO₂/MIL-100(Fe), UV-visible absorption spectroscopy was performed for TiO₂, MIL-100(Fe) and the TiO₂/MIL-100(Fe) composites with different molar ratios. The as-prepared TiO₂ had an absorption wavelength of approximately 400 nm and belonged to the intrinsic absorption band (Fig. 5). In addition, MIL-100(Fe), as a carrier, also played an important role in the light absorption properties of the TiO₂/MIL-100(Fe) composites. MIL-100(Fe) and 10–50% TiO₂/MIL-100(Fe) composites had two absorption edges at 500 nm and 650 nm.

Fig. 5 UV-vis DRS spectra of TiO₂, MIL-100(Fe) and TiO₂/MIL-100(Fe) composites.

3.2 Photocatalytic activities

The photocatalytic properties of TiO₂, MIL-100(Fe) and the TiO₂/MIL-100(Fe) materials with different molar ratios of TiO₂ were investigated by the adsorption degradation of the organic dye MB under visible light. As shown in Fig. 6(a), MIL-100(Fe) and the TiO₂/MIL-100(Fe) materials with different molar ratios of TiO₂ had significant improvement compared with the photocatalyst and pure TiO₂. With the addition of TiO₂, the photocatalytic MB degradation efficiency of the TiO₂/MIL-100(Fe) composites were significantly higher than that of MIL-100(Fe). The order of degradation efficiency was as follows: 30% T/M > 40% T/M > 20% T/M > 50% T/M > 10% T/M > MIL-100(Fe) > TiO₂. The 30% TiO₂/MIL-100(Fe) composite containing {001} and {101} facet co-exposed TiO₂ exhibited the best photocatalytic performance with a degradation efficiency of 99.02% in 30 min. According to the photocatalytic experimental data, a quasi-first-order kinetic equation and the degradation rate constants for MB degradation and adsorption were obtained according to −ln(C/C₀) = kt,^16,17 as shown in Fig. 6(b), (c) and (d). The degradation rate of 30% TiO₂/MIL-100(Fe) was the highest at 0.158 min⁻¹, which was 2.1 and 4.4 folds higher than that of MIL-100(Fe) (0.077 min⁻¹) and TiO₂ (0.036 min⁻¹), respectively (Fig. 6(b)–(d)). This result was observed because the rapid electron transfer between TiO₂ with co-exposed {001} and {101} facets and MIL-100(Fe) allowed the photogenerated electron–hole pairs to be efficiently separated. The MB adsorption kinetics equation (Fig. 6(c)–(d)) showed that the adsorption rates of MIL-100(Fe) and TiO₂/MIL-100(Fe) composites did not significantly improve, which is consistent with the BET results. Pure MIL-100(Fe) had the highest adsorption kinetics rate of 1.4 × 10⁻² min⁻¹. The adsorption capacity of pure TiO₂ was low, and the adsorption amount of MB was approximately 1.9%. Combining TiO₂ and MIL-100(Fe) increased the adsorption kinetics rate of the TiO₂/MIL-100(Fe) complex because the high specific surface area and large pore size facilitated the capture of organic molecules and provided reaction sites. The interface channel also improved the adsorption capacity of the composite. At the same time, the synergy between the co-exposed {001} and {101} crystal facets of TiO₂ could also effectively promote photocarrier separation and further improve the photocatalytic efficiency.

Fig. 6 (a) Control experiments for the photodegradation of MB with respect to different photocatalysts; (b) kinetics of MB degradation as a function of photocatalysts; (c) kinetics of MB adsorption as a function of photocatalysts; (d) catalytic rate constants of MB degradation and adsorption. is the initial concentration of MB before dark adsorption, and C₀ is the concentration of MB before illumination. Reaction conditions: 5 mg photocatalyst, 100 mL of 10 mg L⁻¹ MB aqueous solution.

The photodegradation cycle of the 30% {001} and {101} facet co-exposed TiO₂/MIL-100(Fe) composite was tested four times with MB to evaluate the reusability of the material. In each photodegradation experiment, the suspension was treated in the dark for 1 h to establish an adsorption–desorption equilibrium. The absorbance of MB from 4 cycles of photodegradation showed that the 30% TiO₂/MIL-100(Fe) composite maintained good adsorption and catalytic properties and had good stability (Fig. 7). Only 5 mg of the photocatalyst was slightly damaged by centrifugation and washing, thereby resulting in a slight decrease in the photocatalytic performance and adsorption performance.

Fig. 7 Photocatalytic degradation of MB by 30% TiO₂/MIL-100(Fe) over four consecutive cycles.

According to the above results, the possible photocatalytic degradation mechanism of MB by the {001} and {101} facet co-exposed TiO₂/MIL-100(Fe) complexes was proposed (Fig. 8). Due to the charge transfer occurring between an inorganic semiconductor and the MOF, photogenerated electron–hole pair recombination can be substantially suppressed. Under visible light, photoexcited electrons (e⁻) are transferred from the valence band to the conduction band on TiO₂, thereby forming holes (h⁺) on the valence band.¹⁰ The photoexcited electrons (e⁻) are then transferred from the conduction band of TiO₂ to the conduction band of MIL-100(Fe) by the interactions between the TiO₂ and MIL-100(Fe) interfaces. This phenomenon also effectively suppresses the recombination of the photogenerated electron–hole pairs, thereby further improving the photocatalytic efficiency of the material. The photogenerated hole (e⁻) in the valence band of TiO₂ oxidises MB or directly reacts with OH⁻ in H₂O to form a hydroxyl radical (·OH). The photogenerated electrons in the MIL-100(Fe) and TiO₂ conduction bands react with O molecules to form superoxide radicals (·O²⁻).^4,43 The MB molecules adsorbed on the surface of the composite are photodegraded by ·O²⁻ and ·OH into CO₂, H₂O and other products. The TiO₂/MIL-100(Fe) composite with a larger pore size and specific surface area can effectively adsorb a certain amount of dye molecules and become the base of further photocatalytic reactions. The synergistic effect between the total exposure of the {001} and {101} TiO₂ crystal planes effectively improved the separation efficiency of the photogenerated electron–hole pairs. The special egg-like structure of the material had a larger pore size and specific surface area, which provided an effective channel for photoelectron electron–hole pair separation and a favourable place for the photocatalytic reaction. The interfacial effect of TiO₂ and MIL-100(Fe) also increased the photocatalytic efficiency.

Fig. 8 Proposed mechanism for the photocatalytic degradation of MB by the {001} and {101} facet co-exposed TiO₂/MIL-100(Fe) composite.

4. Conclusion

In summary, the photocatalytic {001} and {101} facet co-exposed TiO₂/MIL-100(Fe) composite material was successfully prepared via a simple two-step hydrothermal method. A TiO₂/MIL-100(Fe) composite with 30% {001} and {101} facet co-exposed TiO₂ that maintained a complete egg-like morphology was prepared for the first time. The material had a higher specific surface area and pore diameter than pure MIL-100(Fe) under the same preparation conditions and exhibited excellent performance in the photocatalytic degradation of the organic dye MB. The efficiency of 30% TiO₂/MIL-100(Fe) in the photocatalytic degradation of MB in 30 min was as high as 99.02%. Photocatalytic experiments with 4 cycles showed that the reduction in the photocatalytic activity of the prepared composites was insignificant and it had good stability. The excellent performance in the photocatalysis was attributed to the synergistic effect between the {001} and {101} planes of TiO₂ and the interfacial effect between TiO₂ and MIL-100(Fe), and the egg-like morphology increased the specific surface area and pore size of the composite, which provided a channel for electron transfer to occur at the interface. To further improve solar energy utilisation, we will continue to conduct research on semiconductors and MOF composites.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 21671026) and Hunan Collaborative Innovation Center of Environmental and Energy Photocatalysis. The authors are also grateful to the aid provided by the Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra06359e

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