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Article

Synthesis, Structure, and Photocatalytic Activity of TiO2-Montmorillonite Composites

by
Yonghui Zhang
1,
Baoji Miao
1,*,
Qiuling Chen
1,
Zhiming Bai
2,*,
Yange Cao
1 and
Basandorj Davaa
3
1
Henan International Joint Laboratory of Nano-Photoelectric Magnetic Materials, Henan University of Technology, Zhengzhou 450001, China
2
Advanced Ceramic Materials Technology Innovation Center, Hefei Innovation Research Institute of Beihang University, Hefei 230012, China
3
Water Research Center, Mongolian University of Science and Technology, Khoroo 8, Sukhbaatar District, Ulaanbaatar 14191, Mongolia
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(5), 486; https://doi.org/10.3390/catal12050486
Submission received: 2 April 2022 / Revised: 19 April 2022 / Accepted: 25 April 2022 / Published: 26 April 2022
(This article belongs to the Special Issue Structural, Optical and Electronic Properties of Photocatalysts Containing Metal Oxide Nanostructures)
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Abstract

:
In the present study, TiO2-montmorillonite (MMT) composites were synthesized hydrothermally under variable conditions, including the TiO2/MMT mass ratio, reaction pH, reaction temperature, and dwelling time. These samples were determined by X-ray photoelectron spectrometry (XPS), ultraviolet–visible spectroscopy% (UV-Vis DRS), electrochemical impedance spectroscopy (EIS), transient photocurrent responses, photoluminescence (PL) spectra, electron paramagnetic resonance (EPR), and N2 adsorption–desorption isotherms. The photocatalytic activity was evaluated as the ability to promote the visible-light-driven degradation of 30 mg/L of aqueous methylene blue, which was maximized for the composite with a TiO2 mass ratio of 30 wt% prepared at a pH of 6, a reaction temperature of 160 °C, and a dwelling time of 24 h (denoted as 30%-TM), which achieved a methylene blue removal efficiency of 95.6%, which was 4.9 times higher than that of pure TiO2. The unit cell volume and crystallite size of 30%-TM were 92.43 Å3 and 9.28 nm, respectively, with a relatively uniform distribution of TiO2 particles on the MMT’s surface. In addition, 30%-TM had a large specific surface area, a strong light absorption capacity, and a high Ti3+ content among the studied catalysts. Thus, the present study provides a basis for the synthesis of composites with controlled structures.

1. Introduction

The global energy crisis and environmental pollution have necessitated the development of materials that can reduce our dependence on nonrenewable energy sources and remove hazardous pollutants from wastewater [1,2,3,4,5]. Recently, photocatalytic degradation has attracted significant attention as a novel, simple, and environmentally friendly wastewater treatment method, which is superior to traditional techniques that rely on adsorption, membrane separation, ion exchange, and oxidation [6,7,8,9]. Titanium dioxide is extensively used for the photocatalytic degradation of pollutants owing to its nontoxicity, low cost, high chemical stability, hydrophilicity, and good catalytic activity [10,11]. However, it exhibits certain drawbacks, such as rapid electron–hole recombination and a wide band gap [12].
Montmorillonite (MMT) is a common lamellar aluminosilicate [13] that is often used as a photocatalyst support owing to its large specific surface area [10], its high adsorption capacity for cations and polar molecules [14], and stable chemical properties [15]. Composites of TiO2-MMT have been reported to exhibit slower electron–hole recombination and promote better oxidative degradation of pollutants by ozone compared with pure TiO2 and MMT [16]. The porosity of these composites, prepared by reacting titanium silicalite with titanium alkoxides at 50–80 °C, strongly influences their ability to adsorb dyes such as methylene blue (MB), which can be adjusted by varying the reaction temperature [17]. A study reporting the preparation of TiO2 gel from titanium tetrachloride (TiCl4) at 30–80 °C demonstrated the photocatalytic performance of the resulting TiO2-MMT composite, which was maximized at a reaction temperature of 70 °C [18]. In another study, TiO2-MMT suspensions were hydrothermally prepared from titanium sulfide (Ti(SO4)2) and MMT at pH 4, and calcined for 2 h at 300–1200 °C [10]. The growth of anatase and rutile TiO2 was inhibited by adding MMT, while the complete transformation of anatase TiO2 into rutile TiO2 was observed at 900 °C. TiO2-MMT composites prepared by combining cation filling, sol-gel processing, and thermal treatment featured a narrow band gap of 2.79 eV and advantageous structural properties [19]. Preparation of a carbon-modified nitrogen-doped TiO2/montmorillonite composite by the sol-gel method increased the absorption range of light and realized the effective separation of photogenerated electron pairs [20]. Mohammad et al. prepared a kind of gray titanium dioxide, which increased the content of Ti3+ and oxygen vacancies in titanium dioxide and increased the rate of charge transfer [21]. Yusuke et al. studied the synergistic effect of layered silicate and TiO2 on the photocatalytic oxidation of benzene to recover phenol with unprecedented efficiency and selectivity [22].
More recently, TiO2-MMT composites were shown to exhibit high photocatalytic activity even at elevated temperatures, which was ascribed to the presence of TiO2 in the anatase phase [16,17,23,24]. Esmail et al. reported a Cl-doped rutile titanium dioxide photocatalyst, which alone could produce a lower effective carrier mass, higher photogenerated electron and hole mobility, and a longer Ti3+ ion interaction lifetime, thus improving the photocatalytic activity [25]. Compared with pure TiO2, TiO2-MMT composites are easier to recover, facilitating their industrial applications [26], and can exhibit reduced absorbance at 220–300 nm [27].
Numerous studies have probed the effects of the photodegradation conditions (such as solution pH, initial dye concentration, reaction atmosphere, and illumination time) on the photocatalytic performance of TiO2-MMT composites [10,16,27,28,29]. However, only a few have examined the corresponding effects of composite preparation conditions, such as the TiO2/MMT mass ratio and pH of the hydrothermal reaction, which may affect the structure and properties of the composite (for example, the phase composition, chemical bonding, the absorption range, and energy bands), thereby influencing the photocatalytic performance. Herein, we hydrothermally synthesized pure TiO2 and TiO2-MMT composites. We examined the effect of TiO2 content, reaction pH, reaction temperature, and dwelling time on the ability of these composites to promote the photodegradation of MB, which was compared with that of pure TiO2 and MMT. Through detection and analysis by X-ray photoelectron spectrometry (XPS), ultraviolet–visible spectroscopy (UV), electrochemical impedance spectroscopy (EIS), transient photocurrent responses, photoluminescence (PL) spectra, electron paramagnetic resonance (EPR), and Brunner–Emmet–Teller measurements (BET), the materials with the best properties under different conditions were explored. Thus, this study describes valuable correlations between synthetic conditions and catalyst properties, and thus provides a basis for synthesizing composites with a controlled structure.

2. Results and Discussion

2.1. Structural, Morphological, and Chemical Bonds

Figure 1 and Figure 2 illustrate the XRD patterns of TiO2-MMT composites prepared under different conditions, demonstrating peaks at 2θ = 25.3°, 37.8°, 48.1°, 53.9°, 55°, and 62.8° that correspond to the (101), (004), (200), (105), (211), and (204) planes of anatase (JCPDS NO.73-1764), respectively. Notably, the anatase (101) surface had a higher intrinsic photocatalytic activity compared with other TiO2 crystal faces [24]. The peak corresponding to the (101) plane was the most intense among those observed previously [28], with the variation in its intensity reflecting the effect of the MMT addition and reaction conditions on the crystallization state of TiO2 and thereby on the photocatalytic performance. Apart from anatase and MMT, the XRD patterns featured relatively sharp peaks of quartz, indicating a large surface area owing to the flaky structure of MMT.
Figure 1a reveals that the relative intensity of the anatase peaks, and thereby the amount of anatase, increases with an increase in the TiO2 content; however, the intensity was less than that observed for pure TiO2. The strongest MMT peaks indicated that the lamellar structure was best preserved at a TiO2 mass ratio of 30 wt%. Table 1 illustrates the unit cell parameters, unit cell volumes (V), and crystallite sizes (D) of the investigated materials, which belong to the tetragonal system. The unit cell volume was calculated as V = 0.866a2c [29,30], and the crystallite size was calculated as D = Kλ/(βcos θ), where θ is the diffraction angle, β is the full width at half-maximum of the most intense peak, λ is the X-ray wavelength (0.15406 nm), and K is the Debye–Scherrer constant (0.89) [14,31,32]. The average crystallite size of the samples described in Figure 1a first increased and then decreased with an increase in the TiO2 content, while the maximum cell volume was obtained at a TiO2 mass ratio of 70 wt% (V = 117.97 Å3).
Figure 1b shows that the anatase (101) peak was the strongest and sharpest at pH 8. Thus, the sample prepared at pH 8 had the highest crystallinity. Moreover, the pH = 6-TM sample featured the strongest MMT (004), (111), (210), and (300) peaks, thus exhibiting the most preserved MMT microstructure. These results, in combination with Table 1 and Figure 2, implied that the particle size of TiO2 in the composite became finer when TiO2 was adsorbed on the MMT surface, which could benefit the photocatalytic performance. The weakest MMT peaks were observed for the sample prepared at pH 2, indicating that the MMT structure was largely destroyed under these conditions. The average crystallite size of the samples described in Figure 1b first decreased and then increased with an increase in pH, while the maximum cell volume was obtained at pH 8 (V = 121.40 Å3).
Figure 2a demonstrates that the intensities of anatase (101), MMT, and quartz peaks reached a maximum at 180 °C; the position of the anatase (101) peak was influenced by the temperature. The average crystallite size of the samples increased steadily with an increase in temperature, and the maximum cell volume was obtained at 140 °C (V = 117.67 Å3).
Figure 2b shows that the intensity of the anatase (101) peak, and thus the crystallinity and photocatalytic activity of TiO2, was maximum at 24 h. In addition, the intensities of quartz and MMT (004) peaks were maximized at 24 h, indicating that this dwelling time was the most suited for preserving the MMT’s microstructure. In contrast, TiO2 particle agglomeration was observed with a dwelling time of 20 h. The average crystallite size of the samples first increased and then decreased with an increase in the dwelling time, and the maximum cell volume was obtained for dwelling times of 18 and 20 h (V = 117.99 Å3).
Figure 3 shows the SEM images of 30%-TM at different magnifications, revealing that the ordered lamellar structure of MMT remained intact after the hydrothermal reaction, and showing the presence of well-dispersed TiO2 nanoparticles on and between the MMT layers [33]. The corresponding particle size distribution (Figure 4) suggested that the TiO2 particles and their aggregates had sizes of 10–30 and 50–100 nm, respectively. These results implied that hybridization with TiO2 did not destroy the ordered interlayer structure of MMT, allowing complete utilization of the large interlamellar and surface area of the latter [34].
Figure 5 and Figure 6 illustrate the FTIR spectra of TiO2-MMT composites, pure TiO2, and MMT. The absorption bands of pure TiO2 were located at 480, 1345, 1377, 1594, 1630, 2821, 2921, and 3423 cm−1 (Figure 6a). The band at 480 cm−1 was ascribed to the tensile vibration of Ti–O bonds [35,36]. Peaks at 1594 and 1630 cm−1 were attributed to the vibration of hydroxyl and water molecular layers [32], whereas the peaks at 2821 and 2921 cm−1 were ascribed to the asymmetric vibration of the C–H bonds. The broad peak at 3423 cm−1 was assigned to the stretching vibration of hydroxyl groups, which were mainly represented by adsorbed moisture and hydroxyl groups on the TiO2’s surface. Pure MMT featured peaks at 472, 525, 775, 1048, 1594, and 3420 cm−1. The peak at 3420 cm−1 was due to the symmetrical O–H stretching of the absorbed moisture, whereas that at 1594 cm−1 was ascribed to the deformation vibration of the interlayered water molecules. The strong peak at 1048 cm−1 was assigned to asymmetric Si–O stretching. The signals at 775, 525, and 472 cm−1 were attributed to Al–O bond stretching, Al–O–Si bond deformation, and Si–O–Si bond deformation, respectively [36,37].
The peak range of 910–930 cm−1 was observed only for the TiO2-MMT composites and was assigned to the Si–O–Ti units produced from the reaction of SiO2 with TiO2 [31,32]. The formation of Si–O bonds increased the number of oxygen vacancies on the surface of the TiO2 attached to the MMT [19], enhancing the catalytic performance of the composites. Figure 6a shows that the characteristic absorption band of MMT is unclear for composites with a TiO2 mass ratio of 50, 60, and 70 wt%. This behavior could reflect the increased occurrence of the reaction of SiO2 with TiO2 with the increase in TiO2 content, which decreased the intensity of the Si–O–Si deformation vibration, Al–O–Si deformation vibration, Si–O tensile vibration, and Ti–O tensile vibration. Figure 5b shows that for pH 2, no peaks were observed at 472 and 525 cm−1, implying that the lamellar structure of MMT was destroyed under strongly acidic conditions, which affected the photocatalytic performance.

2.2. Photocatalytic Activity and Dye Degradation

Figure 7a shows the absorption maxima of MB at 664 nm, which decreased with the degradation time for 30%-TM. Figure 7b,c demonstrates the absorbance spectra obtained for the degradation of MB with different catalysts at degradation times of 120 and 0 min, respectively. Figure 8b illustrates insignificant changes in the absorbance of MB in the blank group (light, no catalyst), suggesting that MB is relatively stable in the absence of a photocatalyst. Figure 8a shows that pure TiO2 had a low MB adsorption capacity in the dark and achieved a fluctuating MB removal efficiency of only 7% after 2 h. However, pure TiO2 achieved a removal efficiency of 19.4% after 2 h when irradiated with light (Figure 8b), indicating that it exhibited a certain photocatalytic activity. In contrast, the MB removal efficiency of MMT did not depend on the lighting conditions because the dye was removed by adsorption only. Figure 8a illustrates the MB adsorption capacity of TiO2-MMT composites, which was lower than that of the pure MMT and required more time to reach an adsorption equilibrium (70 min vs. 60 min for MMT). However, after irradiation for 2 h, a further increase in the removal efficiency was observed upon the hybridization of TiO2 with MMT. The highest degradation efficiency of 95.6% after 2 h was observed for composites at a TiO2 content of 30%. The degradation rate of MB by 30%-TM was superior to many of the previously reported TiO2-based and Ag-based photocatalysts, as shown in Table 2.
Figure 8d shows the MB degradation curves for composites prepared at different pH values, revealing that the highest degradation efficiency of 76.1% after 2 h was obtained for pH 6. Although the sample prepared at pH 2 had the best absorption range and the lowest band gap, it exhibited poor photocatalytic performance. This result can be explained as follows: as a cationic dye, MB mainly exists in the cationic form at pH 8 [38]. Because the zero-charge point of TiO2 is at pH 6.8, the surface of TiO2 is positively charged (pH < 6.8) in acidic solutions and negatively charged (pH > 6.8) in alkaline solutions [32]. Therefore, the surface of TiO2 particles is positively charged at pH 2. In addition, MMT mainly exists as large aggregates with a partially destroyed lamellar structure under acidic conditions (pH 1), whereas the size of these aggregates decreases at pH 7 [39]. Moreover, MMT has a negative potential, which changes insignificantly under acidic conditions. Thus, the electrostatic attraction induces the aggregation of positively charged TiO2 with negatively charged MMT [40] to produce composites with reduced surface potential and surface area available for dye adsorption and photon absorption, thereby seriously affecting the adsorption of MB during its degradation. When hydrothermal synthesis was performed at pH 8, Coulombic repulsion existed between the negatively charged surfaces of TiO2, MMT, and OH− anions. Therefore, the number of TiO2 particles on the surface of the composite prepared at pH 8 was less than that on the surface of the composite prepared at pH 6, thereby exhibiting fewer active TiO2 surface sites and lower performance.
Figure 8e shows the effect of dwelling time, indicating that the MB degradation performance was maximized (95.6% after 2 h) and minimized (33.4% after 2 h) with dwelling times of 24 and 20 h, respectively. The overall intensity of the FTIR peaks of the sample prepared by using a dwelling time of 20 h was lower than that observed for samples with different dwelling times, which may be attributed to the agglomeration of TiO2 particles on the lamellar structure of MMT. This agglomeration also affected the intensity of the characteristic MMT diffraction peaks, although it did not destroy the MMT’s structure. This hypothesis agrees with the maximization of the average crystallite size of TiO2 with a dwelling time of 20 h (10.73 nm).
Figure 8f illustrates the effect of the reaction temperature, exhibiting maximum (95.6%) and minimum (42.8%) MB degradation performance at 160 and 200 °C, respectively. The sample prepared at 200 °C had the largest average TiO2 crystallite size of 9.79 nm. In contrast, the corresponding FTIR spectrum showed no new peaks, which agreed with the results of the optical band gap analysis. Therefore, the poor performance of the sample prepared at 200 °C was ascribed to the agglomeration of TiO2 particles and the absence of heterogeneous TiO2-MMT aggregation.
To probe the mechanism of catalytic MB degradation, we used active species trapping experiments (Figure 9). In particular, 5,5-dimethyl-pyrroline N-oxide (DMPO, a spin trapping agent) reacted with photogenerated holes (h+) to produce the radical cation DMPO+, which subsequently reacted with water molecules to form DMPO-OH•. The signals of spin adducts were predictably weak in the dark and became more intense when irradiated with light. A typical four-line electron paramagnetic resonance (EPR) signal of DMPO-OH• (g = 2.0057, intensity ratio = 1:2:2:1) was observed for all samples after irradiation and was more intense for TiO2-MMT composites than for pure TiO2, which implied that MMT remarkably enhanced the ability of TiO2 to produce ·OH radicals (Figure 9a).
The EPR signal of the DMPO-·O2− adduct (g = 2.009, intensity ratio = 1:1:1:1) was also more intense for TiO2-MMT composites than for pure TiO2 (Figure 9b). This adduct was produced via the reduction of O2 by the electrons in the conduction band of TiO2-MMT (−0.08 V; O2/·O2− reduction potential = −0.046 V). Thus, the high photocatalytic activity of TiO2-MMT was ascribed to the enhanced production of reactive oxygen species on its surface.
Figure 10 shows the capture experiment of active substances in the process of MB degradation. In the degradation process, isopropanol (IPA), disodium ethylenediamine tetraacetate (EDTA-2Na), and p-benzoquinone (BQ) at 1 mmol were added as scavengers of hydroxyl radicals (·OH), holes (h+), and superoxide radicals (·O2−), respectively, to further explore the role of active substances in the process of photodegradation. It was obvious that the addition of IPA, EDTA-2Na, and BQ affected the efficiency of the photodegradation of pollutants, indicating that ·OH, h+, and ·O2− played an important role in the process of photodegradation, and the inhibitory effect of BQ was the most obvious. Therefore, ·O2− played a more important role in the process of photocatalytic degradation. This was consistent with the test results of EPR. As shown in Figure 10, DMPO-·O2− had more obvious peaks under light conditions. In addition, by PL analysis, the recombination rate of the electron hole was lower, and the electron and hole functioned separately for a longer time, so the sample had better photocatalytic efficiency.

2.3. N2 Adsorption–Desorption Isotherms, Light Absorption, and Behavior of Photogenerated Charge Carriers

By testing the N2 adsorption–desorption isotherms of TiO2 and 30%-TM, the specific surface area of these catalysts was obtained. As shown in Figure 11a, it can be seen that TiO2 and 30%-TM displayed characteristic Type IV isotherms, and the shape of the hysteresis loop for these isotherms was Type H3, according to the International Union of Pure and Applied Chemistry (IUPAC) classification [49]. Besides, the BET surface area of 30%-MT was 77.069 m2/g, which was a larger surface area than pure TiO2. The pore volume and pore diameter were 0.2489 cm3/g and 15.192 nm, respectively (Figure 11a and Table 3). Furthermore, Figure 11b shows the pore diameter distribution curves, showing that the 30%-TM featured a shift in the distribution curve to larger pore sizes and the broadening of its shape. Therefore, the surface of 30%-MT can provide more active reaction sites.
Figure 12 illustrates the UV–vis absorption spectra of TiO2-MMT composites prepared under different conditions. Anatase TiO2 had an absorption maximum at 387 nm and an optical band gap of 3.2 eV [50]. The absorption edge of TiO2-MMT shifted in the direction of the wavelength increase, implying a red shift [51], which can be attributed to the production of numerous empty orbitals and defects by the transition metal elements (Fe and Mn) present in MMT. Simultaneously, several studies have reported a blue shift in the absorption edge of TiO2-MMT nanocomposites, ascribing this to the quantum confinement effects [52].
Figure 12a shows that the samples with a TiO2 mass ratio of 30 and 60 wt% featured absorption edges at 437 and 404 nm, which were red-shifted by 50 and 17 nm, respectively, compared with that of anatase TiO2 (387 nm, 3.2 eV). The corresponding red shifts for 40%-TM (425 nm) and 50%-TM (409 nm) were 38 and 22 nm, respectively. Thus, 30%-TM exhibited the largest red shift and featured the highest visible-light-absorbing ability [29], and thus, they potentially had the highest photocatalytic activity [53]. Figure 12b illustrates the effects of pH on the absorption spectra of TiO2-MMT composites. Samples prepared at pH 2, 4, 6, and 8 exhibited red-shifted absorption edges relative to that of the anatase TiO2 by 27, 25, 50, and 18 nm, respectively (Figure 8b). Thus, the sample prepared at pH 2 featured the best visible-light-absorbing ability.
Figure 12c shows the effect of the reaction temperature on recombination light absorption. The samples prepared at 140, 160, 180, and 200 °C have absorption edges at 408, 437, 406, and 405 nm, respectively (the respective red shifts relative to pure anatase TiO2 equaled 21, 50, 19, and 18 nm). Figure 12d demonstrates the effect of dwelling time, revealing that samples prepared using dwelling times of 18, 20, and 24 h had absorption edges of 395 nm, 402 nm, and 437 nm, respectively. Therefore, the sample prepared using a dwelling time of 24 h had the best combination of absorption ability.
In order to further explore the optical properties of the samples, XPS was applied. Figure 13 shows the XPS peak of the pH = 2-TM and the 30%-TM nanoparticles. The Ti 2p peak was measured, as shown in Figure 13a, and the O 1s peak, Si 2p peak, and Ti 2p peak of 30%-TM were detected, as illustrated in Figure 13b–d, respectively. Most studies have pointed out that pure TiO2 only presents the Ti 2p1/2 peak and Ti 2p3/2 peak, and these were attributed to Ti4+ [54]. However, for TM composites, two new peaks appeared (Figure 13a,d), corresponding to Ti3+ 2p1/2 and Ti3+ 2p3/2, respectively. For pH = 2-TM, two peaks appeared at 456.93 eV and 463.7 eV, and two peaks for 30%-TM appeared at 456.8 eV and 463.6 eV. From the corresponding area of each peak, the ratio of Ti3+/Ti4+ of pH = 2-TM and 30%-TM was approximately 18.83% and 20.81%, respectively. Obviously, 30%-TM had the highest Ti3+ content, and the minimum band gap of 30 TM was 2.76 eV (Figure 14), which meant that it had better light absorption capacity and better photocatalytic performance.
As presented in Figure 13b, the photoelectron peaks for O 1s were observed at 530.11, 531.66, and 533.09 eV, provided by carboxyl group, TiO2, and MMT, respectively [19]. Carboxyl can increase the hydrophilicity of materials in the process of photocatalysis. The peak at 102.69 eV in Si 2p can be ascribed to MMT.
Figure 15a shows the EIS of TiO2, 30%-TM and pH = 2-TM, and the PL intensity of TiO2, 30%-TM, 50%-TM, pH = 2-TM and 18 h-TM. In the EIS results, 30%-TM had the smallest radius and had higher charge transfer efficiency, while TiO2 had the highest radius and the smallest charge transfer efficiency. Figure 15b presents the photocurrent versus time (I–t) curve of the prepared samples with off and on rotations of visible light irradiation. The photocurrent intensity of 30%-TM was obviously higher than that of TiO2. This point was also verified by the PL analysis. Figure 15c shows the PL intensity of TiO2, 30%-TM, 50%-TM, pH = 2-TM, and 18 h-TM. The intensity of the PL peak indicated the recombination rate of photogenerated electrons and holes under light irradiation. Therefore, 30%-TM had better charge transfer ability, and a lower recombination rate of photogenerated electrons and holes than the samples prepared under other conditions.

2.4. Mechanism of Photocatalytic

Figure 16 illustrates a plausible mechanism of MB photodegradation by the investigated catalysts. For a better representation of the direct charge transfer in the composite, we estimated the positions of the conduction band (CB) and the valence band (VB) as follows:
EVB = χ − Ee + 0.5Eg,
ECB = EVB − Eg,
where χ, Ee, Eg, EVB, and ECB are the absolute electronegativity of TiO2 (5.8 eV), the energy of electrons at the standard hydrogen electrode (4.8 eV), the band gap of 30%-TM, the VB boundary of 30%-TM, and the CB boundary of 30%-TM, respectively [55]. The values of ECB and EVB obtained using Equations (1) and (2) were 0.08 and 2.68 eV vs. NHE, respectively.
Compared with TiO2, the number of oxygen vacancies in the composites increased markedly, indicating that it was easier to generate oxygen vacancies on the MMT’s surface supported by TiO2. These changes could be attributed to the combination of the Si–O bond (in the Si–O tetrahedron on the MMT’s surface) and the TiO2 hydrosol that formed the Si–O–Ti bond [19,54]. Cheng et al. reported that oxygen vacancies and Ti3+ content affected the band gap of TiO2 matrix composites [19,54,56,57]. Thus, the changes in the band gap of TiO2 matrix composites is mainly related to the number of oxygen vacancies and the content of Ti3+ in the material. In TiO2-MMT composites, TiO2 acts as a photocatalyst, producing electrons (e−) and holes (h+) under illumination. The electrons pass through the band gap and then enter the CB, leaving holes in the VB [58]. Compared with pure TiO2, composites with MMT featured lower band gaps, thus favoring the injection of electrons into the CB. In the composite of TiO2 and MMT, the electrons in the CB react with Ti4+ to form Ti3+, which is considered the most reactive site in the oxidation process because it can produce more oxygen vacancies to facilitate the adsorption of O2 on the TiO2’s surface [59]. The adsorbed O2 molecules then react with electrons to produce highly reactive ·O2− species. Simultaneously, several electrons occupy the empty d-orbitals of metal ions in the MMT’s structure [28], which indirectly delays the electron–hole recombination and then reacts with O2 to produce ·O2− [60]. The holes react with water or adsorbed hydroxyl ions to form hydroxyl radicals (·OH). Both ·O2− and ·OH are strong oxidants that can destroy organic molecules [61] on and near the TiO2′ surface. Thus, the photocatalysis mechanism can be represented by the following equations:
TiO2 + hv → TiO2 + e− + h+,
Ti4+ + e− → Ti3+,
O2 + e− → ·O2−,
H2O + h+ → ·OH + H+,
OH− + h+ → ·OH,
·O2− + H2O → ·OH + H+,
O2−•/h+/·OH + MB → degradation products.
The study of the structure, morphology, and chemical bonds proved that the layered structure of montmorillonite was still intact and that Si-O-Ti was successfully formed. Tao et pointed out in their research that Si-O-Ti could increase Ti3+ content and oxygen vacancies in the composites [19]. In this work, the Ti3+/Ti4+ contentwas determined and simulated, and it increased by 20.81%. In addition, the optical absorption, electron and hole recombination efficiency, photogenerated carrier behavior, and specific surface area proved that 30%-TM had strong optical absorption capacity, low electron–hole recombination efficiency, a large specific surface area, and high Ti3+ content. This work also explored the role of active substances in the degradation process, which fully proved the significant role of ·OH, h+ and ·O2−, and especially the degradation efficiency of ·OH. In a recent study, Thi et al. studied the efficiency and mechanism of photocatalytic degradation of MMT/TiO2 nanotubes [28]. Ami’s group prepared a titanium dioxide composite clay photocatalyst by a microwave hydrothermal (5 min) and calcination method, which proved that a TiO2/bentonite photocatalyst has high photocatalytic efficiency [62]. However, in most research, only a single set of preparation conditions were explored, without the exploration of different preparation conditions. This study will play a guiding role in the compounding of clay and titanium dioxide.

3. Materials and Methods

3.1. Materials

All reagents were of analytical grade and used in accordance with the prescribed requirements. MB (>98.5%), MMT (>98%), tetrabutyl titanate (>98%), and cetyltrimethylammonium bromide (CTAB) (>98%) were purchased from Tianjin Komeo Chemical Reagent Co. Ltd., Tianjin, China. Anhydrous ethanol was purchased from Tianjin Hengxing Chemical Reagent Manufacturing Co. Ltd., Tianjin, China.

3.2. Preparation of TiO2

Tetrabutyl titanate (2.3 mL) and deionized water (30 mL) were mixed and stirred for 30 min to achieve complete homogenization. The homogeneous solution was then transferred to a 100 mL polytetrafluoroethylene-lined stainless steel reactor and heated at 160 °C for 24 h. After washing, the samples were placed on an evaporation dish and oven-dried at 80 °C for 12 h to obtain TiO2.

3.3. Preparation of TiO2-MMT Composites

TiO2-MMT composites with TiO2 mass ratio of 30, 40, 50, 60, and 70 wt% were prepared at different reaction pH values (2, 4, 6, and 8), reaction temperatures (140, 160, 180, and 200 °C), and dwelling times (18, 20, and 24 h) (Figure 17).
Tetrabutyl titanate (1.7, 2.3, 2.8, 3.3, or 3.9 mL) and deionized water (30 mL) were mixed and stirred for 15 min to ensure uniform dispersion. Separately, MMT (1.0 g) was mixed with deionized water (80 mL) in a beaker, which was sealed with clingfilm to prevent evaporation; the suspension was stirred for 4 h. The solution’s pH was adjusted to 2, 4, 6, or 8. The solution was allowed to stand for 5 min to ensure homogeneity and transferred to a 100 mL polytetrafluoroethylene-lined stainless steel reactor. The mixture was heated at 140, 160, 180, or 200 °C for 18, 20, or 24 h, and subsequently cooled to room temperature. After washing, the samples were placed on an evaporation dish and oven-dried at 80 °C for 12 h to obtain TiO2-MMT composites; these composites were denoted as w%-pH = x – y °C-zh-TiO2-MMT, where w, x, y, and z are the TiO2 loading, pH, reaction temperature, and dwelling time, respectively.

3.4. Characterization

The phase composition was probed by X-ray diffraction (XRD; Bruker D8 ADVANCE) in the 2θ range of 5–90°. The morphology and composition were observed by scanning electron microscopy (SEM; FEI INSPECT F50) coupled with energy-dispersive X-ray spectroscopy. Optical absorption was analyzed by ultraviolet–visible spectroscopy (UV-vis; JASCO V-600). Infrared absorption was analyzed by Fourier transform infrared spectroscopy (FTIR; IR Prestige-21) within the range of 400–4000 cm−1. The surface Ti (Si or O) states of the samples were obtained by X-ray photoelectron spectrometry (XPS, Thermo SCIENTIFIC ESCALAB 250Xi). The photoluminescence (PL) spectra for solid samples were obtained using an Shimadzu RF6000 spectrophotometer with excitation wavelength of 305 nm. The surface area of the samples was obtained by applying the Brunner–Emmett–Teller (BET, BELSORP MaxII) method to the N2 adsorption−desorption isotherms.
All electrochemical measurements were carried out in an electrochemical workstation (RST5210F, Shanghai Shiruisi instrument Technology Co., Ltd., Shanghai, China.). The electrochemical impedance spectroscopy (EIS) was conducted using an electrochemical workstation under visible light and a forward bias of 0.5 mV at the frequency range from 1 Hz to 200 Hz. Photocurrent intensity was measured in an electrolyte of 50 mL 0.2 mol/L Na2SO4, 30%-TM, and TiO2 as described above. A xenon lamp equipped with a 420 nm cutoff filter was the light source, and the switching cycle was 20 s.

3.5. Photocatalytic Performance Evaluation

The ability of photocatalysts to (i) adsorb and (ii) photodegrade organic pollutants in wastewater was probed in the dark and under irradiation with visible light, respectively, using MB as a model at room temperature.
The aqueous solution of MB was mixed with the catalyst of choice (0.03 g). In the dark, the mixture was stirred for 30 min for (i), while it was irradiated with a xenon lamp (CEL-PF300-T8) at a power density of 100 mW/cm2 for (ii). Subsequently, 5 mL aliquots of the dispersion were sampled every 20 min and centrifuged at 7000 rpm for 5 min. Finally, to determine the MB removal efficiency, the absorbance of the supernatant was measured; for (ii), the absorbance was measured in the range of 400–800 nm.

4. Conclusions

TiO2-MMT composites with different TiO2 contents were hydrothermally prepared at different pH values, reaction temperatures, and dwelling times, and evaluated as photocatalysts for MB degradation. Instrumental analysis revealed that the best-performing composite (30%-TM; TiO2 content = 30 wt%) had the smallest band gap of 2.76 eV, a unit cell volume of 92.43 Å3, a crystallite size of 9.28 nm, and a relatively uniform distribution of TiO2 particles on the MMT’s surface. The absorption threshold of 30%-TM was red-shifted by 50 nm compared with that of anatase TiO2 (387 nm, band gap = 3.2 eV), which enhanced the visible-light absorption and visible-light-induced activity. The intensity of the PL peak, EIS, and photocurrent indicated that 30%-TM had better charge transfer ability and a lower recombination rate of photogenerated electrons and holes than the samples prepared under other conditions. After irradiation for 2 h with visible light, 30%-TM achieved an MB removal efficiency of 95.6%. Therefore, the compounding and modification of clay materials in this study provides a train of thought for design and research.

Author Contributions

Study design and literature search, Y.Z. and B.M.; investigation and data collection, Z.B., Y.Z., B.M. and Q.C.; artwork and figures, Y.C.; software, Y.Z.; validation, Z.B., Y.Z., B.M. and Q.C.; writing—original draft preparation, B.D.; writing—review and editing, Z.B., Y.Z. and B.M.; data curation, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by the Key Scientific Research projects of Colleges and Universities in Henan Province (21A430009).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank Cai for the electrochemical measurements.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction patterns of TiO2-montmorillonite (MMT) composites prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6, and (b) different pH values at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%.
Figure 1. X-ray diffraction patterns of TiO2-montmorillonite (MMT) composites prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6, and (b) different pH values at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%.
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Figure 2. X-ray diffraction patterns of TiO2-montmorillonite (MMT) composites prepared using (a) different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%, and (b) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%.
Figure 2. X-ray diffraction patterns of TiO2-montmorillonite (MMT) composites prepared using (a) different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%, and (b) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%.
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Figure 3. (a–d) Scanning electron microscopy images of 30%-TM at different magnifications.
Figure 3. (a–d) Scanning electron microscopy images of 30%-TM at different magnifications.
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Figure 4. (a) Representative scanning electron microscopy image of 30%-TM and (b) the related size distribution of TiO2 particles.
Figure 4. (a) Representative scanning electron microscopy image of 30%-TM and (b) the related size distribution of TiO2 particles.
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Figure 5. Fourier transform infrared spectra of TiO2-montmorillonite (MMT) composites prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6, and (b) different pH values at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%.
Figure 5. Fourier transform infrared spectra of TiO2-montmorillonite (MMT) composites prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6, and (b) different pH values at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%.
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Figure 6. Fourier transform infrared spectra of TiO2-montmorillonite (MMT) composites prepared using (a) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt% and (b) different reaction temperatures at a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%.
Figure 6. Fourier transform infrared spectra of TiO2-montmorillonite (MMT) composites prepared using (a) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt% and (b) different reaction temperatures at a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%.
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Figure 7. Ultraviolet–visible absorption spectra of methylene blue solutions degraded by (a) 30%-TM (degradation time = 0–120 min) and (b,c) TiO2-MMT composites synthesized under different conditions (degradation time = (b) 120 min and (c) 0 min).
Figure 7. Ultraviolet–visible absorption spectra of methylene blue solutions degraded by (a) 30%-TM (degradation time = 0–120 min) and (b,c) TiO2-MMT composites synthesized under different conditions (degradation time = (b) 120 min and (c) 0 min).
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Figure 8. Methylene blue degradation curves obtained for (a) TiO2, montmorillonite (MMT), and 30%-TM under dark conditions; (b) the blank group, TiO2, MMT, and 30%-TM under visible light; (c) composites with different TiO2 contents prepared at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%; (d) composites prepared at different pH values with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%; (e) composites prepared with different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%; and (f) composites prepared at different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%.
Figure 8. Methylene blue degradation curves obtained for (a) TiO2, montmorillonite (MMT), and 30%-TM under dark conditions; (b) the blank group, TiO2, MMT, and 30%-TM under visible light; (c) composites with different TiO2 contents prepared at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%; (d) composites prepared at different pH values with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%; (e) composites prepared with different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%; and (f) composites prepared at different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%.
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Figure 9. Electron paramagnetic resonance signals of (a) 5,5-dimethyl-pyrroline N-oxide (DMPO)-·OH and (b) DMPO-·O2− obtained for catalysts irradiated with visible light.
Figure 9. Electron paramagnetic resonance signals of (a) 5,5-dimethyl-pyrroline N-oxide (DMPO)-·OH and (b) DMPO-·O2− obtained for catalysts irradiated with visible light.
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Figure 10. Trapping experiment of active substances during photocatalytic degradation of TiO2-MMT.
Figure 10. Trapping experiment of active substances during photocatalytic degradation of TiO2-MMT.
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Figure 11. (a) Nitrogen adsorption–desorption isotherms of TiO2 and 30%-TM and (b) the pore size distribution of TiO2 and 30%-TM.
Figure 11. (a) Nitrogen adsorption–desorption isotherms of TiO2 and 30%-TM and (b) the pore size distribution of TiO2 and 30%-TM.
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Catalysts 12 00486 g012 550
Figure 12. Ultraviolet-visible absorption spectra of TiO2-montmorillonite (MMT) prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6; (b) different pH levels with a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 content of 30 wt%; (c) different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%; and (d) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%. Inset images show expansions of the 370–450 nm region.
Figure 12. Ultraviolet-visible absorption spectra of TiO2-montmorillonite (MMT) prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6; (b) different pH levels with a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 content of 30 wt%; (c) different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 mass ratio of 30 wt%; and (d) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%. Inset images show expansions of the 370–450 nm region.
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Figure 13. (a) XPS spectra (Ti 2p) of pH = 2-TM, and XPS spectra of 30%-TM. (b) O 1s; (c) Si 2p; (d) Ti 2p spectra.
Figure 13. (a) XPS spectra (Ti 2p) of pH = 2-TM, and XPS spectra of 30%-TM. (b) O 1s; (c) Si 2p; (d) Ti 2p spectra.
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Figure 14. Tauc plots of TiO2-montmorillonite (MMT) composites prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6; (b) different pH values at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%; (c) different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 content of 30 wt%; and (d) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%.
Figure 14. Tauc plots of TiO2-montmorillonite (MMT) composites prepared using (a) different TiO2/MMT mass ratios at a reaction temperature of 160 °C, a dwelling time of 24 h, and a pH of 6; (b) different pH values at a reaction temperature of 160 °C, a dwelling time of 24 h, and a TiO2 mass ratio of 30 wt%; (c) different reaction temperatures with a dwelling time of 24 h, a pH of 6, and a TiO2 content of 30 wt%; and (d) different dwelling times at a reaction temperature of 160 °C, a pH of 6, and a TiO2 mass ratio of 30 wt%.
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Figure 15. (a) EIS Nyquist plots of TiO2, 30%-MT, and pH = 2-MT. (b) Transient photocurrent responses of TiO2 and 30%-MT. (c) Analysis of PL of TiO2, 30%-MT, pH = 2-MT, 50%-TM, and 18 h-TM.
Figure 15. (a) EIS Nyquist plots of TiO2, 30%-MT, and pH = 2-MT. (b) Transient photocurrent responses of TiO2 and 30%-MT. (c) Analysis of PL of TiO2, 30%-MT, pH = 2-MT, 50%-TM, and 18 h-TM.
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Figure 16. Suggested mechanism of methylene blue (MB) photodegradation by 30%-TM.
Figure 16. Suggested mechanism of methylene blue (MB) photodegradation by 30%-TM.
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Figure 17. Schematic synthesis of TiO2-montmorillonite (MMT) composites.
Figure 17. Schematic synthesis of TiO2-montmorillonite (MMT) composites.
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Table
Table 1. Selected structural parameters of the prepared composites and the TiO2 reference (a = b ≠ c, α = β = θ = 90°).
Table 1. Selected structural parameters of the prepared composites and the TiO2 reference (a = b ≠ c, α = β = θ = 90°).
Parametersa (Å)b (Å)c (Å)V (Å3)D (nm)
TiO23.7763.7767.48692.439.74
30%, pH = 6, 160 °C, 24 h TiO2-MMT
(30%-TM)		3.7803.7809.510117.679.28
40%, pH = 6, 160 °C, 24 h TiO2-MMT
(40%-TM)		3.7763.7769.486117.139.28
50%, pH = 6, 160 °C, 24 h TiO2-MMT
(50%-TM)		3.7763.7769.486117.139.42
60%, pH = 6, 160 °C, 24 h, TiO2-MMT
(60%-TM)		3.7833.7839.497117.709.43
70%, pH = 6, 160 °C, 24 h TiO2-MMT
(70%-TM)		3.7843.7849.514117.979.24
pH = 2, 30%, 160 °C, 24 h TiO2-MMT
(pH = 2-TM)		3.7843.7849.514117.979.65
pH = 4, 30%, 160 °C, 24 h TiO2-MMT
(pH = 4-TM)		3.7763.7769.486117.139.00
pH = 6, 30%, 160 °C, 24 h TiO2-MMT
(pH = 6-TM)		3.7803.7809.510117.679.28
pH = 8, 30%, 160 °C, 24 h TiO2-MMT
(pH = 8-TM)		3.7853.7859.785121.4011.76
140 °C, 30%, pH = 6, 24 h TiO2-MMT
(140 °C-TM)		3.7803.7809.510117.678.98
160 °C, 30%, pH = 6, 24 h TiO2-MMT
(160 °C-TM)		3.7803.7809.510117.679.28
180 °C, 30%, pH = 6,24 h TiO2-MMT
(180 °C-TM)		3.8073.8079.090114.099.52
200 °C, 30%, pH = 6, 24 h, TiO2-MMT
(200 °C-TM)		3.8073.8079.090114.099.79
18 h, 30%, pH = 6, 160 °C TiO2-MMT
(18 h-TM)		3.7843.7849.515117.999.994
20 h, 30%, pH = 6, 160 °C TiO2-MMT
(20 h-TM)		3.7843.7849.515117.9910.73
24 h, 30%, pH = 6, 160 °C TiO2-MMT
(24 h-TM)		3.7803.7809.510117.679.28
Table
Table 2. Comparison of the percentage of degradation of 30%-MT with previously reported photocatalysts.
Table 2. Comparison of the percentage of degradation of 30%-MT with previously reported photocatalysts.
MaterialLight SourcePollutant% DegradationTime (min)Reference
CA/TiO2Visible lightMethylene blue85300 [41]
ZnO/AgVisible lightMethyl orange78180 [42]
NiO/AgVisible lightMethyl orange42180 [42]
TiO2/AgVisible lightMethyl orange86180 [42]
Cu-MOF-235Visible lightMethylene blue90480[43]
B-TiO2/MIL100(Fe)Visible lightMethylene blue91.1260 [44]
Cd-TCAAVisible lightMethylene blue81175[45]
ZnO-TiO2Visible lightMethylene blue62120[46]
TiO2/Al2O3@Cu(BDC)Visible lightMethylene blue33.7730[47]
TiO2@Ti(BTC)Visible lightMethylene blue5660[48]
30%-MTVisible lightMethylene blue95.6120This work
Table
Table 3. BET parameters of TiO2 and 30%-TM.
Table 3. BET parameters of TiO2 and 30%-TM.
SampleBET Surface Area (m2/g)BJH Pore Volume (cm3/g)Pore Diameter (nm)
TiO238.9250.1179 10.001
30%-MT77.0690.248915.192
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Zhang, Y.; Miao, B.; Chen, Q.; Bai, Z.; Cao, Y.; Davaa, B. Synthesis, Structure, and Photocatalytic Activity of TiO2-Montmorillonite Composites. Catalysts 2022, 12, 486. https://doi.org/10.3390/catal12050486

AMA Style

Zhang Y, Miao B, Chen Q, Bai Z, Cao Y, Davaa B. Synthesis, Structure, and Photocatalytic Activity of TiO2-Montmorillonite Composites. Catalysts. 2022; 12(5):486. https://doi.org/10.3390/catal12050486

Chicago/Turabian Style

Zhang, Yonghui, Baoji Miao, Qiuling Chen, Zhiming Bai, Yange Cao, and Basandorj Davaa. 2022. "Synthesis, Structure, and Photocatalytic Activity of TiO2-Montmorillonite Composites" Catalysts 12, no. 5: 486. https://doi.org/10.3390/catal12050486

APA Style

Zhang, Y., Miao, B., Chen, Q., Bai, Z., Cao, Y., & Davaa, B. (2022). Synthesis, Structure, and Photocatalytic Activity of TiO2-Montmorillonite Composites. Catalysts, 12(5), 486. https://doi.org/10.3390/catal12050486

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