Abstract
A novel nanocomposite ZnFe2O4-TiO2/MCM-41 (ZTM) was synthesized by a sol-gel method and characterized through X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), N2 adsorption-desorption, Raman spectroscopy, and ultraviolet visible (UV-vis) spectrophotometry. The results confirmed the incorporation of ZnFe2O4-TiO2 nanoparticles inside the pores of the mesoporous MCM-41 host without destroying its integrity. ZnFe2O4 nanoparticles can inhibit the transformation of anatase into rutile phase of TiO2. Incorporation of ZnFe2O4-TiO2 within MCM-41 avoided the agglomeration of nanoparticles and reduced the band gap energy of TiO2 to enhance its visible light photocatalytic activity. UV-vis absorption edges of ZTM nanocomposites redshifted with the increase of Zn/Ti molar ratio. The nanocomposite approach could be a potential choice for enhancing the photoactivity of TiO2, indicating an interesting application in the photodegradation and photoelectric fields.
1. Introduction
TiO2 had been widely used in various applications, such as functional ceramics [1, 2], sensor materials [3–5], cosmetic products [6, 7], photocatalyst [8–10], high grade coatings [11–13], pigment in the paint, and paper and pulp industry [14, 15]. As an interesting photocatalyst, TiO2 had attracted much attention due to high efficiency with low cost, chemical inertness, and photostability. Different kinds of TiO2 were obtained through several procedures: porous TiO2 cryogel fibers through unidirectional freezing of hydrogel [16], wormhole-like mesoporous TiO2 by chemical vapor deposition [17], hollow TiO2 microspheres obtained by a single-step synthesis in ionic liquids [18], and coral-like TiO2 produced by templating polymer gels [17]. However, the widespread use of TiO2 was impaired by some defects of its structure, agglomeration easily occurred during the synthesis process and wide band gap (3.2 eV) which requires ultraviolet irradiation for photocatalytic activation.
In order to improve the photocatalytic properties of TiO2, effective measurements had been taken into experiment. Doping of transition [19] and noble metals [20, 21] can improve the photocatalytic activity of TiO2 to some degree. Zinc ferrite (ZnFe2O4), with a spinel structure and a relatively small band gap (1.9 eV), has a potential application in the conversion of sunlight. However, because of the lower valence band potential and poor property in photoelectric conversion, ZnFe2O4 cannot be directly used in the photocatalytic destruction. It was demonstrated that ZnFe2O4 doping extend the adsorption spectrum to the longer wavelength, this material had high utility of sunlight, high photoactivity, and high efficiency of photoelectric conversion [22–24].
Meanwhile, considering that mesoporous molecular sieves, MCM-41, which possesses a regular hexagonal array of uniform pore openings, we propose that the uniform ordered channels of MCM-41 may be able to control the particle size of TiO2 and efficiently prevent particles from agglomeration. It was a new route to keep nanoparticles inside the pores of MCM-41 material [25]. There are many researches focused on the substitution of silicon by metals such as Ti [26] and Ni [27] and incorporating nanoparticles within the pores of mesoporous materials, including gold nanoparticles within mesoporous silica [28], MCM-41 modified SnO2 [29], and MCM-48 containing TiO2 nanoparticles [30], all of which can enhance the properties of the nanoparticles or the incorporated hosts.
In this paper, we demonstrate a novel route to improve the properties of TiO2 through the addition of ZnFe2O4 and incorporating the ZnFe2O4-TiO2 nanoparticles into the channels of mesoporous MCM-41. The effect of ZnFe2O4 addition on the structure and the properties of ZnFe2O4-TiO2 and ZnFe2O4-TiO2/MCM-41 composites are also investigated. The composites were in detail characterized by XRD, TG-DSC, TEM, BET, Raman, and UV-vis spectroscopy.
2. Experimental
2.1. Materials Synthesis
MCM-41 mesoporous materials were prepared according to our previous work [31]. TiO2/MCM-41 composite (molar ratio of Ti/Si = 0.4) was obtained using tetrabutyl titanate as Ti source. The synthesis procedure of TiO2/MCM-41 composite was as follows: a certain amount of tetrabutyl titanate was added to 40 mL ethanol with uniform stirring. Diethanolamine, used as catalyst and stabilizing agent, was added and constantly stirred for 0.5 h to form a solution S. 1.8 g MCM-41 was added to the above solution S and stirred for 10 min. A mixture of water and ethanol was further dropped, stirred for 4 h and aged for 24 h to obtain a gel. The gel was dried at 80°C overnight and calcined at 600°C to produce TiO2/MCM-41 composites. Pure TiO2 was synthesized according to this procedure without the addition of MCM-41. The synthesis procedure of ZnFe2O4-TiO2 with different Zn/Ti molar ratio was similar to the above procedure as TiO2. Certain amount of ferric nitrate and zinc acetate (molar ratio of Zn/Fe = 0.5) were dissolved in an ethanol solution to obtain a uniform precursor. The precursor was added to the solution S with different Zn/Ti molar ratios (1%, 3%, 5%, and 7%, denoting the corresponding product ZnFe2O4-TiO2 as ZT1, ZT3, ZT5, and ZT7, resp.). When MCM-41 was simultaneously added to the above solution S according to Ti/Si molar ratio of 0.4, the final product ZnFe2O4-TiO2/MCM-41 was correspondingly indicated as ZTM1, ZTM3, ZTM5, and ZTM7, respectively.
2.2. Characterization
X-ray diffraction (XRD) was carried out using a Bruker D8 advance with Cu Kα radiation ( nm) over the scanning range 2 = 1°~10° for small angle XRD (SAXRD) at a voltage of 40 kV and 300 mA and 2 = 1°~10° for wide angle XRD (WAXRD) at a voltage of 40 kV and 200 mA both with a step width of 0.0085°. Nitrogen gas adsorption-desorption isotherms were measured at 77 K using an ASAP 2020 unit. Prior to the sorption experiment, the samples were vacuum-dried at 200°C for 10 h. The specific surface area and pore size distribution were calculated by the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method using the adsorption-desorption isotherms, respectively. The total pore volume was obtained from the maximum amount of nitrogen gas adsorbed at partial pressure (). A Tecnai G220 AEM electron microscope operating at accelerating voltages up to 200 kV was used for the high-resolution TEM (HRTEM) studies. Samples were prepared by suspending MCM-41 material with or without TiO2 nanoparticles in ethanol followed by sonication for 15 min in an ultrasonic bath. The suspension was dripped onto a carbon-coated copper grid and allowed to dry, respectively. Ultraviolet visible (UV-vis) spectrophotometry spectra were collected on a SHIMADZU UV-2450 spectrophotometer at room temperature, and the detection range of wavelength is from 190 nm to 700 nm. The Raman spectra were obtained using Renishaw InVia Raman system which can extend to 100 cm−1. The 514 nm line of an Argon laser was used as the excitation source.
3. Results and Discussion
XRD analysis can provide detailed information on crystallite structure characteristics. WAXRD patterns of pure TiO2 and ZT series samples showed both anatase and rutile phase of TiO2 (Figure 1). The peak intensity of rutile phase decreased with increasing the amount of ZnFe2O4. The percentage of anatase phase in pure TiO2 and ZT series (Zn/Ti from 1% to 7%) were 74%, 76%, 52%, 55%, and 18%, respectively. It was concluded that the addition of ZnFe2O4 can promote the transformation of TiO2 from anatase to rutile phase. Li et al. [32] and Liu et al. [23] obtained similar conclusion from their study. Meanwhile, all the peaks became broadened, suggesting that the grain size decreased with the increasing addition of ZnFe2O4. A weak diffraction peak due to spinel ZnFe2O4 phase appeared at the Zn/Ti molar ratio of 3%, indicating the formation of ZnFe2O4 in the composites, which was in good accordance with the references [33, 34]. The grain sizes of anatase and rutile TiO2 phase were calculated using Jade 5.0 (Figure 2) which all decreased with increasing the amount of ZnFe2O4, ZnFe2O4 seemed to play an important role in inhibiting the growth of TiO2 particles [35, 36].
Figure 3 showed the WAXRD patterns of MCM-41 incorporated within pure TiO2 and ZT nanoparticles. TiO2/MCM-41, ZTM1, and ZTM3 only presented the characteristic peaks of anatase TiO2, while rutile phase was observed in the WAXRD patterns of the other two composites. The peaks corresponding to anatase and rutile became broadened, which was attributed to the confinement of nanoparticles within MCM-41 pore channel. All the patterns did not show any peaks corresponding to spinel ZnFe2O4. The percentage of anatase in ZTM5 and ZTM7 were 69% and 28%, respectively, larger than those of ZT5 and ZT7 samples (55% and 18%), suggesting that MCM-41 had an obvious effect on the anatase-rutile phase transformation of TiO2.
All the samples presented three characteristic peaks of typical MCM-41 in their SAXRD patterns (Figure 4), which was in accordance with the previous report [37], indicating that the pore channel of MCM-41 remained well even after the incorporation of nanoparticles. No change in the position of main peak demonstrated that Ti4+ ions, Zn2+ ions, and Fe3+ ions were not incorporated into the framework of MCM-41. The increase in intensity of (100) peak after incorporation demonstrated that ZnFe2O4 can inhibit the growth of TiO2 particles. However, compared with SAXRD pattern of pure MCM-41 [31], the peak intensity of all samples decreased visibly, one can be attributed to the pore filling of the host material, which reduced the scattering contrast between pore walls and pores, thus leading to a decrease in peak intensity [38]; another is possibly related to the loss of sample integrity [30].
HRTEM images of MCM-41, TiO2/MCM-41, and ZnFe2O4-TiO2/MCM-41 (Zn : Tin = 0.07, ZTM = 7) were collected to find out the location of TiO2 (Figure 5), the ordered mesopores of MCM-41 with an average pore size of 3 nm were clearly observed. After incorporated with nanoparticles, the pore channels were maintained well, and no TiO2 particles were detected on the surface of MCM-41. EDS spectrum of TiO2/MCM-41 (Figure 6) showed the existence of Si, O, and Ti elements with the Ti/Si molar ratio approximatively equal to the experimental value (Ti/Si = 0.4). The HRTEM images of ZTM7 also displayed the ordered hexagonal mesopores without any particles on the surface. Combined with above XRD results, it was concluded that the TiO2 or ZT nanoparticles were incorporated into the pore channel of MCM-41.
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TG-DSC measurements were performed from room temperature to 1100°C to reveal the thermal behavior of three precursors (Figures 7–9). It was well known that the thermal behavior of TiO2 usually depended on the chemical composition, preparation condition, and existing phases [39]. An endothermic peak at 99°C with mass loss of 13.78% was due to the desorption of physically adsorbed water (Figure 7). Three small exothermic peaks at 200~400°C were due to the decomposition and oxidation of organic substances as well as the transformation of TiO2 from amorphous to anatase phase [40]. Peaks at 400~600°C with mass loss of 16.67% were attributed to the oxidation of residual organic substances and the dehydration of structural water. From the above XRD result, TiO2 particles were composed of anatase and rutile phases, therefore, the strong peak at 500~600°C also contained the exothermic peak of phase transformation from anatase to rutile. An exothermic shoulder at 600~800°C could be related to the phase transformation of TiO2 from anatase to rutile. The DSC curve of TiO2/MCM-41 was much simpler than that of TiO2, only one endothermic peak and two exothermic peaks appeared in the DSC curves (Figure 8). The endothermic peak at 70°C was due to the adsorbed water, and the peaks at 323°C and 584°C can be attributed to the oxidation of organic substances and the dehydration of structure water. The whole mass loss of TiO2/MCM-41 precursor was 17.99%. The amount of TiO2 incorporated to MCM-41 can be calculated from the mass loss, and the result was basically in accordance with the experimentally designed values. Since the XRD showed that TiO2/MCM-41 only contained pure anatase phase TiO2 (Figure 3), the exothermic area from 800~1000°C was directly related to the phase transformation from anatase to rutile. The DSC curve of ZTM7 appeared as a sharp peak at 187°C with a big mass loss of 37.26% (Figure 9), probably presenting the oxidation of organic substances and the decomposition of nitrate. The peaks at 200~600°C were due to the oxidation of residual organic substances and the dehydration of precursor, accompanying the crystallization of ZnFe2O4, and the phase transformation of TiO2.
N2 adsorption-desorption isotherms were carried out to investigate the textural characteristics of the samples (Figure 10). All the isotherms exhibited the typical type IV corresponding to the mesophases, indicating that incorporation of ZnFe2O4-TiO2 did not destroy the integrity of mesoporous MCM-41 host. The specific surface area () and pore wall thickness of ZTM series became smaller than that of MCM-41 (Table 1), confirming the integrated incorporation of ZT nanoparticles inside the MCM-41 pore channels. The reason that the N2 gas absorption at is not zero is attributed to likely existing plentiful micropores, which lead to degassing incompletely.
Raman spectroscopy is a powerful technique for the investigation of various phases of titanium oxides. Raman spectrum of pure TiO2 exhibited the vibration modes of anatase phase at 145, 196, 397, 514, 637 cm−1, and rutile phase at 445, 613 cm−1 (Figure 11) [41, 42], but no peaks corresponding to Fe2O3, ZnO, and spinel ZnFe2O4 was observed in the ZT3 sample, the peaks became broadened asymmetrically with the decrease in intensity. One was attributed to the decrease in particle size [41]; another was the breaking of the symmetry of TiO2 molecular structure resulted from the doping of Zn2+ and Fe3+ ions in the lattice of TiO2 [43]. ZTM3 composites only showed the characteristic vibration of anatase, which further demonstrated that MCM-41 can inhibit the phase transformation of TiO2, and the broadening of the peaks obviously indicated the smaller particle size of TiO2.
An obvious redshift in ZT samples was observed compared with pure TiO2 (Figure 12), the redshift increased regularly with increasing the Zn/Ti molar ratio. The energy gaps were 2.86, 2.19, 1.83, 1.79, and 1.71 eV corresponding to pure TiO2 and ZT1, ZT3, ZT5, and ZT7, respectively, the adsorption edges of ZT samples were all in the visible light region, so the addition of ZnFe2O4 exactly reduced the energy gaps and made the ZT active in visible light [44]. Though an obvious blue shift in MCM-41/TiO2 sample was observed compared with pure TiO2 (Figure 13), which illustrated that the effect of MCM-41 is that it can be able to control the particle size of TiO2 and efficiently prevent particles from agglomeration; Figure 13 further showed an obvious redshift in adsorption edge of ZTM compared with pure TiO2, indicating the interesting application of the as-synthesized nanocomposites in the fields of photodegradation and photoelectric devices. The photochemical reactions such as a waste treatment should be developed in the future research.
4. Conclusions
ZnFe2O4-TiO2/MCM-41 (ZTM) nanocomposites with different amount of ZnFe2O4 have been successfully synthesized via a sol-gel method. The addition of ZnFe2O4 inhibited the growth of TiO2 particles and promoted the anatase-rutile phase transformation of TiO2. ZnFe2O4-TiO2 nanoparticles would not destroy the pore structure of MCM-41. The ordered pore structure of MCM-41 can effectively control the growth of TiO2 nanoparticles. UV-vis absorption edge of ZTM shifted to red regularly with the increase of ZnFe2O4 and indicated likely excellent visible-light activity. Our present results showed that the as-synthesized ZTM nanocomposite could be a promising multifunctional material in the fields of photodegradation and photoelectric devices.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (50774095) and the Scientific Research Foundation for ROCS of SEM (2011-1139).