Ordered mesoporous metal oxide-based materials have attracted a wealth of interest because of their large and uniform mesopore size, high specific surface area, particularly the unique electronic optical and catalytic properties relating to applications in photocatalysis, catalysis, advanced batteries and water purification etc. [1-27]. It is well-known that the pores dimensionality is of paramount importance since it can exert a profound effect on the accessibility, adsorption and diffusion behavior of guest molecules within the pore network etc. [9-11]. 3D interconnected mesoporous networks have shown numerous advantages over 2D and 1D counterparts, such as: (1) considerably benefiting ingress/egress of guest species because of possessing far more openings to the internal porous network at the surface, (2) extremely enhancing diffusion efficiencies of guest molecules within the interconnected mesopore networks, (3) largely elevating both the accessibility and availability of the inner surfaces, (4) allowing light to irradiate more inner surfaces through the openings and interlinked mesopore architectures, which can lead to an immense increment of OH radical in number, and (5) enhancing light harvesting efficiency due to the multiple scattering of light in the interpenetrated mesopores [28-30]. The combination of such prominent characteristics can significantly improve photocatalytic oxidation performance.
The syntheses of ordered mesoporous metal oxide-based materials can be roughly divided into "soft template" and "hard template" methods. The syntheses of 3D interconnected mesoporous networks in metal oxide-based materials employing the "soft template" approach have hitherto not been reported, to the best of our knowledge. Conversely, there have been multiple reports detailing the preparation of mesoporous metal oxide-based materials with 3D interlinked voids or mesopores using the "hard template" method [31-35]. However, the main shortcomings of the latter approach are [7]: (1) it is difficult to completely fill the pores of the hard template (such as mesoporous silica or mesoporous carbon), even when multiple impregnation steps are used, and (2) the synthetic approach involves multiple and tedious steps requiring time to generate the template followed by its subsequent removal etc. Hence, the simple preparation of 3D metal oxide-based materials possessing well-established interconnected mesochannels remains a challenge.
Additionally, controlling the crystallinity, phase, and crystal size of the pore walls is an important factor, which determines their performance in practical applications [9, 10]. For example, titania has three crystalline phases with the anatase polymorph showing the highest photocatalytic activity [12, 17, 36]. Both high crystallinity and large nanocrystals can obviously enhance activity [11, 17, 37, 38]. Fortunately, designing ordered mesoporous TiO2-SiO2 nanocomposites with complete anatase crystallization, large nanocrystals and high specific surface areas can be readily achieved as a function of crystallization temperature and time, and Ti/Si ratio. Furthermore, tailored silica nanoparticle sizes residing in the pore walls can also be realized.
In this study, we report the simple preparation of a 3D interconnected mesoporous anatase TiO2-SiO2 nanocomposite. The 3D mesoporous nanocomposite was obtained by using an ordered 2D hexagonal mesoporous anatase 70TiO2-30SiO2 nanocomposite (crystallized at 950 ℃ for 2 h, abbreviated as 70TiO2-30SiO2-950) as a precursor, NaOH as an etchant of silica via a "creating mesopores in the pore walls" approach. Our strategy adopts moderate conditions of creating mesopores such as diluted NaOH solution, appropriate temperature and solid/liquid ratio, in order to ensure the integrities of mesoporous structures and anatase crystals. Under these conditions, the dense and uniform intrawall mesopores with an average size of 3.6 nm were obtained, which highly connect the initial 2D arranged mesochannels into a 3D network. Meanwhile, the mesostructures are retained in the integrity. Rhodamine B (RhB) and methylene blue (MB) dyes were chosen as probe molecules to evaluate the resulting nanocomposite. The 3D interconnected mesoporous nanocomposite exhibits unexpectedly high photocatalytic degradation activities to RhB and MB—significantly higher than that for the precursor sample and a commercial Degussa P25 photocatalyst, in addition to being quite stable and reusable. We believe that this method is generally applicable to other ordered mesoporous metal oxide- based materials, such as niobium and tantalum oxides, opening up a new avenue to design 3D highly interconnected mesoporous architectures with ultra-high performances.
Titanium isopropoxide (Ti(OCH(CH3)2)4, TIPO, ≥ 97%) and tetraethyl orthosilicate (Si(OC2H5)4, TEOS, ≥ 96%) were purchased from Fluka. Pluronic P123 (Mw = 5800, EO20PO70EO20) was received from Sigma-Aldrich. Ethanol (absolute), concentrated HCl (36.5 wt%) and P25 photocatalyst (a commercial nano-crystalline TiO2 consisting of ca. 80% anatase and 20% rutile; BET surface area is ca. 50 m2/g) was kindly supplied by Degussa Corp. RhB (C28H31ClN2O3) was bought from Sigma- Aldrich and MB (C16H18ClN3S路3H2O) was purchased from Sinopharm. The molecular structures of RhB and MB are shown in Fig. 1, each prepared into 2.5 × 10-5 mol/L aqueous solutions with deionized water, respectively. All the chemicals were used as received without any further purification. The pH values of RhB and MB solutions were neither adjusted nor buffered.
The ordered 2D hexagonal mesoporous 70TiO2-30SiO2 nanocomposite was synthesized according to our previous procedure [39]. The as-synthesized sample was calcined at 350 ℃ for 6 h in air to remove the organic template and subsequently crystallized at 950 ℃ for 2 h in air with a heating rate of 1 ℃/min. The obtained ordered 2D hexagonal mesoporous anatase TiO2-SiO2 nanocomposite with a Ti/Si ratio of 70/30 (70TiO2-30SiO2-950) was finely ground and used as the precursor. The precursor was then treated with 0.5 mol/L NaOH solution at 40 ℃ with a solid/liquid ratio of 1/10 (g/mL) [40]. The mixture was isolated and vigorously stirred for 12 h prior to the suspension being centrifuged to recover the solid. The solid was again impregnated with fresh NaOH solution under the same conditions as described. This procedure was repeated a further two times (total 36 h). The final solid was thoroughly washed with deionized water under stirring, centrifuged and dried at 100 ℃ for 24 h before being activated at 300 ℃ for 3-6 h in air at a heating rate of 3 ℃/min.
Small-angle X-ray powder diffraction (SAXRD) patterns were recorded on a German Bruker D4 X-ray diffractometer with Ni-filtered Cu-Kª radiation (40 kV, 40 mA). Wide-angle X-ray diffraction (WAXRD) patterns were collected on a Rigaku D/MAX-rB X-ray powder diffractometer using a high-power Cu-Kª (λ = 0.15418 nm) source operating at 40 kV and 60 mA with a graphite monochromator filter. The average anatase nanocrystal size was estimated using the Scherrer equation at the half-height width of the (101) diffraction peak with silicon as a standard for the instrumental line broadening. The crystallinity of anatase nanocrystals was expressed as a function of the intensity or area of the (101) diffraction peak. Transmission electron microscopy (TEM) images were obtained on a JEM-2011 transmission electron microscope (JEOL Company) combined with energy-dispersive X-ray spectroscopy (EDX) operating at 200 kV. For TEM measurements, the samples were prepared by sonication in ethanol and suspended onto holey carbon grids. The atomic wt% of Ti and Si in the sample were examined using EDX. N2 adsorption-desorption isotherms were collected on a Micromeritics ASAP 2010 Adsorption Analyzer at -196 ℃. All samples were degassed at 250 ℃ for at least 5 h before analyses. The Brunauer-Emmett-Teller (BET) specific surface areas were calculated from adsorption data at a relative pressure range from p/p0 = 0.057-0.20. The total pore volumes (VT) were calculated at a relative pressure of p/p0 = 0.976. Pore size distributions were calculated from adsorption branches using the Barrett-Joyner-Halenda (BJH) model.
Adsorption and photocatalytic oxidation of RhB and MB on mesoporous anatase-silica nanocomposites were investigated in air in a quartz vessel at room temperature according to our previous method [41, 42]. Fifty mL of an aqueous dye solution and 50.0 mg of the finely ground catalyst powders were placed in the quartz vessel, which formed a suspension under stirring. For comparison, all the experiments were performed under identical conditions. First, the suspensions were vigorously stirred in the dark for a desired time to evaluate the adsorption performance. After establishing the adsorption-desorption equilibrium, photocatalytic reactions were initiated by subjecting the suspension to UV light irradiation from a 25-W low-pressure mercury lamp (λ = 254 nm). The radiant flux was measured with a photometer (International Light Model IL1400A). A 1.0-mL aliquot of the suspension was taken at specific time intervals and centrifuged at 15000 r/min for 15 min. Dye concentration was analyzed using a JASCO V-550 UV-Vis spectrophotometer. For comparison, the performance of a commercial P25 photocatalyst was also measured.
Stability and reusability were investigated by repetitive adsorbing and degrading RhB. After the dye was adsorbed and photocatalytically degraded each time, the sample was separated by centrifugation, followed by activating at 300 ℃ in air for 3-6 h. Subsequently, the material was re-used under the same RhB solution concentration.
The SAXRD pattern of the parent sample (70TiO2-30SiO2-950) displays only one peak centered at 2θ = 1.08° (Fig. 2(a)), which can be indexed as the (100) diffraction of a typical 2D hexagonal mesostructure (p6mm space group) [39], demonstrating an ordered arrangement of mesopore channels with a cell parameter (a0) of 9.4 nm. After creating mesopores in the pore walls, both the peak position (2θ = 1.07°) and intensity of the resulting sample have no obvious variation (Fig. 2(a)), indicating that the integrity of the mesostructure remains unchanged.
The WAXRD pattern of the parent sample exhibits the characteristic diffraction peaks of anatase [39] displaying the intense and narrow (101) peak (Fig. 2(b)). The area and intensity of the (101) diffraction peak are ∼323 and ∼288, respectively, and the average size of the nanocrystals is calculated to be ~10.8 nm, which is larger than the cell parameter a0 (9.4 nm). Such sized nanocrystals implies a degree of partially or fully blocked mesochannels as the nanocrystals may protrude into the cylindrical mesochannels. After creation of the intrawall mesopores, the area and intensity of the 101 peak have no significant alterations (345 and 288, respectively), illustrating that the crystal size and crystallinity are not essentially altered (Fig. 2(b)).
TEM micrographs show that the precursor has ordered 2D hexagonal mesopore channels without intrawall pores (Fig. 3(a) and (b)). The cell parameter a0 is evaluated to be ∼9.5 nm, essentially the same as the value (9.4 nm) calculated from SAXRD. The mesochannel sizes are uniform and the average size is ca. 4.1 nm. High-resolution TEM images reveal that the crystals are randomly oriented and link with the amorphous silica nanoparticles to form a "brick-mortar-like" framework structure. The majority of crystals align in the pore wall direction, while some crystals protrude into the mesochannels to produce ink-bottle-shape or blocked channels (Fig. 3(b)). The average nanocrystal size is measured to be ca. 11.3 nm, slightly larger than that (10.8 nm) calculated from WAXRD. The lattice fringes of the nanocrystals can be clearly observed and an average d-spacing is measured to be ∼0.34 nm, which is indexed as the 101 reflection of anatase structure—in agreement with the d101 spacing (0.35 nm) calculated from the WAXRD patterns. The reason for non-transformation of the anatase phase into rutile polymorph when subjecting the material to temperature as high as 950 ℃ is a direct result of the role of SiO2 [39, 40]. Similar results have also been reported previously in the literature [43].
After creating pores in the pore walls, the TEM micrographs of the resultant sample show homogeneously distributed dense pores in the walls connecting the mesochannels to form 3D hexagonal bimodal interconnected mesoporous networks (Fig. 3(c)-(d)). Along the [001] direction, there is evidence of "peanut- shell-like" channel openings. Although the intrawall pores are random in orientation, they always link the 2D mesochannels to form 3D mesoporous networks. The pore size distribution is rather narrow (3.1-4.3 nm), averaging ∼3.6 nm. The amorphous SiO2 nanoparticles linking the anatase nanocrystals clearly disappear, leaving voids in the pore walls along the pore direction. The high-resolution TEM micrographs reveal that some obstructed mesochannels are opened by the intrawall mesopores (marked area in Fig. 3(c)), forming ink-bottle-shaped pores. The average size of the main mesochannels is measured to be 4.2 nm, essentially the same as that of the parent sample. EDX analysis shows that the Ti/Si atomic ratio of the resulting sample is 85.0/15.0 (Fig. 3(c), inset).
The N2 adsorption-desorption isotherm of the precursor shows a typical type IV isotherm with one capillary condensation step at relative pressures (p/p0) of 0.40-0.58 (Fig. 4(a)), suggesting a narrow mesopore size distribution. The hysteresis loop displays H2 type, suggesting an ink-bottle-shaped mesopore geometry. This may be related to any mesochannel blockage from the protruding anatase nanocrystals [39], being in agreement with TEM observations (Fig. 3(b)). The mean pore size is ca. 4.0 nm (Fig. 4(b)), close to the value (4.1 nm) evaluated from high-resolution TEM observations. The calculated BET specific surface area and pore volume are 75 m2/g and0.091 cm3/g, respectively.
It is of interest to observe the presence of two distinct uptake patterns in the resulting sample on the adsorption curve (Fig. 4(a)), which evidently suggests two sets of mesopores of varying size. The new minor N2 uptake at low p/p0 (0.39-0.44) indicates the existence of a smaller mesopores possessing a well-defined pore size distribution. The N2 uptake in the 0.44-0.58 p/p0 region is relatively steep, implying an increment of the mesochannels, which probably come from the opened mesochannels. It is interesting that the desilicated material isotherm consists of two convoluted hysteresis loops. The loop relating to the low p/p0 region results from the intrawall mesopores. The drop of desorbed amount in 0.39-0.48 p/p0 region is sharper than that of the precursor, suggesting an increase of the ink-bottle-shaped mesochannels, which may be ascribed to the opened channels. The pore size distribution curve exhibits two discrete and well-resolved peaks (Fig. 4(b)). The minor peak located at ca. 3.4 nm is sharp, clearly indicating the uniformity of the intrawall mesopore size. The mesochannel mean size (ca. 4.1 nm) is almost the same as that of the precursor. The BET surface area and pore volume (121 m2/g and 0.141 cm3/g) are obviously larger than that of the parent sample, respectively. The textural properties of the materials are in good accordance with those from the TEM and SAXRD measurements.
Using ordered 2D hexagonal mesoporous anatase 70TiO2-30SiO2-950 nanocomposite as a precursor, NaOH as an etchant of silica, the 3D bimodal interconnected mesoporous nanocomposite can successfully be prepared via a "creating mesopores in the pore walls" method (Fig. 5). The precursor was synthesized according to our modified evaporation- induced self-assembly (EISA) process with post heat treatment [39]. The as-synthesized nanocomposite has a uniform and homogeneous framework with well-dispersed silicate. Upon calcination at 350 ℃ to remove the template, the amorphous framework begins to crystallize. At this moment, phase separation occurs and anatase nanocrystals are randomly embedded in the matrices of amorphous TiO2 and SiO2. Increasing the crystallization temperature and/or time results in further growth of the TiO2 nanocrystals [39]. Simultaneously, the amorphous SiO2 nanoparticles also enlarge, which serve as a glue linking the nanocrystals firmly together to form a unique "brick-mortar-like" framework. Furthermore, the silica nanoparticles play a key role in stabilizing the mesoporous structure and limiting the nanocrystals quickly coarsening. In this article, we control pore wall crystallinity, nanocrystal size and the size of the silica nanoparticles by fixing the crystallization temperature (950 ℃) and time (2 h) of the precursor. Under such conditions, we can obtain high crystallinity, large anatase nano- crystals and sufficiently large silica nanoparticles (which is the decisive factor to yield intrawall mesopores). Additionally, the use of mild conditions of creating the intrawall mesopores, such as 0.5 mol/L NaOH concentration and 40 ℃ etc., allows both retention of the mesopore structural integrity and high photocatalytic performance.
The adsorption of RhB on the parent sample proceeds slowly, taking ca. 30 min to essentially reach adsorption- desorption equilibration (Fig. 6(a)). The saturated adsorption amount is ~58.4%. Interestingly, the fast adsorption process observed on the nanocomposite with 3D interconnected mesopores (Fig. 6(a)) achieves a rate ~6 times faster than that of the precursor. The increased adsorption rate is ascribed to the improved diffusion efficiency contributed by the 3D mesoporous architecture [40]. The transport efficiency enhancement resulting from the interlinked mesopores was also observed in the mesoporous silica, SBA-15 material. The diffusivity of n-heptane in SBA-15 with smaller mesopores being dominant in the walls connecting the larger mesopores of the main channels is 3-4 times higher than that in SBA-15 possessing a high content of intrawall micropores linking the mesochannels [44]. The saturated adsorption amount (44.8%) is obviously lower than that on the parent sample and displays a negative correlation with increased surface area and pore volume. This phenomenon results from the decrease of silica composition, which plays an overwhelming role in cationic dye adsorption [41, 42]. Similar trends are also observed with MB (Fig. 6(b)). When subjecting the materials to UV irradiation, the concentration of RhB drops exponentially with photocatalytic degradation time on the precursor, and a pseudo-first-order reaction is observed (Fig. 6(a), (c)). The degradation rate is 0.0597 min-1. The degradation rate observed on the 3D interconnected mesopore sample is extremely high (0.303 min-1), as much as 5.1 times that of the precursor and 16.5 times that of the P25 photocatalyst (0.0184 min-1). Significantly, similar trends also occur with MB (Fig. 6(b), (d)). The degradation rate of MB on the 3D interconnected mesoporous sample is ~5.3 and 24.1 times that of the parent sample and P25 photocatalyst (Fig. 6(d)), respectively. These results fully demonstrate that the 3D interconnected mesoporous structure exhibits unexpectedly high activities to RhB and MB dyes. It is worth noting that both the 3D and 2D nanocomposites exhibit higher catalytic activities for MB photodegradation than for RhB photodegradation. This is related to the higher MB saturated adsorption amounts on both the 3D and 2D nanocomposites compared with RhB, which probably result in the synergistic role of the coupled adsorbing and photocatalytically degrading MB to be closer to the synchronicity. Our previous results have demonstrated that the synchronous role of the coupled adsorption and photocatalytic oxidation generates the optimal activity [41].
The stability and reusability of the 3D interconnected mesoporous anatase TiO2-SiO2 nanocomposite were investigated using RhB. Prior to subjecting the sample to UV light irradiation, the adsorption of RhB onto the sample is rapid, taking only ~5 min to essentially reach adsorption-desorption equilibration in the first cycle (Fig. 7). The saturated adsorption amount is ~44.5%. After UV light irradiation, the concentration of RhB declines exponentially with time and the degradation percentage reaches 99.8% within 20 min. After nine additional cycles, the equilibrium time remains at ca. 5 min, while the saturated adsorption amount is in the range of 41.3%-48.1%, which has no significant change. All degradation percentages are higher than 99.7%. These results fully illustrate that our 3D interconnected mesoporous nanocomposite is quite stable and reusable.
For the 3D interconnected mesoporous anatase TiO2-SiO2 nanocomposite, the anatase nanocrystals and silica nanoparticles co-exist predominantly inside the mesochannels and intrawall mesopores (Fig. 8(a)). During reactions, the RhB and MB molecules are first adsorbed overwhelmingly on the SiO2 nanoparticles on the outer surfaces. Thereafter, the adsorbed molecules diffuse inside the mesopore channels in a quite short time (Fig. 8(b)). Meanwhile, the anatase nanocrystals surfaces generate OH radicals under UV irradiation, which simultaneously react with the adsorbed molecules from all directions inside the mesopore network. As a result, the molecules are in no time degraded into smaller moieties, finally mineralized into CO2, H2O etc. [40-42]. Subsequently, CO2 etc. would depart very easily from the reaction site to the solution outside. All these processes carry out unexpectedly fast because of the 3D interconnected mesoporous architecture.
A 3D interconnected mesoporous anatase crystal-silica nanocomposite has been successfully prepared by using ordered 2D hexagonal mesoporous anatase 70TiO2-30SiO2-950 nanocomposite as a precursor, NaOH as an etchant of SiO2 via a "creating mesopores in the pore walls" method. Our results show that the initial mesochannels in the resultant sample are highly connected by dense and uniform intrawall mesopores while retaining mesostructural integrity. The crystallinity and size of the initial anatase nanocrystals are not significantly altered after creating the intrawall mesopores. The BET specific surface area and pore volume of the sample possessing interconnected mesopores are remarkably higher than those of the precursor. The diffusion rates of RhB and MB molecules through the 3D mesopores system were greatly enhanced by a factor of more than four when compared with the 2D precursor without intrawall pores. The sample possessing interconnected mesopores exhibits significantly higher photocatalytic activity than the parent sample. Unexpectedly high degradation activities for RhB (0.303 min-1) and MB (0.757 min-1) in the 3D mesoporous architecture are as high as 5.1 and 5.3 times that of the precursor (0.0597, 0.144 min-1), respectively, even up to 16.5 and 24.1 times that of a commercial Degussa P25 photocatalyst (0.0184, 0.0314 min-1), respectively. These results fully demonstrate that the 3D interconnected mesoporous network plays a key role in the marked increase in activity. Our sample exhibits excellent photocatalytic degradation activities to RhB and MB when compared with other mesoporous metal oxide- based materials reported in the literature. Importantly, our sample is considerably stable and reusable. Furthermore, this approach paves the way for the preparation of other ordered mesoporous metal oxide-based materials with 3D interconnected mesopores, such as Nb2O5 and Ta2O5, with excellent photocatalytic performances.
The authors declare no competing financial interest.
2016,
Vol. 37 Issue (6): 846-854 
PDF (989 KB)
2016,
Vol. 37 Issue (6): 846-854 PDF (989 KB)