This paper presents a novel synthesis of well characterized nanoporous materials. The development of mesoporous TiO<sub>2</sub> with the use of crosslinked polymer network as structure and surface texture directing agent is reported in this study. Randomly cross-linked DMAEMA-50-PEGMA-50-EGDMA1 was synthesized by radical polymerisation to be used as removable scaffold. The resulting materials were characterized by powder X-ray diffraction (pXRD), nitrogen adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The synthesized oxides morphology was strongly influenced by the polymer network used as removable scaffold. The modified materials exhibited a narrower pore size distribution and marginally higher specific surface area compared to the unmodified samples. The scaffold cross-linking ratio was also found to have a significant effect on the synthesized materials polymorph. The modification has a strong effect on the titania polymorph as the anatase-rutile transformation was observed only for the modified titania samples.
Titania-based materials have rapidly attracted great attention due to important applications in photo-catalysis [
Previous work has been conducted in the porous solids research group of the University of Cyprus, including the synthesis of porous oxides by investigating methods of improvement of these materials properties. These porous metal oxide modifications include the synthesis of mixed ceria with transition metals [
This paper proposes a novel strategy for the design in synthesis of mesoporous materials with the use of swellable polymer networks used as scaffolds. Previous reports use different materials that have a templating effect towards the precursor. In such cases the organic material acts as central formation about which the precursor moieties organize and by this way a material influenced by the template is formed [
Many reports in literature in regards to the synthesis of titania with the use of templates show the growing interest in new synthesis methods for this material over the last years. Additionally, it is in many cases shown that the use of organic supports enhanced the synthesized materials properties by manipulating their porous structure and surface characteristics. In particular, TiO2 fibres were reported to be fabricated under autogenous pressure, using activated carbon fibres as templates with no consumption of organic solvent providing an environmentally friendlier synthesis route in comparison to using organic solvents [
Porous titania which had the anatase structure and high surface area was synthesized with the use of cellulose nanocrystals as template [
The use of these templates played an important role in the formation of the precipitates and improved the structural characteristics of the synthesized materials. In this work, a novel alternative methodology was implemented with the use of swellable polymer networks as removable scaffolds where the material was synthesized into the scaffolds net-like structure using the polymer network itself as both a reaction vessel and a structure directing agent.
The chemical reagents used were reagent grade (Aldrich Germany) and were used without further purification. The compounds used were, 2 (dimethylamino) ethyl methacrylate (DMAEMA, 98%), poly(ethylene glycol) methylether methacrylate (PEGMA 98%, Mw 300 g/mol), the radical initiator 2,2-azobis (isobutyronitrile) (AIBN) and ethylene glycol dimethacrylate (EGDMA 98%), titanuim (IV) butoxide Ti(OCH2CH2CH2CH3)4 97% and ethanol analytical grade ≥99.8%. Tetrahydrofuran (THF) was purchased from Scharlau Spain.
For the preparation of randomly cross-linked network of DMAEMA50-co- PEGMA50-co-EGDMA1, 98 mg (6 × 10−3 mol) of the radical initiator 2, 2-azobis (isobutyronitrile) (AIBN), 5 ml (4.68 g, 0.03 mol) of the monomer 2-(dimethylamino)ethyl methacrylate (DMAEMA), 8.5 ml (8.93 g, 0.03 mole) of the monomer poly(ethylene glycol) methylether methacrylate (PEGMA), 0.11 ml (0.12 g, 6 × 10−4 mol) of the cross-linker ethylene glycol dimethacrylate (EGDMA) and 31.7 ml tedrahydrofuran (THF) were added in a 100 ml round bottom flask under vigorous stirring placed in an oil bath at 70˚C for 24 h until the polymer gel was formed. The resulting polymer gel was dried under vacuum at 60˚C for 48 h.
Titanium (IV) butoxide was diluted in anhydrous ethanol (10% Ti4+/Ethanol). The polymer gel was dispersed in 60 ml of the solution for 24 h under moderate stirring. The swollen gels were washed with anhydrous ethanol in order to remove any precursor traces left outside the gels volume. After the swollen scaffold was washed, it was dispersed in water/ethanol 33% mixture for 24 h and a precipitate was formed inside the gels structure giving the gel a white colour forming the “as-prepared sample”. The resulting “as-prepared sample” was washed several times with ethanol/water mixtures and dried under vacuum at 60˚C overnight. Then the product was calcined at 500˚C for 3 h with a heating rate of 6˚C min−1 and a white solid was obtained. The same procedure was carried out without the use of scaffold so as to obtain the “unmodified TiO2”. The experimental procedure is illustrated in
Nitrogen adsorption isotherm measurements were carried out at 77 K using an ASAP 2010 Micrometrics apparatus. The samples were degassed prior to the measurements at 393 K for 24 h. The BET equation was used to calculate the specific surface areas, while pore size distributions were estimated using density functional theory (DFT) methods using the supporting DFT-plus software employing a slit shaped pore model (meaning a pore length which is much longer than the other two dimensions. Scanning Electron Microscopy images were obtained to examine the materials morphology with the use of a JEOL JSM-5600 instrument. The thermal stability was studied by thermogravimetric analysis (TGA) using a Shimadzu apparatus. The measurements were carried out in air up to 1073 K and the heating rate was 6 K・min−1. Differential scanning calorimetry (DSC) was further used, using a model Q1000 by Thermal Instruments with a heating rate of 6 K・min−1 in the temperature range of 313 - 873 K. FTIR measurements were performed using a Shimadzu spectrometer (FTIR-8501) with KBr powder as diluent. For the polymer networks, ATR-FTIR was used. Powder X ray diffraction measurements were carried out on a Shimadzu 6000 diffractometer using Kα radiation (λ = 0.15478 nm).
The adjustment of the proper calcination conditions was crucial in order to achieve the successful and complete removal of the scaffold. The calcination temperature should be high enough to achieve the clean removal of the scaffold, but at the same time sintering of the synthesized materials needs to be avoided as this would reduce the specific surface area and increase the particle size of the product. Additionally, the material could lose its initial scaffold-fabricated characteristics due to the extensive crystallization. In order to achieve the optimal calcination conditions and to confirm the removal of the scaffold, the materials were characterized by thermogravimetric analysis (TGA/DSC) as shown in
Based on the TGA/DSC trace and the N2 adsorption measurements (to confirm the unblocking of the pores) for the as synthesized Gel-TiO2 samples, the optimal calcination conditions were determined. For the removal of the organic scaffolds, the samples were calcined at 500˚C for 3 h with a heating rate of 6˚C/min.
The crystallite size and the polymorph for each of the synthesized samples were determined by powder XRD.
peaks, characteristic of other impurities are present which confirms the complete removal of the organic scaffold during the calcination process and that the synthesized titania material was of high purity.
Crosslinker Content (%) | Surface Areab (m2/g) | Pore Volumec (cm3/g) | Crystallite sizea (nm) Anatase | Crystallite sizea (nm) Rutile | Mean Pore Width (nm) |
---|---|---|---|---|---|
1% | 47 | 0.085 | 14 | 18 | 8 |
1.5% | 32 | 0.076 | 16 | 22 | 9 |
2% | 27 | 0.09 | 18 | 23 | 12 |
4% | 27 | 0.1 | 19 | 28 | 12 |
Unmodified | 31 | 0.14 | 14 | - | broad 40 |
aCalculated by the Scherer equation. bBET surface area calculated from the linear part of the BET plot. cEstimated using the desorption branch of the isotherm.
crystallite size for the corresponding synthesis conditions. Presumably, the scaffolds ability to promote the mixed anatase-rutile titania phase could be attributed to the steric hindrance caused by the scaffolds net-like formation as the crystallization takes place in the voids formed by the loops of the polymer networks. It is noted that rutile is the polymorph with the higher density (4.25 g・cm−3) compared with anatase (3.89 g・cm−3) (Hanaor and Sorrell, 2011), as obviously crystallization of the dense phase is favoured by a confined space environment. Increase of the cross-linker content leads to a lower degree of swelling, and presumably reducing the space available for the crystallization process. These results suggest that the mechanism by which the scaffold influences the synthesis is not through an effect on the crystallite size, which would impact upon the surface area, but on the way crystallites assemble to form the particles.
The N2 adsorption isotherms and pore size distributions of unmodified samples in comparison with the corresponding modified materials using 1% cross-linker calcined at 500˚C for 3 h are shown in
As can be seen in
and 0.085 cm3/g respectively. BET (Brunauer-Emmett-Teller) equation was used in order to determine the specific surface area from the N2 adsorption measurements [
towards higher partial pressures with increasing cross-linker content. The inflection point moves to higher partial pressures indicating a widening of pore diameter, as confirmed by the pore size distribution presented in
The texture of the material is influenced by the structure of the scaffold, and in particular the distance between the cross-linking moieties, and therefore it is expected that higher cross-linking will result in a product with a more open porous structure and increased average pore diameter. The results obtained in N2 adsorption measurements and XRD measurements are summarised in
Examination of the data in
In order to assess the structural properties of the synthesized samples and to determine the functional groups present FTIR spectroscopy was used. The spectra were obtained in the wavelength region of 4000 - 500 cm−1.
the presence of adsorbed water and structural hydroxyl groups showing surface hydroxylation.
The synthesized, modified and unmodified materials morphology was examined with scanning electron microscopy (SEM) as shown in
It is shown that the modified samples exhibit similar morphologies, with an apparently more loose texture in comparison to the corresponding pristine samples. It is assumed that the slit shaped void spaces have occurred after the removal of the scaffolds and were shaped during the removal of the organic material. It is suggested that the creation of new Ti-O-Ti bonds leading to the synthesis of the titania lattice within the confines of the swollen gel, leads to the exclusion of water from that environment. Since it is the water molecules that cause the swelling, their removal led to shrinkage of the polymer network, which reduced the space available to the growing oxide lattice. This caused the extrusion of the precipitating solid from the inside of the gel, and may explain the observed morphology. The pristine samples seem to have a more rigid structure with less porosity.
The precipitation reaction in the swollen scaffold led, presumably, to the synthesis of the polymorph which is stable at room temperature, anatase, but the anatase to rutile phase transition is known to be facile. As titania crystallites accumulated in the constrained environment, constricting it even further, this provided the impetus for some of the crystallites to undergo an anatase to rutile phase transition. This took place on the surface of small crystallites, where Ti4+ and O2− ions would be hydrated, and moved from an anatase structure arrangement to a rutile one, presumably with the assistance of hydrogen bonded water molecules. This would expose a new surface where further ion movement would be able to take place.
The modified materials showed a narrower pore size distribution, and a
marginally higher specific surface area. Additionally, the modified material contains both anatase and rutile polymorphs while the unmodified titania consisted of anatase phase only. Thus, the modification not only affects the porosity of the material but also offers a control on the polymorph of the product. The results provide a solid indication that the polymer network has a strong influence and in many cases improves the surface properties of the synthesized titania. The effect of the scaffold is attributed to the interaction between the growing titania particles and the polymer network.
Modified titania has been successfully synthesized with the use of the polymer network DMAEMA50-coPEGMA50-co-EGDMA1 as a removable scaffold. We have also studied the precipitation of different metal oxides including ceria where the polymer network had a different effect on the porosity of the synthesized materials, showing that the scaffold-ceria interaction was different from the scaffold-titania [
We appreciate the advice provided by Dr C.S. Patrickios of this Department.
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
We would like to thank the University of Cyprus for financial support, and the Cyprus State Scholarship Foundation for a studentship (Grant number 709 (14)) for CAP.
Papatryfonos, C.A. and Theocharis, C.R. (2018) Using Swellable Polymers as Structure Directing Scaffolds in Mesoporous Titania Synthesis. Materials Sciences and Applications, 9, 211-227. https://doi.org/10.4236/msa.2018.92014