NANO LETTERS
Synthesis of Porous Nanocrystalline TiO2 Foam
2003 Vol. 3, No. 2 249-251
Ioannis M. Arabatzis and Polycarpos Falaras* Institute of Physical Chemistry, NCSR “Demokritos”, 153 10 Aghia ParaskeVi Attikis, Athens, Greece Received November 20, 2002; Revised Manuscript Received December 4, 2002
ABSTRACT Direct decomposition and reaction of hydrogen peroxide inside a titanium dioxide/hexadecylamine slurry dispersion results in a foaming, inorganic/organic hybrid composite material containing TiO2 with intercalated hexadecylamine with extended, sponge-like, porous structure. The complex structure of the synthesized foams can be described by a self-assembly of the amine moieties between the TiO2 nanoparticles. The surfactant molecules, held together by hydrophobic interactions between the alkyl chains, are intercalated inside thin planar TiO2 walls in a way that a lamellar nanostructure is formed. The XRD basal distance of 46.8 Å is consistent with a crystal unit containing two surfactant molecules with alkyl chains perpendicular to the titanium dioxide walls. The hydrophilic amine headgroups are oriented with strong affinity to the hydroxyl groups present on the oxide surface.
There is the tendency for the development of novel, well structured, porous, high surface area and complex forms of titanium dioxide, to be adapted in environmental applications, including sorption media, filters, and photocatalysts for air purification.1,2 For efficiency optimization reasons, the main parameter to be controlled is the nanostructure of the porous material. We report here the synthesis of titanium dioxide foams, starting from a commercially available material, TiO2 Degussa P25. It is well established that titanium dioxide powder can be dissolved by hydrogen peroxide following a reaction which produces surface stabilized superoxide and peroxo anions as well as TiIV(OH)2 species.3 The oxygen molecules, resulting in situ from the decomposition of hydrogen peroxide form bubbles, are captured inside a viscous paste of a hexadecylamine(HDA)-TiO2 mixture and are responsible for the foam material particular structure, as described below. The synthetical procedure includes the following steps: 2 g of 1-hexadecylamine (Fluka, Assay>99%) is dissolved in 4 mL of boiling acetone (Riedel-de Hae¨n, Analytical Reagent). Increased solvent temperature ensures the necessary solubility for the organic surfactant. While the solution is still hot, 0.44 g of TiO2 (Degussa P25) is added. Finally, 50 mL of H2O2 30% (Panreac) is added at once. The long-chain amine, apart from the amphiphilic character of the molecule, provides the necessary alkalinity for hydrogen peroxide to decompose to O2 bubbles. Following the H2O2 addition, white-yellowish foam is generated. The volume increases rapidly and after 1 h 150 mL of foam are produced. Before storing under * Corresponding author. E-mail:
[email protected]. Tel: +30210-6503644. Fax: +30-210-6511766. 10.1021/nl0259028 CCC: $25.00 Published on Web 12/24/2002
© 2003 American Chemical Society
vacuum (20 mbar), the semiconductor foam is dried for 2 days at ambient atmosphere conditions. Scanning electron microscopy (SEM, Leica S440) of the resulting material reveals a porous and sponge-like structure (Figure 1). Low magnification image presents an extended network of features separated by large pores with a diameter of 1-2 µm approximately. Such an open structure has important roughness and complexity. By increasing the SEM magnification, the fine texture of the foam features appears. No existence of grainy matter was observed, proving that a complete reaction occurred. The structure resembles a network of interconnected flakes forming irregular polygonal cavities of 200-500 nm in size, where walls with thickness of about 100 nm are visible. The crystallinity of the titanium dioxide foam was confirmed by the X-ray diffraction (XRD, Siemens D-500, Cu-KR radiation) pattern (Figure 2). The sharp peak at 2θ ) 1.86 degrees, according to Bragg’s law, corresponds to a 001 peak of lamellar structure with a basal distance of 46.8 Å. This value appears increased comparing to the molecular distance between the terminal carbon and the amine group, the latest estimated at about 20.4 Å. The XRD basal distance of 46.8 Å indicates the presence of HDA hydrocarbon chain packing inside the material’s matrix and corresponds to inorganic/organic hybrids of titanium dioxide-hexadecylamine lamellar mesostructures. The XRD pattern from 15 to 85 2θ degrees (not shown here) confirms the presence of nanocrystalline titanium dioxide with intense peaks of anatase (strongest) and rutile crystal phases. This fact is expected as the original Degussa P25 contains both phases4 (∼75% anatase and ∼25% rutile).
Figure 2. X-ray diffraction pattern of TiO2 foam.
Figure 1. SEM images of TiO2 foam. Bar is set at 2 µm for the upper, low magnification image and 300 nm for the lower, high magnification image.
To unambiguously characterize the position of the hexadecylamine in the foam structure, we have undertaken differential scanning calorimetry (DSC, TA Instruments 2920, heating rate: 2 °C/min). The DSC pattern (Figure 3) reveals that the solid is stable until 190 °C where an irreversible destruction takes place. It is important to notice the melting of HDA at 57 °C when heating and 54 °C when cooling. This step is fully reversible with enthalpies of 12.99 J/g and 11.04 J/g, respectively. However, smaller shoulders at 46 °C and 44 °C (heating-cooling) are assigned also to the melting point of 1-hexadecylamine. The difference is attributed to a better stabilization of the intercalated HDA with respect to the ‘‘free” one. Surfactant molecules that form the lamellar structure need elevated temperature to transit in the melted form. The complex structure in the as-synthesized foams can be explained by a self-assembly of the amine moieties between the TiO2 nanoparticles. Our results postulate that the surfactant molecules, held together by hydrophobic interactions between the alkyl chains, are intercalated inside thin planar TiO2 walls in a way that a lamellar mesostructure is formed.5 The crystal unit contains two surfactant molecules with alkyl chains perpendicular to the titanium dioxide walls. The hydrophilic amine headgroups are oriented with strong 250
Figure 3. DSC pattern of TiO2 foam.
affinity to the hydroxyl groups present on the oxide surface, Figure 4. This is further supported by FTIR studies. In the solid HDA FTIR spectrum (not shown here), the NH2 group presents strong vibration bands at 3333 (asymmetric stretch) and 3253 cm-1 (symmetric stretch), which in the foam become broader and shift to lower wavenumbers, at 3314 and 3151 cm-1, respectively. A similar behavior is observed for the 1607 cm-1 NH2 deformation (N-H bending) band. These substantial shifts are indicative of a covalent interaction between the amine head and the inorganic matrix.6 The durability of the titanium oxide foam is relatively poor and the material’s backbone ruins under mechanic crush. However, treatment with ultraviolet light (UV at 254 nm) of the titania foam for 24 h does not affect its morphological and structural characteristics, as indicated by SEM and XRD observations. Other surfactants, such as Triton X-100 in the presence of ammonia (ensuring the necessary conditions for hydrogen peroxide decomposition into O2), have been successfully tested as potential candidates for the titania foam production. In that case the foam-core stiffens, the resulting material is less brittle and shows lower crystallinity. This Nano Lett., Vol. 3, No. 2, 2003
simple and the foam macrostructures were obtained at once from a titanium dioxide commercial powder. Moreover, the foaming process can be extended to other metal oxides. In fact, the preparation of composite macroporous crystalline inorganic oxides (i.e., vanadium oxide foam, possessing ultralight and comb-like structure) was recently reported.8 The obtained TiO2 foams have accessible pores and walls, which can be of benefit for the utilization of semiconducting properties of TiO2. During the foam formation, the novel macroporous materials were introduced (without any separation step) inside one-inch photocatalytic glass tubes and are currently examined as potential photocatalysts for gas-phase pollutant (volatile organic compounds, VOC) decomposition. Acknowledgment. Thanks must be adressed to Delis AE Athens-Greece and Degussa AG Frankfurt-Germany for generously providing the TiO2 Degussa P25 powder. Financial support from NATO (EST.CLG.976641 grant) is greatly acknowledged. Figure 4. Schematic representation of the TiO2/HDA foaming mesostructure.
difference is attributed to the differences in interactions between the hydrophobic parts of the surfactants as well as between the hydrophilic headgroups and the hydroxylated TiO2 surface. Porous titania foam materials consisting of densely packed spherical shells can be formed using the sol-gel processing of titanium alkoxides in nonaqueous emulsions as templates.7 This process requires an additional and extremely delicate step, the removal of the organic matter without damaging the inorganic matrix. On the contrary, our method is very
Nano Lett., Vol. 3, No. 2, 2003
References (1) Fujishima, A.; Hashimoto K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; Bkc Inc.: Tokyo, 1999. (2) Alfano, M.; Bahnemann, D.; Cassano, A. E.; Dilert, R.; Goslich, R. Catal. Today 2000, 58, 199. (3) Murphy, D. M.; Griffiths, E. W.; Rowlands, C. C.; Hancock F. E.; Giamello, E. Chem. Commun. 1997, 2177. (4) Arabatzis, I. M.; Stergiopoulos, T.; Bernard, M. C.; Labou, D.; Neophytides, S. G.; Falaras, P. Appl. Catal. B: EnViron., in press. (5) Miyake Y.; Kondo T. J. Chem. Eng. Jpn. 2001, 34, 319. (6) Dasgupta, S.; Agarwal M.; Datta, A. J. Mater. Chem. 2002, 12, 162. (7) Imhof A.; Pine, D. J. AdV. Mater. 1999, 11, 311. (8) Chandrappa, G. T.; Steunou, N.; Livage, J. Nature 2002, 416, 702.
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