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Journal of Nanomaterials
Volume 2011 (2011), Article ID 597954, 9 pages
Mesoporous Titania Nanocrystals by Hydrothermal Template Growth
Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy
Received 6 May 2010; Revised 30 June 2010; Accepted 22 July 2010
Academic Editor: William W. Yu
Copyright © 2011 Giuseppe Cappelletti et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Mesoporous TiO2 nanocrystals have been synthetized by a classical sol-gel route integrated by an hydrothermal growth step using monomeric (dodecylpyridinium chloride, DPC) or dimeric gemini-like (GS3) surfactants as template directing agents. Adsorption isotherms at the solid/liquid interface of the two surfactants have been obtained on aqueous dispersion of titania; the nature of the oxide/adsorbate interactions and the molecules orientation/coarea are discussed. The effects produced by the presence of the two surfactants on the different morphological (surface area, porosity, and shape) and structural (phase composition and aggregate size) features of the final TiO2 samples, calcined at , are discussed.
In recent years, the achievement of mesoporous titania nanomaterials, characterized by desired grain size, specific shape, high surface area, and tailored pore network, has became fundamental for photocatalytic and photovoltaic applications [1–4].
Several liquid-crystal templating (LCT) techniques, exploiting three-dimensional structures of ionic/nonionic surfactants [5–10] and block copolymers [11–13] have been proposed to synthesize mesoporous titanium oxides. The presence of mesoscopic pores in TiO2 for dye-sensitized solar cells is of great importance; in fact when the pores are filled with a conducting medium, charge carriers can easily percolate across the mesoscopic particle network making the whole internal surface area electronically accessible . However, the fine control of the, often divergent, features of the material is very complex and the presence of the organic templating surfactant with the inherent high decomposition temperature may imply undesired effects. Just for these reasons, the extension of LCT techniques to pure titanium oxide has still remained elusive; the fast grain growth of the oxide, which occurs upon calcination of the surfactant-rich precursor, provokes, in fact, pore collapse with degradation of the mesostructure.
Besides the possible promotion of a controlled pore network, the presence of an amphiphilic molecule can intervene in different ways by tuning the sample final morphology. Very interesting is, for instance, the recent work by Chen et al. , where three-dimensionally ordered arrays of mesoporous titania spheres have successfully been synthesized through opal (ordered closed-packed face-centered cubic lattice of silica or latex spheres) template and triblock copolymer (Pluronic P123) as a mesopore-directing agent. The mean diameter of the titania spheres has been observed to be ca. 165 nm with a narrow distribution of the mesopore size in the range of 2–6 nm.
Together with the surface area/porosity, also the grain size and relative enrichment in the different oxide polymorphs should be controlled through template routes to promote the best efficiency of the TiO2-based semiconductors [9, 10]. Crystallite sizes in the range of 10–80 nm are considered optimal for both optical absorption and electron transport in the conversion of solar light to electric current, by dye-sensitized solar cells [14, 17, 18]. Anatase exhibits the highest activity for photovoltaic applications, while a controlled mixture of anatase and rutile or brookite is reported to produce the best activity in photodegradation experiments [19, 20]. Mohamed et al.  reported that an hydrothermal treatment in the presence of cationic surfactants (cetyltrimethylammonium bromide, CTAB and cetylpyridinum bromide, CPB) as templates may control not only the surface properties (surface area of 240–418 g-1, pore size in the range of 23.2–43.7 Å) and morphology (from nano-sized spheres to cotton fibrils) but also the bulk features (crystallites size in the range of 10.1–18.2 nm) and phase composition (rutile phase at C).
Here we report on the synthesis, of TiO2 nanocrystals, through a simple process implying a sol-gel starting reaction, followed by a growth step in solution, performed, also, in the presence of monomeric or dimeric “gemini-like” alkylpyridinium surfactants. The latter ones are composed by two conventional single-tail amphiphilic moieties chemically connected by spacer groups, usually polymethylene or short poly(oxyethylene) chains [10, 21]. Recently, many studies concerning the practical use of gemini molecules appeared in the literature [22–25], showing that the first speculative interests were followed by practical applications, due to their unusual properties, such as the lower critical micellar concentration (CMC) values, the better adsorption behavior at both the air/water and the solid/water interfaces with respect to their single monomers, and the tendency to form micelles of different shapes and dimensions (i.e., spherical, rodlike, threadlike, vesicles), even at low concentration, when compared with similar nongemini surfactants. In this work, the concentration of surfactants in the ageing solution is varied in order to produce different conditions of self-aggregation between the surfactant molecules themselves. The surfactant/oxide interactions at the solid/liquid interface are evaluated by adsorption isotherms on aqueous suspension of titanium dioxide. The features of the calcined (C) powders are investigated with respect to the phase composition/crystallinity, the surface area/porosity, and morphological aspects to evidence the effects provoked by the conditions of the particle template growth.
2. Experimental Section
All the chemicals were of reagent grade purity and were used without further purification; bi-distilled water passed through a Milli-Q apparatus was used to prepare solutions and suspensions.
2.1. Sample Preparation
TiO2 particles were obtained by following a room-temperature sol-gel reaction starting from Ti(C4H9O)4 and adopting a water/alkoxide molar ratio of 81.7 and a water/propanol molar ratio of 8.5. The xerogel powders were purified by centrifugation-resuspension cycles and then powder fractions were aged at C, at pH 8 (i.e., condition of attractive electrostatic interactions between the oxide and the cationic surfactants), for fixed timelength (5 h) with different surfactants concentrations: 1–100 mM and 0.1–50 mM for dodecylpyridinium chloride, DPC (compound 1) and -didodecil--trimetilendipiridinio-dicloruro, “gemini spacer 3” GS3 (compound 2), respectively.
After the ageing, the suspensions were dried again at C. Finally the powders were thermally treated at C for 6 hours under an oxygen stream.
2.2. Sample Characterization
Room-temperature X-ray powder diffraction (XRPD) patterns were collected between 10 and with a Siemens D500 diffractometer, using Cu radiation. Rietveld refinement has been performed using the GSAS software suite and its graphical interface EXPGUI [26, 27]. The average diameter of the crystallites, <D>, was estimated from the most intense reflection of the anatase (101) and rutile (110) TiO2 phases using the Scherrer equation .
Specific surface areas were determined by the classical BET procedure using a Coulter SA 3100 apparatus. Desorption isotherms were used to determine the pore size distribution using the Barret-Joyner-Halander (BJH) method.
Scanning electron microscopy (SEM) photographs are acquired with a LEO 1430.
The values of the critical micelle concentration (CMC) of DPC and gemini surfactants were obtained by conductometric determinations as a function of the temperature. Therefore, using as a guideline an average increasing slope with the temperature of the CMC of DPC and GS3, obtained from the present results and literature data [15, 21, 29–32], a concentration range roughly corresponding to a possible CMC at C was estimated for both surfactants (18–22 mM, for DPC and 2–4 mM in the case of GS3).
The adsorption isotherm of GS3 at the TiO2 interface was obtained under the following conditions: C; 0.2; equilibration time = 4 h; ionic strength M KCl. At the end of the adsorption time, the surnatant solution was sampled for the residual surfactant concentration by spectrophotometric characterization at 265 nm for the gemini salt. Data on DPC adsorption on , obtained by Koopal et al.  are elaborated and reported for comparison ( C; 0.2; equilibration time 12 h; ionic strength M NaCl). Adsorption isotherms of the two surfactant molecules are reported by plotting the surface excess (Γ) as a function of the final concentration at equilibrium.
3. Results and Discussion
3.1. Adsorption at TiO2-Solution Interface
Figure 1 reports the experimental adsorption isotherms, at the TiO2 solution interface, of both monomeric DPC (Figure 1(a), elaborated from data in ) and dimeric “gemini-like” GS3 (Figure 1(b)) surfactants. The shape of the two curves is markedly different. The monomer isotherm (Figure 1(a)) is S-shaped showing a low adsorbent-adsorbate affinity due to weak interactions between the surfactant and the oxide ; moreover, it can be proposed that the adsorption leads to the formation of surface ion pairs, Ti−O−Py+ (the surface charge of TiO2 at pH 8 is negative), provoking a partial compensation of the surfactant aromatic charge, in accordance with our previous XPS results . The curve pertaining to the gemini (Figure 1(b)), instead, follows the trend classified in the literature as L-type, which is generally associated with coverage of the solid surface by a monolayer of adsorbate molecules and is characterized by a strong adsorbent-adsorbate interaction. The higher affinity of the gemini GS3 for the solid appears also from the larger adsorbed amounts ( = 7.7×10-5 mol cm-2) in the quasiplateau regions with respect to the maximum surface excess (= 4.8 × 10-5 mol cm-2) of the monomeric DPC surfactant. For these reasons, the data relative to the DPC and GS3 isotherms were elaborated on the basis of the Langmuir (strong adsorbent-adsorbate interaction, no lateral interactions between adsorbate molecules) and the Frumkin-Fowler-Guggenheim (FFG, weak adsorbent-adsorbate interaction, electrostatic and nonelectrostatic lateral interactions [33–35]) model equations, respectively. The parameters obtained from the treatment of the data are reported in Table 1. The linear correlation (2) is good in both cases, justifying the choice of the two different model approaches. The adsorption equilibrium constants () and the relative standard adsorption Gibbs energy (∆0) values support the dramatic increase in adsorption at the TiO2 surface passing from the monomer to the dimer . In the case of the DPC isotherm, the lateral interaction parameter () is positive, that is, it represents electrostatic repulsion interactions between the positive charges of the surfactant headgroups, in agreement with what reported by Mehrian et al.  in the case of DPC adsorption on clays. They studied the influence of the electrolyte concentration on the adsorption of DPC on Na-kaolinite; an attractive lateral term was obtained only in the case of high ionic strength (100 mM), while repulsion prevailed at lower electrolyte concentrations (5 and 20 mM).
The limiting areas (å, co-area) for DPC and GS3 (350 and 216 Å2, resp.) at maximum packing () calculated from the surface excess at maximum coverage are higher than those present in the literature. Ottewill et al.  reported two different values of coarea for DPC molecule, obtained by surface tension at the air-water interface, in the case of flat-lying (110 Å2) and vertical (35 Å2) orientations. Thus, on the basis of these considerations, the present size of adsorbed DPC molecule could be interpreted as being due to a flat orientation (sketch inset Figure 1), with the formation of a diluted film, characterized by electrostatic repulsion between the charged pyridinium heads. On the contrary, the lower value of å for the GS3 surfactant could be ascribable to a vertical orientation (see sketch inset Figure 1) in which, in agreement with our previous results of adsorption of gemini zero-spacer on TiO2 , only one pyridinic group is directly involved in the electrostatic interactions with the oxide, whereas the second one is compensated by its counterion (Cl-).
Further information concerning the surfactant adsorption isotherms can be appreciated from the log-log coordinates (Figure 2). The primary advantage of using a log-log plot is that it amplifies the features of the isotherm at low surface excess values. The general form of isotherms plotted in this manner and the morphology of adsorbed structures associated with each region are depicted schematically in the inset of Figure 2. Somasundaran et al. [39, 40] proposed the four-region model attributed, respectively, to the adsorption of (I) monomers by electrostatic interactions, (II) surface aggregates (hemimicelles) up to the substratum charge compensation, (III) headout molecules by chain-chain interactions, and (IV) fully formed bilayer. Further increases in the solution surfactant concentration do not lead to any further increases in the surface excess. From the plot (Figure 2), only three regions are appreciable since the isotherms are limited to monolayer coverage (for both surfactants, CMC) without the complete formation of bilayers. The surfactant concentration relative to the beginning of hemimicelle formation is about 1.8 × 10-4 M and 8.0 × 10-6 M, respectively for DPC (A point) and GS3 (B point); these values could be compared with the bulk CMC values (DPC: 1.6 × 10-2 M, GS3: 1.5 × 10-3 M at C) obtained from conductimetric/tensiometric determinations [15, 21, 29–32], following the criterion reported by Fuerstenau and Jang  that hemimicelles may occur in about 1/100th of the CMC.
The trend between the two isotherms (the shift along the -axis, the variation of the slope in the different regions and the distribution of surface excesses) is the expected one due to the presence of a second homologous pyridinic ring in the case of GS3 species. For both surfactants, the slope of the plot in the Region I (the Henry region) is unity in the ideal situation (∆0 is constant) . The initial slope of the experimental isotherms cannot be determined accurately, but they are, within experimental error, almost equal to unity. The slope of the DPC curve increases passing from Region I to Region II, where hemimicelles form, indicating that the adsorption is enhanced by the formation of surface aggregates. This trend is typical for S-shaped isotherms . In the case of the gemini molecules the slope is, instead, depressed in the region of hemimicelles, possibly due to the steric hindrance.
3.2. Self-Aggregation Features of DPC and GS3 Surfactants
It is well known that many physicochemical properties of a solution show sudden changes when a characteristic concentration of a surfactant is exceeded. This change is attributed to the formation of colloidal aggregates and occurs over a relatively small concentration range characteristic for the surfactant. The formation of direct micelles of DPC in water and in aqueous salt solutions has been the object of numerous studies performed in time by different experimental approaches. By scattering techniques, DPC is reported to form in water, globular micelles with aggregation number of about 20, with a corresponding micellar radius of about 1.9 nm, comparable with the extended length of a C12 chain (1.67 nm) [30–32, 42, 43]. Spherical DPC micelles are not reported to aggregate in tridimensional structures . The corresponding gemini surfactant (GS3) having short spacer () has a particular behavior since it binds its counterions more strongly than the “single” does, as the -- ammonium gemini amphiphiles [44–47]. The lack of conformational freedom could be the result of the sharing of one counterion between the two pyridinium head groups . As a consequence, the short spacer surfactant (GS3) is probably aggregating in nonspherical micelles, but in elongated forms, like rods, as reported in the literature for ammonium gemini surfactants [48–50]. In particular, Manne et al.  found that simmetric gemini C12--C12 (≤6) surfactants composed by linear hydrocarbon tail with quaternary ammonium headgroups may assemble in hexagonal cylinders in silicate mesophase and parallel cylinders at the mica surface with average spacings of 4.2 0.4 nm. Thus, possibly cylinders in hexagonal arrangement for gemini spacer 3 could also occur at high surfactant concentrations.
3.3. Structural and Morphological Results of Surfactant-Assisted Titania Samples
Recently a great interest was devoted to the growth of titania particles with controlled morphology and porosity through template and template-free procedures [51–53]. Yu et al. obtained by a template-free preparation of TiO2 a fine control of macro and mesoporosity up to about C. For higher calcination temperatures, a total collapse of both surface area and porosity took place.
In the present work, the effects provoked by the concentration of both monomeric (DPC) and gemini (GS3) alkylpyridinium salts in the growth solution, on the features of the TiO2 particles are considered. Figure 3 reports the relative phase enrichment (histograms) at different surfactant concentrations of samples submitted to a hydrothermal growth step in the presence of the DPC and GS3, subsequently calcined at C. The anatase and rutile amount in the samples appears to depend on the concentration of the surfactant in the solution adopted for the ageing. When the concentration corresponds to the existence of nonaggregated surfactant units (1, 10 mM for DPC and 1, 2.5 mM for GS3), the anatase polymorph is prevalent; the crystallite sizes (Table 2) appear to be smaller than those in the absence of surfactant. The specific surface areas of the calcined oxide remain quite the same (Table 2). By increasing the surfactant concentration at values larger than the CMC (25 mM for DPC and 5 mM for GS3), the amount of anatase decreases and the anatase and rutile structures are almost equally promoted. The formation of rutile is paralleled by larger crystallite sizes and lower surface areas (Table 2). Moreover, the presence of GS3 at higher concentrations seems to preserve the surface area (11–14 g-1) and to limit the increase in rutile crystallite sizes (~75 nm) with respect to the DPC surfactant (5–8 g-1 and 85 nm, resp.).
These results are considered to be significant since after a calcination at C both the surface area and the total pore volume are still appreciable at variance with samples prepared by template-free procedures [51–53]. The trend in Figure 3 is in agreement with the generally reported lower surface energy of anatase with respect to rutile [54, 55] and with the consistent finding that the phase transformation to rutile occurs after the anatase grains have grown to a certain threshold size, of about 30–40 nm . Actually, during the hydrothermal step particle growth through Ostwald ripening can be expected to occur . When the growth occurs in the presence of a non-aggregated surfactant, the surface of the precursor particles is shielded from the deposition of soluble Ti species by the presence of the surfactant film. The growth is slightly depressed, the particles remain smaller and stable supporting the formation of anatase. At higher concentrations, the micelles may act as carriers for the soluble Ti containing species promoting the growth. Further, the micelles may be, also, adsorbed at the surface of different particles and may therefore promote bridging phenomena leading to larger particles.
These considerations are closely mirrored by the particle morphology as apparent in SEM micrographs (Figure 4). In the case of the sample prepared in the absence of the surfactant (Figure 4(a)), small spheroidal particles with an average size of about 30 nm grouped in raspberry-like aggregates can be appreciated; in the case instead of the sample grown at high DPC concentration (Figure 4b) the particles are much larger, with sizes ranging from about 100 nm (single crystals) and to about 200–300 nm as the result of sintering between crystallites. The large particles in Figure 4(b), show a particularly smooth surface. This effect is presumably the result of a surface annealing provoked by the heat released during the surfactant combustion. In the case of titania particles grown at high GS3 concentration, the shape of the aggregates remains spheroidal, characterized by a diameter of ~ 60–70 nm.
The mesoporosity induced by micelles and further three-dimensional organization of both surfactants can be appreciated by the hysteresis loop of the nitrogen adsorption—desorption isotherm (Figure 5), with respect to the reference material prepared without surfactant. The hydrothermal treatment in the presence of DPC micelles ( = 25 mM, Figure 5 (a)) leads to titania particles with bottle-neck shaped pores mainly in the range of nm (Figure 6 (a)), comparable with the average diameter of the globular DPC micelles (~4 nm) . When the particles are grown at , the shape of hysteresis loops is similar to the reference sample; the total pore volume increases, but the fraction of desired mesopores is not relevant. On the other hand, the presence of GS3 cylindrical micelles and three-dimensional hexagonal arrangements (sketches in Figure 6 (b)) produces an increase of total pore volume at increasing surfactant concentration (Figure 6 (b)), especially the fraction of mesopores with diameters in the range 6 20 nm. The shape of the hysteresis loop is typical of open-ended slit-shaped pores (Figure 5 (a)). A typical pore size distribution curve for the hydrothermally treated TiO2 in the presence of three-dimensional GS3 hexagonal arrangements is shown in Figure 7. A fairly narrow size distribution, 6–20 nm, is achieved for the present sample at variance with the untreated one (calcined at C), which shows almost no porosity. This result indicates that the hydrothermal template treatment of the TiO2 particles leads to their aggregation with evolution of a mesoporosity probably arising from the template structure, which is successively thermally decomposed. Part of the mesoporous structure can be appreciated in the HRTEM image of inset of Figure 7.
These results, together with the surface and morphological aspects, could be explained by invoking the different local heat of combustion (during the calcination step) for DPC and GS3, especially for higher surfactant concentrations. Possible ordered hexagonal GS3 arrangements with respect to unstructured spherical DPC micelles with random size distribution may lead to lower surfactant/oxide ratio in the template growth, even if the GS3 surfactant is formed by double aromatic ring.
The features of the adsorption isotherms of monomeric (DPC) and gemini (GS3) alkylpyridinium surfactants at the TiO2 solution interface are markedly different. DPC gives rise to a very diluted surface film, characterized by weak adsorbate/adsorbent interactions and repulsive lateral interactions. This behaviour is paralleled by a weak self-aggregation tendency, apparent in the sole formation of very small, globular micelles and no long range ordered mesostructures. The behaviour of the gemini, instead, is typical of strong (chemi- and physi-) interactions with TiO2 occurring in the absence of lateral interactions. The vertical orientation of the molecule, suggested on the grounds of the present adsorption data, is fully consistent with the formation of elongated rods and further hexagonal arrangements proposed for gemini surfactant mesostructures.
The above-mentioned specific features of the two surfactants are actually the reason why they produce different effects on the final oxide characteristics, especially on its mesoporosity.
For growths performed at concentrations larger than the CMC, very numerous small DPC micelles are randomly embedded into the oxide, giving rise, during the ensuing calcinations step, to large heats of combustion, particle sintering, and an almost uncontrolled mesoporosity.
In the case of gemini, instead, the relevant surfactant/oxide interactions allow the templating function of the surfactant to significantly occur. The much lower surfactant/oxide ratio than in the case of DPC, inherent in the rods versus globule structure, leads to a lower heat of combustion and to controlled particle shape, size, and mesoporosity.
The authors wish to thank Professor G. Viscardi and Professor P. Quagliotto of Università di Torino for the synthesis of the gemini spacer 3 (GS3) surfactant. This research has been supported by the University of Milan Research Funds (FIRST, PUR).
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