Interplay between Chemistry and Texture in Hydrophobic TiO2

ARTICLE pubs.acs.org/JPCC

Interplay between Chemistry and Texture in Hydrophobic TiO2 Hybrids D. Meroni,†,* S. Ardizzone,† G. Cappelletti,† M. Ceotto,† M. Ratti,† R. Annunziata,‡ M. Benaglia,‡ and L. Raimondi‡ † ‡

Dipartimento di Chimica Fisica ed Elettrochimica, Universit
a di Milano, Via Golgi 19, 20133 Milano, Italy Dipartimento di Chimica Organica Industriale, Universit
a di Milano, Via Venezian 21, 20133 Milano, Italy

bS Supporting Information ABSTRACT: Organic-nano-TiO2 hybrids are successfully employed in different fields, from biomaterials to self-cleaning processes and from sensing to photovoltaics. The specific wetting properties of these materials bear relevance to any application. Here, nano-TiO2 films with a multiscale roughness, as shown by AFM, are derivatized with tri- and bifunctional siloxanes, bearing either alkyl or aromatic end groups. Dynamic contact angle measurements show a marked dependence of the wettability on the siloxane nature, leading to different self-cleaning properties, as assessed by ATR-FTIR. The different behavior is rationalized on the grounds of the structure of the functionalized layer, by employing solid state 29Si CP-MAS NMR and EDX mapping. Semiempirical calculations of hybrids dipole moments in conjunction with solvent surface energy components allow us to explain the strongly different solvents wettability. This work brings attention to the importance of the hydrophobizing molecule nature and its chemical interactions for the interpretation of wettability versus surface texture.

’ INTRODUCTION In recent years, a great deal of effort has been devoted to tailoring the surface features of nanostructured titanium dioxide. This material has potential applications in several fields: from biomaterials to self-cleaning processes and from sensing to photovoltaic applications.1
3 The functionalization of nano-TiO2 by siloxanes takes place by mild reactions, resulting in robust covalent bonds between the oxide and the organic molecule upon hydrolysis of
OR0 moieties.4 The obtained materials are very promising. Organic
inorganic hybrid materials where the organic moieties are covalently bonded to the inorganic framework, are successfully employed as active layers in photovoltaic devices. There, the covalent bond between the chromophores and the silica
titania network appears to be essential in enhancing the electron transfer within the network and also between the network and the conducting substrate.3,5 Siloxanes may also serve as powerful linkers for different functionalities, providing a further possibility to tailor the material properties.6 The specific wetting features of these materials bring direct relevance to any application since the tuned localization of hydrophobic/hydrophilic patches may control numerous physical properties of the system, e.g., condensation of water from the gas phase or the transport/diffusion of ionic species or gas bubbles through porous networks.7,8 Wetting phenomena have been widely studied both theoretically and experimentally and the behavior of liquids wetting a r 2011 American Chemical Society

smooth solid surface is well understood.9 However, when the surface is rough, physics is much less clear. Roughness is unavoidable in real systems and it plays a pivotal role in many practical applications.10,11 Besides inherent roughness, real surfaces are often roughened on purpose in order to promote some physical property, i.e., enhance the extension of the surface area of the solid surface or promote adhesion between different materials. The aim of the present work is to tailor the wettability properties of a rough derivatized surface through a better understanding of the role played by functionalizing molecule structures and chemical interactions. The features of the hydrophobic layers deposited onto rough TiO2 films, composed by homemade nanometric particles, are investigated. The chosen functionalizing agents are three different kinds of siloxanes, both commercial and laboratory-made, with different end-groups. The adopted siloxane contents are selected in order to obtain loosely packed films. These have so far received little attention, despite the fact that they can lead to unique technological applications thanks to their enhanced flexibility and permeability.12,13 The morphology of the hybrid TiO2
siloxane composites and films is investigated by BET method and AFM analysis. The structure of the surface layer and the modes of attachment of Si Received: June 1, 2011 Revised: August 19, 2011 Published: August 19, 2011 18649

dx.doi.org/10.1021/jp205142b | J. Phys. Chem. C 2011, 115, 18649–18658

The Journal of Physical Chemistry C atoms are investigated by combining data of contact angle hysteresis, surface mapping by EDX, and solid state CP/MAS NMR analyses performed on samples with increasing siloxane content. NMR spectroscopy represents an innovative technique to investigate the fine structure of the hydrophobizing layer. This technique provides detailed information on the attachment modes of siloxanes at oxide surfaces, so far poorly studied in the literature. The chemical specificity introduced in the TiO2 hybrids by the different siloxane end-groups is discussed also on the grounds of the interactions of the hybrids with different solvents. A traditional texture approach is integrated with a more chemical one, where polar versus dispersive contributions of each solvent and siloxane dipolar moment are compared. Such fine characterizations will allow us to present a detailed picture of the chemical layers and draw conclusions about the layer structure and the consequent wettabilities behavior.

’ EXPERIMENTAL SECTION All of the chemicals were of reagent grade purity and were used without further purification; doubly distilled water passed through a Milli-Q apparatus was used to prepare solutions and suspensions. Siloxane Derivatives. The adopted siloxanes are both commercial (SILRES BS 1701) and laboratory-made molecules (triethoxy(p-tolyl)silane and dimethoxy(diphenyl)silane).

SILRES BS 1701 (named Si-Alk in the following) is a mixture of isomeric octyltriethoxysilanes with isooctyltriethoxysilane as the main component, which is produced and commercialized by Wacker Chemie AG. Triethoxy(p-tolyl)silane (named Si-Tol in the following) was synthesized as follows. A solution of n-BuLi 1.6 M (1.1 eq, 16.5 mmol) was added to a solution of bromoaryl derivative (1 eq, 15 mmol) in dry THF (20 mL) at
60 °C under nitrogen atmosphere. After 30 min of stirring at this temperature, this solution was added to a solution of tetraethyl orthosilicate (8 eq, 120 mmol) dissolved in dry THF (5 mL) and cooled at
30 °C. After addition was completed, the reaction mixture was allowed to warm to room temperature, stirred for 18 h, quenched by addition of water (10 mL), and diluted with AcOEt (20 mL). The organic layer was then separated, washed with a saturated aqueous solution of NH4Cl (2  10 mL), and dried over Na2SO4; the solvent was removed by rotary evaporation. After drying, a pale yellow oil was obtained that was purified by fractional distillation at reduced pressure.

ARTICLE

Dimethoxy(diphenyl)silane (named Si-biPh in the following) was synthesized according to the following procedure. A solution of diphenyl dichloro silane (100 mmol) in ethanol was stirred at 25 °C for 24 h; the solvent was removed by rotary evaporation. After drying, a pale yellow oil was obtained that was purified by fractional distillation at reduced pressure (89% yield). The 1H and 13C NMR spectra of the synthesized siloxanes were acquired on a Bruker Avance 500 spectrometer, operating at 500.130 MHz (1H) and at 125.00 MHz (13C). The 13C {1H} spectra were obtained using Waltz decoupling and were exponentially multiplied to give 0.8 Hz line broadening before Fourier transformation. A complete numerical listing of 1H/13C NMR peaks in support of the assigned structure is reported in the Supporting Information (SI) section. Siloxane-Clad TiO2 Films. Nanometric TiO2 powders were prepared by a sol
gel synthesis, according to the procedure reported previously by us,14 and subsequently thermally treated at 300 °C for 5 h under O2 stream (9 NL/h). Then, nano-TiO2 was functionalized with siloxane molecules as follows. A total of 0.2 g of TiO2 powder was dispersed in 10 mL of 2-propanol using a sonication treatment. Then a variable amount of siloxane was added to the titania dispersion under vigorous stirring. The amount of added siloxane was calculated on a weight basis, in the range 0
29 wt %. The solvent was removed by means of a vacuum oven (30 min at p = 400 mbar, T = 40 °C). The powders were suspended in 2-propanol and thin films were deposited by spin coating the resulting suspension onto previously cleaned glass slides. Sample names refer to the kind of siloxane used as coating (Si-Alk, Si-Tol, Si-biPh), with the employed siloxane amount (%) reported under brackets. Bare TiO2 nanoparticles are referred to as T. Sample Characterization. Dynamic contact angle (advancing (θa) and receding (θr)) measurements of hybrid siloxaneTiO2 films were performed on a Kr€uss Easy Drop using several high purity solvents (water, toluene, glycerol, ethylene glycol). A drop of 3 μL was produced and then gently placed on the surface, then the drop size was changed with a speed of 15 μL/min. Movies with 150 images were recorded. The drop profile was extrapolated using an appropriate fitting function. Young contact angle θY values were obtained from advancing (θa) and receding (θr) angles using the procedure reported by Tadmor.15 Self-cleaning tests were performed on siloxane-clad titania films. TiO2 layers were stained by an aqueous solution of alizarin red (0.02 M). The dye interaction with the film surface was evaluated by Fourier transform infrared spectroscopy [FTIR; Jasco 4200, accessorized by attenuated total reflectance (ATR) module]. The mode of attachment of the siloxane molecules to the TiO2 surface was investigated by the solid state NMR spectroscopy. 13C and 29Si solid state cross-polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectra were obtained at 125.62 and 99.36 MHz, respectively, on a Bruker Avance 500 spectrometer, equipped with a 4 mm magic angle spinning (MAS) broadband probe (spinning rate up to 4 kHz). The CP/MAS spectra were recorded on solid samples, typically 0.15 g. Each sample was packed into a 4-mm MAS rotor (50 μL sample volume) spinning at 4 kHz and at a temperature of 300 K; no resolution improvement was found at higher spinning rate and/or temperature. Pulse employed in the direct polarization (DP) and cross-polarization (CP) 18650

dx.doi.org/10.1021/jp205142b |J. Phys. Chem. C 2011, 115, 18649–18658

The Journal of Physical Chemistry C

ARTICLE

Figure 1. (a) AFM 3D image of the bare TiO2 on glass substrate; EDX mapping of carbon species for (b) T_Si-Alk, (c) T_Si-Tol, and (d) T_Si-biPh hybrid samples at 5% siloxane content.

experiments were optimized using the Si-AlK sample. A variable amplitude method was used for recording the 29Si spectra, with a CP contact time of 1 ms, a pulse delay of 20.0 s and 20000 scans. The 13C spectra of Si-Alk were acquired in DP proton decoupled mode, with a pulse delay of 2.0 and 10000 scans. In the case of the aromatic substituted materials T_Si-Tol and T_Si-biPh, the CP method was used with CP contact time of 3 ms, a pulse delay of 5.0 s, and 8000 scans. The 13C and 29Si chemical shifts were externally referenced to TMS. The powder 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 Barrett
Joyner
Halenda (BJH) method. Titania films were morphologically characterized by Scanning Electron Microscopy (SEM HITACHI TM-1000) equipped with Energy-Dispersive X-ray spectroscopy (EDX, Hitachi ED3000), and by Atomic Force Microscopy (AFM). For the evaluation of the roughness factor and surface topography of the samples, an AFM microscope (NTMDT Solver PRO-M) working in tapping mode was used; the roughness factor was obtained on areas of 5  5 μm2. Computational Setup. Calculations to assess the siloxane dipole moment were performed at a semiempirical level with the PM6 Hamiltonian (as implemented in the Gaussian 09 package16). All calculations were run for each compound in vacuum for the most stable conformation, as located with molecular mechanics techniques and MMFFs force field with a stochastic Monte Carlo analysis of the potential energy surface. All PM6 minima were characterized as such by performing a vibrational analysis.

’ RESULTS Nano-TiO2 Films Morphological Features. The siloxane addition does not induce bulk structural effects, as siloxane
TiO2 interactions are of surface nature and no thermal treatment follows the deposition of the siloxane layer.4 On the contrary, the progressive siloxane coverage of TiO2 affects noticeably the morphological features of the oxide, such as its specific surface area. For the three siloxanes, the sample surface areas decrease progressively starting from low contents. This effect is much larger for the two aromatic molecules: At 29% coverage the specific surface area of T_Si-Alk is around 80 m2 g
1 (about onethird of the starting value), while for the T_Si-Tol and T_Si-biPh at the same coverage the surface area is reduced to a few square meters per gram (Table 1SI). A marked decrease of the pore volume, especially concerning the smallest mesopores (d < 6 nm), can also be appreciated with increasing siloxane content (Figure 1SI). Interestingly, the larger decrease is shown by the two aromatic molecules. These effects may suggest that the specific loss of surface area can be mainly ascribed to a relevant hindrance produced by the organic moieties of the siloxanes in the pores of the oxide particles. The AFM 3D (Figure 1a) image of the bare TiO2 film shows the complete coverage of the glass substrate by the titania film. The surface presents a multiscale roughness, due to the presence of both nanometric particles and micrometric aggregates. The roughness values were statistically calculated from images taken with a 5 μm  5 μm measurement scale. The average roughness and the root-mean-square (rms) are 120 and 150 nm, respectively. The presence of the siloxane molecules does not significantly modify the roughness features. Instead, when the distribution of 18651

dx.doi.org/10.1021/jp205142b |J. Phys. Chem. C 2011, 115, 18649–18658

The Journal of Physical Chemistry C

ARTICLE

Figure 2. Sketches of (a) ideal Young contact angle and contact angles on rough surfaces according to (b) Wenzel and (c) Cassie
Baxter descriptions.

carbon species is analyzed, striking differences among the different samples can be appreciated, especially at low siloxane content. EDX spectra relative to carbon species of the three siloxane hybrid films at 5% coverage (Figure 1b
d) reveal that the features of the hydrophobizing layers do not simply reflect the texture of the substrate but are instead mainly affected by the molecule arrangement. In the case of T_Si-Alk, carbon species appear to be uniformly distributed over all of the layers; for T_Si-Tol the distribution is again rather uniform, with only few small brighter spots showing the presence of some carbon enriched areas. The film formed by T_Si-biPh, instead, presents large light spots. This suggests the presence of a high level of C-species generated by the nonuniform distribution of the molecule over the TiO2 layer. Wettability of Nano-TiO2
Siloxane Films. Rough surfaces show a different wetting behavior with respect to ideal Young wettability (Figure 2a), arising from the joint effect of the surface chemistry and texture. Due to the presence of in-homogeneities, two different possible conditions can occur: either the spreading of the liquid follows the actual surface profile (Figure 2b) or it leaves air inside the texture (Figure 2c). In the first case (Wenzel description, Figure 2b), the increase in surface area, arising from the presence of the texture, provokes an increase in the material hydrophobicity/hydrophilicity. The relationship between the actual contact angle θw on this surface and the ideal Young contact angle θY (smooth surface) is mediated by the solid roughness r, defined as the ratio between the true surface area over the apparent one (r is a number larger than unity) cos θw ¼ r cos θY

ð3Þ

In a Wenzel mode, the difference between advancing and receding contact angles (contact angle hysteresis, Δθ = θa
θr) can be very large. Indeed, during receding contact angle measurements, as the contact line retreats, it leaves liquid in the cavities. As a result, the receding drop contacts the fraction of liquid left in the textures, yielding significantly lower contact angles. With increasing the surface roughness of a hydrophobic material, the energy required for following the solid surface is much larger than the energy associated with the air pockets. In this state (first suggested by Cassie and Baxter), the liquid only contacts the solid through the top of the asperities (Figure 2c). In this condition the corresponding hysteresis is observed to be very low, since the liquid has scarce interactions with the solid.17,18

Figure 3. Advancing (a and c) and receding (b and d) angles for T_SiAlk(13%) and T_Si-biPh(13%) samples.

In the present case, water drops were deposited on each siloxane-clad TiO2 film, and both the advancing and receding angles, θa and θr, were measured as a function of the type of hydrophobizing molecule and of its content. Figure 3a-d report as an example the advancing and receding angles for T_SiAlk(13%) and T_Si-biPh(13%) samples. The two molecules give rise to sharp differences in contact angle hysteresis: T_Si-Alk(13%) shows almost no hysteresis, while T_Si-biPh(13%) presents a large difference between advancing and receding angles. Interestingly, the three molecules give rise to completely different water contact angle patterns as a function of their content in the hybrid film (Figure 4). The T_Si-Alk shows, starting from low coverage, high hydrophobicity characterized by very high contact angles and a small hysteresis between advancing and receding angles. Conversely, the T_Si-Tol presents at low siloxane content a lower contact angle and an appreciable hysteresis. With increasing the siloxane amount, advancing contact angles get similar to those observed for T_Si-Alk samples, but a larger hysteresis is appreciable for all contents. In the case of the T_Si-biPh, at the lowest siloxane coverage, both advancing and receding angles suggest the formation of a hydrophilic surface. For increasing the siloxane content, both angles increase while the contact angle hysteresis remains relevant (Δθ > 50°). Only at the highest siloxane amount, the contact angle hysteresis becomes small and comparable to those of T_Si-Tol samples. The sharply different behavior shown by the three siloxane molecules in Figure 4 could be explained in terms of different wetting regimes of the solid by water as a consequence of the different surface texture. The trend of T_Si-Alk may represent a condition characterized by high contact angles and a small hysteresis, in which water floats over a solid/gas carpet. The situation for T_Si-Tol is similar to that of T_Si-Alk, with a less uniform disposition of siloxanes at low coverage (