Understanding the Role of Cyclodextrins in the Self-Assembly

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Understanding the Role of Cyclodextrins in the Self-Assembly, Crystallinity, and Porosity of Titania Nanostructures Rudina Bleta, Anthony Lannoy, Cécile Machut, Eric Monflier, and Anne Ponchel* UCCS, UMR-CNRS 8181, Faculté des Sciences Jean Perrin, Université d’Artois, Rue Jean Souvraz, SP 18, F-62307 Lens, France S Supporting Information *

ABSTRACT: A series of mesoporous titania photocatalysts with tailorable structural and textural characteristics was prepared in aqueous phase via a colloidal self-assembly approach using various cyclodextrins (CDs) as structuredirecting agents. The photocatalysts and the structuredirecting agents were characterized at different stages of the synthesis by combining X-ray diffraction, N2-adsorption, field emission scanning electron microscopy, transmission electron microscopy, UV−visible spectroscopy, dynamic light scattering, and surface tension measurements. The results demonstrate that the cyclic macromolecules efficiently direct the selfassembly of titania colloids, resulting in a fine-tuning of the crystal phase composition, crystallite size, surface area, particle morphology, pore volume, and pore size. Depending on the chemical nature of the substituents in the cyclodextrin ring, synergistic or competitive effects arising from the adsorption capacity of these cyclic oligosaccharides onto titania surface, surfaceactive properties, and ability to aggregate in water by intermolecular interactions were found to substantially impact the characteristics of the final material. We propose that, in contrast to the native cyclodextrins, which tend to favor the local agglomeration of titania nanoparticles due to the strong intermolecular interactions, the substitution of hydroxyl groups by a relatively large number of methoxyl or 2-hydropropoxyl ones in the β-CD derivatives allows for creating smoother interfaces, thus facilitating the self-assembly of the colloids in a more homogeneous network. The photocatalytic activity of those titania materials was evaluated in the photodegradation of a toxic herbicide, phenoxyacetic acid, and was correlated to the structural and textural characteristics of the photocatalysts.

1. INTRODUCTION

So far, template-directed colloidal self-assembly has been successfully employed for the preparation of a variety of inorganic materials, especially semiconductor metal oxide nanoparticles, with morphologies depending on the characteristics of both the template and the colloidal nanocrystals.14,15 Among the various semiconductors, titanium dioxide (TiO2) has been applied as one of the most promising photocatalysts for the removal of industrial organic pollutants from water and air16,17 as well as for the photocatalytic water splitting for hydrogen production.18 The three well-known polymorphs of TiO2 are anatase (tetragonal, I41/amd), brookite (orthorhombic, Pbca), and rutile (tetragonal, P42/mnm). Bulk rutile is the only thermodynamically stable phase, while bulk anatase and bulk brookite are metastable.19,20 Nevertheless, under controlled conditions, anatase and brookite can be thermodynamically stabilized when the particle size is below 11 nm for the former and between 11 and 35 nm for the latter.19 Anatase is the most extensively studied polymorph owing to its higher photocatalytic activity under UV light.21,22 However, synergistic effects between anatase and rutile have been noted by several authors and attributed to the triggering of the

Self-assembly has recently emerged as a flexible and powerful strategy for organizing simple entities, such as molecules, macromolecules, and colloidal particles, into ordered structures with a high level of complexity.1 In particular, the colloidal selfassembly approach, employing colloidal particles as basic building blocks, has proved to be hugely successful in generating robust materials with hierarchical order and complexity over several discrete and tunable length scales.2−7 Colloidal self-assembly can be directed, enhanced, or controlled by either tailoring the intrinsic characteristics of the nanoparticles (e.g., their size and shape), applying external directing fields (e.g., electric, magnetic, and flow), or using organic templates to guide the spatial distribution of the nanoparticles (e.g., block copolymers or biopolymers).8−10 In this latter situation, nanoparticles are held together either by weak noncovalent forces, such as hydrogen bonding,10,11 or by strong covalent bonds via different functional groups fixed onto the nanoparticle surface.12,13 Polymers are traditionally used as capping agents to prevent nanoparticle aggregation and flocculation.8,10 The interactions between polymer chains bridging particles and between polymer and colloid surface may be determinant for directing the assembly of the nanoparticles into ordered structures.8 © 2014 American Chemical Society

Received: July 23, 2014 Revised: September 3, 2014 Published: September 15, 2014 11812

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electron and hole transfer between these two phases.23,24 The surface area and degree of crystallinity also play an important role in the photocatalytic process.25−27 Therefore, a high surface area favors the interactions between the organic pollutant and the active sites on the photocatalyst surface, while a high content of the crystalline phase limits the recombination rate of photoexcited electrons and holes. To obtain well-crystallized titania, the material is usually subjected to high thermal treatment temperature during which the crystallites of metastable anatase and brookite coarsen and grow very fast and then transform to rutile when a critical size is reached.20 Such phase transformation is nonreversible due to the greater thermodynamic stability of rutile, which generally provokes a deterioration of the framework and sometimes a complete collapse of the nanostructure.28,29 Utilizing organic templates for directing the self-assembly of nanoparticles not only leads to higher particle stability but also allows for enhanced control over pore structure, pore volume, and surface area. So far, a large variety of soft templates,30,31 hard templates,32,33 or a combination of block copolymer and solid scaffolds such as silica or alumina34 have been explored, generating titania materials with tuned porosity and high thermal stability. Among the large variety of soft templates, cyclodextrin-based assemblies are of great interest owing their multifunctional properties, such as the formation of supramolecular adducts or host−guest inclusion complexes with a large number of molecules of appropriate size and shape.35,36 Cyclodextrins (CDs) are water-soluble cyclic oligosaccharides formed of six (α-CD), seven (β-CD), or eight (γ-CD) glucopyranose units exhibiting a hydrophobic internal cavity and a hydrophilic exterior surface due to the presence of a large number of hydroxyl groups. Although the possibility of utilizing cyclodextrins or supramolecular assemblies formed between block copolymers and cyclodextrin derivatives as soft templates to direct the synthesis of hierarchically structured porous silica37−40 and alumina41,42 has been explored so far, there is a limited number of reports devoted to titania.43,44 Additionally, to our knowledge, no systematic study on the effect of the chemical nature and concentration of the cyclodextrin has been performed so far utilizing the colloidal self-assembly strategy. In the present study, we have undertaken a detailed investigation of the effect of five cyclodextrins, natives (α-CD, β-CD, and γ-CD) or modified [(2-hydroxypropyl)-β-CD and randomly methylated β-CD] [structures available in Figure S1, Supporting Information (SI)], on the porosity, crystallinity, and photocatalytic activity of mesoporous titania. The insights obtained from this study allow a fundamental understanding of the important correlation that exists between the physicochemical properties of these macrocyclic oligosaccharides in water and the structural and textural characteristics of the resulting catalysts. In what follows, these characteristics will be described together with the results obtained on the photocatalytic degradation of phenoxyacetic acid (PAA), a widely utilized herbicide, frequently detected in natural water.45

of molar substitution (DS) of 0.6 and average MW 1380 g/mol] was purchased from Sigma-Aldrich. Randomly methylated β-cyclodextrin (denoted RAMEB, with an average DS of 1.8 and average MW 1310 g/mol) was a gift from Wacker. Titanium isopropoxide [Ti(OiPr)4, MW 284.3 g/mol, d 0.96 g/cm3], nitric acid (HNO3, 68%), and PAA (MW 152.15 g/mol) were procured from Sigma-Aldrich. All chemicals were used as received without further purification. 2.2. Synthesis of Titania Sols. Titanium dioxide nanoparticles were synthesized according to a previously reported sol−gel method.31 Typically, in a 250 mL flask, 30 mL (0.1 mol) of Ti(OiPr)4 was dissolved in 27 mL (0.35 mol) of 2-propanol. Then, 160 mL of hot distilled water was added rapidly at 85 °C under vigorous stirring at a hydrolysis ratio (h = [H2O]/[Ti]) of 88. After 15 min, 1.3 mL of nitric acid ([HNO3]/[Ti] = 0.2) was added dropwise to peptize the hydroxide precipitate. The mixture was maintained under reflux at 85 °C for 16 h. The final product was a stable translucent suspension of titanium dioxide nanoparticles composed of anatase (68%) and brookite (32%) (DRX available in Figure S2, SI). In parallel, 10 mL aliquots of cyclodextrin solution containing various amounts of RAMEB were mixed with 10 mL aliquots of the titania sol (RAMEB/Ti molar ratio = 0.015−0.138). The mixtures were stirred for 30 min and then were allowed to equilibrate at room temperature for 24 h. In some experiments, α-CD, β-CD, γ-CD, and HP-β-CD were utilized as structure-directing agents according to a selected CD/Ti molar ratio of 0.076, except for β-CD (β-CD/Ti = 0.032) due to its low solubility in water (18 mg/mL). Xerogels were recovered after drying the samples by evaporation at 60 °C for 48 h, after which time they were calcined in air at 500 °C for 2 h using a heating ramp of 5 °C/min. The final mesoporous titania materials were denoted CDx, where CD represents the nature of the cyclodextrin used and x the CD/Ti molar ratio multiplied by 1000. For example, RAMEB46 indicates a mesoporous titania prepared with a RAMEB/Ti molar ratio of 0.046, whereas α-CD76 indicates a mesoporous titania prepared with a α-CD/Ti molar ratio of 0.076. 2.3. Characterization Methods. 2.3.1. Powder X-ray Diffraction. Powder X-ray diffraction data were collected on a Siemens D5000 Xray diffractometer in a Bragg−Brentano configuration with a Cu Kα radiation source. Scans were run over the angular domains 10° < 2θ < 80° (for calcined titania) or 5° < 2θ < 80° (for CD/TiO2 xerogels and for neat cyclodextrins) with a step size of 0.02° and a counting time of 2 s/step. Crystalline phases were identified by comparing the experimental diffraction patterns to Joint Committee on Powder Diffraction Standards (JCPDS) files for anatase, brookite, and rutile. The refinement of the diffractograms was performed using the FullProf software and its graphical interface WinPlotr. Profile matching refinement was used to determine unit cell parameters, background, peak shape, and zero shift. The quality of the fit was determined visually by inspection of the difference plot and statistically by the goodness of fit (χ2), defined by χ2 =

∑ wi(yio − yic )2 /(N − P)

where wi is the weight assigned to each observation; yio and yic are the observed and calculated intensities, respectively, at the ith step; N is the number of points used in the refinement; and P is the number of least-squares parameters refined. The refinement was considered satisfactory when χ2 was less than 4. The average crystallite size D was calculated from the Scherrer formula, D = Kλ/(β cos θ), where K is the shape factor (a value of 0.9 was used in this study, considering that the particles are spherical), λ is the X-ray radiation wavelength (1.540 56 Å for Cu Kα), β is the full width at half-maximum (fwhm), and θ is the Bragg angle. In addition, Rietveld refinements over a shorter 2θ range of 20°−35° were also performed to determine the content of each polymorph. In this mode of refinement, only the scale factor was allowed to vary. Typical results obtained from profile matching and Rietveld refinements are shown in Figure S3 (SI). 2.3.2. Nitrogen Adsorption−Desorption. Nitrogen adsorption− desorption isotherms were collected at −196 °C using an adsorption analyzer (Micromeritics Tristar 3020). Prior to analysis, 200−400 mg

2. EXPERIMENTAL SECTION 2.1. Chemicals. Native α-cyclodextrin (denoted α-CD, average MW 973 g/mol) and γ-cyclodextrin (denoted γ-CD, average MW 1297 g/mol) were purchased from Wacker Chemie GmbH (Cavamax, Burghausen, Germany). Native β-cyclodextrin (denoted β-CD, average MW 1135 g/mol) was a gift from Roquette Frères (Lestrem, France). (2-hydroxypropyl)-β-cyclodextrin [denoted HP-β-CD, average degree 11813

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samples were outgassed at 320 °C overnight to remove the species adsorbed on the surface. From N2-adsorption isotherms, specific surface areas were determined by the BET method and pore size distributions were calculated using the NLDFT (nonlocal density functional theory) model assuming a cylindrical pore structure. 2.3.3. Field Emission Scanning Electron Microscopy. Field emission scanning electron microscopy (FE-SEM) observations were performed to examine the morphology of the samples using an FEG Hitachi S-4700 field-emission microscope operating at 5 kV. Before imaging, samples were covered with a thin layer of carbon to reduce the accumulation of charges at high magnification. 2.3.4. Transmission Electron Microscopy. Transmission electron microscopy (TEM) observations were performed on a Tecnai microscope operating at an accelerating voltage of 200 kV at medium magnification. A drop of titania powder suspension dispersed in ethanol was deposited on a carbon-coated copper grid. 2.3.5. Diffuse Reflectance UV−Visible Spectra. Diffuse reflectance UV−visible (DR UV−vis) spectra were collected using a PerkinElmer Lambda 19 UV−Vis NIR spectrometer. BaSO4 was used as the reference. The band gap energies (Eg) were calculated using the equation Eg = (1239/λ) eV, where λ is the wavelength in nm. 2.3.6. Dynamic Light Scattering. Dynamic light scattering (DLS) measurements were performed at 25 °C with a Malvern Zeta Nanosizer instrument equipped with a 4 mW He−Ne laser operating at 633 nm and using a backscattering detection system (scattering angle θ = 173°). Samples were filtered through a 0.2 μm Millipore filter before being analyzed. 2.3.7. Surface Tension Measurements. Surface tension measurements were performed at 25 °C on a Dataphysics OCA 15EC tensiometer using the pendant drop method. Average values of surface tension were obtained from at least three consecutive measurements. The surface tension of double distilled water was measured before each measurement until a value of 72 mN/m was reached. 2.3.8. Photocatalytic Efficiency. The photocatalytic efficiency of the titania materials was evaluated in the photodegradation of PAA.45 The experiments were carried out using a Pyrex cylindrical reactor equipped with a quartz window for the entrance of the UV irradiation. In a typical experiment, 50 mg of photocatalyst (0.25 g/L) was introduced to a 0.15 g/L PAA solution and was maintained under stirring in the dark for 30 min to establish adsorption−desorption equilibrium. Then, UV irradiation was performed using a 6 W UV lamp (Helios Italquartz) with a maximum emission at 352 nm. Aliquots were centrifuged at regular intervals and the substrate concentration in the supernatant was determined by high-performance liquid chromatography (HPLC, PerkinElmer) analyses. The column used was a PerkinElmer Pecosphere C18 (83 mm length × 4.6 mm diameter). A mixture of acetonitrile (85%, v/v) and deionized water (15%, v/v) acidified with 0.2% acetic acid was used as the mobile phase at a flow rate of 1.5 mL/min. Aliquots of 10 μL of the sample were injected and analyzed at a wavelength of 360 nm using a photodiode array detector. The PAA degradation rate, given in percentage, refers to the difference in the PAA concentration before irradiation (C0) and after 7 h of irradiation (C7h) divided by the PAA concentration before irradiation; i.e., 100 × (C0 − C7h)/C0.

Figure 1. XRD patterns of sol−gel titania prepared without template (a) and with increasing RAMEB/Ti molar ratios: 0.015 (b), 0.046 (c), 0.076 (d), 0.107 (e), and 0.138 (f). Samples were calcined at 500 °C. The “A”, “B”, and “R” in the figure denote the anatase, brookite, and rutile phases, respectively.

determined from Rietveld refinement, is high (38% R vs 35% A, and 27% B) (Table 1), which may be due to the small size of the crystallites in the initial titania hydrosol [6.6 nm (A) and 5.3 nm (B), Table S1, SI]. Indeed, Zhang and Banfield19 have shown that the phase transformation in titania is sizedependent. For smaller particles, it takes place at lower temperatures, while with an increase in the initial particle size, the transformation temperature increases. In our materials, such phase transformation is also accompanied by a fast increase in the crystallite size to 36 nm (A), 19 nm (B), and 60 nm (R), as determined from the Scherrer formula (Table 1). Adding the randomly methylated β-CD to the titania hydrosol (RAMEB15 sample) induces a significant decrease in the intensity of the rutile reflections, indicating a delay in the phase transformation. This effect becomes even more pronounced upon addition of increasing amounts of cyclodextrin. For instance, using a RAMEB/Ti molar ratio of 0.076, the rutile content drops sharply to 1.7%, while the anatase content increases to 54%. Interestingly, the size of the rutile crystallites notably decreases to 16 nm, and then the size parameters are not further altered in the range of RAMEB/Ti molar ratios between 0.076 and 0.138 (Table 1). These results indicate that the cyclic molecule creates an efficient barrier to agglomeration of the particles, preventing the grain growth and delaying the phase transformation during heat treatment. The N2-adsorption analyses (Figure 2) show that, for the lowest RAMEB/Ti molar ratio (0.015), the surface area and pore volume are only slightly affected by the cyclic molecule, while the pore size distribution (PSD) is already significantly altered, becoming broadened and showing a shift of the maximum toward larger pores, from 5.3 to 6.8 nm. For a RAMEB/Ti molar ratio of 0.046 and above, a very strong effect is produced. Thus, for the RAMEB76 sample, the

3. RESULTS AND DISCUSSION 3.1. Effect of Addition of the Randomly Methylated βCyclodextrin. The X-ray diffraction (XRD) patterns of the mesoporous titania materials prepared with increasing RAMEB/Ti molar ratios (from 0.015 to 0.138) and calcined at 500 °C are shown in Figure 1. The control sol−gelsynthesized titania (i.e. prepared without cyclodextrin) is added for comparison. It can be noticed that, in addition to the reflections of anatase (A) (JCPDS card no. 00-021-1272) and brookite (B) (JCPDS card no. 01-076-1934), the control sol− gel TiO2 presents an intense, sharp peak at 2θ = 27.4° corresponding to the (110) plane of the rutile (R) (JCPDS card no. 00-034-0180). The rutile content in this sample, 11814

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Table 1. Structural and Textural Parameters of the Different Mesoporous Titania Materials Prepared through CyclodextrinDirected Colloidal Self-Assembly and Calcined at 500 °C anatase sample

CD/Tia

sol−gel TiO2

brookite

csb (nm)

ctc (%)

csb (nm)

36.4

35.3

18.7

RAMEB15 RAMEB46 RAMEB76 RAMEB107 RAMEB138

0.015 0.046 0.076 0.107 0.138

25.3 18.3 14.5 13.8 14.5

47.5 56.8 53.7 50.4 51.3

19.5 14.2 10.7 8.6 8.9

α-CD76 β-CD32 γ-CD76 HP-βCD76

0.076 0.032 0.076 0.076

17.2 18.5 16.3 13.8

47.9 44.7 47.7 52.0

12.6 13.8 12.1 9.8

rutile

ctc (%)

csb (nm)

N2 adsorption ctc (%)

26.5 60.4 38.2 Effect of Addition of RAMEB 30.1 48.2 22.4 38.9 22.1 4.3 44.6 15.7 1.7 46.1 16.9 3.5 45.1 15.6 3.6 Effect of the Nature of the CD 41.6 20.6 10.5 43.6 25.7 11.7 42.5 23.0 9.8 47.0 14.4 1.1

UV−vis

SBETd (m2/g)

pve (cm3/g)

psf (nm)

Egg (eV)

21

0.03

5.3

3.03

31 75 115 80 76

0.05 0.20 0.30 0.23 0.17

6.8 10.7 11.4 12.0 11.9

3.07 3.11 3.16 3.16 3.16

68 52 65 70

0.13 0.10 0.12 0.17

7.0 7.0 7.0 9.5

3.09 3.03 3.09 3.12

a

Cyclodextrin/Ti molar ratio in the sol. bCrystallite size calculated from the Scherrer formula. cPolymorph content determined from Rietveld refinements. dSpecific surface area determined by the BET method in the relative pressure range of 0.1−0.25. ePore volume computed by NLDFT. f Pore size determined by NLDFT. gBand gap energies calculated using the equation Eg = (1239/λ) eV.

show a blue-shift of the absorption edge toward shorter wavelengths, consistent with the delay in rutile formation. Thus, for the titania materials prepared with a RAMEB/Ti molar ratio of 0.046 and above, the extrapolation of the absorption edge yields band gap values in the range of 3.11− 3.16 eV, close to the Eg of anatase (3.2 ± 0.1 eV),46 which is the predominant polymorph (50−57%) in almost all those titania materials (Table 1). Taken together, our results clearly indicate that the randomly methylated β-cyclodextrin acts as an efficient structure-directing agent for generating titania nanostructures with well-defined crystalline framework and tunable pore size, pore volume, and surface area. Interestingly, a control over the characteristics of these materials may be achieved by adjusting the RAMEB/Ti molar ratio in the titania hydrosol. 3.2. Effect of the Nature of the Cyclodextrin. Given that the randomly methylated β-CD alone drastically improves the structural and textural characteristics of titania, we wondered whether other cyclodextrins, natives or modified, could produce similar effects. For that purpose, additional experiments were carried out with three native cyclodextrins (α-CD, β-CD, and γCD) and with a β-CD derivative, the (2-hydroxypropyl)-β-CD (HP-β-CD), in addition to RAMEB. As the sample prepared with a RAMEB/Ti molar ratio of 0.076 presented the best textural characteristics, the effect of the nature of the cyclodextrin was examined using the same CD/Ti molar ratio of 0.076, except for the native β-CD (β-CD/Ti = 0.032) due to its limited solubility in water. From the XRD patterns of titania materials calcined at 500 °C (Figure 3, Table 1), it can be noticed that the crystallite size and phase content are affected to different extents depending on the nature of the cyclodextrin employed. Indeed, while the materials prepared with the three native cyclodextrins contain approximately 10−12% rutile, the content of this polymorph in titania prepared using HP-β-CD and RAMEB drops to 1.1 and 1.7%, respectively. On the other hand, the size of the crystallites gradually increases following the order RAMEB ≈ HP-β-CD < γ-CD ≈ α-CD < β-CD. For example, for the titania material prepared using β-CD, the size of the rutile crystallites is ca. 26 nm, while it decreases to ca. 16 and 14 nm for the materials prepared with RAMEB and HP-βCD, respectively.

Figure 2. N2 adsorption isotherms with corresponding pore size distribution plots (inset) of sol−gel titania prepared without template (a) and with increasing RAMEB/Ti molar ratios: 0.015 (b), 0.046 (c), 0.076 (d), 0.107 (e), and 0.138 (f). Samples were calcined at 500 °C.

surface area is more than quintupled compared to the control TiO2 (115 vs 21 m2/g), the pore volume is multiplied by 10 (0.30 vs 0.03 cm3/g), and the pore size is more than doubled, reaching 11.4 nm. Contrarily, increasing the RAMEB/Ti molar ratio beyond 0.076 produces an opposite trend on the surface area and pore volume, while the PSD is not significantly altered, remaining centered at 11.5−12.0 nm. Further evidence for the effect of the randomly methylated βcyclodextrin on the crystal phase composition is provided from DR UV−vis spectra (Figure S4, SI). Thus, for the control sol− gel TiO2, the energy gap (Eg) calculated from the extrapolation of the absorption edge into the energy axes is found to be 3.03 eV, which is close to the Eg of rutile (3.0 ± 0.1 eV).46 Upon addition of increasing amounts of cyclodextrin, the spectra 11815

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with β-CD and γ-CD. Interestingly, the porosity is further enhanced when the modified cyclodextrins are used as structure-directing agents, and the most relevant textural characteristics are obtained for titania prepared with RAMEB, presenting a specific surface area of 115 m2/g, a pore volume of 0.3 cm3/g, and a pore size of 11.4 nm. The changes observed in crystal phase composition are also confirmed by DR UV−vis spectroscopy (Figure S5, SI). Thus, the blue shift of the absorption edge toward shorter wavelengths upon addition of various cyclodextrins is consistent with the delay in rutile formation evidenced by XRD (Table 1). From the representative FE-SEM and TEM images of the materials prepared without and with cyclodextrins, it can be noticed that the cyclic macromolecule has also a strong effect on the morphology of titania catalyst (Figure 5). Thus, while

Figure 3. XRD patterns of titania prepared without template (a) and with a CD/Ti molar ratio of 0.076 for α-CD (b), γ-CD (d), HP-β-CD (e), and RAMEB (f) and of 0.032 for β-CD (c).

Significant modifications are also observed on the textural characteristics of the photocatalysts (Figure 4, Table 1). For instance, using α-CD, the specific surface area increases from 21 to 68 m2/g, the pore volume from 0.03 to 0.13 cm3/g, and the pore size from 5.3 to 7.0 nm. Similar results are also obtained

Figure 5. Representative FE-SEM images for sol−gel titania prepared without template (a) and for titania prepared with various cyclodextrins: α-CD76 (b), γ-CD76 (c), HP-β-CD76 (d), and RAMEB76 (e). Representative TEM image for the RAMEB76 titania material (f). Samples were calcined at 500 °C.

the control sol−gel TiO2 is comprised of rounded particles densely packed into large aggregates with no regular shape and very low porosity (Figure 5a), the materials prepared using cyclodextrins as structure-directing agents present globally a higher dispersion of the nanoparticles. It can, however, be noticed that the degree of control over the nanoparticle dispersion depends on the nature of the cyclodextrin employed. Therefore, compared to α-CD and γ-CD, which produce some local agglomeration of the nanoparticles (see white circles in Figure 5b,c), HP-β-CD gives rise to better dispersed nanoparticles (Figure 5d), while a highly interconnected pore network is created using RAMEB (Figure 5e). The interparticle porosity in this latter sample is confirmed by TEM observations

Figure 4. N2 adsorption isotherms with corresponding pore size distribution plots (inset) of titania prepared without template (a) and with a CD/Ti molar ratio of 0.076 for α-CD (b), γ-CD (d), HP-β-CD (e), and RAMEB (f) and of 0.032 for β-CD (c). 11816

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Interestingly, when the titania hydrosol and cyclodextrin solutions are mixed together (Figure 6), the agglomeration of

(Figure 5f), which provide evidence of the fact that the porosity results from the aggregation of primary nanoparticles whose average diameter ranges between 10 and 15 nm, in agreement also with our XRD data (Figure 1, Table 1). Overall, our results indicate that, among the five cyclodextrins investigated, RAMEB has the strongest capacity for prohibiting the grain coarsening and enhancing the porosity of titania as compared to the three native cyclodextrins and to HP-β-CD. 3.3. Discussion. Considering our experimental results, the question that arises is whether the effect of the various cyclodextrins on the structural and textural characteristics of titania could be explained by the adsorption capacity of these cyclic oligosaccharides on the nanoparticle surface. Indeed, several studies report that the phase transformation in titania depends on the degree of nanoparticle packing.47−49 When the particles are closely packed, the thermal treatment favors the coalescence between neighboring grains, thus resulting in a very fast sintering during which the metastable anatase and brookite transform to the thermodynamically stable polymorph, i.e., rutile.48,49 The contacts between particles may be hindered by using organic molecules, such as surfactants or polymers, which have the ability to prohibit agglomeration via steric stabilization. For instance, it has been shown that block copolymers, such as Pluronics, physically adsorb through hydrogen bonding on solid−water interfaces, taking the configuration in which the poly(ethylene oxide) PEO groups lie on the nanoparticle surface while the poly(propylene oxide) PPO chains head away from the surface.50 Regarding the cyclodextrins, their binding affinity onto titania is highly dependent on their nature, natives or modified. Thus, the adsorption isotherms of the cyclodextrins onto titania have been shown to follow the Langmuir model,51,52 and adsorption capacities as high as 33 μmol g−1 were obtained with the native β-CD compared to 15.2 μmol g−1 with the 2-O-methyl-β-CD (DS ≈ 4) and 0 μmol g−1 with the permethylated 2,6-di-Omethyl-β-CD (DS ≈ 14).52 Such adsorption was proposed to occur predominantly through the −OH groups located at the secondary ring face of β-CD, which also caused the selective photodegradation of a series of bisphenols by preferential inclusion complexation with the primary ring side.52 In the case of the native α-CD, β-CD, and γ-CD cyclodextrins, the numerous hydroxyl groups, located on both the narrow and the wider ring faces, may favor the interaction of the macrocycle with the surface −OH groups of titania. Conversely, HP-β-CD and RAMEB, possessing fewer surface hydroxyl groups on the ring, are likely to form fewer hydrogen bonds. Additionally, the adsorption capacity of these modified cyclodextrins may be hindered by the steric constraint created by the 2-hydroxypropoxyl and methoxyl groups, thus leaving less room for the interaction with the nonsubstituted hydroxyl groups. To assess the validity of this assumption, DLS measurements were performed on cyclodextrin solutions before and after addition of titania colloids. The size distribution plots shown in Figure S6 (SI) indicate that, in water, α-CD, β-CD, and γ-CD form clusters with an apparent hydrodynamic radius (RH) of 92, 147, and 112 nm, respectively, due to their strong intermolecular interactions.53,54 On the other hand, HP-β-CD and RAMEB present bimodal size distribution plots. Note that clusters are the predominant species in the HP-β-CD solution (RH = 130 nm), while nonassociated cyclodextrins (RH = 0.95 nm) are mostly observed in the RAMEB solution.

Figure 6. Apparent hydrodynamic radius (Rh) distributions of the scattered intensity for titania sols (0.5 M) prepared without organics and with various cyclodextrins for a CD/Ti molar ratio of 0.076 (αCD, γ-CD, HP-β-CD and RAMEB) and of 0.032 (β-CD) at 25 °C.

the colloids is markedly affected by the cyclic molecule. Therefore, it can be seen that the apparent hydrodynamic radius of the TiO2 aggregates decreases sharply, from 110 to 74 nm, due to the adsorption of the cyclodextrins onto the TiO2 surface, producing a steric stabilization of the titania suspension. Moreover, the changes observed appear to be the strongest with α-CD (RH = 74 nm), followed closely by β-CD and γ-CD (RH = 85 nm). By contrast, a very slight shift in the hydrodynamic radius of titania aggregates is noticed with RAMEB and HP-β-CD (RH = 100 nm), indicating a lower adsorption capacity of these β-CD derivatives. From these findings, it appears that the binding affinity of the cyclodextrins onto titania cannot be the only interaction mechanism for explaining the dramatic effect of RAMEB in restructuring the nanoparticle network and enhancing the porosity of the resulting material. Indeed, if adsorption was the only factor at the origin of such effects, materials with better structural and textural characteristics should be obtained with the native cyclodextrins. The substitution of hydroxyl groups by a relatively large number of methoxyl (−OCH 3 ) or 2-hydroxypropoxyl [−OCH2CH(CH3)OH] groups can be another factor that affects the physicochemical properties of the cyclodextrins, implying changes in both the solubility profile (due to the disruption of intermolecular hydrogen bonds) and the interfacial behavior (due to the presence of more marked hydrophobic and hydrophilic microenvironments).35 Thus, our surface tension data shown in Figure 7 indicate that, in contrast to the native cyclodextrins, which are almost not surfaceactive,55 HP-β-CD and RAMEB present surface tension values of 59.9 and 56.8 mN/m, respectively, at the concentration of 38 mM (concentration utilized for the preparation of the mesoporous titania materials). The slightly lower surface activity of HP-β-CD compared to RAMEB may be explained by the lower lipophilic character of the 2-hydroxypropoxyl groups compared to the methoxyl ones. In this sense, the relatively higher surface activity of RAMEB may offer a means to reduce the surface energy of titania nanocrystals, thus 11817

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that the native α-CD and β-CD, before being introduced to the titania hydrosol, present several sharp diffraction lines characteristic of their cage-type crystalline microstructure.56 After interaction with titania colloids, the disappearance of the most intense reflections observed at 12.2°, 14.3°, and 21.6° with the neat α-CD and at 9.0° and 12.5° with the neat β-CD suggests the disruption of the cage-type microstructure due to the adsorption of these macromolecules onto the titania surface. On the other hand, the XRD patterns of the neat β-CD derivatives (HP-β-CD and RAMEB) present only two broad peaks due to their amorphous character.57 Interestingly, from the patterns of the hybrid HP-β-CD/TiO2 and RAMEB/TiO2 materials, it can be noticed that these reflections are still intense, indicating weaker interactions with titania surface, consistent also with our DLS data (Figure 6). It is worth mentioning that the results obtained with titania in this study are also in agreement with those reported on carbon materials, indicating a significantly lower adsorption capacity of HP-β-CD and RAMEB compared to the native cyclodextrins but a higher capacity of these modified cyclodextrins to improve the dispersion of carbon particles in water.58 The overall picture emerging from our experimental data is that among the five cyclodextrins investigated, the randomly methylated β-CD presents the best combination of surface active properties and weak CD−CD intermolecular interactions to efficiently direct the self-assembly of titania nanoparticles in a uniform network. The mechanism suggested for the selfassembly is shown in Figure 9. In the presence of the native

Figure 7. Surface tension plots of various cyclodextrins in water at 25 °C. The solid rectangle shows the domain of CD concentrations under which the titania materials in this study have been prepared.

facilitating their movement, which is critical for their selfassembly. To follow the changes occurring in the structure of cyclodextrin assemblies after interaction with titania colloids and after solvent evaporation, XRD measurements were also performed on the hybrid CD/TiO2 xerogels dried at 60 °C as well as on the neat cyclodextrins. From Figure 8, it can be seen

Figure 8. XRD patterns of the neat CDs (a) and corresponding CD/TiO2 hybrid xerogels (b) prepared with a CD/Ti molar ratio of 0.076 for α-CD, HP-β-CD, and RAMEB and a molar ratio of 0.032 for β-CD. The xerogels were dried at 60 °C. 11818

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Figure 9. Schematic illustration of the mechanism proposed for the self-assembly of titania colloids in the presence of native cyclodextrins (α-CD, βCD, and γ-CD) and modified cyclodextrins (RAMEB and HP-β-CD).

cyclodextrins (α-CD, β-CD, and γ-CD), as the colloid interface is rather rough (due to its high surface energy) and the intermolecular interactions are more important (due to the numerous −OH groups), the interactions between adsorbed cyclodextrins should favor the local agglomeration of titania nanoparticles during solvent evaporation, resulting in a less porous network. By contrast, in the presence of HP-β-CD and RAMEB, it is expected that smoother interfaces will be created by the lipophilic groups present in the macrocycle, leading to a decrease in the surface energy of titania nanocrystals and reorganization of the colloids in a more homogeneous and porous network. The higher separation distance between nanoparticles (i.e., reduced number of contact points) also allows for delaying the phase transformation during thermal treatment, thus resulting in small grain sizes with a fine morphology. 3.4. Photocatalytic Activity. To evaluate the photocatalytic activity of our titania materials prepared using the different cyclodextrins as structure-directing agents, PAA, a toxic herbicide, was chosen as probe molecule for degradation under UV light (360 nm). PAA is a parent molecule of the wellknown 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5trichlorophenoxyacetic acid (2,4,5-T) herbicides.59 Figure 10a shows the PAA degradation rate obtained after 7 h of exposure under UV-light illumination (360 nm) for the materials prepared with various amounts of randomly methylated β-CD. It can be noticed that the photocatalytic activity increases progressively upon addition of increasing amounts of RAMEB. A maximum degradation rate of 86% is reached for a RAMEB/Ti molar ratio of 0.076, which is twice that of the sol−gel TiO2. Further addition of RAMEB, beyond this optimum, leads to a gradual decrease in the photocatalytic activity to 77% and 68% for RAMEB/Ti molar ratios of 0.107 and 0.138, respectively. The chemical nature of the cyclodextrin has also an impact on the photocatalytic activity of titania. Thus, from Figure 10b, it can be seen that α-CD76, β-CD32, and γ-CD76 materials are all photoactive under UV irradiation and give a PAA degradation rate in the range of 58−64%, which is almost 45% higher than that of the sol−gel TiO2 (43%). Further

Figure 10. Photocatalytic degradation rate of phenoxyacetic acid under UV-light irradiation after 7 h on titania materials prepared with increasing amounts of RAMEB (a) and with various CDs at a fixed CD/Ti molar ratio (0.076 for α-CD, γ-CD, HP-β-CD, and RAMEB and 0.032 for β-CD) (b).

enhancement in the photocatalytic activity is noticed with the HP-β-CD76 catalyst, showing a degradation rate of 72%, 11819

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intermediate between those of γ-CD76 (64%) and RAMEB76 (86%). Taken together, our results indicate that the photocatalytic activity of our titania materials may be correlated with their structural and textural characteristics, both of which depend on the concentration and chemical nature of the cyclodextrin employed. Indeed, the strongest effects in the pore volume, surface area, phase composition, and photocatalytic activity are observed when RAMEB is used as structure-directing agent, while the RAMEB/Ti molar ratio is also revealed to play a key role in modulating these parameters. The best photocatalytic activity obtained for the material prepared using RAMEB as structure-directing agent (RAMEB76 sample) may be directly correlated with the specific characteristics of this catalyst. Therefore, the high surface area (115 m2/g) and small crystallite size (11−16 nm) should provide a larger number of adsorption sites and active centers surrounding the electron−hole pairs, thus facilitating the initiation of the photocatalytic reaction. On the other hand, the high pore volume (0.30 cm3/g) may allow for more PAA molecules to be adsorbed on the internal surface of the pores, thus improving the mass transfer of the substrate to the adsorption sites during the photocatalytic process. Finally, the coexistence of the two photoactive crystalline phases in high proportion, i.e., anatase22 (53.7%) and brookite60 (44.6%), may be beneficial for facilitating the electron transfer between the different polymorphs and decreasing the electron−hole recombination rate during the photocatalytic process. Overall, our results indicate that all the above parameters are interlinked and a harmonization between them is necessary to obtain an efficient photocatalyst. To demonstrate the important role played by cyclodextrins in enhancing the porosity and photocatalytic activity of titania, the characteristics of our materials are also compared with those of a similar TiO2 catalyst prepared by the same self-assembly procedure, using Pluronic P123 [PEO20PPO70PEO20, PEO = poly(ethylene oxide) and PPO = poly(propylene oxide)] as structure-directing agent (Figure S7, Table S2, SI). Pluronic P123 is one of the most widely used block copolymers for the preparation of nanostructured titania.34 Its concentration in the titania hydrosol was fixed at 7.8 wt %, which corresponds to an EO/Ti molar ratio of 1. Compared to the sol−gel photocatalyst (SBET = 21 m2/g, pv = 0.03 cm3/g, ps = 5.3 nm), the material prepared using Pluronic P123 presents better textural characteristics (SBET = 89 m2/g, pv = 0.24 cm3/g, ps = 9.2 nm); however, these characteristics are less important than those of the RAMEB76 catalyst (SBET = 115 m2/g, pv = 0.30 cm3/g, ps = 11.4 nm). Similarly, the PAA degradation rate for the mesoporous titania prepared using P123 (78%), although enhanced compared to that of the control sol−gel TiO2 (43%), remains lower when compared to that of RAMEB76 (86%). This particular behavior of RAMEB in directing more efficiently the self-assembly of titania colloids results from a combination of several factors, including the high surface activity of this cyclodextrin, low intermolecular interactions, and sufficient adsorption capacity onto titania surface, parameters that appear to be essential in determining the efficacy of the resulting photocatalyst.

crystalline framework. The addition of increasing amounts of randomly methylated β-CD to the titania hydrosol induced a progressive increase in the porosity of titania as well as a stabilization of the anatase and brookite polymorphs with respect to the rutile phase. Moreover, the utilization of various cyclodextrins, natives (α-CD, β-CD, and γ-CD) or modified [(2-hydroxypropyl)-β-CD and randomly methylated β-CD], allowed for fine-tuning the phase composition, crystallite size, surface area, pore volume, and pore size of the mesoporous titania. The characteristics of our materials were found to depend also on the nature of the cyclic oligosaccharide. On the basis of surface tension measurements, dynamic light scattering, N2-adsorption, X-ray diffraction, and UV−visible spectroscopy, the effects produced on the porosity and crystal phase parameters were correlated to the surface activity of the cyclodextrins employed, as well as to their ability to interact both with titania surface and with other neighboring cyclodextrins. Thus, the stronger effect noticed with the randomly methylated β-CD was attributed to the capacity of this cyclodextrin to decrease the surface energy of titania colloids, creating smoother interfaces and consequently facilitating the self-assembly of the nanoparticles in a homogeneous network. The weaker effects produced with the native cyclodextrins were explained by their negligible surface activity, as well as their ability to strongly adsorb onto titania surface and further aggregate, leaving fewer places for the self-assembly of the nanocrystals. Regarding the (2-hydroxypropyl)-β-CD, this modified cyclodextrin presented an intermediate behavior between those of the native cyclodextrins and the randomly methylated β-CD. Indeed, it showed a relatively high surface activity but, with counterpart, a stronger ability to interact with other adsorbed cyclodextrins, leading to materials with intermediate structural and textural characteristics. Furthermore, the mesoporous titania photocatalysts prepared using the various cyclodextrins as structure-directing agents were shown to efficiently catalyze the photodegradation of the phenoxyacetic acid in water, giving rise to higher conversion rates compared to that of the sol−gel titania particles. Overall, our results revealed that knowledge about the physicochemical behavior of these cyclic oligosaccharides in water and about their interaction with titania colloids is fundamental for understanding and controlling the properties of the resultant photocatalyst. Coupling this understanding with the wellknown ability of the cyclodextrins to selectively bind a variety of metal complexes and form supramolecular adducts or host−guest complexes could open up wide opportunities for designing nanostructured metal-capped TiO2 photocatalysts for energy and environmental challenges.

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ASSOCIATED CONTENT

S Supporting Information *

Chemical structures of the cyclodextrins used in this study, the textural and structural properties of TiO2 xerogel, the DR UV− vis spectra of the titania materials, the DLS measurements performed on cyclodextrin solutions before addition of titania colloids and comparative information on the catalyst prepared with Pluronic P123 in terms of N2 adsorption isotherms and corresponding XRD patterns, PSD, DR UV−vis spectra, and photocatalytic efficiency. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS In this work, we investigated the possibility of utilizing various cyclodextrins as structure-directing agents for the synthesis of a series of mesoporous titania with controlled porosity and 11820

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The European Regional Development Fund (ERDF), the Conseil Regional du Nord-Pas de Calais, the CNRS, and the Ministère de l’Education Nationale de l’Enseignement Supérieur et de la Recherche are acknowledged for funding of the Xray diffractometer. A.L. is grateful to the Region Nord Pas-deCalais and University of Artois for the Ph.D. grant support. The laboratory participates in the Institut de Recherche en ENvironnement Industriel (IRENI), which is financed by the Communauté Urbaine de Dunkerque, the Région Nord Pas-deCalais, the Ministère de l’Enseignement Supérieur et de la Recherche, the CNRS, and the ERDF. We thank Laurence Burylo and Nora Djelal from the University of Lille for technical assistance in XRD measurements and FE-SEM analyses, respectively, as well as Dr. Antonio Da Costa and Dr. Nicolas Kania from the University of Artois for their help with TEM and photocatalytic measurements, respectively.

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dx.doi.org/10.1021/la502911v | Langmuir 2014, 30, 11812−11822