Water−Hydroxyl Interactions on Small Anatase Nanoparticles

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Water-Hydroxyl Interactions on Small Anatase Nanoparticles Prepared by the Hydrothermal Route J. Soria,*,† J. Sanz,‡ I. Sobrados,‡ J. M. Coronado,§ M. D. Herna´ndez-Alonso,† and F. Fresno† Instituto de Cata´lisis y Petroleoquı´mica, CSIC, C/Marie Curie, 2, Cantoblanco, 28049 Madrid, Spain, Instituto de Ciencia de Materiales, CSIC, Cantoblanco, 28049 Madrid, Spain, and IMDEA Energı´a, UniVersidad Rey Juan CarlossLaboratorios III, C/Tulipa´n s/n, 28933 Mo´stoles (Madrid), Spain ReceiVed: June 4, 2010; ReVised Manuscript ReceiVed: July 30, 2010

Modification of metal oxides’ characteristics by decreasing the nanoparticles’ crystal size is usually interpreted in terms of increasing quantum size effects and/or oxygen vacancy concentration. However, some properties of TiO2 nanoparticles, such as water adsorption strength on anatase or photoactivity for toluene mineralization in the gas phase, are optimized when the mean anatase crystal size is close to 6 nm, indicating the action of two opposed effects. Here, we show that these effects are originated by the increasing acidity of bridging hydroxyls with decreasing crystal size. Increasing acidity favors, first, water hydrogen bonding to bridging hydroxyls and, then, simultaneously to bridging and terminal hydroxyls of adjacent particles, favoring nanoparticle agglomeration and interfacial hydroxyls and water stability. A too strong acidic character of bridging hydroxyls favors proton exchange of stabilized hydronium ions with bridging O2- and terminal hydroxyls, facilitating hydroxyl recombination and crystal growth. Introduction The particular properties of anatase TiO2 nanoparticles and their wide range of applications in fields like heterogeneous catalysis, solar energy conversion, environmental remediation, sensor devices, or cell separation,1,2 have generated great interest in determining the origin of anatase characteristics and in developing improved methods to prepare nanoparticles with a very small crystal size. Theoretical models based on the nanoparticles’ small crystal size (quantum size effects)3,4 have often been used, with different degrees of success,5 to explain continuous modifications of anatase properties with decreasing crystal size. The increasing concentration of oxygen vacancies associated with formation of low coordinated Ti4+ cations with decreasing crystal size of small anatase nanoparticles, determined from EXAFS measurements,6,7 can also explain continuous modifications of the nanoparticles’ characteristics.8 However, some anatase nanoparticle properties, such as their photocatalytic activity of anatase TiO2 for mineralization of toluene in the gas phase, are optimized when their mean crystal size is close to 6 nm (P6HT).9 This optimum value suggests the presence of two opposed processes indirectly related to oxygen vacancy concentration. On the basis of the marked improvement of toluene photooxidation conversion and selectivity to CO2 when water was incorporated to the gaseous reaction flow,9 the determination of hydrated anatase characteristics was thought to be essential to understanding the influence of crystal size on anatase surface properties. As a preliminary step, the effects of evacuation on hydrated anatases with a crystal size in the range of 11-20 nm were studied by 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) and Fourier transform infrared (FTIR) spectroscopy. The results showed that the anatase surface * To whom correspondence should be addressed. E-mail: [email protected]. † Instituto de Cata´lisis y Petroleoquı´mica, CSIC. ‡ Instituto de Ciencia de Materiales, CSIC. § IMDEA Energı´a, Universidad Rey Juan Carlos.

SCHEME 1: Hydroxyls and Adsorbed Water on External Surfaces of P11T Anatase: (a) Hydrated and (b) RT Outgassed Sample

characteristics were markedly affected by the adsorbed water.10 Most water was forming molecular arrangements (MAs),11-14 where the first layer, water WA, is strongly bound to low coordinated Ti4+ cations14 via an oxygen lone-pair orbital oriented toward the anatase surface15 (Scheme 1a). The Ti4+ cations’ ionic field induces the MA water polarization and bond ordering.14 The most strongly H-bonded water is tetrahedrally arranged, forming the MAs multilayer water (denoted as water WB), interacting with more weakly H-bonded second layer water (denoted as WB’ in Scheme 1a). The multilayer surface water can be very weakly H-bonded (denoted as WC). After the multilayer removal, the MAs’ second layer, with weakened H bonding, forms the MAs’ surface (denoted as water WC’ in Scheme 1b). In addition, some water (denoted as WO) was H bonded to bridging hydroxyls, increasing its OH acidity. Elimination of water WC’, WO, and most water WA by

10.1021/jp105131w  2010 American Chemical Society Published on Web 09/07/2010

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evacuation at 473 K allowed the observation of isolated bridging and terminal hydroxyls.10 On the other hand, anatases prepared by calcination and by hydrothermal treatments, a liquid phase synthesis method appropriate to prepare anatase with a very small crystal size,16,17 had been studied by electron paramagnetic resonance (EPR), NMR, and FTIR techniques. The results had shown that the anatase characteristics were significantly affected by the preparation method and the crystal size.18-20 To determine the characteristics giving to the hydrothermal P6HT anatase (6 nm) its special properties, in this work, we have studied, by 1H MAS NMR and FTIR spectroscopy, hydroxyls and water adsorbed on two hydrothermal anatases with mean crystal sizes of 4 and 6 nm (P4HT and P6HT). The results obtained in the previous study of P11T, a low defective anatase prepared by calcination with a 11 nm mean crystal size, have been used for comparison purposes.10 Experimental Section Titanium isopropoxide (TIP) was used for the sol-gel synthesis of amorphous titania precursors spheres (100 nm).19 Anatase samples were prepared by heating 0.2 g of titania gel spheres for 8 h in a Teflon-lined autoclave vessel, with 15 mL of H2O and 10 mL of 2-propanol at 373 K (P4HT sample) or with 12 mL of H2O and 13 mL of 2-propanol at 423 K (P6HT sample). The TiO2 powders were analyzed with a Philips PW 1030 X-ray diffraction (XRD) apparatus, equipped with a Cu KR radiation source and a graphite monochromator. TEM images were obtained with a JEOL 2000 FX II system (3.1 Å point resolution). BET surface area values were deduced from N2 adsorption isotherms measured with a Micromeritics ASAP 2010 equipment. TPD experiments were performed in a VG 100-D mass spectrometer, monitoring the m/z )18 and heating about 50 mg of sample at a rate of 10 K min-1 in a flow of 50 cm3 min-1 of Ar. A Nicolet 5ZDX FTIR spectrometer equipped with a MCT detector (4 cm-1 resolution) was used to study TiO2 powders pressed into thin wafers (25 mg · cm-2) placed in an IR Pyrex cell with NaCl windows. Proton MAS NMR spectra were recorded at 400.13 MHz (B0 ) 9.4 T) in an AVANCE 400 (Bruker) spectrometer. Proton NMR spectra were recorded after the irradiation of the sample with a π/2 radio frequency pulse (single pulse technique). In MAS NMR experiments, samples were spun at 10 kHz around an axis inclined 54°44′ with respect to the external magnetic field (magic-angle spinning technique). A conventional vacuum line (residual pressure ) 1 × 10-4 Torr) was used to prepare samples for NMR studies. To preserve the sample outgassing, rotors were filled under a nitrogen atmosphere inside a glovebox. The number of scans, 100, was selected to obtain a signal-tonoise ratio of 40. To avoid saturation effects, the time between successive experiments was chosen to be 5 s. Spectra deconvolution was carried out with the Winfit (Bruker) software package. The intensity, position, and line width of components were determined with a nonlinear least-squares iterative program. Chemical shift values were referred to that of the TMS signal and intensities of NMR components to that of the rotor cap used in MAS experiments. Results Samples Characterization. The XRD patterns of P4HT and P6HT (not presented) show anatase as their only crystalline component. The mean crystal sizes of these anatase samples, calculated with the Scherrer equation from the width of the (101) peak, are 4 and 6 nm, respectively. These values are similar to

Figure 1. Transmission electron micrograph and electron diffraction pattern of hydrated P6HT anatase.

Figure 2. Water temperature-programmed desorption (TPD) patterns of hydrated anatase with a crystal size of (a) 6 and (b) 4 nm.

those deduced from their specific surface areas, SBET, 242 and 160 m2 g-1, respectively. TEM micrographs of P6HT confirmed the presence of anatase and illustrated the homogeneity and agglomeration degree of anatase particles (Figure 1). The crystallite diameter values deduced by TEM match closely those from the XRD patterns. TEM micrographs display less defined agglomerated nanoparticles in the hydrothermally prepared samples than in calcined P11T.10 TPD Experiments. The temperature-progammed desorption (TPD) profiles of water desorbed from samples pretreated with flowing argon at 300 K show two overlapped broad peaks (Figure 2). The maximum of the first one appears at ∼365 K in P6HT and P4HT profiles, while it was formed by two overlapped peaks with maxima at ∼350 and 375 K, with a significantly lower intensity, in the P11T profile.10 The temperatures of these peak maxima are close to that reported by Beck et al., ∼350 K for an anatase with a much lower surface area,11 indicating that this water desorption peak is not significantly affected by the anatase preparation conditions. The increasing intensity of this peak with decreasing anatase crystal

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Figure 4. FTIR difference spectra showing the effect of evacuation at RT and 373 K on P6HT (a, c, respectively) and P4HT (b, d) anatases.

Figure 3. FTIR spectra of P6HT (a) and P4HT (b): (A) hydrated, (B) evacuated at RT, (C) evacuated at 373 K.

size is consistent with the increasing surface area. The second TPD peak, with maxima at 425 and 415 K, was stronger and more extended toward higher temperatures in the P6HT than in the P4HT profile. The much smaller and narrower peak at 430 K in the P11T profile10 and the significantly higher temperature of the second TPD peak observed by Beck, ∼480 K,11 indicate that the intensity of this water desorption peak decreases with increasing anatase crystal size, while its maximum temperature increases. FTIR Study. The dominating feature of the hydrated samples’ FTIR spectra is a broad continuous absorption in the 2000-3700 cm-1 range, stronger in the P6HT than the P4HT spectra, Figure 3A. This absorption, regarded to be characteristic of strongly H-bonded systems, has been attributed to proton transitions strongly coupled with surrounding H-bonded water.21,22 This absorption appears overlapped by broad bands at ∼3450, 2830, and 2570 cm-1 in the P4HT spectrum, Figure 3A, spectrum b. These bands correspond to linear combinations of OH stretching modes of hydrated hydronium ions (Eigen cations), H3O+(H2O)3,23 denoted as WE in Scheme 1a. The band at ∼2800 cm-1 has been considered to be the most characteristic of Eigen cations and used for their identification.23 These spectra also show narrow bands at 3695 and 1645 cm-1, the latter with a shoulder at 1585 cm-1. Although isolated water molecules present two stretching (symmetric and antisymmetric) modes, at 3616 and 3708 cm-1 for monomers surrounded by CCl4,24 the distinction between them breaks down for H-bonded water molecules due to intermolecular couplings.15,24 Thus, water OH groups of hydrated anatase should be discussed in terms of two individual oscillators. The narrow band at 3695 cm-1 has been assigned to the free OH stretching mode of water WC, (Scheme 1a) at the MAs’ surface.10 This type of OH group, protruding into the water-vacuum interface, has been detected in ice melting water25 or in vapor/water and organic/water interfaces.24,26 Considering that the bending mode of water monomers in a nitrogen matrix appears at ∼1600 cm-127 and that H bonding shifts it upward,28 the band at 1645 cm-1 was assigned to the bending mode of the strongly H-bonded multilayer bulk water WB,10 whereas that of weakly H-bonded water WC should

appear close to 1610 cm-1. Water WB and WB’ coupled stretching modes appear at 3130 and 3400 cm-1, respectively.10,14 Evacuation at room temperature (RT) removed the FTIR bands at 3695 and 1645 cm-1, indicating desorption of the multilayer water WC and WB, and the shoulder at 1585 cm-1, corresponding to solvated protons with an Eigen structure23 (Figure 3B). Desorption of strongly H-bonded water WB eliminated the continuous absorption and weakened the H bonding of the second layer water WB’, now forming the MAs’ surface layer (water WC’ in Scheme 1b), shifting its coupled OH stretching and bending modes from ∼3400 and 1645 cm-1 to ∼3450 and 1625 cm-1.10,14,28 The absence of water desorption peaks at RT in TPD profiles is due to the removal of multilayer water during the samples’ pretreatment with flowing inert gas. The removal of the multilayer water lets us see two weaker bands at 3630 (narrow) and 3475 (broad) cm-1, rather weak in P4HT and, also, the strongest and broadest one at ∼3120 cm-1, overlapped by broad bands at its both sides. The two weaker bands have been associated with relatively strong adsorbed water WO29 H bonded to bridging hydroxyls, in low defective anatase10 (adducts denoted as CWO in Scheme 1b). The band at 3630 cm-1 can be assigned to the stretching mode of water WO free OH in CWO adducts because of its small width and wavenumber in the range of OH stretching modes of non-Hbonded water monomers. The lower wavenumber of this mode in water WO than WC, indicates stronger H bonding of water WO to other interfacial water molecules.24 The P6HT FTIR difference spectrum displaying the effect of evacuation at RT showed, besides formation of water WC’ and CWO adducts (Scheme 1b), the growth of narrow bands at 3685 and 3670 cm-1, corresponding to stretching modes of bridging hydroxyls with different acidities,30 and the decrease of solvated proton bands with Eigen structures at 2830 and 1585 cm-1 (Figure 4, spectrum a). These modifications, also observed in the P11T spectrum,10 indicate that the removal of multilayer water induced localization of solvated protons as bridging hydroxyls, forming CWO adducts. However, the growth of the 3120 cm-1 band in the P6HT spectrum, not observed in that of P11T, hid the decrease of the band at 3630 cm-1, a significant feature in the P11T spectrum.10 The P4HT difference spectrum, Figure 4, spectrum b, shows lower formation of acidic hydroxyls and CWO adducts than in the P6HT one due to the stabilization of solvated protons with Eigen and also Zundel (H5O2+)22,31 structures, indicated by the decrease of the narrow bands at ∼1685 and 1670 cm-1. However, in addition to the growth of

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Figure 5. 1H MAS NMR spectra of P6HT: (A) Hydrated (a) and evacuated at RT (b), 373 (c), 423 (d) and 473 (e) K. SSB denotes spinning side bands associated with the sample rotation. (B) Deconvoluted spectrum of P6HT evacuated at 373 (a) and 473 (b) K. Relative intensities of the spectra have been normalized to that of the rotor cap.

Figure 6. 1H MAS NMR spectra of P4HT: (A) Hydrated (a) and evacuated at RT (b), 373 (c), 423 (d) and 473 (e) K. SSB denotes spinning side bands associated with the sample rotation. (B) Deconvoluted spectrum of P4HT evacuated at 373 (a) and 473 (b) K. Relative intensities of the spectra have been normalized to that of the rotor cap.

the band at 3120 cm-1, in this spectrum, there is a significant growth of broad bands at ∼3550 to 3300, 3000-2500, and 1540 cm-1 that can be assigned to hydronium ions H3O+.32 Therefore, these bands indicate that most solvated protons have been stabilized. The FTIR spectra and difference ones displaying the effect of evacuation at 373 K show the decrease of the bands corresponding to water WC’ and CWO adducts and the increased contribution of the narrow bands at 3685, 3670, and 3730 cm-1, indicating that water WC’ and WO removal facilitates detection of terminal and bridging hydroxyls (Figures 3C and 4, spectra c and d). In addition, the decrease of the hydronium cations’ broad bands at 3500, 2850, and 1540 cm-1 and of Zundel cations’ narrow bands in the P4HT spectra indicates decomposition of solvated proton structures. The band at 3120 cm-1 decreases more in the P4HT than in the P6HT spectrum. 1 H NMR Study. Proton MAS NMR spectra of hydrated P6HT and P4HT present a strong line at 5.6 ppm, assigned to protons of anatase adsorbed water10,32 (Figures 5A, spectrum a, and 6A, spectrum a, respectively). The widths of this component, narrowed by water mobility effects, were 0.9 and 1.7 ppm, indicating a higher water mobility in P6HT than in P4HT. The lower water coordination number of solvated protons in P4HT reduces water mobility in this anatase. Water mobility averages proton chemical shifts and cancels dipolar proton-proton interactions. The upward displacement of this line from 4.8 ppm, corresponding to liquid water protons,33 to 5.6 ppm indicates water interaction with acidic hydroxyl protons. The decrease and broadening of the dominant line (3.4 and 5.0 ppm line widths) after outgassing at RT10 (Figures 5A, spectrum b, and 6A, spectrum b) indicates that removal of the strongly H-bonded MAs multilayer water, observed by FTIR, weakens the remaining water H bonding, restricting its mobility. In contrast with the dominating line, the small shoulders at ∼1 and 10 ppm increased their contribution to the spectra. Water desorption by evacuation at 373 K decreased the intensity and further broadened the line at 5.6 ppm in the P6HT spectrum, Figure 5A, spectrum c, while it was replaced by narrower overlapped ones in that of P4HT, Figure 6A, spectrum c. The splitting of the P6HT broad band at 5.6 ppm by evacuation at 423 K (Figure 5A, spectrum d) and the lines

narrowing by evacuation at 473 K (Figure 6A, spectrum e) facilitated the P6HT spectral analysis. The lines pattern fitting the spectrum of P6HT evacuated at 473 K also fitted those obtained after evacuation at 373 and 423 K. The deconvolution of the spectrum central band of the P6HT evacuated at 373 K shows two narrow lines, B and A, at 5.0 and 6.3 ppm, respectively, and two broader ones, DB and DA, at 3.6 and 7.8 ppm (Figure 5B, spectrum a). Meanwhile, the deconvoluted spectrum of P4HT anatase showed two narrow lines, B and A’, at 4.2 and 7.7 ppm and two broader ones, DB and DA, at 4.5 and 7.6 ppm (Figure 6B, spectrum a). The intensity of NMR spectra usually increases with decreasing anatase crystal size but decreases with increasing evacuation temperature. However, a broad line R at ∼1.0 ppm was nearly unaffected by evacuation. This line, assigned to protons of the rotor cap,10,33 has been taken as an internal reference for quantitative determinations. The relative enhancement of the minor outer narrow line PA at 10 ppm with a broad shoulder at ∼11.1 ppm (line PA’) and a narrow line at ∼1.3 ppm (line TB) by evacuating the samples at 373 K, while the anatase lines decreased, indicates their association with secondary species not detected by XRD (Figures 5A, spectrum c, and 6A, spectrum c). The deconvoluted spectrum of P6HT evacuated at 423 K showed narrow lines, B and A, at 5.4 and 6.4 ppm together with a new narrow line A’ at 7.4 ppm after evacuation at 473 K (Figure 5B, spectrum b). In addition, two broad lines, DB and DA, appear at 4.0 and 7.9 ppm and at 3.9 and 7.8 ppm in spectra recorded at these two temperatures. In contrast, evacuation of P4HT anatase at 423 K decreased markedly lines DA and DB, downward shifted line DA toward chemical shifts values of line A, and afforded a new line TA (Figure 6A, spectrum d). Therefore, its deconvoluted spectrum is mainly formed by the narrow line A at 6.7 ppm and broad lines, TA and DB, at 8.6 and 5.4 ppm, respectively (see Table 1). Evacuation at 473 K nearly eliminated line DB and narrowed line DA, by desorption of strongly adsorbed water (Figure 6A, spectrum e). Its deconvoluted spectrum is formed by narrow lines, A’, A, and B, at 7.9, 6.8, and 5.1 ppm and lines TA and TB at 8.9 and 2.4 ppm (Figure 6B, spectrum b). The relative contribution of lines PA and PA’ decreased less than those of anatase with increasing evacuation temperature, whereas the

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TABLE 1: 1H Chemical Shifts and Intensities of Hydroxyl and Water NMR Components of P4HT, P6HT, and P11T Samples Outgassed at 373, 423 and 473 K chemical shifts (ppm) PA’

PA

TA

DA

A’

a

P11T-473K P11T-423Ka P11T-373Ka P6HT-473K P6HT-423K P6HT-373K P4HT-473K P4HT-423K P4HT-373K a

10.9 10.9 11.1 11.3 11.1 11.0

9.8 9.9 10.0 10.2 10.1 9.8

7.2 7.8 7.9 7.8 8.9 8.6

7.2 7.6

7.3 7.4 7.9 7.8 7.7

A

B

6.6 6.7

5.2 5.2 5.3 5.4 5.4 5.0 5.1 4.4 4.2

6.4 6.4 6.3 6.8 6.7

intensities (a.u.) DB

TB

3.9 4.0 3.6

1.2 1.3 1.1 1.4 1.4 1.4 2.4 1.8 1.5

5.4 4.5

rotor 1.6 0.6 0.3

PA’

-0.4 -0.5 -0.2

1.9 1.1 0.8

6.7 7.5 4.5 12.1 6.6 7.4

PA

7.1 6.4 7.7 8.2 10.5 9.4

TA

DA

25.5 21.1 23.0 27.3 13.0 6.6

1.0 28.0

A’

0.9 0.9 2.7 0.6 0.3

A

B

32.5 21.7

17.8 16.2 16.2 7.5 5.6 9.3 4.4 0.3 0.5

3.1 2.5 6.4 33.6 31.8

DB

TB

rotor

26.4 25.1 28.4

7.2 15.2 11.2 8.9 6.6 2.0 10.7 10.8 7.2

42.5 46.5 47.1 15.4 15.2 11.0 12.0 10.9 10.9

13.7 27.6

Quantitative data deduced from spectra previously published (ref 10).

contribution of lines TA and TB increased (Figures 5A, spectrum e, and 6A, spectrum e), indicating that hydrothermally treated samples are formed by three different phases, two amorphous and one crystalline. Besides central components, the 1H MAS NMR spectra obtained after evacuation at 373 K display equally spaced spinning side bands produced by the sample rotation,34 indicated as SSB in Figures 5A and 6A. The outer lines of SSB bands correspond to the 1 and 10 ppm lines. The intensity of the central broad line increases with respect to the outer ones in most separated SSB bands, indicating important dipolar proton-proton interactions. The detection of the highest contribution of the SSB central line after the samples’ evacuation at 423 K (Figures 5A, spectrum d, and 6A, spectrum d), when most water WO and WC’ had been desorbed, suggests that this line corresponds to water WA coordinated to Ti4+ ions, displaying a reduced mobility. The near elimination at 473 K of the central and the corresponding SSB broad bands of the 6.5 ppm component originates the apparent narrowing and upward shift of line A and improves the resolution of the amorphous phase line TA in the P4HT spectrum, Figure 6A, spectrum d. Discussion The elimination of most retained water by evacuation at 473 K facilitates detection of anatase hydroxyls bands in IR and 1H MAS NMR spectra. Taking into account that the line width of the hydroxyls’ NMR lines increases when protons interact with protonated species, narrow lines of outgassed samples spectra have been assigned to isolated hydroxyls. It is also accepted that the increasing acidic/basic character of bridging/terminal hydroxyl protons progressively shifts their 1H NMR line toward more positive/negative values.35 Thus, the higher chemical shift of the narrow line A’ than A in P6HT, but lower than that of line A’ in the P4HT one, indicates that P6HT contained two types of bridging hydroxyls with different acidities, lower than that of the P4HT ones, but some of them more acidic than P11T bridging hydroxyls (Table 1). Similarly, the higher downward shift of the narrow line B in P4HT than in P6HT and P11T anatase spectra indicates a more basic character of P4HT terminal hydroxyls, Table 1. The increasing acidic/basic character of anatase bridging/ terminal hydroxyls with decreasing anatase size can be related to increasing shortening/enlargement of anatase short/long Ti-O distances, induced by anatase lattice relaxation originated by oxygen vacancies accommodation. Modifications of anatase Ti-O distances with decreasing crystal size have been pointed out by EXAFS results, showing distances of ∼0.193 and 0.198 nm in well-crystallized anatase,36 but of 0.185 and 0.199 nm in samples with a crystal size close to 4 nm.37 On the basis of

these considerations, P6HT should contain low defective anatase as P11T, but also some more defective surface regions, with acidic hydroxyls formed by water dissociation at oxygen vacancies.38 In the case of P4HT anatase, particles are more defective than those of P6HT. In agreement with this trend, narrow lines TA and TB (Figure 6A, spectra d and e), with the highest and lowest chemical shift, respectively, can be ascribed to very acidic and basic hydroxyls of highly defective anatase or amorphous TiO2-x. These species, absent in low defective samples prepared by ceramic routes,10 are probably located at the hydrothermal anatase particles’ surface. The high chemical shift values and small line widths of PA and PA’ bands are characteristic of very acidic OH groups of well-defined species. The amount of these species is important in samples prepared by a hydrothermal route, but they are not present in calcined samples.10 The partial elimination of these groups at increasing temperatures (T > 423 K) suggests that part of these species are at the anatase surface, being incorporated to it during sintering processes produced at increasing temperatures. These species can be ascribed to low polymerized titanium oxide precursors. Water Stabilization in Agglomerated Particles. The detection of OH bands in the dehydrated samples’ NMR spectra facilitates the study of the influence of the anatase crystal size on hydroxyl characteristics, hydroxyl-water interactions, and the adsorbed water stabilization observed in TPD profiles (Figure 2). The previously observed transformation of the P11T line DA into line A by evacuation at 473 K,10 while the terminal hydroxyls proton line B was nearly unaffected, indicated that only bridging hydroxyls were significantly interacting with water on low defective anatase. The upward shift of line A by water interaction also indicated the enhancement of bridging hydroxyls’ acidity by H bonding.39 A relatively similar situation is produced in more defective P6HT anatase, where hydroxyl line A is hardly detected because of its broadening and upward shift induced by water H bonding, leading to formation of the line DA.40 However, NMR spectra of this anatase also show that line B is broadened and downward shifted to form line DB, indicating that water interaction enhances the basic character of terminal hydroxyls.41 On the contrary, the spectrum of P4HT evacuated at 373 K shows that water interaction with bridging/terminal hydroxyls displaces lines A/B to positions closer to that of mobile adsorbed water, forming DA/DB lines. Chemical shift values in between those of the interacting species are expected for proton exchanging systems.42 Taking into account the stabilization of solvated protons in P4HT, lines DA/ DB can be ascribed to hydronium species exchanging protons with acidic/basic hydroxyls.

H2O-Hydroxyl Interactions on Small Anatase NPs From the above considerations, two cases can be imagined in analyzed anatases. If the acid and basic strengths of the OH groups are small, their interaction with water further polarizes bridging OH groups. This effect produces an additional shift of the NMR line from that of water (P6HT and P11T samples). If the bridging hydroxyls’ acidity is important, hydronium species exchange protons with bridging and terminal hydroxyls, shifting the NMR line toward that of the liquid water (sample P4HT). On the other hand, very high/low chemical shifts of broad/ narrow lines TA/TB is associated with very acidic/basic hydroxyls on highly defective anatase or amorphous TiO2-x (Figure 6A, spectra d and e). The increasing contribution of the amorphous TiO2-x lines with decreasing anatase crystal size suggests that the preparation conditions of increasingly defective anatase increasingly favors amorphous oxide phase formation. Considering that the intensity, width, and downward shift of the bridging hydroxyls’ stretching mode increase with increasing basic character of the species H bonded to them,39 the relatively strong and broad FTIR band at 3475 cm-1 can be assigned to the stretching mode of bridging hydroxyls H bonded to water WO molecules.10,30 On the same basis, the stronger, broader, and more downward shifted band at 3120 cm-1 in the P6HT spectrum can be assigned to the stretching mode of bridging hydroxyls H bonded to more basic species (hereafter, denoted as CWO’ adducts). Line DA detected in P6HT NMR spectra is mainly originated by these hydroxyls protons. The band at 3120 cm-1 in the P4HT FTIR spectra is consistent with formation of CWO’ species during the process of hydronium cations exchanging protons with bridging O2- and terminal hydroxyls.43 Hydrolysis and condensation of titanium alkoxides, steps of the hydrothermal preparation method, produce small anatase nanoparticles forming large agglomerates of difficult peptization, particularly, when the anatases crystal size is in the range of 3-10 nm.17,44 A high concentration of CWO’ adducts in the particle interfaces, together with amorphous TiO2-x, should contribute to the more poorly defined contour of individual hydrothermally treated nanoparticles, as observed in the P6HT TEM micrograph, Figure 1. The more difficult particle separation in P4HT and P6HT agglomerates than in P11T, with larger particle sizes,44 indicates the effect of strong interparticle interactions. Water-mediated interaction of bridging and terminal hydroxyls located at adjacent nanoparticles can originate their agglomeration and a higher stability of CWO’ adducts in relation to CWO adducts. The more marked decrease of basic than acidic hydroxyls and the significant increment of strongly adsorbed water WA observed in the NMR spectrum of P4HT evacuated at 423 K, besides the decrease of hydronium cation bands in the FTIR spectrum, suggest the reaction of hydronium cations with anatase basic hydroxyls to form titanium coordinated water WA. This reaction is favored by the low basic character of bridging O2ions that hampers formation of bridging hydroxyls. The displacement of P4HT lines A and B to chemical shifts expected for low defective anatases and the removal of water WA, by evacuation at 473 K, indicate that the hydronium cation formation weakens the stabilizing effect of water-mediated interactions of acidic and basic hydroxyls. These cations react with terminal hydroxyls in agglomerated anatase nanoparticles. The fact that the photocatalytic properties of anatase for gasphase toluene mineralization, as well as the stability in the dark of water adsorbed on its surface, vary in a similar way with decreasing crystal size suggest that these anatase characteristics are related. However, the effects of UV irradiation and of toluene

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16539 SCHEME 2: Hydroxyls and Adsorbed Water on Confined Aggregate Surfaces of P4HT and P6HT Anatases: (a) RT Outgassed and (b) 473 K Outgassed Samples

adsorption on hydrated anatase surface properties should be determined in order to confirm this relation. The arrangement of OH groups in adjacent particles’ surfaces could be compared to that detected in lamellar silicic acids, where water molecules were confined between two contiguous layers, forming Si-OH · · · OH2 · · · O-Si associations. The elimination of adsorbed water favored the formation of Si-OH · · · O-Si bridges between contiguous layers.45 The subsequent elimination of OH groups favored the formation of Si-O-Si bonds and the irreversible loss of adsorption capacity. In the case of anatase agglomerates, similar processes can be considered (see Scheme 2a,b). The stabilization of hydroxyl and water molecules at the particle interfaces depends on the nature of hydroxylated particle surfaces. The elimination of hydroxyl groups should favor the particles sintering and an irreversible loss of surface area. Conclusions The presented results show that crystal size influences hydroxyls’ characteristics and water adsorption on anatase nanoparticles. The acidic/basic character of the anatase bridging/ terminal hydroxyls increases with decreasing crystal size in the 4-11 nm range due to the increasing shortening/lengthening of the short/long Ti-O distances at the anatase surface, as a result of the oxygen vacancy accumulation. The bridging hydroxyls’ acidity facilitates their proton solvation on hydrated samples, particularly on anatases with the lowest crystal size, where Eigen structures are stabilized. Mild evacuation at RT removes the most strongly H-bonded water, reversing its solvating effect on acidic hydroxyl protons and stabilizing water H bonded to them in anatases with crystal sizes of 6 nm or higher. The high acidic/basic character of bridging/terminal hydroxyls favors water-mediated interactions between hydroxyls located at adjacent nanoparticles of hydrothermally treated samples. These interactions favor particle agglomeration and the stabilization of water and hydroxyls at the interparticle surfaces. The strong acidity of bridging hydroxyls favors stabilization of solvated protons as hydronium ions when hydrated anatases,

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with mean crystal sizes lower than 6 nm, are evacuated at RT. These cations are simultaneously exchanging protons with bridging O2- anions and terminal hydroxyls. Evacuation at a temperature of 423 K favors that hydronium cations react with terminal hydroxyls, forming water coordinated to Ti4+ cations, whose removal by evacuation at 473 K leads to the formation of oxygen vacancies and crystal growth. Acknowledgment. The authors thank Prof. K. L. Yeung and Dr. A. J. Maira for their contribution to this work. J. Sanz and I. Sobrados thank the Spanish Agency CICYT (project MAT200764486-C07) and the regional Government (project S-505/PPQ0358) for financial support. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (3) Balaji, S.; Diaoued, Y.; Robich, J. J. Raman Spectrosc. 2006, 37, 1416. (4) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (5) Swamy, V. Phys. ReV. B 2008, 77, 195414. (6) Parker, J. C.; Siegel, R. W. J. Mater. Res. 1990, 5, 1246. (7) Yoshitake, H.; Sugihara, T.; Tatsumi, T. Phys. Chem. Chem. Phys. 2003, 5, 767. (8) Mountjoy, G.; Pickup, D. M.; Wallidge, G. W.; Anderson, R.; Cole, J. M.; Newport, R. J.; Smith, M. E. Chem. Mater. 1999, 11, 1253. (9) Maira, A. J.; Yeung, K. L.; Soria, J.; Coronado, J. M.; Belver, C.; Lee, C. Y.; Augugliaro, V. Appl. Catal., B 2001, 29, 327. (10) Soria, J.; Sanz, J.; Sobrados, I.; Coronado, J. M.; Maira, A. J.; Herna´ndez-Alonso, M. D.; Fresno, F. J. Phys. Chem. C 2007, 111, 10590. (11) Beck, D. D.; White, J. M.; Ratcliffe, C. T. J. Phys. Chem. 1986, 90, 3123. (12) Tilocca, A.; Selloni, A. J. Chem. Phys. 2003, 119, 7445. (13) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (14) Du, Q.; Freysz, E.; Ron Shen, Y. Phys. ReV. Lett. 1994, 72, 238. (15) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (16) Zhang, H.; Banfield, J. F. Chem. Mater. 2002, 14, 4154. (17) Maira, A. J.; Yeung, K. L.; Lee, C. Y.; Yue, P. L.; Chan, C. K. J. Catal. 2000, 192, 185. (18) Soria, J.; Sanz, J.; Sobrados, I.; Coronado, J. M.; Fresno, F.; Herna´ndez-Alonso, M. D. Catal. Today 2007, 129, 240. (19) Coronado, J. M.; Maira, A. J.; Conesa, J. C.; Yeung, K. L.; Augugliaro, V.; Soria, J. Langmuir 2001, 17, 5368. (20) Maira, A. J.; Coronado, J. M.; Augugliario, V.; Yeung, K. L.; Conesa, J. C.; Soria, J. J. Catal. 2001, 202, 413.

Soria et al. (21) Zundel, G.; Metzger, H. Z. Phys. Chem.(Muenchem, Germany) 1968, 59, 225. (22) Kim, J.; Schmitt, U. W.; Gruetzmacher, J. A.; Voth, G. A.; Scherer, N. F. J. Chem. Phys. 2002, 116, 737. (23) Amir, W.; Gallot, G.; Hache, F.; Bratos, S.; Leicknam, J. C.; Vuilleumier, R. J. Chem. Phys. 2007, 126, 034511. (24) Scatena, L. F.; Brown, M. G.; Richmond, G. L. Science 2001, 292, 908. (25) Wei, X.; Miranda, P. B.; Shen, Y. R. Phys. ReV. Lett. 2001, 86, 1554. (26) Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Phys. Lett. 1993, 70, 2313. (27) Van Thjel, M.; Becker, F. D.; Pimentel, G. C. J. Chem. Phys. 1957, 27, 486. (28) Yates, D. J. Phys. Chem. 1961, 65, 746. (29) Finnie, K. S.; Cassidy, D. J.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 2001, 17, 816. (30) He, Y.; Tilocca, A.; Dulub, O.; Selloni, A.; Diebold, U. Nat. Mater. 2009, 8, 585. (31) Markovitch, O.; Chen, H.; Izvekov, S.; Paesani, F.; Voth, G.; Agmon, N. J. Phys. Chem. 2008, 112, 9456. (32) Nosaka, A. Y.; Fujiwara, T.; Akutsu, H.; Nosaka, Y. J. Phys. Chem. B 2004, 108, 9121. (33) Colomban, Ph.; Novak, A. In Solid State Protonic Conductors for Fuel Cells and Sensors; Jensen, J., Kleitz, M., Eds.; Odense University Press: Odense, Denmark, 1982. (34) Klinowski, J. In Solid State NMR Spectroscopy. Principles and Applications; Duer, M. J., Ed.; Blackwell Science Ltd.: Oxford, U.K., 2002. (35) Engelhardt, G.; Michel, D. High Resolution Solid State NMR of Silicates and Zeolites; Wiley: New York, 1987. (36) Wu, Z. Y.; Zhang, J.; Ibrahim, K.; Xian, D. C.; Li, G.; Tao, Y.; Hu, T. D.; Zhang, Q. H.; Gao, L.; Chen, Z. Z. Appl. Phys. Lett. 2002, 80, 2973. (37) Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650. (38) Henderson, M. A. Langmuir 1996, 12, 5093. (39) Makarova, M. A.; Ojo, A. F.; Karim, K.; Hunger, M.; Dwyer, J. J. Phys. Chem. 1994, 98, 3619. (40) Pelmenschikov, A. G.; Morosi, G.; Gamba, A. J. Phys. Chem. 1992, 96, 7422. (41) Odde, S.; Pak, Ch.; Lee, H. M.; Kim, K. S. J. Chem. Phys. 2004, 121, 204. (42) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, U.K., 1961. (43) Agmon, N. Chem. Phys. Lett. 1995, 244, 456. (44) Vorkapic, D.; Matsoukas, T. J. Colloid Interface Sci. 1999, 214, 283. (45) Rojo, J. M.; Ruiz-Hitzky, E.; Sanz, J. Inorg. Chem. 1998, 27, 2785.

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