Article pubs.acs.org/JPCC
Influence of Amorphous TiO2−x on Titania Nanoparticle Growth and Anatase-to-Rutile Transformation Jesús Sanz,†,* Javier Soria,‡ Isabel Sobrados,† Sedat Yurdakal,§,∥ and Vincenzo Augugliaro∥ †
Instituto de Ciencia de Materiales, CSIC, C/Sor Juana Inés de la Cruz, 3 Cantoblanco, 28049 Madrid, Spain Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie, 2. Cantoblanco, 28049 Madrid, Spain § Kimya Bölümü, Fen-Edebiyat Fakültesi, Afyon Kocatepe Ü niversitesi, Ahmet Necdet Sezer Kampüsü, 03100 Afyonkarahisar, Turkey ∥ “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Ingegneria Elettrica, Elettronica e delle Telecomunicazioni, di Tecnologie Chimiche, Automatica e Modelli Matematici, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy ‡
ABSTRACT: Amorphous TiO2−x formed together with precursors and anatase nuclei, during TiCl4 hydrolysis at soft conditions, influences both crystal growth and phase stability. The highly defective nanoparticles of anatase grow by reaction of their basic hydroxyls with acidic ones of TiO2 precursor species. The growth of anatase crystals, however, is affected by their interactions with simultaneously formed amorphous TiO2−x which increasingly covers the particles hindering the anatase−precursor contact. The interactions among anatase and amorphous and precursor components have been studied by 1H-MAS (magic angle spinning) NMR spectroscopy. The interaction between acid and basic hydroxyls favors the formation of Ti−O−Ti bonds between amorphous and anatase phases, increasing the agglomeration of anatase nanoparticles. These interactions destabilize anatase crystals and favor anatase-to-rutile transformation as the solution aging progresses.
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INTRODUCTION The possibility of preparing crystalline nanoparticles of metal oxide by controlling not only their uniform crystal size but also the structure they adopt has greatly promoted during the past decade the use of synthesis methods in liquid medium at low temperature.1,2 However, the precursor species generated in the liquid suspensions not only form crystalline oxides but, generally, also amorphous ones. The presence of amorphous oxides, whose characterization is difficult, is usually considered to have no influence on the final sample properties. This simplification, acceptable when the amount of noncrystalline components is low, can be nonvalid if their amount becomes important, as is the case when preparing very small nanoparticles of crystalline oxide.3,4 It has been recently reported that titania samples, obtained by hydrolysis of TiCl4 at room temperature and subsequent aging treatments of the resulting aqueous suspension at 373 K, were predominantly constituted by amorphous phases. After 0.5 h aging, the samples contained anatase with small crystal size; the size slightly grew for 2 h aging, and with progressive aging, it remained nearly unaltered while anatase content decreased and rutile formation increased until almost complete anatase transformation into rutile for 8 h aging.5,6 The hindered growth of anatase crystal size and its transformation to rutile at 373 K, when anatase is usually transformed into rutile by calcination at temperatures higher than 873 K, suggest that thermal effects in the aqueous suspension and/or the presence of amorphous TiO2−x are responsible for the observed phenomena. 1 H-MAS NMR studies of anatase-rich samples have shown that, by increasing the concentration of structural defects of © 2012 American Chemical Society
anatase, their bridging/terminal hydroxyls become more acidic/ basic, particularly the bridging hydroxyls, and their proton lines are upward/downward shifted.7−9 In general, NMR proton lines of acidic/basic hydroxyls are also increasingly upward/ downward shifted by H-bonding to increasingly basic/acidic species.7,10 These lines, however, are displaced toward the position of adsorbed water when hydroxyls exchange protons with water.7 In consequence, NMR chemical shift values can also inform about interactions between OH groups of different phases, when the amount of adsorbed water decreases. Soria et al.7−9 recently investigated anatase-rich samples to relate surface characteristics with sample defects; the samples were prepared by thermal (T) and hydrothermal (HT) treatments of an amorphous TiO2 precursor and showed increasing concentration of defects when particle sizes decreased or temperature increased. After outgassing at 373 K, the NMR spectrum of a low defective anatase with crystal size of 11 nm (T11) presented the lines of bridging hydroxyls H-bonded to water at 7.2 ppm and of isolated terminal ones at 5.4 ppm. Water removal at 473 K allowed the detection of isolated acidic hydroxyls at 6.4 ppm. For an anatase sample with crystal size of 6 nm (HT6), more defective than T11, the lines of isolated acidic and basic hydroxyls appeared at 6.4 and 5.4 ppm (as in the T11 spectrum), but a small line at 7.2 ppm indicated the formation of more acidic bridging hydroxyls at the most defective zones of this sample. Moreover, strong lines at 7.9 and Received: November 21, 2011 Revised: January 31, 2012 Published: January 31, 2012 5110
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NMR experiments, samples were spun at 10 kHz around an axis inclined 54°44′ with respect to the external magnetic field. The time used between successive experiments was 5 s. To obtain S/N ratios higher than 50, 100 scans were accumulated with a filter of 100 kHz. A conventional vacuum line (residual pressure: 1 × 10−4 Torr) was used to evacuate samples. To preserve the evacuation conditions, the rotors were filled under a nitrogen atmosphere inside a glove box. Spectra deconvolution was carried out with the Winfit (Bruker) software package. Chemical shift values were referred to the TMS signal and intensities of NMR lines to that originated by the rotor cap.
5.0 ppm were originated by water-mediated H-bonding of acidic/basic hydroxyls to basic/acidic ones of adjacent anatase particles. These interactions between hydroxyls originated a marked stabilization of adsorbed water and hydroxyls, by preserving the anatase crystal size. After evacuation at 373 K, the spectrum of the anatase sample with crystal size of 4 nm (HT4), more defective than HT6 anatase, showed a marked upward/downward shift of the isolated bridging/terminal hydroxyl lines to 7.8/4.2 ppm, indicating the strengthening of their acidic/basic character with the increment of anatase structural defects. On the other hand, the water-mediated interaction between bridging and terminal hydroxyls of adjacent anatase particles originated lines at 7.2 and 4.6 ppm, respectively, closer to the chemical shift of adsorbed water, indicating processes of proton exchange. Evacuation at 423 and 473 K originated the progressive decrease of the proton lines of anatase hydroxyls and their shift toward the values of low defective anatase. These modifications indicated that the removal of the mediating water facilitated the reaction between hydroxyls of adjacent anatase particles, leading first to formation of water coordinated to Ti cations and then to elimination of oxygen vacancies when water was removed by evacuation. The decrease of proton lines of anatase water and hydroxyls increased the contribution of two bands at 8.8 and 2.4 ppm that were tentatively assigned to acidic and basic hydroxyls of amorphous TiO2−x. In addition, lines at 10.0 and 11.1 ppm, seen in the HT4 and HT6 spectra but not in the T11 one, were attributed to acidic OH groups of TiO2 precursors.11 To analyze possible interactions among the different components of titania nanoparticles, in the present work two samples have been prepared by TiCl4 hydrolysis at soft conditions. The samples, containing anatase with crystal sizes of 5 and 7 nm but, mainly, amorphous phases, have been investigated by 1H-MAS NMR spectroscopy, a technique able of discerning hydroxyl characteristics of different components of samples.12 The sample dehydration has also been used to follow the evolution of amorphous−anatase interactions.
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RESULTS AND DISCUSSION Figures 1 and 2 report the XRD patterns of HP0.5 and HP2 samples, and Table 1 collects the values of some characteristic
Figure 1. XRD patterns of the HP0.5 sample as obtained by the preparation method (a) and after 3 h thermal treatments in air at 473 K (b) and 673 K (c).
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EXPERIMENTAL METHODS The first step of the TiO2 sample synthesis was the preparation of a precursor solution5 at room temperature (RT), by slowly dropping 5 cm3 of TiCl4 into a 200 cm3 beaker containing 50 cm3 of deionized water, during 5 min. The solution was magnetically stirred by a cylindrical bar at 600 rpm. After that, the beaker was closed, and the mixing was prolonged for 12 h at RT, eventually obtaining a clear solution. After the TiCl4 controlled hydrolysis, the solution was transferred to a roundbottom flask having a Graham condenser on its top. The flask was put in boiling water for 0.5 or 2 h, by obtaining white suspensions. The suspensions were then dried at 323 K in a roto-vapor working at 150 rpm, to obtain powdered catalysts; home-prepared (HP) powders are denoted by HP0.5 and HP2. XRD patterns were recorded on a Philips diffractometer, using the Cu Kα radiation and a 2θ scan rate of 1.2° min−1. Specific surface areas (SSA) were measured by the multipoint BET method using a Nova Quantachrome 2000E apparatus. Thermogravimetric analyses were performed by using a PerkinElmer equipment (model STA 6000). The samples were put in an open Pt crucible and heated at 5 K·min−1 up to 870 K in a N2 flow of 20 cm3·min−1. 1 H NMR spectra were recorded after a π/2 radiofrequency pulse irradiation at 400.13 MHz in an AVANCE 400 (Bruker) spectrometer (B0 = 9.4 T). In MAS (magic angle spinning)
Figure 2. XRD patterns of the HP2 sample as obtained by the preparation method (a) and after 3 h thermal treatments in air at 473 K (b) and 673 K (c).
parameters deduced from these patterns. In the absence of any thermal treatment, the samples' patterns (Figures 1a and 2a) show relatively narrow peaks, broader in the HP0.5 pattern than in the HP2 one, overlapped by very broad ones at 2θ = 25.58, 38.08, 48.08, and 54.58°. The narrow peaks indicate the presence of a better crystallized anatase and the broad ones a much more defective anatase, with Ti4+ cation coordination and polyhedra condensation probably lower than in low defective anatase.13 In addition, a small and very broad peak at about 2θ = 27.58°, stronger in the HP2 pattern than in the HP0.5 one, indicates the formation of a highly defective rutile. The anatase 5111
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Table 1. Some Features of HP0.5 and HP2 Samples Determined from XRD Patterns HP0.5 HP0.5a % of anatase % of rutile anatase/(anatase + rutile) rutile/(anatase + rutile) anatase crystal size [nm] rutile crystal size [nm] a
HP0.5b
HP2 HP2a
HP2b
6.8 0.7 0.91
11 1 0.92
20 2 0.89
4 1.4 0.74
12 3.7 0.76
16.2 4.9 0.77
0.09
0.08
0.11
0.26
0.24
0.23
5
17
26
7
13
26
2
---
---
3
---
18
b
Treated in air for 3 h at 473 K. Treated in air for 3 h at 673 K.
crystal sizes in HP0.5 and HP2, determined by means of the Scherrer equation, were 5 and 7 nm, and the average diameters of particle agglomerates, determined from SEM images, were 25 and 23 nm. TEM images obtained in two analyzed samples showed agglomerated particles, where crystalline TiO2 domains of 2−5 nm are maintained together by amorphous material. Specific surface areas (SSAs) were 236 and 220 m2·g−1, showing a small surface decrease with the increment of anatase crystal size. The percentage of crystalline phases, determined with respect to a CaF2 reference,13 was between 5 and 7%. In HP2, the amount of rutile increased, and that of anatase decreased with respect to those of HP0.5. These patterns indicate that the increment of aging time to 2 h improves the crystallinity of anatase, mainly formed during the initial moments of the aging treatment. The progressive increase of the rutile content and the parallel decrease of the anatase one (to almost disappear after 8 h aging)5 indicate that anatase was slowly being transformed into rutile during aging treatment. The thermal treatments of HP0.5 and HP2 samples for 3 h in air at 473 K (Figures 1b and 2b) or at 673 K (Figures 1c and 2c) progressively improved the anatase and rutile crystallinity by lowering of the amount of defects and increasing the particles crystal size (Table 1). In both samples, however, the relative percentage of these two components remained nearly constant, indicating that moderate thermal treatments are not able to induce the anatase−rutile transformation. This result shows that the phase transformation, observed in the course of aging treatments at 373 K, is not determined by the temperature but by the interactions of very defective anatase with other components of the aqueous suspension. Moreover, the finding that the amounts of less defective anatase and rutile increase with the calcination temperature but their relative percentages remain unaltered, suggests that anatase and rutile crystallites grow at the expense of highly defective phases probably formed at the surface of crystalline particles. The thermogravimetric (TG) profiles reported in Figure 3 were recorded after the samples were treated with flowing nitrogen at RT for 2 h to remove the most weakly adsorbed water. The TG profiles show two water desorption peaks, stronger in the HP0.5 pattern than in the HP2 one. The HP0.5 sample presents a total weight loss of about 35%, about 20% of weaker adsorbed water lost at about 358 K and 15% of more strongly adsorbed water lost at 524 K. The HP2 pattern shows a total weight loss of 13%, about 2% of weakly adsorbed water and 11% of more strongly adsorbed water, with maxima losses at about 368 and 553 K, respectively. These peaks should mainly correspond to desorption of water adsorbed on amorphous titania, the main component of HP samples. In titania HT samples prepared by hydrothermal treatment, TPD profiles7 showed that the first desorption peak
Figure 3. Thermogravimetric plots of HP0.5 and HP2 samples.
is split in two overlapped broad peaks (with maxima losses at 365 and 415 K) assigned to water H-bonded to bridging hydroxyls of anatase or forming a second layer of molecular arrangements. In the case of HP samples, the water should mainly be H-bonded to bridging hydroxyls of amorphous titania. The absence of the peak splitting in HP samples indicates that the interactions of the hydroxyls of amorphous particles with hydroxyls of adjacent particles do not lead to water stabilization. Moreover, for both HP samples the second peak of water desorption shows higher intensity and temperature than that of HT samples, this feature indicating that water adsorption is stronger on low coordinated Ti4+ cations of amorphous TiO2−x than on those of defective anatase of HT samples. With respect to the HP0.5 profile, the HP2 one shows lower intensity and higher width and temperature of water desorption peaks, indicating that, by increasing the aging time of the suspension, the reaction between hydroxyls of the amorphous phase is favored while the stabilization of coordinated water decreases. These effects originate removal not only of adsorbed water but also of that coordinated to Ti4+ cations, favoring strong interactions of amorphous titania particles with adjacent ones and eventually an increase of the amorphous phase condensation. The 1H MAS NMR spectra of HP0.5 and HP2 samples are reported in Figures 4 and 5, respectively; Table 2 collects
Figure 4. 1H-MAS NMR spectra of HP0.5 hydrated (a), evacuated at RT (b), and evacuated at 373 K (c).
the chemical shift and relative intensity values of detected proton lines. The spectra of hydrated HP0.5 and HP2 samples 5112
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assigned to protons of the rotor cap, used as reference for quantitative studies, and a very narrow line at 1.4 ppm attributed to organic impurities.15 While in the 1H NMR spectra of anatase-rich HT samples evacuated at RT, line W still dominates, in the spectra of HP samples evacuated at RT (Figures 4b and 5b), line W is replaced by several overlapped broad ones. The removal of water that averaged the hydroxyl characteristics of anatase and amorphous TiO2−x in hydrated samples improves the resolution of hydroxyls bands. The evacuation temperature required by HP samples to resolve the hydroxyl protons lines of anatase and amorphous TiO2−x is lower than that required by anatase-rich HT samples, thus indicating the easier removal from HP samples of the water MAs averaging the characteristics of those hydroxyls. This effect indicates that the high content of amorphous TiO2−x makes HP samples less hydrophilic, probably because the surface characteristics of amorphous TiO2−x do not favor formation of water MAs.7 To homogenize the description of the hydroxyl proton lines in the spectra of HP samples, taking into account the expected prominent contribution of the amorphous TiO2−x hydroxyls to those spectra, the denomination of the NMR lines has been modified with respect to that used in previous studies7−9 to describe anatase hydroxyl interactions. In the present work, the lines will be denoted N, M, or P when they correspond to anatase, amorphous TiO2−x, or precursor hydroxyls, together with A or B if they are acidic or basic ones, with chemical shift values higher or lower than that of the highly mobile adsorbed water. Numeric subscripts, 1 or 2 or 3, stand for hydroxyls interacting with water, with hydronium or with basic centers. Thus, NMR spectra components will be denoted hereafter with two alphabetic characters and, in most cases, a numeric subscript. Taking into account the strongly acidic conditions existing during the synthesis of the HP samples, the acidic hydroxyls of titania precursors would react with basic hydroxyls to form anatase and amorphous TiO2−x particles. So, the spectra of outgassed HP samples should be dominated by the protons line of acidic hydroxyls, particularly those of amorphous titania, the main component of samples. In HP0.5 and HP2, the chemical shift values of this band are ∼8.1 and 7.6 ppm, in between the values of the proton lines displayed in spectra of anatase-rich HT samples by acidic hydroxyls of amorphous TiO2−x and
Figure 5. 1H-MAS NMR spectra of HP2 hydrated (a), evacuated at RT (b), and evacuated at 373 K (c).
(Figures 4a and 5a) are mainly formed by an intense line W at 6.7 ppm with line width of 5.3 and 3.7 ppm, respectively, characteristic of H-bonded adsorbed water forming molecular arrangements (MAs) with limited mobility.7 The absence of proton lines originated by hydroxyls of anatase and amorphous TiO2−x indicates that the water MA is interacting with them, averaging the characteristics of hydroxyl protons of different phases. The chemical shift value of line W (6.7 ppm) is higher in HP samples than in anatase-rich HT ones (5.6 ppm);7,14 this feature indicates that the mobile water is interacting with highly acidic bridging hydroxyls of amorphous TiO2−x, whose protons can be more easily solvated than those of anatase hydroxyls. The broader line W in the HP0.5 spectrum than in the HP2 one indicates that in HP0.5 the mobility of water MAs is more hampered than in HP2, probably because of the stronger interactions of water with the more acidic bridging hydroxyls of the more defective HP0.5 anatase. The spectra also show lines above 9 ppm, assigned to precursor species.7 The fact of not being affected by the averaging effect of adsorbed water indicates a more limited hydrophilic character than the other two components (crystalline and amorphous phases) of the samples. The deconvolution of the HP samples spectra also displays a small broad line at 1.5 ppm,
Table 2. Chemical Shift Values and Relative Intensity of Components Detected in 1H-MAS NMR Spectra of HP0.5 and HP2 Samples Hydrated and Outgassed at RT and 373 K chemical shift values HP2-373 HP2-vac HP2-hyd HP0.5-373 HP0.5-vac HP0.5-hyd
PA3
PA1
PA
MA3
NA1/MA1
W
Wc
NB2/MB2
MBI
rotor
13.0 12.9 19.5 13.0 13.6 13.1
11.1 10.9
10.0 9.9
10.5 9.9
7.9 7.6
6.0 6.3
3.9 3.7
11.1 11.0 11.0
10.0 9.9 9.7
9.9 9.6
8.1 8.1
* * 6.7 * * 6.7
5.7 5.5
4.7 4.4
1.2 1.2 1.4 1.2 1.1 1.3
1.5 1.0 1.5 1.7 1.6 1.9
intensity of components HP2-373 HP2-vac HP2-hyd HP0.5-373 HP0.5-vac HP0.5-hyd
PA3
PA1
PA
MA3
NA1/MA1
W
Wc
NB2/MB2
MBI
rotor
20.3 17.7 2.0 8.1 10.6 2.0
3.8 2.9
1.9 2.0
9.3 5.6
37.7 40.8
9.4 4.3
9.5 12.4
7.1 4.1 2.9
2.7 2.6 2.3
20.7 12.8
28.4 40.6
* * 91.7 * * 86.7
21.9 7.3
6.2 14.6
3.0 4.8 1.9 0.8 1.5 0.9
5.1 9.5 4.4 4.1 5.7 5.2
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anatase, interacting with water.7 This band will be denoted NA1/MA1, as it should be formed by the overlapping of lines NA1 and MA1. The chemical shift value of this band is higher in the HP0.5 spectrum than in the HP2 one, thus indicating that the band is dominated by line MA1 in the HP0.5 spectrum, while in the HP2 one lines NA1 and MA1 contribute significantly to the intensity and the increased width of this band. The interaction of acidic OH groups of anatase and amorphous TiO2−x with water favors the formation of hydronium (H3O+) cations that would interact with basic oxygens of other phases. The decrease of the chemical shift of band NA1/MA1 with increasing aging time is consistent with hydroxyl condensation, lowering the proton acidity of bridging hydroxyls and improving the crystallinity of the anatase phase. The basic hydroxyls display a broad band at 4.4 ppm in the HP0.5 spectrum and at 3.7 ppm in the HP2 one; however, because of the highly acidic conditions of HP sample synthesis, this band shows a considerably lower intensity than that of the band NA1/MA1. This band, that appears in the chemical shift range where the lines of basic hydroxyls of anatase and amorphous titania are observed, is denoted NB2/MB2 as it is formed by overlapped lines NB2 and MB2 ascribed to basic hydroxyls of these two phases. The interaction of hydronium H3O+ cations with basic hydroxyls would stabilize MA1/NA1−NB2/MB2 associations in samples evacuated at room temperature.7,16 The shift of this band toward lower values is higher in the HP2 spectrum than in the HP0.5 one, indicating the weakening of the proton exchange process associated with line MB2, the line that dominates the band NB2/MB2 in both samples. The lower chemical shift of the MB2 line than of the NB2 one is consistent with the longer Ti−O distance deduced by EXAFS spectroscopy in amorphous TiO2−x (0.1984−0.1998 nm)17 than in anatase (0.1934−0.1980 nm) and rutile (0.1949−0.1980) phases.18 As the atomic arrangement of amorphous titania has been described as an assembly of short, up to 1.5 nm, staggered chains of perfect and defective octahedral-like Ti−O units,19 the very narrow line at 1.2 ppm, denoted MBI, may be confidently assigned to terminal hydroxyls of low condensed Ti polyhedra belonging to chains of the amorphous phase. The low intensity of this line and the absence of new lines with lower chemical shift than anatase basic lines, indicate that for the most part basic hydroxyls are exchanging protons with acidic hydroxyls of different phases present in the suspension, i.e., TiO2 precursors, anatase crystals, and amorphous TiO2−x. The interactions of very basic hydroxyls with acidic ones should lead to water formation, unless they are hampered by other effects. On the other hand, the chemical shifts of the proton line MA3 at 9.6 and 9.9 ppm in HP0.5 and HP2 samples, higher than that of line MA1 at about 8.1 ppm, indicate that they are originated by the H-bonding of acidic hydroxyls of amorphous TiO2−x to species more basic than water; these interactions upward shift the hydroxyl proton line to values higher than that of MA1. Taking into account that interaction between acidic and basic hydroxyls should lead to water formation, the band MA3 has been assigned to interaction of acidic hydroxyls with basic oxygens. In HT samples, similar bands were attributed to water-mediated H-bonding of acidic and basic hydroxyls located at adjacent anatase particles. In HP samples these interactions among adjacent particles could also take place between acidic hydroxyls of amorphous TiO2−x and basic ones of anatase or amorphous TiO2−x.
Similarly, lines PA (at 9.9 ppm) and PA1 (at 11.0 and 10.9 ppm) were assigned to acidic OH groups, isolated and Hbonded to water, of the TiO2 precursor.7 However, line PA3, between 12.9 and 13.6 ppm, should correspond to precursor acidic OH groups that are upward shifted by H-bonding to strong basic species of contiguous phases.7,16 The interaction of acidic hydroxyls of precursor species with basic ones of anatase and amorphous TiO2−x usually leads to condensation processes produced during nucleation stages. However, the observation of lines PA3, instead of a water line, indicates that the H-bonding of precursor hydroxyls is stabilized by exchanging protons between precursor OH groups and oxygen anions of amorphous TiO2−x. The much higher intensity of line PA3 in amorphous-rich HP samples than in anatase-rich HT ones and the strong increment of its upward shift suggest that these species could also interact with terminal hydroxyls of amorphous TiO2−x (line MBI). The higher intensity of line PA3 in the HP2 spectrum than in the HP0.5 one suggests that these sites are more abundant in the more condensed, but predominantly amorphous, HP2 sample. An important feature exhibited by the spectra of evacuated samples is the presence of equally spaced spinning side bands7 (denoted ssb). The presence of these bands informs that the detected protons undergo important H−H dipolar interactions with neighboring hydroxyls, enlarging the spectral region occupied by the corresponding components. The calculated chemical shifts of the central ssb components are ∼6 (line Wc) and 9.9 ppm (line MA3), close to those of water molecules and of interacting acidic hydroxyls of amorphous TiO2−x. The high integrated intensity of ssb indicates that they are originated by hydroxyls of amorphous TiO2−x. Water removal produced by outgassing at 373 K (Figures 4c and 5c) originated the decrease of MA1/NA1 and NB2/MB2 lines; however, lines MA3 and Wc increased. This increment, consistent with the increment of the two sets of ssb bands, indicates that dehydration favors the interactions between acidic and basic OH groups of contiguous phases. On the basis of these facts, the set of satellite broader lines centered at ∼6 ppm can be assigned to water formed by the reaction of acidic hydroxyls of highly defective anatase with basic ones of amorphous TiO2−x. This water would remain bound to low coordinated Ti4+ cations of the amorphous phase. However, water of line Wc can be expelled by strong interaction between basic bridging O2− anions of anatase and low coordinated Ti4+ cations of amorphous TiO2−x, favoring the bonding of amorphous titania to the anatase surface and the coverage of anatase particles with amorphous TiO2−x. The higher intensity of line Wc in the HP0.5 spectrum than in the HP2 one is consistent with the stronger peak of the most strongly adsorbed water in the HP0.5 TG profile than in the HP2 one and with the removal of coordinated water and the formation of Ti−O bonds during aging treatments of suspensions. The set of narrower ssb lines, originating lines MA3 at ∼9.9 ppm, could be ascribed to the above-mentioned water-mediated H-bonding of acidic hydroxyls of amorphous titania with basic ones of anatase. In amorphouscovered anatase particles, the basic hydroxyls of amorphous phase could interact with solvated acidic protons of anatase precursor species. In Figure 6, the scheme of all possible interactions occurring between hydroxyls of anatase, the amorphous phase, and precursor species is depicted. The anatase coverage with amorphous titania would impede the anatase crystal growth. The findings that after evacuation at RT line PA3 shows intensity values higher in the HP2 sample 5114
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ACKNOWLEDGMENTS J.S. and I.S. thank the Spanish Agency CICYT (project MAT2010-19837-C06-02) and the regional Government (project S-2009/PPQ 1626) for financial support. S.Y. and V.A. thank Drs. M. Bellardita and S. Megna for help given in catalysts characterization.
■ Figure 6. Scheme of possible interactions occurring between hydroxyls of anatase, the amorphous phase, and precursor species.
than in the HP0.5 one and that anatase crystal size increases only in the first two hours of aging time, maintaining a constant value until the almost complete anatase-to-rutile transformation,5 support this consideration. On the other hand, the strong interaction between these two phases favors the transformation of the metastable anatase into the more stable rutile, observed after 8 h aging. This transformation follows the Ostwald rule that states that first formed compounds are those kinetically favored, transforming into more stable ones as time progresses.20 In small nanoparticles, differences in Gibbs free energy of possible phases decrease when surface energy is considered. In perfect anatase crystals, octahedral Ti−O units are interconnected with other octahedra through four edges, while in rutile crystals the octahedra share only two edges.19 Probably, the interaction of acidic hydroxyls on the highly defective surface of very small anatase particles with basic hydroxyls of low condensed amorphous titania facilitates the transformation of octahedral connections from edges to corners, so determining the formation of rutile.
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CONCLUSIONS The present investigation shows that 1H-MAS NMR spectroscopy allows the simultaneous study of the TiO2 precursor and very small amorphous and anatase nanoparticles. This capability allows us to conclude that, during anatase synthesis by TiCl4 hydrolysis at RT and aging treatments at 373 K, amorphous titania and very defective anatase are first formed, but their mutual strong interaction leads to the progressive covering of the anatase surface by the amorphous phase, impeding the anatase growth and favoring the subsequent slow transformation of anatase into rutile. The formation of amorphous phases is not exclusive of nanocrystalline TiO2 samples. The effects of interactions between amorphous and crystalline oxides must be taken into account when dealing with small nanoparticles of metal oxides prepared by synthesis methods in liquid media.
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The authors declare no competing financial interest. 5115
dx.doi.org/10.1021/jp2112044 | J. Phys. Chem. C 2012, 116, 5110−5115
