Vis Simultaneous Monitoring of the TiO2

Mar 24, 2010 - Weibull Cumulative Distribution Function for Modeling the Isothermal Kinetics of the Titanium-oxo-alkoxy Cluster Growth. Zorica Z. Baro...
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QEXAFS and UV/Vis Simultaneous Monitoring of the TiO2-Nanoparticles Formation by Hydrolytic Sol-Gel Route Jan Sto¨tzel,*,† Dirk Lu¨tzenkirchen-Hecht,† Ronald Frahm,† Celso Valentim Santilli,‡ Sandra Helena Pulcinelli,‡ Renata Kaminski,§ Emiliano Fonda,§ Franc¸oise Villain,§ and Vale´rie Briois§ Department C/Physics, UniVersity of Wuppertal, Gauβstraβe 20, 42097 Wuppertal, Germany, UNESP, Instituto de Quimica, Rua F. Degni, s/n, CP355, 14800-900 Araraquara, Brazil, and Synchrotron SOLEIL, L’Orme des Merisiers Saint Aubin - BP 48, 91192 Gif sur YVette, France ReceiVed: NoVember 3, 2009; ReVised Manuscript ReceiVed: March 10, 2010

The hydrolysis of titanium(IV) tetraisopropoxide and the following condensation and aggregation to titania nanoparticles under slow water solution addition were concomitantly investigated by in situ QEXAFS and UV/vis spectroscopies. The amount of dominant titanate species in the solution was determined as a function of time, temperature, and water addition rate. Resulting from fast hydrolysis and condensation of isopropoxide precursors an intermediate oligomeric species is initially formed at the early stages of the reaction. This oligomeric species has been identified as a dodecatitanate species with Ti11O13 or Ti12O16 molecular framework. At the subsequent stage of the reaction corresponding to slower processes compared to the fast hydrolysis, titania nanoparticles are formed by a cluster-cluster growth mechanism involving the consumption of the dodecatitanate species following a pseudo first-order reaction. Finally, at the advanced stage, sudden aggregation of titania nanoparticles occurs at a given threshold of amount of nanoparticles. Introduction Sol-gel chemistry offers fascinating potentials for the production of tailored nanomaterials with optimized physicochemical properties at modest temperatures. The most common route for preparing suspensions of monodisperse nanometersized particles is the so-called bottom-up route, which starts from molecular precursors in solution giving rise to the formation of solid networks after hydrolysis-condensation reactions.1 The so prepared nanoparticles find numerous technological applications2 as catalyst, optical glasses, dense ceramics, ultrafiltration membranes, gas sensors, and so on. For all those applications it is essential to control the particle growth during the sol-gel process and therefore to have a detailed knowledge about the time scales of the hydrolysis, condensation, and aggregation processes, in particular, concerning their dependencies on water solution addition rate, temperature, and chemical concentration of reactants. Titania (TiO2) is widely used in various technological applications of daily life, such as white pigment and opacifier for paints and cosmetics,3 for dielectric mirrors or energy converter for photovoltaic solar cells,4,5 as catalyst6 for photochemically degrading organic pollutants in so-called self-cleaning coatings and for water and air purification processes.7 The solution sol-gel (SSG) process8,9 used for preparing titania includes the hydrolysis-condensation of titanium alkoxides Ti(OR)4 (R ) Pri, Bun, Prn, But) as precursors forming rapidly titanium oxooligomers of a general formula TixOy(OR)4x-2y. These oligomeric titanium species are in turn chemically active and react to form titania nanoparticles. In the past decade, numerous TixOy(OR)4x-2y oligomeric species obtained for defined hy* Corresponding author. E-mail: [email protected]. † University of Wuppertal. ‡ UNESP. § Synchrotron SOLEIL.

drolysis ratios, h ) [H2O]/[Ti], were identified by nuclear magnetic resonance (NMR) and X-ray diffraction (XRD).10-13 But despite these well-defined crystallographic structures of oligomeric titanium species so-determined, it is noteworthy that no attempt was done to identify in situ the participation of oligomeric species in the process of titania nanoparticle formation under slow water solution addition. In this paper, we report the combination of the QEXAFS14,15 technique and UV/vis spectroscopy for monitoring the main stages involved in the titania nanoparticle formation from titanium tetraisopropoxide (TTIP) precursors, also referenced in literature as Ti{OCH(CH3)2}4, Ti(OPri)4, or tetraisopropyl titanate. The QEXAFS technique is a powerful tool to investigate in situ the structural changes of materials during fast processes such as chemical reactions and phase transitions. The temporal change of the local order around titanium provided by the X-ray absorption near edge structure (XANES) is correlated to the evolution of the nanoscopic size of the particles probed by the light scattering deduced from UV/vis spectroscopy. Experimental Section Materials. Although water and isopropanol are completely miscible, the mechanical mixing process is characterized by a slow kinetic and it takes some time to achieve complete homogeneity at a molecular level. To minimize this time, we did not mix the pure reactants; we are instead mixing two solutions of water and of TTIP in isopropanol. As clearly indicated by simple visual observations and also by flow field simulations,16 the mixing of these two solutions in isopropanol seems a typical mixing process where two fluids with similar properties get in intimate contact. In this work, 22 mL of the TTIP solution (0.16 mol L-1) was loaded into the thermostatted sample cell and maintained under vigorous magnetic stirring (1000 rpm) during the entire experiment. The hydrolysis ratio h was regularly increased during the experiment by the addition

10.1021/jp9105019  2010 American Chemical Society Published on Web 03/24/2010

TiO2-Nanoparticles by the Hydrolytic Sol-Gel Route of a solution of water (18.8 mol L-1) in isopropanol using a peristaltic pump. The volume of one drop of this solution delivered from the peristaltic pump amounted to approximately 0.016 mL, providing 0.30 mmol of H2O to the reactive solution. Depending on the peristaltic pump speed and the duration of the kinetic monitoring, a variation of h ranging from 0 (no added water) to 4.3 and up to 7.5 was considered herein. The experiment was repeated several times to investigate the effect of different parameters like temperature and water solution addition rate on the kinetic processes. For an additional analysis, the precipitates obtained at the end of the monitoring were isolated from the solution, dried, and compacted as a pellet for further XANES characterization. A 0.5 mol L-1 concentrated solution of TTIP in isopropanol was prepared in a nitrogen-filled glovebox and immediately characterized by XANES in a hermetically isolated liquid cell. UV/Vis and X-ray Absorption Spectroscopies. The UV/ vis absorption spectra were measured within the 240-840 nm range using a Cary 50 UV/vis spectrometer from Varian, Inc. An immersion probe in direct contact with the reactive solution was used to measure the transmitted light within an optical path of 1 mm. Data collection was carried out in a continuous way with a scanning speed of 4800 nm min-1, that is, the completion of one spectrum lasted 8 s. X-ray absorption measurements at the titanium K-edge confront several experimental difficulties linked to the low energy of the titanium K-edge (4966 eV). In time-resolved experiments, the high parasitic absorption by air, windows, and solvents cannot be compensated by using long-time measurements. Furthermore, a layer of only 0.1 mm of TTIP solution already absorbs 73% of the photons at the titanium K-edge. Consequently, a special cell design, as that already successfully used for time-resolved experiments at low energies (Ce L3 edge at 5.7 keV),17 is needed. This cell provides a thin layer of solution in the path of the X-rays and enables the user to add water solution through a hole in the top of the cell. This hole is also used to insert the immersion probe of the UV/vis spectrometer inside the solution without disturbing the X-ray experiments and vice versa. Figure 1 shows the experimental setup that made it possible to heat and mix the solution in the cell using a magnetic stirrer with integrated heater, while the water solution could be added drop by drop with a peristaltic pump. The windows of the cell were polyethylene foils with a thickness of 10 µm, and the gap between them was reduced by a thread to a few tenths of a micrometer. The X-ray absorption spectra were acquired at the SAMBA beamline,18 which uses the radiation emitted from a bending magnet with a critical energy of 8.65 keV at the SOLEIL synchrotron radiation source. During the experiments, the ring current amounted to about 150 mA of 2.75 GeV electrons. The Quick-EXAFS monochromator19,20 was temporarily installed in the optical hutch of the beamline about 1 m downstream of the permanently installed sagitally focusing monochromator, about 18 m away from the source and 14 m away from the sample. A first cylindrically bent Pd-coated mirror at a grazing incidence of 8 mrad was used for collimating vertically 6.5 mm of beam onto the Si(111) channel-cut monochromator crystal and a second one also set at 8 mrad was used for focusing vertically the beam to 300 µm at the sample position and for ensuring a fixed-exit of the monochromator. The monochromator crystal resides on a cam-driven tilt table for rapid angular oscillations of the Bragg angle. We used for the first time the upgraded system21 with an angular encoder for coding the Bragg angle oscillations and consequently allowing the collection of XAS

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Figure 1. Experimental setup: the liquid cell on the magnetic stirrer unit is visible as well as the peristaltic pump and the UV-vis immersion probe.

data with a direct energy scale. The channel-cut crystal was tuned to a Bragg angle of 23°. An amplitude of 1° crystal oscillation around this value was chosen to acquire absorption spectra in the energy range of 4861-5278 eV. An oscillation frequency of 0.1 Hz was used for the crystal movements, that is, one spectrum every five seconds, which was sufficient to get an adequate time resolution for the studied reactions. However, the signal-to-noise ratio of the acquired single spectra was limited by electronic noise, and thus, 10 subsequent spectra were averaged in each case for further data analysis. This averaging was done using a moving window in steps of two spectra such that the first averaged spectrum can be assigned to 25 s followed by 1 every 10 seconds. There is a time of uncertainty of a few seconds at the start of the reaction because it is impossible to exactly determine the point in time when the first drop of water solution fell into the TTIP solution. The energy scale of the monochromator was calibrated with a 5 µm thick titanium metal foil located between the second and third ionization chambers and simultaneously measured in transmission with each sample in the cell. For the analysis of the X-ray absorption data, the software package Athena22 was used. Linear combinations (LC) with the spectra of components that were expected in the solution during the reaction were carried out for fitting the XANES spectra at any stage of the kinetic monitoring. An energy range from -20 to 100 eV relative to an energy value E0, which was fixed at 4980 eV for each XANES spectrum, was considered for fitting. The maximum of the first derivative of the titanium reference foil was used to exactly calibrate each data set, so that the E0 was not taken into account as additional parameter for the LC fits. Prior to the LC fits, a linear background was fitted to the pre-edge region and subtracted from the spectra, which were normalized in a consistent way. For the analysis of the EXAFS spectrum of TTIP, first a pre-edge background was removed using a linear function. A postedge background using the Autobk

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Figure 2. Background-subtracted QEXAFS spectra of the first run vs time. Two time regimes can be recognized. For details, see text.

algorithm was applied with a cutoff Rbkg ) 1 and k-weight ) 2 to isolate the EXAFS oscillations χ(k). Then the EXAFS data were Fourier transformed using a k3-weighting Kaiser-Bessel window (3.3-15 Å). Besides, the software package ATOMS23 and FEFF624 integrated in the ARTEMIS graphical interface were employed to generate ab initio phases and amplitude functions for single and multiple scattering paths in a molecular unit TiOCH(CH3)2 deduced from the atomic positions of the same atoms in the crystal [Ti2(O-2,4,6-Me3C6H2)2(OiPr)4(µ-OiPr)2].25 EXAFS fitting of distances and Debye-Waller factors were performed with the ARTEMIS interface to IFEFFIT26 using least-squares refinements. The S02 amplitude reduction parameter, which takes multielectronic effects into account, and the energy shift E0 were calibrated at 1.1 ( 0.1 and 0.4 ( 1.5 eV by fitting the TiO2 anatase crystalline reference, respectively. The reliability of the fit was assessed by a residual factor RF that was minimized. Results and Discussion Time-Resolved Study of the Hydrolytic Sol-Gel Formation of Titania Nanoparticles. The background-subtracted and normalized QXANES spectra recorded for a kinetic monitoring carried out during the controlled water solution addition at 48 °C are presented in Figure 2. During this study, the nominal hydrolysis ratio h increased from 0 to 4.3 with a stepped increase equal to 0.084 each 30 s. The first striking observation is the change of the shape of the XANES spectra observed between the first and last measured spectra, in particular, in the preedge region below 4980 eV. At the 3d transition metal K-edge, electronic transitions are governed in a first approximation by electric dipole selection rules which state that 1s electrons can only transit toward electronic levels built from p-type atomic orbitals. In this framework, the prepeak or pre-edge structure is assigned to a transition of a 1s electron to the first vacant molecular orbital built from 3d atomic orbitals hybrized with p metallic atomic orbitals. Because this hybridization strongly depends on the symmetry around the absorbing atom, group theory considerations show that such hybridization does not occur for absorbing atoms embedded in a centrosymmetric point group, like

Sto¨tzel et al. octahedral symmetry. At the opposite, the hybridization is stronger if the symmetry around the absorbing atom is noncentrosymmetric, like for tetrahedral symmetry. The change observed from the first measured spectrum that displays an intense prepeak at about 4970 eV to the last spectrum that displays a less intense pre-edge is, thus, indicative of a symmetry change around titanium from a noncentrosymmetric into a more centrosymmetric one. This is in good agreement with the chemical reactions involved in the SSG process which transform TTIP into titania nanoparticles. Namely, the titanium atom in the TTIP precursor is well-known to present a tetrahedral symmetry, whereas titania (i.e., TiO2) presents three crystallographic structures, anatase, rutile, and brookite, which are all based on different octahedral arrangements of oxygen atoms around titanium. Besides the symmetry change around titanium clearly evidenced by XANES, two temporal regions are well distinguished upon water solution addition. The first one lasts for approximately 200 s and is characterized by strong modifications of the shape of the XANES resonances upon addition of water solution, in particular a significant decrease of the intensity of the pre-edge resonance located at 4969.7 eV and a change of the relative intensities of resonances I and II at the maximum of the edge. In contrast to that, in the subsequent time range, only slight changes are visible. Nevertheless, these changes can be traced, for example, in the edge structure and they seem to proceed very slowly from 200 s to the end of the data acquisition at 1575 s. The edge position itself remains constant during the reaction indicating, as expected, that the titanium remains at the same oxidation state (4+) during the whole sol-gel process. Figure 3 shows the results of the UV/vis-spectra recorded in situ simultaneously with the XANES spectra shown in Figure 2. Looking at the raw data in Figure 3a for each spectrum, an increasing absorption appears below about 350 nm in agreement with the titania nanoparticle bandgap >3.0 eV. Besides this absorption threshold, the spectra are featureless, displaying only a general absorbance increase during the entire time of data acquisition. The time evolution of the increasing absorption shows three distinct stages in the kinetic in contrast to the two regimes observed with QEXAFS. At the early stage (t < 250 s), a fast increase of absorption is observed, then in the intermediate stage (250 s < t < 1000 s), the absorption varies much more slowly. Finally, at the advanced stage (t > 1000 s), the absorption increases again significantly. These first results clearly demonstrate the importance to combine different techniques in timeresolved experiments. Here the combination of QEXAFS and UV/vis is invaluable to get simultaneous information about the local structure of participating species on the one hand and the optical properties, including the turbidity, of the investigated sol on the other hand. It should be noted that the three-step process observed with UV/vis spectroscopy is in full agreement with time-resolved studies reported in the literature using medium range order techniques like light scattering methods and UV absorption.27-29 Kanaev et al.29 evidenced that (i) the initial hydrolysis-condensation reaction of precursors giving rise to oligomeric titanium species is very fast during the mixing of reagents; (ii) nucleation of 2 nm particles resulting from the reaction of oligomeric species occurs immediately after the start of the reactant mixture; (iii) these nanoparticles slowly grow from 2 to 4 nm and finally explosively grow into large aggregates. Although no identification of the relative populations of hydrolyzed or condensed species arose from these detailed

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I(θ) )

(

)

8π4NV2 n2 - 1 2 (1 + cos2 θ) r2λ4 n2 + 2

(1)

with the scattering angle θ, the particle volume V, the number of contributing particles N, the distance from the particle r, the incident wavelength λ, and the relative real refraction index n ) nparticle/nmedium.30 This relation evidence that both the particle sizes as well as the number of particles lead to an increasing absorption for all wavelengths simultaneously. Accordingly, the observed modifications of absorption properties of the reactive media for 250 s < t < 1000 s are related to changes of the light scattering properties of the solution induced either by an increase of particle size and by an increase of number of particles in the solution. In the advanced stage (t > 1000 s), a distinct increase of the absorption in the UV/vis spectra is observed, and this increase of the absorption starts later for larger wavelengths. Taking into account the work reported by Kanaev et al.,29 which emphasizes a sudden increase in particle size higher than 4 nm at this stage of the reaction, we assume that the observed absorbance increase irrespective of the considered wavelength is mainly governed by the changing size of the titania particles. Because the Rayleigh theory is not valid anymore for larger particle sizes, the well established Mie scattering theory has to be considered at the advanced stage of the reaction.30,31 The extinction of highly reflective titania particles is dominated by the scattering efficiency Qsca, which reveals a characteristic interference pattern as a function of the size parameter F defined by the relation

Figure 3. UV/vis results for kinetic 1. (a) UV/vis raw data for the wavelength range 250-840 nm. The time scale is synchronized with the QEXAFS results. Three regions are distinguishable during the general increase in absorption. (b) UV/vis data at selected wavelengths vs time. The three regions can be assigned to different states of the reaction. For details, see text.

medium range order kinetic studies, they concluded that the hydrolysis condensation of TTIP is complete at low hydrolysis ratio h e 1. Additional details can be extracted from the UV/vis data of selected wavelengths over time, which are depicted in Figure 3b. Whatever the chosen wavelengths, the general trend of temporal evolution is the same, only small differences dealing with the onset of absorbance changes are observed depending on the considered wavelength. Again, the first stage is obvious with a simultaneously increasing absorption at all wavelengths at the beginning of the reaction up to about 200-250 s. This corresponds to the first regime observed in the XANES data and can be assigned to the initial condensation of the hydrolyzed TTIP precursors. Hereafter, there is the already observed slow increase in absorption, which can be assigned to the slow changes in the second stage of the XANES data due to progressive formation of titania nanoparticles. After about 1000 s, the UV/vis-absorption increases significantly while no corresponding effect can be observed in the XANES spectra at this advanced stage. During the initial stages of the hydrolysis and the progressive formation of titania nanoparticles (0 < t < ca. 250 s), the particle size determined by light scattering ranges between 2 and 4 nm,29 which is sufficiently small to consider these particles as Rayleigh scatterers. In the framework of the Rayleigh theory, the scattering intensity of spherical particles can therefore be described by30

F)

(

)

4πm2a m1 -1 λ m2

(2)

with the particle radius a and the complex refraction indices m1,2 of the particles and the medium, respectively. This interference pattern displays a first interference maximum, followed by an oscillatory behavior modulated by ripples, especially for high refraction indices, as is the case here.31 For spherical particles with small contributions of the imaginary part of the refraction index, as expected for titania, the first maximum is located at about F ) 4.4.30 The absorption as a function of time plotted in Figure 3b is closely related to the scattering efficiency as a function of F, however, because the data is getting noisy for higher absorption values, because of the significant decrease in signal intensity and due to the additional rippled structure of the scattering efficiency,31 it is quite difficult to accurately determine the first maximum here. Instead, it is much easier to detect the inflection point of the light absorption, respectively, the scattering efficiency, which appears in the absorbance increase prior to the first maximum and is located at approximately half the size parameter value F ) 2.2. If such a procedure is applied to data presented in Figure 3b, it is thus possible to approximately derive the particle size for the time value, where the inflection point in absorption appears for a specific wavelength λ. This was performed for different values of λ in each run and resulted in the approximated particle size curves shown in Figure 8. It should be stressed here that the particle size derived as described above should only be considered as rough estimates, as discussed below in more detail. First of all, in the theoretical description of the absorption that was used for the data evaluation (eq 2), the polydispersity of the particle size is not included, and accordingly, a particle size distribution can not be determined from the UV/vis data. However, we have performed model calculations assuming different Gaussian distribution functions, especially the mean particle size and the width of the distribution were varied

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systematically. The results show that, in the case of a Gaussian particle size distribution, the position of the inflection point is not affected within the accuracy of the calculations, while the absorption features above the steep increase of the absorption for F > 4.4 are smeared out substantially. This may thus be one of the reasons why no clear oscillatory behavior of the absorption is observed for large time scales t > 1400 s, as expected in the ideal case of monodisperse particles (see ref 30). As a consequence, a closer inspection and modeling of the absorption may allow the determination of the particle size distribution also, however, we restrain on the calculation of an average particle size here. Besides the limitations induced by the experimental conditions, that is, the small optical transmission at large reaction times, a more detailed evaluation of the UV/vis data in terms of the particle size distribution function may also be hampered by a possible influence of the particle shape on the spectra (see, e.g., refs 32, 33). Mie theory provides exact solutions for spherical particles and for some idealized particle shapes such as homogeneous, layered, or ellipsoidal cylinders.32 Much more elaborate calculation techniques have to be used for the modeling of more realistic particle shapes.34-37 However, according to finite-element methods, the influence of the particle shape seems to be almost negligible in the strongly increasing region of the absorbance, which is only considered here, while the region above the first maximum is strongly affected.37 Furthermore, in the present case, at least part of such an impact may cancel out by the random orientation of the titania particles with respect to the optical light path, leading in principle to the same effect in the absorption compared to a broadened particle size distribution.32 In addition, there are indications in the literature that isometric particles are formed during the early stages of particle formation and agglomeration.38 In conclusion, thus, the use of a rather simple approach seems to be justified to obtain an approximated average particle size and enables us at least to compare different runs of the combined time-resolved UV/vis and QEXAFS experiments under different reaction conditions, keeping in mind that the information derived from the UV/vis data is complementary to the structural data obtained by QEXAFS. Identification of Titanium Species. The LC technique for XANES spectra is a valuable approach to get quantitative results about the species arising from the hydrolytic polycondensation process and their proportions during the reaction. Therefore, we need the spectra of the species that are expected to be involved during the kinetic process. Two of those components are already known and have to be the pristine TTIP reactant and the formed titania nanoparticles. The XANES spectrum of the titania nanoparticles isolated at the end of the reaction is presented at the bottom of Figure 4. This spectrum is similar to the spectrum reported by Luca et al.39 for amorphous titania precipitates prepared from the dropwise TTIP addition to water and for which an extensive discussion of the attribution of the multiple resonances appearing in the pre-edge of the so-obtained titania nanoparticles have been given. Despite the widespread use of TTIP for preparing TiO2 nanoparticles, studies reporting the XAS spectra of the starting reactive are scarce.40-42 Furthermore, all the XANES spectra reported in the literature present a pre-edge intensity (∼0.5) far from the normalized pre-edge height (∼0.85 ( 0.15) of the 4-fold coordinated titanium compounds compiled by Farges et al.,43 suggesting that for those samples so investigated TTIP has already partially lost its 4-fold coordination due to the start of hydrolysis-condensation processes. Indeed, TTIP is very

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Figure 4. Reference material used for the linear combination fits: The initial Ti(OPri)4, the final amorphous titania, and the oligomeric intermediate dodecatitanate were used.

reactive and despite the care taken for preserving the solution from air moisture during its characterization, it is reported that the residual water content in isopropanol can be responsible for a hydrolysis ratio ranging from h ∼ 0.1 in the most concentrated up to h ∼ 3 in the most diluted TTIP/isopropanol solution.28 As our LC fitting of data will strongly depend on the pre-edge intensity of the main components in the reactive solution, it is of prime importance to record the spectrum of least hydrolyzed TTIP species possible. The upper part of Figure 4 displays the XANES spectrum measured for a concentrated TTIP solution in isopropanol on the SAMBA beamline using the sagitally focusing monochromator equipped with Si(111) crystals. The pre-edge intensity (∼0.75) is within the range determined phenomenologically by Farges et al.43 for 4-fold Ti coordination. The EXAFS signal for TTIP presented in Figure 5 can be satisfactorily simulated considering the structural parameters of monomeric TTIP species reported in Table 1. The simulation includes three single scattering paths within the TiOCH(CH3)2 molecular unit involving the oxygen atom, the carbon atom linked to the oxygen and the carbon atoms of the two methyl groups considered at the same distance from the central titanium atom in order to limit the number of parameters. Additionally, three multiple scattering paths involving the three quite collinear atoms Ti, O, and C are considered for the simulation. The distances so-refined are consistent with distances calculated from the crystallographic structure where the isopropoxyl group is linked in a terminal way to the titanium atom. It is noteworthy that no crystallographic structure for TTIP is reported in the literature due to the high reactivity of this compound, which makes a crystal preparation difficult. These results reported herein are to our best knowledge the first ones reporting non ambiguous structural parameters for this compound. The results of the LC fits considering only the spectra of pristine TTIP and the final titania precipitate (Figure 4) are not sufficient to describe the QEXAFS spectrum, as evidenced in Figure 6 for the data recorded at t ) 775 s of kinetic monitoring. The deviations between the LC fit and the experimental spectrum clearly indicates the presence of additional titanium species. As shown in the bottom of Figure 6, the inclusion of the XANES spectrum of an oligomeric titanate species leads to an acceptable deviation between the LC fits and the experimental spectra of the monitored system. In the literature, a family of remarkably stable polyoxotitanate species has been proposed by Day et al.10,11 as molecular building blocks for further polymerization

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Figure 6. Fit of the spectrum taken after 775 s with two, that is, TTIP and TiO2, and three (TTIP, TiO2, dodecatitanate) components. The graph at the bottom shows the deviations of the fit from the measured spectrum.

Figure 5. TTIP (a) EXAFS spectrum and (b) corresponding fast Fourier transform compared to the simulation considering the monomeric TTIP species built from the structural parameters gathered in Table 1.

TABLE 1: EXAFS Fit Results for TTIPa shell

N

σ 2 (10-3 Å2)

R (Å)

O C C

4 4 8

3.6 ( 0.5 1.8 ( 0.9 24.3 ( 2.0

1.81 ( 0.01 3.19 ( 0.02 3.73 ( 0.04

a Only parameters for single scattering paths are given in the table, but three additional multiple scattering paths involving the two first shells due to their colinearity with the central titanium atom has been considered in the simulation. The EXAFS reliability factor RF for this fit is equal to 0.020 and its reduced chi-square, χ2υ, is 30.9.

reactions. It deals with the Ti11O13(OPri)18 and Ti12O16(OPri)16 oligomeric species which are structurally very close and formed when the hydrolysis ratio is beyond 0.2. More recently a new oligomeric species Ti17O24(OPri)20 has been reported by Steunou et al.12,13 and assumed by Kanaev et al.44 as being the more probable oligomeric species for explaining their observed light scattering patterns during water/isopropanol solution addition to TTIP/isopropanol solution. Following the synthesis protocol described by Day et al.,10 we prepared Ti11O13(OPri)18 and Ti12O16(OPri)16 molecular building blocks. A yellowish polycrystalline powder was obtained for which Raman results45 and Ti K-edge EXAFS46 investigations agree well with the local order described for this family of compounds.10 In Figure 4 the XANES spectrum of this so-called “dodecatitanate” reference is compared with the two species already considered as components for the LC fits. The energy location of the main

pre-edge resonance (4969.6 eV) is at the same position as the intense pre-edge observed for TTIP, but with a strong reduction of intensity and 1 eV below the peak position of the pre-edge of titania amorphous precipitate isolated at the end of the kinetics. The intensity reduction of the pre-edge is in good agreement with the 5- and 6-fold coordination of titanium present in equal proportions in the dodecatitanate oligomeric species,10 while the difference of 1 eV in peak position compared to the titania can be assigned to the deviation from the octahedral symmetry.40 Specification of Titanium Species versus Time. Considering the three chemical species reported in Figure 4 as components for linear combinations, fits of the reactive solution at any stage of the SSG process were satisfactorily done. The fit results for the first and the last QEXAFS spectra in time are shown in Figure 7 with the deviations between fit and experimental data. It can be observed that the fits reproduce the data with only minor differences in the low percent level. Thus, just a very small fraction of other titanium oligomeric species might be formed during the process. Especially for the first spectrum with a small hydrolysis ratio, some Ti3(µ3O)(OPri)10 species, which are reported by Blanchard et al.47 to be formed for h ≈ 0.05, might be the reason for somewhat larger deviations because we only use the “dodecatitanate” species as oligomers for fitting. Also, the differences in the X-ray absorption near edge spectra of the dried titania nanoparticles used as reference and the titania structure in solution are supposed to contribute to these deviations. However, the LC fits reflect all the features of the spectra sufficiently and thus can be used for a semiquantitative data analysis. Figure 8a shows the time evolution of the three titanium species considered in the LC fitting of each QEXAFS spectrum shown in Figure 2 together with the approximated average particle size determined by Mie scattering theory, as described earlier in this report. These results are plotted in addition as a function of the offset corrected hydrolysis ratio h monitored by water solution addition (top of the x axis). This first experiment was performed with a water solution addition rate of 0.016 mL every 30 s at a temperature of 48 °C. It is remarkable that the LC fraction of TTIP (∼40%) is far from the expected value of 100% at the beginning of the data acquisition prior to any

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Sto¨tzel et al.

Figure 7. Linear combination fit of the first and the last spectrum with the three components shown in Figure 4. Only small deviations, shown at the bottom, are visible. For details, see text.

addition of water. This can be explained with the already mentioned sensitivity of TTIP to residual water content in isopropanol, to air moisture since the reaction takes place in an open cell and to the 1 min period before starting the QEXAFS measurements during which first water solution drops could fall into the cell. As a matter of fact, at the beginning, about 60% of oligomeric species from the “dodecatitanate” family are present, while still no titania nanoparticles could be detected. As condensed species, Ti11O13(OPri)18 and Ti12O16(OPri)16 are characterized by a hydrolysis ratio of 1.18 and 1.33, respectively, we conclude that corresponding to the collection of the first QEXAFS data at t ) 0, the initial hydrolysis ratio in the media is far from 0 but rather equal to a value ranging between 0.7-0.8. This initial hydrolysis ratio (h0 ) 0.75 ( 0.05) will be taken into account in the discussion below. The LC fraction of TTIP is decreasing very fast with water solution added by the peristaltic pump at the early stage, but it is noteworthy that this initial chemical reactive does not totally disappear in solution; the presence of about 3-4% of TTIP can still be identified after 250 s. No titania nanoparticles can be found during the first 100 s (h < 1.02 ( 0.05), however, its concentration increases rapidly to about 24% after 250 s of water solution addition (h < 1.45 ( 0.05). After that, the titania concentration increases slowly but continuously up to 44% at the end of the QEXAFS data acquisition at h ) 5, while the proportion of “dodecatitanate” species decreases with an analogous rate. This finding indicates that the formation and growth of titania nanoparticles occur by a cluster-cluster growth mechanism48 involving the polycondensation reaction between dodecatitanate. The occurrence of cluster-cluster growth process is consistent with the absence of continuous source of monomers revealed by the very low and almost invariant amount of TTIP observed after 250 s. In Figure 8b,c the results of two additional runs with different environmental parameters are shown. Run 2 was performed with a water solution addition rate of one drop every 15 s at a temperature of about 54 °C, while the same water solution addition rate of run 1 was chosen for run 3, with just a slightly lower temperature of about 35 °C. The formation of the titania nanoparticles for the second and third runs starts somewhat later after about 130 s compared to the kinetics of run 1. The higher

Figure 8. Results of linear combination fits for three different runs with varying reaction conditions. Time and corrected hydrolysis rate are included on the horizontal axis while the amount of the three reference materials is assigned to the left axis. Additionally, the approximated average particle size is plotted as a dotted line assigned to the right axis.

initial concentration of TTIP (∼46%) observed for run 2 should be responsible for this delay of TiO2 condensation, whereas the significant decrease of temperature in the third run should be a factor explaining this behavior. In each of the three runs, the titania nanoparticle concentration goes up to about 20-30% in the intermediate stage of the kinetics, which corresponds to the concomitant formation of dodecatitanate oligomeric species and titania nanoparticles, to reach 50% for run 2 after 1000 s and for run 1 after 1500 s. Actually, despite the change of the speed of water solution addition and change of temperature, the general evolutions observed with these additional runs are similar to those discussed previously for run 1. This is especially well evidenced in Figure 8 where the proportions of the different components for the three runs are plotted with a common hydrolysis ratio axis, taking

TiO2-Nanoparticles by the Hydrolytic Sol-Gel Route

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6235 the slope of the particle size curves increases as a function of time and hydrolysis ratio, indicating an increasing growth rate of the particles. This agrees well with the observations of Soloviev et al.28 The comparison of the curves of the different runs evidence that the aggregation starts much earlier for the doubled water solution addition rate and later for lower temperatures in agreement with the already analyzed condensation process. Finally, it clearly appears that the rapid increase in particle size occurs for the three experimental conditions when the same amount of titania nanoparticles is present inside the solution. This amount is equal to about 30% of the total amount of titanium. Conclusions

Figure 9. Plots of (a) [TTIP] vs time for t < 250 s and (b) [TiO2] vs time for t > 300 s for the three different runs with varying reaction conditions.

into account the initial hydrolysis ratio and the nominal added water. The formation of titania nanoparticles occurs when the total hydrolysis ratio is ranged between 1.0 and 1.35. Before any TiO2 nanoparticles appear, a zero-order kinetic law satisfactorily describes the consumption of TTIP to form the more condensed “dodecatitanate” species, as shown in Figure 9a, with the linear dependence of the TTIP proportion as a function of time for t < 240 s. This finding suggests that at this rate of water solution addition the solution contains a continuous source of hydrolyzed alkoxide precursor. It is also remarkable that the formation of TiO2 during the subsequent stage is highly dependent on the speed of water solution addition as can be recognized in Figure 9b with the concentration of TiO2 plotted vs time for t > 300 s. For the doubled water solution addition rate even a slightly more than doubled condensation rate constant is verified what might be a result of the slightly higher temperature in that run. Accordingly, the rate of run 3, which was performed at a lower temperature, is slightly lower than for run 1. These findings strongly suggest that the formation of TiO2 observed at the advanced stage of the kinetic monitoring, for t > 300 s, follows a first order kinetic with respect to the concentration of water. As a consequence, the kinetic of the cluster-cluster growth process of titania nanoparticles can be quantitatively described by a linear time dependency as expected in the so-called reaction limited regime as proposed in early studies.49,50 The evolution of the particle size at the advanced stage of the reaction obtained from UV/vis data within the Mie scattering theory has been reported in Figure 8a-c. It is noteworthy that

The hydrolysis-condensation of titanium(IV) tetraisopropoxide leading to the formation of titania (TiO2) nanoparticles was investigated by a combination of in situ time-resolved X-ray absorption and UV/vis spectroscopies. It has been shown quantitatively that the formation of titania nanoparticles from the reaction of TTIP with water involves the following stepped temporal evolution: (i) At a very early stage, a polynuclear titanium species is produced from the polycondensation reaction of the hydrolyzed alkoxide precursor. This step is completed within the first few minutes, producing the oligomeric species identified as a dodecatitanate species with a Ti11O13 or Ti12O16 molecular framework. (ii) An intermediate stage is achieved when the total hydrolysis ratio is ranged between 1.00 and 1.35. This stage is characterized by the concomitant formation of dodecatitanate oligomeric species and titania nanoparticles. (iii) At the subsequent stage, oligomeric species are very slowly consumed in a polycondensation/aggregation process of growth of titania nanoparticles. The rate of this process is approximately proportional to the instantaneous amount of water addition and increases with higher temperatures. A pseudo first order kinetic of formation of titania from dedecatitanate species is verified. (iv) At the advanced stage, a fast change of the scattering light properties of the system occurs, which is related to the sudden aggregation of titania nanoparticles freshly formed. With the combined techniques of UV/vis spectroscopy and QEXAFS it could be shown that the average particle size, detemined via Mie scattering theory, is linked to the fraction of titania in the solution during the aggregation processes. Acknowledgment. The authors want to thank SOLEIL for providing beamtime at the SAMBA beamline and the financial support of the experiments. Many people, scientists, engineers, and technicians, have contributed to the experiments described here. Especially, we want to thank U. Haake, L. Barthes, and M. Ribbens for their help at the beamline and the staff of SOLEIL for the excellent support and the successful solution of many problems during the installation of the QEXAFS monochromator and its commissioning at the SAMBA beamline. This work was also partially supported by the CAPES/ COFECUB cooperation program (Ph 564/07). References and Notes (1) Schmidt, H. J. Sol-Gel Sci. Technol. 2006, 40, 115. (2) Livage, J. Catal. Today 1998, 41, 3. (3) Clark, R. J. H. The Chemistry of Titanium and Vanadium; Elsevier: Amsterdam, 1968. (4) Mills, A.; Le Hunte, S. J. Photochem. Photobiol., A 1997, 108, 1. (5) O’Regan, B.; Gra¨tzel, M. Nature (London) 1991, 353, 737. (6) Schneider, M.; Baiker, A. Catal. Today 1997, 35, 339. (7) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis Fundaments and Applications; BKC: Tokyo, 1999. (8) Stallings, W. E.; Lamb, H. H. Langmuir 2003, 19, 2989.

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