Intrinsically Formed Trivalent Titanium Ions in Sol–Gel Titania

Eduardo Mun˜oz, Jose´ L. Boldu, Eduardo Andrade, Octavio Novaro, and Xi. Bokhimi

Institute of Physics, The National University of Mexico (UNAM), 01000 Me´xico D. F., Mexico

Tessy Lopez† and Ricardo Go´mez

Department of Chemistry, Universidad Auto´noma Metropolitana–Iztapalapa, 09340 Me´xico D. F., Mexico

Electron paramagnetic resonance (EPR) room-temperatureobservation of trivalent titanium in sol–gel titania has beenreported. This unprecedented detection of stable paramagneticsignals in this titania, which was treated at 200°–800°C,occurred regardless of whether the atmosphere was reducing,inert, or oxidating. The possibility that the paramagneticsignals may originate from an metal impurity other thantitanium was completely excluded by analyzing the samplesusing three different ion-beam-analysis techniques: Ruther-ford backscattering spectroscopy, particle-induced X-rayemission resonance, and energy-recoil detection analysis. Theparamagnetic signals appeared at precisely the temperaturerange of the sample dehydroxylation, which suggests a mech-anism for explaining these Ti31 EPR spectra.

I. Introduction

THE photocatalytic properties of titanium dioxide (TiO2, ortitania) semiconductors are well established.1,2 The continuing

interest in such oxides originates from the fact that TiO2 is anexcellent catalyst for oxidizing a large variety of organic sub-stances.3,4 The capacity of TiO2 to completely oxidize many toxicsubstances to form nontoxic compounds distinguishes it as beingthe most promising material for the photodetoxification of con-taminated water. The semiconductive properties of TiO2 arestrongly dependent on its crystalline structure, nonstoichiometry,and doping procedure.5–8 Stoichiometric titania is a dielectricmaterial; however, it exhibits semiconductor properties when itsstoichiometry is altered (for example, by decreasing its crystallitesize).9,10 Therefore, large TiO2 crystallites are not useful forphotocatalysis. The conductivity and catalytic properties of TiO2

also are related to its reducibility.TiO2 is a reducible oxide; therefore, the valence of the Ti ion

can be changed from the stable tetravalent state (Ti41) to thetrivalent state (Ti31). The latter ion has a 3d1 electronic configu-ration and can be identified using electron paramagnetic resonance(EPR). The formation of Ti31 ions has been observed in or-thophosphate single crystals, titanium phosphate compounds, andreduced samples of platinum/TiO2.

11–13 Traditionally, the Ti31

EPR signal has been observed when the samples are placed inenvironments where the temperature is equal to or less than that ofliquid nitrogen. The charge of the Ti31 centers is balanced by thepresence of hydroxyls in its coordination sphere14 (in the matrix),which suggests that the Ti41 3 Ti31 transformation is stronglydependent on the degree of TiO2 hydroxylation. Large quantitiesof these ions must be observed in highly hydrogenated samples, as

Conesa and Soria13 acknowledged when they reduced TiO2 inhydrogen. These ions also are observed when TiO2 is in thepresence of other metals, such as palladium, platinum, andrhodium. However, the formation of Ti31 ions in the samples fromthe Conesa and Soria study is totally reversible, and no tracesappear at higher temperatures, which raises the following question:would Ti31 ions exist in the conditions of catalytic reactions?

The situation may be quite different for TiO2 that has beenprepared via the sol–gel technique.14–16In this case, when TiO2 isprepared by the hydrolysis of titanium alkoxides, the samples arehighly hydroxylated and the hydroxyls that are bonded to the Tiatoms persist, even when these samples are treated at rather hightemperatures ($600°C). Therefore, in the present study, thesol–gel method has been used to synthesize highly hydroxylatedTiO2, for the purpose of stabilizing the Ti31 ions.

In this paper, differential thermal analysis/thermogravimetricanalysis (DTA/TGA), Fourier transform infrared spectroscopy(FTIR), and EPR studies of the samples that were treated at varioustemperatures suggest that, in such preparations, Ti31 ions werestabilized at room temperature. The EPR signals were clear;however, they could be produced by metal impurities in thesamples. To eliminate this possibility, it was necessary to showthat no significant concentrations of metals other than titaniumwere present in the sample. Accordingly, a particle accelerator wasused to determine the sample atomic composition, with highprecision, which allowed the detection of metal impurities thatmay exist, even at concentrations as low as a few parts per million(ppm). A 5.5 MeV CN Van de Graaff accelerator in our facilities17

was used to perform ion-beam analysis (IBA), including Ruther-ford backscattering spectroscopy (RBS), energy-recoil detectionanalysis (ERDA), and particle-induced X-ray emission (PIXE)analysis. These studies were presumed to discriminate betweensources for the paramagnetic signals and confirmed that Ti31 ionsindeed were present in the sol–gel-derived TiO2, even at atemperature of 600°C. To identify the TiO2 polymorphs in thesamples quantitatively, the specimens were analyzed using powderX-ray diffractometry (XRD) and the crystalline structures wererefined using the Rietveld method.

II. Experimental Procedure

(1) Sample PreparationThe sol–gel-derived TiO2 samples were prepared by dissolving

0.2 mol of titanium tetraethoxide (Ti(OEt)4, 99.9% pure; AldrichChemical Co., Milwaukee, WI) in 50 mL of ethanol (CH3OH) andrefluxing at a temperature of 70°C. Then, 32 mL of water (H2O)and ammonium hydroxide (NH4OH), as a hydrolysis catalyst,were added and the mixture was refluxed and stirred constantlyuntil gelling occurred. The resulting gels were dried at 70°C for12 h and then thermally treated at several temperatures.

(2) Characterization(A) Thermoanalysis: The dried sample was placed in the cell

and then heated at a rate of 20°C/min in flowing nitrogen.

C. J. Brinker—contributing editor

Manuscript No. 189738. Received November 23, 1998; approved January 10,2000.

†Author to whom correspondence should be addressed.

J. Am. Ceram. Soc.,84 [2] 392–98 (2001)

392

journal

DTA/TGA was performed using a DTA/TGA apparatus (ModelDT-30, Shimadzu Co., Kyoto, Japan).

(B) Infrared Spectroscopy: FTIR characterization of thesolid samples was achieved using a spectrometer (Model 170-SX,Nicolet Instrument Corp., Madison, WI) that had a resolution of 2cm21. Self-supported wafers were placed in the quartz (SiO2)infrared cell, which was equipped with potassium bromide (KBr)windows, and activated under vacuum at the desired temperature.FTIR spectra of the calcined samples were recorded at roomtemperature.

(C) Specific Surface Area Analysis:Nitrogen adsorption iso-therms were obtained using a nitrogen adsorption apparatus(Model ASAP 2000, Micromeritics Instrument Group, Norcross,GA). The specific surface areas were calculated from theseisotherms, via the Brunauer–Emmett–Teller (BET) method.

(D) Electron Paramagnetic Resonance Spectroscopy:EPRspectra were obtained at room temperature, using a conventionalreflection-type spectrometer with a cylindrical cavity (ModelJES-RE3X, JEOL, Tokyo, Japan) (in TE011 mode) that wasoperating at a field modulation of 100 kHz. Theg values wereobtained by measuring the resonance field with a calibratednuclear magnetic resonance (NMR) gaussmeter (Model ES-FC5,JEOL) and a frequency counter (Model HP-5350B, Hewlett–Packard, Palo Alto, CA). For the EPR measurements, the thermaltreatments were performed in a controlled atmosphere furnace inwhich the sample was maintained at the appropriate temperature inthe quartz tube, to avoid any influence from the external atmo-sphere. The pressure of the flowing gas was slightly higher thanthe atmospheric pressure during sample cooling to room temper-ature and the respective EPR measurement.

(E) Chemical Composition: Fine TiO2 powder was com-pressed using loads up to 7 metric tons to form specimens in theform of disks 1 cm in diameter and 0.3 cm thick. For the RBS andERDA experiments, a surface-barrier detector and standard elec-tronics were used to measure the particle energy that was producedby the 2 MeV 4He1 bombardment of the sample. The RBSanalysis of the4He1 backscattered particles at an angle ofu 5165° was performed via computer simulation, using the well-known RUMP code18 to obtain the main atomic composition of thesamples: titanium and oxygen. The ERDA technique used tomeasure the hydrogen content in the TiO2 samples was basicallythe same as that used by Doyle and Peercy.19 A 2 MeV 4He1

incident beam on the 75° tilted target and the elastically scatteredprotons were measured at a forward angle ofu 5 30° and analyzedusing a computer code that was developed in our laboratory.20 A6-mm-thick Mylar™ foil (DuPont, Wilmington, DE) was placed infront of the detector, to absorb the4He1 scattered particles andprevent them from reaching the detector. For the PIXE studies, acryogenic Si(Li) detector was used to measure the X-ray energyspectra that was produced by sample bombardment using a 2 MeVproton beam. This detector is only sensitive to elements with anatomic number that is greater than that of sodium, and it canmeasure atomic concentrations in the ppm range.

(F) X-ray Diffractometry: Crystalline structures were ana-lyzed using powder XRD and refined via the Rietveld method withthe DBWS-9411 code.21 XRD patterns from specimens that hadbeen prepared by packing the sample powder into a glass holderwere measured at room temperature with a diffractometer (ModelD8-ADVANCE, Bruker Instruments, Billerica, MA), using CuKaradiation. The peak intensity was measured by step scanning in the2u range of 10°–110°, with a 2u step of 0.02° and a measurementtime of 2 s per point. XRD peak profiles were modeled with apseudo-Voigt function that used the average crystallite size as afitting parameter.22 The standard deviations (which are given inparentheses in the current work) correspond to the variation in thelast decimal place of the measurement. The standard deviationscorrespond to refined parameters; therefore, they are estimates ofthe minimum possible probable errors only, based on the normaldistribution of the parameters, rather than being an estimate of theanalysis as a whole.23

III. Results and Discussion

The parameters that characterize the crystallite structure andsize were obtained from the Rietveld refinement. The followingunit cells and space groups, which are known for each of thecrystalline phases in large crystallite TiO2, were used: for anatase,a tetragonal structure of four TiO2 molecules per unit cell wasused, with the space groupI4i/amd; for rutile, a tetragonal structureof two TiO2 molecules per unit cell was used, with space groupP42/mnm. The positions of the atoms in the anatase and rutilephases are listed in Table I. Both the anatase and rutile phases ofsol–gel-derived TiO2 were titanium-deficient. The average crys-tallite size and density increased when the samples were annealedat high temperature (see Table II).

Figure 1 shows a DTA thermogram with a large endothermicpeak at 110°C that corresponds to desorption of the solvent(ethanol) and the excess H2O in the sample. This temperature alsois related to the sample crystallization (see Table III), because thesamples were amorphous when they were fresh. In the 200°–340°C temperature region, where the samples contain only nano-crystalline anatase (Fig. 2), an endothermic shoulder that isassociated with the onset of dehydroxylation is evident. Moreover,the sharp endothermic peak at 430°C marks the desorption of thestrongly linked hydroxyls, which indicates that the sol–gel-derivedTiO2 is highly hydroxylated. This dehydroxylation occurred dur-ing the anatase3 rutile transformation (see Fig. 2). Completedehydroxylation was achieved only after the sample was annealedat 800°C.

The TGA thermogram in Fig. 1 shows a continuous weight lossfrom 100°C to;480°C; after this point, no further significantmass variation was detected in the TGA study. The evolution ofthe TGA analysis corresponded well with the most importanttransformation that was discussed for the DTA analysis. On theother hand, the BET specific area was constant (;81 m2/g), evenafter the sample was treated at 600°C (see Table III).

The dehydroxylation peaks that are assigned to OH desorption(shown in Fig. 1) are confirmed by the FTIR spectrum that isdepicted in Fig. 3. The curve has two high-energy peaks: onelocated at 3386 cm21 and another located at 3173 cm21 (see curve“(a)”) in Fig. 3). Both peaks correspond to hydroxyl stretchingvibrations. The first peak is due to the TiOOH species that formedduring the gelation; the second peak can be attributed to theinternal OH groups of residual H2O and CH3OH. The peaks thatare located at 2082 and 1401 cm21 resulted from the residualethoxy groups, because the pH level (pH 9) favors polymerizationin the gel. The OH flexion vibration of H2O was present at 1600cm21 but disappeared during the thermal treatment. Indeed, theintensity of the high-energy band that is located at 3386 cm21, aswell as those of the other bands that are associated with TiOOHbonding, were reduced only when the samples were annealed attemperatures of$300°C (see curves “(b),” “(c),” and “(d)” inFig. 3).

The EPR results are shown in Fig. 4. The Ti31 ion has a 3d1

electronic configuration and a2D3/2 ground state. Although, inprinciple, an electric crystalline field would split this ground stateinto a triplet and a doublet, it clearly would be impossible toobserve the Ti31 EPR signal at room temperature, unless theorbital angular momentum is further quenched by rearrangements

Table I. Atomic Fractional Coordinates†

Atom Site

Coordinate

x y z

Anatase, space groupI4i/amdTi 4a 0.0 0.75 0.125O 8e 0.0 0.25 u

Rutile, space groupP42/mnmTi 4a 0.0 0.0 0.0O 8e v v 0.0†Note: According to Table II,u 5 0.0801(4)–0.0831(2) andv 5 0.304(2)–

0.3049(4).

February 2001 Intrinsically Formed Trivalent Titanium Ions in Sol–Gel Titania 393

of the local configuration of the crystalline field, which, indeed,systematically has been the case.11–14 Ti31 ions have beenobserved only at rather low temperatures (2196°C and below),where the splitting of the ground-state energy level is noticeablefor the Ti31 ion, provided that the electron spin relaxation has anadequate value. Even in systems in which noble metals have been

incorporated to reduce the Ti41 ions in TiO2-supported catalysts,temperatures at least as low as that of liquid nitrogen are requiredto observe the Ti31 EPR signal.24

In the present work, we studied samples that were preparedusing the sol–gel technique,25–27which, as evidenced by the DTAand FTIR analyses, induced hydroxyl ions within the TiO2 lattice.The inclusion of hydroxyl ions resulted in vacancies in the TiO2

lattice and apparently provoked a further splitting of the energylevels of the ground-state Ti31 ion, which produced EPR signals athigher temperatures than those reported for conventional TiO2.However, Ti31 EPR signals recently were reported to have beenobserved in phosphate powders at room temperature.12

Even more striking was the phenomena shown in Fig. 4 afterTiO2 was annealed in reducing, neutral, and oxidizing atmo-spheres. For comparison, curve “(a)” in Fig. 4 shows the EPRspectrum of the fresh sample; no appreciable signal was observed(even at a high amplifier gain). Curves “(b),” “(c),” and “(d)” inFig. 4 show the EPR spectra of samples that have been treated at400°C for 15 min in atmospheres of oxygen, argon, and hydrogen,respectively. One single first-derivative absorption line (line widthof 0.6 mT) withg 5 1.9986 0.004 appeared in all three traces asthe only signal over the scanned magnetic field (200 mT). Thespectra of the annealed samples were measured with the sameamplification and field modulation; however, the spectrum for thefresh sample was taken with a gain factor that was 200 times largerthan that of the other samples. In all cases, the signal was similar,albeit of different intensity (compare curves (b)–(d) in Fig. 4).

Naturally, the EPR signal was caused by the hydrogen that wascontained in the sample. The amplitude of the signal for the samplethat was treated in argon diminished substantially and even moresofor that treated in the oxidizing atmosphere. Therefore, we mayconclude that the presence of hydrogen enhanced the formation ofTi31 ions and the presence of oxygen hindered but did not inhibit theproduction of Ti31 ions. Under the current experimental results, this

Table II. Anatase and Rutile Parameters, as a Function of Temperature

Temperature(°C)

Average crystallitesize (Å)

Tioccupancy†

Lattice parameter (nm) Density(g/cm3) ua c

Anatase300 10.1(2) 0.095(1) 0.37838(5) 0.9480(2) 3.35 0.0801(4)600 34.1(8) 0.1124(9) 0.37818(1) 0.95053(4) 3.67 0.0831(2)

Rutile300 50(12) 0.111(6) 0.45904(6) 0.29560(6) 3.97 0.304(2)600 136.2(4) 0.113(1) 0.45901(1) 0.29574(1) 4.01 0.3049(4)

†Note: The Ti occupancy for the stoichiometric composition is 0.125.

Fig. 1. DTA and TGA thermograms of a sol–gel-derived TiO2 sampleheated at a rate of 20°C/min in flowing nitrogen.

Table III. Phase Composition and Specific Surface Area, asa Function of Temperature

Temperature(°C)

Composition (wt%) BET specificsurface area

(m2/g)Mean pore diameter

(nm)Anatase Rutile

300 100.0 0.0 89 8.7600 93.9 6.1 81 12.8

Fig. 2. Powder XRD patterns of samples treated in air at differenttemperatures. The small vertical tick marks under the XRD pattern of thesample that was treated at 900°C correspond to rutile, and those under theXRD pattern of the sample that was treated at 300°C correspond to anatase.

394 Journal of the American Ceramic Society—Mun˜oz et al. Vol. 84, No. 2

is direct evidence of the formation of Ti31 ions, especially consider-ing the PIXE studies (Fig. 6), which excluded the presence of anymetals other than titanium in the sample. This observation is quiteremarkable if we recall that Conesa and Soria14 inevitably had tosubject the sample to highly reducing conditions to obtain a Ti31

signal (observable moreover only at low temperatures).Naturally abundant titanium is composed of different isotopes;

among these species, the even–even isotopes constitute the majorcontribution, although47Ti(I 5 5⁄2) and49Ti(I 5 7⁄2) respectivelyrepresent 7.75% and 5.51% of the total content. Indeed, the signalsthat are reported in Fig. 4 correspond exclusively to even–evenisotopes. The limited signal-to-noise ratio that is ascribed to thesol–gel preparation process contributed to the large spread inmagnetic field of the expected broad hyperfine contribution of thetitanium isotopes, which prevented the observation of hyperfinespectra.

Figure 5 shows the results of the thermal treatment to which thesamples were subjected. The reported intensity represents thepeak-to-peak height of the first derivative of the adsorption EPRcurve, normalized to the weight of the sample. Intensity measure-ments were performed from room temperature up to 800°C, inincrements of 100°C. The sample was held at the specifictemperature for 15 min in the indicated gas flux before its EPRmeasurement. For all three gases, the threshold temperature atwhich Ti31 ions appeared was observed to be 200°C. From 200°Cup to 400°C, the corresponding EPR signal increased monotoni-cally until a maximum was attained. After this maximum wasattained, the intensity of the Ti31 signal decreased and, at 500°C,it began to increase again until a secondary maximum was

attained. Further increases in temperature rapidly decreased theformation of Ti31 ions, which was practically negligible at 800°C.The most significant conclusion of these results is the largestability of the Ti31 signal at such high temperatures, especiallyunder a hydrogen flux, but surprisingly even for the case of thesamples treated in oxygen. To the authors’ knowledge, this result

Fig. 3. FTIR spectra of sol–gel-derived TiO2 ((a) fresh TiO2, (b) TiO2

sample treated at 300°C, (c) sample treated at 600°C, and (d) sampletreated at 900°C).

Fig. 4. Room-temperature EPR spectra of the Ti31 ions in sol–gel-derived TiO2 powder (the first derivative of the adsorption versus appliedmagnetic field). Spectrum “(a)” corresponds to the fresh sample; all othersamples were treated at 400°C for 15 min in either an oxygen flow(spectrum “(b)”), an argon flow (spectrum “(c)”), or a hydrogen flow(spectrum “(d)”). Theg-factor for spectra (b)–(d) was 1.9986 0.004. Forall samples, the magnetic-field modulation was 0.08 mT.

Fig. 5. EPR signal intensity of the Ti31 ions in sol–gel-derived TiO2powder, as a function of annealing temperature; the samples were subjectedto three different gas atmospheres ((F) hydrogen gas, (Œ) argon gas, and(E) oxygen gas.

February 2001 Intrinsically Formed Trivalent Titanium Ions in Sol–Gel Titania 395

has not been observed in traditionally prepared or commercialTiO2. Therefore, the preparation method that has been used in thepresent work must be solely responsible for the existence of theobserved Ti31 signals. The sol–gel technique is characterized bythe high hydroxylation that it induces in oxides. The fact that thefreshly prepared TiO2 did not exhibit an EPR signal also must beconsidered. Indeed, the presence of Ti31 signals was detected atthe temperatures that corresponded to the beginning of the dehy-droxylation process and peaked after the second dehydroxylationtemperature was attained. The signal disappeared when totaldehydroxylation was achieved. The maximum signal intensity inFig. 5 occurred at 400°C, which was closely related to themaximum dehydroxylation. The relative decrease that followedthis maximum signal from 400°C to 600°C was related to theanatase3 rutile transformation (see Fig. 2); yet, at 600°C, 6.1(6)wt% of the anatase was transformed to rutile. Therefore, the excess

hydroxyls from the sol–gel preparation of TiO2 are associated withthe Ti31 signals.

We shall now revisit the question of how one can eliminate thepossibility that the EPR signals could be at least partially due to thepresence of metal impurities. These impurities could be Cr31,Cr51, Mo51, Fe31, etc., which, depending on their coordination,could result in signals similar to those that have been observed.

The determination of the chemical composition of materialsusing ion-beam-accelerator techniques is one of the successfulapplications of low-energy particle accelerators (E , 10 MeV) andhas been demonstrated to be a powerful technique for extremelysensitive measurement of the atomic composition and impurityconcentrations near the surface of solids (at depths of#2 mm).28,29

To perform a complete impurity characterization of the sol–gel-derived TiO2, the following IBA techniques had to be used: (i) aconventional 2 MeV4He RBS analysis, to determine the samplecomposition; (ii) a PIXIE28 method, using a 2 MeV proton beam,which allows the impurity content in the sample to be determined;and (iii) ERDA, using the 2 MeV4He beam, to obtain the1Hconcentration.

In all the samples, the RBS energy spectrum (see Fig. 6)indicated the presence of only two elements: titanium and oxygen,with a composition of Ti1O1.8 (instead of the stoichiometriccomposition of TiO2). However, the XRD analysis indicated acation deficiency.15 To understand this difference, it is importantto note that the XRD technique is much more sensitive to Tivacancies than to O vacancies, which, according to the RBSexperiments, were the most abundant.

A much-more-thorough search for metallic impurities can bederived using the PIXE technique. Figure 7 shows a typical PIXEspectrum for one of the samples; it was produced primarily by Tiatoms. Metal atoms such as chromium, molybdenum, or iron areconspicuously absent or, at most, are present only in a smallfraction (on the order of parts per million).29 Considering that EPRhas the capacity to detect any paramagnetic species, even at theppm level, such impurities cannot be responsible for the EPRsignals.

In turn, the ERDA examination of the samples that were heatedat 400°C for 15 min in a hydrogen, argon, or oxygen flux clearly

Fig. 6. Typical RBS spectrum, where the solid line is theRUMP fit to theexperimental data. It was assumed that only oxygen and titanium contrib-uted to the spectrum. The fit provided a Ti:O atomic ratio of 1:1.8.

Fig. 7. Typical PIXE energy spectrum, where only titanium has a contribution. The positions of the Mo, Cr, and Fe emissions also are indicated, to showthat these elements were not present in the sample. Background for channels of.120 was due to pulse pile-up.

396 Journal of the American Ceramic Society—Mun˜oz et al. Vol. 84, No. 2

showed that they had a high1H content. This content is especiallyhigh for the TiO2 sample that was subjected to a reducingatmosphere (see Fig. 8). The1H concentrations that were obtainedfrom the fitting of the spectra were 142, 17.7, and 1.1 (in units of1020 1H isotopes/cm3) for the samples that were treated inhydrogen, argon, or oxygen, respectively. The large concentrationof protons in the sample may be linked to the titanium deficiencythat was detected using XRD. These deficiencies originate fromthe OH groups in the sol–gel-derived TiO2, which adds oxygenand hydrogen to the final count.

IV. Conclusions

The observation of electron paramagnetic resonance (EPR)signals that are associated with Ti31 ions in sol–gel-derived TiO2is reported; these EPR signals persist at unprecedently hightemperatures (up to.600°C). Careful ion-beam studies usingRutherford backscattering spectroscopy, particle-induced X-rayemission analysis, and energy-recoil detection analysis bar thepossibility that the EPR spectrum could be caused by metalimpurities in the samples. Definitively, the observed EPR signal isproduced by Ti31 ions. Even if room-temperature Ti31 EPRsignals have been observed for powders,12,13 the bulk samplesrequire strong reducing conditions. The well-known Ti31 EPRspectra of aqueous solutions of titanium complexes rarely tran-scend room temperature. Thus, it seems remarkable that solid-state

TiO2 may exhibit a Ti31 EPR intensity, even when treated underoxidizing conditions, as has been shown in this study. Further-more, we have related these results to the exceptional capacity ofsol–gel-derived TiO2 to retain hydroxyls in its lattice at tempera-tures of$600°C. Considering the usefulness of semiconductiveTiO2 for the oxidation of toxic substrates, the present results makethe use of sol–gel-derived TiO2 very promising for such purposes.In particular, the use of sol–gel-derived TiO2 for the photocatalyticdetoxification of contaminated water seems tempting, especiallyconsidering the fact that the semiconductive properties of conven-tional TiO2 are limited by two factors: it must be doped andreduced to remain nonstochiometric. However, the sol–gel-derived TiO2 is quite nonstochiometric, even at high tempera-tures,9–10 and requires no doping or any reducing treatments,maintaining its Ti31 content well beyond what is possible for anyother known solid TiO2 material.

Acknowledgments

The authors would like to acknowledge the technical assistance of J. Barreto, A.Morales, M. Aguilar, and A. Sanchez. They also are grateful to E. P. Zavala and J. C.Pineda for maintaining and operating the accelerator.

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