unraveling the photocatalytic activity of multiwalled hydrogen trititanate and mixed-phase...

Published: January 14, 2011

r 2011 American Chemical Society 2302 dx.doi.org/10.1021/jp112005m | J. Phys. Chem. C 2011, 115, 2302–2313

ARTICLE

pubs.acs.org/JPCC

Unraveling the Photocatalytic Activity of Multiwalled HydrogenTrititanate and Mixed-Phase Anatase/Trititanate Nanotubes:A Combined Catalytic and EPR StudyStefan Ribbens,† Ignacio Caretti,‡,§ Evi Beyers,† Sepideh Zamani,‡ Evi Vinck,‡ Sabine Van Doorslaer,*,‡ andPegie Cool*,†

†Laboratory of Adsorption and Catalysis, University of Antwerpen (UA), Universiteitsplein 1, B-2610 Wilrijk, Belgium‡Department of Physics, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium§Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Campus de Cantoblanco, 28049 Madrid, Spain

bS Supporting Information

ABSTRACT: A detailed study of the photocatalytic activity of hydrogentrititanate nanotubes (H-TNT), formed by a hydrothermal treatment, wascarried out. H-TNT show a limited activity toward pyridinium chloridedegradation under UV-light and even no activity under visible light. Incontrast, H-TNT show activity toward the degradation of rhodamine 6G(R6G), both under UV and visible light. EPR spectroscopy is used to gaininsight into this difference. UV-light excitation of H-TNT leads to thepredominant formation of Ti3þ centers by trapping of electrons at Ti sites, whereas almost no reactive oxygen-based species areformed. Upon visible light excitation of these nanotubes, no light-induced EPR signals are observed. The activity toward R6Gdegradation thus stems from the excitation of R6G (under both UV and visible light) and the subsequent transfer of electrons intothe conduction band of TiO2. After a short calcination process at 623 K, the H-TNT undergo a partial phase transformation intoanatase, without affecting the shape andmorphology of the nanotubes, and the photocatalytic activity increases to a great extent. TheEPR analysis now reveals the formation of different types of species characterized by g values larger than ge, both uponUV and visiblelight excitation. These reactive species, such as O2

- and O-, are known to play an important role in the photocatalytic process.

’ INTRODUCTION

Heterogeneous titanium-dioxide-based photocatalysts arevery promising materials for alternative water and air treatmentssince the photoinduced “advanced oxidation processes” can leadto a completemineralization of most pollutants emitted by indus-trial and domestic activities.1,2 Therefore, a lot of synthesis proce-dures have already been developed to prepare a cheap, stable,mesoporous material with high surface area and excellent photo-catalytic properties.3,4 However, in 1998, Kasuga et al.5 discoveredthe alkaline hydrothermal route for the synthesis of titanium-oxide nanotubes, which allows a complete conversion of an initialraw TiO2 powder to titanate nanotubes

6 at relatively low hydro-thermal temperatures. In the beginning, most studies focused onthe understanding of the formation mechanism7,8 of these nano-tubes, the improvement of the synthesis method,9,10 and theelectrochemical properties.11,12 In this way, the potential of thetitanate-phase nanotubes toward fuel cell technology, super-capacitors, and lithium batteries was investigated. Nowadays,more researchers focus on the photocatalytic properties of thenanotubes.9,13a Whereas nanoparticle-based photocatalysts arehard to remove out of solution, which can have adverse effects onalgae, fish, and higher organisms,13b titanium-oxide nanotubescan be easily recovered from a solution by sedimentation. Despitethe fact that titanate nanotubes are very promising as cheap

photocatalysts toward the photocatalytic degradation of dyes,and allow a facile scaling-up of production, a detailed study of thephotoinduced activity of these Ti-based nanotubes is still awaited.

In this work, the photocatalytic activity of trititanate andmixed-phase (trititanate/anatase) nanotubes was studied byevaluating the photocatalytic degradation of rhodamine 6G(R6G) and pyridinium chloride (PyCl) over long time intervalsusing optical absorption measurements and microvolume totalorganic carbon analysis (TOC). Furthermore, using X-bandlight-induced electron paramagnetic resonance (EPR), the originof the created paramagnetic centers as well as their time evolutionunder illumination is unraveled. In this way, more detailsconcerning charge separation, charge-trapping mechanisms,and the resulting photocatalytic activity are obtained.

’EXPERIMENTAL SECTION

Chemical Reagents. All the products were used as receivedwithout any modification or purification, unless stated other-wise. Ultrapure milli-Q water was used to prepare the differentsolutions.

Received: December 17, 2010

2303 dx.doi.org/10.1021/jp112005m |J. Phys. Chem. C 2011, 115, 2302–2313

The Journal of Physical Chemistry C ARTICLE

Sample Preparation. Trititanate nanotubes (TNTs) arerolled-up mesoporous materials, which can be prepared withoutusing any template. Here, 4.5 g of TiO2 (Fluka) (anatase powder,12 m2/g) was dispersed into 80 mL of 10 M NaOH solutionunder vigorous stirring during 1 h. The mixture was transferredinto a Teflon-lined autoclave and hydrothermally treated at 423K for 48 h. After hydrothermal treatment, the solid was recoveredby centrifugation. The precipitate was washed 3 times withdeionized water. In this way, sodium trititanate nanotubes (Na-TNT) are obtained. To prepare hydrogen trititanate nanotubes(H-TNT), the wet cake was dispersed into 240 mL of 0.1 MHClsolution and stirred for 30 min. The solid was recovered again bycentrifugation and dried for 2 days. Afterward, the dried powderwas dispersed again in 100 mL of 0.1 M HCl for 5 min. Theprecipitate was separated by filtration and washed three timeswith water. Finally, the washed solid was dried at 373 Kfor several days. By calcination at 623 K for 6 h (heating rate278 K min-1), trititanate nanotubes partially recrystallize intoanatase nanotubes. These mixed-phase nanotubes will be hence-forth referred to as H-TNT C623.Samples of H-TNT with adsorbed R6G (hereafter referred to

as H-TNT/R6G) were prepared by dispersing 16 mg of catalystin 50 mL of 4 � 10-5 M R6G solution. After 40 min, the nano-tubes were recovered by filtration, washed with 25mL of distilledwater, and dried at room temperature for 2 days.Instrumentation. UV-vis Absorption Spectroscopy. UV-vis

absorption scans of the aqueous pollutant solution were takenat fixed time intervals (30 min) on a Thermo-electron evolution500 UV-vis double beam spectrometer. In this way, the photo-bleaching of the initial pollutant (oxidation toward intermediates)was evaluated. Using Lambert-Beer’s law, the measured absor-bance can be converted to the corresponding concentration. InTable 1, themolar mass, the absorbancemaximum, and the struc-tural formula of both pollutants are plotted. Figure S1 (Sup-porting Information) shows the optical absorbance spectrum ofR6G and PyCl in the 200-600 nm range.Total Organic Carbon Analysis (TOC). To evaluate the

photomineralization (oxidation to CO2), microvolume TOCanalysis was used. Measurements were performed on a ShimadzuTOC-VCPH equipped with a palladium normal sense catalyst anda Shimadzu designed gas injection kit. Samples are injected with a

high precision syringe of 250 μL (Hamilton 1725 gastight syringewith Chaney Adapter, RN type 2). Combustion of the injectedsamples to CO2 takes place at 953 K. The amount of CO2 ismeasured with an NDIR (nondispersive infrared) detector.UV-vis Diffuse Reflectance Spectroscopy (UV-DR). This

technique was used to get information about the band gap energyand was recorded on a Thermo-electron evolution 500 UV-visspectrometer equipped with a Thermo-electron RSA-UC40 Dif-fuse Reflectance cell.N2-Sorption. The surface area and porosity of the mesopor-

ous nanomaterials were determined on a Quantachrome Quad-rasorb SI automated gas adsorption system with AS-6 degasser.The samples were outgassed at 423 K for 16 h. Subsequent N2-sorption was carried out at 77 K. The Brunauer-Emmet-Teller(BET) method was used to calculate the specific surface area.The volume adsorbed at a relative pressure P/P0 = 0.97 was usedto determine the total pore volume.FT-Raman Spectroscopy. Samples were measured on a Nico-

let Nexus 670 bench equipped with a InGaAs detector in a 180�reflective sampling configuration using a 1064 nmNd:YAG laserand a laser power of 0.8 W.Thermal Gravimetric Analysis/Derivative Thermogravi-

metric Analysis (TGA/DTG). Thermogravimetric analysis givesinformation on the weight loss as a function of the temperature.Measurements were executed on a Mettler Toledo TGA/SDTA851e. Samples were heated until 1073 K with a heating rate of278 K/min.Electron Energy Loss Spectroscopy (EELS). Reference spectra

of trititanate and amorphous TiO2 were recorded. All the refe-rence signals have been background subtracted, deconvoluted,and normalized. The spectra for the unknown samples were theninterpreted as a linear combination of the two references spectra,taking into account a background and multiple scattering.14

Electron Paramagnetic Resonance (EPR). X-band continu-ous wave (CW)-EPR experiments were performed on a BrukerESP300E spectrometer (microwave (mw) frequency 9.45 GHz)equipped with a gas-flow cryogenic system (Oxford, Inc.), allowingoperation from room temperature down to 2.5 K. The magneticfield was measured with a Bruker ER035 M NMR Gaussmeter.The spectra were recorded at 10 K, 100 K, and room temperatureusing a microwave power of 2 or 5 mW, a modulation amplitudeof 0.1 or 0.5 mT, and a modulation frequency of 100 kHz (seedetails in figure captions).Light Sources. For the light-induced (LI)-EPR experiments,

the H-TNT samples were irradiated with UV or visible light froma Kr-ion laser (Spectra Physics 2580) with wavelengths of 350.7and 406.7 nm or with visible light from an Ar-ion laser (SpectraPhysics 2020), with a wavelength of 514.5 nm (see details infigure captions). Laser powers of 5 mW were used.For the photocatalytic activity tests, the following light sources

were used: (1) illumination withUV-light: polymeric light source(equipped with filters, peak at 365 nm), 100 W Hg-lamp(Sylvania Par 38; 21.7 mW/cm2 at 5 cm) and (2) irradiation inthe visible region (395-800 nm): Philips Plusline double-endedlinear halogen lamp with UV-block (0.16 mW/cm2 at 100 cm).Measurement of Photocatalytic Activity by UV-vis Anal-

ysis (Photobleaching). The photocatalytic activity was testedby photodegradation of R6G and PyCl in aqueous solution. Forthe photodegradation of R6G, 8 mg of catalyst was dispersedin 50 mL of 4� 10-5 M R6G (pH of solution is 6.5). If PyCl wasused as a synthetic pollutant, 32 mg of catalyst was added to50 mL of 16 � 10-5 M PyCl (pH of solution is 5). The pH did

Table 1. Molar Mass, Absorbance Maximum, and StructuralFormule of Rhodamine 6G and Pyridinium Chloride



not change when intermediate, mineral acids were convertedtoward CO2. These catalyst amounts and concentrations wereapplied in all experiments, unless stated otherwise. Different con-centrations of pollutant solution were chosen to allow optimumTOC measurements. To maintain the same catalyst/pollutantratio, the amount of catalyst was also adapted. To establish anadsorption-desorption equilibrium between the pollutant mo-lecules and the catalyst surface, the catalyst was stirred in thepollutant solution for 40 min. Then, the solution was irradiatedfor 360 min with UV-light. During illumination, samples with avolume of 5 mL were taken out of the suspension at fixed timeintervals (30 min) and analyzed by UV-vis spectroscopy. Thedetermination of the decolorization efficiency (photobleaching)can be given by the following equation

efficiency ¼ C0 -CC0

ð1Þ

where C0 is the initial concentration of the dye after anadsorption-desorption equilibrium was established and C isthe final concentration after illumination byUV-light for a certaintime. Under the applied experimental conditions, photolysis ofthe pollutant is negligible. All photocatalytic reactions were per-formed at room temperature. The dissolved oxygen demand wasmeasured by an oxygen electrode and determined to be 7.45 (0.04 mg/L for the rhodamine 6G solution and 7.52( 0.16 mg/Lfor the pyridine chloride solution.Measurement of Photocatalytic Activity by Microvolume

(μV)-TOC Analysis (Photomineralization). Although evalua-tion of the photobleaching process (breaking of conjugatedsystem of pollutant) can give interesting information on theactivity of the catalyst, it is also important to evaluate the photo-mineralization (oxidation to CO2) since not all photobleachedmolecules will be fully photomineralized. In this way, detailedinformation on the photodegradation (photobleachingþ photo-mineralization) is obtainend. As mentioned above, a 5 mL samplewas taken every 30 min for the UV-vis analysis. From thissample, 0.5 mL was used for μV-TOC-analysis. The remaining4.5 mL of the sample was brought back to the illuminated testsolution to prevent large changes in volume/catalyst ratio. Usingthis 0.5 mL volume, two separate measurements were performedin which (i) the total amount of carbon (TC) was measured and(ii) the total amount of inorganic carbon (IC) was determined.For both TC and IC measurements, a minimum of threesequential injections was performed. Samples for TC analysiswere injected directly on the combustion oven, using the specialdesigned gas injection kit. Samples for IC analysis were injecteddirectly on the acid-containing vile. The TOC value can becalculated using the following equation

TOC ¼ TC- IC ð2Þ

’RESULTS

Pure trititanate (H-TNT) andmixed-phase anatase/trititanatenanotubes (H-TNT C623) are prepared following the above-described synthesis procedure. As reported in an earlier study,9

calcination of H-TNT at 623 K allows partial recrystallization oftrititanate to anatase without changing the nanotubemorphologyand surface area. Indeed, results of N2-sorption (Table 2) showthat the total pore volume and specific surface area of H-TNTand H-TNT C623 are similar, whereas H-TNT starts to degradeand transform into anatase nanoparticles at 723 K. EELS resultsshow that H-TNT C623 is much more crystalline due to cal-cination compared to noncalcined H-TNT (Table 2). Further-more, the formation of anatase slightly lowers the bandgapenergy of the initial material as shown in the UV-DR spectrum(Figure S2, Supporting Information). Due to the smaller band-gap of H-TNT C623, it can absorb light with a maximum wave-length of 410 nm (violet region of visible light), whereas H-TNTcan absorb light with a maximum wavelength of 398 nm. Althoughthese wavelengths can be absorbed, the absorbance is very limi-ted. In the following sections, the photocatalytic activity of bothcatalysts is evaluated by studying the bleaching and mineraliza-tion of pollutants rhodamine 6G and pyridinium chloride overlong irradiation times of the catalyst-pollutant mixtures. BothUV-light and visible light were used to activate the studied cata-lysts. Detailed EPR measurements are presented that investigatethe formation of photoinduced paramagnetic centers in supportof the information obtained by the photocatalytic tests. Note thata study of the degradation mechanism and identification of inter-mediate reaction products of the pollutants lies beyond the scopeof this article and has already been thoroughly described in theliterature.15-17 The outcome of the latter studies is used here inthe interpretation of the obtained data.Photocatalytic Degradation of Pyridinium Chloride by

H-TNT and H-TNT C623 under UV-Light Illumination.Figure 1 shows the bleaching and mineralization of pyridiniumchloride catalyzed by H-TNT and H-TNT C623 under UV-lightillumination. Here, it can be seen that H-TNT C623 has asomewhat higher adsorption capacity toward PyCl compared toH-TNT (see concentrations at t = 0), although the surface areasof H-TNT and H-TNT C623 are quite similar. Despite the goodinteraction between H-TNT and PyCl, the bleaching of PyClis rather limited. Indeed, after 6 h of intensive UV-irradiation,only 15% of the initial solution has been degraded into smallercompounds byH-TNT. Furthermore, no oxidation to CO2 (0%)takes place. In contrast to H-TNT, high photocatalytic efficiencycan be observed for H-TNT C623. After 6 h, 99% of the initialPyCl molecules have been bleached. The results of the TOC-analysis show that, at this stage, the aqueous solution of PyClcontains only 11% of carbon, which implies that 89% of the initialPyCl molecules and bleached intermediates are completely

Table 2. Surface Area, Total Pore Volume, Morphology, Crystal Phase, and Bandgap Energy of H-TNT, H-TNT C623, andH-TNT C723

SBETa (m2/g) Vp

b (mL/g) morphologyc crystal phased bandgape (ev) crystallinityf(%)

H-TNT 338 1.10 nanotube trititanate 3.3 45

H-TNT C623 318 1.10 nanotube trititanate þ anatase 3.2 79

H-TNT C723 104 0.11 nanotube þ nanoparticles anatase 3.2 -a SBET (specific surface area) deduced from N2-sorption; BET, 77 K. bVp (total pore volume) deduced from N2-sorption; BET, 77 K. cHR-TEM.dCrystal phase determined with FT-Raman spectroscopy. eBandgap energy determined using UV-DR spectroscopy. fCrystallinity determinedby EELS.



converted to CO2. It is clear that H-TNT C623 has excellentphotocatalytic properties, whereas the photocatalytic activity ofH-TNT toward PyCl is rather limited.Photocatalytic Degradation of Rhodamine 6G by H-TNT

and H-TNT C623 under UV-Light Illumination. To gain moreinsight into the photocatalytic activity of H-TNT and H-TNTC623 under UV-light, the photocatalytic degradation of anotherartifical pollutant was studied. In Figure 2, the results of bleachingand mineralization of rhodamine 6G photocatalyzed by H-TNTand H-TNT C623 are shown. Here, it can be seen that thebleaching and mineralization processes of R6G by H-TNT andH-TNT C623 are very different. The UV-vis absorption ana-lysis of the photodegradation of R6G byH-TNT shows that, after120 min of irradiation, 75% of the initial dye solution has beendegraded into smaller compounds, while H-TNT C623 has blea-ched 84% of the dye. This indicates that the rate of bleaching ofR6G by H-TNT C623 is slightly higher compared to H-TNT.After 6 h, the bleaching of R6G by both catalysts is almost comp-lete (99%). In contrast, TOC-analysis shows that, during the first90 min of UV-irradiation, the rate of photocatalytic mineraliza-tion by H-TNT (31%) is significantly higher compared to H-TNTC623 (13%). After 90 min of UV illumination, the photocata-lyzed mineralization by H-TNT stops abruptly, whereas theoxidation of intermediate reaction products by H-TNT C623is a continuous process, taking place in small steps. After 6 h ofirradiation, H-TNTC623 has mineralized 48% of the initial solu-tion, while no additional photomineralization could be observedfor H-TNT after 90 min.Photocatalytic Degradation of Pyridinium Chloride by

H-TNT and H-TNT C623 under Visible Light. Here, thephotocatalytic properties of H-TNT and H-TNT C623 are stu-died under visible light illumination. For these experiments, onlythe photocatalytic bleaching and mineralization of PyCl is studiedbecause dye molecules, such as R6G, may induce photocatalyticactivity under visible light via a fully different mechanism.18-20

Indeed, R6G can absorb visible light (Figure S1, SupportingInformation), which leads to excitation of electrons from π toπ*.Because of the strong interaction between the positively chargedrhodamine molecules and negatively charged trititanate surface,the electron diffusion distance is short allowing the adsorbedphotoexcited dye molecules to inject electrons into the conduc-tion band of the catalyst (Figure 3). This may lead to the forma-tion of radicals, which in turn results in the degradation of R6G.Therefore, only the results of the photocatalytic degradation ofPyCl by H-TNT and H-TNT C623 under visible light are stu-died in detail (see Table 3 and Figure S4, Supporting Information).In Table 3, it can be seen that no bleaching of PyCl takes place

by H-TNT under visible light irradiation, whereas it bleaches14% of the initial PyCl solution under UV-light illumination. Incontrast to H-TNT, the mixed-phase H-TNT C623 can clearlybe photocatalytically activated under visible light. However, thebleaching of PyCl by H-TNT C623 under visible light (15%) is∼6.5 times smaller compared to its performance under UV-light(99%). Furthermore, despite the noticeable bleaching of PyCl byH-TNT C623, no photomineralization is observed under visiblelight irradiation, while 89% of the initial PyCl solution is minera-lized under UV-light illumination. This indicates that photo-induced degradation reactions take place under visible light toconvert the initial PyCl molecules into smaller intermediates butthat the number of degradation reactions is too limited to obtaina large conversion into CO2.EPR Study of Light-Induced Paramagnetic Species. To

gain insight into the paramagnetic species that are formed uponirradiation with UV- and visible light and hence the mechanismof the photocatalysis, LI-EPR experiments were undertaken.Light-Induced EPR Signals in Hydrogen Tritantate Nano-

tubes upon Irradiation with UV-Light (350.7 nm). EPRexperiments were performed at 10 and 100 K, where the recom-bination rate of the photogenerated electron-hole pairs is low.Figure 4A shows the EPR spectra of H-TNT and H-TNT C623

Figure 1. Results of the TOC-analysis and the analysis with UV-vis spectroscopy for 32 mg of H-TNT/H-TNTC623 in 50 mL of 16� 10-5 M PyCl.The experimental procedure is explained in the text. nc = noncalcined.



after irradiation at 10 K with 350.7 nm UV-light during 1 h. Thespectra reveal the formation of several light-induced paramagneticspecies, which are clearly different for H-TNT and H-TNT C623.The EPR spectrum of H-TNT (Figure 4A) is dominated by a

contribution with principal g values of g1 = 1.953, g2 =1.938, andg3 =1.89 and broad linewidths (indicated as species I in Table 4,simulation in Figure S5, Supporting Information). This signal istypical for surface Ti3þ centers21a-21e that are formed when anelectron is trapped at a Ti site

Ti4þ þ e- f Ti3þ ð3ÞSimilar EPR parameters have been reported for Ti3þ centers in col-loidal anatase24c and in partially reduced TiO2 nanoparticles.

22,23

The EPR spectrum of H-TNTC623 (Figure 4B) also containscontributions due to electrons trapped at Ti sites (species II andIII, Table 4, simulation in Figure S5, Supporting Information),although these are only minority species. Species II (Table 4) canagain be assigned to surface Ti3þ centers. Species III (Table 4) is

Figure 2. Results of the TOC-analysis and the analysis with UV-vis spectroscopy for 8 mg of H-TNT/H-TNT C623 in 50 mL of 4� 10-5 M R6G.The experimental procedure is explained in the text. nc = noncalcined.

Figure 3. Energy level diagram of H-TNT and R6G.

Table 3. Percentage of Photodegradation of PyCl by H-TNTandH-TNTC623 under UV-Light and Visible Light for 32mgof H-TNT/H-TNT C623 in 50 mL of 16 � 10-5 M PyCl

UV-analysis (%) TOC-analysis (%)

H-TNT (UV-irradiation) 15 0

H-TNT (visible light irradiation) 0 0

H-TNT C623 (UV-irradiation) 99 89

H-TNT C623 (visible light irradiation) 15 0

Figure 4. X-band LI-EPR signals of (a), (c) H-TNT and (b), (d)H-TNT C623, upon irradiation with (A) UV-light (350.6 nm) and (B)visible light (406.7 nm). The experiments were performed at 10 K underair with modulation amplitude of 0.5 mT (A) or 0.1 mT (B) and a mwpower of 5 mW.



more typical for excess electrons located in the d orbitals of latticeTi atoms.24 The g values are similar to those reported for Ti3þ inan anatase phase.24d,24e This result supports that calcination ofthe tubes leads to anatase formation without affecting their sur-face area and nanotubular shape.9

In addition, both H-TNT and H-TNT C623 show severaloverlappingLI-EPR signals at g> ge afterUV irradiation (Figure 4A).These signals dominate the spectra of H-TNT C623 but are onlyminor in H-TNT. Although the relative intensity of the g > ge versusthe g < ge signals varies slightly upon batch, the overall differencebetween H-TNT and H-TNT C623 was reproduced in all batches.For H-TNT C623, the UV-light-induced signals resemble thoseobserved in anataseTiO2 powder calcined at 523K

24e and in anataseTiO2 nanoparticles.

25 This also corroborates the transformation tothe anatase phase upon calcination of the tubes.Kumar et al.24e interpreted these LI-EPR signals of TiO2

anatase powder at g > ge in terms of a single contribution, withg values of [2.016, 2.012, and 2.002], and assigned it to a holetrapped at a subsurface oxygen center (Ti4þO-Ti4þOH-).Although species with similar g values have often been observedin titania materials,24e,26 their origin is still debated. These gvalues are also typical for O3

- centers24e,27 and have even beenassigned to •OH radicals.26 Furthermore, Kumar et al.24e did notshow spectral simulations, and inspection of the EPR spectrum ofthe 523 K calcined anatase in their paper clearly shows spectralfeatures at g values higher than 2.016.Berger et al.24d interpreted the g > ge part of the CW-EPR

spectrum of TiO2 nanoparticles illuminated under UV-light interms of three species: O2

-[I]: g1 = 2.0248, g2 = 2.0096, g3 =2.0033; O2

-[II]: g1 = 2.0184, g2 = 2.0096, g3 = 2.0033; O-: g1 =2.0121, g2 = 2.0121, g3 = 2.0046. They were able to distinguishdifferent components because the CW-EPR measurements werecarried out at 140 K, whereas Kumar et al.24e recorded the spectraat 4.2 K. Similarly, the line width of the EPR componentsin H-TNT C623 reduces considerably at higher temperatures(Figure S6A, Supporting Information), facilitating the interpre-tation of the spectrum. The spectral features found in UV-illuminated H-TNT C623 could not be fully reproduced usingthe three species observed in TiO2 nanoparticles, and additionalcomponents were included to fit the data (Table S1, SupportingInformation). Due to the high degree of overlap, it was not possi-ble to unambiguously determine all components in the spectrum.However, it is clear that a variety of species are formed. Most ofthe EPR parameters are typical for adsorbed O2

- or O-/O3-

sites (see Supporting Information for an elaborate discussion). Ithas been previously shown that UV irradiation of TiO2 materialsunder air generally results in the formation of O2

- radicals,according to several possible mechanisms,24b such as

Ti3þ þO2 f Ti4þ 3 3 3O2- ð4Þ

Ti4þ þO2 þ e- f Ti4þ 3 3 3O2- ð5Þ

This was also confirmed by a recent study on the influenceof the vacuum annealing temperature of hydrogen trititanatetubes on the formation of O2

- defects under air upon UVillumination.28 It was found that at lower annealing tempera-ture (470 K) the formation of O2

- is higher than the formationof O- sites. These studies show that the presence of O2

-

centers in the tubes is very likely and supports the EPRassignments.Similar results are found for H-TNT. The weak signals at g > ge

can be assigned to a combination of adsorbed O2- or O-/O3

-

sites (Table S3, Supporting Information).Dynamics of the LI-EPR Species under UV-Light. Figure 5

shows the dynamics of the light-induced EPR species underUV-light, recorded at positions (i), (ii), and (iii) (see arrows inFigure 4). Position (i) agrees with surface Ti3þ sites, whereaspositions (ii) and (iii) fall within the g > ge region. In H-TNT, theTi3þ sites build up slowly, whereas the signals at g > ge are formedvery fast and quickly level off. When the laser is switched off, animmediate increase in the intensity of the Ti3þ EPR signal is ob-served, followed by a gradual increase in time. On the contrary,the signals at g > e show an initial small decay, followed by a stabi-lization after the laser is switched off.In H-TNT C623, the EPR intensity at positions (i) and (ii)

builds up fast upon UV irradiation, followed by a slower, conti-nuous increase. After the laser is switched off, signals (i) and (ii)decrease slowly. This was also observed for the Ti3þ sites (position(iii)), contrary to the kinetics of this species inH-TNT.Note thatthe EPR signals at positions (i)-(iv) remain relatively constantup to one hour after switching off the laser. This indicates that thecharge separation is maintained, as was also observed previouslyfor titania nanoparticles.29 The charge separation is still main-tained at 100 K (not shown) but is lost at room temperature,where the depletion of the trapped sites and electron-holerecombination rates becomes too fast.Light-Induced EPR Signals upon Irradiation with Visible

Light. To verify the visible LI-EPR signals, the tubes were irra-diated with laser light of 514.5 and 406.7 nm. As expected,no light-induced EPR signals are observed at 514.5 nm (notshown).As mentioned previously, H-TNT absorbs light at a maximum

wavelength of 398 nm, whereas the band edge maximum of H-TNT C623 lies around 410 nm (Figure S2, Supporting Infor-mation). In parallel, no LI-EPR signals are observed upon illumi-nation at 406.7 nm of H-TNT (Figure 4c), whereas weak butclear signals can be observed in the EPR spectra of H-TNTC623(Figure 4d).Figure S8 (Supporting Information) compares the EPR

spectra of H-TNT C623 illuminated at 406.7 nm at 100 K(A) and 10 K (B). These EPR spectra differ but can be ex-plained in terms of the same four species with varying relativeratio (Table 5).Species III can be ascribed to Ti3þ centers, which are formed

when an electron is trapped by a Ti4þ site22,24e,30,31 (eq 3). Thisspecies was also observed upon UV irradiation of H-TNT C623(Table 4).Species with principal g values similar to species IV have often

been observed in TiO2 materials.24e,26 As mentioned above, theirorigin is still debated, as they were ascribed to a hole trapped bysubsurface oxygen (Ti4þO-Ti4þOH-24d,26c) and even to •OHradicals.26a The g values are, however, also typical for O3

-

radicals.26a,25 The principal g values of O3- centers depend only

slightly on the matrix (Table S2, Supporting Information).

Table 4. EPR Parameters of the Ti3þ Centers Formed uponIllumination of H-TNT and H-TNT C623 with UV LaserLight (λ = 350.6 nm) as Determined from Spectral Simulationa

g1 g2 g3 occurrence

species I 1.953(3) 1.938(3) 1.89(1) H-TNT

species II 1.942(4) 1.942(4) 1.8(5) H-TNT C623

species III 1.982(2) 1.982(2) 1.954(6) H-TNT C623aThe number in brackets represents the error on the last digit.



In contrast, a large range of principal g values have been reportedfor O- radicals.27a,32 O3

- species are formed according to

hVBþ þO2- f O- ð6Þ

O- þO2 f O3- ð7Þ

As the EPR experiments were conducted under air, the assign-ment of species IV to O3

- is quite probable.The EPR characteristics of species V have been ascribed to

O2-,24b,30,33a-33d which are formed according to eqs 4 and 5.

Note that species with similar g values have also been ascribed tosurface-trapped holes (Ti4þO2-Ti4þO-) in solid anatase andrutile calcined at 1023 K.24e However, since these signals werenot found in solid anatase and rutile calcined at temperaturesbelow 700 K, and since all the experiments were performed underair, the assignment to O2

- is most probable.A signal similar to species VI has been observed in vacuum-

annealed titanate tubes34 and in some TiO2 materials.35a-35d

Note that this signal was also observed upon illumination ofH-TNT C623 at room temperature (with 406.7 nm). Althoughmany radical-type species can in principle lead to an isotropicEPR signal near g = ge, the g = 2.0048 signal in titania materials isusually assigned to an electron trapped in an oxygen vacancy(VO), forming the so-called color center F

þ

VO þ e- f Fþ ð8Þ

It has been shown that the synthesis of H-tubes favors the for-mation of oxygen vacancies.9 In most cases, however, the elec-trons in oxygen vacancies in TiO2 materials lead to the reductionof the nearby Ti4þ (formation of a vacancy-Ti3þ site).36 UnlikeMgO, where formation of color centers is standard, these centersare not observed in solid TiO2 and have only been reportedin some TiO2 nanoparticles and nanotubes.34,35 It should benoted that the EPR signal cannot be saturated at low tempera-tures and high microwave powers, in contrast to Fþ centers inother materials.Influence of Rhodamine Adsorption. As mentioned above,

R6G induces photocatalytic activity under visible light, in bothH-TNT and H-TNT C623. This likely proceeds by the transferof photogenerated electrons in R6G to the conduction band ofTiO2, leading to the formation of dye cationic radicals.37,38 Theseelectrons then can further react with dioxygen adsorbed on thesurface of TiO2 and generate a series of reactive oxygen species,which initiate the degradation of the organic dye. EPR experi-ments were performed to trace the radicals that are formed inthese reactions. The EPR spectra of H-TNT R6G withoutillumination (not shown), recorded at 10 K, reveal a broadisotropic EPR line with giso ∼ 2.0048 and ΔHpp ∼ 2 mT (indi-cated as species VII, Table 6). These g values are identical withthose of species VI, but the line width is much broader (linewidth species VI = 0.5 mT), which indicates that signals VI and

Figure 5. UV-light-induced formation of paramagnetic species in (a) H-TNT and (b)H-TNTC623, at 10 K. The EPR signal intensities were measuredunder in situ illumination at positions indicated with (i)-(iv) during 64 min of UV irradiation (350.6 nm) and during 1 h after switching off the laser.

Table 5. EPR Parameters of the Species Formed upon Illumination of H-TNTC623with Visible Laser Light (λ = 406.7 nm) at 100and 10 K as Determined from Spectral Simulation of Figure S8 (Supporting Information)a

principal g values relative contribution to spectrumb

g1 g2 g3 100 K 10 K

species III 1.982(2) 1.982(2) 1.954(6) 33% 12%

species IV 2.017(1) 2.013(1) 2.002(2) 36% 29%

species V 2.025(1) 2.011(2) 2.002(2) 12% 50%

species VI 2.0048(1) 2.0048(1) 2.0048(1) 19% 9%aThe number in brackets represents the error on the last digit. b Full spectral intensity = 100%. The experimental error is ∼5%.



VII correspond to different species. Illumination with visible lightof 514.5 nm increases the intensity of this EPR signal (Figure 6B).A very weak EPR signal corresponding to species VII is alsoobserved in R6G powder and increases upon illumination withvisible light of 514.5 nm (not shown). Therefore, this EPR signalcan be assigned to R6G radicals. Similar EPR signals wereobserved in carotenoid-modified titania nanoparticles and wereassigned to carotenoid cationic radicals.39 The intensity of thelatter signals was also found to increase upon illumination in theUV or visible region. The intensity of species VII is much higherfor H-TNT R6G compared to R6G (more than 50�). Asmentioned in the previous section, no EPR signals are observedupon illumination of bare H-TNT at 514.5 nm (Figure 6A). Thisshows that adsorption of R6G onto H-TNT and subsequentillumination with visible light result in the transfer of electronsfrom R6G to H-TNT with the formation of R6G radicals. Theelectrons injected in H-TNT do not lead to additional EPRsignals and hence are probably conduction-band electrons thatare not detectable by EPR.As R6G absorbs light in the UV region (Figure S1, Supporting

Information), the influence of R6G adsorption on the UV-light-induced EPR signals of H-TNTneeds to be verified as well. UponUV irradiation (350 nm) of H-TNTR6G, the intensity of speciesVII, corresponding to R6G radicals, increases (Figure 6D).Simultaneously, an additional signal corresponding to species I(Ti3þ centers, Figure 6D) appears, which is also observed uponUV illumination of bare H-TNT (Figure 6C). There is nomarked difference in the amount of Ti3þ centers generated inbare H-TNT and in H-TNT R6G. This again indicates thatadsorption of R6G to H-TNT and subsequent illumination withUV-light result in the transfer of electrons from R6G to theconduction band of H-TNT. Similar results (i.e., the formation ofspecies VII) are obtained at 100 K, although in this case species Iis not detected. The EPR signal of species I is indeed difficult ornot detectable at temperatures g100 K.TGA/DTG Experiments. To facilitate the interpretation of

some of the EPR spectra, TGA/DTG experiments wereundertaken (Figure 7). The weight loss between 298 and373 K can be attributed to the removal of physically adsorbedwater and can be observed for all the samples. The removal ofstructural water present in the interlayers of the nanotubestructure can be assigned to a weight loss between 373 and474 K. This signal can be clearly observed for H-TNT but isless pronounced for H-TNT C623. Furthermore, no signal canbe seen in this temperature range for H-TNT C623 O2. Thiscan be explained by the fact that a part of the structural water isremoved due to recrystallization to anatase. Because strongrecrystallization toward anatase takes place under oxygen flow,no removal of structural water can be observed in TGA forH-TNT C623 O2.

’DISCUSSION

Photocatalytic Activity of H-TNT and H-TNT C623 underUV-Light. The photocatalytic activity of trititanate nanotubes(H-TNT) and anatase-containing nanotubes (H-TNT C623)was extensively studied by evaluating the degradation of twodifferent synthetic pollutants (R6G and PyCl) in terms of blea-ching (optical absorption spectroscopy) andmineralization (TOC-analysis) upon light irradiation. The degradation (bleaching þmineralization) was analyzed over a long period of time underUV-illumination and visible-light illumination. Furthermore, thephotoinduced species were characterized using EPR to obtaindetailed information on the differences in photocatalytic activityof both H-TNT and H-TNT C623.To explain the differences in the photocatalytic activity of both

catalysts, the interaction between the catalyst and the pollutantneeds to be studied. Indeed, clear differences in adsorption capa-city are observed (Figure 1 and Figure 2). Besides the surfacearea, the adsorption capacity is also determined by the net chargeon the surface of the catalyst and the crystallinity of the catalyst.Because the surface area and morphology of both catalysts arequasi similar (Table 2), the other factors will have a majorinfluence. When studying the adsorption of PyCl (Figure 1), itbecame clear that the adsorption capacity of H-TNT C623 ishigher compared to H-TNT. In terms of the surface charges onboth catalysts, trititanate is (slightly) negatively charged in thePyCl solution (pItrititanate = 3.5-4.5 < pHPyCl = 5),40 whereasanatase is positively charged (pIanatase = 5.8-6 > pHPyCl = 5).41

Therefore, an electrostatic repulsion will occur between PyCland anatase, while a small electrostatic attraction is presentbetween the cationic PyCl molecules and the weakly negativelycharged trititanate phase. Therefore, it is expected that H-TNTC623 has a lower adsorption capacity compared to H-TNT dueto the presence of anatase. However, opposite results areobserved. Due to calcination, the crystallinity of H-TNT in-creases with 58% (H-TNT, 45% crystalline; and H-TNT C623,79% crystalline; determined by EELS). Therefore, the adsorptioncapacity of H-TNT C623 toward PyCl is higher comparedto H-TNT, although the introduction of anatase reduces the

Table 6. EPR Parameters of the Species Formed uponIllumination of H-TNTR6G at 10 K and 100 K asDeterminedby Spectral Simulationa

species g1 g2 g3 rel. contr. occurence λex/nm

I0 1.961(3) 1.945(4) 1.90(1) 90% HTNT R6G 350.6

VII 2.0048(5) 2.0048(5) 2.0048(5) 10% HTNT R6G 350.6

100% HTNT R6G 514.5aThe number in brackets represents the error on the last digit.

Figure 6. X-band LI-EPR signals of (a,c) H-TNT and (b,d) H-TNTR6G upon irradiation with (a,b) visible light (514.5 nm) and (c,d) UV-light (350.6 nm). The experiments were performed at 10 K under airwith a modulation amplitude of 0.5 mT and a mw power of (a,b) 5 mWand (c,d) 2 mW.



interaction with PyCl molecules. Indeed, crystalline TiO2 has ahigher adsorption capacity compared to amorphous TiO2 be-cause the amorphous phase shows only a short-range order and alot of structural defects (large amount of under- or overcoordi-nated structural Ti units and dangling bonds), whereas the ana-tase phase has a well-formed lattice with a small amount ofstructural defects and high amount of hydroxyl groups. H-TNTand H-TNT C623 show a comparable adsorption capacity towardR6G. Because the aqueous R6G solution has a pH around 6.5,the trititanate phase (pItrititanate = 3.5-4.5) and anatase phase(pIanatase = 5.8-6) will both be negatively charged. However,anatase will only be slightly charged compared to the trititanatephase because the pH of the dye solution approaches theisoelectric point of anatase. Therefore, it is expected thatH-TNT C623 has a smaller adsorption capacity toward R6Gcompared to H-TNT. Again, the weak adsorption of R6G byanatase in H-TNT C623 is compensated by the increased crys-tallinity of H-TNT C623. This results in almost equal adsorptioncapacity for H-TNT and H-TNT C623 toward R6G.Although there is a good interaction between the pollutants

and H-TNT, the photocatalytic degradation of PyCl by H-TNTis rather limited, whereas H-TNT shows high photocatalyticactivity toward the R6G solution. To explain the large differencesin photocatalytic activity of H-TNT toward both pollutants, thepossibility of deactivation of H-TNT by the PyCl solution(influence of acid (pHPyCl = 5) and the influence of Cl-)1 hasto be studied. Therefore, the photodegradation of an acidifiedR6G solution (pH of initial dye solution lowered until pH = 3and pH = 5 by addition of HCl) by H-TNT was examined(Figure 8). As the pH decreases, a decreased adsorption capacityand photocatalytic activity can be observed. Indeed, as the pH ofthe solution approaches the isoelectric point of trititanate, thetrititanate surface becomes neutral, which weakens the interac-tion between H-TNT and R6G. However, bleaching of acidifiedR6G can still be observed, even if the pH is lowered from pH =6.5 to pH = 3. This proves that the acidic environment or thepresence of chloride anions in the PyCl solution does notdeactivate H-TNT. If PyCl does not have a negative effect onthe photocatalytic activity of H-TNT, it may be suggested thatR6G has a positive effect on the photocatalytic activity of H-TNT.

Indeed, the absorption spectrum of R6G (Figure S1, SupportingInformation) shows that R6G molecules absorb light in the visibleregion but also in the UV-region. This implies that the irradiatedUV-light (365 nm) can excite the R6G molecule. Subsequently,the R6G molecule can then inject electrons in the conductionband of H-TNT resulting in the formation of R6G radicals.Indeed, Figure 3 shows that the ELUMO of R6G (∼-1.10 V vsNHE, normal hydrogen electrode)42 is negative enough to injectelectrons into the conduction band of titania (∼-0.3 V vsNHE).43 These electrons can further react with dioxygen ad-sorbed on the surface of TiO2 and generate a series of reactiveoxygen species, which may initiate the degradation of the organicdye. To trace the radicals formed in these reactions, EPR experi-ments were performed. Upon excitation of H-TNT with UV-light (350 nm), predominantly Ti3þ centers (species I) are formed,which are indicative for electron trapping by Ti4þ sites, accordingto reaction scheme 3. When the tubes are loaded with R6G, anEPR signal, typical for R6G radicals, appears (Figure 7 d, speciesVII). Species VII was also observed in R6G after UV illumination,but in much lower amounts. Upon UV illumination of H-TNTR6G, the intensity of species VII increases. These results suggestelectron transfer from the excited R6G dye to H-TNT, whichexplains the high photocatalytic activity of H-TNT toward R6Gcompared to PyCl. Furthermore, this can explain the observedinhibition of mineralization of R6G by H-TNT on the momentthat the dye solution has been completely photobleached. At thisstage, the photocatalytic activity of H-TNT is reduced becausethe intermediates are not able to inject electrons in the conduc-tion band and contribute to the formation of radicals.Whereas the photocatalytic activity of H-TNT is limited,

H-TNT C623 shows high photocatalytic activity toward bothPyCl and R6G due to the presence of the highly photocatalyti-cally active anatase phase. To gain insight in the photocatalysis ofpure, undoped H-TNT and H-TNT C623, the different light-induced paramagnetic centers were studied using EPR spectros-copy. Illumination of H-TNT with UV-light (350.7 nm) pro-duces predominantly surface Ti3þ centers (species I), which areformed by the trapping of electrons at Ti4þ sites (reactionscheme 3). The EPR spectrum of H-TNT C623 also containscontributions due to electrons trapped at Ti sites (species II and III),

Figure 7. TGA/DTG measurements of H-TNT nc, H-TNT C623, and H-TNT C623 O2.



but they are no longer the dominant species. The g values of theTi3þcenters in H-TNT are similar to those reported for colloidalanatase24c and partially reduced TiO2 nanoparticles44,45 butdiffer from those of the Ti3þ centers in H-TNT C623. This isrelated to the high amount of structural water in H-TNT, asrevealed by the TGA/DTG experiments. Two types of Ti3þ

centers are induced by UV-light in H-TNTC623, of which one ischaracteristic of Ti3þ in anatase (species III), consistent with theformation of anatase during the calcination process. In compar-ison, the EPR spectrum of the well-studied Degussa P25 mixedphase TiO2 consists of contributions of two Ti

3þ centers typicalfor its anatase and a rutile phase.24d

The Ti3þ centers exhibit a different dynamic behavior forH-TNT compared to H-TNT C623. The sites build up slowlyupon UV illumination (Figure 5). For H-TNT, an immediateincrease in the intensity of the Ti3þ EPR signal is observed whenthe laser is switched off, followed by a gradual increase in time(Figure 5). In contrast, the Ti3þ EPR signal in H-TNT C623decreases slowly after switching off the laser. This indicates thatUV illumination of H-TNT is accompanied by the excitation oflocalized electrons to the conduction band and the transfer ofelectrons to EPR-inactive sites. When the laser is switched off, afraction of these electrons is released and captured at Ti4þ sites,which explains the sudden increase in intensity of species I. Asimilar light-induced behavior was observed for trapped holes inMgO nanoparticles, which was ascribed to the release of localizedholes from (unspecified) EPR-inactive sites.29 The storage ofelectrons in EPR-inactive sites and subsequent release to Ti4þ

ions does not occur in H-TNT C623. The light-induced chargeseparation pattern of H-TNTC623 parallels that of TiO2 anatasenanoparticles.29 This again confirms the conversion into anataseafter calcination of H-TNT.The photocatalytic process in TiO2 materials is believed to

proceed by the generation of reactive oxygen species, such asO2

-, •O2H, and•OH,26b,46 which are formed when the light-

induced electrons and holes react with dioxygen adsorbed on theTiO2 surface. Molecular oxygen is known to adsorb most fre-quently on or at the vicinity of defect sites such as oxygen vaca-ncies.35d,47 As surface OH groups facilitate the adsorption andactivation of molecular oxygen,48,49 they also play a role in thecatalysis. Calcination at 623 K does not remove all the surfaceOH groups9,35d and leads to recrystallization toward anatase,which is an intrinsically active TiO2 phase toward oxygen

adsorption.50 Apart from the Ti3þ centers, the EPR spectra ofboth H-TNT and H-TNT C623 reveal several overlapping light-induced EPR signals at g > ge, after illumination in the UV region.These signals dominate the spectra of H-TNT C623 but are onlyminor in H-TNT. The g > ge signals can be assigned to acombination of adsorbed O2

- or O-/O3- sites and possibly

Fþ centers. Hence, the current EPR results show that morereactive oxygen species (signals at g > ge) are formed in H-TNTC623, which confirms the higher photocatalytic activity ofH-TNT C623 compared to H-TNT.Photocatalytic Activity of H-TNT and H-TNT C623 under

Visible Light. As already discussed, the photocatalytic activity ofH-TNT toward PyCl is very limited under UV-light compared toH-TNT C623. Using visible light, no photocatalytic activity ofH-TNT is observed, whereas H-TNT C623 displays somephotocatalytic activity. To understand the differences in visiblelight-induced photocatalysis, EPR experiments were undertaken.Upon visible light irradiation at 406.7 nm, no EPR signals are

observed for H-TNT, which can be related to its high band gapenergy (band edge maximum around 398 nm).In contrast, H-TNT C623, which absorbs light up to 410 nm,

shows several light-induced signals at g > ge and minor contribu-tions of species III (Ti3þ in anatase phase). Visible light illumi-nation of pure anatase does not lead to the observation of EPRsignals.24d However, it has been shown for Degussa P25 slurriesthat illumination with visible light leads to the observation ofTi3þ signals in both the anatase and rutile phase.24d This showedthat for P25 the rutile phase plays an active role, whereby chargesproduced on rutile are stabilized through electron transfer tothe lower-energy anatase lattice trapping sites. A similar proce-dure may explain the observation of a small contribution ofspecies III in the H-TNT C623 case after illumination with lightwith 406.7 nm.The g > ge EPR signals involve reactive oxygen species and

possibly also Fþ centers. As mentioned above, the reactiveoxygen species are known to be involved in the photocatalysis.Furthermore, the visible light activity of TiO2 materials has alsobeen related to the presence of Fþ centers.33b,51,52 Serpone andKuznetsov53,54 observed that TiO2 species with absorptionfeatures reported in the visible spectral region usually underwenta heat treatment or photostimulation leading to reduction ofTiO2. They therefore suggest that the red-shift of the absorptionspectrum does not result from a bandgap narrowing but is caused

Figure 8. Results of the adsorption and photocatalytic activity analysis with UV-vis spectroscopy for 16 mg of catalyst in 50 mL of 4� 10-5 M R6G.The photocatalytic activity was induced by UV-light.



by the formation of color centers due to the reduction of TiO2.The observation of the small red-shift in the absorption spectrumof the titania tubes after calcination, in combination with theobservation of the EPR signal of Fþ centers, may suggest that thered-shift is due to a similar mechanism. However, given theobserved photocatalytic activity of H-TNT C623 after illumina-tion with violet light, this would then imply that the F centermustbe involved in the photocatalysis.

’CONCLUSION

In this study, it is shown that pure trititanate nanotubes have alimited photocatalytic activity under UV-light toward the degra-dation of PyCl. EPR studies reveal the predominant formation ofTi3þ centers upon UV excitation of H-TNT, which are formedvia the trapping of electrons at Ti sites (eq 3). The EPR para-meters of these centers are typical for TiO2 powders in a gelphase, which is related to the high amount of structural water inthe untreated tubes as proven by TGA/DTG experiments. Rela-tively few reactive EPR species with g values at g > ge are formed.These reactive species are known to play an important role inphotocatalysis, and hence the EPR results are in agreement withthe low photocatalytic activity. It is proven that the photocatalyticactivity of H-TNT strongly depends on the pollutant. If there is agood interaction between the pollutant and H-TNT, and if thepollutant can be excited by UV/vis-light, as is the case for R6G,electrons can be injected into the conduction band of H-TNT,which leads to an increased photocatalytic activity of H-TNT. Ashort calcination step at relatively low temperature results in theformation of mixed-phase anatase/trititanate nanotubes9 andincreases the photocatalytic activity to a great extent. EPR studiesshow that only a minor amount of Ti3þ sites are formed aftercalcination. The predominant EPR signals have g values at g > geand can be assigned to reactive species, which are known to playan important role in the photocatalysis. The photocatalytic acti-vity is even maintained under visible light. Therefore, calcinedH-TNT can be considered as an interesting candidate forcommercial applications.

’ASSOCIATED CONTENT

bS Supporting Information. Additional experimental de-tails, figures, and tables. This material is available free of chargevia the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Phone: þ32 (0)3265.24.61. Fax: þ32 (0)3 265.24.70. E-mail: [email protected]. Phone: þ32 (0)3 265.23.55. Fax: þ32 (0)3 265.23.74.

’ACKNOWLEDGMENT

This work has been performed in the frame of the FWOprojects (Fund for Scientific Research-Flanders; G.0237.09 andG.0312.05) and the GOA project (Special Fund for Research ofthe University of Antwerp). Sepideh Zamani thanks the Uni-versity of Antwerp for PhD funding via a BOF-NOI grant.Furthermore, Liang Zhang, Jo Verbeeck, and Gustaaf VanTendeloo are gratefully acknowledged for the EELS measure-ments. Financial support from the Hercules Foundation, Flan-ders (contract AUHA013), is gratefully acknowledged.

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