Photocatalysis Involving a Visible Light-Induced Hole Injection in aChromate(VI)−TiO2 SystemJoanna Kuncewicz,† Przemysław Ząbek,† Krzysztof Kruczała,†,‡ Konrad Szaciłowski,†,§

and Wojciech Macyk*,†

†Faculty of Chemistry, Jagiellonian University, ul. R. Ingardena 3, 30-060 Krakow, Poland‡Department of Chemistry and Chemistry Didactics, The Pedagogical University of Cracow, ul. Podchorązych 2, 30-084 Krakow,Poland§Faculty of Non-Ferrous Metals, AGH − University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Poland

ABSTRACT: The photocatalytic activity of materials synthesized bytitanium dioxide impregnation with chromates(VI) was studied in theprocesses of 4-chlorophenol oxidation and photocurrent generation.The materials show measurable activity when excited with visiblelight. Electron paramagnetic resonance (EPR) studies revealed thepresence of chromium(V) species even without irradiation. Detectionof photogenerated reactive oxygen species, together with elucidationof electrochemical properties of the materials, enabled assumption ofa very unique mechanism of TiO2 photosensitization, involving aphotoinduced hole injection from the excited photosensitizer speciesto the valence band. Photoelectrochemical studies revealed thatvisible light induced both hole injection to the valence band and electron injection to the conduction band, depending on theelectrode potential. The former process is responsible for anodic, whereas the latter is responsible for cathodic photocurrentgeneration. This counterintuitive behavior results from a peculiar arrangement of electronic levels in the studied systems.Although the (photo)stability of studied materials, as well as the efficiency of the photosensitization process are moderate, thesystem represents a very unique and therefore interesting mode of titania photosensitization.

■ INTRODUCTION

Irradiation of titanium dioxide (bandgap energy of ∼3.2 eV)with ultraviolet light results in the formation of highly reactiveholes and electrons, which, in the presence of water andoxygen, participate in the generation of reactive oxygen species(ROS) responsible for oxidation of organic matter.1−4 Theactivity of neat titanium dioxide restricted to ultraviolet lightlimits the possible applications of this material. The photo-sensitization is a key approach to extend spectral absorption ofTiO2 to the visible part of solar radiation. Various strategiesleading to TiO2 photosensitization involve bulk doping orsurface modification with various organic and inorganicspecies.3 Independent of the nature of the modifier, photo-sensitization usually results in visible light-induced electroninjection into the conduction band (CB) of titanium dioxide.The most commonly described mechanism of photosensitiza-tion by surface modification encompasses light absorptionfollowed by photoinduced electron transfer (PET) from theexcited sensitizer to the CB of the semiconductor. Alternatively,the direct photosensitization based on optical electron transfer(OET), involving one step ET from the photosensitizer to theCB (e.g., molecule-to-band charge transfer, MBCT), may alsooccur.5 According to our knowledge, there are no detailedstudies on analogous mechanisms involving a photoinducedhole injection into the valence band of TiO2, particularly inphotocatalytic systems. Furtado et al. postulated an electron

transfer from the valence band of TiO2 to the excited state of[Ru3O(Ac)6(py)2(pzCO2H)]PF6 cluster upon excitation at 425nm; however, in this case, the sensitization was used toconstruct photoelectrochemical logic gates XOR and INH.6 Inthis system the observed cathodic photocurrent was the onlyevidence of the assumed mechanism.The scarcity of examples of the hole injection driven

photosensitization of TiO2 is a consequence of a highlypositive potential of the valence band edge (ca. 2.5−2.8 V vsNHE at pH = 77−9). Therefore the hole injection processwould require an extraordinarily strong oxidant generated uponvisible light absorption. Such conditions may be offered bychromate(VI) anions undergoing ligand-to-metal charge trans-fer (LMCT) excitation. The broad LMCT band of chromium-(VI) ions (CrO4

2−) in aqueous solutions, with a maximum at373 nm (at pH = 7), extends to visible light.10 Excitation of theCrO4

2− species at 440 nm (2.8 eV) within the CT band shouldresult in an electron transfer from O−II to CrVI and generationof reactive redox species, CrV and O−I. The redox potential ofthe CrVI/CrV pair is reported to be ca. 0.5511 or 0.6 V12,13 vsNHE. These values are significantly higher than the redoxpotential of the CB edge of TiO2 (ca. −0.6 V vs NHE at pH =

Received: April 27, 2012Revised: August 17, 2012Published: September 18, 2012

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79). Therefore, taking into account the bandgap energy oftitania and the energy necessary for CT excitation ofchromium(VI) ions, the photogenerated oxyl ligand (O−I)might be a sufficiently strong oxidant to oxidize the valenceband of TiO2.Studies on chromate(VI)/TiO2 systems have been under-

taken in the past. There are several reports on UV-light inducedphotocatalytic activity of TiO2 suspended in chromate(VI)solutions.11,14−19 Electrons from the CB of titania are capableof CrVI to CrV reduction. In the presence of some organicspecies undergoing oxidation (such as oxalate, phenols, salicylicacid, and humic acids), the yields of photoinduced CrVI

reduction increase due to an efficient holes consumption bythese organic hole scavengers that suppresses the electron−holerecombination. Parallel reactivity of chromate(VI)/TiO2systems was also observed upon visible light irradiation.20,21

Visible light-induced activity was attributed to the bulk andsurface states localized within the TiO2 bandgap or surfacecharge transfer complexes formed upon organic moleculescoordination to titanium(IV) centers. Recently, we describedthe photocatalytic activity of composites made of TiO2 andinsoluble chromates(VI).22 Photoinduced oxidation of organiccompounds was also observed in chromates(VI) solutions inthe absence of titanium dioxide.23−26 CrVI to CrIII reductionassociated with the oxidation of organic compounds resultsfrom the electron transfer from the electron donor (organicmolecule) to the excited chromate(VI).In this paper we try to verify the mechanism of visible light-

induced activity of chromate(VI)/TiO2 systems, abbreviatedhere as CrVI@TiO2.

■ EXPERIMENTAL SECTIONMaterials. A commercially available titanium P25 dioxide

powder (ca. 80% anatase, 20%, rutile, 50 m2 g−1) supplied byDegussa and Titanhydrat-0 (TH-0, anatase, 330 m2 g−1)purchased from Kerr-McGee were used as received. 4-Chlorophenol (4-CP; Fluka) was purified by distillation atlow pressure (the fraction boiling at 355 K (p = 5 Torr) wascollected). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO; Sigma-Aldrich) was purified before use with active carbon. A stocksolution of DMPO (1.0 mol dm−3, determined spectrophoto-metrically) was used through the studies. All other reagentswere of analytical grade and used without further purification.Milli-Q water, freshly prepared before each experiment, wasused.CrVI@P25 and CrVI@TH-0 materials were prepared by TiO2

powder (P25 and TH-0, respectively) impregnation with 2mmol dm−3 K2Cr2O7 solution (∼200 μmol CrVI per 1 g ofTiO2) for 1 h at room temperature. Measured pH of thesuspensions during impregnation was 5.0−5.1 and 5.6−5.7 inthe case of P25 and TH-0, respectively. The resulted materialswere rinsed with water and centrifuged several times until thesupernatants became colorless. The obtained bright yellowmaterials were dried at 40 °C. All procedures were performedwith limited access of light. K[CrO3F] and [CrO3F]

−@P25were prepared according to procedures described elsewhere.22

Adsorption of Chromate(VI). The amount of adsorbedCrVI species at the TiO2 surface was estimated using UV−visspectroscopy. Fifty milligrams of TiO2 powder was suspendedin 2.5 mL of 2 mmol dm−3 K2Cr2O7 solutions. The suspensionswere sonicated for 15 min and stirred for 1 h. Afterward, thesamples were centrifuged and rinsed several times with thesame amount of water. The supernatants obtained after each

centrifugation were collected. The pH of the obtained solutionswas adjusted to 2.7 by 0.1 mol dm−3 HCl. Concentration of thechromates was calculated basing on the absorption spectra ofthe collected solutions and spectra of the K2Cr2O7 referencesolutions characterized by the same pH of 2.7. The amount ofadsorbed chromium(VI) species was estimated by subtractionof the amount of washed out CrVI from the total amount ofchromate used for impregnation.

Diffuse Reflectance Spectroscopy. The reflectancespectra were recorded with a UV−vis Perkin-Elmer Lambda12 spectrophotometer equipped with 5 cm diameter integratingsphere. Samples were prepared by grinding the testedcomposites with barium sulfate powder (ca. 1:50 weightratio) used also as a reference material. The obtainedreflectance spectra were converted to Kubelka−Munk functionvalues, F(R), defined as F(R) = (1 − R)2/2R, where R standsfor reflectance.

Photocatalytic Activity Tests. Photodegradation of 4-CPin the presence of new materials irradiated with visible light wastested. As blank experiments, irradiation of 4-CP in thechromate solution (in the absence of any TiO2 material) andirradiation of the CrVI@TiO2 materials in aqueous suspension(in the absence of 4-CP) were performed. Experiments werecarried out in an open 20 mL round quartz cuvette (opticalpath 1 cm) at room temperature. Magnetically stirredsuspensions of tested photocatalysts (0.5 g dm−3) in 16 mLof 4-CP solutions (2.5 × 10−4 mol dm−3) were irradiated withan HBO-500 lamp (Osram) equipped with 455 nm cutoff filters(for visible light irradiation) and 10 cm quartz filter filled withCuSO4 solution (1 mol dm−3) for removal of NIR radiation.The intensity of the light reaching the sample was 100 ± 10mW cm−2. The measured initial pH of irradiated suspensionswas around 5.7 ± 0.1. The suspensions were sonicated for 15min prior to irradiation. During the experimental run, samplesof suspension (2 mL) were collected and filtered through aMillipore membrane filter (0.22 μm). The absorption spectra offiltrates were recorded with a UV−vis spectrophotometer(Hewlett-Packard HP8453). The degree of 4-CP decom-position was determined spectrophotometrically monitoringthe absorbance changes at 225 nm. The appearance of a peak atca. 360 nm was evidence of the desorbed chromium(VI)species.

Photoelectrochemical Studies. A typical three-electrodesetup was employed for photoelectrochemical measurements in0.1 mol dm−3 KNO3 solution. The working electrode wasprepared by casting an aqueous photocatalyst suspension onindium−tin oxide (ITO)-covered transparent foil and driedafterward in a stream of warm air (ca. 40−50 °C). Platinum andAg/AgCl were used as auxiliary and reference electrodes,respectively. The measurements were done using the electro-chemical analyzer BAS-50 (Bioanalytical Systems). A 150 WXBO lamp (Osram) equipped with water cooled housing andan LPS 200 power supply (Photon Technology International)was used for irradiation. Working electrodes were irradiatedfrom the backside (through the transparent foil) in order tominimize the influence of thickness of the semiconductor layeron the photocurrent values. An automatically controlledmonochromator and a shutter were applied to chooseappropriate wavelengths of incident light. Photocurrents aredefined as the difference between currents recorded after andbefore shutter opening.

Electron Paramagnetic Resonance (EPR) Measure-ments. The EPR measurements for solid materials, as well as

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spin trapping experiments in suspensions were carried out usingan X-band BrukerElexsys E500 spectrometer operating at 9.7GHz and 100 kHz magnetic field modulation. The EPR spectrawere recorded at room temperature with modulation amplitudeof 0.5 G and microwave power of 20 mW. The measurementswere performed in the dark and upon sample irradiation insidethe spectrometer cavity with a high-pressure HBO-200 mercurylamp equipped with 320 or 455 nm cutoff filters. The EPRspectra were simulated by the EPRSim32 software package.27

The accuracy of EPR parameters determination was 0.0005 forg values and 0.1 G for the hyperfine splitting constant A.Spin trapping experiments were performed using DMPO as a

spin trap. The CrVI@P25 powder was suspended (0.5 g dm−3)in various solvents (DMSO:H2O (95:5), DMSO:H2O (1:5),and H2O) and sonicated for 10 min. Directly beforemeasurements, the DMPO stock solution was added to thesample reaching the final concentration of 100 mmol dm−3.

■ RESULTS AND DISCUSSIONSynthesis of the Materials. The materials were obtained

by impregnation of commercially available P25 and TH-0 withpotassium dichromate. Obtained powders were yellowish.Upon suspending in water, certain amounts of chromatesdesorbed to the solution. The adsorption of CrVI onto TiO2surface and its dependency on temperature, pH, and CrVI

content have been well examined.14,16,18,28 Chromate(VI) ionsadsorb at the titania surface mainly due to electrostaticinteractions. Fourier transform infrared (FTIR) measurementsexcluded the possibility of chromate chemisorption,28 althoughearlier works of Garcia Rodenas et al. postulated the formationof surface Ti−O−CrO3

− complexes.29 Furthermore, Di Iorioet al. presented spectroscopic evidence for a strong interactionbetween CrO4

2− and TiO2, associated with the formation of acharge transfer complex.30 The affinity of CrVI ions to thetitania surface is strongly pH-dependent, since acidity affectsboth the charge of the semiconductor surface and equilibriabetween various CrVI species (eqs 1, 2, 3):31

⇄ + = −+ −H CrO H HCrO log K 0.742 4 4 1 (1)

⇄ + = −− + −HCrO H CrO log K 6.494 42

2 (2)

⇄ + = −− −2HCrO H O Cr O log K 1.524 2 2 72

3 (3)

The influence of the solution acidity on TiO2 surface involvesdetermination of the surface charge. In solutions characterizedby pH values lower than the point of zero charge of TiO2(pHpzc = 6.25−6.5 for P2532,33), the titania surface is positivelycharged, and the adsorption of negatively charged ions isfavored. At applied pH values (5.0−5.6), the TiO2 surfaceenabled adsorption of CrVI anions. The equilibrium betweenadsorbed and desorbed CrVI species also depends on thecharacter of the present chromium(VI) species. Moreover, thetotal chromate(VI) concentration also affects all equilibria. Forinstance, in acidic solutions with CrVI concentration higher than1 mmol dm−3 H2CrO4, HCrO4

−, CrO42−, and Cr2O7

2− speciesmay coexist31 (compare eqs 1−3). Taking into account theconcentration of solutions used for titania impregnation (∼4mmol dm−3 CrVI) and equilibrium constants,31 HCrO4

− can bepointed out as the major chromium(VI) species present insolution when preparing the materials. The approximate CrVI

content in resulted materials amounted to 1.2 and 4.3 mg of Crper gram of TiO2 for CrVI@P25 and CrVI@TH-0 materials,respectively. For comparison, the reported amount of CrVI

adsorbed at hydrous TiO2 material, characterized by the specificsurface area of 185 m2 g−1, was 5 mg of Cr per gram of TiO2 (atpH = 2.5).28

Spectroscopic Measurements. Diffuse reflectance spectraof synthesized materials, neat P25, and TH-0 transformed bythe Kubelka−Munk function, F(R), together with theabsorption spectrum of HCrO4

− in solution, are presented inFigure 1. CrVI@P25 and CrVI@TH-0 materials show an

absorption band characteristic for TiO2, accompanied by tailsreaching ca. 480 and 520 nm, respectively. For both materials,the main absorption onset (below 400 nm) is slightly shiftedtoward lower energies compared to the absorption onsetsobserved for neat TiO2 materials. The batochromic shifts areconsistent with the decrease of bandgap energies (Ebg),calculated from (F(R∞)hν)

0.5 vs hν plots, enabling bandgapenergy estimations for indirect semiconductors (transformationnot shown) − 3.24, 3.20, 3.16, and 3.08 (±0.02) eV for P25,TH-0, CrVI@P25 and CrVI@TH-0, respectively. The absorptionspectrum of HCrO4

− at pH = 2.7 consists of two main bandswith maxima at 260 nm (2600 dm3 mol−1 cm−1) and 350 nm(1600 dm3 mol−1 cm−1),34 attributed to LMCT transitions(O→Cr). Visible light absorption by CrVI@TiO2 materials,extending to ca. 500 nm, has very likely the same character ofO→Cr charge transfer.

Photocatalytic Activity and Photocurrent Generation.The photocatalytic activity of synthesized materials was testedin the reaction of 4-CP degradation upon visible light (λ > 455nm) irradiation. The photodegradation progress was moreefficient in the case of CrVI@TH-0 compared with the activityof CrVI@P25 (Figure 2). The neat titanium dioxide samplesalso demonstrated measurable visible light-induced activity,which may result from impurities, present especially in the caseof TH-0. In addition, the formation of CT complexes between4-CP and titanium(IV) centers acting as TiO2 photosensitizerscan be considered.35 The reaction progress was faster in thepresence of modified materials, pointing at the photosensitiza-tion effect, although efficiencies of this process were not high.The higher activity of CrVI@TH-0 may be attributed to a higherspecific surface area and higher amount of adsorbed chromium-(VI) species. Under applied visible light irradiation conditions,no 4-CP degradation was observed in the system containingchromate alone (in comparable concentrations, 5 × 10−5 mol

Figure 1. Diffuse reflectance spectra (Kubelka−Munk function) ofCrVI@P25 (dotted line), CrVI@TH-0 (dashed line), neat P25 (solidthin line), TH-0 (dash-dot line), and the absorption spectrum ofHCrO4

− aqueous solution (1 × 10−4 mol dm−3, pH = 2.7; solid thickline).

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dm−3) in the absence of TiO2 (data not shown). Although theproducts of 4-CP degradation in the studied systems were notinvestigated, recently we have proven the ability of CrVI@TiO2materials to complete mineralization of acetone and methanolin a gas-phase reaction upon visible light irradiation.36

It should be mentioned that the studied materials arecharacterized by a moderate stability. Residual amounts ofchromate(VI) could be detected in solutions obtained afterfiltration of the irradiated suspensions. The concentration ofdesorbed chromium(VI) ions after 3 h of irradiation was lowerthan 0.01 mM, and 0.006 mM for CrVI@TH0, and CrVI@P25,respectively. The fate of free chromium(VI) species is notcompletely revealed. It is possible that desorbed chromates mayact as electron acceptors from the excited chromates(VI)adsorbed at the titania surface.Further evidence of photosensitization comes from the

results of photocurrent (Iph) measurements. Anodic photo-currents generated by incident visible light are significantlyhigher for the electrode covered with modified titaniacompared with those recorded for neat TiO2, which ispractically inactive upon visible light irradiation (Figure 3a).Even more pronounced photocurrents were recorded whenfluorochromate(VI) was used for P25 impregnation.Fluorochromate(VI) ions, [CrO3F]

−, bind stronger to thetitanium dioxide surface,22 therefore electron exchange betweenthe photosensitizer and TiO2 particle can be more efficient.However, estimated incident photon-to-current efficiency(IPCE) within the 420−460 nm range did not exceed 0.1%.Furthermore, no visible light-induced photocurrents wererecorded for neat chromates(VI) dissolved in the electrolytesolution or applied as a film of insoluble chromates(VI)(BaCrO4 or MnCrO4) at the ITO electrode, preparedaccording to the procedure described elsewhere.22 As seenfrom the plot of Iph(λ,E) (Figure 3b), the photosensitization ofP25 by chromate(VI), resulting in anodic photocurrents, wasobserved when potentials higher than 0.35 V vs Ag/AgCl wereapplied. It corresponds to the redox potential of the CrVI/CrV

pair (E°CrVI/CrV = 0.55−0.6 V vs NHE).11−13 Similar measure-ments performed for the [Fe(CN)6]

3−/2−@TiO2 system showvisible light-induced photocurrents only at potentials lowerthan the potential of FeIII reduction.37,38 In that case,photosensitization results from the FeII→TiIV charge transfer(i.e., electron injection to the CB), which could not be realizedfor the oxidized form of the iron complex. Also in the case of

CrVI@TiO2, cathodic visible light-induced photocurrents wereobserved at lower potentials, when reduced forms of chromiumspecies are formed. Photocurrents recorded for chromium(VI)modified titania in the presence of oxygen give a very similarpicture to that presented in Figure 3b (data not shown). Theresults of photocurrent measurements will be further discussedin the Mechanism of Photosensitization section.Photosensitization effect observed at high potentials must be

associated with LMCT excitation of chromate, O−II→CrVI.Since photocurrents were not observed in the absence of TiO2,the excited chromate(VI) species must exchange electrons withtitania particles. Electron injection to the CB is not possible dueto the insufficient redox potential of the CrVI/CrV pair, howeverthe hole injection to the valence band is plausible. Alternatively,at low potentials, reduced chromium species can participate in aphotoinduced electron injection to the CB, similarly to thephotosensitization mechanism taking place in the case ofhexacyanoferrate(II) chemisorbed at the surface of TiO2.

EPR Studies on the CrVI@TiO2 System. The synthesizedmaterials should contain mainly CrVI adsorbed species.However, EPR measurements also revealed the presence ofCrV. The EPR spectrum recorded in the dark, with thecorresponding simulated spectrum, is shown in Figure 4. Itexhibits a slightly axial symmetry and may be simulated with thefollowing parameters: g⊥ = 1.976 (ΔH = 18.7 G) g|| = 1.956(ΔH = 26.5 G). The parameters are in a good agreement withliterature data for CrV species (d1, S = 1/2).39−41 No signal ofparamagnetic CrIII ion was detected; however, the line width of

Figure 2. Progress of 4-CP photodegradation (λ > 455 nm) in thepresence of neat TiO2 samples (open symbols) and CrVI@TiO2materials (filled symbols). Circles refer to P25, and squares refer toTH-0.

Figure 3. (a) Photocurrent as a function of wavelength of incidentlight recorded in deaerated (saturated with argon) electrolyte.Working electrodes: ITO covered with P25 (black line), CrVI@P25(blue line), [CrO3F]

−@P25 (red line). Applied potential: 0.6 V vs Ag/AgCl; (b) Photocurrent as a function of wavelength of incident lightand electrode potential recorded for CrVI@P25 in deaerated (saturatedwith argon) electrolyte. Anodic photocurrents are depicted in green,while cathodic photocurrents are in yellow and red. Photocurrent ispresented as the difference between current measured upon irradiation(opened shutter) and in the dark (closed shutter).

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the line characteristic for this species is very large (a fewhundred gausses)14,41,42 making the detection limit high, andtherefore the presence of trace amounts of CrIII cannot be ruledout. The results of control EPR measurements made for solidK2Cr2O7 sample and dichromate(VI) solution in DMSO:H2O(95:5) have shown the absence of CrV signal both in the darkand under UV−vis irradiation. Chromium(V) species wasdetected in CrVI@TiO2 irrespective of the material irradiation.EPR spectra of suspension recorded directly after TiO2 additionto the dichromate(VI) solution in the dark show a signal similarto that presented in Figure 4. The results prove an immediategeneration of CrV upon titanium dioxide addition to K2Cr2O7solution. This valence state is stable over the time at roomtemperature: the EPR signal remains practically unchangedwithin weeks after the material preparation.The presence of CrV in modified materials indicates the

reduction of some chromium(VI) species upon the adsorptionprocess. Since TiO2 represents a group of n-type semi-conductors, it offers excessive electrons of near-Fermi levelpotential, which is ca. −0.58 V vs NHE9 measured for P25 atpH = 7. This potential is sufficient to reduce chromates(VI). Insome papers, chromium(V) formation was attributed tothermal treatment of the CrVI@TiO2 system in the presenceof oxygen.40,43 There are also reports describing the generationof CrV species in the presence of irradiated TiO2 as a result ofCrVI reduction by electrons from the CB of titania.11,30,44 Infact, the amount of chromium(V) in CrVI@P25 sample dependson the light dosage, as demonstrated in Figure 5. The CrVI@P25 sample was irradiated with visible light or, simultaneously,with ultraviolet and visible light. It should be noted that appliedirradiation conditions allowed excitation of only a certainfraction of the powder placed inside the cavity. Since the shapeof obtained EPR line remained unchanged, the peak-to-peakheight of the first derivative of the signal was used as anindicator of paramagnetic centers (CrV) concentration.Irradiation of the samples either with visible or UV−vis lightinitially caused a slight increase of the signal (first 25 and 6 minof irradiation at λ > 455 and 320 nm, respectively) followed bya decrease upon further illumination of the sample. Increase ofthe signal intensity in the first stage reflects reduction of certainamounts of chromate(VI). Both a direct TiO2 excitation (UVlight absorption) leading to eCB

−/hVB+ pair generation followed

by a consecutive electron transfer to chromium(VI), and visible

light-induced LMCT (O→Cr), explain the increase of the CrV

signal. Its subsequent decrease may be caused by a furtherreduction of CrV.

Spin-Trapping Experiments. DMPO spin trap was usedfor identification of the reactive species photogenerated in thepresence of the irradiated system. The EPR spectra of CrVI@P25 material suspended in DMSO:H2O (95:5 ratio) recordedupon visible and ultraviolet−visible light irradiation in thepresence of DMPO are presented in Figure 6. The choice ofsolvent was associated with a higher stability of DMPO-OOH•

adduct in DMSO compared to water.45−47 No signal wasrecorded in the dark. Application of visible light (Figure 6a)induced the formation of the 1:2:2:1 quartet with splittingparameters AN = 13.9 G, AH = 13.5 G, and g factor of 2.0063,characteristic for a DMPO−OH• spin adduct.41,48−50 Themeasured values of HFS parameters are remarkably lower thanthose observed for aqueous solutions, what is in accordancewith lower polarity of DMSO compared to water. The signalwas not observed when P25 or potassium dichromate(VI)alone was irradiated with visible light in analogous systems(data not shown). These results indicate a possibility ofhydroxyl radicals generation at photosensitized titaniumdioxide, either in a direct water oxidation process byphotogenerated holes, or in the so-called reductive pathway,involving O2 reduction to superoxide and further generation ofother ROS, including hydrogen peroxide and hydroxyl radicals.Application of UV−vis light instead of visible light gave a

completely different picture (Figure 6b). Recorded spectrainvolved superimposed signals of three components. Two ofthem, with simulated spectra parameters: AN = 13.0 G, AHβ =10.4 G, AHγ = 1.37 G (g = 2.0059), and AN = 15.0 G, AHβ = 21.2G (g = 2.0056), were assigned to DMPO−OOH• and DMPO−CH3

• spin adducts, respectively, in parallel to the literaturedata.41,50,51 The third signal component originates from anunidentified DMPO spin adduct, DMPO−R•. On the basis ofthe hyperfine splitting constant and literature data.52,53 the linemight be assigned to DMPO−sulfur adducts (originating fromDMSO); however, further efforts to solve the structure of theseradicals were not undertaken. The hydroxyl radicals photo-generated in the studied system may react rapidly with DMSOmolecules (the rate constant k = 7.1 × 109 mol−1 dm3 s−1)54

with consecutive formation of methyl radicals, subsequentlytrapped by DMPO.41,54−56 Therefore, the recorded signal ofDMPO−CH3

• adduct is a consequence of OH• generation. On

Figure 4. X-band (9.7 GHz) spectrum of CrV recorded for the CrVI@P25 solid sample (black line, lower) and the corresponding simulatedspectrum (red line, upper).

Figure 5. The changes in CrV relative signal intensity (peak-to-peakheight of the first derivative of the signal) as a function of irradiationtime of the CrVI@P25 solid sample: λ > 320 nm (triangles) and λ >455 nm (squares).

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the other hand, the formation of DMPO-OOH• upon UVirradiation proves the generation of superoxide radical anion inthe process of one-electron reduction of oxygen by eCB

−. O2•−

should be formed also upon visible light irradiation, wheneverthe photosensitization process involves electron injection to theCB of TiO2, as confirmed by spin-trapping experiments inother photosensitized systems, e.g., in [PtIVCl4(H2O)2]@TiO2.

57 Since in the presence of CrVI@P25 visible lightirradiation resulted only in OH• generation and no O2

•− wasdetected (Figure 6a), the photosensitization cannot be basedon electron injection from the photosensitizer to the CB.The DMSO:H2O ratio should influence the EPR spectra due

to the differences in stability of radicals in these two solvents. Inthe case of the DMSO:H2O (1:5) mixture, the EPR spectrumrecorded upon irradiation with UV−vis light shows only twosignals assigned to DMPO−OH• and DMPO−CH3

• (AN =14.8 G, AHβ = 14.4 G, g = 2.0042, and AN = 16.2 G, AH = 23.1G, g = 2.0039, respectively; Figure 7a). The absence of theDMPO−OOH• adduct signal may be explained by a lowerstability of superoxide radical anion in water compared toDMSO. The results of the measurements made for CrVI@P25suspended in water (Figure 7b) differed significantly: the EPRspectra recorded upon irradiation with visible as well as withUV−vis light consist only of a strong signal attributed to theoxidized DMPO trap (DMPOX) with hyperfine couplingconstants AN = 7.3 G, A2H = 4.0 G, and g = 2.0082.57−59

Mechanism of Photosensitization. Photosensitization oftitanium dioxide with chromate(VI) was demonstrated byphotocatalytic and photoelectrochemical tests. Its mechanismdoes not involve electron injection to the CB (quasi-Fermi levelpotential of −0.6 V vs NHE9) since no superoxide radicals(EO

2/O

2− = −0.16 V vs NHE60) can be detected upon irradiationwith visible light. Moreover, chromate(VI) excitation within itsLMCT band results in CrV generation; this species is notcapable of electron injection to the CB due to a too high redoxpotential of the CrVI/CrV pair (E°CrVI/CrV = 0.55−0.6 V vsNHE11−13). Contrarily, a partial reduction of chromate(VI)takes place immediately upon its adsorption at the surface ofTiO2 (originally an n-type semiconductor), proving thepossibility of electron transfer from TiO2 to the CrVI center.Taking into account a relatively high E°CrVI/CrV potential and

energy of the LMCT excitation of chromate(VI) (higher thanca. 2.7−2.8 eV), the potential of the photogenerated O−I ligandcan be estimated to be at least 3.3−3.4 V vs NHE, which ishigher than the potential of the upper edge of TiO2 valenceband (Figure 8). These relative values of potentials facilitate thehole transfer from the excited chromate(VI) to the valenceband. Generation of hydroxyl radicals upon visible lightirradiation was confirmed only in the presence of CrVI@TiO2material, not in the case of chromate(VI) alone. Also, visiblelight-induced photocurrents were recorded for CrVI@TiO2 andnot for chromate(VI) used either in solution or as films of

Figure 6. (a) EPR recorded (black line, bottom) and simulated spectra (red line, top) of DMPO adducts with OH• radicals photogenerated in thepresence of CrVI@P25 and DMPO (100 mmol dm−3) upon visible light irradiation (λ > 455 nm); (b) EPR recorded (black line, bottom), andsimulated (red line, bottom) spectra of DMPO-OOH•, DMPO−CH3

•, and DMPO−R• spin adducts photogenerated in the same system irradiatedwith UV−vis light (λ > 320 nm). Simulated spectra of the components are shown above the recorded ones. Solvent: DMSO:H2O (95:5).

Figure 7. (a) EPR recorded (black line, bottom) and simulated spectra (red line, bottom) of DMPO adducts with OH• and CH3• radicals

photogenerated in the presence of CrVI@P25 suspended in DMSO:H2O (1:5 ratio; λ > 320 nm). (b) EPR recorded (black line, bottom) andsimulated spectra of oxidized DMPO (DMPOX) photogenerated in the presence of CrVI@P25 suspended in water. Simulated spectra of thecomponents are shown above the recorded ones.

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insoluble chromates(VI) at ITO electrode. These experimentsafford additional arguments for the process of hole injectioninto the valence band. Moreover, the hole injection shouldprevent, to some extent, the efficient CrV→O−I recombination.The fate of photogenerated CrV should be also invoked.

Reduction of molecular oxygen by chromium(V) species is notpossible (ECr

VI/Cr

V > EO2/O2−). Therefore another electron

acceptor must be involved in CrV consumption andregeneration of CrVI. One possibility involves reduction oforganic species present in the photolyzed suspension. Anothercomprises CrV disproportionation to CrVI and CrIII. In theabsence of an efficient electron acceptor, recombinationprocesses and further steps of chromium reduction play adominant role and limit efficiency of photosensitization.The presented mechanism explains the generation of anodic

photocurrents induced by visible light at positive potentials(Figure 9). Upon chromate excitation, the electron can betransferred directly to the electrode (Fermi potential of ITO ishigher than the E°CrVI/CrV potential; Figure 9a) while the holeoxidizes an electron donor (e.g., water). In this particular case,the generation of anodic photocurrent does not involve a CB,but electrons are transferred to the conducting support directlyfrom chromium-based surface states. Application of lowerpotentials leads to reduction of CrVI (Figure 9b). Excitation ofelectrogenerated chromium(V) and chromium species at lower

oxidation states may lead to the formation of their excited statescapable of electron injection to the TiO2 CB, followed byreduction of an electron acceptor (e.g., molecular oxygen). Thissituation resembles that observed in the case of the [Fe-(CN)6]

3−/4−@TiO2 system, for which visible light-inducedphotocurrents may only be observed at potentials enabling FeIII

reduction (Figure 9c).37,38 Photosensitization by reducedchromium or iron species leads to cathodic photocurrentgeneration. The formation of chromium species at loweroxidation states may also be responsible for photosensitizationof TiO2 according to a classical mechanism, involving anelectron injection into the CB. Therefore the mechanism ofphotocurrent generation and the photosensitization mode(PET vs OET) depends on the redox state of the surfacemodifier, as it was previously observed in some other cases ofruthenium and iron species at TiO2 surfaces.37,38,61,62 Thepresent systems additionally offer another possibility of redox-based reactivity modulation: hole injection versus electroninjection.

■ CONCLUSIONS

The presented studies point to a very unique mechanism oftitanium dioxide photosensitization involving hole injectionfrom the excited photosensitizer to the valence band. Thisprocess requires a strongly oxidizing agent capable of TiO2

oxidation. Chromates(VI) adsorbed at a titania surface offersuch properties when excited within the LMCT band:photogenerated O−I ligand oxidizes TiO2 particle, while CrV

reduces an external electron acceptor. Although the efficiencyof such a photosensitization process is moderate and the(photo)stability of CrVI@TiO2 materials is unsatisfactory, thestudied system represents a very unusual mode of titaniaphotosensitization. In addition to this mechanism, a classicalelectron injection from the excited reduced surface species mayoperate, as demonstrated by photoelectrochemical studies. Thiscounterintuitive behavior results from a peculiar arrangement ofelectronic levels in the studied systems. The presentedmechanism can explain the photoactivity of chromate(VI)/TiO2 systems described previously in numerous papers. Furtherincrease of visible light activity of CrVI@TiO2 may involve asearch for suitable electron acceptors diminishing efficiencies ofrecombination processes and chromium(V) reduction.

Figure 8. The mechanism of titanium dioxide photosensitization bychromates(VI) involving the hole transfer to the TiO2 valence band.Upon LMCT excitation of chromate(VI), CrV and O−I are generated.“A” denotes an electron acceptor. Approximate values of potentials vsNHE are presented (pH = 7).

Figure 9. The mechanism of photocurrent generation upon visible light application: (a) at electrode covered with CrVI@TiO2 upon application ofpotentials higher than ECrVI/CrV; (b) at electrode covered with Cr

VI@TiO2 upon application of potentials lower than ECrVI/CrV; (c) at electrode coveredwith [Fe(CN)6]

4−@TiO2 upon application of potentials lower than EFeIII/FeII.

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■ AUTHOR INFORMATIONCorresponding Author*E-mail: macyk@chemia.uj.edu.pl; Tel.: +48 126632222.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe support from National Science Centre within the 2011/01/B/ST5/00920 grant is highly acknowledged. Part of themeasurements was carried out with the equipment purchasedthanks to the financial support of the European RegionalDevelopment Fund in the framework of the Polish InnovationEconomy Operational Program (Contract No. POIG.02.01.00-12-023/08).

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