Photocatalysis Involving a Visible Light-Induced Hole Injection in a

Sep 18, 2012 - A. Mickiewicza 30, 30-059 Kraków, Poland. ABSTRACT: The photocatalytic activity of materials synthesized by titanium dioxide impregnati...
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Photocatalysis Involving a Visible Light-Induced Hole Injection in a Chromate(VI)−TiO2 System Joanna Kuncewicz,† Przemysław Ząbek,† Krzysztof Kruczała,†,‡ Konrad Szaciłowski,†,§ and Wojciech Macyk*,† †

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

ABSTRACT: The photocatalytic activity of materials synthesized by titanium dioxide impregnation with chromates(VI) was studied in the processes of 4-chlorophenol oxidation and photocurrent generation. The materials show measurable activity when excited with visible light. Electron paramagnetic resonance (EPR) studies revealed the presence of chromium(V) species even without irradiation. Detection of photogenerated reactive oxygen species, together with elucidation of electrochemical properties of the materials, enabled assumption of a very unique mechanism of TiO2 photosensitization, involving a photoinduced hole injection from the excited photosensitizer species to the valence band. Photoelectrochemical studies revealed that visible light induced both hole injection to the valence band and electron injection to the conduction band, depending on the electrode potential. The former process is responsible for anodic, whereas the latter is responsible for cathodic photocurrent generation. 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, the system represents a very unique and therefore interesting mode of titania photosensitization.

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INTRODUCTION Irradiation of titanium dioxide (bandgap energy of ∼3.2 eV) with ultraviolet light results in the formation of highly reactive holes and electrons, which, in the presence of water and oxygen, participate in the generation of reactive oxygen species (ROS) responsible for oxidation of organic matter.1−4 The activity of neat titanium dioxide restricted to ultraviolet light limits the possible applications of this material. The photosensitization is a key approach to extend spectral absorption of TiO2 to the visible part of solar radiation. Various strategies leading to TiO2 photosensitization involve bulk doping or surface modification with various organic and inorganic species.3 Independent of the nature of the modifier, photosensitization usually results in visible light-induced electron injection into the conduction band (CB) of titanium dioxide. The most commonly described mechanism of photosensitization by surface modification encompasses light absorption followed by photoinduced electron transfer (PET) from the excited 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 the CB (e.g., molecule-to-band charge transfer, MBCT), may also occur.5 According to our knowledge, there are no detailed studies on analogous mechanisms involving a photoinduced hole injection into the valence band of TiO2, particularly in photocatalytic systems. Furtado et al. postulated an electron © 2012 American Chemical Society

transfer from the valence band of TiO2 to the excited state of [Ru3O(Ac)6(py)2(pzCO2H)]PF6 cluster upon excitation at 425 nm; however, in this case, the sensitization was used to construct photoelectrochemical logic gates XOR and INH.6 In this system the observed cathodic photocurrent was the only evidence of the assumed mechanism. The scarcity of examples of the hole injection driven photosensitization of TiO2 is a consequence of a highly positive potential of the valence band edge (ca. 2.5−2.8 V vs NHE at pH = 77−9). Therefore the hole injection process would require an extraordinarily strong oxidant generated upon visible light absorption. Such conditions may be offered by chromate(VI) anions undergoing ligand-to-metal charge transfer (LMCT) excitation. The broad LMCT band of chromium(VI) ions (CrO42−) in aqueous solutions, with a maximum at 373 nm (at pH = 7), extends to visible light.10 Excitation of the CrO42− species at 440 nm (2.8 eV) within the CT band should result in an electron transfer from O−II to CrVI and generation of reactive redox species, CrV and O−I. The redox potential of the CrVI/CrV pair is reported to be ca. 0.5511 or 0.6 V12,13 vs NHE. These values are significantly higher than the redox potential of the CB edge of TiO2 (ca. −0.6 V vs NHE at pH = Received: April 27, 2012 Revised: August 17, 2012 Published: September 18, 2012 21762

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79). Therefore, taking into account the bandgap energy of titania and the energy necessary for CT excitation of chromium(VI) ions, the photogenerated oxyl ligand (O−I) might be a sufficiently strong oxidant to oxidize the valence band of TiO2. Studies on chromate(VI)/TiO2 systems have been undertaken in the past. There are several reports on UV-light induced photocatalytic activity of TiO2 suspended in chromate(VI) solutions.11,14−19 Electrons from the CB of titania are capable of CrVI to CrV reduction. In the presence of some organic species undergoing oxidation (such as oxalate, phenols, salicylic acid, and humic acids), the yields of photoinduced CrVI reduction increase due to an efficient holes consumption by these organic hole scavengers that suppresses the electron−hole recombination. Parallel reactivity of chromate(VI)/TiO 2 systems was also observed upon visible light irradiation.20,21 Visible light-induced activity was attributed to the bulk and surface states localized within the TiO2 bandgap or surface charge transfer complexes formed upon organic molecules coordination to titanium(IV) centers. Recently, we described the photocatalytic activity of composites made of TiO2 and insoluble chromates(VI).22 Photoinduced oxidation of organic compounds was also observed in chromates(VI) solutions in the absence of titanium dioxide.23−26 CrVI to CrIII reduction associated with the oxidation of organic compounds results from the electron transfer from the electron donor (organic molecule) to the excited chromate(VI). In this paper we try to verify the mechanism of visible lightinduced activity of chromate(VI)/TiO2 systems, abbreviated here as CrVI@TiO2.

centrifugation were collected. The pH of the obtained solutions was adjusted to 2.7 by 0.1 mol dm−3 HCl. Concentration of the chromates was calculated basing on the absorption spectra of the collected solutions and spectra of the K2Cr2O7 reference solutions characterized by the same pH of 2.7. The amount of adsorbed chromium(VI) species was estimated by subtraction of the amount of washed out CrVI from the total amount of chromate used for impregnation. Diffuse Reflectance Spectroscopy. The reflectance spectra were recorded with a UV−vis Perkin-Elmer Lambda 12 spectrophotometer equipped with 5 cm diameter integrating sphere. Samples were prepared by grinding the tested composites with barium sulfate powder (ca. 1:50 weight ratio) used also as a reference material. The obtained reflectance spectra were converted to Kubelka−Munk function values, F(R), defined as F(R) = (1 − R)2/2R, where R stands for reflectance. Photocatalytic Activity Tests. Photodegradation of 4-CP in the presence of new materials irradiated with visible light was tested. As blank experiments, irradiation of 4-CP in the chromate solution (in the absence of any TiO2 material) and irradiation of the CrVI@TiO2 materials in aqueous suspension (in the absence of 4-CP) were performed. Experiments were carried out in an open 20 mL round quartz cuvette (optical path 1 cm) at room temperature. Magnetically stirred suspensions of tested photocatalysts (0.5 g dm−3) in 16 mL of 4-CP solutions (2.5 × 10−4 mol dm−3) were irradiated with an HBO-500 lamp (Osram) equipped with 455 nm cutoff filters (for visible light irradiation) and 10 cm quartz filter filled with CuSO4 solution (1 mol dm−3) for removal of NIR radiation. The intensity of the light reaching the sample was 100 ± 10 mW cm−2. The measured initial pH of irradiated suspensions was around 5.7 ± 0.1. The suspensions were sonicated for 15 min prior to irradiation. During the experimental run, samples of suspension (2 mL) were collected and filtered through a Millipore membrane filter (0.22 μm). The absorption spectra of filtrates were recorded with a UV−vis spectrophotometer (Hewlett-Packard HP8453). The degree of 4-CP decomposition was determined spectrophotometrically monitoring the absorbance changes at 225 nm. The appearance of a peak at ca. 360 nm was evidence of the desorbed chromium(VI) species. Photoelectrochemical Studies. A typical three-electrode setup was employed for photoelectrochemical measurements in 0.1 mol dm−3 KNO3 solution. The working electrode was prepared by casting an aqueous photocatalyst suspension on indium−tin oxide (ITO)-covered transparent foil and dried afterward in a stream of warm air (ca. 40−50 °C). Platinum and Ag/AgCl were used as auxiliary and reference electrodes, respectively. The measurements were done using the electrochemical analyzer BAS-50 (Bioanalytical Systems). A 150 W XBO lamp (Osram) equipped with water cooled housing and an LPS 200 power supply (Photon Technology International) was used for irradiation. Working electrodes were irradiated from the backside (through the transparent foil) in order to minimize the influence of thickness of the semiconductor layer on the photocurrent values. An automatically controlled monochromator and a shutter were applied to choose appropriate wavelengths of incident light. Photocurrents are defined as the difference between currents recorded after and before shutter opening. Electron Paramagnetic Resonance (EPR) Measurements. The EPR measurements for solid materials, as well as

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EXPERIMENTAL SECTION Materials. A commercially available titanium P25 dioxide powder (ca. 80% anatase, 20%, rutile, 50 m2 g−1) supplied by Degussa and Titanhydrat-0 (TH-0, anatase, 330 m2 g−1) purchased from Kerr-McGee were used as received. 4Chlorophenol (4-CP; Fluka) was purified by distillation at low pressure (the fraction boiling at 355 K (p = 5 Torr) was collected). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO; SigmaAldrich) was purified before use with active carbon. A stock solution of DMPO (1.0 mol dm−3, determined spectrophotometrically) was used through the studies. All other reagents were of analytical grade and used without further purification. Milli-Q water, freshly prepared before each experiment, was used. CrVI@P25 and CrVI@TH-0 materials were prepared by TiO2 powder (P25 and TH-0, respectively) impregnation with 2 mmol dm−3 K2Cr2O7 solution (∼200 μmol CrVI per 1 g of TiO2) for 1 h at room temperature. Measured pH of the suspensions during impregnation was 5.0−5.1 and 5.6−5.7 in the case of P25 and TH-0, respectively. The resulted materials were rinsed with water and centrifuged several times until the supernatants became colorless. The obtained bright yellow materials were dried at 40 °C. All procedures were performed with limited access of light. K[CrO3F] and [CrO3F]−@P25 were prepared according to procedures described elsewhere.22 Adsorption of Chromate(VI). The amount of adsorbed CrVI species at the TiO2 surface was estimated using UV−vis spectroscopy. Fifty milligrams of TiO2 powder was suspended in 2.5 mL of 2 mmol dm−3 K2Cr2O7 solutions. The suspensions were sonicated for 15 min and stirred for 1 h. Afterward, the samples were centrifuged and rinsed several times with the same amount of water. The supernatants obtained after each 21763

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spin trapping experiments in suspensions were carried out using an X-band BrukerElexsys E500 spectrometer operating at 9.7 GHz and 100 kHz magnetic field modulation. The EPR spectra were recorded at room temperature with modulation amplitude of 0.5 G and microwave power of 20 mW. The measurements were performed in the dark and upon sample irradiation inside the spectrometer cavity with a high-pressure HBO-200 mercury lamp equipped with 320 or 455 nm cutoff filters. The EPR spectra were simulated by the EPRSim32 software package.27 The accuracy of EPR parameters determination was 0.0005 for g 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 before measurements, the DMPO stock solution was added to the sample reaching the final concentration of 100 mmol dm−3.

adsorbed at hydrous TiO2 material, characterized by the specific surface area of 185 m2 g−1, was 5 mg of Cr per gram of TiO2 (at pH = 2.5).28 Spectroscopic Measurements. Diffuse reflectance spectra of synthesized materials, neat P25, and TH-0 transformed by the Kubelka−Munk function, F(R), together with the absorption spectrum of HCrO4− in solution, are presented in Figure 1. CrVI@P25 and CrVI@TH-0 materials show an

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RESULTS AND DISCUSSION Synthesis of the Materials. The materials were obtained by impregnation of commercially available P25 and TH-0 with potassium dichromate. Obtained powders were yellowish. Upon suspending in water, certain amounts of chromates desorbed to the solution. The adsorption of CrVI onto TiO2 surface and its dependency on temperature, pH, and CrVI content have been well examined.14,16,18,28 Chromate(VI) ions adsorb at the titania surface mainly due to electrostatic interactions. Fourier transform infrared (FTIR) measurements excluded the possibility of chromate chemisorption,28 although earlier works of Garcia Rodenas et al. postulated the formation of surface Ti−O−CrO3− complexes.29 Furthermore, Di Iorio et al. presented spectroscopic evidence for a strong interaction between CrO42− and TiO2, associated with the formation of a charge transfer complex.30 The affinity of CrVI ions to the titania surface is strongly pH-dependent, since acidity affects both the charge of the semiconductor surface and equilibria between various CrVI species (eqs 1, 2, 3):31 H 2CrO4 ⇄ H+ + HCrO4 −

log K1 = −0.74

(1)

HCrO4 − ⇄ H+ + CrO4 2 −

log K 2 = − 6.49

(2)

2HCrO4 − ⇄ H 2O + Cr2O7 2 −

log K3 = −1.52

Figure 1. Diffuse reflectance spectra (Kubelka−Munk function) of CrVI@P25 (dotted line), CrVI@TH-0 (dashed line), neat P25 (solid thin line), TH-0 (dash-dot line), and the absorption spectrum of HCrO4− aqueous solution (1 × 10−4 mol dm−3, pH = 2.7; solid thick line).

absorption band characteristic for TiO2, accompanied by tails reaching ca. 480 and 520 nm, respectively. For both materials, the main absorption onset (below 400 nm) is slightly shifted toward lower energies compared to the absorption onsets observed for neat TiO2 materials. The batochromic shifts are consistent with the decrease of bandgap energies (Ebg), calculated from (F(R∞)hν)0.5 vs hν plots, enabling bandgap energy estimations for indirect semiconductors (transformation not 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 absorption spectrum of HCrO4− at pH = 2.7 consists of two main bands with 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 of O→Cr charge transfer. Photocatalytic Activity and Photocurrent Generation. The photocatalytic activity of synthesized materials was tested in the reaction of 4-CP degradation upon visible light (λ > 455 nm) irradiation. The photodegradation progress was more efficient in the case of CrVI@TH-0 compared with the activity of CrVI@P25 (Figure 2). The neat titanium dioxide samples also demonstrated measurable visible light-induced activity, which may result from impurities, present especially in the case of TH-0. In addition, the formation of CT complexes between 4-CP and titanium(IV) centers acting as TiO2 photosensitizers can be considered.35 The reaction progress was faster in the presence of modified materials, pointing at the photosensitization effect, although efficiencies of this process were not high. The higher activity of CrVI@TH-0 may be attributed to a higher specific 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 containing chromate alone (in comparable concentrations, 5 × 10−5 mol

(3)

The influence of the solution acidity on TiO2 surface involves determination of the surface charge. In solutions characterized by pH values lower than the point of zero charge of TiO2 (pHpzc = 6.25−6.5 for P2532,33), the titania surface is positively charged, and the adsorption of negatively charged ions is favored. At applied pH values (5.0−5.6), the TiO2 surface enabled adsorption of CrVI anions. The equilibrium between adsorbed and desorbed CrVI species also depends on the character of the present chromium(VI) species. Moreover, the total chromate(VI) concentration also affects all equilibria. For instance, in acidic solutions with CrVI concentration higher than 1 mmol dm−3 H2CrO4, HCrO4−, CrO42−, and Cr2O72− species may coexist31 (compare eqs 1−3). Taking into account the concentration of solutions used for titania impregnation (∼4 mmol dm−3 CrVI) and equilibrium constants,31 HCrO4− can be pointed out as the major chromium(VI) species present in solution when preparing the materials. The approximate CrVI content in resulted materials amounted to 1.2 and 4.3 mg of Cr per gram of TiO2 for CrVI@P25 and CrVI@TH-0 materials, respectively. For comparison, the reported amount of CrVI 21764

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

dm−3) in the absence of TiO2 (data not shown). Although the products of 4-CP degradation in the studied systems were not investigated, recently we have proven the ability of CrVI@TiO2 materials to complete mineralization of acetone and methanol in a gas-phase reaction upon visible light irradiation.36 It should be mentioned that the studied materials are characterized by a moderate stability. Residual amounts of chromate(VI) could be detected in solutions obtained after filtration of the irradiated suspensions. The concentration of desorbed chromium(VI) ions after 3 h of irradiation was lower than 0.01 mM, and 0.006 mM for CrVI@TH0, and CrVI@P25, respectively. The fate of free chromium(VI) species is not completely revealed. It is possible that desorbed chromates may act 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 photocurrents generated by incident visible light are significantly higher for the electrode covered with modified titania compared with those recorded for neat TiO2, which is practically inactive upon visible light irradiation (Figure 3a). Even more pronounced photocurrents were recorded when fluorochromate(VI) was used for P25 impregnation. Fluorochromate(VI) ions, [CrO3F]−, bind stronger to the titanium dioxide surface,22 therefore electron exchange between the 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 were recorded for neat chromates(VI) dissolved in the electrolyte solution or applied as a film of insoluble chromates(VI) (BaCrO4 or MnCrO4) at the ITO electrode, prepared according to the procedure described elsewhere.22 As seen from the plot of Iph(λ,E) (Figure 3b), the photosensitization of P25 by chromate(VI), resulting in anodic photocurrents, was observed when potentials higher than 0.35 V vs Ag/AgCl were applied. It corresponds to the redox potential of the CrVI/CrV pair (E°CrVI/CrV = 0.55−0.6 V vs NHE).11−13 Similar measurements performed for the [Fe(CN)6]3−/2−@TiO2 system show visible light-induced photocurrents only at potentials lower than 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 realized for the oxidized form of the iron complex. Also in the case of

Figure 3. (a) Photocurrent as a function of wavelength of incident light 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 light and electrode potential recorded for CrVI@P25 in deaerated (saturated with argon) electrolyte. Anodic photocurrents are depicted in green, while cathodic photocurrents are in yellow and red. Photocurrent is presented as the difference between current measured upon irradiation (opened shutter) and in the dark (closed shutter).

CrVI@TiO2, cathodic visible light-induced photocurrents were observed at lower potentials, when reduced forms of chromium species are formed. Photocurrents recorded for chromium(VI) modified titania in the presence of oxygen give a very similar picture to that presented in Figure 3b (data not shown). The results of photocurrent measurements will be further discussed in 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 with titania particles. Electron injection to the CB is not possible due to the insufficient redox potential of the CrVI/CrV pair, however the hole injection to the valence band is plausible. Alternatively, at low potentials, reduced chromium species can participate in a photoinduced electron injection to the CB, similarly to the photosensitization mechanism taking place in the case of hexacyanoferrate(II) chemisorbed at the surface of TiO2. EPR Studies on the CrVI@TiO2 System. The synthesized materials should contain mainly CrVI adsorbed species. However, EPR measurements also revealed the presence of CrV. The EPR spectrum recorded in the dark, with the corresponding simulated spectrum, is shown in Figure 4. It exhibits a slightly axial symmetry and may be simulated with the following parameters: g⊥ = 1.976 (ΔH = 18.7 G) g|| = 1.956 (ΔH = 26.5 G). The parameters are in a good agreement with literature data for CrV species (d1, S = 1/2).39−41 No signal of paramagnetic CrIII ion was detected; however, the line width of 21765

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

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

light-induced LMCT (O→Cr), explain the increase of the CrV signal. Its subsequent decrease may be caused by a further reduction of CrV. Spin-Trapping Experiments. DMPO spin trap was used for identification of the reactive species photogenerated in the presence of the irradiated system. The EPR spectra of CrVI@ P25 material suspended in DMSO:H2O (95:5 ratio) recorded upon visible and ultraviolet−visible light irradiation in the presence of DMPO are presented in Figure 6. The choice of solvent was associated with a higher stability of DMPO-OOH• adduct in DMSO compared to water.45−47 No signal was recorded in the dark. Application of visible light (Figure 6a) induced the formation of the 1:2:2:1 quartet with splitting parameters AN = 13.9 G, AH = 13.5 G, and g factor of 2.0063, characteristic for a DMPO−OH• spin adduct.41,48−50 The measured values of HFS parameters are remarkably lower than those observed for aqueous solutions, what is in accordance with lower polarity of DMSO compared to water. The signal was 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 of hydroxyl radicals generation at photosensitized titanium dioxide, either in a direct water oxidation process by photogenerated holes, or in the so-called reductive pathway, involving O2 reduction to superoxide and further generation of other ROS, including hydrogen peroxide and hydroxyl radicals. Application of UV−vis light instead of visible light gave a completely different picture (Figure 6b). Recorded spectra involved superimposed signals of three components. Two of them, 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 = 21.2 G (g = 2.0056), were assigned to DMPO−OOH• and DMPO− CH3• spin adducts, respectively, in parallel to the literature data.41,50,51 The third signal component originates from an unidentified DMPO spin adduct, DMPO−R•. On the basis of the hyperfine splitting constant and literature data.52,53 the line might be assigned to DMPO−sulfur adducts (originating from DMSO); however, further efforts to solve the structure of these radicals were not undertaken. The hydroxyl radicals photogenerated in the studied system may react rapidly with DMSO molecules (the rate constant k = 7.1 × 109 mol−1 dm3 s−1)54 with consecutive formation of methyl radicals, subsequently trapped by DMPO.41,54−56 Therefore, the recorded signal of DMPO−CH3• adduct is a consequence of OH• generation. On

the line characteristic for this species is very large (a few hundred gausses)14,41,42 making the detection limit high, and therefore the presence of trace amounts of CrIII cannot be ruled out. The results of control EPR measurements made for solid K2Cr2O7 sample and dichromate(VI) solution in DMSO:H2O (95:5) have shown the absence of CrV signal both in the dark and under UV−vis irradiation. Chromium(V) species was detected in CrVI@TiO2 irrespective of the material irradiation. EPR spectra of suspension recorded directly after TiO2 addition to the dichromate(VI) solution in the dark show a signal similar to that presented in Figure 4. The results prove an immediate generation of CrV upon titanium dioxide addition to K2Cr2O7 solution. This valence state is stable over the time at room temperature: the EPR signal remains practically unchanged within weeks after the material preparation. The presence of CrV in modified materials indicates the reduction of some chromium(VI) species upon the adsorption process. Since TiO2 represents a group of n-type semiconductors, it offers excessive electrons of near-Fermi level potential, which is ca. −0.58 V vs NHE9 measured for P25 at pH = 7. This potential is sufficient to reduce chromates(VI). In some papers, chromium(V) formation was attributed to thermal treatment of the CrVI@TiO2 system in the presence of oxygen.40,43 There are also reports describing the generation of CrV species in the presence of irradiated TiO2 as a result of CrVI reduction by electrons from the CB of titania.11,30,44 In fact, the amount of chromium(V) in CrVI@P25 sample depends on 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 applied irradiation conditions allowed excitation of only a certain fraction of the powder placed inside the cavity. Since the shape of obtained EPR line remained unchanged, the peak-to-peak height of the first derivative of the signal was used as an indicator of paramagnetic centers (Cr V) concentration. Irradiation of the samples either with visible or UV−vis light initially caused a slight increase of the signal (first 25 and 6 min of irradiation at λ > 455 and 320 nm, respectively) followed by a decrease upon further illumination of the sample. Increase of the signal intensity in the first stage reflects reduction of certain amounts of chromate(VI). Both a direct TiO2 excitation (UV light absorption) leading to eCB−/hVB+ pair generation followed by a consecutive electron transfer to chromium(VI), and visible 21766

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Figure 6. (a) EPR recorded (black line, bottom) and simulated spectra (red line, top) of DMPO adducts with OH• radicals photogenerated in the presence of CrVI@P25 and DMPO (100 mmol dm−3) upon visible light irradiation (λ > 455 nm); (b) EPR recorded (black line, bottom), and simulated (red line, bottom) spectra of DMPO-OOH•, DMPO−CH3•, and DMPO−R• spin adducts photogenerated in the same system irradiated with 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) and simulated spectra of oxidized DMPO (DMPOX) photogenerated in the presence of CrVI@P25 suspended in water. Simulated spectra of the components are shown above the recorded ones.

the other hand, the formation of DMPO-OOH• upon UV irradiation proves the generation of superoxide radical anion in the process of one-electron reduction of oxygen by eCB−. O2•− should be formed also upon visible light irradiation, whenever the photosensitization process involves electron injection to the CB of TiO2, as confirmed by spin-trapping experiments in other photosensitized systems, e.g., in [PtIVCl4(H2O)2]@ TiO2.57 Since in the presence of CrVI@P25 visible light irradiation resulted only in OH• generation and no O2•− was detected (Figure 6a), the photosensitization cannot be based on 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. In the case of the DMSO:H2O (1:5) mixture, the EPR spectrum recorded upon irradiation with UV−vis light shows only two signals assigned to DMPO−OH• and DMPO−CH3• (AN = 14.8 G, AHβ = 14.4 G, g = 2.0042, and AN = 16.2 G, AH = 23.1 G, g = 2.0039, respectively; Figure 7a). The absence of the DMPO−OOH• adduct signal may be explained by a lower stability of superoxide radical anion in water compared to DMSO. The results of the measurements made for CrVI@P25 suspended in water (Figure 7b) differed significantly: the EPR spectra recorded upon irradiation with visible as well as with UV−vis light consist only of a strong signal attributed to the oxidized DMPO trap (DMPOX) with hyperfine coupling constants AN = 7.3 G, A2H = 4.0 G, and g = 2.0082.57−59

Mechanism of Photosensitization. Photosensitization of titanium dioxide with chromate(VI) was demonstrated by photocatalytic and photoelectrochemical tests. Its mechanism does not involve electron injection to the CB (quasi-Fermi level potential of −0.6 V vs NHE9) since no superoxide radicals (EO2/O2− = −0.16 V vs NHE60) can be detected upon irradiation with visible light. Moreover, chromate(VI) excitation within its LMCT band results in CrV generation; this species is not capable of electron injection to the CB due to a too high redox potential of the CrVI/CrV pair (E°CrVI/CrV = 0.55−0.6 V vs NHE11−13). Contrarily, a partial reduction of chromate(VI) takes place immediately upon its adsorption at the surface of TiO2 (originally an n-type semiconductor), proving the possibility 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 than ca. 2.7−2.8 eV), the potential of the photogenerated O−I ligand can be estimated to be at least 3.3−3.4 V vs NHE, which is higher than the potential of the upper edge of TiO2 valence band (Figure 8). These relative values of potentials facilitate the hole transfer from the excited chromate(VI) to the valence band. Generation of hydroxyl radicals upon visible light irradiation was confirmed only in the presence of CrVI@TiO2 material, not in the case of chromate(VI) alone. Also, visible light-induced photocurrents were recorded for CrVI@TiO2 and not for chromate(VI) used either in solution or as films of 21767

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oxidation states may lead to the formation of their excited states capable of electron injection to the TiO2 CB, followed by reduction of an electron acceptor (e.g., molecular oxygen). This situation resembles that observed in the case of the [Fe(CN)6]3−/4−@TiO2 system, for which visible light-induced photocurrents may only be observed at potentials enabling FeIII reduction (Figure 9c).37,38 Photosensitization by reduced chromium or iron species leads to cathodic photocurrent generation. The formation of chromium species at lower oxidation states may also be responsible for photosensitization of TiO2 according to a classical mechanism, involving an electron injection into the CB. Therefore the mechanism of photocurrent generation and the photosensitization mode (PET vs OET) depends on the redox state of the surface modifier, as it was previously observed in some other cases of ruthenium and iron species at TiO2 surfaces.37,38,61,62 The present systems additionally offer another possibility of redoxbased reactivity modulation: hole injection versus electron injection.

Figure 8. The mechanism of titanium dioxide photosensitization by chromates(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 vs NHE are presented (pH = 7).

insoluble chromates(VI) at ITO electrode. These experiments afford additional arguments for the process of hole injection into the valence band. Moreover, the hole injection should prevent, 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 not possible (ECrVI/CrV > EO2/O2−). Therefore another electron acceptor must be involved in Cr V consumption and regeneration of CrVI. One possibility involves reduction of organic species present in the photolyzed suspension. Another comprises CrV disproportionation to CrVI and CrIII. In the absence of an efficient electron acceptor, recombination processes and further steps of chromium reduction play a dominant 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 be transferred directly to the electrode (Fermi potential of ITO is higher than the E°CrVI/CrV potential; Figure 9a) while the hole oxidizes 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 directly from chromium-based surface states. Application of lower potentials leads to reduction of CrVI (Figure 9b). Excitation of electrogenerated chromium(V) and chromium species at lower

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CONCLUSIONS

The presented studies point to a very unique mechanism of titanium dioxide photosensitization involving hole injection from the excited photosensitizer to the valence band. This process requires a strongly oxidizing agent capable of TiO2 oxidation. Chromates(VI) adsorbed at a titania surface offer such properties when excited within the LMCT band: photogenerated O−I ligand oxidizes TiO2 particle, while CrV reduces an external electron acceptor. Although the efficiency of such a photosensitization process is moderate and the (photo)stability of CrVI@TiO2 materials is unsatisfactory, the studied system represents a very unusual mode of titania photosensitization. In addition to this mechanism, a classical electron injection from the excited reduced surface species may operate, as demonstrated by photoelectrochemical studies. This counterintuitive behavior results from a peculiar arrangement of electronic levels in the studied systems. The presented mechanism can explain the photoactivity of chromate(VI)/ TiO2 systems described previously in numerous papers. Further increase of visible light activity of CrVI@TiO2 may involve a search for suitable electron acceptors diminishing efficiencies of recombination processes and chromium(V) reduction.

Figure 9. The mechanism of photocurrent generation upon visible light application: (a) at electrode covered with CrVI@TiO2 upon application of potentials higher than ECrVI/CrV; (b) at electrode covered with CrVI@TiO2 upon application of potentials lower than ECrVI/CrV; (c) at electrode covered with [Fe(CN)6]4−@TiO2 upon application of potentials lower than EFeIII/FeII. 21768

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +48 126632222. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support from National Science Centre within the 2011/ 01/B/ST5/00920 grant is highly acknowledged. Part of the measurements was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract No. POIG.02.01.0012-023/08).

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