Photocatalytic Activity of TiO2 Modified with Hexafluorometallates

Article pubs.acs.org/JPCC

Photocatalytic Activity of TiO2 Modified with HexafluorometallatesFine Tuning of Redox Properties by RedoxInnocent Anions † ́ Marta Buchalska,*,† Michał Pacia,† Marcin Kobielusz,† Marcin Surówka,† Elzḃ ieta Swiętek, Ewelina Wlaźlak,† Konrad Szaciłowski,*,†,‡ and Wojciech Macyk*,† †

Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland Faculty of Non-Ferrous Metals, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Kraków, Poland

‡

ABSTRACT: A group of selected hexafluorometallates ([AlF6]3−, [TiF6]2−, [ZrF6]2−, and [SiF6]2−) has been adsorbed at the surface of titanium dioxide. Such modification influenced the electronic and chemical properties of TiO2, as well as its photoactivity. The modification with [TiF6]2− inhibited the efficiency of OH• production but enhanced the generation of photocurrent and singlet oxygen. The results of modifications with [ZrF6]2− were the oppositegeneration of 1O2 was not observed in this case and photocurrents were lower; however, due to a better OH• production, degradation of herbicides was faster.

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Generation of multifluorinated titanium moieties ([TiF6]2−) at the surface of TiO2 is often observed upon anodic oxidation of metallic titanium carried out in electrolytes containing fluorides.9,10 Adsorption of hexafluorotitanate at TiO2, similarly to fluorides, should also influence the photocatalytic properties of this oxide. In addition, other hexafluorometallates constitute a group of modifiers that may supply new functionalities of TiO2 photocatalyst. Here we test the photoactivity of titanium dioxide with various adsorbed hexafluorometallate complexes. In particular, we compare the efficiencies of photocatalytic degradation of selected herbicides and generation of OH• and 1 O2 at these materials. A further goal of our studies is to check the applicability of such materials in the process of photocatalytic degradation of two herbicides: 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) (Figure 1). Both tested herbicides have the ability to control the speed of plant growth, and both of them were used to kill weeds in cereal crop plantations.11 Good solubility in water makes them easy to use,

INTRODUCTION Fluorinated titanium dioxide, i.e., TiO2 with surface hydroxyl groups substituted with fluorides, possesses significantly different photocatalytic properties than the unmodified material. The modified material is characterized by a lower pzc (point of zero charge), higher acidity, and higher polarity. These properties affect adsorption of reactants and influence the pathways of photocatalytic reactions photoinduced by the material.1 At both neat TiO2 and fluorinated TiO2 (F−@TiO2), generation of hydroxyl radicals takes place, as proven by electron paramagnetic resonance (EPR) measurements.2,3 In contrast to unmodified titania, F−@TiO2 photocatalyzes decomposition of cyanuric acid, a very inert organic compound.4,5 Two possible mechanisms of this particular activity of F−@TiO2 toward cyanuric acid degradation have been proposed in the literatureone involves generation of the so-called free hydroxyl radicals (not bound to the surface)4,6 and the other one assumes initiation of the degradation process by singlet oxygen.5 Fluoride anions can substitute surface hydroxyl groups of TiO2, forming a strong Ti−F bond.7 Surface fluorination can be achieved successfully in acidic solutions, as the pK of the binding reaction is 6.2.1 The highest efficiency of this process was observed at pH = 4, when 95% of the surface hydroxyl groups were exchanged with F−.1 The opposite process, removal of fluoride, can be achieved in alkaline solutions.1 At higher concentrations of F−, besides terminal Ti−F groups, formation of F−F1 and O−F8 bonds have been detected. Fluoride ions also can substitute oxygen in the TiO2 lattice, in particular those positioned close to the titania surface.1 © 2014 American Chemical Society

Figure 1. Structures of herbicides: 2,4-D and 2,4,5-T. Received: June 3, 2014 Revised: September 19, 2014 Published: October 2, 2014 24915

dx.doi.org/10.1021/jp505449n | J. Phys. Chem. C 2014, 118, 24915−24924

The Journal of Physical Chemistry C

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formed using a Quantachrome Autosorb-6 instrument. The samples were heated at 200 °C for 2 h under vacuum prior to the measurements. The specific surface area of the samples was estimated using the BET equation (nitrogen adsorption at 77 K). The point of zero charge (pzc) measurements were performed using a Malvern Zetasizer NanoZS instrument. The cell temperature was set to 20 °C. The suspensions of tested materials (1 mL of 0.1 g dm−3) were mixed with 1 mL of a buffer solution (acetate or phosphate, depending on the final pH). Tests of the Surface Coverage. Twenty mg of the tested materials were suspended in 2 mL of methanolic solution of catechol (1 mmol dm−3). The suspension was sonicated for 5 min. After centrifugation the concentration of catechol in supernatant solution was determined spectroscopically by measuring the absorbance at λ = 280 nm (Lambda 950, PerkinElmer). Analogous experiments have been done using herbicide solutions (2.5 × 10−3 mol dm−3). The concentration of herbicides was determined by measuring absorbance at 230 and 235 nm for 2,4-D and 2,4,5-T, respectively. Redox Properties of the Materials. Redox properties of the materials were determined using a spectroelectrochemical method,25 which is based on electrochemical measurements combined with UV−vis diffuse reflectance spectroscopy. The changes in reflectance were recorded at λ = 780 nm by a PerkinElmer UV−vis Lambda 12 spectrometer equipped with a 5 cm diameter integrating sphere. The electrochemical measurements were carried out in a three-electrode cell, with platinum wire and Ag/AgCl as counter electrode and reference electrode, respectively. The modified TiO2 materials were previously ground in a mortar and suspended in distilled water. Working electrodes were prepared by casting of tested materials at the surface of platinum foil (ca. 2 cm × 1 cm). Afterward the working electrodes were dried at ∼100 °C. In this way opaque films of the materials were formed on the Pt plate. The electrodes were placed in a quartz cuvette filled with 0.1 mol dm−3 LiClO4 solution in anhydrous acetonitrile. Oxygen was thoroughly removed from the electrolyte by purging with argon before (15 min) and during the experiments. The quartz cuvette was placed in front of the sphere, facing the working electrode (platinum foil with deposited TiO2) toward the light beam. Potential control was provided by the electrochemical analyzer Autolab PGSTAT302N (scan rate of 1 mV s−1). Tests of Photocatalytic Activity. All the tests of photocatalytic activity were done under the same conditions. A 150 W xenon lamp (XBO-150), equipped with a nearinfrared (NIR) filter (0.1 mol dm−3 CuSO4 solution in water) and 320 nm cutoff filter, was used for irradiation. For some tests with α-terpinene, UV−vis−NIR irradiations also were done (without the aqueous filter). Suspensions of the photocatalytic materials were prepared in the solutions of model pollutants (0.5 g dm−3), sonicated for 15 min, and irradiated in a round quartz cuvette (5 cm diameter, 1 cm optical path, 17 mL volume). The samples were collected regularly, filtered through CME syringe filters with a pore size of 0.22 μm, and subjected to analysis. The solutions of herbicides were prepared in water (2.5 × 10−4 mol dm−3). The samples were collected after 0, 2, 5, 10, 15, and 20 min of irradiation. UV−vis absorption spectra of the herbicide solutions were collected in a UV/vis/NIR Lambda 950 (PerkinElmer) spectrophotometer in a 1 cm quartz cuvette. The decomposition of pollutants was calculated from

but on the other hand this property generates a risk to aquatic organisms. 2,4-D and 2,4,5-T relatively easily undergo decomposition. The main problem related to the removal of these materials in environmental conditions is associated with the formation of byproducts, i.e., chlorophenol and chlorobenzene derivatives, which can be more harmful than the herbicides themselves.11 2,4-D and 2,4,5-T can be oxidized in the presence of TiO2 upon sunlight irradiation.12 Hydroxyl radicals play a key role in the process.11,13 Under oxygen-free conditions, oxidation involving valence band holes may take place.14 One of the first steps of degradation is substitution of the −CH2COOH fragment by a hydroxyl group, leading to formation of the corresponding chlorophenols. Photodegradation of polychlorophenols is a more demanding process than oxidation of monochlorophenols.15 Contrary to monochlorophenols oxidized mainly with hydroxyl radicals,16,17 polychlorophenols also react with singlet oxygen.18 The other processes responsible for 2,4-D and 2,4,5-T removal from the environment involve biodegradation. Specific bacterial strains are able to grow in the herbicide-containing media (2,4-D), as reported for Comamonas sp., Pseudomonas putida, Acinetobacter sp., Acinetobacter lwofii, and Klebsiella oxytoca.19 Nocardioides simplex was successfully applied for 2,4,5-T removal,20 while Pseudomonas cepacia was reported as a universal organism for removing both 2,4-D and 2,4,5-T.21,22 Trials of herbicide removal from water by physical adsorption on various sorbers (ion exchange,23 silica gel,24 etc.) were also reported. In this paper we compare electronic properties and photocatalytic activity of TiO2 materials modified with hexafluorometallates. This group of photocatalysts has also been tested in the process of 2,4-D and 2,4,5-T degradation.

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EXPERIMENTAL SECTION Materials. TiO2 (TH0, anatase, 330 m2 g−1, Kerr-McGee; Hombikat UV100, anatase, 300 m2 g−1, Sachtleben Chemie; and P25, anatase/rutile 80:20, 50 m2 g−1, Evonik) has been modified with various fluoride-containing compounds (K3[AlF6], Na2[SiF6], H2[TiF6], H2[ZrF6], and KF; SigmaAldrich). The solutions of these compounds (0.01 mol dm−3) were added to aqueous suspensions of TiO2 (0.75 g dm−3) in a 1:20 modifier/TiO2 molar ratio. The suspensions were stirred magnetically and sonicated for 10 min; afterward they were filtered, washed several times with water, and dried in the air at 80 °C. Within this work the surface-modified materials are abbreviated as [MF6]n−@TiO2. Diffuse reflectance spectra were recorded with a PerkinElmer UV−vis Lambda 12 spectrometer equipped with a 5 cm diameter integrating sphere. Prior to the measurements, the samples were ground in an agate mortar with barium sulfate (ca. 1:50 weight ratio). The reflectances of these mixtures were recorded using BaSO4 as a reference. The work functions of the studied samples were measured using a Kelvin Probe model (KP Technology) with 1 mm stainless steel tip; gold sputtered on aluminum was used as a standard (WFAu = 5.100 eV). Herbicides (2,4-D and 2,4,5-T), terephthalic acid, α-terpinene, and catechol were purchased from Sigma-Aldrich and used as received. Ascaridole standard was purchased from PhytoLab. Characterization of Physicochemical Properties. A scanning electron microscope (Vega 3 LM, Tescan), equipped with an LaB6 cathode, was operated at a voltage of 30 kV. Brunauer−Emmett−Teller (BET) measurements were per24916



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Next, 11 injections were made every 5 min. After the last injection, oxygen concentration was measured in the suspension for another 15 min to determine the final concentration of dissolved oxygen. Theoretical Modeling. The Ti45O99 cluster was cut out of the anatase structure28 along the [001] plane; the edges were substituted with the hydroxyl ligands to achieve a total charge of −3. The geometry of the cluster was optimized using the MM2 method (CaCHE, Fujitsu). Different species (fluoride, water, and hexafluorometallates) were placed in the middle of the upper plane of the cluster; the number of protons was readjusted for the constant charge of −3, and the geometry was reoptimized using tight convergence criteria. Subsequently, the electronic structure of the test systems was computed at the PM7 level of theory using the MOPAC 2012 package.29,30 Population analysis and calculation of density-of-states spectra were performed using the AOMix package.31,32

the absorbance changes at 230 and 235 nm for 2,4-D and 2,4,5T, respectively. The process of conversion of terephthalic acid (TA) to 2hydroxyterephthalic acid (TAOH) was used to compare efficiencies of hydroxyl radicals generation (Figure 2A).26

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Figure 2. Process of (A) TAOH formation in the reaction of terephthalic acid (TA) with hydroxyl radicals;26 (B) formation of ascaridole in the reaction of α-terpinene with singlet oxygen.27

RESULTS AND DISCUSSION Materials. Adsorption of hexafluorometallates ([AlF6]3−, [TiF6]2−, [ZrF6]2−, and [SiF6]2−) and F− at the TiO2 surface did not influence the material color, which remains white. Diffuse reflectance spectra converted to the Kubelka−Munk function, presented in Figure 3, also did not show significant

Photocatalysts were irradiated in TA solution (6 × 10−3 mol dm−3 TA, 0.02 mol dm−3 NaOH, pH = 11) for 30 min. Samples were collected in 5 min intervals. In the reaction of nonfluorescent TA with hydroxyl radicals, the formation of TAOH can be monitored by emission spectra measurements. TAOH shows a broad emission band at λmax = 425 nm when excited at λexc = 315 nm. Fluorescence spectra were measured using a FluoroLog-3 (Horiba JobinYvon) spectrofluorometer in a 1 cm quartz cuvette. Singlet oxygen generation was followed by monitoring the progress of α-terpinene oxidation to ascaridole (Figure 2B).27 The solution of α-terpinene (10−3 mol dm−3) was prepared in methanol. Irradiation was carried out for 2 h. Determination of the changes in the concentration of the substrate has been done with high-performance liquid chromatography (HPLC) analysis (PerkinElmer, Flexar system) equipped with a UV−vis detector. A C18 column (PerkinElmer, Spheri-5, ODS 5 μm, 250 × 4.6 mm, cat. no. 0712-0019) was used with 100% methanol as the eluent (flow rate of 1 mL min−1). Detection was carried out at λ = 220 nm. Under these conditions, the peak of ascaridole occurs at 3.4 min.17 Photoelectrochemical Measurements. Photoelectrochemical measurements were carried out using an Autolab PGSTAT302N and XBO150 xenon lamp with a monochromator (Instytut Fotonowy). Measurements have been done in the three-electrode cell using platinum wire and Ag/AgCl as counter electrode and reference electrode, respectively. A thin layer of material on indium tin oxide (ITO) foil (resistivity = 60 Ω/sq) was used as a working electrode. The electrodes were placed in a quartz cuvette filled with 0.1 mol dm−3 KNO3 solution in water as an electrolyte (pH = 6.0). Irradiation was done in the range 330−450 nm (every 10 nm). Photocurrent generation was measured in the range of −0.2−1 V vs Ag/ AgCl. Oxygen Adsorption. Oxygen adsorption tests were performed using Firesting O2 oxygen meter (PyroScience). For each measurement 5 mL of deoxygenated sample (purged with argon) was poured into a vial equipped with the fluorescence oxygen sensor. After 5 min of stabilizing the oxygen concentration in the sealed vial (completely filled with the suspension, with no gas atmosphere above), a portion of 100 μL of air-saturated water was injected through a septum.

Figure 3. Diffuse reflectance spectra of surface-modified TiO2 (TH0) materials.

differences from the spectrum of starting TiO2 material. In particular, the bandgap energies of all materials are the same. This is fully justified as all hexafluorido complexes do not absorb visible light and cannot yield charge transfer complexes with surface TiIV centers, due to a high oxidation state of the central atoms. Modification of titanium dioxide with hexafluorometallates influenced the surface charge of the particles, as reflected by a faster sedimentation of the surface-modified material from aqueous suspensions. Similarly to fluoride anions, the adsorption of hexafluorometallates is favored from acidic solutions, since in basic media fluorides desorb from the surface.1 Three commercial samples of titanium dioxide have been selected for the studies: TH0 (anatase), UV100 (anatase), and P25 (anatase/rutile mixture). The specific surface areas of the modified materials are in general lower than those measured for unmodified TiO2 (Table 1). This is the result of a lower TiO2 content in the modified samples and a possible blocking of 24917



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Table 1. Specific Surface Area for Materials (m2 g−1; ±2%) surface ligand

TH0

P25

UV100

unmodified F− [AlF6]3− [SiF6]2− [TiF6]2− [ZrF6]2−

352 305 251 246 225 297

58.8 50.0 48.9 52.9 60.0 52.4

321 323 268 272 259 260

More surprisingly, diffuse reflectance spectra of catecholsoaked samples were significantly different (spectra not shown). While the intensity of the main ligand-to-metal charge-transfer (LMCT) band (catechol → TiIV) correlates with the number of accessible sites on the surface and decreases with the increasing amount of hexafluorometallate adsorbed, the energy of this transition changes as well. In the case of TH0-based materials (TH0 is characterized by the highest specific surface area, and therefore the spectra of these materials impregnated with catechol were most intense), the LMCT was observed at 3.084 eV for neat TiO2. A small but significant bathochromic shift was observed upon adsorption of hexafluorometallates, whereas adsorption of fluoride exerts an opposite effect (Table 3). The dominating LMCT character of these transitions is not changed; also the bandwidth, reflecting the vibronic components, is virtually the same.

Table 2. Point of Zero Charge Values for Materials surface ligand

TH0

P25

UV100

unmodified F− [AlF6]3− [SiF6]2− [TiF6]2− [ZrF6]2−

5.77 5.58 5.9 5.56 5.42 5.71

5.98 5.44 6.25 5.38 3.9 5.58

5.69 5.61 5.81 5.51 5.46

Table 3. Energies of the LMCT Transitions at TH0 Surfaces Modified with Fluorometallates and Subsequently with Catechol

pores by the modifiers. The pzc also lowers upon modification (Table 2), which is in agreement with previous reports.1 The lowest values of pzc were observed for [TiF6]2−@TiO2. A slight increase of pzc took place upon TiO2 modification with [AlF6]3− ions. To estimate the titania surface coverage by fluoride and hexafluorometallates, adsorption of catechol was measured. Catechol binds efficiently to the exposed TiO2 surface, forming yellow to orange charge transfer complexes.33 Adsorption of catechol should be significantly weaker at the modified surface, where −OH groups are not fully exposed. Therefore, a comparison of the amounts of surface-bound catechol can be used as a qualitative measure of the surface coverage by the modifiers. The amounts of bound catechol are shown in Figure 4. The comparison of the amounts of adsorbed catechol reveals

surface ligand

LMCT energy (catechol → TiIV)/ eV

bandwidth/ eV

ECBa/ eV

H2O F− [AlF6]3− [SiF6]2− [TiF6]2− [ZrF6]2−

3.084 3.261 3.054 3.052 3.040 3.021

1.009 1.136 0.983 1.128 1.155 1.159

0.70 0.95 0.65 0.65 0.60 0.55

a

Conduction band edge energy as calculated on the PM7 level of theory.

These results suggest an increase of the surface state (or conduction band edge) energies in the case of fluoride adsorption, while hexafluorometallates exert the opposite tendency. The same effect can be observed when surface properties are directly determined using the Kelvin probe technique (Table 4). Modification with fluoride results in a Table 4. Surface Contact Potentials (ECP) versus Standard Hydrogen Electrode (vs SHE; ±0.03 V)

Figure 4. Amounts of catechol adsorbed at the surface of various TiO2 samples. See the Experimental Section for details. The reproducibility of the measurements is within 1−2%.

surface ligand

ECP/V TH0

ECP/V P25

ECP/V UV100

H2O F− [AlF6]3− [SiF6]2− [TiF6]2− [ZrF6]2−

−0.52 −0.56 −0.49 −0.43 −0.43 −0.41

−0.4 −0.23 +0.04 −0.27 −0.07 −0.08

−0.61 −0.50 −0.41 −0.52 −0.53 −0.48

significant decrease of surface contact potential ECP (i.e., increase of the conduction band edge energy), while hexafluorometallates decrease the conduction band energies. This tendency is observed for all studied TiO2 materials. In general, the influence of modification on electronic properties of the materials is more pronounced for the materials with stronger hexafluorometallate−TiO2 interactions. The effect of surface modification is further supported by a computational approach. The Ti45O81(OH)18 cluster of anatase was used as a model of the TiO2 surface. The modifying hexafluorometallates were placed over the center of the cluster. An example of the test structure is shown in Figure 5.

a very efficient surface coverage by hexafluorotitanate and haxafluorozirconate anions. Surprisingly, impregnation with potassium fluoride protected the TiO2 surface from catechol adsorption to a smaller extent, independently of the applied TiO2 material (P25, UV100, and TH0). The differences observed for various TiO2 samples originate from the differences between specific surface areas of these materials. 24918



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the case of titanium dioxide, electrochemical reduction generates electrons trapped close to the conduction band as TiIII centers, characterized by an absorption maximum at 780 nm. The trapped electrons can reduce oxygen according to eq 1, Ti III + O2 → Ti IV + O2•−

(1)

therefore, the measurements should be made under oxygen-free conditions. The relative reflectance signals measured at 780 nm as a function of the potential, recorded for the platinum electrode covered with P25 modified with selected hexafluorometallates, are presented in Figure 7. The measured signals differ from

Figure 5. Geometry of a Ti45O81(OH)18 cluster with one [ZrF6]2− anion electrostatically adsorbed over the central titanium ion.

Semiempirical quantum-chemical calculations at the PM7 level of theory indicate an increase of the conduction band energy upon binding of fluoride by ca. 0.25 eV (Table 3). The interaction with hexafluorometallates results in a significant decrease of the band edge energy. These results are consistent with observed spectral data and Kelvin probe measurements. Scanning electron microscope (SEM) pictures of modified powders are shown in Figure 6. Surface-modified materials

Figure 7. Reflectance changes measured at 780 nm as a function of the electrode potential for selected materials based on TiO2 (UV100) deposited at the surface of platinum plate. The measurements were carried out using 0.1 M LiClO4 in acetonitrile under inert atmosphere (Ar).

each other. The deflection points of recorded curves (at EON) correspond to the onset reduction potential at which electron trapping accelerates. For surface-modified materials and unmodified UV100, two EON potentials can be measured. The first one (EON′, a higher value, between −0.4 and −0.7 V) refers to the electron traps introduced by surface modifiers. The second one (EON″, lower values close to −0.8 V) can be assigned to the electron traps close to the conduction band edge (Table 5). Binding hexafluorometallates to the titania surface apparently changes the character of the surface states; therefore, the measured EON′ shifts to higher potentials. These differences correlate well with the calculated values of ECB and the results of Kelvin probe measurements (Tables 3 and 4).

Figure 6. SEM pictures of UV-100-based materials: (a) unmodified TiO2, (b) [ZrF6]2−@TiO2, (c) [AlF6]3−@TiO2, and (d) [TiF6]2−@ TiO2.

Table 5. EON for the Studied TiO2 Materials (Measured in 0.1 mol dm−3 LiClO4 in Acetonitrile in Inert Atmosphere (Ar) vs Ag/AgCl (±0.05 V))

show a higher tendency to form more compact aggregates when compared to unmodified samples. This observation explains a faster sedimentation of fluorinated materials from aqueous suspensions. Redox Properties of the Materials. Adsorption of hexafluorometallates at the TiO2 surface influences the band edge potentials, so it should also modulate redox properties of this semiconductor. The recently developed spectroelectrochemical method of characterization of redox properties of semiconducting materials has been applied.25 In this method the dependence of reflectance (or absorbance) on the density of electrons in the conduction band of the material is used. In

EON/V TH0 surface ligand unmodified F− [AlF6]3− [SiF6]2− [TiF6]2− [ZrF6]2− 24919

EON′ −0.47 −0.5 −0.43 −0.68

EON″ −1.08 −0.91 −1.03 −0.8 −0.94 −1.13

EON/V P25 EON′

−0.62 −0.67 −0.62

EON/V UV100

EON″

EON′

EON″

−0.80 −0.88

−0.73 −0.44 −0.6 −0.71 −0.66

−1.08 −1.14 −1.02 −0.99 −1.02 −0.87

−1.00 −0.97 −0.82



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Binding hexafluorometallates to the titania surface apparently changes the character of surface states; therefore, the measured EON shifts to higher potentials. The electrons trapped as TiIII may play a crucial role in the process of photodegradation of pollutants, influencing the photoactivity of the material. Photocatalytic ActivityOxidation of Terephthalic Acid. Oxidation of terephthalic acid (TA) to hydroxyterephthalic acid (TAOH) was tested to compare efficiencies of hydroxyl radical generation at different materials. TAOH concentration after 30 min of irradiation is shown in Figure 8. Unmodified photocatalysts, in particular P25, are charac-

Figure 9. Absorption spectra of 2,4-D (thick line) and 2,4,5-T (thin line) (2.5 × 10−4 mol dm−3).

Figure 8. TAOH concentration after 30 min of irradiation of the materials suspended in terephthalic acid solution (λ > 320 nm).

terized by relatively high efficiencies of hydroxyl radical generation. The photoactivities of UV100 and TH0 are similarin the case of both materials, anatase is the predominant form of TiO2 and specific surface area is comparable (300 and 330 m2 g−1 for TH0 and UV100, respectively). Modification with hexafluorometallates influences the activity, mainly that of P25. F−@P25 shows the highest activity, while the efficiencies of OH• formation at P25 modified with [TiF6]2−, [ZrF6]2−, and [SiF6]2− are the lowest. [TiF6]2− reduces also the activity of TH0 and UV100. Hexafluorotitanate ions adsorb at TiO2 surface most efficiently (compare Figure 4) and hinder water oxidation by photogenerated holes. Photocatalytic ActivityDegradation of Herbicides. The absorption spectra of 2,4-D and 2,4,5,-T are presented in Figure 9. Both compounds absorb UV light at wavelengths shorter than ca. 310 nm and do not absorb light used throughout the photocatalytic tests (λ > 320 nm). Therefore, the herbicides are photostable upon irradiation in the absence of any photocatalyst (data not shown). Upon irradiation in the presence of TiO2 materials, a decrease of all bands could be observed, but no new bands were formed. In the case of 2,4-D degradation, the F−@TH0 material appeared to be the most active, while in the case of 2,4,5-T, [AlF6]3−@TH0 and [SiF6]2−@P25 showed the highest degradation rates (Figure 10). In the case of both herbicides, [ZrF6]2−@TH0 also showed a good performance. Modification of P25 and UV100 had a detrimental effect on degradation of 2,4-D and 2,4,5-T, but the photocatalytic activity of TH0 can be improved when surface modification is applied. Although hydroxyl radicals are responsible for degradation of 2,4-D and 2,4,5-T,11 there are clear differences between

Figure 10. Degradation of herbicides in the presence of modified TiO2 after 20 min of irradiation (λ > 320 nm): (A) 2,4-D and (B) 2,4,5-T.

efficiencies of 2,4-D, 2,4,5-T, and TA oxidation. For instance, F−@TH0 appears very active in 2,4-D degradation, but it is not the case for 2,4,5-T and TA oxidation. Conversely, [AlF6]3− modification improves activity of TH0 toward 2,4,5-T degradation but not toward 2,4-D and TA. Moreover, the surface coverage by [AlF6]3− is very inefficient (or adsorption of these ions is weak); thus, the activity of [AlF6]3−@TH0 in every case resembles that of unmodified TiO2, although it is slightly higher for [AlF6]3−@TH0. On the other hand, [TiF6]2− adsorbs very efficiently at TiO2, which reflects in the lowest activities of [TiF6]2−@TiO2 toward TA and 2,4,5-T oxidation but not toward 2,3-D oxidation (in the presence of modified 24920



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[TiF6]2−@TH0 and [TiF6]2−@UV100, oxidation of 2,3-D proceeds as efficiently as in the presence of other photocatalysts). These differences can originate mainly from different and specific adsorption of herbicides at various surfaces (Figure 11). Degradation of 2,4-D proceeds through the attack of

Figure 12. (A) Decrease of the α-terpinene concentration; (B) increase of the ascaridole concentration upon irradiation in the presence of UV100 and [TiF6]2−@UV100. Figure 11. Concentration of herbicides adsorbed at the surface of various surface modified materials: (A) 2,4-D and (B) 2,4,5-T.

Degradation of the substrate is slightly faster in the presence of modified material (Figure 11A), but the efficiency of ascaridole generation is already 2−3 times higher for [TiF6]2−@UV100 than for UV100 (Figure 12B). Upon UV− vis irradiation (320−620 nm), the amount of photogenerated ascaridole increases both when UV100 or [TiF6]2−@UV100 are used as photocatalysts. Simultaneous irradiation with UV−vis and NIR light (λ > 320 nm) accelerates the process. This effect has already been described for other surface-modified TiO2 materials35 and can result either from generation of singlet oxygen in the process of energy transfer from higher excited states or from excitation of electrons trapped in unreactive states to the conduction band, followed by reduction of oxygen to superoxide. Reactions involving oxygen as a reactant should be influenced by O2 adsorption at the surface of a photocatalyst. Therefore, the influence of TiO2 modification on oxygen adsorption was studied in detail. Oxygen Adsorption. A lower concentration of dissolved oxygen in the suspension containing titanium dioxide modified with fluorides and hexafluorometallates indicates a higher concentration of oxygen adsorbed at the surface of these materials. All results are compared to water-saturated sample (dissolved oxygen value = 100%). Hombikat UV100 modified with fluoride anions exhibited ca. 4 times higher oxygen adsorption than the corresponding unmodified sample (Figure 13). In the case of F−@P25, the oxygen adsorption was

hydroxyl radicals, leading to hydroxylation of aromatic ring and removal of the side group.11 Analogous reactions can be expected for 2,4,5-T due to the similarities in the structures of the particles. Because of differences in the degradation of both herbicides in the presence of various materials, further studies of the mechanisms of their mineralization have to be done. Photocatalytic Activityα-Terpinene. The activity of all materials was also studied in the process of α-terpinene photooxidation to ascaridole with singlet oxygen. At short irradiation times (30 min), only [TiF6]2−@UV100 shows a significant activity toward ascaridole production (data not shown). Therefore, longer tests (2 h of irradiation) have been done only with [TiF6]2−@UV100 and unmodified UV100 under various irradiation conditions. Degradation of αterpinene and generation of ascaridole is shown in Figure 12. Singlet oxygen can be generated in the two consecutive redox reactions: the first one involves reduction of oxygen to superoxide, while the second one leads to 1O2 through reoxidation of O2•− with valence band holes.34 However, the energy transfer from higher excited states cannot be ruled outthese states can be populated as a result of NIR light absorption by electrons trapped as TiIII (λmax = 780 nm).35 24921



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[TiF6]2− at TiO2, the behavior of [TiF6]2−@TiO2 at 1 V is surprising. Apparently, under these conditions hexafluorotitanate does not hinder the interfacial electron transfer but may even facilitate this process (in the case of [TiF6]2−@UV100). Adsorption of [AlF6]3− is very weak; therefore, its influence on photocurrent generation is negligible. Significantly better adsorption of [SiF6]2− and F− reflects in more significant changes in photocurrents. In general, fluoride and hexafluorosilicate ions hinder interfacial electron transfer (IFET) processes (with an exception for F−@UV100 and [SiF6]2−@ UV100 at 1 V).

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CONCLUSIONS Impregnation of TiO2 with hexafluorometallates leads to the modification of its surface and strongly influences the photocatalytic activity. Hexafluorotitanate binds most efficiently to all tested materials. This modification decreases rates of TA, 2,4-D, and 2,4,5-T oxidation, due to decreased efficiency of hydroxyl radical formation. On the other hand, the same modification improves yields of singlet oxygen production and photocurrent generation. These results can be explained by an improved interfacial electron transfer responsible for both photocurrent generation and superoxide production. As a consequence, the concluded mechanism of singlet oxygen production involves formation of superoxide, followed by its oxidation with holes.34 [ZrF6]2− adsorbs quite efficiently at titanium dioxide, but it modifies the surface to a lower extent than [TiF6]2−. Therefore, generation of hydroxyl radicals and oxidation of herbicides is slightly faster for [ZrF6]2−@TiO2 than for [TiF6]2−@TiO2. On the other hand, hexafluorozirconate efficiently inhibits photo-

Figure 13. Oxygen concentration after 75 min in the suspensions of tested samples (see Experimental Section for details).

improved by >50%. These results justify a higher activity of [TiF6]2−@UV100 toward singlet oxygen generation, because oxygen availability at this material is better. Photocurrent Generation. Photocurrent measurements revealed significant differences in efficiencies of the photoinduced interfacial electron transfer (Figure 14). When compared to photoactivity of unmodified P25 and UV100 materials, the corresponding [ZrF6]2−@UV100 and [ZrF6]2−@ UV100 appear almost inactive, independent of the electrode potential. On the other hand, hexafluorotitanate diminishes photocurrents at −0.2 V, but at 1 V the photocurrents are comparable with those observed for the unmodified materials. Taking into account a good adsorption of [ZrF6]2− and

Figure 14. Photocurrent generation at electrodes made of the studied materials recorded at 1 V vs Ag/AgCl (A, B) and −0.2 V vs Ag/AgCl (C, D). The legend is valid for all graphs. 24922



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currents (IFET process) and does not enable singlet oxygen generation. The lowest adsorption of hexafluorometallate was observed for [AlF6]3−. Consequently, this modifier shows the smallest impact on the reactivity of titanium dioxide. The studied modifiers, in particular [TiF6]2− and [ZrF6]2−, can significantly influence photocatalytic processes of titanium dioxide. They can be used to tune efficiencies of reactive oxygen species formation and photocurrent generation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to Dr. Zhishun Wei for his help in BET measurements. This work was carried out within the “Activation of small molecules in photocatalytic systems” project supported by the Foundation for Polish Science, cofinanced by European Union, Regional Development Fund (project no. TEAM/2012-9/4). Also a support by the EU-FP7 within the project “4G-PHOTOCAT” (grant no. 309636; photodegradation of herbicides), cofinanced by Polish Ministry of Science and Higher Education (project no. W13/7.PR/ 2013) is highly acknowledged. A part of this work was carried out at 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.02.01.00-12-023/08).

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Photocatalytic Activity of TiO2 Modified with Hexafluorometallates

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