Photocatalytic Activity of TiO2 Modified with Hexafluorometallates – Fine Tuning of Redox...

1 Photocatalytic Activity of TiO2 Modified with2 HexafluorometallatesFine Tuning of Redox Properties by Redox-3 Innocent Anions4 Marta Buchalskadagger Michał Paciadagger Marcin Kobieluszdagger Marcin Surowkadagger Elzbieta Swiętekdagger

5 Ewelina Wlazlakdagger Konrad SzaciłowskidaggerDagger and Wojciech Macykdagger

6daggerFaculty of Chemistry Jagiellonian University ul Ingardena 3 30-060 Krakow Poland

7DaggerFaculty of Non-Ferrous Metals AGH University of Science and Technology al A Mickiewicza 30 30-059 Krakow Poland

8 ABSTRACT A group of selected hexafluorometallates ([AlF6]3minus [TiF6]

2minus [ZrF6]2minus and

9 [SiF6]2minus) has been adsorbed at the surface of titanium dioxide Such modification

10 influenced the electronic and chemical properties of TiO2 as well as its photoactivity The11 modification with [TiF6]

2minus inhibited the efficiency of OHbull production but enhanced the12 generation of photocurrent and singlet oxygen The results of modifications with [ZrF6]

2minus

13 were the oppositegeneration of 1O2 was not observed in this case and photocurrents14 were lower however due to a better OHbull production degradation of herbicides was faster

15 INTRODUCTION

16 Fluorinated titanium dioxide ie TiO2 with surface hydroxyl17 groups substituted with fluorides possesses significantly18 different photocatalytic properties than the unmodified materi-19 al The modified material is characterized by a lower pzc (point20 of zero charge) higher acidity and higher polarity These21 properties affect adsorption of reactants and influence the22 pathways of photocatalytic reactions photoinduced by the23 material1

24 At both neat TiO2 and fluorinated TiO2 (FminusTiO2)25 generation of hydroxyl radicals takes place as proven by26 electron paramagnetic resonance (EPR) measurements23 In27 contrast to unmodified titania FminusTiO2 photocatalyzes28 decomposition of cyanuric acid a very inert organic29 compound45 Two possible mechanisms of this particular30 activity of FminusTiO2 toward cyanuric acid degradation have31 been proposed in the literatureone involves generation of the32 so-called free hydroxyl radicals (not bound to the surface)46

33 and the other one assumes initiation of the degradation process34 by singlet oxygen5

35 Fluoride anions can substitute surface hydroxyl groups of36 TiO2 forming a strong TiminusF bond7 Surface fluorination can be37 achieved successfully in acidic solutions as the pK of the38 binding reaction is 621 The highest efficiency of this process39 was observed at pH = 4 when 95 of the surface hydroxyl40 groups were exchanged with Fminus1 The opposite process41 removal of fluoride can be achieved in alkaline solutions1 At42 higher concentrations of Fminus besides terminal TiminusF groups43 formation of FminusF1 and OminusF8 bonds have been detected44 Fluoride ions also can substitute oxygen in the TiO2 lattice in45 particular those positioned close to the titania surface1

46Generation of multifluorinated titanium moieties ([TiF6]2minus)

47at the surface of TiO2 is often observed upon anodic oxidation48of metallic titanium carried out in electrolytes containing49fluorides910

50Adsorption of hexafluorotitanate at TiO2 similarly to51fluorides should also influence the photocatalytic properties52of this oxide In addition other hexafluorometallates constitute53a group of modifiers that may supply new functionalities of54TiO2 photocatalyst Here we test the photoactivity of titanium55dioxide with various adsorbed hexafluorometallate complexes56In particular we compare the efficiencies of photocatalytic57degradation of selected herbicides and generation of OHbull and58

1O2 at these materials59A further goal of our studies is to check the applicability of60such materials in the process of photocatalytic degradation of61two herbicides 24-dichlorophenoxyacetic acid (24-D) and62 f1245-trichlorophenoxyacetic acid (245-T) (Figure 1) Both63tested herbicides have the ability to control the speed of plant64growth and both of them were used to kill weeds in cereal crop65plantations11 Good solubility in water makes them easy to use

Received June 3 2014Revised September 19 2014

Figure 1 Structures of herbicides 24-D and 245-T

Article

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66 but on the other hand this property generates a risk to aquatic67 organisms 24-D and 245-T relatively easily undergo68 decomposition The main problem related to the removal of69 these materials in environmental conditions is associated with70 the formation of byproducts ie chlorophenol and chlor-71 obenzene derivatives which can be more harmful than the72 herbicides themselves11

73 24-D and 245-T can be oxidized in the presence of TiO274 upon sunlight irradiation12 Hydroxyl radicals play a key role in75 the process1113 Under oxygen-free conditions oxidation76 involving valence band holes may take place14 One of the77 first steps of degradation is substitution of the minusCH2COOH78 fragment by a hydroxyl group leading to formation of the79 corresponding chlorophenols Photodegradation of polychlor-80 ophenols is a more demanding process than oxidation of81 monochlorophenols15 Contrary to monochlorophenols oxi-82 dized mainly with hydroxyl radicals1617 polychlorophenols also83 react with singlet oxygen18

84 The other processes responsible for 24-D and 245-T85 removal from the environment involve biodegradation Specific86 bacterial strains are able to grow in the herbicide-containing87 media (24-D) as reported for Comamonas sp Pseudomonas88 putida Acinetobacter sp Acinetobacter lwof ii and Klebsiella89 oxytoca19 Nocardioides simplex was successfully applied for90 245-T removal20 while Pseudomonas cepacia was reported as a91 universal organism for removing both 24-D and 245-T2122

92 Trials of herbicide removal from water by physical adsorption93 on various sorbers (ion exchange23 silica gel24 etc) were also94 reported95 In this paper we compare electronic properties and96 photocatalytic activity of TiO2 materials modified with97 hexafluorometallates This group of photocatalysts has also98 been tested in the process of 24-D and 245-T degradation

99 EXPERIMENTAL SECTION100 Materials TiO2 (TH0 anatase 330 m2 gminus1 Kerr-McGee101 Hombikat UV100 anatase 300 m2 gminus1 Sachtleben Chemie102 and P25 anataserutile 8020 50 m2 gminus1 Evonik) has been103 modified with various fluoride-containing compounds104 (K3[AlF6] Na2[SiF6] H2[TiF6] H2[ZrF6] and KF Sigma-105 Aldrich) The solutions of these compounds (001 mol dmminus3)106 were added to aqueous suspensions of TiO2 (075 g dm

minus3) in a107 120 modifierTiO2 molar ratio The suspensions were stirred108 magnetically and sonicated for 10 min afterward they were109 filtered washed several times with water and dried in the air at110 80 degC Within this work the surface-modified materials are111 abbreviated as [MF6]

nminusTiO2 Diffuse reflectance spectra were112 recorded with a PerkinElmer UVminusvis Lambda 12 spectrometer113 equipped with a 5 cm diameter integrating sphere Prior to the114 measurements the samples were ground in an agate mortar115 with barium sulfate (ca 150 weight ratio) The reflectances of116 these mixtures were recorded using BaSO4 as a reference The117 work functions of the studied samples were measured using a118 Kelvin Probe model (KP Technology) with 1 mm stainless119 steel tip gold sputtered on aluminum was used as a standard120 (WFAu = 5100 eV) Herbicides (24-D and 245-T)121 terephthalic acid α-terpinene and catechol were purchased122 from Sigma-Aldrich and used as received Ascaridole standard123 was purchased from PhytoLab124 Characterization of Physicochemical Properties A125 scanning electron microscope (Vega 3 LM Tescan) equipped126 with an LaB6 cathode was operated at a voltage of 30 kV127 BrunauerminusEmmettminusTeller (BET) measurements were per-

128formed using a Quantachrome Autosorb-6 instrument The129samples were heated at 200 degC for 2 h under vacuum prior to130the measurements The specific surface area of the samples was131estimated using the BET equation (nitrogen adsorption at 77132K) The point of zero charge (pzc) measurements were133performed using a Malvern Zetasizer NanoZS instrument The134cell temperature was set to 20 degC The suspensions of tested135materials (1 mL of 01 g dmminus3) were mixed with 1 mL of a136buffer solution (acetate or phosphate depending on the final137pH)138Tests of the Surface Coverage Twenty mg of the tested139materials were suspended in 2 mL of methanolic solution of140catechol (1 mmol dmminus3) The suspension was sonicated for 5141min After centrifugation the concentration of catechol in142supernatant solution was determined spectroscopically by143measuring the absorbance at λ = 280 nm (Lambda 950144PerkinElmer)145Analogous experiments have been done using herbicide146solutions (25 times 10minus3 mol dmminus3) The concentration of147herbicides was determined by measuring absorbance at 230 and148235 nm for 24-D and 245-T respectively149Redox Properties of the Materials Redox properties of150the materials were determined using a spectroelectrochemical151method25 which is based on electrochemical measurements152combined with UVminusvis diffuse reflectance spectroscopy The153changes in reflectance were recorded at λ = 780 nm by a154PerkinElmer UVminusvis Lambda 12 spectrometer equipped with a1555 cm diameter integrating sphere The electrochemical156measurements were carried out in a three-electrode cell with157platinum wire and AgAgCl as counterelectrode and reference158electrode respectively The modified TiO2 materials were159previously ground in a mortar and suspended in distilled water160Working electrodes were prepared by casting of tested materials161at the surface of platinum foil (ca 2 cm times 1 cm) Afterward the162working electrodes were dried at sim100 degC In this way opaque163films of the materials were formed on the Pt plate The164electrodes were placed in a quartz cuvette filled with 01 mol165dmminus3 LiClO4 solution in anhydrous acetonitrile Oxygen was166thoroughly removed from the electrolyte by purging with argon167before (15 min) and during the experiments The quartz168cuvette was placed in front of the sphere facing the working169electrode (platinum foil with deposited TiO2) toward the light170beam Potential control was provided by the electrochemical171analyzer Autolab PGSTAT302N (scan rate of 1 mV sminus1)172Tests of Photocatalytic Activity All the tests of173photocatalytic activity were done under the same conditions174A 150 W xenon lamp (XBO-150) equipped with a near-175infrared (NIR) filter (01 mol dmminus3 CuSO4 solution in water)176and 320 nm cutoff filter was used for irradiation For some177tests with α-terpinene UVminusvisminusNIR irradiations also were178done (without the aqueous filter) Suspensions of the179photocatalytic materials were prepared in the solutions of180model pollutants (05 g dmminus3) sonicated for 15 min and181irradiated in a round quartz cuvette (5 cm diameter 1 cm182optical path 17 mL volume) The samples were collected183regularly filtered through CME syringe filters with a pore size184of 022 μm and subjected to analysis185The solutions of herbicides were prepared in water (25 times18610minus4 mol dmminus3) The samples were collected after 0 2 5 1018715 and 20 min of irradiation UVminusvis absorption spectra of the188herbicide solutions were collected in a UVvisNIR Lambda189950 (PerkinElmer) spectrophotometer in a 1 cm quartz190cuvette The decomposition of pollutants was calculated from

The Journal of Physical Chemistry C Article

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191 the absorbance changes at 230 and 235 nm for 24-D and 245-192 T respectively193 The process of conversion of terephthalic acid (TA) to 2-194 hydroxyterephthalic acid (TAOH) was used to compare

f2 195 efficiencies of hydroxyl radicals generation (Figure 2A)26

196 Photocatalysts were irradiated in TA solution (6 times 10minus3 mol197 dmminus3 TA 002 mol dmminus3 NaOH pH = 11) for 30 min198 Samples were collected in 5 min intervals In the reaction of199 nonfluorescent TA with hydroxyl radicals the formation of200 TAOH can be monitored by emission spectra measurements201 TAOH shows a broad emission band at λmax = 425 nm when202 excited at λexc = 315 nm Fluorescence spectra were measured203 using a FluoroLog-3 (Horiba JobinYvon) spectrofluorometer in204 a 1 cm quartz cuvette205 Singlet oxygen generation was followed by monitoring the206 progress of α-terpinene oxidation to ascaridole (Figure 2B)27

207 The solution of α-terpinene (10minus3 mol dmminus3) was prepared in208 methanol Irradiation was carried out for 2 h Determination of209 the changes in the concentration of the substrate has been done210 with high-performance liquid chromatography (HPLC) analysis211 (PerkinElmer Flexar system) equipped with a UVminusvis212 detector A C18 column (PerkinElmer Spheri-5 ODS 5 μm213 250 times 46 mm cat no 0712-0019) was used with 100214 methanol as the eluent (flow rate of 1 mL minminus1) Detection215 was carried out at λ = 220 nm Under these conditions the216 peak of ascaridole occurs at 34 min17

217 Photoelectrochemical Measurements Photoelectro-218 chemical measurements were carried out using an Autolab219 PGSTAT302N and XBO150 xenon lamp with a monochro-220 mator (Instytut Fotonowy) Measurements have been done in221 the three-electrode cell using platinum wire and AgAgCl as222 counterelectrode and reference electrode respectively A thin223 layer of material on indium tin oxide (ITO) foil (resistivity = 60224 Ωsq) was used as a working electrode The electrodes were225 placed in a quartz cuvette filled with 01 mol dmminus3 KNO3226 solution in water as an electrolyte (pH = 60) Irradiation was227 done in the range 330minus450 nm (every 10 nm) Photocurrent228 generation was measured in the range of minus02minus1 V vs Ag229 AgCl230 Oxygen Adsorption Oxygen adsorption tests were231 performed using Firesting O2 oxygen meter (PyroScience)232 For each measurement 5 mL of deoxygenated sample (purged233 with argon) was poured into a vial equipped with the234 fluorescence oxygen sensor After 5 min of stabilizing the235 oxygen concentration in the sealed vial (completely filled with236 the suspension with no gas atmosphere above) a portion of237 100 μL of air-saturated water was injected through a septum

238Next 11 injections were made every 5 min After the last239injection oxygen concentration was measured in the240suspension for another 15 min to determine the final241concentration of dissolved oxygen242Theoretical Modeling The Ti45O99 cluster was cut out of243the anatase structure28 along the [001] plane the edges were244substituted with the hydroxyl ligands to achieve a total charge245of minus3 The geometry of the cluster was optimized using the246MM2 method (CaCHE Fujitsu) Different species (fluoride247water and hexafluorometallates) were placed in the middle of248the upper plane of the cluster the number of protons was249readjusted for the constant charge of minus3 and the geometry was250reoptimized using tight convergence criteria Subsequently the251electronic structure of the test systems was computed at the252PM7 level of theory using the MOPAC 2012 package2930

253Population analysis and calculation of density-of-states spectra254were performed using the AOMix package3132

255 RESULTS AND DISCUSSION256Materials Adsorption of hexafluorometallates ([AlF6]

3minus257[TiF6]

2minus [ZrF6]2minus and [SiF6]

2minus) and Fminus at the TiO2 surface258did not influence the material color which remains white259Diffuse reflectance spectra converted to the KubelkaminusMunk260 f3function presented in Figure 3 also did not show significant

261differences from the spectrum of starting TiO2 material In262particular the bandgap energies of all materials are the same263This is fully justified as all hexafluorido complexes do not264absorb visible light and cannot yield charge transfer complexes265with surface TiIV centers due to a high oxidation state of the266central atoms Modification of titanium dioxide with hexa-267fluorometallates influenced the surface charge of the particles268as reflected by a faster sedimentation of the surface-modified269material from aqueous suspensions Similarly to fluoride anions270the adsorption of hexafluorometallates is favored from acidic271solutions since in basic media fluorides desorb from the272surface1

273Three commercial samples of titanium dioxide have been274selected for the studies TH0 (anatase) UV100 (anatase) and275P25 (anataserutile mixture) The specific surface areas of the276modified materials are in general lower than those measured for277 t1unmodified TiO2 (Table 1) This is the result of a lower TiO2278content in the modified samples and a possible blocking of

Figure 2 Process of (A) TAOH formation in the reaction ofterephthalic acid (TA) with hydroxyl radicals26 (B) formation ofascaridole in the reaction of α-terpinene with singlet oxygen27

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


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279 pores by the modifiers The pzc also lowers upon modificationt2 280 (Table 2) which is in agreement with previous reports1 The

281 lowest values of pzc were observed for [TiF6]2minusTiO2 A slight

282 increase of pzc took place upon TiO2 modification with283 [AlF6]

3minus ions284 To estimate the titania surface coverage by fluoride and285 hexafluorometallates adsorption of catechol was measured286 Catechol binds efficiently to the exposed TiO2 surface forming287 yellow to orange charge transfer complexes33 Adsorption of288 catechol should be significantly weaker at the modified surface289 where minusOH groups are not fully exposed Therefore a290 comparison of the amounts of surface-bound catechol can be291 used as a qualitative measure of the surface coverage by the

f4 292 modifiers The amounts of bound catechol are shown in Figuref4 293 4 The comparison of the amounts of adsorbed catechol reveals

294 a very efficient surface coverage by hexafluorotitanate and295 haxafluorozirconate anions Surprisingly impregnation with296 potassium fluoride protected the TiO2 surface from catechol297 adsorption to a smaller extent independently of the applied298 TiO2 material (P25 UV100 and TH0) The differences299 observed for various TiO2 samples originate from the300 differences between specific surface areas of these materials

301More surprisingly diffuse reflectance spectra of catechol-302soaked samples were significantly different (spectra not shown)303While the intensity of the main ligand-to-metal charge-transfer304(LMCT) band (catecholrarr TiIV) correlates with the number of305accessible sites on the surface and decreases with the increasing306amount of hexafluorometallate adsorbed the energy of this307transition changes as well In the case of TH0-based materials308(TH0 is characterized by the highest specific surface area and309therefore the spectra of these materials impregnated with310catechol were most intense) the LMCT was observed at 3084311eV for neat TiO2 A small but significant bathochromic shift was312observed upon adsorption of hexafluorometallates whereas313 t3adsorption of fluoride exerts an opposite effect (Table 3) The314dominating LMCT character of these transitions is not315changed also the bandwidth reflecting the vibronic compo-316nents is virtually the same

317These results suggest an increase of the surface state (or318conduction band edge) energies in the case of fluoride319adsorption while hexafluorometallates exert the opposite320tendency The same effect can be observed when surface321properties are directly determined using the Kelvin probe322 t4technique (Table 4) Modification with fluoride results in a

323significant decrease of surface contact potential ECP (ie324increase of the conduction band edge energy) while325hexafluorometallates decrease the conduction band energies326This tendency is observed for all studied TiO2 materials In327general the influence of modification on electronic properties328of the materials is more pronounced for the materials with329stronger hexafluorometallateminusTiO2 interactions330The effect of surface modification is further supported by a331computational approach The Ti45O81(OH)18 cluster of anatase332was used as a model of the TiO2 surface The modifying333hexafluorometallates were placed over the center of the cluster334 f5An example of the test structure is shown in Figure 5

Table 1 Specific Surface Area for Materials (m2 gminus1 plusmn2)

surface ligand TH0 P25 UV100

unmodified 352 588 321Fminus 305 500 323[AlF6]

3minus 251 489 268[SiF6]

2minus 246 529 272[TiF6]

2minus 225 600 259[ZrF6]

2minus 297 524 260

Table 2 Point of Zero Charge Values for Materials



3minus 59 625 581[SiF6]

2minus 556 538 551[TiF6]

2minus 542 39 546[ZrF6]

2minus 571 558

Figure 4 Amounts of catechol adsorbed at the surface of various TiO2samples See the Experimental Section for details The reproducibilityof the measurements is within 1minus2

Table 3 Energies of the LMCT Transitions at TH0 SurfacesModified with Fluorometallates and Subsequently withCatechol

surfaceligand

LMCT energy (catechol rarr TiIV)eV

bandwidtheV

ECBa

eV

H2O 3084 1009 070Fminus 3261 1136 095[AlF6]

3minus 3054 0983 065[SiF6]

2minus 3052 1128 065[TiF6]

2minus 3040 1155 060[ZrF6]

2minus 3021 1159 055aConduction band edge energy as calculated on the PM7 level oftheory

Table 4 Surface Contact Potentials (ECP) versus StandardHydrogen Electrode (vs SHE plusmn003 V)

surface ligand ECPV TH0 ECPV P25 ECPV UV100

H2O minus052 minus04 minus061Fminus minus056 minus023 minus050[AlF6]

3minus minus049 +004 minus041[SiF6]

2minus minus043 minus027 minus052[TiF6]

2minus minus043 minus007 minus053[ZrF6]

2minus minus041 minus008 minus048


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335 Semiempirical quantum-chemical calculations at the PM7336 level of theory indicate an increase of the conduction band337 energy upon binding of fluoride by ca 025 eV (Table 3) The338 interaction with hexafluorometallates results in a significant339 decrease of the band edge energy These results are consistent340 with observed spectral data and Kelvin probe measurements341 Scanning electron microscope (SEM) pictures of modified

f6 342 powders are shown in Figure 6 Surface-modified materials

343 show a higher tendency to form more compact aggregates344 when compared to unmodified samples This observation345 explains a faster sedimentation of fluorinated materials from346 aqueous suspensions347 Redox Properties of the Materials Adsorption of348 hexafluorometallates at the TiO2 surface influences the band349 edge potentials so it should also modulate redox properties of350 this semiconductor The recently developed spectroelectro-351 chemical method of characterization of redox properties of352 semiconducting materials has been applied25 In this method353 the dependence of reflectance (or absorbance) on the density354 of electrons in the conduction band of the material is used In

355the case of titanium dioxide electrochemical reduction356generates electrons trapped close to the conduction band as357TiIII centers characterized by an absorption maximum at 780358nm The trapped electrons can reduce oxygen according to eq3591

+ rarr + bullminusTi O Ti OIII2

IV2 360(1)

361therefore the measurements should be made under oxygen-free362conditions363The relative reflectance signals measured at 780 nm as a364function of the potential recorded for the platinum electrode365covered with P25 modified with selected hexafluorometallates366 f7are presented in Figure 7 The measured signals differ from

367each other The deflection points of recorded curves (at EON)368correspond to the onset reduction potential at which electron369trapping accelerates For surface-modified materials and370unmodified UV100 two EON potentials can be measured371The first one (EONprime a higher value between minus04 and minus07 V)372refers to the electron traps introduced by surface modifiers The373second one (EONPrime lower values close to minus08 V) can be374assigned to the electron traps close to the conduction band375 t5edge (Table 5) Binding hexafluorometallates to the titania376surface apparently changes the character of the surface states377therefore the measured EONprime shifts to higher potentials These378differences correlate well with the calculated values of ECB and379the results of Kelvin probe measurements (Tables 3 and 4)

Figure 5 Geometry of a Ti45O81(OH)18 cluster with one [ZrF6]2minus

anion electrostatically adsorbed over the central titanium ion

Figure 6 SEM pictures of UV-100-based materials (a) unmodifiedTiO2 (b) [ZrF6]

2minusTiO2 (c) [AlF6]3minusTiO2 and (d) [TiF6]

2minusTiO2

Figure 7 Reflectance changes measured at 780 nm as a function of theelectrode potential for selected materials based on TiO2 (UV100)deposited at the surface of platinum plate The measurements werecarried out using 01 M LiClO4 in acetonitrile under inert atmosphere(Ar)

Table 5 EON for the Studied TiO2 Materials (Measured in01 mol dmminus3 LiClO4 in Acetonitrile in Inert Atmosphere(Ar) vs AgAgCl (plusmn005 V))

EONV TH0 EONV P25 EONV UV100

surface ligand EONprime EONPrime EONprime EONPrime EONprime EONPrimeunmodified minus108 minus080 minus073 minus108Fminus minus047 minus091 minus088 minus044 minus114[AlF6]

3minus minus05 minus103 minus06 minus102[SiF6]

2minus minus08 minus062 minus100 minus071 minus099[TiF6]

2minus minus043 minus094 minus067 minus097 minus066 minus102[ZrF6]

2minus minus068 minus113 minus062 minus082 minus087


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380 Binding hexafluorometallates to the titania surface apparently381 changes the character of surface states therefore the measured382 EON shifts to higher potentials The electrons trapped as TiIII

383 may play a crucial role in the process of photodegradation of384 pollutants influencing the photoactivity of the material385 Photocatalytic ActivityOxidation of Terephthalic386 Acid Oxidation of terephthalic acid (TA) to hydroxytereph-387 thalic acid (TAOH) was tested to compare efficiencies of388 hydroxyl radical generation at different materials TAOH

f8 389 concentration after 30 min of irradiation is shown in Figuref8 390 8 Unmodified photocatalysts in particular P25 are charac-

391 terized by relatively high efficiencies of hydroxyl radical392 generation The photoactivities of UV100 and TH0 are393 similarin the case of both materials anatase is the394 predominant form of TiO2 and specific surface area is395 comparable (300 and 330 m2 gminus1 for TH0 and UV100396 respectively) Modification with hexafluorometallates influences397 the activity mainly that of P25 FminusP25 shows the highest398 activity while the efficiencies of OHbull formation at P25399 modified with [TiF6]


2minus are the lowest400 [TiF6]

2minus reduces also the activity of TH0 and UV100401 Hexafluorotitanate ions adsorb at TiO2 surface most efficiently402 (compare Figure 4) and hinder water oxidation by photo-403 generated holes404 Photocatalytic ActivityDegradation of Herbicides405 The absorption spectra of 24-D and 245-T are presented in

f9 406 Figure 9 Both compounds absorb UV light at wavelengths407 shorter than ca 310 nm and do not absorb light used408 throughout the photocatalytic tests (λ gt 320 nm) Therefore409 the herbicides are photostable upon irradiation in the absence410 of any photocatalyst (data not shown)411 Upon irradiation in the presence of TiO2 materials a412 decrease of all bands could be observed but no new bands were413 formed In the case of 24-D degradation the FminusTH0414 material appeared to be the most active while in the case of415 245-T [AlF6]

3minusTH0 and [SiF6]2minusP25 showed the highest

f10 416 degradation rates (Figure 10) In the case of both herbicides417 [ZrF6]

2minusTH0 also showed a good performance Modification418 of P25 and UV100 had a detrimental effect on degradation of419 24-D and 245-T but the photocatalytic activity of TH0 can be420 improved when surface modification is applied421 Although hydroxyl radicals are responsible for degradation of422 24-D and 245-T11 there are clear differences between

423efficiencies of 24-D 245-T and TA oxidation For instance424FminusTH0 appears very active in 24-D degradation but it is not425the case for 245-T and TA oxidation Conversely [AlF6]

3minus

426modification improves activity of TH0 toward 245-T427degradation but not toward 24-D and TA Moreover the428surface coverage by [AlF6]

3minus is very inefficient (or adsorption of429these ions is weak) thus the activity of [AlF6]

3minusTH0 in430every case resembles that of unmodified TiO2 although it is431slightly higher for [AlF6]

3minusTH0 On the other hand [TiF6]2minus

432adsorbs very efficiently at TiO2 which reflects in the lowest433activities of [TiF6]

2minusTiO2 toward TA and 245-T oxidation434but not toward 23-D oxidation (in the presence of modified

Figure 8 TAOH concentration after 30 min of irradiation of thematerials suspended in terephthalic acid solution (λ gt 320 nm)

Figure 9 Absorption spectra of 24-D (thick line) and 245-T (thinline) (25 times 10minus4 mol dmminus3)

Figure 10 Degradation of herbicides in the presence of modified TiO2after 20 min of irradiation (λ gt 320 nm) (A) 24-D and (B) 245-T


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435 [TiF6]2minusTH0 and [TiF6]

2minusUV100 oxidation of 23-D436 proceeds as efficiently as in the presence of other photo-437 catalysts) These differences can originate mainly from different

f11 438 and specific adsorption of herbicides at various surfaces (Figuref11 439 11) Degradation of 24-D proceeds through the attack of

440 hydroxyl radicals leading to hydroxylation of aromatic ring and441 removal of the side group11 Analogous reactions can be442 expected for 245-T due to the similarities in the structures of443 the particles Because of differences in the degradation of both444 herbicides in the presence of various materials further studies445 of the mechanisms of their mineralization have to be done446 Photocatalytic Activityα-Terpinene The activity of all447 materials was also studied in the process of α-terpinene448 photooxidation to ascaridole with singlet oxygen At short449 irradiation times (30 min) only [TiF6]

2minusUV100 shows a450 significant activity toward ascaridole production (data not451 shown) Therefore longer tests (2 h of irradiation) have been452 done only with [TiF6]

2minusUV100 and unmodified UV100453 under various irradiation conditions Degradation of α-

f12 454 terpinene and generation of ascaridole is shown in Figure 12455 Singlet oxygen can be generated in the two consecutive redox456 reactions the first one involves reduction of oxygen to457 superoxide while the second one leads to 1O2 through458 reoxidation of O2

bullminus with valence band holes34 However the459 energy transfer from higher excited states cannot be ruled460 outthese states can be populated as a result of NIR light461 absorption by electrons trapped as TiIII (λmax = 780 nm)35

462Degradation of the substrate is slightly faster in the presence463of modified material (Figure 11A) but the efficiency of464ascaridole generation is already 2minus3 times higher for465[TiF6]

2minusUV100 than for UV100 (Figure 12B) Upon UVminus466vis irradiation (320minus620 nm) the amount of photogenerated467ascaridole increases both when UV100 or [TiF6]

2minusUV100 are468used as photocatalysts Simultaneous irradiation with UVminusvis469and NIR light (λ gt 320 nm) accelerates the process This effect470has already been described for other surface-modified TiO2471materials35 and can result either from generation of singlet472oxygen in the process of energy transfer from higher excited473states or from excitation of electrons trapped in unreactive474states to the conduction band followed by reduction of oxygen475to superoxide Reactions involving oxygen as a reactant should476be influenced by O2 adsorption at the surface of a photo-477catalyst Therefore the influence of TiO2 modification on478oxygen adsorption was studied in detail479Oxygen Adsorption A lower concentration of dissolved480oxygen in the suspension containing titanium dioxide modified481with fluorides and hexafluorometallates indicates a higher482concentration of oxygen adsorbed at the surface of these483materials All results are compared to water-saturated sample484(dissolved oxygen value = 100) Hombikat UV100 modified485with fluoride anions exhibited ca 4 times higher oxygen486 f13adsorption than the corresponding unmodified sample (Figure487 f1313) In the case of FminusP25 the oxygen adsorption was

Figure 11 Concentration of herbicides adsorbed at the surface ofvarious surface modified materials (A) 24-D and (B) 245-T

Figure 12 (A) Decrease of the α-terpinene concentration (B)increase of the ascaridole concentration upon irradiation in thepresence of UV100 and [TiF6]

2minusUV100


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488 improved by gt50 These results justify a higher activity of489 [TiF6]

2minusUV100 toward singlet oxygen generation because490 oxygen availability at this material is better491 Photocurrent Generation Photocurrent measurements492 revealed significant differences in efficiencies of the photo-

f14 493 induced interfacial electron transfer (Figure 14) When494 compared to photoactivity of unmodified P25 and UV100495 materials the corresponding [ZrF6]

2minusUV100 and [ZrF6]2minus

496 UV100 appear almost inactive independent of the electrode497 potential On the other hand hexafluorotitanate diminishes498 photocurrents at minus02 V but at 1 V the photocurrents are499 comparable with those observed for the unmodified materials500 Taking into account a good adsorption of [ZrF6]

2minus and

501[TiF6]2minus at TiO2 the behavior of [TiF6]

2minusTiO2 at 1 V is502surprising Apparently under these conditions hexafluorotita-503nate does not hinder the interfacial electron transfer but may504even facilitate this process (in the case of [TiF6]

2minusUV100)505Adsorption of [AlF6]

3minus is very weak therefore its influence506on photocurrent generation is negligible Significantly better507adsorption of [SiF6]

2minus and Fminus reflects in more significant508changes in photocurrents In general fluoride and hexafluor-509osilicate ions hinder interfacial electron transfer (IFET)510processes (with an exception for FminusUV100 and [SiF6]

2minus511UV100 at 1 V)

512 CONCLUSIONS

513Impregnation of TiO2 with hexafluorometallates leads to the514modification of its surface and strongly influences the515photocatalytic activity Hexafluorotitanate binds most efficiently516to all tested materials This modification decreases rates of TA51724-D and 245-T oxidation due to decreased efficiency of518hydroxyl radical formation On the other hand the same519modification improves yields of singlet oxygen production and520photocurrent generation These results can be explained by an521improved interfacial electron transfer responsible for both522photocurrent generation and superoxide production As a523consequence the concluded mechanism of singlet oxygen524production involves formation of superoxide followed by its525oxidation with holes34

526[ZrF6]2minus adsorbs quite efficiently at titanium dioxide but it

527modifies the surface to a lower extent than [TiF6]2minus Therefore

528generation of hydroxyl radicals and oxidation of herbicides is529slightly faster for [ZrF6]

2minusTiO2 than for [TiF6]2minusTiO2 On

530the other hand hexafluorozirconate efficiently inhibits photo-

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

Figure 14 Photocurrent generation at electrodes made of the studied materials recorded at 1 V vs AgAgCl (A B) and minus02 V vs AgAgCl (C D)The legend is valid for all graphs


dxdoiorg101021jp505449n | J Phys Chem C XXXX XXX XXXminusXXXH

531 currents (IFET process) and does not enable singlet oxygen532 generation533 The lowest adsorption of hexafluorometallate was observed534 for [AlF6]

3minus Consequently this modifier shows the smallest535 impact on the reactivity of titanium dioxide536 The studied modifiers in particular [TiF6]

2minus and [ZrF6]2minus

537 can significantly influence photocatalytic processes of titanium538 dioxide They can be used to tune efficiencies of reactive539 oxygen species formation and photocurrent generation

540 AUTHOR INFORMATION541 Corresponding Authors542 E-mail buchalskchemiaujedupl543 E-mail szacilowchemiaujedupl544 E-mail macykchemiaujedupl545 Notes546 The authors declare no competing financial interest

547 ACKNOWLEDGMENTS548 This work was carried out within the ldquoActivation of small549 molecules in photocatalytic systemsrdquo project supported by the550 Foundation for Polish Science cofinanced by European Union551 Regional Development Fund (project no TEAM2012-94)552 Also a support by the EU-FP7 within the project ldquo4G-553 PHOTOCATrdquo (grant no 309636 photodegradation of554 herbicides) cofinanced by Polish Ministry of Science and555 Higher Education (project no W137PR2013) is highly556 acknowledged A part of this work was carried out at the557 equipment purchased thanks to the financial support of the558 European Regional Development Fund in the framework of the559 Polish Innovation Economy Operational Program (contract no560 POIG020100-12-02308)

561 REFERENCES(1)562 Liu S Yu J Cheng B Jaroniec M Fluorinated Semiconductor

563 Photocatalysis Tunable Synthesis and Unique Properties Adv Colloid564 Interface Sci 2012 173 35minus53

(2)565 Ho W Yu J C Lee S Synthesis of Hieratchical Nanoporous566 F-Doped TiO2 Spheres with Visible Light Photocatalytic Activity567 Chem Commun 2006 10 1115minus1117

(3)568 Yang S Halliburton L E Fluorine Donors and Ti3+ Ions in569 TiO2 Crystals Phys Rev B 2010 81 035204(1minus7)

(4)570 Park H Choi W Effects of TiO2 Surface Fluorination on571 Photocatalytic Reactions and Photoelectrochemical Behaviors J Phys572 Chem B 2004 108 4086minus4093

(5)573 Janczyk A Krakowska E Stochel G Macyk W Singlet574 Oxygen Photogeneration at Surface Modified Titanium Dioxide J Am575 Chem Soc 2006 128 15574minus15575

(6)576 Park H Park Y Kim W Choi W Surface Modification of577 TiO2 Photocatalyst for Environmental Applications J Photochem578 Photobiol C 2013 15 1minus20

(7)579 Sun C Selloni A Du A Smith S C Interaction of Water with580 the Fluorine-Covered Anatase TiO2(001) Surface J Phys Chem C581 2011 115 17092minus17096

(8)582 Yamaki T Sumita T Yamamoto S Formation of TiO2‑xFx583 Compounds in Fluorine-Implanted TiO2 J Mater Sci Lett 2002 21584 33minus35

(9)585 Manole C C Stoian A B Picircrvu C Surface Perspective of a586 TiO2 Nanoarchitecture UPB Sci Bull 2010 72 91minus98

(10)587 Kim D Schmidt-Stein F Hahn R Schmuki P Gravity588 Assisted Growth of Self-Organized Anodic Oxide Nanotubes on589 Titanium Electrochem Commun 2008 10 1082minus1086

(11)590 Singh H K Muneer M Photodegradation of a Herbicide591 Derivative 24-Dichlorophenoxyacetic Acid in Aqueous Suspensions of592 Titanium Dioxide Res Chem Intermed 2004 30 317minus329

(12) 593Kamble S P Deosarkar S P Sawant S B Moulijn J A594Pangarkar V G Photocatalytic Degradation of 24-Dichlorophenoxy-595acetic Acid Using Concentrated Solar Radiation Batch and596Continuous Operation Ind Eng Chem Res 2004 43 8178minus8187

(13) 597Kong L Lemley A T Kinetic Modeling of 24-598Dichlorophenoxyacetic Acid (24-D) Degradation in Soil Slurry by599Anodic Fenton Treatment J Agric Food Chem 2006 54 3941minus3950

(14) 600Serpone N Texier I Emeline A V Pichat P Hidaka H601Zhao J Post-Irradiation Effect and Reductive Dechlorination of602Chlorophenols at Oxygen-Free TiO2Water Interfaces in the Presence603of Prominent Hole Scavengers J Photochem Photobiol A 2000 136604145minus155

(15) 605Pandiyan T Martiacutenez-Rivas O Orozco-Martiacutenez J Burillo-606Amezcua G Martiacutenez-Carrillo M A Comparison of Methods for the607Photochemical Degradation of Chlorophenols J Photochem Photobiol608A 2002 146 149minus155

(16) 609Theurich J Lindner M Bahnemann D W Photocatalytic610Degradation of 4-Chlorophenol in Aerated Aqueous Titanium Dioxide611Suspensions A Kinetic and Mechanistic Study Langmuir 1996 126126368minus6377

(17) 613Buchalska M Kras G Oszajca M Łasocha W Macyk W614Singlet Oxygen Generation in the Presence of Titanium Dioxide615Materials Used as Sunscreens in Suntan Lotions J Photochem616Photobiol A 2010 213 158minus163

(18) 617Czaplicka M Photo-Degradation of Chlorophenols in the618Aqueous Solution J Hazard Mater B 2006 134 45minus59

(19) 619Marron-Montiel E Ruiz-Ordaz N Rubio-Granados C620Juarez-Ramiacuterez C Galiacutendez-Mayer C J 24-D-Degrading Bacterial621Consotrium Isolation Kinetic Characterization in Batch and622Continuous Culture and Application for Bioaugmenting an Activated623Sludge Microbal Community Process Biochem 2006 41 1521minus1528

(20) 624Golovleva L A Pertsova R N Evtushenko L I Baskunov625B P Degradation of 245-Trichlorophenoxyacetic Acid by a626Nocardiodes Simplex Culture Biodegradation 1990 1 263minus271

(21) 627Daugherty D D Karel S F Degradation of 24-628Dichlorophenoxyacetic Acid by Pseudomonas Cepacia DBO1-629(pRO101) in a Dual-Substrate Chemostat Appl Environ Microbiol6301994 60 3261minus3267

(22) 631Chatterjee D K Kilbane J J Chakrabarty A M632Biodegradation of 245-Trichlorophenoxyacetic Acid in Soil by a633Pure Culture of Pseudomonas Cepacia Appl Environ Microbiol 198263444 514minus516

(23) 635Ding L Lu X Deng H Zhang X Adsorptive Removal of63624-Dichlorophenoxyacetic Acid (24-D) from Aqueous Solution Using637Miex Resin Ind Eng Chem Res 2012 51 11226minus11235

(24) 638Han D Jia W Liang H Selective Removal of 24-639Dichlorophenoxyacetic Acid from Water by Moleculatly-Imprinted640Amino-Functionalized Silica Gel Sorbent J Environ Sci 2010 22641237minus241

(25) 642Swiętek E Pilarczyk K Derdzin ska J Szaciłowski K Macyk643W Redox Characterization of Semiconductors Based on Electro-644chemical Measurements Combined with UVminusVis Diffuse Reflectance645Specroscopy Phys Chem Chem Phys 2013 15 14256minus14261

(26) 646Hirakawa T Nosaka Y Properties of O2bullminus and OHbull Formed

647in TiO2 Aqueous Suspensions by Photocatalytic Reaction and the648Influence of H2O2 and Some Ions Langmuir 2002 18 3247minus3254

(27) 649Karapire C Kolancilar H Oyman U Icli S Fluorescence650Emission and Photooxidation Studies with 56- and 67-Benzocoumar-651ins and a 56-Benzochromone under Direct and Concentrated Sun652Light J Photochem Photobiol A 2002 153 173minus184

(28) 653Howard C J Sabine T M Dickson F Structural and654Thermal Parameters for Rutile and Anatase Acta Crystallogr Sect B6551991 47 462

(29) 656Stewart J J P Mopac2012 Stewart Computational Chemistry657Ver 14045w httpOpenmopacNet 2012

(30) 658Maia J D C Carvalho G A U Mangueira C P Santana S659R Cabral L A F Rocha G B GPU Linear Algebra Libraries and660GPGPU Programming for Accelerating Mopac Semiempirical


dxdoiorg101021jp505449n | J Phys Chem C XXXX XXX XXXminusXXXI

661 Quantum Chemistry Calculations J Chem Theory Comput 2012 8662 3072minus3081

(31)663 Gorelsky S I Aomix Program for Molecular Orbital Analysis664 University of Ottawa Ottawa Canada 2007 httpwwwsg-chemnet

(32)665 Gorelsky S I Lever A B P Electronic Structure and Spectra666 of Ruthenium Diimine Complexes by Density Functional Theory and667 INDOS Comparison of the Two Methods J Organomet Chem668 2001 635 187minus196

(33)669 Macyk W Szaciłowski K Stochel G Buchalska M670 Kuncewicz J Łabuz P Titanium(IV) Complexes as Direct TiO2671 Photosensitizers Coord Chem Rev 2010 254 2687minus2701

(34)672 Daimon T Nosaka Y Formation and Behavior of Singlet673 Molecular Oxygen in TiO2 Photocatalysis Studied by Detection of674 Near-Infrared Phosphorescence J Phys Chem C 2007 111 4420minus675 4424

(35)676 Buchalska M Łabuz P Bujak Ł Szewczyk G Sarna T677 Mackowski S Macyk W New Insight into Singlet Oxygen678 Generation at Surface Modified Nanocrystalline TiO2The Effect679 of Near-Infrared Irradiation Dalton Trans 2013 42 9468minus9475


dxdoiorg101021jp505449n | J Phys Chem C XXXX XXX XXXminusXXXJ

66 but on the other hand this property generates a risk to aquatic67 organisms 24-D and 245-T relatively easily undergo68 decomposition The main problem related to the removal of69 these materials in environmental conditions is associated with70 the formation of byproducts ie chlorophenol and chlor-71 obenzene derivatives which can be more harmful than the72 herbicides themselves11

73 24-D and 245-T can be oxidized in the presence of TiO274 upon sunlight irradiation12 Hydroxyl radicals play a key role in75 the process1113 Under oxygen-free conditions oxidation76 involving valence band holes may take place14 One of the77 first steps of degradation is substitution of the minusCH2COOH78 fragment by a hydroxyl group leading to formation of the79 corresponding chlorophenols Photodegradation of polychlor-80 ophenols is a more demanding process than oxidation of81 monochlorophenols15 Contrary to monochlorophenols oxi-82 dized mainly with hydroxyl radicals1617 polychlorophenols also83 react with singlet oxygen18

84 The other processes responsible for 24-D and 245-T85 removal from the environment involve biodegradation Specific86 bacterial strains are able to grow in the herbicide-containing87 media (24-D) as reported for Comamonas sp Pseudomonas88 putida Acinetobacter sp Acinetobacter lwof ii and Klebsiella89 oxytoca19 Nocardioides simplex was successfully applied for90 245-T removal20 while Pseudomonas cepacia was reported as a91 universal organism for removing both 24-D and 245-T2122

92 Trials of herbicide removal from water by physical adsorption93 on various sorbers (ion exchange23 silica gel24 etc) were also94 reported95 In this paper we compare electronic properties and96 photocatalytic activity of TiO2 materials modified with97 hexafluorometallates This group of photocatalysts has also98 been tested in the process of 24-D and 245-T degradation

99 EXPERIMENTAL SECTION100 Materials TiO2 (TH0 anatase 330 m2 gminus1 Kerr-McGee101 Hombikat UV100 anatase 300 m2 gminus1 Sachtleben Chemie102 and P25 anataserutile 8020 50 m2 gminus1 Evonik) has been103 modified with various fluoride-containing compounds104 (K3[AlF6] Na2[SiF6] H2[TiF6] H2[ZrF6] and KF Sigma-105 Aldrich) The solutions of these compounds (001 mol dmminus3)106 were added to aqueous suspensions of TiO2 (075 g dm

minus3) in a107 120 modifierTiO2 molar ratio The suspensions were stirred108 magnetically and sonicated for 10 min afterward they were109 filtered washed several times with water and dried in the air at110 80 degC Within this work the surface-modified materials are111 abbreviated as [MF6]

nminusTiO2 Diffuse reflectance spectra were112 recorded with a PerkinElmer UVminusvis Lambda 12 spectrometer113 equipped with a 5 cm diameter integrating sphere Prior to the114 measurements the samples were ground in an agate mortar115 with barium sulfate (ca 150 weight ratio) The reflectances of116 these mixtures were recorded using BaSO4 as a reference The117 work functions of the studied samples were measured using a118 Kelvin Probe model (KP Technology) with 1 mm stainless119 steel tip gold sputtered on aluminum was used as a standard120 (WFAu = 5100 eV) Herbicides (24-D and 245-T)121 terephthalic acid α-terpinene and catechol were purchased122 from Sigma-Aldrich and used as received Ascaridole standard123 was purchased from PhytoLab124 Characterization of Physicochemical Properties A125 scanning electron microscope (Vega 3 LM Tescan) equipped126 with an LaB6 cathode was operated at a voltage of 30 kV127 BrunauerminusEmmettminusTeller (BET) measurements were per-

128formed using a Quantachrome Autosorb-6 instrument The129samples were heated at 200 degC for 2 h under vacuum prior to130the measurements The specific surface area of the samples was131estimated using the BET equation (nitrogen adsorption at 77132K) The point of zero charge (pzc) measurements were133performed using a Malvern Zetasizer NanoZS instrument The134cell temperature was set to 20 degC The suspensions of tested135materials (1 mL of 01 g dmminus3) were mixed with 1 mL of a136buffer solution (acetate or phosphate depending on the final137pH)138Tests of the Surface Coverage Twenty mg of the tested139materials were suspended in 2 mL of methanolic solution of140catechol (1 mmol dmminus3) The suspension was sonicated for 5141min After centrifugation the concentration of catechol in142supernatant solution was determined spectroscopically by143measuring the absorbance at λ = 280 nm (Lambda 950144PerkinElmer)145Analogous experiments have been done using herbicide146solutions (25 times 10minus3 mol dmminus3) The concentration of147herbicides was determined by measuring absorbance at 230 and148235 nm for 24-D and 245-T respectively149Redox Properties of the Materials Redox properties of150the materials were determined using a spectroelectrochemical151method25 which is based on electrochemical measurements152combined with UVminusvis diffuse reflectance spectroscopy The153changes in reflectance were recorded at λ = 780 nm by a154PerkinElmer UVminusvis Lambda 12 spectrometer equipped with a1555 cm diameter integrating sphere The electrochemical156measurements were carried out in a three-electrode cell with157platinum wire and AgAgCl as counterelectrode and reference158electrode respectively The modified TiO2 materials were159previously ground in a mortar and suspended in distilled water160Working electrodes were prepared by casting of tested materials161at the surface of platinum foil (ca 2 cm times 1 cm) Afterward the162working electrodes were dried at sim100 degC In this way opaque163films of the materials were formed on the Pt plate The164electrodes were placed in a quartz cuvette filled with 01 mol165dmminus3 LiClO4 solution in anhydrous acetonitrile Oxygen was166thoroughly removed from the electrolyte by purging with argon167before (15 min) and during the experiments The quartz168cuvette was placed in front of the sphere facing the working169electrode (platinum foil with deposited TiO2) toward the light170beam Potential control was provided by the electrochemical171analyzer Autolab PGSTAT302N (scan rate of 1 mV sminus1)172Tests of Photocatalytic Activity All the tests of173photocatalytic activity were done under the same conditions174A 150 W xenon lamp (XBO-150) equipped with a near-175infrared (NIR) filter (01 mol dmminus3 CuSO4 solution in water)176and 320 nm cutoff filter was used for irradiation For some177tests with α-terpinene UVminusvisminusNIR irradiations also were178done (without the aqueous filter) Suspensions of the179photocatalytic materials were prepared in the solutions of180model pollutants (05 g dmminus3) sonicated for 15 min and181irradiated in a round quartz cuvette (5 cm diameter 1 cm182optical path 17 mL volume) The samples were collected183regularly filtered through CME syringe filters with a pore size184of 022 μm and subjected to analysis185The solutions of herbicides were prepared in water (25 times18610minus4 mol dmminus3) The samples were collected after 0 2 5 1018715 and 20 min of irradiation UVminusvis absorption spectra of the188herbicide solutions were collected in a UVvisNIR Lambda189950 (PerkinElmer) spectrophotometer in a 1 cm quartz190cuvette The decomposition of pollutants was calculated from


dxdoiorg101021jp505449n | J Phys Chem C XXXX XXX XXXminusXXXB









3minus257[TiF6]





















3minus 251 489 268[SiF6]

2minus 246 529 272[TiF6]

2minus 225 600 259[ZrF6]

2minus 297 524 260




3minus 59 625 581[SiF6]

2minus 556 538 551[TiF6]

2minus 542 39 546[ZrF6]

2minus 571 558



surfaceligand


bandwidtheV

ECBa

eV

H2O 3084 1009 070Fminus 3261 1136 095[AlF6]

3minus 3054 0983 065[SiF6]

2minus 3052 1128 065[TiF6]

2minus 3040 1155 060[ZrF6]
















IV2 360(1)







2minusTiO2























3minus

























2minusUV100








2minus and







512 CONCLUSIONS



































































3minus257[TiF6]





















3minus 251 489 268[SiF6]

2minus 246 529 272[TiF6]

2minus 225 600 259[ZrF6]

2minus 297 524 260




3minus 59 625 581[SiF6]

2minus 556 538 551[TiF6]

2minus 542 39 546[ZrF6]

2minus 571 558



surfaceligand


bandwidtheV

ECBa

eV

H2O 3084 1009 070Fminus 3261 1136 095[AlF6]

3minus 3054 0983 065[SiF6]

2minus 3052 1128 065[TiF6]

2minus 3040 1155 060[ZrF6]
















IV2 360(1)







2minusTiO2























3minus

























2minusUV100








2minus and







512 CONCLUSIONS







































































3minus 251 489 268[SiF6]

2minus 246 529 272[TiF6]

2minus 225 600 259[ZrF6]

2minus 297 524 260




3minus 59 625 581[SiF6]

2minus 556 538 551[TiF6]

2minus 542 39 546[ZrF6]

2minus 571 558



surfaceligand


bandwidtheV

ECBa

eV

H2O 3084 1009 070Fminus 3261 1136 095[AlF6]

3minus 3054 0983 065[SiF6]

2minus 3052 1128 065[TiF6]

2minus 3040 1155 060[ZrF6]
















IV2 360(1)







2minusTiO2























3minus

























2minusUV100








2minus and







512 CONCLUSIONS
































































IV2 360(1)







2minusTiO2























3minus

























2minusUV100








2minus and







512 CONCLUSIONS







































































3minus

























2minusUV100








2minus and







512 CONCLUSIONS








































































2minusUV100








2minus and







512 CONCLUSIONS
































































2minus and







512 CONCLUSIONS



























































Photocatalytic Activity of TiO2 Modified with Hexafluorometallates – Fine Tuning of Redox...

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