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Aiming at discerning the role of fluorine from that of nitrogen as a dopant in N,F-codoped TiO2, a series of HF-doped TiO2 photocatalysts were investi...
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Fluorine-Doped TiO2 Materials: Photocatalytic Activity vs TimeResolved Photoluminescence Maria Vittoria Dozzi,† Cosimo D’Andrea,‡,§ Bunsho Ohtani,∥ Gianluca Valentini,‡,§ and Elena Selli*,† †

Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, I-20133 Milano, Italy Dipartimento di Fisica, Politecnico di Milano, IFN-CNR, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy § Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, I-20133 Milano, Italy ∥ Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan ‡

ABSTRACT: Aiming at discerning the role of fluorine from that of nitrogen as a dopant in N,F-codoped TiO2, a series of HF-doped TiO2 photocatalysts were investigated in the decomposition of formic and acetic acid in aqueous suspensions, also as a function of the irradiation wavelength (action spectra analysis), in comparison with recent results obtained with an analogous series of NH4F-doped TiO2 photocatalysts. Visible light absorption around 420 nm, which was found to be inactive in acetic acid decomposition, is definitely shown to be associated with nitrogen doping, whereas the enhanced absorption at ca. 365 nm, increasing with increasing calcination temperature, can be unambiguously attributed to structural modifications induced by fluorine doping. Action spectra analysis confirms that this absorption is active in acetic acid decomposition, in both HF- and NH4F-doped TiO2 photocatalysts. From time-resolved photoluminescence (PL) spectroscopy analysis, a clear correlation is outlined between the photoactivity of the materials and the long-lasting component of the PL signal, which increases with the calcination temperature and is related to the formation of surface defects. Thus, fluorine doping, followed by calcination at high temperature, increases the amount of surface traps originating the long-lasting PL signal, which are beneficial in photoactivity by ensuring long-living photoproduced charge couples. nanoparticle deposition.7 The photocatalytic activity of moderately doped materials was found to be higher than that of pure TiO2 prepared by the same route and unexpectedly increased with increasing calcination temperature of these full anatase materials, although their surface area in parallel decreased, with an identical trend as a function of the dopant content in all investigated reactions.5−7 Furthermore, the photocatalytic oxidation of acetic acid was investigated as a function of the irradiation wavelength, by collecting so-called action spectra. By comparing the shapes of these latter with the absorption spectra of the investigated photocatalysts, a clear distinction between inactive light absorption and effective photoactivity could be achieved.5 On the other hand, both X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) revealed the presence of residual nitrogen-containing species6 in NH4Fdoped TiO2, even if calcined at 700 °C, that might be responsible for the absorption features and photoactivity of such materials. Aiming at discerning the role of fluorine from that of nitrogen as a dopant in TiO2, a series of doped photocatalysts have now been prepared by the same synthetic procedure (sol− gel method), employing different amounts of HF instead of NH4F as the dopant source, thus avoiding the copresence of nitrogen in the material, followed by calcination at different

1. INTRODUCTION Many efforts have been devoted in the past decade to overcome the two major drawbacks of stable and cheap TiO2 as a photocatalytic material, i.e., its relatively high energy band gap (3.2 eV), which limits its absorption mainly to the UV region, and the low quantum efficiency of photocatalyzed reactions, occurring in competition with photogenerated electron−hole pair recombination, highly probable in semiconductor photocatalysts. Indeed, only a small portion (less than 5%) of sunlight can be exploited for photocatalytic processes employing TiO2, and this represents a great limitation to its use, particularly in solar to chemical energy conversion for solar fuel production. Among the different routes so far explored to activate TiO2 under visible light,1−3 anion doping with p-block elements has been widely investigated in recent years, either to introduce newly created midgap energy states or to narrow the semiconductor band gap itself. However, the effects of doping titanium dioxide with such elements are still rather controversial,4 mainly because visible light absorption does not always guarantee effective photoactivity. Furthermore, the insertion of dopant impurities into the oxide structure may also increase the rate of the undesired recombination of photogenerated charge carriers, an effect becoming less important with increasing crystallinity of the oxide structure. The photocatalytic behavior of an extended series of NH4Fdoped TiO2 photocatalysts has been recently investigated by some of us in both liquid- and gas-phase oxidation reactions,5,6 as well as in hydrogen production from methanol−water vapor mixtures, upon photocatalyst modification by noble-metal © 2013 American Chemical Society

Received: September 25, 2013 Revised: November 9, 2013 Published: November 11, 2013 25586

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irradiated aqueous suspensions contained 0.1 g L−1 photocatalyst and an initial formic acid concentration of 1.0 × 10−3 mol L−1. After preliminary ultrasound treatment for 30 min, they were magnetically stirred in the dark for 15 min to attain adsorption equilibrium of the substrate on the photocatalyst surface, before irradiation was started. Stirring was continued during the runs. The irradiation source, a commercial Osram lamp (λ > 340 nm, intensity on the reactor 2.5 × 10−7 einstein s−1 cm−2), was always switched on at least 30 min before the beginning of irradiation. At different time intervals during the runs, 2.0 mL samples of the suspension were withdrawn from the reactor and centrifuged. The surnatant was analyzed for its residual formic acid content by ion chromatography with conductivity detection (Metrohm 761 Compact IC instrument), after calibration for formate ion concentration in the 0− 50 ppm range. All kinetic runs were performed without modification of the natural pH of the suspensions, up to ca. 70% substrate degradation, and repeated at least twice to check their reproducibility. The pH of the suspensions increased during the runs from an initial value around 3.7 to ca. 4.8, as a consequence of formic acid mineralization to CO2 and H2O. 2.3. Acetic Acid Photocatalytic Decomposition under Polychromatic Irradiation. These photocatalytic tests were performed at 25 °C in a ca. 35 mL glass tube, as already described.5 Each photocatalyst powder (50 mg) was suspended in an aqueous solution (5.0 mL) containing 5.0 vol % acetic acid. This photocatalyst amount in the suspension ensured total light absorption. The pH value was 2.5 and did not vary either after TiO2 addition or under irradiation. After the glass tube was sealed with a double-capped rubber septum, the suspensions were irradiated, under vigorous magnetic stirring (1000 rpm), by a 400 W high-pressure mercury lamp (Eikosha), with λ > 290 nm and an average emission power of 390 mW. At regular time intervals during the runs, 0.2 mL samples of the gas in the tube were withdrawn with a gastight syringe and analyzed using a Shimadzu GC-8A gas chromatograph with a TC detector, equipped with MS-5A and Porapak-Q columns. The molar amount of evolved carbon dioxide was calibrated considering the increase in pressure in the reaction tube consequent to the increased amount of gas-phase molecules. All kinetic runs were monitored for 80 min and repeated at least twice to check their reproducibility. 2.4. Acetic Acid Photocatalytic Decomposition: Action Spectra Analysis. The photocatalytic decomposition of acetic acid in aerated aqueous suspensions was investigated as a function of irradiation wavelength in ca. 10.5 mL quartz cells, as in previous studies.5 Each photocatalyst powder (20 mg) was suspended in an aqueous solution (2.0 mL) containing 5.0 vol % acetic acid. The suspensions were stirred in the dark for 15 min to attain adsorption equilibrium and then irradiated for 80 min with stirring by means of a 300 W xenon lamp (Hamamatsu Photonics C2578-02) within a diffraction gratingtype illuminator (Jasco CRM-FD). The irradiation wavelength was varied in the 370−460 nm range, with a full width at halfmaximum intensity smaller than ca. 17 nm. The irradiation power, measured by a Hioki 3664 optical power meter, was in the 15−20 mW range. All other conditions were similar to those of photocatalytic activity tests under high-pressure mercury lamp irradiation. The wavelength-dependent apparent quantum efficiency Φapp was calculated as the ratio between the rate of photogenerated hole consumption and the flux of incident

temperatures (500−700 °C). The activity of the so-obtained TiO2-based photocatalysts has been investigated in the photooxidation of transparent formic acid and acetic acid in aqueous suspensions, also as a function of the irradiation wavelength (action spectra analysis). To verify if and how the dynamics of the photogenerated charge couples can be related to the peculiar structure of the full anatase F-doped TiO2 materials and to their photocatalytic activity, time-resolved photoluminescence (PL) spectroscopy measurements have been performed on the picosecond time scale. To the best of our knowledge, such an analysis, which provided direct evidence that the photocatalytic activity is related to the population of the long-lasting trap sites, has never been attempted so far with this type of doped TiO2 photocatalytic materials.

2. EXPERIMENTAL SECTION 2.1. Fluorine-Doped TiO2 Preparation and Characterization. All photocatalysts were prepared by the sol−gel method, according to the procedure already described for the preparation of NH4F-doped materials.5,6 Briefly, an anhydrous ethanol solution (100 mL) containing 10 mL of dissolved titanium(IV) isopropoxide (Aldrich 97%) was heated at 30 °C. Then a fresh water solution containing different amounts of hydrofluoric acid, employed as a dopant source, was added dropwise under vigorous stirring to obtain a 1/58 specific molar Ti/H2O ratio and F/Ti nominal percent molar ratios equal to 3, 5, 12, and 25. An immediate exothermic hydrolysis reaction occurred. After being stirred and refluxed for 1 h, the suspension was concentrated under reduced pressure at 35 °C and kept at 70 °C overnight to remove organic compounds. Precursor powders were then calcined under a 100 mL min−1 air flow at different temperatures (500, 600, and 700 °C) for 4 h, with an initial heating ramp of 5 °C min−1. The so-obtained materials were labeled as HF_Y_Z, with HF referring to the dopant, Y to the nominal dopant/Ti percent molar ratio, and Z to the calcination temperature (°C). Reference undoped materials, prepared by exactly the same synthetic route in the absence of any dopant source, are referred to as the HF_0_Z photocatalysts. All reagents were purchased from Aldrich and employed as received. The Brunauer−Emmett−Teller (BET) specific surface area was measured by N2 adsorption/desorption at liquid nitrogen temperature in a Micromeritics Tristar II 3020 V1.03 apparatus, after outgassing at 300 °C for 1 h under a N2 stream. X-ray powder diffraction (XRD) patterns were recorded on a Philips PW3020 powder diffractometer by using Cu Kα radiation (λ = 1.54056 Å). Quantitative phase analysis was done by the Rietveld refinement method.8 XPS analysis was performed by using a JEOL JPS-9010MC spectrometer with Mg Kα radiation, 10 eV pass energy, and 0.1 eV energy step. The analysis area of the sample pellets was about 6 mm2 and the depth about 1−2 mm. The charging effect on the analysis was corrected by fixing the binding energy of adventitious carbon (C 1s) at 284.7 eV. Diffuse reflectance (R) spectra of the photocatalyst powders were recorded on a Jasco V-670 spectrophotometer equipped with a PIN-757 integrating sphere, using barium sulfate as a reference, and then converted into absorption (A) spectra (A = 1 − R). 2.2. Formic Acid Photocatalytic Decomposition. All formic acid photodegradation runs were performed under atmospheric conditions in a magnetically stirred 60 mL cylindrical quartz reactor, as already described.5,6,9 The 25587

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according to eq 1 was much longer than both the laser period t0 and the width of the measurement time window, which sets the time axis t. In this case, the convergence of the fitting procedure with eq 1 may be critical and lead to an unreliable estimation of A3 and τ3. From the experimental point of view, a widening of the time scale would have been beneficial to attain complete excitation decay between successive laser pulses. Indeed, this was attempted by reducing the laser repetition rate through a pulse peaker, yet the signal reduced accordingly and fell below the noise threshold, thus preventing this approach. Nevertheless, still good results were obtained from PL data, provided that a suitable approximation was applied to eq 1. In fact, a series expansion of the exponential functions in the third exponential term leads to the following expression:

photons by taking into account that eight electrons are transferred in the reaction CH3COOH + 2O2 → 2CO2 + 2H2O. 2.5. Time-Resolved Photoluminescence Spectroscopy and Data Analysis. Emission lifetime measurements were carried out by means of time-resolved PL instrumentation based on a streak camera, which allows one to acquire temporal decay profiles at different wavelengths.10 The pulsed excitation light was provided by a tunable (680−1080 nm) Ti:sapphire laser (Chameleon Ultra II, Coherent), emitting light pulses of approximately 140 fs, at a repetition rate of 80 MHz and with a maximum pulse energy of about 50 nJ. The optical signal was frequency-doubled by focusing the light pulses into a type I βbarium borate crystal. In the present experiments, a central wavelength of 350 nm was selected by properly tuning the Ti:sapphire laser. The IR radiation was removed by a short-pass filter (Schott). The light pulses at 350 nm were focused on the sample surface through a home-built epifluorescence microscope based on a dichroic mirror (cutoff at 400 nm) and a long working distance 5× objective lens (NA = 0.15). The luminescence signal was collected by the same objective, filtered by a long-pass filter with cutoff at 400 nm (Comar Optics Inc.) and focused into the entrance slit of an imaging spectrometer (focal length 300 mm, f/3.9, 50 lines/mm grating, Acton SP2300, Princeton Instruments). The spectrometer was coupled to a streak camera detector (C5680, Hamamatsu) working in synchroscan operation mode at 80 MHz with a temporal resolution of about 20 ps in the selected 2 ns time range. To trigger the streak camera, a small portion of the laser beam at 700 nm was split off and directed to a photodiode. The measurements were performed at ambient temperature on an open-face aluminum sample holder over which the samples were placed in the form of circular pellets of powder (1 cm diameter). The PL decay profiles at selected spectral ranges of interest were normalized to the maximum intensity value and fitted using the curve-fitting tool cftool (Matlab), with a nonlinear least-squares interpolation, to retrieve both PL amplitudes and lifetimes. Triexponential decay curves, assumed as model functions, were found to adequately fit the signal decay profiles. In some cases, i.e., when the relaxation dynamics of the excitation (lifetime) was longer than the period of the laser pulses (12 ns in our case), a pileup effect was observed in the temporal decay curve. Consequently, the PL emission did not vanish, and the time-resolved curves measured by the streak camera tended to a constant plateau. Nevertheless, the incomplete fluorescence decay could be accounted for by a proper analytical model.11 More specifically, the following model function was used to fit the normalized PL decay profiles: I(t ) = A1 exp( −t /τ1) + A 2 exp( −t /τ2) A3 + exp( −t /τ3) 1 − exp( −t0/τ3)

I(t ) = A1 exp( −t /τ1) + A 2 exp(−t /τ2) +

A3τ3 At − 3 t0 t0 (2)

According to eq 2, the constant plateau in the I(t) decay curves is ascribed to the A3τ3/t0 term, which is proportional to the number of states undergoing the long-lasting radiative decay.

3. RESULTS AND DISCUSSION 3.1. Photocatalyst Structural Features. The main structural features of the homemade investigated photocatalysts, obtained by XRD and BET analyses, are collected in Table 1. As clearly shown by XRD results, HF doping, even if Table 1. Phase Composition and Crystallite Dimensions (dA), Obtained from XRD Analysis by Assuming the Absence of Amorphous Phase, and Specific Surface Area (SSA), Obtained from BET Analysis, of the Investigated Photocatalyst Series sample

F/Ti molar ratio (%)

anatase content (%)

HF_0_500 HF_0_600 HF_0_700 HF_3_500 HF_3_600 HF_3_700 HF_5_500 HF_5_600 HF_5_700 HF_12_500 HF_12_600 HF_12_700 HF_25_500 HF_25_600 HF_25_700

0 0 0 3 3 3 5 5 5 12 12 12 25 25 25

99 90 38 100 100 100 100 100 100 100 100 100 100 100 100

rutile content (%) 10 62

dA (nm)

SSA (m2 g−1)

15 32 65 23 41 56 22 40 55 20 50 57 20 49 85

15 9 7 44 25 20 45 28 22 34 16 18 21 11 8

at a very low level (e.g., the HF_3 photocatalyst series), inhibits the anatase to rutile phase transformation. In fact, at difference with respect to pure TiO2 materials obtained by the same route in the absence of any dopant species (HF_0 series), all doped samples, even if calcined at 700 °C, consisted of almost pure anatase phase. This confirms the crystal-phase-controlling role played by fluoride, as well as by other anions, that hampers the condensation of spiral chains of rutile TiO6 octahedra at calcination temperatures above 500 °C.12,13 The dimensions of anatase crystallites, dA, calculated according to the Scherrer

(1)

where Ai and τi (i = 1, 2, 3) are the amplitudes and lifetimes of the three luminescent components. The last term includes a factor that depends on the ratio between the laser period t0 and the longest lifetime τ3 and corrects the amplitude A3 for the incomplete decay (pileup). The correction factor was included only in the third exponential term, since lifetimes τ1 and τ2 are much smaller than t0 in our experiments (vide infra). Moreover, for some samples the τ3 lifetime resulting from the data fit 25588

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equation,14 did not sensibly depend on the nominal content of fluorine and increased, as expected, with increasing calcination temperature. In parallel, BET analysis showed a decrease in the specific surface area (SSA) of the TiO2-based photocatalysts with increasing calcination temperature for a fixed nominal dopant amount, as a consequence of particle sintering. On the other hand, materials with larger surface areas were obtained upon doping TiO2 with HF (Table 1). A similar trend in particle dimensions and SSA values was observed in the case of the previously investigated5 NH4F-doped TiO2 materials (labeled as D_TiO2 photocatalyst series), though smaller dA values and larger SSA values were always obtained for D_TiO2 samples5 calcined at the same temperature and containing the same nominal dopant amount as the HF-doped photocatalysts listed in Table 1. XPS analysis evidenced that both the Ti 2p doublet signal, with the two components peaking at 465.1 and 459.5 eV, assigned to Ti 2p1/2 and Ti 2p3/2, respectively, and the line shape of the XPS signals associated with O 1s, with a main band peaking at 530.6 eV, typical of oxygen bound to Ti, were very similar for the undoped and the doped HF_12 samples. The XPS signals in the F 1s binding energy region of HF_12_500 and HF_12_700, shown in Figure 1, consist of only one band,

Also the absorption properties of the here investigated HFdoped TiO2 photocatalysts should be analyzed in comparison to those of the NH4F-doped (D_TiO2) photocatalysts, which also consisted of almost pure anatase phase.5 Some examples are shown in Figure 2. As a matter of fact, by a difference absorption spectral analysis, the absorption spectra of the D_TiO2 series were shown to contain a peak A, in the near-UV region, with a maximum at ca. 365 nm, the contribution of which increased with increasing calcination temperature, and a peak B, with a maximum at 420 nm, the contribution of which decreased with increasing calcination temperature.5 The maximum position of both A and B peaks was not affected by either the calcination temperature and/or the dopant content, whereas their maximum intensity increased with increasing nominal dopant amount (see the 400−500 nm region of the absorption spectra of the D_25 series, shown in Figure 2). Furthermore, by comparing the absorption spectra of the D_TiO2 photocatalysts with their action spectra in acetic acid decomposition, peak A was found to be active in acetic acid decomposition, whereas peak B was demonstrated to be totally inactive.5 The absorption spectra of the HF-doped TiO2 materials shown in Figure 2 exhibit no absorption contribution due to peak B. To better enlighten this fact, we calculated the difference between the absorption spectra of NH4F-doped samples (D_5 and D_25 series) and the absorption spectra of the corresponding photocatalysts of the HF-doped series with the same nominal dopant amount (5% and 25%, respectively). Such absorption difference spectra are shown in Figure 3. In this way one can easily recognize the contribution of peak B in the absorption spectra of D_TiO2 photocatalysts, which is particularly evident for samples prepared starting from NH4F as the dopant source in relatively high amount (25%) and calcined at 500 °C (see the D_25_500 − HF_25_500 difference spectrum in Figure 3b). At the same time, HF-doped samples calcined at 700 °C appear to absorb more than NH4F-doped TiO2 in the near-UV region, where peak A is located (negative absorption in the difference spectra of Figure 3). This definitely confirms the attribution of (inactive) peak B mainly to nitrogen doping,5 this absorption contribution being absent in HFdoped materials (Figure 2a,b), whereas the shorter wavelength absorption contribution in the near-UVA region (peak A, for the HF-TiO2 series slightly more intense than for the D_TiO2 series) appears to be related to structure effects induced by the presence of the fluorine dopant. By considering that all doped samples, even those calcined at 700 °C, consist of pure anatase phase, any artifact in the difference spectra due to rutile phase absorption can be excluded, as already outlined.5 3.2. Photocatalytic Oxidation of Formic and Acetic Acid under Polychromatic Irradiation. Formic acid and acetic acid were chosen as photocatalytic degradation substrates mainly because they do not absorb in the visible region and thus allow a straightforward verification of the photoactivity of doped materials under visible light.19,20 Furthermore, formic acid undergoes direct photomineralization without forming any stable intermediate species, which simplifies the interpretation of kinetic results. The photocatalytic degradation of these test compounds always occurred at a constant rate, i.e., according to a zeroth-order rate law, as in previous studies.5−7,9,14,21,22 Therefore, the photocatalytic activity of the here investigated HF-TiO2 photocatalysts can be compared in terms of the zeroth-order rate constant values k0 reported in Figures 4 and 5,

Figure 1. High-resolution XPS spectra in the F 1s binding energy region of (a) HF_12_500 and (b) HF_12_700 before (solid lines) and after (dotted lines) etching with Ar ions.

peaking around 684 eV, attributed to fluoride anions adsorbed on the photocatalyst surface.15,16 This F 1s signal decreased after etching (Figure 1), almost completely disappearing in the case of HF_12_700. These results, in line with those previously obtained for NH4F-doped TiO2,5 exclude the possibility of detecting fluorine in bulk HF-TiO2, especially if calcined at high temperature. No XPS signal at 688 eV, assigned to substitutional ions in the F-TiO2 lattice,15,17 appeared either before or after etching, possibly always being below the detection limit of the XPS technique. The effective F/Ti molar ratios determined by quantitative XPS analysis were 1.7% and 0.9% for HF_12_500 and HF_12_700, respectively, i.e., far lower than the nominal value according to the synthesis (12%) and also the fluorine content detected in NH4F-doped D_12_500 and D_12_700 (7.1% and 4.4%, respectively).5 This confirms that the copresence of nitrogen and fluorine ensures the insertion of a greater dopant amount, because of charge compensation effects.18 In both cases, the amount of fluorine was found to decrease with increasing calcination temperature. 25589

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Figure 2. Absorption spectra of the (a) HF_5 and (b) HF_25 photocatalyst series, in comparison with the absorption spectra of the NH4F-doped (c) D_5 and (d) D_25 photocatalyst series.

Figure 4. Zeroth-order rate constants of formic acid photomineralization on HF-doped photocatalysts containing different fluorine amounts and calcined at different temperatures.

Figure 3. Difference absorption spectra of NH4F-doped (labeled as DTiO2) samples and the corresponding HF-doped TiO2 samples (containing no nitrogen dopant). Nominal dopant content: (a) 5% and (b) 25%. Figure 5. Zeroth-order rate constants of CO2 photoevolution during acetic acid decomposition on HF-doped photocatalyst series containing different fluorine amounts and calcined at different temperatures.

obtained from formic and acetic acid photocatalytic degradation tests, respectively. First, with undoped TiO2 both reactions were confirmed to proceed at a lower rate with respect to that attained on moderately doped TiO2. Moreover, with the undoped (HF_0) series the reaction rate decreased with increasing calcination temperature of the photocatalysts, most probably as a consequence of the anatase to rutile transformation (Table

1). On the other hand, the rate of both photocatalytic oxidation reactions increased with increasing calcination temperature of the doped materials within each HF series, although their surface area decreased (Table 1), a result which is perfectly in 25590

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line with those obtained with NH4F-doped TiO2 (D_TiO2 series).5 This marked similarity suggests that the beneficial effects obtained for doped samples calcined at high temperature should be related to intrinsic structural modifications induced by fluorine rather than by the presence of nitrogen as a codopant in the D_TiO2 series. Furthermore, we verified that nitrogen doping of TiO2, starting from NH3 as the dopant source, does not inhibit the anatase to rutile phase transition up to 700 °C. In fact, a TiO2 sample prepared under identical conditions in the presence of 5 mol % NH3 and calcined at 700 °C was found to contain only 19% anatase, 81% being the rutile phase, and to have a SSA of 7 m2 g−1. The corresponding nitrogen-doped sample calcined at 500 °C was a full anatase material, with an SSA of 72 m2 g−1. 3.3. Action Spectra Analysis of Acetic Acid Decomposition. This type of analysis was shown to be very powerful to determine the effective wavelength-dependent response and activity of a photocatalyst.19 In the present study it was restricted to full anatase TiO2 materials, the rutile phase being expected to be less photoactive than anatase in acetic acid decomposition.23,24 Figure 6 shows the action spectra in the

Figure 7. Difference action spectra of the HF_5 photocatalyst series. The action spectrum of the sample calcined at 500 °C was subtracted from those of samples calcined at a higher temperature.

obtained for the corresponding NH4F-doped series.5 In this way an unequivocal confirmation is obtained that fluorine, and not nitrogen, is mainly responsible for the observed photoactivity increase in the UVA region of F-doped and N,Fcodoped TiO2 photocatalysts, as a consequence of an increased absorption on the band gap long-wavelength side (peak A). This may be ascribed to extrinsic absorption originated from surface defects,25−27 most probably subsurface Ti3+ centers formed upon the introduction of 3-coordinated surface F atoms.26,27 The photoactivity increase in the 370−410 nm range observed with increasing calcination temperature may be simply a consequence of the higher crystallinity28 of the Fdoped or codoped samples, persisting in the anatase phase also after calcination at high temperature, with respect to those calcined at 500 °C. In the attempt to discriminate between these two possibilities, we performed a time-resolved PL analysis on the picosecond time scale. The aim of this study was to obtain information on both the shape and intensity of the PL signals originating from our photocatalytic materials to correlate them to the presence and amount of surface defects and, more importantly, to obtain information on the lifetimes of the photogenerated electron−hole couples to correlate them with the observed photocatalytic activity and action spectra of the F-doped materials. 3.4. Time-Resolved Photoluminescence Analysis. The PL response of pure (undoped) TiO2 has been explored so far, mainly in micropowder or single-crystal form or recently even in nanotube arrays,29 and generally found to be sensitive to the fabrication method and/or surface treatments introducing nearsurface defect states.30,31 The shape and intensity of the PL spectrum of nanocrystalline TiO2 may strongly depend on the contacting solvent,32 on the nanoparticle morphology,33 or on specific treatments under aerobic or anaerobic conditions.34−37 Time-resolved PL studies on pure TiO2 have mainly been performed at low temperature so far and limited to the microsecond time scale.38−40 We investigated the PL signal intensity, shape, and decay profile after excitation at 350 nm, i.e., within the TiO2 band gap, of HF-doped TiO2 photocatalysts, in comparison with the undoped and some NH4F-doped TiO2 materials. To obtain information on the dynamics of photogenerated species under conditions similar to those adopted in our photocatalytic test reactions, all time-resolved PL experiments were performed at room temperature and under atmospheric conditions, i.e., in the presence of oxygen, which notoriously acts as an electron scavenger, quenching anatase TiO2 PL emission.41 TiO2 emission resulting from the recombination of conduction band electrons with valence band holes is very

Figure 6. Action spectra of acetic acid decomposition in the 370−460 nm wavelength range obtained with the HF_5 photocatalyst series.

370−460 nm range obtained with the HF_5 TiO2 photocatalysts in the photooxidation of the transparent acetic acid substrate. No photoactivity can be outlined in the visible light region at λ > 420 nm, as expected, these samples exhibiting no visible light absorption (see Figure 2a,b). On the other hand, the data reported in Figure 6 also show that a progressively higher calcination temperature ensured a higher apparent quantum efficiency in the near-UVA region (370−410 nm). Also this result perfectly parallels those obtained from the action spectra analysis of N- and F-codoped D_TiO2 in the same spectral range,5 thus evidencing, for both F-doped and N,F-codoped TiO2 photocatalysts, a peculiar photoefficiency increase under irradiation in the 370−410 nm range with increasing calcination temperature of the materials. To better compare the results obtained with F-doped TiO2 to those obtained with the corresponding N,F-codoped systems containing the same nominal dopant amount (5%), the action spectra subtraction procedure, already described5 in the case of the NH4F-doped materials, was applied also to the full anatase HF_5 series. Figure 7 shows the difference action spectra obtained by subtracting the action spectrum of the sample calcined at 500 °C from those of samples calcined at 600 and 700 °C. This subtraction procedure clearly demonstrates that HF-doped TiO2 calcined at 600 and 700 °C exhibits increased photocatalytic activity in the UVA region, with maximum values and relative ΔΦapp values in excellent agreement with those 25591

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weak at room temperature, TiO2 being an indirect semiconductor, though relaxed selection rules for localized defect states permit the radiative recombination of trapped electrons and holes.42 The spectrum and intensity of PL signals are thus mainly related to the presence of surface traps induced by defective TiO2 crystal structures. Moreover, our PL investigation essentially involved full anatase TiO2 materials. According to a recent analysis carried out continuously under 350 nm excitation at room temperature,32,33 the broad visible PL of nanocrystalline anatase TiO2 can be resolved into two main contributions: (i) a type 1 or “green” PL, centered around 540 nm and associated with radiative transitions involving mobile electrons (i.e., those in the conduction band or in shallow bulk traps) recombining with hole-trapped states (located 0.7−1.4 eV above the valence band edge); (ii) a type 2 or “red” PL, centered around 650 nm and related to the radiative recombination of trapped electrons, about 0.7−1.6 eV below the conduction band edge, with valence band holes. The green PL peak intensity was found to increase upon vacuum annealing, a phenomenon related to oxygen vacancy formation. Indeed, the removal of a bridging oxygen atom leaves behind two electrons. Such electrons can reduce two neighboring Ti4+ ions to Ti3+, which could act as trap sites for the holes generated under UV irradiation. Figure 8 shows a comparison among the steady-state PL spectra of undoped HF_0_500 and fluorine-doped HF_5_500

Fluorine-doped TiO2 materials clearly exhibit a more intense, broad PL signal, centered around 500 nm (Figure 8), i.e., corresponding to a lower energy emission transition with respect to that of undoped TiO2. Therefore, fluorine doping seems to introduce a range of energy states within the band gap that work as trapping sites of photoproduced charged species, thus significantly affecting the electron−hole recombination dynamics. Moreover, as shown in Figure 8, the PL intensity increased with increasing nominal dopant amount. The PL signal here obtained for HF_TiO2 samples can be attributed to surface defects leading to the already mentioned formation of Ti3+ centers, consequent to the insertion of fluorine.26,27 The PL maximum peak of our full anatase HF_5_500 and HF_12_500 materials appears to be slightly blue-shifted (Figure 8) with respect to the green PL described for pure anatase TiO2 materials,32 but closely resembles the visible luminescence band, usually centered at 505 nm, ascribed to oxygen vacancies associated with Ti3+ in anatase TiO2.47 Slightly different energetic distributions of surface traps, which are highly dependent on sample preparation and history, may account for such small differences in the PL signal shape. Most interestingly, also the photocatalysts of the D_5 series, containing both fluorine and nitrogen dopants, showed a PL signal shape almost identical to those of the HF-doped TiO2 series (see, for example, the inset of Figure 8). This indicates that, although NH4F-doped TiO2 samples, especially if calcined at 500 °C, contain filled energy levels above the valence band (VB) introduced by nitrogen doping which originate visible light absorption (peak B; see Figures 2c,d and 3), they have an emission behavior almost identical to that of singly fluorine doped TiO2 materials. On the other hand, the PL emission observed with doped and codoped samples upon band gap excitation corresponds to an energy gap smaller than that of undoped TiO2 luminescence (Figure 8), the latter being most probably related to light emission upon electron transition from the conduction band (CB) to the VB. PL emission from Fdoped or N,F-codoped TiO2 should consequently involve intra band gap trap states. As a matter of fact, two main possible mechanisms of anatase visible light PL involving intra band gap trap states may be envisaged for the investigated doped or codoped TiO2 materials. The PL signal can result from electron−hole recombination taking place either between Ti3+ trap states, localized just a few tenths of an electronvolt below the bottom of the CB, and VB holes (case A in Scheme 1) or between CB electrons and intra band gap states located just above the VB,25 mainly acting as VB hole traps (case B in Scheme 1). Scheme

Figure 8. Photoluminescence spectra of HF_0_500, HF_5_500, and HF_12_500 after excitation at 350 nm. Inset: comparison between the PL spectra of HF_5_700 and D_5_700.

and HF_12_500. Steady-state emissions were obtained from time-resolved data by time integration. The undoped sample exhibits a PL spectrum in the 400−450 nm range. Unfortunately, we are not able to assign a precise PL maximum position, because of the presence of a dichroic filter (laser removal) cutting the PL signal below 400 nm. This kind of PL spectrum may be assigned to the radiative recombination of bulk self-trapped excitons at intrinsic TiO6 octahedra of anatase and rutile TiO2 crystal structures.30,43−45 Identical PL spectra, both in shape and in low intensity, were obtained from the other undoped TiO2 samples of the HF_0 series. This may appear surprising, when considering that HF_0_600 and particularly HF_0_700 contained considerable amounts of rutile phase (Table 1). Most probably, only the PL emission of their anatase fraction could be detected under the adopted experimental conditions. In fact, the PL emission from rutile has been recently reported to become observable only upon strong laser excitation46 and to be in general not easily detectable, especially in the case of mixed-phase materials.30,33

Scheme 1. Schematic Model for Trap-State-Originated PL in Full Anatase F-Doped TiO2 Materialsa

a

Straight and wavy lines indicate radiative and nonradiative transitions, respectively. Case A: Ti3+ trap state below the CB. Case B: acceptor level above the VB.

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1A appears most reasonable in the case of our present PL results obtained with HF-doped TiO2, the existence of a trap energy level above the VB (case B) still needing to be demonstrated in the case of F-doped TiO2. On the other hand, both cases A and B certainly need to be considered for N,Fcodoped TiO2, due to the presence of the nitrogen dopant. However, the very similar shape of the PL signals we obtained with HF- or NH4F-doped TiO2 provides an indication that Scheme 1A applies also in the case of our N,F-codoped materials. A more detailed investigation on the effects that donor or acceptor species in contact with the materials have on the PL signal might provide information on the energy level of the trap states involved in the PL mechanism. Thus, the PL spectra obtained with fluorine-doped or -codoped materials have very similar shapes, regardless of their dopant type and nominal content or calcination temperature (see, for example, Figure 9, referring to the

Figure 10. Photoluminescence decay measured for the (a) HF_5 and (b) HF_12 photocatalyst series.

Table 2. Parameters Obtained from Time-Resolved PL Decay Curves According to a Triexponential Decay Figure 9. Normalized photoluminescence spectra of the HF_5 photocatalyst series after excitation at 350 nm.

HF_5 series), suggesting the presence of similar emitting species in all samples. However, their PL decay within the 2 ns time window was markedly different, depending on the calcination temperature of the sample, as clearly shown in Figure 10 reporting the PL signal decay after excitation for the HF_5 and HF_12 series. The PL lifetime generally increased with increasing calcination temperature within each series of full anatase HF-TiO2 materials. The triexponential decay fitting mentioned in section 2.5 was adequately assessed for the PL signal decay in all investigated materials and led to the decay parameters reported in Table 2. All the PL curves show a very fast (subnanoseconds) relaxation, which can be separated into two exponential decay components, with amplitudes A1 and A2 and lifetimes τ1 and τ2, respectively, related to the fast recombination of excitons, occurring in tens to hundreds of picoseconds, and a relatively long-lasting emission with amplitude A3 and lifetime τ3. This last component, which has been characterized by the A3τ3 product by data fitting according to eq 2, accounts for most of the detected photons and also determines the shape of the PL spectrum. The PL decay for samples calcined at higher temperature tends to be progressively slower, independently of the nominal dopant amount. Moreover, while the PL signal decays associated with the fast components (A1, τ1 and A2, τ2) do not exhibit any specific trend either with the calcination temperature or with the nominal dopant amount (Table 2), the long-living component (A3τ3) clearly increases with the calcination temperature and reaches its maximum value for the highly crystalline photocatalyst samples calcined at 700 °C.

sample

A1

τ1 (ps)

A2

τ2 (ps)

A3τ3 (ps)

HF_0_500 HF_0_600 HF_0_700 D_5_500 D_5_600 D_5_700 HF_5_500 HF_5_600 HF_5_700 HF_12_500 HF_12_600 HF_12_700

0.59 0.36 0.20 0.35 0.06 0.14 0.26 0.09 0.16 0.35 0.20 0.16

25 40 33 68 24 30 77 35 22 62 47 6

0.20 0.17 0.26 0.54 0.51 0.27 0.56 0.48 0.22 0.50 0.43 0.25

269 401 295 323 469 280 350 424 290 293 423 239

4463 6999 7130 1053 5336 7897 1106 4964 7650 442 3542 7205

Actually, beyond the arguments concerning the fitting procedure, there are good reasons for paying attention to the A3τ3 term since an increase in both the number of trapped states, which might correlate with A3, and in their average lifetime τ3 is expected to favor the photocatalytic activity of the materials, since they indicate an increased permanence of longliving excited states. As a matter of fact, the A3τ3 product always increases with increasing calcination temperature within each photocatalyst series with a fixed nominal dopant amount, accounting for ca. 99% of the emitted photons in all doped samples calcined at 700 °C. Fluorine doping thus seems to play a beneficial effect in increasing the number of long-living charged trapped species and their persistence, as a natural consequence of the introduction of a new type of surface defective sites for doped TiO2 samples with respect to the undoped materials. Moreover, the A3τ3 values clearly increase with increasing calcination temperature within each doped photocatalyst series, with a trend perfectly parallel to the photoactivity trend in 25593

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as a complementary powerful tool for determining the distribution of trap states and their effects in carrier dynamics, which are very helpful for understanding how the photocatalytic processes depend on the specific structural features of the photoactive materials.

formic and acetic acid decomposition (Figures 4 and 5). In particular, by taking into account the A3τ3 values reported in Table 2 and the formic acid degradation rate constants collected in Figure 4, a clear correlation can be outlined, as shown in Figure 11, between the long-lasting PL decay

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AUTHOR INFORMATION

Corresponding Author

*Phone: +39 02 50314237. Fax: +39 02 50314300. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The skillful collaboration of Dr. Luca Artiglia, University of Padova, on XPS measurements is gratefully acknowledged. This work was partly financed by the Italian MIUR through the 2009PASLSL PRIN Project.

Figure 11. Correlation between the long-lasting PL component and the surface-normalized photocatalytic reaction rates of formic acid degradation for the HF_5 and HF_12 photocatalyst series.

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REFERENCES

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4. CONCLUSIONS Doping TiO2 with fluorine induces both bulk and surface modification. In fact, it hampers the anatase to rutile phase transformation, thus leading to highly crystalline and photoactive full anatase materials even after calcination at 700 °C, and also facilitates the formation of surface defects, which might be at the origin of the photoactive enhanced absorption in the near-UV region (ca. 365 nm), increasing with increasing calcination temperature. On the other hand, nitrogen, and not fluorine, is definitely responsible for the visible light, though photocatalytically inactive, absorption observed in N,F-codoped TiO2 materials. Furthermore, fluorine doping (or codoping) favors the formation of long-living (tens of nanoseconds or more) luminescent surface trapping sites. Their relative amount and lifetime with respect to other shorter living luminescent components increase with increasing calcination temperature of these materials, which is expected to play a major role also in increasing their crystallinity degree. Such long-living surface traps are beneficial for photoactivity. In fact, if photogenerated electrons or holes are efficiently trapped at specific defective sites, the probability that they interact with adsorbed oxidizable or reducible species largely increases,46,49 with a parallel decrease of the undesired electron−hole recombination. The present results, together with those very recently obtained under different conditions with core−shell nanostructured materials,50 confirm time-resolved PL spectroscopy 25594

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