Article pubs.acs.org/JPCC
Evaluating the Mechanism of Visible Light Activity for N,F-TiO2 Using Photoelectrochemistry Jeremy W.J. Hamilton,*,† J. Anthony Byrne,† Patrick S.M. Dunlop,† Dionysios D. Dionysiou,‡ Miguel Pelaez,‡ Kevin O’Shea,§ Damian Synnott,∥ and Suresh C. Pillai⊥ †
Nanotechnology and Integrated BioEngineering Centre (NIBEC), University of Ulster, Shore road Newtownabbey, BT37 0QB, Northern Ireland, United Kingdom ‡ Environmental Engineering and Science program, University of Cincinnati, 2600 Clifton Ave., Cincinnati, Ohio 45221, United States § Department of Chemistry and Biochemistry, Florida International University, 11200 SW 8th St., Miami, Florida 33199, United States ∥ Centre for Research in Engineering Surface Technology (CREST), FOCAS Institute, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland ⊥ Department of Environmental Science, School of Science, Institute of Technology Sligo, Ash Lane, Sligo, Ireland S Supporting Information *
ABSTRACT: The improvement of the solar efficiency of photocatalytic materials is important for solar driven environmental remediation and solar energy harvesting applications. Photoelectrochemical characterization of nitrogen and fluorine codoped titanium dioxide (N,FTiO2) was used to probe the mechanism of visible light activity. The spectral photocurrent response under visible irradiation did not correlate with the optical absorption spectrum of the N,F-TiO2; however, open-circuit photopotential measurements provided better correlation to the optical absorption spectra. These observations suggest that electrons excited to the conduction band from the N-induced midgap state are rapidly trapped by defect levels below the conduction band. Reactive oxygen species (ROS) can be produced via the reduction of molecular oxygen by conduction band electrons leading to the oxidative degradation of organic pollutants, and singlet oxygen may play a role. If there is no loss in the band gap activity, as compared to undoped titania, then any additional visible light activity may give an overall improvement in the solar efficiency. The photocurrent response should not be used as a direct measure of photocatalytic activity for doped titania as the oxygen reduction pathway is vitally important for the generation of ROS, whereas hole transfer from dopant midgap states may not be so critical.
■
INTRODUCTION Photocatalytic materials are important for environmental remediation, including air and water treatment, and solar energy harvesting (water splitting and CO2 reduction).1−3 Titanium dioxide (TiO2) is the most common material employed for heterogeneous photocatalysis.4 TiO2 has a wide band gap and absorbs in UV domain (anatase Ebg = 3.2 eV); however, only around 4% of solar photons are in the UV range. The development of visible light active photocatalytic materials with improved solar efficiency has become the “holy grail” for researchers in the field.5 The doping of TiO2 with other elements to elicit visible light activity (VLA) has been the objective of intense research activities in recent years.3,6 The term “doping” is used loosely as many researchers employ large concentrations of other elements in protocols which may result in new materials or composites. Visible light sensitization of titania by the incorporation of nitrogen was first reported by Sato over 25 years ago.7 Despite this early report of visible light activity by nonmetal doping, metal ion doping received the most attention in subsequent years.8 Renewed interest was created for nitrogen doping of titania for visible light activity following a publication by Asahi © 2014 American Chemical Society
et al., in 2001, where they reported improved photocatalytic activity for the degradation of methylene blue and gaseous acetaldehyde.9 Other researchers have questioned the value of nitrogen modification to titania, reporting unsuccessful degradation of organic pollutants under visible light, leading to some debate within the community.10 The questions for debate include the nature of the incorporated nitrogen (substitutional or interstitial) and the suitability of photocatalytic test protocols.3,11 Chemical substrates used in photocatalytic test protocols can significantly alter the photocatalytic mechanism making comparison between some activity tests inappropriate. For example, dye degradation studies may not be appropriate for the measurement of visible light activity as a dye-sensitized reaction could be occurring. The mechanism of photocatalytic degradation of organic species in water using UV excited undoped TiO2 is generally accepted to involve the formation of reactive oxygen species (ROS). The valence band holes (UVB = +3.0 V (standard Received: December 10, 2013 Revised: April 18, 2014 Published: May 16, 2014 12206
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
The Journal of Physical Chemistry C
Article
other contaminants of emerging concern using N-TiO2 and N,F-TiO2 under visible light irradiation.20−24 In these studies, N,F-TiO2 showed higher removal of microcystin-LR than Evonik Aeroxide P25 (formerly Degussa P25)24 and comparable removal rates to commercial VLA titania (Kronos 7000)21 under visible light irradiation. The UV activity for the N,F-TiO2 was found to be much higher than the visible light activity.23 In this work, we have utilized photoelectrochemical techniques and new insights from the recent literature to help elucidate the mechanism of visible light activity for the N,FTiO2.
hydrogen electrode, SHE) at pH 0) should be able to oxidize water (or hydroxyl ion) to give hydroxyl radical or to directly oxidize adsorbed organics at the surface. The oxidation pathway involving valence band holes is critically important for good efficiency of UV photocatalysis on TiO2. Conduction band electrons (UCB = −0.2 V (SHE) at pH 0) should be able to reduce molecular oxygen to yield superoxide radical anion (or hydroperoxyl radical) with subsequent conduction band electron reduction reactions leading to hydrogen peroxide and hydroxyl radical. The reduction pathway is essential to maintain electrical neutrality, reducing recombination of charge carriers, but is also important in the generation of ROS. The ROS produced can react with other ROS or the parent semiconductor complicating the mechanism of photocatalysis. For instance, superoxide generated under UV reduction of molecular oxygen by conduction band electrons can be oxidized by the valence band holes to yield singlet oxygen, a weaker but longer lifetime ROS than hydroxyl radical.12 The photocatalytic mechanism for N-doped (or other nonmetal doped) titania is not as well understood. Research into the active centers for light absorption and the fate of photogenerated charge carriers remains an interesting area for research. It was initially proposed that visible light absorption in N-TiO2 is caused by a narrowing of the band gap of pristine TiO2 (e.g., anatase, Ebg = 3.2 eV; absorption edge ca. λ ≤ 387 nm).9 Others subsequently suggested the appearance of intragap localized states because of dopants that absorb visible irradiation.13 Kuznetsov and Serpone proposed that doping TiO2 results in formation of oxygen vacancies and the advent of color centers (e.g., F, F+, F++, and Ti3+) responsible for absorption of visible radiation but were uncertain if photoexcited electrons were transferred to the conduction band.14 Oxygen vacancies (Vo) and color centers (Ti3+) have been proposed as the origin of visible light photocatalytic activity in both doped and undoped TiO2.15−18 If electrons excited from a midgap state reach the conduction band, they may participate in the reductive pathway leading to ROS generation. The hole reaction for the midgap state remains largely undetermined. The hole generated in the midgap state is at a less positive electrochemical reduction potential than the valence band and, therefore, will be less efficient for the oxidation of water to yield hydroxyl radical therefore limiting the efficiency of photocatalysis under visible light irradiation as compared to UV irradiation. RengifoHerrera et al. reported that the hydroxyl radical does not play a major role in photocatalysis with nonmetal-doped titania under visible irradiation, and they found evidence of singlet oxygen generation formed by the oxidation of superoxide by the dopant-induced midgap state.19 Previously, Nakamura et al. reported on the mechanism of VLA in N-TiO2 using photoelectrochemistry to probe the position of the proposed N-induced midgap state with a range of redox species with different equilibrium redox potentials (Eeq).13 They observed an increase in the incident photon to current conversion efficiency (IPCE) under visible light irradiation with the addition of hydroquinone and iodide but not with other electron donors with a more positive Eeq, which in theory could be oxidized by such a midgap state. They concluded that the observed visible light photocurrent depends on the reaction mechanism involved and not simply on the redox potential of the electron donor. Our previous work has reported on the photocatalytic degradation of water contaminants such as algal toxins and
■
EXPERIMENTAL SECTION Electrode Preparation. N,F-TiO2 was prepared via a sol− gel route utilizing a Zonyl fluorosurfactant and ethylenediamine as the fluoride- and nitrogen-doping agents. Detailed preparation and characterization of these materials have been described previously.24 Films of N,F-TiO2 were prepared by dip-coating the sol gel on indium tin oxide coated glass (ITO). In addition, nanoparticulate electrodes were prepared by spray coating suspensions (5% suspension in methanol from lab spray gun) of N,F-TiO2 powder or Evonik Aeroxide P25 (referred to as P25 from here on) on either ITO or Ti foil to give a catalyst loading of 1 mg cm−2. The geometric area coated with TiO2 was 4 cm2 for all electrodes. Electrical connection was made to an uncoated area of ITO or titanium foil using copper wire and silver epoxy (CW2400 manufactured by Circuit Works). All areas of the electrodes excluding the area coated with TiO2 were insulated with a negative photoresist (SU8-2000.5 manufactured by Microchem). All chemicals used for sol−gel preparation, electrolytes or additives, were of reagent grade and were used without further purification and were purchased from Sigma-Aldrich. Photoelectrochemistry. Electrochemical measurements were carried out using an electrochemical cell with a quartz window containing the working electrode (TiO2 coated on ITO glass or titanium foil), a platinum basket counter electrode, and a saturated calomel reference electrode (SCE). All potentials were reported versus SCE. Air (via an aquarium pump) or oxygen-free nitrogen (OFN, BOC UN1066) was sparged into the cell using a Pasteur pipette. The irradiation source was 450 W xenon (Horiba, Jobin Yvon FL-1039/40) with or without UV cutoff filters λ > 420 nm (FGG-42050) and λ > 435 nm (FGG-43550) from UQG optics. For monochromatic irradiation, an integrated HRmicro monochromator was used (bandwidth 20 nm). In some experiments, the irradiation source was chopped using a Uniblitz (VMM-T1) chopper with a response time 387 nm for anatase), a different mechanism occurs as valence band electrons cannot be excited to the conduction band. It has been argued that the absorbance by doped materials at longer wavelengths than the parent band gap could be explained by two distinct scenarios. First, electrons are promoted from a midgap state to the conduction band using sub-band gap irradiation; second, electrons promoted from the midgap state (under sub-band gap irradiation) are only promoted to states beneath the conduction band.15 In this latter case, visible light irradiation is not expected to give any photocurrent as there would be no free electrons in the conduction band. Figure 1 shows the current−potential response of the N,FTiO2 and P25 electrodes under chopped polychromatic UV− vis irradiation with supporting electrolyte alone (NaClO4). The electrodes showed a typical n-type response with no significant dark current at potentials more positive than the flat band potential (for anatase, TiO2 at pH 5.0 Efb ∼ −0.7 vs SCE). Under unfiltered xenon irradiation, the anodic current increases substantially because of UV photon absorption and the formation of electron/hole pairs. Because these experiments were performed in electrolyte alone, it is assumed that the hole reaction is the oxidation of water. The difference between the light and dark current measurements is the photocurrent. The photocurrent for the N,F-TiO2 is lower than that for the P25 under UV−vis irradiation in these conditions, but the onset for anodic current is more negative for the N,F-TiO2. The more negative onset potential observed for anodic current with the N,F-TiO2 (as compared to the P25) could be due to a higher donor density in the N,F-TiO2, induced by doping. Experiments increasing donor density by chemical reduction or electron bombardment have been reported to yield more negative onset potentials as a result of those treatments.25−27 Figure 2 shows the current−potential response under the same conditions but with visible light only irradiation (λ > 400 12209
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
The Journal of Physical Chemistry C
Article
Figure 4. Current−time response for N,F-TiO2 and P25 at fixed potential with chopped monochromatic irradiation. Applied fixed potential +1.0 V; electrolyte was 0.1 M NaClO4; monochromatic irradiation using a 20 nm band-pass. Nanoparticle electrodes using Ti foil as the supporting substrate.
Figure 5. Current−time response for N,F-TiO2 at fixed potential (+1.0 V) with and without the addition of KI. Electrolyte was 0.1 M NaClO4; chopped monochromatic irradiation using a 20 nm band-pass. Inset shows IPCE% vs wavelength (nm). Data shows measurements taken 350 s onward.
published work reporting a maximum quantum efficiency around 320 nm for anatase TiO2.31,32 As the irradiation wavelength is increased beyond 330 nm, the photocurrent response decreases. The photocurrent response in the visible (λ > 400 nm) is only a fraction of the UV (band gap) response. The data in Figure 3 has not been normalized for light intensity; however, the photon flux actually increased with wavelength above 320 nm (8.5 × 1019 photons m−2 at 320 nm; 1.7 × 1020 photons m−2 at 400 nm, see SA2 of the Supporting Information). An expanded current scale is necessary to show the photocurrent at wavelengths in the visible domain (Figure 4). There is a significant (but relatively small compared to UV) photocurrent response from P25 at λ < 420 nm, while N,F-
Monochromatic irradiation was used to investigate the spectral photocurrent response of the electrodes. To have a sufficient photon flux, a 20 nm band-pass with 10 nm increments of wavelength from 250 to 700 nm was used. Figure 3 shows the spectral photocurrent response for the N,FTiO2 and the P25 electrodes in supporting electrolyte alone. Current measurement began in the dark at a fixed applied potential (+1.0 V). The monochromator was set to the desired wavelength, and the light was chopped on and then off. The monochromator was set to the next wavelength during the light-off stage, and the measurement was repeated until the desired wavelength range was covered. The peak in photocurrent magnitude for both the N,F-TiO2 and the P25 was observed around 320 nm which is consistent with other 12210
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
The Journal of Physical Chemistry C
Article
Figure 6. Current−time response for N,F-TiO2 at fixed potential (+1.0 V) with and without the addition of KBr. Electrolyte was 0.1 M NaClO4; chopped monochromatic irradiation using a 20 nm band-pass. Data shows measurements taken 350 s onward. Inset shows IPCE% vs wavelength (nm).
induced midgap state is able to oxidize the superoxide to yield singlet oxygen, one should observe an increase in the visible light photocurrent upon the addition of superoxide to the electrolyte. Figure 7 shows the current−potential response
TiO2 shows a larger photocurrent response under visible irradiation. In line with the previous work of Nakamura et al.,13 hole acceptors (electron donors) were added to the electrolyte to probe the position of the N-induced midgap state. In theory, any species with an equilibrium redox potential more negative than N-induced midgap state can be oxidized. A small enhancement in the visible light photocurrent for N,F-TiO2 was observed in the presence of iodide (Figure 5) and hydroquinone (data not shown), but no enhancement was observed in the presence of bromide (Figure 6). These results correlate reasonably well with those of Nakamura et al.;13 however, the enhancement in IPCE% was not as pronounced as that previously reported. With the addition of hole acceptors, the visible light photocurrent increased but remained a small fraction of the UV photocurrent, despite the higher photon flux under visible light irradiation. The addition of hole acceptors, such as KI, to enhance the VLA photocurrent in N-TiO2 has been studied by Beranek and Kisch.33 In their study, N-TiO2 was exposed to chopped monochromatic light of increasing wavelength. The spectral photocurrent response observed in their study extended beyond 506 nm (equivalent to a midgap state 2.45 eV below the conduction band) to wavelengths as long as 690 nm equivalent to an electron promoted from a state ∼1.8 eV below the conduction band. Results such as these suggest that there are other photoactive centers in doped TiO2, most likely the oxygen vacancies and color centers, as suggested by Serpone.15 One of the proposed mechanisms for visible light activity on nonmetal-doped titania is the formation of singlet oxygen by oxidation of superoxide by the midgap state.19 Superoxide radical anion (2O2·−) can be generated by the reduction of molecular oxygen (3O2) by conduction band electrons, and the oxidation of 2O2·− can yield singlet oxygen (1O2). If the N-
Figure 7. LSV for N,F-TiO2 with and without the addition of KO2 under chopped visible light irradiation (UV 435 nm cutoff filter). Electrolyte was 0.1 M HClO4. SR = 5 mV s−1.
under visible light only irradiation in the presence and absence of superoxide (added as solid KO2 directly to the electrolyte). A very small increase in the visible light photocurrent was observed in the presence of superoxide which is further evidence of the mechanism proposed by Rengifo-Herrera et al.19 To further investigate visible light mechanism, the opencircuit photopotential (EOCP, open-circuit potential under irradiation minus the dark open circuit potential) was measured 12211
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
The Journal of Physical Chemistry C
Article
Figure 8. Open-circuit photopotential vs wavelength for N,F-TiO2 and P25 electrodes. Electrolyte was OFN sparged 0.1 M HClO4. The inset shows the optical absorption spectra of P25 and N,F-TiO2.
Figure 9. Steady-state open-circuit potential vs wavelength for N,F-TiO2 electrodes in the presence (air-sparged) and absence (OFN sparged) of oxygen. Electrolyte was 0.1 M HClO4.
as a function of wavelength (Figure 8). An EOCP for P25 was only observed under band gap irradiation. For the N,F-TiO2, a negative EOCP was observed under irradiation up to 600 nm, a longer wavelength than that proposed for the transition between the nitrogen dopant level and the conduction band. This photopotential spectrum correlated reasonably well with the optical absorption spectrum of the N,F-TiO2 (Figure 8 inset). To probe the transfer of photoexcited electrons to oxygen, photopotential measurements were performed in the presence and absence of oxygen (see Figure 9). Comparing the EOC in the dark and under irradiation, more negative potentials are maintained in the absence of oxygen. The observed less negative irradiated EOCP with air sparging is consistent with
dissolved oxygen acting as an electron scavenger. This result shows that electrons promoted under visible light irradiation reach potentials sufficiently close to the conduction band to reduce oxygen. It was proposed, on the basis of density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) measurements, that electrons could be promoted from nitrogen centers to the conduction band by visible light and that these empty N states could be repopulated by electron transfer from Ti3+ states.34 Subsequent EPR work from the same group now also suggests refilling of the nitrogen center from the valence band under IR irradiation.35 In this work, IR irradiation was absent under monochromatic irradiation, and as such, the earlier mechanism is expected to be dominant. 12212
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
The Journal of Physical Chemistry C
Article
The photocurrent response spectrum for the N,F-TiO2 does not correlate to the absorption spectra for these materials; however, the photopotential spectrum gives a better correlation. The lack of observed visible light photocurrent and the observed photopotential spectrum suggests that electrons excited from the N-induced midgap state have a short lifetime in the conduction band and are rapidly trapped by defect states below the conduction band. The more positive photopotential observed in the presence of oxygen indicates that some photogenerated electrons can be passed to molecular oxygen to form superoxide radical anion (with subsequent reduction reactions giving other ROS). The addition of hole acceptors with equilibrium potentials in favorable positions to allow oxidation by the N-induced midgap state did not yield a marked increase in the visible light photocurrent. A small increase in the visible light photocurrent was observed following the addition of KO2 as a source of superoxide radical anion and is further evidence of oxidation of superoxide by the midgap state to yield singlet oxygen. Therefore, visible light photocatalytic activity in nonmetal-doped titania is predominantly due to the formation of ROS via the reduction of molecular oxygen by conduction band electrons and possibly singlet oxygen by oxidation of superoxide by holes in the Ninduced midgap state. This complex mechanism involving reactive oxygen species may yield a small visible light photocatalytic activity which if superimposed upon the band gap activity could yield an overall improvement in the solar efficiency (assuming no loss of the band gap activity). Photocurrent response spectra should not be used as a direct measure of photocatalytic activity for nonmetal-doped titania (or metal-ion-doped titania) because of the complexity of the system involving cycling of electrons in defect states and the oxygen reduction pathway to ROS. Photopotential spectra give a better correlation to the absorption spectra for the materials. More work is needed to compare the photocatalytic activity spectra with the photopotential and photocurrent spectra.
The refilling of holes trapped on nitrogen centers is a dynamic process cycling electrons trapped on defect sites within the band gap.34 Under sub-band gap irradiation, the percentage of empty acceptor states (trapped holes) inside the band gap will increase, relative to band gap excitation, and in doing so will increase the probability of eCB− trapping by these states. This mechanism would explain the decreased lifetime of eCB−, as observed by Yamanaka and Morikawa,29 resulting in a lower photocurrent and a more positive onset potential for anodic current under visible light excitation (as compared to band gap excitation), as was observed in this work. The presence of fluorine as a dopant induces formation of shallow Ti3+ donor levels a few tenths of electron volts below the conduction band.36 Calculations suggest that the codoping N-TiO2 with fluorine reduces the number of oxygen vacancies as compared to nitrogen doping alone.37 Therefore, we expect that visible light excitation will lead to promotion of electrons from the nitrogen midgap state to the conduction band. The empty N states can be repopulated by electron transfer from Ti3+ states, which can then accept conduction band electrons. Oxygen vacancies (although reduced in number because of F doping) can also transfer electrons to repopulate the empty N states, and conduction band electrons can then repopulate the oxygen vacancy site. Thus, there is a cycling of excited electrons from the N midgap state, to the conduction band, and then to Ti3+ or Ovac with eventual repopulation of the excited empty N states (Figure 10). The negative shift of the open-circuit
■
CONCLUSIONS N,F-TiO2 has been previously shown to be effective for the degradation of organic contaminants in water, for example, cyanotoxins, under visible light irradiation. In this work, we used photoelectrochemistry to probe the mechanism of visible light activity as compared to UV activity. The measurement of photocurrent for the N,F-TiO2 showed very small response under polychromatic visible irradiation as compared to band gap irradiation. The onset potential for anodic current was more positive under visible irradiation than under UV irradiation because of a less negative shift in the Fermi energy level. The N,F-TiO2 gave a very small visible light photocurrent response at fixed potential under monochromatic irradiation, as compared to band gap irradiation. The IPCE% measured under visible irradiation was a small fraction of that measured under band gap excitation. The addition of hole acceptors, KI and hydroquinone, gave a small increase in photocurrent, consistent with earlier reports. The addition of KO2 as a source of superoxide yielded a small increase in the visible light photocurrent which is evidence of the oxidation of superoxide by the N-induced midgap state to yield singlet oxygen. Opencircuit photopotential measurements gave a better correlation to the optical absorption spectra for the N,F-TiO2. Our results in combination with recent literature in the area suggest that visible light activity is predominantly due to reduction reactions involving conduction band electrons with molecular oxygen
Figure 10. Visible light excitation of N,F-TiO2 and refilling of empty N states by electron transfer from either Ti3+ or Ovac.
potential under visible light irradiation is evidence that excitation of electrons occurs to higher energy levels, and recombination (repopulation) of the N states does not occur instantly with a net negative shift of the Fermi energy. The less negative open circuit photopotential in the presence of oxygen is evidence that some of the electrons excited to the conduction band can be passed on to oxygen to form superoxide before being trapped by Ti3+ or Ovac. The subsequent reduction reactions involving conduction band electrons can lead to the formation of hydrogen peroxide and hydroxyl radical. Also, there is some evidence of oxidation of superoxide which would lead to the formation of singlet oxygen. 12213
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
The Journal of Physical Chemistry C
Article
Dioxide Photocatalysts For Environmental Applications. Appl. Catal., B 2012, 125, 331−349. (4) Herrmann, J. Titania-based True Heterogeneous Photocatalysis. Environ. Sci. Pollut. Res. 2012, 19, 3655−3665. (5) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141−145. (6) Kumar, S. G.; Devi, L. G. Review on Modified TiO 2 Photocatalysis Under UV/Visible Light: Selected Results and Related Mechanisms on Interfacial Charge Carrier Transfer Dynamics. J. Phys. Chem. A 2011, 115, 13211−13241. (7) Sato, S. Photocatalytic Activity of NOx-Doped TiO2 in the Visible Light Region. Chem. Phys. Lett. 1986, 123 (1−2), 126−128. (8) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. A Review and Recent Developments in Photocatalytic Water-splitting using TiO2 for Hydrogen Production. Renewable Sustainable Energy Rev. 2007, 11, 401−425. (9) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269−271. (10) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. Oxidative Power of Nitrogen-Doped TiO2 Photocatalysts under Visible Illumination. J. Phys. Chem. B 2004, 108, 17269−17273. (11) Viswanathan, B.; Krishanmurthy, K. R. Nitrogen Incorporation in TiO2: Does It Make a Visible Light Photo-Active Material? Int. J. Photoenergy 2012, 269654 DOI: 10.1155/2012/269654. (12) Daimon, T.; Nosaka, Y. Formation and Behavior of Singlet Molecular Oxygen in TiO2 Photocatalysis Studied by Detection of Near-Infrared Phosphoresence. J. Phys. Chem. C 2007, 111, 4420− 4424. (13) Nakamura, R.; Tanaka, T.; Nakato, Y. Mechanism for Visible Light Responses in Anodic Photocurrents at N-Doped TiO2 Film Electrodes. J. Phys. Chem. B 2004, 108, 10617−10620. (14) Kuznetsov, V. N.; Serpone, N. On the Origin of the Spectral Bands in the Visible Absorption Spectra of Visible-Light-Active TiO2 Specimens Analysis and Assignments. J. Phys. Chem. C 2009, 113, 15110−15123. (15) Serpone, N. Is the Band Gap of Pristine TiO2 Narrowed by Anion and Cation Doping of Titanium Dioxide in Second-Generation Photocatalysts? J. Phys. Chem. B 2006, 110, 24287−24293. (16) Wang, Y.; Feng, C.; Zhang, M.; Yang, J.; Zhang, Z. Enhanced Visible Light Photocatalytic Activity of N-doped TiO2 in Relation to Single-Electron-Trapped Oxygen Vacancy and Doped-Nitrogen. Appl. Catal., B 2010, 100, 84−90. (17) Hamilton, J. W. J.; Byrne, J. A.; McCullagh, C.; Dunlop, P. S. M. Electrochemical Investigation of Doped Titanium Dioxide. Int. J. Photoenergy 2008, 631597 DOI: 10.1155/2008/631597. (18) Lo, H.-H.; Gopal, N. O.; Ke, S.-C. Origin of Photoactivity of Oxygen-deficient TiO2 Under Visible Light. Appl. Phys. Lett. 2009, 95, 083126 DOI: 10.1063/1.3216585. (19) Rengifo-Herrera, J. A.; Pierzchała, K.; Sienki, A.; Forro, L.; Kiwi, J.; Pulgarin, C. Abatement of Organics and Escherichia coli by N, S Co-doped TiO2 Under UV and Visible Light. Implications of the Formation of Singlet Oxygen (1O2) Under Visible Light. Appl. Catal., B 2009, 88, 398−406. (20) Choi, H.; Antonio, M. G.; Pelaez, M.; De La Cruz, A. A.; Shoemaker, J. A.; Dionysiou, D. D. Mesoporous Nitrogen-Doped TiO2 for the Photocatalytic Destruction of the Cyanobacterial Toxin Microcystin-LR Under Visible Light Irradiation. Environ. Sci. Technol. 2007, 41 (21), 7530−7535. (21) Pelaez, M.; de la Cruz, A. A.; Stathatos, E.; Falaras, P.; Dionysiou, D. D. Visible Light Activated N-F-Codoped TiO2 Nanoparticles for the Photocatalytic Degradation of Microcystin-LR in Water. Catal. Today 2009, 144, 19−25. (22) Barndok, H.; Pelaez, M.; Han, C.; Platten, W. E.; Campo, P.; Hermosilla, D.; Blanco, A.; Dionysiou, D. D. Photocatalytic Degradation of Contaminants of Concern with Composite NF-TiO2 Films Under Visible and Solar Light. Environ. Sci. Pollut. Res. 2013, 20, 3582−3591.
leading to the formation of reactive oxygen species. The relatively low visible light photocurrent, but with observed open-circuit photopotential, suggests rapid trapping of conduction band electrons by states below the conduction band. It is unlikely that holes in the midgap state play a dominant role in the oxidation of organics at the interface, but the N-induced midgap state is positive enough to oxidize superoxide radical anion to yield singlet oxygen. This complex mechanism involving reactive oxygen species may yield a small visible light photocatalytic activity which if superimposed upon the band gap activity could yield an overall improvement in the solar efficiency, assuming no loss of the band gap activity. Photocurrent response spectra should not be used as a direct measure of photocatalytic activity for nonmetaldoped titania (or metal-ion-doped titania) because of the complexity of the system involving cycling of electrons in defect states and the oxygen reduction pathway to ROS. Photopotential spectra give a better correlation to the absorption spectra for the materials. More work is needed to compare the photocatalytic activity spectra with the photopotential and photocurrent spectra.
■
ASSOCIATED CONTENT
S Supporting Information *
Plot of the measured transmission through 420 and 435 nm cutoff filters as a function of wavelength and plot of monochromatic irradiation intensity (photon flux) as a function of wavelength used in this study. This material is available free of charge via the Internet at http://pubs.acs.org
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through equal contributions by all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was funded as part of a collaborative project “Degradation Mechanism of Cyanotoxins Using Novel Visible Light-Activated Titania (TiO2) Photocatalysts” funded under the U.S.−Ireland Initiative as follows: Department for Employment and Learning Northern Ireland (University of Ulster, DELNI USI 021), The National Science Foundation (University of Cincinnati and Florida International University (NSF CBET-Award1033317)), and Science Foundation Ireland (DIT and Institute of Technology Sligo, Ireland (SFI grant number 10/US/I1822)).
■
REFERENCES
(1) Rajeshwar, K. Solar Energy Conversion and Environmental Remediation Using Inorganic Semiconductor Liquid Interfaces: The Road Traveled and the Way Forward. J. Phys. Chem. Lett. 2011, 2, 1301−1309. (2) Agrios, A. G.; Pichat, P. State of the Art and Perspectives on Materials and Applications of Photocatalysis Over TiO2. J. Appl. Electrochem. 2005, 35, 655−663. (3) Pelaez, M.; Nolan, N. T.; Pillai, S. C.; Seery, M. K.; Falaras, P.; Kontos, A. G.; Dunlop, P. S. M.; Hamilton, J. W. J.; Byrne, J. A.; O’Shea, K.; et al. A Review on the Visible Light Active Titanium 12214
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
The Journal of Physical Chemistry C
Article
(23) Pelaez, M.; de la Cruz, A. A.; O’Shea, K.; Falaras, P.; Dionysiou, D. D. Effects of Water Parameters on the Degradation of MicrocystinLR Under Visible Light-Activated TiO2 Photocatalyst. Water Res. 2011, 45, 3787−3796. (24) Pelaez, M.; Falaras, P.; Likodimos, V.; Kontos, A. G.; de la Cruz, A. A.; O’Shea, K.; Dionysiou, D. D. Synthesis, Structural Characterization and Evaluation of Sol−Gel-Based NF-TiO2 Films with Visible Light-Photoactivation for the Removal of Microcystin-LR. Appl. Catal., B 2010, 99, 378−387. (25) Kang, Q.; Cao, J.; Zhang, Y.; Liu, L.; Xu, H.; Ye, J. Reduced TiO2 Nanotube Arrays for Photoelectrochemical Water Splitting. J. Mater. Chem. A 2010, 1, 5766−5774. (26) Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y. Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 3026−3033. (27) Kim, M. S.; Jo, W. J.; Lee, D.; Baeck, S.-H.; Shin, J. H.; Lee, B. C. Enhanced Photocatalytic Activity of TiO2 Modified by e-Beam Irradiation. Bull. Korean Chem. Soc. 2013, 34, 1397−1400. (28) Ohtani, B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R. What is Degussa (Evonik) P25? Crystalline Composition Analysis, Reconstruction From Isolated Pure Particles and Photocatalytic Activity Test. J. Photochem. Photobiol., A: Chem. 2010, 216 (2−3), 179−182. (29) Yamanaka, K.; Morikawa, T. Charge-Carrier Dynamics in Nitrogen-Doped TiO2 Powder Studied by Femtosecond TimeResolved Diffuse Reflectance Spectroscopy. J. Phys. Chem. C 2012, 116, 1286−1292. (30) Sivula, K. Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis. J. Phys. Chem. Lett. 2013, 4, 1624−1633. (31) Rensmo, H.; Lindström, H.; Soderaren, S.; Willstedt, A.-K.; Solbrand, A.; Hagfeldt, A.; Lindquist, S.-E. Photocurrent Losses in Nanocrystalline/Nanoporous TiO2 Electrodes Due to Electrochemically Active Species in the Electrolyte. J. Electrochem. Soc. 1996, 143, 3173−3178. (32) Lin, H.; Rumaiz, A. K.; Schulz, M.; Wang, D.; Rock, R.; Huang, C. P.; Shah, S. I. Photocatalytic Activity of Pulsed Laser Deposited TiO2 Thin Films. Mater. Sci. Eng., B 2008, 151, 133−139. (33) Beranek, R.; Kisch, H. Surface-modified Anodic TiO2 Films for Visible LightPhotocurrent Response. Electrochem. Commun. 2007, 9, 761−766. (34) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Di Valentin, C.; Pacchioni, G. Origin of Photoactivity of Nitrogen-Doped Titanium Dioxide under Visible Light. J. Am. Chem. Soc. 2006, 128, 15666−15671. (35) Barolo, G.; Livraghi, S.; Chiesa, M.; Paganini, M. C.; Giamello, E. Mechanism of the Photoactivity under Visible Light of N-Doped Titanium Dioxide. Charge Carriers Migration in Irradiated N-TiO2 Investigated by Electron Paramagnetic Resonance. J. Phys. Chem. C 2012, 116, 20887−20894. (36) Di Valentin, C.; Pacchioni, G.; Selloni, A. Reduced and n-Type Doped TiO2: Nature of Ti3+ Species. J. Phys. Chem. C 2009, 113, 20543−20552. (37) Di Valentin, C.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Czoska, A. M.; Paganini, M. C.; Giamello, E. Density Functional Theory and Electron Paramagnetic Resonance Study on the Effect of N-F Codoping of TiO2. Chem. Mater. 2008, 20, 3706−3714.
12215
dx.doi.org/10.1021/jp4120964 | J. Phys. Chem. C 2014, 118, 12206−12215
