Article pubs.acs.org/JPCC
Photogenerated Charge Carriers and Paramagnetic Species in (W,N)Codoped TiO2 Photocatalysts under Visible-Light Irradiation: An EPR Study Andrea Folli,*,†,‡ Jonathan Z. Bloh,‡ Eva-Panduleni Beukes,‡ Russell F. Howe,‡ and Donald E. Macphee‡ †
Danish Technological Institute, Gregersensvej 4, 2630 Taastrup, Denmark Department of Chemistry, University of Aberdeen, Meston Building, Meston Walk, AB24 3UE Aberdeen, United Kingdom
‡
S Supporting Information *
ABSTRACT: (W,N)-codoped TiO2 has recently attracted interest due to substantial band gap narrowing in conjunction with good visible-light photocatalytic activities. A complete picture of the fundamental mechanism at the origin of their photoactivity is, however, far from being understood. We present an EPR study on (W,N)-codoped titanias by recording spectra in the dark and under 550 nm visible-light irradiation to identify the major species involved in the formation and migration of photogenerated charge carriers. Interstitial N−lattice O (formally NO2−) paramagnetic groups with and without a specific close range coupling interaction with a neighboring W, that is, [NiO]W● and [NiO]●, respectively, are observed when visible light excites electrons from diamagnetic intraband gap states located at ∼2.30 V versus RHE (formally NO3−) to the conductance band at around −0.05 V versus RHE. The [NiO]W● EPR signal, reported here for the first time, is characterized by a much stronger hyperfine interaction between the unpaired electron and the 14N nucleus than ordinary [NiO]● found in N-doped TiO2, leading to a much higher spin density on the N center. The overall signal also contains superhyperfine coupling of the unpaired electron with the 183W nuclide (natural abundance 14.31%), the only naturally occurring nuclide of W with a nonzero nuclear spin.
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INTRODUCTION Doping TiO2 crystals with p-block elements is a practice that has been systematically adopted to extend the photocatalytic activity of titania samples into the visible-light region of the electromagnetic spectrum. The main goal remains the narrowing of the semiconductor band gap (TiO2 is sensitized by UV radiation) while ensuring significant quantum yields.1,2 Nitrogen doping has without doubt received the greatest interest due to its ability to introduce intraband gap donor states2−5 and ensure acceptable photocatalytic activity when used to decompose hazardous compounds often found as pollutants in contaminated air and water.6−8 The introduction of N in the TiO2 crystal seems to be highly dependent on the synthetic route adopted. High-temperature nitridation treatments of TiO2 often induce N substitution of lattice O.2,4 The new intraband gap states are in this case found to lie ∼0.14 eV above the TiO2 valence band.2 On the contrary, wet chemistry synthetic routes (e.g., sol−gel) preferentially generate interstitial N, chemically bound to a lattice O ion forming an N−O group in the bulk of the solid.2,4,5 This second species is responsible for introducing intraband gap states deeper in the band gap, ∼0.74 eV above the valence band.2,3 Simultaneous (W,N) codoping of TiO2 has recently attracted interest due to substantial band gap narrowing in conjunction with good visible-light photocatalytic activities.9,10 Although © 2013 American Chemical Society
highly promising on the basis of such experimental evidence, a clear and complete picture of the fundamental mechanisms for their photoactivity is far from being understood. As pointed out by Marquez et al.,11 there is still a significant lack of detailed microscopic information about the effect of the codoping on the structure and electronic properties of TiO2. Marquez at al.11 and Ç elik et al.12 recently proposed computational studies to derive crystal and band structure information to complete a structure−photochemistry relationship. Although the data derived surely contribute to improve the knowledge about (W,N)-codoped titanias, their simulations are not representative of all the possible structures. For example, the band structures and density of states calculated refer to substitutional N and do not deal in great detail with interstitial N. Furthermore, these and other computational studies need to be backed up by detailed and precise experimental evidence. We attempt to provide a few elements of this necessary experimental insight. Specifically, we adopt a combination of UV−vis diffuse reflectance spectroscopy, impedance spectroscopy, and electron paramagnetic resonance (EPR) to measure optical band gaps of two (W,N)-codoped TiO2 samples, to Received: August 15, 2013 Revised: September 18, 2013 Published: September 18, 2013 22149
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of the WT_1:1000_400 and WT_1:100_400 samples are shown in Figure 1. Both of the samples clearly show patterns
identify the redox potentials of their band edges and intraband gap states as well as to characterize the photochemistry, including exciton formation and trapping under visible-light illumination.
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EXPERIMENTAL SECTION (W,N)-codoped TiO2 semiconductors were prepared according to a sol−gel route. Titanium isopropoxide (10 mL), Ti(OiC3H7)4 (≥97%, Sigma-Aldrich), was dissolved in 10 mL of anhydrous ethanol. Under constant stirring, a stoichiometric amount of deionized water (milli-Q, 18 MΩ·cm) equal to 5 mL was slowly dropped into the mixture. The white sol formed was aged for 1 h at room temperature under stirring. Subsequently, the sol was buffered at pH 10 by adding 20 mL of NH3·H2O/ NH4Cl buffer solution (5% ammonia, Sigma-Aldrich), and 50 mL of an ammonium tungstate, (NH4)2WO4, solution was slowly dropped into the mixture with a dropping funnel. The ammonium tungstate solution was prepared by dissolving ammonium tungstate salt (BDH Chemicals) in 50 mL of warm deionized water (milli-Q, 18 MΩ·cm). The amount of salt used was calculated to be the stoichiometric amount necessary to obtain either a 1:1000 or 1:100 W:Ti molar ratio in the final product. The resulting sol was aged for 24 h under constant stirring. After Buchner filtration and washing with deionized water (milli-Q, 18 MΩ·cm), the precipitate was dried for 24 h in an oven at 373.15 K and then calcined for 4 h at 673.15 K. Before and after calcination, the solid was accurately ground to a fine powder. To confirm mineralogy and crystallinity, X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance diffractometer equipped to deliver CuKα1 X-ray radiation (1.54 Å) at room temperature. Band-gap information on the synthesized powders was obtained from light absorption measurements using a modular UV−vis diffuse reflectance spectrometer StellarNet EPP2000. Spectra were processed according to the Kubelka−Munk transform approach for indirect semiconductors. The electrochemical impedance spectroscopy (EIS) measurements was performed using a three-electrode cell, the working electrode consisting of the sample ((W,N)-codoped TiO2) coated onto a FTO glass slide, a coiled platinum wire counter electrode, and an Ag/AgCl reference electrode (3 M NaCl, +205 mV vs RHE). The electrodes were fixed in the measurement cell in a way that only the coated part of the FTO glass (circular contact area of 1.77 cm2) was in contact with the 0.1 M sulphuric acid electrolyte. Impedance spectra were collected with a Zahner IM6e potentiostat in a frequency range of 1 × 105 to 1 × 10−1 Hz with 11 steps per decade and amplitude of 10 mV. From the impedance spectra, flat band potentials and hence conductance band edges were derived using Mott−Schottky analysis. EPR spectra were recorded with a JEOL JES-FA200 spectrometer operating at 80 K using an Oxford Instruments liquid-nitrogen cryostat. Before each measurement, the samples were evacuated at room temperature and under high vacuum, ∼10−6 bar. Spectra were recorded in darkness and under in situ irradiation (in the spectrometer cavity) with an Oriel 500 W Xe arc light source, filtered using a 550 nm band-pass filter (Andover Corporation, 550FS80-50 (Q097-05)).
Figure 1. XRD patterns for the (W,N)-codoped TiO2s at different W content: W/Ti = 1/1000 and W/Ti = 1/100. The XRD pattern of an anatase TiO2 is reported as a reference.
typical of the anatase structure; the pure anatase TiO2 pattern is also shown for comparison. The diffuse reflectance UV−vis spectra of the N- and W-codoped samples (Figure 2) exhibit
Figure 2. Optical absorption spectra for the (W,N)-codoped TiO2 at different W content: W/Ti = 1/1000 and W/Ti = 1/100. The optical absorption profile of an anatase TiO2 is reported as a reference.
the typical profile corresponding to the band-gap transition of anatase overlapped with an edge in the visible field between 400 and 550 nm. The absorption edge of pure anatase is again included for reference. The corresponding Tauc plot (Figure 3) for the codoped samples shows that the portion of the optical absorption profile in the UV corresponds to band gaps of 3.15 ± 0.03 and 3.13 ± 0.02 eV for WT_1:1000 and WT_1:100, respectively, and these can be attributed to the electronic transition (Ti 3d ← O 2p) of anatase. The portion of the absorption edge in the visible field corresponds to electron-transition energies of 2.32 ± 0.02 and 2.37 eV ± 0.03 eV for WT_1:1000 and WT_1:100, respectively, attributable to an electronic transition from an
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RESULTS AND DISCUSSION Crystal Phases, Optical Band Gaps, and Band-Edge/ Intraband Gap-State Redox Potentials. The XRD patterns 22150
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Visible-Light Sensitization and Paramagnetic Centers. The specific nature of the visible-light active species can be deduced using EPR. Although N-doped TiO2 is a well-studied and characterized oxide semiconductor,2,3,5,13,14 to the best of our knowledge we are the first to report well-resolved EPR spectra for W/N-codoped anatase. For simplicity, we present and discuss the EPR data for the WT_1:1000_400 sample only, which is also representative of the WT_1:100_400 sample. Figure 5 shows the spectra of the WT_1:1000_400 sample in the dark (a) and under in situ visible light irradiation (550 nm) (b). Upon illumination, the EPR signal increases in intensity by a factor of around three and is modified compared with that from the nonilluminated sample. SIM 32 simulation software was used to fit the spectra in Figure 5. The closeness of fit is shown in Figures 6 and 7 for the dark and illuminated spectrum, respectively, and enables the characteristic EPR parameters to be extracted. The nature of the EPR signals recorded can be described on the basis of three different EPR active centers, presented in Figure 8. The paramagnetic center b1 can be attributed to adsorbed molecular NO,2,14 often detected in the cavities of sol−gel N-TiO2.14 This species exhibits an anisotropic g tensor with rhombic symmetry, gxx ≠ gyy ≠ gzz. The coupling of the unpaired electron with 14N (nuclear spin I = 1; signal multiplicity 2I + 1 = 3) develops a hyperfine structure characterized by 3 × 3 lines. g and A tensors are reported in Table 2. The presented values are in good agreement with those reported elsewhere.2,14 The paramagnetic center b2 consists of an interstitial N chemically bound to a lattice O ion forming a [NiO]●17 group carrying one electron in the singly occupied molecular orbital (SOMO). [NiO]● is effectively an intraband gap NO2− state also exhibiting an anisotropic g tensor with rhombic symmetry, gxx ≠ gyy ≠ gzz. g tensor components and coupling constants are shown in Table 2. The paramagnetic centers b1 and b2 have been discussed in more detail elsewhere.2−5,17 However, a third paramagnetic center, b3, is present that we believe has not previously been reported. It is generated by the coupling of paramagnetic NO2− with W and is denoted [NiO]W●. W possesses five naturally occurring nuclides: 180W (0.12%), 182W (26.50%), 183W (14.31%), 184W (30.64%), and 186W (28.43%); the bracketed
Figure 3. Tauc plots used to derive band gap values for the (W,N)codoped TiO2 at different W content: W/Ti = 1/1000 and W/Ti = 1/ 100. The Tauc plot of an anatase TiO2 is reported as a reference.
intraband gap state, due to the presence of N doping,2,13 to the CB. This is responsible for the yellow color of the codoped samples. Mott−Schottky analysis (see Supporting Information) positions the conductance band edge at −0.05 ± 0.03 and −0.04 ± 0.01 V versus RHE for WT_1:1000_400 and WT_1:100_400, respectively, representing a significant shift of the conduction band when compared with the undoped anatase TiO2 (at −0.20 ± 0.01 V vs SHE). These data, together with the band gap data, allow the construction of the band structure diagram (Figure 4), which shows band gap narrowing (CB−VB) by 0.12 and 0.14 eV for WT_1:1000_400 and WT_1:100_400, respectively, and intraband gap states at 0.83 and 0.76 eV above the valence bands of WT_1:1000_400 and WT_1:100_400, respectively. The positions of these intraband gap states are in line with the interstitial N doping model,2,3 once again confirming the preferential implantation of N in interstitial positions when wet chemistry synthetic routes are employed. A summary of the characterization data is presented in Table 1.
Figure 4. Band structure diagram obtained for the (W,N)-codoped TiO2 at different W content: W/Ti = 1/1000 and W/Ti = 1/100. The band structure of an anatase TiO2 is reported as a reference. 22151
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Table 1. Crystal Phase, Band Edges, and Optical Band Gap Data sample
polymorph
ECB vs RHE (V)
EVB vs RHE (V)
reference anatase WT_1:1000_400 WT_1:100_400
anatase anatase anatase
−0.20 ± 0.01 −0.05 ± 0.03 −0.04 ± 0.01
3.07 ± 0.05 3.10 ± 0.06 3.08 ± 0.03
Figure 5. EPR spectra in the dark (a) and under in situ irradiation with 550 nm wavelength visible light (b) for WT_1:1000_400 sample.
ENOx− vs RHE (V)
band gap (eV)
mid band gap (eV)
2.27 ± 0.05 2.33 ± 0.04
3.27 ± 0.03 3.15 ± 0.03 3.13 ± 0.02
2.32 ± 0.02 2.37 ± 0.03
Figure 7. Experimental (b exp) and simulated (b sim) in situ light irradiation spectra for WT_1:1000_400 sample.
Figure 6. Experimental (a exp) and simulated (a sim) dark spectra for WT_1:1000_400 sample. (The scale has been enlarged three times to show the good fitting between experiment and simulation.)
Figure 8. Simulated single spectra of the three EPR active centers. b1 corresponds to an adsorbed molecular paramagnetic NO;2,14 b2 corresponds to an interstitial N chemically bound to a lattice O ion forming a [NiO]●5,8,15−20 group carrying one electron in the singly occupied molecular orbital; b3 is the signal of the newly recorded [NiO]W● species, that is, chemically equivalent to b2 but influenced by a short-range interaction with W. The intensities in this Figure are relative to the components alone.
term indicates natural abundance. Although five [NiO]W● isotopomers are possible, four are EPR-equivalent due to their zero nuclear spins (180W, 182W, 184W, and 186W). Only 183 W is able to generate super hyperfine structure when coupling with the free NO2− electron. The overall signal of the b3 center is therefore the combination of a close-range WNO2− signal without super hyperfine couplings (related to the four equivalent isotopomers, Figure 9 (b3′)) and a W-close range NO2− signal with super hyperfine splitting, accounting for the 14.31% of 183W that couples to the unpaired electron (Figures 9 (b3″)). g and A tensors are reported in Table 2. Having identified the nature of the EPR active species, we can now explain the change from the dark to the light-irradiated spectra in Figure 5. As in the case of N doping only, a large fraction of the nitrogen centers is diamagnetic before light irradiation, carrying two paired electrons in the highest
occupied molecular orbital. They can be seen as a [NiO]−, intraband gap NO3− states,3 and, similarly, [NiO]W− and intraband gap NO3− states characterized by close range N−W coupling. The dark spectrum (Figure 5a) is therefore dominated by the signal of adsorbed molecular NO2,14 (b1 center). Upon irradiation with visible light, electron transitions NO3− → CB occur, leaving behind a much larger quantity of paramagnetic NO2− states that can be easily detected by EPR. The signal of the adsorbed molecular NO does not significantly change, but the considerable formation of paramagnetic 22152
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0.107
ρ5dz2(183W)
0.006
ρ6s(183W)
0.500
0.707
0.021
0.035
≈0
0.056
ρ2py(14N) ρ2pz(14N) ρ2s(14N)
8 8 50 2.002 2.001 2.000
x = 180, 182, 184, 186. bOverall abundance of the four nuclides together.
[NiO]● and [NiO]W● centers is responsible for the new profile of the EPR spectrum (b in Figure 5) together with the increased intensity. After switching the light off, the intensity of the [NiO]● and [NiO]W● signals decreases very slowly, while, once again, the component assigned to the adsorbed molecular NO does not show a significant change; however, even after more than 30 min in the dark the overall spectrum is still dominated by the [NiO]● and [NiO]W● centers and the initial spectrum before irradiation is not recovered. Hyperfine Coupling and Orbital Spin Densities. The hyperfine tensor A is generated by an isotropic Fermi contact term, Ai, together with an anisotropic spin − dipole (Ad) contribution: A = Ai + Ad
(1)
In the case of 14N ([He] 2s2 2p3), the isotropic hyperfine interaction arises from the 2s orbital, and the dipolar interaction arises from 2p orbitals. Considering a contribution from two different p orbitals, the hyperfine interaction matrix can be written as:
a
W
183
99.632 14N, 14.31
99.632 14N, 85.69b xW [14NiO]xW●a
[14NiO]183W● [NiO]W● b3
99.632 14N [14NiO]● [NiO]● b2
xx yy zz xx yy zz xx yy zz 99.632 14N NO● 14
molecular NO b1
Figure 9. Simulated single EPR profiles of: (b3′) four equivalent [NiO]xW● paramagnetic isotopomers carrying a null spin xW nuclide, x = 180, 182, 184, 186; (b3″) [NiO]183W● paramagnetic isotopomer carrying 183W nuclide, I = 1/2.
xx yy zz
