Article pubs.acs.org/Langmuir
Differing Photo-Oxidation Mechanisms: Electron Transfer in TiO2 versus Iron-Doped TiO2 Faith M. Dukes, Elizabeth Iuppa, Bryce Meyer, and Mary Jane Shultz* Laboratory for Water and Surface Studies, Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States S Supporting Information *
ABSTRACT: Low-level iron doping has been found to alter the photo-oxidation mechanism of TiO2 by efficiently activating molecular oxygen. In the absence of iron, TiO2 either reduces water or stores the electron. Quantitatively, 0.5% Fe-TiO2 is nearly three times as efficient as undoped TiO2; 0.1% Fe-TiO2 is twice as efficient. It is found that the efficiency boost primarily results from a more effective use of the absorbed UV photons. Extension of absorption into the visible region due to iron doping increases the efficiency by a factor of only 1.03 compared with UV-only irradiation. Characterization of these small particles reveals that particle size, crystal structure (anatase in all cases), and exposed faces are all insensitive to iron doping. Iron modifies the electronic states of TiO2 by introducing an interband state and enhancing population of a newly identified state at +1.48 eV that acts as an efficient electron−hole recombination site. Activation of molecular oxygen effectively competes with electron−hole pair recombination.
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undoped TiO2. Low-level doping, ∼0.5%, improves efficiency almost three times relative to that of its undoped counterpart. This paper is organized as follows: The Experimental Section describes nanoparticle synthesis and characterization. This is followed by experimental details of the dual-arm reactor designed to facilitate catalyst comparison. The product analysis protocol is outlined. These are followed by a description of the results including characterization of the sensitivity to dissolved oxygen, separation of UV and visible irradiation, particle size, crystal structure, and exposed face analysis. Fluorescence collected along with Raman spectra locates a newly identified interband state that plays a central role in quenching photooxidation. Digestion of the results and support for the conclusions is contained in the Discussion section. The Conclusion section summarizes the results and indicates their significance.
INTRODUCTION
The semiconductor TiO2 has great potential for use in solar energy collection, catalytic, and photocatalytic reactions. TiO2 is environmentally benign; it is used as a colorant in everyday items such as milk, toothpaste, white paint, and creamy salad dressings. Consequently, TiO2 has been the subject of numerous studies,1−4 several focusing on its potential to photocatalytically oxidize organic waste.4−6 While the literature has shown that TiO2 is a suitable agent for wastewater treatment,7−10 it is not currently viable due to its low efficiency when exposed to condensed water. Additionally its wide band gap results in utilization of only a narrow slice of the solar spectrum. Many reports thus focus on improving the effectiveness of TiO2 by extending its band gap into the visible through modification with transition metal elements, nitrogen or carbon.11−14 This report focuses specifically on the transition metal, iron. Iron doping introduces a visible absorption, but it is found that visible photons contribute little to oxidation. Instead, iron improves the photo-oxidation rate by efficiently transferring the photo-generated electron to molecular oxygen. Indeed, iron-doped TiO2 requires molecular oxygen for oxidation. In contrast, undoped TiO2 either reduces water or stores the electron. These mechanisms are quite different. Interestingly, the TiO2 literature differs on whether doping with iron improves15−20 or degrades21−25 the photocatalyst. This report concludes that avoiding the formation of a separate Fe2O3 phase by low-level doping and the use of ultranano (≤2 nm) particles leads to a consistent improvement relative to © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Sample Preparation and Analysis. Nanoparticulate TiO2 and Fe-TiO2 were synthesized using a modified procedure of Wang.26 Briefly, 2.1 mL of cooled titanium tetrachloride (TiCl4) was added dropwise to 600 mL of iron(III) chloride (FeCl3) solution held at 0 °C using a dry ice and ice bath. Sufficient FeCl3 was used to achieve 0.5% and 1.0% Fe:Ti mole ratios. Following addition of TiCl4, hydrolysis continued for 40 min. The resulting solution was dialyzed in a cellulose tubular membrane (Cellusep, 25 Å pore size, 25 °C, in a 40 L bath) to a pH of 2.2 and aged for at least seven days at 0 °C before Received: September 25, 2012 Revised: November 1, 2012
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dx.doi.org/10.1021/la303848g | Langmuir XXXX, XXX, XXX−XXX
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rotary evaporation to dryness. All solutions used UV irradiated ≥18 MΩ nanopure water (Barnstead model D11911). Samples were analyzed with X-ray fluorescence (XRF), micro-Raman, and ultraviolet−visible (UV−vis) spectroscopy. XRF was conducted by the Evans Analytical Group (molybdenum X-ray tube, 47 kV, ∼100 μm beam diameter, 1.5 × 1.5 mm sample) to measure the percentage of Fe, Ti, and other elements incorporated in the particles. Pure titanium and stainless steel were used to quantify elemental Ti, Cl, Fe, and contaminants. Colorimetric measurement27 indicates that iron is in the +3 oxidation state for oxygenated, irradiated solutions and +2 for oxygen-depleted solutions. The crystal structure of TiO2 and Fe-TiO2 was determined by Raman spectroscopy (Jasco NRS-3100 Laser Raman spectrophotometer, 785-nm, 22.1 mW excitation). Spectra were collected using CCD (charge-coupled device) detection (air cooled to −65 °C, 1024 × 128 pixels, 1 cm−1 resolution). The instrument was fitted with a notch filter (cut off 100 cm−1 below excitation). Polypropylene was used as a calibration standard. All spectra were taken at room temperature. Particle size was calculated from the band gap (UV−vis spectroscopy, Jasco model V-570). In measuring the band gap, all solutions contained 0.5 g/L catalyst. Kinetic experiments used a catalyst loading of 0.3125 g/L to ensure saturation of the absorption; methanol was added in 20-fold excess relative to TiO2 to ensure saturation of the particles. Irradiation used a 1000 W, ozone-free xenon lamp (Oriel model 66921). Short-bandpass irradiation used a 365 nm, 50 × 3.5 mm Maxlamp mercury-line filter (Semrock Incorporated) fitted between the sample and the xenon lamp. Visible irradiation used a 400 nm, 50 mm2 long-pass, edge filter (Andover Corporation, transmittance 90% at ≥408 nm; below the TiO2 band gap). A dual-arm photolysis cell consisted of a 3.8 cm diameter by 5 cm long Pyrex cylinder separated lengthwise into two equal compartments by a glass partition. Each arm holds up to 30 mL. One compartment was loaded with undoped TiO2 suspended in water to serve as a reference. UV grade silica windows sealed the cell. The measured light energy at the sample was 750 ± 5 W/m2. A 17.75 cm water filter between the photolysis cell and the lamp limited heating due to infrared radiation. Nanoparticulate solutions were irradiated for 30 min prior to the addition of liquid methanol to eliminate contaminants adsorbed on the TiO2 surface. All TiO2 reference arms were saturated with oxygen gas (ultrahigh purity, Airgas East). Iron-doped nanoparticles were saturated with oxygen or nitrogen (ultrahigh purity, Airgas East) as noted. Each kinetic experiment was run for 100 min with aliquots collected at ten-minute intervals. For nitrogen-purge experiments, irradiation was stopped during the addition of methanol and the solution was purged for an additional 20 min to remove residual oxygen dissolved in the methanol prior to restarting irradiation. Formaldehyde analysis followed a modified EPA standard protocol.28 The following materials were used without further purification: sodium hydrogen phosphate (Fisher Scientific, ACS grade), (2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride (PFBOA) (Sigma Aldrich, 99.0%), difluorobenzene (Sigma Aldrich, 98%), hexane (Fischer Scientific, ACS grade), and 1,2-difluorobenzene (Sigma Aldrich, 98%). Aliquots were treated with 0.75 M sodium hydrogen phosphate to neutralize and precipitate particles. The mixture was centrifuged for 5 min; 0.4 mL of the supernatant solution was transferred to a glass vial, and 2.5 mL PFBOA (0.2 g/L) was added. The PFBOA−H2CO complex was allowed to equilibrate for 8 h, and then extracted with hexane spiked with 1,2-difluorobenzene that served as an internal standard. The complex was analyzed with gas chromatography-mass spectrometry (GC-MS) (Shimadzu model 17A QP5050A MS with Shimadzu CI-50 negative ion detector); parameters include an injection temperature of 250 °C and an interface temperature of 200 °C. The initial oven temperature was held at 50.0 °C for 2.00 min before being ramped to 105 °C at 7.0 degrees/ min. The helium carrier gas (Airgas East) flow was set at 84.4 mL/min, and the detector voltage was 1.5 kV. Charge-to-mass ratios 88, 93, 99, 111, 117, 161, 167, 181, and 182 were analyzed.
Article
RESULTS Photo-Oxidation. There is considerable disagreement in the literature concerning the effect of dopants on the photo efficiency of TiO2.18,21,22,24,29−39 Faced with literature uncertainty, this work first focused on generating a reproducible, undoped starting material. The use of ambient-temperature hydrolysis without sintering or other high-temperature treatments and limiting particle growth to ∼2 nm eliminates batchto-batch efficiency variations. In addition, the dual-arm reactor provides for a standard facilitating run-to-run comparisons: One arm is loaded with a standard sample, the other with the modified catalyst. Simultaneous irradiation ensures that both sample and reference receive the same flux. This work uses photocatalytic oxidation of methanol to formaldehyde to evaluate photo-efficiency. Further, since the primary goal is to compare the photon-to-oxidation conversion efficiency, reactions are run in a photon-limited regime; solutions are saturated with the catalyst, and the catalyst surface is saturated with methanol. Product concentration is thus linear in time. Figure 1
Figure 1. Photo-oxidation rate with 0.5% Fe-TiO2 (⬒) and 1.0% FeTiO2 (red ◓) compared with undoped TiO2 (solid top, green triangle). When irradiated with the full lamp spectrum, 0.5% Fe-TiO2 is 2.95 times more efficient than the undoped photocatalyst and nearly 1.5 times more efficient than 1.0% Fe-TiO2. Each experiment was conducted at least three separate times to produce error bars.
shows the results of a typical kinetic run. Consistent with previously reported data,40 Fe-TiO2 (black squares and red circles) is more efficient than undoped TiO2 (green triangles). Comparing the doping level, 0.5% Fe-TiO2 nanoparticles are more efficient than the 1.0% particles. Specifically, the 0.5% particles are 2.95 times more efficient than undoped particles; 1.0% Fe-TiO2 is just twice as efficient as undoped TiO2. The remainder of this results section presents data that separates efficiency due to the boost resulting from extension of the absorption into the visible region from that due to greater efficiency with UV photons. Additional data characterize the particles and examine the role of molecular oxygen in the reaction mechanism. Doping with iron introduces an absorption in the visible region of the spectrum, so it is instructive to separate improved UV photon efficiency from visible absorption. UV photon efficiency is determined by placing a short-pass filter between the lamp and the sample. The short-band-pass filter (360 to 372 nm: just greater than the bulk anatase band edge, 387 nm) transmits less than 1% for wavelengths longer than 375 nm. Hence, with the short-pass filter, only photons above the undoped band gap are transmitted. The data in Figure 2a show that with UV irradiation, both 0.5% and 1.0% Fe-doped TiO2 B
dx.doi.org/10.1021/la303848g | Langmuir XXXX, XXX, XXX−XXX
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Figure 2. Photo-oxidation with ultraviolet and visible wavelengths for 0.5% Fe-TiO2 (⬒) and 1.0% Fe-TiO2 (red ◓) compared to undoped (solid top, green triangle) particles. (a) A 365 nm mercury-line filter transmits 90% of the light between 360 and 372 nm, just above the bulk anatase band gap of 387 nm. With UV irradiation, 0.5% Fe-TiO2 is 2.88 times more efficient than undoped TiO2 indicating that most of the increase is due to a more efficient conversion of UV photons to oxidations. Note that the concentration scale is expanded by a factor of 3 compared with Figure 1. (b) A 400 nm edge filter transmits 90% at ≥408 nm. Note the 40 times expanded vertical scale compared with Figure 1. Fe-TiO2 (0.5%) is consistently the most efficient. The expanded vertical scale compared with Figure 1 reflects the low absorbance and low efficiency in the longer wavelength region of the spectrum.
where h is Planck’s constant, R the radius, μ the reduced mass of an exciton, ∈o the vacuum permittivity, ∈r (=12)43 the dielectric constant, and e the charge of an electron. For TiO2, the bulk anatase band gap is 3.2 eV (387 nm). The reduced mass was calculated from the effective masses of the electron and hole, which are 10me and 0.8me,43 respectively. The UV− vis spectra (Figure 3) indicates that for undoped TiO2 the band
have increased efficiency (by a factor of 2.88 and 1.88) relative to undoped TiO2. Note that the short-band-pass filter also reduces the UV flux; reduced flux is reflected in the reduced reaction rate for undoped TiO2. Nonetheless, the dual-arm reactor enables ready comparison. Both 1.0% and 0.5% Fe-TiO2 are more efficient than undoped TiO2, thus doped particles utilize ultraviolet photons more efficiently than do undoped particles. As a wide band gap material, TiO2 absorbs very little visible light. Thus, one thrust of TiO2 photocatalytic work entails extension of the band gap into the visible region of the spectrum. To characterize the visible-light oxidation efficiency, a 400 nm edge filter was used. At 400 nm, the absorbance of undoped TiO2 is less than 5%. As a result (Figure 2b), the undoped TiO2 formaldehyde production is almost nonexistent. In contrast, 0.5% Fe-TiO2 is much more efficient than is undoped TiO2. Note that the production rate is less than 3% of the UV photo-oxidation rate. Hence, visible photons are not very effective. Adding visible photons to UV photons provides only a miner boost relative to UV-only irradiation. Characterization. Size. It is challenging to determine the size of very small (
