LETTER pubs.acs.org/JPCL
Role of Water Oxidation Catalyst IrO2 in Shuttling Photogenerated Holes Across TiO2 Interface Benjamin H. Meekins and Prashant V. Kamat* Radiation Laboratory and Departments of Chemistry and Biochemistry and Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States
bS Supporting Information ABSTRACT: Iridium oxide, a water oxidation cocatalyst, plays an important role in mediating the hole transfer process of a UV-irradiated TiO2 system. Spectroscopic identification of trapped holes has enabled their characterization in colloidal TiO2 suspension and monitoring of the transfer of trapped holes to IrO2. Titration of trapped holes with potassium iodide yields an estimate of three holes per particle during 7 min of UV irradiation of TiO2 suspension in ethanol containing 5% acetic acid. The hole transfer to IrO2 occurs with a rate constant of 6 105 s1. Interestingly, IrO2 also catalyzes the recombination of trapped holes with reduced oxygen species. The results discussed here provide a mechanistic and kinetic insight into the catalytic role of IrO2 in the photogenerated hole transfer process. SECTION: Energy Conversion and Storage
T
he semiconductor-assisted water splitting reaction has dominated photocatalytic research for the past few decades.15 Although precious metals such as Pt promote the production of hydrogen,610 finding a suitable cocatalyst for oxygen generation at the photoanode remains a challenge.11 Metal oxides such as IrO2,12,13 RuO2,1416 and cobalt phosphate17,18 have been shown to promote water oxidation at the semiconductor/electrolyte interface with high turnover rates. In addition, direct photoexcitation of these cocatalysts (e.g., IrO2) may also play an important role in the oxidation of water.19 The semiconductor-assisted photocatalytic water splitting scheme (Scheme 1) shows the transfer of electrons and holes to Pt and IrO2 cocatalysts that promote the reduction and oxidation processes. The purpose of having such catalysts is two-fold: (i) to improve charge separation and (ii) to decrease the overpotential for corresponding redox processes. Whereas most studies of photocatalytic water oxidation are focused on developing new strategies to extend the photoresponse into the visible by using a sensitized dye2022 or short bandgap semiconductor,2326 less attention has been paid to the role of cocatalysts in water oxidation. Our recent studies have focused on understanding mechanistic and kinetic details of electron transfer and charge equilibration processes in semiconductor metal systems.2733 In a previous study,34,35 it was shown that OH• radical interaction with TiO2 and hole trapping produces surface bound species with similar spectral identity. The trapped holes exhibit a characteristic absorption maximum around 360 nm. Other interfacial charge transfer studies have also shed light onto interfacial charge transfer processes.3644 The recent thrust to design oxygen generation cocatalysts in many laboratories has prompted us to better understand how r 2011 American Chemical Society
Scheme 1. Photoinduced Charge Separation in a TiO2 Semiconductor Particle Coupled to Pt and IrO2 Co-Catalyst Particlesa
a
et and ht represent trapped electrons and holes, respectively.
oxides such as IrO2 intercept photogenerated holes from semiconductor nanoparticles and mediate the hole transfer process. The steady-state and transient absorption studies that highlight the mediating role of IrO2 in the interfacial hole transfer process are described. Characterization of Trapped Holes. TiO2 colloidal suspensions prepared in ethanol undergo charge separation when illuminated with UV light (300350 nm). The photogenerated charge carriers mostly undergo charge recombination, but a small fraction of the electrons are trapped at Ti4+ sites to produce Ti3+ centers. Holes Received: June 23, 2011 Accepted: August 24, 2011 Published: August 24, 2011 2304
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Figure 1. (a) 16 mM TiO2 colloidal solution (5/95 vol % acetic acid/ethanol) after illumination under N2 atmosphere, (b) absorbance spectrum of trace a after equilibration with air, and (c) 16 mM TiO2 colloidal solution (ethanol solution) after illumination under a N2 atmosphere.
are usually scavenged by the ethanol. Interestingly, if acetic acid is present during the synthesis of the TiO2 colloidal suspension, then it introduces surface defects to trap holes. This aspect of electron and hole trapping was discussed in a previous study.45 These trapped holes usually exist as oxygen anion radicals covalently bound to titanium ions, TiIVOTiIVO•.4648 Electron paramagnetic resonance (EPR) studies have shown that the nature of these trapped holes in alcoholic medium exhibits association with alcoholic groups.49,50 The quintet EPR signal of the trapped holes was ascribed to TiOTiO•CHCH3 on the TiO2 surface. The reactions that follow UV excitation of TiO2 colloids are given below (reactions 14)
trapped holes are less energetic than valence band holes (as evidenced by their inability to oxidize ethanol). However, these trapped holes are part of the overall oxidative scheme as they remain in equilibrium with charge separation and hole discharge at the electrolyte interface during steady-state irradiation. Understanding the properties of IrO2 by capturing trapped holes provides an opportunity to elucidate its behavior as a water oxidation catalyst. To obtain further quantitative information, we employed iodide oxidation reaction to titrate the trapped holes in UVirradiated TiO2. KI solution is colorless, but upon oxidation, it provides an intense dark-orange-brown color as a result of I3 formation. The oxidation of I under semiconductor52 as well as sonolysis53 conditions has already been documented (reactions 5 and 6)
TiO2 f TiO2 ðeÞ þ TiO2 ðhÞ
ð1Þ
TiO2 ðeÞ þ TiO2 ðhÞ
ð2Þ
I þ ht f I• ðI Þ f I2 •
ð5Þ
TiO2 ðeÞ f TiO2 ðet Þ
ð3Þ
2I2 • f I3 þ I
ð6Þ
TiO2 ðhÞ f TiO2 ðht Þ
ð4Þ
Figure 1 shows absorbance spectra and corresponding color changes following the UV-irradiated TiO2 suspension in ethanol under different experimental conditions. If the TiO2 colloids are prepared in ethanol containing 5% acetic acid, then we observe a green color upon UV irradiation of deaerated suspension. The absorption spectrum a in Figure 1 shows an absorption band around 360 nm and a broad band in the 500800 nm region. We assign these two bands to trapped holes and trapped electrons, respectively. Upon exposure to air, the absorption in the red region (500800 nm) disappears, leaving behind only the 360 nm absorption band (spectrum b in Figure 1). The insensitivity of the 360 nm band to oxygen confirms its origin to be from the trapped holes. The fact that these trapped holes survive in ethanol medium demonstrates that they have less oxidizing power than the valence band holes. The UV irradiation of deaerated TiO2 suspension in ethanol produces blue color that is characteristic of trapped electrons (spectrum c in Figure 1). This color disappears if we expose the solutions to air or introduce an electron acceptor species. In a previous study, we have confirmed hundreds of trapping sites per particle and extracted an extinction coefficient of 760 M1 cm1 for trapped electrons.51 The spectral changes in Figure 1 indicate that the presence of acetic acid in ethanol is crucial in creating defects necessary for trapping holes. It should be noted that the
The initially formed I• radical quickly complexes with excess I to produce I2• radical (reaction 5), which in turn is quickly disproportionate to produce I3 (reaction 6). The details of the chemistry of II bond formation and breaking can be found elsewhere.54 Thus, the formation of each I3 species corresponds to two trapped holes in TiO2. Figure 2A shows the absorption spectra recorded prior to and following the UV irradiation of TiO2 suspension in ethanol/ acetic acid (spectra a and b). Only in the case of TiO2 suspension in ethanol/acetic acid do we observe the absorption band at 360 nm. A known amount of KI solution was added to both of these suspensions, and the spectra were recorded. Whereas little change in the absorption is seen in unirradiated samples (spectrum a0 in Figure 2A), the UV-irradiated, air-exposed (spectrum b0 in Figure 2A) suspension shows absorption corresponding to I3. It should be noted that the spectral shape of trapped holes appears to be similar to that of I3, but the peak position is separated by ∼15 nm. The identity of the spectral band was also independently confirmed from the oxidation of KI with H2O2. (See the Supporting Information.) These experiments further provide evidence of the oxidative nature of hole trap sites (corresponding to absorbance band at 360 nm) in TiO2 colloidal suspension in ethanol/acetic acid. We further extended the iodide oxidation to determine the concentration of trapped holes in TiO2 suspension. The aerated TiO2 colloidal suspension in ethanol/acetic acid was irradiated 2305
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Figure 2. (A) Absorbance of TiO2 (a) before and (b) after UV illumination under a N2 atmosphere and then exposed to air. The spectra (a0 ) and (b0 ) correspond to the changes following the addition of 170 μL of 60 mM KI to the solutions of a and b, respectively (final sample volume 3.17 mL). (B) Triiodide concentration (left axis) and corresponding number of trapped holes per TiO2 nanoparticle (right axis) versus measured absorbance of the trapped holes at 360 nm. The reciprocal of the slope of the linear fit (black line) corresponds to the extinction coefficient of the trapped holes.
Figure 3. TEM images of (A) 2 to 3 nm IrO2 nanoparticles and (B) an individual IrO2 nanoparticle. (C) (a) Absorption spectra of trapped holes (obtained after UV irradiation of 16 mM TiO2 solution in air) and (bf) after addition of IrO2 nanoparticle solution to sample a. The concentrations of IrO2 corresponded to (b) 0.04, (c) 0.08, (d) 0.12, (e) 0.157, and (f) 0.193 mM. (g) Absorbance spectrum of 0.193 mM IrO2 with 16 mM TiO2 in acetic acid/ethanol taken as the reference.
for different times (0 to 7 min in 1 min intervals). The absorbance at 360 nm corresponding to trapped holes was noted. A known amount of KI solution was then added to each of these samples, and the suspension was allowed to equilibrate for 1 min before the absorbance of I3 was recorded. Figure 2B shows the relationship between trapped hole absorbance and tri-iodide concentration as well as the relationship between the absorbance of the hole peak and the number of trapped holes per TiO2 nanoparticle. From the slope of this plot, we estimate the extinction coefficient of trapped hole in TiO2 to be 11 235 M1 cm1. In previous studies, we have estimated the particle size of TiO2 colloids in ethanol to be around 35 nm. The number of holes trapped per TiO2 particle for this range of diameter was estimated to be between 1 and 3. (See the Supporting Information for the details of this calculation.) Hole Transfer to IrO2 Water Oxidation Catalyst. One of the major hurdles in achieving photocatalytic water oxidation at the semiconductor interface is the generation of O2. Significantly lower oxidation rates for the proton-coupled four-electron reaction limit the overall quantum efficiency of the water splitting process. IrO2 nanoparticles and iridium-based complexes are often coupled to semiconductor photocatalysts to promote the
hole transfer during water oxidation.55,56 Accordingly, it is of interest to obtain mechanistic insights into the hole transfer between TiO2 and IrO2. Figure 3A,B shows TEM images of IrO2 nanoparticles prepared using a literature method. (See the Experimental Methods.) The particles are 2 to 3 nm in diameter with a high degree of crystallinity. Figure 3C shows the influence of the addition of IrO2 to a previously irradiated TiO2 suspension under hole-trapping conditions. With increasing IrO2 addition, we see a gradual decrease in the 360 nm peak until the two systems attain hole equilibration. At this point, the two systems (viz., TiO2 and IrO2) remain at an isoenergetic point. Further addition of IrO2 does not cause any more decrease in the absorption at 360 nm. A nearly 20% decrease in the absorption peak reflects the ability of IrO2 to capture a fraction of the trapped holes from TiO2. The ability of IrO2 to shuttle the photogenerated holes across the photocatalyst interface is important in facilitating water oxidation. The inability of excess IrO2 to scavenge completely trapped holes likely arises from (i) attaining a charge equilibration between TiO2 and IrO2 or (ii) inaccessibility of holes caught in deep traps. To estimate further the time scale of the hole transfer process, we carried out nanosecond transient absorption measurements. 2306
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Figure 4. (A) Transient absorption spectra recorded 2 μs after 308 nm excimer laser pulse excitation of 23 mM TiO2 (5% acetic acid/95% ethanol) containing (a) 0, (b) 0.02, (c) 0.04, and (d) 0.06 mM IrO2. (B) Normalized absorptiontime profiles recorded at 380 nm following the excitation of 23 mM TiO2 (a) without IrO2 and (b) with 0.06 mM IrO2.
Figure 4A shows transient absorption spectra recorded immediately after the 308 nm laser pulse excitation of TiO2 colloids in ethanol/acetic acid. The difference absorption spectrum recorded in the absence of IrO2 shows a peak at 380 nm and a broad absorption in the IR. As discussed in the previous section, these peaks correspond to trapped holes and trapped electrons, respectively. Previous studies have shown that trapping of these carriers occurs on the picosecond-to-nanosecond time scale.5760 The difference absorption spectrum recorded following the addition of IrO2 shows significantly lower absorption. Figure 4B shows absorbancetime profiles monitored at 380 nm in the absence and presence of IrO2. The absorbance at 380 nm remains steady during the time scale of 15 μs in the absence of IrO2. However, a rapid decay in the absorbance (380 nm) is seen in the presence of IrO2. The lifetime of 1.53 μs obtained from the first-order kinetics yields a rate constant of hole transfer as 6 105 s1. It should be noted that the hole transfer estimated from the nanosecond laser flash photolysis experiment should be regarded as a lower limit as the calculated rate constant corresponds only to trapped holes in TiO2. Direct hole transfer from the valence band of TiO2 is expected to be significantly higher than this value. It should also be noted that even this lower limit time scale is orders of magnitude faster than measured electron transfer from IrO2 to photoactive species such as dyes (∼2.2 ms),20 making this alternative reaction path significantly competitive with the desired reaction path.
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Figure 5. (A) Absorbance spectra of trapped holes following (a) 2, (b) 6, and (c) 10 min of UV illumination of aerated 16 mM TiO2 colloidal suspension. (B) Decrease in the absorbance of the trapped holes of the previously irradiated TiO2 suspension (part A, spectrum c) in the presence of 0.193 mM IrO2 with (a) 0, (b) 2, (c) 4, (d) 6, and (e) 8 min of further UV illumination.
Reaction of Trapped Holes with Reduced Oxygen Species. The charge trapping studies presented in the previous sections indicate that the trapped holes are sufficiently protected from recombination with trapped electrons. As shown in the spectrum a in Figure 1, absorption peaks corresponding to both trapped electrons and trapped holes appear as we irradiate the TiO2 suspension under deaerated conditions. If we carry out the same experiment under aerated (or oxygenated) conditions (without IrO2), then only an absorbance increase corresponding to trapped holes could be seen (Figure 5A). As one would expect, electron scavenging by adsorbed oxygen causes no accumulation of electrons. When we UV-irradiate aerated TiO2 suspension in the presence of IrO2, we do not observe the absorption peak corresponding to trapped holes at 360 nm. (See Figure S2 in the Supporting Information.) To establish the role of oxygen for suppressing the accumulation of trapped holes in TiO2/IrO2, we first trapped holes in TiO2 with UV irradiation of an aerated TiO2 suspension and then added a known amount of IrO2 (spectrum a in Figure 5B). If we re-excite this TiO2/IrO2 suspension containing trapped holes with UV light, then we observe a decrease in the absorption peak at 360 nm (Figure 5, spectra be). These results indicate that IrO2 is influential in promoting the discharge of holes under UV irradiation. It should be noted that the reduced oxygen species are generated during UV irradiation as photogenerated electrons are scavenged by the oxygen (reaction 7).61 Through the mediating role of IrO2, these reduced oxygen species are capable of scavenging 2307
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Scheme 2. Charge Transfer Events That Follow UV Excitation of TiO2/IrO2 Containing Trapped Holes (left) in the Presence of Nitrogen and Oxygena
a Yellow-colored solution turns to green under a N2 atmosphere as electrons and additional holes are trapped. However, it turns colorless if O2 is present during UV irradiation as the trapped holes are scavenged by reduced oxygen species.
the accumulated holes and preventing further accumulation of holes (reaction 8). TiO2 ðeÞ þ O2 f TiO2 þ O2 IrO2
O2 þ TiO2 ðhÞ sf TiO2 þ O2
ð7Þ ð8Þ
Because we see accumulation of electrons and holes in the absence of oxygen, followed by the stability of trapped holes in aerated solution, we rule out the possibility of direct participation of O2 in the disappearance of trapped holes in Figure 5B. However, the presence of O2 during UV irradiation can indirectly scavenge the holes via reactions 7 and 8 (Scheme 2). The interesting observation is that the reduced oxygen scavenges trapped holes only when IrO2 is present. Under deaerated conditions in the presence of IrO2, the absorbance peaks corresponding to trapped holes and electrons are both observed when a TiO2 colloidal suspension is excited with UV light. These observations raise an important issue regarding catalyst design for the oxidation of water. Whereas IrO2 is quite effective for transfer of holes at a UV-irradiated semiconductor system and to lower the overpotential for water oxidation, one needs to evaluate its role in promoting the discharge of holes through reactions with other reduced species in solution. EPR studies of water cleavage on TiO2 revealed that hole traps are formed during photolysis.62 This issue becomes more important in systems in which semiconductor/IrO2 photocatalysts are dispersed in solution. The adverse discharge of holes to reduced species can be suppressed if the anodic and cathodic processes are separated (e.g., through a photoelectrolysis cell). Such a configuration allows the separation of the products (viz. H2 and O2) of the water splitting reaction.9 The mechanistic aspects of hole trapping and hole transfer presented in this work provide insights that should prove valuable in the design of efficient water oxidation catalysts.
’ EXPERIMENTAL METHODS Titanium isopropoxide (98%+) was purchased from Acros and stored in a desiccator until use. Potassium hexachloroiridate was purchased from Aldrich. Potassium hydroxide pellets were purchased from Fisher Scientific. KI solutions were made by first dissolving the KI in a small amount of DI water and diluting with ethanol to the desired concentration. TiO2 colloidal suspensions were prepared by hydrolysis of titanium isopropoxide in ethanol or 95/5% ethanol/acetic acid. Titanium isopropoxide was added dropwise to the ethanol or ethanol/acetic acid mixture with vigorous stirring, resulting in a clear solution. IrO2 colloidal suspensions were prepared by a modified hydrolysis method from the Mallouk group.13 In brief, an aqueous solution of K2IrCl6 was heated to 90 °C. The pH was then adjusted to 13 with KOH while stirring the solution. After ∼30 min, a color change to blue signaled the formation of IrO2. The solution was cooled to room temperature and then placed in an ice bath. After cooling further in the ice bath, concentrated nitric acid was added to adjust the pH of the solution to 1. UVvisible absorption spectra were collected on a Shimadzu UV-3101PC in dual beam mode. The slit width was 3.0 nm, and the scan rate was 1.0 nm/sec. The reference sample was TiO2 with the same concentration and volume. For experiments where the interaction of KI or IrO2 with TiO2 was examined, the same aliquot added to the working cell was added to the reference cell to eliminate any scattering effects. Nanosecond flash photolysis transient spectroscopy was conducted using a Lambda Physik Compex 102 excimer laser (308 nm) operated at 5 mJ/pulse. Laser excitation was performed at a right angle to the monitoring white light beam. Transient absorption time profiles were recorded at 380 nm. The detection system consisted of a xenon lamp (1 kW) as the monitoring light source, SPEX 270 M monochromator, and Hamamatsu R955 photomultiplier. The photomultiplier signal was processed by a LeCroy 7200 digital storage oscilloscope and a PC-AT compatible computer. A rectangular quartz (0.5 cm 1 cm) flow cell was used, and a minimum of seven laser shots were averaged to generate the kinetic traces. All experiments were performed in an air atmosphere. Transmission electron microscopy (TEM) of prepared IrO2 colloids was performed on a Titan 80-300 (FEI Company, 300 kV). A drop of IrO2 colloidal solution was allowed to dry on a copper grid for imaging. ’ ASSOCIATED CONTENT
bS
Supporting Information. Absorbance spectra of TiO2 colloids with and without IrO2 under N2, TiO2 colloids irradiated in air atmosphere in the presence of IrO2, and potassium iodide before and after oxidation with hydrogen peroxide. Equations to calculate hole concentrations and holes per TiO2 particle. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT P.V.K. acknowledges the support by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy 2308
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The Journal of Physical Chemistry Letters Sciences of the U.S. Department of Energy through grant DEFC02-04ER15533. B.H.M. acknowledges the fellowship provided by the Center for Sustainable Energy at University of Notre Dame. This is contribution number NDRL 4892 from the Notre Dame Radiation Laboratory.
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