Article pubs.acs.org/JPCC
Behavior and Energy States of Photogenerated Charge Carriers on Pt- or CoOx‑Loaded LaTiO2N Photocatalysts: Time-Resolved Visible to Mid-Infrared Absorption Study Akira Yamakata,*,†,‡ Masayuki Kawaguchi,† Naoyuki Nishimura,§ Tsutomu Minegishi,§ Jun Kubota,§ and Kazunari Domen*,§ †
Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡
S Supporting Information *
ABSTRACT: Femtosecond to second time-resolved visible to mid-infrared absorption spectroscopy was applied to investigate the behavior of photogenerated electrons and holes on a Pt- or CoOx-loaded LaTiO2N photocatalyst. CoOx-loaded catalyst exhibits the highest activity for water oxidation under visible light (900 K) and the observed depth of the traps are comparable, oxygen vacancies could be responsible for the presence of deep electron traps in LaTiO2N. 3.2. Effects of Pt Loading. The effects of Pt loading on the behavior of photogenerated charge carriers were examined. Pt is the most frequently used cocatalyst for enhancing H 2 evolution.5 As shown in Figure 1B, absorption peaks appeared at 17 000 and 6000 cm−1, as in the case of bare LaTiO2N (Figure 1A). However, their relative intensity was somewhat different: the intensity at 17 000 cm−1 was slightly increased while that at 6000 cm−1 was slightly decreased. These results suggest that the relative density of electrons and holes in LaTiO2N was altered by Pt loading. The detailed decay processes for photogenerated holes and trapped and free electrons were examined by observing the intensity changes at 17 000, 6000, and 2000 cm−1, respectively. Intensity change at 2000 cm−1 involves contribution from trapped electrons. However, by comparison of the change of trapped electrons at 6000 cm−1, the effects on the free electrons can be extracted. As shown in Figure 3, loading of 0.5 wt % Pt slightly accelerates the decay of free electrons at 0−100 μs, while that of holes is slightly decelerated at 0−300 μs. In our previous papers, we proposed that Pt 28 and NiO 32 capture electrons from TiO2 and NaTaO3, respectively. The proposed mechanism is based solely on the accelerated electron decay resulting from cocatalyst loading, since the decay of holes is not measured, and the possibility of accelerated recombination is ignored. In the case of LaTiO2N, it is clear that the acceleration of electron decay is due to electron capture by Pt, because the decay of holes is decelerated. If the loaded Pt enhanced recombination, the decay of holes would also be accelerated. Thus, the loaded Pt decelerates recombination. When electrons are captured by Pt, the holes in LaTiO2N seldom recombine with the electrons in Pt. Therefore, it can be deduced that the loaded Pt enhances the spatial separation of electrons and holes in Pt and LaTiO2N, respectively, and hence prevents recombination. Free electrons show larger variations than trapped electrons, as seen in Figure 3B and Figure 3C). This is expected because free electrons have much higher mobilities than trapped electrons. It is observed that the effects of Pt on LaTiO2N are not as large as in the cases of TiO2 28 and NaTaO3.32 It is known that LaTiO2N has a high activity for O2 evolution when CoOx is loaded (∼900 μmol h−1 at >420 nm irradiation), but the activity for H2 evolution is low despite the Pt loading (20 μmol h−1 at >420 nm irradiation).11 The low activity for H2 evolution is caused by the sluggish and inefficient
Figure 3. Decay of transient absorption of Pt-loaded LaTiO2N photocatalysts in a vacuum. The transient absorption was measured at 17 000 cm−1 (A), 6000 cm−1 (B), and 2000 cm−1 (C), as indicated in the plots. The sample was excited by 355 nm UV laser pulses (6 ns duration, 0.5 mJ cm−2, 1 Hz).
electron transfer to Pt, which in turn could be due to the low mobility of electrons caused by deep trapping. As the Pt loading is increased from 0.5 to 2 wt %, the number of surviving electrons at 0−200 μs decreases monotonically, and the decay of free and trapped electrons within 1 μs is accelerated. This suggests that the electron transfer to Pt becomes faster. However, the decay of holes involves an increasingly complex process as the Pt loading is increased. For instance, the number of holes in 0.5 and 2 wt % Pt loaded samples is almost the same in the early microsecond time range, but the decay of holes is accelerated at 2 wt % Pt loading compared to 0.5 wt % Pt loading. As a result, the number of holes in bare and 2 wt % Pt loaded samples becomes nearly identical after about 30 μs. The decrease in the number of holes at 2 wt % loading is due to the holes captured by Pt, suggesting that the excess Pt induces an undesirable capture of holes by Pt particles. Theoretical calculations predict that Pt loaded on TiO2 can capture both electrons and holes.24 We have reported that Pt particles loaded on n-type GaN capture holes as well as electrons, i.e., some of the Pt particles capture electrons, while others capture holes.40 The selectivity for the capture of electrons vs holes presumably depends on the properties of the Pt particles, which are affected by the amount of Pt loading. It is well-known that loading of excess Pt decreases the overall photocatalytic activity, which is thought to be due to the hindrance of light absorption by the excess Pt. However, the 23900
dx.doi.org/10.1021/jp508233z | J. Phys. Chem. C 2014, 118, 23897−23906
The Journal of Physical Chemistry C
Article
in the microsecond region. On the other hand, the number of surviving trapped and free electrons increased within the time resolution (∼2 μs), and their decay curves are almost parallel across the entire microsecond region. These results suggest that the hole capture by CoOx occurs within the time resolution of the spectrometer (∼2 μs). In this experiment, the possibility of enhanced recombination can be ruled out because the lifetime of electrons becomes longer. The prolonged lifetime of electrons can be explained as follows: when the holes are captured by CoOx, the electrons in LaTiO2N seldom recombine with the holes in CoOx. As a result, the lifetime of electrons becomes longer; i.e., electrons and holes are spatially separated in LaTiO2N and CoOx, respectively. It is noted that the number of free electrons increases more dramatically than does the number of trapped electrons. The increase in the intensity at 2000 cm−1 is more than 10-fold compared to that obtained on the bare catalyst, suggesting that the lifetime of free electrons becomes longer than that of deeply trapped electrons. As a result, a considerable number of electrons survive in the seconds region shown in the inset of Figure 4C. Similarly, a prolongation of the lifetime of electrons is often observed in the presence of hole scavengers, such as MeOH on TiO2.8,27−32 In the case of LaTiO2N, MeOH is not as effective as TiO2 at capturing holes (Figure 2), implying that CoOx is a more effective hole scavenger than MeOH. It is this effective hole capture by CoOx that is responsible for the enhancement of water oxidation. When the amount of CoOx loading increases from 0.5 to 2 wt %, the number of holes decreases monotonically, but the number of surviving electrons at 2 wt % CoOx loading is lower than that at 0.5 wt % CoOx loading. In this case, the size and the oxidation state of loaded CoOx do not differ, but the number of CoOx particles increases. Therefore, these results suggest that excess CoOx induces unfavorable recombination, as in the case of Pt. The hole capture by CoOx is supported by the appearance of a new band at 12 500 cm−1 (800 nm), as shown in Figure 1C. This band is absent for bare and Pt-loaded LaTiO2N (Figure 1A and Figure 1B, respectively), and only appears upon CoOx loading. This band is sensitive to the oxidation states of Co; it appears only when Co is partly oxidized. It is important to note that metallic Co can capture holes, but this does not result in a 12 500 cm−1 peak, as shown in Figure 1D. It has been reported19,20,41 that the color of CoOx changes from yellow to gray upon the oxidation of Co(II) to Co(III). Thus, the appearance of the 12 500 cm−1 peak confirms that the holes are captured by CoOx with an increase in the oxidation number of the Co ion. Oxidation and reduction of Co ions are reversible. Therefore, this highly oxidized CoOx could serve as a reaction center for the oxidation of water. We find that CoOx has an additional function that aids photocatalytic reactions. As shown in Figure 1C, CoOx loading shifts the peak top position of the trapped electrons from 6000 (0.74 eV) to 4000 cm−1 (0.49 eV). Since CoOx-loaded LaTiO2N has no absorption peak around 6000−4000 cm−1 (1600−2500 nm) in the ground state (Figure S3), the peak shift is not due to a change in CoOx but rather a change in the depth of the electron traps in LaTiO2N: the traps become shallower, their depth decreasing from 0.74 to 0.49 eV. Loading of CoOx increases the number of surviving electrons, and hence, the depth of the trap can be shallower by filling the electrons into the deep trap. However, as will be shown later (Figure 5), the peak position of the electron trap is not related to the number of surviving electrons. Therefore, we propose
present results confirm that an undesirable hole transfer from LaTiO2N to the Pt particles is another factor contributing to the decrease in the steady-state activity. 3.3. Effects of CoOx Loading. The effects of CoOx loading on the behavior of electrons and holes were next examined. CoOx is a recently discovered cocatalyst for O2 evolution,18 and its effects on the behavior of charge carriers are not yet fully understood. Figure 1C shows transient absorption spectra for CoOx-loaded LaTiO2N, obtained after band gap photoexcitation. Three peaks appeared at 17 000, 12 500, and 4000 cm−1. The bands at 17 000 and 4000 cm−1 were also observed for bare and Pt-loaded catalysts, but the top peak position for trapped electrons shifted from 6000 to 4000 cm−1 with increasing bandwidth. It is notable that the relative intensities of these two bands were significantly changed: the band intensity at 17 000 cm−1 was decreased, while that at 4000 cm−1 was greatly increased. These results suggest that the relative densities of holes and electrons in LaTiO2N changed significantly upon CoOx loading. The decay processes for photogenerated electrons and holes were examined in detail. As seen in Figure 4A, the loading of 0.5 wt % CoOx accelerated the hole decay: the number of surviving holes was already decreased within the time resolution of the spectrometer (∼2 μs) and stayed at parallel
Figure 4. Decay of transient absorption of CoOx-loaded LaTiO2N photocatalysts in a vacuum. The transient absorption was measured at 17 000 cm−1 (A), 6000 cm−1 (B), and 2000 cm−1 (C), as indicated in the plots. The sample was excited by 355 nm UV laser pulses (6 ns duration, 0.5 mJ cm−2, 1 Hz). The inset shows the decay in the second region (pump pulse repetition rate at 0.01 Hz). 23901
dx.doi.org/10.1021/jp508233z | J. Phys. Chem. C 2014, 118, 23897−23906
The Journal of Physical Chemistry C
Article
shorter. In general, band bending plays an essential role in the separation of charge carriers in photoelectrochemical systems. In addition, we have also reported that the lifetime of electrons became longer when the upward band bending was increased in n-type GaN photoelectrodes.33 However, the behavior of photogenerated charge carriers in powder photocatalysts is not always the same as that in photoelectrochemical systems even in the absence of a bias voltage. In the case of powder photocatalysts, the effects of band bending are often disregarded because the thickness of the space charge layers is not large enough to induce effective band bending. Furthermore, band bending does not always improve the photocatalytic activity, since the enhancement of band bending pushes electrons into the bulk and these confined electrons cannot react with the adsorbed molecules at the surface. Therefore, the upward band bending model cannot always be applied to powder photocatalysts. In the case of the LaTiO2N powder photocatalyst, sufficient band bending would be unlikely to occur because the particle size of the catalysts (few 100 nm length with a porous and brick-like morphology)11 would not be large enough to separate electrons and holes effectively. As described above, the hole-capturing function of CoOx reasonably explains the results obtained in this work. Several authors maintain that this hole-capture process is also essential in photoelectrochemical systems.22,23 Particularly in the case of the TaON electrode,23 CoOx loading not only increases the activity but also prevents self-oxidation of the electrode material. When the lifetime of holes becomes longer in the absence of the capturing process induced by CoOx, the self-oxidation of TaON is more extensive degrading the electrode. Therefore, the hole capture by CoOx is an important function for several photoelectrochemical systems. 3.4. Initial Processes of Electron Capture and Hole Capture by Pt and CoOx. The initial process of charge separation by loaded Pt and CoOx was studied further by using femtosecond TR vis and IR absorption spectroscopy. Since free electrons show more dramatic changes than trapped electrons, the absorption changes at 2000 cm−1 (free electrons) and 17 000 cm−1 (holes) were measured. In the picosecond region, the recombination rate increases as the number of generated electrons increases with laser power (Figure S7), although the rate of electron decay does not have a significant dependence on the number of electrons in the microsecond region (Figure S8: negligible influence of pump power and excitation wavelength). This disparity stems from the difference in the average number of photogenerated charge carriers per photocatalyst particle; more than one electron−hole pair is generated under irradiation with an intense femtosecond pump laser, which subsequently persist in the picosecond region. However, in the microsecond region, most of the charge carriers recombine and less than one pair is present. Thus, the recombination kinetics follows second- and first-order rate equations in the picosecond and microsecond regions, respectively: it was reported by Rothenberger et al. that the recombination rate law switched from second-order to firstorder by decreasing electron−hole pair concentration in colloidal particles of TiO2.42 Hence, the recombination kinetics in the picosecond region cannot be compared directly with that in the microsecond experiments. However, when the laser power was fixed in each experiment, useful information on the charge transfer processes to the cocatalysts could be extracted. In the beginning, the effects of Pt loading on the recombination kinetics were examined, as shown in Figure 5B. However, no
Figure 5. Decay of transient absorption at 17 000 cm−1 (A) and 2000 cm−1 (B) for bare and CoOx- and Pt-loaded LaTiO2N photocatalysts measured in air. The sample was excited by 500 nm laser pulses (90 fs duration, 6 μJ, 500 Hz).
that the change of the electronic structure of the surface defects is responsible for the change of the electron trap depth. CoOx is in contact with the LaTiO2N surface and does not penetrate the bulk. Thus, it is the bonding of CoOx to the surface defects that changes the electron trap depth. As described in section 3.1, deep electron traps consist of defects such as oxygen vacancies. The attachment of CoOx can partially fill the surface defects and hence affect their electronic properties. Especially, we consider the interaction of Co with the coordinately unsaturated Ti atoms is responsible. Reducing the depth of electron traps increases the reactivity of the electrons. Even for the oxidation of water, the reactivity of electrons affects the overall photocatalytic activity. Therefore, the effect of CoOx in increasing the energy level of the trapped electrons also plays an important role in enhancing water oxidation. Similar experiments were conducted by Barroso et al. to study the effects of CoOx (Co-Pi) on the hole decay kinetics on α-Fe2O3 photoelectrodes using TR-vis.19,20 They reported that the lifetime of holes was prolonged by more than 3 orders of magnitude upon CoOx loading. They proposed that CoOx enhanced the upward band bending and then separated the electrons and holes via the space charge layers in the Fe2O3; hole capture by CoOx was not the main factor. This band bending model explains well the enhanced water oxidation on Co-Pi loaded WO3 photoelectrochemical systems.21 However, the behavior of holes in the Fe2O3 photoelectrode is totally opposite to our results, where the lifetime of holes becomes 23902
dx.doi.org/10.1021/jp508233z | J. Phys. Chem. C 2014, 118, 23897−23906
The Journal of Physical Chemistry C
Article
difference was observed: the decay curves for free electrons on bare and Pt-loaded samples (0.5 and 2 wt %) were almost identical. The decay curves for holes were not affected by Pt loading, either (Figure 5A). These results suggest that the electron transfer to Pt does not occur between 0 and 1000 ps. The microsecond experiments revealed that the electron transfer to Pt proceeded relatively slowly, in 0−100 μs. The present results obtained in the picosecond region are consistent with those obtained in the microsecond experiment, where the electron transfer to Pt on LaTiO2N is rather slow. In the case of TiO2, it is reported that the electron transfer to Pt proceeds within 10 ps.43 The disparity of the effects of Pt loading on LaTiO2N and TiO2 is due to the difference in the mobility and energy state of the photogenerated electrons: most of the electrons in TiO2 are free or shallowly trapped,27 while those in LaTiO2N are deeply trapped at the trap sites. Therefore, the mobility of electrons in LaTiO2N is lower than in TiO2. This deep trapping of electrons is what leads to the slow electron transfer to Pt particles. In contrast to Pt, CoOx loading causes drastic changes in the picosecond region. As shown in Figure 5A, the decay of holes is accelerated by CoOx loading: the number of holes just after irradiation at 0 ps is identical for all the catalysts, but the rate of hole decay decreases in a few picoseconds (inset of Figure 5A). The deviations between these decay curves become larger with elapsed time. Furthermore, the decay of holes accelerates as the amount of CoOx is increased from 0.5 to 2 wt %. Electron decay is also greatly affected by CoOx loading: the number of photogenerated electrons at 0 ps is identical for all the catalysts, but the electron decay decelerates within a few picoseconds, and the difference increases with elapsed time (Figure 5B). The number of surviving electrons increases as the amount of loaded CoOx increases from 0.5 to 2 wt %. These results suggest that the hole capture by CoOx starts within a few picoseconds, during which time recombination is prevented. The present picosecond results are consistent with the results of the microsecond experiments, in which hole capture proceeds faster than the time resolution of the spectrometer (∼3 μs). The numbers of holes and electrons are already decreased and increased within the time resolution of the spectrometer, respectively. CoOx loading dramatically increases the activity for water oxidation. The present results confirm that the highly rapid hole capture by CoOx separates electrons and holes effectively and greatly prolongs the lifetime of electrons, bringing them into the second region (inset of Figure 4C). The driving force for the charge capture by the cocatalysts is the light-induced sudden change in the quasi-Fermi level of the photocatalysts. When LaTiO2N is irradiated, the quasi-Fermi level for holes drops below the Fermi level of CoOx, and hole transfer proceeds until equilibrium is reached. In the absence of reactant molecules, holes are not consumed in the reaction, and hence the hole transfer stops at equilibrium. In the microsecond experiment (Figure 4), the decay curves for electrons and holes on bare and CoOx-loaded samples are almost parallel across the entire microsecond region, suggesting that the Fermi level for holes reached equilibrium in the microsecond region. The number of holes that transferred from LaTiO2N to CoOx increased with increasing amount of loaded CoOx, as shown in Figures 3A and 4A; consistent results were obtained in both the picosecond and microsecond experiments. However, the number of electrons exhibits a complex behavior in the picosecond and microsecond regions. In the picosecond region, it increases monotonically with increasing amount of CoOx
loading, while in the microsecond region, it decreases slightly at 2 wt % (compared to 0.5 wt %). These results demonstrate that after 1000 ps, undesired electron transfer to CoOx proceeds at 2 wt % loading. In steady-state water oxidation, the photocatalytic activity first increases and then decreases with increasing amount of loaded CoOx. The present TR experiment suggests that the decrease in activity is due to undesired electron capture by CoOx. As we previously reported,11 a loading of 2 wt % CoOx shows the highest activity. Although unfavorable electron capture proceeds at 2 wt %, more holes are captured by CoOx, which enhances water oxidation. Since the steady-state reaction involves many steps, the optimum amount of cocatalysts is determined by the competition between hole capture and undesired electron capture by the cocatalysts. 3.5. Effects of Coloading of Pt and CoOx. We expected that coloading of Pt and CoOx would further enhance the charge separation and thus increase the photocatalytic activity. Therefore, the behavior of photogenerated electrons and holes on Pt and CoOx coloaded LaTiO2N was examined. In this experiment, 3 wt % Pt-loaded LaTiO2N was prepared, and then CoOx was gradually postloaded. Figure 6 shows the transient absorption spectra of Pt and CoOx coloaded catalysts. When 0.05 wt % CoOx was added to the 3 wt % Pt-loaded sample, the intensity of holes at 17 000 cm−1 slightly decreased and that of trapped electrons at 5000 cm−1 increased (Figure 6B). These results suggest that the holes are captured by CoOx and the
Figure 6. Time-resolved absorption spectra of Pt and CoOx coloaded LaTiO2N photocatalyst irradiated by UV laser pulses (355 nm, 6 ns duration, 0.5 mJ cm−2, 5 Hz) in a vacuum. (A) 3 wt % Pt, (B) 3 wt % Pt with 0.05 wt % CoOx, and (C) 3 wt % Pt with 2 wt % CoOx were loaded on LaTiO2N. 23903
dx.doi.org/10.1021/jp508233z | J. Phys. Chem. C 2014, 118, 23897−23906
The Journal of Physical Chemistry C
Article
lifetime of the electrons becomes longer. However, further loading of CoOx to 2 wt % decreases both the intensity of holes (17 000 cm−1) and electrons (4000 cm−1), as shown in Figure 6C. It is notable that the peak position of the trapped electrons is red-shifted from 6000 to 4000 cm−1 upon coloading with CoOx, although the intensity at 4000 cm−1 is also decreased. This shows that the peak top position is not sensitive to the absorption intensity; i.e., the change in depth does not depend on the population of trapped electrons but rather on the structure of the surface defects, as described in section 3.3 (Figure 1C). The decay processes for photogenerated electrons and holes are next examined in further detail (see Figure 7). It is found
Figure 8. Decay curves of transient absorption for bare, 3 wt % Pt, and CoOx coloaded LaTiO2N photocatalysts. The amount of coloaded CoOx is indicated in the plots. The transient absorption was measured in air at 17 000 cm−1 (A) and 2000 cm−1 (B). The sample was excited by 500 nm laser pulses (90 fs duration, 6 μJ, 500 Hz).
suggests that an unfavorable new recombination process is induced by coloading. CoOx extracts holes from the catalysts, but Pt captures electrons. In the samples used in this study, Pt and CoOx are deposited randomly on the LaTiO2N surface. Therefore, some of the Pt and CoOx particles are deposited in close proximity. In such instances, the probability of a collision between electrons and holes would be higher, which would accelerate recombination. It is often reported that the coloading of cocatalysts decreases the steady-state activity. In the case of LaTiO2N, the activity of O2 evolution decreases from 900 to 390 μmol h−1 upon coloading of 3 wt % Pt on 2 wt % CoOxloaded LaTiO2N. The combination of 3 wt % Pt and 0.05 wt % CoOx results in longer-lived electrons; however, 0.05 wt % CoOx is not enough to maximize O2 evolution (180 μmol h−1).11 Thus, it can be concluded that coloading is not effective in enhancing the steady-state activity. The present timeresolved measurement shows that the effects of cocatalysts on the behavior of charge carriers are complex: in some cases, recombination is accelerated and thus the steady-state activity decreases. It is necessary to control the deposition of cocatalysts. If Pt and CoOx can be deposited separately on opposite surfaces of the catalyst particles,44 charge separation would be enhanced, leading to improved catalytic activity.
Figure 7. Decay curves for transient absorption of 3 wt % Pt and CoOx coloaded LaTiO2N photocatalysts. The amounts of coloaded CoOx were indicated in the plots. The transient absorption was measured in a vacuum at 17 000 cm−1 (A) and 2000 cm−1 (B). The sample was excited by a 355 nm UV laser pulse (6 ns duration, 0.5 mJ cm−2, 1 Hz). The insets show the intensities at 10 μs.
that the intensity of trapped holes decreases monotonically with increasing amount of loaded CoOx from 0 to 2 wt % (Figure 7A), while that of free electrons (2000 cm−1) shows a two-step change: the number of surviving electrons in the microsecond region increases up to 0.05 wt % but decreases upon further loading (Figure 7B). In the picosecond region, a similar dependence on CoOx loading is observed, as seen in Figure 8: the decay of holes accelerates monotonically (Figure 8A), while the decay of electrons shows a two-step change, namely, the lifetime of electrons is prolonged upon 0.05 wt % CoOx loading but further loading to 2 wt % accelerates electron decay (Figure 8B). Since electrons do not transfer to Pt particles within 1000 ps, the accelerated electron decay at 2 wt % CoOx loading
4. CONCLUSION In this work, we have elucidated the effects of CoOx loading on the behavior of photogenerated charge carriers in LaTiO2N photocatalysts by using femtosecond to second time-resolved 23904
dx.doi.org/10.1021/jp508233z | J. Phys. Chem. C 2014, 118, 23897−23906
The Journal of Physical Chemistry C
Article
(4) Maeda, K.; Domen, K. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. C 2007, 111, 7851−7861. (5) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−278. (6) Inoue, Y. Photocatalytic Water Splitting by RuO2-Loaded Metal Oxides and Nitrides with D(0)- and D(10)-Related Electronic Configurations. Energy Environ. Sci. 2009, 2, 364−386. (7) Takata, T.; Furumi, Y.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Photocatalytic Decomposition of Water on Spontaneously Hydrated Layered Perovskites. Chem. Mater. 1997, 9, 1063−1064. (8) Kato, H.; Asakura, K.; Kudo, A. Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped Natao3 Photocatalysts with High Crystallinity and Surface Nanostructure. J. Am. Chem. Soc. 2003, 125, 3082−3089. (9) Hisatomi, T.; Miyazaki, K.; Takanabe, K.; Maeda, K.; Kubota, J.; Sakata, Y.; Domen, K. Isotopic and Kinetic Assessment of Photocatalytic Water Splitting on Zn-Added Ga2O3 Photocatalyst Loaded with Rh2−YCryO3 Cocatalyst. Chem. Phys. Lett. 2010, 486, 144−146. (10) Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen from Water Enhancing Catalytic Performance Holds Promise for Hydrogen Production by Water Splitting in Sunlight. Nature 2006, 440, 295− 295. (11) Zhang, F. X.; Yamakata, A.; Maeda, K.; Moriya, Y.; Takata, T.; Kubota, J.; Teshima, K.; Oishi, S.; Domen, K. Cobalt-Modified Porous Single-Crystalline LaTiO2N for Highly Efficient Water Oxidation under Visible Light. J. Am. Chem. Soc. 2012, 134, 8348−8351. (12) Ma, S. S. K.; Hisatomi, T.; Maeda, K.; Moriya, Y.; Domen, K. Enhanced Water Oxidation on Ta3N5 Photocatalysts by Modification with Alkaline Metal Salts. J. Am. Chem. Soc. 2012, 134, 19993−19996. (13) Kato, H.; Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 2002, 106, 5029−5034. (14) Konta, R.; Ishii, T.; Kato, H.; Kudo, A. Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation. J. Phys. Chem. B 2004, 108, 8992−8995. (15) Domen, K.; Naito, S.; Soma, M.; Onishi, T.; Tamaru, K. Photocatalytic Decomposition of Water-Vapor on an NiO-SrTiO3 Catalyst. J. Chem. Soc., Chem. Commun. 1980, 543−544. (16) Harriman, A.; Pickering, I. J.; Thomas, J. M.; Christensen, P. A. Metal-Oxides as Heterogeneous Catalysts for Oxygen Evolution under Photochemical Conditions. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2795−2806. (17) Meekins, B. H.; Kamat, P. V. Role of Water Oxidation Catalyst IrO2 in Shuttling Photogenerated Holes across TiO2 Interface. J. Phys. Chem. Lett. 2011, 2, 2304−2310. (18) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (19) Barroso, M.; Cowan, A. J.; Pendlebury, S. R.; Gratzel, M.; Klug, D. R.; Durrant, J. R. The Role of Cobalt Phosphate in Enhancing the Photocatalytic Activity of Alpha-Fe2O3 toward Water Oxidation. J. Am. Chem. Soc. 2011, 133, 14868−14871. (20) Barroso, M.; Mesa, C. A.; Pendlebury, S. R.; Cowan, A. J.; Hisatomi, T.; Sivula, K.; Gratzel, M.; Klug, D. R.; Durrant, J. R. Dynamics of Photogenerated Holes in Surface Modified Alpha-Fe2O3 Photoanodes for Solar Water Splitting. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 15640−15645. (21) Seabold, J. A.; Choi, K. S. Effect of a Cobalt-Based Oxygen Evolution Catalyst on the Stability and the Selectivity of PhotoOxidation Reactions of a WO3 Photoanode. Chem. Mater. 2011, 23, 1105−1112. (22) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with "Co-Pi"-Coated Hematite Electrodes. J. Am. Chem. Soc. 2012, 134, 16693−16700.
visible to mid-infrared absorption spectroscopy. We observed for the first time that the loaded CoOx captures holes effectively within a few picoseconds and prolongs the lifetimes of electrons dramatically, up to the order of seconds. Most of the electrons are trapped in the mid-gap states created 0.74 eV below the conduction band in bare LaTiO2N. However, CoOx loading reduces the depth of the electron trap to 0.49 eV. Loading of CoOx on LaTiO2N photocatalysts increases the activity for water oxidation markedly, from 25 to 900 μmol h−1, owing to rapid hole capture by CoOx and the reduction in electron trap depth. In contrast, Pt loading has little effect on the decay kinetics: Pt captures electrons and prolongs the lifetime of holes, but the process is inefficient, occurring within 0−100 μs. This low efficiency of electron capture by Pt is the reason why the activity for H2 evolution (∼20 μmol h−1) does not improve as much as the activity for O2 evolution (∼900 μmol h−1) does upon CoOx loading. Further, the effects of coloading Pt and CoOx were examined. Coloading accelerated the recombination and decreased the steady-state activity. The effects of cocatalysts on carrier dynamics are complex: CoOx and Pt capture holes and electrons, respectively, but undesired recombination is enhanced under high loading and coloading. Femtosecond to second time-resolved vis to mid-IR absorption measurements reveal detailed information about the behaviors and energy states of photogenerated charge carriers, which govern the photocatalytic activities.
■
ASSOCIATED CONTENT
* Supporting Information S
Time-resolved spectrometers, diffuse reflectance spectra of LaTiO2N, femtosecond time-resolved IR spectra, and laser power and wavelength dependence of the decay kinetics of photogenerated electrons. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*A.Y.: e-mail,
[email protected]. *K.D.: e-mail,
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the PRESTO/JST program “Chemical Conversion of Light Energy”. The authors also acknowledge the Grant-in-Aid for Specially Promoted Research (Grant No. 23000009) and Basic Research (B) (Grant No. 23360360) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. One of the authors (A.Y.) thanks the Nippon Sheet Glass Foundation for Materials Science and Engineering for funding support.
■
REFERENCES
(1) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Photocatalysis on TiO2 SurfacesPrinciples, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735−758. (2) Thompson, T. L.; Yates, J. T. Surface Science Studies of the Photoactivation of TiO2New Photochemical Processes. Chem. Rev. 2006, 106, 4428−4453. (3) Hoertz, P. G.; Mallouk, T. E. Light-to-Chemical Energy Conversion in Lamellar Solids and Thin Films. Inorg. Chem. 2005, 44, 6828−6840. 23905
dx.doi.org/10.1021/jp508233z | J. Phys. Chem. C 2014, 118, 23897−23906
The Journal of Physical Chemistry C
Article
(23) Higashi, M.; Domen, K.; Abe, R. Highly Stable Water Splitting on Oxynitride TaON Photoanode System under Visible Light Irradiation. J. Am. Chem. Soc. 2012, 134, 6968−6971. (24) Muhich, C. L.; Zhou, Y.; Holder, A. M.; Weimer, A. W.; Musgrave, C. B. Effect of Surface Deposited Pt on the Photoactivity of TiO2. J. Phys. Chem. C 2012, 116, 10138−10149. (25) Pankove, J. I. Optical Processes in Semiconductors; Dover: New York, 1975. (26) Basu, P. K. Theory of Optical Processes in Semiconductors; Oxford University Press: New York, 1997. (27) Yamakata, A.; Ishibashi, T.; Onishi, H. Time-Resolved Infrared Absorption Spectroscopy of Photogenerated Electrons in Platinized TiO2 Particles. Chem. Phys. Lett. 2001, 333, 271−277. (28) Yamakata, A.; Ishibashi, T.; Onishi, H. Water- and OxygenInduced Decay Kinetics of Photogenerated Electrons in TiO2 and Pt/ TiO2: A Time-Resolved Infrared Absorption Study. J. Phys. Chem. B 2001, 105, 7258−7262. (29) Yamakata, A.; Ishibashi, T.; Onishi, H. Electron- and HoleCapture Reactions on Pt/TiO2 Photocatalyst Exposed to Methanol Vapor Studied with Time-Resolved Infrared Absorption Spectroscopy. J. Phys. Chem. B 2002, 106, 9122−9125. (30) Heimer, T. A.; Heilweil, E. J. Direct Time-Resolved Infrared Measurement of Electron Injection in Dye-Sensitized Titanium Dioxide Films. J. Phys. Chem. B 1997, 101, 10990−10993. (31) Yamakata, A.; Ishibashi, T.; Onishi, H. Microsecond Kinetics of Photocatalytic Oxidation on Pt/TiO2 Traced by Vibrational Spectroscopy. Chem. Phys. Lett. 2003, 376, 576−580. (32) Yamakata, A.; Ishibashi, T.; Kato, H.; Kudo, A.; Onishi, H. Photodynamics of NaTaO3 Catalysts for Efficient Water Splitting. J. Phys. Chem. B 2003, 107, 14383−14387. (33) Yamakata, A.; Yoshida, M.; Kubota, J.; Osawa, M.; Domen, K. Potential-Dependent Recombination Kinetics of Photogenerated Electrons in n- and p-Type GaN Photoelectrodes Studied by TimeResolved IR Absorption Spectroscopy. J. Am. Chem. Soc. 2011, 133, 11351−11357. (34) Furuhashi, K.; Jia, Q.; Kudo, A.; Onishi, H. Time-Resolved Infrared Absorption Study of SrTiO3 Photocatalysts Codoped with Rhodium and Antimony. J. Phys. Chem. C 2013, 117, 19101. (35) Bahnemann, D.; Henglein, A.; Lilie, J.; Spanhel, L. FlashPhotolysis Observation of the Absorption-Spectra of Trapped Positive Holes and Electrons in Colloidal TiO2. J. Phys. Chem. 1984, 88, 709− 711. (36) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. Charge Carrier Dynamics at TiO2 Particles: Reactivity of Free and Trapped Holes. J. Phys. Chem. B 1997, 101, 4265−4275. (37) Yoshihara, T.; Katoh, R.; Furube, A.; Tamaki, Y.; Murai, M.; Hara, K.; Murata, S.; Arakawa, H.; Tachiya, M. Identification of Reactive Species in Photoexcited Nanocrystalline TiO2 Films by WideWavelength-Range (400−2500 nm) Transient Absorption Spectroscopy. J. Phys. Chem. B 2004, 108, 3817−3823. (38) Tamaki, Y.; Furube, A.; Murai, M.; Hara, K.; Katoh, R.; Tachiya, M. Direct Observation of Reactive Trapped Holes in TiO 2 Undergoing Photocatalytic Oxidation of Adsorbed Alcohols: Evaluation of the Reaction Rates and Yields. J. Am. Chem. Soc. 2006, 128, 416−417. (39) 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. (40) Yoshida, M.; Yamakata, A.; Takanabe, K.; Kubota, J.; Osawa, M.; Domen, K. ATR-SEIRAS Investigation of the Fermi Level of Pt Cocatalyst on a GaN Photocatalyst for Hydrogen Evolution under Irradiation. J. Am. Chem. Soc. 2009, 131, 13218−13219. (41) Fonseca, C. N. P. d.; Paolia, M.-A. D.; Gorenstein, A. Electrochromism in Cobalt Oxide Thin-Films Grown by Anodic Electroprecipitation. Sol. Energy Mater. Sol. Cells 1994, 33, 73. (42) Rothenberger, G.; Moser, J.; Gratzel, M.; Serpone, N.; Sharma, D. K. Charge Carrier Trapping and Recombination Dynamics in Small Semiconductor Particles. J. Am. Chem. Soc. 1985, 107, 8054−8059.
(43) Iwata, K.; Takaya, T.; Hamaguchi, H.; Yamakata, A.; Ishibashi, T.; Onishi, H.; Kuroda, H. Carrier Dynamics in TiO2 and Pt/TiO2 Powders Observed by Femtosecond Time-Resolved Near-Infrared Spectroscopy at a Spectral Region of 0.9−1.5 μm with the Direct Absorption Method. J. Phys. Chem. B 2004, 108, 20233−20239. (44) Li, R. G.; Han, H. X.; Zhang, F. X.; Wang, D. G.; Li, C. Highly Efficient Photocatalysts Constructed by Rational Assembly of DualCocatalysts Separately on Different Facets of BiVO4. Energy Environ. Sci. 2014, 7, 1369−1376.
23906
dx.doi.org/10.1021/jp508233z | J. Phys. Chem. C 2014, 118, 23897−23906
