Time-Resolved Infrared Absorption Study of NaTaO3 Photocatalysts

Jul 1, 2009 - E-mail: [email protected]., †. Kobe University. , ‡. Tokyo University of Science. ACS AuthorChoice - This is an open access article p...
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J. Phys. Chem. C 2009, 113, 13918–13923

Time-Resolved Infrared Absorption Study of NaTaO3 Photocatalysts Doped with Alkali Earth Metals Motoji Maruyama,† Akihide Iwase,‡ Hideki Kato,‡ Akihiko Kudo,‡ and Hiroshi Onishi*,† Department of Chemistry, Kobe UniVersity, Rokkodai, Nada, Kobe 657-8501, Japan and Department of Chemistry, Tokyo UniVersity of Science, Kagurazaka, Shinjuku, Tokyo, 162-8601, Japan ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: June 7, 2009

Electron-hole recombination kinetics was observed in NaTaO3 photocatalysts doped with Ca, Sr, Ba, and La using time-resolved infrared absorption. The recombination rate was compared with the ultraviolet lightderived H2 production rate in the water splitting reaction to estimate the electron-to-H2 conversion efficiency. The conversion efficiency was sensitive to the nanometer-scale topography of the photocatalyst surface. The particularly high efficiency on the nondoped and 0.5 mol % Sr-doped photocatalysts was related to the flat (100) crystalline surfaces exposed on the photocatalyst particles. The mobility of holes was suggested to be restricted by doping on the basis of a kinetic simulation of the recombination rate. 1. Introduction There is rising interest in producing CO2-free sources of energy. The photocatalytic splitting reaction of water is one of the promising processes when driven by solar lights. Efforts have been made to prepare efficient photocatalysts.1-5 The quantum efficiency of the conversion is more than 50% on Ladoped NaTaO3 driven by ultraviolet light.6 As demonstrated by that example, doping with a heteroelement provides a common and efficient way to enhance the rate of H2 production. Doping Ca, Sr, and Ba on NaTaO3 was further examined, yielding enhanced H2 production rates.7 It is important to know how the dopant atoms positively affect catalytic performances. In the present study, electron-hole recombination kinetics was observed using time-resolved infrared (IR) absorption in NaTaO3 photocatalysts doped with Ca, Sr, Ba, and La. The observed recombination rate was compared with the H2 production rate to estimate the electron-to-H2 conversion efficiency. Sensitive detection of excited electrons by monitoring IR absorption has been achieved, free from the scattering problem with micrometer-sized photocatalyst particles.8,9 Photoinduced electron kinetics has been studied on Pt/TiO2 (P25) exposed to a vacuum,10 oxygen, water, and methanol vapor,11-14 and propanol liquid.15 Electron kinetics in more complex objects, including TiO2 from nine suppliers,16 TiO2 doped with Cr and Sb,17 sulfur-doped TiO2,18 dye-sensitized TiO2 films,19,20 K3Ta3B2O12,21 and La-doped NaTaO322 was successfully examined in a microsecond time domain. 2. Experimental Section NaTaO3 photocatalysts doped with Ca, Sr, Ba, or La were prepared by calcining mixtures of starting materials. Na2CO3, Ta2O5, CaCO3, SrCO3, BaCO3, and La2O3 were mixed at a desired molar ratio and calcined in a Pt crucible at 1173 K for 1 h and then 1423 K for 10 h. The fraction of Na2CO3 was adjusted in excess by 5% over the desired ratio. The loss of Na * To whom correspondence should be addressed. E-mail: oni@ kobe-u.ac.jp. † Kobe University. ‡ Tokyo University of Science.

in the calcination process was thus compensated. The calcined photocatalysts were washed with water to remove excess Nacontaining species. The H2 production rate was determined with each photocatalyst of 0.5 g suspended in water and irradiated with a 400 W high-pressure Hg lamp. The size and shape of the photocatalysts were observed with scanning electron microscopes (Hitachi, S-5000 and Jeol, JSM-6700F), while their surface area was determined by the BET method. The calcined photocatalysts were fixed on a 1 mm thick CaF2 plate with a density of 2 mg cm-2. The photocatalyst on the plate was heated at 573 K in an open furnace for 3 h to remove organic contaminants, and it was then placed in a gas cell. The base pressure of the cell was 5 Pa. The IR light from a ceramic light source (JASCO, LS-4000) was focused on the photocatalyst with ellipsoidal mirrors, and a transmitted light was monochromatized with gratings, 300 lines mm-1 for 4000-2000 cm-1 and 150 lines mm-1 for 2300-980 cm-1. A mercury-cadmiumtelluride (MCT) detector (Kolmar, KMPV11-1-J1/DC) received the monochromatized light. The MCT output was amplified with an ac-coupled amplifier (NF circuit, NF5307 or Stanford Research Systems, SR560) and accumulated with a digital oscilloscope (LeCroy, LT264M). Absorbance change as small as 10-6 was detectable. The time resolution of the spectrometer was limited to 50 ns by the response of the MCT. The NF amplifier of 10 MHz bandwidth was used to record responses faster than 1 µs, while the SR amplifier of 1 MHz bandwidth was employed in the time domain of one microsecond to milliseconds. The photocatalyst was excited by the forth harmonic light pulses of a Q-switched Nd:YAG laser (Lotis TII, LS-2139). The wavenumber, time width, and spot size of the pump pulses were 266 nm, 20 ns, and 6 mm, respectively. The irradiated photocatalyst was heated by the pump light. The pulse energy was tuned at 0.4 mJ pulse-1 to prevent the blackbody radiation overlapping the IR probe light. The intensity of the transmitted IR light at a fixed wavenumber was recorded as a function of the time delay from the pump pulse. A photodiode received the reflected pump pulse in order to send a trigger to the oscilloscope. Three thousand responses were accumulated to make the intensity trace at one wavenumber. Transient absorp-

10.1021/jp903142n CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

Time-Resolved IR Study of Alkali Earth-Doped NaTaO3

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13919

TABLE 1: NaTaO3 Photocatalysts Doped with Ca, Sr, Ba, or La dopanta

particle diameter (µm)

surface area (m2 g-1)

H2 production rate (µmol h-1)

none 2 mol % La 2 mol % Ca 5 mol % Ca 0.5 mol % Sr 2 mol % Sr 1 mol % Ba 2 mol % Ba

2–4 0.1–0.7 0.1–0.5 0.1–0.5 0.1–0.3 0.1–0.3 0.1–0.3 0.1–0.5

0.3 3.2 2.6 2.6 2.6 3.4 3.4 3.0

157 404 204 205 1090 530 436 618

a

The concentration is in the starting material.

tion spectra at different time delays were reconstructed from the traces at different wavenumbers of 20 cm-1 intervals. The repetition rate of the pump pulses was reduced to 1 Hz to minimize the contribution of electrons accumulated in the photocatalyst. 3. Results and Discussion 3.1. Steady-State H2 Production Rate. The rate of H2 production of the series of photocatalysts is listed in Table 1, together with the particle size and surface area. Doping with alkali earth elements and La enhanced the H2 production rate by a maximum of seven times. The particle size was reduced by an order of magnitude. The primary purpose of this study is to compare the recombination kinetics in the photocatalysts doped with different elements. This can be done by comparing four photocatalysts doped by 2 mol % with each element. However, the dopant concentration to achieve the optimum H2 production rate was different from one dopant to another: 5 mol % with Ca, 0.5 mol % with Sr, and 1 mol % with Ba. The recombination kinetics in the photocatalysts of the optimum concentrations was also examined. 3.2. Steady-State IR Absorption in the Dark. Figure 1 shows the steady-state IR transmittance spectra observed in the dark. The transmittance was monotonously reduced at high wavenumbers. The reduced transmittance is ascribed to light scattering by photocatalyst particles. On the photocatalysts of high surface areas, absorption bands appeared at 3400, 2900, 2400, 1600, 1300, and 1100 cm-1. The broadband at 3400 cm-1 and weak band at 1300 cm-1 are from the O-H stretch and X-O-H deformation modes of adsorbed hydroxyl species. Multiple peaks at 2900 and 1100 cm-1 are assigned to the C-H stretch and fingerprint modes of organic compounds adsorbed on the photocatalyst. Twin peaks at 2400 cm-1 are assigned to CO2 in the light pass. 3.3. IR Absorption Induced by Electronic Excitation. Infrared absorption was induced by pump light pulses. Figure 2 presents transient absorbance spectra of the eight photocatalysts observed at different time delays. The common feature in the eight spectra (Figure 2a-h) was the monotonously increased absorbance from 1800 to 1000 cm-1. This is due to the absorption induced by the photoexcited electrons, according to our earlier study of NaTaO3.22 The decay of the excited electrons is quantitatively traced as a function of the time delay in Section 3.4. Additional features on the spectra are considered here. Negative peaks appearing at 1500-1700 cm-1 are attributed to fingerprint vibrations of organic compounds residing on the photocatalysts. Twin negative peaks are caused at 2300 cm-1 by the absorption of CO2 in the IR light pass. With higher wavenumbers, three broad bands appeared in the negative direction at 2700, 3100, and 3400 cm-1. The 2700 cm-1 bands

Figure 1. IR transmittance spectra of the NaTaO3 photocatalysts doped with (a) none, (b) La of 2 mol %, (c) Ca of 2 mol %, (d) Ca of 5 mol %, (e) Sr of 0.5 mol %, (f) Sr of 2 mol %, (g) Ba of 1 mol %, and (h) Ba of 2 mol %.

Figure 2. Transient IR absorbance spectra of the NaTaO3 photocatalysts doped with (a) none, (b) La of 2 mol %, (c) Ca of 2 mol %, (d) Sr of 2 mol %, and (e) Ba of 2 mol %. The gas cell was evacuated, and the photocatalysts were irradiated at a zero time delay with pump pulses. Absorbance curves at time delays of 1, 2, 3, 5, and 8 µs are shown from the top to the bottom in each panel.

correspond to the C-H stretching vibrations of organic adsorbates. The two bands at 3100 and 3400 cm-1 are assigned to O-H stretching vibrations of adsorbed hydroxyl species, which have some efficient hydrogen bonds and some less efficient

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Figure 4. Rate of H2 production as a function of ∆abs(10 µs)/∆abs(0): 0, nondoped NaTaO3; 9, 0.5 mol % Sr-doped photocatalyst, and b, other six doped photocatalysts listed in Table 1.

Figure 3. Normalized absorbance change at 2000 cm-1 as a function of the time delay. Nondoped photocatalysts and photocatalysts doped with none, La of 2 mol %, Ca of 2 mol %, Sr of 2 mol %, and Ba of 2 mol % are shown in panel a. Photocatalysts doped with Ca of 5 mol %, Sr of 0.5 mol %, and Ba of 1 mol % are shown in panel b.

hydrogen bonds. Those negative responses of the O-H stretching, C-H stretching, and fingerprint vibrations suggest that organic compounds, water and hydroxyl species adsorbed on the photocatalyst, were reduced by the pump light irradiation. Thermally induced desorption and photochemically induced decomposition are possible means to reduce the adsorbates. 3.4. Electron-Hole Recombination in the Doped Photocatalysts. We assigned the monotonous absorption feature to the electrons that were band gap-excited in each photocatalyst. The absorbance at 2000 cm-1, which is free from adsorbateinduced features, is normalized at a zero time delay and shown in Figure 3 as a function of the delay. The absolute absorbance of each photocatalyst was distributed by a factor of 0.3 due to different film thicknesses. The absorbance in Figure 3 was observed in vacuum, where the electrons decay by recombination, not by reactions at the surface. The number of electrons not yet recombined should be proportional to absorbance. By doping the alkali earth elements, we retarded the electron-hole recombination. On the photocatalysts doped with 2 mol %, the absorbance at time delays of 1 µs or later was enhanced, and the extent of the enhancement was an order of (large) Sr, Ba and Ca (small). This is interpreted with the retarded recombination in the doped photocatalysts. The H2 production rates in Table 1 present the same order of enhancement. The enhanced electron population in the bulk leads to efficient material conversion at the surface. The recombination rate was sensitive to the dopant concentration. In Figure 3b, the IR absorbance of the photocatalysts with optimum H2 production is shown. The 1 mol % Ba-doped and 0.5 mol % Sr-doped photocatalysts presented the enhanced electron population when compared with the 2 mol %-doped photocatalysts. The 5 mol % Ca-doped photocatalyst exhibited almost the same absorbance as that of the 2 mol %-doped photocatalyst. Here, we quantitatively consider the relationship of the recombination rate and the H2 production rate. The recombination rate is represented by the absorbance at 10 µs relative to

Figure 5. Scanning electron microscope images of the (a) nondoped, (b) 0.5 mol % Sr-doped, (c) 2 mol % Sr-doped, (d) 2 mol % Ca-doped, (e) 1 mol % Ba-doped, and (f) 2 mol % La-doped NaTaO3 photocatalysts.

the absorbance at zero delay, ∆abs(10 µs)/∆abs(0). A small ratio of ∆abs(10 µs)/∆abs(0) indicates efficient recombination. Figure 4 shows the rate of H2 production of the eight photocatalysts in Table 1 as a function of the ratio. Two photocatalysts, nondoped and 0.5 mol % Sr-doped, are shifted away from the other six photocatalysts. The six photocatalysts present a linear relationship in Figure 4. Because the particle size and surface area are similar on the six photocatalysts, the linear relationship suggests that a common fraction of the electrons observed at 10 µs participate in the reaction to produce H2. The six photocatalysts are homogeneous in how efficiently the excited electrons are utilized in the desired reactions at the surface. The other two, nondoped and 0.5 mol % Sr-doped, are excluded from the six photocatalysts. They are characterized by more efficient electronto-H2 conversion than those on the six photocatalysts, i.e., more H2 production with the same number of excited electrons. We propose that the conversion efficiency is affected by the nanometer-scale topography of the photocatalyst surface. If a large number of photons were absorbed by the two particular photocatalysts, the H2 production rate would be enhanced. However, this was not the case. The band gap of the nondoped and seven doped photocatalysts was in a small range of 4.0-4.1 eV. Figure 5 shows scanning electron microscope images of the photocatalysts. The photocatalyst particles were cubic, reflecting the perovskite structure of NaTaO3. The two highly efficient photocatalysts presented flat crystalline surfaces,

Time-Resolved IR Study of Alkali Earth-Doped NaTaO3

Figure 6. Reduction rate of IR absorbance as a function of the absorbance. Absorbance of the photocatalysts doped with (a) none, (b) La of 2 mol %, (c) Ca of 2 mol %, (d) Ca of 5 mol %, (e) Sr of 0.5 mol %, (f) Sr of 2 mol %, (g) Ba of 1 mol %, and (h) Ba of 2 mol % was numerically differentiated with the time delay. Dotted lines show the fitted results.

TABLE 2: Absorbance Decay Fitted with a Kinetic Model dopant

a (106 s-1)

b (103 s-1)

range of fit (10-3)

none 2 mol % La 2 mol % Ca 5 mol % Ca 0.5 mol % Sr 2 mol % Sr 1 mol % Ba 2 mol % Ba

4.5 2.8 2.8 2.9 3.4 2.6 2.6 2.4

–1.7 –2.7 –2.1 –3.8 –4.1 –2.9 –2.7 –2.0

0.42–0.69 1.11–1.27 0.82–0.94 1.50–1.72 1.33–1.51 1.25–1.35 1.12–1.25 0.94–1.05

whereas terraces of 10 nm widths were separated by straight steps on the other photocatalysts. Similar steps of singlenanometer heights were found on La-doped NaTaO3.23 The electron-to-H2 conversion efficiency is sensitive to the presence of the step structures. The high efficiencies on the flat crystalline surfaces indicate efficient electron transfer at the intrinsic (100) surface of NaTaO3. With increasing concentration of the dopants, the lattice of the host oxide experiences strains. More surface is required to release the stress, and the nanometerscale steps and terraces appear to respond to this requirement. The one-to-one aspect ratio of the step height over the terrace width is favorable to increase the surface area. The results shown in Figure 4 suggest that the (100) surface modified with the steps is, for some reason, less efficient than the intrinsic surface in producing H2. The segregated dopants may reduce the fraction of surface reaction sites. The dopant concentration increasing from the bulk to the surface may induce an electrostatic field in the submicrometer-sized photocatalyst particles. The transport of either electrons or holes is restricted. Dopant segregation to the surface happened, according to an X-ray photoelectron spectroscopy study.24 On the series of 2 mol %-doped photocatalysts, the surface concentration of Ca,

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13921 Sr, Ba, and La was 12, 14, 15, and 9 mol %, respectively. The transmission electronmicrograph of the La-doped photocatalyst, which was reported in Figure 4 of ref 23 shows convex truncations of terraces. The convex terraces are possibly modified with the segregated dopant. In our earlier studies on 2 mol % La-doped NaTaO3 loaded with NiO,22,23 enhanced H2 production was ascribed to the step structures on the doped photocatalysts. Ultrafine particles of NiO appearing on step edges offer efficient electron transfer to produce H2, while the grooves facilitate multiple injections of holes in order to produce O2. The edges and grooves are separated on the stepped surfaces of the La-doped photocatalyst. The water formation reaction, which consumes H2 and O2, is restrained because the two reactants produced at separate sites are less likely to encounter each other. The findings and interpretation of the current study are consistent with those of the earlier conclusions. The rate of H2 production shown in Table 1 was determined on the doped photocatalysts in the absence of NiO. By loading NiO, the H2 production rate of the doped photocatalysts was enhanced by 10-30 times.23 The reduction and oxidation half reactions take place at the same rate in the water splitting reaction. When the rate of the two complementary half reactions are not balanced, the photocatalyst is poisoned by electrons or by holes accumulated in the photocatalyst.25 The efficient hole injection on the grooves is an intrinsic property of the stepped surface. In the absence of the cocatalyst, the whole reaction rate is determined by the H2 production rate. By putting the cocatalyst on the photocatalyst with stepped surfaces, we fully improved the H2 production rate to balance efficient hole injection. The particle size of the NaTaO3 photocatalysts was sensitive to the presence of dopants. By doping La, Ca, Sr, and Ba, we increased the surface area of NaTaO3 by 9-11 times, and the particle diameter reduced accordingly. The particle size can affect the H2 production rate in two different manners. Small photocatalyst particles are favorable for efficient charge carrier transport from the bulk to the surface. However, H2 production is a four-hole assisted reaction, and multiple hole attachments to a reactant are expected on the surface. Small particles are not favorable to hole accumulation. The H2 production rate of 0.5 mol % Sr-doped NaTaO3 was enhanced by seven times compared with that of the nondoped NaTaO3. This significant enhancement may be additionally affected by the efficient transport in the small photocatalyst particles. 3.5. Kinetic Simulation of the Recombination. The observed decay of IR absorbance is simulated by assuming that the electron-hole recombination is mediated by trap sites. The absorbance ∆abs(t) is proportional to the number of electrons not yet recombined, and it is reduced with the time delay. The reduction rate of the absorbance, -d∆abs(t)/dt, was determined by numerically differentiating the observed ∆abs(t). The reduction rate is plotted in Figure 6 as a function of ∆abs(t). We identified a straight portion in each curve and fitted that portion with a formula, -d∆abs(t)/dt ) a∆abs(t) + b. Two parameters, a and b, are listed in Table 2 with the range of fitted ∆abs(t). The nondoped and 0.5 mol % Sr-doped photocatalysts presented a large a, 4.5 and 3.4, respectively, relative to those of the other photocatalysts. Here the electrons and holes are assumed to be trapped and then recombined. k1

e- + Te {\} etrk-1

(1)

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k2

h+ + Th {\} htr+

(2)

k-2

k3

etr- + h+ 98 Te

(3)

k4

e- + htr+ 98 Th

(4)

e- and h+ are a free electron in the conduction band and a free hole in the valence band. etr- and htr+ represent an electron and hole trapped on an electron trap site Te and on a hole trap site Th. k1, k-1, k2, and k-2 are the rate constants of trapping and detrapping of the charge carriers. The trapped electrons recombined with free holes with a rate constant of k3. k4 is the recombination rate constant of the trapped holes and free electrons. The rate equations of the six components are as follows

d[e-] ) -k1[e-][Te] + k-1[etr-] - k4[e-][htr+] dt

(5)

d[h+] ) -k2[h+][Th] + k-2[htr+] - k3[etr-][h+] dt

(6)

d[etr-] ) k1[e-][Te] - k-1[etr-] - k3[etr-][h+] dt

(7)

d[htr+] ) k2[h+][Th] - k-2[htr+] - k4[e-][htr+] dt

(8)

[Te] ) [Te]0 - [etr-]

(9)

[Th] ) [Th]0 - [htr+]

(10)

[n] is the number of n, and [n]0 is the number of n in the dark. Here, we apply a steady-state approximation on the two intermediate states, etr- and htr+. The number of trap sites is further assumed to be much less than the number of electrons and holes. Hence, [e-] equals [h+]. The rate of electron decay, r ) -d[e-]/dt, is given in these assumptions -

r ) k1[Te]0[e ] -

k1k-1[Te]0[e-] + k12[Te]0[e-]2 (k1 + k3)[e-] + k-1

+

k2k4[Th]0[e-]2 (k2 + k4)[e-] + k-2

(11)

When [e-] is much larger than k-1/(k1 + k3) and k-2/(k2 + k4), eq 11 is simplified to

r)

(

)

k2k4[Th]0 k1k-1[Te]0 k1k3[Te]0 + [e ] k1 + k3 k2 + k4 k1 + k3

(12)

The experimentally obtained decay rate is compared to simplified eq 12. A large a indicates fast recombination with the same number

of electrons. As shown in Table 2, the nondoped and 0.5 mol % Sr-doped photocatalysts are characterized by a large a. Considering possible origins of the large a, it is difficult to expect a large concentration of electron trap sites [Te]0 and hole trap sites [Th]0 in the nondoped and slightly doped photocatalysts. We assumed that there were mobile electrons in the Ta5dderived conduction band and less mobile holes in the O2pderived valence band, i.e., k1 . k3 and k4 . k2. The first term of eq 12 is further simplified to be (k3[Te]0 + k2[Th]0)[e-]. The large a thus suggests a large k3[Te]0 or a large k2[Th]0. The common step in the two terms is free hole diffusion. Mobile holes present large k3 and k2, leading to a large a in the two photocatalysts. Less mobile holes are suggested in the other six photocatalysts with a small a. We propose that the mobility of electrons and holes is reduced by doping. Sodium and tantalum cations are 12-fold and 6-fold coordinated in NaTaO3. The ionic radius of the dopants, Ca2+, Sr2+, Ba2+, and La3+ is 0.13, 0.14, 0.16, and 0.14 nm, respectively.26 These cations are too large to substitute for Ta5+. Instead, Na+ of 0.14 nm is substituted by the divalent or trivalent cations. Sodium cation vacancies can additionally appear to maintain the neutral ionic charge. The periodic potential of the host crystal is disturbed at random by the dopants and Na-vacancies. The mobility of holes and electrons decreased as a result. The Ta-derived conduction band and O-induced valence band produce a low density of states on the dopant cations. If this is not the case, the dopants and Na vacancies efficiently trap charge carriers, leading to recombination. The fraction of charge carriers trapped on the dopant-induced states may be experimentally observed in time-resolved absorption of visible wavelengths as has been done on TiO2.27 The mobility of electrons and holes is not known in NaTaO3. Hall effect measurements have been performed on a nondoped KTaO328 crystal and a 1 atom % Ca-doped KTaO3 film.29 They exhibited n-type semiconducting behavior, and the Hall mobility of electrons was determined at room temperature to be 30 and 0.3 cm2 V-1 s-1 on the nondoped and doped materials, respectively. It is natural on NaTaO3 to expect that the mobility is reduced by doping. Another possible mechanism of the reduced recombination rate is efficient separation of electrons and holes. As mentioned in Section 3.4, the dopants were segregated to the surface. The uneven distribution of the dopants can induce an electric field in the micrometer-sized photocatalyst particles. Electrons and holes are driven in opposite directions in the field to reduce the probability of encounters. 3.6. Electron Decay in O2 or Methanol Vapor. The recombination reaction observed in the vacuum is dominated by intrinsic bulk properties of the photocatalysts. The kinetics of the carrier attachment at the surface was examined on the photocatalysts of optimum dopant concentration placed in methanol or O2 vapor. Figure 7 shows the electron-induced IR absorbance observed in methanol vapor of 1.3 × 103 Pa in comparison with the response in the vacuum. It is expected and actually observed on TiO2,11,17 K3Ta3B2O12,21 and NaTaO3,22 that photoexcited holes transfer to adsorbed methoxy species,

CH3O-(a) + h+ f CH3O(a) The hole transfer competes with the recombination reaction. On the alkali earth-doped photocatalysts, the electron decay was retarded by the presence of methanol vapor. This indicates that

Time-Resolved IR Study of Alkali Earth-Doped NaTaO3

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13923 La-doped NaTaO3 photocatalysts. The rate of electron-hole recombination was quantitatively observed by monitoring the absorption decay, and it was related to the rate of H2 production in the absence of any cocatalyst. The efficiency of the electronto-H2 conversion was affected by the nanometer-scale topography of the photocatalyst surface. The particularly high efficiency on the nondoped and 0.5 mol % Sr-doped photocatalysts was related to the intrinsic (100) surfaces exposed on the photocatalyst particles. It was suggested that the free hole mobility was restricted in the doped photocatalysts on the basis of the recombination kinetics simulated with a trap and recombination mechanism. Acknowledgment. This work was supported by Grant-inAids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (17GS0209). References and Notes

-1

Figure 7. Transient absorbance at 2000 cm observed in different atmospheres: (a) nondoped, (b) 2 mol % La-doped, (c) 5 mol % Cadoped, (d) 0.5 mol % Sr-doped, and (e) 1 mol % Ba-doped NaTaO3 photocatalysts were irradiated with 0.4 mJ pulses at a zero time delay. Curves observed in methanol vapor of 1.3 × 103 Pa and O2 of 1.3 × 103 Pa are presented with the curve in the vacuum.

the holes are transferred at the surface of the doped photocatalysts. The extent of retardation is small when compared with the methanol-induced electron decay on TiO2 (P25).11,17 This is possibly because the NaTaO3-based photocatalyst particles are much larger than the P25 photocatalyst particles. IR absorbance curves were observed also in O2 of the same pressure. On the nondoped and La-doped photocatalysts, the electron decay was accelerated as expected. The excited electrons are transferred to adsorbed molecular oxygen

O2(g) + e- f O2-(a) Oxygen-induced decay of electrons has been observed on TiO211,17 and K3Ta3B2O12.21 However, the decay was retarded on the photocatalysts doped with Ca, Sr, and Ba. The particular retardation with the alkali earth dopants is interpreted by hole transfer to an O2-induced adsorbate, ozonide. An electron paramagnetic resonance study30 showed that ozonide ions, O3-, were formed on polycrystalline CaO exposed to O2 as

O2(g) + O2- + h+ f O3-(a) Photoexcited holes can be consumed by this reaction, contrary to what happened on the nondoped and La-doped NaTaO3. The electron-hole recombination was thus retarded by being exposed to the O2 atmosphere. This is consistent with the dopant segregation mentioned in Section 3.4. 4. Conclusions The NaTaO3 photocatalysts doped with Ca, Sr, and Ba displayed monotonous IR absorption when irradiated by 266 nm light pulses. The absorption was assigned to excited electrons, following the previous results on the nondoped and

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