Dynamics of Photogenerated Charge Carriers on Ni- and Ta-Doped

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Dynamics of Photogenerated Charge Carriers on Ni- and Ta-Doped SrTiO3 Photocatalysts Studied by Time-Resolved Absorption and Emission Spectroscopy Akira Yamakata,*,†,‡ Masayuki Kawaguchi,†,§ Ryosuke Murachi,† Masahiro Okawa,†,∥ and Itaru Kamiya† †

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

‡

S Supporting Information *

ABSTRACT: The behavior of photogenerated charge carriers on SrTiO3 photocatalysts doped with transition metals (such as Ni and Ta) was examined by time-resolved visible to mid-IR absorption and emission spectroscopy. When SrTiO3 was co-doped with Ni and Ta, the catalyst absorbed visible light and exhibited photocatalytic activity under visible light irradiation. However, activity under UV light was decreased significantly compared to that before doping. The results of time-resolved measurements showed that monodoping of Ni or Ta accelerated the recombination but co-doping Ni with Ta increased the lifetime of charge carriers compared to those without doping. Furthermore, electrons excited by a visible laser pulse had longer lifetimes compared to those excited by a UV laser pulse. Time-resolved photoluminescence measurements suggested that doped Ni cations act as recombination centers, giving a luminescence peak at ∼8000 cm−1 due to the downward d−d transition at Ni2+. However, the lifetime of the emission was much shorter than that of free or shallowly trapped electrons. These results suggest that recombination at the Ni cations is not the dominant process. In addition, the reactivity of photogenerated electrons was decreased dramatically by doping; electrons did not react with exposed O2, although holes maintained reactivity with MeOH. These results confirm that the decrease in the steady-state activity of doped SrTiO3 under UV light irradiation is responsible for the decrease in reactivity of photogenerated electrons.

1. INTRODUCTION Photocatalysts have attracted attention due to their potential applications for water-splitting and degradation of pollutants using solar energy. TiO21−7 and SrTiO38−11 have been used as photocatalysts but are only compatible with UV light. Doping of nitrogen,12 sulfur,13 or transition metals10,14,15 into these UV-responsive photocatalysts creates midgap states which allow absorption of visible (Vis) light. These doped catalysts exhibit photocatalytic activity under visible light; therefore, band gap modification by doping has been investigated for developing visible-light-driven photocatalysts. However, the intrinsic photocatalytic activity is decreased by doping; photocatalytic activity increases under visible light irradiation but decreases under UV light.15 As a result, doping does not increase the overall photocatalytic activity under sunlight (UV + Vis). Doping of lanthanides16 and alkali earth metals17,18 has been reported to enhance the photocatalytic activity of several UV-responsive photocatalysts. However, the doping of transition metals often decreases the photocatalytic activity because partly filled d-orbitals of transition metals can capture both electrons and holes and act as recombination centers.2−7 The actual effects of dopants on the recombination processes © 2016 American Chemical Society

have not been fully elucidated. To increase the overall photocatalytic activity under sunlight, the effects of doping on the behavior of photogenerated charge carriers need to be elucidated. Time-resolved (TR) visible to mid-IR spectroscopy is useful for examining the behavior of photogenerated charge carriers. Recombination processes on TiO 2 , 19−28 SrTiO 3 , 11,29 NaTaO3,17,30,31 LaTiO2N,32 GaN,33 and Fe2O334,35 have been investigated using these spectroscopic methods. For example, free and shallowly trapped electrons produced structureless broad absorptions in the mid-IR region,26,36 indicating that the behavior of photogenerated electrons can be elucidated by observing transient absorptions in the mid-IR region. In the UV to near-IR (NIR) region, deeply trapped electrons and holes often give transient absorptions; therefore, their behavior has been studied by observing these transient absorptions. In addition, emission peaks often appear during the recombination in the visible to NIR region. By observing emission spectra and Received: February 12, 2016 Revised: March 29, 2016 Published: March 29, 2016 7997

DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004

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The Journal of Physical Chemistry C

TiO2 (Nippon Aerosil Co., Ltd., P-25), SrCO3 (Kanto Chemical Co., Inc.), NiO (Kanto Chemical Co., Inc.), and Ta2O5 (Kanto Chemical Co., Inc.). The mixed materials were calcined in air at 1270 K for 10 h. Diffuse reflectance absorption spectra were obtained using a Jasco V-670 spectrometer equipped with an integrating sphere in diffuse reflection mode. The size and shape of the powder catalysts were determined using a scanning electron microscope (SU6600, Hitachi). The steady-state photocatalytic activity of the catalysts was examined by observing the rate of H2 evolution in 10 vol % aq MeOH. The powder catalyst (200 mg) was added to the MeOH solution (200 mL) and deaerated by Ar gas bubbling in Pyrex reactors. The catalysts were irradiated by a 300 W Xe lamp (Excelitas Technologies Co., Ltd., Cermax PE300BUV) equipped with a long-pass filter for visible irradiation (>420 nm, Sigma-Koki Co., Ltd., SCF-50S-42L) and without the filter for UV + Vis (>250 nm) irradiation. The amount of H2 gas generated was analyzed by gas chromatography (Shimadzu, GC8A). For the time-resolved measurements, the powder photocatalyst was fixed on a CaF2 plate at a density of 2 mg cm−2, and placed in an optical cell that allowed the introduction of reactant gases such as O2 and MeOH vapor.

their decay kinetics, the behavior of charge carriers at the recombination center can be studied. Therefore, transient absorption and emission measurements from the visible to midIR region provide useful information for understanding the behavior of photogenerated charge carriers. In the present study, we have determined the effects of doping on the behavior of photogenerated charge carriers in SrTiO3 photocatalysts using time-resolved absorption and emission spectroscopy. The Ni and Ta were doped in SrTiO3 because this catalyst exhibits a visible-light response in photocatalytic reactions.15 However, the UV activity drastically decreases upon doping.15 The mechanism of this degradation must be elucidated for the further enhancement of the photocatalytic activity. Photocatalytic activity is determined by the competition between recombination and chargeconsuming reactions by the reactant molecules. Therefore, the effects of doping on these processes were investigated. The catalyst modifications for the deceleration of the recombination and for the acceleration of the charge-consuming reactions are different; therefore, this information is useful to improve the photocatalytic activity. The photocatalytic activity of H2 evolution from water was determined, and the change in steady-state activity was examined in relation to the behavior of photogenerated charge carriers.

3. RESULTS AND DISCUSSION 3.1. Absorption Spectra and Photocatalytic Activities of Ni and Ta Doped SrTiO3. The absorption spectra and photocatalytic activity of the synthesized catalysts were first examined to clarify the property of the sample used in this work. Diffuse reflection absorption spectra of the catalysts are shown in Figure 1. The undoped SrTiO3 absorbed UV light below 380 nm. However, doping with 1% Ni induced strong absorption in the visible region from 400 to 900 nm. A broad absorption band was observed at 520 nm, which was assigned to the upward d−d transition of Ni3+ cations,39 and indicates that the doping form of Ni was Ni3+. Co-doping of Ni with 2% Ta5+ also produced an absorption band within the visible range of 400−900 nm, but the intensity of the broad absorption at 520 nm decreased, which suggests that the Ni3+ was changed to Ni2+. It is reported15 that co-doping of Ni with high-valent cations such as Ta5+ and Nb5+ reduce the formation of Ni3+ and oxygen vacancy with overall charge compensation. The doped Ni2+ produces an absorption band below 500 nm, which is assigned to excitation of electrons from the d-orbitals of Ni2+ to

2. EXPERIMENTAL SECTION Microsecond time-resolved visible to mid-IR measurements were performed using home-built spectrometers as described previously.26,37 In the IR region from 6000 to 1000 cm−1, experiments were conducted in transmittance mode, with the IR light from a MoSi coil focused on the sample (5 mm in diameter) and transmitted light dispersed by the spectrometer. The monochromated light was detected by an MCT detector, and the output electric signal was amplified using an ACcoupled amplifier. The transient absorption change was recorded using a digital oscilloscope (Teledyne Lecroy, HDO4034) after photoexcitation of the catalysts by an Nd:YAG laser (Continuum Surelite II and Surelite OPO, 6 ns duration, 0.1−10 Hz, laser power is 0.5 and 10 mJ pulse−1 and beam diameters are 6 and 10 mm, for 355 nm and 425− 650 nm, respectively). In the visible and NIR regions from 25000 to 6000 cm−1, experiments were performed in reflection mode, with the probe light from a halogen lamp focused on the sample (5 mm in diameter), and diffuse reflected light dispersed by the spectrometer. The monochromated output was detected using a Si-photodiode or InGaAs detector. For the calculation of time-resolved spectra and decay curves for both transmission and reflection modes, probe-light intensity (I) without pump pulse and intensity change (ΔI) induced by the pump pulse irradiation were measured. The absorbance change (ΔAbsorbance) was calculated by the equation, ΔAbsorbance = −log(1 + ΔI/I).38 Emission spectra (Iemission) were measured by using the above-described time-resolved spectrometers. The experimental procedures were the same with the absorption measurements, except for turning off the light source. The decay curves of emitted light intensity (ΔI) from the sample after the laser pulse irradiation were recorded by a digital oscilloscope. In order to compensate the wavelength-dependent sensitivity of the spectrometers, the recorded light intensities (ΔI) were normalized by the intensities of light from halogen lamp (I) without pump pulse irradiation (Iemission = ΔI/I). The SrTiO3 powder doped with Ni and Ta was synthesized by mixing the starting materials. The metal oxides used were

Figure 1. Diffuse reflection absorption spectra of synthesized catalysts. Spectra of undoped, 1% Ni-doped, and 1% Ni and 2% Ta co-doped SrTiO3 are shown. Inset: Scanning electron microscope image of Ni/ Ta co-doped SrTiO3. 7998

DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004

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The Journal of Physical Chemistry C the conduction band (CB).15 This optical transition is essential for the photocatalytic reactions under visible light irradiation. The photocatalytic activity of the synthesized photocatalysts was examined by loading 0.6 wt % Pt on the catalysts and measuring the rate of H2 evolution in 10 vol % MeOH. Under visible light irradiation at >420 nm, little H2 (250 nm) light irradiation also was examined (Figure 2B). The undoped SrTiO3 powder produced H2 at 241 ± 6 μmol h−1 in the initial 1 h. However, the activity drastically decreased to 22 ± 1 μmol h−1 upon Ni/Ta co-doping, which is only 10% of the amount produced by undoped SrTiO3. A similar decrease in UV activity also has been reported previously.15 Therefore, these results confirm that the doped catalysts absorb light in the UV to visible region, but the overall photocatalytic activity under UV + Vis irradiation dramatically decreases upon Ni/Ta doping. 3.2. Transient Absorption Spectra and Recombiation Kinetics of Photogenerated Electrons and Holes in Undoped SrTiO3 Powder. Transient absorption spectra of undoped powder SrTiO3 were measured after UV laser pulse irradiation. The similar experiments were performed for several commercial SrTiO3 powders;29,40 however, the results for our sample were different from those of commercial powders. As shown in Figure 3A, at least three absorption bands were observed at 25000 cm−1, 11000 cm−1, and below ∼4000 cm−1 with a small peak at 2500 cm−1. As reported previously, a strong absorption below ∼4000 cm−1 was attributed to free electrons in the CB and/or shallowly trapped electrons in the midgap states.29,40 In contrast, the absorption bands from 25000 to 5000 cm−1 were assigned to the charge carriers trapped in the midgap states that arose from the defects on the powder

Figure 3. Time-resolved absorption spectra of (A) undoped SrTiO3, (B) 1% Ni-doped SrTiO3, (C) 1% Ta-doped SrTiO3, and (D) 1% Ni and 2% Ta co-doped SrTiO3. Irradiation by 355 nm UV laser pulses (0.5 mJ pulse−1, 6 ns duration, 5 Hz).

particles, because they were absent from defect-free single crystalline SrTiO3 but present on defect-rich polycrystalline powder SrTiO3.29,40 The spectral shape and assignments in the visible to NIR region are also reportedly very sensitive to the morphology of the SrTiO3 powder and surface treatments. Hence, even for commercial SrTiO3 powders, the spectral shapes from powder obtained from Aldrich,29 Kojundo Co.,29 and Wako Pure Chemicals40 were different. The SrTiO3 powder synthesized by solid-state reaction in the present study was also different from these commercial powders. These results confirm that the assignment of these absorption bands as well as the energy states of the trapped charge carriers were very sensitive to the synthetic methods of the SrTiO3 powder. Detailed decay processes of the absorption maxima at 20000, 11000, and 2000 cm−1 were investigated in the presence and absence of O2 gas or MeOH vapor. The decrease in absorption intensity at 20000 cm−1 was accelerated by exposure to MeOH but slowed down by exposure to O2 gas (Figure 4A). Since O2 gas and MeOH vapor consume photogenerated electrons and holes, respectively, these results suggest that the band intensity at 20000 cm−1 mainly reflects the quantity of holes. The band intensity at 11000 cm−1 also showed similar changes upon exposure to O2 and MeOH (Figure 4B), suggesting this absorption also reflects the quantity of holes. However, the band intensity at 2000 cm−1 showed the opposite trend: it was decreased by exposure to O2 and increased upon exposure to MeOH, indicating that this absorption reflects the quantity of electrons. These results confirm that the behavior of photogenerated electrons and holes can be examined using transient absorption measurements and both the electrons and the holes generated in undoped SrTiO3 are active in the photocatalytic reactions.

Figure 2. Photocatalytic H2 evolution from 10 vol % aq MeOH over 0.6 wt % Pt undoped and Ni/Ta co-doped SrTiO3 under (A) VIS (>420 nm) and (B) UV + VIS light (>250 nm). Reaction conditions: 0.1 g catalyst, 200 mL reactant solution, 300 W Xe lamp light source, Pyrex reactor. 7999

DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004

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greater than 10-fold larger than that of any other sample. Note that the absorption intensities at 20000 and 2500 cm−1 were increased greatly compared to the Ni monodoped sample. These results suggest that co-doping of Ni and Ta produced different effects than did monodoping of either Ni or Ta. The decay kinetics of photogenerated electrons excited by a 355 nm UV laser pulse was further examined by observing the intensity change at 2000 cm−1 (Figure 5). For undoped SrTiO3, most of the electrons recombined within ∼100 μs, and a portion of the electrons recombined slowly because they had a lifetime longer than 1 ms: the absorption intensity at 1 ms was 5.2 × 10−5. For Ni monodoped SrTiO3, the absorption intensity decreased, even at 0 μs. As a result, the quantity of surviving electrons at 1 ms (ΔAbs = 3.0 × 10−5) decreased compared to that of undoped SrTiO3. Monodoping of Ta also accelerated the recombination, and the absorption intensity at 1 ms (ΔAbs = 6.6 × 10−6) was much less than that of undoped and Ni monodoped SrTiO3. However, when Ni was co-doped with Ta, the absorption intensity at 1 ms increased to 9.6 × 10−4; the quantity of surviving electrons increased by greater than 10-fold. These results suggest that monodoping of Ni or Ta accelerates the recombination but co-doping of Ni with Ta significantly decelerates recombination. Thus, the quantity of surviving electrons in the microsecond region becomes larger than that in an undoped sample, which confirms that co-doping of Ni and Ta in SrTiO3 does not accelerate the recombination but instead increases the lifetime. A similar co-doping effect was reported for Cr/Sb co-doped TiO241 and Rh/Sb co-doped SrTiO3.11 In these cases, monodoping of Cr or Rh accelerated the recombination but Sb monodoping and Cr/Sb or Rh/Sb co-doping increased the lifetime, which supports that co-doping is necessary to increase photocatalytic activity. The energy dependence of the pump pulse on the excitation of electrons into the CB was examined by changing the wavelength of the pump laser pulse from 355 to 650 nm, and observing the intensity change at 2000 cm−1 to investigate the decay kinetics of the excited electrons. As shown in Figure 6, no signal was observed upon irradiation of the pump pulse from 575 to 650 nm, although the sample absorbed visible light from 400 to 900 nm (Figure 1). This result indicates that visible light irradiation at wavelengths longer than 575 nm did not excite electrons into the CB. Instead, the absorbed light energy was consumed for the upward d−d transition of electrons in the

Figure 4. Reactivity of photogenerated electrons and holes in undoped SrTiO3 irradiated by 355 nm laser pulses. Intensity changes at (A) 20000 cm−1, (B) 11000 cm−1, and (C) 2000 cm−1 were measured under a vacuum, 20 Torr O2, and 20 Torr MeOH vapor. Observed data and fitted curves are presented together.

The lifetimes of charge carriers were estimated by multiexponential curve fitting. In order to fit the observed decay curves properly, at least three components were necessary. The estimated values are summarized in Table S1, but the comparison of the three components is not easy. Therefore, we compared the absorption intensities at 1 ms for each decay curve. By the exposure to O2, the absorption intensities at 20000, 11000, and 2000 cm−1 become 1.8, 2.3, and 1/3.2 times compared to that in a vacuum. On the other hand, by the exposure to MeOH, the absorption intensities at 20000, 11000, and 2000 cm−1 become 1/1.7, 1/1.3, and 2.0 times compared to that in a vacuum. These results suggest that the band intensity at 2000 cm−1 is more sensitive to the exposure to O2 and MeOH. 3.3. Transient Absorption Spectra of Ni/Ta Doped SrTiO3 and Recombination Kinetics of Photogenerated Electrons in Undoped and Doped SrTiO3. Transient absorption spectra of transition metal doped SrTiO3 were obtained. For Ni monodoped SrTiO3 (Figure 3B), very broad absorption was observed in the entire region from 25000 to 5000 cm−1. However, the signal shape was different than that of the undoped sample; specifically, a new peak appeared at 20000 cm−1 and the absorption below 5000 cm−1 was significantly lower. The Ta monodoping also produced a different spectral shape (Figure 3C), where the absorption intensity below 5000 cm−1 was lower but the peak intensity at 20000 cm−1 was not as large as that of Ni-doped sample. For Ni/Ta co-doping (Figure 3D), the spectral shape was similar to that of the Ni monodoped sample, but the overall absorption intensity was

Figure 5. Decay curves of photogenerated electrons in undoped, 1% Ni, 1% Ta, and 1% Ni and 2% Ta co-doped SrTiO3 measured in a vacuum at 2000 cm−1. The catalysts were irradiated by 355 nm UV laser pulses at 0.5 Hz. Observed data and fitted curves are presented together. 8000

DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004

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Figure 7. Emission spectra of Ni/Ta co-doped SrTiO3 irradiated by 355 nm UV laser pulses (0.5 mJ pulse−1, repetition rate 5 Hz) in a vacuum. Figure 6. Decay curves of photogenerated electrons in Ni/Ta codoped SrTiO3 irradiated by UV (355 nm) and visible (425−650 nm) laser pulses (laser power was 0.5 and 10 mJ pulse−1 for 355 nm and visible light, respectively). Transient absorption at 2000 cm−1 was measured in a vacuum, and absorption intensity at 5 μs is plotted as a function of pump laser pulse wavelength. Observed data and fitted curves are presented together.

respectively. Therefore, the emission peak observed at 8000 cm−1 on Ni/Ta doped SrTiO3 also was assigned to the downward d−d transition represented in eq 1. The decay process of the emission from Ni/Ta-doped SrTiO3 was further investigated by comparing the decay curves of photogenerated electrons in the CB. Figure 8 shows the decay curves of emission at 8000 cm−1 and transient absorption intensity at 2000 cm−1. The emission almost completely diminished within 0.5 ms. The decay curve fit a doubleexponential function with lifetimes of 11.2 ± 0.2 and 243 ± 3 μs (Table S6). However, a considerable quantity of free or shallowly trapped electrons survived longer than 1 ms. These results confirm that the lifetime of free or shallowly trapped electrons is much longer than that of the emission from the doped Ni cations. The emission from Ni2+ could be induced via two pathways: the direct photoexcitation of Ni2+ and the indirect photoexcitation of the band gap of SrTiO3. For direct photoexcitation, the doped Ni2+ cations showed several absorption bands at ∼380, ∼630, and ∼1050 nm, which were assigned to the upward d−d transitions, 3A2g(3F) → 3T1g(3P), 3A2g(3F) → 3 T1g(3F), and 3A2g(3F) → 3T2g(3F), respectively.42,43 Therefore, the doped Ni2+ in SrTiO3 could be directly excited by the 355 nm laser pulse irradiation via 3A2g(3F) → 3T1g(3P). For Ni2+doped SrTiO3, these absorption bands were not clearly observed (Figure 1). Furthermore, the Ni2+ was embedded in the SrTiO3 particles; hence, most of the irradiated UV light

doped Ni cations, and did not contribute to the photocatalytic reactions.15,39 In contrast, the visible light pump pulses from 425 to 550 nm produced a transient absorption at 2000 cm−1, as shown in Figure 6. These results indicate that electrons are excited into the CB by these pump pulses. The quantity of photogenerated electrons increased with a decrease in wavelength of the pump pulse, although the laser power remained constant at 10 mJ pulse−1 (Figure 6, inset). The rate constant of the electron decay (the slope of the decay) was not dependent on the quantity of photogenerated charge carriers, where the slope of the decay curves was nearly parallel, which shows that the recombination follows first order kinetics. However, the lifetime of electrons excited by 355 nm UV pulse was much shorter than that excited by visible light pulses, which confirms that visible light is more effective for the reaction because the lifetime of electrons excited by visible light is longer than that of electrons excited by UV pulses. The absorption intensity excited by 355 nm pulse seems to be smaller than that excited by visible pulse, but this is ascribed to the weaker intensity of the irradiated laser pulse (0.5 mJ pulse−1). 3.4. The Role of Ni2+ as a Recombination Center Determined by Photoluminescence Spectroscopy. Doped transition metals are widely believed to act as recombination centers because transition metals can capture both electrons and holes. For Ni2+ in an octahedral site, recombination can be identified from the emissions, because the excited electrons in Ni2+ are relaxed via a downward d−d transition with the release of excess energy as photons, as shown in eq 1:42,43 Ni 2 +[3T2g (3F)] → Ni 2 +[3A 2g (3F)] + hν (downward d−d transition)

(1)

In this experiment, we measured emission spectra from 25000 to 1000 cm−1, and observed one emission band appeared at 8000 cm−1 (1250 nm) after 355 nm laser pulse irradiation (Figure 7). Similar emission spectra have been observed on Ni 2+ -doped Li 2 O−Ga 2 O 3 −SiO 2 glass-ceramics 42 and MgGa2O4,43 giving emission peaks at 1300 and 1135 nm,

Figure 8. Comparison of decay curves of transient absorption at 2000 cm−1 and emission at 8000 cm−1 in Ni/Ta co-doped SrTiO3. The catalyst was excited by 355 nm laser pulse (0.5 mJ pulse−1, repetition rate 0.5 Hz) in a vacuum. Observed data and fitted curves are presented together. 8001

DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004

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The Journal of Physical Chemistry C would be used to excite the band gap of the SrTiO3 powder. Therefore, this direct excitation process would not be the dominant process. However, if the electrons in Ni2+ were directly excited by UV light, they would relax from 3T1g(3P) to 3 T2g(3F) and produce emission at 8000 cm−1 via eq 1. For the indirect pathway, the band gap photoexcitation of SrTiO3 produced the emission from Ni2+ [3T2g(3F)] via holetrapping and subsequent electron-trapping, as follows: Ni 2 + + h+(VB) → Ni 3 +

(h+ trapping from VB)

(2)

Ni 3 + + e−(CB) → Ni 2 +[3T2g (3F)] (e− trapping from CB)

(3)

It is very difficult to distinguish between the direct and indirect pathways from the shape of emission spectra only, since the shape of the emission spectra of Ni2+ via these two processes should be identical. However, the analysis of the decay behaviors provides more detailed information about the difference. It is deduced that the doped Ni2+ cannot be the dominant recombination center that determines the overall lifetime of charge carriers in SrTiO3, because the lifetime of free and shallowly trapped electrons was obviously much longer than that of the emission. Furthermore, the quantity of surviving electrons was increased by Ni/Ta doping (Figure 5). These results confirm that co-doping Ni2+ with Ta5+ into SrTiO3 does not accelerate recombination. The doped Ni2+ acts as a recombination center when it traps an electron after holetrapping, as described in eqs 2 and 3. However, the probability of hole-trapping with subsequent electron-trapping by the same Ni atom is not great. In most cases, each Ni cation traps either electrons or holes and reduces the probability of encountering additional electrons and holes. Therefore, the Ni cation acts to increase the lifetime rather than accelerate the recombination. 3.5. Reactivity of Photogenerated Electrons and Holes in Ni/Ta Co-Doped SrTiO3. The results showed that the lifetime of photogenerated electrons increased by co-doping with Ni and Ta. However, the steady-state activity for H2 evolution under UV irradiation was drastically decreased. Photocatalytic activity is determined not only by the rate of recombination but also by the rate of charge transfer to the reactant molecules. Therefore, the reactivity of photogenerated electrons and holes was examined. Decay curves of transient absorption at 20000, 11000, and 2000 cm−1 of Ni/Ta-doped SrTiO3 after 355 nm laser irradiation were examined. As shown in Figure 9C, the decay of electrons producing the absorption band at 2000 cm−1 was decelerated in the presence of MeOH vapor. However, the decay was not accelerated by exposure to O2, as it was for undoped SrTiO3. These results suggest that the reactivity of holes is maintained but reactivity of electrons was decreased. Similar results were observed for intensity changes at 20000 and 11000 cm−1;, i.e., decay was decelerated by exposure to MeOH vapor but not by exposure to O2, which indicates that the absorption intensities at 20000 and 11000 cm−1 reflect the quantity of electrons. In each case, the electrons did not react with exposed O2. This result suggests that the decrease in reactivity of electrons is the main reason for the drastic decrease in steady-state activity for H2 evolution under UV irradiation. Several mechanisms are responsible for the decrease in reactivity of the photogenerated electrons. A frequently proposed mechanism involves deep electron trapping at defects such as oxygen vacancies or doped Ni cations. Substitution of Ti4+ with low-valence cations, such as Ni2+ and Ni3+, induces

Figure 9. Reactivity of photogenerated electrons and holes in Ni/Ta co-doped SrTiO3 irradiated by 355 nm laser pulses (0.5 mJ pulse−1, repetition rate 0.5 Hz). Intensity changes at (A) 20000 cm−1, (B) 11000 cm−1, and (C) 2000 cm−1 were measured under a vacuum, 20 Torr O2, and 20 Torr MeOH vapor. Observed data and fitted curves are presented together.

oxygen vacancies to maintain the charge balance, even when high-valence cations, such as Ta5+ or Sb5+, are co-doped.39 The deep electron trapping at the Ni3+ (eq 3) and Ti4+ at the oxygen vacancy drastically decreases their reactivity. The photogenerated holes also should be trapped at the defects; however, the position of the valence band is high enough for the oxidation of MeOH, so trapping of electrons rather than holes causes the drastic decrease in steady-state reactivity. Deeply trapped electrons can be identified from their transient absorption spectra. Highly reactive free and shallowly trapped electrons produce absorption bands in the mid-IR region, but the deeply trapped electrons produce absorption bands in the visible to NIR region.29,40 As shown in Figure 9A and B, band intensity at 20000 and 11000 cm−1 reflected the quantity of deeply trapped electrons. Furthermore, the peak intensity at 20000 cm−1 was increased by Ni- and Ni/Tadoping (Figure 3B and D), which suggests that the defects trapping electrons are created by Ni-doping. Note that the absorption intensity at 20000 and 11000 cm−1 of undoped SrTiO3 mainly reflects the quantity of holes but that, for Ni/Tadoped SrTiO3, the assignment was the opposite and the absorptions were assigned to electrons. These results suggest that the number of defects that deeply trap electrons increased by Ni/Ta-doping. This curious behavior was induced by a change in the properties of the defects, and has been observed for other commercial SrTiO329,40 and TiO225 powders. Absorption assignments in the visible-to-NIR region were 8002

DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004

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complicated because the contribution from both trapped electrons and holes was present. However, this analysis is still useful for determining the properties of the defects on the catalysts. In the mid-IR region, a broad absorption band appears below 5000 cm−1, which was attributed to free or shallowly trapped electrons. A small peak appeared at 2500 cm−1, suggesting that a portion of the electrons were shallowly trapped at defects located below ∼0.3 eV (2500 cm−1) from the CB. However, the spectral shape was similar to that of the undoped sample. Therefore, some of the electrons in the Ni/Ta-doped SrTiO3 must be energetically reactive as they are in the undoped SrTiO3. Other factors may be responsible for the low reactivity of electrons. The spatial distribution of electrons and holes in the powder particles is involved, as well as the size of the particles created by the solid-state reaction, which is larger than hundreds of nanometers (Figure 1, inset). In these large particles, formation of band-bending would not be negligible. In principle, doped Ni cations act as acceptors for p-type conductivity. However, as mentioned above, Ni-doping induces oxygen vacancies with increasing electron density, which enhances n-type conductivity. As a result, Ni-doping results in n-type doping44 and upward band bending occurs at the interface that enhances the charge separation. Thus, electrons and holes are forced to migrate into the bulk and the surface, respectively, increasing the lifetime of charge carriers. In this situation, holes accumulated at the surface readily reacted with adsorbed molecules, but electrons were not accessible to reactant molecules on the surface. Thus, the lifetime of electrons increased, but reactivity decreased. These results can be reasonably explained in this band-bending model.

SanDisk Corporation 800, Yamanoisshiki-cho, Yokkaichi, Mie, 512-8550, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the PRESTO/JST program “Chemical Conversion of Light Energy”. The authors would also like to acknowledge the Grant-in-Aid for Specially Promoted Research (No. 23000009) and Basic Research (B) (No. 23360360) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. One of the authors (A.Y.) also thanks the Nippon Sheet Glass Foundation and the Nagai Foundation for Science and Technology.

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4. CONCLUSION In this work, we have studied the recombination kinetics and the reactivity of photogenerated electrons and holes in Ni/Ta co-doped SrTiO3 photocatalysts. Ni and Ta co-doping enhanced photocatalytic activity under visible light irradiation but decreased the activity under UV light irradiation. The results showed that the doping extended the lifetime of photogenerated charge carriers but decreased the reactivity of photogenerated electrons. Since overall photocatalytic activity is determined by competition between recombination and charge transfer to the reactant molecules, the deactivation of the electron-consuming reaction was responsible for the decrease in overall photocatalytic activity. Therefore, enhancement of the reactivity of electrons is required for further improvement in steady-state photocatalytic activity under solar light irradiation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01494. Curve fitting results of decay curves (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-52-809-1828. Present Addresses §

Daihatsu Motor Co., Ltd. 3000, Yamanoue, Ryuo-cho, Gamogun, Shiga, 520-2593, Japan. 8003

DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004

Article

The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.6b01494 J. Phys. Chem. C 2016, 120, 7997−8004