Behavior and Energy State of Photogenerated Charge Carriers in

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Behavior and Energy State of Photogenerated Charge Carriers in Single-Crystalline and Polycrystalline Powder SrTiO Studied by TimeResolved Absorption Spectroscopy in the Visible to Mid-Infrared Region 3

Akira Yamakata, Junie Jhon M. Vequizo, and Masayuki Kawaguchi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510647b • Publication Date (Web): 22 Dec 2014 Downloaded from http://pubs.acs.org on January 10, 2015

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The Journal of Physical Chemistry

Behavior and Energy State of Photogenerated Charge

Carriers

in

Single-crystalline

and

Polycrystalline Powder SrTiO3 Studied by Timeresolved Absorption Spectroscopy in the Visible to mid-Infrared Region

Akira Yamakata1, 2*, Junie Jhon M. Vequizo1, and Masayuki Kawaguchi1

1

Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku,

Nagoya 468-8511, Japan 2

Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and

Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.

Corresponding Author *Akira Yamakata, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The effects of defects on the behavior of photogenerated charge carriers in SrTiO3 (STO) are studied by time-resolved absorption spectroscopy from the visible to mid-IR region. In the case of defects-free single-crystalline STO, free and shallowly trapped electrons are dominant, but they recombine within 50 ns. By contrast, in the case of defect-rich powder STO, the electron lifetime is much longer than 1 ms. The transient absorption spectra show that most of the charge carriers in powder STO are trapped in the defects, which elongates their lifetime. We found that these trapped carriers nevertheless reactivity toward O2 or CH3OH that depends on the trap depth. The steady-state photocatalytic activity is strongly correlated with the lifetime and the reactivity of the trapped charge carriers: the energy state of electrons can be deduced from the spectral shape, especially in the mid-IR region.

KEYWORDS: photocatalysis • carrier dynamics • recombination • trap • time-resolved absorption spectroscopy


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1. Introduction Photocatalysts based on strontium titanate (SrTiO3; STO) have been intensively studied in recent years because of their potential applications in the solar-driven water splitting reaction.1-4 The properties of STO can be easily controlled by doping, and thus, STO has been widely used in many electronic devices, such as thermoelectric conversion elements,5 capacitors,6, 7 transparent electrodes,8 gas sensors,9 and solar cells.10-12 For electronic devices, more often than not, defect-free single-crystalline STO (STO-SC) is used, since defects have negative effects on the optical properties and carrier dynamics.13,

14

15

However, for

photocatalysts, polycrystalline powder STO (STO-P) with a particle size of less than a few microns has been preferred because it has a larger specific surface area than STO-SC, which enhances the photocatalytic activity. However, powder particles are rich in surface defects, which capture charge carriers: charge trapping is believed to accelerate the electron-hole pair recombination and decrease the photocatalytic activity; however, powder STO is still suitable for photocatalysts. This contradiction comes from the uncertainty of the effects of defects on photocatalytic activity. Therefore, for the development of highly efficient photocatalysts, the effects of the defects on the energy states as well as on the behavior of photogenerated charge carriers should be investigated. Time-resolved absorption spectroscopy (TAS) in the visible (Vis) and mid-infrared (IR) regions is useful for examining the decay kinetics of photogenerated charge carriers because the photogenerated holes and electrons exhibit TA signals in the Vis and mid-IR regions, respectively.16 By observing the absorption peaks and the intensity change of the TA spectra, trapping of charge carriers as well as the recombination kinetics can be studied. These methods have been used to investigate the behavior of photogenerated electrons and holes in TiO2,17-21


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NaTaO3,22,

23

GaN,24 SrTiO3,4 and LaTiO2N,25,

26

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however, most of these studies have been

performed in a very limited spectral region, namely, the Vis or IR region. TA measurements performed in the wide spectral region from Vis to mid-IR (400 nm to 10 µm) provide useful information, not only about the decay kinetics but also the energy states of trapped charge carriers:20, 26 free, shallowly trapped, and deeply trapped electrons can be distinguished since this wide region covers the energy range from 0.1 to 3 eV. Thus, many optical transitions induced by trapped charge carriers can be detected. By using this method, we have recently succeeded in observing that CoOx-loading of LaTiO2N photocatalysts reduces the electron trap depth and increases the reactivity of the trapped electrons.26 In the present work, the effects of defects on the behavior of photogenerated charge carriers are investigated by means of TAS in the Vis to mid-IR region. Defect-free STO-SC and two different commercial defect-rich STO-Ps were used, and the behavior of charge carriers and the steady-state photocatalytic activity were examined. Through analysis of the TA spectra, we found that free and/or shallowly trapped electrons are dominant in STO-SC, but their lifetime is shorter than 50 ns. In the case of STO-P, most of the electrons and holes are trapped in mid-gap states that arise from defects, and the lifetimes of photogenerated electrons and holes become longer than several ms. It has been widely believed that defects accelerate recombination, but the present study clearly demonstrated the opposite trend. It is noted that these trapped electrons still show reactivity for the photocatalytic reactions. Shallow trapping reduces the reactivity to some extent, but it can decelerate the recombination and enhance the overall photocatalytic activity. We have also shown in the present work that the reactivity of the surviving electrons can be predicted by analyzing the transient absorption spectra, especially in the mid-IR region.


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2. Experimental section TAS measurements were performed by using laboratory-built spectrometers as described in our previous papers.18, 19, 24, 26 Briefly, in the IR region, IR light from a MoSi coil was focused on the sample and the transmitted light was dispersed by the spectrometer. The monochromated light was detected by a photovoltaic MCT detector (Kolmar), and the output electric signal was amplified by AC-coupled amplifiers (NF Corporation NF5307 with 10 MHz bandwidth; Stanford Research Systems SR560 with 1 MHz bandwidth). The time resolution of these spectrometers is limited to ~50 ns and 1 µs by the bandwidth of the amplifiers, respectively. The transient absorption change was recorded by a digital oscilloscope after band gap photoexcitation using 355 nm pulses from a Nd:YAG laser (Continuum, Surelite I, 6 ns duration, 10-0.01 Hz). In the visible and near-IR (NIR) region, the probe light from a halogen lamp (50 W) was focused on the sample, and the transmitted or diffuse reflected light was dispersed by the spectrometer. The monochromated output was detected by Si-photodiode or InGaAs detectors. Undoped single-crystalline STO (orientation: [100]; dimensions: 10×10×0.5 mm) was purchased from Shinkosha Co., Ltd. As for STO-P, two commercial polycrystalline powders with different particle sizes were used, with the expectation that their defect properties would be different: these were labeled STO-P1 (Aldrich, Co.; particle size ≈ 100 nm; specific surface area: 99 m2 g-1) and STO-P2 (Kojundo Co., Ltd.; particle size: a few micrometers; specific surface area: 3.5 m2 g-1). Each STO-P was fixed on a CaF2 plate at a density of 2 mg cm-2 and placed in an IR cell for TAS measurements. The steady-state photocatalytic activities of the STO-P were determined by measuring the rate of H2 evolution. Pt was loaded on the STO-P (0.6 wt%) by photodeposition and dispersed in 200 mL of 10 vol% aqueous CH3OH solution. The STO-P catalysts were then irradiated by a 300 W


5


Xenon lamp (Excelitas Technologies, Inc., PE300BUV). The amount of H2 gas generated was analyzed by gas chromatography with TCD detectors (Shimadzu, GC8A).

3. Results and Discussion 3.1 Transient absorption spectra of Single-crystalline SrTiO3 The TA spectrum of STO-SC measured at 0 ns after the band gap (3.2 eV) photoexcitation using 355 nm (3.5 eV) pulses is shown in Fig. 1. Clearly, a structureless broad absorption was observed upon UV irradiation in the entire wavenumber region from 25000 to 2500 cm-1 (400 nm to 4 µm). The absorption intensity increased monotonically with decreasing wavenumber. These features are characteristics of the intra-band transition of free electrons in the conduction band (CB) and/or direct excitation of shallowly trapped electrons to the CB. A broad peak appears at ~2500 cm-1 which implies that shallow trap is present on the sample with depths shallower than 0.3 eV (~2500 cm-1) to the CB: it is noted that the energy from the electron trap maximum to the CB minimum is much smaller

10

Therefore, the absorption is

assigned to the free electrons in the CB and shallowly trapped electrons below CB. It is important to note that the deeply trapped carriers exhibit discernable absorption peaks in the Vis to NIR region.16 As seen in Fig. 1, no peaks are observed in the TA spectrum from 25000 to 2500 cm-1. The absence of any peaks in the TA spectrum suggests that the populations of deeply

8 6 4

∆Absorbance / x 10-3

than 0.3 eV.

16, 27

∆Absorbance / 10-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8

Vacuum O2

4

CH3OH

0 0 20 0 400 Delay Time / ns

2 0 25000

20000

1 5000

10000

5000

1000

Wavenumber / cm-1 Fig. 1 Transient absorption spectrum for singlecrystalline SrTiO3 measured at 0 ns after irradiation by -1 355 nm laser pulses (0.5 mJ pulse with a repetition rate of 10 Hz). Inset shows the decay curves for the -1 photogenerated electrons measured at 2500 cm in vacuum, 20-Torr O2 and CH3OH.


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trapped electrons and holes are much smaller compared to those of free and shallowly trapped electrons. The decay kinetics of photogenerated electrons in STO-SC is examined by observing the intensity change at 2500 cm-1, as shown in the inset of Fig. 1. The rise time of the transient absorption and its decay are comparable (~50 ns). This bilaterally symmetric shape of the decay curve confirms that the lifetime of electrons is shorter than the time resolution of the spectrometer (~50 ns). The reactivity of photogenerated charge carriers is also examined by exposing the sample to reactive gases such as O2 and CH3OH vapor. However, the resulting decay curves were identical to those obtained in a vacuum. This suggests that no chargeconsuming reactions occur with O2 or CH3OH. The low activity can presumably be attributed to the extremely small specific surface area of the single crystal: the amount of adsorbates is too small for the carriers to meet with the reactant molecules at the surface within their much shorter lifetime. Therefore, most of the charge carriers recombine before they are consumed by the reactant molecules. The small specific surface area is the most serious disadvantage of the application of single crystals to photocatalysts.

3.2 Transient Absorption spectra of polycrystalline powder SrTiO3 The measured TA spectra for the commercial powders STO-P1 and STO-P2 are shown in Fig. 2. The shapes of the TA spectra obtained for STO-P1 and STO-P2 differ completely from that of STO-SC, where at least two peaks appear at 20000 and 11000 cm-1 that are absent in the case of STO-SC. Therefore, these peaks are ascribed to the photogenerated carriers trapped at defects on the powder catalysts. Defects are present on the STO-SC surface as well, but the defect density is considerably lower than that on STO-P because the specific surface area of STO-SC is much


7


smaller than those of STO-P1 and STO-

5 µs

(a) STO-P1

-3

∆ A bs orban ce / x10

-3

∆Abs = 1 x 10

P2. Thus, the absorption intensity of the 10 µs

trapped charge carriers would be lower in

0.6

5 µs

0.4

1 ms

0.0 6 0 00

the case of STO-SC. It should be noted

20 µs

that the spectral shape of the trapped

100 µs

4 0 00

2 0 00

W ave num be r / cm

-1

∆Abs = 1 x 10

-3

1 ms

carriers

is

sensitive

to

the

0 2 5000

2 0000

1 5000

5000

1000

-1

properties of the powder particles. Indeed,

Wavenumber / cm

(b) STO-P2

as seen in Fig. 2, the shapes of the TA

∆Abs = 2 x 10

-3

spectra for STO-P1 and STO-P2 are 5 µs

different: from 6000 to 1000 cm-1, the

2 .0

5 µs

1 .0

0 .0

1 ms 6 0 00

absorption intensity for STO-P1 increases

4 0 00

2 0 00

10 µs

W aven um ber /cm

20 µs

∆Abs = 2 x 10

-1

-3

100 µs

with decreasing wavenumber, while that 0

for STO-P2 decreases. Furthermore, the

10000

-3

charge

∆ A b sorba nce / x 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 ms

2 5000

2 0000

1 5000

1 0000

5000

1000

-1

absorption peak at 11000 cm-1 for STOP2 is stronger than that for STO-P1. The

Wavenumber / cm

Fig. 2 Transient absorption spectra for commercial SrTiO3 powders (a) STO-P1 and (b) STO-P2, irradiated by UV (355 nm) laser pulses under vacuum. Pump energy is 0.5 mJ per pulse, and repetition rate is 5 Hz. Inset is an expansion of the mid-IR region.

dissimilarity of the spectra for STO-P1 and STO-P2 is due to the difference in the energy states of the trapped charge carriers, which in turn is attributable to the difference in the structure of the defects on the STO particles. The decay kinetics of photogenerated charge carriers in STO-P1 and STO-P2 is further examined by observing the intensity change at 2500 cm-1. As seen in Fig. 3, the lifetimes of charge carriers on STO-P1 and STO-P2 are both longer than milliseconds. It is widely believed that surface defects work as recombination centers, however, unexpectedly, the lifetime of charge carriers in defect-rich STO-P is much longer than that in defect-free STO-SC. These


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results imply that surface defects serve to elongate the lifetime of charge carriers. It is simulated by Nelson et al.

28, 29

that the charge trapping at the defects can decelerate recombination:

trapping and detrapping from the traps limits the transportation of the charge carriers and then reduces the probability to encounter electrons and holes. This process is experimentally well studied by Nelson et al. 29 and Wang et al. 30 In addition, the signal intensity of STO-P1 at 0-10 µs is larger than that of STO-P2, but decays faster at < 10 µs. This difference in lifetime is also attributable to the dissimilarity in the structure of the surface defects, i.e., trapping affects the energy states as well as the lifetime of charge carriers.

3.3 Decay kinetics of photogenerated charge carriers in polycrystalline powder SrTiO3 Information about the reactivity of long-lived charge carriers is important for determining the steady-state activity of photocatalytic reactions. This information can be obtained by exposing the photocatalyst to reactant molecules such as O2 gas and CH3OH vapor, which consume electrons and holes, respectively. Figure 4 shows the decay curves of transient STO-P1

absorption at 20000, 11000, and 2500 cm-1 for STO-P1. As seen in Fig. 4a, upon exposure to O2, the decay of the band intensity at 2500 cm-1 is accelerated at 0-3

∆Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10-3

STO-P2

µs, suggesting that the intensity at 2500 cm1

reflects the number of electrons and the

electron-consuming reaction proceeds at 0-3 µs. By contrast, when STO-P1 is exposed to

10 ns 10 0 ns 1 µs

10 µs 100 µs 1 m s

T im e Delay Fig. 3 Decay curves of transient absorption intensity at -1 2500 cm for commercial SrTiO3 powders STO-P1 and STO-P2, irradiated by UV (355 nm, 0.1 Hz, .5 mJ per pulse) laser pulses under vacuum.


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CH3OH vapor, the decay of electrons is decelerated at 0-3 µs owing to the hole-consuming reaction by CH3OH, which prevents the recombination of electrons and thus elongates their lifetime. A similar electron-consuming reaction by O2 and hole-consuming reaction by CH3OH are reported to take place on TiO219, 31 and NaTaO3,22 and gives similar intensity changes of electrons at 2000 cm-1. In the case of the band intensity at 11000 cm-1 (Fig. 4b), the decay is similar to that at 2500 cm-1, where the decay STO-P1

is accelerated in O2 and decelerated in CH3OH. These results suggest that this band intensity reflects the number of electrons. However, the decay at 20000 cm

-1

exhibits

∆ Absorbance

(a) 2500 cm-1 CH3OH 10-3 va cu u m

O2

10-4

the opposite trends: the decay at 0-1 ms is

1 µs

1 0 µs 1 00 µs 1 m s

T im e D elay

decelerated in O2 and accelerated in CH3OH

at 20000 cm-1 reflects the number of holes in STO-P1.

∆ Absorbance

(Fig. 4c). This suggests that the band intensity

-2

10

-3

10

STO-P1, the band intensity at 2500 cm-1 decreases (increases) upon exposure to O2 (CH3OH), whereas that at 20000 cm-1 increases slightly upon exposure to O2. However, the reactivity of electrons and holes

CH3OH

va cu u m

1 µs

1 0 µs

10 0 µs

1 ms

T im e D ela y

carriers with O2 and CH3OH on STO-P2 was also examined (see Fig. 5). As in the case of

(b) 11000 cm-1

O2

The reaction of photogenerated charge

-1 10-2 (c) 20000 cm

∆ Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O2

va cu u m 10-3

C H 3O H 1 µs

10 µs

10 0 µs

1 ms

T im e D elay Fig. 4 Decay curves of transient absorption of the powder STO-P1 irradiated by 355 nm laser pulses (0.5 mJ per pulse at 0.1 Hz). The decay curves were measured at -1 -1 -1 2500 cm (A), 11000 cm (B), and 20000 cm (C) in vacuum, 20 Torr O2, and CH3OH.


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in STO-P2 is different from that in STO-P1: the rate of the electron-consuming reaction on STOP2 (Fig. 5a) is less than 10 µs, and the amplitude of the intensity change is much lower than on STO-P1 (Fig. 4a). Furthermore, the reactant-induced hole decay at 20000 cm-1 is negligibly small (Fig. 5c). These results confirm that the charge carriers in STO-P2 have a lower activity than those in STO-P1. A more dramatic difference in the behavior of charge carriers in STO-P2 can be inferred from the intensity change at

decay,

where

the

intensity

decreases

(increases) upon exposure to CH3OH (O2), is

-1

∆ Absorbance

11000 cm-1: the reactant-induced charge

STO-P2 (a) 2500 cm

C H 3O H

10-3

va cuu m O2

exactly opposite that on STO-P1 (Fig. 4b).

1 µs

10 µs

This result suggests that in STO-P2, the -2

10

mainly

reflects the number of holes, whereas in the case of STO-P1, it reflects the number of

∆ Absorbance

absorption intensity at 11000 cm

-1

100 µs

1 ms

T im e D elay (b) 11000 cm-1 O2 vacuu m 10-3 C H 3O H

electrons. These opposite assignments are

1 µs

10 µs

100 µs 1 m s

Time Delay

attributable to structural differences between the defects in STO-P1 and STO-P2. The structure of the powder particles and the impurity amount vary depending on the synthesis method. Therefore, the structure of

10-2 (c) 20000 cm-1

∆ Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O2 vac uu m 10-3 C H 3O H

1 µs

10 µs

100 µs 1 m s

Time Delay

the defects on STO-P1 and STO-P2 must be different. It is widely believed that oxygen vacancies

(Vo)

and

Fig. 5 Decay curves of transient absorption of the powder STO-P2 irradiated by 355 nm laser pulses (0.5 mJ per pulse at 0.1 Hz). The decay curves were measured at -1 -1 -1 2500 cm (A), 11000 cm (B), and 20000 cm (C) in vacuum, 20 Torr O2, and CH3OH.

coordinatively


11


unsaturated Ti4+ serve as electron traps, while the surface hydroxyl groups serve as hole traps. Moreover, Vo and Ti3+ in the complex (Ti3+- Vo -Ti3+) are proposed to capture electrons and holes, respectively.14 The depth and properties of these traps are sensitive to the structure of the defects. Thus, each trapped electron and hole would give absorption peaks at different wavenumbers. As seen in Figure 2, the shape of the transient absorption spectra of photocarriers in STO-P1 and STO-P2 are different, implying that several different peaks are involved. This confirms that the energy state of trapped charge carriers depends on the structure of the defects. In the present stage, it is very difficult to discuss the detailed structure of the defects, but this difference should bring difference in the reactivity of charge carriers on STO-P1 and STO-P2.

3.4 Steady-state photocatalytic activity on different two polycrystalline powder SrTiO3 Steady-state photocatalytic activities on STO-P1 and STO-P2 are compared by monitoring the H2 evolution from an aqueous CH3OH solution (as illustrated in Fig. 6). The activity of H2 evolution obtained from STO-P1 is higher than that from STO-P2; the measured rates are 173 and

54

µmol

h-1,

respectively.

This

difference in photocatalytic activity is ascribed to the difference in behavior of the photogenerated charge carriers. As shown in Fig. 3, the number of electrons surviving in STO-P1 at 0-10 µs is larger than that in

A m oun t o f e volved H2 / µmol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 18

4 00

3 00

STO-P1

2 00 STO-P2

1 00

0 0.0

0.5

papers that photocatalytic activity has a relatively strong correlation with lifetime of

1.0

1.5

2.0

T im e / h

STO-P2. We have reported in our previous

Fig. 6 Time courses of photocatalytic H2 evolution from 10 vol% aqueous CH3OH solution over powder SrTiO3, STOP1, and STO-P2. Pt (0.6 wt%) was loaded on STO and irradiated by a 300-W Xe lamp.


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photogenerated charge carriers in the oxide photocatalysts tested: TiO2,32 NaTaO3,22 and BiWO3.33 In the present case, the consistent result was obtained in the relation between the steady-state activity and the lifetime of charge carriers within 10 µs. However, it should be noted that the steady-state activity is not solely determined by the lifetime of charge carriers: the reactivity of photogenerated electrons and holes should be also considered. It is evident from Figures 4a and 5a that the electrons in STO-P1 have a higher activity than in STO-P2: the electron-consuming reaction by O2 occurs within 3 µs on STO-P1 (Figure 4a), whereas it lasts longer than 10 µs on STO-P2 (Figure 5a). The photocatalytic activity is determined by the competition between the recombination and the charge-consuming reaction by the reactant molecules. Therefore, the activity increases as the charge transfer rate increases. The higher reactivity of photogenerated carriers in STO-P1 compared to that in STO-2 is also responsible for the higher activity of STO-P1. It is noted that STO-P2 retains more than 30% activity of the STO-P1 despite of the markedly slow charge consuming reactions and smaller specific surface area. This result is accounted for by considering the larger number of surviving charge carriers after 10 µs in STO-P2 as compared to that on STO-P1. Since the charge consuming reaction on STO-P2 proceeds after 10 µs, the longer lifetime of charge carriers can partly compensate the disadvantage of the slow charge consuming reactions. This result confirms us again that the photocatalytic activity is not determined solely by the lifetime or the rate of the charge consuming reactions: both should be equally considered. Photocatalysts with a larger specific surface area often have a higher activity because the amount of adsorbed reactant is larger, and thus, charge carriers are more accessible to the reactant molecules. In the present case, STO-P1 has a larger specific surface area than STO-P2,


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and therefore the higher activity of STO-P1 can be partly attributed to the specific surface area. However, the enhancement in photocatalytic activity cannot always be accounted for by a larger specific surface area. The behavior and energy states of photogenerated charge carriers should be considered, because they determine the quantum efficiency of the photocatalytic reactions. Especially, the depth of the trap should be considered because it affects the mobility and energy level of the charge carriers needed to reach the redox potentials to induce the reactions. In principle, the energy states of photogenerated electrons can be deduced from an analysis of the shapes of the transient absorption spectra, especially in the mid-IR region: free electrons give structureless broad absorption with increasing the intensity at lower wavenumbers but deeply trapped electrons give peaks. In the case of highly reactive STO-P1, the spectral shape exhibits characteristics ascribable to free and shallowly trapped electrons: the absorption intensity increases from 6000 to 1000 cm-1 with a small peak at 2500 cm-1. But for poorly reactive STOP2, the spectral shape is the opposite: the absorption intensity decreases monotonically from 6000 to 1000 cm-1. We note that several broad absorption bands are involved in the TA spectra and they are difficult to separate distinctively. In principle, free and shallowly trapped electrons should be more reactive than the deeply trapped electrons; therefore information of the energy state of charge carriers is very important to understand the photocatalytic activities.

4. Conclusion In summary, we have elucidated the effects of defects in single-crystalline SrTiO3 and polycrystalline powder SrTiO3 (STO-SC and STO-P, respectively) on photocatalytic activity. Time-resolved visible to mid-IR absorption spectroscopy is utilized to investigate the effects of defects on the energy states as well as the behavior of photogenerated charge carriers. Defect-


14

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free STO-SC and defect-rich STO-P were compared. Band gap photoexcitation on STO-SC exhibits a structureless broad absorption from 25000 to 2500 cm-1 with increasing intensity at lower wavenumbers. This feature is ascribed to free and shallowly trapped electrons. However, in the case of the powder SrTiO3 photocatalyst, broad peaks appeared at 20000 and 11000 cm-1, which are associated with trapped charge carriers at surface defects. Even though STO-P is richer in surface defects than STO-SC, the lifetime of photogenerated charge carriers is much longer in the former. This is because the trapping and detrapping of electrons and holes by surface defects decreases their mobility and thus decelerates recombination. It is noted that trapped charge carriers still show reactivity toward O2 and CH3OH. It is widely believed that trapping of charge carriers dramatically decreases the photocatalytic activity; however, the results presented in this work confirm that trapping does not always degrade photocatalytic activity. Shallow trapping reduces the reactivity to some extent, but it can decelerate recombination and enhance the overall photocatalytic activity. We have demonstrated in this work that time-resolved absorption measurements, especially in the mid-IR region, provide useful information about the energy states as well as the reactivity of photogenerated charge carriers. 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.) thanks the Nippon Sheet Glass Foundation for Materials Science and Engineering for funding support.


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Single-Crystalline SrTiO3 trapped e- & h+

Transient Absorption

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e-

h+

Powder SrTiO3 free eeh+

Visible

Wavenumber / cm-1

mid-IR


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Behavior and Energy State of Photogenerated Charge Carriers in

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