Correlation between Photocatalytic Activity and Carrier Lifetime: Acetic

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Correlation between Photocatalytic Activity and Carrier Lifetime: Acetic Acid on Single-Crystal Surfaces of Anatase and Rutile TiO

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Kenichi Ozawa, Susumu Yamamoto, Ryu Yukawa, Ro-Ya Liu, Naoya Terashima, Yuto Natsui, Hiroo Kato, Kazuhiko Mase, and Iwao Matsuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02259 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Correlation between Photocatalytic Activity and Carrier Lifetime: Acetic Acid on Single-Crystal Surfaces of Anatase and Rutile TiO2 Kenichi Ozawa,∗,† Susumu Yamamoto,‡ Ryu Yukawa,‡ Ro-Ya Liu,‡ Naoya Terashima,¶ Yuto Natsui,¶ Hiroo Kato,¶ Kazuhiko Mase,§,∥ and Iwao Matsuda‡ †Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan ‡Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan ¶Department of Advanced Physics, Hirosaki University, Hirosaki, Aomori 036-8561, Japan §Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan ∥SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801, Japan E-mail: [email protected]

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Abstract Photocatalytic activity and lifetime of photoexcited carriers on well-defined single crystalline anatase and rutile TiO2 surfaces with different surface orientation have been systematically studied by photoelectron spectroscopy. Photocatalytic activity, evaluated with reference to the photocatalytic degradation of acetic acid, has a positive and linear correlation with carrier lifetime at the crystal surface, which was determined by following the time evolution of the ultraviolet-induced surface photovoltage. This indicates that the carrier lifetime is a prime factor for the photocatalytic activity so that it can be viewed as the origin of the crystal-surface-orientation dependence of the photocatalytic activity.

Introduction The development of laser-assisted time-resolved measurement techniques has facilitated significant advances in our understanding of photoinduced charge carrier dynamics, stimulating fruitful discussion on the unique photoresponsive functionalities of metal oxide materials in the scientific community. The various photocatalytic properties that TiO2 crystals exhibit may be explained in terms of the behavior of photoexcited carriers in the bulk and on the surface of the crystals. It has long been recognized that the photocatalytic activity of anatase TiO2 (a-TiO2 ) is, in general, higher than that of rutile TiO2 (r-TiO2 ). 1–5 Previous studies have concluded that this polymorph dependence arises due to the different carrier lifetimes in a-TiO2 and r-TiO2 , which is a result of their different bulk band gap types, i.e., indirect and direct for a-TiO2 and r-TiO2 , respectively. On the other hand, it has also been recognized that photocatalytic activity depends on the orientation of the crystal surface. For example, the (101) surface of single-crystal a-TiO2 is more active than the (001) surface in various photocatalytic oxidation and reduction reactions. 6,7 Regarding the r-TiO2 surfaces, the photocatalytic degradation of methyl orange is highest on the (011) surface, followed by the (110), (001), and (100) surfaces. 4 Contrastingly, 2


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the order of activity for the photooxidation of methanol is r-TiO2 (001) > (100) > (110). 8 The face-dependence of properties such as the redox potential, 6 anisotropic diffusion of the charge carriers in the bulk, 4 electron-trapping probability, 7 an efficiency of photogenerated electron-hole pair utilization, 9 etc. have been proposed as the origin of the face-dependent activities. Nevertheless, these carrier-oriented mechanisms do not comprehensively explain the reported results. Therefore, a more detailed picture of the carrier behavior on each crystal surface must be drawn to further understand this phenomenon. Photogenerated carriers at a semiconductor surface behave under the strong influence of the electric field in the space-charge layer (SCL), which is formed when a charge imbalance exists between the surface and the bulk. 10 The SCL is sensitive to the environment surrounding the crystal because adsorption of foreign molecules leads to modification of the surface charge. Thus, in order to elucidate the relationship between photocatalytic activity and carrier dynamics, both must be assessed under the same experimental conditions. However, although many previous studies have addressed the activity-dynamics relationships of TiO2 photocatalysts, 5,11,12 systematic and comparative analyses, especially those using well-defined single-crystal TiO2 surfaces in a controlled environment, are limited to a few studies. 5 This is a major obstacle to our understanding of the origin of the face-dependent activity of TiO2 . In the present study, we systematically assessed photocatalytic activities and photocarrier lifetimes on single-crystal a-TiO2 and r-TiO2 surfaces under ultrahigh vacuum (UHV) conditions by X-ray photoelectron spectroscopy (XPS) and time-resolved XPS (TRXPS). The photocatalytic activity was evaluated by an rate constant of a photocatalytic decomposition and desorption of acetic acid. The carrier lifetime on each TiO2 surface was determined from a time evolution of the ultraviolet (UV)-induced surface photovoltage (SPV). A linear and positive correlation is found between the photocatalytic activity and the carrier lifetime.

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Experimental Section XPS measurements The photocatalytic activity was determined using acetic acid as a probe molecule. The degradation of acetic acid adsorbed on TiO2 was induced by irradiation with an UV laser and was qualitatively evaluated from the decrease in the C 1s peak intensity as obtained by XPS at beamline BL-13B of the Photon Factory, High Energy Accelerator Research Organization (KEK). 13 XPS spectra were acquired by a hemispherical electron-energy analyzer (Gamma Data/Scienta SES200) with a total energy resolution of 60 meV and 350 meV at photon energies (hν) of 100 eV and 753 eV, respectively. An angle between the analyzer lens axis and the synchrotron radiation (SR) light was 65◦ , and the TiO2 sample was set so that the normal direction was parallel to the lens axis. Since the operation mode for the SR source was a storage mode during the XPS measurements, the SR flux varied with time. To compensate the light intensity variation, the intensity of the XPS spectra was normalized by a mirror current of a focusing mirror at the final stage of the beamline. The XPS measurements were carried out at room temperature. The binding energy of the spectra was referenced to zero at the Fermi edge of a Ta foil. Photocatalytic activity of the TiO2 surfaces was verified by examining a photocatalytic decrease of the amount of adsorbed acetic acid. A continuous-wave UV laser (375 nm = 3.31 eV, 2.4 W cm−2 ) was used to induce photocatalytic desorption/decomposition of adsorbed acetic acid. The laser was irradiated on the TiO2 surfaces covered with a saturation amount of acetic acid using the laser diode module (Obis 375LX, Coherent), which was mounted on an x-y stage and was attached to a MgF viewport at the back side of the SES200 analyzer. Thus, the laser was injected on to the sample surface through an analyzer slit and the analyzer lens. An overlap between the laser and the SR light was checked by a laser-induced SPV shift of a Si 2p XPS peak [see Supporting Information (SI) for details].

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TRXPS measurements The TRXPS measurements were carried out at beamline BL07LSU of SPring-8. 14 A pumpprobe method was employed for the time-resolved measurements. A second harmonic of an amplified Ti:sapphire laser pulse (3.06 eV) was used for the pump light. The pulse duration was 35 fs and a repetition rate was 1 kHz. The laser power density was between 12 and 48 mJ cm−2 pulse−1 . For the probe light, the SR at hν = 600 eV was used. The pulse duration was 50 ps, which restricted the time resolution of the measurement system. The pulse interval was 4.79 µs (an H-mode operation) and 342 ns (an F-mode operation). The TRXPS spectra were acquired by a time-of-flight electron energy analyzer (VG Scienta ARTOF 10k). The binding energy of the TRXPS spectra was referenced to a Au 4f7/2 peak position (84.0 eV) of a gold foil. The laser-SR overlap was visually checked using a fluorescent plate and was confirmed by the SPV shift of the Si 2p spectrum of Si(111). The TRXPS measurements were done at room temperature. The UV-induced SPV on the TiO2 was assessed from the temporal shift of the Ti 2p3/2 peak position as a function of a time interval between the UV pulse and the SR pulse (a delay time).

Sample preparation For the XPS measurements, commercially available a-TiO2 and r-TiO2 single crystals were used. The a-TiO2 crystals with (101) and (001) orientation were natural crystals (Surface Preparation Laboratory) with a purity of about 99.95%. Mn and Fe were main impurities with a concentration of < 10 ppm. R-TiO2 with (110), (100), and (011) orientation (MTI Co.) and with (001) orientation (Furuuchi Chemical Co.) were undoped crystals with a purity of > 99.99%. For the TRXPS measurements, commercially available r-TiO2 (110) and r-TiO2 (011) (MTI Co.) were used, whereas a-TiO2 (001) was an epitaxial thin film (the thickness of 1 µm) grown on LaTiO3 (001) by pulsed laser deposition. 12 Cleaning of the TiO2 surfaces was carried out under UHV conditions. Ar+ sputtering (1– 2 keV, 1 × 10−4 Pa Ar, 10 min) and annealing at 900 K in O2 atmosphere (between 1 × 10−4 5



Pa and 2.5 × 10−4 Pa, 10 min) were repeated until sharp low energy electron diffraction spots were obtained and no carbon contaminants were observed in the XPS spectra. We also prepared defective r-TiO2 (110) surfaces to assess the effect of surface O defects on the photocatalytic activity and the carrier lifetime. The clean r-TiO2 (110) surface was subjected to Ar+ sputtering (1.25 kV in 6 × 10−6 Pa Ar for 10 min) at room temperature. Although the same sputtering condition was employed in the experiments at BL-13B and BL07LSU, the density of the surface O defects was slightly different. The estimated density was 2 × 1014 cm−2 and 3 × 1014 cm−2 for the samples prepared at BL-13B and BL07LSU, respectively. Details are given in the SI. Adsorption of acetic acid was done in a loadlock chamber which was connected to the preparation chamber. Acetic acid (99.7%, Kanto Chemical Co.) was contained in the glass vial that was attached to a variable leak valve, from which a vapor of acetic acid was admitted to the loadlock chamber. Before use, acetic acid was degassed by several freeze-pump-thaw cycles. Adsorption of acetic acid on the TiO2 surfaces was carried out by exposing the surfaces to the acetic acid vapor at room temperature. The pressure during exposure was 1.3 × 10−5 Pa, and the total amount of exposure was between 10 L and 20 L (Langmuir; 1 L = 1.3 × 10−4 Pa s). These amounts were enough to reach adsorption saturation at room temperature. 15–17

Results Photocatalytic activity Figure 1A shows C 1s XPS spectra of acetic acid on r-TiO2 (110) at different UV-irradiation times. Acetic acid is known to adsorb on r-TiO2 (110) in the form of acetate (CH3 COO− ), with the two O atoms being bonded to two five-coordinated Ti atoms in a bidentate configuration (Figure 1B). 16,18 A saturation coverage (θsat ) has been estimated to be 0.5 monolayer 16 (ML, where 1 ML is defined as the density of the five-coordinated Ti atoms, i.e., 5.2 × 1014 6


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cm−2 ). Naturally, the adsorption configuration and θsat differ depending on the crystal surface orientation. 16,17,19 The acetate-covered TiO2 surfaces present C 1s peaks at 286 and 289 eV, which correspond to the methyl carbon and the carboxylate carbon, respectively. Although the doublepeak structure persists even after 55-min UV irradiation, the peak intensity is slightly decreased, indicating elimination of acetate from the surface. It should be noted that a peak intensity is also decreased by soft X-ray irradiation (Figure 1C) because of the acetatephotoelectron interaction. 18 However, UV irradiation accelerates the intensity decrease. Figure 1D compares the C 1s peak intensity decrease with and without UV irradiation. The peak intensity is reduced in the first one hour to ∼90% without UV irradiation and to ∼80% with UV irradiation. Suppose that the intensity reduction follows a single exponential function, i.e., I(t) = I(0) exp(−t/τ ), where I(t) and τ are a C 1s peak intensity at irradiation time t and a decay constant, respectively, least-square fitting gives τ values of 260 min and 490 min with and without the UV laser irradiation, respectively. Smaller τ means a faster reduction of the peak intensity. Thus, the UV laser stimulates additional desorption and decomposition of acetate. Since the energy of the UV laser (3.31 eV) is too low to generate photoelectrons, the intensity reduction must be caused by a photocatalytic decomposition/desorption reaction, not by the acetate-photoelectron interaction. To extract the effect of the UV laser, the C 1s intensity of the UV-on data I(t)on was subtracted by the UV-off data I(t)off , which is given as I(t)off = I(0)off exp(−t/490 min). The re[

]

sult is shown in Figure 1E. Here, the normalized intensity is calculated by I(t)on − I(t)off /I(0)on + 1. The normalized intensity is reduced to 85–90% in the first one hour. A solid line in Figure 1E is obtained by fitting with IC1s = exp(−t/τph ) with τph = 370 min. Thus, UV irradiation induces desorption/decomposition of acetate with a time constant of 370 min on r-TiO2 (110). Dashed lines exhibit faster and slower limits of the intensity decay with τph = 300 min and 600 min, respectively. These values are obtained if each experimental data takes the upper and lower values of the error bar.

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The above mentioned procedure is equivalent to the assumption that the overall C 1s intensity reduction is reproduced by a double-exponential function with τ and τph , which represent the time constants of SR-induced and UV-induced degradation, respectively, of acetate. Photocatalytic degradation of acetate is a hole-mediated reaction because acetate adopts an anionic form on the surface. The initial reaction step is considered to be CH3 COO− + h+ → CH3 + CO2 , which is analogous to the photocatalytic decomposition of trimethyl acetate. 20 The methyl fragments do not remain on the surface because both the methyl and carboxylate carbon peaks are attenuated at the same rate. The photogenerated electrons may facilitate the formation of CH4 from the methyl fragments via CH3 + H+ + e− → CH4 , 21 where H+ is a dissociated species upon acetic acid adsorption. Thus, the UV-stimulated C 1s intensity decrease provides a suitable index to assess photocatalytic activity. The same peak intensity analysis was carried out for other six TiO2 surfaces. Figure 2 shows C 1s spectra of the acetate-saturated TiO2 surfaces acquired with and without UV irradiation, and the C 1s intensity variations with time are indicated in the inset. A net intensity reduction by UV irradiation on each surface is summarized in Figure 3. Solid lines are the best-fitted results of fitting by the exponential function. Among the r-TiO2 surfaces, the (110) surface is most active with τph = 370 min, followed by the (001) surface with τph = 580 min. R-TiO2 (011) and r-TiO2 (100) are inactive (τph = ∞). Regarding aTiO2 , both (101) and (001) surfaces have nearly the same activity, but with a slightly higher activity for a-TiO2 (101) (τph = 480 min) than for a-TiO2 (001) (τph = 600 min). The τph values are summarized in Table 1, together with 1/τph , which is a rate constant kph and, thus, a direct measure of photocatalytic activity. Also listed in Table 1 is θsat for each surface determined from the C 1s intensity. Considering that θsat reflects surface chemical activity, r-TiO2 (011) is the least active surface for acetic acid adsorption, whereas r-TiO2 (100) is the most active one. It is apparent that there is no correlation between kph and θsat , indicating that chemical activity is not a determining factor for the face-dependence of photocatalytic

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activity.

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(A)

(B) acetate

five-coordinated Ti bridging O

UV on

C 1s

Time (min)

Intensity (arb. units)

hn = 753 eV

55 43 25 12 Ti

0 295

290

285

280

UV off

hn = 753 eV Ti O Ti O


C 1s

Time (min)

C CH3

0

(E)

38 19

290

285

C

H

280

UV off

UV on

2

56

0 295

4

Normalized intensity

(C)

O

(D)6 C 1s peak intensity

Binding Energy (eV)


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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0

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0

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Reaction Time (min)

UV-induced change 1.0 0.9 0.8

0

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Reaction Time (min)

Binding Energy (eV)

Figure 1: (A) C 1s XPS spectra of acetate-saturated r-TiO2 (110) under UV irradiation. Lines formed by the dots are the raw data, and results of curve fitting using a polynomial function for the background and two Gaussian functions for the peaks are shown by solid lines. (B) An adsorption model of acetic acid on r-TiO2 (110). (C) The same as Figure 1A, but without UV irradiation. (D) Time dependent changes of integrated intensities of the C 1s spectra acquired with and without UV irradiation. Solid lines are results of curve fitting by an exponential function with τ = 490 min and 260 min for the UV-off and UV-on data, respectively. (E) UV-induced decrease in the C 1s peak intensity. A solid line is a result of exponential fitting with τph = 370 min, and dashed lines are faster and slower limits of the exponential decay with τph = 300 min and 600 min, respectively.

10


0

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50 Time (min)

Time (min)

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295 290 285 295 290 285 Binding Energy (eV) Binding Energy (eV)

UV−off

8 Intensity (arb. units)

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50 Time (min)


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(E) anatase TiO2(101)

(F) anatase TiO2(001)

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Time (min)

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(C) rutile TiO2(100) Intensity (arb. units)

UV−on

8


UV−off

(B) rutile TiO2(011)


(A) rutile TiO2(001)


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4

0

0

50 Time (min)


Figure 2: Changes in C 1s spectra of acetate-saturated TiO2 surfaces acquired with and without UV laser irradiation. An inset in each panel shows a temporal variation of the C 1s integrated intensity. All spectra were measured at hν = 753 eV.

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(011) (100)

1.0

Normalized Intensity

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

0.8

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(001) (110)

rutile TiO2 0

20

40

60

80

anatase TiO2 1.0

(001) (101)

0.8 0

20

40

60

80

Reaction Time (min) Figure 3: UV-induced intensity decrease of the C 1s peaks from acetate on the TiO2 surfaces. Symbols with error bars are experimental data and solid lines are results of least-square fitting by IC1s = exp(−t/τph ).

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XPS τph (min) kph (min−1 ) θsat (ML) Vs,Ac (eV)d τ∞,Ac (ns)e r-TiO2 (110) 370 2.7 × 10−3 0.50 0.44 (3.44) 450 r-TiO2 (001) 580 1.7 × 10−3 0.45 r-TiO2 (011) 1.3 0.50 (3.50) ∞ 0 0.41 0.56 (3.56) 7 r-TiO2 (100) ∞ 0 0.63 a-TiO2 (101) 480 2.1 × 10−3 0.56 c −3 a-TiO2 (001) 90 0.20 (3.40) 600 1.7 × 10 0.54 0.25 (3.45) 360 sp. r-TiO2 (110) a 230 0.55 (3.55) 2100 0.5 × 10−3 0.66 0.50 (3.50) 60 a b c d sp. stands for sputtered. Values are taken from refs. 12 and 24. τ∞ was reexamined using η = 1.4. Vs and Vs,Ac at the surfaces are obtained by subtracting the band gap (3.0 and 3.2 eV for r-TiO2 and a-TiO2 , respectively) from the VBM under the assumption that the Fermi level lies just below the conduction band minimum in the bulk. The VBM values are given in parentheses. e τ∞,Ac is calculated by Eq. 3.

TRXPSb τ∞ (ns) Vs (eV)d 150 c 0.40 (3.40)

Table 1: Carrier lifetime (τ∞ ), surface potential barrier height (Vs ), decay time constant of the C 1s XPS peak intensity (τph ), the rate constant of the photocatalytic degradation of acetate (kph ), acetate-saturation coverage (θsat ), potential barrier height on the acetate-saturated surface (Vs,Ac ), and carrier lifetime on the acetate-saturated surface (τ∞,Ac ) for different TiO2 surfaces.

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0.0

2

0.1 0.0

r−TiO2(011) clean acetic acid x 30

a−TiO2(001)

a−TiO2(001) 2

12 mJ/cm

0.1 0.0 0.1

x 30

r−TiO2(011) 40 mJ/cm

0.2

clean acetic acid

Intensity

0.1

0.2

r−TiO2(110)

2

12 mJ/cm 2 40 mJ/cm

Intensity

0.2

(B) valence band

r−TiO2(110)

clean acetic acid

Intensity

SPV (eV)

SPV (eV)

(A) SPV relaxation

SPV (eV)

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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x 10

9

1 10 100 Delay Time (ns)

3 6 Binding Energy (eV)

0

Figure 4: (A) Change in the SPV as a function of a delay time. Dots are the experimental data and the solid lines are the results of analysis based on the thermionic emission model. (B) Valence band spectra of the clean (solid lines) and acetic-acid saturated (dashed lines) TiO2 surfaces. The photon energy was 100 eV.

Carrier lifetime Next, we turned our attention to the lifetime of the photogenerated carriers. Figure 4A shows a time-dependence of the UV-induced SPV on adsorbate-free r-TiO2 (110), r-TiO2 (011), and a-TiO2 (001), The SPV was determined from a transient shift of the Ti 2p3/2 peak measured at hν = 600 eV, a surface sensitive condition with a probing depth of about 1.8 nm. Figure 4B shows valence band spectra of the TiO2 surfaces with and without acetic acid. The measurement condition is also a surface sensitive one with a probing depth of ∼1.7 nm. On all the surfaces examined in the present study, an accumulation-type SCL is formed, as revealed by the valence band maximum (VBM) positions in the spectra, because the VBM position on each TiO2 surface is located at the binding energies larger than 3.0 eV or 3.2 eV in the cases of the rutile and anatase TiO2 surfaces, respectively (the VBM values are listed in Table 1). Thus, electrons and holes generated within the SCL drift towards the surface

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and the bulk, respectively, along the potential gradient. These hot carriers are involved in photocatalytic reactions more than those generated deep inside the bulk where the flat band condition is robust and the electron-hole recombination swiftly proceeds. The charge separation in the SCL induces a counter potential to cancel out intrinsic band bending, and this is the origin of the SPV. The magnitude of the UV-induced band bending equals the SPV. The SPV diminishes on the TiO2 surfaces in a nanosecond timescale (Figure 4A) as a result of thermal diffusion of the holes from the bottom of the SCL to the surface and subsequent recombination with the electrons that are reached to the surface before the holes. Delay time dependent changes in the SPV were analyzed to deduce the carrier lifetimes τ∞ within the framework of the thermionic emission model. 22–24 In this model, a number of the charged particles trapped at the surface immediately after the charge separation (the electrons in the present case) is decreased by recombination with the counterpart particles (the holes) that diffuse from the bottom of the SCL and reach the surface by overpassing the surface potential barrier. The SPV shift VSPV (t) is diminished with time t; {

[

(

V0 VSPV (t) = −ηkB T ln 1 − 1 − exp − ηkB T

)]

}

e

−t/τ∞

.

(1)

Here, η is an ideality factor in a Schottky diode model. 25 kB and T are the Boltzmann constant and the sample temperature, respectively. V0 is the SPV at t = 0. τ∞ defines how fast the SPV is diminished and, thus, represents the carrier lifetime. Solid lines in Figure 4A are results of least-squares fitting to Eq. 1, while V0 and τ∞ were treated as variables. The best-fitted curves were obtained with τ∞ = 150 ns, 1.3 ns and 90 ns for r-TiO2 (110), r-TiO2 (011) and a-TiO2 (001), respectively. These τ∞ values are obtained if we assume η = 1.4, which was evaluated from the laser-power dependence of the magnitude of the SPV. 24 On the other hand, theoretically predicted η ranges between 1 and 2. 25 Thus, if the upper and lower limits of η are 1 and 2, respectively, τ∞ should take a value between 90 ns and 310 ns for r-TiO2 (110), i.e., τ∞ = 150+160 −60 ns. Similarly, τ∞ for r-TiO2 (011) is

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+180 1.3+0.7 −0.4 ns, and that for a-TiO2 (001) is 90−50 ns.

The carrier lifetime depends on the surface potential barrier height (Vs ), i.e., the magnitude of band bending, as follows: 24 (

τ∞

Vs WSCL = exp Sp ηkB T

)

(2)

WSCL and Sp are the width of the SCL and the surface recombination velocity, respectively. Equation 2 tells us that a small change of Vs leads to a large variation of τ∞ , while WSCL and Sp have a smaller effect. This is why the activity-lifetime relationship must be verified under the same environmental conditions. The TiO2 surfaces subjected to the TRXPS study have Vs = 0.4 eV, 0.5 eV, and 0.2 eV for r-TiO2 (110), r-TiO2 (011), and a-TiO2 (001), respectively. Upon saturation with acetate, the barrier heights are increased to Vs,As = 0.44 eV, 0.56 eV, and 0.25 eV as judged from the VBM shift towards the higher binding energy side (Figure 4B). Under the assumption that η is unaffected by acetate and that small changes of WSCL and Sp are neglected, the lifetime on acetate-saturated TiO2 (τ∞,Ac ) is estimated by an equation;

(

τ∞,Ac

)

Vs,Ac − Vs , = τ∞ exp ηkB T

(3)

which can be derived from Eq. 2. τ∞,Ac are 450 ns, 7 ns, and 360 ns for r-TiO2 (110), r-TiO2 (011), and a-TiO2 (001), respectively. Although the carrier lifetimes were evaluated for only three surfaces, both kph and τ∞,Ac follow the order r-TiO2 (110) > a-TiO2 (001) > r-TiO2 (011). In a pioneering work by Xu et al., 5 close correlation between photocatalytic activity and carrier lifetime was found for CO oxidation over a-TiO2 (101) and r-TiO2 (110). The present study demonstrates that the activity-lifetime correlation is extendable to the acetate adsorption system.

16


hν = 100 eV

(A) valence band

x 30

acetic acid sputtered r−TiO2(110)

x 30

pristine r−TiO2(110)

(B) C 1s

5 Binding Energy (eV)

hν = 753 eV sputtered pristine

1.1 Normalized Intensity

10

Intenstiy (arb. units)

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 17 of 26

5 0 Relative Binding Energy (eV)

0

(C)

sputtered

1.0 0.9 pristine 0.8 0

20 40 60 80 Reaction Time (min)

Figure 5: (A) Valence band spectra of pristine and sputtered r-TiO2 (110) surfaces (solid lines) and those saturated with acetate (dashed lines). Vertical lines indicate the VBM positions. (B) C 1s XPS spectra of acetate-saturated surfaces. (C) Comparison of the photocatalytic activities of the pristine and sputtered surfaces. Experimental data are indicated by triangles, and the solid lines were derived by exponential fitting.

Effect of surface modification We further investigated the effect of an intentional surface modification on photocatalytic activity and carrier lifetime. Here, r-TiO2 (110) was subjected to Ar+ sputtering to form a defective surface. Figure 5A shows valence band spectra before and after sputtering (1.25 kV in 6 × 10−6 Pa Ar for 10 min). Sputtering leads to an increase in the gap state at 0.8 eV (Ti 3d of Ti3+ ), indicating the formation of surface O defects. These defects stimulate acetic acid adsorption with θsat = 0.66 ML (Figure 5B). However, the photocatalytic activity is suppressed with τph = 2100 min (the C 1s spectra are shown in Figure 2D). The photocatalytic degradation of acetate is more than five-times slower on the sputtered surface than on the pristine surface (Figure 5C). The carrier lifetime on sputtered r-TiO2 (110) saturated with acetate (τ∞,Ac ) can be again

17


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

Photocatalytic Activity −3 −1 kph (x 10 min )


r−TiO2(011)

Page 18 of 26

r−TiO2(110)

3

2 a−TiO2(001)

1 sputtered r−TiO2(110)

0 0

200 400 Carrier Lifetime τ∞,Ac (ns)

600

Figure 6: Correlation between the carrier lifetimes (τ∞,Ac ) and photocatalytic activities (kph ) of four TiO2 surfaces. evaluated from a surface potential barrier height on this surface (Vs,Ac ) and the carrier lifetime (τ∞ ) on the acetate-free sputtered r-TiO2 (110) surface having a barrier height Vs . In the previous study, 24 we prepared sputtered r-TiO2 (110) with Vs = 0.55 eV, and the carrier lifetime was determined to be τ∞ = 230 ns. On the other hand, the sputtered surface prepared in the present study has a lower barrier height of 0.50 eV (lower Vs is due to the lower O defect density). This barrier height is hardly affected by adsorption of acetic acid (Figure 5A), i.e., Vs,Ac = 0.50 eV. Thus, τ∞,Ac = 60 ns is evaluated by Eq. 3. The lifetime is much shorter than that on the pristine surface saturated with acetate (450 ns). Thus, the lower photocatalytic activity of the sputtered surface can be explained by the shorter carrier lifetime. The activity-lifetime correlation holds even for the sputtered surface.

Discussion A photocatalytic reaction is initiated by an interaction between adsorbates and photoexcited carriers at the crystal surface. To enhance the photocatalytic activity, the density of either the adsorbates or the carriers arriving at the surface, or both, should be increased. The present study reveals that, regarding photocatalytic decomposition of acetate on TiO2 , there is a correlation between kph and τ∞,Ac . The correlation is actually linear and positive, as 18


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shown in Figure 6. On the other hand, no such correlation is found between kph and θsat . This result is understandable as follows; if the carriers survive longer, more carriers have a chance to encounter adsorbates on the surface to initiate the reaction. Since the carrier lifetime strongly depends on the surface barrier height (Eq. 2) and the barrier height depends on the surface orientation as well as the surface chemical condition, it is natural to consider that the carrier lifetime depends on crystal-surface orientation. Therefore, the orientationdependence of the photocatalytic activity should originate from the orientation-dependent carrier lifetime rather than the orientation-dependent chemical activity. It is worth noting a possible effect of shallow trap states in TiO2 on the carrier lifetime as well as the photocatalytic activity. It is known that the photoexcited electrons in the conduction band populate the shallow trap states. 26–28 This process corresponds to the formation of polarons. Thus, the electrons in the conduction band diffuse in the crystal as the polarons, and this should result in an increasing recombination probability because of a moderate diffusion velocity. The parameter η in Eqs. 1, 2, and 3 includes the degree of the electron-hole recombination rate in the bulk; the η value larger than 1.0 means that some of the photoexcited electrons and holes are lost before they reach the crystal surface. The shallow trap state is formed not on the crystal surface but in the bulk. 27 Moreover, the state is formed at ∼0.1 eV below the CBM in both anatase and rutile TiO2 crystals. 27,28 These facts imply that the effect of the trap state on the electron diffusion behavior should not be crystal-structure-dependent nor crystal-surface-dependent. Although the formation of the polarons surely affects the carrier lifetime and the photocatalytic activity, we consider that the effect on the linear lifetime-activity correlation is rather limited. Our finding of the linear kph –τ∞,Ac correlation is in line with the conclusion drawn by Morris Hotsenpiller et al., 9 who compared photoreduction rates of Ag+ to Ag0 on oriented rutile TiO2 surfaces. They have concluded that the surface-dependence arises from the efficiency of the electron-hole pairs that are used to initiate photochemistry. The Ag+ photoreduction is found to be higher on r-TiO2 (011), (001) and (111) than r-TiO2 (110) and (100). This

19



reactivity order, however, is different from that observed in the present study. Such a difference is naturally understood if the carrier-adsorbate interaction is a rate limiting step. On the other hand,iIf a rate limiting step is not a carrier-adsorbate interaction step, but rather carrier diffusion, the difference of adsorbed species and the type of the reaction may not be responsible for the difference in the reactivity order. In the photoreduction study, 9 the reaction was carried out in an AgNO3 aqueous solution, which should change a band bending structure, i.e., Vs , of the TiO2 surface from that in the UHV. Therefore, though not proved experimentally, it is possible that the correlation between the photoreduction activity and the carrier lifetime is persistent even in the aqueous environment. This hypothesis should be verified in future works.

Summary Photocatalytic decomposition/desorption of acetic acid and the lifetime of the photogenerated carriers on single-crystal r-TiO2 and a-TiO2 surfaces have been investigated under the UHV condition by XPS and TRXPS utilizing synchrotron radiation as X-ray sources. The photocatalytic activity on the r-TiO2 surface is the highest on the (110) surface followed by the (001) surface. Both r-TiO2 (011) and (100) are inactive. Regarding a-TiO2 , the (101) and (001) surfaces exhibit a similar activity but a slightly higher activity for (101) than (001). Although the r-TiO2 (110) surface has the highest activity among the TiO2 surfaces investigated, the introduction of the surface O vacancies largely suppresses the activity. These crystal-surface-dependence and the surface-modification-dependence of the photocatalytic activity is should originate from the difference in the carrier lifetime on each TiO2 surface, because there is a positive and linear correlation between the photocatalytic activity and the carrier lifetime. The present study suggests that the carrier lifetime should be a prime factor to determine the photocatalytic activity of TiO2 .

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Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at DOI: Laser-SR spatial overlap and surface O defect density (PDF) AUTHOR INFORMATION Corresponding Author ∗

E-mail: [email protected] (K.O.)

Present Addresses R. Y.: Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan R.-Y. L.: Institute of Physics, Academia Sinica, Taipei City, 11574, Taiwan Notes The authors declare no competing financial interests.

Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research (Grant No. 16H03867) from MEXT, Japan. The XPS study at BL-13B was performed under the approval of the Photon Factory Advisory Committee (Proposal Nos. 2012S2-006 and 2015S2-008). The TRXPS study at BL07LSU was conducted using the facilities of the Synchrotron Radiation Research Organization, The University of Tokyo (Proposal Nos. 2012A7426, 2012B7433, 2013A7444, 2014A7463, and 2015A7487).

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2015–2022. (2) Tanaka, K.; Capule, M. F. V.; Hisanaga, T. Effect of Crystallinity of TiO2 on Its Photocatalytic Action Chem. Phys. Lett. 1991, 187, 73–76. (3) Sclafani, A.; Herrmann, J. M. Comparison of the Photoelectronic and Photocatalytic Activities of Various Anatase and Rutile Forms of Titania in Pure Liquid Organic Phases and in Aqueous Solutions J. Phys. Chem. 1996, 100, 13655–13661. (4) Luttrel, T. Halpegamage, S.; Tao, J.; Kramer, A.; Sutter, E.; Batzill, M. Why is Anatase a Better Photocatalyst than Rutile? - Model Studies on Epitaxial TiO2 Films Sci. Rep. 2014, 4, 4043. (5) Xu, M.; Gau, Y.; Moreno, E. M.; Kunst, M.; Muhler, M.; Wang, Y.; Idriss, H.; Wöll, C. Photocatalytic Activity of Bulk TiO2 Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy Phys. Rev. Lett. 2011, 106, 138302. (6) Pan, J.; Liu, G.; Lu, G. Q.; Cheng, H.-M. On the True Photoreactivity Order of {001}, {010}, and {101} Facets of Anatase TiO2 Crystals Angew. Chem. Int. Ed. 2011, 50, 2133–2137. (7) Tachikawa, T.; Yamashita, S.; Majima, T. Evidence for Crystal-Face-Dependent TiO2 Photocatalysis from Single-Molecule Imaging and Kinetic Analysis J. Am. Chem. Soc. 2011, 133, 7197–7204. (8) Ahmed, A. Y.; Kandiel, T. A.; Oekermann, T.; Bahnemann, D. Photocatalytic Activities of Different Well-defined Single Crystal TiO2 Surfaces: Anatase versus Rutile J. Chem. Phys. Lett. 2011, 2, 2461–2465. (9) Morris Hotsenpiller, P. A.; Bolt, J. D.; Farneth, W. E.; Lowekamp, J. B.; Rohrer, G. S. Orientation Dependence of Photochemical Reactions on TiO2 Surfaces J. Chem. Phys. B 1998, 102, 3216–3226. 22


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(19) Heckel, W.; Elsner, B. A. M.; Schulz, C.; Muller, S. The Role of Hydrogen on the Adsorption Behavior of Carboxylic Acid on TiO2 Surfaces J. Phys. Chem. C 2014, 118, 10771–10779. (20) Ohsawa, T.; Lyubinetsky, I. V.; Henderson, M. A.; Chambers, S. A. Hole-Mediated Photodecomposition of Trimethyl Acetate on a TiO2 (001) Anatase Epitaxial Thin Film Surface J. Phys. Chem. C 2008, 112, 20050–20056. (21) Kraeutler, B.; Bard, A. J. Heterogeneous Photocatalytic Decomposition of Saturated Carboxylic Acids on Titanium Dioxide Powder. Decarboxylative Route to Alkanes J. Am. Chem. Soc. 1978, 100, 5985–5992. (22) Bröcker, D.; Gießel, T.; Widdra, W. Charge Carrier Dynamics at the SiO2 /Si(100) Surface: a Time-resolved Photoemission Study with Combined Laser and Synchrotron Radiation Chem. Phys. 2004, 299, 247–251. (23) Spencer, B. F.; Graham, D. M.; Hardman, S. J. O.; Seddon. E. A.; Cliffe, M. J.; Syres, K. L.; Thomas, A. G.; Stubbs, S. K.; Sirotti, F.; Silly M. G.; et al. Time-resolved Surface Photovoltage Measurements at n-type Photovoltaic Surfaces: Si(111) and ZnO(1010) Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 195301. (24) Ozawa, K.; Yamamoto, S.; Yukawa, R.; Liu, R.; Emori, M.; Inoue, K.; Higuchi, T.; Sakama, H.; Mase, K.; Matsuda, I. What Determines the Lifetime of Photoexcited Carriers on TiO2 Surfaces? J. Phys. Chem. C 2016, 120, 29283–29289. (25) Sah, C.; Noyce, R. N.; Shockley, W. Carrier Generation and Recombination in P -N Junctions and P -N Junction Characteristics Proc. IRE 1957, 45, 1228–1243. (26) Panayotov, D. A.; Yates, Jr. J. T.; n-Type Doping of TiO2 with Atomic HydrogenObservation of the Production of Conduction Band Electrons by Infrared Spectroscopy Chem. Phys. Lett. 2007, 436, 204–208.

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Graphical TOC Entry UV

CO2, CH4, etc.

drift

(× 10•• min••)

diffusion

e- h+

Photocatalytic Activity

acetate

drift drift

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 26 of 26

3

2

1

0 0

h+

200

400

600

Carrier Lifetime (ns)

Schematic illustration of photocatalytic decomposition of acetate on TiO2 and an experimentally determined correlation between carrier lifetime and the photocatalytic activity.

26


Correlation between Photocatalytic Activity and Carrier Lifetime: Acetic

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