Transfer of Photoinduced Electrons in Anatase–Rutile TiO2

May 28, 2014 - It has been a long-standing debate about the photogenerated charge transfer in anatase–rutile mixed phase TiO2. In this work, we inve...
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Transfer of Photoinduced Electrons in Anatase−Rutile TiO2 Determined by Time-Resolved Mid-Infrared Spectroscopy Shuai Shen,†,‡ Xiuli Wang,† Tao Chen,† Zhaochi Feng,† and Can Li*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: It has been a long-standing debate about the photogenerated charge transfer in anatase−rutile mixed phase TiO2. In this work, we investigated this issue by studying anatase, rutile, and anatase−rutile mixed phase TiO2 with time-resolved mid-IR (MIR) spectroscopy (TR-MIR) in a vacuum or methanol vapor. Anatase TiO2 shows transient MIR absorption on microsecond time scale after 355 nm excitation, which is attributed to photoinduced electrons in the shallow states of anatase. Conversely, there is no transient MIR absorption detected for rutile TiO2. For anatase−rutile mixed phase TiO2, the initial MIR absorption intensity decreases relative to that calculated from its phase composition, indicating that electron transfer takes place in the anatase−rutile phase junction of mixed phase TiO2. Our results suggest that the photogenerated electrons in the shallow states of anatase transfer to rutile across the anatase−rutile interface, but this does not exclude the possibility that the net electron transfer from rutile to anatase due to the complicated energy levels and kinds of trapped states in mixed phase TiO2.

1. INTRODUCTION Titania (TiO2) is one of the most extensively studied materials for its wide applications, particularly in photocatalysis and solar cells, because of its unique optical and chemical properties. TiO2 has been well studied from different aspects, such as phase structure, morphology, doping, sensitization, surface structure, electronic structure, and carrier dynamics.1,2 Anatase and rutile are the two main crystalline structures of TiO2 with band gap of 3.2 and 3.0 eV, respectively. Generally, anatase TiO2 is reported to be more active than rutile TiO2 in photocatalysis, especially in environment photocatalysis.3,4 However, Degussa P25 composed of rutile and anatase phase was found to be more active than TiO2 with sole phase. It was also demonstrated in our previous work and many other researchers that anatase− rutile mixed-phase TiO2 are more active than anatase or rutile in photocatalytic reactions.5−8 It is proposed that the formation of anatase/rutile phase junction is responsible for the improved photoactivity of mixed phase TiO2. The anatase/rutile phase junction is supposed to improve charge separation and then prolong charge lifetime for photocatalytic reactions. It is very crucial to characterize the charge transfer process at the anatase/rutile phase junction to confirm the role of phase junction in photocatalysis. Many researchers are devoted to understanding the role of anatase/rutile phase junction in photocatalysis.9−13 Hurum et al. claimed that the high photocatalytic activity of Degussa P25 TiO2 is due to the rapid electron transfer from rutile to anatase in the transition points between anatase and rutile,9,10 while photoinduced electron transfer from anatase to rutile in © XXXX American Chemical Society

partially reduced P25 is proposed by Komaguchi et al. based on ESR experiments.14 Kawahara et al. investigated the patterned TiO2(anatase)/TiO2(rutile) bilayer-type photocatalyst with TEM and regarded that the high photocatalytic activity of P25 is caused mainly by the increase in chargeseparation efficiency resulting from interfacial electron transfer from TiO2(A) to TiO2(R).11 To understand the charge transfer at interfaces of anatase and rutile, theory calculation is widely used to obtain the band alignment between anatase and rutile. Kang et al. claimed that the valence band maxima of anatase and rutile are close to each other, whereas the conduction band minimum of anatase is found to be about 0.2 eV higher than that of rutile, which indicates that the electron transfer from anatase to rutile is more likely.15 On the other hand, Scanlon et al. reported that a type II, staggered, band alignment of 0.4 eV exists between anatase and rutile with anatase possessing the higher electron affinity or work function, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission experiments.16 To date, it is not confirmed that charge transfer do exist at anatase−rutile junction, and the charge transfer direction is still controversial as discussed above. Time-resolved spectroscopic techniques, which can give the information on separation, recombination, and transfer of photogenerated carriers in photocatalysis, have also been Received: March 24, 2014 Revised: April 30, 2014

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pulse width of 355 nm laser is 6−8 ns, and the repetition rate was set to 1 Hz. The laser energy was about 1−2 mJ/pulse, and the spot size of the laser is about 4−5 mm. The stepping mirror was stopped to one fixed position. The dc output (20 MHz to 0 Hz) of the MCT amplified by an ac-coupled SR560 preamplifier (1 MHz to 0.03 Hz) was used to measure the transient signals. The synchronization between laser excitation and data acquisition was achieved with a Stanford Research Model DG 535 pulse generator. To prevent the scattered light from reaching interferometer and the MCT detector, two ARcoated Ge plates were placed in the openings of the sample chamber. 2.3. Structure Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku MiniFlex diffractometer with Cu Kα radiation source. The 2θ range is 20°−80° at a step size of 0.02° and a scan speed of 5° min−1. Raman spectra were obtained on a homemade Raman spectrograph with a triple-stage spectrograph at a resolution of 2 cm−1. The excitation laser line at 532 nm from a semiconductor laser was used as excitation source. The morphologies of the as-prepared samples were observed by scanning electron microscopy (SEM, Quanta 200 FEG). UV− vis diffuse reflectance spectra (DRS) were recorded on a UV− vis spectrophotometer (JASCO V-550) equipped with an integrating BaSO4 sphere.

applied in the study of mixed phase TiO2 besides the steady state characterization. Carneiro et al. studied the carrier dynamics of anatase and mixed phase TiO2 with time-resolved microwave conductance (TRMC) spectroscopy.17 The presence of rutile in the mixed-phase TiO2 improved charge separation as derived from the increased levels of conductivity and electron lifetimes. They claimed that photogenerated holes were captured by rutile in mixed phase TiO2. But they did not compare these results with that of rutile, which may have much higher level of conductivity and electron lifetimes. Moreover, TRMC cannot distinguish between electrons and holes in TiO2. Thus, time-resolved spectroscopy which can identify charge transfer between anatase and rutile directly is still highly demanded. Although there are lots of publications studying the role of anatase−rutile phase junction, the charge transfer process at the interface still needs to be investigated and confirmed directly by carrier kinetics study. This is because the published results by the steady-state techniques cannot characterize charge transfer processes directly, while photogenerated electrons and holes in TiO2 cannot be distinguished by TRMC. More importantly, photoinduced electrons or holes should be identified separately in either anatase or rutile TiO2 in order to clarify the charge transfer direction. Time-resolved mid-IR (MIR) spectroscopy (TR-MIR) has proven to be a powerful tool to monitor kinetics of photogenerated electrons in semiconductor photocatalyst18−20 because photogenerated electrons can usually absorb MIR light while photogenerated holes cannot absorb MIR light. In this work, we investigated the kinetics of photoinduced electrons in anatase, rutile, and mixed phase TiO2 using timeresolved MIR spectroscopy. It is found that transient IR absorption decays of anatase and rutile are fairly different. Anatase shows a transient MIR absorption signal, and rutile does not display a detected MIR absorption on the microsecond time scale. Based on the transient MIR dynamics of anatase−rutile mixed phase TiO2, charge transfer process is confirmed at the anatase−rutile phase junction, and the electron transfer from anatase to rutile is proposed at the interface of anatase/rutile junction in mixed phase TiO2.

3. RESULTS AND DISCUSSION 3.1. Structure Characterization. Figure 1A shows XRD patterns of TiO2 calcined at different temperatures. The narrow and intense peaks at 2θ = 25.5°, 37.9°, 48.2°, 53.8°, and 55.0° are the standard X-ray diffraction peaks of anatase TiO2, while the narrow and intense peaks at 2θ = 27.6°, 36.1°, 41.2°, and 54.3° demonstrate that the phase structure is rutile TiO2. TiO2 calcined at 500 °C is pure anatase, while TiO2 calcined at 800 °C is pure rutile, based on the XRD patterns. TiO2 samples calcined at 600 (TiO2-600) and 700 °C (TiO2-700) are in the anatase−rutile mixed phase. The weight fraction of rutile phase in the mixed phase TiO2, WR, can be estimated from the XRD peak intensities using the formula22

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. TiO2 was prepared by the precipitation method.21 Solution A was prepared by adding 20 mL of tetrabutyl titanate (Ti(OBu)4) into 100 mL of anhydrous ethanol. Solution B was a 100 mL mixture solution of deionized water and anhydrous ethanol. The molar ratio of water/Ti(OBu)4 was 75. Then solution A was added to solution B drop by drop. The white precipitate was stirred continuously for 24 h, and then it was filtered and washed twice with deionized water and anhydrous ethanol, respectively. Finally, the sample was dried at 100 °C and calcined at 500, 600, 700, or 800 °C in air for 3 h. The TiO2 powders are named as TiO2-T (T = 500, 600, 700, and 800). TiO2-500 and TiO2800 are used as anatase and rutile, respectively, as discussed below. Mechanically mixed phase TiO2 were prepared with TiO2-500 and TiO2-800 to obtain A/R-600 and A/R-700, which have the same phase composition as TiO2-600 and TiO2700, respectively. 2.2. Time-Resolved MIR Spectroscopy. Time-resolved MIR absorption spectra were recorded on Nicolet 870 FT-IR spectrometer with a fast-response (20 MHz) photovoltaic MCT detector. 355 nm light of Q-switched Nd:YAG pulse laser (Spectra-Physics) was used to excite TiO2 photocatalysts. The

WR =

1 1 + 0.884(A ana /A rut )

where Aana and Arut are the integrated intensities of anatase (101) and rutile (110) X-ray diffraction peaks, respectively. The rutile contents estimated for TiO2-600 and TiO2-700 are 9 and 49 wt %, respectively. The phase compositions were also calculated with other two formulas as listed in Table S1.23−25 The above formula was chosen due to its good agreement with Raman results as discussed below. Phase structures of TiO2 samples are confirmed by Raman spectroscopy, as shown in Figure 1B. TiO2 calcined at 500 °C shows the characteristic Raman bands at 144, 197, 395, 516, and 638 cm−1 of anatase TiO2, while TiO2 calcined at 800 °C displays the characteristic Raman bands at 144, 235, 446, and 611 cm−1 of rutile TiO2. TiO2 calcined at 600 and 700 °C exhibit the characteristic Raman bands of both anatase and rutile. Therefore, TiO2 calcined at 500 and 800 °C are anatase and rutile TiO2, respectively, while TiO2 calcined at 600 and 700 °C are in anatase−rutile mixed phase. Based on our previous visible Raman results,21 a linear relationship between the band area ratios and the weight ratios of anatase phase to rutile phase in the mixture is obtained. The Raman results B

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Figure 2. Transient IR absorption decays of anatase and rutile TiO2 in (A) vacuum and (B) CH3OH vapor, excited by 355 nm laser pulse of 6−8 ns duration.

Figure 1. XRD patterns (A) and Raman spectra (B) of TiO2 calcined at 500, 600, 700, and 800 °C. The excitation source for Raman spectroscopy is a 532 nm laser.

possible reasons for the absence of transient MIR absorption in rutile: one is that the photoinduced electrons in rutile recombine with holes in less than 50 ns which is the time resolution limitation of the spectrometer, and the other one is that the MIR absorption of electrons in rutile is extremely low. Figure 2B displays the transient MIR absorption decays of anatase and rutile TiO2 in the presence of CH3OH vapor. The initial MIR absorbance of anatase is increased 6 times by exposure to CH3OH, and no decay is observed in 90 μs. The enhancement in the electron population is ascribed to the effective capture of photogenerated holes by methanol-derived adsorbates.20,28 On the other hand, rutile still does not show any transient MIR absorption on the microsecond time scale even when methanol is used to produce long-lived electrons. Methanol has been widely utilized as hole scavenger for H2 evolution in photocatalytic reactions and study of long-lived electrons in photocatalyst.20,28,29 The capture reaction of photoinduced holes by CH3OH complete within 1 ns, and lifetime of photogenerated electrons can be prolonged to seconds.28 This is the reason for the long-lived electrons in anatase exposed to methanol. The absence of transient MIR absorption in rutile may be due to the extremely low extinction coefficient of electrons in rutile. Anyway, our experimental results confirm that rutile does not show any transient MIR absorption on the microsecond time

indicate that the rutile content in TiO2-600 and TiO2-700 is about 9% and 54%, respectively, which are similar to those calculated from XRD patterns with the above formula. We name and utilize TiO2 calcined at 500 and 800 °C as anatase and rutile in this work, respectively. TiO2 calcined at 600 and 700 °C with rutile contents of 9 and 49 wt %, respectively, are used as anatase−rutile mixed phase TiO2 to study anatase− rutile phase junction. 3.2. Dynamics of Photogenerated Electrons in Anatase and Rutile TiO2. Figure 2A shows transient MIR absorption decays of anatase and rutile TiO2 in a vacuum. The transient MIR absorption of anatase TiO2 exhibits a fast decay in 1 μs followed by a slow decay. The fast decay is attributed to the recombination of electrons with free holes. The slow decay is assigned to the recombination of electrons with trapped holes that are captured by surface hydroxyl groups.19,20,26 Conversely, there is no transient MIR absorption on rutile TiO2 at microsecond time scale after excitation. It should be mentioned that rutile TiO2 prepared with several synthetic methods were also tested, and no transient MIR absorption is observed for all the different rutile samples. Yamakata et al. also reported that no transient MIR absorption was observed on rutile in microsecond time scale, especially in oxygen.27 There are two C

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TiO2 is assumed to not absorb MIR light as rutile itself. Thus, if there is no charge transfer in mixed phase TiO2, the initial MIR absorption intensity of mixed phase TiO2 should be in proportion to relative content of anatase phase. Therefore, the initial absorption intensity ratio of mixed phase TiO2 to anatase should be Ianatase:ITiO2−600:ITiO2−700 = 1:0.91:0.51 based on the phase composition of mixed phase TiO2. In fact, the initial absorption intensity ratio, as listed in Table 1, is Ianatase:ITiO2−600:ITiO2−700 = 1:0.81:0.42. It should be mentioned that based on the UV−vis diffuse reflectance spectra in Figure S1, the MIR absorption of photogenerated electrons was normalized by the absorption at 355 nm, which was used as excitation light, in order to eliminate the difference in photogenerated carriers. Apparently, the initial absorption intensities of mixed phase TiO2 samples decrease and are less than that calculated from the phase compositions. This result demonstrates that it is reasonable to assume that rutile in mixed phase TiO2 still does not show MIR absorption, because the initial absorption intensity of mixed phase TiO2 should be increased if there is MIR absorption of rutile. More importantly, the decreased initial MIR absorption in mixedphase TiO2 illustrates that charge transfer does exist at interfaces of the anatase/rutile phase junction. Moreover, the charge transfer process can be discussed based on the above results. If the charge transfer is the electron transfer from rutile to anatase, the initial MIR absorption has to increase relative to that calculated from the phase composition of mixed phase TiO2, which is opposite to our results. If the charge transfer is the hole transfer from anatase to rutile, the initial MIR absorption should also increase due to the decrease recombination rate in anatase. Therefore, the charge transfer should be either the electron transfer from anatase to rutile or the hole transfer from rutile to anatase on the pre-microsecond time scale, which results in the decreased initial MIR absorption in the mixed phase TiO2. The charge transfer percentages of TiO2-600 and TiO2-700 in a vacuum, calculated from the decreased initial MIR absorption, are about 18% and 24%, respectively, as listed in Table 1. Apparently, the charge transfer percentage of TiO2700 is larger than that of TiO2-600. This means that the charge transfer between anatase and rutile is dependent on phase composition of mixed phase TiO2, which may due to the different amount of anatase/rutile interfaces in mixed phase

scale while photoinduced electrons in anatase absorb MIR light obviously. On the basis of the significantly different electron properties of anatase and rutile, we are able to investigate and discuss the electron dynamics in anatase−rutile mixed phase TiO2. 3.3. Dynamics of Photogenerated Electrons in Anatase−Rutile Mixed-Phase TiO2. Figure 3 displays

Figure 3. Transient MIR absorption decays of (a) anatase, (b) anatase−rutile mixed phase TiO2-600, (c) anatase−rutile mixed phase TiO2-700, and (d) rutile in a vacuum excited by 355 nm laser pulse of 6−8 ns duration.

transient MIR absorption decays of anatase, mixed phase TiO2 (TiO2-600, TiO2-700), and rutile in a vacuum. The initial MIR absorption intensity originates from photogenerated electrons after excitation and is proportional to the number of the electrons in conduction band and/or shallowly trapped states. As shown in the inset of Figure 3, the initial absorption intensity of mixed phase TiO2 decreases with the relative content of anatase phase decreasing. Considering photoinduced electrons in rutile do not exhibit transient MIR absorption, time-resolved MIR absorption in mixed phase TiO2 is supposed to only originate from photogenerated electrons in anatase if rutile in mixed phase

Table 1. Relative Initial MIR Absorption Intensity of TiO2 Samples and Their Electron Transfer Efficiency from Anatase to Rutile sample

relative content of anatase phasea (%)

relative initial MIR absorption intensity in a vacuumb

electron transfer percentage from anatase to rutilec (%)

relative initial MIR absorption intensity in CH3OH vapord

electron transfer percentage from anatase to rutilee (%)

anatase TiO2-600 A/R-600 TiO2-700 A/R-700 rutile

100 91 91 51 51 0

1 0.75 0.88 0.39 0.49 0

18 3 24 4

1 0.71 0.91 0.29 0.47 0

22 0 43 8

Relative content of anatase phase is based on the XRD of mixed phase TiO2. bThe absorbance at 0 μs of a sample measured in a vacuum after excitation. Based on UV−vis diffuse reflectance spectra (Figure S1), the MIR absorption of photogenerated electrons was normalized by the absorbance at 355 nm, which was used as excitation light, in order to eliminate the difference in photogenerated carriers. cElectron transfer percentage = (I1 − I2)/I1, where I1 is the absorption intensity due to relative content of anatase phase and I2 is the absorption intensity of TiO2 in a vacuum. dThe absorbance at 0 μs of a sample measured in CH3OH vapor after excitation. eElectron transfer percentage = (I1 − I2)/I1, where I1 is the absorption intensity due to relative content of anatase phase and I2 is the absorption intensity of TiO2 in CH3OH vapor. a

D

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TiO2. The increase of anatase/rutile interfaces has to improve the charge transfer process in mixed phase TiO2. In order to confirm the effect of anatase−rutile phase junction, mechanically mixed phase TiO2, A/R-600 and A/R700, which have the same phase compositions as TiO2-600 and TiO2-700, respectively, were also studied by time-resolved MIR spectroscopy (Figure 4). The initial absorption intensity of

Figure 5. Transient MIR absorption decays of mixed phase TiO2: anatase, A/R-600, TiO2-600, A/R-700, TiO2-700, and rutile in 20 Torr of CH3OH excited by 355 nm laser pulse of 6−8 ns duration.

initial MIR absorption intensities of TiO2-600 and TiO2-700 are apparently weaker than that of A/R-600 and A/R-700, respectively, which are in consistent with the results in a vacuum. These results further confirm the charge transfer process at the anatase/rutile interfaces. The charge transfer percentages in all mixed phase TiO2 in CH3OH vapor were also calculated, as listed in Table 1. The charge transfer percentage of A/R-600 in CH3OH vapor is about 0, while it is about 3% in a vacuum. This result indicates that transfer percentage of 3% is almost the experiment error. The transfer percentage of 8% in A/R-700 suggests that charge transfer just starts happening in mechanically mixed phase TiO2 due to the weak interaction between anatase and rutile. Amazingly, the transfer percentages of TiO2-600 and TiO2-700 in 20 Torr of CH3OH vapor are about 22% and 43%, respectively, which are much larger than those in a vacuum. This increase of the charge transfer percentage in methanol vapor demonstrates that the charge transfer is the electron transfer from anatase to rutile, not the hole transfer from rutile to anatase without electron transfer in the mixed phase TiO2. This is because if the charge transfer is only the hole transfer, the efficient consumption of photogenerated holes captured by methanol in 1 ns must decrease the hole transfer rate, resulting in a higher electron population of anatase in methanol vapor and a lower transfer percentage. However, the charge transfer percentages are increased by the addition of methanol vapor, illustrating the electron transfer from anatase to rutile. But this does not mean the hole transfer from rutile to anatase is impossible; the hole transfer from rutile to anatase may coexist with the electron transfer from anatase to rutile in mixed phase TiO2. Especially for TiO2-700, the electron transfer percentage in CH3OH vapor increases nearly 20% in comparison with that in a vacuum. The exposure to CH3OH vapor prolongs lifetimes of photoinduced electrons of anatase TiO2 as shown in Figure 2B. The long-lived electrons should have more time to be trapped by shallow trap states in anatase and then transfer to rutile in mixed phase TiO2. Thus, the increased electron transfer percentage should be contributed by both increased free electrons in conduction band and also by shallowly trapped electrons. 3.4. Electron Transfer from Anatase to Rutile in Anatase−Rutile Mixed Phase TiO2. The mixed phase

Figure 4. Transient MIR absorption decays of mixed phase TiO2: (a) A/R-600, (b) TiO2-600, (c) A/R-700, and (d) TiO2-700 excited by a 355 nm laser pulse of 6−8 ns duration.

mechanically mixed samples is a little weaker than that calculated from the phase composition as well, but apparently much stronger than that of mixed-phase samples with the same phase composition prepared by calcination at high temperatures. This result further confirms the charge transfer process at anatase/rutile interfaces in mixed phase TiO2-600 and TiO2700. If the decreased initial MIR absorption of mechanically mixed phase TiO2 is also assumed to due to charge transfer at anatase/rutile interface, the charge transfer percentages in A/R600 and A/R-700 are only 3% and 4%, respectively, as listed in Table 1. The low charge transfer percentage in mechanically mixed phase TiO2 may be just due to experiment error but not the real charge transfer process, or it is due to weak contact between anatase and rutile if there is charge transfer. Thus, the charge transfer percentage between anatase and rutile is sensitively related to the interface of anatase−rutile phase junction. The interaction between anatase and rutile in TiO2600 and TiO2-700 should be much stronger than that in A/R600 and A/R-700. The strong interaction between anatase and rutile makes more efficient charge transfer. Therefore, the charge transfer percentages of TiO2-600 and TiO2-700 are much larger than that of A/R-600 and A/R-700. Figure 5 shows the MIR absorption decay curves of TiO2 in 20 Torr of CH3OH vapor. The electron-induced absorption of MIR light in mixed phase TiO2 remains constant in 90 μs as that in pure anatase, indicating that no charge transfer takes place on the microsecond time scale in mixed phase TiO2. The charge transfer must complete within 50 ns, which is the time resolution limitation of the spectrometer. Thus, it is reasonable to calculate charge transfer percentage of mixed phase TiO2 using the initial MIR absorption intensity. The initial absorption intensities of all mixed phase TiO2 in CH3OH vapor are also weaker than that calculated from the relative content of anatase in mixed phase TiO2. Moreover, the E

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junction was identified by HRTEM using the TiO2(A)/ TiO2(R)-n sample and the mixed phase TiO2 prepared from Degussa P25 in our previous papers.5,30 In a summary, the quasi-Fermi level equilibration, recombination, and electron transfer rates determine the amount of electron transfer for the mixed phase TiO2 with a specific phase composition and structure. It is demonstrated that the electron transfer percentage is highly dependent on the phase composition and the preparation method of the mixed phase TiO2. The different anatase/rutile junction obtained in TiO2 prepared by different authors may be one reason for their differently proposed charge transfer process in the mixed phase TiO2. Moreover, another possible reason is because the different characterization method and measurement conditions investigate the different charge carrier aspects, like the EPR experiments at 10 K reported by Hurum et al. mainly studied transfer process of the trapped electrons in mixed phase TiO2.9 Anyway, the electron transfer from anatase to rutile does exist in the mixed phase TiO2. But this does not mean that the other charge transfer process, such as the hole transfer process, does not proceed in the mixed phase TiO2. Actually, the charge transfer process at the anatase/ rutile interface may be very complicated. Further work with combination of multiple time-resolved spectroscopics is being done to fully understand the complete charge transfer process. Electrochemical impedance analysis established that the flatband potential of anatase (101) is shifted negatively by 0.2 V vs the flat-band potential of rutile (001),31 although different band alignments of anatase−rutile mixed phase TiO2 are obtained by theory calculation.15,16,32,33 We tentatively propose the charge transfer process in mixed phase TiO2 with the supposed band alignment of higher flat-band potential of anatase, as shown in Scheme 1. Photoinduced electrons in anatase exhibit apparent

TiO2 were prepared by calcination at different temperatures. The calcination process changes not only the phase composition but also the surface structure, surface area, optical property, and crystallinity of TiO2 samples. All these factors, which influence the photogenerated carrier dynamics of TiO2, must be considered when we discuss the charge transfer in anatase−rutile mixed phase TiO2. The particle diameters of TiO2 increase with the calcination temperature increasing. The particle diameters are about 50, 70, 120−150, and 200−300 nm for anatase (TiO2-500), TiO2-600, TiO2-700, and rutile (TiO2800), respectively, as shown in the SEM images in Figure S2. Correspondingly, the surface area decrease dramatically with calcination temperature increasing. As reported in our previous work, the surface area of TiO2-700 decreases to 9.3 m2/g from 54.3 m2/g for anatase.5 Moreover, the crystallinity of TiO2 increases with calcination temperature increasing, evidenced by the increased diffraction intensity in XRD patterns. The decrease of surface area, the change of surface structure, and crystallinity are indeed possible reasons for the decrease of the electron population in mixed-phase TiO2 relative to the electron population calculated with phase composition. Now we discuss this possibility using the MIR absorption change of the TiO2-700 sample relative to anatase. If we assume the 24% decrease of the initial MIR absorption intensity of TiO2-700 in a vacuum is only attributed to the fact that the decrease of surface area or the change of surface structure and crystallinity accelerate the recombination rate of electrons and holes, the effect of surface area, surface structure, and crystallinity on the recombination rate in methanol vapor will dramatically decrease due to the effective capture of photogenerated holes by methanol-derived adsorbates.20,28 For TiO2-700, the decrease extent of mid-IR absorption in methanol vapor should be the same as or less than that in a vacuum due to lower recombination rates. However, the decrease extent increases from 24% to 43% by the addition of methanol. Therefore, further decrease of electron population is mainly ascribed to the electron transfer from anatase to rutile. Although the photogenerated electrons are still present on the anatase in microseconds as shown in transient MIR absorption, the electron transfer from anatase to rutile on the microsecond time scale is not observed even in methanol vapor (Figure 5). Thus, the photoinduced electrons in anatase transfer to rutile in 50 ns after excitation in mixed phase TiO2. The reason why electrons do not transfer in microsecond time scale may be that the charge equilibration between anatase and rutile is achieved before microsecond time scale. The electron transfer from anatase to rutile is due to that the quasi-Fermi level of anatase is higher than that of rutile. With the electron transfer from anatase to rutile across the anatase−rutile interface, the quasi-Fermi level of anatase becomes lower whereas that of rutile gets higher. When the electron transfer makes the quasi-Fermi level equilibration, there will no more electrons transfer occurring. No electron transfer is observed on the microsecond time scale in mixed phase TiO2, indicating that the quasi-Fermi level equilibration is already achieved in 50 ns even in methanol vapor. The electron transfer and recombination are competitive processes in TiO2. The electron transfer percentage of TiO2700 is 24% in a vacuum. With the dramatically decrease of recombination rates by methanol, the electron transfer percentage can be increased to 43% for TiO2-700. Moreover, the electron transfer from anatase to rutile is sensitive to the structure of the anatase/rutile junction. The anatase/rutile

Scheme 1. Schematic Band Structure of Anatase and Rutile under UV Light Irradiation

transient MIR absorbance on the microsecond time scale, while photogenerated electrons in rutile do not show any transient MIR absorption. In mixed phase TiO2, the transient MIR absorption is assumed to reflect the electron dynamics in only anatase. The decreased initial MIR absorption in mixed phase TiO2 demonstrates the charge transfer process at anatase/rutile interfaces, and the photoinduced electrons in anatase transfer to rutile in 50 ns after excitation in mixed phase TiO2. The F

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electron transfer percentage is highly dependent on phase composition and close interaction at interface between anatase and rutile, and hole scavenger methanol improves effectively the electron transfer efficiency from anatase to rutile by prolonging electron lifetimes in TiO2, which might make electron transfer from shallowly trapped states of anatase to rutile. The electron transfer percentage is about 43% in methanol in TiO2-700, which has the relative content of anatase about 51%. The electron transfer improves charge separation and increases carrier lifetime in mixed phase TiO2. This is most possibly the reason for enhanced photoactivity of mixed phase TiO2.

4. CONCLUSION Photogenerated electron dynamics in anatase, rutile, and mixed phase TiO2 were investigated by time-resolved MIR spectroscopy. Photoinduced electrons in anatase can be detected with a transient MIR absorption on the microsecond time scale, while no transient MIR absorption is observed in rutile in microsecond time scale. The decreased initial MIR absorption intensity in mixed-phase TiO2 demonstrates the charge transfer process at anatase/rutile interfaces of mixed phase TiO2. The photogenerated electrons in conduction band and shallow trap states of anatase transfer to rutile in 50 ns after excitation. The electron transfer percentage highly depends on phase composition and anatase−rutile interface in mixed phase TiO2. The charge transfer process improves charge separation of photogenerated carriers and then enhances the photocatalytic activity of mixed phase TiO2. The identification of charge transfer process at the anatase/rutile phase junction can help to understand the role of TiO2 phase junction in photocatalysis.

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ASSOCIATED CONTENT

* Supporting Information S

Tables S1 and S2; Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel 86-411-84379070, Fax 86-41184694447 (C.L.). Author Contributions

S.S. and X.W. made similar contributions to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21373209, 21203185), the National Basic Research Program of China (973 Program, 2014CB239400), and the Knowledge Innovative Program of The Chinese Academy of Sciences (KGCX2-EW-310-1).

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