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Explaining the enhanced photoelectrochemical behavior of highlyordered TiO2 nanotube arrays: anatase/rutile phase junction ai changzhi, Pengcheng Xie, Xidong Zhang, XuSheng Zheng, Jin Li, Andreas Kafizas, and Shiwei Lin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06219 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Sustainable Chemistry & Engineering

Explaining the enhanced photoelectrochemical behavior of highly-ordered TiO2 nanotube arrays: anatase/rutile phase junction Changzhi Ai†, §, Pengcheng Xie†, §, Xidong Zhang†, §, Xusheng Zheng||, Jin Li§*, Andreas Kafizas‡*, Shiwei Lin†, §* †State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, P. R. China §College of Materials and Chemical Engineering, Hainan University, Haikou 570228, P. R. China ‡Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. ||

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China.

Correspondence

should

be

addressed

to:

[email protected]

[email protected] (A. Kafizas), [email protected] (S. Lin).

1

ACS Paragon Plus Environment

(J.

Li),

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ABSTRACT: The effect of calcination temperature on the photoelectrochemical properties of TiO2 nanotube arrays (TNTAs) has been investigated in many studies. Most work focused on improving the photoelectrochemical properties through optimization of the microstructure. In this paper, however, an anatase/rutile phase junction formed in TiO2 nanotubes has been demonstrated to account for the enhancement of the photoelectrochemical performance. Observations by the UVvisible diffuse reflectance spectra, glancing incidence angle X-ray diffraction (GIAXRD) and electrochemical impedance spectroscopy indicate that the rutile fraction is at the bottom of the nanotubes while the anatase fraction at the body of the nanotubes. The TNTAs with the coexistence of about 60% anatase and 40% rutile exhibit the optimal performance and show the 1.4 times improved photocurrent density compared with the pure anatase TNTAs. Detailed synchrotron radiation photoemission spectroscopy further confirms the existence and effect of the phase junction. The results suggest photogenerated electrons transfer from the rutile phase to anatase phase in the nanotubes due to the band edge alignment, which facilitates the photogenerated carriers separation and transport along the nanotubes and leads to apparent enhancement of the photoelectrochemical behavior. Keywords: TiO2 nanotube arrays; phase junction; photoelectrochemical properties; thermal treatment

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INTRODUCTION Since Fujishima and Honda found photocatalytic water splitting on TiO2 electrodes in 1972 1, lots of researchers began to study TiO2 nanomaterials, which has been applied to many promising applications in the fields of solar cell, photocatalytic water splitting, photocatalytic degradation, sensors and electrochromic devices.2-11 TiO2 has three crystalline phases, which are rutile, anatase and brookite. Anatase (band gap of 3.2 eV) and rutile (3.0 eV) are the two most common crystalline phases. And anatase is generally considered to have higher photocatalytic activity than rutile. Nevertheless, TiO2 photocatalyst Degussa P-25 consisting of 22% rutile and 78% anatase phases was discovered to possess better photocatalytic activity than pure rutile or anatase phase. Our research group and other groups have also demonstrated that mixed crystal phases had better photocatalytic activity.12-15 And the existence of anatase/rutile phase junction in TiO2 nanomaterials has been proposed to be the main reason for improving photocatalytic activity due to the favorable energy-level alignment for photo-generated electrons transfer between their corresponding conduction and valence bands.16 -20 1D TiO2 nanotubes have been extensively studied in the catalytic field for their excellent photogenerated charge separation and transport properties. The highly-ordered TiO2 nanotube arrays (TNTAs) could be conveniently obtained by a potentiostatic anodization of Ti foils.21 This technique can yield nanotubes with controllable diameters, wall thicknesses and lengths. In general, the as-fabricated TNTAs are amorphous, and the thermal annealing is needed to turn the amorphous nanotubes into crystalline ones before different applications. At elevated temperatures, grain size, surface morphology and crystal phase composition of TNTAs will change, and have significant influences on the photoelectrochemical properties.22 Among these factors, the crystallinity and fraction of anatase and rutile phases play the most important 3 ACS Paragon Plus Environment


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roles.23,24 Meanwhile, with increasing calcination temperature, anatase in the bottom of the TNTAs begins to transform into rutile, whereas the anatase phase remains in the body of nanotube.25-27 Finally, all of anatase turns into rutile. During this process, an anatase/rutile phase junction is formed. So far, various optimal rutile ratio (0-90 wt%) of the maximum photocatalytic activity have been reported.28 However, there is not much comprehensive research on the effect of anatase/rutile phase junction on the photoelectrochemical properties of TNTAs. Especially, it is still lack of a convincing evidence for the energetic alignment of the band edges of the rutile and anatase phases in the TNTAs and its influence on the separation and transfer of photoexcited charge carriers between the two phases, which is directly related to the photocatalytic activity. Such information is not only very important to clarify the underlying mechanism but also to design the high-performance photoelectrodes for photoelectrochemical applications. In comparison to the nanoparticles system mostly reported before, the highly-ordered TiO2 nanotubes provide a well-defined platform to in situ form the anatase/rutile phase junction by easily controlling the calcination temperature. In this paper, therefore, we demonstrated the unique anatase/rutile phase junction in TNTAs using the UV-visible absorption spectra, glancing incidence angle X-ray diffraction (GIA-XRD) and electrochemical impedance spectra. Such configuration, where the rutile phase is existed in the bottom of the nanotubes and the anatase phase in the body of nanotubes, facilitates the separation of photogenerated electron-hole pairs, and leads to an enhancement of photoelectrochemical properties of TNTAs. Furthermore, synchrotron radiation photoemission spectroscopy (SRPES) was used to further understand the role of anatase/rutile junction. EXPERIMENTAL SECTION 4 ACS Paragon Plus Environment

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Fabrication of TiO2 nanotube arrays Highly ordered TiO2 nanotube arrays on Ti foils were prepared following our previous method.29,30 Briefly, after chemically polishing and ultrasonically cleaning, a Ti foil (1.5 cm × 5 cm, 99.4% purity, 0.2 mm thickness) was anodized in glycerol solution containing 0.27 mol/L NH4F at 25 V for 3 h. The voltage was provided by a DC power supply. The anodization was carried out in a two-electrode system in which the Ti foil was used as the working electrode and a stainless steel foil was used as the counter electrode. After anodization, the samples were annealed at 350, 450, 500, 550 and 650oC for 3 h with a heating rate of 2 oC /min in ambient air, respectively. Characterization and photoelectrochemical measurements Surface and cross-sectional morphologies of TNTAs were evaluated using a field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The crystal phase of the TNTAs was detected using X-ray diffractions (XRD, Bruker D8) with a monochromatic Cu Kα radiation in an angle interval of 20-70o (2θ). Raman spectra were characterized by using Renishaw InVia micro-Raman spectrometer with 514 nm laser excitation. The diffuse reflectance spectra were recorded by a UV-vis spectrophotometer (Persee, TU-1901) using BaSO4 powder as reference. Synchrotron radiation photoemission spectroscopy were applied at the Catalysis and Surface Science Endstation in National Synchrotron Radiation Laboratory (NSRL), Hefei. Synchrotronradiation light with 170 eV was utilized and 10 V bias was applied. The photoelectrochemical properties were characterized with an electrochemical workstation (Zahner Zennium, Germany) using the three-electrode system with the prepared samples as the working electrodes. A platinum foil and a saturated calomel electrode (SCE) were the counter electrode and the reference electrode, respectively. The electrolyte was 0.1 M sodium sulfate 5 ACS Paragon Plus Environment


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(Na2SO4) solution, and a UV-LED light source (wavelength 370 nm and light intensity 0.3 mW/cm2) was used to illuminate the TNTAs electrodes. Electrochemical impedance spectroscopy (EIS) was performed at open circuit potential under illumination conditions, with an amplitude of 10 mV and frequencies varying between 1 Hz to 100 kHz. RESULTS AND DISCUSSION Figure 1a displays the typical SEM image of the uncalcined TNTAs sample, indicating that the uniform tubular structure can be achieved by using anodization. The as-prepared sample has an average inner diameter of about 90 nm and a wall thickness of about 20 nm. The length is about 1.7 μm. The inset of Figure 1a represents a cross-sectional view of the TNTAs, revealing that the TNTAs are made up of highly-ordered nanotubes. The SEM images of the TNTAs annealed at different temperatures in Figures 1b-f reveal that such nanoscale tubular structure did not apparently change with the increase of the temperature up to 500 oC, beyond which the structure begins to change at 550 oC and collapses at 650 oC.

Figure 1. Top-view SEM images of the TNTAs before (a) and after the annealing treatment at (b) 350 oC, (c) 450 oC, (d) 500 oC, (e) 550 oC, and (f) 650 oC. The insets of (a)-(e) show the cross-sectional SEM images. 6 ACS Paragon Plus Environment

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Figure 2. (a) XRD patterns and (b) Raman spectra of the TNTAs samples annealed at different temperatures. (c) Transient photocurrent responses and (d) J-V characteristics curves of the samples calcined at different temperatures under the UV-LED illumination. Figure 2a displays the XRD patterns of the as-prepared TNTAs and the TNTAs calcined at different temperatures. The crystallite sizes and fractions of corresponding phases for each TNTAs sample were calculated using the Scherrer and Spurr-Myers formulas, respectively.31,32 The results are listed in Table 1. The as-prepared TNTAs are amorphous before annealing. For the TNTAs sample annealed at 350 oC, two diffraction peaks corresponding to anatase (101) and (200) faces (2θ=25.38° and 48.18°, JPCDS 21-1272) could be clearly observed. When the 7 ACS Paragon Plus Environment


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temperature increases to 450 oC, the diffraction peak of rutile (110) face (2θ=27.58°, JPCDS 87920) begins to appear, indicating that phase transformation from anatase to rutile occurs. It can be found that 16.2% of the anatase phase turns into rutile phase and the average crystalline size of anatase slightly increases at this moment. The peak intensity of rutile increases as the temperature increases to 500 oC, suggesting that more anatase turns into rutile. In addition, we can find that 41.2% of the anatase phase turns into rutile phase and the crystallite size increases to 29.2 nm. As the annealing temperature increases to 550 oC, the peak intensity of rutile phase dominates, demonstrating that most of anatase transforms into rutile. The mass ratio of rutile phase increases to 91.0% with the crystallite size of 33.2 nm. Almost all anatase phase (98.7%) transforms to rutile when the temperature rises to 650 oC. Table 1. Effects of treatment temperatures on crystallite size, phase composition and bandgap of TNTAs. Temperature

Crystalline size (nm)a

Fraction (%)b

(oC) Rutile

Bandgap (eV)c

Anatase

Rutile

Anatase

350

25.1

-

100

0

3.19

450

25.8

14.8

83.8

16.2

3.16

500

27.0

29.2

58.8

41.2

3.15

550

31.4

33.2

9.0

91.0

3.02

650

52.6

41.8

1.3

98.7

2.98

a

calculated by Scherrer equation 𝑑 = 0.94𝜆/𝛽𝑐𝑜𝑠 𝜃, where d is the crystalline size, 𝜆 is the wavelength of the X-ray, 𝛽 is the full width at half maximum of the (101) and (110) peaks, and 𝜃 is the half angle of the diffraction peak on the 2𝜃 scale. b

calculated by Spurr-Myers formulas 𝜒 = 1/[1 + 0.8(𝐼𝐴/𝐼𝑅)], where 𝜒 is the mass ratio of rutile in the mixed phase, IA and IR are the diffraction peak intensities of anatase phase (101) and rutile phase (110) faces, respectively. c

calculated by the Tauc plots, (αhv)2 versus hv, from the UV-vis absorption spectra in Figure 3a, where α is the absorption coefficient, h is Planck constant and v is light frequency. 8 ACS Paragon Plus Environment

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The samples annealed at different temperatures was further characterized by Raman spectroscopy, as presented in Figure 2b. For the TNTAs annealed at 350 oC, five typical Raman peaks of the anatase phase could be observed at 145, 199, 396, 518 and 634 cm-1, which can be ascribed to Eg, Eg, B1g, A1g(B1g), and Eg modes, respectively.33 Apart from that, there are no other Raman peaks, suggesting the existence of only anatase phase at low annealing temperature. The Raman peak intensity increases as the annealed temperature increases, indicating the improved crystallinity of anatase. As the treatment temperature rises to 500 oC, three different Raman peaks appear at 230, 447, and 607 cm-1, which are the characteristic peaks of the rutile phase and can be ascribed to multi-photon process, Eg, and A1g modes, respectively.34 The Raman spectroscopy results are consistent with the XRD results. The photoelectrochemical behavior of the TNTAs annealed at different temperatures was characterized under an intermittent UV-LED irradiation. Figure 2c shows the transient light responses of the TNTAs under a potential of 0 V versus SCE. Due to the amorphous nature, the as-prepared TNTAs shows neglectable photocurrent response, which is not shown here. The photocurrent density increases from 50 to 70 μA/cm2 when the treatment temperature rises from 350 to 500 oC. As the calcination temperature further increases, the photocurrent density gradually decreases. At 650 oC, the photocurrent observed is negligible. This may be caused by the collapse of tubular structure and the formation of coarsened rutile film as shown in Figure 1f. It is worth noting that the photocurrent density of the calcined sample at 500 oC is 1.4 times that of the calcined sample at 350 oC. In order to further affirm the high photocurrent density of the 500 oC-annealed sample, the photocurrent densities of the TNTAs annealed at different temperatures were measured as a function of the applied voltage under the same conditions. In comparison to the photocurrent in 9 ACS Paragon Plus Environment


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Figure 2d, the dark currents for the samples calcined at different temperatures are negligible as shown in Figure S1. The photocurrent density obviously enhances as the treatment temperature rises from 350 to 500oC. However, when the calcination temperature continues to increase, the photocurrent density decreases instead. In addition, the onset potentials appear at -0.5 V for the sample calcined at 350 oC. Then, as the treatment temperature increases, the onset potentials shift to more negative voltages and reaches -0.6 V for the 500 oC-annealed sample. As reported previously, the negative onset shift is indicative of the positive effect of the coexistence of anatase and rutile phases.35,36 Also, no significant decrease could be observed in the photocurrent density of the 500 oC-annealed sample for 7200 s UV-LED light irradiation (Figure S2), indicating its good photoelectrochemical stability. The photoelectrochemical performance is closely linked with the optical absorption properties of the TNTAs. And in order to elucidate the effect of the phase composition on the photoelectrical behavior, light absorption properties of the TNTAs samples calcined at different temperatures were studied, and the results are shown in Figure 3a. The corresponding band gap (Eg) could be calculated from Figure 3a and listed in Table 1. The 350 oC-calcined sample with pure anatase phase has a weak UV absorption. With the increasing treatment temperature, the rutile phase emerges, and the UV absorption enhances. At the temperature of 500 oC, the TNTAs contains about 60% anatase and 40% rutile. After that, further increasing the treatment temperature would decrease the UV absorption. Except for the UV absorption intensity, a close look at Figure 3a can find that the UV absorption edge has small redshift and a small peak emerges at the wavelength around 410 nm when the annealing temperature increases to 500 oC. This may be related to the phase transformation from anatase to rutile phase at the bottom of the nanotubes. As the annealing temperature keeps increasing, this small absorption peak increases, 10 ACS Paragon Plus Environment

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suggesting that the portion of rutile phase grows upward with the temperature. For the 650 oCannealed sample, the absorption spectrum might be only contributed by the rutile phase, and the low absorption intensity could be due to the collapse of tubular structure.

Figure 3. (a) UV-vis absorption spectra of the TNTAs samples annealed at different temperatures. (b) UV-vis absorption spectra of the 500 oC-annealed TNTAs illuminated from the top (blue line) and bottom (red line), respectively. The TNTAs with short tube length was used as a control sample (black line). (c) GIA-XRD patterns of the 500 oC-annealed TNTAs under various grazing incidence angles. (d) Anatase (101): rutile (110) ratio calculated from (c). In order to verify a phase junction was formed after high-temperature annealing, Figure 3b shows three carefully collected UV-vis absorption spectra. We first tested the top-illuminated 11 ACS Paragon Plus Environment


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absorption property of the TNTAs films annealed at 500 oC as shown in Figure 3b (blue line). To exam the light absorption property of the TNTAs films from the bottom as depicted in Figure 3b (red line), we separated the 500 oC-annealed TNTAs from the substrate by applying a small reverse voltage, and then reversed and stuck them on the substrate to fabricate the inverted TNTAs as shown in Figure 4a. Raman spectroscopy of the top and bottom of the TNTAs are shown in Figure 4b. It can directly prove that the top is anatase phase and the bottom is rutile phase. The corresponding SEM images of the top, cross-sectional and bottom views of the TNTAs are present in Figures 4c. Interestingly, a significant redshift was observed when the TNTAs was illuminated from the bottom as shown in Figure 3b. The absorption edges of the spectra observed from the top and the bottom illumination are around 390 and 410 nm, respectively. According to the Tauc plots (Figures 6c and 6f), (αhv)2 versus hv, from the UV-vis absorption spectra in Figure 3b, the bandgap of the TNTAs top is about 3.15 eV, and that of the TNTAs bottom is about 2.97 eV, which are in good agreement with the bandgaps of anatase and rutile phases, respectively. For comparison, a TNTAs control sample with short tube length was prepared under the same conditions, and characterized by the absorption spectrum (black line in Figure 3b). The purpose of preparing the short nanotubes is to allow light to shine on both the top and bottom when characterized by diffuse reflectance spectra. It is worth noting that the short TNTAs sample exhibits an intermediate absorption characteristic. In addition, the broad absorption peak from 250 to 375 nm looks like the top-illuminated absorption spectrum of the 500 oC-annealed TNTAs, while the small peak at the right side in the range between 375 and 425 nm is similar to the bottom-illuminated spectrum. Thereby, we could confirm that the anatase and rutile phases can coexist in the TNTAs. The arrangement of anatase and rutile in TiO2 nanotubes might be in 12 ACS Paragon Plus Environment

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the form of layered structure. With the increasing treatment temperature, rutile phase begins to grow at the interface of the nanotubes and Ti substrate, and the anatase phase remains in the body of nanotubes, forming an anatase/rutile phase junction in the nanotubes.27 The rutile part would grow and move toward the anatase as the annealing temperature increases. At the meanwhile, it should be noted that it is not easy to find the exact location of anatase-rutile interface by a visualized characterization technology, such as TEM, because the nanotubes would be easily broken into pieces during the tested sample preparation. Direct observation of the anatase-rutile interface requires other advanced methods in the future.

Figure 4. (a) Schematic process for fabricating the inverted TNTAs after the annealing treatment at 500 oC. (b) Raman spectra of the top and bottom TNTAs annealed at 500 oC. (c) SEM images of top, sectional and bottom views of the TNTAs at 500 oC. To further prove the existence of the phase junction after high-temperature annealing, grazing incidence angle X-ray diffraction (GIA-XRD) patterns of the 500oC-annealed TNTAs were 13 ACS Paragon Plus Environment


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measured (Figure 3c). Figure 3d shows the ratio of the anatase (101) intensity to the rutile (110) intensity by calculating from the GIA-XRD patterns. With the increasing grazing incidence angle, the anatase (101): rutile (110) ratio gradually decreases. As more anatase is observed at less shallow grazing incidence angles, this indicates that the anatase component is more predominant at the top of the nanotubes (i.e. the body), and that the rutile component is more predominant towards the bottom (i.e. near the substrate). Therefore, this further confirms the existence of an anatase/rutile phase junction architecture. From the experimental results above, we can contribute the enhanced photocurrent response of the 500 oC-annealed TNTAs in Figures 2c and 2d to the following reasons. Firstly, the 500 oCannealed sample possesses good crystallinity. Secondly, the 500 oC-annealed sample has a relatively high UV absorption property. Thirdly, the 500 oC-annealed sample is composed of both anatase (~60%) and rutile (~40%) phases, and it is accepted that the mixed phases are beneficial of separation of electron-hole pairs and thus improve the photocurrent density.37-39 And the arrangement of anatase and rutile phases in TiO2 nanotubes may be in the form of layered structure. But how the anatase/rutile phase junction affects the photoelectrochemical performance is still exactly unclear. Next, we will seek to clarify the role of the phase junction on the photoelectrical properties of the TNTAs. Electrochemical impedance spectroscopy was applied to further provide additional evidence for the existence of the anatase/rutile phase junction and investigate its effects on the photoelectrochemical performance. Figure 5a shows the Bode phase plots of the TNTAs annealed at different temperatures. The data was recorded under open-circuit condition in the frequency range of 1 Hz to 100 kHz, where the frequency peaks are associated with the electron transfer in the photoanode and charge reaction at the photoanode/electrolyte interface.40 For the 14 ACS Paragon Plus Environment

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350 oC-annealed TNTAs, the peak at the low-frequency region can be assigned to the charge transfer at the interface of the electrolyte/anatase-phase TNTAs. As the thermal treatment temperature increases to 500 oC, a new high-frequency peak emerges, corresponding to the charge transfer at the interface of the electrolyte/rutile-phase TNTAs, while the low-frequency peak corresponds to not only the charge transfer at the interface of electrolyte/anatase-phase TNTAs but also the charge transfer across the anatase/rutile phase junction evolving in the photoanode as indicated by its movement toward high frequency. This thus suggests the formation of the anatase/rutile phase junction in TNTAs. As the annealing temperature further increases to 650 oC, the anatase/rutile phase junction almost disappears, leading to the absence of the low-frequency peak, and the phase angle at high-frequency region reaching a maximum value. In Figure 5b, we reported the impedance spectra in the Nyquist representation relating to the TNTAs calcined at different temperatures. The semi-circle of high frequency part is related to the electron transfer across the electrolyte/TNTAs interface, and the linear part at low frequency represents the mass-transfer process of solute.41,42 The equivalent circuit shown in Figure S3 is used to fit the experimental data. The ohmic serial resistance R0 corresponds to the electrolyte resistance. R1 and C1 refer to the resistance and capacitance of space charge layer at the anatase/rutile phase junction, respectively. R2 and CPE represent the charge-transfer resistance and double charge layer capacitance at the electrode/electrolyte interface, respectively. W0 is the Warburg component. The parameters obtained by fitting the electrochemical impedance spectra are summarized in Table 2. As can be seen, the values of R1 increase in the order of 500 oCannealed sample (98.61 Ω) < 450 oC-annealed sample (160.2 Ω) < 550 oC-annealed sample (201.6 Ω). The lowest R1 value from the TNTAs annealed at 500 oC is due to the existence of 15 ACS Paragon Plus Environment


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anatase/rutile phase junction. It demonstrates that photo-generated electrons can easily transfer across the anatase and rutile phase junction along the nanotubes and thus improve the photoelectrochemical performance. It can be validated by the photocurrent response experiments in Figure 2, where the sample calcined at 500 oC shows the highest photocurrent density. In addition, the R2 value of the 650 oC-annealed sample (1643 Ω cm-2) is larger than that of the 550 oC-

annealed sample (1221 Ω cm-2), demonstrating the advantages of tubular structure: the

increased surface area and the accelerated carrier transportation.43,44

Figure 5. (a) Bode phase plots and (b) EIS Nyquist plots for the TNTAs samples calcined at different temperatures. Table 2. Parameters obtained by fitting the impedance spectra of the TNTAs samples using the equivalent circuit in Figure S3. 350 oC

450 oC

500 oC

550 oC

650 oC

R0 (Ω cm-2)

9.06

8.698

5.34

6.271

12.1

R1 (Ω cm-2)

-

160.2

98.61

201.6

-

R2 (Ω cm-2)

199.1

32.28

44.91

1221

1643 16


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Figure 6. Valence-band spectra (a, d), secondary electron cutoff (b, e) and Tauc plots (c, f) of the top (a-c) and bottom (d-f) 500 oC-annealed TNTAs measured by SRPES.

Scheme 1. Schematic diagram of the transfer direction of photogenerated electrons and holes across the anatase/rutile phase junction. To illuminate the migration direction of electrons in the anatase/rutile phase junction, we carefully investigated the energy-band structure of the TNTAs calcined at 500oC using 17 ACS Paragon Plus Environment


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synchrotron radiation photoemission spectroscopy. Quite a few previous studies were devoted to investigating the migration direction of carriers at anatase/rutile interface. But two opposite views have been reported. Electrons transfer from rutile to anatase had been reported in the literature by using the hybrid density functional theory calculations,20 X-ray photoelectron spectroscopy,45 spatially resolved surface photovoltage spectroscopy,46 electron paramagnetic resonance spectroscopy,17 the branching point energies,47 etc. However, the opposite view that electrons migrates from anatase to rutile was also proposed in the investigation by the timeresolved mid-IR spectroscopy,15 Ag deposition,18 passivated quantum dots,48 electrochemical measurements,49 surface photovoltage and transient photovoltage techniques,50 etc. Possible reasons for the two opposite views are discussed in the Supporting Information with Figure S4 and Tables S1-S3. Here we carefully analyzed the SRPES spectra to investigate the charge transfer properties. As shown in Figure 6, the valence-band spectra and secondary electron cutoff of SRPES of the top and bottom 500 oC-annealed TNTAs were measured, respectively, and excitation energy of 169.4 eV was utilized.51 The ionization potential (equivalent to the valence band energy) of the top TNTAs was calculated to be 6.66 eV versus the vacuum level by subtracting the width of the entire spectra from the excitation energy (Figures 6a and 6b). In the same way, the valence band energy of the bottom TNTAs was calculated to be 6.44 eV versus the vacuum level (Figures 6d and 6e). Furthermore, the band gap of the top and bottom TNTAs could be estimated from the Tauc plots from the UV-vis absorption spectra in Figure 3b. Figures 6c and 6f show that the bandgaps of the top and bottom TNTAs are 3.15 eV and 2.97 eV, respectively. Consequently, the energy positions of conduction band of the top and bottom TNTAs are further calculated to be at 3.51 eV and 3.47 eV versus the vacuum level, respectively. Eventually, the positions of the 18 ACS Paragon Plus Environment

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valance and conduction bands of the top and the bottom TNTAs can be determined as shown in Scheme 1. Then, we could speculate that charge transfer from the TNTAs bottom to the TNTAs top. In other words, photogenerated electrons migrate from rutile to anatase in the anatase/rutile phase junction due to the proper band alignment in Scheme 1. Accordingly, we can clarify the effects of the phase junction in the TNTAs modulated by various annealing temperature on the photoelectrochemical performance. When the treatment temperature increases to 350 oC, only TiO2 anatase phase forms. As the temperature increases to 450 oC, transformation from anatase to rutile begins to happen from the bottom of the nanotubes. When the treatment temperature rises to 500 oC, more anatase phase continues to transform to rutile from the bottom to the top. At this moment, an anatase/rutile phase junction forms in the middle of the nanotubes. According to the SRPES analysis above, there exists electron transfer from rutile to anatase as shown in Scheme 1, resulting electron accumulation at the top of the nanotubes. Thus, a built-in electric field is setup with the direction from rutile to anatase, which can accelerate the photo-generated carriers separation and facilitates the carriers transport along the nanotubes, corresponding to the enhancement of the photoelectrochemical performance. In comparison to the photocatalytic activity of similar TiO2 photoelectrode reported before, the 500 oC-annealed

TNTAs show a superior normalized photocurrent of 233 mA/W as shown in Table

S4. When the temperature further increases to 550 oC, the effect from the phase junction decreases because almost all of the anatase phase have been transformed into the rutile phase. The collapse of tubular structure at 650 oC leads to further reduction of the photoelectrochemical performance. CONCLUSIONS

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In summary, the effect of the annealing temperature on the photoelectrochemical properties of TNTAs was studied. At elevated temperature, the unique anatase/rutile junction was observed, where rutile phase is formed mainly in the bottom of the nanotubes and the anatase phase remains in the body. The anatase/rutile junction is responsible for the enhanced photocurrent density. The optimum mixture is found to be about 60% anatase and 40% rutile when the TNTAs annealed at 500 oC. Further SRPES analysis determined the energy band alignment, which indicates the charge transfer from rutile to anatase in the anatase/rutile junction. Such an anatase/rutile phase junction in TNTAs is beneficial to the separation of photogenerated electronhole pairs which effectively improve the photocurrent response. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. J-V characteristics of the samples in dark; potentiostatic plot of the 500 oC-annealed sample; equivalent circuits for EIS analysis; theoretical analysis on possible reasons for the two opposite views about electron transfer direction between anatase and rutile TiO2; schematic diagram of work functions of common crystal faces of anatase and rutile phases; electrostatic potentials, Fermi energies and calculated work functions for anatase and rutile with common crystal surfaces; photocatalytic activity comparison of different TiO2 samples. ACKNOWLEDGMENTS This work is supported by the Key Research and Development Program of Hainan Province (ZDYF2017166) and the National Natural Science Foundation of China (51462008, 61764003, 11574157).

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For Table of Contents Use Only

An anatase/rutile phase junction has been demonstrated in TiO2 nanotubes to facilitate the photoexcited charge separation and electron transfer from rutile to anatase, corresponding to the enhanced photoelectrochemical performance.


rutile phase junction - ACS Publications

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