Directly Probing Charge Separation at Interface of TiO2 Phase Junction

Letter pubs.acs.org/JPCL

Directly Probing Charge Separation at Interface of TiO2 Phase Junction Yuying Gao,†,‡,§ Jian Zhu,†,§ Hongyu An,†,‡,§ Pengli Yan,†,‡ Baokun Huang,† Ruotian Chen,†,‡,§ Fengtao Fan,*,†,§ and Can Li*,†,§ †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Zhongshan Road 457, Dalian 116023, China ‡ University of Chinese Academy of Science, Beijing, 100049, China § Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Zhongshan Road 457, Dalian 116023, China S Supporting Information *

ABSTRACT: Phase junction is often recognized as an effective strategy to achieve efficient charge separation in photocatalysis and photochemistry. As a crucial factor to determine the photogenerated charges dynamics, there is an increasingly hot debate about the energy band alignment across the interface of phase junction. Herein, we reported the direct measurement of the surface potential profile over the interface of TiO2 phase junction. A built-in electric field up to 1 kV/cm from rutile to anatase nanoparticle was detected by Kelvin Probe Force Microscopy (KPFM). Home-built spatially resolved surface photovoltage spectroscopy (SRSPS) supplies a direct evidence for the vectorial charge transfer of photogenerated electrons from rutile to anatase. Moreover, the tunable anatase nanoparticle sizes in TiO2 phase junction leads to high surface photovoltage (SPV) by creating completely depleted space charge region (SCR) and enhancing the charge separation efficiency. The results provide a strong basis for understanding the impact of built-in electric field on the charge transfer across the interface of artificial photocatalysts.

P

accumulation of holes in rutile and electrons in anatase, respectively.15 Xie et al. investigated charge transfer using Kelvin probe and they found that the Fermi level of rutile is 0.22 eV higher than that of anatase, and photogenerated electrons transfer from rutile to anatase upon band gap illumination.24 However, the charge transfer processes in these models were often derived from energy levels of pure anatase and rutile under equilibrium state. Convincing evidence on the charge transfer and electric field at the interface of a real anatase/rutile junction are still lacking. To make this issue clear, a direct probe of energy band alignment at the interface of TiO2 phase junction at nanoscale is highly desirable but challenging, because the phase junction structures are often buried at interface and therefore high spatial resolution techniques are required. Herein, Kelvin probe force microscopy (KPFM)25−28 was employed to directly probe the energy band alignment across the interface of a model rutile/anatase phase junction on nanometer scale. An internal built-in electric field with upward band bending from anatase to rutile across the rutile/anatase interface is clearly revealed. Moreover, a home-built spatially resolved surface photovoltage spectroscopy (SRSPS)29 directly

hase junction has received a great deal of attention in artificial photosynthesis since its unique feature of same chemical composition but different crystal structure that can significantly improve the photocatalytic performance.1−8 An excellent example is that Degussa P25 composed of anatase/ rutile mixed phase exhibited higher photocatalytic activity than either pure anatase or rutile nanocrystals.8,9 It was also found that the appropriate phase alignment is essential for higher photo-to-electron conversion efficiency in photoelectrochemical (PEC) cell.10 In recent years, as a standard model system, there is a growing interest to explore the energy band alignment and charge transfer in TiO2 phase junction.11−14 It is widely accepted that the small difference between energy bands in the different phases of TiO2 might be the sole driving force to control the charge transfer direction and thus improve the charge separation efficiency. The charge transfer in TiO2 phase junction have been theoretically studied with first-principle calculations11,12,15,16 and experimentally studied by photoluminescence spectroscopy (PL),17,18 electron paramagnetric resonance spectroscopy (EPR),19−21 X-ray photoelectron spectroscopy (XPS),11,12,22 etc. The positions of the conduction band (CB) and valence band (VB) of components in the mixed-phase TiO2 are generally believed to play a key role in determining the charge transfer direction.23 Frauenheim et al. recently reported that the calculated EVB and ECB of rutile lied higher than those of anatase and thus lead to the © 2017 American Chemical Society

Received: February 6, 2017 Accepted: March 14, 2017 Published: March 14, 2017 1419

DOI: 10.1021/acs.jpclett.7b00285 J. Phys. Chem. Lett. 2017, 8, 1419−1423

Letter

The Journal of Physical Chemistry Letters reveals that photogenerated electrons transfer from rutile nanorods (NRs) to anatase nanoparticles (NPs) under UV light illumination. We demonstrated that the size of anatase NPs in the phase junction significantly affect the surface photovoltage and charge transfer process due to the variation of charge depletion layer and built-in electric field at the interface. A quantitative understanding of SCR width will further broaden the application of phase junction in photocatalysts where interface engineering is required to optimize the photocatalytic activity. The TiO2 phase junction sample was prepared by two-step method (see Experimental Methods in the Supporting Information). SEM images show that the TiO2 NRs vertically grew on FTO substrate (Figure 1a,b). XRD analysis (Figure

Figure 2. (a) Topographic and (b) 3D surface potential image of the cross section of rutile and anatase phase junction corresponding to the region as labeled in panel a. (c) The built-in potential distribution of rutile/anatase phase junction was derived from panel b. (d) The intensity distribution of built-in electric field across the interface of rutile/anatase phase junction.

observed. Figure 2b shows the 3D surface potential at the interface of TiO2 phase junction. It is noted that the rutile NRs and anatase NPs show obvious difference in surface potential. Surface potential variation presents gradual change across the interface from rutile to anatase. The CPD of rutile phase is about 30 mV lower than that of anatase. The result is consistent with the predication by Weng et al.13 using the newly developed transient infrared absorption-excitation energy scanning spectroscopy (TIRA-ESS), they suggested that the difference between the bottom of CB for anatase and rutile should be around one kT (26 meV). In KPFM measurement, the rutile/anatase sample was grounded without illumination and their Fermi levels were aligned at thermal equilibrium state. Therefore, the CPD represents the local vacuum energy level variations relative to Fermi level positions at TiO2 phase junction. Usually, the depletion layer in semiconductor will weaken the built-in electric field due to the formation of SCR perpendicular to the interface. However, as for the current case, the depletion layer within the nanorod (Figure 1a) is far from completely depleted, and thus will not significantly affect the measured result. Thus, the result demonstrates that work function of rutile is 30 meV higher than that of anatase. Consequently, it can be deduced that there should be an internal built-in electric field and an upward band bending from anatase to rutile across the rutile/anatase interface. The depletion layer width (W) at rutile/anatase interface can be also estimated to be 450 nm (Figure 2c). Since the surface potential distribution at phase junction is dependent on the doping concentration and dielectric coefficient, it is possible to derive the charge density from the potential drop. In thermodynamic equilibrium, the SCR is charge neutrality. The relations between the surface potential and the position in the SCR can be derived from the Poisson equation

Figure 1. SEM images of rutile NRs on FTO substrate: (a) top view and (b) cross section view. SEM images of rutile NRs/anatase NPs: (c) top view and (d) cross section view. (e) TEM image of rutile NRs/ anatase NPs. (f) HRTEM image of rutile NRs/anatase NPs.

S1) reveals that the NRs are indexed to the pure rutile phase. The NRs are tetragonal in shape with square top facets (insert in Figure 1a) and present the expected growth fashion for the tetragonal crystal structure.30 The length of the NRs were estimated to be about 3 μm from SEM images. The diameter of NRs is about 100 nm. The lattice fringe is found to be approximately 0.25 nm (Figure S2), corresponding to d101 spacing of rutile TiO2 crystal. After depositing TiO2 NPs on rutile NRs, the top view of the NPs shows a cauliflower-like morphology (Figure 1c). The XRD pattern in Figure S1 indicates that the sputtered TiO2 NPs are in anatase phase. As shown in Figure 1d, the thickness of the rutile/anatase sample is increased by 300 nm because of deposited anatase NPs on the tips of rutile NRs. The TEM image in Figure 1e shows that the top of aligned NRs are decorated and closely packed with NPs. The HRTEM image (Figure 1f) shows that the NPs exhibit lattice fringes index to the (101) plane of anatase. The above results clearly show that the anatase NPs are successfully coated with rutile NRs to form intimate TiO2 phase junction structure, which serves as an ideal model system for detailed investigation of phase junction. KPFM is based on the noncontact AFM mode for detecting contact potential difference (CPD) between the work function of AFM tip and the sample.31,32 KPFM could directly image the local work function of rutile/anatase phase junction due to its nanometer scale spatial resolution and millivolt sensitivity.33 Figure 2a shows the overlaid AFM morphology and phase image. At the bottom of the image, a well-defined interface between rutile nanorod arrays and anatase NPs is clearly 1420

φ (x ) =

qNR (x − x0)2 , 2ε0εR

when − WR ≤ x ≤ 0

φ (x ) =

qNA (x − x0)2 , 2ε0εA

when 0 ≤ x ≤ WA

(1)

(2)

DOI: 10.1021/acs.jpclett.7b00285 J. Phys. Chem. Lett. 2017, 8, 1419−1423

Letter

The Journal of Physical Chemistry Letters where φ(x) is the surface potential distribution as a function of the coordinate across the interface, NA and NR are the charge densities of anatase and rutile in the SCR, and WA and WR are the depletion layer widths of anatase and rutile, respectively. The relative dielectric constants of εA and εR are 28 and 115, respectively.34 The coordinate x = 0 corresponds to the interface of rutile and anatase, which can be obtained from the phase image. The charge depletion width of the rutile and anatase phase is estimated to be 150 and 300 nm, respectively. The surface potential of the interfacial phase junction can be well fitted by eqs 1 and 2, as shown in Figure 2c. The relative error of fitting values at x = 0 from eq 1 and 2 is less than 1%, which satisfied the continuity requirement of surface potential at SCR. The result reflects the formation of well-defined interface in the prepared rutile/anatase sample. The fitting of the curves also provides the carrier concentration of rutile and anatase as 5.51 × 1018 cm−3 for NR and 5.52 × 1017 cm−3 for NA, respectively. By differentiating the fitting profile, the built-in electric field intensity can be obtained, as shown in Figure 2d. The maximum electric field intensity at the anatase/rutile interface is calculated to be ∼1 kV/cm, which is comparable but slightly less than that in the heterojunction structure.35 According to the surface potential and the direction of the built-in electric field, it is convincingly demonstrated that photogenerated electrons could transfer from rutile to anatase across the phase junction interface. In order to further disclose the role of phase junction in the charge transfer, a reference sample with anatase NPs covering only half of rutile NRs was prepared by sputtering. The asymmetric TiO2 phase junction structure was composed with the right part consisting of rutile NRs and the left part consisting of rutile NRs and anatase NPs, as confirmed by Raman spectra and confocal Raman imaging in Figure S3. Figure 3a displays the AFM height image of the asymmetric

spectra show a SPV response at photoexcitation wavelength below 420 nm, which is consistent with optical absorption band edge of TiO2 (Figure S4), suggesting that the SPV results caused by the band-to-band excitation of TiO2. The intensity of the SPV of mix-phase TiO2 is significantly stronger than that of bare rutile NRs by 6 times (4.3 vs 0.7), indicating that phase junction can improve charge separation comparing to pure rutile phase. It should be noted that, for bare anatase NPs, no SPS signal is detected under the same experimental condition (Figure S5). The result can be attributed to the extremely weak built-in electric field due to the negligible band bending in the SCR of the nanoscale anatase NPs. 38 Therefore, the significantly enhanced SPV response in mixed phases TiO2 solely comes from the contribution of internal built-in electric field at interface. Moreover, it is interesting to note that the feature phase values of bare rutile NRs and mix-phase TiO2 are quite different from each other (Figure 3d). The phase values of bare rutile NRs show a typical response of n-type semiconductors around 150°, while the phase values of anatase in mix-phase TiO2 show a typical response of p-type semiconductors around −50°, suggesting that the type of accumulated photogenerated carriers at the surface is changed after the formation of phase junction. The result indicates that the presence of space charge layer associated with built-in electric field at interface can separate electrons and holes and drive electron and hole to anatase and rutile, respectively. This observation provides direct evidence for the aforementioned conclusions that a built-in electric field from anatase to rutile is formed and photogenerated electron transfer from rutile to anatase upon UV light excitation. To ensure the formation of an effective built-in electric field, the widths of any phases need to be sufficient to contain the whole SCR. However, large anatase NPs will lead to the unwanted charge recombination outside the SCR. The relation between the surface photovoltage and the size of anatase NPs was studied by SPV spectroscopy (Figure 4). The SPV strongly

Figure 4. SPV of rutile NRs/anatase NPs sample with different particle size of anatase. The inset shows the SPV of anatase/rutile phase junction as a function of particle size. Figure 3. (a) AFM topographic image of rutile NRs/anatase NPs. (b) Height profile as marked by the red line in a. SRSPS (c) amplitude and (d) phase spectra obtained at different locations on TiO2 sample as labeled in panel a.

varied with the size of anatase NPs. The maximum SPV is obtained in the TiO2 phase junction with 300 nm anatase NPs (inset of Figure 4). It should be noted that the depletion width as measured by KPFM in anatase region is 300 nm, which is an effective region for driving charge separation.38 Therefore, SCRs in the anatase NPs smaller than 300 nm are incompletely depleted and the increased anatase NPs size could actually give rise to the growth of the depletion width. Under UV light illumination, the photogenerated electron−hole pairs can be separated effectively in SCR. It is clearly observed that further increasing the anatase NPs size to 500 nm will lead to the

anatase/rutile sample. The height of the mixed phase TiO2 at the left is 300 nm higher than the bare rutile NRs (Figure 3b). Then, SRSPS based on a lock-in amplifier is employed to characterize charge transfer at the nanoscale.29,36,37 Figure 3c shows surface photovoltage spectra of the TiO2 mixed phase sample measured at different locations in Figure 3a. All the 1421

DOI: 10.1021/acs.jpclett.7b00285 J. Phys. Chem. Lett. 2017, 8, 1419−1423

The Journal of Physical Chemistry Letters

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decreased SPV. Since the photon penetration depth is much greater than the size of anatase NPs,39 the TiO2 phase junction interface was excited. We attributed this to the recombination of the photogenerated carriers outside the SCR due to the lack of driving force. This result reveals that the selection of proper size of anatase NPs based on depleted width is quite important for charge separation efficiency when fabricating the phase junction structures. On the basis of these results, a proposed mechanism for charge separation of rutile/anatase phase junction is shown in Scheme 1. The alignment of Fermi levels in TiO2 phase

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00285. Experimental details, TEM, XRD measurements, Raman spectra/imaging, UV−vis spectra, and surface photovoltage (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(F.T.F.) E-mail: [email protected]. *(C.L.) E-mail: [email protected].

Scheme 1. Schematic of Using Cross-Section KPFM to Obtain the Energy Band Alignment of a Rutile/Anatase Phase Junction and the Transfer Direction of Photogenerated Electrons and Holes at the Interface of a TiO2 Phase Junction

ORCID

Can Li: 0000-0002-9301-7850 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21633015, 21373212), the National Key Basic Research Program of China (973 Program, Grant No. 2014CB239403), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB01020300).

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DOI: 10.1021/acs.jpclett.7b00285 J. Phys. Chem. Lett. 2017, 8, 1419−1423

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