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Multiple Heterojunction in Single Titanium Dioxide Nanoparticles for Novel Metal-free Photocatalysis Yoonjun Cho, Sungsoon Kim, Bumsu Park, Chang-Lyoul Lee, Jung Kyu Kim, KugSeung Lee, Il Yong Choi, Jong Kyu Kim, Kan Zhang, Sang Ho Oh, and Jong Hyeok Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01245 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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Multiple Heterojunction in Single Titanium Dioxide Nanoparticles for Novel Metal-free Photocatalysis Yoonjun Cho†, Sungsoon Kim†, Bumsu Park‡, §, Chang-Lyoul Lee⊥, Jung Kyu Kim∥, KugSeung Lee#, Il Yong Choi‡, Jong Kyu Kim‡, Kan Zhang†, ∇ *, Sang Ho Oh§* and Jong Hyeok Park†* †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea ‡

Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea §

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea

∥

School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea

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Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea

#

Beamline Division, Pohang Accelerator Laboratory, Pohang 790-834, Republic of Korea

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College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

*Correspondence to: Kan Zhang ([email protected]), Sang Ho Oh ([email protected]), and Jong Hyeok Park ([email protected])

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ABSTRACT

Despite a longstanding controversy surrounding TiO2 materials, TiO2 polymorphs with heterojunctions composed of anatase and rutile outperform individual polymorphs because of the type-II energetic band alignment at the heterojunction interface. Improvement in photocatalysis has also been achieved via black TiO2 with a thin disorder layer surrounding ordered TiO2. However, localization of this disorder layer in a conventional single TiO2 nanoparticle with the heterojunction composed of anatase and rutile has remained a big challenge. Here we report the selective positioning of a disorder layer of controlled thicknesses between the anatase and rutile phases by a conceptually different synthetic route to access highly efficient novel metal-free photocatalysis for H2 production. The presence of localized disorder layer within a single TiO2 nanoparticle was confirmed for the first time by high-resolution transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS) and inline electron holography. Multiple heterojunctions in single TiO2 nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile layers give the nanoparticles superior electron/hole separation efficiency and novel metal-free surface reactivity, which concomitantly yields an H2 production rate that is ~11 times higher than that of Pt decorated conventional anatase and rutile single heterojunction TiO2 systems.

Keywords: Photocatalysts, Disorder-engineering, Multiple-heterojunction, Charge separation, Surface reactivity, Water splitting


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Introduction Since Fujishima and Honda’s discovery in 1972, titanium dioxide (TiO2) has been the most widely studied material for various photocatalysis applications because of its low cost and high UV-driven activity1-5. Heterogeneous photocatalysis over TiO2 mainly occurs on the surface or the surface phases of the two polymorphs (anatase and rutile), and the polymorphs significantly influence the photocatalytic behaviors of TiO26. In particular, the photocatalytic performance of TiO2 heterojunctions composed of both anatase and rutile are superior to those composed of either anatase or rutile alone. This performance is due to the type-II band alignment of the favored charge separation7,8, as well as the additional interface region that facilitates photocatalytic reactions6,9. More recently, TiO2 surfaces disordered by hydrogenation, in which the crystalline TiO2 was surrounded by an ~1 nm-thick disordered shell, exhibited impressive reactivities in photocatalytic H2 generation10. The thinner disordered shell differing from atomic defects (oxygen vacancy and Ti3+ doping)11,12, significantly tuned electronic structure of the crystalline TiO2, which narrows the bandgap of crystalline TiO2, enabling to absorb light in infrared and microwave region13,14. Since then, apart from the conventional polymorph interfaces, surface-disordered TiO2 has been the subject of much interest and is considered an unpredictable strategy for the synthesis of effective photocatalysts15,16. Studies regarding surface disorder engineering of TiO2 have also confirmed that the order/disorder heterojunction in a TiO2 single nanoparticle strongly suppressed electron/hole recombination17,18. In addition, the order/disorder interface in disordered TiO2 showed co-catalytic H2 generation activity beyond the conventional interface of two polymorphs17-20. Accordingly, introduction of the disorder layer in TiO2 and its photocatalytic performance is of great significance, but localization of this


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disorder layer in conventional TiO2 nanoparticles with the heterojunctions composed of anatase and rutile has remained a big challenge. Herein, we report highly efficient co-catalyst free photocatalytic H2 generation with TiO2 nanoparticles by selective positioning of a disorder layer between crystalline anatase/crystalline rutile polymorphs. In an extension of our previous work on phase selective disorder-engineering, further structural modification is achieved via simple control of annealing temperature under air condition to induce oxygen-mediated diffusion of oxygen vacancies17. Multiple heterojunctions in a single P25 nanoparticle is therefore realized with thermal outward-recrystallization of a disordered rutile phase. The schematic for this defect engineering process is illustrated in Figure 1a. First, the rutile phase of P25 was fully disordered selectively to generate an ordered anatase/disordered rutile interface, and this system was labeled Blue-P2517. Second, bulk disordered rutile was partially recrystallized to generate ordered anatase, disordered rutile, and ordered rutile mixed phases with multiple heterojunctions in a single TiO2 nanoparticle (denoted DE-P25). Results and Discussion The crystalline phases of P25, Blue-P25, and DE-P25 were determined by X-ray diffraction (XRD) patterns, as shown in Figure 1b. The diffraction peaks of P25 could be clearly attributed to the mixed phases of anatase and rutile. The diffraction peaks assigned to the rutile phase completely disappear in the XRD spectrum of Blue-P25, which is consistent with our previously reported work on rutile phase-selective disordering by an Li/EDA solution13. Interestingly, the rutile peaks that were not seen in the spectrum of Blue-25 begin to reappear in the spectrum of DE-P25 after thermal annealing at 200 °C in air, and that temperature is much lower than the


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phase transition temperature from amorphous TiO2 to the rutile phase (generally, > 600 °C)18. Moreover, the original intensity of the rutile peaks in P25 can be mostly recovered by thermal annealing at 500 °C (Figure S1); however, the diffraction peaks corresponding to the anatase phase did not change regardless of the annealing temperature. Since the residual Li is thoroughly quenched and washed after Li-EDA treatment, it can be assured that there is no intercalated or residual Li on disordered phase17,21,22. This indicates that the theoretical transition from amorphous to crystalline TiO2 polymorphs is not applicable to the rutile nature-containing disordered TiO2. Therefore, the disordered rutile phase in Blue-P25 seems to retain its original short-range order even though no rutile phase-related XRD peaks were observed. Further investigations on the DE-P25 were conducted by high-resolution transmission electron microscopy (HR-TEM), and the results of those tests are shown in Figure 1c-e. The HR-TEM image of pristine P25 (Figure 1c) clearly shows a well-defined anatase and rutile junction in which the rutile part has a distinct lattice space of 3.25 Å (yellow square). The HR-TEM image of Blue-P25 (Figure 1d) reveals a disordered rutile portion that is intimately connected to a crystalline anatase portion. The texture of Blue-P25 together with disappearance of the characteristic rutile peaks in the XRD spectrum can be attributed to selective structural disordering in P25 after solution reduction. The re-oxidation of Blue-P25 at moderate temperature (200 °C) in air allowed the disordered rutile phase to recover to its original crystalline rutile phase in direction from the core towards the surface. It seems that the vacancy diffusion mechanism with O2 exposure induces thermal outward-recrystallization, so that the reoxidized Blue-P25 can have a crystalline rutile core surrounded by a 2~3 nm-thick disordered shell (Figure 1e). To further determine the thermal induced transition from disorder to order, blue P25 annealed at 200 °C with different time is investigated. As shown in Figure S2, the intensities


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of rutile peaks do not change obviously as prolonging the annealing time, which indicate that the annealing temperature is the dominated factor for rutile phase regeneration, also imply that the final disordered shell is thermodynamically stable at the setting temperature. In short, the localized disordered structure of DE-P25 has the layer preferentially formed both on the surface of the rutile TiO2 and the interface between ordered anatase and rutile. To clearly confirm the homogeneity of the outward recrystallization in disordered rutile TiO2, a pure rutile TiO2 nanorod was synthesized and subjected to the same defect engineering process (Figure S3). The interatomic structural properties of the TiO2 during the synthetic process were determined by extended X-ray absorption fine structure (EXAFS) analysis. As shown in Supplementary Table 1, the coordination number of Blue-P25 dramatically decreased due to the disordering of rutile TiO2 at the atomic level. For DE-P25, this number rebounded to a moderate level between the former pristine and disordered phases, which is in perfect agreement with HRTEM and XRD observations. Thus, ordered anatase/disordered rutile/ordered rutile multiple heterojunctions were achieved by re-forming interatom interactions, which recrystallizes the bulk material while leaving open coordination sites on the surface. However, the bond lengths of P25, Blue-P25 and DE-P25 are not noticeably changed; for the most part, they are all similar. Therefore, the determined coordination number only concerns the oxygen vacancies or deficient states of TiO2. Moreover, the open coordination sites on the disordered surface might be stabilized by bridging hydroxyl groups, as evidenced by the O 1s X-ray photoelectron spectroscopy (XPS) spectra. The peaks indicative of bridging hydroxyl groups (531.1 eV) in the O 1s spectra of Blue-P25 and DE-P25 have similar magnitudes, while the corresponding peak in the P25 spectra is smaller (Figure S4).


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The Ti-L2,3 edge EELS mapping of DE-P25 was carried out to give detailed insight into valence state distributions along the ordered anatase/disordered rutile/ordered rutile multiple heterojunctions. As shown in Figure 2a, the transitions in the fine structure of the Ti-L2,3 edge at fourteen sequential points (shown in Figure 2b) are observed in EELS spectra images because of the phase and oxygen-deficiency variations. The relative intensity ratios of the fine features at each Ti-L2,3 white line in the disorder layer (points #6~8) are significantly different from those of the crystalline rutile and anatase polymorphs, and the ratios appear to be very similar to those of the oxygen deficient states of the titanium oxide crystals23. Specifically, unlike in the bulk crystals, the positions of four peaks (a and b of Ti L3 and c and d of Ti L2) are shifted around the disordered region (Figure S5), so that the intensity ratios (b/a and d/c) are distinct at the disorder layer. This confirms the existence of multiple heterojunction interfaces as visualized in Figure 2c. To investigate the distribution of electrostatic properties, the section crossing the ordered anatase/disordered rutile/ordered rutile region is again highlighted in potential and charge density mappings. Phase and thickness profile calculations at the identical region were used to calibrate the potential distributions (Figure S6). For the framed area of Figure 3a, a drastic decrease in potential is observed for both interface regions of crystalline/disordered crystals (Figure 3b). These built-up potential gradients within the interface regions of crystal defect boundaries induce interfacial polarization. The migration pathways of majority and minority carriers are in different directions, since the directions of potential drop at the disorder/anatase and rutile/disorder are identical. Either ordered or disordered TiO2 can be regarded as n-type semiconductor, the higher surface potential will attract minority carriers (holes), and vice versa for majority carriers (electrons). In this regard, the diagram of charge transfer shows a vectoral charge transfer that is facilitated by disorder interfacial polarization, as depicted in Figure S7. The charge mapping and


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charge density profiles show corresponding trend at a disorder interface (Figure 3c,d). The potential gradient along the interface region clearly builds up space charge region, where the charge density deviations create electric field. The overall charge accumulation and interfacial polarization are illustrated in Figure 3e, as well as the relative electronic band-edge positions. The electrostatic characteristics throughout the ordered anatase/disordered rutile/ordered rutile multiple interface channels of DE-P25 provide comprehensive evidence for, and aid our understanding of highly efficient interfacial charge transfer processes. To understand the thermodynamics of charge migration behavior, a time-correlated singlephoton counting (TCSPC) measurement was conducted. In this study, the photoluminescence (PL) decay profiles by TCSPC experiments were characterized at low temperature (20 K) to exclude negative interferences from the defect states of the metal oxides. The acquired PL decay profiles were fitted with a bi-exponential function, which suggested that the PL decay occurred through two relaxation pathways, one of which was much faster than the other (Figure 4a). The fitted parameters, including the PL lifetimes and fractional intensities, are listed in Supplementary Table 2. The amplitude weighted average exciton lifetime (τavr) was shorter in Blue-P25 than it was in P25 (0.38 ns and 0.86 ns, respectively). After re-oxidation, there was a significant decrease in the τavr DE-P25 (0.29 ns), which is roughly one third the lifetime of P25. This result can be attributed to improved charge separation efficiency throughout the ultrafast charge migration. The dramatic decrease in the PL lifetime is caused by the rapid PL decay of the more dominant fast component (τ1), which is due to the direct formation of free excitons. Although the values of the slow component (τ2) were approximately ten-fold larger than those of the fast component, the fraction of long-lived excited species was much smaller than that of the short-lived excited species, so the average exciton lifetime (τavr) would not be noticeably


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impacted. τ2 is attributed to the indirect formation of self-trapped excitons along with trapped electrons. This indicates that the rutile phase containing a disorder layer in the TiO2 nanoparticle induces dominant direct exciton formation and reduces the self-trap of charge carriers, thus suppressing electron-hole recombination. Consequentially, in line with the EELS mapping analysis, the potential distribution throughout the multiple interfaces perfectly coincides with the improvement in the interfacial charge separation efficiency. Another aspect regarding the reaction activity at the surface of a disorder layer was elucidated by an electrochemical polarization curve. As shown in Figure 4b and c, both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) activities are improved by the positioning of a disorder layer within the TiO2 particle either through a decrease in the overpotential or through an increase in the current density. The enhanced surface activities can be attributed to the presence of disordered phases that alter the intrinsic chemical reactivity of the surface in terms of its co-catalytic properties25-27. The overall co-catalyst free hydrogen generation by DE-P25 in methanol containing electrolyte (See Supplementary in detail) had a significantly higher rate of H2 generation (3.9940 µmol/cm2·h) than Pt decorated P25 or Blue-P25 (0.3507 µmol/cm2·h and 0.9590 µmol/cm2·h, respectively, Figure 4d). Moreover, the H2 production rates of DE-P25 under alternating light irradiation and dark condition were still steady (Figure S8), suggesting that the DE-P25 does not be significantly influenced during photocatalysis. The optical properties of P25, Blue-P25 and DE-P25 were analyzed by their UV-Vis absorption spectra. As shown in Figure 4e, pure P25 showed a characteristic absorption band at 410 nm, which corresponded to an intrinsic bandgap of 3.02 eV. For Blue-P25, absorption throughout the visible and near-infrared regions was significantly enhanced. Interestingly, DE-P25 exhibited optical properties that were very similar


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to those of conventional hydrogenated TiO2, in which an absorption tail starting from the intrinsic absorption edge of P25 stretching into the near-infrared region is observed. However, the contribution of the improved absorbance above 400 nm in DE-P25 had a negligible impact on solar water splitting as shown in Figure S9. Therefore, the efficient charge separation and surface reactivity of DE-P25 enable it to be an efficient novel metal-free photocatalyst for H2 generation from light28,29. The effectiveness of multiple interfaces in our DE-P25 was compared with other types of interface engineering reported in recent years, notes that the references investigated are limited to TiO2 photocatalyst, co-catalyst free photocatalytic H2 generation and AM1.5 light source. As shown in Table S3, the photocatalytic H2 production rate of our DE-P25 with multiple ordered anatase/disordered rutile/ordered rutile interfaces was several times higher than others.

Conclusion

In summary, we have developed a method to position a disorder layer a few nanometers thick at the interface between the anatase and rutile crystalline phases for multiple heterojunctions in a single TiO2 nanoparticle to use in highly efficient Pt-free photocatalysis. The unique multiple heterojunctions found in TiO2 nanoparticles with multiple ordered anatase/disordered rutile/ordered rutile interfaces were eventually synthesized by thermal outward-recrystallization of the disordered bulk rutile phase. Various analyses confirmed that the multiple heterojunction TiO2 nanoparticles created via the selective positioning of a disorder layer with a few nanometers-thick in P25 could achieve ultrafast charge separation and prominent co-catalytic surface reactivity, thus exhibiting an H2 generation rate that was about 11 times higher than that of Pt decorated P25. We believe that the selective positioning of a disorder


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layer in a single nanoparticle photocatalyst will be new avenue for the commercialization of photocatalytic solar water splitting.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at http://pubs.acs.org.

XRD and XPS spectra, TEM images, EELS values, electron holography mappings, proposed schematics, recyclability, quantum efficiency, and statistic charts of EXAFS fittings, PL decay lifetimes, H2 generation in comparison of different interface engineering of TiO2 photocatalysts. AUTHOR INFORMATION * Corresponding author:, [email protected] (Prof. K. Zhang), and [email protected] (Prof. S. H. Oh), [email protected] (Prof. J. H. Park)

Author Contributions J.H. P. conceived and directed this project. S.H. O. and K. Z. co-directed this project. Y. C. and S. K. performed all of the experiments, supported by B. P. Author B. P. conducted and analyzed EELS and inline holography measurements. Author C.L. L. conducted and analyzed the lowtemperature PL and TCSCP measurements with J.K. K. Author K.S. L. and I.Y. C. conducted and analyzed the EXAFS measurements, and Y. C., S. K., K. Z., and J.H. P. wrote the manuscript with feedback from all co-authors. ACKNOWLEDGMENT This work was supported by the NRF of Korea Grant funded by the Ministry of Science, ICT & Future Planning (2016R1A2A1A05005216, 2016M3D3A1A01913254, 2015M1A2A2074663). The TEM work at SKKU was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and


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Future Planning (NRF-2015R1A2A2A01007904). K.S. Lee acknowledges the support by NanoMaterial Fundamental Technology Development program (2017M3A7B4049173) through the National Research Foundation of Korea (NRF). K. Zhang acknowledges the support by “the Fundamental Research Funds for the Central Universities”, No.30918011106.


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Competing Interests The authors declare no competing financial interests.


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Figure 1. (a) Schematic diagram of defect engineering, (b) X-Ray Diffraction spectra; P25 (black), Blue-P25 (blue), DE-P25 (red), and HR-TEM images with selected-area electron diffraction pattern of (c) P25, (d) Blue-P25, and (e) DE-P25; scale bar: 10 nm.


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Figure 2. (a) Ti EELS spectra of DE-P25 at fourteen sequential points, (b) Fourteen sequential points specified in 2-D EELS spectrum from dark-field STEM image; scale bar: 20 nm, (c) Intensity ratio at each point; b/a of Ti L3 and d/c of Ti L2.


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Figure 3. (a) Potential map of DE-P25 at rutile/disorder layer/anatase multiple heterojunction, (b) Corresponding averaged potential vs distance plot of DE-P25 with in selected region, (c) Charge density map of DE-P25 derived from potential map, (d) Charge density graph versus distance at the same region, and (e) Proposed interfacial polarization across the multiple heterojunction and their relative positions of the electronic band structure. CBE: conduction band edge, VBE: valence band edge.


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Figure 4. (a) PL decay profiles at 410 nm wavelength of P25 (black), Blue-P25 (blue), and DEP25 (red), (b) Electrochemical HER activity comparison of P25 without disorder layer (black) and with disorder layer (red), (c) Electrochemical OER activity comparison of P25 without disorder layer (black) and with disorder layer (red), (d) Hydrogen production rate of P25 (black), Blue-P25 (blue), DE-P25 (red), as a function of time, (e) UV-vis absorption spectra within 300~1600 nm wavelength of P25 (black), Blue-P25 (blue), and DE-P25 (red).


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Multiple Heterojunction in Single Titanium Dioxide ... - ACS Publications

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