Effects of Defects on Photocatalytic Activity of Hydrogen-Treated

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Effects of Defects on Photocatalytic Activity of Hydrogen-Treated Titanium Oxide Nanobelts Scott K Cushing, Fanke Meng, Junying Zhang, Bangfu Ding, Chih Kai Chen, ChihJung Chen, Ru-Shi Liu, Alan D Bristow, Peng Zheng, Joeseph Bright, and Nianqiang Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02177 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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Effects of Defects on Photocatalytic Activity of Hydrogen-Treated Titanium Oxide Nanobelts

Scott. K. Cushing,a,b Fanke Meng, a Junying Zhang,c Bangfu Ding,c Chih Kai Chen,d Chih-Jung Chen,d Ru-Shi Liu,d,e,* Alan D. Bristow,b Joeseph Bright,a Peng Zheng,a Nianqiang Wua,*

a

Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown,

WV 26506-6106,USA b

Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506-

6315,USA c

Department of Physics, Beihang University, Beijing 100191, China

d

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

e

Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology,

National Taipei University of Technology, Taipei 106, Taiwan *Corresponding

authors:

TEL:

+1-304-293-3326,

FAX:

+1-304-293-6689,

E-Mails:

[email protected] (for NW); TEL: +886-2-3366-8671, FAX: +886-2-3366-8671, [email protected] (for RSL)

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Abstract Previous studies have shown that hydrogen treatment leads to formation of blue to black TiO2, which exhibits different photocatalytic activity from white pristine TiO2. However, the underlying mechanism remains poorly understood. Herein density-functional theory is combined with comprehensive analytical approaches such as X-ray absorption near edge structure (XANES) spectroscopy and transient absorption spectroscopy to gain fundamental understanding of the correlation among the oxygen vacancy, electronic band structure, charge separation, charge carrier lifetime, reactive oxygen species (ROS) generation, and photocatalytic activity. The present work reveals that hydrogen treatment results in chemical reduction of TiO2, inducing surface and sub-surface oxygen vacancies, which create shallow and deep sub-band gap Ti(III) states below the conduction band. This leads to blue color but limited enhancement of visiblelight photocatalytic activity up to 440 nm at the cost of reduced ultraviolet photocatalytic activity. The extended light absorption spectral range for reduced TiO2 is ascribed to both the defect-to-conduction band transitions and the valence band-to-defect transitions. The photogenerated charge carriers from the defect states to the conduction band have lifetime too short to drive photocatalysis. The Ti(III) deep and shallow trap states below the conduction band are also found to reduce the lifetime of photogenerated charge carriers under ultraviolet-light irradiation. The ROS generated by the reduced TiO2 are less than those by pristine TiO2. Consequently, the reduced TiO2 exhibits worse ultraviolet-responsive photocatalytic activity than pristine TiO2. This report has shown that increasing the light absorption spectral range of a semiconductor by doping or introduction of defects does not necessarily guarantee an increase in photocatalytic activity.

Key words: photocatalyst, titanium oxide, band structure, oxygen vacancy, charge dynamics

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Introduction Semiconductor-based photocatalysts have a great potential in renewable fuel production by utilizing solar energy1-4. The optimal band gaps of semiconductors for water splitting fall within the visible-light wavelength range5. Unfortunately, most semiconductors with optimal band gaps such as III-V compounds suffer from photodegradation, which hinders their practical application6–11. This has prompted continual research on metal oxide semiconductors because metal oxides are inexpensive and very stable during operation. Titanium dioxide, which generally exhibits white color, is a prime example of metal oxide photocatalysts. Its excellent surface chemistry and charge mobility make it one of the most popular photocatalysts12–15. However, its large band gap (3.2 eV for anatase TiO2) limits its light absorption spectral range within the ultraviolet (UV) region of solar radiation1–4. Hence various approaches such as doping,16–19 and introducing vacancies or defects,20–30 especially chemical reduction by hydrogen annealing, have been used to extend the light absorption spectral range of TiO231,32. This has tuned the color of TiO2 from white to blue20 to red24 to black25. Given that blue and black TiO2 have much wider light absorption spectral range than white TiO2, they should exhibit much better photocatalytic activity than the white one under the full spectrum of solar radiation. Disappointingly, only a small enhancement in photocatalytic activity was observed for blue and black TiO2. This has motivated us to understand the underlying mechanism of this dilemma. It is well known that photocatalytic activity of semiconductors is dependent on four important processes: light absorption, charge separation, charge recombination and radical species generation. The electronic band structure, especially the sub-band gap states, plays a pivotal role in governing these four processes. In order for the sub-band gap states to be photocatalysisactive, they must have a favorable energetic location for the desired reaction, be mobile or located on the surface to allow diffusion of excited charge carriers, and have sufficient lifetimes for excited carriers to participate in a photochemical reaction4,33–37. This complex interplay makes the impact of sub-band gap states on photocatalysis difficult to study, and as such, the misconception that increased light absorption guarantees increased photoactivity persists in literature. Hence this study will investigate the electronic band structure of chemically reduced blue TiO2, and study the correlation of the band structure with these four important processes.

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In this work, pristine TiO2 (P-TiO2) is annealed in a hydrogen-containing atmosphere to produce the chemically reduced blue TiO2 (R-TiO2), leading to generation of Ti3+ in R-TiO2. The photocatalytic activity of P-TiO2 and R-TiO2 toward methyl orange decomposition is evaluated comparatively. The difference in the electronic band structure between P-TiO2 and R-TiO2 is unraveled by density-functional theory (DFT), X-ray absorption near edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS). The effects of oxygen vacancies, confirmed by electron paramagnetic resonance (EPR), are tracked from electronic structure through photocatalysis, correlating the changes in light absorption, charge carrier lifetime, and reactive oxygen species (ROS) to the new sub-band gap states. DFT and XANES analyses confirm that reduction of Ti4+ to Ti3+ introduces the surface and sub-surface oxygen vacancies, adding a set of shallow and deep trap states below the conduction band (CB). The sub-band gap states open new defect-to-conduction band and valence band-to-defect state transitions, corresponding to the enhanced visible light absorption and a shift in the TiO2 color from white to blue. New inter-band transitions are found at ~0.1 to 0.2 eV below the CB, corresponding to the Ti3+ levels. These mid-gap states extend the photocatalysis-active wavelength range by 40 nm, but transient absorption measurements show this is at the expense of the reduced lifetimes of photogenerated charge carriers under UV irradiation. The defect-to-conduction band transitions, which comprise most of the color change, are found to have too short lifetimes to support evident photocatalysis. Overall, the introduction of oxygen vacancies into TiO2 increases the visible-light photocatalytic activity to a limited extent, but leads to the trapping of electrons below the CB, thus reducing the UV-responsive photocatalytic activity, leading to little net gain.

Methods The synthesis procedure of pristine TiO2 nanobelts (P-TiO2) was described previously31,32,38. After synthesis, the dried white powder of single-crystalline anatase TiO2 nanobelts were placed in a quartz-tube furnace equipped with a gas-flow-controlled system for hydrogen treatment. The powder was heated at 600 °C for 3 h in a H2 flow (0.2 L/min) at a ramp rate of 10 °C/min, producing reduced anatase TiO2 nanobelts (R-TiO2). The color of the nanobelts became greyblue after chemical reduction. The P-TiO2 and R-TiO2 nanobelts were characterized using a field-emission scanning electron microscope (FE-SEM) (JEOL 7600F) and X-ray diffraction (XRD, X’ Pert Pro PW3040-Pro, 4 ACS Paragon Plus Environment

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Panalytical Inc.) with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) experiments were conducted with PHI 5000 Versa Probe system (Physical Electronics, MN), calibrated according to the reference of C 1s peak at 284.8 eV. UV-Visible absorption spectra were measured using diffuse reflection with an integrating sphere (Shimadzu 2550 UV-vis spectrometer). Electron paramagnetic resonance (EPR) was measured at 100 K using a microwave frequency of 9.64 GHz, modulation frequency of 100 kHz, microwave power of 1 mW, and modulation amplitude of 10 G. EPR was performed using a Bruker EMX/plus spectrometer equipped with a dual mode cavity (ER 4116DM). Low temperatures were achieved and controlled using a liquid He quartz cryostat (Oxford Instruments ESR900) with a temperature and gas flow controller (Oxford Instruments ITC 503). The photocatalytic activity was evaluated in a photoreactor (LUZ-4 V, Luzchem) consisting of fourteen 8 W lamps, illuminating in either the UV (light spectrum centered at 350 nm, LZCUVA, Luzchem) or visible light (light spectrum centered at 420 nm, LZC-420, Luzchem). The visible light lamp spectrum had a tail in the short-wavelength range, the effect of which was excluded by checking the results with 1.4 mW monochromic light (420 nm) from an Oriel Cornerstone 130 Monochromator. Decomposition of methyl orange at 464 nm was evaluated by 10 mg of TiO2 added into 10 mL of methyl orange solution in a UV-light-transparent polyethylene tube. The initial methyl orange concentrations were 20 mg/L and 5 mg/L for UV and visible light radiation, respectively. The suspensions were preserved in dark for 1 h to achieve the adsorption equilibrium and rotated in a carousel during illumination. For superoxide radical (ܱଶ•ି ) monitoring, the absorption peak at ~340 nm of 4-Chloro-7nitrobenzofurazan (NBD-Cl, 25455, Sigma-Aldrich) was used with acetonitrile (576956, SigmaAldrich) as the solvent39. 20 mg of the TiO2 nanobelts were added into a polyethylene tube with 5 mL of 0.1 mM NBD-Cl solution to form a suspension, then sonicated and allowed to equilibrate in the dark for 3 h. Similarly, for the hydroxyl radical (•OH) monitoring, hydroxyl radical and peroxynitrite sensor (HPF, H36004, Invitrogen Inc.) was dissolved into phosphate buffered saline (PBS) solution (1760426, MP Biomedicals, LLC) to form a homogeneous 5 µM solution40. The characteristic emission peak of HPF at 515 nm was detected using a fluorescence spectrophotometer (Hitachi, F-7000). 10 mg of the TiO2 nanobelts were added into a polyethylene tube with 5 mL of 5 µM HPF solution to form suspensions.

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Theoretical calculations were carried out using the plane wave pseudopotential spin-polarized DFT method as implemented in the VASP code41. The Perdew-Burke-Ernzerhof

(PBE)

exchange-correlation functional based on the generalized gradient approximation42 (GGA) was employed with a plane-wave basis cutoff energy of 500 eV and a Monkhorst-Pack k-point mesh of 1×1×1.The nanobelt dominated by (101) surface was created using a slab, adding 12 Å vacuum to a 120-atom supercell containing five TiO2 layers. The geometry of the TiO2 nanobelt was further optimized by fixing the middle layer and letting the surface layers move freely. Structural relaxations were performed until all the residual forces on atoms were less than 0.03 eV/Å, and the self-consistent convergence accuracy was set at 10-4 eV/atom. The self-consistent electronic properties were calculated by GGA+U functional43. The effective U values of O 2p and Ti 3d were set as 0.0 eV and 9.5 eV, respectively. The zero energy point was set to the Fermi level of pristine TiO2 nanobelts. The systems with defects were corrected by aligning the electrostatic potential of Ti located far from the defects and that of the same element in the pristine TiO2 nanobelt44,45. XANES measurements were performed in the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu City, Taiwan. The O K-edge and Ti L-edge XAS spectra were conducted at the 20A1 beamline, and Ti K-edge XAS spectra were characterized at 01C1 and 17C1 beamlines. Transient absorption measurements were performed using a 1 kHz Ti:Sapphire regenerative amplifier (Libra, Coherent). The fundamental 800 nm beam was down-converted to the 325 nm and 400 nm pump and probe beams by using an optical parametric amplifier (OPerA Solo, Coherent) and ߚ-Barium Borate nonlinear crystals. The pump and probe were delayed using a mechanical stage, and the resulting transient absorption measured using an optical chopper and lock-in amplifier. The incident power was kept at 0.3 mJ/cm2 for each pump wavelength.

Results and Discussion The hydrothermally prepared TiO2 showed a typical morphology of nanobelts, with width below 500 nm and length up to 15 µm (Fig. S1a). The crystal structure of the TiO2 nanobelts did not change from the monolithic anatase phase after reduction in H2 at 600 ºC (Fig. S1b). The TiO2 nanobelts were single-crystalline and grew along the direction of [101], with the dominant facet being (101) as explained in our previous publication31,38. The introduced Ti3+ defects 6 ACS Paragon Plus Environment

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consisted of surface oxygen vacancies compensated by O- and bulk Ti3+ as confirmed by the EPR peaks at g-values ranges of 2.0-2.02 and 1.96-1.99, respectively (Figure S1c-d), consistent with Reference 46 and 47, wherein such oxygen vacancies were shown stable up to 10 months. The pristine sample has a large concentration of O- compared to Ti3+, consistent with the compensated surface vacancies expected in the nanobelt geometry. During the hydrogen treatment, additional surface Ti3+ sites were created, which migrated into the bulk region of the nanobelt. The hydrogen-treated TiO2 thus shows a large concentration of Ti3+ compared to the surface compensated oxygen vacancies, consistent with the reduced form of the blue TiO2.

Figure 1. Density of states (DOS) with vacancies for pristine TiO2 (TiO2) and reduced TiO2 nanobelts (R-TiO2). (a) GGA+U prediction shows that surface vacancies add deep and shallow traps below the conduction band, with multiple surface vacancies replicating disorder adding to this density of states. Here, the DOS have been aligned using the semicore levels of the DFT functional to better represent x-ray transitions. (b, c) XPS and O-Kedge XANES, respectively, show that in the synthesized reduced TiO2, the valence band does not shift but the conduction band shifts by -0.1 eV, consistent with the introduction of sub-surface oxygen vacancies.

DFT calculations predicted a DOS for the pristine TiO2 as shown in Fig. 1a, with the valence band (VB) composed of primarily O 2p orbitals and the conduction band (CB) composed of 7 ACS Paragon Plus Environment

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primarily Ti 3d orbitals. To represent the different possibilities for introduction of oxygen vacancies, three vacancy locations were calculated as indicated in the inset of Fig. 1a and shown in detail in Fig. S2. The partial DOS for all oxygen vacancies calculated is shown in Figure S3. For 3 ns decay (Fig. 3c). The long-lifetime component corresponded to photocatalytically active carriers. For R-TiO2, the recombination time decreased to 930 ps, at which point the signal returned to the background levels, which indicated that almost all excited carriers left the CB states. The quicker recombination time reveals that excited VB-CB carriers almost immediately fall into the added Ti3+ trap states, where they then recombine to reduce the photocatalytic activity. The change in lifetime was consistent with time resolved-fluorescence measurements in Figure S8, which gave a decrease in the radiative lifetime from 3.6 ns to 1.2 ns due to the increased non11 ACS Paragon Plus Environment

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radiative relaxation through the oxygen vacancies. The reduced excited carrier lifetimes were responsible for the decreased UV-responsive photocatalytic activity for R-TiO2.

Figure 4. Reactive oxygen species photo-generated by the TiO2 nanobelts. (a) Superoxide radical produced by R-TiO2 less than P-TiO2 under UV excitation, (b) but more under visible-light excitation, consistent with the photocatalysis results and addition of Ti3+ states below the conduction band. (c, d) hydroxyl radical production was relatively similar between UV and visible-light excitation, consistent with the valence band remaining unchanged after reduction of TiO2.

The effect of the trap states is further confirmed by using a 400 nm pump to excite VB-CB edge transitions in the Ti3+ states and an 800 nm probe (Fig. 3d). The 800 nm probe is sensitive to occupation of the oxygen vacancy-induced trap states57-59. In the pristine TiO2, excitation produces a sharp initial thermalization and relaxation period as the trap states fill, followed by a recombination time >3 ns. The quick thermalization and relaxation occurs at excitation wavelengths that match the non-photocatalytically active absorption of Fig. 2, confirming that these states do not have sufficient lifetimes to significantly drive photocatalysis. In R-TiO2, the trap state filling is prolonged to a single exponential of lifetime 1.2 ns. This is consistent with more carriers filling the trap states. Thermalization and relaxation is again followed by a >3 ns recombination time. Correspondingly, the visible-light photocatalytic activity of R-TiO2 was better than that of P-TiO2 (Fig 3b). Production rates of superoxide radicals (ܱଶ•ି ), which came from the photogenerated electrons, was consistent with the conclusions drawn from transient absorption measurement. Compared to 12 ACS Paragon Plus Environment

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pristine TiO2, the Ti3+ states reduced the superoxide production under UV excitation (Fig. 4a) but increased the superoxide production under visible-light excitation (Fig. 4b). On the other hand, the production of hydroxyl radicals (•OH), which came from the photogenerated holes, was relatively unchanged between P-TiO2 and R-TiO2 for UV and visible excitation (Fig. 4c and 4d). This further confirmed the added Ti3+ states increased the charge recombination of the excited electrons but kept the VB structure and hole kinetics unchanged. The effect of surface and subsurface oxygen vacancies in reduced TiO2 is summarized in Figure 5.

Figure 5. Schematic illustration showing the effect of oxygen vacancies on the band structure and charge separation in TiO2 under visible irradiation (a) and UV irradiation (b).

Conclusions This report has shown that increasing the light absorption spectral range of a semiconductor by doping or introduction of defects does not guarantee an increase in photocatalytic activity. The blue color of reduced TiO2 was shown to originate from the Ti3+ states and the sub-surface and surface oxygen vacancies. These new states in R-TiO2 enhanced the light absorption through the interband transitions into Ti3+ defect states and the defect state-CB transitions. However, only the former are shown to be catalytically active, with low mobility and reduced lifetimes, resulting in the limited visible-light photocatalytic activity. However, these new mid-gap states served as the charge recombination centers and reduced the photoactivity under UV irradiation.

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The results in the present work deals with the defect states near the CB edge and the deep mid-gap trap states in blue TiO2 created by oxygen vacancies. But it is worth noting that the color of TiO2 can also be changed by various surface processes25-27,60-62. Recently, it has been shown that high pressure hydrogen treatment63 can lead to an increased photocatalytic activity in the UV region. While the findings of this paper are for the as-synthesized blue TiO2 and deviations will exist for different synthesis approaches, the findings of this paper support that regardless of synthesis approach, if the light absorption range of large band gap semiconductors is to be extended, continuum bands must be formed instead of localized states. This will preserve the excited carrier lifetimes and mobility64, preventing the tradeoff between visible-light and UV photocatalysis rates. ■ ASSOCIATED CONTENT Supporting Information The following file is available free of charge on the ACS Publications website at DOI: xxxxxxx. Figures S1-S10 are seen in the Supporting Information. ■ Notes The authors declare no competing financial interest.

Acknowledgements The resources used in this work were partially supported by the National Science Foundation (CBET-1233795). Cushing was supported by NSF Graduate Research Fellowship under Grant No. (1102689). Dr. Liu was grateful for the financial supports from the Ministry of Science, Technology of Taiwan (Contract No. MOST 104-2113-M-002-012-MY3), Academia Sinica (Contract No. AS-103-TP-A06) and National Taiwan University (104R7563-3).

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