Characterization of Oxygen Vacancy Associates within Hydrogenated

This paper introduces a novel method for characterizing the oxygen vacancy associates in hydrogenation-modified TiO2 by using a positron annihilation ...
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Characterization of Oxygen Vacancy Associates within Hydrogenated TiO2: A Positron Annihilation Study Xudong Jiang,† Yupeng Zhang,† Jing Jiang,† Yongsen Rong,† Yancheng Wang,† Yichu Wu,† and Chunxu Pan*,†,‡ †

School of Physics and Technology, Hubei Nuclear Solid Physics Key Laboratory and MOE Key Laboratory of Artificial Micro- and Nano-structures, Wuhan University, Wuhan 430072, China ‡ Center for Electron Microscopy, Wuhan University, Wuhan 430072, China ABSTRACT: This paper introduces a novel method for characterizing the oxygen vacancy associates in hydrogenationmodified TiO2 by using a positron annihilation lifetime spectroscopy (PALS). It was found that a huge number of small neutral Ti3+−oxygen vacancy associates, some larger size vacancy clusters, and a few voids of vacancy associates were introduced into hydrogenated TiO2. The defects blurred the atomic lattice high-resolution transmission electron microscopy (HRTEM) images and brought about the emergence of new Raman vibration. X-ray photoelectron spectroscopy (XPS) measurement indicated that the concentration of oxygen vacancies was 3% in the TiO2 lattice. The photoluminescence (PL) spectroscopy, photocurrent, and degradation of methylene blue indicated that the oxygen vacancy associates introduced by hydrogenation retarded the charge recombination and therefore improved the photocatalytic activity remarkably.

1. INTRODUCTION As one of the promising semiconductors, titania (TiO2) has grabbed appreciable attention over the past decades, due to its powerful potential applications in photovoltaics, biomedicine, and photocatalysis.1−3 However, the photocatalytic efficiency of TiO2 is limited by its wide band gap and the low efficiency of recombination of photogenerated electrons and holes.4 Herein, the methods for enhancing the photocatalytic activity of TiO2 are mainly based on decreasing the band gap and inhibiting the recombination of photo-generated electrons and holes. In general, the nonmetal dopants for TiO2, such as N,5 C,6 7 F, and S,8 were widely used for shifting the photoresponse toward longer wavelengths. Meanwhile, numerous surface modifications have been devoted to reduce the recombination of photogenerated electrons and holes, for example, TiO2/ semiconductor composites,9,10 metallic doping,11 heavy metal deposition upon TiO2,12 and so on. More current evidence has demonstrated that oxygen vacancy defects on the TiO2 surface play an essential part in acting as electron scavengers when electron−hole pair creation occurs by photoexcitation in the TiO2.13 Recently, hydrogenation modified TiO2 has triggered extensive research interest and is considered to have great potential.14−18 Chen et al.14 reported the preparation of black hydrogenated TiO2 nanoparticles in a 20 bar hydrogen atmosphere at 200 °C for 5 days. It exhibited a greatly enhanced absorption in the visible light and near-infrared region, due to its narrowed band gap around 1.0 eV, which was ascribed to the surface disorder of TiO2 nanoparticles. © 2012 American Chemical Society

However, the unclear detailed image of the disorder and the role of the hydrogenation as well as the mechanism of high photoactivity are still ambiguous. In addition, surface-hydrogenated anatase TiO2 nanowire microspheres were also reported by Zheng and his coworkers.17 They ascribed the enhanced photocatalytic activities to the improved optical absorption and efficient photogenerated electron−hole separation induced by the surface Ti−H bonds as well as the unique structure. Simultaneously, Naldoni et al.18 fabricated the black TiO2 nanoparticles with crystalline core/disordered shell morphology, and ascribed its narrowed bandgap to the synergistic presence of oxygen vacancies and surface disorder. More examples showed the decisive role of defects such as oxygen vacancy in hydrogenated TiO2. However, few studies concern the detection of oxygenvacancies induced by hydrogenation in TiO2. Positron annihilation lifetime spectroscopy (PALS) provides a sensitive method for investigating vacancy-type defects, and it has been proven to be useful in determining the intrinsic defects in semiconductors.19,20 When positrons inject inside bulk materials, they get thermalized and annihilated with electrons, and result in the emission of γ rays, which convey information of the lifetime of positrons. Compared to the bulk of the material, positrons preferentially distribute in the regions where electron density is low, e.g., vacancy type defects, Received: July 31, 2012 Revised: September 30, 2012 Published: October 2, 2012 22619

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(160 W), which generates light with maximum intensity at 365 nm, was used as a light source away from the working electrode with a distance of 10 cm. The intensity of photocurrent was measured by an electrochemical workstation (CHI660C, Chenhua, China). 2.4. Measurement of Photocatalytic Activities. The photocatalytic activity was evaluated by detecting the degradation rate of methylene blue (MB). In brief, a 250 W high pressure mercury lamp was used as a light source for photocatalytic reaction, 100 mg of photocatalyst was dispersed into 100 mL of 10 mg/L MB solution, and then the mixture was stirred constantly under the light at a distance of 250 mm. The concentration of MB solution was measured by the UV− vis spectrophotometer (UV-2550, Shimadzu, Japan) every 30 min.

vacancy clusters, and microvoids. Herein, the electron-positron annihilation photons allow measurement of the lifetime of positrons, which provides information regarding various defects.21,22 In this research, commercial P25 was chosen as the pristine TiO2 photocatalyst and annealed at 400 °C in H2 under atmospheric pressure for 10 h. The positron annihilation characterization revealed the existence of a large number of various oxygen vacancy associates in the hydrogenated TiO2, which would play the key role of inhibiting the recombination of holes and electrons during photocatalyzing.

2. EXPERIMETAL SECTION 2.1. Preparation of Hydrogenated TiO2. Commercial pure white P25 TiO2 nanoparticles purchased from Degussa were used for the pristine sample. The hydrogenation was carried out in a tube furnace filled with ultrahigh purity hydrogen gas (99.99%) under atmospheric pressure, and the pristine P25 was annealed at 400 °C for 10 h, as described in earlier reports.15,18 The flow rate of hydrogen is 10 sccm. The obtained taupe powders were denoted as H-P25, distinguished from the pristine white P25. 2.2. Characterization. The morphologies of P25 and HP25 nanoparticles were observed by using high-resolution transmission electron microscopy (HRTEM, JEM 2010FEF HRTEM, JEOL, Japan). An X-ray diffraction spectrometer (XRD) (D8 Advanced XRD, Bruker AXS, Germany) with a Cu Kα source was employed to analyze their crystalline structures. The scanning speed was 4°/m in the range of 20−80°. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Multilab2000 spectrometer to obtain information on the chemical binding energy of the samples. The ultraviolet−visible diffuse reflectance spectra (UV−vis DRS) of the samples were measured by using a diffuse reflectance accessory of a UV−vis spectrophotometer (UV2550, Shimadzu, Japan). The Raman spectra were obtained using a LabRAM HR spectrometer (HORIBA Jobin Yvon, France) with a laser excitation of 488 nm. Photoluminescence (PL) spectra were measured at room temperature on a fluorescence spectrophotometer (F-4600, Hitachi, Japan) with an excitation wavelength of 300 nm, a scanning speed of 240 nm/min, and a photomultiplier tube (PMT) voltage of 700 V. PALS was measured using a conventional ORTEC-265 fast− fast coincident system at room temperature. The coincidence spectrometer used had a prompt time resolution of 150 ps (fwhm) for the γ-rays from a 60Co source selected under the experimental conditions. Three hundred milligrams of sample powder was pressed into a disk (diameter: 13 mm, thickness: 1 mm). The source (∼1.3MBq) of 22Na was sandwiched between two identical sample disks. The positron lifetime spectrum containing 106 counts was analyzed by the PATFIT program to decompose several lifetime components. 2.3. Photocurrent Tests. Five milligrams of photocatalysts was dispersed in 5 mL of ethanol and ultrasonically vibrated for 30 min. The 0.25 mL resultant slurry was then dip-coated onto a 10 × 20 mm indium−tin oxide (ITO) glass electrode and dried under high-pressure mercury lamp irradiation to eliminate ethanol. The prepared TiO2/ITO electrode, platinum electrode, and saturation calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. The electrolyte was 0.5 mol/L Na2SO4 aqueous solution. The working electrode was activated in the electrolyte for 0.5 h before measurement. A high-pressure mercury lamp

3. RESULTS AND DISCUSSION Figure 1 illustrates the XRD patterns of the pristine and hydrogenated P25 TiO2 nanoparticles. The strong XRD

Figure 1. XRD patterns of P25 before (a) and after (b) hydrogenation.

diffraction peaks indicated that the pristine P25 was highly crystallized with a mixture of anatase (70−80%) and rutile (20−30%), which has been reported elsewhere,23 and no obvious change was observed for the H-P25 TiO2. The anatase grain sizes of the P25 TiO2 nanoparticles were around 16 nm calculated from the Scherrer. By contrast, Figure 2 shows the absorption spectra of the pristine and hydrogenated P25 TiO2 nanoparticles. The hydrogenation had little effect on the absorption of P25 in the UV region, while H-P25 exhibited a remarkable absorption in the visible light region, which was also similar to the results of Chen et al.14 Nevertheless, the gray coloration of H-P25 in our experiment was due to the use of slow cooling rate after hydrogenation.18 HRTEM observations are shown in Figure 3 and Figure 4, respectively, with related fast Fourier transform (FFT) analysis images in the inset. It revealed clear lattice fringes and a sharp FFT image in pristine P25 TiO2, which demonstrated a high level of crystallization. However, after hydrogenation, the lattice images and the FFT image of H-P25 became blurred, which indicated the distortion of the TiO2 atomic lattice structures during hydrogenation. Raman spectroscopy is a spectroscopic technique to measure molecular vibrations. It can be used for detecting the structural 22620

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Figure 2. UV−vis DRS of P25 and H-P25.

Figure 3. HRTEM micrographs of P25 nanoparticles. Figure 5. Raman spectra of P25 before and after hydrogenation: (a) survey; (b) magnified.

cm−1 might result from the hydrogenation-induced structural changes at the surface. XPS analysis of the Ti 2p region evidences a slight shift to low binding energy. Figure 6 shows the XPS spectra of the Ti 2p doublet region after peak deconvolution and background removal. Comparing with pristine P25 with symmetrical Gauss distribution (Figure 6a), the 2p doublet peaks of the hydrogenated P25 exhibited a tail in the region of lower binding energy, which indicated the presence of lower valence states of Ti, as shown in Figure 6b. The small peaks at 457.6 eV for Ti 2p3/2 and 463.3 eV for Ti 2p1/2 clearly indicate the presence of Ti3+. In addition, the main sharp peaks of Ti 2p3/2 located at 458.3 eV and Ti 2p3/2 located at 464.0 eV were assigned to Ti4+ in TiO2. The Ti3+/Ti4+ ratio was calculated to be approximately 14% from the peak areas. In general, the existence of Ti3+ in TiO2 indicates the oxygen vacancies will be generated to maintain electrostatic balance according to the following chemical equation:

Figure 4. HRTEM micrographs of the hydrogenated P25 nanoparticles.

changes on the surface of P25 TiO2 before and after hydrogenation. As shown in Figure 5, pristine P25 TiO2 exhibited Eg modes (143 cm−1, 198 cm−1 and 639 cm−1), a B 1g mode (399 cm −1 ) and an A 1g mode (515 cm −1 superimposed by 519 cm−1), in good agreement with a former report.24 However, besides the peaks appearing in pristine P25, the Raman peaks of the hydrogenated P25 TiO2 became fluctuated more, and two weak peaks emerged at 316 cm−1 and 810 cm−1, as shown in Figure 5b. The band centered at about 320 cm−1 should be assigned to second-order scattering, which always showed an A1g symmetry component.25 In addition, the band at 810 cm−1 still could not be assigned to any type of anatase Raman-active modes. A new band at 321.1 cm−1 was also observed in the surface-hydrogenated anatase TiO 2 nanowire microspheres.17 Therefore, the phenomena of the emerging of the second-order scattering and new band at 810

4Ti4 + + O2 − → 4Ti4 + + 2e−/□ + 0.5O2 → 2Ti4 + + 2Ti 3 + + □ + 0.5O2

The □ represents an empty position originating from the removal of O2− in the lattice. It can be deduced that an oxygen vacancy generated companied with two Ti3+ ions from the equation. Therefore, the concentration of oxygen vacancy in the TiO2 lattice is calculated to be 3% in H-P25. Naldoni et al.18 reported a 5% concentration of oxygen vacancy, which 22621

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Figure 7. Positron lifetime spectra with fitting lines of P25 (blue) and H-P25 (red).

Table 1. Positron Lifetimes and Relative Intensities with Their Deviations of Pristine P25 and H-P25 sample P25 H-P25

value deviation value deviation

τ1 (ps)

τ2 (ps)

τ3 (ns)

I1 (%)

I2 (%)

I3 (%)

140.5 14.1 188.1 19.9

376.9 2.1 378.7 3.8

NA NA 19.3 10.7

11.22 1.08 14.30 2.74

88.78 1.08 85.45 2.75

NA NA 0.25 0.07

a straight line, because they closed to the time resolution. The slop of the straight line was according to λ = Δ ln D(t) /Δt strictly. The λ represents annihilation rate, which is the reciprocal of the τ. The shorter component (τ1) is generally due to the free annihilation of positrons in a defect-free crystal, denoted as the first lifetime of positrons.27 In a disordered system, smaller vacancies (such as monovacancies, etc.) or shallow positron traps (such as oxygen vacancies associated in CeO2) may reduce the surrounding electron density,19 which increases the lifetime of τ1. In pristine P25, some monovacancies exist naturally in the crystal lattice. Herein, the τ1 for pristine P25 was 140.5 ps, which could be ascribed to the inherent free positron lifetime in pristine TiO2. However, the τ1 for H-P25 was remarkably longer at 188.1 ps, 33% greater than in pristine P25. Therefore, the prolonged τ1 of H-P25 demonstrates that a huge number of small neutral Ti3+−oxygen vacancy associates were introduced into the TiO2 lattice by hydrogenation. Correspondingly, the relative intensity (I1) of H-P25 was also increased dramatically, i.e., from 11.22% in pristine P25 to 14.3% in H-P25. The longer lifetime component (τ2) is attributed to positrons captured by defects of larger size.27 The average electron density in larger-sized defects is lower than that in small size defects, which decreases the annihilation rate and increases the positron lifetime correspondingly. Therefore, the values of τ2 were much larger than the values of τ1. In our samples, both P25 and H-P25, the positions could diffuse throughout the lattice and reach the grain boundaries, where there existed a huge number of boundary-like defects. So the longer lifetime was generated with larger proportion. For pristine P25, the τ2 was 376.9 ps with the intensity (I2) of 88.78%. Obviously, the boundary-like defect accounts for more proportion than that of natural monovacancies in pristine P25 due to I2 > I1. For H-

Figure 6. Ti 2p XPS spectra of P25 (a) and H-P25 (b).

indicated the fast cooling rate could be a plus to keep the concentration of oxygen vacancy in hydrogenation. PALS provides information including size, type, and relative concentration of various vacancies according to the lifetime of the positrons.21,22 Generally, the positions would undergo rapid thermalization by dissipation of their energy to the surrounding medium and then diffuse through the medium with an average diffusion length after entering into solid. For anatase TiO2, the position diffusion length is approximately 45 nm referring to the previous report,26 which is much longer than the grain size of our sample (about 16 nm of anatase). Herein, the positron can diffuse throughout the lattice and reach the grain surface to annihilation. The time-dependent positron decay spectrum D(t) = ∑iIi exp(−t/τi) was analyzed as the sum of exponential decay components convoluted with the Gaussian resolution function of the spectrometer. Figure 7 illustrates the typical positron lifetime spectra from the samples P25 and H-P25. The positron lifetime spectrum N(t) = ∑iIi/τi exp(−t/τi) and the absolute value of the time were the deconvolution of the positron decay spectrum D(t) by the PATFIT program. The fitting results are listed in Table 1 with three positron lifetime components, τ1, τ2, and τ3, and the relative intensities, I1, I2, and I3, for pristine P25 and H-P25. The obtained lifetime components τ2 for both P25 and H-P25 and τ3 for H-P25 were added as straight lines in the plots for illustration. The τ1 components were not indicated as 22622

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P25, the τ2 was increased to 378.7 ps because of hydrogenation, which demonstrated that some larger size vacancy clusters were induced inside the hydrogenated TiO2, which resulted from interaction between the small neutral Ti3+−oxygen vacancy associates. However, the increase of τ2 was only 0.4% for the monovacancies introduced predominately by hydrogenation. The decrease of I2 in H-P25 was due to the dramatic increase of I1, because of the relationship of I1 + I2 + I3 = 100%. The longest lifetime component (τ3) was probably assigned to the annihilation of orthopositronium atoms formed in the large voids (nanoscale) in materials.27 For the present work, τ3 = 19.3 ns was detected only in H-P25, and no τ3 single in pristine P25. However, the intensity of τ3 (I3) was also so slight that it was only 0.25% in the hydrogenated TiO2. That is to say, few voids of oxygen vacancy associates in nanoscale were formed in the hydrogenated TiO2 crystalline grains. Hydrogen gas (H2) is generally used as a reducing agent. By heating TiO2 in a hydrogen atmosphere, numerous oxygen vacancies, associated with Ti3+ were introduced in H-P25. Thus the shorter lifetime component (τ1) of positrons was prolonged by the oxygen vacancy associates, where the surrounding electron density decreased. Some of the oxygen vacancy associates tended to flock to larger size vacancy clusters and few voids in nanoscale, which increased the τ2 and generated τ3 accordingly. Herein, the concentration of the small neutral Ti3+−oxygen vacancy associates, larger size vacancy clusters, and voids of vacancy associates decreased subsequently. The vacancy associates also disordered the surface structure of modified TiO2 and thus blurred the atomic lattice HRTEM images, as shown in Figure 4. Moreover, the surface structural variations were caused by vacancies in the modification of hydrogenation, thereby the emergence of the second-order scattering and new band at 810 cm−1 of Raman vibration were brought about in the hydrogenated TiO2. Oxygen vacancy defects on the surface of TiO2 play an essential part in governing the adsorption of O2 molecules, which interact with Ti3+ sites or act as electron scavengers.13 Additionally, the Ti3+ species from the removal of oxygen atoms would act as holes scavengers. These behaviors could retard the charge recombination and therefore improve the photocatalytic activity. The measurement of PL emission was performed to reveal the efficiency of charge carrier trapping, migration, transfer, and separation, and to understand the fate of photogenerated electrons and holes in semiconductors, since PL emission results from the recombination of free carriers.28 Figure 8 shows the comparison of PL spectra of pristine and hydrogenated P25 TiO2 in the wavelength range of 350−550 nm with the excitation at 300 nm. The main emission peaks of pristine P25 and H-P25 appeared at 400 and 412 nm, which were attributed to the emission of bandgap transition with the energy of emission corresponding to the bandgap energy of anatase (3.2 eV) and rutile (3.0 eV), respectively. The PL emission intensity of H-P25 at 400 and 412 nm are only onethird of the pristine P25, which indicated that the recombination rate of photogenerated electrons and holes had been inhibited considerably in H-P25, because of the formation of oxygen vacancies during the hydrogenation. The oxygen vacancies actually served as electron capture traps, and hence separated the charge carriers and reduced the recombination significantly. The secondary PL emission peaks at 451 and 469 nm were equivalent to 2.75 and 2.65 eV, respectively. These PL signals were attributed to surface oxygen vacancies and defects of the

Figure 8. PL spectra of P25 and H-P25.

TiO2 samples.28 For pristine P25, the intensities of the secondary peaks (at 451 and 469 nm) are lower than their main peaks (at 400 and 412 nm). However, for H-P25, the relative intensity of the secondary peaks was enhanced remarkably, when compared with the main peaks. Particularly, the intensity of peak at 469 nm reached the summit of the survey spectrum, which demonstrated the formation of oxygen defects in the TiO2 lattice by hydrogenation. However, for the lower intensity of background, the absolute intensity of the secondary PL peaks of H-P25 was lower than that of pristine P25. It has been well-known that there is a positive correlation between e−−h+ separation efficiency, photocurrent, and photocatalytic activity. Figure 9 illustrates the photocurrent

Figure 9. Photocurrents of P25 and H-P25.

profiles of pristine P25 and H-P25. It was found that value of the H−P25 photocurrent was 3 times higher than that of pristine P25, in good agreement with the PL test. Figure 10 shows the time profiles of ln(C0/C) under UV−vis irradiation for degradation of MB aqueous solution, where C is the concentration of MB at irradiation time t, and C0 is the concentration in absorption equilibrium of photocatalysts before irradiation. The linear relationship between the ln(C0/ C) and irradiation time confirmed the stability of the TiO2 before and after hydrogenation. The line slope of H-P25 was 2 22623

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(12) Wang, C.; Pagel, R.; Bahnemann, D. W.; Dohrmann, J. K. J. Phys. Chem. B 2004, 108, 14082−14092. (13) Thompson, T. L.; Yates, J. T. Top. Catal. 2005, 35, 197−210. (14) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746− 450. (15) Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11, 3026−3033. (16) Zhang, Z.; Bai, M.; Guo, D.; Hou, S.; Zhang, G. Chem. Commun. 2011, 47, 8439−8441. (17) Zheng, Z.; Huang, B.; Lu, J.; Wang, Z.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M. Chem. Commun. 2012, 48, 5733−5735. (18) Naldoni, A.; Allieta, M.; Santangelo, S.; Marcello., Marelli; Fabbri, F.; Cappelli, S.; Bianchi, C. L.; Psaro, R.; Santo, V. D. J. Am. Chem. Soc. 2012, 134, 7600−7603. (19) Liu, X.; Zhou, K.; Wang, L.; Wang, B.; Li, Y. J. Am. Chem. Soc. 2009, 131, 3140−3141. (20) Kong, M.; Li, Y.; Chen, X.; Tian, T.; Fang, P.; Zheng, F.; Zhao, X. J. Am. Chem. Soc. 2011, 133, 16414−16417. (21) Cruz, R. M.; Pareja, R.; Gonzalez, R. Phys. Rev. B 1992, 45, 6581−6586. (22) Dutta, S.; Chattopadhyay, S.; Jana, D.; Banerjee, A.; Manik, S.; Prahan, S. K.; Sutradar, M.; Sarkar, A. J. Appl. Phys. 2006, 100, 114328−114333. (23) Zhang, J.; Zhang, Y.; Lei, Y.; Pan, C. Catal. Sci. Technol. 2011, 1, 273−278. (24) Zhang, W. F.; He, Y. L.; Zhang, M. S.; Yin, Z.; Chen, Q. J. Phys. D 2000, 33, 912−916. (25) Giarola, M.; Sanson, A.; Monti, F.; Mariotto, G. Phys. Rev. B 2010, 81, 174305−174311. (26) Zhang, Y.; Ma, X.; Chen, P.; Li, D.; Pi, X.; Yang, D.; Coleman, P. G. Appl. Phys. Lett. 2009, 95, 252102−252104. (27) Sanyal, D.; Banerjee, D. Phys. Rev. B 1998, 58, 15226−15230. (28) Xiang, Q.; Lv, K.; Yu, J. Appl. Catal., B 2010, 96, 557−564.

Figure 10. Photocatalytic degradation of MB by P25 and H-P25.

times higher than that of pristine P25, which demonstrated the superior photocatalytic property of H-P25.

4. CONCLUSIONS As a sensitive approach for measuring vacancy-type defects, PALS provides an effective route to qualitatively characterize the oxygen-vacancy-induced defects in TiO2. It will be of significance to evaluate and understand the importance of oxygen vacancies, the associated with Ti3+ sites in TiO2 as electron and hole scavengers, which thus separated the charge carriers and remarkably improved the photocatalytic activity of TiO2.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-27-68752481 ext. 5201; Fax.: +86-27-68752003; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Basic Research Program of China (973 Program) (No. 2009CB939705 and No. 2009CB939704).

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