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Comment on “Characterization of Oxygen Vacancy Associates within Hydrogenated TiO: A Positron Annihilation Study,” by Jiang, J. et al. 2
Riley E Rex, Fritz J. Knorr, and Jeanne L. McHale J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp402351u • Publication Date (Web): 25 Mar 2013 Downloaded from http://pubs.acs.org on March 25, 2013
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Comment on “Characterization of Oxygen Vacancy Associates within Hydrogenated TiO2: a Positron Annihilation Study,” by Jiang, J. et al. Riley E. Rex, Fritz J. Knorr and Jeanne L. McHale Department of Chemistry and Materials Science and Engineering Program Washington State University Pullman, WA 99164-4630 In a recent paper1 investigating the formation of oxygen vacancies on hydrogenation of nanocrystalline TiO2 (P25), Jiang et al. present the photoluminescence (PL) spectrum of the nanoparticles before and after hydrogenation. While we do not wish to dispute the general conclusions of this work, we are compelled to point out that the PL spectra reported in Ref. 1, used to infer a change in oxygen vacancy content with hydrogenation, are erroneous. The putative PL spectra shown in Fig. 8 of Ref. 1 exhibit structured emission with distinct peaks at violet and blue wavelengths superimposed on a continuous background of visible wavelengths. Though this spectrum is in agreement with several reports of what is referred to as PL from TiO2 nanoparticles, 2,3,4,5,6,7,8,9,10,11,12 we note the strong resemblance of the “PL spectra” of Refs. 1-12 to the emission spectrum of the Xe lamp likely used as the source in their fluorimeter. The use of a conventional fluorimeter for the determination of TiO2 photoluminescence is challenging because the emission is quite weak at room temperature and strongly quenched by air. In addition, the highly scattering nature of a typical nano-TiO2 sample requires great care to reject stray excitation light that masquerades as PL, as is apparent in Refs. 1-12. A typical Xe lamp shows a broad continuous “grey body” emission on which the structured atomic emission lines of Xe are superimposed.13 A number of sharp Xe emission lines are observed in the 400-500 nm region, and these are likely the source of the features assigned by the authors of Ref. 1 to band gap emission and oxygen vacancies. Fig. 1 shows the results of attempting to measure the PL spectrum of a nanocrystalline P25 TiO2 film in air using a typical benchtop fluorimeter (PTI Quanta Master equipped with a 75 W Xe lamp source) using an excitation wavelength of 325 nm. No filters were used to reject scattered light. The green trace is the P25 TiO2 film and the pink trace was obtained by replacing the TiO2 sample with a MgO reflectance standard. The only significant difference in the data for the TiO2 and MgO samples is the diminished relative intensity at ultraviolet wavelengths in the TiO2 sample that results from absorption by the band gap transition. It is this enhanced absorption at UV wavelengths that results in the appearance of a peak at a near-UV wavelength that the authors of Ref. 1 assigned to band gap emission. Regardless of experimental detail, such a claim should be viewed with suspicion since the band gap emission is forbidden for an indirect semiconductor such as TiO2. No signal is observed from either sample if a UV band pass filter is used on the excitation side to remove stray light. This is expected since the sensitivity of our fluorimeter is insufficient to detect the weak emission of TiO2 in air at room temperature. Overlaid in Fig. 1 is the PL of P25 TiO2 reported in Ref. 1, before (blue) and after (red) hydrogenation. The coincidence of the structured peaks in the data of Ref. 1 with those seen our data for both the TiO2 and the MgO sample clearly shows that the data reported as PL in Ref. 1 result from scattering of the weak Xe lamp emission passed by the excitation monochromator.
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Figure 1. Signals recorded by PTI fluorimeter using 325 nm Xe lamp excitation of P25 TiO2 (green trace) and MgO (pink) overlaid by Figure 8 of Ref. 1 (including x-axis) showing putative PL of P25 TiO2 (blue and red). The data from Ref. 1 are reprinted with permission from Jiang, X.; Zhang, Y.; Jiang, J.; Rong, Y.; Wang, Y.; Wu, Y.; Pan, C. J. Phys. Chem. C 2012, 116, 22619-22624. Copyright 2012, American Chemical Society.
Fig. 2 shows the room-temperature PL spectra of the same TiO2 and MgO samples in air obtained using 350 nm excitation from a Kr ion laser, backscattering geometry and a single monochromator equipped with a CCD detector. A longpass filter was placed after the sample to reject the elastically scattered laser light. The laser power at the sample was roughly 250 mW/cm2, which is below the threshold for power-induced spectral shifts, discussed below. In agreement with our previous reports, for example Ref. 14, the P25 TiO2 spectrum shows a peak at about 530 nm which is strongly quenched in air, hence the poor signal to noise in the data of Fig. 2. Since the excitation wavelength of 350 nm, equivalent to 3.5 eV, is well below the almost 8 eV band gap of MgO,15 no PL from this sample is expected or observed, clearly showing that the “PL spectra” of MgO and TiO2 in Fig. 1 are artifacts. We have reported widely14,16,17,18,19,20,21,22,23 on the weak PL of nanocrystalline TiO2 in the anatase and rutile phases as well as that of mixed-phase P25 (~75% anatase, 25% rutile) particles such as those used in Ref. 1. The visible emission of TiO2 is easily recorded using laser excitation at room temperature when air is excluded and care is taken to reject stray light. The intensity of anatase TiO2 PL is the same order of magnitude as a typical Raman peak, reflecting a fairly low luminescence quantum efficiency, while the PL from rutile nanoparticles is brighter by several orders of magnitude. Based on studies in which the PL of TiO2 was examined as a function of particle morphology, phase, and environment, we have formulated the hypothesis that the 2 ACS Paragon Plus Environment
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visible emission from the anatase polymorph results from two overlapping emission spectra. One is assigned to the radiative recombination of trapped electrons with valence band holes, peaking in the red, and the other results from radiative recombination of conduction band electrons with trapped holes, peaking in the green. The latter type of PL reflects oxygen vacancies and is the dominant PL from the P25 nanoparticles. Both types of PL are quenched in air as a result of efficient electron scavenging by O2. A review of our ongoing work on trap state luminescence of TiO2 can be seen in Ref. 17.
Figure 2. Photoluminescence of P25 TiO2 (green) and MgO (pink) in air using 350 nm excitation from a Kr ion laser. A number of other reports of TiO2 PL are in agreement with those from our lab. These include the work from the group of Can Li,24,25 and from the Yates group.26,27 Tachikawa and Majima have observed similar PL to ours from single TiO2 nanoparticles,28,29 highlighting the influence of carrier transport in limiting the PL from electrically connected nanoparticles. (See also our Ref. 20.) Sham et al.30 used X-ray excitation to observe PL from anatase and rutile similar to what is observed in our lab using UV excitation. Cavigli et al.,31 working at cryogenic temperatures, observed the typical anatase PL that we have reported. M. Anpo and co-workers have published a number of reports on the visible luminescence of TiO2 powders and nanoparticles.32,33 Properly measured, the PL spectra of TiO2 anatase and rutile nanoparticles are similar to what is seen from bulk crystals of the same polymorph.34,35 Many of the above references use laser excitation (Ref. 25) and others employ an ordinary fluorimeter at low temperature (Refs. 24, 26, 27, 31, 32, 33) which results in slower nonradiative decay and brighter PL. Vacuum conditions in Refs. 26 and 27 further enhance the PL intensity by reducing quenching by O2. It is worthwhile to remark that the power density for exciting the PL is an important consideration, since higher power densities result in a blue-shifted PL spectrum, as shown in Supporting Information for Ref. 20. There are several possible reasons for this, including saturation of redemitting electron traps and a Burstein-Moss shift that results from an increase in the pseudoFermi level for electrons at higher excitation powers. Thus in addition to attention to possible artifacts from stray excitation light, the observation and report of TiO2 PL should take into ac3 ACS Paragon Plus Environment
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count the incident power density. We believe that careful measurement of PL is an excellent approach for investigating the oxygen vacancies that were the subject of Ref. 1, and we encourage the authors to reexamine their samples more carefully to corroborate the results of their positron annihilation work. Acknowledgment This work was supported by the National Science Foundation, CHE-1149013. References 1. Jiang, X.; Zhang, Y.; Jiang, J.; Rong, Y.; Wang, Y.; Wu, Y.; Pan, C. J. Phys. Chem. C 2012, 116, 22619-22624. 2. Xiang, Q.; Lv, K.; Yu, J. Appl. Catal. B 2010, 96, 557-564. 3. Yang, L.; Zhang, Y.; Ruan, W.; Zhao, B.; Xu, W.; Lombardi, J. R. J. Raman Spectr. 2010, 41, 721-726. 4. Xiao, Q.; Si, Z.; Zhang, J.; Xiao, C.; Yu, Z.; Qiu, G. J. Mater. Sci. 2007, 42, 9194-9199. 5. Nair, R.G.; Paul, S.; Samdarshi, S.K. Solar Energy Materials & Solar Cells 2011, 95, 19011907 6. Liu, X.; Liu, Z.; Zheng, J.; Yan, X.; Li, D.; Chen, S.; Chu, W. Journal of Alloys and Compounds 2011, 509, 9970-996. 7 Xin, H.; Ma, R.; Wang, L.; Ebina, Y.; Takada, K. Appl. Phys. Letters 2004, 85, 4187-4189 8. Liqiang, J.; Yichun, Q.; Baiqi, W.; Shudan, L.; Baojiang, J; Libin, Y.; Wei, F.; Honggang, F.; Jiazhong, S. Solar Energy Materials & Solar Cells 2006, 90, 1773-1787. 9. Tu, Y. F.; Huang, S.Y.; Sang, J. P.; Zou, X.W. Journal of Alloys and Compounds 2009, 482, 382-387. 10. Patel, S. K. S.; Gajbhiye, N. S. Materials Chemistry and Physics 2012, 132, 175-179 11. Liu, B.; Wang, X.; Cai, G.; Wen, L.; Song, Y.; Zhao, X. Journal of Hazardous Materials 2009, 169, 1112-1118. 12. Li, J.G.; Wang, X.; Watanabe, K.; Ishigaki, T. J. Phys. Chem. B 2006, 110, 1121-1127 13. Moore, J. H.; Davis, C. C.; Coplan, M. A. Building Scientific Apparatus, Addison-Wesley, London, 1983. 14. Knorr, F. J.; Mercado, C. C.; McHale, J. L. J. Phys. Chem. C 2008, 112, 12786-94. 15. Yang, S.-H; Balke, B.; Papp. C.; Dӧring, S.; Berges, U.; Plucinski, L.; Westphal, C.; Schneider, C. M.; Parkin, S. S. P.; Fadley, C. S. Phys. Rev. B 2011, 84, 184410/1-9. 16. Knorr, F. J.; Zhang, D.; McHale, J. L. Langmuir 2007, 23, 8686-8690. 17. McHale, J. L.; Knorr, F. J. “Photoluminescence and Carrier Transport in Nanocrystalline TiO2,” Ch. 13 in the Handbook of Luminescent Semiconductor Materials, Taylor and Francis, 2011. 18. Mercado, C. C.; Seeley, Z.; Bandyopadhyay, A.; Bose, S.; McHale, J. L. ACS Applied Materials & Interfaces 2011, 3, 2281-2288. 19. Mercado, C. C.; Knorr, F. J.; McHale, J. L.; Usmani, S. M.; Ichimura, A. S.; Saraf, L. V. J. Phys. Chem. C 2012, 116, 10796-10804. 20. Mercado, C. C.; Knorr, F. J.; McHale, J. L. ACS Nano 2012, 6, 7270-7280. 21. Rich, C. C.; Knorr, F. J.; McHale, J. L. Mater. Res. Soc. Symposium Proc. 2010, 1268, EE0308. 22. Mercado, C. C.; McHale, J. L. Mater.Res. Soc. Symposium Proc. 2010, 1268, EE03-10. 4 ACS Paragon Plus Environment
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23. Knorr, F. J.; McHale, J. L. J. Phys. Chem. C., submitted. 24. Wang, X.; Feng, Z.; Shi, J.; Jia, G.; Shen, S.; Zhou, J.; Li, C. Phys. Chem. Chem. Phys. 2010, 12, 7083-7090. 25. Shi, J.; Chen, J.; Feng, Z.; Chen, T.; Lian, Y.; Li, C. J. Phys. Chem. C 2007, 111, 693-699. 26. Stevanovic, A.; Buሷttner, M.; Zhang, Z; Yates, J. T. Jr J. Am. Chem. Soc. 2012, 134, 324−332. 27. Stevanovic, A.; Yates, J. T. Jr. Langmuir 2012, 28, 5652-5659. 28. Tachikawa, T.; Majima, T. J. Am. Chem. Soc. 2009, 131, 8485–8495. 29. Tachikawa, T.; Ishigaki, T.; Li, J.-G.; Fujisuka, M.; Majima, T. Angew. Chem. Int. Ed. 2008, 47, 5348-5352. 30. Liu, L.; Chan, J.; Sham, T-K. J. Phys. Chem. C 2010, 114, 21353–21359. 31. Cavigli, L.; Bogani, F.; Vinattieri, A.; Faso, V.; Baldi, G. J. Appl. Phys. 2009, 106, 053516/1-8. 32. Zhou, J.; Takeuchi. M; Ray, A. K.; Anpo, M.; Zhao, X. S. J. Colloid and Interface Sciences 2007, 311, 497-501. 33. Anpo, M.; Tomonari, M.; Fox, M. A. J. Phys. Chem. 1989, 93, 7300-7302. 34. Tang, H.; Berger, H.; Schmid, P.; Lévy, F.; Burri, G. Solid State Commun. 1994, 92, 267271. 35. Addiss, R. R. Jr.; Ghosh, A. K.; Wakim, F. G. Appl. Phys. Lett. 1968, 12, 397-400.
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