Baker, D. B.

Bunton, P. H.

Fujishima, A.

Halliburton, L. E.

Horn, S.

Klemm, M.

Manivannan, A.

Yang, Shan

English

The Research Repository @ WVU (West Virginia University)

Faculty Scholarship2009Photoinduced Electron Paramagnetic ResonanceStudy Of Electron Traps In Tio2 Crystals: OxygenVacancies And Ti3+ IonsShan YangL. E. HalliburtonA. ManivannanP. H. BuntonD. B. BakerSee next page for additional authorsFollow this and additional works at: https://researchrepository.wvu.edu/faculty_publicationsThis Article is brought to you for free and open access by The Research Repository @ WVU. It has been accepted for inclusion in Faculty Scholarshipby an authorized administrator of The Research Repository @ WVU. For more information, please contact ian.harmon@mail.wvu.edu.Digital Commons CitationYang, Shan; Halliburton, L. E.; Manivannan, A.; Bunton, P. H.; Baker, D. B.; Klemm, M.; Horn, S.; and Fujishima, A., "PhotoinducedElectron Paramagnetic Resonance Study Of Electron Traps In Tio2 Crystals: Oxygen Vacancies And Ti3+ Ions" (2009). FacultyScholarship. 634.https://researchrepository.wvu.edu/faculty_publications/634AuthorsShan Yang, L. E. Halliburton, A. Manivannan, P. H. Bunton, D. B. Baker, M. Klemm, S. Horn, and A.FujishimaThis article is available at The Research Repository @ WVU: https://researchrepository.wvu.edu/faculty_publications/634Photoinduced electron paramagnetic resonance study of electron trapsin TiO2 crystals: Oxygen vacancies and Ti3+ ionsShan Yang,1 L. E. Halliburton,1,a A. Manivannan,1,2 P. H. Bunton,3 D. B. Baker,3M. Klemm,4 S. Horn,4 and A. Fujishima51Department of Physics, West Virginia University, Morgantown, West Virginia 26506, USA2National Energy Technology Laboratory, Morgantown, West Virginia 26507, USA3Department of Physics, William Jewell College, Liberty, Missouri 64068, USA4Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany5Kanagawa Academy of Science and Technology, Takatsu Ku, Kanagawa 2130012, JapanReceived 19 December 2008; accepted 3 April 2009; published online 24 April 2009Electron paramagnetic resonance EPR is used to identify photoinduced titanium-associatedelectron traps in TiO2 crystals rutile. Defect production occurs at low temperature with 442 nmlaser light. Spectra with S=1 /2 and S=1 are assigned to singly ionized and neutral oxygenvacancies, respectively. These oxygen vacancies have their unpaired spins localized on the twoneighboring titanium ions aligned along the c axis. A Ti3+ ion next to a Si4+ ion, a Ti3+ self-trappedelectron, and a self-trapped hole shared by two adjacent oxygen ions are also observed. Isolatedsubstitutional Fe3+ and Cr3+ ions serve as hole traps. © 2009 American Institute of Physics.DOI: 10.1063/1.3124656Titanium dioxide TiO2 is a wide-band-gap semicon-ductor with an established record as a versatilephotocatalyst.1,2 Although this important material has beenwidely studied for more than five decades, a detailed under-standing of the role of point defects in controlling its opticaland electrical properties is still emerging.3–7 Many of theprevious investigations of defects have focused on reducedcrystals where oxygen vacancies, introduced during or aftergrowth, significantly increase the electrical conductivity.8–11In contrast, our current report describes results obtained fromcolorless fully oxidized TiO2 crystals that contain smallconcentrations of doubly ionized oxygen vacancies. Thesedoubly ionized vacancies provide charge compensation forthe trace amounts of trivalent transition-metal-ion impuritiesthat are inadvertently present. When describing the differentcharge states of the oxygen vacancy, we use doubly ionizedVO++, singly ionized VO+ , and neutral VO0  to refer to avacancy that is unoccupied, singly occupied, and doubly oc-cupied, respectively, with trapped electrons.In the present paper, electron paramagnetic resonanceEPR is used to identify the singly ionized S=1 /2 andneutral S=1 charge states of oxygen vacancies in TiO2crystals. These centers appear when crystals containing dou-bly ionized oxygen vacancies are exposed at low temperatureto 442 nm laser light. Electrons trapped at the oxygen vacan-cies are localized on two of the three neighboring titaniumions. Additional photoinduced EPR signals are assigned toTi3+ ions next to Si4+ ions, to Ti3+ ions with no nearby de-fects, and to self-trapped holes shared by two oxygen ions.Ionic notation is used for the cations.The two TiO2 crystals used in this investigation have therutile structure and were grown by CrysTec Germany andby Namiki Japan. Their dimensions were 3.85.00.5 mm3 and 2.53.51.2 mm3, respectively. Both ofthe as-received samples were colorless at room temperature,having been annealed in air after growth. The EPR data weretaken with a Bruker EMX spectrometer operating near 9.427GHz. Approximately 15 mW of 442 nm light from a He–Cdlaser was incident on the sample during the low temperatureilluminations.At room temperature, the as-received TiO2 samplesshowed several narrow and intense EPR signals from Fe3+and Cr3+ ions substituting for Ti4+ ions.12–15 With the mag-netic field parallel to the c axis, a signal due to Cr3+ ions wasobserved at 1354 G and two signals due to Fe3+ ions wereobserved at 829 and 8904 G. It is generally accepted16 thatthe isolated substitutional impurities responsible for theseEPR lines are charge compensated by remotely located oxy-gen vacancies i.e., one doubly ionized oxygen vacancy com-pensates two trivalent transition-metal ions replacing Ti4+ions. It is also possible that some of the substitutional Fe3+and Cr3+ ions are charge compensated by interstitial hydro-gen in a positive charge state. Our EPR results show that theNamiki sample has a larger concentration of Fe3+ and Cr3+than the CrysTec sample. From EPR, the combined concen-tration of these impurities is approximately one part per mil-lion in the Namiki sample and one-tenth of this value in theCrysTec sample. The increased number of Fe3+ and Cr3+ ionsin the Namiki sample suggests that it will have more oxygenvacancies than the CrysTec sample. These EPR estimates ofthe absolute concentrations of Fe3+ and Cr3+ ions are basedon a comparison with a weak pitch sample supplied byBruker error limits are 30%.Our TiO2 samples did not show EPR signals at low tem-perature 10 to 40 K in the magnetic field region from 3300to 4000 G when they were initially cooled in the dark. Sub-sequent illumination at low temperature, however, immedi-ately produced easily observed trapped-electron centers. TheEPR data in Fig. 1 were taken from the Namiki sample withthe magnetic field parallel to the c axis. Exposure of thesample to 442 nm laser light at 26 K produced three sharpEPR lines two lines are shown in Fig. 1a while a thirdEPR line near 3958 G is not shown. The lowest-field lineaAuthor to whom correspondence should be addressed. Electronic mail:larry.halliburton@mail.wvu.edu.APPLIED PHYSICS LETTERS 94, 162114 20090003-6951/2009/9416/162114/3/$25.00 © 2009 American Institute of Physics94, 162114-1Downloaded 26 Jan 2010 to 137.250.81.116. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspnear 3419 G and the highest-field line near 3958 G notshown belong to a S=1 center with gc=1.826. We use gc todenote the g value when the magnetic field is parallel to thec axis. These two lines represent the MS=−1 to 0 and theMS=0 to +1 transitions. A detailed angular dependencestudy, to be reported later, provides complete sets of spin-Hamiltonian parameters for this and the other defects de-scribed in the present paper. We assign this S=1 spectrum tothe neutral charge state of the oxygen vacancy VO0 . It wasproduced in our study when a doubly ionized oxygen va-cancy VO++ trapped two photoinduced electrons. The EPRangular dependence indicates that these two electrons arelocalized on the two neighboring titanium ions aligned alongthe c axis. We suggest that these trapped electrons form apair of neighboring Ti3+ ions that couple ferromagnetically toproduce a spin-triplet S=1 ground state. In support of ourassignment, we found that a much more intense S=1 signalfrom the neutral oxygen vacancy was present at 26 K with-out light in a slightly reduced TiO2 crystal i.e., a crystalheld for 30 min at 600 °C in flowing nitrogen gas. Thereduction treatment creates uncompensated neutral oxygenvacancies as oxygen atoms are removed, and laser light isnot needed to photoinduce the S=1 spectrum.We identify the EPR signal near 3476 G in Fig. 1a as aTi3+–Si4+ center. This is a S=1 /2 defect with gc=1.938, andis formed when a photoinduced electron is trapped on a regu-lar Ti4+ ion that has a substitutional Si4+ ion at a nearestcation site along the c axis. Figure 2 shows the hyperfinestructure accompanying this Ti3+–Si4+ center. In addition tothe eight lines caused by the 47Ti and 49Ti isotopes, there isan extra pair of hyperfine lines separated by approximately2.0 G and symmetrically located about the center line. Theleft line is slightly more intense than the right line because itis overlapped by one of four small closely spaced lines.Note: Several weak four-line EPR spectra possibly associ-ated with interstitial Li+ or Na+ ions were observed in theNamiki sample. The extra pair of hyperfine lines in Fig. 2requires the responsible nucleus to have I=1 /2, and thesmall hyperfine constant of 2.0 G suggests that this nucleusis located at a neighboring site. A comparison of the intensi-ties of these two hyperfine lines with the intensities of the47Ti and 49Ti hyperfine lines shows that the isotopic abun-dance of the I=1 /2 nucleus must be slightly less than 5%.The only possible candidate is 29Si with an abundance of4.67%. Elemental analyses of impurities in TiO2 crystals of-ten reveal the presence of silicon.10,17Figure 1b shows the EPR spectrum taken at 26 Kshortly after the laser light was removed from the sample. Anadditional line, representing a S=1 /2 defect with gc=1.824,appeared near 3693 G and quickly became the dominant sig-nal. At the same time, the intensities of the VO0 and Ti3+–Si4+signals decreased significantly. When the laser light was re-stored while still keeping the sample at 26 K, the line at 3693G disappeared almost instantly and the VO0 and Ti3+–Si4+signals returned to their initial intensities. We assign this sig-nal at 3693 G to a VO+ center. Specifically, we suggest thatthis defect is a singly ionized oxygen vacancy with thetrapped electron equally shared by the same two titaniumions that form the S=1 VO0 center i.e., the two titaniumneighbors aligned along the c axis. The VO0 center and theVO+ center have similar g matrices gc=1.826 and gc=1.824,respectively, which supports our oxygen vacancy models.Conversion of a VO0 center to a VO+ center occurs when theshallow VO0 center thermally releases one of its two trappedelectrons at temperatures near or above 26 K. In general, theequilibrium concentration of VO0 centers produced during anillumination represents a “balance” between the formationand thermal decay rates and thus depends sensitively on boththe temperature and the intensity and wavelength of the light.As expected, the VO0 and VO+ centers were produced in theCrysTec sample at 26 K with 442 nm laser light, but theirintensities were much lower than in the Namiki sample. Thisis consistent with the room-temperature EPR results thatshowed reduced amounts of Fe3+ and Cr3+ ions in the Crys-Tec sample. In an early study, Kerssen and Volger10 observedthe EPR spectra that we have assigned to VO+ and VO0 centers,and they proposed models for these centers that consisted ofsubstitutional Ti3+ ions with two nearby Al3+ ions. Theircomplex models are, however, questionable because they didnot observe any hyperfine splittings due to the 100% abun-FIG. 1. EPR spectra taken at 26 K from a TiO2 crystal. The magnetic fieldis parallel to the c axis. a Upper spectrum was taken with 442 nm laserlight on. b Lower spectrum was taken approximately 2 min after turningoff the laser light.FIG. 2. EPR spectrum of the Ti3+–Si4+ center taken at 26 K with themagnetic field parallel to the c axis. The upper and lower stick diagramsshow the 29Si and the 47Ti and 49Ti hyperfine lines, respectively. Additionalweak lines in the spectrum are unidentified.162114-2 Yang et al. Appl. Phys. Lett. 94, 162114 2009Downloaded 26 Jan 2010 to 137.250.81.116. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspdant 27Al nuclei. It is now known18 that an Al3+ ion substi-tuting for a Ti4+ ion will trap a hole on an adjacent oxygenion, and will not produce electronlike Ti3+ centers.The mechanism by which below-band-gap light 442nm produces trapped-electron centers in TiO2 is not fullyunderstood. The most likely scenario involves the direct ex-citation of electrons from Fe3+ and Cr3+ ions to the conduc-tion band. The Fe3+ and Cr3+ ions, acting as deep singlyionized acceptors, are expected to have near-edge opticaltransitions i.e., broad absorption bands that correspond toan electron moving from the acceptor to the conductionband. A decrease of about 50% in the intensities of the Fe3+and Cr3+ EPR signals occurred when the Namiki sample wasilluminated at 26 K with 442 nm laser light. This decreasecoincided with the production of the VO0 and Ti3+–Si4+ cen-ters in Fig. 1a. The trivalent impurity ions convert to Fe4+and Cr4+ ions and serve as the compensating hole centers forthe photoinduced electron centers, thus allowing the crystalto remain electrically neutral. EPR signals from Fe4+ andCr4+ ions in TiO2 have not been reported.Two additional S=1 /2 EPR centers appeared in theNamiki sample during an illumination with 442 nm laserlight at 14 K. These EPR lines, shown in Fig. 3a, are at3345 and 3443 G when the magnetic field is parallel to the caxis. The lower-field signal is holelike gc=2.013 and thehigher-field signal is electronlike gc=1.956. Both of thesecenters had very low thermal stabilities. Their EPR signalswere much smaller when the illumination occurred at 18 K,instead of 14 K. The two signals disappeared instantly whenthe light was removed at 18 K, but decayed more slowly inminutes when the light was removed at 14 K as in Fig.3b. Neither defect could be produced with 442 nm lightwhen the temperature was above 25 K. These signals at 3345and 3443 G in Fig. 3 had long spin-lattice relaxation times inthe 14–25 K range and were very easily saturated at highermicrowave powers.The two c-axis lines at 3345 and 3443 G were the domi-nant photoinduced EPR signals in the CrysTec sample whenit was illuminated with 442 nm laser light while being held at14 K. The VO0 and VO+ centers were much less intense in theCrysTec sample during low temperature illuminations be-cause the crystal contained fewer oxygen vacancies. Basedon their low thermal stabilities and their appearance in boththe Namiki and the CrysTec samples, we propose modelsinvolving the self-trapping of photoinduced charges for theEPR signals at 3345 and 3443 G. The holelike signal has noresolved hyperfine structure, which indicates that there is nonearby trivalent cation impurity that serves to “stabilize” thehole.18 This leads us to assign the EPR signal at 3345 G inTiO2 to a “self-trapped” hole, i.e., a hole shared by two ad-jacent oxygen ions that have relaxed toward each other. Ananalogous defect has been observed in KD2PO4 crystals.19Similarly, we assign the signal at 3443 G to a “self-trapped”electron in the form of an isolated Ti3+ ion, i.e., an electrontrapped at a Ti4+ ion in the otherwise perfect lattice.In summary, doubly ionized oxygen vacancies in color-less oxidized TiO2 crystals have been converted to their neu-tral charge state by illumination with 442 nm laser light at26 K. The neutral oxygen vacancy VO0  has S=1 with thetwo trapped electrons localized on the pair of titanium neigh-bors aligned along the c axis. If the temperature is near orabove 26 K, these neutral oxygen vacancies will thermallyrelease an electron and become singly ionized oxygen vacan-cies VO+  when the light is removed. This latter defect hasS=1 /2 with the one trapped electron equally shared by thesame two titanium ions that form the S=1 VO0 center i.e., thetwo titanium neighbors aligned along the c axis.This work was supported at West Virginia University byGrant No. DMR-0804352 from the National Science Foun-dation and at William Jewell College by a Cottrell CollegeScience Award from the Research Corporation. Support atthe University of Augsburg was provided by the DeutscheForschungsgemeinschaft DFG through Sonderforschungs-bereich SFB 484. The authors acknowledge helpful discus-sions with Anderson Janotti and Chris Van de Walle.1F. A. Grant, Rev. Mod. Phys. 31, 646 1959.2A. Fujishima, X. Zhang, and D. A. Tryk, Surf. Sci. Rep. 63, 515 2008.3G. Pacchioni, ChemPhysChem 4, 1041 2003.4V. N. Kuznetsov and N. Serpone, J. Phys. Chem. B 110, 25203 2006.5N. Serpone, J. Phys. Chem. B 110, 24287 2006.6C. Di Valentin, E. Finazzi, G. Pacchioni, A. Selloni, S. Livraghi, M. C.Paganini, and E. Giamello, Chem. Phys. 339, 44 2007.7M. K. Nowotny, L. R. Sheppard, T. Bak, and J. Nowotny, J. Phys. Chem.C 112, 5275 2008.8P. F. Chester, J. Appl. Phys. 32, 2233 1961.9P. I. Kingsbury, W. D. Ohlsen, and O. W. Johnson, Phys. Rev. 175, 10911968.10J. Kerssen and J. Volger, Physica Amsterdam 69, 535 1973.11M. Aono and R. R. Hasiguti, Phys. Rev. B 48, 12406 1993.12H. J. Gerritsen, S. E. Harrison, H. R. Lewis, and J. P. Wittke, Phys. Rev.Lett. 2, 153 1959.13D. L. Carter and A. Okaya, Phys. Rev. 118, 1485 1960.14G. J. Lichtenberger and J. R. Addison, Phys. Rev. 184, 381 1969.15S. S. Kim, S. S. Jun, and M. J. Park, J. Korean Phys. Soc. 23, 73 1990.16B. W. Faughnan and Z. J. Kiss, Phys. Rev. Lett. 21, 1331 1968.17T. Purcell and R. A. Weeks, J. Chem. Phys. 54, 2800 1971.18D. Zwingel, Solid State Commun. 20, 397 1976.19K. T. Stevens, N. Y. Garces, L. E. Halliburton, M. Yan, N. P. Zaitseva, J.J. DeYoreo, G. C. Catella, and J. R. Luken, Appl. Phys. Lett. 75, 15031999.FIG. 3. EPR spectra taken at 14 K from a TiO2 crystal. The magnetic fieldis parallel to the c axis. a Upper spectrum was taken with 442 nm laserlight on. b Lower spectrum was taken approximately 2 min after turningoff the laser light.162114-3 Yang et al. Appl. Phys. Lett. 94, 162114 2009Downloaded 26 Jan 2010 to 137.250.81.116. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Photoinduced Electron Paramagnetic Resonance Study Of Electron Traps In Tio2 Crystals: Oxygen Vacancies And Ti3+ Ions

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Photoinduced Electron Paramagnetic Resonance Study Of Electron Traps In Tio2 Crystals: Oxygen Vacancies And Ti3+ Ions

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