2007-2008 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Chen, WP

Wang, Y

Applied Physics Letters

PolyU Institutional Repository

Hydrogen: A metastable donor in TiO2 single crystalsW. P. Chen,1,a Y. Wang,2 and H. L. W. Chan21Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong,People’s Republic of China and Department of Physics, Wuhan University, Wuhan 430072,People’s Republic of China2Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University,Hong Kong, People’s Republic of ChinaReceived 10 February 2008; accepted 29 February 2008; published online 20 March 2008The properties of TiO2 110 single crystals are greatly changed after hydrogen is doped into themthrough electrochemical hydrogen charging, in which the crystals have been placed in 0.01M NaOHsolution to deposit hydrogen on their electrodes through the electrolysis of water. The changes inproperties, including great increases in leakage current, strong dielectric dispersions over102–106 Hz, and a strong aging phenomenon suggest that hydrogen is a donor in TiO2 while ionizedhydrogen is not bound to an oxygen site by a strong O–H bond. This behavior of hydrogen, whichcan be characterized as a metastable donor, is different from the usual donor behavior of hydrogenin oxides and should be interesting for a better understanding of the behavior of hydrogen inoxides. © 2008 American Institute of Physics. DOI: 10.1063/1.2900957The behavior of hydrogen in oxides has been a topic ofextensive investigations. On one hand, hydrogen is a ubiqui-tous impurity in various oxides. It can be introduced intooxides from precursors, from annealing gases,1 or even fromaqueous vapor in ambient atmosphere.2 On the other hand,hydrogen behaves quite differently from one oxide to an-other. In some oxides, hydrogen forms deep gap states inall of its three charge states, H+, H0, and H−; while in otheroxides, hydrogen gives rise to a shallow level at theconduction band edge and acts as a donor.3–5 As an example,hydrogen has been predicted theoretically6 and provenexperimentally7 as a shallow donor in ZnO. It must possessa high stability in ZnO to survive high-temperature sinteringor crystal growth. As a matter of fact, it is generally believedthat as a donor, hydrogen ionizes to H+ with the electronbeing delocalized in the oxide conduction band and then H+forms a single strong O–H bond to an O site; in contrast,when hydrogen is a deep impurity, it will only form somelonger, weaker bonds to more than one neighboring oxygenand exist as a metastable interstitial.5 Metastable interstitialhydrogen has been observed in proton-exchanged LiNbO3. Asharp peak at around 3500 cm−1 can always be observed inthe infrared absorption spectra of proton exchange process–treated LiNbO3, which is the characteristic absorption peakfor the stretch mode of the O–H bond, as hydrogen has re-placed some lithium ions in LiNbO3. When the acidity of themelt in the proton exchange process is high enough, anotherbroad peak at 3280 cm−1 can also be observed, which is onlymetastable and will disappear through an annealing process.8Obviously, these two peaks should correspond to two differ-ent states of hydrogen with quite different stabilities. Thebroad peak should indicate the existence in LiNbO3 of hy-drogen with a much lower stability, which is possibly a deepimpurity forming only long and weak bonds to neighboringoxygen sites. In a previous investigation, hydrogen has beendoped into TiO2 single crystals through an electrochemicalmethod.9 Presently, a study on the behavior of hydrogen inTiO2 single crystals has been conducted in terms of dielectricand conduction analyses, which strongly suggest that hydro-gen acts a donor in TiO2 while ionized hydrogen is notbound to an oxygen site by a strong O–H bond. Such behav-ior of hydrogen in oxides, which can be characterized asmetastable donor, is different from the usual donor behaviorof hydrogen in oxides and should help in understanding morefully the behavior of hydrogen in oxides.Rutile 110 single crystals, 10100.5 mm3 and twosides polished, were used in the investigation. Circular silverelectrodes DuPont 1183 with a diameter of 2 mm werefired onto the centers of the two major surfaces of the crys-tals. The crystals were immersed in a 0.01M NaOH solution,and a 4.5 V dc voltage was imposed between the silver elec-trodes of the crystals and a counter Pt electrode in the solu-tion, with the former acting as the cathode and the latteracting as the anode. The imposed dc voltage induced elec-trolysis of water, and hydrogen was deposited on the silverelectrodes of the crystals. Hydrogen generated in this elec-trochemical process can diffuse and react with oxides; here-after, this process is referred to as electrochemical hydrogencharging. The solution was kept stirred by a magnetic agita-tor, and the temperature was kept constant at 20 °C. Thehydrogen charging was interrupted from time to time and thecrystals were taken out, washed with de-ionized water andacetone, and dried for measurement. Using the two silverelectrodes of the crystals, the frequency spectra of capaci-tance and dielectric loss of the crystals were measured on animpedance analyzer Agilent 4294 A, and the I-V character-istics of the crystals were recorded through an electrometer/high resistance meter Keithley 6517. All measurementswere conducted at ambient temperature 20 °C.It should be pointed out that the single crystals werechemically stable in water. There were no discerniblechanges in the properties of the crystals after they had beenimmersed in the 0.01M NaOH solution for dozens of hourswithout application of any voltages. The changes induced byelectrochemical hydrogen charging can be reliably ascribedto the reaction of hydrogen. Some representative results ob-tained for a single crystal are reported in the following. Fig-ures 1 and 2 show the frequency spectra of capacitance andaElectronic mail: chenwp66@yahoo.com.APPLIED PHYSICS LETTERS 92, 112907 20080003-6951/2008/9211/112907/3/$23.00 © 2008 American Institute of Physics92, 112907-1Downloaded 28 Sep 2008 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspdielectric loss and I-V characteristics of the crystal measuredas received and after different periods of electrochemical hy-drogen charging, respectively. To be consistent, every timethat electrochemical hydrogen charging was interrupted, thefrequency spectra were measured after the crystal was stabi-lized for 0.1 h and the I-V curve was measured after thecrystal was stabilized for 0.2 h. Obviously, electrochemicalhydrogen charging had induced great changes in both thedielectric properties and the conductivity of the single crys-tal. The leakage current was increased by more than fourorders of magnitude after 50 h of electrochemical hydrogencharging. It is reasonable to conclude that some free chargecarriers were formed in the single crystal through electro-chemical hydrogen charging. In a previous investigation, apeak at 3280 cm−1 had been observed in the infrared absorp-tion spectra of hydrogen-charged TiO2 single crystals,8which gives a direct evidence for the incorporation of hydro-gen in the TiO2 lattice. As predicted by some theoreticalinvestigations,3–5 hydrogen is a shallow donor in TiO2, and ithas been proposed that free electrons are formed in the fol-lowing way:Hads→ Hi• + e, 1where Hads represents an adsorbed hydrogen atom generatedfrom electrochemical hydrogen charging and Hi• is an ionizedhydrogen in an interstitial site of TiO2 lattice. The great in-crease in leakage current can be explained well by the donorbehavior of hydrogen in TiO2.For the as-received crystal, the capacitance showed alittle dispersion over the testing frequency range and the di-electric loss peak was around 3104 Hz. For the hydrogen-charged single crystal, the capacitance showed much stron-ger dispersions and the dielectric loss peak occurred at somehigher frequencies. For example, after 50 h of electrochemi-cal hydrogen charging, the capacitance was increased bymore than two times at 102 Hz and by only 40% at 106 Hz,and the dielectric loss peak was around 2105 Hz. Amongthe various polarization mechanisms in oxide crystals,10 ori-entation polarization is responsible for dispersions over thetesting frequency range 102–106 Hz, which occurs whentwo or more equivalent positions for an ion are present.There must be some defects, such as oxygen vacancy andtitanium vacancy, in small concentrations in the as-receivedsingle crystal. The defects must be associated in some wayswith a formed dipole moment, and orientation polarization ofthe dipole moment is accomplished through ion jump, lead-ing to the observed little dispersion. Accordingly, the strongdispersions in the hydrogen-charged crystal must be due toorientation polarization of hydrogen-related dipoles throughjumping of H+ between some equivalent positions. There ex-ists a strong aging behavior in the properties of hydrogen-charged single crystals. The results obtained for the abovesingle crystal after 50 h of electrochemical hydrogen charg-ing are shown in Figs. 3 and 4. The moment when electro-chemical hydrogen charging was finally stopped was takenas the starting point of aging, and its dielectric properties andI-V characteristics were successively measured after a seriesof periods of aging time. As shown in Fig. 3, the dielectricproperties of the single crystal were found to change system-atically in the course of aging. With increasing time of aging,the slope of the capacitance-frequency curve, or dispersion,was gradually decreased; while the dielectric loss peak wasmoving to the low frequency side and its value was graduallydecreased. After 500 h of aging, the frequency spectra ofcapacitance and dielectric loss were almost the same to thoseobtained for the as-received crystal. At the same time, theleakage current was gradually decreased in the course of ag-ing. After 500 h of aging, the leakage current was only twotimes as big as that measured for the as-received crystal. Sothe effects of hydrogen on the properties of TiO2 single crys-tals gradually weakened and finally disappeared through theFIG. 1. Frequency spectra of capacitance and dielectric loss of a TiO2 singlecrystal measured at different times: as received and after a series of periodsof electrochemical hydrogen charging.FIG. 2. I-V characteristics of a TiO2 single crystal measured at differenttimes: as received and after a series of periods of electrochemical hydrogencharging.FIG. 3. Color online Frequency spectra of capacitance and dielectric lossof a hydrogen-charged TiO2 single crystal in the course of the aging process.112907-2 Chen, Wang, and Chan Appl. Phys. Lett. 92, 112907 2008Downloaded 28 Sep 2008 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspaging process, which indicates that hydrogen is only meta-stable in TiO2 and will gradually diffuse out. A possiblemechanism for the outdiffusion of hydrogen from TiO2 canbe expressed as2Hi• + 2e→ H2. 2With the outdiffusion of hydrogen, the free electrons gener-ated by the ionization of hydrogen and hydrogen-related di-poles were eliminated, so the dielectric and conduction prop-erties of TiO2 were both restored.The question now arises on the nature on the interactionbetween H+ and neighboring oxygen sites. As a donor, hy-drogen has been expected to form a single strong O–H bondperpendicular to the Ti–O bonding plane after ionization inTiO2.4 The aging behavior shown in Fig. 3, however, sug-gests something quite unexpected. It is well-known that thefrequency at the dielectric loss peak represents a dielectricrelaxation frequency denoted fc,10 which is the frequencyfor ion jump between equivalent positions. Frequency fc fol-lows an Arrhenius law expressed asfc = f0 exp− U/kBT , 3where U denotes the activation energy required for the ionjump. Obviously, fc shifts to a higher frequency with increas-ing temperature.11,12 It is strange that for the hydrogen-charged crystals in this study, the dielectric loss peak wasmoving to the low frequency side with increasing aging timeat a constant temperature. It seems that the activation energyfor the jumping of H+ is not a constant in this system. Asmentioned above, the concentration of hydrogen in the singlecrystals is gradually decreased in the course of aging. Al-though an exact mechanism is still lacking, it is reasonable toassume that the activation energy for the jumping of H+ de-pends on the concentration of hydrogen in TiO2. The lowerthe concentration is, the bigger the activation energy is andthe smaller the jumping frequency fc is. Such activation en-ergy should not result from an O–H bond, which is indepen-dent of the concentration of hydrogen. The possibility thathydrogen could act as a donor while it is not bound to anoxygen site by an O–H bond has to be considered. As amatter of fact, the low stability of hydrogen in TiO2 hasalready implied the absence of a strong O–H bond for hy-drogen.A similar aging behavior has been observed in electro-chemically hydrogen-charged BaTiO3 and SrTiO3 singlecrystals,13,14 whose dielectric loss peaks are also moving tothe low frequency side in the course of aging. Hydrogendoped through electrochemical hydrogen charging may alsoact as a metastable donor in BaTiO3 and SrTiO3. Severalinvestigations have shown that hydrogen can be doped intoperovskite titanates through annealing in atmospheres con-taining H2.1,12,15 It is believed that hydrogen doped throughannealing acts as a donor in the titanates with a strong O–Hbond formed and a heat treatment at some elevated tempera-tures is needed to remove hydrogen from them.15 There havebeen no reports on any obvious aging behavior at ambienttemperature induced by hydrogen in them. Some researchershave pointed out that hydrogen doped at different times orthrough different ways may behave quite differently inoxides.4,15 So depending on the doping methods, there maybe two different donor states, namely, stable donor dopedthrough annealing and metastable donor electrochemicallydoped, for the behavior of hydrogen in oxides. Further in-vestigations are being conducted on the relationship betweenthese two different donor states.In summary, the properties of electrochemical hydrogencharging–treated TiO2 110 single crystals indicate that hy-drogen acts as a metastable donor in TiO2; that is, it ionizesto H+ with a free electron formed, while H+ is not bound toan oxygen site by a strong O–H bond. The great increases inleakage current must be due to free electrons ionized fromhydrogen, the strong dielectric dispersions over the fre-quency range of 102–106 Hz should be explained by orien-tation polarization of hydrogen-related dipoles, and thestrong aging behavior indicates variable activation energy forthe jumping of H+ and a low stability of hydrogen in TiO2lattice. Hydrogen as a metastable donor may be differentfrom a donor bound to an oxygen site by a strong O–H bondin oxides, and further investigations should be carried out.This work has been supported by the National NaturalScience Foundation of China under Grant No. 50772077 andthe Centre for Smart Materials of The Hong Kong Polytech-nic University Project 1-BBZ3.1S. Aggarwal, S. R. Perusse, C. W. Tipton, R. Ramesh, H. D. Drew, T.Venkatesan, D. B. Romero, V. B. Podobedov, and A. Weber, Appl. Phys.Lett. 73, 1973 1998.2J. D. Baniecki, J. S. Cross, M. Tsukada, and J. Watanabe, Appl. Phys. Lett.81, 3837 2002.3C. Kilic and A. Zunger, Appl. Phys. Lett. 81, 73 2002.4P. W. Peacock and J. Robertson, Appl. Phys. Lett. 83, 2025 2003.5K. Xiong, J. Robertson, and S. J. Clark, J. Appl. Phys. 102, 0837102007.6C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 2000.7D. M. Hofmann, A. Hofstaetter, F. Leiter, H. Zhou, F. Henecker, B. K.Meyer, S. B. Orlinskii, J. Schmidt, and P. G. Baranov, Phys. Rev. Lett. 88,045504 2002.8P. Baldi, M. P. De Micheli, K. El Hadi, S. Nouh, A. C. Cino, P. Aschieri,and D. B. Ostrowsky, Opt. Eng. Bellingham 37, 1193 1998.9W. P. Chen, Y. Wang, J. Y. Dai, S. G. Lu, X. X. Wang, P. F. Lee, H. L. W.Chan, and C. L. Choy, Appl. Phys. Lett. 84, 103 2004.10W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceram-ics, 2nd ed. Wiley, New York, 1976.11J. B. Wu, C. W. Nan, Y. H. Lin, and Y. Deng, Phys. Rev. Lett. 89, 2176012002.12P. Gonon and F. El Kamel, Appl. Phys. Lett. 90, 232902 2007.13W. P. Chen, Y. Wang, H. L. W. Chan, and H. S. Luo, Appl. Phys. Lett. 88,202906 2006.14W. P. Chen, J. Y. Dai, Y. Wang, and H. L. W. Chan, Appl. Phys. A: Mater.Sci. Process. 81, 631 2005.15P. C. McIntyre, J.-H. Ahn, R. J. Becker, R.-V. Wang, S. R. Gilbert, L.Wills Mirkarimi, and M. T. Schulberg, J. Appl. Phys. 89, 6378 2001.FIG. 4. I-V characteristics of a hydrogen-charged TiO2 single crystal in thecourse of the aging process.112907-3 Chen, Wang, and Chan Appl. Phys. Lett. 92, 112907 2008Downloaded 28 Sep 2008 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Hydrogen : a metastable donor in TiO₂ single crystals

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