Scanning tunneling microscopy has been used to identify the adsorption site of H₂O on TiO₂(110)-(1 x 1) at 150 K, and to monitor the site of the dissociation products at 290 K. Water adsorbs onto the rows of fivefold coordinated Ti atoms at 150 K, dissociating by 290 K to form bridging but not terminal hydroxyls. This points to the involvement of bridging O vacancies in the dissociation pathway

Brookes, IM

Muryn, CA

Thornton, G

English

UCL Discovery

VOLUME 87, NUMBER 26 PHYSICAL REVIEW LETTERS 24D ECEMBER 2001Imaging Water Dissociation on TiO2  110  I.M. Brookes, C.A. Muryn, and G. ThorntonSurface Science Research Centre and Chemistry Department, Manchester University,Manchester M13 9PL, United Kingdom(Received 2 July 2001; published 6 December 2001)Scanning tunneling microscopy has been used to identify the adsorption site of H2Oo nT i O 2110-1 3 1 at 150 K, and to monitor the site of the dissociation products at 290 K. Water adsorbs onto therows of ﬁvefold coordinated Ti atoms at 150 K, dissociating by 290 K to form bridging but not terminalhydroxyls. This points to the involvement of bridging O vacancies in the dissociation pathway.DOI: 10.1103/PhysRevLett.87.266103 PACS numbers: 68.43.Fg, 68.37.Ef, 68.47.GhMolecular processes on oxide surfaces are important ina number of applications, including catalysis and environ-mental control. The interaction of water with TiO2 sur-faces, the subject of this Letter, has particular relevance tophotocatalysis.Oxide surface reactivity has long been thought to bedominated by minority sites in the form of defects [1], al-though a detailed understanding of these processes still hasto be achieved. The principal experimental obstacle to thisgoal has been associated with the spatially averaging na-ture of nonimaging techniques. Experience with prototypesurfaces, particularly TiO2110 indicates that a mixtureof minority sites is often present [2]. The effect producedby each type of minority site will be summed in a non-imaging measurement. In principle, scanning tunnelingmicroscopy(STM) offers the possibility to focuson the be-havior of one type of minority site. Here we employ STMto study the reactivity to H2Oo faT i O 2110-1 3 1 sur-face withpoint defects in the form of bridging Ovacancies.As judged by scanning probe microscopy images, a typicalTiO2110-1 3 1 surface contains about 3% monolayer(ML) of bridging O vacancies, where 1 ML is deﬁned asthe number of ﬁvefold coordinate Ti41 atoms at the sur-face [3].Temperature programmed desorption (TPD) spectrafrom TiO2110-1 3 1 after exposure to water containfour peaks centered at 155, 190, 295, and 490–540 K [4].The ﬁrst three have been assigned to molecular desorptionfrom multilayer, second layer, and ﬁrst layer states. As forthe higher temperature feature, this has been assigned torecombinative desorption. The corresponding adsorbatecoverage was in the region of that of the bridging-Ovacancies [4]. This implicates these vacancies in the H2Odissociation process, a suggestion that is further supportedby isotope substitution measurements [4].Although there is general agreement that the maximumcoverage of OH species on TiO2110 resulting from H2Odissociation is in the region of 0.1 ML [5–7], a consensusstill has to be reached regarding the temperature at whichH2O dissociates. A photoemission study concluded thatwater adsorbs molecularly at 160 K, with some dissocia-tion associated with O vacancies as the temperature is in-creased to 200 K[5]. Incontrast, a highresolutionelectronenergy loss spectroscopy (HREELS) study found that wa-ter dissociates at 130 K at defect sites to give a coverageof about 0.1 ML, while subsequent adsorption is molecu-lar [7]. This apparent discrepancy may reﬂect a differencein the O vacancy density.A ﬁrst principles density functional theory calculationand molecular dynamics simulations of water adsorptionon TiO2110 have been reported for a defect free surface[8]. This was based on a substrate structure determined bysurface x-ray diffraction [9]. The calculations predict dis-sociative adsorption below 0.5 ML, and mixed dissociativeand molecular adsorption with H bonding above this cov-erage [8].In this Letter we describe a variable temperature STMstudy of water adsorption on TiO2110 at #150 K and theeffect of increasing the substrate temperature to 290 K.Adsorbate ﬁngerprinting is provided by complementaryphotoemissionmeasurements. Theresults pointto molecu-lar adsorption on ﬁvefold coordinated Ti atoms at 150 Kwith thermally activateddissociation at bridgingOvacancysites to form only bridging OH species.STM images were recorded using an Omicron VariableTemperature UHV STM. Images of TiO2110 wererecorded in the constant current mode with the samplepositively biased with respect to the tip. As in earlierstudies we were unable to image the sample at negativebias [10]. A 1.5 V sample bias and a tunneling current of1 nA were used to record all of the images reported here.Photoemission measurements employed a VSW HA100MCD electron energy analyzer and VG He lamp. Both thephotoemission and STM instruments had base pressuresof #2 3 10210 mbar during this work.The TiO2110 samples (Pi-Kem) were cleaned by re-peated Ar1 sputtering (1 kV) and annealing to 900 K untiljudged clean and ordered by Auger electron spectroscopy,low energy electron diffraction, and STM. The samplewas exposed to the vapor of distilled H2O after further pu-riﬁcation of the liquid using freeze-pump-thaw cycles. Anin situ mass spectrometer was used to conﬁrm the absenceof signiﬁcant impurity levels in the water vapor. The doserinlet on the photoemission chamber was in direct line of266103-1 0031-90070187(26)266103(4)$15.00 © 2001 The American Physical Society 266103-1VOLUME 87, NUMBER 26 PHYSICAL REVIEW LETTERS 24D ECEMBER 2001sight to the sample, which explains the smaller exposuresto give the same coverage of H2O.STM images of the clean TiO2110-1 3 1 surfacerecorded in the range 150–300 K resemble the room tem-perature images reported in the literature [2,3]. Thesecontain bright [001] direction rows that arise from tunnel-ing into ﬁvefold coordinated Ti atoms, with the dark rowscorresponding to the positions of bridging O atoms [2,3].A representative STM image of TiO2110 after expo-sure to 0.5 L (1 Langmuir  1.32 mbar ? s) of water at120 K and warming to 150 K is shown in Fig. 1. It is clearthat adsorbate induced features are situated on the brightTi rows. Although individual Ti atoms in the row can bediscerned, it is not possible from the data recorded thusfar to determine whether the adsorbate induced features lieatop Ti. The line proﬁle in Fig. 1 shows that the adsor-FIG. 1. A 60 Å 3 60 Å STM image of TiO2110-1 3 1after exposure to 0.5 L H2O at 120 K and subsequent heatingto the measurement temperature of 150 K. The line within theimage shows the path of the line proﬁle. Features identiﬁed inthe image and line proﬁle are discussed in the text.bates have an apparent height of about 1 Å above the Tirows with intermolecular separations between molecules 1and 2, 2 and 3, and 3 and 39 of about 9, 12, and 3 Å, re-spectively. These separations are integer multiples of the2.96 Å unit cell length along [001].The upper photoemission difference spectrum in Fig. 2was recorded from the STM sample, after recleaning andreexposing to 0.2 L at 150 K. This spectrum can be seento be characteristic of molecular water by comparison withthe gas phase spectrum of water that is shown in Fig. 2for comparison. The 0.4 eV bonding shift of the 3a1peak is consistent with chemisorbed H2O [5]. On the basisFIG. 2. He II (hn  40.8 eV) photoemission differencespectra of TiO2110-1 3 1 recorded at normal emission.Difference spectra were obtained by subtracting a clean surfacespectrum from spectra recorded after exposing the surface to0.2 L H2O at 150 K and subsequently warming in steps to300 K. The binding energy scale was aligned to the Fermilevel recorded from the Ta sample holder. For comparison, agas phase spectra of water [12] has been included, aligned atthe 1b1 orbital, as well as the relative positions of the OHsand OHp features [11]. We can estimate the H2O coveragecorresponding to the 150 K spectrum if we assume that the OHand H2O valence band intensity is proportional to the numberof adsorbate O atoms. On the basis of an 0.08 ML coverageof OH in the 290 K spectrum (see text), the H2O coverage at150 K is 0.7 ML. For comparison, a difference spectrum isincluded which was recorded following an exposure to 0.02 LH2O at 150 K.266103-2 266103-2VOLUME 87, NUMBER 26 PHYSICAL REVIEW LETTERS 24D ECEMBER 2001of the photoemission results, features in the STM imagein Fig. 1 can be associated with chemisorbed H2O. Themolecules are expected to be bonded atop Ti atoms onthe basis of theoretical calculations [8]. A small butdiscernible 1b2 feature is observed in the photoemissiondifference spectrum at the lowest H2O exposure studied,namely 0.02 L (see Fig. 2), suggesting that on this surfacethere may be little dissociation at 150 K.The STM image in Fig. 3(a) indicates that an H2Oe x -posure of 3.5 L at 150 K results in a coverage of about0.7 ML, with the features remaining above the Ti rows.The STM images in Figs. 3(b) and 3(c) show the effect ofwarming the sample imaged in Fig. 3(a) from 150 K. At190 K [Fig. 3(b)] the apparent coverage ofwater decreasesto about 0.3 ML.Increasing the temperature of the overlayer to 290 K re-sults in a quite different signature in the photoemissiondifference spectrum (see Fig. 2). Now only two featuresare observed, which appear at binding energies consistentwith photoemission from the s and p orbitals of OH [11].The integrated adsorbate valence region intensity has de-creased by a factor of about 10, consistent with waterdesorption. In the STM image, water dissociation is re-FIG. 3. STM images of TiO2110-1 3 1 exposed to 3.5 Lof H2O at 150 K, and consecutively heated to the measurementtemperatures of the following: (a) 150 K (240 Å 3 120 Å);(b) 195 K (240 Å 3 120 Å); and (c) 290 K (200 Å 3 145 Å).ﬂected by the appearance of adsorbate induced features onthe dark rows, which correspond to the position of bridg-ing O atoms. The apparent height of the adsorbates isabout 1.2 Å above Ti rows, and they appear at a cover-age of 0.08 ML. This is close to the density of bridgingOH species observed (0.07 ML) on an as-prepared sampleof TiO2110-1 3 1 [3], presumably because of adventi-tious reaction with water in the vacuum system.As in the photoemission spectra, the STM images re-ﬂect desorption of water on increasing the temperature to290 K. Annealing to above 500 K is sufﬁcient to removeall adsorbate contributions to the STM image, leaving aclean 1 3 1 substrate.The observation of what have been termed bridginghydroxylspecies [8] resultingfrom thermally activateddis-sociation of chemisorbed water is signiﬁcant. On the basisof the calculations for water interacting with the perfectsurface, dissociation on regular lattice sites should giverise to both terminal and bridging hydroxyl species, whereterminal OH groups lie atop ﬁvefold coordinated Ti atoms[8]. That only bridging OH species are observed pointsto water dissociation to ﬁll a bridging O vacancy with abridging OH and releases a H atom to form an additionalbridging OH. An autocorrelation analysis of the positionof OH species reveals no evidence of pairing, suggestinga certain mobility of the H atoms. This is in line with theisotope substitution measurements [4]. The behavior de-scribed above is summarized in the model shown in Fig. 4.Implicit in the description above is the absence of terminalOH groups which are invisible to STM. This is discountedon two grounds. First, the reasonable argument that ifbridging OH groups are STM visible then it is likely thatFIG. 4. Schematic diagram showing the mechanism deducedfrom our STM and photoemission measurements for the interac-tion of H2O with TiO2110 at 150 K and its thermally activateddissociation. “Temperature” increases from left to right in thediagram. The mechanism takes into account the predictions oftheoretical calculations which point to molecular adsorption onTi atoms [8]. Water initially adsorbs on Ti atoms with dissocia-tion at bridging O vacancies to form bridging hydroxyl species.Small spheres are Ti atoms, while large spheres are O atoms.266103-3 266103-3VOLUME 87, NUMBER 26 PHYSICAL REVIEW LETTERS 24D ECEMBER 2001terminal OH would also be visible. Second, the HREELSdata are consistent with only one type of OH (see below).If we now inspect the results of nonimaging techniquesin light of our STM work, we ﬁnd a number of featureswhich provide aconsistentpicturewiththe model inFig. 4.First, the TPD results [4], which have a desorption peak at270 K for molecular desorption and recombinative peak at490–540 K, are broadly in line with our expectations. Ourobservation of molecular adsorption at 150 K is in agree-ment with previous photoemission work [5] but not withHREELS [7]. However, the observation of a sharp singlenOH loss in HREELS spectra recorded from a surface ex-posed to H2O at 130 K and heated to 275 K [7] is consis-tent with the presence of only one type of OH species.In summary, we have used STM to follow the reactionof water with TiO2110-1 3 1, using photoemissionspectroscopy to identify the adsorbates. Molecular wateradsorbed at 150 K occupies sites on the Ti rows of thesubstrate. Theoretical studies indicate that water will sitatop the Ti atoms in the row. Heating an 0.7 ML H2Ooverlayer to 290 K results in the formation of 0.08 ML ofbridging OH, with no adsorbate species on the Ti rows ofthe substrate. On the basis of theoretical studies that in-dicate that dissociation on regular lattice sites will resultin both terminal and bridging OH, this result indicates thatwater dissociates at bridging-O vacancy sites.This work was funded by the U.K. Engineering andPhysical Sciences Research Council.[1] V.E. Henrich and P.A. Cox, The Surface Science ofMetal Oxides (Cambridge University Press, Cambridge,1994).[2] U. Diebold, in The Chemical Physics of Solid Surfaces,Oxide Surfaces, Vol. 9, edited by D.P. Woodruff (Elsevier,Amsterdam, 2001).[3] S. Suzuki, K. Fukui, H. Onishi, and Y. Iwasawa, Phys. Rev.Lett. 84, 2156 (2000).[4] M.A. Henderson, Langmuir 12, 5093 (1996); M.A.Henderson, S. Otero-Tapia, and M.E. Castro, FaradayDiscuss. 114, 313 (1999).[5] R.L. Kurtz, R. Stockbauer, T. E. Madey, E. Román, andJ.L. de Segovia, Surf. Sci. 218, 178 (1989).[6] D. Brinkley, M. Dietrich, T. Engel, P. Farrall, G. Gant-ner, A. Schafer, and A. Szuchmacher, Surf. Sci. 395, 292(1998).[7] M.A. Henderson, Surf. Sci. 355, 151 (1996).[8] P.J.D.Lindan, N.M. Harrison, and M.J. Gillan, Phys. Rev.Lett. 80, 762 (1998).[9] G. Charlton, P.B. Howes, C.L. Nicklin, P. Steadman,J.S.G. Taylor, C.A. Muryn, S.P. Harte, J. Mercer,R. McGrath, D. Norman, T.S. Turner, and G. Thornton,Phys. Rev. Lett. 78, 495 (1997).[10] P.W. Murray, N.G. Condon, and G. Thornton, Phys.Rev. B 51, 10989 (1995).[11] J.A. Connor, M. Considine, I.H. Hillier, and D. Briggs,J. Electron Spectrosc. Relat. Phenom. 12, 143 (1977).[12] D.W. Turner, C. Baker, A.D. Baker, and C.R.Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, New York, 1970).266103-4 266103-4

Imaging Water Dissociation on TiO₂(110)

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