Acetone Chemistry On Oxidized Rutile TiO2(110)

Patrick Murray,∗,† Yaobiao Xia,‡ Zhenrong Zhang,‡ Ken Park,‡ and Ke Zhu‡

CASPER NSF Fellow, Baylor University, and Department of Physics, Baylor University

E-mail: pcmurra1@asu.edu

AbstractSurface defects are known to dominate both theelectronic and chemical properties of metal oxidesurfaces, an object of much study for use in cat-alytic processes. In order to study surface chem-istry in detail, an ultra-high vacuum (UHV) cham-ber was designed to combine a LEED and STMsystem. Construction is underway, with a reportedinitial base pressure of 10−9 Torr revealing no ma-jor leaks. On a fully functioning system, we stud-ied the relationship between acetone and oxygenadatoms on rutile TiO2(110) using scanning tun-neling microscopy (STM). We report an acetone-assisted oxygen adatom diffusion mechanism. TheSTM images from the same area reveal that ace-tone molecules preferentially adsorb to oxygen va-cancy sites at room temperature. Sequences ofisothermal STM images directly show that diffu-sion of acetone over oxygen adatom sites causesapparent motion of the adatoms. We propose amechanism whereby the acetone diffuses along theTi5c row and exchanges oxygen with the adatom,creating an apparent motion of the defect.

IntroductionThe understanding of point defects is essential tothe understanding of the physics and chemistryon the surface of metal oxides. In heterogeneouscatalysis between gaseous reagents and a solid cat-alyst, the adsorption and subsequent dissociationof the molecules usually occurs at surface defect

∗To whom correspondence should be addressed†Arizona State University‡Baylor University

sites. Defects change the electron configurationof the surface, altering the reactivity of the sur-face.1–3 The diffusion of molecules on the surfaceis a key step in most surface mediated reactions.Oxygen is one of the main oxidizing reagents, asmany catalytic reactions rely on the equilibriumbetween the addition and removal of oxygen atomsattached to the surface.

TiO2 is an important metal oxide used incatalysis, solar cells, gas sensors, pigment, andcorrosion-protective coating, among other things.1

Due to its widespread use, it has been thoroughlystudied and become regarded as a general modelfor metal oxide surfaces. The rutile (110) surfaceof titania is the most stable and well character-ized surface, making it ideal for surface chemistryresearch. The effect of surface defects on theadsorption and following dissociation of variousadsorbates on this surface has been extensivelystudied.1,4,5 It is established that the prevalentsurface defect of the vacancy of a bridge-bondedoxygen site (VO) facilitates the adsorption andsubsequent dissociation of water and various alco-hols.3,6,7 More recently, it has been observed thatTi interstitials below the surface also play a role inthe surface chemistry of TiO2(110).8

This study examines the simplest ketone, ace-tone ((CH3)2CO), to study the effects ketones canplay on surface defects and how these defects canaffect the adsorption of the molecule. Acetone rep-resents not only a common reactant, intermediate,and product in many organic catalytic reactions,but is also commonly present as a contaminantin the air. The acetone interaction with surfacedefects has been studied before, with our group,Xia and co-workers, reporting on the acetone-mediated diffusion of bridging bonded oxygen va-

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cancy sites, VO. They report that adsorbed acetoneon vacancy sites react with adjacent Ob atoms,leading to the formation of a η2-diolate diffusionintermediate. The alkyl group can then move tothe next Ob atom via creation of a carbonyl bond.During this process, acetone loses its oxygen andheals the vacancy. This alkyl group can then pickup any Ob atom and migrate to the Ti5c row as aacetone molecule, taking its oxygen with it andcreating a new vacancy, leading to the diffusion ofthe original vacancy.9 The purpose of this study isto further research acetone’s interaction with pointdefects by focusing on its interaction with Oa onthe rutile TiO2(110) surface.

In order to obtain a molecular-level visual rep-resentation of surface reaction processes, we em-ployed the variable temperature scanning tunnel-ing microscope (STM) to study the surface reac-tion. Before using an STM to study surface sci-ence at the molecular level, however, the properenvironment must be created. Under normal con-ditions, there are about 2 ×1025 molecules/m3 inthe air, traveling with a mean velocity of around500 m/s. This causes numerous collisions of gasmolecules with the surface of a solid, leading toa high number of adsorbates and reactions on thesurface. To adequately compare theory to exper-iment, and to limit observations to those eventscaused by the surface’s specific chemistry, theseside reactions must be minimized. To this end, aUHV (ultra-high vacuum) system is typically em-ployed. UHV systems reach pressures below 1×10−9 Torr.

LEED/STM Chamber

Chamber Design and ConstructionIn order to obtain detailed information about boththe surface and structure of TiO2 and other materi-als, we designed and began construction on a UHVsystem to house a low energy electron diffraction(LEED) system and RHK STM together. Whilethe STM gains information only about the sur-face structure, the LEED provides insights belowthe surface, detecting the atomic structure throughcomparison to simulations. It was decided to sep-arate the two systems into a preparation cham-

ber and a STM chamber, as seen in Figure 1.The preparation chamber contains the load-lock,LEED, ion and electron guns. Care was takento position the LEED out of the line of Ar ionsused when sputtering. In this chamber, we canload the sample, then clean it through annealingor sputtering, before analyzing it for its cleanli-ness and structure with LEED. A bellows, yet to beinstalled, will connect the two chambers. As theRHK sample holder was not compatible with theone for the LEED, a system of movable arms wasdesigned to transfer the sample from the LEED tothe load lock holder, through the bellows to theSTM. The entire system rests on an air-lag table toisolate the STM from mechanical vibrations. TheLEED, ion gun, and other ports on the preparationchamber have been installed, and the ion and turbopumps have been connected.

Figure 1: Chamber on left is the preparation chamber, hous-ing the LEED and ion gun. Chamber on right is the STMchamber.

Chamber Testing and ResultsAfter fully connecting the preparation chamber,we surrounded the chamber and ion pump in heat-ing tape and Al foil for insulation and baked thechamber at 160◦C for one day. After degassingthe ion pump, the chamber was pumped for a day,reaching the mid 10−9 Torr range. This is in thehigh-vacuum range, and approaching that of ultra-high-vacuum. This indicates that the chamber isfree of any major leaks, and provides an optimisticview for the future of the chamber.

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Brief Explanation of STMThe basis behind a Scanning Tunneling Micro-scope (STM) is based on the concept of quantumtunneling. It consists of an atomically sharp tipover the surface to be studied. A bias is then in-duced between the sample and the tip, allowingelectrons to tunnel from the tip to the sample (pos-itive bias) or from the sample to the tip (negativebias).10 This tunneling current is approximated bythe following relation, where It is the tunnelingcurrent, E is the energy of the electrons, and κ

is a constant related to the applied bias. Z is thedistance from the sample to the tip.

It ∝ ρs(Z,E)e−2κZ (1)

Here, ρs(Z,E) refers to the local density of states,as tunneling requires an empty level on the sam-ple/tip of the same energy as the electron attempt-ing to tunnel. Due the the exponential dependenceof the tunneling current to position, very smallchanges in distance between the tip and samplecan be observed as measurable changes in tunnel-ing current, allowing the topography of the sur-face to be inferred from It . In reality, however,as the local density of states differs near differentelements and molecules, an STM scan of a non-homogeneous surface reveals a convolution of theelectronic structure and topography of the surface.

As the new system is not yet operational, the ex-periment and all images were taken on the systemdescribed in the following section.

Experimental SectionThe STM experiments were conducted in an UHVsystem (base pressure 2×10−11 Torr) equippedwith a variable-temperature STM (SPECS), a massspectrometer (SPECS), and ion gun (MKS). Thesingle-crystal rutile TiO2(110) surface (PrincetonScientific) was cleaned by repeated cycles of Arion sputtering at 2 keV and UHV annealing from800-900 K, with the sample’s cleanliness deter-mined by STM imaging. Special care was takento minimize background water contamination, in-cluding baking the chamber to 160◦C. At the startof each experiment, the sample was flashed to600 K to evaporate off water adsorbed from back-

Figure 2: Ball-and-stick model of the (110) surface of rutileTiO2. Titanium (blue) atoms are 6-fold-coordinated in thebulk, with surface atoms having 5-fold coordination. Oxy-gen (red) atoms possess 3-fold coordination in general, withbridge-bonded oxygen (Ob) having 2-fold coordination.

ground molecules. The initial stages of oxygenadsorption were studied by introducing oxygenthrough backfilling, with experiments carried outat 300 K and 100 K. We analyzed the same areasof the surface before and after oxygen exposureto monitor changes in point defects caused by ad-sorption of individual molecules. After exposureto oxygen, acetone was allowed to adsorb, with thesame surface area being analyzed before and afteradsorption in order to monitor how defect concen-tration and configuration affects acetone adsorp-tion. Acetone was introduced using a directionaldoser which releases acetone 3 mm from the sam-ple. The STM tip was etched tungsten and pre-pared by Ar sputtering. STM images presentedwere collected in constant current (.1 nA) modeat positive sample bias voltage of 0.9 - 1.5 V.

Results and DiscussionFigure 3 shows a typical STM image of aTiO2(110) surface at 300 K after exposure tooxygen. In these images, the low, five-fold-coordinated Ti4+ ions are imaged as high hills,while the rows of bridging oxygen ions are imagedas valleys, in spite of the physical topography ofthe surface, in which the Ob rows are 0.1 nm closerto the tip, as depicted in Figure 2. This is due to thelocal density of states around each type of atom.The conduction band of the Ti atoms allows for alarger number of unoccupied states for electrons totunnel into, whereas the oxygen atoms’ 2p charac-ter valence states are very limited. In this case, theelectronic configuration plays a larger role than

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Figure 3: STM image of reduced TiO2(110) surface afterexposure to 1×10−8 Torr of O2 for 1 min at 300 K. Some ofthe oxygen vacancies (VO), single oxygen adatoms (Oa) andpaired oxygen adatoms (Oa −Oa) are shown. An example ofa bridging hydroxyl (OHb) and terminal hydroxyl (OHt) arealso marked.

the topographic configuration in the appearanceof STM images.11 Bridging oxygen vacancy sitesappear as bright spots on the dark oxygen rows, asmarked in Figure 3. Oxygen adatoms are revealedas slightly brighter dots on the Ti rows surroundedby a slightly darker region. These features ap-pear both in pairs and singletons depending on thechannel of adsorption.12,13 Hydroxyl moleculesare also sometimes present due to the dissocia-tion of background water molecules on the surfaceat room temperature.3 Both varieties, the termi-nal (OHt) and bridging (OHb) types are shownso as to differentiate them from adsorbed acetonemolecules.

Upon dosing acetone on the oxidized surface,we observed bright protrusions centered on the Obrow, as seen in Figure 4b. We assign these fea-tures to be acetone molecules, as supported bydensity functional theory (DFT) calculations re-ported by Xia and co-workers.9 Figure 4(a)-(b)demonstrates how acetone was observed to pref-erentially adsorb at vacancy sites. This is contraryto the lack of a temperature programmed desorp-tion (TPD) peak for such a state, reported by Hen-derson5 implying that there is no preference foracetone adsorption at such sites, but in agreementwith the recent findings of Xia and co-workers.9

We observed the acetone species to be mobile at300 K. Both diffusion along the row and hop-ping across rows were observed from sequences of

Figure 4: STM image of same area before and after acetonedosing. Dotted circles on left indicate vacancy sites whichact as active sites for adsorption of acetone, seen in right im-age. The bright features prevalent on the right are adsorbedacetone molecules.

isothermal images of the same area for periods oftime up to 45 min. Interestingly, when an acetonemolecule diffuses along the row past where a Oais, the Oa is observed to shift as well, by one lat-tice constant opposite to the motion of the acetonemolecule. For every observed diffusion of an Oa,an acetone molecule was observed to diffuse pastit. Figure 5(a-c) presents a sequence of imagesshowing one such event. The acetone molecule(green dot) moves along the row, leaving a va-cancy behind and causes a shift in the Oa position.From (b)-(c), it then diffuses back, ending in a va-cancy near its original position, resulting in a shiftof the Oa back to its original position. Water as-sisted Oa “pseudodiffusion” has been reported pre-viously,14 but Oa diffusion has never before beenobserved from acetone, as surface point defects aremostly stationary at 300 K. Analysis of the entire18 minute movie from which Figure 5 was takenreveals 107 adatoms on the approximately 20 nm× 20 nm imaged surface. In this area, there were45 acetone molecules, 20 of which were in mo-tion. There were a relatively high vacancy cover-age, with over a hundred VO sites observed withno bonded acetone. Statistically, 9% of the Oawerein motion, and for 70% of those in motion,an acetone molecule was observed to move acrossthe Oa site simultaneously with the Oa diffusion.Xia and co-workers describe two channels for ace-tone diffusion on rutile TiO2(110), as described inthe introduction. When acetone diffuses along theOb row, it leaves its oxygen behind, healing thevacancy it sat on. On the other hand, when ace-tone diffuses along the Ti5c row, it remains bonded

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Figure 5: Acetone molecule labeled with the dot diffusesalong Ti5c row from (a) to (c). White lines are on sameposition in all images to show movement. The motion ofthe adatom by one lattice constant toward the origin of theacetone is easily observable between (a) and (b). Whenthe molecule returns to its initial position in frame (c), theadatom again moves toward where the acetone was. Thelower right frame (d) illustrates our proposed method for thisdiffusion. The red circles are the Ti5c row atoms, the greenan Oa, and the blue the oxygen from the acetone.

to an oxygen, leaving a vacancy behind.9 As thevacancy is observed to be left behind by the ace-tone in these Oa diffusion situations, we can in-fer that the acetone moves along the Ti5c row. Wepropose a mechanism for acetone assisted Oa dif-fusion similar to that proposed for H2O by Duand co-workers.14 Figure 5d illustrates this mech-anism. Acetone binds at Ti4+ sites via the lonepair on the oxygen atom becoming a η1-acetonespecies. Acetone can then diffuse on the Ti5c rowwith a small diffusion barrier (~0.3eV9). When itreaches the Oa they interact, creating a temporaryO-C-O bond as an acetone diolate species, whichthen reverts to a C = O bond on one side. The ace-tone molecule then continues to diffuse, only nowbonded to the oxygen that used to be the Oa, leav-ing behind its oxygen atom as a new Oa, causingthe apparent motion of the Oa on the surface.

ConclusionsWe report that acetone molecules preferentiallyadsorb at VO sites on oxidized TiO2(110) surface,despite the lack of a TPD peak for this state re-ported by other groups.5 From here, acetone is freeto diffuse at 300 K, demonstrating both along-rowand across-row diffusion. Sequences of isothermalSTM images from the same area reveal that uponacetone adsorption at room temperature, oxygenadatoms begin to move on the surface. The STMmovies directly show a adsorbate-mediated oxy-gen adatom diffusion mechanism. Acetone diffus-ing along the Ti5c row reacts with the adsorbedoxygen, bonding to it and continuing to diffuse,leaving its original oxygen behind, one lattice con-stant from where the Oa was found.

Acknowledgement The author thanks Dr. Tru-ell Hyde, Dr. Lorin Matthews, and everyone atBaylor University and CASPER who worked tomake the Research Experience for Undergraduatesprogram the success it is. The author also thanksNSF for funding this research through grant no.PHY-1002637

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