Films: Growth and Characterization - American Chemical Society

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J. Phys. Chem. C 2007, 111, 4765-4773

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Reactive Ballistic Deposition of Porous TiO2 Films: Growth and Characterization David W. Flaherty,† Zdenek Dohna´ lek,*,‡ Alice Dohna´ lkova´ ,§ Bruce W. Arey,§ David E. McCready,§ Nachimuthu Ponnusamy,§ C. Buddie Mullins,† and Bruce D. Kay*,‡ UniVersity of Texas at Austin, Department of Chemical Engineering, Texas, Materials Institute, 1 UniVersity Station CO400, Austin, Texas 78712-0231, Pacific Northwest National Laboratory, Fundamental Sciences Directorate, Chemical and Material Sciences DiVision, P.O. Box 999, K8-88, Richland, Washington 99352, and Pacific Northwest National Laboratory, EnVironmental Molecular Sciences Laboratory, P.O. Box 999, K8-88, Richland, Washington 99352 ReceiVed: NoVember 17, 2006; In Final Form: January 18, 2007

Nanoporous, high-surface area films of TiO2 are synthesized by reactive ballistic deposition of titanium metal in an oxygen ambient. Auger electron spectroscopy (AES) is used to investigate the stoichiometric dependence of the films on growth conditions (surface temperature and partial pressure of oxygen). Scanning and transmission electron microscopies show that the films consist of arrays of separated filaments. The surface area and the distribution of binding site energies of the films are measured as functions of growth temperature, deposition angle, and annealing conditions using temperature programmed desorption (TPD) of N2. TiO2 films deposited at 50 K at 70° from substrate normal display the greatest specific surface area of ∼100 m2/g. In addition, the films retain greater than 70% of their original surface area after annealing to 600 K. The combination of high surface area and thermal stability suggests that these films could serve as supports for applications in heterogeneous catalysis.

I. Introduction and Background As a material titania (TiO2) has attracted much attention due to its wide range of applications. Titania is implemented in heterogeneous catalysis, gas sensors, photocatalysis, photolysis of water, optical coatings, and pigments. Correspondingly, it is the most studied metal-oxide in surface science.1 Recently, there has been great interest in the growth and chemical characterization of metal-oxide-supported metal particles and thin films as model systems for industrial catalytic systems.2 One particular system of interest has been TiO2-supported gold nanoclusters and thin films which are well documented as being highly active for CO oxidation.3-10 An important step to furthering these investigations has been the growth of thin, crystalline TiO2 films on conductive metal substrates to create model catalysts, increase the number of compatible spectroscopic techniques, and improve temperature control. To this end, dense, highly ordered titania thin films of the two most technologically relevant phases of TiO2 (highly stable rutile and metastable anatase) have been grown on a variety of substrates. For some time now, rutile TiO2 thin films have been synthesized on polycrystalline Pt,11 Pt(111),12,13 Ni(110),14 W(100),15 Mo(100),16 Mo(110),17 Mo(112),18 Cu(100),19 and TiO2(110),20 while more recently, anatase films have been deposited onto SrTiO3(001).21,22 The study of crystalline titania smooth surfaces has proven invaluable for elucidating the fundamental chemistry of TiO2 surfaces and the catalytic behavior of metal particle/film decorated TiO2(110).3-10 However, in practical heterogeneous catalytic systems, titania is implemented as a high surface area * Corresponding authors. E-mail: [email protected], Bruce.Kay@ pnl.gov. † University of Texas at Austin. ‡ Pacific Northwest National Laboratory, Fundamental Sciences Directorate. § Pacific Northwest National Laboratory, Environmental Molecular Sciences Laboratory.

support such as porous particles and pellets or fine powders. Since the structure of the titania support (both crystallographic and morphological) will affect its chemical behavior, there is a clear lack of connection between the smooth, terraced surfaces commonly used for surface science studies and those used in high surface area catalytic studies. This material gap demonstrates the need for a technique to produce TiO2 media which more closely resemble those utilized for industrial catalysis, while remaining compatible with ultrahigh vacuum techniques such as thermal desorption and electron-based spectroscopies. The growth of metal-oxide thin films has been partially motivated by a desire to study fundamental chemistry on these surfaces. Because many metals form a native oxide in atmospheric conditions, the behavior of metal-oxide surfaces is of great interest. However, a large number of metal-oxides, particularly those of greater technological relevance (MgO, SiO2, Al2O3, etc.), are electrical insulators which complicates characterization with electron-based spectroscopies. The solution has been to deposit high-purity, highly oriented, crystalline metaloxide thin films on conducting substrates.23 One approach for growing such films is to dose the necessary components with molecular beams onto the surface of a heated crystal, a technique referred to as molecular beam epitaxy (MBE).23 By heating the substrate during deposition, high-quality crystalline thin films with highly ordered, low-energy surfaces can be deposited. However, if the surface is cooled to low temperatures, surface diffusion can be strongly limited and nonequilibrium surfaces can be generated. This limited mobility is a key factor for a film growth technique frequently called ballistic deposition (BD).24 Ballistic deposition refers to a growth scheme in which deposited particles travel from randomly positioned origins, along straight-line trajectories to the surface, where the particles are incorporated in close proximity to their original landing

10.1021/jp067641m CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007

4766 J. Phys. Chem. C, Vol. 111, No. 12, 2007 site.24 Initial surface “roughening” occurs due to the inherently random nature of the deposition process, and the subsequent development of the film is highly dependent on the deposition angle. At oblique deposition angles, topographically elevated points, created randomly, preferentially intercept the incoming flux while shadowing lower regions from the incoming adatoms (see Abelmann and Lodder for more discussion).25 This “selfshadowing” growth process results in porous, columnar films. Brett et al. have grown highly sculptured films consisting of columns, zig-zags, and helices using a similar process termed glancing angle deposition (GLAD).26 Capabilities of BD can be further extended by directionally depositing metal in a reactive background gas (O2). In this way metal-oxide thin films can be grown through the surface reaction of the oxidizing specie and the metal adatoms. This technique is called reactive ballistic deposition (RBD).27 Using RBD or GLAD, highly structured films have been grown from a number of materials including Pd,28 Cr,29 Cu,29 Fe,30 Ti,31 TiO2,32 Pt,33 Mn,34 MgO,27 Ta2O5,35 WO3,35 SiOx,36,37 MgF2,34 CaF2,34 and amorphous solid water (ASW).38-40 The resulting films often have optical, electronic, and magnetic properties which differ from dense, nonstructured films of similar chemical composition. In addition, the films demonstrate unique chemi-physical surface properties; however, surface characterization studies have only been conducted for porous films of Pd,28 MgO,27 and ASW.38-40 Structural properties and chemical activity for TiO2 thin films deposited by processes similar to RBD or GLAD have been surveyed. Brett and co-workers studied the crystallographic and morphological changes induced by annealing in columnar films of TiO2.32 Suzuki et al. studied the aqueous-phase photobleaching of methylene blue over sculptured thin films of TiO2 and annealing of the films.41,42 Weinberger and Garber investigated the oxidation of C2H4 and C2Cl3H under 350 nm illumination in the presence of porous TiO2 films produced from reactive magnetron sputtering of a titanium target with an O2/Ar mixture.43 In short, these studies suggest that the structured TiO2 films have enhanced photocatalytic activity and may be thermally robust. However, no surface science investigations exist for RBD/GLAD type TiO2 films. In this study, we explore high surface area, porous TiO2 thin films grown employing the RBD technique and characterize their chemi-physical properties using a combination of Auger electron spectroscopy, electron microscopy, X-ray diffraction, and temperature programmed desorption (TPD) of N2. These films are of interest from the aspect of fundamental chemistry as well as for the study of surfaces that better emulate those employed in industrial catalytic systems. II. Experimental Methods All films were grown in an ultrahigh vacuum (UHV) apparatus with a base pressure of ∼1 × 10-10 Torr. The chamber has been described in detail28 and has capabilities for Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), quadrupole mass spectrometry (QMS), physical vapor deposition via a high-temperature Knudsen cell (CreaTec), and deposition rate measurement by quartz crystal microbalance (QCM, Inficon). In addition, the chamber contains nozzles and apertures for three separate molecular beams which are used in conjunction with the QMS for TPD. A polished polycrystalline tantalum plate (10 × 10 × 1 mm3) is spot-welded to a 1 mm tantalum wire loop and mounted on a closed cycle helium cryostat. The sample temperature is measured with a type-K (chromel-alumel) thermocouple spot-welded to the backside of the Ta plate and controlled by resistive heating over the range

Flaherty et al. 22-1400 K. The absolute temperature ((2 K) is calibrated using known multilayer desorption temperatures of adsorbates (N2 ≈ 28 K and H2O ≈165 K). Titania films are deposited by evaporating 2 mm titanium metal wire (Aldrich, 99.9+ %) from a graphite crucible using the high-temperature Knudsen cell in an oxygen ambient. AES confirms that the deposited films contain only titanium and oxygen. The Knudsen cell is mounted pointing upward such that the minimum film deposition angle is 48° (all deposition angles are in reference from substrate normal). Rotation of the sample around the vertical axis allows the overall deposition angle to vary from 48°-90°. TiO2 is deposited at a rate of 3-4 ML/min (1 ML TiO2 ≈ 1.0 × 1015 Ti atoms/cm2 as calculated from the bulk density), measured by the QCM. The stoichiometry of the deposited films is found to be dependent on the oxygen pressure and sample temperature during film growth, and these relationships are determined with AES and the QCM (see Results and Discussion) allowing for the reproducible deposition of films with known composition. For the deposition rates used in this study, an oxygen background pressure of 1.5 × 10-7 Torr is sufficient to deposit fully oxidized films. Prior to each deposition, the substrate is annealed to 1400 K to provide a relatively smooth surface for film growth by completely densifying the previously grown TiO2 film. Following deposition, each film is preannealed to 200 K to drive off any gases (O2, CO) which may adsorb to the sample and the cryostat during growth. TPD of N2 (over the temperature range 24200 K) from annealed surfaces yields highly reproducible spectra, regardless of the number of previous depositions, indicating that surface roughness is not increasing significantly with multiple depositions. Following deposition, the surface area and binding site energy distribution of the porous films are determined using in situ TPD of N2. The N2 gas (from liquid N2 boil-off) is dosed normal to the sample with a triply differentially pumped, quasi-effusive, 300 K molecular beam. For all N2 desorption experiments, the sample is linearly heated at 0.6 K/s as the N2 desorption rate is monitored with a quadrupole mass spectrometer (UTI). To improve the signal-to-noise ratio, the mass spectrometer is fitted with an integrating cup directed toward the sample surface from a distance of ∼5 cm. Selected TiO2 films are analyzed ex situ with scanning electron microscopy (SEM), transmission electron microscopy (TEM) accompanied by selected area diffraction (SAD), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). III. Results and Discussion Film Composition. The effectiveness of the RBD technique in growing porous materials relies on limited surface diffusion of adatoms during growth, so it is advantageous to deposit the films at the lowest temperature possible. However, temperatures that are too low may kinetically limit the reaction of surface species (e.g., oxidation of titanium). Several groups have reported on the growth of TiO2 thin films in UHV11-20,41-44 by the direct evaporation of Ti or TiO2 with either simultaneous surface oxidation (by an oxygen ambient) or postdeposition oxidation. In general, the procedures used to achieve stoichiometric TiO2 rely on elevated surface temperatures during film growth followed by high temperature annealing, but these elevated temperatures must be avoided in order to grow porous films. To address this issue we investigated the composition of the RBD-grown films as a function of deposition temperature and O2 pressure using AES and QCM as well as ex situ XPS. In each experiment 15 ML of metallic titanium were deposited at a rate of 3 ML/min at 48° (the minimum deposition angle

Growth and Characterization of Porous TiO2 Films

Figure 1. (A) Ratio of OKLL/TiLMM AES peak-to-peak intensities for TiOx films deposited at a rate of 3 ML/min by thermal evaporation of Ti onto Ta as a function of oxygen background pressure and substrate temperature. The absolute films stoichiometry from AES was calibrated using the AES OKLL/TiLMM ratio from TiO2(110) single crystal (dashed line). (B) TiOx film stoichiometry as a function of oxygen background pressure for titanium deposited onto the quartz crystal microbalance at ∼300 K. Stoichiometry is determined by the difference in mass deposition rate between pure Ti deposition without oxygen and after oxygen is leaked in at the indicated pressure. Titanium metal was deposited at 1.8 ML/min (9) and 3.8 ML/min (O). The solid lines are provided to guide the eye.

possible) over a range of substrate temperatures (100, 300, 400, and 600 K) in an oxygen background varying from 4 × 10-9 Torr to 2 × 10-6 Torr. To ensure that the AES signal originates only from the most recently deposited film (and not the underlying layers), each sample deposition was 15 ML thick, well above the escape depth for Auger electrons from the substrate.45 The stoichiometry of each film was quantified by the direct comparison of peak intensities at 383 and 508 eV, corresponding to the TiLMM and OKLL AES transitions, respectively.46 An AES spectrum taken from a clean, commercially made TiO2(110) crystal was used as a standard to calibrate the stoichiometry. The TiO2(110) crystal displayed AES spectra with an oxygen to titanium peak intensity ratio of 1.6:1 which corresponds with nearly stoichiometric TiO2. Film stoichiometry for each sample as calculated from AES spectra, as well as the ratio of peak-to-peak intensities for the deposited films, shown in Figure 1A, reveal that the degree of titanium oxidation is largely independent of the substrate temperature, over the temperature range investigated. In addition, we have grown thin films of titania by vapor deposition of Ti atoms into a stoichiometric excess of O2 multilayers adsorbed on the sample surface at 20 K. After deposition, the sample was slowly heated to 300 K desorbing unreacted oxygen and leaving what we

J. Phys. Chem. C, Vol. 111, No. 12, 2007 4767 believe is fully oxidized titanium. Films grown in this manner show Auger spectra nearly identical to that of TiO2(110). The QCM was also employed to determine stoichiometry of the deposited films. To do so, titanium metal was deposited over a similar range of oxygen background pressures onto the QCM, assumed to be at ∼300 K. By establishing a baseline deposition rate of metallic titanium followed by the introduction of oxygen through a leak valve, the difference in the two mass deposition rates can be related to the amount of oxygen contained within the film. The QCM data was taken with titanium deposition rates comparable to those used for the film growths and the resulting film stoichiometry is plotted with respect to the ambient oxygen pressure in Figure 1B. The apparent stoichiometry acquired by AES and QCM agree quite well. Both sets of data have a sigmoidal shape and plateau at a partial oxygen pressure of ∼ 9 × 10-8 Torr indicating that films grown at this or greater oxygen pressures are fully oxidized. At this pressure, O2 molecules strike the film surface at a rate of 5.4 × 1015 O2 molecules cm-2 min-1 in comparison to a titanium deposition rate of 3 × 1015 Ti atoms cm-2 min-1. For these fluxes, the amount of oxygen required to fully oxidize each film is approximately twice the stoichiometrically required amount. Selected films deposited employing oxygen pressures of ∼1.5 × 10-7 Torr at 100 and 300 K were examined with XPS after exposure to ambient conditions. The resulting XPS spectra were compared to the spectra from a clean, commercially made TiO2(110) crystal. The 2p XPS spectra from the RBD TiO2 samples displayed a symmetric XPS peak with a binding energy of 458.9 eV. Based on comparison with the spectra from the TiO2(110) we assign this peak to Ti4+ 2p3/2. The absence of a shoulder on the lower binding energy side of the Ti4+ 2p3/2 peak indicates that all titanium was in the Ti4+ oxidation state. Therefore, we believe that under the fully oxidizing deposition conditions employed here, all the films described below consist of stoichiometric TiO2. Ex Situ Film Structure Analysis. Panels A (side view) and B (top view) of Figure 2 show SEM images for a TiO2 film ∼750 ML thick deposited in an oxygen background of 2 × 10-7 Torr at 85° from substrate normal and a sample temperature of 300 K. The sample was removed from the molecular beam apparatus and thus exposed to ambient air prior to imaging in the SEM. The images confirm that the film consists of an array of separated filaments, which grow toward the physical vapor deposition source. While it is difficult to determine the exact growth angle of the filaments with respect to the surface normal, it is clear that the growth angle is significantly less than the incident vapor deposition angle of 85°. Figure 2C shows a film deposited at 85° and 100 K which appears very similar to the film grown at 300 K. The images indicate that an increase in temperature from 100 to 300 K does not increase the mobility of adatoms enough to affect the morphology of the film. Figure 2D shows a film deposited at 70° and 300 K. Decreasing the deposition angle from 85° to 70° has a dramatic effect on film structure, as seen in Figure 2D. On this length scale, a film deposited at 70° appears dense, but films deposited at this angle actually prove to have the greatest surface area, as shown by N2 desorption later in this paper. Transmission electron microscopy (TEM) was performed on filaments scraped from the tantalum plate and placed on amorphous lacey carbon grids. The filaments grown at both 100 and 300 K reveal nanoscale features within the filaments when viewed under high magnification (see Figure 3A). Selected area diffraction (SAD) of individual filaments indicates that the

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Figure 2. SEM images of TiO2 films deposited Via RBD. (A) Top and (B) side views of a film grown at 300 K and at a deposition angle of 85°. (C) Side view of a film grown at 100 K and 85°, (similar in appearance to the film grown at 300 K). (D) Side view of a film grown at 300 K and at 70°.

Figure 3. TEM images of TiO2 films accompanied by insets with selected area diffraction (SAD). (A) A single, amorphous filament from a film deposited at 85° and 300 K. (B) A cluster of filaments from the same film as (A) displaying a SAD pattern corresponding to polycrystalline rutile TiO2. (C) A portion of a film grown at 70° and 300 K with a high degree of crystallinity.

filaments are predominantly amorphous; however, in some cases randomly oriented groups of filaments display the well-defined diffraction patterns seen in the insets of Figure 3B,C. D-spacing values extracted from these SAD patterns are indicative of the rutile phase. To further assess the crystallinity of these films, we conducted XRD, in both glancing incidence and 2θ modes, on intact films as well as on the powder produced by scraping the filaments from the substrate. All XRD scans indicate only the presence of amorphous TiO2. Therefore, we conclude that the dominant portion of the film is amorphous. This finding is in agreement with the result from a previous study of porous TiO2 films grown by the direct evaporation of TiO2 at oblique angles, where the as-deposited filaments were amorphous prior to annealing at 773 K. The 773 K annealing resulted in the formation of the metastable anatase phase as demonstrated by SAD and XRD.32

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Figure 4. N2 TPD spectra from a 75 ML thick, porous TiO2 deposited at 70° and 100 K and annealed to 200 K. Various initial N2 coverages were used and are shown in relation to saturation coverage at 28.5 K, corresponding to a single layer of adsorbed N2. For each spectrum, N2 was dosed at 24 K, and the sample was heated at a linear rate of 0.6 K/s from 20 to 200 K.

Temperature-Programmed Desorption of N2. Characteristics of N2 Physisorption from TiO2 Films. Figure 4 displays a series of N2 TPD spectra from a porous RBD-grown TiO2 sample (75 ML of TiO2 deposited at 100 K, 70°, and preannealed to 200 K) prepared by impinging a beam of N2 normal to the surface (it was determined that variations in the dosing angle have no effect on TPD results) onto the sample at a surface temperature of 24 K. At 24 K, the amount of adsorbed N2 is proportional to the dose time, because N2 is able to form multiple adsorbate layers. However, at 28.5 K multilayers of N2 do not form on flat surfaces with the employed N2 flux. Therefore, only a single layer of N2 can adsorb on the films. The desorption spectrum from the sample co-incident with saturation coverage at 28.5 K (i. e. a single layer of adsorbed N2) is indicated in Figure 4 by a dashed line. This data set reveals several important properties of the film. First, the majority (∼ 75%) of N2 desorption within the first adsorbate layer (all desorption above 28.5 K) occurs over a broad temperature range from 35-100 K, and with increasing N2 exposure the spectra fill in from the high-temperature end due to preferential N2 adsorption on the unoccupied surface sites with the highest binding energy. Thus, the TPD spectra indicate that transport of physisorbed N2 within the film is rapid, even at 24 K, allowing for fast diffusion of molecules throughout the film. Second, the N2 desorption spectra contain distinct peaks at ∼28 K and at ∼33 K. The lower temperature peak, ∼28 K, exhibits zero-order desorption kinetics and is assigned to the formation of a second layer of adsorbed N2. A feature similar to that at ∼33 K has been observed in our prior studies of other nanoporous films of amorphous solid water (ASW),38-40 MgO,27 and Pd28 and has been attributed to the unique structure of the BD films, specifically desorption of N2 condensed within nanometer-size pores in the films, predicted by the Kelvin equation.47 Aside from the pore condensation feature at 33 K, the distribution of binding energies for N2 on the porous TiO2 surface differs from those on a flat TiO2(110) single crystal.48 Figure 5 shows two N2 TPD spectra for a single layer of N2 adsorbed (i) on TiO2(110) (acquired by Dohna´lek et al.48) and (ii) on a porous TiO2 film (from this investigation). For better

Growth and Characterization of Porous TiO2 Films

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Figure 5. TPD spectra of N2 from porous TiO2 (solid line) and from TiO2(110) (dashed line).48 The spectrum from the porous TiO2 film was taken after saturating the sample with N2 at 28.5 K. The spectrum from TiO2(110) corresponds to a N2 saturation coverage (1 N2 per surface Ti4+ + 1/2 N2 per surface bridge-bonded oxygen). The spectra are normalized by their corresponding integrals to allow for easier comparison.

Figure 6. N2 TPD spectra after saturating 75 ML TiO2 films at 28.5 K. The films were deposited at 100 K and at various angles from substrate normal (48°, 60°, 70°, 85°) and annealed to 200 K prior N2 adsorption. N2 TPD for densified TiOx samples (Figure 6(v)) are obtained after annealing the film at 1400 K for 2 min. The spectra for densified samples are independent of the original deposition angle and temperature.

comparison, the spectra are scaled to one another by normalizing the TPD integral. Both spectra display the greatest desorption rates at the low-temperature range of the spectra, followed by a long desorption tail. For the TiO2(110) surface, the broad feature at ∼90 K is associated with adsorption of N2 molecules on exposed Ti4+ sites. Strong repulsive interactions between N2 molecules on neighboring sites results in the activation energy for desorption being highly dependent on coverage, thus this feature covers a wide temperature range. Because the porous TiO2 film has a notably smaller portion of binding sites at temperatures greater than 70 K, the porous film may have significantly fewer exposed cationic sites which are available for N2 adsorption. The fact that the porous film shows an overall lower distribution of binding energies is somewhat surprising, considering that the porous films should have increased concentrations of high-energy, defect sites (e.g., step-edges, kink sites, vacancies) in comparison to the TiO2(110). Such defects are expected to bind N2 more tightly than the highly coordinated terrace sites, as observed in our prior studies of N2 adsorption on other nanoporous thin films.27,28 Beyond illustrating the energy distribution of binding sites, N2 physisorption can also be used to investigate other aspects of the film structure resulting from different growth (deposition angle and temperature and film thickness) and post-growth (annealing treatment) parameters. Since N2 multilayers desorb at ∼28 K, their formation can be prevented by holding the sample above this temperature which still allows for condensation within the pores as well as surface saturation. The integral of TPD spectra acquired in this manner (limited to single layer adsorption) corresponds directly to the amount of N2 adsorbed within the film. Then, by comparing the amounts of N2 adsorbed in porous films to the amount of N2 corresponding to monolayer adsorption on a dense, planar sample the surface area of the porous materials can be calculated. Throughout this study, N2 uptake in porous films is quantified as multiples of the amount of N2 contained in a monolayer, on a planar surface. To summarize, the comparison of the integrated desorption spectra and the desorption spectra line shapes provide a quantitative

measure of surface area (analogous to BET isotherm) and a qualitative description of the films’ morphology. Deposition Angle Dependent Morphology of Films. The structure of films grown by RBD is inherently sensitive to the particular angle of deposition.28,38-40 Figure 6 displays N2 TPD spectra taken from porous TiO2 films grown by depositing the equivalent of 75 ML of dense film at various deposition angles at a substrate temperature of 100 K. The spectra are taken after each film is saturated with N2 at 28.5 K. The N2 TPD spectrum of a dense TiOx (x < 2) film, Figure 6(v), is included as a basis of comparison to illustrate the enhanced surface area of the asdeposited films. Dense TiOx films are prepared by annealing a particular TiO2 sample at 1400 K for 2 min. The annealing treatment results in a surface comparable in area to smooth TiO2(110) as confirmed by N2 TPD. The films annealed in this manner show reproducible N2 TPD spectra, independent of prior deposition conditions (i.e., deposition angle and temperature). The TPD spectra from the porous TiO2 films (Figure 6(i)-(iv)) differ noticeably from those for dense, 1400 K annealed TiOx films (Figure 6(v)) and TiO2(110) (dashed line, Figure 5). The N2 desorption spectra from all porous TiO2 films are relatively similar containing three distinct regions: the low-temperature end of the spectra is dominated by a single peak at ∼33 K; followed by a region with a relatively constant desorption rate (40-50 K); and a final region of monotonically decreasing desorption which vanishes at ∼100 K. As mentioned above, the low-temperature peak (∼33 K) is associated with condensation of N2 within nanometer-size pores.27,28,39 Integration of the N2 desorption spectrum for each deposition angle provides a quantitative measure of the total N2 uptake. Figure 7 displays the amount of adsorbed N2 for 75 ML TiO2 films grown at 100 K and varying deposition angles as a multiple of monolayer adsorption on the dense film, Figure 6(v). The total amount of adsorbed N2 increases with increasing deposition angle, peaking at ∼70°, above which the N2 uptake decreases by ∼25%. In contrast, earlier work on Pd films showed a monotonic increase of N2 uptake with increasing film deposition angle over the range of 45°-85°.28 ASW films, on the other hand, also displayed a maximum in the N2 uptake at 70°.38,39 The origin of these

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Figure 7. The amount of N2 adsorbed on porous TiO2 films grown at 100 K as a function of the titanium deposition angle. The amount adsorbed is expressed in terms of monolayer equivalents, where 1 ML is the amount of N2 desorbed from a dense TiOx film saturated at 28.5 K.

respective maxima is not fully understood. However, ballistic deposition simulations suggest that as the deposition angle tends toward more oblique angles the resulting distribution of pore diameters will show a greater number of large pores.49 Then following the Kelvin equation, an increase in the average pore radii will lead to a decrease in the fraction of pores which condense N2 multilayers. Effect of Annealing on Film Structure. Porous metal-oxide films, such as the TiO2 films described in this paper, have potential applications in a number of heterogeneous catalyst systems, many of which employ elevated temperatures. Therefore, information concerning their thermal stability is particularly relevant. To investigate the thermal stability of nanoporous TiO2 films prepared via RBD, we conducted a series of N2 TPD experiments on films which were annealed incrementally from 200 to 1300 K, and densified completely at 1400 K. Figure 8A displays a representative set of N2 desorption spectra from a single porous TiO2 film grown at 100 K and 70° which was annealed to higher temperatures in 100 K increments. As Figure 8A illustrates, annealing of the film to subsequently higher temperatures gradually decreases the surface area and changes the line shape of N2 desorption. The low-temperature region (30 - 40 K) of the TPD spectra originally displays a single peak which evolves to form two separate peaks by 800 K, Figure 8A(iv). Further annealing to 900 K, Figure 8A(v), causes the low-temperature features to disappear, indicating the collapse of the pore network. Figure 8B emphasizes relative changes in the high-temperature tail of the N2 TPD spectra; therefore, in Figure 8B the spectra have been normalized by setting their integrals equal so that the relative number of high-energy binding sites can be easily compared. Figure 8B demonstrates that the high-temperature tail of the TPD spectra shifts from 100 K (Figure 8B(i)) to 160 K (Figure 8B(vi)) after annealing to 1400 K. Recall that N2 molecules are expected to bind most tightly to Ti4+ sites,48,50,51 so that an increase in the number of accessible Ti4+ adsorption sites on the surface is expected to lead to a corresponding fractional increase in high-energy binding site coverage. Therefore, we note that the extension of desorption to higher temperatures is evidence of adsorption on an increasing number of cationic sites on the films. High temperature

Figure 8. (A) N2 TPD spectra after saturating a 75 ML TiO2 film deposited at 100 K and 70° with N2 at 28.5 K, after annealing the film to indicated temperatures (note that TPD spectra for TiO2(110) and for the 1400 K annealed TiOx sample have been enlarged by a factor of 3). (B) High-temperature region of normalized N2 TPD spectra for films shown in Figure 8A, illustrating the development of higher energy binding sites produced by annealing. The spectra in Figure 8B are normalized by their corresponding integrals to allow for better comparison.

annealing (TAnneal g 900 K) is known to create oxygen vacancies on the TiO2(110) surface which would lead to an increase in the number of available cationic sites for adsorption, and this could explain changes in TPD line shape for samples annealed in this temperature range. However, significant reduction of the film should not occur for samples annealed at much lower temperatures. Therefore, we believe that the appearance of additional cationic sites on films annealed to 400 K (Figure 8B(ii)) and 600 K (Figure 8B(iii)) is not due to reduction of the films’ bulk. Rather, this effect may be a result of the metastable RBD grown surface relaxing and restructuring without significant loss of oxygen.

Growth and Characterization of Porous TiO2 Films

Figure 9. Total amounts of N2 (expressed at monolayer equivalents) saturated at 28.5 K on 75 ML TiO2 films grown at 100 K as a function of annealing temperature (200-1300 K) and deposition angle (48°, 60°, 70°, and 85°).

A direct comparison of N2 uptake as a function of annealing treatment and deposition angle (48°, 60°, 70°, and 85°) is shown in Figure 9. As discussed in the previous section, films grown at 70° have the greatest total N2 uptake, and this trend persists regardless of annealing conditions. Films deposited at all angles show little decrease in the amount of N2 adsorbed prior to being annealed to 500 K. The films lose approximately 60% of their N2 uptake after annealing between 500 and 900 K. By 900 K, the internal surface area of the films has disappeared leaving behind a rough surface which undergoes very few additional changes up to the final temperature of 1400 K (films annealed to 1300 K still exhibit excess surface area (20-70%) as compared to the dense films likely due to surface roughness). Effect of Deposition Temperature on Film Structure. The structure of films grown by RBD, including the TiO2 films grown in this study, is a result of kinetically limited surface diffusion of adatoms during growth. This prevents material from reaching ‘shadowed’ regions of the surface and results in the formation of well separated filaments. The rate of surface diffusion is a strong function of the temperature which makes surface temperature an important deposition variable to examine. Figure 10 displays total N2 uptake (obtained via integration of the TPD spectra) for a series of TiO2 films deposited at 70° over a range of deposition temperatures (50, 100, 300, and 500 K). As mentioned in the Experimental Methods section, each film is preannealed to 200 K to drive off gases adsorbed during deposition which may interfere with adsorption of N2. Each curve traces the total N2 uptake as a function of annealing temperature up to 1300 K. It is evident from the data presented in Figure 10 that the film surface area decreases more dramatically with the surface deposition temperature (dashed line) than with the post-growth annealing temperature (solid lines). For example, a film deposited at 50 K and preannealed to 500 K has a surface area 8 times greater than a dense film, but a film grown at 500 K has a surface area only 2-3 times greater than a dense film. These results are consistent with a lower activation barrier for surface diffusion than that for bulk diffusion, as expected. The greatest amount of N2 uptake occurred on porous TiO2 deposited at 70° at 50 K (see Figure 10) which adsorbed ∼10 times the amount of annealed dense TiO2 films. Assuming that the number of N2 molecules in a monolayer on annealed dense TiO2 is equivalent to that on TiO2(110) (7.8 × 1014 N2/cm2 (

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Figure 10. Saturation coverage of N2 adsorbed at 28.5 K on 75 ML TiO2 films deposited at 70° as a function of annealing temperature (200-1300 K) and deposition temperature (50, 100, 300, and 500 K). The dashed line connects the as-grown, 200 K preannealed films for different deposition temperatures.

10%), then the specific surface area of the film can be estimated by dividing the integrated N2 uptake of a porous film by the N2 uptake of the dense film and the mass of deposited material. Calculated in this way, porous TiO2 deposited at 70° and 50 K has a specific surface area of ∼ 100 m2/g. This is in comparison to maximum specific surface areas of ∼ 3000 m2/g for ASW grown at 22 K,38,39 ∼ 1000 m2/g for MgO grown at 100 K,27 and ∼120 m2/g for Pd grown at 22 K.28 Considering the wide range of molecular masses for H2O (ASW), MgO, Pd, and TiO2 (18 amu - 106 amu), a more revealing comparison is the ratio of adsorbed N2 molecules per substrate unit (e.g., H2O, MgO, or TiO2). Simple calculations reveal that the TiO2 film adsorbs roughly 1 N2 molecule for every 10 TiO2 units (an adsorption ratio of 0.1) compared to N2 adsorption ratios of 0.5, 0.5, and 0.2 for H2O, MgO, and Pd, respectively. We do not fully understand the cause of the notably lower N2 adsorption ratio for TiO2 in comparison to MgO; however, it is probably rooted in the relative mobility of adatoms during film growth. The mobility of the adatoms on both surfaces (TiO2 and MgO) will be strongly affected by two factors: kinetic energy of the incident atoms from the effusive source, and localized heating due to the oxidation of metal on the surface. Evaporated metal atoms have an average kinetic energy proportional to the temperature of the Knudsen cell (1923 and 600 K for Ti and Mg, respectively). Thus, the surface must accommodate three times the energy for an incident Ti atom than for a Mg atom before the adatom can come to rest. Support for this argument is provided by recent work on the growth of porous ASW films which showed that the observed porosity of deposited ASW films decreases dramatically with increasing kinetic energy of the incident water molecules.52 For metal-oxide films specifically, we expect that localized heating from oxidation could also result in transient mobility of adatoms. Oxidation reactions of both Ti and Mg are highly exothermic (944 and 601 kJ/mol, respectively) although the heat of formation for TiO2 is notably greater by ∼ 60%. This additional energy may also be partially responsible for the lower N2 adsorption ratio on porous TiO2 films. EVolution of Pore Structure with Film Thickness. The selfshadowing process responsible for the high surface area of films deposited at oblique angles has an inherent induction period

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Flaherty et al. from dense TiO2 as well as films 7, 15, 38, and 150 ML thick. Dashed lines represent the TPD spectrum from the 7 ML thick film (Figure 12(v)) normalized at 40 K to each of the thicker films and are included to help illustrate the development of the pore condensation feature at 33 K while preserving the rest of the N2 TPD line shape. The peak at 33 K is absent for the dense film and barely detectable after 7 ML of deposition. Following 15 ML of deposition, there is a noticeable change in the line shape which grows into a distinct peak by 38 ML. Thus, it seems at least 15 ML of deposition is necessary before a film develops ‘pores’ with radii small enough to induce pore condensation. IV. Summary

Figure 11. Saturation coverage of N2 adsorbed at 28.5 K on TiO2 films deposited at 100 K and 70° as a function of film thickness (7150 ML). The slope (M) and y-intercept (Yo) of the fitted line are found to be 0.10 [ML N2]/[ML TiO2] and 0.85 [ML N2], respectively.

Figure 12. N2 TPD spectra after saturating TiO2 films deposited at 100 K and 70° with N2 at 28.5 K as a function of film thickness. Dashed lines represent the N2 TPD spectra from the 7 ML film normalized to each spectrum at 40 K to emphasize the desorption due to N2 condensed in the film pores.

before the films develop pores. BD simulations49 illustrate that after the deposition of only a few monolayers the substrate surface should remain mostly smooth and have a surface area approximately the same as the flat substrate. However, further deposition onto the surface leads to greater surface roughness, the formation of pores, and a notable increase in surface area. Once the pores have fully developed, the N2 uptake of the films should scale linearly with film thickness. Porous TiO2 films (deposited at 70° and 100 K) display this characteristic as shown in Figure 11. The total N2 uptake (obtained via TPD integration) of the films increases nearly linearly with film thickness over the range 15-150 ML. Greater insight into pore evolution within the films can be gathered from examining the N2 TPD spectra from films of various thicknesses. Figure 12 displays selected N2 TPD spectra from the data set used to calculate the total N2 uptake, shown in Figure 11. Figure 12(ii) is a N2 TPD spectrum from a 75 ML TiO2 film containing the low-temperature peak at ∼ 33 K indicative of desorption from pores. Figure 12 also shows spectra

We have investigated the deposition of porous TiO2 using the RBD technique and the effects of varying growth parameters including deposition angle (48°-85°), surface temperature (50500 K), and film thickness (7-150 ML) as well as post-growth annealing. In addition, film composition was examined as a function of growth temperature and oxygen pressure, and the development of film porosity was illustrated. Films deposited at 70° from the surface normal at 50 K in 1.5 × 10-7 Torr O2 exhibited the greatest surface area ∼100 m2/g. The high surface area is the product of limited surface mobility of film adatoms at low temperatures and high deposition rates. Stoichiometry of the deposited films was nearly independent of temperature over the range investigated (100-600 K) and monotonically dependent on O2 pressure. Incremental annealing of the films suggests that they are thermally stable to ∼600 K, above which they rapidly densify and lose all porosity by 900 K. Investigation of pore development indicates that at least 15 ML of deposition are required before pore formation is evident and nearly 38 ML of deposition are necessary for the pore structure to fully develop. Because porous TiO2 films can be deposited reliably over a range of conditions, we expect that these films could be useful as supports for catalytic materials or may be tailored for catalytic activity themselves. It is reasonable to expect that the films may be chemically modified by adding dopant molecules to the background ambient during growth. Also, TiO2 films could be decorated with metal clusters by simultaneously co-dosing the substrate with a second metal source during film deposition. Acknowledgment. We thank Mark Engelhard for performing XPS analysis of the porous TiO2 films. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Chemical and Material Sciences Divisions, and it was performed at the W. R. Wiley Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract No. DE-AC06-76RLO 1830. CBM acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, the Welch Foundation (F1436), the National Science Foundation (CTS-0553243), and the U.S. Department of Energy (DE-FG02-04ER15587). References and Notes (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (3) Haruta, M.; Yamada, N.; Kobayashi, T.; Ijima, S. J. Catal. 1989, 115, 301. (4) Chen, M. S.; Goodman, D. W. Nature 2004, 306, 252.

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