J. Phys. Chem. 1993,97, 5965-5972
5965
Scanning Tunneling Microscopic and Auger Electron Spectroscopic Characterization of a Model Catalyst: Rhodium on TiOz(001) G. E. Poirier,+B. K. Hance, and J. M. White’ Department of Chemistry, University of Texas, Austin, Texas 78712 Received: November 20, 1992; In Final Form: February 17, 1993
Rhodium films were vapor deposited on Ti02(001) rutile and examined in situ at various coverages and following various annealing procedures by using scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Rh deposition at ambient temperature leads to STM images that indicate nucleation and growth of three-dimensional particles. The particles have a narrow size distribution centered around 30 A with an aspect ratio of 0.3 and are not arranged with long-range order. AES results corroborate the film thickness measured by STM. Annealing in vacuum decreases both the Rh/Ti Auger intensity ratio and the number density of particles, indicating particlecoalescence at the onset of surface diffusion. Simultaneously, the apparent included volume of the particle film increases, an effect that is consistent with Rh particle encapsulation but may also be due to tip convolution. The utility of STM in morphologic characterization of oxide-supported metal particles, such as those found in catalysts and gas sensors, is demonstrated.
I. Introduction
Volmer-Weber growth is indicated by the variation of the morphology with total rhodium coverage. After deposition at ambient temperature, changes of the particles, brought on by annealing, were followed with STM and AES. A model in which particle coalescenceand encapsulation both occur at the onset of surface diffusion is consistent with both STM and AES changes; however, a tip convolution artifact is also considered.
Noble metals, finely dispersed and supported on oxides, are useful in a broad class of applications including catalysis, photocatalysis, and gas sensing. In these systems, the primary purpose of the support material is to maintain the high surfacearea-to-volume ratio that is economically necessary when using noble metals. Titania andother reducible transition-metal oxides have come under intense scrutiny because they chemically interact 11. Experimental Section with the dispersed metal. For example, annealing noble metals supported on titania greatly reduces their capacity for CO and A single-crystal rutile boule (Atomergic) was cut within 0.5O H2 adsorption, an effect not seen on supports such as A1203or of (001) with a diamond saw. The surfaces were abraded and Si02.1-3 This behavior has been termed a strong metal-support polished to give a 1-cm-diameter by 1-mm-thick sample. The interaction (SMSI) and has been explained variously in terms of final polish was with 0.05-pm alumina. All experimentsdescribed an electronic effect, particle coalescence, particle encapsulation, here were carried out in an ion-pumpedultrahigh vacuum chamber and metal-support alloying. A wide variety of techniques have with a base pressure of 1 X l t 9 Torr. The available tools are been applied to this interesting system including electron mass spectrometry, Auger electron spectrometry, scanning micro~copy,~.”~ electrical conductivity? thermal desorption,%12 tunneling microscopy, low-energy electron diffraction, ion sputultrahigh vacuum ion and electron s p e c t r o ~ c o p y , ~ ~ ~ J ~ Jtering, ~ - * ~ e-beam heating, and dosing of gases and metals. The infrared,*+27 chemical titration,lJ212&3’Jchemical r e a ~ t i o n , ~ l - ~ ~STM and other techniques are arranged in a circle about the X-ray absorption,7.24,30,34 magnetic resonance,*J5 self-consistent instrument’s novel rotating sample holder. Auger measurements field theory,36 and broken-bond model calculation~.3~J* With were made with a Physical Electronics coaxial-gun single pass the exception of electron microscopy, all of these techniques CMA operating at 2-eV modulation, 3000-eV primary energy, provided results representing a spatial average over perhaps 1Olo and 8-pA nominal beam current. Derivative mode spectra were particles. While the electron microscopy studies were enlightacquired in less than 10 s to minimize e-beam damage. Sputter ening, application of STM offers the ability to develop additional cleaning was accomplished with a Kratos narrow-beam ion gun insight; this tool offers exquisite spatial resolution in three at off-normal incidence. The 1.0-keV, 0.33-pA Ar+ beam was dimensionsand is, potentially, also sensitiveto electronic structure. rastered over a 1.5 cm X 1.5 cm area. Salmeron et al. have applied STM to the related system TiO, on The custom-madeSTM was described thoroughly in a previous Rh(l1 l).39 article.40 Briefly, it uses a fine-thread screw and lever arm device The purpose of the experiments described here was to for the mechanical approach and a piezo-tube scanner for the tip. demonstrate the use of STM for a metal on metal oxide system. A pneumatictable and internal Viton O-ring separated steel plates, Rhodium on titania was chosen because it has shown interesting at the STM sample holder, constitute the vibration isolation. chemical interactions. The (001) face of titania was chosen Tips were fashioned from 0.25-mm-diametertungsten wire etched because it is thermodynamically unstable and roughens when at 10 VAC in 2 M KOH. Etching was commenced with the wire annealed in vacuum at temperatures exceeding 650 K. Though submerged 0.2 in. and was terminated just before the receding mostly (001) planes are still exposed, roughened surfaces likely tip reached the solution surface. These tips have a conical shape are more representative of the polycrystalline powder supports with a small (1 5-20°) macroscopic cone angle and were screened used in technical materials. We present STM topographs which with an optical microscope prior to installation. They generally indicate that individual Rh particles are indeed resolvable. gave high-resolution images on the first scan but occasionally had to be conditioned by scanning at -1-V sample bias.41 The To whom correspondence should be addressed. piezo-scanner was calibrated by using an ultraviolet diffraction t Currentaddress: Chemical ScienceandTechnologyLaboratory,National grating and monoatomic steps on graphite and Au( 11 1). Unless Institute of Standards and Technology, Gaithersburg, MD 20899. 0022-3654/93/2097-5965$04.00/0
0 1993 American Chemical Society
Poirier et al.
5966 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 6 5
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Figure 1. Evolution of the AE spectrum during vapor deposition of rhodium on room temperature titania (001) and subsequent annealing. Rhodium dose increases from scan (a) to scan ( f ) . Scan (g) was collected after annealing the sample 30 min at 675 K. The decrease in Rh/Ti ratio is a signature of the "SMSI"state.
noted otherwise, images were acquired in constant-current mode at a sample bias of +2.0 to +2.5 V and tunneling current of 0.25 nA. It was seldom possible to obtain good images with the bias voltage between -2.0 and +2.0 V. Voltages below -2.0 V were slightly better but the best images were obtained with the sample biased +2.0 to +3.0 V. Tunneling barrier heights were determined in two ways: (1) by measuring current vs tunneling gap and (2) by modulating the tip height (h0.9 A at 1.6 kHz) and using phase-sensitive detection of the log of the tunneling current. Both methods of measurement yielded barrier heights between 3 and 5 eV. The sample was heated from behind with a thoriated tungsten filament formed into a 1-cm square grid and biased -700 V relative to the grounded sample. The sample temperature was monitored with a chromel-alumel thermocouple held onto the front of the sample envelope by a tantalum clip and was controlled with an analog feedbackcircuit. Large uncertainties in measured surface temperature have been reported for metal o ~ i d e s . ~ 2Since ~ 4 the threshold temperatures for various phenomena investigated with our system (no LEED pattern LEED pattern, nonfaceted -, faceted) were within 50 K of established literature values,'3,45,46 we believe our temperatures are accurate to h50 K. Unless noted otherwise, thesubstrate was prepared for filmdeposition by sputter cleaning then annealing to 675 K for 30 min. The sample had an opaque blue-black color and chargedvery little (
