6390
J. Phys. Chem. C 2008, 112, 6390-6397
Controlling the Size and Distribution of Pd-Au Nanoparticles on TiO2(110) Patrick Han and D. Wayne Goodman* Department of Chemistry, Texas A & M UniVersity, College Station, Texas 77843-3255 ReceiVed: October 16, 2007; In Final Form: February 25, 2008
Variation in the extent of annealing of a TiO2(110) surface was used to control the distribution of deposited Pd, Au, and Pd-Au clusters at room temperature. Programming with respect to the metal deposition and thermal sintering was used to optimize the size distribution and cluster density of Pd-Au mixed-metal clusters. The influence of varying the evaporation sequence on the cluster distribution is demonstrated, and the results explained using known sintering mechanisms.
Introduction Supported bimetallic nanoparticles play a central role in many catalytic processes.1-3 Important examples are Pd-Au catalysts whose applications include CO removal in car exhaust,4 trichloroethene production,5 hydrogen peroxide synthesis,6 and vinyl acetate manufacturing.7 The synthesis of alloy nanoparticles typically use two recipes: (i) ex situ fabrication before deposition on the substrate4,8-10 or (ii) in situ epitaxial growth by vapor deposition.11-14 Approach i focuses on the monodispersity of size, shape, and metal composition of the supported clusters; however, little real-space information regarding the dynamics of the clusters is available. Approach ii uses local probes to study epitaxially grown clusters; however, this method often results in metal clusters with a broad size distribution and ill-defined composition. A notable example is described in ref 14 where the authors use a nanostructured alumina film on Ni2Al(111) to produce a monodispersed array of Pd-Au particles with a diameter of ∼4 nm. In the present work, surface oxygen vacancies are induced by changing the annealing conditions of a TiO2(110) substrate. This, in turn, controls the size distribution and density of subsequently nucleated Pd and Au clusters. The TiO2(110) substrate was also used to synthesize Pd-Au clusters by thermal sintering. By control of the substrate, the sequence of deposition, thermal annealing, and the initial distribution of the deposited metal, well-dispersed Pd-Au clusters can be obtained with an average size of ∼5 nm. The interplay of the synthesis conditions on the final cluster density and size distribution are discussed and explained using known sintering mechanisms. Experimental Section The scanning tunneling microscopy (STM) experiments were performed in ultrahigh vacuum at room temperature, using a RHK 300 STM. All images were scanned in the constant-current mode using an electrochemically etched W tip. All bias voltages are reported with reference to the sample. An ordered TiOx(110) surface was obtained through sputtering and anneal cycles (1 keV Ar+ sputtering for 10 min, annealing at 850 °C) on a TiO2(110) single crystal. Pd, Au, and Pd/Au alloy clusters were grown by vapor deposition followed by annealing the substrate at 630 °C. The sample temperature was measured * To whom correspondence should be addressed. E-mail: goodman@ mail.chem.tamu.edu.
Figure 1. (A) STM image of a clean TiO2(110) surface after 10 min of Ar+ sputtering (E ) 1 keV), followed by 3 min of annealing at 850 °C (300 nm × 300 nm, Vs ) 2.62 V, It ) 74 pA). Bright areas in the inset are believed to be missing oxygen atoms (100 nm × 100 nm, Vs ) 2.62 V, It ) 96 pA). (B) STM image of the same sample after additional annealing for 3 min at 850 °C (300 nm × 300 nm, Vs ) 2.44 V, It ) 63 pA). The inset shows the 1 × 1 surface structure at atomic resolution, as well as some added rows (30 nm × 30 nm, Vs ) 2.44 V, It ) 58 pA).
optically using an infrared pyrometer, and the cluster volumes were estimated from the STM images. Results and Discussion Clean TiO2(110) Surface. Figure 1 shows two sets of STM images of clean surfaces of TiO2(110) at various stages of sample preparation. Following the initial 10 min of sputtering with Ar+ and an anneal at 850 °C for 3 min, bright spots believed to be missing O atoms are evident (Figure 1A and inset). A defect-free surface showing the (1 × 1) structure can
10.1021/jp7100607 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008
Size and Distribution of Pd-Au Nanoparticles on TiO2(110) be obtained by annealing once more (850 °C for 3 min) without sputtering (Figure 1B and inset). The appearance of an addedrow structure associated with the (1 × 2) reconstruction along with straightening of the step edges by annealing is apparent in the images of Figure 1. It is well-known that Pd deposited on a clean, defect-free TiO2(110)-(1 × 1) surface preferentially nucleates at step edges.16 Au shows a similar trend at low deposition coverages (
