Nanoscale Fabrication for Enhanced Properties

Don R. Baer & Yong Liang

Project Description

This research project investigates the formation and properties of nanoscale functional structures with the ultimate objective of integrating active nanometer-sized components into "smart" microtechnology devices. The focus of this project is the creation of ordered nanosized clusters with a potential for catalytic studies. We investigated using self-assembled processes to form organized Pt nanoclusters on TiO₂ surfaces. Com-pared to a lithography method, self-assembly has the advantages of a higher yield and lower cost. Our results show that Pt interacts preferentially with the (1x2) phase on TiO₂ surfaces and forms organized strings of nano-clusters along the step edges of the (1x2) domains on TiO₂ surfaces.

Technical Accomplishments

This effort on nanoscale component development uses a range of new experimental tools recently made available in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). These tools enhance our ability to work in the nanometer dimension. We have demonstrated the general ability to create the desired types of surfaces and form organized strings of platinum nanoclusters on these surfaces using self-assembled processes.

Creation of TiO₂ surfaces with different domains

TiO₂ is an important catalyst support material. A flat, well-defined surface is needed to enable detailed characterization of the properties of metal clusters attached to this surface. It is well established that the TiO₂ (110) surface can form two different structures, (1x1) and (1x2). However, recent work using the scanning tunneling microscope (STM) has shown that these structures can exist in different domains on a single surface. Because of their different geometric and electronic structures, one would expect that these two domains exhibit different interactions with metal clusters (supported metal catalysts) and thus result in different catalyst formations. During FY 1998, we demonstrated that Rh grew along the step edges of the TiO₂ (1x1) domain. One objective in FY 1999 was to create better defined steps and surface structures on TiO₂ (110) surfaces. We were able to obtain well-characterized TiO₂ (1x1) surfaces and surfaces with both (1x1) and (1x2) domains (as shown in Figure 1). These were ideal substrates for examination of the inter-action of Pt with these two different domains. We used STM and low energy electron diffraction (LEED) to examine atomic structures and symmetry of TiO₂ (110) surfaces under different processing conditions. Results showed that after annealing the TiO₂ (110) surfaces at 800°C or higher temperatures for an extended period of time, the surface underwent the following phase transi-tions: (1x1) phase -> mixed (1x1) and (1xn) phases -> mixed (1x1) and (1x2) phases -> (1x2) phase. Here, the (1xn) phase represents a TiO₂ (110) surface covered by line defects with various atomic spacings.


Figure 1. (a) A 400Åx400Å STM Image of a TiO2(110) Surface of Alternating (1x1) and (1x2) Domains; (b) A LEED pattern of a TiO₂(110) surface with mixed (1x1) and (1x2) phases.

Figure 1 shows an STM image and the related LEED pattern of a TiO₂ (110) surface annealed at 850°C. While the LEED shows that the surface exhibits mixed (1x1) and (1x2) domains, the STM image reveals that (1x1) and (1x2) domains alternate on the surface with distinct boundaries between the two. Because different catalysts may have different interactions with these domains or their associated step edges, a significant implication of these alternating domains is that one could create an organized, multicomponent catalyst system supported on TiO₂(110) surfaces.

Pt Nanoclusters and Strings on TiO2 Surfaces

We used vapor deposition method to create Pt nano-clusters on TiO₂ surfaces. By controlling the amount of Pt deposited on the surface, we were able to control the average size of Pt clusters on the surfaces. Figure 2, an STM image of Pt deposited on a TiO₂ (110) (1x1) surface, shows that Pt clusters are randomly distributed on the surface. In contrast, strings of Pt clusters along the step edges of TiO₂ (1x2) domains were found on surfaces with alternating (1x1) and (1x2) domains, as shown in Figure 3. The fact that Pt clusters preferentially formed along the edges of (1x2) domains suggests a stronger interaction of Pt with the TiO₂ (1x2) domain than the (1x1) domain.

Figure 2. A 400Å x 400Å STM Image Showing Pt Nanoclusters
Randomly Distributed on the TiO2 (110) (1x1) Phase.

Figure 3. A 400Å x 400Å STM Image Showing Formation of Strings
of Pt Nanoclusters along the Step Edges of (1x2) domains.

In addition to the preferential growth of Pt nanoclusters along the steps of the (1x2) phase, under the same amount of Pt deposition, the average size and distribution of Pt clusters is smaller and narrower on the TiO₂ (1x2) domain than on the (1x1) domain. Figure 4 is the size distribution of Pt clusters along the step edges of (1x2) domains. The average Pt cluster size is 1.4 nm compared with ~2.5 nm on a (1x1) domain. Our current effort is to test the reactivity of Pt/TiO₂ (1x1) and (1x2) using temperature program desorption method.

Figure 4. Size Distribution of Pt Nanoclusters
Stabilized by the Steps of (1x2) Domains.

Publications and Presentations

Liang Y, J Daschbach, A Joly, D Baer, M Na, and H Luo. "Enhancement of catalytic properties using surface and interface engineering." November 1998. MRS Fall meeting, Boston.

Liang Y, J Daschbach, Y Su, D Baer, M Na, and H Luo. February 1999. "Using tailored surfaces and interfaces to enhance catalytic properties." DOE Catalysis Workshop.

Liang Y, A Grant, D Baer, and S Gan. October 1999. "Formation and properties of Pt on TiO2 rutile and anatase surfaces." AVS Meeting, Seattle.

Liang Y, J Daschbach, S Chambers, Y Su, A Joly, D Baer, M Na, and H Luo. May 1999. "Use of surface and interface engineering to enhance catalytic properties." 195th Meeting of the Electrochemical Society, Seattle.