Noncontact Atomic Force Microscopy: An Emerging Tool for Fundamental Catalysis Research

Abstract

CONSPECTUS:Although atomic force microscopy (AFM) was rapidly adopted as a routine surface imaging apparatus after its introduction in 1986, it has not been widely used in catalysis research. The reason is that common AFM operating modes do not provide the atomic resolution required to follow catalytic processes; rather the more complex noncontact (NC) mode is needed. Thus, scanning tunneling microscopy has been the principal tool for atomic scale catalysis research. In this Account, recent developments in NC-AFM will be presented that offer significant advantages for gaining a complete atomic level view of catalysis. The main advantage of NC-AFM is that the image contrast is due to the very short-range chemical forces that are of interest in catalysis. This motivated our development of 3D-AFM, a method that yields quantitative atomic resolution images of the potential energy surfaces that govern how molecules approach, stick, diffuse, and rebound from surfaces. A variation of 3D-AFM allows the determination of forces required to push atoms and molecules on surfaces, from which diffusion barriers and variations in adsorption strength may be obtained. Pushing molecules towards each other provides access to intermolecular interaction between reaction partners. Following reaction, NC-AFM with CO-terminated tips yields textbook images of intramolecular structure that can be used to identify reaction intermediates and products. Because NC-AFM and STM contrast mechanisms are distinct, combining the two methods can produce unique insight. It is demonstrated for surface-oxidized Cu(100) that simultaneous 3D-AFM/STM yields resolution of both the Cu and O atoms. Moreover, atomic defects in the Cu sublattice lead to variations in the reactivity of the neighboring O atoms. It is shown that NC-AFM also allows a straightforward imaging of work function variations which has been used to identify defect charge states on catalytic surfaces and to map charge transfer within an individual molecule. These advances highlight the potential for NC-AFM-based methods to become the cornerstone upon which a quantitative atomic scale view of each step of a catalytic process may be gained. Realizing this potential will rely on two breakthroughs: (1) development of robust methods for tip functionalization and (2) simplification of NC-AFM instrumentation and control schemes. Quartz force sensors may offer paths forward in both cases. They allow any material with an atomic asperity to be used as a tip, opening the door to a wide range of surface functionalization chemistry. In addition, they do not suffer from the instabilities that motivated the initial adoption of complex control strategies that are still used today.

Surface phase, morphology, and charge distribution transitions on vacuum and ambient annealedSrTiO3(100)

“…In contrast, NC-AFM is sensitive to anything that produces a spatially varying force on the tip, including trapped charges that can be several nanometers below the surface. [68] Thus, NC-AFM reveals that even SrTiO 3 surfaces prepared using standard procedures that yield sharp electron diffraction patterns still contain plentiful surface and near surface defects that can trap charge.…”

Ultrahigh-resolution imaging of water networks by atomic force microscopy

“…H bonds between organic molecules are also potentially visualized 29 , even though the results of these imagings remain controversial 28 30 . This imaging technique has already been applied to various organic molecules 31 32 and non-carbon materials such as iron clusters 33 and metal chalcogenide thin films 34 35 . In contrast to the rigid and stable materials, water molecules in the first layers would form various adsorption geometries differing from the monomeric configurations, owing to H bonds 16 18 22 36 .…”

Three-dimensional interaction force and tunneling current spectroscopy of point defects on rutile TiO2(110)

“…3,4 The combination of the method with the complementary recording of tunneling current (3D-AFM/STM) leads to the ability to simultaneously probe chemical interactions as well as electronic properties with local specificity as a function of tip-sample distance, making it ideal for fundamental catalysis research. 5,6 Transition metal oxides are widely used as catalysts. 7,8 Among the large family of transition metal oxide surfaces, the (110) face of rutile titanium dioxide (TiO 2 ) is considered prototypical due to the fact that the types of defects commonly encountered on this surface are well known.…”