Surface Chemistry in Nanoscale Materials

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Materials 2009, 2, 2404-2428; doi:10.3390/ma2042404
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materials
ISSN 1996-1944
www.mdpi.com/journal/materials
Review

Surface Chemistry in Nanoscale Materials
Jürgen Biener 1,*, Arne Wittstock 2, Theodore F. Baumann 1, Jörg Weissmüller 3,4, Marcus Bäumer 2 and Alex V. Hamza 1
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Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory, Livermore, USA; E-Mails: baumann2@llnl.gov (T.F.B.); hamza1@llnl.gov (A.V.H.) Institut für Angewandte und Physikalische Chemie, Universität Bremen, Bremen, Germany; E-Mails: wittstock1@gmail.com (A.W.); mbaeumer@uni-bremen.de (M.B.) Institut für Nanotechnologie, Karlsruher Institut für Technologie, Karlsruhe, Germany; E-Mail: joerg.weissmueller@kit.edu (J.W.)

Technische Physik, Universität des Saarlandes, Saarbrücken, Germany

* Author to whom correspondence should be addressed; E-Mail: biener2@llnl.gov; Tel.: +1-925-422-9081; Fax: +1-925-422-7098.
Received: 31 October 2009; in revised form: 5 December 2009 / Accepted: 14 December 2009 / Published: 16 December 2009

Abstract: Although surfaces or, more precisely, the surface atomic and electronic structure, determine the way materials interact with their environment, the influence of surface chemistry on the bulk of the material is generally considered to be small. However, in the case of high surface area materials such as nanoporous solids, surface properties can start to dominate the overall material behavior. This allows one to create new materials with physical and chemical properties that are no longer determined by the bulk material, but by their nanoscale architectures. Here, we discuss several examples, ranging from nanoporous gold to surface engineered carbon aerogels that demonstrate the tuneability of nanoporous solids for sustainable energy applications.

Keywords: nanoporous materials; nanoporous Au; carbon aerogel; surface chemistry; surface stress; atomic layer deposition; catalysis; actuation; hydrogen storage

Materials 2009, 2

2405

1. Introduction
Enrico Fermi reputedly said “God made the solid state. He left the surface to the devil” to describe the fact that surfaces and interfaces are difficult to treat theoretically due to their complex nature. On the other hand, one can exploit this complexity to design tunable interface controlled materials with high surface area such as nanoparticles, nanowires and low-density nanoporous materials. Their surface-to-volume ratio increases with deceasing feature size (Figure 1a), and for small enough feature sizes, their properties are no longer dominated by the bulk of the material but by surface atoms. Based on coordination, one can differentiate between three types of surface atoms which, in the order of decreasing coordination, are terrace atoms, step edge atoms, and kink sites (Figure 1b). Simple geometrical considerations reveal that the curved surfaces, which are typical for most high surface area materials (Figure 1c), are dominated by the more undercoordinated step edge and kink site surface atoms. It is indeed the undercoordination which gives rise to new properties. For example, surface stress is the consequence of an electronic relaxation (Figure 1d) by transferring electronic charge into in-plane bonds [1]. This surface stress induces a pressure in the bulk (Figure 1e) that then further affects the chemical, physical and mechanical properties of the material. For example, theoretical studies have shown that tensile strain can make gold less noble by increasing the ability of gold to bond simple adsorbates more strongly [2,3]. Thus high surface area materials open a new dimension in material design for a multitude of technological important areas, including energy storage and conversion, sensing and catalysis. Beyond morphology, the properties of high surface area materials can be further tuned by surface engineering.

Figure 1. A characteristic property of high-surface area materials...
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