Introduction to Nanoporous Gold

ARNE WITTSTOCK, JÜRGEN BIENER AND MARCUS BÄUMER

1.1 Nanoporous Gold

Nanoporous gold is a corrosion-derived bulk nanostructured material. It is generated by the corrosion of an alloy of Au and a less noble metal, such as Ag or Cu. By electrochemical removal (dealloying) of the less noble constituent, the remaining gold undergoes a self-organization process forming a three-dimensional bicontinuous porous network of interconnected ligaments (Figure 1.1). Depending on the preparation conditions, the resulting pores and ligaments can be as small as 5 nm, but are typically around 30 to 40 nm. By IUPAC definition, the as-prepared material is mesoporous. Due to its high porosity and small feature size, this material has a specific surface area in the range of 10 m2 g-1. The void or pore volume in the resulting material mostly depends on the concentration of the less noble metal (e.g. Ag) in the starting compound. Because of fundamental limitations for bulk dealloying, such as the 'parting limit' (see Chapter 2) and the stability of the evolving porous network, alloys containing between 60 at.% and 80 at.% Ag are most viable. Processing (dealloying) of according alloys results in pore volumes between about 60% and 80%.

Early experimental work on corrosion-derived nanoporous Au by Pickering and Swann in the 1960s and by Forty in the 1970s focused on the corrosion aspect using this material and its starting alloys, respectively, as a model system for studies on the molecular mechanism of alloy corrosion. With the onset of nanotechnology in the late 1990s and the early 2000s, researchers revealed and developed the potential of this material for a variety of techno- logical aspects. As a consequence, the number of publications dealing with nanoporous Au has increased steeply by about 40% per year, from about 11 publication in the year 2001 to more than 150 in the year 2010. One of the reasons for the success of this material is the comparatively simple preparation of this nanomaterial using bench-top corrosion techniques to generate bulk samples several millimeters in size and even larger. By avoiding financially demanding techniques, such as electron beam lithography, this material became available to a variety of research groups working on the optical or mechanical properties, the catalysis or the electrochemistry of the material.

Besides the availability of the material for different research groups, another crucial factor fuelling interest in this material is its structural and chemical flexibility (see Figure 1.1). Microfabrication of the material using fast ion bombardment has been used to generate various micrometer-sized patterns and structures of interest for mechanical tests, for example. Temperature-activated ripening of the nanostructures opens the door to materials with pores and ligaments in the size regime between about 30 nm and several micrometers, without losing the typical bicontinuous structure of the material. By using templating techniques, such as slip casting of alloy-coated polystyrene beads and subsequent removal of the template, hierarchical nanoporous Au can be generated as well with relative densities as low as 2 to 3%. In addition to these structural variations, the materials surface can be chemically modified with metals, organic entities or metal oxides bringing forward its applications in electrochemistry (e.g. fuel-cell applications), sensorics, and catalysis.

Although the term 'nanotechnology' is rather new, the use of nanomaterials can be dated back several hundreds or even thousands of years. The first reports on the use of corrosion to generate nanoporous gold can be related to pre-Columbian civilizations, such as the Incans (see Chapter 2). Here, the superficial dealloying and subsequent burnishing of a comparably cheaper Au–Cu alloy (removal of the Cu from the alloy surface) was used to generate a shiny gold surface, giving the work piece the allure of pure gold. Undoubtedly, this must have caused severe frustration in the Spanish conquistadores when melting the looted, apparently pure, gold pieces back in Spain. However, artisans throughout the centuries have used this superficial enrichment of Au alloys as a means of gilding. The technique was thus dubbed depletion gilding or 'mis-en-colour', accordingly. For these reasons, when dealing with nanoporous gold, we speak of an ancient material yet with a novel technological impact.

1.2 Gold — Some Facts

Gold is an element that has inspired mankind at all times. Around 700 BC, the famous Greek poet Hesiod described the five ages of mankind in his poem Works and Days. The first age he calls the golden age of mankind, free from later gradual deterioration of moral values. Indeed, gold was the first metal recognized by humans even before bronze and iron. Traces of gold can be found in early human settlements (~8000 BC) in the Euphrat and Tigris river system, an area that is today part of the Iraq. Archeological findings of gold from later high civilizations such as Egypt and Mesopotamia can be dated back as early as 4000 BC. Back in those days, gold was already used as a means of payment, in the form of rings (about 2700 BC) and later in the form of coins (since 600 BC). The earliest craftsmanship, such as the funeral mask of the Egyptian Pharaoh Tutankhamun (1223 BC) or Solomon's famous temple in Jerusalem (build around 950 BC), allegedly overlaid with gold, bears testimony to this early and lasting fascination with gold.

The belief in gold as the embodiment of value continued throughout the centuries. Today, the drastically increasing demand for gold as a safe investment very much reflects this fact. One reason for investing in gold as a safe haven of treasure and investment is its nobleness and obvious inability to corrode like iron. Gold stays in its metallic form, apparently unaffected by dirt and corrosion. Another reason is that gold is rare. Elements heavier than iron (56Fe) cannot be generated by fusion reactions that occur in the sun but result from neutron-capture reactions as in supernovae, a comparatively rare astrophysical event. In the galaxy, elements such as gold are thus inherently rare.

The...