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2 Facets Engineering on Catalysts
Jian (Jeffery) Pan
The University of New South Wales, School of Chemical Engineering, Department of Particles and Catalysis Research Group, Sydney, NSW, 2052, Australia
2.1 Introduction
Crystal facets engineering has become one of the most effective strategies to enhance performance of nanomaterials in many applications, such as heterogeneous catalysis, gas or liquid sensing, photocatalysis, electrocatalysis, fuel cell, solar cell, and lithium‐ion batteries [1–4]. The reactions occurred at the surface or interface of nanomaterials are extremely sensitive to the exposed surface atomic structures and their respective physical and chemical properties. The well‐defined crystal surface has unusual properties compared to the bulk, due to the termination of periodic crystal lattices. The various properties of different facets of a single crystal are attributed to crystal anisotropy. As a consequence, the whole behavior of a faceted nanomaterial would be dramatically affected by the surface, especially when the particle size shrinks to a nanoscale and the surface/bulk atomic ratio can no longer be negligible.
This chapter is mainly focused on faceted single crystals of metals and semiconductors in heterogeneous catalysis. Metal catalysts are mainly used in thermal catalysis and electrocatalysis, such as producing chemicals, petroleum refining, and in fuel cells. Studies of metal catalyst surfaces are comprehensive and began much earlier than that of semiconductors in photo‐related catalysis. In the past two decades, significant progress has been made to synthesize metal nanocrystals in a variety of shapes and enhanced performance [5]. Although the miniaturization of catalysts to single (metal) atom catalysts and metal cluster catalysts have become popular in recent years due to the strong desire to attain 100% atom utilization efficiency especially when using some of the least abundant elements [6, 7], the advantage of facets engineering of metal catalysts continues to be an essential and indispensable tool. For example, in photocatalysis where optical absorption is a bulk property that determines the rate of charge carriers generation, it is desirable to apply facets engineering to the bulk crystals to enhance surface charge transfer efficiencies.
Being the core fundamental of heterogeneous catalysis, the studies of surface reactivities have long been appreciated by the community. However, it is only in the last two decades that the tremendous development in the synthesis and characterization of crystal facets further paved the way for significant achievements of facets engineering in catalytic applications. As such, this chapter briefly introduces the mechanisms of facets engineering, the anisotropic properties of crystal facets, and the effects of facets engineering.
2.2 Mechanisms of Facets Engineering
The faceted nanocrystals, whether metals or semiconductors, can be achieved through many synthesis methods, including solution‐phase, vapor‐phase, and solid‐phase methods. The solid‐phase methods include gas oxidation route, topotactic transformation method, and crystallization transformation method [8, 9]; the vapor‐phase methods include thermal decomposition method, metal–organic chemical vapor deposition [10, 11]; and the solution‐phase methods are more powerful and versatile than others. It includes basic wet chemical route, sol–gel method, hydrothermal [12, 13] and solvothermal [14] methods, microwave treatments [15, 16], electrochemical [17] and photochemical [18, 19] methods. It should be noted that the synthesis of metal crystals is quite different from semiconductor crystals, although they sometimes share the same methods. Metal crystal catalysts are composed of single or binary metal atoms. Their synthesis process, including the steps from ions to nuclei or cluster, to seed, and then to nanocrystals, might be far different from the synthesis of semiconductors, which are composed of metal and nonmetal elements. However, no matter metals or semiconductors, and regardless of the synthesis methods, the one common feature is the spontaneous reaction that dictates the crystal formation is under thermodynamic control.
The excess energy at the surface of a material compared with the bulk is defined as the surface energy. Without facets engineering, the most stable free surface will occupy the greatest portion of the crystal surface as it attempts to minimize the total surface energy in a given growth condition. According to the Gibbs–Wulff theorem, the facets with higher surface energies always grow faster and end up with a tiny fraction or even vanish [20]. The Wulff construction method predicts the shape of a crystal with the lowest surface energy, and the most stable form is called the equilibrium shape of a crystal. Owing to the anisotropic surface energy of crystal facets, the final shape of a single crystal is usually enclosed with the facets of lowest surface energy and smallest surface area in a given volume. For example, for a metal crystal that contains NA atoms and each bulk (i.e., nonsurface) atom that is defined by a coordination number of CN, there will be (NA × CN)/2 bonds in the crystal. As such, the energy of each bond can be calculated as
where ΔHs is the molar enthalpy of the metal sublimation.
A face‐centered cubic (fcc) structured metal crystal typically has three low‐index facets, i.e. {111}, {100}, and {110} facets. While each bulk atom has a CN of 12 (6 around the side perimeter of the atom and 3 each on top and bottom), depending on the direction of the surface cut, which, in turn, dictates the exposed facets, the atom on terminated surface would lose a certain number of atoms and result in dangling bonds.
As shown in Figure 2.1a, the CN of the atoms at the {111} surface is 9, which means 3 bonds are broken. Therefore, the energy required per atom to form the {111} surface can be calculated as
Then, the surface energy of {111} facet γ{111} can be calculated as follows:
where
Similarly, the CN of the atoms at the {100} surface is 8, which means 4 bonds are broken (Figure 2.1b). Therefore, the energy required per atom to form the {100} surface can be calculated as
Then, the surface energy of {100} facet γ{100} can be calculated as follows:
The {110} surface contains two layers of atoms