Nanometric chemical clocks

Abstract

Field ion microscopy combined with video techniques and chemical probing reveals the existence of catalytic oscillatory patterns at the nanoscale. This is the case when a rhodium nanosized crystalconditioned as a field emitter tip-is exposed to hydrogen and oxygen. Here, we show that these nonequilibrium oscillatory patterns find their origin in the different catalytic properties of all of the nanofacets that are simultaneously exposed at the tip's surface. These results suggest that the underlying surface anisotropy, rather than a standard reaction-diffusion mechanism, plays a major role in determining the self-organizational behavior of multifaceted nanostructured surfaces. Surprisingly, this nanoreactor, composed of the tip crystal and a constant molecular flow of reactants, is large enough for the emergence of regular oscillations from the molecular fluctuations.field ion microscopy ͉ heterogeneous catalysis ͉ nanopatterns ͉ nonequilibrium oscillations T he Belousov-Zhabotinskii reaction is probably the most famous example of an oscillating chemical reaction in the liquid phase (1, 2). However, studies of oscillations in heterogeneous catalysis begun in the 1970s (3, 4). One decade later, Ertl et al. demonstrated for the first time (5, 6) that oscillatory surface reactions are associated with the occurrence of specific pattern formations ranging from tens to hundreds of micrometers. More recently, oscillations have been discovered on the nanoscale in field electron and field ion microscopes by using video techniques (FEM and FIM, respectively) (7-10). At this length scale of tens of nanometers, self-sustained oscillatory patterns are observed about which little is known. This is certainly the case for the catalytic water production when exposing oxygen and hydrogen to a rhodium field emitter tip (11,12). Our purpose in the present article is to show that this nonequilibrium self-organizational behavior can be understood by taking into account the structural anisotropy of the crystalline tip that results in different catalytic properties on the various nanofacets. Moreover, we demonstrate that the external electric field, as applied in a FIM (of the order of 10 V/nm), promotes surface oxidation thus giving way to a feedback mechanism to explain self-sustained rate oscillations in the H 2 /O 2 /Rh system.The understanding of self-sustained oscillations at the macroscale was pioneered by Prigogine who showed that such phenomena are consistent with thermodynamics as long as the system is open and far from equilibrium (13). Such selforganization phenomena are understood in terms of reactiondiffusion processes, which also explains the mesoscopic patterns observed on oriented metal single-crystal surfaces (14, 15). However, the nanopatterns observed during a catalytic reaction in a FIM have a spatial scale smaller than typical diffusion lengths. Moreover, one may wonder how the molecular fluctuations that manifest themselves in such small systems (16, 17) affect the oscillations. Indeed, their regularity disappears...

Nonlinear Dynamics of Reactive Nanosystems: Theory and Experiments

“…The bistability is made visible by FIM via abrupt and unpredictable modifications of the surface state of these quadrants flipping in an independent manner from the oxidized to the metallic states, and vice-versa. A kinetic phase diagram has been established earlier for this system [13,14]. For high temperatures, i.e.…”

“…Although the probability is indeed bimodal as expected, one should make sure that it is indeed the chemistry of the reaction that is responsible for the bistability and the transition between the states. To show this, we consider here a "backbone" model for the reaction, which is a simplification of the complex, realistic model developed in [13,14]. We first take into account the fact that oxygen adsorbs dissociatively on the surface: (6.18) an equation in which * stands for an available active site on the surface of the metal.…”

Effect of ultrafast diffusion on adsorption, desorption, and reaction processes over heterogeneous surfaces

“…Since the underlying solid is a crystal, the different points of the surface correspond to different crystalline orientations, which are labeled by Miller indices (h,k,l). It is known that properties such as the activation energies E j , the prefactors k 0 j , or the dipole coefficients d j depend on the orientation of the crystalline surface and thus on the location r at the surface [10][11][12].…”

“…Such a field deeply influences the surface kinetics as well as the gas mixture of reactants around the catalyst. In this system, the surface diffusion of hydrogen turns out to be much faster than the reaction and the diffusion of oxygen [10][11][12]. Accordingly, the hydrogen atoms have a fast motion on the surface, quickly leading toward a quasiequilibrium distribution with a spatial dependence on the local crystalline facets as well as on the electric field.…”

“…An example of such complex systems is provided by the reaction of water formation on rhodium in heterogeneous catalysis [10][11][12]. Under the conditions of field ion microscopy, the reaction proceeds on a highly heterogeneous medium, which is a metal needle presenting multiple crystalline facets and surrounded by a nonuniform electric field.…”