Reactions of liquid and solid aluminum clusters with N2: the role of structure and phase in Al114 (+), Al115 (+), and Al117 (+).

Kinetic energy thresholds have been measured for the chemisorption of N2 onto Al114 (+), Al115 (+), and Al117 (+) as a function of the cluster's initial temperature, from around 200 K up to around 900 K. For all three clusters there is a sharp drop in the kinetic energy threshold of 0.5-0.6 eV at around 450 K, that is correlated with the structural transition identified in heat capacity measurements. The decrease in the thresholds corresponds to an increase in the reaction rate constant, k(T) at 450 K, of around 10(6)-fold. No significant change in the thresholds occurs when the clusters melt at around 600 K. This contrasts with behavior previously reported for smaller clusters where a substantial drop in the kinetic energy thresholds is correlated with the melting transition.

Activation of dinitrogen by solid and liquid aluminum nanoclusters: a combined experimental and theoretical study.

Cross sections for chemisorption of N2 onto Al44(+/-) cluster ions have been measured as a function of relative kinetic energy and the temperature of the metal cluster. There is a kinetic energy threshold for chemisorption, indicating that it is an activated process. The threshold energies are around 3.5 eV when the clusters are in their solid phase and drop to around 2.5 eV when the clusters melt, indicating that the liquid clusters are much more reactive than the solid. Below the melting temperature the threshold for Al44(-) is smaller than for Al44(+), but for the liquid clusters the anion and cation have similar thresholds. At high cluster temperatures and high collision energies the Al44N2(+/-) chemisorption product dissociates through several channels, including loss of Al, N2, and Al3N. Density functional calculations are employed to understand the thermodynamics and the dynamics of the reaction. The theoretical results suggest that the lowest energy pathway for activation of dinitrogen is not dynamically accessible under the experimental conditions, so that an explicit account of dynamical effects, via molecular dynamics simulations, is necessary in order to interpret the experimental measurements. The calculations reproduce all of the main features of the experimental results, including the kinetic energy thresholds of the anion and cation and the dissociation energies of the liquid Al44N2(+/-) product. The strong increase in reactivity on melting appears to be due to the volume change of melting and to atomic disorder.

Melting of size-selected aluminum nanoclusters with 84-128 atoms.

Heat capacities have been measured as a function of temperature for isolated aluminum nanoclusters with 84-128 atoms. Most clusters show a single sharp peak in the heat capacity which is attributed to a melting transition. However, there are several size regimes where additional features are observed; for clusters with 84-89 atoms the peak in the heat capacity is either broad or bimodal. For Al(115) (+), Al(116) (+), and Al(117) (+) there are two well-defined peaks, and for Al(126) (+), Al(127) (+), and Al(128) (+) there is a dip in the heat capacity at lower temperature than the peak. The broad or bimodal peaks for clusters with 84-89 atoms are not significantly changed by annealing to 823 K (above the melting temperature), but the dips for Al(126) (+), Al(127) (+), and Al(128) (+) disappear when these clusters are annealed to 523 K (above the temperature of the dip but below the melting temperature). Both the melting temperatures and the latent heats change fairly smoothly with the cluster size in the size regime examined here. There are steps in the melting temperatures for clusters with around 100 and 117 atoms. The step at Al(100) (+) is correlated with a substantial peak in the latent heats but the step at Al(117) (+) correlates with a minimum. Since the latent heats are correlated with the cluster cohesive energies, the substantial peak in the latent heats at Al(100) (+) indicates this cluster is particularly strongly bound.

Metal clusters with hidden ground states: Melting and structural transitions in Al115(+), Al116(+), and Al117(+).

Heat capacities measured as a function of temperature for Al(115)(+), Al(116)(+), and Al(117)(+) show two well-resolved peaks, at around 450 and 600 K. After being annealed to 523 K (a temperature between the two peaks) or to 773 K (well above both peaks), the high temperature peak remains unchanged but the low temperature peak disappears. After considering the possible explanations, the low temperature peak is attributed to a structural transition and the high temperature peak to the melting of the higher enthalpy structure generated by the structural transition. The annealing results show that the liquid clusters freeze exclusively into the higher enthalpy structure and that the lower enthalpy structure is not accessible from the higher enthalpy one on the timescale of the experiments. We suggest that the low enthalpy structure observed before annealing results from epitaxy, where the smaller clusters act as a nucleus and follow a growth pattern that provides access to the low enthalpy structure. The solid-to-solid transition that leads to the low temperature peak in the heat capacity does not occur under equilibrium but requires a superheated solid.

Phase coexistence in melting aluminum clusters.

The internal energy distributions for melting aluminum cluster cations with 100, 101, 126, and 127 atoms have been investigated using multicollision induced dissociation. The experimental results can be best fit with a statistical thermodynamic model that incorporates only fully solidlike and fully liquidlike clusters so that the internal energy distributions become bimodal during melting. This result is consistent with computer simulations of small clusters, where rapid fluctuations between entirely solidlike and entirely liquidlike states occur during the phase change. To establish a bimodal internal energy distribution, the time between the melting and freezing transitions must be longer than the time required for equilibration of the energy distribution (which is estimated to be around 1-2 micros under our conditions). For Al(100)(+) and Al(101)(+), the results indicate that this criterion is largely met. However, for Al(126)(+) and Al(127)(+), it appears that the bimodal energy distributions are partly filled in, suggesting that either the time between the melting and freezing transitions is comparable to the equilibration time or that the system starts to switch to macroscopic behavior where the phase change occurs with the two phases in contact.

Melting dramatically enhances the reactivity of aluminum nanoclusters.

The kinetic energy threshold for chemisorption of N(2) on Al(100)(+) has been measured as a function of the nanocluster's temperature from 440 to 790 K. When the Al(100)(+) cluster melts at 620-660 K, the threshold drops by approximately 1 eV (approximately 96 kJ/mol). A decrease in the activation energy of this magnitude causes a 10(8)-fold increase in the reaction rate at the melting temperature. The decrease in the activation energy may result from the mobility of the surface atoms on the liquid cluster, which allows them to move to a lower energy arrangement as the N(2) approaches.

Metal clusters that freeze into high energy geometries.

Heat capacities measured for isolated aluminum clusters show peaks due to melting. For some clusters with around 60 and 80 atoms there is a dip in the heat capacities at a slightly lower temperature than the peak. The dips have been attributed to structural transitions. Here we report studies where the clusters are annealed before the heat capacity is measured. The dips disappear for some clusters, but in many cases they persist, even when the clusters are annealed to well above their melting temperature. This indicates that the dips do not result from badly formed clusters generated during cluster growth, as originally suggested. We develop a simple kinetic model of melting and freezing in a system consisting of one liquidlike and two solidlike states with different melting temperatures and latent heats. Using this model we are able to reproduce the experimental results including the dependence on the annealing conditions. The dips result from freezing into a high energy geometry and then annealing into the thermodynamically preferred solid. The thermodynamically preferred solid has the higher freezing temperature. However, the liquid can bypass freezing into the thermodynamically preferred solid (at high cooling rates) if the higher energy geometry has a larger freezing rate.