ABSTRACT
Ultra-small chromium oxide nanoclusters produced by laser vaporization in a fast flow tube reactor are ligand-coated by gas-phase reactions with acetonitrile vapor and then captured in a cold trap and transferred to solution. The resulting clusters are characterized with mass spectrometry, UV-visible absorption and emission spectroscopy, infrared spectroscopy, and surface-enhanced Raman spectroscopy. According to mass spectrometry, clusters of the form Cr xO y(MeCN) z are produced in the size range of x ≤ 10 and y < 25. The ligand-coated clusters in solution exhibit a limited number of prominent sizes, with the same preferences for specific stoichiometries seen in earlier gas-phase studies of ligand-free clusters. Computational studies provide structures and predicted spectra for these systems. The intrinsic stability of these clusters is confirmed by their production under different laser ablation conditions and by their significant shelf lives (several months) without aggregation or decomposition. UV-visible spectra indicate that these clusters contain highly oxidized chromium. Theory and previous experiments indicate that compact cages are favored for ligand-free clusters. However, infrared and Raman spectra suggest that ring and chainlike structures become prominent for ligand-coated clusters. Consistent with these observations, theory also indicates that these more open structures are energetically favored for ligated clusters. Apparently, ligand binding induces a structural transformation of the compact oxide core clusters, producing more extended ring and chain structures.
ABSTRACT
Cerium oxide cluster cations, CexOy(+), are produced via laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. The mass spectrum displays a strongly preferred oxide stoichiometry for each cluster with a specific number of metal atoms x, with x ≤ y. Specifically, the most prominent clusters correspond to the formula CeO(CeO2)n(+). The cluster cations are mass selected and photodissociated with a Nd:YAG laser at either 532 or 355 nm. The prominent clusters dissociate to produce smaller species also having a similar CeO(CeO2)n(+) formula, always with apparent leaving groups of (CeO2). The production of CeO(CeO2)n(+) from the dissociation of many cluster sizes establishes the relative stability of these clusters. Furthermore, the consistent loss of neutral CeO2 shows that the smallest neutral clusters adopt the same oxidation state (IV) as the most common form of bulk cerium oxide. Clusters with higher oxygen content than the CeO(CeO2)n(+) masses are present with much lower abundance. These species dissociate by the loss of O2, leaving surviving clusters with the CeO(CeO2)n(+) formula. Density functional theory calculations on these clusters suggest structures composed of stable CeO(CeO2)n(+) cores with excess oxygen bound to the surface as a superoxide unit (O2(-)).
ABSTRACT
Cobalt and nickel oxide cluster cations, Co(x)O(y)(+) and Ni(x)O(y)(+), are produced by laser vaporization of metal rods in a pulsed nozzle cluster source and detected using time-of-flight mass spectrometry. The mass spectra show prominent stoichiometries of x = y for Co(x)O(y)(+) along with x = y and x = y - 1 for Ni(x)O(y)(+). The cluster cations are mass selected and multiphoton photodissociated using the third harmonic (355 nm) of a Nd:YAG laser. Although various channels are observed, photofragmentation exhibits two main forms of dissociation processes in each system. Co(x)O(y)(+) dissociates preferentially through the loss of O(2) and the formation of cobalt oxide clusters with a 1:1 stoichiometry. The Co(4)O(4)(+) cluster seems to be particularly stable. Ni(x)O(y)(+) fragments reveal a similar loss of O(2), although they are found to favor metal-rich fragments with stoichiometries of Ni(x)O(x-1). The Ni(2)O(+) fragment is produced from many parent ions. The patterns in fragmentation here are not nearly as strong as those seen for early or mid-period transition-metal oxides studied previously.
