Laser Synthesis and Spectroscopy of Ligand-Coated Chromium Oxide Nanoclusters

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 O(MeCN) 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.

Laser Synthesis and Spectroscopy of Molybdenum Oxide Nanorods

“…Because this wall-deposited material was not entrained in the gas flow and not exposed to the ligand vapor, it was insoluble; its analysis was consistent with larger MoO 3 material (see Figure S2 in the Supporting Information). Under all conditions employed, mass spectrometry measurements showed that these colored solutions contained no small ligand-coated clusters in the molecular size range like those seen previously for titanium, vanadium, or chromium oxides. ,, …”

“…To produce clusters or particles of molybdenum oxide in the laser vaporization flow reactor, − a metal rod was ablated with a 248 nm high power (100–140 mJ/pulse)/high repetition rate (300–400 Hz) excimer laser (Coherent LPX Pro 240F, KrF). The laser was focused to a spot size of 3 × 10 mm under a flow of 2–10% O 2 in helium.…”

“…The synthesis of ligand-coated clusters and nanoparticles with the laser vaporization flow reactor has been described previously. − This method produces gas-phase metal vapor with laser ablation in a flowing inert gas mixture, where collisional cooling and energy transfer lead to particle growth. Subsequently, gas-phase reactions lead to ligand coating, followed by cryogenic trapping and transfer to solution.…”

“…Subsequently, gas-phase reactions lead to ligand coating, followed by cryogenic trapping and transfer to solution. This method has been used to synthesize molecular-sized ligand-coated metal oxide clusters of titanium, vanadium, or chromium, ,, as well as larger, ligand-coated nanoparticles of silver . In the present study, we investigate the molybdenum oxide clusters and/or nanoparticles that may be produced by these methods.…”

The emergence of mass spectrometry for characterizing nanomaterials. Atomically precise nanoclusters and beyond

“…[251][252][253] Combining laser desorption and spray as new « out-of-equilibrium » media will allow for reactivity within droplets and/or laser ablation plume and can lead to new synthetic routes for nanomaterials. 219,[254][255][256]…”