Manganese(II) Oxide Nanohexapods:  Insight into Controlling the Form of Nanocrystals

Abstract

Cross-shaped and octahedral nanoparticles (hexapods) of MnO in size, and fragments thereof, are created in an amine/carboxylic acid mixture from manganese formate at elevated temperatures in the presence of water. The nanocrosses have dimensions on the order of 100 nm, but with exposure to trace amounts of water during the synthesis process they can be prepared up to about 300 nm in size. Electron microscopy and X-ray diffraction results show that these complex shaped nanoparticles are single crystal face-centered cubic MnO. In the absence of water, the ratio of amine to carboxylic acid determines the nanocrystal size and morphology. Conventionally shaped rhomboehdral/square nanocrystals or hexagonal particles can be prepared by simply varying the ratio of tri-n-octylamine/oleic acid with sizes on the order of 35-40 nm in the absence of added water. If the metal salt is rigorously dried before the synthesis, then "flower-shaped" morphologies on the order of 50-60 nm across are observed. Conventional squareshaped nanocrystals with clearly discernible thickness fringes that also arise under conditions producing the nanocrosses mimic the morphology of the cross-shaped and octahedral nanocrystals and provide clues to the crystal growth mechanism(s), which agree with predictions of crystal growth theory from rough, negatively curved surfaces. The synthetic methodology appears to be general and promises to provide an entryway into other nanoparticle compositions.

Organic Reaction Pathways in the Nonaqueous Synthesis of Metal Oxide Nanoparticles

“…Nowadays, nonaqueous synthesis approaches have become a valuable alternative to aqueous systems and the family of metal oxide nanoparticles synthesized by nonhydrolytic reaction routes is growing rapidly, [74][75][76][77] including TiO 2 , [78][79][80][81][82][83][84][85] FeO x , [86][87][88][89][90][91][92][93][94][95][96][97][98][99] MnO, [86,90,[100][101][102][103][104] Mn 3 O 4 , [84,100,105] CeO 2 , [106] CuO x , [105,107] Cr 2 O 3 , [90] Co 3 O 4 , [90,108] CoO, [105,109] NiO, [90,102,110] ReO 3 , [111] ZrO 2 , [112,…”

Shell pressure on the core of MnO/Mn3O4core/shell nanoparticles

“…31 The formation of brownish oxidized MnO NPs and nanopods was also reported after the decomposition of H 2 O:Mn-ac in different ratios of trioctylamine:oleic acid. 32 Another example is the decomposition of Mn-acetate in trioctylamine and oleic acid under N 2 , leading to the formation of MnO NPs with sizes from 7 to 20 nm. 33 Again, the authors mention the formation of a black dispersion at high temperature suggesting that the MnO NPs are surface oxidized.…”

Shape control of new FexO–Fe3O4and Fe1–yMnyO–Fe3–zMnzO4 nanostructures

“…[24] The shaping process, as seen in other similar systems, agrees with predictions of crystal growth theory from rough, negatively curved surfaces coupled with a digestive ripening mechanism. [17] Such processes lead to symmetrical shapes like stellated polyhedra or Archimedean solids such as the tetracubes obtained here.…”

“…4). [17] The lattice spacing was determined to be 4.383 Å for the Fe 1-x Mn x O phase and 8.402 Å for the Fe 3-z Mn z O 4 phase by powder X-ray diffraction. An expanded view on a point on the FeMnOX NP clearly shows the atomic planes ( Fig.…”

“…The need to understand NP shape formation has prompted investigations of shape-control syntheses and the development of new growth-mechanism theories. [15,[17][18][19][20][21][22][23][24] Understanding the growth mechanisms of anisotropic and multibranched NPs is essential in order to control their production for technological applications. We report herein an investigation on shape control of new shapes of iron and iron-manganese oxide NPs as a function of reaction parameters: precursor type and concentration, surfactant concentration, water concentration, reaction time, and temperature.…”