Strong Chiroptical Activity in Cobalt Oxide/Hydroxide Nanoparticles Passivated by Chiral Nonthiol Amino Acid Proline

Abstract

Semiconducting transition metal oxide-based nanoarchitectures with chirality are a newly developed research area attracting significant attention because of their wide variety of structures with tunability and their versatile optical/chiroptical properties. In this Article we report the successful synthesis of chiroptically active cobalt oxide/hydroxide nanoparticles passivated by chiral nonthiol ligand proline using a solvothermal approach. Crystalline phases and optical/chiroptical properties of the obtained nanoparticles are dependent on the amount of chiral proline added (or concentration) in the synthesis: In the absence of proline, crystalline cubic-phase Co 3 O 4 nanocubes are obtained, but the use of proline changes their phase to a mixture of Co 3 O 4 , CoO, and β-Co(OH) 2 with amorphous nature, which can be confirmed by X-ray photoelectron and X-ray absorption spectroscopy. Notably, chiroptical responses of the nanoparticles increase with increasing the proline concentration, resulting in the maximum anisotropy factor (g-factor) of about 0.014 in the visible light region. We discuss the origin of the observed chiroptical activity from a viewpoint of chiral distortions of lattices in the nanoparticle. We then anticipate that such chiral metal oxide-based nanomaterials synthesized using proline will be developed as a powerful motif for high-performance asymmetric catalysis and devices in the future.

Increasing the Catalytic Activity of Co3O4 via Boron Doping and Chemical Reduction for Enhanced Acetone Detection

“…The bandgap of 1.74 eV is associated with the O 2− → Co 2+ charge transfer (valence to conduction band excitation), whereas the bandgap of 2.52 eV can be assigned to O 2− → Co 3+ charge transfer (the Co 3+ level located below the conduction band). [ 49 ] Similarly, E g1 and E g2 of B‐Co 3 O 4 ‐350‐R were 1.48 and 2.22 eV, respectively (Figure 5d). The band alignments of the energy band structures of B‐Co 3 O 4 ‐350 and B‐Co 3 O 4 ‐350‐R are shown in Figure 5e (See the detailed calculation process in Supplementary Note S2, Supporting Information).…”

“…The bandgap of 1.74 eV is associated with the O 2− → Co 2+ charge transfer (valence to conduction band excitation), whereas the bandgap of 2.52 eV can be assigned to O 2− → Co 3+ charge transfer (the Co 3+ level located below the conduction band). [49] Similarly, E g1 and E g2 of B-Co 3 O 4 -350-R were 1.48 and 2.22 eV, respectively (Figure 5d). This phenomenon is attributed to the introduction of additional oxygen vacancies, resulting in the expansion of the impurity energy level into the impurity energy band, thereby reducing the ionization energy.…”

Size and Magnetic-Field Dependence of MCD Spectra of Magnetite (Fe3O4) Nanoparticles Synthesized by an l-Proline-Mediated Solvothermal Approach: Effect of Surface Spin Disorder

“…As can be seen, the average diameter of the nanoparticles decreased with an increase in the L-Pro concentration involved, indicating that L- Pro is very effective for size-regulating the magnetite nanoparticles. 18 Figure 3a and 3b represents IR absorption spectra of nanoparticle samples P00−P20 in the range of 400−4000 (full range) or 400−900 cm −1 , respectively. All samples exhibited very similar IR spectra to each other: (i) Two typical bands at ∼590 and ∼440 cm −1 correspond to vibrations of Fe 3+ −O and Fe 2+ −O bonds, respectively, corroborating the formation of magnetite with a mixed-valence state.…”

“…(iv) We could observe a weak shoulder at ∼1670 cm −1 especially for sample P20, corresponding to the bending mode of R 2 N−H, which suggests the minor presence of L-Pro onto the nanoparticle surfaces. 18 Hence in the present synthesis protocol, L-Pro definitely contributes to the growth suppression or size regulation during the magnetite nanoparticle formation, but it would mostly be removed via washing/purification procedures.…”

“…12,15−17 In this Article, we first report successful synthesis of sizetunable Fe 3 O 4 nanoparticles modified by a biocompatible molecule, L-proline ((S)-pyrrolidine-2-carboxylic acid). 18 L-Proline is one of the well-known amino acid organocatalysts for various stereoselective organic synthesis, and we find that a solvothermal reaction in the presence of iron(II) acetate, Lproline, and tert-butyl alcohol (tert-BuOH) provides well sizecontrolled Fe 3 O 4 nanoparticles. 19,20 The solvothermal method using tert-BuOH is known to be effective for obtaining small crystalline oxide nanodots, 19,20 and addition of L-proline then improves the size-tunability as well as dispersibility in polar solvents such as water and methanol.…”