Theoretical and Experimental Evaluation of the Reduction Potential of Straight-Chain Alcohols for the Designed Synthesis of Bimetallic Nanostructures

Abstract

Recently, the development of bimetallic nanoparticles with functional properties has been attempted extensively but with limited control over their morphological and structural properties. The reason was the inability to control the kinetics of the reduction reaction in most liquid-phase syntheses. However, the alcohol reduction technique has demonstrated the possibility of controlling the reduction reaction and facilitating the incorporation of other phenomena such as diffusion, etching, and galvanic replacement during nanostructure synthesis. In this study, the reduction potential of straight-chain alcohols has been investigated using molecular orbital calculations and experimentally verified by reducing transition metals. The alcohols with a longer chain exhibited higher reduction potential, and 1-octanol was found to be the strongest among alcohols considered. Furthermore, the experimental evaluation carried out via the synthesis of metallic Cu, Ni, and Co particles was consistent with the theoretical predictions. The reaction mechanism of metallic particle formation was also studied in detail in the Ni–1-octanol system, and the metal ions were confirmed to be reduced via the formation of nickel alkoxide. The results of this investigation were successfully implemented to synthesize Cu–Ni bimetallic nanostructures (core–shell, wire, and tube) via the incorporation of diffusion and etching besides the reduction reaction. These results suggest that the designed synthesis of a wide range of bimetallic nanostructures with more refined control has become possible.

Strategy to Design-Synthesize Bimetallic Nanostructures Using the Alcohol Reduction Method

“…The synthesis of metallic Cu and Co nanoparticles has been carried out using acetate salts in both 1-heptanol and 1-octanol. 26 Thus, the synthesis of Cu−Co alloy nanoparticles was attempted via coreduction under similar experimental conditions. The reactions were carried out nearly at the boiling points of 1-heptanol and 1-octanol, 170 and 190 °C, respectively.…”

“…Although the behavior was similar in both solvents, the particles precipitated faster in the case of 1-octanol due to the higher reducing power. 26 Figure 2 shows the scanning electron microscopy (SEM) image, EDS mapping images, and X-ray diffraction (XRD) patterns of the samples obtained using 1heptanol and 1-octanol.…”

“…Neither Cu and Co alkoxides nor their respective oxide phases were observed. 26 Generally, Co crystallizes in a hcp structure under standard atmospheric conditions, 32 while it crystallizes in fcc due to two effects: crystallite size 33,34 and epitaxial growth. 35 In either case, the fcc structure has not been observed unless the crystallite size of Co is smaller than ∼20 nm.…”

“…28,29,39 However, either the complexing or protecting effect becomes productive only when the reducing solvent is comparatively weak, such as 1-heptanol compared to 1-octanol. 26 Thus, the effect of oleylamine during the formation of Cu−Co alloy in 1heptanol is discussed in the next paragraphs. Figure 2a,biii,iv shows the XRD patterns and the SEM and elemental mapping images of the Cu−Co samples synthesized using 1-heptanol in the presence of 30 and 140 mmol of oleylamine.…”

“…Recently, the reduction power of straight-chain alcohols such as 1-heptanol and 1-octanol has been investigated theoretically using molecular orbital calculations and experimentally verified by reducing transition metals such as Cu, Ni, and Co. 26 Hence, alcohol results in a milder reducing agent than polyol and possesses the adequate potential to reduce transition metals. Consequently, the size and shape of monometallic particles can be tuned up by controlling the kinetics of the reduction reaction.…”