Solvent dependent growth of one-dimensional crystalline ß-FeOOH nanorods

Abstract

Several authors have reported on the use of alcohols – water /or mixed solvents to synthesise metal oxide nanoparticles. However, no systematic study has been carried out to evaluate the effect of mixed solvent on the particle characteristics, although considerable research has been reported, a gap still exists with regard to the effect of the alcohols as solvents on the growth kinetics of nanoparticles. To address these issues, four different alcohols, namely, methanol (MeOH), ethanol (EtOH), propanol (PrOH) and butanol (BuOH) were used as solvents in the synthesis of β-FeOOH particles. The effect of organic solvents on the growth kinetics of β-FeOOH nanorods has been evaluated for the first time in this study. Two-stage growth of akaganeite nanorods has been observed in BuOH and PrOH. The first growth stage follows a typical power law representing Ostwald ripening (OR) kinetic. The second stage was found to be asymptotic and obeyed oriented attachment (OA) kinetic. The proof of the OA kinetic also comes from the HRTEM images of the synthesised particles. Simultaneous occurrence of the two mechanisms was observed in the growth of the particles synthesised in EtOH and MeOH. The rate constants for OR kinetic, KOR, was found to be higher than the rate constant for OA kinetic, KOA, for different solvents used. Preamble

The use of a mixed solvent is a new approach in the synthesis and processing of materials. Various researchers have stated that the surface tension of the solvent plays an important role in the formation of uniform nanorods. However, the effect of surface tension was not correlated with the particle growth, earlier, though the dielectric properties of the mixed solvents were only taken into account. Additionally, no quantitative or qualitative relationship was presented between surface tension and particle growth in the literature. In this work an attempt to correlate these two parameters (surface tension and particle growth) with the concentration of the precursor and temperature was made, resulting in an exponential relationship between KOR for the particle growth and surface tension of the alcohols. Furthermore, the relationship between surface tension and particle growth was validated by an independent study using statistically designed experiments to account for the influence of various process variables on the particle growth. The findings in this study obtained from both theoretical and experimental work provides an insight into the relationship between solvent surface tension and particle growth interactions, producing a new piece of information that will further promote our understanding of the formation mechanisms of β- FeOOH growth. The transformation temperature of akaganeite (β-FeOOH) nanorods to hematite (α-Fe2O3) particles was found to be solvent dependent. Thermogravimetric analysis and differential scanning calorimetry were performed to evaluate the effect of alcohol on the thermodynamic stability of the particles. Alcohol as solvent played a significant role in the dehydration property of the synthesised particles. The percentage mass loss of the particles at 300°C decreases linearly with increasing carbon number in the linear alkyl chain of the solvent. The effect of alcohol type on the particle morphology was found to be more pronounced at higher FeCl3 concentrations (>0.5M). Splitting of β-FeOOH nanorods was observed at FeCl3 concentration of 0.7M in BuOH. In PrOH, rectangular morphologies were obtained whereas nanoribbons resulted in surfactant-free conditions. It was found that the nature of anions (chloride vs. nitrate and sulphate) in the precursor salt also influenced the morphology.