Part I: NiMoO4 Nanostructures Synthesized by the Solution Combustion Method: A Parametric Study on the Influence of Synthesis Parameters on the Materials' Physicochemical, Structural, and Morphological Properties

Abstract

The impact of process conditions on the synthesis of NiMoO₄ nanostructures using a solution combustion synthesis (SCS) method, in which agar powder and Ni(NO₃)₂ were utilized as fuel and as the oxidant, respectively, was thoroughly studied. The results show that the calcination temperature had a significant implication on the specific surface area, phase composition, particle size, band gap, and crystallite size. The influence of calcination time on the resulting physicochemical/structural/morphological properties of NiMoO₄ nanostructures was found to be a major effect during the first 20 min, beyond which these properties varied to a lesser extent. The increase in the Ni/Mo atomic ratio in the oxide impacted the combustion dynamics of the system, which led to the formation of higher surface area materials, with the prevalence of the β-phase in Ni-rich samples. Likewise, the change in the pH of the precursor solution showed that the combustion reaction is more intense in the high-pH region, entailing major implications on the physicochemical properties and phase composition of the samples. The change in the fuel content showed that the presence of agar is important, as it endows the sample with a fluffy, porous texture and is also vital for the preponderance of the β-phase.

Keywords: agar; nanostructures; nickel molybdate; parametric study; physicochemical properties; solution combustion synthesis.

Figure 1

XRD patterns of NiMoO₄ samples produced at different calcination temperatures; (α) α-NiMoO₄; (β) β-NiMoO₄, (*): MoO₂, (♦) MoO₃, and (Δ) NiO. Calcination time: 6 h, pH = 4.57, and fuel-to-oxidant ratio φ = 1.

Figure 2

The effect of calcination temperature on the (a) crystallite size, (b) specific surface area and pore volume, (c) BJH pore size distribution, and (d) band gap of the synthesized NiMoO₄ materials. Calcination time: 6 h, pH = 4.57, and φ = 1.

Figure 3

FTIR spectra of NiMoO₄ samples produced at different calcination temperatures. Calcination time: 6 h, pH = 4.57, and φ = 1.

Figure 4

Representative SEM images illustrating the effect of calcination temperature on the surface morphology of NiMoO₄ samples. Calcination time: 6 h, pH = 4.57, and φ = 1.

Figure 5

The effect of calcination time on the (a) crystallite size, (b) specific surface area and pore volume, (c) BJH pore size distribution, and (d) band gap of the synthesized NiMoO₄ materials. Calcination temperature: 500 °C, pH = 4.57, and φ = 1.

Figure 6

Variation in the theoretically calculated adiabatic temperature and amount of gas evolved as a function of (a) x, where x stands for the atomic fraction of Ni in Ni_xMo_1-x-oxide, (b) pH of the precursor solution, and (c) fuel-to-oxidant ratio (φ).

Figure 7

The effect of composition (depicted by x, which stands for the atomic fraction of Ni in Ni_xMo_1-x-oxide) on the (a) crystallite size, (b) specific surface area and pore volume, (c) BJH pore size distribution, and (d) band gap measurements of the synthesized of Ni_xMo_1-x-oxide samples. Calcination temperature: 500 °C, calcination time: 6 h, pH = 4.57, and φ = 1.

Figure 8

The effect of the precursor solution’s pH on the (a) crystallite size, (b) specific surface area and pore volume, (c) BJH pore size distribution, and (d) band gap of the synthesized NiMoO₄ materials. Calcination temperature: 500 °C, calcination time: 6 h, and φ = 1.

Figure 9

The effect of fuel-to-oxidant ratio (φ) on the (a) crystallite size, (b) specific surface area and pore volume, (c) BJH pore size distribution, and (d) band gap of the synthesized NiMoO₄ materials. Calcination temperature: 500 °C, calcination time: 6 h, and pH = 4.57.