Controlling the reduction rate and oxidative etching for high yield green synthesis of palladium nanorods in segmented millifluidic flow reactors (SMFRs)

Abstract

Recent studies have demonstrated the effectiveness of Pd nanorods (PdNRs) in a wide variety of applications, which makes them highly desired nanocrystals. Green synthesis methods have resulted in very low PdNR yields (12% to 15%) in the past, therefore, making them less attractive for PdNR synthesis compared to conventional, less sustainable synthesis methods such as the polyol process. The lower yield of PdNRs is partly caused by oxidative etching that reduces the proportion of multiple-twinned-structures produced during nucleation, which act as the seeds for the growth of PdNRs. The rate of reduction also plays an essential role in determining the morphology of synthesized palladium (Pd) nanostructures. In this study, PdNRs were synthesized in segmented millifluidic flow reactors (SMFRs) using L-ascorbic acid as a green reducing agent. Application of SMFRs minimizes oxidative etching, while improves mass and heat transfer control to maximize the PdNR yield. Pd precursors containing different halides demonstrate varying levels of oxidative etching. Each precursor is also reduced at a different rate by L-ascorbic acid. Both oxidative etching and the rate of reduction can be adjusted by using different Pd precursors as a novel approach to further increase the yield of PdNRs in SMFRs. Reactions were conducted using Pd(NO₃)₂, PdCl₂, Na₂PdCl₄, and Na₂PdBr₄ and their combination as Pd precursors for the first time in SMFRs to the best of our knowledge. The synthesized Pd nanostructures were characterized using transmission electron microscopy (TEM) to compare the PdNR yields resulting from each reaction. The rate of reduction in each reaction was determined using kinetic studies by measuring the remaining palladium species concentration in the aqueous reaction solution using ultraviolet-visible (UV-vis) spectroscopy. It was found that the combination of two palladium precursors at an appropriate molar ratio would provide a better control of oxidative etching and the reduction rate, simultaneously. The synthesis process using Pd(NO₃)₂ and Na₂PdBr₄ in a 1 : 1 molar ratio as palladium precursors yielded the highest percentage of PdNRs over 90%.