Branched metal and alloy nanoparticles for plasmonic and electrochemistry applications
Tran, Thi Nhung
Date of Issue2017
School of Materials Science and Engineering
Noble metal nanoparticles are fundamental to modern science and technology and have been utilized in many applications such as sensing, energy, biology, and medicine. Manipulation of the morphology and composition of noble metal nanoparticles enables the control over their optical/electrochemical properties and also to improve their performance in targeted applications. Among different classes of morphology-engineered nanoparticles, branched nanoparticles have attracted tremendous research attention recently due to their unique optical and electrochemical properties. The high charge density and charge polarization at tip extremities of branched nanoparticles generate a huge enhancement of the local electromagnetic (EM) field and a tunable localized surface plasmon resonance (LSPR) response over a wide spectral range. In addition, branched nanoparticles also exhibit pronounced electrocatalytic efficiency attributed to their large surface area, the exposure of certain high-index facets at tip edges/corners, and the synergetic effect arisen from the alloying character. However, the anisotropic growth of highly branched nanoparticles is not energetically favored in solution because of their large surface energy and the high crystalline symmetry of face centered cubic (fcc) metal nanocrystals. The growth of highly branched nanoparticles often required multi-steps, high pressure/temperature treatment, or the addition of capping agents which led to tedious and time-consuming purification steps. Besides, the synthesis of alloy nanoparticles composing of multi-metals is also challenging because of the mismatches in the reduction potential, solubility, and lattice of all metal precursors. Therefore, my aim is to develop new and facile routes for the synthesis of complex branched metallic particles and further explore their applications in plasmonics and electrocatalysis. Herein, I developed a one-step, aqueous-based, and surfactant-free method to synthesize highly branched gold mesoflowers (AuMFs) with multilevel long and sharp tips. The method enables me to control the size, morphology, and the optical properties of the AuMFs by manipulating the reaction condition. I also demonstrated that chlorite ions Clˉ played a critical role in the formation of long and sharp tips and the tip branching by regulating the formation of multiple twin defects in the crystal structure of growing particles. The branched AuMFs exhibit a LSPR peak maximum in the NIR to MIR region (~1800-3000 nm), depending on the tip morphology. The simulation analysis based on finite-difference time domain (FDTD) model was also conducted to elucidate the effect of tip morphology on the optical property of the AuMFs. The simulation was done in collaboration with researchers in prof. Li Shuzou’s group, MSE, NTU with the assistance on the simulation. Besides, the galvanic replacement reaction was often used in the seed mediated growth approaches to form hollow alloy metallic nanoparticles. I also established a new way to synthesize highly branched trimetallic hollow PdAgCu nanoparticles by coupling galvanic replacement with co-reduction reaction. The particles exhibited a high density of multiple tips on the surface, hollow interiors, and a tunable LSPR response in the NIR region. I also investigated the refractive index (RI) sensitivity and electrocatalytic activity of branched trimetallic PdAgCu nanoparticles towards ethanol oxidation reaction (EOR). The results showed that highly branched trimetallic PdAgCu nanoparticles exhibited a higher RI sensitivity and pronounced electrocatalytic improvement compared to that of branched bimetallic PdAg and PdCu nanoparticles. This work was done in collaboration with researchers in Prof. Jason’s group, MSE, NTU with the assistance on electrocatalytic measurement and discussion.