Self-assembled gold nanochain based materials as building blocks to fabricate flexible devices and electrocatalysts

Abstract

1D metal nanostructures are being actively researched as a component in next-generation electronic devices and as highly efficient electrocatalysts. 1D gold nanomaterials can be designed by controlling the aspect ratio and introducing multiple chemical elements in their structure. In this work, gold nanochains are self-assembled using electrostatic interactions by controlled addition of cations into a colloidal solution of citrate stabilized Au NPs. The UV-Vis spectrum shows the shift in the LSRP due to the assembly of the Au NPs into micrometer scale chains. The self-assembled chains have a gap of 1 to 2 nm between adjacent Au NPs. This gap can serve as a quantum tunneling barrier for the electrons transport between adjacent NPs. In part A, we study the gold chains with two kinds of morphologies: 1) the AuCa NCs with 1 to 2 nm gaps between adjacent Au NPs; 2) AuPt NWs with continuous structure and highly branched morphology. In Chapter 2, AuCa NCs are mainly investigated. The chains are used as building blocks to fabricate a range of flexible devices to monitor human physiology signals based on the modulation of the tunneling barrier, including temperature, artery pulsation, and ECG by simple filtration method. In Chapter 3, the two kinds of 1D Au nanomaterials are researched to understand the relationship between the morphology and conductivity under tensile strain. We develop an easy process to produce stretchable devices through a filtration and transfer process. During the transfer process, microcracks are introduced, which facilitate the transport pathway for electrons under strain. AuPt NWs devices with continuous structure can endure larger strain than AuCa NCs devices because of the extended wire-like continuous morphology. We also integrate AuPt NWs as electrodes with perovskite to produce photodetectors, which are highly bendable and stretchable. In part B, multiple elements are integrated into Au nanochains to explore their electrocatalytic performance for various applications. Stable bifunctional water splitting catalysts can simplify the development of alkaline medium electrolysers. In this work, a catalytic material with controlled nanoscale domains of Pt and Ni is formed by a self-assembly process at room temperature. The final structure of the catalyst is achieved through two stage transformation, first formation of Pt-Ni nanoscale domains and then inducing Ni to higher oxidation states. The material has a nanowire like morphology at the macroscale, which ensures rapid kinetics and mass transfer. The results prove that the nanoscale domains of Pt, Ni and Ni2+ and Ni3+ with close interfacing are crucial for the performance of the electrocatalyst and ensure the presence of Ni in a high oxidation state, leading to both HER and OER activity. The catalyst has a low overpotential and ultralow Tafel slope for both HER and OER, 13.7 mV dec-1 and 32 mV dec-1, respectively, crucial for high power applications. Similarly, in Chapter 5, Pt and Ni integrated gold chains are investigated as a dopamine sensor in neutral pH. The size of Pt and Ni domain can be tuned in the range of complete homogenous mixing to 2-3 nm size nanoscale domains. The AuNiAuPt-R sample with nanoscale domains shows the best detection performance with high sensitivity of 1279.3 μA mM-1 cm-2 in the dopamine concentration range of 0.1-36.5 μM. In Chapter 6, Ni and Cu integrated gold chains are designed as a glucose detector in 0.1 M NaOH. The AuNiAuCu-R sample with nanoscale domains demonstrates the highest sensitivity of 643.9 μA mM-1 cm-2 with high stability, high selectivity, reproducibility, and low limit of detection (LoD). The detection range fits the glucose level in saliva, sweat, and blood, which is possible for the practical analytical application. The research in part B shows that nanometer scale elemental domains are critical for efficient electrocatalysis. This work presents two studies: first, research and understanding the electrical performance of two kinds of micron scale 1D gold nanomaterials by using a simple filtration method to fabricate their films and devices. Second, the synthesis process can control the spatial distribution of elements to design the materials for specific electrocatalytic applications. The work presents a simple method to prepare flexible and stretchable devices and a new facile self-assembly avenue for the use of electrical double layer as new composite catalysts.