Optical Properties of Centric Diatom Frustules and Their Potential in Solar Interfacial Evaporation for Water Purification
Authors:   , Wu Jiazun 
Published: June 2023

<p><b>Diatoms, the unicellular algae capable of photosynthesis, are inhabitants of both freshwater and marine environments. These entities have their cellular protoplasm encapsulated within an amorphous silica cell wall, also known as frustules. Intriguing optical properties emerge from the atomic and micro-level structures of these amorphous silica frustules, paving the way for a plethora of potential applications. This thesis delineates the optical properties of two species of centric diatom frustules, namely \emph{Actinocyclus sp.} and \emph{Coscinodiscus sp.}, which have been subjected to rigorous cleansing procedures. The focus of this investigation was to accentuate the optical properties of \emph{Actinocyclus sp.} frustules augmented with plasmonic silver nanoparticles, with an overarching aim to cultivate an advanced material capable of facilitating high solar water interfacial evaporation rates, thereby catering to the effective solar-powered evaporation of water.</b></p>
<p>The thesis represents a pioneering effort to ascertain the absolute reflectance, transmittance, and absorptance spectra of a single-layered, close-packed array of meticulously cleaned frustules. This necessitated the innovation of a method for assembling an array of frustules on a planar fused silica substrate, and the creation of a purpose-built spectrometer proficient in measuring strongly scattering samples of several hundred micrometres in diameter.</p>
<p>Luminescence spectral characteristics of diatom frustules have been demystified in this research, focusing on photoluminescence (PL), cathodoluminescence (CL), and thermoluminescence (TL). By juxtaposing these with the widely recognized PL and CL properties of defects in fused quartz and synthetic silica nanoparticles synthesized via hydrolysis of tetraethylorthosilicate (sans adventitious metal ion impurities), luminescent defects associated with Al\textsuperscript{3+} ions were identified within the silica of diatom frustules. Additionally, the TL properties of frustules were characterized for the first time, revealing similar quantities and types of traps as those present in fused silica and silica nanoparticles.</p>
<p>The hierarchical porous silica structures of diatom frustules, enhanced with plasmonic metal nanoparticles, hold considerable potential for applications in solar-powered interfacial water evaporation. A novel one-pot synthesis method was developed for creating a single layer of plasmonic silver nanoparticles on a planar quartz surface with a controlled area filling fraction. This method resulted in the observation of localized surface plasmonic resonances with wavelength maxima increasing monotonically with the filling fraction. Surface-enhanced Raman scattering and photoluminescence were also noted. The same synthesis method facilitated the decoration of porous frustule silica structures with a dense monolayer of silver nanoparticles. The enhanced frustules exhibited potent, broadband light absorption across the solar spectrum due to an array of silver nanoparticle sizes and shapes, and the inherent hierarchical porous structures of the frustules. The application of silver nanoparticle decorated frustules on the surface of water droplets engendered a significantly amplified solar-powered interfacial water evaporation rate, approximately five times higher than that of a bare water droplet.</p>