Thin-Film Exsolution of Metal Nanoparticles and Their Galvanic Restructuring for Plasmonically Enhanced Photocatalytic Activity

The exsolution and precipitation of metal nanoparticles (MNPs) from the bulk of a host material have been studied extensively due to their ability to create well-socketed particles on the host material’s surface. This method is particularly useful in catalysis, as these supported catalysts generally have a longer lifetime due to the better adhesion with the substrate. This has been extensively applied to the field of solid oxide fuel cells for electrochemical energy conversion. But, could we apply it to semiconducting materials and improve their photoelectrocatalytic properties?<br/><br/>Most metals considered for exsolution (e.g., Ni, Fe, Co) do not possess notable plasmonic properties, rendering the possibility of increasing the efficiency of wide band gap semiconductors, such as strontium titanate (STO) through the surface plasmon resonance of socketed and plasmonically active nanoparticles. However, silver (Ag) and copper (Cu) can be substituted into STO and forced to exsolve by introducing a reducing environment at elevated temperatures. Moreover, utilizing the principle of galvanic replacement/deposition reaction allows the introduction of other plasmonically active elements, such as gold (Au), despite their inability to be exsolved. We have recently demonstrated this principle by exsolving nickel (Ni) and replacing it with the plasmonically active Au [1].<br/><br/>The direct exsolution or exsolution with subsequent galvanic replacement offer not only the ability to create well-socketed particles, but the latter also ensures that the precious metals are only present on the surface, reducing the amount of precious metals significantly. Moreover, the combination of exsolution and galvanic replacement allows the synthesis of materials with two different MNPs, one located in bulk and one sticking out of the surface. The combination of two plasmonically active elements enables an additional degree of freedom to adjust the absorption spectrum of the final device. In this work, Ag-doped thin films and Cu-doped thin films have been exsolved. Additionally, Ni MNPs created by exsolution have been galvanically replaced by Ag and Au. The samples were characterized by X-Ray Diffraction and electron microscopy (both scanning electron microscopy and scanning transmission electron microscopy), including energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. These techniques allowed the study of thin films on the nanometer scale. Additionally, the photoelectrochemical performance was determined, and a clear improvement was found between a sample with Au MNPs, and samples without MNPs.<br/><br/>Moreover, the shape, geometry, and the STO-matrix in proximity influence the peak position drastically. Combined with Finite Difference Time Domain (FDTD) calculations, the results indicate that hot charge carriers are mainly responsible for enhancing photocatalytic properties. Recently, we considered MNPs of two elements close to each other, where one is plasmonically active and the other inactive (i.e., Au and Ni). The plasmonically inactive MNPs showed enhanced absorption while near the plasmonically active MNPs [1]. This configuration is evidence of a reactor-antenna formation, further increasing the possible structures obtained by our proposed methodology.<br/><br/>In brief, thin films with well socketed MNPs were created by exsolution and galvanic replacement. The particles were well dispersed and plasmonically active. The photoelectrochemical response was enhanced in comparison to thin films without MNPs.<br/><br/>[1] Kang, X., Reinertsen, V. M., Both, K. G., Galeckas, A., Aarholt, T., Prytz, Ø., Norby, T., Neagu, D., Chatzitakis, A., Galvanic Restructuring of Exsolved Nanoparticles for Plasmonic and Electrocatalytic Energy Conversion. <i>Small</i> 2022, 2201106. https://doi.org/10.1002/smll.202201106