Ordered porous gold/titanium dioxide (Au/TiO2) hybrid nanostructured films are specifically interesting in large-scale applications using localized surface plasmon resonances (LSPRs) and surface-enhanced Raman scattering (SERS). Deposition of Au nanoparticles via sputter deposition is one of the promising technologies to establish optically active sites at low cost in combination with nanostructured TiO2 films. In this work, we investigate the optical response of sputter-deposited Au/TiO2 nanohybrid thin films with a focus on the plasmonic response and application as molecular sensors. The LSPR peak red shifts with an increasing thickness of deposited Au. The Raman intensity of deposited molecules, probed with rhodamine 6G (R6G), depends on the deposited gold thickness. It has its maximum at an effective Au thickness of 3.4 nm. To elucidate the origin of this behavior, we apply in situ grazing-incidence small-angle X-ray scattering (GISAXS) to investigate the growth kinetics of Au on a TiO2 template during sputter deposition. On the basis of time-resolved GISAXS, the growth characteristics of sputter-deposited Au on a TiO2 template with a final effective Au layer thickness around the percolation threshold is described with the well-known four-stage model of nucleation and cluster formation, diffusion-mediated growth, adsorption-mediated growth, and grain growth. The maximum in SERS intensity is corroborated by the existence and optimal size of hot spots in the narrow space occurring between the sputter-deposited Au clusters, on staying below the percolation threshold. On the basis of the growth laws extracted, we give a guideline for tailoring the ordered porous Au/TiO2 nanohybrid thin films for SERS sensor applications.
Nafion ionomer, composed of hydrophobic perfluorocarbon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for electrochemical CO2 reduction, but its impact on the performance of CO2 electrolysis remains poorly understood. Here, we systematically investigate the role of the catalyst ink formulation on CO2 electrolysis using commercial CuO nanoparticles as the model pre-catalyst. We find that the presence of Nafion is essential for achieving stable product distributions due to its ability to stabilize the catalyst morphology under reaction conditions. Moreover, the Nafion content and solvent composition (water/alcohol fraction) regulate the internal structure of Nafion coatings, as well as the catalyst morphology, thereby significantly impacting CO2 electrolysis performance, resulting in variations of C2+ product Faradaic efficiency (FE) by >3×, with C2+ FE ranging from 17 to 54% on carbon paper substrates. Using a combination of ellipsometry and in situ Raman spectroscopy during CO2 reduction, we find that such selectivity differences stem from changes to the local reaction microenvironment. In particular, the combination of high water/alcohol ratios and low Nafion fractions in the catalyst ink results in stable and favorable microenvironments, increasing the local CO2/H2O concentration ratio and promoting high CO surface coverage to facilitate C2+ production in long-term CO2 electrolysis. Therefore, this work provides insights into the critical role of Nafion binders and underlines the importance of optimizing Nafion/solvent formulations as a means of enhancing the performance of electrochemical CO2 reduction systems.
Molybdenum nitride (MoN x ) was perceived as carrier-selective contacts (CSCs) for crystalline silicon (c-Si) solar cells due to having proper work functions and excellent conductivities. However, the poor passivation and non-Ohmic contact at the c-Si/MoN x interface endow an inferior hole selectivity. Here, the surface, interface, and bulk structures of MoN x films are systematically investigated by X-ray scattering, surface spectroscopy, and electron microscope analysis to reveal the carrier-selective features. Surface layers with the composition of MoO2.51N0.21 form upon air exposure, which induces the overestimated work function and explains the origin of inferior hole selectivities. The c-Si/MoN x interface is confirmed to adopt long-term stability, providing guidance for designing stable CSCs. A detailed evolution of the scattering length density, domain sizes, and crystallinity in the bulk phase is presented to elucidate its superior conductivity. These multiscale structural investigations offer a clear structure–function correlation of MoN x films, providing key inspiration for developing excellent CSCs for c-Si solar cells.
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