Metal Oxide Nanoparticle Mediated Enhanced Raman Scattering and Its Use in Direct Monitoring of Interfacial Chemical Reactions

Abstract

Metal oxide nanoparticles (MONPs) have widespread usage across many disciplines, but monitoring molecular processes at their surfaces in situ has not been possible. Here we demonstrate that MONPs give highly enhanced (×10 4 ) Raman scattering signals from molecules at the interface permitting direct monitoring of their reactions, when placed on top of flat metallic surfaces. Experiments with different metal oxide materials and molecules indicate that the enhancement is generic and operates at the single nanoparticle level. Simulations confirm that the amplification is principally electromagnetic and is a result of optical modulation of the underlying plasmonic metallic surface by MONPs, which act as scattering antennae and couple light into the confined region sandwiched by the underlying surface. Because of additional functionalities of metal oxides as magnetic, photoelectrochemical and catalytic materials, enhanced Raman scattering mediated by MONPs opens up significant opportunities in fundamental science, allowing direct tracking and understanding of application-specific transformations at such interfaces. We show a first example by monitoring the MONPassisted photocatalytic decomposition reaction of an organic dye by individual nanoparticles. KEYWORDS: Metal oxide, plasmons, surface-enhanced Raman scattering, photocatalysis, interface T ransition-metal oxides, due to the strong correlations of their d electrons, give rise to a wide variety of phenomena such as magnetism, ionic conduction, metal−insulator transitions, multiferroicity, and superconductivity. 1 As a result, they have an extensive range of applications that include fuel cells, batteries, catalysts, sensors, and microelectronics. 1 Despite the resulting importance of molecular binding and surface reactivity, their utilization in plasmonic applications has been prevented by the tuning of their localized surface plasmon resonance (LSPR) into the infrared. 2−6 Surface-enhanced Raman scattering (SERS) is a popular plasmonic application utilizing ultraviolet (UV), visible (VIS), or near-infrared (NIR) excitation, which overcomes the extremely small scattering cross section (∼10 −30 cm 2 per molecule) in conventional Raman scattering 7 to yield a technique that offers noninvasive and nondestructive fingerprint characterization 8 with extensive applications in chemical and biological sensing. The amplification in SERS stems primarily from the electromagnetic (EM) enhancement (up to 10 14 ) 9 obtained by excitation of SPR. 10 This is accompanied by typically smaller and system-dependent chemical enhancement as a result of formation of chargetransfer complexes between adsorbate and the surface. 11 Therefore, for efficient and sensitive SERS detection of molecules, nanoscale structures fabricated entirely with coinage metals (especially Ag and Au) have been the materials of choice since their SPR is easily excited in the vis or NIR regions. On the other hand, use of metal oxide nanoscale materials for enhanced Raman scattering has remained confi...

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