Thomas E. Furtak

Surface-Enhanced Raman Scattering

The increasing applications of surface-enhanced Raman scattering (SERS) has led to the development of various SERS-active platforms (SERS substrates) for SERS measurement. This work reviews the current optimization techniques available for improving the performance of some of these SERS substrates. The work particularly identifies self-assembled-monolayer(SAM-) based substrate modification for optimum SERS activity and wider applications. An overview of SERS, SAM, and studies involving SAM-modified substrates is highlighted. The focus of the paper then shifts to the use of SAMs to improve analytical applications of SERS substrates by addressing issues including long-term stability, selectivity, reproducibility, and functionalization, and so forth. The paper elaborates on the use of SAMs to achieve optimum SERS enhancement. Specific examples are based on novel multilayered SERS substrates developed in the author’s laboratory where SAMs have been demonstrated as excellent dielectric spacers for improving SERS enhancement more than 20-fold relative to conventional single layer SERS substrates. Such substrate optimization can significantly improve the sensitivity of the SERS method for analyte detection.

A critical analysis of theoretical models for the giant Raman effect from adsorbed molecules

Abstract We analyze several theoretical models which have recently been proposed in an attempt to explain the anomalously high Raman intensity from molecules adsorbed onto metal surfaces. We classify the theories according to their primary mechanism, pointing out similarities and differences, some which have not previously been recognized in print. We present a critical analysis of the conceptual basis and an outline of the experimental implications of each theory. We conclude that several theories have serious problems in their associated assumptions, and that other theories have been brought into question by recent experimental results. We propose additional experiments which would help clarify the situation.

Cobalt-phosphate (Co-Pi) catalyst modified Mo-doped BiVO4 photoelectrodes for solar water oxidation

A cobalt-phosphate based oxygen evolution catalyst (Co-Pi OEC) was electrochemically deposited onto the surface of a porous bismuth vanadate electrode doped with 2 atom% Mo (BiV0.98Mo0.02O4). The porous BiV0.98Mo0.02O4electrode was prepared using a surfactant assisted metal–organic decomposition technique at 500 °C. The comparison of the photocurrent–voltage characteristics of the BiV0.98Mo0.02O4electrodes with and without the presence of Co-Pi catalyst demonstrated that the Co-Pi catalyst enhanced the anodic photocurrent of the BiV0.98Mo0.02O4electrode with its effect more pronounced at lower potentials. A stable photocurrrent density of 1.0 mA cm−2 at 1.0 V vs.Ag/AgCl was achieved under standard AM 1.5 illumination using 0.5M Na2SO4 aqueous solution in phosphate buffer at pH7. Relative to the BiV0.98Mo0.02O4electrode, a sustained enhancement, nearly doubled photocurrent density was observed at 1.0 V vs.Ag/AgCl for Co-Pi/BiV0.98Mo0.02O4 composite photoelectrode. Significant performance gains are obtained on BiV0.98Mo0.02O4electrodes upon modification with Co-Pi water oxidation catalyst.

Voltage-induced shifting of charge-transfer excitations and their role in surface-enhanced Raman scattering

Abstract Changes in the incident photon energy cause the maximum of the surface-enhanced Raman intensity versus the electrochemically applied voltage to shift for the case of pyridine adsorbed on Ag. This is interpreted as the result of charge-transfer excitation from a filled metal state to an unoccupied defect state or molecular level which increases the effective polarizability of the system through electronic resonance. Variations in the local electrostatic potential difference across the interface are found to be larger by 50% than the externally applied changes.

Anomalously intense Raman scattering at the solid-electrolyte interface

Abstract The enhanced Raman effect is used to characterize cyanide on silver at the interface with 0.1 M Na 2 SO 4 + 0.01 M KCN solution. We conclude the bond is polarized with considerable backbonding. The scattering intensity dependence on electrode voltage supports enhancement models which involve wavefunction overlap between the metal and the adsorbate.

Surface Modification of ZnO Using Triethoxysilane-Based Molecules

Zinc oxide (ZnO) is an important material for hybrid inorganic-organic devices in which the characteristics of the interface can dominate both the structural and electronic properties of the system. These characteristics can be modified through chemical functionalization of the ZnO surface. One of the possible strategies involves covalent bonding of the modifier using silane chemistry. Whereas a significant body of work has been published regarding silane attachments to glass and SiO2, there is less information about the efficacy of this method for controlling the surface of metal oxides. Here we report our investigation of molecular layers attached to polycrystalline ZnO through silane bonding, controlled by an amine catalyst. The catalyst enables us to use triethoxysilane precursors and thereby avoid undesirable multilayer formation. The polycrystalline surface is a practical material, grown by sol-gel processing, that is under active exploration for device applications. Our study included terminations with alkyl and phenyl groups. We used water contact angles, infrared spectroscopy, and X-ray photoemission spectroscopy to evaluate the modified surfaces. Alkyltriethoxysilane functionalization of ZnO produced molecular layers with submonolayer coverage and evidence of disorder. Nevertheless, a very stable hydrophobic surface with contact angles approaching 106 degrees resulted. Phenyltriethoxysilane was found to deposit in a similar manner. The resulting surface, however, exhibited significantly different wetting as a result of the nature of the end group. Molecular layers of this type, with a variety of surface terminations that use the same molecular attachment scheme, should enable interface engineering that optimizes the chemical selectivity of ZnO biosensors or the charge-transfer properties of ZnO-polymer interfaces found in oxide-organic electronics.

Evidence for Ag cluster vibrations in enhanced Raman scattering from the Ag/electrolyte interface

Abstract Low-frequency vibrational features are observed in the enhanced Raman spectra from an electrochemically roughened Ag electrode. Evidence is found that these features are related to resonant Raman scattering from the internal vibrations of small Ag clusters stabilized on the surface. Comparison of the experimental data with the results of a simple normal mode calculation suggests that a possible identity for such clusters is pyramidal Ag+4. These clusters may be responsible for some of the “chemical” factor in SERS from coadsorbed molecules.

Light induced water oxidation on cobalt-phosphate (Co-Pi) catalyst modified semi-transparent, porous SiO2-BiVO4 electrodes.

A facile and simple procedure for the synthesis of semi-transparent and porous SiO2-BiVO4 electrodes is reported. The method involves a surfactant assisted metal-organic decomposition at 500 °C. An earth abundant oxygen evolution catalyst (OEC), cobalt phosphate (Co-Pi), has been used to modify the SiO2-BiVO4 electrode by electrodeposition (ED) and photoassisted electrodeposition (PED) methods. Modified electrodes by these two methods have been examined for light induced water oxidation and compared to the unmodified SiO2-BiVO4 electrodes by various photoelectrochemical techniques. The PED method was a more effective method of OEC preparation than the ED method as evidenced by an increased photocurrent magnitude during photocurrent-potential (I-V) characterizations. Electrode surfaces catalyzed by PED exhibited a very large cathodic shift (∼420 mV) in the onset potential for water oxidation. The chopped-light I-V measurements performed at different intervals over 24-hour extended testing under illumination and applied bias conditions show a fair photostability for PED Co-Pi modified SiO2-BiVO4.

Current understanding of the mechanism of surface enhanced Raman scattering

Abstract On Cu, Ag, Au, and Li, geometrically defined surface optical resonances are a major contributor to surface enhanced Raman scattering (SERS). This explains the incident photon energy dependence, the variation among these metals from metal to metal, the extension of the enhancement to molecules removed from the surface, and the sensitivity that SERS demonstrates toward surface roughness on the 5 to 200 nm scale. Additional features of the enhancement, such as short range effects in the first monolayer of the adsorbate, molecular specificity, and the demonstration of enhanced scattering for molecules on Ni and Pt are associated with active sites. These active sites, which may be adatoms, and which in some cases involve complex formation, lead to electronic resonance which is similar to the normal resonant Raman effect but which requires the association between the metal and the molecule.

Efficient photoelectrochemical water oxidation over cobalt-phosphate (Co-Pi) catalyst modified BiVO4/1D-WO3 heterojunction electrodes

We report the design, synthesis and photoelectrochemical characterization of cobalt phosphate (Co-Pi) oxygen evolution catalyst modified heterojunction photoelectrodes consisting of one-dimensional WO3 nanorods (1D-WO3) and highly porous BiVO4 layers. The 1D-WO3 nanorods were prepared by the decomposition of the tetrabutylammonium decatungstate precursor in the presence of poly(ethylene glycol) as a binding agent. The porous BiVO4 layers were spray deposited using a surfactant assisted metal-organic decomposition method. The Co-Pi oxygen evolution catalyst was deposited onto the BiVO4/1D-WO3/FTO heterojunction electrode using a photoassisted electrodeposition method. The Co-Pi catalyst modified heterojunction electrodes exhibited a sustained enhancement in the photocurrent compared to the unmodified BiVO4/1D-WO3/FTO heterojunction electrodes. The improved photoelectrochemical properties profited from the enhanced charge carrier separation achieved through the integration of highly porous BiVO4 layers on top of 1D-WO3 nanorods and from the superior kinetics due to the presence of the Co-Pi oxygen evolution catalyst on top of BiVO4/1D-WO3/FTO heterojunction electrodes.