•A facile template-free electrodeposition technique is developed for large-scale fabrication of Ag nanorods.•The diameter, length, and surface coverage of Ag NRs are dependent on the electrodeposition time and choice of substrates.•This technique can be extended to other materials for obtaining vertically aligned NR structures.•Vertically standing NRs in large scale have the potential to increase photocatalysis efficiency.A facile template-free electrodeposition technique is developed for large-scale fabrication of vertically standing silver nanorods (NRs) on transparent and conductive substrates. The diameter, length, and surface coverage of Ag NRs are dependent on the electrodeposition time and choice of substrates. The growth mechanism of vertically standing Ag NRs is investigated by tracking their morphology evolution as a function of deposition time. Because of their large specific surface area, oriented alignment, broad range light scattering, and light absorption tunability, these vertically standing NRs can be used as ideal substrates for thin layer photocatalysts for enhancing light absorption and charge collection. Preliminary tests on photoelectrochemical performance of bare Ag NRs, CdS modified Ag NRs, and Ag NRs converted to Ag2S NRs are presented. This simple NR fabrication method has been successfully applied to prepare Fe NRs as well. This technique can be extended to other conductive substrates and other materials for obtaining vertically standing NR structures.Download high-res image (309KB)Download full-size image

Electrochemical reduction method is used for the first time to significantly improve the photo-electrochemical performance of α-Fe2O3 photoanode prepared on fluorine-doped tin oxide substrates by spin-coating aqueous solution of Fe(NO3)3 followed by thermal annealing in air. Photocurrent density of α-Fe2O3 thin film photoanode can be enhanced 25 times by partially reducing the oxide film to form more conductive Fe3O4 (magnetite). Fe3O4 helps facilitate efficient charge transport and collection from the top α-Fe2O3 layer upon light absorption and charge separation to yield enhanced photocurrent density. The optimal enhancement can be obtained for <50 nm films because of the short charge transport distance for the α-Fe2O3 layer. Thick α-Fe2O3 films require more charge and overpotential than thinner films to achieve limited enhancement because of the sluggish charge transport over a longer distance to oxidize water. Electrochemical reduction of α-Fe2O3 in unbuffered pH-neutral solution yields much higher but unstable photocurrent enhancement because of the increase in local pH value accompanied by proton reduction at a hematite surface.Keywords: electrochemical reduction; hematite; magnetite; solar energy; water splitting;

Because of the current increase in consumption of fossil fuels and its negative impact on the environment, clean energy technologies such as solar cells are highly desirable to address this global energy challenge. Among these, dye-sensitized solar cells (DSSCs) have emerged as potential substitutes to traditional silicon-based solar cells. In this study, a series of boron dipyrromethene (BODIPY)-based dyes (1–5) which contain thiophene and/or triphenylamine (TPA) as redox relays of chromophorebridges are synthesized and characterized using electrochemical and optical spectroscopic methods for their potential applications in DSSCs. Their electrochemical and photophysical properties are investigated and compared with the computational results. DSSCs made of these BODIPY-based dyes exhibit incident photon-to-current conversion efficiencies (IPCE) that correspond to their absorption profiles. BODIPY dye 5 bearing TPA provides the highest power efficiency because of its reversible redox activities, while the dyes bearing thiophene yield a decrease in overall solar cell efficiency because of the irreversible oxidation and electropolymerization of thiophene. Despite their low overall conversion efficiencies, these dyes show interesting structural dependence in their DSSC performance. TiO2 electrodes loaded with these BODIPY dyes are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared analysis to illustrate the surface bonding characteristics of these dyes.

Developing high-efficiency, durable, and low-cost catalysts based on earth-abundant elements for the oxygen evolution reaction (OER) is essential for renewable energy conversion and energy storage devices. In this study, we report a highly active nanostructured electrode, NanoCOT, which contains carbon, oxygen, and titanium, for efficient OER in alkaline solution. The NanoCOT electrode is synthesized from carbon transformation of TiO2 in an atmosphere of methane, hydrogen, and nitrogen at a high temperature. The NanoCOT exhibits enhanced OER catalytic activity in alkaline solution, providing a current density of 1.33 mA/cm2 at an overpotential of 0.42 V. This OER current density of a NanoCOT electrode is about 4 times higher than an oxidized Ir electrode and 15 times higher than a Pt electrode because of its nanostructured high surface area and favorable OER kinetics. The enhanced catalytic activity of NanoCOT is attributed to the presence of a continuous energy band of the titanium oxide electrode with predominantly reduced defect states of Ti (e.g., Ti1+, Ti2+, and Ti3+) formed by chemical reduction with hydrogen and carbon. The OER performance of NanoCOT can also be further enhanced by decreasing its overpotential by 150 mV at a current density of 1.0 mA/cm2 after coating its surface electrophoretically with 2.0 nm IrOx nanoparticles.

The spectroelectrochemical properties of individual luminescent, plasmonic silver nanoparticles (Ag NPs) are investigated using the combined methods of dark-field scattering (DFS) and photoluminescence (PL) spectroelectrochemistry. Individual NP light scattering and PL intensities are measured while the substrate’s electrochemical potential is controlled to produce and oxidize the NPs. The spectroelectrochemical responses of individual NPs are used to study heterogeneities in their redox properties not visible in bulk voltammetric measurements. Our studies show that the Ag NPs exhibit a range of redox potentials, and their statistical distribution is dependent on the electrolyte system used. No variations in the spectral profile of bulk NP samples are observed, implying no correlation between the redox potentials of individual NPs and the energy of emitted photons from fluorescent sites on Ag NPs. This is due to a negligible difference in the redox potentials for individual emissive sites on a given Ag NP and/or the shrinking of the polarizable bulk of the Ag NP.

We present single molecule fluorescence and spectroelectrochemistry characteristics of 4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) bearing two carboxylic acid groups at its 2 and 6 positions. Our study shows a heterogeneous half redox potential distribution for the BODIPY molecules embedded in polystyrene film because of the heterogeneity in their charge transfer rates. Single molecules adsorbed onto a TiO2 surface with ordered nanostructures show surprising fluorescence blinking activity with the shortest ON duration time in comparison to bare glass and indium-tin oxide (ITO) surfaces. Single molecule stability tests show longer ON duration time and a stable fluorescence feature when dispersed in polystyrene thin film than molecules exposed to air. Shorter ON times are observed for molecules. In intimate contact with ITO in comparison to glass substrates. Such a decrease in their fluorescence stability or intensity is explained by charge transfer activities from the dye molecules to the metal oxide surface. Electron transfer and back transfer rates are calculated to illustrate the substrate effects by using a well-established model.