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This paper presents a review of the research progress in the carbon-metal oxide composites for supercapacitor electrodes. In the past decade, various carbon-metal oxide composite electrodes have been developed by integrating metal oxides into different carbon nanostructures including zero-dimensional carbon nanoparticles, one-dimensional nanostructures (carbon nanotubes and carbon nanofibers), two-dimensional nanosheets (graphene and reduced graphene oxides) as well as three-dimensional porous carbon nano-architectures. This paper has described the constituent, the structure and the properties of the carbon-metal oxide composites. An emphasis is placed on the synergistic effects of the composite on the performance of supercapacitors in terms of specific capacitance, energy density, power density, rate capability and cyclic stability. This paper has also discussed the physico-chemical processes such as charge transport, ion diffusion and redox reactions involved in supercapacitors.

Plasmonic metal nanostructures have been incorporated into semiconductors to enhance the solar-light harvesting and the energy-conversion efficiency. So far the mechanism of energy transfer from the plasmonic metal to semiconductors remains unclear. Herein the underlying plasmonic energy-transfer mechanism is unambiguously determined in Au@SiO(2)@Cu(2)O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement. The gold core converts the energy of incident photons into localized surface plasmon resonance oscillations and transfers the plasmonic energy to the Cu(2)O semiconductor shell via resonant energy transfer (RET). RET generates electron-hole pairs in the semiconductor by the dipole-dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor), which greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone. RET from a plasmonic metal to a semiconductor is a viable and efficient mechanism that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices.

Experiments combined with the density functional theory (DFT) calculation have been performed to understand the underlying photocatalysis mechanism of the nitrogen-doped titania nanobelts. Nitrogen-doped anatase titania nanobelts are prepared via hydrothermal processing and subsequent heat treatment in NH(3). Both the nitrogen content and the oxygen vacancy concentration increase with increasing the NH(3) treatment temperature. Nitrogen doping leads to an add-on shoulder on the edge of the valence band, the localized N 2p levels above the valence band maximum, and the 3d states of Ti(3+) below the conduction band, which is confirmed by DFT calculation and X-ray photoelectron spectroscopy (XPS) measurement. Extension of the light absorption from the ultraviolet (UV) region to the visible-light region arises from the N 2p levels near the valence band and from the color centers induced by the oxygen vacancies and the Ti(3+) species. Nitrogen doping allows visible-light-responsive photocatalytic activity but lowers UV-light-responsive photocatalytic activity. The visible-light photocatalytic activity originates from the N 2p levels near the valence band. The oxygen vacancies and the associated Ti(3+) species act as the recombination centers for the photoinduced electrons and holes. They reduce the photocatalytic activity although they contribute to the visible light absorbance.

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In Förster resonance energy transfer (FRET), energy non-radiatively transfers from a blue-shifted emitter to a red-shifted absorber by dipole-dipole coupling. This study shows that plasmonics enables the opposite transfer direction, transferring the plasmonic energy towards the short-wavelength direction to induce charge separation in a semiconductor. Plasmoninduced resonance energy transfer (PIRET) differs from FRET because of the lack of a Stoke's shift, non-local absorption effects and a strong dependence on the plasmon's dephasing rate and dipole moment. PIRET non-radiatively transfers energy through an insulating spacer layer, which prevents interfacial charge recombination losses and dephasing of the plasmon from hot-electron transfer. The distance dependence of dipole-dipole coupling is mapped out for a range of detuning across the plasmon resonance. PIRET can efficiently harvest visible and near-infrared sunlight with energy below the semiconductor band edge to help overcome the constraints of band-edge energetics for single semiconductors in photoelectrochemical cells, photocatalysts and photovoltaics. Balancing the semiconductor bandgap against heating losses 1 in photovoltaics and photocorrosion 2 in photoelectrochemical cells limits the absorption of the solar spectrum. Semiconductor-semiconductor heterostructures, hot carriers and photosensitizers extend the absorption range by transferring charge between two dissimilar bandgap materials, but band alignment and interfacial charge-transfer issues limit the realizable enhancement 3-5 . Optically extracting energy by non-radiative dipole-dipole transfer could overcome these problems. However, dipole-dipole coupling in Förster resonance energy transfer (FRET) occurs in the incoherent and downwards energy-transfer limit to a red-shifted acceptor 6-9 , which prevents charge separation to a higher bandgap material.It is not that dipole-dipole coupling cannot extend light absorption. If the dipole is coherent before the Stoke's shift, a symmetric energy transfer is observed on short timescales for strong coupling between excitons and plasmons 10 , atomic vapour 11 , photosynthesis 12 and spin-triplet excitons 13 . Unfortunately, these conditions are difficult to meet in candidate solar materials such as quantum dots and fluorophores because the radiative lifetime is longer than that of thermal relaxation. Plasmons, however, have a large dipole moment when the collective electron oscillations are coherent, which creates the possibility of a strong blue-shifted transfer.If plasmonics can modulate the energy flow direction in dipole-dipole coupling, this opens a third possible process (the other two are light trapping 14,15 and hot-electron injection 15-22 ) for plasmon-enhanced solar energy harvesting. In this case, the plasmonic metal absorbs sunlight, then transfers the absorbed energy from a metal to a semiconductor via dipole-dipole coupling, which generates electron-hole pairs below and near the semiconductor band edge 23-26 . We denote this process as pl...

Surface plasmon resonance (SPR) has found extensive applications in chemi-sensors and biosensors. Plasmons play different roles in different types of optical sensors. SPR transduces a signal in a colorimetric sensor through shifts in the spectral position and intensity in response to external stimuli. SPR can also concentrate the incident electromagnetic field in a nanostructure, modulating fluorescence emission and enabling plasmon-enhanced fluorescence to be used for ultrasensitive detection. Furthermore, plasmons have been extensively used for amplifying a Raman signal in a surface-enhanced Raman scattering sensor. This paper presents a review of recent research progress in plasmon-enhanced optical sensing, giving an emphasis on the physical basis of plasmon-enhanced sensors and how these principles guide the design of sensors. In particular, this paper discusses the design strategies for nanomaterials and nanostructures to plasmonically enhance optical sensing signals, also highlighting the applications of plasmon-enhanced optical sensors in health care, homeland security, food safety and environmental monitoring.

This perspective article describes the barrier, progress and future direction of research on the photocatalytic and photoelectrochemical solar fuel generation.