Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

We use photoluminescence excitation to study the electronic spectrum of CdSe quantum dots ranging from \ensuremath{\sim}12 to \ensuremath{\sim}53 \AA{} in radius. We follow the size evolution of ten quantum dot absorption features. Comparison of the spectra with theoretical predictions allows us to confidently assign six of these transitions. We discuss the most likely assignments for the remaining four. We find that the ${\mathit{n}}_{\mathit{hS}3/2}$1${\mathit{S}}_{\mathit{e}}$ and ${\mathit{n}}_{\mathit{hS}1/2}$1${\mathit{S}}_{\mathit{e}}$ transitions dominate the spectra, accounting for half of the observed features. Our size-dependent data exhibit two strong avoided crossings, demonstrating the importance of valence-band structure in the description of the excited states. \textcopyright{} 1996 The American Physical Society.

http://www.fizyka.umk.pl/~wj/EDU/Pracownia_Projektowa/CdSe_QD_spectra.pdf?origin=publication_detail

Measurement and assignment of the size-dependent optical spectrum in cdse quantum dots

This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.

Quantum crystallites and nonlinear optics

We study the luminescence of surface modified CdSe nanocrystallites There has been much speculation as to the origin of the band edge emission in these quantum confined structures Because of their large surface to volume ratios it has been suggested that the emission originates from surface-related states However, recent theory suggests that the band edge luminescence arises from an optically inactive fine structure state or “dark” exciton To address this issue we modify the surface of CdSe nanocrystallites with a variety of organic and inorganic ligands We then monitor the effect changing the surface has on the energetics of the band edge luminescence through photoluminescence and fluorescence line narrowing experiments Our results are compared with theoretical predictions for the nonresonant and resonant luminescence We find good agreement between experiment and theory for CdSe nanocrystallites passivated with trioctylphosphine oxide, ZnS, 4-picoline, 4-(trifluoromethyl)thiophenol, and tris(2-eth

The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state

We use transient optical hole burning and photoluminescence to investigate the static and dynamic electronic properties of 32-\AA{} CdSe quantum dots. We observe a number of discrete electronic transitions, resolve LO-phonon progressions, and obtain homogeneous linewidths and electron--LO-phonon couplings. We find that the band-gap luminescence is not from the exciton state, but from a surface trapped state. Rapid (\ensuremath{\sim}160 fs) trapping into these surface states results in long-lived (\ensuremath{\sim}10--100 ns) bleach and induced-absorption features in pump-probe experiments.

Electronic structure and photoexcited-carrier dynamics in nanometer-size CdSe clusters.

Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition. >

Femtosecond optical nonlinearities of CdSe quantum dots

An all‐optical logic gate consisting of a GaAs‐GaAlAs multiple quantum well structure inserted in a 1.3‐μm‐thick Fabry–Perot cavity is demonstrated to perform with subpicosecond on and off switching time. The use of a purely optical field effect allows for a recovery time as rapid as the switch‐on time.

Ultrafast all-optical gate with subpicosecond on and off response time

The speed of a GaAs‐AlGaAs optical logic gate is time resolved using a 100‐fs laser system. It shows that the gating operation can be performed in ≂1 ps, the fastest reported for such a low‐energy optical device. The lowest incident switching for such a device is ≲3 pJ.

One-picosecond optical nor gate at room temperature with a GaAs-AlGaAs multiple-quantum-well nonlinear Fabry-Perot étalon

Attract-Femtosecond differential absorption measurements of the quantum confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The bole width increases with pump intensity and the magnitude of the induced transparency saturates at our highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition.

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Femtosecond optical nonlinearities of CdSe quantum dots

Ultrafast all-optical gate with subpicosecond on and off response time

One-picosecond optical nor gate at room temperature with a GaAs-AlGaAs multiple-quantum-well nonlinear Fabry-Perot étalon

Femtosecond optical nonlinearities of CdSe quantum dots