Energy Conversion of Visible Light and Interfacial Charge Transfer Processes on Semiconductor-Based Photocatalytic Nanomaterials

Abstract

The broad energy distribution of solar spectrum is one of the intrinsic factors limiting the efficiency of photocatalytic systems. Artificial photosynthesis using a Z-schematic mechanism of green plants is regarded as one of the most attractive technologies for efficiently converting energy of visible light into other energy sources. An all-solid-state Z-schematic system, Au@CdS/TiO1.96C0.04, has been reported for the efficient H2 generation from water under visible-light irradiation. However, the vectorial charge-carrier flow at the interface was not fully investigated from the standpoint of kinetics and mechanism. In this study, spectroscopic tools were developed for interrogating their properties. Electron pathways were constructed on the basis of steady-state photoluminescence (PL) spectral data, and the rate constants for charge transfer were calculated from time-resolved PL spectra. The PL results revealed that Au core played an important role in capturing the photoexcited electrons in the conduction band (CB) of TiO1.96C0.04 and accelerating the electron transfer to the valence band (VB) of CdS, leading to an efficient quenching of the holes left in the VB of CdS shell. The minimum energy pathways for H2 production on the surfaces of TiO1.96C0.04(101) and CdS(101) were elucidated through first-principle calculations, indicating that the CdS shell has a lower energy barrier (2.81 eV) for the surface reaction than that (3.34 eV) of TiO1.96C0.04. Consequently, Au@CdS/TiO1.96C0.04 showed a vectorial electron transfer of TiO1.96C0.04 → Au → CdS in the form of the letter Z, which allowed the photoexcited electrons to be shuttled to a higher energy level, thereby producing substantial level of H2 on the CdS(101) surface. Interestingly, although the Z-schematic system generates photoexited electrons with thermodynamically strong redox characteristics, the core-shell nanomaterial produces little photocurrent in photoelectronchemical (PEC) system as compared to the amount of H2 generataion. The reason for low photocurrent can be understood from the results of the previous fundamental research. The photoexcited electrons accumulated in the CB of CdS shell have to reach the indium-tin oxide (ITO) circuit for photocurrent generation. However, there is no any pathway to allow the electrons to be transferred to the ITO circuit. Therefore, a significant fraction of the electrons are recombined at the interparticles. Perovskite SrTiO3 nanoparticles (NPs) were used as a high performance electron filter instead of the PS II material, TiO1.96C0.04, thereby entirely reversing the Z-schematic charge transfer direction. The nanocomposte enabled the efficient production of H2 as well as electron harvest within a range of visible light. The findings showed that the ultrafast decay of hot electrons across Au nanoparticles can be significantly reduced by strong coupling with CdS quantum dots and by a Schottky junction with perovskite SrTiO3 nanoparticles. The designed plasmonic nanostructure created a hot-electron-assisted energy cascade for the favorable electron pathway involving CdS → Au → SrTiO3, as demonstrated through steady-state and time-resolved PL spectroscopies. This results provide a new approach for overcoming the low efficiency typically associated with plasmonic nanostructures. Of the light energy delivered by the sun to the surface of the earth, ~95% is in the visible and infrared region. However, it is difficult to utilize a significant fraction of solar photons with wavelengths longer than 500 nm for working photocatalysts and photovoltaics. This is because the photon energy is not sufficiently potent to activate desired reactions through electronic excitation processes of semiconductors or molecules. The loss of this otherwise-wasted portion of sunlight can be ameliorated by shifting two or more low-energy photons into higher-energy ones. This novel process, which is referred to as photon upconversion showing an anti-Stokes type emission, is known to one of the promising routes for amplifying frequencies of sub-band-gap light (E < Eg). Here I show a fluorescence resonance energy transfer (FRET)-assisted upconversion process across three components for efficiently utilizing sub-band-gap light. In the process, a highly luminescent rhodamine B (RhB) makes a FRET pair with naked carbon nanodots (NC-dots) of 3.4 nm, which are not passivated by any insulating molecules but show excitation-wavelength dependent upconversion behavior. The FRET effect enhances the upconverted light intensity of NC-dots. These upconverted photons are subsequently used to sensitize Ag3PO4 particles and thus generate 18 times higher photocurrent at λex > 500 nm than before adding the RhB molecules. The reason for increase in photocurrent is fundamentally unveiled by steady-state and time-resolved PL spectroscopies. The findings provide a new approach for the utilization of light energy in the visible and infrared region.