Surface Passivation of TiO₂ Nanowires Using a Facile Precursor-Treatment Approach for Photoelectrochemical Water Oxidation

We developed a facile precursor-treatment approach for effective surface passivation of rutile TiO₂ nanowire photoanode to improve its performance in photoelectrochemical (PEC) water oxidation. The approach was demonstrated by treating rutile TiO₂ nanowires with titanium precursor solutions (TiCl₄, Ti­(OBu)₄, or Ti­(OiP)₄) followed by a postannealing process, which resulted in the additional deposition of anatase TiO₂ layer on the nanowire surface. Compared to pristine TiO₂, all the precursor-treated TiO₂ nanowire electrodes exhibited a significantly enhanced photocurrent density under white light illumination. Among the three precursor-treated samples, Ti­(OBu)₄-treated TiO₂ nanowires achieved the largest enhancement of photocurrent generation, which is approximately a 3-fold increase over pristine TiO₂. Monochromatic incident photon-to-electron conversion efficiency (IPCE) measurements showed that the improvement of PEC performance was dominated by the enhanced photoactivity of TiO₂ in the UV region. The photovoltage and electrochemical impedance spectroscopy (EIS) measurements showed that the enhanced photoactivity can be attributed to the improved charge transfer as a result of effective surface state passivation. This work demonstrates a facile, low-cost, and efficient method for preparing highly photoactive TiO₂ nanowire electrodes for PEC water oxidation. This approach could also potentially be used for other photoconversion applications, such as TiO₂ based dye-sensitized solar cells, as well as photocatalytic systems

Au@Cu₂O Core–Shell and Au@Cu₂Se Yolk–Shell Nanocrystals as Promising Photocatalysts in Photoelectrochemical Water Splitting and Photocatalytic Hydrogen Production

In this work, we demonstrated the practical use of Au@Cu2O core–shell and Au@Cu2Se yolk–shell nanocrystals as photocatalysts in photoelectrochemical (PEC) water splitting and photocatalytic hydrogen (H2) production. The samples were prepared by conducting a sequential ion-exchange reaction on a Au@Cu2O core–shell nanocrystal template. Au@Cu2O and Au@Cu2Se displayed enhanced charge separation as the Au core and yolk can attract photoexcited electrons from the Cu2O and Cu2Se shells. The localized surface plasmon resonance (LSPR) of Au, on the other hand, can facilitate additional charge carrier generation for Cu2O and Cu2Se. Finite-difference time-domain simulations were carried out to explore the amplification of the localized electromagnetic field induced by the LSPR of Au. The charge transfer dynamics and band alignment of the samples were examined with time-resolved photoluminescence and ultraviolet photoelectron spectroscopy. As a result of the improved interfacial charge transfer, Au@Cu2O and Au@Cu2Se exhibited a substantially larger photocurrent of water reduction and higher photocatalytic activity of H2 production than the corresponding pure counterpart samples. Incident photon-to-current efficiency measurements were conducted to evaluate the contribution of the plasmonic effect of Au to the enhanced photoactivity. Relative to Au@Cu2O, Au@Cu2Se was more suited for PEC water splitting and photocatalytic H2 production by virtue of the structural advantages of yolk–shell architectures. The demonstrations from the present work may shed light on the rational design of sophisticated metal–semiconductor yolk–shell nanocrystals, especially those comprising metal selenides, for superior photocatalytic applications

Au Nanostructure-Decorated TiO₂ Nanowires Exhibiting Photoactivity Across Entire UV-visible Region for Photoelectrochemical Water Splitting

Here we demonstrate that the photoactivity of Au-decorated TiO₂ electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV–visible region from 300 to 800 nm by manipulating the shape of the decorated Au nanostructures. The samples were prepared by carefully depositing Au nanoparticles (NPs), Au nanorods (NRs), and a mixture of Au NPs and NRs on the surface of TiO₂ nanowire arrays. As compared with bare TiO₂, Au NP-decorated TiO₂ nanowire electrodes exhibited significantly enhanced photoactivity in both the UV and visible regions. For Au NR-decorated TiO₂ electrodes, the photoactivity enhancement was, however, observed in the visible region only, with the largest photocurrent generation achieved at 710 nm. Significantly, TiO₂ nanowires deposited with a mixture of Au NPs and NRs showed enhanced photoactivity in the entire UV–visible region. Monochromatic incident photon-to-electron conversion efficiency measurements indicated that excitation of surface plasmon resonance of Au is responsible for the enhanced photoactivity of Au nanostructure-decorated TiO₂ nanowires. Photovoltage experiment showed that the enhanced photoactivity of Au NP-decorated TiO₂ in the UV region was attributable to the effective surface passivation of Au NPs. Furthermore, 3D finite-difference time domain simulation was performed to investigate the electrical field amplification at the interface between Au nanostructures and TiO₂ upon SPR excitation. The results suggested that the enhanced photoactivity of Au NP-decorated TiO₂ in the UV region was partially due to the increased optical absorption of TiO₂ associated with SPR electrical field amplification. The current study could provide a new paradigm for designing plasmonic metal/semiconductor composite systems to effectively harvest the entire UV–visible light for solar fuel production