With the capability of localizing optical energy via surface plasmon resonance (SPR), plasmonic Au nanostructures hold great promise for enhancing the solar water splitting of semiconductor ...photocatalysts. While the content of Au plays a critical role in mediating interfacial charge transfer, its quantitative influence on the efficiency of plasmon-assisted water splitting is still not fully understood. This work aimed to explore the correlations among plasmonic metal content, SPR-mediated charge transfer and electromagnetic response, and the resultant photoactivity enhancement toward photoelectrochemical (PEC) water splitting. The PEC system was constructed by employing Au particle-decorated ZnO nanocrystals (ZnO–Au) as the plasmonic photoanode. Time-resolved photoluminescence spectroscopy and finite-difference time-domain simulations were utilized to evaluate the optimal Au content which attained effective charge separation and imposed a significant SPR effect for achieving the largest photoactivity enhancement. The charge transfer at the photoanode/electrolyte interface and its dependence on the Au content were examined with electrochemical impedance analysis, which manifested the effectiveness of the optimal Au content in facilitating the hole transfer kinetics. The present study reports a technical advance in the realization of the quantitative effect of Au for designing sophisticated plasmonic PEC systems that enabled efficient solar-to-fuel energy conversion.
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.
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.
With porous shells and mobile cores, yolk-shell nanostructures provide great structural advantage for mass transport-related applications such as photocatalysis. In this work, Au–Cu7S4 yolk-shell ...nanostructures are synthesized from Au–Cu2O core-shell templates. The Cu7S4 shell is then converted to CdS through a cation exchange process to produce Au–CdS yolk-shell photocatalysts for hydrogen generation. Ultrafast transient absorption and finite-difference time-domain simulation are used to investigate electronic interaction between Au nanoparticle core and the surrounding CdS shell. Additionally, a new method is presented to simulate chemical transport and quantitatively compare diffusion kinetics by monitoring mass transport through the porous CdS shell with dye molecules as optical probes. The highest hydrogen generation rate of 3390 μmol g−1 h−1, corresponding to an adequate apparent quantum yield of 4.22% at 420 nm, is achieved for Au–CdS with the largest void size. The enhancement in photocatalytic performance with increase in void size is mostly attributed to improved mass transport kinetics, with additional gains from more efficient charge transfer and stronger surface plasmon resonance-mediated near-field effects. This comprehensive study demonstrates that void size is a critical structural parameter in optimizing the performance of yolk-shell nanostructures for photocatalysis or other mass-transport related applications.
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•Yolk-shell nanostructures are demonstrated as an emerging photocatalyst paradigm.•The activity of hydrogen generation is enhanced with increase in void size.•Efficient charge transfer and strong SPR-mediated near-field effects are observed.•Control over the void size can improve mass transport kinetics across the shell.
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.
Abstract Structural engineering has proven effective in tailoring the photocatalytic properties of semiconductor nanostructures. In this work, a sophisticated double‐hollow yolk@shell nanostructure ...composed of a plasmonic, mobile, hollow Au nanosphere (HGN) yolk and a permeable, hollow CdS shell is proposed to achieve remarkable solar hydrogen production. The shell thickness of HGN@CdS is finely adjusted from 7.7, 18.4 to 24.5 nm to investigate its influence on the photocatalytic performance. Compared with pure HGN, pure CdS, a physical mixture of HGN and CdS, and a counterpart single‐hollow cit‐Au@CdS yolk@shell nanostructure, HGN@CdS exhibits superior hydrogen production under visible light illumination (λ = 400–700 nm). The apparent quantum yield of hydrogen production reaches 8.2% at 320 nm, 6.2% at 420 nm, and 4.4% at 660 nm. The plasmon‐enhanced activity at 660 nm is exceptional, surpassing the plasmon‐induced photoactivities of the state‐of‐the‐art plasmonic photocatalysts ever reported. The superiority of HGN@CdS originates from the creation of charge separation state at HGN/CdS heterojunction, the considerably long‐lived hot electrons of plasmonic HGN, the magnified electric field, and the advantageous features of double‐hollow yolk@shell nanostructures. The findings can provide a guideline for the rational design of versatile double‐hollow yolk@shell nanostructures for widespread photocatalytic applications.
We demonstrated for the first time that Ag-nanoparticle-decorated SiO2 nanospheres (NSs) may display noticeable photocatalytic activities upon surface plasmon resonance (SPR) excitation. The samples ...were prepared by reacting SiO2 NSs with AgNO3 in the seed-mediated growth process, from which the Ag particle size and decoration density can be readily controlled. The dependence of the SPR-mediated photocatalytic performance of Ag-decorated SiO2 NSs on the Ag morphology was investigated and presented. The as-prepared Ag-decorated SiO2 NSs showed a significantly red shifted and relatively broad SPR absorption when compared with the individually dispersed Ag nanoparticles. Owing to the considerably broad SPR absorption that spanned from the visible to the near-infrared region, Ag-decorated SiO2 NSs surpassed N-doped P-25 TiO2 powder and individually dispersed Ag nanoparticles in photocatalytic activity, demonstrating their potential as an active photocatalyst in nearly all the current photocatalysis applications. Furthermore, the result of performance evaluation under natural sunlight shows that the present Ag-decorated SiO2 NSs can be used as highly efficient photocatalysts that may practically harvest energy from sunlight. The current study provides a new paradigm for designing plasmonic metal nanostructures that can effectively absorb the entire solar spectrum and beyond for solar fuel generation.
Heterostructure provides a powerful route in manipulating electrical transport, optical response, electrolytic water splitting and water treatment of complex oxides. As a model for noble metal/ ...complex oxide heterostructures, we have successfully prepared Au/BiVO4 (BVO) heterostructures in which the Au nanoparticles (NPs) with various sizes and densities were uniformly deposited on the {001} facets of epitaxial BVO thin films. The heterostructures exhibit significantly enhanced photoactivities in both dye degradation and electrolytic water splitting. By employing X-ray photoelectron spectroscopy, the energy band alignment of Au/BVO heterojunction suggests a charge separation at their interfaces, that can manipulate the photoexcited electron–hole pairs and photocatalytic efficiency of the heterostructures. Photogenerated carrier injection, which mainly affects the photoactivity of photocatalysis, was detected across Au/BVO interfaces by ultrafast dynamics spectroscopy. This study delivers a general approach to probe and understand the photochemistry of noble metal-complex oxide heterostructures for photoconversion applications.
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•Crystal Au NPs with good control of size and density were deposited on epitaxial BVO film•Energy band alignment of Au/BVO heterojuction can be tuned by controlling Au NPs size and density•The photoactivity enhancement of the Au/BVO is due to effective interfacial charge separation
Capsule endoscopes are pill-sized devices that physicians can utilize for diagnosis of a patient's small intestine. For diagnostic requirements, excellent image quality, large field of view (FOV) and ...compact size are crucial. In this paper, a multi-camera capsule endoscope (MCCE) composed of two wafer level optic modules and microprism elements which can offer both a large FOV and clear image quality within limited space conditions is described. The properties of the microprism needed to ensure the necessary image quality are further discussed, including working efficiency, the ability to increase the FOV, and the inhibition of chromatic aberration. Finally, the functioning of the MCCE is verified through an optical simulation and preliminary experiments. The results show that the max half FOV of the MCCE could be increased from 28° to 37°, and the relative illumination could be kept at 80% without a dark corner effect.
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