Progress and Perspectives of Plasmon-Enhanced Solar Energy Conversion

Purely Visible-Light-Induced Photochromism in Ag-TiO2 Nanoheterostructures

We report titania nanoheterostructures decorated with silver, exhibiting tuneable photochromic properties for the first time when stimulated only by visible white light (domestic indoor lamp), with no UV wavelengths. Photochromic materials show reversible color changes under light exposure. However, all inorganic photochromic nanoparticles (NPs) require UV light to operate. Conventionally, multicolor photochromism in Ag-TiO₂ films involves a change in color to brownish-gray during UV-light irradiation (i.e., reduction of Ag⁺ to Ag⁰) and a (re)bleaching (i.e., (re)oxidation of Ag⁰ to colorless Ag⁺) upon visible-light exposure. In this work, on the contrary, we demonstrate visible-light-induced photochromism (ranging from yellow to violet) of 1-10 mol % Ag-modified titania NPs using both spectroscopic and colorimetric CIEL*a*b* analyses. This is not a bleaching of the UV-induced color but a change in color itself under exposure to visible light, and it is shown to be a completely different mechanism-driven by the interfacial charge transfer of an electron from the valence band of TiO₂ to that of the Ag_xO clusters that surround the titania-to the usual UV-triggered photochromism reported in titania-based materials. The quantity of Ag or irradiation time dictated the magnitude and degree of tuneability of the color change, from pale yellow to dark blue, with a rapid change visible only after a few seconds, and the intensity and red shift of surface plasmon resonance induced under visible light also increased. This effect was reversible after annealing in the dark at 100 °C/15 min. Photocatalytic activity under visible light was also assessed against the abatement of nitrogen oxide pollutants, for interior use, therefore showing the coexistence of photochromism and photocatalysis-both triggered by the same wavelength-in the same material, making it a multifunctional material. Moreover, we also demonstrate and explain why X-ray photoelectron spectroscopy is an unreliable technique with such materials.

Optical response of heterogeneous polymer layers containing silver nanostructures

This work is focused on the study of the optical properties of silver nanostructures embedded in a polymer host matrix. The introduction of silver nanostructures in polymer thin films is assumed to result in layers having adaptable optical properties. Thin film layers with inclusions of differently shaped nanoparticles, such as nanospheres and nanoprisms, and of different sizes, are optically characterized. The nanoparticles are produced by a simple chemical synthesis at room temperature in water. The plasmonic resonance peaks of the different colloidal solutions range from 390 to 1300 nm. The non-absorbing, transparent polymer matrix poly(vinylpyrrolidone) (PVP) was chosen because of its suitable optical and chemical properties. The optical studies of the layers include spectrophotometry and spectroscopic ellipsometry measurements, which provide information about the reflection, transmission, absorption of the material as well as the complex optical indices, n and k. Finite difference time domain simulations of nanoparticles in thin film layers allow the visualization of the nanoparticle interactions or the electric field enhancement on and around the nanoparticles to complete the optical characterization. A simple analysis method is proposed to obtain the complex refractive index of nanospheres and nanoprisms in a polymer matrix.

Plasmon-Sensitized Graphene/TiO2 Inverse Opal Nanostructures with Enhanced Charge Collection Efficiency for Water Splitting

In this contribution we have developed TiO₂ inverse opal based photoelectrodes for photoelectrochemical (PEC) water splitting devices, in which Au nanoparticles (NPs) and reduced graphene oxide (rGO) have been strategically incorporated (TiO₂@rGO@Au). The periodic hybrid nanostructure showed a photocurrent density of 1.29 mA cm^-2 at 1.23 V vs RHE, uncovering a 2-fold enhancement compared to a pristine TiO₂ reference. The Au NPs were confirmed to extensively broaden the absorption spectrum of TiO₂ into the visible range and to reduce the onset potential of these photoelectrodes. Most importantly, TiO₂@rGO@Au hybrid exhibited a 14-fold enhanced PEC efficiency under visible light and a 2.5-fold enrichment in the applied bias photon-to-current efficiency at much lower bias potential compared with pristine TiO₂. Incident photon-to-electron conversion efficiency measurements highlighted a synergetic effect between Au plasmon sensitization and rGO-mediated facile charge separation/transportation, which is believed to significantly enhance the PEC activity of these nanostructures under simulated and visible light irradiation. Under the selected operating conditions the incorporation of Au NPs and rGO into TiO₂ resulted in a remarkable boost in the H₂ evolution rate (17.8 μmol/cm²) compared to a pristine TiO₂ photoelectrode reference (7.6 μmol/cm²). In line with these results and by showing excellent stability as a photoelectrode, these materials are herin underlined to be of promising interest in the PEC water splitting reaction.

Metal nanoparticles induced photocatalysis

Photocatalysis induced by light absorption of metal nanoparticles (NPs) has emerged as a promising strategy for exploiting efficient visible-light-responsive composites for solar-energy conversion. In this review, we first introduce the light absorption of metal NPs and the mechanisms proposed in metal-induced photocatalysis (MIP). Then, its applications in water splitting, artificial photosynthesis and inert molecular activation are summarized. To address the challenge of low efficiency in this field, strategies in promoting catalytic activity are reviewed, and particular attention is paid to the particle-size effect of metal. Finally, the challenges and possible development directions of MIP are briefly discussed.

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials, thereby enabling the fabrication of bioconjugates and heterogeneous catalysts. Accurate size control of LSPC-synthesized materials ranging from quantum dots to submicrometer spheres and recent upscaling advancement toward the multiple-gram scale are helpful for extending the applicability of LSPC-synthesized nanomaterials to various fields. By discussing key reports on both the fundamentals and the applications related to laser ablation, fragmentation, and melting in liquids, this Article presents a timely and critical review of this emerging topic.

Hot electron-driven hydrogen evolution using anisotropic gold nanostructure assembled monolayer MoS2

Plasmonic nanostructures attracting particular interest in plasmon-induced highly energetic electrons, also known as hot electrons, play a fundamental role in photocatalysis for solar energy conversion. Plasmon-induced hot electron excitation, relaxation, transport, and injection to two-dimensional semiconductors are necessary to clearly understand the efficient plasmon-induced chemical reaction. Herein, we use a plasmonic photocatalyst composed of anisotropic gold nanostructures as the electron donor assembled on two-dimensional molybdenum dichalcogenide, monolayer MoS₂, as the electron acceptor in order to unveil the plasmon-induced interfacial hot electron transfer for the hydrogen evolution reaction (HER). Single-particle confocal fluorescence microscopy, computational calculation of finite-difference-time-domain (FDTD) simulation, and time-resolved transient absorption measurements revealed that anisotropic gold nanostructures with strong plasmon resonance exhibit interfacial hot electron transfer to monolayer MoS₂, giving the charge separated state with a long lifetime of 800 ps which is responsible for efficient HER. This is the first example to show the plasmon-induced interfacial hot electron transfer from anisotropic Au nanostructures to two-dimensional materials.

Plasmon-Induced Hot Electrons on Mesoporous Carbon for Decomposition of Organic Pollutants under Outdoor Sunlight Irradiation

In this study, a 4 in. CMK-8-Nafion membrane was fabricated using three-dimensional cubic ordered mesoporous carbon CMK-8 blended with a Nafion polymer. Plasmon-resonance hot electrons and holes from Au nanoparticles (NPs) combined with this CMK-8-Nafion membrane resulted in the effective decomposition of methyl orange (MO) due to the synergetic work of hot carriers with mesoporous carbon; a sample of Au/CMK-8-Nafion exposed to outdoor sunlight radiation for 150 min successfully removed 97% of MO. Fourier transform infrared spectroscopy (FTIR) was employed to examine the generation of hydroxyl groups (OH-) during decomposition. Finally, the spatial distribution of hydroxyl groups was also investigated across the different coverage densities of plasmonic Au NPs.

Plasmon enhanced light absorption in aluminium@Hematite core shell hybrid nanocylinders: the critical role of length

The absorbed photon flux in cylindrical α-Fe₂O₃ shells can be enhanced by filling it with an Al core and tailoring its length.

Microwave-solvothermal synthesis of various TiO2 nano-morphologies with enhanced efficiency by incorporating Ni nanoparticles in an electrolyte for dye-sensitized solar cells

A novel systematic approach is demonstrated to enhance the efficiency of dye-sensitized solar cells by impregnating Ni-nanoparticles into I⁻/I₃⁻ electrolyte with various TiO₂ nanomorphologies-based photo-anodes synthesized via microwave-solvothermal process.

Observing the dynamic “hot spots” on two-dimensional Au nanoparticles monolayer film

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays.

In-Situ Hydrothermal Synthesis of Bi-Bi2O2CO3 Heterojunction Photocatalyst with Enhanced Visible Light Photocatalytic Activity

Bismuth containing nanomaterials recently received increasing attention with respect to environmental applications because of their low cost, high stability and nontoxicity. In this work, Bi-Bi₂O₂CO₃ heterojunctions were fabricated by in-situ decoration of Bi nanoparticles on Bi₂O₂CO₃ nanosheets via a simple hydrothermal synthesis approach. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to confirm the morphology of the nanosheet-like heterostructure of the Bi-Bi₂O₂CO₃ composite. Detailed ultrafast electronic spectroscopy reveals that the in-situ decoration of Bi nanoparticles on Bi₂O₂CO₃ nanosheets exhibit a dramatically enhanced electron-hole pair separation rate, which results in an extraordinarily high photocatalytic activity for the degradation of a model organic dye, methylene blue (MB) under visible light illumination. Cycling experiments revealed a good photochemical stability of the Bi-Bi₂O₂CO₃ heterojunction under repeated irradiation. Photocurrent measurements further indicated that the heterojunction incredibly enhanced the charge generation and suppressed the charge recombination of photogenerated electron-hole pairs.

Optical response of silver clusters and their hollow shells from linear-response TDDFT

We present a study of the optical response of compact and hollow icosahedral clusters containing up to 868 silver atoms by means of time-dependent density functional theory. We have studied the dependence on size and morphology of both the sharp plasmonic resonance at 3-4 eV (originated mainly from sp-electrons), and the less studied broader feature appearing in the 6-7 eV range (interband transitions). An analysis of the effect of structural relaxations, as well as the choice of exchange correlation functional (local density versus generalised gradient approximations) both in the ground state and optical response calculations is also presented. We have further analysed the role of the different atom layers (surface versus inner layers) and the different orbital symmetries on the absorption cross-section for energies up to 8 eV. We have also studied the dependence on the number of atom layers in hollow structures. Shells formed by a single layer of atoms show a pronounced red shift of the main plasmon resonances that, however, rapidly converge to those of the compact structures as the number of layers is increased. The methods used to obtain these results are also carefully discussed. Our methodology is based on the use of localised basis (atomic orbitals, and atom-centered and dominant-product functions), which bring several computational advantages related to their relatively small size and the sparsity of the resulting matrices. Furthermore, the use of basis sets of atomic orbitals also allows the possibility of extending some of the standard population analysis tools (e.g. Mulliken population analysis) to the realm of optical excitations. Some examples of these analyses are described in the present work.