Plasmonic Noble Metal Nanoparticles Research Articles

Surface-Plasmon-Mediated Alloying for Monodisperse Au-Ag Alloy Nanoparticles in Liquid.

Plasmonic noble-metal nanoparticles with broadly tunable optical properties and catalytically active surfaces offer a unique opportunity for photochemistry. Resonant optical excitation of surface-plasmon generates high-energy hot carriers, which can participate in photochemical reactions. Although the surface-plasmon-driven catalysis on molecules has been extensively studied, surface-plasmon-mediated synthesis of bimetallic nanomaterials is less reported. Herein, we perform a detailed investigation on the formation mechanism and colloidal stability of monodisperse Au-Ag alloy nanoparticles synthesized through irradiating the intermixture of Au nanochains and AgNO3 solution with a nanosecond pulsed laser. It is revealed that the Ag atoms can be extracted from AgNO3 solution by surface-plasmon-generated hot electrons and alloy with Au atoms. Particularly, the obtained Au-Ag alloy nanoparticles without any surfactants or ligands exhibit superior stability that is confirmed by experiments as well as DLVO-based theoretical simulation. Our work would provide novel insights into the synthesis of potentially useful bimetallic nanoparticles via surface-plasmon-medicated alloying.

(Digital Presentation) Characterization of Ultrafast Dynamics of Refractory Metal Nanoparticles of Interest as Photocatalysts

Plasmonic noble metal nanoparticles are of interest as photocatalysts for several reasons, including a very large absorption cross section and the creation of a greatly enhanced evanescent electromagnetic field at the surface of the nanoparticles when excited at the plasmon resonance. Furthermore, the plasmon resonance is tunable over a wide wavelength range as a function of the size, shape, and composition of the nanoparticles. When these properties are combined with the known photocatalytic activity of noble metals for technologically important targets such as solar fuels, it is not surprising that these nanoparticle systems are widely studied. Noble metals do, however, possess a weakness in that they can be easily melted upon photoexcitation, due to the fact that noble metals dissipate absorbed photon energy efficiently through ultrafast electron-phonon coupling, in roughly one picosecond, to produce heat. As a result, the study of new classes of plasmonic materials, such as refractory metals, with much higher melting points than noble metals is growing rapidly. While there are still challenges in the size-selective synthesis of refractory metal nanoparticles, characterization efforts on these materials are of strong interest. We report here a study on the time-resolved spectroscopy of electron-phonon coupling in refractory metal nanoparticles. There are very significant variations in the efficiency and time-scales of electron-phonon coupling in refractory metal nanoparticles as compared to noble metal nanoparticles. Furthermore, due to the fact that many refractory metals are oxidized under ambient conditions to create a thin oxide layer on the surface of the nanoparticles, these materials also demonstrate dynamics related to electron injection from the metal to the oxide layer that noble metals do not show. Far from being a negative, the added electron injection dynamics offer new opportunities for photocatalysis that we discuss. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Metal oxides as electrocatalysts for water splitting: On plasmon‐driven enhanced activity

AbstractMany technological approaches have been searched in order to overcome the main challenges concerning the world energy crisis and global environmental issues. Among them, plasmon‐driven photoelectrochemical reactions towards water electrolysis attract great attention due to their capacity to efficiently harvest solar energy. Synergism between tunable optical features and catalysts active sites of plasmonic nanomaterials gives rise to a singular perspective for photochemical processes. Through resonant photonic excitation, hot carriers’ motion facilitates the charge transfer process on the catalyst surface for chemical reactions. In this minireview, recent experimental research with emphasis on water splitting reactions have been summarized with the purpose of understanding the mechanistic hot electrons generation and transfer on the plasmonic noble metal nanoparticles (MNPs) and transition metal oxides (MOs) heterostructures. Examples of plasmonic nanomaterials are highlighted and compared for both water electrolysis semi reactions. Finally, this work concludes by describing the remaining challenges and gives some perspectives regarding the promising future of plasmon‐driven reactions investigations.

Dual Photolytic Pathways in an Alloyed Plasmonic Near-Perfect Absorber: Implications for Photoelectrocatalysis

Plasmonic noble metal nanoparticles exhibit intense interactions with light but are inherently chemically unreactive. Alloying plasmonic gold with catalytic palladium forms a hybrid catalyst incorp...

Tunable thickness of mesoporous ZnO-coated metal nanoparticles for enhanced visible-light driven photoelectrochemical water splitting

The insufficient utilization of sunlight of ZnO, due to its broad band gap, results in low efficiency for photocatalytic hydrogen production. In this work, plasmonic noble metal nanoparticles (NPs) with different shapes (spheres and rods) were combined with mesoporous ZnO forming core-shell nanostructure to enhance the photocatalytic efficiency of ZnO in visible-light region. The photoelectrochemical water splitting activities of the metal@ZnO core-shell nanocomposites (NCs) were investigated. The photocurrent response of metal@ZnO NCs was found higher than pure ZnO or the mixture of metal NPs and ZnO ascribed to the effective charge transfer mechanism. It was also found that the photocurrent of metal@ZnO NCs was related to the thickness of ZnO and there was optimized shell for each kind of metal cores. Moreover, the introduction of Ag shell can get a higher photoelectrocatalytic efficiency compared to pure Au NPs core due to lower Schottky barrier between Ag and ZnO and wider extinction range in the visible light of Au@Ag NPs.

(Invited) Enhancement of Plasmon-Induced Charge Separation through Plasmon Coupling

Plasmon-induced charge separation (PICS)1,2 involves electron transfer from plasmonic noble metal nanoparticles or compound nanoparticles to an n-type semiconductor such as TiO2. Oxidation reactions involved in PICS are based on a charge accumulation mechanism (Figure a) or a hole ejection mechanism (Figure b).3 The latter enables nanoscale local oxidation at relatively positive potentials.4-7 This can be applied to photoinduced nanofabrication beyond the diffraction limit of light. For instance, chiral plasmonic nanostructures can be prepared by PICS under right or left circularly polarized light (Figure c).8,9 Some other applications of the plasmonic hole ejection will also be mentioned. Enhancement of PICS with hole ejection through plasmon coupling10 and its application to photocatalysis will also be described.References Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci., 8, 3325 (2017) [review].T. Tatsuma and H. Nishi, Nanoscale Horiz., 5, 597-606 (2020) [review].E. Kazuma, N. Sakai, and T. Tatsuma, Chem. Commun., 47, 5777 (2011).I. Tanabe and T. Tatsuma, Nano Lett., 12, 5418 (2012).K. Saito, I. Tanabe, and T. Tatsuma, J. Phys. Chem. Lett., 7, 4363 (2016).H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun., 54, 11741 (2018).K. Saito and T. Tatsuma, Nano Lett., 18, 3209 (2018).K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano, 14, 3603 (2020).T. Ishida and T. Tatsuma, J. Phys. Chem. C, 122, 26153 (2018). Figure 1

Mechanism insight of dual synergistic effects of plasmonic Pd-SrTiO3 for enhanced solar energy photocatalysis

This study presents the integration of UV-active semiconductor with plasmonic noble metal nanoparticles for enhanced solar energy photocatalysis. Nanocubes strontium titanate (SrTiO3) is synthesized via a simple hydrothermal process. Then palladium (Pd) nanoparticles will be deposited onto the surface of SrTiO3 by simple photochemical deposition route. The deposition of plasmonic Pd nanoparticles significantly increased the light absorption, especially in visible and near-infrared region and enhanced charge separation efficiency. The photocatalytic performance of Pd-deposited SrTiO3 is assessed by photodegradation of bisphenol A (BPA) and 4-chlorophenol (4CP) under solar light. The results confirm that the existence of Pd nanoparticles in SrTiO3 has improved the photocatalysis efficiency compared to pure SrTiO3. The higher weight percentage of Pd loading achieved better photocatalytic performance compared to lower weight percentage of Pd loading. This improvement can be deduced from the dual localized surface plasmon resonance effects that led to higher photoresponse and generation of free electrons. Moreover, the existence of Pd nanoparticles further retards the recombination rate of electron and hole pairs. This leads to the excess presence of electrons that contributed to the formation of active radicals that enhanced the oxidation of BPA and 4CP. Thus, this study will provide a new mechanism insight and approach to modify visible and near-infrared light-driven photocatalysts in degrading various organic pollutants.

Nonvolatile Electrical Bistability Behaviors Observed in Au/Ag Nanoparticle-Embedded MOFs and Switching Mechanisms.

Electrically bistable devices play an important role in the next generation of information materials. Plasmonic noble metal nanoparticles (Au and Ag NPs) with diameters <6 nm were embedded into 3-D Cd-based metal-organic framework (MOF) matrixes via the photoreduction method to generate Au (Ag) NPs@MOF composites. Electrical bistability measurements on the sandwiched ITO/NPs@MOF/silver devices indicate that two switchable conductivity states with nonvolatile memory behaviors can be observed. The ITO/Au NP@2/Ag device with neutral matrix possesses the highest ON/OFF current ratio of 104, which can be attributed to its higher electron tunneling efficiency because of the better dispersity of Au NPs in the MOF matrix. A mechanism regarding the electric-field-induced charge-transfer process assisted by conformational change in the active layer was proposed.

Multi-Layered Mesoporous TiO2 Thin Films: Photoelectrodes with Improved Activity and Stability

This work aims at the identification of porous titanium dioxide thin film (photo)electrodes that represent suitable host structures for a subsequent electrodeposition of plasmonic nanoparticles. Sufficient UV absorption and electrical conductivity were assured by adjusting film thickness and TiO 2 crystallinity. Films with up to 10 layers were prepared by an evaporation-induced self-assembly (EISA) method and layer-by-layer deposition. Activities were tested towards the photoelectrochemical oxidation of water under UV illumination. Enhanced activities with each additional layer were observed and explained with increased amounts of immobilized TiO 2 and access to more active sites as a combined effect of increased surface area, better crystallinity and improved transport properties. Furthermore, films display good electrochemical and mechanical stability, which was related to the controlled intermediate thermal annealing steps, making these materials a promising candidate for future electrochemical depositions of plasmonic noble metal nanoparticles that has been further demonstrated by incorporation of gold.

Plasmon-Driven Catalysis on Molecules and Nanomaterials

As a new class of photocatalysts, plasmonic noble metal nanoparticles with the unique ability to harvest solar energy across the entire visible spectrum and produce effective energy conversion have been explored as a promising pathway for the energy crisis. The resonant excitation of surface plasmon resonance allows the nanoparticles to collect the energy of photons to form a highly enhanced electromagnetic field, and the energy stored in the plasmonic field can induce hot carriers in the metal. The hot electron-hole pairs ultimately dissipate by coupling to phonon modes of the metal nanoparticles, resulting in a higher lattice temperature. The plasmonic electromagnetic field, hot electrons, and heat can catalyze chemical reactions of reactants near the surface of the plasmonic metal nanoparticles. This Account summarizes recent theoretical and experimental advances on the excitation mechanisms and energy transfer pathways in the plasmonic catalysis on molecules. Especially, current advances on plasmon-driven crystal growth and transformation of nanomaterials are introduced. The efficiency of the chemical reaction can be dramatically increased by the plasmonic electromagnetic field because of its higher density of photons. Similar to traditional photocatalysis, energy overlap between the plasmonic field and the HOMO-LUMO gap of the reactant is needed to realize resonant energy transfer. For hot-carrier-driven catalysis, hot electrons generated by plasmon decay can be transferred to the reactant through the indirect electron transfer or direct electron excitation process. For this mechanism, the energy of hot electrons needs to overlap with the unoccupied orbitals of the reactant, and the particular chemical channel can be selectively enhanced by controlling the energy distribution of hot electrons. In addition, the local thermal effect following plasmon decay offers an opportunity to facilitate chemical reactions at room temperature. Importantly, surface plasmons can not only catalyze chemical reactions of molecules but also induce crystal growth and transformation of nanomaterials. As a new development in plasmonic catalysis, plasmon-driven crystal transformation reveals a more powerful aspect of the catalysis effect, which opens the new field of plasmonic catalysis. We believe that this Account will promote clear understanding of plasmonic catalysis on both molecules and materials and contribute to the design of highly tunable catalytic systems to realize crystal transformations that are essential to achieve efficient solar-to-chemical energy conversion.

Plasmonic Ag nanoparticles and p-type CuO-modified ZnO nanorods for efficient photoelectrochemical water splitting

The light absorption range of semiconductor materials and the separation rate of electron–hole pairs are significant challenge in photoelectrochemical (PEC) water splitting. In this work, we first prepared a ternary heterojunction of ZnO/CuO/Ag by chemical bath deposition method. The formation of the heterojunction can improve the absorption range and increase the electron–hole pairs’ separation rate, and the loading of plasmonic Ag nanoparticles can expand the absorption range of visible light and generate more photogenerated electrons through surface plasmon resonance (SPR). The ZnO/CuO/Ag heterojunction photoelectrode reveals an efficient photocurrent density of 3.45 mA cm−2 at 1.23 V vs. RHE, which is 3.08 and 1.12 times higher compared with ZnO and ZnO/CuO photoelectrode, respectively. This study shows that the simultaneous reaction of heterojunction and plasmonic noble metal nanoparticles can synergistically improve the photoelectric properties of photoanodes in photoelectrochemical water-splitting system.

Role of various nanoparticles in photodynamic therapy and detection methods of singlet oxygen.

In this review article we described the applications of various nanoparticles that can be used for photodynamic therapy (PDT), such as: plasmonic nanoparticles, quantum dots and upconversion nanoparticles. In comparison with typical organic photosensitizers such as hematoporphyrins, they exhibit higher photostability and resistance to enzymatic degradation, and hence, in some cases, they could replace organic photosensitizers in PDT therapy. It has also been found that the presence of plasmonic noble metal nanoparticles increases the efficiency of conjugated standard photosensitizers. Therefore, one can expect that, due to their very promising optical properties, plasmonic nanoparticles, plasmonic composites, and upconversion nanoconjugates will have a significant impact on the detection and treatment of cancer in the near future. Various methods of detecting the singlet oxygen produced are also reviewed.

Shape Tailored TiO₂ Nanostructures and Their Hybrids for Advanced Energy and Environmental Applications: A Review.

Shape tailored TiO₂ nanostructures with various dimensionality (zero to three dimension) have unique physicochemical and functional properties that facilitates its efficient energy and environment applications, e.g., solar light driven photocatalytic hydrogen generation and decontamination of organic/inorganic toxic pollutants, CO₂ reduction into the hydrocarbon fuels, solar cells, supercapacitors and lithium-ion batteries etc. However, the wide band gap nature and the fast recombination of the photogenerated charge carriers in TiO₂ usually limit its overall performance under solar light illumination. In this review, we present a state of the art on the fabrication techniques of shape tailored TiO₂ nanostructures and the strategies employed to make the system catalytically more efficient. Though shape tailored TiO₂ nanostructures with large specific surface area and highly energetic (001) facet exposed TiO₂ nanostructures (2D and 3D) can enhance the photocatalytic efficiency to a reasonable extent, further surface engineering is needed for the modification of the electronic band arrangement, visible light sensitization and efficient charge separation. Herein, TiO₂ heterostructures (HSs) with metal/non-metal doping, surface fluorination, plasmonic noble metal nanoparticles (NPs) and coupling with the narrow band gap suitable semiconductor (type-II) are discussed in details covering from zero dimensional to three dimensional heterostructures. The synthesis strategies, charge transfer mechanism and their participation in the photocatalysis are elaborated. Though one dimensional TiO₂ HSs have been widely studied, we present the recent development of critical surface engineering strategies of two and three dimensional systems, which give rise to the excellent properties including the enlargement of surface area, light absorption capability and efficient separation of electrons/holes resulting in the superior performance in advanced applications. Based on recent breakthroughs in the field, future directions and outlook of the field are presented at the end.

Redox-Active AIEgen-Derived Plasmonic and Fluorescent Core@Shell Nanoparticles for Multimodality Bioimaging.

Multimodality imaging is highly desirable for accurate diagnosis by achieving high sensitivity, spatial-temporal resolution, and penetration depth with a single structural unit. However, it is still challenging to integrate fluorescent and plasmonic modalities into a single structure, as they are naturally incompatible because of significant fluorescence quenching by plasmonic noble-metal nanoparticles. Herein, we report a new type of silver@AIEgen (aggregation-induced emission luminogen) core-shell nanoparticle (AACSN) with both strong aggregated-state fluorescence of the AIEgen and distinctive plasmonic scattering of silver nanoparticles for multimodality imaging in living cells and small animals. The AACSNs were prepared through a redox reaction between silver ions and a redox-active AIEgen, which promoted synergistic formation of the silver core and self-assembly of the AIEgen around the core. The resulting AACSNs exhibited good biocompatibility and high resistance to environmental damage. As a result, excellent performance in fluorescence imaging, dark-field microscopy, and X-ray computed tomography-based multimodality imaging was achieved.

Coupling plasmonic noble metal with TiO2 for efficient photocatalytic transfer hydrogenation: M/TiO2 (M = Au and Pt) for chemoselective transformation of cinnamaldehyde to cinnamyl alcohol under visible and 365 nm UV light

M/TiO2 (M = Au and Pt) were prepared by a solvothermal method combined with an in situ photo-reduction process and their photocatalytic performance for transfer hydrogenation of cinnamaldehyde (CAL) to form cinnamyl alcohol (COL) in the presence of isopropanol was investigated under visible light and 365 nm UV light. Although the photocatalytic transformation of CAL to COL can be realized over both Au/TiO2 and Pt/TiO2, they show different activity order under visible light and 365 nm UV light, ie, Au/TiO2 show superior activity than Pt/TiO2 under visible light, while an opposite activity order was observed under 365 nm UV light. The hydrogenation to form COL over Pt/TiO2 reached a turnover frequency (TOF) of 197 h−1 under 365 nm UV light, which is among the highest in all the already reported systems for hydrogenation and transfer hydrogenation of CAL to form COL. The superior performance observed over Pt/TiO2 at 365 nm can be ascribed to a synergistic effect of plasmonic Pt and the direct excitation of TiO2. This study not only provides a green method for the transfer hydrogenation of CAL to form COL, but also highlights the great potential of coupling of plasmonic noble metal nanoparticles with semiconductor photocatalysts for light-induced organic transformations.

Two-dimensional molybdenum disulfide (MoS2) with gold nanoparticles for biosensing of explosives by optical spectroscopy

As one of the most important transition-metal dichalcogenides (TMDs) materials, molybdenum disulfide (MoS2) has gained extensive attention for its marvelous optoelectronic properties. In this study, nanocomposites of two-dimensional (2D) molybdenum disulfide (MoS2) with plasmonic noble metal nanoparticles were synthesized by a one-step green process. Under irradiation, 2D MoS2 in the nanocomposites absorbed photons and generated carriers in a wide spectrum. Then MoS2 coupled to gold nanoparticles and produced the enhanced electromagnetic field in the nanosturctures, leading to outstanding optical properties. In virtue of bio-compatibilities of the particles, the nanocomposites were chemically modified with specific peptides to construct an optical biosensor for explosive detection. When exposed to 2,4,6-trinitrotoluene (TNT), the biosensor showed significant absorption peak changes in visible spectra with a concentration-dependent behavior. The biosensor could detect TNT at the concentration as low as 2 × 10−7 M. For selectivity, the biosensor could differentiate TNT from other nitroaromatic explosives that have extremely similar structures to TNT, such as 4-nitrotoluene (4-NT) and 2,6-dinitrotoluene (DNT). All the results suggested that the biosensor based on the nanocomposites of 2D MoS2 coupling to plasmonic nanoparticles exhibited remarkable optical performance, which provided a promising approach to design versatile biosensors to detect biochemical molecules.

Plasmonic behaviour and plasmon-induced charge separation of nanostructured MoO3-x under near infrared irradiation.

Plasmon-induced charge separation (PICS) allows direct conversion of localized surface plasmon resonance (LSPR) to electron flows and photoelectrochemical reactions. However, PICS has only been achieved using plasmonic noble metal nanoparticles, not with compound nanoparticles. In order to achieve compound PICS, MoO3-x nanostructures were prepared that exhibit LSPR in the near infrared region by using metal oxides or metal nanoparticles as templates. Solid-state cells based on the MoO3-x nanostructure were developed. Their photoresponse to 700-1400 nm infrared light was investigated and analyzed on the basis of their PICS mechanisms.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals.

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

Modeling of the surface plasmon resonance tunability of silver/gold core-shell nanostructures.

Tunable plasmonic noble metal nanoparticles are indispensable for chemical sensors and optical near field enhancement applications. Laser wavelengths within the absorption spectrum of the nanoparticle and Localized Surface Plasmon Resonances (LSPR) in the visible and near infrared range are the key points to be met for the successful utilization in the field of aforementioned high sensitivity sensors. This way, Surface Enhanced Raman Spectroscopy (SERS) has been pushed to the sensitivity level of single molecule. The tunability, i.e. the modulation of the surface plasmon resonance wavelength as a function of the ambient refractive index is one of the important criteria to be understood clearly. Among various noble metals, gold and silver nanoparticles have the strongest surface enhancement factors for the Raman signal and their tunability for many practical applications has been experimentally demonstrated. We present a comprehensive numerical investigation by means of a finite element analysis on Ag/Au core–shell nanospheres including agglomerated and non-agglomerated dimers. Tunability as a function of shell thickness, total nanosphere radius and fraction of overlap between the dimer is discussed. Our studies show that tunability is considerably affected by the nanosphere radius rather than the shell thickness. These findings may be helpful in the synthesis of nanoplasmonic structures, especially related to an optimized use of gold as the shell material for the targeted application.

On-a-chip Biosensing Based on All-Dielectric Nanoresonators.

Nanophotonics has become a key enabling technology in biomedicine with great promises in early diagnosis and less invasive therapies. In this context, the unique capability of plasmonic noble metal nanoparticles to concentrate light on the nanometer scale has widely contributed to biosensing and enhanced spectroscopy. Recently, high-refractive index dielectric nanostructures featuring low loss resonances have been proposed as a promising alternative to nanoplasmonics, potentially offering better sensing performances along with full compatibility with the microelectronics industry. In this letter we report the first demonstration of biosensing with silicon nanoresonators integrated in state-of-the-art microfluidics. Our lab-on-a-chip platform enables detecting Prostate Specific Antigen (PSA) cancer marker in human serum with a sensitivity that meets clinical needs. These performances are directly compared with its plasmonic counterpart based on gold nanorods. Our work opens new opportunities in the development of future point-of-care devices toward a more personalized healthcare.