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Plasmonic color generation has attracted much research interest because of the unique optical properties of plasmonic nanocrystals that are promising for chromatic applications, such as flat-panel displays, smart windows, and wearable devices. Low-cost, monodisperse plasmonic nanocrystals supporting strong localized surface plasmon resonances are favorable for the generation of plasmonic colors. However, many implementations so far have either a single static state or complexities in the particle alignment and switching mechanism for generating multiple displaying states. Herein, we report on a facile and robust approach for realizing the electrochemical switching of plasmonic colors out of colloidal plasmonic nanocrystals. The metal nanocrystals are coated with a layer of polyaniline, whose refractive index and optical absorption are reversibly switched through the variation of an applied electrochemical potential. The change in refractive index and optical absorption results in the modulation of the plasmonic scattering intensity with a depth of 11 dB. The electrochemical switching process is fast (∼5 ms) and stable (over 1000 switching cycles). A device configuration is further demonstrated for switching plasmonic color patterns in a transparent electrochemical device, which is made from indium tin oxide electrodes and a polyvinyl alcohol solid electrolyte. Our control of plasmonic colors provides a favorable platform for engineering low-cost and high-performance miniaturized optical devices.

Conductive polymers are attracting much attention for realizing active plasmonics on conventional static plasmonic nanostructures because of their variable dielectric functions. Combining organic conductive polymers with inorganic plasmonic nanostructures allows for the creation of active devices, such as active metasurfaces, reconfigurable metalenses and dynamic plasmonic holography. However, the complexity of such a combination, together with the poor control in polymer thickness and morphology, has limited the advancement of active plasmonics. Herein we report on the electrochemical coating of conductive polymers on pre-grown metal nanocrystals. Robust control of the polymer thickness and morphology is accomplished through the variation of the applied electrochemical potential. Various types of conductive polymers are coated on different metal nanocrystals, including Au, Pd and Pt. Active plasmonic color switching and H2O2 sensing are demonstrated with polyaniline-coated Au nanorods.

The interference between spectrally overlapping superradiant and subradiant plasmon resonances generates plasmonic Fano resonance, which allows for attractive applications such as electromagnetically induced transparency, light trapping, and refractometric sensing with high figures of merit. The active switching of plasmonic Fano resonance holds great promise in modulating optical signals, dynamically harvesting light energy, and constructing switchable plasmonic sensors. However, structures enabling the active control of plasmonic Fano resonance have rarely been achieved because of the fabrication complexity and cost. Herein we report on the realization of active plasmonic Fano resonance switching on Au nanosphere-nanoplate heterodimers. The active switching is enabled by varying the refractive index of a layer of polyaniline that fills in the gap between the Au nanosphere and the Au nanoplate. A reversible spectral shift of 20 nm is observed on the individual heterodimers during switching. The maximal spectral shift decreases as the interparticle gap distance is enlarged, showing a strong dependence of the spectral shift on the local electric field intensity enhancement in the gap region. This trend agrees with the predicted dependence of the refractive index sensitivity on the local field intensity enhancement. Our results provide insights into the development of plasmonic structures supporting actively switchable Fano resonances, which can lead to new technological applications, such as switchable cloaking and display, dynamic coding of optical signals, color sorting and filtering. The Au heterodimers with polyaniline in the gap can also be applied for the sensing of local environmental parameters such as pH values and heavy metal ions.

Transition metal phosphides have sparked considerable interest as electrocatalysts owing to their excellent catalytic activities towards water splitting. However, the development of a general strategy for the preparation of metal phosphides has remained challenging. In this work, we demonstrate a two-step, aerosol spray-based strategy for the synthesis of various mesoporous monometallic and multimetallic phosphide microspheres. Four monometallic (FeP, CoP, Ni2P, and Cu3P), one bimetallic (Co6Ni4P5) and one trimetallic (Co4Ni3Cu2P10) phosphide samples are successfully prepared, suggesting the generality of our method. In addition, the CoP sample is found to be an excellent material for the hydrogen evolution reaction over a wide pH range through the introduction of corrosion-resistant Cr2O3. On the other hand, its catalytic activity can be switched to the oxygen evolution reaction in combination with Fe2O3. The overpotential for the oxygen evolution reaction to generate a current density of 10 mA cm−2 is as low as 302 mV. This activity can be maintained for at least 50 h. More importantly, a two-electrode electrolyzer constructed from 10% Cr–CoP and 30% Fe–CoP samples delivers a current density of 100 mA cm−2 at a cell voltage of merely 1.67 V, showing the use of these cost-effective materials for bifunctional water splitting.

Gold nanorings are attractive as plasmonic metal nanocrystals because they have a hollow inner cavity. Their enhanced electric field inside the ring cavity is accessible, which is highly desirable for assembling with other optical components and studying their plasmon-coupling behaviors. However, the lack of robust methods for synthesizing size-controllable and uniform Au nanorings severely impedes the study of their attractive plasmonic properties and plasmon-driven applications. Herein, an improved wet-chemistry method is reported for the synthesis of monodisperse colloidal Au nanorings. Using circular Au nanodisks with different thicknesses and diameters as templates, Au nanorings are synthesized with thicknesses varied from ≈30 to ≈50 nm and cavity sizes varied from ≈90 to ≈40 nm. The produced Au nanorings are assembled with colloidal Au nanospheres to yield Au nanoring-nanosphere heterodimers in sphere-in-ring and sphere-on-ring configurations on substrates. The sphere-in-ring heterodimers exhibit the interesting feature of plasmonic Fano resonance upon the excitation of the dark quadrupolar plasmon mode of the Au nanorings. The open cavity in a nanoring holds a great promise for studying plasmon-coupled systems, which will facilitate the construction of advanced metamaterials and high-performance Fano-based devices.

Plasmonic metal nanocrystals, owing to their high sensitivity to the dielectric changes in the surrounding environment, can allow for the direct probing and monitoring of molecular binding on their surfaces. Anisotropic Au nanoplates possess high refractive index sensitivities, with their nanoscale sensing volumes located at their sharp tips and edges. One of the main challenges in molecular detection based on localized plasmon resonance is to increase the detection capability at the single-particle level. Vertically oriented Au nanoplates are more attractive candidates for developing ultrasensitive plasmonic sensors than horizontally oriented ones, as vertical Au nanoplates allow for more analyte molecules to access their sharp tips and edges. However, few reports have studied the sensing performance of vertically oriented, elongated, individual metal nanocrystals. Herein we report on the deposition of vertically oriented, hexagonal Au nanoplates on substrates and the study of their plasmonic and sensing properties. The vertically aligned nanoplates are compared with the horizontally oriented ones both experimentally and through numerical simulations. The vertically oriented nanoplates possess shorter plasmon resonance wavelengths and narrower peak widths than the horizontally oriented ones of similar sizes. The shorter plasmon wavelengths and smaller peak widths are also confirmed by knocking down the vertical Au nanoplates through mild perturbation and performing the scattering measurements beforehand and afterwards. Further scattering measurements of the Au nanoplates upon the adsorption of positively charged polyelectrolyte reveal that the vertical Au nanoplates are more sensitive to the polyelectrolyte molecules than the horizontal ones.

The fixation of atmospheric N2 to NH3 is an essential process for sustaining life. One grand challenge is to develop efficient catalysts to photofix N2 under ambient conditions. Herein we report an...

The photochemical production of fuels using sunlight is an innovative way for meeting the quickly increasing energy demands. One of the largest challenges is to develop high-performance photocatalysts that can meet the requirements of practical applications. Owing to their intriguing localized surface plasmon resonances, noble metal nanoparticles and nanostructures show a great potential for enhancing the photocatalytic efficiency and thereby have attracted rapidly growing interest recently. Here, for the first time, the latest achievements in the utilization of plasmons in driving CO2 reduction and N2 fixation into high-value products are comprehensively described. The involved plasmonic enhancement mechanisms in the two types of reactions are fully illustrated. A particular emphasis is given to the outlook on the direction and prospects for future work in this topic.

High-performance electrochemical plasmonic switching is realized on both single-particle and ensemble levels by coating polyaniline on colloidal gold nanocrystals through surfactant-assisted oxidative polymerization. Under small applied potentials, the core@shell nanostructures exhibit reversible plasmon shifts as large as 150 nm, a switching time of less than 10 ms, and a high switching stability.