Ag Nanocubes Research Articles

Third order nonlinear optics in Ag nanocubes: local and nonlocal optical responses as a function of excitation wavelength and particle size.

Colloidal silver nanocubes in solution were analyzed by the Z-scan technique in the resonant and non-resonant regimes. Three different systems with particle sizes of 56, 99 and 215 nm, and concentrations of 0.378, 0.831 and 0.715 mg/mL, respectively, were obtained by the polyol method. Nonlinear excitation of the samples was performed with laser pulses of 26 ps, at a repetition rate of 10 Hz, and using three different wavelenghts (355, 532 and 1064 nm), exciting only the electronic part of the optical nonlinearity, and avoiding induced thermal loading of the samples. Whenever observable, samples showed saturable absorption for all wavelengths, which was dependent, in general, on concentration and incident intensity. For samples featuring sizes of 55 and 99 nm, saturable absorption could be observed for wavelengths close to their dipolar surface plasmon resonances; while, for samples with sizes of 99 and 215 nm, saturable absorption and positive nonlinear optical refraction (only for size of 215 nm) could be observed at 1064 nm. Besides, for some samples, nonlinear optical response followed the incident intensity profile at wavelengths close to resonance. On the contrary, for wavelengths out of resonance, the nonlinear optical behavior showed to be nonlocal, ie, its profile was narrower than the incident one.

Plasmonic molybdenum oxide nanosheets supported silver nanocubes for enhanced near-infrared antibacterial activity: Synergism of photothermal effect, silver release and photocatalytic reactions

Plasmonic MoO3−x nanosheets supported Ag nanocubes were designed as high-efficient Near-infrared (NIR) light driven antibacterial agent. The as-designed MoO3−x-Ag not only significantly reduces the cost and toxicity of Ag, but also greatly enhances the antibacterial activity towards E. coli and S. aureus in comparison with bare MoO3−x and Ag upon NIR laser irradiation. Three effects are believed to contribute to this enhancement: (1) the plasmonic MoO3−x-Ag exhibits excellent NIR light absorption and photothermal conversion abilities, which help to kill bacteria; (2) NIR light induced heat in MoO3−x also diffuses to Ag nanocubes and triggers the release of more Ag+, leading to bacterial cell death; (3) under NIR light irradiation, hot electrons and holes are generated and separated through MoO3−x-Ag interface, which produces reactive species for the oxidation of out membrane of bacteria. The synergism of NIR light driven photothermal effect, silver release and photocatalytic reaction makes the designed plasmonic MoO3−x-Ag a promising antibacterial agent for cleaning the microbial contaminated water environment.

Decrease of amplified spontaneous emission threshold achieved by core–shell Ag nanocube@SiO2 with ultrasmall shell thicknesses

The amplified spontaneous emission (ASE) threshold reduction of R6G/PMMA film with a quasi-waveguide structure was observed by doping core–shell Ag nanocube@SiO2. When the SiO2 shell thickness was only 3 nm, the ASE threshold declined from 0.8 to 0.2 mJ cm−2, presenting a large decreased ratio of 75%. With the enhancement of the emission intensity, the pulse width of the ASE spectra also narrowed from 8 to 6.8 nm after doping. An efficient coupling between the R6G media and Ag nanocube through the proper ultrathin spacer was concluded as the main reason for such phenomena.

Microchemical Plant in a Liquid Droplet: Plasmonic Liquid Marble for Sequential Reactions and Attomole Detection of Toxin at Microliter Scale.

Miniaturizing the continuous multistep operations of a factory into a microchemical plant offers a safe and cost-effective approach to promote high-throughput screening in drug development and enforcement of industrial/environmental safety. While particle-assembled microdroplets in the form of liquid marble are ideal as microchemical plant, these platforms are mainly restricted to single-step reactions and limited to ex situ reaction monitoring. Herein, we utilize plasmonic liquid marble (PLM), formed by encapsulating liquid droplet with Ag nanocubes, to address these issues and demonstrate it as an ideal microchemical plant to conduct reaction-and-detection sequences on-demand in a nondisruptive manner. Utilizing a two-step azo-dye formation as our model reaction, our microchemical plant allows rapid and efficient diazotization of nitroaniline to form diazonium nitrobenzene, followed by the azo coupling of this intermediate with target aromatic compound to yield azo-dye. These molecular events are tracked in situ via SERS measurement through the plasmonic shell and further verified with in silico investigation. Furthermore, we apply our microchemical plant for ultrasensitive SERS detection and quantification of bisphenol A (BPA) with detection limit down to 10 amol, which is 50 000-fold lower than the BPA safety limit. Together with the protections offered by plasmonic shell against external environments, these collective advantages empower PLM as a multifunctional microchemical plant to facilitate small-volume testing and optimization of processes relevant in industrial and research contexts.

Direct observation of the nanoscale Kirkendall effect during galvanic replacement reactions

Galvanic replacement (GR) is a simple and widely used approach to synthesize hollow nanostructures for applications in catalysis, plasmonics, and biomedical research. The reaction is driven by the difference in electrochemical potential between two metals in a solution. However, transient stages of this reaction are not fully understood. Here, we show using liquid cell transmission electron microscopy that silver (Ag) nanocubes become hollow via the nucleation, growth, and coalescence of voids inside the nanocubes, as they undergo GR with gold (Au) ions at different temperatures. These direct in situ observations indicate that void formation due to the nanoscale Kirkendall effect occurs in conjunction with GR. Although this mechanism has been suggested before, it has not been verified experimentally until now. These experiments can inform future strategies for deriving such nanostructures by providing insights into the structural transformations as a function of Au ion concentration, oxidation state of Au, and temperature.

Hot Electron Emission Can Lead to Damping of Optomechanical Modes in Core–Shell Ag@TiO2 Nanocubes

Interactions between light and metal nanostructures are mediated by collective excitations of free electrons called surface plasmons, which depend primarily on geometry and dielectric environment. Excitation with ultrafast pulses can excite optomechanical modes that modulate the volume and shape of nanostructures at gigahertz frequencies. Plasmons serve as an optical handle to study the ultrafast electronic dynamics of nanoscale systems. We describe a method to synthesize core–shell Ag@TiO2 nanocubes—while successfully maintaining the size and shape of the nanocube. Transient absorbance spectroscopy (TAS) is used to track photophysical processes on multiple time scales: from the ultrafast creation of hot carriers to their decay into phonons and the formation of optomechanical modes. Surprisingly, the TiO2 shell surrounding the Ag nanocubes caused no appreciable change in the frequency of the optomechanical mode, indicating that mechanical coupling between the core and shell is weak. However, the optomecha...

Bimetallic nanoplasmonic gap-mode SERS substrate for lung normal and cancer-derived exosomes detection

Abstract Surface-enhanced Raman scattering (SERS) has received significant attention for analytical and diagnostic applications in cancer research due to its potency for detecting low abundance of biomarkers at high sensitivity. Cell-secreted exosomes play an important role is not only physiological but also pathological conditions by carrying various key biomolecules such as proteins, lipids, and nucleic acid of their cells of origin and are involved in intercellular transport. They are thus possible biomarkers for many diseases, including cancer. Herein, we fabricated a strong plasmonic gap-mode SERS substrate using Ag nanocubes (NCs) on an Au nanorod (NR) array substrate in order to investigate exosomes. A strong plasmonic cavity effect was obtained from individual Ag NCs assembled on the hot-ring diameter (HR) area on the NR surface. The SERS signal of a low concentration of Rhodamine 6 G upon NCs on NR substrate was amplified. The enhancement factor of the optimized Ag NCs on Ag NR array substrate reached 9.11 × 10 8 . The developed SERS substrate was applied to investigate exosomes derived from lung normal (from NL-20, BEAS-2B, and L929) and cancer (from PC-9, H1975, and HCC827) cell lines with limited sample input. A significant difference was found in the SERS signals from proteins, lipids, and nucleic acid not only between lung normal and cancer exosomes but also between fibroblast and epithelium exosomes. For the lung cancer exosomes, a strong protein SERS signal was observed under time-dependent SERS analysis, whereas for the normal exosomes, the SERS signal mostly corresponded to proteins, lipids, and nucleic acid. The attachment of Ag NCs on the HR area of Au NRs is presumably key for enhancing the electromagnetic effect and thus the SERS signal of exosomes.

Graphene “bridge” in transferring hot electrons from plasmonic Ag nanocubes to TiO2 nanosheets for enhanced visible light photocatalytic hydrogen evolution

The integration of plasmonic metal with wide-bandgap semiconductor is a promising approach to utilize the visible light without compromise of the redox ability of photogenerated charge carriers. However, a larger work function of metal than that of semiconductor is indispensable to enable the injection of hot electrons from plasmonic metal to semiconductor. In this paper, we demonstrated that reduced graphene oxide (rGO) nanosheets as conductive “bridge” can breakthrough the restriction and transfer hot electrons from Ag of smaller work function to TiO2 of larger work function. In the design, both of the Ag nanocubes and TiO2 nanosheets are co-deposited on the surface of rGO nanosheets to form Ag-rGO-TiO2 structure, which was characterized by XRD, TEM, Raman and XPS spectra. On one hand, the Ag-rGO interface facilitates the transfer of hot electrons from Ag to rGO through conductor–conductor contact. On the other hand, the new formed Schottky junction on the rGO-TiO2 interface further pumps the transferred electrons to the surface of TiO2 for photocatalytic reduction reaction resulted from the larger work function of rGO than that of TiO2. Enabled by this unique design, the hydrogen production activity achieved under visible light irradiation is dramatically enhanced in comparison with that of Ag-TiO2 counterpart with the direct contact between the same Ag nanocubes and TiO2 nanosheets. This work represents a step toward the rational interfacial design of plasmonic metal-semiconductor hybrid structures for broad-spectrum photocatalysis.

Surface-enhanced Raman spectroscopy competitive binding biosensor development utilizing surface modification of silver nanocubes and a citrulline aptamer.

A point-of-care (PoC) device with the ability to detect biomarkers at low concentrations in bodily fluids would have an enormous potential for medical diagnostics outside the central laboratory. One method to monitor analytes at low concentrations is by using surface-enhanced Raman spectroscopy (SERS). In this preliminary study toward using SERS for PoC biosensing, the surface of colloidal silver (Ag) nanocubes has been modified to test the feasibility of a competitive binding SERS assay utilizing aptamers against citrulline. Specifically, Ag nanocubes were functionalized with mercaptobenzoic acid, as well as a heterobifunctional polyethylene glycol linker that forms an amide bond with the amino acid citrulline. After the functionalization, the nanocubes were characterized by zeta-potential, transmission electron microscopy images, ultraviolet/visible spectroscopy, and by SERS. The citrulline aptamers were developed and tested using backscattering interferometry. The data show that our surface modification method does work and that the functionalized nanoparticles can be detected using SERS down to a 24.5 picomolar level. Last, we used microscale thermophoresis to show that the aptamers bind to citrulline with at least a 50 times stronger affinity than other amino acids. Download PDF SAVE FOR LATER

Controlling energy flow in multimetallic nanostructures for plasmonic catalysis

It has been shown that photoexcitation of plasmonic metal nanoparticles (Ag, Au and Cu) can induce direct photochemical reactions. However, the widespread application of this technology in catalysis has been limited by the relatively poor chemical reactivity of noble metal surfaces. Despite efforts to combine plasmonic and catalytic metals, the physical mechanisms that govern energy transfer from plasmonic metals to catalytic metals remain unclear. Here we show that hybrid core-shell nanostructures in which a core plasmonic metal harvests visible-light photons can selectively channel that energy into catalytically active centres on the nanostructure shell. To accomplish this, we developed a synthetic protocol to deposit a few monolayers of Pt onto Ag nanocubes. This model system allows us to conclusively separate the optical and catalytic functions of the hybrid nanomaterial and determine that the flow of energy is strongly biased towards the excitation of energetic charge carriers in the Pt shell. We demonstrate the utility of these nanostructures for photocatalytic chemical reactions in the preferential oxidation of CO in excess H2. Our data demonstrate that the reaction occurs exclusively on the Pt surface.

Tuning Molecular-Level Polymer Conformations Enables Dynamic Control over Both the Interfacial Behaviors of Ag Nanocubes and Their Assembled Metacrystals

In surface chemistry-directed nanoparticle self-assembly, it remains challenging to continuously modulate nanoparticle behavior at the oil/water interface without replacing surface functionality or particle morphology. Here, we utilize solvent-tunable molecular-level polymer conformation changes to achieve “multiple metacrystals using one nanoparticle with one chemical functionality”. We use Ag nanocubes functionalized with a mixed monolayer of thiol-terminated poly(ethylene glycol) (PEG) and hexadecanethiol (C16). We continuously modulate PEG conformation from swollen to coiled states by decreasing solvent polarity, whereas C16 promotes nanocube dispersion in organic carrier solvents. Such PEG conformation changes drive Ag nanocubes to adopt tilted, standing, and planar configurations at the oil/water interface, with their interfacial positions changing from halfway across the interface to almost immersed within the oil phase. We also identify four specific polarities which enable Ag nanocubes to assembl...

Enriching Silver Nanocrystals with a Second Noble Metal

Noble-metal nanocrystals have received considerable interests owing to their fascinating properties and promising applications in areas including plasmonics, catalysis, sensing, imaging, and medicine. As demonstrated by ample examples, the performance of nanocrystals in these and related applications can be augmented by switching from monometallic to bimetallic systems. The inclusion of a second metal can enhance the properties and greatly expand the application landscape by bringing in new capabilities. Seeded growth offers a powerful route to bimetallic nanocrystals. This approach is built upon the concept that preformed nanocrystals with uniform, well-controlled size, shape, and structure can serve as seeds to template and direct the deposition of metal atoms. Seeded growth is, however, limited by galvanic replacement when the deposited metal is less reactive than the seed. The involvement of galvanic replacement not only makes it difficult to control the outcome of seeded growth but also causes degradation to some properties. We have successfully addressed this issue by reducing the salt precursor(s) into atoms with essentially no galvanic replacement. In the absence of self-nucleation, the atoms are preferentially deposited onto the seeds to generate bimetallic nanocrystals with controlled structures. In this Account, we use Ag nanocubes as an example to demonstrate the fabrication of Ag@M and Ag@Ag-M (M = Au, Pd, or Pt) nanocubes with a core-frame or core-shell structure by controlling the deposition of M atoms. A typical synthesis involves the titration of Mn+ (a precursor to M) ions into an aqueous suspension containing Ag nanocubes, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. In one approach, aqueous sodium hydroxide is introduced to increase the initial pH of the reaction system. At pH = 11.9, ascorbic acid is dominated by ascorbate monoanion, a much stronger reductant, to suppress the galvanic replacement between Mn+ and Ag. In this case, the M atoms derived from the reduction by ascorbate monoanion are sequentially deposited on the edges, corners, and side faces to generate Ag@M core-frame and then core-shell nanocubes. The other approach involves the use of ascorbic acid as a relatively weak reductant while Mn+ is cotitrated with Ag+ ions in the absence of sodium hydroxide. At pH = 3.2, when the molar ratio of Ag+ to Mn+ is sufficiently high, the added Ag+ ions can effectively push the galvanic reaction backward and thus inhibit it. As a result, coreduction of the two precursors by ascorbic acid produces Ag and M atoms for the generation of Ag@Ag-M core-frame nanocubes with increasingly thicker ridges. The Ag@Ag-Pd core-frame nanocubes can serve as a dual catalyst to promote the stepwise reduction of nitroaromatics to aminoaromatics and then oxidation to azo compounds. The consecutive reactions can be monitored using surface-enhanced Raman scattering (SERS). The Ag@Au core-shell nanocubes with Au shells of three or six atomic layers exhibit plasmonic peaks almost identical to those of the Ag nanocubes while the chemical stability and SERS activity are substantially augmented. For both types of bimetallic nanocubes, the Ag cores can be selectively removed to generate nanoframes and nanoboxes.

Effects of Pt content on the crystallinity and optical properties of Ag/Pt nanoboxes: from solid to single and polycrystalline mesoporous nanostructures

Synthetic methods to obtain hollow-like systems based on galvanic reaction are of wide importance due to the simplicity and efficiency of the method, yet due to fast and spontaneous reduction, the reaction for platinum-based structures is usually not well controlled. Systematic studies over the evolution of the galvanization process are still important in order to determine key feature on the structural transformation. On this paper, electron microscopy techniques were used to correlate the crystallinity of Ag/Pt nanoboxes of 78 nm in size with low and high-index facets surfaces from Ag nanocubes template. It was observed that 1.6 μmol addition of platinum precursor is enough to produce Ag/Pt nanoboxes. However, the crystallinity of the walls was tune from single crystal to polycrystalline by increasing the amount of platinum. More importantly, electron tomography reconstructions allow us to identify the concavities sides on the polycrystalline double-layer porous sample. Platinum content and nanostructure has been also associated to the optical properties were an extinction of the absorption band is observed after the solid template has been fully transformed to a nanobox. The final Ag/Pt porous/hollow nanoboxes with concave sides were tested on a model reaction for the reduction of 4-ntp to 4-amp.

Simulations of the Surface Plasmon Resonances of Au@Ag Core-Shell Nanocubes: Effect of Core-Shell Size Ratio

This paper reports a systematic investigation on the optical properties of Au@Ag core-shell nanocubes as a function of the core-shell size ratio with the boundary element method (BEM).The absorption cross sections (nm2) for different light wavelengths are calculated using the full Maxwell equations. Also, the related field enhancements for Au@Ag core-shell nanocubes with an incoming plane wave with a polarization along the x-axis are computed. The surface charge at the resonance wavelength of 400 nm and then the electric fields outside and inside the core-shell nanocubes are mapped. A comparison of plasmonic modes between Au@Ag core-shell and single Ag nanocubes shows that there are minor differences between their responses where the surface plasmonic resonance peak position remains constant with varying the core-shell nanocubes ratio up to 50% in most cases under consideration. However, the maximum values of absorption cross sections increase with increasing the core-shell sizes ratio up to core-shell size ratio equal to 50%. The core-shell size ratio dependence of optical properties described here can be utilized in designing plasmonic nanostructures for applications in biosensors, nonlinear spectroscopy, near-field lithography and plasmonic solar cells.

Observing the Overgrowth of a Second Metal on Silver Cubic Seeds in Solution by Surface-Enhanced Raman Scattering.

We report the development of an isocyanide-based molecular probe for in situ characterizing the overgrowth of a second metal on silver nanocrystal seeds in solution by surface-enhanced Raman scattering (SERS). As the first demonstration, we elucidate that the vibrational frequency of 2,6-dimethylphenyl isocyanide (2,6-DMPI) can serve as a distinctive reporter for capturing the nucleation of Pt on the edges of Ag nanocubes in the aqueous solution containing a Pt precursor, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. Our success relies on the difference in stretching frequency for the NC bond when the isocyanide group binds to the Ag and Pt atoms. Specifically, σ donation from the antibonding σ* orbital of the -NC group to the d-band of Ag would strengthen the NC bond, blue shifting the stretching frequency. In contrast, π-back-donation from the d-band of Pt to the π* antibonding orbital of the -NC group would weaken the NC bond, leading to a red shift of stretching frequency. Therefore, it is feasible to in situ characterize the outermost surface that consists of both newly deposited Pt atoms and remaining Ag atoms by following the stretching frequencies and intensities of 2,6-DMPI in real time. Because the SERS hot spots on the edges of Ag nanocubes coincide with the {110} facets preferred for the nucleation of Pt atoms, this technique is capable of resolving 27 Pt atoms being deposited on each edge of a 39 nm Ag nanocube in the original growth solution. Collectively, in situ SERS, with its consummate sensitivity to molecular structure and bonding of isocyanide-based molecular probe, could elucidate the mechanistic details involved in the seeded overgrowth of a catalytically significant metal, such as Pt, Pd, Ir, Rh, and Ru, on the surface of a Ag or Au nanocrystal seed.

Mechanistic Roles of Hydroxide in Controlling the Deposition of Gold on Colloidal Silver Nanocrystals

This article describes a systematic study of the roles played by hydroxide in controlling the deposition of Au on Ag nanocubes for the fabrication of diversified Ag–Au bimetallic nanocrystals. The synthesis simply involves the titration of aqueous HAuCl4 into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (H2Asc), NaOH, and poly(vinylpyrrolidone) at room temperature. The OH– ions from NaOH can affect the reduction kinetics of the Au(III) precursor in a number of ways and thereby the deposition pathways of the Au atoms. First of all, the OH– can accelerate the reduction kinetics by neutralizing H2Asc into ascorbate monoanion (HAsc–), the true player behind the reduction power of ascorbic acid. Second, the OH– can neutralize the added HAuCl4 and progressively transform AuCl4– into AuCl3(OH)−, AuCl2(OH)2–, AuCl(OH)3–, or Au(OH)4– through ligand exchange, generating Au(III) precursors with increasingly lower reduction potentials and thus lower probability for galvanic replacement react...

Wideband zero-index metacrystal with high transmission at visible frequencies

Materials with zero refractive index exhibit unprecedented optical properties including no spatial phase change and infinitely large phase velocity. Several zero-index material designs including artificial layered metallic/dielectric medium were proposed and demonstrated at microwave, terahertz, and IR wavelengths. However, realizing a zero-index material with low-losses, none-dispersion, and relatively wide bandwidth operation at visible frequencies is quite challenging due to optical losses in metals. Here, we propose and numerically demonstrate a three-dimensional zero-index metacrystal (ZIM) with low loss, low dispersion, and wide bandwidth at visible frequencies. The ZIM simply consists of periodic Ag nanocube arrays embedded inside a dielectric medium with same lattice constant in all directions. The calculated effective refractive index using a parameter retrieval method reveals a relatively wide band (∼40  nm) of near-zero index (<0.02) and achromatic behavior for designed metacrystal in the visible frequency. Using full-field electromagnetic (EM) simulations, we have theoretically demonstrated that the EM wave always propagates normal to the ZIM–air interface in spite of oblique incidence cases or any arbitrary wavefront of illumination. Our proposed zero-index metacrystal for visible frequencies could find use in many practical applications of wide-bandwidth and low-loss achromatic photonic devices for steering light propagation, arbitrary wavefront conversion, directional emission, and obstacle-free light guiding.

Shape-selective catalysis and surface enhanced Raman scattering studies using Ag nanocubes, nanospheres and aggregated anisotropic nanostructures.

Three morphologies of silver nanoparticles (Ag NPs) such as nanocubes, aggregated anisotropic Ag NPs, and nanospheres were prepared using polystyrene sulfonate (PSS) and citrate as stabilizing agents utilizing a simple wet-chemical and microwave heating route respectively. Ag nanocubes were prepared within one min through microwave heating whereas anisotropic Ag NPs and spherical Ag NPs via 5 and 30min of normal stirring at room temperature (RT) respectively. The shape effect of three different morphologies of Ag NPs were examined in catalysis reaction and in surface enhanced Raman scattering (SERS) studies. For catalysis experiments, reduction of various nitroaromatics was done taking excess NaBH4 in presence of those morphologically different Ag NPs as catalyst and the corresponding catalytic activity is ordered as: Ag nanospheres>aggregated anisotropic Ag NPs>Ag nanocubes. The highest catalytic rate of ∼1.34×10-1min-1 was observed with citrate capped Ag nanospheres. SERS study was done taking methylene blue (MB) as the Raman probe where a highest enhancement factor (EF) of ∼1.05×107 was observed with Ag nanospheres and the order of EF values is as follows: Ag nanospheres>Ag nanocubes>aggregated anisotropic Ag NPs. The highest catalytic and SERS activity of citrate stabilized spherical Ag NPs are attributed due to the fast electron transfer in catalysis and creation of more number of surface active 'hot spots' in SERS studies. In future, the overall process we highlighted here might found potential application for the preparation of other varieties of nanomaterials applicable to catalysis reaction and in SERS-based trace analysis of various biologically important molecules and fine chemicals.

Plasmonic Control and Stabilization of Asymmetric Light Scattering from Ag Nanocubes on TiO2.

When plasmonic nanoparticles are placed on a highly refractive semiconductor substrate, we can expect three different effects: (i) resonance mode splitting, (ii) asymmetric light scattering based on the split modes, and (iii) site-selective nanoetching due to plasmon-induced charge separation (PICS) at the nanoparticle-semiconductor interface. Here, we develop novel photofunctional materials by taking advantage of those three effects. More specifically, we control the asymmetric scattering of Ag nanocubes on TiO2 by PICS, so as to develop materials for photodrawing of one-way visible translucent images and multicolor scattering images. The one-way visible translucent images, which are translucent scattering images visible only from the back side, are drawn by anaerobic bottom-selective etching of the Ag nanocubes. For drawing the multicolor scattering images, a scattering color of Ag nanocubes is changed from yellow to green by the anaerobic bottom-selective etching and from yellow to red by aerobic nonselective etching. We also theoretically and experimentally examined the contribution of a possible thermal effect to the nanoetching, and revealed that the contribution is negligible; Ag nanocubes on TiO2 are stable even at 473 K for 2 h in the dark, whereas the theoretically expected temperature increase under light is less than 1 K. In addition, we developed methods to stabilize the Ag nanocubes by inactivating PICS. When Ag nanocubes on TiO2 are coated with a thin polymer layer, PICS is decelerated and the stability is improved. Replacing TiO2 with diamond, which does not accept electrons from plasmonic nanoparticles, is also an effective method to stabilize the nanocubes.

Synthesis of ultrathin carbon dots-coated iron oxide nanocubes decorated with silver nanoparticles and their excellent catalytic properties

A facile ultrasonic method has been successfully developed for the fabrication of multifunctional Fe3O4@carbon dot/Ag (Fe3O4@C-dot/Ag) nanocubes (NCs), and the resulting materials are well characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), Vibrating sample magnetometer (VSM) and fluorescence measurements. The Ag nanoparticles (NPs) are uniform and well dispersed on the surface of Fe3O4@C-dot, while maintaining the shape and the size of the core-shell Fe3O4@C-dot NCs. In addition, its catalytic activities are evaluated by measuring the reduction of p-nitroaniline (p-NA) and crystal violet (CV), and the composite materials exhibit excellent catalytic activity towards reduction of p-NA and CV dye, which is superior to most reported catalysts. The good catalytic performance of Fe3O4@C-dot/Ag NCs may be attributed to the specific characteristics of its nanostructure and the synergistic effect on the delivery of electrons between Ag NPs and Fe3O4@C-dot NCs. Furthermore, the as-prepared catalysts also show good activity for the reduction of other nitrobenzene analogs. The effect of solvent and reducing agent was also studied on the catalytic activity of Fe3O4@C-dot/Ag NCs. Most importantly, the Fe3O4@C-dot/Ag catalyst shows excellent recycling stabilities, which can be potentially applied in the fields of catalysis and green chemistry.