Nanoporous gold with networks of interconnected ligaments and highly porous structure holds stimulating technological implications in fuel cell catalysis. Current syntheses of nanoporous gold mainly revolve around de-alloying approaches that are generally limited by stringent and harsh multistep protocols. Here we develop a one-step solution phase synthesis of zero-dimensional hollow nanoporous gold nanoparticles with tunable particle size (150-1,000 nm) and ligament thickness (21-54 nm). With faster mass diffusivity, excellent specific electroactive surface area and large density of highly active surface sites, our zero-dimensional nanoporous gold nanoparticles exhibit ~1.4 times enhanced catalytic activity and improved tolerance towards carbonaceous species, demonstrating their superiority over conventional nanoporous gold sheets. Detailed mechanistic study also reveals the crucial heteroepitaxial growth of gold on the surface of silver chloride templates, implying that our synthetic protocol is generic and may be extended to the synthesis of other nanoporous metals via different templates.

Graphene oxide (GO) is an emerging material for surface-enhanced Raman scattering (SERS) due to its strong chemical enhancement. Studying the SERS performance of plasmonic nanoparticle/GO hybrid materials at the single particle level is crucial for direct probing of the chemical effect of GO on plasmonic nanoparticles. In this work, we integrate GO and shape-controlled Ag nanoparticles to create hybrid nanomaterials, and the chemical enhancement arising from GO is investigated using single-particle SERS measurements. Ag nanoparticle@GO hybrid nanostructures are prepared by assembling Ag nanoparticles, including spheres, cubes and octahedra with GO sheets. The SERS behaviors of the hybrid nanostructures are characterized, and 2-3 times enhanced SERS intensities are detected from the Ag nanoparticle@GO hybrid nanostructures as compared to pure Ag nanoparticles. Furthermore, we probe the mechanism of SERS enhancement in the hybrid nanostructures by changing the surface coverage of GO on Ag octahedra, by using reduced GO in place of GO as well as by using probe molecules of different electronegativities. This hybrid system is an excellent candidate for single-particle SERS sensors. Sub-nanomolar levels of aromatic molecules are detected using a single Ag/GO hybrid nanomaterial. This as-prepared GO and shape-controlled Ag nanoparticle hybrid is capable of serving as a high performance SERS platform, providing new opportunities for efficient chemical and biological sensing applications.

Single-phase Cu2ZnSnS4 (CZTS) is an essential prerequisite toward a high-efficiency thin-film solar cell device. Herein, the selective phase formation of single-phase CZTS nanoparticles by ligand control is reported. Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time as a characterization tool for nanoparticles to differentiate the mixed compositional phase (e.g., CZTS, CTS, and ZnS), which cannot be distinguished by X-ray diffraction. Due to the superior selectivity and sensitivity of SERS, the growth mechanism of CZTS nanoparticle formation by hot injection is revealed to involve three growth steps. First, it starts with nucleation of Cu(2-x)S nanoparticles, followed by diffusion of Sn(4+) into Cu(2-x)S nanoparticles to form the Cu3SnS4 (CTS) phase and diffusion of Zn(2+) into CTS nanoparticles to form the CZTS phase. In addition, it is revealed that single-phase CZTS nanoparticles can be obtained via balancing the rate of CTS phase formation and diffusion of Zn(2+) into the CTS phase. We demonstrate that this balance can be achieved by 1 mL of thiol with Cu(OAc)2, Sn(OAc)4, and Zn(acac)2 metal salts to synthesize the CZTS phase without the presence of a detectable binary/ternary phase with SERS.

The effective number of surface-enhanced Raman spectroscopy (SERS) active hot spots on plasmonic nanostructures is the most crucial factor in ensuring high sensitivity in SERS sensing platform. Here we demonstrate a chemical etching method to increase the surface roughness of one-dimensional Ag nanowires, targeted at creating more SERS active hot spots along Ag nanowire's longitudinal axis for increased SERS detection sensitivity. Silver nanowires were first synthesized by the conventional polyol method and then subjected to chemical etching by NH(4)OH and H(2)O(2) mixture. The surfaces of silver nanowires were anisotropically etched off to create miniature "beads on a string" features with increased surface roughness while their crystallinity was preserved. Mapping of single-nanowire SERS measurements showed that the chemical etching method has overcome the limitation of conventional one-dimensional Ag nanowires with limited SERS active area at the tips to produce etched Ag nanowires with an increase in Raman hot spots and polarization-independent SERS signals across tens of micrometers length scale.

The electrochemical nitrogen reduction reaction (NRR) offers a sustainable solution towards ammonia production but suffers poor reaction performance owing to preferential catalyst–H formation and the consequential hydrogen evolution reaction (HER). Now, the Pt/Au electrocatalyst d‐band structure is electronically modified using zeolitic imidazole framework (ZIF) to achieve a Faradaic efficiency (FE) of >44 % with high ammonia yield rate of >161 μg mgcat−1 h−1 under ambient conditions. The strategy lowers electrocatalyst d‐band position to weaken H adsorption and concurrently creates electron‐deficient sites to kinetically drive NRR by promoting catalyst–N2 interaction. The ZIF coating on the electrocatalyst doubles as a hydrophobic layer to suppress HER, further improving FE by >44‐fold compared to without ZIF (ca. 1 %). The Pt/Au‐NZIF interaction is key to enable strong N2 adsorption over H atom.

We demonstrate a molecular-level observation of driving CO molecules into a quasi-condensed phase on the solid surface of metal nanoparticles (NP) under ambient conditions of 1 bar and 298 K. This is achieved via a CO accumulation in the interface between a metal-organic framework (MOF) and a metal NP surface formed by coating NPs with a MOF. Using real-time surface-enhanced Raman scattering spectroscopy, a >18-fold enhancement of surface coverage of CO is observed at the interface. The high surface concentration leads CO molecules to be in close proximity with the probe molecules on the metal surface (4-methylbenzenethiol), and transforms CO molecules into a bent conformation without the formation of chemical bonds. Such linear-to-bent transition of CO is unprecedented at ambient conditions in the absence of chemical bond formation, and is commonly observed only in pressurized systems (>10 bar). The molecular-level observation of a quasi-condensed phase induced by MOF coating could impact the future design of hybrid materials in diverse applications, including catalytic CO conversion and ambient solid-gas operation.

Current synthesis of gold nanoframes has only demonstrated morphological control over wall thickness and wall length. Here, we demonstrate the ability to control the nanoscale porosity of these nanoframes, using a templated seed-mediated approach. The porosity on these nanoporous gold nanoframes (NGNs) is tuned by controlling the crystallite size of Au nanoparticles deposited on the AgCl templates. The yield of the NGNs approaches 100%. Despite its minimalist architectural construction, the NGNs are mechanically robust, retaining its morphology even after multiple centrifugation and sonication rounds. We further highlight that decreasing the porosity on the NGN leads to improved surface-enhanced Raman scattering (SERS) enhancement. Increasing the constituent Au crystallite size decreases the porosity, but increases the surface roughness of NGN, hence leading to greater SERS enhancement. The introduction of porosity in a gold nanoframe structure through our synthesis method is novel and generic, suggesting the extendibility of our method to other types of templates.

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 104 field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM).