Periodic arrays of structurally complex oxide nanoshells and their use as substrate-confined nanoreactors.

Sacrificial templates present an effective pathway for gaining high-level control over nanoscale reaction products. Atomic layer deposition (ALD) is ideally suited for such approaches due to its ability to replicate the surface topography of a template material through the deposition of an ultrathin conformal layer. Herein, metal nanostructures are demonstrated as sacrificial templates for the formation of architecturally complex and deterministically positioned oxide nanoshells, open-topped nanobowls, vertically standing half-shells, and nanorings. The three-step process sees metal nanocrystals formed in periodic arrays, coated with an ALD-deposited oxide, and hollowed out with a selective etch through nanopores formed in the oxide shell. The procedure is further augmented through the use of a directional ion beam that is used to sculpt the oxide shells into bowl- and ring-like configurations. The functionality of the so-formed materials is demonstrated through their use as substrate-confined nanoreactors able to promote the growth and confinement of nanomaterials. Taken together, the work expands the design space for substrate-based nanomaterials, creates a platform for advancing functional surfaces and devices and, from a broader perspective, advances the use of ALD in forming complex nanomaterials.

Insights into the Photoelectrocatalytic Behavior of gCN-Based Anode Materials Supported on Ni Foams.

Graphitic carbon nitride (gCN) is a promising n-type semiconductor widely investigated for photo-assisted water splitting, but less studied for the (photo)electrochemical degradation of aqueous organic pollutants. In these fields, attractive perspectives for advancements are offered by a proper engineering of the material properties, e.g., by depositing gCN onto conductive and porous scaffolds, tailoring its nanoscale morphology, and functionalizing it with suitable cocatalysts. The present study reports on a simple and easily controllable synthesis of gCN flakes on Ni foam substrates by electrophoretic deposition (EPD), and on their eventual decoration with Co-based cocatalysts [CoO, CoFe2O4, cobalt phosphate (CoPi)] via radio frequency (RF)-sputtering or electrodeposition. After examining the influence of processing conditions on the material characteristics, the developed systems are comparatively investigated as (photo)anodes for water splitting and photoelectrocatalysts for the degradation of a recalcitrant water pollutant [potassium hydrogen phthalate (KHP)]. The obtained results highlight that while gCN decoration with Co-based cocatalysts boosts water splitting performances, bare gCN as such is more efficient in KHP abatement, due to the occurrence of a different reaction mechanism. The related insights, provided by a multi-technique characterization, may provide valuable guidelines for the implementation of active nanomaterials in environmental remediation and sustainable solar-to-chemical energy conversion.

Exploring multielement nanogranular coatings to forestall implant-related infections.

Introduction: As we approach the post-antibiotic era, the development of innovative antimicrobial strategies that carry out their activities through non-specific mechanisms could limit the onset and spread of drug resistance. In this context, the use of nanogranular coatings of multielement nanoparticles (NPs) conjugated to the surface of implantable biomaterials might represent a strategy to reduce the systemic drawbacks by locally confining the NPs effects against either prokaryotic or eukaryotic cells. Methods: In the present study, two new multielement nanogranular coatings combining Ag and Cu with either Ti or Mg were synthesized by a gas phase physical method and tested against pathogens isolated from periprosthetic joint infections to address their potential antimicrobial value and toxicity in an in vitro experimental setting. Results: Overall, Staphylococcus aureus, Staphylococcus epidermidis and Escherichia coli displayed a significantly decreased adhesion when cultured on Ti-Ag-Cu and Mg-Ag-Cu coatings compared to uncoated controls, regardless of their antibiotic resistance traits. A dissimilar behavior was observed when Pseudomonas aeruginosa was cultured for 30 and 120 minutes upon the surface of Ti-Ag-Cu and Mg-Ag-Cu-coated discs. Biofilm formation was mainly reduced by the active effect of Mg-Ag-Cu compared to Ti-Ag-Cu and, again, coatings had a milder effect on P. aeruginosa, probably due to its exceptional capability of attachment and matrix production. These data were further confirmed by the evaluation of bacterial colonization on nanoparticle-coated discs through confocal microscopy. Finally, to exclude any cytotoxic effects on eukaryotic cells, the biocompatibility of NPs-coated discs was studied. Results demonstrated a viability of 95.8% and 89.4% of cells cultured in the presence of Ti-Ag-Cu and Mg-Ag-Cu discs, respectively, when compared to negative controls. Conclusion: In conclusion, the present study demonstrated the promising anti-adhesive features of both Ti-Ag-Cu and Mg-Ag-Cu coatings, as well as their action in hampering the biofilm formation, highlighting the safe use of the tested multi-element families of nanoparticles as new strategies against bacterial attachment to the surface of biomedical implants.

Synergistic Effect of Sn and Fe in Fe-Nx Site Formation and Activity in Fe-N-C Catalyst for ORR.

Iron-nitrogen-carbon (Fe-N-C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O2 in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe-N-C materials were co-doped with Sn as a secondary metal center. Sn-N-C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H2SO4 electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn-N-C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe-N4 site density, whereas the possible synergistic interaction of Fe-N4 and Sn-Nx found no confirmation. The presence of Fe-N4 and Sn-Nx was thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe-N-C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe-N-C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (jk = 2.11 vs 1.83 A g-1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests.

A Bipolar CdS/Pd Photocatalytic Membrane for Selective Segregation of Reduction and Oxidation Processes.

A photocatalytically active bipolar membrane consisting of a CdS photocatalyst and Pd electrocatalyst has been constructed to carry out environmentally relevant oxidation and reduction processes. The ion exchange property of a bipolar membrane (BPM) has allowed us to load the CdS photocatalyst on one side and Pd electrocatalyst on the other side. By inserting the photocatalytic BPM-CdS/Pd membrane between the two compartments of an H-cell, we can separate the reduction and oxidation processes. Following visible light excitation of CdS in the BPM-CdS/Pd membrane, we can induce vectorial electron transfer from CdS to Pd and to an electron acceptor (4-nitrophenol). The holes generated at CdS are scavenged by ethanol or 4-chlorophenol. The photocatalytic reduction rate dependence on the Pd loading in the membrane as well as its effect on modulating the rates of electron and hole transfer processes are discussed. The design of a semiconductor and metal loaded membrane paves the way for improving selectivity and efficiency of photocatalytic processes.

Mechanical Properties of Nanoporous Metallic Ultrathin Films: A Paradigmatic Case.

Nanoporous ultrathin films, constituted by a slab less than 100 nm thick and a certain void volume fraction provided by nanopores, are emerging as a new class of systems with a wide range of possible applications, including electrochemistry, energy storage, gas sensing and supercapacitors. The film porosity and morphology strongly affect nanoporous films mechanical properties, the knowledge of which is fundamental for designing films for specific applications. To unveil the relationships among the morphology, structure and mechanical response, a comprehensive and non-destructive investigation of a model system was sought. In this review, we examined the paradigmatic case of a nanoporous, granular, metallic ultrathin film with comprehensive bottom-up and top-down approaches, both experimentals and theoreticals. The granular film was made of Ag nanoparticles deposited by gas-phase synthesis, thus providing a solvent-free and ultrapure nanoporous system at room temperature. The results, bearing generality beyond the specific model system, are discussed for several applications specific to the morphological and mechanical properties of the investigated films, including bendable electronics, membrane separation and nanofluidic sensing.

The Role of Substrate on Thermal Evolution of Ag/TiO2 Nanogranular Thin Films.

In multicomponent thin films, properties and functionalities related to post-deposition annealing treatments, such as thermal stability, optical absorption and surface morphology are typically rationalized, neglecting the role of the substrate. Here, we show the role of the substrate in determining the temperature dependent behaviour of a paradigmatic two-component nanogranular thin film (Ag/TiO2) deposited by gas phase supersonic cluster beam deposition (SCBD) on silica and sapphire. Up to 600 °C, no TiO2 grain growth nor crystallization is observed, likely inhibited by the Zener pinning pressure exerted by the Ag nanoparticles on the TiO2 grain boundaries. Above 600 °C, grain coalescence, formation of However, the two substrates steer the evolution of the film morphology and optical properties in two different directions. anatase and rutile phases and drastic modification of the optical absorption are observed. On silica, Ag is still present as NPs distributed into the TiO2 matrix, while on sapphire, hundreds of nm wide Ag aggregates appear on the film surface. Moreover, the silica-deposited film shows a broad absorption band in the visible range while the sapphire-deposited film becomes almost transparent for wavelengths above 380 nm. We discuss this result in terms of substrate differences in thermal conductivity, thermal expansion coefficient and Ag diffusivity. The study of the substrate role during annealing is possible since SCBD allows the synthesis of the same film independently of the substrate, and suggests new perspectives on the thermodynamics and physical exchanges between thin films and their substrates during heat treatments.

Photocatalytic Activity of Cellulose Acetate Nanoceria/Pt Hybrid Mats Driven by Visible Light Irradiation.

A photocatalytic system for the degradation of aqueous organic pollutants under visible light irradiation is obtained by an innovative approach based on ceria/platinum (Pt) hybrid nanoclusters on cellulose acetate fibrous membranes. The catalytic materials are fabricated by supersonic beam deposition of Pt nanoclusters directly on the surface of electrospun cellulose acetate fibrous mats, pre-loaded with a cerium salt precursor that is transformed into ceria nanoparticles directly in the solid mats by a simple thermal treatment. The presence of Pt enhances the oxygen vacancies on the surface of the formed ceria nanoparticles and reduces their band gap, resulting in a significant improvement of the photocatalytic performance of the composite mats under visible light irradiation. Upon the appropriate pretreatment and visible light irradiation, we prove that the most efficient mats, with both ceria nanoparticles and Pt nanoclusters, present a degradation efficiency of methylene blue of 70% and a photodegradation rate improved by about five times compared to the ceria loaded samples, without Pt. The present results bring a significant improvement of the photocatalytic performance of polymeric nanocomposite fibrous systems under visible light irradiation, for efficient wastewater treatment applications.

Antimicrobial Nanostructured Coatings: A Gas Phase Deposition and Magnetron Sputtering Perspective.

Antimicrobial coatings are a promising strategy to counteract the spreading of multi-drug-resistant pathogens through cross-contamination of surfaces. Coatings with nanostructured characteristics can exploit the different antimicrobial mechanisms of nanomaterials provided the composition, the morphology and the mechanical properties of the film can be tuned by the specific synthesis methods. This review addresses the synthesis of antibacterial nanostructured coatings with a focus on physical synthesis methods. After a short description of the bacteria-NP interaction mechanism, leading to the killing of cells, paradigmatic examples of coatings, obtained by magnetron sputtering and supersonic cluster beam deposition, are discussed, with an emphasis on the possibility of combining different elements into the coating to widen the bactericidal spectrum.

Ag@TiO2 nanogranular films by gas phase synthesis as hybrid SERS platforms.

The synthesis of hybrid metallic-dielectric substrates as reliable SERS platforms relies on core-shell nanoparticles, obtained by wet chemistry, with an outer dielectric shell composed of SiO2 or TiO2. Apart from the shell composition, the nanoparticle density and aggregation type strongly affect the surface-enhanced SERS. Going beyond a single layer by building random aggregates of hybrid NPs would result in a step forward in the production of reliable hybrid SERS platforms. Here we achieve the fabrication of a 3D nanogranular film of Ag metallic cores not fully enclosed in a TiO2 capping layer, defined as a Ag@TiO2 quasi-shell-isolated Raman substrate (Ag@TiO2 QuaSIRS) by an environmentally friendly gas phase synthesis technique (SCBD). The Ag core drives the electromagnetic enhancement with plasmonic hotspots while the TiO2 shell passivates it and leads to different possible surface functionalization. The SERS capabilities of the Ag@TiO2 QuaSIRS peak at a film thickness of 60 nm providing a detection limit of 10-9 M concentration for Methylene Blue at 632.81 nm. The importance of the nanogranular 3D morphology is evidenced by the very good detection of analytes dispersed in aqueous solutions, since the liquid can penetrate the pores hence exploiting most of the plasmonic hotspots present in the film. The versatility of SCBD to deposit such reliable hybrid SERS platforms by a single step at room temperature over different substrates provides an opportunity to design a new generation of hybrid SERS-active substrates based on hybrid nanoparticles.

Tailored Ag-Cu-Mg multielemental nanoparticles for wide-spectrum antibacterial coating.

Bactericidal nanoparticle coatings are very promising for hindering the indirect transmission of pathogens through cross-contaminated surfaces. The challenge, limiting their employment in nosocomial environments, is the ability of tailoring the coating's physicochemical properties, namely, composition, cytotoxicity, bactericidal spectrum, adhesion to the substrate, and consequent nanoparticles release into the environment. We have engineered a new family of nanoparticle-based bactericidal coatings comprising Ag, Cu, and Mg and synthesized by a green gas-phase technique. These coatings present wide-spectrum bactericidal activity on both Gram-positive and Gram-negative reference strains and tunable physicochemical properties of relevance in view of their "on-field" deployment. The link between material and functional properties is rationalized based on a multidisciplinary and multitechnique approach. Our results pave the way for engineering biofunctional, fully tunable nanoparticle coatings, exploiting an arbitrarily wide number of elements in a straightforward, eco-friendly, high-throughput, one-step process.

Photoacoustic Sensing of Trapped Fluids in Nanoporous Thin Films: Device Engineering and Sensing Scheme.

Accessing fluid infiltration in nanogranular coatings is an outstanding challenge, of relevance for applications ranging from nanomedicine to catalysis. A sensing platform, allowing quantifying the amount of fluid infiltrated in a nanogranular ultrathin coating, with thickness in the 10-40 nm range, is here proposed and theoretically investigated by multiscale modeling. The scheme relies on impulsive photoacoustic excitation of hypersonic mechanical breathing modes in engineered gas-phase-synthesized nanogranular metallic ultrathin films and time-resolved acousto-optical read-out of the breathing modes frequency shift upon liquid infiltration. A superior sensitivity, exceeding 26 × 103 cm2/g, is predicted upon equivalent areal mass loading of a few ng/mm2. The capability of the present scheme to discriminate among different infiltration patterns is discussed. The platform is an ideal tool to investigate nanofluidics in granular materials and naturally serves as a distributed nanogetter coating, integrating fluid sensing capabilities. The proposed scheme is readily extendable to other nanoscale and mesoscale porous materials.

Exploring the Optical and Morphological Properties of Ag and Ag/TiO2 Nanocomposites Grown by Supersonic Cluster Beam Deposition.

Nanocomposite systems and nanoparticle (NP) films are crucial for many applications and research fields. The structure-properties correlation raises complex questions due to the collective structure of these systems, often granular and porous, a crucial factor impacting their effectiveness and performance. In this framework, we investigate the optical and morphological properties of Ag nanoparticles (NPs) films and of Ag NPs/TiO2 porous matrix films, one-step grown by supersonic cluster beam deposition. Morphology and structure of the Ag NPs film and of the Ag/TiO2 (Ag/Ti 50-50) nanocomposite are related to the optical properties of the film employing spectroscopic ellipsometry (SE). We employ a simple Bruggeman effective medium approximation model, corrected by finite size effects of the nano-objects in the film structure to gather information on the structure and morphology of the nanocomposites, in particular porosity and average NPs size for the Ag/TiO2 NP film. Our results suggest that SE is a simple, quick and effective method to measure porosity of nanoscale films and systems, where standard methods for measuring pore sizes might not be applicable.

Interaction of iron with a wagon wheel-like ultrathin TiOx film grown on Pt(111).

The structure and thermal evolution of Fe nanoparticles deposited on a wetting TiOx ultrathin film epitaxially grown on Pt(111) has been characterized by various surface science techniques. Combining the results obtained it is shown that, at room temperature, metallic Fe nucleates randomly and oxidizes at the interface. A thermal treatment causes Fe migration through the TiOx layer, forming a mixed oxide and a new hexagonal ultrathin film phase. Finally, the pristine TiOx phase motif is restored, due to the complete diffusion of Fe into the Pt substrate.

Highly bactericidal Ag nanoparticle films obtained by cluster beam deposition.

The recent emergence of bacterial pathogens resistant to most or all available antibiotics is among the major global public health problems. As indirect transmission through contaminated surfaces is a main route of dissemination for most of such pathogens, the implementation of effective antimicrobial surfaces has been advocated as a promising approach for their containment, especially in the hospital settings. However, traditional wet synthesis methods of nanoparticle-based antimicrobial materials leave a number of key points open for metal surfaces: such as adhesion to the surface and nanoparticle coalescence. Here we demonstrate an alternative route, i.e. supersonic cluster beam deposition, to obtain antimicrobial Ag nanoparticle films deposited directly on surfaces. The synthesized films are simple to produce with controlled density and thickness, are stable over time, and are shown to be highly bactericidal against major Gram positive and Gram negative bacterial pathogens, including extensively drug-resistant strains. FROM THE CLINICAL EDITOR: The use of silver nanoparticle in health care is getting more widespread. The authors here describe the technique of cluster beam deposition for spraying silver on surfaces used in health care sectors. This may open a new avenue for future anti-bacterial coatings.

Polyvinyl alcohol electrospun nanofibers containing Ag nanoparticles used as sensors for the detection of biogenic amines.

Polyvinyl alcohol (PVA) electrospun nanofibers containing Ag nanoparticles (NPs) have been deposited on glass substrates. The aim of the work was to test the feasibility of this approach for the detection of biogenic amines by using either the Ag localized surface plasmon resonance quenching caused by the adsorption of amines on Ag NPs or by detecting the amines by surface enhanced Raman spectroscopy (SERS) after adsorption, from the gas phase, on the metal NPs. Two different approaches have been adopted. In the first one an ethanol/water solution containing AgNO3 was used directly in the electrospinning apparatus. In this way, a simple heat treatment of the nanofibers mat was sufficient to obtain the formation of Ag NPs inside the nanofibers and a partial cross-link of PVA. In the second procedure, the Ag NPs were deposited on PVA nanofibers by using the supersonic cluster beam deposition method, so that a beam of pure Ag NPs of controlled size was obtained. Exposure of the PVA mat to the beam produced a uniform distribution of the NPs on the nanofibers surface. Ethylendiamine vapors and volatile amines released from fresh shrimp meat were chemisorbed on the nanofibers mats. A SERS spectrum characterized by a diagnostic Ag-N stretching vibration at 230 cm(-1) was obtained. The results allow to compare the two different approaches in the detection of ammines.

Tracking thermally-activated transformations in a nanostructured metal/oxide/metal system.

We present experimental and theoretical evidence of sequential redox processes and structural transformations occurring by increasing temperature in a metal/oxide/metal system obtained via deposition of Fe atoms onto a z'-TiO(1.25)/Pt(111) ultrathin film in UHV. The initial reduction of the z'-TiO(x) phase by Fe at room temperature is followed by Fe diffusion and partial penetration into the substrate at intermediate temperatures. This triggers the formation of a bi-component material in which mixed FeO/TiO(2) nanoislands coexist on a h-TiO(1.14) ultrathin film, notably restructured (from rectangular to hexagonal) and reduced (from Ti : O = 1 : 1.25 to 1 : 1.14) with respect to the original TiO(1.25) phase. Further heating recovers the pristine z'-TiO(x) phase while Fe completely dissolves into the substrate.

Stability and chemisorption properties of ultrathin TiO(x)/Pt(111) films and Au/TiO(x)/Pt(111) model catalysts in reactive atmospheres.

The stability of three ultrathin TiO(x)/Pt(111) films with different stoichiometry and defectivity and the corresponding Au/TiO(x)/Pt(111) model catalysts in CO or a CO-O(2) (1 : 1) gas mixture up to a pressure of 100 mbar has been investigated. According to previous studies, the ultrathin films proved to be effective substrates to deposit in UHV Au nanoparticles with specific morphologies and lateral sizes ranging between 1 and 6 nm. The films have been characterized before and after the exposure using X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). Additional in situ measurements of the CO chemisorption behavior were performed using polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS). A fully oxidized film is stable in CO and CO-O(2) (1 : 1) ambient, while the reduced films undergo an oxidative dewetting process at RT in the latter atmosphere. This process ultimately produces a nano-composite surface, where very tiny (from 0.5 to 3 nm lateral sizes) titania nanograins are mixed with open, uncovered areas of the Pt substrate. IRAS measurements on the corresponding Au/TiO(x)/Pt(111) model catalysts demonstrated that the CO chemisorption strongly depends on the Au nanoparticle size and morphology, while the actual Ti oxidation state of the oxide support does not seem to play a significant role.

Directed assembly of Au and Fe nanoparticles on a TiOx/Pt(111) ultrathin template: the role of oxygen affinity.

The essential role of O affinity in the directed assembly of size-selected Au and Fe nanoparticles (NPs) on a TiO(x)/Pt(111) ultrathin oxide phase, an effective template for size selected metal NP growth, is revealed through scanning tunneling microscopy and density-functional calculations. A weakly interacting element (Au) diffuses rapidly and gets trapped in the vacancy defects (picoholes) located inside parallel rows (troughs, spaced 1.44 nm apart) peculiar to the film structure, producing size-selected NPs arranged in regular linear arrays aligned along the troughs. In contrast, an element with greater O affinity (Fe) experiences higher diffusion barriers, and the growth is dominated by kinetic effects, with a less effective preferential nucleation and the appearance of irregular NP morphologies.