Templated 2D Polymer Heterojunctions for Improved Photocatalytic Hydrogen Production.

2D polymers have emerged as one of the most promising classes of organic photocatalysts for solar fuel production due to their tunability, charge-transport properties, and robustness. They are however difficult to process and so there are limited studies into the formation of heterojunction materials incorporating these components. In this work, a novel templating approach is used to combine an imine-based donor polymer and an acceptor polymer formed through Knoevenagel condensation. Heterojunction formation is shown to be highly dependent on the topological match of the donor and acceptor polymers with the most active templated material found to be between three and nine times more active for photocatalysis than its constituent components. Transient absorption spectroscopy reveals that this improvement is due to faster charge separation and more efficient charge extraction in the templated heterojunction. The templated material shows a very high hydrogen evolution rate of >20 mmol h-1 m-2 with an ascorbic acid hole scavenger but also produces hydrogen in the presence of only water and a cobalt-based redox mediator. This suggests the improved charge-separation interface and reduced trapping accessed through this approach could be suitable for Z-scheme formation.

Biocompatibility and Physiological Thiolytic Degradability of Radically Made Thioester-Functional Copolymers: Opportunities for Drug Release.

Being nondegradable, vinyl polymers have limited biomedical applicability. Unfortunately, backbone esters incorporated through conventional radical ring-opening methods do not undergo appreciable abiotic hydrolysis under physiologically relevant conditions. Here, PEG acrylate and di(ethylene glycol) acrylamide-based copolymers containing backbone thioesters were prepared through the radical ring-opening copolymerization of the thionolactone dibenzo[c,e]oxepin-5(7H)-thione. The thioesters degraded fully in the presence of 10 mM cysteine at pH 7.4, with the mechanism presumed to involve an irreversible S-N switch. Degradations with N-acetylcysteine and glutathione were reversible through the thiol-thioester exchange polycondensation of R-SC(═O)-polymer-SH fragments with full degradation relying on an increased thiolate/thioester ratio. Treatment with 10 mM glutathione at pH 7.2 (mimicking intracellular conditions) triggered an insoluble-soluble switch of a temperature-responsive copolymer at 37 °C and the release of encapsulated Nile Red (as a drug model) from core-degradable diblock copolymer micelles. Copolymers and their cysteinolytic degradation products were found to be noncytotoxic, making thioester backbone-functional polymers promising for drug delivery applications.

Oleylamine Aging of PtNi Nanoparticles Giving Enhanced Functionality for the Oxygen Reduction Reaction.

We report a rapid solution-phase strategy to synthesize alloyed PtNi nanoparticles which demonstrate outstanding functionality for the oxygen reduction reaction (ORR). This one-pot coreduction colloidal synthesis results in a monodisperse population of single-crystal nanoparticles of rhombic dodecahedral morphology with Pt-enriched edges and compositions close to Pt1Ni2. We use nanoscale 3D compositional analysis to reveal for the first time that oleylamine (OAm)-aging of the rhombic dodecahedral Pt1Ni2 particles results in Ni leaching from surface facets, producing aged particles with concave faceting, an exceptionally high surface area, and a composition of Pt2Ni1. We show that the modified atomic nanostructures catalytically outperform the original PtNi rhombic dodecahedral particles by more than two-fold and also yield improved cycling durability. Their functionality for the ORR far exceeds commercially available Pt/C nanoparticle electrocatalysts, both in terms of mass-specific activities (up to a 25-fold increase) and intrinsic area-specific activities (up to a 27-fold increase).

Characterising porosity in platinum nanoparticles.

Accurately determining the morphology and hence the true surface areas of catalytic nanoparticles remains challenging. For many chemically synthesised nanoparticle suspensions conventional BET surface area measurements are often not feasible due to the large quantities of material required. For platinum, a paradigmatic catalyst, this issue is further complicated by the propensity of this metal to form porous aggregate structures comprised of smaller (ca. 2-5 nm) crystallites as opposed to continuous solid structures. This dendritic/porous particulate morphology leads to a large but poorly defined 'active' surface which is difficult to measure accurately. Here we compare, single nanoparticle electrochemistry with three dimensional (3D) electron tomography and quantitative 2D high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) analysis to yield insights into the porosity and chemically accessible surface area of a 30 nm diameter commercial Pt nanoparticle catalyst. Good quantitative agreement is found between 2D and 3D STEM-based measurements of the particle morphology, density and size distribution. Both 3D STEM tomography and single nanoparticle electrochemical measurements allow quantification of the surface area but the electrocatalytic surface area is found to be 2.8× larger than is measured in STEM; indicating the importance of the atomic scale roughness and structure (<2 nm) in contributing to the total catalytic surface area of the nanomaterial.

Porosity controls the catalytic activity of platinum nanoparticles.

Dendritic/mesoporous nanoparticle structures arise naturally and result from aggregation based growth mechanisms. The resulting porous particles exhibit high total surface areas (internal and external) but determining the magnitude of the interface remains challenging. Furthermore, assessing the chemical accessibility of the catalytic interface presents an additional difficulty. Taking three structurally related but different sized platinum nanoparticle samples (30-70 nm), we demonstrate how the catalytic rate of two archetypal surface limited reactions scale not with the square of the particle radius but with a power law of 2.6-2.9. This power law directly reflects the mesoporosity of the nanoparticles; the internal surface of the nanoparticles is both chemically accessible and contributes to the catalytic activity. For the 70 nm particles, up to 60% of the catalytic surface is contained in the internal structure of the particle.

Tannic acid capped gold nanoparticles: capping agent chemistry controls the redox activity.

We report the key role of the capping agent in the detection of metal cations using tannic acid (TA) capped gold nanoparticles at both ensembles (using cyclic voltammetry) and with individual particles (using oxidative and reductive nanoimpacts). The results show that the capping agent complexes with Zn2+ and Hg2+ in a reversible and Langmuirian manner in both cases. The sensitivity of detection is determined by the amount of capping agent present on the nanoparticles with similar values seen for both oxidation and reduction reactions. The optimisation of the capping agent loading is therefore key to metal ion detection.

Imaging Three-Dimensional Elemental Inhomogeneity in Pt-Ni Nanoparticles Using Spectroscopic Single Particle Reconstruction.

The properties of nanoparticles are known to critically depend on their local chemistry but characterizing three-dimensional (3D) elemental segregation at the nanometer scale is highly challenging. Scanning transmission electron microscope (STEM) tomographic imaging is one of the few techniques able to measure local chemistry for inorganic nanoparticles but conventional methodologies often fail due to the high electron dose imparted. Here, we demonstrate realization of a new spectroscopic single particle reconstruction approach built on a method developed by structural biologists. We apply this technique to the imaging of PtNi nanocatalysts and find new evidence of a complex inhomogeneous alloying with a Pt-rich core, a Ni-rich hollow octahedral intermediate shell and a Pt-rich rhombic dodecahedral skeleton framework with less Pt at ⟨100⟩ vertices. The ability to gain evidence of local surface enrichment that varies with the crystallographic orientation of facets and vertices is expected to provide significant insight toward the development of nanoparticles for sensing, medical imaging, and catalysis.

Electrochemical impacts complement light scattering techniques for in situ nanoparticle sizing.

We show that the electrochemical particle-impact technique (or 'nano-impacts') complements light scattering techniques for sizing both mono- and poly-disperse nanoparticles. It is found that established techniques - Dynamic Light Scattering (DLS) and Nanoparticle Tracking Analysis (NTA) - can accurately measure the diameters of '30 nm' silver particles assuming spherical shapes, but are unable to accurately size a smaller '20 nm' sample. In contrast, nano-impacts have a high accuracy (<5% error in effective diameters) and are able to size both individual '20 nm' and '30 nm' silver NPs in terms of the number of constituent atoms. Further study of a '20 nm and 30 nm' bimodal sample shows that the electrochemical technique resolves the two very similar sizes well, demonstrating accurate sizing regardless of particle size polydispersity, whereas due to inherent limitations of light scattering measurements this is not possible for DLS and NTA. Electrochemical sizing is concluded to offer significant attractions over light scattering methods.

Understanding gold nanoparticle dissolution in cyanide-containing solution via impact-chemistry.

The electrochemical dissolution of citrate-capped gold nanoparticles (AuNPs) was studied in cyanide (CN-) containing solutions. It was found that the gold nanoparticles exhibited different dissolution behaviours as ensembles compared to the single particles. At the single particle level, a nearly complete oxidation of 60 nm AuNPs was achieved at concentrations greater than or equal to 35.0 mM CN- and at a potential of 1.0 V. Mechanistic insights and rate data are reported.

Simultaneous activity and surface area measurements on single mesoporous nanoparticle aggregates.

The underpotential deposition of hydrogen and the hydrogen evolution reaction is studied at individual mesoporous nanoparticles. This work shows how the electroactive surface area and catalytic activity of these individual particles can be simultaneously measured.

In Situ Atomic-Level Studies of Gd Atom Release and Migration on Graphene from a Metallofullerene Precursor.

We show how gadolinium (Gd)-based metallofullerene (Gd3N@C80) molecules can be used to create single adatoms and nanoclusters on a graphene surface. An in situ heating holder within an aberration-corrected scanning transmission electron microscope is used to track the adhesion of endohedral metallofullerenes (MFs) to the surface of graphene, followed by Gd metal ejection and diffusion across the surface. Heating to 900 °C is used to promote adatom migration and metal nanocluster formation, enabling direct imaging of the assembly of nanoclusters of Gd. We show that hydrogen can be used to reduce the temperature of MF fragmentation and metal ejection, enabling Gd nanocluster formation on graphene surfaces at temperatures as low as 300 °C. The process of MF fragmentation and metal ejection is captured in situ and reveals that after metal release, the C80 cage opens further and fuses with the surface monolayer carbon glass on graphene, creating a highly stable carbon layer for further Gd adatom adhesion. Small voids and defects (∼1 nm) in the surface carbon glass act as trapping sites for Gd atoms, leading to atomic self-assembly of 2D monolayer Gd clusters. These results show that MFs can adhere to graphene surfaces at temperatures well above their bulk sublimation point, indicating that the surface bound MFs have strong adhesion to dangling bonds on graphene surfaces. The ability to create dispersed single Gd adatoms and Gd nanoclusters on graphene may have impact in spintronics and magnetism.

Shape and size of non-spherical silver nanoparticles: implications for calculating nanoparticle number concentrations.

The international drive to measure accurate number concentrations of nanoparticles is impeded by the typically heterogeneous populations of non-spherical nanoparticles. The irregular shape and size of "50 nm" silver nanoparticles is studied using Electron Tomography. It is evidenced that even for highly symmetrical particles the volume can be over 20% less than that of the circumscribed sphere; more irregularly shaped particles can have volumes of over 45% less. On this basis, criteria are provided to determine the particle sphericity from 2D projections obtained from Electron Microscopy, including an empirical method for particle volume estimation. The results allow the visualisation of irregularly shaped particles, revealing the presence of previously unseen voids in the nanoparticle structure. Comparison of tomographic data with other commonly used particle-sizing methods exposes the limitations of these methods in studying nanoparticle populations that exhibit heterogeneity.

Coupled Optical and Electrochemical Probing of Silver Nanoparticle Destruction in a Reaction Layer.

The oxidation of silver nanoparticles is induced to occur near to, but not at, an electrode surface. This reaction at a distance from the electrode is studied through the use of dark-field microscopy, allowing individual nanoparticles and their reaction with the electrode product to be visualized. The oxidation product diffuses away from the electrode and oxidizes the nanoparticles in a reaction layer, resulting in their destruction. The kinetics of the silver nanoparticle solution-phase reaction is shown to control the length scale over which the nanoparticles react. In general, the new methodology offers a route by which nanoparticle reactivity can be studied close to an electrode surface.

A quantitative methodology for the study of particle-electrode impacts.

Herein we provide a generic framework for use in the acquisition and analysis of the electrochemical responses of individual nanoparticles, summarising aspects that must be considered to avoid mis-interpretation of data. Specifically, we threefold highlight the importance of the nanoparticle shape, the effect of the nanoparticle diffusion coefficient on the probability of it being observed and the influence of the used measurement bandwidth. Using the oxidation of silver nanoparticles as a model system, it is evidenced that when all of the above have been accounted for, the experimental data is consistent with being associated with the complete oxidation of the nanoparticles (50 nm diameter). The duration of many single nanoparticle events are found to be ca. milliseconds in duration over a range of experiments. Consequently, the insight that the use of lower frequency filtered data yields a more accurate description of the charge passed during a nano-event is likely widely applicable to this class of experiment; thus we report a generic methodology. Conversely, information regarding the dynamics of the nano redox event is obscured when using such lower frequency measurements; hence, both data sets are complementary and are required to provide full insight into the behaviour of the reactions at the nanoscale.

Electrochemical Hg2+ detection at tannic acid-gold nanoparticle modified electrodes by square wave voltammetry.

We report the electrochemical sensing of Hg2+ based on tannic acid capped gold nanoparticle (AuNP@TA) complexes. At optimal conditions using square wave voltammetry, the presented analytical method exhibits a "measurable lower limit" of 100.0 fM. This limit is considerably below the permissible level of 30.0 nM for inorganic mercury in drinking water, specified by the World Health Organization (WHO). The effect of potentially interfering ions, such as Zn2+ and Al3+, was studied and results indicate an excellent selectivity for Hg2+. The transfer of the proposed strategy onto AuNP@TA modified screen-printed electrodes demonstrates its applicability to routine monitoring of Hg2+ in tap water.

Multiscale Engineered Si/SiO x Nanocomposite Electrodes for Lithium-Ion Batteries Using Layer-by-Layer Spray Deposition.

Si-based high-capacity materials have gained much attention as an alternative to graphite in Li-ion battery anodes. Although Si additions to graphite anodes are now commercialized, the fraction of Si that can be usefully exploited is restricted due to its poor cyclability arising from the large volume changes during charge/discharge. Si/SiO x nanocomposites have also shown promising behavior, such as better capacity retention than Si alone because the amorphous SiO x helps to accommodate the volume changes of the Si. Here, we demonstrate a new electrode architecture for further advancing the performance of Si/SiO x nanocomposite anodes using a scalable layer-by-layer atomization spray deposition technique. We show that particulate C interlayers between the current collector and the Si/SiO x layer and between the separator and the Si/SiO x layer improved electrical contact and reduced irreversible pulverization of the Si/SiO x significantly. Overall, the multiscale approach based on microstructuring at the electrode level combined with nanoengineering at the material level improved the capacity, rate capability, and cycling stability compared to that of an anode comprising a random mixture of the same materials.

Single Oxidative Collision Events of Silver Nanoparticles: Understanding the Rate-Determining Chemistry.

The oxidative dissolution of citrate-capped silver nanoparticles (AgNPs, ∼50 nm diameter) is investigated herein by two electrochemical techniques: nano-impacts and anodic stripping voltammetry. Nano-impacts or single nanoparticle-electrode collisions allow the detection of individual nanoparticles. The technique offers an advantage over surface-immobilized methods such as anodic stripping voltammetry as it eliminates the effects of particle agglomeration/aggregation. The electrochemical studies are performed in different electrolytes (KNO3 , KCl, KBr and KI) at varied concentrations (≤20 mm). In nano-impact measurements, the AgNP undergoes complete oxidation upon impact at a suitably potentiostated electrode. The frequency of the nanoparticle-electrode collisions observed as current-transient spikes depends on the electrolyte identity, its concentration and the potential applied at the working electrode. The frequencies of the spikes are significantly higher in the presence of halide ions and increase with increasing potentials. From the frequency, the rate of AgNP oxidation as compared with the timescale the AgNP is in electrical contact with the electrode can be inferred, and hence is indicative of the relative kinetics of the oxidation process. Primarily based on these results, we propose the initial formation of the silver (I) nucleus (Ag+ , AgCl, AgBr or AgI) as the rate-determining process of silver oxidation on the nanoparticle.

Fluorescence Electrochemical Microscopy: Capping Agent Effects with Ethidium Bromide/DNA Capped Silver Nanoparticles.

Fluorescence microscopy and electrochemistry were employed to examine capping agent dynamics in silver nanoparticles capped with DNA intercalated with ethidium bromide, a fluorescent molecule. The capped NPs were studied first electrochemically, demonstrating that the intercalation of the capping agent promotes oxidation of the silver core, occurring at 0.50 V (vs. Ag, compared with 1.15 V for Ag NPs capped in DNA alone). Second, fluorescence electrochemical microscopy revealed that the electron transfer from the nanoparticles is gated by the capping agent, allowing dynamic insights unobservable using electrochemistry alone.

Electrochemical Detection of Ultratrace (Picomolar) Levels of Hg2+ Using a Silver Nanoparticle-Modified Glassy Carbon Electrode.

Ultratrace levels of Hg2+ have been quantified by undertaking linear sweep voltammetry with a silver nanoparticle-modified glassy carbon electrode (AgNP-GCE) in aqueous solutions containing Hg2+. This is achieved by monitoring the change in the silver stripping peak with Hg2+ concentration resulting from the galvanic displacement of silver by mercury: Ag(np) + 1/2Hg2+(aq) → Ag+(aq) + 1/2Hg(l). This facile and reproducible detection method exhibits an excellent linear dynamic range of 100.0 pM to 10.0 nM Hg2+ concentration with R2 = 0.982. The limit of detection (LoD) based on 3σ is 28 pM Hg2+, while the lowest detectable level for quantification purposes is 100.0 pM. This method is appropriate for routine environmental monitoring and drinking water quality assessment since the guideline value set by the US Environmental Protection Agency (EPA) for inorganic mercury in drinking water is 0.002 mg L-1 (10 nM).

Understanding nanoparticle porosity via nanoimpacts and XPS: electro-oxidation of platinum nanoparticle aggregates.

The porosity of platinum nanoparticle aggregates (PtNPs) is investigated electrochemically via particle-electrode impacts and by XPS. The mean charge per oxidative transient is measured from nanoimpacts; XPS shows the formation of PtO and PtO2 in relative amounts defined by the electrode potential and an average oxidation state is deduced as a function of potential. The number of platinum atoms oxidised per PtNP is calculated and compared with two models: solid and porous spheres, within which there are two cases: full and surface oxidation. This allows insight into extent to which the internal surface of the aggregate is 'seen' by the solution and is electrochemically active.