The influence of crystal structures on the performance of CoMoO4 battery-type supercapacitor electrodes.

CoMoO4 is a promising battery-type supercapacitor electrode material that can offer relatively high storage capacity and cycle stability. In this work, we investigate the role of the crystalline phase of CoMoO4 in determining these performance parameters. The hydrate phase of CoMoO4 was synthesized on a nickel foam substrate via hydrothermal reaction with subsequent annealing under an inert atmosphere leading to the formation of the ß-phase CoMoO4. Similar nanoplate morphologies were observed in all of the samples. The hydrate-phase CoMoO4 demonstrates larger specific capacity than the annealed ß-phase CoMoO4. Besides, the samples synthesized at lower temperatures have better rate capability than the sample annealed at higher temperatures. However, the hydrate phase had worse long-term stability compared to the ß-phase samples.

Electrochemical Polarization of Disparate Catalytic Sites Drives Thermochemical Rate Enhancement.

Supported bimetallic catalysts commonly exhibit higher rates of reaction compared to their monometallic counterparts, but the origin of these enhancements is often poorly defined. The recent discovery that cooperative redox enhancement effects in Au-Pd systems promote bimetallic catalysis in thermochemical oxidation is an important development in this field. This effect aligns two important research fields, thermo- and electrocatalysis, but questions relating to the generality and origin of the effect remain. Here, we demonstrate that these effects can be observed in reactions over a range of bimetal combinations and reveal the origin using a combination of electrochemical and material characterization. We disclose that the observed activity enhancement in thermochemical systems is a result of the electrochemical polarization of two disparate catalytic sites. This forms an alternative operating potential for a given bimetallic system that increases the driving force of each of the composite half reactions in oxidative dehydrogenation. We therefore uncover the physicochemical descriptors that dictate whether these enhancement effects will be exhibited by a particular combination of supported metal catalysts and determine the magnitude of the effect.

Bioinspired, Non-Enzymatic, Aqueous Synthesis of Size-Tunable CdS Quantum Dots for Sustainable Optoelectronic Applications.

The enzymatic production of hydrogen sulfide (H2S) from cysteine in various metabolic processes has been exploited as an intrinsically "green" and sustainable mode for the aqueous biomineralization of functional metal sulfide quantum dots (QDs). Yet, the reliance on proteinaceous enzymes tends to limit the efficacy of the synthesis to physiological temperature and pH, with implications for QD functionality, stability, and tunability (i.e., particle size and composition). Inspired by a secondary non-enzymatic biochemical cycle that is responsible for basal H2S production in mammalian systems, we establish how iron(III)- and vitamin B6 (pyridoxal phosphate, PLP)-catalyzed decomposition of cysteine can be harnessed for the aqueous synthesis of size-tunable QDs, demonstrated here for CdS, within an expanded temperature, pH, and compositional space. Rates of H2S production by this non-enzymatic biochemical process are sufficient for the nucleation and growth of CdS QDs within buffered solutions of cadmium acetate. Ultimately, the simplicity, demonstrated robustness, and tunability of the previously unexploited H2S-producing biochemical cycle help establish its promise as a versatile platform for the benign, sustainable synthesis of an even wider range of functional metal sulfide nanomaterials for optoelectronic applications.

Insights into the Effect of Metal Ratio on Cooperative Redox Enhancement Effects over Au- and Pd-Mediated Alcohol Oxidation.

The aerobic oxidation of alcohols and aldehydes over supported heterogeneous catalysts can be considered as comprising two complementary and linked processes: dehydrogenation and oxygen reduction. Significant rate enhancements can be observed when these processes are catalyzed by independent active sites, coupled by electron transport between the two catalysts. This effect, termed cooperative redox enhancement (CORE), could significantly influence how researchers approach catalyst design, but a greater understanding of the factors which influence it is required. Herein, we demonstrate that the Au/Pd ratio used in physical mixtures of monometallic catalysts and phase-separated Au and Pd bimetallic catalysts dramatically influences the degree to which CORE effects can promote alcohol oxidation. Perhaps more interestingly, the roles of Au and Pd in this coupled system are determined to be interchangeable. Preliminarily, we hypothesize that this is attributed to the relative rates of the coupled reactions and demonstrate how physical properties can influence this. This deeper understanding of the factors which influence CORE is an important development in bimetallic catalysis.

Sequential, low-temperature aqueous synthesis of Ag-In-S/Zn quantum dots via staged cation exchange under biomineralization conditions.

The development of high quality, non-toxic (i.e., heavy-metal-free), and functional quantum dots (QDs) via 'green' and scalable synthesis routes is critical for realizing truly sustainable QD-based solutions to diverse technological challenges. Herein, we demonstrate the low-temperature all-aqueous-phase synthesis of silver indium sulfide/zinc (AIS/Zn) QDs with a process initiated by the biomineralization of highly crystalline indium sulfide nanocrystals, and followed by the sequential staging of Ag+ cation exchange and Zn2+ addition directly within the biomineralization media without any intermediate product purification. Therein, we exploit solution phase cation concentration, the duration of incubation in the presence of In2S3 precursor nanocrystals, and the subsequent addition of Zn2+ as facile handles under biomineralization conditions for controlling QD composition, tuning optical properties, and improving the photoluminescence quantum yield of the AIS/Zn product. We demonstrate how engineering biomineralization for the synthesis of intrinsically hydrophilic and thus readily functionalizable AIS/Zn QDs with a quantum yield of 18% offers a 'green' and non-toxic materials platform for targeted bioimaging in sensitive cellular systems. Ultimately, the decoupling of synthetic steps helps unravel the complexities of ion exchange-based synthesis within the biomineralization platform, enabling its adaptation for the sustainable synthesis of 'green', compositionally diverse QDs.

Au-Pd separation enhances bimetallic catalysis of alcohol oxidation.

In oxidation reactions catalysed by supported metal nanoparticles with oxygen as the terminal oxidant, the rate of the oxygen reduction can be a limiting factor. This is exemplified by the oxidative dehydrogenation of alcohols, an important class of reactions with modern commercial applications1-3. Supported gold nanoparticles are highly active for the dehydrogenation of the alcohol to an aldehyde4 but are less effective for oxygen reduction5,6. By contrast, supported palladium nanoparticles offer high efficacy for oxygen reduction5,6. This imbalance can be overcome by alloying gold with palladium, which gives enhanced activity to both reactions7,8,9; however, the electrochemical potential of the alloy is a compromise between that of the two metals, meaning that although the oxygen reduction can be improved in the alloy, the dehydrogenation activity is often limited. Here we show that by separating the gold and palladium components in bimetallic carbon-supported catalysts, we can almost double the reaction rate compared with that achieved with the corresponding alloy catalyst. We demonstrate this using physical mixtures of carbon-supported monometallic gold and palladium catalysts and a bimetallic catalyst comprising separated gold and palladium regions. Furthermore, we demonstrate electrochemically that this enhancement is attributable to the coupling of separate redox processes occurring at isolated gold and palladium sites. The discovery of this catalytic effect-a cooperative redox enhancement-offers an approach to the design of multicomponent heterogeneous catalysts.

Tailored Coupling of Biomineralized CdS Quantum Dots to rGO to Realize Ambient Aqueous Synthesis of a High-Performance Hydrogen Evolution Photocatalyst.

Nanocomposite photocatalysts offer a promising route to efficient and clean hydrogen production. However, the multistep, high-temperature, solvent-based syntheses typically utilized to prepare these photocatalysts can limit their scalability and sustainability. Biosynthetic routes to produce functional nanomaterials occur at room temperature and in aqueous conditions, but typically do not produce high-performance materials. We have developed a method to produce a highly efficient hydrogen evolution photocatalyst consisting of CdS quantum dots (QDs) supported on reduced graphene oxide (rGO) via enzyme-based syntheses combined with tuned ligand exchange-mediated self-assembly. All preparation steps are carried out in an aqueous environment at ambient temperature. Size-controlled CdS QDs and rGO are prepared through enzyme-mediated turnover of l-cysteine to HS- in aqueous solutions of Cd-acetate and graphene oxide, respectively. Exchange of cysteamine for the native l-cysteine ligand capping the CdS QDs drives self-assembly of the now positively charged cysteamine-capped CdS (CdS/CA) onto negatively charged rGO. The use of this short linker molecule additionally enables efficient charge transfer from CdS to rGO, increasing exciton lifetime and, subsequently, photocatalytic activity. The visible-light hydrogen evolution rate of the resulting CdS/CA/rGO photocatalyst is 3300 µmol h-1 g-1. This represents, to our knowledge, one of the highest reported rates for a CdS/rGO nanocomposite photocatalyst, irrespective of the synthesis method.

In Situ Biomineralization of CuxZnySnzS4 Nanocrystals within TiO2-Based Quantum Dot Sensitized Solar Cell Anodes.

CuZnSnS (CZTS) quantum dots (QDs) have potential application in quantum dot sensitized solar cells (QDSSCs); however, traditional synthesis approaches typically require elevated temperatures, expensive precursors, and organic solvents that can hinder large-scale application. Herein we develop and utilize an enzymatic, aqueous-phase, ambient temperature route to prepare CZTS nanocrystals with good compositional control. Nanoparticle synthesis occurs in a minimal buffered solution containing only the enzyme, metal chloride and acetate salts, and l-cysteine as a capping agent and sulfur source. Beyond isolated nanocrystal synthesis, we further demonstrate biomineralization of these particles within a preformed mesoporous TiO2 anode template where the formed nanocrystals bind to the TiO2 surface. This in situ biomineralization approach facilitates enhanced distribution of the nanocrystals in the anode and, through this, enhanced QDSSC performance.

Low temperature aqueous synthesis of size-controlled nanocrystals through size focusing: a quantum dot biomineralization case study.

Traditional quantum dot synthesis techniques rely on the separation of nucleation and growth to control nanocrystal size. However, the same goal can be achieved through slow and continuous introduction of reactive precursors to keep the growth mechanism in the size focusing regime throughout synthesis. In this work, we demonstrate the efficacy of this approach within the framework of functional material biomineralization where, despite simultaneous nucleation and growth of particles, this growth mechanism enables size-controlled nanocrystal synthesis. Herein, the single enzyme cystathionine γ-lyase (CSE) is utilized to biomineralize CdS nanocrystals via the slow, but continuous turnover of the amino acid l-cysteine to produce H2S. Nanocrystal nucleation and growth theories confirm that consistent addition of monomers will result in a high supersaturation term, driving the nanocrystal growth mechanism into the size focusing regime. We further confirm this theory by mimicking biomineralization via chemical routes and demonstrate the influence of varying supersaturation, to further control the average nanocrystal size. Finally, altering the chelation strength of the capping agent l-cysteine is found to play a key role in balancing nanocrystal growth in solution and long-term stability.

Enzymatic biomineralization of biocompatible CuInS2, (CuInZn)S2 and CuInS2/ZnS core/shell nanocrystals for bioimaging.

This work demonstrates a bioenabled fully aqueous phase and room temperature route to the synthesis of CuInS2/ZnS core/shell quantum confined nanocrystals conjugated to IgG antibodies and used for fluorescent tagging of THP-1 leukemia cells. This elegant, straightforward and green approach avoids the use of solvents, high temperatures and the necessity to phase transfer the nanocrystals prior to application. Non-toxic CuInS2, (CuInZn)S2, and CuInS2/ZnS core/shell quantum confined nanocrystals are synthesized via a biomineralization process based on a single recombinant cystathionine γ-lyase (CSE) enzyme. First, soluble In-S complexes are formed from indium acetate and H2S generated by CSE, which are then stabilized by l-cysteine in solution. The subsequent addition of copper, or both copper and zinc, precursors then results in the immediate formation of CuInS2 or (CuInZn)S2 quantum dots. Shell growth is realized through subsequent introduction of Zn acetate to the preformed core nanocrystals. The size and optical properties of the nanocrystals are tuned by adjusting the indium precursor concentration and initial incubation period. CuInS2/ZnS core/shell particles are conjugated to IgG antibodies using EDC/NHS cross-linkers and then applied in the bioimaging of THP-1 cells. Cytotoxicity tests confirm that CuInS2/ZnS core/shell quantum dots do not cause cell death during bioimaging. Thus, this biomineralization enabled approach provides a facile, low temperature route for the fully aqueous synthesis of non-toxic CuInS2/ZnS quantum dots, which are ideal for use in bioimaging applications.

Single Enzyme Direct Biomineralization of CdSe and CdSe-CdS Core-Shell Quantum Dots.

Biomineralization is the process by which biological systems synthesize inorganic materials. Herein, we demonstrate an engineered cystathionine γ-lyase enzyme, smCSE that is active for the direct aqueous phase biomineralization of CdSe and CdSe-CdS core-shell nanocrystals. The nanocrystals are formed in an otherwise unreactive buffered solution of Cd acetate and selenocystine through enzymatic turnover of the selenocystine to form a reactive precursor, likely H2Se. The particle size of the CdSe core nanocrystals can be tuned by varying the incubation time to generated particle sizes between 2.74 ± 0.63 nm and 4.78 ± 1.16 nm formed after 20 min and 24 h of incubation, respectively. Subsequent purification and introduction of l-cysteine as a sulfur source facilitates the biomineralization of a CdS shell onto the CdSe cores. The quantum yield of the resulting CdSe-CdS core-shell particles is up to 12% in the aqueous phase; comparable to that reported for more traditional chemical synthesis routes for core-shell particles of similar size with similar shell coverage. This single-enzyme route to functional nanocrystals synthesis reveals the powerful potential of biomineralization processes.

Direct Single-Enzyme Biomineralization of Catalytically Active Ceria and Ceria-Zirconia Nanocrystals.

Biomineralization is an intriguing approach to the synthesis of functional inorganic materials for energy applications whereby biological systems are engineered to mineralize inorganic materials and control their structure over multiple length scales under mild reaction conditions. Herein we demonstrate a single-enzyme-mediated biomineralization route to synthesize crystalline, catalytically active, quantum-confined ceria (CeO2-x) and ceria-zirconia (Ce1-yZryO2-x) nanocrystals for application as environmental catalysts. In contrast to typical anthropogenic synthesis routes, the crystalline oxide nanoparticles are formed at room temperature from an otherwise inert aqueous solution without the addition of a precipitant or additional reactant. An engineered form of silicatein, rCeSi, as a single enzyme not only catalyzes the direct biomineralization of the nanocrystalline oxides but also serves as a templating agent to control their morphological structure. The biomineralized nanocrystals of less than 3 nm in diameter are catalytically active toward carbon monoxide oxidation following an oxidative annealing step to remove carbonaceous residue. The introduction of zirconia into the nanocrystals leads to an increase in Ce(III) concentration, associated catalytic activity, and the thermal stability of the nanocrystals.

Single-enzyme biomineralization of cadmium sulfide nanocrystals with controlled optical properties.

Nature has evolved several unique biomineralization strategies to direct the synthesis and growth of inorganic materials. These natural systems are complex, involving the interaction of multiple biomolecules to catalyze biomineralization and template growth. Herein we describe the first report to our knowledge of a single enzyme capable of both catalyzing mineralization in otherwise unreactive solution and of templating nanocrystal growth. A recombinant putative cystathionine γ-lyase (smCSE) mineralizes CdS from an aqueous cadmium acetate solution via reactive H2S generation from l-cysteine and controls nanocrystal growth within the quantum confined size range. The role of enzymatic nanocrystal templating is demonstrated by substituting reactive Na2S as the sulfur source. Whereas bulk CdS is formed in the absence of the enzyme or other capping agents, nanocrystal formation is observed when smCSE is present to control the growth. This dual-function, single-enzyme, aerobic, and aqueous route to functional material synthesis demonstrates the powerful potential of engineered functional material biomineralization.

Is the surface oxygen exchange rate linked to bulk ion diffusivity in mixed conducting Ruddlesden-Popper phases?

The possible link between oxygen surface exchange rate and bulk oxygen anion diffusivity in mixed ionic and electronic conducting oxides is a topic of great interest and debate. While a large body of experimental evidence and theoretical analyses support a link, observed differences between bulk and surface composition of these materials are hard to reconcile with this observation. This is further compounded by potential problems with simultaneous measurement of both parameters. Here we utilize separate techniques, in situ neutron diffraction and pulsed isotopic surface exchange, to examine bulk ion mobility and surface oxygen exchange rates of three Ruddlesden-Popper phases, general form A(n-1)A(2)'B(n)O(3n+1), A(n-1)A(2)'B(n)X(3n+1); LaSrCo(0.5)Fe(0.5)O(4-δ) (n = 1), La(0.3)Sr(2.7)CoFeO(7-δ) (n = 2) and LaSr3Co(1.5)Fe(1.5)O(10-δ) (n = 3). These measurements are complemented by surface composition determination via high sensitivity-low energy ion scattering. We observe a correlation between bulk ion mobility and surface exchange rate between materials. The surface exchange rates vary by more than one order of magnitude with high anion mobility in the bulk of an oxygen vacancy-rich n = 2 Ruddlesden-Popper material correlating with rapid oxygen exchange. This is in contrast with the similar surface exchange rates which we may expect due to similar surface compositions across all three samples. We conclude that experimental limitations lead to inherent convolution of surface and bulk rates, and that surface exchange steps are not likely to be rate limiting in oxygen incorporation.

The scrub practitioner and the first assistant role.

An audit of the activities undertaken by scrub nurses was performed at Ninewells Hospital, Dundee during November 1999. The result showed that a high percentage of scrub practitioners are undertaking first assistant activities. In order to prepare staff for their role within specialised areas of practice an ongoing program of Fitness for Practice modules is being developed collaboratively between Tayside University Hospitals Trust and the University of Dundee. This article describes their experiences.