Wipula P. R. Liyanage

Cobalt Selenide Nanostructures: An Efficient Bifunctional Catalyst with High Current Density at Low Coverage

Electrodeposited Co7Se8 nanostructures exhibiting flake-like morphology show bifunctional catalytic activity for oxygen evolution and hydrogen evolution reaction (OER and HER, respectively) in alkaline medium with long-term durability (>12 h) and high Faradaic efficiency (99.62%). In addition to low Tafel slope (32.6 mV per decade), the Co7Se8 OER electrocatalyst also exhibited very low overpotential to achieve 10 mA cm(-2) (0.26 V) which is lower than other transition metal chalcogenide based OER electrocatalysts reported in the literature and significantly lower than the state-of-the-art precious metal oxides. A low Tafel slope (59.1 mV per decade) was also obtained for the HER catalytic activity in alkaline electrolyte. The OER catalytic activity could be further improved by creating arrays of 3-dimensional rod-like and tubular structures of Co7Se8 through confined electrodeposition on lithographically patterned nanoelectrodes. Such arrays of patterned nanostructures produced exceptionally high mass activity and gravimetric current density (∼68 000 A g(-1)) compared to the planar thin films (∼220 A g(-1)). Such high mass activity of the catalysts underlines reduction in usage of the active material without compromising efficiency and their practical applicability. The catalyst layer could be electrodeposited on different substrates, and an effect of the substrate surface on the catalytic activity was also investigated. The Co7Se8 bifunctional catalyst enabled water electrolysis in alkaline solution at a cell voltage of 1.6 V. The electrodeposition works with exceptional reproducibility on any conducting substrate and shows unprecedented catalytic performance especially with the patterned growth of catalyst rods and tubes.

Textured NiSe 2 Film: Bifunctional Electrocatalyst for Full Water Splitting at Remarkably Low Overpotential with High Energy Efficiency

Herein we have shown that electrodeposited NiSe2 can be used as a bifunctional electrocatalyst under alkaline conditions to split water at very low potential by catalyzing both oxygen evolution and hydrogen evolution reactions at anode and cathode, respectively, achieving a very high electrolysis energy efficiency exceeding 80% at considerably high current densities (100 mA cm−2). The OER catalytic activity as well as electrolysis energy efficiency surpasses any previously reported OER electrocatalyst in alkaline medium and energy efficiency of an electrolyzer using state-of-the-art Pt and RuO2 as the HER and OER catalyst, respectively. Through detailed electrochemical and structural characterization, we have shown that the enhanced catalytic activity is attributed to directional growth of the electrodeposited film that exposes a Ni-rich lattice plane as the terminating plane, as well as increased covalency of the selenide lattice which decreases the Ni(II) to Ni(III) oxidation potential. Thereby, the high efficiency along with extended stability makes NiSe2 as the most efficient water electrolyzer known to-date.

CdS–CdTe heterojunction nanotube arrays for efficient solar energy conversion

We have established a protocol for the fabrication of CdTe–CdS, lateral p–n junction nanotube arrays, configured akin to either substrate or superstrate geometries. The protocol involves confined electrodeposition of p-type CdTe on lithographically patterned nanoelectrodes defined on conducting substrates, where the substrate surface has been coated with the n-type CdS layer deposited through chemical bath deposition. Post-lithography the underlying CdS layer could be revealed at selective positions, which is the key factor in defining the nature of the formed CdS–CdTe heterojunction. The aspect ratio of the CdTe nanotubes could be controlled by doing pulsed electrodeposition as well as altering the thickness of the polymeric resist used for lithography and tuning the diameter of the nanoelectrodes. Photoelectrochemical analysis in a liquid junction electrolyte has been performed to characterize the photoconductivity response of nanotube arrays. Both the substrate and superstrate p–n junction arrangements of CdS–CdTe nanotube arrays showed photocurrent comparable to that obtained from a bulk film covering a much larger surface area compared to the nanodevice. Typically it was observed that for the heterojunction CdS–CdTe nanotube device less than 10% coverage with the photoabsorber layer was required compared to the bulk film, in order to produce the same amount of photocurrent. Specifically, the photoconversion efficiency was increased by 50% on changing the morphology from bulk film (6.3%) to tubular (9.6%). Additionally, the advantage of the “holey” architecture in the photoabsorber layer was very evident as the photocurrent obtained from the nanotube arrays was larger than that obtained from the nanorod arrays electrodeposited under similar conditions. These observation suggest that these nanotube architectures combining the advantages of both the “holey” and nanopillar geometries might lead to an optimal nanostructured solar cell. The growth method of the nanotube arrays is simple and versatile which can be readily adapted to produce complex photoabsorber layers including the ternary and quaternary chalcogenides.

Fabrication of multifunctional ferromagnetic Au3Pd–CoSe nanoparticles

We have synthesized multifunctional anisotropic Au3Pd–CoSe nanoparticles on Si substrate through a catalyst aided chemical vapour deposition technique. The technique utilized volatile cobalt acetylacetonate and elemental selenium as precursors while sputter coated Au–Pd (3:2) film acted as a catalyst. The typical growth conditions led to clear segregation of the hetero-compositions (i.e. Au3Pd and CoSe) in the product nanostructures thereby preserving the functionality of both the phases. The degree of crystallinity of the individual phases in the composite nanostructure was fairly high. The bifunctional nanoparticles show soft ferromagnetic behaviour at room temperature and optical activity making them ideal for opto-magnetic applications.

Nickel telluride as a bifunctional electrocatalyst for efficient water splitting in alkaline medium

Designing efficient electrocatalysts has been one of the primary goals for water electrolysis, which is one of the most promising routes towards sustainable energy generation from renewable sources. In this article, we have tried to expand the family of transition metal chalcogenide based highly efficient OER electrocatalysts by investigating nickel telluride, Ni3Te2 as a catalyst for the first time. Interestingly Ni3Te2 electrodeposited on a GC electrode showed very low onset potential and overpotential at 10 mA cm−2 (180 mV), which is the lowest in the series of chalcogenides with similar stoichiometry, Ni3E2 (E = S, Se, Te) as well as Ni-oxides. This observation falls in line with the hypothesis that increasing the covalency around the transition metal center enhances catalytic activity. Such a hypothesis has been previously validated in oxide-based electrocatalysts by creating anion vacancies. However, this is the first instance where this hypothesis has been convincingly validated in the chalcogenide series. The operational stability of the Ni3Te2 electrocatalyst surface during the OER for an extended period of time in alkaline medium was confirmed through surface-sensitive analytical techniques such as XPS, as well as electrochemical methods which showed that the telluride surface did not undergo any corrosion, degradation, or compositional change. More importantly we have compared the catalyst activation step (Ni2+ → Ni3+ oxidation) in the chalcogenide series, through electrochemical cyclic voltammetry studies, and have shown that catalyst activation occurs at lower applied potential as the electronegativity of the anion decreases. From DFT calculations we have also shown that the hydroxyl attachment energy is more favorable on the Ni3Te2 surface compared to the Ni-oxide, confirming the enhanced catalytic activity of the telluride. Ni3Te2 also exhibited efficient HER catalytic activity in alkaline medium making it a very effective bifunctional catalyst for full water splitting with a cell voltage of 1.66 V at 10 mA cm−2. It should be noted here that this is the first report of OER and HER activity in the family of Ni-tellurides.

Scalable Nanomanufacturing of Metasurfaces Using Nanosphere Photolithography

We report on using Nanosphere Photolithography (NPL) for submicron patterning of Frequency Selective Surfaces (FSS). NPL is a combination of two techniques; colloidal nanolithography — where nanospheres form a self-assembled hexagonal close-packed (HCP) array when dispensed on a surface, and photonic jets — which are created when light is incident onto a microsphere in contact with a surface. NPL creates a mask-free HCP hole array in the photoresist. This pattern can be used with evaporation and lift-off to create an array of antenna elements, constituting the FSS. Alternatively, electrodeposition techniques can be used to deposit the metal elements. The later is particularly appealing as it lends itself to reel-to-reel fabrication techniques. Finally, we demonstrate that geometries other than simple hole arrays can be patterned in the photoresist by exposing the microsphere array with off normal incidence light.Copyright