SnSe has emerged as one of the most promising materials for thermoelectric energy conversion due to its extraordinary performance in its single-crystal form and its low-cost constituent elements. ...However, to achieve an economic impact, the polycrystalline counterpart needs to replicate the performance of the single crystal. Herein, we optimize the thermoelectric performance of polycrystalline SnSe produced by consolidating solution-processed and surface-engineered SnSe particles. In particular, the SnSe particles are coated with CdSe molecular complexes that crystallize during the sintering process, forming CdSe nanoparticles. The presence of CdSe nanoparticles inhibits SnSe grain growth during the consolidation step due to Zener pinning, yielding a material with a high density of grain boundaries. Moreover, the resulting SnSe–CdSe nanocomposites present a large number of defects at different length scales, which significantly reduce the thermal conductivity. The produced SnSe–CdSe nanocomposites exhibit thermoelectric figures of merit up to 2.2 at 786 K, which is among the highest reported for solution-processed SnSe.
The use of high pH electrolytes requires protective layers to avoid corrosion in photoanodes based on semiconductors like silicon. NiO is one of the materials that comply with the requirements for ...transparency, conductivity, chemical stability and catalysis on the surface in contact with the electrolyte. Here, NiO layers have been deposited by atomic layer deposition (ALD) at low temperatures, and their stability is analyzed over 1000 hours. Due to the layer structure characteristics, the best overall performance was achieved at 100 °C deposition temperature. By electrochemical measurements progressive time dependent degradation under anodic working conditions is observed, attributed to the formation of higher nickel oxidation states at the electrode/electrolyte interface as a main degradation mechanism, resulting in an OER overpotential increase. Another minor degradation mechanism affects the optical surface quality and gives rise to a loss of photon absorption efficiency on the scale of hundreds of hours. A regeneration process based on
in situ
periodic cyclic voltammetry, bringing the electrodes to cathodic conditions every 3, 12 or 48 hours, has been shown to partially reverse the main degradation mechanism achieving 85% stability over 1000 hours in a study with over 10 mA cm
−2
photocurrent densities.
The degradation mechanisms of ALD-grown NiO protective layers over 1000 h under anodic alkaline conditions are identified and a recovery mechanism is presented.
Au-catalyzed III–V nanowire heterostructures based on the group III interchange usually grow straight only in one of the two growth sequences, whereas the other sequence produces kinked geometries; ...thus, the realization of double heterostructures remains challenging. Here, we investigate the growth of Au-assisted InAs–GaAs and GaAs–InAs axial nanowire heterostructures. A detailed study of the heterostructure morphology as a function of growth parameters and chemical composition of the catalyst nanoparticle is performed by means of scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray analysis. Our results clearly demonstrate that the nanoparticle composition, rather than other growth parameters, as postulated so far, controls the growth mode and the resulting nanowire morphology. Although GaAs easily grows straight on InAs, straight growth of InAs on GaAs is achieved only if the nanoparticle composition is properly tuned. We find that straight InAs segments on GaAs require high group III-to-Au ratios in the nanoparticle (greater than 0.8); otherwise, the droplet wets the sidewalls and the nanowire kinks. We discuss the observed behavior within a theoretical model that relates the nanoparticle stability to the group III-to-Au ratio. Based on this finding, we demonstrate the growth of straight nanowire heterostructures for both sequences. The proposed strategy can be extended to other III–V nanowire heterostructures based on the group III interchange, allowing for straight morphology regardless of the growth sequence, and ultimately for designing nanowire heterostructures with the required properties for different applications.
We demonstrate rapid deterministic (seeded) growth of large single-crystals of graphene by chemical vapour deposition (CVD) utilising pre-patterned copper substrates with chromium nucleation sites. ...Arrays of graphene single-crystals as large as several hundred microns are grown with a periodicity of up to 1 mm. The graphene is transferred to target substrates using aligned and contamination- free semi-dry transfer. The high quality of the synthesised graphene is confirmed by Raman spectroscopy and transport measurements, demonstrating room-temperature carrier mobility of 21 000 cm2 V−1 s−1 when transferred on top of hexagonal boron nitride. By tailoring the nucleation of large single-crystals according to the desired device geometry, it will be possible to produce complex device architectures based on single-crystal graphene, thus paving the way to the adoption of CVD graphene in wafer-scale fabrication.
Zirconium-based Metal-Organic Frameworks (MOF) precursors were used to synthesize nanopowders of ZrCxOy zirconium oxycarbide. The reaction mechanisms involved in the thermal transformation of two ...different MOF precursors obtained by using fumaric (H2FUM) and terephthalic (H2BDC) acid as linkers were investigated. The transformation of these two precursors, characterized by two different C/Zr ratio proceeds through an intermediate zirconia phase formed during heating, in association with turbostratic carbon. The zirconia is shown to nucleate within carbon areas coming from the decomposition of the precursors. Zirconia is mainly observed under its tetragonal form in the Zr-BDC carbon rich system (C/Zr = 8) while the monoclinic form predominates in the Zr-FUM low carbon system (C/Zr = 4). The transformation then finally appears very similar to the classical carbothermal reaction between zirconia and carbon black. The reconstructive transformation of zirconia into an oxycarbide phase being highly energy consuming, high temperature heat treatment are required precluding the possibility to obtain the expected nanocrystalline oxycarbide phase by such a way of synthesis. Coupled XRD and TEM study reveals that Zr-BDC system is considered as carbon saturated and the Zr-FUM system appears as carbon deficient. In the Zr-BDC system, zirconia first disappears during the thermal reaction and the oxygen rich oxycarbide that is first formed then reacts with residual carbon to provide progressive carbon enrichment until the saturation of the ZrCxOy oxycarbide solid solution. On the opposite, in the Zr-FUM system, the carbon is the rate-limiting factor of the reaction and the residual zirconia is shown to react with the oxycarbide earlier formed, suggesting the occurrence of a peritectoid like transformation.
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•Oxycarbide compounds were obtained directly from Metal Oxide Frameworks precursors.•The transformation of two different precursors was studied by TEM, XRD and ATD.•The transformation involves the formation of an intermediate zirconia phase.•The reaction mechanisms involved are likely to those of the carbothermal reaction.
Four crystal structures are solved from precession electron diffraction data using either the highest‐quality experimental data available or voluntarily deteriorated data. The data quality was ...lowered by changing the intensity of each individual reflection by as much as a factor of 2, even for strong reflections, while taking care to keep strong reflections strong and weak reflections weak. The crystals have been chosen for their different characteristics, ranging from small to large unit cells, low to high symmetry, and containing heavy and light atoms. In each case the structure solution succeeded with the deteriorated data provided that the completeness of the data was high enough. The accuracy of the atom positions obtained for the cations was comparable to that for the best experimental data. Light‐atom positions were sometimes less accurate but still satisfactory for a possible subsequent refinement.
The synthesis of HfC
x
O
y
oxycarbides through the carbothermal reaction of hafnia with carbon black was undertaken. The obtained powders at different rates of advancement were studied by TEM and XRD ...in order to investigate the reaction mechanisms involved during such a transformation. The contact between the two starting reactants is shown to be non-reactive, attesting to the transformation operating through solid-gas reactions. The hafnia phase is destabilized by the CO
(g)
rich atmosphere and is consumed by the migration of ledges at the surface of the crystals acting as a zipper mechanism that liberates HfO
(g)
and CO
2(g)
species. The carbon dioxide thus released is used in return to oxidize the carbon black forming carbon monoxide through the Boudouard equilibrium. The liberated HfO
(g)
then reacts with the ambient CO
(g)
to form the oxycarbide phase which is shown to nucleate in the carbon black areas. The oxycarbide nuclei display a core-shell microstructure which is formed by a single crystal core embedded in an oxygen rich amorphous phase. During the final stage of the reaction, the atmosphere, which, saturated in CO
(g)
, progressively reduces the oxygen rich gangue until it finally disappears. The accurate determination of the cell parameter of the oxycarbide phase during the reaction indicates that the first formed compound is nearly saturated in carbon, comparable to the metallic carbide. The small change in the lattice parameter indicates that the chemical composition is very restricted, so the solid solution of oxygen within the hafnium oxycarbide seems to be very limited.
The synthesis of HfC
x
O
y
oxycarbides through the carbothermal reaction of hafnia with carbon black was undertaken.
ZrC
x
O
y
oxycarbides were synthesized for the first time by using metal-organic framework (MOF) compounds as precursors. These MOFs, based on zirconium carboxylates, are derived from the UiO-66 type ...structure. Three different Zr-MOF compounds were synthesized, differing by their C/Zr ratio, due to the use of terephthalic acid (C/Zr = 8, or UiO-66), naphthalene-2,6-dicarboxylic acid (C/Zr = 12) and biphenyl-4,4′-dicarboxylic acid (C/Zr = 14 or UiO-67). The oxycarbide crystallites obtained through appropriate thermal treatments (under Ar atmosphere) of the Zr-MOF precursors show an average size of a few hundred microns. They are surrounded by an outer rim of turbostratic carbon, whose thickness is directly relevant to the C/Zr ratio coming from the nature of the organic linker. The composition of the oxycarbides obtained was estimated on the basis of the cell parameters refined from the powder XRD patterns. The oxycarbides synthesized from naphtalene-2,6-dicarboxylic acid and biphenyl-4,4′-dicarboxylic acid are close to ZrC
0.925
O
0.075
, while that of the oxycarbide obtained from the terephthalic acid precursor is ZrC
0.944
O
0.056
. It results that the carbon richer oxycarbide product is observed for the UiO-66 precursor synthesized with terephthalic acid, and exhibiting the lower C/Zr ratio of the series. The composition of the oxycarbide powders is shown to be directly relevant to the mechanisms of decomposition of the Zr-MOF compound involved during heating.
ZrC
x
O
y
oxycarbides were synthesized for the first time by using metal-organic framework (MOF) compounds as precursors.
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