Metal Additive Distribution in TiO2 and SnO2 Semiconductor Gas Sensor Nanostructured Materials

Recently, there has been an increasing interest in the electronics world for those aspects related to semiconducting gas sensor (SGS) materials. In view of the increasingly strict legal limits for pollutant gas emissions, there is a great interest in developing high performance gas sensors for applications such as controlling air pollution and exhaust gases. In this way, semiconductor gas sensors offer good advantages with respect to other gas sensor devices (such as spectroscopic and optic systems), due to their simple implementation, low cost and good reliability for real-time control systems. In the present work, we have been especially interested in the study of the different ways of metal additive distribution in the most common SGS materials used nowadays and furthermore in the physical and chemical sensing properties they can achieve

Engineering surface states of hematite based photoanodes for boosting photoelectrochemical water splitting

Hematite-based photoanodes are promising candidates for photoelectrochemical water splitting. However, the performance of pristine hematite semiconductors is unsatisfactory due to charge recombination occurring at different interfaces: Back contact, bulk and semiconductor/electrolyte interfaces. Increasing efforts have been focused on enhancing the performance of hematite based photoanodes via nanostructure control, doping, heterojunction construction, and surface modification with a secondary semiconductor or an oxygen evolution electrocatalyst. Most of the previous studies attributed the enhanced PEC water splitting performance to the changes in the donor density via doping, the formation of type II heterojunction via a secondary semiconductor coating and the improved water oxidation kinetics via coating oxygen evolution electrocatalysts. However, the role of surface states presented at the semiconductor/electrolyte interfaces of hematite-based photoanodes has been overlooked in previous investigations, which virtually play a critical role in determining the photoelectrochemical water oxidation process. In this review, we summarize the recent progress of various techniques employed for the detection of surface states of hematite photoanodes and highlight the important role of modifying surface states in the development of high performance hematite based photoanodes for photoelectrochemical water splitting application. The challenges and future prospects in the study of hematite based photoanodes are also discussed

Metal-organic framework-derived single atom catalysts for electrocatalytic reduction of carbon dioxide to C1 products

Electrochemical carbon dioxide reduction reaction (eCO RR) is an efficient strategy to relieve global environmental and energy issues by converting excess CO from the atmosphere to value-added products. Single-atom catalysts (SACs) derived from metal-organic frameworks (MOF), which feature unique active sites and adjustable structures, are emerging as extraordinary materials for eCO RR. By modulating the MOF precursors and their fabrication strategy, MOF-derived SACs with specific-site coordination configuration have been recently designed for the conversion of CO to targeted products. In the first part of this review, MOF synthesis routes to afford well-dispersed SACs along with the respective synthesis strategy have been systematically reviewed, and typical examples for each strategy have been discussed. Compared with traditional M-N active sites, SACs with regulated coordination structures have been rapidly developed for eCO RR. Secondly, the relationship between regulation of the coordination environment of the central metal atoms, including asymmetrical M-N sites, heteroatom doped M-N sites, and dual-metal active sites (M-M sites), and their respective catalytic performance has been systematically discussed. Finally, the challenges and future research directions for the application of SACs derived from MOFs for eCO RR have been proposed

High aspect ratio gold nanorods grown with platinum seeds

Using Au chloride as precursor, Pt nanocrystals as seeds, ascorbic acid as a reducer, and CTAB as surfactant and complexing agent, extremely long Au nanorods have been grown. The influence of different parameters such as the composition of the seeds, the amount of Pt, or the type of Pt present in solution has been analyzed. These large Au NRs have been exhaustively characterized by (S)TEM, SEM and optical microscopy as well as UV-vis spectroscopy and their morphology correlated with the growth mechanism

Metal island film-based structures for sensing using spectrophotometry and ellipsometry

Metal island films (MIF) are good candidates for sensors due to the strong sensitivity of the localised surface plasmon resonance to the environment refractive index. The strong near field enhancement in the vicinity of the island surface can be even higher if a metal layer (ML) is placed close to a MIF. Structures containing MIF with and without ML are prepared and sensitivities of spectrophotometric and ellipsometric features of the measurements are compared. It is shown that simple MIF is preferable for ellipsometry-based sensing and the one including ML in the case of spectrophotometric measurements

Colloidal Silicon-Germanium Nanorod Heterostructures

Colloidal nanorods with axial Si and Ge heterojunction segments were produced by solution-liquid-solid (SLS) growth using Sn as a seed metal and trisilane and diphenylgermane as Si and Ge reactants. The low solubility of Si and Ge in Sn helps to generate abrupt Si-Ge heterojunction interfaces. To control the composition of the nanorods, it was also necessary to limit an undesired side reaction between the Ge reaction byproduct tetraphenylgermane and trisilane. High-resolution transmission electron microscopy reveals that the Si-Ge interfaces are epitaxial, which gives rise to a significant amount of bond strain resulting in interfacial misfit dislocations that nucleate stacking faults in the nanorods

Degradation and regeneration mechanisms of NiO protective layers deposited by ALD on photoanodes

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 photocurrent densities

Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy - The influence of Si- and Mg-doping

The self-assembled growth of GaN nanorods on Si (111) substrates by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions is investigated. An amorphous silicon nitride layer is formed in the initial stage of growth that prevents the formation of a GaN wetting layer. The nucleation time was found to be strongly influenced by the substrate temperature and was more than 30 min for the applied growth conditions. The observed tapering and reduced length of silicon-doped nanorods is explained by enhanced nucleation on nonpolar facets and proves Ga-adatom diffusion on nanorod sidewalls as one contribution to the axial growth. The presence of Mg leads to an increased radial growth rate with a simultaneous decrease of the nanorod length and reduces the nucleation time for high Mg concentrations

Reduction of Thermal Conductivity in Nanowires by Combined Engineering of Crystal Phase and Isotope Disorder

Nanowires are a versatile platform to investigate and harness phonon and thermal transport phenomena in nanoscale systems. With this perspective, we demonstrate herein the use of crystal phase and mass disorder as effective degrees of freedom to manipulate the behavior of phonons and control the flow of local heat in silicon nanowires. The investigated nanowires consist of isotopically pure and isotopically mixed nanowires bearing either a pure diamond cubic or a cubic-rhombohedral polytypic crystal phase. The nanowires with tailor-made isotopic compositions were grown using isotopically enriched silane precursors SiH, SiH, and SiH with purities better than 99.9%. The analysis of polytypic nanowires revealed ordered and modulated inclusions of lamellar rhombohedral silicon phases toward the center in otherwise diamond-cubic lattice with negligible interphase biaxial strain. Raman nanothermometry was employed to investigate the rate at which the local temperature of single suspended nanowires evolves in response to locally generated heat. Our analysis shows that the lattice thermal conductivity in nanowires can be tuned over a broad range by combining the effects of isotope disorder and the nature and degree of polytypism on phonon scattering. We found that the thermal conductivity can be reduced by up to ∼40% relative to that of isotopically pure nanowires, with the lowest value being recorded for the rhombohedral phase in isotopically mixed Si Si nanowires with composition close to the highest mass disorder (x ∼ 0.5). These results shed new light on the fundamentals of nanoscale thermal transport and lay the groundwork to design innovative phononic devices

Hydrothermal Fabrication of Carbon-Supported Oxide-Derived Copper Heterostructures : A Robust Catalyst System for Enhanced Electro-Reduction of CO2 to C2H4

Anthropogenic CO can be converted to alternative fuels and value-added products by electrocatalytic routes. Copper-based catalysts are found to be the star materials for obtaining longer-chain carbon compounds beyond 2e products. Herein, we report a facile hydrothermal fabrication of a highly robust electrocatalyst: in-situ grown heterostructures of plate-like CuO−CuO on carbon black. Simultaneous synthesis of copper-carbon catalysts with varied amounts of copper was conducted to determine the optimum blend. It is observed that the optimum ratio and structure have aided in achieving the state of art faradaic efficiency for ethylene >45 % at −1.6 V vs. RHE at industrially relevant high current densities over 160 to 200 mA ⋅ cm. It is understood that the in-situ modification of CuO to CuO during the electrolysis is the driving force for the highly selective conversion of CO to ethylene through the *CO intermediates at the onset potentials followed by C−C coupling. The excellent distribution of Cu-based platelets on the carbon structure enables rapid electron transfer and enhanced catalytic efficiency. It is inferred that choosing the right composition of the catalyst by tuning the catalyst layer over the gas diffusion electrode can substantially affect the product selectivity and promote reaching the potential industrial scale