Achieving large two-dimensional (2D) sheets of any metal is challenging due to their tendency to coalescence or cluster into 3D shapes. Recently, single-atom-thick gold sheets, termed goldene, was reported. Here, we ask if goldene can be extended to include multiple layers. The answer is yes, and trilayer goldene is the magic number, for reasons of electronegativity. Experiments are made to synthesize the atomically laminated phase Ti 4 Au 3 C 3 through substitutional intercalation of Si layers in Ti 4 SiC 3 for Au. Density functional theory calculations suggest that it is energetically favorable to insert three layers of Au into Ti 4 SiC 3 , compared to inserting a monolayer, a bilayer, or more than three layers. Isolated trilayer goldene sheets, ~100 nanometers wide and 6.7 angstroms thick, were obtained by chemically etching the Ti 4 C 3 layers from Ti 4 Au 3 C 3 templates. Furthermore, trilayer goldene is found in both hcp and fcc forms, where the hcp is ~50 milli–electron volts per atom more stable at room temperature from ab initio molecular dynamic simulations.

The synthesis of monolayer gold has so far been limited to free-standing several-atoms-thick layers, or monolayers confined on or inside templates. Here we report the exfoliation of single-atom-thick gold achieved through wet-chemically etching away Ti3C2 from nanolaminated Ti3AuC2, initially formed by substituting Si in Ti3SiC2 with Au. Ti3SiC2 is a renown MAX phase, where M is a transition metal, A is a group A element, and X is C or N. Our developed synthetic route is by a facile, scalable and hydrofluoric acid-free method. The two-dimensional layers are termed goldene. Goldene layers with roughly 9% lattice contraction compared to bulk gold are observed by electron microscopy. While ab initio molecular dynamics simulations show that two-dimensional goldene is inherently stable, experiments show some curling and agglomeration, which can be mitigated by surfactants. X-ray photoelectron spectroscopy reveals an Au 4f binding energy increase of 0.88 eV. Prospects for preparing goldene from other non-van der Waals Au-intercalated phases, including developing etching schemes, are presented. Atomically thin gold nanosheets are predicted to have interesting properties but their synthesis is challenging. Here the exfoliation of two-dimensional single-atom-thick gold, termed goldene, is achieved through wet-chemically etching Ti3C2 from Ti3AuC2. The synthesized goldene has promising properties as a heterocatalyst.

Urea-SCR is generally considered the most efficient method for reducing nitrogen oxides (NOx) emissions from marine diesel engines. As ammonia (NH3), formed from the urea injected into the exhaust, reacts with nitrogen oxides to form nitrogen and water in the Selective Catalytic Reduction (SCR) catalyst, the urea dosing should be controlled to match engine-out NOx levels. Monitoring the NOx concentrations upstream and the NOx and NH3 concentrations downstream the catalyst offers one alternative to realize such dosing control. As SiC MOSFET-based gas sensors from previous studies have shown promising ammonia sensing ability, the characterization and performance evaluation of iridium-gate MOSFETs for selective monitoring of NH3 downstream a marine diesel SCR catalyst has been the focus of this study.Both laboratory measurements of NH3 sensitivity, selectivity, and cross-sensitivity to typical diesel exhaust constituents such as nitrogen oxides (NO, NO2), carbon monoxide (CO), and hydrocarbons, and field measurements downstream a diesel SCR system on-board a ferry in normal operation were performed. Except in the case of NO2, the cross-sensitivities between NH3 and NO/NO2/CO/ hydrocarbons were found to be practically insignificant for the sensor operation temperature corresponding to optimum NH3 sensitivity. The cross-sensitivity to nitrogen oxides observed under controlled conditions did not seem to have a significant impact on the exhaust ammonia measurement when evaluated in the real application. Besides promising performance for ammonia slip monitoring in the concentration range useful for urea dosing control, iridium-gate MOSFET ammonia sensors may also be able to act as a urea dosing/ control unit diagnostics tool.

Abstract The quest to make free-standing monolayer gold has hitherto been limited to free-standing several-atoms-thick layers, or monolayers but confined on or inside another template. Examples are monolayers diffused into double hydroxides1, membranes framed in alloys under electron irradiation2, nano-ribbons suspended in graphene3, quantum dots on hexagonal BN4, monolayers in between SiC wafers and monolayer graphene5, and fragments produced via thermal dewetting on sapphire6. Here, we report the synthesis of free-standing single-atom-thick 2D gold (named goldene) by wet-chemically etching away Ti3C2 from Ti3AuC2, a nano-laminated MAX-phase ceramic initially formed by substituting Si in Ti3SiC2 with Au7. The free-standing goldene layers are revealed by scanning transmission electron microscopy. While ab initio molecular dynamics simulations show that 2D goldene is inherently stable, the experiments reveal a tendency for curling and agglomeration at edges. Prospects for preparing goldene from a series of non-van der Waals Au-intercalated MAX-phases, including developing etching schemes, are also presented.

Abstract The quest to make monolayer gold has hitherto been limited to a few atomic layers stabilized on or inside another material. Examples are monolayers diffused into double hydroxides1, sheets framed in Au-Ag alloys under electron irradiation2, nano-ribbons suspended in graphene3, single-atom-thick quantum dots on hexagonal BN4, monolayers in between SiC wafers and monolayer graphene5, and fragments produced via thermal dewetting on substrates6. Here, we report the synthesis of free-standing single-atom-thick 2D gold (named goldene) by wet-chemically etching away Ti3C2 from Ti3AuC2, a nano-laminated MAX-phase ceramic initially formed by substitution of Si in Ti3SiC2 with Au7. The free-standing goldene layers are revealed by scanning transmission electron microscopy. While ab initio molecular dynamics simulations show that 2D goldene is inherently stable, the experiments reveal a tendency for curling and agglomeration at edges. Prospects for preparing goldene from a series of non-van der Waals Au-intercalated MAX-phases, including developing etching schemes, are also presented.

Intercalation of noble metals into non-van der Waals solids provides a new avenue to synthesize novel nanolaminated compounds with distinct material properties. Herein, we use solid-state reaction at 400 °C to prepare Cr2AuC from two Cr-based Mn+1AXn phase precursors and demonstrate the formation of Cr2AuC upon full replacement of Ga layers with Au in Cr2GaC thin films via thermal substitution reaction. The resulting Cr2AuC exhibits 2.7% lattice expansion relative to the original Cr2GaC, whereas Ge in a Cr2GeC thin film was sparsely replaced by Au, as revealed by electron microscopy and x-ray diffraction analysis. To explain the observed differences, using ab initio calculations, we consider the bonding properties of Cr2GaC and Cr2GeC, and the energetic driving forces for substitution by evaluating the mixing free energy of Au on both A-sites of the MAX phases, and of both A-elements in the surrounding Au lattice. The results suggest that it is somewhat easier to mix Au on the A-site in Cr2GaC than in Cr2GeC, and substantially easier to mix Ga into the Au-lattice than Ge. Finally, we discuss how the gained insights can be consulted for exploring a wider class of Mn+1AXn phases with intercalated noble metals.

Following tightened regulations, selective catalytic reduction (SCR) of nitrogen oxides (NOx) by ammonia (NH3) has over the last couple of decades found wider adoption as a means of reducing NOx emissions from e.g. power production and district heating plants. As in the SCR process NH3 injected into the flue gas reacts with and reduces NOx to nitrogen (N2) and water (H2O) on the surface of a specific catalyst, the NH3 injection has to be dynamically adjusted to match both instant and long-term variations in flue gas nitrogen oxide concentration in order to minimize NOx and NH3 emissions. One possibility of realizing such NH3 dosing control would be the real-time monitoring and feedback of downstream flue gas NOx and NH3 concentrations to the NH3 injection control unit. In this study the sensing characteristics and performance of SiC-based Metal Oxide Semiconductor Field Effect Transistor (MOSFET) sensors with a structurally tailored gas-sensitive gate contact of iridium (Ir) for in situ NH3 monitoring downstream from the SCR catalyst in a combined heat and power (CHP) plant have therefore been investigated and evaluated. The sensor's NH3 sensitivity and selectivity as well as the cross-sensitivity to common flue gas components – oxygen (O2), water vapour (H2O), nitric oxide (NO), nitrogen dioxide (NO2), carbon monoxide (CO), and a model hydrocarbon, ethene (C2H4) – were thereby investigated for relevant concentration ranges under controlled conditions in the laboratory. While, at the prescribed sensor operation temperature of 300 ∘C, the influence of H2O, CO, and C2H4 on the sensor's NH3 concentration reading could be regarded as practically insignificant, a moderate cross-sensitivity was observed between NH3 and NO2 and, to a lesser extent, between NH3 / NO and NH3 / O2. As the NOx concentration downstream from the SCR catalyst under normal SCR and power plant operation is expected to be considerably smaller than the NH3 concentration whenever any appreciable ammonia slip occurs, the observed NH3 / NOx cross-sensitivities may, however, be of less practical significance for ammonia monitoring in real flue gases downstream from the SCR catalyst. Furthermore, if required, the small influence of O2 concentration variations on the sensor reading may also be compensated for by utilizing the signal from a commercially available oxygen sensor. Judging from in situ measurements performed in a combined heat and power plant, the structurally tailored Ir gate field effect sensors also exhibit good NH3 sensitivity over the relevant 0–40 ppm range when directly exposed to real flue gases, offering an accuracy of ±3 ppm as well as low sensor signal drift, the latter most likely to further improve with regular zero-point calibration and thereby make the Ir gate MOSFET ammonia sensor a promising alternative for cost-efficient real-time ammonia slip monitoring or SCR system control in heat and/or power production plants.

In this study, the performance of MOSFET CO and NOx sensors for their applicability in on-line monitoring of flue gas composition and the automatic control of combustion for individual gas burners, producing heat to an industrial process, has been evaluated. In comparing the readings from pre-calibrated sensors with the concentrations measured by a commercially available flue gas analysis instrument and an oxygen sensor, promising sensor characteristics were obtained for the in-situ flue gas CO and NOx level monitoring, suggesting their possible applicability in automatic, self-adapting industrial process control and future related Industrial IoT applications.

In order to investigate the necessary device improvements for high-temperature CO sensing with SiC metal insulator semiconductor field effect transistor (MISFET)-based chemical gas sensors, devices employing, as the gas-sensitive gate contact, a film of co-deposited Pt/Al2O3 instead of the commonly used catalytic metal-based contacts were fabricated and characterized for CO detection at elevated temperatures and different CO and O2 levels. It can be concluded that the sensing mechanism at elevated temperatures correlates with oxygen removal from the sensor surface rather than the surface CO coverage as observed at lower temperatures. The long-term stability performance was also shown to be improved compared to that of previously studied devices.

To fulfil today's requirements, gas sensors have to become more and more sensitive and selective. Temperature-cycled operation has long been used to enhance the sensitivity and selectivity of metal-oxide semiconductor gas sensors and, more recently, silicon-carbide-based, gas-sensitive field-effect transistors (SiC-FETs). In this work, we present a novel method to significantly enhance the effect of gate bias on a SiC-FET's response, giving rise to new possibilities for static and transient signal generation and, thus, increased sensitivity and selectivity. A tungsten trioxide (WO3) layer is deposited via pulsed laser deposition as an oxide layer beneath a porous iridium gate, and is doped with 0.1 AT% of lithium cations. Tests with ammonia as a well-characterized model gas show a relaxation effect with a time constant between 20 and 30 s after a gate bias step as well as significantly increased response and sensitivity at +/- 2V compared to 0V. We propose an electric field-mediated change in oxygen surface coverage as the cause of this novel effect.

The performance of Silicon Carbide based field effect gas sensor devices, modified to enable long-term reliable operation with improved sensitivity to e.g., NH3 and CO at relevant temperatures for exhaust/flue gas emissions monitoring and control of combustion processes as well as after-treatment systems in automotive/stationary applications has been investigated with promising results.

This study concerns the tailoring of gas-sensitive FET sensor devices for the development of hightemperature NOx and NH3 sensors applicable for the control of urea-SCR when used as a NOx abatement measure in automotive applications through the investigation of the interaction between different series of gas-sensitive materials and NH3, NO, and NO2 at different temperatures.

Static and dynamic responses of a silicon carbide field-effect transistor gas sensor have been investigated at two different gate biases in several test gases. Especially the dynamic effects are gas dependent and can be used for gas identification. The addition of ultraviolet light reduces internal electrical relaxation effects, but also introduces new, temperature-dependent effects.

N.B.: When citing this work, cite the original publication. Fashandi, H., Dahlqvist, M., Lu, J., Palisaitis, J., Simak, S., Abrikosov, I., Rosén, J., Hultman, L., Andersson, M., Lloyd Spetz, A., Eklund, P., (2017), Synthesis of Ti3AuC2, Ti3Au2C2 and Ti3IrC2 by noble metal substitution reaction in Ti3SiC2 for high-temperature-stable Ohmic contacts to SiC, Nature Materials, 16(8), 814-818. https://doi.org/10.1038/NMAT4896

Data from a silicon carbide based field-effect transistor were recorded over a period of nine days in a ventilated school room. For enhanced sensitivity and selectivity especially to formaldehyde, ...

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti3AuC2 and Ti3Au2C2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Ti3IrC2 is subsequently produced by a substitution reaction of Ir for Au in Ti3Au2C2. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 °C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti 3 AuC 2 and Ti 3 Au 2 C 2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti 3 SiC 2 single-crystal thin films with simultaneous out-diffusion of Si. Ti 3 IrC 2 is subsequently produced by a substitution reaction of Ir for Au in Ti 3 Au 2 C 2 . These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 °C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.

Metal Oxide Semiconductor (MOS) capacitor devices comprised of monolayer iron oxide-coated as well as non-coated polycrystalline Pt deposited on oxidized silicon carbide substrates have been fabricated and their usefulness as realistic model systems in catalyst studies development was evaluated. The CO oxidation characteristics of both iron oxide- and non-coated Pt catalysts were investigated using mass spectrometry, monitoring the carbon dioxide production rate for different combinations of carbon monoxide (CO) and oxygen concentrations at various temperatures. Additionally, the output capacitance of the MOS model catalysts was recorded for each individual CO oxidation activity. A low-temperature shift in CO oxidation characteristics for the monolayer-coated compared to the non-coated Pt catalysts was observed, similar to that previously reported for monolayer iron oxide grown on single-crystalline Pt substrates. A strong correlation between the output capacitance of the MOS structures and the CO oxidation characteristics was found for both monolayer- and non-coated model catalysts. Furthermore, the devices exhibit retained MOS electrical output and CO oxidation characteristics as well as an unaffected catalyst surface composition, as confirmed by photoelectron spectroscopy, even after 200 h of continuous model catalyst operation. In addition to the implications on practical applicability of monolayer iron oxide coating on widely used polycrystalline Pt films in real-world catalysts and sensors, the findings also point to new possibilities regarding the use of MOS model systems for in situ characterization, high throughput screening, and tailoring of e.g. catalyst- and fuel-cell-electrode materials for specific applications. (C) 2016 Elsevier Inc. All rights reserved.