R.F. Frindt

Single-layer MoS2

Abstract MoS 2 has been exfoliated into monolayers by intercalation with lithium followed by reaction with water. X-ray diffraction analysis has shown that the exfoliated MoS 2 in suspension is in the form of one-molecule-thick sheets. X-ray patterns from dried and re-stacked films of exfoliated MoS 2 indicate that the layers are randomly stacked. Exfoliated MoS 2 has been deposited on alumina particles in aqueous suspension, enabling recovery of dry exfoliated MoS 2 supported on alumina.

Structure and staging of intercalated AgxTaS2 and AgxTiS2

The crystal structure of the intercalation compounds AgxTaS2 (0≤x≤23) and AgxTiS2 (0≤x≤0.42) are reported. For both intercalated dichalcogenides only stages n=2 and 1 are observed and there is no evidence for higher staging. In stage 1 Ag23TaS2, the silver ions reside in tetrahedral sites and the 2Hb−TaS2 structure changes to that of MoS2. For stage 2 Ag13TaS2, we observe a six layer unit cell with the silver ions again in tetrahedral sites. In stage 1 Ag0.42TiS2 and stage 2 Ag0.2TiS2, the silver ions are in octahedral sites and there is no change of the host lattice stacking. The samples were prepared both thermally and electrolytically and the same lattice parameters were obtained in both cases.

Inclusion compounds of MoS2

Abstract By adsorbing monolayers of various species on MoS 2 while the sulfide is suspended as single layers and then recrystallizing, a new group of compounds is formed which can be called inclusion compounds or inclusion solids. In the re-crystallized or re-stacked materials, the inter-layer spacing can be expanded or contracted compared to MoS 2 . If the included species is deposited as precipitated particles rather than as an adsorbed monolayer, in some cases the B.E.T. surface area can be made much higher than re-stacked MoS 2 with no inclusion. The new compounds have potential application in catalysis, gas storage, intercalation batteries and perhaps lubricants.

The gas-sensing properties of bismuth molybdate evaporated films

Abstract A gas sensor that operates by the detection of changes in the bulk conductivity of a bismuth molybdate semiconductor catalyst is presented. The reduction of the catalyst by organic vapours produces highly mobile oxygen vacancies, which have a direct effect on the carrier density throughout the sample. This reaction is completely reversible in air. The sensor can be prepared as a mixture of the alpha and gamma phases of bismuth molybdate evaporated onto a quartz substrate. The prototype sensors studied show long-term stability of response and insensitivity to water vapour. A degree of selectivity to alcohols and ketones and some alkenes over other reducing agents such as alkanes, hydrogen and carbon monoxide is shown. At an operating temperature of 330 °C, increases in conductivity of a factor of 30 after exposure to 200 ppm of ethanol were commonly observed, with sensitivities down to 5 ppm. Eventually this class of sensors may find use in breathalyser devices.

Thin oriented films of molybdenum disulphide

Single molecular layers of molybdenum disulphide in suspension in water can be collected at a water-organic liquid interface if mixed well with an immiscible organic liquid. Techniques are described for depositing thin (30–350A˚), highly oriented optical quality films of restacked MoS 2 on a variety of substrates. Characterization of the MoS 2 films with X-ray diffraction and optical absorption are described. The same techniques are successfully applied in depositing oriented films of unexfoliated MoS 2 and WS 2 powder.

Exfoliated single molecular layers of Mn0.8PS3 and Cd0.8PS3

The layered compounds MnPS 3 and CdPS 3 were exfoliated to form single molecular layers of Mn 0.8 PS 3 and Cd 0.8 PS 3 in suspension in water by ion exchange. The x-ray diffraction patterns of the two single-layer suspensions showed profound differences in some of the Bragg peaks, and we demonstrated that the differences are not due to the quality or size of the single layers, but are caused by structure factor modulations of the Warren tail for two-dimensional systems. We also demonstrated that the Cd or Mn vacancies generated in the exfoliation process are not ordered at long range, in contrast to an earlier report of vacancy ordering on intercalated MnPS 3 .

Intercalation of Ag in TaS2 and TiS2

Abstract Very thin, optically transparent crystals have been used to study the electro-intercalation of Ag in TaS2 and TiS2. The optical observations suggest that staging occurs in both systems, and this has been verified by X-ray diffraction studies. The optical data is correlated with the open-circuit potential of the Ag-TaS2 and Ag-TiS2 cells. Intercalation rates and diffusion rates over a range of temperatures have also been determined from the optical data. The diffusion rates are comparable to other intercalation systems, but the intercalation rate for Ag in TaS2 is anomalously high. A strong thickness dependence of intercalation rate for Ag in both TaS2 and TiS2 has been observed: the intercalation rate increases with decreasing crystal thickness. We suggest that similar thickness effects will be observed for all intercalation systems.

Transition metal dichalcogenide/polymer nanocomposites

Single molecular layers of transition metal dichalcogenides (TMDs) such as MoS2, MoSe2 and WS2, have been formed in suspension in water by lithium intercalation of crystalline powders and then exfoliation in water. The two-dimensionality of such systems is readily identified using powder X-ray diffraction, where the strong asymmetric shape of the (hk0) peaks and the absence of (00l) peaks and mixed (hkl) peaks are observed. Water-soluble polymers, such as poly(ethylene oxide), poly(ethylene glycol) and poly(vinylpyrrolidone), can be encapsulated in TMD layers to form TMD/polymer lamellar nanocomposites. TMD/polymer nanocomposites are turbostratically restacked with the TMD layers parallel to each other with an expanded interlayer spacing due to the polymer insertion. The expansion of interlayer spacing varies from 8 to 22 A and can be remarkably uniform. Highly oriented TMD/polymer films can be formed on glass substrates.

Positive ion emission from a platinum hot wire gas sensor

Abstract It was found that hot (500 °C to 1000 °C) metal wires in contact with organic vapors or contaminated by surface carbon will emit positive ions in air. With appropriate bias and collection geometry, currents up to 100 nA can be detected. As the carbon burns on a contaminated wire and the wire becomes cleaner, the current decays to zero. A clean platinum wire that is a good oxidation catalyst produces a steady positive ionic current in the presence of organic vapors. A number of different vapors were tested and it was concluded that higher responses were obtained for vapors with higher numbers of carbon atoms per molecule and greater ease of oxidation by the wire. Oxidizable gases with little or no carbon produced little or no ionic response. The Saha-Langmuir equation is used to calculate the ionization energies required to emit positive ions from the surface of the hot metal wire. This gave ionization potentials of about 6 eV, which are too low to represent ionization potentials for carbon itself or an oxide of carbon and therefore must represent some, as yet unknown, intermediate of the oxidation reaction. It is concluded that both the clean and carbon-contaminated wire responses can be used to design selective gas sensors. At 800 °C, for example, a clean platinum wire works as a highly reproducible gas sensor, giving a linear response from about 10 ppm to 1% vapor concentration of acetone.