Sunita Dey

Charge‐Transfer Interaction between Few‐Layer MoS2 and Tetrathiafulvalene

Graphene has emerged to be a material of great interest because of its unique electronic structure and properties associated with its two-dimensional structure. Single-layer graphene is well known for properties such as quantum Hall effect, ambipolar electric field effect, and ballistic conduction of charge carriers. Graphene exhibits significant changes in electronic structure and properties on introduction of electrons or holes by electrochemical means. Such doping is reported to stiffen the Raman G band (frequency of the Raman band increases). Electron and hole doping can also be achieved by molecular charge transfer through interaction with electron donor and acceptor molecules, respectively. Molecular charge transfer with graphene has been investigated in detail by using Raman spectroscopy and other techniques. Charge-transfer interaction with an electron donor molecule like tetrathiafulvalene (TTF) softens the G band of graphene, whereas stiffening occurs upon interaction with an electron acceptor like tetracyanoethylene (TCNE), the Raman G band frequency will increase and decrease due to interaction with TCNE and TTF, respectively. These changes in the Raman G band are different from those found with electrochemical doping. We were interested to explore the interaction of electron donor and acceptor molecules with a two-dimensional layered material such as MoS2 to explore the occurrence of charge transfer, if any. With this purpose, we have studied the interaction of few-layer MoS2 material with TTF and TCNE. It is to be noted that MoS2 layers consist of Mo atoms sandwiched between two layers of chalcogen atoms, where the adjacent sheets are stacked by van der Waals interactions. We have observed the occurrence of charge transfer of fewlayer MoS2 material with TTF, but not with TCNE. Electronic absorption spectroscopic measurements indicate the formation of TTF radical cation by the interaction of TTF with few-layer MoS2 material, accompanied by the stiffening of the A1g mode of MoS2 in the Raman spectrum. This shift in the Raman A1g mode is opposite to that found in electrochemical doping. We have carried out detailed first-principle calculations to understand the results. The XRD pattern of the few-layer MoS2 material does not exhibit the (002) reflection, thus confirming the presence of only a few layers and the graphene-like nature of the material. The AFM images and the corresponding height profiles also confirm the presence of two to three layers with an average thickness of 2.44 nm. Figure 1 shows

Ln0.5 A0.5 MnO3 (Ln=Lanthanide, A= Ca, Sr) Perovskites Exhibiting Remarkable Performance in the Thermochemical Generation of CO and H2 from CO2 and H2 O.

Perovskite oxides of the Ln0.5 A0.5 MnO3 (Ln=lanthanide, A=Sr, Ca) family have been investigated for the thermochemical splitting of H2 O and CO2 to produce H2 and CO respectively. The amounts of O2 and CO produced strongly depend on the size of the rare earth ions and alkaline earth ions. The manganite with the smallest rare earth possessing the highest distortion and size disorder as well as the smallest tolerance factor, gives out the maximum amount of O2 , and, hence, the maximum amount of CO. Thus, the best results are found with Y0.5 Sr0.5 MnO3 , which possesses the highest distortion and size disorder. Y0.5 Sr0.5 MnO3 shows remarkable fuel production activity even at the reduction and oxidation temperatures as low as 1200 °C and 900 °C, respectively.

Solar thermochemical splitting of water to generate hydrogen

Solar photochemical means of splitting water (artificial photosynthesis) to generate hydrogen is emerging as a viable process. The solar thermochemical route also promises to be an attractive means of achieving this objective. In this paper we present different types of thermochemical cycles that one can use for the purpose. These include the low-temperature multistep process as well as the high-temperature two-step process. It is noteworthy that the multistep process based on the Mn(II)/Mn(III) oxide system can be carried out at 700 °C or 750 °C. The two-step process has been achieved at 1,300 °C/900 °C by using yttrium-based rare earth manganites. It seems possible to render this high-temperature process as an isothermal process. Thermodynamics and kinetics of H2O splitting are largely controlled by the inherent redox properties of the materials. Interestingly, under the conditions of H2O splitting in the high-temperature process CO2 can also be decomposed to CO, providing a feasible method for generating the industrially important syngas (CO+H2). Although carbonate formation can be addressed as a hurdle during CO2 splitting, the problem can be avoided by a suitable choice of experimental conditions. The choice of the solar reactor holds the key for the commercialization of thermochemical fuel production.

Visible light-induced hydrogen generation using colloidal (ZnS)0.4(AgInS2)0.6 nanocrystals capped by S2− ions

(ZnS)0.4(AgInS2)0.6 nanocrystals (NCs) capped by S2− ions exhibit interesting electronic properties, absorbing visible light along with a suitable band alignment as well as properties such as high surface area providing active reaction sites for photocatalytic H2 evolution. A high activity of 5.0 mmol g−1 h−1 is achieved using non-toxic (ZnS)0.4(AgInS2)0.6 NCs as photocatalysts without the need for a noble metal co-catalyst.

Solar photochemical and thermochemical splitting of water.

Artificial photosynthesis to carry out both the oxidation and the reduction of water has emerged to be an exciting area of research. It has been possible to photochemically generate oxygen by using a scheme similar to the Z-scheme, by using suitable catalysts in place of water-oxidation catalyst in the Z-scheme in natural photosynthesis. The best oxidation catalysts are found to be Co and Mn oxides with the e1g configuration. The more important aspects investigated pertain to the visible-light-induced generation of hydrogen by using semiconductor heterostructures of the type ZnO/Pt/Cd1−xZnxS and dye-sensitized semiconductors. In the case of heterostructures, good yields of H2 have been obtained. Modifications of the heterostructures, wherein Pt is replaced by NiO, and the oxide is substituted with different anions are discussed. MoS2 and MoSe2 in the 1T form yield high quantities of H2 when sensitized by Eosin Y. Two-step thermochemical splitting of H2O using metal oxide redox pairs provides a strategy to produce H2 and CO. Performance of the Ln0.5A0.5MnO3 (Ln = rare earth ion, A = Ca, Sr) family of perovskites is found to be promising in this context. The best results to date are found with Y0.5Sr0.5MnO3.

Significant reduction in the operating temperature of the Mn(II)/Mn(III) oxide-based thermochemical water splitting cycle brought about by the use of nanoparticles

Among the many efforts to devise thermochemical cycles to generate H2 by water splitting, the Mn(II)/Mn(III) oxide based cycle operating at 850 °C is a significant one and involves no toxic and corrosive materials. The essential process in this cycle is the shuttling of Na+ ions in and out of Mn oxides. In an effort to bring down the temperature of this cycle, we have found that the use of Mn3O4 nanoparticles is particularly effective. Ball milling has been applied to decrease the particle size of commercial Mn3O4 to less than 500 nm. Thus the solid state reaction between Na2CO3 and Mn3O4 nanoparticles occurs at a temperature 200 °C lower than with bulk samples. One of the challenges of this particular cycle lies in its slow H2 evolution. It has been possible to operate this cycle and generate H2 at a much faster rate at 750 °C and even at 700 °C by this means. Furthermore, the step involving hydrolysis of NaMnO2 can be performed at 50 °C instead of 100 °C.