[Show abstract][Hide abstract] ABSTRACT: 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 e(g)(1) 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 H-2 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 H-2 when sensitized by Eosin Y. Two-step thermochemical splitting of H2O using metal oxide redox pairs provides a strategy to produce H-2 and CO. Performance of the Ln(0.5)A(0.5)MnO(3) (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.
Preview · Article · Feb 2016 · Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences
[Show abstract][Hide abstract] ABSTRACT: High-resolution neutron powder diffraction is used to demonstrate that the Eu2+ moments in EuZrO3 are oriented along the c axis, in contrast to EuTiO3 where the moments lie within the ab plane. By applying a Landau theory analysis to the Ti and Zr system we are able to contrast the differing magnetoelectric coupling symmetries resulting from the magnetic space groups of the two ordered structures. The observed order parameter is consistent with the asymptotic three-dimensional Ising model.
[Show abstract][Hide abstract] ABSTRACT: Generation of H2 and CO by splitting H2O and CO2 respectively constitutes an important aspect of the present-day concerns with energy and environment. The solar thermochemical route making use of metal oxides is a viable means of accomplishing these reduction reactions. The method essentially involves reducing a metal oxide by heating and passing H2O or CO2 over the nonstoichiometric oxide to cause reverse oxidation by abstracting oxygen from H2O or CO2. While ceria, perovskites and other oxides have been investigated for this purpose, recent studies have demonstrated the superior performance of perovskites of the type Ln1−xAxMn1−yMyO3 (Ln=rare earth, A=alkaline earth, M=various +2 and +3 metal ions), in the thermochemical generation of H2 and CO. We present the important results obtained hitherto to point out how the alkaine earth and the Ln ions, specially the radius of the latter, determine the performance of the perovskites. The encouraging results obtained are exemplefied by Y0.5Sr0.5MnO3 which releases 483 µmol/g of O2 at 1673 K and produces 757 µmol/g of CO from CO2 at 1173 K. The production of H2 from H2O is also quite appreciable. Modification of the B site ion of the perovskite also affects the performance. In addition to perovskites, we present the generation of H2 based on the Mn3O4/NaMnO2 cycle briefly.
[Show abstract][Hide abstract] ABSTRACT: Graphene and other two-dimensional materials have been used as catalysts in the oxygen reduction reaction (ORR) in fuel cells with a few limitations in terms of the onset potential and number of electrons transferred per oxygen molecule.Wefind that heavily nitrogenated graphene with a nitrogen content of 14-18 wt.% and a high proportion of pyrrolic nitrogen, prepared under microwave irradiation, shows excellent activity towards ORR, comparable to platinum, with the transferred electron number of 4.
[Show abstract][Hide abstract] ABSTRACT: Physical and chemical properties of graphene can be tailored by nitrogen doping. As a consequence of the continuous race to achieve the highest possible amount of doping there is a growing tendency to assume that all N species are incorporated within the graphene lattice (doping). Here we show that this is not always the case and employ several complementary techniques that allow a proper assessment of the type of nitrogen present. The nature of the nitrogen atoms has been tuned by ammonolysis of graphene oxide in the range of 100 °C-800 °C. This allows us to expand the capabilities of the synthetic approach to afford not only N-doping (at high temperature) but also to introduce amine and amide moieties at 100 °C; the latter presenting a much higher dispersability in aqueous media than graphene oxide. Interestingly, the sample with the highest amount of nitrogen (14.7 wt. %) cannot be referred to as N-doped graphene since it also contains N-bearing aliphatic moieties.
[Show abstract][Hide abstract] ABSTRACT: Electrochemical generation of hydrogen by non‐precious metal electrocatalysts at a lower overpotential is a thrust area of research directed towards sustainable energy. The exorbitant costs associated with Pt based catalysts is the major bottleneck associated with the commercial scale hydrogen generation. Hence strategies for the synthesis of cost effective and stable catalysts is craving its way for prospective 'hydrogen economy'. In this report, we highlight the novel and general strategy to enhance the electrochemical activity of molybdenum disulfide (MoS 2) in the fullerene structure (IF‐). In particular, pristine (undoped) and Rhenium‐doped nanoparticles of MoS 2 with fullerene‐like structure (IF‐MoS 2) were studied and their performance as catalysts for hydoregen evolution reaction (HER) was compared to that of 2H‐MoS 2 particles (platelets). The current density of IF‐MoS 2 is higher by one order of magnitude as compared to few‐layer (FL‐) MoS 2 due to the enhanced density of edge sites. Furthermore, Re‐doping as low as 100 ppm in IF‐MoS 2 decreases the onset potential by 60‐80 mV and increases the activity by 60 times compared to FL‐MoS 2. The combined synergistic effect of Re‐doping and the IF‐structure not only changes the intrinsic nature of MoS 2 but also increases its reactivity. This strategy highlights the potential use of IF‐structure and Re‐doping in electrocatalytic hydrogen evolution using MoS 2 based catalysts.
Full-text · Article · Aug 2015 · Dalton Transactions
[Show abstract][Hide abstract] ABSTRACT: Mechanical properties of single-walled carbon nanohorn (SWNH) and SWNH plus few-layer graphene (EG) reinforced polyvinyl alcohol (PVA) matrix composites have been estimated using the nanoindentation technique. Elastic modulus, E, and hardness, H, of PVA were found to improve by ~315% and ~135% respectively upon addition of just 0.4 wt.% SWNH. These properties were found to be comparable to those obtained upon addition of 0.2 wt.% single-walled nanotube (SWNT) to PVA. Further, on binary addition of 0.2 wt.% EG and 0.4 wt.% SWNH to PVA, benefits in the form of ~400% and ~330% synergy in E and H respectively were observed, along with an increased resistance to viscoelastic creep. The reasons for these improvements are discussed in terms of the dimensionality of nanocarbon, the effectiveness of nanocarbon and polymer matrix interaction, and the influence of nanocarbon on the degree of crystallinity of the polymer. The results from SWNH reinforcement in this study demonstrate the scope for a novel and, in contrast to SWNT composites, a commercially feasible opportunity for strengthening polymer matrices.
[Show abstract][Hide abstract] ABSTRACT: In the last four to five years, there has been a great resurgence of research on two-dimensional inorganic materials, partly because of the impetus received from graphene research. Unlike graphene, which is a gap-less material, most inorganic layered materials are semiconductors or insulators. Some of them, as exemplified by MoS2, exhibit unexpected properties, not unlike graphene, with possible applications. Thus, layered metal chalcogenides are being explored intensely, and MoS2 is emerging as a wonder material. In this article, we present the synthesis and properties of nanosheets composing single or few layers of these fascinating materials. Besides metal chalcogenides, boron nitride, borocarbonitrides (BxCyNz), metal oxides, and metal-organic frameworks are also discussed.
No preview · Article · Jul 2015 · Annual Review of Materials Research
[Show abstract][Hide abstract] ABSTRACT: Chemical vapor deposition (CVD) is a process commonly used to produce high-purity solid materials. This chapter gives examples of the synthesis of some of the important inorganic materials by CVD. The common gases used for CVD synthesis of SiO2 are silane and oxygen, dichlo-rosilane (SiCl2H2) and nitrous oxide (N2O), or tetraethylorthosilicate (Si(OC2H5)4). Atomic layer deposition (ALD) is a self-limiting (the amount of film material deposited in each reaction cycle is constant), sequential surface chemistry technique that deposits thin films of materials on solid substrates. The chemistry of ALD is similar to that of CVD, except that the reaction in ALD breaks the CVD reaction into two half-reactions, keeping the precursor materials separate. The chapter provides a few examples of important ALD reactions. ALD of Al2O3 is a model system. ALD of Al2O3 is usually performed using trimethylaluminum (TMA) and H2O.
[Show abstract][Hide abstract] ABSTRACT: A solid-state reaction is said to be topochemically controlled when the reactivity is controlled by the crystal structure rather than by the chemical nature of the constituents. The products obtained in many solid-state decompositions are determined by topochemical factors, especially when the reaction occurs within the solid without the separation of a new phase. In topotactic solid-state reactions, the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Orientational relations between the parent and the product phases are generally found. Many developments in solid-state chemistry owe much to the investigations carried out on MoO3 and WO3 (for example, crystallographic shear planes). Topochemical dehydration has been used for sometime to prepare new metastable solids (for example, the synthesis of Ti2Nb2O9 from HTiNbO5). This strategy has been extended to perovskites. Intercalation reactions of solids involve the insertion of a guest species (ion or molecule) into a solid host lattice without any major rearrangement of the solid structure. A variety of layered structures act as hosts. The general feature of these structures is that the interlayer interactions are weak while the intralayer bonding is strong. Alkali metal intercalation in dichalcogenides is achieved by direct reaction of the elements around 1070 K in sealed tubes. Alkali metal intercalation compounds with dichalcogenides form hydrated phases. Metal phosphorus trisulfides undergo redox intercalation reactions just as the dichalcogenides and also ion exchange reactions. Pillaring is another intercalation reaction that enables synthesis of metastable oxide material. Pillaring refers to intercalation of robust, thermally stable, molecular species that prop the layers apart and convert the two-dimensional interlayer space into micropores of molecular dimensions, similar to the pores in zeolites. Ion exchange in fast-ion conductors such as β-alumina is well known. It can be carried out in aqueous as well as molten salt media conditions. Ion exchange in inorganic solids is a general phenomenon, not restricted to fast-ion conductors alone. Kinetic and thermodynamic aspects of ion exchange in inorganic solids were examined by England. Their results reveal that ion exchange is a phenomenon that occurs even when the diffusion coefficients are as small as ˜10-12cm2/s, at temperatures far below the sintering temperatures of solids. Ion exchange occurs at a considerable rate in stoichiometric solids as well. A variety of inorganic solids have been exchanged with protons to give new phases, some of which exhibit high protonic conduction. Ion exchange chemistry of layered metal chalcogenides is not explored much compared to that of metal oxides. These are by and large limited to alkali ion-containing transition metal dichalcogenides. Use of molten salts as reactive fluxes is a non-topochemical route that enables the synthesis of metastable phases, especially at intermediate temperatures (150 to 500 degrees Celsius) between those employed in the hydrothermal route and the conventional ceramic route. Strong alkaline media, either in the form of solid fluxes or molten (or aqueous) solutions, enable the synthesis of novel oxides. The alkali flux stabilizes higher oxidation states of metals by providing an oxidizing atmosphere. Alkali carbonate fluxes have been traditionally used to prepare transition metal oxides such as LaNiO3 with Ni in the +3 state. A good example of an oxide synthesized in a strongly alkaline medium is the pyrochlore, Pb2(Ru2-xPbx)O7-y, where Pb is in the +4 state. This oxide is a bifunctional electrocatalyst. The procedure for preparation involves bubbling oxygen through a solution of Pb and Ru salts in strong KOH at 320 K. The sol-gel method has provided a very important means of preparing inorganic oxides. It is a wet chemical method and a multistep process involving both chemical and physical processes such as hydrolysis, polymerization, drying and densification. Important features of the sol-gel method are better homogeneity compared to the traditional ceramic method, high purity, lower processing temperature, more uniform phase distribution in multicomponent systems, better size and morphological control, the possibility of preparing new crystalline and non-crystalline materials and, lastly, easy preparation of thin films and coatings. The six important steps in sol-gel synthesis include: hydrolysis, polymerization, gelation, drying, dehydration and densification. The sol-gel technique has been used to prepare sub-micrometer metal oxide powders with a narrow particle size distribution and unique particle shapes. Electrochemical methods have been employed to advantage for the synthesis of many solid materials. Typical materials prepared in this manner are metal borides, carbides, suicides, oxides and sulfides. Vanadate spinels of the formula MV2O4 as well as tungsten bronzes A5WO3 have been prepared by the electrochemical route. Electrochemical oxidation has been employed to prepare oxygen-excess La2CuO4 and other related materials. Thin films of BaTiO3 and lead zirconate titanate have been prepared by cathodic reduction. Intercalation of alkali metals in host solids is readily accomplished electrochemically. In recent years, hydrothermal synthesis has been employed to prepare various inorganic materials such as metal oxides, chalcogenides, metal-organic frameworks, porous materials and nanomaterials. Hydrothermal high-pressure synthesis under closed system conditions has been employed for the preparation of higher-valence metal oxides. Solvothermal synthesis is similar to hydrothermal synthesis but uses organic solvents such as toluene, decalin and octadecene instead of water. Solvothermal reactions have been extensively employed to prepare inorganic nanocrystals. In solvothermal synthesis, the size and shape of nanocrystals are controlled by the concentration of precursors and the reaction temperature. Ionothermal synthesis involves the use of an ionic liquid as the solvent in the synthesis of novel inorganic compounds.
[Show abstract][Hide abstract] ABSTRACT: Perovskite oxides of the composition La1-xCaxMnO3 (LCM) have been investigated for the thermochemical splitting of H2O and CO2 to produce H2 and CO respectively. The study was carried out in comparison with La1-xSrxMnO3, CeO2 and other oxides. The LCM system exhibits superior characteristics in high-temperature evolution of oxygen, and in reducing CO2 to CO and H2O to H2. The best results are found with La0.5Ca0.5MnO3 whose performance is noteworthy compared to that of other oxides including ceria. The orthorhombic structure of LCM seems to be a crucial factor.
Full-text · Article · Nov 2014 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The presence of nitrogen heteroatoms within the lattice of reduced graphene oxide sheets induces a remarkable increase in the thermal stability against oxidation by air. This phenomenon is represented by a nitrogen-doped (blue spheres) graphene sheet withstanding a flame. For more details, see the Communication by C. N. R. Rao, G. Tobias et al. on page 11999 ff.
Preview · Article · Sep 2014 · Chemistry - A European Journal
[Show abstract][Hide abstract] ABSTRACT: ZnO/NiO/Cd1-xZnxS (x= 0.0, 0.2) heterostructures have been prepared by a simple solution-based procedure using ZnO/NiO heterostructures prepared by different methods. We obtain good hydrogen evolution activity only with ZnO/NiO heterostructures and not with Zn1-yNiyO solid solutions. The hydrogen evolution activities of ZnO/NiO/CdS and ZnO/NiO/Cd1-xZnxS are 2.2 and 8.2 mmol/h/g respectively with apparent quantum yields of 2.3 and 14% under visible-light irradiation. These values of activity are comparable or superior to those obtained with ZnO/Pt/ Cd1-xZnxS and ZnO/Au/ Cd1-xZnxS heterostructures. With UV-visible irradiation, the activity found with ZnO/NiO/Cd1-xZnxS is 14-17 mmol/h/g with an apparent quantum yield in the range of 12-15%.
Full-text · Article · Jul 2014 · Chemical Physics Letters