Electrochemical Synthesis of Photoactive Mos2

The growth of polycrystalline films and single crystals of molybdenum disulfide by electrolytic reduction of a sodium tetraborate-sodium fluoride melt containing molybdenum trioxide and sodium sulfate has been examined and physical properties of those deposits studied. Polycrystalline n-type Mos2is readily obtained from a melt at 800°C while larger single-crystals are obtained if the melt temperature is raised to 900°C. MoS2is a semiconductor with bandgap well matched to the solar spectrum and with unusual resistance to photoelectrochemical corrosion. Electrochemically-grown single crystals and polycrystalline films were employed as photoanodes in aqueous iodide-triiodide electrolytes under ~40 mW/cm2tungsten-halogen illumination. Open-circuit photopotentials of ~150 mV were observed for polycrystalline MoS2. For single crystal electrolytic MoS2an open-circuit photopotential of 210 mV and ~2 mA/cm2short circuit current were reached. The use of electrochemical methods to prepare these materials offers the advantages of relatively low growth temperatures, controlled thickness, and excellent adaptability to producing large-area films inexpensively

ECS Classics: Making the Phone System More Reliable: Battery Research At Bell Labs

Research played a key role by bringing together the evolving technical needs and opportunities with experimental science and applications. Electrochemical processes were widely used in the manufacture of much of the equipment used in the Bell System. Batteries were used in every Bell System central office to provide load leveling and backup power. Research on batteries began in Bell Laboratories in 1930. The main focus of the early battery work involved lead-acid batteries, rechargeable batteries that offered low cost, together with high power density. Research at Bell Laboratories in the late 1970s focused on cathode materials for high energy density lithium rechargeable batteries. The search was for an electronically conducting material that could be reversibly oxidized and reduced with little change to its shape and structure. Other research taking place at Bell Laboratories focused on different aspects of a possible lithium battery, especially conductive organic-based electrolytes and a commercially viable anode material. Starting in the late 1970s, Samar Basu and coworkers worked on these problems aiming to create a viable commercial lithium battery. Ultimately Sony Engineers combined the Bell Labs patents with a cobalt oxide cathode material pioneered by John Goodenough and his colleagues then at Oxford University to produce the first commercial lithium-ion battery, the basis for the batteries which power most modern rechargeable portable electronic devices

InP and CdS Photoanodes in Concentrated Aqueous Iodide Electrolytes

Conditions for the efficient photo-oxidation of iodide for both n-CdS and n-InP are demonstrated by theoretical collection properties in rotating ring-disk electrode configurations. A specific I-CdS surface interaction promoting hole transfer kinetics is evidenced by open-circuit potential shifts of illuminated disks with iodide concentration and nearly quantitative disk iodine production at low iodide concentrations. For n-InP, however, high concentrations of CaI2 and control of hydrogen ion levels are both needed to maintain a thin, charge conducting semiconductor-oxide film and adequate kinetics for solution iodide oxidation. Acidified concentrated iodide is the first electrolyte with an n-InP photoanode to handle solar current densities

N-Type Molybdenum Diselenide-Based Photoelectrochemical Cells: Evidence for Fermi Level Pinning and Comparison of the Efficiency for Conversion of Light to Electricity with Various Solvent/Halogen/Halide Combinations

Interfacial energetics for n-type MoSe2 (Eg = 1.4 eV, direct) and photoelectrochemical conversion of light to electrical energy in the presence of Xn-/X- (X = Cl, Br, I) have been characterized in CH3CN electrolyte solution. Data for MoSe2 in H20/I3-/I- are included for comparison, along with a comparison of MoSe2-based cells with MoS2- (Eg = 1.7 eV, direct) based cells. Cyclic voltammetry for a set of reversible (at Pt electrodes) redox couples whose formal potential, E. spans a range -0.8 to +1.5 V vs. SCE has been employed to establish the interface energetics of MoSe2. For the redox couples having E° more negative than ~-0.1 V vs. SCE, we find reversible electrochemistry in the dark at n-type MoSe2. When E° is somewhat positive of -0.1 V vs. SCE, we find that oxidation of the reduced form of the redox couple can be effected in an uphill sense by irradiation of the n-type MoSe2 with \u3eEg light; the anodic current peak is at a more negative potential than at Pt for such situations. The extent to which the photoanodic current peak is more negative than at Pt is a measure of the output photovoltage for a given couple. For E° more positive than ~+0.7 V vs. SCE it would appear that this output photovoltage is constant at ~0.4 V. For a redox couple such as biferrocene (E°(BF+/BF) = +0.3 V vs. SCE) we find a photoanodic current onset at -0.2 V vs. SCE; a redox couple with E° = 1.5 V vs. SCE shows an output photovoltage of 0.43 V under the same conditions. The ability to observe (i) photoeffects for redox reagents spanning a range of E°\u27s that is greater than the direct Eg and (ii) constant photovoltage for a range of E°\u27s evidences an important role for surface states or carrier inversion such that a constant amount of band bending (constant barrier height) is found for a couple having E° more positive than ~+0.7 V vs. SCE. Conversion of \u3eg light to electricity can be sustained in CH3CN solutions of Xn-/X- (X = Cl, Br, I) with an efficiency that is ordered Cl \u3e Br \u3e I where n-type MoSe2 is used as a stable photoanode. In aqueous solution n-type MoSe2 is not a stable anode in the presence of similar concentrations of Br2/Br- or C12/C1-, showing an important role for solvent in thermodynamics for electrode decomposition. In CH3CN, efficiency for conversion of 632.8-nm light to electricity has been found to be up to 7.5% for C12/C1-, 1.4% for Br2/Br-, and 0.14% for Br2/Br-. Differences among these redox systems are output voltage and short-circuit current, accounting for the changes in efficiency. In H20, I3-/I- yields a stable n-type MoSe2-based photoelectrochemical cell with an efficiency for 632.8-nm light a little lower than that for the CH3CN/C12/C1- solvent/redox couple system. Data for MoS2-based cells in the CH3CN/Xn-/X- solvent/redox couple systems show that the efficiency again depends on X: Cl \u3e Br \u3e I. In H20/I3-/I- an efficiency a little lower than that for the CH3CN/C12/C1- is obtained. MoSe2-based cells are somewhat more efficient than MoS2-based cells, but significant variations in efficiency are found depending on the electrode sample used

Flat-Band Potential of N-Type Semiconducting Molybdenum Disulfide by Cyclic Voltammetry of Two-Electron Reductants: Interface Energetics and the Sustained Photooxidation of Chloride

Cyclic voltammetry has been used to locate the band edges of n-type MoS2 in CH3CN/ and EtOH/[n-Bu4N]ClO4 solutions. The crucial experiments concern the study of the cyclic voltammetry of biferrocene (BF) and N,N,N\u27,N\u27-tetra-methyl-p-phenylenediamine (TMPD) each of which has two, reversible, one-electron waves at Pt. At MoS2, the first oxidation is reversible in the dark, whereas the second oxidation is observed only upon illumination of the MoS2. The dark oxidation BF → BF+ and the photoanodic BF+ → BF2+ are separated by only ~150 mV, allowing us to assign an uncommonly accurate flat-band potential of +0.30 + 0.05 V vs. SCE to MoS2. This flat-band potential reveals that the valence band edge is at ca. +1.9 V vs. SCE showing that photooxidations workable at TiO2 are thermodynamically possible at illuminated MoS2 as well. As an example of the ruggedness of MoS2, we demonstrate the ability to effect the sustained oxidation of Cl- at illuminated n-type MoS2. Conclusions from BF are fully supported by those from TMPD and one-electron systems ferrocene, acetylferrocene, 1,1\u27-diacetylferrocene, and [Ru(2,2\u27-bipyridine)3]2+. Oxidation of [Ru(2,2\u27-bipyridine)3]2+ can be effected \u3e0.5 V more negative than at Pt by illumination of MoS2

Electric and Magnetic Anomalies At the Charge-Density-Wave Transition in Niobium-Substituted Vanadium Diselenides

A systematic study has been made of the effect of niobium substitution on the CDW transition in VSe2, especially in the low-doping region. Transitions were monitored by measurements of static magnetic susceptibility, which has been found to be particularly sensitive for both onset and lock-in discontinuities, and by measurements on resistivity and Hall effect. Results, discussed in terms of the model of Chan and Heine and in terms of McMillan\u27s phenomenological model, suggest that structural changes due to the presence of the niobium dopant lead to enchanced electron-phonon coupling thus raising the CDW onset temperature

Band Electronic Structure of the Molybdenum Blue Bronze A0.30moo3(a = K, Rb)

The electronic structure of the blue bronze A0.30MoO3(A = K, Rb) was examined by performing tight-binding band calculations on a number of model chains and an Moi0O30slab. When normalized to A3Mo10O30(i.e., half the unit cell), the bottom two d-block bands of an Mo10O30slab are partially filled. The Fermi surfaces of these two bands are open along the interchain direction, in agreement with the experimental fact that the blue bronze is a pseudo-one-dimensional metal with good electrical conductivity along the chain direction b. The Fermi surfaces of the two bands are curved due to interactions between adjacent Mo10O32chains, but the curvatures of the Fermi surfaces are opposite for the two bands. Thus the two pieces of the first-band Fermi surface are nested to those of the second-band Fermi surface by a single wave vector qb≃ 0.75b*, which explains why only one charge density wave occurs in the blue bronze. For an Mo10O30slab, the bottom of the third d-block band is calculated to lie above, but very close to, the Fermi level (i.e., 0.012 eV above ef). This feature is responsible for the temperature dependence of qbin the blue bronze, which increases gradually from-0.72b* at room temperature to-0.75b* below the metal-to-semiconductor phase-transition temperature