M. A. Butler

Photoelectrolysis and physical properties of the semiconducting electrode WO2

The behavior of semiconducting electrodes for photoelectrolysis of water is examined in terms of the physical properties of the semiconductor. The semiconductor‐electrolyte junction is treated as a simple Schottky barrier, and the photocurrent is described using this model. The approach is appropriate since large‐band‐gap semiconductors have an intrinsic oxygen overpotential which removes the electrode reaction kinetics as the rate‐limiting step. The model is successful in describing the wavelength and potential dependence of the photocurrent in WO3 and allows a determination of the band gap, optical absorption depth, minority‐carrier diffusion length, flat‐band potential, and the nature of the fundamental optical transition (direct or indirect). It is shown for WO3 that minority‐carrier diffusion plays a limited role in determining the photoresponse of the semiconductor‐electrolyte junction. There are indications that the diffusion length in this low carrier mobility material is determined by diffusion‐c...

Optical fiber hydrogen sensor

A new type of hydrogen sensor is reported in which an optical fiber is used as the sensing element. The fiber is coated with palladium which expands on exposure to hydrogen. This changes the effective optical path length of the fiber, which is detected by interferometric techniques. Preliminary experiments have demonstrated the effect and suggest a high sensitivity and a wide dynamic range for this kind of sensor. The experimental results are compared to calculated optical path length changes. Application of this kind of sensor to the detection of other chemicals appears feasible.

Micromirror optical-fiber hydrogen sensor

Abstract In a recent letter (M.A. Butler, J. Electrochem. Soc., 138 (1991) L46) a new hydrogen sensor was reported that operates by measuring changes in the reflectivity of thin palladium films deposited on the end of an optical fiber when exposed to hydrogen gas. In this paper we explore in more detail the mechanisms of operation of this sensor. Two effects are observed: a reversible micro-blistering of the films on exposure to hydrogen and changes in optical constants with hydriding. Use of a thin nickel layer under the palladium to improve adhesion results in large changes in optical response. These changes are attributed to clamping effects on the film.

The photoelectrolysis of water using iron titanate anodes

Single‐crystal and polycrystalline samples of the iron titanates FeTiO3, Fe2TiO4, and Fe2TiO5 have been studied as anodes for the photoelectrolysis of water. Band gaps and flatbands for the various compositions have been determined and aging studies have been conducted which indicate electrochemical leaching of iron atoms from the surface.

Hydrogen sensing with palladium-coated optical fibers

Palladium‐coated, single‐mode optical fibers have been used to detect hydrogen in a concentration range 20 parts per billion to 2% in 1 atm of nitrogen. When the coated fibers are exposed to hydrogen, a hydride is formed with an expanded lattice constant, which stretches the optical fiber. This change in optical path length is measured using a Mach–Zehnder interferometer. A simple one‐dimensional model is presented and shown to accurately reproduce the fiber response. Using electrodeposited palladium films, a factor of 7 enhancement in the hydrogen solubility is observed, as well as a deeply bound site which is saturated in the parts per million concentration range. The data suggest that this site may be due to hydrogen binding to a free surface of the palladium.

Photoelectrolysis with YFeO3 electrodes

YFeO3 is shown to work as a photoanode for photoelectrolysis of water. Conduction is n type as indicated by its use as an anode. The optical band gap is an indirect transition of 2.6 eV, somewhat larger than in Fe2O3. However, the smaller electron affinity of YFeO3 means that it requires less bias for photoelectrolysis than Fe2O3.

Gas sensing with acoustic devices

A survey is made of acoustic devices that are suitable as gas and vapor sensors. This survey focuses on attributes such as operating frequency, mass sensitivity, quality factor (Q), and their ability to be fabricated on a semiconductor substrate to allow integration with electronic circuitry. The treatment of the device surface with chemically-sensitive films to detect species of interest is discussed. Strategies for improving discrimination are described, including sensor arrays and species concentration and separation schemes. The advantages and disadvantages of integrating sensors with microelectronics are considered, along with the effect on sensitivity of scaling acoustic gas sensors to smaller size.

Flexural plate wave resonator excited with Lorentz forces

A flexural plate wave resonator was constructed by patterning current lines on a silicon nitride membrane suspended on a rectangular silicon frame. Eigenmodes of the rectangular membrane were excited using Lorentz forces generated between alternating surface currents and a static in-plane magnetic field. Preferential coupling to a particular membrane mode was achieved by positioning current lines along longitudinal mode antinodes. An equivalent-circuit model was derived which characterizes the input impedance of a one-port device and the transmission response of a two-port device over a range of frequencies near a single membrane resonance. Experiments were performed to characterize the device’s response to changes in dc magnetic field strength, ambient gas composition, gas pressure, and input power.

The Mars Oxidant experiment (MOx) for Mars '96

The MOx instrument was developed to characterize the reactive nature of the martian soil. The objectives of MOx were: (1) to measure the rate of degradation of organics in the martian environment; (2) to determine if the reactions seen by the Viking biology experiments were caused by a soil oxidant and measure the reactivity of the soil and atmosphere: (3) to monitor the degradation, when exposed to the martian environment, of materials of potential use in future missions; and, finally, (4) to develop technologies and approaches that can be part of future soil analysis instrumentation. The basic approach taken in the MOx instrument was to place a variety of materials composed as thin films in contact with the soil and monitor the physical and chemical changes that result. The optical reflectance of the thin films was the primary sensing-mode. Thin films of organic materials, metals, and semiconductors were prepared. Laboratory simulations demonstrated the response of thin films to active oxidants.

Hg adsorption on optically thin Au films

Monitoring the reflectivity of optically thin Au films on the ends of multimode optical fibers during exposure to Hg vapor provides a new means of determining the nature and extent of chemisorption and reaction of Hg with Au. Upon Hg vapor exposure, a freshly deposited Au film shows an initial increase in reflectivity, followed after some time by a substantial reflectivity decrease. The initial reflectivity increase is a result of the chemisorption of Hg on the Au, augmenting the film’s optical thickness. The subsequent reflectivity decrease, which is observed only for saturated or near‐saturated Hg vapor concentrations (p/psat >0.1), is a consequence of amalgamation of the Au by the Hg. The amalgamation process, which begins at grain boundaries when multilayers of Hg exist on the surface of the Au, results in the formation of large voids in the Au film. Energy‐dispersive x‐ray fluorescence spectroscopy shows the resulting amalgam to have the approximate composition Au2 Hg3. Reflectivity measurements have...