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It is becoming increasingly important to find renewable carbon-free energy sources. Harnessing the relatively large potential of solar radiation is currently the subject of considerable research. We have found a relatively strong thermal activation process for photocatalytic water splitting. Our measurements indicate an order of magnitude increase in hydrogen generation rates for a temperature rise of 50°C, suggesting an approach to practical efficiencies. Generation rates are stable over time. This process, which makes use of ultraviolet, visible, and infrared components of the solar spectrum, is analyzed via first-principles computations.Producing hydrogen via solar radiation has long 1 been a goal. Fujishima and Honda 2 showed that a TiO 2 electrode in a galvanic cell can absorb sunlight to promote H 2 production. More recently, a photocatalytic approach to produce H 2 via water splitting from ultraviolet and even visible sunlight has evolved. 3-5 Despite considerable progress, desired solar energy to H 2 fuel energy efficiencies have yet to be obtained, and processes are not always stable over time. At the same time, solar thermal 6-8 energy is being employed to produce H 2 at operating temperatures of 700-2500°C. The challenge there is to find materials that survive high temperatures and often aggressive chemical environments.We have found promising results by combining thermal and photocatalytic effects to produce H 2 . The goal is a stable process, producing H 2 generation rates high enough to be of practical interest within a noncorrosive environment and with lower operating temperatures than those of solar thermal processes. We investigated this thermal-photocatalytic process via a dispersion of CuAlO 2 catalyst nanoparticles in water and sunlight. A schematic of our apparatus is shown in Fig. 1. Natural sunlight is the source of solar radiation and the axis of the cylindrical hydrogen generation container is aligned with the sun. In this first experiment, solar heating is supplemented by electrical resistance heating to increase the water temperature. There are no electrodes in the water and hence no potential for ion currents leading to photocorrosion. In fact, as will be discussed below, the process 9 is quite stable. After typical runs of ϳ1 h to several hours, the cylinder is connected to a gas chromatograph to measure the H 2 content.CuAlO 2 catalyst powders are prepared by solid-state reaction of CuO and Al 2 O 3 , followed by grinding between each calcining step for temperatures in the range of 800-1160°C. X-ray diffraction ͑XRD͒ spectra of the final powder showed excellent agreement with the computed CuAlO 2 spectrum. The Brunauer-Emmett-Teller 10 ͑BET͒ surface area per gram of powder was found to be 22.7 m 2 / g. The helium pycnometer 11 powder mass density was found to be 5.33ϫ 10 6 g / m 3 . These two measurements imply an average particle diameter of 49.4 nm, consistent with the XRD grain size of 30-60 nm.It has been found ͑see, e.g., Refs. 12 and 13͒ that CuAlO 2 band gaps can depend on mat...

Potential automotive applications of electrorheological (ER) fluids have stimulated renewed interest in their development. The inherent simplicity of a suspension of particles in a fluid that responds to an applied electric field with a two- or three-order-of-magnitude increase in viscosity is clearly attractive. However, there are some important parameters that are not well delineated. Among them is the ER-fluid response time, which needs to approach the millisecond range for effective device operation. We report results from recent measurements of response times for glass spheres suspended in silicone oil.