Photocatalytic Reactions on Model Single Crystal TiO2 Surfaces

Abstract

Light from the sun is by far the most abundant source of energy on earth. Yet, at present, less than 0.05% of the total power (15 000 GW annual) used by humans is generated from the sun (excluding solar heating, which contributes around 0.6%). The estimated practical and convertible power that the earth surface receives is equivalent to that provided by 600 000 nuclear reactors (one nuclear power plant generates, on average, 1 GW power) or about 40 times the present global need. 1 One mode of solar energy utilization is the use of sunlight to generate energy carriers, such as hydrogen, from renewable sources (e.g., ethanol and water) using semiconductor photocatalysts.The photoassisted splitting of water into hydrogen and oxygen was first achieved by Fujushima and Honda [1], who showed that hydrogen and oxygen could be generated in an electrochemical cell containing a titania photoelectrode, provided an external bias was applied. Since that time, numerous researchers have explored ways of achieving direct water dissociation without the need for an external bias. Much work has been conducted, a large fraction of which is discussed in a recent review [2]. Among the many issues affecting direct water splitting is the need to separate hydrogen from oxygen and the relatively low hydrogen evolution rates so far achieved. These, in addition to the need for using UV light (>3eV) to excite TiO 2 and other related materials, has been one of the main obstacles for practical applications. Many authors have sought modified photocatalysts which, unlike pure TiO 2 , respond to visible (sunlight) excitation, with limited success to date; see some of these materials in ref.[2]. 1 The total amount of sunlight reaching the earth surface is orders of magnitude higher than the quoted figure.