Most transport fuels are derived from fossil fuels, generate greenhouse gases, and consume significant amounts of
water in the extraction, purification, and/or burning processes. The generation of hydrogen using solar energy to split
water, ideally from abundant water sources such as sea water or other non-potable sources, could potentially provide an
unlimited, clean fuel for the future. Solar, electrochemical water splitting typically combines a photoanode at which
water oxidation occurs, with a cathode for proton reduction to hydrogen. In recent work, we have found that a
bioinspired tetra-manganese cluster catalyzes water oxidation at relatively low overpotentials (0.38 V) when doped into
a Nafion proton conduction membrane deposited on a suitable electrode surface, and illuminated with visible light. We
report here that this assembly is active in aqueous and organic electrolyte solutions containing a range of different salts
in varying concentrations. Similar photocurrents were obtained using electrolytes containing 0.0 - 0.5 M sodium
sulfate, sodium perchlorate or sodium chloride. A slight decline in photocurrent was observed for sodium perchlorate
but only at and above 5.0 M concentration. In acetonitrile and acetone solutions containing 10% water, increasing the
electrolyte concentration was found to result in leaching of the catalytic species from the membrane and a decrease in
photocurrent. Leaching was not observed when the system was tested in an ionic liquid containing water, however, a
lower photocurrent was generated than observed in aqueous electrolyte. We conclude that immersion of the membrane
in an aqueous solution containing an electrolyte concentration of 0.05 - 0.5M represent good conditions for operation
for the cubium/Nafion catalytic system.
In previous work we have demonstrated that selective masking, or modulation, of digital images can be used to create documents and transparent media containing covert or optically variable, overt images. In the present work we describe new applications and techniques of such "modulated digital images" (MDI's) in document security. In particular, we demonstrate that multiple hidden images can be imperceptibly concealed within visible, host images by incorporating them as a new, half-tone, printing screen. Half-toned hidden images of this type may contain a variety of novel features that hinder unauthorized copying, including concealed multiple images, and microprinted-, color-, and various fadeeffects. Black-and-white or full color images may be readily used in this respect. We also report a new technique for the embossing of multiple, covert- or optically variable, overt-images into transparent substrates. This method employs an embossing tool that is prepared using a combination of electron beam and greytone lithography. Two approaches may be used: (i) a double-sided "soft" emboss into curable, transparent, lacquer layers, and (ii) a single-sided "hot" emboss in which multiple, dithered images consisting of distinctly-sloped microprisms are impressed into the substrate. Technique (ii) requires a novel, electron-beam-originated master dye.
There are, in general, two ways for an observer to deal with light that is incorrect in some way (e.g. which is partially out of focus). One approach is to correct the error (e.g. by using a lens to selectively bend the light). Another approach employs selective masking to block those portions of the light which are unwanted (e.g. out of focus). The principle of selective masking is used in a number of important industries. However it has not found widespread application in the field of optical security devices. This work describes the selective masking, or modulation, of digital images as a means of creating documents and transparent media containing overt or covert biometric and other images. In particular, we show how animation effects, flash-illumination features, color-shifting patches, information concealment devices, and biometric portraiture in various settings can be incorporated in transparent media like plastic packaging materials, credit cards, and plastic banknotes. We also demonstrate the application of modulated digital images to the preparation of optically variable diffractive foils which are readily customized to display biometric portraits and information. Selective masking is shown to be an important means of creating a diverse range of effects useful in authentication. Such effects can be readily and inexpensively produced without the need, for example, to fabricate lenses on materials which may not be conducive in this respect.