In this paper, we demonstrate the use of continuous hydrothermal flow synthesis (CHFS) technology to rapidly produce a library of 56 crystalline (doped) zinc oxide nanopowders and two undoped samples, each with different particle properties. Each sample was produced in series from the mixing of an aqueous stream of basic zinc nitrate (and dopant ion or modifier) solution with a flow of superheated water (at 450 °C and 24.1 MPa), whereupon a crystalline nanoparticle slurry was rapidly formed. Each composition was collected in series, cleaned, freeze-dried, and then characterized using analytical methods, including powder X-ray diffraction, transmission electron microscopy, Brunauer-Emmett-Teller surface area measurement, X-ray photoelectron spectroscopy, and UV-vis spectrophotometry. Photocatalytic activity of the samples toward the decolorization of methylene blue dye was assessed, and the results revealed that transition metal dopants tended to reduce the photoactivity while rare earth ions, in general, increased the photocatalytic activity. In general, low dopant concentrations were more beneficial to having greater photodecolorization in all cases.

Continuous hydrothermal flow synthesis (CHFS) has been used as a rapid and clean, synthetic method to produce a range of crystalline nanoparticles in the Ti-Zn-O binary system. The nanopowders were prepared from aqueous solutions of titanium(IV) bis(ammonium lactato)dihydroxide (TIBALD) and hydrated zinc nitrate, respectively, using a CHFS reactor which uses superheated water (at 400 C and 24.1 MPa) as a reagent and crystallizing medium. The resulting nanopowders were heat-treated at 850 C for five hours in air to give photoactive semiconductor mixtures of rutile and zinc oxide and/or zinc titanates. The as-prepared powders and their corresponding heat-treated mixed phase photocatalysts were characterized using powder X-ray synchrotron diffraction, transmission electron microscopy, BET surface area measurement, X-ray photoelectron spectroscopy and UV-Vis spectrophotometry. Because of the interest for these materials in UVA and UVB attenuating materials, the UV-Vis profiles of the nanocomposites and solid solutions were studied. Photocatalytic activity of the samples towards the decolourisation of methylene blue dye was also assessed.

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A triggered oxygen indicator, formulated from a combination of electrochrome, titanium dioxide and EDTA, was evaluated for use in food packaging. Methylene blue was not an ideal electrochrome due to its slow reduction to the leuco form and fast subsequent oxidation by oxygen present at low concentrations, <0.5%. Polyviologen electrochromes showed much faster reduction after exposure to UV light.Thionine and 2,2'-dicyano-1,1'-dimethylviologen dimesylate, which have more anodic reduction potentials compared to methylene blue, can be used to produce oxygen indicators with decreased sensitivity to oxygen. These indicators can be used to detect oxygen even when levels increase up to 4.0%.

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