Chemical vapor deposition of a thin titanium dioxide (TiO 2 ) fi lm on lightweight native nanocellulose aerogels offers a novel type of functional material that shows photoswitching between water-superabsorbent and water-repellent states. Cellulose nanofi brils (diameters in the range of 5-20 nm) with native crystalline internal structures are topical due to their attractive mechanical properties, and they have become relevant for applications due to the recent progress in the methods of their preparation. Highly porous, nanocellulose aerogels are here fi rst formed by freeze-drying from the corresponding aqueous gels. Well-defi ned, nearly conformal TiO 2 coatings with thicknesses of about 7 nm are prepared by chemical vapor deposition on the aerogel skeleton. Weighing shows that such TiO 2 -coated aerogel specimens essentially do not absorb water upon immersion, which is also evidenced by a high contact angle for water of 140 ° on the surface. Upon UV illumination, they absorb water 16 times their own weight and show a vanishing contact angle on the surface, allowing them to be denoted as superabsorbents. Recovery of the original absorption and wetting properties occurs upon storage in the dark. That the cellulose nanofi brils spontaneously aggregate into porous sheets of different length scales during freeze-drying is relevant: in the water-repellent state they may stabilize air pockets, as evidenced by a high contact angle, in the superabsorbent state they facilitate rapid water-spreading into the aerogel cavities by capillary effects. The TiO 2 -coated nanocellulose aerogels also show photooxidative decomposition, i.e., photocatalytic activity, which, in combination with the porous structure, is interesting for applications such as water purifi cation. It is expected that the present dynamic, externally controlled, organic/inorganic aerogels will open technically relevant approaches for various applications.

Atomic layer deposition (ALD) of metal selenide and telluride thin films has been limited because of a lack of precursors that would at the same time be safe and exhibit high reactivity as required in ALD. Yet there are many important metal selenide and telluride thin film materials whose deposition by ALD might be beneficial, for example, CuInSe2 for solar cells and Ge2Sb2Te5 for phase-change random-access memories. Especially in the latter case highly conformal deposition offered by ALD is essential for high storage density. By now, ALD of germanium antimony telluride (GST) has been attempted only using plasma-assisted processes owing to the lack of appropriate tellurium precursors. In this paper we make a breakthrough in the development of new ALD precursors for tellurium and selenium. Compounds with a general formula (R3Si)2Te and (R3Si)2Se react with various metal halides forming the corresponding metal tellurides and selenides. As an example, we show that Sb2Te3, GeTe, and GST films can be deposited by ALD using (Et3Si)2Te, SbCl3, and GeCl2 x C4H8O2 compounds as precursors. All three precursors exhibit a typical saturative ALD growth behavior and GST films prepared at 90 degrees C show excellent conformality on a high aspect-ratio trench structure.

Titanium dioxide thin films were grown by atomic layer deposition (ALD) at 200±400 C from a new titanium precursor, titanium tetramethoxide, and water. As compared with other titanium alkoxides studied earlier, titanium methoxide shows the highest stability with respect to thermal decomposition, and can thus be used over the widest range of temperatures. The films deposited at 250 C and above were polycrystalline with the anatase structure, whereas those deposited at 200 C were amorphous. Except for the film deposited at 200 C, the films contained only minor amounts of carbon and hydrogen residues. The crystalline films were shown to have photocatalytic activity in decomposing both methylene blue in aqueous solution, and solid stearic acid coated on the film surface.

No abstract

TiO2 replicas of filter paper with nanometer-level accuracy were prepared by atomic layer deposition of thin conformal TiO2 coating, followed by a removal of the paper by air-anneal at 450 degrees C. Photocatalytic anatase TiO2/cellulose composites were also made by leaving the paper intact. The TiO2 films were deposited from Ti(OMe)4 and H2O at 150-250 degrees C. The photocatalytic activity of the TiO2/cellulose composite was verified by photocatalytic reduction of Ag(I) from an aqueous solution to Ag nanoparticles on the TiO2 surface. The TiO2/cellulose composites are mechanically more stable than the free-standing TiO2 replicas and are therefore potentially suitable as lightweight, high surface area photocatalysts.