Synthesis of TiO₂ Nanocoral Structures in Ever-Changing Aqueous Reaction Systems

Abstract

A far-from-equilibrium strategy is developed to synthesize coral-like nanostructures of TiO(2) on a variety of surfaces. TiO(2) nanocoral structures consist of anatase base film and rutile nanowire layers, and they are continuously formed on substrates immersed in aqueous TiOSO(4)-H(2)O(2). The sequential deposition of TiO(2) starts with hydrolysis and condensation reactions of titanium peroxocomplexes in the aqueous phase, resulting in deposition of amorphous film. The film serves as adhesive interface on which succeeding growth of rutile nanowires to occur. This initial deposition reaction is accompanied by shift in pH of the reaction media, which is favorable condition for the growth of rutile nanocrystals. During the growth of rutile nanocoral layers, the amorphous base films are transformed to anatase phase. These sequential deposition reactions occur at temperatures as low as 80 °C, and the mild synthetic condition allows the use of a wide range of substrates such as ITO (indium tin oxide), glass, and even organic polymer films. The thickness of nanocoral layer is controllable by repeating the growth reaction of rutile nanocorals. TiO(2) nanocorals show photocatalytic activity as demonstrated by site-specific reduction of Ag(I) ions, which proceeds preferentially on the rutile nanowire layer. The rutile nanowire layer also shows photocatalytic decomposition of acetaldehyde, which is promoted upon increase of the thickness of the nanowire layer. The use of temporally transforming reaction media allows the formation of biphasic TiO(2) nanocoral structures, and the concept of nonequilibrium synthetic approach would be widely applicable to developing structurally graded inorganic nanointerfaces.

TiO2 Nanocoral Structures as Versatile Substrates for Surface‐Assisted Laser Desorption/Ionization Mass Spectrometry

“…TNCs were prepared by a previously reported low‐temperature (<100 °C) one‐pot method . Field‐emission scanning electron microscopy (SEM) images of TNCs grown on Si wafers are shown in Figure a–b.…”

“…Such reusability originates from two characteristics of the TNC substrates: super‐hydrophilicity and high hardness. The TNCs on the Si wafer show super‐hydrophilicity without UV irradiation due to the capillary effect exerted by their structures . Thus, droplets of water quickly spread, and water penetrated through the surface of TNC structures, resulting in rapid and simple removal of angiotensin II.…”

“…50 nm, and laser irradiation penetrates the TiO 2 layer. The surface of the transparent FTO film became translucent after TiO 2 growth, indicating that visible light penetrates TNCs . The holes in the n ‐Si diffuse to the interface of the n ‐Si and anatase film layers and combine with electrons from the anatase layer.…”

“…A 2 mL sample solution (0.5 mL×4 times) containing a target analyte (10 mM) was spotted on the surface of TNC substrates and the sample plate was dried under reduced pressure. Droplets of the sample solutions spread on the surface of TNC substrates due to the capillary effect exerted by nanocoral structures . SALDI mass spectrometry was performed using a SHIMADZU, AXIMA Confidence TOF mass spectrometer with a pulsed nitrogen laser (337 nm).…”

“…We recently reported a simple synthetic route to hierarchical TiO 2 nanoarrays with a high surface area, TiO 2 nanocorals (TNCs) . These consist of a bilayer of anatase base film with arrayed rutile nanowire layers.…”