Abstract
The synthesis of nanostructured sub-microspheres of TiO2 anatase with hierarchical nano- and mesoporosity was successfully achieved by using an innovative approach that applies the principles of acidic digestion to microwave (MW) solvothermal synthesis. This process, termed flash microwave-assisted solvothermal (FMS) synthesis, facilitates the formation of spherical particles without surfactants or templating agents, exploiting the rapid reaction kinetics engendered by MW heating. Unlike many other MW-assisted solvothermal methods, the application of constant MW power leads to a rapid increase of the autogenous pressure, inducing burst-nucleation of small primary crystallites and subsequent rapid agglomeration into secondary particles, with reaction times reduced to minute-timescales. The use of non-aqueous polar solvents such as ethanol is key to the production of regular spheres with a narrow size distribution, composed of nanocrystallites. Morphology, porosity, specific surface area, phase composition, crystallite size and optical properties of the particles can be controlled via a judicious selection of physical and chemical synthesis parameters, especially precursor choice and acid concentration. The complex structure of the particles leads to surface areas of up to ca. 500 m2 g−1 with intergranular mesoporosity. The as-synthesised FMS particles show increased adsorption under dark conditions and selective de-ethylation of rhodamine B under visible light compared to a commercial photocatalyst (Degussa P25). The photodegradation mechanism hinges on the capacity of the spheres to accept electrons from the photoexcited state of molecules at the particle surface, with the large sphere surface area maximising adsorption capacity and improving the efficiency of the photocatalytic processes. The singular characteristics and properties of the particles could pave the way for further applications in water purification and optoelectronic devices.
Highlights
Titanium dioxide (TiO2) is one of the most versatile materials available with everyday essential uses[1,2,3] to very speci c technological applications in photocatalysis, water splitting and energy conversion devices such as dye-sensitized solar cells (DSSCs).[4]
We propose a “Flash Microwave Solvothermal (FMS)” route to synthesise spherical mesoporous microspheres
Calcining the flash microwaveassisted solvothermal (FMS)–TiO2 led to narrower peaks in the Powder X-ray diffraction (PXD) patterns and improved the sample crystallinity
Summary
Titanium dioxide (TiO2) is one of the most versatile materials available with everyday essential uses (as a pigment/additive in sunscreen, paints and toothpaste)[1,2,3] to very speci c technological applications in photocatalysis, water splitting and energy conversion devices such as dye-sensitized solar cells (DSSCs).[4] Due to its low toxicity and high chemical and thermal stability, TiO2 has gained favour in environmental applications; notably in gas and water puri cation.[5]. Microspheres produced by the self-assembly of nanoparticles (NPs) are useful mesoporous materials with. O en the main challenge in the use of micron-scale particles is to prevent reduction of surface area and, for example, a corresponding loss of photocatalytic efficiency. Mesopores increase the speci c surface area and the density of reactive sites, which are. An improvement of photoactivity can be achieved in the presence of macropores, which provide a light-transfer path for the distribution of photon energy.[14]
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