(283h) Aerosol Synthesis of Oxygen-Deficient Titania in a Hot-Wall Reactor

Titania is one of the most widely applied oxide materials in daily life. Its high refractive index makes it an excellent white pigment used in paintings, plastics and sunscreens among others. Another attractive property of titania is its photocatalytic activity, enabling it to disintegrate organic substances or even water splitting when exposed to sunlight [1]. During the last years attempts, have been made to increase this catalytic performance by doping with other metals, forming composites e.g. with carbon or inducing oxygen deficiencies [2]. The latter method leads to a partial reduction of the tetravalent titanium cation and the formation of a series of complex titanium oxides called Magnéli phases[3]. Their stoichiometry can be generally written as Ti_nO_2n-1 (with n between 4 and 9)and compounds like Ti₄O₇ and Ti₅O₉ belong to this group. Within this work, a one-step method to produce well-defined oxygen-deficient titania nanoparticles via a facile gas-phase process is presented and carefully evaluated.

Titanium (IV)-tetraisopropoxide (TTIP) is used as a precursor and supplied via a temperature-controlled bubbler. The steam-laden gas flow is then mixed with hydrogen or air. Furthermore, nitrogen is added to adjust the residence time in the reactor, which consists of a three-zone tube furnace. Consequently, TTIP undergoes decomposition to form titania and afterwards partial reduction. After passing the reaction zone the hot aerosol stream is quenched consequently the particles are deposited onto a fiber filter. The admixture of nitrogen into the system also allows for feeding water vapor into the system by letting it pass through a second temperature-controlled bubbler. This leads to a shift in particle morphology from nanoparticles to submicron spheres.

In this work, the degree of oxygen deficiency, which can be introduced under different process conditions, was investigated. Therefore, different reactor temperatures, residence times and hydrogen contents in the reactor were used to synthesize substoichiometric titania. The XRD results evidence that both higher temperatures and residence times increase the content of Magnéli phases whereas less anatase is produced. The onset of titania reduction occurs above 700 °C. At very high temperatures Ti₄O₇ becomes the predominant phase and, according to Rietveld refinement, or is even present as the single crystalline phase. Regarding the crystallite sizes, values range between 20 and 30 nm depending on the reactor temperature. This is in good agreement with results from scanning electron microscopy which indicates that these particles are single crystals. For elevated reactor temperatures the particles tend to be much larger which is due to strong sintering of the primary particles. Notably, this sintering is much stronger than for unreduced titanium dioxide synthesized at otherwise similar process conditions. The smallest particles are obtained between 700 °C and 800 °C which is in good agreement with [4]. A further reduction of the particle size can be achieved by a lower residence time of the aerosol in the reactor. Under optimized conditions, a decrease of x_50,0 from 46 nm to 32 nm can be achieved.

All produced powders were stable over several months at ambient conditions and even up to 80 °C. Depending on their degree of oxygen deficiency, their color varies between white and dark blue. They showed the ability to catalytically oxidize methylene blue upon UV irradiation (380 nm). Here, samples with a high degree of oxygen deficiency generally showed a higher catalytic activity compared to samples produced below the onset temperature of titania reduction. Strongly sintered, pure Ti₄O₇ was relatively weakly active under UV irradiation as it was also found in [5].

Summarizing, a one-step approach for the gas-phase synthesis of titania with a variable degree of oxygen-deficiency was demonstrated. A precise control of the process parameters enables fine tuning of the produced particles’ properties and can lead to production of promising materials for future applications in the field of photocatalysis.

This work was supported by the German Research Council (DFG) and the Cluster of Excellence “Engineering of Advanced Materials” (EAM).

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