Photocatalytic Nanoparticulate Zr_xTi_1‐xO₂ Coatings with Controlled Homogeneity of Elemental Composition

Abstract

The mixed oxide ZrxTi1‐xO2 nanoparticulate photocatalysts of different compositions (0≤x≤1) were prepared via sol‐gel method in a rapid micromixing reactor followed by chemical colloid deposition on borosilicate glass beads, drying and heat treatment at temperatures between 400 to 650 °C. The point‐like reaction conditions in the regime of low Damköhler numbers resulted in nucleation of size‐selected reactive zirconium‐titanium oxo‐alkoxy (ZTOA) nanoparticles from zirconium (IV) propoxide and titanium (IV) isopropoxide precursors, which enable strong covalent bonds with hydrophilic supports. The ZTOA nanoparticles with compositions 0≤x≤0.2 convert after a heat treatment to nanoporous monocrystals in distorted anatase phase. The photocatalytic activity of the prepared nanoparticulate coatings on gaseous ethylene decomposition attained maximum at x=0.043 after heat treatment at 500 °C, which was two times higher than that of pure anatase TiO2. A net correlation between the material photocatalytic activity and amplitude of the slow power decay of photoinduced charges was observed. A model is proposed explaining the power law decay by a thermal release of the localised charges from shallow surface Urbach states. The number density of these states is connected to the specific surface area of the photocatalyst.

Photocatalytic nanocomposite anatase–rutile TiO2 coating

“…5). Except for the very beginning related to the photoinduced charges thermalisation, the both curves follow the power-law decay I=At -k , which has been previously related to the relaxation of a series of localised states observed via transient absorption spectroscopy [48] and later attributed to the liberation of charges from shallow traps (Ei) of the Urbach tail (EU) to the conduction band (Eg) with power constant k expressed as k=kBT/Ei [49], where Eg, EU and Ei are band gap, Urbach and trap states energies, kB is Boltzmann constant and T is ambient temperature. According to our measurements, the decay constants of the both materials were very similar and correspond to Eg-Ei  70 meV, which is twice larger than EU at room temperature [50].…”

Mixed Metal Oxide W-TiO2 Nanopowder for Environmental Process: Synergy of Adsorption and Photocatalysis

“…A competition between an increase in the specific surface area and a decrease in the activity per adsorption site has been also confirmed in the compounds with W content ≤ 6 mol% [81]. However, the elemental composition can greatly affect the specific surface area of these materials, as it has been shown in an example of Zr x Ti 1−x O 2 nanopowders [23]. This offers an opportunity to control adsorption and photocatalysis processes in the same material.…”

“…The decay curves of the photoinduced charges in pure and W-doped TiO 2 nanopowders are shown in Figure 11. The observed signal has been previously attributed to the liberation of charges from shallow traps with binding energy E * following a power law decay I = At α with parameter α = k B T/E * , where k B is the Boltzmann constant and T is temperature [23]. The shallow states have attracted much attention over the last decade because of their key influence on the photocatalysts' activity [98,99].…”

“…There have been many attempts made to improve the photocatalytic activity of TiO 2 by changing the size of the particles, modifying the band gap energy, postponing the recombination of electrons and holes, combining it with adsorbents (e.g., activated carbon [20]), etc. As a method to improve its activity, the mixing of TiO 2 with other metal oxides, such as CuO, V 2 O 5 , ZrO 2 , Co 3 O 4 , Fe 2 O 3 , ZnO, MnO 2 and WO 3 , has been undertaken [21][22][23][24][25][26][27][28][29][30]. These inorganic materials are reuseable and highly sustainable.…”

Solvothermal sol–gel synthesis of TiO2-cellulose nanocrystalline composites

“…This is an important characteristic showing potentiality of the material application in the photocatalytic process. It can be shown that for a relatively small mass and first order process kinetics,  is proportional to the reaction rate constant R normalized on the photocatalyst mass [Cheng et al 2018;Bouslama et al 2012]. This normalized rate reflects the material effectiveness when the photocatalyst mass is relatively small and decreases at the larger masses due to the saturation of light absorbance in accordance with the Beer-Lambert law.…”