Stability of mixed-oxide titanosilicates: dependency on size and composition from nanocluster to bulk.

Nanostructured titanosilicate materials based upon interfacing nano-TiO2 with nano-SiO2 have drawn much attention due to their huge potential for applications in a diverse range of important fields including gas sensing, (photo)catalysis, solar cells, photonics/optical components, tailored multi-(bio)functional supports and self-cleaning coatings. In each case it is the specific mixed combination of the two SiO2 and TiO2 nanophases that determines the unique properties of the final nanomaterial. In the bulk, stoichiometric mixing of TiO2 with SiO2 is limited by formation of segregated TiO2 nanoparticles or metastable glassy phases and more controlled disperse crystalline mixings only occur at small fractions of TiO2 (<15 wt%). In order to more fully understand the stability of nano-SiO2 and nano-TiO2 combinations with respect to composition and size, we employ accurate all-electron density functional calculations to evaluate the mixing energy in (Tix Si1-x O2 )n nanoclusters with a range of sizes (n = 2-24) having different titania molar fractions (x = 0-1). We derive all nanoclusters from a dedicated global optimisation procedure to help ensure that they are the most energetically stable structures for their size and composition. We also consider a selection of representative intimately mixed crystalline solid phase (Tix Si1-x O2 )bulk systems for comparison. In agreement with experiment, we find that homogeneous mixing of SiO2 and TiO2 in bulk crystalline phases is energetically unfavourable. Conversely, we find that SiO2 -TiO2 mixing is energetically favoured in small (Tix Si1-x O2 )n nanoclusters. Following the evolution of mixing energy with nanocluster size and composition we find that mixing is most favoured in nanoclusters with a diameter of 1 nm with TiO2 molar fractions between 0.3-0.5. Thereafter, mixed nanoclusters with increasing size have progressively less negative mixing energies up to diameters of approximately 1.5 nm. We propose some chemical-structural principles to help rationale this energetically favourable nanoscale mixing. As a guide for experimentalists to observe and characterize these mixed nano-species we also provide two measurable signatures of mixing based on their unique vibrational and structural characteristics.