Group 13 Precursor Structures and Their Effect on Oxide Nanocrystal Formation

Abstract

Precursor structures (PSs) in solution are expected to influenceb oth nanocrystal formation mechanisms, as well as crystallization of specific polymorphs. Herein, Group 13 PS structures determined by paird istribution functiona nd extended X-ray absorption fine structure analysis are reported.C orner-sharing octahedral dimers form from the metal nitrates dissolved in either water,i sopropanol, or ethanol at room temperature contradicting previous studies that suggested monomers or larger Keggin clusters. Because all crystalline indium oxides have octahedral coordination, crystalsc an easily nucleate from the observed PSs. Similarly,M OOH (M = Al and Ga) with octahedral Mc oordination is expected to form readily from the PSs, whereas formation of g-M 2 O 3 requires ap artial conversion to tetrahedral Mcoordination. This explains the long-standing observation of initial AlOOHf ormation as ab ottleneck for g-Al 2 O 3 synthesis. Different indium polymorphs crystallize from the variouss olvents, and thus there is no obvious link between the PSs and observed polymorphism.The Group 13 oxides are extremely important in technological applications. [1] Indeed, huge amounts of g-Al 2 O 3 are produced every month worldwide as catalyst support material, watertreatment agent, or image-enhancementa dditive, [1a, 2] whereas Ga 2 O 3 and In 2 O 3 are essential materials, for example, in the semiconductor industry. [1b,c] Al, Ga, and In form relatedo xides and hydroxides, such as M(OH) 3 ,MOOH and M 2 O 3 ,but the individual systems exhibit complex polymorphism with many different crystal structures. In general, the physicala nd chemical properties are controlled not only by the exact crystals tructure, but also the crystal morphology and size, [1a] and it is challenging to achieve control of all these characteristics. For all three systems, it is possible to calcine MOOH to form various M 2 O 3 polymorphs, but for industrial applications,i ti sd esirable to avoid the energy-consuming calcination step.M oreover,t he resultingm orphologya nd size of the M 2 O 3 crystals typically reflect the intermediate MOOH, and in addition high-temperatures fosters crystalliteg rowth, which further limits the control. Thus,d irect synthesis pathways are targeted. This wasa chieved by Lock et al. [3] andN oguchi et al. [4] who reported onestep synthesis of g-Al 2 O 3 (withint he time resolution of the experiments) at very specific solvent conditions or at very high reactiont emperature.C orrelation between the water content/ solventa nd the resulting oxide phase has also been reported for the In system. [5] However, to controla nd further develop Group13 oxide synthesis, atomics cale understandingo ft he crystal formation processes is required.During the last decade it has becomei ncreasingly clear that classical crystal nucleation theory established for supersaturated solutions [6] and homogenous melts [7] is too simplistic in relation to the complex chemical solutione nvironments presenti n formation of many modernm aterials. [8] Understan...

Pair distribution function and 71Ga NMR study of aqueous Ga3+ complexes

“…It appears that on balance the best model has eight oxygen atoms, but since the system is highly dynamic it is reasonable to expect a significant range of solvent molecules. The dimer as reported by Sommer et al 17 gives a narrow signal for the 4 Å peak, which is similar to the crystal model for the monomer (Figure S4b). For a better description refinement of atomic positions was done…”

“…An octahedral dimer model has previously been suggested for this feature, which describes both the distances and the intensity ratio of the 1.95 and 4 Å peaks when refined (Figure 3a). 17 We have reanalysed the dimer model, and significantly better fits of the 4 Å peak can be obtained if the atoms of the dimer are allowed to relax where R w changes from 0.499 for the model published by Sommer et al 17 to 0.412 for a relaxed model (see supporting information Figure S3 and S4). However, the improved dimer model comes at the expense of increasing the number of refined parameters.…”

“…Building on pioneering early PDF analysis, [14][15][16] in recent years a range of solution structures have been refined with atomic resolution using these techniques. 4,17,18 These advances inspired us to revisit central, and previously characterised solution systems, to provide updated structural information. Here we re-examine the aqueous Ga 3+ system using X-ray TS to unravel its complex solution chemistry.…”

“…Such understanding rests on the structure of the ions in solution, sometimes coined the Precursor Structure (PS), since that is the origin point of the formation reaction. 17,24 Aqueous Ga 3+ chemistry is also interesting for medical applications as both radioactive and stable gallium nitrates are used for diagnostic and therapeutic purposes in cancer and bone diseases, while other gallium compounds have anti-inflammatory and antimicrobial effects. 25,26 Hence, the aqueous chemistry of Ga 3+ must be understood on a structural level including the effects of concentration, pH and counter ions.…”

There's no place like real-space: elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

“…[203][204][205][206] In situ PDF studies, on the other hand, allow for identication and characterization of disordered or amorphous intermediate structures and species in solution, which are important in nanomaterials. For example, in studies of nanomaterial synthesis, in situ PDF analysis gives the possibility for studying the atomic structure of the species present before, during and aer crystallization of nanoparticles, 183,[207][208][209][210][211][212][213][214][215][216][217][218][219] which is essential for gaining a deeper understanding of material nucleation processes, where atomic scale mechanistic information generally is scarce. Knowledge of the relation between chemical synthesis and the resulting atomic structure of the material is crucial in the advancement of nanoscience.…”