Influence of Aqueous Precursor Chemistry on the Growth Process of Epitaxial SrTiO₃ Buffer Layers

In this Article, epitaxial thin films of SrTiO₃ were prepared on single crystalline (100) LaAlO₃ by an aqueous chemical solution deposition method. By using different chelating agents to stabilize the metal ions in water, the impact of the precursor chemistry on the microstructural and crystalline properties of the films was studied. Thorough investigation of the precursor by means of infrared and Raman spectroscopy as well as thermogravimetric analysis revealed that stable precursors can be obtained in which strontium ions can be either free in the solution or stabilized by one of the chelating agents. This stabilization of strontium ions appeared to be essential in order to obtain single phase SrTiO₃ films. Precursors in which Sr²⁺ remained as free ions showed SrO microcrystal segregation. Precursors in which both metal ions were stabilized gave rise to strongly textured, dense, and terraced SrTiO₃ films, allowing subsequent deposition of YBa₂Cu₃O_7‑δ with superior superconducting performances

Direct Conversion of Syngas to Olefins over a Hybrid CrZn Mixed Oxide/SAPO-34 Catalyst: Incorporation of Dopants for Increased Olefin Yield Stability

A bifunctional catalyst was developed utilizing a physical mixture of a CrZn-based mixed metal oxide and zeotype SAPO-34 for the direct conversion of syngas to short-chain olefins. A series of promoted CrZn-M (M = Fe, Ga, Al) mixed oxide catalysts were synthesized by coprecipitation and calcined at different temperatures. CrZn-Fe-SAPO-34 catalysts calcined at 400 °C selectively converted syngas to C2–C4 olefins, while maintaining high CO conversion and olefin stability over time. The high olefin yield is ascribed to the stabilization effect of iron on inversed spinel phase ZnCr2O4 and to reduction of the detrimental ZnO phase formed during syngas conditions. At a higher calcination temperature of 600 °C, the stabilization effect is less pronounced. Ga and Al-doped CrZn oxides enabled high and stable olefin selectivity of the hybrid catalysts CrZn-Ga-SAPO-34 and CrZn-Al-SAPO-34, regardless the applied calcination temperature. Spectroscopy analysis demonstrated that these promoters are able to scavenge free ZnO formed on the catalyst, thus stabilizing the inversed spinel. This work demonstrates that a rational design of mixed metal oxide components of the hybrid catalyst process is required to maximize olefin yield and catalyst stability. The selection of dopants capable of stabilizing an inversed spinel phase and scavenging detrimental ZnO is a critical step in successful catalyst design

Highly Crystalline Nanoparticle Suspensions for Low-Temperature Processing of TiO₂ Thin Films

In this work, we present preparation and stabilization methods for highly crystalline TiO2 nanoparticle suspensions for the successful deposition of transparent, photocatalytically active TiO2 thin films toward the degradation of organic pollutants by a low temperature deposition method. A proof-of-concept is provided wherein stable, aqueous TiO2 suspensions are deposited on glass substrates. Even if the processing temperature is lowered to 150–200 °C, the subsequent heat treatment provides transparent and photocatalytically active titania thin layers. Because all precursor solutions are water-based, this method provides an energy-efficient, sustainable, and environmentally friendly synthesis route. The high load in crystalline titania particles obtained after microwave heating opens up the possibility to produce thin coatings by low temperature processing, as a conventional crystallization procedure is in this case superfluous. The impact of the precursor chemistry in Ti4+-peroxo solutions, containing imino-diacetic acid as a complexing ligand and different bases to promote complexation was studied as a function of pH, reaction time and temperature. The nanocrystal formation was followed in terms of colloidal stability, crystallinity and particle size. Combined data from Raman and infrared spectroscopy, confirmed that stable titanium precursors could be obtained at pH levels ranging from 2 to 11. A maximum amount of 50.7% crystallinity was achieved, which is one of the highest reported amounts of anatase nanoparticles that are suspendable in stable aqueous titania suspensions. Decoloring of methylene blue solutions by precipitated nanosized powders from the TiO2 suspensions proves their photocatalytic properties toward degradation of organic materials, a key requisite for further processing. This synthesis method proves that the deposition of highly crystalline anatase suspensions is a valid route for the production of photocatalytically active, transparent films on heat-sensitive substrates such as polymers

Support Effect and Surface Reconstruction in In₂O₃/<i>m-</i>ZrO₂ Catalyzed CO₂ Hydrogenation

We investigate the chemical and structural dynamics at the interface of In2O3/m-ZrO2 and their consequences on the CO2 hydrogenation reaction (CO2HR) under reaction conditions. While acting to enrich CO2, monoclinic zirconia (m-ZrO2) was also found to serve as a chemical and structural modifier of In2O3 that directly governs the outcome of the CO2HR. These modifying effects include the following: (1) Under reaction conditions (above 623 K), partially reduced In2O3, i.e., InOx (0 x < 1.5), was found to migrate in and out of the subsurface of m-ZrO2 in a semireversible manner, where m-ZrO2 accommodates and stabilizes InOx by serving as a reservoir. The decreased concentration of surface InOx under elevated temperatures coincides with significantly decreased selectivity toward methanol and a sharp increase of the reverse water–gas shift reaction. The reconstruction-induced variation of InOx concentration appears to be one of the most important factors contributing to the altered catalytic performance of CO2HR at different reaction conditions. (2) The strong interactions and reactions between m-ZrO2 and In2O3 result in the activation of a pool of In–O bonds at the In2O3/m-ZrO2 interface to form oxygen vacancies. On the other hand, the high dispersity of In2O3 nanostructures onto m-ZrO2 prevents their over-reduction under catalytically relevant conditions (up to 673 K), when bare In2O3 is unavoidably reduced into the metallic phase (In0). The relationship between the extent of reduction of In2O3 and catalytic performance (CO2 conversion, CH3OH selectivity, or yield of CH3OH) suggests the presence of an optimum coverage of surface InOx and oxygen vacancies under reaction conditions. The conventional model that links catalytic performance solely to the coverage of oxygen vacancies appears invalid in the present case. In situ analysis also allows the observation of surface reaction intermediates and their interconversions, including the reduction of CO3* into formate, a precursor for the formation of methanol and CO. The combinative ex situ and in situ study sheds light on the reaction mechanism of the CO2HR on In2O3/m-ZrO2-based catalysts. Our findings on the large-scale surface reconstructions, support effect, and the reaction mechanism of In2O3/m-ZrO2 for CO2HR may apply to other related metal oxide catalyzed CO2 reduction reactions