L. Tóth

Gold nanoparticles: effect of treatment on structure and catalytic activity of Au/Fe2O3 catalyst prepared by co‐precipitation

Abstract1 wt% Au/Fe2O3 catalyst was prepared by a co‐precipitation method. The structure of the sample in the as prepared, oxidized and reduced states was investigated by means of X‐ray photoelectron spectroscopy (XPS), transition electron microscopy (TEM), electron diffraction (ED) and X‐ray diffraction (XRD). The structure of the samples after various treatments and their activity in the CO oxidation were compared. The results show the stability of the gold particle size during the treatments. However, after oxidation, a slight shift in the Au 4f binding energy towards lower values points to the formation of an electron‐rich state of the metallic gold particles compared to that revealed in the as prepared sample. Simultaneously, a goethite phase in the Fe2O3 support is present, which is not observed in the “as prepared” and reduced samples. In the reduced sample the presence of a crystalline maghemite‐c phase indicates a change in the support morphology. In the CO oxidation the oxidized sample shows the highest activity and it might be the result of the cooperative effect of goethite, FeO and the electron‐rich metallic gold nanoparticles. We suggest that a structural transformation occurs along the gold/support perimeter during the treatments and we propose a possible mechanism for the effect of the oxidation treatment.

Silica-supported Au nanoparticles decorated by TiO2 : Formation, morphology, and CO oxidation activity

Au-TiO(2) interface on silica support was aimed to be produced in a controlled way by use of Au hydrosol. In method A, the Au colloids were modified by hydrolysis of the water-soluble Ti(IV) bis(ammoniumlactato)dihydroxide (TALH) precursor and then adsorbed on Aerosil SiO(2) surface. In method B, Au sol was first deposited onto the SiO(2) surface and then TALH was adsorbed on it. Regular and high-resolution transmission electron microscopy (TEM and HRTEM) and energy dispersive spectrometry (EDS) analysis allowed us to conclude that, in method A, gold particles were able to retain the precursor of TiO(2) at 1.5 wt % TiO(2) loading, but at 4 wt % TiO(2) content the promoter oxide appeared over the silica surface as well. With method B, titania was detected on silica at each TiO(2) concentration. In Au-TiO(2)/SiO(2) samples, the stability of Au particles against sintering was much higher than in Au/TiO(2). The formation of an active Au-TiO(2) perimeter was proven by the greatly increased CO oxidation activity compared to that of the reference Au/SiO(2).

Growth of GaN layers onto misoriented (0001) sapphire by metalorganic chemical vapor deposition

The growth process of GaN layers grown by metalorganic chemical vapor deposition on sapphire is characterized by transmission electron microscopy and atomic force microscopy. The nitridation of the sapphire substrate and GaN buffer layers as well as film structure and the nature of defects are studied. Nitridation causes the formation of a 4 nm thick AlN layer on sapphire. GaN buffer layers grown at 510 °C are found to be hexagonal single crystals in their as-grown state with a mosaic structure. Annealing of the buffer layers leads to substantial smoothening of their surfaces due to the coalescence of the grains. GaN layers themselves are single crystalline, hexagonal, and epitaxial to the substrate. Layers grown on exactly oriented (0001) type substrate as well as on miscut substrate are compared. Smooth surfaces have been achieved on exactly oriented and on miscut substrates as well, but the range of the deposition parameters is wider when miscut substrates are used.

Sealing of hard CrN and DLC coatings with atomic layer deposition

Atomic layer deposition (ALD) is a thin film deposition technique that is based on alternating and saturating surface reactions of two or more gaseous precursors. The excellent conformality of ALD thin films can be exploited for sealing defects in coatings made by other techniques. Here the corrosion protection properties of hard CrN and diamond-like carbon (DLC) coatings on low alloy steel were improved by ALD sealing with 50 nm thick layers consisting of Al2O3 and Ta2O5 nanolaminates or mixtures. In cross sectional images the ALD layers were found to follow the surface morphology of the CrN coatings uniformly. Furthermore, ALD growth into the pinholes of the CrN coating was verified. In electrochemical measurements the ALD sealing was found to decrease the current density of the CrN coated steel by over 2 orders of magnitude. The neutral salt spray (NSS) durability was also improved: on the best samples the appearance of corrosion spots was delayed from 2 to 168 h. On DLC coatings the adhesion of the ALD sealing layers was weaker, but still clear improvement in NSS durability was achieved indicating sealing of the pinholes.

Thermal conductivity of ultrathin nano-crystalline diamond films determined by Raman thermography assisted by silicon nanowires

The thermal transport in polycrystalline diamond films near its nucleation region is still not well understood. Here, a steady-state technique to determine the thermal transport within the nano-crystalline diamond present at their nucleation site has been demonstrated. Taking advantage of silicon nanowires as surface temperature nano-sensors, and using Raman Thermography, the in-plane and cross-plane components of the thermal conductivity of ultra-thin diamond layers and their thermal barrier to the Si substrate were determined. Both components of the thermal conductivity of the nano-crystalline diamond were found to be well below the values of polycrystalline bulk diamond, with a cross-plane thermal conductivity larger than the in-plane thermal conductivity. Also a depth dependence of the lateral thermal conductivity through the diamond layer was determined. The results impact the design and integration of diamond for thermal management of AlGaN/GaN high power transistors and also show the usefulness of the nanowires as accurate nano-thermometers.

Luminescence efficiency enhancement in laser soaked hydrogenated amorphous carbon films

An unexpected effect of laser soaking was observed on the properties of annealed a‐C:H films. The photoluminescence (PL) intensity was increased up to six times and, the spectrum was blue shifted in samples exposed to an unfocused Ar+ laser beam. The time dependence of the PL intensity increase was found to be exponential up to saturation with characteristic times around 103 s. The observed changes were influenced by illumination intensity and sample properties. These results are corroborated by a PL mechanism when excitation and recombination take place on π‐bonded clusters.

Single-crystal hexagonal and cubic GaN growth directly on vicinal (001) GaAs substrates by molecular-beam epitaxy

Single-crystal hexagonal and cubic GaN thin films have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy directly on vicinal (001) GaAs substrates, misoriented by 2° toward [100], without using an incident As beam during oxide desorption or the following stages of growth. Both the GaAs nitridation and GaN growth conditions were found to control the structure of the layers. Cubic layers could be grown only without nitridation and under stoichiometric N/Ga flux ratio conditions. N-rich conditions favored the growth of hexagonal layers, which exhibited significantly higher photoluminescence intensities compared to cubic ones. Hexagonal single crystalline GaN films were grown with (1012) planes and presented characteristic surface roughness striations along a 〈110〉 substrate direction. On the contrary, a stepped surface morphology was observed for cubic GaN.

Metal-related gate sinking due to interfacial oxygen layer in Ir/InAlN high electron mobility transistors

We report on an annealing-induced “gate sinking” effect in a 2-nm-thin In0.17Al0.83N/AlN barrier high electron mobility transistor with Ir gate. Investigations by transmission electron microscopy linked the effect to an oxygen containing interlayer between the gate metal and the InAlN layer and revealed diffusion of oxygen into iridium during annealing. Below 700 °C the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700 °C increased the gate capacitance over a factor 2, shifted the threshold voltage from +0.3 to +1 V and increased the transconductance from 400 to 640 mS/mm.

Electron microscopic and AES studies on thin layers of NiCr

Abstract In order to understand the parameters affecting the properties of NiCr resistors prepared by various methods, the annealing properties of NiCr thin films were studied. TEM and electron diffraction were used to determine the structure of the films deposited onto SiOx coated microgrids, while electrical measurements and AES indepth profiling were carried out on the samples deposited under identical conditions onto silica substrates. The effect of a SiOx protective coating on the structural changes was investigated during high temperature annealing in vacuum. In the NiCr samples annealed without a protective layer the selective oxidation of chromium led to a rather metallic type of conduction, while in the protected samples the stabilization of the conduction was observed after the same annealing. By hindering oxidation and surface diffusion processes the protective layer resulted in the development of a more stable structure in the NiCr films.

Study of morphology and composition of Pt-black catalysts: I. Analytical electron microscopy of Pt-black

Abstract Sintering and impurities of Pt-black catalysts of various treatment has been followed by analtical electron microscopy. Silicon contamination of H 2 PtCl 6 has been incorporated into Pt-black reduced from it in aqueous medium. Sintering in hydrogen between 470 and 750 K produced polydisperse Pt. The silicon content decreased monotonically with increasing sintering temperature. Correlation has been found between specific surface and Si percentage. A mechanism for removal of Si from Pt has been proposed consisting of the Pt catalyzed enhancement of reduction of silicon by hydrogen resulting, ultimately in SiH 4 . The segregation of carbon above about 670 K was observed leading to a two-dimensional carbonaceous overlayer of graphite character.