Ranjith Karuparambil Ramachandran

Bio: Ranjith Karuparambil Ramachandran is an academic researcher from Ghent University. The author has contributed to research in topics: Atomic layer deposition & Thin film. The author has an hindex of 15, co-authored 41 publications receiving 629 citations.

Independent tuning of size and coverage of supported Pt nanoparticles using atomic layer deposition.

Abstract: Synthetic methods that allow for the controlled design of well-defined Pt nanoparticles are highly desirable for fundamental catalysis research. In this work, we propose a strategy that allows precise and independent control of the Pt particle size and coverage. Our approach exploits the versatility of the atomic layer deposition (ALD) technique by combining two ALD processes for Pt using different reactants. The particle areal density is controlled by tailoring the number of ALD cycles using trimethyl(methylcyclopentadienyl)platinum and oxygen, while subsequent growth using the same Pt precursor in combination with nitrogen plasma allows for tuning of the particle size at the atomic level. The excellent control over the particle morphology is clearly demonstrated by means of in situ and ex situ X-ray fluorescence and grazing incidence small angle X-ray scattering experiments, providing information about the Pt loading, average particle dimensions, and mean center-to-center particle distance. The performance of supported nanoparticle catalysts is closely related to their size, shape and interparticle distance. Here, the authors introduce an atomic layer deposition-based strategy to independently tune the size and coverage of platinum nanoparticles with atomic-level precision.

Low-Temperature Atomic Layer Deposition of Platinum Using (Methylcyclopentadienyl)trimethylplatinum and Ozone

Abstract: Thermal atomic layer deposition (ALD) of platinum is usually achieved using molecular oxygen as the reactant gas and deposition temperatures in the 250–300 °C range. In this work, crystalline thin films of metallic Pt have been grown by ALD at temperatures as low as 100 °C using (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3) as the Pt precursor and ozone as the reactant gas. The novel process is characterized by a constant growth rate of 0.45 A per cycle within the 100–300 °C temperature window. The Pt films are uniform with low impurity levels and close-to-bulk resistivities even at the lowest deposition temperature. We show that the initial growth on SiO2 surfaces is nucleation-controlled and islandlike and demonstrate the good conformality of the low-temperature ALD process by Pt deposition on anodic alumina nanopores and mesoporous silica thin films.

Plasma enhanced atomic layer deposition of Ga2O3 thin films

Abstract: Amorphous Ga2O3 thin films have been grown on SiO2/Si substrates by atomic layer deposition (ALD) using tris (2,2,6,6-tetramethyl-3,5-heptanedionato) gallium(III) [Ga(TMHD)3] as a gallium source and O2 plasma as reactant. A constant growth rate of 0.1 A per cycle was obtained in a broad temperature range starting from 100 to 400 °C. X-ray photoelectron spectroscopy (XPS) analysis revealed stoichiometric Ga2O3 thin films with no detectable carbon contamination. A double beam – double monochromator spectrophotometer was used to measure the transmittance of Ga2O3 thin films deposited on a quartz substrate and analysis of the adsorption edge yielded a band gap energy of 4.95 eV. The refractive index of the Ga2O3 films was determined from spectroscopic ellipsometry measurements and found to be 1.84 at a wavelength of 632.8 nm. Atomic force microscopic (AFM) analysis showed surface roughness values of 0.15 and 0.51 nm for films deposited at 200 and 400 °C, respectively. Finally, all the films could be crystallized into a monoclinic β-Ga2O3 crystal structure by a post deposition annealing in He as indicated by X-ray diffraction (XRD) measurements.

Hydrogen Evolution at the Buried Interface between Pt Thin Films and Silicon Oxide Nanomembranes

Abstract: This paper reports the performance of hydrogen evolution reaction (HER) electrocatalysts based on Pt thin film electrodes that are encapsulated by silicon oxide (SiOx) nanomembranes. This membrane-coated electrocatalyst (MCEC) architecture offers a promising approach to enhancing electrocatalyst stability while incorporating advanced catalytic functionalities such as poison resistance and tunable reaction selectivity. Herein, a room-temperature ultraviolet (UV) ozone synthesis process was used to systematically control the thickness of SiOx overlayers with nanoscale precision and evaluate their influence on the electrochemically active surface area (ECSA) and HER performance of the underlying Pt thin films. Through detailed characterization of the physical and electrochemical properties of the SiOx-encapsulated electrodes, it is shown that proton and H2 transport occur primarily through the SiOx coating such that the HER takes place at the buried Pt|SiOx interface. Increasing the thickness of the SiOx ove...

Plasma enhanced atomic layer deposition of Fe2O3 thin films

Abstract: Atomic layer deposition of Fe2O3 is generally performed at temperatures above 350 °C. In this work, Fe2O3 thin films were deposited by remote plasma enhanced atomic layer deposition using tertiary butyl ferrocene (TBF) and O2 plasma in a broad temperature range starting from 150 to 400 °C. A maximum growth rate of 1.2 A per cycle was achieved between 300 and 350 °C. Below 300 °C, the saturated growth per cycle was found to depend on the temperature of the sample. All the deposited films were pure with no significant amount of carbon contamination. Films deposited at 250 °C and above were crystalline with an α-Fe2O3 crystal structure, while the low temperature films were crystallized by a post-deposition annealing in He. Annealing in H2 induced the formation of metallic iron.

Molecular Interactions Driving the Layer-by-Layer Assembly of Multilayers

Abstract: This work received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. REGPOT-CT2012-316331-POLARIS. The work was also funded by FEDER through the Competitive Factors Operational Program (COMPETE) and by National funds through the Portuguese Foundation for Science and Technology (FCT) in the scope of the projects PTDC/FIS/115048/2009 and PTDC/CTM-BIO/1814/2012. The authors gratefully acknowledge Dr. Luca Gasperini (3B's Research Group, University of Minho, Portugal) for his help with the figures.

Multicomponent electrocatalyst with ultralow Pt loading and high hydrogen evolution activity

Abstract: Platinum is the most effective electrocatalyst for the hydrogen evolution reaction in acidic solutions, but its high cost limits its wide application. Therefore, it is desirable to design catalysts that only require minimal amounts of Pt to function, but that are still highly active. Here we report hydrogen production in acidic water using a multicomponent catalyst with an ultralow Pt loading (1.4 μg per electrode area (cm2)) supported on melamine-derived graphitic tubes (GTs) that encapsulate a FeCo alloy and have Cu deposited on the inside tube walls. With a 1/80th Pt loading of a commercial 20% Pt/C catalyst, in 0.5 M H2SO4 the catalyst achieves a current density of 10 mA cm−2 at an overpotential of 18 mV, and shows a turnover frequency of 7.22 s−1 (96 times higher than that of the Pt/C catalyst) and long-term durability (10,000 cycles). We propose that a synergistic effect between the Pt clusters and single Pt atoms embedded in the GTs enhances the catalytic activity. Although Pt is highly active for electrocatalytic production of H2 from water, its cost limits its wide application. Here, the authors prepare a high-performing catalyst that is supported on graphitic tubes, containing Fe, Co and Cu, and requires only a small amount of Pt.

Direct and continuous strain control of catalysts with tunable battery electrode materials

Abstract: We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.

Iron Oxide-Decorated Carbon for Supercapacitor Anodes with Ultrahigh Energy Density and Outstanding Cycling Stability

Abstract: Supercapacitor with ultrahigh energy density (e.g., comparable with those of rechargeable batteries) and long cycling ability (>50000 cycles) is attractive for the next-generation energy storage devices. The energy density of carbonaceous material electrodes can be effectively improved by combining with certain metal oxides/hydroxides, but many at the expenses of power density and long-time cycling stability. To achieve an optimized overall electrochemical performance, rationally designed electrode structures with proper control in metal oxide/carbon are highly desirable. Here we have successfully realized an ultrahigh-energy and long-life supercapacitor anode by developing a hierarchical graphite foam–carbon nanotube framework and coating the surface with a thin layer of iron oxide (GF–CNT@Fe2O3). The full cell of anode based on this structure gives rise to a high energy of ∼74.7 Wh/kg at a power of ∼1400 W/kg, and ∼95.4% of the capacitance can be retained after 50000 cycles of charge–discharge. These pe...