Väino Sammelselg - Academia.edu (original) (raw)

Papers by Väino Sammelselg

Mikrochimica Acta, Apr 28, 2006

Quantitative electron probe microanalysis of highly insulating materials is a complicated problem... more Quantitative electron probe microanalysis of highly insulating materials is a complicated problem, partially solved by coating samples with grounded thin conductive layers or using novel scanning electron microscopy (SEM) techniques, such as lowvoltage and=or variable pressure SEM. In this work, some problems of quantitative X-ray microanalysis of thin HfO 2 films, in particular the possibility to determine mass thickness correlated to the density of the layer material, are discussed. For comparison, Al 2 O 3 , Ta 2 O 5 and TiO 2 films grown onto both semiconductive Si and insulating quartz substrates were also analysed. All the films studied were synthesized by atomic layer deposition method.

Thin Solid Films, May 1, 1995

Thin tantalum oxide films were deposited using atomic layer deposition from TaCl5 and H2O at temp... more Thin tantalum oxide films were deposited using atomic layer deposition from TaCl5 and H2O at temperatures in the range 80500° C. The films deposited at temperatures below 300° C were predominantly amorphous, whereas those grown at higher temperatures were ...

Nanoscale Advances, 2021

High ORR activity and durability of Pt nanoparticles deposited on graphenated alumina nanofibers ... more High ORR activity and durability of Pt nanoparticles deposited on graphenated alumina nanofibers by plasma-assisted synthesis and sonoelectrochemical methods are demonstrated.

Optical Materials, 2019

For the first time we report the synthesis of nanocrystalline powders of Na 3 Al 2 Li 3 F 12 phas... more For the first time we report the synthesis of nanocrystalline powders of Na 3 Al 2 Li 3 F 12 phase, both pure and doped with Mn 4+ using slow room temperature co-precipitation in methanol-water mixtures. Optical properties of obtained highly crystalline nanoparticles with the size in the range of 40-80 nm are studied. The synthesized Mn 4+ doped fluoride garnet nanophosphors show typical Mn 4+ luminescence spectrum in the red region (620-630 nm) under both photo-and 10 keV electron excitation, which is optimal emission wavelength for red phosphors in warm white LEDs. The relaxation dynamics of Mn 4+ luminescence in this host has a complicated nature. Under the intra-centre excitation of Mn 4+ ions and charge-transfer band excitation the decay is nonexponential in the studied temperature range, which indicates that there exists some additional mechanism of quenching, which is not related to intrinsic thermal quenching of Mn 4+ luminescence.

Preparation of electrocatalysts often relies on the use of multiple starting materials-inorganic ... more Preparation of electrocatalysts often relies on the use of multiple starting materials-inorganic salts or organometallic precursors, nanostructured carbon supports, organic additives, dopants and carbonization under modifying atmospheres (e.g. NH3 or H2)-with the examples of electrocatalysts arising from a single precursor being much less common. Herein, we have surveyed a series of heterobivalent scaffolds to identify an iron/benzimidazole-based metalorganic framework as a uniform starting material. By merging the catechol and imidazole units together, we get direct entry into a highly efficient bifunctional oxygen electrocatalyst, which alleviates the need for additional dopants and modifying conditions (ORR: Eon = 1.01 V, E1/2 = 0.87 V vs. RHE in 0.1 M KOH; OER: 1.60 V @10 mA cm-2 in 0.1 M KOH; ∆E = 0.73 V). We demonstrate that by fine-tuning the chemical nature of an organic linker, one is able modulate the electrochemical properties of a single precursor-derived electrocatalyst material. Main Text Metal-organic frameworks (MOFs) are a great platform for designing and building novel materials with a foray into applications in energy storage and conversion, e.g. water splitting, fuel cells and metal-air batteries. [1,2] MOFs incorporate both the metal centres and organic ligands, which serve as essential building blocks. [3] Hence, having an intrinsic ability to modify their combinations may lead to more efficient, modular electrocatalyst materials. The oxygen reduction reaction (ORR) electrocatalysts have been exemplary with different MOFs having been traditionally employed as carbon-scaffolds and nitrogen-rich sources. [2] However, they often necessitate using dopants to raise the overall performance of the underlying electrocatalyst. [2] To date, only a few MOF-derived materials were shown to serve as highly efficient electrocatalysts

International Journal of Hydrogen Energy, 2018

Acid-treated multi-walled carbon nanotubes (MWCNTs) were decorated with TiO 2 using the atomic la... more Acid-treated multi-walled carbon nanotubes (MWCNTs) were decorated with TiO 2 using the atomic layer deposition (ALD) technique followed by uniform distribution of platinum nanoparticles (PtNPs) through magnetron sputtering. Surface analyses were performed by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical decontamination and characterization of the Pt-TiO 2 /MWCNT electrodes were carried out by CO stripping followed by cyclic voltammetry in acid media. The oxygen reduction reaction (ORR) was studied in O 2saturated 0.05 M H 2 SO 4 solution using the rotating disk electrode (RDE) method. Durability of the prepared catalysts was examined by repetitive potential cycling. Electrochemical data obtained was analyzed and compared to that of the commercial Pt/C catalyst. It was revealed that the Pt-TiO 2 /MWCNT catalysts possess higher ORR activity and better durability as compared to that of the commercial Pt/C.

Electrocatalysis, 2015

M a n g a n e s e o x i d e-m o d i f i e d g r a p h e n e nanosheet-supported silver nanocataly... more M a n g a n e s e o x i d e-m o d i f i e d g r a p h e n e nanosheet-supported silver nanocatalyst (Ag-MnO x /G) was prepared via two-step chemical and electrochemical deposition. Surface characterization of the prepared Ag-MnO x /G catalyst was performed by X-ray photoelectron spectroscopy, scanning electron microscopy, as well as Xray fluorescence techniques, and the electrocatalytic activity toward the oxygen reduction reaction (ORR) in alkaline media was studied using cyclic voltammetry and the rotating disk electrode (RDE) method. The onset potential of the ORR of the prepared catalyst material shifted positive about 40 mV, and the half-wave potential 20 mV compared to those of the bulk Ag electrode. After 1000 potential cycles between 0.05 and 1.1 V for accelerated aging tests, high stability of the Ag-MnO x /G catalyst in the ORR was observed with the half-wave potential of the ORR shifting negatively only about 0.04 V. RDE studies displayed unconditional improvement of electrochemical activity and long-term durability for the Ag-MnO x /G composite material.

The corrosion inhibition of metallic substrates is a prime issue for many potential applications ... more The corrosion inhibition of metallic substrates is a prime issue for many potential applications where corrosion plays a crucial role. The development of carbon based on functionalized coatings could increase the lifetime of metallic substrates by inhibiting the corrosion process. Present work is an effort to develop a corrosion inhibiting composite coating of graphene oxide and polypyrrole for AISI (American Iron and Steel Institute) type 304 stainless steel substrates. The electrochemical galvanostatic deposition process was applied for coating development. The coating morphology and ability to cover the substrate surface was analyzed with a high-resolution scanning electron microscope. The coating's structural and electronic properties were analyzed with Raman spectroscopy. The investigation of corrosion inhibition involved open circuit potential, Tafel, and voltammetry analysis. The standard salt test ASTM (American Society for Testing and Materials) G48A for stainless steel...

Electrochimica Acta, 2015

This work focuses on investigating the electrografting of chemical vapour deposition (CVD) graphe... more This work focuses on investigating the electrografting of chemical vapour deposition (CVD) graphene electrodes grown onto Ni foil (Ni/Gra) with different diazonium salts (including azobenzene diazonium tetrafluoroborate, Fast Garnet GBC sulphate salt, Fast Black K salt, 4-bromobenzene diazonium tetrafluoroborate and 4-nitrobenzenediazonium tetrafluoroborate). Various grafting conditions (e.g. "normal" electrografting in the narrow potential range and redox grafting in the wider potential range) were used. The electrochemical grafting behaviour was similar for all diazonium compounds used, except for the 4-nitrobenzenediazonium tetrafluoroborate when redox grafting was applied. The X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy results confirmed the presence of the corresponding aryl layers on Ni/Gra surfaces. The formation of multilayers on Ni/Gra substrates was in evidence since the thickness of different aryl layers varied from few to 30 nm depending on the modification procedures as well as the diazonium compounds used and the XPS analysis revealed a peak at about 400 eV for all aryl-modified Ni/Gra samples suggesting the multilayer formation also through azo linkages.

Open Physics, 2011

Atomic layer deposition of HfO2 on unmodified graphene from HfCl4 and H2O was investigated. Surfa... more Atomic layer deposition of HfO2 on unmodified graphene from HfCl4 and H2O was investigated. Surface RMS roughness down to 0.5 nm was obtained for amorphous, 30 nm thick hafnia film grown at 180°C. HfO2 was also deposited in a two-step temperature process where the initial growth of about 1 nm at 170°C was continued up to 10–30 nm at 300°C. This process yielded uniform, monoclinic HfO2 films with RMS roughness of 1.7 nm for 10–12 nm thick films and 2.5 nm for 30 nm thick films. Raman spectroscopy studies revealed that the deposition process caused compressive biaxial strain in graphene, whereas no extra defects were generated. An 11 nm thick HfO2 film deposited onto bilayer graphene reduced the electron mobility by less than 10% at the Dirac point and by 30–40% far away from it.

Thin Solid Films, 2014

ABSTRACT Atomic layer deposition of ZrO2 films from tris(dimethylamino)cyclopentadienylzirconium ... more ABSTRACT Atomic layer deposition of ZrO2 films from tris(dimethylamino)cyclopentadienylzirconium CpZr(NMe2)3 and H2O, was investigated using real-time characterization of the growth process and post-growth measurements of the films. Self-limited nature of the deposition process was observed at substrate temperatures ranging from 120 to 350 °C. In this temperature range growth rate of 0.08–0.1 nm per cycle was obtained on silicon substrates. The films deposited on silicon substrates at 200 °C and higher temperatures contained tetragonal and monoclinic phases of ZrO2. The phase composition of the films depended on the deposition temperature as well as on the film thickness. The concentration of carbon residues decreased with increasing deposition temperature and did not exceed 0.9 at.% in the films deposited at 250 °C and higher temperatures. The refractive indices and densities of films grown from CpZr(NMe2)3 and H2O at 250–350 °C ranged from 2.15 to 2.20 (at a wavelength of 633 nm) and 5.6 to 6.0 g/cm3, respectively, being close to the highest values obtained for films deposited from ZrCl4 and H2O. The former process ensured, however, more uniform nucleation of ZrO2 on graphene than the latter process did.

physica status solidi c, 2011

MgO nano‐powder was synthesized using the combustion method and was studied by low temperature VU... more MgO nano‐powder was synthesized using the combustion method and was studied by low temperature VUV spectroscopy under synchrotron radiation excitation. A considerable blue shift (by ∼0.6 eV, see the figure) of UV luminescence excited in intrinsic absorption was revealed in nanocrystalline MgO ceramics. This dense ceramics consists of large (10‐20 µm size) blocks and can be formed already at 1600 °C, i.e. far below the melting point of MgO. According to XRD studies, thermal treatment increased the size of nanocrystallites from 24 to 80 nm. In bulk MgO single crystals the UV luminescence (≥5 eV) is due to the recombination of electrons and holes near V‐centres, whereas nanocrystalline MgO ceramics with high surface to volume ratio facilitate the formation of the surface self‐trapped excitons, which decay radiatively. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ECS Transactions, 2013

The electrospinning method as a straightforward way to produce nanofibrous membranes from polymer... more The electrospinning method as a straightforward way to produce nanofibrous membranes from polymer solutions or melts under high electric field conditions has been applied on this work. Polymer fibers/membranes can be used in a wide variety of applications, starting from electrochemical devices to textiles and composite reinforcement materials. Poly(vinylidene fluoride) (PVDF) electrospun membranes were prepared from different (15, 20, 22.5 and 25% w/w) solutions of PVDF in N,N−dimethylformamide and acetone mixture (80:20 w/w %). PVDF fibrous membranes with fiber diameter within the range from 100 to 2200 nm and different polymorphism of PVDF were produced by electrospinning method and tested electrochemically.

International Journal of Electrochemistry, 2012

Metallic impurity free solid carbon nanorod “Whiskers” (SCNR Whiskers), a derivative of carbon na... more Metallic impurity free solid carbon nanorod “Whiskers” (SCNR Whiskers), a derivative of carbon nanotubes, are explored in the fabrication of a Prussian Blue composite electrode and critically evaluated towards the mediated electroanalytical sensing of H2O2. The sensitivity and detection limits for H2O2on the paste electrodes containing 20% (w/w) Prussian Blue, mineral oil, and carbon nanorod whiskers were explored and found to be 120 mA/(M cm2) and 4.1 μM, respectively, over the concentration range 0.01 to 0.10 mM. Charge transfer constant for the 20% Prussian Blue containing SCNR Whiskers paste electrode was calculated, for the reduction of Prussian Blue to Prussian White, to reveal a value of1.8±0.2 1/s (α=0.43,N=3). Surprisingly, our studies indicate that these metallic impurity-free SCNR Whiskers, in this configuration, behave electrochemically similar to that of an electrode constructed from graphite.

Synthetic Metals, 2007

The stability of oxidised polypyrrole (PPy) doped with mobile anion (ClO 4 −) was studied in aque... more The stability of oxidised polypyrrole (PPy) doped with mobile anion (ClO 4 −) was studied in aqueous media. The influence of various components of the solution (dissolved O 2 , indifferent electrolyte, organic compounds) on the stability of PPy was characterised by cyclic voltammetry and electron-probe X-ray microanalysis (EPMA). It was established that dissolved O 2 favours the PPy overoxidation-the rate determining process of the electrochemical corrosion. The high concentration of the indifferent electrolyte on the other hand enhances the stability of the PPy in aqueous solutions to an essential extent. The more effective inhibitors of the PPy corrosion are the surface-active anions (dodecyl sulfate) and polar aromatic compounds (benzyl alcohol, benzonitrile).

Electrocatalysis, 2014

The electrocatalytic activity of electrodeposited palladium-gold (PdAu) alloys towards the oxygen... more The electrocatalytic activity of electrodeposited palladium-gold (PdAu) alloys towards the oxygen reduction reaction (ORR) was studied in 0.05 M H 2 SO 4 and 0.1 M KOH solutions using the rotating disc electrode (RDE) method. The electrochemical deposition was carried out at a constant potential and the ratio of concentrations of the precursors in the deposition bath was varied. The surface morphology of the deposits was studied using scanning electron microscopy (SEM), and the average particle size for PdAu alloys was determined to be about 6 nm. The X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) studies suggest that the resulting PdAu coatings are alloyed. The ORR activities of the electrodes were compared by calculating the specific activities on the basis of the real electroactive surface area of Pd. The specific activity did not depend on the composition of the alloy in sulphuric acid solution, but in alkaline medium it increased with decreasing Pd content.

Graphene - Synthesis, Characterization, Properties and Applications, 2011

Synthetic Metals, 2006

PPy|SO 4 and PPy|ClO 4 films have been synthesized and investigated in K 2 SO 4 , ZnSO 4 and NaCl... more PPy|SO 4 and PPy|ClO 4 films have been synthesized and investigated in K 2 SO 4 , ZnSO 4 and NaClO 4 aqueous solutions by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and electron probe microanalysis (EPMA) methods. On the basis of obtained data and calculated impedance parameters as the potential functions, the role of different processes (diffusion of ions, double-layer charging, adsorption and charge transfer) in oxidized, partially reduced and reduced PPy films is estimated. The lowest pseudocapacitance values (from n × 10 −6 to n × 10 −4 F cm −2 for 1 m film), independent of solution concentration, were established for PPy|SO 4 in ZnSO 4. This phenomenon is related with strongly aggravated film reduction process in the solution of double-charged cations. In the case of PPy|ClO 4 in NaClO 4 and PPy|SO 4 in K 2 SO 4 ,where the mono-charged cations participate in redox process, the capacitance values are in the range from: n × 10 −3 to n × 10 −2 F cm −2 and even somewhat higher for PPy|ClO 4 system at oxidized state. The calculated effective diffusion coefficients of ions D remain inside the range from n × 10 −12 to n × 10 −14 cm 2 s −1 for PPy|SO 4 in 0.1 M K 2 SO 4 and PPy|ClO 4 in 0.1 M NaClO 4 aqueous solution. In the case of PPy|SO 4 film in ZnSO 4 solution the D values are essentially lower.

Sensors and Actuators B: Chemical, 2014

We have synthesized single-layer graphene on Cu foils using chemical vapor deposition method and ... more We have synthesized single-layer graphene on Cu foils using chemical vapor deposition method and transferred the graphene to the top of a Si/SiO 2 substrate with a pair of prefabricated Ti/Au electrodes. A resistive graphene-based gas sensor prepared in this way revealed n-type oxygen response at room temperature and we have successfully fitted the data obtained with varying oxygen levels using a two-site Langmuir model. P-type oxygen response of our sensor was observed after the temperature was raised to 100 o C, with a reversible transition to n-type behaviour when the temperature was lowered back to room temperature. Such inversion of the gas response type with temperature was interpreted as a result of interplay between the adsorbate-induced charge transfer and charge carrier scattering. The transduction function was derived, which relates the electrical response to surface coverage through both the induced mobility and charge density changes.

physica status solidi (a), 2013

Real time monitoring of atomic layer deposition by quartz crystal microbalance (QCM) was used to ... more Real time monitoring of atomic layer deposition by quartz crystal microbalance (QCM) was used to follow the growth of ZrO 2 thin films on graphene. The films were grown from ZrCl 4 and H 2 O on graphene prepared by chemical vapor deposition method on 100-nm thick nickel film or on Cu-foil and transferred onto QCM sensor. The deposition was performed at a substrate temperature of 190 8C. The growth of the dielectric film on graphene was significantly retarded compared to the process carried out on QCM without graphene. After the deposition of dielectric films, the basic structure of graphene was retained.