Anita Grozdanov

Production, Purification, Characterization, and Application of CNTs

Nanomaterials and nanotechnologies play a significant role in many fields of modern science: material science, chemistry, catalysis, molecular biology, and medicine. They exhibit novel, better physical and chemical properties than the bulk materials. This is a result of the change in the electronic structure, which is responsible for electroconductivity, optical absorption, chemical reactivity, catalytically activity and mechanical properties. The subject of this work is the production, characterization, purification and application of carbon nanotubes (CNTs).

Improvement of the Catalytic Activity of Pt through Synergetic Interaction with Co

This study is concerned with the preparation and characterization of electrocatalysts containing Magneli phases as a support material, and different metallic systems (Co, Pt, and CoPt, Co:Pt = 1:1 wt.), as catalytic phase. The main idea was to reduce the amount of Pt with the addition of Co as a non-noble metal. The studied electrocatalysts were tested for water electrolysis in aqueous electrolytes, i.e. for hydrogen evolution reactions (HER) and for oxygen evolution reactions (OER). They were also tested in PEM fuel cells for oxygen reduction reaction (ORR). The order of activity for HER and OER was the following: CoPt > Pt > Co, and for ORR: CoPt ≈ Pt > Co. Co has been shown as promoter for reducing Pt particles (the most active monometallic phase), so the reduction of the Pt quantity in the metallic phase was compensated by smaller particles. Also, the interaction between the metallic phases increases the intrinsic catalytic activity for all reactions (HER, OER and ORR).

Electrochemical Synthesis of PANI/Graphene Nanocomposites Aimed for Sensors

This study is concerned with the electrochemical synthesis of polyaniline (PANI) based composites reinforced by carbon nanostructures (CNSs) such as graphene. To determine the optimal conditions for electropolymerization of the aniline to PANI, an electrochemical characterization was performed by means of cyclic voltammetry, steady state polarization and galvanostatic measurements. Within the CV spectra all steps of the transformation from aniline to polyaniline were observed with corresponding potential regions. It was found that the optimal potential for electropolymerization of the PANI based nanocomposites is 0.75 V vs. saturated calomel electrode (SCE). The electropolymerization rate was observed for two ways of involving of carbon nanostructures in the polymer matrix: dispersion in the electrolyte and previous deposition at the working electrode. The produced nanocomposites were characterized by means of Scanning Electron Microscopy (SEM) and Raman Spectroscopy. Strong interaction between the quinoidal structure of PANI and carbon nanostructures were detected. Electrical properties of the nanocomposite tablets were tested using the four probe method. These measurements show the great potential of the studied materials for sensors.

Removal of Heavy Metal Ions from Wastewater Using Bio- and Nanosorbents

The rapid industrial development and urbanization have intensified environmental pollution and caused deterioration of ecosystems by accumulation of many pollutants, especially heavy metals. Most of the heavy metals are toxic, and their ions are not biodegradable with the tendency to accumulate in the soil, water resources and the living organisms; hence, they are significant environmental pollutants. Therefore, the treatment of the heavy metal ions and their elimination from water and wastewater is very important for environmental protection and thus the public health. In the frame of this work, the adsorption abilities of natural and nanosorbents, particularly natural peanut husks, expanded perlite and graphene, to remove Ni(II), Pb(II), and Fe(II) ions from water systems, were investigated. The influence of the pH (4–8) of the solution, the amount of adsorbent (0.5–5.5 g/l), the initial metal ion concentration (0.3–2.0 mg/l), and the contact time (5–180 min.) on the efficiency of removal of metal ions was investigated. Thus, the optimal conditions for achieving maximal effectiveness for heavy metals removal were determined. The characterization of the sorbents was performed utilizing the following techniques: SEM and TGA. Adsorption equilibrium of the systems was analyzed using the following isotherms: Langmuir, Freundlich, Langmuir–Freundlich, and Redlich–Peterson. The maximal adsorption capacity of the peanut husks, perlite and graphene for Ni(II), Fe(II), Pb(II) was obtained, and the percentage of removal was determined. A comparative analysis for the efficiency of all used sorbents for Ni(II), Pb(II), and Fe(II) ions removal from the three component systems was conducted at the end. The expanded perlite gave the best results for the removal of Ni (II) and Pb (II) ions, while graphene proved to be excellent adsorbent for Fe(II) ions with an efficiency of 100%.

MWCNT/Polyaniline Nanocomposites Used for pH Nanosensors of Marine Waters

Polymer nanocomposites of conductive polymer, polyaniline (PANI) with multi-walled carbon nanotubes (MWCNTs), have gained a great interest for their application in environmental and water quality monitoring (where pH value becomes one of the reliable data). Compared to the inorganic counterparts, conducting polymers have advantage in achieving high sensitivity and selectivity by virtue of their chemical and structural diversity. In the framework of FP7 project COMMON SENSE (OCEAN 2013.2-614155), screen-printed electrodes as a pH nanosensors based on nanocomposites of conductive polymer matrix-PANI and MWCNT were prepared by electrochemical polymerization. Characterization was performed by several spectroscopic techniques and electrical measurements. Electrochemical synthesis of the PANI-based composites was performed at 0.75 V vs. saturated calomel electrode (SCE) for 40 and 60 minutes. The working conditions were determined using electrochemical steady-state polarization measurements. Morphology of the produced composites was observed using scanning electron microscope (SEM), structural characteristics were studied using Raman spectroscopy, while thermal stability was determined using thermal gravimetric analysis (TGA/DTA). The results point out on fibrous and porous structure of PANI-based composites, with strong interaction between quinoidal structure of PANI with carbon nanostructures via p–p stacking according to Raman spectroscopy measurements. TGA coupled with DTA showed the increased thermal stability of the studied composites. The obtained nanocomposites exhibited a high value of conductivity which attributed to the synergy effect of the conductive polymer matrix and carbon nanostructure. Resistivity (i.e., conductivity) changes were measured at different pHs (4 to 10) as well as in different marine regions.

MWCNT/PANI Screen Printed Electrodes for Gas Sensors

During the last 10 years, CNT based nanocomposites belong to a group of new materials that have intensively been tested for gas sensing, and great efforts have been spent for the development of gas sensors. The construction of SOx sensors is an important task because SOx containing oxides are dangerous having a negative influence on the environment and humans. Most of the literature data are related to MWCNTs based sensors for gases such as water vapors, NH3, CO2 and CO. In the present work, a promising application of screen printed electrodes with MWCNT/PANI nanocomposites prepared by a direct electro-polymerization method as a gas sensor was tested. Resistivity variations were found for different acid concentration. Surface changes of the SPE sensors before and after acid exposure, were followed by SEM. Polymer/CNTs interactions and their changes due to the acid vapors were studied by FTIR-ATR spectroscopy. The obtained results confirmed polymer/CNT – SO42− interactions and their characteristic band-shifting. SEM photos show the formation of typical oxid nanorods.

Characterization of Graphene Produced by Electrolysis in Aqueous Electrolytes

This work is concerned with the production of graphene using electrolysis in aqueous electrolytes with a reverse change of the potential. As electrodes and precursors for the graphene production highly oriented graphite was used. The electrolytes used were: H2SO4 (pH = 0.5); H2SO4 + KOH (pH = 1.2) and H2SO4 + NaOH (pH = 1.2). The produced graphene samples were characterized by means of scanning and transmission electron microscope (SEM and TEM) and Raman spectroscopy. The size of the crystallites and the number of layers of the studied graphene samples was determined. It was found that by the proposed electrochemical method graphene with few layers only can be produced.

Pathways for the Production of Non-stoichiometric Titanium Oxides

The subject of this study is the development of a bottom-up method for the preparation of nano-scaled Magneli phases. There are two steps involved; the first step is sol-gel preparation of Ti(OH)4 using titanium tetraisopropoxide as organometallic precursor, and the second step is its thermal decomposition to TiO2 or Magneli phases. Thermal treatment in an oxidative atmosphere (air) was performed at different temperatures to produce TiO2 in order to determine the regions of stability of anatase and rutile. Next, thermal treatment in a reductive (10 % H2 + 90 % N2) atmosphere at temperature of rutile was performed to produce Magneli phases. Furthermore, a top-down approach was applied to produce non-stoichiometric titanium oxides by mechanical activation of commercial Magneli phases known as Ebonex. Examples of the application of top-down produced Magneli phases in electro- and photocatalysis are shown.