Evangelia A. Pavlatou

D.S. Tsoukleris, A. Zoikis-Karathanasis, P. Gyftou, A.G. Kontos, E.A. Pavlatou, P. Falaras

63rd Annual Meeting of the International Society of Electrochemistry-Electrochemistry for Advanced Materials, Technologies and Instrumentation (2012), 19-24 August, Prague, Czech Republic (poster)

TiO₂ and related semiconducting materials have been widely used as photocatalysts in order to decompose environmental pollutants. For a number practical applications, the photocatalyst, usually immobilized in the form of a thin film on an inert substrate, has to show low stability over long time periods, including low resistance to corrosion, as well as to mechanical stresses. This work describes the synthesis of efficient Ni-P titania photocatalysts, which have a high stability, an excellent corrosion resistance and a photocatalytic action against liquid pollutants. Ni-P TiO₂ films has generally been prepared through TiO₂ and Ni–P matrix from an additive-free modified Watts type bath by applying pulse electroplating. The impact of pulse plating parameters, such as duty cycle and frequency of applied current pulses over a wide range of values, as well as the hydrodynamic conditions of electrolyte on the composition, microstructure, morphology and microhardness of the Ni–P TiO₂ composite coatings were examined. Composite deposits were also produced under direct current conditions for comparison. The resulting materials were subsequently characterized using X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), SEM and Raman spectroscopy.The films chemical stability was investigated as a function of calcination temperature, pH and immersion time in aqueous solutions. The relationship between film morphology and thickness has been elucidated. In addition, structural and hardness modifications were examined. A critical film thickness below which films were crackfree was determined. Existing models for the mechanical stability of the films were also examined.

D.S. Tsoukleris, T. Stergiopoulos, A. Zoikis-Karathanasis, A. Karantonis, E.A. Pavlatou, P. Falaras

63rd Annual Meeting of the International Society of Electrochemistry-Electrochemistry for Advanced Materials, Technologies and Instrumentation (2012), 19-24 August, Prague, Czech Republic (poster)

Dye-sensitized solar cells (DSCs) are promising materials for light-to-energy conversion devices due to their low-cost, easy fabrication and relative high conversion efficiency. Further cost reduction, which lies in finding cheaper raw materials and simplifying the manufacturing procedure, is one of those key-issues during the development of DSCs. As an important component of DSCs, the cathode or counter electrode (CE) collects electrons arriving from the external circuit and promotes the regeneration of I^- from I₃^-. The conventional CEs are based on standard Pt catalysts deposited on fluorine-doped tin oxide (FTO) glass substrates (usually by thermal decomposition), which have shown excellent performance and stability. However, the large-scale manufacturing of DSCs may be impeded since Pt is one of the most expensive materials available in the world. Ni-based cathodes (e.g. NiP, NiS) with high corrosion resistance and sufficient conductivity have already been used as an attractive low-cost substitute. In this work, a nickel plating technique, used to prepare effective Ni-P cathodes is described. To take advantage of the low sheet resistance and the high light reflectivity of Ni-P CEs, Ni-P was deposited on two different substrates namely Ti foil and FTO glass. Different preparation conditions were tested by changing the time of electroplating, the applied voltage etc. The performance characteristics of DSCs with Ni-P CEs were studied and compared with standard Pt electrodes prepared by sputtering. The resulting CEs were characterized using XRF, X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), SEM and Raman spectroscopy. The study of unlubricated uni-directional sliding on the films was performed using a ball-on-disc tribometer of CSM Instruments. Furthermore, measurements of the Vickers microhardness (HV in GPa) were performed on the surface of the composite deposits by using a Wilson Instruments InstronCompany microhardness tester.

A. Zoikis-Karathanasis, S. Spanou, A. Mitrokotsa, P. Gyftou, E.A. Pavlatou

5th European Pulse Plating Seminar (2012), 9 March, Perchtoldsdorf-Austria (oral)

Nickel electrodeposition is one of the most widely used surface finishing processes for decorative, engineering and electroforming applications. The increasing technological demand to develop metallic or metal matrix composite components for micromechanical systems necessitates enhanced control of the production of Ni-based deposits with specific nano-structures and well-defined properties. Moreover, during the recent years, nanotechnology has attracted increasingly scientific and technological interest concerning the fabrication of nanostructures and the study of their properties and potential applications in nanoscale systems. The challenge in the fabrication of nanostructures is about controlling their size, shape and periodicity in order to produce efficient devices to a smaller scale with ameliorated performance, which mainly depends on predefined conditions and parameters of the fabrication process. Among many different nanostructured materials, several works have been dedicated to the fabrication of nickel one-dimensional nanostructures (nanotubes/nanowires) by utilizing electrodeposition in porous anodic alumina membranes, as a fast, controllable and low cost procedure. Under this framework, composite coatings of Ni and Ni-P matrix enhanced by TiO₂ nano-particles (mean diameter~21 nm), SiC (mean diameter 1 μm and ~200nm) and WC nano-particles (mean diameter~200 nm) were produced under direct (DC) and pulse current (PC) conditions from an additive-free Watts’ type bath. The influence of the variable electrolysis parameters (Τon, Toff, frequency and duty cycle of pulses, pH and current density) on the surface morphology, structure, mechanical and photo-induced properties-study of photocatalytic activity and photo-induced hydrophilicity in case of TiO₂ embedding in matrix- as well as on the tribological performance of the deposits will be demonstrated. Hardness and wear resistance were correlated with the crystalline orientation of the matrix, the existence of amorphous and crystalline phases as well as, the codeposition percentage of embedding particles. According to the overall experimental findings it is proved that the imposition of specific pulse current conditions leads to more compact coatings with increased embedding percentage, more homogenous distribution of nanoparticles in the matrix and increased micro-hardness, compared to those prepared under direct current conditions. Specifically, it seems that low duty cycles values and medium to high frequencies favor the codeposition and lead to grain refinement accompanied by enhanced microhardness values and wear resistance. Additionally, pure Ni nanocrystalline electrodeposits were produced from a Watts type bath with the simultaneous addition of brightener agents, such as 2-butyne-1,4-diol and cis 2-buten-1,4-diol, and application of pulse current conditions. The surface morphology, the preferred orientation and grain size of Ni deposits were determined as a function of the concentration of the additive in bath, as well as of the frequency of imposed current pulses. Moreover, the micro-hardness of the coatings was also assessed from the viewpoint of crystal orientation, grain size, presence and concentration of additive and type of imposed current. In general, the overall experimental data reveal that the proper selection of pulse parameters could lead to the production of pure Ni nano-structured “macro-coatings”, nanostructured Ni matrix composites and uniform one-dimensional Ni nanostructures exhibiting superior mechanical, wear resistance and self cleaning properties.

A. Zoikis-Karathanasis, E.A. Pavlatou

EAST – MINDE FORUM (2012), 8 March, Fachhochschule Wiener Neustadt, Austria (oral)

The total annual estimated direct cost of corrosion and wear in the worlds is estimated at about 4% of the world’s Gross Domestic Product (GDP). Protective coatings serve to prevent wear and corrosion and thus reduce the total loss from corrosion and wear. Hard chromium plating is one of the most widely used techniques for production of such coatings. However due to EU regulations, health issues in the plating personnel, functional problems of the coatings as well as to low current efficient, the substitution of hard chrome is compelling. In the present study Ni-P and Ni-P matrix composite coatings have been examined as alternatives to hard chrome deposits. Thus, pure Ni-P and Ni-P matrix composite coatings reinforced by either micro-SiC or submicro-WC particles have been produced from a modified Watts bath, utilizing both direct and pulse current regime. Regarding Ni-P coatings, it has been found that by applying suitable combinations of pulse plating parameters (frequency and duty cycle) alteration of P content is feasible in the range of 11-18wt.% (DC~13.8 wt%). Moreover, high values of duty cycle and low values of frequency of imposed pulses as well as intense hydrodynamic conditions lead to reduced P content in the deposit. As long as the structure of the coatings, the majority of the produced coatings are characterized as amorphous, while under specific electrolytic condition crystalline Ni12P5 was also produced. Finally, thermal treatment of the coatings at 400^oC has as a result crystallization of amorphous phase in Ni, Ni₃P and Ni₂P. In the case of Ni-P/SiC composite coatings, alteration of electrolytic parameters of pulse plating (duty cycle and frequency) as well as of hydrodynamic conditions has as a result the variation of codeposition rate in the range of 0.5-22wt.%. The imposition of pulse current lead to composite coatings with more homogeneous dispersion of SiC particles compared to direct current regime. Although the presence of SiC particles results in reduction of P content in the Ni-P matrix compared to pure Ni-P coatings, the matrix is still characterized as amorphous, while, after thermal treatment the matrix is crystallized in the phase of Ni and Ni₃P. The Ni-P/WC composite coatings are characterized by significant high codeposition rate of WC nanoparticles (10-30wt.%). The matrix is characterized as amorphous and the presence of WC nanoparticles does not affect it in compared with pure Ni-P. The application of pulse plating has as a result more compact coating with lower roughness compared to the direct current. Thermal treatment of coating at 400^οC has as a result the crystallization of the amorphous matrix in the phases of Ni, Ni₃P and Ni₂P. Concerning mechanical properties, Ni-P coatings present hardness values in the range of 4.5-5 GPa. The hardness is maximized after thermal treatment at 400^οC (9-12 GPa). This increase is due to the crystallization of the amorphous matrix and mainly in the precipitation of Ni₃P phase. Further thermal treatment in higher temperatures results in decrease of hardness. Ni-P/SiC composite coatings present microhardness values: 5-7 GPa. Higher incorporation rate leads to higher hardness. After thermal treatment composite coatings exhibit hardness 10-13 GPa. Precipitation hardness mechanism predominates dispersion hardening. Finally, Ni-P/WC composite coatings presents hardness in the range of 6.3-7 GPa for a wide range of WC nanoparticles codeposition rate (10-30wt.%). However, after thermal treatment at 400^oC hardness is significantly increased (15-19 GPa). Further thermal treatment in higher temperatures results in sharp decrease of the hardness. Tribological tests showed that the friction coefficient is decreased in pulse plated Ni-P coatings. Further decreased is observed with the codeposition of SiC and WC particles in the matrix. Thermal treatment, and the accompanied increase in hardeness, resulted in decrease of friction coefficient. The wear rate depends on the micrhardness values both in as plated and thermal treatment coatings. It should be mentioned that Ni-P/WC exhibited extremely high wear resistance.

D.S. Tsoukleris, E.A. Pavlatou, D.D. Dionysiou, P. Falaras

3rd IC4N2011-Third International Conference- From nanoparticles and nanomaterials to nanodevices and nanosystems (2011), 26-30 June, Crete Island, Greece (poster)

Α. Mitrokotsa, S. Spanou, P. Gyftou, E.A. Pavlatou

8th International Workshop on Electrodeposited Nanostructures – EDNANO 8 (2011), 17-19 March, Milan-Italy (oral)

E.A. Pavlatou, S. Spanou, A. Zoikis-Karathanasis, P. Gyftou, A.I. Kontos, P. Falaras

61st Annual Meeting of the International Society of Electrochemistry (2010), 26 September-1 October, Nice, France (oral)

E.A. Pavlatou, A. Zoikis-Karathanasis, S. Spanou, P. Gyftou

EAST Forum – MINDE Workshop (2010), 24-15 June, Schwäbisch Gmünd, Germany (oral)

Composite coatings of Ni and Ni-P matrix enhanced by TiO₂ nano-particles (mean diameter ~21 nm), SiC (mean diameter 1 μm) and WC nano-particles (mean diameter ~200 nm) were produced under direct (DC) and pulse current (PC) conditions from an additive-free Watts type bath. Pure Ni and NiP deposits were also produced under the same experimental conditions for comparison. The influence of the variable electrolysis parameters (type of current, Τon, Toff, frequency and duty cycle of pulses, pH and current density) on the surface morphology, structure, mechanical and photo-induced properties of the deposits will be demonstrated. In the case of the Ni-based composite coatings with dispersed TiO₂ nanoparticles the observed mechanical and self-cleaning properties are associated with the efficient immobilization of the TiO₂ particles in the metal matrix and the specific structural modifications of Ni crystallites (for the Ni-matrix deposits) provoked by the adsorption - desorption phenomena occurring on the metal surface, induced by the presence of particles. It has been revealed that the overall self-cleaning character of the Ni based surfaces was improved following the titania codeposition percentage, as both hydrophility and photocatalytic activity were present. Ni-P/TiO₂ composites are amorphous and exhibit higher microhardness values and reduced volumetric wear factors compared to the Ni/nano-TiO₂ ones. However, when ascribing the observed strengthening effect of composites with Ni, not only grain refinement and dispersion strengthening mechanisms, but also preferred crystalline orientation should be taken into consideration. Regarding the Ni-P, Ni-P/SiC and NiP/WC coatings the main study was focused on the effect of pulse parameters (duty cycle and frequency) on the structure, microhardness and tribological behaviour of the deposits, as well as the influence of heat treatment on the above properties. For both kind of deposits the experimental findings proved that the imposition of pulse current conditions led to more compact coatings with increased embedded particles, more homogenous distribution of the increased micro-hardness and reduced volumetric wear factors, compared to those produced under direct current conditions. Furthermore, heat treatment resulted in an enhancement of the hardness and wear resistance of deposits. Abrasive and adhesive wear mechanisms were mainly detected on worn surfaces, while fatigue was also present in some coatings. Overall, Ni-P/WC composite coatings performed superior wear resistance accompanied by considerable high microhardness values (maximum ~19 HV) compared to corresponding of pure Ni-P and composite Ni-P/SiC deposits.

A. Zoikis-Karathanasis, S. Spanou, P. Gyftou, E.A. Pavlatou

2nd Regional Symposium on Electrochemistry – South East Europe (2010), 6-10 June, Belgrade, Serbia (poster)

A.Zoikis-Karathanasis, E.A. Pavlatou

2nd International Conference on Functional Nanocoatings (2010), March 28-31, Dresden, Germany (oral)

A. Mitrokotsa, A. Zoikis-Karathanasis, S. Spanou, P. Gyftou, E.D. Koronaki, G. Pashos, A.G. Boudouvis, E.A. Pavlatou

2nd International Conference on Functional Nanocoatings (2010), March 28-31, Dresden, Germany (poster)

S. Spanou, E.A. Pavlatou, A.I. Kontos, P. Falaras

2nd International Conference on Functional Nanocoatings (2010), March 28-31, Dresden, Germany (oral)

A.Zoikis-Karathanasis, S. Spanou, E.A. Pavlatou

6th International Conference on the Ιnstrumental Μethods of Analysis: Modern Trends and Applications (2009), October 4-8, Athens, Greece (poster)

S. Spanou, P. Gyftou, E.A. Pavlatou

6th International Conference on the Ιnstrumental Μethods of Analysis: Modern Trends and Applications (2009), October 4-8, Athens, Greece (poster)

A.Zoikis – Karathanasis, S. Spanou, P. Gyftou, E.A. Pavlatou, N. Spyrellis

EuroInterfinish 2009 - Pulse plating and other Electrochemical Techniques (2009), September 23-25, Bremen, Germany (oral)

S. Spanou, E.A. Pavlatou, A.I. Kontos, P. Falaras

2nd European Conference on Environmental Applications of Advanced Oxidation Processes- EAAOP2 (2009), September 9-11, Nicosia, Cyprus (CD-rom) (poster)

M. Giallousi, V. Gialamas, E.A. Pavlatou

European Science Education Research Association (ESERA) Conference (2009), 31 August -4 September, Istanbul, Turkey (poster)

S. Spanou, E.A. Pavlatou, A.I. Kontos, P.Falaras, N. Spyrellis

EAST FORUM 2008-Challenging and Testing to Improve Surface Technology (2008), October 23-24, Trento, Italy (oral)

X. Vamvakeros, E.A. Pavlatou, N. Spyrellis

9th European Conference on Research in Chemical Education (ECRICE) (2008), 6-9 July, Istanbul, Turkey (oral)

S. Spanou, A. Zoikis – Karathanasis, E.A. Pavlatou, N. Spyrellis

INTERFINISH 2008 –Nanotechnology and Innovative Coatings (2008), June 16-19, Pusan, Korea (oral)