Dr.-Ing. Alexandra Wittmar

Technical Chemistry
University of Duisburg-Essen

Author IDs

  • Preparation of {N}-doped carbon materials from cellulose:chitosan blends and their potential application in electrocatalytic oxygen reduction
    Wittmar, A.S.M. and Ropertz, M. and Braun, M. and Hagemann, U. and Andronescu, C. and Ulbricht, M.
    Polymer Bulletin 80 (2023)
    view abstract10.1007/s00289-022-04429-2
  • Preparation of N-doped carbon materials from cellulose:chitosan blends and their potential application in electrocatalytic oxygen reduction
    Wittmar, A.S.M. and Ropertz, M. and Braun, M. and Hagemann, U. and Andronescu, C. and Ulbricht, M.
    Polymer Bulletin (2022)
    view abstract10.1007/s00289-022-04429-2
  • Cotton as Precursor for the Preparation of Porous Cellulose Adsorbers
    Wittmar, A.S.M. and Baumert, D. and Ulbricht, M.
    Macromolecular Materials and Engineering 306 (2021)
    Natural biopolymer-based porous spherical adsorbers from cellulose have good efficiency for removal of metal ion pollutants from aqueous media. However, high purity celluloses, most commonly used as precursors for preparation of the adsorber spheres, require complex synthesis processes, which consume energy and chemicals, and may thus lead to other types of pollution. In this work, the possibility to prepare cellulose-based porous spherical adsorbers directly from cotton, using an ionic liquid-based platform is analyzed in detail. The dissolution of microcrystalline cellulose (MCC), as reference, and of cotton in ionic liquid-based solvents and the properties of the obtained polymer solutions are investigated in order to evaluate their processability toward porous macrospheres using the drop shaping cum non-solvent induced phase separation process. The properties of the prepared spheres are assessed. The dissolution of cotton is more difficult than the dissolution of MCC and the formed cotton-based solutions are considerably more viscous, which makes their processability possible only after careful adjustment of the cotton solution concentration. The maximum adsorption capacities toward Cu2+ are ≈110 and ≈72 mg/g for the porous cotton-based spheres prepared from 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]):dimethylsulfoxide (DMSO) = 2:1 and 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]):DMSO = 2:1 solutions, respectively. © 2021 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH
    view abstract10.1002/mame.202000778
  • A swift technique to hydrophobize graphene and increase its mechanical stability and charge carrier density
    Madauß, L. and Pollmann, E. and Foller, T. and Schumacher, J. and Hagemann, U. and Heckhoff, T. and Herder, M. and Skopinski, L. and Breuer, L. and Hierzenberger, A. and Wittmar, A. and Lebius, H. and Benyagoub, A. and Ulbricht, M. and Joshi, R. and Schleberger, M.
    npj 2D Materials and Applications 4 (2020)
    Despite the improvement of the quality of CVD grown single-layer graphene on copper substrates, transferring the two-dimensional layer without introducing any unintentional defects still poses a challenge. While many approaches focus on optimizing the transfer itself or on necessary post-transfer cleaning steps, we have focused on developing a pre-treatment of the monolayer graphene on copper to improve the quality and reproducibility of the transfer process. By pressing an ethylene-vinyl acetate copolymer foil onto the monolayer graphene on copper using a commercially available vacuum bag sealer graphene is stabilized by the attachment of functional carbon groups. As a result, we are able to transfer graphene without the need of any supporting layer in an all-H2O wet-chemical transfer step. Despite the general belief that the crumbling of graphene without a support layer in a H2O environment is caused due to differences in surface energy, we will show that this assumption is false and that this behavior is caused rather by the polar interactions between graphene and water. Suppressing these interactions protects graphene from ripping and results in extremely clean, highly crystalline graphene with a coverage close to 100%. © 2020, The Author(s).
    view abstract10.1038/s41699-020-0148-9
  • Cellulose/chitosan porous spheres prepared from 1-butyl-3-methylimidazolium acetate/dimethylformamide solutions for Cu2+ adsorption
    Wittmar, A.S.M. and Klug, J. and Ulbricht, M.
    Carbohydrate Polymers 237 (2020)
    A series of cellulose/chitosan blend porous spheres were prepared by dropping cum phase separation from 1-butyl-3-methylimidazolium acetate/dimethylformamide based solutions via coagulation in water. Special attention was given to the investigation of the phase separation process initiated with water in relation with the different chitosan content in polymer solutions. The increase of the chitosan fraction in the biopolymer solution led to a destabilization of the solutions by lower amounts of water. Both viscosity of the polymer and stability of the solution played a very important role to the spheres structure formation. It was observed that the SBET seems to decrease at the increase of the chitosan content in the porous material. Despite the decrease of porosity, the increase of the chitosan fraction in the blend had a beneficial influence on the adsorber properties of the spheres, due to the amino groups in the chitosan units. © 2020 Elsevier Ltd
    view abstract10.1016/j.carbpol.2020.116135
  • Development of a bone substitute material based on additive manufactured Ti6Al4V alloys modified with bioceramic calcium carbonate coating: Characterization and antimicrobial properties
    Surmeneva, M.A. and Chudinova, E.A. and Chernozem, R.V. and Lapanje, A. and Koptyug, A.V. and Rijavec, T. and Loza, K. and Prymak, O. and Epple, M. and Wittmar, A. and Ulbricht, M. and Surmenev, R.A.
    Ceramics International 46 (2020)
    This investigation shows that composite structures based on additive manufactured electron beam melted Ti6Al4V scaffolds coated with calcium carbonate particles can be used as a potential biocomposites for bone substitutes. A continuous bioceramic coating of CaCO3 was deposited on additive manufactured titanium alloy under the influence of ultrasound. XRD analysis revealed the formation of a mixture of calcite and vaterite phases. CaCO3 coating led to decreasing roughness of additively manufactured (AM) scaffolds and improved surface hydrophilicity. In vitro assay demonstrated enhanced inorganic bone phase formation on the surface of CaCO3-coated AM scaffolds compared to as-manufactured ones. The short-term adhesion of S. aureus onto sample surface was evaluated by fluorescent microscopy 0, 3, and 72 h after cell seeding. It revealed that the surface modification resulted in the decreased number of bacteria attached to the surface after CaCO3 deposition. The morphology, roughness, solubility and superhydrophilic character of the CaCO3 coated EBM-manufactured Ti6Al4V alloy surface are suggested as factors contributing to preventing S. aureus adhesion. Thus, the developed biocomposites based on additively manufactured Ti6Al4V alloy scaffolds and CaCO3 coating can be successfully used in bone tissue regeneration providing the effective growth of inorganic bone phase and preventing the bacteria adhesion. © 2020 Elsevier Ltd and Techna Group S.r.l.
    view abstract10.1016/j.ceramint.2020.07.041
  • Factors affecting the nonsolvent-induced phase separation of cellulose from ionic liquid-based solutions
    Wittmar, A.S.M. and Koch, D. and Prymak, O. and Ulbricht, M.
    ACS Omega 5 (2020)
    In the present work, we report for the first time an in-depth study of the factors influencing porous cellulose film structure formation during the nonsolvent-induced phase separation (NIPS) process from biopolymer solutions in ionic liquid-based solvents. The length of the alkyl chain of the ionic liquid's cation, the solvent/co-solvent ratio, and the type of the cellulose precursor used were found to have great influence both on cellulose solution formation and properties and to the NIPS process with water acting as nonsolvent. In the undiluted form, both studied ionic liquids proved to dissolve almost equally well the cellulose; however, due to differences in viscosities of the formed biopolymer solutions and due to differences in miscibility with water of the two ionic liquids, the used ionic liquid had a strong influence on the film's porous structure formation. The use of increasing amounts of an aprotic co-solvent, here dimethylsulfoxide, improved biopolymer solubilization and also led to the formation of a more pronounced macroporous structure during the NIPS process. The cellulose type also affected the porous structure generation during the NIPS process: with the increase of the molecular weight of the precursor, the viscosity of the formed biopolymer solution increased and the tendency to generate macroporous structures decreased. © 2020 American Chemical Society
    view abstract10.1021/acsomega.0c03632
  • One-step preparation of porous cellulose/chitosan macro-spheres from ionic liquid-based solutions
    Wittmar, A.S.M. and Böhler, H. and Kayali, A.L. and Ulbricht, M.
    Cellulose 27 (2020)
    Abstract: Environmentally friendly and easily biodegradable porous adsorbents for water purification based on cellulose and chitosan blends have been prepared by a green technology utilizing room temperature ionic liquids as main solvent component. The dissolution and re-precipitation mechanisms for the two biopolymers and of their mixtures have been studied in detail, targeting to optimize the composition of the casting solutions and make them adequate for preparation of porous polymer spheres by the “dropping cum phase separation” technique. With the increase of the chitosan content in the sample the time required for the polymer dissolution was increasing and the amount of water necessary to destabilize the formed casting solution was decreasing. Further on, the viscosities of the casting solutions had a strong impact on the formation of the porous structure during the non-solvent induced phase separation process. For the blend spheres prepared from 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]) based solutions materials with higher specific surface areas and improved Cu2+ adsorption capacity were obtained at the increase of the chitosan fraction in the polymer blend. For the samples prepared from 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) based solutions no such a trend has been observed. Graphic abstract: [Figure not available: see fulltext.]. © 2020, Springer Nature B.V.
    view abstract10.1007/s10570-020-03165-y
  • Adhesion, proliferation, and osteogenic differentiation of human mesenchymal stem cells on additively manufactured Ti6Al4V alloy scaffolds modified with calcium phosphate nanoparticles
    Chudinova, E.A. and Surmeneva, M.A. and Timin, A.S. and Karpov, T.E. and Wittmar, A. and Ulbricht, M. and Ivanova, A. and Loza, K. and Prymak, O. and Koptyug, A. and Epple, M. and Surmenev, R.A.
    Colloids and Surfaces B: Biointerfaces 176 (2019)
    In the present study, biocomposites based on 3D porous additively manufactured Ti6Al4V (Ti64) scaffolds modified with biocompatible calcium phosphate nanoparticles (CaPNPs) were investigated. Ti64 scaffolds were manufactured via electron beam melting technology using an Arcam machine. Electrophoretic deposition was used to modify the scaffolds with CaPNPs, which were synthesized by precipitation in the presence of polyethyleneimine (PEI). Dynamic light scattering revealed that the CaP/PEI nanoparticles had an average size of 46 ± 18 nm and a zeta potential of +22 ± 9 mV. Scanning electron microscopy (SEM) revealed that the obtained spherical CaPNPs had an average diameter of approximately 90 nm. The titanium-based scaffolds coated with CaPNPs exhibited improved hydrophilic surface properties, with a water contact angle below 5°. Cultivation of human mesenchymal stem cells (hMSCs) on the CaPNPs-coated Ti64 scaffolds indicated that the improved hydrophilicity was beneficial for the attachment and growth of cells in vitro. The Ti6Al4V/CaPNPs scaffold supported an increase in the alkaline phosphatase (ALP) activity of cells. In addition to the favourable cell proliferation and differentiation, Ti6Al4V/CaPNPs scaffolds displayed increased mineralization compared to non-coated Ti6Al4V scaffolds. Thus, the developed composite 3D scaffolds of Ti6Al4V functionalized with CaPNPs are promising materials for different applications related to bone repair. © 2018 Elsevier B.V.
    view abstract10.1016/j.colsurfb.2018.12.047
  • Functionalization of titania nanotubes with electrophoretically deposited silver and calcium phosphate nanoparticles: Structure, composition and antibacterial assay
    Chernozem, R.V. and Surmeneva, M.A. and Krause, B. and Baumbach, T. and Ignatov, V.P. and Prymak, O. and Loza, K. and Epple, M. and Ennen-Roth, F. and Wittmar, A. and Ulbricht, M. and Chudinova, E.A. and Rijavec, T. and Lapanje, A. and Surmenev, R.A.
    Materials Science and Engineering C 97 (2019)
    Herein TiO2 nanotubes (NTs) were fabricated via electrochemical anodization and coated with silver and calcium phosphate (CaP) nanoparticles (NPs) by electrophoretic deposition. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) revealed that Ag and CaP NPs were successfully deposited onto the TiO2 NTs. Using X-ray diffraction, only anatase and Ti were observed after deposition of Ag and CaP NPs. However, X-ray photoelectron spectroscopy (XPS) analysis revealed that the binding energy (BE) of the Ag and CaP NP core levels corresponded to metallic Ag, hydroxyapatite and amorphous calcium phosphate, based on the knowledge that CaP NPs synthesized by precipitation have the nanocrystalline structure of hydroxyapatite. The application of Ag NPs allows for decreasing the water contact angle and thus increasing the surface free energy. It was concluded that the CaP NP surfaces are superhydrophilic. A significant antimicrobial effect was observed on the TiO2 NT surface after the application of Ag NPs and/or CaP NPs compared with that of the pure TiO2 NTs. Thus, fabrication of TiO2 NTs, Ag NPs and CaP NPs with PEI is promising for diverse biomedical applications, such as in constructing a biocompatible coating on the surface of Ti that includes an antimicrobial effect. © 2018 Elsevier B.V.
    view abstract10.1016/j.msec.2018.12.045
  • Photocatalytic and magnetic porous cellulose macrospheres for water purification
    Wittmar, A.S.M. and Fu, Q. and Ulbricht, M.
    Cellulose (2019)
    Abstract: In this work, we report the preparation of photocatalytically active and easy to recycle porous cellulose-based spheres from polymer solutions in ionic liquid/dimethylsulfoxide mixtures by using the dropping cum phase separation technique. The factors affecting the sphere structure formation in relation to their efficiency as photocatalysts have been studied in detail. It was found that the increase of the nanoparticulate dopant fraction (TiO 2 and/or Fe 3 O 4 ) in the casting solution led to the formation of nanocomposites with a higher specific surface area as well as with enhanced photocatalytic activity. The embedment of the TiO 2 nanoparticles in the polymeric matrix did not change the bandgap of the photocatalyst. Furthermore, the co-doping with Fe 3 O 4 had no negative impact on the photocatalytic activity of the TiO 2 doped porous cellulose spheres. The addition of a moderate amount of dimethylsulfoxide led to an improvement of the photocatalytic activity of the formed nanocomposites, due to an increase of the matrix porosity without an agglomeration of the active nanoparticles. However, higher fractions of dimethylsulfoxide led to the agglomeration of the photocatalytic nanoparticles and therefore a decrease of the photocatalytic activity of the hybrid materials. The obtained porous spheres could be successfully recycled and reused in at least five consecutive cycles for the photocatalytic degradation of the model organic pollutant Rhodamine B in aqueous solution. Additionally, the prepared porous spheres also exhibited good adsorber properties toward Cu 2+ ions which were used in this study as model metal ion pollutant in water. Graphical abstract: [Figure not available: see fulltext.]. © 2019, Springer Nature B.V.
    view abstract10.1007/s10570-019-02401-4
  • Ionic Liquid-Based Route for the Preparation of Catalytically Active Cellulose-TiO2 Porous Films and Spheres
    Wittmar, A. and Ulbricht, M.
    Industrial & Engineering Chemistry Research 56 (2017)
    The present work evaluates the possibilities of processing cellulose with ionic liquids and functional nanoparticles like TiO2 toward a new generation of porous nanocomposites, shaped as films or spheres, which may find direct application in water purification, catalysis, and selfcleaning materials. The focus was set on the factors controlling the formation of the porous film structure during the nonsolvent induced phase separation process from polymer solutions in ionic liquids via immersion in water and during the porous film drying step. Temperature and cosolvent addition facilitate cellulose solubilization and help control the phase separation by improving the mass transfer. The complex relation between the catalytic activity of the porous TiO2-cellulose nanocomposite materials obtained under different processing conditions and their structure has been studied during the photodegradation of model organic dyes like rhodamine B and methylene blue. After drying, the catalytic activity of the nanocomposites decreases as a consequence of the reformation of the intra- and intermolecular hydrogen bonds in cellulose which diminish the flexibility and the mobility of the fine cellulose fibrils network.
    view abstract10.1021/acs.iecr.6b04720
  • Photocatalytic and Magnetic Porous Cellulose-Based Nanocomposite Films Prepared by a Green Method
    Wittmar, A. and Fu, Q. and Ulbricht, M.
    ACS Sustainable Chemistry and Engineering 5 (2017)
    The present work expands our previous studies related to cellulose processing with room-temperature ionic liquids and simultaneous integration of functional nanoparticles toward photocatalytically active and easily recyclable nanocomposite porous films based on a renewable matrix material. Porosity can be tuned by the selection of phase separation conditions for the films obtained from the casting solutions of cellulose in ionic liquids or their mixture with an organic co-solvent. TiO2 nanoparticles confer to the nanocomposite photocatalytic activity, while Fe3O4 nanoparticles make it magnetically active. The photocatalytic activity of the cellulose film containing 10 mg of TiO2 was 1 order of magnitude lower than that of the same amount of pure TiO2 nanopowder, due to the reduction of the active catalytic surface which can be reached by UV irradiation after embedment in the polymer matrix. However, this fixation in a solid polymer support allows facile recovery of the catalyst after use. The rate constant when using the cellulose nanocomposite doped with TiO2 and Fe3O4 (k ≈ 0.0019 min-1) is very close to that for the corresponding composite containing only TiO2 (k ≈ 0.0017 min-1), suggesting that co-doping with Fe3O4 nanoparticles did not diminish the photocatalytic activity of the final composite, which can be easily separated from solution with a magnet. Additionally, by Fe3O4 doping, the composite material's temperature can be homogeneously increased by ∼12 K via exposure to a high-frequency alternating magnetic field (AMF) for 5 min. For an optimal thermal response to AMF, the magnetite nanoparticles have to be homogeneously dispersed within the polymer matrix. The preparation method for the casting solution has been found to play an essential role for the one-step fabrication of multifunctional cellulose-based nanocomposite materials. © 2017 American Chemical Society.
    view abstract10.1021/acssuschemeng.7b01830
  • Correlation between surface properties and wettability of multi-scale structured biocompatible surfaces
    Gorodzha, S.N. and Surmeneva, M.A. and Prymak, O. and Wittmar, A. and Ulbricht, M. and Epple, M. and Teresov, A. and Koval, N. and Surmenev, R.A.
    IOP Conference Series: Materials Science and Engineering 98 (2015)
    The influence of surface properties of radio-frequency (RF) magnetron deposited hydroxyapatite (HA) and Si-containing HA coatings on wettability was studied. The composition and morphology of the coatings fabricated on titanium (Ti) were characterized using atomic force microscopy (AFM) and X-ray diffraction (XRD). The surface wettability was studied using contact angle analysis. Different geometric parameters of acid-etched (AE) and pulse electron beam (PEB)-treated Ti substrates and silicate content in the HA films resulted in the different morphology of the coatings at micro- and nano- length scales. Water contact angles for the HA coated Ti samples were evaluated as a combined effect of micro roughness of the substrate and nano-roughness of the HA films resulting in higher water contact angles compared with acid-etched (AE) or pulse electron beam (PEB) treated Ti substrates. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/98/1/012026
  • Nano-hydroxyapatite-coated metal-ceramic composite of iron-tricalcium phosphate: Improving the surface wettability, adhesion and proliferation of mesenchymal stem cells in vitro
    Surmeneva, M.A. and Kleinhans, C. and Vacun, G. and Kluger, P.J. and Schönhaar, V. and Müller, M. and Hein, S.B. and Wittmar, A. and Ulbricht, M. and Prymak, O. and Oehr, C. and Surmenev, R.A.
    Colloids and Surfaces B: Biointerfaces 135 (2015)
    Thin radio-frequency magnetron sputter deposited nano-hydroxyapatite (HA) films were prepared on the surface of a Fe-tricalcium phosphate (Fe-TCP) bioceramic composite, which was obtained using a conventional powder injection moulding technique. The obtained nano-hydroxyapatite coated Fe-TCP biocomposites (nano-HA-Fe-TCP) were studied with respect to their chemical and phase composition, surface morphology, water contact angle, surface free energy and hysteresis. The deposition process resulted in a homogeneous, single-phase HA coating. The ability of the surface to support adhesion and the proliferation of human mesenchymal stem cells (hMSCs) was studied using biological short-term tests in vitro. The surface of the uncoated Fe-TCP bioceramic composite showed an initial cell attachment after 24 h of seeding, but adhesion, proliferation and growth did not persist during 14 days of culture. However, the HA-Fe-TCP surfaces allowed cell adhesion, and proliferation during 14 days. The deposition of the nano-HA films on the Fe-TCP surface resulted in higher surface energy, improved hydrophilicity and biocompatibility compared with the surface of the uncoated Fe-TCP. Furthermore, it is suggested that an increase in the polar component of the surface energy was responsible for the enhanced cell adhesion and proliferation in the case of the nano-HA-Fe-TCP biocomposites. © 2015 Elsevier B.V.
    view abstract10.1016/j.colsurfb.2015.07.057
  • Routes towards catalytically active TiO2 doped porous cellulose
    Wittmar, A. and Thierfeld, H. and Köcher, S. and Ulbricht, M.
    RSC Advances 5 (2015)
    Cellulose-TiO<inf>2</inf> nanocomposites have been successfully prepared by non-solvent induced phase separation, either from cellulose solutions in ionic liquids or from cellulose acetate solutions in classical organic solvents followed by deacetylation ("regeneration"). Commercially available titania nanoparticles from gas phase synthesis processes have been used and processed as dispersions in the respective polymer solution. The used TiO<inf>2</inf> nanoparticles have been characterized by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD), and their dispersions in ionic liquids and organic solvents have been evaluated by dynamic light scattering (DLS) and advanced rheology. The intermediate polymer solutions used in the phase separation process have been studied by advanced rheology. The resulting nanocomposites have been characterized by means of scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Special attention has been given to the complex relationship between the characteristics of the phase separation process and the porous structure of the formed nanocomposites. Two catalytic tests, based on the photocatalytic degradation of model organic dyes under UV irradiation, have been used for the characterization of the TiO<inf>2</inf> doped nanocomposites. The proof-of-concept experiments demonstrated the feasibility of photocatalyst immobilization in porous cellulose via phase separation of nanoparticle dispersions in polymer solutions, as indicated by UV-activated dye degradation in aqueous solution. © The Royal Society of Chemistry.2015.
    view abstract10.1039/c5ra03707g
  • Two step and one step preparation of porous nanocomposite cellulose membranes doped with TiO2
    Wittmar, A. and Vorat, D. and Ulbricht, M.
    RSC Advances 5 (2015)
    Cellulose-TiO2 nanocomposites have been successfully prepared by non-solvent induced phase separation from cellulose acetate solutions in classical organic solvents followed by deacetylation ("regeneration"). The cellulose deacetylation has been performed either sequentially, i.e. after the completion of the phase separation process, or simultaneously, i.e. during the phase separation process. Commercially available titania nanoparticles from gas phase synthesis processes have been used and processed as a dispersion in the respective polymer solutions. The resulting nanocomposites have been characterized by means of scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Special attention has been given to the complex relation between the conditions of the deacetylation process, the structure of the resulting TiO2 doped cellulose membranes and their corresponding catalytic activities. Two catalytic activity tests, based on the photocatalytic degradation of model organic dyes under UV irradiation, have been used for the functional characterization of the TiO2 doped nanocomposites. The performed experiments demonstrated the successful photocatalyst immobilization in porous cellulose acetate together with good catalytic activity of this nanocomposite intermediate. By simply varying the conditions of the cellulose deacetylation, nanocomposite cellulose membranes with different structures and properties have been obtained. However after the regeneration of cellulose a partial decrease of the catalytic activity was observed. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5ra16337d
  • Stable zinc oxide nanoparticle dispersions in ionic liquids
    Wittmar, A. and Gautam, D. and Schilling, C. and Dörfler, U. and Mayer-Zaika, W. and Winterer, M. and Ulbricht, M.
    Journal of Nanoparticle Research 16 (2014)
    The influence of the hydrophilicity and length of the cation alkyl chain in imidazolium-based ionic liquids on the dispersability of ZnO nanoparticles by ultrasound treatment was studied by dynamic light scattering and advanced rheology. ZnO nano-powder synthesized by chemical vapor synthesis was used in parallel with one commercially available material. Before preparation of the dispersion, the nanoparticles characteristics were determined by transmission electron microscopy, X-ray diffraction, nitrogen adsorption with BET analysis, and FT-IR spectroscopy. Hydrophilic ionic liquids dispersed all studied nanopowders better and in the series of hydrophilic ionic liquids, an improvement of the dispersion quality with increasing length of the alkyl chain of the cation was observed. Especially, for ionic liquids with short alkyl chain, additional factors like nanoparticle concentration in the dispersion and the period of the ultrasonic treatment had significant influence on the dispersion quality. Additionally, nanopowder characteristics (crystallite shape and size as well as the agglomeration level) influenced the dispersion quality. The results indicate that the studied ionic liquids are promising candidates for absorber media at the end of the gas phase synthesis reactor allowing the direct preparation of non-agglomerated nanoparticle dispersions without supplementary addition of dispersants and stabilizers. © Springer Science+Business Media 2014.
    view abstract10.1007/s11051-014-2341-2
  • Wettability of Thin Silicate-Containing Hydroxyapatite Films Formed by RF-Magnetron Sputtering
    Gorodzha, S.N. and Surmeneva, M.A. and Surmenev, R.A. and Gribennikov, M.V. and Pichugin, V.F. and Sharonova, A.A. and Pustovalova, A.A. and Prymack, O. and Epple, M. and Wittmar, A. and Ulbricht, M. and Gogolinskii, K.V. and Kravchuk, K.S.
    Russian Physics Journal 56 (2014)
    Using the methods of electron and atomic force microscopy, X-ray structural analysis and measurements of the wetting angle, the features of morphology, structure, contact angle and free surface energy of silicon-containing calcium-phosphate coatings formed on the substrates made from titanium VT1-0 and stainless steel 12Cr18Ni10Ti are investigated. It is shown that the coating - substrate system possesses bimodal roughness formed by the substrate microrelief and coating nanostructure, whose principal crystalline phase is represented by silicon-substituted hydroxiapatite with the size of the coherent scattering region (CSR) 18-26 nm. It is found out that the formation of a nanostructured coating on the surface of rough substrates makes them hydrophilic. The limiting angle of water wetting for the coatings formed on titanium and steel acquires the values in the following ranges: 90-92 and 101-104°, respectively, and decreases with time. © 2014 Springer Science+Business Media New York.
    view abstract10.1007/s11182-014-0157-2
  • Influence of the cation alkyl chain length of imidazolium-based room temperature ionic liquids on the dispersibility of TiO2 nanopowders
    Wittmar, A. and Gajda, M. and Gautam, D. and Dörfler, U. and Winterer, M. and Ulbricht, M.
    Journal of Nanoparticle Research 15 (2013)
    The influence of the length of the cation alkyl chain on the dispersibility by ultrasonic treatment of TiO2 nanopowders in hydrophilic imidazolium-based room temperature ionic liquids was studied for the first time by dynamic light scattering and advanced rheology. TiO2 nanopowders had been synthesized by chemical vapor synthesis (CVS) under varied conditions leading to two different materials. A commercial nanopowder had been used for comparison. Characterizations had been done using transmission electron microscopy, X-ray diffraction, nitrogen adsorption with BET analysis, and FT-IR spectroscopy. Primary particle sizes were about 6 and 8 nm for the CVS-based and 26 nm for the commercial materials. The particle size distribution in the dispersion was strongly influenced by the length of the cation alkyl chain for all the investigated powders with different structural characteristics and concentrations in the dispersion. It was found that an increase of the alkyl chain length was beneficial, leading to a narrowing of the particle size distribution and a decrease of the agglomerate size in dispersion. The smallest average nanoparticle sizes in dispersion were around 30 nm. Additionally, the surface functionality of the nanoparticles, the concentration of the solid material in the liquid, and the period of ultrasonic treatment control the dispersion quality, especially in the case of the ionic liquids with the shorter alkyl chain. The influence of the nanopowders characteristics on their dispersibility decreases considerably with increasing cation alkyl chain length. The results indicate that ionic liquids with adapted structure are candidates as absorber media for nanoparticles synthesized in gas phase processes to obtain liquid dispersions directly without redispergation. © 2013 Springer Science+Business Media Dordrecht.
    view abstract10.1007/s11051-013-1463-2
  • Preparation of a silicate-containing hydroxyapatite-based coating by magnetron sputtering: Structure and osteoblast-like MG63 cells in vitro study
    Surmeneva, M.A. and Kovtun, A. and Peetsch, A. and Goroja, S.N. and Sharonova, A.A. and Pichugin, V.F. and Grubova, I.Y. and Ivanova, A.A. and Teresov, A.D. and Koval, N.N. and Buck, V. and Wittmar, A. and Ulbricht, M. and Prymak, O. and Epple, M. and Surmenev, R.A.
    RSC Advances 3 (2013)
    Silicate-containing hydroxyapatite-based coatings with different structure and calcium/phosphate ratios were prepared by radio-frequency magnetron sputtering on silicon and titanium substrates, respectively. Scanning electron microscopy, X-ray diffraction and IR spectroscopy were used to investigate the effect of the substrate bias on the properties of the silicate-containing hydroxyapatite-based coatings. The deposition rate, composition, and microstructure of the deposited coatings were all controlled by changing the bias voltage from grounded (0 V) to -50 and -100 V. The biocompatibility was assessed by cell culture with human osteoblast-like cells (MG-63 cell line), showing a good biocompatibility and cell growth on the substrates. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3ra40446c
  • Dispersions of silica nanoparticles in ionic liquids investigated with advanced rheology
    Wittmar, A. and Ruiz-Abad, D. and Ulbricht, M.
    Journal of Nanoparticle Research 14 (2012)
    The colloidal stabilities of dispersions of unmodified and surface-functionalized SiO 2 nanoparticles in hydrophobic and hydrophilic imidazolium-based ionic liquids were studied with advanced rheology at three temperatures (25, 100, and 200 °C). The rheological behavior of the dispersions was strongly affected by the ionic liquids hydrophilicity, by the nanoparticles surface, by the concentration of the nanoparticles in the dispersion as well as by the temperature. The unmodified hydrophilic nanoparticles showed a better compatibility with the hydrophilic ionic liquid. The SiO 2 surface functionalization with hydrophobic groups clearly improved the colloidal stability of the dispersions in the hydrophobic ionic liquid. The temperature increase was found to lead to a destabilization in all studied systems, especially at higher concentrations. The results of this study imply that ionic liquids with tailored properties could be used in absorbers directly after reactors for gas-phase synthesis of nanoparticles or/and as solvents for their further surface functionalization without agglomeration or aggregation. © Springer Science+Business Media B.V. 2012.
    view abstract10.1007/s11051-011-0651-1
  • Dispersions of various titania nanoparticles in two different ionic liquids
    Wittmar, A. and Ulbricht, M.
    Industrial and Engineering Chemistry Research 51 (2012)
    The dispersibility of different lab-made and commercial TiO 2 nanoparticles prepared by gas-phase processes in room temperature ionic liquids was for the first time studied by dynamic light scattering and advanced rheology. The characterization of the nanopowders has been done with transmission electron microscopy, X-ray diffraction analysis, nitrogen adsorption, and Brunauer-Emmett-Teller (BET) analysis and FT-IR spectroscopy. The colloidal stabilities of the resulting dispersions were strongly influenced by particle characteristics such as aggregation level, mean particle size, and surface functionality. The period of the ultrasound treatment, the powder concentration in the dispersion, and the hydrophilicity of the ionic liquid were also important influences. It was found that most types of powders disperse better in the hydrophilic ionic liquid because of the hydroxyl groups and adsorbed water present on the powders' surfaces. The best dispersions over a broader concentration range were obtained for a lab-made powder produced by chemical vapor synthesis (aerosol method) which had the smallest nonaggregated particles. © 2012 American Chemical Society.
    view abstract10.1021/ie203010x
  • Hybrid sol-gel coatings doped with transition metal ions for the protection of AA 2024-T3
    Wittmar, A. and Wittmar, M. and Ulrich, A. and Caparrotti, H. and Veith, M.
    Journal of Sol-Gel Science and Technology 61 (2012)
    The protective capabilities of sol-gel coatings are determined by their physical barrier properties. For an effective protection, a homogenous crack-free material is required, which prevents from attacks of corrosive species. When the coating is damaged, active corrosion protection is usually achieved by the use of inhibitors. Among the different inhibitors rare earth ions and especially cerium have shown effective inhibiting properties. Due to the complexity of the corrosion processes, a combination of inhibitors is expected to be superior to a monocomponent inhibiting. The aim of this study was to prove which other ions, used in combination with cerium, can improve the corrosion protection abilities of hybrid silica based inorganic- organic sol-gel coatings applied on aluminium alloy 2024 substrates. Mixtures of cerium nitrate with two other potential inhibitor substances were incorporated into a sol- gel matrix and their behaviour in neutral salt spray test and during EIS measurements was investigated. The Ce-P-Pr inhibitor combination (Ce3+, PO4 3-, Pr3+) has shown the best long-term corrosion protection properties at low doping levels. © Springer Science+Business Media, LLC 2011.
    view abstract10.1007/s10971-011-2666-6
  • Hybrid sol-gel silica films with (TiO 2-CeO 2) binary nanopowders
    Zaharescu, M. and Nicolescu, M. and Gartner, M. and Barau, A. and Predoana, L. and Anastasescu, M. and Stoica, M. and Szekeres, A.
    Journal of Physics: Conference Series 356 (2012)
    In the present work the preparation of hybrid sol-gel silica coatings doped with binary TiO 2-CeO 2 nanopowders was studied. The oxide powder was embedded in the hybrid matrix either by in-situ generation or by previously prepared powder dispersion. The main objective of the work was to establish a correlation between the method of generation of the dopant particles in the system and the properties of the films. The films were deposited on silicon wafer and glass substrates by the 'dip-coating' method and characterized in the as-prepared stage and after annealing at 120 °C. The optical and morphological properties of the films deposited on glass and silicon wafer were determined by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/356/1/012018
  • Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy
    Kozhukharov, S. and Kozhukharov, V. and Schem, M. and Aslan, M. and Wittmar, M. and Wittmar, A. and Veith, M.
    Progress in Organic Coatings 73 (2012)
    The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol-gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce2(SO 4)3, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al2O3 nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al2O3 nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.porgcoat.2011.09.005
  • Simple preparation routes for corrosion protection hybrid sol-gel coatings on AA 2024
    Wittmar, A. and Caparrotti, H. and Wittmar, M. and Veith, M.
    Surface and Interface Analysis 44 (2012)
    In recent years, many hybrid inorganic-organic systems have been proposed in order to replace the traditional conversion coatings on metals like aluminum, and some results have been promising. However, many proposed solutions are based on complicated processes which are not easy to be adapted to industrial scale. The aim of this study was to establish a simple process leading to the production of highly efficient corrosion protective hybrid sol-gel coating systems for the aluminum alloys as replacement for the highly hazardous conventional chromate conversion coatings. Hybrid coatings have been realized by means of the sol-gel process. CeO 2 and ZnO have been introduced as dispersions of nanoparticles in the system and used as corrosion inhibitors. The aim of this work was to obtain pore-free coatings with increased barrier properties using nanoparticles that possess the double function of pore fillers and corrosion inhibitors. The proposed processes led to coating materials with good adherence to the aluminum substrate and an extremely long life in the accelerated neutral salt spray test according to DIN ISO 9227. Electrochemical impedance spectroscopy approves these results by high impedance values in the low-frequency region of the Bode plot. © 2011 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.3771
  • The influence of the inhibitor particle sizes to the corrosion properties of hybrid sol-gel coatings
    Wittmar, A. and Wittmar, M. and Caparrotti, H. and Veith, M.
    Journal of Sol-Gel Science and Technology 59 (2011)
    The influence of the inhibitor particle size (nano and micro cerium dioxide) embedded in several hybrid sol-gel coating systems for the corrosion protection of aluminium AA2024 alloy was studied, as well as the influence of other parameters like the inhibitor loading level and the method of reticulation. The properties of the obtained coatings were evaluated by means of transmission electron microscopy, accelerated salt spray test and electrochemical impedance spectroscopy. All the varied parameters proved to have an important influence on the corrosion mechanism, and an improvement induced by the use of nanometric inhibitors in comparison with the micrometric ones was demonstrated. In the case of the strong reticulated matrix, an inhomogeneous dispersion of the inhibiting species (micro inhibitor) favours the tension formation, making the coating more vulnerable to the corrosion attack. For the same doping level it was observed that the reticulation with 1-methylimidazole (MI) leads to a slightly better corrosion protection. Concomitantly, it was shown that high loadings with inhibitors have an adverse effect on the corrosion protection. © Springer Science+Business Media, LLC 2011.
    view abstract10.1007/s10971-011-2536-2
  • hybrid nanocomposites

  • ionic liquids

  • nanoparticles

  • porous nanocomposites

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