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HISTORY:

The “Nanofunctional and Nanocomposite Materials” Lab emerged from the “Phylosil” Lab after its reorganization in 2006.


OBJECTIVE:

The laboratory is focused on the development of nanostructured materials with targeted functionalities and properties for environment and energy applications.


CURRENT INTERESTS

  • TiO2 and new photocatalytic materials
  • semiconductor morphology control, doping, surface modification and composites’ synthesis for efficient photocatalysts;
  • applications in photocatalytic air pollutants’ degradation, H2 production from water splitting and CO2 reduction to hydrocarbons;
  • Graphene and 2D materials
  • different synthetic routes, surface modification for endowing with different functionalities;
  • applications in supercapacitors;
  • graphene/polymer composites for tailoring O2 and H2O barrier properties.

MAIN ACTIVITIES

  1. Photoactive Materials for Environmental Applications

Morphosynthesis of anatase nanocrystals with exposed {001} facets for environmental applications

The anatase phase of TiO2 has in most cases higher photocatalytic activity than the other crystalline allotropies. Recently several studies have shown that the (001) surface of anatase is more reactive than the thermodynamically stable (101) surface. This implies that anatase structures with dominant {001} crystal facets will exhibit enhanced photocatalytic properties.

In our laboratory, the formation of dominant {001} crystal facets was managed by combining hydrofluoric acid and other non-toxic substances as a capping agent, which facilitates the formation of these crystal facets by lowering their free energy and making them more stable. The anatase structures produced using hydro/solvothermal method are micro/nanoplates and hollow microspheres composed of crystals with {001} dominant facets.

Nanofunctional_and_Nanocomposite Materials_Img2 SEM micrograph of TiO2 anatase microcrystals Nanofunctional_and_Nanocomposite Materials_Img3 SEM micrograph of anatase microspheres Nanofunctional_and_Nanocomposite Materials_Img4 TEM micrograph of the TiO2 anatase nanocrystals

The TiO2 samples with mentioned morphologies exhibited higher photonic efficiency in NO oxidation and acetaldehyde decomposition than Evonik Degussa P25, which was used as a reference.

Decoration of TiO2 Anatase Nanoplates with Silver Nanoparticles on the {101} Crystal Facets

Ag nanoparticles were photodeposited on the {101} crystal facets of the TiO2 anatase nanoplates. This was achieved with the photoreduction of AgNO3 in methanol solution in which TiO2 anatase nanoplates were suspended while UVA irradiation was performed. The manipulation of their size was achieved by controlling the UVA light irradiation period of time. Under illumination, electron and hole pairs are created which move to the surface of the nanoplates and are spatially separated. The electrons migrate to the {101} facets while the holes migrate to the {001} facets. As a result, Ag clusters are formed exclusively located on the {101} facets of the nanoplates.

Nanofunctional_and_Nanocomposite Materials_Img5 TEM micrographs of the Ag-decorated TiO2 anatase nanoplates for the shortest illumination period t=1min (a) and the longest t=20min (b).

All Ag-decorated TiO2 nanoplates exhibited enhanced photocatalytic properties, regarding NO oxidation and acetaldehyde decomposition, in comparison to the pure TiO2 nanoplates due to the lower recombination rate of the photogenerated electrons and holes.

Morphosynthesis of magnesium and manganese ions doped anatase nanocrystals with exposed {001} facets

The doped and undoped nanocrystals with exposed {001} crystal facets were synthesized by a solvothermal method. The morphological structure of TiO2 nanocrystals (a,b) changed with Mg2+ doping. The nanoplates became larger and pilled one on top of the other forming this way a separate bigger nanocrystal (c,d). As the content of Mg2+ ions increases, more nanoplates pile up making the crystal more dense and thicker (e,f). With the further increase of Mg2+ content in the anatase lattice the nanoplates are densely stocked together forming a sea-urchin morphological structure (g,h). The corresponding SAED patterns show how the nanoplates gradually lose their perfect alignment and perfect single crystal structure and become randomly oriented one on top of the other for the highest Mg2+ content.

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TEM micrographs of the pure TiO2 anatase nanoplates, in the insert corresponding SAED pattern (a,b), 2 at% Mg2+/TiO2, in the insert corresponding SAED pattern (c,d), 5.1 at% Mg2+/TiO2 in the insert corresponding SAED pattern(e,f), 6.2 at% Mg2+/TiO2 anatase nanoplates, in the insert corresponding SAED pattern (g,h).

The TEM micrographs of the doped nanocrystals with Mn2+ ions in different atomic ratios reveal that the obtained doped TiO2 product consists of uniform, well-defined plate-shaped structures possessing a rectangular outline. The HRTEM images (f,g) of the anatase samples viewed along the [001] and [010] crystallographic directions, as well as the SAED pattern (h) confirm that the square surfaces are {001} facets of the single-crystalline nanoplate (e).

Nanofunctional_and_Nanocomposite Materials_Img7 TEM micrographs of the pure TiO2 anatase nanoplates (a,e), 2 at% Mn2+/TiO2 nanoplates (b), 6 at% Mn2+/TiO2 nanoplates (c), 7 at% Mn2+/TiO2 nanoplates (d), individual nanoplate recorded along [001] (f), individual nanoplate recorded along [010] (g), SAED pattern of the nanoplate (h).

The photocatalytic tests that were performed to all samples revealed that Mg2+ and Mn2+ ion doping in small amounts causes better separation of electrons and holes thus enhancing the photocatalystic activity in comparison to the pure TiO2 nanoplates. Lastly, the manganese doped nanoplates showed photocatalytic activity towards the visible light region in comparison to the pure anatase sample.

TiO2 surface modification: TiO2 core/inorganic SiO2 shell structures

Core-shell nanostructures with TiO2 core and aerogel SiO2 shell with controlled shell thickness and porosity were developed. Such surface modification aims protection and immobilization of TiO2 photocatalyst in different construction material substrates. Partial covering and porous shell structure allow "access" of the reactants and decomposition products to and from the TiO2 photocatalyst core.

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Schematic total & partial cover of TiO2 with SiO2 aerogel (a) and TEM micrograph of partial cover of TiO2 nanoparticles with SiO2 aerogel (b) and (c).

The photonic efficiency in NO oxidation and NOx removal was higher in the samples with higher SiO2/TiO2 mass ratios equal to 0.3 and 0.5.

TiO2 surface functionalization with organic compounds

Treatment with organic compounds such as oleic acid, oleylamine and organosilane endow titania nanonoparticles with specific properties like hydrophilicity or hydrophobicity, reduced mean size, improved dispersibility in polar and non-polar solvents, improved photocatalytic efficiency even under visible light irradiation.

TiO2 nanoparticles modified with hydrophilic organosilane compound (3-(2-aminoethylamino)propyltrimethoxysilane) depict the best photocatalytic activity for NO oxidation and NOx removal.

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TiO2 nanoparticles grafted simultaneously with hydrophilic 3-(2-aminoethylamino) propyltrimethoxysilane and hydrophobic octadecyltriethoxysilane showed excellent photoactivity concerning De-NOx process in relation to bare photocatalyst. The incorporation of the TiO2 nanoparticles modified with the two mentioned modifiers in paints with low resin content, gave the best result in comparison with other modifiers as well as improved photocatalytic properties of the paint in comparison with the incorporation of bare photocatalyst.

In order to obtain anisotropic and controllable hydrophobic properties biphase toluene/water emulsion processing was used. When amphiphilically modified nanoparticles with hydrophilic polyethylene glycol and hydrophobic oleylamine were incorporated in cement matrix, they depict excellent photocatalytic activity. The attached hydrophobic groups facilitate the accumulation of TiO2 at the cement surface and its exposure to irradiation and air pollutants, while the attached hydrophilic OH groups of polyethylene glycol enhance the De-NOx ability.

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Photonic efficiencies of photocatalytic paint with TiO2 nanoparticles modified with hydrophilic and hydrophobic modifiers. Photonic efficiency of photocatalytic cement with TiO2 modified with hydrophilic and hydrophobic modifiers in biphase solvent.

Nanomaterials by Flame Spray Pyrolysis

The flame spray pyrolysis (FSP) is a fast (one step) and versatile process for the production of a wide variety of oxide nanoparticles. A homemade FSP set-up was constructed and utilized to synthesize TiO2 pure and composite materials.

Precursor solution is injected through the centre capillary of the FSP nozzle by a syringe pump at different rates. Oxygen is fed through the surrounding annulus as dispersion gas and a supporting CH4/O2 flame surrounding the oxygen gas annulus stabilizes the spray flame. A sintered metal plate ring provides an additional sheath flow (oxygen or nitrogen) surrounding the spray flame. The resulting powders are collected on glass microfiber filters with the aid of a vacuum pump. The relationship between the particle properties and the synthesis conditions (the combustion environment - excess or restrictive oxygen, the pump/precursor rate and the enthalpy combustion of the fuel) is studied.

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Although the surface specific area of the TiO2/perlite composites was lower than that of P25 sample, their photocatalytic activity in NO oxidation was comparable and even higher for some of them. This result was attributed to the presence of perlite glassy substrate that facilitates photocatalyst dispersion as well as to the synergistic effect between two crystalline phases.

Composite TiO2/ZnO films

Composite TiO2/ZnO films were prepared targeting novel materials with improved photocatalytic and optical properties. The films were synthesized via sol-gel technique using Ti alkoxide and Zn acetate as metal precursors and dip coating deposition on quartz substrate.

The optical characteristics were assessed via measurements of UV-vis spectra of the films and via modeling of their complex refractive index in the frames of popular dispersion models (Forouhi-Bloomer (FB) and Tauc-Lorentz (TL). The photocatalytic properties of the films were investigated by monitoring their photoinduced hydrophilicity and NOx oxidation activity under UV irradiation. Modelling of the films’ optical constant via FB and TL models allowed determination of their energy band gap and thickness. The recorded blue shift of the absorption edge of composite films Ti25 and Ti50 was attributed to Burstein-Moss effect concerning heavy doped semiconductors. Under UV illumination, the hydrophilicity of all the films gradually increased with the change for the ZnO containing films to be more prominent. The pure TiO2 film exhibited the highest photocatalytic activity especially in NO to NO2 oxidation.

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Graphene/TiO2 composite materials

Exfoliated graphene and graphene oxide were separately introduced in TiO2 matrix in order to increase the photocatalytic efficiency of the photocatalyst through enhanced charge carriers separation. The structural properties and the photocatalytic activities of TiO2/G and TiO2/GO composites in various ratios were investigated. The TiO2/GO materials exhibited significant improvement in the NOx removal from ambient air especially under visible light irradiation. Coupling of graphene with TiO2 in the form of anatase with highly active {001} crystallographic facets was achieved via hydrothermal processing.

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One-step fabrication of graphitic carbon nitride (g-C3N4) hybrids

Graphitic carbon nitride was prepared by direct pyrolysis of melamine under Ar atmosphere. Various temperatures and heating durations were tested, until the optimization of the final product was achieved. The obtained material appears to be really hard and revealed an interesting tube-like morphology.

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Melamine was mixed with TiO2 in various ratios and pyrolyzed in order to receive g-C3N4/TiO2 hybrids. These hybrids showed a really good and stable photocatalytic activity which opens a whole new range of possibilities in the field of air pollutants removal

  1. Graphene based materials for energy applications

Non-activated high surface area expanded graphite oxide

The preparation of graphene by chemical methods offers the possibility of producing it on a large scale and, at the same time, of controlling its quality. This depends on the properties of (i) the pristine graphite, (ii) the oxidation method used, and (iii) the final reduction of the graphene oxide (GO) to graphene.

In the present study some of these factors were investigated. The natural graphite without pretreatment and expandable graphite pretreated in a microwave oven (850 W) were used for the preparation of GO according to a modified Staudenmaier method. After oxidation the produced graphite oxide (GtO) were subsequently subjected to microwave irradiation since during this treatment both exfoliation and reduction occurs. All the types of graphite that were tested had high BET surface areas ranging from 940 to 2490 m2/g. According to cyclic voltammetry (CV), all samples exhibited supercapacitor behavior.

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SEM images of GOG2L (natural graphite 10 mesh twice oxidated) (a), of MWGOG2L that are GOG2L after microwave exfoliation/reduction (b), magnification of MWGOG2L, (c) TEM image of MWGOG2L (d).

Single-step hydrothermal synthesis of reduced graphene oxide and Fe2O3/reduced graphene oxide composites

Hydrothermal process constitutes a facile, one-step, low cost and environmental friendly approach that results in production of freestanding reduced graphene oxide (rGO) sheets as well as intercalated with nanoparticles rGO sheets to prevent restacking. In this study, rGO sheets and their intercalated with Fe2O3 analogues were prepared using this treatment process.

TEM image presents rGO sheets decorated by Fe2O3 nanoparticles (a). In HRTEM image (b), a few layer graphene material is observed supporting our claim about hydrothermal exfoliation to a significant extend. CV measurements indicated that the sample treated with K2CO3 had the best performance in terms of capacitance.

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Graphitic materials decorated with Ag nanoparticles

The electrical properties of graphenes can be improved by decoration with metal nanoparticles. The incorporated particles also prevent aggregation and further restacking of graphenes thus acting as both conductor and nanospacer. In our lab the activities were focused on deposition of silver (Ag) on GO sheets.

Ag nanoparticles were deposited via chemical reduction of Ag salts using NaBH4 and photo-reduction using solar light irradiation. The shift from GtO prepared by Hummers method to rGO was evidenced by XRD, FT-IR, Raman and XPS analysis. The CV measurements revealed enhancement in the specific capacitance with the increase of the reduction degree. The photo-reduction of GO using solar light that can be easily complemented with simultaneous metal nanoparticles deposition provides a green sustainable route towards preparation of graphene-based heterostructures for applications beyond that of supercapacitors.

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Crumpled graphene oxide by Spray Pyrolysis (SP)

Restacking of graphene/graphene oxide sheets is a serious problem of these materials. To avoid restacking except for chemical routes based on pillaring procedures, the sheets’ crumpling using spray pyrolysis is proposed.

In our lab, spray pyrolysis system was constructed consisting of three main parts: (1) an atomizer, which atomizes the precursor solutions and converts them to droplets, (2) a vertical furnace (length: 130 cm, diameter: 5 cm) with controllable temperature until 1200oC, and (3) a filter to collect prepared nanoparticles.

GtO was prepared using Hummer’s synthetic route. Dried GtO powder was added to water at ~1.5 mg/L forming dispersion that was sprayed using an aerosol generator. Passing through a furnace heating zone at 300 degrees, the droplets formed crumpled GO preventing GO sheets from restacking.

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Graphene/polymer composite materials

In order oxygen barrier property and controlled water vapor permeability to be granted to the polymer, microwave exfoliated graphite (GNP) and graphene oxide were introduced in polypropylene (PP) matrix. The quantities, composition and the method of introduction of the graphene component were investigated targeting the mentioned properties, the homogeneity and the mechanical properties of the graphene/PP materials. Although increased oxygen barrier ability was achieved after addition of GNP, the mechanical properties were not improved. To overcome this obstacle, functionalization of graphene is in progress.

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Electrochemical deposition of few-layer graphene films on ITO substrates

Electrochemical deposition (ECD) is a process based on the movement of charged particles in suspension toward an oppositely charged electrode and their deposition on it. The EDP of graphene films is possible from GO suspensions due to presence of oxygen-containing groups like epoxides and carboxylic acids on GO surface. These groups endow the GO flakes with hydrophilic properties and therefore contribute to creation of stable aqueous suspensions which is important aspect in ECD. Also, the deprotonation of carboxylic groups in water leads to formation negatively charged moieties that can be deposited on positively charged electrode.

In our lab, GO films were deposited on ITO substrate under application of +4 V electrical potential and controlling deposition time and pH of the GO (1 mg/mL) aqueous suspensions. The Raman spectra exhibit small decrease in ID/IG ratio with the increase of pH that is attributed to partial simultaneous reduction of GO films during deposition procedure.

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