Saara Heinonen

Applications of supercritical carbon dioxide in materials processing and synthesis

Supercritical carbon dioxide (scCO2) is carbon dioxide that is held beyond supercritical conditions of 31.1 °C and 7.4 MPa. As a non-toxic and environmentally benign green solvent, it has been widely used in the food and pharmaceutical industries for extraction. However, scCO2 also has many unique properties and thus has great potential for advanced, green materials processing. This concise review focuses on its use as a solvent and an anti-solvent in materials processing and synthesis. Different experimental routes are described that are used to synthesize bulk materials, thin films, coatings, particle suspensions and powders. Examples from the literature are highlighted to illustrate the different experimental set-ups and applications of the resulting materials. This review endeavours to reveal the potential and versatility of scCO2 in materials processing and synthesis, aiming to encourage a wider application of scCO2 to open more opportunities in innovative green processing of both traditional and functional materials.

Bacterial growth on a superhydrophobic surface containing silver nanoparticles

The antibacterial effect of silver can be exploited in the food and beverage industry and medicinal applications to reduce biofouling of surfaces. Very small amount of silver ions are enough to destructively affect the metabolism of bacteria. Moreover, superhydrophobic properties could reduce bacterial adhesion to the surface. In this study we fabricated superhydrophobic surfaces that contained nanosized silver particles. The superhydrophobic surfaces were manufactured onto stainless steel as combination of ceramic nanotopography and hydrophobication by fluorosilane. Silver nanoparticles were precipitated onto the surface by a chemical method. The dissolution of silver from the surface was tested in an aqueous environment under pH values of 1, 3, 5, 7, 9, 11 and 13. The pH value was adjusted with nitric acid and ammonia. It was found that dissolution rate of silver increased as the pH of the solution altered from the pH of de-ionized water to lower and higher pH values but dissolution occurred also in de-ionized water. The antimicrobial potential of this coating was investigated using bacterial strains isolated from the brewery equipment surfaces. The results showed that the number of bacteria adhering onto steel surface was significantly reduced (88%) on the superhydrophobic silver containing coating.

Photocatalytically active titanium dioxide nanopowders: Synthesis, photoactivity and magnetic separation

Two approaches were used to obtain nanocrystalline titanium dioxide (TiO2) photocatalyst powders. Firstly, low-temperature synthesis method and secondly liquid flame spraying. The structural properties of the produced powders were determined with X-ray diffraction, transmission electron microscopy and nitrogen adsorption tests. The photocatalytic properties of the powders were studied with methylene blue (MB) discoloration tests. After discolorations tests, TiO2 was coagulated with magnetite particles using FeCl3·6 H2O at a fixed pH value. Magnetic separation of coagulated TiO2 and magnetite was carried out by a permanent magnet. The obtained results showed that the particle size of the powders synthesized at low-temperature was very small and the specific surface area high. The phase content of the powder was also shown to depend greatly on the acidity of the synthesis solution. Powder synthesized by liquid flame spraying was mixture of anatase and rutile phases with essentially larger particle size and lower specific surface area than those of low-temperature synthesized powders. The MB discoloration test showed that photocatalytic activity depends on the phase structure as well as the specific surface area of the synthesized TiO2 powder. The magnetic separation of TiO2–magnetite coagulate from solution proved to be efficient around pH:8.

Effect of temperature and concentration of precursors on morphology and photocatalytic activity of zinc oxide thin films prepared by hydrothermal route

Zinc oxide (ZnO) is an important semiconductive material due to its potential applications, such as conductive gas sensors, transparent conductive electrodes, solar cells, and photocatalysts. Photocatalytic activity can be exploited in the decomposition of hazardous pollutants from environment. In this study, we produced zinc oxide thin films on stainless steel plates by hydrothermal method varying the precursor concentration (from 0.029 M to 0.16 M) and the synthesis temperature (from 70 °C to 90 °C). Morphology of the synthesized films was examined using field-emission scanning electron microscopy (FESEM) and photocatalytic activity of the films was characterized using methylene blue decomposition tests. It was found that the morphology of the nanostructures was strongly affected by the precursor concentration and the temperature of the synthesis. At lower concentrations zinc oxide grew as thin needlelike nanorods of uniform length and shape and aligned perpendicular to the stainless steel substrate surface. At higher concentrations the shape of the rods transformed towards hexagon shaped units and further on towards flaky platelets. Temperature changes caused variations in the coating thickness and the orientation of the crystal units. It was also observed, that the photocatalytic activity of the prepared films was clearly dependent on the morphology of the surfaces.

Dissolution-Induced Nanowire Synthesis on Hot-Dip Galvanized Surface in Supercritical Carbon Dioxide

In this study, we demonstrate a rapid treatment method for producing a needle-like nanowire structure on a hot-dip galvanized sheet at a temperature of 50 °C. The processing method involved only supercritical carbon dioxide and water to induce a reaction on the zinc surface, which resulted in growth of zinc hydroxycarbonate nanowires into flower-like shapes. This artificial patina nanostructure predicts high surface area and offers interesting opportunities for its use in industrial high-end applications. The nanowires can significantly improve paint adhesion and promote electrochemical stability for organic coatings, or be converted to ZnO nanostructures by calcining to be used in various semiconductor applications.

Synthesis of ZnO nanowires with supercritical carbon dioxide and post heat treatment

Zinc oxide (ZnO) nanowires are used in applications such as gas sensors and solar cells. This work presents a novel synthesis route for ZnO nanowires using supercritical carbon dioxide (scCO2) and post heat treatment. The method used scCO2 and a precursor solution as reactants to form nanowires on a galvanized surface. After the scCO2 treatment, the substrate was heat-treated. The surfaces were characterized with SEM, TEM, EDS, FTIR, XRD and optical spectroscopy. The FTIR results showed that the surface structure had changed from zinc hydroxycarbonate to ZnO during the heat treatment. The nanowires were slightly bent due to the heat treatment according to the SEM images. The presence of ZnO was further confirmed with XRD. The bandgap of the structure was determined by reflectance measurements and showed a value of 3.23 eV. The synthesis method presented in this study offers a unique approach into the formation of ZnO nanowires in a facile, rapid and environmentally friendly process.

Tailoring of Versatile Surface Morphologies on Hot Dip Galvanized Steel in Wet CO2: Aspects on Formation, Barrier Properties, and Utilization as a Substrate for Coatings

Zinc carbonate and a mixed-phase zinc carbonate were precipitated selectively on hot dip galvanized steel in the presence of CO2 and water. The zinc carbonate was precipitated as a uniform layer with cubic superficial appearance, while the mixed-phase zinc carbonate was precipitated as nanowires. The distinct structures could be formed separately or as a dual structure with nanowires on the outermost surface. The barrier properties were improved by the both patina forms; a significant increase in surface hydrophobicity was obtained. The dual patina structure was successfully coated with an organic coating, and the intact wet CO2-induced patina with both structures was confirmed within the coating. The formed carbonates can be further converted to zinc oxide by calcination, preserving the delicate structures, which opens a wide range of potential applications for the nanostructured ZnO in a variety of future electronic and optoelectronic devices.

Structured ZnO films: Effect of copper nitrate addition to precursor solution on topography, band gap energy and photocatalytic activity

ZnO is a widely studied semiconductor material with interesting properties such as photocatalytic activity leading to wide range of applications, for example in the field of opto-electronics and self-cleaning and antimicrobial applications. Doping of photocatalytic semiconductor materials has been shown to introduce variation in the band gap energy of the material. In this work, ZnO rods were grown on a stainless steel substrates using hydrothermal method introducing copper nitrate into the precursor solution. Zinc nitrate and hexamethylenetetramine were used as precursor materials and the growth was conducted at 90 °C for 2 h in order to achieve a well-aligned evenly distributed rod structure. Copper was introduced as copper nitrate that was added in the precursor solution in the beginning of the growth. The as-prepared films were then heat-treated at 350 °C and band gap measurements were performed for prepared films. It was found that increase in the copper concentration in the precursor solution decreased the band gap of the ZnO film. Methylene blue discolouration tests were then performed in order to study the effect of the copper nitrate addition to precursor solution on photocatalytic activity of the structured ZnO films.

Synthesis and crystallization of titanium dioxide in supercritical carbon dioxide (scCO2)

In this work, a simple and low-temperature method to synthesize titanium dioxide (TiO2) particles with supercritical carbon dioxide is presented. The particles were synthesized by measuring 5 ml of tetra-n-butyl orthotitanate precursor to the supercritical chamber. The pressure was maintained at 15.0 MPafor all experiments. Reaction temperatures used were 50 °C or 70 °C. After reaching treatment parameters 10 ml of deionized water was introduced to the chamber with a co-solvent pump. A mixer was used inside the chamber to ensure proper mixing of water and precursor. Reaction times of 10, 60 and 300 min were used. Characterization of the particle crystal phase was determined by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The specific surface areas were measured with nitrogen adsorption tests (BET). The results showed that the particles synthesized with reaction times of 10 and 60 min contained brookite as the crystalline phase. With longer reaction time of 300 min the phase shifted to anatase. In most experiments there was also significant amount of amorphous phase present. The specific surface areas varied between 274.3–566.6 m2/g.