Research Program of the Centre NANOPIN

Three interlinked priority areas are thoroughly investigated in the NANOPIN Research Centre. These involve preparation of various forms of nanocrystalline photoactive materials (mostly titanium oxide and using nanotechnologies for synthesis), their physical and chemical characterisation and finally their functional characterisation in practical photocatalytic processes.

These areas could be in principle characterised by defining their specific objectives:

Synthesis of highly active and resistant photocatalysts sensitive also to the visible light
Preparation of photoactive thin layers (transparent) of titanium oxide with a high degree of internal organisation on various supports
Correlations between materials` properties and "the preparation imprinted" photoactivity
Design of standard photoactivity testing methods
Construction, testing and optimisation of operation conditions of special types of photoreactors for water and air decontamination and other applications
Kinetics and mechanism study of photocatalytic model reactions

1. Synthesis of highly active and resistant photocatalysts

A number of methods will be designed for preparation of active photocatalysts based on metal oxides (mostly titanium dioxide). These methods will vary in principles, however, always should produce a resistant material with significantly enhanced photoactivity, sophisticated internal structure and specially designed selectivity pattern for a particular process.

Colloidal solutions of extremely small particles (diameter of 1-2 nm), both pure (Q-TiO₂) and doped (by various amounts of three valence iron (Q-TiO₂ (Fe³⁺)) will be synthesized at J. Heyrovsky Institute of Physical Chemistry (JHIPCh). This process will employ the controlled hydrolysis of TiCl₄ in aqueous solutions containing corresponding concentrations of FeCl₃. To prevent the precipitation, ionic strength of the prepared colloidal solutions will be lowered by performing a dialysis. Another approach, developed at Institute of Inorganic Chemistry (IICh), comprises the homogeneous precipitation of the aqueous solution of TiOSO₄ with urea. Calcination at controlled temperatures is proposed to adjust the ratio of anatase and rutile with the aim to improve the material's photoactivity. Ordered structure nanoparticles of titania will be also prepared at JHIPCh and Department of Organic Technology, ICT (DOT) by the process of templating incorporated into the sol-gel method. This technique should yield a material characteristic by its structural homogeneity, surface uniformity and predictable properties. To extend the materials` spectral sensitivity (into the visible light area) nanoparticles of chemically doped or modified anatase will be also produced. In order to broaden the application possibilities, new types of photocatalysts based on modified nanocrystalline hydroxyapatite will be synthesised in parallel. These structures will embody, besides the photo-activity, also a very high ability to adsorb impurities (e.g. coatings).

Prepared materials will by characterised by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), by determining the BET surface area and the pore size distribution (BJH method) using nitrogen adsorption. Measurements of the diffusion reflective spectra in ultra-violet, visible and near infra-red areas will also be considered. Titania band gap energies will be estimated measuring UV/VIS absorption spectra of its colloidal solutions. Redox properties will be determined by polarography and cyclic voltammetry. Charge transfer processes will be studied by means of the laser flash photolysis performing time resolved kinetic and spectroscopic measurements.

The obtained material characteristics will be correlated with experimental conditions of their synthesis (concentration of reactants, temperature of the reaction mixture, reaction time, temperature of calcinations, templates structure, etc.) with the aim to develop a reproducible method for titanium oxide nanoparticles preparation and potentially also applicable in a pilot plant scale.

2. Preparation of photoactive thin layers

Deposition (immobilisation) of the nanoparticulate photocatalysts for the use in (mostly) self-constructed plate photoreactors will be, among others, also based on a repeated sedimentation of the powder material from its acidified aqueous suspension followed by drying and heating. Conditions such as concentration, pH and temperature will be controlled to optimise both mechanical stability and photoactivity.

Highly organised layers of nanocrystalline anatase will be also prepared form liquid precursors (by dip or spin coating). This method will involve spatial blocking of hydrolysis of the initial metallic alkoxide with micelles forming surfactant molecules. After calcination (thermal decomposition of organic molecules) very specific materials could be yielded. The characteristic features of this kind of a material are the high activity, surface organisation and homogeneity, significant photoactivity and adjustable selectivity profiles according to the desired applications. TiO₂ sol-gel layers will be mainly prepared by the hydrolysis of Ti(IV) isopropoxide. Attention will be paid to the synthesis of highly organized nanocrystalline anatase by supramolecular templating with poly(alkylene oxide) block copolymers (JHIPCh, DOT). Another possibility represents the approach which does not involve any thermal treatment steps and thus allows layers preparation/application together with thermally non stable materials. For this step extraction with supercritical carbon dioxide was suggested and recently successfully tested (DOT).

Procedures outgoing from aqueous solutions will be also tested to prepare nanocrystalline titanium oxide layers at IICh. The method involves synthesis of anatase nanoparticles with homogenous precipitation from TiOSO₄ aqueous solutions analogous to that for colloidal particles. The anatase nanoparticulate material synthesised at IICh will be processed into the form of thin layers in laboratories of Department of Inorganic Technology, ICT (DIT). The preparation procedure is based on the repetitive sedimentation of the material from aqueous suspension followed by drying. Final fixation of the layer on the photoreactor basement associated with the improvement of the layers` mechanical properties will be achieved by controlled heating. Alternatively "hybrid" layers (combination of this procedure with "sol-gel") will be also produced. Films with high photoactivity (typical for particulate films) and high mechanical stability (typical for sol-gel layers) are expected.

The nanocrystalline titanium dioxide will be also deposited on various substrates by using PECVD and/or PVD methods at Technical University of Liberec (TUL) equipped with the plasma deposition facility. The films properties will be optimised by systematic variation of the deposition conditions (chemical composition of the precursor, pressure and composition of the working gas, substrate temperature, electric discharge parameters, geometrical configuration and the type of the discharge, deposition time etc). Plasma diagnostics will be used to ensure the reproducible deposition conditions and to clarify the dependence of the film properties on the microscopic discharge conditions. This knowledge is necessary for the effective process up-scaling. Additionally to the preparation of pure TiO₂, deposition of doped and/or structured films will be investigated. The principal advantages of the plasma based processes, mainly the low substrate temperature, deposition of compact films and good film adhesion (even to hydrophobic substrates) including plastics are the main motivations for these experiments. Mechanical properties of the films including the abrasive resistance will be also optimised (TUL).

As the second task, methods of the film deposition on substrates with complex shapes superior for applications in photocatalytic reactors will be developed (e.g. special textile fabrics or glass bullets). The general goal of the subproject is to elaborate photoactive films with high activity and proper mechanical properties on different substrates (TUL).

Highly variable separation properties could be expected from using the discussed structures as photoactive components of membranes (e.g. based on zeolites on which an active layer of nanocrystalline titanium oxide is deposited). This part of the research would be studied at JHIPCh in cooperation with DOT and DIT. The multicomponent and multifunctional membranes will be first characterised in a standard way (adsorption properties, porosity, particle size, etc.), then their functional properties will be specified.

3. Correlations between materials` properties and "the preparation imprinted" photoactivity

The search for the relations between structural characteristics and photoactivity of the nanocrystalline titanium oxide and elucidation of the photocatalytically induced superhydrophility are important (and priority) topics of the proposed research (ATG, IICh, JHIPCh, DIT, DOT, ATG). Photocatalytic activity of titanium oxide nanoparticles will be tested in specifically designed model systems. Similar methods will be used for characterisation of nanocrystalline titanium oxide layers. Effect of a number of factors will be examined and many parameters evaluated (aquatic drops contact angle, effect of the chemical composition, role of porosity, spectral bands in the near infra-red area typical for surface hydroxyl groups and adsorbed water molecules, parameters of irradiation such as time, intensity and spectral characteristics of the light source, etc.). Analysis of the data obtained will enable to define and understand principles of anatase enhanced photoactivity and its photo induced superhydrophility.

4. Design of standard photoactivity testing methods

For the assessment of the produced photocatalysts activity model testing methods will be designed (DIT, JHIPCh). The photoactivity tests will be performed for colloidal solutions of quantum-sized particles, aqueous suspensions of powder materials and for the nanoparticulate thin layers deposited on various supports. Both photooxidation and photoreduction properties of the synthesised photocatalysts might be examined by measuring the initial transformation rates of appropriate model reactants in two different types of reaction systems: either electron donor (e.g. CHCl₃) in excess of electron acceptor (O₂ or H₂O₂) or electron acceptor (Ag⁺, O₂ or CCl₄) in excess of electron donor (e.g. CH₃OH). Overall photocatalytic activity will be estimated applying the standard procedure of the degradation of 4-chlorophenol in the corresponding photoreactor. Besides the reactant transformation an extent of mineralisation will be determined (using the TOC analyzer).

The measurements with colloids and powders will be carried out in self-constructed laboratory photoreactors, magnetically stirred and continuously irradiated with black light lamps (λ = 365 nm). Concerning the colloids, influence of the particle size and doping on the photoreactivity will be examined. Structural parameters of powder materials will be correlated with photooxidation and photoreduction properties as well as with the overall photocatalytic activity. This standard procedure is based on monitoring the degradation kinetics of 4-chlorophenol in aqueous suspension of photocatalyst in the tube batch photoreactor, magnetically stirred and irradiated by monochromatic light of wavelength 365 nm will be considered. This approach promises to find the relationship between physical characteristics of the semiconductor material and its photocatalytic activity.

For testing the photocatalytic activity of prepared thin layers a plate photoreactor with laminar flow of the aqueous solution will be designed. The main part of the reactor represents a plate of standard dimensions (e. g. 10×15 cm) with the photoactive surface irradiated with polychromatic UV light in the wavelength range from 300 to 400 nm. Both long-time stability of the synthesised photocatalysts and their photocorrosion resistance will be tested. In addition the photocatalytic degradations based on solar irradiation will be considered.

The photocatalytic experiments with highly oriented layers of nanocrystalline anatase will be carried out in a small photoreactor based on a standard spectroscopic cell. A quartz plate with the photocatalyst layer will be fixed inside the cell filled with the aqueous solution of an appropriate reactant composition (e.g. Ag⁺, CHCl₃, CCl₄ or CH₃OH). The liquid will be magnetically stirred. During the monochromatic irradiation, both pH and concentration of chloride and oxygen will be continuously recorded. Moreover, the UV/VIS absorption spectra in two perpendicular directions, across the layer and parallel to it, will be scanned at selected irradiation times. This arrangement will enable a complex kinetic observation of occurring photocatalytic reactions (e.g. formation of HCl, deposition of Ag, consumption of O₂) and also a precise determination of their quantum yields. In addition, spectroscopic characterisation of the anatase layer, determination of its acid-base properties and estimation of its sorption capability, including the formation of charge transfer complexes, could be performed.

Another possibility is the electrochemically assisted photocatalysis using polarisation of the prepared layer by external voltage. Effect of the bias potential on the degradation rate will be investigated. The flow rate, light intensity and concentrations of both organic substance and oxygen will be varied and optimized. Besides the artificial irradiation (Eversun tubes, Osram), both direct and diffuse solar light will be applied. Colloidal solutions of Q-particles of photoactive semiconductors will be used in a model study of the reductive photocorrosion. For the process of photocorrosion extremely small particles of Fe³⁺ are considered. The involved mechanism has not been fully understood yet. The photocatalysis using titania and the process of photocorrosion of Fe³⁺ could significantly improve, due to the expected synergism, the efficiency of decomposition processes.

A new method based on monitoring the photocatalytic degradation of a suitable dye will be developed. The key parameter is the choice of a suitable dye. Its absorption band in the visible part of the spectrum should reveal a reasonably high molar absorption coefficient (10⁴-10⁵ mol^-1dm³cm^-1). Its absorption at the irradiation wavelength 365 nm (area of the catalyst absorption) should be preferably low. At the same time the dye must not obey the direct photolysis. Any of the degradation intermediates must not absorb visible light in the wavelength region where the original dye absorbs and thus the concentration of the dye during degradation can be precisely determined by absorption spectroscopy. Furthermore the ionic polarity of dye should be the same as the charge of titanium dioxide surface. It will prevent the adsorption of the dye and therefore the photocatalytic degradation is not initiated by the direct charge transfer from the semiconducting particle but by the attack of primarily photogenerated hydroxyl radicals. This mechanism is typical for many organic contaminants.

Antibacterial properties of the photoactive thin layers will be evaluated at Department of Water Technology (DWT) and Department of Biochemistry (DB, both ICT) using spectrophotometric and cultivation methods. A column with a photocatalytic bed will be operated continuously. Water contaminated in a lab in parallel with bacteria rich waters from natural biological environments will be used. Initial concentration of bacteria will be determined by means of the cultivation method and the data compared with the spectral analysis. Samples will be regularly withdraw on certain sampling points (e.g. before and after the column) and the effect of the photocatalyst discussed. Role of the operation conditions and structural properties of the catalyst will also be studied. Genetic analysis will be performed to assess the effect of the strong UV on the mutation activity of the bacteria population (16SrDNA and temperature gradient gel electrophoresis).

5. Construction, testing and optimisation of operation conditions of special types of photoreactors for water and air decontamination and other applications

Laboratory photoreactors for model cleaning of water, soil and air will be constructed in laboratories at ATG, DIT, DOT and JHIPCh. For fast (qualitative) tests of photoactivity a simple method based on the photocatalytic degradation of a described dye will be worked out. Plates of standard dimension (e. g. 5×5 cm) will be placed into Petri dishes (filled with the same volumes of aqueous solution of the dye). After irradiation time the decay of dye will be determined spectrophotometrically. The dye decay will be the measure of the photocatalytical activity of a particular layer. This method will be developed mainly for the fast preliminary examination of layers prepared by plasma deposition in the TUL laboratories and templated layers synthesised at DIT, DOT.

For a specific purpose of "natural" water decontamination, solar energy plate photoreactors with photocatalyst layers immobilised on glass or a metal plate will be designed. The contaminated aqueous solution will be pumped from the holding tank to a higher vessel. The overflow will produce a liquid film flowing over the immobilised TiO₂ layer. In order to optimise working conditions suitable model compounds will be used (e. g. 4-chlorophenol or oxalic acid). The effect of key parameters (reactant concentrations, flow rate and incident light intensity, etc.) will be systematically studied.

A model photoreactor for water purification (decontamination) has recently been constructed (DIT, JHIPCh). It consists of two coaxial quartz tubes. In the annular space, small glass beads coated with a TiO₂ photocatalyst are positioned (standard sol-gel for the coating). A peristaltic pump is used to circulate the purified water from a reservoir through the photoreactor and back to the reservoir. The flow rate can be adjusted in a broad range of three orders of magnitude (from milliliters to liters per minute). A magnetic stirrer is used to mix the stock solution in the reservoir. As irradiation source, a black light fluorescent lamp (SYLVANIA Black light - Blue F8W/BLB, 8 W, 30 cm long) is employed, which dominantly emits mercury line at 365 nm. It is placed coaxially inside the narrower quartz tube. An aluminum foil wraps around the broader quartz tube to protect escaping light from the photoreactor. Moreover, the aluminum surface reflexes UV radiation well and thus, multireflections occur that enable a nearly quantitative utilisation of the emitted irradiation at 365 nm for the photocatalytic process.

The photocatalytic method of water purification is based on an oxidative degradation of organic substances, which are totally mineralised, i.e. decomposed to inorganic compounds such as carbon dioxide, water and mineral acids (JHIPCh, DIT). To test the efficiency of this general photoprocess in the constructed photoreactor and to optimise its working conditions, an azo dye orange II was chosen as a model compound. Orange II, as a natrium salt of sulphonated azo dye, profits of several advantages. As an anion, it is not adsorbed on the negatively charged surface of titanium dioxide in neutral aqueous solution, where the surface hydroxyl groups are deprotonated. That is why no direct charge transfer from TiO₂ photocatalyst to orange II is possible as an initial reaction step. Instead of it, photogenerated hydroxyl radicals mediate the oxidative degradation of orange II that is a general reaction way typical for the majority of organic compounds. As an azo dye, orange II has an optical absorption with a maximum at 483 nm. Moreover, there is an absorption minimum around the irradiation wavelength of 365 nm. Therefore, orange II absorbs only a small part of the emitted radiation while its majority is absorbed by the photocatalyst und utilised for the degradation of orange Using a flow-through cell, direct analysis without any manual sampling is possible. It will also enable to operate several parallel photocatalytic experiments in an automatic regime (JHIPCh, DIT).

Kinetic and mechanistic studies of photocatalytic degradation of volatile pollutants in gas phase will be studied using the FTIR spectroscopy. A self-constructed gas phase photoreactor consists of two coaxial glass tubes with a black light fluorescent lamp (power 40 W, length 1200 mm, diameter 38 mm, maximum wavelength 352 nm) centred in the axis. The TiO₂ photocatalyst is immobilised on inner side of the outer tube and/or on quartz sand filling the internal space between both tubes. The gas air phase containing a pollutant passes through the photocatalyst bed. Samples are taken using a valve system with a set of differently long capillaries reaching to various distances along the photoreactor tubes and analysed by FTIR spectrometer employing a Nicolet Nexus GC/IR Interface. Model compounds of three main groups of air pollutants (halogenated hydrocarbons, aromatic hydrocarbons and nitrogen oxides) will be investigated for kinetics and mechanism of their oxidative degradation on the irradiated TiO₂ surface.

Photoactivity of larger surfaces is tested using a standard plate photoreactor, in which aqueous solution containing a model organic compound (azo dye orange II) circulates over a desk with the photoactive coating illuminated by fluorescent black light lamps. Orange II, a natrium salt of sulphonated azo dye, has several advantages. As an anion, it is not adsorbed on the negatively charged surface of titanium dioxide in neutral aqueous solution. That is why no direct charge transfer from the organic molecule to illuminated TiO₂ surface proceeds and thus the oxidative degradation of orange II should be only induced by primary photogenerated hydroxyl radicals. This is a general degradation way typical for majority of organic structures. Orange II has absorption band with a maximum at 483 nm and absorption minimum at irradiation wavelengths around 350 nm. Therefore, orange II absorbs only a small part of the emitted radiation while the majority of it is absorbed by the photocatalyst. Moreover, none of degradation intermediates of orange II absorbs at wavelengths longer than 300 nm. That is why the absorption spectroscopy method can be used for precise determination of the concentration changes of orange II during the whole photodegradation process (JHIPCh, DIT).

Special applications are also considered. Among others efficient purification microunits involving high surface area templated layers with a specific selectivity profile are considered. Optical fibres transmitting UV are supposed "to bring" the photoflux to such a unit. Model photoreactors for land decontamination will be also constructed with special attention to the effective decomposition of dioxins and polychlorinated biphenyls.

6. Kinetics and mechanism study of photocatalytic model reactions

The effectiveness of photocatalytic purifications of waste water, polluted air in industrial vents or contaminated land has been evidenced widely during the recent years. However, fundamental kinetic and mechanistic studies are rather rare. From principle reasons there is an urgent need of a complex research focused on mechanisms of decomposition reactions, on identification of intermediates, their toxicity evaluation and conditions of their further transformation. In the proposed project special attention will be paid (DIT, DOT, JHIPCh) to the detailed study of kinetics and decomposition mechanisms of a series of real pollutants both in gas and liquid phase (among others substituted phenols, herbicides, carboxylic acids, organic solvents, aromatic compounds, chlorinated hydrocarbons, etc.).

Mechanism of the photocatalytic "deactivation" of microorganisms will be studied (DWT, DM) first on model bacteria systems, then on samples from real, natural environment. Calibration will be based on evaluating the dependency of the amount of model microorganisms on the illumination time with light of certain intensity. Genetic changes will also be analysed.