Marko Kete

ZnO Nanorod Arrays by Plasma‐Enhanced CVD for Light‐Activated Functional Applications

The growth of one-dimensional (1D) semiconducting materials for advanced functional applications is the current focus of various cutting-edge research activities. In this context, 1D ZnO nanosystems, such as nanowires and nanorods (NRs), are extremely appealing targets due to their broad variety of attractive properties, resulting from the synergy between their ultrahigh surface-to-volume ratio, inherent anisotropy and quantum confinement of charge carriers. Such unique characteristics render 1D ZnO nanosystems appealing multi-functional modules for various utilizations, such as electron field emitters, gas sensors, lasers, piezoelectrics and photovoltaic cells. Recently, ZnO NR arrays have also received considerable attention for their advanced photo-activated functions, regarding, in particular, photo-induced superhydrophilicity (PSH) and photocatalysis (PC), that pave the way to the development of stimuliresponsive systems. In this context, the growth of supported ZnO NRs with tailored properties plays a strategic role, since such arrays represent very favorable architectures for light trapping and reduce undesired particle aggregation frequently observed for powdered materials. In addition, for PSH and PC applications, a high surface-to-volume ratio enables a faster arrival of photo-generated electrons and holes to the NRs surface, reducing detrimental recombination phenomena. The functional performance of ZnO NRs is directly dependent on the availability of proper synthetic strategies enabling a fine control of their morphology and spatial organization. 5a,6] Among the various approaches, chemical vapor deposition (CVD) is one of the most versatile techniques due to its intrinsic experimental flexibility and to the use of metalorganic precursors endowed with ad hoc chemical and physical properties. In particular, activation of both gas-phase and surface processes by means of non-equilibrium plasmas (plasma-enhanced CVD, PE-CVD) promotes alternative reaction pathways under softer conditions than in thermal CVD, enabling one to modulate deposition surface chemistry and to obtain 1D nanostructures with tailored properties. Herein, we report on the PE-CVD of supported ZnO nanorod arrays on Si(100) substrates from two recently developed bis(ketoiminato) zinc (II) compounds, Zn[(R’)NC(CH3)= C(H)C(CH3)=O]2, with R’= (CH2)2OCH3 (1) or (CH2)3OCH3 (2). The selective growth of ZnO NR arrays was performed from Ar/O2 plasmas at 200 and 300 8C (see Figure 1, Table 1 and Supporting Information), the lowest temperatures reported in the literature for the PE-CVD of such 1D architectures. Special attention was devoted to the interrelations between PSH and PC activity of the obtained 1D ZnO systems and their morphological characteristics. PSH properties were analyzed by monitoring the evolution of water contact angle (CA) as a function of the UV irradiation time. The self-cleaning activity of the synthesized NRs was investigated through a novel quantitative and highly sensitive PC method recently developed by some of us, consisting of the determination of a terephthalic acid (TPA) oxidation product, that is, hydroxyterephthalic acid (HOTPA).

Photocatalytic degradation of gaseous toluene by using immobilized titania/silica on aluminum sheets

PurposeThe aim of this study was to prepare a highly active immobilized titania/silica photocatalyst and to test its performance in situ toward degradation of toluene as one of the major toxic indoor contaminants.MethodsIn this work, two different titania layers immobilized on Al sheets were synthesized via low temperature sol–gel method employing presynthesized highly active titania powders (Degussa P25 and Millennium PC500, mass ratio 1:1): (a) with a silica/titania binder and a protective layer and (b) without the binder. The photocatalysts were characterized by X-ray diffraction, nitrogen sorption measurements, scanning electron microscopy (SEM), infrared spectroscopy, and UV–vis diffuse reflectance spectroscopy (DRS). The in situ photocatalytic degradation of gaseous toluene was selected as a probe reaction to test photocatalytic activity and to verify the potential application of these materials for air remediation.ResultsResults show that nontransparent highly photocatalytically active coatings based on the silica/titania binder and homogeneously dispersed TiO2 powders were obtained on the Al sheets. The crystalline structure of titania was not altered upon addition of the binder, which also prevented inhomogeneous agglomeration of particles on the photocatalyst surface. The photoactivity results indicate that the adsorption properties and photocatalytic activity of immobilized photocatalysts with the silica/titania binder and an underlying protective layer were very effective and additionally, they exhibited considerably improved adhesion and uniformity.ConclusionWe present a new highly photocatalytically active immobilized catalyst on a convenient metallic support, which has a potential application in an air cleaning device.

Solar-driven photocatalytic treatment of diclofenac using immobilized TiO2-based zeolite composites

The study is aimed at evaluating the potential of immobilized TiO2-based zeolite composite for solar-driven photocatalytic water treatment. In that purpose, TiO2-iron-exchanged zeolite (FeZ) composite was prepared using commercial Aeroxide TiO2 P25 and iron-exchanged zeolite of ZSM5 type, FeZ. The activity of TiO2-FeZ, immobilized on glass support, was evaluated under solar irradiation for removal of diclofenac (DCF) in water. TiO2-FeZ immobilized in a form of thin film was characterized for its morphology, structure, and composition using scanning electron microscopy/energy-dispersive x-ray spectroscopy (SEM/EDX). Diffuse reflectance spectroscopy (DRS) was used to determine potential changes in band gaps of prepared TiO2-FeZ in comparison to pure TiO2. The influence of pH, concentration of hydrogen peroxide, FeZ wt% within the composite, and photocatalyst dosage on DCF removal and conversion efficiency by solar/TiO2-FeZ/H2O2 process was investigated. TiO2-FeZ demonstrated higher photocatalytic activity than pure TiO2 under solar irradiation in acidic conditions and presence of H2O2.

The Synthesis of Anatase Nanoparticles and the Preparation of Photocatalytically Active Coatings Based on Wet Chemical Methods for Self-Cleaning Applications

We report on an improved sol-gel method for the production of highly photocatalytic titanium dioxide (TiO2) anatase nanoparticles which can provide appropriate control over the final characteristics of the nanoparticles, such as particle size, crystallinity, crystal structure, morphology, and also the degree of agglomeration. The synthesized anatase nanoparticles were characterized using various techniques, such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and were tested in coatings for self-cleaning glass and ceramic surfaces. The coatings were prepared using a soft chemistry route and are completely transparent to visible light and exhibit a high photocatalytic effect, which was determined by contact-angle measurements. Finally, it is worth mentioning that both the sol-gel synthesis method and the coating-preparation method are based on a wet chemical process, thus presenting no risk of handling the TiO2 anatase nanoparticles in their potentially hazardous powder form at any stage of our development. Low-price, easy-to-handle, and nontoxic materials were used. Therefore, our work represents an important contribution to the development of TiO2 anatase nanoparticle coatings that provide a high photocatalytic effect and can thus be used for numerous applications.

Properties and Application Perspective of Hybrid Titania-Silica Patterns Fabricated by Inkjet Printing

A hybrid titania-silica cold-setting sol has been developed that can be deposited onto a wide variety of surfaces without the need for high-temperature fixing and that is suitable for material printing deposition. Thin hybrid titania-silica coatings were patterned onto glass and PET substrates by inkjet printing. Well-defined hybrid titania-silica patterns, with thicknesses ranging from 40 to 400 nm, were fabricated by overprinting 1 to 10 layers. Excellent mechanical, optical, and photocatalytic properties were observed, making the reported material well suited for the fabrication of transparent self-cleaning coatings both on mineral and organic substrates. The printed patterns exhibit photoelectrochemical activity that can be further improved by thermal or photonic curing. A concept of fully printed interdigitated photoelectrochemical cells on flexible PET substrates utilizing the reported hybrid photocatalyst is disclosed as well.

Testing of Photocatalytic Activity of Self-Cleaning Surfaces

Due to relatively successful application of TiO2 photocatalysis in the field of self-cleaning surfaces, a reliable and appropriate quantitative method for determining the self-cleaning efficiency of the products (photocatalyst films on different supports) should be widely recognized and established. Currently, the two standard methods are based on photobleaching of methylene blue aqueous solution in contact with thin solid catalyst layer, and on photodegradation of a solid fatty deposit (e.g. stearic acid) over catalyst layer followed indirectly by water contact angle (CA) measurements. Another method proposed recently is based on the entrapment of an organic dye in a solid polymer matrix deposited over the photocatalyst layer. Upon illumination, the dye (e.g. resazurin) is reduced by the photogenerated electrons to the form of a different colour or to the bleached form. Recently, a new method for determination of self-cleaning activity of photocatalytic surfaces was developed in our laboratory. It is based on the deposition of a transparent solid layer comprising terephthalic acid over the photocatalytic surface. When such a system is irradiated, among the other degradation products also a hydroxyterephthalic acid is formed due to a reaction between photoexcited TiO2 and terephthalic acid. Hydroxyterephthalic acid is a highly fluorescent molecule and can be easily detected by HPLC-FLD or spectrofluorimeter. Many different samples were tested using this method and using the standard method based on photodegradation of a solid fatty deposit over the catalyst layer. Regarding the sample (substrate) type, the focus was given to the self-cleaning ceramic tiles coated with TiO2-SiO2 thin films that were produced in cooperation with the major manufacturer of floor ceramic tiles in Slovenia, Martex d.o.o.. In case of self-cleaning ceramics and some other sample types, the advantages of the new method over standard methods were highlighted and critically evaluated.

Solution-derived photocatalytic films for environmental cleaning applications

When photocatalytic water treatment is concerned, suspended catalyst in the aqueous phase is usually more efficient than immobilized on an inert support, but in the former case an undesirable separation/recycling step is needed. We have therefore concentrated on the preparation of immobilized catalysts in the form of films on glass and aluminium supports. The low-temperature sol-gel processing route to obtain transparent thin TiO2/SiO2 films for self- cleaning purposes and thicker TiO2/SiO2 coatings for efficient removal of pollutants in water and air are presented. The synthesis is based on a production of a nanocrystalline titania sol with a silica binder that after deposition does not require thermal treatment at high temperatures. Depending on the target application, some specific synthesis parameters and photocatalytic activity testing conditions are illustrated. For water-cleaning coatings fast kinetics is required, which was achieved by addition of a highly active titania powder into the sol. The same preparation procedure was used to prepare efficient air-cleaning coatings. On the other hand, self-cleaning films were thinner and transparent to keep the original appearance of the substrate and they solidified at ambient conditions. Advanced methodologies to evaluate photocatalytic activity of the films were applied.

Self-Cleaning and Anti-Fogging Surfaces Based on Nanostructured Metal Oxides

Self-cleaning and anti-fogging technology is already used in a variety of the products today, among which glazing products prevail. Their function is based on the two principles, either hydrophobic (Lotus effect) or photocatalytic hydrophilic principle. In the latter case, self-cleaning action is based on the combination of photocatalysis and photoinduced superhydrophilicity, where dirt removal by uniform spreading of water over the whole surface is improved with photocatalytic decomposition of organic contaminants. There is a big room to play and a huge potential to improve the properties of existing self-cleaning and anti-fogging surfaces by designing and tailoring new nanoarchitectures of the light-activated thin films. In this lecture we report about the latest achievements and relevance of nanostructured metal oxide surfaces for specific application in photocatalytic self-cleaning. The emphasis is given to the preparation and structure-property relationship of nanocomposite materials based on TiO2 (TiO2-SiO2) and ZnO (ZnO/Co3O4). The emphasis is given also to determination of photocatalytic self-cleaning activity by quantitative and sensitive methods such as fluorescence detection of the first degradation product of terephthalic acid.