Wettability conversion of colloidal TiO2 nanocrystal thin films with UV-switchable hydrophilicity

Under pulsed laser UV irradiation, thin-film coatings made of close-packed TiO2 nanorods individually coated with surfactants can exhibit a temporary increase in their degree of surface hydroxylation without any apparent photocatalytic removal of the capping molecules. This mechanism provides a basis for achieving light-driven conversion from a highly hydrophobic to a highly hydrophilic, metastable state, followed by extremely slow recovery of the original conditions under dark ambient environment. A deeper insight into the wetting dynamics is gained by time-dependent water contact-angle and infrared spectroscopy monitoring of the film properties under different post-UV storage conditions. Our study reveals that, for reversible switchability between extreme wettability excursions and long-term repeatability of such changes to be achieved, specific modifications in the polar and nonpolar components of the TiO2 films need to be guaranteed along with preservation of the original geometric arrangement of the nanocrystal building blocks. The application of moderate vacuum is found to be an effective method for accelerating the post-UV hydrophilic-to-hydrophobic conversion, thereby enabling fast and cyclic hydrophilization/hydrophobicization alternation without any detrimental signs of significant fatigue

Reliable palladium nanoparticle syntheses in aqueous solution: the importance of understanding precursor chemistry and growth mechanism

Reliable protocols for the synthesis of palladium nanoparticles (Pd-NPs) in aqueous solution are rarely found and the corresponding growth mechanisms often remain unknown. Furthermore, syntheses of Pd-NPs always demand the use of stabilizing agents which are often unfavorable for catalytic applications. In this contribution, the importance of the palladium precursor chemistry as a prerequisite for any reliable Pd-NP synthesis in aqueous solution is shown. This includes a detailed study of the influence of the precursor chemistry on the nanoparticle growth mechanism. The findings enable the controlled modification of a common synthetic protocol (i.e. the reduction of a palladium precursor with NaBH4) to obtain sub-5 nm Pd-NPs without the use of any stabilizing agent. In addition, it is also shown that such mechanistic studies are not only of great importance to the development of novel synthetic procedures. Exemplarily, the successful transfer of the synthesis from lab-to large-scale is demonstrated.BMBF, 03EK3009, Design hocheffizienter Elektrolysekatalysatore

Reversible wettability of hybrid organic/inorganic surfaces of systems upon light irradiation/storage cycles

In this work we present hybrid organic/inorganic structures that can exhibit reversible surface wettability, altered in a controllable manner. In particular, we use the method of photo-patterning to produce polymeric SU-8 pillars of specific geometries, onto which we subsequently deposit colloidal TiO2 nanorods. In this way, we combine the microroughness of the polymeric pillars with the nanoroughness of the nanorod-coating to create highly hydrophobic surfaces. The hydrophobicity of these systems can be changed reversibly into hydrophilicity upon irradiation of the hybrid structures with pulsed UV laser light. This behaviour is due to the well-known property of TiO2, that becomes superhydrophilic upon UV light irradiation. This property is reversible and we monitor the recovery of our hybrid polymeric/inorganic-nanorods structures to their initial hydrophobic character upon dark storage and heating. The wetting behaviour has been modelled and analysed according to the surface geometry. The direct implementation of such structures into microfluidics devices is demonstrated. Copyright © 2010 Inderscience Enterprises Ltd

The Effect of Irradiation Wavelength on the Quality of CdS Nanocrystals Formed Directly into PMMA Matrix

UV laser irradiation of PMMA films containing Cd thiolate precursors results in the spatially selective formation of CdS crystalline nanoparticles in the host matrix. Here we investigate the effect of the irradiation wavelength on the quality of the formed nanocrystals. Fluorescence topography and XPS studies reveal that the polymer matrix contributes to the trap states formation on the surface of the nanocrystals. When the latter are formed upon irradiation at 266 nm, they exhibit broad emission spectra, ascribed to the high degree of photodegradation of the polymer. In contrast, the irradiation at 355 nm does not chemically modify the matrix, resulting in the formation of CdS nanocrystals with narrow emission, i.e. high emission quality. This is further confirmed by fluorescence lifetime topography studies giving a mean fluorescence nanocrystal lifetime as short as 200 ps at room temperature. Thus, the optimized combination of irradiation wavelength with polymer matrix gives nanocomposite materials inco..

Controlled Swapping of Nanocomposite Surface Wettability by Multilayer Photopolymerization

Single-layered photopolymerized nanocomposite films of polystyrene and TiO2 nanorods change their wetting characteristics from hydrophobic to hydrophilic when deposited on substrates with decreasing hydrophilicity. Interestingly, the addition of a second photopolymerized layer causes a swapping in the wettability, so that the final samples result converted from hydrophobic to hydrophilic or vice versa. The wettability characteristics continue to be swapped as the number of photopolymerized layers increases. In fact, odd-layered samples show the same wetting behavior as single-layered ones, while even-layered samples have the same surface characteristics as double-layered ones. Analytical surface studies demonstrate that all samples, independently of the number of layers, have similar low roughness, and that the wettability swap is due to the different concentration of the nanocomposites constituents on the samples surface. Particularly, the different interactions between the hydrophilic TiO2 nanorods and the underlying layer lead to different amounts of nanorods exposed on the nanocomposites surface. Moreover, due to the unique property of TiO2 to reversibly increase its wettability upon UV irradiation and subsequent storage, the wetting characteristics of the multilayered nanocomposites can be tuned in a reversible manner. In this way, a combination of substrate, number of photopolymerized layers, and external UV light stimulus can be used in order to precisely control the surface wettability properties of nanocomposite films, opening the way to a vast number of potential applications in microfluidics, protein assays, and cell growth

Light-controlled directional liquid drop movement on TiO2 nanorods-based nanocomposite photopatterns.

Patterned polymeric coatings enriched with colloidal TiO(2) nanorods and prepared by photopolymerization are found to exhibit a remarkable increase in their water wettability when irradiated with UV laser light. The effect can be completely reversed using successive storage in vacuum and dark ambient environment. By exploiting the enhancement of the nanocomposites hydrophilicity upon UV irradiation, we prepare wettability gradients along the surfaces by irradiating adjacent surface areas with increasing time. The gradients are carefully designed to achieve directional movement of water drops along them, taking into account the hysteresis effect that opposes the movement as well as the change in the shape of the drop during its motion. The accomplishment of surface paths for liquid flow, along which the hydrophilicity gradually increases, opens the way to a vast number of potential applications in microfluidics

Determination of surface properties of various substrates using TiO2 nanorod coatings with tunable characteristics

We present a novel approach to cover different substrates with thin light-sensitive layers that consist of organic-capped TiO2 nanorods (NRs). Such NR-based coatings exhibit an increasing initial hydrophobicity with increasing NR length, and they demonstrate a surface transition from this highly hydrophobic state to a highly hydrophilic one under selective UV–laser irradiation. This behaviour is reversed under long dark storage. Infrared spectroscopy measurements reveal that light-driven wettability changes are accompanied by a progressive hydroxylation of the TiO2 surface. The surfactant molecules that cover the NRs do not appear to suffer for any significant photocatalytic degradation

Localized formation and size tuning of CdS nanocrystals upon irradiation of metal precursors embedded in polymer matrices

We present a method of spatial and dimensional controlled formation of CdS quantum dots into polymer matrices by light irradiation. The initial samples consist of Cd thiolate precursors doped in TOPAS polymer matrix. Under pulsed UV laser irradiation the precursors are photolysed driving to the nucleation of CdS nanocrystals, with increasing size and concentration, related to the number of UV pulses. The formed quantum dots are localized in the irradiated area, while the host polymer remains macroscopically unaffected by the UV irradiation. In this study we investigate how the formation of the nanocrystals (size, dimensions, and concentration) is affected by the use of different irradiation conditions (wavelength, number of pulses), revealing information about the different pathways followed during the formation. The change of the size of the dots results in the change of the peak of their emission due to the quantum size effect, which is studied by fluorescence measurements. The results are reinforced by TEM microscopy and by XRD measurements. The main advantages of the presented method are the size tuning of the produced dots and their spatial confinement inside the host matrix, not possible by the other methods used until now (thermal annealing, mixing etc.)

Comparison between laser-induced nucleation of ZnS and CdS nanocrystals directly into polymer matrices

The nucleation of two kinds of crystalline nanoparticles, zinc sulfide (ZnS), and cadmium sulfide (CdS), is achieved directly into specific sites of polymer matrices after their irradiation with UV laser pulses. The starting samples consist of polymers doped with precursors of Zn or Cd thiolate that are proved to decompose after the absorption of UV light, resulting into the nanoparticles formation. The growth of the crystalline nanostructures is followed throughout the irradiation of the samples with successive incident pulses, by different methods, such as transmission electron microscopy, atomic force microscopy, confocal microscopy, and X-ray diffraction. Special attention is paid to the difference of the formation pathways of the two kinds of nanoparticles studied, because the Cd thiolate precursor exhibits much higher absorption efficiency than the Zn thiolate one, at the applied UV wavelength. Indeed, CdS nanoparticles become evident after the very first incident UV pulses, whereas the formation of ZnS nanocrystals requires rather prolonged irradiation, always through a macroscopically nondestructive procedure for the polymer matrix. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineer

Reversibly Light-Switchable Wettability of Hybrid Organic/Inorganic Surfaces With Dual Micro-/ Nanoscale Roughness

Here, an approach to realize ''smart'' solid substrates that can convert their wetting behavior between extreme states under selective light irradiation conditions is described. Hybrid organic/inorganic surfaces are engineered by exploiting photolithographically tailored SU-8 polymer patterns as templates for accommodating closely packed arrays of colloidal anatase TiO2 nanorods, which are able to respond to UV light by reversibly changing their surface chemistry. The TiO2-covered SU-8 substrates are characterized by a dual micro-/nanoscale roughness, arising from the overlapping of surfactantcapped inorganic nanorods onto micrometer-sized polymer pillars. Such combined architectural and chemical surface design enables the achievement of UV-driven reversible transitions from a highly hydrophobic to a highly hydrophilic condition, with excursions in water contact angle values larger than 1008. The influence of the geometric and compositional parameters of the hybrid surfaces on their wettability behavior is examined and discussed within the frame of the available theoretical models