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

Comparative Raman Study of Organic-Free and Surfactant-Capped Rod-Shaped Anatase TiO2 Nanocrystals

Excitation of lattice vibrations in nanostructured anatase TiO2 frequently occurs at energy values differing from that found for the corresponding bulk phase. Particularly, investigations have long aimed at establishing a correlation between the low-frequency E-g(1) mode and the mean crystallite size on the basis of phonon-confinement models. Here, we report a detailed Raman study, supported by X-ray diffraction analyses, on anatase TiO2 nanocrystals with rod-shaped morphology and variable geometric parameters, prepared by colloidal wet-chemical routes. By examining the anomalous shifts of the E-g(1) mode in the spectra of surfactant-capped nanorods and in those of corresponding organic-free derivatives (obtained by a suitable thermal oxidative treatment), an insight into the impact of exposed facets and of the coherent crystalline domain size on Raman-active lattice vibrational modes has been gained. Our investigation offers a ground for clarifying the current lack of consensus as to the applicability of phonon-confinement models for drawing information on the size of surface-functionalized TiO2 nanocrystals upon analysis of their Raman features

Picosecond photoluminescence decay time in colloidal nanocrystals : The role of intrinsic and surface states

Picosecond time-resolved photoluminescence measurements were performed on CdSe core and CdSe/ZnS core/shell colloidal quantum dots (QDs). Photoluminescence (PL) emission is observed to originate from intrinsic ±1U and ±1L bright states with lifetimes of 60 and 450 ps, respectively, and from a long living component with nanosecond lifetimes. The latter is attribuited to the emission from surface states (ss) approximately 16 and 13 meV below the ±1L state for core and core/shell QDs, respectively. We show that in the temperature range between 15 and 70 K the three recombination processes compete and they are thermally populated through different pathways (±1L → ±1U and ss → ±1L)

Nonhydrolytic synthesis of high-quality anisotropically shaped brookite TiO2 nanocrystals.

A surfactant-assisted nonaqueous strategy, relying on high-temperature aminolysis of titanium carboxylate complexes, has been developed to access anisotropically shaped TiO2 nanocrystals selectively trapped in the metastable brookite phase. Judicious temporal manipulation of precursor supply to the reaction mixture enables systematic tuning of the nanostructure geometric features over an exceptionally wide dimensional range (30-200 nm). Such degree of control is rationalized within the frame of a self-regulated phase-changing seed-catalyzed mechanism, in which homogeneous nucleation, on one side, and heterogeneous nucleation/growth processes, on the other side, are properly balanced while switching from the anatase to the brookite structures, respectively, in a continuous unidirectional crystal development regime. The time variation of the chemical potential for the monomer species in the solution, the size dependence of thermodynamic structural stability of the involved titania polymorphs, and the reduced activation barrier for brookite nucleation onto initially formed anatase seeds play decisive roles in the crystal-phase- and shape-tailored growth of titania nanostructures by the present approach

Selective reactions on the tips of colloidal semiconductor nanorods

A strategy to access several types of Au-tipped dumbbell-like nanocrystal heterostructures is presented, which involves the selective oxidation of either PbSe or CdTe sacrificial domains, initially grown on CdSe and CdS nanorods, with a Au(III) : surfactant complex. The formation of gold patches is supported by TEM, XRD and elemental analysis. This approach has allowed us to grow Au domains onto specific locations of anisotropically shaped nanocrystals for which direct metal deposition is unfeasible, as for the case of CdS nanorods. We believe that this strategy may be of general utility to create other types of complex colloidal nanoheterostructures, provided that a suitable sacrificial material can be grown on top of the starting nanocrystal seeds

An Insight into Chemistry and Structure of Colloidal 2D-WS2 Nanoflakes: Combined XPS and XRD Study

The surface and structural characterization techniques of three atom-thick bi-dimensional 2D-WS2 colloidal nanocrystals cross the limit of bulk investigation, offering the possibility of simultaneous phase identification, structural-to-morphological evaluation, and surface chemical description. In the present study, we report a rational understanding based on X-ray photoelectron spectroscopy (XPS) and structural inspection of two kinds of dimensionally controllable 2D-WS2 colloidal nanoflakes (NFLs) generated with a surfactant assisted non-hydrolytic route. The qualitative and quantitative determination of 1T’ and 2H phases based on W 4f XPS signal components, together with the presence of two kinds of sulfur ions, S22− and S2−, based on S 2p signal and related to the formation of WS2 and WOxSy in a mixed oxygen-sulfur environment, are carefully reported and discussed for both nanocrystals breeds. The XPS results are used as an input for detailed X-ray Diffraction (XRD) analysis allowing for a clear discrimination of NFLs crystal habit, and an estimation of the exact number of atomic monolayers composing the 2D-WS2 nanocrystalline samples

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

Dynamical formation of spatially localized arrays of aligned nanowires in plastic films with magnetic anisotropy.

We present a simple technique for magnetic-field-induced formation, assembling, and positioning of magnetic nanowires in a polymer film. Starting from a polymer/iron oxide nanoparticle casted solution that is allowed to dry along with the application of a weak magnetic field, nanocomposite films incorporating aligned nanocrystal-built nanowire arrays are obtained. The control of the dimensions of the nanowires and of their localization across the polymer matrix is achieved by varying the duration of the applied magnetic field, in combination with the evaporation dynamics. These multifunctional anisotropic free-standing nanocomposite films, which demonstrate high magnetic anisotropy, can be used in a wide field of technological applications, ranging from sensors to microfluidics and magnetic devices

Reversible Wettability Changes in Colloidal TiO2 Nanorod Thin-Film Coatings under Selective UV Laser Irradiation

We demonstrate the light-induced, reversible wettability of homogeneous nanocrystal-based, thin-film coatings composed of closely packed arrays of surfactant-capped anatase TiO2 nanorods laterally oriented on various substrates. Under selective pulsed UV laser excitation, the oxide films exhibit a surface transition from a highly hydrophobic and superoleophilic state (water and oil contact angles of 110° and less than 8°, respectively) to a highly amphiphilic condition (water and oil contact angles of 20° and 3°, respectively). A mechanism is identified according to which the UV-induced hydrophilicity correlates with a progressive increase in the degree of surface hydroxylation of TiO2. The observed wettability changes are not accompanied by any noticeable photocatalytic degradation of the surfactants on the nanorods, which has been explained by the combined effects of the intense and pulsed irradiation regime and of the rodlike nanocrystal morphology. The organic ligands on the oxide are instead assumed to rearrange conformationally in response to the lightdriven surface reconstruction. The amphiphilic state of the UV-irradiated TiO2 films is then considered as the macroscopic wetting result of alternating hydrophilic and oleophilic surface domains of nanoscale extension. Upon prolonged storage in the dark, ambient oxygen removes the newly implanted hydroxyl groups from the TiO2 surfaces and consequently affects again the conformations of ligands such that the films are allowed to recover their native hydrophobic/superoleophilic properties

Colloidal semiconductor/magnetic heterostructures based on iron-oxide-functionalized brookite TiO2 nanorods

A flexible colloidal seeded-growth strategy has been developed to synthesize all-oxide semiconductor/magnetic hybrid nanocrystals (HNCs) in various topological arrangements, for which the dimensions of the constituent material domains can be controlled independently over a wide range. Our approach relies on driving preferential heterogeneous nucleation and growth of spinel cubic iron oxide (IO) domains onto brookite TiO2 nanorods (b-TiO2) with tailored geometric parameters, by means of time-programmed delivery of organometallic precursors into a suitable TiO2-loaded surfactant environment. The b-TiO2 seeds exhibit size-dependent accessibility towards IO under diffusion-controlled growth regime, which allows attainment of HNCs individually made of a single b-TiO2 section functionalized with either one or multiple nearly spherical IO domains. In spite of the dissimilarity of the respective crystal-phases, the two materials share large interfacial junctions without significant lattice strain being induced across the heterostructures. The synthetic achievements have been supported by a systematic morphological, compositional and structural characterization of the as-prepared HNCs, offering a mechanistic insight into the specific role of the seeds in the control of heterostructure formation in liquid media. In addition, the impact of the formed b-TiO2/IO heterojunctions on the magnetic properties of IO has also been assessed