Simona Badilescu

Study of sol-gel prepared nanostructured WO3 thin films and composites for electrochromic applications

Abstract Nanostructured tungsten oxide (WO 3 ) thin films have been prepared by the sol–gel method following the inorganic route in which alcoholic solutions containing tungsten salts have been used as precursors. The WO 3 films were deposited by dipcoating on glass substrates. In order to create nanoporosity in some of these films, the films were deposited on glass substrates coated with a layer of polystyrene (PS) microspheres. This polymer was then removed either by thermal treatment or by solvent extraction, generating a high density of pores within the tungsten framework. The characterization of these films has been carried out by atomic force microscopy (AFM), optical studies and ATR–FTIR spectroscopy. The lithium intercalation of these WO 3 films has been carried out using a dry (in vacuo) technique. Results on the comparative study of the optical and electrochromic properties of these different types of WO 3 are presented here.

Study of Anatase to Rutile Phase Transition in Nanocrystalline Titania Films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel dip-coating process. The effect of acetylacetone, diethanolamine and polyethylene glycol on the structure of the heat-treated titania films was examined by Raman and FTIR spectroscopy and X-ray diffraction. The effect the complexing agents have on the anatase to rutile phase transition during the heat treatment process is studied. The understanding of this effect is expected to enhance our capacity to tailor the composition and morphology of films and thus their properties. The Raman and the infrared spectra of nanocrystalline titania films and the changes induced by the heat treatment were also investigated. We have also studied the size of the crystallites in TiO2 films and its dependence on the type of complexing agent used.

Low Temperature Sol-Gel Preparation of Nanocrystalline TiO2 Thin Films

TiO2 nanocrystalline thin films with varying degree of porosity have been prepared using a low temperature method. TiO2 films of the anatase form have been obtained by using a polyethylene glycol (PEG) modified sol-gel method. Densification and crystallization of the films was found to result from the thermal treatment of the dip coated films in boiling water. The films have been characterized by Raman, XRD, FTIR, AFM and optical methods. Highly transparent films with transmission in excess of 85% and porosity as high as 58% are formed predominantly of anatase crystallites of dimensions of the order of 5 nm. Initial results on lithium intercalation into these films resulting in an efficient optical modulation in the visible and near infrared regions demonstrate a good potential of these films for electrochromic applications.

Self-assembly of colloidal spheres on patterned substrates

We report here on the self-assembly of polystyrene colloidal spheres on patterned substrates. The substrate was a grating with a periodic one-dimensional-height profile. The two-dimensional (2D) arrays of colloidal spheres were obtained by the convective self-assembly method. It is shown that these structures strongly depend on the ratio between the diameter of the sphere (d) and the period of the grating (p) (x=d/p). When 1<x<1.15, and 1.15<x<2, the 2D array showed a centered-rectangular symmetry. For x=1.15 and 2, the ordered 2D array had a hexagonal symmetry structure. The angle between the grating groove direction and the crystal lattice vector (θ) is given by θ=sin−1(d/p). It is suggested that the variety of the 2D structures are formed by the capillary forces and the self-shadowing effects during the drying process.

Integration of gold nanoparticles in PDMS microfluidics for lab-on-a-chip plasmonic biosensing of growth hormones

Gold nanoparticles were synthesized in a poly(dimethylsiloxane) (PDMS) microfluidic chip by using an in-situ method, on the basis of reductive properties of the cross-linking agent of PDMS. The proposed integrated device was further used as a sensitive and low-cost LSPR-based biosensor for the detection of polypeptides. Synthesis of nanoparticles in the microfluidic environment resulted in improvement of size distribution with only 8% variation, compared with the macro-environment that yields about 67% variation in size. The chemical kinetics of the in-situ reaction in the microfluidic environment was studied in detail and compared with the reaction carried out at the macro-scale. The effect of temperature and gold precursor concentration on the kinetics of the reaction was investigated and the apparent activation energy was estimated to be Ea*=30 kJ/mol. The sensitivity test revealed that the proposed sensor has a high sensitivity of 74 nm/RIU to the surrounding medium. The sensing of bovine growth hormone also known as bovine somatotropin (bST) shows that the proposed biosensor can reach a detection limit of as low as 3.7 ng/ml (185 pM). The results demonstrate the successful integration of microfluidics and nanoparticles which provides a potential alternative for protein detection in clinical diagnostics.

Sol-gel-prepared ITO films for electrochromic systems

Abstract Indium tin oxide (ITO) films have been prepared by the sol-gel method using both organic and inorganic precursors. A computer-controlled dip-coating unit is designed and fabricated in our laboratory for a precise control of the parameters during the dip-coating process. These films have been characterized by X-ray diffraction optical and electrical study and also by atomic absorption spectroscopy. The optimized coatings exhibit a sheet resistance of ara und 100 Ω/□ and an average visible solar transmission of around 85%. A five-layer electrochromic system using these ITO layers as transparent electrodes was fabricated and tested. The performance of the electrochromic system indicates the high potential of these films for such applications, especially for large area coaling.

In situ Raman spectroscopic-electrochemical studies of lithium-ion battery materials: a historical overview

In this review, the recent advances in the development of in situ Raman spectroscopy and electrochemical techniques and their application for the study of lithium-ion batteries are revisited. It is demonstrated that, during a relatively short period of time (1995–2013), the spectroelectrochemical techniques used for the investigation of battery components, benefited directly from the tremendous advances of Raman technology. The most important step was the implementation of confocal Raman microscopy in the battery research, which opened the way to new and more sophisticated applications. This review shows how the discovery of new Raman techniques such as surface-enhanced Raman scattering, tip-enhanced Raman spectroscopy, spatially offset Raman spectroscopy as well as the integration of Raman spectrometers into non-optical microscopes, for example AFM and SEM, allowed to perform two or more analytical techniques on the same sample region, with an exceptionally high resolution. All these progresses led to new insights into battery materials and components such as electrodes and electrolytes, and helped to understand the electrode/electrolyte interface phenomena. This enhanced understanding allowed a deeper insight into important phenomena, as e.g., battery aging and the dynamic nature of the solid electrolyte interfaces in lithium batteries. The high relevance of the information provided by these techniques in the progress of battery modeling is another positive contribution. Another area of high practical significance for the battery field is the screening of electrode materials, which is facilitated by the availability of the data provided by spectroscopic methods.

Synthesis and Characterization of Macroporous Tungsten Oxide Films for Electrochromic Application

Macroporous tungsten oxide films have been prepared by combining a nonhydrolytic sol-gel method with a molecular assembly templating strategy. The material has been prepared by hydrolysis of an ethanolic solution of tungsten ethoxide in the presence of polyethylene glycol (PEG), followed by calcination of the dip-coated films. AFM images indicated that an important morphological diversity can be obtained by simply varying the amount of PEG in the coating solution and the conditions of the heat-treatment. The formation of nanostructures of controlled shapes and patterns (fibrils or striped phases) with relatively uniform channel spacings is accounted for by strong hydrogen bonding interactions between the PEG and the partially hydrolyzed tungsten oxide oligomers. XRD and FTIR data showed that PEG delays the crystallization of WO3. When compared to sol-gel prepared tungsten oxide fims prepared without PEG, the coloration efficiency of the macroporous films appears to be significantly improved especially in the near-infrared region.

Enhanced infrared ATR spectra of o-, m-, and p-nitrobenzoic acid with Ag films

The dependence of enhancement of the infrared bands by thin Ag films on the molecular structure of the organic compound was investigated. A different enhancement behavior for the three isomers—ortho-, meta-, and paranitrobenzoic acids—was observed and accounted for by structural features. The presence of the ionized species of the acids in the monolayer bonded to Ag was found, and the spectral features of the monolayer and bulk compound were compared. The splitting of the antisymmetric stretching band of the nitro group for metanitro benzoic acid strongly interacting with Ag was observed. The correlation of the morphology of the Ag surface to the magnitude of the achieved enhancement is discussed.

Study of the phase transition and the thermal nitridation of nanocrystalline sol–gel titania films

Abstract Nanocrystalline titania films were prepared by a complexing agent-assisted sol–gel method and converted to titanium nitride by a thermal nitridation process. The effect of acetylacetone (AcAc), diethanolamine (DEA) and acid catalysts (HCl and HNO 3 ) on the structure and morphology of the heat-treated titania films and on their nitridation products was examined by FTIR spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). The carbothermal reduction of titania during the nitridation process with the formation of carboxynitrides has been considered. The results showed that the oxide to nitride transition strongly depends on the complexing agent used to prepare the titania films. The XRD results indicated the dependence of the lattice parameter of the nitridation product on the complexing agent or acid catalyst: AcAc and DEA lead to TiN x with a lattice parameter α close to the theoretical value, while with HCl the lattice parameter was found sensibly lower showing the presence of an oxynitride. These results are accounted for by the effect of complexing agents and acid catalysts on the size of both TiO 2 and TiN grains and the different reactivity of the anatase and rutile phases. The possibility of tailoring the composition and morphology of TiN films by using complexing agents is envisaged.