One-Step Synthesis of TiO2/SiO2-np Nanocomposite Photocatalytic Multilayer Films: Effect of Incorporation Time Sequences of SiO2 Nanoparticles during the TiO2 Film Growth.

In this article, the TiO2/SiO2-np nanocomposite multilayer films were synthesized in a single step by reactive magnetron sputtering combined with a nanoparticle aerosol jet. The SiO2 nanoparticles (SiO2-np) were introduced into a growing TiO2 thin film with different time sequences during deposition for a fixed duration. The SiO2-np acting as impurities are introduced into the TiO2 to willingly disturb its growth and to cause growth defects in order to increase the specific surface area of the photocatalytic film. In reason of the non-photoactive properties of the SiO2 nanoparticles, their introduction allows us to study only the effects induced on the film morphology, microstructure, and photocatalytic properties by their incorporation. The fractographies and topographies reveal strong changes in the morphologies depending on the time sequence of the nanoparticle introduction in the thin films. The introduction of SiO2-np from the beginning of the TiO2 film growth leads to the formation of high and large growth defects resulting in a highly diffusive surface. In addition, XRD analysis shows that the crystallite size tends to decrease as the composite film layer gets closer to the surface. Their photocatalytic performance is obtained by following the degradation of orange G dye under UV-visible irradiation. The photocatalytic performance is not only related to the specific surface area of the catalyst film, and the coverage of the photoactive phase on the surface, but also to the crystal quality of the photoactive phase. Furthermore, the samples exhibit good photostability, maintaining the same activity after four degradation cycles. In the specific case of TiO2/SiO2-np, it is demonstrated that the introduction of the nanoparticles only at the beginning of the film growth is more efficient than a continuous introduction. This result suggests that this original process allows the use of a relevant strategy for the nanoparticle introduction according to the required functionality.

Aerosol-based functional nanocomposite coating process for large surface areas.

The incorporation of nanometric-sized objects in conventional coatings can improve the properties of the matrix alone or give rise to new functionalities brought by the nanostructures. Current processes call on various shaping technologies that depend on the nature of the nano-inclusions and the matrix considered. Here, we present an integrated process based on the incorporation of nanoparticles using the aerosol route. It combines divergent nanoparticle jets with a uniform spatial profile and Physical Vapor Deposition (PVD). The chemical nature of the nanoparticles is then independent of that used for the matrix. First samples show that the morphology of nanocomposites is strongly dependent on the particle density in the films. Moreover, using several aerodynamic lens arrays combined with smart masking demonstrate the ability for coating on large surface area (40 cm2) substrates. These extended possibilities for developing new types of nanocomposites on any type of substrate and on large surface areas at low temperatures proves to be of strategic interest for various applications.

Experimental determination of the curvature-induced intra-wall polarization of inorganic nanotubes.

Inspired by a natural nano-mineral known as imogolite, aluminosilicate inorganic nanotubes are appealing systems for photocatalysis. Here, we studied two types of synthetic imogolites: one is completely hydrophilic (IMO-OH), while the other has a hydrophilic exterior and a hydrophobic interior (IMO-CH3), enabling the encapsulation of organic molecules. We combined UV-Vis diffuse reflectance spectroscopy of imogolite powders and X-ray photoelectron spectroscopy of deposited imogolite films and isolated nanotubes agglomerates to obtain not only the band structure, but also the quantitative intra-wall polarization of both synthetic imogolites for the first time. The potential difference across the imogolite wall was determined to be 0.7 V for IMO-OH and around 0.2 V for IMO-CH3. The high curvature of the nanotubes, together with the thinness of their wall, favors efficient spontaneous charge separation and electron exchange reactions on both the internal and external nanotube surfaces. In addition, the positions of their valence and conduction band edges make them interesting candidates for co-catalysts or doped catalysts for water splitting, among other possible photocatalytic reactions relevant to energy and the environment.

Water adsorption on TiO2 surfaces probed by soft X-ray spectroscopies: bulk materials vs. isolated nanoparticles.

We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs' surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.

One-step synthesis of Si@C nanoparticles by laser pyrolysis: high-capacity anode material for lithium-ion batteries.

Carbon-covered silicon nanoparticles (Si@C) were synthesized for the first time by a one-step continuous process in a novel two stages laser pyrolysis reactor. Crystallized silicon cores formed in a first stage were covered in the second stage by a continuous shell mainly consisting in low organized sp(2) carbon. At the Si/C interface silicon carbide is absent. Moreover, the presence of silicon oxide is reduced compared to materials synthesized in several steps, allowing the use of such material as promising anode material in lithium-ion batteries (LIB). Auger Electron Spectroscopy (AES) analysis of the samples at both SiKLL and SiLVV edges proved the uniformity of the carbon coating. Cyclic voltammetry was used to compare the stability of Si and Si@C active materials. In half-cell configuration, Si@C exhibits a high and stable capacity of 2400 mAh g(-1) at C/10 and up to 500 mAh g(-1) over 500 cycles at 2C. The retention of the capacity is attributed to the protective effect of the carbon shell, which avoids direct contact between the silicon surface and the electrolyte.

X-ray Photoelectron Spectroscopy of Isolated Nanoparticles.

X-ray photoelectron spectroscopy (XPS) is a very efficient and still progressing surface analysis technique. However, when applied to nano-objects, this technique faces drawbacks due to interactions with the substrate and sample charging effects. We present a new experimental approach to XPS based on coupling soft X-ray synchrotron radiation with an in-vacuum beam of free nanoparticles, focused by an aerodynamic lens system. The structure of the Si/SiO2 interface was probed without any substrate interaction or charging effects for silicon nanocrystals previously oxidized in ambient air. Complete characterization of the surface was obtained. The Si 2p core level spectrum reveals a nonabrupt interface.