Project stage 1 - Preliminary activities of synthesis and characterization of the MOS-NP systems regarding the mesoporous structure, nature and quantity of the deposited material on the surface.

Performed activities

1. Preparation of the mesoporous metal oxide semiconductor (MOS) structures and their decoration with catalytic nanoparticles (MOS-NP systems) by solvothermal synthesis from polymeric precursors and wet impregnation method. Mesoporous structures of SnO₂, SnO₂:Zn and SnO₂:In decorated with Pd/Fe based nanoparticles have been prepared.
2. Preliminary evaluation of the synthesized MOS-NP systems by XRD, SEM, EDS techniques, from a structural, morphological and compositional point of view. The formation of nanostructured tetragonal phase of SnO₂ (cassiterite) has been evidenced by XRD. The mean grain size varies between 5-10 nm, depending on the doping level. Our measurements evidenced that in general, doping inhibits the crystal growth, resulting in nanoparticles with a lower size. SEM-EDS shows that palladium, as decorating element, is segregated in clusters of nanoparticles of 100-200 nm presumably embedded into the mesoporous SnO₂ matrix.
3. Microstructural and spectroscopic properties of the MOS-NP systems spatially resolved by HRTEM, STEM, EFTEM, EELS, EDS. Transmission electron microscopy confirms the formation of the tetragonal SnO2 phase consisting in nanocrystals with a mean size in the range 5-10 nm, depending on the sample. STEM-EDS observations have proven that doping with Zn or In is uniform at nanometric level, while Pd is non-uniformly distributed, forming clusters of tens or hundreds of nm size.
4. 3D morphology at nanometric scale by electron tomography: optimizing the acquisition parameters of the tomographic series. Series of tomographic images have been acquired showing that the Pd clusters are actually attached to the surface of large mesoporous grains of SnO₂.
5. Complementary spectroscopic characterization of surface and volume properties by XPS and EPR. XPS measurements evidenced the surface oxidation of the Pd-rich nanoparticles. EPR measurements have evidenced the presence of several types of point defects, such as a substitutional N center or substitutional Mn²⁺ center (in the case of the Zn-doped samples).

Project stage 2 – Advanced 3D+ morpho-structural characterization of MOS-NP systems

Performed activities

1. Fine tuning of the synthesis parameters for the increase of the specific surface area and the uniform dispersion of the Fe/Pd-based nanoparticles. Two synthesis routes have been applied for fabricating mesoporous supports from SnO₂ (pure or doped with In) using either organic or inorganic precursors and two types of surfactants (Brij 52 or CTAB). Wet and semi-dry impregnation methods have been used for decorating the mesoporous SnO₂-based supports with Pd or Fe-based nanoparticles.
2. Global structural, morphological and compositional evaluations by XRD, SEM and EDS measurements. The formation of nanostructured tetragonal phase of SnO₂ (cassiterite) has been evidenced by XRD. The mean grain size varies between 4-16 nm, depending on the doping level, type of surfactant and thermal treatment. Our measurements evidenced that in general, doping (with Zn or In) inhibits the crystal growth, resulting in nanoparticles with a lower size. SEM-EDS shows that palladium, as decorating element, is segregated in clusters of nanoparticles of 100-200 nm presumably embedded into the mesoporous SnO₂ matrix. However, SEM-EDS measurements onto Fe decoration by semi-dry impregnation reveals a more uniform distribution of Fe with respect to the SnO₂ matrix.
3. Surface and volume distribution of the metallic ions by EPR spectroscopy in correlation with XPS measurements. Measurements of EPR spectroscopy at variable temperature have evidenced the presence of NO radicals adsorbed onto Sn⁴⁺ cationic positions. The Zn-doped samples reveal the presence of Mn²⁺ centers as non-intentional doping element (traces below 0.01%) usually associated with zinc. High-resolution XPS measurements have evidenced 3 valence states for Pd: metallic Pd, Pd²⁺ and Pd⁴⁺.
4. Study of the morphological and structural stability of the synthesized MOS-NP systems at variable temperature by in situ heating TEM experiments. In situ heating has been performed during the TEM observations up to 600 ^oC. Although no structural modifications have been evidenced, morphological changes do occur at temperatures above 500 ^oC concerning the aggregation of the Pd-rich decorating nanoparticles.
5. Nanoscale 3D reconstruction and by electron tomography of the mesoporous MOS-NP systems (part 1). The 3D reconstruction procedure has been optimized using several algorithms (Weighted Back-Projection - WBP, Simultaneous Iteration Reconstruction Technique - SIRT and Simultaneous Algebraic Reconstruction Techniques - SART). The procedure has been applied to reveal the morphological relation between the Pd-rich decorating nanoparticles and the SnO₂ mesoporous support.

Project stage 3 – Connection between the morphological, structural, spectroscopic and gas sensing properties

Foreseen activities

1. . Fabrication of gas sensors and the characterization of their electrical properties under controlled atmosphere of NO₂, ethanol or CO.
2. 3D reconstruction and nanoscale modeling by electron tomography on heterogeneous mesoporous MOS-NP systems.
3. Correlation between the nanoscale 3D information and the atomic structure of the nanoparticles obtained by TEM/HRTEM investigations and electron tomography.
4. Connection between the spectroscopic information at atomic level and the global information from the EPR, XPS, EELS and EDS measurements.
5. Understanding the growth and assembling mechanisms of the heterogeneous mesoporous MOS-NP systems from the 3D morphological, microstructural and spectroscopic investigations in relation with the methods and parameters of chemical synthesis.
6. Chemical and physical sensing mechanisms resulting from the nanoscale 3D, microstructural and spectroscopic information on the MOS-NP systems in connection with their electrical properties under controlled atmosphere.
7. Dissemination

Cosmin Marian ISTRATE

6-9 February 2018 - Attended the courses of the FELMI/European EELS & EFTEM School organized by FELMI-ZFE, Graz University of Technology, Austria

Aurel Mihai VLAICU

26-30 November 2018 - Attended the courses of the international school Materials Characterization by the Combined Analysis MAUD 2018 organized by University of Trento, Italy

Cosmin Marian ISTRATE

01.10.2018 - was admitted as PhD student at the University of Bucharest, Faculty of Physics, with the thematic “Local structural and spectroscopic information in nanostructured materials ”, under the supervision of Dr. Valentin Serban Teodorescu