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Scanning probe energy loss spectroscopy of Nanoscale Alloy Particles for heterogeneous CATalysis

Periodic Reporting for period 1 - SNAPCAT (Scanning probe energy loss spectroscopy of Nanoscale Alloy Particles for heterogeneous CATalysis)

Periodo di rendicontazione: 2016-05-01 al 2018-04-30

Most heterogeneous catalysts take the form of catalytically active nanoparticles dispersed over a support medium. To improve catalytic function and reduce waste requires improving the homogeneity of these nanoparticles, whether it be their size, shape or composition. Measurements of individual nanoparticles require access to expensive instrumentation such as a scanning transmission electron microscope (STEM). The project applies a novel and relatively inexpensive technique, scanning probe energy loss spectroscopy (SPELS), to study the composition, size and shape of individual size-selected Pt-based alloy nanoparticles, deposited using an inert gas-aggregation source, with a view to better understanding and improving their catalytic performance. The study is timely given the current interest in developing clean energy alternatives to fossil fuels. New methodologies and fundamental insights in this area can lead to significant economic and environmental impact.

The overall objectives of the project have been: RO1 To quantify the sensitivity and resolution of SPELS by mapping plasmons on well-defined Ag nanostructures. RO2 To investigate Pt alloys formed on single crystal surfaces. RO3 To produce size-selected nanoparticles of Pt alloys. RO4 To measure the composition of individual alloy nanoparticles with SPEL. RO5 To identify target materials and optimise deposition parameters for alloy nanoparticle production.

Due to the early termination of the project not all of the anticipated research objectives were reached. However, many valuable insights have been gained from the work done. Using a home-built retarding field analyser, it was possible to detect the plasmon loss feature at 2.6 eV on the Au(111) surface with SPELS. Field-induced roughening of the Au surface was also observed. SPELS was used to image size-selected Pt and Au nanoparticles in both constant field emission current (CFEC) mode and backscattered electron (BSE) mode. Nanoparticles appear as protrusions in constant field emission current images and as troughs in the corresponding backscattered electron images. A small lateral offset was found between the field emission and backscattered electron images that is independent of the scan direction. This can be ascribed to the fact that backscattered electrons are not detected from the region directly under the tip apex due to field suppression. This is the first experimental demonstration of this effect and also represents the highest imaging resolution obtained with SPELS to date.
WP1 SPELS calibration on e-beam lithography samples: Task 1 – prepare nanostructures by e-beam lithography. The instrument used for e-beam lithography broke during the project. Instead, Ag nanoplates with well-defined plasmon resonances were purchased from NanoComposix. These triangular nanoplates are 10 nm thick and 40 nm to 130 nm wide, and have different plasmon modes at their centres and corners. Task 2 – preliminary SPELS measurements on thermally evaporated Ag nanostructures. STM measurements could not be obtained on polyvinylpyrrolidone (PVP) capped Ag nanoplates due to the polymer, a smaller stabilising molecule (lipoic acid) was used. STM images were obtained but SPELS measurements of these nanostructures have not yet been performed. A new cylindrical sector analyser, having improved energy resolution, was designed in conjunction with LK technologies. Task 3 – statistical process control over tip production. A procedure for preparing electrochemically etched tungsten field emitters was developed, using an in-vacuum tip preparation stage tips were flash annealed in order to remove surface oxides, followed by field-induced sharpening.

WP2 SPELS measurements on epitaxial alloy films: Task 1 – build a temperature programmed desorption setup. Due to the need to design and assemble a working electron energy analyser, building the temperature programmed desorption was put on hold. Task 2 – SPELS measurements on epitaxial thin films. SPELS measurments were performed on the Au(111) surface where the characteristic plasmon loss feature at 2.6 eV was observed. A home-built electron beam evaporator was assembled and tested for in-situ thin film deposition. Task 3 – microfabrication of co-axial tips. No co-axial tips were microfabricated during the reporting period.

WP3 SPELS on nanoparticles: Task 1 – Produce size-selected Pt clusters. Pt923, Au923 and Pt147 nanoparticles were deposited onto graphite substrates using the inert gas aggregation source. Task 2 – Produce alloy nanoparticles. Co-deposition of Pt and Au nanoparticles produced PtAu nanostructures. Task 3 – SPELS of alloy nanoparticles. The BSE signal from the nanoparticles was lower than that obtained from the surrounding graphite substrate.The Pt and Au could not be distinguished in BSE images due to their similar atomic numbers. A small offset was observed between the same nanoparticle features in simultaneously acquired CFEC and BSE images, which was independent of the scan direction. This confirms that the BSE are detected from a probe area adjacent to the tip, and not from the region directly under the tip apex due to field suppression. Preliminary SPELS measurements of the energy loss spectrum of the PtAu nanostructures could not resolve the plasmon loss feature attributed to Au. Task 4 – optimise magnetron targets for producing homogeneous nanoparticle ensembles. This work was not completed due to the early termination of the project.

WP4 Training & Dissemination activities: Task 1 – Website. A website related to the project has been established at Task 2 – Public outreach. Dr. Murphy established a profile on ResearchGate with details of the project and relevant publications. Task 3 – Training & secondment. Dr. Murphy received some training in the operation of the gas aggregation cluster source. He has also learned some programming in Python in order to process the SPELS data. He attended several seminars by invited speakers who are experts in the field of nanoparticle synthesis and characterisation and microscopy. Task 4 – Conference. Dr. Murphy has recently attended the Microscience and Microscopy Congress (mmc2017) in Manchester. Task 5 – Papers. A paper reporting the preliminary results on imaging size-selected nanoparticles with SPELS is in preparation.
The main results that move beyond the current state-of-the-art in SPELS are: (1) The observed negative contrast in backscattered electron images of nanoparticles of different materials deposited on graphite. (2) The observation of an offset between nanoparticle features in the simultaneously acquired field emisson and backscattered electron images. (3) The observation of field-induced roughening of the Au(111) surface when imaged with SPELS. Insights such as these are essential if SPELS is to be developed as a tool to characterise materials, such as catalytic nanoparticles, that have societal and economic importance.
Dr. Murphy has started a new permanent position as a lecturer at the Institute of Technology Tallaght (ITT), Ireland. He is being supported by the University of Birmingham through an equipment loan that will allow him to continue the work begun during his fellowship with a view to completing the workplan outlined in the project.
Topographic and backscattered electron images along with energy loss spectra of the Au(111) surface.
Field emission and backscattered electron images of size-selected Pt nanoparticles imaged with SPELS
STM images showing the field-induced multilayer roughening of the Au(111) surface by SPELS.