Metal Nanocatalyst Sintering Interrogated at Complementary Length Scales.

Metal nanoparticle (NP) sintering is a prime cause of catalyst degradation, limiting its economic lifetime and viability. To date, sintering phenomena are interrogated either at the bulk scale to probe averaged NP properties or at the level of individual NPs to visualize atomic motion. Yet, "mesoscale" strategies which bridge these worlds can chart NP populations at intermediate length scales but remain elusive due to characterization challenges. Here, a multi-pronged approach is developed to provide complementary information on Pt NP sintering covering multiple length scales. High-resolution scanning electron microscopy (HRSEM) and Monte Carlo simulation show that the size evolution of individual NPs depends on the number of coalescence events they undergo during their lifetime. In its turn, the probability of coalescence is strongly dependent on the NP's mesoscale environment, where local population heterogeneities generate NP-rich "hotspots" and NP-free zones during sintering. Surprisingly, advanced in situ synchrotron X-ray diffraction shows that not all NPs within the small NP sub-population are equally prone to sintering, depending on their crystallographic orientation on the support surface. The demonstrated approach shows that mesoscale heterogeneities in the NP population drive sintering and mitigation strategies demand their maximal elimination via advanced catalyst synthesis strategies.

Atomic layer deposition of ternary ruthenates by combining metalorganic precursors with RuO4 as the co-reactant.

In this work, the use of ruthenium tetroxide (RuO4) as a co-reactant for atomic layer deposition (ALD) is reported. The role of RuO4 as a co-reactant is twofold: it acts both as an oxidizing agent and as a Ru source. It is demonstrated that ALD of a ternary Ru-containing metal oxide (i.e. a metal ruthenate) can be achieved by combining a metalorganic precursor with RuO4 in a two-step process. RuO4 is proposed to combust the organic ligands of the adsorbed precursor molecules while also binding RuO2 to the surface. As a proof of concept two metal ruthenate processes are developed: one for aluminum ruthenate, by combining trimethylaluminum (TMA) with RuO4; and one for platinum ruthenate, by combining MeCpPtMe3 with RuO4. Both processes exhibit self-limiting surface reactions and linear growth as a function of the number of ALD cycles. The observed saturated growth rates are relatively high compared to what is usually the case for ALD. At 100 °C sample temperature, growth rates of 0.86 nm per cycle and 0.52 nm per cycle are observed for the aluminum and platinum ruthenate processes, respectively. The TMA/RuO4 process results in a 1 : 1 Al to Ru ratio, while the MeCpPtMe3/RuO4 process yields a highly Ru-rich composition with respect to Pt. Carbon, hydrogen and fluorine impurities are present in the thin films with different relative amounts for the two investigated processes. For both processes, the as-deposited films are amorphous.

In situ study of the thermal stability of supported Pt nanoparticles and their stabilization via atomic layer deposition overcoating.

Downscaling of supported Pt structures to the nanoscale is motivated by the augmentation of the catalytic activity and selectivity, which depend on the particle size, shape and coverage. Harsh thermal and chemical conditions generally required for catalytic applications entail an undesirable particle coarsening, and consequently limit the catalyst lifetime. Herein we report an in situ synchrotron study on the stability of supported Pt nanoparticles and their stabilization using atomic layer deposition (ALD) as the stabilizing methodology against particle coarsening. Pt nanoparticles were thermally annealed up to 850 °C in an oxidizing environment while recording in situ synchrotron grazing incidence small angle X-ray scattering (GISAXS) 2D patterns, thereby obtaining continuous information about the particle radius evolution. Al2O3 overcoat as a protective capping layer against coarsening via ALD was investigated. In situ data proved that only 1 cycle of Al2O3 ALD caused an augmentation of the onset temperature for particle coarsening. Moreover, the results showed a dependence of the required overcoat thickness on the initial particle size and distribution, being more efficient (i.e. requiring lower thicknesses) when isolated particles are present on the sample surface. The Pt surface accessibility, which is decisive in catalytic applications, was analyzed using the low energy ion scattering (LEIS) technique, revealing a larger Pt surface accessibility for a sample with Al2O3 overcoat than for a sample without a protective layer after a long-term isothermal annealing.

The co-reactant role during plasma enhanced atomic layer deposition of palladium.

Atomic layer deposition (ALD) of noble metals is an attractive technology potentially applied in nanoelectronics and catalysis. Unlike the combustion-like mechanism shown by other noble metal ALD processes, the main palladium (Pd) ALD process using palladium(ii)hexafluoroacetylacetonate [Pd(hfac)2] as precursor is based on true reducing surface chemistry. In this work, a thorough investigation of plasma-enhanced Pd ALD is carried out by employing this precursor with different plasmas (H2*, NH3*, O2*) and plasma sequences (H2* + O2*, O2* + H2*) as co-reactants at varying temperatures, providing insights in the co-reactant and temperature dependence of the Pd growth per cycle (GPC). At all temperatures, films grown with only reducing co-reactants contain a large amount of carbon, while an additional O2* in the co-reactant sequence helps to obtain Pd films with much lower impurity concentrations. Remarkably, in situ XRD and SEM show an abrupt release of the carbon impurities during annealing at moderate temperatures in different atmospheres. In vacuo XPS measurements reveal the remaining species on the as-deposited surface after every exposure. Links are established between the particular surface termination prior to the precursor pulse and the observed differences in GPC, highlighting hydrogen as the key growth facilitator and carbon and oxygen as growth inhibitors. The increase in GPC with temperature for ALD sequences with H2* or NH3* prior to the precursor pulse is explained by an increase in the amount of hydrogen species that reside on the Pd surface which are available for reaction with the Pd(hfac)2 precursor.

Plasma-Enhanced Atomic Layer Deposition of Nanostructured Gold Near Room Temperature.

A plasma-enhanced atomic layer deposition (PE-ALD) process to deposit metallic gold is reported, using the previously reported Me3Au(PMe3) precursor with H2 plasma as the reactant. The process has a deposition window from 50 to 120 °C with a growth rate of 0.030 ± 0.002 nm per cycle on gold seed layers, and it shows saturating behavior for both the precursor and reactant exposure. X-ray photoelectron spectroscopy measurements show that the gold films deposited at 120 °C are of higher purity than the previously reported ones (<1 at. % carbon and oxygen impurities and <0.1 at. % phosphorous). A low resistivity value was obtained (5.9 ± 0.3 µΩ cm), and X-ray diffraction measurements confirm that films deposited at 50 and 120 °C are polycrystalline. The process forms gold nanoparticles on oxide surfaces, which coalesce into wormlike nanostructures during deposition. Nanostructures grown at 120 °C are evaluated as substrates for free-space surface-enhanced Raman spectroscopy (SERS) and exhibit an excellent enhancement factor that is without optimization, only one order of magnitude weaker than state-of-the-art gold nanodome substrates. The reported gold PE-ALD process therefore offers a deposition method to create SERS substrates that are template-free and does not require lithography. Using this process, it is possible to deposit nanostructured gold layers at low temperatures on complex three-dimensional (3D) substrates, opening up opportunities for the application of gold ALD in flexible electronics, heterogeneous catalysis, or the preparation of 3D SERS substrates.