This review addresses the inconsistencies in interpreting measurements of intrinsic catalyst properties using lab-scale devices. Any experiment must be analyzed in the framework of a model for... Show moreThis review addresses the inconsistencies in interpreting measurements of intrinsic catalyst properties using lab-scale devices. Any experiment must be analyzed in the framework of a model for which the choice and assumptions regarding the necessary parameters must be based on critical reasoning. Physical intuition about the properties of the system is required for both rigorous 3D computational and simplified analytic descriptions. Any divergence between hypothesis and characteristics of the systems affects both the investigation of intrinsic catalytic properties and the later industrial design where parameters are extrapolated outside their obtained operating range. In this work, we make an overview of the underlying physics of photocatalytic reactions, while focusing on pertinent hypothesis, and discuss the consequences for the most basic reactor designs for which guidelines and criteria are provided to meet their premise. Show less
Grillo, F.; Moulijn, J.A.; Kreutzer, M.T.; Ommen, J.R. van 2018
In industrial catalysis, the sintering of supported nanoparticles (NPs) is often associated with the loss of catalyst activity and thus with periodic plant downtime and economic burdens. Yet,... Show moreIn industrial catalysis, the sintering of supported nanoparticles (NPs) is often associated with the loss of catalyst activity and thus with periodic plant downtime and economic burdens. Yet, sintering mechanisms are at play also during the synthesis of the catalyst itself. They can, in fact, determine the size distribution of the NPs, and thus the activity and the stability of the catalyst. Here, we examine the role of nanoparticle sintering in a technique borrowed from the semiconductor industry that promises to reconcile atomic-scale precision with scalability: atomic layer deposition. By modeling the cyclic influx of single atoms in concomitance with NP sintering via either dynamic coalescence or Ostwald ripening, we establish the "signature" of different growth regimes: the size distribution. In contrast, we show that integral quantities such as the mean diameter, the number of NPs per unit area, and the material loading are poor indicators of the underlying growth mechanism. In particular, a constant number of NPs cannot be interpreted as a sign of no sintering. Finally, we argue that NP sintering, if properly understood, can open up new avenues for the control over the size distribution of NPs, and thus over their catalytic activity and stability. Show less
Grillo, F.; La Zara, D.; Mulder, P.; Kreutzer, M.T.; Ommen, J.R. van 2018
Understanding the spontaneous organization of atoms on well-defined surfaces promises to enable control over the shape and size of supported nanostructures. Atomic layer deposition (ALD) boasts... Show moreUnderstanding the spontaneous organization of atoms on well-defined surfaces promises to enable control over the shape and size of supported nanostructures. Atomic layer deposition (ALD) boasts atomic-scale control in the synthesis of thin films and nanoparticles. Yet, the possibility to control the shape of ALD-grown nanostructures remains mostly unexplored. Here, we report on the bottom-up formation of both linear and V-shaped anatase TiO2 nanorods (NRs) on graphene nanoplatelets during TiCl4/H2O ALD carried out at 300 degrees C. NRs as large as 200 nm form after only five ALD cycles, indicating that diffusional processes rather than layer-by-layer growth are behind the NR formation. In particular, high-resolution transmission electron microscopy reveals that the TiO2 NRs and graphene nanoplatelets are in rotational alignment as a result of lattice matching. Crucially, we also show that individual nanocrystals can undergo in-plane oriented attachment. Show less
Grillo, F.; Bui, H.V.; La Zara, D.; Aarnink, A.A.I.; Kovalgin, A.Y.; Kooyman, P.; ... ; Ommen, J.R. van 2018
A fundamental understanding of the interplay between ligand-removal kinetics and metal aggregation during the formation of platinum nanoparticles (NPs) in atomic layer deposition of Pt on TiO2... Show moreA fundamental understanding of the interplay between ligand-removal kinetics and metal aggregation during the formation of platinum nanoparticles (NPs) in atomic layer deposition of Pt on TiO2 nanopowder using trimethyl(methylcyclo-pentadienyl)platinum(IV) as the precursor and O-2 as the coreactant is presented. The growth follows a pathway from single atoms to NPs as a function of the oxygen exposure (P-O2 x time). The growth kinetics is modeled by accounting for the autocatalytic combustion of the precursor ligands via a variant of the Finke-Watzky two-step model. Even at relatively high oxygen exposures (<120 mbar s) little to no Pt is deposited after the first cycle and most of the Pt is atomically dispersed. Increasing the oxygen exposure above 120 mbar s results in a rapid increase in the Pt loading, which saturates at exposures >> 120 mbar s. The deposition of more Pt leads to the formation of NPs that can be as large as 6 nm. Crucially, high P-O2 (>= 5 mbar) hinders metal aggregation, thus leading to narrow particle size distributions. The results show that ALD of Pt NPs is reproducible across small and large surface areas if the precursor ligands are removed at high P-O2. Show less
Bui, H.V.; Grillo, F.; Kulkarni, S.; Bevaart, R.; Thang, N.V.; Linden, B. van der; ... ; Ommen, J.R. van 2017
We tailored the size distribution of Pt nanoparticles (NPs) on graphene nanoplatelets at a given metal loading by using low-temperature atomic layer deposition carried out in a fluidized bed... Show moreWe tailored the size distribution of Pt nanoparticles (NPs) on graphene nanoplatelets at a given metal loading by using low-temperature atomic layer deposition carried out in a fluidized bed reactor operated at atmospheric pressure. The Pt NPs deposited at low temperature (100 degrees C) after 10 cycles were more active and stable towards the propene oxidation reaction than their high-temperature counterparts. Crucially, the gap in the catalytic performance was retained even after prolonged periods of time (>24 hours) at reaction temperatures as high as 450 degrees C. After exposure to such harsh conditions the Pt NPs deposited at 100 degrees C still retained a size distribution that is narrower than the one of the as-synthesized NPs obtained at 250 degrees C. The difference in performance correlated with the difference in the number of facet sites as estimated after the catalytic test. Our approach provides not only a viable route for the scalable synthesis of stable supported Pt NPs with tailored size distributions but also a tool for studying the structure-function relationship. Show less
In this work, we report a method to change the surface finish of a standard polyester-based powder coating paint, from gloss to matt, by depositing ultrathin films of Al2O3 on the powder coating... Show moreIn this work, we report a method to change the surface finish of a standard polyester-based powder coating paint, from gloss to matt, by depositing ultrathin films of Al2O3 on the powder coating particles. The coating experiments were performed in a fluidized bed reactor at 1 bar and 27 degrees C, using a gas-phase coating process of alternating exposure of the particles to the two precursors (trimethylaluminium and water), similar to atomic layer deposition (ALD). We varied the number of coating cycles (1, 2, 3, 5, 7 and 9 cycles) to obtain film thicknesses of the alumina shell ranging from 1 to 30 nm. The average growth per cycle of the process is 3.5 nm, significantly larger than the one for pure self-limiting ALD. When the average alumina shell was thicker than 6 nm, the shell prevented the flow of the core particles, even though the powder particles did soften above the glass transition temperature. With the particles morphology intact, this resulted in a rough and matte surface finish of the coating after curing. The surface roughness, with a value around 9 pm determined by surface profilometry, is associated to the alumina coated particles as observed with SEM and EDX analysis. In addition, the matte finish coating showed mechanical resistance similar to that of uncoated powder particles. (C) 2017 The Authors. Published by Elsevier B.V. Show less
The article by Andrea Fabre et al. (Environ. Sci.: Nano, 2017, 4, 670–678) was published with an incorrect title (‘Modeling thesize distribution in a fluidized bed of nanopowder’). The correct... Show moreThe article by Andrea Fabre et al. (Environ. Sci.: Nano, 2017, 4, 670–678) was published with an incorrect title (‘Modeling thesize distribution in a fluidized bed of nanopowder’). The correct article title is ‘Entrainment of nanosized clusters from a nanopowder fluidized bed’. Show less
We present an atomistic understanding of the evolution of the size distribution with temperature and number of cycles in atomic layer deposition (ALD) of Pt nanoparticles (NPs). Atomistic modeling... Show moreWe present an atomistic understanding of the evolution of the size distribution with temperature and number of cycles in atomic layer deposition (ALD) of Pt nanoparticles (NPs). Atomistic modeling of our experiments teaches us that the NPs grow mostly via NP diffusion and coalescence rather than through single-atom processes such as precursor chemisorption, atom attachment, and Ostwald ripening. In particular, our analysis shows that the NP aggregation takes place during the oxygen half-reaction and that the NP mobility exhibits a size and temperature dependent scaling. Finally, we show that contrary to what has been widely reported, in general, one cannot simply control the NP size by the number of cycles alone. Instead, while the amount of Pt deposited can be precisely controlled over a wide range of temperatures, ALD-like precision over the NP size requires low deposition temperatures (e.g., T<100 degrees C) when growth is dominated by atom attachment. Show less
The release of nanosized particles from fluidized beds of ceramic oxide nanopowders, namely, TiO2 (P25), Al2O3 (AluC) and SiO2 (A130) has been assessed for the first time. Previous models and... Show moreThe release of nanosized particles from fluidized beds of ceramic oxide nanopowders, namely, TiO2 (P25), Al2O3 (AluC) and SiO2 (A130) has been assessed for the first time. Previous models and experiments for processing engineered nanoparticles (ENP) using fluidized beds reported only the formation of micron-sized cluster agglomerates in the gas phase. In this work, aerosol spectrometry techniques such as scanning mobility particle sizing (SMPS) and optical particle counting (OPC) have been combined with powder technologies, such as the borescope high-speed camera system, to determine the particle size distribution from 5 nm to 1 mm above a fluidized bed. Furthermore, the morphology of nanoparticulate aerosol at different locations in the bed was determined by offline electron microscopy. The results demonstrate that free nano- and micron-sized particles are released from fluidized beds. Since the structures found above the bed are also expected to be present within fluidized beds, a revision of existing nanoparticle fluidization models, and improved safety and control measures in reactors for gas-phase ENP processing are needed to avoid nanoparticle release. Show less
Fabre, A.; Salameh, S.; Kreutzer, M.T.; Ommen, J.R. van 2017
Fluidization is a technique used to process large quantities of nanopowder with no solvent waste and a large gas-solid contact area. Nonetheless, nanoparticles in the gas phase form clusters,... Show moreFluidization is a technique used to process large quantities of nanopowder with no solvent waste and a large gas-solid contact area. Nonetheless, nanoparticles in the gas phase form clusters, called agglomerates, due to the relatively large adhesion forces. The dynamics within the fluidized bed influence the mechanism of formation, and thus, the morphology of the agglomerates. There are many theoretical models to predict the average size of fluidized agglomerates; however, these estimates of the average lack information on the whole size range. Here, we predict the agglomerate size distribution within the fluidized bed by estimating the mode and width using a force balance model. The model was tested for titania (TiO2), alumina (Al2O3), and silica (SiO2) nanopowders, which were studied experimentally. An in-situ method was used to record the fluidized agglomerates for size analysis and model validation. (C) 2017 Elsevier B.V. All rights reserved. Show less
Nanoparticles surrounded by gas agglomerate in a hierarchical fashion. From production until powder processing in the gas phase, nanoparticles go from individual particles to aggregates, simple... Show moreNanoparticles surrounded by gas agglomerate in a hierarchical fashion. From production until powder processing in the gas phase, nanoparticles go from individual particles to aggregates, simple agglomerates, and complex agglomerates. Even though the structures at each level have unique properties, they are commonly assessed as a whole. Additionally, the effect of external factors on the morphology of these structures during gas processing is not well understood and challenging to study due to the limited techniques for in situ analysis of the dynamic phenomenon. Here, we study three materials in their hydrophobic and hydrophilic version. We describe the structural characteristics of each hierarchical level of complex agglomerate formation obtained from two in situ techniques. The first scale, namely aggregates, are open structures with a fractal dimension of about 1.5, which then form simple agglomerates with a fractal dimension close to 3, that later cluster into complex agglomerates that present a fractal dimension of about 2. Furthermore, gas dynamics were found to densify the simple agglomerates, increasing their fractal dimension by more than 0.1. Show less
Efficient nanopowder processing requires knowledge of the powder's mechanical properties. Due to the large surface area to volume ratio, nanoparticles experience relatively strong attractive... Show moreEfficient nanopowder processing requires knowledge of the powder's mechanical properties. Due to the large surface area to volume ratio, nanoparticles experience relatively strong attractive interactions, leading to the formation of micron-size porous structures called agglomerates. Significant effort has been directed towards the development of models and experimental procedures to estimate the elasticity of porous objects such as nanoparticle agglomerates; however, none of the existing models has been validated for solid fractions below 0.1. Here, we measure the elasticity of titania (TiO, 22 nm), alumina (AlO, 8 nm), and silica (SiO, 16 nm) nanopowder agglomerates by Atomic Force Microscopy, using a 3.75 m glass colloid for the stress-strain experiments. Three sample preparations with varying degree of powder manipulation are assessed. The measured Young's moduli are in the same order of magnitude as those predicted by the model of Kendall et al., thus validating it for the estimation of the Young's modulus of structures with porosity above 90 %. Show less
Stability of quantum dot (QD) films is an issue of concern for applications in devices such as solar cells, LEDs, and transistors. This paper analyzes and optimizes the passivation of such QD films... Show moreStability of quantum dot (QD) films is an issue of concern for applications in devices such as solar cells, LEDs, and transistors. This paper analyzes and optimizes the passivation of such QD films using gas-phase deposition, resulting in enhanced stability. Crucially, we deposited alumina at economically attractive conditions, room temperature and atmospheric pressure, on (1,2-ethanediamine) capped PbSe QD films using an approach based on atomic layer deposition (ALD), with trimethylaluminum (TMA) and water as precursors. We performed coating experiments from 1 to 25 cycles on the QD films, finding that alumina formed from the first exposure of TMA. X-ray photoelectron spectroscopy points to the presence of oxygen-rich compounds on the bare QD films, most likely from entrapped solvent molecules during the assembly of the QD films. These oxygenated compounds and the amine groups of the organic ligands react with TMA in the first cycle, resulting in a fast growth of alumina. Using 10 cycles resulted in a QD film that was optically stable for at least 27 days. Depositing alumina at ambient conditions is preferred, since the production of the QD films is also carried out at room temperature and atmospheric pressure, allowing combination of both processes in a single go. Show less
We have enhanced the radio-activation efficiency of SiC (silicon carbide) particles, which by nature have a poor affinity towards F-18 ions, to be employed as tracers in studies using PEPT ... Show moreWe have enhanced the radio-activation efficiency of SiC (silicon carbide) particles, which by nature have a poor affinity towards F-18 ions, to be employed as tracers in studies using PEPT (Positron Emission Particle Tracking). The resulting SiC-Al2O3 core-shell structure shows a good labelling efficiency, comparable to y-Al2O3 tracer particles, which are commonly used in PEPT. The coating of the SiC particles was carried at 27 +/- 3 degrees C and 1 bar in a fluidized bed reactor, using trimethylaluminium and water as precursors, by a gas phase technique similar to atomic layer deposition. The thickness of the alumina films, which ranged from 5 to 500 nm, was measured by elemental analysis and confirmed with FIB-TEM (focused ion beam - transmission electron microscope), obtaining consistent results from both techniques. By depositing such a thin film of alumina, properties that influence the hydrodynamic behaviour of the SiC particles, such as size, shape and density, are hardly altered, ensuring that the tracer particle shows the same flow behaviour as the other particles. The paper describes a general method to improve the activation efficiency of materials, which can be applied for the production of tracer particles for many other applications too. (C) 2015 Elsevier B.V. All rights reserved. Show less
We present a multiscale dynamic model as a means of understanding and optimizing the precursor utilization during atomic layer deposition (ALD) on nanoparticles and micron-sized nano-porous... Show moreWe present a multiscale dynamic model as a means of understanding and optimizing the precursor utilization during atomic layer deposition (ALD) on nanoparticles and micron-sized nano-porous particles in fluidized bed reactors. We used as case study the deposition of alumina using trimethylaluminum and water on both, titania nanoparticles and micron-sized nano-porous gamma-alumina particles under low (similar to 1 mbar) and atmospheric pressure. In doing so, we assess the effect of the precursor transport, from the inlet of the reactor to the particles active surface, on the precursor utilization efficiency. Our results show that, at proper operating conditions, fast ALD reaction kinetics enables the saturation of the particles surface area with hardly any loss of precursors. Finally, simple scaling rules for the optimization of the precursor utilization are proposed. (C) 2015 Elsevier B.V. All rights reserved. Show less
Valdesueiro, D.; Meesters, G.M.H.; Kreutzer, M.T.; Ommen, J.R. van 2015
We have deposited aluminium oxide films by atomic layer deposition on titanium oxide nanoparticles in a fluidized bed reactor at 27 +/- 3 degrees C and atmospheric pressure. Working at room... Show moreWe have deposited aluminium oxide films by atomic layer deposition on titanium oxide nanoparticles in a fluidized bed reactor at 27 +/- 3 degrees C and atmospheric pressure. Working at room temperature allows the coating of heat-sensitive materials, while working at atmospheric pressure would simplify the scale-up of this process. We performed 4, 7 and 15 cycles by dosing a predefined amount of precursors, i.e., trimethyl aluminium and water. We obtained a growth per cycle of 0.14-0.15 nm determined by transmission electron microscopy (TEM), similar to atomic layer deposition (ALD) experiments at a few millibars and ~180 degrees C. We also increased the amount of precursors dosed by a factor of 2, 4 and 6 compared to the base case, maintaining the same purging time. The growth per cycle (GPC) increased, although not linearly, with the dosing time. In addition, we performed an experiment at 170 degrees C and 1 bar using the dosing times increased by factor 6, and obtained a growth per cycle of 0.16 nm. These results were verified with elemental analysis, which showed a good agreement with the results from TEM pictures. Thermal gravimetric analysis (TGA) showed a negligible amount of unreacted molecules inside the alumina films. Overall, the dosage of the precursors is crucial to control precisely the growth of the alumina films at atmospheric pressure and room temperature. Dosing excess precursor induces a chemical vapour deposition type of growth due to the physisorption of molecules on the particles, but this can be avoided by working at high temperatures. Show less
