Liquid flow control: Unlock untapped research capabilities within in situ LPEM

Liquid flow control: Unlock untapped research capabilities within in situ LPEM

Via the unique on-chip microfluidic channel of the DENSsolutions Stream system, researchers were able to create a highly controlled chemical environment for visualizing the nanoscale metallic electrodeposition of copper crystals.

Original article by Cheng et al.

Liquid phase transmission electron microscopy (LPTEM) enables the observation of time-resolved dynamics in liquid state at high spatial resolution. The technique has gained exponential popularity over the last decade, and has contributed greatly to a wide range of fields, including materials science, chemistry and life science. With LPEM, researchers can explore the dynamical evolution of key materials and uncover fundamental insights into nucleation and growth. Only in recent years have researchers been able to control the chemical environment within an in situ LPEM experiment, owing to the award-winning innovation that is the DENSsolutions Stream system. In a recent publication, researchers including Dr. Ningyan Cheng from Anhui University utilized the Stream system to visualize the metallic electrodeposition of copper crystals in a highly controlled chemical environment. This was made possible due to the unique on-chip flow channel of Stream, which enables numerous advantages such as the ability to flush away beam-induced species, explore flow-dependent liquid dynamics and easily change electrolyte composition.

On-chip microfluidic channel

The core of the DENSsolutions Stream system is our patented Nano-Cell, which consists of a top and bottom chip, together forming a sealed compartment that enables users to safely perform liquid experiments inside the TEM. The bottom chip contains spacers, an integrated liquid inlet, flow channel and an outlet. Via pressure-based pumps, a liquid sample can be driven from the inlet through the field of view and then through the outlet. This process is demonstrated in the video below. Importantly, users can independently control the pressure at the inlet and outlet of the Nano-Cell, and therefore the absolute pressure in the microfluidic channel. This then enables full control over the liquid flow rate within the cell.

Movie 1: Animation depicting the microfluidic channel of the Stream Nano-Cell

Efficient liquid flow

Before observing any liquid phenomena in the TEM, Dr. Cheng and her fellow collaborators first had to ensure that the flow was efficient and well-controlled. To do this, the researchers first assembled a dry Nano-Cell. The flow was then initiated by turning on the pressure-based pump, while keeping all imaging parameters constant. After 30 seconds, the imaging contrast changed abruptly, implying that the liquid had definitely flowed into the Nano-Cell. This process is shown in the video below. The time taken to completely fill the Nano-Cell ranges anywhere from tens of seconds to just 3 minutes when a flow rate of 8 μl/min is applied.

Movie 2: In situ TEM movie showing the liquid flow into the Nano-Cell in just 30 seconds

Removal of beam-induced species

A key benefit of controlling the liquid flow within an LPEM experiment is the ability to remove beam-induced particles. The researchers first generated particles by increasing the electron flux on purpose through changing the spot size from 5 to 1. The process of removing the beam-induced species in this experiment is detailed in the video below, with the direction of the flow going from top to bottom. As soon as the flow was cut off at 6.4s, the particles started to form and grow on the membrane. The flow was then switched on again at 11.7 seconds, which is when the particles that were stuck to the membrane started to peel off and move from the top to the bottom area in the field of view. It took just 2 minutes to fully flush away the particles, which is a reproducible process. The direction of the particles’ movement is the same as the direction of the flow (top to bottom), confirming the effectiveness and power of the liquid flow control. 

Movie 3: Removal of beam-induced species via liquid flow control

Capturing flow-dependent liquid dynamics

The next step for the researchers was to explore the effect of the flow rate on the electrochemical copper crystallization and dissolution processes in real time. They first observed the effect of using a higher flow rate of 1.4 μl/min on the Cu deposition and dissolution processes, which showed to be reversible. The protocol included the initial electrode cleaning, deposition (−0.9 V, 10 s), dissolution (+0.4 V, 15 s) and repeating the process for 4 cycles. As demonstrated in the video below, the researchers found that uniform copper deposition can be obtained at a higher liquid flow rate (~1.4 μl/min), whereas at a lower liquid flow rate (0.1 μl/min), the growth of copper dendrites was observed. 

Movie 4: Copper electrodeposition at a flow rate of 1.4 μl/min (left) and 0.1 μl/min (right)

Changing electrolyte composition

Besides exploring the effects of altering the flow rate, a major point of interest in this study was observing the effect of adding foreign ions, such as phosphates, on the electrodeposition. Such additives can affect the electrochemically deposited crystals by, for example, changing the nuclei structures. The researchers first studied the electrodeposition of copper from a pure CuSO₄ aqueous solution. In this case, no obvious dendritic morphology was observed and instead only granules were formed (see below Figure 1, Left).

After the experiment, the electrolyte in the sample source was replaced by a mixture of CuSO and KHPO solution. The liquid was kept flowing with a flow rate of 3 μl/min for 15 min, which enabled the researchers to directly study the electrolyte effect on the depositions in the same liquid cell by excluding all the uncertainties during different cell assembly. Contrastingly in this case, copper dendrites were observed to grow and the addition of HPO− ions in the electrolyte led to the formation of Cu-phosphate complexes (see Figure 1, Right). These results further confirm the importance of being able to modulate the electrolyte composition, and demonstrate the effectiveness of the environment control that Stream enables.

Figure 1: The effect of phosphate addition on Cu electrodeposition

Exploiting electrode design to alter the chemical environment

When studying Cu electrodeposition in the previous experiment, the researchers saw that dendrite formation can be further promoted by the in situ addition of foreign ions, such as phosphates. In order to confirm the generality of this technique, they also took a look at Zn electrodeposition in an aqueous solution of ZnSO. The figure below shows the total growth of the zinc layer at b) a lower potential of −0.9 V (versus Pt) in the first 10 seconds, and c) a higher potential of −1.1 V (versus Pt) in the next 10 seconds. In d) the total growth of the Zn depositions in the first 20 seconds is shown. The researchers observed in the first 10 seconds (−0.9 V) that the deposition on the inner edge is rougher compared to the outer edge. In the following growth at −1.1 V, dendritic depositions are nucleated and grown on the previous outer edge, while no further growth can be observed in the inner edge. This experiment demonstrates that the special electrode design of Stream enables the exploration of rich liquid dynamics within different chemical environments. 

Figure 2: Zinc electrodeposition in b) the first 10 seconds with a potential of −0.9 V and c) in the next 10 seconds with a potential of −1.1 V. d) shows the total Zn growth in the 20 seconds.

Conclusion

Through this study, it is shown that Stream’s distinctive ability to enable liquid flow control opens the doors for researchers to truly alter the chemical environment within the liquid cell. By controlling the liquid flow, a user can flush away beam-induced species, explore flow-dependent liquid dynamics and easily change electrolyte composition. Moreover, the unique design of the electrodes in the Stream system allows researchers to explore complex liquid dynamics within different chemical environments within the same liquid cell. Importantly, the direct observations made by Cheng et al. not only provide new insights into understanding the nucleation and growth, but also give guidelines for the design and synthesis of desired nanostructures for specific applications, such as high performance electrocatalysis for energy conversion and electrodes for secondary batteries. 

Hanglong Wu portrait

Image of Dr. Ningyan Cheng from Max-Planck-Institut für Eisenforschung GmbH

“The DENSsolutions Stream System not only provides a useful means to study a wide range of dynamics in solution, but also enables systematic studies of the effect of the chemical environment on the corresponding reactions through precise control of the flow rate, liquid composition and other significant parameters.” 

Prof. Dr. Ningyan Cheng   Associate Professor  |  Anhui University

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Climate helps uncover phase coexistence and structural dynamics of redox metal catalysts

Using our Climate system, scientists are able to interrelate the atomic-scale structural dynamics of redox metal catalysts to their activity.

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DENSsolutions has installed yet another Climate system in the U.S. at Alfred University

DENSsolutions has installed yet another Climate system in the U.S. at Alfred University

We are proud to announce that DENSsolutions has installed another Climate system in the United States, at Alfred University, which is located in the west of New York State. In this article, we interview Dr. Kun Wang, Assistant Professor at the Inamori School of Engineering in Alfred University, to learn more about their microscopy facility, its research direction, as well as how our Climate system is advancing their research.

Can you tell me more about the microscopy facility at Alfred University?

Alfred University has numerous research facilities that boast a wide range of high-tech equipment. There are dedicated facilities for materials characterization, mechanical and physical testing, biological evaluation of materials, spectroscopy, materials synthesis and processing as well as imaging and microscopy. The Imaging and Microscopy facility is equipped with a scanning electron microscope, an atomic force microscope and a fluorescent optical microscope, among many other tools. Just last summer, we had our new transmission electron microscope installed, the TFS Talos F200X. This microscope is equipped with a super X-ray detector which enables us to perform high resolution chemical analyses in a highly efficient manner.”

What type of applications are the users at Alfred using the Climate system for?

“Users of the facility are interested in a couple of applications, now enabled via the use of our newly acquired DENSsolutions Climate system. Via Climate, we would like to perform in situ oxidation and reduction experiments on batteries and catalyst materials. Moreover, we are interested in performing in situ high-temperature oxidation experiments for aerospace materials and nuclear matter in order to better understand these materials and their behavior under varying temperature conditions. We are also interested in performing energy-dispersive X-ray spectroscopy (EDX) in those experiments to get a better idea of the elemental composition of a given sample.”

What particular features of the DENSsolutions Climate solution attracted you to the system?

“Aside from the ability of the system to combine gas and heating functions, it was particularly important for me to use an in situ system that could handle high temperatures. Specifically, I was looking for a system that could handle high temperatures while still maintaining the stability of the holder. This is particularly what attracted me most to the Climate system.”

Can you tell me about the grant that was won to acquire the system?

“The grant was actually awarded several years ago, from an institute called the New York State’s Empire State Development, which provides numerous services and resources for education, healthcare, military and other fields.”

DENSsolutions Prof. Jungwon Park

Dr. Kun Wang
Assistant Proffessor | Inamori School of Engineering, Alfred University

Dr. Kun Wang received his Ph.D. degree in Materials Science and Engineering from the Swiss Federal Institute of Technology Lausanne (EPFL). He used to work as a Postdoctoral Research Associate at the Nuclear Materials Science and Technology group of the Oak Ridge National Laboratory (ORNL). Currently, he is working as an Assistant Professor at the Inamori School of Engineering, Alfred University. His research focuses on study of structural materials under extreme environmental conditions.

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A comprehensive guide for data synchronization in operando gas and heating TEM

A comprehensive guide for data synchronization in operando gas and heating TEM

A team of experts develop a data synchronization method for operando gas and heating TEM via time delay calibration, enabled by the unique ability of the DENSsolutions Climate Nano-Reactor to perform nano-calorimetry.

Original article by Fan Zhang, Merijn Pen, Ronald G. Spruit, Hugo Perez Garza, Wei Liu, Dan Zhou

In recent years, operando gas and heating transmission electron microscopy (TEM) has become recognized as a powerful tool for creating time-resolved correlations between the environment, reaction products, energy transfer and material structures during reaction processes. Via operando gas and heating TEM, a detailed understanding of the relationship between the structural evolution of a catalyst and its performance can be achieved. Despite its many benefits, one inevitable issue with the technique is the intrinsic time delays that occur between different parameter measurement locations. These time delays must be calibrated in order for researchers to draw valid correlations and conclusions. Correlations without time delay calibration could lead to, for example, over/under-estimating critical temperatures and generating misleading relationships between catalytic structure and activity.

In recent research performed at the Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences, where our advanced Climate G+ system is installed, Fan Zhang, Dr. Wei Liu and our own team of experts, including Merijn Pen, Ronald G. Spruit, Dr. Hugo Perez Garza and Dr. Dan Zhou, developed a data synchronization method to account for time delays in operando gas and heating TEM. Specifically, they systematically explored the relationship between delayed time and reaction conditions such as gas pressure, flow rate and gas composition. Based on the results, they developed open source scripts that can be used to achieve reliable and automated data synchronization via time delay characterization and calibration. The authors also developed a general protocol to perform automatic time delay calibration for all kinds of TEM setups.

The experimental setup

The experimental setup used in the study included the DENSsolutions Climate Gas Supply System (GSS), Climate in situ TEM holder, Climate Nano-Reactor, Gas Analyzer and Thermo Fisher Scientific Themis ETEM. In Figure 1a) below, a schematic view of the operando gas and heating TEM setup is presented. Generally, a gas will first travel from the GSS into the TEM, and then from the TEM into the Mass Spectrometer (MS), also known as the Gas Analyzer. This process is depicted in Movie 1 below. The different colors in the video visualize what happens when the gas composition is changed, where a new color represents a new gas mixture. In this visual depiction, it is shown that the gas composition measurement at the GSS is actually ahead in time of the measurements at the TEM. This is because the gas takes time to reach the sample. Moreover, the measurements from the MS are also delayed because the gas needs time to flow from the TEM to the MS. A more detailed account of the gas path and the components involved is provided below.

The GSS contains three gas bottles as well as three flow controllers that measure and regulate the gasses’ flow rates. The gasses are mixed using a unique on-the-fly mixing technique, where a user can change the gas composition dynamically as well as alter the gas environment within a matter of seconds. The mixed gas is then guided into the Nano-Reactor in the TEM through the holder, and then to the MS, which derives the outlet gas composition by measuring the partial pressures of reactants and products. Expectedly, as the gas travels through this path, considerable time delays occur. In fact, the researchers found that a user-set gas composition change in the GSS will only show changes in the MS after 79.1 seconds (see Figure 1b) below). 

Movie 1: Movie depicting the gas path and time delays associated with operando gas and heating TEM.

Figure 1: (a) A schematic view of a gas cell based operando TEM gas path; (b) Illustration of time delay between GSS and MS. Here GSS data are measured by flow meter and MS by measuring the ionized gasses’ mass to charge ratio.

In Figure 2 below, a diagram of the operando gas and heating TEM setup used in this work is presented. The setup can be grouped in two ways: from a hardware perspective and from a gas path/TEM investigation perspective. In the case of the former grouping, the setup can be divided into GSS, TEM and MS. In the case of the latter grouping, the setup can be divided into pre-TEM, in-TEM and post-TEM. The researchers were able to synchronize the data from pre-, in- and post-TEM by first measuring and then calibrating the time delays involved. 

Figure 2: A diagram of the operando gas and heating TEM setup.

Calorimetry-based time delay calibration

The DENSsolutions Climate system offers users the unique ability to perform nano-calorimetry, which comes in as a convenient feature in the calibration of time delays. Specifically, when a new gas type enters the Nano-Reactor, this is detected by the microheater because it is very sensitive to even the most minute changes in heat. Users can monitor the gas changing process due to the difference in the thermal properties of the different gasses flown through the Nano-Reactor, enabling the detection of what time the new gas has reached the sample. The unique on-chip calorimetry feature of our Nano-Reactor allows for calibration of the time delays, and therefore enables perfect synchronization between gas compositional changes, TEM imaging and spectroscopy data.

Relationship between time delay and various parameters

The next step for the researchers was to systematically explore the relationship between the delayed time and reaction conditions such as gas pressure, flow rate and gas composition. They found that the delayed time between different parts, in either of the two grouping mechanisms, is determined by the gas pressure and flow rate in the Nano-Reactor, as well as the total gas path length. They also found that the time delay has little dependence on the gas type. The investigation of the relationship between the time delay and the above-mentioned critical parameters allowed the researchers to develop a functional relationship. Using the established functional relationship, they were able to lay the foundation for manually and automatically calibrating the time delays.

Automation of data synchronization

Based on the investigations, the authors developed algorithms and scripts to enable the automatic data synchronization in operando gas and heating TEM. Specifically, two open source Python scripts have been written for characterizing and calibrating the time delays in experiments. 

The first script characterizes the time delay curve, utilizing DENSsolutions Impulse API and ImpulsePy library to control and retrieve the measurements from the GSS, the heating control unit and the MS. It does this by alternating between different gas mixtures and measuring the time delays until these gas composition changes are detected in the calorimetry data of the microheater and in the partial pressure data from the Mass Spectrometer. It performs multiple gas composition switches at different flow-rates and pressures to collect enough data points to be able to fit the two characterization curves through them. These curves are the pre-TEM to in-TEM delay and the in-TEM to post-TEM delay characterization curves of that specific system. The characterization only needs to be performed once, after which the calibration file can be used repeatedly on any dataset measured by the same system.

The second script removes the time delays from a dataset by first splitting the system parameters into different sets: pre-TEM, in-TEM and post-TEM. Because the pressure and flow parameters can change over the duration of the experiment, the time delay correction amount is calculated and applied for every measurement in the dataset individually. The script then saves three corrected datasets (pre-TEM, in-TEM and post-TEM) individually with their own time resolution, and creates a synchronized log file in which the pre-TEM and post-TEM parameters are interpolated towards the in-TEM dataset timestamps. In Figure 3 below, an example of the synchronized data output from the time delay calibration script is presented. The data is then synchronized with the TEM imaging and spectroscopy data to achieve the complete operando gas and heating TEM data synchronization.

Figure 3: The synchronized data corrected by the time delay correction scripts.

General operation protocol

With the above conclusions and scripts, the authors have created a step-by-step guide for performing automatic time delay calibration for your own operando TEM setup. This protocol is summarized in Figure 4 below.

Figure 4: Schematic view of the operation protocol.

Conclusion

Time delay is an issue inherent to every gas supply system used in an operando gas and heating TEM setup. In a collaborative effort between DICP and DENSsolutions, the researchers have developed a data synchronization method to effectively address this problem. Specifically, by developing a functional relationship between delayed time and reaction conditions like gas pressure, flow rate and gas composition, the authors were able to develop open source scripts that characterize and calibrate the time delay automatically. On top of this, a conductive protocol is described such that any researcher can apply the data synchronization method to their own unique operando gas and heating experimental setup. Overall, this work is a major step forward in ensuring that researchers conducting operando gas and heating experiments are able to make valid correlations between critical parameters and provide the right conclusions and insights.

“The DENSsolutions Climate system is able to create an accurate working environment of what practical catalysts go through. Not only does the system facilitate the real-time observation of dynamic catalytic behavior, but it also enables the simultaneous monitoring of the performance of a catalyst by detecting the resulting products within the Nano-Reactor. By enabling this elaborate strategy of data synchronization in operando gas and heating TEM, Climate initiates the distinct functionality of correlating catalytic property with microstructural changes, which adds core value to what the technique of in situ TEM can reveal about a dynamic reaction.”

Prof. Dr. Wei Liu
Professor  |  Dalian Institute of Chemical Physics, CAS

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Scientists explore complex metal-support interactions under redox conditions using our Climate system

Via the DENSsolutions Climate system, a team of scientists uncover the dynamic interplay between platinum nanoparticles and titania support under reaction conditions.

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Scientists explore complex metal-support interactions under redox conditions using our Climate system

Scientists explore complex metal-support interactions under redox conditions using our Climate system

Via the DENSsolutions Climate system, a team of scientists uncover the dynamic interplay between platinum nanoparticles and titania support under reaction conditions.

Original article by H. Frey, A. Beck, X. Huang, J.A. van Bokhoven and M. G. Willinger

For catalysts consisting of metal nanoparticles and an oxide support, understanding the synergistic reactions between the metal and support is paramount. In the case of reducible supports, the so-called strong metal-support interaction (SMSI) provides a means of tuning the chemisorption and catalytic properties of supported metal particles. SMSI involves the encapsulation of nanoparticles by a thin layer of partially reduced support material. The encapsulation is typically induced during high-temperature reductive “activation”, i.e., treatment in hydrogen. Notably, the direct imaging of this encapsulated state has mostly been achieved ex situ. Little is known about SMSI under catalytic working conditions, where the application of in situ electron microscopy is invaluable. While environmental transmission electron microscopy (TEM) is generally limited to chamber pressures of around 20 mbar, the DENSsolutions Climate Nano-Reactor can handle pressures 100 times higher, unlocking unprecedented research possibilities for the study of catalysts in their native environment.

In recent research performed at ScopeM, ETH Zurich, where our advanced Climate G+ system is installed, Hannes Frey, Arik Beck, Dr. Xing Huang, Prof. Dr. Jeroen Anton van Bokhoven and Prof. Dr. Marc Georg Willinger investigated the dynamic interplay between metal nanoparticles and oxide support under reaction conditions. More specifically, the scientists revealed the working state of a model catalyst, Pt–TiO, by directly observing the synergistic interactions related to SMSI between the platinum nanoparticles and titania support.

Switching to a redox-active H₂–O₂ mixture

Frey and his fellow collaborators first induced the classical SMSI state by heating the titania-supported platinum nanoparticles (NPs) in H₂. The nanoparticles were then transferred into an O₂ atmosphere via inert gas purging. Interestingly, this treatment resulted in the platinum NPs incurring a nonclassical oxidized SMSI state. After preparing the system, the researchers then exposed it to a redox active atmosphere through the addition of H₂ into the O₂ flow. With the Climate G+ system, the researchers were able to mix the gasses on the fly, while continuing with the high-resolution observation. 

In Figure 1 and Movie 1 below, the morphological change of the platinum NPs upon transition into the redox-active regime is presented. Increasing the partial pressure of H₂ in the Climate Nano-Reactor resulted in the gradual change in the encapsulated state of the Pt NPs. Moreover, this resulted in the ultimate disappearance of the overlayer as soon as the gas composition reached a set mixture of 60 mbar H₂ and 700 mbar O₂ after ~180 s. Once the overlayer was fully removed, the particle was observed to experience particle dynamics like restructuring and migration (shown in Figure 1E and 1F). 

Figure 1: Image series depicting the morphological changes observed when a titania-supported platinum nanoparticle in a nonclassical SMSI state is exposed to a redox-active atmosphere. The composition of the gas in the Nano-Reactor was gradually changed from 700 mbar O₂ to a mixture of 60 mbar H₂ plus 700 mbar O₂. t0 is the time at which the H₂ flow was turned on.

Movie 1: TEM Movie depicting the morphological changes observed when a titania-supported platinum nanoparticle in a nonclassical SMSI state is exposed to a redox-active atmosphere.

Particle and interfacial dynamics in the redox-active regime

The researchers then set out to explore the response of a collection of nanoparticles to reaction conditions. They discovered that the degree of structural dynamics and mobility differed greatly among the nanoparticles. Whereas some NPs remained static and stationary, others incurred structural fluctuations and migrated across the substrate surface. The researchers decided to follow three representative cases of nanoparticles with different orientations. In all cases, it is observed that the redox chemistry at the interface is the driving force for particle reconstruction and migration. In Figure 2 below, the respective structural dynamics of the three selected nanoparticles are presented.

Figure 2: (A–C) Pt NP oriented with (111) planes perpendicular to the interface. (G to J) Pt NP oriented with (111) planes parallel to the interface. (K to N) Pt NP that has its (111) planes inclined toward the interface. The blue shapes indicate the respective positions of the Pt NP in the previous frames.

Case 1

The first platinum NP that was considered was oriented with (111) planes perpendicular to the interface. In Movie 2 below, the recorded time series of this platinum NP at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂ is shown. Here, the NP developed pronounced structural dynamics that involved twin formation and shearing along (111) planes in an up-down motion, perpendicular to the interface. 

Movie 2: Recorded time series of the Pt NP presented in Figure 2A–C) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Case 2

The second Pt NP that was considered was oriented with (111) planes parallel to the interface. Movie 3 below shows the image series acquired for this nanoparticle, at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂. Here, a continuous step flow motion of the (111)-type facet in contact with the interface is observed.

Movie 3: Image series of the Pt NP presented in Figure 2G–J) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Case 3

The third platinum NP that was considered was oriented with (111) planes inclined towards the interface. Movie 4 below shows the image series acquired for this nanoparticle, at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂. In this case, the nanoparticle is observed to engage in redox chemistry-driven directional surface migration which is caused by restructuring at the interface.

Movie 4: Image series of the Pt NP presented in Figure 2K–N) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Retraction of H₂ and reformation of the oxidic SMSI overlayer

The next step for Frey and his fellow researchers was to switch the gas composition from a reactive feed back to a purely oxidizing environment by turning off the H₂ flow. This change in the gas composition resulted in the encapsulated state of the platinum NPs to be reestablished. In Figure 3 below, the morphological change of a Pt NP upon switching the gas from a redox-active environment to a purely oxidizing environment is presented. This switching of the gas composition led to the reformation of a classical particle overgrowth. It is seen that the nanoparticles first adopted a spherical morphology (Figure 3A–C). Then, as soon as H₂ was fully removed from the Nano-Reactor, the support material migrated onto the Pt NPs and the overlayer reformed (Figure 3D–F).

Figure 3: Image series depicting the morphological changes of a Pt NP observed when switching from a redox-active to purely oxidizing environment at 600°C. t0 is the time at which the H₂ flow was set to zero.

Conclusion

The aim of this paper was to reveal the working state of a catalyst via direct observation and to study possible synergistic interactions related to SMSI between metal nanoparticles and a reducible oxide support. The use of our Climate system enabled the researchers to capture in exceptional detail the dynamic and complex metal-support interactions of Pt–TiO₂ under reactive catalytic conditions. This is especially thanks to the advanced capacity of the system to handle pressures of up to 2 bar. 

A key finding of this paper is that the stable configurations of static Pt particles exhibiting encapsulation were observed to exist either in pure H₂ (the classical SMSI state) or in pure O₂ (the nonclassical SMSI state), but not in an environment where both gases were simultaneously present. Indeed, the exposure to the redox-active environment led to the removal of the overlayer and the subsequent emergence of pronounced particle dynamics. Moreover, the in situ observations show that the particle restructuring and migration behavior is dependent on the relative orientation of the particle on the support, and therefore the configuration of the interface. Overall, these findings advance our comprehension of SMSI-induced encapsulation of metal nanoparticles, which can in turn help us better tune the chemisorption and catalytic properties of catalysts. 

Hanglong Wu portrait

“The DENSsolutions Climate System allows us to reveal the so-far unseen: Looking at not only how the gas-phase is changed in the presence of a catalyst, but also studying how the interaction between gas-phase and catalyst leads to the emergence of catalytic function. Direct real-space observation is essential for our understanding of working catalysts and the development of new processes that are urgently needed in view of climate change and limited natural resources.”

Prof. Dr. Marc-Georg Willinger
Professor  |  Technical University of Munich

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Climate helps uncover phase coexistence and structural dynamics of redox metal catalysts

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Hydrogen vs. syngas: The battle of the reducing gases

Hydrogen vs. syngas: The battle of the reducing gases

Using our Climate system, scientists are able to unveil the most effective method to obtain sintering-resistant metallic cobalt nanoparticles at lower temperatures.

Original article by Ofentse A. Makgae, Tumelo N. Phaahlamohlaka, Benzhen Yao, Manfred E. Schuster, Thomas J. A. Slater, Peter P. Edwards, Neil J. Coville, Emanuela Liberti, and Angus I. Kirkland

The Fischer-Tropsch synthesis (FTS) is an established catalytic chemical process that occurs when carbon monoxide and hydrogen are converted into hydrocarbons. These hydrocarbons can be further developed into high-quality fuels, lubricants as well as raw materials for the production of plastics, rubbers and industrial chemicals. In FTS, supported cobalt catalysts are commonly used, which are active in their reduced metallic state. Many factors can affect the reduction process of cobalt oxide (Co₃3O₄) to metallic cobalt, including the reduction temperature, the composition of the reducing gas, the catalyst promoter, catalyst support and particle size. Despite this, little is known about the effect of using H₂ versus syngas (H₂ & CO) as the reducing agent on carbon-supported cobalt catalysts.

In recent research performed at the David Cockayne Centre for Electron Microscopy and the electron Physical Science Imaging Centre (ePSIC), a number of scientists from highly renowned universities and institutes, including University of Oxford, Johnson Matthey, Diamond Light Source and Cardiff University, investigate the atomic structure and valence state of cobalt nanocrystals on carbon supports under syngas versus hydrogen reduction. Specifically, using ex situ and in situ high-resolution aberration-corrected analytical electron microscopy with our Climate system, the researchers were able to explore the effect of H₂ versus syngas (H₂ & CO) on the reducibility of cobalt oxide nanoparticles supported on hollow carbon spheres. By uncovering highly effective methods to obtain sintering-resistant metallic Co nanoparticles at lower temperatures on hollow carbon sphere supports, this work provides valuable insights that can inform important industrial processes.

Synthesis and reduction of cobalt oxide

Dr. Ofenste Makgae and his fellow collaborators first synthesized the Co₃3O₄ nanoparticles before preparing the hollow carbon sphere supports (HCSs). The synthesized Co₃3O₄ nanoparticles were then loaded on the surface of the HCSs to prepare the precursor catalyst.

The researchers then initiated the reduction process of the Co₃3O₄ nanoparticles for both the ex situ and in situ analysis. For the ex situ analysis, the precursor was reduced via H₂ and syngas, respectively, at 350 °C at atmospheric pressure for 20 hours. For the in situ analysis, the catalysts were studied using the DENSsolutions Climate system in a probe-corrected JEOL ARM200F at ePSIC operated at 200 kV. Notably, the DENSsolutions Climate system is capable of real-time dynamic mixing of 3 gases over a temperature range from room temperature to 1000 °C. Via the Climate Gas Supply System, Dr. Makgae and his fellow collaborators were able to provide a 0.2mL/min flow of gas at a pressure of 500mbar for both the H₂ and syngas reduction. The temperature was ramped from room temperature to 150 °C under a N₂ flow to remove contaminants. Next, it was ramped to 350 °C at 10 °C/min under a reducing environment. Finally, the temperature was held at 350 °C for 2 hours for catalyst activation.

Ex situ post-reduction morphology analysis

The researchers then set out to analyze the overall morphology of the carbon support post reduction, to ensure that its integrity was not compromised during the reduction process. In Figure 1 below, (high-resolution) STEM-ADF images of the as-prepared Co₃3O₄ and ex situ syngas- and H₂-reduced HCS-supported nanoparticles are shown. The researchers were able to determine that the morphology of the HCSs remained intact after 20 hours of reduction at 350 °C.

Figure 1: STEM–ADF images of the Co nanoparticles (a) before and after the ex situ reduction in (b) syngas (H₂/CO = 2) and (c) H₂ at 350 °C. (d–f) are the corresponding high-resolution STEM–ADF images of the as-prepared Co₃3O₄, syngas-, and H₂-reduced nanoparticles.

EELS spectra and elemental maps

Aside from morphological changes, the oxidation state of cobalt also changes during the reduction process as a function of the reducing gas composition. Using STEM–EELS, the researchers were able to thoroughly investigate the Co-oxidation state changes. Figure 2 below compares the EELS spectra and elemental maps of single particles of the as-prepared Co₃3O₄, and the syngas-reduced and H₂-reduced nanoparticles. Via the EELS spectra analysis, the researchers determined that there was indeed a significant decrease in the oxidation state of Co.

Figure 2: EELS spectra of the as-prepared Co₃3O₄ (green), syngas- (orange), and H₂- (blue) ex situ reduced nanoparticles at 350 °C. EELS elemental maps of (b–d) as-prepared, (e–g) syngas-reduced, and (h–j) H₂-reduced Co₃3O₄ nanoparticles.

Importantly, the EELS spectra observations also show that the degree of reduction varies amongst the two methods (H₂ and syngas). To explore this further, the researchers collected additional spectra to compare the ex situ syngas-reduced and H₂-reduced nanoparticles at 350 °C, with the as-prepared Co₃3O₄ as a reference. From this further analysis, it is observed that the Co in the case of the syngas-reduced nanoparticles’ spectra is in a near-metallic state as compared to the H₂-reduced nanoparticles. These results are consistent with the elemental maps obtained, showing that a more intense Co signal (green) is observed in the elemental map from a syngas-reduced nanoparticle (Figure 2g), compared to a more diffuse Co signal in the elemental map of a H₂-reduced nanoparticle (Figure 2j). Ultimately, in the absence of air surface reoxidation, the results of the in situ STEM–EELS and ADF analysis show that syngas achieves a higher degree of Co reduction than H₂.

High-resolution STEM-ADF analysis

The next step for Dr. Makgae and his collaborators was to perform high-resolution STEM–ADF imaging in order to investigate the differences in the atomic structure associated with the observed differences in Co-oxidation states in the syngas- and H₂-reduced nanoparticles. They explored the reduction of the syngas- and H₂-reduced nanoparticles at two different temperatures, 350 °C and 600 °C.

1) At 350 °C

In Figure 3a–e below, the high-resolution STEM–ADF images of syngas-reduced nanoparticles (a–c) and H₂-reduced nanoparticles (d,e) are presented. The results were found to be consistent with the EELS data, showing that syngas reduction produces a majority of metallic Co nanoparticles, whereas H₂ produced particles with a majority of the Co in the 2+ oxidation state. 

Figure 3: High-resolution STEM–ADF images of ex situ (a–c) 350 °C syngas-reduced nanoparticles and (d,e) H₂-reduced nanoparticles.

2) At 600 °C

The temperature was then increased to 600 °C to investigate the effect of temperature on reducing HCS-supported Co₃3O₄ nanoparticles under H₂ versus syngas. It is observed that both the H₂- and syngas-reduced nanoparticles exhibit an atomic structure consistent with cobalt (Figure 4a–d). Indeed, the results are consistent with the EELS data from the 600 °C reduced nanoparticles, which shows the presence of metallic cobalt. Conclusively, in order to achieve a complete reduction of Co₃3O₄ under H₂, the HCS-supported nanoparticles must be reduced at high temperatures. Importantly, it is observed that the high temperature of 600 °C resulted in severe sintering of the nanoparticles reduced in H₂ compared to syngas. The sinter-resistance in syngas-reduced nanoparticles is attributed to the encapsulation of nanoparticles with a carbon shell (indicated by the orange arrow in Figure 4b). This encapsulation prevents the coalescence of the nanoparticles at high temperatures. Interestingly, in the case of the H₂-reduced nanoparticles, a carbon shell is not observed (Figure 4d).

Figure 4: High-resolution STEM–ADF images of 600 °C ex situ (a,b) syngas-reduced nanoparticles and (c,d) H₂-reduced Co nanoparticles.

Conclusively, the presence of CO in syngas is observed to play a critical role in the conversion of Co₃3O₄ to metallic Co at 350 °C. In contrast, the full reduction of Co₃3O₄ to active metallic Co in H₂ is not achieved at 350 °C. Increasing the activation temperature to 600 °C does drive the complete reduction, however, significant particle sintering was still observed. Therefore, the presence of CO in a syngas mixture is indeed crucial to achieving complete Co₃3O₄ reduction at 350 °C.

Conclusion

This paper comprehensively illuminates the most effective method to obtain sintering-resistant metallic cobalt nanoparticles at lower temperatures under carbon supports. By conducting a thorough ex situ and in situ electron microscopy analysis using the DENSsolutions Climate system, Dr. Makgae and his fellow collaborators were able to observe under nanometer resolution how the reduction process under H₂ versus syngas affects the physicochemical properties of the reduced nanoparticles. The findings show that syngas achieves a higher reduction at an industrially relevant Fischer-Tropsch reduction temperature of 350 °C compared to H₂ on carbon supports. Importantly, lower reduction temperatures are more efficient as operating costs are reduced, which is a significant consideration for organizations involved in industrial processes. The discovery of such findings could not be possible without the application of in situ electron microscopy techniques, for which our Climate system has proved to be remarkably valuable.

“The DENSsolutions Climate system allowed us to directly evaluate the reducibility of Fischer-Tropsch catalysts under industrially relevant temperatures. Such observations provide a unique insight into the evolution of catalysts under activation conditions.

Dr. Ofentse Makgae
Post-doctoral Research Fellow  |  Lund University

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Introducing the Stream Liquid Supply System: An integrated solution offering unmatched flexibility

Introducing the Stream Liquid Supply System: An integrated solution offering unmatched flexibility

An interview with DENSsolutions Mechanical Engineer Alejandro Rozene about our latest addition to the Stream product line: the Liquid Supply System (LSS).

DENSsolutions introduces its latest solution: the Stream Liquid Supply System (LSS): an integrated solution designed to offer you ease-of-use, flexibility and reproducibility in your in situ liquid experiments. In this article, we interview our Mechanical Engineer Alejandro Rozene to learn all about the LSS, from what inspired its development, its unique capabilities and the many applications that will benefit from its creation.

What led to the development of the Stream Liquid Supply System?

“Our Liquid Phase Electron Microscopy (LPEM) solutions have been used by researchers around the world in a plethora of research fields, such as protein studies, battery research and molecular self-assembly. With our Stream system, users can fully control the microfluidic environment inside the Nano-Cell, our environmental MEMS sample carrier, while biasing or heating their sample. Considering the wide range of capabilities the system offers and the complexity of LPEM, Stream is quite the advanced system. This is precisely what inspired the next step in the innovation process: the development of an integrated solution designed to offer greater flexibility and ease-of-use. The Liquid Supply System (LSS) is a single scientific instrument that allows microscopists to carry out imaging experiments in fewer steps while introducing new features. The LSS is also designed to be the basic unit of a modular system that can be configured for different research applications.” 

What are the main benefits of the LSS for users?

“The introduction of the LSS to the Stream product line brings forth numerous advantages for your in situ liquid experiments, including greater flexibility, reliability and reproducibility. These benefits are detailed below:

1) Ease of use: Thanks to the clever architecture of the LSS, particularly the moveable base, you can easily relocate, store and set it up in various locations. It is also possible to control the microfluidic environment of the Nano-Cell and to apply different stimuli using a single interface in Impulse, our in situ experiment control and automation software. All of the sensor data is collected centrally. The LSS therefore simplifies the LPEM workflow and allows you to shift the focus from the hardware to the imaging experiment. This will potentially open the door to more elaborate experimental workflows.

2) High resolution imaging and meaningful analytical analysis: It is widely known that LPEM suffers from limited resolution caused by the thickness of the liquid layer inside the sample chamber. Even a 500 nm liquid layer can limit imaging resolution. With the LSS, the capability of controllable and inert gas purging is introduced. This means that you can easily displace the liquid in the Nano-Cell and effortlessly cycle between dry and liquid environments. This can be done with air or with an inert gas for air-sensitive samples. Via purging, you can easily get rid of excess liquid in the sample, thereby allowing you to achieve high resolution imaging, image in electron diffraction mode and perform spectroscopy techniques: EDS and EELS.

3) Reliable and reproducible results: The components of the LSS, such as the inlet and outlet pressure-based pumps as well as the liquid flow meter, introduce an unprecedented level of control during your liquid phase microscopy experiments. The LSS offers the unique ability to actively measure the liquid flow, making it possible for you to compare results from different experiments. Moreover, this means that you can easily detect potential clogs in the system and act fast, allowing you to spend your TEM time efficiently and effectively. Ultimately, the combination of our LSS and unique Nano-Cell design, having an on-chip inlet and outlet, enables the liquid delivery to be both reliable and reproducible with a success rate of more than 95%.”

Which applications will benefit most from the LSS?

“The LSS is beneficial for any researcher who wants to use LPEM to observe hydrated samples, matter suspended in liquid or liquid itself in a dynamic environment. Some of the many fields that will benefit from the system include:

  1. Life Science: DNA imaging, biomineralization, cell imaging and protein studies
  2. Electrochemistry: studies of aqueous electrolytes and electrocatalysis
  3. Material Science: studies of nanoparticle formation, self-assembly and growth

These are just a few examples of the several fields that can benefit from the Stream system. Furthermore, the door is open to any researcher who may want to exploit the unique capabilities of the LSS for any liquid-based study outside of the TEM.”

What kind of challenges were tackled during development?

“The LSS is a state-of-the-art scientific instrument. As with any other mechatronic development, integrating the sensing, actuating and microfluidic components into a single, robust machine was in itself a challenge. Also, our market is especially demanding since the users of this instrument already work with some of the most advanced equipment out there, which undoubtedly sets the bar very high. However, it was a very enjoyable experience thanks to the incredible teamwork and collaboration of our MEMS Project Manager, Tijn van Omme, and our Software Developer, Emil Svensson, as well as our manufacturing partners.”

What is the compatibility of the LSS?

“The LSS is an integral part of our Stream product line. It is therefore fully compatible with the modular Stream sample holder, the Stream liquid biasing and liquid heating Nano-Cells, as well as with Impulse. With regards to the microscopes, compatibility is given by the holder type. We currently offer compatibility with JEOL and Thermo Fisher Scientific microscopes.”

Has the LSS already been installed?

“Yes, the Liquid Supply System has been installed at the University of Alberta Nanofabrication and Characterization Facility (nanoFAB) in Canada. The nanoFAB is a national, open-access training, service, and collaboration centre, focused on academic and industrial applications in micro- and nanoscale fabrication and characterization. The installation was carried out by our MEMS Project Manager, Tijn van Omme, with the support of our close distributor Colt Murray from Nanoscience Instruments. It is the first of several installations planned in the upcoming months.”

From left to right: XueHai Tan, Colt Murray, Tijn van Omme and Haoyang Yu

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