DENSsolutions’ Lightning system helps uncover the interaction mechanism in reactive metal-ceramic system, Al-SiC

DENSsolutions’ Lightning system helps uncover the interaction mechanism in reactive metal-ceramic system, Al-SiC

Using the DENSsolutions Lightning system, researchers were able to provide an electrical, chemical and structural analysis of the Al–amorphous SiO₂–SiC interface at high temperatures.

Original article by Adabifiroozjaei et al.

The use of hybrid materials containing both metals and ceramics has become increasingly popular within manufacturing and microelectronic industries due to their optimized and well-balanced properties. Aluminum-silicon carbide (Al-SiC) is a widely known metal-ceramic composite material, commonly used in microelectronic packaging for automotive and aerospace applications. In Al-SiC an amorphous oxide layer (AOL) of SiO₂ is known to exist between the Al and SiC. Notably, the mechanism of interaction between the reactive metal (Al) and ceramic (SiC) and the AOL (SiO₂) under the heat-treatment process is still not well-understood. In fact, numerous theories about the interaction mechanism have been proposed over the past few decades. The major problem is that the studies conducted so far, regardless of the mechanism proposed in them, were ex situ and therefore not capable of resolving the atomic-scale nanostructural and chemical changes occurring at the interfaces during the heat-treatment process. In a recent paper published in the Journal of Materials Science, involving our valued users at TU Darmstadt, Dr. Esmaeil Adabifiroozjaei and Dr. Leopoldo Molina-Luna, the DENSsolutions Lightning system was utilized to reveal the evolution mechanism of the Al–AOL–SiC system under heating and biasing conditions. This study involved a team of researchers from institutes all over the world, including the University of Tabriz in Iran, NIMS and Shibaura Institute of Technology in Japan, and UNSW Sydney in Australia. 

Sample preparation

The first step for Dr. Adabifiroozjaei and his fellow collaborators was to carefully prepare the Al-SiC sample. After ultrasonically cleaning the SiC wafer, removing the oxide layer and allowing its regrowth by inserting the wafer into a desiccator, an Al layer with a thickness of ~1 µm was sputtered on the wafer using Shibaura’s CFS-4EP-LL sputtering machine. Next, in order to prepare the lamella, the researchers applied focused ion beam milling using JEOL’s JIB-4000 FIB. The prepared lamella was then loaded onto the DENSsolutions Lightning Nano-Chip (see Figure 1a). The low- and high-magnification scanning electron microscopy (SEM) images of the chip and the loaded lamella are shown below in Figure 1b) and 1c), respectively. Next, an Au lamella was prepared by FIB and connected to Al–AOL–SiC lamella and chip in order to ensure electrical current passes through Al–AOL–SiC lamella.

Figure 1: a) DENSsolutions Lightning Nano-Chip used for the in situ heating and biasing experiment, b) low- c) and high-magnification SEM images of the loaded lamella on the Nano-Chip, respectively.

Experimental results

The researchers performed EDX and EELS elemental mapping to determine the chemical composition of the phases across the Al–AOL–SiC interface. The EDS mapping of the interface is shown in Figure 2a), while the high-resolution EELS elemental mapping of the interface is shown in Figure 3b) – both of which reveal the consistent presence of a narrow oxide layer with a thickness in the range of 3–5 nm. 

Figure 2: a) EDS elemental mapping of Al–AOL–SiC interface, showing the presence of the AOL, b) STEM-HAADF image of Al–AOL–SiC interface and its EELS map profile.

Next, the researchers began with the in situ heating and biasing experiment to study the electrical characteristics of the lamella. First, a compliance current was set to 3 nA, then the voltage required to reach such a current was recorded at each temperature. The acquired I–V curves for room temperature, 500 ° and 600 °C after 30 minutes of application of the field are presented in Figure 3a–c), respectively. The I–V curves and high resolution TEM images (shown in Figure 3d–f) indicate that the resistivity of the Al–AOL–SiC device decreased three orders of magnitudes at 500 °C with no apparent change in the nanostructure. 

Figure 3: a), b), and c) show the I–V curves of Al–AOL–SiC interface measured at room temperature, 500° and 600 °C, respectively. d), e), and f) show the high-magnification images of Al–AOL–SiC interface from a small area of low-magnification images.

The chemical changes occurring at the interface during the heating process were investigated on another lamella using the same DENSsolutions Lightning holder, but on a Wildfire (heating-only) Nano-Chip. HAADF-STEM images and EELS chemical profiles were acquired and the results are shown in Figure 4 below. 

Figure 4: a), b), c ) and d) show changes in chemistry (line profiles of Al (Aqua), Si (Violet), C (Lime), and O (Yellow)) of Al–AOL–SiC interface at room temperature (25°), 550°, 500° and 600 °C, respectively.

During this analysis, the researchers observed that at 550 °C, the AOL width was reduced, which was specifically due to AOL dissolution into the Al. Moreover, the analysis of the structural changes at the interface nanostructure at 600 °C showed that the dissolution of the SiO₂ amorphous layer resulted in the formation of α-AlO and Si within the Al. In contrast, the elemental interdiffusion (Al³⁺ ⇄ Si⁴⁺) between Al and SiC was observed to occur, resulting in formation of AlC. From the results, we can infer that the reaction mechanism between Al and crystalline SiC is different with that between Al and SiO₂ amorphous phase.

Conclusion

Dr. Adabifiroozjaei and his fellow collaborators performed a comprehensive in situ STEM heating and biasing study using the DENSsolutions Lightning system, investigating the electrical, chemical and microstructural features of the interface of a Al–AOL–SiC system. Performing this study under an ultrahigh resolution of 4 Å allowed the researchers to confirm, for the first time in literature, that the reaction mechanism between reactive Al and crystalline SiC is different than between Al and amorphous SiO₂. Specifically, they found that whereas the reaction between SiO₂ and Al follows the dissolution mechanism, the reaction between SiC and Al proceeds through elemental interdiffusion. Importantly, these findings might be applicable to other reactive metal-ceramic systems that are currently used in manufacturing and electronic industries.

“With the stability and accuracy provided by DENSsolutions Lightning system, we could reveal features of an interfacial interaction in a common metal-ceramic system (Al-SiC) that were not previously observed. Such studies at very high resolution are absolutely necessary for the understanding and future development of composite materials at elevated temperatures.” 

Prof. Dr. Leopoldo Molina-Luna   Professor  |  TU Darmstadt

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

Using the DENSsolutions Stream system, researchers were able to create a highly controlled chemical environment for visualizing the nanoscale metallic electrodeposition of copper crystals.

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DENSsolutions successfully installs another Climate system in Japan, at JFCC

DENSsolutions successfully installs another Climate system in Japan, at JFCC

Top row – from left to right: Mr. Suzuki (Nano Tech Solutions), Mr. Anada (JFCC), Dr. Lars van der Wal (DENSsolutions) and Mr. Hirai (JEOL). Bottom row – from left to right: Mr. Fukunaga (JEOL), Mr. Jinbo (JEOL) and Mr. Hisada (JEOL).

We are proud to announce that DENSsolutions has installed another Climate system in Japan, at the Japan Fine Ceramics Center, located in Nagoya, a highly populated Japanese port city. In this article, we interview Dr. Satoshi Anada, Senior Researcher at the Nanostructures Research Lab in JFCC, 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 about Japan Fine Ceramics Center and its research and development initiatives?

Japan Fine Ceramics Center (JFCC) was established back in 1985, with the goal of improving the quality of fine ceramics mainly through integrated testing and evaluation systems. JFCC has numerous business activities, one of which is the research and development (R&D) of materials, manufacturing technology and evaluation technology. Our R&D initiatives are focused on obtaining technological solutions to problems related to the environment, energy and safety. We have two main laboratories: 1) the Materials R&D Lab and 2) the Nanostructures Research Lab. The Materials R&D Lab focuses on the development of highly functional and novel materials (mainly ceramics) by improved process control, whereas the Nanostructures Research Lab focuses on the development and enhancement of state-of-the-art electron microscopy and related technologies. At the Nanostructures Research Lab, we have a high-end electron microscope – the JEOL JEM-ARM300F2 Grand ARM. This microscope enables us to observe samples at ultra-high spatial resolution with highly sensitive analysis over a wide range of accelerating voltages.”

What type of applications are the users at the Nanostructures Research Lab interested in using the Climate G+ system for?

“Users at the Nanostructures Research Lab are interested in applying the DENSsolutions Climate system to record operando TEM observations of battery and catalyst materials. We aim to understand where and how reactions take place, and which conditions enhance the performance of those materials. Moreover, we are interested in the electrochemical oxidation of materials in reaction with oxidants such as oxygen and hydrogen.”

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

“In order to understand factors and mechanisms related to the performance of battery and catalyst materials, it is important to observe their reactions in the actual environments in which they are used. The Climate system has the ability to flexibly and rapidly adjust gas composition, temperature, flow and pressure, which enables us to observe our battery and catalyst materials under various experimental conditions. This is capability is particularly what attracted us to the solution.”

In your experience so far, how have you found working with the Climate G+ system?

“The preliminary processes including the assembly of the Climate Nano-Reactor and leak testing are quite straightforward, assisted by the well-established Climate manual and software. With the Climate system, we have been able to perform numerous experiments without running into any leakage issues. Moreover, we are particularly impressed with the stability of the system even at extremely high temperatures.”

Dr. Satoshi Anada
Senior Researcher | Japan Fine Ceramics Center

Dr. Satoshi Anada received his Ph.D. degree in Engineering, Material Science, from Osaka University. Previously, he was working as a Specially Appointed Assistant Professor in the Research Center for Ultra-High Voltage Electron Microscopy at Osaka University. Currently, Dr. Anada is working as a Senior Researcher in the Japan Fine Ceramics Center (JFCC). His research was focused on the electromagnetic analysis of functional materials and devices using transmission electron microscopy, and now particularly on different microscopic measurement informatics.

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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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