Controlled Environment Specimen Transfer

© MICROSCOPY SOCIETY OF AMERICA 2014

Controlled Environment Specimen Transfer

Christian D. Damsgaard,1,2,*Henny Zandbergen,3Thomas W. Hansen,1Ib Chorkendorff,2and Jakob B. Wagner1

1

Center for Electron Nanoscopy, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark 2

CINF, Department of Physics, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark 3

Kavli Institute of Nanoscience, Delft University of Technology, 2628 CJ Delft, The Netherlands

Abstract: Specimen transfer under controlled environment conditions, such as temperature, pressure, and gas composition, is necessary to conduct successive complementary in situ characterization of materials sensitive to ambient conditions. The in situ transfer concept is introduced by linking an environmental transmission electron microscope to an in situ X-ray diffractometer through a dedicated transmission electron microscope specimen transfer holder, capable of sealing the specimen in a gaseous environment at elevated temperatures. Two catalyst material systems have been investigated; Cu/ZnO/Al2O3catalyst for methanol synthesis and a Co/Al2O3catalyst for Fischer–Tropsch synthesis.

Both systems are sensitive to ambient atmosphere as they will oxidize after relatively short air exposure. The Cu/ZnO/ Al2O3catalyst, was reduced in the in situ X-ray diffractometer set-up, and subsequently, successfully transferred in a

reactive environment to the environmental transmission electron microscope where further analysis on the local scale were conducted. The Co/Al2O3catalyst was reduced in the environmental microscope and successfully kept reduced

outside the microscope in a reactive environment. The in situ transfer holder facilitates complimentary in situ experiments of the same specimen without changing the specimen state during transfer.

Key words: specimen holder, in situ, high-resolution, environmental TEM, gas reaction, microscopy, catalysis, specimen transfer

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In situ studies of functional materials are of utmost impor-tance in order to give fundamental insight into the state of the material under working conditions. Even though several in situ characterization techniques have been developed and used extensively over the past decades the operational conditions of the instrument do not completely resemble the working conditions. Compromises on specimen geometry, gas pressure, gas composition, etc. are often necessary, thus introducing material and pressure gaps. In situ transmission electron microscopy (TEM) or environmental TEM (ETEM) have rather strict conditions concerning specimen geometry (has to be electron transparent) and gas path length (Hansen et al., 2010). Environmental studies can be performed in an ETEM using conventional specimen holders (Hansen et al., 2001; Helveg et al., 2004; Kim et al., 2010; Simonsen et al., 2010; Yoshida et al., 2012) or in a traditional TEM by use of a dedicated specimen holder with a pressure cell (Creemer et al., 2008; Yokosawa et al., 2012).

In order to fully exploit the potential of ETEM, com-plementary experiments and characterization techniques are conducted. Normally, this is done in parallel with experiments separated in time and space (Hansen et al., 2002) or mimicking a reactor bed by changing the feed gas composition according to reactivity and conversion measured in dedicated catalyst set-ups (Chenna et al., 2011). In both strategies the

ETEM defines the conditions (gas, pressure, and tempera-ture), which normally are far from the operando conditions of, e.g., heterogeneous catalysis, but also different from more model-based research in the ultra-high vacuum (UHV) regime used in traditional surface science techniques.

Although the above mentioned strategies have proven useful, it will be beneficial to take it one step further and use the same specimen geometry in all complementary experi-mental studies and thereby (in principle) use the very same specimen transferred under reaction conditions.

Our efforts focus on establishing specimen transfer under controlled environment conditions (in situ transfer) to complementary characterization/synthesis techniques that are compatible with conditions closer to the specimen working conditions. This work presents the use of a dedi-cated TEM specimen holder capable of performing in situ transfer between compatible set-ups.

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In order to minimize specimen exposure to ambient atmos-phere during transfer between experimental facilities, vacuum transfer TEM specimen holders have been used (Kooyman et al., 2001). The specimens were sealed in the reactor set-up and transferred to a glove box, where the specimen, under inert atmosphere, was dispersed on a TEM grid and mounted in the dedicated vacuum transfer TEM holder. The holder was then sealed in the inert atmosphere of the glove box and transferred to the TEM. Expanding the concept of controlled atmosphere transfer by allowing transfer between set-ups in a controlled

*Corresponding author. cdda@cen.dtu.dk

gas composition (reaction gas) and at elevated temperature makes it possible to, for instance, study a catalyst specimen at the atomic scale during catalyst synthesis and activity testing by in situ transfer of the specimen between synthesis set-up and TEM without changing the specimen state during transfer. As TEM is a time consuming and expensive character-ization technique requiring human interaction, following specimen evolution over extended periods of time, from days to weeks, is usually impractical. In situ transfer makes it possible to treat the specimen in set-ups better suited for long-term experiments, e.g., in situ X-ray diffraction (XRD), and allows intermittent study of materials transferred to the ETEM without changing the environment during transfer. Such experiments could include formation and aging of catalysts under close to operando conditions, while con-tinuously measuring the specimen state with XRD and the catalytic activity by mass spectrometry. Local information can be extracted by transferring the specimen to the ETEM facility at various stages of the process without altering the environment.

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To validate the concept of in situ transfer, two well established catalyst materials systems were been investigated; Cu/ZnO/ Al2O3catalyst for methanol synthesis (Baltes et al., 2008) and a Co/Al2O3 catalyst for the Fischer–Tropsch process (Dry, 2002; Dehghan et al., 2011). The systems have previously been investigated in the ETEM and thus are well characterized with respect to electron beam sensitivity and reduction and oxidation conditions. Both systems are sensitive to ambient atmosphere as they will (at least partly) oxidize after relatively short air exposure. The Co/Al2O3catalyst is particularly suited for testing the sealing efficiency of the in situ transfer TEM specimen holder as oxidation in the ETEM occurs even in H2 flow at temperatures lower than ~300°C, owing to residual oxygen in the column (Dehghan et al., 2011).

Proof-of-concept studies of in situ transfer between set-ups were performed by investigating specimen phase changes and degree of oxidation before and after transfer. The in situ transfer has been conducted from an in situ XRD to an ETEM. Furthermore, transfer experiments were con-ducted solely on the ETEM, by investigating the specimen before and after a simulated in situ transfer, where the in situ transfer TEM holder was stored outside the microscope for a given time. Specimen transfer in ambient atmosphere was investigated as reference measurements.

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TEM S

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The in situ transfer TEM specimen holder combinesα and β tilt with the ability of heating the specimen to ~900°C. A Si-based microelectromechanical-system (MEMS) is used as specimen hot plate (van Huis et al., 2009), which makes it possible to store the specimen in a small sealed compartment at the tip of the holder, while heating in a controlled envir-onment, e.g., gas or vacuum during transfer. The tip of the

holder is illustrated in Figure 1 in the open and closed (sealed) configuration. By manually rotating a collar near the base of the holder, the specimen part of the tip slides into a Viton O-ring sealed compartment (Fig. 1c). In the open state (Figs. 1a, 1b) the specimen can be tilted up to± 23 and − 16 to 30° in theα and β-direction, respectively. In this study the sealing efficiency of the compartment was investigated in a pressure range from 120 to 105Pa.

Water adsorbed on the surface of the holder is inevitably introduced to the column upon inserting the holder into the microscope. In order to limit the amount of oxygen (water) in the column, when unsealing the specimen, the column was pumped to a pressure of ~10− 4Pa, before setting the conditions at which the specimen was unsealed and investi-gated. A pressure of 10− 4Pa was reached after half an hour of pumping. If necessary 10− 6Pa can be reached by con-tinuously pumping for several hours.

This holder makes it possible to transfer samples between spatially separated set-ups without changing the reactive environment during transfer. The reactive environ-ment (gas composition and pressure) during transfer is determined by the conditions in which the sample is sealed. During the transfer, the temperature is maintained at a given set-point by a feedback loop. As the different set-ups have different restrictions concerning the reactive environment, the effects on the sample to the changes in the reactive conditions when unsealing has to be addressed.

In practice, experiments that demand high pressure reactive environments should be performed in a dedicated

Figure 1. Drawing of the in situ transfer holder in open [(a) and (b)] and closed (sealed) (c) configuration. The sealing between the compartment and the specimen is performed by a Viton O-ring. The position of the sample in sealed configuration is illustrated transparent in (c).

high pressure set-up. The sample is then transferred under the same high pressure conditions to an ETEM to study the sample at the local scale under a controlled reactive environment. However, ETEM is restricted regarding gas composition and pressure and it is of high importance to consider how the sample reacts to these restrictions. If the sample changes state during unsealing because of differences in the reactive environment conditions (gas composition or pressure) the transfer is purposeless. In situ XRD study of a large volume sample at different reactive conditions provides a good benchmark for probing such possible changes of state.

ETEM

The FEI Titan 80–300 ETEM used in this study is described in detail by Hansen et al. (2010) and Hansen & Wagner (2012). The gas pressure is maintained around the specimen by a controlledflow directly into the pole piece gap. With a similar type of microscope it is possible to acquire atomically resolved images at pressures up to 2·103Pa (Yoshida et al., 2007). Using dedicated specimen holders (commercially available or custom built) in situ experiments can be performed in a controlled gaseous environment while maintaining a high spatial resolu-tion. The techniques used to study the specimens in this work includes electron diffraction (ED), electron energy-loss spectroscopy (EELS), and high-resolution TEM (HRTEM).

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XRD

The in situ XRD set-up, schematically shown in Figure 2, consists of a gas mixing part with five separate gas lines. Each line is equipped with mass flow controllers (MFC’s), with an operating range of 1–100 Nml/min, to establish an atmosphere of controlled composition. The mixed gas is injected into an Anton Paar XRK900 reaction chamber (APC) mounted in a Panalytical X’pert X-ray diffractometer. The APC is able to heat the specimen to 900°C and has a reaction volume of 4·10− 4_m3 _{and a specimen volume of 2·10}− 5_m3_. From here the gas is injected into the TEM holder dock (specimen dock, SD), where the gasflows around the surface of the TEM specimen. A three-dimensional drawing of the SD

with a TEM specimen holder inserted can be seen in Figure 2b. The pressure on the outlet of the APC and SD is controlled by a pressure controller and either a turbo pump or a rotary pump. The pressure range can be set from 100 to 2·105Pa. The chemical composition of the gas is analyzed by a Pfeiffer Vacuum QMS 422 quadrupole mass spectrometer (MS) connected to the gas line by a glass capillary.

The treatment and in situ characterization is performed by placing a large volume of the specimen in the APC and a small volume of the specimen on the MEMS hot plate in the TEM specimen holder. Equal specimen environment (temperature and pressure) is thus ensured by setting the same temperature in the APC as well as on the TEM MEMS hot plate. We can therefore assume that the XRD and MS data, primarily determined by the larger specimen volume in the APC, describes the crystal structure and catalytic activity of the specimen in the TEM holder as well. When a certain specimen state has been observed in the XRD or MS data the experiment can be paused, the TEM holder sealed, and subsequently transferred in a reactive environment to the ETEM for further analysis on the local scale under relevant conditions.

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Cu/ZnO/Al

O

Catalyst

The specimen was prepared according to Baltes et al. (2008). A small amount of the specimen was dispersed on the MEMS hot plate and mounted in the TEM specimen holder. The specimen was studied in the ETEM under vacuum (10− 4Pa) at room temperature (RT) before any specimen treatment. The EEL spectrum obtained at RT at 10− 4Pa of the untreated specimen, shown in Figure 3a, indicates that the initial state of the Cu is oxidic as the spectrum shows pro-nounced features at the L2and L3ionization edges at 951 and 931 eV, respectively. These features, known as“white lines”, results from transitions from the 2p core level to the 3d states (Laffont et al., 2006). This correlates with the rotationally averaged ED pattern, shown in Figure 3b. The strongest peak is observed around 4 nm− 1, corresponding to the f111g

Figure 2. a: Schematic diagram of the in situ X-ray diffraction APC: Anton Paar reaction cell; SD: specimen dock; MFC: massflow controller; PC: pressure controller; RP: roughing pump; QMS: quadropole mass spectrometer; XRD: X-ray diffractometer. b: Drawing of the transmission electron microscopy specimen holder dock SD. Arrows indicate theflow in- and out-let.

reflection of CuO (Asbrink & Norrby, 1970) at 3.96 nm− 1 and the f111g reflection of Cu2O at 4.05 nm− 1(Foo et al., 2006). A large volume of same specimen was loaded in the APC reactor chamber. The in situ XRD pattern (Fig. 4) of the large volume specimen obtained at RT in H2 also supports the EELS and ED data as it reveals a mixture of Cu2O and CuO.

The two fractions of the specimen were reduced in the same gas stream, at the same experimental conditions; 2% H2 in He at 105Pa at aflow of 100 Nml/min at 230°C for ~10 h. XRD patterns and MS data were obtained during reduction. The large volume specimen in the APC was reduced to metallic Cu as observed in Figure 4. The MS data shows a decrease in the H2signal and an increase in the H2O for-mation during reduction. This is expected as the hydrogen reacts with the oxygen from the reducing Cu oxide nano-particles (NP) forming H2O.

After reduction the holder was sealed in the gas stream while maintaining the elevated temperature. The TEM speci-men was transferred from the in situ XRD system to the ETEM in a reactive environment. The transfer took ~15 min.

Just before inserting the holder in the microscope the temperature was decreased to RT. After evacuating the microscope column to 10− 4Pa the holder was opened and the specimen was studied by ED, EELS, and HRTEM. Pre-vious results show that the Cu/ZnO/Al2O3catalyst maintains its metallic state when cooling down to RT in H2 and decreasing the pressure to 10− 4Pa.

The temperature of the specimen could also be main-tained while opening the holder in aflow of H2. However, in this proof-of-concept study the aim was to see if the specimen was oxidized during transfer and was thus investigated in nonreducing or nonoxidizing environments in the microscope. In order to test if the sample remains in the reduced state during transfer in this proof-of-concept study it was necessary to decrease the temperature to RT in vacuum in the ETEM. Opening the transfer holder at elevated temperature in a H2 gas flow in the ETEM resulted in instant reduction of the sample and thereby compromise the purpose of the proof-of-concept test.

An EEL spectrum and rotational averaged ED pattern of the TEM specimen after in situ transfer, acquired at RT

Figure 3. Electron energy-loss spectrum [(a) and (c)] and rotational averaged ED (electron diffraction) pattern [(b) and (d)] of Cu/ZnO/Al2O3catalyst before reduction [(a) and (b)] and after in situ transfer to the environmental

transmission electron microscopy [(c) and (d)]. The data is obtained at room temperature at 10−4Pa. Theoretical reflections corresponding to metallic Cu ■, CuO ●, and Cu2O ♦ are indicated. The insets shows the obtained ED

at 10− 4Pa, are shown in Figure 3c and 3d. In metallic Cu the 3d-band is full, which results in a step-like feature in the EEL spectrum at the L2 and L3 ionization edges, as observed in Figure 3c. The ED pattern (Fig. 3d) supports the EELS data, as it shows peaks from metallic Cu at 4.76 and 5.48 nm− 1 corresponding to the {111} reflection at 4.78 nm− 1 and {002} reflection at 5.53 nm− 1_{, respectively (Otte, 1961). No} apparent features of Cu oxides are observed.

HRTEM images acquired at RT at 10− 4Pa after the in situ transfer show lattice fringes corresponding to metallic Cu. Figure 5a shows a Cu NP along the [110] zone axis. The lattice spacings and internal angles determined from the fast Fourier transformation (FFT) (Fig. 5b) correspond to the fcc Cu {111}, {002}, and {113} reflections at 0.208, 0.181, and 0.109 nm, and internal angles 70.53 and 54.74°, respectively (Otte, 1961). No surface oxide layers were observed.

A reference transfer experiment at 150°C in air shows the Cu specimen to be fully oxidized after 15 min.

This experiment shows that it is possible to treat a spe-cimen at high pressure conditions and subsequently transfer it in a reactive environment to the ETEM without changing the specimen state—something that is not possible under ambient atmosphere. Furthermore, it shows correlation between the XRD patterns obtained from a large volume specimen and the ED patterns obtained from the TEM specimen. This suggests that the two specimens were exposed to the same conditions during the reaction as intended in the in situ XRD set-up.

The next step is to investigate the catalyst during methanol synthesis in syngas (H2/CO/CO2gas mixture), i.e., test the catalyst activity in the in situ XRD and subsequently, transfer the active catalyst to the ETEM in a reactive environment.

Co/Al

O

Catalyst

The composition of the investigated specimen is 12 wt%Co/ 0.5 wt%Re/α-Al2O3. At temperatures above 450°C, bulk metallic cobalt has the fcc structure, while at lower tem-peratures the hcp cobalt structure is stable (Troiano & Tokich, 1948; Bulavchenko et al., 2009). However, supported cobalt NPs tend to form with both fcc and hcp cobalt structures at low temperatures, even below the bulk transi-tion temperature (Bulavchenko et al., 2009; Ducreux et al., 2009). In this experiment the specimen was reduced in the ETEM at 120 Pa H2 at 500°C. HRTEM images and ED patterns were obtained before and after the in situ transfer. Figure 6 shows HRTEM images of the same particle, before reduction (a) acquired at RT in 120 Pa H2, and after reduc-tion (b) acquired at 500°C in 120 Pa H2. The untreated specimen shows a core/shell structure. The FFT of the core in Figure 6a corresponds to fcc Co along the [110] zone axis. The shell appears mostly amorphous, but crystalline areas of the shell can be found as shown in the insets of Figure 6a.

Figure 5. High-resolution transmission electron microscopic image (a) and reduced fast Fourier transformation (b) corresponding to the [110] zone axis of metallic fcc Cu. The image is obtained at room temperature at 10− 4Pa after in situ transfer.

Figure 4. X-ray diffractometer patterns of the large volume specimen before and after reduction (10 h) in 2% H2in He at

105Pa at room temperature and 230°C, respectively. Theoretical reflections corresponding to metallic Cu ■, CuO●, and Cu2O♦

The fringe spacing of 2.85 Å corresponds to the spacing between {220} planes in Co3O4, while the spacing of 2.43 Å corresponds to the spacing between {311} planes in Co3O4or {111} planes in CoO, suggesting that the shell is a Co oxide as shown in the work of Dehghan et al. (2011) of a specimen from the same batch. After reduction at 500°C in 120 Pa H₂, most of the shell disappeared and the size of the metallic Co part increased by ~2 nm, corresponding to 50% of the shell width. The expected decrease in volume when reducing Co3O4 (21.91 Å3/Co atom) to metallic fcc Co (10.00 Å3/Co atom) or hcp Co (11.07 Å3/Co atom) is 54 and 49%, respectively. Parts of the shell remains on some NPs after the reduction as shown in Figures 6b and 7.

The holder was subsequently closed in the H2gas stream (keeping the specimen at 500°C), removed from the micro-scope and left at ambient conditions for 15 min, simulating an in situ transfer between set-ups. The holder was then

re-inserted into the column and the H2flow was re-established at 120 Pa. One could keep the high temperature conditions, but since this experiment was designed to determine any oxidation of the specimen the temperature was decreased to a tempera-ture lower than the reduction temperatempera-ture, but higher than the oxidation temperature, 350°C suited this purpose and was set just before opening the holder. Figure 7 shows a reduced metallic Co particle before and after in situ transfer. The images were acquired at 500 and 350°C in 120 Pa H2, respectively. FFT analysis suggests that the NP maintains structure and orienta-tion along the [101] zone axis of metallic hcp Co, after the transfer. The structure seen on the right part of Figure 7b is part of the support that moved during handling.

The nature of the particle overlayer observed in Figures 6b and 7 is still under investigation and beyond the scope of this paper. However, when lowering the temperature from 500 to 200°C a clear (~4 nm) oxide shell re-appears.

Figure 6. High-resolution transmission electron microscopic image before (a) and after (b) reduction. The images are obtained in 120 Pa H2at room temperature and 500°C, respectively.

Figure 7. High-resolution transmission electron microscopic image before (a) and after (b) in situ transfer. The data are obtained in 120 Pa H2at 500°C and 350°C, respectively.

Figure 7 shows that by using the in situ transfer holder it is possible to keep the same specimen state and similar NP morphology during in situ transfer.

ED and HRTEM of the oxidized specimen during cooling to RT was obtained as a reference measurement. The influence of the electron beam on the reduction using similar experi-mental conditions was verified by Dehghan et al. (2011) by blank reduction experiments (in the absence of electron beam). The reduction was, subsequently confirmed by EELS analysis.

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Proof-of-concept of in situ transfer, i.e., transfer in a reactive environment (temperature and pressure) is shown for two relevant catalytic systems for methanol synthesis and Fischer-–Tropsch synthesis, respectively, by use of an in situ transfer TEM specimen holder. The study shows that in situ transfer makes it possible to study specimens over a wider pressure range by combining specimen characterization and treatment performed on different set-ups with complementary cap-abilities. In this case, an in situ XRD with a pressure limit of 2·105_{Pa and an ETEM with a pressure limit of 2·10}3_Pa. Fur-thermore, it is shown that a transition metal, like Co that oxi-dizes readily, maintains its metallic phase during in situ transfer at elevated temperatures in H2. This shows that the transfer holder seals efficiently and opens up new possibilities regarding specimen transfer at elevated temperatures in vacuum or in gas (reactive or inert). Currently, in situ transfer from an UVH cluster source to the ETEM is being investigated. This would present an efficient way of studying annealing effects of mass-selected NP without breaking vacuum and annealing temperature during transfer.

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For funding of this work we gratefully acknowledge The Danish Ministry of Science’s UNIK initiative CAtalysis for Sustainable Energy (CASE) and The Danish National Research Foundation’s Center for Individual Nanoparticle Functionality (DNRF54). The A.P. Møller and Chastine Mc-Kinney Møller Foundation is gratefully acknowledged for the contribution toward the establishment of the Center for Electron Nanoscopy at the Technical University of Denmark. FEI is acknowledged for co-financing the devel-opment of the specimen holder. We thank Dr. John C. Walmsley from SINTEF Materials and Chemistry, Dr. Roya Dehghan from the Statoil Research Centre in Trondheim for preparing the Co/Al2O3 catalyst, Dr. Irek Sharafutdinov from DTU Physics for preparing the Cu/ZnO/Al2O3catalyst, and Mr. Eef Grafhorst from Delft University of Technology for assembling and optimizing the in situ transfer holder.

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