A facile heating cell for in situ transmittance and fluorescence X-ray absorption spectroscopy investigations

A facile heating cell has been designed for in situ transmittance and fluorescence X-ray absorption spectroscopy (XAS) measurements up to 1273 K under vacuum or an inert atmosphere. These high temperatures are achieved using a tantalum heating element by ohmic heating. Because of the small specific heat capacity, the temperature can be changed in a matter of minutes from room temperature to high temperature. Furthermore, a commercial power controller was adapted to provide stable temperature control. The construction of the heat shielding system provides a novel approach to reducing the beam's path length and the cell's size. The cell is inexpensive and easy to build. Its performance was evaluated by in situ XAS measurements of the temperature-dependent structure of ceria nanocrystals. Some preliminary results for the structural mechanism in ceria nanocrystal redox applications are given.

1. Introduction

The design, synthesis and manufacture of new materials with optimized functionalities require a sufficiently good understanding of the microstructure in the materials. X-ray absorption spectroscopy (XAS) is a powerful method for local geometric and electronic characterization in crystalline compounds, amorphous substances, glasses and liquids (Koningsberger & Prins, 1987). In many cases, structural information can be collected in situ, i.e. under well known temperature and atmospheric conditions. In situ high-temperature XAS measurements are of fundamental interest for a detailed understanding of temperature-dependent structures in phase-change materials (Kolobov et al., 2004; Waychunas et al., 1988). This method can also be used, for instance, to trace the growth of nanocrystals and redox reaction dynamics induced by temperature changes (Chen et al., 2008; Kong et al., 2012; Nashner et al., 1998) and to reveal the relationship between a catalyst's structure and its reactivity (Frenkel et al., 2012; Newton et al., 2007; Yamamoto et al., 2007).

To accomplish these tasks, several design approaches have been reported for versatile XAS cells (Bare et al., 2006; Farges et al., 1995; Girardon et al., 2005; Guilera et al., 2009; Huwe & Fröba, 2004; Longo et al., 2005; Meitzner et al., 1998). For example, Farges et al. (1995) applied the Mysen–Frantz heating-wire technique to design an XAS high-temperature cell for the study of metal oxide phase transitions. Girardon et al. (2005) proposed an XAS cell dedicated to in situ and operando experiments in heterogeneous catalysis. The cell can be used at ambient or elevated temperatures under the flow of a large variety of oxidizing or reducing gas mixtures. Huwe & Fröba (2004) designed and tested a high-temperature cell for in situ transmission and fluorescence XAS investigations. Furthermore, Guilera et al. (2009) built a miniature maxthal furnace for XAS experiments up to 1273 K.

Despite these recent advances, it is still a great challenge to design a high-quality cell that utilizes an inexpensive heater, convenient windows, a stable temperature controller and a compact volume. Here, we present a facile heating cell for in situ transmission and fluorescence XAS measurements up to 1273 K in a vacuum or an inert atmosphere. The high temperature is achieved by ohmic heating. Because of its high melting point and chemical stability, a tantalum heating element can also serve as a sample holder. The temperature can be changed in a matter of minutes from room temperature to high temperature and back because of the small heat capacity. The cell uses a commercial power controller and a silicon-controlled rectifier (SCR) voltage regulator module, so the temperature control precision of the system is ±1‰ over the range from room temperature to 1273 K. More importantly, a special X-ray window is stacked with kapton and carbon foils to form a heat shield system; this system provides a novel approach to reducing the beam's path length and to making the cell more compact. The cell is inexpensive and easy to build. Combined with quick-scanning XAS (QXAS) techniques (Frahm, 1989; Prestipino et al., 2011; Frenkel et al., 2012), it is suitable for performing in situ investigations as demanded, e.g. metallic melts (Lou et al., 2013), nanoparticles phase-change, and the solidification process of crystalline materials. Here, the cell's performance was evaluated by in situ XAS measurements of the temperature-dependent structure of ceria nanocrystals. The initial results for the structural mechanism in ceria nanocrystal redox applications are given.

2. In situ cell

The in situ X-ray absorption cell is displayed in Fig. 1. The cell consists of three main parts: (i) an aluminium chamber, which accommodates (ii) the heater and the sample holder system, and (iii) the temperature control system. The aluminium chamber is equipped with three windows to accommodate the incoming, transmitted and fluorescence X-rays. Special films made of stacks of kapton foil with a 10 µm thickness and graphite paper with a 100 µm thickness were fixed on each window with an O-ring. The black graphite paper can absorb thermal radiation from the heat element and prevents the direct exposure of the kapton foil to strong thermal radiation that would otherwise cause aging and cracking of the kapton foil. Fig. S1 of the supporting information¹ shows the effect of the carbon heat shield layer on the heating cell. When the cell is at high temperature without the heat shield, thermal radiation can be visually observed, and this radiation would eventually destroy the cell window. In addition, such a window is almost transparent to hard X-rays because its composition is mostly the light elements carbon, hydrogen and oxygen. Using this window, the beam's path length can be reduced, and the cell is more compact. The top/inner part of the cell is the heart of the cell. As shown in Fig. 2, the heater and sample holder can be removed to enable sample loading. The cell consists of current feedthroughs, a stainless steel fixture, a molybdenum fixture with a channel, and a tantalum-sheet heating element. Important cell parameters are listed in Table 1.

Table 1
Important cell parameters

Dimensions (length × width × height)	70 × 70 × 120 mm
Sample–window distance	35 mm
Temperature range	Room temperature to 1273 K
Heating power (at 1273 K)	75 W
Heating resistance (at 1273 K)	0.31 Ω
Heating electric current (at 1273 K)	56 A
Thermocouple	Chromel/alumel

Figure 1 Structure and appearance of the in situ cell.

Figure 2 Top/inner part of the cell.

As depicted in Fig. 2, the heating element is made up of two pieces of 100 µm-thick tantalum sheets. The two tantalum sheets are welded together to create a pocket to mount the test sample. The size of the tantalum elements is 12 mm × 42 mm. An elongated hole is reserved in the middle of the tantalum sample pocket to allow the incident and outgoing X-rays to pass through. Such a pure metal heater has a small electrical resistivity. Therefore, a large current is needed to heat the sample to high temperature. Two copper poles, 10 mm in diameter, are used as current feedthroughs. To minimize heat outflow from the copper poles, a molybdenum column and stainless steel bracket are used to decrease the thermoconductivity. They connect the copper poles to the tantalum heat element as show in Fig. 2. An additional tantalum sheet with a 0.1 mm thickness is present on the top of the sample holder, which further shields the thermal radiation from the heating system and localizes the heat intended to warm the test sample. A K-type thermocouple, 0.1 mm in diameter, is welded to the tantalum heater to monitor the temperature in the heating zone.

Because of the small heat capacity of the tantalum heater element, the temperature could be raised from room temperature to 1273 K in less than one minute and could drop back to room temperature in ten minutes. On the other hand, the heater element has a small resistance, so a large current is needed to drive it to reach high temperatures. To achieve system accuracy and stability, an optimized heating control system was introduced. This control system is made of a proportional-integral-differential (PID) controller (AIJ 908, Xiameng Yudian Automation Technology), a silicon-controlled rectifier (SCR) voltage regulator module (SSR-V2240A, Hangzhou Guojing Deji) and a transformer (220 VAC to 3 VAC). As shown in Fig. S2, the PID controller automatically adjusts the output power using the SCR voltage regulator according to the measured temperature. Fig. 3 shows the temperature curves when the cell is operated at the possible heating speeds under actual experiment conditions. The controlled temperature precision of the system is 1‰ over the range from room temperature to 1273 K, suggesting the temperature control system has the ability to heat the cell stably under actual experiment conditions.

Figure 3 Temperature curves recorded under actual experiment conditions (i.e. at the sample position, under vacuum, with water cooling and an aligned sample holder).

As shown in Fig. 2, the thermocouple is used to monitor the temperature in the heating zone. However, the sample center temperature should always be lower than that of the monitor spot, due to a lacking tantalum heat element covering in the sample center. In order to calibrate the temperature difference between the center of the sample and the monitor spot on the tantalum heater, two thermocouples were used in the calibration experiment. One thermocouple was welded to the tantalum heater to monitor the temperature in the heating zone. The other was inserted into the center of a boron nitride pellet of 1 mm thickness and 10 mm diameter to show the actual temperature of the sample. At each set temperature point the temperatures of the thermocouples were recorded at the same time, as shown in Fig. 4. The value of the temperature difference R is lower than 1% from room temperature to 1000 K, and is lower than 4% when the temperature is between 1000 K and 1273 K, since the thermal radiation diffusing from the hot sample enlarges the temperature difference in the higher temperature range.

Figure 4 Temperature difference between the sample center and monitor spot on the tantalum heater.

Further, to avoid fast oxidation of the tantalum heating element in air, the heating cell can only be used under vacuum or an inert atmosphere. Vacuum was achieved using a rotary mechanical pump, and the operation pressure is several Pascals. The life time of the tantalum heating element is several hundred hours.

3. XAS measurements

The in situ cell was tested on the 1W1B beamline of the Beijing Synchrotron Radiation Facility, which was operating at 250 mA of stored current. For the in situ high-temperature measurements, ceria samples were pressed into a boron nitride pellet with a diameter of 10 mm and a thickness of ∼0.5–1 mm. The heating speed was 5 K min⁻¹. The XAS spectra were scanned in the range 5.5–6.2 keV, which covers the L₃-edge absorption of cerium atoms. Figs. 5(a) and 5(b) show the Ce L₃-edge XANES spectra of the ceria in nanocrystal form and in bulk. Based on the literature (Douillard et al., 1995; Kaindl et al., 1988; Modeshia et al., 2007; Soldatov et al., 1994), the standard CeO₂ XANES spectrum from 5710 to 5750 eV contains four peaks (labeled A, B, C and D). The pre-edge structure, labeled D, is assigned to final states with delocalized d-character at the bottom of the conduction band. Due to the cubic crystal-field splitting of Ce 5d states, features B and C are associated with the transitions of Ce 2p to the Ce 4f¹5d_egL and Ce 4f¹5d_t2gL states, where L denotes an oxygen ligand 2p hole and 4f¹ refers to an electron going from an oxygen 2p orbital to a cerium 4f orbital (charge-transfer-like transition). The energy separation between peaks B and C is approximately 3.6 eV, in agreement with previous works (Soldatov et al., 1994). Feature A is attributed to the contribution of a different final state configuration, 4f⁰5d. Generally, the Ce spectrum has a distinct double-peaked structure (A and B) with the higher lying peak corresponding to the Ce⁴⁺ valence state (Kaindl et al., 1988; Soldatov et al., 1994). As shown in Fig. 5(c), a significant change is observed in the ceria nanocrystals after an increase of the temperature. When the temperature rises to 1273 K, feature A disappears and a single peak appears near C, which is shifted to a lower energy than B and suggests a change in the cerium oxidation state from +4 to +3 (Douillard et al., 1995; Modeshia et al., 2007). More interestingly, the spectroscopic valence changed back to +4 after the heated sample was kept under normal room conditions for several days. Additionally, the structural transformation is reversible when the sample is heated again. This reversibility is important because it provides a structural mechanism for the storage and release of oxygen that takes place in ceria in redox applications.

Figure 5 Effect of temperature on the Ce L3-edge XANES spectra of ceria. (a) Nanoceria. (b) Bulk ceria. (c) Valence reversibility in nanoceria.

4. Conclusions

A facile heating cell was designed, constructed and applied. The cell can reach temperatures up to 1273 K and can record X-ray absorption spectra in transmission and fluorescence modes. The heat shielding system provides a novel approach to reducing the beam's path length and the cell size. Because of the small heat capacity of the tantalum heater element, the temperature of this system could be raised from room temperature to 1273 K in less than one minute and could drop back to room temperature in ten minutes. Using an optimized heating control system, the heater temperature precision was ±1‰ over the range from room temperature to 1273 K. The cell was successfully used to analyze the structural evolution of oxygen storage and release in ceria nanocrystals. Further, combined with QXAS techniques, the heating cell is very suitable for investigation of the temperature-dependent structure transition as demanded, e.g. metallic melts, nanoparticles phase-change and the solidification process of crystalline materials.

Footnotes

¹Supporting information for this paper is available from the IUCr electronic archives (Reference: HF5239 ).