Method for analyzing the electronic state of transition metal compounds

Abstract

To provide a method of analyzing electronic states of transition metal compounds.SOLUTION: A method of analyzing the electronic state of a transition metal compound comprises a spectrum measurement step of measuring a photoelectron spectrum of a sample by the X-ray photoelectron spectroscopy, the sample being a transition metal compound. In the spectrum measurement step, the amount of photons per unit area of X-rays with which the sample is irradiated is in a range of 7.13×104 to 1.78×106 photons / (μm2 sec), inclusive.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for analyzing the electronic state of a transition metal compound. [Background technology]

[0002] Recently, various materials have been developed, and in order to design more appropriate materials, analysis of the electronic state of the materials is sometimes required.

[0003] As a method for analyzing the electronic state of a material, X-ray photoelectron spectroscopy has been conventionally used, as disclosed in, for example, Patent Document 1.

[0004] X-ray photoelectron spectroscopy is a method for qualitative, semi-quantitative, and chemical state analysis of the outermost surface of a sample by irradiating the sample with X-rays and obtaining the spectrum of electrons (photoelectrons) emitted by the photoelectric effect. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-146416 Summary of the Invention [Problem to be solved by the invention]

[0006] When measuring photoelectron spectra that reflect the electronic state of a material, one possible way to improve the S / N ratio and accurately measure even minute peaks is to increase the intensity of the irradiated X-rays. However, when the sample is a transition metal compound, for example, increasing the X-ray intensity can cause the sample to change color and change state, which can make accurate measurements impossible.

[0007] Therefore, one aspect of the present invention aims to provide a method for analyzing the electronic state of a transition metal compound. [Means for solving the problem]

[0008] In one aspect of the present invention, a spectrum measuring step of measuring a photoelectron spectrum of a sample by X-ray photoelectron spectroscopy is provided, the sample is a transition metal compound; In the spectrum measurement step, the photon amount per unit area of ​​the X-ray irradiated onto the sample is 7.13 × 10 4 Photons / (μm 2 ·sec) or more 1.78×10 6 Photons / (μm 2 sec) or less the law of nature, The energy step when measuring the low binding energy region below 30 eV is set to 0.025 eV or more and 0.09 eV or less, and the energy step when measuring the binding energy region above 30 eV is set to 0.1 eV or more. A method for analyzing the electronic state of a transition metal compound is provided. [Effects of the Invention]

[0009] In one aspect of the present invention, a method for analyzing the electronic state of a transition metal compound can be provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 shows photoelectron spectra obtained in Examples 1 to 4. [Figure 2] FIG. 2 shows photoelectron spectra obtained in Examples 1 to 4. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. [Method for analyzing the electronic state of transition metal compounds] The method for analyzing the electronic state of a transition metal compound according to this embodiment may include the following steps.

[0012] A spectrum measurement step of measuring a photoelectron spectrum of the sample by X-ray photoelectron spectroscopy.

[0013] In the above spectrum measurement step, the sample is a transition metal compound, and the photon amount per unit area of ​​the X-ray irradiated onto the sample is 7.13 × 10 4 Photons / (μm 2 ·sec) or more 1.78×10 6 Photons / (μm 2 ·sec) or less.

[0014] Hereinafter, the method for analyzing the electronic state of a transition metal compound according to this embodiment will be described step by step. (1) Spectral measurement process (About the measurement sample) In the spectrum measurement step, the photoelectron spectrum of the sample can be measured by X-ray photoelectron spectroscopy (XPS).

[0015] The sample is preferably a transition metal compound, more preferably a transition metal oxide, for which accurate photoelectron spectrum measurement has been difficult using conventional analytical methods. The transition metal compound may contain multiple transition metals or may also contain elements other than the transition metals.

[0016] There are no particular limitations on the method for preparing the sample to be subjected to the measurement, and the sample can be prepared by the same procedures and methods as those used in ordinary XPS measurements. (Measurement conditions) As described above, when measuring a photoelectron spectrum, increasing the intensity of the irradiated X-rays is considered to increase the S / N ratio and accurately measure even minute peaks. However, when the sample is a transition metal compound, for example, increasing the X-ray intensity may cause the sample to discolor or change state, which may result in inaccurate measurement. Therefore, in the spectrum measurement step of the method for analyzing the electronic state of a transition metal compound according to this embodiment, the photon quantity per unit area of ​​the X-rays irradiated onto the sample is set within a predetermined range, thereby suppressing damage to the sample and increasing the S / N ratio of the obtained photoelectron spectrum.

[0017] In the spectrum measurement process, the photon amount per unit area of ​​the X-ray irradiated on the sample is 7.13 × 10 4 Photons / (μm 2 ·sec) or more 1.78×10 6 Photons / (μm 2 ·sec) or less, and 5 Photons / (μm 2 ·sec) or more 7.93×10 5 Photons / (μm 2 It is more preferable that the time is set to 1 / 2 sec or less.

[0018] The photon dose per unit area of ​​the X-rays irradiated on the sample in the spectrum measurement process is 7.13 × 10 4 Photons / (μm 2 ·sec) or more, the S / N ratio of the obtained photoelectron spectrum is improved, and measurements can be performed with high accuracy. In addition, the photon amount per unit area of ​​the X-ray irradiated to the sample in the spectrum measurement process is set to 1.78 × 10 6 Photons / (μm 2 ·sec) or less, damage to the sample due to X-ray irradiation can be suppressed, and changes to the sample during analysis can be suppressed.

[0019] The photon quantity per unit area of ​​the X-rays irradiated onto the sample can be adjusted to the desired value by adjusting the X-ray output, i.e., the power applied to the X-ray source, and the diameter of the X-rays emitted from the X-ray source (X-ray diameter).

[0020] The X-ray diameter can be selected depending on the characteristics of the X-ray source used, but is preferably 100 μmφ or more and 500 μmφ or less, and more preferably 150 μmφ or more and 400 μmφ or less.

[0021] In the spectrum measurement step, the shape and size of the area to be measured on the sample are not particularly limited, but the measurement area is preferably, for example, 300 μm square to 1000 μm square, and more preferably 400 μm square to 700 μm square.

[0022] By making the area of ​​the measurement region 300 μm square or more, photoelectron spectra can be measured with higher accuracy, and by making the area of ​​the measurement region 1000 μm square or less, the time required for measurement can be reduced.

[0023] From the photoelectron spectrum measured in the spectrum measurement step, it is also possible to analyze the electronic state, for example, from the valence band to the conduction band. From the viewpoint of performing such an analysis of the electronic state with particularly high precision and accurately measuring and analyzing the peak shape, peak top position, etc. of the photoelectron spectrum, it is preferable to set the energy step in measuring the photoelectron spectrum within a predetermined range.

[0024] For example, in the spectrum measurement step, the energy step when measuring the photoelectron spectrum is preferably set to 0.025 eV or more and 0.09 eV or less, and more preferably 0.025 eV or more and 0.05 eV or less.

[0025] By setting the energy step when measuring the photoelectron spectrum to 0.09 eV or less, the peak shape and peak top position of the obtained photoelectron spectrum can be measured particularly accurately. Furthermore, by setting the energy step when measuring the photoelectron spectrum to 0.025 eV or more, the time required for measurement can be reduced.

[0026] For example, when measuring a low binding energy region of 30 eV or less, the above energy step can be used, and when measuring a region with binding energy greater than 30 eV, the energy step can be larger than the above range. By using the above energy step in the low binding energy region, it becomes possible to analyze the electronic state from the valence band to the conduction band with high accuracy. Furthermore, by using an energy step larger than the above range when measuring a region with binding energy greater than 30 eV, for example, 0.1 eV or more, the time required for measurement can be reduced. However, in order to avoid excessively reducing measurement accuracy, it is preferable to use an energy step of 0.2 eV or less when measuring a region with binding energy greater than 30 eV.

[0027] In the spectrum measurement step, the pass energy when measuring the photoelectron spectrum is not particularly limited, but is preferably 15 eV to 50 eV, and more preferably 20 eV to 40 eV. The pass energy refers to the magnitude of the electric field applied between the deflection electrodes inside the analyzer.

[0028] By setting the pass energy to 15 eV or more, the measurement sensitivity can be increased, and by setting the pass energy to 50 eV or less, the resolution when measuring the photoelectron spectrum can be increased.

[0029] The method for analyzing the electronic state of a transition metal compound according to the present embodiment may further include any steps other than the above-described spectrum measurement step. For example, it may include a correction spectrum measurement step and a correction step, which will be described below. (2) Correction spectrum measurement process, correction process The method for analyzing the electronic state of a transition metal compound according to this embodiment may further include the following steps.

[0030] A correction spectrum measurement process for measuring the photoelectron spectrum of a correction sample having a noble metal film formed on the surface of the sample.

[0031] a correction step of correcting the photoelectron spectrum of the sample obtained in the spectrum measurement step, using the photoelectron spectrum of the correction sample obtained in the correction spectrum measurement step; (Correction spectrum measurement process) In the correction spectrum measuring step, a correction sample can be prepared by first forming a noble metal film on the surface of a sample prepared under the same conditions as the sample prepared in the spectrum measuring step.

[0032] For the calibration sample, it is preferable to select film formation conditions such as film formation time so that the precious metal film does not completely cover the surface of the sample (measurement sample). In other words, it is preferable to select film formation conditions so that at least a portion of the sample surface is exposed between the precious metal films. The film formation method is not particularly limited, and the film can be formed by vapor deposition or the like. The type of precious metal used for the precious metal film is not particularly limited, but it is preferable that it is a chemically stable material, such as one or more selected from Au (gold), Pt (platinum), Pd (palladium), Rh (rhodium), Ir (iridium), etc. Note that the precious metal may be a simple metal or an alloy.

[0033] The photoelectron spectrum of the correction sample can be measured under the same conditions as in the above-described spectrum measurement step, except that the correction sample is used. By measuring the photoelectron spectrum of the correction sample, a photoelectron spectrum containing the spectra of the noble metal and the sample can be obtained. (correction process) In the correction step, the photoelectron spectrum of the sample obtained in the spectrum measurement step can be corrected using the photoelectron spectrum of the correction sample obtained in the correction spectrum measurement step.

[0034] Specifically, the difference between the binding energy of a peak in the photoelectron spectrum of a noble metal contained in the photoelectron spectrum of the correction sample and the binding energy of a peak reported in literature for the noble metal is calculated. Then, the binding energy of the photoelectron spectrum of the sample obtained in the spectrum measurement step can be corrected based on this difference. The peak in the photoelectron spectrum of the noble metal used to calculate the difference is not particularly limited, but it is preferable to use, for example, the peak with the maximum intensity for the noble metal.

[0035] As described above, by correcting the binding energy of the photoelectron spectrum of the sample obtained in the spectrum measurement step, a particularly accurate photoelectron spectrum can be measured. [Example]

[0036] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples. [Example 1] (1) Sample preparation Cesium carbonate (Cs2CO3) and tungsten trioxide (WO3) were weighed, mixed, and kneaded to a molar ratio of Cs2CO3:WO3 = 2:11, and the resulting mixture was placed in a carbon boat. It was then heated twice in a tubular furnace in the atmosphere at 850°C for 20 hours, yielding Powder A, a very lightly greenish white powder. Note that after heating at 850°C for 20 hours, the powder was removed, crushed, mixed, and reheated under the same conditions.

[0037] The X-ray powder diffraction pattern of the obtained powder A showed a slight Cs6W 11 O 36 There are some mixed in, but it's mostly Cs4W 11 O 35 It was identified as a single phase (ICDD 00-51-1891). That is, it was confirmed that cesium tungsten oxide, a transition metal compound, was obtained. It was also confirmed that cesium tungsten oxide, a transition metal compound, was obtained in the following Examples 2 to 4. (2) Analysis of electronic states The electronic state of the obtained Powder A was analyzed, specifically, the photoelectron spectrum was measured, according to the following procedure.

[0038] The photoelectron spectrum of the sample Powder A was measured by X-ray photoelectron spectroscopy (spectrum measurement step).

[0039] The photoelectron spectrum was measured under the conditions shown in Table 1. Specifically, monochromatic AlKα rays were used as the X-ray source, the X-ray diameter was 200 μmφ, and the X-ray output was 50 W. The photon dose per unit area of ​​the X-ray irradiated on the sample was 4.46 × 10 5 Photons / (μm 2 ·sec).

[0040] The measurement area was 500 μm × 500 μm, i.e., a 500 μm square. The energy step was 0.025 eV in the region where the binding energy was 30 eV or less, and 0.1 eV in the region where the binding energy was greater than 30 eV. The pass energy was 23.5 eV.

[0041] Next, a calibration sample was prepared by depositing an Au (gold) film on the surface of the sample powder A, and the photoelectron spectrum of the calibration sample was measured (calibration spectrum measurement step). The photoelectron spectrum was measured under the same conditions as in the spectrum measurement step, except that the calibration sample was used as the measurement sample.

[0042] Then, the photoelectron spectrum of the sample obtained in the spectrum measuring step was corrected using the photoelectron spectrum of the correction sample obtained in the correction spectrum measuring step (correction step).

[0043] Specifically, the difference between the binding energy of the peak with the maximum intensity in the photoelectron spectrum of Au contained in the photoelectron spectrum of the correction sample and the binding energy of the peak with the maximum intensity among peaks reported in the literature for Au was first calculated, and the photoelectron spectrum obtained in the spectrum measurement step was then corrected using this difference.

[0044] The photoelectron spectrum obtained after correction is shown as spectrum 11 in Figures 1 and 2. Figure 1 shows the photoelectron spectrum in the region where the binding energy is 15 eV or less. Figure 1(B) is an enlarged view of a portion of Figure 1(A).

[0045] Figure 2 shows the photoelectron spectrum of W4f in the binding energy range of 32 eV to 42 eV. [Example 2] (1) Sample preparation Powder A obtained in Example 1, Cs4W 11 O 35The powder was spread thinly and evenly on a carbon boat, placed in a tubular furnace, and heated from room temperature to 800°C in an Ar gas stream. While maintaining the temperature at 800°C, the stream was switched to a mixture of 1% H2 gas with Ar gas as a carrier, and reduction was carried out for 15 minutes. After that, the H2 gas was stopped and the mixture was slowly cooled to 100°C using only Ar gas. The Ar gas stream was then stopped and the mixture was slowly cooled to room temperature, and Powder B was removed. The removed Powder B was blue in color. (2) Analysis of electronic states Photoelectron spectrum measurement was carried out in the same manner as in Example 1, except that powder B prepared in this example was used as the measurement sample. The measurement results are shown as spectrum 12 in Figures 1 and 2. [Example 3] (1) Sample preparation Powder A obtained in Example 1, Cs4W 11 O 35 The powder was spread thinly and evenly on a carbon boat, placed in a tubular furnace, and heated from room temperature to 800°C in an Ar gas stream. While maintaining the temperature at 800°C, the stream was switched to a mixture of 1% H2 gas with Ar gas as a carrier, and reduction was carried out for 30 minutes. After that, the H2 gas was stopped and the mixture was slowly cooled to 100°C using only Ar gas. The Ar gas stream was then stopped and the mixture was slowly cooled to room temperature, and Powder C was removed. The removed Powder C was dark blue in color. (2) Analysis of electronic states Photoelectron spectrum measurement was carried out in the same manner as in Example 1, except that Powder C prepared in this example was used as the measurement sample. The measurement results are shown as spectrum 13 in Figures 1 and 2. [Example 4] (1) Sample preparation Powder A obtained in Example 1, Cs4W 11 O 35 The powder was spread thinly and evenly on a carbon boat, placed in a tubular furnace, and heated from room temperature to 800°C in an Ar gas stream. While maintaining the temperature at 800°C, the stream was switched to a mixture of 1% H2 gas with Ar gas as a carrier, and reduction was carried out for 60 minutes. After that, the H2 gas was stopped and the mixture was slowly cooled to 100°C using only Ar gas. The Ar gas stream was then stopped and the mixture was slowly cooled to room temperature, and Powder D was removed. The removed Powder D was dark blue in color. (2) Analysis of electronic states Photoelectron spectroscopy was carried out in the same manner as in Example 1, except that the powder D prepared in this example was used as the measurement sample. The measurement results are shown as spectrum 14 in Figures 1 and 2. In Figure 2, spectrum 14 also shows the results of peak separation. 6+ 7 / 2 , W 6+ 5 / 2 indicates the hexavalent component of W4f, and W 5+ 7 / 2 , W 5+ 5 / 2 indicates the pentavalent component of W4f. The subscripts 7 / 2 and 5 / 2 indicate the splitting of the 4f electron orbitals due to the spin-orbit interaction, which corresponds to the total angular momentum quantum number calculated from the azimuthal and spin quantum numbers.

[0046] [Table 1] (Consideration) As shown in Figure 1, the lower part of the valence band (VB) and the conduction band (CB), as well as the band gap between them, were clearly observed. In addition, a small peak due to W-5d, indicated by arrow A in Figure 1(A), was observed in the range of 0 to 2 eV. Figure 1(B) shows the E F This is an enlarged view of the vicinity, and it can be seen that a small peak originating from W-5d can also be clearly observed.

[0047] The photoelectron spectrum shown in Figure 2 is a W4f spectrum, and Cs4W 11 O 35 It can be seen that the photoelectron spectrum shows almost two peaks. Furthermore, as the reduction time increases, from spectrum 11 to spectrum 14, the shoulder on the low binding energy side increases.

[0048] In Examples 1 to 4, when the condition of the sample was checked after the XPS measurement was completed, no discoloration or the like was observed, and it was confirmed that the photoelectron spectrum could be measured with high accuracy while suppressing damage caused by X-ray irradiation.

Claims

1. a spectrum measurement step of measuring a photoelectron spectrum of the sample by X-ray photoelectron spectroscopy; the sample is a transition metal compound; In the spectrum measurement step, the photon amount per unit area of ​​the X-ray irradiated onto the sample is 7.13×10 ４ Photons / (μm ２ ・sec) or more 1.78×10 ６ Photons / (μm ２ sec) or less, A method for analyzing the electronic state of a transition metal compound, in which the energy step when measuring a low binding energy region of 30 eV or less is 0.025 eV or more and 0.09 eV or less, and the energy step when measuring a region with a binding energy higher than 30 eV is 0.1 eV or more.

2. a correction spectrum measurement step of measuring a photoelectron spectrum of a correction sample having a noble metal film formed on the surface of the sample; 2. The method for analyzing the electronic state of a transition metal compound according to claim 1, further comprising a correction step of correcting the photoelectron spectrum of the sample obtained in the spectrum measurement step using the photoelectron spectrum of the correction sample obtained in the correction spectrum measurement step.

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