JPH07115864B2 - Selenium storage zeolite and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a selenium-containing material as an electronic material having a specific performance as a semiconductor, a photoelectric device, a superconducting device or an optical semiconductor catalyst, and a method for producing the same. And is effectively used in the fields of electronic industry and chemical industry.

(Prior Art) Crystalline selenium has a chain structure in which atoms are connected by a covalent bond and are spirally connected. Of the four P electrons of selenium,
Two occupy bond orbitals that make covalent bonds, and the other two occupy loan-pair orbitals. This loan-pair orbit occupies the uppermost part of the valence band, and there is a band gap of about 2 eV with the conduction band formed by the antibonding orbit, so selenium acts as a semiconductor. This semiconducting property is largely governed by the interaction between adjacent chains.

However, hitherto, there has been no attempt to realize a semiconductor having a peculiar property consisting of an isolated one-dimensional chain of selenium or a small number of clusters. Further, since it was extremely difficult to dope the crystalline selenium with the impurity element, no crystalline selenium having excellent semiconductor characteristics existed.

(Problems to be Solved by the Invention) From such a background, it is possible to control the electronic, optical and chemical properties by changing the band gap or the state in the gap of crystalline selenium significantly and variously. It is an object of the present invention to obtain new materials such as semiconductors and photoactive catalysts having excellent electronic characteristics.

(Means for Solving the Problems) With respect to the above technical problem, the present inventor has conducted various studies on the method, and as a result, if the Angstrom-order pores or cavities present in the zeolite crystal are used, That is, they have found that the problem can be solved by storing selenium in zeolite, and completed the present invention.

The present invention will be described in detail below.

Crystalline selenium is a crystal that belongs to a hexagonal system and has a chain structure in which covalent chains are connected with three-fold spiral axis symmetry. The diameter of the helix is about 2.5 Å, the adjacent chains are relatively tightly bound, and the band gap is about 2 eV.

The present inventor has found that zeolite may be used as a means for changing the band gap. As the concrete means, for example, the following three methods are possible.

(1) By confining isolated selenium chains in zeolite having one-dimensional pores, the interaction between adjacent chains is removed.

(2) Selenium clusters consisting of a small number of atoms and independent of each other are present in the pores or cavities of the zeolite crystal.

(3) Selenium is occluded in the pores or cavities of zeolite that has undergone ion exchange with exchangeable cations in the zeolite, especially transition metal ions. The above three methods can be combined by changing the type of zeolite and the exchanged ion species.

Next, the zeolite and its pore structure will be described.

Zeolite is a type of crystalline aluminosilicate,
Its structure is characterized by having a three-dimensional network structure in which AlO ₄ tetrahedra and SiO ₄ tetrahedra are bonded by sharing all O atoms, and the intracrystalline pores with a diameter of about 3 to 10Å Have. The size of the pores differs depending on the difference in crystal structure, and there are zeolites having a space larger than the pores, that is, a so-called cavity, inside the pores. Furthermore, the structure of pores in crystals is roughly classified into those in which pores extending in one dimension do not intersect with other pores, those in which pores have intersections in two dimensions, and pores in pores. There are three types of intersections in the three-dimensional direction. These pore structures also depend on the crystal structure of the zeolite.

The zeolite used in the present invention may have any of the above-mentioned pore structures, and the type of zeolite is selected according to which of the above methods for changing the band gap of selenium is used.

The zeolite may be synthetic or naturally occurring.

Zeolites with one-dimensional pores include mordenite, L,
Omega, analcime, bikitite and the like, and zeolite having two-dimensional pores include levinite and the like. Zeolites having three-dimensional pores include A, X, Y, chamber site, and erionite, among which A,
X and Y have large voids in the pores.

Some of these zeolites can change the Si / Al ratio in the crystal largely without changing the crystal structure like model night. When such a zeolite is used, the number of exchangeable ion sites and the adsorption property can be controlled by changing the Si / Al ratio to change the semiconductor property of the stored selenium.

The zeolite crystal AlO ₂ ^- are contained normal metal cations to the negative charges and to balance, the cations can be replaced by readily other cations and ion exchange. The type of cation in the zeolite is not particularly limited, but is selected for the purpose of controlling the electronic characteristics of the selenium that has been occluded. For example, when an alkali metal ion exchanger such as Na ion is used, the band gap of selenium increases, but a transition metal ion exchanger such as an ion exchanger of chromium, manganese, iron, cobalt, nickel, copper, etc. When is used, the band gap disappears or a new state appears in the band gap. Therefore, the electronic characteristics can be arbitrarily controlled by changing the types of these exchanged ion species and their exchange rates.

Conventionally, it has been impossible to control the doping amount of metal ions, particularly transition metal ions, in crystalline selenium, so that the effect of the present invention by using zeolite is remarkable.

Furthermore, if a rare earth ion exchanger is used, new magnetic properties will appear.

The method for obtaining these metal ion exchangers is generally carried out by putting zeolite in an aqueous soluble salt solution of the target metal. Usually, the ion exchange temperature is in the range of room temperature to less than 100 ° C, and the exchange time is several hours to several days. Further, preferably, the replacement is carried out while stirring. Ion exchange rate control is based on the amount of zeolite used,
It is carried out by a combination of the concentration of the soluble salt aqueous solution and the amount used.

Next, a method of occluding selenium in these zeolite pores will be described.

Here, occlusion refers to a state in which selenium taken into zeolite is not easily desorbed under normal reversible desorption conditions.

The zeolite is dehydrated and deaerated before occluding selenium, and heating in a vacuum exhaust device is a preferred method. The heating temperature at this time is 200 ° C or higher, a temperature at which zeolite crystals are not destroyed, usually 250 to 600 ° C.
The time is generally in the range of 1 to 10 hours. At this time, the degree of vacuum is sufficient if it is about 1 × 10 ⁻³ mmHg or less.

The sample is then contacted with selenium vapor under heating for a sufficient time. The contact temperature at this time is in the range of 250-600 ℃,
The contact time is in the range of 10 minutes to several hundred hours. At this time, it is preferable to heat selenium in the range of 250 to 600 ° C. in order to increase the partial pressure of selenium vapor. Also, the atmosphere to be contacted is such that there is no gas component other than selenium vapor,
That is, it is preferable to carry out under the same vacuum as when dehydration and deaeration. This is because the presence of a gas other than selenium vapor reduces the selenium storage speed and storage amount. After the contact, the zeolite is gradually cooled to complete the storage of selenium in the zeolite.

One of the methods to confirm that selenium has entered the zeolite crystal pores is to change the color of the zeolite. For example, Na-type zeolite changes from white to orange. In the case of Co-type zeolite, it turns black. Yet another method is weight change.

The maximum amount of selenium occluded is about 23 wt%, and can be arbitrarily controlled below this. The smaller the occlusion amount, the larger the band gap, which can be controlled by the occlusion amount.

Also, confirm that the crystalline selenium obtained by the above method does not exist outside the zeolite crystal only inside the pores of the zeolite crystal by confirming that no new diffraction line appears in the X-ray diffraction image. Can be done.

(Operation and effect) The atomic arrangement of selenium in the selenium-occluded zeolite obtained by the above method can be determined by EXAFS (extensive X-ray absorption fine structure) measurement. The coordination number of selenium in the zeolite crystal pores is almost 2, and the average interatomic distance is 2.33 ± 0.01 Å regardless of the exchange metal ion species, which is significantly shorter than that of crystalline selenium.

Electronic properties of selenium-occluded zeolite are PAS (photoacoustic spectroscopy)
Characterized by measurements and ESR (electron spin resonance absorption) measurements. For example, the band gap of selenium in mordenite obtained by the method (1) is 2.7 eV, which is about 1 eV higher than that of ordinary crystalline selenium.
Confirmed by measurement. Further, if the amount of selenium stored in mordenite is reduced by the method (1), the band gap further increases. The same increase in bandgap is also observed for selenium obtained by the method (2).

In the case of selenium in mordenite ion-exchanged with Cu ²⁺ according to the method of (3), the 0.4eV band gap becomes smaller than that in the case of using Na-type mordenite. Also M
In the case of n ²⁺ type mordenite, the size of the band cap does not change as compared with the case of using Na type mordenite, but an impurity level appears in the gap. On the other hand, in Co ²⁺ type mordenite, the gap disappears and the sample appears black and exhibits metallic properties.

When these samples are irradiated with light having energy above the band gap at low temperature, the absorption edge is red-shifted and a new absorption band appears in the gap, so-called photo-blackening phenomenon is observed. When this sample is returned to room temperature, it returns to its original state.
No signal is observed even if ESR measurement is performed when light is not irradiated, but an ESR signal is observed when an optical blackening phenomenon occurs. This indicates that the selenium chain is cleaved within the mordenite crystal pores and a radical is formed at the selenium chain end. The number of the radicals can be controlled by the energy of irradiation light and the irradiation time. Further, selenium in zetolite, which is ion-exchanged with a transition metal, is transformed from a semiconductor to a metal by light irradiation.

As described above, it is possible to obtain selenium having electronic properties which have not been heretofore obtained by storing selenium in zeolite in which various cations have been exchanged. In particular, the characteristic of being sensitive and reversible to light is a noteworthy feature. Such characteristics can be exhibited for the first time by occluding selenium in the pores of zeolite crystals of the Angstrom order.

Therefore, the selenium storage zeolite according to the present invention is a semiconductor,
It can be effectively used as an optical storage element, a switching element, a photocatalyst, a dielectric having a unique frequency characteristic, a superconducting material, a magnetic material, a pollution prevention material, and the like.

(Examples) Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1 A sodium-type mordenite powder having a SiO ₂ / Al ₂ O ₃ molar ratio of 20 was set in a vacuum device and placed under a vacuum of 10 ⁻⁶ mmHg or less.
Dehydration and degassing were performed at 350 ° C. for 5 hours. The mordenite was then heated to 400 ° C. under the same vacuum and contacted with metal selenium vapor heated to the same temperature for 16 hours.
Then, it was gradually cooled to room temperature.

The mordenite before contacting with selenium vapor was white, but after contacting it turned orange. The weight increase of mordenite was 23 wt%. When the X-ray diffraction images before and after contact with the selenium vapor were compared, the intensity of the diffraction line on the (200) plane was remarkably reduced, but no new diffraction line was observed. The Se-Se distance obtained by EXAFS measurement is 2.33 ±
It was 0.01Å and the coordination number was 2.0. FIG. 1 shows the radial distribution function F (r) derived from EXAFS measurement.

PAS of selenium storage mordenite thus obtained
As a result of the measurement, it was confirmed that the band gap of selenium in the pores of the mordenite crystal was wider than that of crystalline selenium by about 1 eV. When the sample is irradiated with light corresponding to the band gap at a temperature of 20 K, the absorption edge is red-shifted and a new absorption band appears in the gap. This is shown in FIG.

Example 2 The same mordenite powder as that used in Example 1 was subjected to ion exchange using a 1N ammonium nitrate aqueous solution to obtain mordenite having an ammonium exchange rate of 100%. This mordenite is fired in air at 500 ° C for 2 hours for deammonium treatment to remove the cations in the mordenite to H ^+.
And This zeolite was placed in a vacuum device, and Example 1 was used.
The same operation was performed to obtain selenium-occluding mordenite.

ESR signal of this mordenite was observed by bandgap irradiation at a low temperature of 80 K, and it was confirmed that the selenium chain was broken. Also, when the temperature was returned to room temperature, it returned to the state before irradiation. This is shown in FIG.

Examples 3 and 4 The same mordenite powder as that used in Example 1 was Mn ²⁺ using 1N aqueous solution of manganese nitrate and cobalt nitrate.
Exchanged mordenite (Example 3) and Co ²⁺ exchanged mordenite (Example 4) were obtained. These zeolites were placed in a vacuum apparatus and the same operation as in Example 1 was performed to obtain selenium-occluding mordenite.

As a result of PAS measurement of these samples, in the case of Mn ²⁺ exchanged mordenite, almost no movement of the absorption edge was observed, but a new absorption band appeared in the gap. On the other hand, in the case of Co ²⁺ exchanged mordenite, almost no wavelength dependence of PAS signal was observed and the gap disappeared.
This is shown in FIG. For comparison, the figure also shows the PAS measurement results of Na ⁺ -type mordenite.

[Brief description of drawings]

FIG. 1 is a graph showing the dynamic distribution function F (r) of selenium obtained from EXAFS of the selenium storage mordenite obtained in Example 1. FIG. 2 is a graph showing changes in PAS signals of the selenium storage mordenite obtained in Example 1 with and without light irradiation.
FIG. 3 is a graph showing the photoinduced ESR of the selenium storage mordenite obtained in Example 2. FIG. 4 is a graph showing changes in PAS signals of Na-type mordenite and selenium-occluding mordenite obtained in Examples 3 and 4.

Claims (3)

[Claims] 1. A zeolite in which selenium is occluded in a zeolite crystal. 2. A zeolite obtained by storing selenium in a crystal of a zeolite having a transition metal ion as at least a part of ion-exchangeable cations. 3. A method for producing a zeolite in which selenium is occluded in crystalline pores of zeolite, which comprises contacting dehydrated and degassed zeolite with selenium vapor under heating and then cooling.