J. Chem. Soc., Faraday Trans. I , 1987, 83 ( I 0), 3 107-3 1 14 Doping Effect of Sodium on y-Irradiated Magnesium Oxide Tsuneo Matsuda," Koji Y amada, Y asumasa Shibata, Hiroshi Miura and Kazuo Sugiyama Faculty of Engineering, Saitarna University, 255 Shirno-ohkubo, Urawa 338, Japan The concentration of one-electron donor centres increases markedly when y-irradiated MgO is treated with sodium amide (NaNH,), followed by calcination at 500 "C for 2 h. The drastic increase in the concentration of one-electron donor centres is attributed to the electron-donating effect of Na produced by the decomposition of NaNH, at the anionic vacancy sites which are formed during irradiation. In contrast, the concentration of one- electron donor sites decreases if NaNH, is previously introduced into MgO before y-irradiation.A similar result is obtained with MgO doped with NaNO,. Thus in both cases when NaNH, was doped into the irradiated MgO and when MgO-NaNH, was irradiated, the concentration of one- electron donor centres changed considerably. This is explained by transfer of electrons trapped at anion vacancies to lattice oxygen atoms in MgO. The electrical conductivity was measured at temperatures between 50 and 300 "C. As the sodium content increases the electrical conductivity also increases. The thermal activation energy, described by the conductivity, differs on going from the low- to the high-temperature region, indicating that there exist two kinds of electron levels. The correlation between electroconductivity, one-electron donor and basicity properties is dis- cussed.Defects are produced in an MgO crystal lattice by irradiation with y- or u.v.- irradiati~n.'-~ As a result of irradiation, MgO turns blue, the change of colour being attributed to the formation of F-centres. The F-centre, an electron trapped at an anionic vacancy in the MgO lattice, behaves as a one-electron donor centre.6 According to Tench et al.,' the lattice oxygen ions on the magnesium oxide surface may act as electron donors. Cordishi et a1.8 have also mentioned that electron donor sites consist of 02- ions in a coordinatively unsaturated site and OH- on the magnesium oxide surface. Thus there is still no definite explanation of one-electron donor centres. In either case they occur at sites which are able to transfer electrons to molecules having a high electron affinity, such as nitrobenzene, tetracyanoethylene (TCNE) etc.Lunsford et aL4. investigated the effect of ultraviolet and neutron irradiation on MgO using electron spin resonance (e.s.r.), concluding that irradiation has a more pronounced effect on insulators such as MgO than on metals or semiconductors. Consequently it would be useful to study the effect of irradiation of MgO, especially in connection with the electron-donating effect of sodium. Furthermore, it is expected that one-electron donor centres will increase in concentration on irradiation. However, the experimental results we obtained were contrary to this expectation, i.e. the number of one-electron donor centres decreased remarkably on irradiation. The cause may be that electrons trapped at anion vacancy sites on the surface transfer to oxygen molecules which are produced by the irradiation.' If sodium is doped into pre-irradiated MgO, which is presumed to have a larger concentration of anion vacancies, it can be assumed that the number of one-electron donor centres will increase through the electron- 31073108 Na Doping of y-Irradiated MgO T a- u Fig.1. Apparatus to produce the sample for e.s.r. measurements a, sample; b, quartz tube for e.s.r. measurements; c, quartz-Pyrex glass join; d, greaseless joint with O-ring; e, trap to seal the grease vapour which is dipped in liquid N, during the preparation; f, to vacuum line; g, connection with the air-tight benzene solution of TCNE; 0, greaseless stop valve. donating properties of sodium. This study was carried out in order to verify the above consideration.The basic site in magnesium oxide lattice is O2-.l0 Consequently it is of interest to examine whether one-electron donor centres and the basic site will be related by electric conductivity. On the basis of this consideration the electric conductivity of MgO-NaNO, was measured at various temperatures and at different concentrations of NaNO,. Experimental The magnesium oxide mainly used in this study (purity > 99.9%; surface area 146 m2 g-') was kindly donated by the Kohnoshima Chemical Co. Ltd. Another source of magnesium oxide from Wako Pure Chemicals was also used (surface area 40 m2 g-'). Ca. 7 g MgO was placed in a glass tube of 2 cm outer diameter, outgassed at 500 "C for 3 h and then sealed under vacuum.The MgO in the glass tube was irradiated with y-rays from a 6oCo source at room temperature with either 5 x lo-, or 8 x 10-1 Mrad irradiation dosages. The MgO samples doped with sodium compounds such as NaNH, and NaNO, were also irradiated using the same irradiation dosages as in the case of MgO. Before irradiation the mixture of MgO and the sodium compound was calcined at 500 "C for 3 h under evacuation. Doping the irradiated MgO with NaNH, was carried out as follows; after the irradiation of MgO the seal of the glass tube was broken in a dry box filled with pure N,. NaNH, (which decomposes to give metallic sodium at ca. 400 "C) was mixed with the MgO and then a portion of the mixture (ca. 0.1 g) was placed in glass tube ('A') of fig.1. The glass tube was sealed with a rubber stopper before removal from the box.T. Matsuda, K. Yamada, Y. Shibata, H. Miura and K. Sugiyama 3109 Fig. 2. Apparatus for electrical conductivity measurements; a, thermocouple; b, sample; c, electric furnace; d; metal stop valve. While dry, pure N, was allowed to flow from the narrow glass tube (B), near (d) in fig. 1, the stopper of tube (A) was rapidly removed and the tube connected with (B) through joint (d); thus MgO-NaNH, mixture did not come into contact with air. The MgO- NaNO, sample was prepared by impregnating MgO in an NaNO, aqueous solution, followed by drying at 110 "C and calcining at 700 "C for 5 h in flowing N,. Each sample of MgO, MgO-NaNH, and -NaNO, was evacuated at 500 "C for 2 h in tube (A).The samples doped with sodium as described above were further treated with a benzene solution of TCNE, which flowed down from the top tube (B) into the bottom of tube (A). After the TCNE solution was adequately adsorbed onto the sample, by leaving it for about half a day at room temperature, the benzene was removed by evaporation at 70 "C. A small portion of the prepared sample was transferred into the e.s.r. sample probe [(quartz tube, 0.d. 4 mm, see (b) in fig. 13 and then sealed off. The e.s.r. spectra of the samples were recorded at room temperature in a dual-cavity spectrometer. In the cavity two quartz tubes were prepared; one contained the standard probe, DPPH diluted with sodium chloride, and the other the sample. By using the dual cavity a mode check could be easily operated at the same time for the two samples, and the e.s.r.spectra could be measured under the same conditions of modulation amplitude and microwave frequency. The e.s.r. spectrometer, operating at X-band frequency, was a Varian model E-9 instrument equipped with a 100 kHz modulation unit. The relative spin concentration was determined by comparison of each integrated peak area of the samples and using DPPH. Contamination from grease vapour was prevented by using a liquid-nitrogen cold trap and greaseless stop valves in both the sample preparation and the following electrical conductivity measurements. With the apparatus shown in fig. 2 the electrical conductivity31 10 Na Doping of y-Irradiated MgO I I Na/mmol g i i o Fig.3. Relative spin concentration of the irradiated MgO-NaNH, and MgO-NaNO, us. the sodium content. (a) Sample prepared by the doping of NaNH, in the preirradiated MgO, 0, 0, A, 5 x lo-, Mrad h-l; 0 , 8 x lo-' Mrad h-l. (ti) MgO-NaNO, irradiated with prays. ( c ) A, MgO from Wako Pure Chemicals (left-hand ordinate). was measured by raising the temperature from 50 to 300 "C at the rate of 100 "C h-l. For electrical-conductivity measurements of MgO-NaNO,, the powder was pressed into a thin disc 2 mm thick by 2 cm diameter and then broken into 5 x 5 mm squares; then on both sides gold contacts ( > 99.9% purity) were attached by an evaporation method. Before measurement, the square disc was heated at 700 "C for 1 h in the apparatus. The basicity of the samples, dissolved in a benzoic acid-benzene solution (0.1 mmol), was measured by titration using indicators with various pK, values.Results and Discussion The e.s.r. spectra exhibited a single differential absorption of a shape similar to those cited in the literature.6 However, the g values of the TCNE radical were 2.0035 for MgO and between 2.0027 and 2.0030 for MgO doped with sodium. There are small differences in these g values between our results and those in the literature. The dependence of the relative spin concentration Gel of one-electron donor centres in MgO-NaNH, and MgO-NaNO, as a function of the sodium content is shown in fig. 3. Cel of the sample in which NaNH, was added in the irradiated MgO, followed by calcination at 500 "C for 2 h [denoted as MgO (I)] is remarkably large and is 40 times greater than the value of Gel of irradiated MgO-NaNO, calcined at 700 "C for 5 h [cf.fig. 3(a) and (b)]. Gel of an irradiated sample [MgO (11)] of MgO-NaNH, already calcined at 500 "C also showed the same behaviour as that of the irradiated MgO-NaNO, sample, as shown in fig. 3 (b). A notable decrease in Gel due to irradiation was found with MgO. Thus theT. Matsuda, K. Yamada, Y. Shibata, H. Miura and K . Sugiyama 3111 A I I I I 1 2 4 6 8 I ( Na/mmol g i i o Fig. 4. Electrical conductivity of MgO doped with NaNO, at various temperatures: 0, 100; a, 200; A, 300 "C. addition of sodium compounds followed by calcination greatly affects to the value of Gel of the one-electron donor centres. The cause of the increase of the Gel of MgO (I) may be as follows: the donation of Na electrons into the anion vacancies formed by y-irradiation will produce one-electron donor centres.This result also indicates that these centres are stable after evacuation at 500 "C. On the other hand, in the case of MgO (11) the electrons already trapped in one- electron donor centres will be ejected by y-irradiation, resulting in a decrease in Gel. The ejected electrons may be transfered to the oxygen atoms in the MgO lattice to form the basic site, 02-. One might expect an increase in the basicity, but no increase could be detected in this study. The number of basic sites is ca. 1019 g&,,ll and that of one- electron donor centres is lo1' or 10l6 spin g-l from the results of this study and others.12 Even if the electrons in a one-electron donor centre were transferred to the production of the basic site, the contribution to the basicity would give rise to an increase of only 1 or 0.1 O/O, which would be taken up by the error range of the basicity measurement. Another cause may be considered as follows.According to Wy~ocki,~ oxygen is released during y-irradiation in the initial stage of the process and then readsorbed on the MgO surface, mainly as 0;. The adsorbed site of 0, is neither a one- electron donor centre nor a basic site. If the 0, is produced by y-irradiation with the31 12 Na Doping of y-Irradiated MgO 2.0 2.5 3.0 103 K I T Fig. 5. Plot of electric conductivity vs. the reciprocal of temperature in an MgO-NaNO, sample. Sodium content per g MgO (mmol) as follows: (1) 3.2, (2) 1.6, (3) 1.0, (4) 0.35, ( 5 ) 0.consumption of the electron trapped at the anion vacancy, a remarkable decrease in the concentration of centres would be expected, as shown in fig. 3 (b) in comparison with fig. 3(a). However, this speculation seems to be doubtful, because such spectra as 0, and 0; indicated by Wysocki' could not be detected in this study, perhaps owing to the contamination of Na ions in MgO (I) and (11) samples. Normally, no 0; ions can be directly produced when oxygen is contacted with MgO, but are formed through the agency of presorbed species such as pyridine,l3l1* h ydrogen, ethylene15 etc.16 Consequently, it seems to be difficult to form 0, ions without an oxygen source. The fact that Gel increases with increasing sodium content is a consequence of the high electron- donating ability of Na.This will also contribute to the formation of one-electron donor centres, as in the case of MgO (I). In this case the sodium electrons will be trapped in anion vacancies formed by y-irradiation to produce one-electron donor centres. With increasing sodium content Gel attains a maximum [fig. 3(a)] and then gradually decreases irrespective of the irradiation dosage. The decrease in the Cel signal may be ascribed to sodium covering the MgO surface, especially near anion vacancy sites, thus diminishing the adsorption of TCNE. The surface area of MgO and the corresponding samples doped with sodium compounds did not change before and after y-irradiation. Consequently, one may conclude that irradiation is responsible for the effect occurring only at one-electron donor centres.The surface of zinc oxide irradiated with y-rays sinter~,~' but no sintering was observed in this study of MgO. The most likely cause is probably the higher melting point of MgO compared with that of ZnO.T. Matsuda, K. Yamada, Y. Shibata, H . Miura and K. Sugiyama 3113 0 2 4 6 Na/mmol g,',, Fig. 6. Relationship between the activation energy for electric conduction and sodium content in MgO-NaNO, in two temperature regions : 0, high-temperature region (130-300 "C); A, low- temperature region (50-130 "C). The basic site of MgO is 02- on the electron-rich MgO surface." The basicity was increased by the doping of sodium compounds in MgO, followed by calcination." The electrons on one-electron donor centres or on the basic sites can be considered to be involved with electrical conductivity.The electrical conductivity of MgO doped with NaNO, at various temperatures was examined, and the results are shown in fig. 4. With progressively increasing doping in MgO and on raising the measurement temperature the electrical conductivity increases, which is similar to some extent to semiconductors. The temperature dependence of the average electrical conductivity of the samples is shown in fig. 5, from which the thermal activation energy of the electrical conductivity is obtained. As shown in fig. 5, a plot of temperature dependence against electrical conductivity exhibits a discontinuity. The activation energy in the high-temperature region, (i.e.above 130 "C) attained a constant value of 0.9 eV following the increase in sodium content (see fig. 6). In the low temperature region (below 130 "C) the activation energy is constant (0.4 eV) and independent of the sodium content. It is clear from fig. 5 and 6 that there are two different conduction-electron energy levels. Electrons in the higher energy level can easily jump into the conduction band in the low-temperature region with a low activation energy. On the other hand, the jump from the lower energy level in the high-temperature region requires a higher activation energy. The precise mechanism for electric conduction in Na-doped MgO is not yet clear, and other electrical conductivity measurements need to be made. By comparing the results in fig. 3(a) and 4 the electrical conductivity appears to correlate with the Gel of one-electron donor centres.On the other hand, the relationship between basicity and sodium content goes through a maximum at 0.87 mmol sodiumll g&, which is unlike the result of fig. 3(b). This shows that the electrons in one-electron donor centres will contribute to the electrical conduction. The electrical conductivity of the sample doped with NaNH, in pre-irradiated MgO was not measured in this study since it was difficult to prepare the sample without exposing pre-irradiated MgO to air. This sample will later be examined to study the effect of moisture in the air on the conductivity.31 14 Na Doping of y-Irradiated MgO We thank the Japanese Ministry of Education, Science and Culture for financial support in the form of a Grant in Aid for Special Research, and the Kohnoshima Chemical Co.Ltd for donating the magnesia samples. References 1 R. L. Nelson, A. J. Tench and B. J. Harmsworth, Trans. Faraday SOC., 1967, 63, 1427. 2 C. Naccache and M. Che, Proc. Fifth Int. Congr. Catal. (Amsterdam, 1972), 105-1. 3 J. H. Lunsford, J. Colloid Interface Sci., 1962, 26, 355. 4 J. H. Lunsford and T. W. Leland, J. Phys. Chem., 1968, 66, 2591. 5 J. H. Lunsford, J. Phys. Chem., 1964, 68, 2312. 6 J. Kijenski and S. Malinowski, Bull. Acad. Polon Sci., Ser., Sci. Chim., 1977, 25, 329; 501. 7 A. J. Tench and R. L. Nelson, Trans. Faraday SOC., 1967, 63, 2254. 8 D. Cordishini, V. Indovina and A. Cimino, J. Chem. Soc., Faraday Trans. I, 1974, 70, 2189. 9 S. Wysoki, J. Chem. SOC., Faraday Trans. 1 , 1986, 82, 715. 10 K. Tanabe, Shokubai no Hataraki (Kagakudohjin Pub. Co., Japan, 1974), p. 63. 11 T. Matsuda, Z. Minami, Y. Shibata, S. Nagano, H. Miura and K. Sugiyama, J. Chem. Soc., Faraday 12 S. Coluccia, A. Barton and A. J. Tench, J. Chem. Soc., Faraday Trans, I , 1981, 77, 2203. 13 T. Iizuka and K. Tanabe, Bull. Chem. SOC. Jpn, 1975, 48, 2527. 14 T. Iizuka, Chem. Lett., 1973, 891. 15 D. Cordischi, V. Indovina and M. Occhinzzi, J. Chem. Soc., Faraday Trans. 1, 1978, 74, 456. 16 E. Garrone, A. Zecchina and F. S. Stone, J. Catal., 1980, 62, 396. 17 W. Wysocki and H. Sugier, Radiochem. Radioanal. Lett., 1975, 20, 191. Trans. I, 1986, 82, 1357. Paper 611751; Received 29th August, 1986