Poeitron annihilation in Cd+* ion-exchanged and CdS loaded zeolite Y Yuan-Jin He, Yong Hu Department of Physics; Tsinghua University,Beijing 1OOO84, China The lifetime of positron annihilation in Nay, Na(Cd+a)Y, Na(CdS)Y can be decomposed to four components, the third component (2-4ns) ia contributed by Ps trapped in B-cages, while the fourth component (the longest one ) ia an average lifetime of Ps in supercages and more open spaces between zeolite grain particles. By magnetic and 0, quenching one can obtain the Ps lifetime values for B-cage and supercage in NaY zeolite structures, namely 2.71~and 18ns respectively, which are reasonably close to what expected by 'free volume model'.
1.INTRODUCTION Increasing activities have been aimed at the development of intrazeolite semiconductor quantum dots materials [I1. In developing a reproducible way to load semiconductor dots into the accessible void spaces of zeolite host lattices, the direct information of the occupancy of different cages in a zeolite is important. As one of sensitive microprobe of voids on nanometer scale, positron annihilation has been used to characterize zeolite-based materialsC2-e1. In zeolite materials positrons can be slowed down internally and form para-or otho-positronium (p-Ps,o-Ps). The lifetime of self-annihilated o-Ps is very long in vacuum (*134ns), but in zeolite cages, it will be shortened by pick-off annihilation which ia an annihilation process of the the positron in o-Ps with a surrounding electron. In principle, free volume of each kind of cages could be well correlated to Ps lifetime and the changes of intensities of corresponding lifetime
components could give the information about the extent of semiconductor dots loading. In previous works two longest lifetime components ( T T *) have been observed in zeolite Y, but it is still arguable whether T a or T is responsible to Ps annihilation i n 0 cage or supercage. In this paper we report on the results of positron annihilation in ion-exchanged zeolite Y and CdS loaded zeolite Y. We found that T a is responsible to Ps annililation in 0-cage and positron annihilation may be a very interesting technique to characterize intrazeolite materiala. 2. EXPERIMENTAL
Zeolite NaY powder was employed as the starting material for preparing Na(Cd+2)Y and Na(CdS)Y. Cd+2 impregnating was carried out in Cd(N08), solutions. The NaY powders were soaked i n Cd(N08), solution of different concentrations. The pH values of the solutions were adjusted to 6. The concentrations were selected i n that way so that the ratio of the total amounts of Cd(N08), in the solutions over the weights of NaY zeolite powders are 0.07, 0.21, and 0.48 mol/Kg respectively. After being pre-heated, the powders were sintered at 4OOC in O1 for 12 hours. In order to load CdS molecules into the zeolite cages, gaseous H2S was used. The extent of CdS loading can be controlled by the pressure and flow rate of HIS gas. In this work HIS gas was obtained by reaction of FeS with dilute HC1 solution. The loading process lasted about 2 hours, the colours of the zeolites changed from white at the begining to yellow progressively. Positron lifetimes were measured in a fast timing system with a time resolution of -3OOps. In order to determine the lifetime values with a reasonable accuracy, two time windows were chosen: 0.4nslchannel for T *, the longest lifetime, and O.lns/channel for T T and T a. The longest lifetime was obtained by 3-component free fitting of the lifetime spectra taken with O.4ns/channel time window and then was inputed as the fixed T~ component in 4-component fitting of the spectra taken with O.lns/channel time window.
3. RESULTS AND DISCUSSION
In general, 4 components could be resolved from the lifetime spectra of positron annihilation in intrazeolite materials used in this study. Typically T is about 0.2118, which is mainly due to p-Ps annihilation, T is about 0.66-0.76ns, which is due to the annihilation of positrons bounded to the zeolite grain surface, T a (3-4118) and T (16-70ns) are both due to o-Ps pick-off annihilation, this is convinced by magnetic quenching and oxygen quenching experiments of this study. 3.1 Magnetic quenching In order to confirm the nature of the long lifetimes, positron annihilation lifetime was measured on Nay, Na(Cd+)Y, and Na(CdS)Y samples under different intensity of magnetic field. The p-Ps state is a singlet state I O,O> with m=O, where m is the magnetic quantum number, while o-Ps state has three components I l,l>,I 1,0>, and 11, -1> (m=1, 0,-1) respectively. Under finite magnetic field I O,O> and 11, O> are mixed and the lifetime of I1,0> o-Ps state is decreased to 1 X ’ *lo+x2 X p 4 where X o( X ), is the o-Ps (p-Ps)annihilation rate at zero-field, and x is the magnetic field intensity devided by the internal magnetic field of Ps. In zeolites, the o-Ps lifetime will be decreased further via pick-off annihilation even at zero-field. In fact there was no significant magnetic effect on 1 a, since 1 8 is dominated by a relatively high pick-off annihilation rate and the lifetime decreasing hy magnetic quenching was not pronounced. But for T the magnetic quenching effect is very clear (see Fig.1). Among the three components of o-Ps, only 11, O> state will be quenched by magnetic field, while positrons annihilated from other two thirds will not affected by magnetic field at all. Therefore in analysing data we took T and 214/3 of the longest lifetime of the zero-field spectrum as the fixed parameters, and looked the variation of the so obtained second longest lifetime T 4’ with magenetic fields. When pick-off rate is small, T 4’ can be roughly viewed as I 1, O> o-Pe lifetime .
Fig.1 The lifetime T 4' (see text) ae a function of magnetic field for Nay( 0 ),Na(Cd+2)Y (0.2lmol/Kg) ( 0 ), and Na(CdS)Y (O.almol/kg)(A).
In Fig. 1 one can see that the lifetime of I1,0> state of o-Pa can be magnetically quenched. The different 1 4' values at zero-field for Nay, Na(Cd+)Y, and Na(CdS)Y samples indicate that positronium pick-off annihilation rate is different in different samples. These data may relate to the size of free volume in the sample and also to the local chemical environment where Ps annihilated. The T ~ ' lifetime decreased very rapidly at low field, and saturated at about 1OkG. The ultimate lifetime at strong magnetic quenching seems around 6ns. This may explain why magnetic quenching on 1 8 (3-4ne) was not observable.
3.2 Oxygen quenching Fig.2 and Fig.3 show the Os quenching effect observed in this work. Both 1 a and 1 have undergone big changes upon introducing oxygen since oxygen has unpaired electron which leads the positron in o-Ps annihilate with the electron of opposite spin
v H m
Fig. 2 Variation of lifetime and intensity of 3rd component vs. concentration of ion-exchanged (a) and of CdS loaded (b) Zeolite Y: (.)lifetime in 0,; (U)lifetime in vacuum; (A)intensity in vacuum; (A)intensity in 0,.
I 0 2
I 0 3
Fig. 3 Variation of lifetime and intensity of 4th component VB.concentration of ion-exchanged (a) and of CdS loaded (b) Zeolite Y: (0)lifetime in vacuum; (.)lifetime in 0%; (A)intemity in vacuum; (A)intensity in O1.
According to free volume model'c71, the Pa lifetime has a linear relation to the cavity volume as : s -7.80V +1.29 (ns) (1) where V is the free volume in unit of . For NaY zeolite the volume of 8-cage is 0.16(nm)s, then the lifetime of Pa located in B -cage should be 2.611s according to equation (l), this value is very close to the T a value observed in Nay, 2.7ns. Therefore it is reasonable to contribute T a to positronium annihilation in B -cage. From Fig.2 one can see that 1 a increases with the concentrations of Cd+2 ions. This effect could simply explained by the increasing of the volume of B-cage when the two Na' ions were replaced by one Cd+2 ion. A rough estimation of the volume of B-cage of ion -exchanged Y zeolite could be made according to equation (1) and But this point needs to be our data, the 'value is 0.31 clarified by more examination since other possibilities, for instance, the uncertainty of data manipulation, may also have to be put into account. The lifetime of Pa in supercages is expected to be 14ns according to free volume model (eq.(l)), but what we observed in NaY i n is an average value of the vaccum is -70ns, implies that T lifetime of Ps in supercages, and Ps in some more open space, e.g., the free volume between small zeolite grain particlea. Interesting fact is that,the Pa lifetime when quenched with O2 becomes -18ns for NaY zeolite and Na(Cd+2)Y samples. From this fact one would suggest that O2 quenching is very effective to Pa located in open space between zeolite particles and is not effective to Ps trapped in supercages, and that the lifetime of Ps trapped i n supercage is about 18ns, which is reasonably close to the prediction of free volume model'. The big difference between the results for Na(Cd+=)Y and for and L for Na(CdS) Y samples is the additional decreasing of T latter case with CdS concentration increasing (see Fig.3). This can easily be explained by suggesting that CdS molecules are more effective than Cd+z in bloking the free volume in supercages and the open space between particles so as to reducing positronium
formation. In conclusion, all these results indicate that positron annihilation will be a very promising microprobe to characterize intrazeolite materials.
REFERENCES [l] G.A.Ozin, Adv. Matter. 4 (1992) 612.  K. Venkateswaran, K. L. Cheng and Y. C. Jean, in Positron Annihilation (World scientific Publ.Co., Singapore) (1986), P.199.  M. Debowska, J. Ch. Abbe and G. Duplatre, Phys. Stat. Sol.(b) 146 (1988) 91.  Y. Ito, T. Takano and M. Hasegawa,Appl. Phys.,A46 (1988) 193.  Y. Ito, M. Hirose and Y. Tabata, Appl. Phys., As0 (1990) 39.  2. Gao, X. Yang, J. Cui, Y. Wang, Zeolites, 11 (1991) 607.  Y. C. Jean, T. C. Sandreczki and D. P. Ames, J. Polym. Sci B, (1986) 1247.
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