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Materials Rssearcb Bullek Vol. 30. No. 9. pp. 11614171.1995 cqqTi&t 0 1995 Blscvim soia Ltd F’rintcd in the USA All rights emved

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PREPARATION OF NEW ORGANIC-INORGANIC NANOCOMPOSITE BY INTERCALATION OF ORGANIC COMPOUNDS INTO MOO,

BY ULTRASOUND

Hideyuki Tagaya*, Kazumasa Takeshi, Kenusuke Ant, Jun-ichi Kadokawa, Masa Karasu and Koji Chiba

Department of Materials Science and Engineering Yamagata University, Yonezawa, Yamagata 992, Japan

(Received February 3,1995; Refereed)

ABSTR4CT 4-Substituted pyridines were reacted with the layered host lattice MOO, using ultrasonic wave irradiation. XRD patterns and TG spectra showed the formation of new organic-inorganic nanocomposites. XPS spectra showed that no reduction of Mo metal occurred through the reaction. These results and IR spectra indicated that pyridimium ion interacted with oxygen of MOO, by hydrogen bonding.

MATERIALS INDEX: molybdenum oxide, pyridine, molybdic acid.

ODUCTION

The interclllation of orgauic molecules into layered inorganic solids has attracted considerable interest in recent years because such organic intercalates are organic-inorganic nano- composites and candidates for electronic devices and heterogeneous catalysts (l-4). Among inorganic host materials, oxides are expected to form the stable intercalation compounds. Layered MOO, is a member of such a host; however, MOO, is actually not very reactive and syntheses are often lengthy (weeks) and require elevated temperatures. It is believed that electron transfer from guest species to MOO, is the first step of the intercalation reaction. We have found that organic compounds such as alkylpyridines, methylviologen and azo

l Author tc whom all correspondence should be addressed.

1161

1162 H. TAGAYA et al Vol. 30, No. 9

Y 0

Guest size (A) (Pm

1. R=H 3.96 (8.75) 2. R=CH3 4.86 (7.92) 3. R=CH2CH,CH3 6.60 (7.86) 4. R=CaH5 6.73 (8.69) 5. R=CH&& 7.27 (8.22) 6. R=COOH 5.30 (8.45)

PIG. 1. Compounds used in this study.

compounds are intercalated easily into MOO, layers by an ion exchange in which pre-reduced MoO,“(Na+), is reacted with organic compounds. Reductions of Mo atom by the chemical reduction and intercalation of amines were confirmed by X-ray photoelectron spectroscopy (XPS) (5). We have also attained electrochemical intercalation of organic compounds into the MOO, layers (6).

The effectiveness of ultrasonic wave power for intercalation has been reported (7-9). Intercalation of organic compounds into inorganic hosts is a heterogeneous reaction. Acceleration of heterogeneous reactions by ultrasonic wave irradiation is well-documented (10). It has been suggested that ultrasonic wave irradiation does not increase intercalation rates through improvement of mass transport, and the high temperature rise associated with the solution is primarily responsible for the rate enhancement.

In this work, we used ultrasonic wave irradiation to study the formation and characteristics of organic intercalation compounds into MOO, nanocomposites.

The organic solvents used were of analytical grade. If necessary, they were dried and fractionated prior to use. Molybdic acid was synthesized from ammonium molybdate (11). All other materials used were from commercial suppliers and used as received. Pyridine, 4-methylpyridine, 4-propylpyridine, 4-phenylpyridine, 4-benzylpyridine and 4-carboxy- pyridine (compounds l-6) were used as the reactants with MOO,. They are weak bases, with pKb values from 7.92 to 8.75.

. . . of4-sub-. Direct thermal intercalations into MOO, were

carried out at 80” C for 2-24 h under a dry nitrogen atmosphere. Intercalation of organic compounds into MOO, by ultrasound was carried out by irradiating with ultraaoni~ wave (3OOW, 2OKHz) at room temperature for 5-l 50 h. Reaction products were filtered and washed with acetone.

on of the rew nroductg. Powder X-ray di%kaction spectra were recorded

Vol. 30, No. 9 MOLYBlBNUMOXIDENANOCOMPOS~ 1163

on a Rigalcu powder diffractometer unit using CuK a (filtered) radiation at 40KV and 2OmA. XPS spectra were collected on a Shimazu ESCA, using a monochromatic Mg K a X-ray source. ,XPS spectra of the intercalation compounds were taken after etching by Argon ion beam. Themial analyses (TGiDTA) were done on a SEIKO SSCSOOO thermal analysis system, using a heating rate of 10” Wmin. IR measurements were performed using Horiba FTIR.

DISCUSSIOl’l

. . R action of sdwtituted qadws with MOO, m. It was already reported that 30 dlys at 160- 180 o C was necessary to ensure complete reaction of pyridine with MOO, ( 12). Certainly no reaction products were obtained by the reactions of 4-methylpyridine and 4-propylpyridine with Moos at room temperature for up to 72 h. However, after irradiating

6.9 A

10 20

Two theta (-)

FIG. 2. XRD patterns of(a) Moo, and the reaction products of(b) pyridine, (c) 4-methylpyridiue, aud (d) 4-propyIIr@iine with MOO,.

1164 H. TAGAYA et al Vol. 30, No. 9

the mixture of substituted pyridines with ultrasonic wave, swelling of MOO, was observed, except for 4-phenylpyridine.

. XRD patterns of MOO, and the reaction products are shown in Fig. 2. Interlayer spacing of Moo, is 6.9 A. Spectra of the reaction products depended on the kind of substituted pyridines. Spectra of the reaction products of 4methylpyridine and 4-propylpyridine were diierent from those of intercalation compounds by ion exchange reactions (S), but the patterns were similar to those of the organic intercalates from the layered solid MoO,oOH, (13). By using irradiating ultrasonic waves, swelbng of reactauts was observed as described above. The swelling resulted in insufficient irradiation to part of the reactants. As shown in Fig. 2 (b), (c) and (d), we could not avoid the presence of unreacted MOO,, probably because of insuEcient irradiation to the reactants caused by the swelling.

Thermal analyses of Moo,, the reaction product of 4-methylpyridine with MOO, for 72 h, and 4-methylpyridine alone are shown in Fig. 3. The weight of MOO, did not change up to 600 o C, although the weight due to 4-methylpyridine was lost completely by 125 o C. The weight loss of the reaction product of 4-methylpyridine with MOO, continued from 120” C to 600“ C and finally reached 34.5%. For the elemental analysis of the reaction product, hydrogen content was larger than expected, which was calculated from the nitrogen content. In the case of the thermal pyridine intercalation into TaS,, proton formation by oxidation of pyridine into bipyridine has been reported (14). Bipyridine formation, however, was not confirmed in this reaction by ultrasound. Therefore, we considered the presence of water in the reaction products. The composition was calculated as MOO,.

(4-methylpyridine)o.5,~(l&O)o.i, from the elemental analysis. The maximum weight loss corresponded to the desorption of 4-methylpyridine and water was expected as 28.0% from the composition.

0

Temperature (“c)

FIG. 3. Thermal analyses of (a) MOO,, (b) the reaction product of 4methylpyridine, and (c) 4-methylpyridine.

Vol. 30, No. 9 MOLYBDENUMOXIDENANOCOWOSITEB 1165

grtl=- tw&ment of the reaction product of 4-methylpyridine with Moo, to 150 o C, XRD pattern changed slightly, as shown in Fig. 4(a). The color of the powder, however, turned from white to black by the thermal treatment up to 600” C, indicating a change of valence state ofMo metal in MOO,. A new strong peak was observed in the XRD pattern, as shown in Fig. 4(b). The peak agreed with that of Moo,. Removal of the one oxygen from MOO, in the reaction product of 4-methylpyridine corresponded to the weight loss of 8%. As shown above, weight loss up to 600” C of the 4-methylpyridine product reached to 34S%The results indicated that the difference between 34.5% and 28.0% corresponded to the &sorption of oxygen in MOO,.

Weight losses up to 600” C of the reaction products of pyridine, 4-propylpyridine, 4-benzylpyridine and 4-carboqpyridine reached 27.5,42.5,60.0 and 34.0%, respectively, as shown /in Table 1.

p. It was reported that MOO, showed MO-O-MO bridging bands at 520- 590 cm-‘. Bands at 980 cm-’ and 865 cm-’ were assigned to MO-krminal oxygen bauds (15). The lR spectrum of the reaction product of 4-methylpyridine with MOO, indicated

10.4 A 1

3.5 A

10 20

Two theta (-)

FIG. 4. XRD patterns of (a) the reaction product after baking at 150” C of 4methylpyridine with MOO,, (b)bakingat6OO”C,and(c)productsatterbakingat6OO0Cinaoxygenflow.

1166 H. TAGAYA et al Vol. 30, No. 9

TABLE 1

Characterization of the reaction of products of substituted pyridines with MOO,

Subsituted pyridine

wt loss . . Comnositiop * IR spectra XPS to 600°C x Y (cm-l) (eV)

Moo3 0.0 980,865,520-590 238.0

233.3

Pyridine 27.5 0.56 0.20 939,906,846,713,675,607 238.4

1448,1487,1535,1637 235.3

4-methylpyridine 34.5 0.57 0.17 933,850,715,663,605 238.0

1381,1441,15l2,1635 234.7

4-propylpyrid.ine 42.5 0.38 0.11 937,906,816,713 238.4

1390,1456,1508,1637 234.8

4-benzylpyridine 60.0

4-carboxypyridine 34.0

* x, y in MoOs+ubstitutod pyridine),*(HsO),

that v (Mo-terminal oxygen) bands shifted from 865,980 to 933-850 cm-’ and v (MO-O-MO) bridging bands shifted from 520-590 to 715-605 cm-’ (Fig. 5). Such shifts suggested the presence of co-ordination to oxygen of Moo,. Similar results were also observed for other reaction products, as shown in Table 1.

The absorption bands in the 1400-1700 cm-’ region showed the state of substituted pyridines (I 6). Bands at 1562 cm-’ and 14 13 cm-’ disappeared by the reaction with MOO, as shown in Fig. 6(c). By the reaction, a strong new band appeared at 15 12 cm“. The same peak was observed in lR spectrum of protonated 4-methylpyridine by hydrochloric acid, as shown in Fig. 6(b). The same results were obtained in the other substituted pyridines.

Valence state of MO metal in MOO, was measured using Xl%. In the case of allqlamine inter&ate, the peaks fairly shifted to reduction state (5), 234.0 and 230.8 eV in the case of thermal intercalation of tetradecylamine. However, no shifts were observed in the case of substituted pyridines as shown in Table 1. This suggests that the products obtained by the reactions of MOO, by ultrasound in this study were different from those of intercalation by chemical and electrochemical reduction. These results suggest that the substituted pyridinium ions were co-ordinated to MOO, oxygen as shown in Fig. 7.

. . . of 4--. XRD spectra of the substituted pyridine

reaction products with MOO, by ultrasound were similar to those of the organic intercalates li-om the layered solid MoO,mO&. Elemental analysis and IR spectra of the reaction products indicated the presence of proton.

To clarify the reaction mechanism, molybdic acid was prepared and reacted with 4-methylpyridine. It is well-known that MOO, structure is constructed by the dehydration of molybdic acid as shown in Fig. 8 (17). XRD pattern of the MoO,oOHr is shown in Fig. 9(a).

Vol. 30. No. 9 MOLYBDENUMOXlDENANOCOh@OSITES 1167

1

865 520490

Wavcnumbfx (Cd) FIG. 5.

500

IR spectra of(a) kne&ylpyri& (IQ MJA& and(c) the reaction product of 4-methylpyridine with MOO,.

The pattern of the reaction product of 4-methylpyridine with MoO,oOH, was similar to that with MOO, by ultrasound, as shown in Fig. 9 (b) and (c).

The results suggested the participation of water in the reaction. The reaction was carried out by using dehydrated reactants. However, we could not deny the presence of a very small amount ofwater in the reaction. No reaction of 4-methylpyridine with MOO, occurred in the conditions of strictly anhydrous reactants for 72 h. The 4-substituted pyridines might react with MOO, through molybdic with the aid of ultrasonic wave irradiation.

1168 H. TAGAYA et al Vol. 30, No. 9

1500

Wavenumber (cd)

FIG. 6.

1000

IR spectra of (a) 4-mct&ylpykhe, (b) protonated 4-methylpyridine with HCl, and (c) the xaction product of 4-methylpyridine with MOO,.

CON-

New organic-inorganic nanocomposites were prepared by the reaction of the 4-substituted pyridines with MOO,, using ultrasonic wave irradiation. The structures of the products indicated that the 4-substituted pyridinium ions were coordinated to MOO, oxygen. Ultrasonic

Vol. 30, No. 9 MOLYBDENUM OXIDE NANOCOMPOSITES 1169

b

C IL a

0 0 6 0 I/ I/ I/ I/

yyo-o~h;lo-o /MO-O /MO-O

0 0 A A FIG. 7.

Propod ,&udure of the reaction product of 4-substituted pyridines with MOO, by ultrasound.

wave irradiation is essential for these reactions. In the preparation of the nanocomposite with MOO,, ultrasonic wave irradiation was highly effective.

The authors express their thanks to H. Morioka for his technical support.

Dehydration

MOOJ l OH2 MOO3

FIG. 8. Dehydration reaction of MoO,.O& to MOO,.

1170 H. TAGAYA et al Vol. 30, No. 9

9.4 A

10 20

Two theta (-)

FIG. 9. XRD p&tans of(a) h4dpO& (b) the reaction pduct of 4-methylpydine with MoOp~ ad (c)thereactionproductof4-metfrylpyridinewithMoo,byul~

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Vol. 30, No. 9 MOLYBDENUMOXIDENANOCOMPOSITES 1171

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