Spectral Characteristics and Surface Morphology of Organic Polymer Films Containing Vanadium Pentoxide Nanoparticles

Russian Journal of Electrochemistry, Vol. 40, No. 3, 2004, pp. 259–266. Translated from Elektrokhimiya, Vol. 40, No. 3, 2004, pp. 294–302. Original Russian Text Copyright © 2004 by Svechnikov, Pokhodenko, Guba, Fenenko, Smertenko, Prokopenko, Grebinskaya, P. Lytvyn, Piryatinskii, O. Lytvyn, Ol’khovik.

Spectral Characteristics and Surface Morphology of Organic Polymer Films Containing Vanadium Pentoxide Nanoparticles S. V. Svechnikova, V. D. Pokhodenkob, N. F. Gubab,z, L. I. Fenenkoa, P. S. Smertenkoa, I. V. Prokopenkoa, L. N. Grebinskayab, P. M. Lytvyna, Yu. P. Piryatinskiic, O. P. Lytvyna, and G. P. Ol’khovika a Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, pr. Nauki 45, Kiev, 03028 Ukraine b Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, pr. Nauki 31, Kiev, 03039 Ukraine c Physics Institute, National Academy of Sciences of Ukraine, pr. Nauki 46, Kiev, 03050 Ukraine

Received February 26, 2003

Abstract—Films of nanosized composites of poly(N-epoxypropylcarbazole) (pEPC) and poly(3,6-di-Br-Nepoxypropylcarbazole) (pdBEPC) with vanadium pentoxide (V2O5) are produced for the first time ever. The electroconduction in dark and when illuminated, absorption spectra, steady-state photoluminescence spectra, and time-resolved photoluminescence spectra for the composite films are studied. The results are compared with relevant data for the pEPC, pdBEPC, and V2O5films. A conclusion about the formation of donor–acceptor –σ

–σ

complexes with incomplete charge transfer [pEPC+σ···V2 O 5 ] and [pdBEPC+σ···V2 O 5 ] is made. The surface morphology of the composites differs in the size of V2O5 fibers and polymer grains; the polymer inclusions in the composites are of different character. A surface morphology study reveals that the composite constituents— polymer base and V2O5 fibers—are nanosized. Key words: polymer film, poly(N-epoxypropylcarbazole), vanadium pentoxide, composite, nanosized particles, surface morphology, charge-transfer complexes, photoluminescence

INTRODUCTION Of late, the organic polymers exhibiting metallic and semiconductor conduction and composites on their basis with nanosized oxides of transition metals (V2O5, TiO2, Fe3O4, WO3, MoO3) have attracted an ever closer attention of the researchers [1–5]. The combination of good film formation properties with the simplicity of the technology for manufacturing films and structures that would have the desired physicochemical, spectral, electrophysical, and other characteristics, taken together with the possibility of controlling these characteristics by varying the polymer structure, the film composition, or the technological conditions are obvious advantages of such materials. Inserting nanosized oxides of transition metals into polymers substantially alters certain physicochemical characteristics of composites, thus allowing one to achieve new levels of functional application of composites [1–5]. Electronconducting polymers and composites based on these are promising materials for electronics and optoelectronics (diodes, transistors, light-emitting diodes, converters of the energy of light into electric energy, and so on). They may also be utilized as working materials of chemical power sources and light-sensitive materials for information recording. The carbazole-containing z Corresponding

author, e-mail: [email protected]

materials belong precisely with this class of polymers [6–11]. This work is devoted to obtaining thin films of nanosized composites of poly(N-epoxypropylcarbazole) (pEPC) and poly(3,6-di-Br-N-epoxypropylcarbazole) (pdBEPC) with vanadium pentoxide (V2O5) and studying their spectral characteristics and the surface morphology. EXPERIMENTAL The samples were prepared from solutions of pEPC (I) and pdBEPC (II) in acetone and a water sol of V2O5, which was obtained using the Bilts method: H2C CH O

H2C CH O

n

CH2

CH2

N

N Br

I

n

Br II

To synthesize the composites, the V2O5 sol was added into the pEPC (pdBEPC) solution at a 1 : 1 volume ratio, which corresponded to a V2O5 content of 33.82 (24.80) wp % in the composite. The films were formed as “sandwich” structures, by pouring the pEPC + V2O5 (pdBEPC + V2O5) solution onto a glass substrate

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Table 1. Electroconductivity of films in dark and when illuminated Electroconductivity, S/cm Film pEPC pEPC + V2O5(33.82 wp %) 3,6-pdBEPC pdBEPC + V2O5(24.80 wp %)

in dark

when illuminated

1.2 × 10–10 2.1 × 10–6 3.7 × 10–11 5.1 × 10–6

2.9 × 10–8 1.1 × 10–5 3.7 × 10–7 5.6 × 10–6

covered with a layer of sputtered ITO. The latter is an electron-conducting, transparent in the visible range, coating based on a mixture of SnO2 and In2O3 (up to 10%). The ITO layer was ~0.15 µm thick and its resistance was 100 ohm/䊐. When dried in air at room temperature, the composite films were 0.48–1.08 µm thick. Polymer films of composites PVS OH –

– ( –CH 2 –CH– )– n , (III)

containing hydrolysis groups with nanosized V2O5 were prepared as follows. The V2O5 sol was added into a PVS solution so that the V2O5 content in the film be 10 wp %. The films were formed as “sandwich” structures, by pouring the PVS + V2O5 composite solution onto a glass substrate covered with a layer of sputtered ITO ~0.15 µm thick and a resistance of 100 ohm/䊐. When dried in a vacuum at room temperature, the composite films were 1–2 µm thick. For the second contact when measuring electroconduction of the films we used indium ~1 mm thick with a working area of 1 mm2. The electroconduction of films of pEPC and pdBEPC and their composites with V2O5 was measured in dark and when illuminating samples by nonfiltered light from a source of type A with the color temperature 2850°ë. Absorption spectra for the films were obtained on an SF-20M spectrophotometer. The steady-state photoluminescence spectra (SSPS) and the time-resolved photoluminescence spectra (TRPS) were obtained on an MDR-12 monochromator with a photoelectric adapter. To record TRPS we used a stroboscopic system with a time window of 0.1 ns. To raise the TRPS intensity at the initial instant of their excitation, the spectra were recorded at the leading edge of the laser pulse. As the pulse was very steep, the excitation time of samples was equal to 0.7 ns at td = –4 ns, where td is the delay time relative to the maximum of a laser pulse. As the luminescence “flames up” throughout a laser pulse, its maximum lags behind the pulse, while the delay time is counted from the maximum, and negative values of td correspond to a measurement taken in the leading (steep) edge of the luminescence front. The photoluminescence was excited by radiation of a nitrogen laser

with a wavelength of 337.1 nm. The films' morphology was studied using a Model (AFM)-Nanoscope IIIa atomic force microscope (Digital Instruments). RESULTS AND DISCUSSION As follows from the data compiled in Table 1, the electroconductivity of films of pEPC + V2O5 and pdBEPC + V2O5 composites both in dark and when illuminated is higher by several orders of magnitude than that of the pEPC and pdBEPC films containing no nanosized V2O5. This increase is presumably due to the formation of donor–acceptor complexes with incomplete charge transfer. We also examined absorption spectra for films of V2O5, pEPC, and pdBEPC and their composites pEPC + V2O5 and pdBEPC + V2O5 deposited on glass substrates covered with an ITO layer in the range 300 to 800 nm at 300 K (Fig. 1). The absorption spectrum for the V2O5 film is a broad band at 300–525 nm, whose optical density practically vanishes at 400–525 nm and is characterized by a maximum at 308 nm and a blurred maximum at 375 nm. The absorption band for the pEPC film exhibits a clearcut maximum at 332 nm. The absorption band for the pEPC + V2O5 film displays at 332 nm a new broad absorption band of high intensity with a well pronounced maximum at 375 nm, with optical density D = 1.80 at 375 nm. Note that the optical density of the absorption band for the V2O5 and pEPC films at 375 nm equals 0.67. Comparing absorption bands for films of V2O5, pEPC, and pEPC + V2O5 and optical densities of these films at a certain wavelength suggests that mixing solutions of pEPC and V2O5 makes V2O5 interact with the carbazole fragments in the polymer chain of pEPC, leading to the formation of a donor–acceptor complex –σ with incomplete charge transfer [pEPC+σ···V2 O 5 ], in which V2O5 acts as the electron acceptor. A confirmation of this assumption is that, with increasing V2O5 content in the pEPC + V2O5 composite, the intensity of the absorption band for the composite at 375 nm first increases and then decreases. Yet another confirmation is supplied by the data on the electroconduction of pEPC and pEPC + V2O5 films in dark and when illuminated (Table 1). Figure 1b shows absorption spectra for the V2O5, pdBEPC, and pdBEPC + V2O5 films. The band for pdBEPC has a maximum at 353 nm with D = 0.60. Inserting V2O5 into the polymer gives rise to a wide intensive absorption band with λmax = 384 nm and D384 = 1.59, which points to the formation of complex –σ

[pdBEPC+σ···V2 O 5 ], just as is the case with unsubstituted pEPC. To confirm the formation of donor–acceptor com–σ –σ plexes [pEPC+σ···V2 O 5 ] and [pdBEPC+σ···V2 O 5 ], we examined SSPS and TRPS for the films (Figs. 2, 3). We

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established that films of V2O5 on a glass substrate covered by an ITO layer emit no photoluminescence at 300 K. The SSPS for the pEPC film exhibit a narrow band at λmax = 319 nm and a wide nonstructured bans at 337–725 nm with a maximum at 450 nm (Fig. 2, curve 1). The SSPS for the pEPC + V2O5 film differs from that for pure pEPC, displaying a pronounced red shift (Fig. 2, curve 2). The SSPS for the pEPC + V2O5 film exhibits an intensive band at λmax = 525 nm with two barely pronounced maximums at 328 and 412 nm. That the V2O5 film emits no photoluminescence and the band of photoluminescence emitted by the pEPC + V2O5 film is displaced as compared with that for pEPC points to the formation of a new compound upon mixing solutions of pEPC and V2O5. Presumably, this is

D 2.0

3

2 1.0 1

0

Figure 2 shows that TRPS (td = –5 ns) for the pEPC + V2O5 film exhibit a marginal shift of maximums in the absorption bands into the ultraviolet (UV) range (curve 3) and their intensity rises as compared with the SSPS for this film (curve 2). As a result, the TRPS band is broader that SSPS. The maximums in the bands occur at 518, 459–475 (shoulder), 403, and 328 nm. The TRPS (td = –5 ns) for the pEPC + V2O5 film reveal bands with maximums 459–475 (shoulder), 403, and 328 nm, which are more distinct than those at td = 7 ns (curve 4) and those in SSPS (curve 2). We attribute such bands to complexes of oligomers of pEPC I (with n < 5) with V2O5. We established the presence of these oligomers in pEPC by methods of liquid chromatography and field mass spectroscopy. Figure 3 presents SSPS and TRPS for the pdBEPC and pdBEPC + V2O5 films. The pdBEPC film is characterized by a band with λmax = 405 nm (curve 1), whereas the photoluminescence spectrum for pdBEPC + V2O5 exhibits two bands with λmax of 400 and 517 nm and a plateau at 450–485 nm (curve 2). We refer the band with λmax = 400 nm to pdBEPC and that with λmax = 517 nm, to pdBEPC + V2O5. The band of TRPS (td = −4 ns) for pdBEPC + V2O5 (curve 3) shifts into the UV range relative to the photoluminescence bands for both pdBEPC and pdBEPC + V2O5, and λmax = 394 nm. At td = –4 ns, the pdBEPC + V2O5 film emits no photoluminescence at 500–550 nm. At td = 10 ns, the pdBEPC + V2O5 spectrum is markedly different (curve 4) and displays three bands. The band with λmax = 497 nm –σ

belongs to complex [pdBEPC+σ···V2 O 5 ]. The band with λmax = 466 nm we refer to complexes of oligomers of pdBEPC (where n < 5) with V2O5. And the band with λmax = 394 nm belongs to pdBEPC. At td = 10 ns, the bands with λmax of 497 and 394 nm are displaced into RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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400

300

–σ

donor–acceptor complex [pEPC+σ···V2 O 5 ]. This assumption is confirmed by the data on the electroconduction in dark and when illuminated and by the absorption and photoluminescence spectra for these films.

261

500

600

700

5 1.5

1.0 1 4

0.5

0

300

400

500

600

700

800 λ, nm

Fig. 1. Absorption spectra for (1) V2O5, (2) pEPC, (3) pEPC + V2O5 (33.82 wp %), (4) pdBEPC, and (5) pdBEPC + V2O5 (24.80 wp %); 300 K, comparison—glass substrate with ITO layer.

the UV range relative to the bands observed in SSPS for pdBEPC + V2O5 (curves 4, 2). As seen in Fig. 3, the photoluminescence bands for films of pdBEPC + V2O5 at 400 nm are substantially more intensive than the intensity for pdBEPC, i.e. V2O5 enhances the photoluminescence in composites. That the intensity of bands at 400 nm increases and they shift into UV range we attribute to the formation of complexes of pdBEPC with V2O5 (with a lower degree of charge transfer), which are weaker than the complexes that are responsible for the photoluminescence at 450−700 nm. In stronger donor–acceptor complexes of pdBEPC with V2O5, vanadium pentoxide probably interacts with two neighboring carbazole fragments of the polymer chain, while in weaker complexes (with a smaller degree of charge transfer) V2O5 presumably interacts with one carbazole fragment. There is a chance that the formation of such complexes might reduce the deactivating action of heavy bromine atoms on the photoluminescence of composites at 400 nm. That the intensity of photoluminescence bands for pdBEPC and its composite with V2O5 is two orders of No. 3

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3 0.5 1 4 2 0

400

500

700 λ, nm

600

Fig. 2. (1, 2) SSPS and (3, 4) TRPS for films of (1) pEPC and (2–4) pEPC + V2O5 (33.82 wp %) on glass substrates with ITO layer at 300 K; td is equal to (3) –5 and (4) 7 ns.

Photoluminescence, rel. units 0.8

2 4 3

0.4

0

1

400

500

600

700 λ, nm

Fig. 3. (1, 2) SSPS and (3, 4) TRPS for films of (1) pdBEPC and (2–4) pdBEPC + V2O5 (24.80 wp %) on glass substrates with ITO layer at 300 K; td is equal to (3) –4 and (4) 10 ns.

magnitude as low as that of pEPC and its composite with V2O5 (Figs. 2, 3), we explain by the deactivation of excited states in pdBEPC and complex –σ

[pdBEPC+σ···V2 O 5 ] by heavy bromine atoms that are present as substituents in positions 3 and 6 of carbazole fragments.

It seemed of interest to investigate the behavior of nanosized V2O5 in a polymer of different nature, which is capable of forming complexes with V2O5 at the expense of functional groups. To this end we selected PVS containing hydroxyl groups. For a film of composite PVS + V2O5 (10 wp %), the absorption spectrum (Fig. 4) exhibits a broad absorption band at 325–660 nm

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with barely distinguishable resolved maximums at 344, 370 (most intensive), 380, and 392 nm, whose shape and the spectral sensitivity range are similar to those for films of composites pEPC + V2O5 and pdBEPC + V2O5 (Fig. 1). Hence, in the case of PVS there also occurs the formation of a donor–acceptor complex with incom–σ plete charge transfer, specifically, [PVS+σ···V2 O 5 ]. The SSPS and TRPS (td = 6 ns) for the PVS + V2O5 films at both 300 K (Fig. 5a) and 4.2 K (Fig. 5b) develop in the same spectral range of 325 to 650 nm. At 4.2 K, the observed bands of SSPS and TRPS (td = 6 ns) for the PVS + V2O5 composite are practically identical, with λmax = 514 nm (Fig. 5b) and substantially wider than those at 300 K. The SSPS and TRPS bands for the PVS + V2O5 composite at 300 K are also practically identical at 444 nm (Fig. 5a). Thus, upon going from room temperature to 4.2 K, the photoluminescence bands for the PVS + V2O5 composite become substantially broader and shift into the long-wave range. The above data suggest that SSPS and TRPS (td = 6 ns) for PVS + V2O5 at 4.2 K (Fig. 5b; curves 1, 3) result from the superposition of two bands corresponding to different complexes of PVS with V2O5. To verify this assumption, we examined TRPS (td = –4 ns) for the PVS + V2O5 film at 4.2 K. The obtained spectrum (Fig. 5b, curve 2) is situated at 325–650 nm, with λmax = 493 nm. Subtracting curve 1 out of curve 2 in Fig. 5b reveals still another photoluminescence band for PVS + V2O5 with λmax = 525 nm (curve 4). In this case we can compare only positions of the photoluminescence maximums, rather than the intensities, because the measurements were taken at different widths of the slit and different, and low, radiation intensities. Thus, broad SSPS and TRPS (Fig. 5b; curves 1, 3) do result from the superposition of two bands. We refer the bands with λmax of 493 and 525 nm to PVS complexes with V2O5 of types, respectively, IV and V: O

=

H O (H2C–CH)n

O

H H O O (H2C–CH—CH2–CH)n

–

O

O

··· – –

··· – –

= = ··· – –

V–O–V

– =

O

O = =

=

O

–

V O V

–

=

O

–

–

IV

V

263

D 1.2

0.6

0

300

400

500

600 λ, nm

Fig. 4. Absorption spectrum for PVS + V2O5 (10 wp %) film on glass substrate with ITO layer at 300 K.

However, to confirm the formation of such complexes and determine their structure, it is necessary to conduct additional studies. Figure 6 shows the surface morphology of films of V2O5, pEPC + V2O5 (33.82 wp %), and pdBEPC + V2O5 (24.80 wp %). The height of the surface points is reflected by different shades of gray; white color corresponds to a maximum height, and dark color refers to a minimum height. The size of V2O5 fibers and polymer grains in the composites appears in Table 2. The surface of the V2O5 films (Figs. 6a, 6b) consists of ordered V2O5 fibers 50–125 nm in diameter and 300– 1500 nm long. Fibers of V2O5 in the films of both composites are disordered and much smaller. The pEPC + V2O5 surface (Fig. 6c) comprises chaotically interwoven V2O5 fibers 30–50 nm in diameter and 300–400 nm long, which are braiding polymer inclusions in the form of isolated ellipse-shaped grains of size 250 by 350 nm. The pdBEPC + V2O5 surface (Fig. 6d) also comprises chaotically interwoven V2O5 fibers 20–30 nm in diameter and 200–300 nm long, which are braiding polymer inclusions in the form of grain clusters of approximate size 70 by 150 nm. As follows from Table 2, the composite films have different morphology: both the size of V2O5 fibers and polymer grains in pdBEPC + V2O5 is smaller than that in pEPC + V2O5. The character of polymer inclusions also differs: in pEPC + V2O5 these are present as relatively large grains and in pdBEPC + V2O5, as clusters

Table 2. Size of V2O5 fibers and polymer grains in composites Composite

Diameter of V2O5 fibers, nm

Length of V2O5 fibers, nm

Size of polymer grains, nm

Most typical size of polymer grains, nm

pEPC + V2O5 (33.82 wp %) pdBEPC + V2O5 (24.80 wp %)

30–50 20–30

300–400 200–300

250 × 350 70 × 150

250 125

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0.5

2

1

0 400

1

1.0

500

600

λ, nm

(b)

3

2 0.5

4 0 400

500

600

700

λ, nm

Fig. 5. (1) SSPS and (2–4) TRPS for films of PVS + V2O5 (10 wp %) on glass substrates with ITO layer at (a) 300 and (b) 4.2 K; td is equal to (2) –4 and (3) 6 ns; curve 4 is constructed by subtracting curve 2 out of curve 1.

of small grains. This is probably due to differences in the nature of polymers and in the character of their interaction with V2O5. The data listed in Table 2 confirm the assumption that V2O5 and polymers in the composites are nanosized. We infer from the obtained results that the surface morphology of composites has a fundamentally different nature. It is clear that there is a link between the surface morphology and both the film composition and their properties. The photoluminescence emitted by the pEPC + V2O5 composite is more intensive than that of pdBEPC + V2O5. As the V2O5 fibers are arranged in both composites chaotically, it makes sense to assume

that the difference between the composites is due to the nature of the polymer and to its distribution and interaction with V2O5 in the composite. CONCLUSIONS We have obtained films of composites pEPC + V2O5 and pdBEPC + V2O5 for the first time ever, examined their electroconduction in dark and when illuminated, as well as their absorption spectra and steady-state and time-resolved photoluminescence spectra, and compared the obtained data with that for individual films of pEPC, pdBEPC, and V2O5. The comparison points to

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265

(b) 5.2

nm 100 50

2.5

0

2.5 µm

0

0 5.0

(c)

0

500 0 nm

0

500 nm

(d)

1 0 µm

1 µm

0

1 0 µm

1 µm

Fig. 6. Film surface morphology for (a) V2O5, area 5 by 5 µm; (b) V2O5, area 500 by 500 nm; (c) pEPC + V2O5, area 1 by 1 µm; and (d) pdBEPC + V2O5, area 1 by 1 µm. To the right of panels b–d we give a contrast treatment of the same micrographs.

the formation of donor–acceptor complexes with –σ incomplete charge transfer [pEPC+σ···V2 O 5 ] and –σ

[pdBEPC+σ···V2 O 5 ]. That the steady-state and timeresolved photoluminescence spectra for the pEPC + V2O5 and pdBEPC + V2O5 films exhibit several photoluminescence bands points to the formation of different donor–acceptor complexes of pEPC and pdBEPC with V2O5. These include weak complexes, where V2O5 interacts with one carbazole fragment of the polymer chain, and strong complexes, where V2O5 interacts probably with two neighboring carbazole fragments. The photoluminescence spectra for pEPC + V2O5 and pdBEPC + V2O5 display also bands we referred to complexes of oligomers of pEPC and pdBEPC with V2O5. The presence of such oligomers was established by methods of liquid chromatography and field mass spectroscopy. The intensity of the observed bands of photoluminescence emitted by films of pEPC and pEPC + V2O5 exceeds that emitted by pdBEPC and pdBEPC + V2O5 by more than two orders of magnitude, which we attributed to the deactivation of excited states in pdBEPC and pdBEPC + V2O5 by heavy bromine atoms present as substituents in positions 3 and 6 of carbazole fragments in the polymer chain. RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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Nanosized V2O5 interacts with PVS and also forms complexes at the expense of hydroxyl groups in the polymer. The PVS + V2O5 photoluminescence spectrum exhibits two bands with λmax of 493 and 525 nm. We referred the 493-nm band to the PVS + V2O5 complex where the V2O5 molecule interacts with one hydroxyl group and the 525-nm band, to the PVS + V2O5 complex where V2O5 interacts with two closely situated hydroxyl groups of polymer III. The surface of films of V2O5 and its composites with pEPC and pdBEPC consists of chaotically intertwined fibers of V2O5 that surround the polymer inclusions in the form of individual (in the case of pEPC) or clustered ellipse-shaped grains (in the case of pdBEPC). The surface morphology of composites differs in the size of V2O5 fibers and polymer grains and in the character of pEPC and pdBEPC inclusions. This is probably due to different nature of the two polymers and their different interactions with V2O5, which is revealed in spectral characteristics of the composites. The experimental data on the surface morphology of composite films confirm the assumption that the composite constituents (V2O5, pEPC, pdBEPC) are nanosized. No. 3

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