Preparation and photoelectronic properties of Q-CdS/polyaniline (PANI) nanocomposites

Received Mar. 4, 2004.

1. INTRODUCTION
In recent years, the studies of synthesis, characterization and application of Q-CdS nanoparticles with narrow size distribution have attracted much more attention owing to their size-dependent photophysical, photochemical and non-linear optical properties, etc ^[1-4]. In contrast to the bulk solids, Q-CdS nanoparticles exhibit different absorption and emission in UV-vis spectra and fluorescent spectra etc, which varies with particle size. However, these particles have a strong tendency to coalescence for their large surface-to-volume ratio ^[5]. In order to solve this problem, some strategies including encapsulation in sol-gels and in polymer matrixes etc, have been investigated ^[6]. Polyaniline compounds (PANIs) are being considered as a kind of conducting polymer substance and have wide potential application in conducting materials, light emitting diodes (LEDs), optical devices, etc ^{[7, 8]} and they show many advantages in recombining nanomaterials compared to other polymers matrixes. Some studies on the optical and electronic properties of Q-CdS/PANI nanocomposites have been reported in recent years ^{[6, 9-12]}.
    CdS nanoparticles have been generated by many methods including molecular beam epitaxy, MOCVD, microlithography, and direct chemical preparation, etc ^[5]. In this paper, preparation and photoelectronic properties of Q-CdS particles, PANI and the Q-CdS/PANI nanocomposites have been reported. A kind of freshly Q-CdS nanoparticles with narrow distribution were successfully prepared by using n-octyl mercaptan as ligand and its dimension was determined by UV-Vis spectra to be about 2.4 nm. Then, the nanoparticles were dispersed into freshly PANIs synthesized by emulsion polymerization and a series of Q-CdS/PANI nanocomposites with different ratio were successfully obtained. Furthermore, their photoelectronic properties have been characterized by UV-Vis spectra, conductance measurement and fluorescent spectra, respectively.

2. EXPERIMENTAL
2.1 Measurement and chemicals
n-Octyl mercaptan have been obtained from J&K Chemical Ltd. Aniline was redistilled prior to use. Unless noted, other reagents such as CdSO₄, n-hexadecyltrimethylammonium bromide, dodecylbenzene sulfonic acid (DBSA), N-methyl-pyrrolidone(NMP), (NH₄)₂S₂O₈, xylene, toluene, acetone etc were of analytical grade and without further purification. Saturated H₂S solution was prepared in the laboratory.
    UV-Vis spectra were measured on UV-Vis 8500 spectrophotometer (Shanghai) in NMP solution. Fluorescent spectra were recorded with Hitachi F-2500 spectrophotofluorometer. Conductivity measurements were carried out in DDS-11A conductometer using DJS-1 conductivity electrode in NMP solutions at room temperature.
2.2 Preparation of Q-CdS nanoparticles
The particles were prepared according to the literature method ^[4]. n-Hexadecyl trimethylammonium bromide (5 mmol) was slowly added to the mixture of CdSO₄ (10 mmol) and n-octyl mercaptan (20 mmol) under vigorous stirring. The particle size was controlled by the quantity of saturated H₂S solution. After 6 hrs, the crude mixture was extracted repeatedly by using a mixture of toluene and acetone, and then post-precipitation techniques were applied to separate Q-CdS particles with different sizes.
2.3 Preparation of conductive Polyaniline(PANI)
Synthetic route of PANI was shown in Scheme 1^[8]. A solution of (NH₄)₂S₂O₈ (0.02 mol) in 10 mL water was added dropwise to the mixture of redistilled aniline (0.05 mol) and DBSA (0.075 mol) in xylene/H₂O solution. Reactive temperature was steadily controlled in the range of 0 - 5°C. After stirring for 12 hrs, the precipitate of polyaniline-dodecylbenzene sulfonate was formed by adding acetone into the emulsion. After filtrating and washing, the blackish green solid was obtained and dried in vacuum for 48 hrs.

         
                                                                   (PANI-DBSA adulterants)
Scheme 1 Synthetic Route of the conductive PANI-DBSA adulterants

2.4 Preparation of Q-CdS/PANI nanocomposites
The mixture of Q-CdS nanoparitcles and PANIs with different ratio were dissolved in N-methyl-pyrrolidone (NMP) and stirred vigorously at room temperature for 2 hrs.

3. RESULTS AND DISCUSSION
3.1 UV-Vis spectra of the samples               
Absorption peaks in different position would be observed in UV-Vis spectra for Q-CdS particles with different size. So, UV-Vis spectrum is a kind of the basic technique of measuring the particle size from the absorption peaks and also estimating the size distribution from the sharpness of peaks ^{[9, 13]}. In general, the Brus equation ^[14] was used to calculate the particle size from the absorption peak,

    Where, E_R^* is the energies of excited state, E_R^* = hc / l . With respect to CdS, the value of the electron effective mass (m_e) is 0.19 electron masses; the value of the hole mass (m_h) is 0.8; the value of dielectric constant (e ) is 5.7 and 2R is the particle size. Finally, E_g must be identified with bulk band gap of CdS, 2.58 eV.
    UV-Vis spectra of Q-CdS nanoparticles, PANI and Q-CdS/PANI nanocomposites are shown in Fig.1, respectively. Absorption peak in 292 nm is assigned for the characteristic absorption band of the Q-CdS nanoparticles and the size is determined to about 2.4 nm according to the above-mentioned Brus equation.

Fig.1 UV-Vis spectra of Q-CdS nanoparticles, PANI and Q-CdS/PANI nanocomposites

    The spectrum of conductive PANI shows two strong peaks in Fig. 1. The peak at about 320 nm is assigned to the p-p * transition of benzene rings, and the other at around 630 nm represent the transition of the quinoid rings in long PANI chains^[15], that is to say that characteristic PANI were successfully prepared by emulsion polymerization.
    The absorption spectrum turns much greater changes when the Q-CdS nanoparticles were dispersed in PANI matrix. Compared to Q-CdS and PANI, the spectrum of Q-CdS/PANI nanocomposite shows two strong peaks with one weak shoulder. Very strong and sharp absorption in about 292 nm can be attributed to the characteristic absorption of Q-CdS particles, however, the absorption peak at about 320 nm that be assigned to aryl p-p* transition becomes much weaker than that of single PANI sample. It can be explained that surface deficiencies of the Q-CdS samples decreased and their stabilities increased greatly when the CdS particles were embedded with PANI. The transition from the quinoid rings of the PANI (about 630 nm) becomes weaker than before.
3.2 Conductivity measurement
Conductivity of Q-CdS, PANI and Q-CdS/PANI nanocomposite in NMP solution are listed in Table 1. Q-CdS nanoparticle shows weak conductivity before mixed. In comparison, Q-CdS/PANI nanocomposites show stronger conductivity than that of the pure samples. Moreover, the conductivity increases gradually with the content of Q-CdS particles in the nanocomposites. It would be explained that when Q-CdS particles were dispersed in the PANI, the surface of PANI would be modified to have high conductivity for Q-state particles have much more activity surface. The properties that the conductivity would be easily enhanced and altered by adding the different content of Q-CdS may play an important role in assembling the nanocomposite electrodes and be attracted wide attention in many research fields.

Table 1 Conductivity of Q-CdS, PANI and Q-CdS/PANIs nanocomposite in NMP solution

3.3.2. Fluorescent spectra of the Q-CdS/PANI nanocomposites with different contents of PANI
Fluorescent spectra of Q-CdS/PANI nanocomposites with different contents of PANI are shown in Fig.3. Similar spectra are observed for all samples in despite of having different ratios. In the first instance (from a to b), the emission intensities increased obviously with increasing the content of PANI; then, the intensities yet decreased step by step with the ratio of PANI (from b to d). When increasing to 8:1, the fluorescence of the nanocomposite was almost quenched. The phenomenon may be explained that boundary effect, which derived from the change of refractive index in Q-CdS particle surface owing to the modification of PANI to Q-CdS, would play crucial role for the fluorescent intensity of Q-CdS/PANI.

Fig.3 Fluorescent spectra of Q-CdS/PANI composites in different ratios between PANI and Q-CdS. (a) 1:1, (b) 2:1, (c) 5:1, (d) 8:1

4 CONCLUSIONS
The Q-CdS nanoparticles with narrow distribution were obtained by using post-precipitation techniques and the average size determined from the band-edge emission by using Brus equation is about 2.4 nm. Then, the Q-CdS nanoparticles were dispersed into freshly PANI under vigorously stirring and a series of Q-CdS/PANI nanocomposites with different ratios were formed. Furthermore, their photoelectronic properties have been investigated by UV-Vis spectra, conductance measurement and fluorescent spectra. Conductivity of the composites turns stronger than that of the Q-CdS particles and PANI. Fluorescent intensities of the samples changed continuously with the ratios due to the recombination of excitons and the emission from trap states. The band-gap emission (- 440 nm) intensities of all the nanocomposite samples increased greatly and these peaks changed to sharpness, however, the intensities of the surface state emission (- 550 nm) slightly decreased, which indicated that long chain of PANI modified the surface of Q-CdS and reduced the surface defect when Q-CdS nanoparticles were embedded in PANI.

REFERENCES
[1] Wang Y, Suna A, Mchugh J. J. Chem. Phys., 1990, 92 (11): 6927.
[2] Alivisatos P. Science, 1996, 271: 933.
[3] Soloviev V N, Eichhofer A, Fenske D et al. J. Am. Chem. Soc., 2001, 123 (10): 2354.
[4] Zhang W G, Zhong Y, Fan J et al. Science in China (series B), 2003, 46 (2): 196.
[5] Premachandran R, Banerjee S, John V T et al. Chem. Mater., 1997, 9 (6): 1342.
[6] Ma X Y, Lu G X, Yang B J. Applied Surface Science, 2002, 187: 235.
[7] Zhu D B, Wang F S. Organic Solids (Youji Guti). Shanghai Science and Technology Press, 1999, 12.
[8] Osterholm J E, Cao Y, Klavetter F. Polymer, 1994, 13 (35): 2902.
[9] Pethkar S, Patil R C, Kher J A et al. Thin. Solid. Films, 1999, 349: 105.
[10] Godowsky D Y, Varfolomeev A E, Zaretsky D F. J. Mater. Chem, 2001, 11: 2465.
[11] Li D S, Lu G X. Acta Phys. Chim. Sin.(Wuli Huaxue Xuebao), 2001, 17 (3): 252.
[12] Li D S, Lu G X. Chem. J. Chin. Univ.,(Gaodeng Xuexiao Huaxue Xuebao), 2002, 23 (4): 685.
[13] Borreli N F, Hall D W, Holland H J, et al. Appl. Phys., 1987, 61, 5399.
[14] Brus L. J. Phys. Chem., 1986, 90 (12): 2555.
[15] Stilwell D E, Park S M. J. Electrochem. Soc., 1989, 136, 427.

Q-CdS/聚苯胺纳米复合物的制备及光电性能研究
范军 纪欣 章伟光^* 闫云辉
(华南师范大学化学系, 广州, 510631)
摘要   以辛基硫醇为配体，制备出单分散的、粒径约为2.4 nm的Q-CdS纳米粒子；采用乳液聚合法，选用十二烷基苯磺酸(DBSA)作为掺杂剂，得到溶解性能较好的聚苯胺(PANI)；将Q-CdS与PANI按一定比例复合得到Q-CdS/聚苯胺纳米复合物；通过紫外光谱、电导率测定及荧光光谱等对杂化物的光电性能进行研究。结果表明，复合物在紫外光谱上表现出明显的量子尺寸效应；其电导率随Q-CdS含量的增加而增大；在与PANI复合后，Q-CdS纳米粒子的激子发射峰(440 nm)强度明显增强，峰变尖锐，而表面态发射峰(-550 nm)强度变弱，说明PANI对Q-CdS纳米粒子表面进行有效的修饰，使粒子表面缺陷减少。
关键词  Q-CdS, 聚苯胺(PANI)，纳米杂化物，光电性能

[ Back ] [ Home ] [ Up ] [ Next ]