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Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration
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Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration Mehmet Cabuk a,∗ , Bayram Gündüz b a b
Department of Chemistry, Faculty of Arts and Sciences, Süleyman Demirel University, Isparta 32260, Turkey Department of Science Education, Faculty of Education, Mus¸ Alparslan University, Mus¸ 49250, Turkey
a r t i c l e
i n f o
Article history: Received 24 February 2017 Accepted 1 March 2017 Available online xxx Keywords: Polyaniline Boric acid APS Optical properties Optical band gap Refractive index
a b s t r a c t In this study, polyaniline doped by boric acid (PAni:BA) conducting polymers were chemically synthesized by oxidative polymerization method using (NH4 )2 S2 08 (APS) as initiator. Pani:BA conducting polymers were synthesized by using two different APS/aniline molar ratios as 1:1 and 2:1. Their results were compared with PAni doped by HCl (PAni) conducting polymer. Structural properties of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were characterized by using FTIR, SEM, TGA, particle size and apparent density measurements. Effects of doping agents and initiator concentrations on optical properties were investigated in detail. The optoelectronic parameters such as absorption band edge, molar extinction coefficient, direct allowed band gap, refractive index, optical conductance and electrical conductance of the PAni, PAni:BA (1:1) and PAni:BA (2:1) were determined. The absorption band edge and direct allowed band gap of PAni were decreased with doping BA and increasing APS ratio. Also, the refractive index values of the materials were calculated from experimental results and compared with obtained results from Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh relations. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Increasing usage of conducting polymers has accelerated studies on polymer applications in recent years. Conducting polymers with long -conjugated structures have unique properties such as flexibility, thermal and electrical stability, ease of synthesis and durability [1]. The conductivity of the conjugated polymers is achieved through chemical oxidation or reduction reactions by using a series of simple anionic or cationic species called dopant [2]. Also, in order to modify the transport, optical and mechanical properties of the conducting polymers, dopant agents can be added directly to this polymers [3,4]. Polyaniline (PAni) is one of the most popular one among the conducting polymers due to environmental stability, ease of synthesis, adjustable conductivity, amazing chemical, electrical and optical properties [5]. PAni and its derivatives are used in various industrial areas such as electrochromic devices, sensors, toners, conductive paints, drug delivery, rechargeable batteries electrolytes, transistors, solar cells and electrorheological applications [5–8].
∗ Corresponding author at: Süleyman Demirel University, Faculty of Arts and Sciences, Department of Chemistry, Isparta 32260, Turkey. E-mail addresses: [email protected], [email protected] (M. Cabuk).
Among the doping agents, boric acid (BA) particles have been used in various studies for boron doping process. Sevinis et al. have reported the optoelectronic properties of soluble copolymers composed of dithienothiophenes (DTT), DTT-4,4-dioxide (DTT-S,S-O2 ) donor and esitylboryl acceptor units. Their results showed that the band gap values increased to 2.46–3.21 and 2.18–2.88 eV due to an intramolecular charge transfer transition between the donor DTT units and the boron acceptor atoms, respectively [9]. Yagci et al. Synthesized the poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) doped with BA layers and investigated their effects on the solar cells based organic molecules. Their results showed that the fill factor (FF) and open-circuit voltage (Voc ) values increased with interaction of the PEDOT:PSS layers with BA dopant [10]. Subramanian and Wang have reported the dye sensitized solar cells based on TiO2 nanotubes doped with BA. The conduction band of doped nanotubes shifted to lower values due to introduction of boron into the interstitial sites of TiO2 lattice [11]. Wu et al. synthesized the monolayer graphene doped by boron and urea (nitrogen). Their results showed that the doping of graphene by nitrogen and boron caused to increase in the electrical and optical performance of the graphene sheets [12]. According to these studies, it was concluded that BA is an effective doping agent to improve electrical and optical properties of the materials.
http://dx.doi.org/10.1016/j.apsusc.2017.03.010 0169-4332/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Cabuk, B. Gündüz, Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration, Appl. Surf. Sci. (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.010
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Optical parameters have a significant role on various properties of organic electronic, photonic and optoelectronic applications such as displays, lasers, light emitting diodes, diodes, solar cells, photovoltaics, transistors, photodetectors, electroluminescent sensors, optical waveguides, radio frequency identification tags and smart cards [13]. The molar extinction coefficient is parameter to define the absorbance intensity at a given wavelength per molar concentration [14,15]. The optical band gap and refractive index are fundamental parameters for optoelectronic materials [16,17]. The optical band gap gives many useful clues about various properties of the materials. The imaginary part of the refraction index gives the intensity attenuation inside the medium, while the real part of the refraction index is inversely proportional to the wave propagation velocity [18]. Effects of the APS initiator concentration on optical properties of the soluble PAni:BA conducting polymers have not hitherto been reported in the literature. Therefore, the aim of the present study was to investigate the optical parameters of PAni:BA conducting polymers synthesized in two different initiator ratios (nAPS /naniline = 1:1 and 2:1). Since PAni is generally doped in HCl medium, the results obtained were compared with that of PAni doped by 1 M HCl(aq) (nAPS /naniline = 1:1). 2. Experimental 2.1. Materials Aniline monomer (C6 H5 NH2 ) was used after vacuum distillation (E. Merck, Germany). Boric acid, B(OH)3 , was purchased from Aldrich (Germany) and used as doping agent. All the other chemicals used in the experiments (HCl, APS, dimethylsulfoxide (DMSO) etc.) were purchased from E. Merck (Germany) with analytical grade and used as received without further purification. 2.2. Synthesis of PAni and PAni:BA conducting polymers 0.05 mol aniline was dissolved in 1 M B(OH)3 and stirred for 30 min at 0–5 ◦ C temperature. Then, pre-cooled (0–5 ◦ C) APS solution by taking nAPS /naniline = 1:1 was prepared in 1 M B(OH)3 . This solution was added dropwise to the aqueous solution containing aniline and B(OH)3 at 0–5 ◦ C temperature. The mixture was kept stirring under N2 atmosphere at 0–5 ◦ C for 16 h under reflux. Then, the crude product in dark green color was filtered, washed with distilled water and dried at 60 ◦ C under vacuum. Also, PAni doped by BA by taking nAPS /naniline = 2:1 was synthesized with the same method. The synthesized PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained with 93% and 95% yield, respectively. To compare the effects of BA with HCl doping agent, PAni was synthesized with the same method in 1 M HCl(aq) at 0–5 ◦ C by using APS (nAPS /naniline = 1:1). PAni doped by HCl was obtained with 87% yield and coded as PAni. Thus, PAni and PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were used for the determination of optical parameters.
of the samples were examined by using a JEOL JSM 5500LV (Japan) scanning electron microscope (SEM). Hydrodynamic particle size of the samples was determined by dynamic light scattering (DLS) using Malvern Zeta-Sizer Nano ZS (England). The self-optimization routine (laser attenuation and data collection time) in the ZetaSizer software was used for all the measurements. The samples were turned into pellets using a steel die of 13 mm diameter and their apparent densities were calculated from masses and volumes of the pellets. 2.4. Preparation of the solutions The solubility of the materials is very important for preparation of the optoelectronic devices. A defined amount of PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were taken to prepare their solutions with a AND-GR-200 Series Analytical Balance and dissolved homogeneously in 12 mL volume of DMSO solvent using digital vortex mixer (Four E’s Scientific CO., Ltd.). 2.5. The UV measurements of the solutions A cylindrical bathtub (Hellma QS-100) whose optical path length is 10 mm and volume is 3.5 mL was used for all the solutions of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. The UV measurements of the materials were recorded with a Shimadzu model UV-1800 Spectrophotometer at room temperature. 3. Results and discussion 3.1. Characterization results FTIR spectra of the PAni, PAni:BA (1:1) and PAni:BA (2:1) were depicted in Fig. 1. As shown that PAni had characteristic stretching bands of NH (3250 cm−1 ), aliphatic CH (2950 cm−1 ), C C and C N (1700 cm−1 ), and aromatic benzene (820 cm−1 ). PAni:BA (1:1) and PAni:BA (2:1) conducting polymers showed similar spectra and the peaks at 3200–3450 cm−1 due to N H stretching; at 2920 cm−1 due to aliphatic C H stretching; at 1580–1450 cm−1 due to C C and C N stretching of quinoid and benzenoid rings of PAni; 1100–1300 cm−1 due to aromatic stretching; 750–800 cm−1 due to C H bending of 1,4-disubstituted benzene ring. When the absorption bands of PAni-BA (2:1) were compared with that of PAni-BA (1:1), some shifts were observed in the peak values and intensities. These shifts can be attributed to increasing polymerization ratio of aniline with increasing initiator molecules in the solution and hydrogen-bonded interactions between conducting PAni chains. Similar effects were observed in literature for polyaniline-graft-chitosan copolymer [19] and self-assembled nano/microstructured polyaniline–clay nanocomposite [20]. The
2.3. Characterizations To obtain small and homogeneous particle size distributions, the materials were ground milled by using a Retsch MM400 model milling machine (Germany) and subjected to the following characterizations. FTIR spectra of the materials were conducted as KBr discs using a Perkin Elmer Spectrometer BX FTIR system (England). Thermal analyses of the materials were performed with a Perkin Elmer Diamond thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) thermal analysis instrument (U.S.A). The specimens were heated at a rate of 10 ◦ C/min under N2 atmosphere from room temperature to 900 ◦ C. The surface morphology
Fig. 1. FTIR spectra of the conducting polymers.
Please cite this article in press as: M. Cabuk, B. Gündüz, Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration, Appl. Surf. Sci. (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.010
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Fig. 2. Chemical polymerization of aniline (a) doping with HCl (b) doping with BA.
As reflected from SEM images in Fig. 4(a–c), PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers have clearly different morphologies. PAni has nonporous and clustered form. PAni:BA (1:1) shows granular morphology consisted of particles with diverse sizes (Fig. 4(b)). The morphology of PAni:BA (2:1) has also granular structure and consists of more homogeneously distributed particle sizes (3–5 m) as seen in Fig. 4(c). The changes in the morphology of the PAni:BA (2:1) can be attributed to the successful polymerization of aniline monomers due to doping with BA and increasing the (NH4 )2 S2 08 ratio. Similar changes in the surface morphology were reported for physically cross-linked Laponite-PAni composites [21]. Also, an increase in the (NH4 )2 S2 08 ratio could be caused to an increase in electrical conductivity of PAni:BA (2:1) compared with PAni-BA (1:1). The hydrodynamic average diameters of the PAni:BA (2:1) particles were found to be bigger (18 m) than the PAni:BA (1:1) particles (10 m) as expected. Also, the hydrodynamic average particle sizes of the particles obtained from DLS experiments were bigger than that of SEM image. It can be attributed to swelling of particles in aqueous dispersion. Apparent densities of the PAni:BA (1:1) and PAni:BA (2:1) were found almost the same (d (1:1) = 1.18 gcm3− and d (2:1) = 1.21 g cm3− ). Fig. 3. TGA curves of the conducting polymers.
3.2. Optical properties of the solutions proposed polymerization reactions of PAni doped by HCl and PAni doped by BA conducting polymers are given in Fig. 2. Fig. 3 shows the TGA curves of the PAni, PAni-BA (1:1) and PAni-BA (2:1). All the polymers showed two-step weight losses between 25 and 900 ◦ C temperature range. The first weight loss was observed between 25 and 120 ◦ C and attributed to the loss of moisture, adsorbed solvent, and unreacted species. The second weight loss was observed between 430 and 550 ◦ C and attributed to the removal of dopant anions (Cl− or BO3− ) and thermal decomposition of PAni and PAni-BA conducting polymer chains. The residual amounts of the PAni, PAni:BA (1:1) and PAni:BA (2:1) at the end of 900 ◦ C were found as 36 wt.%, 43 wt.% and 55 wt.%, respectively. It is clearly seen that thermal stability of PAni:BA conducting polymers increased with both increasing APS initiator concentration and changing doping agent from HCl to B(OH)3 , as targeted. It can be attributed to the enhanced intermolecular and intramolecular H-bonds in the polymer structure and increasing chain length of PAni due to the presence of more initiator molecules in the mixture. These thermal stability results of materials are suitable for availability as optical material in potential industrial applications.
Absorbance (A) is the measurement of the amount of light absorbed. Absorption occurs when the energy is absorbed by an electron resulting in a transition from ground state to an excited state. One of the vital intrinsic parameters can be seen as the molar extinction coefficient (), which plays a key role for determining the electronic transition of a material [22]. The values of the solutions of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained from well-known equation, Beer–Lambert relation [23]; ε=
A cl
(1)
where c is the molar concentration and l is length of optical path of the used cylindrical bathtub. The plot versus wavelength () of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers is given in Fig. 5. As seen in Fig. 5, the values are the highest in the near ultraviolet (NUV) region and vary from about 101 L mol−1 cm−1 to 9 × 103 L mol−1 cm−1 . Obtained results suggest that the molar extinction coefficient increases with increasing molar ratio of APS.
Please cite this article in press as: M. Cabuk, B. Gündüz, Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration, Appl. Surf. Sci. (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.010
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Fig. 4. SEM images of the conducting polymers.
Table 1 The optoelectronic parameters of the materials. Materials
max (nm)
Ee-A (eV)
Egd (eV)
PAni PAni:BA (1:1) PAni:BA (2:1)
402 456 458
3.085 2.719 2.707
3.058 2.852 2.783
The first derivation (dT/d) of the transmittance is calculated to get the absorption band edge of materials. Fig. 6(b) indicates the dT/d vs curves of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. The maximum peak (max ) and absorption band edge (Ee-A ) values are determined from Fig. 6(b) and given in Table 1. As seen in Table 1, the Ee-A value varies from 3.085 to 2.707 eV. Obtained results show that the absorption band edge of the PAni decreases with doping BA particles and with increasing ratio of the APS. Optical band gap (Eg ) of optical transitions has been computed by benefitting from well-known relation, Tauc relation [24]; (␣h)m = A(E − Eg ) Fig. 5. The molar extinction coefficient () plot versus wavelength () of the conducting polymers.
The transmittance (T) spectra were recorded to examine the optical properties of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. Fig. 6(a) indicates the T curves vs . As seen in Fig. 6(a), the T values are quite high and the highest in the visible (V) region. Obtained results suggest that the T value of the PAni varies with doping boric acid particles and the transmittance decreases with increasing molar ratio of APS.
(2)
where ␣ is absorption coefficient, A is a constant, both h and E are photon energy and m is a parameter, which measure type of band gaps. Fig. 7 indicates the (␣h )2 plot vs E of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. The direct allowed band gap (Egd ) values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained from the linear regions of the (␣h )2 curves in Fig. 7 and given in Table 1. As seen that the Egd value shifts from 3.058 to 2.783 eV. Obtained results indicate that the direct allowed band gap of the PAni decreases with doping BA particles and with increasing ratio of nAPS /naniline from 1:1 to 2:1. It can be attributed to the interactions between the PAni matrix and BA doping agent. When these interactions increase, the conductiv-
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Fig. 6. The (a) transmittance and (b) dT/d plots vs. wavelength of the conducting polymers.
Table 2 The refractive index parameters obtained from experimental (Exp) results, Moss (M), Ravindra (Ra), Herve-Vandamme (H-V), Reddy (Re) and Kumar-Singh (K-S) relations of the materials. Materials
Refractive index (n) values
PAni PAni:BA (1:1) PAni:BA (2:1)
Exp
M
Ra
H-V
Re
K-S
2.406 2.415 2.429
2.361 2.402 2.417
2.188 2.316 2.359
2.311 2.372 2.394
2.750 2.805 2.825
2.348 2.402 2.421
Moss relation is given by n4 =
95eV Eg
(4)
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Moss relation were calculated from Eq. (4) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.361 (for PAni) to 2.417 (for PAni:BA (2:1)). Ravindra relation is given by n = 4.084 − 0.62Eg Fig. 7. The (␣h )2 plot vs photon energy (E) of the conducting polymers.
ity of the particles increase and number of charge carriers increase and the band edge value decrease. The refractive index (n) is another significant parameter in optoelectronic parameters. The n values of the semiconductors can be calculated by [25,26],
n=
4R (R − 1)2
1/2 − k2
R+1 − R−1
(3)
The n values calculated experimentally of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were given in Table 2. As seen in Table 2, n value shifts from 2.406 (for PAni) to 2.429 (for PAni:BA (2:1)). Obtained result suggests that the refractive index of the PAni increases with doping BA particles and with increasing ratio of nAPS /naniline from 1:1 to 2:1. The n and Eg are of fundamental parameters in optoelectronic parameters. The relation between the Eg and n can be given with many relations such as Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh [27].
(5)
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Ravindra relation were calculated from Eq. (5) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.188 (for PAni) to 2.359 (for PAni:BA (2:1)). Herve-Vandamme relation is given by
2
n =1+
A Eg + B
2 (6)
where A is the hydrogen ionization energy 13.6 eV and B = 3.47 eV is a constant assumed to be the difference between the band gap energy and UV resonance energy. The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with HerveVandamme relation were calculated from Eq. (6) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.311 (for PAni) to 2.394 (for PAni:BA (2:1)). Reddy relation is given by
n=
1/4
154
Eg − 0.365
(7)
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Reddy relation were calculated from Eq. (7) and
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and,
elec =
Fig. 8. The refractive index plots of the conducting polymers obtained from various relations.
obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.750 (for PAni) to 2.825 (for PAni:BA (2:1)). Kumar-Singh relation is given by n=
3.3668
0.32234
(8)
Eg
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Kumar-Singh relation were calculated from Eq. (8) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.348 (for PAni) to 2.421 (for PAni:BA (2:1)). Hence, the n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained from experimental results and as well as the Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh relations. Fig. 8 shows the n values obtained with different relations. As seen that, the refractive index values obtained from Reddy relation are the highest, while the refractive index values obtained from Ravindra relation are the lowest. The optical conductance (op ) and electrical conductance (elec ) of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were calculated with following equations [28], op =
␣nc 4
(9)
2op ␣
(10)
where c is the velocity of light. Fig. 9(a,b) indicates the op and elec plot vs. E of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers, respectively. As seen in Fig. 9(a) and (b), the op values of the conducting polymers are in the order of 1010 S, while elec values of the conducting polymers are in the order of 102 S. It is observed that the op values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers are higher than that of the elec values and the op and elec values of the PAni varies with doping boric acid particles and the op and elec increases with increasing ratio of APS. Another interesting result is that the op and elec curves of the PAni exhibit two peaks, while the op and elec curves of the PAni:BA (1:1) and PAni:BA (2:1) conducting polymers exhibit three peaks.
4. Conclusions PAni doped by HCl (nAPS /naniline = 1:1) and PAni doped by BA (nAPS /naniline = 1:1 and 2:1) conducting polymers were chemically synthesized and their structures were confirmed with various characterization techniques. Thermal stability of PAni doped by HCl increased from 36 wt.% to 55 wt.% with the BA and increasing APS molar ratio (2:1). Effects of the APS molar ratio on the optical properties of PAni:BA were investigated in detail. It was observed that the molar extinction coefficient, refractive index, optical conductance and electrical conductance values increased with increasing ratio of nAPS /naniline . On the other hand, the transmittance, absorption band edge and direct allowed band gap values decreased with increasing ratio of nAPS /naniline . Similarly, the absorption band edge and direct allowed band gap of the PAni decreased with doping BA particles from 3.085 eV and 3.058 to 2.707 eV and 2.783, respectively. The refractive index of the PAni doped by BA was higher than that’s of PAni doped by HCl. The highest refractive index value was obtained from Reddy relation as 2.825 for PAni:BA (2:1). As a result, soluble PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with enhanced optical properties could be a good candidate for fabrication of many optoelectronic devices such as diode and transistor.
Fig. 9. The (a) optical conductance (op ) and (b) electrical conductance (elec ) plot vs. E of the conducting polymers.
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Acknowledgement This work was supported by the Scientific and Technological Research Council of Turkey [grant number: 214Z199].
[14]
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Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration Mehmet Cabuk a,∗ , Bayram Gündüz b a b
Department of Chemistry, Faculty of Arts and Sciences, Süleyman Demirel University, Isparta 32260, Turkey Department of Science Education, Faculty of Education, Mus¸ Alparslan University, Mus¸ 49250, Turkey
a r t i c l e
i n f o
Article history: Received 24 February 2017 Accepted 1 March 2017 Available online xxx Keywords: Polyaniline Boric acid APS Optical properties Optical band gap Refractive index
a b s t r a c t In this study, polyaniline doped by boric acid (PAni:BA) conducting polymers were chemically synthesized by oxidative polymerization method using (NH4 )2 S2 08 (APS) as initiator. Pani:BA conducting polymers were synthesized by using two different APS/aniline molar ratios as 1:1 and 2:1. Their results were compared with PAni doped by HCl (PAni) conducting polymer. Structural properties of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were characterized by using FTIR, SEM, TGA, particle size and apparent density measurements. Effects of doping agents and initiator concentrations on optical properties were investigated in detail. The optoelectronic parameters such as absorption band edge, molar extinction coefficient, direct allowed band gap, refractive index, optical conductance and electrical conductance of the PAni, PAni:BA (1:1) and PAni:BA (2:1) were determined. The absorption band edge and direct allowed band gap of PAni were decreased with doping BA and increasing APS ratio. Also, the refractive index values of the materials were calculated from experimental results and compared with obtained results from Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh relations. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Increasing usage of conducting polymers has accelerated studies on polymer applications in recent years. Conducting polymers with long -conjugated structures have unique properties such as flexibility, thermal and electrical stability, ease of synthesis and durability [1]. The conductivity of the conjugated polymers is achieved through chemical oxidation or reduction reactions by using a series of simple anionic or cationic species called dopant [2]. Also, in order to modify the transport, optical and mechanical properties of the conducting polymers, dopant agents can be added directly to this polymers [3,4]. Polyaniline (PAni) is one of the most popular one among the conducting polymers due to environmental stability, ease of synthesis, adjustable conductivity, amazing chemical, electrical and optical properties [5]. PAni and its derivatives are used in various industrial areas such as electrochromic devices, sensors, toners, conductive paints, drug delivery, rechargeable batteries electrolytes, transistors, solar cells and electrorheological applications [5–8].
∗ Corresponding author at: Süleyman Demirel University, Faculty of Arts and Sciences, Department of Chemistry, Isparta 32260, Turkey. E-mail addresses: [email protected], [email protected] (M. Cabuk).
Among the doping agents, boric acid (BA) particles have been used in various studies for boron doping process. Sevinis et al. have reported the optoelectronic properties of soluble copolymers composed of dithienothiophenes (DTT), DTT-4,4-dioxide (DTT-S,S-O2 ) donor and esitylboryl acceptor units. Their results showed that the band gap values increased to 2.46–3.21 and 2.18–2.88 eV due to an intramolecular charge transfer transition between the donor DTT units and the boron acceptor atoms, respectively [9]. Yagci et al. Synthesized the poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) doped with BA layers and investigated their effects on the solar cells based organic molecules. Their results showed that the fill factor (FF) and open-circuit voltage (Voc ) values increased with interaction of the PEDOT:PSS layers with BA dopant [10]. Subramanian and Wang have reported the dye sensitized solar cells based on TiO2 nanotubes doped with BA. The conduction band of doped nanotubes shifted to lower values due to introduction of boron into the interstitial sites of TiO2 lattice [11]. Wu et al. synthesized the monolayer graphene doped by boron and urea (nitrogen). Their results showed that the doping of graphene by nitrogen and boron caused to increase in the electrical and optical performance of the graphene sheets [12]. According to these studies, it was concluded that BA is an effective doping agent to improve electrical and optical properties of the materials.
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Optical parameters have a significant role on various properties of organic electronic, photonic and optoelectronic applications such as displays, lasers, light emitting diodes, diodes, solar cells, photovoltaics, transistors, photodetectors, electroluminescent sensors, optical waveguides, radio frequency identification tags and smart cards [13]. The molar extinction coefficient is parameter to define the absorbance intensity at a given wavelength per molar concentration [14,15]. The optical band gap and refractive index are fundamental parameters for optoelectronic materials [16,17]. The optical band gap gives many useful clues about various properties of the materials. The imaginary part of the refraction index gives the intensity attenuation inside the medium, while the real part of the refraction index is inversely proportional to the wave propagation velocity [18]. Effects of the APS initiator concentration on optical properties of the soluble PAni:BA conducting polymers have not hitherto been reported in the literature. Therefore, the aim of the present study was to investigate the optical parameters of PAni:BA conducting polymers synthesized in two different initiator ratios (nAPS /naniline = 1:1 and 2:1). Since PAni is generally doped in HCl medium, the results obtained were compared with that of PAni doped by 1 M HCl(aq) (nAPS /naniline = 1:1). 2. Experimental 2.1. Materials Aniline monomer (C6 H5 NH2 ) was used after vacuum distillation (E. Merck, Germany). Boric acid, B(OH)3 , was purchased from Aldrich (Germany) and used as doping agent. All the other chemicals used in the experiments (HCl, APS, dimethylsulfoxide (DMSO) etc.) were purchased from E. Merck (Germany) with analytical grade and used as received without further purification. 2.2. Synthesis of PAni and PAni:BA conducting polymers 0.05 mol aniline was dissolved in 1 M B(OH)3 and stirred for 30 min at 0–5 ◦ C temperature. Then, pre-cooled (0–5 ◦ C) APS solution by taking nAPS /naniline = 1:1 was prepared in 1 M B(OH)3 . This solution was added dropwise to the aqueous solution containing aniline and B(OH)3 at 0–5 ◦ C temperature. The mixture was kept stirring under N2 atmosphere at 0–5 ◦ C for 16 h under reflux. Then, the crude product in dark green color was filtered, washed with distilled water and dried at 60 ◦ C under vacuum. Also, PAni doped by BA by taking nAPS /naniline = 2:1 was synthesized with the same method. The synthesized PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained with 93% and 95% yield, respectively. To compare the effects of BA with HCl doping agent, PAni was synthesized with the same method in 1 M HCl(aq) at 0–5 ◦ C by using APS (nAPS /naniline = 1:1). PAni doped by HCl was obtained with 87% yield and coded as PAni. Thus, PAni and PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were used for the determination of optical parameters.
of the samples were examined by using a JEOL JSM 5500LV (Japan) scanning electron microscope (SEM). Hydrodynamic particle size of the samples was determined by dynamic light scattering (DLS) using Malvern Zeta-Sizer Nano ZS (England). The self-optimization routine (laser attenuation and data collection time) in the ZetaSizer software was used for all the measurements. The samples were turned into pellets using a steel die of 13 mm diameter and their apparent densities were calculated from masses and volumes of the pellets. 2.4. Preparation of the solutions The solubility of the materials is very important for preparation of the optoelectronic devices. A defined amount of PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were taken to prepare their solutions with a AND-GR-200 Series Analytical Balance and dissolved homogeneously in 12 mL volume of DMSO solvent using digital vortex mixer (Four E’s Scientific CO., Ltd.). 2.5. The UV measurements of the solutions A cylindrical bathtub (Hellma QS-100) whose optical path length is 10 mm and volume is 3.5 mL was used for all the solutions of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. The UV measurements of the materials were recorded with a Shimadzu model UV-1800 Spectrophotometer at room temperature. 3. Results and discussion 3.1. Characterization results FTIR spectra of the PAni, PAni:BA (1:1) and PAni:BA (2:1) were depicted in Fig. 1. As shown that PAni had characteristic stretching bands of NH (3250 cm−1 ), aliphatic CH (2950 cm−1 ), C C and C N (1700 cm−1 ), and aromatic benzene (820 cm−1 ). PAni:BA (1:1) and PAni:BA (2:1) conducting polymers showed similar spectra and the peaks at 3200–3450 cm−1 due to N H stretching; at 2920 cm−1 due to aliphatic C H stretching; at 1580–1450 cm−1 due to C C and C N stretching of quinoid and benzenoid rings of PAni; 1100–1300 cm−1 due to aromatic stretching; 750–800 cm−1 due to C H bending of 1,4-disubstituted benzene ring. When the absorption bands of PAni-BA (2:1) were compared with that of PAni-BA (1:1), some shifts were observed in the peak values and intensities. These shifts can be attributed to increasing polymerization ratio of aniline with increasing initiator molecules in the solution and hydrogen-bonded interactions between conducting PAni chains. Similar effects were observed in literature for polyaniline-graft-chitosan copolymer [19] and self-assembled nano/microstructured polyaniline–clay nanocomposite [20]. The
2.3. Characterizations To obtain small and homogeneous particle size distributions, the materials were ground milled by using a Retsch MM400 model milling machine (Germany) and subjected to the following characterizations. FTIR spectra of the materials were conducted as KBr discs using a Perkin Elmer Spectrometer BX FTIR system (England). Thermal analyses of the materials were performed with a Perkin Elmer Diamond thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) thermal analysis instrument (U.S.A). The specimens were heated at a rate of 10 ◦ C/min under N2 atmosphere from room temperature to 900 ◦ C. The surface morphology
Fig. 1. FTIR spectra of the conducting polymers.
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Fig. 2. Chemical polymerization of aniline (a) doping with HCl (b) doping with BA.
As reflected from SEM images in Fig. 4(a–c), PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers have clearly different morphologies. PAni has nonporous and clustered form. PAni:BA (1:1) shows granular morphology consisted of particles with diverse sizes (Fig. 4(b)). The morphology of PAni:BA (2:1) has also granular structure and consists of more homogeneously distributed particle sizes (3–5 m) as seen in Fig. 4(c). The changes in the morphology of the PAni:BA (2:1) can be attributed to the successful polymerization of aniline monomers due to doping with BA and increasing the (NH4 )2 S2 08 ratio. Similar changes in the surface morphology were reported for physically cross-linked Laponite-PAni composites [21]. Also, an increase in the (NH4 )2 S2 08 ratio could be caused to an increase in electrical conductivity of PAni:BA (2:1) compared with PAni-BA (1:1). The hydrodynamic average diameters of the PAni:BA (2:1) particles were found to be bigger (18 m) than the PAni:BA (1:1) particles (10 m) as expected. Also, the hydrodynamic average particle sizes of the particles obtained from DLS experiments were bigger than that of SEM image. It can be attributed to swelling of particles in aqueous dispersion. Apparent densities of the PAni:BA (1:1) and PAni:BA (2:1) were found almost the same (d (1:1) = 1.18 gcm3− and d (2:1) = 1.21 g cm3− ). Fig. 3. TGA curves of the conducting polymers.
3.2. Optical properties of the solutions proposed polymerization reactions of PAni doped by HCl and PAni doped by BA conducting polymers are given in Fig. 2. Fig. 3 shows the TGA curves of the PAni, PAni-BA (1:1) and PAni-BA (2:1). All the polymers showed two-step weight losses between 25 and 900 ◦ C temperature range. The first weight loss was observed between 25 and 120 ◦ C and attributed to the loss of moisture, adsorbed solvent, and unreacted species. The second weight loss was observed between 430 and 550 ◦ C and attributed to the removal of dopant anions (Cl− or BO3− ) and thermal decomposition of PAni and PAni-BA conducting polymer chains. The residual amounts of the PAni, PAni:BA (1:1) and PAni:BA (2:1) at the end of 900 ◦ C were found as 36 wt.%, 43 wt.% and 55 wt.%, respectively. It is clearly seen that thermal stability of PAni:BA conducting polymers increased with both increasing APS initiator concentration and changing doping agent from HCl to B(OH)3 , as targeted. It can be attributed to the enhanced intermolecular and intramolecular H-bonds in the polymer structure and increasing chain length of PAni due to the presence of more initiator molecules in the mixture. These thermal stability results of materials are suitable for availability as optical material in potential industrial applications.
Absorbance (A) is the measurement of the amount of light absorbed. Absorption occurs when the energy is absorbed by an electron resulting in a transition from ground state to an excited state. One of the vital intrinsic parameters can be seen as the molar extinction coefficient (), which plays a key role for determining the electronic transition of a material [22]. The values of the solutions of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained from well-known equation, Beer–Lambert relation [23]; ε=
A cl
(1)
where c is the molar concentration and l is length of optical path of the used cylindrical bathtub. The plot versus wavelength () of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers is given in Fig. 5. As seen in Fig. 5, the values are the highest in the near ultraviolet (NUV) region and vary from about 101 L mol−1 cm−1 to 9 × 103 L mol−1 cm−1 . Obtained results suggest that the molar extinction coefficient increases with increasing molar ratio of APS.
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Fig. 4. SEM images of the conducting polymers.
Table 1 The optoelectronic parameters of the materials. Materials
max (nm)
Ee-A (eV)
Egd (eV)
PAni PAni:BA (1:1) PAni:BA (2:1)
402 456 458
3.085 2.719 2.707
3.058 2.852 2.783
The first derivation (dT/d) of the transmittance is calculated to get the absorption band edge of materials. Fig. 6(b) indicates the dT/d vs curves of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. The maximum peak (max ) and absorption band edge (Ee-A ) values are determined from Fig. 6(b) and given in Table 1. As seen in Table 1, the Ee-A value varies from 3.085 to 2.707 eV. Obtained results show that the absorption band edge of the PAni decreases with doping BA particles and with increasing ratio of the APS. Optical band gap (Eg ) of optical transitions has been computed by benefitting from well-known relation, Tauc relation [24]; (␣h)m = A(E − Eg ) Fig. 5. The molar extinction coefficient () plot versus wavelength () of the conducting polymers.
The transmittance (T) spectra were recorded to examine the optical properties of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. Fig. 6(a) indicates the T curves vs . As seen in Fig. 6(a), the T values are quite high and the highest in the visible (V) region. Obtained results suggest that the T value of the PAni varies with doping boric acid particles and the transmittance decreases with increasing molar ratio of APS.
(2)
where ␣ is absorption coefficient, A is a constant, both h and E are photon energy and m is a parameter, which measure type of band gaps. Fig. 7 indicates the (␣h )2 plot vs E of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers. The direct allowed band gap (Egd ) values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained from the linear regions of the (␣h )2 curves in Fig. 7 and given in Table 1. As seen that the Egd value shifts from 3.058 to 2.783 eV. Obtained results indicate that the direct allowed band gap of the PAni decreases with doping BA particles and with increasing ratio of nAPS /naniline from 1:1 to 2:1. It can be attributed to the interactions between the PAni matrix and BA doping agent. When these interactions increase, the conductiv-
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Fig. 6. The (a) transmittance and (b) dT/d plots vs. wavelength of the conducting polymers.
Table 2 The refractive index parameters obtained from experimental (Exp) results, Moss (M), Ravindra (Ra), Herve-Vandamme (H-V), Reddy (Re) and Kumar-Singh (K-S) relations of the materials. Materials
Refractive index (n) values
PAni PAni:BA (1:1) PAni:BA (2:1)
Exp
M
Ra
H-V
Re
K-S
2.406 2.415 2.429
2.361 2.402 2.417
2.188 2.316 2.359
2.311 2.372 2.394
2.750 2.805 2.825
2.348 2.402 2.421
Moss relation is given by n4 =
95eV Eg
(4)
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Moss relation were calculated from Eq. (4) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.361 (for PAni) to 2.417 (for PAni:BA (2:1)). Ravindra relation is given by n = 4.084 − 0.62Eg Fig. 7. The (␣h )2 plot vs photon energy (E) of the conducting polymers.
ity of the particles increase and number of charge carriers increase and the band edge value decrease. The refractive index (n) is another significant parameter in optoelectronic parameters. The n values of the semiconductors can be calculated by [25,26],
n=
4R (R − 1)2
1/2 − k2
R+1 − R−1
(3)
The n values calculated experimentally of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were given in Table 2. As seen in Table 2, n value shifts from 2.406 (for PAni) to 2.429 (for PAni:BA (2:1)). Obtained result suggests that the refractive index of the PAni increases with doping BA particles and with increasing ratio of nAPS /naniline from 1:1 to 2:1. The n and Eg are of fundamental parameters in optoelectronic parameters. The relation between the Eg and n can be given with many relations such as Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh [27].
(5)
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Ravindra relation were calculated from Eq. (5) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.188 (for PAni) to 2.359 (for PAni:BA (2:1)). Herve-Vandamme relation is given by
2
n =1+
A Eg + B
2 (6)
where A is the hydrogen ionization energy 13.6 eV and B = 3.47 eV is a constant assumed to be the difference between the band gap energy and UV resonance energy. The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with HerveVandamme relation were calculated from Eq. (6) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.311 (for PAni) to 2.394 (for PAni:BA (2:1)). Reddy relation is given by
n=
1/4
154
Eg − 0.365
(7)
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Reddy relation were calculated from Eq. (7) and
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and,
elec =
Fig. 8. The refractive index plots of the conducting polymers obtained from various relations.
obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.750 (for PAni) to 2.825 (for PAni:BA (2:1)). Kumar-Singh relation is given by n=
3.3668
0.32234
(8)
Eg
The n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with Kumar-Singh relation were calculated from Eq. (8) and obtained n values are imparted in Table 2. As seen in Table 2, n value shifts from 2.348 (for PAni) to 2.421 (for PAni:BA (2:1)). Hence, the n values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were obtained from experimental results and as well as the Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh relations. Fig. 8 shows the n values obtained with different relations. As seen that, the refractive index values obtained from Reddy relation are the highest, while the refractive index values obtained from Ravindra relation are the lowest. The optical conductance (op ) and electrical conductance (elec ) of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were calculated with following equations [28], op =
␣nc 4
(9)
2op ␣
(10)
where c is the velocity of light. Fig. 9(a,b) indicates the op and elec plot vs. E of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers, respectively. As seen in Fig. 9(a) and (b), the op values of the conducting polymers are in the order of 1010 S, while elec values of the conducting polymers are in the order of 102 S. It is observed that the op values of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers are higher than that of the elec values and the op and elec values of the PAni varies with doping boric acid particles and the op and elec increases with increasing ratio of APS. Another interesting result is that the op and elec curves of the PAni exhibit two peaks, while the op and elec curves of the PAni:BA (1:1) and PAni:BA (2:1) conducting polymers exhibit three peaks.
4. Conclusions PAni doped by HCl (nAPS /naniline = 1:1) and PAni doped by BA (nAPS /naniline = 1:1 and 2:1) conducting polymers were chemically synthesized and their structures were confirmed with various characterization techniques. Thermal stability of PAni doped by HCl increased from 36 wt.% to 55 wt.% with the BA and increasing APS molar ratio (2:1). Effects of the APS molar ratio on the optical properties of PAni:BA were investigated in detail. It was observed that the molar extinction coefficient, refractive index, optical conductance and electrical conductance values increased with increasing ratio of nAPS /naniline . On the other hand, the transmittance, absorption band edge and direct allowed band gap values decreased with increasing ratio of nAPS /naniline . Similarly, the absorption band edge and direct allowed band gap of the PAni decreased with doping BA particles from 3.085 eV and 3.058 to 2.707 eV and 2.783, respectively. The refractive index of the PAni doped by BA was higher than that’s of PAni doped by HCl. The highest refractive index value was obtained from Reddy relation as 2.825 for PAni:BA (2:1). As a result, soluble PAni:BA (1:1) and PAni:BA (2:1) conducting polymers with enhanced optical properties could be a good candidate for fabrication of many optoelectronic devices such as diode and transistor.
Fig. 9. The (a) optical conductance (op ) and (b) electrical conductance (elec ) plot vs. E of the conducting polymers.
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Acknowledgement This work was supported by the Scientific and Technological Research Council of Turkey [grant number: 214Z199].
[14]
[15]
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Please cite this article in press as: M. Cabuk, B. Gündüz, Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration, Appl. Surf. Sci. (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.010