Electrical transport and morphological studies of polyaniline nanostructures.
Conducting polymers are a class of functional materials having alternating single and double carbon-carbon bonds along the polymeric chains. The highly conjugated polymers possess reversible chemical, electrochemical, and physical properties which can be controlled by doping/de-doping process. The attractive electronic and optoelectronic properties arise from the extended [pi]-electron system along the polymer backbone . Nanostructuring of conducting polymers enhances their unique electrical, optical, and chemical properties that lead to wide range of technological applications . The conductivity and morphology of the structures are greatly influenced by the nature of the doping acid, doping levels, and dopant to monomer molar ratio . In conducting polymers solitons, polarons and bipolarons are the charge carriers, which are generated by the process of doping and contribute to the conduction by the phonon assisted variable range hopping (VRH) [4, 5] or by fluctuation-induced tunneling (FIT) . Among the different conducting polymers, Polyaniline (PAni) has received most interest for its special characteristics such as wide range of conductivity from insulating to metallic regime, unique redox tunability, good environmental stability, biocompatibility, ability to be produced as bulk powder, films or fibers, ease of synthesis, and low cost , Some of its promising applications are metallic corrosion protection, actuators, sensors, electrostatic discharge, and so forth. . The PAni can mainly occur in three forms, fully reduced leucoemeraldine, partially oxidized emeraldine base, and fully oxidized pernigraniline. Electrical and optical properties of PAni vary with different oxidation states. It can be controlled to conduct across a wide range from insulating to highly conductive for different electrical purposes . In the recent years along with the progress in processing of conducting polymers, the nanostructures of these materials in the form of nanoparticles, nanofibers, nanotubes, nanospheres, and so forth, have been synthesized. The nanostructures provide large surface area and enhanced charge transport rate, which leads to remarkable development in different field and gives opportunity for applications in electronic nanodevices, biomedical technologies, and so forth . During the doping process due to the incorporation of [H.sup.+] PAni transforms from emeraldine base to emeraldine salt. The solubility of PAni crucially depends on the choice of dopant. Although PAni is doped with HCl it shows poor solubility and increased instability upon exposure to moisture. Using functionalized protonic acid such as camphorsulfonic acid as a dopant it is possible to obtain enhanced solubility and stability of the nanostructures . Besides, larger molecular size of CSA helps in obtaining more ordered nanostructures as compared to other protonic acid such as HCl, PTSA, HCl[O.sub.4], and so forth.
In the past several years, researchers are working to understand the charge transport in PAni nanostructures. The low temperature conductivity measurements can determine the dimensionality of transport mechanism. The electrical, morphological, and optical properties of PAni nanostructures are not completely understood where the properties are expected to be different as compared to that of the bulk due to the quantum confinement. In this work, we have studied the electrical transport and morphological properties of self-assembled CSA doped polyaniline nanostructures at low temperatures by varying the dopant to monomer molar ratio to control the morphology, crystallinity, optical properties, and dc conductivity of the nanostructures.
Aniline (Ani) monomer and ammonium peroxydisulfate (APS) were purchased from Merck. Doping agent camphorsulfonic acid (CSA) was obtained from Aldrich. Methanol (Merck) and acetone (Merck) were used to wash the synthesize nanostructure.
Synthesis of Polyaniline Nanostructures
In a typical synthesis procedure, Ani monomer (2 mmol) and CSA (0.5 mmol) were mixed in 20 ml of deionized water with magnetic stirring at room temperature for 1/2 h. The mixture was reacted and formed a transparent solution of CSA-Ani salt. Before oxidant is added, the solution was cooled in an ice bath. An aqueous solution of APS (2 mmol in 10 ml of distilled water), cooled in advance, was then slowly added to the above cooled CSA-Ani salt solution. After adding the oxidant, the mixture was left to react for 15 h at 0-5[degrees]C. After 15 h, the collected precipitate was washed several times with DD water, methanol, and acetone, and then dried in vacuum at room temperature for 24 h. Four sets of samples were synthesized by taking the [CSA]/[Ani] molar ratio as 1:4, 2:4, 3:4, and 1:1, which are coded as P1, P2, P3, and P4, respectively.
The morphologies of the CSA doped PAni nanostructures were examined by high resolution transmission electron microscope (JEM-2100, 200 kV, Jeol). The X-ray scattering of the nanostructures was performed using Rigaku X-ray Diffractometer (model MINIFLEX 200) with Cu[K.sub.[alpha]] radiation ([lambda] = 1.5406 [Angstrom]) in the 2[theta] range of 0[degrees]-40[degrees]. Optical spectra of the samples were recorded on a Shimadzu Corporation, Japan/UV2450 spectrometer. Infrared spectra in the range of 400-4000 [cm.sup.-1] on the samples pellets made with KBr were measured by means of an infrared spectrometer (Nicolet Impact I-410). Micro-Raman spectroscopy was performed to study the different molecular interactions using Renishaw In-Via micro-Raman spectrometer (Renishaw, Wotton-under-Edge, UK) at a resolution of 0.3 [cm.sup.-1]. [Ar.sup.+] laser of 514.5 nm wavelength was used for excitation. Thermal stability of the nanostructures was investigated by Perkin-Elmer thermogravimetric analyzer (TGA, Model 6000) from 50 to 700[degrees]C under nitrogen atmosphere at the heating rate of 30[degrees]C/min. The temperature dependence of dc conductivity has been measured on rectangular shapes (~3.2 x 2.5 x 2.1 [mm.sup.3]) cut from the disk shaped pellets in the temperature range 50-300 K using standard four-probe method. Ohmic electrical contacts were made using silver paste. A helium cryostat with computer controlled measuring system was used to measure the dc conductivity. The current was adjusted at each temperature to keep the power dissipation in the sample less than 1 [micro]W to avoid sample heating. Lakeshore temperature controller 340, Keithley source meter 2400, and Keithley digital voltmeter 180 were used for the measurements.
RESULTS AND DISCUSSION
High resolution transmission electron microscopy (HRTEM) is a powerful tool used to infer information about the shape, size, and the size distribution of nanostructures, which are the key parameters for determining their physical properties. HRTEM images of polyaniline nanostructures obtained at different dopant to monomer [[CSA]/[Ani]] molar ratio are shown in Fig. 1. The average diameter of the PAni nanofibers for the sample P1 is 26 nm and average diameter of nanotubes for the samples P2, P3, and P4 is 98 nm, 106 nm, and 96 nm, respectively. The HRTEM micrographs reveal that formation of the nanostructures is greatly influenced by the [CSA]/[Ani] molar ratio. When [CSA]/[Ani] molar ratio is 1:4, the system has CSA-Aniline salt micelle and free aniline. The protonated aniline can adsorb on the micellar surface and helps the linear growth. The excess free aniline diffuses inside the hydrophilic micelle and the polymerization takes place inside the micelles resulting in the formation of nanofibers (Fig. 1a) . When APS is added to the CSA-Ani salt solution initially nanoparticles are formed due to the excess amount of protonated aniline present in the solution. The CSA-Ani micelles act as substrates to grow the nanoparticles and results nanofibers at lower [CSA/ Ani] molar ratio because of the presence of excess free aniline. At higher value of [CSA]/[Ani] molar ratio as there is no free aniline the nanoparticles aggregate on the micelle substrate due to the hydrogen bonds and [pi]-[pi] interactions and the phenyl rings provide variety of active sites for nucleation, organization, and binding of the nanoparticles . The side-on attachment of the nanoparticles results in increased diameter of the nanostructures resulting in the formation of the nanotubes. The well-ordered nanotubes are observed in Fig. 1c for 3:4 dopants to monomer molar ratio. For higher [CSA]/[Ani] molar ratio, the amount of free aniline decreases and the reaction is dominated on the surface of the micelles leading to the formation of nanotubes. When the dopant concentration is about 1 M or higher, the formation probability of PAni nanotubes again decreases (Fig. 1d) that can be attributed to the prevention of secondary growth at higher dopant concentration. For 1:1 [CSA]/[Ani] ratio, free aniline molecules disappear in the aniline/acid aqueous solution and most of aniline monomers are transformed into amphiphilic anilium ions and the deformation of PAni nanotubes at higher concentration may be related to the effect of salts on the micelle structures . The amphiphilic anilinium ion micelles were polymerized with the morphological transition from spherical to cylindrical morphology owing to the salt ions such as [S.sub.2][O.sup.2-.sub.8], N[H.sup.+.sub.4], and [H.sup.+] present in the reaction medium. Besides these the H-bonding interaction between the benzenoid amine and [H.sub.2]O, and electrostatic interaction between the S[O.sup.-.sub.3] anion of CSA and the quinoid imine coexists in the PAni doped with CSA .
X-ray Diffraction Analysis
X-ray diffraction pattern of the polyaniline nanostructures is shown in Fig. 2 and it exhibits the partial crystalline nature of the samples. The diffractions peaks at 2[theta] = 19[degrees] and 2[theta] = 25.5[degrees] can be attributed to the parallel periodicity and perpendicular periodicity of the polymer chains , respectively. The sharp peak at 2[theta] = 6.2[degrees] (d = 14.26 [Angstrom]) is assigned to the periodicity distance between the dopant atom and the N atom on the adjacent main chains. Thus, the presence of the peak at 2[theta] = 6.2[degrees] indicates the short range ordering between the chiral counter anion and the polymer chains . The enhancement of the diffraction peak at 2[theta] = 6.2[degrees] intensity with increasing dopant concentration indicates the increase in nanostructures ordering. To find out the relative degree of crystallinity, the intensity ratio [I.sub.2[theta]=25[degrees]/[I.sub.2[theta]=19[degrees]]] the PAni nanostructures has been calculated and their values are tabulated in Table 1. From the intensity ratio, it is observed that the peak intensity values increase from 0.50 to 0.61 with increasing the [CSA]/[Ani] molar ratio, becomes maximum at 3:4 molar ratio and again decreases to 0.57 for [CSA]/[Ani] molar ratio of 1:1. The higher peak intensity ratio for [CSA]/[Ani] molar ratio of 3:4 sample indicates the formation of highly ordered nanostructures and it is also confirmed from HRTEM micrographs .
The UV-Vis absorption spectrum of PAni nanostructures at different [CSA]/[Ani] molar ratio is shown in Fig. 3. The optical absorption is associated with the electronic transitions from highest occupied molecular orbital (HOMO) [pi]-band to lowest unoccupied molecular orbital (LUMO) [[pi].sup.*]-band of electronic states. In the absorption spectra, two absorption peaks at about 330 nm and 680 nm are observed. The peak at 330 nm can be attributed to the [pi]-[pi]* electron excitation within the benzenoid segments and the absorption band at 680 nm corresponds to the excitation from the nitrogen lone pair to [[pi].sup.*] band , From the spectra it is observed that with increasing the [CSA]/[Ani] molar ratio the 680 nm absorption peak shifts towards higher wavelength side. The red shifting of this peak indicates the increase in the extent of [pi]-conjugation length, that is, enhancement of ordering in nanostructures.
Figure 4 shows the FTIR spectra of CSA doped PAni nanostructures for different [CSA]/[Ani] molar ratio. A broad peak around 3400 [cm.sup.-1] is due to the N-H stretching of PAni. Vibrational band at 810 [cm.sup.-1] is an indication of C-H out-of-plane bending vibration , The band at 1297 [cm.sup.-1] is attributed to the stretching of C-N secondary aromatic amines and displacement of [pi] electrons induced by the acid doping of the polymer . Presence of the two bands in the vicinity of the 1576 [cm.sup.-1] and 1476 [cm.sup.-1] is assigned to the nonsymmetrical [C.sub.6] ring stretching modes. The vibration band at 1576 [cm.sup.-1] corresponds to the quinoid ring units whereas lower frequency vibration band at 1476 [cm.sup.-1] depicts the presence of benzenoid ring units. The presence of these two bands indicates that the PAni is composed of amine and imine units [19, 20]. In particular, the band at 503 [cm.sup.-1] is ascribed to the absorption of -S[O.sub.3]H group indicating that the PAni is in salt form . The band at 1111 [cm.sup.-1] is characteristic of the conducting form of PAni, which can be ascribed to the charge delocalization on the polymer backbone .
Raman spectroscopy is a powerful tool which can measure the vibrational spectra of nonpolar bonds such as crystal-lattice vibrations and carbon-carbon bonds. This technique, in combination with FTIR spectroscopy, can give a more complete picture of the sample bonding structure. The micro-Raman spectra of CSA doped PAni nanostructures are shown in Fig. 5. The Raman bands observed at 1177 and 1600 [cm.sup.-1] can be assigned to the in-plane deformation of the C-C bond of the quinoid ring and C-C stretching of the quinoid ring, respectively. Bands at 569 and 606 [cm.sup.-1] correspond to the deformation of the benzene ring in the PAni backbone and cross-linking between the PAni chains, respectively . The band at 1463 [cm.sup.-1] is assigned to the formation of bipolarons. Raman bands at 1247 and 1401 [cm.sup.-1] are attributed to the C-N stretching of the polaronic units and C-C stretching of quinoid units and ring stretching vibrations of phenazine like structures , Band at 1557 [cm.sup.-1] is assigned to the C=C stretching vibrations of quinoid. The small band at 1812 [cm.sup.-1] is ascribed to the benzene ring deformation , The band at 1336 [cm.sup.-1] in the spectrum gives the information about the C-[N.sup.+.] vibration of the delocalized polaronic structures. The enhancement of intensity with increasing [CSA]/[Ani] molar ratio suggests the increase in the degree of delocalization of C-[N.sup.+.] segments .
The thermal stability of the CSA doped PAni nanostructures was investigated by TGA analysis in nitrogen atmosphere and the results are depicted in Fig. 6. The TGA curves exhibit three-step weight loss patterns. The first weight loss below 100[degrees]C corresponds to the loss of water, the second weight loss in the temperature range from 250 to 450[degrees]C is assigned to the loss of acid dopant and last weight loss starting at around 500[degrees]C is attributed to the degradation of the backbone units of PAni chains . As a comparison of the third weight loss of PAni it is observed that with increasing the [CSA]/[Ani] molar ratio the decomposition temperature shifted towards higher value, which indicates the increased thermal stability of PAni nanostructures at higher doping level. The enhancement of thermal stability may be due to formation of more ordered nanostructures at higher doping level which is also confirmed from HRTEM micrographs.
The study of temperature and doping level dependence of dc conductivity is a powerful tool to investigate the nature of hopping transport in conjugated polymers. As dc conductivity is determined by the weakest link in the conducting path spanning the sample, the study of [[sigma].sub.dc] (7) gives insight into the slowest transport processes in the system . According to Mott and Davis , the temperature dependence of the hopping conductivity follows the relation
[sigma] = [[sigma].sub.o] exp [-[([T.sub.o]/T).sup.[gamma]]
with, [gamma] = 1/(1 + d), (1)
where [T.sub.o] is the characteristic Mott temperature that corresponds to the hopping barrier for the charge carriers and measures the degree of disorder present in the system, [[sigma].sub.o] is the conductivity at T [right arrow] [infinity], and d is dimensionality. The value of [gamma] = 1/2, 1/3, and 1/4 for 1D, 2D, and 3D VRH in the system. In case of highly heterogeneous systems, conduction takes place by electronic tunneling through the non-conducting regions separating metallic islands rather than between the localized states. If the metallic islands are sufficiently large that the electrostatic charging energy is smaller than kT for accessible temperatures, tunneling can occur between metallic states of the same energy on different sides of the barrier without thermal excitation. As the temperature increases the fluctuation in the voltage across the tunneling junction gives rise to the increase of tunneling current. The conductivity according to fluctuation-induced tunneling (FIT) model can be expressed as :
[sigma](T) = [[sigma].sub.t] exp [[-T.sub.t]/(T + [T.sub.S])], (2)
where [T.sub.t] is the temperature at which fluctuations in the voltage across the barrier become sufficiently large to make the electron energy higher than the barrier height and [T.sub.s] is the temperature above which thermally activated over-barrier conductivity becomes possible. For small metallic islands, the FIT model leads to the form:
[sigma](T) [approximately equal to] -[T.sup.-0.5] (3)
which is similar to one-dimensional (ID) VRH. The transport properties of conducting polymers are strongly dependent on the degree of disorder present in the material. The disorder in the materials comes from many factors including (1) the nonequivalence of conjugated fragments, (2) the presence of both crystalline and noncrystalline phases, (3) impurities, (4) substitution at the monomer unit, and so forth. Moreover in the quantum picture of disorder, the conducting polymers are regarded as a randomly distributed polaron/bipolaron lattice. When disorder is introduced into a system electronic states change so as to give rise to a scattering process that would decrease the amplitude of the charge carriers for moving from one localized state to other , The extent of disorder is generally characterized in terms of the temperature dependence of the conductivity, more specifically in terms of conductivity ratio, [[sigma].sub.r] = [sigma] (300 K)/[sigma] (50 K). In the present samples P1, P2, P3, and P4, the conductivity ratio is 1827, 1003, 256, and 358, respectively. The decrease of conductivity ratio with increasing the molar ratio indicates the enhancement of ordering in the system. There are three regimes of carrier transport depending upon the degree of disorder : (i) metallic region ([[sigma].sub.r] < 2), (ii) critical region (2 < [[sigma].sub.r] < 6), and (iii) insulating region ([[sigma].sub.r] > 6). The higher value of conductivity ratio indicates that the samples fall in the insulating region. Figure 7 shows the temperature dependence of dc conductivity of the PAni nanostructures in the temperature range of 50-300 K for different dopant to monomer molar ratio. In the whole temperature range the sample shows semiconducting behavior, that is, conductivity increases with increasing temperature. From the conductivity plot it is observed that with increasing CSA concentration, the electrical conductivity has a rising trend until it reaches the maximum conductivity for [CSA]/[Ani] molar ratio of 3:4 followed by decrease at higher ratio of 1:1. In conducting polymers polarons, bipolarons and solitons are the charge carriers that are generated upon doping. In conductive PAni, polarons are the main charge carriers and they not only transfer along the chains but also jump between chains . With increasing CSA concentration, more [H.sup.+] ions may form bonds with =N- and more polarons can be formed leading to the enhanced electrical conductivity. At higher doping level the excess CSA saturates the 1, 4 positions of the quinoid ring resulting in their strong effects on the charge distribution on the chains leading to the structural distortion . The structural distortion could be the reason for decrease of conductivity when dopant to monomer molar ratio exceeds 3:4. HRTEM micrographs confirm that most ordered nanotubes are formed at dopant to monomer ratio of 3:4 (Fig. 1c) and at higher dopant to monomer ratio of 1:1 the nanotubes become distorted (Fig. 1d).
In crystalline materials, the temperature dependence of the conductivity gives the activation energy, and this method is typically used to differentiate metals from semiconductors and insulators. Metals and insulators have positive and negative values of (d ln[[sigma](T)/d ln(T), respectively. In case of conducting polymers, the transport properties are dominated by the disorder, which results in the charge carriers' localization. For such systems, the insulating, critical, and metallic behaviors can be distinguished with the help of temperature dependence of the reduced activation energy (W) expressed as :
W = d(ln [sigma])/d(ln T). (4)
Positive, zero, and negative slopes of the W-T plot correspond to the metallic, critical, and insulating regimes, respectively. The negative slopes in the present case indicate that the samples fall in insulating regimes. The value of the exponent [gamma] can be obtained from the slope of log-log plot of W versus T (the inset of Fig. 8a-d). The values of [gamma] are found to be 0.47, 0.31, 0.26, and 0.28 for samples PI, P2, P3, and P4, respectively, from Fig. 8. For Mott's VRH mechanism the value of [gamma] is 0.25 for 3D, 0.33 for 2D, and 0.5 for ID hopping. From the obtained [gamma] values, a transition from ID to 3D hopping is observed by changing the dopant to monomer molar ratio. The crossover from 1D to 3D hopping with increasing the dopant to monomer molar ratio can be attributed to the enhancement of the interchain connectivity. When a conducting polymer is doped, the charge carriers are generated and the dopant counter ions are incorporated into the system. The counter ions locally decrease the interchain potential encountered by the charge carriers resulting in the enhancement of hopping rate . The Fermi energy is shifted upwards and the counter ions can be considered as the hopping sites when the energy difference of the dopant energy level to the Fermi level becomes of the order of hopping energy . As the dopant concentration increases, the density of hopping sites increases making the variable range hopping in the direction perpendicular to the polymer chains (enhancement of interchain charge transfer integral) resulting in the 3D hopping at higher dopant concentration.
The transport parameters such as most probable hopping distance (R) and average charge hopping energy can be calculated for all three dimensions using the following relations [4, 35, 36]:
[R.sub.hop] = [([T.sub.o]/T]).sup.1/2] ([[alpha].sup.-1]/4) and [[DELTA].sub.hop] = Zk [T.sub.o]/16 (5)
[R.sub.hop] = [[alpha].sup.-1] [([T.sub.o]/T]).sup.1/3] and [[DELTA].sub.hop] = kT[([T.sub.o]/T]).sup.1/3] (6)
and for 3D,
[R.sub.hop] = (3/8) [([T.sub.o]/T]).sup.1/4] [[alpha].sup.-1] and [[DELTA].sub.hop] = (1/4)kT[([T.sub.o]/T]).sup.1/4], (7)
where Z is the number of nearest neighbor chains (~4), [[alpha].sup.-1] is the localization length, and k is Boltzmann constant. The value of characteristic Mott temperature [T.sub.o] is obtained from the slope of the linear fit of the plots shown in Fig. 8a-d. The hopping distances ([R.sub.hop]) and average hopping energy ([[DELTA].sub.hop]) calculated for temperature T = 100 K are presented in Table 1. The localization length [[alpha].sup.-1] has been taken to be 75 [Angstrom] , It is observed from the table that the average hopping distance ([R.sub.hop]), the average hopping energy ([[DELTA].sub.hop]), and characteristic Mott temperature ([T.sub.o]) decrease with increasing dopant concentration. The decrease of the transport parameters with increasing dopant concentration indicates the enhancement of hopping rate between the localized states leading to the increased conductivity.
In summary, we have successfully synthesized polyaniline nanostructures by self-assembly method using CSA as dopant and APS as the oxidant. The structural and transport properties of PAni nanostructures are strongly affected by the [CSA]/[Ani] molar ratio. HRTEM micrographs show a transition from nanofibers to nanotubes and more ordered nanotubes are formed at [CSA]/[Ani] molar ratio of 3:4. Enhancement of the peak intensity at 2[theta] = 6.2[degrees] in XRD diffraction pattern indicates the increase of ordering between the counter anion and the polymer chains with increasing dopant concentration. Temperature dependence dc conductivity shows semi-conducting behavior in the entire temperature range of 50-300 K. From the conductivity plot it is observed that with increasing CSA concentration, the electrical conductivity exhibits maximum conductivity for [CSA]/[Ani] molar ratio of 3:4, which may be due to formation of ordered nanotubes as confirmed from HRTEM. In FTIR spectra the band at 1111 [cm.sup.-1] is the characteristic of the conducting form of PAni that can be ascribed to the charge delocalization on the polymer backbone. The enhancement of the 1336 [cm.sup.-1] band intensity in the micro-Raman spectra with increasing [CSA]/[Ani] molar ratio suggests the increase of delocalization degree of C-[N.sup.+.] segments. The red shifting of the absorption band at 680 nm with increasing [CSA]/[Ani] molar ratio in UV-Vis absorption spectra can be attributed to the increase of [pi]-conjugation length. Thermal study shows the enhancement of thermal stability with increasing the dopant to monomer ratio.
Authors sincerely thank Dr. R. Rawat, Scientist UGC-DAECSR Indore centre for his help in conductivity measurements. Authors gratefully acknowledge the financial support provided by UGC-DAE-CSR Indore Centre, India through project grant No. CSR-I/CRS-50.
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P. Chutia, A. Kumar
Materials Research Laboratory, Department of Physics, Tezpur University, Tezpur 784028, Assam, India
Correspondence to: A. Kumar; e-mail: email@example.com
Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Structural and transport parameters of CSA doped PAni nanostructures. Parameters P1 P2 P3 P4 [I.sub.2[theta]=25[degrees]]/ 0.50 0.57 0.61 0.57 [I.sub.2[theta]=19[degrees]] [T.sub.o] (K) 15225.5 14728.4 12,331 13572.8 [R.sub.hop] ([Angstrom]) 231.4 389.5 93.7 95.9 [[DELTA].sub.hop] (meV) 328.3 44.8 7.2 7.4
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|Author:||Chutia, P.; Kumar, A.|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 2015|
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