Experimental and Computational Study of the Effect of Temperature on the Electro-Polymerization Process of Thiophene ()
1. Introduction
Conducting polymers are promising materials for innovative technological applications in research topics like electronic and optoelectronic science. They have been successfully implemented in light emitting diodes (LED’s), solar cells, transistors, nanoswitches, imaging materials and chemical sensors [1].
The electrochemical synthesis of conducting polymers, also known as electro-polymerization, is a very complex process where many factors are involved. During the last decades, the electrochemical synthesis of conducting polymers on metallic substrates has been thoroughly studied, however, the formation mechanism of this process is not completely clear.
The proposed model for the electro-deposition of conducting polymers [2-4] establishes that the nucleation process and subsequent growth on the electrode surface depends on the degree of saturation of the high density oligomeric region, HDOR, i.e. is determined by the solubility of the oligomers present at the electrode-solution interface. Thus, when oligomers reach a critical chain length they become insoluble, precipitate on the electrode surface and originate nuclei that cause polymer film growth.
Electro-polymerization depends of different experimental factors such as the chemical nature and concentration of monomer and electrolyte, the solvent, the physical and chemical nature of the electrode substrate, and the conditions of the electrical perturbation applied to the interface [4]. Among literature, it is possible to find some studies about the effect of experimental factors on the nucleation and growth mechanism (NGM), but only a few of them point to a systematic study regarding temperature effect on the NGM during the electro-synthesis of polymer films on metal substrates [3].
According to experimental evidence reported by Mostany and coworkers [5], a temperature increment during the electro-crystallization process, will not produce any modification in the nucleation and growth mechanism of the metal. This can be explained by the Arrhenius equation:
(1)
where A and A0 are kinetic factors and Eb is the required energy to overcome the energetic barrier of any event, which in general is independent of the temperature. Consequently, the rate A is directly proportional to the temperature; an increase of the temperature will raise the rate of the event. Then, in a deposition process, temperature increments will not change the nature of the process; there will be only a higher probability of some events to occur.
In this work, we performed a computational and an experimental study of the electro-polymerization of poly (thiophene) (PTh), in order to identify the temperature effect on its electrochemical synthesis. The computational simulation method was based on a kinetic Monte Carlo algorithm [6] and reproduced key processes such as diffusion, oligomerization, and the precipitation of oligomers onto the electrode surface. Simulation results were compared to the experimental evidence to test the model and evaluate its prediction performance. Theoretical finds had excellent agreement with the experimental results and demonstrated how computational simulations can help to understand and predict the electrochemical process of conducting polymers formation.
2. Experimental and Computational Methodology
2.1. Experimental Details
A conventional three-compartment, three-electrode cell was employed throughout this work. A polycrystalline platinum disk (0.07 cm2 geometric area) was used as working electrode. The counter electrode was a coiled Pt wire with a surface area at least ten times greater than the working electrode. It was separated from the electrolytic solution by a sintered glass. All potentials quoted in this paper are referred to the saturated calomel electrode (SCE) [7]. All reagents were provided by Aldrich. The electro-polymerization of thiophene (≥99% purity) was carried out in anhydrous acetonitrile (99.8% purity, 0.001% water) using a concentration of 0.1 mol·L−1. Tetrabutyl ammonium hexafluorophosphate (TBAPF6, 0.1 mol·L−1) dried at 110˚C and kept into a dryer was used as supporting electrolyte.
Prior to all the experiments, solutions were purged with high-purity argon during 30 min and an argon atmosphere was maintained over the solution during the measurements. The optimal electro-polymerization conditions (monomer and TBAPF6 concentration and potential scanning range) and the potential range of chronoamperometry experiments were selected by potentiodynamic methodologies (cyclic voltammetry, CV). Potentiostatic measurements were performed to study nucleation and growth process of PTh films.
The electrochemical work was conducted on a PGP100 Voltalab potenciostat/galvanostat. Data was recorded by means of a compatible computational program (Voltamaster 4). Temperature was regulated and kept constant using a thermal bath controlled by a HAAKE G thermostat-cryostat that enables an ethylene glycol-water mixture to flow through the electrochemical cell jacket. The temperatures of work selected were 263, 273, 283, 293 and 303 K.
2.2. Model and Simulation Method
As described in our first report [8], a Fortran77 code based on kinetic Monte Carlo rules, was developed to model the electro-polymerization of thiophene. Monomers were represented as spheres and the electrolyte was not included explicitly. 3D reticular model was used (Figure 1) with a total of [s·s·(h + r)] positions, each representing a cube with an edge length d.
The size of d was selected according to the crystallographic center-center distance of the rings of the thiophene dimer [9] (d = 3.89 Å). The simulation cell contains at the upper end a reservoir of monomers, labeled A
Figure 1. Schematic representation of the simulation box.
in Figure 1. During the simulation monomers were added to A as needed in order to keep a constant number of particles. The region B, the reaction zone, is delimited by region A at the top, and at the bottom by a box representing the working electrode. Periodic boundary conditions were implemented along the code in the x and y directions parallel to the electrode surface. The reservoir was placed at a distance sufficiently large so that the monomers in A do not interact with particles near the surface. Particle entrance from zone B to A was not allowed.
Based on an experimental study of thiophene [10] (Th), the critical chain length was established as three units. The simulation started with the diffusion of monomers from the reservoir to the electrode. Every movement was selected randomly, and its probability was directly proportional to the magnitude of its rate in the processes catalogue, i.e. higher rate values have greater probabilities of being selected. The time corresponding to a movement Γ was calculated from a randomly chosen number μ, selected by a code’s subroutine [11], uniformly distributed between 0 and 1 according to Equation (2):
(2)
where R corresponds to the total sum of all rates included in the processes catalogue.
In the model proposed, when a monomer reaches the electrode or gets in contact with already deposited oligomers, is automatically labeled as a radical cation, and can diffuse inside the zone B again. This radical monomer can react with other activated monomer creating a neutral dimer. Dimers can also become radical species in the same way as monomers, and can react with cationic monomers or dimers producing neutral trimers and tetramers, respectively. In accordance to experimental evidence, which indicates that the film grows parallel to the metal surface [12], the generation of oligomeric chains with geometry parallel to the z axis was not allowed. This consideration was reflected in a lower production of trimers, tetramers, pentamers and hexamers. The formation of branched or nonlinear chains was not permitted, in order to preserve the planarity and linearity of the oligomers. Triple linkage can also occur when radicals move up or down in the z axis and find two activated oligomers lined up on the electrode surface, generating trimers, tetramers, pentamers and hexamers. When the size of the chain exceeds two units, the oligomer can still diffuse inside the zone B, however, upward movements in the z direction are not allowed to guarantee the precipitation on the electrode. When insoluble oligomers reach the bottom of the simulation box or touch already deposited oligomers they become immediately fixed and thus, dendritic growth is expected. The diffusion of precipitated species was not permitted. The concentration set in the simulation (0.01 mol·L−1) was lower than the one used at the experimental study (0.1 mol·L−1) to reduce computational cost and calculation time.
Diffusion rates calculated by Molecular Dynamic (MD) simulations through the Einstein-Stokes expression [13] have been introduced for the oligomers (1 Th - 6 Th) present in the simulation. Each oligomer has the same rate in the three axes. Table 1 resumes these values.
The temperatures of the MD diffusion coefficients are considerably distant from the temperatures of the experimental part. However, the relevant factor in this study is the effect of temperature rising and not the quantity of increment.
3. Results and Discussion
3.1. Experimental Measurements
3.1.1. Cyclic Voltammetry
Figure 2 depicts voltammetric profiles recorded during the thiophene electro-polymerization at the extreme working temperatures: 268 and 303 K, respectively. No relevant differences related to the nucleation and growth potential range (closer to anodic inversion potential) were observed at these temperatures. Thus, in both cases the growth mechanism was studied between 1720 and 1770 mV.
Voltammetric profiles recorded at the same successive scan number were similar at both temperatures. This indicates that under the same electrochemical conditions, temperature has no critical influence in the electrochemical deposition mechanism of PTh. There is only a difference between the charges involved. The current recorded is higher at higher temperature, accounting for an increased deposition rate.
3.1.2. Potentiostatic Measurements
The deposition process of PTh by a potentiostatic perturbation (fixed potential) was also study. Figure 3
Table 1. Diffusion coefficient (105·cm2·s−1) of thiophene (Th) oligomers at different concentrations and temperatures*.
Figure 2. Potentiodynamic profiles of Th at (a) 263 and (b) 303 K. Interface: Pt| 0.1 mol·L−1 Th + 0.1 mol·L−1 TBAPF6, CH3CN. Scanning potential limits Es,c = 0.30 V and Es,a = 1.85 V.
Figure 3. Time-current transient of Th at (a) 263 and (b) 303 K. Interface: Pt| 0.1 mol·L−1 Th + 0.1 mol·L−1 TBAPF6, CH3CN.
shows the current-time (i-t) profiles at 263 and 303 K in a potential range between 1720 mV and 1770 mV. All profiles were recorded till constant current intensity.
At both temperatures the i-t transients exhibited a complex mechanism, with two clear steps: the first where PTh deposits on the bare surface of the working electrode, and a second step where a new layer of PTh layer grows on the first one. As previously reported [14], only the initial stage of the transients was considered for the analysis of the PTh deposition mechanism. This zone of the i-t profile delivers relevant information about the modification of the surface of the working electrode, first nuclei deposition and morphology development. As the working temperature increased, the second step appeared at shorter time. This fact suggests that all implicated processes come earlier.
Table 2 summarizes the induction time (τ) of the first