MSCE  Vol.6 No.8 , August 2018
Optical, Photoelectrochemical, and Electrochemical Impedance Studies on Photoactive Organic/Inorganic/Interface Assemblies of Poly 2,2 Bithiophene/Poly 3-(2-Thienyl) Aniline (PThA)/TiO2
ABSTRACT
Particles of TiO2 modified with poly 3-(2-thienyl) aniline (PThA) and occluded in poly 2,2 bithiophene (PBTh), were subjected to optical, electrochemical impedance spectroscopic (EIS) and photoelectrochemical (PEC) investigation in aqueous, acetate, citrate, and phosphate electrolytes. EIS studies revealed that the assembly film of TiO2/PThA/PBTh possess porous-type structure. They also confirmed the approximate value of Ef obtained from electrochemical studies. Both EIS and optical studies indicated that ac conductivity is much greater than dc conductivity. Guided by the properties of PBTh, no large changes in the energy band structure occurred due to occlusion of TiO2 in PBTh films. Occlusion of TiO2/PThA into the network structure of PBTh inhibits the energy dissipation process and impeded charge polarization process of the material. Photoelectrochemical outcome suggested possible band alignments between the organic film and TiO2 and formation of hybrid sub-bands. Inclusion of TiO2 in the thiophene-based polymers enhanced the charge separation and consequently charge transfer processes and widen the absorption in visible light range.

1. Introduction

An important method for creation or elimination of defects in solid materials is surface or bulk modification of inorganic/organic interfaces. This method will also alter the donor/acceptor character of these interfaces. The charge production, separation, and transfer at these interfaces were the subject of several investigations. Hybrid interfaces, such as those at the hetero-junction of inorganic/organic interfaces (IOI) recently became the focus of several research efforts [1] - [6] .

Because of its chemical stability, TiO2 was subjected to many investigations compared to other photoactive metal oxides. TiO2-based heterojunction assemblies were included in several applications [7] - [20] . Enhancing the photoelectrical performance of a dye-sensitized solar cell was achieved by doping SrTiO3: Sm3+/SiO2 core-shell nanoparticles in the cell’s photoanode [7] [17] . Several applications used TiO2-based interfaces for water splitting and hydrogen production [8] [15] , photocatalysis [9] [13] , and solar cells [12] . Involving TiO2 in photoactive self-cleaning polymer coatings was also recently reported [14] [20] .

Occlusion electrodeposition (OE) is one of the most effective methods for building photoactive assemblies of hybrid thin film interfaces. OE has been used to build composite films containing occluded TiO2 [21] [22] [23] [24] or CdS [25] [26] particles within other matrices.

In this article, we investigated difference(s) in optical, electrical properties, and photoelectrochemical behaviors caused by the occlusion of TiO2 surface modified with PThA in organic polymers Poly Bithiophene (PBTh). In particular, we studied the changes in the photocurrent generation as an indicator for this assembly’s ability to cause the photoinduced charge separation. Further electrochemical impedance spectroscopy (EIS) studies were used to investigate changes in electrical properties, such as dielectric constants and electrical conductivity. The host matrix was produced by electro-polymerization of 2,2 bithiophene (BTh) which forms polymeric networks suitable for efficient occlusion. The optical parameters such as the optical conductivity (σopt), optical absorption coefficient (α), refractive index (n), real dielectric constants (εr), and imaginary dielectric constants (εi) were also investigated.

2. Experimental

2.1. Reagents

The monomers 2,2 bithiophene (BTh), and 3-(2-thienyl) aniline (ThA) (Alfa Aesar) were used to prepare their corresponding polymers; poly 2,2 bithiophene (PBTh) and poly 3-(2-thienyl) aniline (PThA), respectively. All of the chemicals used were of analytical grade and used as received from the vendors. Unless otherwise stated all of the solutions were prepared using deionized (DI) water.

2.2. Preparations

Surface modified TiO2 nanoparticles were prepared as previously described [27] , briefly; suspensions of TiO2/P2ThA interface were prepared as follows: 0.05 g of TiO2 nanoparticles were suspended in the solution of 2ThA in acetonitrile. The mixture was subjected to a 10 minute sonication followed by stirring for 1.0 hour to allow maximum adsorption of 2ThA on the TiO2 nanoparticles. The excess 2ThA was removed by centrifugation. The IOI thin films were prepared using occlusion method; thin films of TiO2 modified with PTHA/PBTh were generated electrochemically using cyclic voltammetry (CV) by repetitive cycling of the FTO electrode potential between −0.5 and 1.7 V vs Ag/AgCl in an acetonitrile suspension (1 mg/mL) of TiO2, 1 mM of the BTh monomer, and 0.5 M LiClO4.

2.3. Instrumentation

A conventional three-electrode cell consisting of a Pt wire as a counter electrode, a Ag/AgCl reference electrode, and FTO with surface area 2.0 cm2 as working electrode was used for electrochemical studies. Photoelectrochemical studies on the thin solid films were performed on the experimental setup as described in previous work [27] . A Solartron 2101A was used for EIS studies. A BAS 100W electrochemical analyzer (Bioanalytical Co.) was used to perform the electrochemical studies. Optical parameters were calculated based on the steady state reflectance spectra, measured by a Shimadzu UV-2101PC spectrophotometer. An Olympus BX-FL reflected light fluorescence microscope, working with polarized light at wavelengths ranging between 330 and 550 nm was used to visualize the surface imaging of the film. Irradiation was performed with a solar simulator 300-watt xenon lamp (Newport, NJ) with an IR filter. All measurements were performed at 298 K.

3. Results and Discussion

3.1. Optical Studies

Optical parameters such as σopt, α, skin factor, n, εr and εi have been calculated and plotted as a function of photon energy. The results are displayed in Figures 1-5.

3.1.1. Optical Band Gap Studies

The absorption spectra of the TiO2/PThA/PBTh assembly displayed in Figure 1(A) indicates that occlusion of TiO2 shifts the absorption peak to higher photon energies than that of the host polymer PBTh. Figure 1(B) and Figure 1(C) were prepared after treatment of the absorption data as plots of α 1/2 vs photon energy (hυ) and (α*hυ)2 vs hυ, respectively, as described in previous study [28] . The value of α was calculated using a film thickness of 1.0 μm. Figure 1(B) and Figure 1(C) indicated that the absorption behavior of the host film was dominating the assembly behavior. Both the host polymer, PBTh, and the assembly showed direct and indirect band gaps. This is because the occlusion of TiO2, modified with PThA, created hybrid sub-bands with smaller band gaps between the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular

Figure 1. (A) Absorption spectra (B) α1/2 (cm−1/2) vs photon energy (C) (α*hυ)2, (eV・cm)2 vs photon energy for (a) PBTh and (b) TiO2/PThA/PBTh.

Figure 2. Plot of Ln α vs photon energy for (a) PBTh and (b) TiO2/PThA/PBTh.

Figure 3. (A) Refractive index n vs photon energy, (B) 1/(n2 − 1) vs (hυ)2, and (C) n2 vs λ2 for (a) PBTh and (b) TiO2/PThA/PBTh.

orbitals (LUMO) of the host polymer. Results shown in Figure 1 also indicate existence of absorption band tails attributed to energy band tail, also known as Urbach energy [29] .

Figure 2 shows the plot of lnα vs photon energy for the host polymer and for the assembly. The rising linear portion of the plot (indicated by colors) exhibits slopes of 1.536 and 2.454 for the assembly and the host respectively. These values

Figure 4. (A) Real εr, and (B) imaginary εi components of dielectric constant for (a) PBTh and (b) TiO2/PThA/PBTh.

Figure 5. Conductivity (optical σopt & electrical σele) vs photon energy, (a) σopt and, (a') σele for PBTh, and (b) σopt and (b') σele for TiO2/PThA/PBTh.

correspond to Urbach energy (energy band tail) values of 0.651 eV and 0.407 eV for the assembly and for the host PBTh, respectively. These value of energy band tail reflects the amorphous nature of the material; the greater the energy band tail, the greater the amorphous nature of the material. This indicates that occlusion of TiO2 modified particles into PBTh increased the degree of amorphousness of the assembly.

3.1.2. Optical Parameters

1) Refractive index, n

Figure 3(A) displays the plot of refractive index (n) vs photon energy. Although both materials exhibit a large increase in n when the photon’s energy is greater than 2.0 eV, the value of n for TiO2/PThA/PBTh is smaller than that of PBTh up to ≈2.5 eV, after which both n values are approximately equivalent. Figure 3(A) shows that both PBTh and TiO2/PThA/PBTh exhibits a normal dispersion region up to 2 eV or at λ 620 nm. At this region, both systems obey a single oscillator model. At λ shorter than 620 nm, an anomalous dispersion (multi-oscillator model) can be applied.

At region of normal dispersion, the following equation can be applied [30] :

[ n 2 ( h ν ) 1 ] 1 = 1 E o E d ( h ν ) 2 + E o E d (1)

where Eo is oscillator energy, and Ed is the dispersion energy. Plotting the values of 1/(n2 − 1) vs (hυ)2 in the region of single oscillator model, the values of Eo and Ed can be obtained from the slope and the intercept of the obtained line. Figure 3(B) displays the plots for both PBTh and TiO2/PThA/PBTh. The calculated Eo and Ed values for PBTh are 3.179 and 11.65 eV, respectively, while Eo and Ed for TiO2/PThA/PBTh are 2.58 and 2.035 eV respectively. As Ed is a measure of the inter band intensity, it can be concluded that occlusion of TiO2/PThA into PBTh reduced this intensity evident from the lower Ed of TiO2/PThA/PBTh than that of PBTh.

Figure 3(C) was created following the relation [31] :

n 2 = ε L [ e 2 Π C 2 ] [ N m * ] λ 2 (2)

The intercepts of the linear equations displayed in the Figure 3(C) denote to ε L lattice dielectric constant. These intercepts are 11.216 and 18.021 for PBTh and for TiO2/PThA/PBTh, respectively. This indicates that occlusion of TiO2/PThA into PBTh increased the lattice dielectric constant.

2) Dielectric constants, real εr, and imaginary εi

Figure 4 displays the plots of the calculated εr and εi against photon energy. The plot of the εr vs photon energy is displayed in Figure 4(A). This figure shows a pattern similar to that displayed in Figure 3(A). As εr was calculated from the relation εr = n2 − k2, and as n k , we can approximate that εr is directly proportional to n. On the other hand, Figure 3(B) shows the change in εi vs photon energy. It can be noticed that εi for the host polymer is greater than that of the hybrid assembly. The εi started increasing around the absorption edge and reach its maximum value when photon energy reached ≈2.5 eV for PBTh, and about 2.8 eV for TiO2/PThA/PBTh.

The results displayed in Figure 4(A) show that the εr of PBTh and that of TiO2/PThA/PBTh assembly have closer values around a photon energy range between 2.2 to 3.0 eV. Above and below this range the real dielectric part for TiO2/PThA/PBTh was less than that of PBTh. As the real part of the dielectric is related to polarization and anomalous dispersion, the εr indicates how much occlusion of TiO2/PThA/PBTh enhanced the speed of light in the material [32] .

The results displayed in Figure 4(B) show that: the εi of TiO2/PThA/PBTh assembly is less than that calculated for the host polymer PBTh. Such behavior can be explained considering that the TiO2/PThA nanoparticles occluded into PBTh inhibit the energy dissipation process [33] . Because εi is associated with dissipation of energy into the medium, the εi signifies the influence of dipole motion on energy absorption by the dielectric material from an electric field.

3) Optical conductivity σopt and Electrical conductivity σele

Both σopt and σele were calculated using the following formulas [34] [35] [36] :

σ o p t = α n c 4 Π (3)

and

σ e l e = 2 λ σ o p t α (4)

The plots of σopt and σele vs photon energy for PBTh and for TiO2/PThA/PBTh is displayed in Figure 5.

Figure 5 clearly shows that 1) σopt for PBTh is greater than that of TiO2/PThA/ PBTh, 2) σopt increases with increasing photon energy up to 2.5 eV for PBTh, and up to 3.0 eV for CdS/PThA/PBTh. The lower optical conductivity of TiO2/PThA/PBTh than PBTh is due to the presence of modified TiO2/PThA nanoparticles as a dopant in PBTh network structure. Figure 5(B) indicates that the dopant lacks the ability to provide the host polymer with an additional charge transfer [34] . Incident light interacts with charges of the material as a result of absorption of photon energy by the assembly. The presence of TiO2/PThA impeded the charge polarization process of the material. This means that the TiO2/PThA/PBTh negatively affected the dissipation of energy into the host PBTh film. This is consistent with the results displayed for the εi vs photon energy displayed in Figure 4(B). Figure 5(a') and Figure 5(b'), also shows that σele. for each of PBTh, and for TiO2/PThA/PBTh are smaller than the corresponding σopt. However, they increase slightly with decreasing the photo energy. Such behavior can be explained on the bases of the Drude model [37] . As electrical conductivity is considered as optical conductivity in a lack of alternating field (frequency), at lower photon energy optical conductivity will be under lower frequency.

3.2. Photoelectrochemical Behavior

The previous investigation done on the host polymer PBTh [27] was used to compare and drive conclusion on the contribution of the occluded TiO2/PThA to the photo activity outcome of the TiO2/PThA/PBTh assembly. Unless otherwise noted, the photoelectrochemical behavior was investigated in the dark and under illumination by cycling the potential of FTO/TiO2/PThA/PBTh between −1.0 to 1.0 V vs. Ag/AgCl at a scan rate of 0.10 V/s in a given electrolyte. The electrode surface area was kept at 2.0 cm2.

3.2.1. Electrochemical Behavior of the TiO2/PThA/PBTh Assembly in Aqueous Acetate Electrolyte

The behavior of the FTO/TiO2/PThA/PBTh assemblies was investigated in 0.2 M acetate electrolyte (pH 8). Figure 6(A) shows that the recorded photocurrent is greater than the current recorded in the dark in the cathodic scan at ≈0.30 vs Ag/AgCl. This means that the approximate Efb (flat band potential) of the assembly is at ≈0.30 V or 0.50 V vs SHE. (Table 1). The photocurrent-time

Figure 6. Photoelectrochemical behavior of TiO2/PThA/PBTh in 0.2 M acetate electrolyte (A) CV at 0.1 V/s, in (a) dark (b) illumination; (B) Photocurrent vs time curve at −0.5 V vs Ag/AgCl (c) in the presence of O2, (d) after purge with N2.

Table 1. Photoelectochemical data for the TiO2/PThA/PBTh.

IP = ionization potential, Eg = band gap, EA = electron affinity.

curve displayed in Figure 6(B-c), Figure 6(B-d) was generated by subjecting the FTO/CdS/PThA/PBTh assembly to illumination at constant potential (−0.5 V vs Ag/AgCl). Upon illumination of an oxygenated electrolyte, a sharp current spear shown in the first trail followed by steady small changes for longer time Figure 6(B-c). This behavior was reproducible but with a smaller magnitude in the following trials. Such behavior is indication of fast charge recombination due to hole accumulations at the outermost layers of the assembly/electrolyte interface [38] . When the experiment was repeated in deoxygenated electrolyte (using N2 gas), the illumination generated much less photocurrent (Figure 6(B-d)). Figure 6(B) also shows a reduction in the capacitive current in the deoxygenated electrolyte compare to that in presence of oxygen. These results assume that O2 plays an important role in enhancing charge separation during the illumination period.

3.2.2. Electrochemical Behavior of the TiO2/PThA/PBTh Assembly in Aqueous Citrate Electrolytes

Figure 7 displays the electrochemical behavior of FTO/TiO2/PThA/PBTh in aqueous citrate electrolyte (pH 8). This figure shows that at ≈0.5 V vs Ag/AgCl, the photocurrent exceeds the current recorded in the dark for citrate electrolyte (Figure 7(A)) in the cathodic scan, we assume that the value of the hybrid sub-band is at ≈0.7 V vs SHE.

The photocurrent vs time curve in Figure 7(B) shows a behavior comparable to that observed in Figure 6(B) (acetate electrolyte). However, upon illumination

Figure 7. Photoelectrochemical behavior of FTO/TiO2/PThA/PBTh in 0.2 M Citrate electrolyte (pH 8) (A) CV at 0.1 V/s a) dark, (b) illumination; (B) Photocurrent vs time curve at −0.5 V vs Ag/AgCl (c) in the presence of O2, and (d) after purge with N2.

of the oxygenated citrate electrolyte (Figure 7(B)), a reproducible larger sharp anodic current spear is observed. Such phenomena were more noticeable in the deoxygenated electrolyte (Figure 7(B-b)). When the light is off there is evidence for reversed transient current, as evident by the small cathodic current spike at the first few seconds in dark. This is due to backflow of electrons from the substrate FTO to the assembly body.

When the electrolyte was deoxygenated, illumination generated much less photocurrent. This behavior was reproducible through multiple cycles of illumination and darkness. The photocurrent generated in citrate is greater than that generated in acetate.

3.2.3. Electrochemical Behavior of the TiO2/PThA/PBTh Assembly in Aqueous Phosphate Electrolyte

Figure 8 displays the electrochemical behavior of TiO2/PThA/PBTh in 0.2 M phosphate electrolyte (pH 6) in dark and under illumination. Figure 8(A) shows that at ≈0.4 V vs Ag/AgCl (0.6 V vs SHE), the recorded photocurrent exceeds that measured in the dark during the cathodic scan. The manual chopping of light experiment indicates that the assembly is highly responsive to the illumination-dark cycles. Furthermore, Figure 8(B) shows the photocurrent-time curve under a constant potential (ca −0.500 V vs Ag/AgCl) with illumination for a longer period of time. Upon illumination in the oxygenated phosphate electrolyte (Figure 8(B-c)), a sharp anodic current spike, similar to that observed in the citrate was obtained. In darkness, there is no evidence for reversed transient current. This means that no backflow of electrons from the substrate FTO to the assembly body took place. When the electrolyte was deoxygenated using nitrogen gas (Figure 8(B-d)), much less photocurrent was recorded with behavior similar to that observed in the oxygenated solution.

Upon illumination of the oxygenated phosphate electrolyte (Figure 8(B-a)), a sudden increase in the photocurrent was recorded followed by a steady decrease in photocurrent to constant quantity. The initial decay reflects some e/h recombination. The photocurrent vs time curve for the host polymer PBTh only [27] is

Figure 8. Photoelectrochemical behavior of TiO2/PThA/PBTh in 0.2 M phosphate electrolyte (pH 6). (A) CV at 0.1 V/s (a) dark, (b) illumination; (B) Photocurrent vs time curve at −0.5 V vs Ag/AgCl (c) in the presence of O2, and (d) after purge with N2.

smaller than that observed in Figure 8(B). This indicates that occlusion of TiO2 enhanced the photocurrent generation as a result of improvement of the photo-induced charge separation.

We further investigated the effect of changing the pH on the Efb of this assembly. No changes in Efb were observed within the pH 5 - 8 range. A change of approximate 2 pH units did not affect the position of EFb in the sulfur-based assembly. The relation between EFb and pH in oxide-based semiconductors, changes by 25 mV per change in 1 pH unit.

Oxygen involvement in the photochemical activities is in the electron consummation processes illustrated by the equation:

O 2 + 2e + 2H + H 2 O 2 (5)

As PBTh act as p-type semiconductor where holes are the charge carrier. When the outermost layer of the assembly is hit by suitable photon energy, this creates a shorter diffusion course to photogenerated holes to reach the adsorbed anions on the surface of the assembly. This makes the hole consummation by the used electrolytes anions is important step in the mechanism of charge separation.

The following explains the oxidation of the studied anions at the electrolyte TiO2/PThA/PBTh/electrolyte interface.

For oxidation of phosphate anion, a formation of phosphate radical anion [39] can prevent the e/h recombination process according to Equation (6),

HPO 4 2 ( aq ) + h HPO 4 ( aq ) (6)

Involvement of both oxygen and phosphate in the charge separation process that lowers the e/h recombination is explained by Equations (5) and (6).

In case of carboxylic anions, a Kolb-type reaction [40] causes photooxidation of carboxylate anions according to the following equation:

R ( COO ) ( aq ) + h R ( aq ) + CO 2 ( gas ) (7)

3.2.4. Electrochemical Impedance Spectroscopic Studies

Impedance spectra of the FTO/TiO2/PThA/ or FTO/PBTh was measured and analyzed on three-electrode cell containing liquid electrolytes, between 105 - 10−2 Hz utilizing Solartron 1201A, MX-studio ECS software. Impedance complexes (Nyquist plot) generated from these studies are displayed in Figure 9. This Figure shows both kinetic control at high frequency and diffusional control at low frequency. The shape of unconcentrated semicircle in at high frequencies and existence of Warburg impedance reflects the film porosity [41] . The calculated Cdl was 7.43 × 10−5 F. The maxima of the semicircle corresponded to relaxation frequency of 1.25Hz, which is 0.79 s relaxation time.

Using equivalent circuit and modeling approach by Randel [42] , the reaction rate at the assembly interface can be calculated. The difference between Rct and the intercepts of the tangent line of Warburg diffusional region equals to [(Rct ×

λ)2 × Cdl], where λ = k D , k is rate constant, and D is the diffusion coefficient.

Knowing (λ) and D, k can be calculated For Warburg frequency region (the very low frequencies), plotting the Z'' vs 1/ω (Figure 9 inset) generates a straight line with slop = 1/CL) = 191 F−1. Substituting the approximate RL value of 5000 ohm, the diffusion coefficient can be determined using the following equation:

R L = 1 C L L 2 3 D C T (8)

For L = 1 µm, the calculated D was = 5.65 × 10−10 cm2/s. The calculated k under dark condition is 1.89 × 10−5 cm/s, while under illumination k is 2.22 × 10−5 cm/s.

1) Dielectric constants

Figure 10 Shows that dielectric constants increased at very low frequencies. As frequency increased, the values of the dielectric constants decreased. Such

Figure 9. Nyquist plot of 3 µm, TiO2/PThA/PBTh film on FTO in 0.2 M acetate electrolytes (pH 6) at 0.5 V vs Ag/AgCl (a) dark, (b) light.

Figure 10. Plot of dielectric constant ε vs log ω (a) real dielectric component (εr), and (b) imaginary component (εi) under illumination, (a') εr and (b') εi, in dark, in acetate electrolyte (pH 8).

behavior was previously observed and attributed to the inability of the electric dipoles to comply with variation of an applied a.c. electric field [43] . Materials that possess conducting grains, but with poor conducting boarders causes charge carriers accumulated at these boarders, when external external electric field (low frequency) is applied. This creates large polarization and consequently a high dielectric constant [44] .

2) AC conductivity σac

The σac was calculated using the following equation [45] .

σ a c = L A Z Z 2 + Z 2 (9)

According to the following equation [46] :

σ a c = σ d c + A ω S (10)

where A is the strength of polarizability, s is temperature dependent parameter which can be determined from the slope of line of the plot of logσac vs logω.

log σ a c = log σ d c + log A + S log ω (11)

Figure 11 was constructed to show the plot of calculated log σac vs logω at different frequencies. This figure clearly shows the positive correlation between σac and ω at the high dispersive region of high frequencies range up to several kHz. The slope of the line (s) was = 0.7901, which indicates the hopping due to the translational motion [47] . Figure 11 also shows that the conductivity at very low frequency (ca 10−2 Hz) which is corresponds to σdc, and it is much smaller than σac. The energy required to remove one electron from one site to another within the film structure (Wm) or binding energy, can be calculated from the following relation [48] :

Figure 11. logσac vs logω for TiO2/PThA/PBTh/FTO in acetate electrolyte (pH 6) at 298 K, in dark.

W m = 6 k B T 1 s (12)

The obtained s value is corresponding to Wm of 1.233 × 10−19 J or 0.76 eV. The minimum hopping distance Rmin can be calculated as follow [48] :

R min = 2 e 2 π ε ε o W m (13)

The hopping distance Rmin corresponding to the calculated Wm is 15 nm. Both Wm and Rmin are temperature dependent, they generally decreases as temperature increases if s decreases with increasing temperature. The data plotted in Figure 11 were closer to those reported under illumination. No change in s value was reported.

3.3. Band-Energy Map of TiO2/PThA/PBTh

Mott-Schottky plot of TiO2/PThA/PBTh in acetate electrolyte was generated using 1 KHz with a sinusoidal signal of 10 mV peak to peak amplitude (Figure 12). The slope indicates a carrier density (holes) ND = 2.93 × 1019. The intercept indicates the position of flat band potential (Ef) at 0.40V vs Ag/AgCl or at 5.4 eV on vacuum scale. In similar studies on the host film PBTh, the value for ND was 9.67 × 1019, with no changes in Ef values. This indicates that the only change that occlusion of TiO2/PThA in PBTh caused was a lowering of the carrier density. Closer look at the CV’s displayed in Figures 6-8, the current recorded upon illumination exceeds that recorded in dark at ≈0.4 V vs Ag/AgCl or at ≈0.6 V vs SHE. This potential was assumed to be Ef, and it is confirmed by Mott-Schottky plot. This also indicates that Ef did not change by changing the electrolyte.

The data listed in Table 1 were used to generate an energy map displayed in Figure 13. This figure illustrates the formation of a hybrid sub-band energy level that organizes the charge transfer at the TiO2/PThA/PBTh. Hybridization between hole-like and electron-like sub-band states takes place in close proximity

Figure 12. Mott Schuttky plot of TiO2/PThA/PBTh in 0.2 M acetate electrolyte pH 6.

Figure 13. Energy map for TiO2/PThA/PBTh interface.

to the Fermi level. The difference between the energy level of the hybrid band and valance band (VB) represents a hole barrier height of TiO2 being ~2.2 eV. That is more negative than the VB of TiO2. When the magnitude of the hybrid band has more negative potential (~2.2 V) than VB, attraction to the hole is stronger than at a less negative potential. This makes the charge transfer process via hole transfer more likely. This rule out electrons participation in the charge transfer process. This is because the electron barrier height is ~1.3 eV which is ~1.3 V more positive than the potential of the electrons in the LUMO of the modifier PThA. At the PThA/TiO2, the electrons are concentrated at a lower energy level than the LUMO. This causes the electron barrier height to be even lower than the calculated value (~1.3 eV).

4. Conclusion

EIS studies revealed that the assembly film of TiO2/PThA/PBTh possesses a porous-type structure. It also confirmed the approximate value of Ef obtained from electrochemical studies. Guided by the properties of the host PBTh, some optical properties such as (Eo) oscillator energy, and (Ed) dispersion energy, σopt and σele (≡σdc) were calculated. EIS was used to calculate σac and σdc. Both EIS and optical studies indicated that ac conductivity is much greater than dc conductivity. Data listed in Table 1 indicate that no large changes in the energy band structure due to the occlusion of TiO2 in organic films occurs. The fact that the σopt of the assembly is less than σopt of PBTh indicates that occlusion of modified TiO2 nanoparticles into the network structure of PBTh; 1) inhibited the energy dissipation process, and 2) impeded charge polarization process of the material. Photoelectrochemical results show that the behavioral outcome of the assemblies was dominated by poly bithiophene. Possible band alignments between the organic film and TiO2 nanoparticles, cause formation of hybrid sub-bands. Furthermore, inclusion of TiO2 in the thiophene-based polymers enhanced the charge separation, and consequently charge transfer processes. The PBTh, PThA, and amorphous TiO2 have band gaps that allow absorption of broad wavelengths in the blue zone which makes both materials and their I/O/O/I assemblies potentially useful in solar energy harvesting systems.

Acknowledgements

The authors acknowledge Office of the Academic Affairs, at Indiana University Kokomo for supporting this project.

Cite this paper
Kasem, K. , Sadou, H. , Worley, H. and Wenger, J. (2018) Optical, Photoelectrochemical, and Electrochemical Impedance Studies on Photoactive Organic/Inorganic/Interface Assemblies of Poly 2,2 Bithiophene/Poly 3-(2-Thienyl) Aniline (PThA)/TiO2. Journal of Materials Science and Chemical Engineering, 6, 50-67. doi: 10.4236/msce.2018.68005.
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