Optical , Photoelectrochemical , and Electrochemical Impedance Studies on Photoactive Organic / Inorganic / Interface Assemblies of Poly 2 , 2 Bithiophene / Poly 3-( 2-Thienyl ) Aniline ( PThA ) / TiO 2

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.


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].
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 TiO 2 [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 TiO 2 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.

Preparations
Surface modified TiO 2 nanoparticles were prepared as previously described [27], briefly; suspensions of TiO 2 /P2ThA interface were prepared as follows: 0.05 g of TiO 2 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 TiO 2 nanoparticles.The excess 2ThA was removed by centrifugation.The IOI thin films were prepared using occlusion method; thin films of TiO 2 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 TiO 2 , 1 mM of the BTh monomer, and 0.5 M LiClO 4 .

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 cm 2 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.

Optical Studies
Optical parameters such as σ opt , α, skin factor, n, ε r and ε i have been calculated and plotted as a function of photon energy.The results are displayed in Figures 1-5.

Optical Band Gap Studies
The absorption spectra of the TiO 2 /PThA/PBTh assembly displayed in Figure 1(A) indicates that occlusion of TiO 2 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 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 Journal of Materials Science and Chemical Engineering  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 TiO 2 modified particles into PBTh increased the degree of amorphousness of the assembly.

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 TiO 2 /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 TiO 2 /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]: where E o is oscillator energy, and E d is the dispersion energy.Plotting the values of 1/(n 2 − 1) vs (hυ) 2 in the region of single oscillator model, the values of E o and E d can be obtained from the slope and the intercept of the obtained line.
The intercepts of the linear equations displayed in the Figure 3(C) denote to L ε lattice dielectric constant.These intercepts are 11.216 and 18.021 for PBTh and for TiO 2 /PThA/PBTh, respectively.This indicates that occlusion of TiO 2 /PThA into PBTh increased the lattice dielectric constant.
2) Dielectric constants, real ε r , and imaginary ε i Figure 4 displays the plots of the calculated ε r and ε i against photon energy.The plot of the ε r vs photon energy is displayed in Figure 4(A).This figure shows a pattern similar to that displayed in Figure 3(A).As ε r was calculated from the relation ε r = n 2 − k 2 , and as n k  , we can approximate that ε r is directly proportional to n.On the other hand, Figure 3(B) shows the change in ε i vs photon energy.It can be noticed that ε i for the host polymer is greater than that of the hybrid assembly.The ε i 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 TiO 2 /PThA/PBTh.The results displayed in Figure 4(A) show that the ε r of PBTh and that of TiO 2 /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 TiO 2 /PThA/PBTh was less than that of PBTh.As the real part of the dielectric is related to polarization and anomalous dispersion, the ε r indicates how much occlusion of TiO 2 /PThA/PBTh enhanced the speed of light in the material [32].
The results displayed in Figure 4(B) show that: the ε i of TiO 2 /PThA/PBTh assembly is less than that calculated for the host polymer PBTh.Such behavior can be explained considering that the TiO 2 /PThA nanoparticles occluded into PBTh inhibit the energy dissipation process [33].Because ε i is associated with dissipation of energy into the medium, the ε i signifies the influence of dipole motion on energy absorption by the dielectric material from an electric field.
3) Optical conductivity σ opt and Electrical conductivity σ ele Both σ opt and σ ele were calculated using the following formulas [34] [35] [36]: and The plots of σ opt and σ ele vs photon energy for PBTh and for TiO 2 /PThA/PBTh is displayed in Figure 5.
Figure 5 clearly shows that 1) σ opt for PBTh is greater than that of TiO 2 /PThA/ PBTh, 2) σ opt 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 TiO 2 /PThA/PBTh than PBTh is due to the presence of modified TiO 2 /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 TiO 2 /PThA impeded the charge polarization process of the material.This means that the TiO 2 /PThA/PBTh negatively affected the dissipation of energy into the host PBTh film.This is consistent with the results displayed for the ε i vs photon energy displayed in Figure 4(B).Figure 5(a') and Figure 5(b'), also shows that σ ele .for each of PBTh, and for TiO 2 /PThA/PBTh are smaller than the corresponding σ opt .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.

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 TiO 2 /PThA to the photo activity outcome of the TiO 2 /PThA/PBTh assembly.Unless otherwise noted, the photoelectrochemical behavior was investigated in the dark and under illumination by cycling the potential of FTO/TiO 2 /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 cm 2 .

Electrochemical Behavior of the TiO2/PThA/PBTh Assembly in
Aqueous Acetate Electrolyte The behavior of the FTO/TiO 2 /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 E fb (flat band potential) of the assembly is at ≈0.30 V or 0.50 V vs SHE.(Table 1).The photocurrent-time  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 N 2 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 O 2 plays an important role in enhancing charge separation during the illumination period.

Electrochemical Behavior of the TiO2/PThA/PBTh Assembly in
Aqueous Citrate Electrolytes Figure 7 displays the electrochemical behavior of FTO/TiO 2 /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 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.

Electrochemical Behavior of the TiO2/PThA/PBTh Assembly in
Aqueous Phosphate Electrolyte Figure 8 displays the electrochemical behavior of TiO 2 /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 smaller than that observed in Figure 8(B).This indicates that occlusion of TiO 2 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 E fb of this assembly.No changes in E fb were observed within the pH 5 -8 range.A change of approximate 2 pH units did not affect the position of E Fb in the sulfur-based assembly.The relation between E Fb 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: 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 TiO 2 /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), 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: Using equivalent circuit and modeling approach by Randel [42], the reaction rate at the assembly interface can be calculated.The difference between R ct and the intercepts of the tangent line of Warburg diffusional region equals to

Electrochemical Impedance Spectroscopic Studies
where , 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/C L ) = 191 F −1 .Substituting the approximate R L value of 5000 ohm, the diffusion coefficient can be determined using the following equation: For L = 1 µm, the calculated D was = 5.65 × 10 −10 cm 2 /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.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].
According to the following equation [46]: 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 logσ ac vs logω.log log log log Figure 11 was constructed to show the plot of calculated log σ ac vs logω at different frequencies.This figure clearly shows the positive correlation between σ ac 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 σ dc , and it is much smaller than σ ac .The energy required to remove one electron from one site to another within the film structure (W m ) or binding energy, can be calculated from the following relation [48]: The obtained s value is corresponding to W m of 1.233 × 10 −19 J or 0.76 eV.The minimum hopping distance R min can be calculated as follow [48]: The hopping distance R min corresponding to the calculated W m is 15 nm.Both W m and R min 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.

Band-Energy Map of TiO2/PThA/PBTh
Mott-Schottky plot of TiO 2 /PThA/PBTh in acetate electrolyte was generated using 1 KHz with a sinusoidal signal of 10 mV peak to peak amplitude (Figure 12).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 E f , and it is confirmed by Mott-Schottky plot.This also indicates that E f 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 TiO 2 /PThA/PBTh.Hybridization between hole-like and electron-like sub-band states takes place in close proximity  That is more negative than the VB of TiO 2 .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/TiO 2 , 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).

Conclusion
EIS studies revealed that the assembly film of TiO 2 /PThA/PBTh possesses a porous-type structure.It also confirmed the approximate value of E f obtained from electrochemical studies.Guided by the properties of the host PBTh, some optical properties such as (E o ) oscillator energy, and (E d ) dispersion energy, σ opt and σ ele (≡σ dc ) were calculated.EIS was used to calculate σ ac and σ dc .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 TiO 2 in organic films occurs.The fact that the σ opt of the assembly is less than σ opt of PBTh indicates that occlusion of modified TiO 2 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 TiO 2 nanoparticles, cause formation of hybrid sub-bands.Furthermore, inclusion of TiO 2 in the thiophene-based polymers enhanced the charge separation, and consequently charge transfer processes.The PBTh, PThA, and amorphous TiO 2 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.

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 TiO 2 , modified with PThA, created hybrid sub-bands with smaller band gaps between the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular

Figure 3 (
B) displays the plots for both PBTh and TiO 2 /PThA/PBTh.The calculated E o and E d values for PBTh are 3.179 and 11.65 eV, respectively, while E o and E d for TiO 2 /PThA/PBTh are 2.58 and 2.035 eV respectively.As E d is a measure of the inter band intensity, it can be concluded that occlusion of TiO 2 /PThA into PBTh reduced this intensity evident from the lower E d of TiO 2 /PThA/PBTh than that of PBTh.
Impedance spectra of the FTO/TiO 2 /PThA/ or FTO/PBTh was measured and Journal of Materials Science and Chemical Engineering analyzed on three-electrode cell containing liquid electrolytes, between 10 5 -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 C dl 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.

Figure 10 Figure 9 .
Figure 10 Shows that dielectric constants increased at very low frequencies.As frequency increased, the values of the dielectric constants decreased.Such

6
The slope indicates a carrier density (holes) N D = 2.93 × 1019 .The intercept indicates the position of flat band potential (E f ) 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 N D was 9.67 × 1019 , with no changes in E f values.This indicates that the only change that occlusion of TiO 2 /PThA in PBTh caused was a lowering of the carrier density.