Electrochemical and optical studies on photoactive BiVO 4 -TiO 2 / /poly 3,4-ethylenedioxythiophene assemblies in gel electrolyte: Role of inorganic/organic interfaces in surface functionalization

Inorganic/ o rganic interface assemblies were created from poly 3,4-ethylenedioxythio-phene (PEDOT) interfaced with amorphous BiVO 4 and with BiVO 4 -TiO 2 . Electrochemical cells-based thermoplastic gel electrolytes containing KI/I 2 were used to study the photoelectrochemical behavior of the Inorganic/organic interface electrodes. Optical studies show that doping BiVO 4 with TiO 2 narrowed the optical band gap to allow more absorption in the visible region and increases solar energy conversion. Evidence for both direct and indirect band gaps was observed. Refractive index data indicates that BiVO 4 and BiVO 4 /TiO 2 obey the anomalous dispassion multiple-oscillator model. Chronoampero-metry of these assemblies shows the phenomena of dark current, which correlates to the presence of random electron/hole generation in the depletion layer. PEDOT enhances the photoactivity of BiVO 4 only. Electrochemical impedance spectroscopy studies indicated that both kinetic and diffusional control at high and low frequencies, respectively. Furthermore, studies show that as frequency increases, the conductivity increases due to dispersion and charge carrier hopping. All photoactivity outcomes were reproducible.


Introduction
Effective photoactive materials possess a narrow bandgap for visible light activities, noticeable electron-hole separation, longer electron lifetime, and longer charge carrier diffusion length.Among these materials are transition metal oxides, which possess several functional properties.Their crystal structure, composition, intrinsic defects, and doping may determine their optical, dielectric, and catalytic outcomes.To have the desired band gaps and conduction band (CB) edge positions that can be used in a needed photoelectrochemical process, a selective method of preparation must be used.The process's parameters strongly govern the morpho-structural characteristics and, therefore, the physicochemical properties of metal oxides [1,2].
The photochemical stability of TiO2 drove the attention to couple it with another metal oxide to create narrower band gap assemblies.Examples of such work includes NiO/TiO2 [10], Ag2O/TiO2 [11], Bi2O3/TiO2 [12], SnO2/TiO2 [13], Fe2O3/TiO2 [14].Narrow or small band gap semiconductors increase the absorption of visible light.It also inhibits photo-generated electron-hole recombination in a semiconductor heterojunction structure.To overcome the intrinsic limitations of binary metal oxides, several studies were directed to investigate the use of multiple cations to build multi-ternary oxides.These multi-ternary oxide assemblies, which contain some post-transition metal elements, should possess band gap edges that can be aligned with some desired redox systems.Mixed-valence transition-metal oxides with a spinal structure were found to be catalytically active for oxygen reduction reactions in alkaline conditions [15].Further, the catalytic activities of Bi20TO32 [16], SnNb2O6 [17], and BiVO4 [18,19] with visible light were investigated.Some studies show that the overall performance of BiVO4 is limited by poor carrier transport properties [20,21].Such poor carrier transport lengths were explained on the basis that some doping strongly decreases carrier mobility by introducing intermediate-depth donor defects as carrier traps [22].
Most studies on BiVO4 took place in liquid electrolytes to investigate its effectiveness in water splitting to produce hydrogen and oxygen.Recently, BiVO4 has been used as a competitive anode in lithium-ion batteries [23,24].Some metal oxide composites in connection with reduced graphene (rGO) or other conducting polymers in what is known as pseudocapacitors have been studied [25][26][27].
The lack of studies on the photoelectron-chemical (PEC) behavior of BiVO4 in gel electrolytes or under immobilization conditions drove our interest in exploring the PEC behavior of pure BiVO4 and BiVO4-TiO2 /poly 3,4-ethylene-dioxythiophene (PEDOT) interfaces in thermoplastic gel electrolyte (TPGE).The objective is to study the contribution of the BiVO4 /PEDOT interface to the PEC outcome in TPGE.One of the advantages of gel electrolytes is their potential to the creation of safe electrochemical (EC) devices (batteries or supercapacitors).Both BiVO 4, and PEDOT are electrochemically stable, and TPGE has no hazardous effect.Our goal is to have a stable and reproducible electrochemical system that provides useful data for future applications.

Reagents
The monomer 3,4-ethylenedioxythiophene (EDOT) (Alfa Aesar Inc., USA) was used to prepare poly 3,4-ethylenedioxythiophene (PEDOT).All other chemicals used were of analytical grade.Unless otherwise stated, all the solutions were prepared using the appropriate solvents, and deionized (DI) water was used for the aqueous solution.

Preparations
Both BiVO4 and BiVO4-TiO2 were prepared as previously described [19], briefly, BiVO4 and/or BiVO4/TiO2 composites are synthesized by adopting a sol-gel method and a facile hydrothermal route.A Teflon-lined stainless steel autoclave of 100 mL capacity was used and maintained at 190 °C for 8 h to get the needed composite.The obtained products were collected by centrifugation and washed several times with deionized water and dried at 90 °C for 12 h.Suspensions of BiVO4 or BiVO4-TiO2 in acetonitrile were dispersed evenly on the surfaces of fluorinated tin oxide (FTO) to create FTO/BiVO4 and FTO/BiVO4-TiO2 and allowed to dry at 40 °C.at ambient conditions for 4 hours.The inorganic/organic interface (IOI) thin films of BiVO4 and BiVO4-TiO2 deposited on FTO occluded in PEDOT were prepared electrochemically, using cyclic voltammetry (CV) technique, by repetitive cycling (3 cycles) of the FTO glass electrode potential between -0.5 V and 1.7 V vs. Ag/AgCl in an acetonitrile suspension (1 mg/mL) of the inorganic materials, 5 mM of the EDOT monomer and 0.2 M LiClO4.

Preparation of thermoplastic gel electrolyte (TPGE)
Thermoplastic gel electrolyte (TPGE) was prepared following the published procedure [28].Briefly, 0.65M KI and 0.065M I2 were dissolved in 10 mL polycarbonate (PC), and then 8.5 g of PEG (M-20 000) was added to the mixture.The mixture was heated at 100 °C under continuous stirring for ca. 12 h in a flask under an inert atmosphere.The mixture was hydrothermally treated at 180 °C for 14 h in a Teflon autoclave container.

Instrumentation
A conventional three-electrode cell consisting of a Pt sheet as a counter electrode, an Ag/AgCl reference electrode, and FTO with a surface area of 2.0 cm 2 was used as the working electrode for occlusion electrodeposition of BiVO4 or BiVO4-TiO2 during electrochemical polymerization of EDOT.Photoelectrochemical studies of the thin solid films in gel electrolyte were performed using the experimental setup described in Figure 1, where the FTO covered with the photoactive material was a working electrode and platinized FTO (Pt-FTO) served as both reference and counter electrode.The gel electrolyte was poured on top of the working electrode, and the counter electrode was pressed on top of the gel electrolyte to make a uniform electrolyte thickness of 10 μm (Figure 1).A Solartron 2101A, USA, was used for the electrochemical impedance spectroscopy (EIS) studies in a frequency range between 100 kHz to 0.01 Hz.A BAS 100W electrochemical analyzer (Bioanalytical Co. IN, USA) was used to perform electrochemical studies.Optical parameters were calculated based on the steady-state reflectance spectra measured by the Shimadzu UV-2101PC spectrophotometer.Irradiations were performed with a solar simulator 300-watt xenon lamp (Newport, NJ, USA) with an IR filter.All measurements were performed at 298 K.

Optical bandgap
The absorption spectra of BiVO4 and BiVO4-TiO2 were investigated along with other optical parameters such as refractive index (n) and optical conductivity (σopt).Both n and σopt have been calculated and plotted as a function of photon energy, the results are displayed in Figures 2, 3, and 4. Figure 2 A indicates that doping BiVO4 with TiO2 narrowed the band gap to allow absorption in the visible region.This behavior is related to the formation of surface states at the interface of BiVO4-TiO2 heterojunction, as it was also reported in other composite materials such as ZrO2/TiO2, SnO2/TiO2 and WO3/BiVO4 [13].Furthermore, Figures 2B and 2C were obtained after treatment of the absorption data in Figure 2A, as plots of α ½ vs. photon energy [hν] and (αhv) 2 vs. hν respectively, where α is the absorption coefficient as described in previous studies [29].Figures 2B and 2C indicate the existence of both direct and indirect band gaps in the studies assemblies.The estimated values of these band gaps are listed in Table 1.While the absorption spectrum for PEDOT is not displayed, the bandgap, direct, and indirect bandgap values are listed for comparison.The existence of direct or indirect bandgaps may indicate the creation of interfacial hybrid sub-bands with the PEDOT interface.This also indicates that doping BiVO4 with TiO2 shifts the absorption peak to photon energies smaller than the undoped BiVO4.The contribution of the hard/soft acid-base characteristics (HSAB) to the reactivities of these composites on FTO can be evaluated from a calculation of ΔN (the fraction of electrons transferred between the assemblies and the FTO interface) [30].The calculated hardness (η), electronegativity (), softness (š), and ΔN are listed in Table 1.The calculated η of BiVO4-TiO2 is less than that of BiVO4, while that of PEDOT is the least.Thus, š increases in this order PEDOT> BiVO4-TiO2> BiVO4.Such an increase in the acid softness character reflects strong adsorption on the substrate.Such adsorption facilitates the electron transfer process.The values of ΔN listed in the table are calculated as published elsewhere [30], indicating the fraction of electron transfer between the substrate (FTO glass) and any studied composites.The increase of ΔN is evidence of increasing electron transfer at the thiophene-substrate interface.

Refractive index
The optical, electronic, and optoelectronics properties of a semiconductor (SC) material are dictated by its refractive index (n) and its energy gap (Eg).The refractive index is a measure of the SC transparency to the incident spectral radiation [31].The greater the polarizability of the interface the greater the n.On that basis, the refractive index can reflect the purity of the system and the coexistence of multiple phases.The greater the deviation of n from the accepted measured value for a pure substance, the greater the existence of multiple phases.
Equations ( 1) and ( 2) [32,33] are based on the general assumption that all energy is scaled down by 1/ԑeff, where ԑeff is the dielectric constant. (1) where λ is the wavelength, nm.The plot of refractive index (n) vs. photon energy is displayed in Figure 3.The wavelength corresponding to the absorption edge is 450 nm for BiVO4 and 550 nm for BiVO4-TiO2.According to Equation 1, the calculated n value for BiVO4 is 2.43, while n for BiVO4-TiO2 is ≈2.55. Figure 3 indicates that these n values correspond to an Eg of 2.0 eV for BiVO4-TiO2 and Eg ≈2.6 eV for BiVO4.These Eg values support the presence of an indirect bang gap (cf. Figure 2B).This validates the approximation of Equation 2, as applying the indirect Eg values in this equation generates n = 2.47 for BiVO4 and n = 2.46 for BiVO4-TiO2.These values agree with previous studies [34].Both σopt and σele were calculated using equations ( 3) and (4) [35,36]: Figure 4 clearly shows that 1) σopt increases with increasing photon energy up to 2.4 eV for BIVO4-TiO2, while σopt for BiVO4, increases up to 3.4 eV, after which they remain constant for both compounds: 2) BIVO4-TiO2 has a greater conductivity than BiVO4 and 3) the great rise around the band gap of each assembly can be attributed to the electron excitation in this range of photon energy.Figure 4 also indicates that optical conductivity is much greater than electrical conductivity.Such behavior can be explained based on the J.T. Edwards [37] or the Drude model [38].However, electrical conductivity for BiVO4 increases with increasing photon energy, while that of BIVO4-TiO2 reaches a maximum at photon energy ca 2.4 eV and then decreases with increasing photon energy.

Photoelectrochemical (PEC) behavior
All electrochemical studies performed in gel electrolyte took place in the electrochemical cell described in Figure 1, where the modified FTO with either inorganic or organic PEDOT both acts as working electrode, thermoplastic KI/I3 -gel acts as an electrolyte, and platinized FTO (Pt/FTO) acts as both counter and reference electrode.

Effect of TiO2 on the EC behavior of BiVO4 thin film
PEC studies were performed in the dark and under illumination, with a scan rate of 0.10 V s -1 , between -1.5 to 2.0 V unless otherwise stated.The results are displayed in Figure 5.This figure shows that upon illumination, an increase in the photocurrent for FTO/BiVO4 (trace 3 or FTO/BiVO4-TiO2 (trace 4) in both cathodic and anodic scans.Furthermore, FTO/BiVO4-TiO2 gives the highest photocurrent, which indicates that TiO2 enhances the charge separation and decreases the (e/h) recombination.This is consistent with the values of ΔN (listed in Table 1), as BiVO4-TiO2 has greater ΔN than BiVO4.Effect of PEDOT on the EC behavior of BiVO4 thin film FTO-modified PEDOT occluded with BiVO4 or BiVO4-TiO2 was subjected to CV studies in the dark and under illumination, the results are shown in Figures 6 and 7. Figure 6 shows that PEDOT increases the photocurrent of BiVO4 during cathode scanning.On the other hand, Figure 7A shows that PEDOT did not increase the BiVO4-TiO2 photocurrent in the cathode scan (shaded traces 2 and 3).This can be explained if consideration is given to the possibility of the formation of a broken-band alignment between PEDOT and BiVO4-TiO2 with n-type heterojunction.Figure 7B shows that a larger photocurrent was recorded in the anode scan for BiVO4-TiO2/PEDOT (trace 3) than for solely BiVO4-TiO2 (trace 2).The band alignments between PEDOT and BiVO4-TiO2 (Figure 8) show higher HOMO energy for PEDOT than that of the conduction band of BiVO4-TiO2.In broken-band alignments, the current transport [39] can be achieved with three mechanisms: 1) interface tunneling, 2) drift-diffusion, and 3) ballistic tunneling.Any of these mechanisms can negatively affect the photocurrent outcome of the assembly.The fact that such behavior was not reported with BiVO4/PEDOT suggests that the presence of TiO2 as a dopant created a different subband at the interface with PEDOT.9A) and for FTO/BiVO4-TiO2 and FTO/PEDOT-BiVO4-TiO2 (Figure 9B).It can be noticed that in all studied assemblies, the gradual increase in photocurrent and the lack of sudden photocurrent rise indicate that the studied interfaces did not generate hole accumulation.On the other hand, under dark conditions, no sudden drop in the measured current instead a gradual drop of the measured current took place for ≈200 s (shaded area).The current generated in the absence of incident light photons is known as dark current, which may reflect the random generation of electrons and holes within the depletion region at the interface [40,41].Figure 9A and B shows that such behavior is reproducible with less photocurrent generation.

Effect of TiO2 doping in BiVO4
Nyquist plots as well as conductivity measurements for FTO/BiVO4 and FTO/BiVO4-TiO2, are displayed in Figures 10 and 11, respectively.Figure 10 shows both kinetic and diffusional control across the studied frequency range.The figure also shows that illumination increases the resistance (real impedance) and decreases the imaginary component of the studied assembly.This may be attributed to the fact that upon illumination, the photogenerated charges affect the charge transport properties.The generated electron/hole (e/h) pairs change the charge carriers' density, this will lead to a decrease in its mobilities and therefore increases the impedance.The fact that illumination increases the real impedance (true resistance) and decreases the imaginary impedance may reflect fewer effects on the inductive and capacitive impedances but more effects on the resistance.Furthermore, the doping of BiVO4 with TiO2 results in increases in the impedance.The shape of an un-concentered semicircle at high frequencies and the existence of Warburg impedance reflect the film porosity [42].The dielectric behavior of the generated assemblies was explored at 25 °C, by further treatment of EIS data for each assemblies.The AC conductivity was calculated using the following equation [43]: where ac is ac conductivity, L is film thickness, m, and a is electrode surface area, m 2 .Z' and Z" are the real and imaginary impedance, respectively.Figure 11 is the plot of the log ac vs. log .This figure clearly shows that under illumination the ac conductivity increases by raising the frequency for FTO/BiVO4 up to 100 Hz (Figure 11, trace 1, 2) after which the conductivity becomes independent of frequency.However, the illuminated BiVO4 shows less conductivity than that measured in the dark.Similar behavior was reported with FTO/BiVO4-TiO2 (Figure 10, trace 3,4) showing that, in the dark, the conductivity of BiVO4-TiO2 increases by raising the frequency to 100 Hz.However, under illumination, the conductivity continues to increase by increasing frequency up to 10 kHz (Figure 11, trace 4). Figure 11 shows that the addition of TiO2 to BiVO4 causes a decrease in the AC conductivity in comparison to that of BiVO4.This decrease in conductivity may be attributed to the increase in the capacitive reactance of the assembly.This leads to an increase in the impedance and consequently decreases the AC conductivity.The conductivity can also increase due to the hopping of charge carriers at high frequencies.Similar results were obtained when FTO/BiVO4 was interfaced with PEDOT (Figures 12 and 13).Figure 12A (trace 1, 2) shows that interfacing BiVO4-TiO2 with PEDOT results in similar results to that without PEDOT (Figure 10 A, trace 1, 2), where the illuminated interface shows increasing both real and imaginary impedance of the assembly.The doping of BiVO4 with TiO2 and interfacing it with PEDOT alters the equivalent circuits of this interface to include more inductive and capacitive elements (Figure 12 A, trace 3, 4).decreases the AC conductivity, especially at frequencies greater than 100 Hz.Such behavior can be attributed to the decrease in conductivity due to the increase in the capacitive reactance of the assembly.

Conclusion
Photoelectrochemical behavior (PEC) of mixed transition and post-transition metal oxides with or without interfacing with PEDOT in thermoplastic gel electrolyte (TPGE) shows that gel electrolyte has proven validity in electrochemical measurements.In comparison with the studies took place in liquid or aqueous electrolytes, the results obtained in gel electrolytes were reproducible and consistent.This confirms our assumption of the validity of gel electrolytes in electrochemical studies with mixed metal oxide systems.The noticeable photoactivity outcome confirms that mixed-metal oxide composites are excellent photocatalysts even in gel electrolytes.The results obtained in this study will add to the data bank of electrochemical studies in gel electrolytes.

Declaration of competing interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Table 1 .
The optical band gap and other acids/base characters for the studied compounds