Photo-Electrochemical Investigation of Inorganic / Organic Interfaces Assembly Consisting of Zn-Doped WO 3 / Poly 4-( Thiophen-3-yl ) Aniline

Photoactivities at inorganic/Organic/Interfaces (IOI) consisting of Zn-doped WO3 (Zn-WO3) /Poly 4-(Thiophen-3-yl) aniline (P3ThA) assemblies were investigated in nanoparticle suspensions and in thin solid film forms. The effects of P3ThA modifier on the photoelectrochemical behavior of the IOI were investigated using [Fe(CN)6] as a photoactive donor of hydrated electrons. Results show that the adsorption process of [Fe(CN)6] (photolysis product) controls the photoactivity outcomes of the IOI assemblies. P3ThA shows a greater heterogeneous photochemical response than native Zn-WO3. The band alignment between Zn-WO3 and P3ThA is of a p-p junction straddling gap type, where the charge transfer process is achieved through a hole transfer mechanism. The interface activities were explained by analyzing the IOI junction characteristics such as electron affinity, work function and hole/electron barrier heights. The creation of hybrid sub-band states close to the Fermi energy level at the interface was suggested. The aqueous nano-systems retained moderate stability as indicated by the reproducibility of their photocatalytic activities. Both [Fe(CN)6] and P3ThA contributed to the stability of the native Zn-WO3 surfaces.


Introduction
The quest for improving the solar to electrical energy conversion efficiencies in photovoltaic assemblies involved many techniques including the creation of the hybrid Inorganic/Organic interfaces or [IOI] (Wang Y. et al 2011, Kasem K. et al, 2008, Thomas K.G. et al 2003).The hetero-junction at the IOI assembly can affect its chemical, electrochemical, optical, magnetic and mechanical properties.Creating effective IOI requires energy coordination between the organic and inorganic interfaces for efficient charge transfer and separation as in heterojunction-type assemblies.Information about physical quantities such as electronegativity [ϰ ], Electron affinity [EA] , work function [Ф], barrier height [φ] and band gap [E g ] for components of the IOI will help understanding how the heterojunction assemblies work.
Surface modification caused by formation of IOIs, can be a very effective way to create or eliminate defects and alter the energy band at inorganic /organic interfaces.This will also alter the donor /acceptor character of the IOI assemblies.Recent studies show that binary oxides can provide more efficient charge separation, increased lifetime of charge carriers and enhanced interfacial charge transfer to absorbed substrates (Wang C. et al, 2002, Liao D.L. et al, 2008, Wang Y. et al, 2011, Kasem K. et al, 2008, Thomas K.G. et al, 2003, Kasem K. et al, 2009, Zhang, Qichun et al, 2008 ).Some metal chalcogenides modified with poly-aniline, poly-pyrrole, or other organic semiconductors were studied (Wang Y. et al, 2011, Kasem K. et al, 2008, Thomas K.G. et al, 2003, Kasem K. et al, 2009, Zhang, Qichun et al, 2008).Special assemblies of narrow band gap semiconductor nanostructures can be convenient systems for capturing visible light energy.Metal/ chalcogenides/ oxide semiconductors absorb only solar radiation that matches their band gaps.However, the spectral range can be widened if the metal sulfides surfaces are modified with agent/s that can absorb or become excited by greater radiation energies, such as UV.Some conjugated organic semiconductors absorb UV radiation and then re-emit radiation at longer wavelengths.
Several studies have been carried out in searching for organic semiconductors that can act as a modifier in IOI assemblies.Copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene were found to generate low-band gap materials (Kim, Ji-Hoon et al, 2014).Opto-electrochemical properties of selenopheno [3,2-c]thiophene as a low band gap conjugated polymer has been investigated (Jo, Yu-Ra et al, 2011).Furthermore, electrochemical copolymerizations of thiophene derivatives were used in building photovoltaic devices (Kumar, Prajwal et al, 2011), or Ion selective sensors (Pengchao Si, et al 2007) .Synthesis, thermal and optoelectrochemical properties of symmetrical conjugated thiophene and tri-phenylamine have been investigated (Vacareanu, Loredana et al, 2012).A study (Bondock, Samir et al, 2010) shows that the title compound is related to category of compounds that has a proven antimicrobial activity.
The compound 4-[thiophen-3-yl] aniline [3ThA] consists of thiophene and aniline that can be polymerized by chemical, photo-, and electro-chemical methods.The proposed structure of P3ThA is illustrated in Figure 1 inset C. Both polymer based aniline and polymer based thiophene have moderate to low band gap characters.No previous studies are made on the photoelectrochemical behavior of 3ThA.In the present work we report some of the photoelectrochemical behavior of 3ThA as the organic part of IOI assembly consists of Zn-doped WO 3 /P3ThA nanoparticles.Furthermore, the effectiveness of this assembly in hydrogen production during the photolysis of aqueous suspensions of nanoparticles of this assembly containing [Fe [CN]

Reagents
All the reagents were of analytical grade.All of the solutions were prepared using deionized water, unless otherwise stated.Zn-WO 3 / P3ThA were either in nanoparticles form or thin solid films.

Preparations
P3ThA: This polymer was prepared by both electrochemical and photochemical techniques: Polymer thin films were generated electrochemically using cyclic voltammetry (CV) by repetitive cycling of the FTO electrode potential at a scan rate 0.10V/s between -1.0 and 2.0 V vs Ag/AgCl in acetonitrile of 1 mM of 3ThA and 0.5M LiClO 4 .
B-(Occlusion Method): Thin films of Zn-WO 3 / (P3ThA) were generated electrochemically using cyclic voltammetry (CV) by repetitive cycling of the FTO electrode between -1.0 and 2.0 V vs Ag/AgCl in acetonitrile suspension of Zn-WO 3, 1 mM of the monomer and 0.5M LiClO 4 .

C-Preparation of Zn-Doped WO 3 / P3ThA / Interface:
Colloidal suspensions of Zn-doped WO 3 / P3ThA interface were prepared as follows: 0.05 g of Zn-doped WO 3 nanoparticles prepared as reported previously (Kalyanasundaram K. et al 1998) were suspended in the solution of 3-ThA in acetonitrile.The mixture was subjected to a 10 minute sonication followed by stirring for 1.0 hour to allow maximum adsorption of P3ThA on the Zn-doped WO 3 nanoparticles.The excess P3ThA was removed by centrifugation.Zn-doped WO 3 with adsorbed P3ThA was re-suspended in deionized water containing few drops of H 2 O 2 and subjected to UV radiation under constant stirring for 3 hours.The resultant Zn-Doped WO 3 / P3ThA was rinsed with deionized water several times and allowed to dry at 120 o C for 2 hours.

D-Deposition of Zn-Doped WO 3 / P3ThA Thin Solid Films:
Thin solid films of Zn-Doped WO 3 particles, modified with P3ThA (prepared as described in C) were suspended in acetonitrile solution of poly vinyl pyridine (PVP).The suspension was evenly spread over fluorine doped Tin Oxide (FTO) slides (12.5 x75 mm) and dried at 120 o C for 2 hours.The assembled electrode was transferred to a three-electrode cell containing the chosen buffer as the electrolyte and a Ag/AgCl and Pt electrode as reference and counter electrode, respectively.

Instrumentation
All electrochemical experiments were carried out using a conventional three-electrode cell consisting of a Pt wire as a counter electrode, Ag/AgCl as a reference electrode, and Pt gauze as an electron collector.A BAS 100W electrochemical analyzer (Bioanalytical Co.) was used to perform the electrochemical studies.Steady state reflectance spectra were performed using Shimadzu UV-2101 PC.Irradiation was performed with a solar simulator 300-watt xenon lamp (Newport) with an IR filter.Photoelectrochemical studies on thin solid films were performed on an experimental set as illustrated in Diagram 1A). www.ccsen

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