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Insights on tunneled electrons for electrical and photoelectric behaviors in conducting multilayer films.

INTRODUCTION

Layer-by-layer (LbL) assembly has an ability to fine-tune the composition of nanostructured films, which are preferred in modern nanoscience and nanotechnology [1-6]. The merits of LbL assembly technique on simplicity, universality, and controllability facilitate their employment in electronic, microelectronic, electrochemical, and microelectromechanical nanodevices [7, 8]. However, traditional techniques give a limit of fabricating devices in submicro level, which can not meet the nowadays needs in high-tech fields. As a versatility approach, LbL assembly provides regular molecular-level architectures in a facile manner. To make such nanodevices, multifunctional substances, such as conducting polymers, graphene, and quantum dots are usually assembled into the nanostructures [9-11]. Among diverse functional compounds, polypyrrole (PPy) as well as its derivatives are attracted growing interests because of their high conductivity, easy synthesis, excellent photoelectric properties, good environmental-stability, irreversible redox, and nontoxicity [12].

To date, PPy has been successfully used in nanoscaled molecular junctions, supercapacitors, photovoltaic cells, sensors, lithium batteries, and photocatalysts [13, 14]. However, the conventional techniques such as the electropolymerization or chemical polymerization uniformly make architectures with intertwisted molecular conformation can no longer form an interconnected network inside the nanodevices, which is highly desirable to reduce the electronic resistance to improve the conduction, electrochemical stability and also to achieve rapid charge-transfer kinetics [15], By addressing these issues, fabrication of well-defined mono-/multi-layers of PPy on electrode is of high importance [16, 17], The electrical performances, in particular, the extraordinary charge transfer by a tunneling mode is important for designing versatile electrical devices. However, there is no report on the charge tunneling effect in self-assembly multilayers.

Nanodevices from PPy multilayers are comparatively rare because of the poor solubility of PPy in aqueous solution. Increase of PPy dosage in per multilayer can significantly enhance their functionality in nanodevices, especially in molecular junctions. Our goal was that of producing well-defined multilayers of poly(vinyl alcohol) solubilized polypyrrole (PVA/PPy) and negatively charged poly(styrcne sulfonate) (PSS) using LbL electrostatic adsorption technique. Here we give an insight on electrical and photoelectric behaviors in conducting multilayer films. Interesting multifunctional properties of consolidated LbL films are demonstrated by electrochemical characterizations.

EXPERIMENTAL

Reagents and Materials

All the reagents were purchased from Sinopharm Chemical Reagent.

Synthesis of Soluble PPy

1 g of poly(vinyl alcohol) (PVA, alcoholysis degree: 87.0-89.0%) and 7.788 g of Fe[Cl.sub.3] x [H.sub.2]O (purity: >98%) was dissolved in 60 ml of deionized water at 90[degrees]C to form a well-dispersed solution. Under vigorous agitation, a mixture from 1 ml of pyrrole and 10 ml anhydrous ethanol was added into colloidal solution of PVA and Fe[Cl.sub.3] within 30 min. Finally, the reactant was kept at 0[degrees]C for 12 h to obtain homogeneous PVA/PPy solution, which maintain chemical stability about 6 months.

Assembly of [(PSS-PVA/PPy).sub.n] Ultratliin Films

LbL assembly of [(PSS-PVA/PPy).sub.n] ultrathin films was carried out on cleaned glass substrates. Prior to the LbL assembly, the glass slide was cleaned with piranha solution [7:3 (v:v) concentrated [H.sub.2]S[O.sub.4]/[H.sub.2][O.sub.2]] for 1 h, followed by thoroughly rinsing with deionized water, followed by anhydrous ethanol and deionized water again. For the LbL assembly, the clean glass slide was immersed in PVA/PPy solution for 5 min, rinsed with deionized water for 2 min and dried by [N.sub.2] gas stream, then immersed in 3 mM polystyrene sulfonate) (PSS, [M.sub.w] = 70,000) aqueous solution for 5 min, then rinsed again for 2 min and dried by [N.sub.2] gas stream. The cycle was repeated to obtain various bilayers of [(PSS-PVA/PPy).sub.n] films.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Electrochemical Characterizations

The electrical behaviors were evaluated by recording the acimpedance spectroscopy on a CHI660E Electrochemical Workstation in a frequency range of 0.1 Hz to 1 MHz and an ac amplitude of 10 mV at room temperature in 0.5 M [H.sub.2]S[O.sub.4] solution. Glassy carbon rod ([phi] = 5 mm) coated [(PSS-PVA/PPy).sub.n] films was employed as working electrode, Pt sheet with a dimension of 1 x 1 x 0.3 [cm.sup.3] was as counter electrode and Ag/AgCl was reference electrode. The ohmic resistance associated with the film was determined from high-frequency intersection of the complex impedance spectrum with the [Z.sup.l] axis. Cyclic voltammetry (CV) was also conducted on the same cell. Before the measurement, the electrolyte was deoxygenated by nitrogen bubbling for 5 min.

Photoelectronic Test

To record the photoelectronic performances of resulting [(PSS-PVA/PPy).sub.n] multilayer, the films were assembly on an fluorine doped tin oxide (FTO) glass substrate (12 [ohms] [Square.sup.-1]). Prior to assembly, the FTO glass substrate was ultrasonic treated in deionized water for 15 min, acetone/[H.sub.2]O (v/v = 1/1) for 15 min, ethanol for 15 min, and again deionized water for 15 min. Cleaned FTO glass slide was dried by [N.sub.2] gas stream and immersed in concentrated [H.sub.2]S[O.sub.4]/[H.sub.2][O.sub.2] (v/v = 7/3) for 24 h and subsequently dried by [N.sub.2] gas stream. The pretreated substrate was immersed in PVA/PPy and PSS solution alternatively, similar to described above. Photoelectronic test was carried out using a CHI660E Electrochemical Workstation on FTO glass supported [(PSS-PVA/PPy).sub.n] films, Pt sheet, and Ag/AgCl as working electrode, counter electrode, and reference electrode, respectively. All the measurement was tested in a rectangular quartz vessel with a dimension of 5 x 5 x 7 [cm.sup.3]. The thickness of wall was 2.5 mm. The on-off photocurrent density performances were determined by repeatedly recording the i-t curve at dark and irradiation of simulated sunlight (100 mW [cm.sup.-2]) from a 500 W Xe lamp (CHF-XM-500 W).

[FIGURE 3 OMITTED]

Other Characterizations

UV-vis absorption measurements were taken using a Mapada 3200 UV-visible spectrometer.

RESULTS AND DISCUSSION

The PVA/PPy complex was found to be highly soluble in water. Two distinct bands at 450 nm and above 1000 nm were detected in UV-vis spectrum, as shown in Fig. 1. The insert of Fig. 1 is the photograph of homogeneous PVA/PPy aqueous solution.

UV-vis absorption spectroscopy was employed to gain further insight into the assembly mode of [(PSS-PVA/PPy).sub.n] multilayers (Fig. 2a). Because PSS is almost transparent in the UV-vis spectral range, the elevation in absorbance is attributed to the absorption by PPy as in the previously self-assembled PSS layer. The UV-vis spectra of the multilayer films exhibited two characteristic features that can be used as a means of identification: a maximum at 450 and 1025 nm, corresponding to transitions from valence band to bipolarons and anti-bipolarons of the oxidized form of PPy [18]. The absorbances of [(PSS-PVA/PPy).sub.n] at 450 and 1025 nm increased linearly with the PSS-PVA/PPy bilayer (Fig. 2b), which indicates that the amounts of PSS or PVA/PPy deposited in each dipping cycle are the same and the multilayer is formed in a regular manner [19]. Self-assembly of the multilayers has a dependence on adsorption time, as shown in Fig. 2c. As the adsorption time increased from 0.5 to 20 min, the absorbances of PSS-PVA/PPy bilayers at 450 and 1025 nm increased sharply until the achievement of a saturation at 5 min, as shown in Fig. 2d, which indicates dense, homogeneous multilayers were assembled [20], Plots of -ln(A) vs adsorption time were shown in the insert. The poor linear correlation was observed suggesting that the adsorption of PPy obeys higher order kinetics (second or third) [21].

Figure 3a shows the CVs of glassy carbon pole coated with [(PSS-PVA/PPy).sub.n] multilayer after each assembly of PSS (that is, the outside layer was always PSS) in 0.5 M [H.sub.2]S[O.sub.4] aqueous solution. Well-reversible broad redox waves were recorded at ~0.52 V and ~0.30 V, corresponding to the redox reaction of

PPy, which indicated that PPy molecules were assembled. From the redox reaction

PPy + ([Cl.sup.-]) + e + [H.sup.+] [left and right arrow] [PPy.sup.0] + ([H.sup.+] [Cl.sup.-]) (1)

one can conclude that protons from [H.sub.2]S[O.sub.4] participate in the redox reaction of PPy. High proton concentration is facile to accelerate the reduction of [PPy.sup.+] to [PPy.sup.0] and oxidation of [PPy.sup.0] to PPy'. Therefore, the CV recorded in the [H.sub.2]S[O.sub.4] solution with higher concentration give higher peak current densities, as shown in Fig. 3b. A standard criterion to determine the charge-transfer mechanism is recording the CV curves at various scan rates (Fig. 3c), peak current densities plotted against square roots of scan rates for the PSS-PVA/PPy multilayer, as shown in Fig. 3d. The linearity of the peak current densities ([i.sub.p]) with the square root of scan rates indicates that charge transfer in the redox step is controlled by the diffusion of charges in the films as described empirically by the empirical Randles-Sevcik theory [22]:

[i.sub.p] = (2.69 X [10.sup.5])[n.sup.3/2] [AD.sup.1/2.sub.ct] [v.sup.1/2][C.sub.0] (2)

where [D.sub.ct] is the charge transport diffusion coefficient, n is number of electrons, A is active area, v is seal rate, and [C.sub.0] is the concentration of electroactive sites. The [i.sub.p] can be determined by [D.sub.ct][tau]/[d.sup.2], where [tau] is the experimental time scale (the time for the potential to traverse the wave) and d is the film thickness. [D.sub.ct][tau]/ [d.sup.2] [much greater than] 1 means charger-transfer rate is significantly high compared to the experimental time, indicates that there is a linear relation between [i.sub.p] and scan rate. However, [i.sub.p] is directly proportional to square root of scan rate at [D.sub.ct][tau]/[d.sup.2][much less than] 1, in this case, the charge-transfer ability is low.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

EIS has been successfully used to demonstrate the charge transfer between PPy layer in [(PSS-PVA/PPy).sub.n] multilayer, as shown in Fig. 4a and b. [R.sub.s] describes the resistance of inside and outside of the thin film (i.e., film resistance). There is a characteristic that the [R.sub.s] is an approximate constant instead of depending on bilayer number. However, the film thickness is in a linear manner, therefore, sheet conductivity of [(PSS-PVA/PPy).sub.n] multilayer has increasing linear correlation with number of bilayer, as is shown in Fig. 5a.

Electron tunneling effect is of significance in understanding the conductivity of conducting composites by integration an insulator polymer with conductive fillers [23-25]. It is believed that the electron tunneling effect is also applicable to [(PSS-PVA/PPy).sub.n] multilayer, where electrons jump from the bottom PPy layer to the upper PPy layer across the insulating PSS is caused by the voltage on the multilayer. Electron tunneling always occurs in the composites with distance of conducting component-conducting component in several to hundreds of nanometers. The tunneling assisted charges (Q) are expressed as [26]:

Q [varies] exp(-Ad) (3)

where A and d represent the tunnel parameter and tunnel distance, respectively. A higher d indicates that the electron tunneling across the insulting PSS meet with higher resistance. Moreover, the well-defined structure between conducting and insulting phases allows a robust electron tunneling and therefore increased accumulative charges on upper PPy layer. It has been known that the self-assembled multilayer films are characterized as highly ordered multilayer architecture, and schematic diagram of electron tunneling within [(PSS-PVA/PPy).sub.n] multilayer is shown in Fig. 6. Positively charged PVA/PPy in several nanometers is adsorbed on negatively charged FTO substrate with subsequent stacking of negatively charged PSS on PVA/PPy. In sequence, conducting multilayers with alternated PSS and PVA/PPy can be fabricated, in which PPy are believed to be parallel to FTO substrate because of positive charge configuration and ordering nature of self-assemble technique. Electron tunneling occurs at the conjugated structure of PPy chains. The electrical behaviors of [(PSS-PVA/PPy).sub.n] multilayer is the reflection of accumulative charges on PVA/PPy layer. As depicted in Fig. 5, charges on the first PPy layer is believed from the intrinsic PPy chains themselves, however, the charges transfer on the top PPy layer is an accumulative effect of electrons from themselves and tunneled electrons from the bottom PPy layers. From the linear increase in conductivity, we can conclude that there may be a linear charge accumulation with increment of bilayer. The extraordinary electron tunneling effect is expected to give remarkable electrical and photoelectric behaviors.

From Fig. 5b, there is an effect of [H.sub.2]S[O.sub.4] concentration on recorded [R.sub.s] because of the promotion of electrolyte on charge transfer, as is shown in Eq. 1. As shown in Fig. 4c and d, the resistance decreased with an increase of [H.sub.2]S[O.sub.4] concentration, giving an increased accumulation in electrons and therefore sheet conductivity (Fig. 5b).

Photocurrent responses of the [(PSS-PVA/PPy).sub.n] multilayers with varied bilayer numbers (n = 1, 2, 3) were probed to assess the dependence of accumulative photogenerated charges on GO layers. Figure 7a shows that photocurrent responses of [(PSS-PVA/PPy).sub.n] multilayers are highly dependent on deposition cycles of PPy, in which photocurrent of [(PSS-PVA/PPy).sub.1] exhibits the minimum intensity and subsequently readily increases with deposition cycle. This indicates photogenerated electrons have also a tunneling effect from bottom to top PPy layers, giving an enhanced photocurrent density. The tunneling mechanism is similar to that of electrical conductivity because of linear elevation in photocurrent density with bilayer (Fig. 7b). The linear increment in accumulative charges significantly promotes the application of conducting multilayer films in photovoltaic cells, photocatalysts, batteries, and supercapacitors.

CONCLUSIONS

In summary, we have demonstrated the fabrication of conducting [(PSS-PVA/PPy).sub.n] multilayer films via a LbL technique. The uniform deposition process of the multilayer obeys higher order kinetics. The [(PSS-PVA/PPy).sub.n] multilayer gives a linear increment in conductivity, which is a reflection of accumulative charges on PPy chains. Electron tunneling mode is potential mechanism in disclosing linearly increased charge quantity from bottom to top PPy layer across the insulating PSS layer. Similar phenomenon also occurs in photocurrent response. These fancy behaviors along with profound advantages in easy synthesis, versatile electrical and photoelectric performances promise the conducting multilayers to be excellent electrode materials in photovoltaic cells, photocatalysts, batteries, and supercapacitors.

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Hongyuan Cai, Qunwei Tang, Benlin He, Shuangshuang Yuan

Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, Shandong Province, People's Republic of China

Correspondence to: Qunwei Tang; e-mail: tangqunwei@hotmail.com

Contract grant sponsor; Fundamental Research Funds for the Central Universities; contract grand numbers: 201313001, 201312005; Contract grant sponsor: Shandong Province Outstanding Youth Scientist Foundation Plan; contract grand number: BS2013CL015; Contract grant sponsor: Doctoral Fund of Ministry of Education of China; contract grand number: 20130132120023; Contract grant sponsor: Shandong Provincial Natural Science Foundation; contract grand number: ZR2011BQ017; Contract grant sponsor: Research Project for the Application Foundation in Qingdao; contract grand number: 13-1-4-198-jch; Contract grant sponsor: Ocean University of China.

DOI 10.1002/pen.23880
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Author:Cai, Hongyuan; Tang, Qunwei; He, Benlin; Yuan, Shuangshuang
Publication:Polymer Engineering and Science
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Date:Jan 1, 2015
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