Internal photoemission in polyaniline revealed by photoelectrochemistry

Synthetic Metals 123 (2001) 321±325

Internal photoemission in polyaniline revealed by photoelectrochemistry H.G. Huang, Z.X. Zheng, J. Luo, H.P. Zhang, L.L. Wu, Z.H. Lin* State Key Laboratory for Physical Chemistry of the Solid Surface, Department of Chemistry, Institute of Physical Chemistry, Xiamen University, Xiamen 361005, PR China Accepted 12 January 2001

Abstract After self-assembling a p-aminothiophenol (PATP) monolayer on bare Au electrode, polyaniline (PANI) ®lms were prepared by electropolymerization on the modi®ed Au electrode and three forms of PANI (the emeraldine salt, the leucoemeraldine and the pernigraniline) were obtained by electrochemical method. The cyclic voltammograms of the probe (K3Fe(CN)6/K4Fe(CN)6) for the three forms of PANI show that three different oxidized forms can be found and all of them are stable under our experiment conditions. The potential dependence of magnitude and the spectra of the photocurrent of the PANI ®lms were observed. When the spectra of photocurrent are displayed in a Fowler plot (the square-root of the incident photon-to-current ef®ciencies versus the photon energy), the functional form appears linear, which is interpreted as indication of internal photoemission of holes or electrons from metal into the appropriate electronic band of the semiconductor or insulator. The band gap energy of insulating matrix in the emeraldine salt is determined as 3.33 eV by the Fowler plots. The ¯at-band potential, in the order of 0.63 V versus SCE, is obtained from Mott±Schottky plots. A photoelectrochemical process based on internal photoemission in the emeraldine salt, which agrees with the model of granular metal island that assumes metallic polymer particles are embedded in the insulating matrix, is proposed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline ®lms; Internal photoemission; Emeraldine salt

1. Introduction Polyaniline (PANI) as one of heteroatomic conducting polymers, has attracted considerable attention because of its interesting electrical conductivity, novel electronic structure [1±5] and mechanism of electrical conductivity [6±11] as well as the possibility for applications as a new electronic material. In aqueous acid solutions both electrochemical redox and doping±undoping can induce transition between conducting and insulating form of PANI [12±14]. The cyclic voltammogram of PANI in acid aqueous solutions, as characterization of both its electrochemical redox and doping±undoping, commonly presents two sets of redox peaks: the ®rst peak at more negative potential corresponding to a redox couple of the leucoemeraldine (reduced form of PANI) and the emeraldine salt (the fully protonated composition and partially oxidized form of PANI), the second peak at more positive potential corresponding to a redox couple of the emeraldine * Corresponding author. E-mail address: [email protected] (Z.H. Lin).

salt and the pernigraniline (fully oxidized form of PANI), as shown in Fig. 1. The leucoemeraldine has been proposed to have an energy gap (Eg ˆ 3 4 eV) originating predominantly from extrinsic effects involving the overlap of molecule orbits of the neighboring phenyl rings and the nitrogen atoms, and its valence bands (VB) and conduction bands (CB) are composed of p antibonding molecule orbit and p bonding molecule orbit, respectively [1,5]. Similar to the leucoemeraldine, the emeraldine base has a large extrinsic gap. The pernigraniline has been proposed to have an energy gap (Eg  1:4 eV) that is intrinsic in origin, which is due to the electron±phonon interaction. The VB of the pernigraniline originates mostly in the phenyl rings and the major contribution for the conduction band (CB) comes from the N=quinoid=N fragment. These bands are slightly asymmetric with respect to the Fermi level [4]. The leucoemeraldine, the emeraldine base and the pernigraniline are insulators, but the emeraldine salt exhibits metallic conductivity. The conducting emeraldine salt supports the formation of polarons and can be described as possessing a half-®lled metallic polaron energy band in the energy gap. The Fermi level is set in the half-®lled polaron band

0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 2 9 8 - 3

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sub-band gap spectra of photocurrent, and found that the spectra followed Fowler's rule which is commonly used to interpret results of internal photoemission [31]. In this paper, we also present the electron transport performance of three forms of PANI using electron transfer probe, the potential dependence of their capacitance, and eventually band structure of the emeraldine salt. 2. Experimental section

Fig. 1. (a) Cyclic voltammogram of PANI in 0.5 mol/l HClO4 solution; sweep rate 100 mV/s; (b) The peaks in the cyclic voltammogram are associated with interconversion among the three forms of PANI: the leucoemeraldine, the emeraldine salt and the pernigraniline.

[2,5]. A large number of physical studies supported the granular metal island model for the conduction mechanism of the emeraldine salt. The model assumes that the threedimensional metal island of polymer, originating from the formation of metallic polaron lattice with disorder, is embedded in the nonmetallic matrix. The conductivity is thought to be due to the charging energy limiting tunneling between the metal islands [6±11]. Recently, photoelectrochemical properties and the doping conditions of PANI have been widely studied by photoelectrochemistry. The photoelectrochemical transient response of PANI has been obtained by a number of authors [15±27], but few of them observed the spectra of photocurrent directly. As it is well known, a lot of thin metal oxide (semiconductor or insulator) covered metals possess internal photoemission effect [28±30]. The granular metal island model of PANI is similar to them. Does the emeraldine salt possess the internal photoemission effect? In our steady photoelectrochemical experiments of the emeraldine salt, we observed its

Reagent grade PATP was obtained from Sigma and used without further puri®cation. Other regents used in the experiments were of analytical reagent quality. The working electrode was a pure gold disc (0.41 cm2), with a platinum as sheet counter electrode. The PATP monolayer was self-assembled on bare Au polycrystalline electrodes after the appropriate surface treatment [32]. The PANI ®lms on PATP/Au substrate were obtained by cycling the electrode potential between 0.2 and 0.7 V in the solution containing 0.05 mol/l aniline and 1.0 mol/l HClO4. Films with thickness 50±100 nm were prepared with 100 scan cycles. The PANI ®lms had a better adhesion to the PATP/Au electrode than those prepared by other methods. When this PANI ®lm was controlled at applied potentials of 0.4, 0.35 and 1.0 V in 1.0 mol/l HClO4 solution for 10 min, the leucoemeraldine, the emeraldine salt and the pernigraniline were obtained respectively. After rinsed with a large volume of deionized water and dried with N2 gas, electrochemical and photoelectrochemical measurements of the PANI ®lms proceeded immediately. The electrochemical experiments were carried out using a computer controlled electrochemical workstation (Model 660, CH Instruments). For photocurrent measuring, a home-made combined UV±VIS spectroelectrochemical measurement system [33] was used. The light source was a 150 W Xe lamp with a high-throughput monochromator (ARC SpectraPro-275) and a chopper of frequency of 18 Hz. The incident intensity was calibrated using an Rk-5710 power radiometer (LaserProbe Inc.) with an RkP575 probe and an RkP576a probe. 3. Results and discussion The electron transfer probe (K3Fe(CN)6/K4Fe(CN)6) was selected to examine the electric conductivity of three forms of PANI because this probe does not contain insertable ions. The cyclic voltammograms of the probe on the PANI ®lms and bare gold electrode are shown in Fig. 2. The cyclic voltammogram of the probe on the emeraldine salt ®lm is considerably similar to the one on bare gold electrode apart from more negative peak potentials and rather small peak currents. The cyclic voltammogram of the probe on the leucoemeraldine ®lm with quite small peak current is

H.G. Huang et al. / Synthetic Metals 123 (2001) 321±325

Fig. 2. Cyclic voltammograms of 0.05 mol/l K3Fe(CN)6/K4Fe(CN)6 solution for the PANI films and bare gold electrode; sweep rate: 50 mV/s.

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is a little greater than that of the pernigraniline, while the emeraldine salt has much larger photocurrent than both the leucoemeraldine and the pernigraniline. We ®nd that when forwarding potential sweep, the potential dependence of photocurrent for the emeraldine salt ®lm is rather different from that when reversing potential sweep. The possible reason is that some ions of ClO4 and H‡ in the emeraldine salt can be expulsed and then inserted again to a different extent in a more positive or more negative potential region even though there are no insertable ions in K3Fe(CN)6/ K4Fe(CN)6 solution. The spectra of photocurrent for the PANI ®lms in a 0.05 mol/l K3Fe(CN)6/K4Fe(CN)6 solution is presented in Fig. 4. The incident photon-to-current ef®ciencies (IPCE) are de®ned by the Eq. (1): IPCE ˆ

jhc lPq

(1)

irreversible. It may be hard to understand why the peak currents can be detected in the cyclic voltammogram on the leucoemeraldine ®lm which is insulating. One possible reason is that it is dif®cult to avoid the leucoemeraldine polymer prepared by electropolymerization from containing some emeraldine salt The phenomenon is also observed in the following photoelectrochemical experiments. On the cyclic voltammogram of the pernigraniline ®lm, there is no current peak observed. These results show that the emeraldine salt has a good electron transport performance while the conductivity of the leucoemeraldine and the pernigraniline is not so good. Fig. 2 shows that three forms of PANI can be found and are stable under our experimental condition. The cathodic and anodic photocurrents of three forms of PANI can be obtained in differently applied potential region in a 0.05 mol/l K3Fe(CN)6/K4Fe(CN)6 solution as shown in Fig. 3. At more negative (positive) potential than about 0.4 V, the cathodic (anodic) photocurrent is observed. At the same potential, the photocurrent of the leucoemeraldine

where j is the photocurrent density, h the Planck constant, c the velocity of light, l the wavelength, P the light density, and q the elemental charge. The relative value of IPCE is presented because we have not corrected accurately the value of P. The anodic and cathodic photocurrent bands of the pernigraniline (appeared within 1.45±2.8 eV), the emeraldine salt and the leucoemeraldine (both appeared within 1.7±2.8 eV) are observed. The onset energy of the emeraldine salt is smaller than the value of 3±4 eV given by references, and obviously the subband gap photocurrent are obtained. And since the leucoemeraldine prepared by electropolymerization contains some emeraldine salt, the spectra of photocurrent for the leucoemeraldine is similar to that of the emeraldine salt except for much smaller IPCE. The onset energy of the pernigraniline (about 1.45 eV) is considerably consistent with theoretical calculation [1,4,5]. The differences between the spectra of photocurrent for three forms of PANI also show that the three forms can be distinguished and are stable under our experimental conditions.

Fig. 3. The potential dependence of magnitude of photocurrent for the PANI films in 0.05 mol/l K3Fe(CN)6/K4Fe(CN)6 solution. (a) the leucoemeraldine; (b) the emeraldine salt; (c) the pernigraniline.

Fig. 4. The spectra of photocurrent for the PANI films in 0.05 mol/l K3Fe(CN)6/K4Fe(CN)6 solution. (a) the emeraldine salt (0.7 V) ;(b) the leucoemeraldine (0.7 V); (c) the pernigraniline (0.7 V); (d) the emeraldine salt (0 V) ;(e) the leucoemeraldine (0 V) (f) the pernigraniline (0 V).

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Fig. 6. Mott±Schottky plots for PANI. (a) the leucoemeraldine; (b) the emeraldine salt.

Fig. 5. IPCE1/2 vs. hn plot derived from spectra of photocurrent for the emeraldine salt.

When the anodic and cathodic spectra of photocurrent for the emeraldine salt are displayed in a Fowler plot (the square-root of the IPCE versus the photon energyhn), its functional form appears linear as shown in Fig. 5. According to the following relation led by Fowler's hypothesis for internal photoemission from metal into semiconductor IPCE1=2 ˆ A…hv

Et †

(2)

where Et is the threshold energy for photoemission, h the Planck's constant, n wave velocity, A the constant [31], the threshold energies of 1.69 and 1.64 eV can be determined by means of extrapolating line a and line b, respectively. Linear Flower plots can be an indication of internal photoemission of holes or electrons from the metal islands into appropriate electronic band of insulating matrix. The threshold energies represent a minimum energy required for the promotion of a hole (electron) from the Fermi level of the metal island and the transition to the valence band (conduction band) of the insulating matrix. The sum of both the anodic and the cathodic photocurrent threshold energies should approximate to the band gap energy of insulating matrix, presuming the Fermi level of the metal island does not shift with respect to the insulating matrix film band edges as a function of applied potential [30]. In order to obtain the ¯at-band potential (Efb) at the interfaces of the leucoemeraldine ®lm-solution and the emeraldine salt ®lm-solution, the potential dependent capacitance data of the leucoemeraldine and the emeraldine salt in 1.0 mol/l HClO4 solution (pH ˆ 0:43) were measured over a potential region without Faradaic current under dark. From these data the Mott±Schottky plots of the potential dependence of the capacitance for the leucoemeraldine and the emeraldine salt are obtained as shown in Fig. 6. According to Mott±Schottky equation that it could be applied to conducting polymer is presumed, 1 1:41  1020 …E Efb ˆ 2 C eN

kT=q†

(3)

where q is the element charge, N the concentration of carriers, e the dielectric constant of the semiconductor, k the Boltzmann's constant and T the temperature [34]. The flat-band potentials (Efb) of the leucoemeraldine and the emeraldine salt in 1.0 mol/l HClO4 solution, which were calculated as the intersection of plots with the x-axis, are almost the same as 0.63 V, but the slope of the Mott± Schottky plots for the leucoemeraldine is lager than that for the emeraldine salt. According to the relation between pH and Efb [35], the Efb of the leucoemeraldine and the emeraldine salt in 0.05 mol/l K3Fe(CN)6/K4Fe(CN)6 solution (pH ˆ 8:52) are obtained with the value of 0.13 V (versus SCE). Furthermore, the negative slopes of Mott± Schottky plots of them show that we can categorize them as p-type semiconductors. Per the results stated above, we can construct the energy band diagram of the emeraldine salt shown in Fig. 7. When the emeraldine salt is excited by light having appropriate energy, electrons and holes will emit from the metal islands of the emeraldine salt. On the one hand, the electrons will

Fig. 7. Energy band diagram of the emeraldine salt.

H.G. Huang et al. / Synthetic Metals 123 (2001) 321±325

emit from the Fermi level of metal island and transfer to conduction band of the insulating matrix of the emeraldine salt, then move into the solution where they can be trapped by oxidized species to produce cathodic photocurrent. On the other hand, the holes will emit from the Fermi level of metal islands and transmit to valence band of insulating matrix, then move into the solution where they can be trapped by reduced species to produce anodic photocurrent. The more negative potential applied, the more electrons will be trapped by the oxidized species in the solution to produce greater cathodic photocurrent, and vice versa. 4. Conclusion The cyclic voltammograms of the electron transfer probe [K3Fe(CN)6/K4Fe(CN)6] for the PANI ®lms show that the three forms of PANI are stable in the experiments, the emeraldine salt has a excellent electron transport performance, and the conductivity of the leucoemeraldine and the pernigraniline is not so good. The potential dependence of magnitude and spectra of photocurrent of the PANI ®lms were obtained. The cathodic and anodic photocurrents were observed in different potential regions for the three forms of PANI. The Fowler plots of the emeraldine salt are interpreted as an indication of internal photoemission of holes or electrons from the metal islands into appropriate electronic band of insulating matrix. The ¯at-band potential of the emeraldine salt in 0.05 mol/l K3Fe(CN)6/K4Fe(CN)6 is calculated as 0.37 V(SHE). An Energy band diagram of the emeraldine salt is presented and a photoelectrochemical process based on internal photoemission in the emeraldine salt is also explained. Acknowledgements The support of this work by the National Nature Science Foundation of China (29833060 and 20023001) and Ministry of Education (99177) are gratefully acknowledged. References [1] D.S. Boudreaux, R.R. Chance, J.F. Wolf, L.W. Shacklette, J.L. Bredas, B. Themans, J.M. Andre, R.J. Silbey, Chem. Phys. 85 (1986) 4584. [2] S. Stafstrom, J.L. Bredas, A.J. Epstein, H.S. Woo, D.B. Tanner, W.S. Huang, A.G. MacDiarmid, Phys. Rev. Lett. 59 (1987) 1464.

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