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Effect of TiO Nanoparticles on Polyaniline Films Electropolymerized at Different pH Valfrido F. L. Filho, Giovanna Machado, Ronaldo Junio Campos Batista, Jaqueline S Soares, Alan Barros de Oliveira, Claudia de Vasconcelos, André A. Lino, and Taíse Matte Manhabosco J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04919 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Effect of TiO2 Nanoparticles on Polyaniline Films Electropolymerized at Different pH Valfrido F. L. Filhoa, Giovanna Machadob, Ronaldo J. C. Batistaa, Jaqueline S. Soaresa, Alan B. de Oliveiraa, Cláudia de Vasconcelosc, André A. Linod, Taíse M. Manhaboscoa*. a

Departamento de Física, Universidade Federal de Ouro Preto, Campus Universitário

Morro do Cruzeiro ICEB/DEFIS, 35400-000, Ouro Preto, Minas Gerais, Brazil b

Centro de Tecnologias Estratégicas do Nordeste - CETENE, Av. Prof. Luiz Freire, 01,

50740-540, Recife, Brazil c

Departamento de Física e Química, PUC Minas, Belo Horizonte, MG, 30535-901,

Brazil d

Departamento de Física, Universidade Federal do Piauí, 64049-550, Teresina,

Piauí, Brazil * Corresponding author telephone number: (+55)(31)35591675 fax number: (+55)(31)35591667 E-mail address: [email protected]

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Abstract In this work hybrids of titanium nanoparticles and polyaniline are obtained by pulsed electrodeposition at different pH (1.5, 3.9 and 5.9) and characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, cyclic voltammetry, ultraviolet-visible, and Raman spectroscopies. We found that films deposited at pH 5.9 with nanoparticles incorporation are composed of emeraldine meanwhile films without nanoparticles are composed of pernigraniline. As a result, films deposited with nanoparticles incorporation present conductivity 6 times higher than that of films deposited without nanoparticles. Films deposited at pH 3.9 with or without nanoparticles incorporation are both made of pernigraniline. Even though films with nanoparticles incorporation still present higher conductivity. To explain such a result we performed first-principles calculations on polyaniline/TiO2 interface. The calculations predict a metallic polyaniline/TiO2 interface in spite of polyaniline and TiO2 being semiconductors. At pH 1.5 the presence of nanoparticles has negligible effect on films characteristics. We believe that at low pH (pH 1.5) H atoms tend to bind TiO2 surface resulting in positively charged nanoparticles, which are further screened by SO4-2 anions. Such a screening layer prevents the physical contact between nanoparticles and polyaniline monomers diminishing the effects of nanoparticles presence.



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1. Introduction Polyaniline receive great attention due to its good environmental and thermal stability, interesting redox properties, easy synthesis, controllable electrical conductivity and processability.

1,2

Polyaniline posses various oxidation states like

emeraldine base (half-oxidized), leucoemeraldine base (fully reduced) and pernigraniline base (fully oxidized). Emeraldine base is composed of two benzoid units and one quinoid unit that alternate. Emeraldine base can be protonated with an acid to become the called emeraldine salt, the conductive form (σ ~ 1-5 S/cm) widely employed in technological applications. quinoid units, is easily degraded

2

1

Pernigraniline form, composed of

and it is the only known polymer besides

polyacetylene that exibit a two-fold degenerate ground state. 3 The association of doped polyaniline (PANI) with nanomaterials to form hybrid materials has open up the possibility of new applications like in sensors, capacitors.

6,7

dye-sensitized solar cells

8

and lithium-ion batteries

9

4,5

(which has

shown excellent electrochemical performance with a stable capacity during 100 cycles and excellent rate capability). In fact, the range of possible applications of hydrids of PANI and nanomaterials is very broad because of the large number of nanomaterials that exist. For instance, nanoparticles only can be made of several different materials (gold, silver, TiO2, etc), their physical and chemical properties are size dependent 10 and can be modified by several kinds of functionalization.

11

Applications of hybrids of PANI and

nanoparticles have already been reported.

12-16

Gold nanoparticles and polyaniline 3


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have been employed as a cortisol biosensor that could detect cortisol in the range of 0.001-100 nM with a sensitivity of 1.63 µAM-1. 12 Also, titanium dioxide nanoparticles (which has been largely studied because of their potential for applications in photocatalysis, sunscreens and pharmaceuticals)

13,14

have been entrapped on

polyaniline matrix to be used as hydrogen gas sensors.

4

Those sensors have

presented better sensing performance than those with no nanoparticles incorporated.

Besides, the association of polyaniline with titanium dioxide

nanoparticles increases photocurrent values, electrochemical stability and AC conductivity. 15,16 In this work we studied the effects of titanium dioxide nanoparticles incorporation on the optical, electrical and morphological properties of electrodeposited PANI films deposited at different pH (1.5, 3.9 and 5.9). This work is organized as follows: materials and methods as presented in section 2; results and discussion are presented in section 3; and the conclusion are summarized in section 4.

2. Experiments: Material and Methods 2.1 Deposition of PANI films PANI films were deposited on an indium-tin oxide (ITO – Delta Technologies Corporation) glass electrode with a specific resistance of 4-8 Ω/square. Prior to deposition, ITO glass was washed with detergent (Aquet – Bel Art) followed by a treatment in ultrasonic bath with acetone, alcohol and bidistilled water. After, ITO



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glass was immersed in a 20% v/v ethanolamine solution at 80 ˚C for 20 minutes, washed with bidistilled water and dried with nitrogen gas. Depositions were performed in a three-electrode electrochemical cell with a saturated calomel electrode (SCE) as reference electrode. A spiral platinum wire served as counter-electrode. Pulsed potentiostatic depositions were carried out at a constant potential of 1.1 VSCE for the duration of 15 min. The pulse frequency was 0.1 Hz, with the pulse varying between 0 VSCE and 1.1 VSCE. Electrodeposition solutions with different pH were prepared by adjusting the amount of H2SO4 to 0.1 M of aniline (Sigma Aldrich). The monomer aniline was distilled under vacuum before use. For the TiO2 nanoparticles (STS-100, Mw = 80 g mol−1, 15.4 wt % in titanium/Ishihara Sangyo Kaisha Ltd.), a solution with a concentration of 0.025 mol L−1 was used to electrodeposition in order to incorporate nanoparticles to the film. 2.2 Characterization of PANI films Electrochemical characterization The electrochemical behavior of the films was evaluated by cyclic voltammetry with a PGSTAT 128N potentiostat (Metrohm-Autolab). Potential was cycled between -0.4 VSCE and 0.8 VSCE at 0.02 V/s in a 0.1 M H2SO4 solution. Optical absorption spectra characterizations Optical absorption spectra were recorded by using a Cary 300 (Agilent Technologies) spectrophotometer over a wavelength range of 200-900 nm.


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Raman characterization Raman spectra were recorded with T6400 Horiba Raman Instrument with the excitation wavelength of 514 nm. The laser power was kept at 1.6 mW to avoid sample burning. Scanning electron microscopy characterization Scanning Electron Microscopy (SEM) images and Energy Dispersive Spectroscopy (EDS) analysis were performed in a Quanta 200 FEG at an acceleration voltage of 20 KV. A gold coating was used to cover PANI samples.

2.3 First-principles calculations Our first-principles methodology is based on the density functional theory (DFT) as implemented in the SIESTA program.

17

We used the generalized gradient

approximation (GGA) as parameterized in the Perdew–Burke–Ernzerhof scheme (PBE) 18 for the exchange–correlation functional. Troullier–Martins pseudopotentials were employed to represent the ionic core potentials.

19

The fineness of the real-

space grid integration was defined by a minimal energy cutoff of 150 Ryd. The geometries were fully optimized using the conjugate gradient algorithm until all the force components were smaller than 0.05 eV/Å . The Kohn–Sham (KS) eigenfunctions were expanded as a linear combination of pseudo-atomic orbitals of finite range consisting of double- zeta radial functions per angular momentum plus polarization orbitals (DZP). The range of each atomic orbital was determined by a common confinement energy shift of δE = 0.001 Ryd. 6 ACS Paragon Plus Environment


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Our model to represent the TiO2 nanoparticle surface consists in a (101) anatase film, see the left panel of Figure 1. Along the (101) direction the employed orthorhombic supercell is large enough to avoid interaction between periodic film images. Parallel to film surface the supercell dimensions are 11.1 x 21.3 Å. Because the flat forms of PANI are about 10.65

Å long

commensurates with two PANI monomers, Figure 1.

20

the employed supercell Therefore, the stretching

effects due to differences in lattices parameters are negligible.

Figure 1. Model for TiO2. Left panel: unit cell side view. Right panel: unit cell front view.

Because the nanoparticles dimensions are big in comparison to a PANI

monomer we neglect the curvature of the nanoparticle’s surface.

The employed DFT based methodology predicts band gaps of 2.7, 2.3 and 0.8 eV for TiO2, leucoemeraldine and emeraldine, respectively, which are in agreement with previous calculations. 20,21 In addition, such a methodology has successfully predicted a broken gap heterojunction (analogous to the one predicted in this work) formed by


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a semiconductor carbon nanotube deposited on diamond. 22,23 Such a prediction has been confirmed experimentally by means of atomic force microscopy. 23

3. Results and discussion 3.1 SEM and EDS The influence of pH on the films morphology can be seen in the supporting file. SEM imagens do not provide the observation of nanoparticles due to the technique resolution. In order to identify the presence of TiO2 in the electrodeposited films EDS analyses were performed. Figure 2 (a), (b) and (c) presents the EDS analysis for films deposited at pH 1.5, 3.9 and 5.9 with incorporation of nanoparticles, respectively. It is possible to observe in all spectra the Ti peak confirming the presence of the nanoparticles in the electrodeposited films. Ti peak varies in intensity for different films due to the amount of film deposited; higher the thickness, higher the amount of TiO2 incorporated in the film. As it will be possible to observe in UV-Vis results (section 3.3), films deposited at pH 1.5 are thicker, followed by films deposited at pH 5.9 and 3.9. N and C peaks show that the main components of polyaniline are present. Once again, N and C peaks are less intense for films deposited at 3.9. S is observed only for films deposited at pH 1.5. As it is further discussed in the text, SO42

anions tend to screen positively charged TiO2 nanoparticles at low pH (below pH 2).

The peak of S is about 4 times less than Ti peak due to the fact of SO4-2 ions bind only the surface of nanoparticles, which contains only a fraction of the atom of particles bulk. In peak came from ITO.



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Figure 2. EDS analysis of PANI films with incorporation of TiO2 deposited at pH 1.5 (a), 3.9 (b) and 5.9 (c).

3.2 Cyclic voltammetry PANI films were deposited at pH 1.5, 3.9 and 5.9 with and without incorporation of TiO2. The obtained films were cycled between –0.4 VSCE and 0.8 VSCE at 0.02 V/s in 0.1 M H2SO4 solution. Films deposited at pH 1.5, with and without the incorporation of TiO2, present four redox processes (Figure 3 (a)). In the anodic scan a first oxidation peak is localized at 0.26 VSCE corresponding to polyaniline transformation from its reduced form (leucoemeraldine) to semi-oxidized form (emeraldine) with 9 ACS Paragon Plus Environment

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the consequent formation of radical cations. 24 A second oxidation peak is located at 0.66 VSCE, it is related to PANI transformation from semi-oxidized form to fully oxidized form (pernigraniline). In the reverse scan, a reduction peak at approximately 0.5 VSCE and a reduction band corresponding to pernigranilineemeraldine and emeraldine-leucoemeraldine transition are observed, respectively. The first and second oxidation peaks are merged in films deposited at pH 3.9 and 5.9 (Figure 3 (b) and (c)) indicating the non-stability of radical cations due to deprotonation. The presence of TiO2 nanoparticles can affect the film characteristics depending on the pH in which the films were deposited. For low pH, 1.5, the nanoparticles have no apparent effect on film characteristics but for pH 3.9 and 5.9 the presence of nanoparticles may alters films oxidation state, thickness, and conductivity. Let us first discuss situations where the presence/absence of nanoparticles is important. Results regarding depositions without nanoparticles incorporation show that, in comparison to films deposited at pH 1.5, films deposited at pH 3.9 and 5.9 (Figure 3 (b) e (c)) develop small values of current densities in the cyclic voltammograms. Such smaller values of current densities are obtained because films deposited at pH 3.9 and 5.9 (pernigraniline) present lower conductivity than films deposited at 1.5 (emeraldine). The incorporation of TiO2 nanoparticles in the deposition at pH 3.9 and 5.9, Figure 3 (b) and Figure 3 (c), leads to an increase by a factor of 1.5 and 6.0, respectively, in current density in comparison to that of films where TiO2 nanoparticles are absent. We will discuss such results in detail further in the text.



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On the other hand, for pH 1.5 the effects of nanoparticles in the film characteristics are negligible. Depositions with and without nanoparticles incorporation, Figure 3 (a), exhibit so similar voltammograms that is not possible to ascribe considerable effects to the nanoparticles presence, in contrast to previous results for pH 3.9 and 5.9. From the results above it is natural to question why the effects of TiO2 nanoparticles on PANI films characteristics are pH dependent. We believe that the answer to this question relies on the fact that in very acid environments (pH lower than 2) the TiO2 nanoparticles become positively charged, see Figure 8 (b) of Paramasivan et al.

25

Therefore, negatively charged ions in electrodeposition solution, i.e. SO4-2, should screen the titanium dioxide nanoparticles. Such a screening of nanoparticles could be inferred from EDS measurements because sulfur is observed in the spectrum obtained for films deposited at pH 1.5 while it absent in EDS spectrum for films deposited at higher pH. Such screened nanoparticles should weakly interact with PANI monomers, which would explain their negligible effect on PANI films properties. Another natural question that arises from the presented so far is why the current densities increase up to 6 times with nanoparticle incorporation for higher pH. Two factors may account for the observed increase: (i) the formation of a more conducting form of PANI due to the presence of TiO2 nanoparticles during the deposition process as observed by UV-Vis (Figure 5); (ii) the interaction between PANI and TiO2 nanoparticles results in a more conducting film.


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Depositions at pH 5.9 may produce films composed of either pernigraniline (without nanoparticles incorporation) or emeraldine (with nanoparticles incorporation). Because emeraldine is more conducting than pernigraniline films deposited at pH 5.9 with addition of nanoparticles are expected to present higher current densities than those without nanoparticles incorporation shown in Figure 3 (c). Films deposited at pH 3.9 are composed of pernigraniline no matter nanoparticles are incorporated or not. Thus, the observed increase in current density cannot be attributed to a more conducting form of PANI. We argue, then, that the TiO2 nanoparticle interaction with PANI results in a more conducting film. To give support to such an argument we performed ab-initio calculations on PANI interacting with TiO2 nanoparticles. Because the nanoparticles dimensions are big in comparison to a PANI monomer we neglected the curvature of the nanoparticle’s surface and represented the PANI+nanoparticle system by the model shown in Figure 1. The upper panels of Figure 4 show the band structures of three forms of PANI and the lower panels show band structure of such molecules deposited on TiO2 according to the model described in Figure 1. It can be seen in the upper panels that the three forms of PANI are semiconductors, which is observed experimentally. However, the lower panels of Figure 4 shows that PANI in contact with TiO2 forms the so-called broken gap heterojunction,

22,23

which is the interface of two different

semiconductor crystalline materials where the top of the valence band of one material is above the bottom of the conduction band of the other material. Such kind of junction occurs, for instance, when semiconductor carbon nanotubes are deposited on diamond. 22,23



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In that junction the valence band of PANI is above the TiO2 conduction band resulting in a metallic interface where electrons are transferred from PANI to TiO2.

Figure 3. Cyclic voltammetry curves of different PANI films in 0.1 M H2SO4. (a) PANI films with and without nanoparticles deposited at pH 1.5, (b) PANI films with and without nanoparticles deposited at pH 3.9 and (c) PANI films with and without nanoparticles deposited at pH 5.9. Scanning rate 10 mV/s.


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Figure 4. Upper panels: band structures of leucoemeraldine, emeraldine and pernigraniline . Lower panels: band structures of leucoemeraldine, emeraldine and pernigraniline on TiO2 . Due to the deposition the molecules become flat with negligible twist angles. The unit cell of emeraldine on TiO2 is shown in Figure 1, which is twice as large as the unit cells of pernigraniline and leucoemeraline. In all panels the Fermi level was set to zero.

3.3 UV-Vis spectroscopy Figure 5 shows the UV-Vis absorption spectra of PANI films deposited at different pH (1.5, 3.9 and 5.9) with and without the incorporation of nanoparticles. Films deposited at pH 1.5 present two characteristic bands from emeraldine salt, one band



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localized in the range 410-460 nm and another one at wavelengths higher than 800 nm. The band localized at 410-460 nm is assigned to radical cation (polaron) in the polymeric matrix, while the band at >800 nm is attributed to free carrier electron band for conducting polymer (delocalized radical cations). 26 Films deposited at pH 3.9 present a single band located about 550-600 nm, which is assigned to Peierls gap transition.

27

This band is characteristic for PANI in the full

oxidized form (pernigraniline). Films deposited at pH 5.9 have a single band located between 450 and 550 nm that could be assigned to Peierls gap transition in a full oxidized PANI (pernigraniline). The band is displaced to lower wavelengths probably due to the oxidation state of the polymer. More oxidized polymers present the Peierls gap transition at lower wavelengths. 27 Films deposited at pH 1.5 and 3.9 with incorporation of TiO2 nanoparticles presented no changes in UV-Vis absorption spectra, Figure 5. However, when nanoparticles are add to films deposited at pH 5.9 a change in the oxidation state is observed. Films present a band around 410-460 nm characteristic of doped film

26

indicating the

influence of nanoparticles at higher pH (5.9). Thus, films deposited at pH 5.9 with nanoparticles incorporation (half oxidized or emeraldine) are more conductive than films without nanoparticles (fully oxidized or pernigraniline), which explains the obtained cyclic voltammetry curves shown in section 3.2. Films deposited at pH 1.5 have higher absorption due to the superior amount of deposited material. The absorbance is an indirect measurement of the thickness of the film. 28


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Figure 5. UV-Vis absorption spectra of PANI films with and without incorporation of nanoparticles deposited at different pH.

3.4 Raman spectroscopy For completeness, we have also employed Raman spectroscopy characterization of PANI films in order to determine the role played by TiO2 nanoparticles. Figure 6 shows the Raman analysis of the same films discussed earlier. To investigate the homogeneity of PANI films with or without incorporation of TiO2, we took spectra at different locations for all films. Spectra in Figure 6 (a), (b) and (c) are representative Raman spectra of these films.



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Figure 6. Raman spectroscopic analysis of PANI films deposited at different pH with and without incorporation of TiO2 nanoparticles. (a) pH 1.5 without and with TiO2. (b) pH 3.9 without and with TiO2. (c) pH 5.9 with TiO2 (spectrum I); pH 1.5 without TiO2 (spectrum II) – same spectrum of (a) for comparison; and pH 5.9 without TiO2 (spectrum III).

Analysis of the spectral features in Figure 6 (a) and (b) shows no apparent effect of nanoparticles on films for pH 1.5 and 3.9, respectively, in agreement with UV-Vis results. The Raman spectra in Figure 6 (a) show peaks which represent out of plane


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ring deformations at 414 cm-1, ring deformation at 574 cm-1 and 608 cm-1, amine deformation at 812 cm-1, 29 the C-H bending of the quinoid ring at 1180 cm-1, 27 C-N stretching in polaronic units at 1250 cm-1, 30 C–N+ stretching vibrations at 1340 cm-1, 30

C–C stretching of structures intermediate between quinoid and semiquinoid

structures at 1567 cm-1, 30 C–C stretching modes of semiquinoid units at 1597 cm-1, 28 and phenazine, phenoxazine, safranine like segments at 1636 cm-1.

30

Figure 6 (b)

shows the peak at 1606 cm−1, which corresponds to quinonediimine units. 31 Figure 6 (c) shows the Raman proﬁles of PANI films at pH 5.9 with (spectrum I) and without (spectrum III) TiO2, whereas spectrum (II) was taken on PANI films at pH 1.5 for comparison. An overview of Figure 6 (c) shows that the spectral changes observed are due to presence of TiO2 nanoparticles (see spectrum I) for pH 5.9, when we compare spectra I and II. These aspects of Raman spectroscopy confirm cyclic voltammetry and UV-Vis results. Raman spectrum for hybrid PANI films at pH 5.9 (see Figure 6 (c) spectrum I) shows changes when compared with PANI at pH 5.9 without TiO2, and shows a similar spectrum to that one obtained for deposition at pH 1.5. As mentioned earlier, depositions at pH 1.5 may produce films composed of emeraldine while depositions at pH 5.9 may produce films composed of either pernigraniline

(without

nanoparticles

incorporation)

or

emeraldine

(with

nanoparticles incorporation), which are observed with Raman spectroscopy and shown in Figure 6 (c). Therefore, the Raman results shown in Figure 6 corroborate the influence of nanoparticles incorporation in PANI films.



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4. Conclusion According to first principles calculations, TiO2 nanoparticles and PANI form a broken gap heterojunction. Thus, the physical contact between PANI and TiO2 nanoparticles results in a metallic interface, which may contribute to increase films conductivity. The influence of nanoparticles on optical and electrical properties of the films is pH dependent. In acid environments (pH 1.5) H atoms tends to bind TiO2 surface and therefore, screened by SO4-2 anions present in the solution. In this case no effects of nanoparticles are observed. In less acid environments (pH 3.9 and 5.9) the TiO2 surface is not completely screened by anions, allowing electrons transfer. The interaction between PANI and TiO2 nanoparticles results in more conducting films, which was confirmed by cyclic voltammetry experiments. At higher pH (pH 5.9) the nanoparticles influence in the state of oxidation of PANI films as confirmed by UV-Vis and Raman spectroscopy.


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Acknowledgments The authors are grateful for financial support from the following Brazilian agencies: Fundação de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG – Proc. APQ01284-13), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The authors also thanks the Federal University of Ouro Preto for the Auxílio Pesquisador program. Furthermore, authors are very grateful to Prof. Marcos A. Pimenta for the Raman measurements.

Supporting Information. SEM images and discussion of pH influence in morphology of PANI films. Atomic coordinates (xyz format) of structures used in the first principle calculations.



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Figure 5. UV-Vis absorption spectra of PANI films with and without incorporation of nanoparticles deposited at different pH. 279x215mm (300 x 300 DPI)



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Effect of TiO2 Nanoparticles on Polyaniline Films ... - ACS Publications

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