Electrochemistry Communications 48 (2014) 164–168

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Electrochemistry Communications

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Short communication

QCM-D studies of polypyrrole influence on structure stabilization of βphase of Ni(OH)2 nanoparticles during electrochemical cycling

Fernando H.C. Miguel, Tânia M. Benedetti ⁎, Roberto M. Torresi, Susana I. Córdoba de Torresi ⁎Instituto de Química, Universidade de São Paulo, CP 26077, 05513-970 São Paulo, Brazil

⁎ Corresponding authors.E-mail addresses: tania@iq.usp.br (T.M. Benedetti), sto

(S.I. Córdoba de Torresi).

http://dx.doi.org/10.1016/j.elecom.2014.09.0121388-2481/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 August 2014Received in revised form 8 September 2014Accepted 12 September 2014Available online 19 September 2014

Keywords:Nickel hydroxidePolypyrroleQuartz crystal microbalance with dissipation

Pure β badly crystallized, βbc-Ni(OH)2, and hybrid βbc-Ni(OH)2/polypyrrole nanoparticles (NPs) were synthe-sized and immobilized by electrophoretic deposition. The influence of polypyrrole (Ppy) on the structural stabil-ity with cycling was evaluated by Quartz Crystal Microbalance with Dissipation. The results show that Ppystabilizes the NP structure avoiding stress features provoked by the formation of γ-NiOOH. Ppy remains oxidizedduring the entire process and, as it is doped with a big counter anion such as dodecylbenzenesulfonate (DBS−)which is not removed during cycling, avoids volumetric changes of Ni(OH)2 leading to higher cyclability.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Nickel hydroxide (Ni(OH)2) is employed in electrochromic devices,[1] energy storage [2] and electrocatalysis [3]. It exists in two arrange-ments, α and β, that can be interconverted in strong alkaline media orupon electrochemical cycling, causing large volumetric changes thatcompromise their performance [4]. In order to stabilize their structures,approaches such as partial substitution of Ni by other atoms [5,6] andthe preparation of nanostructures [7] have been studied. In this contri-bution we show that the incorporation of polypyrrole (Ppy) by poly-merization of pyrrole (py) in situ during the synthesis of Ni(OH)2nanoparticles (NPs) stabilizes its β badly crystallized phase (βbc) [8,9]along cycling in alkaline media, preventing volumetric changes causedby the conversion into more opened structures.

2. Experimental section

All solutions were prepared in deionized water (UHQ Elga). For thesynthesis of pure Ni(OH)2 NPs, an ultrasonic probe (Sonics Vibra-Cell)was dipped into a water bath containing a beaker with 10 mL of0.01 mol L−1 Ni(NO3)2 (Aldrich). Then, 200 μL of 1 mol L−1 NH4OH(Synth) was added under 2 s pulsed sonication at 20 W, being main-tained for 5 min. For the synthesis of Ni(OH)2/Ppy NPs, the processwas the same but adding 10 μL of 0.05 mol L−1 py (Aldrich) and0.02 mol L−1 of dodecylbenzenesulfonic acid (DBSA — Aldrich)solutions.

rresi@iq.usp.br

The NPs were immobilized on AT-cut 5 MHz piezoelectric quartzcrystals coated with platinum electrodes (Qsense) by electrophoresisusing a Pt counter electrode under an electric field of 0.5 V cm−1. Forthe electrochemical characterization, a 0.1 mol L−1 KOH solution wasemployed as electrolyte and Ptmesh and Ag/AgClKClsat were the counterand the reference electrodes, respectively. The experiments wereperformed with an Autolab PGSTAT30 potentiostat and were concomi-tantly monitored with a QCM-D (QSense model E4).

For the Attenuated Total Reflectance Infrared Spectroscopy (ATR-FTIR — Bomem MB100) analysis, the NPs were immobilized on a Ptsheet by casting. The NPs as powder were characterized by X-ray dif-fraction (XRD — Rikagu Miniflex) and thermogravimetry (TGA — STAi1500). HRTEM and FESEM images were taken using Jeol Microscopes(models: JEM 2100 and JSM-7401F respectively).

3. Results and discussion

3.1. Structural characterization of Ni(OH)2 nanoparticles

Fig. 1(a and b) shows the TEM images of pure and hybrid NPs(ca. 5 nm in diameter) obtained. The diffractograms of both materialsare nearly identical, being mainly assigned to β-Ni(OH)2, with peaksat 2θ= 15.7, 33.2, 38.6, 52.3, 59.3, 62.5, 69.6 and 72.7 [10,11]. However,peaks assigned toα-Ni(OH)2 are also present at 6.5 and 20.7, indicatingan intermediate structure between the well crystallized β-Ni(OH)2 andthe disorderedα-Ni(OH)2. It was confirmed by FTIRwhere a sharp peakat 3645 cm−1 from the ν(OH) stretching vibration of the β phase, in ad-dition to a broad band centered at 3270 cm−1 and a peak at 1640 cm−1

assigned to the stretching and bending vibrations of water moleculesfrom the α phase, are observed [12]. The formation of the α-Ni(OH)2phase is observed for reactions at mild temperatures [9,12]. In our

Fig. 1. TEM image of (a) pureNi(OH)2 and (b) hybrid Ppy/Ni(OH)2 NPs; (c) Nyquist plots at frequency range from10kHz to 10mHz; amplitude (RMS): 10mV; E=0.5 V vs.Ag/Ag+KClsat.in 0.1 mol L−1 KOH electrolytic solution; (d) mass of electrophoretically deposited NPs as a function of time at electric field= 0.5 V cm−1. (○) Pure Ni(OH)2 and (●) hybrid Ni(OH)2/Ppy.

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case, the formation of βbc structure is explained by the sonicationmethod that generates high local temperature causing the transitionfrom α to β phase [13]. In a previous work, the sonication for 90 minresulted in β phase NPs [14]. In the present case, as a shorter sonica-tion timewas employed, the badly crystallized structurewas formed.The high local temperature also generates the radicals H• and OH•

from water, promoting the polymerization of the py monomer(Eqs. (1) and (2)) [15,16]. The polymerization reaction is initiatedby the H2O2 formed by combination of two hydroxyl radicals gener-ated by the sonolysis.

H2O→ÞÞH � þOH� ð1Þ

OH � þOH �→H2O2: ð2Þ

The TGA results show that pure and hybrid NPs present an initialmass decrease of 1.8% and 3.7%, respectively, from the removal ofadsorbed water, followed by a large decrease around 250 °C of 15.5%and 18.2% respectively, from the decomposition to NiO [17]. The massdecrease is higher for the hybrid material, once the polymer decompo-sition occurs at this same temperature range. Based on this, the weightpercentage of polymer is estimated in approximately 3%. The lowpolymer content in the composite explains the reason why thediffractograms and FTIR spectra of both pure and hybrid NPs are nearlyidentical.

3.2. Electrophoretic deposition of the Ni(OH)2 NPs

Due to the incompatibility of the QCM-D with the system employedfor the EPD, the frequency (f) and dissipation (D) changes during thedeposition were measured point to point at different times buildingup the frequency–dissipation vs. time curves. As D shifts are negligiblewith respect to the f changes for harmonics (n) ranging from n = 3 ton = 11, both films can be considered rigid [18]. Thus, the mass can be

calculated from the Sauerbrey equation: Δf = −Δm / CA [19], whereΔf is the shift in frequency (Hz),Δm is themass change (ng), A is the ac-tive area of the quartz crystal (0.2 cm−2) and C is the mass sensitivity(17.7 ng cm−2 Hz−1).

The mass of the films after 1 h of EPD was 25.2 μg cm−2 and31.4 μg cm−2 for pure and hybrid NPs, respectively. The higher conduc-tivity of the hybrid material, as demonstrated by impedance measure-ments (Fig. 1c), leads to a longer time for passivating the substratesurface due to the EPD process [20], resulting in higher depositedmass. In fact, the depositing mass change is practically the same atshorter times being the deviation for lower values observed afterabout 20 min of deposition for the pure NPs (Fig. 1d).

From FESEM images (data not shown), no difference in morphologywas observed for both films. Although the spherical geometry of theNPs, thefilms are fibrouswhichwas observed in a previous contributionfor V2O5 NPs [21], leading to assume that the filmmorphology is associ-ated with the tendency of the NPs to agglomerate into fibers under theapplied electric field.

3.3. Electrochemical/QCM-D characterization

The f andD changesweremonitored byQCM-Dduring theCVexper-iments. The Δfn and Dn data were collected from n = 3 to n = 11. AsD × 10−6 shifts are higher than 5% of f shifts in addition to significantdifferences in the values among the different n, the data cannot be con-verted into mass values using the Sauerbrey equation once viscoelasticchanges are also taking place [18]. Therefore, the −Δf5 data was takento evaluate the charge compensation behavior in Fig. 2 being its profileassociated with the flow of species into and out of the films.

For bothmaterials, in the first cycles−Δf5 abruptly decreases duringoxidation, being recovered to its initial value upon reduction, in agree-mentwithwhatwas expected for β-Ni(OH)2 that loseswatermoleculesupon reduction due its compact structure [22,23] (Eq. (3)).

β−Ni OHð Þ2 þ OH−→β−NiOOHþH2Oþ e−: ð3Þ

Fig. 2. j and−Δf5 as a function of E at the 5th cycle (a and d), 20th cycle (b and e) and 50th cycle (c and f) for thin films obtained from pure (a, b and c) and hybrid NPs (d, e and f);v = 10 mV s−1; electrolyte: KOH 0.1 mol L−1 aqueous solution.

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For thefilm of pureNPs, an inversion of the charge compensation oc-curs upon cycling, being accompanied by slight dislocation of oxidationand reduction peaks from 0.53 V to 0.51 V and from 0.30 V to 0.32 V re-spectively, ascribed to the conversion ofβ-NiOOH into themore opened

Fig. 3. FTIR spectra of films of (a) pure and (b) hybrid NPs before (\) and after (—) 20voltammetric cycles at 10 mV s−1 in KOH 0.1 mol L−1.

γ-NiOOH that occurs upon overcharge [4], tending to be reduced back toα-Ni(OH)2 with similar lattice structure. The formation of the moreopened structure allows the insertion of both cations and water mole-cules into the layers [22] (Eq. (4)) andwas confirmed by FTIR spectrumtaken after 20 cycles (Fig. 3a).

α−Ni OHð Þ2 þ ðKþ;H2OÞ þ OH−→β−NiOOHðKþ

;H2OÞ þH2Oþ e−: ð4Þ

On the other hand, for the films of hybrid NPs, the charge compensa-tion behavior is maintained upon cycling, as well as the oxidation andreduction peak potentials at 0.50 V and 0.30 V respectively, meaningthat in the presence of Ppy the structure is maintained as corroboratedby the FTIR spectrum taken after 20 cycles (Fig. 3b).

Fig. 4 shows the changes in f and D along time during cycling.For both cases, initiallyΔf is the same for all n andD is relatively low,

meaning that there are no significant changes in viscoelasticity. Theoverall f tends to decrease and its variance tends to increase, as well asD increases, which can be due to increasing swelling of the films. Thisbehavior persists, tending to the stabilization (insets in Fig. 4b andd) for the film of hybrid NPs. However, for the film of pure NPs, f trendstarts to invert from the 7th cycle, being accompanied by a significantincrease inD (insets in Fig. 4a and c). After about the 40th cycle, thema-terial starts to detach from the substrate, as indicated by the increase off. This is associatedwith significant volumetric changes caused by phasetransition during cycling.

Based on these observations, we clearly note that Ppy is stabilizingthe NPstructure. At the applied electrochemical window the Ppy is atits oxidized state during the whole process. The oxidized polymerprevents the hydroxide layers to be held off, avoiding the conversionof the β-NiOOH into γ-NiOOH which is associated with volumetricchanges that compromises the film stability along cycling. In addition,the DBS− counter anion, due to its size, is not removed from the struc-ture [24,25] as observed for small anions that are exchanged by OH−

in alkaline solution, compromising the conductivity [26]. The impor-tance of the presence of the big anion is supported by the fact that

Fig. 4. Δf (a and b) and D × 10−6 (c and d) as a function of t during the 20 first voltammetric cycles with n = 3 to n = 11 for pure (a and c) and hybrid (b and d) NPs; v= 10 mV s−1;electrolyte: KOH 0.1 mol L−1. Insets: data for 100 cycles.

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when a small anion such as Cl− is employed as counter ion, the resultinghybrid NP structure is not maintained within cycling (data not shown).

4. Conclusions

We have demonstrated that βbc-Ni(OH)2 NP structure is stabilizedby the incorporation of Ppy doped with a large counter ion such asDBS−, thatminimizes volumetric changes along cycling, as demonstrat-ed by following structural and viscoelastic changes taking place alongthe process. This material can be applied in devices based on Ni(OH)2where the prevention against mechanical stress due to the β-Ni(OH)2/γ-NiOOH redox reaction is of crucial importance.

Conflict of interest statement

The authors declares no conflict interests.

Acknowledgments

Financial support was provided by FAPESP (2009/53199-3) andCNPq. TMB thanks FAPESP for the fellowship (2012/02117-0).

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