Controlled Growth of Polyaniline Fractals on HOPG through

Letter pubs.acs.org/Langmuir

Controlled Growth of Polyaniline Fractals on HOPG through Potentiodynamic Electropolymerization Dhrubajyoti Bhattacharjya‡ and Indrajit Mukhopadhyay*,† †

School of Solar Energy, Pandit Deendayal Petroleum University, Raisan Gandhinagar 382007, Gujarat, India Central Salt and Marine Chemicals Research Institute (CSIR), G. B. Marg Bhavnagar 364002, Gujarat, India

‡

S Supporting Information *

ABSTRACT: Polyaniline (PANI) in fractal dimension has been electrodeposited reproducibly on highly oriented pyrolytic graphite (HOPG) from 0.2 M aniline in 1 M aqueous HCl solution by potentiodynamic sweeping in the range of −0.2 to 0.76 V vs Ag/AgCl at room temperature. Fractal growth of PANI dendrimers is affected by diffusion limited polymerization (DLP) at a sweep rate of 15 mV s−1 for 43 min. This type of PANI dendrimer is prepared for the first time on such large area HOPG substrate by electrochemical technique using rather simple cell setup. The fractal dimension has been determined by chronoamperometry (CA) and box counting technique and is found to vary from 1.4 to 1.9 with the duration of electropolymerization. The sweep rate, terminal oxidation potential, and the diverse surface anisotropy of the HOPG surface are found to be crucial factors in controlling the growth of such PANI fractals.

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INTRODUCTION Studies on the electrochemical and surface properties of conducting polymers has been a matter of intense investigation in last few decades due to their potential in wide variety of applications like in sensors,1,2 actuators,3 supercapacitors,4−6 electrochromic display devices,7,8 and microelectronic devices.9,10 Of these conducting polymers, polyaniline (PANI) has elicited the most interest due to its wide range of conductivity from insulating to metallic regime, unique redox tunability, good environmental stability, low cost, ease of synthesis, and promising applications.11−16 Various synthetic routes have been employed for controlled growth of PANI microstructure, such as microemulsion,17 interfacial polymerization,18 template synthesis,19 and self-assembly.20 Electrochemical methods are the versatile, cost-effective, and simple methods to synthesize nanoand microstructures of PANI. Cyclic voltammetry, potentiostatic, galvanostatic methods,21,22 and pulse potentiostatic methods23 are the classical electrochemical techniques employed to synthesize PANI having different morphologies. Dendrimer is a class of architecture having highly branched structural design. This kind of architecture is a matter of scientific attraction due to the possibility of structure allied novel physical and chemical property. The diffusion-limited aggregation (DLA) model has been widely used to explain and analyze the formation of dendritic structure. The DLA model considers the growth of aggregate by random walk of particle on a lattice containing a seed and then rapid growth of these clusters toward the exposed end than other perimeter sites.24 Mandelbort promulgated the description of this complex pattern in terms of fractal geometry.25 Its research object is the rough and © 2012 American Chemical Society

nondifferential body in a nonlinear system which cannot be solved in a Newtonian system. Due to strong interaction between molecules, conjugated conductive polymers have a tendency for self-aggregation and it is easy to form fractal aggregates.26 Electrodeposition is known to give rise to fractal structures under certain experimental conditions. Fractal growth of polypyrrole in an area of 1.5 cm2 by electropolymerization was first reported by Melroy et al. using a custom designed complicated electrochemical cell.27 B. Villeret et al. evidenced the preparation of polyaniline fractals by elctropolymerization by analyzing only the cyclic voltametric data.28 Although a number of theoretical works have been carried out on the diffusion limited polymerization of various polymers, only a few experimental reports were found dealing with the same subject. Haberko et al. reported on the synthesis of dendrites and pillars of PANI through spin-casting.29 Detailed work on the dispersion of symmetric triblock copolymer based on PANI has been reported by Knaapila et al.30 Small angle X-ray scattering (SAXS) and electrochemical methods have been employed by Neves and Fonseka for the determination of fractal dimension of PANI.31 In the present Article, we report on controlled and reproducible synthesis of large scale fractal aggregates of polyaniline by conventional electropolymerization using potentiodynamic sweeping method on HOPG substrate at room temperature. The fractal dimension of the aggregates is shown to vary with the time of polymerization. The crucial role of the terminal Received: July 8, 2011 Revised: March 20, 2012 Published: March 21, 2012 5893

dx.doi.org/10.1021/la3006184 | Langmuir 2012, 28, 5893−5899

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polymerization potential and the substrate surface chemistry on the aggregate structure is also revealed.

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EXPERIMENTAL SECTION

Aniline (99.5%) was purchased from Sigma Aldrich and purified by distillation and then stored in a nitrogen glovebox (with moisture and oxygen content ≤2 ppm) prior to use. Analytical reagent grade HCl was used to make a monomer solution of aniline in MilliQ water. HOPG substrate (ZYH grade, 12 × 12 mm2 area and 2 mm thick) was purchased from Advanced Ceramics Corporation. The surfaces of substrate were cleaved and detached by adhesive tape to get a fresh surface prior to each experiment. The fluctuation in the number of defects of the new graphite surface exposed during cleavage is about 10−20%.32 The electrochemical experiments were performed by using a bipotentiostat (Pine Instrument Company). A vertical Teflon electrochemical cell with a three electrode configuration was employed for electropolymerization. The working HOPG electrode was sealed to the Teflon cell by a Teflon coated O-ring with an active surface area of 3.6 × 10−5 m2.33 A Pt ring was used as counter electrode and Ag/AgCl (1 N KCl) (CH instruments) served as reference electrode. The cell and counter electrode were cleaned with freshly prepared 1:1 (v/v) H2O2/H2SO4 solution followed by ultrasonication in MilliQ water (18 MΩ·cm resistivity) prior to each experiment. 2% aniline in aqueous 1 M HCl (v/v) solution was used as monomer for electropolymerization. Electropolymerization was performed by potentiodynamic sweeping from −200 to 760 mV at a scan rate of 15 mV s−1 for different times (no. of cycles). After each experiment, the remaining electrolyte on the electrode surface was soaked with filter paper and then kept for drying. The morphology of polyaniline microstructures was investigated by scanning electron microscopy (SEM, LEO series 1430 VP). It is important to mention here that the microstructures of PANI were always produced by terminating the applied potential at the initial value. The surface coverage of the fractals was calculated by analyzing the SEM images using standard image analysis software with a maximum digital resolution of 512 × 512. Harmonic and Fractal Image Analyzer software (HarFA 5.4) developed by Zmeškal et al. of Institute of Physical and Applied Chemistry, Brno University of Technology, Czech Republic was used for measurement of fractal dimension of the electropolymerized polyaniline microstructures. The standard box counting method was used in this software and all the images processed were of identical digital maximum resolution of 512 × 512.34 The fractal dimensions were calculated by linear regression analysis. In an electrochemical system with high rate constants, the time dependence of diffusion limited current is expressed by the conventional Cottrell equation35 which is given by I(t ) =

Figure 1. Cyclic voltammogram of 0.2% aniline in 1 M HCl on HOPG substrate obtained at a sweep rate of 0.015 V s−1 at room temperature. The highest current peaks belong to the 20th (the last) cycle.

The voltammogram shows prominent redox couple “a1−c1” which may be attributed to the electroformation and reduction of leucoemeraldine form of PANI.37 The current rise at the extreme anodic scan, a3 is generally attributed to the formation of polyemeraldine.37 The weak redox couple a2−c2 indicates that there is no significant formation of benzoquinone type decomposition product due to restriction of terminal oxidation potential to 0.76 V. The initial cycle shows very low peak current for the polymer oxidation and reduction processes. The morphology of the HOPG substrate prior to electropolymerization was found to be associated with the basal plane and steps. The morphology of the deposits after various length of electropolymarization is shown in Figure 2. It can be seen from Figure 2a that the polyaniline forms discrete dendrites and the clusters are arranged in a treelike morphology, typical for the diffusion limited polymerization (DLP) process. In electropolymerization, nucleation and growth are two important competitive processes. The result of this competition is reflected on the morphology of the deposited polymer. Two important parameters that effectively determine the outcome of the competition are surface diffusion (area migrated by an adatom in unit time) and deposition flux (atoms deposited per unit area per unit time).36 An adatom stops diffusing when it hits a stable aggregate as it condenses. The nucleation density increases with increasing coverage until it reaches a saturated value. Thereafter, the impinging atoms condense only on the existing aggregates. At this stage, the aggregates migrate an average distance Λa. The morphology (shape) of these aggregates is determined by the directional anisotropy of Λa and the average diffusion length of atoms adsorbed on the perimeter of aggregate Λ1.38 In the case of DLP (or DLA) atoms attaching to an aggregate stick where they hit and as a result of subsequent propagation, a fractal structure is formed.38 Hence, generation of the fractal surface by potentiodynamic electropolymerization is strongly influenced by the employed sweep rate, which directly effect the deposition flux. In the present investigation, we found that the fractals can be obtained at an optimum sweep rate of 15 mV s−1. At higher sweep rate, a tubular bush of bulk PANI is found to form on HOPG, while at lower sweep rate the coverage of fractal dendrimers reduced. In the case of DLP similar to that of DLA, the integrated two point correlation function, M(r) should be scaled with the

nFD1/2C π1/2t1/2

However, for a rough (fractal) electrode, the Cottrell equation transforms to an extended form:

I(t ) = σF t −α where σF is a proportionality factor, known as the Warburg coefficient, and α is a fractal parameter.36 It is thus observed that the fractal parameter can be calculated for a system with constant σF if the current varies as the power law of time in a certain time range. The fractal parameter is related to fractal dimension (D) by: α = (D − 1)/2. Hence, fractal dimension can be calculated from the slope of the current−time relationship plotted in a log−log scale.35 All the potentials are mentioned with respect to standard normal Ag/AgCl reference electrode.

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RESULTS AND DISCUSSION The cyclic voltammogram for the electropolymerization of Ph-NH2·HCl at a sweep rate of 15 mV s−1 is shown in Figure 1. 5894


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Figure 2. SEM image of the PANI fractals obtained by potentiodymanic diffusion controlled electropolymerization at a sweep rate of 0.015 V s−1 for (a) 15, (b) 20, (c) 25, and (d) 30 cycles.

radius of the aggregates, r, by M(r) = rD in two dimensions, with D being the fractal dimension.27 It can be seen in Figure 2 that the inner core or the center of the tree-structure is less dense in comparison to the growing branches. After 32 min of electropolymerization (about 15 cycles), the surface coverage is found to be about 13% (Supporting Information Figure 1). As the time of electropolymerization is increased to 42.6 min (20 cycles) under identical conditions, the relatively empty inner core is found to be covered by the polymer deposits almost to the same degree (Figure 2b) as that of the outer growing branches and the surface coverage increases to 61% (Supporting Information Figure 2). At this point, it is interesting to note that the growth of the outer branches is terminated at the extreme edges leaving an uncovered HOPG surface in between. We attribute this termination process to two probable reasons. Since DLP generally occurs under overpotential deposition (OPD) conditions, the monomer concentration becomes effectively zero inside the fractals and at the growing surface.27 Hence, after a certain duration of electropolymerization, a typical empty region is expected to appear in between two growing fractals due to lack of sufficient number of monomers,. The other reason, which will be evidenced later, may be the existence of high surface energy zone on HOPG in between two adjacent growing fractal faces, which is not favoring nucleation of new cluster for further growth. When the time of electropolymerization is increased to around 53 min (25 cycles), the fractal domains of polyaniline dendrimers become more defined and the denuded zones between the edges of the

domains are observed clearly in the SEM image (see Figure 2c). Although the coverage is increased to 73% (Supporting Information Figure 3), the braches of the fractal become more dense and compact. The interesting observation from Figure 2c is the enhanced growth of terminal polymer clusters. The bright feature of the micrograph can be due to increased height of the edges, or it may be related to excess surface charge density. The overgrowth of PANI takes place when the polymerization time is increased to 64 min (30 cycles). The fractal dendrimeric morphology starts to disappear and a coillike structure of PANI is formed. The surface coverage increased to 87% (Supporting Information Figure 4), and the growth in the vertical direction is also evident from Figure 2d. The fractal dimension of the PANI dendrimer obtained by electropolymerization for 53 min (25 cycles) was determined by chronoamperometry (CA) and was found as 1.92 (the correlation coefficient for this evaluation based on the best straight line plotted for the experimental data was 0.992, Supporting Information Figure 5) which was close to the calculated value of 1.89 by box counting method. It is seen that the CA method is based on the right choice of the time domain where current is a power law function of time.35 Since the overall current is influenced by the double layer charging current at shorter time domain and perturbation of planar diffusion takes place at long time domain, the measured fractal dimension by CA is more prone to error.35 Hence, we have adopted the box counting method for the determination of fractal dimension, which is based on the analysis of the image of fractals that already exist on the substrate. The variation of fractal dimension of the 5895


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PANI films over HOPG substrate for various length of polymerization is shown in Figure 3. The micrograph in Figure 2a

mass, the reason of which is not very clear at present. The morphology changes entirely when the terminal oxidation potential is made 20 mV anodic as compared to 760 mV. It can be seen from Figure 4b that the entire HOPG surface is covered with a tubular bush of PANI with localized growth of PANI clusters in the vertical direction over the tubular structures. The primary reason may be associated with the usual over potential deposition process where three-dimensional PANI clusters are found to exist over the localized tubular structures. It is further noted that the growth rate and surface coverage of PANI fractals at higher sweep rate is enhanced (see Supporting Information Figure 6). The role of diverse surface anisotropy of HOPG in facilitating the growth of PANI fractals was disclosed from the fact that, under identical experimental conditions, no PANI fractals were formed on Au(111) substrate (see Supporting Information Figure 7). It is well-known that HOPG having very low surface energy (35 dyn cm−1) shows extremely anisotropic electrochemical properties like defect free terraces that are electrochemically inert or nearly so while the step edges behave like linear microelectrodes. Hence, it became very crucial to ascertain whether the observed fractal growth in the present investigation is a genuine case of diffusion limited polymerization or anisotropic HOPG surface assisted nucleation and growth.30 Figure 5a shows the usual feature of the HOPG substrate with very large terraces and step edges. Further confirmation that the observed fractals are not formed by usual step edge decoration is provided in the SEM image of Figure 5b. It can be clearly seen that, under identical electropolymerization conditions, the step edges are decorated with larger clusters of PANI and the fractals are grown preferably on the basal plane. The nature of nucleation and growth over the HOPG surface was also followed from the current time (I−t) transients. Figure 6i shows the I−t transients when the potential was stepped to 200 and 760 mV vs Ag/AgCl. Both the transients reflect a typical nature matching with two-dimensional nucleation and growth.40−42 In order to know about the specific nucleation process of PANI over HOPG, the I−t transient obtained at a step potential of 200 mV was analyzed for the shorter time domain. Figure 6ii shows the dimensionless plot and reveals that the instantaneous nucleation of PANI occurs on the HOPG surface.40 When the similar task is carried out for the I−t transient obtained at a step potential of 760 mV, we do find (see Figure 6iii) that the response is not exactly matching with

Figure 3. Variation of fractal dimensions with the time of polymerization.

and its fractal dimension, D, of 1.59 led us to conclude that the PANI formation proceeds via two dimension diffusion limited aggregation (DLA) or DLP processes.38,39 As the polymerization time is increased, the fractal dimension increases sequentially which is clearly supported by the micrographs in Figure 2b−d where enlarged inner cores with successively dense radial structures are observed. We have next followed the effect of terminal oxidation potential of monomers on the dendrimer structure and also the DLA process as a whole. For this purpose, we polymerize monomer by potentiodynamic method for 42 min in the potential window where the maximum anodic potential was restricted, separately, to 740 and 780 mV, keeping all other deposition parameters identical. As we can see in Figure 4a that the restriction of terminal anodic potential by 20 mV in the cathodic side, that is, at 740 mV compared to 760 mV, well patterned PANI fractals are formed (see Figure 2b) with less overall coverage of the HOPG substrate. Monomer oxidation at low applied potential leads to less number density of PANI clusters, and hence, DLP proceeds with the formation of localized fractals. The edges of the fractal domains are also found to be terminated by the accumulated excess polymer

Figure 4. SEM images obtained by potentiodynamic polymerization of aniline on HOPG at a sweep rate of 0.015 V s−1 by limiting the anodic potential to (a) 0.74 and (b) 0.78 V. 5896


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Figure 5. SEM image of (a) HOPG surface prior to electropolymerization and (b) fractal grown on the basal plane as well as on the step edge of HOPG indicating diffusion limited polymerization process.

Figure 6. Current−time transients (I−t) for the electropolymerization process: (i) the step potential is at (a) 0.2 V and (b) 0.76 V. Dimensionless plots of I/Im vs t/tm for the shorter time domain when the step potential was at (ii) 0.2 V and (iii) 0.76 V. Empty (○) and filled (●) symbols indicate the experimental and simulated data, respectively.

instantaneous nucleation but a major share (about 90%) of the nucleation process belongs to it.36 The two-dimensional instantaneous nucleation and diffusion limited polymerization can be further supported from a SEM image (see Figure 7) that

was acquired at very early stage of (after third cycle) polymerization process. The narrow size distributions of the PANI clusters in Figure 7 indicate the instantaneous nucleation, whereas the growth of the fractals supports the DLP process.

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CONCLUSIONS

In summary, we have demonstrated that the controlled synthesis of PANI fractals on HOPG by DLP is possible using a simple electrochemical cell by potentiodynamic method. It is shown that the surface coverage and the nature of the fractals change to an observable extent with the duration of potential sweeping. The fractal dimension changes from 1.56 to 1.94 as the time of electropolymerization increased from 15 to 30 cycles. It is established through SEM studies that the fractal structure and consequently, the fractal dimension can be nicely controlled by tuning the monomer oxidation potential limit in the present method. The role of HOPG surface in facilitating the fractal growth by DLP is also revealed. Moreover, the DLP process is established by excluding the possibility of usual step edge assisted growth on HOPG through independent experiment. Since the method is highly reproducible and PANI fractals are grown over large surface, it is believed that these structures can be used easily for many interesting awaited studies.

Figure 7. SEM images revealing the nature of the PANI fractals at an early stage of electropolymerization indicating dominant instantaneous nucleation process. 5897


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(14) Chandrakanthi, N.; Careem, M. A. Thermal stability of polyaniline. Polym. Bull. 2000, 44, 101−108. (15) Persaud, K. C. Polymers for chemical sensing. Mater.Today. 2005, 8, 38−44. (16) Lu, W.; Smela, E.; Adams, P.; Zuccarello, G.; Mattes, B. R. Development of Solid-in-Hollow Electrochemical Linear Actuators Using Highly Conductive Polyaniline. Chem. Mater. 2004, 16, 1615− 1621. (17) Jang, J.; Ha, J.; Kim., S. Fabrication of polyaniline nanoparticles using microemulsion polymerization. Macromol. Res. 2007, 15, 154− 159. (18) Detsri, E.; Dubas, S. T. Interfacial Polymerization of WaterSoluble Polyaniline and Its Assembly Using the Layer-By-Layer Technique. JOM 2009, 19, 39−44. (19) Bucholz, T.; Sun, Y.; Loo, Y. L. Near-monodispersed polyaniline particles through template synthesis and simultaneous doping with diblock copolymers of PMA and PAAMPSA. J. Mater. Chem. 2008, 18, 5835−5842. (20) Tang, Q.; Wu, J.; Sun, X.; Li, Q.; Lin, J.; Huang, M. Templateless self-assembly of highly oriented polyaniline arrays. Chem. Commun. 2009, 2166−2167. (21) Thyssen, A.; Hochfeld, A.; Kessel, R.; Meyer, A.; Schulzte, J. W. Anodic polymerisation of aniline and methyl substituted derivatives: ortho and para coupling. Synth. Met. 1989, 29, 357−362. (22) Lyons, M. E. G. Polymer Electrochemistry; Plenum: New York, 1994. (23) Tang, Z.; Liu, S.; Wang, Z.; Dong, S.; Wang, E. Electrochemical synthesis of polyaniline nanoparticles. Electrochem. Commun. 2000, 2, 32−35. (24) Witten, T. A. Jr.; Sander, L. M. Diffusion-Limited Aggregation, a Kinetic Critical Phenomenon. Phys. Rev. Lett. 1981, 47, 1400−1403. (25) Mandelbort, B. B. The fractal geometry of nature; Freeman: New York, 1983 (26) Wu, Q.; Wu, L.; Qi, Z.; Wang, F. The fractal structures in highly sulfonated polyaniline. Synth. Met. 1999, 105, 13−16. (27) Kaufman, J. H.; Baker, C. K.; Nazzal, A. I.; Flickner, M.; Melroy, O. R.; Kapitulnik, A. Statics and Dynamics of the Diffusion-Limited Polymerization of the Conducting Polymer Polypyrrole. Phys. Rev. Lett. 1986, 56, 1932−1935. (28) Villeret, B.; Nechtchein, M. Growth of polyaniline films: Evidence for fractal surface. Solid State Commun. 1987, 64, 435−437. (29) Haberko, J.; Bemasik, A.; tużny, W.; Rackowska, J.; Rysz, J.; Budkowski, A. Dendrites and pillars in spin cast blends of polyaniline or its oligomeric analogue. Synth. Met. 2010, 160, 2459−2466. (30) Knaapila, M; Garamus, M. V.; Almasy, L.; Pang, S. J.; Forster, M.; Gutacker, A.; Scherf, U.; Monkman, A. P. Fractal Aggregates of Polyfluorene−Polyaniline Triblock Copolymer in Solution State. J. Phys. Chem B 2008, 112, 16415−16421. (31) Neves, S.; Fonseka, C. P. Determination of fractal dimension of polyaniline composites by SAXS and electrochemical techniques. Electrochem. Commun. 2001, 3, 36−43. (32) Zoval, J. V.; Stiger, R. M.; Biernacki, P. R.; Penner, R. M. Electrochemical Deposition of Silver Nanocrystallites on the Atomically Smooth Graphite Basal Plane. J. Phys. Chem. 1996, 100, 837−844. (33) Mukhopadhyay, I.; Freyland, W. Electrodeposition of Ti Nanowires on Highly Oriented Pyrolytic Graphite from an Ionic Liquid at Room Temperature. Langmuir 2003, 19, 1951−1953. (34) Zmeškal, O.; Veselý, M.; Nežad́ al, M.; Buchníček, M. HarFA Harmonic Fractal Image Anal. 2001, 3−5. (35) Iftekhari, A. Influence of atomic-scale irregularities in fractal analysis of electrode surfaces. Appl. Surf. Sci. 2004, 227, 331−340. (36) Pazkossy, T. Electrochemistry at fractal surfaces. J. Electroanal. Chem. 1991, 300, 1−11. (37) Vela, M. E.; Zubimendi, J. L.; Ocon, P.; Herrasti, P.; Salvarezza, R. C.; Vazquez, L.; Arvia, A. V. Structural configurations of thin dried polyaniline films on gold(111) from scanning tunneling microscopy. Electrochim. Acta 1996, 41, 1891−1903.

ASSOCIATED CONTENT

S Supporting Information *

Image analysis for the determination of surface coverage. Determining the fractal dimension by using CA method. SEM image of the PANI fractals obtained at different sweep rate on HOPG surface. SEM image revealing the effect of substrate on the morphology of the electrochemically synthesized PANI. SEM showing the absence of PANI in the fractal boundary. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-79-23275030. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support for carrying out this work has been received from CSIR net work project (NWP 0010) and DST sponsored project (SR/S1/PC-01/2010).

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REFERENCES

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