self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

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Synthetic Metals 160 (2010) 1325–1334

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Synthetic Metals

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Self-assembled polyaniline nanotubes and nanoribbons/titanium dioxidenanocomposites

Marija Radoicic a, Zoran Saponjic a, Jovan Nedeljkovic a, Gordana Ciric-Marjanovic b,∗, Jaroslav Stejskal c

a Vinca Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbiab Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12–16, 11158 Belgrade, Serbiac Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic

a r t i c l e i n f o

Article history:Received 18 November 2009Received in revised form 29 March 2010Accepted 14 April 2010Available online 13 May 2010

Keywords:NanocompositesNanoribbonsNanostructuresNanotubesPolyanilineTitanium dioxide

a b s t r a c t

Self-assembled polyaniline (PANI) nanotubes, accompanied with nanoribbons, were synthesized by theoxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous medium, in thepresence of colloidal titanium dioxide (TiO2) nanoparticles of 4.5 nm size, without added acid. Themorphology, structure, and physicochemical properties of the PANI/TiO2 nanocomposites, prepared atvarious initial aniline/TiO2 mole ratios, were studied by scanning (SEM) and transmission (TEM) electronmicroscopies, FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies,elemental analysis, X-ray powder diffraction (XRPD), conductivity measurements, and thermogravi-metric analysis (TGA). The electrical conductivity of PANI/TiO2 nanocomposites increases in the range3.8 × 10−4 to 1.1 × 10−3 S cm−1 by increasing aniline/TiO2 mole ratio from 1 to 10. The morphology ofPANI/TiO2 nanocomposites significantly depends on the initial aniline/TiO2 mole ratio. In the morphol-ogy of the nanocomposite synthesized using aniline/TiO2 mole ratio 10, nanotubes accompanied withnanosheets prevail. The nanocomposite synthesized at aniline/TiO2 mole ratio 5 consists of the network ofnanotubes (an outer diameter 30–40 nm, an inner diameter 4–7 nm) and nanorods (diameter 50–90 nm),accompanied with nanoribbons (a thickness, width, and length in the range of 50–70 nm, 160–350 nm,and ∼1–3 �m, respectively). The PANI/TiO2 nanocomposite synthesized at aniline/TiO2 mole ratio 2contains polyhedral submicrometre particles accompanied with nanotubes, while the nanocompositeprepared at aniline/TiO2 mole ratio 1 consists of agglomerated nanofibers, submicrometre and nanopar-ticles. The presence of emeraldine salt form of PANI, linear and branched PANI chains, and phenazineunits in PANI/TiO2 nanocomposites was proved by FTIR and Raman spectroscopies. The improved thermalstability of PANI matrix in all PANI/TiO2 nanocomposites was observed.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Polyaniline (PANI) is one of the most important conduct-ing polymers, frequently studied due to its simple synthesis [1],unique doping/dedoping chemistry [2], low cost, high conductivity,and excellent environmental stability. The crystallinity, solubil-ity, thermal stability, electrical, magnetic and optical propertiesof PANI mainly depend on its oxidation state and protonationdegree [3]. The green emeraldine salt of PANI reaches conduc-tivity of ∼1–10 S cm−1 for ordinary granular PANI powders [1a],∼102 S cm−1 for PANI powders with nanospherical morphology[1b], and ∼103 S cm−1 for PANI films [2]. It contains, depend-ing on synthetic and isolation procedure, various proportions ofdiamagnetic [(–B–NH+ Q NH+–)n(–B–NH–)2n](A−)2n and param-agnetic units [(–B–NH•+–B–NH–)n](A−)n, where B, Q and A−denote

∗ Corresponding author. Tel.: +381 11 3336623; fax: +381 11 2187133.E-mail address: [email protected] (G. Ciric-Marjanovic).

a benzenoid ring, quinonoid ring and dopant anion, respectively.The blue emeraldine base [(–B–N Q N–)n(–B–NH–)2n], colour-less leucoemeraldine base [(–B–NH–)n], and violet pernigranilinebase form [(–B–N Q N–)n] are insulating. The preparation of bulkquantities of conducting granular PANI is usually carried out bythe oxidative polymerization of aniline with ammonium peroxy-disulfate (APS) in an acidic aqueous solution (initial pH < 2.0) [1].Colloidal PANI nanoparticles are prepared by dispersion polymer-ization of aniline in the presence of various colloidal stabilizers[4,5].

In recent years, various one-dimensional (1D) (nanofibers,nanorods, nanoneedles, rice-like nanostructures, nanotubes),two-dimensional (2D) [nanobelts (nanoribbons), nanosheets(nanoplates, nanoflakes, nanodisks, leaf-like nanostructures),nanorings], as well as three-dimensional (3D) PANI nanostruc-tures (hollow nanospheres, dendritic and polyhedral nanoparticles,flower-like, rhizoid-like, brain-like, urchin-like, star-like nanos-tructures, etc.) were synthesized and characterized. Thesenanostructures are in the focus of intense research owing to

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1326 M. Radoicic et al. / Synthetic Metals 160 (2010) 1325–1334

their significantly improved dispersibility and processibility, andsubstantially enhanced performance in many applications in com-parison with granular and colloidal PANI [6–8]. The uniquephysical and chemical properties of PANI nanostructures, whichare quite different in comparison to the ordinary granular andcolloidal PANI, open up possibility for the synthesis of a newclass of nanocomposite materials for different applications. Wher-ever a high interfacial area between PANI and its environment isimportant, nanostructured PANI offers the possibility of enhancedperformance.

Extensive research in the field of synthesis and charac-terization of nanostructured composite materials, particularlythose consisting of nanostructured PANI matrix and microme-tre/submicrometre and nanoparticles of numerous metals, oxides,as well as organic and bioorganic compounds, develops inten-sive targeting at novel design of composite nanomaterials withunique physicochemical properties and applicability. During lasttwo decades, an interest in development of conducting PANI/TiO2nanocomposites has significantly grown [9–51]. Encapsulation ofnanocrystalline TiO2 into PANI matrix can improve mechanicalproperties, heat resistance, dielectric and magnetic properties,microwave absorption properties, and processibility of PANI. Themajority of the work was performed by using TiO2 nanoparti-cles of anatase crystal form, that are commercially available orprepared by various methods [9–36,38–48]. The TiO2 nanopar-ticles with the rutile crystal structure [37] or as a mixture ofanatase and rutile [49] phases were used rarely. For the synthesis ofPANI/TiO2 nanocomposites several methods have been employed.Chemical oxidative polymerization of aniline in the presence oftitanium dioxide [10,12,14,16–19,24–28,30–35,37–41,43,46–52]was the most frequently reported. The electrochemical poly-merization of aniline [9,11,45], the polymerization of anilineusing reverse micelles [22,23], ultrasonic [13] and �-irradiationmethods [42], sol–gel methods [20], and physical mixing [44]were studied, too. The polymerization of aniline in the pres-ence of TiO2 nanoparticles was carried out using hydrochloricacid [9,10,13,14,17–22,24,25,27–29,31,33–40,42,43,47–49,51,52],sulfosalicylic acid [11], �-naphthalenesulfonic acid [16], salicylicacid [26], sulfuric acid [45], n-hexanoic acid [41,46], and para-toluene sulfonic acid [50] as dopant acids. The reports that aredevoted to the preparation of PANI nanotubes/nanorods, by in situpolymerization of aniline, in the presence of TiO2 nanoparticles anddopant amphiphilic acids (�-naphthalenesulfonic acid, n-hexanoicacid), are scarce [16,41,46].

The combination of desirable properties such as electrical con-ductivity of PANI and UV-sensitivity of TiO2, opens the possibilityfor various applications of PANI/TiO2 nanocomposite materials,such as: piezoresistivity devices [10], electrochromic devices,nonlinear optical systems, photoelectrochemical devices [12,45],photovoltaic cells [13,30], solar cells [13,19,29], photocatalysis[21,32,43,44,47], gas sensors [25,30,34,38], microbial fuel cells [40],catalysis [49], microwave absorption [46], and corrosion protection[51].

In the present study, we report, for the first time, the dopant-free template-free synthesis of conducting PANI nanotubes andnanoribbons/TiO2 nanocomposites. Applied reaction route offersa wide range of opportunities for the production of variousnanocomposites based on conducting self-assembled PANI nanos-tructures and inorganic materials [53,54]. It should be noticedthat this facile synthetic method not only excludes hard-templateusage and post-treatment processing for template removal, butalso simplifies the selection of reagents. The prepared PANI/TiO2nanocomposites were characterized by FTIR, Raman and ICP-OESspectroscopies, SEM and TEM microscopies, XRPD, TGA, elementalanalysis and electrical conductivity measurements. The influence ofinitial TiO2/aniline mole ratio on the molecular and supramolecular

structure, as well as on physicochemical properties of synthesizedPANI/TiO2 nanocomposites was studied.

2. Experimental

2.1. Materials

Aniline (p.a., > 99.5%, Centrohem, Serbia), was distilled underreduced pressure and stored at room temperature, under argon,prior to use. Ammonium peroxydisulfate (analytical grade, Centro-hem, Serbia) was used as received.

2.2. Synthesis of TiO2 colloid

The colloidal solution of TiO2 nanoparticles was prepared inaccordance with the procedure of Rajh et al. [55,56]. All the chem-icals for this synthesis were analytical grade and used as receivedwithout further purification (Aldrich, Fluka). As a solvent Milli-Qdeionized water was used. The titanium(IV) chloride (6 ml), cooledto −20 ◦C, was added drop-wise to water (200 ml), at 4 ◦C, undervigorous stirring and then kept at this temperature 30 min. Slowgrowth of the particles was achieved by using dialysis against waterat 4 ◦C until the pH of the solution has reached 3.5. The concentra-tion of prepared TiO2 colloidal solution (0.17 M) was determinedfrom the concentration of the peroxide complex obtained afterdissolving the particles in concentrated sulfuric acid.

2.3. Syntheses of PANI/TiO2 nanocomposites

The aqueous solution of APS (0.4 M, 25 ml) was poured into theaqueous solution of aniline (0.32 M, 25 ml). Thereupon diluted col-loidal TiO2 solution of various concentrations (0.016–0.16 M, 50 ml)was added into the aniline/APS solution. The molar concentra-tions of reactants thus were [aniline] = 0.08 M, [APS] = 0.10 M, and[TiO2] = 0.008–0.08 M. The reaction mixture was stirred for 3 h atroom temperature. The precipitated PANI/TiO2 nanocomposite wascollected on a filter, rinsed with ethanol acidified with sulfuric acid(5 × 10−3 M), and dried in vacuum at 60 ◦C for 3 h. The PANI/TiO2nanocomposites prepared at initial [aniline]/[TiO2] mole ratios of1, 2, 5 and 10, are designated as PT-1, PT-2, PT-5 and PT-10, respec-tively. As a reference sample, pure PANI was prepared using thesame procedure without TiO2 (50 ml of distilled water was addedinto aniline/APS solution instead of the colloidal TiO2 solution).The pH values of the starting reacting mixtures were in the range6.0–6.5, while final pH value was 1.3–1.5.

2.4. Isolation of PANI bases from PANI/TiO2 nanocomposites

The deprotonation of PANI in PANI/TiO2 nanocomposite wasperformed by treating 1 g of composite with 200 ml of 0.5 M sodiumhydroxide with stirring for 1 h. The composite which containsdeprotonated PANI was then separated by filtration, washed with0.5 M sodium hydroxide and distilled water (until the pH value ofthe filtrate reached ∼6), and dried in vacuum at 60 ◦C for 2 h. Thedeprotonated composite was treated with N-methyl-2-pyrrolidone(NMP), and PANI base dissolved in NMP was separated from TiO2by centrifugation at 13,000 rpm for 10 min. PANI base was precip-itated from obtained supernatant liquid with methanol in excess,separated by centrifugation (at 13,000 rpm for 10 min), and finallydried in vacuum at 60 ◦C for 3 h. Resulting deprotonated forms ofPANI extracted from the PANI/TiO2 composites PT-1, PT-2, PT-5 andPT-10 are denoted as PANI-base-1, PANI-base-2, PANI-base-5 andPANI-base-10, respectively. The sample obtained by deprotonationof pure PANI sulfate/hydrogen sulfate is designated as PANI-base.


M. Radoicic et al. / Synthetic Metals 160 (2010) 1325–1334 1327

2.5. Characterization

A scanning electron microscope JEOL JSM 6460 LV and a trans-mission electron microscope Tecnai G2 Spirit (FEI, Brno, CzechRepublic) have been used to characterize the morphology of thePANI and PANI/TiO2 samples. Powdered materials were depositedon adhesive tape fixed to specimen tabs and then ion sputter coatedwith gold using a BAL-TEC SCD 005 Sputter Coater prior to SEMmeasurements. A transmission electron microscope JEOL 100CX,operating at 100 kV, has been used to determine the sizes of theused TiO2 nanoparticles. The sample was prepared by drop-wiseplacing volume of 6 �l onto a holey carbon film supported on a cop-per grid after 10 min of ultrasound treatment of TiO2 nanoparticlesdispersion. The specimen was air-dried overnight. The conductivityof PT powders compressed between stainless pistons was mea-sured at room temperature by means of an ac bridge (WaynneKerr Universal Bridge B 224), at fixed frequency of 1.0 kHz. Duringthe measurement, pressure was maintained at 124 MPa. The TGAwas carried out using SETSYS Evolution 1750 ThermogravimetryAnalyzer with air purging gas, at a flow rate of 20 ml min−1 andat a heating rate of 10 ◦C min−1. Elemental analysis (C, H, N, andS) was performed using an Elemental Analyzer VARIO EL III (Ele-mentar). The content of titanium in PT samples was determinedby ICP-OES, using a Thermo Scientific iCAP 6500 Duo ICP spec-trometer. Prior to analysis by ICP-OES, the microwave-assisted aciddigestion of samples was performed (Digestion Application NoteDG-GE-36) by means of ETHOS 1 Advanced Microwave DigestionSystem (Milestone, Italy) using HPR-1000/10S high pressure seg-mented rotor. ICP-OES analysis was performed at emission line TiII 336.121 nm. FTIR spectra of the powdered samples, dispersed inKBr and compressed into pellets, were recorded using a Nicolet6700 FTIR Spectrometer (Thermo Scientific) at 2 cm−1 resolution,in the range of 400–4000 cm−1. Raman spectra excited with adiode-pumped solid state high-brightness laser (532 nm) were col-lected on a Thermo Scientific DXR Raman microscope, equippedwith a research optical microscope and a CCD detector. The laserbeam was focused on the sample using objective magnification50× (∼1.1 �m laser spot size). The powdered sample was placedon an X–Y motorized sample stage. The scattered light was ana-lyzed by a spectrograph with grating 900 lines mm−1. Laser powerwas in the range of 0.1–0.5 mW on the sample. The XRPD patternswere obtained by using a Philips PW 1050 powder diffractome-ter with Ni filtered Cu K� radiation (� = 1.5418 Å). The diffractionintensity was measured by the scanning technique in the range of2� = 10–70◦ by a step size of 0.05◦ and a counting time of 50 s perstep.

3. Results and discussion

3.1. The course of chemical oxidative polymerization of aniline inaqueous dispersions of colloidal TiO2

The oxidation of aniline with APS in water, in the presenceof colloidal TiO2 nanoparticles proceeds in two rapid exothermicphases (I and III, Fig. 1a). Those two phases are well separated byan athermal period (II, Fig. 1a), similarly to corresponding oxidationof aniline without presence of colloidal TiO2 nanoparticles [57a].In the post-polymerization period (IV, Fig. 1a) the temperature ofthe reaction medium decreased. The acidity of the reaction mix-ture continuously increases during the oxidative polymerizationof aniline with peroxydisulfate (Fig. 1b) due to the formation ofhydronium ions [54].

The first exothermic phase corresponds to the fast oxidativeoligomerization of nonprotonated aniline molecules, which aresignificantly prevalent over anilinium cations (pKa = 4.6) at the

Fig. 1. (a) Temperature and (b) pH change during the oxidation of aniline (0.08 M)with APS (0.1 M) in water: without colloidal TiO2 nanoparticles (©) and for the syn-thesis of PT-1 nanocomposite (N). The individual phases I–IV refer to the preparationof a nanocomposite.

beginning of the oxidation process (pH = 6.0–6.5). Nonprotonatedfully oxidized oligoanilines, which contain linear pernigraniline-like as well as branched phenazine-like units, are formed [58–61].Formation of the mixtures of phenazine-containing Michael-typeadducts of aniline and benzoquinone monoimine (a byproductof Boyland–Sims oxidation of aniline), has also been recentlyproposed to occur during the oligomerization phase in the aniline-peroxydisulfate system in basic, neutral or weakly acidic aqueousconditions [62].

Because the anilinium cation, which became prevalent overnonprotonated aniline at pH ≤ 4.6, is much weaker reductant thannonprotonated aniline molecule [58–60], and the nonprotonatedfully oxidized oligoanilines are much weaker oxidants than peroxy-disulfate, the oxidative oligomerization of remaining C6H5NH2 andC6H5NH3

+ shows considerable slowdown until it is almost stoppedat pH ∼ 3.5. At this point low-reactive anilinium cations became sig-nificantly prevalent over reactive nonprotonated aniline molecules([C6H5NH3

+]/[C6H5NH2] ∼ 10 at pH ∼ 3.5).From the mechanistic point of view, a subsequent athermal

phase is quite similar to the induction period of the oxidative poly-merization of aniline, which starts in an acidic aqueous solution.The presence of colloidal TiO2 nanoparticles causes an accelerationof redox reactions during this polymerization phase at pH < 3.5,i.e. the athermal periods during the syntheses of all compositesbecomes shorter than that of pure PANI (Fig. 1a), due to the sub-stantial increase of the surface area of oligoanilines in the presenceof colloidal TiO2. For that reason, the oxidative polymerization ofaniline in aqueous dispersions of colloidal TiO2 was completed con-siderably faster than in water, without added colloidal TiO2.



Fig. 2. The overall reaction scheme for the oxidation of aniline with APS in water, which yields the mixture of PANI hydrogen sulfate (1) and PANI sulfate (2) in emeraldinesalt form, with ammonium hydrogen sulfate and ammonium sulfate as byproducts.

The auto-accelerated polymerization of aniline takes place dur-ing the second exothermic phase at pH ≤ 2.5 (III, Fig. 1a) [57], whenthe protonation of fully oxidized oligoanilines causes the significantincrease of their oxidant power and solubility [58,59]. This phase isvery similar to the single exothermic phase of the oxidative poly-merization of anilinium sulfate with APS in an aqueous solution[60]. The protonated, completely oxidized oligoanilines and resid-ual peroxydisulfate react with remaining nonprotonated anilinemolecules, anilinium cations as well as reduced segments of partlyoxidized oligoanilines via the exothermic redox equilibrating pro-cess. This reaction route leads toward the formation of PANI chainsin the form of emeraldine salt (PANI sulfate/hydrogen sulfate) withprevalent N–C4 coupling between aniline units (Fig. 2).

3.2. Morphology of PANI/TiO2 nanocomposites

TEM micrographs revealed that the average particle diameter ofcolloidal TiO2 used for the syntheses of PANI/TiO2 nanocompositesis 4.5 nm (Fig. 3). The pure PANI sulfate/hydrogen sulfate, synthe-sized without TiO2 nanoparticles, mainly consists of nanotubes andnanoribbons (Fig. 4). The PANI nanotubes with an average outerdiameter of 20–80 nm, an inner diameter of 5–15 nm, and a lengthin the range 0.3 to ∼3.0 �m (Fig. 4B) form a network (Fig. 4A).The nanoribbons have a thickness in the range of 50–140 nm, awidth in the range 230–370 nm and a length extending up to∼3.0 �m (Fig. 4C and D). In addition, twisted nanoribbon can beseen (Fig. 4D). Some sheet-like structures ∼130–140 nm thick arealso detected in pure PANI sample (Fig. 4A). The observed nanos-tructured morphology of PANI differs from that recently obtainedin more concentrated aqueous solutions of aniline, characterized bythe presence of PANI nanotubes with significantly higher averagediameter, accompanied with nanorods and nanosheets [57a].

SEM and TEM images of PANI/TiO2 nanocomposites showthat their morphology is significantly affected by the initial[aniline]/[TiO2] mole ratio (Figs. 5–7). All the composites exhibitnonuniform morphology, which consists of different types ofnanophases. Nanotubes and nanosheets predominate in the mor-phology of PT-10, which has been prepared at the highest mole ratioaniline/TiO2 (Fig. 5). Nanotubes have an average outer diameter of40–260 nm, an inner diameter of 10–130 nm, and a length extend-ing from 0.3 to 2.0 �m, as revealed by TEM (Fig. 5B and C). Fig. 5Cshows an example of branched PANI nanotube with a fluctuatingdiameter. The thickness of nanosheets is in the range 65–100 nm. Inthe sample PT-5 the significant amount of nanoribbons, along with

Fig. 3. TEM image of colloidal TiO2 used for the syntheses of PANI/TiO2 nanocom-posites.

the network of nanotubes (outer diameter 30–40 nm, inner diam-eter 4–7 nm) and nanorods (diameter 50–90 nm) can be observed(Fig. 6). The thickness, widths, and lengths of nanoribbons are inthe range of 50–70 nm, 160–350 nm, and ∼1–3 �m, respectively.As shown in Fig. 6A and B, nanoribbons have tendency to formhelices by twisting along the length axis, indicating their good flex-ibility. The nanoribbons frequently grow from the same startingpoint (Fig. 6B). There have been few reports on the preparation ofPANI nanoribbons [63–67]. To our best knowledge, in present work,PANI nanoribbons are synthesized for the first time by dopant-freetemplate-free method, and this is the first preparation of nanorib-bons in the composites of PANI and TiO2.

With further decrease of [aniline]/[TiO2] mole ratio, polyhe-dral submicrometre particles are observed in the sample PT-2(Fig. 7A), accompanied with nanotubes of an average outer diam-eter 10–90 nm (Fig. 7B). The sample PT-1, prepared by using theequimolar quantities of aniline and TiO2, contains agglomeratednanofibers, submicrometre- and nanoparticles (Fig. 7C and D). Theheterogeneous morphology of synthesized PANI/TiO2 nanocom-



Fig. 4. SEM (A, C, D) and TEM (b) images of pure PANI prepared without colloidal TiO2 nanoparticles. Nanotubes are marked by a in Figs. A and B (a network of nanotubesis shown in Fig. A); sheet-like structures are marked by b in Fig. A; nanoribbons are marked by c in Figs. A, C and D. A rectangular cross-section of nanoribbon is shown byarrow in Fig. D.

posites could be useful for their potential application as microwaveabsorbing materials, as it has been shown for PANI/TiO2 nanocom-posite prepared in the presence of hexanoic acid [46].

We proposed that the growth of PANI nanostructures[nanotubes (PT-2, PT-5 and PT-10), nanosheets (PT-10), nanorib-bons (PT-5) and nanorods (PT-5)] occurs in the bulk of theaqueous dispersion of colloidal TiO2. During the first exother-mic phase of the oxidative polymerization of aniline in waterwithout added acid, nonprotonated pernigraniline-like andpseudomauveine-like oligoanilines are precipitated as hydropho-bic crystallites [53,54,59–61], which do not adhere efficiently to thehydrophilic surfaces of colloidal TiO2. The non-conducting needle-like nanocrystallites, with high content of substituted phenazines,which have tendency to build columnar aggregates by stacking,become coated with a conducting PANI sulfate/hydrogen sulfatefilm during the third polymerization phase at pH ≤ 2. This leadsto the formation of core (non-conducting oligoaniline)/shell (con-ducting PANI sulfate/hydrogen sulfate) structured nanorods. PANInanotubes are formed by the dissolution of the cores of nanorods,induced by the protonation of fully oxidized oligoanilines at pH ≤ 2.In the case of equimolar mixture of colloidal TiO2 and aniline (PT-1), the dominant process is oxidative polymerization of anilinemolecules adsorbed on TiO2 nanoparticles, resulting in the forma-tion of PANI/TiO2 nanoparticles comprised of TiO2 core and PANIshell, which are partly agglomerated in submicrometre particles.

3.3. Thermogravimetric analysis

The content of TiO2, PANI sulfate/hydrogen sulfate and water inPANI/TiO2 nanocomposites was determined by TGA (Fig. 8, Table 1).The weight loss of the PANI/TiO2 nanocomposites from 25 to 200 ◦Ccorresponds to the release of residual water. The weight loss in

the temperature range from ∼200 to 780 ◦C is attributed to therelease of sulfuric acid from the PANI matrix, followed by the pro-gressive degradation and combustion of PANI. The combustion ofPANI present in the PANI/TiO2 nanocomposites was completedat ∼780 ◦C, and the residual weight refers to the content of TiO2in the composite. The fact that pure PANI sulfate/hydrogen sul-fate sample is completely combusted at 720 ◦C indicates improvedthermal stability of PANI in the nanocomposites. In addition, ithas been found that the extent of thermal decomposition of PANIin nanocomposites, at temperatures higher than ∼500 ◦C, becamelower than that of pure PANI sulfate/hydrogen sulfate. It is inter-esting to note that the thermal stability of PANI in PANI/TiO2nanocomposites decreases by increasing the TiO2 content. Forexample, the extent of thermal decomposition of PANI at 600 ◦Camounts to 81.5, 76.1, 75.0, 72.0 and 69.7 wt% for pure PANI sul-fate/hydrogen sulfate, PT-1, PT-2, PT-5 and PT-10 nanocomposites,respectively.

Taking into account the fact that the amount of aniline usedin the syntheses of all PANI/TiO2 nanocomposites was kept con-stant, it was revealed that the yield of PANI sulfate/hydrogen sulfateincreased by increasing the initial TiO2/aniline mole ratio (Table 2).

Table 1Content of TiO2, PANI sulfate/hydrogen sulfate and water in PANI/TiO2 nanocom-posites, determined by TGA.

sample Content (wt%)

TiO2 PANI sulfate/hydrogen sulfate H2O

PT-1 35.0 57.2 7.7PT-2 23.6 68.2 8.1PT-5 12.5 78.6 9.0PT-10 7.0 84.7 8.3PANI 0 91.0 9.0



Fig. 5. SEM (A) and TEM (B and C) images of the nanocomposite PT-10. Nanosheetsare designated by a in Fig. A; a branched cavity of PANI nanotube and the diameterfluctuation are marked by arrows in Fig. C.

Fig. 6. SEM images of the nanocomposite PT-5: twisted nanoribbons are marked byarrows in Fig. A; nanoribbons growing from the same starting point are shown inFig. B; nanotubes/nanorods are shown in Fig. C with higher magnification.

The lowest yield of PANI was obtained in the polymerization pro-cess that was performed without colloidal TiO2 nanoparticles. Asimilar observation has recently been reported for PANI/Co3O4composites, synthesized by in situ polymerization of aniline in thepresence of Co3O4 nanoparticles and HCl as a dopant [68].

3.4. Elemental analysis

The elemental composition analysis of PANI/TiO2 nanocompos-ites (Table 3) shows substantially lower C/N weight ratio for thePT-1 sample (C/N = 4.14), and somewhat lower C/N weight ratiofor the PT-2 sample (C/N = 4.75), in comparison with the theoreti-cally expected value of C/N weight ratio for all redox and acid–baseforms of PANI (5.14). The decrease in the C/N weight ratio, as well



Fig. 7. SEM (A, C) and TEM (B, D) images of PT-2 (A, B) and PT-1 (C, D) nanocomposites.

as the increase of the S/C ratio, with the increase of the initialTiO2/aniline ratio is probably a consequence of the presence ofadsorbed ammonium sulfate and ammonium hydrogen sulfate onsurfaces of colloidal TiO2 particles, entrapped in PANI matrix. Thisadsorption is negligible for PANI/TiO2 nanocomposites with lowTiO2 content (PT-5: C/N = 5.12, PT-10: C/N = 5.19). In order to find

Fig. 8. TGA curves for PANI/TiO2 composites, pure PANI and pure colloidal TiO2,recorded in an air stream.

real C/N weight ratio in PANI present in PANI/TiO2, the nanocom-posites are deprotonated and PANI bases are isolated from thedeprotonated composites. The elemental analysis revealed thatall PANI bases show C/N weight ratio close to the theoreticallyexpected value (Table 4). It is also interesting to note that PANIbases contain ∼ 1.0–1.3 wt% of sulfur. Sulfur in PANI base cannotoriginate from the sulfate and hydrogen sulfate counter-ions, whichare efficiently removed in the process of deprotonation. It may orig-inate only from covalently bound groups containing sulfur, such assulfonate [57] and/or sulfate esters groups.

It should be stressed that the content of titanium in nanocom-posites determined by ICP-OES technique corresponds well to theresults obtained by TGA (Table 3).

3.5. Conductivity

The PANI/TiO2 nanocomposites exhibited the electrical conduc-tivity on a semiconductor level, ∼10−3 to 10−4 S cm−1 (Table 2).

Table 2The reaction yield of PANI sulfate/hydrogen sulfate in the nanocomposites, calcu-lated using the corresponding content (wt%) of PANI sulfate/hydrogen sulfate (TGAdata, Table 1), and conductivity of PANI/TiO2 nanocomposites.

Sample Mass of thesample (g)

Calculated mass ofPANI in the sample (g)

Conductivity(S cm−1)

PT-1 1.419 0.812 3.8 × 10−4

PT-2 1.053 0.719 4.8 × 10−4

PT-5 0.845 0.664 5.8 × 10−4

PT-10 0.770 0.652 1.1 × 10−3

PANI 0.667 0.607 9.0 × 10−4



Table 3The elemental composition of PANI/TiO2 nanocomposites and PANI sul-fate/hydrogen sulfate determined by the elemental analysis (C, H, N and S), TGAand ICP-OES measurements (Ti), and by difference (O).

Sample Content (wt%)

C H N S Tia Tib O

PT-1 27.45 3.34 6.63 6.36 21.00 19.62 35.22PT-2 36.74 3.86 7.74 6.23 14.17 13.86 31.26PT-5 46.02 4.26 8.99 6.19 7.47 6.99 27.07PT-10 50.67 4.50 9.75 6.18 4.21 3.68 24.69PANI 55.92 4.63 10.77 5.54 – – 23.14

a The content of Ti determined by TGA.b The content of Ti determined by ICP-OES.

Table 4The elemental composition of PANI bases isolated from deprotonated PANI/TiO2

composites and PANI base obtained by the deprotonation of PANI sulfate/hydrogensulfate, determined by the elemental analysis (C, H, N and S), and by difference (O).

Sample Content (wt%)

C H N S O

PANI-base-1 64.71 5.33 12.81 1.28 15.87PANI-base-2 64.29 5.51 12.81 1.25 16.14PANI-base-5 64.78 5.78 12.49 1.02 15.93PANI-base-10 63.34 5.39 12.82 1.18 17.27PANI-base 65.40 5.58 13.18 1.21 14.63

The conductivity of nanocomposites continuously decreases withdecreasing the mole ratio [aniline]/[TiO2] from 10 (PT-10) to 1(PT-1). The nanocomposite PT-10 has the highest conductivity,1.1 × 10−3 S cm−1, which is slightly higher than that of pure PANIprepared in the absence of colloidal TiO2.

3.6. Structural characterization

3.6.1. FTIR spectroscopyFTIR spectroscopy has proved the presence of emeraldine salt

form of PANI in all PANI/TiO2 nanocomposites. PANI emeraldinesalt bands are observed in the spectra of nanocomposites (Fig. 9) at1569 cm−1 [quinonoid (Q) ring stretching], 1496–1490 cm−1 [ben-zenoid (B) ring stretching], 1308–1303 cm−1 (the C–N stretching ofsecondary aromatic amine), 1249–1243 cm−1 (the C–N+• stretch-ing in polaron form of PANI emeraldine salt), ∼1146 cm−1 (thestretching vibration of–NH+= in the B–NH+ Q segment in bipolaronform of PANI emeraldine salt), and at 825–821 cm−1 [the aromaticC–H out-of-plane deformation vibration of 1,4-disubstituted ben-zene ring, �(C–H), in linear N–C4 coupled PANI chains] [57,69–71].The weak band at ∼877 cm−1 in the spectra of nanocompositescan be assigned to �(C–H) vibration of 1,2,4-trisubstituted ben-zene ring, and indicates branching of PANI chains [57,69,71]. In theFTIR spectra of nanocomposites, the PANI bands are positioned atvery close wavenumbers to those observed in the spectrum of purePANI (Fig. 9). The exception is the spectrum of PT-1 sample withthe bands at 1303 and 1243 cm−1 which are red-shifted relative tothe corresponding bands positioned at 1308 and 1249 cm−1 in thespectrum of pure PANI. This feature can be indication that someinteraction exists between PANI chains and TiO2 nanoparticles inthe sample PT-1, possibly via O· · ·H–N hydrogen bonding. In thespectra of all PANI/TiO2 nanocomposites, the bands at ∼1041 and∼588 cm−1 correspond to hydrogen sulfate counter-ion, while theband at ∼618 cm−1 can be attributed to hydrogen sulfate and/orsulfate counter-ions [71]. The asymmetric SO3 stretching vibra-tion in hydrogen sulfate anions also contributes to the band at∼1146 cm−1. Actually, in the spectra of PT-10, PT-5, PT-2 nanocom-posites and pure PANI, two maxima are observed within one broadband, at 1146 and ∼ 1120 cm−1, the latter maximum may be dueto sulfate counter-ion vibration [71]. Along with the bands of

Fig. 9. FTIR spectra of PANI/TiO2 nanocomposites, PANI and colloidal TiO2 nanopar-ticles.

ordinary (granular) PANI, the band at ∼1404 cm−1 is observed inthe FTIR spectra of the samples PT-1 and PT-2. This band can beattributed to the substituted phenazine-like units, formed by theoxidative intramolecular cyclization of branched oligoaniline andPANI chains [57]. The shoulder at ∼1451 cm−1, also non-typicalfor ordinary PANI, is present in the spectra of all composites andpure PANI, and can be assigned to the C C stretching vibration ofaromatic ring (most probably the monosubstituted and/or 1,2,4-trisubstituted rings) [71,72].

Long “absorption tail” above ∼1700 cm−1 is present in the spec-tra of all nanocomposites, associated with properties of electricalconductivity and high degree of electron delocalization in PANI[70]. The slope of “absorption tail” increases with the decreasingof TiO2 content in the nanocomposites, in accordance with theincrease of nanocomposites conductivity. In the high-frequencyregion, the spectra of PT-10, PT-5 and PT-2 nanocomposites andpure PANI show the band at ∼3200 cm−1 that is characteristic forthe different types of intra- and intermolecular hydrogen-bondedN–H stretching vibrations of secondary amine [57a]. The bandpositioned at 3136 cm−1 in the spectrum of PT-1 is due to mixedcontributions of hydrogen-bonded N–H stretching vibrations inPANI and O–H stretching mode of strongly hydrogen-bonded wateron the TiO2 nanoparticles [73]. The spectrum of pure colloidalTiO2 nanoparticles shows a very strong broad band centered at3134 cm−1 due to the O–H stretching vibration mode.

3.6.2. Raman spectroscopyThe Raman spectra of PT-2 nanocomposite, pure PANI and

TiO2 nanoparticles are shown at Fig. 10. Raman spectral profilesof other PANI/TiO2 nanocomposites are similar to that of PT-2.The characteristic Raman bands of PANI emeraldine salt struc-



Fig. 10. Raman spectra of PT-2 nanocomposite, PANI and colloidal TiO2; excitationwavelength 532 nm. The bands marked with arrow are assigned to TiO2 (anatase).

tural units are observed in the spectrum of PT-2 at 1582 cm−1 (theC C and C∼C stretching vibrations of the quinonoid and semi-quinonoid (SQ) rings, respectively, where ‘∼’ denotes the bondintermediate between the single and double bond), 1340 cm−1

(the C∼N•+ stretching vibration of delocalized polaronic structure),and 1229 cm−1 (C–N stretching in benzenoid units). A shoulderobserved at ∼1176 cm−1 is due to the C–H bending in-plane vibra-tion of SQ rings [61]. The bands observed at 1561, 1387 and569 cm−1 in the Raman spectrum of PT-2, and at 1557, 1383 and568 cm−1 in the spectrum of pure PANI, have been associated withthe substituted phenazine structural units which are important forthe formation of nanostructured PANI morphology [61,69,74–76].The shoulder at 1625 cm−1 in the spectrum of PT-2 correspondsto the C∼C stretching vibrations of benzenoid units, with possiblecontribution of substituted phenazine units [61,74].

The Raman spectra confirmed the presence of anatase TiO2nanoparticles in the PANI/TiO2 nanocomposites. The Raman spec-trum of pure colloidal TiO2 nanoparticles had characteristic bandsof anatase at 155, 402, 498 and 631 cm−1 (Fig. 10). The strong bandat 155 cm−1 assigned as the Eg phonon of the anatase structureappears in the spectrum of PT-2 nanocomposite at 140 cm−1. Thebands at 402, 498 and 631 cm−1 are assigned as B1g, A1g, and Eg

modes of the anatase phase, respectively [16]. These bands arelocated at 391, 511 and 624 cm−1 in the spectrum of PT-2 nanocom-posite.

Fig. 11. XRPD patterns of PT-1 and PT-10 nanocomposites, pure TiO2 and pure PANI.

3.6.3. X-ray powder diffractionPure nanostructured PANI sulfate/hydrogen sulfate is partly

crystalline (Fig. 11) with strongest diffraction peaks centered at2� ∼ 13.2◦, 17.6◦, 18.4◦, 19.0◦, 20.4◦, 25.8◦, and 29.0◦ which aresuperimposed on the amorphous halo. The peaks at ∼21◦ and26◦ have been ascribed to (1 0 0) and (1 1 0) reflections of “stan-dard” emeraldine salt [77]. Additional peaks in XRPD pattern ofpure PANI are observed at 2� ∼ 38.9◦, 41.1◦, 48.5◦, and 50.0◦.The XRPD patterns of PT-1 and PT-10 nanocomposites are morecomplex (Fig. 11) than those previously reported for PANI/TiO2nanocomposites [13,14,16,17,20,26,39,50]. These differences incrystalline structure are most probably induced by the presence ofbranched and phenazine-like structural segments, besides the ordi-nary PANI emeraldine salt segments, in PANI nanostructures whichare present in nanocomposites. The XRPD pattern of PT-1 exhib-ited main PANI peaks at 2� ∼17.7◦, 18.4◦, 19.0◦, 20.6◦ and 29.4◦. Inthe diffraction pattern of nanocomposite PT-10 the low angle PANIpeaks appear at similar positions, followed by not well-resolvedpattern in 2� range between 35◦ and 58◦, that is a consequence ofthe presence of crystalline TiO2 nanoparticles in low concentration(7 wt%, Table 1).

All diffraction peaks observed in the XRPD pattern of colloidalTiO2 nanoparticles correspond to the anatase crystalline phase[78,79]. It should be notice that the diffraction peaks of anataseTiO2 are relatively broad due to the nanosize of the crystallineparticles [78]. All characteristic peaks of anatase TiO2 crystallinestructure presented in XRPD pattern of PT-1 nanocomposite, pre-served almost unchanged intensities ratios related to pure TiO2,that is an indication of existence of unmodified anatase crystallinestructure in the nanocomposite. In XRPD spectra of PT-1 and PT-10nanocomposites dual contribution of signals diffracted from (1 0 1)crystal plane of anatase TiO2 and (1 1 0) crystal face of PANI todiffraction peak at 2� ∼ 25.4◦ can be recognized [35,77,78].

4. Conclusions

Novel semiconducting PANI/TiO2 nanocomposites were suc-cessfully synthesized through the oxidative polymerization ofaniline with ammonium peroxydisulfate in aqueous dispersionsof colloidal TiO2 nanoparticles of ∼4.5 nm size without addedacid. The oxidation proceeds in two rapid exothermic phases wellseparated by an athermal period. The presence of colloidal TiO2nanoparticles causes an acceleration of redox reactions duringthe athermal phase of aniline oxidation, i.e. it shortens athermalperiods during the syntheses of PANI/TiO2 nanocomposites in com-parison with corresponding synthesis of pure PANI, because of the



substantial increase of the surface area of precipitated oligoanilinesin the presence of colloidal TiO2. The yield of PANI in nanocompos-ites increases, while the electrical conductivity of nanocompositesdecreases (1.1 × 10−3 → 3.8 × 10−4 S cm−1) with decreasing the ini-tial aniline/TiO2 mole ratio from 10 to 1. The initial aniline/TiO2mole ratio significantly affected the morphology of nanocom-posites. All the nanocomposites exhibit nonuniform morphology,which consists of different nanophases. The nanocomposite ofPANI nanotubes/nanorods/nanoribbons and TiO2 nanoparticles hasbeen synthesized at aniline/TiO2 mole ratio 5. Nanotubes (an outerdiameter 30–40 nm, an inner diameter 4–7 nm) and nanorods(diameter 50–90 nm) form a network. The nanoribbons have athickness, width, and length in the range of 50–70 nm, 160–350 nm,and ∼1–3 �m, respectively. In the morphology of the nanocom-posite produced using the aniline/TiO2 mole ratio 10, nanotubesaccompanied with nanosheets predominate. The nanocompositeprepared at aniline/TiO2 mole ratio 2 contains polyhedral submi-cron particles accompanied with nanotubes, while the morphologyof nanocomposite synthesized at aniline/TiO2 mole ratio 1 consistsof agglomerated nanofibers, submicrometre- and nanoparticles.FTIR and Raman spectroscopies showed that PANI is present in thenanocomposites in its emeraldine salt form, and it contains linear,branched and phenazine-like structural elements. The presence ofTiO2 enhanced thermal stability of PANI in the nanocomposites.The results of XRPD and Raman spectroscopy confirmed anatasecrystalline structure of colloidal TiO2 nanoparticles and partly crys-talline structure of PANI in the PANI/TiO2 nanocomposites.

Acknowledgements

The authors thank the Ministry of Science and TechnologicalDevelopment of Serbia (142047 and 142066) and Czech GrantAgency (202/09/1626) for financial support. The thanks are due toDr. M. Mitric from Vinca Institute of Nuclear Sciences, Belgrade,Serbia, for XRPD measurements and to Dr. S. Trifunovic and Dr.D. Manojlovic from Faculty of Chemistry, Belgrade, Serbia, for theelemental analysis and ICP-OES measurements.

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self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

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