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Polymer 55 (2014) 2496e2500

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Polymer

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Platinumepolymereclay nanocomposite hydrogels via exfoliatedclay-mediated in situ reduction

Kazutoshi Haraguchi*, Dharmesh VaradeKawamura Institute of Chemical Research, 631 Sakado, Sakura-shi, Chiba 285-0078, Japan

a r t i c l e i n f o

Article history:Received 12 February 2014Received in revised form18 March 2014Accepted 22 March 2014Available online 29 March 2014

Keywords:Nanocomposite gelsPlatinum nanoparticlesClays

* Corresponding author.E-mail address: hara@kicr.or.jp (K. Haraguchi).

http://dx.doi.org/10.1016/j.polymer.2014.03.0400032-3861/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Pt nanoparticles (Pt NPs) are currently used in many areas of nanoscience and technology. Numerousstudies have been reported on the design of Pt and Pt-based nanomaterials with different sizes, shapes,and compositions. Here, we report the synthesis, structure, and properties of a novel hydrogel-basednanostructured Pt material, Pt-NC gel, consisting of ultrafine Pt NPs strongly immobilized within aunique polymer�clay network. Pt-NC gels were synthesized through exfoliated clay-mediated in situreduction of Pt ions in the NC gel at ambient temperature. Pt NPs were trapped on the clay surface,probably at the edges of the clay nanoplatelets. Ultrafine Pt NPs were also obtained as a stable suspensionfrom the NC gel, without any stabilizing agents. The combination of ultrafine Pt NPs and mechanicallytough NC gel may open up new possibilities for designing functional Pt-gel materials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Since the first synthesis of nanocomposite hydrogels (NC gels)with a unique organic (polymer)/inorganic (clay) network structurewas reported [1a], NC gels have received a great deal of attention assuper hydrogels that can overcome many of the serious disadvan-tages associated with conventional chemically crosslinked hydro-gels (OR gels) [1b,1c]. In particular, NC gels consisting ofpoly(N-alkylacrylamide)s such as poly(N-isopropylacrylamide)(PNIPA) or poly(N,N-dimethyl acrylamide) (PDMAA), and exfoliatedinorganic clay such as hectorite or montmorillonite, have beenshown to have extraordinary optical, mechanical, and swelling/de-swelling properties [1de1g]. Furthermore, NC gels can be preparedeasily at ambient temperature in an aqueous system, providing avariety of shapes, sizes, and surface morphologies, along with novelsurface characteristics such as unique sliding friction, ultrahighhydrophobicity, support for stem cell proliferation, self-healingability, and non-toxicity, which allows for their use in many newapplications [1h-1j]. Moreover, NC gels have contributed to wide-ranging developments in soft and wet materials [1k,1l]. In thepresent study, we report a new class of highly functional soft ma-terials, NC gel-based nanostructured Pt materials.

Noble metal nanoparticles are currently used in many areas ofnanoscience and nanotechnology [2]. In particular, Pt nanoparticles

(Pt NPs) play a critical role as high performance catalysts in variousreactions and applications [3], including fuel cells [3a], sensors [3b],automobile exhaust systems [3c], and petroleum cracking [3d]. Todate, numerous studies have been reported on the design of noblemetal-based nanomaterials including NP-hydrogel composites thatfabricated by exploiting the interspatial area between crosslinkingpoints as a nanoreactor or nanocarrier [4]. However, there havebeen very limited studies on Pt NP�hydrogel composites [4d,4e]because it is difficult to prepare a material with fine and well-dispersed Pt NPs immobilized within a hydrogel. Furthermore,since the inherent properties of Pt NPs strongly depend on theirsize, dispersion, and the supporting material, new nanostructuredPt materials with tailored morphologies and performances are stillgreatly desired [4f]. Here, we report the synthesis, structure, andproperties of NC gel-based nanostructured Pt materials (Pt-NCgels), inwhich ultrafine Pt NPs were synthesized through exfoliatedclay-mediated in situ reduction and then effectively immobilized inthe polymer�clay network.

2. Experimental section

2.1. Synthesis

The NC gels were prepared according to a previously reportedprocedure [5,7]. For the N-NC5 gel, a reaction mixture consisting ofNIPA (1.13 g), clay (Laponite XLG: [Mg5.34Li0.66Si8O20(OH)4]Na0.66:Rockwood Ltd, UK) (0.4 g), H2O (10 g), N,N,N0,N0-tetramethylethy-lenediamine (8 mL), and K2S2O8 (0.01 g) was prepared, and the free-

K. Haraguchi, D. Varade / Polymer 55 (2014) 2496e2500 2497

radical polymerization was allowed to proceed in a water bath at20 �C for 20 h. Throughout the synthesis, O2 was excluded from thesystem. The resulting transparent N-NC5 gel was washed for30 minwith a change of H2O every 3 min, and then immersed in anaqueous solution of K2PtCl4 (10 mL, 10 mM) at 25 �C for 60 h. Thegel and surrounding solution slowly changed from transparent toblack. The black N-NC5 gel (Pt-NC5 gel) was rinsed with pure watertwice and then subjected to a swelling�deswelling test by alter-nating the temperature between 25 and 50 �C and awashing test byimmersion in H2O at 25�90 �C for 3�100 h. The effect of a cationicsurfactant absorbed by the gel was also evaluated. The N-NC5 gelwas first immersed in an aqueous solution of cationic surfactant(cetyl pyridinium chloride (CPC): 1 wt%) and subsequentlyimmersed in an aqueous solution of K2PtCl4. Another NC gel (D-NCgel) and a chemically crosslinked OR gel (N-OR gel) were similarlyprepared by using DMAA and BIS (organic crosslinker),respectively.

2.2. Materials characterization

EM images were obtained for ultrathin films (60 nm) of thedried Pt-NC gel and Pt NPs in the surrounding solution using a high-resolution field-emission transmission electron microscope (JEM-2200TFE, JEOL) operating at 200 kV. Energy-dispersive X-rayspectroscopy (EDS) was performed using a scanning transmissionelectron microscopy (STEM) detector fitted to a JEM-2200TFE sys-tem. XRD patterns were obtained using a Rigaku SmartLab X-raydiffractometer with monochromated Cu Ka radiation (40 kV,100 mA). UVevis absorption spectra were acquired in a 1 mmquartz cuvette at room temperature using a Hitachi U-4100 UVevisdouble-beam spectrometer. Inductively coupled plasma atomicemission spectroscopy (ICP-AES, Perkin Elmer Optima 4300 DV)was used to measure the concentrations of Pt in various solutionsand H2O washings (Table 1).

2.3. Catalytic properties

A 0.5 mL sample of NaBH4 solution (60 mM) was added to2.5 mL of 4-nitrophenol solution (0.12 mM) contained in a glassvessel. Dried Pt-NC5 gel powder (3.5 mg) or dried Pt NPs (1 mg)obtained from the surrounding solution was subsequently added,and UV spectra were collected immediately and then at 1 min in-tervals in the range 250e550 nm at 25 �C.

3. Results & discussion

For the synthesis of Pt-NC gels, NC gels consisting of PNIPA andinorganic clay (synthetic hectroite: Laponite XLG) were firstlyprepared according to a previously described protocol [1e,1f]. Theuniform and transparent N-NCn gel obtained is shown in Fig. 1a(left: N-NC5 gel), where the prefix N- and the numerical value n

Table 1Concentration of Pt (ppm), measured by ICP-AES for the original K2PtCl4 solution, Pt-NC gel, surrounding solution, and H2O in which the Pt-NC gel was stored.

NCgela

K2PtCl4Solnb

SurroundingSoln.

(Pt-NC gel)c H2O washing

N-NC5 11,000 230 (870) 0.350(25 �C,100 h)

0.130(50 �C,3 h)

D-NC5 11,000 110 (990) 0.400(25 �C,100 h)

0.280(90 �C,3 h)

a The NC gel from which Pt-NC gel was prepared.b Aqueous solution of K2PtCl4 (10 mM).c Pt content of the Pt-NC gel was calculated from the difference between the Pt

content of the K2PtCl4 solution and the surrounding solution.

represent the constituent polymer (PNIPA) and the clay concen-tration (Cclay ¼ n � 10�2 mol L�1 in H2O ¼ 0.762 � n wt%),respectively. The N-NC5 gel showed excellent mechanical proper-ties (tensile strength ¼ 120 kPa, elongation ¼ 1010%), considerableswelling in water (Wgel/Wdry ¼ 50), and a thermoresponsive phasetransition (lower critical solution temperature (LCST) ¼ 32 �C).After thorough washing, the N-NC5 gel was immersed in anaqueous solution of K2PtCl4 (10 mM) and then maintained in thedark at 25 �C for 60 h, after which it became swollen and black, asshown in Fig. 1a (right). In addition, the aqueous solution sur-rounding the gel in the vial also turned black (Fig. S1a in theSupporting Information). These drastic color changes indicate theformation of Pt NPs within the gel and the surrounding solution.Similar observations were made with thermostable D-NC gelscomposed of PDMAA and clay. It was therefore concluded that thePt NPs were synthesized in the NC gel at ambient temperature,without the addition of any reducing agent, regardless of the typeof starting NC gel. From here on, this black NC gel is referred to asPt-NC gel.

Fig. 1b shows the changes in the appearance of a cross section ofan NC gel during the synthesis of the Pt-NC gel. Here, the thermallystable D-NC5 gel that consisted of a PDMAA�clay network wasused in order to investigate the effect of preparation temperature.At 25 �C (Fig.1b(i)), during the first 4 h, Pt ions gradually penetratedthe NC5 gel. As reduction of Pt ions takes place slowly at ambienttemperature, the Pt ions were able to infiltrate the entire gel before

Fig. 1. (a) Color changes of the N-NC5 gel kept in an aqueous solution of K2PtCl4 in thedark at 25 �C for 60 h. (b) Time-dependent color changes of D-NC5 gel (cross section)in an aqueous solution of K2PtCl4 at (i) 25 �C and (ii) 60 �C.

Fig. 3. EDX mapping of (i) Pt-NC5 gel containing clay (Mg or Si detected), and (ii) thesurrounding Pt NP solution with no clay (no Mg or Si detected). The inset photo showsthe Pt-NC5 gel and the surrounding solution.

K. Haraguchi, D. Varade / Polymer 55 (2014) 2496e25002498

being reduced. The subsequent gradual color change up to 20 h wasa result of in situ reduction to form the Pt NPs. The gel turnedalmost black by 20 h, with a further increase in blackness up tow60 h. When the Pt-NC gel was prepared at 60 �C (Fig. 1b(ii)), thepenetration and reduction of Pt ions would proceed simultaneouslyas the higher temperature would speed up the reduction. As aresult, a gradient of blackness was observed in the early stages(0.5 h). At this temperature, the entire NC gel was filled with Pt NPswithin a few hours.

Fig. 2a shows a transmission electron microscopy (TEM) imageof an ultrathin section of the dried black N-NC5 gel. It can be seenthat very fine Pt NPs were distributed throughout the gel sample.The inset (Fig. 2a) shows a HR-TEM image depicting lattice fringeswith 0.20 nm periodicity, which coincides with the (111) d-spacingof Pt crystals [5], and a histogram showing the distribution ofparticle sizes, as derived from measurements of over 100 Pt parti-cles in the TEM images. The values for average diameter (dave) andstandard deviation (s) of the Pt NPs were 1.75 nm and 0.93 nm,respectively. In order to elucidate the specific sites on which the PtNPs became trapped, a Pt-NC gel was synthesized using an N-NC0.5gel, which had a low concentration of clay (1/10) and K2PtCl4 (1/10).TEM images of the dried Pt-NC0.5 gel are shown in Fig. 2b, whereultrafine and uniformly sized particles (dave ¼ 1.25 nm,s ¼ 0.64 nm) appear to be located along a curved line (the inset ofFig. 2b). This clearly suggests that the specific sites for trapping PtNPs may exist at the edge of the clay platelets. This would alsoexplain the unique network-like morphology of Pt NPs seen for thePt-NC5 gel (Fig. 2a). The ultrafine Pt NPs, and the narrow size dis-tribution, may be due to the specific conditions of reduction and NPformation on the surface of the clay platelets, as will be discussedlater in the article.

During the preparation of the Pt-NC gel, Pt NPs were alsoobserved in the surrounding solution (Fig. S1a). Using TEM imaging(Fig. 2c), it was found that these dispersed Pt NPs also had a smalland uniform size (dave ¼ 2.0 nm, s¼ 0.79 nm). Energy-dispersive X-ray spectroscopy (EDX) confirmed the formation of Pt NPs in theclay (Mg silicate)�polymer matrix (Fig. 3(i)) and surrounding so-lution (Fig. 3(ii)). Here, EDX analysis revealed that no clay wascontained in the surrounding solution, confirming that the reduc-tion of Pt ions and formation of Pt NPs proceeded within the NC gel.In contrast, for the chemically crosslinked PNIPA hydrogel (N-ORgel), with the same compositions as the N-NC gel except for thetype of crosslinker, i.e., organic crosslinker (N,N0-methyl-enebis(acrylamide): BIS) instead of inorganic clay (hectorite), thecolors of the gel and surrounding solution did not change at allunder the same experimental conditions (Fig. S1b), indicating thatno Pt NPs were produced. The formation of Pt NPs in the N-NC5 gelwas therefore attributed to the clay, and more specifically, to the

Fig. 2. (a,b) show the TEM images of the dried Pt-NC5 gel and dried Pt-NC0.5 gel, respectiveNC0.5 gels. The inset of (a) shows the HR-TEM image, revealing the lattice fringes of crystallinTEM image and histogram of Pt NPs contained in the aqueous solution surrounding the Pt

exfoliated clay-mediated in situ reduction of Pt ions. This isconsistent with a previous report [6], where clay platelets werefound to be capable of inducingmild reduction of Pt ions in aqueousmedia. In the NC gel, because individual clay nanosheets made up athree-dimensional network swollen in aqueous media, reductionwould occur effectively on the clay surface, resulting in the newnanostructured Pt material.

The proposed mechanism underlying the formation of Pt NPs inthe NC gel is shown in Fig. 4. In an aqueous solution of K2PtCl4, thePtCl42� ions undergo the following solvolysis reaction: [7]

PtCl2�4 þ2H2O%PtCl3ðH2OÞ�þCl�þH2O%PtCl2ðH2OÞ2þ2Cl�

(1)

As shown in the schematic in Fig. 4a and b, Pt ions (PtCl2)penetrate the NC gel and interact with the silanol groups (SieOH)on the clay surface to form a complex. Subsequently, the reductionof Pt ions is induced on the clay surface (Equation (2)), whichpossibly occurs via successive proton�electron transfer processes.[6]

2SiOHþ PtCl2/SiOSiþ Ptþ 2HCl (2)

Pt atoms then migrate on the clay surface and collide to formclusters. The resulting Pt NPs are trapped at specific sites on the clayplatelets (Fig. 4c). Consequently, in the final Pt-NC gel, the Pt NPs

ly. In each figure, the histogram shows the Pt NP size distribution in the Pt-NC5 and Pt-e Pt NPs. The inset of (b) shows Pt NPs located along a curved line (edge of the clay). (c)-NC5 gel.

Fig. 4. Schematic representation of the formation of the Pt-NC gel: (a) Pt ions penetrate the N-NC gel, (b) Pt ions (PtCl2) interact with the silanol groups on the clay surface and arereduced to Pt0, (c) Pt NPs are formed by the migration of Pt, and are subsequently trapped on the clay surface.

K. Haraguchi, D. Varade / Polymer 55 (2014) 2496e2500 2499

are attached to the clay surface and not suspended in the H2Owithin the polymereclay network. This was confirmed byimmersing the Pt-NC5 gel in pure H2O, with no Pt NPs releasedeven after extended periods of time or at high temperatures(Table 1 and Fig. S2).

As clay is essential for Pt NP formation, i.e., Pt NPs were notformed in the absence of the NC gel (Fig. S1b and c), it was deducedthat Pt NPs in the surrounding solution were also formed by theeffect of the clay platelets in the NC gel. Pt ions interacted with theclay platelets near the outer surface of the NC gel and were reducedto form Pt NPs. It is possible that some of these NPs were nottrapped on the clay, and hence, they diffused into the surroundingsolution (arrow in Fig. 4b). Interestingly, the Pt NPs dispersed in thesolution surrounding the Pt-NC gel were quite stable for more thanaweek, with no aggregation or precipitation observed. On standingfor a long time (e.g., one month), the Pt NPs gradually sedimented;however, they could be readily re-dispersed by mechanical vibra-tion or sonication. An astonishing fact was that such fine Pt NPswere formed in H2O, and they exhibited high dispersion stability,unlike previous studies which required stabilizing or cappingagents to prevent NP aggregation [8]. To the best of our knowledge,the Pt NPs contained within the solution surrounding the Pt-NCgels in the present study is the first example of an aqueousdispersion of ultrafine Pt NPs with high stability, obtained withoutthe use of stabilizing or capping agents. Based on the results ofinductively coupled plasma�atomic emission spectrometry (ICP-

Fig. 5. Reversible volume changes of Pt-NC5 gel due to the thermoresponsive coil-to-globule transition of PNIPA at the LCST (32.5 �C). The third swelling time is shorter by8 h than the first and second ones.

AES for Pt, Table 1), it was deduced that the Pt contained within thePt-NC gel and the surrounding solution was found to be 90% and10% (from D-NC5 gel), and 79% and 21% (from N-NC5 gel), respec-tively. The critical role of clay in the formation of Pt NPs in the NCgels was further investigated by employing an organically modifiedNC gel (Fig. S3). The N-NC5 gel modified by a cationic surfactant(cetyl pyridinium chloride) (Fig. S3(ii)) did not turn black on im-mersion in aqueous K2PtCl4, but turned white, probably because ofthe absorption of surfactant ions and ions from K2PtCl4. Thisdemonstrated that covering the clay surface (blocking) beforehandwith an organic molecule prevented Pt ion reduction.

The novel Pt-NC gel obtained exhibited interesting characteris-tics. For example, the Pt-NC5 gel retained the thermoresponsivephase transition of the PNIPA. As shown in Fig. 5, Pt-NC5 gelshowed reversible swelling�deswelling behavior on alternatingthe temperature above and below the LCST. Moreover, as shown in

Fig. 6. Catalytic reduction of 4-nitrophenol by NaBH4 in the presence of (a) dried Pt-NC5 gel powder (3.5 mg), and (b) dried Pt NPs (1 mg) obtained in the surroundingsolution. The strong UV absorption peak at 400 nm corresponds to the nitrophenolateions.

K. Haraguchi, D. Varade / Polymer 55 (2014) 2496e25002500

the inset optical images in Fig. 4 and Table 1, Pt NPs were notreleased from the Pt-NC gel during either the swelling or deswel-ling processes. The Pt-NC gel also exhibited excellent catalyticproperties as a result of the ultrafine nature of the Pt NPs. This wasdemonstrated by the reduction of 4-nitrophenol by dried Pt-NC gelpowder in the presence of NaBH4. While the reduction of 4-nitro-phenol was very slow in the presence of NaBH4 alone, it occurredrapidly in the presence of Pt-NC5 gel (Fig. 6a). The Pt NPs in thesurrounding solution also exhibited superb catalytic propertiesbecause of the ultrafine size and high degree of dispersion (Fig. 6b).It is highly likely that other useful reactions would also be accel-erated by the action of the Pt-NC gels and Pt NPs obtained in thesurrounding solution, demonstrating the wide applicability ofthese materials.

4. Conclusions

We succeeded in preparing NC gel-based nanostructured Ptmaterials, Pt-NC gel, consisting of ultrafine Pt NPs strongly immo-bilized within a polymer�clay network. Pt-NC gels were synthe-sized through exfoliated clay-mediated in situ reduction of Pt ions inthe NC gel at ambient temperature. The Pt ions probably interactedwith the silanol groups of the clay platelets and were slowlyreduced to Pt, forming crystalline Pt NPsmainly located at the edgesof the clay platelets. This trapping of theNPsmeant that they did notmove out of the Pt-NC gel, even upon repeated swelling anddeswelling of the gel in water. Furthermore, the present workrevealed a new possibility for the preparation of a stable aqueousdispersion of ultrafine Pt NPs without the need for stabilizing orcapping agents. The Pt-NC gels exhibited many interesting charac-teristics such as thermoresponsive swellingedeswelling behavior,high mechanical toughness, and exceptional stability; further, theycould be produced in various forms and sizes, and with differentsurface morphologies. The gels and Pt NPs also showed excellentcatalytic properties, which further expands the scope of theirapplication in many advanced research fields. Thus, the presentstudy reveals a new concept, polymereclayeplatinum NC gels, andthe formation of ultrafine Pt NPs through inorganic clay-mediatedin situ reduction in the network. This novel combination of ultra-fine Pt NPs and a mechanically tough NC gel opens up many newpossibilities in the design of functional Pt and Pt-based nano-materials for widespread use in various reactions and applications.

Acknowledgments

This work was supported by the Ministry of Education, Science,Sports and Culture of Japan (Grant-in-Aid 23350117). The authors

thank DIC analysis center (DIC Corp.) for the TEM, ICP, and XRDmeasurements. The authors also thank Dr. F. Li and Ms. N.Kobayashi for their support with the experiments.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2014.03.040.

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