We report a one-pot route to Au/CeO2 nanocomposites. A readily-available biopolymer, sodium alginate, is exploited for controlled formation and stabilisation of gold nanoparticles followed by in situ growth of a sponge-like network of CeO2 nanoparticles. The flexible nature of this method as a general route to mixed metal/metal oxide nanocomposites is also demonstrated

Hall, SR

Hollamby, M

Manna, S

Schnepp, Z

English

Hall, S.R

Hollamby, M.J

Mann, Stephen

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A flexible one-pot route to metal/metal oxide nanocomposites

Zoë Schnepp

Simon R. Hall

Martin J. Hollamby

Stephen Mann

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Green Chemistry

Schnepp, Zoë

Hall, Simon R.

Hollamby, Martin J.

University of Birmingham Research Portal

Keele Research Repository

Green Chemistry Dynamic Article LinksCite this: Green Chem., 2011, 13, 272www.rsc.org/greenchem COMMUNICATIONA flexible one-pot route to metal/metal oxide nanocomposites†Zoë Schnepp, Simon R. Hall, Martin J. Hollamby and Stephen Mann*Received 19th July 2010, Accepted 1st December 2010DOI: 10.1039/c0gc00338gWe report a one-pot route to Au/CeO2 nanocomposites. Areadily-available biopolymer, sodium alginate, is exploitedfor controlled formation and stabilisation of gold nanopar-ticles followed by in situ growth of a sponge-like network ofCeO2 nanoparticles. The flexible nature of this method as ageneral route to mixed metal/metal oxide nanocompositesis also demonstrated.IntroductionA key area of focus in current materials synthesis is the formationof nano-structured composites of two or more materials. In thisway, it is possible to combine the properties of the individualcomponents, or even generate entirely new effects throughsurface interactions. Of particular interest are composites ofmetal nanoparticles with a metal oxide support.1–3 These haveshown promise in a wide range of applications, includingmedical imaging, sensors and catalysis. However, current routesto metal/metal oxide nanocomposites are typically multi-step,involving pre-fabrication of the metal oxide support via e.g.sol–gel,4 microemulsion5 or hydrothermal6 syntheses. These sup-ports often require numerous washing cycles to remove solventor surfactant, before the introduction of metal nanoparticles asa separate step. As such, a one-pot route to metal/metal oxidenanocomposites would have clear, considerable advantages.Recently, biopolymers have been shown to exert a remark-able degree of control over the nucleation and growth ofcrystalline metal oxides.7 Many biopolymers contain metal-binding functional groups which can be used to pre-organizeaqueous metal cations within a gel structure. Alginate, forexample, is the structural biopolymer found in seaweed andcontains blocks of guluronate monomers (Fig. 1a) that arereadily crosslinked by strong electrostatic bonds to multivalentmetal cations. In this way, the polymer forms an extended gelnetwork in water, with metal cations dispersed throughout thestructure. Another characteristic of many biopolymers is theirhigh thermal stability.8,9 As a result, they are able to maintainthe pre-organized dispersion of metal cations to relatively highSchool of Chemistry, University of Bristol, Bristol, UK BS81TS.E-mail: S.Mann@bristol.ac.uk† Electronic supplementary information (ESI) available: TEM of goldnanoparticles, EDXA of CeO2 and Au, SAED of CeO2 sponge and datafor a control sample. See DOI: 10.1039/c0gc00338gFig. 1 (a) Structure of alginate showing guluronate (G) and man-nuronate (M) epimers, and (b) schematic showing the synthesis pro-cedure for Au/CeO2 nanocomposites.temperatures. In solid state synthesis, this means that they caninfluence the nucleation and growth of crystalline phases. Thishas already been demonstrated in the formation of nanowiresand nanoparticles of several metal oxides.In this paper, we couple the metal binding and structure-directing capacity of the biopolymer alginate with its ability toreduce precious metal salts to nanoparticles. In this way, wehave developed a simple, one-pot route to metal/metal oxidenanocomposites. The concept is demonstrated in the synthesisof an Au/CeO2 nanocomposite (Fig. 1b). Au supported onCeO2 is widely studied, particularly in the field of oxidationcatalysis,10,11 but to the best of our knowledge this is the firstexample of the formation of such a system via a one-potprocess. In this synthesis, the biopolymer first reduces aqueousAu3+ and stabilises the resulting nanoparticles. On heating, thepre-organization of Ce3+ cations within the polymer gel leadsto constrained nucleation of CeO2 nanoparticles. Finally, thedecomposition products of the biopolymer restrict sintering ofthe Au and CeO2 nanoparticles before the final transformationto the composite product. The flexibility and general nature ofthis method is then demonstrated in the preparation of Au/ZrO2nanocomposites.Results and discussionAddition of a mixture of aqueous Ce(NO3)3 and HAuCl4 to asolution of sodium alginate resulted in the formation of a gelthrough crosslinking of the polymer. On drying, the gel formeda clear, pink-purple film (Fig. 2a) indicating reduction of theAu3+ precursor by alginate to form stabilized gold nanoparticles.272 | Green Chem., 2011, 13, 272–275 This journal is © The Royal Society of Chemistry 2011Published on 23 December 2010. Downloaded by Keele University on 20/05/2014 13:10:44. View Article Online / Journal Homepage / Table of Contents for this issueFig. 2 Optical images of (a) as-prepared alginate/Au/Ce(NO3)3 filmand (b) corresponding Au/CeO2 composite after calcination (theoreticalAu mass of 5%). (c) PXRD profile of the sample (i) before and (ii) afterwashing in dilute HCl showing indexed peaks for CeO2 and markedpeaks for Na2CO3 (*). (d) SEM image of the Au/CeO2 nanostructure. (e)TEM image of a typical Au/CeO2 nanostructure (5% Au by weight) witharrows showing gold nanoparticles and (f) a single gold nanoparticle.(g) High resolution TEM of a single CeO2 nanoparticle.Transmission electron microscopy (TEM) of a sample redis-persed in water confirmed the formation of gold nanoparticles(Fig. S1†). The colour of the film became more intense withincreasing concentration of HAuCl4 in the precursor solution(Fig. S2†). In a similar system, Pal et al.,12 reported that theconcentration of HAuCl4 added to a sodium alginate solutiondid not affect the size of the resulting nanoparticles. Calcinationof the dried film in air resulted in the formation of a light,delicate pink-purple solid (Fig. 2b). The colour of the compositeagain became more intense with increasing Au concentration(Fig. S2†). Powder X-ray diffraction (PXRD) indicated theprimary phase to be cubic CeO2 (Fm3m, ICDD 04-011-8938)with minor peaks corresponding to Na2CO3 (Fig. 2ci). ThisNa2CO3 phase, resulting from decomposition of the sodiumalginate biopolymer could be simply removed by washingthe sample in 0.1 M HCl, leaving a pure Au/CeO2 sponge(Fig. 2cii). Nitrogen porosimetry indicated a specific surfacearea for the sample before and after washing of 4 m2 g-1and 31 m2 g-1 respectively. Scanning and transmission electronmicroscopy (SEM, TEM) images of the sample showed asponge-like structure of interconnected, monodisperse sphericalCeO2 particles (Fig. 2d–f), with a mean diameter of 20 nm(s = 25%). The sample containing 5 wt% Au showed discretegold nanoparticles (diameter range 20–100 nm, mean 50 nm)dispersed through the reticulated structure. TEM images of the1 wt% and 2 wt% Au samples showed Au nanoparticles of asimilar size range, in agreement with the work of Pal et al.Energy dispersive X-ray analysis (EDXA) detected peaks for Cein the bulk network and for Au in the darker gold nanoparticles(Fig. S3†). Corresponding selected area electron diffraction(SAED) displayed a series of rings characteristic of cubic CeO2,confirming the crystalline nature of the nanoparticle sponge(Fig. S4†). Furthermore, high-resolution TEM of individualnanoparticles showed a regular pattern of fringes correspondingto the (111) and (1̄11) set of planes and an interplanar anglecharacteristic of cubic CeO2, indicating that each nanoparticlewas a single crystal (Fig. 2g). A PXRD pattern of a controlsample calcined in the absence of alginate also showed peakscharacteristic of CeO2 (Fig. S5b†). However, SEM and TEMimages of the control showed a more densely packed structurewith no evidence of a sponge-like network of nanoparticles (Fig.S5c,d†).To investigate the mechanism of formation of the CeO2/Ausponge, a series of samples were quenched at 100 ◦C intervalsduring calcination. PXRD patterns of the samples indicatedthat the onset of CeO2 formation was between 300 ◦C and400 ◦C and the broad peaks were consistent with the observationof nanoparticles (Fig. 3a). The Scherrer equation was usedto estimate the mean CeO2 particle size from XRD patternsfor samples quenched at 50 ◦C intervals from 400 ◦C. Thesedata showed a slight increase in the CeO2 particle size duringcalcination (Fig. 3b), with the final particle diameter consistentwith that observed by TEM.Fig. 3 (a) PXRD profile of samples quenched at 100 ◦C intervals, (b)plot showing CeO2 particle size with increasing temperature estimatedusing the Scherrer equation, and (c) TGA (left side upper plot) and DSC(left side lower plot) profiles.Thermogravimetric analysis (TG) and differential scanningcalorimetry (DSC) offered further insight into the mechanismThis journal is © The Royal Society of Chemistry 2011 Green Chem., 2011, 13, 272–275 | 273Published on 23 December 2010. Downloaded by Keele University on 20/05/2014 13:10:44. View Article Onlineof CeO2 nanoparticle formation (Fig. 3c). The initial mass lossof 4.5% was ascribed to the loss of bound water molecules. Asecond mass loss of 40%, between 180 ◦C and 300 ◦C with astrong exothermic peak at 272 ◦C, corresponded to the maindecomposition step of the biological polymer. The group ofexothermic peaks at 583 ◦C were possibly associated with thefinal decomposition stage of residual carbonaceous products.Although there are no previous studies of the decomposition ofcerium alginate, the above TGA and DSC values were similarto those previously determined for a range of metal alginatecompounds published by Said et al.9Based on the observations described above, we propose thatthe alginate biopolymer has three key functions in the formationof the Au/CeO2 nanocomposite. Firstly, the carboxylate-richstructure enables reduction of the Au3+ precursor and stabili-sation of the resulting gold nanoparticles. In the well-knowncitrate method, it has been suggested that the reduction of goldoccurs via a process of oxidative decarboxylation with remainingcarboxylate groups stabilizing the resulting gold nanoparticlesand it seems likely that the mechanism of alginate reduction ofgold proceeds via a similar route. Secondly, the biopolymer actsas a template for the nucleation of CeO2. As mentioned above,alginate is known to strongly bind multi-valent cations througha mechanism known as the egg-box model.13 By complexing anddispersing the Ce3+ ions into microcrystalline regions within thegel, the initial nucleation of CeO2 is constrained to nanoparticles.Finally, since metal-alginate compounds are known to be stableto relatively high temperatures, it is likely that the decompositionproducts of the polymer help to prevent significant mass-transport and sintering of these Au and CeO2 nanoparticlesbefore complete combustion of organic material to leave aporous Au/CeO2 sponge.To investigate the scope of this method, a gel was preparedfrom ZrO(NO3)2, HAuCl4 and sodium alginate. On drying, thegel turned a pink-purple colour, which was retained on calcina-tion to 600 ◦C. TEM images (Fig. 4a) showed Au nanoparticles(in the same size range as the Au/CeO2 composites) dispersed ina matrix of ZrO2 nanoparticles of mean diameter 9 nm. PXRD(Fig. 4b) showed peaks characteristic of ZrO2 (ICDD 04-006-0278) and Na2CO3 which could be removed as before by washingwith 0.1 M HCl. Nitrogen porosimetry indicated a surface areafor the sample before and after washing of 3 m2 g-1 and 47 m2 g-1,respectively.Finally, alternative biopolymer precursors were employedto study the effect on the structure of the CeO2 and also tocircumvent the formation of Na2CO3. Fig. 4 shows TEM imagesand PXRD patterns for samples of CeO2 synthesized from (c,d)agar and (e,f) ammonium alginate. Agar produced a gel-likenetwork with a surface area of 13 m2 g-1. Ammonium alginategenerated clustered nanoparticles of mean diameter 11 nm andsurface area of 10 m2 g-1. The use of the sodium-free biopolymerseliminated the sodium carbonate byproduct and so may offer aroute to optimise the synthesis and structure of metal/metaloxide composites via this simple method.ConclusionsIn summary, we have demonstrated a remarkably simple, one-pot route to the synthesis of a nanoparticulate Au/CeO2 spongeFig. 4 (a) TEM image of an Au/ZrO2 composite synthesized fromalginate. (b) PXRD profile of the sample (i) before and (ii) afterwashing in dilute HCl showing indexed peaks for ZrO2 and markedpeaks for Na2CO3 (*). (c) TEM image and (d) PXRD pattern for CeO2synthesized from agar and (e) TEM image and (f) PXRD pattern forCeO2 synthesized from ammonium alginate.using a readily available biopolymer alginate template. Themulti-functionality of the alginate biopolymer in reducing theAu3+ precursor and controlling and stabilising CeO2 nucleationhas been discussed. Furthermore, the potential scope of themethod has been demonstrated through the synthesis of anAu/ZrO2 nanocomposite. This simple and flexible approachoffers considerable potential for future development in thesynthesis of metal/metal oxide composites. There are numerousexamples in the literature of metal nanoparticles, such as Au, Pt,Ru, Rh, Pd and Ag, supported on porous metal oxide supports(e.g. ZrO2, CuO, ZnO) and our biopolymer-mediated, one-potroute should offer significant advantages over many previoussynthesis techniques.ExperimentalBiopolymer solutions were prepared at 80 ◦C from 0.1 g ofsodium alginate (FMC Biopolymer, LF20/40, 65–75% gu-luronate) in 10 mL of deionised water. Gels were prepared bypremixing 2 mL of a solution of Ce(NO3)3 (20 mM in deionisedwater) with a solution of HAuCl4 (at a range of concentrationsin deionised water) and vigorously mixing the resulting solutionwith the alginate solution. The desired concentration of gold wascalculated assuming complete conversion of 4 ¥ 10-5 moles ofCe(NO3)3 to CeO2. Thus, a 1% by mass Au on CeO2 compositewas prepared using 0.018 mL of aqueous HAuCl4 (20 mM), 2%from 0.035 mL and 5% from 0.088 mL. The gel was dried atroom temperature for 48 h to form a flexible pink or purple film274 | Green Chem., 2011, 13, 272–275 This journal is © The Royal Society of Chemistry 2011Published on 23 December 2010. Downloaded by Keele University on 20/05/2014 13:10:44. View Article Onlinethat was calcined in air at 1 ◦C min-1 to 600 ◦C for two hours.Control syntheses in the absence of sodium alginate were alsocarried out. ZrO2/Au nanocomposites were prepared in thesame way using aqueous ZrO(NO3)2 (0.02 M). Agar solution(1% by weight) was prepared by dissolving agar (Aldrich) inboiling water. Ammonium alginate was prepared by stirringalginic acid (FMC Biopolymer) in water and adjusting the pHto 7 with ammonium hydroxide.Samples for TEM were prepared by dispersing in ethanol anddropping onto a carbon coated nickel grid. TEM analysis wascarried out using a JEOL JEM 1200EX microscope equippedwith an Oxford EDX detector and high resolution TEM carriedout on a JEOL JEM 2010 microscope. Samples for SEM werecoated with platinum/palladium and analysed using a JEOLJSM 6330F Field Emission SEM. PXRD was carried out ona Bruker D8 Advance diffractometer equipped with a Lynxeyeposition sensitive detector.AcknowledgementsThe authors thank the University of Bristol for financial supportand FMC Biopolymer for samples of alginate. We are gratefulto Prof. G. Hutchings and Dr P. Miedziak for useful discussion.Notes and references1 R. J. White, R. Luque, V. L. Budarin, J. H. Clark and D. J.Macquarrie, Chem. Soc. Rev., 2009, 38, 481–494.2 J. Beckers and G. Rothernberg, Green Chem., 2010, 12, 939–948.3 X. Wang, D. R. G. Mitchell, K. Prince, A. J. Atanacio and R. A.Caruso, Chem. Mater., 2008, 20, 3917–3926.4 O. Rosseler, M. V. Shankar, M. Karkmaz-Le Du, L. Schmidlin, N.Keller and V. Keller, J. Catal., 2010, 269, 179–190.5 M. Sahnchez-Dominguez, L. F. Liotta, G. Di Carlo, G. Pantaleo,A. M. Venezia, C. Solans and M. Boutonnet, Catal. Today, 2010,158, 35–43.6 R. Si and M. Flytzani-Stephanopoulos, Angew. Chem., Int. Ed., 2005,44, 7411–7414.7 Z. Schnepp, S. C. Wimbush, S. Mann and S. R. Hall, CrystEngComm,2010, 12, 1410–1415.8 J. M. Nieto and C. Peniche-Covas, Thermochim. Acta, 1991, 176,63–68.9 A. A. Said and R. M. Hassan, Polym. Degrad. Stab., 1993, 39, 393–397.10 M. Cargnello, C. Gentilini, T. Montini, E. Fonda, S. Mehraeen, M.Chi, M. Herrera-Collado, N. D. Browning, S. Polizzi, L. Pasquatoand P. Fornasiero, Chem. Mater., 2010, 22, 4335–4345.11 S. Carrettin, P. Concepcion, A. Corma, J. M. Lopez Nieto and V. F.Puntes, Angew. Chem., Int. Ed., 2004, 43, 2538–2450.12 A. Pal, K. Esumi and T. Pal, J. Colloid Interface Sci., 2005, 288,396–401.13 G. T. Grant, E. R. Morris, D. A. Rees, P. J. C. Smith and D. Thom,FEBS Lett., 1973, 32, 195–198.This journal is © The Royal Society of Chemistry 2011 Green Chem., 2011, 13, 272–275 | 275Published on 23 December 2010. Downloaded by Keele University on 20/05/2014 13:10:44. View Article Online

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A flexible one-pot route to metal/metal oxide nanocomposites

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