Au@Pd Core−Shell Nanoparticle Incorporated Alumina Sols and

May 5, 2009 - Michel Pellarin , Inas Issa , Cyril Langlois , Marie-Ange Lebeault , Julien Ramade , Jean Lermé , Michel Broyer , and Emmanuel Cottancin...
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J. Phys. Chem. C 2009, 113, 9101–9107

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Au@Pd Core-Shell Nanoparticle Incorporated Alumina Sols and Coatings: Transformation of Au@Pd to Au-Pd Alloy Nanoparticles Debrina Jana, Anirban Dandapat, and Goutam De* Sol-Gel DiVision, Central Glass and Ceramic Research Institute, 196, Raja S.C. Mullick Road, Kolkata 700 032, India ReceiVed: December 4, 2008; ReVised Manuscript ReceiVed: April 6, 2009

Au@Pd nanoparticles (NPs) of size range 4.5-13 nm (〈D〉 ≈ 7.2 nm) were generated inside the alumina matrix sol without using any additional reducing agent. Partially acetylacetonato (acac) complexed aluminum tri-sec-butoxide has been used to prepare the matrix alumina sol. First, Au NPs of size range 3-9 nm (〈D〉 ≈ 5.3 nm) were generated in the alumina sol following our reported method (J. Mater. Chem. 2008, 18, 2816). The addition of Pd-acetate in this Au NPs containing sol resulted in formation of Pd shells onto the Au NPs. The previously formed Au NPs were acting as seeds and the mild reducing environment of the reaction medium (due to Al-alkoxide) reduced the Pd ions to form Pd shells onto the Au NPs. The strong plasmon band observed at 520 nm in the case of Au NPs containing sol has been significantly dampened due to the formation of Pd shells. Grazing incidence X-ray diffraction and transmission electron microscopy (including scanning TEM, EDS using nanoprobe, and HRTEM) analyses revealed the core-shell structure of the NPs. The above sol can be used to prepare Au@Pd core-shell NPs incorporated alumina coatings on glass substrates. The Au@Pd NPs were transformed to Au-Pd alloy NPs inside the film matrix after heat treatment at 500 °C in air. Introduction Bimetallic nanoparticles (NPs) attract great interest due to their unique catalytic, electronic, and optical properties different from those of the corresponding monometallic components.1-9 Core-shell NPs with a strained shell attain widespread use in various applications that include plasmonics, biological sensing, and catalysis, which places them on the frontier of advanced materials chemistry.9-11 Among the various bimetallic systems, e.g. Au-Cu,12,13 Au-Ag,14 Co-Ni,15 Au-Pt,1,2,16,17 Cu-Pt,18 Cu-Ni,19 Pd-Ni,7 Pd-Pt,9 etc., Au-Pd4-6,8,20-33 have a special interest because of their interesting catalytic activities and magnetic and optical properties. Au-Pd bimetallic nanoclusters are employed for the direct synthesis of hydrogen peroxide from H2 and O2,11,20b,c primary alcohol to aldehyde,11,20a ethylene/ acetylene hydrogenation,24 carbon monoxide reduction,27 and the hydrogenation of aldehyde to alcohol.30 Most of the studies concerning the Au-Pd systems were confined in solution and employed a successive4,26,33 or coreduction4,20,31,32 approach. However, synthesis of Au-Pd core-shell NPs inside a matrix alumina sol without using any additional reducing agent, which in turn is usable for the deposition of transparent films with the embedded Au-Pd core-shell and alloy NPs, is not documented in the literature. Many studies concerning the synthesis of Au-Pd NPs have been made (mostly in solution) and their structure is still of interest.11,21,22,24,33 In most of the studies suppression of the Au plasmon band has been considered as a proof of the formation of Au@Pd NPs in solution. However, the optical study is unable to provide the information regarding the composition, structure (core-shell or alloy), and segregation of individual components of the bimetallic system. Some recent studies have demonstrated the structure of Au@Pd NPs.11,21,22,24b Recently Ferrer et al.33 * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +91 33 24838086. Fax: +91 33 24730957.

showed a very clear 3-layer core-shell structure of Au-Pd bimetallic NPs synthesized by the successive reduction method, using the transmission electron microscopy that includes high resolution and scanning tunneling modes. Very recently we have reported a synthetic protocol to prepare Au nanoparticles (Au NPs) in alumina sols34 derived from partially acetylacetonato complexed aluminum secondary butoxide (ASB). When chloroauric acid solution is added to the alumina sol Au NPs are generated spontaneously through the reduction of Au3+ fAu+f Au0 by Al-alkoxide. The generated Au NPs remain protected by the acac chelates attached to Al, and as a result, the Au NPs containing sol can be stored for months without any growth or aggregation of the NPs and can be used to deposit Au NPs incorporated coatings. We attempted to synthesize Pd NPs in such alumina sols using this synthetic protocol. We thought that the reducing sol-environment could reduce Pd2+ to the Pd nanometal; however, the attempt failed because the reducing environment of the alumina sol was not strong enough. So we attempted to deposit Pd nanoshells onto the prereduced Au NPs in the alumina sols. The idea behind this is that the preformed Au NPs could act as a seed nanocrystal for the deposition of Pd shells35 onto it and the mild reducing environment of the sol could facilitate this process. Using the above idea Au@Pd NPs have been generated in alumina matrix sols without using any additional reducing agent. The use of these sols to prepare Au@Pd NPs incorporated alumina coatings and the transformation of Au@Pd to Au-Pd alloy NPs in such a film matrix by thermal treatment have been accomplished. The formation of these core-shell and alloy NPs in sols and coatings has been studied and characterized by the systematic analyses of optical absorption measurements, grazing incidence X-ray diffraction, and transmission electron microscopy (HRTEM, STEM, and EDS with nanoprobe).

10.1021/jp810673x CCC: $40.75  2009 American Chemical Society Published on Web 05/05/2009

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Experimental Section Materials. All chemicals were used as received. Aluminumtri-sec-butoxide Al[OCH(CH3)C2H5]3 (ASB) and palladium acetate Pd(CH3COO)2 were supplied by Sigma-Aldrich, while HAuCl4 · 3H2O, acetylacetone (acac), 2-butanol, and n-propanol were obtained from s.d. fine-chem limited and Ranbuxy fine chemicals. Preparation of Au Nanoparticle Incorporated Sol. The preparation of Au nanoparticle incorporated alumina sol has been reported in our previous paper.34 First a partially acetylacetonato complexed ASB stock solution with an acac/ASB molar ratio of 0.66 was prepared (ASB0.66acac) and to this solution a calculated amount of HAuCl4 · 3H2O solution was added to obtain Au NPs incorporated alumina sol. The Au concentration was maintained at 0.5 mol % with respect to the equivalent amount of AlO1.5 (1/2Al2O3) present in the sol. About 4.5 equiv wt % of AlO1.5 was maintained in the final sol. Au@Pd Core-Shell Nanoparticle Incorporated Alumina Sols. The calculated amount of Pd(CH3COO)2 dissolved in a small amount of acetone was added to the above in situ generated Au NPs incorporated alumina sol with stirring and the solution was kept undisturbed in closed condition for 2 h. The formation of Au@Pd core-shell nanoparticle in the alumina sol was monitored through UV-visible spectra from time to time up to 2 d. The molar ratio of equivalent Au:Pd in the sol was maintained as 1:1 and 1:2, respectively. After the addition of Pd(CH3COO)2 solution, the color of the sol changed from deep red to brownish. Preparation of Coatings. Prior to the coating deposition, the soda-lime and silica glass slides were first cleaned following the method reported earlier.14a The coatings were prepared by using the dipping technique (Dip-master 200, Chemat Corporation) with a withdrawal velocity of 6-8 in. min-1. The coatings were dried at 60 °C for 30 min followed by heat treated at 500 °C (ramp of 1 and 2 deg min-1) in air with a holding time of 1 h. The thickness of the coatings was measured by a Surfcorder SE-2300 profilometer (Kosaka Laboratory Ltd., Japan). The UV-visible spectra of the coatings deposited on silica glass substrates were obtained with a Cary 50 scan spectrophotometer. Infrared absorption spectra of the sols (by placing the liquid sample between two KBr windows) and films (deposited on Si-wafers) were recorded by FTIR spectrometer (Nicolet, 380) with 64-128 scans for each sample. Grazing incidence X-ray diffraction (GIXRD) patterns of the films were recorded by using Rigaku SmartLab (operating at 9 kW using the rotating anode and cross-beam optics to enhance the X-ray intensity) using Cu KR (λ ) 1.5406 Å) radiation. Transmission electron microscopic (TEM) measurements were carried out with a JEOL 2010 operating at 200 kV (IACS, Kolkata) and Tecnai G2 30ST (FEI) operating at 300 kV. High-resolution TEM (HRTEM), single-particle EDS using a nanoprobe in STEM mode and highangle annular dark-field (HAADF) images were recorded with the Tecnai G2 30ST transmission electron microscope. TEM samples were prepared by drop casting of diluted sols and scratched off coatings onto the carbon-coated grids. Results and Discussion When HAuCl4 solution is added to the partially acac complexed ASB0.66acac solution a deep red color has been observed due to the spontaneous generation of Au NPs.34 In this work 0.5 mol of Au with respect to the equivalent AlO1.5 present in the sol has been used. Following this synthetic protocol Au NPs of average diameter ∼5.3 nm (see ref 34 for

Figure 1. Optical absorption spectral evolution showing the dampening of the Au surface plasmon peak after the addition of Pd acetate solution into the Au containing alumina sols. Two systems have been studied: (a) Au:Pd ≈ 1:2 and (b) Au:Pd ≈ 1:1. The spectra have been recorded with respect to time as indicated in the body of the figure. All spectra were taken after diluting (10 times) the sols with 1-propanol.

details) are formed in the alumina sol and remain protected by the acac chelates attached with alumina structure. This in situ generated Au NPs incorporated sol shows an intense absorption peak near 520 nm (Figure 1a; marked by Au0.5). However, the addition of Pd acetate solution into the ASB0.66acac solution results in no reduction of Pd ions. As a proof we presented the XRD patterns of these Pd-incorporated (2 mol equiv of Pd with respect to the total AlO1.5) alumina gels after drying the whole mixture at 60 °C in Figure S1 (Supporting Information). This 60 °C dried gel showed broad peaks in the range 20-26° 2θ and 35-50° 2θ most probably due to the poorly crystalline aluminum oxide hydrates (JCPDS card 05-0355) along with some sharp peaks due to the formation of some Al-acac and/or Pd-acac chelated compounds.36 However, no signals due to Pd metals have been observed. When these gels are heated at 350 and 500 °C, peaks corresponding to the crystalline phases of PdO (tetragonal) are observed (Figure S1, Supporting Information). This experiment confirms that this “ASB0.66acac sol-system” could reduce the Au3+; however, it was not strong enough to reduce the Pd2+ ions. The reason of this difference in reduction is due to the smaller potential of reduction of palladium than gold. It may be noted here that the ionization potentials of gold (9.2 eV) and Pd (8.3 eV) are different. Interestingly when Pd acetate solution is added to the Au NPs containing alumina sols, we observed significant dampening of the Au surface plasmon (SP) band. Figure 1 shows spectral evolution showing the dampening of the Au SP band after the addition of Pd acetate solution. Two systems have been studied: Au:Pd (molar) ≈ 1:2 (Figure 1a) and Au:Pd ≈ 1:1 (Figure 1b). The spectra have been recorded with respect to time up to 2 d. We also observed that the Au SP, which absorbed very strongly at 520 nm, has been blue-shifted to about 508 nm and the plasmon peak gradually further weakens with respect to time. The dampening and blueshifting of Au-SPR are in agreement with the formation of an

Transformation of Au@Pd to Au-Pd Alloy Nanoparticles

Figure 2. Transmission electron microscopy images of Au@Pd NPs obtained by the deposition of Pd shells onto the pregenerated Au NPs in alumina sol: low magnification images showing the (a) Au NPs before addition of Pd acetate (size distribution is given in the inset); (b) Au@Pd NPs along with the size distributions and selected area electron diffraction (SAED) pattern (inset).

Au@Pd core-shell type structure because in this situation the surface electron density in the Au core will increase at the expense of electrons from the outer Pd shell, because Pd has a higher electron chemical potential, and as a result electrons will flow from Pd to Au once the two metals contact each other until the equilibrium of electron chemical potential is established.24,26b So in the present system the previously generated Au NPs acted as seeds and onto this Pd shells grow with respect to time. In the case of the Au:Pd ≈ 1:2 system, after about 2 d the dampening of the plasmon peak is found to be much more pronounced than that of the Au:Pd ≈ 1:1 system (see Figures 1, parts a and b, for comparison). For this reason the detailed characterization of Au@Pd NPs formed in the alumina sol in the case of the Au:Pd ≈ 1:2 system after about 2 d has been undertaken. A TEM study of Au NPs generated in such alumina sol derived from ASB0.66acac (before the addition of Pd-acetate) has already been reported by us.34 For a comparative study of the TEM of Au NPs before the addition of Pd-acetate is presented in Figure 2a along with the size distribution. As shown in Figure 2a, Au NPs of size ranges ∼3-9 nm (average 5.3 nm) are visible. Figure 2b shows the TEM image of the Au@Pd NPs formed after about 2 d of Pd-acetate addition to the previously generated Au NPs in alumina sol. In this case NPs of size ranges 4.5-13 nm (〈D〉 ≈ 7.2 nm) can be observed (Figure 2b). Clearly the size of the NPs has been increased compared to that of the bare Au NPs. This observation gives an idea that the Pd shell of average thickness ∼1.9 nm has been deposited on Au NPs in alumina sol. The selected area electron diffraction (SAED) pattern obtained from such Au@Pd NPs show faint lines of fcc Au; however, lines corresponding to the Pd are not clearly visible, most probably due to the low thickness of Pd shells. Figure 3 shows the high-resolution (a) and scanning TEM (b) images of one individual Au@Pd NP. It is possible to distinguish the core-shell structure in Figure 3a where the core is darker. The high angle annular dark field (HAADF) image also shows clear contrast. From these images a core-shell structure of the NPs could be established. To estimate the composition of the NPs, some of the individual NPs were analyzed by the STEM-EDS technique. The equipped STEMEDS nanoprobe system is capable of elemental analysis. The results are summarized in Figure 4. Figure 4a shows the HAADF image of the NPs and the individual particles which are analyzed by the nanoprobe are marked by 1-4 (also indicated by black filled circles). The nanoprobe was focused at the center of the NPs. A representative EDS spectrum obtained from the NP #1 is presented in Figure 4b. Apart from signals of Au and Pd the

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Figure 3. Au@Pd NPs at high resolution showing its core-shell structure: (a) HRTEM and (b) scanning TEM images.

Figure 4. Quantitative elemental composition of the individual NPs using the STEM-EDS. The nanoprobe used for such analysis has a spot size of about 5 nm. (a) STEM images showing the analyzed NPs with marking, along with compositions in tabular form; (b) EDS obtained from the nanoparticle marked as 1. The signals of Al, O, and C are from the matrix sol and Cu appears from the grid.

spectrum also shows signals of Al, O, and C (from the matrix sol) and Cu (from the copper grid). The results of elemental analyses concerning the atomic weight percent of Au/Pd in the individual NPs are given in a tabular form in Figure 4a. It can be seen that Au:Pd atomic ratios of the NPs are varied from 1:0.33 to 1:1 with an average value of Au:Pd ≈ 1:0.61. This value is, however, less than the added total Pd to the system. As we did not observe any individual Pd NPs by TEM analysis, it is expected that some part of the Pd remains in the matrix sol in ionic form. To confirm this, EDS analyses from large areas of samples were also undertaken. Such EDS analyses gave an average Au/Pd atomic ratio of 1:1.6. So some Pd which did not form shells on Au NPs remained in the alumina sol as Pd2+. We have studied the FTIR of Au and Au@Pd incorporated alumina sols, and the corresponding coatings dried at room temperature. The results are presented in Figure 5. We could not distinguish any Pd-acetate related peaks (FTIR of Pd acetate is given as reference in Figure 5) in the spectra of Pd containing sols or coatings after comparing the spectra of the Au and Au@Pd sols and the corresponding coatings obtained from these sols. This observation suggests decomposition of the Pd acetate in the sol. The acetone peak that is observed in the case of Pd containing sol (acetone has been used to dissolve Pd acetate) has disappeared in the case of the corresponding room temperature dried coating. The combined peaks originating from acac

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Figure 5. FTIR spectra of Au and Au@Pd incorporated alumina sols, and the corresponding coatings dried at room temperature. After addition of Pd acetate solution, the Au@Pd incorporated sol was aged for 2 d and the spectra of this aged sol and the corresponding coating deposited on an Si wafer were recorded. The Y-axis has been shifted for clarity. The spectrum of Pd-acetate (KBr pellet method) has been given as a reference.

chelates (shown in rectangular green box) appeared at similar positions in cases of both the sols and room temperature dried coatings. We found slight shifting of absorption (originating from Al-acac chelates in the case of Au doped sol) from about 1604 (relatively broad in nature) to 1596 cm-1 (marked with red circles and a blue dashed line in Figure 5) after addition of Pd acetate and subsequent aging (2 d). This shifting indicates the possibility of formation of some Pd-acac linkages (exchange of acac from Al to Pd) because Pd could form stronger chelates with acac.37 So part of the added Pd that did not form a shell on Au is expected to remain attached with acac in the composite sol as well as in the dried coatings. To understand further the structure of Au@Pd, one of the NPs (relatively bigger in size) has been assessed by using the line scanning analysis in the STEM-EDS mode. The result of such a line scan analysis is presented in Figure 6a,b. The analyzed NP and the direction of analysis are shown in the inset of Figure 6a. This analysis reveals the presence of both Au and Pd in the same nanoparticle. As the concentration of Au is higher in this NP, the signal of Au is more intense than that of the Pd. However, careful observation also reveals that the signal of Pd is slightly broader than that of the Au. This indicates the existence of Pd as shell and Au as core. The internal structure of the Au@Pd NPs has also been assessed by high-resolution transmission electron microscopy (HRTEM). Figure 7a shows the HRTEM of the Au@Pd NPs. The figure shows a truncated octahedron shaped nanoparticle with distinct lattice fringes characteristic of the fcc structure. As the Pd remains as a shell the measurement of lattice fringes shows the Pd〈111〉 lattice spacing of 0.225 nm (JCPDS card 46-1043)26a throughout the NP. However, careful observation also shows the existence of lattice spacing equal to 0.235 nm characteristic of the Au〈111〉 plane (JCPDS card 04-0784).26a The Fourier diffractogram obtained from the nanoparticle as marked in Figure 7a clearly shows two sets of spots characteristics of Au and Pd (see the inset of Figure 7a). The Fourier filtering images obtained from such HRTEM image are shown in Figure 7, parts b and c. The Fourier filtering image

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Figure 6. Elemental distribution of Au and Pd across a single nanoparticle using the STEM-EDS line scanning technique. The inset shows the analyzed area and direction of analysis.

Figure 7. (a) High-resolution TEM image displaying the lattice fringes of the Au@Pd core-shell NPs. The Fourier diffractogram shown in the inset clearly shows spots corresponding to the Au〈111〉 and Pd〈111〉 lattice planes; (b and c) Fourier filtering images considering the Pd and Au〈111〉 spots, respectively.

considering to the 〈111〉 spots of Pd (Figure 7b) shows that the Pd has covered almost the whole NP. However, considering the Au〈111〉 spots the FFT filtering image (Figure 7c) shows the presence of Au〈111〉 planes in some corners (a few are marked by red circles in the Figure 7c), as well as some scattered areas throughout the NP. This analysis gives the idea that Pd

Transformation of Au@Pd to Au-Pd Alloy Nanoparticles

Figure 8. Optical absorption spectra of Au@Pd NPs, and the corresponding Au-Pd alloy NPs (formed after heat treatment at 500 °C) incorporated alumina films. The spectrum of the corresponding Au NPs incorporated alumina film (this film was prepared before addition of Pd) is also shown.

has been deposited on the Au. So the transmission electron microscopy study unequivocally confirmed that a core-shell type composite structure has been formed after the addition of Pd solution in the previously formed Au NPs containing alumina sol. The suppression of the Au plasmon peak as observed by optical absorption spectroscopy (Figure 1a) can thus be explained by using the TEM (including STEM, EDS, line scanning of single NP, high resolution) investigations. One interesting part of this work is that the Au@Pd NPs containing sols can be used to prepare coatings on glass substrates. We thought that this type of coating material could be useful as a catalyst because this type of catalyst material in the form of a thin film could be separated easily after the reaction, and reusable.38 As the metal loading is comparatively less and one layer film yielded a thickness of ∼100 nm (after heat treatment at 500 °C in air), we prepared coatings by multiple dipping to characterize this film material. One such representative 6-times coated film has been prepared both from before (i.e., Au NPs containing alumina sol) and after the addition of Pd-acetate followed by 2 days of aging. Figure 8 shows optical spectra of such coatings before and after heating. The 60 °C dried Au NPs incorporated coating shows an Au SP peak at 530 nm. The position of this peak remains almost unaffected after heat treatment at 500 °C (not shown in the figure). Similar results were also obtained in our earlier work.34 The shifting of the Au SP position in the case of film (530 nm) and that of sol (520 nm) is due to the increase of the refractive index of the embedding medium of the Au NPs. In case of the film the refractive index is higher than that of the sol and accordingly the plasmon has been red-shifted following Mie theory. This we have explained in our previous paper.34 The corresponding Au@Pd NPs incorporated alumina film shows significant dampening of the Au SP band (Figure 8) supporting the existence of the core (Au)-shell (Pd) NPs. In this case a weak peak at about 515 nm (blue-shifted compare to that of pure Au-doped film) has been observed. The cause of such blueshifting of plasmonic absorptions, as also observed in the case of sols (see Figure 1), has already been discussed.24,26b When this 60 °C dried Au@Pd NPs incorporated film is heat treated at 500 °C (ramp of 1 or 2 deg min-1) in air we found further dampening of the Au SP band (Figure 8) suggesting some kind of reactions took place due to such heat treatment. We have performed the GIXRD analysis of the film materials and the results are presented in Figure 9. In the case of 60 °C dried films clear fcc peaks of Au at 38.22 (Au〈111〉) and 44.47° 2θ

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Figure 9. GIXRD patterns of the Au@Pd NPs incorporated films dried at 60 °C (a) and the corresponding heat-treated (500 °C) films (b). The Y-axis of part b is shifted for clarity.

(Au〈200〉) are observed (Figure 9a). A weak broad peak at about 40.1° 2θ has also been observed due to Pd〈111〉 (Figure 9a). Interestingly the corresponding heat-treated coating (500 °C/ air) shows peaks at 38.87 and 45.12° 2θ (Figure 9b).39 In this case additional broad peaks near 33.9 and 55.1° 2θ due to the existence of tetragonal PdO phases are also observed. The shifting of peaks from 38.22 to 38.87° and 44.47 to 45.12° 2θ gives a clear indication of forming Au-Pd solid solution.40 Applying Vegard’s law the composition of the Au-Pd fcc solid solution (alloy) has been estimated from the Bragg diffractions at 38.87° 2θ to be Au0.59Pd0.41 which gives an Au:Pd atomic ratio close to 1:0.69. It should be pointed out here that we estimated an average Au:Pd atomic ratio of 1:0.61 in case of the generated Au@Pd NPs in sol by STEM-EDS analysis (see Figure 4). So this ratio is found to be maintained in the Au-Pd alloy NPs formed after the heat treatment. It seems that some part of the Pd that was not reduced and remained in the matrix sol as well as in the films transformed into the tetragonal PdO nanocrystallites. The formation of such PdO is quite logical because we showed that PdO crystallites were formed in the Pd incorporated alumina after heat treatment in air (see Figure S1 in the Supporting Information). In the present case the X-ray reflections of PdO (Figure 9b) are less intense and broad suggesting formation of very small nanocrystallites. The structures of the Au-Pd alloy NPs have also been analyzed by TEM. Figure 10 shows low-magnification (a) and high-resolution (b) TEM images. The low-magnification image shows the existence of NPs of average diameter ∼7.6 nm embedded in the alumina film matrix. The corresponding HRTEM shows well-defined lattice planes of spacing close to 0.231 nm, which matches well with the Au0.59Pd0.41 〈111〉 Bragg diffraction (Figure 9b). Thus the Au@Pd core-shell NPs transforms into the Au-Pd alloy NPs inside the film matrix after the heat treatment at 500 °C in air. It is clear that the Pd shell adhered with Au forms a solid solution with the Au. In the TEM study of the film, however, we could not locate the PdO crystallites: most probably either they are very small in size and embedded in films or not resolvable because of the alumina matrix and high contrast of the Au-Pd alloy NPs. Conclusions In this work we showed Pd shell formation onto the Au NPs in a partially acac complexed ASB derived alumina sol medium. The combined effect of Au NP seeding and the reducing

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Jana et al. authors thank Dr. S. Bysakh of CGCRI for helping in TEM studies and S. Bhattacharya for some technical help. We thank Director, CGCRI for his kind permission to publish this work. A.D. thanks CSIR for awarding a Junior Research Fellowship. Supporting Information Available: XRD patterns of Pd incorporated alumina powders heat treated at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. Low- (a) and high-resolution (b) TEM images of Au-Pd alloy NPs incorporated alumina film obtained after heat treatment at 500 °C. The size distribution of Au-Pd alloy NPs is shown in the inset of (a).

environment of the sol (due to Al-alkoxide) help in generating Pd shells on the Au NPs; as a result, Au@Pd core-shell NPs are formed. The strong plasmon band of Au NPs has been dampened significantly due to the formation of Pd shells on the Au NPs. The detailed optical, XRD, and TEM studies reveal the core-shell structure of the NPs. This in situ generated Au@Pd NPs containing alumina sol is useful to prepare transparent alumina coatings embedded with Au@Pd NPs on glass substrates. When heat treated in air at 500 °C the core-shell structure has been converted into the Au-Pd alloy. These Au@Pd and Au-Pd alloy NPs incorporated alumina coatings could find useful applications in the fields of nonlinear optics and catalysis. Acknowledgment. Financial support from the Department of Science and Technology (DST), Government of India under the Nano Mission program is thankfully acknowledged. The

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