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Green synthesis and characterization of Au@Pt core–shell bimetallic nanoparticles using gallic acid
Author’s Accepted Manuscript Green synthesis and characterization of Au@Pt CORE-shell bimetallic nanoparticles using gallic acid Guojun Zhang, Hongmei Zheng, Ming Shen, Lei Wang, Xiaosan Wang www.elsevier.com/locate/jpcs
PII: DOI: Reference:
S0022-3697(14)00320-5 http://dx.doi.org/10.1016/j.jpcs.2014.12.012 PCS7448
To appear in: Journal of Physical and Chemistry of Solids Received date: 26 July 2014 Revised date: 28 November 2014 Accepted date: 23 December 2014 Cite this article as: Guojun Zhang, Hongmei Zheng, Ming Shen, Lei Wang and Xiaosan Wang, Green synthesis and characterization of Au@Pt CORE-shell bimetallic nanoparticles using gallic acid, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2014.12.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Green synthesis and characterization of Au@Pt core-shell bimetallic nanoparticles using gallic acid Guojun Zhang, Hongmei Zheng, Ming Shen *, Lei Wang, Xiaosan Wang
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, Jiangsu Province, P. R. China
ARTICLE INFO Article history: Received
2014
Received in revised form Accepted Available online
2014
2014 2014
Keywords: A. Nanostructures A. Inorganic compounds B. Chemical synthesis C. X-ray diffraction D. Optical properties
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ABSTRACT In this paper, a facile and benign green-synthesis was developed to successfully fabricate well-dispersed urchin-like Au@Pt core-shell nanoparticles (NPs) by using gallic acid (GA) as both a reducing and protecting agent. The proposed one-step synthesis utilized the differences in reduction potentials of AuCl4 and PtCl62 to make AuCl4 ions preferentially reduced to Au cores and then make PtCl62 ions to be continuously deposited onto the Au core surface as Pt shell. The as-prepared Au@Pt NPs were characterized by TEM, HRTEM, SEM, XRD, UV-vis and FTIR. Moreover, we had systematically investigated the effects of some experimental parameters, such as reaction temperature, the molar ratios of HAuCl4/H2PtCl6 and the amount of GA, on the formation of the Au@Pt NPs. When PVP was used as a protecting agent, the Au@Pt core-shell NPs with better-disperse and smaller size was obtained through this green synthesis method. In the reaction of NaBH4 reducing p-nitrophenol (PNP) to p-aminophenol (PAP), the as-prepared Au@Pt NPs showed better catalytic activity. However, the results showed that the Au@Pt bimetallic NPs had lower catalytic activity than that of pure Au NPs obtained with the same method, which just confirmed the formation of Au@Pt core-shell nanostructures because the active sites on the surface of Au NPs were covered with later-formed Pt-shell.
*Corresponding author.Tel.:+86 514 87975590 9405; fax: +86 514 87975244. E-mail address: [email protected] (M. Shen).
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1. Introduction Noble metal nanoparticles (NPs) have found widespread use in several technological applications [1-3], and various wet chemical synthesis methods have been reported [4-6]. Noble metal-based composite nanomaterials integrate two or more distinct functionalities into a single entity with superior and sometimes unprecedented properties. Compared with the individual components or bulk metals, bimetallic nanostructures have become a new class of attractive advanced materials with unique physicochemistry properties [7,8]. Bimetallic NPs are generally categorized into different forms of core-shell, heterostructure and intermetallic or alloyed nanostructures [9-11], among which the core-shell nanostructure has specially aroused considerable interest due to its fascinating versatility in optics, magnetism, electronics, catalysis, anticorrosion [12], etc. The core/shell type NPs can be broadly defined as the combination of a core (inner material) and a shell (outer layer material), and their properties are strongly dependent on the core and shell dimensions and composition [13]. Meanwhile, the change of either the constituting materials or the core to shell ratio can produce their novel properties. Up to now, varieties of core/shell NPs have been reported, especially when both the core and shell materials are made of noble metals, such as Au@Ag [14], Au@Pd [15], Au@Pt [16], Ag@Pd [17] and Ag@Pt [18]. The well-known Pt nanostructures with high surface area and catalytic activity have been used as an antitumor agent in biomedical applications together with other metal NPs [19]. With such important applications, the development of preparing Pt–based materials is at present receiving significant attention. The core-shell nanostructures can not only improve the utilization efficiency but also reduce the Pt usage. Recently, Yamauchi and Wang reported one-step synthesis of unique Au@Pd@Pt triple-layered core-shell structured nanoparticles consisting of a Au core, Pd inner layer and nanoporous Pt outer shell through the step-by-step depositions of Au, Pd, and Pt completing temporal separations [20]. They developed a new synthetic route for preparing bimetallic colloids with a Au core and dendritic Pt shell as well, and optimized Pt shell thickness by changing the Pt/Au molar ratio. Their results showed that the electrocatalytic activity of the prepared Au@Pt NPs for methanol
3
oxidation reaction was ~4 times as large as that of the individual Pt nanocrystals [21]. In another study made by Wang’s group [22], bifunctional Au@Pt hybrid nanorods had been prepared via a wet chemical route, in which the seed (core) of Au nanorods were firstly formed and then PtCl62 ions were chemically reduced on the core surface to form a Pt shell layer. Moreover, by adjusting the ratio of seeds to shell metal ions, they made the fine controlling of the nanostructure possible and achieved distinct shell thickness and finally the obtained Au@Pt hybrid nanorods had been successfully applied to investigate the SERS effects and catalytic activities. It is well known that green chemistry maintains the goal of development and implementation of chemical products and process to reduce and eliminate the use and generation of substances hazardous to human health and environmental protection [23]. Currently, various methods have appeared to fabricate noble metal-based nanomaterial according to green chemistry principles [24,25]. For instance, the polyphenol can chelate with metal ions in solution and further reduce them to the corresponding metal [26]. In fact, many plant extracts and plant-biomass, being in rich polyphenols, have been extensively employed as ecofriendly alternatives to chemical and physical methods for synthesis of noble metal NPs. Researchers have made many efforts preparing such noble metals as silver, Au and Pd using plant extracts [27, 28]. Varma and co-workers firstly reported the green synthesis of noble metal NPs like Ag and Pd using tea/coffee extracts [26] and developed a convenient green synthesis of Au nanostructures by employing biodegradable plant-based surfactants [29]. Song and Kim had proposed an environmentally friendly method to synthesize bimetallic Au/Ag nanoparticles using Persimmon leaf extracts [30]. Our group had developed a benign method using tannic acid as both a reducing and protecting agent to fabricate Au NPs and proved that cetyltrimethylammonium bromide (CTAB) had a strong deaggregating effect on tannic-capped gold nanoaggregates [31]. However, there is relatively little knowledge concerning the synthesis of Pt NPs using plant extracts because of the lower deposition potential of PtCl62- ions [32], the lower reaction rate in the green synthesis system of preparing Pt NPs, and the severe aggregation of Pt NPs produced by plant extracts reducing, as 4
was described in Song and Kim’s paper [33]. For many years, people have always been trying to find a new green synthetic method to effectively and speedily fabricate Pt NPs with better dispersion and uniformity. Therefore, in this paper, we have developed a new green synthetic method of “one-pot” reaction for the fabrication of Au@Pt core-shell nanostructures using gallic acid (GA) as both a reducing and protecting agent. This facile and green synthesis can obviously accelerate the kinetics of reduction of Pt precursor by inducing of HAuCl4 in the solution and readily prepare urchinlike Au@Pt core-shell nanocrystals with less Pt loading. What’s more, the benign method can effectively speed up the formation of Pt NPs and obtain better dispersed Au@Pt nanostructures.
2. Experimental section 2.1. Chemicals and Reagents Gallic acid(GA, A.R.), hydrogen tetrachloroaurate tetrahydrate (HAuCl44H2O, A.R.), hydrogen hexachloroplatinate hexahydrate (H2PtCl66H2O, A.R.), and polyvinylpyrrolidone K-30 (PVP, A.R.) were purchased from Sinopharm Chemical Reagent Co. Ltd, China and were used without further purification before the experiments. Doubly distilled water was used for all solution preparations and experiments. 2.2. Synthesis of individual Au and Pt nanocolloids The Au nanocolloids were prepared by the reduction of HAuCl4 aqueous solution with GA as both a protecting and reducing agent. A typical procedure for preparing gold NPs was as follows. 18.0 mL doubly distilled water and 0.5 mL of 9.70 10-3 M HAuCl4 solution were added into a 50 mL of round-bottom flask and heated to boiling under magnetic stirring. Then 1.5 mL of 5.0 10-3 M GA solution was added quickly, accompanied by a solution color changing from initial pale yellow, light red, purplish to eventual wine-red in a short moment, which suggested the formation of gold NPs. After continuous stirring for 3h, GA-stabilized gold hydrosol were obtained and the reaction formula was exhibited as follows [25]:
5
COOH 3
COOH + 2 HAuCl4
OH
3
OH
+ 2 Au0 + 8 HCl
O
OH
OH O
The typical GA-protected and -reduced Pt NPs were simulated as what was used in the synthesis of Au nanocolloids: 17.5 mL doubly distilled water and 0.5 mL of 9.6810-3 M H2PtCl6 solution were added into a 50 mL of round-bottom flask continuously and heated to boiling under magnetic stirring. Then 2.0 mL of 5.0103 M GA solution was quickly added, accompanied by a gradual color evolution of pale yellow, light orange, yellowish-brown and brown, which indicated the formation of Pt colloidal solution, and the whole color process took about 5 h. The reaction formula was exhibited as follows: COOH 2
COOH + H2PtCl6
OH
+ Pt0 + 6 HCl
2
OH
O
OH
OH O
2.3. Synthesis of Au@Pt core-shell nanostructures In a typical procedure for the one-step synthesis of Au@Pt core-shell nanocolloids (Au/Pt molar ratio R=1:9 ), two metal precursor aqueous solutions of HAuCl4 (9.70103 M) and H2PtCl6 (9.68103 M) (in a total volume of 0.5 mL) were mixed in 17.5 mL of doubly distilled water in a round-bottom flask of 50 mL, and heated to boiling under magnetic stirring, when 2.0 mL of 5.0103 M GA solution was rapidly added into the blend. For a better understanding of the influence of protecting agent, another series of experiments were conducted by using a procedure similar to what was used in the above-mentioned typical synthesis of Au, Pt and Au@Pt core-shell nanocolloids (R=9:1, 1:1 and 1:9), except for adding 1.0 mL of 0.10 gL-1 PVP aqueous into the mixture. 2.4. Catalytic activity of Au@Pt core-shell nanostructures In a typical procedure for the catalytic reaction of Au@Pt core-shell nanostructures, 30.0 mL of 1.5×10-4 M PNP solution was added into a 50 mL of round-bottom flask, and then 2.0 mL of freshly-prepared NaBH4 (2.5 M) aqueous 6
solution was added under magnetic stirring. After then, another 2.0 mL Au@Pt colloidal solution, which was obtained after a series of treatment of centrifugating, washing and redispersing from the original 2.0 mL Au@Pt (R=1:1) colloidal solution prepared in the experimental section 2.3, was added into the above blend of PNP and NaBH4 under vigorous stirring. The catalytic activity of Au@Pt core-shell NPs was confirmed by tracing the change of PNP absorbance with the reaction time using the UV-vis absorption spectrum. Another comparative experiment was made, which was about the catalytic activity of pure Au hydrosol prepared in the experimental section 2.2 with the equivalent amount of Au@Pt colloidal solution. 2.5. Characterization Studies of morphology and particle size were measured using S-4800 Ⅱ field emission scanning electron microscope (FE-SEM, Hitachi Company, Japan) operating at 30 kV, Tecani-12 transmission electron microscope (TEM, Philips Company, Holland) manipulating at an accelerating voltage of 120 kV and Tecnai G2 F30 S-Twin TEM high-resolution transmission electron microscope (HR-TEM, FEI Company, USA) with 300 kV working voltage. The samples for FE-SEM, TEM and HR-TEM analysis were prepared by dripping the as-obtained noble metal nanocolloids onto the mica substrates, conductive glass or tin foil and carbon-coating copper grids, respectively, and air-dried at room temperature. UV-vis absorption spectra were investigated at room temperature on a UV-2501 PC UV-vis spectrometer from 200 to 900 nm using 1 cm path length quartz cuvette. Wide-angle powder X-ray diffraction (XRD) data were taken with a graphite monochromator and Cu Κα radiation ( = 0.1541 nm) on a D8 Advance Superspeed powder X-ray diffractometer (Bruker Company, Germany), operated in the -2 mode primarily in the 20-80 (2) range and at a step scan of 2=0.04°. The operating voltage was 80 kV, and the operating current was 200 mA. The analysis sample for XRD detection was obtained by spreading the redispersed colloidal solution onto the glass substrate and dried naturally to form the film. Fourier transmission infrared spectra (FT-IR) were collected in the transmission mode on a Nicolet 740 FT-IR spectrometer by blending the dried product with KBr powders and pelleting into a thin plate.
3. Results and Discussion 3.1. Formation and Characterization of Au@Pt core-shell nanostructures 7
Fig. 1 presents a typical Au@Pt nanostructure prepared under the typical condition (R=1:9, T=100 °C). TEM image in Fig. 1a shows the Au@Pt nanostructures with strikingly uniform morphology and particle size of about 50 nm without observing other isolated Pt or Au NPs, which demonstrates the high yield synthesis (~100%) of the Au@Pt NPs. The urchin-like Au@Pt NPs possess a dark center with a gray shell, and this kind of contrast indicates the formation of a core-shell structure (see Fig. 1a and its inset). HR-TEM is employed to investigate the crystal structure for further study of the Au@Pt nanostructure. The overgrowth of Pt NPs on the Au cores can be seen in HR-TEM images (Fig. 1b), but there is no obvious boundary between Au cores and Pt shells due to their minute differences in atomic number and slight attenuation of electrons [34]. HR-TEM image portrays clear lattice fringes, which suggests good crystal morphology. The obvious fringes with ~0.226 nm and 0.196 nm are assigned to the (111) and (200) planes of Pt crystal structure of face-centered cubic (fcc), respectively, which indicates that the growth direction of the Pt shell is along the facet of (111) and (200). And the lattice fringes with ~0.235 nm and ~0.204 nm interplanar separation corresponding to the (111) and (200) planes of Au fcc, respectively, which suggests they are directionally oriented over the entire area. The selected-area electron diffraction (SAED) pattern (Fig. 1c) of the Au@Pt core-shell nanostructures displays the ring patterns with intense spots assigned to (111), (200), (220) and (311) plane of typical Au and Pt fcc crystals, which shows the polycrystalline nature of the as-prepared NPs.
Fig. 1.
As depicted in the typical preparation of individual Pt nanocolloids, a longer reaction time is needed to obtain Pt NPs with smaller size and intense aggregation (see in Fig. 9e). The proposed one-step green synthesis can effectively shorten the reaction time and form better-dispersed Au@Pt bimetallic NPs. The essence of the synthesis of Au@Pt bimetallic NPs utilizes the difference in standard reduction potential [21] of the two metal salts (H2PtCl6 and HAuCl4). In the present reaction system, AuCl4－and PtCl62－can be reduced as the following:
8
AuCl4－
Au + 4Cl－
+1.00 eV vs SHE (1)
[PtCl6] 2－ + 2e-
[PtCl4]2－ + 2Cl－
+ 0.68 eV vs SHE (2)
[PtCl4] 2－ + 2e-
Pt + 4Cl－
+ 0.76 eV vs SHE (3)
+ 3e-
The large difference of the deposition potential of Au and Pt is critical to provide a driving force for ideal one-step synthesis of core-shell structure. In the mixture of HAuCl4 and H2PtCl6, Au NPs are expected to be firstly formed through GA reducing with AuCl4－ ions as the favorable seeds to induce the reaction of H2PtCl6 with GA so as to further shorten the reaction time. Fig. 2a shows the XRD pattern of the as-prepared Au@Pt core-shell bimetallic NPs. The crystal structure of face-centered cubic (fcc) phase is confirmed from the diffraction peaks, which are characterized by the (111), (200), and (220) diffraction peaks, in comparison with fcc structure of both bulk Au (JCPDS file No. 87-0720) and bulk Pt (JCPDS File No. 040802) with regard to 2θ value of 39.64°, 45.98° and 67.36°, respectively [35-37]. The XRD result is consistent with the above HR-TEM image and SAED study. In Fig. 2b, the FE-SEM image of the as-prepared Au@Pt core-shell NPs without Au or Pt sputtering treatment displays a uniform and well-dispersed system. The chemical composition of Au@Pt core-shell NPs is determined by using the energy-dispersive X-ray spectroscopy (EDX) (shown in Fig. 2c). The EDX spectrum with only two peaks (Au and Pt) is clearly presented, indicating that the core-shell nanostructures are made up of metallic gold and platinum. And the elemental molar ratio of Au to Pt is close to that in the starting solution of 1:9.
Fig. 2.
3.2. Effect of reaction temperature on the Au@Pt core-shell nanostructures Fig. 3 shows the TEM images of Au@Pt NPs prepared by using GA as both a reducing and protecting agent at 30 °C, 60 °C and 100 °C, respectively. The synthetic process can be easily monitored through the solution color evolution with time at
9
30 °C. The reaction is triggered within 1 min, accompanied by a color change from pale yellow to colorless and to light pink, which indicates that Au3+ ions are firstly reduced to Au+ and then to Au0. Then the system color goes through pale orange, light blue, bluish green and finally becomes dark brown, which indicates the formation of Au@Pt nanocrystals with dark centers corresponding to Au cores and light edges corresponding to the Pt shell (see in Fig. 3a). As discussed above, the initially-formed Au nanocrystals play the role of seeds for the overgrowth of Pt shell. Enhancing the reaction temperature can obviously shorten the starting reaction time (from 1 min of 30°C to 10 s of 100°C) and decrease the particle size (from 80 nm of 30°C to ~60 nm of 60°C and ~50 nm of 100°C, respectively), because it can promote quick nucleation and vigorous molecular thermal motion and intensify molecules to collide into the secondary particles [38]. Thus, higher reaction temperature generates more Au nuclei with smaller size and larger specific area, providing enough nucleation sites for the deposition of Pt atoms to form high-density shell.
Fig. 3.
3.3 Effect of the Au/Pt molar ratios on the Au@Pt NPs Fig. 4 shows TEM images of Au@Pt NPs prepared from different Au/Pt molar ratios at 100 C using GA as both a reducing and protecting agent. The average size of the nanocrystals is about 40 nm at R=1:9 in Fig. 4a, in contrast, the average size is around 50 nm and 60 nm of R at 1:1 and 9:1, respectively (see Fig. 4b and c). The results show that the particle size of Au@Pt nanocrystals slightly increase and the dispersion becomes poorer with R increasing. Especially, in the case of 9:1, both the uniformity and dispersion show significant differences, and the nanocrystals dominating the Au cores of polygonal nanodisk exhibit larger size and thinner Pt shell. In addition, the experiments also show that the reduction rate of R at 1:9 sharply becomes slower compared with that of 1:1 and 9:1 cases.
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Fig. 4.
The UV-vis spectra of Au@Pt colloids using different molar ratios at 100C are shown in Fig. 5. The absorption spectrum of Au@Pt nanocolloid at R= 9:1 displays a weak absorption peak at 522 nm (see Fig. 5a) corresponding to the characteristic surface plasmon absorption (SPR) of Au NPs [31,36] and a tailing peak at <350 nm corresponding to the formation of Pt thin shell on the surface of Au NPs. Upon increasing R, the UV-vis spectrum of Au@Pt nanocolloid at R=1:1 shows a wider absorption band in the visible region of about 600 nm, while the tailing peak still exists (in Fig. 5b), corresponding to the relative decrease of absorption from Au NPs and the relative increase of the thickness of Pt shell. When the Au/Pt molar ratio decreases to 1:9, the typical absorption peak of Au nanoparticles at 500~550 nm becomes invisible while the tailing peak from Pt NPs still exists (in Fig. 5c), which indicates that the formerly-formed Au NPs have been completely covered by high-density Pt shell. Therefore, in Fig. 5c, there appears no characteristic absorption in the visible-near-infrared (Vis-NIR) spectroscopy [39]. In addition, the scattering of the Pt shell may be another important factor for changing the dielectric that surrounds the gold [40]. The results of UV-vis spectroscopy analysis match well with the above TEM images.
Fig. 5.
Fig. 6A displays the XRD patterns of the as-prepared individual Au and Pt NPs and Au@Pt nanocrystals with various Au/Pt molar ratios (denoted as Au(9)Pt(1), Au(1)Pt(1) and Au(1)Pt(9)), respectively. All the XRD patterns in Fig. 6A exhibit similar appearance and shape, but the diffraction peak locations of Au@Pt core-shell NPs, with regard to 2θ value of around 39°, 45° and 67°, respectively, appear a continuous shift towards Pt peak locations at their XRD diagrams, which indicates that the bimetallic NPs diffraction peak locations are closer to that of pure Pt as the molar ratio of Au to Pt gradually decreases. The clear XRD evidence shows that the PtCl62 ions have been slowly reduced to Pt in the presence of GA and deposited on the surface of Au core. The characteristic XRD result is in agreement with the above 11
SAED pattern shown in Fig. 1c. Each ring pattern arranged from the inner to outer in SAED pattern is corresponding to a certain measured d spacing, and the lattice distance d of Au@Pt nanocolloids neither matches with that of Au nor Pt, but between the two, which is equivalent to the diffraction peak counterparts (calculated from the index of plane) between Au and Pt peak locations. Fig. 6B displays an excellent plot summarizing the relationship between the 2θ value (indexed to the crystal facet (111) of the mentioned five samples) and the molar fraction of Pt (xPt). While increasing the amount of Pt, the diffraction angles (2θ) tend to shift from that of Au to Pt. Setting the 2θ as ordinate, and xPt as horizontal ordinate to plot the curve, a perfect linear relationship is expressed. The results fully testify the fact that Au@Pt core-shell nanostructures are successfully fabricated when GA as both a reducing and protecting agent co-reduces HAuCl4 and H2PtCl6 in water at 100C.
Fig. 6.
3.4. Effect of GA concentration on the Au@Pt core-shell nanostructures When R is fixed as 1:9 and GA/(HAuCl4/H2PtCl6) molar ratio (denoted as R1) is changed from 1:2 to 1:1 and 10:1, TEM images of Au@Pt core-shell NPs, fabricated by GA green reducing, are shown in Fig.7, which displays a nonsignificant shape change of as-synthesized Au@Pt nanostructures. The results indicate that, with increasing the molar ratio of GA to (Au/Pt), the NPs tend to be uniform and particle size increases from 30 nm (at R1=1:2) to 50 nm (at R1=1:1 and 10:1). When the amount of GA is not sufficient or just stoichiometric (R1=1:2 or 1:1), the as-prepared nanocrystals tend to aggregate and grow into relatively large size, due to the insufficient GA protection originating from electrostatic adsorption effect and steric effect of the phenolic hydroxyl group in GA. However, when R1 is added up to 10:1, the pH value of the starting solution dramatically decreases, which results in the decrease of the reduction activity of GA, and finely-dispersed Au@Pt NPs with smaller size are prepared, thanks to the sufficient protection of GA as a capping agent on the Au@Pt particles can efficiently prevent the aggregation of Au@Pt NPs.
Fig. 7.
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Fig. 8 shows the FTIR spectra of pure GA powder and the obtained Au@Pt NPs by GA reduction of AuCl4- and PtCl62- species. In Fig. 8a, the spectrum at 3600~3000 cm-1 appears a strong and broad absorption band corresponding to the –OH group stretching vibration in GA, and a strong but narrow peak at 1705 cm-1 indicates the existence of carbonyl group (C=O). And typical stretching vibrations of C=C bonds can be observed from three peaks of 1616, 1541 and 1429 cm-1. There are several peaks ranging from 300 cm-1 to 1000 cm-1 region that could be assigned to be the stretching vibration and bending vibration of C–O and O–H bond of GA, respectively. It is reported that phenols are easily oxidized to form quinones and the quinoid compound might be produced by GA reduction of HAuCl4 and absorbed on the surface of gold NPs [25]. In Fig. 8b, compared with the spectrum of pure GA shown in Fig. 8a , the stretching vibration of C–O bond and bending vibration of O–H bond in 1300~1000 cm-1 region is still retained, but the intensity obviously decreases, and the stretching vibration peak of C=O group shifts from 1702 to 1632 cm-1, and the stretching vibration of C=C bond at 1616 and 1541 cm-1 is covered with a broad and middle intensity band at around 1632 cm-1. Similarly, our FTIR analysis also indicates that the quinoid compound with keto-enol system could be produced by GA reduction of AuCl4- and PtCl62- species and absorbed on the surface of Au@Pt NPs. On the other hand, the significant shift of C=O group from 1702 to 1632 cm-1 observed from Fig. 8b can be ascribed to the possibly existing inductive effect between the nucleophilic compound and the electron dense Au@Pt core-shell NPs.
Fig. 8.
3.5. Effect of PVP using as protecting agent on the Au@Pt nanocrystals As is well known, PVP is a better commonly used hydrophilic protecting reagent for preparing noble NPs, which is used in our study to investigate the effect of adding protecting reagent on preparing Au@Pt core-shell nanostructures through GA green reducing. Fig. 9 and 10 respectively show the TEM images of prepared samples from the typical and PVP-protected synthesis. As is displayed in Fig. 10a, the as-prepared Au nanocrystals with a better dispersity have no obvious change in the shape (including prominent sphere-like NPs of around 30 nm, nano-rods, triangle plates, and 13
hexagonal sheets), and the NPs size decreases compared with the PVP-free case shown in Fig. 9a. The PVP-protected Au/Pt NPs of R=9:1 case obtain a multi-shape mixture with larger size of about 50 nm (see Fig. 10 b), in contrast with the PVP-free case in Fig. 9b, these NPs take the form of smooth edges without the adhesion of smaller particles on NPs surface. When R is 1:1, these PVP-protected Au@Pt core-shell nanocrystals present smaller diameter size and thinner Pt layer (see Fig. 10c), along with better dispersion. The experiment shows that the introduction of PVP in the typical reaction system really influences the growth of either Au or Au@Pt core-shell in terms of the diameter size and dispersion, by adsorbing onto the surface of the formed NPs to prevent them from aggregating and further to decrease the size. Meanwhile, compared with Fig. 9d (at R=1:9), after adding PVP, quasi-monodisperse urchin-like Au@Pt core-shell nanocrystals exhibit strikingly uniform shape and size, and the average size decreases to approximately 40 nm (see Fig. 10d), and the stability of the colloidal solution is also enhanced dramatically. Compared with the PVP-protected cases of 9:1 and 1:1 (shown in Fig. 10b and c), Au cores wear thicker Pt shell resulting in a clearer contrast of dark center and gray shell in Fig. 10d. Fig. 10e shows that PVP-protected pure Pt NPs have better dispersity, even though fewer dark Pt aggregations are still formed, which is consistent with the reported literature [41]. Fig. 9.
Fig. 10.
3.6. Catalytic activity of Au@Pt nanocrystals in the reduction of p-nitrophenol to p-aminophenol by NaBH4 The catalytic activity of gold NPs is usually evaluated by the reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) using NaBH4 as a reducing agent. In this paper, the same method is adopted to evaluate the catalytic activity of as-prepared Au@Pt nanocrystals and the individual Au NPs obtained through this green synthesis. Fig.11 A a, b, c and d show the UV-vis absorption spectra of primary PNP solution, the blend of PNP and NaBH4 aqueous solution, the mixture introducing the as-prepared Au nanocolloids into PNP before and after adding NaBH4, respectively. An obvious absorption peak locates at 399 nm (in Fig. 11A b), indicating the 14
formation of p-nitrophenolic ions under alkaline condition[42, 43], and then disappears after adding NaBH4 in the presence of the as-prepared Au nanocrystals, accompanied by the appearance of signatures located at 311 nm, which suggests the reduction of NO2 group of PNP to NH2 group of PAP [44]. Therefore, the catalytic reduction of gold/ Au@Pt nanoparticles can also be studied by simply monitoring the change of intensity and location of typical absorption peak, associated with PNP and PAP, or tracing the UV-vis absorption spectra of the system with the evolution of reaction time. As depicted in Fig. 11(B) and (C), both the magnitude of the absorption peak at 399 nm gradually decreases at different rates. Fig. 11(B) shows that, under the condition of individual Au-based catalysis, the intensity of absorption peak decreases at 399 nm and increases at 311 nm within 3 min, and only 40 min is needed to complete the whole catalytic process. However, under the condition of Au@Pt-based catalysis, it takes about 2 h to complete the whole catalytic process. As proved in this study, the individual Au NPs obtained through GA as both a protecting and reducing agent performs superior catalytic activity compared with the as-prepared Au@Pt bimetallic NPs. The initially-reduced Au cores are covered with a thick Pt shell coating or the exposed Au atoms for catalytic reduction become fewer, which can be used to explain the lower catalytic activity of Au@Pt nanostructures. In reverse, the study of PNP-PAP catalytic reduction, as an indirect evidence, just confirms the formation of Au@Pt core-shell nanostructures. Some further researches involving the catalytic activity of nano-Pt coming from Au@Pt core-shell structures and individual Pt nanocrystals are on the way.
Fig.11.
4. Conclusion This paper puts forward a new and effective method to fabricate Au@Pt nanocrystals through green synthesis method by using gallic acid as both a reducing and protecting agent, which overcomes the barrier of plant extracts reducing H2PtCl6 to individual Pt NPs under the usual condition. When GA is used as both a reducing and protecting agent, urchin-like Au@Pt core-shell NPs by chemical reduction of
15
HAuCl4 and H2PtCl6 salts can be successfully fabricated, in which the seeds (cores) of Au NPs are firstly formed and then PtCl62 ions are chemically reduced on the core surface to form a Pt shell layer. In this research, the effects of the reaction temperature and Au/Pt molar ratios on the Au@Pt core-shell NPs are systematically investigated. The results show that, with the increase of the reaction temperature, the reduction rate obviously accelerates and the particle size decreases along with fine dispersion. And upon lowering the molar ratio of Au/Pt, the morphology of Au@Pt nanocrystals varies from Au polygon to Pt sphere and the Pt shell becomes thicker, accompanied by the 2θ locations indexed to the crystal facet (111) shift from Au to Pt. The characteristic SPR of Au nanocrystals gradually disappears owning to the Pt shell effect in the corresponding
UV-vis
spectrum.
Meanwhile,
after
adding
PVP,
the
quasi-monodisperse urchin-like Au@Pt NPs possessing smaller particle size are obtained, compared with PVP-free ones. In the reaction of NaBH4 reducing p-nitrophenol (PNP) to p-aminophenol (PAP), the as-prepared Au@Pt core-shell NPs have better catalytic activity, however, this catalytic activity is lower than that of pure Au NPs obtained with the same method, which just confirms the formation of Au@Pt core-shell nanostructures because of the coverage of Pt-shell on the surface of Au core with high catalytic activity.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (Grant No. 21273194) and Priority Academic Program Development of Jiangsu Higher Education Institutions, China. The authors also give thanks to Test Center of Yangzhou University for helping the tests of nanomaterial structure.
16
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[23] P.T. Anastas, J.C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York, 1998, p. 30. [24] M.N. Nadagouda, R.S. Varma, Green Chem. 9 (2007) 632-637. [25] W.X. Wang, Q.F. Chen, C. Jiang, Colloids Surf. A 301 (2007): 73-79. [26] M.N. Nadagouda, R.S. Varma, Green Chem. 10 (2008) 859-862. [27] J.Y. Song, B.S. Kim, Bioprocess Biosyst Eng. 32 (2009) 79-84. [28] J.Y. Song, H.K. Jang, B.S. Kim, Process Biochem. 44 (2009) 1133-1138. [29] M.N. Nadagouda, G. Hoag, J. Collins, R.S. Varma, Crystal Growth & Design 9 (2009) 4979-4983. [30] J.Y. Song, B.S. Kim, Korean J. Chem. Eng. 25 (2008) 808-811. [31] M. Shen, J.T. Dai, J. Zhu, J.M. Yang, Chinese J. Inorg. Chem. 27 (2011) 308-314. [32] X.Y Han, D.W. Wang, D. Liu, J.S. Huang , T.Y. You, J. Colloid Interface. Sci. 367 (2012) 342–347. [33] J.Y. Song, E.Y. Kwon, B.S. Kim, Bioprocess Biosyst Eng. 33 (2010) 159-164. [34] Y.Y. Ma, W.Y. Li, E.C. Cho, Z.Y. Li, T. Yu, J. Zeng, Z. X.Xie, Y. N. Xia, ACS Nano 4 (2010) 6725-6734. [35] K.Okitsu, M. Ashokkumar, F. Grieser, J. Phys. Chem. B 109 (2005) 20673-20675. [36] M. Shen, Y.K. Du, P. Yang, L. Jiang, J. Phys. Chem. Solids 66 (2005) 1628-1634. [37] V.L. Nguyen, M. Ohtaki, T. Matsubara, M.T. Cao, M. Nogami, J. Phys. Chem. C 116 (2012) 12265-12274. [38] N.R. Jana, L. Gearheart, C.J. Murphy, Adv. Mater. 13 (2001) 1389−1393. [39] S.J. Guo, Y.X. Fang, S.J. Dong, E.K, Wang, J. Phys. Chem. C 111 (2007) 17104-17109. [40] J.H. Song, F. Kim, D. Kim, P.D. Yang, Chem. Eur. J. 11 (2005) 910-916. [41] W. Chen, S.G. Sun, Z.Y. Zhou, J. Phys. Chem. B 107 (2003) 9809-9812. [42] S.H. Sun, D.Q. Yang, D. Villers, G.X. Zhang, E. Sacher, J.P. Dodelet, Adv. Mater. 20 (2008) 571-574. [43] C. Wang, H. Daimon, T. Onodera, T. Koda, S.H. Sun, Angew. Chem. Int. Ed. 47 (2008) 3588-3591. [44] H.X. Wu, P. Wang, H.L. He, Y.D. Jin, Nano Res. 5 (2012) 135-144.
18
(a)
(c)
(b)
(111)
Fig. 1. TEM image (a), HR-TEM image (b) and Selected-area electron diffraction (SAED) (c) patterns of as-synthesized Au@Pt nanostructures. The scale bars in (a) and (b) are 100 nm and 2 nm, respectively.
(b)
(220)
(200)
(a)
20
30
40
50
2 (degree)
60
70
80
(c)
Fig. 2. XRD pattern (a), FE-SEM image(b) and EDX image(c) of the as-prepared Au@Pt core-shell nanostructures prepared from the typical synthesis.
19
(a)
(b)
(c)
Fig. 3. TEM images of Au@Pt nanostructures prepared (R=1:1) using GA as both a reducing and protecting agent at different temperatures (a) 30C, (b) 60C, (c) 100C. (All scale bars are 100 nm.)
(b)
(a)
(c)
Fig. 4. TEM images of Au@Pt nanostructures prepared using GA as both a reducing and protecting agent at 100 C, R= 1:9 (a), 1:1 (b) and 9:1 (c). (All scale bars are 100 nm.)
20
1.0
Absorbance(a.u.)
0.8 0.6
(a) (c)
0.4
(b)
0.2 0.0 300
400
500
600
700
800
900
Wavelength (nm)
Fig. 5. UV-vis absorption spectra of the Au@Pt nanocolloids prepared using GA as
40.0
(220)
(e) (d) (c)
39.8
(B)
39.6
2D e g re e
39.4
(311)
(A)
(200)
(111)
both a reducing and protecting agent at 100C, R= 9:1 (a), 1:1 (b) and 1:9 (c).
(b)
39.2 39.0 38.8 38.6 38.4 38.2
(a)
38.0 0.0 20
30
40
50
2 (degree)
60
70
80
0.2
0.4
0.6
0.8
1.0
xPt
Fig. 6. (A) XRD patterns of Au (a), Au(9)Pt(1) (b), Au(1)Pt(1) (c), Au(1)Pt(9) (d), Pt (e) and (B) The relationship between 2θ value (indexed to the crystal facet (111)) and the molar fraction of Pt(xPt).
21
(b)
(a)
(c)
Fig. 7. TEM images of the Au@Pt nanocrystals prepared by varying GA/(Au/Pt) molar ratios (R1) from 1:2 (a) to 1:1 (b) and 10:1 (c), R=1:9. (All scale bars are 100 nm.)
3500
3000
2500
2000
1500
1078 1022 866
1383
1705 1616 1541 1429 1313
3448 3500
4000
3275
(a)
1632
(b)
1000
500
-1
Wavenumber (cm )
Fig. 8. FTIR spectra of GA(a) and GA-protected Au@Pt nanocrystals(b).
22
(a)
(b)
(d)
(c)
(e)
Fig. 9. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the typical synthesis using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm.)
(a)
(b)
(c)
(e)
(d)
Fig. 10. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the PVP (CPVP=0.010 g/L) introduction system, using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm except for (e) of 50 nm.)
23
2.5
(A)
2.0
Absorbance(a.u.)
Absorbance(a.u.)
2.5 2.0 1.5
(a)
(b)
1.0
(d)
1.5 1.0
2.5
(B)
0 min 1 min 3 min 5 min 10 min 20 min 30 min 40 min
Absorbance(a.u.)
3.0
0.5
0.0 200
300
400
500
600
Wavelength (nm)
700
800
0.0 200
2.0
1.5
1.0
(C)
0.5
0.5
(c)
0 min 1 min 5 min 10 min 20 min 30 min 40 min 1 h 1.5 h 2 h
300
400
500
600
Wavelength (nm)
700
800
0.0 200
300
400
500
600
700
800
Wavelength (nm)
Fig.11. (A) The UV-vis absorption spectra of the PNP solution (a), the blend of PNP and NaBH4 (b), adding Au colloids into PNP solution (c), complete reaction system after catalytic reduction(d). (B), (C) The UV-vis absorption spectra of the catalytic system taken at different reaction times, using as-prepared Au colloidal solution and Au@Pt nanocrystals as catalyst, respectively.
24
Fig. 1. TEM image (a), HR-TEM image (b) and Selected-area electron diffraction (SAED) (c) patterns of as-synthesized Au@Pt nanostructures. The scale bars in (a) and (b) are 100 nm and 2 nm, respectively.
Fig. 2. XRD pattern (a), FE-SEM image(b) and EDX image(c) of the as-prepared Au@Pt core-shell nanostructures prepared from the typical synthesis.
Fig. 3. TEM images of Au@Pt nanostructures prepared (R=1:1) using GA as both a reducing and protecting agent at different temperatures (a) 30C, (b) 60C, (c) 100C. (All scale bars are 100 nm.)
Fig. 4. TEM images of Au@Pt nanostructures prepared using GA as both a reducing and protecting agent at 100 C, R= 1:9 (a), 1:1 (b) and 9:1 (c). (All scale bars are 100 nm.)
Fig. 5. UV-vis absorption spectra of the Au@Pt nanocolloids prepared using GA as both a reducing and protecting agent at 100C, R= 9:1 (a), 1:1 (b) and 1:9 (c).
Fig. 6. (A) XRD patterns of Au (a), Au(9)Pt(1) (b), Au(1)Pt(1) (c), Au(1)Pt(9) (d), Pt (e) and (B) The relationship between 2θ value (indexed to the crystal facet (111)) and the molar fraction of Pt(xPt).
Fig. 7. TEM images of the Au@Pt nanocrystals prepared by varying GA/(Au/Pt) molar ratios (R1) from 1:2 (a) to 1:1 (b) and 10:1 (c), R=1:9. (All scale bars are 100 nm.)
Fig. 8. FTIR spectra of GA(a) and GA-protected Au@Pt nanocrystals(b). 25
Fig. 9. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the typical synthesis using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm.)
Fig. 10. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the PVP (CPVP=0.010 g/L) introduction system, using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm except for (e) of 50 nm.)
Fig.11. (A) The UV-vis absorption spectra of the PNP solution (a), the blend of PNP and NaBH4 (b), adding Au colloids into PNP solution (c), complete reaction system after catalytic reduction(d). (B), (C) The UV-vis absorption spectra of the catalytic system taken at different reaction times, using as-prepared Au colloidal solution and Au@Pt nanocrystals as catalyst, respectively.
Highlights 1. A facile green-synthesis was developed to fabricate Au@Pt core-shell nanoparticles. 2. Gallic acid was used as both a reducing and protecting agent in green-synthesis. 3. The initially-formed Au NPs played a role of seeds in HAuCl4 and H2PtCl6 mixture. 4. Adding PVP could decrease the size and enhance the monodispersity of Au@Pt NPs. 5. Au@Pt core-shell NPs have better catalytic activity for NaBH4 reducing PNP to PAP.
26
Green synthesis and characterization of Au@Pt core-shell bimetallic nanoparticles using gallic acid Guojun Zhang, Hongmei Zheng, Ming Shen *, Lei Wang, Xiaosan Wang
(a)
(c)
(b)
27
PII: DOI: Reference:
S0022-3697(14)00320-5 http://dx.doi.org/10.1016/j.jpcs.2014.12.012 PCS7448
To appear in: Journal of Physical and Chemistry of Solids Received date: 26 July 2014 Revised date: 28 November 2014 Accepted date: 23 December 2014 Cite this article as: Guojun Zhang, Hongmei Zheng, Ming Shen, Lei Wang and Xiaosan Wang, Green synthesis and characterization of Au@Pt CORE-shell bimetallic nanoparticles using gallic acid, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2014.12.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Green synthesis and characterization of Au@Pt core-shell bimetallic nanoparticles using gallic acid Guojun Zhang, Hongmei Zheng, Ming Shen *, Lei Wang, Xiaosan Wang
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, Jiangsu Province, P. R. China
ARTICLE INFO Article history: Received
2014
Received in revised form Accepted Available online
2014
2014 2014
Keywords: A. Nanostructures A. Inorganic compounds B. Chemical synthesis C. X-ray diffraction D. Optical properties
1
ABSTRACT In this paper, a facile and benign green-synthesis was developed to successfully fabricate well-dispersed urchin-like Au@Pt core-shell nanoparticles (NPs) by using gallic acid (GA) as both a reducing and protecting agent. The proposed one-step synthesis utilized the differences in reduction potentials of AuCl4 and PtCl62 to make AuCl4 ions preferentially reduced to Au cores and then make PtCl62 ions to be continuously deposited onto the Au core surface as Pt shell. The as-prepared Au@Pt NPs were characterized by TEM, HRTEM, SEM, XRD, UV-vis and FTIR. Moreover, we had systematically investigated the effects of some experimental parameters, such as reaction temperature, the molar ratios of HAuCl4/H2PtCl6 and the amount of GA, on the formation of the Au@Pt NPs. When PVP was used as a protecting agent, the Au@Pt core-shell NPs with better-disperse and smaller size was obtained through this green synthesis method. In the reaction of NaBH4 reducing p-nitrophenol (PNP) to p-aminophenol (PAP), the as-prepared Au@Pt NPs showed better catalytic activity. However, the results showed that the Au@Pt bimetallic NPs had lower catalytic activity than that of pure Au NPs obtained with the same method, which just confirmed the formation of Au@Pt core-shell nanostructures because the active sites on the surface of Au NPs were covered with later-formed Pt-shell.
*Corresponding author.Tel.:+86 514 87975590 9405; fax: +86 514 87975244. E-mail address: [email protected] (M. Shen).
2
1. Introduction Noble metal nanoparticles (NPs) have found widespread use in several technological applications [1-3], and various wet chemical synthesis methods have been reported [4-6]. Noble metal-based composite nanomaterials integrate two or more distinct functionalities into a single entity with superior and sometimes unprecedented properties. Compared with the individual components or bulk metals, bimetallic nanostructures have become a new class of attractive advanced materials with unique physicochemistry properties [7,8]. Bimetallic NPs are generally categorized into different forms of core-shell, heterostructure and intermetallic or alloyed nanostructures [9-11], among which the core-shell nanostructure has specially aroused considerable interest due to its fascinating versatility in optics, magnetism, electronics, catalysis, anticorrosion [12], etc. The core/shell type NPs can be broadly defined as the combination of a core (inner material) and a shell (outer layer material), and their properties are strongly dependent on the core and shell dimensions and composition [13]. Meanwhile, the change of either the constituting materials or the core to shell ratio can produce their novel properties. Up to now, varieties of core/shell NPs have been reported, especially when both the core and shell materials are made of noble metals, such as Au@Ag [14], Au@Pd [15], Au@Pt [16], Ag@Pd [17] and Ag@Pt [18]. The well-known Pt nanostructures with high surface area and catalytic activity have been used as an antitumor agent in biomedical applications together with other metal NPs [19]. With such important applications, the development of preparing Pt–based materials is at present receiving significant attention. The core-shell nanostructures can not only improve the utilization efficiency but also reduce the Pt usage. Recently, Yamauchi and Wang reported one-step synthesis of unique Au@Pd@Pt triple-layered core-shell structured nanoparticles consisting of a Au core, Pd inner layer and nanoporous Pt outer shell through the step-by-step depositions of Au, Pd, and Pt completing temporal separations [20]. They developed a new synthetic route for preparing bimetallic colloids with a Au core and dendritic Pt shell as well, and optimized Pt shell thickness by changing the Pt/Au molar ratio. Their results showed that the electrocatalytic activity of the prepared Au@Pt NPs for methanol
3
oxidation reaction was ~4 times as large as that of the individual Pt nanocrystals [21]. In another study made by Wang’s group [22], bifunctional Au@Pt hybrid nanorods had been prepared via a wet chemical route, in which the seed (core) of Au nanorods were firstly formed and then PtCl62 ions were chemically reduced on the core surface to form a Pt shell layer. Moreover, by adjusting the ratio of seeds to shell metal ions, they made the fine controlling of the nanostructure possible and achieved distinct shell thickness and finally the obtained Au@Pt hybrid nanorods had been successfully applied to investigate the SERS effects and catalytic activities. It is well known that green chemistry maintains the goal of development and implementation of chemical products and process to reduce and eliminate the use and generation of substances hazardous to human health and environmental protection [23]. Currently, various methods have appeared to fabricate noble metal-based nanomaterial according to green chemistry principles [24,25]. For instance, the polyphenol can chelate with metal ions in solution and further reduce them to the corresponding metal [26]. In fact, many plant extracts and plant-biomass, being in rich polyphenols, have been extensively employed as ecofriendly alternatives to chemical and physical methods for synthesis of noble metal NPs. Researchers have made many efforts preparing such noble metals as silver, Au and Pd using plant extracts [27, 28]. Varma and co-workers firstly reported the green synthesis of noble metal NPs like Ag and Pd using tea/coffee extracts [26] and developed a convenient green synthesis of Au nanostructures by employing biodegradable plant-based surfactants [29]. Song and Kim had proposed an environmentally friendly method to synthesize bimetallic Au/Ag nanoparticles using Persimmon leaf extracts [30]. Our group had developed a benign method using tannic acid as both a reducing and protecting agent to fabricate Au NPs and proved that cetyltrimethylammonium bromide (CTAB) had a strong deaggregating effect on tannic-capped gold nanoaggregates [31]. However, there is relatively little knowledge concerning the synthesis of Pt NPs using plant extracts because of the lower deposition potential of PtCl62- ions [32], the lower reaction rate in the green synthesis system of preparing Pt NPs, and the severe aggregation of Pt NPs produced by plant extracts reducing, as 4
was described in Song and Kim’s paper [33]. For many years, people have always been trying to find a new green synthetic method to effectively and speedily fabricate Pt NPs with better dispersion and uniformity. Therefore, in this paper, we have developed a new green synthetic method of “one-pot” reaction for the fabrication of Au@Pt core-shell nanostructures using gallic acid (GA) as both a reducing and protecting agent. This facile and green synthesis can obviously accelerate the kinetics of reduction of Pt precursor by inducing of HAuCl4 in the solution and readily prepare urchinlike Au@Pt core-shell nanocrystals with less Pt loading. What’s more, the benign method can effectively speed up the formation of Pt NPs and obtain better dispersed Au@Pt nanostructures.
2. Experimental section 2.1. Chemicals and Reagents Gallic acid(GA, A.R.), hydrogen tetrachloroaurate tetrahydrate (HAuCl44H2O, A.R.), hydrogen hexachloroplatinate hexahydrate (H2PtCl66H2O, A.R.), and polyvinylpyrrolidone K-30 (PVP, A.R.) were purchased from Sinopharm Chemical Reagent Co. Ltd, China and were used without further purification before the experiments. Doubly distilled water was used for all solution preparations and experiments. 2.2. Synthesis of individual Au and Pt nanocolloids The Au nanocolloids were prepared by the reduction of HAuCl4 aqueous solution with GA as both a protecting and reducing agent. A typical procedure for preparing gold NPs was as follows. 18.0 mL doubly distilled water and 0.5 mL of 9.70 10-3 M HAuCl4 solution were added into a 50 mL of round-bottom flask and heated to boiling under magnetic stirring. Then 1.5 mL of 5.0 10-3 M GA solution was added quickly, accompanied by a solution color changing from initial pale yellow, light red, purplish to eventual wine-red in a short moment, which suggested the formation of gold NPs. After continuous stirring for 3h, GA-stabilized gold hydrosol were obtained and the reaction formula was exhibited as follows [25]:
5
COOH 3
COOH + 2 HAuCl4
OH
3
OH
+ 2 Au0 + 8 HCl
O
OH
OH O
The typical GA-protected and -reduced Pt NPs were simulated as what was used in the synthesis of Au nanocolloids: 17.5 mL doubly distilled water and 0.5 mL of 9.6810-3 M H2PtCl6 solution were added into a 50 mL of round-bottom flask continuously and heated to boiling under magnetic stirring. Then 2.0 mL of 5.0103 M GA solution was quickly added, accompanied by a gradual color evolution of pale yellow, light orange, yellowish-brown and brown, which indicated the formation of Pt colloidal solution, and the whole color process took about 5 h. The reaction formula was exhibited as follows: COOH 2
COOH + H2PtCl6
OH
+ Pt0 + 6 HCl
2
OH
O
OH
OH O
2.3. Synthesis of Au@Pt core-shell nanostructures In a typical procedure for the one-step synthesis of Au@Pt core-shell nanocolloids (Au/Pt molar ratio R=1:9 ), two metal precursor aqueous solutions of HAuCl4 (9.70103 M) and H2PtCl6 (9.68103 M) (in a total volume of 0.5 mL) were mixed in 17.5 mL of doubly distilled water in a round-bottom flask of 50 mL, and heated to boiling under magnetic stirring, when 2.0 mL of 5.0103 M GA solution was rapidly added into the blend. For a better understanding of the influence of protecting agent, another series of experiments were conducted by using a procedure similar to what was used in the above-mentioned typical synthesis of Au, Pt and Au@Pt core-shell nanocolloids (R=9:1, 1:1 and 1:9), except for adding 1.0 mL of 0.10 gL-1 PVP aqueous into the mixture. 2.4. Catalytic activity of Au@Pt core-shell nanostructures In a typical procedure for the catalytic reaction of Au@Pt core-shell nanostructures, 30.0 mL of 1.5×10-4 M PNP solution was added into a 50 mL of round-bottom flask, and then 2.0 mL of freshly-prepared NaBH4 (2.5 M) aqueous 6
solution was added under magnetic stirring. After then, another 2.0 mL Au@Pt colloidal solution, which was obtained after a series of treatment of centrifugating, washing and redispersing from the original 2.0 mL Au@Pt (R=1:1) colloidal solution prepared in the experimental section 2.3, was added into the above blend of PNP and NaBH4 under vigorous stirring. The catalytic activity of Au@Pt core-shell NPs was confirmed by tracing the change of PNP absorbance with the reaction time using the UV-vis absorption spectrum. Another comparative experiment was made, which was about the catalytic activity of pure Au hydrosol prepared in the experimental section 2.2 with the equivalent amount of Au@Pt colloidal solution. 2.5. Characterization Studies of morphology and particle size were measured using S-4800 Ⅱ field emission scanning electron microscope (FE-SEM, Hitachi Company, Japan) operating at 30 kV, Tecani-12 transmission electron microscope (TEM, Philips Company, Holland) manipulating at an accelerating voltage of 120 kV and Tecnai G2 F30 S-Twin TEM high-resolution transmission electron microscope (HR-TEM, FEI Company, USA) with 300 kV working voltage. The samples for FE-SEM, TEM and HR-TEM analysis were prepared by dripping the as-obtained noble metal nanocolloids onto the mica substrates, conductive glass or tin foil and carbon-coating copper grids, respectively, and air-dried at room temperature. UV-vis absorption spectra were investigated at room temperature on a UV-2501 PC UV-vis spectrometer from 200 to 900 nm using 1 cm path length quartz cuvette. Wide-angle powder X-ray diffraction (XRD) data were taken with a graphite monochromator and Cu Κα radiation ( = 0.1541 nm) on a D8 Advance Superspeed powder X-ray diffractometer (Bruker Company, Germany), operated in the -2 mode primarily in the 20-80 (2) range and at a step scan of 2=0.04°. The operating voltage was 80 kV, and the operating current was 200 mA. The analysis sample for XRD detection was obtained by spreading the redispersed colloidal solution onto the glass substrate and dried naturally to form the film. Fourier transmission infrared spectra (FT-IR) were collected in the transmission mode on a Nicolet 740 FT-IR spectrometer by blending the dried product with KBr powders and pelleting into a thin plate.
3. Results and Discussion 3.1. Formation and Characterization of Au@Pt core-shell nanostructures 7
Fig. 1 presents a typical Au@Pt nanostructure prepared under the typical condition (R=1:9, T=100 °C). TEM image in Fig. 1a shows the Au@Pt nanostructures with strikingly uniform morphology and particle size of about 50 nm without observing other isolated Pt or Au NPs, which demonstrates the high yield synthesis (~100%) of the Au@Pt NPs. The urchin-like Au@Pt NPs possess a dark center with a gray shell, and this kind of contrast indicates the formation of a core-shell structure (see Fig. 1a and its inset). HR-TEM is employed to investigate the crystal structure for further study of the Au@Pt nanostructure. The overgrowth of Pt NPs on the Au cores can be seen in HR-TEM images (Fig. 1b), but there is no obvious boundary between Au cores and Pt shells due to their minute differences in atomic number and slight attenuation of electrons [34]. HR-TEM image portrays clear lattice fringes, which suggests good crystal morphology. The obvious fringes with ~0.226 nm and 0.196 nm are assigned to the (111) and (200) planes of Pt crystal structure of face-centered cubic (fcc), respectively, which indicates that the growth direction of the Pt shell is along the facet of (111) and (200). And the lattice fringes with ~0.235 nm and ~0.204 nm interplanar separation corresponding to the (111) and (200) planes of Au fcc, respectively, which suggests they are directionally oriented over the entire area. The selected-area electron diffraction (SAED) pattern (Fig. 1c) of the Au@Pt core-shell nanostructures displays the ring patterns with intense spots assigned to (111), (200), (220) and (311) plane of typical Au and Pt fcc crystals, which shows the polycrystalline nature of the as-prepared NPs.
Fig. 1.
As depicted in the typical preparation of individual Pt nanocolloids, a longer reaction time is needed to obtain Pt NPs with smaller size and intense aggregation (see in Fig. 9e). The proposed one-step green synthesis can effectively shorten the reaction time and form better-dispersed Au@Pt bimetallic NPs. The essence of the synthesis of Au@Pt bimetallic NPs utilizes the difference in standard reduction potential [21] of the two metal salts (H2PtCl6 and HAuCl4). In the present reaction system, AuCl4－and PtCl62－can be reduced as the following:
8
AuCl4－
Au + 4Cl－
+1.00 eV vs SHE (1)
[PtCl6] 2－ + 2e-
[PtCl4]2－ + 2Cl－
+ 0.68 eV vs SHE (2)
[PtCl4] 2－ + 2e-
Pt + 4Cl－
+ 0.76 eV vs SHE (3)
+ 3e-
The large difference of the deposition potential of Au and Pt is critical to provide a driving force for ideal one-step synthesis of core-shell structure. In the mixture of HAuCl4 and H2PtCl6, Au NPs are expected to be firstly formed through GA reducing with AuCl4－ ions as the favorable seeds to induce the reaction of H2PtCl6 with GA so as to further shorten the reaction time. Fig. 2a shows the XRD pattern of the as-prepared Au@Pt core-shell bimetallic NPs. The crystal structure of face-centered cubic (fcc) phase is confirmed from the diffraction peaks, which are characterized by the (111), (200), and (220) diffraction peaks, in comparison with fcc structure of both bulk Au (JCPDS file No. 87-0720) and bulk Pt (JCPDS File No. 040802) with regard to 2θ value of 39.64°, 45.98° and 67.36°, respectively [35-37]. The XRD result is consistent with the above HR-TEM image and SAED study. In Fig. 2b, the FE-SEM image of the as-prepared Au@Pt core-shell NPs without Au or Pt sputtering treatment displays a uniform and well-dispersed system. The chemical composition of Au@Pt core-shell NPs is determined by using the energy-dispersive X-ray spectroscopy (EDX) (shown in Fig. 2c). The EDX spectrum with only two peaks (Au and Pt) is clearly presented, indicating that the core-shell nanostructures are made up of metallic gold and platinum. And the elemental molar ratio of Au to Pt is close to that in the starting solution of 1:9.
Fig. 2.
3.2. Effect of reaction temperature on the Au@Pt core-shell nanostructures Fig. 3 shows the TEM images of Au@Pt NPs prepared by using GA as both a reducing and protecting agent at 30 °C, 60 °C and 100 °C, respectively. The synthetic process can be easily monitored through the solution color evolution with time at
9
30 °C. The reaction is triggered within 1 min, accompanied by a color change from pale yellow to colorless and to light pink, which indicates that Au3+ ions are firstly reduced to Au+ and then to Au0. Then the system color goes through pale orange, light blue, bluish green and finally becomes dark brown, which indicates the formation of Au@Pt nanocrystals with dark centers corresponding to Au cores and light edges corresponding to the Pt shell (see in Fig. 3a). As discussed above, the initially-formed Au nanocrystals play the role of seeds for the overgrowth of Pt shell. Enhancing the reaction temperature can obviously shorten the starting reaction time (from 1 min of 30°C to 10 s of 100°C) and decrease the particle size (from 80 nm of 30°C to ~60 nm of 60°C and ~50 nm of 100°C, respectively), because it can promote quick nucleation and vigorous molecular thermal motion and intensify molecules to collide into the secondary particles [38]. Thus, higher reaction temperature generates more Au nuclei with smaller size and larger specific area, providing enough nucleation sites for the deposition of Pt atoms to form high-density shell.
Fig. 3.
3.3 Effect of the Au/Pt molar ratios on the Au@Pt NPs Fig. 4 shows TEM images of Au@Pt NPs prepared from different Au/Pt molar ratios at 100 C using GA as both a reducing and protecting agent. The average size of the nanocrystals is about 40 nm at R=1:9 in Fig. 4a, in contrast, the average size is around 50 nm and 60 nm of R at 1:1 and 9:1, respectively (see Fig. 4b and c). The results show that the particle size of Au@Pt nanocrystals slightly increase and the dispersion becomes poorer with R increasing. Especially, in the case of 9:1, both the uniformity and dispersion show significant differences, and the nanocrystals dominating the Au cores of polygonal nanodisk exhibit larger size and thinner Pt shell. In addition, the experiments also show that the reduction rate of R at 1:9 sharply becomes slower compared with that of 1:1 and 9:1 cases.
10
Fig. 4.
The UV-vis spectra of Au@Pt colloids using different molar ratios at 100C are shown in Fig. 5. The absorption spectrum of Au@Pt nanocolloid at R= 9:1 displays a weak absorption peak at 522 nm (see Fig. 5a) corresponding to the characteristic surface plasmon absorption (SPR) of Au NPs [31,36] and a tailing peak at <350 nm corresponding to the formation of Pt thin shell on the surface of Au NPs. Upon increasing R, the UV-vis spectrum of Au@Pt nanocolloid at R=1:1 shows a wider absorption band in the visible region of about 600 nm, while the tailing peak still exists (in Fig. 5b), corresponding to the relative decrease of absorption from Au NPs and the relative increase of the thickness of Pt shell. When the Au/Pt molar ratio decreases to 1:9, the typical absorption peak of Au nanoparticles at 500~550 nm becomes invisible while the tailing peak from Pt NPs still exists (in Fig. 5c), which indicates that the formerly-formed Au NPs have been completely covered by high-density Pt shell. Therefore, in Fig. 5c, there appears no characteristic absorption in the visible-near-infrared (Vis-NIR) spectroscopy [39]. In addition, the scattering of the Pt shell may be another important factor for changing the dielectric that surrounds the gold [40]. The results of UV-vis spectroscopy analysis match well with the above TEM images.
Fig. 5.
Fig. 6A displays the XRD patterns of the as-prepared individual Au and Pt NPs and Au@Pt nanocrystals with various Au/Pt molar ratios (denoted as Au(9)Pt(1), Au(1)Pt(1) and Au(1)Pt(9)), respectively. All the XRD patterns in Fig. 6A exhibit similar appearance and shape, but the diffraction peak locations of Au@Pt core-shell NPs, with regard to 2θ value of around 39°, 45° and 67°, respectively, appear a continuous shift towards Pt peak locations at their XRD diagrams, which indicates that the bimetallic NPs diffraction peak locations are closer to that of pure Pt as the molar ratio of Au to Pt gradually decreases. The clear XRD evidence shows that the PtCl62 ions have been slowly reduced to Pt in the presence of GA and deposited on the surface of Au core. The characteristic XRD result is in agreement with the above 11
SAED pattern shown in Fig. 1c. Each ring pattern arranged from the inner to outer in SAED pattern is corresponding to a certain measured d spacing, and the lattice distance d of Au@Pt nanocolloids neither matches with that of Au nor Pt, but between the two, which is equivalent to the diffraction peak counterparts (calculated from the index of plane) between Au and Pt peak locations. Fig. 6B displays an excellent plot summarizing the relationship between the 2θ value (indexed to the crystal facet (111) of the mentioned five samples) and the molar fraction of Pt (xPt). While increasing the amount of Pt, the diffraction angles (2θ) tend to shift from that of Au to Pt. Setting the 2θ as ordinate, and xPt as horizontal ordinate to plot the curve, a perfect linear relationship is expressed. The results fully testify the fact that Au@Pt core-shell nanostructures are successfully fabricated when GA as both a reducing and protecting agent co-reduces HAuCl4 and H2PtCl6 in water at 100C.
Fig. 6.
3.4. Effect of GA concentration on the Au@Pt core-shell nanostructures When R is fixed as 1:9 and GA/(HAuCl4/H2PtCl6) molar ratio (denoted as R1) is changed from 1:2 to 1:1 and 10:1, TEM images of Au@Pt core-shell NPs, fabricated by GA green reducing, are shown in Fig.7, which displays a nonsignificant shape change of as-synthesized Au@Pt nanostructures. The results indicate that, with increasing the molar ratio of GA to (Au/Pt), the NPs tend to be uniform and particle size increases from 30 nm (at R1=1:2) to 50 nm (at R1=1:1 and 10:1). When the amount of GA is not sufficient or just stoichiometric (R1=1:2 or 1:1), the as-prepared nanocrystals tend to aggregate and grow into relatively large size, due to the insufficient GA protection originating from electrostatic adsorption effect and steric effect of the phenolic hydroxyl group in GA. However, when R1 is added up to 10:1, the pH value of the starting solution dramatically decreases, which results in the decrease of the reduction activity of GA, and finely-dispersed Au@Pt NPs with smaller size are prepared, thanks to the sufficient protection of GA as a capping agent on the Au@Pt particles can efficiently prevent the aggregation of Au@Pt NPs.
Fig. 7.
12
Fig. 8 shows the FTIR spectra of pure GA powder and the obtained Au@Pt NPs by GA reduction of AuCl4- and PtCl62- species. In Fig. 8a, the spectrum at 3600~3000 cm-1 appears a strong and broad absorption band corresponding to the –OH group stretching vibration in GA, and a strong but narrow peak at 1705 cm-1 indicates the existence of carbonyl group (C=O). And typical stretching vibrations of C=C bonds can be observed from three peaks of 1616, 1541 and 1429 cm-1. There are several peaks ranging from 300 cm-1 to 1000 cm-1 region that could be assigned to be the stretching vibration and bending vibration of C–O and O–H bond of GA, respectively. It is reported that phenols are easily oxidized to form quinones and the quinoid compound might be produced by GA reduction of HAuCl4 and absorbed on the surface of gold NPs [25]. In Fig. 8b, compared with the spectrum of pure GA shown in Fig. 8a , the stretching vibration of C–O bond and bending vibration of O–H bond in 1300~1000 cm-1 region is still retained, but the intensity obviously decreases, and the stretching vibration peak of C=O group shifts from 1702 to 1632 cm-1, and the stretching vibration of C=C bond at 1616 and 1541 cm-1 is covered with a broad and middle intensity band at around 1632 cm-1. Similarly, our FTIR analysis also indicates that the quinoid compound with keto-enol system could be produced by GA reduction of AuCl4- and PtCl62- species and absorbed on the surface of Au@Pt NPs. On the other hand, the significant shift of C=O group from 1702 to 1632 cm-1 observed from Fig. 8b can be ascribed to the possibly existing inductive effect between the nucleophilic compound and the electron dense Au@Pt core-shell NPs.
Fig. 8.
3.5. Effect of PVP using as protecting agent on the Au@Pt nanocrystals As is well known, PVP is a better commonly used hydrophilic protecting reagent for preparing noble NPs, which is used in our study to investigate the effect of adding protecting reagent on preparing Au@Pt core-shell nanostructures through GA green reducing. Fig. 9 and 10 respectively show the TEM images of prepared samples from the typical and PVP-protected synthesis. As is displayed in Fig. 10a, the as-prepared Au nanocrystals with a better dispersity have no obvious change in the shape (including prominent sphere-like NPs of around 30 nm, nano-rods, triangle plates, and 13
hexagonal sheets), and the NPs size decreases compared with the PVP-free case shown in Fig. 9a. The PVP-protected Au/Pt NPs of R=9:1 case obtain a multi-shape mixture with larger size of about 50 nm (see Fig. 10 b), in contrast with the PVP-free case in Fig. 9b, these NPs take the form of smooth edges without the adhesion of smaller particles on NPs surface. When R is 1:1, these PVP-protected Au@Pt core-shell nanocrystals present smaller diameter size and thinner Pt layer (see Fig. 10c), along with better dispersion. The experiment shows that the introduction of PVP in the typical reaction system really influences the growth of either Au or Au@Pt core-shell in terms of the diameter size and dispersion, by adsorbing onto the surface of the formed NPs to prevent them from aggregating and further to decrease the size. Meanwhile, compared with Fig. 9d (at R=1:9), after adding PVP, quasi-monodisperse urchin-like Au@Pt core-shell nanocrystals exhibit strikingly uniform shape and size, and the average size decreases to approximately 40 nm (see Fig. 10d), and the stability of the colloidal solution is also enhanced dramatically. Compared with the PVP-protected cases of 9:1 and 1:1 (shown in Fig. 10b and c), Au cores wear thicker Pt shell resulting in a clearer contrast of dark center and gray shell in Fig. 10d. Fig. 10e shows that PVP-protected pure Pt NPs have better dispersity, even though fewer dark Pt aggregations are still formed, which is consistent with the reported literature [41]. Fig. 9.
Fig. 10.
3.6. Catalytic activity of Au@Pt nanocrystals in the reduction of p-nitrophenol to p-aminophenol by NaBH4 The catalytic activity of gold NPs is usually evaluated by the reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) using NaBH4 as a reducing agent. In this paper, the same method is adopted to evaluate the catalytic activity of as-prepared Au@Pt nanocrystals and the individual Au NPs obtained through this green synthesis. Fig.11 A a, b, c and d show the UV-vis absorption spectra of primary PNP solution, the blend of PNP and NaBH4 aqueous solution, the mixture introducing the as-prepared Au nanocolloids into PNP before and after adding NaBH4, respectively. An obvious absorption peak locates at 399 nm (in Fig. 11A b), indicating the 14
formation of p-nitrophenolic ions under alkaline condition[42, 43], and then disappears after adding NaBH4 in the presence of the as-prepared Au nanocrystals, accompanied by the appearance of signatures located at 311 nm, which suggests the reduction of NO2 group of PNP to NH2 group of PAP [44]. Therefore, the catalytic reduction of gold/ Au@Pt nanoparticles can also be studied by simply monitoring the change of intensity and location of typical absorption peak, associated with PNP and PAP, or tracing the UV-vis absorption spectra of the system with the evolution of reaction time. As depicted in Fig. 11(B) and (C), both the magnitude of the absorption peak at 399 nm gradually decreases at different rates. Fig. 11(B) shows that, under the condition of individual Au-based catalysis, the intensity of absorption peak decreases at 399 nm and increases at 311 nm within 3 min, and only 40 min is needed to complete the whole catalytic process. However, under the condition of Au@Pt-based catalysis, it takes about 2 h to complete the whole catalytic process. As proved in this study, the individual Au NPs obtained through GA as both a protecting and reducing agent performs superior catalytic activity compared with the as-prepared Au@Pt bimetallic NPs. The initially-reduced Au cores are covered with a thick Pt shell coating or the exposed Au atoms for catalytic reduction become fewer, which can be used to explain the lower catalytic activity of Au@Pt nanostructures. In reverse, the study of PNP-PAP catalytic reduction, as an indirect evidence, just confirms the formation of Au@Pt core-shell nanostructures. Some further researches involving the catalytic activity of nano-Pt coming from Au@Pt core-shell structures and individual Pt nanocrystals are on the way.
Fig.11.
4. Conclusion This paper puts forward a new and effective method to fabricate Au@Pt nanocrystals through green synthesis method by using gallic acid as both a reducing and protecting agent, which overcomes the barrier of plant extracts reducing H2PtCl6 to individual Pt NPs under the usual condition. When GA is used as both a reducing and protecting agent, urchin-like Au@Pt core-shell NPs by chemical reduction of
15
HAuCl4 and H2PtCl6 salts can be successfully fabricated, in which the seeds (cores) of Au NPs are firstly formed and then PtCl62 ions are chemically reduced on the core surface to form a Pt shell layer. In this research, the effects of the reaction temperature and Au/Pt molar ratios on the Au@Pt core-shell NPs are systematically investigated. The results show that, with the increase of the reaction temperature, the reduction rate obviously accelerates and the particle size decreases along with fine dispersion. And upon lowering the molar ratio of Au/Pt, the morphology of Au@Pt nanocrystals varies from Au polygon to Pt sphere and the Pt shell becomes thicker, accompanied by the 2θ locations indexed to the crystal facet (111) shift from Au to Pt. The characteristic SPR of Au nanocrystals gradually disappears owning to the Pt shell effect in the corresponding
UV-vis
spectrum.
Meanwhile,
after
adding
PVP,
the
quasi-monodisperse urchin-like Au@Pt NPs possessing smaller particle size are obtained, compared with PVP-free ones. In the reaction of NaBH4 reducing p-nitrophenol (PNP) to p-aminophenol (PAP), the as-prepared Au@Pt core-shell NPs have better catalytic activity, however, this catalytic activity is lower than that of pure Au NPs obtained with the same method, which just confirms the formation of Au@Pt core-shell nanostructures because of the coverage of Pt-shell on the surface of Au core with high catalytic activity.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (Grant No. 21273194) and Priority Academic Program Development of Jiangsu Higher Education Institutions, China. The authors also give thanks to Test Center of Yangzhou University for helping the tests of nanomaterial structure.
16
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18
(a)
(c)
(b)
(111)
Fig. 1. TEM image (a), HR-TEM image (b) and Selected-area electron diffraction (SAED) (c) patterns of as-synthesized Au@Pt nanostructures. The scale bars in (a) and (b) are 100 nm and 2 nm, respectively.
(b)
(220)
(200)
(a)
20
30
40
50
2 (degree)
60
70
80
(c)
Fig. 2. XRD pattern (a), FE-SEM image(b) and EDX image(c) of the as-prepared Au@Pt core-shell nanostructures prepared from the typical synthesis.
19
(a)
(b)
(c)
Fig. 3. TEM images of Au@Pt nanostructures prepared (R=1:1) using GA as both a reducing and protecting agent at different temperatures (a) 30C, (b) 60C, (c) 100C. (All scale bars are 100 nm.)
(b)
(a)
(c)
Fig. 4. TEM images of Au@Pt nanostructures prepared using GA as both a reducing and protecting agent at 100 C, R= 1:9 (a), 1:1 (b) and 9:1 (c). (All scale bars are 100 nm.)
20
1.0
Absorbance(a.u.)
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(a) (c)
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(b)
0.2 0.0 300
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Fig. 5. UV-vis absorption spectra of the Au@Pt nanocolloids prepared using GA as
40.0
(220)
(e) (d) (c)
39.8
(B)
39.6
2D e g re e
39.4
(311)
(A)
(200)
(111)
both a reducing and protecting agent at 100C, R= 9:1 (a), 1:1 (b) and 1:9 (c).
(b)
39.2 39.0 38.8 38.6 38.4 38.2
(a)
38.0 0.0 20
30
40
50
2 (degree)
60
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0.2
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0.6
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xPt
Fig. 6. (A) XRD patterns of Au (a), Au(9)Pt(1) (b), Au(1)Pt(1) (c), Au(1)Pt(9) (d), Pt (e) and (B) The relationship between 2θ value (indexed to the crystal facet (111)) and the molar fraction of Pt(xPt).
21
(b)
(a)
(c)
Fig. 7. TEM images of the Au@Pt nanocrystals prepared by varying GA/(Au/Pt) molar ratios (R1) from 1:2 (a) to 1:1 (b) and 10:1 (c), R=1:9. (All scale bars are 100 nm.)
3500
3000
2500
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1500
1078 1022 866
1383
1705 1616 1541 1429 1313
3448 3500
4000
3275
(a)
1632
(b)
1000
500
-1
Wavenumber (cm )
Fig. 8. FTIR spectra of GA(a) and GA-protected Au@Pt nanocrystals(b).
22
(a)
(b)
(d)
(c)
(e)
Fig. 9. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the typical synthesis using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm.)
(a)
(b)
(c)
(e)
(d)
Fig. 10. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the PVP (CPVP=0.010 g/L) introduction system, using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm except for (e) of 50 nm.)
23
2.5
(A)
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Absorbance(a.u.)
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Fig.11. (A) The UV-vis absorption spectra of the PNP solution (a), the blend of PNP and NaBH4 (b), adding Au colloids into PNP solution (c), complete reaction system after catalytic reduction(d). (B), (C) The UV-vis absorption spectra of the catalytic system taken at different reaction times, using as-prepared Au colloidal solution and Au@Pt nanocrystals as catalyst, respectively.
24
Fig. 1. TEM image (a), HR-TEM image (b) and Selected-area electron diffraction (SAED) (c) patterns of as-synthesized Au@Pt nanostructures. The scale bars in (a) and (b) are 100 nm and 2 nm, respectively.
Fig. 2. XRD pattern (a), FE-SEM image(b) and EDX image(c) of the as-prepared Au@Pt core-shell nanostructures prepared from the typical synthesis.
Fig. 3. TEM images of Au@Pt nanostructures prepared (R=1:1) using GA as both a reducing and protecting agent at different temperatures (a) 30C, (b) 60C, (c) 100C. (All scale bars are 100 nm.)
Fig. 4. TEM images of Au@Pt nanostructures prepared using GA as both a reducing and protecting agent at 100 C, R= 1:9 (a), 1:1 (b) and 9:1 (c). (All scale bars are 100 nm.)
Fig. 5. UV-vis absorption spectra of the Au@Pt nanocolloids prepared using GA as both a reducing and protecting agent at 100C, R= 9:1 (a), 1:1 (b) and 1:9 (c).
Fig. 6. (A) XRD patterns of Au (a), Au(9)Pt(1) (b), Au(1)Pt(1) (c), Au(1)Pt(9) (d), Pt (e) and (B) The relationship between 2θ value (indexed to the crystal facet (111)) and the molar fraction of Pt(xPt).
Fig. 7. TEM images of the Au@Pt nanocrystals prepared by varying GA/(Au/Pt) molar ratios (R1) from 1:2 (a) to 1:1 (b) and 10:1 (c), R=1:9. (All scale bars are 100 nm.)
Fig. 8. FTIR spectra of GA(a) and GA-protected Au@Pt nanocrystals(b). 25
Fig. 9. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the typical synthesis using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm.)
Fig. 10. TEM images of (a) Au nanocrystals, (b-d) Au@Pt nanocrystals and (e) Pt nanocrystals prepared from the PVP (CPVP=0.010 g/L) introduction system, using various Au/Pt molar ratios (b) 9:1, (c) 1:1, (d) 1:9. (All scale bars are 100 nm except for (e) of 50 nm.)
Fig.11. (A) The UV-vis absorption spectra of the PNP solution (a), the blend of PNP and NaBH4 (b), adding Au colloids into PNP solution (c), complete reaction system after catalytic reduction(d). (B), (C) The UV-vis absorption spectra of the catalytic system taken at different reaction times, using as-prepared Au colloidal solution and Au@Pt nanocrystals as catalyst, respectively.
Highlights 1. A facile green-synthesis was developed to fabricate Au@Pt core-shell nanoparticles. 2. Gallic acid was used as both a reducing and protecting agent in green-synthesis. 3. The initially-formed Au NPs played a role of seeds in HAuCl4 and H2PtCl6 mixture. 4. Adding PVP could decrease the size and enhance the monodispersity of Au@Pt NPs. 5. Au@Pt core-shell NPs have better catalytic activity for NaBH4 reducing PNP to PAP.
26
Green synthesis and characterization of Au@Pt core-shell bimetallic nanoparticles using gallic acid Guojun Zhang, Hongmei Zheng, Ming Shen *, Lei Wang, Xiaosan Wang
(a)
(c)
(b)
27