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Synthesis of hybrid Au–ZnO nanoparticles using a one pot polyol process
Materials Chemistry and Physics xxx (2014) 1e8
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Synthesis of hybrid AueZnO nanoparticles using a one pot polyol process Amine Mezni a, b, Adnen Mlayah b, Virginie Serin b, Leila Samia Smiri a, * a b
Unit e de recherche “Synth ese et Structure de Nanomat eriaux” UR11ES30, Facult e des Sciences de Bizerte, Universit e de Carthage, 7021 Jarzouna, Tunisia Centre d'Elaboration de Mat eriaux et d'Etudes Structurales, CNRS, UPR 8011, Universit e de Toulouse, 29 Rue Jeanne Marvig, 31055 Toulouse, France
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Hybrid AueZnO nanoparticles were synthesized by a novel one-pot synthesis method that makes use of 1,3propanediol. The polyol solvent 1,3-propanediol plays the roles of the reducing agent and the stabilizer layer. The AueZnO nanoparticles exhibit a strong localized surface plasmon resonance.
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Article history: Received 2 September 2013 Received in revised form 29 March 2014 Accepted 10 May 2014
In this work, we report on the synthesis of hybrid AueZnO nanoparticles using a one-pot chemical method that makes use of 1,3-propanediol as a solvent, a reducing agent and a stabilizing layer. The produced nanoparticles consisted of Au cores decorated with ZnO nanoparticles. The structure and morphology of the nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDX) and Raman spectroscopy. Optical extinction measurements, combined with numerical simulations, showed that the AueZnO nanoparticles exhibit a localized surface plasmon resonance (SPR) clearly red-shifted with respect to that of bare Au nanoparticles (AuNPs). This work contributes to the emergence of multi-functional nanomaterials with possible applications in surface plasmon resonance based biosensors, energy-conversion devices, and in water-splitting hydrogen production. © 2014 Published by Elsevier B.V.
Keywords: Chemical synthesis Composite materials Nanostructures Raman spectroscopy and scattering Optical properties
1. Introduction Hybrid metalesemiconductor nanoparticles have attracted great attention from both, the fundamental basic scientific and technological points of view [1e4]. These nanocomposite materials not only combine the unique properties of the metal and the semiconductor, but can also generate new properties due to the metalesemiconductor interface in coreeshell and Janus
* Corresponding author. E-mail address: [email protected] (L.S. Smiri).
nanoparticles for instance. Indeed, the presence of such an interface can enhance the light absorption in the semiconductor and promote effective charge separation which is required for efficient photocatalytic applications [5e8]. AueZnO nanoparticles are nontoxic, biocompatible and chemically stable, and can thus be used in diverse areas such as multi-modal biological detection [9], catalysis [10], solar energy conversion [11] and opto-electronics applications [12]. Several chemistry-based methods have been proposed to synthesize hybrid metalesemiconductor nanoparticles [13e28]. The main difficulty in preparing such particles lies in the weak interaction between the semiconductor (e.g., ZnO, CdSe and TiO2) and
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Please cite this article in press as: A. Mezni, et al., Synthesis of hybrid AueZnO nanoparticles using a one pot polyol process, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.022
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the metal (e.g., Au, Ag, Pt, and Pd). The key to form AueZnO hybrid nanomaterials is to promote the heterogeneous growth of ZnO (or AuNPs) on Au (or ZnO) seeds. Recently, Li et al. [15] successfully prepared AueZnO hybrid nanoparticles with hexagonal pyramidlike structure. The controlled synthesis of AueZnO hybrid NPs was based on the heterogeneous nucleation and the selective growth of ZnO on pre-synthesized Au seeds dispersed in hexane. Lee et al. [18] synthesized AueZnO nanoparticles through the nucleation and decomposition of zinc hydroxide on the surface of NaBH4-reduced AuNPs in ethanol. Xin Wang et al. [25] used ZnO nanocrystals as the seeding material for the nucleation and growth of citrate-reduced gold to form water-soluble dumbbell-shaped ZnOeAuNPs. Another strategy for the synthesis of AueZnO nanoparticles consists in using specific ligands as linkers with the affinity of their functional groups to AuNPs and ZnO surfaces. Xianghong Liu et al. [26] reported a facile method for the assembly of noble metal (Au and Pt) nanoparticles onto ZnO rods using a green non-toxic reagent amino acid lysine, with two amino functional groups as the capping agent. Exploiting the affinity of thiol functional groups to AuNPs and ZnO surfaces, Whitten et al. [27] reported a simple strategy to synthesize AueZnO nanoparticles by using dithiol as the linker ligand. So, in most cases, the synthesis of stable colloids of AueZnO nanoparticles requires the use of stabilizers or capping agents, several reagents and the reaction is generally carried out in several steps. In this work, we report the synthesis of ZnO-decorated AuNPs formed in one-pot in 1,3-propanediol by using only gold (III) and zinc (II) precursors without the addition of any other reagents, template or complex metal ligand. The polyol solvent plays the role of both a complexant and a surfactant agent which adsorbs on the nanoparticles' surface, thus preventing their agglomeration.
flask equipped with a condenser, a mechanical stirrer and a thermograph. 0.038 mmol of HAuCl4$3H2O (purchased from Aldrich) were mixed with 0.17 mmol of zinc acetate dehydrate [(Zn(OAc)2$2H2O), Aldrich, AR grade] in 50 ml of 1,3-propanediol (ACROS Organics, 98%) with vigorous stirring. First, the mixture was slowly heated to 150 C and kept at that temperature for 10 min, then heated to 160 C and kept at this temperature for 1 h. A violet homogeneous colloidal suspension was obtained. After cooling down to room temperature, the product was separated by centrifugation, washed several times with ethanol/acetone (2:1) and re-dispersed in ethanol. After the centrifugation, the supernatant solution became clear and colorless, indicating that no AuNPs were present in the solution.
2. Experimental procedure
The optical absorption spectra were acquired using a PerkineElmer Lambda 11 UV/VIS spectrophotometer. The Raman experiments were carried out using a HoribaeJobineYvon XY spectrometer. We used the 363 nm Argon laser line to excite the Raman spectra close to the band gap of ZnO. The laser beam was focused onto the sample using a 50X objective. The laser intensity was kept as low as possible in order to avoid sample heating and degradation.
2.1. Synthesis of ZnO-decorated AuNPs The synthesis of the ZnO-decorated AuNPs was achieved in a one-pot chemical process where gold salt HAuCl4 and zinc (II) acetate were mixed together in 1,3-propanediol (C3H8O2). The sequential reactions were thermally controlled. Au nanoparticles were initially formed by thermal reduction of the Au precursor at a moderate temperature. The thermal decomposition of zinc acetate dehydrate (Zn(OAc)2$2H2O) onto the surface of the preformed Au nanoparticles occurred at a high temperature. The synthesis was carried out in a 100 ml three-neck
2.2. Structural and morphological characterization The crystallographic structure of the so-obtained powder was characterized by X-ray diffraction (XRD) (using an INEL diffractometer equipped with a cobalt anticathode (l ¼ 1.7890Å). The crystallite sizes were calculated using Scherrer's relation L ¼ 0.94l/ bcos q, where l is the wavelength of the X-ray radiation, q and b are, respectively, the Bragg angle and full-width at half-maximum (FWHM) of the diffraction peak. The morphology of the particles was determined by transmission electron microscopy (TEM) performed using a JEOL-JFC 1600 microscope operating at 100 keV accelerating voltage. The chemical composition of the ZnOdecorated AuNPs was determined by energy-dispersive X-ray spectroscopy (EDX) attached to the TEM. High-resolution TEM (HRTEM) (Philips Tecnai F-20 SACTEM operating at 200 kV) images provided further insight into the structure of the AueZnO NPs. 2.3. Optical characterization
2.4. DDA simulations Simulations of the optical extinction spectra were performed using the discrete dipole approximation (DDA) implemented in
Fig. 1. XRD patterns of pure ZnO NPs and ZnO-decorated AuNPs.
Please cite this article in press as: A. Mezni, et al., Synthesis of hybrid AueZnO nanoparticles using a one pot polyol process, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.022
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Fig. 2. Raman scattering spectra of pure ZnO NPs and ZnO-decorated AuNPs excited at 363 nm.
DDSCAT 7.2.2 software [29,30]. The wavelength-dependent refractive indices of gold and bulk zinc oxide were taken from Johnson and Christy [31] and from Hisashi et al. [32], respectively. 3. Results and discussion 3.1. Structural characterization The crystalline phase of the nanoparticles was determined by XRD. Fig. 1 shows the XRD patterns of pure ZnO NPs and ZnOdecorated AuNPs. All the diffraction peaks of ZnO can be indexed to hexagonal wurtzite ZnO with the strong (100), (002), (101)
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characteristic peaks (JCPDS No. 36-1451). The XRD pattern of the ZnO-decorated AuNPs is very similar to that of pure ZnO NPs, thus indicating that the formation of Au in the reaction process has no influence on the crystal structure of the ZnO. Besides the diffraction peaks of ZnO NPs, three additional diffraction peaks were present at 2q ¼ 38.33, 44.21 and 64.79 and were assigned, respectively, to the diffraction lines of the (111), (200) and (220) planes of FCC gold (JCPDS No. 65-2870) [25]. The XRD results indicated that the synthesized nanoparticles have good crystallinity. The average size of the crystallites, estimated from the FWHM of the (100) diffraction peak using Scherrer's relation, was of the order of 3.5 nm for the ZnO NPs. Further evidence for the formation of wurtzite ZnO around the Au cores was provided by the Raman scattering spectra shown in Fig. 2. In wurtzite ZnO, phonons with A1, E1 and E2 symmetries are Raman active. Under resonant excitation, first and second order resonant Raman scattering by E1(LO) phonons is dominant [33,34] as seen in Fig. 2. These Raman peaks are clearly visible thus attesting the good crystal quality of the ZnO nanoparticles surrounding the Au cores [35]. However, their linewidths are larger (nearly by a factor of 2) than those observed in pure ZnO nanoparticles. Moreover, the Raman peaks of the AueZnO nanoparticles are shifted towards lower wavenumbers as compared to pure ZnO nanoparticles; this is more pronounced for the 2E1(LO) second harmonic peak. The fact that the Raman peaks are broader and shifted in the case of AueZnO nanoparticles can be attributed to the AueZnO interface. Indeed, it is well known that impurities, structural defects and interfaces may lead to phonon localization that results in the Raman peak broadening and shift, and which are directly connected with the phonon dispersion curve [36]. In the case of E1(LO) phonons, the vibrational frequency decreases with increasing wavevector [37]. Hence, a down shift and a broadening of the Raman line are expected in the case of E1(LO) phonons. It is interesting to notice that the Raman scattering is enhanced in the
Fig. 3. TEM images (a, b) of the ZnO-decorated AuNPs. The histogram in (c) shows the size distribution of the Au core.
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Fig. 4. EDX spectrum of the ZnO-decorated AuNPs.
AueZnO NPs relative to pure ZnO NPs. Wang et al. [25] also observed the enhancement of the Raman scattering in bifunctional ZnOeAu nanocomposites. The reason may be the electric field at the AueZnO interface which increases the electronephonon interaction and hence the Raman intensity [38]. As shown by the transmission electron microscopy, presented in Fig. 3, the nanoparticles consist mainly of faceted quasi-spherical gold cores (around 10e40 nm) surrounded by closely packed zinc oxide NPs (3e5 nm) forming a decoration. The size distribution of the Au core is shown in Fig. 3c. The average size of the Au core is around 23.2 nm. The EDX spectrum presented in Fig. 4, indicates that the nanoparticles are of a high purity, since only Au, O and Zn elements are detected. The presence of Cu is due to the copper grid used for the
TEM/EDX experiments, the Fe and Co elements are due to the pole components of the microscope. The high-resolution TEM (HRTEM) images of the ZnO-decorated AuNPs are shown in Fig. 5. A strong interface contiguity between the Au and the ZnO nanoparticles is observed. From the HRTEM image (Fig. 5c, d), the interplanar spacing of the ZnO (002) atomic plane is 0.26 nm, in agreement with the wurtzite ZnO structure [25]. On the other hand, it is evident from the HRTEM images that the AuNPs ” fringes which suggest that the Au and ZnO lattices exhibit “moire are gradually displaced from each other or rotated. This indicates that the Au/ZnO interface is not coherent, which is often the case in lattice-mismatched metal-oxide systems [39]. As a matter of fact, the epitaxial nucleation and growth of ZnO on the surface of Au was not observed in this study. The reason why hybrid AueZnO NPs do
Fig. 5. Low resolution TEM image (a) and HRTEM of ZnO-decorated AuNPs (bed).
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Fig. 6. Schematic of the formation mechanism of the ZnO-decorated AuNPs in 1,3-propanediol.
Fig. 7. (a) TEM images of AueZnO NPs prepared with a higher zinc acetate concentration (10), (b) selected area diffraction (SAED) patterns of AueZnO, (c) high-resolution transmission electron microscope (HRTEM) images and (d) typical EELS spectra from selected nanoparticles.
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Fig. 8. Secondary electrons STEM image and EDX maps at the Zn, O and Au signals. The lower panel shows the EDX profile of the Zn (green), O (blue) and Au (red) across the line shown in top right panel. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
not form a core/shell structure is the difference between their crystalline structures, i.e., Au is cubic and ZnO is hexagonal [18]. 3.2. Formation mechanism The observations showed that the solution color changed from yellow to purple at 100 C. This coloration was visible until the temperature reached 150 C. Above this temperature, and up to 160 , the solution became milky with some turbidity. These observations suggest a sequential formation process. First, the change of color from yellow to purple observed at 100 C reveals the formation of Au nanoparticles. Indeed, based on our previous study [40] and many other reports [41,42], the formation of AuNPs in a polyol medium is accompanied by a change in the colloidal solution color (e.g. from yellow to pale in 1,2
propanediol reflecting the formation of gold nanoplates [41], from yellow to blue in triethylene glycol [40], from light yellow to pinkish violet in glycerol [42]). The stability of these nanoparticles is ensured by the polyol molecules (1,3-propanediol in our case) grafted to the Au surface via electrostatic bonds [40]. With increasing temperature, the zinc acetate starts to decompose at the reactive Au surface to form an intermediate zinc hydroxide [43]. Indeed, at 150 C the solution becomes milky with some turbidity, this could be attributed to the spontaneous formation of zinc hydroxide arising from the reaction of 1,3propanediol with the zinc acetate [43]. The zinc hydroxide at the Au surface acts as a nucleation site for the growth of zinc oxide via inorganic polymerization reactions [44] thus leading to ZnO nuclei [43]. At 160 C, further nucleation and growth of the ZnO takes place. Because heterogeneous nucleation is preferred over
Please cite this article in press as: A. Mezni, et al., Synthesis of hybrid AueZnO nanoparticles using a one pot polyol process, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.022
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homogeneous nucleation [45], Zn ions tend to condense on preexisting zinc oxide seeds already present at the surface of the AuNPs, rather than forming new nuclei in the solution. We believe that the 1,3-propanediol solvent plays a crucial role in the formation of ZnO-decorated AuNPs. As a matter of fact, when the synthesis was conducted in ethylene glycol (EG) or diethylene glycol (DEG) using the same precursors, individual ZnO NPs and AuNPs were formed instead of hybrid AueZnO NPs. We suggest that the formation of the hybrid AueZnO NPs is based on an initial production of the Au nanoparticles followed by a hydroxylationecondensation process of an intermediate zinc hydroxide, onto the surface of the preformed Au nanoparticles. The proposed chemical reaction mechanism is described in Fig. 6. It can be divided into 4 steps: formation of AuNPs in the polyol solvent (a), decomposition of the Zn (II) precursor at the surface of the preformed AuNPs and formation of an intermediate zinc hydroxide (b), nucleation of ZnO seeds (c) and growth of the ZnO at the AuNPs surface leading to hybrid AueZnO NPs (d). When the reaction was carried out under the same experimental conditions but with the zinc acetate concentration increased by a factor 10, nanoparticles around 140 nm in size and with strong surface roughness are observed. The selected area electron diffraction (SAED) ring patterns clearly show the FCC Au and wurtzite ZnO phases (Fig. 7b). The High-Resolution TEM (HRTEM) image in Fig. 7c shows well-defined ZnO crystal planes thus corroborating the crystalline structure of the formed particles. The EELS spectra show (Fig. 7d) loss peaks corresponding to zinc (Zn L23) and oxygen (O K) only. In order to investigate more precisely the composition of these nanoparticles, we have performed a mapping of the EDX signal (Fig. 8). As can be seen, the nanoparticles consist mainly in ZnO. The Au atomic percentage does not exceed 1e2% as evidenced by the concentration profile in Fig. 8. It may reach 5% at some particular locations of the nanoparticle but in general the Au concentration is small. This shows that the control of the zinc acetate versus Au precursor concentration is critical since increasing the zinc acetate concentration does not result in a coreeshell AueZnO nanoparticles with a larger ZnO coverage.
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3.3. Optical properties The measured optical extinction spectrum of the ZnO-decorated AuNPs (corresponding to Figs. 3 and 5) is shown in Fig. 9. The spectrum of 25 nm spherical bare gold NPs is also shown for comparison. The extinction spectrum of the ZnO-decorated AuNPs clearly shows a surface plasmon resonance peaking at around 568 nm, red-shifted by 35 nm with respect to the surface plasmon resonance of the bare AuNPs. This red-shift is due to the increase in the optical index of the medium surrounding the gold cores [46]. Indeed, the optical index of (bulk) ZnO is around 2.1 (orientation averaged) which is much larger than that of ethanol (1.36). This red-shift is confirmed by the DDA simulations performed for 25 nm spherical AuNPs surrounded by a 10 nm spherical ZnO shell. The spectra calculated for the bare AuNPs and for the AueZnO coreeshell NPs are in satisfactory agreement with the measured ones. However, the experimental linewidth of the surface plasmon resonance is clearly larger than the simulated one probably because of inhomogeneous broadening due to size and shape fluctuations of the nanoparticles. 4. Conclusion Summing up, we have reported the synthesis of hybrid AueZnO nanoparticles with a controlled morphology and high crystalline quality using a one pot polyol process, without adding any other reagents, template or complex metal ligand. The 1,3-propanediol solvent plays the role of both a reducing agent and a stabilizing layer. The formation mechanism of ZnO NPs in 1,3-propanediol facilitates and guides the reaction to the formation of the ZnOdecorated AuNPs. The temperature control of the spontaneous nucleation step followed by a well-separated growth step, explain the homogeneity in size and morphology of the obtained hybrid nanoparticles. Because of the simplicity of the process, we believe that this strategy is suitable for the scalable production of hybrid nanocomposite materials. On the other hand, because of their unique structure, the ZnO-decorated AuNPs are expected to provide new insights into various application fields including plasmonic enhanced photocatalysis, hydrogen production via solar-light induced water splitting and for chemical and biological sensing, as well as for medical applications [47,48]. Acknowledgments Amine Mezni, gratefully acknowledges the support of the Ministry of Higher Education and Scientific Research of Tunisia. This work has also been supported by the CALMIP high-performance computing facilities center at the Paul Sabatier University of Toubastien Joulie for the EDX louse. The authors are also grateful to Se mapping measurements. References
Fig. 9. Measured and calculated Extinction spectra of bare Au nanoparticles and ZnOdecorated AuNPs dispersed in ethanol. The surface plasmon bands, in the measured spectra (continuous line) of the bare Au nanoparticles and ZnO-decorated AuNPs are peaking at 533 and 568 nm, respectively. The extinction spectra of bare Au nanoparticles and ZnOeAu coreeshell nanoparticles simulated using DDA are plotted as dashed lines. In the simulations, the diameter of the spherical Au core is 25 nm and the ZnO shell thickness is 10 nm.
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Please cite this article in press as: A. Mezni, et al., Synthesis of hybrid AueZnO nanoparticles using a one pot polyol process, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.022
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of hybrid AueZnO nanoparticles using a one pot polyol process Amine Mezni a, b, Adnen Mlayah b, Virginie Serin b, Leila Samia Smiri a, * a b
Unit e de recherche “Synth ese et Structure de Nanomat eriaux” UR11ES30, Facult e des Sciences de Bizerte, Universit e de Carthage, 7021 Jarzouna, Tunisia Centre d'Elaboration de Mat eriaux et d'Etudes Structurales, CNRS, UPR 8011, Universit e de Toulouse, 29 Rue Jeanne Marvig, 31055 Toulouse, France
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hybrid AueZnO nanoparticles were synthesized by a novel one-pot synthesis method that makes use of 1,3propanediol. The polyol solvent 1,3-propanediol plays the roles of the reducing agent and the stabilizer layer. The AueZnO nanoparticles exhibit a strong localized surface plasmon resonance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2013 Received in revised form 29 March 2014 Accepted 10 May 2014
In this work, we report on the synthesis of hybrid AueZnO nanoparticles using a one-pot chemical method that makes use of 1,3-propanediol as a solvent, a reducing agent and a stabilizing layer. The produced nanoparticles consisted of Au cores decorated with ZnO nanoparticles. The structure and morphology of the nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDX) and Raman spectroscopy. Optical extinction measurements, combined with numerical simulations, showed that the AueZnO nanoparticles exhibit a localized surface plasmon resonance (SPR) clearly red-shifted with respect to that of bare Au nanoparticles (AuNPs). This work contributes to the emergence of multi-functional nanomaterials with possible applications in surface plasmon resonance based biosensors, energy-conversion devices, and in water-splitting hydrogen production. © 2014 Published by Elsevier B.V.
Keywords: Chemical synthesis Composite materials Nanostructures Raman spectroscopy and scattering Optical properties
1. Introduction Hybrid metalesemiconductor nanoparticles have attracted great attention from both, the fundamental basic scientific and technological points of view [1e4]. These nanocomposite materials not only combine the unique properties of the metal and the semiconductor, but can also generate new properties due to the metalesemiconductor interface in coreeshell and Janus
* Corresponding author. E-mail address: [email protected] (L.S. Smiri).
nanoparticles for instance. Indeed, the presence of such an interface can enhance the light absorption in the semiconductor and promote effective charge separation which is required for efficient photocatalytic applications [5e8]. AueZnO nanoparticles are nontoxic, biocompatible and chemically stable, and can thus be used in diverse areas such as multi-modal biological detection [9], catalysis [10], solar energy conversion [11] and opto-electronics applications [12]. Several chemistry-based methods have been proposed to synthesize hybrid metalesemiconductor nanoparticles [13e28]. The main difficulty in preparing such particles lies in the weak interaction between the semiconductor (e.g., ZnO, CdSe and TiO2) and
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the metal (e.g., Au, Ag, Pt, and Pd). The key to form AueZnO hybrid nanomaterials is to promote the heterogeneous growth of ZnO (or AuNPs) on Au (or ZnO) seeds. Recently, Li et al. [15] successfully prepared AueZnO hybrid nanoparticles with hexagonal pyramidlike structure. The controlled synthesis of AueZnO hybrid NPs was based on the heterogeneous nucleation and the selective growth of ZnO on pre-synthesized Au seeds dispersed in hexane. Lee et al. [18] synthesized AueZnO nanoparticles through the nucleation and decomposition of zinc hydroxide on the surface of NaBH4-reduced AuNPs in ethanol. Xin Wang et al. [25] used ZnO nanocrystals as the seeding material for the nucleation and growth of citrate-reduced gold to form water-soluble dumbbell-shaped ZnOeAuNPs. Another strategy for the synthesis of AueZnO nanoparticles consists in using specific ligands as linkers with the affinity of their functional groups to AuNPs and ZnO surfaces. Xianghong Liu et al. [26] reported a facile method for the assembly of noble metal (Au and Pt) nanoparticles onto ZnO rods using a green non-toxic reagent amino acid lysine, with two amino functional groups as the capping agent. Exploiting the affinity of thiol functional groups to AuNPs and ZnO surfaces, Whitten et al. [27] reported a simple strategy to synthesize AueZnO nanoparticles by using dithiol as the linker ligand. So, in most cases, the synthesis of stable colloids of AueZnO nanoparticles requires the use of stabilizers or capping agents, several reagents and the reaction is generally carried out in several steps. In this work, we report the synthesis of ZnO-decorated AuNPs formed in one-pot in 1,3-propanediol by using only gold (III) and zinc (II) precursors without the addition of any other reagents, template or complex metal ligand. The polyol solvent plays the role of both a complexant and a surfactant agent which adsorbs on the nanoparticles' surface, thus preventing their agglomeration.
flask equipped with a condenser, a mechanical stirrer and a thermograph. 0.038 mmol of HAuCl4$3H2O (purchased from Aldrich) were mixed with 0.17 mmol of zinc acetate dehydrate [(Zn(OAc)2$2H2O), Aldrich, AR grade] in 50 ml of 1,3-propanediol (ACROS Organics, 98%) with vigorous stirring. First, the mixture was slowly heated to 150 C and kept at that temperature for 10 min, then heated to 160 C and kept at this temperature for 1 h. A violet homogeneous colloidal suspension was obtained. After cooling down to room temperature, the product was separated by centrifugation, washed several times with ethanol/acetone (2:1) and re-dispersed in ethanol. After the centrifugation, the supernatant solution became clear and colorless, indicating that no AuNPs were present in the solution.
2. Experimental procedure
The optical absorption spectra were acquired using a PerkineElmer Lambda 11 UV/VIS spectrophotometer. The Raman experiments were carried out using a HoribaeJobineYvon XY spectrometer. We used the 363 nm Argon laser line to excite the Raman spectra close to the band gap of ZnO. The laser beam was focused onto the sample using a 50X objective. The laser intensity was kept as low as possible in order to avoid sample heating and degradation.
2.1. Synthesis of ZnO-decorated AuNPs The synthesis of the ZnO-decorated AuNPs was achieved in a one-pot chemical process where gold salt HAuCl4 and zinc (II) acetate were mixed together in 1,3-propanediol (C3H8O2). The sequential reactions were thermally controlled. Au nanoparticles were initially formed by thermal reduction of the Au precursor at a moderate temperature. The thermal decomposition of zinc acetate dehydrate (Zn(OAc)2$2H2O) onto the surface of the preformed Au nanoparticles occurred at a high temperature. The synthesis was carried out in a 100 ml three-neck
2.2. Structural and morphological characterization The crystallographic structure of the so-obtained powder was characterized by X-ray diffraction (XRD) (using an INEL diffractometer equipped with a cobalt anticathode (l ¼ 1.7890Å). The crystallite sizes were calculated using Scherrer's relation L ¼ 0.94l/ bcos q, where l is the wavelength of the X-ray radiation, q and b are, respectively, the Bragg angle and full-width at half-maximum (FWHM) of the diffraction peak. The morphology of the particles was determined by transmission electron microscopy (TEM) performed using a JEOL-JFC 1600 microscope operating at 100 keV accelerating voltage. The chemical composition of the ZnOdecorated AuNPs was determined by energy-dispersive X-ray spectroscopy (EDX) attached to the TEM. High-resolution TEM (HRTEM) (Philips Tecnai F-20 SACTEM operating at 200 kV) images provided further insight into the structure of the AueZnO NPs. 2.3. Optical characterization
2.4. DDA simulations Simulations of the optical extinction spectra were performed using the discrete dipole approximation (DDA) implemented in
Fig. 1. XRD patterns of pure ZnO NPs and ZnO-decorated AuNPs.
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Fig. 2. Raman scattering spectra of pure ZnO NPs and ZnO-decorated AuNPs excited at 363 nm.
DDSCAT 7.2.2 software [29,30]. The wavelength-dependent refractive indices of gold and bulk zinc oxide were taken from Johnson and Christy [31] and from Hisashi et al. [32], respectively. 3. Results and discussion 3.1. Structural characterization The crystalline phase of the nanoparticles was determined by XRD. Fig. 1 shows the XRD patterns of pure ZnO NPs and ZnOdecorated AuNPs. All the diffraction peaks of ZnO can be indexed to hexagonal wurtzite ZnO with the strong (100), (002), (101)
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characteristic peaks (JCPDS No. 36-1451). The XRD pattern of the ZnO-decorated AuNPs is very similar to that of pure ZnO NPs, thus indicating that the formation of Au in the reaction process has no influence on the crystal structure of the ZnO. Besides the diffraction peaks of ZnO NPs, three additional diffraction peaks were present at 2q ¼ 38.33, 44.21 and 64.79 and were assigned, respectively, to the diffraction lines of the (111), (200) and (220) planes of FCC gold (JCPDS No. 65-2870) [25]. The XRD results indicated that the synthesized nanoparticles have good crystallinity. The average size of the crystallites, estimated from the FWHM of the (100) diffraction peak using Scherrer's relation, was of the order of 3.5 nm for the ZnO NPs. Further evidence for the formation of wurtzite ZnO around the Au cores was provided by the Raman scattering spectra shown in Fig. 2. In wurtzite ZnO, phonons with A1, E1 and E2 symmetries are Raman active. Under resonant excitation, first and second order resonant Raman scattering by E1(LO) phonons is dominant [33,34] as seen in Fig. 2. These Raman peaks are clearly visible thus attesting the good crystal quality of the ZnO nanoparticles surrounding the Au cores [35]. However, their linewidths are larger (nearly by a factor of 2) than those observed in pure ZnO nanoparticles. Moreover, the Raman peaks of the AueZnO nanoparticles are shifted towards lower wavenumbers as compared to pure ZnO nanoparticles; this is more pronounced for the 2E1(LO) second harmonic peak. The fact that the Raman peaks are broader and shifted in the case of AueZnO nanoparticles can be attributed to the AueZnO interface. Indeed, it is well known that impurities, structural defects and interfaces may lead to phonon localization that results in the Raman peak broadening and shift, and which are directly connected with the phonon dispersion curve [36]. In the case of E1(LO) phonons, the vibrational frequency decreases with increasing wavevector [37]. Hence, a down shift and a broadening of the Raman line are expected in the case of E1(LO) phonons. It is interesting to notice that the Raman scattering is enhanced in the
Fig. 3. TEM images (a, b) of the ZnO-decorated AuNPs. The histogram in (c) shows the size distribution of the Au core.
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Fig. 4. EDX spectrum of the ZnO-decorated AuNPs.
AueZnO NPs relative to pure ZnO NPs. Wang et al. [25] also observed the enhancement of the Raman scattering in bifunctional ZnOeAu nanocomposites. The reason may be the electric field at the AueZnO interface which increases the electronephonon interaction and hence the Raman intensity [38]. As shown by the transmission electron microscopy, presented in Fig. 3, the nanoparticles consist mainly of faceted quasi-spherical gold cores (around 10e40 nm) surrounded by closely packed zinc oxide NPs (3e5 nm) forming a decoration. The size distribution of the Au core is shown in Fig. 3c. The average size of the Au core is around 23.2 nm. The EDX spectrum presented in Fig. 4, indicates that the nanoparticles are of a high purity, since only Au, O and Zn elements are detected. The presence of Cu is due to the copper grid used for the
TEM/EDX experiments, the Fe and Co elements are due to the pole components of the microscope. The high-resolution TEM (HRTEM) images of the ZnO-decorated AuNPs are shown in Fig. 5. A strong interface contiguity between the Au and the ZnO nanoparticles is observed. From the HRTEM image (Fig. 5c, d), the interplanar spacing of the ZnO (002) atomic plane is 0.26 nm, in agreement with the wurtzite ZnO structure [25]. On the other hand, it is evident from the HRTEM images that the AuNPs ” fringes which suggest that the Au and ZnO lattices exhibit “moire are gradually displaced from each other or rotated. This indicates that the Au/ZnO interface is not coherent, which is often the case in lattice-mismatched metal-oxide systems [39]. As a matter of fact, the epitaxial nucleation and growth of ZnO on the surface of Au was not observed in this study. The reason why hybrid AueZnO NPs do
Fig. 5. Low resolution TEM image (a) and HRTEM of ZnO-decorated AuNPs (bed).
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Fig. 6. Schematic of the formation mechanism of the ZnO-decorated AuNPs in 1,3-propanediol.
Fig. 7. (a) TEM images of AueZnO NPs prepared with a higher zinc acetate concentration (10), (b) selected area diffraction (SAED) patterns of AueZnO, (c) high-resolution transmission electron microscope (HRTEM) images and (d) typical EELS spectra from selected nanoparticles.
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Fig. 8. Secondary electrons STEM image and EDX maps at the Zn, O and Au signals. The lower panel shows the EDX profile of the Zn (green), O (blue) and Au (red) across the line shown in top right panel. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
not form a core/shell structure is the difference between their crystalline structures, i.e., Au is cubic and ZnO is hexagonal [18]. 3.2. Formation mechanism The observations showed that the solution color changed from yellow to purple at 100 C. This coloration was visible until the temperature reached 150 C. Above this temperature, and up to 160 , the solution became milky with some turbidity. These observations suggest a sequential formation process. First, the change of color from yellow to purple observed at 100 C reveals the formation of Au nanoparticles. Indeed, based on our previous study [40] and many other reports [41,42], the formation of AuNPs in a polyol medium is accompanied by a change in the colloidal solution color (e.g. from yellow to pale in 1,2
propanediol reflecting the formation of gold nanoplates [41], from yellow to blue in triethylene glycol [40], from light yellow to pinkish violet in glycerol [42]). The stability of these nanoparticles is ensured by the polyol molecules (1,3-propanediol in our case) grafted to the Au surface via electrostatic bonds [40]. With increasing temperature, the zinc acetate starts to decompose at the reactive Au surface to form an intermediate zinc hydroxide [43]. Indeed, at 150 C the solution becomes milky with some turbidity, this could be attributed to the spontaneous formation of zinc hydroxide arising from the reaction of 1,3propanediol with the zinc acetate [43]. The zinc hydroxide at the Au surface acts as a nucleation site for the growth of zinc oxide via inorganic polymerization reactions [44] thus leading to ZnO nuclei [43]. At 160 C, further nucleation and growth of the ZnO takes place. Because heterogeneous nucleation is preferred over
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homogeneous nucleation [45], Zn ions tend to condense on preexisting zinc oxide seeds already present at the surface of the AuNPs, rather than forming new nuclei in the solution. We believe that the 1,3-propanediol solvent plays a crucial role in the formation of ZnO-decorated AuNPs. As a matter of fact, when the synthesis was conducted in ethylene glycol (EG) or diethylene glycol (DEG) using the same precursors, individual ZnO NPs and AuNPs were formed instead of hybrid AueZnO NPs. We suggest that the formation of the hybrid AueZnO NPs is based on an initial production of the Au nanoparticles followed by a hydroxylationecondensation process of an intermediate zinc hydroxide, onto the surface of the preformed Au nanoparticles. The proposed chemical reaction mechanism is described in Fig. 6. It can be divided into 4 steps: formation of AuNPs in the polyol solvent (a), decomposition of the Zn (II) precursor at the surface of the preformed AuNPs and formation of an intermediate zinc hydroxide (b), nucleation of ZnO seeds (c) and growth of the ZnO at the AuNPs surface leading to hybrid AueZnO NPs (d). When the reaction was carried out under the same experimental conditions but with the zinc acetate concentration increased by a factor 10, nanoparticles around 140 nm in size and with strong surface roughness are observed. The selected area electron diffraction (SAED) ring patterns clearly show the FCC Au and wurtzite ZnO phases (Fig. 7b). The High-Resolution TEM (HRTEM) image in Fig. 7c shows well-defined ZnO crystal planes thus corroborating the crystalline structure of the formed particles. The EELS spectra show (Fig. 7d) loss peaks corresponding to zinc (Zn L23) and oxygen (O K) only. In order to investigate more precisely the composition of these nanoparticles, we have performed a mapping of the EDX signal (Fig. 8). As can be seen, the nanoparticles consist mainly in ZnO. The Au atomic percentage does not exceed 1e2% as evidenced by the concentration profile in Fig. 8. It may reach 5% at some particular locations of the nanoparticle but in general the Au concentration is small. This shows that the control of the zinc acetate versus Au precursor concentration is critical since increasing the zinc acetate concentration does not result in a coreeshell AueZnO nanoparticles with a larger ZnO coverage.
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3.3. Optical properties The measured optical extinction spectrum of the ZnO-decorated AuNPs (corresponding to Figs. 3 and 5) is shown in Fig. 9. The spectrum of 25 nm spherical bare gold NPs is also shown for comparison. The extinction spectrum of the ZnO-decorated AuNPs clearly shows a surface plasmon resonance peaking at around 568 nm, red-shifted by 35 nm with respect to the surface plasmon resonance of the bare AuNPs. This red-shift is due to the increase in the optical index of the medium surrounding the gold cores [46]. Indeed, the optical index of (bulk) ZnO is around 2.1 (orientation averaged) which is much larger than that of ethanol (1.36). This red-shift is confirmed by the DDA simulations performed for 25 nm spherical AuNPs surrounded by a 10 nm spherical ZnO shell. The spectra calculated for the bare AuNPs and for the AueZnO coreeshell NPs are in satisfactory agreement with the measured ones. However, the experimental linewidth of the surface plasmon resonance is clearly larger than the simulated one probably because of inhomogeneous broadening due to size and shape fluctuations of the nanoparticles. 4. Conclusion Summing up, we have reported the synthesis of hybrid AueZnO nanoparticles with a controlled morphology and high crystalline quality using a one pot polyol process, without adding any other reagents, template or complex metal ligand. The 1,3-propanediol solvent plays the role of both a reducing agent and a stabilizing layer. The formation mechanism of ZnO NPs in 1,3-propanediol facilitates and guides the reaction to the formation of the ZnOdecorated AuNPs. The temperature control of the spontaneous nucleation step followed by a well-separated growth step, explain the homogeneity in size and morphology of the obtained hybrid nanoparticles. Because of the simplicity of the process, we believe that this strategy is suitable for the scalable production of hybrid nanocomposite materials. On the other hand, because of their unique structure, the ZnO-decorated AuNPs are expected to provide new insights into various application fields including plasmonic enhanced photocatalysis, hydrogen production via solar-light induced water splitting and for chemical and biological sensing, as well as for medical applications [47,48]. Acknowledgments Amine Mezni, gratefully acknowledges the support of the Ministry of Higher Education and Scientific Research of Tunisia. This work has also been supported by the CALMIP high-performance computing facilities center at the Paul Sabatier University of Toubastien Joulie for the EDX louse. The authors are also grateful to Se mapping measurements. References
Fig. 9. Measured and calculated Extinction spectra of bare Au nanoparticles and ZnOdecorated AuNPs dispersed in ethanol. The surface plasmon bands, in the measured spectra (continuous line) of the bare Au nanoparticles and ZnO-decorated AuNPs are peaking at 533 and 568 nm, respectively. The extinction spectra of bare Au nanoparticles and ZnOeAu coreeshell nanoparticles simulated using DDA are plotted as dashed lines. In the simulations, the diameter of the spherical Au core is 25 nm and the ZnO shell thickness is 10 nm.
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