Plasma Based Synthesis, Electron Microscopy, and Optical

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Plasma based Synthesis, Electron Microscopy and Optical Characterisation of Au-, Ag and Ag/Au- Core-Shell Nanoparticles Sandra Peglow, Marga-Martina Pohl, Angela Kruth, and Volker Bruser J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508818z • Publication Date (Web): 04 Dec 2014 Downloaded from http://pubs.acs.org on December 6, 2014

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The Journal of Physical Chemistry

Plasma Based Synthesis, Electron Microscopy and Optical Characterisation of Au-, Ag- and Ag/AuCore-Shell Nanoparticles Sandra Peglow 1, Marga-Martina Pohl2, Angela Kruth1 and Volker Brüser*1 1

Leibniz-Institute for Plasma Science and Technology e.V., Felix-Hausdorff-Str. 2, 17489

Greifswald 2

Leibniz-Institute for Catalysis e.V., Albert-Einstein-Str. 29a, 18059 Rostock

ABSTRACT Metal nanoparticles are commonly used for a wide range of applications. This paper presents a novel synthesis method using plasma-vapour-deposition (PVD) and a subsequent thermal treatment to produce homogenous particles within the nanometer range. A combination of high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) studies as well as ultraviolet/visible light (UV/Vis) transmission measurements are employed in order to characterise crystallographic, microstructural and optical properties of gold, silver and core-shell bimetallic nanostructures on a titania surface. The nanostructural size and composition of the core-shell nanostructures and therefore their optical properties could be directly adjusted by the plasma process parameters and/or the post-deposition thermal treatment of the samples.

ACS Paragon Plus Environment

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KEYWORDS plasma-vapour-deposition (PVD), thermal annealing, bimetallic core-shell nanoparticle, solid state dewetting 1. Introduction At nanometer scale, the concept of quantum mechanics and the influence of the surface become increasingly dominant, leading to new physical and chemical properties that differ greatly from the bulk materials1. Such metal nanoparticles are therefore used in a variety of applications such as cancer treatment2, surface plasmon resonance spectroscopy3, environmental protection4, biochemical sensing5 information technology6 and photocatalytic applications7. Methods for preparations of nanoparticles can be roughly divided into three main synthesis pathways: (i) mechanically commencing from a solid bulk material, (ii) via chemical reaction of liquid precursors and (iii) by vapour condensation8. The most common way of producing gold nanoparticles is via chemical methods. Amongst them are established techniques such as the Turkevich method9 where a gold hydrochlorate (H[AuCl4]) solution is reduced and stabilised by citrate under high temperature. For the production of gold nanoparticles in organic liquids the Brust method10 is applied, with chlorauric acid solution being reduced by NaBH4 and tetraoctylammonium bromide (TOABr) being responsible for the phase transfer and the stabilisation11. Similar approaches are taken for the silver nanoparticle synthesis12. In the 1990s, the term "core-shell" was widely established for concentric multilayer nanoparticles13. In contrast to single component nanoparticles, core-shell particle consist of a core and an outer layer composed of a different material from that of the shell. Typical core-shell materials beside core shell semiconductor nanocrystals are metals such as gold and silver exhibiting similar lattice parameters enabling a good match of the crystal lattices and preventing


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interfacial stress14. The shape of such hybrid particles may vary from spherical to cubic, prismatic, hexagonal or octahedral and from a disk to a wire or a tube. The combination sequence and elemental ratio of the two (or more) materials that are used in core-shell structures depends on the application. So, for instance, the surface properties of the core such as its reactivity, stability, functionality and dispersibility of the nanostructures in solvents can be changed by applying the outer shell layer. Also, by coating noble metal shell layers on top of inexpensive cores, the amount of precious metal needed for certain applications can be decreased.

Common fields of application for core-shell particles are biomedicine (bio-imaging15, drug delivery16 and controlled release17), pharmacy18, catalysis19, electronics, photonic crystals20 and photoluminescence15,

21

. A gold shell is often used to protect the core from oxidation and

corrosion; furthermore it is biocompatible and shows preferred optical properties8 whereas a silver outer layer has antibacterial effects22. Using two metals that show plasmonic behaviour enables a shift of the plasmon resonance frequency of the core-shell particle between the resonance frequencies of both materials involved. By tuning of the elemental ratio, the resonance frequency may even be shifted to a desired region of the absorption band. However, with regard to absorption properties, the influence of the shells is usually dominant over the core material23. For the synthesis of core-shell particles electron beam lithographic techniques24, laser-beam25, mechanical

processing26,

chemical

vapour

deposition27,

galvanic

replacement28,

coprecipitation29, templated growth30, laser-induced assembly8, self-assembly due to physical or chemical influence and other methods for colloidal agglomeration and growth are used. Different synthesis processes can be combined so that the core maybe synthesised following procedure


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that is different from that for the covering shell. Furthermore, it is possible to remove the core of a core-shell particle by dissolution or calcination in order to synthesise hollow nanoparticles. Additional to their low weight and their function as an electrical and thermal insulator, such particles can be used as a vessel for pharmaceuticals, as an absorbent and as a catalyst8. This paper introduces a new and simple monometallic and bimetallic core-shell nanoparticle synthesis method, combining a conventional magnetron sputtering process and thermal annealing. It takes advantage of a process referred to as solid state dewetting describing the formation of nanoparticles due to the annealing of thin metal films31 (see figure 1). Different thermal expansion coefficients of the metal film and the underlying substrate results in nonhomogeneously distributed interfacial stress. This leads to a metal particle migration towards relaxed areas leading to the formation of hillock and the development of voids, preferably at grain boundaries. Continuing energy supply supports surface diffusion and the growth of holes in film. These holes percolate resulting in the manifestation of isolated metal islands. In case of high temperature and expanded annealing time the islands seek a spherical shape as to reduce their surface energy31,32.


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Figure 1: solid state dewetting. a) Annealing of a thin gold layer leads to thermal stress (b) promoting particle migration to relaxed areas. Hillocks and holes (c) develop. Hole growth is preferred at grain boundaries and supported by surface diffusion. Finally, isolated metal nanoparticles are produced (d). To reduce their surface energy, a spherical shape is aspired.

Radio frequency (RF)-magnetron sputtering has the advantage of producing homogeneous layers33 with an excellent control of nominal layer thickness and nanoparticle composition. The dimensions of the initial metal layer thickness directly influence the nanoparticle size after annealing34. The optical properties of the synthesised nanoparticles can be optimised for desired applications by adjusting the elemental composition. Regarding catalytical applications such as photocatalytic water splitting where the plasmon resonance effect of metal nanoparticles is used to enhance the visible light photoactivity, the systems Au/TiO2 and Ag/TiO2 have been comprehensively investigated in the literature35-36. Since visible light photocatalysis is one of the


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recently emerging application fields of such materials, Au, Ag and AuAg nanoparticles were deposited onto titanium dioxide substrates in our study.

2. Experimental section 2.1 Preparation of TiO2 substrates In this work, nanoparticles are deposited onto PVD synthesised TiO2 anatase layers on two different substrates. For SEM work and grazing incidence x-ray diffraction (GIXRD) as well as UV-Vis measurements, commercially available soda lime glass substrates covered with a rough fluorine doped tin oxide (FTO, TCO 22-7, Solaronix) layer and dimensions, 25 mm × 25 mm × 2 mm, are used as substrate for TiO2 deposition. A 270 nm layer of TiO2 is deposited onto the FTO by a reactive direct current (DC) magnetron sputtering process in an O2/N2/Ar atmosphere (6 sccm O2, 3 sccm N2 and 60 sccm Ar) and a working pressure of 3 Pa. To remove all impurities arising from surface oxidation during pre-runs, the Ti target (Ti-133, Bekaert Advanced Coatings NV, Belgium) was sputter-cleaned in an Ar atmosphere at 8 kW for 5 min. Thereafter, the process conditions are stabilised for 8 min in an O2/N2/Ar atmosphere, as described above, prior to deposition. Finally, the layer deposition was carried out at a magnetron power of about 5.3 kW and a magnetron voltage of 450 V. For transmission electron microscopy (TEM) work, a 30 nm thick TiO2 layer is sputter-deposited onto silicon nitride grids using the same PVD method. Afterwards, these layers were annealed as described in section 2.5.2.

2.2 RF magnetron sputtering for nanoparticle deposition For depositing Au and Ag metals onto TiO2 a RF magnetron sputtering process involving two 2 inch magnetrons is used; one magnetron with a 3 mm thick gold sputtering target, the other with


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a 3 mm thick silver sputtering target (both 99,999 %, MaTeck). The distance between the sputter target and the substrate is 8 cm for gold and 5 cm for silver. The sputtering is performed in a 5 Pa argon atmosphere (15 sccm gas flow) at a power of 50 W. In initial experiments, high sputtering rates were observed resulting in fairly thick layers even at short deposition times. To optimise the nominal layer thickness of the sputtered metal for production of nanoparticles, a reduction of the sputtering rate is necessary, allowing thin layer deposition at reasonable sputtering times.

Conventionally, small sputtering rates may be

achieved by using an unbalanced magnetron and hence control the spreading of the magnetic field lines and their local density. As an eligible and simple alternative to unbalanced magnetrons, a 1 mm thick iron disk (99,95 %, MaTeck) can be placed between the magnetron and the metal sputter target. This approach was carried out in this work. The reduction of the magnetic field due to the shielding effect by the Fe-disk led to more easily controllable sputtering times which are in the order of 30 s to 300 s for depositing layers of less than 5 nm of nominal thickness.

2.3 Thermal annealing of RF sputtered Nanoparticles on TiO2 To create nanoparticles, the as synthesised metal/ TiO2 samples are annealed in a quartz tube located in a tube furnace (Zirox GmbH) with a thermal controller (Eurotherm 2416), connected to gas inlet and outlet and a gas flow controller (Multi Gas Controller 647B, MKS Instruments). Au/TiO2, Ag/TiO2 and Ag/Ag/TiO2 are annealed for 30 min in an O2 atmosphere (0.050 slm). Pure gold and Ag/Au/TiO2 samples are put straight into the furnace at 400 °C, whereas silver layers were annealed at 200 °C (see section 2.4). No temperature ramp was applied during thermal annealing.


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2.4 Synthesis steps for production of monometallic and bimetallic nanoparticles For synthesis of monometallic nanoparticles, repetitions of two alternating steps are applied. First of all, a thin metal layer is deposited by magnetron sputtering (parameters as in section 2.2). To create nanoparticles, the sample is subsequently annealed at high temperature as described in section 2.3. Two repetitions of the synthesis steps were required to produce homogenous sized nanoparticles. For a gold layer, a deposition time of 300 s results in a nominal thickness of 4 nm. After performing two synthesis cycles, a particle size distribution of 10 nm to 30 nm is observed. To deposit a silver layer with a nominal thickness of 4 nm, a sputtering time of 60 s is applied. To homogenise the size distribution of the nanoparticles, the sample is annealed at 200 °C without repetition of the two synthesis steps. For the production of AgAu core-shell structures, a gold layer is deposited on top of a silver layer. The nominal thickness of the bilayer is 5 nm. The Ag/Au ratio is varied from 1:3 (19 s Ag and 293 s Au) to 1:1 (36 s Ag and 188 s Au) and 3:1 (57 s Ag and 98 s Au). Finally, the layer is annealed following the heating procedure described in section 2.3.

2.5 Analytical techniques 2.5.1 Scanning Electron Microscopy To estimate the distribution of the nanoparticles on the titania substrate and to evaluate their size as well as size distribution a scanning electron microscope (SEM), JSM 7500F JEOL, is used. It utilises a field-emission gun, a semi-in-lens conical objective lens and a secondary electron inlens detector to provide high-resolution images of 1.0 nm at acceleration voltages of 15 keV.


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2.5.2 Transmission Electron Microscopy To investigate the formation of core-shell structures and the size distribution of the nanoparticles, a transmission electron microscope (TEM) is used. The TEM measurements are performed at 200 kV with an aberration-corrected JEM-ARM200F (JEOL, Corrector: CEOS). The microscope is equipped with a JED-2300 (JEOL) energy-dispersive x-ray-spectrometer (EDX) for chemical analysis. The aberration corrected scanning transmission electron microscope (STEM) imaging (HighAngle Annular Dark Field (HAADF) and Annular Bright Field (ABF)) are performed under the following conditions. HAADF and ABF are both done with a spot size of approximately 0.13 nm, a convergence angle of 30 °- 36 ° and collection semi-angles for HAADF and ABF of 90 mrad - 170 mrad and 11 mrad - 22 mrad, respectively. For TEM measurements, silicon nitride membranes (Plano GmbH) were covered with a 30 nm TiO2 layer using the same sputtering process. The resulting largely amorphous titania layer was transformed into crystalline anatase by post-deposition sintering of the sample at 400 °C for 60 min using a heating rate of 1 °C/min in an O2 atmosphere at a gas flow rate of 0.050 slm. The nanoparticles were produced as described above.

2.5.3 Grazing Incident X-ray diffraction Phase identity and crystallite size of the Au, Ag and AuAg nanoparticles are investigated by the means of a Bruker D8 Advance Diffractometer with measurements performed at an incident angle of ω = 0.5° over a range of 2Θ of 20 ° to 80 ° with a step width of 0.02 ° and 5 s per step. Crystallite sizes are calculated from the (200) reflection for the Au phase, using a single line


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method based on the Stokes and Wilson model as well as the Variance model, according to de Keijser et al.37

2.5.4 UV/Vis-Measurements To investigate the optical properties of the metal nanoparticles, a PerkinElmer Lambda UV/Vis 850 spectrophotometer with a L6020322 150 mm integrated sphere was used. To ensure high reflectivity inside the sphere calibrated Spectralon Reflectance Standards (>99% R, USRS-99020, Perkin-Elmer Inc.) are attached. The recordings were performed in the visible light region between 250 nm and 850 nm. All measurements are carried out in transmission mode and calculation of the absorbance is undertaken assuming that no reflection occurs in samples.

3. Results and discussion

3.1 Morphology 3.1.1 Au nanoparticles The morphology of the subjacent substrate, TiO2, is likely to influence the microstructure of the particles. The TiO2 substrate prepared for our SEM, GIXRD and UV/Vis experiments (see section 2.1) was deposited onto rough FTO, leading to a high surface roughness of TiO2 which is found to agglomerate into nanocolumns of ca. 200 nm in diameter. As described in section 2.4, a thin gold layer with a nominal thickness of 4 nm is deposited onto the sputtered TiO2 substrate. Thin metal film growth on non-metal surfaces usually occurs by the Volmer-Weber mechanism. Initially droplet-shaped islands are formed that become elongated with increasing deposition time. Finally, the islands grow together to form a closed but rough


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layer38. SEM images of our samples as seen in figure 2 illustrate that due to the rough morphology of the substrate, the deposited metal does not form a fully continuous layer.

Figure 2: SEM image of 4 nm nominal Au layer on TiO2. The surface of the semiconductor is not completely covered by the metal layer.

After annealing, the semi-continuous Au film transformed into discrete Au particles that appear facetted, as seen in figure 3. Sizes are observed to increase with temperature and annealing time. Simultaneously, the amount of surface coverage decreases. These observations suggest that socalled Ostwald ripening occurs. Particles which exceed a critical radius will continue to grow whilst smaller particles will undergo surface diffusion and agglomerate into bigger particles. This leads to formation of depletion zones around the growing clusters38. It appears that there is a dependence of particle size on the location of the cluster within the 3-dimensional structure of


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the TiO2 surface, i.e. large particles are located on top of the TiO2 columns and smaller particles form in the inter columnar space. To confirm this, cross sections are needed to be investigated in further work.

Figure 3: SEM image of gold nanoparticles on TiO2 after two cycles of subsequent PVD and annealing steps. PVD step: deposition of Au with 4 nm nominal thickness. Annealing step: 30 min at 400°C in O2 atmosphere.

3.1.2 Ag nanoparticles The deposition of silver onto TiO2/FTO at similar loadings as for gold resulted into formation of separated islands. This observation differs from that found for gold deposition where a nearly


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continuous metal film was formed as described in the previous section. Without subsequent annealing, a PVD-deposited silver layer with a nominal thickness of 4 nm consists of particles exhibiting a broad size distribution from 10 nm to 40 nm as shown in figure 4. The observation had no coalescence and layer formation has occurred yet suggest that the process of particle nucleation and layer growth is only commencing at a nominal thickness of 4 nm39. Annealing of the deposited particles at 200 °C, however, resulted in a more homogenous size and narrower range of distribution from 5 nm to 10 nm. This process can be explained by solid state dewetting as described in section 1. As well as a decrease in particle size, the concentration of the Ag particles at the top of the TiO2 nanocolumns is also significantly decreased. After annealing at 400 °C only very few Ag nanoparticles are still located at the tops of the TiO2 nanocolumns. Instead, several large Ag particles of several hundreds of nm in size are found to agglomerate within the inter columnar spaces. Clearly, this can be attributed to Ag diffusion commencing at 200 °C and occurring more quantitatively at 400 °C. Table 1 compiles the average diameter of silver particles with and without annealing. (a)


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(b)


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(c)


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Figure 4: SEM images of a silver layer (4 nm nominal thickness) on TiO2 after different thermal treatment. (a): without post-deposition annealing step. (b): silver nanoparticles formed after annealing for 30 min at 200°C in O2. (c): Annealing for 30 min at 400°C. Due to its high mobility, the silver starts creeping into inter columnar space of TiO2.

SEM images

Average particle diameter (nm)

Standard deviation (nm)

Ag without annealing

17

10

Ag annealed at 200°C

8

3

Table 1: Average particle diameter and standard deviation for as-deposited and post annealed Ag derived by manual SEM image analysis (horizontal dimensions considered). Due to the limited resolution of SEM images, particles below 3 nm may not be visible. The non-spherical


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geometry and broad size distribution of as-deposited Ag particles are reflected in a high standard deviation of the diameter.

3.1.3 Ag/Au core-shell nanoparticles Particles with three different Ag/Au ratios (1:3; 1:1; 3:1) have been prepared as described in the experimental. After PVD and subsequent annealing for 30 min, SEM results (figure 5) show the formation of nanostructures with main sizes varying from 10 nm to 20 nm but also several small particles with diameters around and below 3 nm are observed. In the case of pure Ag and a small Au content, fewer numbers of the large particles are present, whereas at higher Au content and for pure Au, a higher number of the large particles is observed. A comparison of the average particle diameter for different Ag and Au ratios derived from SEM images is presented in table 2. However, a broad size distribution and deviation from a spherical shape limits the expressiveness of an average value for the particle size. (a)


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(b)


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(c)


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Figure 5: SEM images of Ag/Au core-shell nanoparticles with different elemental ratios. (a): Ag/Au (1:3), (b): Ag/Au (1:1), (c): Ag/Au (3:1).

SEM images

Average particle diameter (nm) Standard deviation (nm)

Au

14

8

Ag/Au (1:3)

18

10

Ag/Au (1:1)

16

8

Ag/Au (3:1)

11

7

Table 2: Average particle diameter and standard deviation determined by manual SEM image analysis. Particles below 3 nm may not be visible due to limited resolution. For diameter


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determination the horizontal dimension of particles is considered. With SEM no differentiation of gold and silver is possible. Considerable standard deviation indicates a broad size distribution due to the morphologically determined particle growth.

Using STEM high angle annular dark field (HAADF) measurements with an aberration corrected microscope one is enabled to detect both z-contrast that means contrast depending on the atomic number combined with atomic resolution. Thus enables the imaging of core-shell structures not only by EDXS but also by different contrast within systems like AgAu nanoparticles. Silver (atomic number 47) gives less contrast than gold (atomic number 79), which gives a brighter contribution, see figure 6. (a)


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(b)


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(c)


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Figure 6: STEM-HAADF images of Ag/Au core-shell nanoparticles with different elemental ratios. (a): Ag/Au (1:3), (b): Ag/Au (1:1), (c): Ag/Au (3:1).

TEM images

Ag/Au core-shell particles

Ag nanoparticles

Average particle Standard

Average particle Standard

diameter (nm)

diameter (nm)

deviation (nm)

deviation (nm)


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Ag/Au (1:3)

43

14

2

1

Ag/Au (1:1)

26

8

2

1

Ag/Au (3:1)

37

17

4

2

Table 3: Average particle diameter and standard variation. Due to the formation of two different particle types with diverse properties there is a differentiation between Ag/Au core-shell NP and smaller Ag NP. At a Ag/Au ratio of (3:1) the core-shell particles get irregular geometries making comparison of average particle diameter challenging. The diameter is measured in horizontal dimension using TEM images.

From TEM images in figure 6a, at low silver content, Ag/Au(1:3), large particles with sizes ranging from 20 to 50 nm are observed, as well as very few small particles with sizes below 5 nm. Increasing the silver content in Ag/Au (1:1) is observed to lead to a decrease in size of the larger particles to ca. 10 to 40 nm (see figure 6b), however, a slightly larger quantity of small particles appears. Interestingly, at a high silver content, Ag/Au (3:1), large irregularly shaped particles of 20-80 nm in size and a high number of small sized particles are additionally observed (figure 6c). In table 3, a compilation of average particle sizes extracted from TEM images can be found. Due to their qualitative difference, a distinction between Ag/Au core-shell particles and monometallic Ag nanoparticles is made. Nevertheless the limitations of comparing average diameters for irregular particle geometries have to be considered.

EDX maps that are overlaid with TEM images in figure 7 confirm that bigger particles consist of a gold core (red), whereas silver (green) is mainly located in the shell. The smaller particles were found to consist almost entirely of silver.


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During the sputtering process silver atoms migrate on the substrate to agglomerate to small nanoparticles. Further sputtering would lead to the growth and coalescence of the particle and to the formation of a thin film39. Due to short sputtering time the substrate is still covered in isolated silver particles that gain increased mobility by thermal annealing40. From the EDX measurements, it may be discussed, that during tempering some of the smaller silver particles finally migrate towards the gold core forming the nanoshell. This process is favoured as opposed to formation of Ag/titania interface because of Au and Ag being isostructural41. With increasing silver content, the thickness of the silver shell increases whereas the diameter of the gold core decreases. Additionally some of the smaller silver particles also agglomerated onto the surface of the core shell structures. The TEM image in figure 8 shows in high resolution the contact region between the gold core with a diameter of 40 nm (bright array) surrounded by a silver shell of 2-3 nm thickness (darker grey array). (a)


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(b)


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(c)


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Figure 7: EDX map overlaid with TEM images of Au/Ag core-shell nanoparticles. Silver is depicted in green, gold in red. (a): Ag/Au (1:3), (b): Ag/Au (1:1), (c): Ag/Au (3:1).

As well as the earlier described formation of small monometallic silver particles at a high silver content (Ag/Au 3:1), the large particles of sizes above 10 nm are observed to contain multicores. The multicore nanoparticles consist of several small gold particles of 2 nm - 3 nm in diameter


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that are encapsulated in one silver shell as shown in figure 9. This phenomenon might be a result of Ag/Au core-shell nanoparticle agglomeration.

Figure 8: STEM-HAADF image showing an Au rich nanoparticle core with a 2-3 nm thick Ag shell for a AgAu(1:3) particle. High resolution of nanoparticle seen in figure 6a, indicated as “003” in.


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Figure 9: STEM-HAADF image showing a multicore-shell nanostructure forming at a high silver content in Ag/Au 3:1.

The previously discussed SEM images depicted in figure 3 show that the PVD-deposited Au nanoparticles form fairly spherical but slightly faceted nanostructures. Facets are even more pronounced in bimetallic AgAu nanoparticles, as shown in figure 5 and top picture of figure 10. It is discussed in the literature that wet-chemically deposited gold nanoparticles are spherical at low temperature but are known to evolve from spherical to facetted shapes during annealing42.


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Due to the additional thermal energy arising from the annealing procedure, the nanoparticle restructures from a spherical to a faceted shape to form a thermodynamically stable low energy surface, with (111) and the (100) crystallographic planes exhibiting the lowest energy for the face-centered cubic crystal lattice symmetry of gold and silver42. Decahedrons, cubes, triangular prism and icosahedrons may appear as a result of the surface stabilisation42. The TEM image in the bottom picture of figure 10 reveal that the PVD-deposited and annealed bimetallic nanostructures form decahedrons.

(a)

(b)


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Figure 10: (a): SEM image of a decahedron (Au/Ag 1:3); (b): STEM-HAADF image of a decahedral nanoparticle (Ag/Au 1:1).

(a)


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(b)


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Figure 11: GIXRD measurements, showing in (a): patterns for Au, Ag/Au (1:3), Ag/Au (3:1) and Ag on TiO2/FTO with open squares, closed circles and triangles marking peaks arising from Au, TiO2 and FTO, respectively. (b): comparison of the (200) reflection for Au, Ag und AgAu on a large scale.

GIXRD patterns in Figure 11 show a number of peaks that could be attributed to Au, Ag or AgAu as well as TiO2 anatase and the FTO substrate. The peaks that correspond to the metal were indexed in space group Fm3� m with face-centred cubic crystal symmetry, exhibiting lattice

parameter a =4.075 Å(Au), 4.087 Å(Ag), 4.077 Å(Ag/Au=3:1), 4.076 Å (Ag/Au=1:1) and 4.076 Å(Ag/Au 1:3). Interestingly, all Au-containing samples showed similar lattice parameters


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whereas the lattice of the sample with pure Ag nanoparticle deposition had a slightly reduced parameter. For all samples, a (111) preferred orientation was observed for the metallic phase. Preferred orientations in Au crystals on a TiO2 support are often observed and discussed in a number of epitaxial orientation models for Au/TiO243. The peak area of the (200) reflection, bottom of figure 11 inset, was found to generally increase with increasing Au content, commencing with comparably low intensity peaks for pure Ag and Ag/Au 3:1. This may be due to the higher electron density of the Au compared to Ag but also due to the size variation of the crystalline domains for different Ag/Au compositions as discussed earlier from TEM results. The size of crystalline domains was calculated from the peak width of the (200) reflection at ca. 44.3 ° 2Θ, taking into account partial overlap with the reflection from the SnO2 casserite phase at ca. 42.6 ° 2 Θ. Values were observed to range from 5 nm to 9 nm, with the trend increasing with increasing Au content. Also, a peak shift towards smaller values of 2 Theta was observed for pure Ag as compared with pure Au and bimetallic samples. The peak shift towards lower diffraction angles is associated with observed increase in unit cell parameter for this sample.

3.2 Optical properties 3.2.1 Au nanoparticles


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Figure 12: UV/Vis image of gold nanoparticles on TiO2, prepared by different numbers of alternating cycles of sputtering and annealing. a: sputtered for 300 s; b: sputtered for 300 s and annealed at 400 °C for 30 min (one cycle); c: two cycles.

The presence of gold nanoparticles leads to absorption in the visible spectrum of the light (figure 12). The absorption peak at 520 nm is due to the plasmon resonance for isolated spherical gold nanoparticles. The broad absorption band around 620 nm is based on gold particles that are embedded in the pores of the TiO2 (matrix effect). A repetition of the alternating deposition and annealing processes shifts this absorption band towards higher wavelengths.


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3.2.2 Ag nanoparticles Silver exhibits a plasmon resonance peak at 420 nm, although, this peak cannot be observed in this system due to the optical cut-off from the titanium dioxide substrate.

3.2.3 Ag/Au-core-shell nanoparticles According to the Mie scattering theory23 the plasmon resonance frequency of an Au/Ag coreshell particle lies between the plasmon resonance of silver (400 nm) and of gold (520 nm). Increasing the silver content of the particle leads to a blue shift whereas a greater gold ratio shifts the absorption frequency to red23. The diagram in figure 13 shows the calculated absorbance spectra for core-shell particles with different elemental ratios. Particles containing a large proportion of gold exhibit an absorption peak that is red-shifted to that of the pure gold particles, as discussed in section 3.2.1., due to change in particle size, density and the tendency for non-spherical shapes. Additionally, the surface of the core-shell particle can easily be oxidised to silver oxides changing the dielectric constant of the gold core environment. It is known from previous experiments, that annealing of silver nanoparticle under oxygen atmosphere shifts the corresponding plasmonic peak to higher wavelengths44. These factors oppose the blue shift predicted by Mie theory due to the increased influence of silver having a plasmonic peak at lower wavelengths30. Having equal proportions of both metals lead to a decrease in absorbance up to 700 nm and an additional absorbance at higher wavelengths. The trend of decreasing absorbance continues for an increased silver enrichment. Unfortunately, due to the cut-off frequency of the titanium dioxide substrate, the absorbance behaviour around 400 nm is not observable in this study. However, an effect of the particle composition on their optical properties is still apparent.


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Investigations by Lu et. al45 showed that adding a silver shell of ca. 20 at % of the overall metal content shifted the plasmon resonance frequency from the gold core from 523 nm to 511 nm. When a thicker shell is synthesised, e.g. relating to a 30 at % of the overall metal, a new peak occurs at 391 nm which is attributable to Ag. Adding even more Ag, an increase of peak intensity and a red-shift of peak position is observed. On the other hand, the peak that correlates to Au continues to shift towards the blue region of the spectrum, with a decrease in intensity. It can be discussed, that a critical layer thickness exists at atomic ratios of 20 % to 30 % of silver for appearance of plasmonic electron oscillations in silver. In the literature a threshold was reported at 2 nm - 3 nm. Furthermore, the plasmon resonance band for gold cores are no longer observed when the shell thickness exceeds 4 nm45. These findings support the results found in this paper, given that the average silver layer thickness observed in this study is approximately 2 nm.


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Figure 13: UV-Vis diagram showing Au NP, Ag NP and Ag/Au core-shell nanoparticle with different ratios.

4. Conclusion Monometallic and bimetallic core-shell nanoparticles could be synthesised by a combination of a simple and reproducible conventional magnetron sputtering process and thermal annealing. The nanostructure and optical properties are easily adjustable by the process parameters. Alternating sputtering and annealing of deposited gold and silver produces facetted core-shell nanoparticles, with preferred crystallographic orientation. By variation of the elemental ratio during the sputtering process, the thickness of the shell and corresponding band structure can be tuned directly. This may be of high value of the new synthesis process for nanoparticle applications


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where band structure engineering is required in order to achieve the most favourable absorption properties.

AUTHOR INFORMATION Corresponding Author * Corresponding author, e-mail address: [email protected], Tel.: +49 3834 554 3808 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was funded by the German Research Foundation (DFG). ACKNOWLEDGMENT I would like to thank Daniel Köpp for PVD of the TiO2 substrate, Anja Albrecht for XRD measurements and Dr. Harm Wulff from University of Greifswald for the crystallite size calculation. ABBREVIATIONS PVD, plasma vapour deposition; HAADF, high angle annular dark field, TEM, transmission electron microscope; EDX energy-dispersive x-ray-spectrometer; STEM scanning transmission electron microscope; GIXRD, grazing incident x-ray diffraction REFERENCES


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Table of content

1. Introduction 2. Experimental Section 2.1 Preparation of TiO2 substrates 2.2 RF magnetron sputtering for nanoparticle deposition 2.3 Thermal annealing of RF sputtered Nanoparticles on TiO2 2.4 Synthesis steps for production of monometallic and bimetallic nanoparticles 2.5 Analytical techniques 2.5.1

Scanning Electron Microscopy

2.5.2

Transmission Electron Microscopy

2.5.3 Grazing Incident X-ray diffraction 2.5.4 UV/Vis-Measurements


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3. Results and discussion 3.1 Morphology 3.1.1

Au nanoparticles

3.1.2

Ag nanoparticles

3.1.3

Ag/Au core-shell nanoparticles

3.2 Optical properties 3.2.1

Au nanoparticles

3.2.2

Ag nanoparticles

3.2.3

Ag/-Au-core-shell nanoparticles

4. Conclusion Acknowledgement References


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This paper introduces a new and reproducible PVD method to produce Au, Ag and Ag/Au coreshell nanoparticles with certain morphological and optical properties.


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Plasma Based Synthesis, Electron Microscopy, and Optical

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