- Email: [email protected]
Deposition of Ag and Au–Ag alloy nanoparticle films by spray pyrolysis technique with tuned plasmonic properties
Journal of Alloys and Compounds 585 (2014) 312–317
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Deposition of Ag and Au–Ag alloy nanoparticle films by spray pyrolysis technique with tuned plasmonic properties Neetesh Kumar ⇑, Firoz Alam, Viresh Dutta Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e
i n f o
Article history: Received 11 August 2013 Received in revised form 8 September 2013 Accepted 20 September 2013 Available online 1 October 2013 Keywords: Surface plasmons Alloy Nanoparticles Thin films Spray pyrolysis Electric field
a b s t r a c t Silver (Ag) and gold–silver (Au–Ag) alloy nanoparticle films have been prepared on the transparent substrates using a simple inexpensive spray pyrolysis technique. The surface plasmon resonance (SPR) properties of Ag films were investigated by varying spray solution volume. Also, the tunability of SPR properties using an electric field to the nozzle was demonstrated in this work. The reduced full width at half maximum and blue shift in the SPR band position were observed with DkFW 35 nm and DkP 30 nm at the applied voltage of 2 kV. The extinction spectra of alloy nanoparticle films show only one plasmon absorption it is concluded that mixing of gold and silver precursors leads to a homogeneous formation of alloy nanoparticles. The maximum of the SPR band tuned continuously from kmax 414– 525 nm with increasing gold content. The SEM observations show the variable size and spherical structure of nanoparticle films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, noble metal nanoparticles (NPs) and nanostructured metals (Au, Ag, pt etc.) films have generated great interest in different fields of science and technology because of their localized and propagating surface plasmons resonance (SPR). The plasmonic metal nanoparticles have interesting applications in inorganic and excitonic photovoltaic [1–3], photocatalysis [4], plasmon waveguides [5–6], nanoscale optical antennas [7], plasmonic lasers [8–9], surface- plasmon-enhanced light-emitting diodes[10], plasmonic integrated circuits nanoscale switches [11], imaging and materials with negative refractive index[12–13]. The metal surface plasmons (MSP) have opened up the possibility to enhance the efficiency of all types of photovoltaic devices [1–3,16–20].The metal alloy nanoparticles have very different properties from the properties of the pure metal nanoparticles. The alloy nanoparticle adds new aspect in customizing the properties of nanomaterials [14–15]. Nowadays, there are well-established applications of SPs that increase rapidly with the development of our capabilities to fabricate and manipulate nanomaterials with enhanced properties. For example, 20 nm monometallic Ag and Au nanoparticles have SPR band at kmax
⇑ Corresponding author. Address: Photovoltaic Laboratory, Room No-350, Block– V, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India. Tel.: +11 2659 6417; fax: +11 26659 1261. E-mail address: [email protected] (N. Kumar). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.145
400 nm and 520 nm respectively, while the Au–Ag alloy nanoparticles the SPR properties can be continuously tuned from 400 to 520 nm simply by changing the composition. Several methods have been reported for the preparation of Ag and Au–Ag nanoparticle films; most of the methods used to deposit metal nanoparticles onto the substrate involve either spin coating [16], dewetting [17], sonochemical [18], sol–gel [19], electrode position [20], chemical vapor deposition [21] or complicated physical techniques such as thermal evaporation [22] sputtering [23–25] and laser assisted vacuum deposition [26]. All of these techniques add time, cost, multistep synthesis and complexity to the overall process of fabricating metal and alloy particle films. Hence, the deposition of metal and alloy nanoparticles films without using costly setup in a single step by excluding extra purification processes become very challenging. The spray pyrolysis technique has the advantage over above methods. Here in an attempt to prepare Ag and Au–Ag alloy nanoparticle films with tunable SPR bands using inexpensive and simple pneumatic as well as electric field-assisted spray pyrolysis technique is reported. The spray pyrolysis is a straight forward, single step technique that creates and deposits Ag and Au–Ag alloy nanoparticle on the substrate in the form of film. In this technique, the most critical parameter is the size of droplets generated at the nozzle and their distribution over the preheated substrates for pyrolytic reaction. Apart from the air-pressure for the droplet generation, an electric field can also play a major role in tuning the droplet sizes due the Coulomb fission [27] so as to improve the film quality [28].
N. Kumar et al. / Journal of Alloys and Compounds 585 (2014) 312–317
This paper reports the spray deposition of silver nanoparticle films without and with the electric field and Au–Ag alloy nanoparticle films showing the effect on SPR due to changes in the nanoparticle size, distribution and composition respectively. 2. Experimental details 2.1. Materials Silver nitrate (AgNO3, >99.9%), chloroauric acid (HAuCl4 99%) were used which were obtained from Alfa Aesar and Molychem respectively. Deionized water (14.0 MX cm 1) obtained from mili-Q.
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2.2. Deposition of Ag and Au–Ag alloy nanoparticle films The Ag and Au–Ag alloy nanoparticle films were deposited inside a thermally insulating and gas leak-proof spray chamber, made of fiber reinforced plastic (FRP) material, reported elsewhere [28,29]. The films were deposited onto glass substrates of the size 4 cm 4 cm. Prior to deposition these substrates were cleaned with soap solution, ultrasonically cleaned in DI water and acetone for 15 min. each respectively. For depositing the silver nanoparticles films a 5 mM concentration solution of silver nitrate (AgNO3) in DI water was used. The deposition was done for varying spray solution volume 5.0, 8.0, 10.0 and 12.0 ml the corresponding sample named as Ag5, Ag8, Ag10, and Ag12 respectively. The other parameters were as follows: deposition temperature 450 °C, spray rate 3.0 ml/ min, nozzle to substrate distance 20.0 cm. Nitrogen was used as carrier gas with 8.0 L/min flow rate at a pressure of 2.0 kgf/cm2. After deposition, the films were removed instantly from hot plate and kept in N2 filled spray chamber to cool down. For electric field assisted spray deposition, an additional electric pressure on N2 gas generated droplets was produced by applying high voltage between the nozzle (+ve) and a circular metal electrode ( ve) placed 2.0 mm below it. The deposition for 12 ml spray volume, keeping other spray parameters the same as mentioned above, was performed without voltage (0 V) and with voltage at 0.5, 1, 1.5 and 2.0 kV. The samples were named Ag0, Ag0.5, Ag1.0, Ag1.5 and Ag2.0 respectively. For depositing the Au–Ag alloy nanoparticle films mixed precursor solution of different molar concentrations of silver nitrate and chloroauric acid (HAuCl4) in DI water was used keeping other parameters same as above.
2.3. Characterization
Fig. 1. XRD pattern of Ag (a), Au (b) and Au–Ag alloy (c) nanoparticle films on glass substrate and inset showing SEAD patterns.
The phase constituents and crystallinity of the samples were carried out by glancing incidence angle X-ray diffraction (GIXRD) using X-ray diffractometer (Phillips X’PERT PRO), having Cu Ka incident beam (k = 1.54056 Å). The surface morphology of the films was observed by ZEISS EVO-50 model scanning electron microscope (SEM). Phillips CM12 120 kV transmission electron microscope (TEM) was used for particle size analysis. For TEM measurement nanoparticles were dispersed in ethanol by ultra-sonication, small amount of ethanol solution drop casted on the carbon coated copper grids (200-mesh) and air dried for about 2 h. The optical properties of the films were studied by using UV–Vis–NIR spectrophotometer (Perkin–Elmer Lamda-1050).The mean thickness of the films was measured from the Dektak surface profiler (DektakXT) using 2 lm stylus tip.
Fig. 2. Normalized extinction spectra of Au NP films on glass substrate of samples Ag5 (a), Ag8 (b), Ag10 (c) and Ag12 (d). Inset: histogram showing size distribution.
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3. Results and discussion 3.1. Crystal structure Bulk silver and gold nanocrystals both have a face centered cubic (fcc) structure. However, when the particle size lies in the cluster regime, drastic changes occur to the particle structure. Ag, Au and Au–Ag alloy nanoparticle films shows a well defined diffraction pattern with cubic structure (Fig. 1), having lattice constants of a = b = c = 4.077 Å, and it is in agreement with JCPDS card No. 087-0720 and 089-3697 respectively. The XRD and electron diffraction pattern of Au, Ag and Au–Ag alloy are identical because all three have same fcc structure. The average crystallite size (t) estimated using the Debye–Scherrer formula, t = 0.94k/b1/2 cosh, is around 50 nm, where, the k is wavelength of X-ray used is 1.54056 Å. The inset of Fig. 1 shows the selected area electron diffraction pattern having diffraction rings corresponding to the crystal planes observed in the X-ray diffraction pattern. 3.2. Microstructure The silver nanoparticle films deposited for 5, 8, 10 and 12 ml spray volume (samples- Ag5, Ag8, Ag10 and Ag12) were investigated by SEM to determine the morphology, size and density of the as-fabricated nanoparticles. For sample Ag5, (Fig. 2a) a
monolayer of spherical Ag nanoparticles was clearly observable on the surface of the glass substrate with isolated particles, whose average diameter is 45 nm. In the sample Ag8, Ag10, and Ag12 (Fig. 2b, c and d) dense Ag nanoparticles were observed, whose average diameters is 50, 55, 60 nm respectively. The increase in particle size and distribution is clearly reflected in the optical properties observed that the red shift in SPR band position and increase in FWHM. The Fig. 3 shows the morphology and particle size distribution of the Ag nanoparticle films deposited under electric field (without and with applied voltage). The nanoparticle films are not continuous but made of discrete nanoparticles with size range of 30–80 nm. It is observed that as voltage increases the nanoparticle become finer and uniformly distributed on the substrates with little agglomeration. The histogram plots indicate that fine tuning in the particle size distribution with voltage. The SEM images of NP films showing some aggregations of the nanoparticles because of low resolution. Some very small nanoparticles and the nanoparticles that are very closely approached might not be distinguished. But they are important to the analysis of the SPR band. The Fig. 4 TEM images of the Ag nanoparticles deposited without and with applied voltage (a) 0 kV, (b) 0.5 kV, (c) 1.0 kV and (d) 2.0 kV. TEM images confirmed the poly-dispersed Ag NPs having spheroidal morphology. The nanoparticles are well isolated because of ultrasonic dispersion in ethanol solution.
Fig. 3. SEM morphology of the Ag nanoparticle films deposited without and with applied voltage (a) 0 kV, (b) 0.5 kV, (c) 1.0 kV and (d) 2.0 kV. Inset: histogram showing size distribution.
N. Kumar et al. / Journal of Alloys and Compounds 585 (2014) 312–317
3.3. Optical (surface plasmon resonance) properties The silver nanoparticles are extraordinarily efficient at absorbing and scattering light and depend upon the size and the shape of the particle. When nanoparticles excited with specific wavelength due to strong interaction of silver nanoparticles with light, the conduction electron on the metal surface undergo a collective oscillations. Thus, plasmons are collective oscillations of the free electron gas density or a quantum of plasma oscillation and surface plasmons are those plasmons that are confined to surfaces that interact strongly with light. The SPR properties can be easily tuned by changing the synthesis conditions. For the metal nanoparticles the SPR band position, intensity and FWHM are very sensitive to the particle size, shape and inter-particle spacing. The generation of dipoles/multipoles oscillations results from the interaction of electromagnetic wave with the isolated metal nanoparticles in the process of light matter interaction. The interaction among generated dipoles/multipoles is mutual and reciprocal. For high concentration of metal nanoparticles 10% this interaction become very important. Both, theory [30] and experiment [31] indicate that the increased volume fraction of silver NPs raises the SPR band intensity and leads to a red shift of its position. Thus, there are several means for tuning the SPR spectral properties, namely, (1) NP shape, (2) NP size, and (3) NP concentration. In order to obtain tunable SPR absorption properties, we need to have control over some of the above mentioned parameters. Therefore, a study of the SPR absorption
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features has been done by changing both size and distribution of silver nanoparticles using the electric field effect in the spray process. 3.4. Effect of spray volume on the SPR properties The normalized extinction spectra (defined as log [T]) of nanostructured silver films (Ag5, Ag8, Ag10 and Ag12) are shown in Fig. 5. The Ag nanoparticles of average diameter of 45 nm are observed for sample Ag5 the corresponding SPR band at kmax = 408 nm. For sample Ag8 dense Ag particles with average diameter of 50 nm are observed and corresponding SPR band at kmax = 415 nm. The shift in SPR band position is due to near-field particle–particle interactions. Further increase in spray volume red shift is observed in the SPR band position. For the Ag10 and Ag12 samples the SPR band observed at kmax 421 nm and 432 nm respectively. The SPR band is obtained in the visible range which matches well with others result [22–24]. Here, it was observed that as the spray volume increases there is a red shift in the SPR band position as well as an increase in bandwidth, due to the increase in nanoparticle concentration, surface coverage and further aggregation, as shown in SEM observation Fig. 2. 3.5. Effect of electric field on SPR properties The SPR absorption spectra of silver films deposited in the absence/presence of the electric field for the 12 ml spray volume
Fig. 4. TEM images of the Ag nanoparticles deposited without and with applied voltage (a) 0 kV, (b) 0.5 kV, (c) 1.0 kV and (d) 2.0 kV.
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1.0
408 nm
Ag5
432 nm
Normalized Extinction (a.u.)
Ag8 Ag10 0.8
Ag12
0.6
0.4
0.2
0.0 350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 5. Normalized extinction spectra of silver NP films for 5 ml, 8 ml, 10 ml and 12 ml spray volume.
406 nm
434 nm
Ag 0kV
1.0
Table 1 SPR properties of Ag and Ag–Au alloy nanoparticle films.
Normalized Ectinction (a.u.)
Ag 0.5kV
0.8
Ag 1.0kV
Sample
kmax (nm)
FWHM (nm)
Ag 1.5kV
Ag5 Ag8 Ag10 Ag12
408 415 421 432
53 58 64 68
Ag0.0 Ag0.5 Ag1.0 Ag1.5 Ag2.0
436 426 420 413 406
79 73 67 60 54
415 440 460 490 525
60 70 – – 76
Ag 2.0kV 0.6
0.4
0.2
0.0 350
400
450
500
550
600
3.5.1. Optical (SPR) properties of Au–Ag alloy nanoparticles The as-prepared Au–Ag alloy nanoparticle films from the mixed precursor solution were first verified by the UV–visible extinction spectra. Only one surface plasmon band appeared in the visible absorption spectra for each of Au–Ag-3:1, 1:1 and 1:3 and the absorption maximum was always positioned between the SPR of pure Ag (415 nm) and pure Au (525 nm) as shown in Fig. 7. This is a proof that the nanoparticle films are made of only alloy nanoparticles rather than core–shell nanoparticles, or a mixture of pure Au and Ag nanoparticles. It is known that any system of physically mixed pure Au and Ag has two SPR band due to their individual absorption. The SPR band intensity increases with increasing concentration of such component. The core–shell type nanoparticles also exhibit two absorption bands and the absorption intensity depends on the concentration of the each component [32–35]. These observations are similar with the experimental and theoretical predictions in preceding papers based on Mie theory [36,37]. From the above observations it is clear that the spray deposition technique has an ability to fabricate silver nanoparticle films with tuned SPR properties. Previous investigations showed that the morphology of the nanoparticle assembly is also strongly dependent on the nature of the substrate [24]. Since both changes on the size and interparticle spacing can induce significant changes on the plasmonic properties of the nanoparticle films. With the
650
700
Ag–Au Ag–Au Ag–Au Ag–Au Ag–Au
(1:0) (3:1) (1:1) (1:3) (0:1)
Wavelength (nm) Fig. 6. Normalized extinction spectra of Ag–Au alloy NP films deposited without (0 kV) and with applied voltage (0.5–2.0 kV).
415 nm
525 nm
Ag-Au (1:0) Ag-Au (3:1)
1.0
Ag-Au (1:1)
Normalized Extinction (a.u.)
are shown in the Fig. 6. It is observed that as the voltage increases from 0 kV to 2.0 kV, with a step of 500 V, the SPR band gradually shifts to the blue side of the extinction spectrum from kmax 434 nm to 406 nm, and also the SPR bandwidth (FWHM) decreases from k 76 nm to 54 nm. This is expected since due to the applied voltages, the spray droplets become finer and nearly uniform size [27], which then produce smaller and uniform sized nanoparticles on the substrate after the thermal decomposition of silver salt. The Coulomb fission describes that if the surface charge density on the spray droplet is sufficiently high, the electrostatic force adds to overcoming of surface tension and the droplet disrupts into a fine aerosol. The consistent reduction in the droplet size with the increase in applied voltage is clearly reflected in the extinction spectra by the stepwise reduction in FWHM, as well as blue shift in the SPR band. The SPR band FWHM decreases by 30 nm and an approximately 35 nm blue shift is observed at 2.0 kV, compared to the results without electric field (Ag0). Also, the SPR band position and FWHM of Ag2.0 are nearly equal to Ag5 sample. The details of each extinction spectrum are given in the table 1.
Ag-Au (1:3) Ag-Au (0:1)
0.8
0.6
0.4
0.2
0.0 400
500
600
700
800
Wavelength (nm) Fig. 7. Normalized extinction spectra of Ag–Au alloy NP films with various molar ratios of Au and Ag.
N. Kumar et al. / Journal of Alloys and Compounds 585 (2014) 312–317
increase of the deposition volume from 5 ml to 12 ml, the wavelength of their SPR band increase monotonously from 408 nm to 432 nm and which can be further extended by increasing the spray volume. But there is increase in the particle coalescence with increasing the spray volume as inspected with SEM (Fig. 2). Here we have used two different ways to attain large modulation in the SPR properties one is electric field and another is the alloying process. The electric field reduces the finer droplets by Coulomb fission which then produces smaller and uniform sized silver nanoparticles with high density on the substrate after the thermal decomposition of silver salt as observed in the SEM morphology Fig. 3. But large SPR modulation can be done simply by alloying Ag with Au. The Ag–Au alloy nanoparticle films show the SPR modulation from 414 nm to 525 nm with increasing the Au fraction as given in Table 1. This method of film preparation has several notable advantages: (1) low and high nanoparticle density films can be prepared; (2) large area deposition on flexible substrates; (3) monometallic and alloy nanoparticle films; (4) size tuning by electric field. 4. Conclusions The method used to produce silver and Au–Ag alloy nanoparticles is feasible, and does not require expensive or difficult to control equipment. The SPR band position and FWHM can be easily controlled by voltage during the spray. The SPR band characteristic of Au–Ag alloy nanoparticles was detected; the band shifts towards the red as a function of Au concentration. These particles were observed by scanning electron microscopy; the particles having a spheroidal morphology. Since the spray technique is amenable to large area deposition, the technology developed can be useful for making such layers for a variety of applications inexpensively. Acknowledgements This research work was performed under the project ‘‘Design and Fabrication of Organic Solar Cells using Organic-Inorganic Hybrid Absorber’’ sponsored by the Ministry of New and Renewable Energy (MNRE) of India. References [1] S. Pillai, K.R. Catchpole, T. Trupke, M.A. Green, J. Appl. Phys. 101 (2007) 093105. [2] I.K. Ding, J. Zhu, W. Cai, S.J. Moon, N. Cai, P. Wang, S.M. Zakeeruddin, M. Gratzel, M.L. Brongersma, Y. Cui, M.D. McGehee, Adv. Energy Mater. 1 (2011) 52–57.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Deposition of Ag and Au–Ag alloy nanoparticle films by spray pyrolysis technique with tuned plasmonic properties Neetesh Kumar ⇑, Firoz Alam, Viresh Dutta Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e
i n f o
Article history: Received 11 August 2013 Received in revised form 8 September 2013 Accepted 20 September 2013 Available online 1 October 2013 Keywords: Surface plasmons Alloy Nanoparticles Thin films Spray pyrolysis Electric field
a b s t r a c t Silver (Ag) and gold–silver (Au–Ag) alloy nanoparticle films have been prepared on the transparent substrates using a simple inexpensive spray pyrolysis technique. The surface plasmon resonance (SPR) properties of Ag films were investigated by varying spray solution volume. Also, the tunability of SPR properties using an electric field to the nozzle was demonstrated in this work. The reduced full width at half maximum and blue shift in the SPR band position were observed with DkFW 35 nm and DkP 30 nm at the applied voltage of 2 kV. The extinction spectra of alloy nanoparticle films show only one plasmon absorption it is concluded that mixing of gold and silver precursors leads to a homogeneous formation of alloy nanoparticles. The maximum of the SPR band tuned continuously from kmax 414– 525 nm with increasing gold content. The SEM observations show the variable size and spherical structure of nanoparticle films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, noble metal nanoparticles (NPs) and nanostructured metals (Au, Ag, pt etc.) films have generated great interest in different fields of science and technology because of their localized and propagating surface plasmons resonance (SPR). The plasmonic metal nanoparticles have interesting applications in inorganic and excitonic photovoltaic [1–3], photocatalysis [4], plasmon waveguides [5–6], nanoscale optical antennas [7], plasmonic lasers [8–9], surface- plasmon-enhanced light-emitting diodes[10], plasmonic integrated circuits nanoscale switches [11], imaging and materials with negative refractive index[12–13]. The metal surface plasmons (MSP) have opened up the possibility to enhance the efficiency of all types of photovoltaic devices [1–3,16–20].The metal alloy nanoparticles have very different properties from the properties of the pure metal nanoparticles. The alloy nanoparticle adds new aspect in customizing the properties of nanomaterials [14–15]. Nowadays, there are well-established applications of SPs that increase rapidly with the development of our capabilities to fabricate and manipulate nanomaterials with enhanced properties. For example, 20 nm monometallic Ag and Au nanoparticles have SPR band at kmax
⇑ Corresponding author. Address: Photovoltaic Laboratory, Room No-350, Block– V, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India. Tel.: +11 2659 6417; fax: +11 26659 1261. E-mail address: [email protected] (N. Kumar). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.145
400 nm and 520 nm respectively, while the Au–Ag alloy nanoparticles the SPR properties can be continuously tuned from 400 to 520 nm simply by changing the composition. Several methods have been reported for the preparation of Ag and Au–Ag nanoparticle films; most of the methods used to deposit metal nanoparticles onto the substrate involve either spin coating [16], dewetting [17], sonochemical [18], sol–gel [19], electrode position [20], chemical vapor deposition [21] or complicated physical techniques such as thermal evaporation [22] sputtering [23–25] and laser assisted vacuum deposition [26]. All of these techniques add time, cost, multistep synthesis and complexity to the overall process of fabricating metal and alloy particle films. Hence, the deposition of metal and alloy nanoparticles films without using costly setup in a single step by excluding extra purification processes become very challenging. The spray pyrolysis technique has the advantage over above methods. Here in an attempt to prepare Ag and Au–Ag alloy nanoparticle films with tunable SPR bands using inexpensive and simple pneumatic as well as electric field-assisted spray pyrolysis technique is reported. The spray pyrolysis is a straight forward, single step technique that creates and deposits Ag and Au–Ag alloy nanoparticle on the substrate in the form of film. In this technique, the most critical parameter is the size of droplets generated at the nozzle and their distribution over the preheated substrates for pyrolytic reaction. Apart from the air-pressure for the droplet generation, an electric field can also play a major role in tuning the droplet sizes due the Coulomb fission [27] so as to improve the film quality [28].
N. Kumar et al. / Journal of Alloys and Compounds 585 (2014) 312–317
This paper reports the spray deposition of silver nanoparticle films without and with the electric field and Au–Ag alloy nanoparticle films showing the effect on SPR due to changes in the nanoparticle size, distribution and composition respectively. 2. Experimental details 2.1. Materials Silver nitrate (AgNO3, >99.9%), chloroauric acid (HAuCl4 99%) were used which were obtained from Alfa Aesar and Molychem respectively. Deionized water (14.0 MX cm 1) obtained from mili-Q.
313
2.2. Deposition of Ag and Au–Ag alloy nanoparticle films The Ag and Au–Ag alloy nanoparticle films were deposited inside a thermally insulating and gas leak-proof spray chamber, made of fiber reinforced plastic (FRP) material, reported elsewhere [28,29]. The films were deposited onto glass substrates of the size 4 cm 4 cm. Prior to deposition these substrates were cleaned with soap solution, ultrasonically cleaned in DI water and acetone for 15 min. each respectively. For depositing the silver nanoparticles films a 5 mM concentration solution of silver nitrate (AgNO3) in DI water was used. The deposition was done for varying spray solution volume 5.0, 8.0, 10.0 and 12.0 ml the corresponding sample named as Ag5, Ag8, Ag10, and Ag12 respectively. The other parameters were as follows: deposition temperature 450 °C, spray rate 3.0 ml/ min, nozzle to substrate distance 20.0 cm. Nitrogen was used as carrier gas with 8.0 L/min flow rate at a pressure of 2.0 kgf/cm2. After deposition, the films were removed instantly from hot plate and kept in N2 filled spray chamber to cool down. For electric field assisted spray deposition, an additional electric pressure on N2 gas generated droplets was produced by applying high voltage between the nozzle (+ve) and a circular metal electrode ( ve) placed 2.0 mm below it. The deposition for 12 ml spray volume, keeping other spray parameters the same as mentioned above, was performed without voltage (0 V) and with voltage at 0.5, 1, 1.5 and 2.0 kV. The samples were named Ag0, Ag0.5, Ag1.0, Ag1.5 and Ag2.0 respectively. For depositing the Au–Ag alloy nanoparticle films mixed precursor solution of different molar concentrations of silver nitrate and chloroauric acid (HAuCl4) in DI water was used keeping other parameters same as above.
2.3. Characterization
Fig. 1. XRD pattern of Ag (a), Au (b) and Au–Ag alloy (c) nanoparticle films on glass substrate and inset showing SEAD patterns.
The phase constituents and crystallinity of the samples were carried out by glancing incidence angle X-ray diffraction (GIXRD) using X-ray diffractometer (Phillips X’PERT PRO), having Cu Ka incident beam (k = 1.54056 Å). The surface morphology of the films was observed by ZEISS EVO-50 model scanning electron microscope (SEM). Phillips CM12 120 kV transmission electron microscope (TEM) was used for particle size analysis. For TEM measurement nanoparticles were dispersed in ethanol by ultra-sonication, small amount of ethanol solution drop casted on the carbon coated copper grids (200-mesh) and air dried for about 2 h. The optical properties of the films were studied by using UV–Vis–NIR spectrophotometer (Perkin–Elmer Lamda-1050).The mean thickness of the films was measured from the Dektak surface profiler (DektakXT) using 2 lm stylus tip.
Fig. 2. Normalized extinction spectra of Au NP films on glass substrate of samples Ag5 (a), Ag8 (b), Ag10 (c) and Ag12 (d). Inset: histogram showing size distribution.
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3. Results and discussion 3.1. Crystal structure Bulk silver and gold nanocrystals both have a face centered cubic (fcc) structure. However, when the particle size lies in the cluster regime, drastic changes occur to the particle structure. Ag, Au and Au–Ag alloy nanoparticle films shows a well defined diffraction pattern with cubic structure (Fig. 1), having lattice constants of a = b = c = 4.077 Å, and it is in agreement with JCPDS card No. 087-0720 and 089-3697 respectively. The XRD and electron diffraction pattern of Au, Ag and Au–Ag alloy are identical because all three have same fcc structure. The average crystallite size (t) estimated using the Debye–Scherrer formula, t = 0.94k/b1/2 cosh, is around 50 nm, where, the k is wavelength of X-ray used is 1.54056 Å. The inset of Fig. 1 shows the selected area electron diffraction pattern having diffraction rings corresponding to the crystal planes observed in the X-ray diffraction pattern. 3.2. Microstructure The silver nanoparticle films deposited for 5, 8, 10 and 12 ml spray volume (samples- Ag5, Ag8, Ag10 and Ag12) were investigated by SEM to determine the morphology, size and density of the as-fabricated nanoparticles. For sample Ag5, (Fig. 2a) a
monolayer of spherical Ag nanoparticles was clearly observable on the surface of the glass substrate with isolated particles, whose average diameter is 45 nm. In the sample Ag8, Ag10, and Ag12 (Fig. 2b, c and d) dense Ag nanoparticles were observed, whose average diameters is 50, 55, 60 nm respectively. The increase in particle size and distribution is clearly reflected in the optical properties observed that the red shift in SPR band position and increase in FWHM. The Fig. 3 shows the morphology and particle size distribution of the Ag nanoparticle films deposited under electric field (without and with applied voltage). The nanoparticle films are not continuous but made of discrete nanoparticles with size range of 30–80 nm. It is observed that as voltage increases the nanoparticle become finer and uniformly distributed on the substrates with little agglomeration. The histogram plots indicate that fine tuning in the particle size distribution with voltage. The SEM images of NP films showing some aggregations of the nanoparticles because of low resolution. Some very small nanoparticles and the nanoparticles that are very closely approached might not be distinguished. But they are important to the analysis of the SPR band. The Fig. 4 TEM images of the Ag nanoparticles deposited without and with applied voltage (a) 0 kV, (b) 0.5 kV, (c) 1.0 kV and (d) 2.0 kV. TEM images confirmed the poly-dispersed Ag NPs having spheroidal morphology. The nanoparticles are well isolated because of ultrasonic dispersion in ethanol solution.
Fig. 3. SEM morphology of the Ag nanoparticle films deposited without and with applied voltage (a) 0 kV, (b) 0.5 kV, (c) 1.0 kV and (d) 2.0 kV. Inset: histogram showing size distribution.
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3.3. Optical (surface plasmon resonance) properties The silver nanoparticles are extraordinarily efficient at absorbing and scattering light and depend upon the size and the shape of the particle. When nanoparticles excited with specific wavelength due to strong interaction of silver nanoparticles with light, the conduction electron on the metal surface undergo a collective oscillations. Thus, plasmons are collective oscillations of the free electron gas density or a quantum of plasma oscillation and surface plasmons are those plasmons that are confined to surfaces that interact strongly with light. The SPR properties can be easily tuned by changing the synthesis conditions. For the metal nanoparticles the SPR band position, intensity and FWHM are very sensitive to the particle size, shape and inter-particle spacing. The generation of dipoles/multipoles oscillations results from the interaction of electromagnetic wave with the isolated metal nanoparticles in the process of light matter interaction. The interaction among generated dipoles/multipoles is mutual and reciprocal. For high concentration of metal nanoparticles 10% this interaction become very important. Both, theory [30] and experiment [31] indicate that the increased volume fraction of silver NPs raises the SPR band intensity and leads to a red shift of its position. Thus, there are several means for tuning the SPR spectral properties, namely, (1) NP shape, (2) NP size, and (3) NP concentration. In order to obtain tunable SPR absorption properties, we need to have control over some of the above mentioned parameters. Therefore, a study of the SPR absorption
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features has been done by changing both size and distribution of silver nanoparticles using the electric field effect in the spray process. 3.4. Effect of spray volume on the SPR properties The normalized extinction spectra (defined as log [T]) of nanostructured silver films (Ag5, Ag8, Ag10 and Ag12) are shown in Fig. 5. The Ag nanoparticles of average diameter of 45 nm are observed for sample Ag5 the corresponding SPR band at kmax = 408 nm. For sample Ag8 dense Ag particles with average diameter of 50 nm are observed and corresponding SPR band at kmax = 415 nm. The shift in SPR band position is due to near-field particle–particle interactions. Further increase in spray volume red shift is observed in the SPR band position. For the Ag10 and Ag12 samples the SPR band observed at kmax 421 nm and 432 nm respectively. The SPR band is obtained in the visible range which matches well with others result [22–24]. Here, it was observed that as the spray volume increases there is a red shift in the SPR band position as well as an increase in bandwidth, due to the increase in nanoparticle concentration, surface coverage and further aggregation, as shown in SEM observation Fig. 2. 3.5. Effect of electric field on SPR properties The SPR absorption spectra of silver films deposited in the absence/presence of the electric field for the 12 ml spray volume
Fig. 4. TEM images of the Ag nanoparticles deposited without and with applied voltage (a) 0 kV, (b) 0.5 kV, (c) 1.0 kV and (d) 2.0 kV.
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1.0
408 nm
Ag5
432 nm
Normalized Extinction (a.u.)
Ag8 Ag10 0.8
Ag12
0.6
0.4
0.2
0.0 350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 5. Normalized extinction spectra of silver NP films for 5 ml, 8 ml, 10 ml and 12 ml spray volume.
406 nm
434 nm
Ag 0kV
1.0
Table 1 SPR properties of Ag and Ag–Au alloy nanoparticle films.
Normalized Ectinction (a.u.)
Ag 0.5kV
0.8
Ag 1.0kV
Sample
kmax (nm)
FWHM (nm)
Ag 1.5kV
Ag5 Ag8 Ag10 Ag12
408 415 421 432
53 58 64 68
Ag0.0 Ag0.5 Ag1.0 Ag1.5 Ag2.0
436 426 420 413 406
79 73 67 60 54
415 440 460 490 525
60 70 – – 76
Ag 2.0kV 0.6
0.4
0.2
0.0 350
400
450
500
550
600
3.5.1. Optical (SPR) properties of Au–Ag alloy nanoparticles The as-prepared Au–Ag alloy nanoparticle films from the mixed precursor solution were first verified by the UV–visible extinction spectra. Only one surface plasmon band appeared in the visible absorption spectra for each of Au–Ag-3:1, 1:1 and 1:3 and the absorption maximum was always positioned between the SPR of pure Ag (415 nm) and pure Au (525 nm) as shown in Fig. 7. This is a proof that the nanoparticle films are made of only alloy nanoparticles rather than core–shell nanoparticles, or a mixture of pure Au and Ag nanoparticles. It is known that any system of physically mixed pure Au and Ag has two SPR band due to their individual absorption. The SPR band intensity increases with increasing concentration of such component. The core–shell type nanoparticles also exhibit two absorption bands and the absorption intensity depends on the concentration of the each component [32–35]. These observations are similar with the experimental and theoretical predictions in preceding papers based on Mie theory [36,37]. From the above observations it is clear that the spray deposition technique has an ability to fabricate silver nanoparticle films with tuned SPR properties. Previous investigations showed that the morphology of the nanoparticle assembly is also strongly dependent on the nature of the substrate [24]. Since both changes on the size and interparticle spacing can induce significant changes on the plasmonic properties of the nanoparticle films. With the
650
700
Ag–Au Ag–Au Ag–Au Ag–Au Ag–Au
(1:0) (3:1) (1:1) (1:3) (0:1)
Wavelength (nm) Fig. 6. Normalized extinction spectra of Ag–Au alloy NP films deposited without (0 kV) and with applied voltage (0.5–2.0 kV).
415 nm
525 nm
Ag-Au (1:0) Ag-Au (3:1)
1.0
Ag-Au (1:1)
Normalized Extinction (a.u.)
are shown in the Fig. 6. It is observed that as the voltage increases from 0 kV to 2.0 kV, with a step of 500 V, the SPR band gradually shifts to the blue side of the extinction spectrum from kmax 434 nm to 406 nm, and also the SPR bandwidth (FWHM) decreases from k 76 nm to 54 nm. This is expected since due to the applied voltages, the spray droplets become finer and nearly uniform size [27], which then produce smaller and uniform sized nanoparticles on the substrate after the thermal decomposition of silver salt. The Coulomb fission describes that if the surface charge density on the spray droplet is sufficiently high, the electrostatic force adds to overcoming of surface tension and the droplet disrupts into a fine aerosol. The consistent reduction in the droplet size with the increase in applied voltage is clearly reflected in the extinction spectra by the stepwise reduction in FWHM, as well as blue shift in the SPR band. The SPR band FWHM decreases by 30 nm and an approximately 35 nm blue shift is observed at 2.0 kV, compared to the results without electric field (Ag0). Also, the SPR band position and FWHM of Ag2.0 are nearly equal to Ag5 sample. The details of each extinction spectrum are given in the table 1.
Ag-Au (1:3) Ag-Au (0:1)
0.8
0.6
0.4
0.2
0.0 400
500
600
700
800
Wavelength (nm) Fig. 7. Normalized extinction spectra of Ag–Au alloy NP films with various molar ratios of Au and Ag.
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increase of the deposition volume from 5 ml to 12 ml, the wavelength of their SPR band increase monotonously from 408 nm to 432 nm and which can be further extended by increasing the spray volume. But there is increase in the particle coalescence with increasing the spray volume as inspected with SEM (Fig. 2). Here we have used two different ways to attain large modulation in the SPR properties one is electric field and another is the alloying process. The electric field reduces the finer droplets by Coulomb fission which then produces smaller and uniform sized silver nanoparticles with high density on the substrate after the thermal decomposition of silver salt as observed in the SEM morphology Fig. 3. But large SPR modulation can be done simply by alloying Ag with Au. The Ag–Au alloy nanoparticle films show the SPR modulation from 414 nm to 525 nm with increasing the Au fraction as given in Table 1. This method of film preparation has several notable advantages: (1) low and high nanoparticle density films can be prepared; (2) large area deposition on flexible substrates; (3) monometallic and alloy nanoparticle films; (4) size tuning by electric field. 4. Conclusions The method used to produce silver and Au–Ag alloy nanoparticles is feasible, and does not require expensive or difficult to control equipment. The SPR band position and FWHM can be easily controlled by voltage during the spray. The SPR band characteristic of Au–Ag alloy nanoparticles was detected; the band shifts towards the red as a function of Au concentration. These particles were observed by scanning electron microscopy; the particles having a spheroidal morphology. Since the spray technique is amenable to large area deposition, the technology developed can be useful for making such layers for a variety of applications inexpensively. Acknowledgements This research work was performed under the project ‘‘Design and Fabrication of Organic Solar Cells using Organic-Inorganic Hybrid Absorber’’ sponsored by the Ministry of New and Renewable Energy (MNRE) of India. References [1] S. Pillai, K.R. Catchpole, T. Trupke, M.A. Green, J. Appl. Phys. 101 (2007) 093105. [2] I.K. Ding, J. Zhu, W. Cai, S.J. Moon, N. Cai, P. Wang, S.M. Zakeeruddin, M. Gratzel, M.L. Brongersma, Y. Cui, M.D. McGehee, Adv. Energy Mater. 1 (2011) 52–57.
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