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Electrochemical tuned scattering of gold nanostructure
Applied Surface Science 265 (2013) 802–809
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrochemical tuned scattering of gold nanostructure Yu Huang a,∗ , Liangping Xia a,b , Zheng Yang a,b , Yuan Liu a , Wanyi Xie a , Hua Zhang a a b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 401122, China Institute of Optics and Electronics, Chinese Academy of Sciences, P. O. Box 350, Chengdu, 610209, China
a r t i c l e
i n f o
Article history: Received 30 July 2012 Received in revised form 30 October 2012 Accepted 23 November 2012 Available online 30 November 2012 Keywords: Gold nanostructure Potential sweep transient Potential step transient Adsorptive ion
a b s t r a c t The ultrathin gold film and gold nanoprism immobilized on ITO electrodes displayed strong perturbed plasmonic scattering as the result of potential linear sweep and step applications. Although nanoscale surface structure strongly determines the potential-dependent plasmonic response of gold nanostructure, the universality of exponential fit for the differential scattering induced by the potential linear sweep indicates the same mechanism could be widely used to explain the effect of potential on the gold nanostructure. The larger scattering changed by the anodic potential step than cathodic counterpart is attributed to increased double layer capacitance due to the presence of adsorptive ion. The presence of potential controlled scattering indicates the tunability of localized surface plasmon resonance of gold nanostructure. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nobel metal nanostructures have attracted the interest of scientists for generations because of their unique optical properties due to tunable localized surface plasmon resonance (LSPR). Interacted with electromagnetic radiation, the nanostructures exhibit plasmon-enhanced light scattering, absorption and enhancement of the local electromagnetic field in the immediate environment [1–4]. The dependence of plasmon resonance on the local environment provides the basis for development of nanostructures as chemical and biological sensors [1,5,6]. The application of an electric potential on nanostructure in an electrochemical environment forms a double layer capacitance at the particle-electrolyte interface, which is almost several orders of magnitude higher than that in air [7]. Potential-induced spectral changes are achieved by donating or withdrawing of electrons from nanostructures through this double layer capacitance. The blue or red shifts of resonant wavelengths have been observed in several early studies [7–12]. Various modes of potential applied on nanostructure could help us to understand the particle’s spectral respond to the potential from different perspectives. In our previously report, the scattering of gold nanostructures with various morphologies in 0.1 M NaCl under potential modulation was detected from the dark-field imaging spectrometer [13]. The change in scattering is dependent on the wavelengths
∗ Corresponding author. E-mail address: [email protected] (Y. Huang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.122
where it is observed. The shoulder of spectrum is an ideal position to detect the scattering modulated by potential since the scattering contrast is highest there. The 16 nm thickness gold film and 16 nm height, 466 nm in-plane length gold nanoprism exhibited high voltage sensitivity compared to other gold nanostructures studied in the same condition. = 618 nm and = 640 nm were the locations of highest sensitivity shown by 16 nm gold film and 16 nm height gold nanoprism respectively. We herein focus on the scattering response transient of 16 nm thickness gold film and 16 nm height gold nanoprism under potential sweep and step application. Their scattering modulated by various potential controlled modes was observed at single wavelengths which their largest voltage sensitivity was at. The track of accurate scattered light transient is helpful to explore the interfacial electron transfer originated from the charging-discharging process between gold and electrolyte.
2. Experimental methods Indium-tin-oxide (ITO) coverslips (surface resistivity 15–30 /sq, Structure Probe, Inc.) were used as substrates and they were cleaned in isopropanol and acetone in an ultrasound bath and rinsed copiously with Milli-Q 18 M water prior to the gold nanostructure preparation [14]. 16 nm thickness gold film, 16 nm height and 466 nm in-plane length gold nanoprism were fabricated on the ITO substrate by using thermal evaporation and nanosphere lithography respectively [15–17]. The electrochemical cell is composed of Teflon cages. Gold nanostructure immobilized on ITO coverslips was working electrode connected to a potentiostat (Princeton Applied Research). An coiled
Y. Huang et al. / Applied Surface Science 265 (2013) 802–809
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Fig. 1. (a) Dark field image of 16 nm thickness gold film recorded at wavelength of 618 nm. (b) Dark field image of 16 nm height and 466 nm in-plane length gold nanoprism recorded at wavelength of 640 nm. (c) SEM image of 16 nm thickness gold film. (d) SEM image of 16 nm height and 466 nm in-plane length gold nanoprism.
Au and an Ag wire were counter and reference electrodes respectively. The dark-field imaging spectrometer described elsewhere [13] was used to monitor gold nanostructures’ scattering modulated by potential in 0.1 M NaCl. The epi-illuminated spectrometer enables the objective lens to collect the back scattered light from gold nanostructure. The single wavelength light scattered by the gold film and nanoprism was selected by a movable linear variable interference filter and was recorded on the camera. The exposure time was dependent on the scattering strength of gold nanostructure. The acquisition time of 3.6 s for one image of gold film and 1.4 s for one image of gold nanoprism and their SEM images are shown in Fig. 1.
3. Results and discussion 3.1. Scattering controlled by linear potential sweep The capacitances value of gold film and gold nanoprism determined from the impedance measurements are shown in Fig. 2. The location of dip capacitance of gold film is slightly negative with respect to the 0 V while that of nanoprism shifts to positive direction. The value of point zero charge (pzc) of gold nanostructure is dependent both on the nature of gold nanostructure and ions. The morphology of gold nanostructure determines that the capacitance of 16 nm gold film is higher than that of 16 nm height gold
Fig. 2. Capacitance of gold film (a) and gold nanoprism (b) as a function of electrode potential.
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Fig. 3. Transition of the scattering of (a, b, c) 16 nm Au film and (d, e, f) 16 nm height gold nanoprism during the cyclic voltammetry. The voltage scan rates used in (a, b,c) and (d, e, f) were 0.0112 V/s, 0.0056 V/s, 0.0028 V/s and 0.0276 V/s, 0.0138 V/s, 0.0071 V/s respectively. The initial potentials for red squares and blue circles were 0.2 V and −0.2 V respectively.
nanoprism in the solvent of sodium chloride, which is partly contributed by the specific adsorption of chloride ions. The degree of chloride ions adsorption on the gold film is stronger than that of gold nanoprism. The presence of adsorption of anions will negatively shift the value of pzc [18]. As a result, the pzc of gold film is slightly negative than that of gold nanoprism. When linear potential between 0.2 V and −0.2 V were swept on gold film and gold nanoprism, a gradually increasing rate of change in scattering during each linear scan was clearly observed no matter which direction and scan rate were. The averaged scattering strength of 20 regions on the gold film taken at = 618 nm, each corresponding to a sample area of approximately 0.5 m2 , and scattering collected from 60 to 70 16 nm height gold nanoprisms,
recording at = 640 nm are shown in Fig. 3. Determining the rate of change in scattering within the ±200 mV potential linear sweep was achieved by differentiating the measured samples in these linear potential scans, shown in Fig. 4. The scattering transients were fitted to the equation I(t) = a(1 − e−t/ ) + b
(1)
where a is the change of scattering at the final steady state, b is the initial change of scattering after the application of the potential sweep, t is the duration time and is the time constant. Other types of function of time were also used to fit the scattering transient, but Eq. (1) always gave rise to the best fit.
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Fig. 4. Differential scattering of 16 nm Au film in response to the potential scan at (a) 0.0112 V/s, (b) 0.0056 V/s (b) and (c) 0.0028 V/s and differential scattering of 16 nm height Au nanoprisms in response to the potential scan at (d) 0.0276 V/s, (e) 0.0138 V/s and (f) 0.0071 V/s.Subtracting the i+1th scattering sampling from the ith sampling can get the change rate of scattering within the fixed exposure time. Repeating the subtraction at each single scan and averaging twelve of them can obtain the change rate of scattering represented as absolute value by red squares lines in ±200 mV voltage window. The blue circles lines are the fitting curve for Eq. (1).
The red solid squares lines and blue solid circles lines in Fig. 4 are the experimental differentiating scattering step and their corresponding fitting curves respectively. The gold film sets value of (a, b, ) for the scan rate 0.0112 V/s, 0.0056 V/s and 0.0028 V/s are (101.6, 14.56, 17.54 s), (53.65, 6.695, 33.87 s), (28.47, 4.628, 76.15 s) and gold nanoprism set value of (a, b, ) for the scan rate 0.0276 V/s, 0.0138 V/s and 0.0071 V/s are (44.7, −7.524, 3.607 s), (23.19, −1.335, 8.796 s) and (9.612, 0.3883, 13.92 s) respectively. It is noticeable that both the ratio of three time constants of gold film and gold nanoprism, 17.54:33.87:76.15 = 1:1.93:4.34 and 3.607:8.796:13.92 = 1:2.43: 3.86, closely matches the reciprocal ratio of their corresponding scan rates 0.0112:0.0056:0.0028 = 1:1/2:1/4 and 0.0276:0.0138:0.0071 = 1:1/2:1/3.88.
The inversely proportional relationship between time constants and potential sweep rates discovered in gold nanoprism and gold film implies the universality of the interpretation for the potential’s perturbation on gold nanostructures. At the beginning of potential application, the accumulation of counterions outside the gold surface and the assembly of excess charges inside the gold surface construct the double layer at the metal-electrolyte interface. An external electric field induced by the presence of the double layer tends to penetrate into the gold. Meanwhile, an internal electric field is created by the movement of the electrons in the gold’s space charge region to resist the influence of the external electric field [19,20]. Equilibrium was achieved after a certain time depending on the potential scan rate. The balanced interaction between the internal and the external electric fields is reflected by the scattered
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Fig. 5. The scattering of (a, b, c, d) 16 nm gold film and (e, f, g, h) 16 nm gold nanoprism in response to a potential step as the sequence in (a, e) and (c, g) were 0→0.05→−0.05→0 V and 0→0.2→-0.2→0 V, represented as red squares and lines. The potential sequences in (b, f) and (d, h) are revered as 0→-0.05→0.05→0 V, 0→0.2→0.2→0 V, represented as blue circles and lines.
light, whose rate of change in intensity tends to match the scan rate of the potential. The faster the potential scan rate is, the earlier the equilibration is established and the smaller the time constant will be.
3.2. Scattering controlled by step potential The scattering from the gold film and gold nanoprism changing with time after the application of ±50 mV and ±200 mV potential
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Fig. 6. The rate of change of scattering from (a, b) 16 nm gold film and (c, d) 16 nm height gold nanoprism during the application of a potential step. The potential sequence of the red square lines in (a, c) and (b, d) are 0→0.05→-0.05→0 V, 0→0.2→-0.2→0 V and the blue circle lines in (a, c) and (b, d) are 0→-0.05→0.05→0 V, 0→-0.2→0.2→0 V. Each point was obtained by subtracting the i+1th scattering sampling from the ith scattering sampling.
steps are shown in Fig. 5. Their scattering had almost no distinctive feature in the first duration of 0 V. Once a polarized potential step was applied, the scattering was perturbed to change with the trace of potential. As the potential step size increased, the level of scattered light stabilized more quickly. The different scattering intensity at the initial and final applications of 0 V was due to the slow relaxation of charge convection after the gold nanostructure suffered a long application of potential, although the application time of the positive and negative potentials was equal. The quantity of charge transferred was limited by the surface area of the gold. Application of a larger potential results in faster allocation of large charges and an earlier achievement of a balanced electric field at the gold-solution interface. The gold film’s differentiation of scattering is presented in Fig. 6 (a) and (b). The anion Cl− adsorbing at the gold film surface in the case of positive potential is stronger than the cation Na+ adsorbing in the case of negative potential [21]. As a result, the application of positive potential induces larger capacitance than negative potential on the electrode surface when the potential was altered between the positive and zero. Since the exchange quantity of chloride and sodium ions adsorbed on the electrode surface induced by the switch of two polarized potentials are almost equal, the same change in the magnitude of adsorption capacitance results in the same size of spikes. To explore the penetration depth of electric field of gold nanostructure into the solvent, the electric field outside the gold film and the gold nanoprisms was calculated, choosing the wavelength to be that associated with the experimental condition. Spherical particles with 5 nm diameter are assumed to embed on the surface of gold film. Fig. 7(a) and (b) show the top surface view plots of the square of the normalized electric field for gold film
and the base 10 logarithm of square of the normalized electric field for gold nanoprisms respectively. The incident wavevector is perpendicular to the surface upon which the nanostructure sits (not shown), and the polarization vector is parallel to the surface and along y direction. The induced electric field at the vertices of gold nanoprisms is much higher than that at rough surface edge of gold film, although it is smaller at the central part of gold nanoprism. To further study the decay length of the induced electric field, Fig. 7(c) shows the integrated electric field as a function of distance from the top surface of gold nanostructure. In the case of gold film, the electric field falls off very rapidly with distance from the surface, while for the gold nanoprism, the electric field decreases slowly and its value becomes higher than that of gold film at the distance from 15 nm. This indicates that the electric field extends further out from the 16 nm gold nanoprism than that of 16 nm gold film, due to its sharp curvature [22,23]. Although the specific adsorption anions Cl− is capable of increasing the capacitance of the gold nanoprism’s surface in the case of positive potential, more cations Na+ at the vicinity of gold nanoprism’s surface are detectable due to the longer EM field decay length in the case of negative potential. The presence of more detected cations is transduced as an increase of local refractive index and consequently increases the scattering strength when the negative potential is used. The field enhancement resulting from the gold nanoprism’s sharp curvature is considered to compensate the smaller scattered light in the case of negative potential and enhances it to match the scattered light caused by the positive potential. As a result, the change of scattering of 16 nm height gold nanoprism was almost equal when the potential were switched between 0 V and any other levels as shown in Fig. 6(c) and (d).
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the reciprocal ratio of the three potential sweep rates reveals that the electrons in gold nanostructure have the same mechanism to respond to dynamic potential. The universality of the exponential fit indicates the mechanism of potential on the gold nanostructure is the same, whatever the morphologies are. The adsorptive anion Cl− enhances the change in scattering strength significantly at positive potentials for gold film. However, due to the special curvature of gold nanoprism, the effect of adsorption anion on the change in scattering is less comparable to that of gold film. The different performances of scattered light between gold nanoprisms and gold film under potential control confirms the size and shape effect on the localized surface plasmons and the important role morphology plays in the determination of potential detection capability. These results could provide a gateway to investigate the sensor of gold nanostructure in the electrochemical application. Acknowledgements This work is finally supported by Chongqing Key Scientific and Technological Program Project (cstc2011ggB0015) of China, National Natural Science Foundation of China (Grant No. 61275061), West Light Foundation of The Chinese Academy of Sciences. References
Fig. 7. Plots of (a) the square of the normalized electric field for gold film and (b) the base 10 logarithm of square of the normalized electric field for gold nanoprisms at the top surface. (c) Integrated normalized |E|2 E as a function of distance from the top surface of gold film and nanoprisms.
4. Conclusions Time dependent scattering of 16 nm thickness gold film and 16 nm height gold nanoprisms has been studied by the application of potential cyclic voltammetry and potential steps. An exponential expression for the differential scattering induced by the potential cyclic voltammetry can be applied to gold film and gold nanoprism. The faster the potential scan is, the earlier steady state of the scattering transients will settle. The similar ratio of time constants deduced from the scattering’s exponential fits as
[1] P.K. Jain, X.H. Huang, I.H. El-Sayed, M.A. El-Sayed, Noble metals on the nanoscale: optical and photothermal properties and some applications in imaging, sensing, biology, and medicine, Accounts of Chemical Research 41 (2008) 1578–1586. [2] H. Tamaru, H. Kuwata, H.T. Miyazaki, K. Miyano, Resonant light scattering from individual Ag nanoparticles and particle pairs, Applied Physics Letters 80 (2001) 1826–1828. [3] O. Muskens, D. Christofilos, N. Del Fatti, F. Vallee, Optical response of a single noble metal nanoparticle, Journal of Optics 8 (2006) S264–S272. [4] E.S. Kooij, B. Poelsema, Shape and size effects in the optical properties of metallic nanorods, Physical Chemistry Chemical Physics 8 (2006) 3349–3357. [5] A.V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G.A. Wurtz, R. Atkinson, R. Pollard, V.A. Podolskiy, A.V. Zayats, Plasmonic nanorod metamaterials for biosensing, Nature Materials 8 (2009) 867–871. [6] P.K. Jain, M.A. Ei-Sayed, Surface plasmon resonance sensitivity of metal nanostructures: Physical basis and universal scaling in metal nanoshells, Journal of Physical Chemistry C 111 (2007) 17451–17454. [7] C. Novo, A.M. Funston, A.K. Gooding, P. Mulvaney, Electrochemical Charging of Single Gold Nanorods, Journal of the American Chemical Society 131 (2009) 14664–14666. [8] R. Chapman, P. Mulvaney, Electro-optical shifts in silver nanoparticle films, Chemical Physics Letters 349 (5–6) (2001) 358–362. [9] X.Y. Zhang, E.M. Hicks, J. Zhao, G.C. Schatz, R.P. Van Duyne, Electrochemical Tuning of silver nanoparticles fabricated by nanosphere lithography, Nano Letters 5 (2005) 1503–1507. [10] T. Ung, M. Giersig, D. Dunstan, P. Mulvaney, Spectroelectrochemistry of colloidal silver, Langmuir 13 (1997) 1773–1782. [11] A. Toyota, T. Sagara, Particle size dependence of the charging of Au nanoparticles immobilized on a modified ITO electrode, Electrochimica Acta 53 (2008) 2553–2559. [12] J.Y. Zhang, T. Atay, A.V. Nurmikko, Optical detection of brain cell activity using plasmonic gold nanoparticles, Nano Letters 9 (2009) 519–524. [13] Y. Huang, M. Pitter, M. Somekh, Morphology-Dependent voltage sensitivity of a gold nanostructure, Langmuir 27 (2011) 13950–13961. [14] S. Szunerits, V.G. Praig, M. Manesse, R. Boukherroub, Gold island films on indium tin oxide for localized surface plasmon sensing, Nanotechnology 19 (2008) 195712–195721. [15] C.L. Haynes, R.P. Van Duyne, Nanosphere lithography: a versatile nanofabrication tool for studies of size-dependent nanoparticle optics, Journal of Physical Chemistry B 105 (2001) 5599–5611. [16] J.C. Hulteen, R.P. Vanduyne, Nanosphere lithography: a materials general fabrication process for periodic particle array surfaces, Journal of Vacuum Science and Technology A 13 (1995) 1553–1558. [17] S. Zhang, L. Berguiga, J. Elezgaray, T. Roland, C. Faivre-Moskalenko, F. Argoul, Surface plasmon resonance characterization of thermally evaporated thin gold films, Surface Science 601 (2007) 5445–5458. [18] J. Lyklema, Surface chemistry of colloids in connection with stability, in: K.J. Ives (Ed.), The Scientific Basis of Flocculation, Kluwer Academic Publishers, Alphen aan den Rijn, 1978, pp. 3–16.
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[22] C. Tabor, R. Murali, M. Mahmoud, M.A. El-Sayed, On the use of plasmonic nanoparticle pairs as a plasmon ruler: the dependence of the near-field dipole plasmon coupling on nanoparticle size and shape, Journal of Physical Chemistry A 113 (2009) 1946–1953. [23] A.J. Haes, S.L. Zou, G.C. Schatz, R.P. Van Duyne, A nanoscale optical biosensor: the long range distance dependence of the localized surface plasmon resonance of noble metal nanoparticles, Journal of Physical Chemistry B 108 (2004) 109–116.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrochemical tuned scattering of gold nanostructure Yu Huang a,∗ , Liangping Xia a,b , Zheng Yang a,b , Yuan Liu a , Wanyi Xie a , Hua Zhang a a b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 401122, China Institute of Optics and Electronics, Chinese Academy of Sciences, P. O. Box 350, Chengdu, 610209, China
a r t i c l e
i n f o
Article history: Received 30 July 2012 Received in revised form 30 October 2012 Accepted 23 November 2012 Available online 30 November 2012 Keywords: Gold nanostructure Potential sweep transient Potential step transient Adsorptive ion
a b s t r a c t The ultrathin gold film and gold nanoprism immobilized on ITO electrodes displayed strong perturbed plasmonic scattering as the result of potential linear sweep and step applications. Although nanoscale surface structure strongly determines the potential-dependent plasmonic response of gold nanostructure, the universality of exponential fit for the differential scattering induced by the potential linear sweep indicates the same mechanism could be widely used to explain the effect of potential on the gold nanostructure. The larger scattering changed by the anodic potential step than cathodic counterpart is attributed to increased double layer capacitance due to the presence of adsorptive ion. The presence of potential controlled scattering indicates the tunability of localized surface plasmon resonance of gold nanostructure. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nobel metal nanostructures have attracted the interest of scientists for generations because of their unique optical properties due to tunable localized surface plasmon resonance (LSPR). Interacted with electromagnetic radiation, the nanostructures exhibit plasmon-enhanced light scattering, absorption and enhancement of the local electromagnetic field in the immediate environment [1–4]. The dependence of plasmon resonance on the local environment provides the basis for development of nanostructures as chemical and biological sensors [1,5,6]. The application of an electric potential on nanostructure in an electrochemical environment forms a double layer capacitance at the particle-electrolyte interface, which is almost several orders of magnitude higher than that in air [7]. Potential-induced spectral changes are achieved by donating or withdrawing of electrons from nanostructures through this double layer capacitance. The blue or red shifts of resonant wavelengths have been observed in several early studies [7–12]. Various modes of potential applied on nanostructure could help us to understand the particle’s spectral respond to the potential from different perspectives. In our previously report, the scattering of gold nanostructures with various morphologies in 0.1 M NaCl under potential modulation was detected from the dark-field imaging spectrometer [13]. The change in scattering is dependent on the wavelengths
∗ Corresponding author. E-mail address: [email protected] (Y. Huang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.122
where it is observed. The shoulder of spectrum is an ideal position to detect the scattering modulated by potential since the scattering contrast is highest there. The 16 nm thickness gold film and 16 nm height, 466 nm in-plane length gold nanoprism exhibited high voltage sensitivity compared to other gold nanostructures studied in the same condition. = 618 nm and = 640 nm were the locations of highest sensitivity shown by 16 nm gold film and 16 nm height gold nanoprism respectively. We herein focus on the scattering response transient of 16 nm thickness gold film and 16 nm height gold nanoprism under potential sweep and step application. Their scattering modulated by various potential controlled modes was observed at single wavelengths which their largest voltage sensitivity was at. The track of accurate scattered light transient is helpful to explore the interfacial electron transfer originated from the charging-discharging process between gold and electrolyte.
2. Experimental methods Indium-tin-oxide (ITO) coverslips (surface resistivity 15–30 /sq, Structure Probe, Inc.) were used as substrates and they were cleaned in isopropanol and acetone in an ultrasound bath and rinsed copiously with Milli-Q 18 M water prior to the gold nanostructure preparation [14]. 16 nm thickness gold film, 16 nm height and 466 nm in-plane length gold nanoprism were fabricated on the ITO substrate by using thermal evaporation and nanosphere lithography respectively [15–17]. The electrochemical cell is composed of Teflon cages. Gold nanostructure immobilized on ITO coverslips was working electrode connected to a potentiostat (Princeton Applied Research). An coiled
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Fig. 1. (a) Dark field image of 16 nm thickness gold film recorded at wavelength of 618 nm. (b) Dark field image of 16 nm height and 466 nm in-plane length gold nanoprism recorded at wavelength of 640 nm. (c) SEM image of 16 nm thickness gold film. (d) SEM image of 16 nm height and 466 nm in-plane length gold nanoprism.
Au and an Ag wire were counter and reference electrodes respectively. The dark-field imaging spectrometer described elsewhere [13] was used to monitor gold nanostructures’ scattering modulated by potential in 0.1 M NaCl. The epi-illuminated spectrometer enables the objective lens to collect the back scattered light from gold nanostructure. The single wavelength light scattered by the gold film and nanoprism was selected by a movable linear variable interference filter and was recorded on the camera. The exposure time was dependent on the scattering strength of gold nanostructure. The acquisition time of 3.6 s for one image of gold film and 1.4 s for one image of gold nanoprism and their SEM images are shown in Fig. 1.
3. Results and discussion 3.1. Scattering controlled by linear potential sweep The capacitances value of gold film and gold nanoprism determined from the impedance measurements are shown in Fig. 2. The location of dip capacitance of gold film is slightly negative with respect to the 0 V while that of nanoprism shifts to positive direction. The value of point zero charge (pzc) of gold nanostructure is dependent both on the nature of gold nanostructure and ions. The morphology of gold nanostructure determines that the capacitance of 16 nm gold film is higher than that of 16 nm height gold
Fig. 2. Capacitance of gold film (a) and gold nanoprism (b) as a function of electrode potential.
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Fig. 3. Transition of the scattering of (a, b, c) 16 nm Au film and (d, e, f) 16 nm height gold nanoprism during the cyclic voltammetry. The voltage scan rates used in (a, b,c) and (d, e, f) were 0.0112 V/s, 0.0056 V/s, 0.0028 V/s and 0.0276 V/s, 0.0138 V/s, 0.0071 V/s respectively. The initial potentials for red squares and blue circles were 0.2 V and −0.2 V respectively.
nanoprism in the solvent of sodium chloride, which is partly contributed by the specific adsorption of chloride ions. The degree of chloride ions adsorption on the gold film is stronger than that of gold nanoprism. The presence of adsorption of anions will negatively shift the value of pzc [18]. As a result, the pzc of gold film is slightly negative than that of gold nanoprism. When linear potential between 0.2 V and −0.2 V were swept on gold film and gold nanoprism, a gradually increasing rate of change in scattering during each linear scan was clearly observed no matter which direction and scan rate were. The averaged scattering strength of 20 regions on the gold film taken at = 618 nm, each corresponding to a sample area of approximately 0.5 m2 , and scattering collected from 60 to 70 16 nm height gold nanoprisms,
recording at = 640 nm are shown in Fig. 3. Determining the rate of change in scattering within the ±200 mV potential linear sweep was achieved by differentiating the measured samples in these linear potential scans, shown in Fig. 4. The scattering transients were fitted to the equation I(t) = a(1 − e−t/ ) + b
(1)
where a is the change of scattering at the final steady state, b is the initial change of scattering after the application of the potential sweep, t is the duration time and is the time constant. Other types of function of time were also used to fit the scattering transient, but Eq. (1) always gave rise to the best fit.
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Fig. 4. Differential scattering of 16 nm Au film in response to the potential scan at (a) 0.0112 V/s, (b) 0.0056 V/s (b) and (c) 0.0028 V/s and differential scattering of 16 nm height Au nanoprisms in response to the potential scan at (d) 0.0276 V/s, (e) 0.0138 V/s and (f) 0.0071 V/s.Subtracting the i+1th scattering sampling from the ith sampling can get the change rate of scattering within the fixed exposure time. Repeating the subtraction at each single scan and averaging twelve of them can obtain the change rate of scattering represented as absolute value by red squares lines in ±200 mV voltage window. The blue circles lines are the fitting curve for Eq. (1).
The red solid squares lines and blue solid circles lines in Fig. 4 are the experimental differentiating scattering step and their corresponding fitting curves respectively. The gold film sets value of (a, b, ) for the scan rate 0.0112 V/s, 0.0056 V/s and 0.0028 V/s are (101.6, 14.56, 17.54 s), (53.65, 6.695, 33.87 s), (28.47, 4.628, 76.15 s) and gold nanoprism set value of (a, b, ) for the scan rate 0.0276 V/s, 0.0138 V/s and 0.0071 V/s are (44.7, −7.524, 3.607 s), (23.19, −1.335, 8.796 s) and (9.612, 0.3883, 13.92 s) respectively. It is noticeable that both the ratio of three time constants of gold film and gold nanoprism, 17.54:33.87:76.15 = 1:1.93:4.34 and 3.607:8.796:13.92 = 1:2.43: 3.86, closely matches the reciprocal ratio of their corresponding scan rates 0.0112:0.0056:0.0028 = 1:1/2:1/4 and 0.0276:0.0138:0.0071 = 1:1/2:1/3.88.
The inversely proportional relationship between time constants and potential sweep rates discovered in gold nanoprism and gold film implies the universality of the interpretation for the potential’s perturbation on gold nanostructures. At the beginning of potential application, the accumulation of counterions outside the gold surface and the assembly of excess charges inside the gold surface construct the double layer at the metal-electrolyte interface. An external electric field induced by the presence of the double layer tends to penetrate into the gold. Meanwhile, an internal electric field is created by the movement of the electrons in the gold’s space charge region to resist the influence of the external electric field [19,20]. Equilibrium was achieved after a certain time depending on the potential scan rate. The balanced interaction between the internal and the external electric fields is reflected by the scattered
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Fig. 5. The scattering of (a, b, c, d) 16 nm gold film and (e, f, g, h) 16 nm gold nanoprism in response to a potential step as the sequence in (a, e) and (c, g) were 0→0.05→−0.05→0 V and 0→0.2→-0.2→0 V, represented as red squares and lines. The potential sequences in (b, f) and (d, h) are revered as 0→-0.05→0.05→0 V, 0→0.2→0.2→0 V, represented as blue circles and lines.
light, whose rate of change in intensity tends to match the scan rate of the potential. The faster the potential scan rate is, the earlier the equilibration is established and the smaller the time constant will be.
3.2. Scattering controlled by step potential The scattering from the gold film and gold nanoprism changing with time after the application of ±50 mV and ±200 mV potential
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Fig. 6. The rate of change of scattering from (a, b) 16 nm gold film and (c, d) 16 nm height gold nanoprism during the application of a potential step. The potential sequence of the red square lines in (a, c) and (b, d) are 0→0.05→-0.05→0 V, 0→0.2→-0.2→0 V and the blue circle lines in (a, c) and (b, d) are 0→-0.05→0.05→0 V, 0→-0.2→0.2→0 V. Each point was obtained by subtracting the i+1th scattering sampling from the ith scattering sampling.
steps are shown in Fig. 5. Their scattering had almost no distinctive feature in the first duration of 0 V. Once a polarized potential step was applied, the scattering was perturbed to change with the trace of potential. As the potential step size increased, the level of scattered light stabilized more quickly. The different scattering intensity at the initial and final applications of 0 V was due to the slow relaxation of charge convection after the gold nanostructure suffered a long application of potential, although the application time of the positive and negative potentials was equal. The quantity of charge transferred was limited by the surface area of the gold. Application of a larger potential results in faster allocation of large charges and an earlier achievement of a balanced electric field at the gold-solution interface. The gold film’s differentiation of scattering is presented in Fig. 6 (a) and (b). The anion Cl− adsorbing at the gold film surface in the case of positive potential is stronger than the cation Na+ adsorbing in the case of negative potential [21]. As a result, the application of positive potential induces larger capacitance than negative potential on the electrode surface when the potential was altered between the positive and zero. Since the exchange quantity of chloride and sodium ions adsorbed on the electrode surface induced by the switch of two polarized potentials are almost equal, the same change in the magnitude of adsorption capacitance results in the same size of spikes. To explore the penetration depth of electric field of gold nanostructure into the solvent, the electric field outside the gold film and the gold nanoprisms was calculated, choosing the wavelength to be that associated with the experimental condition. Spherical particles with 5 nm diameter are assumed to embed on the surface of gold film. Fig. 7(a) and (b) show the top surface view plots of the square of the normalized electric field for gold film
and the base 10 logarithm of square of the normalized electric field for gold nanoprisms respectively. The incident wavevector is perpendicular to the surface upon which the nanostructure sits (not shown), and the polarization vector is parallel to the surface and along y direction. The induced electric field at the vertices of gold nanoprisms is much higher than that at rough surface edge of gold film, although it is smaller at the central part of gold nanoprism. To further study the decay length of the induced electric field, Fig. 7(c) shows the integrated electric field as a function of distance from the top surface of gold nanostructure. In the case of gold film, the electric field falls off very rapidly with distance from the surface, while for the gold nanoprism, the electric field decreases slowly and its value becomes higher than that of gold film at the distance from 15 nm. This indicates that the electric field extends further out from the 16 nm gold nanoprism than that of 16 nm gold film, due to its sharp curvature [22,23]. Although the specific adsorption anions Cl− is capable of increasing the capacitance of the gold nanoprism’s surface in the case of positive potential, more cations Na+ at the vicinity of gold nanoprism’s surface are detectable due to the longer EM field decay length in the case of negative potential. The presence of more detected cations is transduced as an increase of local refractive index and consequently increases the scattering strength when the negative potential is used. The field enhancement resulting from the gold nanoprism’s sharp curvature is considered to compensate the smaller scattered light in the case of negative potential and enhances it to match the scattered light caused by the positive potential. As a result, the change of scattering of 16 nm height gold nanoprism was almost equal when the potential were switched between 0 V and any other levels as shown in Fig. 6(c) and (d).
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the reciprocal ratio of the three potential sweep rates reveals that the electrons in gold nanostructure have the same mechanism to respond to dynamic potential. The universality of the exponential fit indicates the mechanism of potential on the gold nanostructure is the same, whatever the morphologies are. The adsorptive anion Cl− enhances the change in scattering strength significantly at positive potentials for gold film. However, due to the special curvature of gold nanoprism, the effect of adsorption anion on the change in scattering is less comparable to that of gold film. The different performances of scattered light between gold nanoprisms and gold film under potential control confirms the size and shape effect on the localized surface plasmons and the important role morphology plays in the determination of potential detection capability. These results could provide a gateway to investigate the sensor of gold nanostructure in the electrochemical application. Acknowledgements This work is finally supported by Chongqing Key Scientific and Technological Program Project (cstc2011ggB0015) of China, National Natural Science Foundation of China (Grant No. 61275061), West Light Foundation of The Chinese Academy of Sciences. References
Fig. 7. Plots of (a) the square of the normalized electric field for gold film and (b) the base 10 logarithm of square of the normalized electric field for gold nanoprisms at the top surface. (c) Integrated normalized |E|2 E as a function of distance from the top surface of gold film and nanoprisms.
4. Conclusions Time dependent scattering of 16 nm thickness gold film and 16 nm height gold nanoprisms has been studied by the application of potential cyclic voltammetry and potential steps. An exponential expression for the differential scattering induced by the potential cyclic voltammetry can be applied to gold film and gold nanoprism. The faster the potential scan is, the earlier steady state of the scattering transients will settle. The similar ratio of time constants deduced from the scattering’s exponential fits as
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