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Electroless Deposition of Pd Nanostructures for Multifunctional Applications as Surface Enhanced Raman Scattering Substrate and Electrochemical Non-Enzymatic Sensor Abhijit Roy, Shib Shankar Singha, Sumit Majumder, Achintya Singha, Sangam Banerjee, and Biswarup Satpati ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00420 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Applied Nano Materials

Electroless Deposition of Pd Nanostructures for Multifunctional Applications as Surface Enhanced Raman Scattering Substrate and Electrochemical Non-Enzymatic Sensor Abhijit Roy,a Shib Shankar Singha,b Sumit Majumder,a Achintya Singha b, Sangam Banerjee a and Biswarup Satpati*,a ,

a

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, HBNI,

1/AF Bidhannagar, Kolkata-700064, India b

Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Kolkata

700009, India

*E-mail: [email protected]

ABSTRACT: Herein, we report a method to produce surface-enhanced Raman scattering (SERS) active and excellent non-enzymatic glucose and ascorbic acid (AA) sensing substrate by electroless deposition technique. Palladium (Pd) nanoparticles were deposited on different semiconductor (Si and Ge) and patterned (pyramidal Si) surfaces without any use of surfactant. Growth rate and the final morphology of the Pd nanostructures are observed to be dependent heavily on surface energy of the substrate and number of defects present on the substrate surface. Highest SERS enhancement is observed for Pd nanoparticles deposited on pyramidal Si substrate. Finite-difference time-domain (FDTD) simulation substantiate the experimental observation by showing that the sharp tip and the gap between the shafts are main contributing

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factor to the large enhancement of the incident electric field. Our result shows superior SERS enhancement compared to previously reported literature using pure Pd nanoparticles and also Pd nanoparticles deposited on different substrates. The substrates showed very good sensing properties for glucose and AA detection. The highest sensitivity (18.67 μA mM-1 cm-2) for AA is observed for Pd deposited on Ge substrate for 60 minutes in the linear range of 20 µΜ to 40 mM and for glucose the highest sensitivity (2.658 μA Mm-1 cm-2) is also observed for the same substrate in the linear range of 1 mM to 40 mM. Lowest detection limit for AA and glucose is 2.19 µΜ and 7.19 µΜ, respectively for the same substrate. Substrates we prepared are very useful for multifunctional applications like SERS and electrochemical non-enzymatic sensor.

KEYWORDS: Electroless deposition, Pd nanoparticles, Surface enhanced Raman scattering, Electron microscopy, FDTD simulation, Non enzymatic sensor.

1. INTRODUCTION During last four decades, research interest have been grown immensely in developing surface-enhanced Raman scattering (SERS) active substrate.1 In case of SERS, the Raman scattering cross sections of molecules adsorbed on metal surfaces can increase dramatically.2 The ability of SERS to detect molecular concentration up to single molecule level has made this phenomenon really fascinating.3 Besides biological application, SERS has also been used in , catalysis, chromatographic separation and sensors.4-6


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Silver (Ag) and Gold (Au) are the two metals used extensively as SERS active substrate due to their strong localized surface plasmon resonance (LSPR) at the surface of these metallic nanostructures to produce large enhancement effect in the UV-visible region.7-9 However, this limits the involvement of other materials as SERS active substrate. Recently, noble metals like platinum (Pt) and palladium (Pd) are also observed to be SERS active,10-11 although the enhancement factor (EF) is found to be lower than that of Au and Ag.12-13Since Pd shows high catalytic activity it finds application in hydrogen storage, chemical sensors, fuel cells, which motivate researchers to find processes where Pd can be used as improved SERS active material.14-17As SERS effect depends on various parameters like size, shape and inter-particle distances, different fabrication processes lead to different morphologies which leads to different enhancement factor.17 SERS effect of Pd was first studied by Srnova et al. where they reported an EF of 190.18 EF of 103 was obtained on electrodeposited Pd.11 Different other morphologies like hexagon, cubes were tried and an average EF of 104 was achieved. Pd nanoparticles of 60 nm in size were observed to show better SERS effect than 30 nm or 110 nm nanoparticles.19 Fang et al. observed a decrease in SERS signal with increasing thickness of Pd shell.20 Gutes et al. showed that Pd nanoparticles deposited by galvanic displacement process do not undergo any compositional changes after several weeks.21 This is a very important observation towards the production of stable SERS substrate. However, Pd nanoparticles deposited by electrochemical or electroless process showed very low SERS effect (~103) which motivates us to work with Pd nanoparticle on semiconductor substrate to improve the enhancement factor.11,18,22-24 E. Ringe et al. have shown size dependent narrow and strongly localized LSPR that can be formed at Pd rich tips and can couple with the dielectric substrate underneath which would be very much beneficial for SERS.25 Now, various applications demand that the particles should strongly adhere to the substrate otherwise uneven coagulation may occur which can lead to undesirable result. Hence, it is important to grow the



nanostructures directly onto the substrates.21 Between various deposition techniques, electroless deposition is a very simple, clean and cost-effective process to deposit Pd nanoparticles on solid substrates.26 Here, the difference in the reduction potential between two materials is the driving force of the reaction, which finally leads to uniform deposition of one material on top of the other.26-27 A clean interface is obtained between the substrate and the film as no external reducing agent is used. Also, metal films deposited by electroless process shows excellent adhesion with the substrate. Still, there are very few reports in the literature regarding SERS enhancement using Pd nanoparticles produced by electroless deposition. 22-24 To the best of our knowledge, there is no report where researchers have studied the effect of different substrate and deposition time on the SERS effect of Pd nanoparticles. The standard reduction potential of Pd2+/Pd pair (0.83 V vs. standard hydrogen electrode) is higher than Si4+/Si (-1.2 V) and the reaction equation is shown below,11 2− 0 2PdCl2− 4 (aq.) + Si (s) +6F (aq) → 2Pd (s) + SiF6 (aq.) + 8Cl

HF is very much essential in this reaction to produce SiF62− , a water-soluble product and can be taken away to give direct contact between Pd and Si. Two main features which affect the SERS are (i) Electromagnetic (EM) enhancement and (ii) Chemical (CM) enhancement. EM plays a dominant role in the enhancement process and can enhance the SERS effect to 108-1011 order of magnitude28-29 on the other hand CM can enhance the SERS effect up to 10-100 times.30-31 The EM enhancement heavily depends on size, shape, inter particle distance, and different interfaces. On the other hand, CM enhancement contributes in this process mostly in the form of charge transfer mechanism between adsorbed molecules and nanoparticles. Xu et al. fabricated Pd nanoneedle on ITO coated substrate by electrodeposition method and obtained EF of 103.32 However, to control the morphology of the Pd nanoparticles deposited on semiconductor substrates various surfactants like PVP, CTAB


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were used in these cases.11,32 Therefore, it may not be easy to separate these surfactants completely from nanoparticles and hence may affect the SERS enhancement process. Diabetes mellitus is one of the major health problems nowadays for modern world. It arises from the deficiency of insulin from human body and can leads to serious health complications and reduces life expectancy. The normal range of sugar in human blood is 80-120 mg/dl but anything over 200 mg/dl is sign of diabetes and should be treated accordingly. 33 So, detection of glucose is very much important for detection and treatment of diabetes. On the other hand, ascorbic acid (AA) which is an anti-oxidant is a very vital vitamin for human diet. It has efficient application in infertility, common cold, mental illness, HIV and in cancer also.34-35 So, detection of glucose and AA are important not only in biological and neurochemistry field but they are equivalently important for pathological and diagnostic research also. Various enzymes-based sensors can be able to detect glucose and AA efficiently.36-38 But enzymesbased sensors always show poor stability and it is also affected vastly by other electro-oxidized species. Introduction of non-enzymatic amperometric sensors has changed the way a lot. Metal nanoparticles have observed to be very much effective for the production of stable nonenzymatic sensor.39-40 Besides, various methods of detection, electrochemical method are fast, low-cost and can be easily fabricated.41 Electrochemical sensors based on metal nanoparticles (Au, Ag, Pt, Pd, Fe, Mn) have already observed to show sensitivity towards glucose, AA and H2O2 detection.41-50 However sensors consists of nanoparticle is needed to be drop casted on conducting substrate for electrochemical detection, which most of the cases does not provide a good adhesion to the substrate and can affect the signal when sample is inserted into the electrolyte. However, in GD process a chemical bond is created between the deposited metal and the substrate. This helps to obtain a stronger adhesion between them (metal and substrate). Hence disturbance in the signal will be less compared to the sensors prepared based on drop casting of nanoparticles.



In this work, we show that the size and shape of the Pd nanoparticles deposited on semiconductor substrates can be controlled by changing the deposition time, substrate material and without using any surfactant that may influence the SERS activity. Also, the substrates hence produced will be able to detect AA and glucose efficiently with significant sensitivity and detection limit. Different substrate morphology was studied in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and optical property by Raman spectroscopy. SERS technique was employed to study the morphology dependent sensing of the Pd nanostructures. The experimental results are supported by the Finitedifference time-domain (FDTD) computer simulation. Electrochemical analysis was performed to show sensing property towards AA and glucose. 2. EXPERIMENTAL PROCESS: 2.1. Deposition of Pd: Si (100) [p-type] and Ge (100) [p-type] substrates were used to deposit Pd nanoparticles by electroless deposition. Initially, both the substrates (Si and Ge) were subjected to ultrasonic cleaning using ethanol, acetone and de-ionized (DI) water for 5 minutes respectively. To produce pyramidal Si (P-Si) array saw-damage removal (SDR) process was used. In this process, the substrates were first immersed into a solution of 30 wt% NaOH solution in DI water at 75ºC for 30 minutes. In this step 5-6 µm thick layer from both the sides of the Si substrates were removed. In next step, the substrates were again immersed in a mixed solution of NaOH (3 wt%) and isopropyl alcohol (IPA, 10%) in DI water at 80ºC for 45 minutes to finally P-Si arrays were produced by anisotropic etching. Plan Si (100) substrates were subjected to RCA cleaning process to finally produce a layer of SiO 2 at the Si surface.51 P-Si (100) and Si (100) substrates were immersed in 2%HF solution in DI water for 1 minute to remove the native oxide layer from the surface. The Galvanic displacement plating solution was prepared by mixing the appropriate amount of PdCl2 (Aldrich,99%) and HF (48%, Merck) dissolved in 0.3 M KCl (Aldrich,99%) solution to obtain a final Pd2+ concentration of


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0.15 M and F- concentration of 20 mM. Excess KCl was required to dissolve PdCl2 and finally to form the PdCl42- species. Ge (100) substrates were directly inserted into the plating solution after ultrasonic cleaning. Similarly, Si (100) and P-Si(100) were also inserted into the plating solution. All three different substrates were immersed for three different times to observe the time-dependent morphology. Henceforth for convenience, we will use different notations for different samples grown for different time on different substrates. 10 minutes, 20 minutes and 60 minutes Pd deposited on Ge (100) substrate will be called as G10, G20 and G60, respectively. Similarly, 10 minutes, 20 minutes and 60 minutes Pd deposited on Si (100) substrate will be called as S10, S20 and S60, respectively and 10 minutes, 20 minutes and 60 minutes Pd deposited on P-Si (100) substrate will be called as PS10, PS20 and PS60, respectively. The electroless process of Pd deposition and their morphological variation with time on different substrates is shown in the schematic (Figure 1).

Figure 1. Schematic representing different kinds of formation of Pd nanostructures on different substrates.



2.2. SEM and TEM Characterization: For scanning electron microscopy (SEM) we have used a ZEISS SUPRA 40 field emission gun (FEG) microscope operated at 20 kV. Crosssectional TEM (XTEM) measurements were done using FEI Tecnai G2 F30-ST microscope operated at 300 kV. The TEM is also equipped with high-angle annular dark field (HAADF) detector from Fischione (model 3000) and energy dispersive X-ray spectrometer from EDAX Inc. for compositional analysis.27 XTEM samples are prepared by mechanical thinning followed by low energy (3.0 keV) Ar+ ion milling using precision ion polishing system (PIPS, Gatan) and cleaning at 1.5 keV. TEM images were recorded when substrate was aligned along [110] zone axis condition. 2.3. SERS Measurement: SERS experiment was performed using a micro Raman Spectrometer (LabRam HR, Jobin Yvon) equipped with Peltier cooled CCD detector at room temperature. He-Ne laser with wavelength of 632.8 nm was used as excitation light source and a 100× objective lens was used to focus the laser on the sample and to collect the Raman signal. The laser spot size was 1 μm × 1 μm and laser power was 100 μW. The data were recorded for 10s integration time. Assuming R6G to be deposited on Si substrate uniformly the number of molecules within the focal volume (NBULK) and the number of molecules probed in the SERS substrate (NSERS) was determined. 2.4. Electrochemical Measurement: Electrochemical measurements were done using CHI 660C electrochemical workstation (CH instrument, USA). The electrochemical measurements were done using a three-electrode configuration with the substrates as the working electrode, Ag/AgCl as the reference electrode and Pt wire as the counter electrode. The substrates were washed by acetone and ethanol and dried before the measurements. 1M Na2SO4 solution was used as an electrolyte due to its high ionic conductivity and low cost.


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2.5. FDTD Simulation: 3D finite-difference time-domain (FDTD) simulations (from Lumerical Solutions, Canada) were used to understand the near-field intensity distribution and to calculate the SERS EF from different nanostructures of Pd. The nanostructures were laid on planar Si, Ge and P-Si to mitigate the structures observed using electron microscope. The coupled systems were excited by a total-field scattered-field (TFSF) source with negative zaxis as the propagation direction and x- axis as the polarization direction. The total simulation region was set as 500 × 500 × 500 nm3 and perfectly matched-layer (PML) boundary conditions were used. The simulation time was set to 500fs so that the total internal field decay completely. The mesh size was set to 1.5 × 1.5×1 nm during all numerical simulations. Field monitors were placed at x-z and y-z plane around the nanostructures and 632 nm excitation wavelength with 100 nm span on both sides were used. The geometries of different structures and dielectric constant values of Si, Ge and Pd used for the simulation is shown in Fig. S11, SI and in Fig. S12, SI, respectively.

3. RESULTS & DISCUSSIONS 3.1. Morphology: SEM Analysis. Figure 2 together with Figure S1, SI shows the detailed morphology of the deposited Pd nanoparticles on different substrates for various deposition times. For 1 min deposition on Ge particle size is 41 ± 4 nm [Figure S1a, SI]. The particles are consisting of spherical shaped core and shaft on it. With increasing deposition time, the size of the spherical core increases but the length of the shafts does not increase significantly [Figure S1b, SI]. The deposited particles took a spherical shape (for 10 minutes deposition time) on which sharp metal edges are produced and almost all the particles form a spherical shape core (Figure 2a). A continuous layer of Pd is observed in this case and confirmed by the TEM observation. With increasing deposition time (20 minutes) Pd nanoparticles form cluster of



dendritic structures (nanoflake) having a very rough surface as shown in Figure 2b. An almost continuous layer in combination with spherical shaped particle of diameter ranging from 300500 nm is observed for 60 minutes deposition time. However, the surface of the spherical shape particle is rough as observed from high resolution SEM images [Figure S1e, SI], thereby producing more electroactive surface area. Uniform particle growth is observed for all substrates when observed in larger scale. We have observed a very slow deposition rate and also a different nanostructure of Pd on Si (100) as observed from Figure S1f, SI and Figure S1g, SI. For 1 minute and 2 minutes deposition, it is observed that sharp urchin like structures are formed with relatively smaller and asymmetric shape core. With increasing deposition time, the inner core increases in size but they are smaller compared to the sphere formed on Ge (100). For 10 minutes deposition time, the surface coverage is low with an average particle diameter of 35 nm (Figure 2d). With increasing deposition time (20 minutes), the particles grow larger in size and some of them form spherical cores on which sharp metal edges are formed (Figure 2e and Figure S1i, SI). The numbers of sharp metal edges increase and it took a sharp petallike shape and finally cover the whole spherical core for deposition time of about 60 minutes (Figure 2f, Figure 2j). We have observed faster growth rate of Pd on P-Si compared to planar Si. From Figure 2g it is observed that the growth rate is highest along the ridge of the pyramid (brighter portions) due to increased surface energy. Here also the particles took an urchin-like shape. With increasing deposition time, the surface coverage of Pd nanoparticles increases keeping its rough features at the top (Figure 2h). Uniform and coverage of almost 76% is obtained for 60-minute deposition time (Figure 2i). However, unlike Ge substrate the sharp metal edges are produced here are different in nature and helpful to produce efficient SERS enhancement in combination with the pyramid structure of Si. Figure 2 (panels j-l) shows the magnified images of G60, S60 and PS60, respectively.


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Figure 2. Top view secondary electron images. (a), (b) and (c) represent SEM of G10, G20, and G60 samples, respectively. (d), (e) and (f) represent SEM of S10, S20 and S60 samples, respectively. (g), (h) and (i) represent SEM of PS10, PS20 and PS60 samples, respectively. (j), (k) and (l) show magnified views of G60, S60 and PS60 samples, respectively.



Figure 3. Bright-field XTEM (Cross-sectional TEM) images:(a), (b) and (c) represent samples G10, G20, G60, respectively. (d), (e) and (f) represent samples S10, S20, PS60, respectively. (g), (h) and (i) represent samples PS10, PS20, PS60, respectively. (j), (k) and (l) show magnified images of G60, P60 and PS60 samples, respectively. The insets show the magnified image of the Pd nanostructures. 3.2. Structural Study: TEM Analysis. To study the structures in detail we have done extensive TEM analysis. The bright-field TEM image of Pd deposited on Ge (100) for 10 minutes is shown in Figure 3a. From the magnified view (inset), one can observe that Pd nanoparticles form flower or urchin-like structure which can be considered as a spherical core on which dendritic metal edges are formed. The STEM-HAADF image and the elemental


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mapping confirm the presence of urchin shaped Pd nanoparticles on Ge (Figure S2, SI). The height and width of the urchin structure is 117.8± 18.5 nm and 150.2±25.2 nm. The dendritic shaped shafts have sharp metal corners which can enhance the incident EM field effectively.32 The shafts have an average length of 37.5 nm with FWHM of 15.6 nm. The average tapered angle is 38.52º. A continuous layer of Pd of 15.9 ± 2.3 nm thickness is observed at the bottom of the structure on which the urchin structures are formed. The film is formed with Pd (111) plane as observed from the HRTEM images and the FFT pattern [Figure S3, SI]. The HRTEM image and the FFT pattern from the metal edges indicate that the edges are consists of atomic plane with inter-planar spacing of 2.25 Å,1 which is close to (111) inter planer spacing of fcc Pd. When Pd nanoparticles are deposited for 20 minutes (Figure 3b), individual flower-like feature demolished due to agglomeration but the dendritic structure remains intact. The thickness of the continuous layer is 64.4± 3.1 nm with a decrement in the average length of the shafts (28.5±7.5 nm). The width of the shafts also decreases to 9.9 nm. The magnified image of a single dendrite is shown in the inset which shows the formation of some small bumps on its sides. For 60 minutes deposition time, the shafts of Pd nanoparticles lost their sharpness and obtained a flat top morphology as shown in Figure 3c. The thickness of the Pd layer becomes 300 nm for 60 minutes deposition time. The growth of Pd is totally different on planar Si compared to Ge. The bright-field TEM image (Figure 3d) shows a clear view of the Pd nanoparticles on Si for 10 minutes deposition and it is observed that the rate of growth of Pd on Si is very less compared to Ge. The EDX spectra and line spectrum together confirm the presence of Pd on Si [Figure S4, SI]. Pd is observed to form in the form of nanorod which finally agglomerates to form an urchin like structure as observed from the SEM images previously. The average length of the nanorod is 29.5±6.6 nm. The inset of Figure 3d shows the high magnification image of the primary stage of the nanorod



growth from the clusters having 6 nm height and 10.68 nm width. The initial clusters are of spherical shape having 5-7 nm diameter. Which grow in a particular direction to finally form the nanorod. The magnified view shows that the cluster contains line dislocations and strain induced lattice distortion [Figure S5, SI]. When the deposition time is increased the length of the Pd nanorod combination increases (Figure 3e). The average length of the Pd rods increases to 38.3± 5.9 nm and the width decreases to 7.2 nm. So, in both the cases with increasing deposition time the length of the shaft or rod increases while there is a decrease along the lateral direction. The coverage of Pd also increases as shown in Figure 3e. Increased deposition time also leads to urchin-like shape with increased shaft length (Figure 3f). The shafts are of 77.5± 12.5 nm length and 17.1± 2.1 nm in width. When Pd nanoparticles are deposited on P-Si, the growth rate is observed to be much faster compared to planar Si. The bright-field TEM image shows the structure of the Pd nanoparticles on P-Si (Figure 3g). Morphology of Pd on the ridges and at the base of the pyramid is shown in Figure S6, SI (panels a-b) respectively. The height of the Pd shafts decreases along the ridge of the pyramid as shown by arrow direction in Figure S6, SI (panel a). The thickness of the agglomerated portions increases at the valley region of the pyramid (Figure S6, SI). For 10minute deposition time almost a continuous layer of Pd of thickness 45 nm is observed on PSi (Figure 3g). From the high magnification image (inset) it is observed that they took a flowerlike morphology containing smaller size shafts. Higher deposition time (20 minutes) leads to formation of shafts along the perpendicular direction of the substrate (Figure 3h). Inset shows formation of twin boundary at the mid-portion of the Pd shaft and extends from top to the bottom of the structure. The magnified image at the interface shows that the shafts are single twinned structure. Where the twin plane is at the mid-section of the shafts and the twinning starts from the base of the Pd film. This shows directly that the Pd shafts are formed from single twinned nanocluster. However, for 60 minutes deposition the thickness of the Pd nanoparticle


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layer increases up to 95.2± 16.6 nm with revival of flower-like structure due to agglomeration (Figure 3i). Figure 3 (panels j-l) shows the magnified XTEM images of G60, S60 and PS60 respectively. Figure 3k shows some small side branches are formed from the shafts. The top angle of the shafts is 28.32º. However, some shafts are merged together to form a continuous layer at the bottom on the substrate. 3.3. Analysis of crystal structure: Figure 4a shows HRTEM image of a single Pd shaft for 10 minutes Pd deposition on Ge (100). The fast Fourier transform (FFT) pattern and inverse FFT (IFFT) images show the presence of Pd (111) plane (Figure 4b). Figure 4c shows SAED pattern taken along [110] zone axis of Si which shows polycrystalline nature of Pd nanostructure and diffraction pattern corresponds to (111), (220), (200) and (311) plane of Pd. HRTEM image of initially deposited Pd cluster on planar Si (100) is shown in Figure 4d. The IFFT image indicates that the cluster consists of (111) plane and contains lattice defects like dislocation (Figure 4e). The SAED pattern along [110] zone axis indicates the formation of Pd (111), (200) and (311) plane. The HRTEM image of the Pd shaft shows formation of twin boundary along the mid-section of it and extends almost to the top of the shaft (Figure 4g). When deposited on P-Si the twin boundary is observed to form at the bottom of the Pd shafts (Figure 4i) and Pd (111) plane is observed to make an angle of 24˚ with the Si (111) plane which indicates formation of tilted epitaxy in this case (Figure 4j).



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Figure 4. (a) HRTEM image of a single Pd shaft of G10 sample. The inset shows FFT pattern from the portion marked by red rectangle in (a). (b) IFFT image shows Pd (111) plane and its direction w.r.t substrate. (c) SAED pattern taken along [110] zone axis showing diffraction spots corresponds to Si and Pd. (d) HRTEM image of a single Pd cluster (S10) from which nanorod shaped Pd nanoparticles were developed shows formation of dislocations and grain boundary and strain-induced defects. (e) IFFT image (f) SAED pattern from S60 showing good crystallinity. (g) HRTEM image of a single Pd shaft showing formation of twin boundary at the mid portion. (h) SAED pattern taken along [110] zone axis showing a formation of tilted heteroepitaxy between Pd and Si for PS20 sample. (i) and (j) HRTEM image of a single Pd shaft

and

FFT

from

the

interface

of

PS20,

respectively.

Surface defects and surface energy at the substrate surface plays the most dominant role to determine the final morphology and growth rate of deposited film. The surface energy of H-


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passivated Si is 0.3 J/m2and surface energy of SiO2 is also same.52 However, the surface energy of Ge is 1.06-1.835 J/m2and of Pd is 2.01 J/m2.53-54 Hence it is obvious that same metal (Pd) to be deposited on both the substrate, growth rate will be higher on Ge compare to Si. The Pd2+ ions initially tend to reduce at different defect centre (nucleation centre) on the substrate. Initially Pd ions reduce slowly on Si substrate to form Pd nanocluster. The Si substrate and the nanocluster act as a local anode and cathode in an electro-chemical redox reaction. The Pd ions then reduced on the surface of the clusters. Once the critical size of the nucleus is reached, the oxidation (of Si) and reduction (of Pd) ions increases concurrently facilitating the anisotropic formation of dendritic shaft on the clusters leading to the formation of urchin-like structures.11 Hallide ion (Cl–) plays an important role in the formation of such anisotropic nanostructure. The Cl– ion tend to adsorb on the Pd (111) plane and leads the formation of anisotropic growth of Pd.55 Song et al. observed that in presence of SO42- ion dendritic Pd nanostructure was formed.56 So, different kind of Pd nanostructure can be made by just controlling the deposition rate, time and by choosing proper precursor solution. J. Kim et al. was able to produce Pd nanoflake and Pd nanorod structures using electrodeposition process in the absence of any external reducing agent. Depending on the applied potential they varied the reaction rate which finally leads to different morphology of Pd nanostructures.23-24 Song et al. also observed that a lower reduction potential could produce thick and short branches of dendritic nanowire due to slow growth rate.56 We have also observed on Si formation of nanorod like structure indicating a slower growth rate. On Si metal clusters tend to take a nanorod like shape indicating their growth rate is higher along the vertical direction compared to the horizontal direction. Columnar or nanorod shaped growth of Pd nanostructure was observed by Xia et al. using chemical synthesis method.57 Pd was observed to form nanoneedle like shape using electrodeposition method.32 All these studies indicate higher growth rate along the vertical direction on Si. In Ge, the Pd particles tend to form a spherical shape core which was observed previously



when PVP surfactant was used but in our case, we could modify the shape using different substrates and also using electroless deposition.11 As P-Si is formed by the process of anisotropic etching of Si, the surface of pyramid become rougher and also contain more defect spots which can act as a nucleation centre, justify higher growth rate of Pd on P-Si.58 Conrad et al. have observed that clean substrates, free from electron beam damage results a very small number density of crystallites and at higher defect rates, the number density of crystallites gets increased by a factor of 10.59 Lower number density of Pt is observed on Si surfaces containing fewer defects.59 The number of increased defect spots is confirmed in our study as the number of particles increases while a reduced agglomeration is observed. Ostwald ripening is the dominant process behind the agglomeration of the nanoparticle26but in a surface where roughness is more the mobility of the particles get reduced while defect spots act as a nucleation centre for deposition. In earlier work, control on structures was obtained by adjusting the deposition voltage but in this work, we use the surface energy and defects of different substrates to vary the growth rate and to obtain different nanostructures. 3.4. Measurement of SERS Activity: Rhodamine 6G (R6G) was chosen as a probe molecule to evaluate the SERS performance of the above-mentioned substrates. The SERS substrates were prepared by drop casting 50 μL of 10-6 M R6G solution in DI water and then drying overnight. The corresponding SERS spectra of the substrates are shown in Figure 5. Same procedure was followed for all other concentrations. The characteristics Raman peaks of R6G at 610, 772, 1182, 1312, 1364, 1512, 1572 and 1650 cm-1 can be seen clearly.60 The inplane xanthene ring deformation and out of plane C-H bonding is responsible for the peak at 610 and 772 cm-1 respectively. In-plane xanthene ring deformation (C-H bending, N-H bending), in plane xanthene ring breathing (N-H bending, CH2 wagging) and in-plane xanthene ring stretching is responsible for the peak at 1182, 1312 and 1364 cm-1, respectively. The bands at 1512, 1572 and 1650 cm-1 are due to in-plane xanthene ring stretching (C-H bending, N-H


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bending and C-H stretching), in-plane xanthene ring stretching (N-H bending), in-plane xanthene ring stretching (C-H bending), respectively.60 Raman spectra were calibrated using the characteristic Raman peak of Si at 525cm-1. To measure the enhancement of Raman signal of R6G we used the following formula61 EF = [(ISERS) / (IBULK)] × [(NBULK)] / (NSERS)] Where, ISERS is the integral intensity of a particular Raman peak of the analyte adsorbed on the sample and IBULK is the same for the analyte alone. NBULK is the number of analyte molecule present in the solution and NSERS is the number of molecules adsorbed and probed on the SERS substrate. The SERS spectrum of different substrates is shown in Figure 5. Highest EF was obtained for S60 sample and also lowest detection limit of 10-10 M R6G (Figure 5d). The other three samples we can detect R6G up to 10-9 M. For S60 the intensity of the Raman bands at 1364, 1512 and 1650 cm-1 are 2.45, 3.14 and 2.57 times higher than S20 and 1.76, 2.04 and 1.74 times higher than G20. To check the uniformity of the SERS substrates the Raman spectra were taken from 3 different positions of the samples [Figure S7, SI] and the average enhancement factor with standard deviation is listed in Table 1in SI. The EF obtained for S60 for the peak at 1650 cm-1 is (8.54± 2.11) ×105. This is comparable to the previously obtained EF of both chemically synthesized Pd nanoparticle and Pd nanoparticles deposited on substrates.18, 27-29, 62-64 However, in previous reports various surfactants like CTAB, PVP were used to control the size and shape of the nanoparticles. Surfactants are not very easy to remove from the particle surface and must have an effect on Raman spectra, but in our case we could able to form different nanostructure (size and shape) by changing the substrates or by changing the surface morphology. EF obtained for G20 and G60 is (3.48±0.27) ×105 and (3.41 ±0.86) ×105 for the peak at 1650cm-1. The high Raman enhancement of R6G peaks for S60 sample can be attributed as follows. In planar Si (100) there are cluster of nanorods (Figure 3k) with



sharp tips and also have nano-gaps in between the rods which leads to the most desirable condition for EM field enhancement.65-66 The enhancement of incident EM field in S10 is also due to the same reason but the lower surface coverage of nanoparticles leads to the decrement in the intensity. For Ge surface the growth rate is much faster than Si as stated previously and leads to a continuous Pd film on which spherical shaped particles containing shaft are situated. The ratio of the shaft structure to the spherical shaped core is lower in this case and hence does not provide favourable condition for SERS enhancement. This fact is also observed from FDTD simulation and discussed in the later part of this paper.


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Figure 5. (a) and (b) show SERS spectra of G20 and G60, respectively for different concentration of R6G. (c) and (d) show SERS spectra of S20 and S60, respectively for different concentration of R6G.

The relationship between SERS intensity of the peaks at 1364, 1512 and 1650 cm -1 with the concentration of R6G for different samples is shown in Figure S8, SI. All the samples show a linear behavior with concentration. The value of R2 (standard deviations) for different samples shows that all the samples can analyze R6G concentration quantitatively. The R2 value for S60 shows least standard deviations indicating increasing uniformity with increasing deposition time.



Figure 6. (a) and (b) show SERS spectra of G20, S20,PS20 and G60, S60.PS60 substrates taken with 10-6 M concentration of R6G. (c) and (d) show the intensity of Raman spectra for 10-10 M and 10-11 M concentration of R6G for PS20 and PS60 sample, respectively. When Pd on P-Si substrate was used as SERS substrate we observed almost 3-7 times enhancement in the SERS signal compared to G20, G30 and S60 for all the characteristic peaks of R6G (Figure 6). As there is no shift in the peak positions and also the substrate material remains the same (both are Si) it is obvious that the huge enhancement results from EM enhancement of the incident radiation. From the SERS spectra it is observed that the relative intensity of 1312 cm-1/ 1364 cm-1 is increased while that of 1572 cm-1/ 1650 cm-1 decreases. It


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is mentioned previously that the peak at 1312 cm-1 has different origin compared to the peaks at 1364, 1572 and 1650 cm-1. The relative change in the intensity of 1312/ 1364 cm-1 and 1572/ 1650 cm-1 for P-Si substrate is due to change in adsorption geometry of R6G molecule on P-Si compared to planar Si and Ge. The high intensity of 1364 cm-1, 1650 cm-1 indicates R6G align their long axis across Pd surface (The ethylamine up configuration) and interacts with the aromatic ring resulting higher charge transfer. The decrement in the intensity of 1572 cm -1 indicates a decrement in the charge transfer when they are adsorbed on P-Si surface.60 However; a detailed experimental and theoretical understanding is needed to explain these features correctly. Figure 6a and 6b show the comparative SERS spectra of PS20 and PS60 for 10-6 M R6G. The EF obtained for 1364 cm-1, 1512 cm-1 and 1650 cm-1peak using PS20 is 8.31×105, 1.52×106, and 1.06×106, respectively while that for PS60 is 1.91×106, 3.82×106 and 1.91×106, respectively for the above-mentioned peaks. These values of EF are much higher than the previously reported literature. The huge signal enhancement in case of Pd nanostructures on P-Si substrate is due to the following reasons. Firstly, 3D Si itself provides an enhancement of incident EM field due to its sharp edges (also shown from the FDTD simulation). Lee et al. have observed that the SERS signal enhancement measured at the 3D plasmonic nanostructures was 3.9 times the signal measured at the 2D plasmonic nanostructures.65 Secondly, 3D Si substrate with well-separated pyramid arrays can effectively make the incident laser oscillate between the pyramidal valleys, which will further give rise to local enhancement of the incident laser.66 The top of the Pd nanostructures on P-Si has sharp triangular feature compare to the nanostructures formed on planar Si and this is another cause for huge signal enhancement. Figure 6c and 6d show the SERS spectra with 10-10 M and 10-11 M concentration of R6G using P-Si substrates. Both the substrates show significant enhancement even at such a low concentration of R6G for all characteristic peaks, which is remarkable using Pd nanostructures P-Si substrate. The stability of the substrate is an important



factor for SERS. We have tested the stability of the substrates by taking the SERS spectra of the substrates (Figure S9, SI) after keeping them in ambient atmosphere for one month. The intensity is reduced with time for all the substrates but still they show significant enhancement for all the characteristic peaks of R6G. The least reduction is observed for PS20 (30%). PS60 also shows almost similar stability (32% reduction). However, the reduction is most for G60 and S20 (52%). Which indicates P-Si substrate is superior compared to the others in terms of EF and stability. The comparative graph of the SERS intensity for 3 different peaks of R6G is plotted for different samples and is shown in Figure S10, SI.

Figure 7. FDTD calculated electric field distribution of (a) Pd on Ge (100) (b) Pd on Si (100) and (c) Pd on P-Si. Calculated EF in log scale for the same three substrates is shown in (d), (e) and (f) respectively.


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3.5. Electromagnetic Field Distribution: FDTD Simulation: We also have performed extensive FDTD simulation to compare the experimentally obtained EF for Pd nanostructures deposited on Ge, Si and P-Si substrate. Figure 7a shows near field intensity distribution of Pd deposited on Ge substrate. It is clear that the enhancement of incident electric field occurs at the sharp tips of Pd shafts. However, almost no significant enhancement occurs at the spherical particle surface. EF is proportional to (E/E0)4, where E0 is the intensity of incident electric field and E is the magnitude of the local electric field.27 The corresponding EF obtained is about 1.6 ×105 at 632.8 nm wavelength (Figure. 7d). This is slightly lower than the observed value. This can be attributed to the fact that in real sample the number of shafts on spherical Pd particles is different on different spheres. Again, when two shafts are situated very closely, the incident EM field will enhance at the middle portions of these shafts and also the gaps between two spheres. So, when two spheres lie very close to each other incident EM radiation can also get enhanced between them. We have done further FDTD simulation using finer mesh and more than one particle for all the substrates and indeed observed large enhancement of the incident EM radiation in between the particles [Figure S13, SI]. For Pd nanoparticles on Ge, we have considered only four shafts on sphere but from TEM images one can observe 7-8 number of shafts on a single sphere which will definitely further enhance the incident EM field. We have to restrict ourselves for four shafts due to limited computer facility. On Si also, we have considered a cluster made of only 3 nanorods but in real sample a single cluster is made of 8-9 number of rods. The scale bar in this case (from FDTD simulation) though indicate maximum enhancement of ̴ 105-106 for Ge and Si and ̴ 109 for Pd on P-Si. The log normal distribution also shows formation of “hot spots” is very much widespread. The maximum EF obtained from experimental EF calculation is lower than this and contributed by the average enhancement of the incident EM field. Overall the variation of the magnitude of the local electric field for different sample is clearly evident from the Figure 7 and Figure S13, SI. Figure 7b and Figure



7c show the electric field intensity in the x-z plane for Pd on Si and Pd on P-Si. The dominant enhancement is observed between the rods (in case of Pd on Si) and at the edges of the shaft and P-Si. One has to note that enhancement of incident electric field also occurs due to pyramidal shape of Si (as observed from FDTD simulation) and contributes to the overall enhancement. 3.6. Electrochemical Cyclic Voltammetry Study: Cyclic voltammetry provides the accumulation of electron and interfacial charge transfer at an electrode/ electrolyte interface. Figure S14 (supplementary) depicts CV plots of all the electrodes using 1M Na2SO4 electrolyte at a scan rate of 60 mV/s. It has been observed that the current density enhances in PS60 and G60 electrode compared to S20. So, the electrochemical response of PS60 and G60 electrodes are better than that of S20 electrode and we have performed further biosensing study by using PS60 and G60 electrodes.


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Figure 8. (a) and (b) cyclic voltammetry (CV) study with different molar concentrations of glucose using PS60 and G60 substrates as working electrodes, respectively. (c) and (d) cyclic voltammetry (CV) study with different molar concentrations of AA using PS60 and G60 substrates as working electrodes, respectively. 3.7. Glucose and Ascorbic Acid Sensing Study: Non-enzymatic electrochemical sensing property of glucose and AA was conducted in 1M Na2SO4 solution using PS60 and G60 electrode as working electrode, is shown in Figure 8. The oxidation peak near 0V is the oxidation peak of Pd which was previously observed by Wang et al.67 for Pd nanodendrites and also by Burke et al. for bulk Pd.68 They also observed that the reduction potential of Pd oxide occurs at about -0.37 V in the negative scan. An increment in the oxidation/ reduction peak current with increasing methanol concentration was also reported by them. From Figure 8 (a) and (b) it is observed that the detection of glucose is prominent at the reduction potential of H2O2. So, detection of glucose is occurring indirectly through the reduction of H2O2 at -0.6 V.47,69 From Figure 10. (c) and (d) the anodic peak near 0 V and the cathodic peak at -0.3 V indicates Pd oxidation/ reduction cycle and the reduction current of Pd oxide increases significantly with increasing AA concentration. PS60 and G60 electrode adsorbs AA molecule from the electrolytic mixture and then by hydrolysis process AA gets oxidized and converted to de-hydroascorbic acid.69 This process releases two protons and two electrons which helps to reduce the Pd2+ to Pd0. The increment in the peak current density of PdO/Pd redox system on G60 is stronger than PS60 indicating greater electrocatalytic activity and larger electroactive surface area.67 As the reduction current of Pd oxide formed at the surface dominates the C-V response hence the surface adsorbed species is the most dominating here. We have also observed a linear change in reduction current with the scan rate [Figure S16, SI and Figure S17, SI] thereby indicating a surface confined process.49



Amperometric sensing study towards glucose and AA was performed at -0.6V and -0.3 V, respectively, which is shown in Figure 9. Figure 9(a) and (b) shows amperometric current density- time curve for glucose sensing using PS60 and G60 as working electrode, respectively. When glucose and AA were added in the electrolyte, stable current steps were obtained. Figure 9 (c) and (d) shows amperometric current density-time curve for AA sensing using PS60 and G60 as working electrode, respectively. The calibration curve of the amperometric sensing study for different substrates is shown in Fig S18, SI with the correlation coefficient. The calibration curve for glucose sensing using PS60 and G60 as working electrodes can be fitted by equations Ips60,glu= -19.273 - 1.04Cglu and Ig60,glu= -58.75 -2.65Cglu respectively and the calibration curve for AA sensing using PS60 and G60 as working electrode can be fitted by equations Ips60,aa= -17 – 0.92 Cglu and Ig60,aa= -33.29 – 18.67Caa respectively. The highest values of sensitivity for glucose and AA both were obtained for G60 (2.658 μA/mM/ cm 2 and 18.67 μA/mM/ cm2 respectively). The detection limit for PS60 and G60 for AA are 4.35 μΜ and 2.01 μΜ, respectively and the detection limit for the same two substrates for glucose are 2.86 μΜ and 7.19 µm, respectively (S/N =3).


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Figure 9. (a) and (b) amperometric current density- time curve for glucose sensing using PS60 and G60 as working electrode, respectively. (c) and (d) amperometric current density- time curve for AA sensing using PS60 and G60 as working electrode, respectively.

The comparison of different analytical factors of our substrates with other non-enzymatic amperometric sensors reported earlier is listed in Table. 2. This shows that our system is very much capable to detect AA and glucose efficiently. The higher current response for G60 sample is attributed to the presence of more active catalysis site as we previously observed that the rate of growth of Pd on Ge substrate is higher compared to the other substrates.



3.8. Interference Study: One of the most important properties for any sensor is its selectivity of the desired sensing material over the other electro-active elements. For our sample the selectivity response study was done using uric acid (UA), dopamine (DA), tap water, H2O2, ascorbic acid (AA) and glucose [Figure 10]. The current density-time (j-t) plot for all the four sensors indicates a significant change in the current density in presence of AA (working potential -0.3V) and glucose (working potential -0.6V). Disturbance in current is also observed when other electro-active materials are introduced into the electrolyte but the current density regains its previous value within some seconds. The lesser change in current in presence of other electro-active materials may be attributed to the fact that UA, DA, tap water and H2O2 does not have a reduction peak at the working potential.

Figure 10. (a) and (b) selectivity of glucose (working potential -0.6 V) over other electroactive materials using PS60 and G60 as working electrode. (c) and (d) selectivity of AA (working potential -0.3 V) over other electroactive materials using PS60 and G60 as working electrode.


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The comparison of different analytical factors (linear range, detection limit, sensitivity) for nonenzymatic glucose and AA sensor reported earlier is reported in Table 1 and Table 2 respectively. This shows that our system is very much capable to detect glucose and AA effectively. Table 1. Comparison of the analytical performance of the proposed glucose biosensor with other glucose biosensors reported previously. Electrode

Mesoporous Pt

Sensitivity

Linear

Detecti

Reference

(µAcm−2mM-1)

range

on limit

9.6

Up to

N.A

41

0.0001-

0.06

42

0.0188 mM

µM

10mM NiTiO3/ NiO nanoparticles

1454 & 52.86

and 0.042.07 mM Glucose oxidease- nanoporous

12.1

Gold Bimetallic PtM (M=Ru, Pd and

10.7

Au) Nanoparticles on Carbon

50µM-

1.02

10mM

µM

Up to

0.05

15mM

mM

43

44

Nanotubes – ionic Liquid Composite Film GD of Pd on Ge for 60 minutes

2.66

1-40 mM


7.19 μM This Work


Page 32 of 45

Table 2. Comparison of the analytical performance of the proposed AA biosensor with other AA biosensors reported previously. Electrode

Mesopore-rich active carbon-

Sensitivity

Linear

Detection

(μAcm-2mM-1)

range

limit

2.27

modified pyrolytic graphite

0.5-2000

Reference

0.3 μM

45

N.A

46

7μM

47

µM

electrode Pd nanoparticle supported

6.18

μM

graphene oxide Poly/N-methylpyrrole/ Pd-

20-2280

5.6

0.05-1mM

8.9

20-900 μM

0.11 μM

48

20 μM –

2.01 μM

This work

nanocluster Graphene anchored with Pd-Pt nanopartcles GD of Pd on Ge for 60 minutes

18.67

40000 μM

4. CONCLUSIONS In conclusion, we have studied the growth morphology and sensing properties of Pd nanostructures on different substrates and substrate with patterning using a very clean method of deposition without any use of surfactants. We have observed that the growth rate and size and shape of nanoparticle grown depend heavily on substrate material. The SERS activity of different substrates is also heavily dependent on the structure. We have observed two order magnitude (106) higher enhancements compared to previously reported result (104) using Pd


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nanoparticle and substrate combination. 3D surface (pyramidal structure) produces higher enhancement of the SERS signal of R6G compared to planer substrate. Efficient enhancement of the incident EM radiation is the main cause behind the enhancement. The substrates are also examined to show very good sensing properties for the detection of glucose and AA. The highest sensitivity (18.67 μA mM-1 cm-2) for AA is observed for Pd deposited on G60 in the linear range of 20 µΜ to 40 mM and for glucose the highest sensitivity (2.658 μA Mm-1 cm-2) is also observed for the same substrate in the linear range of 1 mM to 40 mM. The lowest detection limit for AA and glucose is 2.19 µΜ and 7.19 µΜ, respectively for the same substrate. Clearly the substrates we prepared have strong implication in identifying new SERS substrate using low cost materials as well as electrochemical non-enzymatic sensor. It would be much more beneficial if the smaller interference from other electro-active material can be minimized further by introducing some structural or chemical modification to the Pd nanostructures.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. High magnification secondary electron (SE) images of Pd nanoparticles deposited on different substrates for different deposition times (Figure S1); Elemental mapping of G10 using the STEM-HAADF-EDX technique (Figure S2); HRTEM image showing the interface of G10 sample (Figure S3); STEM-HAADF image and EDX spectra of S10 sample (Figure S4); HRTEM image of a Pd cluster showing various defects (Figure S5); Bright-field TEM image of PS20 showing nature of Pd thin film along the ridge and the base of the P-Si (Figure S6); SERS spectra of 10−6 M R6G at three random sites for all the substrates. (Figure S7); Intensity versus concentration graph for G20, G60, S20 and S60 showing linear relationship (Figure S8);



SERS spectra of the samples after 1 month (Figure S9); SERS intensity for different samples for 3 different peaks of R6G (Figure S10); SERS enhancement factor calculation; Average enhancement factor (EF) with standard deviations from SERS measurement for different Raman peak of 10-6M R6G (Table S1). Geometries of Pd nanoparticle on (a) Pd (b) Si and (c) P-Si used for FDTD simulation (Figure S11); Variation of real and imaginary part of dielectric function for Si, Ge and Pd over the FDTD simulated wavelength range (Figure S12); FDTD calculated electric field distribution for more than one Pd nanoparticle for different substrates (Figure S13); Comparative C-V plot for (a) S10, S20 and S60 (b) PS10, PS20 and PS60 (c) G10, G20 and G60 in 1M Na2SO4 solution as electrolyte and at a scan rate of 60 mV/s. (Figure S14); C-V comparison graph for 20 mM AA using three best samples from each category as working electrode. (Figure S15); Variation of CV characteristic with scan rate for glucose different electrodes for detection of glucose and AA (Figure S16); Linear response of peak reduction current with scan rate for glucose and AA detection using PS60 and G60 as working electrode respectively (Figure S17); Linear response of glucose and AA concentration (with residual errors) with the current density for PS60 and G60 electrodes (Figure S18).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS Financial support from the CENSUP-II project (PIC No: 12-R&D-SIN-5.09-0102), DAE, Govt. of India is gratefully acknowledged. The authors also acknowledge the FESEM facility at the Centre of Excellence in Advanced Materials, NIT Durgapur for a part of SEM characterization. The authors sincerely thank Prof. Tapas Kumar Chini, SINP, Kolkata for the access of FDTD simulation software and SEM and Mr. Debraj Dey, SINP, Kolkata for his assistance SEM measurement.

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