This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Thin Solid Films 544 (2013) Contents lists available at ScienceDirect Thin Solid Films journal homepage: Graphene thin film electrodes synthesized by thermally treating co-sputtered nickel carbon mixed layers for detection of trace lead, cadmium and copper ions in acetate buffer solutions Zhaomeng Wang, Pui Mun Lee, Erjia Liu School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore article info abstract Available online 5 March 2013 Keywords: Graphene Solid-state carbon diffusion Square wave anodic stripping voltammetry Trace lead, copper and cadmium Acetate buffer solution Bismuth modification Graphene thin films were synthesized using a solid-state carbon diffusion method by thermally treating (heating at 1000 C for 3 min and then cooling) nickel carbon (Ni C) mixed layers co-sputtered on silicon (Si) substrates with or without a silicon dioxide layer. Field-emission scanning electron microscopy, high resolution transmission electron microscopy and Raman spectroscopy were used to characterize the structure and composition of both the as-deposited and thermally treated Ni C coated samples with respect to the C concentration of the Ni C thin films. The graphene thin films were used as working electrodes in the simultaneous detection of trace heavy metal ions (Pb 2+,Cd 2+ and Cu 2+ ) in acetate buffer solutions modified with bismuth ions (Bi 3+ ). The enhancing effect of Bi 3+ on the electroanalytical performance of the graphene electrodes was studied Elsevier B.V. All rights reserved. 1. Introduction Contamination and mismanagement of water resources have released toxic heavy metals such as mercury (Hg), lead (Pb), cadmium (Cd) and copper (Cu) into the environment. The presence of these toxic metals in aquatic ecosystems affects directly or indirectly biota and human being [1]. Hence, fast detection and determination of trace toxic heavy metals in aqueous solutions are necessary to reduce fatal cases due to misconsumption of polluted water. Anodic stripping voltammetry has been widely used for detection of heavy metals in a solution due to its remarkably low detection limit (nanogram per liter) [2], capability of simultaneous detection of multi-elements, low operating power and relatively low cost [3]. Square-wave anodic stripping voltammetry (SWASV) has been recognized as a powerful technique for detection of trace heavy metals in various aqueous solutions, because of its unique accumulation/preconcentration of analyte species contained in the solutions. Glassy carbon electrode (GCE) has been widely used in electroanalytical applications because of its robust and smooth surface nature, as well as a large potential window. However, its electroanalytical performance frequently suffers from gradual loss of surface activity [4]. In order to reliably measure toxic metals in solutions, GCE materials have been modified. For example, bismuth (Bi) film coated GCEs modified with polyaniline interlayers were developed for simultaneous determination of trace Cd 2+ and Pb 2+ ions at nm levels [5]. Nitrogen-doped diamond like carbon thin film electrodes showed Corresponding author. Tel.: ; fax: address: mejliu@ntu.edu.sg (E. Liu). improved electrocatalytic activities [6], offered wide potential windows with different types of solutions, and improved signal-to-background and signal-to-noise ratios, repeatability of voltammograms and longtime response stability [7]. Nickel-doped DLC thin film electrodes promoted glucose oxidation, hence facilitating the electroanalytical process [8]. Recently, graphene-based electrochemical sensors have also been developed to trace toxic heavy metal ions in aqueous solutions. Graphene possesses various unique properties with its atomic carbon layers, such as nanometer layer thickness, high electrical conductivity, fast transfer of electrons, and alleviation of the fouling effect of surfactant [9]. Graphene-based electrochemical sensors can be modified with nafion to improve their sensitivities in detection of heavy metal ions, thus greatly enhancing stripping current signals [9]. There are several viable methods for fabrication of doped graphene based electrode materials, such as chemical vapor deposition, physical vapor deposition and spin coating that are usually followed by high temperature carbonization [4]. In this work, nickel (Ni) and carbon (C) mixed layers were co-sputtered on silicon (Si) substrates without or with a silicon dioxide (SiO 2 ) layer, followed by rapid thermal processing (RTP). During high temperature heating, the C atoms dissolved into the Ni atom seas. However, during rapid cooling, the solubility of C atoms in Ni sharply reduced, leading to the precipitation of excess C atoms and the formation of graphene thin films on the outer surfaces of the Ni C layers. The structure of the graphene films was characterized with respect to the C concentration of the Ni C mixed layers. The Si substrate surface conditions (with or without a SiO 2 layer) were also investigated with respect to the performance of the graphene /$ see front matter 2013 Elsevier B.V. All rights reserved.

3 342 Z. Wang et al. / Thin Solid Films 544 (2013) electrodes. In order to enhance the sensitivity of graphene electrodes, bismuth ions (Bi 3+ ) were added to the electrolyte together with the target metal ions. The Bi modified graphene electrodes showed significantly enhanced performance in the simultaneous detection of Pb 2+, Cd 2+ and Cu 2+ ions in the acetate buffer solutions. 2. Experimental details 2.1. Sample preparation P Si (111) wafers (boron doped, resistivity: Ω cm, thickness: ~525 μm) without and with a thermally oxidized SiO 2 layer of about 300 nm in thickness were used as substrates, which were designated as Si substrates and SiO 2 /Si substrates, respectively. Before the sputtering process, the substrates were ultrasonically cleaned with acetone for 3 min and deionized water for 3 min and then dried with compressed air. Inside the deposition chamber and before the film deposition, an argon (Ar + ) plasma with an RF power of 50 W was used to etch the substrates for 10 min. During the plasma etching and film deposition, an Ar gas flow of 10 sccm and a vacuum pressure of about 5 mtorr (about Pa) were maintained. Ni C mixed layers with varying C contents were then deposited on the substrate surfaces via DC magnetron co-sputtering deposition, with a DC power of 50 W applied to a pure Ni target ( 99.99% Ni) and varying DC powers from 25 to 200 W applied to a pure graphite target ( 99.99% C). The duration of the co-sputtering deposition of the Ni C films was maintained at 90 min. The deposited samples were then left to cool in the vacuum chamber for 1 h to prevent any thermal shock to the Ni C thin films [10]. The as-deposited samples were thermally annealed via RTP at 1000 C for 3 min, with both heating and cooling rates of about 20 C/s. In order to prevent oxidation of the samples during the RTP, a continuous Ar gas flow of 200 sccm was maintained in the chamber [11]. The thermally treated samples were designated as graphene electrodes that were used as the working electrodes for detection of trace Pb 2+,Cd 2+ and Cu 2+ ions in acetate buffer solutions Characterization Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7600F, 10 kv), high resolution transmission electron microscopy (HR-TEM, JEOL 2010, 200 kv) and Raman spectroscopy (RENISHAW 1000, He Ne laser of 633 nm wavelength) were used to study the morphology and structure of the samples [8]. For a HR-TEM measurement, the Ni in a sample was dissolved in a 0.25 M Fe(NO 3 ) 3 solution, leaving thin jasper-colored flocculent C flakes floating in the solution. The C flakes were then transferred onto a Cu grid and made ready for the TEM measurement. An electrochemical workstation (CHI 660C) with a cell consisting a graphene working electrode (7.5 mm in diameter), a reference electrode of Ag/AgCl (with saturated KCl) and a counter electrode of a platinum mesh was used to perform all the electrochemical experiments at room temperature (RT, ~22 C). To achieve the equilibrium concentrations of the solutions, the testing solutions were stirred at 400 rpm using a magnetic stirrer when needed. In SWASV experiments, all the electrodes were immersed into 0.1 M acetate buffer solutions (ph 5.3) containing 0.1 M KNO 3 and predetermined concentrations of target Cu 2+,Pb 2+ and Cd 2+ ions with or without Bi 3+ ions. In the solutions containing Bi 3+, the Bi 3+ concentrations were varied from 0 to 2.5 μm (with a step increment of 0.25 μm) with the Pb 2+ concentration maintained at 0.1 μm to optimize the SWASV performance. In an electroanalytical measurement, a preconcentration potential of 1 V was firstly applied to the working electrode with continuous stirring for 180 s. Then, a quiet time of 30 s was taken to stabilize the solution. Further, anodic stripping was performed from 1.1 to 0.2 V with an increment of 5 mv/s at a frequency of 50 Hz and an amplitude of 50 mv. Meanwhile, a voltammogram was recorded for analysis. For repetitive measurements, the residual metals on a working electrode from a preceding measurement were thoroughly removed by applying a voltage of 0.2 V for 180 s with continuous stirring before next measurement. For the SWASV experiments with the Bi 3+ ions dissolved in the electrolyte, the graphene electrodes used were designated as Bi/graphene electrodes. 3. Results and discussion The synthesis process of graphene films used in this study is similar to the one for graphene films via metal-catalyzed crystallization of amorphous carbon (a-c) through thermal annealing [12], where a Ni/a-C bilayer deposited on a SiO 2 /Si substrate was thermally treated at C [12]. As schematically shown in Fig. 1, during heating at 1000 C for 3 min, the C atoms can be dissolved into the Ni lattices, while during cooling the C solubility in the Ni lattices sharply reduces and the excess C atoms precipitate on the surface of the Ni C layer to form a graphene film. This is because at the temperature far below melting point, the surface diffusion is more favorable than the diffusion via lattices, grain boundaries and dislocations [13]. Fig. 2a shows the lattice structure of the graphene film formed on the thermally treated Ni C mixed layer deposited on the Si substrate measured by using HR-TEM. It can be observed that the graphene film has a planer shape, instead of cylindrical shape (e.g., carbon nanotube) or spherical shape (e.g., fullerance). Fig. 2b shows an electron diffraction pattern of the thermally treated Ni C mixed layer, which indicates the hexagonal lattice structure of the graphene layer formed in which a central dot was surrounded by six others. The lattice structure and electron diffraction pattern are similar to the graphene film formed on the SiO 2 /Si substrate. The lattice structure deduced from the TEM measurements in this study is in agreement with the literature [14]. Fig. 3 shows the Raman spectra of the thermally treated Ni C mixed layers deposited on both Si and SiO 2 /Si substrates, in which the G and D bands are located at about 1600 cm 1 and 1350 cm 1, respectively. The G peak is due to the stretching mode of all pairs of sp 2 bonds in both rings and chains while the D peak is due to the breathing mode of sp 2 bonds in rings. The D and G peak intensity ratio, I D /I G, indicates the quantity of defects in a graphitic material [15]. The I D /I G ratios are about 1.83 and 0.29 for the Si and SiO 2 /Si based samples, respectively. The smaller I D /I G ratio of the SiO 2 /Si based sample indicates that its graphene film has fewer defects than the one formed on the Si substrate. Thus, the graphene film formed on the SiO 2 /Si substrate has better electrical or optical properties. The higher I D /I G ratio of the Si based sample can be explained by the oxidation reactions between the Si atoms diffused out from the substrate and the residual oxygen in the RTP chamber during the thermal processing with the Ni in the Ni C layer as a catalyst [16 18], which produce gaseous products and SiO 2 species that are trapped underneath the graphene layer and distort the film structure [19], thus leading to more sp 2 defects [20]. The surface distortion due to those defects can be further confirmed by the FE-SEM images as shown in Fig. 4, where the graphene film formed on the SiO 2 /Si substrate is smoother than that formed on the Si substrate. However for electroanalysis, a graphene film with more surface active sites (edge defects and sp 2 defects) where the local surfaces are relatively rougher (higher surface aspect ratios) is more favorable. The 2D bands of the Raman spectra in Fig. 3 confirm the formation of the graphene films, which are located at around cm 1 and cm 1 for the Si and SiO 2 /Si based samples, respectively. According to the double resonance theory [21], there should not be a shift in 2D peak position unless an environmental effect does exist. Hence, the shift of the 2D band observed in Fig. 3 may be caused by the different surface conditions of the Si and SiO 2 /Si substrates. The I 2D /I G ratio can be related to

4 Z. Wang et al. / Thin Solid Films 544 (2013) Fig. 1. A model for fabrication of graphene thin films with thermal processing of a Ni C mixed layer co-sputtered on a Si substrate. the number of graphene sheets formed [22]. The I 2D /I G for the graphene formed on the Si substrate (~0.67) is bigger than that of the graphene formed on the SiO 2 /Si substrate (0.45), which means that the graphene film formed on the Si substrate has fewer graphene sheets. It can be seen from Fig. 5 that the background current of the SWASV curve of Pb 2+ (0.1 μm) measured by the graphene electrode formed on the SiO 2 /Si substrate is very high with a broad and weak peak, which may be due to the poor conductivity of the SiO 2 layer of the SiO 2 /Si substrate. On the other hand, the anodic stripping peak of Pb 2+ measured by the graphene electrode formed on the Si substrate, which is positioned at around V, is stronger and sharper (full width at half maximum (FWHM) of about V), indicating that this is a much more effective electrode. The excellent performance of the Si substrate based graphene electrode can also be explained by the higher sp 2 defects that offer more surface active sites for the electrochemical reactions to take place. Therefore, the following discussion will be focused on the results measured with only the Si substrate-based graphene electrodes. The C concentrations in the as-deposited Ni C mixed layers with respect to C sputtering powers ranging from 25 to 200 W are summarized in Table 1. With a fixed Ni sputtering power of 50 W, the C concentration is almost linearly proportional to the C sputtering power. Since the C concentrations are small, a mean deposition rate of the mixed layers is about 3 nm/min, which is mainly controlled by the sputtering power of Ni. Fig. 6 shows the Raman spectra of the graphene films formed from the Ni C layers of different C concentrations. With 0.7 at.% C (Fig. 6a) and 1.8 at.% C (Fig. 6b), D, G and 2D Raman peaks are hardly resolved, which may be due to the insufficient C for precipitation during cooling. The C atoms can also be depleted by the oxidization with the residual oxygen in the RTP chamber. However, with the C concentrations higher than 3.5 at.%, the 2D peaks gradually diminish (Fig. 6d f) compared to that shown in Fig. 6c, indicating the increasing number of graphene sheets, which means that the higher C concentrations in the Ni C layers can promote the saturation of C in the Ni lattices during heating and then the precipitation of C on the outer surface of the Ni C layers in the form of graphene during cooling. However, if the number of graphene sheets is too large, the electronic band structure tends to approach that of graphite [23]. Therefore, I 2D decreases as the number of graphene sheets increases. The 2D peak of the Ni C film with 3.5 at.% C is the strongest with Intensity (arb. units) On Si substrate On SiO 2 /Si substrate Fig. 2. (a) HR-TEM image and (b) electron diffraction pattern of the graphene film formed on the thermally processed Ni C layer (3.5 at.% of C in the as-deposited mixed layer) deposited on a Si substrate Raman Shift (cm -1 ) Fig. 3. Raman spectra of the thermally treated Ni C layers (3.5 at.% of C in the as-deposited mixed layers) deposited on Si substrates without and with a SiO 2 coating.

5 344 Z. Wang et al. / Thin Solid Films 544 (2013) Table 1 C atomic concentrations with respect to C sputtering powers. DC sputtering power on C target (W) DC sputtering power on Ni target (W) C concentration in as-deposited Ni C layer (at.%) Fig. 4. FE-SEM micrographs of the thermally treated Ni C layers (3.5 at.% of C in the as-deposited mixed layers) deposited on SiO 2 /Si (a) and Si (b) substrates, respectively. the D and G peaks less overlapped (Fig. 6c), indicating an optimal graphene film structure. Thus, the following discussion will be related to the results measured using the graphene electrodes formed from the Ni C layers having 3.5 at.% C. The Bi 3+ ions dissolved in the electrolyte can enhance the anodic stripping peak currents of the heavy metals as shown in Fig. 7, where the anodic stripping peak currents of the Pb 2+ ions measured with a Bi/graphene electrode are almost linearly proportional to the Bi 3+ concentrations. In Fig. 7a, the FWHMs and peak positions are all similar with respect to the Bi 3+ concentrations ranging from 0 to 2.5 μm. According to a previous study [24], Bi 3+ can also be reduced (Bi 3+ +3e Bi 0 ) and deposited, together with the target metals, on the electrode surface during preconcentration. Thus, those reduced metals can form binary- or multi-component alloys that have a strong adsorptive ability to facilitate the further reduction of the target metal ions. Usually, with the addition of Bi in electroanalysis, there is a peak at about 0.1 V in the SWASV curves, which is attributed to the Bi 3+ ions. The Bi 3+ peak position may overlap with some target metals (e.g., Cu 2+ ) [5], hence, making the detection of such target metals difficult. Nevertheless, the Bi 3+ stripping peaks shown in Fig. 7a have very small amplitudes, which may not fully prevent the electrode from sensing the target metals that have overlapping stripping peaks with Bi at around 0.1 V. Fig. 8 shows the anodic stripping peak currents of Pb 2+ (1 μm) measured by a Bi/graphene electrode with respect to the preconcentration potentials with the stripping potential of Pb 2+ located at around V. With relatively high preconcentration potentials (> 0.7 V), it is difficult for Pb 2+ to be reduced and deposited on the electrode surface, and thus the anodic stripping currents are near zero (Fig. 8b). However, with preconcentration potentials lower than 0.7 V, more Pb 2+ ions can be reduced to their neutral state (Pb 2+ +2e Pb 0 ), leading to greatly increased anodic stripping peak currents. As shown in Fig. 8a, the FWHMs increase from about V to V with decreasing preconcentration potentials from 0.8 to 1.2 V, which are still quite small. However, to avoid hydrogen evolution (H + +2e H 2 (g)) that usually occurs at a low potential and can degrade the surface activities of the electrodes, I ( A) [Pb 2+ ] = 0.1 M On SiO 2 /Si substrate On Si substrate Intensity (arb. units) D G 2D (f) (e) (d) (c) (b) (a) E (V) vs. Ag/AgCl Raman Shift (cm -1 ) Fig. 5. Anodic stripping peak currents of Pb 2+ (0.1 μm) measured with the thermally treated Ni C layers (3.5 at.% of C in the as-deposited mixed layers) deposited on Si substrates without or with a SiO 2 coating. Fig. 6. Raman spectra of thermally treated Ni C layers with respect to C concentrations in the as-deposited mixed layers: (a) 0.7 at.% C, (b) 1.8 at.% C, (c) 3.5 at.% C, (d) 4.9 at.% C, (e) 6.1 at.% C, and (f) 9.8 at.% C.

6 Z. Wang et al. / Thin Solid Films 544 (2013) a a b b Fig. 7. (a) SWASV curves and (b) anodic stripping peak currents of Pb 2+ (1 μm) measured with a Bi/graphene electrode with respect to Bi 3+ concentrations from 0 to 2.5 μm. Fig. 8. (a) SWASV curves and (b) anodic stripping peak currents of Pb 2+ (1 μm) measured with a Bi/graphene electrode with respect to preconcentration potentials. the following analyses will be concentrated on the results measured with the optimized preconcentration potential of 1 V. Similarly, the preconcentration time is also optimized as shown in Fig. 9, where the anodic stripping peak currents are almost linearly proportional to the preconcentration time. This is because with a longer deposition time, more metal ions can be reduced and deposited on the electrode surface. Then, during the anodic stripping period, those deposited metals are oxidized (Pb 0 Pb 2+ +2e ), resulting in higher anodic stripping peak currents. On the other hand, to avoid the oversaturation of the target metals on the electrode surfaces, the preconcentration time is optimized as 180 s. Fig. 10a shows the voltammograms for the simultaneous detection of Pb 2+ (1 1.7 μm), Cd 2+ (0 0.7 μm) and Cu 2+ (0 0.7 μm), which are measured with a step increment of 0.1 μm. When the concentration of Cu 2+ is 0, a small peak located at about 0.05 V is attributed to the Bi 3+ ions. In Fig. 10b, both the Cu 2+ and the Cd 2+ stripping peak currents are almost linearly proportional to their concentrations in the solutions, which can be expressed as: I ¼ 0:29 þ 23:5 C Cd 2þ and I ¼ 22:12 þ 196:68 C Cu 2þ where I is the anodic stripping peak current in μa, and C Cd 2þ and C Cu 2þ are the concentrations of Cd 2+ and Cu 2+ in μm, respectively. The ð1þ ð2þ regression coefficients (R) of the above two equations are and 0.998, respectively, indicating the good matches of the above two calibration equations with the experimental results. Thus, the detection limits for both Cd 2+ and Cu 2+ can be deduced to be lower than 0.1 μm, at which their stripping peak currents can still be resolved as shown in Fig. 10b. With only Pb 2+ dissolved in the electrolyte, as shown in Fig. 10c, the anodic stripping peak currents of Pb 2+ almost linearly increase with the increasing Pb 2+ concentrations in the ranges of μm and μm. Their calibration curves can be expressed as: I ¼ 49:21 þ 1375:78 C Pb 2þ and I ¼ 372:16 þ 203:99 C Pb 2þ where C Pb 2þ is the concentration of Pb 2+ in μm in the ranges of μm and μm, for Eqs. (3) and (4), respectively. The regression coefficients of the above two equations are and 0.999, respectively, indicating the good matches of the above two calibration equations with the experimental results. The turningpoint of the calibration curve at 0.3 μm as shown in Fig. 10c can be possibly explained as that for the Pb 2+ concentration below 0.3 μm, only a monolayer of Pb atoms can be deposited on the graphene electrode surface during the preconcentration period, which is a condition of so-called under-potential deposition (UPD) [25]. However, ð3þ ð4þ

7 346 Z. Wang et al. / Thin Solid Films 544 (2013) a a b b Fig. 9. (a) SWASV curves and (b) anodic stripping peak currents of Pb 2+ (1 μm) measured with a Bi/graphene electrode with respect to preconcentration time. c for a Pb 2+ concentration higher than 0.3 μm, a multilayer of Pb atoms is further deposited on the previously formed Pb monolayer, which is termed as bulk deposition (BD) [25]. For the UPD, the metals are directly deposited on the graphene film surface. But for the BD, the metals are deposited on the previously formed metal monolayer that may have a lower surface activity than the initial graphene film, leading to a smaller slope in the calibration curve as shown in Fig. 10c. Measured with the UPD condition, the detection limit of Pb 2+ should be smaller than 0.03 μm, at which the stripping peak current is measurable as shown in Fig. 10c. Along with Cd 2+ (0 0.7 μm) and Cu 2+ (0 0.7 μm) added in the solutions, the anodic stripping peak currents of Pb 2+ (1 1.7 μm) read from Fig. 10a are also shown in Fig. 10c for comparison. The good matching of the above peak currents with those measured without any Cd 2+ and Cu 2+ in the solutions indicates that the interferences of Cd 2+ and Cu 2+ with Pb 2+ are quite limited during the simultaneous detection. The repeatability of the Bi/graphene electrodes in the acetate buffer solution containing 1 μm Pb 2+ is excellent with a standard deviation of about 3.58 as shown in Fig Conclusions In this paper, graphene thin films were synthesized using a solid-state carbon diffusion method, which involved the co-sputtering deposition of Ni C mixed layers on Si substrates with or without a SiO 2 layer, followed by RTP at 1000 C for 3 min. It was found that the graphene films formed on the SiO 2 /Si substrates had a less structural Fig. 10. (a) SWASV curves and (b and c) anodic stripping peak currents with respect to concentrations of (b) Cd 2+ and Cu 2+ (0 0.7 μm) and (c) Pb 2+ (1 1.7 μm). The anodic stripping peak currents measured with only Pb 2+ (0 1.7 μm) dissolved in electrolyte are also showed in (c) (for comparison). All the results were measured with a Bi/ graphene electrode in 0.1 M acetate buffer solutions (ph 5.3) with a step increment of 0.1 μm of each target metal. disorder, while the graphene films formed on the Si substrates showed a much better performance in electroanalysis of trace heavy metals in acetate buffer solutions. The addition of Bi 3+ in the solutions could greatly enhance the anodic stripping peak currents of the target metal ions. The Bi modified graphene electrodes formed on the Si substrates were used in the simultaneous detection of the Pb 2+,Cd 2+ and Cu 2+

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