Porous GaN electrode for anodic stripping voltammetry of silver(I)

Talanta 165 (2017) 540–544

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Porous GaN electrode for anodic stripping voltammetry of silver(I) a,b

MARK

a,⁎

Miao-Rong Zhang , Ge-Bo Pan a b

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, PR China University of Chinese Academy of Sciences, 100049 Beijing, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: Porous GaN Anodic stripping voltammetry Silver

Here we demonstrate porous GaN electrode can be applied for trace Ag(I) detection. Compared to traditional planar electrodes, porous GaN electrode can detect lower concentration of Ag(I) as it possesses more deposition sites (crystal defects) and larger surface area. Under the optimum conditions, porous GaN electrode shows a linear voltammetric response in the Ag(I) concentration range from 1 to 100 ppb with the detection limit of 0.5 ppb. Such an unmodified, high-porosity and chemically stable electrode is promising to operate in real samples.

1. Introduction With the widespread use of silver compounds and silver-containing processes in industrial activities, the ever-increasing silver content in ecological environment and biological organisms has drawn much attention due to its potential threat to human health [1–3]. In order to determine the concentration level of Ag(I), different analytical techniques such as fluorimetry [4], atomic absorption spectroscopy (AAS) [5], inductively couple plasma mass spectrometry (ICP-MS) [6] have been developed and applied. However, these methods are not suitable for in-situ monitoring applications. In this respect, electrochemical technique has advantages over the above approaches owing to its easy operation, low cost, high sensitivity and fast response. Among them, stripping voltammetric analysis has been widely used for measuring trace heavy metals [7–9]. The mercury-film and hanging mercury drop electrodes had played a great role in the development of stripping voltammetry [10]. Nevertheless, because of the toxicity of mercury, environment-friendly coating materials such as bismuth [11,12] and antimony [13] replaced mercury and became new generation electrodes. Whereas the addition of coating materials such as mercury and bismuth into samples makes the determination process more complicated. Ideally, non-film electrode materials are favored over above film electrodes due to their simplified measuring operation [14–17]. Gallium nitride (GaN) as a direct wide band gap semiconductor has been long prized for its superior optical properties [18,19]. Nevertheless, GaN, with its adjustable carrier concentration and conductivity type, has the potential to be a good electrode substrate. Furthermore, GaN possesses excellent chemical stability under harsh

⁎

conditions, which is vital for some practical applications. Last but not least, the characteristic of high electron mobility means GaN can generate less noise and thus spot smaller signals. Above facts endow GaN with capacities to be a highly accurate biosensor or chemical sensor [20]. In this work, we demonstrate porous GaN can be a good electrode material for Ag(I) detection using anodic stripping voltammetric technique. This porous GaN electrode without any surface modification is more favorable industrially due to its easy electrode fabrication and measurement process. The low limit of detection (0.5 ppb) and good anti-interference ability prove porous GaN electrode has the potential to be applied in real water samples. 2. Experimental 2.1. Fabrication of porous GaN electrode Single-crystal n-type GaN(0001) film was grown on sapphire(0001) substrate by hydride vapor phase epitaxy (HVPE). The Si-doped GaN layer is 5 µm thick with the carrier concentration of 4.8×1018 cm−3. The size of planar GaN electrode is 1.3 ×0.3 cm. Porous GaN electrode was fabricated by photo-assisted electrochemical etching technique. Planar GaN with front-side ohmic contact made by indium point and platinum plate were used as the anode and cathode, respectively. The front-side of planar GaN electrode was illuminated by a 300 W Xenon lamp; the applied etching voltage and time were 5 V and 5 min, respectively. Ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (C7H11F3N2O3S) was used as the etchant. The schematic diagram of etching device is shown in Fig. 1. After etching porous GaN electrode was obtained by completely washing. The surface morphology

Corresponding author. E-mail address: [email protected] (G.-B. Pan).

http://dx.doi.org/10.1016/j.talanta.2017.01.016 Received 27 September 2016; Received in revised form 27 December 2016; Accepted 6 January 2017 Available online 07 January 2017 0039-9140/ © 2017 Elsevier B.V. All rights reserved.

Talanta 165 (2017) 540–544

M.-R. Zhang, G.-B. Pan

surements were performed as follows. (a) The preconcentration process was carried out at the applied potential of −0.5 V for 2 min under magnetic stirring (800 rpm). (b) The stripping voltammogram was recorded by applying a positive-going square-wave voltammetric potential scan from −0.1 to 0.4 V with a frequency of 20 Hz, amplitude of 25 mV, and potential step of 5 mV. (c) The regeneration step was executed at the potential of +0.6 V for 5 min under magnetic stirring (800 rpm), prior to the next measurement.

3. Results and discussion Fig. 2 shows SEM images of porous GaN under different stages of SWASV process. The surface morphology of freshly prepared GaN is shown in Fig. 2a. The pore density is estimated up to 6.5 ×109 per square centimeter. The pore diameter is between 20 and 160 nm, and the average pore diameter is 58 nm. Moreover, the pore shape is hexagonal, which is consistent with the wurtzite structure (hexagonal system) of GaN. The etching mechanism of ionic liquid has been speculated in our previous report [21]. The pore morphology of porous GaN after preconcentration procedure demonstrates Ag(I) can enter into the inner pores and the deposited Ag fills most of the pores of GaN, which is shown in Fig. 2b. This result also proves porous GaN electrode can be fully wetted by the electrolyte. Fig. 2c exhibits there is still a significant amount of Ag in the pores of GaN, this result suggests the stripping process can not wipe off all Ag deposit. The previous report had demonstrated the Ag deposit could not all be stripped voltammetrically [22]. The main reason may be ascribed to the unique interface between Ag and GaN, which needs a more positive potential and longer time to oxidize Ag to Ag(I). Fig. 2d shows porous GaN recovers to its

Fig. 1. Schematic diagram of fabricating porous GaN electrode by photo-assisted electrochemical etching using ionic liquid as the etchant.

of porous GaN electrodes was characterized by scanning electron microscopy (SEM, Hitachi S-4800). CHI 660D (CH Instruments) was used in all electrochemical measurements.

2.2. Analytical procedure The analysis of Ag(I) was carried out using porous GaN, platinum wire and Ag/AgCl electrodes as working, counter and reference electrodes, respectively. The three electrodes were immersed into a 50 mL electrochemical cell, containing 0.1 M acetate buffer (pH 4.5) and different concentrations of Ag(I). All measurements were implemented at room temperature in the presence of dissolved oxygen. The detailed square-wave anodic stripping voltammetric (SWASV) mea-

Fig. 2. SEM images of porous GaN (a) cleanly, (b) after preconcentration step, (c) after stripping step, and (d) after regeneration step.

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Fig. 3. (a) Square-wave anodic stripping voltammograms (SWASVs) of planar and porous GaN electrodes in acetate buffer solution containing 50 ppb Ag(I). (b) SWASVs of planar and porous GaN electrodes in acetate buffer solution containing 1 ppb Ag(I). (c) SWASVs of porous GaN electrode in acetate buffer solution containing 20 ppb Ag(I). The black curve represents the anodic sweep after regeneration step at +0.6 V for 5 min (d) SWASVs of porous GaN electrode in acetate buffer solution containing 20 ppb Ag(I) in three successive full measurement processes.

Fig. 4. (a) SWASVs of porous GaN electrode in acetate buffer solution containing 1, 10, 20, 50, 100 ppb Ag(I). (b) Linear calibration curve of anodic peak currents towards Ag(I) concentrations. (c) SWASVs of porous GaN electrode in acetate buffer solution containing 20 ppb Ag(I) (blank control group), and 10 times concentration of Na+, K+, Fe3+, Ca2+, Mg2+, Al3+, Pb2+, Cu2+(interference group). (d) Anodic peak currents of five porous GaN electrodes towards 10 ppb Ag(I) in acetate buffer solution.

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errors can be attributed to the unique high-porosity structure and the surface state change of porous GaN electrode as the number of measurement times increases. Some common metal ions were added into the sample to evaluate the anti-interference ability of porous GaN electrode (Fig. 4c). Compared to blank control group, the relative deviation of peak current for interference group is 0.95%, which means porous GaN electrode has good anti-interference ability. When the Ag(I) concentration is 20 ppb, ϕAg+/Ag is calculated to be 0.34 V by Nernst equation. Even with the overpotential, Ag(I) can be electrodeposited onto GaN electrode at −0.5 V. However, the electrodeposition of interfering ions needs larger negative potential, which is the reason why we selected −0.5 V as the preconcentration potential. For instance, when the Pb(II) concentration is 200 ppb, ϕPb2+/Pb is calculated to be −0.53 V through Nernst equation, so Pb(II) cannot be deposited onto GaN electrode at −0.5 V even excluding the overpotential. The same is applicable to Na(I), K(I), Ca(II), Mg(II), Fe(III) and Al(III). Our previous report demonstrated the instantaneous nucleation of Cu particles was observed only at the large applied potential of −1.1 V [24]. Furthermore, the stripping potential (0.2 V) of Ag is different from other metals, that is the principle why anodic stripping voltammetric technique can detect many metal ions in one measurement process. Moreover, after regeneration step, all deposited metals can be removed. The detection performance is not affected in the next measurement. The reproducibility of porous GaN electrode is of another great concern for practical applications (Fig. 4d). The anodic peak currents of five porous GaN electrodes fabricated by the same process towards 10 ppb Ag(I) are 0.0412, 0.0453, 0.0392, 0.0426 and 0.0443 µA with the relative standard deviation (RSD) of 5.72%. This result can be ascribed to the tiny difference of pore morphology. Table 1 shows the comparison of analytical performance of porous GaN electrode with other electrode materials. The detection limit of porous GaN electrode is lower than other planar film electrodes. Previous theoretical discussions demonstrated porous electrodes have beneficial properties and lower detection limits compared to planar electrodes [27,28]. To fully utilize the potential of porous GaN electrode in other metal ions detection, further optimizations in pore structure and morphology are required to enhance its repeatability and reproducibility. Related works are already in progress.

Table 1 Comparison of analytical performance with other electrode materials. Electrode

Procedure

Linear range (ppb)

Detection limit (ppb)

Reference

Graphite felt Hybrid diamond/ graphite Boron-doped diamond 3C-SiC Porous GaN

ASV ASV

2.7–134 10–1000

2.7 5.8

[14] [15]

DPASV

1–1000

1.0

[25]

ASV SWASV

10–1000 1–100

4.0 0.5

[26] This work

initial morphology, which suggests the applied regeneration condition ( +0.6 V, 5 min) can totally remove Ag deposit. As shown in Fig. 3a, the stripping peak current of porous GaN electrode is much larger than that of planar GaN electrode of the same geometric area. Note that the base current of porous GaN electrode is much larger than that of planar GaN electrode. The reason may be that the electrolyte can enter into the pore structure, which increases the interfacial contact area of solid-liquid. Compared to planar GaN, porous GaN has a much higher active area, then more Ag(I) can be deposited onto porous GaN electrode under the same Ag(I) concentration, which also suggests porous GaN electrode can response to lower concentration of Ag(I). To further verify this point, we observe porous GaN electrode shows obvious stripping peak towards 1 ppb Ag(I), whereas planar GaN electrode exhibits no stripping peak (Fig. 3b). Above results demonstrate porous GaN electrode can detect lower Ag(I) concentration than planar GaN electrode. In order to prove the good surface renewability of porous GaN electrode, after regeneration step, porous GaN electrode underwent another stripping process to test whether there are any Ag deposits in the pores of GaN. No peak current is observed in the black curve shown in Fig. 3c, suggesting that the accumulated Ag has been fully removed. Combined with the result of SEM image (Fig. 2d), this result demonstrates porous GaN electrode has good self-renewal capacity, which is vital for long-term practical measurements. In order to further verify the recovery ability of porous GaN electrode, as shown in Fig. 3d, the anodic peak currents of porous GaN electrode are almost unchanged in three successive full measurement processes of 20 ppb Ag(I), which indicates porous GaN electrode has good repeatability. Fig. 4a shows a series of square-wave anodic stripping voltammetric determinations towards different concentrations of Ag(I) under the optimized experimental parameters, which was used to roughly evaluate the linear relationship of stripping peak currents and Ag(I) concentrations. However, it is obvious that the base currents increase with the increase of Ag(I) concentrations. The probable reason is that the deposited Ag increases the conductivity of porous GaN electrode, moreover, the deposition amount of Ag increases as Ag(I) concentrations increase, which leads to the ever-increasing interfacial contact area between GaN and Ag. The peak current can be clearly seen even for the Ag(I) concentration as low as 1 ppb, furthermore, the stripping peaks shift to more positive potentials as the concentrations of Ag(I) increase. It can be ascribed to the increased reduction equilibrium potential of Ag+/Ag couple and the growing quantity of Ag deposit with increasing concentrations of Ag(I) [23]. The calibration curve of anodic peak currents towards Ag(I) concentrations ranging from 1 to 100 ppb is shown in Fig. 4b. The peak current was obtained by calculating arithmetic average of ten successive measurements of the same Ag(I) concentration. The fitted linear regression equation is I (µA) =0.02977+0.00106 C (µg/L) with R2 =0.9967. The limit of detection (LOD) is 0.5 ppb with a signal-to-noise ratio of 3. Half the length of error bars equals to the value of uncertainty, which is the maximum of the differences of measured and average peak currents. The source of

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