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ZrO2 nanotube electrode for detection of heavy metal ions
Electrochemistry Communications 110 (2020) 106614
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Zr/ZrO2 nanotube electrode for detection of heavy metal ions George-Octavian Buica, Andrei Bogdan Stoian, Claudiu Manole, Ioana Demetrescu, ⁎ Cristian Pirvu
T
University Polytechnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Polizu, 011061 Bucharest, Romania
ARTICLE INFO
ABSTRACT
Keywords: ZrO2 nanotubes Photoreduction Sensing Metal ions
An integrated system for photo-accumulation (photoreduction) and sensing, based on a Zr/ZrO2 electrode, has been developed for the detection of trace heavy metals. The nanostructuring of the Zr/ZrO2 electrode was achieved by anodizing, which produced a nanotubular structure. Solutions containing Cd(II), Pb(II), Cu(II) and Hg(II) were used for analysis. Linear calibration plots were obtained in the concentration ranges from 8 × 10−8 to 10−5 M for Pb(II), 5 × 10−8 to 2 × 10−6 M for Cu(II) and 4 × 10−8 to 10−5 M for Hg(II).
1. Introduction It is necessary to detect heavy metals (including Pb, Cd, Hg and Cu) in environmental media because these ions represent a major threat to human health [1,2]. To analyze ions from environmental samples it is desirable that the detection should be performed rapidly, that multiple ions be detected simultaneously on site with a high sensitivity, and that the detection method should have low running costs. This can be achieved by electrochemical sensors based on modified electrodes coupled with a simple and sensitive electroanalytical method such as anodic stripping voltammetry (ASV). The materials commonly used as electrodes for electrochemical detection of heavy metal ions can be separated into three main categories: organic materials, biomaterials and inorganic materials [3]. An important factor in the choice of inorganic materials is their high level of sorption of metal ions [4] and relatively low cost. In particular, inorganic nanomaterials (e.g. metal oxides, carbon and silica) [5] represent excellent candidates for sensors for metal ion detection due to their good structural and thermal stability, their large real surface area relative to the geometric one, their high surface activity and the suitability of the surface for adsorption [5,6]. Zirconium (Zr) with an oxide layer of specific thickness with surface nanostructure represents a very promising material for sensors [7,8] and photocatalysts [9,10]. Zr can form a thin native passive oxide film ZrO2 (zirconia) which is responsible for its electrochemical stability. However, for many applications, this self-passivation is not enough and anodizing under various conditions is a practical way of tailoring oxide properties. Different anodizing procedures have been performed, leading to the fabrication of self-organized porous layers of ZrO2 [11] or
⁎
nanotubes [12,13] with various dimensions depending on the anodizing conditions [14]. A ZrO2 nanotube material prepared by anodization methods has been shown to exhibit high catalytic activity [15,16]. ZrO2 is suitable for photocatalytic applications involving metal photodeposition [17,18] due its wide band gap (~5.0 eV), the lowest potential of the conduction band being −1.0 V (compared with −0.1 V for TiO2) and the highest potential of the valence band being ca. +4.0 V (compared with +3.1 V for TiO2) [19]. The high negative value of the conduction band promotes the photogeneration of electrons and provides strong reducing ability [20]. ZrO2 also has other advantages, such as good stability towards photocorrosion, non-toxicity, low cost and being environmentally friendly. The possibility of metal ions being accumulated under photo-irradiation and quantified by anodic stripping voltammetry has already been demonstrated by Suárez et al. [21]. The classical approach involves the accumulation of the metal on the electrode surface by means of electrochemical reduction of the metal ion in solution (which is rarely selective) or sorption of metal ions in open circuit on electrodes previously modified with chelates (a more selective approach) followed by metal ion reduction and stripping. In the case of photo-accumulation, the accumulation of metal ions and their reduction occur in a single step. Moreover, Ma et al. [22] hypothesizes that matching the potential of metal ions and the energy levels of the conduction band of a semiconductor could promote the selectivity and sensitivity of electrode materials for the electrochemical detection of heavy metal ions. Using the important features of nanostructured ZrO2, an integrated system of photo-accumulation (photoreduction) and sensing based on a Zr/ZrO2 nanotube electrode has been developed for the detection of trace heavy metals for the first time. The enhancement of sensitivity for
Corresponding author. E-mail address: [email protected] (C. Pirvu).
https://doi.org/10.1016/j.elecom.2019.106614 Received 18 October 2019; Received in revised form 14 November 2019; Accepted 15 November 2019 Available online 18 November 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 110 (2020) 106614
G.-O. Buica, et al.
metal ion detection under UV light irradiation has been reported for TiO2 films [21] and Ti/TiO2 electrodes [22], but not for Zr/ZrO2 electrodes. Solutions containing Cd(II), Pb(II), Cu(II) and Hg(II) with a potential higher than the potential of the conduction band of ZrO2 were used for analysis.
nanostructuring can only be advantageous for metal detection. Based on the intensity of the main peaks from the XRD patterns (Fig. 2) and using Scherrer’s equation, the mean size of the crystallites could be computed. The ZrO2 pattern can be indexed to a face-centered cubic structure (Fm-3m – 225 with a unit parameter of a = b = c = 5.0900 Å), which is consistent with the standard pattern of bulk ZrO2 [PDF Card No. 01-072-2742]. The ZrO2 pattern shows diffraction features appearing at 2θ = 30.3°, 35.1°, 50.52°, 60.1°, 74.4° and 84.9°, corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0) and (4 2 0) planes of the zirconium oxide phase.
2. Material and methods 2.1. Zr/ZrO2 nanotubes electrode preparation Zirconium disks (99% trace metals, Sigma Aldrich) with a diameter of 6 mm were polished with abrasive SiC paper up to a final porosity size of 4000 and the polishing finished with diamond paste (0.25 μm from Presi). After that, the Zr disks were cleaned by sonicating in acetone and ethanol and rinsing with ultrapure deionized water. The Zr/ZrO2 nanotube electrodes were obtained under potentiostatic conditions at 20 V in (NH4)2SO4 (1 M) + 0.5% NH4F [23] for 45 min (unless otherwise specified) followed by rinsing with purified water and drying under Ar.
3.2. Adsorption and photoreduction of metals The selectivity and sensitivity of electrode materials towards reduction and detection of heavy metal ions can be tuned by fitting the potential of the metal ions with the conduction band energy levels of the semiconductor. It can be seen from Fig. 3 that the potentials of Cd (II), Pb(II), Cu(II) and Hg(II) ions are higher than the potential of the conduction band. Thus, spontaneous photoreduction (M2+ + 2e− → M0) takes place under UV irradiation. Anodic stripping using DPV was used to highlight the photoreduction process on the Zr/ZrO2 nanotube surface. The roles of both ZrO2 and UV irradiation are essential in the adsorption and reduction of metal ions. Thus, if the unmodified Zr electrode was exposed to UV light, the photo-reduction of the metal ions did not take place, and no peak was seen in DPV (Fig. 4). If the Zr/ZrO2 electrodes are not exposed to UV light, only two small stripping peaks at −0.5 V and 0 V attributed to Pb and Cu, respectively, were found by DPV (Fig. 4). This weak adsorption is most likely due to the interaction of oxygen with the metal ions. By contrast, when the Zr/ZrO2 nanotube electrodes were exposed to UV light irradiation, enhanced sorption of these ions onto the electrode surface occurred, which is clearly shown in the DPV curves. When the electrode was irradiated with UV light, the stripping peak currents for Pb and Cu increased. In addition, a new oxidation peak appeared at ~0.38 V, attributed to the redissolution of reduced mercury. This confirms the beneficial effect of UV light irradiation of the electrode surface. However, the presence of Cd (II) ions could not be observed. In the case of Cd(II) photoreduction is theoretically possible, the redox potential of the Cd2+/Cd being −0.40 V (vs. NHE) but the thermodynamic driving force is small. The absence of Cd ions is probably due to competitive adsorption at binding sites, as previously shown by Yang et al. [26]. This shows that the adsorption of Cd ions is strongly affected by the presence of Pb(II) and Cu(II) ions. Moreover, Pb ions show higher adsorption competitiveness and a higher adsorption capacity than Cd, Cd or Ni.
2.2. Apparatus A DC power supply (Matrix MPS-7505L-1) was used for potentiostatic anodizing of the Zr disks. An AUTOLAB PGSTAT 204 potentiostat was used to record electrochemical experiments performed in a threeelectrode cell (Zr/ZrO2 nanotube disk – working electrode; Pt mesh – counter electrode; Ag/AgCl, 3 M KCl – reference electrode). The MottSchottky experiments were performed with an AUTOLAB PGSTAT 302 N in the +0.15 V to +1.2 V range at a frequency of 1 kHz, with an amplitude of 0.01 V RMS [24]. A Quanta 650 FEG scanning electron microscope (SEM) from Thermo Fisher Scientific (USA) was used to investigate the morphology of the samples. XRD measurements were performed with a Rigaku SmartLab X-ray diffractometer Cu: Kβ1 = 1.39217 Å, Kα1 = 1.540598 Å and Cu Kα2 = 1.544426 Å. 2.3. Sensing of metal ions The photo-accumulation of metal ions on the Zr/ZrO2 nanotubes was achieved by immersing the electrode in a quartz cuvette containing 25 mL aqueous acetate buffer solutions containing Cd(II), Pb(II), Cu(II) and Hg(II) ions, under UV irradiation (λ = 254 nm). The electrode was then rinsed with purified water (18.2 MΩ cm) to remove the unattached ions and transferred into a 0.1 M acetate buffer supporting electrolyte (with the same pH as the accumulation solution). Further, the reduced ions on the surface of the Zr/ZrO2 nanotubes were stripped to metal ions by anodic stripping using the differential pulse voltammetry (DPV) technique (20 mV s−1, 25 mV amplitude and 0.5 s pulse periods). The oxygen found in aqueous solutions could generate a current which may interfere with the stripping process due to its relatively low reduction potential [25], so in order to reduce the influence of the oxygen, the DPV curves were recorded under Ar.
3.3. Detection of metal ions Because the electrode response towards Pb(II), Cu(II) and Hg(II) ions produced well-defined and separated peaks, the response of the Zr/ ZrO2 nanotube electrodes was further studied to improve simultaneous detection of these ions. To optimize the sensing properties of the Zr/ ZrO2 nanotube electrodes towards simultaneous detection of the heavy metal ions, the influence of UV light irradiation time, the anodizing time (thickness of ZrO2 nanotubes) and the pH of the solution of metal ions were studied. Fig. 5A shows that increasing the time of irradiation with UV light leads to an increase in the stripping currents. Thus, the sensitivity of the Zr/ZrO2 nanotube electrodes is enhanced due to the increased number of photoelectrons accumulated [2], leading to the deposition of a larger amount of reduced ions on the surface of the electrodes. This direct proportionality between the incident radiation and the oxidation current highlights the catalytic mechanism of photoreduction. The anodizing time influences the length of the ZrO2 nanotubes. In Fig. 5B it can be seen that the stripping peak currents for Pb, Cu, and Hg increase with anodizing time up to 45 min. After this time no major
3. Results and discussion 3.1. Zr/ZrO2 surface characterization The surface of the Zr electrode (Fig. 1A) was completely covered with ZrO2 nanotubes with inner diameters ranging between ~38 and 55 nm. The cross section of the sample (Fig. 1B) shows that the nanotubes have straight walls and are well organized. The length of the nanotubes was ~13 μm and the outer diameter was ~100 nm. Based on these measurements the nanotube wall thickness is approximately ~25 nm and the aspect ratio (length-to-diameter ratio) is approximately ~130:1. During photocatalysis, photo-induced reactions take place at the surface. Thus, increasing the active surface of the electrode by 2
Electrochemistry Communications 110 (2020) 106614
G.-O. Buica, et al.
Fig. 1. SEM micrographs of ZrO2 nanotubes: (A, B) top view; (C, D) cross section.
Fig. 3. Schematic reaction mechanism during UV irradiation.
protonation of functional groups which for metal oxides are generally hydroxyl groups, and competitive adsorption at complexing sites between protons and metal ions. As the pH increases, more hydroxyl groups are deprotonated and electrostatic interaction between the metal cations and hydroxyl groups occurs. Moreover, the optimum pH of 6 found for the metal ions under study is in the range of the point of zero charge of ZrO2, which according to Kosmulski et al. [27] is between 4 and 8. According to Muhammad et al. [28], the surface becomes more negatively charged at higher pH values, thus positively influencing the complexation of metal ions by zirconium oxide.
Fig. 2. X-ray diffraction (XRD) pattern of the Zr/ZrO2 nanotube surface.
signal improvement was observed. The influence of pH on Pb, Cu, and Hg stripping peak currents was checked in the range 2–6.5 in 0.1 M acetate buffer solutions (Fig. 5C). The maximum peak current response for all ions was obtained at pH 6. For smaller values, a lower response was observed, probably due to the 3
Electrochemistry Communications 110 (2020) 106614
G.-O. Buica, et al.
nanotubes (e.g. from 10 min and above) can be mainly attributed to the oxide self-ordering as nanotubes, see Fig. 6B. The relative number of charge carrier density in the nanotubes remains reasonably constant after 10 min. of anodization, at an average of 6.6 × 1015 with a variation of ± 1.1 × 1015 suggesting that the semiconductor character is not influenced by the increase in the length of the nanotubes. The improved effect observed in Fig. 6B could be associated with an increase in the effective surface and the number of metal ion binding sites on increasing the length of the nanotubes. Thus, the increase in the sensitivity of the Zr/ZrO2 nanotube electrode led to an increase in the stripping currents for Pb(II), Cu(II) and Hg(II). The Mott-Schottky data shows that the charge carrier density is not a contributing factor in this performance. On the other hand, Fig. 6B shows that immediately after anodizing and formation of a ZrO2 nanostructured film, the flat band potential suddenly decreases from −0.2 V to −1.4 V. The values are close to those presented in the literature for a Zr/ZrO2 passive film (−0.1 V) [29], annealed F doped ZrO2 nanotubes (−1.09) [20] and pure ZrO2 (−1.25 V) [30]. This could explain the lack of signal on the Zr/ZrO2 (passive layer) electrode and the appearance of clear peaks after the formation of ZrO2, probably due to the better absorption of the metal ions (Me2+) and the Zr/ZrO2 surface being negatively affected by the formation of the oxide. The high negative value of the flat band indicates the preservation of the high negative conduction position of the
Fig. 4. DPV curves on Zr/ZrO2 nanotube electrodes without (solid line) and with (dotted line) UV light irradiation. The accumulation of reduced metal ions was performed in 5 × 10−6 M Cd(II), Pb(II), Cu(II) and Hg(II) for 30 min.
A Mott Schottky analysis was used to evaluate the semiconductor character of Zr/ZrO2 nanotube electrodes and the influence of the charge carrier density and flat band potential on photoreduction and electrode sensitivity. The decrease in the carrier charge density after growth of the
Fig. 5. The effects of various factors which can influence the photodeposition of metal ions on Zr/ZrO2 nanotubes: (A) irradiation time under UV light at 254 nm, (B) anodizing time (nanotube length), and (C) pH of acetate buffer solutions. 4
Electrochemistry Communications 110 (2020) 106614
G.-O. Buica, et al.
Fig. 6. Mott-Schottky results (a) as obtained experimentally with a linear fit regression; (b) calculated values for the carrier charge density (Nd) and flat band potential (Efb) for different anodizing times (nanotube lengths).
Zr/ZrO2 nanotube electrode which is important for photogeneration of electrons in order to reduce metal ions in the photo-accumulation step.
signal/noise ratio of 5 × 10−8, 4 × 10−8, and 10−7 for Pb, Cu and Hg, respectively.
3.4. Simultaneous detection of metal ions
4. Conclusions
The performance of the Zr/ZrO2 nanotube electrode towards simultaneous detection of Pb(II), Cu(II) and Hg(II) ions at different concentrations was evaluated under the optimized conditions (Fig. 7). The peak currents increase with the concentrations of metal ions and their linear dependence on concentration is given in Fig. 7 (inset). Linear calibration plots were obtained in the concentration ranges 8 × 10−8–10−5 M for Pb(II), 5 × 10−8–2 × 10−6 M for Cu(II) and 4 × 10−8–10−5 M for Hg(II), with detection limits based on 3 times
A new integrated system of photoaccumulation and sensing based on Zr/ZrO2 nanotube electrodes was developed for the detection of trace amounts of heavy metals. The obtained results show that the deposition by photoreduction of metal ions on Zr/ZrO2 nanotube electrodes under UV irradiation can lead to simultaneous detection of Pb(II), Cu(II) and Hg(II) ions in aqueous solution.
Fig. 7. DPV curves at Zr/ZrO2 nanotube electrodes for various concentrations of Pb(II), Cu(II) and Hg(II) ions. Inset: linear concentration domain for the studied metal ions. 5
Electrochemistry Communications 110 (2020) 106614
G.-O. Buica, et al.
Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by CNCSIS–UEFISCSU, project number PN-III-P4-ID PCE 2016-0316. SEM analyses were possible due to European Regional Development Fund through Competitiveness Operational Program 2014-2020, Priority axis 1, Project No. P_36_611, MySMIS code 107066. References [1] K.A. Heys, R.F. Shore, M.G. Pereira, K.C. Jones, F.L. Martin, Risk assessment of environmental mixture effects, RSC Adv. 6 (2016) 47844–47857. [2] P. Koedrith, H. Kim, J.I. Weon, Y.R. Seo, Toxicogenomic approaches for understanding molecular mechanisms of heavy metal mutagenicity and carcinogenicity, Int. J. Hyg. Environ. Health 216 (2013) 587–598. [3] L. Cui, J. Wu, H.X. Ju, Electrochemical sensing of heavy metal ions with inorganic, organic and bio-materials, Biosens. Bioelectron. 63 (2015) 276–286. [4] S. Sen Gupta, K.G. Bhattacharyya, Kinetics of adsorption of metal ions on inorganic materials: a review, Adv. Colloid Interface 162 (2011) 39–58. [5] P.F. Schuster, K.M. Schaefer, G.R. Aiken, R.C. Antweiler, J.F. Dewild, J.D. Gryziec, A. Gusmeroli, G. Hugelius, E. Jafarov, D.P. Krabbenhoft, L. Liu, N. Herman-Mercer, C.C. Mu, D.A. Roth, T. Schaefer, R.G. Striegl, K.P. Wickland, T.J. Zhang, Permafrost stores a globally significant amount of mercury, Geophys. Res. Lett. 45 (2018) 1463–1471. [6] C. Gao, X.Y. Yu, S.Q. Xiong, J.H. Liu, X.J. Huang, Electrochemical detection of arsenic(III) completely free from noble metal: Fe3O4 microspheres-room temperature ionic liquid composite showing better performance than gold, Anal. Chem. 85 (2013) 2673–2680. [7] L.H. Zhang, Q.W. Wang, Y. Qi, L. Li, S.T. Wang, X.H. Wang, An ultrasensitive sensor based on polyoxometalate and zirconium dioxide nanocomposites hybrids material for simultaneous detection of toxic clenbuterol and ractopamine, Sens. Actuator B: Chem. 288 (2019) 347–355. [8] M.R. Mohammadi, D.J. Fray, Synthesis and characterisation of nanosized TiO2-ZrO2 binary system prepared by an aqueous sol-gel process: physical and sensing properties, Sens. Actuator. B: Chem. 155 (2011) 568–576. [9] R.B. Anjaneyulu, B.S. Mohan, G.P. Naidu, R. Muralikrishna, ZrO2/Fe2O3/RGO nanocomposite: good photocatalyst for dyes degradation, Physica E 108 (2019) 105–111. [10] A. Majedi, F. Davar, A. Abbasi, A. Ashrafi, Modified sol-gel based nanostructured zirconia thin film: preparation, characterization, photocatalyst and corrosion behavior, J. Inorg. Organomet. Pols. Mater. 26 (2016) 932–942. [11] A.B. Stoian, M. Vardaki, I. Demetrescu, Micro and nanostructure surface and interface characterization of anodized Zr in two different electrolytes, Acta Chim.
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Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Zr/ZrO2 nanotube electrode for detection of heavy metal ions George-Octavian Buica, Andrei Bogdan Stoian, Claudiu Manole, Ioana Demetrescu, ⁎ Cristian Pirvu
T
University Polytechnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Polizu, 011061 Bucharest, Romania
ARTICLE INFO
ABSTRACT
Keywords: ZrO2 nanotubes Photoreduction Sensing Metal ions
An integrated system for photo-accumulation (photoreduction) and sensing, based on a Zr/ZrO2 electrode, has been developed for the detection of trace heavy metals. The nanostructuring of the Zr/ZrO2 electrode was achieved by anodizing, which produced a nanotubular structure. Solutions containing Cd(II), Pb(II), Cu(II) and Hg(II) were used for analysis. Linear calibration plots were obtained in the concentration ranges from 8 × 10−8 to 10−5 M for Pb(II), 5 × 10−8 to 2 × 10−6 M for Cu(II) and 4 × 10−8 to 10−5 M for Hg(II).
1. Introduction It is necessary to detect heavy metals (including Pb, Cd, Hg and Cu) in environmental media because these ions represent a major threat to human health [1,2]. To analyze ions from environmental samples it is desirable that the detection should be performed rapidly, that multiple ions be detected simultaneously on site with a high sensitivity, and that the detection method should have low running costs. This can be achieved by electrochemical sensors based on modified electrodes coupled with a simple and sensitive electroanalytical method such as anodic stripping voltammetry (ASV). The materials commonly used as electrodes for electrochemical detection of heavy metal ions can be separated into three main categories: organic materials, biomaterials and inorganic materials [3]. An important factor in the choice of inorganic materials is their high level of sorption of metal ions [4] and relatively low cost. In particular, inorganic nanomaterials (e.g. metal oxides, carbon and silica) [5] represent excellent candidates for sensors for metal ion detection due to their good structural and thermal stability, their large real surface area relative to the geometric one, their high surface activity and the suitability of the surface for adsorption [5,6]. Zirconium (Zr) with an oxide layer of specific thickness with surface nanostructure represents a very promising material for sensors [7,8] and photocatalysts [9,10]. Zr can form a thin native passive oxide film ZrO2 (zirconia) which is responsible for its electrochemical stability. However, for many applications, this self-passivation is not enough and anodizing under various conditions is a practical way of tailoring oxide properties. Different anodizing procedures have been performed, leading to the fabrication of self-organized porous layers of ZrO2 [11] or
⁎
nanotubes [12,13] with various dimensions depending on the anodizing conditions [14]. A ZrO2 nanotube material prepared by anodization methods has been shown to exhibit high catalytic activity [15,16]. ZrO2 is suitable for photocatalytic applications involving metal photodeposition [17,18] due its wide band gap (~5.0 eV), the lowest potential of the conduction band being −1.0 V (compared with −0.1 V for TiO2) and the highest potential of the valence band being ca. +4.0 V (compared with +3.1 V for TiO2) [19]. The high negative value of the conduction band promotes the photogeneration of electrons and provides strong reducing ability [20]. ZrO2 also has other advantages, such as good stability towards photocorrosion, non-toxicity, low cost and being environmentally friendly. The possibility of metal ions being accumulated under photo-irradiation and quantified by anodic stripping voltammetry has already been demonstrated by Suárez et al. [21]. The classical approach involves the accumulation of the metal on the electrode surface by means of electrochemical reduction of the metal ion in solution (which is rarely selective) or sorption of metal ions in open circuit on electrodes previously modified with chelates (a more selective approach) followed by metal ion reduction and stripping. In the case of photo-accumulation, the accumulation of metal ions and their reduction occur in a single step. Moreover, Ma et al. [22] hypothesizes that matching the potential of metal ions and the energy levels of the conduction band of a semiconductor could promote the selectivity and sensitivity of electrode materials for the electrochemical detection of heavy metal ions. Using the important features of nanostructured ZrO2, an integrated system of photo-accumulation (photoreduction) and sensing based on a Zr/ZrO2 nanotube electrode has been developed for the detection of trace heavy metals for the first time. The enhancement of sensitivity for
Corresponding author. E-mail address: [email protected] (C. Pirvu).
https://doi.org/10.1016/j.elecom.2019.106614 Received 18 October 2019; Received in revised form 14 November 2019; Accepted 15 November 2019 Available online 18 November 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 110 (2020) 106614
G.-O. Buica, et al.
metal ion detection under UV light irradiation has been reported for TiO2 films [21] and Ti/TiO2 electrodes [22], but not for Zr/ZrO2 electrodes. Solutions containing Cd(II), Pb(II), Cu(II) and Hg(II) with a potential higher than the potential of the conduction band of ZrO2 were used for analysis.
nanostructuring can only be advantageous for metal detection. Based on the intensity of the main peaks from the XRD patterns (Fig. 2) and using Scherrer’s equation, the mean size of the crystallites could be computed. The ZrO2 pattern can be indexed to a face-centered cubic structure (Fm-3m – 225 with a unit parameter of a = b = c = 5.0900 Å), which is consistent with the standard pattern of bulk ZrO2 [PDF Card No. 01-072-2742]. The ZrO2 pattern shows diffraction features appearing at 2θ = 30.3°, 35.1°, 50.52°, 60.1°, 74.4° and 84.9°, corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0) and (4 2 0) planes of the zirconium oxide phase.
2. Material and methods 2.1. Zr/ZrO2 nanotubes electrode preparation Zirconium disks (99% trace metals, Sigma Aldrich) with a diameter of 6 mm were polished with abrasive SiC paper up to a final porosity size of 4000 and the polishing finished with diamond paste (0.25 μm from Presi). After that, the Zr disks were cleaned by sonicating in acetone and ethanol and rinsing with ultrapure deionized water. The Zr/ZrO2 nanotube electrodes were obtained under potentiostatic conditions at 20 V in (NH4)2SO4 (1 M) + 0.5% NH4F [23] for 45 min (unless otherwise specified) followed by rinsing with purified water and drying under Ar.
3.2. Adsorption and photoreduction of metals The selectivity and sensitivity of electrode materials towards reduction and detection of heavy metal ions can be tuned by fitting the potential of the metal ions with the conduction band energy levels of the semiconductor. It can be seen from Fig. 3 that the potentials of Cd (II), Pb(II), Cu(II) and Hg(II) ions are higher than the potential of the conduction band. Thus, spontaneous photoreduction (M2+ + 2e− → M0) takes place under UV irradiation. Anodic stripping using DPV was used to highlight the photoreduction process on the Zr/ZrO2 nanotube surface. The roles of both ZrO2 and UV irradiation are essential in the adsorption and reduction of metal ions. Thus, if the unmodified Zr electrode was exposed to UV light, the photo-reduction of the metal ions did not take place, and no peak was seen in DPV (Fig. 4). If the Zr/ZrO2 electrodes are not exposed to UV light, only two small stripping peaks at −0.5 V and 0 V attributed to Pb and Cu, respectively, were found by DPV (Fig. 4). This weak adsorption is most likely due to the interaction of oxygen with the metal ions. By contrast, when the Zr/ZrO2 nanotube electrodes were exposed to UV light irradiation, enhanced sorption of these ions onto the electrode surface occurred, which is clearly shown in the DPV curves. When the electrode was irradiated with UV light, the stripping peak currents for Pb and Cu increased. In addition, a new oxidation peak appeared at ~0.38 V, attributed to the redissolution of reduced mercury. This confirms the beneficial effect of UV light irradiation of the electrode surface. However, the presence of Cd (II) ions could not be observed. In the case of Cd(II) photoreduction is theoretically possible, the redox potential of the Cd2+/Cd being −0.40 V (vs. NHE) but the thermodynamic driving force is small. The absence of Cd ions is probably due to competitive adsorption at binding sites, as previously shown by Yang et al. [26]. This shows that the adsorption of Cd ions is strongly affected by the presence of Pb(II) and Cu(II) ions. Moreover, Pb ions show higher adsorption competitiveness and a higher adsorption capacity than Cd, Cd or Ni.
2.2. Apparatus A DC power supply (Matrix MPS-7505L-1) was used for potentiostatic anodizing of the Zr disks. An AUTOLAB PGSTAT 204 potentiostat was used to record electrochemical experiments performed in a threeelectrode cell (Zr/ZrO2 nanotube disk – working electrode; Pt mesh – counter electrode; Ag/AgCl, 3 M KCl – reference electrode). The MottSchottky experiments were performed with an AUTOLAB PGSTAT 302 N in the +0.15 V to +1.2 V range at a frequency of 1 kHz, with an amplitude of 0.01 V RMS [24]. A Quanta 650 FEG scanning electron microscope (SEM) from Thermo Fisher Scientific (USA) was used to investigate the morphology of the samples. XRD measurements were performed with a Rigaku SmartLab X-ray diffractometer Cu: Kβ1 = 1.39217 Å, Kα1 = 1.540598 Å and Cu Kα2 = 1.544426 Å. 2.3. Sensing of metal ions The photo-accumulation of metal ions on the Zr/ZrO2 nanotubes was achieved by immersing the electrode in a quartz cuvette containing 25 mL aqueous acetate buffer solutions containing Cd(II), Pb(II), Cu(II) and Hg(II) ions, under UV irradiation (λ = 254 nm). The electrode was then rinsed with purified water (18.2 MΩ cm) to remove the unattached ions and transferred into a 0.1 M acetate buffer supporting electrolyte (with the same pH as the accumulation solution). Further, the reduced ions on the surface of the Zr/ZrO2 nanotubes were stripped to metal ions by anodic stripping using the differential pulse voltammetry (DPV) technique (20 mV s−1, 25 mV amplitude and 0.5 s pulse periods). The oxygen found in aqueous solutions could generate a current which may interfere with the stripping process due to its relatively low reduction potential [25], so in order to reduce the influence of the oxygen, the DPV curves were recorded under Ar.
3.3. Detection of metal ions Because the electrode response towards Pb(II), Cu(II) and Hg(II) ions produced well-defined and separated peaks, the response of the Zr/ ZrO2 nanotube electrodes was further studied to improve simultaneous detection of these ions. To optimize the sensing properties of the Zr/ ZrO2 nanotube electrodes towards simultaneous detection of the heavy metal ions, the influence of UV light irradiation time, the anodizing time (thickness of ZrO2 nanotubes) and the pH of the solution of metal ions were studied. Fig. 5A shows that increasing the time of irradiation with UV light leads to an increase in the stripping currents. Thus, the sensitivity of the Zr/ZrO2 nanotube electrodes is enhanced due to the increased number of photoelectrons accumulated [2], leading to the deposition of a larger amount of reduced ions on the surface of the electrodes. This direct proportionality between the incident radiation and the oxidation current highlights the catalytic mechanism of photoreduction. The anodizing time influences the length of the ZrO2 nanotubes. In Fig. 5B it can be seen that the stripping peak currents for Pb, Cu, and Hg increase with anodizing time up to 45 min. After this time no major
3. Results and discussion 3.1. Zr/ZrO2 surface characterization The surface of the Zr electrode (Fig. 1A) was completely covered with ZrO2 nanotubes with inner diameters ranging between ~38 and 55 nm. The cross section of the sample (Fig. 1B) shows that the nanotubes have straight walls and are well organized. The length of the nanotubes was ~13 μm and the outer diameter was ~100 nm. Based on these measurements the nanotube wall thickness is approximately ~25 nm and the aspect ratio (length-to-diameter ratio) is approximately ~130:1. During photocatalysis, photo-induced reactions take place at the surface. Thus, increasing the active surface of the electrode by 2
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Fig. 1. SEM micrographs of ZrO2 nanotubes: (A, B) top view; (C, D) cross section.
Fig. 3. Schematic reaction mechanism during UV irradiation.
protonation of functional groups which for metal oxides are generally hydroxyl groups, and competitive adsorption at complexing sites between protons and metal ions. As the pH increases, more hydroxyl groups are deprotonated and electrostatic interaction between the metal cations and hydroxyl groups occurs. Moreover, the optimum pH of 6 found for the metal ions under study is in the range of the point of zero charge of ZrO2, which according to Kosmulski et al. [27] is between 4 and 8. According to Muhammad et al. [28], the surface becomes more negatively charged at higher pH values, thus positively influencing the complexation of metal ions by zirconium oxide.
Fig. 2. X-ray diffraction (XRD) pattern of the Zr/ZrO2 nanotube surface.
signal improvement was observed. The influence of pH on Pb, Cu, and Hg stripping peak currents was checked in the range 2–6.5 in 0.1 M acetate buffer solutions (Fig. 5C). The maximum peak current response for all ions was obtained at pH 6. For smaller values, a lower response was observed, probably due to the 3
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nanotubes (e.g. from 10 min and above) can be mainly attributed to the oxide self-ordering as nanotubes, see Fig. 6B. The relative number of charge carrier density in the nanotubes remains reasonably constant after 10 min. of anodization, at an average of 6.6 × 1015 with a variation of ± 1.1 × 1015 suggesting that the semiconductor character is not influenced by the increase in the length of the nanotubes. The improved effect observed in Fig. 6B could be associated with an increase in the effective surface and the number of metal ion binding sites on increasing the length of the nanotubes. Thus, the increase in the sensitivity of the Zr/ZrO2 nanotube electrode led to an increase in the stripping currents for Pb(II), Cu(II) and Hg(II). The Mott-Schottky data shows that the charge carrier density is not a contributing factor in this performance. On the other hand, Fig. 6B shows that immediately after anodizing and formation of a ZrO2 nanostructured film, the flat band potential suddenly decreases from −0.2 V to −1.4 V. The values are close to those presented in the literature for a Zr/ZrO2 passive film (−0.1 V) [29], annealed F doped ZrO2 nanotubes (−1.09) [20] and pure ZrO2 (−1.25 V) [30]. This could explain the lack of signal on the Zr/ZrO2 (passive layer) electrode and the appearance of clear peaks after the formation of ZrO2, probably due to the better absorption of the metal ions (Me2+) and the Zr/ZrO2 surface being negatively affected by the formation of the oxide. The high negative value of the flat band indicates the preservation of the high negative conduction position of the
Fig. 4. DPV curves on Zr/ZrO2 nanotube electrodes without (solid line) and with (dotted line) UV light irradiation. The accumulation of reduced metal ions was performed in 5 × 10−6 M Cd(II), Pb(II), Cu(II) and Hg(II) for 30 min.
A Mott Schottky analysis was used to evaluate the semiconductor character of Zr/ZrO2 nanotube electrodes and the influence of the charge carrier density and flat band potential on photoreduction and electrode sensitivity. The decrease in the carrier charge density after growth of the
Fig. 5. The effects of various factors which can influence the photodeposition of metal ions on Zr/ZrO2 nanotubes: (A) irradiation time under UV light at 254 nm, (B) anodizing time (nanotube length), and (C) pH of acetate buffer solutions. 4
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Fig. 6. Mott-Schottky results (a) as obtained experimentally with a linear fit regression; (b) calculated values for the carrier charge density (Nd) and flat band potential (Efb) for different anodizing times (nanotube lengths).
Zr/ZrO2 nanotube electrode which is important for photogeneration of electrons in order to reduce metal ions in the photo-accumulation step.
signal/noise ratio of 5 × 10−8, 4 × 10−8, and 10−7 for Pb, Cu and Hg, respectively.
3.4. Simultaneous detection of metal ions
4. Conclusions
The performance of the Zr/ZrO2 nanotube electrode towards simultaneous detection of Pb(II), Cu(II) and Hg(II) ions at different concentrations was evaluated under the optimized conditions (Fig. 7). The peak currents increase with the concentrations of metal ions and their linear dependence on concentration is given in Fig. 7 (inset). Linear calibration plots were obtained in the concentration ranges 8 × 10−8–10−5 M for Pb(II), 5 × 10−8–2 × 10−6 M for Cu(II) and 4 × 10−8–10−5 M for Hg(II), with detection limits based on 3 times
A new integrated system of photoaccumulation and sensing based on Zr/ZrO2 nanotube electrodes was developed for the detection of trace amounts of heavy metals. The obtained results show that the deposition by photoreduction of metal ions on Zr/ZrO2 nanotube electrodes under UV irradiation can lead to simultaneous detection of Pb(II), Cu(II) and Hg(II) ions in aqueous solution.
Fig. 7. DPV curves at Zr/ZrO2 nanotube electrodes for various concentrations of Pb(II), Cu(II) and Hg(II) ions. Inset: linear concentration domain for the studied metal ions. 5
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Declaration of Competing Interest
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