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Theoretical and experimental insights into the electrochemical heavy metal ion sensing with nonconductive nanomaterials
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Electrochemistry
ScienceDirect Review Article
Theoretical and experimental insights into the electrochemical heavy metal ion sensing with nonconductive nanomaterials Jianjun Liao1,2, Zui Tao1,3 and Shiwei Lin1,3 Abstract
Nonconductive nanomaterial is a type of modifiers widely used in electrochemical heavy metal ion detection. Despite a large number of studies devoted to the electrochemical stripping behaviors of modifiers, a clear picture regarding the structure–performance relationship is still lacking. Recently, benefiting from the development of fine-structure characterization techniques and density functional theory calculations, the atomic details on how the surface interaction between heavy metal ions and the modifiers leads to its high sensitivity have attracted much attention. This short review discusses the development and challenge of nanomaterial-based stripping behaviors in the determination of heavy metal ions and highlights the structure–performance relationship at the atomic level.
electrochemical detection. For the past two decades, mercury-based electrodes have been widely adopted [4,5]. However, mercury poses great risks to the environment and human health, and thus, the European Union regulations will ban its use by 2020 [6]. Bismuth is recognized as an alternative to mercury because it exhibits the same excellent properties as mercury and much lower toxicity [7,8]. Unfortunately, the bismuth electrode suffers from the narrow potential window and instability in air [9].
Keywords Electrochemical detection, Heavy metal ions, Structure–performance relationship, Atomic-level insights.
To obtain high sensitivity and selective detection, the environmentally friendly nanomaterials (nonconductive nanomaterials) were widely used to modify electrodes [10e13]. The role of nonconductive nanomaterials is much similar to solid-phase extraction [14], which can efficiently preconcentrate heavy metal ions onto the electrodes, thus enhancing the stripping signals. Among them, metal oxides have gained significant interest because of their good adsorption properties for heavy metal ions, including MgO [15], Co3O4 [16,17], MnO2 [11], SnO2 [18], Fe2O3 [13], Fe3O4 [19], MgSiO3 [20], and NiCo2O4 [21]. And the underlying mechanism became gradually clear when some fine-structure techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), were included into the studies. Especially combined with density functional theory (DFT) calculations, the atomic details of the interaction between heavy metal ions and nanomaterials are of great benefit for the scientific understanding of the structureeperformance relationship (for the mechanisms of DFT calculations, please refer to the studies by Liao et al. and Matanovic et al. [22,23]). Therefore, this review summaries the recent development of nanomaterial-based stripping behaviors in the determination of heavy metal ions and highlights the structureeperformance relationship at atomic-scale insight.
Introduction
Detection mechanism
Trace analysis of heavy metal ions is of great importance because of their high toxicity on human health. Anodic stripping voltammetry is considered a promising technology [1]. It offers the characteristics of simplicity, high sensitivity, good selectivity, and low cost [2,3]. Proper choice of the working electrodes is crucial for sensitive
When nonconductive nanomaterials are used as sensing materials, the stripping behaviors of heavy metal ions can be depicted using an adsorption-release model. As shown in Figure 1a, heavy metal ions (M2þ) in water are first adsorbed onto the surfaces of the nanomaterials, and then M2þ ions desorb and diffuse to the electrode
Addresses 1 State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China 2 College of Ecology and Environment, Hainan University, Haikou 570228, China 3 College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China Corresponding author: Lin, Shiwei ([email protected])
Current Opinion in Electrochemistry 2019, 17:1–6 This review comes from a themed issue on Environmental electrochemistry Edited by Xing-Jiu Huang For a complete overview see the Issue and the Editorial Available online 17 April 2019 https://doi.org/10.1016/j.coelec.2019.04.006 2451-9103/© 2019 Elsevier B.V. All rights reserved.
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Figure 1
The adsorption-release model illustrating the electrochemical stripping behaviors. (a) Describes the adsorption and diffusion processes of heavy metal ions when nonconductive nanomaterials are used as modifiers. (b) Shows the generation of stripping signals due to the oxidation of heavy metal. Reprinted with permission from Liao et al. [24].
surface because of the concentration gradient. When deposition potential is applied, M2þ ions are reduced on the electrode surface (M2þ þ 2e / M0). Subsequent scanning potential is applied, M0 is reoxidized to M2þ (M0 - 2e / M2þ), and the stripping current signal is observed (Figure 1b).
sensitivities of wires NiO, flakes NiO, and balls NiO toward Pb(II) were 4.72, 10.3, and 13.46 mA mM1, respectively [27]. The excellent electrochemical performance of balls NiO was ascribed to their larger specific surface area and lower electron transfer resistance. Crystal phase
It can be seen that the adsorption and desorption abilities of modifiers are two critical features for the excellent electrochemical signals. Large adsorption ability indicates that more heavy metal ions in water can be absorbed onto the modifiers, and the low desorption energy barrier indicates that the absorbed heavy metal ions can be easily transferred to the electrode surface from modifiers. To achieve good electroanalytical properties, large adsorption ability and low desorption energy barrier are necessary. Morphology
Morphology not only affects the surface area but also affects the number of adsorption sites on the surface of nanomaterials. Liao et al. [25] compared the stripping behaviors of NiCo2O4 nanoplatelets with NiCo2O4 nanoparticles. They found that the sensitivity of nanoplatelets toward Pb(II) was NiCo2O4 39.47 mA mM1, which was 1.7 times that of NiCo2O4 nanoparticles. The large electrochemical sensitivity was ascribed to the high BET surface area of NiCo2O4 nanoplatelets because the large specific surface area can provide more adsorption sites for heavy metal ions.
Iron oxide has two common crystal phases in nature: aand g-Fe2O3. Li et al. [29] studied their difference in electroanalysis of Pb(II) from crystal structure. A high sensitivity of 197.82 mA nM1 cm2 in the range of 0.1e 1.0 nM with the detection limit of 0.092 nM was exhibited by g-Fe2O3, whereas a low sensitivity of 137.23 mA mM1 cm2 in the range of 0.1e1.0 mM with the detection limit of 0.090 mM was exhibited by aFe2O3. It can be seen that all the sensitivity, linear range, and detection limit of g-Fe2O3 showed 3 orders of magnitude better than those of a-Fe2O3. Furthermore, XPS and XAFS techniques were used to clarify the reasonable mechanism from a microscopic crystal structure. XPS results showed that more hydroxyl groups were covered on the surface of g-Fe2O3, and these surface hydroxyl group can efficiently increase the adsorption capacity of g-Fe2O3. XAFS indicated that the ˚, interatomic distance of PbeO on g-Fe2O3 was 2.28 A ˚ whereas the PbeO distance on a-Fe2O3 was 2.25 A. The long PbeO distance made the desorption of Pb(II) from g-Fe2O3 more easy, leading to a higher electrochemical response. Crystal surface
Morphology-dependent stripping behaviors were also found in NiO, MnO2, and Fe2O3 [26e28]. Li et al. fabricated three different morphologies of NiO (rods, flakes, and balls) by hydrothermal method, and the Current Opinion in Electrochemistry 2019, 17:1–6
Facet-dependent stripping behaviors are interesting topic in electroanalysis. The related research has been found on Cu2O, Fe3O4, and TiO2 nanocrystals [24,30,31]. As shown in Figure 2aec, TiO2 nanocrystals www.sciencedirect.com
Theoretical and experimental insights Liao et al.
3
Figure 2
SEM images of TiO2 nanocrystals with different ratios of exposed (001) and (101) facets. (a) HF0, 7%. (b) HF1.5, 35%. (c) HF3, 80%. (d) The comparison of sensitivity toward Pb(II). Reprinted with permission from Liao et al. [24]. SEM, scanning electron microscopy.
of three different (001)/(101) ratios (designated as HF0, HF1.5, and HF3) were fabricated. The corresponding percentages of exposed (001) facets in HF0, HF1.5, and HF3 were about 7%, 35%, and 80%, respectively. The sensitivities of Pb(II) and Cd(II) increased with the increasing of (001)/(101) ratio on TiO2 nanocrystals (Figure 2d). Furthermore, DFT calculations were used to understand the adsorption/ desorption behaviors of Pb(II)/Cd(II) on TiO2 (001) and (101) facets. The adsorption energies of Pb(II) on TiO2 (001) and TiO2 (101) facets were 2.59 eV and 2.35 eV, respectively. The desorption energy barrier of Pb(II) on TiO2 (001) and TiO2 (101) facets were 0.92 eV and 1.30 eV, respectively. The synergistic effects of large adsorption energy and low desorption energy barrier led to its high electrochemical sensitivity, which is in accordance with the detection mechanism shown in Figure 1. Liu et al. [31] synthesized three different Cu2O microcrystalseexposed (100), (111), and (110) facets, respectively. The electrochemical results indicated that the sensitivities of stripping analysis of Pb(II) followed the order (111) > (100) > (110). They explained the different sensitivities from the adsorption and www.sciencedirect.com
desorption abilities of crystal planes. XPS results showed the adsorption capacity was sequenced as (111) > (100) > (110). The desorption current results indicated the desorption energy barrier followed (111) ＜ (100) ＜ (110). It can be concluded that the high adsorption energy and low desorption energy barrier led to the high electrochemical sensitivity of Cu2O (111). Furthermore, DFT calculations were used to understand the adsorption behaviors of Pb(II) on Cu2O (100), (111), and (110) facets. The calculated adsorption energies agreed with the sensitivity sequence well: Pb/ Cu2O (111), 5.742 eV > Pb/Cu2O (100), 4.952 eV > Pb/ Cu2O (110), 4.761 eV. However, Jin et al. [32] demonstrated that the desorption energy barrier was more critical for better electrochemical performance than adsorption energy. They compared the electrochemical heavy metal ionesensing performance of SnO2 with two different facets ((221) and (110) facets) and found that SnO2 (110) facet showed higher electrochemical sensitivities toward heavy metal ions. The sensitivity of (110) facetedominated SnO2 toward Pb(II) was about fivefold that of (221) facetedominated SnO2 and about sixfold toward Cd(II) than that of (221) facetedominated SnO2. Current Opinion in Electrochemistry 2019, 17:1–6
4 Environmental electrochemistry
However, DFT calculations indicated that the adsorption energy of (221) facet was 1.29 eV, which was much larger than that of (110) facet (0.19 eV). That is to say, according to adsorption energy, (221) facet should have better heavy metal ions capture capability than (110) facet and showed better stripping behaviors. The large adsorption energy seemed to become a hurdle for the mobility of metal ions in this work, which reduced the electrochemical sensitivity. The calculated diffusion energy barriers demonstrated this speculation. Much larger diffusion energy barriers were observed on SnO2 (110) facet. The diffusion energy barriers of Pb(II) and Cd (II) on SnO2 (110) facet were 0.63 eV and 0.17 eV, respectively, whereas the diffusion energy barriers of Pb(II) and Cd (II) on SnO2 (221) facet were 1.88 eV and 2.00 eV, respectively. Dopant and defect
The introduction of foreign elements into the lattice of nanomaterials can modulate their surface electronic state and thus affects their electroanalytical properties. Zhang et al. [33] systematically studied the enhanced mechanism of electroanalytical properties resulted from the nonmetallic element doping. As shown in Figure 3a, after fluoride (F), sulfur (S), and iodine (I) doping, the sensitivity of Pb(II) increased by 102%, 35.3%, and 95.4%, respectively, and the detection limit decreased by 54.5%, 45.5%, and 54.5%, respectively. The improved reasons are as follows: 1) the electron transfer rate was greatly enhanced after doping. For example, the electron transfer resistance has been decreased from 580 U to 60 U after fluoride doping. 2) Larger adsorption energy has been obtained. The adsorption energy was increased from 2.23 eV to 2.65 eV after fluoride doping. 3) The desorption energy barrier was decreased, which is favorable for the diffusion of Pb(II) to the electrode.
Compared with bare TiO2 nanosheets, the desorption current decreased by 83.94% after fluoride doping. Besides, oxygen vacancy (OV) can also enhance the electrochemical sensing performance [34,35]. Zhou et al. [34] fabricated four different oxygen-deficient TiO2 nanosheets (TiO2-x nanosheets) and investigated their electrochemical sensing performance toward Hg(II). Four different TiO2-x nanosheets were named according to the annealing temperature: 25 C, t-1; 300 C, t-2; 600 C, t-3; 800 C, t-4. As shown in Figure 3b, although T-1, T-2, and T-3 samples exhibited the same morphology and crystal phase, the sensitivity toward Hg(II) decreased with the decrease of surface OV concentration (T-3 ＜ T-2 ＜ T-1). When the annealing temperature increased to 800 C, the sensitivity of T-4 increased again because of the different phase structure (t-3 is pure anatase, and t-4 is anatase þ rutile phases). Further XPS and XAFS analysis demonstrated the improved sensitivity was ascribed to the catalysis of OV surface (inset of Figure 2b). The O2 molecules adsorbed on the surface OVs is beneficial for the formation of superoxide radical (O 2 ), which can serve as active sites for the adsorption of electropositive Hg(II).
Conclusions and outlook By modifying the sensing nanomaterials on the working electrodes, the electrochemical detection of toxic metal ions with high sensitivity and selectivity has been confirmed. Despite numerous studies focusing on the stripping behaviors of heavy metal ions at the nonconductive nanomaterials, the structureeperformance relationship is far from understanding. For example, how does the selective adsorption lead to the selective response of metal ions? And how does the surface-
Figure 3
Effect of dopant and defect on the eletrochemical sensitivity toward heavy metal ions. (a) The effect of element doping on the electrochemical sensitivity toward Pb(II). Reprinted with permission from Zhang et al. [33]. (b) The effect of oxygen vacancy on the electrochemical sensitivity toward Hg(II). Inset illustrates the different stripping path when considering the oxygen vacancy in the defective surface. Reprinted with permission from Zhou et al. [34].
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Theoretical and experimental insights Liao et al.
electronic-state modulation affect the adsorption/ desorption abilities of modifiers? Future studies require in situ spectroscopic/microscopic methods to observe the detailed interaction at the atomic level. Adsorption energy and desorption energy barrier of modifier are two intrinsic factors affecting the electroanalytical performance. To maximize the detection sensitivity and selectivity, how to balance the adsorption energy and desorption energy barrier is an important issue that needs to be deeply studied in future work. For practical application, more future work should pay attention to it.
Conflict of Interest The authors declare no conflict of interest.
Acknowledgements This work is supported by the National Natural Science Foundation of China (21866012, 61764003), Major Science and Technology Planning Project of Hainan Province (ZDKJ201810), and Scientific Research Foundation of Hainan University (kyqd1659).
References Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest
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21. Yu X-Y, Yao X-Z, Luo T, et al.: Facile synthesis of urchin-like NiCo2O4 hollow microspheres with enhanced electrochemical properties in energy and environmentally related applications. ACS Appl Mater Interfaces 2014, 6:3689–3695. 22. Liao J, Yang L, Wang CZ, Lin S: Tuning Cd adsorption behaviours on graphene by introducing defects: a firstprinciples study. Mater Technol 2017, 32:840–844. 23. Matanovic I, Artyushkova K, Atanassov P: Understanding PGMfree catalysts by linking density functional theory calculations and structural analysis: perspectives and challenges. Curr Opin Electrochem 2018, 9:137–144. 24. Liao J, Yang F, Wang C-Z, Lin S: The crystal facet-dependent electrochemical performance of TiO2 nanocrystals for heavy metal detection: theoretical prediction and experimental proof. Sens Actuators, B 2018, 271:195–202. Facet-dependent electroanalytic property was investigated by experiment and theoretical calculations. 25. Liao J, Zhang J, Wang C-Z, Lin S: Electrochemical and density functional theory investigation on the differential behaviors of core-ring structured NiCo2O4 nanoplatelets toward heavy metal ions. Anal Chim Acta 2018, 1022:37–44. 26. Zhang Q-X, Peng D, Huang X-J: Effect of morphology of aMnO2 nanocrystals on electrochemical detection of toxic metal ions. Electrochem Commun 2013, 34:270–273.
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ScienceDirect Review Article
Theoretical and experimental insights into the electrochemical heavy metal ion sensing with nonconductive nanomaterials Jianjun Liao1,2, Zui Tao1,3 and Shiwei Lin1,3 Abstract
Nonconductive nanomaterial is a type of modifiers widely used in electrochemical heavy metal ion detection. Despite a large number of studies devoted to the electrochemical stripping behaviors of modifiers, a clear picture regarding the structure–performance relationship is still lacking. Recently, benefiting from the development of fine-structure characterization techniques and density functional theory calculations, the atomic details on how the surface interaction between heavy metal ions and the modifiers leads to its high sensitivity have attracted much attention. This short review discusses the development and challenge of nanomaterial-based stripping behaviors in the determination of heavy metal ions and highlights the structure–performance relationship at the atomic level.
electrochemical detection. For the past two decades, mercury-based electrodes have been widely adopted [4,5]. However, mercury poses great risks to the environment and human health, and thus, the European Union regulations will ban its use by 2020 [6]. Bismuth is recognized as an alternative to mercury because it exhibits the same excellent properties as mercury and much lower toxicity [7,8]. Unfortunately, the bismuth electrode suffers from the narrow potential window and instability in air [9].
Keywords Electrochemical detection, Heavy metal ions, Structure–performance relationship, Atomic-level insights.
To obtain high sensitivity and selective detection, the environmentally friendly nanomaterials (nonconductive nanomaterials) were widely used to modify electrodes [10e13]. The role of nonconductive nanomaterials is much similar to solid-phase extraction [14], which can efficiently preconcentrate heavy metal ions onto the electrodes, thus enhancing the stripping signals. Among them, metal oxides have gained significant interest because of their good adsorption properties for heavy metal ions, including MgO [15], Co3O4 [16,17], MnO2 [11], SnO2 [18], Fe2O3 [13], Fe3O4 [19], MgSiO3 [20], and NiCo2O4 [21]. And the underlying mechanism became gradually clear when some fine-structure techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), were included into the studies. Especially combined with density functional theory (DFT) calculations, the atomic details of the interaction between heavy metal ions and nanomaterials are of great benefit for the scientific understanding of the structureeperformance relationship (for the mechanisms of DFT calculations, please refer to the studies by Liao et al. and Matanovic et al. [22,23]). Therefore, this review summaries the recent development of nanomaterial-based stripping behaviors in the determination of heavy metal ions and highlights the structureeperformance relationship at atomic-scale insight.
Introduction
Detection mechanism
Trace analysis of heavy metal ions is of great importance because of their high toxicity on human health. Anodic stripping voltammetry is considered a promising technology [1]. It offers the characteristics of simplicity, high sensitivity, good selectivity, and low cost [2,3]. Proper choice of the working electrodes is crucial for sensitive
When nonconductive nanomaterials are used as sensing materials, the stripping behaviors of heavy metal ions can be depicted using an adsorption-release model. As shown in Figure 1a, heavy metal ions (M2þ) in water are first adsorbed onto the surfaces of the nanomaterials, and then M2þ ions desorb and diffuse to the electrode
Addresses 1 State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China 2 College of Ecology and Environment, Hainan University, Haikou 570228, China 3 College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China Corresponding author: Lin, Shiwei ([email protected])
Current Opinion in Electrochemistry 2019, 17:1–6 This review comes from a themed issue on Environmental electrochemistry Edited by Xing-Jiu Huang For a complete overview see the Issue and the Editorial Available online 17 April 2019 https://doi.org/10.1016/j.coelec.2019.04.006 2451-9103/© 2019 Elsevier B.V. All rights reserved.
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Current Opinion in Electrochemistry 2019, 17:1–6
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Figure 1
The adsorption-release model illustrating the electrochemical stripping behaviors. (a) Describes the adsorption and diffusion processes of heavy metal ions when nonconductive nanomaterials are used as modifiers. (b) Shows the generation of stripping signals due to the oxidation of heavy metal. Reprinted with permission from Liao et al. [24].
surface because of the concentration gradient. When deposition potential is applied, M2þ ions are reduced on the electrode surface (M2þ þ 2e / M0). Subsequent scanning potential is applied, M0 is reoxidized to M2þ (M0 - 2e / M2þ), and the stripping current signal is observed (Figure 1b).
sensitivities of wires NiO, flakes NiO, and balls NiO toward Pb(II) were 4.72, 10.3, and 13.46 mA mM1, respectively [27]. The excellent electrochemical performance of balls NiO was ascribed to their larger specific surface area and lower electron transfer resistance. Crystal phase
It can be seen that the adsorption and desorption abilities of modifiers are two critical features for the excellent electrochemical signals. Large adsorption ability indicates that more heavy metal ions in water can be absorbed onto the modifiers, and the low desorption energy barrier indicates that the absorbed heavy metal ions can be easily transferred to the electrode surface from modifiers. To achieve good electroanalytical properties, large adsorption ability and low desorption energy barrier are necessary. Morphology
Morphology not only affects the surface area but also affects the number of adsorption sites on the surface of nanomaterials. Liao et al. [25] compared the stripping behaviors of NiCo2O4 nanoplatelets with NiCo2O4 nanoparticles. They found that the sensitivity of nanoplatelets toward Pb(II) was NiCo2O4 39.47 mA mM1, which was 1.7 times that of NiCo2O4 nanoparticles. The large electrochemical sensitivity was ascribed to the high BET surface area of NiCo2O4 nanoplatelets because the large specific surface area can provide more adsorption sites for heavy metal ions.
Iron oxide has two common crystal phases in nature: aand g-Fe2O3. Li et al. [29] studied their difference in electroanalysis of Pb(II) from crystal structure. A high sensitivity of 197.82 mA nM1 cm2 in the range of 0.1e 1.0 nM with the detection limit of 0.092 nM was exhibited by g-Fe2O3, whereas a low sensitivity of 137.23 mA mM1 cm2 in the range of 0.1e1.0 mM with the detection limit of 0.090 mM was exhibited by aFe2O3. It can be seen that all the sensitivity, linear range, and detection limit of g-Fe2O3 showed 3 orders of magnitude better than those of a-Fe2O3. Furthermore, XPS and XAFS techniques were used to clarify the reasonable mechanism from a microscopic crystal structure. XPS results showed that more hydroxyl groups were covered on the surface of g-Fe2O3, and these surface hydroxyl group can efficiently increase the adsorption capacity of g-Fe2O3. XAFS indicated that the ˚, interatomic distance of PbeO on g-Fe2O3 was 2.28 A ˚ whereas the PbeO distance on a-Fe2O3 was 2.25 A. The long PbeO distance made the desorption of Pb(II) from g-Fe2O3 more easy, leading to a higher electrochemical response. Crystal surface
Morphology-dependent stripping behaviors were also found in NiO, MnO2, and Fe2O3 [26e28]. Li et al. fabricated three different morphologies of NiO (rods, flakes, and balls) by hydrothermal method, and the Current Opinion in Electrochemistry 2019, 17:1–6
Facet-dependent stripping behaviors are interesting topic in electroanalysis. The related research has been found on Cu2O, Fe3O4, and TiO2 nanocrystals [24,30,31]. As shown in Figure 2aec, TiO2 nanocrystals www.sciencedirect.com
Theoretical and experimental insights Liao et al.
3
Figure 2
SEM images of TiO2 nanocrystals with different ratios of exposed (001) and (101) facets. (a) HF0, 7%. (b) HF1.5, 35%. (c) HF3, 80%. (d) The comparison of sensitivity toward Pb(II). Reprinted with permission from Liao et al. [24]. SEM, scanning electron microscopy.
of three different (001)/(101) ratios (designated as HF0, HF1.5, and HF3) were fabricated. The corresponding percentages of exposed (001) facets in HF0, HF1.5, and HF3 were about 7%, 35%, and 80%, respectively. The sensitivities of Pb(II) and Cd(II) increased with the increasing of (001)/(101) ratio on TiO2 nanocrystals (Figure 2d). Furthermore, DFT calculations were used to understand the adsorption/ desorption behaviors of Pb(II)/Cd(II) on TiO2 (001) and (101) facets. The adsorption energies of Pb(II) on TiO2 (001) and TiO2 (101) facets were 2.59 eV and 2.35 eV, respectively. The desorption energy barrier of Pb(II) on TiO2 (001) and TiO2 (101) facets were 0.92 eV and 1.30 eV, respectively. The synergistic effects of large adsorption energy and low desorption energy barrier led to its high electrochemical sensitivity, which is in accordance with the detection mechanism shown in Figure 1. Liu et al. [31] synthesized three different Cu2O microcrystalseexposed (100), (111), and (110) facets, respectively. The electrochemical results indicated that the sensitivities of stripping analysis of Pb(II) followed the order (111) > (100) > (110). They explained the different sensitivities from the adsorption and www.sciencedirect.com
desorption abilities of crystal planes. XPS results showed the adsorption capacity was sequenced as (111) > (100) > (110). The desorption current results indicated the desorption energy barrier followed (111) ＜ (100) ＜ (110). It can be concluded that the high adsorption energy and low desorption energy barrier led to the high electrochemical sensitivity of Cu2O (111). Furthermore, DFT calculations were used to understand the adsorption behaviors of Pb(II) on Cu2O (100), (111), and (110) facets. The calculated adsorption energies agreed with the sensitivity sequence well: Pb/ Cu2O (111), 5.742 eV > Pb/Cu2O (100), 4.952 eV > Pb/ Cu2O (110), 4.761 eV. However, Jin et al. [32] demonstrated that the desorption energy barrier was more critical for better electrochemical performance than adsorption energy. They compared the electrochemical heavy metal ionesensing performance of SnO2 with two different facets ((221) and (110) facets) and found that SnO2 (110) facet showed higher electrochemical sensitivities toward heavy metal ions. The sensitivity of (110) facetedominated SnO2 toward Pb(II) was about fivefold that of (221) facetedominated SnO2 and about sixfold toward Cd(II) than that of (221) facetedominated SnO2. Current Opinion in Electrochemistry 2019, 17:1–6
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However, DFT calculations indicated that the adsorption energy of (221) facet was 1.29 eV, which was much larger than that of (110) facet (0.19 eV). That is to say, according to adsorption energy, (221) facet should have better heavy metal ions capture capability than (110) facet and showed better stripping behaviors. The large adsorption energy seemed to become a hurdle for the mobility of metal ions in this work, which reduced the electrochemical sensitivity. The calculated diffusion energy barriers demonstrated this speculation. Much larger diffusion energy barriers were observed on SnO2 (110) facet. The diffusion energy barriers of Pb(II) and Cd (II) on SnO2 (110) facet were 0.63 eV and 0.17 eV, respectively, whereas the diffusion energy barriers of Pb(II) and Cd (II) on SnO2 (221) facet were 1.88 eV and 2.00 eV, respectively. Dopant and defect
The introduction of foreign elements into the lattice of nanomaterials can modulate their surface electronic state and thus affects their electroanalytical properties. Zhang et al. [33] systematically studied the enhanced mechanism of electroanalytical properties resulted from the nonmetallic element doping. As shown in Figure 3a, after fluoride (F), sulfur (S), and iodine (I) doping, the sensitivity of Pb(II) increased by 102%, 35.3%, and 95.4%, respectively, and the detection limit decreased by 54.5%, 45.5%, and 54.5%, respectively. The improved reasons are as follows: 1) the electron transfer rate was greatly enhanced after doping. For example, the electron transfer resistance has been decreased from 580 U to 60 U after fluoride doping. 2) Larger adsorption energy has been obtained. The adsorption energy was increased from 2.23 eV to 2.65 eV after fluoride doping. 3) The desorption energy barrier was decreased, which is favorable for the diffusion of Pb(II) to the electrode.
Compared with bare TiO2 nanosheets, the desorption current decreased by 83.94% after fluoride doping. Besides, oxygen vacancy (OV) can also enhance the electrochemical sensing performance [34,35]. Zhou et al. [34] fabricated four different oxygen-deficient TiO2 nanosheets (TiO2-x nanosheets) and investigated their electrochemical sensing performance toward Hg(II). Four different TiO2-x nanosheets were named according to the annealing temperature: 25 C, t-1; 300 C, t-2; 600 C, t-3; 800 C, t-4. As shown in Figure 3b, although T-1, T-2, and T-3 samples exhibited the same morphology and crystal phase, the sensitivity toward Hg(II) decreased with the decrease of surface OV concentration (T-3 ＜ T-2 ＜ T-1). When the annealing temperature increased to 800 C, the sensitivity of T-4 increased again because of the different phase structure (t-3 is pure anatase, and t-4 is anatase þ rutile phases). Further XPS and XAFS analysis demonstrated the improved sensitivity was ascribed to the catalysis of OV surface (inset of Figure 2b). The O2 molecules adsorbed on the surface OVs is beneficial for the formation of superoxide radical (O 2 ), which can serve as active sites for the adsorption of electropositive Hg(II).
Conclusions and outlook By modifying the sensing nanomaterials on the working electrodes, the electrochemical detection of toxic metal ions with high sensitivity and selectivity has been confirmed. Despite numerous studies focusing on the stripping behaviors of heavy metal ions at the nonconductive nanomaterials, the structureeperformance relationship is far from understanding. For example, how does the selective adsorption lead to the selective response of metal ions? And how does the surface-
Figure 3
Effect of dopant and defect on the eletrochemical sensitivity toward heavy metal ions. (a) The effect of element doping on the electrochemical sensitivity toward Pb(II). Reprinted with permission from Zhang et al. [33]. (b) The effect of oxygen vacancy on the electrochemical sensitivity toward Hg(II). Inset illustrates the different stripping path when considering the oxygen vacancy in the defective surface. Reprinted with permission from Zhou et al. [34].
Current Opinion in Electrochemistry 2019, 17:1–6
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Theoretical and experimental insights Liao et al.
electronic-state modulation affect the adsorption/ desorption abilities of modifiers? Future studies require in situ spectroscopic/microscopic methods to observe the detailed interaction at the atomic level. Adsorption energy and desorption energy barrier of modifier are two intrinsic factors affecting the electroanalytical performance. To maximize the detection sensitivity and selectivity, how to balance the adsorption energy and desorption energy barrier is an important issue that needs to be deeply studied in future work. For practical application, more future work should pay attention to it.
Conflict of Interest The authors declare no conflict of interest.
Acknowledgements This work is supported by the National Natural Science Foundation of China (21866012, 61764003), Major Science and Technology Planning Project of Hainan Province (ZDKJ201810), and Scientific Research Foundation of Hainan University (kyqd1659).
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