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Letter Cite This: Anal. Chem. 2019, 91, 3195−3198
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Pore Confined Liquid−Vacuum Interface for Charge Transfer Study in an Electrochemical Process Jun-Gang Wang, Xin Hua,* Hai-Lun Xia, and Yi-Tao Long* Key Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China
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S Supporting Information *
ABSTRACT: A pore confined liquid−vacuum interface was created in liquid secondary ion mass spectrometry analysis in order to study the charge transfer in electrochemical reactions. The interfacial processes such as the critical diameter, influence of aperture properties on the morphology of the liquid−vacuum interface, pressure field, concentration field, and electric field were revealed by finite element simulation. The correlation between numerical study of the chemical changes at the electrode−electrolyte interface and experimental results during the dynamic potential scan was built successfully. Better understanding of these interfacial processes could promote further applications of liquid secondary ion mass spectrometry in energy storage and electrochemical catalysis.
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the electrode−electrolyte interface. However, owing to the complicated physical and chemical processes during in situ liquid SIMS analysis and lack of appropriate analytical techniques,23 detailed information, such as the critical diameter of the aperture, the influence of the interfacial property (i.e., hydrophobicity and hydrophilicity of the aperture) on morphology of the liquid−vacuum interface, mass transport in the aperture, etc., is not clear so far. The goal of this work is to answer these questions based on electrodynamics and physics via finite element simulation, which will give more insights into the understanding of the confined interfacial processes in liquid SIMS analysis. The fundamental process of in situ liquid SIMS combined with electrochemistry is shown in Figure 1. When the membrane was penetrated, a significant increase of representative ion signal (K+) in the electrolyte was observed. When the aperture diameter from both negative and positive total ions images was compared with the theoretical value, excellent consistency was obtained indicating the well controllability for the penetration process (Figure S1). Owing to the small size of the aperture and hydrophobicity of SiNx membrane, the liquid can be confined in the aperture by surface tension without obvious deterioration of the vacuum system.24 When beyond the critical size of the aperture, the liquid−vacuum interface could not be withheld in the aperture due to the disturbance of the balance between the surface tension and the pressure difference.25 Although the increase of aperture size can
nderstanding of electrochemical reactions and ion solvation as well as mass transfer at the electrode− electrolyte interface during electrochemical processes is crucial in energy storage and biological sensing.1,2 In order to identify the chemical compositions and monitor the reactions near the electrode surface, numerous in situ spectroelectrochemical techniques such as in situ infrared spectroscopy (IR),3 in situ surface-enhanced Raman scattering (SERS),4 in situ X-ray absorption spectroscopy (XAS),5 in situ X-ray diffraction (XRD),6 and in situ nuclear magnetic resonance (NMR)7 have been presented and great progress has been made in providing structural details of molecules adsorbed on the electrode surface.8,9 Compared with these approaches, direct electrochemistry in an electrochemical cell combined with mass spectrometry (MS) can provide direct molecular information at the electrode−electrolyte interface, which could benefit in better understanding of electrochemical processes. For example, electron ionization (EI),10,11 electrospray ionization (ESI),12,13 and desorption electrospray ionization (DESI)14 have been widely used to couple with electrochemistry to monitor redox reaction and provide molecular information unavailable solely from traditional ex situ electrochemical or spectroscopic approaches.15−18 Owing to the high surface sensitivity, high depth resolution, and fast response, in situ liquid secondary ion mass spectrometry (SIMS) combined with electrochemistry has been developed to investigate complex chemistries occurring at the electrode−electrolyte interfaces.19,20 It has been successfully applied to identify transient intermediates in redox reactions and the formation mechanism of the solid electrolyte interphase in lithium ion batteries.21,22 Moreover, it has the potential to acquire information about small ions solvation and transportation at © 2019 American Chemical Society
Received: November 2, 2018 Accepted: January 17, 2019 Published: January 17, 2019 3195
DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198
Letter
Analytical Chemistry
Figure 1. (A) Schematic of the in situ liquid SIMS combined with electrochemistry. (B) ToF-SIMS depth profiles of representative positive ion species (Si2N+, Au+, and K+) in 2 mM KNO3 at open circuit potential. The inset image shows the three-dimensional overlay of Si2N+ (blue), Au+ (orange), and K+ (pink).
Figure 3. (A) Schematic of the hydrophilic and hydrophobic regions of the apertures (a−e). (B) The evolution of the liquid−vacuum interface height with changing interfacial properties of the apertures with diameter of 1−3 μm. Vacuum region, SiNx/Au film, and liquid region are indicated by orange, blue, and green color, respectively. (C) The corresponding morphology of the liquid−vacuum interface in an aperture with diameter of 2 μm with different interfacial properties (a−e). (D) The relative pressure on the liquid−vacuum interface in an aperture with diameter of 2 μm with different interfacial properties (a−e). The distance covered from the liquid− vacuum interface to the center of the aperture. (E) The liquid− vacuum interface morphology in a 2 μm aperture corresponding to interfacial property (c). The concentration field is shown in the inset.
Figure 2. (A) Height of liquid−vacuum interface as a function of the aperture radius. The inset shows the definition of the height of the liquid−vacuum interface which covers the lower surface of the gold electrode to the highest point of the liquid−vacuum interface. (B) The comparison of interface height in the aperture with radius at 3304 nm and 3305 nm as a function of time. The inset shows the morphology of the liquid−vacuum interface at 6 ms in the aperture with radius at 3305 nm (left) and 3304 nm (right). (C) The pressure distribution on the liquid−vacuum interfaces in apertures with various radius. The distance covered from the liquid−vacuum interface to the center of the aperture. (D) The pressure at the contact point between the liquid−vacuum interface and the aperture wall as a function of aperture radius.
3305 nm which was considered as the critical size for the aperture (Figures 2A and S3). When compared with the transient curve of interfacial height at 3304 nm, an obvious increase of the interface height was found at 3305 nm within 1.3 ms. Below the critical size, the interfacial height remains stable without obvious fluctuation (Figure 2B). Moreover, the obvious difference in the concentration field at the critical size and 3304 nm further proves that the critical size was located at 3305 nm during in situ liquid SIMS analysis (Figure S4). After validating the critical size, a further investigation of the distribution of pressure on the liquid−vacuum interface below the critical size was performed (Figure 2C,D). The pressure between the liquid−vacuum interface and the rim of aperture reaches a maximum value at the aperture with 2 μm diameter, indicating a strengthened suction effect at the rim was achieved.27 Hence, in order to avoid the leakage of liquid, the aperture size at 2 μm was chosen in practical analysis and further finite element models (FEM) simulation. Appropriate design of the microfluidic device with desired interfacial properties will improve the confinement of the liquid−vacuum interface in the aperture during in situ liquid SIMS analysis. Considering the significant influence of hydrophobic and hydrophilic properties of the aperture on the morphology of the liquid−vacuum interface, the FEM was further conducted under various interfacial conditions (Figure 3A). Here, the initial contact angles of hydrophilic and hydrophobic portion were fixed at 67° and 130°, respectively.28,29 Obviously, with the increase of the hydrophilic
improve the ion intensity and provide more spatial information in the aperture, the larger size of the aperture will induce the overflow of the liquid and lead to the attenuation of the vacuum system. Considering the probability of the damage on the SIMS system induced by the leakage of liquid, the critical size of the aperture at which the abrupt change of liquid− vacuum interface occurred was elucidated by finite element simulation. Given the circular hole structure of an aperture, a twodimensional axisymmetric geometry can be used to simplify its three-dimensional geometry (Figure S2). The transport fluid through the aperture involves the two-phase flow dynamics which was governed by surface tension, wall adhesive forces, and pressure difference.26 A detailed description of the equations for finite element calculation could be found in the Supporting Information. When the aperture radius was enlarged, an abrupt change of the interfacial height arises at 3196
DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198
Letter
Analytical Chemistry
Figure 4. Schematic of the locations adopted to calculate the concentration of K+ during dynamic potential scan. (A) The volumes with different thickness of liquid layer. (B) The interface region includes the liquid−vacuum interface (LVI) and different lengths of electrode−electrolyte interface (EEI). FEM simulated distributions of K+ concentration in (C) various volumes with different thickness of the liquid layer (a−g: 1, 3, 6, 12, 18, 24, and 30 nm; h: the whole liquid volume in the aperture) corresponding to (A), and (D) interface regions (a: EEI; b−i: LVI plus EEI with various length (i.e., the whole electrode, 30, 24, 18, 12, 6, 3, and 1 nm); j: LVI) corresponding to (B) during dynamic potential scan from −0.20 to +0.18 V at a scan rate of 10 mV s−1. The inset in (D) shows the enlarged results for b−j. (E) Schematic of K+ accumulation at the electrode−electrolyte interface under cathodic polarization is shown in the inset I. Comparison between the differential curves of the experimental K+ mass intensity (dashed line) and simulated K+ concentration trends (solid line) at various interfacial regions (a−j, a is shown in the inset II) as a function of polarized potential.
simulated by the Nernst−Planck−Poisson (NPP) equations.30 The potassium ions were attracted or repelled under cathodic or anodic polarized potential, respectively, which was in good accordance with electric double layer theory (Figure S5). To correlate the in situ liquid SIMS signal with the chemical information from the electrode−electrolyte interface, finite element models were built and compared with the experimental observation. In the first case, the liquid layers with various thickness from the liquid−vacuum interface were considered (Figure 4A). Figure 4C shows that the potassium concentration decreased under anodic polarization and increased under cathodic polarization. With the increase of the thickness of liquid layer, the concentration of potassium is closer to the bulk value (2 mM), indicating that the applied dynamic potential scan cannot induce the drastic fluctuation of potassium concentration in the aperture. In the second case (Figure 4B), owing to the important role of electrode− electrolyte interface in electric double layer theory, where the accumulation and repulsion of potassium ion mainly occurred, the potassium concentration at the liquid−vacuum interface and the electrode surface with different lengths was calculated. Figure 4D shows that the concentration of potassium change with the dynamic potential scan and a U-shape can be found. The more the electrode−electrolyte interface was included, the more noticeable was the fluctuation of the concentration of potassium, confirming that the accumulation of potassium ion mainly occurred at the electrode−electrolyte interface. Comparing these modeling results with experimental observations, the trends of potassium concentration change during dynamic potential scan at the electrode−electrolyte interface agreed well with the in situ liquid SIMS results, which further validated the correlation between the in situ liquid SIMS signal
region, the liquid−vacuum interface height increases as well in various apertures with diameter between 1 and 3 μm (Figure 3B). In the whole hydrophobic aperture, a concave meniscus can be seen, which was induced by the hydrophobic interaction between the inner wall and the liquid. On the other hand, the whole hydrophilic aperture causes the noticeable leakage of the liquid out of the aperture, further confirming the influence of the interfacial properties on the morphology of the liquid−vacuum interface. For the aperture constituted by the hydrophilic SiNx (upper region) and hydrophilic Au (lower region), the liquid−vacuum interface was well confined in the aperture and the interface height reached 106 nm at steady state, which is lower than the depth of the aperture (170 nm) (Figure 3C). At this case (c), the pressure on the liquid−vacuum interface becomes greater than other cases (a, b, d, and e) and is responsible for the enhanced confining effect (Figure 3D). Meanwhile, the rim of the liquid−vacuum interface contacted with the Au rather than the SiNx at 2 μm radius (Figure 3E), indicating that the liquid− vacuum interface has a good contact with the working electrode. On the basis of the above modeling results, surface modifications to make the inner wall more hydrophilic and upper rim more hydrophobic will be favorable to confine the aqueous liquid in the aperture. Meanwhile, for nonaqueous systems, it is advisable to change the material of the aperture to make the lower part of the aperture hydrophobic and upper part hydrophilic. The above design will help to confine the liquid−vacuum interface of the nonaqueous system in the aperture. These modifications will be helpful to the successful design of the vacuum compatible microfluidic electrochemical device. Under dynamic electric field, the ion mass transfer and charge transfer at the electrode−electrolyte interface is 3197
DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198
Letter
Analytical Chemistry
(5) Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 8525−8534. (6) Favaro, M.; Jeong, B.; Ross, P. N.; Yano, J.; Hussain, Z.; Liu, Z.; Crumlin, E. J. Nat. Commun. 2016, 7, 12695. (7) Cao, S.-H.; Ni, Z.-R.; Huang, L.; Sun, H.-J.; Tang, B.; Lin, L.-J.; Huang, Y.-Q.; Zhou, Z.-Y.; Sun, S.-G.; Chen, Z. Anal. Chem. 2017, 89, 3810−3813. (8) Tripathi, A. M.; Su, W.-N.; Hwang, B. J. Chem. Soc. Rev. 2018, 47, 736−851. (9) León, L.; Mozo, J. D. TrAC, Trends Anal. Chem. 2018, 102, 147− 169. (10) Bruckenstein, S.; Gadde, R. R. J. Am. Chem. Soc. 1971, 93, 793− 794. (11) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. Science 2012, 337, 563. (12) Odijk, M.; Olthuis, W.; van den Berg, A.; Qiao, L.; Girault, H. Anal. Chem. 2012, 84, 9176−9183. (13) Abd-El-Latif, A.; Bondue, C.; Ernst, S.; Hegemann, M.; Kaul, J.; Khodayari, M.; Mostafa, E.; Stefanova, A.; Baltruschat, H. TrAC, Trends Anal. Chem. 2015, 70, 4−13. (14) Brown, T. A.; Chen, H.; Zare, R. N. J. Am. Chem. Soc. 2015, 137, 7274−7277. (15) Wang, J.-G.; Fossey, J. S.; Li, M.; Li, D.-W.; Ma, W.; Ying, Y.-L.; Qian, R.-C.; Cao, C.; Long, Y.-T. J. Electroanal. Chem. 2016, 781, 257−264. (16) Wang, J.-G.; Fossey, J. S.; Li, M.; Xie, T.; Long, Y.-T. ACS Appl. Mater. Interfaces 2016, 8, 8305−8314. (17) Wang, J.-G.; Jing, C.; Long, Y.-T. Single-Nanoparticle Plasmonic Spectroelectrochemistry. In Frontiers of Plasmon Enhanced Spectroscopy Volume 2; American Chemical Society: Washington, DC, 2016; Vol. 1246, pp 57−96. (18) Wang, J.; Cao, X.; Wang, X.; Yang, S.; Wang, R. Electrochim. Acta 2014, 138, 174−186. (19) Lu, J.; Hua, X.; Long, Y.-T. Analyst 2017, 142, 691−699. (20) Liu, B.; Yu, X.-Y.; Zhu, Z.; Hua, X.; Yang, L.; Wang, Z. Lab Chip 2014, 14, 855−859. (21) Zhu, Z.; Zhou, Y.; Yan, P.; Vemuri, R. S.; Xu, W.; Zhao, R.; Wang, X.; Thevuthasan, S.; Baer, D. R.; Wang, C.-M. Nano Lett. 2015, 15, 6170−6176. (22) Wang, Z.; Zhang, Y.; Liu, B.; Wu, K.; Thevuthasan, S.; Baer, D. R.; Zhu, Z.; Yu, X.-Y.; Wang, F. Anal. Chem. 2017, 89, 960−965. (23) Dwyer, J. R.; Harb, M. Appl. Spectrosc. 2017, 71, 2051−2075. (24) Yang, L.; Yu, X.-Y.; Zhu, Z.; Thevuthasan, T.; Cowin, J. P. J. Vac. Sci. Technol., A 2011, 29, No. 061101. (25) Zhou, Y.; Yao, J.; Ding, Y.; Yu, J.; Hua, X.; Evans, J. E.; Yu, X.; Lao, D. B.; Heldebrant, D. J.; Nune, S. K.; Cao, B.; Bowden, M. E.; Yu, X.-Y.; Wang, X.-L.; Zhu, Z. J. Am. Soc. Mass Spectrom. 2016, 27, 2006−2013. (26) Tian, H.; Shao, J.; Ding, Y.; Li, X.; Liu, H. Langmuir 2013, 29, 4703−4714. (27) Zang, D.; Li, L.; Di, W.; Zhang, Z.; Ding, C.; Chen, Z.; Shen, W.; Binks, B. P.; Geng, X. Nat. Commun. 2018, 9, 3546. (28) Smith, T. J. Colloid Interface Sci. 1980, 75, 51−55. (29) Desai, S.; Kaware, R. D. Int. J. Nanomanuf. 2014, 10, 432−452. (30) Wang, H.; Thiele, A.; Pilon, L. J. Phys. Chem. C 2013, 117, 18286−18297.
and the chemical information from the electrode−electrolyte interface (Figures 4E and S6). In conclusion, the critical size of the apertures during in situ liquid SIMS analysis is achieved according to the simulation results. On the basis of the influence of surface properties of aperture on the morphology of the liquid−vacuum interface, it is advised to make the lower part of the aperture hydrophilic and upper part hydrophobic for an aqueous system. Moreover, the correlation between MS signal and the chemical information from the electrode−electrolyte interface was built successfully. This work represents an improvement in understanding of the liquid interface processes where the depletion layer is constrained and the mass transport is confined in the aperture. It will provide more insights into the design of the vacuum compatible electrochemical microdevice and understanding of the physical and chemical processes at the electrode−electrolyte interface.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b05051.
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Chemicals and reagents; fabrication of the microelectrochemical cell; instrumentation and procedures; numerical simulation; detailed information about SEM images of punched apertures; in situ liquid SIMS experiment (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. ORCID
Jun-Gang Wang: 0000-0002-1938-3991 Xin Hua: 0000-0003-1064-083X Yi-Tao Long: 0000-0003-2571-7457 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (21705046, 21421004), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-02-E00023), the Fundamental Research Funds for the Central University (222201718001, 222201717003), Shanghai Sailing Program (17YF1403000), and Shanghai Natural Science Foundation (17ZR1407700).
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REFERENCES
(1) Gauthier, M.; Carney, T. J.; Grimaud, A.; Giordano, L.; Pour, N.; Chang, H.-H.; Fenning, D. P.; Lux, S. F.; Paschos, O.; Bauer, C.; Maglia, F.; Lupart, S.; Lamp, P.; Shao-Horn, Y. J. Phys. Chem. Lett. 2015, 6, 4653−4672. (2) Kimmel, D. W.; LeBlanc, G.; Meschievitz, M. E.; Cliffel, D. E. Anal. Chem. 2012, 84, 685−707. (3) Li, J.-T.; Zhou, Z.-Y.; Broadwell, I.; Sun, S.-G. Acc. Chem. Res. 2012, 45, 485−494. (4) Wang, Y.-H.; Liang, M.-M.; Zhang, Y.-J.; Chen, S.; Radjenovic, P.; Zhang, H.; Yang, Z.-L.; Zhou, X.-S.; Tian, Z.-Q.; Li, J.-F. Angew. Chem., Int. Ed. 2018, 57, 11257−11261. 3198
DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198
pubs.acs.org/ac
Pore Confined Liquid−Vacuum Interface for Charge Transfer Study in an Electrochemical Process Jun-Gang Wang, Xin Hua,* Hai-Lun Xia, and Yi-Tao Long* Key Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China
Downloaded via 49.49.236.30 on October 21, 2019 at 13:31:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: A pore confined liquid−vacuum interface was created in liquid secondary ion mass spectrometry analysis in order to study the charge transfer in electrochemical reactions. The interfacial processes such as the critical diameter, influence of aperture properties on the morphology of the liquid−vacuum interface, pressure field, concentration field, and electric field were revealed by finite element simulation. The correlation between numerical study of the chemical changes at the electrode−electrolyte interface and experimental results during the dynamic potential scan was built successfully. Better understanding of these interfacial processes could promote further applications of liquid secondary ion mass spectrometry in energy storage and electrochemical catalysis.
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the electrode−electrolyte interface. However, owing to the complicated physical and chemical processes during in situ liquid SIMS analysis and lack of appropriate analytical techniques,23 detailed information, such as the critical diameter of the aperture, the influence of the interfacial property (i.e., hydrophobicity and hydrophilicity of the aperture) on morphology of the liquid−vacuum interface, mass transport in the aperture, etc., is not clear so far. The goal of this work is to answer these questions based on electrodynamics and physics via finite element simulation, which will give more insights into the understanding of the confined interfacial processes in liquid SIMS analysis. The fundamental process of in situ liquid SIMS combined with electrochemistry is shown in Figure 1. When the membrane was penetrated, a significant increase of representative ion signal (K+) in the electrolyte was observed. When the aperture diameter from both negative and positive total ions images was compared with the theoretical value, excellent consistency was obtained indicating the well controllability for the penetration process (Figure S1). Owing to the small size of the aperture and hydrophobicity of SiNx membrane, the liquid can be confined in the aperture by surface tension without obvious deterioration of the vacuum system.24 When beyond the critical size of the aperture, the liquid−vacuum interface could not be withheld in the aperture due to the disturbance of the balance between the surface tension and the pressure difference.25 Although the increase of aperture size can
nderstanding of electrochemical reactions and ion solvation as well as mass transfer at the electrode− electrolyte interface during electrochemical processes is crucial in energy storage and biological sensing.1,2 In order to identify the chemical compositions and monitor the reactions near the electrode surface, numerous in situ spectroelectrochemical techniques such as in situ infrared spectroscopy (IR),3 in situ surface-enhanced Raman scattering (SERS),4 in situ X-ray absorption spectroscopy (XAS),5 in situ X-ray diffraction (XRD),6 and in situ nuclear magnetic resonance (NMR)7 have been presented and great progress has been made in providing structural details of molecules adsorbed on the electrode surface.8,9 Compared with these approaches, direct electrochemistry in an electrochemical cell combined with mass spectrometry (MS) can provide direct molecular information at the electrode−electrolyte interface, which could benefit in better understanding of electrochemical processes. For example, electron ionization (EI),10,11 electrospray ionization (ESI),12,13 and desorption electrospray ionization (DESI)14 have been widely used to couple with electrochemistry to monitor redox reaction and provide molecular information unavailable solely from traditional ex situ electrochemical or spectroscopic approaches.15−18 Owing to the high surface sensitivity, high depth resolution, and fast response, in situ liquid secondary ion mass spectrometry (SIMS) combined with electrochemistry has been developed to investigate complex chemistries occurring at the electrode−electrolyte interfaces.19,20 It has been successfully applied to identify transient intermediates in redox reactions and the formation mechanism of the solid electrolyte interphase in lithium ion batteries.21,22 Moreover, it has the potential to acquire information about small ions solvation and transportation at © 2019 American Chemical Society
Received: November 2, 2018 Accepted: January 17, 2019 Published: January 17, 2019 3195
DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198
Letter
Analytical Chemistry
Figure 1. (A) Schematic of the in situ liquid SIMS combined with electrochemistry. (B) ToF-SIMS depth profiles of representative positive ion species (Si2N+, Au+, and K+) in 2 mM KNO3 at open circuit potential. The inset image shows the three-dimensional overlay of Si2N+ (blue), Au+ (orange), and K+ (pink).
Figure 3. (A) Schematic of the hydrophilic and hydrophobic regions of the apertures (a−e). (B) The evolution of the liquid−vacuum interface height with changing interfacial properties of the apertures with diameter of 1−3 μm. Vacuum region, SiNx/Au film, and liquid region are indicated by orange, blue, and green color, respectively. (C) The corresponding morphology of the liquid−vacuum interface in an aperture with diameter of 2 μm with different interfacial properties (a−e). (D) The relative pressure on the liquid−vacuum interface in an aperture with diameter of 2 μm with different interfacial properties (a−e). The distance covered from the liquid− vacuum interface to the center of the aperture. (E) The liquid− vacuum interface morphology in a 2 μm aperture corresponding to interfacial property (c). The concentration field is shown in the inset.
Figure 2. (A) Height of liquid−vacuum interface as a function of the aperture radius. The inset shows the definition of the height of the liquid−vacuum interface which covers the lower surface of the gold electrode to the highest point of the liquid−vacuum interface. (B) The comparison of interface height in the aperture with radius at 3304 nm and 3305 nm as a function of time. The inset shows the morphology of the liquid−vacuum interface at 6 ms in the aperture with radius at 3305 nm (left) and 3304 nm (right). (C) The pressure distribution on the liquid−vacuum interfaces in apertures with various radius. The distance covered from the liquid−vacuum interface to the center of the aperture. (D) The pressure at the contact point between the liquid−vacuum interface and the aperture wall as a function of aperture radius.
3305 nm which was considered as the critical size for the aperture (Figures 2A and S3). When compared with the transient curve of interfacial height at 3304 nm, an obvious increase of the interface height was found at 3305 nm within 1.3 ms. Below the critical size, the interfacial height remains stable without obvious fluctuation (Figure 2B). Moreover, the obvious difference in the concentration field at the critical size and 3304 nm further proves that the critical size was located at 3305 nm during in situ liquid SIMS analysis (Figure S4). After validating the critical size, a further investigation of the distribution of pressure on the liquid−vacuum interface below the critical size was performed (Figure 2C,D). The pressure between the liquid−vacuum interface and the rim of aperture reaches a maximum value at the aperture with 2 μm diameter, indicating a strengthened suction effect at the rim was achieved.27 Hence, in order to avoid the leakage of liquid, the aperture size at 2 μm was chosen in practical analysis and further finite element models (FEM) simulation. Appropriate design of the microfluidic device with desired interfacial properties will improve the confinement of the liquid−vacuum interface in the aperture during in situ liquid SIMS analysis. Considering the significant influence of hydrophobic and hydrophilic properties of the aperture on the morphology of the liquid−vacuum interface, the FEM was further conducted under various interfacial conditions (Figure 3A). Here, the initial contact angles of hydrophilic and hydrophobic portion were fixed at 67° and 130°, respectively.28,29 Obviously, with the increase of the hydrophilic
improve the ion intensity and provide more spatial information in the aperture, the larger size of the aperture will induce the overflow of the liquid and lead to the attenuation of the vacuum system. Considering the probability of the damage on the SIMS system induced by the leakage of liquid, the critical size of the aperture at which the abrupt change of liquid− vacuum interface occurred was elucidated by finite element simulation. Given the circular hole structure of an aperture, a twodimensional axisymmetric geometry can be used to simplify its three-dimensional geometry (Figure S2). The transport fluid through the aperture involves the two-phase flow dynamics which was governed by surface tension, wall adhesive forces, and pressure difference.26 A detailed description of the equations for finite element calculation could be found in the Supporting Information. When the aperture radius was enlarged, an abrupt change of the interfacial height arises at 3196
DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198
Letter
Analytical Chemistry
Figure 4. Schematic of the locations adopted to calculate the concentration of K+ during dynamic potential scan. (A) The volumes with different thickness of liquid layer. (B) The interface region includes the liquid−vacuum interface (LVI) and different lengths of electrode−electrolyte interface (EEI). FEM simulated distributions of K+ concentration in (C) various volumes with different thickness of the liquid layer (a−g: 1, 3, 6, 12, 18, 24, and 30 nm; h: the whole liquid volume in the aperture) corresponding to (A), and (D) interface regions (a: EEI; b−i: LVI plus EEI with various length (i.e., the whole electrode, 30, 24, 18, 12, 6, 3, and 1 nm); j: LVI) corresponding to (B) during dynamic potential scan from −0.20 to +0.18 V at a scan rate of 10 mV s−1. The inset in (D) shows the enlarged results for b−j. (E) Schematic of K+ accumulation at the electrode−electrolyte interface under cathodic polarization is shown in the inset I. Comparison between the differential curves of the experimental K+ mass intensity (dashed line) and simulated K+ concentration trends (solid line) at various interfacial regions (a−j, a is shown in the inset II) as a function of polarized potential.
simulated by the Nernst−Planck−Poisson (NPP) equations.30 The potassium ions were attracted or repelled under cathodic or anodic polarized potential, respectively, which was in good accordance with electric double layer theory (Figure S5). To correlate the in situ liquid SIMS signal with the chemical information from the electrode−electrolyte interface, finite element models were built and compared with the experimental observation. In the first case, the liquid layers with various thickness from the liquid−vacuum interface were considered (Figure 4A). Figure 4C shows that the potassium concentration decreased under anodic polarization and increased under cathodic polarization. With the increase of the thickness of liquid layer, the concentration of potassium is closer to the bulk value (2 mM), indicating that the applied dynamic potential scan cannot induce the drastic fluctuation of potassium concentration in the aperture. In the second case (Figure 4B), owing to the important role of electrode− electrolyte interface in electric double layer theory, where the accumulation and repulsion of potassium ion mainly occurred, the potassium concentration at the liquid−vacuum interface and the electrode surface with different lengths was calculated. Figure 4D shows that the concentration of potassium change with the dynamic potential scan and a U-shape can be found. The more the electrode−electrolyte interface was included, the more noticeable was the fluctuation of the concentration of potassium, confirming that the accumulation of potassium ion mainly occurred at the electrode−electrolyte interface. Comparing these modeling results with experimental observations, the trends of potassium concentration change during dynamic potential scan at the electrode−electrolyte interface agreed well with the in situ liquid SIMS results, which further validated the correlation between the in situ liquid SIMS signal
region, the liquid−vacuum interface height increases as well in various apertures with diameter between 1 and 3 μm (Figure 3B). In the whole hydrophobic aperture, a concave meniscus can be seen, which was induced by the hydrophobic interaction between the inner wall and the liquid. On the other hand, the whole hydrophilic aperture causes the noticeable leakage of the liquid out of the aperture, further confirming the influence of the interfacial properties on the morphology of the liquid−vacuum interface. For the aperture constituted by the hydrophilic SiNx (upper region) and hydrophilic Au (lower region), the liquid−vacuum interface was well confined in the aperture and the interface height reached 106 nm at steady state, which is lower than the depth of the aperture (170 nm) (Figure 3C). At this case (c), the pressure on the liquid−vacuum interface becomes greater than other cases (a, b, d, and e) and is responsible for the enhanced confining effect (Figure 3D). Meanwhile, the rim of the liquid−vacuum interface contacted with the Au rather than the SiNx at 2 μm radius (Figure 3E), indicating that the liquid− vacuum interface has a good contact with the working electrode. On the basis of the above modeling results, surface modifications to make the inner wall more hydrophilic and upper rim more hydrophobic will be favorable to confine the aqueous liquid in the aperture. Meanwhile, for nonaqueous systems, it is advisable to change the material of the aperture to make the lower part of the aperture hydrophobic and upper part hydrophilic. The above design will help to confine the liquid−vacuum interface of the nonaqueous system in the aperture. These modifications will be helpful to the successful design of the vacuum compatible microfluidic electrochemical device. Under dynamic electric field, the ion mass transfer and charge transfer at the electrode−electrolyte interface is 3197
DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198
Letter
Analytical Chemistry
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and the chemical information from the electrode−electrolyte interface (Figures 4E and S6). In conclusion, the critical size of the apertures during in situ liquid SIMS analysis is achieved according to the simulation results. On the basis of the influence of surface properties of aperture on the morphology of the liquid−vacuum interface, it is advised to make the lower part of the aperture hydrophilic and upper part hydrophobic for an aqueous system. Moreover, the correlation between MS signal and the chemical information from the electrode−electrolyte interface was built successfully. This work represents an improvement in understanding of the liquid interface processes where the depletion layer is constrained and the mass transport is confined in the aperture. It will provide more insights into the design of the vacuum compatible electrochemical microdevice and understanding of the physical and chemical processes at the electrode−electrolyte interface.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b05051.
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Chemicals and reagents; fabrication of the microelectrochemical cell; instrumentation and procedures; numerical simulation; detailed information about SEM images of punched apertures; in situ liquid SIMS experiment (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. ORCID
Jun-Gang Wang: 0000-0002-1938-3991 Xin Hua: 0000-0003-1064-083X Yi-Tao Long: 0000-0003-2571-7457 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (21705046, 21421004), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-02-E00023), the Fundamental Research Funds for the Central University (222201718001, 222201717003), Shanghai Sailing Program (17YF1403000), and Shanghai Natural Science Foundation (17ZR1407700).
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DOI: 10.1021/acs.analchem.8b05051 Anal. Chem. 2019, 91, 3195−3198