Surface Analysis of Materials in Aqueous Solution by Localized

Here it is demonstrated that this effect can be exploited for surface analysis of compound materials in solution by making localized alternating curre...
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Surface Analysis of Materials in Aqueous Solution by Localized Alternating Current Impedance Measurements Piotr Michal Diakowski and Miao Chen* CSIRO Process Science and Engineering, Clayton, Victoria, 3168, Australia S Supporting Information *

ABSTRACT: Differences in electrical conductivity provide a basis for identification of different components present at a material surface. Here it is demonstrated that this effect can be exploited for surface analysis of compound materials in solution by making localized alternating current impedance measurements.

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to distinguish chalcopyrite (CuFeS2) and pyrite (FeS2) unequivocally. Chalcopyrite is the most abundant copper sulfide mineral in the world, and it is often accompanied by pyrite and pyrrhotite (Fe(1−X)S), which affects its processing performance.17,18 Scanning electron microscopy (SEM) with energy dispersive X-ray detectors (EDX),16,19 laser-induced plasma spectroscopy (LIPS),20 Raman spectroscopy,21 or X-ray absorption near edge structure spectroscopy (XANES)22 can also be used for reliable mineral characterization, but these methods are expensive and often require well equipped laboratory facilities. Because the proposed method relies on ac measurements, common drawbacks of amperometric SECM are eliminated. Specifically, measurements are carried out in the absence of redox mediator, and topographical effects on the observed response are minimized. Our method is based on differences in electrical conductivity exhibited by various mineral phases.23 Basic principles of the method are depicted in Figure 1. In brief, a mineral specimen embedded in epoxy resin is polished flat, placed in an SECM cell, and immersed in electrolyte solution. A sinusoidal voltage waveform is applied between the tip and auxiliary electrodes, and the resulting flow of ac current between the auxiliary and tip electrodes is measured. In the bulk solution or in close proximity to an insulating surface, the ac current can reach the tip electrode by flowing exclusively through the electrolyte solution, following current path i, Figure 1a. This case can be approximated by a simple combination of solution resistance, RT, and double-layer capacitance of the tip electrode, CT, Figure 1b. Consequently, flow of ac current can be hindered by proximity to a nonconductive surface, resulting from an increase in the RT component. When the solid sample is conductive, the ac current can also reach the tip electrode through an alternative

canning electrochemical microscopy (SECM) is a powerful instrumental method capable of providing localized electrochemical and topographical information at the micrometer scale.1 Despite the growing popularity of SECM as a research tool, routine analytical application of the method outside the research laboratory remains elusive. This is largely due to fundamental limitations of the amperometric mode SECM as it relies on the presence of a freely diffusing redox mediator for its operation. Consequently, the redox mediator may affect the investigated surface by changing its corresponding Nernst potential and result in the disturbance of the chemical equilibrium.2 To overcome this limitation, alternating current-scanning electrochemical microscopy (ac-SECM) has been advocated in recent years as a powerful alternative to SECM.3−8 The presence of a redox mediator is not required for operation of ac-SECM, making this method suitable for investigation of complex systems that cannot be studied in the presence of electroactive species. Furthermore, in contrast to conventional SECM, topographical and chemical information can be deconvoluted.9,10 Also, several instrumental methods to decouple topography sample reactivity were reported, for instance, atomic force microscopy-SECM (AFM-SECM),11 SECM-scanning ion conductance microscopy (SECMSICM),12 shear force SECM,13 and intermittent contact SECM.14 However, AFM-SECM and SECM-SICM rely on special dual-probes, and shear force SECM and intermittent contact SECM require instrumentation to vibrate the probe. In this technical note, we demonstrate the capability of acSECM as a surface characterization tool in aqueous solutions by applying it to the identification of minerals in a mixed metal sulfide system. Efficient mining and mineral processing require adequate ore characterization as properties of the ore strongly affect processing performance.15 Traditionally used, manual techniques such as point counting and optical microscopy are time-consuming and dependent on the operator skills, which makes them prone to errors.16 Additionally, it is often not easy Published 2012 by the American Chemical Society

Received: May 22, 2012 Accepted: August 27, 2012 Published: August 27, 2012 7622

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Figure 1. Depiction of ac-SECM principles: (a) ac current can reach the tip electrode by flowing through electrolyte solution, path i, and by flowing through a conductive sample, path i1. (b) Equivalent circuit approximation of ac-SECM measurement. RT represents solution resistance between auxiliary and the tip electrodes, CT is double-layer capacitance of the tip, RA‑S solution resistance between auxiliary electrode and sample, CS′ doublelayer capacitance of a large sample section not covered by the tip, RS resistance of the sample under the tip, CS double-layer capacitance of the small section of the sample covered by the tip, and RT‑S solution resistance between the sample and the tip.

Figure 2. ac-SECM visualization of a chalcopyrite/pyrrhotite sample at 90 kHz and 0.2 Vp‑p amplitude in 10 mM NaClO4 (a), 3 kHz and 0.1 Vp‑p amplitude (b). Optical image of the sample (c) and SEM backscattered electron micrograph (d).

path flowing through the sample, current path i1, Figure 1a. An equivalent circuit model approximation of this scenario is shown in Figure 1b, current path i1. Flowing from the auxiliary to the tip electrode, ac current passes through solution resistance between the auxiliary electrode and a section of conductive sample not covered by the tip, RA‑S; double-layer capacitance of the sample not covered by the tip, CS′; resistance of the sample, RS; double-layer capacitance of the small section of the sample covered by the tip, CS; solution resistance that develops between the tip and the sample, RT‑S; and double-layer capacitance of the tip, CT. Accordingly, the contributions of current paths i and i1 to the overall ac current flow depend on their respective impedances. The impedance of path i is to a large extent determined by RT, which is strongly dependent on tip−sample separation distance. Impedance of the path i1 is determined by the resistance of the sample material, RS, and impedance of the capacitive element CS and distance dependent RT‑S. The impedance of a capacitive circuit element C is given by XC = 1/ωC, where ω represents the angular frequency. Consequently, when capacitive contribution to the impedance of current path i1 is large (i.e., low frequency and/or high concentration of supporting electrolyte), the ac current provides topographical information. When the capacitive

contribution to the impedance of current path i1 is small (i.e., high frequency and/or low concentration of supporting electrolyte), the magnitude of the resulting ac current depends to a large extent on the nature of the sample material. Similar equivalent circuit models have been proposed in the past.5,6 Such equivalent circuit models properly illustrate the most important properties of the system but at the same time involve very significant oversimplifications. It has to be noted that rigorous circuits for such systems cannot be represented by a finite number of circuit elements connected in a finite number of points.

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EXPERIMENTAL SECTION A detailed description of experimental procedures, instrumentation, materials used, and data treatment is given in the Supporting Information.

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RESULTS AND DISCUSSION In order to investigate the feasibility of this method for characterization of naturally occurring metal sulfides, a specimen consisting of chalcopyrite (CuFeS2) and pyrrhotite (Fe(1−X)S) was placed in a solution containing 10 mM NaClO4 as a supporting electrolyte. The microelectrode tip was then 7623

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rastered in to close proximity of the sample and the resulting ac current recorded. A representative ac-SECM image obtained at 90 kHz is shown in Figure 2a. Variations in the ac current observed above different areas of the sample can be related to differences in the nature of the sample material (EDX analysis of the sample is included as Supporting Information). Nonconductive material is represented by relatively low ac currents (blue color in Figure 2a). The elongated low-current feature visible in the bottom-left corner of the ac-SECM image corresponds to epoxy resin. The chalcopyrite phase is represented by intermediate current values (yellow color), and pyrrhotite is characterized by high ac current (red color). It should be emphasized that a relatively small difference in conductivity of chalcopyrite and pyrrhotite23 results in clear ac current contrast in the ac-SECM image, making unequivocal identification of mineral phases feasible. There is a spread in literature values of these two metal sulfides, but their conductivities probably differ by no more than a factor of 2.23 The ac-SECM image of the chalcopyrite/pyrrhotite specimen obtained at 3 kHz is shown in Figure 1b. At low frequencies the impedance of current path i1 (Figure 1a) becomes relatively large and the majority of the ac current reaches the tip through the electrolyte solution. As a result, current flow is determined by sample topography. The acSECM image shown in Figure 2b displays two elevated current areas, which represent recessed features on the sample surface. For comparison, optical and back scattered SEM micrographs are shown in parts c and d of Figure 2, respectively. Clearly, optical and SEM images confirm ac-SECM based identification of chalcopyrite and pyrrhotite domains and the presence of two depressed features on the sample surface. In order to further evaluate the flow of ac currents, we performed approach curve experiments by vertically approaching the tip electrode toward different sections of the sample, which produced characteristic plots of tip ac current vs tip− sample separation distance, shown in Figure 3.

The pyrrhotite displayed the most positive ac feedback current indicating relatively high electrical conductivity of Fe(1−X)S. As expected, less positive feedback was recorded on approach toward chalcopyrite. For both positive feedback approach curves, the ac current initially increased until approximately L = 1 and the normalized tip−sample separation distance was reached (separation distance, d, was normalized by the tip radius, a). This initial current rise resulted from gradually increasing the contribution of the current path i1 (Figure 1a) to the overall ac current flow with decreasing tip− sample separation. For separation distances smaller than approximately L = 1, the ac current decreased due to the impediment of the current path i (Figure 1) by proximity of the sample surface. On another hand, the nonconductive section of the sample (epoxy) produced purely negative feedback current, indicating that only current path i was active in this case. On the basis of the assumption that equations describing solution resistance between the SECM tip and the auxiliary electrode are analogous to Fick’s equations for amperometric SECM,24 a theoretical approach curve for insulating sample and the relevant experimental conditions was obtained and is shown as a solid line in Figure 3. The very good fit between experimental and theoretical approach curves for the insulating sample allowed the tip−sample separation in ac-SECM imaging experiments to be calculated as approximately 4 μm. Approach curves shown in Figure 3 indicate that sufficient differences in the ac current to unambiguously discriminate different mineral phases can be observed at tip−sample separations smaller than approximately L = 2. Importantly, under given experimental conditions (i.e., excitation frequency and supporting electrolyte concentration) approach curves are characteristic for a given material and can be used for unequivocal discrimination of mineral phases. For mineralogical assessment, ore is often crushed and set with epoxy to form a hardened sample, and cross sections of the sample are used for characterization.19 To further demonstrate relevance of ac methodology in the mineralogical context, a composite sample consisting of ground chalcopyrite and pyrite (FeS2) set in epoxy resin was prepared. Fixing sample in epoxy resin also prevented possible galvanic interaction between chalcopyrite and pyrite particles. An acSECM image of the composite sample obtained at 90 kHz is shown in Figure 4a and the ac current profile in Figure 4b. An optical image and EDX data for the composite mineral sample are included in the Supporting Information. The ac current map shown in Figure 4 indicates the presence of different domains on the sample surface. High ac current values (red color) correspond to the pyrite phase, and intermediate ac currents (green and yellow colors) represent chalcopyrite. Epoxy resin is characterized by low ac currents (blue color). Standard image analysis techniques can be applied to extract parameters of mineralogical significance (i.e., composition, relative amounts, and size distribution) from the ac-SECM image presented in Figure 4. Using commercial image analysis software (details in the Supporting Information), we have estimated that approximately 34% of the sample surface corresponds to pyrite and approximately 8% to chalcopyrite, which translates to approximately 81% pyrite and 19% chalcopyrite content in the ore sample.

Figure 3. Normalized ac approach curves observed at 90 kHz and 0.2 Vp‑p amplitude above ◇, pyrrhotite; ○, chalcopyrite; and Δ, epoxy resin. The solid line represents a predicted approach curve for insulating sample, and the vertical dashed line corresponds to tip− sample separation in imaging experiments. Separation distance, d, normalized by tip radius, a.

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CONCLUSIONS In summary, we have demonstrated that localized ac current measurements can be employed for unequivocal discrimination 7624

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REFERENCES

(1) Bard, A. J.; Mirkin, M. V. Scanning Electrochemical Microscopy; Marcel Dekker: New York, 2001. (2) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (3) Eckhard, K.; Schuhmann, W. Analyst 2008, 133, 1486−1497. (4) Gebala, M.; Schuhmann, W.; La Mantia, F. Electrochem. Commun. 2011, 13, 689−693. (5) Eckhard, K.; Erichsen, T.; Stratmann, M.; Schuhmann, W. Chem.Eur. J. 2008, 14, 3968−3976. (6) Baranski, A. S.; Diakowski, P. M. J. Solid State Elect. 2004, 8, 683− 692. (7) Alpuche-Aviles, M. A.; Wipf, D. O. Anal. Chem. 2001, 73, 4873− 4881. (8) Diakowski, P. M.; Baranski, A. S. Electrochim. Acta 2006, 52, 854−862. (9) Diakowski, P. M.; Ding, Z. Electrochem. Commun. 2007, 9, 2617− 2621. (10) Zhao, X.; Diakowski, P. M.; Ding, Z. Anal. Chem. 2010, 82, 8371−8373. (11) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276− 285. (12) Takahashi, Y.; Shevchuk, A. I.; Novak, P; Murakami, Y.; Shiku, H.; Korchev, J. E.; Matsue, T. J. Am. Chem. Soc. 2010, 132, 10118− 10126. (13) Katemann, B. B.; Schulte, A.; Schuhmann, W. Chem.Eur. J. 2003, 9, 2025−2033. (14) McKelvey, K. M.; Edwards, M. A.; Unwin, P. R. Anal. Chem. 2010, 82, 6334−6337. (15) Hunt, J.; Berry, R.; Bradshaw, D. Miner. Eng. 2011, 24, 1271− 1276. (16) Schouwstra, R. P.; Smit, A. J. Miner. Eng. 2011, 24, 1224−1228. (17) Watling, H. R. Hydrometallurgy 2006, 84, 81−108. (18) Rohwerder, T.; Gehrke, T.; Kinzler, K.; Sand, W. Appl. Microbiol. Biot. 2003, 63, 239−248. (19) Fandrich, R.; Gu, Y.; Burrows, D.; Moeller, K. Int. J. Miner. Process. 2007, 84, 310−320. (20) Kaski, S.; Hakkanen, H.; Korppi-Tommola, J. J. Miner. Eng. 2003, 16, 1239−1243. (21) White, S. N. Chem. Geol. 2009, 259, 240−252. (22) Xia, J.-l.; Yang, Y.; He, H.; Zhao, X.-j.; Liang, C.-l.; Zheng, L.; Ma, C.-y.; Zhao, Y.-d.; Nie, Z.-y. N.; Qiu, G.-z. Hydrometallurgy 2010, 100, 129−135. (23) Pridmore, D. F.; Shuey, R. T. Am. Mineral. 1976, 61, 248−259. (24) Horrocks, B. R.; Schmidtke, D.; Heller, A.; Bard, A. J. Anal. Chem. 1993, 65, 3605−3614.

Figure 4. ac-SECM visualization of chalcopyrite and pyrite composite sample at 90 kHz and 0.2 Vp‑p amplitude (a) and ac current profile (b).

of metal sulfide phases based on their characteristic electrical conductivity. The proposed method is fast and cost-effective due to the simple instrumentation and minimal sample preparation and minimal use of consumables. Furthermore, standard image analysis techniques allow extraction of mineralogically significant information from ac-SECM data. The method presents a viable alternative to inaccurate optical methods and high-cost SEM-based techniques often used for identification of naturally occurring metal sulfides. Proper identification of metal sulfides can greatly benefit the mining industry in terms of cost and environmental impact reduction. It is especially important in low-cost operations in remote areas and developing countries, where environmental impact of such operations is often neglected. The method could be applied to surface analysis, in solution, of any composite material with components of different conductivity and/or double-layer capacitance.

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Technical Note

ASSOCIATED CONTENT

S Supporting Information *

Experimental details, optical images, EDX analysis, and AFM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 3 9545 8748. Fax: +61 395628918. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the funding from the CSIRO CEO Science Leader program. Discussions with Roger Horn are gratefully acknowledged. We thank Mathew Glenn for help with the SEM-EDX analysis. 7625

dx.doi.org/10.1021/ac3019944 | Anal. Chem. 2012, 84, 7622−7625