Electrochemical Behaviors of Single Microcrystals of Iron

Article pubs.acs.org/ac

Electrochemical Behaviors of Single Microcrystals of Iron Hexacyanides/NaCl Solid Solution Dongping Zhan,* Dezhi Yang, Bing-sheng Yin, Jie Zhang, and Zhong-Qun Tian College of Chemistry and Chemical Engineering, and State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China ABSTRACT: Solid-state solution with excellent ionic conductivity and redox activity has potential applications in electrochemical microdevices, such as transistors, switches, sensors, and actuators, due to its controllable assembly, integration, and package in microchips. In this paper, we present an electrochemical method to synthesize single microcrystals of an iron hexacyanides/NaCl solid solution and to assemble them into microdevices based on scanning electrochemical cell microscopy. The redox behaviors of the single microcrystals were investigated systematically, especially in the “all-in-solid-state”, that is, without exposure to any external liquid environment. The unique metal/solid solution interface has similar electrochemical properties as the conventional metal/liquid solution interface. The apparent concentration of ion hexacyanides (1.05 × 10−3 mol/L), the diffusion coefficient of the counterion Na+ (8.05 × 10−8 cm2/s), and the electron transfer rate in the lattice (1.22 × 10−4 cm/s) were evaluated from the data obtained through all-in-solid-state cyclic voltammetry and electrochemical impedance. All the results are comparable to the conventional metal/solution theory of electrochemistry.

E

Solid electrolyte solutions with good electronic and ionic conductivities are crucial in constructing all-in-solid-state electrochemical microdevices. A fast electron transfer rate across the electrode/solid solution interface (i.e., good electronic conductivity) is essential to avoid electrochemical polarization. Good ionic mobility inside the solid solution (i.e., good ionic conductivity) is important to decrease Ohm drop and to compensate redox charges. When an all-in-solid-state electrochemical system works, the ionic and electronic currents should match to each other. Excellent properties of charge transfer will make the response of solid-state electrochemical microdevices fast and reversible. Scanning electrochemical microscopy (SECM) is a scanning probe microscopy (SPM) technique by using an ultromicroelectrode (UME) as the scanning probe.15 If the scanning tip is replaced by a microcapillary, it is termed recently as scanning electrochemical cell microscopy (SECCM). In SECCM, a conductive substrate contacts with the microcapillary orifice through a ∼picoliter or ∼femtoliter drop of electrolyte and serves as the working electrode while the counter and reference electrodes are placed inside the microcapillary. This moveable electrochemical microsystem has been proved powerful in the investigations of local corrosion,16−18 surface imaging,19−24 electrocatalysis,25−27 underpotential deposition,28 microsynthesis,29 and electron transfer kinetics.30,31 Since the electrolyte microdrop is exposed to the atmosphere, it is crucial to avoid

lectrochemical technologies have played a decisive role in the phenomenal development of energy storage, chip interconnectors, microelectronic packaging, MEMS systems, sensors, actuators, and other microelectronic and micromechanic components.1−4 One of the challenges for the miniaturization of electrochemical devices is to ensure they work in absence of liquid electrolytes, which is essential for their assembly, integration, and package. However, solid electrolyte solutions usually have high Ohm resistance and low electron transfer rate, which results in slow responses and therefore hinders their practical application.5 To meet the needs of micromanufacture, it is essential to develop qualified materials of redox solid solution with both high electron transfer rate and high ionic mobility. It is a long history that scientists have paid attention to the electrochemical analysis of solid materials, from electrograph of minerals to electrocatalysis of powder microelectrodes.6,7 Electrochemistry of solid materials and particles are comprehensively reviewed.5,8 For example, the solid-state electrochemistry of Prussian blue and its derivatives have been well studied as materials for sensors, electrocolorimeters, and power sources.9−12 Although single molecular devices are still far away from practical applications at present, Bard’s recent investigations on the catalysis of single nanoparticles provide a new approach to develop electrochemical microdevices with microor nanoparticles due to the feasibility at the technical level.13,14 Nevertheless, most of the investigations on the so-called “solidstate” electrochemistry were still performed in a liquid electrolyte environment involving “solid” particles or “solid” redox material. © 2012 American Chemical Society

Received: July 20, 2012 Accepted: October 1, 2012 Published: October 1, 2012 9276

dx.doi.org/10.1021/ac302053x | Anal. Chem. 2012, 84, 9276−9281

Analytical Chemistry

Article

Figure 1. Schematic diagrams of SECCM: a microcapillary with a micrometer sized orifice was employed as the scanning tip and the electrochemical microcell, the reference and counter electrodes were inserted in the microcapillary, while a conductive substrate acts as the working electrode; thus, a electrochemical microsystem was formed.

Figure 2. Cyclic voltammograms of the SECCM microsystem, where the concentrations of Na4Fe(CN)6 were 1.0 × 10−5 mol/L (a) and 1.0 × 10−7 mol/L (b), the supporting electrolyte was 0.05 mol/L NaCl, and the scanning rate was 0.1 V/s. The diameters of the microcapillary orifice were 5 μm (a) and 11 μm (b), respectively. (c) The optic image of a blocked micropipet used in (b) after the SECCM microsystem achieved the steady state.

microcrystalline solution. All electrochemical experiments were performed with the SECM workstation CHI920c (CHI Instrument Co., USA). SECCM Setup. The SECCM technique was developed from SECM as shown in Figure 1. A micropipet with a micrometer sized orifice acts as both the scanning tip and the electrolytic cell. An Ag/AgCl wire was inserted into the micropipet as both the counter electrode and the reference electrode. A conductive substrate, such as ITO, Au, or Pt thin-film-coated glass slide, was the working electrode. With the help of the video camera, the micropipet was moved to contact with the conductive substrate. Since the tip and substrate contact through a ∼picoliter or ∼femtoliter drop of electrolyte solution, electrochemical reactions were confined in the small volume of electrolyte between the tip and the substrate. Actually, the spatial resolution of SEECM depends on the size of the microdrop. When the tip was scanning, the whole electrochemical microsystem moved.

the evaporation of water. Therefore, SECCM works usually in either a damp environment or a closed system. Recently, with the aid of evaporation, we have synthesized single NaCl microcrystals doped with iron hexacyanides, which was proved to be a solid solution with excellent ionic and electronic conductivity.29 In this paper, we focus on the electrochemical properties of single microcrystals of iron hexacyanides/NaCl solid solution, including their electrochemical microsynthesis, redox behaviors in the SECCM microsystem, especially, charge transfer kinetics across the metal/solid-solution interface.

■

EXPERIMENTAL SECTION Chemicals, Materials, and Instruments. All chemicals used in the experiments (NaCl, Na4Fe(CN)6, and Na3Fe(CN)6) were analytical grade or better (Sigma−Aldrich Co.). All aqueous solutions were prepared with deionized water (18.2 MΩ, Milli-Q, Millipore Corp.). The borosilicate micropipets (o.d., 1.2 mm; i.d., 0.8 mm) with orifice of 3−10 μm diameter were prepared with a programmed laser puller PS-2000 (Sutter Co., USA) as reported previously.29,32 The Au and Pt thin-filmcoated glass slides were prepared through magnetron sputter plating (JC500-3/D, Chengdu Vacuum Equipment Co., China). The ITO glass slides are a kind gift from Prof. Bin Ren at Xiamen University. Before experiments, the slides were cleaned with acetone and deionized water for several times and dried with pure nitrogen gas. A scanning electron microscope (SEM, Hitachi High-Technologies Co., Japan) was employed to obtain geometric topography and elementary analysis of the single microcrystals. Confocal Raman spectrum experiments were performed with a Renishaw inVia Raman microscope (Renishaw Co., British) to confirm the composition of the

■

RESULTS AND DISCUSSION Abnormal Electrochemical Behavior of the SECCM Microsystem in Ambient Environment. In the experiment of Figure 2a, the microcapillary contained an aqueous solution of 1.0 × 10−5 mol/L Na4Fe(CN)6 and 0.5 mol/L NaCl. The cyclic voltammograms shown here are the every fifth cycles. The symmetric current peaks show a typical characteristic of a confined redox couple at the electrode/electrolyte interface. It took a few minutes for the electrochemical microsystem to be stable. In Figure 2b, the concentration of Na4Fe(CN)6 is decreased to 1.0 × 10−7 mol/L, and the cyclic voltmmograms are the every 200th cycles. At the beginning, the cyclic voltammograms show a characteristic of a diffusion controlled 9277

dx.doi.org/10.1021/ac302053x | Anal. Chem. 2012, 84, 9276−9281

Analytical Chemistry

Article

cubic microcrystal was formed during a liner scan (5 s), where the orifice radius is 3 μm and the solution contains 1.0 × 10−5 mol/L Na4Fe(CN)6 and 0.05 mol/L NaCl. The SEM and the optical microscopy images of the microcrystal array are shown in Figure 3b and c, respectively. The well-defined interference rings observed in Figure 3c indicate that the surface of the microcrystal is perfect smooth. The STM scrapping mode experimental results show that the microcrystals are actually solid without holes or hollow space inside (Figure 3d). The components of the single microcrystals were investigated through EDS and confocal Raman spectra experiments. The EDS results (not shown) show that the main component of the single microcrystals is NaCl. As shown in Figure 4, the Raman

process. With the scan going on, the Faraday current keeps increasing until a steady state was achieved. Under the steady state, the redox peaks become more symmetric and similar to Figure 2a. In general, the electrochemical microsystem with a higher bulk concentration of Na4Fe(CN)6 in the microcapillary needs shorter time to reach the steady state. From Figure 2c, it can be observed that the tip was blocked by white crystals. As mentioned above, the symmetric voltammograms of the SECCM indicate that the redox species are confined in a thin layer at the electrode/solution interface. From the following equation:33

ip =

n2F 2vVCo 4RT

(1)

where ip is the peak current, n is the charge number, F is the Faraday constant, R is the gas constant, T is the absolute temperature, V is the volume of single microcrystal, v is the scanning rate, and Co is the concentration of iron hexacyanides. Suppose the distance between the tip and the substrate is equal to the diameter of the tip orifice, the peak current should be 3.8 × 10−13 A which is 2−3 orders lower than those in Figure 2a and difficult to be sensed. That means the true concentration in the electrolyte microdrop must be much higher than the bulk concentration in the microcapillary. It should be noted that the electrochemical microsystem was confined in a volume of ∼picoliter or ∼femtoliter electrolyte microdrop, which was exposed to the ambient atmospheric environment. Because of the evaporation of the water, both iron hexacyanides and NaCl were concentrated. When the supersaturation was met, the microcrystal began to grow. Consequently, iron hexacyanides were trapped in the crystals, and the cyclic voltammetric behavior presented the characteristics of the “thin-layer” electrolysis. Electrochemical Synthesis and Characterization of Single Microcrystals. Because of water evaporation, the microcapillary orifice was blocked by the salt-out crystal. Could the size and topography of the crystal be controllable? The simplest way to synthesize a well-shaped microcrystal is to shorten the growth time. As shown in Figure 3a, a well-defined

Figure 4. Confocal Raman spectra of the single microcrystals synthesized with the aqueous solution containing 0.05 M NaCl and different bulk concentrations of Na4Fe(CN)6 in the microcapillary.

bands in the region from 2000 to 2200 cm−1 are attributed to the CN stretching, confirming the existence of both Fe(CN)64− and Fe(CN)63− in the single NaCl microcrystals. The characteristic Raman spectrum of Fe−C bands near 300 and 530 cm−1 of Prussian blue (PB) are obtained same as the previous report.34 The results also show that the strength of the Raman spectrum is relevant to the initial concentration of Na4Fe(CN)6 in the electrolyte solution, which predicts that the doping amount of iron hexacyanides is adjustable. The mechanism of crystallization has been discussed in detail in our previous work, and a solid solution of iron hexacyanides/ NaCl was formed actually.29 It should be noted that electrochemistry plays an important role in the microsynthesis of the single microcrystals. If we do not apply potential scan and leave the SECCM microsystem alone, it takes a longer time for crystal growth, and the shape is difficult to control. Obviously, the electrocapillarity caused by the applied potential changes the interfacial tension and accelerates water evaporation. Second, the potential scan will alter the components in the vicinity of the substrate electrode surface. The existence of multivalent irons in the crystal is the prerequisite for the all-in-solid-state electrochemistry of the microcrystal. In other words, the electron donor and acceptor should exist together in the crystal lattices so that electron propagation can occur. Third, from the Raman spectrum, the characteristic peak for Prussian blue (PB) at 530 cm−1 can be observed. That means PB was produced during the potential scan. It is well-known that PB is very insoluble in aqueous solution. The precipitation of PB provides the “nucleation” seeds, which accelerates the nucleation and crystal growth processes. Moreover, lattice defects were introduced when NaCl65− units were substituted by iron hexacyanides, which results in a good ionic conductivity. Electrochemical Behavior of the Single Microcrystals in the SECCM Microsystem. To promote the electro-

Figure 3. (a) Linear scanning voltammogram recorded for the formation of one single microcrystal, with scanning rate of 0.1 V/s; (b) SEM images and (c) the optical microscopy images of the single microcrystals; (d) AFM contact mode image of one single microcrystal indicates it is solid. The aqueous solution inside the microcapillary contains 1.0 × 10−5 mol/L Na4Fe(CN)6 and 0.05 mol/L NaCl. 9278

dx.doi.org/10.1021/ac302053x | Anal. Chem. 2012, 84, 9276−9281

Analytical Chemistry

Article

chemical responses, the aqueous solution containing 1.0 × 10−3 mol/L Na4Fe(CN)6 and 0.05 mol/L NaCl was optimized to prepare the single microcrystals. When the single microcrystal arrays were prepared, the old micropipet was replaced by a new one containing an aqueous solution with 0.05 mol/L NaCl but without Na4Fe(CN)6. The new micropipet was positioned on one of the single microcrystals, and the redox behavior of the microcrystal in the SECCM microsystem was investigated. Curve 1 in Figure 5 is a cyclic voltammogram when the new

apparent concentration of iron hexacyanides in the microsystem is obtained as 1.9 × 10−3 mol/L. If the scanning rate keeps increasing, the peak current will be no longer in proportion to the scanning rate. The peak current is now in proportion to the square root of the scanning rate (Figure 6c and d) according to the Randles−Sevcik equation:33 1/2 i p = 2.69 × 105n3/2ADapp Cov1/2

(2)

where A is the electrode area, Dapp is the apparent diffusion coefficient of active species, and the other parameters are same as those in eq 1. The apparent diffusion coefficient can be calculated as 3.8 × 10−5 cm2/s. Note that this value is much higher than those of iron hexacyanides33 and matches to Na+ in aqueous or aqueous mixed solution (from 1.0 to 4.5 × 10−5 cm2/s).35,36 By comparing these data, there should exist free Na+ ion, and the rate-determining step (rds) might be the diffusion of the Na+ ion at high scanning rates. When scanning rates are low, the counterion Na+ always has enough time to balance the charge caused by the redox of iron hexacyanides, which results in the symmetric voltammograms. If there does exist free Na+ ion, the microcrystal exposure to aqueous solution should be a kind of “colloid crystal”, that is, a highly ordered array of smaller crystal particles surrounded by concentrated NaCl solution. To elucidate the existence of a “colloid” state, the microcapillary is moved to the next place of the substrate to perform another cycle of voltammetry. The subsequent cyclic voltmmograms were recorded as shown in Figure 7. The Faraday currents kept decreasing and disappeared

Figure 5. Cyclic voltammograms obtained in the SECCM microsystem in which a microcapillary containing 0.05 M NaCl was fixed on the ITO substrate (curve 1) and one single microcrystal (curve 2); scanning rate of 0.1 V/s; the diameter of the microcapillary orifice is 5 μm.

micropipet contacted with the ITO substrate, which shows no Faraday current but a well-defined charging current of the double layer. When the new micropipet was fixed on the top of a single microcrystal, a symmetrical Faraday current was observed. The cyclic voltammograms of the first 20 cycles are shown as curve 2 in Figure 5. The pretty stable current response shows that there is equilibrium between the evaporation and supplement of water in the SECCM microsystem. When the scanning rate is lower than 100 mV/ s, the Faraday current is in positive proportion to the scanning rate (Figure 6a and b). The results show the characteristic of a “thin-layer” electrolysis, indicating that the redox couples, iron hexacyanides, are well confined in a thin-layer solution outside the single microcrystal due to its good solubility. From eq 1, the

Figure 7. Series of cyclic voltammograms recorded when the microcapillary containing 0.05 mol/L NaCl was moved from the top of one single microcrystal to the different spots on ITO substrate continuously; scanning rate of 0.1 V/s; the diameter of the microcapillary orifice is 5 μm.

finally indicating that the redox species, iron hexacyanides, were taken away when the microcapillary moved. The results elucidate that the microcrystal will partially dissolve to form a “colloid crystal” under the SECCM microenvironment. All-in-Solid Electrochemistry of the Single Microcrystals. The single microcrystals are a solid solution in which iron hexacyanides units substitute NaCl65− units as the electron transfer mediator. In the meantime, the lattice infects including vacancies and interstitials make them ionic conductive. This property indicates that the single microcrystal may have all-in-solid-state electrochemistry, that is, the redox behavior without any liquid electrolyte environment. To investigate its all-in-solid-state electrochemical properties, one single microcrystal was first assembled in a microchip as shown in Figure 8. A microcapillary containing an aqueous solution of 1.0 × 10−3 mol/L Na4Fe(CN)6 and 0.05 mol/L NaCl was positioned on the pair of gold microelectrodes with a gap of 2 μm. After a few cycles of voltammetry, one single microcrystal was synthesized and assembled in situ at the gap. Then, the prepared microchip was placed in a vacuum drier to remove the

Figure 6. (a, c) Cyclic voltammograms obtained in the SECCM microsystem in which a microcapillary containing 0.05 M NaCl was fixed on one single microcrystal; (b, d) relationship between peak current and scanning rate. Scanning rates are lower than 100 mV/s (a) and (b), while higher than 100 mV/s in (c) and (d); the diameter of the microcapillary orifice is 6 μm. 9279

dx.doi.org/10.1021/ac302053x | Anal. Chem. 2012, 84, 9276−9281

Analytical Chemistry

Article

where Ru is the Ohm resistance of the single microcrystal of the solid solution, Rct is the charge transfer resistance, Z′ is the real part of the impedance, and Z′′ is the imaginary part of the impedance. It can be read out that Ru is 0.08 MΩ and Rct is 65 MΩ. From Rct, the apparent charge transfer rate can be obtained as ks = 1.22 × 10−4 cm/s. In the low frequency region, it should abide by the following:33 Z″ = 2σ 2Cd − (R u + R ct) + Z′ Figure 8. Optical microscopy images of microchip (a) and the single microcrystal synthesized at the gap of the pair Au electrodes (b).

where

residential water overnight. Finally, all-in-solid-state electrochemical measurements were performed in a drybox. Figure 9

σ=

⎛ 1 RT 1 ⎞ ⎟⎟ ⎜ × + ⎜ 1/2 n2F 2A 2 DR1/2C R ⎠ ⎝ DO Co

(5)

Since the redox species are immobilized in the lattice of the single microcrystal, DO and DR are actually the apparent diffusion coefficient of couterion Na+ in the solid solution; thus, DO = DR = DNa+. Suppose CironIII = CironII (1.05 × 10−3 mol/L) in the lattice, the apparent diffusion coefficient of the counterion Na+ can be derived as DNa+ = 8.05 × 10−8 cm2/s. Suppose the diffusion layer (δ) is approximately equal to the size of the Au electrode (2 μm), the mass transfer rate (DNa+/δ) matches well with the charge transfer rate (ks). The result indicates it is reasonable that we deal with the all-in-solid-state cyclic voltammetric behavior as a reversible process. Further Discussion on the Charge Transfer in the Microcrystals of Solid Solution. Since the redox sites are actually trapped in the microcrystal lattices, the electron transfer occurs through an “electron hopping” way similar to the cases of modified electrodes, conductive polymers, and the redox monolayer modified on nonconducting surfaces.37−40 In the above-mentioned investigations, the conductive polymer or redox monolayer is exposed to liquid electrolyte solution, in which there exist a large amount of free counterions. When the electron transfer occurs, ionic compensation is always fast and adequate. It is reasonable to attribute the physicochemical parameters to electron transfer because diffusion is not the ratedetermining step (rds). The apparent diffusion coefficient, Dapp, can be very high. As reported in refs 39 and 40, the electron transfer rate is determined to be as high as 0.2 cm/s. The diameter of the SECM tip used in these studies is 5 μm. Suppose the diffusion distance is 10 μm (two times that of the UME diameter), the Dapp for electron hopping can reach to 2 × 10−4 cm2/s (ks = Dapp/δ), since in the steady state, all element steps are proceeding at the same rate. However, there is little free counterion in the solid microcrystal discussed here. The charge compensation is realized through the counterion diffusion between the crystal lattices. Consequently, both electron transfer and ionic diffusion can be the rds. When an all-in-solid-state electrochemical system works, the ionic and electronic currents should match to each other. In the case of the all-in-solid-state, we obtained a Dapp value of 8.05 × 10−8 cm2/s. In the meantime, Dapp is derived as 3.8 × 10−5 cm2/s in the case of the “colloid” state. Note that electron hopping occurs between the iron hexacyanides fixed in the crystal lattice. The Dapp should be an “intrinsic” property of the microcrystal. That means the values should be comparable if the Dapp is the electron hopping coefficient. Obviously, the deduction conflicts with the experimental results. The reasonable explain is that the Dapp difference is caused by the different mobility of couterions, Na+ in our experiments, in the “colloid” and all-in-solid-state statuses. The electron hopping (or transfer) should be fast,

Figure 9. (a) All-in-solid-state cyclic voltammograms of one single microcrystal without exposure to any liquid environment; scan rates are from 0.05 to 0.4 V/s. (b) The linear relationship between the peak currents and the scanning rates.

recorded the all-in-solid-state cyclic voltammograms of one single microcrystal. In contrast to the electrochemical behavior reported above, in the all-in-solid-state, the peak Faraday current is in positive proportion to the scanning rate in the region from 0.01 to 0.5 V/s. The results show that iron hexacyanides are confined in the lattice of the single microcrystal, where the electron hopping process coupled by Na+ diffusion occurs during the polarization of the metal/solid solution interface. From eq 1, the apparent concentration of iron hexacyanides in the solid solution can be calculated as 1.05 × 10−3 mol/L by dealing the redox behavior as a reversible process. To figure out the all-in-solid-state electrochemical properties of the single microcrystals, electrochemical impedance was performed as shown in Figure 10. Based on the equivalent circuit (insert in Figure 10), in the high frequency region, it should abide by the following:33 (Z′ − R u − R ct /2)2 + Z′′2 = (R ct /2)2

(4)

(3)

Figure 10. All-in-solid-state electrochemical impedance spectroscopy of one single microcrystal performed within a frequency range of 0.1 Hz to 100 kHz at the potential of 0.2 V; the amplitude of the alternate voltage was 10 mV. The insert is the equivalent circuit. 9280

dx.doi.org/10.1021/ac302053x | Anal. Chem. 2012, 84, 9276−9281

Analytical Chemistry

Article

(11) Itaya, K.; Uchida, I. Acc. Chem. Res. 1986, 19, 162−168. (12) Imanishi, N.; Morikawa, T.; Kondo, J.; Takeda, Y.; Yamamoto, O.; Kinugasa, N.; Yamagishi, T. J. Power Sources 1999, 79, 215−219. (13) Xiao, X.; Bard, A. J. J. Am. Chem. Soc. 2007, 129, 9610−9612. (14) Xiao, X.; Fan, F.-R. F.; Zhou, J.; Bard, A. J. J. Am. Chem. Soc. 2008, 130, 16669−16677. (15) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001. (16) Böhni, H.; Suter, T.; Schreyer, A. Electrochim. Acta 1995, 40, 1361−1368. (17) Suter, T.; Böhni, H. Electrochim. Acta 1997, 42, 3275−3280. (18) Suter, T.; Böhni, H. Electrochim. Acta 1998, 43, 2843−2849. (19) Krawiec, H.; Vignal, V.; Akid, R. Electrochim. Acta 2008, 53, 5252−5259. (20) Lohrengel, M. M.; Moehring, A.; Pilaski, M.; Fresenius, J. Anal. Chem. 2000, 367, 334−339. (21) Cortes-Salazar, F.; Trauble, M.; Li, F.; Busnel, J.-M.; Gassner, A.L.; Hojeij, M.; Wittstock, G.; Girault, H. H. Anal. Chem. 2009, 81, 6889−6896. (22) Cortes-Salazar, F.; Momotenko, D.; Girault, H. H. Anal. Chem. 2011, 83, 1493−1499. (23) Williams, C. G.; Edwards, M. A.; Colley, A. L.; Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2009, 81, 2486−2495. (24) Ebejer, N.; Schnippering, M.; Colburn, A. W.; Edwards, M. A.; Unwin, P. R. Anal. Chem. 2010, 82, 9141−9145. (25) Schwamborn, S.; Stoica, L.; Chen, X.; Xia, W.; Kundu, S.; Muhler, M.; Schuhmann, W. ChemPhysChem 2010, 11, 74−78. (26) Chen, X.; Eckhard, K.; Zhou, M.; Bron, M.; Schuhmann, W. Anal. Chem. 2009, 81, 7597−7603. (27) Guadagninia, L.; Maljuschb, A.; Chen, X.; Neugebauerb, S.; Tonelli, D.; Schuhmann, W. Electrochim. Acta 2009, 54, 3753−3758. (28) Yan, J. W.; Sun, C. F.; Zhou, X. S.; Tang, Y. A.; Mao, B. W. Electrochem. Commun. 2007, 9, 2716−2720. (29) Yang, D.; Han, L.; Yang, Y.; Zhao, L. B.; Zong, C.; Huang, Y.-F.; Zhan, D.; Tian, Z.-Q. Angew. Chem., Int. Ed. 2011, 50, 8679−8682. (30) Patten, H. V.; Meadows, K. E.; Hutton, L. A.; Iacobini, J. G.; Battistel, D.; McKelvey, K.; Colburn, A. W.; Newton, M. E.; Macpherson, J. V.; Unwin, P. R. Angew. Chem., Int. Ed. 2012, 51, 1−6. (31) Güell, A. G.; Ebejer, N.; Snowden, M. E.; Macpherson, J. V.; Unwin, P. R. J. Am. Chem. Soc. 2012, 134, 7258−7261. (32) Zhan, D.; Fan, F.-R. F.; Bard, A. J. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 12118−12122. (33) Bard, A. J.; Faulker, L. R. Electrochemical Methods Fundamentals and Methods, 2nd ed.; John Wiley & Sons: New York, 2001. (34) Edwards, H. G. M.; Wolstenholme, R.; Wilkinson, D. S.; Brooke, C.; Pepper, M. Anal. Bioanal. Chem. 2007, 387, 2255−2262. (35) Passiniemi, P. J. Solution Chem. 1983, 12, 801−813. (36) Schifino, J.; Olegario, R. M. J. Braz. Chem. Soc. 1993, 4, 102− 107. (37) Lee, W.-Y.; Majda, M.; Brezesinski, G.; Wittek, M.; Mobius, D. J. Phys. Chem. B 1999, 103, 6950−6956. (38) Ghilane, J.; Martin, P.; Janin, M.; Randriamahazaka, H.; Hapiot, P.; Lacroix, J.-C. Electrochem. Commun. 2009, 11, 2304−2307. (39) Hauquier, F.; Ghilane, J.; Fabre, B.; Hapiot, P. J. Am. Chem. Soc. 2008, 130, 2748−2749. (40) Zigah, D.; Herrier, C.; Scheres, L.; Giesbers, M.; Fabre, B.; Hapiot, P.; Zuilhof, H. Angew. Chem., Int. Ed. 2010, 49, 3157−3160.

and the Dapp values obtained from the experiments are actually the apparent diffusion coefficients for Na+ counterions.

■

CONCLUSIONS SECCM was employed to synthesize the single microcrystals of an iron hexacyanides/NaCl solid solution. The single microcrystals had a good redox behavior of “thin-layer” electrolysis in the SECCM microsystem due to the dissolution/evaporation equilibrium in the solid/liquid/gas microenvironment. Because iron hexacyanides substituted the NaCl65− units in the crystal lattice, the single microcrystals had excellent ionic and electronic conductivities. Consequently, the single microcrystals presented excellent all-in-solid-state electrochemistry, in which the electron hopping between the iron hexacyanide sites was coupled by the diffusion of the counterions Na+. The apparent concentration of iron hexacyanides, diffusion coefficient of the counterions Na+ in the single microcrystals, and the electron transfer rate across the metal/solid solution interface were obtained as 1.05 × 10−3 mol/L, 8.05 × 10−8 cm2/s, and 1.22 × 10−4 cm/s, respectively, by using the conventional electrochemical theory of the metal/liquid interface. All the results showed that the unique redox microcrystals have a prospective application in the all-in-solid-state electrochemical microdevices. Furthermore, SECCM was proved as an effective method for the microsynthesis and in situ assembly of functional materials into microdevices.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +865922185797; fax: +865922181906. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support by the National Science Foundation of China (20973142, 21061120456, 21021002), the National Basic Research Program of China (2012CB932900, 2011CB933700), National Project 985 of High Education, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry) is appreciated.

■

REFERENCES

(1) Korvink, J.; Haber, O. MEMS: A Practical Guide to Design, Analysis, and Applications; William Andrew Publishing: New York, 2005. (2) Andersson, H.; Van den Berg, A. Lab-on-Chips for Cellomics: Micro and Nanotechnologies for Life Science; Kluwer: London, 2004. (3) Feringa, B. J. Molecular Switches; Wiley-VCH: Weinheim, Germany, 2001. (4) Balzani, V.; Credi, A.; Venturi, M. Molecular Devices and MachinesA Journey into the Nano World; Wiley-VCH: Weinheim, Germany, 2003. (5) Gellings, P. J.; Bouwmeester, H. J. M. The CRC Handbook of Solid State Electrochemistry; CRC Press: New York, 1997. (6) Jirkovsky, R. Mikrochemie 1934, 15, 331−342. (7) Cha, C. S.; Li, C. M.; Yang, H. X.; Liu, P. F. J. Electroanal. Chem. 1994, 368, 47−50. (8) Scholz, F.; Schroder, U.; Gulaboski, R. Electrochemistry of Immobilized Particles and Droplets; Springer-Verlag: Berlin, Germany, 2005. (9) Karyakin, A. A. Electroanalysis 2001, 13, 813−819. (10) Kulesza, P. J.; Cox, J. A. Electroanalysis 1998, 10, 73−80. 9281

dx.doi.org/10.1021/ac302053x | Anal. Chem. 2012, 84, 9276−9281