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Short Communication

Electrochemical Formation of Prussian Blue in Natural Iron-Intercalated Clay and Cinder Matrixes Jyh-Myng Zen,* Annamalai Senthil Kumar, and Huey-Wen Chen Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan; e-mail: [email protected] Received: October 22, 1999 Final version: December 9, 1999 Abstract

Natural iron-intercalated clays and waste cinder (from steel industry) were converted into an ef®cient catalyst by electrochemical formation of Prussian blue (PB) directly inside the matrix. The basic electrochemical features for the PB-modi®ed electrodes were investigated by means of cyclic voltammetry technique, in terms of Efwhm , surface charge (q), surface excess (G), site-site interaction model, and ®lm resistivity (Rf ), etc. The iron intercalated PB electrodes show superior electrochemical activity and stability over the classical GCE/PB electrodes and were analyzed by the interaction parameter `r'. The more ef®cient electrocatalytic function of the iron intercalated PB electrodes than the GCE/PB was demonstrated with guanine oxidation reaction. Keywords: Clay, Cinder, Prussian blue, Guanine

We report here the ®rst electrochemical evidence for the formation of Prussian blue (PB) directly inside the natural ironriched materials of nontronite and cinder. Earlier studies on clay modi®ed electrodes (CMEs) in the presence of Fe(CN)63 focussed only on the interfacial surface characterization of the modi®ed clays [1±8]. Neither the clay ®lm formed by rapid drying nor the spin-coated CMEs showed the PB formation on the intercalated iron ions [1±8]. Since only a very small amount of clay can be deposited on the electrode surface, the concentration of iron available for PB formation is even lower. Furthermore, the CME prepared by simple evaporation technique has highly porous aerogel structures and the ®lms eventually ``sloughed off'' as platelet to platelet distance increased to a minimum of energy interaction due to osmotic swelling in aqueous solutions [9]. Indeed, the nature of making the electrode, the amount of the material, and its operating concentration, etc., make a crucial modi®cation in the interfacial-structures that may in¯uence the PB formation. In this study, we solve the above problems by preparing the clay=carbon paste electrode (clay=CPE) and cinder=carbon paste electrode (cinder=CPE), which contain a relatively higher amount

of clay or cinder. Two natural iron-riched modi®ers of nontronite, [(Si7.25Al0.75)(Fe2.753‡Al0.85Mg0.33Ti0.05)O20(OH)4X0.98‡, SWa1] [10], and industrial waste cinder were speci®cally used in this study. The iron content of cinder was experimentally determined as 7272.8 ppm=g by atomic absorption spectroscopy. To verify the iron-carbon intercalated nature of the cinder powders, same measurements were also done for 1 M HNO3 treated (ca. 2 h at 60  C) cinder powders. The result of 6921.1 ppm=g is only about a 5 % decrease in iron content con®rming the expectation. Another one of the most studied smectite clays, [montmorillonite, (Si7.84Al0.16)(Fe0.263‡Al3.22Mg0.44Fe0.12)O20(OH)4X0.68‡, SWy1], was also chosen to prepare the SWy-1=CPE for comparison. Note that, even though the amount of iron centers in nontronite is about 7.2 times greater than that in montmorillonite, nontronite has isomorphous substitution in the tetrahedral sheet and montmorillonite is a dioctahedral mineral with substitution mainly in the octahedral sheet [10, 11]. Figure 1 shows the continuous cyclic voltammetric (CV) response of the SWa-1=CPE, SWy-1=CPE, and cinder=CPE in 2 mM K3[Fe(CN)6] and 0.1 M KCl=HCl (pH 2) solution. Scheme 1 illustrates the formation of PB directly into the iron

Scheme 1. # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 1040±0397/00/0704±0542 $17.50‡:50=0

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intercalated matrix. Two redox peaks, A1=C1 and A2=C2, at equilibrium potentials (Eo ) of 0.23 V and 0.85 V were noticed at all three electrodes. For comparison, the PB ®lm on GCE, designated as GCE=PB, was also prepared by electrochemical method at the same conditions except with additional 2 mM Fe3‡ in solution [9]. Since no PB formation was observed without Fe3‡ ions and with Al3‡ ions in 2 mM K3[Fe(CN)6] on both a bare GCE and CPE, the possibility of the formation of PB from Al3‡ (which is abundantly present in the clay) and pure carbon paste or even due to Fe(CN)63 decomposition was ruled out. Note that, once the SWa-1=PB, SWy-1=PB, and cinder=PB were formed, virtually the same peaks were observed when cycled in the absence of Fe(CN)63 . Most important of all, the modi®ed PB electrodes showed a long-term stability, which is critical to the application point of view, and it cannot be expected for the classical GCE=PB electrodes. Moreover, it has been already noticed that continuous leaching of PB was possible on the GCE=PB [12], and this problem could also be fully overcome in the present investigation. The obtained CV spectra of the GCE=PB shows two similar redox couples to those of the SWa-1=CPE, SWy-1=CPE, and cinder=CPE implying the formation of PB ®lms in the interfacial galleries of SWa-1, SWy-1, and cinder materials. It is well known for the classical PB electrodes that the A1=C1 and A2=C2 corresponds to the reversible electrochromic one-electron redox processes of PB to Everitts salt (ES) and PB to Prussian yellow (PY), respectively, in conjunction with the K‡ insertion reactions as shown below [13±15]: A1yC1 : KFeIII ‰Fe2‡ …CN†6 Š ‡ K‡ ‡ e ˆ K2 FeII ‰Fe2‡ …CN†6 Š …PByBlue† Fig. 1. Cyclic voltammograms of the SWa-1=CPE, SWy-1=CPE and cinder=CPE in 2 mM K3[Fe(CN)6] and 0.1 M KCl=HCl (pH 2.0), and GCE in the same solution above with additional 2 mM Fe3‡ at a scan rate of 50 mV=s for 20 cycles.

…ESycolorless†

…1†

A2yC2 : KFeIII ‰Fe2‡ …CN†6 Š ˆ FeIII Fe3‡ …CN†6 ‡ K‡ ‡ e …PByBlue†

…PYyyellow†

…2†

Table 1. Electrochemical features of the PB-modi®ed electrodes. PB-modi®ed Electrodes PB=ES Parameters o

E (mV) [a] DEp (mV) [a] a Efwhm (mV) [a] c Efwhm (mV) [a] ipc yipa q…log ipa †yq…log v† q…log ipc †yq…log v† qa (mC=cm2) [a] qc (mC=cm2) [a] ipa v 1 (mAV 1 s) [a] ipc v 1 (mAV 1 s) [a] Ga (610 8 mol cm 2) [a] Gc (610 8 mol cm 2) [a] ra (6108 cm2 mol 1) [a] rc (6108 cm2 mol 1) [a] Rfa (6103O [b] Rfc (6103O) [b]

PB=PY

SWa-1

SWy-1

Cinder

GCE

SWa-1

SWy-1

Cinder

GCE

228 46 206 66 2.4 0.49 0.69 1.19 1.23 2041 4890 1.23 1.27 1.60 1.53 3.2 1.0

227 34 162 69 2.4 0.68 0.73 1.09 1.13 1540 3923 1.13 1.17 1.75 1.67 2.1 0.6

248 24 126 83 1.6 0.53 0.74 7.19 8.29 1874 3088 7.45 8.59 26.80 23.20 2.2 0.9

188 35 46 164 1.9 0.61 0.58 1.01 1.13 3625 6941 1.05 1.17 1.87 1.64 2.2 1.4

844 34 130 99 1.0 0.61 0.57 1.44 1.29 2505 2426 1.49 1.34 1.33 1.48 5.6 3.9

855 33 147 87 1.0 0.54 0.55 1.24 0.92 1896 1989 1.29 9.57 1.54 2.06 5.9 4.8

856 21 146 68 1.0 0.67 0.49 8.13 7.29 5610 5790 8.43 7.55 23.60 26.30 1.9 1.8

889 41 23 103 1.1 0.70 0.64 1.10 1.05 1441 1610 1.14 1.09 1.74 1.82 5.7 4.6

[a] Based on slow scan rate CV (v ˆ 2 mV=s) after loading; sub-/superscripts: a ˆ anodic and c ˆ cathodic. [b] Calculated from the DEp vs. ip plot at different scan rate. Electroanalysis 2000, 12, No. 7

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The basic electrochemical features for the SWa-1=PB, SWy1=PB, cinder=PB, and GCE=PB measured by CV are listed in Table 1. At slow scan rates (v510 mV=s), the SWa-1=PB, SWy1=PB, and cinder=PB behave as a surface-con®rmed process, and beyond that, they obey a diffusion-controlled behavior. This is indeed the characteristic feature of the operation of an electron hoping process similar to that in ionomers, redox polymers [16], and semiconducting systems [14, 17], and also close to the behavior of the conventional PB electrodes [13±15]. The quantitative electrochemical features of the PB in the ironintercalated matrixes at different scan rates were also similar to that of the classical PB electrodes. The site-site interaction parameter (r) [18, 19] which is calculated from ip ˆ nFyRTqv…4 2rGc †, were positive (i.e., attractive) for all three PB-modi®ed electrodes with the Efwhm values 590yn. Note that q and Gc are surface charge and concentration of the total adsorbed species in the ®lm, respectively. However, some 490yn values were also observed indicating the nonideal electrochemical behavior of these electrodes. This also is evident from the slope of DEp vs. log(v), which is about 150 mV=decade. The nonidealities of the modi®ed electrodes were further expressed in terms of ®lm resistance (Rf ) determined from the slope of the DEp vs. ip plot. These results together with the obtained qc, ipc yv, and Gc indicated the more favorable redox transition of the cathodic processes than that of the anodic one. Note that the SWa-1=PB, SWy-1=PB, and cinder=PB matrices show marked quantitative difference in Efwhm at the cathodic process of A1=C1. The low Efwhm value indicates a more effective formation of PB than that of the bare GCE in the beginning, while the higher q, ip , and Gc values for cinder than those for SWa-1 and SWy-1 reveal the superior rate of formation especially in cinder=CPE. The reason can be explained as follows. Caused by the existence of negative charge on the clay interfacial surface, the electrostatic repulsion with inserting Fe(CN)63 reduces the attractive interaction between the Fe3‡ in the interfacial site and hence hinders the PB formation. The competition between these two forces always operates during the potential cycling in CV. In the anodic cycles, due to the addition of positive charge on the electrode surface, more insertion of Fe(CN)63 occurs and in turn leads to the formation of PB. On the other hand, no such marked charge competitions were ruling in the interfacial sites for the cinder, and therefore, the sole-existed attraction force leads to a high yield of PB. Note that the phenomenon is also con®rmed from the much more positive `r' values for the cinder=PB over GCE=PB. Figure 2 demonstrates one of the catalytic features of the PBmodi®ed electrodes towards the guanine oxidation reaction. As can be seen, both the cinder=PB and SWa-1=PB electrodes show marked enhancement in the peak currents and same catalytic behavior to a much lesser extent was observed at the GCE=PB electrode. The obtained catalytic currents for the guanine oxidation on GCE, cinder, and SWa-1 modi®ed PB electrodes are 19.22, 480.2, and 320.8 mA, respectively. The catalytic current values on cinder=PB and SWa-1=PB were ca. 25 and ca. 15 times higher than the GCE=PB which further corroborates the high potential catalytic application of these reported materials. Note that the strategy used here to prepare the PB ®lm can also be applied to other iron intercalated analogues and one can extend the same increase in the catalytic action for the other compounds like cysteine, nitrite, hydrazine, etc. The analytical assays for the detection of guanine on the cinder=PB using ¯ow injection analysis are in progress. Finally, because of the interfacial stability of the PB ®lm, the SWaElectroanalysis 2000, 12, No. 7

J. Zen et al.

Fig. 2. Cyclic voltammograms with (solid line) and without (dotted line) 1 mM guanine in 0.1 M KCl=HCl (pH 2.0) at the SWa-1=PB, cinder=PB and GCE=PB. Scan rate: 50 mV=s.

1=PB and cinder=PB have wide and potential scope in the research ®elds of chemical sensors, material science, photoelectrochemistry, catalysis, and electrochromism, etc. Numerous applications can readily be imagined from the results and materials discussed above.

Experimental K3[Fe(CN)6] and KCl were ACS certi®ed reagents and used without any further puri®cation. Graphite powder (Aldrich) with a diameter of 1±2 mm and mineral oil (Mallinckrodt) of viscosity 25 cs were used in making carbon paste electrode. Aqueous solutions were prepared with doubly distilled deionized water. Unless otherwise mentioned, a 0.1 M KCl of pH 2 (adjusted with dilute HCl) solution was used for all the electrochemical measurements. Industrial waste cinder was collected as solid cinders enriched with carbon and iron from a local steel mill. The solid cinder was ®rst powdered and then thoroughly washed with doubly distilled water to remove the washable dust materials followed by drying at 50± 55  C for 24 h.

Prussian Blue in Cinder Matrixes Carbon paste of the iron enriched cinder materials were prepared by mixing in the ratio of 5 : 3 : 2 of carbon, oil, and modi®er with a total amount of 1 g. The mixture was homogenized and packed into the electrode holder of 3 mm diameter, which was an impregnated conducting graphite rod enclosed in a Te¯on tube. Then, the electrode surface was smoothed using the ¯at surface of a glazed tile or plate. A suitable check on the achievement of homogenous distribution of the sample particles in the paste was the reproducibility of the peak currents obtained for a number of re®lls with the paste. CHI 660All the electrochemical workstation. Experiments were carried out with a conventional three electrodes setup and a BAS mBAS 100B electrochemical analyzer (Bioanalytical systems, West Lafayette, USA). A BAS Model VC-2 electrochemical cell was employed in these experiments. The three electrodes consisted of either a SWa-1=PB system, cinder=PB-carbon paste electrode or GCE as working electrode, an Ag=AgCl reference electrode (Model RE-5, BAS), and a platinum wire auxiliary electrode. Since dissolved oxygen contributed, no in¯uence, no deaeration was performed. The GCE surface pretreatment was performed using BAS polishing kit followed by ultrasonic cleaning.

Acknowledgement The authors gratefully acknowledge ®nancial support from the National Science Council of the Republic of China under Grants NSC 89-2113-M-005-019 and NSC 89-2113-M-005-020.

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