Electrodeposited Cobalt-Iron Alloy Thin-Film for Potentiometric Hydrogen Phosphate-Ion Sensor

A cobalt-iron alloy thin-film electrode-based electrochemical hydrogen-phosphate-ion sensor was prepared by electrodepositing on an Au-coated Al2O3 substrate from an aqueous solution of metal-salts. The use of a cobalt-iron alloy electrode greatly improved the hydrogen-ion sensor response performance, i.e., the sensor worked stably for more than 7 weeks and showed a quick response time of several seconds. Among the cobalt and iron alloy systems tested, the electrodeposited Co58Fe42 thin-film electrode showed the best EMF response characteristics, i.e., the sensor exhibited a linear potentiometric response to hydrogen-phosphate ion at the concentration range between 1.0 × 10 and 1.0 × 10 M with the slope of –43 mV/decade at pH 5.0 and at 30 ̊C. A sensing mechanism of the Co-based potentiometric hydrogen-phosphate ion sensor was proposed on the basis of results of instrumental analysis.


Introduction
Phosphate is an important element that is used widely for various industrial applications such as metal plating and food processing.However, phosphate has been a major factor of eutrophication in closed natural systems such as lakes, rivers and bays.Thus, determination of the exact amounts of (hydrogen)-phosphate ions in water systems has become important for environmental analysis.The conventional spectrophotometric method that is widely used has a high level of accuracy but is not suitable for on-site monitoring because of complex procedures including formation of a phosphate-molybdate complex.Considerable efforts have been made to develop highperformance ion sensors for monitoring hydrogenphosphate-ion concentration, such as ion-selective electrodes [1][2][3][4][5][6][7], bio-related systems [8], hydroxyapatite-based electrodes [9], electrochromic devices [10,11], and amperometric devices [12,13].Among these phosphate-ion sensors, ISE (ion-selective electrode)-based sensors using polymer membranes, with calixarene or molybdenumacetylacetonate [14,15] are effective but still have problems in selectivity and/or stability.Xiao et al. reported that a cobalt metal electrode exhibited good potentiometric response and selectivity to dihydrogen-phosphate ion ( 2 4 H P ) among common anions in an acidic medium [4].Meruva  PO  ) ions in different pH solutions [3].It was revealed that a Co-wire-based hydrogen-phosphate ion sensor had a low concentration detection limit and a broad detection range [16][17][18].A disposable on-chip hydrogen-phosphate ion sensor with planar cobalt microelectrodes on a polymer substrate has also been reported [19].These Co-based sensors are particularly attractive as hydrogen-phosphate ion sensors; however, a Co metal-based sensor seems to still lack stability for practical use.The sensing mechanism of the Co-based potentiometric sensor has still not been clarified.
In this study, it was found that the use of a cobalt-iron metal-alloy electrode drastically improved not only the stability of the sensor device but also its sensitivity to hydrogen-phosphate ion.A sensing mechanism of the Co-based potentiometric hydrogen-phosphate ion sensor was also proposed on the basis of characterization of the materials.

Experimental
A commercial alumina (Al 2 O 3 ) sintered plate (SSA-T, 10 × 20 × 0.5 t : mm, Nikkato Ltd., Japan) was used as a substrate for the sensor devices.An Au coating (10 × 10 mm 2 ) on Al 2 O 3 as an electrode for electrodeposition was prepared by painting with Au-paste (TKK Ltd., Japan) and heating at 900˚C.The prepared Au-attached alumina substrate and Pt plate (10 × 20 × 1 t : mm) were used as cathode and anode electrodes, respectively, for electrodepositing.The distance between the two electrodes was The deposited metal films were characterized by means of X-ray diffractometry (XRD, JDX-3500K JEOL), X-ray photoelectron spectroscopy (XPS, JPS-90SX JEOL), and FT-IR spectroscopy (Magna760, Nicolet).Thicknesses of the metal-alloy electrodes were determined by scanning electron microscopy (FE-SEM, ERA8800, ELIONIX).Elemental composition of the prepared metal electrodes was determined by using an X-ray micro-analysis system (EDX, Genesis XM2 EDAX).
An electrochemical cell assembly (Metal electrode| sample solution||Ag/AgCl, KCl (sat.) ) was used for potentiometric sensor measurements.Electromotive force (EMF) was measured by using an electrometer (R8240, Advantest Ltd.) in 0.1 M phthalate buffer solution with various anion concentrations between 1.0 × 10 -6 and 1.0 × 10 -2 M at 30˚C using their potassium salts.Figure 2 shows SEM images of cross-sectional views of the Co and Co 58 Fe 42 thin-film electrodes.They are adhered tightly onto the Au layer.The thicknesses of the Co and Co 58 Fe 42 thin-film electrodes were about 0.8 and 1.0 μm, respectively.

Sensing Performance of the Metal Film Electrode
The Co metal thin-film electrode showed relatively good EMF response characteristic to 2 4 , as has already been reported by many researchers [4,[16][17][18][19].The prepared Co-only sensor responded well to  tion ratio are summarized in Table 1.It was shown out that the sensitivity and stability to 2 4 depends on the composition of the electrode materials.The sensitivity here was expressed in terms of slope in the EMF change (mV/decade), as well as the 90% response time from 1 mM to 3 mM 2 4 .In the Co-Fe system, the slope increased with increasing Fe content in Co-Fe alloy up to 42 at % Fe.Lower concentration limit was obtained using triple the background noise.Faster response time and long-term stability were also observed with increase in Fe content in the Co-Fe alloy up to 42 at %Fe.On the other hand the sensor performances (slope, response time and stability) of the Co-Fe system decreased at higher Fe content of 64 at %Fe (Co 36 Fe 64 ) in the Co-Fe alloy.Among the Co-Fe systems tested, the Co 58 Fe 42 alloy thin-H PO  H PO  was also found that the use of a Co-Fe alloy system electrode improved the response performance.As shown in  H PO .At this point, the sensing mechanism is a hypothesis.The tendency was similar for the magnetic relation for a Co/Fe alloy system [21].This should be come from the electric inter-molecule interaction between the Co/Fe electrode and the hydrogen-phosphate ion which has 3 electrons in a 3p orbital in the P-atom.This might be due to the high hydrogen-phosphate ion adsorption when Co/Fe alloys were introduced, and magnetic properties might therefore be important for the phosphate-ion sensing.
Phosphate-ion response characteristics of the Co 58 Fe 42 thin-film electrode at various pH values are shown in  H PO s of th electrode before and after measurements.and then the Co 2+ state in o-Fe alloy was more sta--film, the C bilized than that in the Co-only electrode.It was also revealed that the state of Co 3+ after sensor measurements was richer than that of Co 2+ in the Co Fe thin 58 42 although they also showed both Co 2+ and Co 3+ states.Figure 8 shows Fe 2p peaks of the Co 58 Fe 42 electrode before and after measurements.Fe peaks before the measurement appeared at 709.4 eV and 711.4 eV, indicating the presence of both states of Fe 2+ and Fe 3+ on the surface of the film.After the sensor measurement, the peak at 709 eV decreased and m nsists of the peak at 711.6 eV of Fe 3+ .Thus, it turned out that the surface of the Co Fe 42 thin-film electrode was in the state of coexistence of Fe 2+ and Fe 3+ before the measurement, and it changed to a richer state of Fe 3+ than that of Fe 2+ after exposure to the electrolyte.
FT-IR spectra of the Co 58 Fe 42 thin-film electrode before and after tively, appeared before the measurement [23,24].Absorptions at around 1030, 1100 and 1600 cm -1 should be indicated for ν( and ν(OH -) from the electrolyte, respectively.Large absorption at 1360 cm -1 for ν(MOOH) should be come from FeOOH, as also observed in Figure 1.
After the sensor measurement, broad rptio ν(Co-O) appeared at around 482 and 553 cm -1 .Absorption of ν(Fe-O) also appeared at 752 and 810 cm -1 .The M-O (M = Co, Fe) band shifted to a higher w abso ns of ave number, indicating that lower-valence metal ions (Co 2+ , Fe 2+ ) are oxidized to higher-valence metal ions (Co 3+ , Fe 3+ ), as also shown by the XPS results.
Moreover, absorption of ν( 2 4 H PO  ) newly appeared at 1040 cm -1 [24].The Co-site seems to be more sensitive to hydrogen-phosphate ion than that of Fe in the Co-Fe alloy, and the sensitivity to was thus decreased at the highest Fe content in alloy, as shown in Table 1.When the XPS and FT-IR spectra were taken into consideration, the surface of the Co-Fe electrode before measurement formed Co 3 O 4 (Co(II), Co(III)) and Fe 3 O 4 (Fe(II), Fe(III)) and changed after measurement to Co(H 2 PO 4 ) 3 (Co(III)) and Fe(H 2 PO 4 ) 3 (Fe(III)) (or FeOOH), respectively.
The slope of the line for EMF versus The two cathodic and anodic electrochemical reactions form a local cell and determine the mixed electrode potential.Figure 10 shows schematic polarization curves for both the anodic and cathodic reactions.As a sensor device, the sensing signal of the Co-Fe electrode should be a more accurate value than the Co-onl the slope of EMF versus    Thus, the Co-Fe alloy electrode should show quick response and long-term stability.However, further investigations are needed to determine the sensing mechanisms of the present electrochemical device.

Conclusion
Potentiometric hydrogen-phosphate ion-se teristics of various Co-based alloy electrodes were vestigated.Among the alloys tested, the Co 58 Fe 42 thin-film electrode showed good hydrogen-phosphate ion-sensing characteristics of fast response rate, high selectivity, and high reproducibility.Electrochemical reactions for the mixed potential mechanism of the Co-based potentiometric sensor of the rforme authors are grateful to the Center for Instrumental Analysis, Kyushu Institute of Technology for XRD, SEM and FT-IR measurements.
fixed at 10 mm.The aqueous solution (250 mL) used for Co-only electrodeposition consisted of 1.1 mol/L CoSO 4 •7H 2 O, 0.022 mol/L H 3 BO 3 , and 0.051 mol/L NaCl with pH of 5.04.For Co-Fe alloy electrodeposition, metal sulfate solutions (A, B, and C) were firstly prepared as follows.Concentration ratios [CoSO 4 •7H 2 O] (mol/L)/[FeSO 4 •7H 2 O] (mol/L) in solutions A, B, and C were 0.090/0.270,0.200/0.166,and 0.267/0.089,respectively.Metal sulfate solution (A, B, or C) consisting of 0.041 mol/L H 3 BO 3 and 0.51 mol/L NaCl with pH of 2.00 and total volume of 250 mL was finally prepared for Co-Fe alloy electrodeposition.All metal thin-film electrodes were electrochemically deposited at a constant current of 100 mA/cm 2 for 180 s at room temperature.

3. 1 .
Structure of the Metal Film Electrode X-ray microanalysis of Co-Fe alloy electrodeposited films prepared from three different solutions (A, B, and C) revealed that the element compositions of Co and Fe in the alloys were at Co 36 Fe 64 , Co 58 Fe 42 and Co 75 Fe 25 , respectively, depending on the Co concentration in the electrolyte bath.

Figure 2 .
Figure 2. SEM images of cross-sectional views of (a) Co and (b) Co 58 Fe 42 thin-film electrodes.

Figure 3 (Figure 3 .Figure 4 .
Figure 3. EMF response tra t of the Co 58 Fe 42 thin-film Figure 4. Stability of (a) Co and (b) Co 58 Fe 42 thin-film sensors to hydrogen-phosphate ion at 30˚C.nsien electrode to hydrogen-phosphate ion at 30˚C.

Figure 6 .
Figure 6.EMF responses of the Co 58 Fe 42 thin-film el trode to various anions.

Figure 7 .
Figure 7. XPS spectra of cobalt on the surfaces of (a) Co and (b) Co 58 Fe 42 thin-film electrodes before and after measurements for hydrogen-phosphate ion sensing.

Figure 8 .
Figure 8. XPS spectra of iron on the surface of the Co 58 Fe 42 thin-film electrode before and after measurements for hydrogen-phosphate ion sensing.

Figure 9 .
Figure 9. FT-IR spectra of the Co 58 Fe 42 thin-film electrode efore and after measurements for hydrogen-phosphate ion b sensing.

Figure 10 .
Figure 10.Schematic polarization curves for cathodic and anodic electrochemical reactions of the sensor electrode.means a Tafel slope of the anodic reaction (1), on a Co(II)-rich surface as like CoO.The Co(II)-rich surface should be stabilized by Fe in the alloy by chemial reaction (3).Reaction (3) should consist of (3-1) a c (3-2).
and Meyerhoff showed that a Co-wire electrode responded well to hydrogen-phosphate (

Table 1 . Hydrogen-phosphate ion sensing characteristics of the Co-Fe thin-film device with various elemental ratios.
 istic to2 4