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Electrochromic Phosphate-Ion Sensor Based on Nickel-Oxide Thin-Film Electrode

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2000 Jpn. J. Appl. Phys. 39 L384

(http://iopscience.iop.org/1347-4065/39/4B/L384)

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Jpn. J. Appl. Phys. Vol. 39 (2000) pp.L 384–L 386Part 2, No. 4B, 15 April 2000c©2000 The Japan Society of Applied Physics

Electrochromic Phosphate-Ion Sensor Based on Nickel-Oxide Thin-Film ElectrodeYouichi SHIMIZU ∗, Tohru YAMASHITA and Satoko TAKASE

Department of Applied Chemistry, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata, Kitakyushu 804-8550, Japan

(Received November 10, 1999; accepted for publication February 28, 2000)

An electrochromic phosphate-ion sensor was developed with a nickel-oxide thin-film electrode. The nickel-oxide (NiO)thin-film electrode exhibited a remarkable change of the transmittance at 530 nm, under a positive potential at+0.55 V vssaturated calomel electrode (SCE), which was dependent on the HPO2−

4 concentration. The change of transmittance at 530 nm,the sensor signal, was almost linear to the logarithm of the HPO2−

4 concentration between 1.0× 10−5 and 1.0× 10−2 M. The90% response time, when the electrode potential was changed from+0.55 to 0 V vs SCE at 1.0× 10−2 M, was about 40 s atroom temperature.

KEYWORDS: electrochromic sensor, nickel-oxide, thin-film, phosphate-ion sensor, opto-electrochemical, optical sensor

∗Corresponding author. E-mail address: shims@che.kyutech.ac.jp

L 384

1. Introduction

It has been becoming very important to detect the exactamount of phosphate-ions, one of the main causes of eu-trophication of closed natural systems, for the protection ofthe global environment. The importance of a phosphate-ionsensor is also increasing in the areas of metal electroplating,dyeing processes, food additive research, and biorelated pro-cesses. Chemical analysis using analytical instruments suchas UV absorption or chemiluminescence is commonly car-ried out for the practical monitoring of phosphate-ion concen-tration, however the system is still complicated and remainsunsuitable for application to feedback control. Many typesof phosphate-ion sensors based on ion-selective electrodes(ISEs) based on metal salts,1–3) metal oxides,4,5) organometalcompounds,6–9) and biorelated systems10) have been investi-gated. However, they still have some problems with respect tosensitivity, selectivity, and stability. We have been attemptingto develop high-performance phosphate-ion sensors based onmetal-oxide ceramics,11,12) which have high chemical and/orthermal stability. Recently, we have developed a new type ofopto-electrochemical phosphate-ion sensors based upon theelectrochromic reaction of metal oxides and ions.13–15) Theelectrochromic sensor based on Co3O4

15) thin-films exhib-ited high sensitivity and high selectivity to other anions suchas Cl− or NO−3 . NiO thin-film is well known as a goodelectrochromic device similar to Co3O4-based devices whichwork with the anodic reactions. Therefore, we have been at-tempting to develop a NiO thin-film electrode. Here, we re-port on the sensing properties as well as the sensing mecha-nism of the electrochromic-type ion sensor based on a nickel-oxide thin-film electrode.

2. Experimental

Figure 1 shows the schematic diagram of an electrochromicsensor device based on a metal-oxide thin-film electrode.Nickel-oxide was prepared by a sol-gel method.15,16) A so-lution of nickel 2-ethylhexanoate (Nickel-octoate: KishidaChemical) was spin-coated onto indium-tin-oxide (ITO:10Ä/sq) glass, and after drying at 160◦C, it was calcined at450◦C for 2 h in an ambient atmosphere. The spin-coatingwas carried out to provide only a single coating on the sen-sor element. The products were characterized by an X-ray

diffraction (XRD: XD-D1, Shimadzu Ltd.) analysis for fivespin-coatings film and scanning electron microscopy (SEM:JEOL, JSM6320F) for the single-coated NiO film.

Electrochemical and optical properties of these oxide thin-film electrodes for sensing phosphate-ions were investigatedat room temperature in a quartz electrochemical cell whichwas placed in a spectrophotometer (UV-200, Shimadzu Ltd.)and connected to a potentiostat (PS-14, Toho Technical Re-search). A saturated calomel electrode (SCE) (or Ag/AgCl)and a Pt-wire were used as a reference and a counter elec-trode, respectively. A sample solution was prepared by mix-ing K2HPO4 with 0.1 M ammonium acetate-ammonia buffersolution adjusted to pH= 9.3.

3. Results and Discussion

3.1 Preparation of NiO thin-filmFigure 2 shows the XRD pattern of the NiO thin-film elec-

trode prepared on an ITO glass at 450◦C. X-ray diffractionanalysis of the NiO film after five spin-coatings revealed that

Fig. 1. Schematic diagram of an optical sensor device using a nickel-oxidethin-film electrode.

the oxide thus prepared exhibited well-crystallized and almostsingle-phase NiO. As shown in Fig. 3, SEM observation ofthe NiO thin-film with one spin-coating revealed that the sur-face of the film was relatively smooth and consisted of finehomogeneous grains of dimensions of about 20–30 nm. Thethickness of the film with one spin-coating was estimated to200–400 nm by SEM.

3.2 Opto-electrochemical propertiesThe oxide (NiO) thin-film electrodes exhibited a change

in the transmittance at 400–800 nm, under an open-circuitpotential, which was dependent on the HPO2−

4 concentration.The reason for this is not clear yet, however it seems to arisefrom the desorption of the HPO2−4 ion to the surface of theNiO thin-film.

Further more, it was found that the change in the transmit-tance was markedly enhanced by the application of positivepotential. Figure 4 shows the change in transmittance spectraof the NiO-based element between 0 V and 0.55 V (vs SCE) atthe HPO2−

4 concentration of 1.0× 10−2 M. The change in thetransmittance spectrum between 400 and 800 nm which wasdependent on both applied potentials was observed for theNiO-based element. The largest change in the transmittancewhich was dependent on the applied potential was observedat around 530 nm. These phenomena which depended on theHPO2−

4 concentration were also observed and this could beapplicable to a new type of opto-electrochemical phosphate-ion sensor.

Figure 5 shows the change in the transmittance at 530 nm(1T) for the NiO electrode as a function of K2HPO4 concen-tration. Each data (1T) were normalized to the transmittanceat 0 V vs SCE;1T = T0 V − T0.55 V, whereT0 V or T0.55 V arethe transmittance at 0 V or 0.55 V vs SCE for 530 nm, respec-tively. As shown in Fig. 5, the change in the transmittance(1T) was almost linear to the logarithm of the HPO2−

4 con-centration between 1.0×10−5 and 1.0×10−2 M. As shown inFig. 6, the sensor responded reversibly and the 90% responsetime, when the electrode potential was changed from+0.55to 0 V vs SCE at 1.0× 10−2 M, was about 40 s at room tem-perature.

Further investigations on the selectivity as well as the sta-bility of NiO-based devices are currently underway.

Fig. 2. XRD pattern of NiO thin-film prepared on an ITO glass with fivespin-coatings after sintering at 450◦C.

Fig. 3. SEM image of the surface of the NiO thin-film electrode.Fig. 6. Response transient of the element using the NiO thin-film electrode

on switching the electrode potential.

Fig. 5. Dependence of1T of the element using the NiO thin-film electrodeon the H2PO4 concentration. (1T = T0 V − T0.55 V; T0 V: Transmittanceat 0 V vs SCE for 530 nm;T0.55 V: Transmittance at 0.55 V vs SCE for530 nm)

Fig. 4. Transmittance spectra of the NiO thin-film electrode under the po-tential at (a): 0 V or (b):+0.55 V vs SCE.

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 2, No. 4B Y. SHIMIZU et al. L 385

valuable.

4. Conclusions

An optical phosphate-ion sensor based on the elec-trochromic properties of NiO thin-film electrode was pro-posed. The transmittance change at 530 nm, the sensor signal,was changed reversibly depending on the HPO2−

4 concentra-tion between 1.0 × 10−5 and 1.0 × 10−2 M at +0.55 V vsSCE. The 90% response time, when the electrode potentialwas changed from+0.55 to 0 V vs SCE at 1.0×10−2 M, wasabout 40 s at room temperature.

Acknowledgements

This study was partially supported by a Grant-in-Aid forScientific Research on Priority Area (No. 282) from the Min-istry of Education, Science, Sports, and Culture of Japan.

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though the presented reactions are highly speculative ones.

NiO+OH− = NiOOH+ e− (1)

NiOOH+ HPO2−4 = NiO2+ H2PO−4 + e− (2)

NiO+OH− + HPO2−4 = NiO2+ H2PO−4 + 2e− (3)

Further investigation on the sensing mechanism may prove

3.3 Sensing mechanismLinear-sweep cyclic voltammograms for the NiO-based el-

ement at various HPO2−4 concentrations were studied fur-ther in order to discuss the electrode reaction. As shown inFig. 7, the electrode exhibited too small a current to observean electrode reaction at the negative electrode potential be-tween−0.8 V and−0.5 V vs SCE. On the other hand, theelectrode exhibited two reversible peaks in the positive poten-tial region. The rapid increase in current> +0.7 V seems toresult from oxygen evolution. The current at around+0.55 Vwas increased with increasing HPO2−

4 concentration. TheNiO electrode exhibited an anodic electrochemical reactionincluding phosphate-ions. Electrode reactions (1–3) with theelectrochromic reaction of NiO are tentatively proposed, al-

Fig. 7. I –V characteristic of the NiO thin-film electrode for various con-centrations of K2HPO4 (sweep rate 5 mV/s).

L 386 Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 2, No. 4B Y. SHIMIZU et al.