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Characterization and electrochemical response of sonogel carbon electrode modified with nanostructured zirconium dioxide film
Sensors and Actuators B 137 (2009) 291–296
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Characterization and electrochemical response of sonogel carbon electrode modified with nanostructured zirconium dioxide film Yongjun Chen a , Suzanne Lunsford b,∗ , Dionysios D. Dionysiou a,∗∗ a b
Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, USA Department of Chemistry, Wright State University, Dayton, OH 45435, USA
a r t i c l e
i n f o
Article history: Received 4 September 2008 Received in revised form 16 October 2008 Accepted 21 October 2008 Available online 5 November 2008 Keywords: Zirconium dioxide Sensor Sonogel carbon Catechcol Acetylacetone Electrode
a b s t r a c t A new modified carbon electrode was developed by coating sol–gel derived zirconium dioxide film on the top of graphite carbon electrode, which showed good electrochemical activity and reversibility in the redox process of catechol in acidic aqueous solution (0.1 M H2 SO4 ), even in the presence of ascorbic acid (a common interferent). Good electrochemical behavior, including relatively small oxidation and reduction peak separation (Ep = 71 mV), improved sensitivity, and good operation stability and reproducibility, can be explained by specific physicochemical properties of sol–gel derived ZrO2 film. Such properties include robust and interconnected crystal network structure with high ZrO2 purity, relatively high BET surface area/pore volume, larger interfacial contact area and good adhesion between ZrO2 film and sonogel graphite carbon material. This study proved that zirconium dioxide film prepared by such a sol–gel route is a promising electrode material for the development of novel conducting electrodes based on sonogel graphite carbon for the detection of neurotransmitters. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Development of accurate and smart electrochemical sensors for in situ detection of neurotransmitters (i.e., catechol and dopamine) has been recognized as an important aspect in the clinical and pharmaceutical field because the nervous system can be greatly affected by the change in the concentration of such chemicals in the human body [1–4]. Currently, there is an increased interest in employing sonogel carbon conducting matrix as electrode material because of its many advantages such as high conductivity, relative chemical inertness, and amenable chemical and biological modification [3,5,6]. Recently, we have successfully developed a new class of TiO2 film-modified sonogel carbon electrodes using a sol–gel method to coat porous TiO2 films on top of sonogel graphite carbon electrode [3,7]. Porous anatase TiO2 film has been proved to be a suitable semiconductor metal oxide to detect neurotransmitters (i.e., catechol and dopamine) due to its specific texture for good adsorption of neurotransmitters, good electrical conductivity and robust mechanical strength for long-term electrode stability [3,7]. However, some metal oxides with certain promising properties, such as high hardness, good thermal stability, chemical inertness, and good conductivity, have not yet been explored. Since there are many chal-
∗ Corresponding author. Tel.: +1 937 775 2855; fax: +1 937 775 2717. ∗∗ Corresponding author. Tel.: +1 513 556 0724; fax: +1 513 556 2599. E-mail addresses: [email protected] (S. Lunsford), [email protected] (D.D. Dionysiou). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.10.022
lenging requirements to guarantee reliability in the application of electrochemical sensors in various environments, including sensitivity, selectivity and stability, it is necessary to further explore and develop novel metal oxides as effective electrode materials, so as to improve our current electrode fabrication method. Recently, carbon electrodes modified by ZrO2 or doped ZrO2 have been studied by few research groups in the fields of electrochemical sensors and solid electrochemistry [8–11]. Considering the fact that ZrO2 as electrode material has many advantages, including low toxicity, chemical inertness, good thermal stability, high hardness and good wear resistance [12–14], which are beneficial properties to the enhancement of electrode endurance, especially at extreme conditions, in this study, we report a new sonogel carbon electrode modified by sol–gel derived ZrO2 film. Unlike anatase TiO2 with “narrow” band gap energy (i.e., 3.2 eV), which may have oxidizing effects on the detected chemicals when it is activated by the UV fraction (i.e., wavelength from 300 to 380 nm) of the solar light [15,16], ZrO2 film possesses another advantage as electrode material due to its wide band gap energy (i.e., 5.78 eV for t-ZrO2 ) [17,18], which ensures almost no photocatalytic activity under solar light radiation. 2. Experimental part 2.1. Synthesis of ZrO2 films Zirconium(IV) propoxide solution 70 wt.% in 1-propanol (Aldrich), ethanol (200 proof, Aldrich), acetylacetone (99%, Fisher
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Scientific) and deionized water were used for the preparation of the zirconium sol. The molar ratio of zirconium(IV) propoxide:water:acetylacetone: ethanol was 2:4:1:62. The final zirconium propoxide concentration was 0.43 M. The materials and method to prepare sonogel carbon (SGC) electrode were similar to those of our former publication [7]. The ZrO2 gel was coated on the tip of SGC electrode by dip coating. The relative humidity and room temperature in the lab were ∼50% and ∼23 ◦ C, respectively. After coating, the coated carbon electrodes were calcined in a programmable high temperature furnace. The furnace temperature was incremented at a ramp rate of 15 ◦ C min−1 to 100 ◦ C and hold for 15 min, then continued to increase at a ramp rate of 3.0 ◦ C min−1 until 500 ◦ C and was held at this value for 30 min. Finally, the films were cooled naturally to room temperature. 2.2. Characterization of ZrO2 films and sonogel carbon material The crystal phase composition of the ZrO2 coated on sonogel carbon material was determined by X-ray diffraction (XRD) using a Siemens Kristalloflex D500 diffractometer with Cu K␣ radiation. Powder samples used for XRD and TEM analysis were obtained from calcined sonogel graphite carbon powder mixed with zirconium sol (about 3 ml zirconium sol to impregnate 1 g sonogel graphite carbon powders). The heat treatment (calcination) procedure was the same as that for the electrode preparation. The morphology of ZrO2 and that of the sonogel carbon material was characterized by an environmental scanning electron microscope (ESEM, Philips XL 30 ESEM-FEG). The crystal and pore morphology of the films were determined by a JEM-2010F (JEOL) High Resolution-Transmission Electron Microscope (HR-TEM) with field emission gun at 200 kV. All powder samples were dispersed in methanol (High Performance Liquid Chromatography (HPLC) grade, Pharmco) using an ultrasonic cleaner (2510R-DH, Bransonic) for 5 min and fixed on a carboncoated copper grid (LC200-Cu, EMS). The specific surface area and pore structure of the ZrO2 film and graphite carbon material were measured using a Micromeritics TriStar 3000 Gas Adsorption Analyzer. Powder samples were prepared at the same conditions as the electrode. 2.3. Electrochemical sensor response Electrochemical sensor response to catechol was carried out using an Electrochemical Workstation (Epsilon, Bioanalytical Systems) based on Cyclic voltammetry (CV) with three electrode compartment cell. The electrochemical compartment cell was composed of a Pt auxiliary electrode (Bioanalytical Systems), a Ag/AgCl reference electrode (Bioanalytical Systems), and the modified working ZrO2 /SGC electrode. 5 mM catechol (C6 H4 (OH)2 , Fluka) solution was prepared in 0.1 M sulfuric acid (Aldrich) with deion-
Fig. 1. X-ray diffraction spectra of (a) carbon electrode and (b) ZrO2 film modified carbon electrode.
ized water. 5 mM catechol solution mixed with 5 mM ascorbic acid (C6 H8 O6 , Aldrich), a common interferent, was also prepared in 0.1 M sulfuric acid (Aldrich) with deionized water. The scan rate of CV was 100 mV s−1 . 3. Results and discussion Fig. 1 shows the X-ray diffraction results of (a) sonogel carbon material and (b) ZrO2 coated on the sonogel carbon material. The four sharp peaks at 2-theta angle of 26.4◦ , 43.5◦ , 44.2◦ , and 54.5◦ are assigned to (0 0 2), (1 0 0), (1 0 1) and (0 0 4) planes of graphite, respectively [19,20], while three sharp peaks at 2 angle of 30.2◦ , 50.2◦ , and 58.9◦ are assigned to (1 0 1), (2 0 0), and (1 0 3) planes of tetragonal zirconium dioxide (t-ZrO2 ) (Fig. 1(b)) [17]. The relatively broad peak width at half maximum intensity (FWHM) at 30.2◦ suggested a fine crystalline size, which has been confirmed based on Scherrer equation (14.3 nm) and TEM image of ZrO2 particles attached on the sonogel carbon material. From Fig. 2(a) and (b), it can be observed that graphite has a larger particle size, typically above 500 nm; small nanoparticles with size from 10 to 18 nm attached on the larger particles are ZrO2 particles. Consequently, it is reasonable to conclude that small nanoparticles are ZrO2 crystallites. It has been reported that, after 500 ◦ C calcination, zirconium acetylacetonate can be completely crystallized and most organic composition can be eliminated [21]. As a result, it is believed that sol gel derived ZrO2 coated on the sonogel carbon electrode can be well crystallized into tetragonal phase with high purity. In order to obtain information on the optical properties of the ZrO2 film, UV–vis spectroscopy was used for its characterization. Fig. 3 shows UV–vis spectra of the ZrO2 film coated on the borosilicate glass.
Fig. 2. TEM images of (a) carbon material and (b) ZrO2 mixed with carbon material.
Y. Chen et al. / Sensors and Actuators B 137 (2009) 291–296
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Fig. 3. UV–vis spectra of ZrO2 film coated on the borosilicate glass (four layers and two dip coatings).
Before sample analysis, bare borosilicate glass was used as “blank”, so as to eliminate the effect of the substrate on the optical properties of the ZrO2 film. The transmittance of ZrO2 film was above 85% in the wavelength range of 300–800 nm, which suggested that the as-prepared ZrO2 film on the top of graphite carbon electrode was of high optical transparency. It has been well established that good light absorbance is an important prerequisite to obtain high catalytic activity of the semiconductor film (i.e., TiO2 film); a weak UV–vis light absorbance is in fact beneficial to the maintenance of catalytic inertness to avoid degradation of detected chemicals under UV–vis light radiation. Fig. 4 shows the surface morphology of ZrO2 film coated on the sonogel carbon material. The surface morphology of ZrO2 film with one dip coating on the sonogel carbon material is similar to that of sonogel carbon without ZrO2 (figure is not shown). This can be explained by (a) very small film thickness of ZrO2 with one dip coating, and (b) high optical transparency of ZrO2 film (refer to Fig. 3). When multiple dip coating layers (i.e., two dip coating layers) of ZrO2 film are coated on the surface of graphite material, smooth ZrO2 film on-top of graphite carbon electrode can be observed. During the period of aging process for the coated zirconium alkoxide sol, hydrolysis and condensation reactions take place, which lead to the formation of Zr–O–Zr network. As a result, the coated ZrO2 film has in fact interconnected tetragonal crystalline network after high calcination temperature (i.e., 500 ◦ C). On the other hand, many “cracks” of the ZrO2 film are also observed. Formation of such “cracks” was due to the uneven surface of graphite carbon material. Therefore, the structural integrity of the as-prepared ZrO2 film
Fig. 5. Element composition of the surface of the sonogel carbon electrode modified by ZrO2 film.
can be greatly affected by the surface morphology of the graphite carbon material. Fig. 5 shows the EDS composition analysis on the surface of the sonogel carbon electrode modified by ZrO2 film. Four types of elements can be observed: carbon, zirconium, silicon, and oxygen. The total molar ratio of oxygen (O) to (silicon (Si) + zirconium (Zr)) is near 2, which may suggest that there are ZrO2 and SiO2 on the surface of carbon electrode. Combined with the result of XRD crystalline phase (Fig. 1), EDS results further confirm that there is indeed ZrO2 film on the top of carbon electrode. On the other hand, it can be noticed that the total molar ratio of oxygen to (silicon + zirconium) was in fact 2.34, slightly higher than 2. This may be due to the existence of residue compositions containing C–O bond, induced by incomplete pyrolysis of organic compositions (i.e., acetylacetone) in the coated gel. Fig. 6(i) and (ii) shows the results of pore size distribution and nitrogen adsorption and desorption isotherms of (a) carbon material and (b) ZrO2 , respectively. Based in Fig. 6(ii) (a), sonogel carbon material is classified as type I (IUPAC), which suggests predominately microporous structure. A remarkable uptake and high-adsorbed volume at relative pressure from 0.01 to 0.1 were attributed to the existence of high volume of micropores in the carbon material [22]. On the other hand, based in Fig. 6(ii) (b), ZrO2 material was classified as type IV (IUPAC), which indicates mesoporous structure. ZrO2 film exhibited a narrow pore size dis-
Fig. 4. Scanning electron micrographs of the surface of ZrO2 film modified sonogel carbon electrodes.
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Fig. 6. (i) Pore size distribution curve of (a) carbon material and (b) ZrO2 (500 ◦ C calcination) and (ii) nitrogen adsorption and desorption isotherms of (a) carbon material and (b) ZrO2 .
Fig. 7. Geometry configuration of ZrO2 film immobilized on top of sonogel carbon electrode.
tribution with 2.6 nm of BJH adsorption average pore diameter (refer to Fig. 6(i) (b)). Formation of such a mesoporous structure was in fact attributed to inter-particle pore formation induced by “uniform” particle size [23,24]. The total pore volume of graphite carbon material was 0.0400 cm3 /g (see results reported in Table 1), while the micropore volume was 0.0223 cm3 /g. Compared with mesoporous volume, the slightly higher micropore volume of sonogel graphite carbon material suggests that there is a larger number of micropores in the sonogel graphite carbon, which is also in agreement with the result that the highest BJH adsorption pore volume corresponds to a pore size of less than 2 nm (refer to Fig. 6(i) (a)). In addition, considering some important factors, including low surface tension of ethanol solvent and high capillary force induced by large number of micropores in the sonogel graphite material, are beneficial to filling the pores of the graphite electrode by the precursor zirconium sol, it is believed that good interlocking of ZrO2 in the micro-mesopores of sonogel graphite carbon is another important reason for the good mechanical stability of the ZiO2 film (see Fig. 7). Moreover, good filling of ZiO2 into the pores of graphite leads to an enhanced interfacial contact area between ZrO2 crystallites and
graphite material, which is beneficial to effective electron transfer between them. Fig. 8(b–d) shows results of the electrochemical characterization of sonogel carbon electrode modified by zirconium dioxide for the detection of catechol. Sharp redox peak currents can be observed at such an electrode, even in the existence of ascorbic acid (a common interferent). The anodic and cathodic peak potentials for catechol were at 507 mV (Epa , anodic potential) and 436 mV (Epc , cathodic potential), respectively. The peak separation (Ep ) of catechol was only 71 mV, which is comparable to that in our former report [3]. From Fig. 8(c and d), it can be observed that the oxidation peaks of catechol and ascorbic acid are exceptionally resolved. Control test results showed that weak voltammetric redox peaks of catechol could be occasionally recorded at bare sonogel carbon electrodes. As an example, Fig. 8(a) shows one of the cyclic voltammograms of catechol at the bare sonogel carbon electrode. Therefore, the existence of ZiO2 film on the top of sonogel carbon electrode can lead to a good reversible electrochemical response to catechol. In addition, cyclic voltammogram of catechol at the sonogel carbon electrode modified by zirconium dioxide is similar in shape to that at the sonogel carbon electrode modified by porous titanium dioxide film [3,7], which suggests that there is the same reaction pathway at these electrodes [25]. Small Ep of catechol suggests fast heterogeneous electron transfer [26], which is induced by several factors, including high purity of tetragonal ZrO2 crystallites, interconnected crystal network, and larger interface contact area between ZrO2 nanocrystallites and graphite carbon material (see Fig. 7). Because catechol has groups containing oxygen, which can lead to a good affinity of this chemical to ZrO2 [13,27,28], strong interaction of catechol with the active sites of the as-prepared nanostructured ZrO2 film was an important prerequisite to lead to such a good electrochemical sensor behavior. Moreover, based on the above discussion on the properties of electrode materials, it is reasonable to conclude that good adhesion between ZrO2 film
Table 1 BET surface area and pore structure of ZrO2 film and sonogel graphite carbon matrix. BET surface area (m2 /g) ZrO2 Graphite a b c d
69.0 62.2
Total pore volumea (cm3 /g) 0.0531 0.0400
Single point adsorption total pore volume. Based on adsorption average pore width (4 V/A by BET). Based on density of 5.9 g/cm3 of ZrO2 . Based on density of 2.1 g/cm3 of graphite.
t-Plot micropore volume (cm3 /g) – 0.02227
Porosity c
23.9% 7.8%d
Pore sizeb (nm) 3.1 2.6
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Fig. 8. Cyclic voltammogram of bare sonogel carbon electrode (a) 5 mM catechol solution, 2 scans and cyclic voltammograms of ZrO2 modified sonogel carbon electrode (b) 5 mM catechol solution, 20 scans; (c) 5 mM catechol solution mixed with 5 mM ascorbic acid solution, 2 scans; (d) 5 mM catechol solution mixed with 5 mM ascorbic acid solution, 20 scans; scan rate 100 mV/s, electrolyte 0.1 M sulfuric acid.
and the graphite carbon matrix as well as the formation of ZrO2 film with special microstructure, including high BET surface area (69 m2 /g) and tetragonal crystal phase for high electrical conductivity, are other important reasons for good electrochemical behavior, excellent stability and reproducibility, good sensitivity, and less edge effect. Considering that the texture of ZrO2 film can be further improved by optimizing the preparation parameters in such a sol–gel route, further studies are underway to develop porous ZrO2 films with enhanced BET surface area and pore volume to improve the sensitivity of this kind of electrode for its application as electrochemical sensor. In addition, optimizing the texture of sonogel carbon matrix to improve interfacial contact area and structural integrity of ZrO2 film should not be ignored in the development of this type of electrode in the future. 4. Conclusions Sol–gel derived ZrO2 film formed from zirconium alkoxide precursor proved to be a valuable wide band gap metal oxide on the detection of catechol. Good electrochemical response is correlated to good physicochemical properties of as-prepared tetragonal ZrO2 film including robust crystal structure, high purity and interconnected crystal network of ZrO2 film, larger interfacial contact area and good adhesion between ZrO2 and graphite carbon material, and relatively high BET surface area for the adsorption of large number of catechol molecules. This study provided an alternative metal oxide based electrode material, which has been proven to be valuable in the fabrication of graphite carbon based electrode in the detection of neurotransmitters.
Acknowledgements This work was funded in part by the Ohio State University Research Foundation and in part by an NSF CAREER Award (BES # 0448117) to D.D. Dionysiou. References [1] H.B. Mark, N.F. Atta, Y.L. Ma, L.L. Petticrew, H. Zimmer, Y. Shi, S.K. Lunsford, J.F. Rubinson, A. Galal, The electrochemistry of neurotransmitters at conducting organic polymer electrodes: electrocatalysis and analytical applications, Bioelectrochem. Bioenerg. 38 (1995) 229. [2] N.F. Atta, I. Marawi, K.L. Petticrew, H. Zimmer, H.B. Mark Jr., A. Galal, Electrochemistry and detection of some organic and biological molecules at conducting polymer electrodes. Part 3. Evidence of the electrocatalytic effect of the heteroatom of the poly (hetetroarylene) at the electrode/electrolyte interface, J. Electroanal. Chem. 408 (1996) 47–52. [3] S.K. Lunsford, H. Choi, J. Stinson, A. Yeary, D.D. Dionysiou, Voltammetric determination of catechol using a sonogel carbon electrode modified with nanostructured titanium dioxide, Talanta 73 (1 (August 15)) (2007) 172–177. [4] M.D.P.T. Sotomayor, A.A. Tanaka, L.T. Kubota, Development of an enzymeless biosensor for the determination of phenolic compounds, Anal. Chim. Acta 455 (2002) 215–223. [5] Cordero-Rando FM.d.M., J.L. Hidalgo-Hidalgo de Cisneros, E. Blanco, I. Naranjo-Rodrıˇıguez, The sonogel-carbon electrode as a sol–gel graphite-based electrode, Anal. Chem. 74 (2002) 2423–2427. [6] H. Zejli, J.L. Hidalgo-Hidalgo de Cisnerosc, I. Naranjo-Rodriguez, B. Liu, K.R. Temsamani, J.L. Marty, Phenol biosensor based on sonogel-carbon transducer with tyrosinase alumina sol–gel immobilization, Anal. Chim. Acta 6 (12) (2008) 198–203. [7] Yongjun Chen, Suzanne Lunsford, D.D. Dionysiou, Photocatalytic activity and electrochemical sensor response of TiO2 film with macro/mesoporous texture, Thin Solid Films 516 (2008) 7930–7936. [8] Z.Y. Sun, X.R. Zhang, N. Na, Z.M. Liu, B.X. Han, G.M. An, Synthesis of ZrO2 –carbon nanotube composites and their application as chemiluminescent sensor material for ethanol, J. Phys. Chem. B 110 (2006) 13410–13414.
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Biographies Yongjun Chen received his Ph.D. degree in environmental engineering from the Department of Civil and Environmental Engineering at University of Cincinnati, United States, in 2007. During the time of this project, Dr. Yongjun Chen was a postdoctoral fellow working in the groups of Dr. Dionysiou and Dr. Lunsford. Recently, Dr. Yongjun Chen obtained the qualification of “Outstanding Young Teacher” in the College of Environmental Science and Engineering at the South China University of Technology in China. His current research interests include electrochemical sensors, advanced oxidation technologies, and dye-sensitized solar cells. Suzanne K. Lunsford received her B.S. degree in Chemistry from Xavier University in 1990. She received her Ph.D. from the University of Cincinnati under the tutelage of Dr. Harry B. Mark, Jr., in 1995. She is presently an Associate Professor in the Department of Chemistry at Wright State University in Dayton, Ohio. Her current research involves the development of modified conducting, and nanoparticle (sol–gel methods)-based working sensor electrodes to detect common biological molecules of interest utilizing electrochemical skills. Dionysios (Dion) D. Dionysiou is a Professor of Environmental Engineering and Science in the Department of Civil and Environmental Engineering at the University of Cincinnati, Cincinnati, Ohio. He received a B.S. degree (Diploma) from the National Technical University of Athens (N.T.U.A.) Greece, and an M.S. degree from Tufts University, both in Chemical Engineering. He received his Ph.D. degree in Environmental Engineering from the University of Cincinnati in 2001. He performs research in the fields of (i) advanced oxidation technologies, (ii) water quality, monitoring, treatment and purification, and (iii) environmental nanotechnology.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Characterization and electrochemical response of sonogel carbon electrode modified with nanostructured zirconium dioxide film Yongjun Chen a , Suzanne Lunsford b,∗ , Dionysios D. Dionysiou a,∗∗ a b
Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, USA Department of Chemistry, Wright State University, Dayton, OH 45435, USA
a r t i c l e
i n f o
Article history: Received 4 September 2008 Received in revised form 16 October 2008 Accepted 21 October 2008 Available online 5 November 2008 Keywords: Zirconium dioxide Sensor Sonogel carbon Catechcol Acetylacetone Electrode
a b s t r a c t A new modified carbon electrode was developed by coating sol–gel derived zirconium dioxide film on the top of graphite carbon electrode, which showed good electrochemical activity and reversibility in the redox process of catechol in acidic aqueous solution (0.1 M H2 SO4 ), even in the presence of ascorbic acid (a common interferent). Good electrochemical behavior, including relatively small oxidation and reduction peak separation (Ep = 71 mV), improved sensitivity, and good operation stability and reproducibility, can be explained by specific physicochemical properties of sol–gel derived ZrO2 film. Such properties include robust and interconnected crystal network structure with high ZrO2 purity, relatively high BET surface area/pore volume, larger interfacial contact area and good adhesion between ZrO2 film and sonogel graphite carbon material. This study proved that zirconium dioxide film prepared by such a sol–gel route is a promising electrode material for the development of novel conducting electrodes based on sonogel graphite carbon for the detection of neurotransmitters. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Development of accurate and smart electrochemical sensors for in situ detection of neurotransmitters (i.e., catechol and dopamine) has been recognized as an important aspect in the clinical and pharmaceutical field because the nervous system can be greatly affected by the change in the concentration of such chemicals in the human body [1–4]. Currently, there is an increased interest in employing sonogel carbon conducting matrix as electrode material because of its many advantages such as high conductivity, relative chemical inertness, and amenable chemical and biological modification [3,5,6]. Recently, we have successfully developed a new class of TiO2 film-modified sonogel carbon electrodes using a sol–gel method to coat porous TiO2 films on top of sonogel graphite carbon electrode [3,7]. Porous anatase TiO2 film has been proved to be a suitable semiconductor metal oxide to detect neurotransmitters (i.e., catechol and dopamine) due to its specific texture for good adsorption of neurotransmitters, good electrical conductivity and robust mechanical strength for long-term electrode stability [3,7]. However, some metal oxides with certain promising properties, such as high hardness, good thermal stability, chemical inertness, and good conductivity, have not yet been explored. Since there are many chal-
∗ Corresponding author. Tel.: +1 937 775 2855; fax: +1 937 775 2717. ∗∗ Corresponding author. Tel.: +1 513 556 0724; fax: +1 513 556 2599. E-mail addresses: [email protected] (S. Lunsford), [email protected] (D.D. Dionysiou). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.10.022
lenging requirements to guarantee reliability in the application of electrochemical sensors in various environments, including sensitivity, selectivity and stability, it is necessary to further explore and develop novel metal oxides as effective electrode materials, so as to improve our current electrode fabrication method. Recently, carbon electrodes modified by ZrO2 or doped ZrO2 have been studied by few research groups in the fields of electrochemical sensors and solid electrochemistry [8–11]. Considering the fact that ZrO2 as electrode material has many advantages, including low toxicity, chemical inertness, good thermal stability, high hardness and good wear resistance [12–14], which are beneficial properties to the enhancement of electrode endurance, especially at extreme conditions, in this study, we report a new sonogel carbon electrode modified by sol–gel derived ZrO2 film. Unlike anatase TiO2 with “narrow” band gap energy (i.e., 3.2 eV), which may have oxidizing effects on the detected chemicals when it is activated by the UV fraction (i.e., wavelength from 300 to 380 nm) of the solar light [15,16], ZrO2 film possesses another advantage as electrode material due to its wide band gap energy (i.e., 5.78 eV for t-ZrO2 ) [17,18], which ensures almost no photocatalytic activity under solar light radiation. 2. Experimental part 2.1. Synthesis of ZrO2 films Zirconium(IV) propoxide solution 70 wt.% in 1-propanol (Aldrich), ethanol (200 proof, Aldrich), acetylacetone (99%, Fisher
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Scientific) and deionized water were used for the preparation of the zirconium sol. The molar ratio of zirconium(IV) propoxide:water:acetylacetone: ethanol was 2:4:1:62. The final zirconium propoxide concentration was 0.43 M. The materials and method to prepare sonogel carbon (SGC) electrode were similar to those of our former publication [7]. The ZrO2 gel was coated on the tip of SGC electrode by dip coating. The relative humidity and room temperature in the lab were ∼50% and ∼23 ◦ C, respectively. After coating, the coated carbon electrodes were calcined in a programmable high temperature furnace. The furnace temperature was incremented at a ramp rate of 15 ◦ C min−1 to 100 ◦ C and hold for 15 min, then continued to increase at a ramp rate of 3.0 ◦ C min−1 until 500 ◦ C and was held at this value for 30 min. Finally, the films were cooled naturally to room temperature. 2.2. Characterization of ZrO2 films and sonogel carbon material The crystal phase composition of the ZrO2 coated on sonogel carbon material was determined by X-ray diffraction (XRD) using a Siemens Kristalloflex D500 diffractometer with Cu K␣ radiation. Powder samples used for XRD and TEM analysis were obtained from calcined sonogel graphite carbon powder mixed with zirconium sol (about 3 ml zirconium sol to impregnate 1 g sonogel graphite carbon powders). The heat treatment (calcination) procedure was the same as that for the electrode preparation. The morphology of ZrO2 and that of the sonogel carbon material was characterized by an environmental scanning electron microscope (ESEM, Philips XL 30 ESEM-FEG). The crystal and pore morphology of the films were determined by a JEM-2010F (JEOL) High Resolution-Transmission Electron Microscope (HR-TEM) with field emission gun at 200 kV. All powder samples were dispersed in methanol (High Performance Liquid Chromatography (HPLC) grade, Pharmco) using an ultrasonic cleaner (2510R-DH, Bransonic) for 5 min and fixed on a carboncoated copper grid (LC200-Cu, EMS). The specific surface area and pore structure of the ZrO2 film and graphite carbon material were measured using a Micromeritics TriStar 3000 Gas Adsorption Analyzer. Powder samples were prepared at the same conditions as the electrode. 2.3. Electrochemical sensor response Electrochemical sensor response to catechol was carried out using an Electrochemical Workstation (Epsilon, Bioanalytical Systems) based on Cyclic voltammetry (CV) with three electrode compartment cell. The electrochemical compartment cell was composed of a Pt auxiliary electrode (Bioanalytical Systems), a Ag/AgCl reference electrode (Bioanalytical Systems), and the modified working ZrO2 /SGC electrode. 5 mM catechol (C6 H4 (OH)2 , Fluka) solution was prepared in 0.1 M sulfuric acid (Aldrich) with deion-
Fig. 1. X-ray diffraction spectra of (a) carbon electrode and (b) ZrO2 film modified carbon electrode.
ized water. 5 mM catechol solution mixed with 5 mM ascorbic acid (C6 H8 O6 , Aldrich), a common interferent, was also prepared in 0.1 M sulfuric acid (Aldrich) with deionized water. The scan rate of CV was 100 mV s−1 . 3. Results and discussion Fig. 1 shows the X-ray diffraction results of (a) sonogel carbon material and (b) ZrO2 coated on the sonogel carbon material. The four sharp peaks at 2-theta angle of 26.4◦ , 43.5◦ , 44.2◦ , and 54.5◦ are assigned to (0 0 2), (1 0 0), (1 0 1) and (0 0 4) planes of graphite, respectively [19,20], while three sharp peaks at 2 angle of 30.2◦ , 50.2◦ , and 58.9◦ are assigned to (1 0 1), (2 0 0), and (1 0 3) planes of tetragonal zirconium dioxide (t-ZrO2 ) (Fig. 1(b)) [17]. The relatively broad peak width at half maximum intensity (FWHM) at 30.2◦ suggested a fine crystalline size, which has been confirmed based on Scherrer equation (14.3 nm) and TEM image of ZrO2 particles attached on the sonogel carbon material. From Fig. 2(a) and (b), it can be observed that graphite has a larger particle size, typically above 500 nm; small nanoparticles with size from 10 to 18 nm attached on the larger particles are ZrO2 particles. Consequently, it is reasonable to conclude that small nanoparticles are ZrO2 crystallites. It has been reported that, after 500 ◦ C calcination, zirconium acetylacetonate can be completely crystallized and most organic composition can be eliminated [21]. As a result, it is believed that sol gel derived ZrO2 coated on the sonogel carbon electrode can be well crystallized into tetragonal phase with high purity. In order to obtain information on the optical properties of the ZrO2 film, UV–vis spectroscopy was used for its characterization. Fig. 3 shows UV–vis spectra of the ZrO2 film coated on the borosilicate glass.
Fig. 2. TEM images of (a) carbon material and (b) ZrO2 mixed with carbon material.
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Fig. 3. UV–vis spectra of ZrO2 film coated on the borosilicate glass (four layers and two dip coatings).
Before sample analysis, bare borosilicate glass was used as “blank”, so as to eliminate the effect of the substrate on the optical properties of the ZrO2 film. The transmittance of ZrO2 film was above 85% in the wavelength range of 300–800 nm, which suggested that the as-prepared ZrO2 film on the top of graphite carbon electrode was of high optical transparency. It has been well established that good light absorbance is an important prerequisite to obtain high catalytic activity of the semiconductor film (i.e., TiO2 film); a weak UV–vis light absorbance is in fact beneficial to the maintenance of catalytic inertness to avoid degradation of detected chemicals under UV–vis light radiation. Fig. 4 shows the surface morphology of ZrO2 film coated on the sonogel carbon material. The surface morphology of ZrO2 film with one dip coating on the sonogel carbon material is similar to that of sonogel carbon without ZrO2 (figure is not shown). This can be explained by (a) very small film thickness of ZrO2 with one dip coating, and (b) high optical transparency of ZrO2 film (refer to Fig. 3). When multiple dip coating layers (i.e., two dip coating layers) of ZrO2 film are coated on the surface of graphite material, smooth ZrO2 film on-top of graphite carbon electrode can be observed. During the period of aging process for the coated zirconium alkoxide sol, hydrolysis and condensation reactions take place, which lead to the formation of Zr–O–Zr network. As a result, the coated ZrO2 film has in fact interconnected tetragonal crystalline network after high calcination temperature (i.e., 500 ◦ C). On the other hand, many “cracks” of the ZrO2 film are also observed. Formation of such “cracks” was due to the uneven surface of graphite carbon material. Therefore, the structural integrity of the as-prepared ZrO2 film
Fig. 5. Element composition of the surface of the sonogel carbon electrode modified by ZrO2 film.
can be greatly affected by the surface morphology of the graphite carbon material. Fig. 5 shows the EDS composition analysis on the surface of the sonogel carbon electrode modified by ZrO2 film. Four types of elements can be observed: carbon, zirconium, silicon, and oxygen. The total molar ratio of oxygen (O) to (silicon (Si) + zirconium (Zr)) is near 2, which may suggest that there are ZrO2 and SiO2 on the surface of carbon electrode. Combined with the result of XRD crystalline phase (Fig. 1), EDS results further confirm that there is indeed ZrO2 film on the top of carbon electrode. On the other hand, it can be noticed that the total molar ratio of oxygen to (silicon + zirconium) was in fact 2.34, slightly higher than 2. This may be due to the existence of residue compositions containing C–O bond, induced by incomplete pyrolysis of organic compositions (i.e., acetylacetone) in the coated gel. Fig. 6(i) and (ii) shows the results of pore size distribution and nitrogen adsorption and desorption isotherms of (a) carbon material and (b) ZrO2 , respectively. Based in Fig. 6(ii) (a), sonogel carbon material is classified as type I (IUPAC), which suggests predominately microporous structure. A remarkable uptake and high-adsorbed volume at relative pressure from 0.01 to 0.1 were attributed to the existence of high volume of micropores in the carbon material [22]. On the other hand, based in Fig. 6(ii) (b), ZrO2 material was classified as type IV (IUPAC), which indicates mesoporous structure. ZrO2 film exhibited a narrow pore size dis-
Fig. 4. Scanning electron micrographs of the surface of ZrO2 film modified sonogel carbon electrodes.
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Fig. 6. (i) Pore size distribution curve of (a) carbon material and (b) ZrO2 (500 ◦ C calcination) and (ii) nitrogen adsorption and desorption isotherms of (a) carbon material and (b) ZrO2 .
Fig. 7. Geometry configuration of ZrO2 film immobilized on top of sonogel carbon electrode.
tribution with 2.6 nm of BJH adsorption average pore diameter (refer to Fig. 6(i) (b)). Formation of such a mesoporous structure was in fact attributed to inter-particle pore formation induced by “uniform” particle size [23,24]. The total pore volume of graphite carbon material was 0.0400 cm3 /g (see results reported in Table 1), while the micropore volume was 0.0223 cm3 /g. Compared with mesoporous volume, the slightly higher micropore volume of sonogel graphite carbon material suggests that there is a larger number of micropores in the sonogel graphite carbon, which is also in agreement with the result that the highest BJH adsorption pore volume corresponds to a pore size of less than 2 nm (refer to Fig. 6(i) (a)). In addition, considering some important factors, including low surface tension of ethanol solvent and high capillary force induced by large number of micropores in the sonogel graphite material, are beneficial to filling the pores of the graphite electrode by the precursor zirconium sol, it is believed that good interlocking of ZrO2 in the micro-mesopores of sonogel graphite carbon is another important reason for the good mechanical stability of the ZiO2 film (see Fig. 7). Moreover, good filling of ZiO2 into the pores of graphite leads to an enhanced interfacial contact area between ZrO2 crystallites and
graphite material, which is beneficial to effective electron transfer between them. Fig. 8(b–d) shows results of the electrochemical characterization of sonogel carbon electrode modified by zirconium dioxide for the detection of catechol. Sharp redox peak currents can be observed at such an electrode, even in the existence of ascorbic acid (a common interferent). The anodic and cathodic peak potentials for catechol were at 507 mV (Epa , anodic potential) and 436 mV (Epc , cathodic potential), respectively. The peak separation (Ep ) of catechol was only 71 mV, which is comparable to that in our former report [3]. From Fig. 8(c and d), it can be observed that the oxidation peaks of catechol and ascorbic acid are exceptionally resolved. Control test results showed that weak voltammetric redox peaks of catechol could be occasionally recorded at bare sonogel carbon electrodes. As an example, Fig. 8(a) shows one of the cyclic voltammograms of catechol at the bare sonogel carbon electrode. Therefore, the existence of ZiO2 film on the top of sonogel carbon electrode can lead to a good reversible electrochemical response to catechol. In addition, cyclic voltammogram of catechol at the sonogel carbon electrode modified by zirconium dioxide is similar in shape to that at the sonogel carbon electrode modified by porous titanium dioxide film [3,7], which suggests that there is the same reaction pathway at these electrodes [25]. Small Ep of catechol suggests fast heterogeneous electron transfer [26], which is induced by several factors, including high purity of tetragonal ZrO2 crystallites, interconnected crystal network, and larger interface contact area between ZrO2 nanocrystallites and graphite carbon material (see Fig. 7). Because catechol has groups containing oxygen, which can lead to a good affinity of this chemical to ZrO2 [13,27,28], strong interaction of catechol with the active sites of the as-prepared nanostructured ZrO2 film was an important prerequisite to lead to such a good electrochemical sensor behavior. Moreover, based on the above discussion on the properties of electrode materials, it is reasonable to conclude that good adhesion between ZrO2 film
Table 1 BET surface area and pore structure of ZrO2 film and sonogel graphite carbon matrix. BET surface area (m2 /g) ZrO2 Graphite a b c d
69.0 62.2
Total pore volumea (cm3 /g) 0.0531 0.0400
Single point adsorption total pore volume. Based on adsorption average pore width (4 V/A by BET). Based on density of 5.9 g/cm3 of ZrO2 . Based on density of 2.1 g/cm3 of graphite.
t-Plot micropore volume (cm3 /g) – 0.02227
Porosity c
23.9% 7.8%d
Pore sizeb (nm) 3.1 2.6
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Fig. 8. Cyclic voltammogram of bare sonogel carbon electrode (a) 5 mM catechol solution, 2 scans and cyclic voltammograms of ZrO2 modified sonogel carbon electrode (b) 5 mM catechol solution, 20 scans; (c) 5 mM catechol solution mixed with 5 mM ascorbic acid solution, 2 scans; (d) 5 mM catechol solution mixed with 5 mM ascorbic acid solution, 20 scans; scan rate 100 mV/s, electrolyte 0.1 M sulfuric acid.
and the graphite carbon matrix as well as the formation of ZrO2 film with special microstructure, including high BET surface area (69 m2 /g) and tetragonal crystal phase for high electrical conductivity, are other important reasons for good electrochemical behavior, excellent stability and reproducibility, good sensitivity, and less edge effect. Considering that the texture of ZrO2 film can be further improved by optimizing the preparation parameters in such a sol–gel route, further studies are underway to develop porous ZrO2 films with enhanced BET surface area and pore volume to improve the sensitivity of this kind of electrode for its application as electrochemical sensor. In addition, optimizing the texture of sonogel carbon matrix to improve interfacial contact area and structural integrity of ZrO2 film should not be ignored in the development of this type of electrode in the future. 4. Conclusions Sol–gel derived ZrO2 film formed from zirconium alkoxide precursor proved to be a valuable wide band gap metal oxide on the detection of catechol. Good electrochemical response is correlated to good physicochemical properties of as-prepared tetragonal ZrO2 film including robust crystal structure, high purity and interconnected crystal network of ZrO2 film, larger interfacial contact area and good adhesion between ZrO2 and graphite carbon material, and relatively high BET surface area for the adsorption of large number of catechol molecules. This study provided an alternative metal oxide based electrode material, which has been proven to be valuable in the fabrication of graphite carbon based electrode in the detection of neurotransmitters.
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Biographies Yongjun Chen received his Ph.D. degree in environmental engineering from the Department of Civil and Environmental Engineering at University of Cincinnati, United States, in 2007. During the time of this project, Dr. Yongjun Chen was a postdoctoral fellow working in the groups of Dr. Dionysiou and Dr. Lunsford. Recently, Dr. Yongjun Chen obtained the qualification of “Outstanding Young Teacher” in the College of Environmental Science and Engineering at the South China University of Technology in China. His current research interests include electrochemical sensors, advanced oxidation technologies, and dye-sensitized solar cells. Suzanne K. Lunsford received her B.S. degree in Chemistry from Xavier University in 1990. She received her Ph.D. from the University of Cincinnati under the tutelage of Dr. Harry B. Mark, Jr., in 1995. She is presently an Associate Professor in the Department of Chemistry at Wright State University in Dayton, Ohio. Her current research involves the development of modified conducting, and nanoparticle (sol–gel methods)-based working sensor electrodes to detect common biological molecules of interest utilizing electrochemical skills. Dionysios (Dion) D. Dionysiou is a Professor of Environmental Engineering and Science in the Department of Civil and Environmental Engineering at the University of Cincinnati, Cincinnati, Ohio. He received a B.S. degree (Diploma) from the National Technical University of Athens (N.T.U.A.) Greece, and an M.S. degree from Tufts University, both in Chemical Engineering. He received his Ph.D. degree in Environmental Engineering from the University of Cincinnati in 2001. He performs research in the fields of (i) advanced oxidation technologies, (ii) water quality, monitoring, treatment and purification, and (iii) environmental nanotechnology.