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Study on the Electrochemistry of a Na3[Ti2P2O10F] · xH2O-Modified Glassy Carbon Electrode Xiaopei Miao,†,‡ Guobao Li,† Jingbo Hu,‡ Zhiwei Zhu,† Yuanhua Shao,† and Meixian Li*,† College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China, and Department of Chemistry, Beijing Normal UniVersity, Beijing 100875, China ReceiVed: NoVember 25, 2008; ReVised Manuscript ReceiVed: January 20, 2009
A new oxyfluorinated titanium phosphate Na3[Ti2P2O10F] · xH2O (TiP)-modified electrode was prepared by casting the dispersion of TiP on a glassy carbon electrode. The electrochemical behavior of the modified electrode was investigated by cyclic voltammetry, and a couple of reduction/reoxidation peaks corresponding to the reduction of Ti (IV) were found at -1.29 V versus SCE in the 0.1 M NaCl solution, which was supported by UV-vis and FTIR characterization. The modified electrode showed electrocatalysis toward the reduction of H2O2. Furthermore, the addition of phosphate in the supporting electrolyte resulted in an easier reduction of Ti(IV) and direct electron transfer of cytochrome c at the modified electrode due to changes in the surface structure of the TiP films. The results indicate that TiP can act as not only a good electrontransfer mediator but also an electron-transfer promoter. 1. Introduction Inorganic phosphorus-containing materials have received much attention during the past few decades.1,2 Among these materials, titanium phosphates (TiPOs) have attracted more attention because they showed various properties and had various applications, such as in nonlinear optics,3,4 ion exchange,5 ion conductivity,6 and redox catalysis.7,8 In the TiPO structure, Ti and P atoms are octahedrally and tetrahedrally coordinated and linked via sharing corners to form various frameworks, including layers,9 open frameworks,10,11 mesoporosity,12 and amorphousness.13 The phosphate modification of titania has also been reported. Yu et al. found that the photocatalytic activity of TiO2 increased because of phosphate modification.14 Korosi et al. reported that phosphate groups bound to the surface of titania are responsible for enhancing photo-oxidation on P-TiO2 samples.15 Furthermore, phosphatebased molecular sieves16-18 with mostly a neutral framework have attracted considerable attention from academia and industry. Kapoor et al. obtained hexagonal mesoporous titanium aluminophosphate molecular sieves using a fluoride route.19 However, most of these reports emphasize only the structural features of these compounds. The electrochemical data related to them are limited to the literature.20,21 Our research interest is to investigate the electrochemical properties of titanium phosphates and explore their potential applications in electrocatalysis. In this work, a new oxyfluorinated titanium phosphate with an ionic conductive property whose structure was described in the literature,10 a Na3[Ti2P2O10F] · xH2O (TiP)-modified glassy carbon electrode (GCE), was prepared. Their electrochemistry was investigated, and their potential application was also explored. Cytochrome c (cyto c) is a very basic redox heme protein playing a crucial role in many biological processes in aerobic organisms such as photosynthesis, cell respiration, and apoptosis.22 Finding efficient promoters to accelerate the electron transfer between cyto c and the electrode is still a vital topic. * To whom correspondence should be addressed. E-mail: lmwx@ pku.edu.cn. Tel: +86-10-62757953. Fax: +86-10-62751708 † Peking University. ‡ Beijing Normal University.
Herein, we have investigated the direct electron transfer of cyto c in NaCl/PBS solution on the TiP-modified GCE. 2. Experimental Section 2.1. Chemicals. TiP has been synthesized under hydrothermal conditions in our laboratory according to our previous report.10 Its purity was 99%. Horse heart cytochrome c was purchased from Sigma. Phosphate buffer (PBS) (1/33 mol · L-1, pH 7) was made by mixing 1/33 mol · L-1 solutions of disodium hydrogen phosphate/ sodium dihydrogen phosphate (Merck). Other reagents were analytical grade. Water was triply distilled with a quartz apparatus. All chemicals were used without further purification. 2.2. Preparation of the Film-Modified Electrodes. A GCE was used as the working electrode. Before being used in electrochemical experiments and also before modification, the surface of the GCE was polished with fine emery paper and chamois leather soaked in an Al2O3 slurry and then rinsed thoroughly with distilled water and ethanol in an ultrasonic bath for a few minutes, respectively. Indium tin oxide (ITO)-coated glasses were employed as substrates for UV-vis and FTIR spectroscopic characterization. The ITO substrates were cleaned by sonication in acetone, ethanol, and ultrapure water for 15 min each and stored in water until use. The white solid powder of TiP was dispersed with the aid of ultrasonic agitation in water for about 1 h to give a 0.4 mg · mL-1 white suspension. The films were prepared by dropping a few microliters of suspension onto the GCE and air drying. The experiments showed that there was an optimal CV response for a certain amount of suspension. Below the optimal value, the current became insignificant, and above the value, the resistance of the films increased and the peak shape became worse. Thus, 35 µL of the suspension of TiP dripped onto the surface of the GCE was chosen (denoted as TiP/GCE). Similarly, the filmmodified electrodes were prepared by dropping 20 µL of the suspension onto the ITO substrates (the area is about 1 × 1 cm2) and drying under an infrared lamp (denoted as TiP/ITO). 2.3. Instrumentation. A CHI 660A electrochemical workstation (CH Instruments) with a conventional three-electrode cell was used to perform electrochemical measurements. The
10.1021/jp8103672 CCC: $40.75 2009 American Chemical Society Published on Web 02/23/2009
Electrochemistry of a Glassy Carbon Electrode
Figure 1. Multiple-scan CV of the TiP/GCE in 0.1 M NaCl. The cycles are 1, 2, 20, 50, and 70. The scan rate is 0.1 V/s.
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Figure 2. CV of the TiP/GCE in 0.1 M NaCl at scan rates of (a) 1, (b) 0.7, (c) 0.5, (d) 0.3, (e) 0.2, (f) 0.1, (g) 0.07, and (h) 0.05 V/s.
working electrode was a GCE with a diameter of 4 mm. A KCl saturated calomel electrode in an aqueous solution was used as the reference electrode, and a platinum electrode was used as the auxiliary electrode, respectively. All electrochemical experiments were conducted at ambient temperature (20 ( 2 °C). Prior to electrochemical experiments, the solutions were routinely deaerated by purging with high-purity nitrogen. Scanning electron microscopy (SEM) was done with a Quanta 200F SEM instrument. UV-vis spectroscopic measurements were performed using a UV-3100 spectrometer. FTIR spectra were recorded on a Nicolet Magna-IR 750 spectrometer. 3. Results and Discussion 3.1. Electrochemistry of TiP Films in NaCl Solution. Figure 1 displays cyclic voltammograms (CV) of the TiP/GCE in an aqueous solution containing 0.1 M NaCl as the supporting electrolyte in the potential range of 0 to -1.60 V. One pair of quasi-reversible redox peaks at E1/2 ) -1.29 V versus SCE is observed. Following the subsequent scanning process, the reduction peak current decreases first and then increases slowly while the reoxidation peak current increases the entire time. They tend to be relatively stable after about 70 cyclic scans. Compared to CV peaks, much better developed peaks were obtained by differential pulse voltammetry (Figure S1 in Supporting Information), in which the background current was largely eliminated. Sakai et al. reported that the titania nanosheet electrode underwent reduction/reoxidation of Ti4+/Ti3+ at around -1.2 to -0.9 V in a propylene carbonate solution containing 0.1 mol · dm-3 LiClO4,21 and we conclude that the redox peaks correspond to the one-electron transfer reduction of Ti(IV) to Ti(III), which is supported by UV-vis and FTIR spectroscopies discussed below. The cyclic voltammograms for the TiP/GCE at different scan rates were examined, as shown in Figure 2. The reduction peak current is linear, with the square root of the scan rate in the range of 0.05-1 V/s, indicating a diffusion-controlled electrochemical process. When continuous potential cycling between 0 and -1.6 V at a scan rate of 0.1 V/s for the TiP/GCE was tested, no obvious change in the peak currents was observed after more than 100 complete CV cycles. Moreover, the relative standard deviation of electrochemical responses is 2.4% for six replicate preparations of the TiP/GCE. These results suggest that the modified electrode has good stability and reproducibility. In view of the well-defined electrochemical properties of the TiP/GCE in 0.1 M NaCl aqueous solution as mentioned above, the electrocatalysis of H2O2 at the modified electrode was also explored.
Figure 3. CV of the TiP/GCE in 0.1 M NaCl in the (a) absence and in the presence of (b) 0.2 mM H2O2 and (c) 0.6 mM H2O2.
CVs obtained with the TiP/GCE between 0 and -1.7 V in 0.1 M NaCl with the addition of H2O2 are shown in Figure 3. The voltammograms showed an increase in the reduction peak current and a decrease in the reoxidation peak current in the presence of H2O2, which indicate that TiP at the modified electrode can act as an electron-transfer mediator for the electrocatalysis of H2O2. However, the structure of the TiP film will be damaged by the higher concentration of H2O2, leading to the collapse of the structural framework of TiP, as described in the literature.23,24 3.2. Direct Electrochemistry of Cytochrome c at the TiP/ GCE. The study of the direct electron transfer of proteins at the modified electrodes has been a hot topic in understanding the redox properties of proteins and also in developing biosensors without mediators.25-29 Herein, we investigated the electrochemistry of cyto c at the TiP/GCE due to the good framework of TiP. Almost no redox peaks were observed at the TiP/GCE in 0.1 M NaCl solution containing 4 × 10-5 M cyto c, which indicates that TiP at the modified electrode cannot promote the electron-transfer rate of cyto c under such conditions, possibly because of protein denaturation at the electrode surface leading to extremely slow electron-transfer kinetics. However, when adding some amount of phosphate buffer solution to 0.1 M NaCl to maintain a constant pH, one pair of redox peaks at E1/2 ) 25 mV versus SCE corresponding to the one-electron reduction and oxidation of cyto c was observed, as shown in Figure 4. Oxidation and reduction peak currents were almost identical, and they are both proportional to the square root of scan rate in the range of 0.01-0.1 V/s, indicating a diffusion-controlled electrochemical process. Moreover, the
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Figure 4. CV of 4 × 10 0.03 M PBS (pH 6.5).
M cyto c at the TiP/GCE in 0.1 M NaCl/
Figure 5. Correlation between Epc and the pH of NaCl/PBS (pH 5.1, 5.9, 6.5, and 7.8).
peak currents underwent no obvious decrease after continuously scanning 20 circles at a scan rate of 0.1 V s-1. These results indicated that the TiP at the TiP/GCE could act as an electrontransfer promoter to accelerate the electron-transfer rate of cyto c in the presence of phosphate. In the meantime, it was found that the redox peak potentials for the TiP/GCE shifted a little toward the more positive direction with the addition of phosphate, which suggests that the reduction of Ti(IV) tends to be easier. To elucidate whether pH or phosphate had an effect on the shift of redox peak potentials, the effects of pH and phosphate on the electrochemistry of the TiP/GCE was investigated. To keep the TiP’s structure stable, we tested pH values in the range of 5-8. The pH values of the electrolyte solution were adjusted by adding different pH values of phosphate to 0.1 M NaCl, but the concentration of phosphate was kept constant. It was found that the reduction peak potential Epc was linear with pH value in the test range; that is, Epc shifted positively with decreasing pH, as shown in Figure 5. The linear regression equation was Epc(V) ) -0.205-0.12pH, and the variation of the reduction potential shift with the change in pH is larger than the theoretical value of the Nernst response (59 mV/pH, 25 °C) for the equivalent proton-coupled equivalent electron-transfer reaction, which indicates that the proton might be involved in the formation of hydrogen bonds as a result of the existence of fluorine atoms and oxygen atoms, leading to the change in surface structure of the TiP films and the easier
Miao et al.
Figure 6. SWV of TiP/GCE in NaCl/PBS (pH 5.9) with different concentrations of H2PO4-/HPO42-: (a) 0 M, (b) 10-5 M, (c) 6 × 10-5 M, (d) 10-4 M, (e) 2 × 10-4 M, (f) 4 × 10-4 M, (g) 10-3 M, and (h) 10-2 M.
reduction of Ti(IV). However, when the electrolyte solution was 0.1 M NaCl (pH ∼6) without phosphate, the reduction peak potential appeared at -1.29 V, which exceeded the expected value in Figure 5. We conclude that the existence of phosphate plays an important role in the electrochemical reduction of the TiP films. To verify this, we investigated the effect of the concentration of phosphate on the peak potentials. Figure 6 shows square wave voltammograms of the TiP/GCE in 0.1 M NaCl solution containing different concentrations of PBS with a constant pH value at 5.9, and the effect of the proton is essentially excluded. It was found that the peak potential shifted positively with the increase in the concentration of phosphate and then reached a plateau when the concentration of phosphate was more than 1.0 × 10-3 M, which indicates that the existence of phosphate is helpful for the reduction of TiP. One possible reason is that the structure of the TiP films changes during the electrochemical reduction process in the presence of phosphate. To confirm this further, we performed SEM characterization. Figure 7 displays SEM images of the TiP films in 0.1 M NaCl before and after 10 CV scans as well as the films in 0.1 M NaCl containing 0.03 M PBS (pH 5.1) after 10 CV scans. Compared with SEM images of the TiP films in 0.1 M NaCl, the surfaces of particles originating from the aggregation of TiP in 0.1 M NaCl containing 0.03 M PBS (pH 5.1) became rough, and the edges of the particles were irregular, which indicates that the change in the form of the TiP aggregates leads to the change in the structure of the TiP films, similar to literature reports.30,31 This is one possible reason that the reduction of the TiP films is easier and direct electron transfer of cyto c is obtained in the electrolyte solution containing phosphate. To confirm the electrode reaction mechanism of the TiP films, the TiP/ITO was electrolyzed in the NaCl/PBS (pH 5.9) solution using controlled potential electrolysis at -1.0 V for about 1.5 h. After electrolysis, the TiP films on the ITO electrode underwent a significant color change. The initially colorless films turned gray-dark blue, similarly to those reported in the literature,32 which indicates the reduction of Ti(IV). To verify further the existence of Ti3+ in the films after electrolysis, we performed UV-vis characterization. Figure 8 shows UV-vis spectra for the TiP films on the ITO electrode before and after electrolysis. The experimental results demonstrate that the initial TiP films almost have no absorption in the visible region (>400 nm) whereas the TiP films have significant absorption between 600 and 350 nm after electrolysis. These are consistent with those
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Figure 7. SEM of TiP films (a) before the electrochemical scan and after 10 CV scans in (b) 0.1 M NaCl or (c) 0.1 M NaCl/0.03 M PBS (pH 5.1).
Figure 8. UV-vis of TiP/ITO (a) before and (b) after electrolysis in the solution of NaCl/PBS (pH 5.9) using a controlled potential at -1.0 V for about 1.5 h.
reported in the literature,33-35 that is, the existence of oxygen vacancies corresponding to Ti3 + helps the absorption edge extend to visible light, which suggests the one-electron transfer reduction of Ti(IV) to Ti(III) in the TiP films during the electrochemical process. To assist the identification of the electrolysis products, FTIR was also used as a characterization technique based on the changes in several different absorption bands.
Figure 9. Micro-FTIR of TiP/ITO (a) before and (b) after electrolysis in the solution of NaCl/PBS (pH 5.9) using controlled potential electrolysis at -1.0 V for about 1.5 h.
As shown in Figure 9a, before electrolysis, TiP/ITO has a weak, broad band in the hydroxyl region with its maximum at 3480 cm-1 corresponding to O-H stretching vibrations of the residual water, OH-, and defective OH-.36 After electrolysis in the solution of NaCl/PBS (pH 5.9) using a controlled potential at -1.0 V for about 1.5 h, the absorption band of hydroxyls for both dissociated water and molecularly adsorbed water shifts to 3301 cm-1, as shown in Figure 9b. The characteristic band
4596 J. Phys. Chem. C, Vol. 113, No. 11, 2009 of H2O at 1654 cm-1, pertaining to H-O-H bending for adsorbed water molecules,37-39 also becomes relatively larger and more intense than that for the sample before electrolysis. These indicate the existence of Ti3+ defect sites after electrochemical reduction. The created Ti3+ defect sites were healed by the adsorption of the hydroxyls (dissociated water) that coexisted with the additionally adsorbed molecular water. In addition, strong spectra in the region centered at 1053 cm-1 from Ti-O-P skeletal stretching vibrations and 874 cm-1 from Ti-O-Ti decrease significantly.12 These can be ascribed to the formation of oxygen vacancies diminishing the contribution of the Ti-O bonds.40 4. Conclusions The TiP films prepared by casting the dispersion of TiP on the GCE showed a quasi-reversible reduction peak corresponding to the reduction of Ti(IV) in the TiP films, which was verified by UV-vis and FTIR characterization. The effect of pH values and the concentration of phosphate in the electrolyte solution on the reduction of the TiP films were investigated, and the experimental results indicated that the increase in the concentrations of the proton and phosphate changed the surface structure of the TiP films, resulting in easier reduction of the TiP films. Furthermore, direct electron transfer of cyto c at the TiP/GCE was observed with the addition of phosphate. The TiP films also showed electrocatalysis toward the reduction of H2O2. These suggest that TiP can act not only as a good mediator but also as a good promoter, showing potential application in catalysis. Acknowledgment. This project was supported by the National Nature Science Foundation of China (grants 20875005 and 20735001). Supporting Information Available: Differential pulse voltammogram of the TiP/GCE in 0.1 M NaCl at a scan rate of 20 mV/s. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Clearfield, A. Annu. ReV. Mater. Sci. 1984, 14, 205. (2) Clearfield, A., Ed. Inorganic Ion Exchange Materials; CRC Press: Boca Raton, FL, 1982. (3) Masse, R.; Grenier, J. C. Bull. Soc. Fr. Mineral. Cristallogr. 1971, 94, 437. (4) Satyanarayan, M. N.; Deepthy, A.; Bhar, H. L. Crit. ReV. Solid State Mater. Sci. 1999, 24, 103.
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